the role of the interphase in 3d printed photo-cured polymers€¦ · materials engineering and...
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POLITECNICO DI MILANO
School of Industrial and Information Engineering
Department of Chemistry, Materials and Chemical Engineering “Giulio
Natta”
Master of Science in
Materials Engineering and Nanotechnology
The role of the interphase in 3D printed photo-cured
polymers
Supervisors:
Prof. Luca ANDENA
Prof. Francesco BRIATICO VANGOSA
Lorenzo DE NONI
876133
Academic Year 2017 – 2018
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Ringraziamenti
Vi ringrazio, Luca e Francesco, per la disponibilità e la gentilezza con cui mi avete seguito nel mio percorso
di tesi e per gli stimoli che mi avete trasmesso. Un ringraziamento anche a Stefano e Marco, per il vostro
aiuto. Tutto ciò non è né dovuto né scontato.
Un abbraccio va ai voi, papà e mamma, vi ringrazio per la forza con cui mi siete sempre stati vicino. Ti
ringrazio mamma per avermi donato il tuo bellissimo sorriso, non poteva esserci accoglienza migliore a fine
giornata. Un bacione va anche a te, papà, so quanto tu tenga a noi. Un pensiero va a voi, Andrea e Francesca.
Quante volte abbiamo riso, quante abbiamo litigato, mi avete sempre insegnato molto. Grazie per avermi
ascoltato e per ascoltarmi quando ne ho bisogno. Ringrazio tutti voi per la fiducia che mi dimostrate.
Voglio ringraziare anche te, Giacomo. Se tutto ciò per i miei familiari poteva essere spontaneo, ebbene
sembra lo sia stato anche per te. Mi hai insegnato il valore dell’amicizia. Un mio pensiero anche a te,
Stefano: grazie per il supporto, i consigli e la fiducia.
Ringrazio infine tutti coloro con cui ho condiviso questi 6 anni.
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Contents
1. Introduction ............................................................................................................................................. 10
1.1 State of the art ...................................................................................................................................... 10
1.2 3D printing applications ........................................................................................................................ 13
1.2.1 Medical applications ...................................................................................................................... 13
1.2.2 Bio-inspired applications ............................................................................................................... 13
1.3 Polyjet 3D printer .................................................................................................................................. 15
1.4 Aim and goals ........................................................................................................................................ 17
2. Preliminary results ...................................................................................................................................... 18
2.1 Introduction ........................................................................................................................................... 18
2.2 Equipment and material ........................................................................................................................ 18
2.3 Sample preparation ............................................................................................................................... 20
2.3.1 Samples for nano-indentation test .................................................................................................. 20
2.3.2 Samples for dynamic mechanical analysis: 3-point bending configuration ................................... 21
2.4 Experimental procedure ........................................................................................................................ 21
2.4.1 Nano-indentation test ..................................................................................................................... 21
2.4.2 Optical microscopy ......................................................................................................................... 22
2.4.3 DMA test ......................................................................................................................................... 22
2.5 Results and discussion ........................................................................................................................... 23
2.5.1 Nano-indentation and SEM results ................................................................................................. 23
2.5.2 DMA results .................................................................................................................................... 23
2.6 Discussion and aim ................................................................................................................................ 26
2.7 Composite models .................................................................................................................................. 27
3. Experimental procedure .............................................................................................................................. 29
3.1 Materials and equipment ....................................................................................................................... 29
3.2 Samples .................................................................................................................................................. 29
3.2.1 VeroWhitePlus RGD835 sample..................................................................................................... 30
3.2.2 TangoBlackPlus FLX930 sample ................................................................................................... 31
3.2.3 Composite sample ........................................................................................................................... 31
3.3 Optical microscopy ................................................................................................................................ 35
3.4 Dynamic Mechanical Analysis (DMA) .................................................................................................. 38
4. Preliminary results ...................................................................................................................................... 40
4.1 Effect of curing conditions ..................................................................................................................... 40
4.2 Effect of calibration of UV lamp ............................................................................................................ 41
4.3 Effect of the layer arrangement ............................................................................................................. 43
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5. Results and discussions ............................................................................................................................... 44
5.1 Samples composition ............................................................................................................................. 44
5.2 Constituent materials ............................................................................................................................. 45
5.3 Composite samples ................................................................................................................................ 47
5.3.1 AL samples ...................................................................................................................................... 47
5.3.2 AT samples ..................................................................................................................................... 48
5.3.3 AW samples .................................................................................................................................... 50
5.3.4 AT samples with varying composition ............................................................................................ 53
6. Mechanical model ....................................................................................................................................... 55
6.1 Mathematical procedure ....................................................................................................................... 55
6.2 Results and discussion ........................................................................................................................... 55
6.3 Further considerations .......................................................................................................................... 61
7. Conclusions ................................................................................................................................................. 62
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Index of figures
Figure 1. Process development cycle [2]. ....................................................................................................... 10
Figure 2. Example of faceted representation of the exterior surface of the object [3]. .................................. 10
Figure 3. Example of CAD design and creation of STL file [4]. ..................................................................... 11
Figure 4. Rapid prototyping growth rate in the last two decades [2]. ............................................................ 11
Figure 5. Classification of AM processes [6]. ................................................................................................. 12
Figure 6. Radiographic images can be translated into .stl CAD files [9]. ...................................................... 13
Figure 7. Examples of TPMS IPC structures [12]. ......................................................................................... 14
Figure 8. Picture of Polyjet 3D printing process [24] .................................................................................... 15
Figure 9. Example of functionally graded structure with gradient transitions of material A and B [27]. ..... 17
Figure 10. Interface is formed before (on the left) or after (on the right) curing step [personal
communication]. .............................................................................................................................................. 18
Figure 11. Bi-material cylinders for nano-indentation test [personal communication]. ................................ 20
Figure 12. Beam-like sample for dynamic mechanical analysis [personal communication]. ......................... 21
Figure 13. Nano-indentation test parameters [personal communication]. ..................................................... 22
Figure 14. Nano-indentation test configuration [personal communication]. ................................................. 22
Figure 15. 3-point bending configuration for dynamic mechanical analysis [personal communication] ...... 22
Figure 16. Nano-indentation and optical microscope results: a) sharp interphase; b) thick interphase
[personal communication]. ............................................................................................................................. 23
Figure 17. Relative volumetric fraction of components in samples where a sharp or wide interphase is
obtained. .......................................................................................................................................................... 24
Figure 18. VeroWhitePlus RGD835 damping and elastic properties (on the left); TangoBlackPlus FLX930
damping and elastic properties (on the right). ................................................................................................ 24
Figure 19. Damping and elastic properties of laminate samples with: a) 2 layers (brown curve); b) 2 and 4
layers (orange curve); c) 2, 4 and 6 layers (light green curve); d) 2, 4, 6 and 8 layers (green curve). Dotted
lines represent pure rubbery and glassy materials as a reference [personal communication]. ..................... 25
Figure 20. Damping and elastic properties of laminate samples with: a) 2 layers (brown curve); b) 2 and 4
layers (orange curve); c) 2, 4 and 6 layers (light green curve); d) 2, 4, 6 and 8 layers (green curve). Dotted
lines represent pure rubbery and glassy materials as a reference [personal communication]. ..................... 26
Figure 21. Idealization in “solid blocks” of fibers and matrix in composites: uniaxially aligned fiber in
continuous matrix. ........................................................................................................................................... 27
Figure 22. Same component can be printed in: "flat-printing" mode on the left; “wall-printing” mode on the
right. ................................................................................................................................................................ 30
Figure 23. From CAD file to final component ................................................................................................ 30
Figure 24. "Along thickness" sample. ............................................................................................................. 31
Figure 25. "Along width" sample. ................................................................................................................... 32
Figure 26. "Along length" sample. .................................................................................................................. 32
Figure 27. Example of symmetric sample. ...................................................................................................... 33
Figure 28. Example of AW sample with separated glassy and rubbery layers. .............................................. 35
Figure 29. Transmitted light microscope: "flat-printed" AW samples with 16 layers. ................................... 36
Figure 30. Transmitted light microscope: "flat-printed" AW samples with 16 layers. ................................... 36
Figure 31. ImageJ analysis: RGB intensity plot for "flat-printed" AW samples. ............................................ 37
Figure 32. Transmitted light microscope: “flat-printed” AW samples with 32 layers. .................................. 37
Figure 33. DMA input and output signals [30]. .............................................................................................. 38
Figure 34. Example of data repeatability: superimposed storage modulus curve of "flat-printed" AW samples
with 8 layers (on the left); average curve with standard deviation (on the right). .......................................... 40
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Figure 35. Storage modulus trends for AT sample with 2 layers: first DMA test (light grey); second DMA test
(grey) . ............................................................................................................................................................. 41
Figure 36. "Before"(BC) and "after" (AC) calibration samples: elastic and damping properties results. ..... 42
Figure 37. "Before" (BC) and "after calibration" (AC) samples: damping properties. .................................. 42
Figure 38. "Before" (BC) and "after calibration" (AC) samples: elastic (left) and damping (right) properties.
......................................................................................................................................................................... 43
Figure 39. Elastic and damping properties of two variants of AT samples with 2 layers: glassy and rubbery
outer plies. ....................................................................................................................................................... 43
Figure 40. Volumetric content of the interphase as a function of the sample configuration and printing
direction. .......................................................................................................................................................... 44
Figure 41. Strain sweep test: rubbery material’s modulus vs strain. .............................................................. 46
Figure 42. Rubbery material: damping and elastic properties. ...................................................................... 46
Figure 43. Comparison of elastic properties of AL samples and rubbery material. ....................................... 47
Figure 44. Damping properties of AL samples with 2, 4 and 8 layers. ........................................................... 48
Figure 45. Comparison of storage modulus of AT samples with 2, 4 and 8 layers. ........................................ 49
Figure 46. Damping properties of AT samples with 2, 4 and 8 layers. ........................................................... 49
Figure 47. Comparison between storage modulus curves of constituent materials (solid lines) and "wall-
printed" AW samples. ...................................................................................................................................... 50
Figure 48. Damping properties of AT samples with 2, 4 and 8 layers. ........................................................... 51
Figure 49. Comparison between storage modulus curves of constituent materials (solid lines) and “flat-
printed” AW samples with 8, 16 and 32 layers. .............................................................................................. 52
Figure 50. Tan delta curves of "flat-printed" AW samples. ............................................................................ 53
Figure 51. Samples with volumetric fraction of the rubbery material equal to 0,5, 0,625, 0,750, 0,875, 0,92
and 0,95: damping properties. ........................................................................................................................ 54
Figure 52. Samples with volumetric fraction of the rubbery material equal to 0,5, 0,625, 0,750, 0,875, 0,92
and 0,95: damping properties. ........................................................................................................................ 54
Figure 53. Theoretical model for the AL sample with 2 layers. ...................................................................... 56
Figure 54. Theoretical model for the AL sample with 4 layers. ...................................................................... 56
Figure 55. Theoretical model for the AL sample with 8 layers. ...................................................................... 57
Figure 56. Theoretical model for the "wall-printed" AW sample with 8 layers. ............................................. 57
Figure 57. Theoretical model for the "wall-printed" AW sample with 16 layers. ........................................... 58
Figure 58. Theoretical model for the "wall-printed" AW sample with 32 layers. ........................................... 58
Figure 59. Theoretical model for the "flat-printed" AW sample with 16 layers. ............................................ 59
Figure 60. Theoretical model for the "flat-printed" AW sample with 16 layers. ............................................ 60
Figure 61. Comparison between theoretical model for AT and AW samples. ................................................. 61
Figure 62. Example of the obtained AW sample with 2 separated layers. ...................................................... 61
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Index of tables
Table 1. Material properties for Polyjet Stratasys's machine [6]. .................................................................. 16
Table 2. Datasheet of Objet 260 Connex2 3D printer [25]. ............................................................................ 19
Table 3. Datasheet of VeroWhitePlus (RGD835) material [25]. .................................................................... 20
Table 4. Datasheet of TangoBlackPlus (FLX930) [25]................................................................................... 20
Table 5. Glassy material: sample dimensions. ................................................................................................ 31
Table 6. Rubbery material: sample dimensions. ............................................................................................. 31
Table 7. "Along length": sample dimensions. .................................................................................................. 33
Table 8. "AW" sample dimensions. .................................................................................................................. 34
Table 9. Dimensions of samples with different volumetric composition. ........................................................ 34
Table 10. Dimensions of the AW samples with separated layers. ................................................................... 35
Table 11. ImageJ analysis: interphase dimension. .......................................................................................... 37
Table 12. Volumetric fraction of the components in AL samples. ................................................................... 44
Table 13. Volumetric fraction of the components in AT samples. ................................................................... 44
Table 14. Volumetric fraction of the component in AW samples..................................................................... 45
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Summary Additive manufacturing (AM), in particular 3D printing, gained a lot of interest in the past few years. This
work is focused in particular on the Polyjet 3D process by means of which photo-curable polymers with
strongly different physical and mechanical properties can be injected (in the form of liquid droplets) and
cured through the use of a UV lamp. This technique is especially useful for the production of complex
structures, such as so called “interpenetrating phase composites” (IPCs) in which two or more phases are
interconnected in three dimensions and construct a topologically continuous network throughout the
microstructure. A recent work showed features which had been completely neglected in previous researches:
an inter-diffusion phenomenon between constituent photo-curable polymers in their liquid state that occurs
during the deposition process of liquid droplets as a function of the printing setup. As a consequence, an
interphase forms. The present work is the continuation of the study of the effect of the interphase on the
overall mechanical properties of 3D printed composite materials.
Two photo-curable polymers, a glassy polymer (VeroWhitePlus RGD835) and a rubbery one
(TangoBlackPlus FLX930), were used for the production of the desired specimens. Samples were designed
in the form of laminates in different configurations and printed along different printing directions in order to
generally evaluate the role of the “interphase”. The surface of the laminates was analyzed through the use of
an optical microscope. Dynamic mechanical analysis (DMA) in tensile configuration was carried out to
evaluate the elastic properties and the main thermal transition (i.e. glass transition temperature) of the
composite laminates. An oscillatory strain (0,1%) was applied in order to evaluate the storage modulus
values as a function of the temperature within polymer linear regime. To do so, a temperature ramp was
applied from -50 °C to 80 °C. An analytical model was used to predict theoretical elastic properties from the
obtained experimental values. Laminate damping properties were evaluated, too. The present research opens
the way for further studies in order to better characterize the mechanical behavior of 3D printed photo-
curable polymers.
Sommario
L’“Additive manufacturing” (AM) (produzione additiva), nello specifico la stampa 3D, ha attratto l’interesse
dei ricercatori negli ultimi anni. In particolare, questo lavoro di tesi si focalizza sul processo 3D Polyjet,
attraverso il quale polimeri foto-polimerizzabili, con proprietà fisiche e meccaniche significativamente
differenti, possono essere depositati su un piano di lavoro e polimerizzati mediante l’uso di un’apposita
lampada UV. Questa tecnica risulta essere molto utile per produzione di strutture complesse, il cui esempio
principale è rappresentato dai cosiddetti “compositi a fasi interpenetrate” (“interpentrating phase
composite”, IPCs), nei quali due o più fasi sono interconnesse nello spazio e costituiscono un network
continuo attraverso l’architettura del componente. Una recente ricerca ha evidenziato aspetti interessanti che
erano stati completamente trascurati fino ad oggi: si è dimostrata la presenza di un fenomeno di inter-
diffusione, in funzione della direzione di stampaggio, tra i foto-polimeri quando depositati allo stato liquido
sotto forma di gocce. Conseguenza di ciò è la formazione di un’”interfase”. Il seguente lavoro di tesi
prosegue nel lavoro di studio dell’effetto di questa “interfase” sulle proprietà meccaniche macroscopiche di
materiali compositi ottenuti per stampa Polyjet 3D.
Due polimeri foto-polimerizzabili, un polimero vetroso (VeroWhitePlus RGD835) e uno gommoso
(TangoBlackPlus FLX930) , sono stati usati per la produzione dei campioni. I campioni sono stati progettati
in forma di laminati, con configurazioni e direzioni di stampaggio differenti, per studiare nella maniera più
generale l’effetto dell’”interfase” sulle proprietà meccaniche. La superficie dei campioni è stata analizzata
con un microscopio ottico. L’analisi dinamico-meccanico è stata eseguita in trazione al fine di valutare le
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proprietà elastiche e le principali transizioni termiche (in particolare, la temperatura di transizione vetrosa)
dei campioni compositi. Una deformazione oscillatoria (0,1%) è stata applicata allo scopo di analizzare le
proprietà elastiche in funzione della temperatura in campo lineare. Per fare ciò, i campioni sono stati
analizzati da -50 °C a 80 °C applicando un’apposita rampa di temperatura. Le proprietà elastiche teoriche
sono state previste dai dati sperimentali attraverso l’utilizzo di un apposito modello analitico. In parallelo, si
è eseguita l’analisi delle proprietà dissipative. La ricerca di seguito presentata apre la via a studi futuri per
meglio caratterizzare le proprietà meccaniche di compositi composti da polimeri foto-polimerizzabili ottenuti
tramite stampa 3D (Polyjet).
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1. Introduction
1.1 State of the art
Additive manufacturing (AM), also referred to “3D printing” or “rapid prototyping” (RP), was first
introduced by Hull [1] in the 1980’s, when the idea of generating physical objects through layer-by-layer
deposition of material was developed [2]. Hence, in contrast with the more common subtractive techniques,
which involve removing sections of material by cutting or machining it away, 3D printing consists in the
addition of layers of material to produce the desired component. The term “rapid” also relates to the ease of
making an object due to the simplicity of designing it through a “Computer-Aided Design” (CAD) software
that controls the layer-by-layer shape production. Instead, the word “prototyping” first referred to this
process as too slow for its use in mass production (in contrast with other technologies, such as injection
molding, which allows for large quantities production at low cost per-unit product) [3]. The production
process can be briefly described as depicted in Figure 1 [2] .
Figure 1. Process development cycle [2].
i) Creation of a 3D computer model with a CAD software.
ii) The 3D component model is subsequently converted into a Surface Tessellation Language (STL)
file, which is a triangular representation of the 3D surface geometry. This means that the surface
is “tessellated” into a series of oriented triangles containing spatial information of the object
(Figure 2) [4]. The main disadvantage of using polygons to “tessellate” object surfaces is related
to the level of precision the solid object is approximated to.
Figure 2. Example of faceted representation of the exterior surface of the object [3].
iii) An AM-machine-software slices the STL file into several layers (Figure 3) [4], which are then
used to produce the component layer-by-layer via the selective deposition of the chosen material.
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Figure 3. Example of CAD design and creation of STL file [4].
iv) Layer-by-layer deposition of chosen material and the consolidation of the deposited layers (each
layer is representing a slice of the STL file) [4]. Subsequently, the so-produced prototype (hence,
“rapid prototyping”) is finally tested in order to evaluate whether it meets design criteria or not;
the process allows for reduction of cost and time of production as a single prototype can be
easily produced without the need of post-processing.
As reported by Wong et al. [2], AM techniques have gained interest among the scientific community over
the years. According to Wohler report (2010) [5], the growth rate began to significantly increase since the
2000’s and in 2010 was about 24% higher with respect to the previous year (figure 4).
Figure 4. Rapid prototyping growth rate in the last two decades [2].
As a matter of fact, the enormous development of this advanced technology has outgrown its prototyping
roots in the past few years. In this sense, a large number of 3D printing techniques have been developed and
improved over the past three decades. As stated by Lee et al. [6], 3D printing processes can be classified into
seven categories (Figure 5):
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.
Figure 5. Classification of AM processes [6].
1) Powder bed fusion, in which a thermal source such as laser is used to induce partial or complete melting
between powder particles followed by a roller or blade re-coater to add and smooth another powder layer.
The binding mechanism is mainly sintering or melting.
2) Sheet lamination, in which sheet material are cut by a laser; each laminate sheet can be considered as
one of the cross-sectional layer of the final object.
3) Vat polymerization, in which photocurable resins are exposed to a laser and undergo a chemical process
(photo-polymerization) in order to solidify.
4) Binder jetting, in which material is deposited in the form of powder particles and are joined through the
selective deposition of a liquid bonding agent, resulting in the generation of the component due to the gluing
of powders.
5) Directed energy absorption, used for metals, in which a high power laser is focused on a continuous
stream of material in order to melt the material as it is deposited.
6) Material extrusion, in which material is pulled out through a nozzle by the application of a pressure
onto a substrate where solidification occurs.
7) Material Jetting (Polyjet technology), in which liquid droplets of constituent materials are deposited
layer-by-layer on the working tray, where they subsequently solidify as one piece. Sub-chapter 1.3 of this
thesis will be focused on the main features of this technique.
The significant interest in 3D printing techniques is probably due to the several advantages these techniques
feature. As reported by Gao et al. [7], each of the above presented process has its own advantages, which can
generally be gathered into the following points by comparing them to the conventional subtractive
manufacturing technique:
- Design flexibility, i.e. the ability to produce shapes with a high degree of complexity since fewer
constrains have to be imposed during working process, as compared to the ones typical of subtractive
manufacturing technique. These include the need of fixtures, the possibility of having tooling contact or
difficulty of the cutter to reach deeper zones. These features enable the making of topologically
optimized structures.
- Low cost of geometric flexibility, since the geometrical complexity has no additional needs in terms of
further tooling operations, increased operator expertise or fabrication time.
- No need for assembly, i.e. the production of single-assembly that would otherwise require the assembly
of multiple components. Parts are printed in place and supported by “support material”, which can be
removed at the end of the process.
- Time and cost efficiency in production run, suited for “low-part production”; AM can lower inventory
costs and reduce the cost of supply chain and delivery. The “buy-to-fly” ratio, i.e. the ratio of the
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purchased material over the amount of material found in the final component, is very low meaning very
little wasted material.
To conclude, it is expected that AM will revolutionize the manufacturing field, providing a large number of
benefits to the society, such as [8]:
- healthcare customized products as a function of the individual needs;
- reduced raw material usage and low energy consumption, coupled with low amount of waste materials
for an “environment friendly” technique;
- on-demand manufacturing.
In light of this, the next chapter of this thesis focuses on the way AM process abilities could be exploited.
1.2 3D printing applications
1.2.1 Medical applications
3D manufacturing has been primarily used for decades in the production of prototypes or molds. Advances in
this field have enabled this technology to be exploited in a wider range of applications. For instances, 3D
printing technology has become affordable by consumers since it is cheap, user-friendly and many
downloadable CAD software are nowadays available at low price [9]. In the meantime, many companies
employing 3D printers for production processes emerged. In the medical field, 3D printing allows for
custom-made implants, living human tissue, fixtures and surgical tools for use in the operating rooms having
a positive impact on the success of surgery procedures. As a matter of fact, 3D printing is used for the
bio-printing of organs and tissues. Organ transplantation requires tissue matching between the donor and
patient: by 3D printing the replacement organ with cells taken from organ transplant patient’s own body, the
problem could be easily eliminated minimizing the risk of tissue rejection. Furthermore, implants and
prostheses can be made in any possible geometry through the translation of X-ray, computerized tomography
(CT) or magnetic resonance imaging (MRI) scans into “.stl” CAD files [8] allowing for excellent
dimensional accuracy of the printed component (figure 6).
Figure 6. Radiographic images can be translated into .stl CAD files [9].
3D printing technology is also exploited in the production of anatomical models available for physicians to
study or to make practice with. Personalized 3D-printed drugs can also be produced with particular benefits
for patients who are known to have a pharmacogenetic polymorphism. Generally speaking, the medical field
is a breeding ground for 3D printing applications.
1.2.2 Bio-inspired applications
Nowadays, a new way to exploit 3D printers is attracting the interest of scientists, taking inspiration from
natural structures. For instance, the structure of wood is a clear example of a multi-phase soft core-hard shell
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composite made of an inner core, which allows the transportation of water and other nutrients, in addition to
an outer layer whose main goal is to mechanically support the structure. Its properties are therefore strictly
dependent on its co-continuous composite structure. In this sense, the idea is to produce lightweight co-
continuous composite structures via 3D printing, that exhibits a strong relationship between topology
(architecture) and properties, i.e. the macroscopic physical properties are controlled by constituent materials
and by the exhibited cellular architecture, in the same way occurring in wood structures. 3D printers
nowadays look like the best technique to create this new kind of co-continuous composite materials, which
are also known as interpenetrating phase composites (IPC) (i.e. “two phases or more that are interconnected
in three dimensions and construct a topologically continuous network throughout the microstructure”) [10].
Of great interest among IPC structures are the triply periodic minimal surfaces (TPMS), which are “surfaces
which have a minimum area with zero mean curvature H = (k1+k2)/2 at every point” (where k1 and k2 are the
principal curvatures) and “which are periodic in three independent directions” [11]. TPMS structures divide
the space into two continuous and labyrinthine sub-volumes (co-continuous structures); each of them is a
single, connected and infinite component which is self-supporting when the second phase is eliminated.
Different families of TPMS exist and some examples are reported below (figure 7) [12], in which one of the
two (or more) phases is highlighted. Even if geometrically complex, these kind of structures can be easily
produced via 3D printing techniques, without the need of any further post-processing.
Figure 7. Examples of TPMS IPC structures [12].
Most of the researches presented below deal with Polyjet 3D printing technique, which allows the production
of such complex multi-phase architectures. In 2011, Wang et al. [13] first described co-continuous ordered
structures (IPCs) composed of a rigid photopolymer (VeroWhite) and a rubbery one (TangoPlus), reporting
enhancement of the mechanical properties of the structure in terms of stiffness, strength and energy
absorption when compared to conventional particle-reinforced composites of similar composition. In the
meantime, Kapfer et al. [12] showed the advantages of TPMS design for scaffold production with the aim of
finding the most suitable design. Dalaq et al. [14] studied the mechanical properties of TMPS IPC composed
by a rubbery matrix (TangoPlus) reinforced with a glassy phase (VeroPlus) comparing different TPMS IPCs
structures and showing how properties can be tailored by coupling geometry and materials. In 2017, Al-
Ketan et al. [15] tested TPMS IPC composed by a glassy photopolymer (VeroWhite) as the matrix,
reinforced by a rubbery polymer (TangoPlus), both injected and cured by UV light. An enhancement of the
mechanical properties in terms of stiffness, strength and toughness was shown and the ability of TPMS
structure to confine crack initiation and propagation resulted in non-catastrophic failure events, which could
make them suitable for damage tolerant and vibration controlling systems. Al-Ketan et al. [16] also
compared two IPC structures with similar geometry, one being TPMS whereas the other not being
characterized by surface minimization property, in which both phases were made out of polymeric materials.
A superior design of TPMS structure was devised, mainly due to the elimination of sharp edges and corners,
which in turn strongly reduce stress concentration phenomena and increase the overall mechanical
properties. Maskery et al. [17] highlighted, numerically and experimentally, the relationship between
geometry and mechanical properties to create better TPMS IPC lattice designs to meet the requirements for
mechanical and biomedical applications. More recently, Al-Ketan et al. [18] characterized IPC with a
gyroidal (TPMS) structure made of a glassy photopolymer (VeroWhite) and a rubbery polymer
(TangoGray). They demonstrated that a range of mechanical properties can be obtained by varying the
15
composition of the two materials. In particular, they obtained toughness values much higher than those of the
individual components. They also demonstrated the possibility to control the spatial composition of the co-
continuous composite, which allows further studies to optimize and tune the properties for specific
engineering applications. Many other studies have been published in 2018 focusing on which field this kind
of structure could be applied in. Li et al. [19] demonstrated that IPCs show improved fracture toughness with
respect to other conventional structured composites. They also showed that the interaction between soft and
hard phases could be tailored to get significant stiffness with superior toughness. Again, Al-Ketan et al. [20]
performed mechanical tests on TPMS composite structure, obtaining higher mechanical properties as
compared to that of metal-coated polymers and ceramic-coated polymers cellular composites. Sreedhar et al.
[21] showed higher performances for membrane-based systems (such as microfiltration, ultra-filtration and
reverse osmosis) with TPMS morphology produced 3D Polyjet technology. Thomas et al. [22] designed feed
spacers with a TPMS topology for membrane distillation, showing higher water flux and heat transfer
coefficient as compared to the commercial ones. To conclude, Schubert et al. [23] briefly summarized the
state-of-the-art of the 3D printing topic for medical applications. The next sub-chapter will be focused on 3D
Polyjet technique in order to better understand how it works.
1.3 Polyjet 3D printer
This chapter briefly deals with material jetting processes, particularly the Polyjet 3D printing technique, in
which liquid droplets of build material (photo-polymers) are selectively deposited by a print head onto a
working platform (“build tray”). As depicted in figure 8, if necessary, deposited material can be sustained by
a support material; then a “leveling roller” immediately removes the excess of material on each pass of the
inkjet head and subsequently a curing process is performed with a UV lamp to harden each layer [24].
Figure 8. Picture of Polyjet 3D printing process [24]
Several working parameters have to be considered during production of specific components. As stated by
Lee et al. [6], the minimum ideal resolution volume of this technique is the volume of the deposited droplet
(Eq. 1):
𝑉𝑟 = 𝜋
6𝑑𝑑3
(Eq. 1)
16
where dd is the diameter of the droplet. The “image time” (timage) per layer is function of the droplet volume
Vd, droplet release rate f, layer area A and layer thickness l (Eq. 2):
𝑡𝑖𝑚𝑎𝑔𝑒 = 𝐴∗𝑙
𝑓∗𝑉𝑑
The fabrication speed is instead given by Eq. 3:
𝑣 = 𝑙
(𝐴∗𝐿
𝑓∗𝑉𝑑+ 𝑡𝑟𝑒𝑠𝑒𝑡)
where treset is the time needed to begin the deposition of the second layer. Some of the most common
commercially available machines are those belonging to the Polyjet Stratasys’s company [25] such as
Objet260 Connex2, Objet260 Connex3, Objet300 Plus, etc.. These machines are able to print a wide range
(in terms of properties) of materials. Some of the available ones for jetting processes are listed in Table 1.
Table 1. Material properties for Polyjet Stratasys's machine [6].
Polyjet 3D printing is the only AM technique capable of jetting multiple materials at once, exploiting both
flexible and rigid polymers within a single build, to obtain multi-material, complex objects [12]. For
instance, by simultaneously jetting VeroWhitePlus (a glassy photo-polymer) and TangoBlackPlus (a rubbery
photo-polymer), a product with tailored properties can be designed, as stated by Barclift et al. [26]. In this
sense, it’s easily understandable why TPMS IPCs, which have been discussed in the previous sub-chapter,
were produced via Polyjet technology. Moreover, this technique also allows for voxel-based deposition
processes (called “dithering”) whereby material is gradually suspended within the matrix of another material
across a structure’s volume [27]. By finely controlling the composition of the composite material at the
voxel-level, functionally graded materials can be created, i.e. components consisting of two or more
constituent materials that spatially vary in their content across the volume of the structure, as depicted in
figure 9. In this sense, while producing A and B components (example in figure 9) one could promote the
spontaneous formation of a transition region whose properties could be completely different from that of
bulk A and B components.
(Eq. 2)
(Eq. 3)
17
Figure 9. Example of functionally graded structure with gradient transitions of material A and B [27].
To sum up, material jetting techniques (in particular, Polyjet techniques) have gained a lot of interest within
the scientific community in the last few years due to the ability to couple desired material properties with the
advantages derived from the complexity of a specific geometry/shape. In fact, it is known that composite
materials are ideally suited to achieve multi-functionality since the best features of different components can
be combined to form a new material that has a broader spectrum of desired properties. Moreover, the
possibility to produce systems composed of two or more interpenetrating structures via 3D printing has
proven to give remarkably improved mechanical properties, which are strictly related to the complex
architecture and topology, as shown in past years [28].
1.4 Aim and goals
In view of the above, the Polyjet 3D technique seems to emerge as one of the most promising technologies
among AM ones. In particular, the Polyjet technique allows for the production of complex architectures,
such as TPMS co-continuous composites, coupled with the use of constituent materials with significant
different macroscopic properties. Hence, it seems interesting to deeper investigate its practical operation: our
research fits into this general interest. The simultaneous deposition of multiple constituent materials in their
liquid state (in form of droplets) for the production of the desired component can promote an inter-diffusion
process with the spontaneous formation of an interface between them. This phenomenon has been
completely ignored in the researches above presented. However, a previous work carried out by Davide
Ruffoni and Laura Zorzetto [29] tried to evaluate its role on the overall mechanical properties of 3D printed
components: they highlighted interesting features, which occur during multi-material deposition process,
proving the presence of the previously mentioned interface between different constituent materials. As a
matter of fact, they also showed that the interface can significantly modify the mechanical properties of the
resulting composite. A brief summary is attached in chapter 2: Preliminary results. In light of that, the goal
of this thesis will be clear.
18
2. Preliminary results
2.1 Introduction
The present work is the continuation of a recent research presented by Zorzetto et al. [29] in which a 3D
multi-material Polyjet deposition process has been examined. The authors showed that, during the 3D
deposition of two materials via Polyjet technique, a sort of inter-diffusion process can occur between the two
photopolymers (a soft rubbery and stiff glassy photo-polymer were used) before or after the curing step as
function of the printing setup. As a matter of fact, the same component can be produced along different
printing directions as depicted in Figures 10. When two layers made of different materials are simultaneously
injected (in figure 10 on the left), an inter-diffusion process may occur between the liquid materials before
the curing process is completed. This phenomenon seems to be significant and leads to the formation of a
relatively thick interface between the two building materials. Conversely, the diffusion process occurs only
after the curing step has been carried out when the deposition process of the constituent layers (still made of
different materials) is alternating (i.e. the deposition of the second layer is performed only once deposition
and curing of the first one are completed) (figure 10 on the right). As a result, the generated interface is
thinner with respect to one generated when simultaneous deposition occurs.
Figure 10. Interface is formed before (on the left) or after (on the right) curing step [personal communication].
In both cases, a sort of diffusion process seems to lead to the formation of an interface, whose geometrical
properties are therefore function of the printing direction. Nano-indentation tests were performed to evaluate
the size of the interface depending on printing direction (Figures 10) and dynamic mechanical analysis
(DMA) was subsequently carried out to assess its effect on the dynamic mechanical behavior of simple
multi-layered samples. Nano-indentation test was fundamental in view of understanding results of the
following dynamic mechanical analysis.
2.2 Equipment and material
The commercial 3D printer Objet260 Connex 2, produced by Stratasys [25], was used to obtain the desired
specimens. The printer properties as described in its technical sheet are listed in Table 2.
19
Table 2. Datasheet of Objet 260 Connex2 3D printer [25].
The key-feature of Objet260 Connex2 lies in the possibility to simultaneously print two different materials
among those reported in the Table 2. It’s worth noting that constituent materials can be combined in various
(predefined) ratios by simultaneously depositing both liquid materials at the same time with the goal of
creating gradient material blends (“dithering” process) with properties ranging along the continuum between
the two extremes. Materials which could be used for Polyjet printing process are usually miscible, allowing
for the phenomenon just explained. The generation of an interface between two constituent materials can be
understood in light of this characteristic of the printer. The curing process is performed through the use of a
UV lamp, which is periodically calibrated. This process consists in the measurement of the intensity of the
emitted light through the use of a suitable sensor: if the value does not lie within the recommended range, the
intensity of the UV lamp is increased to match the proper specifications.
The 3D printer is used to inject and cure materials, as previously described. The materials were supplied by
Stratasys: a glassy photopolymer VeroWhitePlus (RGD835) and a rubbery photopolymer TangoBlackPlus
(FLX930), whose properties are listed in Table 3 and 4 [25].
20
Table 3. Datasheet of VeroWhitePlus (RGD835) material [25].
Table 4. Datasheet of TangoBlackPlus (FLX930) [25]
2.3 Sample preparation
2.3.1 Samples for nano-indentation test
Bi-material cylinders of 1 mm diameter were produced in two configurations as described in Figures 11 and
corresponding to two different printing directions, in analogy with what is shown in figure 10.
TangoBlackPlus (FLX930) and VeroWhitePlus (RGD835) will be represented by “black” and “white” color,
respectively.
Figure 11. Bi-material cylinders for nano-indentation test [personal communication].
21
It’s worth noting that rubbery TangoBlackPlus (FLX930) and glassy VeroWhitePlus (RGD835) materials are
simultaneously injected and the generation of interface occurs before curing step to get the cylinder in figure
11 (a). Contrarily, the deposition process of constituent materials in figure 11 (b) is sequential, which means
that deposition and curing process of VeroWhitePlus (RGD835) follow the deposition and curing process of
TangoBlackPlus (FLX930) material. Samples were obtained by cryo-sectioning a slice perpendicularly to the
interface from cylinders.
2.3.2 Samples for dynamic mechanical analysis: 3-point bending configuration
Cylindrical samples of TangoBlackPlus FLX930 were produced for dynamic mechanical analysis (DMA) in
compression configuration with diameter equal to 2 cm and height of 2 cm. This test setup was chosen since
beam-shaped TangoBlackPlus (FLX930) wouldn’t be stiff enough to be tested in 3-point bending
configuration. In turn, acceptable values of compliance were obtained by testing it in compression. Beam-
like sample of pure VeroWhitePlus (RGD835) were produced with size 1,5mm x 12mm x 44mm. Beam-like
composite samples were produced by stacking a glassy layer on the top of a rubbery layer along the
thickness of the sample itself as depicted in Figure 12. To evaluate how the interface affects the mechanical
properties of the laminate, sample with 2, 4, 6 and 8 layers were produced with size 1,5mm x 12mm x
44mm. VeroWhitePlus (RGD835) layer was stacked on top of each TangoBlackPlus (FLX930) layer.
Specimens were printed as to obtain samples with thicker and thinner interfaces (such as in figures 12 and
13, respectively).
Figure 12. Beam-like sample for dynamic mechanical analysis [personal communication].
2.4 Experimental procedure
2.4.1 Nano-indentation test
TI 950 TriboIndenter was used to perform the tests in displacement controlled mode. The machine was
equipped with a sphero-conical probe with tip radius of 5 µm. As depicted in the following Figure 13, test
parameters were:
- penetration depth = 1 µm;
- hold time = 20 s;
- displacement rate = 100 nm/s;
- lift height = 2 µm;
- unloading = 2 µm.
22
Figure 13. Nano-indentation test parameters [personal communication].
The indent array is shown in Figure 14, in which spacing between indents (represented by red dots) was 25
µm, whereas spacing between lines was around 100 µm.
Figure 14. Nano-indentation test configuration [personal communication].
2.4.2 Optical microscopy
An optical microscope with reflected light was used on the above described cylindrical samples in order to
investigate the microstructural features (topological) of the interface.
2.4.3 DMA test
Dynamic mechanical analysis (DMA) was performed to evaluate the storage modulus and thermal transition
properties. Specimens were tested with RSA III Rheometrics System Analyzer in 3 point bending
configuration as shown in Figure 15. An oscillatory displacement resulting in 0,1% strain was applied, with 1
Hz frequency, 0.5 N pre-loading and temperature varied from -35 °C to 65 °C. TangoBlackPlus FLX930
samples were tested in compression with the same parameters.
Figure 15. 3-point bending configuration for dynamic mechanical analysis [personal communication]
Time [s]
0 20 40 60 80 100 120
Dep
th [
nm
]
-2000
-1000
0
1000
23
2.5 Results and discussion
2.5.1 Nano-indentation and SEM results
Nano-indentation tests show the presence of an interface whose properties are significantly different from the
bulk material ones. When TangoBlackPlus (FLX930) and VeroWhitePlus (RGD835) are sequentially
deposited and cured (Figure 16a), the interface thickness is about 50 µm (or smaller), a value comparable to
the resolution of the printer, around 42 µm (600 dpi). Whereas, as shown in figure 16b, when VeroWhitePlus
(RGD835) and TangoBlackPlus (FLX930) materials are simultaneously injected a wider interface seems to
be generated before the curing step is performed, with a characteristic length-scale around 175 µm, which is
4-5 times higher than printer resolution (42 µm). Optical microscopy also validates the nano-indentation
results as shown in figure 19, which highlights the formation of a wide region between the rubbery and
glassy components, which has a distinct appearance.
Figure 16. Nano-indentation and optical microscope results: a) sharp interphase; b) thick interphase [personal communication].
Thus, depending on the printing direction according to which the component is produced, the thickness of
the interface region varies from 50 to 175 µm. The interface is characterized by a modulus which is closer to
that of the glassy material (VeroWhitePlus (RGD835)). As a matter of fact, our specimens are composed by
TangoBlackPlus (FLX930), VeroWhitePlus (RGD835) and a third phase, the “interphase”. From here on out,
the term “interphase” will be used instead of the ideal “interface”. Once the presence of an interphase had
been assessed, the authors tried to evaluate its effect on the mechanical properties of laminate samples.
2.5.2 DMA results
In light of nano-indentation results, it must be taken into account that beam-like specimens for DMA are
characterized by a different interphase size and volumetric fraction: increasing the number of base material
layers, the interphase volumetric fraction clearly becomes much more significant (sample dimension
remaining unchanged, with a total thickness of 1500 µm). Figure 17 shows the characteristic theoretical
24
volumetric fraction of base materials (equal to 0,5) and of the interphase as a function of the printing
direction.
2 4 6 8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Volu
metr
ic fra
ction
number of layers
"Wall-printed": Interphase
"Flat-printed": Interphase
Figure 17. Relative volumetric fraction of components in samples where a sharp or wide interphase is obtained.
Results of elastic and damping properties from dynamic mechanical analysis for the pure rubbery and glassy
material are shown in figures 18.
Figure 18. VeroWhitePlus RGD835 damping and elastic properties (on the left); TangoBlackPlus FLX930 damping and elastic
properties (on the right).
As previously mentioned, the relative volumetric fraction of the interphase increases with increasing number
of layers. DMA results for specimens with a sharp interphase are shown in Figure 19 (dotted lines represent
the behavior of the pure rubbery and glassy material shown in figure 18):
25
Figure 19. Damping and elastic properties of laminate samples with: a) 2 layers (brown curve); b) 2 and 4 layers (orange curve); c)
2, 4 and 6 layers (light green curve); d) 2, 4, 6 and 8 layers (green curve). Dotted lines represent pure rubbery and glassy materials
as a reference [personal communication].
As it can be noted, the interphase seems not to significantly affect the elastic properties of the sample since
no differences of storage modulus behavior are evident among the 4, 6 and 8 layers. Therefore, this printing
mode seems to only slightly change the properties of the component. No differences in damping properties
were detected, too. By changing the printing direction, a wider interphase between glassy and rubbery layers
is obtained as shown in figure 17. Again, the results for specimens with a wider interphase are shown in
Figure 20 (dotted lines still represent the properties of the pure rubbery and glassy material, for reference
purposes).
26
Figure 20. Damping and elastic properties of laminate samples with: a) 2 layers (brown curve); b) 2 and 4 layers (orange curve); c)
2, 4 and 6 layers (light green curve); d) 2, 4, 6 and 8 layers (green curve). Dotted lines represent pure rubbery and glassy materials
as a reference [personal communication].
By considering the trend of storage modulus E’, it is clear how the modulus curves increase with increasing
number of layers up to a 6-layers configuration. There are no differences between 6 and 8 layers sample
since the whole sample becomes made of interphase mainly (as it is clearly highlighted in figure 17).
Therefore, the increase of the volumetric fraction of the interphase seems to strongly affect the composite
elastic properties. Moreover, damping properties (highlighted by tan δ trend in figure 21) show a peculiar
trend: two main distinct peaks are detected for samples made of two layers, around -10 °C and 55 °C.
Increasing the number of layers to 4, tan δ curve shows the appearance of a third peak, between the ones we
previously mentioned, which should be related to the presence of interphase (with a volumetric fraction
higher with respect 2-layers case). The maximum of these peaks shifts up to being one peak only when
sample is composed of 8 layers, i.e. when it should be made (almost) of interphase only, as depicted by the
green curve in figure 20 (point d).
2.6 Discussion and aim
3D printing, in particular Polyjet technology, seems an intriguing technique among AM ones due to the
variety of ways it could applied (Chapter 1: Introduction). In light of the increasing interest around this field,
27
the research just presented highlights interesting features which have been so far completely neglected. The
generation of the interphase as a function of the printing setup was assessed (via
nano-indentation analysis) as well as its effect on the overall mechanical properties of the component (via
dynamic mechanical analysis). In this sense, the core of our work is to carry on with a deeper analysis on the
effect of the interphase on the overall elastic and damping properties of the component. The results obtained
by 3-point bending DMA test are difficult to interpret through the use of theoretical models because of the
relatively complex stress state the samples are subjected to (bending of the laminate). In light of this, tensile
configuration should a priori allow us a better understanding of the results and to more easily interpret the
obtained data through the development of simple mechanical models, such as the one presented in next sub-
chapter 2.7. Therefore, the work will be carried on by performing dynamic mechanical analysis in tensile
configuration, followed by a theoretical interpretation of results.
2.7 Composite models
“Composite materials” are engineered from two constituent materials with significantly different chemical or
physical properties which remain separate phases within the final structure. Often constituent materials can
have a different role: a “matrix” which surrounds and supports the “reinforcement material” that is instead
imparting its special mechanical and physical properties to enhance the matrix properties. Reinforcements
can be in different shapes: in the form of particle, of short- or long-fibers, etc… For instance, fibers may be
oriented randomly within the matrix, but they can also be arranged in a preferential direction that is expected
to have the highest mechanical properties. Such a material is said to be anisotropic (i.e. different properties in
different directions), and control of the anisotropy is an important means of optimizing the material for
specific applications. At the microscopic level, the properties of the composite are determined by the
orientation and by the distribution of these fibers, as well as by the properties of both matrix and
reinforcement. Let’s consider a continuous fiber composite, uniaxially aligned in a preferential direction: the
considered component contains a certain volumetric fraction Vf of fiber reinforcement. Hence, the volumetric
fraction of matrix is equal to Vm=(1-Vf). This component can be idealized by gathering both matrix and fibers
in solid “blocks” with volumes proportional to their relative abundance (i.e. their volumetric fraction) in the
composite (figure 21).
Figure 21. Idealization in “solid blocks” of fibers and matrix in composites: uniaxially aligned fiber in continuous matrix.
If a stress σ is applied along the direction of the fibers, both phases (matrix and fibers) act in parallel to
withstand the applied load. In these fibers configuration, the strain in each phase must be the same, therefore
the strain ε1 along fiber direction can be written as:
𝜀𝑚 = 𝜀𝑓 = 𝜀1
28
Instead, the sum of the forces in each phase must balance the total applied load on the material. Therefore,
we can write an equation in order to balance the forces acting on single phases:
𝜎1 = 𝜎𝑚 ∗ 𝑉𝑚 + 𝜎𝑓 ∗ 𝑉𝑓 = 𝐸𝑚 ∗ 𝜀𝑚 ∗ 𝑉𝑚 + 𝐸𝑓 ∗ 𝜀𝑓 ∗ 𝑉𝑓
The stiffness in the fiber direction is obtained by dividing the equation by the strain, thus getting:
𝐸1 =𝜎1
𝜀1= 𝑉𝑚 ∗ 𝐸𝑚 + 𝑉𝑓 ∗ 𝐸𝑓
This equation is known as Rule of Mixtures prediction of the overall mechanical properties of composite
materials in the direction of fiber orientation. The previous equation can be generally extended to i-phases,
thus getting equation 4:
𝐸1 = 𝛴𝑉𝑖 ∗ 𝐸𝑖
Instead, when the stress is applied perpendicularly to the direction of the fibers, they act in series to support
the applied load. In this case, matrix and fibers are subjected to the same stresses and the overall strength of
material in this direction is:
1
𝐸2=
𝑉𝑚
𝐸𝑚+
𝑉𝑓
𝐸𝑓
By generalizing the equation to i-phases (equation 5):
1
𝐸2= 𝛴
𝑉𝑖
𝐸𝑖
(Eq. 4)
(Eq. 5)
29
3. Experimental procedure
Methodology
Material and printer properties are the same reported in sub-chapter 2.2. For the reader, it would be
recommended to understand how the two different printing modes affect the generation of the interphase
(sharper or wider) and to remember the nomenclature used for distinguishing the sample configuration. Since
the composition of the constituent materials is potentially subject to change by the manufacturer, base
materials and composite samples were printed at the same time.
3.1 Materials and equipment
Objet260 Connex2 was used to print VeroWhitePlus RGD835 and TangoBlackPlus FLX930 materials. Their
properties are described in the above presented tables 2, 3 and 4, respectively.
3.2 Samples
The process of sample designing was carried out by taking into account that specimens have to be tested in
tensile configuration bearing in mind the following general considerations:
a) Geometry
TA Instrument RSA3 specifies the recommended specimen dimensions related to the clamping system. Since
tests with temperature ramp are performed, the dimensions of the oven chamber must be taken into
consideration too: the effective maximum sample length was 40 mm.
b) Compliance
Minimum (1,02x10-2
mm/N) and maximum (1,02 mm/N) compliance limit values are given by the
manufacturer in order to get an accurate measurement. Therefore, samples were designed to have a
compliance value lying between these two limits. This goal was achieved by tailoring sample thickness,
width and length. The layer stacking sequence had also to be considered, as it affects the final compliance
value of the sample. Expected values were evaluated by considering preliminary DMA results (3-point
bending test) previously carried out by Zorzetto et al. [28].
c) Printing direction
Samples can be printed in two possible directions: “flat-printed” mode means that width and length of the
samples lie on the printing tray plane (x-y direction), whereas the thickness extends along z-direction, normal
to x-y plane. Vice versa, “wall-printed” samples have thickness and length lying on x-y plane, whereas width
extends along the perpendicular to x-y plane. Figure 22 depicts an example of “flat-printing configuration”
and “wall-printing configuration”, respectively.
30
Figure 22. Same component can be printed in: "flat-printing" mode on the left; “wall-printing” mode on the right.
Either a sharp or a wide interphase can be generated depending on sample design and orientation, as will be
discussed later on.
d) Printer resolution
To comply with the printer spatial resolution, it is recommended to design the size of the samples as follows:
the x-y dimensions have to be multiple of 85 µm, whereas the z-size has to be multiple of 29 µm.
As explained in the Introduction (chapter 1.1), specimens were designed using a CAD software (Autodesk
Inventor Professional 2016). Files were subsequently converted into STL format, which is used by the 3D
printer for the layer-by-layer production process, as depicted in the following example (Fig. 23).
Figure 23. From CAD file to final component
Characteristics of the individual types of samples produced will now be presented.
3.2.1 VeroWhitePlus RGD835 sample
For the sake of simplicity, VeroWhitePlus (RGD835) was “flat-printed”: as a matter of fact, this printing
setup avoids the use of any mechanical support during the deposition process, which would be otherwise
required for the “wall-printing” mode. In this sense, VeroWhitePlus (RGD835) was produced as a
rectangular bar whose dimensions are described in table 5. The total length incudes two clamping regions
made of VeroWhitePlus (RGD835), each of length equal to 15 mm, in order to avoid deformation in the
clamping system. Three samples were produced both before and after calibration of the UV lamp.
31
Table 5. Glassy material: sample dimensions.
Thickness [mm] Width [mm] Length [mm]
VeroWhitePlus 1,015 5,440 69,950
3.2.2 TangoBlackPlus FLX930 sample
TangoBlackPlus (FLX930) was also “flat-printed” for the same reason explained in sub-chapter 3.2.1. Since
the elastic and damping properties had to be studied far below and above the glass transition temperature of
rubbery material, two different samples were produced: one to be tested below rubber glass transition
temperature (around 5 °C from previous analysis), the other to be designed considering its properties above
the glass transition. Sample dimensions are listed in table 6: sample width was doubled to get a stiffer sample
above its glass transition. The total length incudes two clamping regions made of VeroWhitePlus (RGD835),
each of length equal to 15 mm, in order to place the sample into the clamping system. Three samples were
produced both before and after calibration of the UV lamp.
Table 6. Rubbery material: sample dimensions.
Thickness [mm] Width [mm] Length [mm]
TangoBlackPlus (I) 1,015 5,440 69,950
TangoBlackPlus (II) 1,015 10,880 69,950
3.2.3 Composite sample
Samples were designed in the form of laminate, by sequentially stacking on top of each other a certain
number of VeroWhitePlus (RGD835) layers onto an equal number of TangoBlackPlus (FLX930) layers.
This means that each laminate contains a volumetric fraction of glassy and rubbery material equal to 0,5
(unless specified otherwise). Various stacking sequences were chosen, grouped in the following three main
categories:
a) “Along thickness” (AT) stacking sequence, where the sequence of layers lies along the thickness of
the sample (z-axis in figure 24).
Figure 24. "Along thickness" sample.
32
b) “Along width” (AW) stacking sequence, where layers are stacked along the width of sample itself (x-
axis in figure 25).
Figure 25. "Along width" sample.
c) “Along length” (AL) stacking sequence, where layers are assembled along the length of the sample
(y-axis in figure 26).
Figure 26. "Along length" sample.
Figures 24, 25 and 26 depict the three configurations, where “x-axis” represents sample width, “y-axis” is the
sample length and the thickness increase along “z-axis”. Deeper in the discussion, we will exploit this simple
nomenclature in order to identify specimens. To be as clear as possible, this chapter is divided into three
sub-chapters, indicating different composite samples which will be useful for the analysis of the topic. Each
sub-chapter specifies the group of samples, the reason why they have been produced, indicates the choice of
the dimensions and the number of layers specimens are made of, the printing mode and the resulting
volumetric fraction of interphase.
3.2.3.1 AL samples
“AL” samples were designed in order to evaluate the effect of the interphase on laminate mechanical
properties when the stress is applied in the direction transverse to the layers. Specimen dimensions were
selected in order to meet design criteria a, b, and c. Criterium d) had not yet been considered when samples
were produced. Sample sizes of AL specimens are reported in Table 7. The total length includes two
clamping regions made of VeroWhitePlus (RGD835), each equal to 15 mm, in order to firmly place the
sample into the clamping system. The number of layers was selected by taking into account the results
shown in chapter 2. AL samples with 2, 4 and 8 layers were made. The volumetric fraction of interphase
33
increases with the number of layers (for instance, as previously described in figure 17). The whole set of
specimens was designed to be symmetric with respect to their mid-plane in order to avoid in-plane bending
coupling. This implies that outer layer is divided into two halves, as depicted in the example in the following
picture 27. AL were “flat-printed” resulting in the generation of a wide interphase (ca. 175 µm) since
deposition process of constituent materials is simultaneous. Samples were produced before and after the
calibration of the UV lamp (used for the curing process). Three samples for each type were obtained.
Table 7. "Along length": sample dimensions.
Sample Thickness [mm] Width [mm] Length [mm]
AL 1,5 12,5 65
Figure 27. Example of symmetric sample.
3.2.3.2 “AT” and “AW” samples
AT and AW samples were designed to evaluate the effect of the interphase on mechanical properties of the
laminate when the stress is applied in the direction parallel to the layers. Dimensions are listed in the Table 8.
The total length incudes two clamping regions made of VeroWhitePlus (RGD835), each of length equal to
15 mm, in order to place the sample into the clamping system. Samples were “flat-printed”: this entails that
the generation of a sharp interphase (ca. 50 µm) is promoted in AT samples, whereas a wider interphase (ca.
175 µm) is generated in AW samples. AT samples were designed with 2, 4 and 8 layers. To evaluate the
effect the of the external layer, the AT samples with 2 layers were produced in two variants with either
glassy or rubbery material as the external layer. Instead, AW samples were produced with 8, 16 and 32
layers. Afterwards, AW samples were also “wall-printed”, generating a sharp interphase (ca. 50 µm) in order
to have a comparison with the previous cases. Specimens were symmetric with respect to their mid-plane to
avoid in-plane bending coupling. Samples were produced only after the calibration of the UV lamp. Three
samples for each configuration were obtained.
34
Table 8. "AW" sample dimensions.
Thickness [mm] Width [mm] Length [mm]
“Flat-printed” AT 1,015 5,440 79,950
“Flat-printed” AW 1,015 5,440 69,950
“Wall-printed” AW 1,020 5,104 69,950
3.2.3.3 AT samples with varying composition
A different type of AT sample was also produced: previous cases showed specimens with volumetric fraction
for both constituent materials equal to 0,50. To evaluate the potential effect of a different volumetric
composition, AT samples with 2 layers were produced, varying the volumetric fraction of rubbery material
(0,625, 0,750 and 0,875). Dimensions are reported in table 9. In order to increase the volumetric fraction of
rubbery materials as much as possible, in a second stage we also produced sample in which rubber relative
content was further increased up to 0,92 and 0,95. Again, the whole set of samples is symmetric with respect
to their mid-plane. Since we wanted to evaluate only the effect of rubbery material volumetric fraction,
samples were “flat-printed” in order to reduce the presence of interphase as much as possible. They were
produced after the calibration of the UV lamp only. Three specimens for each type have been tested.
Table 9. Dimensions of samples with different volumetric composition.
Thickness [mm] Width [mm] Length [mm]
0,625 / 0,750 / 0,875 1,015 5,440 39,950
0,92 / 0,95 1,280 5,440 39,950
3.2.3.4 AW samples with separated layers
A different kind of AW sample was also produced: in order to completely avoid the formation of the
interphase, a sample with separated glassy and rubbery layers was obtained (figure 28). Sample were
produced in AW configuration with 2 layers. Glassy and rubbery layers were separated by 500 µm in order
to completely avoid any possible diffusion process leading to the generation of an interphase. The active
cross section of the sample is still 1,015mm x 5,440mm with a length of 39,950mm. Hence, the overall
sample dimensions are listed in table 10, complying with all criteria above presented (sub-chapter 3.2).
Again, it is symmetric with respect to its mid-plane. Sample were printed in “flat-printing” mode due to the
presence of the separating space between layers. Three samples were obtained after the calibration of the UV
lamp.
35
Table 10. Dimensions of the AW samples with separated layers.
Thickness [mm] Width [mm] Length [mm]
AW 1,015 6,440 39,950
Figure 28. Example of AW sample with separated glassy and rubbery layers.
3.3 Optical microscopy
An optical microscope (Olympus BX60) was used to evaluate surface topography of the AW samples with 8,
16 and 32 layers. The use of visible light allowed us to clearly distinguish the three different components
characterized by different colors. VeroWhitePlus RGD835 contains titanium dioxide, giving its white color;
TangoBlackPlus FLX930 is filled with carbon black; the interphase is expected to be grey in color since
constituent materials are miscible. Transmitted signal was analyzed to study “flat-printed” AW samples
described in sub-chapter 3.2.3.2. The examples are below presented. The images taken from “wall-printed”
AW samples with 16 layers are shown in figure 29 (a, b, c and d): the images are not very clear, but the
interphase seems not to be very wide. The analysis showed that the interphase thickness is around 40 µm,
qualitatively comparable with the nano-indentation datum. “Flat-printed” AW sample surfaces were also
analyzed, in which the interphase thickness should be approximately 175 µm. ImageJ software was used to
analyze the intensity of the RGB color to have a qualitative idea of the surface layer dimensions: an example
of the analysis (of the picture 30d) is shown in figure 31. The green curve was arbitrarily selected as
reference as well as the intensity threshold (140). The results are listed in table 11. The qualitative results are
comparable with the ideal interphase thickness, obtained via nano-indentation test by Zorzetto et al. [29].
36
Figure 29. Transmitted light microscope: "flat-printed" AW samples with 16 layers.
Figure 30. Transmitted light microscope: "flat-printed" AW samples with 16 layers.
37
-200 0 200 400 600 800 1000 1200 1400 1600 1800
0
20
40
60
80
100
120
140
160
180
200
220
240
260
Inte
nsity
distance (m)
Red
Green
Blue
Figure 31. ImageJ analysis: RGB intensity plot for "flat-printed" AW samples.
Table 11. ImageJ analysis: interphase dimension.
Image 1 Image 2 Image 3 Image 4
Interphase
dimension [µm]
152.5
151.5
147.5
134.3
The surface of AW samples with 32 layers was also analyzed: an example is shown in figure 32. The ImageJ
analysis can’t be performed on these images since the separation in layers is not evident. Again, the features
highlighted are interesting in view of the following since the “flat-printed” AW sample with 32 should be
theoretically made of interphase only.
Figure 32. Transmitted light microscope: “flat-printed” AW samples with 32 layers.
38
3.4 Dynamic Mechanical Analysis (DMA)
Dynamic mechanical analysis (DMA) is a common laboratory technique to characterize bulk properties as
well as to supply information about material transitions not readily identifiable with other techniques.
Dynamic mechanical testing can be easily described as the application of a sinusoidal stimulus to a sample
analyzing material’s response to that stimulus. It typically involves the application of an oscillatory strain to
a sample. The resulting sinusoidal stress is measured as well as the viscous and elastic properties of the
sample by correlating the output signal against the input strain (figure 33).
Figure 33. DMA input and output signals [30].
As a matter of fact, the mechanical response would completely be in phase for an elastic material, whereas it
would be 90° out of phase for purely viscous materials. Actually, the phase angle shift (δ) between stress
(output) and strain (input) occurs somewhere between the elastic and viscous extremes for a viscoelastic
material. This fact implies that the stress output signal generated by a viscoelastic material can be separated
into two components: an elastic component which is in phase with strain, and a viscous stress which is 90°
out of phase with strain. The ratio of the elastic stress to strain is referred to as the elastic (or storage)
modulus (Ε'), which is related to the ability of a material to elastically store energy. Vice versa, the ratio of
viscous stress to strain is referred to as the viscous (or loss) modulus (Ε''), and it is the measure of a
material's ability to dissipate energy. The complex modulus (Ε*) is a measure of the overall resistance of a
material to deformation. These different moduli allow a better characterization of the material because they
allow to examine the capability of the material to return (E′) or dissipate energy (E″), and the ratio of these
effects (tan δ), which is called damping.). One advantage of DMA is that we can obtain data while from the
sinusoidal loading while sweeping across a temperature or frequency range. For example, samples can be
scanned during a constant heating ramp (e.g. 5-10 °C/min). By mean of a temperature ramp major, secondary
and tertiary transitions can be measured (e.g. glass transition). A TA RSA III Rheometrics System Analyzer
was used to perform the analysis. It was equipped with a furnace chamber allowing for thermal
measurements. Bearing in mind preliminary analysis results (sub-chapter 2), dynamic mechanical analysis
was performed in tensile configuration in order to evaluate the effect of the interphase on the overall elastic
properties of the sample and to define the glass transition temperatures by mean of temperature scans. The
presence of a third phase (“interphase”) and the complex state of stress characteristic of three-point bending
configuration, lead us to choose the tensile configuration for further analysis. In this case, the easier state of
stress should a priori allow us to better comprehend the results and to more easily forecast the obtained data
through theoretical modelling such as via (parallel/series) composite mixture rule. An oscillatory
displacement resulting in 0,1% strain was applied to the sample, in order to remain within the linear
viscoelastic region. The analysis was performed in strain-controlled condition. Standard 1 Hz frequency was
39
set, according to the previous analysis performed by Zorzetto et al. [29]. The dynamic mechanical analysis
was performed coupled with a temperature ramp from -50 °C to 80 °C with 5 °C/min heating rate to evaluate
the transition temperatures of the two materials. Sample were cooled from room temperature to -50 °C,
stabilized at that temperature for at least 5 minutes, and subsequently heated up to 80 °C. The machine
collects data every 12 seconds. It must be underlined that TangoBlackPlus (FLX930) samples (below its
glass transition temperature) were cooled from room temperature to -50 °C, stabilized for more than 5
minutes and then heated according to the previous procedure (same parameters) from -50 °C to 30 °(above
its glass transition temperature). Vice versa, TangoBlackPlus (FLX930) (above its glass transition
temperature) were heated from room temperature (20 °C) up to 80 °C, applying an oscillatory displacement
that resulted in 0,5% strain (instead of previous 0,1%), whereas other parameters remain unchanged. In order
to verify being in the linear polymer regime, a “strain-sweep” test at room temperature (approximately 20
°C) was also performed varying the applied strain from 0,01% up to 1%. To get the whole curve from -50
°C to 80 °C, data of two experiments are joined. Since the two curves are superimposed in the range of
temperature between 20 °C and 30 °C, the obtained data were mathematically averaged to obtain the whole
trend.
40
4. Preliminary results
Some work had to be done before venturing in the analysis of the role of the interphase on the mechanical
properties of 3D printed composites: the next sub-chapters deal with these preliminary steps.
Three samples for each configuration were tested to verify data repeatability as shown in figure 34. The
whole set of data is shown in Appendix A. For the sake of simplicity, only averaged curves will be presented
in the following. Data analysis was performed through the use of OriginPro software. Trends for each
material are represented by different colors:
a) VeroWhitePlus (RGD835) and TangoBlackPlus (FLX930) are represented by solid lines, named
“glassy” and “rubbery”, respectively.
b) Composite data are pictured in different shades of grey. The labels 2L, 4L and 8L indicate the
number of layers the sample is made of: 2, 4 and 8 respectively.
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ean
(P
a)
T (°C)
Figure 34. Example of data repeatability: superimposed storage modulus curve of "flat-printed" AW samples with 8 layers (on the
left); average curve with standard deviation (on the right).
4.1 Effect of curing conditions
Samples described in sub-chapter 3.2.3.2 were tested. Figure 35 shows storage modulus curves for AT
samples with 2 layers where the outer plies are made of the glassy material. The storage modulus curve
indicated as “1st” in figure 35 shows an abrupt decrease above 60 °C. Another test (with the same
parameters) was carried out on the same sample: the relevant curve indicates that the abrupt decrease in the
elastic properties is no more present (as shown by the curve named as “2nd
” in figure 35). This behavior
(also verified for other samples by comparison of 1st and 2nd scans) can be ascribed to the completion of the
polymerization process of the glassy component. It can be stated that the first test promotes completion of
the curing process for the glassy material, meaning that the sample has been subjected to a sort of “thermal
treatment”. The completed polymerization process leads to the typical rubbery plateau, as we would have
expected. It’s worth noting that the manufacturer suggests to carry out a thermal treatment (as a post-process)
once the component has been subjected to all typical production steps of a Polyjet 3D printer (described in
chapter 1: introduction). Within the following, only data obtained after this virtual “thermal treatment” will
be shown.
41
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (1st)
E' (2nd)
Figure 35. Storage modulus trends for AT sample with 2 layers: first DMA test (light grey); second DMA test (grey) .
4.2 Effect of calibration of UV lamp
In light of what highlighted in the previous section, the following step aims at understanding how the
efficiency of the photo-polymerization process may be affected by UV lamp parameters. The tested samples
had no problem related to the completion of the curing process: no abrupt decrease was detected. Then, the
idea is to produce samples before and after re-calibration of the lamp (described in sub-chapter 2.2). Both
constituent materials and composite samples were tested.
VeroWhitePlus (RGD835) results
Glassy samples were printed before the UV lamp calibration: test results are reported in the plot 36 below
(hollow curves, “BC”). A sharp peak in tan δ trend approximately at 56 °C was detected. The damping
properties of samples fabricated after the calibration of the UV lamp (“AC” in the graph) are still shown in
figure 36: a slight shift from 56 °C to 59 °C was detected in tan δ peaks. Hence, by re-calibrating the UV
lamp, the efficiency of the photo-polymerization process is improved. The storage modulus curves are
strictly comparable below room temperature. Above it, higher values for the “after-calibration” (AC)
samples are detected due to a more efficient polymerization process.
42
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
BC__
__AC
Figure 36. "Before"(BC) and "after" (AC) calibration samples: elastic and damping properties results.
TangoBlackPlus FLX930 results
Two samples (described in sub-chapter 3.2.2) produced before and after UV lamp calibration were tested.
No significant differences were detected (figure 37): the sharp peak in tan δ remains unchanged at about 5 °C
after the calibration of the UV lamp.
Figure 37. "Before" (BC) and "after calibration" (AC) samples: damping properties.
Composite results
AT samples with 2 layers were printed before and after the UV lamp calibration. Figure 38 shows damping
and elastic properties: the high-temperature peak (related to glassy material) in tan δ curve shifts from
43
55,5 °C to 57,5 °C after the calibration of the UV lamp, as described in previous analysis. The low-
temperature peak remains unchanged.
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
BC__
__AC
Figure 38. "Before" (BC) and "after calibration" (AC) samples: elastic (left) and damping (right) properties.
4.3 Effect of the layer arrangement
In order to evaluate the potential confinement effect of the glassy material on the rubbery layer, AT samples
with 2 layers were produced in two configurations in which the outer plies are made of either glassy (G) or
rubbery (R) material (described in sub-chapter 3.2.3.1). No significant differences in the elastic properties
and damping properties were detected (figure 39). For convenience, glassy material was chosen as external
layer.
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (R)
E' (G)
E' (P
a)
Temp (°C)
-60 -40 -20 0 20 40 60 80
0.00
0.25
0.50
0.75
1.00
1.25
tan (R)
tan (G)
tan
Temp (°C)
Figure 39. Elastic and damping properties of two variants of AT samples with 2 layers: glassy and rubbery outer plies.
44
5. Results and discussions
The core of the analysis pertains the effect the interphase has on the overall elastic and damping properties of
the laminate.
5.1 Samples composition
The relative amount of interphase as a function of the sample configuration and of the printing direction is
depicted in figure 40. The theoretical numeric values of the relative content of the individual components
(interphase, glassy and rubbery phase) in AL, AT and AW samples are listed in table 12, 13 and 14,
respectively.
0 5 10 15 20 25 30 35
0
20
40
60
80
100
Inte
rpha
se c
onte
nt
(%)
number of layers
Flat-printed AW
Wall-printed AW
Flat-printed AT
Flat-printed AL
Figure 40. Volumetric content of the interphase as a function of the sample configuration and printing direction.
Table 12. Volumetric fraction of the components in AL samples.
Vol. Fraction [Ad.] 2 layers AL 4 layers AL 8 layers AL
Glassy phase 0, 495 0,49 0,48
Rubbery phase 0, 495 0,49 0,48
Interphase 0,01 0,02 0,04
Table 13. Volumetric fraction of the components in AT samples.
Vol. Fraction [Ad.] 2 layers AT 4 layers AT 8 layers AT
Glassy phase 0,45 0,40 0,30
Rubbery phase 0,45 0,40 0,30
Interphase 0,10 0,20 0,40
45
Table 14. Volumetric fraction of the component in AW samples.
Vol. Fraction
[Ad.]
8 layers AW
(flat)
16 layers AW
(flat)
8 layers AW
(wall)
16 layers AW
(wall)
32 layers AW
(wall)
Glassy phase 0,37 0,24 0,46 0,42 0,34
Rubbery phase 0,37 0,24 0,46 0,42 0,34
Interphase 0,26 0,52 0,08 0,16 0,32
5.2 Constituent materials
Before analyzing the behavior of composite laminates, constituent materials alone were tested in order to
obtain reference values. Storage modulus and damping properties of the glassy material have already been
presented in figure 36 (sub-chapter 4.2): a low-temperature glassy plateau around 3,5x109 Pa was detected.
Storage modulus decreases to approximately 107 Pa (high-temperature plateau) when temperature increases
above glass transition temperature, which is approximately at 59 °C for the glassy phase.
As mentioned in the experimental procedure (sub-chapter 3.2.2), two different samples of rubbery material
were produced in order to lie within machine minimum and maximum compliance: damping and elastic
properties had to be tested far below and above its glass transition temperature. Since the rubbery material
wasn’t stiff enough (above its glass transition temperature) with respect to the maximum measurable
compliance, the applied strain was increased up to 0,5%. Before that, strain sweep tests at room temperature
were carried out to verify whether changing this parameter affects the values we obtained. Since there were
no significant differences in the detected storage modulus E’ (figure 41), we can confirm that measurements
lie within the linear polymer regime. Hence, data from samples tested by applying 0,5% strain can be
compared with the other results.
46
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 110
5
106
107
E' (P
a)
strain (%)
Figure 41. Strain sweep test: rubbery material’s modulus vs strain.
.
Elastic and damping properties of the rubbery material were thus obtained by joining results below and
above glass transition temperature (figure 42). As explained in the experimental procedure, in the range of
temperature between 20 °C and 30 °C, data were mathematically averaged to obtain a unique curve. The tan
δ curve (figure 42) shows a sharp peak, whose maximum at 5 °C corresponds to its glass transition
temperature; as concerns elastic properties, the storage modulus strongly decreases from the low-temperature
plateau around 3x109 Pa to the high-temperature plateau approximately at 0,5x10
6 Pa. Properties of
constituent materials will be useful to analyze those of composite laminates.
Figure 42. Rubbery material: damping and elastic properties.
47
5.3 Composite samples
5.3.1 AL samples
The storage modulus behavior for AL samples is depicted in figure 43. It’s worth noting that rubbery and
glassy stacked layers act in series with respect to the applied stimulus. As matter of fact, the glassy material
(at room temperature) is much stiffer than the rubbery one; therefore, when samples are subjected to an
external stimulus, the applied deformation acts mostly on the rubbery component. In this sense, the results
obtained are close to the properties of rubbery material alone. Indeed, since the applied deformation acts only
on the volumetric fraction of the rubbery material which is equal to 0,5 (50%) in AL sample, the apparent
storage modulus values are twice those obtained for the pure rubbery sample (where the whole sample is
made of rubbery component, 100%) as shown in figure 43. The elastic modulus strongly decreases above
room temperature as depicted in the picture 42 above. This hypothesis is also confirmed by the tan δ
behavior, showing a sharp peak around 5 °C (figure 44) in correspondence of the pure rubbery glass
transition temperature. This configuration is not very helpful for the study of the effect of the interphase on
the overall elastic properties of composites but may be useful for a comparison with the properties of pure
rubbery material.
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
E' (2L)
E' (4L)
E' (8L)
Figure 43. Comparison of elastic properties of AL samples and rubbery material.
48
Figure 44. Damping properties of AL samples with 2, 4 and 8 layers.
.
5.3.2 AT samples
AT samples described in sub-chapter 3.2.3.1 were tested. The theoretical volumetric fraction of the
interphase is shown in figure 40. The results of the individual tests are reported in Appendix A. The averaged
curves for each specimen type are reported in figure 45. The storage modulus decreases from the low-
temperature plateau around 3x109 Pa to the rubbery plateau around 4x10
6 Pa. The results differ from the
preliminary ones presented in chapter: 2: by increasing the number of layers, i.e. increasing the relative
volumetric fraction of interphase, the overall elastic properties of the composite samples seem not to improve
when a tensile load is applied. However, the results are in strong agreement with the theoretical model in
which the presence of the interphase has been taken into account, as will be demonstrated later on (chapter:
6). Hence, the simpler stress state, characteristic of the tensile configuration, coupled with the symmetry of
the produced samples allow us a better understanding of the results. Still, the amount of interphase could be
too low to affect the overall elastic properties. Then, the idea is to tailor the design of the composite samples
in order to increase the volumetric content of interphase as much as possible: hence, AW samples were
subsequently produced as described in following sub-chapters 5.2.3.
49
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (
Pa)
T (°C)
E' (2L)
E' (4L)
E' (8L)
Figure 45. Comparison of storage modulus of AT samples with 2, 4 and 8 layers.
Three peaks were detected in the tan δ trend: a low-temperature peak, an intermediate-temperature peak and
a high-temperature peak (figure 46). The intermediate-temperature peak is an experimental artifact which
does not appear in other sample configurations, as will be demonstrated later on; its origin is still uncertain.
The high-temperature peak, instead, corresponds to the glass transition temperature of the glassy component
(Tg ≈ 59 °C) in the composite. The slight decrease is probably due to the diffusion phenomenon of the
rubbery material inside VeroWhitePlus (RGD835) layers. A low-temperature peak, too, was detected which
should correspond to the rubbery component inside the composite samples. Actually, pure TangoBlackPlus
(FLX930) bulk glass transition temperature is around 5 °C: this means that the apparent maximum of the tan
δ curve shifts from 5 °C (of pure rubbery sample) to -9 °C (of the rubbery component in the composite
sample). Yet, the peak is heavily suppressed since its maximum (slightly higher of 0,1) is much lower than
the maximum of tan δ peak of pure rubbery material (higher than 2,0 as depicted in figure 42).
Figure 46. Damping properties of AT samples with 2, 4 and 8 layers.
50
5.3.3 AW samples
5.3.3.1 “Wall-printed”
The idea is to maximize the interphase content by properly tailoring the design of composite samples. Before
proceeding with this analysis, AW samples were “wall-printed” with 8, 16 and 32 layers (as described in
sub-chapter 3.2.3.2), in order to verify the hypothesis made for AT samples. This printing setup in fact
promotes the generation of amount of interphase which is comparable to that of AT samples tested in the
previous sub-chapter 5.2.2: relative volumetric fraction of the components are still reported in 40. In this
sense, no variation of the storage modulus behavior should be expected as the number of layers is increased.
This statement is verified by the data reported in figure 47, where storage modulus curves are clearly
superimposed with no significant differences.
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (8L)
E' (16L)
E' (32L)
Glassy
Rubbery
Figure 47. Comparison between storage modulus curves of constituent materials (solid lines) and "wall-printed" AW samples.
The considerations made for damping properties of AT samples hold for “wall-printed” AW samples too
(figure 48). The intermediate temperature peak does not appear.
51
Figure 48. Damping properties of AT samples with 2, 4 and 8 layers.
5.3.3.2 “Flat-printed”
The effect of a higher amount of interphase on the elastic properties of composite samples was subsequently
evaluated. To do so, samples described in sub-chapter 3.2.3.2 were tested (AW samples with 8, 16 and 32
layers). The theoretical volumetric fraction of interphase for the 8-layers, 16-layers and 32-layers samples
are still reported in figure 40. The latter should theoretically be a homogeneous material whose properties are
characteristic of the interphase. Results are shown in figure 49, in which the storage modulus behavior of
constituent materials are also reported (solid lines). Slightly different behaviors were observed for 8 layers
samples (“low-interphase content”) and 16 and 32 layers samples (“high-interphase content”). This
phenomenon has to be ascribed to a sort of homogenization process as the number of layers is increased. As
already mentioned the 32-layers samples is a mixture of rubbery and glassy base constituents, thus being a
completely different material. Therefore, we can now state that the interphase is only slightly affecting
mechanical properties of the composite laminate (above room temperature). Whereas, at low or high
temperature when both base materials are either glassy or rubbery (i.e. below TangoBlackPlus FLX930 glass
transition temperature and above VeroWhitePlus RGD835 glass transition temperature), no clear differences
were detected.
52
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (8L)
E' (16L)
E' (32L)
Glassy
Rubbery
Figure 49. Comparison between storage modulus curves of constituent materials (solid lines) and “flat-printed” AW samples with 8,
16 and 32 layers.
To sum up, a low volumetric fraction of interphase, such as in case of AT samples and AW “wall-printed”
samples (equal to 0,32 at maximum), has no significant effect on the elastic properties of the laminate when
a tensile load is applied. Again, when the interphase volumetric fraction increases, up to 0,52 in case of AW
“flat-printed” specimen with 16 layers, there’s only a slight change of the overall elastic properties of the
composite sample. Hence, we would like to verify whether the experimental data obeys the rule of mixture or
not. In this sense, since the load is applied parallel to the plies of both AT and AW specimens, we will try
later to estimate the elastic properties by applying a simple mechanical model.
“Flat-printed” AW samples show two peaks in the tan δ curves (figure 50): the low-temperature peaks are
still very suppressed (around -10 °C and -6 °C). The 32-layers sample should be made of interphase only: as
a matter of facts, only one main peak is detected around 51 °C. In this case, the low-temperature is almost
negligible. This characteristic curve should therefore be interpreted as the characteristic damping properties
of the interphase. Samples with varying volumetric composition (described in sub-chapter 3.2.3.3) were
tested. The analysis is reported in the next sub-chapter.
53
Figure 50. Tan delta curves of "flat-printed" AW samples.
5.3.4 AT samples with varying composition
The volumetric fraction of components was varied in order to evaluate potential shifts of the tan δ peak when
the volumetric content of rubbery component is higher than 0,5. Elastic properties are shown in figure 51.
Damping properties results are shown in figure 52, compared with constituent materials damping properties
(solid lines). The intensity of the peak increases with the increasing volumetric content of the rubbery
material as we would have expected: dissipative phenomena are much more significant as the volume of the
rubbery material increases. Yet, the peak is heavily suppressed, even when the rubbery material volumetric
fraction is equal to 0,95: the damping properties trends for rubbery material alone and in the composite
sample are significantly different. The temperature at which the maximum of the peak is detected is still
lower than that of pure rubbery material; however, this value is apparent only due to the significant
suppression of the peak when rubbery material is combined with glassy layers. It would be useful to
understand which phenomena contribute to the intensity of the tan δ curve for pure rubbery material in order
to evaluate how can be hindered by the presence of surrounding glassy layers.
54
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
E' (0,500)
E' (0,625)
E' (0,750)
E' (0,875)
E' (0,920)
E' (0,950)
Glassy
Rubbery
Figure 51. Samples with volumetric fraction of the rubbery material equal to 0,5, 0,625, 0,750, 0,875, 0,92 and 0,95: damping
properties.
-60 -40 -20 0 20 40 60 80
0,0
0,5
1,0
1,5
2,0
2,5
tan
T (°C)
tan (0,500)
tan (0,625)
tan (0,750)
tan (0,875)
tan (0,920)
tan (0,950)
Rubbery
Glassy
Figure 52. Samples with volumetric fraction of the rubbery material equal to 0,5, 0,625, 0,750, 0,875, 0,92 and 0,95: damping
properties.
55
6. Mechanical model
The simple mechanical model introduced in sub-chapter 2.7 was used to describe the obtained results.
Equations 4 and 5 were used:
𝐸1 = 𝛴𝑉𝑖 ∗ 𝐸𝑖
1
𝐸2= 𝛴
𝑉𝑖
𝐸𝑖
where Vi and Ei are the volumetric fraction and the elastic modulus of the i-component, respectively.
6.1 Mathematical procedure
The test setup has been previously described in the experimental section: the load was applied in parallel to
the plies for AW and AT samples; in series for AL samples. The prediction of the elastic properties thus
follows from the subsequent considerations for our laminate samples:
a) AT and AW laminates can be generally described by Eq. 4 since the plies act in parallel to support
the applied stimulus. Instead, AL samples can be described by Eq. 5.
b) Laminates can be considered as composed of three phases: interphase, glassy and rubbery material.
c) The properties of the interphase were obtained from dynamic mechanical analysis on 32-layers AW
laminate samples, which can be considered as completely made of interphase.
Therefore, we will try to compare the ideal case in which the presence of the interphase is neglected, i.e. the
laminates are made of two phases only (rubbery and glassy ones), with respect to the case described in
previous point b).
6.2 Results and discussion
The analysis is divided into separate sub-chapters for the different sample configurations. In each graph the
blue curve represents the simplified theoretical model in which the presence of the interphase was neglected.
Vice versa, the red curve represents the expected storage modulus values obtained when the presence of the
interphase is taken into account. The relative error was evaluated for both cases exploiting the equation 6
below and by taking the maximum value over the whole range of temperature (from -50 °C to 80 °C).
𝑒𝑟𝑟𝑜𝑟 (%) =𝐸𝑚𝑜𝑑𝑒𝑙−𝐸𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙
𝐸𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙∗ 100
a) 2, 4 and 8 layers AL samples
The interphase content is very low as depicted in the above figure 3. The effect of the interphase on the
overall elastic properties is not observable: the foreseen curves are superimposed. Figures 53, 54 and 55
represent AL samples with 2, 4 and 8 layers, respectively. The maximum relative errors for blue and red
curves are comparable around 10-30%.
(Eq. 4)
(Eq. 5)
(Eq. 6)
56
-20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
2L experimental
2L model without interphase
2L model with interphaseGlassy
Rubbery
Figure 53. Theoretical model for the AL sample with 2 layers.
.
-20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
4L experimental
4L model without interphase
4L model with interphase
Glassy
Rubbery
Figure 54. Theoretical model for the AL sample with 4 layers.
57
-20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
8L experimental
8L model without interphase
8L model with interphase
Glassy
Rubbery
Figure 55. Theoretical model for the AL sample with 8 layers.
b) 8 layers “wall-printed” AW samples (figure 56)
The maximum relative errors for the blue and red lines are about 16% and 11%, respectively.
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
8L experimental
8L model without interphase
8L model with interphaseGlassy
Rubbery
Figure 56. Theoretical model for the "wall-printed" AW sample with 8 layers.
58
c) 16 layers “wall-printed” AW samples (figure 57)
The maximum relative errors for blue and red curves are 38% and 28%, respectively.
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
16L experimental
16L model without interphase
16L model with interphaseGlassy
Rubbery
Figure 57. Theoretical model for the "wall-printed" AW sample with 16 layers.
d) 32 layers “wall-printed” AW samples (figure 58)
The maximum errors for the blue and red curves are about 61% and 32%, respectively.
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
32L experimental
32L model without interphase
32L model with interphaseGlassy
Rubbery
Figure 58. Theoretical model for the "wall-printed" AW sample with 32 layers.
59
e) 8 layers “flat-printed” AW samples (figure 59)
The maximum errors for the blue and red curves are about 68% and 33%, respectively.
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
8L experimental
8L model without interphase
8L model with interphaseGlassy
Rubbery
Figure 59. Theoretical model for the "flat-printed" AW sample with 16 layers.
f) 16 layers “flat-printed” AW samples
16-layers “flat-printed” AW sample is the most interesting case since characterized by the higher content of
interphase. A strong agreement between ideal case and experimental data was obtained for low-temperature
and high-temperature plateaus. Instead, within the 30-60 °C temperature range the prediction of the elastic
properties does not match with the experimental results. By taking into account the weighted effect of the
interphase a better agreement between experimental and the model can be obtained as shown in figure 60.
The maximum errors for blue and red curves are 232% and 96%, respectively.
60
-60 -40 -20 0 20 40 60 80
106
107
108
109
E' (P
a)
T (°C)
16L experimental
16L model without interphase
16L model with interphaseGlassy
Rubbery
Figure 60. Theoretical model for the "flat-printed" AW sample with 16 layers.
Taking into account the presence of the interphase for the evaluation of the overall elastic properties of the
composites allows a better agreement between the experimental data and the theoretical ones. This works
especially where interphase volumetric fraction is significant, such as for AW “flat-printed” samples with 8
and 16 layers. Limitations to the applied procedure are listed in the following points:
a) The calculated volumetric fraction is theoretical only: by knowing the printing direction and
consequently the (ideal) interphase thickness, we were able to get the volumetric fraction value for
each sample. Still, there is a degree of uncertainty in the determination of the values of 175 µm for
“flat-printed” samples and 50 µm for the “wall-printed” ones.
b) “Flat-printed” AW sample with 32 layers was considered as homogeneous, with results
representing the interphase properties. This might not be correct, in particular if the interphase size is
different from what we assumed so far.
In any case, bearing in mind the main limitations just explained, the results are still interesting since the
whole set of tests shows that the expected storage modulus is in a better agreement with the experimental
data when the interphase properties are taken into account in the Eq. 2 (Rule of Mixture).
A comparison between the AT sample with 2 layers and the AW sample with 16 layers is reported is figure
61: a focus 30-60 °C range is shown. The red curves much better approximate the experimental data.
61
30 35 40 45 50 55 60 65 70 75 8010
5
106
107
108
109
E' (P
a)
T (°C)
2L experimental
model without interphase
2L model with interphase
16L experimental
16L model with interphase
Rubbery
Glassy
Figure 61. Comparison between theoretical model for AT and AW samples.
6.3 Further considerations
Samples described in sub-chapter 3.2.3.4 were produced: an example is shown in picture 62. The rubbery
layer seems to be strongly compressed due to presence of the external glassy ones. This means that:
1) The dimensional stability constituent materials is different after the polymerization process occurred
(performed at room temperature).
2) The stress state of the rubbery layer is not what we would have expected.
The simple mechanical model has to be modified taking into account this phenomenon in order to better
evaluate the overall mechanical response when an external deformation is applied to the sample.
Figure 62. Example of the obtained AW sample with 2 separated layers.
62
7. Conclusions and future developments
Additive manufacturing (AM) gained a lot of interest in the past decades. In particular, the Polyjet
technology is an intriguing technique to produce complex architectures (such as “interpenetrating phase
composites”, IPCs, introduced in sub-chapter 1.2.2) whereby materials with completely different physical
and mechanical properties can be combined. Using the Polyjet technique, photo-polymers (building
materials) can be injected in the form of liquid droplets and subsequently cured through the use of an
external UV lamp. A recent work [29] highlighted interesting aspects which have been completely neglected
before: the deposition of the building materials in their liquid state leads to the formation of an “interphase”
before the curing step is completed. The dimensions and the elastic properties of this interphase were
analyzed by means of nano-indentation and dynamic mechanical analysis (DMA, in 3-point bending
configuration), respectively, showing that its characteristic dimensions vary as a function of the printing
direction: 175 µm when the two building materials were simultaneously injected; 50 µm when they were
sequentially deposited. In the present work, samples were produced in the form of laminates, by alternating
layers of the glassy and rubbery photo-polymer. Dynamic mechanical analysis (DMA) in tensile
configuration was performed in order to investigate the mechanical behavior and interpret it through the use
of simple mechanical model. Elastic properties were only slightly changing above room temperature due to a
sort of homogenization process occurring when the number of layers was increased. The role of the
interphase was assessed through the use of a simple mechanical model (Rule of Mixture for parallel/series
composites) with a good agreement with the experimental data, once the presence of the generated
interphase was correctly taken into account. Damping properties of composite samples showed not-trivial
trends: a high-temperature peak was detected in tan δ curves which was related to the glassy component in
the composite sample. A low-temperature peak was also detected, in turn related to the rubbery component,
which was strongly suppressed. The maximum of the peak is apparent only: the reason of this phenomenon
could be ascribed to a sort of confinement phenomenon of the rubbery material occurring due to the presence
of glassy layers. Yet, the phenomenon was not clearly understood. To sum up, in light of the increasing
interest shown about this technique, this analysis opens the way for further studies since some phenomena
still have to be more clearly understood. In this sense, it is worth noting that the effect of the interphase on
the overall mechanical properties could be even more significant in the so-called “interpenetrating phase
composites” (IPCs) with respect to the simple laminate samples considered in this study. By recalling the
survey presented in sub-chapter 1.2.2, the generation of the interphase could be fundamental by affecting the
deformation mechanism of these complex architectures. Moreover, the crack confinement behavior typical of
these complex structures could be affected by the presence of the generated interphase. These phenomena
could be the next steps in the analysis of the role of the interphase in the mechanical behavior of 3D printed
components.
63
64
Appendix A
a) AL samples with 2, 4 and 8 layers (from the top to the bottom, respectively)
-20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-20 0 20 40 60 8010
5
106
107
108
109
E' m
ean (
Pa)
T (°C)
-20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-20 0 20 40 60 8010
5
106
107
108
109
E' (
Pa
)
T (°C)
-20 0 20 40 60 8010
5
106
107
108
109
E' (
Pa
)
T (°C)
E' (I)
E' (II)
E' (III)
-20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
65
b) AT with 2, 4 and 8 layers (from the top to the bottom, respectively)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (
Pa
)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
66
c) “Flat-printed” AW samples with 8, 16 and 32 layers (from the top to the bottom, respectively)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ea
n (
Pa
)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (
Pa
)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ean (
Pa)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ean (
Pa)
T (°C)
67
d) “Wall-printed” AW samples with 8, 16 and 32 layers (from the top to the bottom, respectively)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ea
n (
Pa
)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ea
n (
Pa
)
T (°C)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' (P
a)
T (°C)
E' (I)
E' (II)
E' (III)
-60 -40 -20 0 20 40 60 8010
5
106
107
108
109
E' m
ea
n (
Pa
)
T (°C)
68
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