eva compatiblizer

8/18/2019 EVA Compatiblizer

1/5

Process abi l i t y and Therm al Proper t ies of

Blends of High Dens i ty Polyethy lene,

Poly Ethy1ene Terep h th alate) , and Ethyl Viny l

Acetate Compat i i ze

C . CHEN

and F.

S. LAI

Department

of

Plastics Engineering

Universityof Massachusetts at Lowell

Lowell, Massachusetts

01

854

Because of differences in chemical structu re a nd rheological characteristics,

high density polyethylene (HDPE) and poldethylene terephthalate) (PET) are in-

compatible when blended dur ing recycling of PET soft drink bottles. To improve

the properties of the blends, ethylene vinyl acetate copolymer (EVA) was used as a

compatibilizer. Based on torque rheometer t ests , the higher the concentration of

PET in the blends, the higher the initial loading torque. Blends of

50%

HDPE and

50%PET had the lowest equilibrium torque. Equilibrium torque was highest at 5%

EVA. The presence of EVA made only a slight difference in the glass transition

tempera tures of HDPE/PET blends. Higher EVA content in the blend resulted in a

lower melting endotherm. Thermogravimetric analysis showed that thermal stabil-

ity was independent of EVA content: b ut the more PET in the blend, the lower the

final weight loss.

INTRODUCTION

he wide variety of applications of plastics in in-

T ustry and the continuing increase in household

plastics consumption has led to serious waste dis-

posal problems. A good recycling program would not

only alleviate environmental pollution, but also re-

duce the cost of materials.

In

recent years, use of PET soft drink bottles h as

skyrocketed. Since poly(ethy1ene terephthalate) (PET)

ha s many good physical and chemical properties s uc h

as high streng th, good impact strength, light weight,

safe toxicology, and t ransparency, it is often selected

a s a container material. However, the round base

of the PET bottle is often made of high density poly-

ethylene (HDPE), which is incompatible with PET.

Therefore, ethylene/vinyl acetate copolymer (EVA) is

usually used

as

a compatibilizer to increase the com-

patibility of HDPE and PET.

The main purpose of this research was to study the

processability and thermal properties of HDPE/PET

blends with an d without EVA compatibilizer. The prc-

cessability of the HDPE/PET blend was est imated

from the experimental results of the torque rheo-

meter, from which the processing conditions for in-

jection molding, extrusion, etc. could then be

determined.

Thermal analysis was used to characterize the mis-

cibility and thermal stability of the blends, and the

effects of the EVA compatibilizer on them.

In many practical applications, miscible polymer

blends are not desirable: multi-phase blends are

among the most important commodities in the plas-

tics industry ( 1 , Z ) . However, poor blending of two

immiscible polymers will cause poor morphology,

weak interfaces, and unacceptable properties. This

problem can be addressed with the use of additives

referred to

as

compatibilizers, which a lter the interfa-

cial conditions between the different phases. Practical

compatibility may be achieved even when, in

a

ther-

modynamic sense , two polymers are not miscible 3) .

High crystallinity prevents compatibility with plas-

ticizers, because they are unable to separate the poly-

mer molecules sufficiently to penetrate between them

(4).

Thus, copolymers are frequently used as compati-

bilizers since each different segment adheres better

to one or the other of the blend ingredients. This

behavior leds to sufficient compatibility in the poly-

mer blend system.

Polymer blends consisting of two polymers th at

show miscibility usually exhibit one Tg.An immisci-

ble polyblend may show two separate Tgs and have

mechanical properties that depend in a nonlinear

way on the composition 3) .The crystallinity of ma-

472

POLYMER ENGINEERING AND SCIENCE MAR CH 1994 Vol. 34 No. 6


2/5

Blends

of

HDPE, PET and EVA

terials can be evaluated by observing the melting

transition and calculating the enthalpy change. The

material's thermal stability

is

usually judged by its

decomposition temperature, which is obtained by

plotting weight loss

against

temperature.

EXPERIMENTAL APPROACH

Materials

Three materials were

used in this

research:

high

density polyethylene (HDPE) sold by Dow Chemical

Co. under the designation 32060C; polfiethylene

terephthalate) (PET) manufactured by Eastman

Ko-

dak under

the

designation 9902; and ethylene vinyl

acetate (EVA) with

36

vinyl acetate content, manu-

factured by Du Pont

as

Elvax. Table

1

summarizes

rheological

and

thermal properties

of

HDPE

and

PET.

Equipment and Experimental Procedure

Blends of HDPE and PET were prepared in ratios of

75/25,

50/50,

25/75. The EVA compatibilizer was

added to these blends

in

concentrations of

0.3,

5,

7 ,

and 10 .Because the PET and EVA were sensitive to

moisture, they were dried in a vacuum oven prior to

blending, to prevent hydrolyhc degradation of the

polymers. The PET is also sensitive to heat and the

melt temperatures of HDPE and PET differ greatly.

Thus, when HDPE and PET were blended in the

Haake Torque Rheometer, the processing tempera-

tures were programmed in five steps: 1)processing at

250°C for 7 min, 2) reducing the temperature to

200°C over 2 min,

3)

processing at 200°C for

3 min,

4)

raising the temperature back to 250°C over 2 min,

and

5)

processing a t 250°C for 3

min.

The Haake Torque Rheometer was also used to

investigate the rheological properties of the blends.

The analysis was conducted with a System 40 soft-

ware copyrighted by Haake Buchler Instruments, Inc.

Four different EVA contents, 0. 2 , 5, and 10%.were

blended with 50 HDPE and 50%PET to investigate

the effects of EVA content on the rheological proper-

ties of the blends. In addition, three rotor speeds, 40,

60, and 80 rpm, were studied.

For all blends, differential scanning calorimetry

(DSC) was used to study the melting behavior. In th is

Table

1.

Torque Behavior and Thermal Properties

of

HDPE and PET.

Torque Behavior HDPE PET

Loading torque (G-M) 3369 4045

Minimum torque (G-M) 1037 96

Loading temperature( C) 141 247

Minimum temperature

( C)

190 272

Thermal Properties HDPE PET

80.7

Melting emp. ( C) 125.1 239.4

Melting Endotherm (J/g) 185.0 36.8

Glass transition temp. ( C)

Onset decomposition emp. ( C) 408.1 395.5

Weight

Loss

( ) 99.2 84.2

Maximum weight

loss

rate 23.4 19.9

Wrnin)

research, the Du Pont DSC 2910 and Du Pont Ther-

mal Analysis 2000 Controller were used. The cooling

system was supplied with liquid nitrogen which al-

lowed investigation at temperature as low as 170°C.

The heating rate was set

at

lO C/min and the tem-

perature continuously increased from 170 to

300 C.

A Du Pont TGA 2950 and the Du Pont 2000 Con-

troller were used to study the thermal stability

of

all

the blends. The heating rate was set

at

lO C/min,

and samples were heated from room temperature to

800°C.

RESULTS

AM

DISCUSSION

Haake Torque

Rheometer

To study the effect of EVA copolymer

on

the rheo-

logical properties of HDPE/PET blends, increasing

amounts of EVA were added to the HDPE/PET 50/50

blend. Based on

the

results ofprocessing torque mea-

surements by the Haake Torque Rheometer as shown

in Fig. 1 it was determined that the equilibrium

torque increased from

0

to 5% EVA concentrations,

suggesting

that

specific interactions were created

be-

tween the polymers. These interactions led to an in-

crease of resistance to flow. At 10%EVA, the viscosity

decreased, probably due to increasing free volume.

Observation of initial loading torques,

m.

2,

showed

that

they increased with increasing PET con-

tent. This was because the original PET material was

harder than the HDPE, leading to

a

high loading

torque created

as

high concentrations of PET in the

blend were put into the mixing chamber.

After processing for

a

couple of minutes, the torque

approached a stable and equilibrium value for

all

blends. The equilibrium torque was lowest for the

HDPE/PET

50/50

blend. This result is shown in

Rg.

and is consistent with the observation by Khelifi

and

L a i

(5)who used the capillary rheometer to mea-

sure the viscosities of similar blends with different

amounts of compatibilizers.

The higher rotor speeds led to higher torques since

the higher rotor speed was accompanied by a higher

5608 0

-

W

5 4 0

m

520

m

0

500

g 4 8 0

Li

6 460

;

4 0

f

:

2 0

w

4 0 0

2

380 4

0

2

4

6 8

EVA

Fig.

1 .

Equilibrium

torque

of

HDPE/ PET 50/ 50

as

a

func

tion of

E V A content.

POLYMER ENGlNEERlNG AND SCIENCE MARCH

1994

YO\.

34,

NO. 6

473


3/5

C. Chen and

F S.

i

6 5 0

-

L

2 6 2 5 -

m

E

6 0 0

m

'1 5 7 5

-

L

5 5 0

-

5 2 5 -

5 0 0 -

*

4 1 5

-

4 5 0 -

4 2 5

-

4

LT

4 6 0 0

h

4000

3 4 0 0

3 2 0 0

10

-1

2 3 0 0 0

2 0 0 0

u

+

2 6 0 0

%PET

IQg. 2.

Initial loading torque

as

a function of

PET

content at

60 rpm rotor speed.

o o

4 0 0

2 5

5 0 5

1

P E T

FUJ.

3.

Equilibrium torque

as

a function of

PET

content at

different rotor speeds.

shear rate. This can be explained by the power law

behavior commonly observed for polymer melts:

T = K j

( 1 )

where

T is

the shear stress,

y is

the shear rate, and

K and n are the power law parameters.

With a few assumptions, Blyler and Daane o b

served and derived an equation which

is

similar to

E q

1 (6.7):

M =

C(

n)KS

( 2 )

where

M

=equilibrium torque

n = power law index

C(

n)= a function which appears to be weakly depen-

dent on n

K = onstant in the power law shear stress/shear

rate relationship

S

= rotor speed

Figure

4

hows the plot of log torque

us.

log rpm for

various blend compositions. Straight lines were o b

2 8 5

2 8 0

-

m

2.15

z

0

a

-

5 2 . 7 0

0

2 . 6 5

2 . 6 0

1 . 6 1 7 1 8

1 9

LogIRotor Speed lrprnll

Rg.

4.

og (torque)

us.

log

(rotor speed, for

H D P E / P E T

blends with 5%

EVA.

tained, and the slope of the plot gave the power law

index n Different blends had different power law

indices. This result was not in accordance with the

observation by Blyler and Daane (6) and Abraham,

et

aL

7).This phenomenon may be explained by the

immiscibility of HDPE/PET blends an d the effect

of

EVA.

Higher PET conten ts resulted in a larger power

law index. This illustrated the effect of rpm on the

equilibrium torque, increasing with increasing con-

centration of PET in the blends

with

compatibilizers.

It

is

important to predict general processing condi-

tions for processing equipment. In this study, specific

parameters such as the screw speed, melt tempera-

ture, and barrel temperature, in recycling PET soft

drink bottles, might be estimated from the results

measured by the torque rheometer. Also, energy con-

sumption was calculated from the area under the

curve of torque

us.

time. Thus cost of manufacturing

could be reduced

if

the proper processing conditions

were selected.

The energy required to process plastic materials at

a given temperature and a given shear rate may be

calculated from the following formula

7,8):

W = 2 ~ n ~ ~ M d t

1

3)

where

n = the number of revolutions of the rotor

t , = the initial time

t2= the final time

M

= the torque

Table 2 shows the energy required to process the

blends of HDPE and PETwith

5

EVA. These calcula-

tions demonstrate that higher rotor speeds and

greater process stable torques would require more

energy.

474

POLYMER ENGINEERING AND SCIENCE MARCH 1994,Vol.

34,

No. 6


4/5

Differential

Scanning

Calorimetry DSC)

It was

difficult

to detect the glass transition temper-

ature of HDPE because of complications that are

introduced by crystallinity. The glass transition tem-

perature of HDPE

is

usually reported to be between

120 to -20°C.

In HDPE/PET 75/25 and 50/50 blends, the glass

transition temperatures

of

PET could not be detected

by DSC, because the amount of PET in the blends

was too small, and the location of the PET glass

transition was affected by the HDPE melting curve.

However, the glass transition temperature of PET at

75% PET was readily observed, as shown in Table 3,

and

was

almost the same

as

that of pure PET. This

indicated that HDPE and PET remained immiscible,

even after addition of EVA.

The melting endotherms of the blends are

a

mea-

sure of the degree of crystallinity of the polymers in

the blend. The decrease

in

degree of crystallinity of

HDPE and PET implies that there

is

some interaction

created by the EVA copolymer.

Fig

5

illustrates that

higher concentrations of EVA in blends resulted in

lower heats of fusion, suggesting increased EVA prcr

duced interaction between the materials, causing the

crystallinity to decrease. However, this does not mean

that increasing the EVA content in the blends would

necessarily produce better properties. Since EVA acts

like

a

plasticizer, it might increase the impact pro-

perties but reduce the modulus and the ultimate

strength. The determination of compatibility of blends

in industry is dependent upon the properties needed

for each end use. The EVA content in the blend for

each specific use would be determined by

a

combina-

tion of processability, thermal properties, and me-

chanical testing.

ThermogravhetricAnalysis (TGA)

The Thermogravimetric Analyzer

=A)

was used to

characterize the thermal stabilities of the blends of

HDPE and PET. Table

4

shows tha t the initial decom-

TaMe 2. The Energies Required o Process he Blends

of

HDPE and PET with 5% EVA at Different Rotor Speeds,

and to Process he Blends

of 50

HDPEISO PET With

and Wrthout EVA at 60 pm.

Blends ofHDPE, PET andEV

POLYMER ENGINEERING AND SCIENCE, MARCH 1994, Vol.

34, NO. 6

475

Energy Required (Joules)

Rotor

75 HDPEl

50

HDPEl

25 HDPEI

speed 25 PET

50 PET 75 PET

40

pm

1919 1617 1651

60 pm

3302 2986

3025

80

pm

4886 4479 4675

EVA

0

2% 5% 10%

50HDPE/50PET 2482 2686 2986 2302

Table 3.

Glass

Transition Temperatures

of

PET in

HDPE/PET 25/75 Blends With and Without EVA.

EVAConc.

0

3% 5% 7%

10%

T-

PC

80.9

80.5

79.9 80.4 79.9

position temperatures (9-1 1) of all blends were near

that of PET. In addition, Table 5 shows that the final

weight loss of PET was less than that of HDPE, and

the EVA compatibilizer had little effect on final weight

loss.

CONCLUSIONS

The addition of EVA caused

a

change

in

rheological

properties of the HDPE/PET blends, suggesting that

it affected the specific interaction between the poly-

mers. As the concentration of EVA increased from 0

to

5%,

the equilibrium torque increased. However,

when the EVA content increase to 10 of the blend,

the torque decreased. Again, probably due to the fact

that EVA acted like a high molecular weight plasti-

cizer, creating more free volume and thus reducing

the viscosity of the blend.

An increase in the rotor speed led to an increase in

process torque because of the resistance to shear.

Also,

it was found that

a

higher PET concentration

caused

a

higher initial loading torque. The HDPE/PET

100

, , 2 6

:

HDPE

x : PET

t

::

2 5

\

21

0

2

4 i 8

50

8EVA

Flg. 5. Melting

endothem

of HDPE

nd

PET

as afunction

of

EVA

in 50 H.DPE/ 50 PET blends.

Table4 Onset Decomposition Temperatures in C

of

HDPE

and PET Blends With and Without EVA.

(Onset Decomposition Temperature

of

pure PET 395.VC.)

75HDPE 50HDPE 25HDPE

EVA Conc. 25 PET

50

PET 75 PET

0% 395.5 395.8

396.4

3% 395.2

395.2 394.9

5% 395.8 395.9

394.4

7% 398.3

396.5 395.0

0%

397.6 395.6 395.0

Table 5. Weight Loss

( I of

HDPE and PET Blends With

and Without EVA. (Temp. Range: Room Temp. to

SOOT.

75HDPE 50HDPE 25HDPE

EVA Conc. 25 PET

50

PET 75 PET

0%

96.4 92.3 87.8

3% 96.3 93.2

90.0

5% 96.5 93.2

89.5

7% 97.3 93.7 88.8

10% 97.3 93.3 88.6


5/5

C . Chen and F.S.

Lai

50/50

blend had the lowest equilibrium torque, re-

quiring less energy for processing.

Increasing EVA content in the blends led to de-

creased melting endotherms, indicating lower degree

of crystallinity of the blends.

EVA had little effect on thermal stability. The main

factor affecting thermal decomposition was the

HDPE/PET ratio.

ACKNOWLEDGMENT

The authors would like to express their special

thanks for Dr.

R

D. Deanin of the Department of

Plastics Engineering at the University of Mas-

sachusetts Lowell for his invaluable help and useful

suggestions.

REFERENCES

1.

J.

A. Brydson,

Plastics Materials

Butterworth Scientific,

Boston (

1982).

476

2. L.

A.

Utracki,

Polymer Alloys

and

Blends-Themody

namics and Rheology Hanser, New York

(1990).

3. M. Kryszewski. in Polymer Blends M. Martuscelli and

R. Palumbo, eds., Plenum P ress, New York

(1980).

4. R D. Deanin,

Polymer StructureProperties

and

Applica-

tions,Cahner s, Boston

(1972).

5.

A. Khelifi and F. Lai,

SPEANTEC

Tech Papers,34,

1824

(1988).

6.

L. L.

Blyler and J . J . Daane,

Polyym

Eng. Sci., 7 , 178

(1967).

7.

D. Abraham, K. E. George, and D.J. Francis, Eur. Polym

J.,

6

(2). 197 (1990).

8.

Z.

Bartha, P. Erdos, an d J . Matis,

Int

Polyym Sci. Tech

nol, 1 61, 50 (1983).

9.

M.

E.

Brown, Introduction to Thermal Analysis Teck

niques

and

Applications

Chapman and Hall, New York

(

1988).

10.

W.

W. Wendlandt and P.

K.

Gallagher, in Thermal

Char-

acterization of Polymeric Materials

E. A.

Tun,

d., Aca-

demic Press, New York (1981).

1 1. W. W. Wendlandt, Thermal

Analysis

Jo hn Wiley & Sons,

New York (

1986).

POLYMER ENGINEERING AND SCIENCE, MARCH 1994, Vol. 34, No. 6

eva compatiblizer

Documents