effect of screw speed on morphology, tensile and thermal properties...

© 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: [email protected]

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.


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


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


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)


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


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.

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