simultaneous in-situ analysis of instabilities and first ... · ited by the onset of instabilities,...

Tailoring polymer nanocomposite microstructure by controlling orientation,dispersion and exfoliation of GnP in LDPE via extrusion flow

Govindan Induchoodan and Roland Kádár

Chalmers University of Technology, 41258 Gothenburg, Sweden

ABSTRACTThe orientation of low density polyethylene -graphite nanoplatelets polymer nanocompos-ites during extrusion flow was considered inthis study, with the purpose of controlling thenanocomposite orientation and dispersion fordesired extrudate properties. The emphasis wason orientation of the nanofillers as a function ofthe composition of the nanocomposite, die ap-parent shear rate and draw ratio. Scanning elec-tron microscopy analysis of the extrudates re-vealed a strong orientation of the nanoplateletsin the flow direction provided the concentrationof fillers is low enough to avoid agglomeration.The achieved orientation was independent ofthe applied shear rates and filler type, however,a slight de-orientation could be achieved by im-posing of draw ratios greater than 1. The orien-tation of the fillers in the flow direction could beobserved as early as the transition zone duringscrew channel flow, with the platelets orientedalong the secondary flows therein formed.

INTRODUCTIONGraphene stands out as potential nanofiller inpolymeric melts due to its outstanding me-chanical, dielectric, barrier, thermal, etc. prop-erties.1 However, significant challenges re-main to be overcome in the development ofgraphene-based consumer products. In the caseof polymer-based nanocomposites, three mainchallenges can be envisioned: (i) graphenepreparation, (ii) its incorporation into poly-mers and (iii) tailoring the microstructure dur-ing processing to achieve the desired proper-ties. The main issues regarding the prepara-tion of graphene refers to the absence of af-fordable large quantities of defect-free single

layer graphene.2 Thus, most efforts are directedtoward investigated few-layers thick graphenenanoplatelets also called graphite nanoplatelets(GnP). The incorporation of graphene intopolymers is a critical step for the propertiesof the nanocomposites and several options areavailable, such as in-situ polymerization, solu-tion blending, melt blending etc.3 Melt blend-ing is a fast and simple route for preparing mas-ter batches for polymer melts albeit its exfolia-tion is typically extremely low. Overall, the de-gree of exfoliation and the quality of dispersionis a function of the preparation of the grapheneand its incorporation into polymers. Thereafter,the processing stage is of utmost importancefor obtaining graphene-enhanced properties inpolymer nanocomposites. By method of pro-cessing, the microstructure can be tailored toattain the desired material properties by de-agglomerating the particles, improving disper-sion and, ensuring the desired orientation of thenanofillers in the melt. For example, to attaingood electrical properties the platelets must bepercolated and randomly oriented, whereas theplatelets must be oriented perpendicular to thedirection of gas diffusion to attain good bar-rier properties. Thus, with respect to process-ing it is of utmost importance to understandthe flow field - filler interaction in polymermelt nanocomposites. In this context, in thispublication we explore the orientation dynam-ics during the extrusion flow of polyethylene -graphite nanoplatelets nanocomposites.

EXPERIMENTAL

Two variants (M5 and M25) of Grade-MXGNP graphite nanoplatelets (GnP) were used

Simultaneous In-situ Analysis of Instabilities and First Normal StressDifference during Polymer Melt Extrusion Flows

Roland Kádár1,2, Ingo F. C. Naue2 and Manfred Wilhelm2

1 Chalmers University of Technology, 41258 Gothenburg, Sweden2 Karlsruhe Institute of Technology - KIT, 76128 Karlsruhe, Germany

ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 24, 2016

ABSTRACTA high sensitivity system for capillaryrheometry capable of simultaneously de-tecting the onset and propagation of insta-bilities and the first normal stress differ-ence during polymer melt extrusion flowsis here presented. The main goals of thestudy are to analyse the nonlinear dynam-ics of extrusion instabilities and to deter-mine the first normal stress difference inthe presence of an induced streamline cur-vature via the so-called ’hole effect’. Anoverview of the system, general analysisprinciples, preliminary results and overallframework are herein discussed.

INTRODUCTIONCapillary rheometry is the preferredrheological characterisation method forpressure-driven processing applications,e.g. extrusion, injection moulding. Themain reason is that capillary rheometry isthe only method of probing material rheo-logical properties in processing-like condi-tions, i.e. high shear rate, nonlinear vis-coelastic regime, albeit in a controlledenvironment and using a comparativelysmall amount of material.1 Thus, it isof paramount importance to develop newtechniques to enhance capillary rheome-ters for a more comprehensive probing ofmaterial properties. Extrusion alone ac-counts for the processing of approximately35% of the worldwide production of plas-tics, currently 280⇥ 106 tons (Plastics Eu-rope, 2014). This makes it the most im-portant single polymer processing opera-

tion for the industry and can be found ina variety of forms in many manufacturingoperations. Extrusion throughput is lim-ited by the onset of instabilities, i.e. prod-uct defects. Comprehensive reviews on thesubject of polymer melt extrusion insta-bilities can be found elsewhere.4,6 A re-cent method proposed for the detectionand analysis of these instabilities is that ofa high sensitivity in-situ mechanical pres-sure instability detection system for cap-illary rheometry.8,10 The system consistsof high sensitivity piezoelectric transducersplaced along the extrusion slit die. In thisway all instability types detectable, thusopening new means of scientific inquiry. Asa result, new insights into the nonlinear dy-namics of the flow have been provided.9,14

Moreover, the possibility of investigatingthe reconstructed nonlinear dynamics wasconsidered, whereby a reconstructed phasespace is an embedding of the original phasespace.2,14 It was shown that a positive Lya-punov exponent was detected for the pri-mary and secondary instabilities in lin-ear and linear low density polyethylenes,LDPE and LLDPE,.14 Furthermore, it wasdetermined that Lyapunov exponents aresensitive to the changes in flow regime andbehave qualitatively different for the iden-tified transition sequences.14 It was alsoshown that it is possible to transfer thehigh sensitivity instability detection sys-tem to lab-sized extruders for inline ad-vanced processing control and quality con-trol systems.13

A very recent possibility considered

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and their main characteristics are presented inTable 1, as provided by the supplier. A pack-aging grade polyethylene with a long chainbranched molecular architecture, i.e. low den-sity polyethylene was used as matrix. Masterbatches of high filler concentrations (e.g. 16.7wt%) were produced via melt mixing in a twinsscrew extruder with the melt zone temperatureset to 190�C. The effect of concentration wasstudied by diluting the master batch to concen-trations of 7.5 wt% and 1.5 wt% of both M25and M5. The dilution was performed in a sin-gle screw extrusion with a compression screw(compression ratio of 2:1) and Maillefer screwwith a Saxton mixing element and a 3 mm die.The dilution was achieved by several runs foreach batch. A compression screw (compressionratio of 2:1) was used for the final extrusionflow study, equipped with dies of dimensions10 mm x 0.45 mm and 20 mm x 0.45 mm.

For both dilution and shaping, a Brabendersingle screw extruder 19/25D (barrel diameterof 25 mm and barrel length of 25 ⇥ 19) wasused. The extruder was equipped with Terwin2000 series (model 2076) melt pressure sen-sors with maximum pressure of 700 bar forinline rheometry. Shear viscosity and normalstress difference measurements via the ’holeeffect’4, 5 were performed, however they arenot discussed in this publication. The extrusionsetup is complemented by two Brabender con-veyor belts with belt speeds of 6 m/s and 18m/s, respectively and rollers to impose a drawratio, and an inline visualization system formonitoring the onset of instabilities.4 Overall,the influence of the apparent (die) shear rate,

Table 1. The main characteristics of thegraphite nanoplatelets (GnP) used in the

present study, as provided by the supplier.

Grade

Averageparticlediameter

µm

Surfacearea (BET)

m2g�1

Averagethickness

nm

Densitykg/cm3

M5 5 120 - 160 6 - 8 2.2

M25 25 120 - 160 6 - 8 2.2

defined6 as ga = 6Q/WH2, where Q is the vol-umetric flow rate entering the die and W and Hare the width and height of the slit die, and ofthe draw ratio defined as v2/v1, where v2 is thelinear velocity between the rollers and v1 is theaverage velocity inside the extrusion die. Scan-ning electron microscopy (SEM) analysis wasperformed on the extruded films in order to as-sess the orientation of the GnP.

RESULTS AND DISCUSSIONBy controlling the flow and drawing deforma-tion rates and the material composition dif-ferent microstructures can be obtained. In thecase of filler concentration over 7.5wt% e.g.the master batches a strongly agglomerated mi-crostructure was observed, e.g. see Fig. 1(a).The high filler concentration signifies also thatthe fillers are distributed until close to the sur-face of the films thus affecting their appear-ance. Any orientation effects observable can berelated to the orientation of agglomerates alongtheir principal axes to the flow direction. Thiswas characteristic of the microstructure inde-pendent of the shear rates, ga � [35,350] s�1,draw ratios applied, the maximum draw ratiobeing v2/v1 = 5 for ga = 35 s�1 and the GnPgrade. It should be noted that the ability of theextrudates to withstand the applied draw ra-tios was reduced for the filled samples com-pared to the unfilled samples, e.g. for LDPEthe maximal draw ratio applied at ga = 35 s�1

was v2/v1 = 13, limited only by the speedof the conveyor belt, whereas in the case ofthe LDPE-GnP nanocomposite sample fracturewas recorded above v2/v1 = 5. For diluted GnPconcentrations, i.e. 7.5wt% and 1.5wt% and inthe absence of an applied draw ratio well dis-persed and perfectly oriented GnP in the flowdirection for all shear rates investigated, e.g.Fig. 1(b), (c) independent the GnP grade. Itshould be noted that even at the lowest shearrates investigated (ga = 35 s�1) the orienta-tion and de-agglomeration of the fillers wasobserved, which can be related to the onsetof nonlinear deformations in the melt (belowthe shear rats applied during extrusion) and thelong residence time distribution associated to

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5 μm 5 μm50 μm

2 μm 20 μm

(a)

<date><location>

5 μm 5 μm50 μm

2 μm 20 μm

(b)

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5 μm 5 μm50 μm

2 μm 20 μm

(c)

Figure 1. Nanocomposite morphology at various compositions and in various flow conditions: (a)highly agglomerated structures with a composition of 16.5wt% GnP - M5, ga = 264 s�1,

v2/v1 = 2, (b) highly oriented GnP - M25 in the flow direction with a composition of 7.5wt%,ga = 97.3 s�1 and (c) highly oriented GnP - M5 in the flow direction ga = 1351 s�1. The z-axis in

the images corresponds to the flow direction.

extrusion processing. By subjecting the com-posite melt to drawing in the belt zone the fillerorientation was further altered. As the draw ra-tio increases, i.e. v2/v1 > 1, the platelets exhib-ited a tendency to change their orientation fromthe direction of the flow to match thinning pro-file of the films i.e. an orientation with an an-gle relative to the flow direction was induced.The onset of orientation was traced in-situ asfar as the towards the middle of the meteringzone, Fig. 2(a), zone that is responsible for themelting of the feed and due to the screw chan-nel height, corresponds to the minimal shearrates existing in barrel. Thus, in a screw chan-nel cross-section, the GnP are oriented in thedirection of the secondary flows, Fig. 2(b).

From flow stability point of view, the onsetof instabilities in the LDPE, surface distortionscharacterized by one characteristic frequency,was at apparent shear rates of around ga = 80s�1. The presence of the GnP has a stabilizingeffect on the flow, with the onset of instabilitiesis delayed toward apparent shear rates of over300 s�1.

SUMMARY AND CONCLUSIONSThe processing stage is of utmost importancefor obtaining graphene-enhanced properties inpolymer nanocomposites. By method of pro-cessing, the microstructure can be tailored toattain the desired material properties by de-agglomerating the particles, improving disper-










189

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i ii iii

/rpm

/rpm

(a)

<date><location>

i ii iii

(b)

Figure 2. In-situ qualitative observations of the orientation dynamics towards the end of themetering zone, (ii), of a compression screw, compression ratio 2:1, (a), during the extrusion flow

of PE-GnP nanocomposites showing the orientation of GnP along the streamlines of the secondaryflows, (b). The following notations were used for the screw zones: (i) solid conveying zone, (ii)

transition zone and (iii) metering zone.

sion and, ensuring the desired orientation ofthe nanofillers in the melt. Thus, the orientationof low density polyethylene (LDPE) - graphitenanoplatelets (GnP) polymer nanocompositesduring extrusion flow was considered in thisstudy with emphasis on orientation. A strongorientation of the nanoplatelets was achievedorientation for filler concentrations lower than16.5 wt%, independent of the applied die shearrates and filler type. However, a de-orientationcould be achieved with the application of adraw ratio. The orientation of the fillers inthe flow direction could be observed as earlyas the transition zone during screw channel

flow, with the platelets oriented along the sec-ondary flows therein formed. Overall, extru-sion flow is shown to be capable of successfullyde-agglomerating the nanofillers and achievingorientation in LDPE - GnP systems, while exfo-liation must be sought in the preparation stageof the nanocomposites.

ACKNOWLEDGEMENTS

The initial trials have been funded through theVinnova Grant SIO Grafen Grant - Graphene asbarrier and packaging material. The authors aregrateful to Kristina Karlsson for SEM support.

G. Induchoodan and R. Kadar

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REFERENCES1. Ferrari, A.C., Bonaccorso, F., Fal’ko, V.,et al. (2015) “Science and technology roadmapfor graphene, related two-dimensional crystals,and hybrid systems.” Nanoscale, 7, 4598–4810.2. Paton, K.R., Varrla, E., Backes, C., et al.(2014) “Scalable production of large quantitiesof defect-free few-layer graphene by shear ex-foliation in liquids.” Nat. Mat., 13, 624–630.3. Janas, D. and Koziol, K.K. (2014) “A reviewof production methods of carbon nanotube andgraphene thin films for electrothermal applica-tions.” Nanoscale, 6, 3037–3045.4. Kádár, R., Naue, I.F.C., and Wilhelm, M.(2014) “Simultaneous in-situ analysis of insta-bilities and first normal stress difference duringpolymer melt extrusion flows.” Annu. Trans. ofthe Nordic Rheol. Soc., 22, 153–160.5. Teixeira, P.F., Hilliou, L., Covas, J.A., et al.(2013) “Assessing the practical utility of thehole-pressure method for the in-line rheologi-cal characterization of polymer melts.” Rheol.Acta, 52, 661–672.6. Walters, K. (1975) "Rheometry". Chapmanand Hall, London.










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