effect of screw speed on morphology, tensile and thermal properties...
Post on 07-Jun-2020
0 Views
Preview:
TRANSCRIPT
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO71
Effect of Screw Speed on Morphology, Tensile and Thermal Properties of
In-situ Fibrillation PLA/LLDPE Blend Kantapong Samleekaew
1,2, Chanchai Thongpin
1,2*
1Department of Materials Science and Engineering, Faculty of Engineering and Industrial Technology,
Silpakorn University, Sanamchandra Palace Campus, Nakhon Pathom, 73000, Thailand. 2Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University Research
Building, Bangkok 10330, Thailand
*E-mail: THONGPIN_C@su.ac.th
Abstract:
In-situ fibrillation of polymer blends is the technique that can be used to improve
mechanical properties of polymers. It gives advantage over conventional composite because
the minor phase is in the form of filaments dispersed in the continuous phase. This research is
aimed to study effect of screw speed on in-situ fibrillation during extrusion cast film of
PLA/LLDPE blends. Processing conditions of screw speed were varied at 30, 60, 90, 120 and
150 rpm. The compositions of LLDPE in PLA were varied from 5 to 20 wt%. Morphology,
tensile and thermal properties of PLA/LLDPE blend were investigated. The results indicated
that, Young’s modulus, tensile strength of film blends increased with screw speed. This was
due to the length and the diameter change of LLDPE fibril. However, the mentioned
properties were lowered than those of neat PLA. The research pointed out that tensile
properties of PLA/LLDPE could be improved by the process conditions.
1. Introduction
Polylactic acid (PLA) is the most
interesting biodegradable polymer that can
be produced from renewable resource such
as corn starch and cassava starch. PLA is
usually used in packaging due to its good
mechanical properties in term of strength,
stiffness at room temperature but it is
limited for the brittleness.1 According to the
problem, PLA can be improved by blending
with soft polymer such as rubber and
polyolefin. Polyolefin such as polyethylene
has been widely used as commodity plastic
due to their relative low density, good
chemical resistance, good electrical
insulation and low cost.2 The addition of low
stiffness polymer inevitably brought
polymer blends to the lowered stiffness.3
Reinforcement was usually added to the
blends in order to maintain the stiffness.
The agglomeration of filler is usually
unavoidable. In-situ fibrillation of added
dispersed polymer phase in polymer matrix
is a technique that could improve
mechanical properties of polymer blends.4 In
addition fibers produced during melt
processing have advantage over
conventional composite because the form of
filaments are not entangles. Thus, the
agglomeration of fibrils is diminished. The
occurring of fiber from minor phase is
dependent not only upon viscosity ratio
between major phase and minor phase but
also the processing conditions.
This research is aimed to study the
effect of screw speed on in-situ fibrillation
during extrusion cast film of PLA/LLDPE
blends. The effect of screw speed on
morphology, tensile and thermal properties
were investigated.
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO72
2. Materials and Methods
2.1 Materials
Polylactic acid, PLA with the grade of
4043D (IngeoTM
Biopolymer 4043D) having
MFR of 6 g/10 min testing at 210°C with
weight of 2.16 kg and melting point of 145-
160 °C was manufactured from Nature
Works and supplied by BC polymer
marketing (Thailand). Linear low density
polyethylene, LLDPE (LL8420A) having
MFR of 20 g/10 min testing at 190 °C and
weight of 2.16 kg, density 0.924 g/cm3 and
melting point 123 °C was manufactured by
PTT polymer marketing (PTTPM) and
supplied by Global connections, Thailand.
2.2 Experimental method
Before melt compounding, PLA and
LLDPE were dried at 50°C for 8 h in hot air
oven. The film of PLA/LLDPE blends were
prepared by cast film extrusion using single
screw extruder, Labtech engineering,
Thailand. The compositions of LLDPE in
PLA were varied from 5 to 20 %. Processing
conditions in term of screw speed were
varied at 30, 60, 90, 120 and 150 rpm. The
temperature profile of screw to die in cast
film extrusion machine was set at 190 to 210
°C. Furthermore, the temperature of die and
chill roll were set at 195°C and 80°C
respectively.
2.3 Testing
Morphology of blends film was
investigated using scanning electron
microscopy, SEM, Hitachi, TM3030, Japan.
The specimens were coated with platinum
before investigation and scanned with 15 kV
electron beam produced form tungsten.
Crystallization of both PLA and
LLDPE in the films were followed using
differential scanning calorimeter, DSC
(METTLER TOLEDO, Switzerland). About
8 mg of sample was filled in a pan and
scanned in non-isothermal mode under
nitrogen atmosphere. The step of testing is
as followed: firstly the sample was heated
from 0 to 200 °C with heating rate 2°C/min.
The sample was held at 200 °C for 1 min
before cooling down to 0 °C with the
cooling rate of 2°C/min and the temperature
was held for 1 min. Then sample was
secondly heated to 200 °C with the same
heating rate as the first step. The
crystallinity of polymers (Xc) were
calculated using equation shown below.
1000
mΔHw
ccΔH-
mH
cX
(1)
Where Hm, Hcc and w are enthalpy of
melting, enthalpy of cold crystallization and
weight fraction of polymers in polymer
blends respectively. Hm0 is enthalpy of
100% crystalline polymer. The Hm0 of
PLA and LLDPE are 93.0 J/g and 289 J/g
respectively.4, 5
Tensile properties, i.e. Young’s
modulus, tensile strength and tensile strain
at yield of the films were investigated using
Universal Testing Machine (Instron 5969,
USA). The tested specimen were cut as
rectangular shape from casted film in
machine direction and tested at room
temperature according to ASTM D882 and
using 5 kN load cell with the crosshead
speed of 12.5 mm/min. At least 20 samples
were tested for each reported values.
3. Results & Discussion
3.1 Morphology
SEM micrographs of cryogenic
fractured surface in cross sectional and
longitudinal direction of various
PLA/LLDPE film are shown in Figure 1 and
2. It is important to bear in mind that PLA
and LLDPE are immiscible polymers thus
these 2 polymers should occur as phase
separated blends in which LLDPE appeared
as dispersed phase in PLA matrix, as seen
from the cross sectional SEM image in
Figure 1. In addition, fibrillation of minor
LLDPE phase would occur depending on
viscosity ratio between PLA and PE phase.3
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO73
Upon shearing in the metering zone of
single screw barrel, elongational and shear
flows were produced.6 This resulted in fibril
formation of minor phase. Thus the higher
screw speed will lead to smaller fibrils. As
shown in Figure 2, the higher the screw
speed the small fibril diameter is showed. In
term of LLDPE content, at the same rotor
speed, high LLDPE content led to slightly
larger LLDPE fiber. This is due to the
surface tension of the LLDPE and hence the
breaking up of LLDPE was rather difficult.
Also the effect of screw speed on fibrillation
was found very clear in every LLDPE
contents. Nonetheless, at rotor speed higher
than 90 rpm, LLDPE minor phase appeared
as sheet, as indicated in white circle in
Figure 2. This could be due to the
coalescence of LLDPE during high shear.6
Figure 1. SEM micrographs of cryogenic fractured surface in cross section of PLA/LDPE
blend processed with screw speed.
Figure 2. SEM micrographs of cryogenic fractured surface in longitudinal direction of
PLA/LLDPE blend processed with screw speed
Composition
of
PLA/LLDPE
Screw speed
30 rpm 60 rpm 90 rpm 120 rpm 150 rpm
95/5
90/10
85/15
80/20
Composition
of
PLA/LLDPE
Screw speed
30 rpm 60 rpm 90 rpm 120 rpm 150 rpm
95/5
90/10
85/15
80/20
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO74
3.2 Mechanical properties
Tensile properties are presented in
Figure 3. Considering the relation between
Young’s modulus and LLDPE contents in
the film, the results show that Young’s
modulus were decreased with the content of
LLDPE which owing to the incorporation of
low stiffness materials into stiff PLA.
Interestingly, at as high as 20 % LLDPE
content, Young’s modulus turned to
increased slightly compared to 15 %
LLDPE. This was thought to be the effect of
crystallinity of the LLDPE phase, as
depicted in Table 1. In term of screw speed
difference, at higher screw speed, PLA was
affected by the decreasing of Young’s
modulus. This was caused by the thermal
degradation of PLA upon heating and
shearing.7 In this context, tensile strength at
yield was interested as during tensioning,
plastic deformation of the film had occurred.
Tensile strength at yield was then reported.
It was found that yield strength was
decreased with the LLDPE contents
meanwhile strain at yield was increased with
LLDPE content. As it is generally known
that LLDPE is ductile semi-crystalline
thermoplastic that could perform large
plastic deformation under tension force.
There must be stress that had been
transferred from PLA phase to LLDPE
phase via interfacial bonding even though
the interaction was thought to be so weak
via van de Waals force. As the fibrils are
small then surface area should be so large
that induced the stress transfer from PLA
matrix to LLDPE minor phase. The plastic
deformation under tensioning of LLDPE
were then able to perform and resulting in
high yield strain. Having concentrating on
screw speed of single screw, higher screw
speed, fibril with smaller diameter was
obtained. The stress transferred should be
efficient leading to slightly increased in the
yield strength. Likewise, the elongation at
yield or yield strain was increased with
LLDPE contents. In this case of screw speed
did not significantly affect the elongation at
break except for LLDPE itself.
Figure 3 Young’s modulus (a), yield
strength (b) and strain at yield (c) of
PLA/LLDPE blends film.
(a) (b)
(c)
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO75
3.3 Crystallization of PLA/LLDPE film
The Tg, Tcc, of PLA, Tm and Xc of
PLA and LLDPE which were evaluated
from the 1st heating scan were shown in
Table 1. The Tg of PLA was found
unchanged and appeared at approximate
57°C except for the blends at 20% LLDPE
content in which the Tg of PLA was found
decreased about 2-4 °C. This could be due to
the degradation of LLDPE upon heating
under shear. LLDPE molecular chain
contains a large number of tertiary carbons
which are sensitive to heat shear. The
shearing force can induce chain breaking
under free radical formation of tertiary
carbon free radicals and hence chain
breaking could occurred,8 as also evidence
from the elongation at yield of LLDPE with
the screw speed. This short degraded
LLDPE segments could act as plasticizers
for PLA. The effect was very significant at
high LLDPE content.
Table 1. Data obtained from 1st
heating scan on DSC. PLA/LLDPE: 95/5
Screw speed Tg (°C)
(PLA)
Tcc (°C)
(PLA)
Tm (°C)
(LLDPE)
Xc (%)
(LLDPE)
Tm (°C)
(PLA)
Xc (%)
(PLA)
30 57.01 99.75 123.49 13.43 144.83/152.19 3.20
60 57.31 99.48 123.56 20.07 144.78/151.87 5.46
90 57.28 99.27 123.72 18.06 144.63/151.99 2.77
120 57.47 98.69 123.47 23.11 144.56/151.76 7.37
150 57.31 98.71 123.89 12.53 144.70/151.76 6.23
PLA/LLDPE: 90/10
30 57.16 99.55 122.66 18.79 144.65/152.21 6.14
60 57.23 98.76 123.54 15.50 144.74/152.13 2.33
90 57.29 98.35 122.94 27.68 144.37/151.73 5.81
120 57.76 98.09 123.94 16.92 144.80/152.22 3.89
150 57.47 97.71 123.76 12.53 144.55/151.64 1.42
PLA/LLDPE: 85/15
30 57.41 98.90 123.38 19.15 144.38/151.97 5.86
60 57.48 98.58 123.70 19.33 144.41/151.77 4.49
90 57.28 98.46 123.57 20.78 144.74/151.46 6.00
120 57.87 97.04 123.83 18.27 144.70/151.69 4.49
150 57.64 96.77 123.86 22.75 144.53/151.39 6.86
PLA/LLDPE: 80/20
30 55.23 99.20 123.41 21.28 144.28/152.01 7.27
60 55.79 98.17 123.72 18.70 144.89/150.87 6.18
90 53.79 96.97 123.46 20.10 143.83/151.06 5.60
120 54.65 96.86 123.62 17.98 143.89/151.65 4.84
150 54.53 96.50 123.45 21.18 143.76/151.32 6.25
© The 2019 Pure and Applied Chemistry International Conference (PACCON 2019) PO76
4. Conclusion
The PLA/LLDPE blend films were
shown as the immiscible blend from
scanning electron microscope (SEM). The
fibrillation of dispersed LLDPE PLA matrix
phase was affected by both content and
screw speed in addition to the viscosity
ratio. The content of fibril is important for
tensile behavior especially for the elongation
ability. The plastic deformation of LLDPE
phase could enhance the ductility of PLA
especially at the content as high as 15 %
otherwise the high LLDPE content caused
modulus to be inferior. Moreover, screw
speed would facilitate LLDPE to be well
dispersed and elongated hence stress transfer
from PLA matrix to LLDPE to perform
plastic deformation hence enhanced ductility
of PLA.
Acknowledgements The authors are in debt to the Center of
Excellence on Petrochemical and Materials
Technology (PETROMAT), The Petroleum
and Petrochemical College (PPC),
Chulalongkorn University, Thailand for the
financial support. Also sincere thank and
gratitude must be given to the Department of
Materials Science and Engineering, Faculty of
Engineering and Industrial Technology,
Silpakorn University, Thailand for the
research work.
References 1. Balakrishnan, H.; Hassan, A. and Wahit,
M.U. Journal of Elastomers & Plastics.
2010, 42, 223-239.
2. Hamad, K.; Kaseem, M. and Deri, F.
Asia-Pacific Journal of Chemical
Engineering. 2012, 7, 310-316.
3. Yi, X.; Xu, L.; Wang, Y.-L.; Zhong,
G.J.; Ji, X. and Li, Z.M. European
Polymer Journal. 2010, 46, 719-730.
4. Taib, R. M.; Ghaleb, Z. A. and Mohd
Ishak, Z. A. Journal of Applied Polymer
Science 2012, 123, 2715-2725.
5. Luyt, A. S. and Hato, M. J. Journal of
Applied Polymer Science. 2005, 96,
1748-1755.
6. Sundararaj, U.; Macosko, C. W.
Macromolecules. 1995, 28, 2647–2657.
7. Signoria, F.; Coltellib, M.B. and
Broncoa, S. Polymer Degradation and
Stability. 2009, 94, 74-82.
8. Mark, J.E., Erman, B., Roland, M.
Science and Technology of Rubber.
2013; PP 816.
top related