Chapter 6

Poly(acry lonitrile-co-butadiene-co-styrene)/

Acry Zonitri le-Butadiene Rubber

Binary Blends

B lends of acrylonitrile-butadiene-styrene and nitrile rubber are expected

to possess outstanding oil-resistance, impact and mechanical strength.

ABS copolymer has gained commercial importance because it combines the

impact strength of polybutadiene rubber and the tensile strength and heat

stability of the styrene-acrylonitrile copolymer (SAN) matrix.' In this chapter

the tensile, tear and impact properties, and morphological characteristics of

binary ABSINBR system are discussed. The melt-flow properties of the

blends were analysed using a capillary rheometer. The glass transition

temperature of the blends was measured by differential scanning calorimetry.

The degradation characteristics o f the blends were studied by

thermogravimetry.

'l'he properlies of ABS and NUll are given in 'l'able 6.1. 'I'he

surface cnergy mismatch (Ay,) for AI3S-NBR systcln is 1 mN/m

(= Y C , N H H - Y C , A U S ) . The low value of surface energy mismatch indicates low

interfacial tension in ABSlNBR system. The viscosity ratio of NBR and

ABS is 1.39 (= T N U R I T A B ~ ) . This viscosity mismatch between NBR and

ABS indicates phase segregation in the blend.

The density of ABS/NBR blends is shown in Fig 6.1. The addition of

ABS increases the density of the blends. ABSso blend shows a positive

deviation from the weight-average value while the ABS30 and ABS70 blends

show negative deviation. A positive deviat ion o f density from the

Table 6.1: Properties of the homopolymers in ABSiNBR blends

Polymer Solubility Critical surface tension Steady torque,'

parametera, for wettingb, t (Nm)

6 (cal f i cm'31?) YC (mN/m)

ABS 9.1 38 10.12

NBR 8.9 39 14.09

a: experimentally determined; b: obtained from ref. 2

0.90 , , , , , , , , , , , , , , , ,

0 20 40 60 80 100

Wt.% of ABS in the blend

203

- experimental - - 0 . - - theoretical

,

Fig. 6.1 : Density as a function of ABS concentration in ABSlNBR blends.

additive contribution o f respective component indicates increased

interfacial adhesion between the homopolymer phases in the blend.334

Thus, moderate level of adhesivity between the homopolymer phases is

present in ABSxo blcnds.

The properties of rubber-plastic blends are determined by (1) material

properties of rubber and plastic phases, (2) rubberlplastic proportions, (3) the

phase morphology, and (4) the interaction in the i ~ ~ t e r ~ h a s e . ~ The stress-strain

curves of the ABSloo, ABS70, ABSso, ABSjo and ABSo blends are shown in

Fig. 6.2. 'l'hc differcnce in the deformation characteristics of thc blends under

an applied load is evident from the stress-strain curves. The curves o f ABS5o

have distinct elastic and inelastic regions. The elastic moduli of the blends

Strain, %

Fig. 6.2: Stress-strain curves of ABSlNBR blends (posterior part of the curve of ABSO has not been shown in order to enhance the clarity other curves).

are found to be considerably reduced with the increase in rubber

concentration. I'he increased rubbery nature of ABS30 blend compared to

ABSSo can be explained in terms of the change in morphology of blends. In

ABS3" blend, ABS is dispersed in the continuous NBR phase.

The effect of ABS concentration o n the Young's modulus of the blends

is shown in Fig. 6.3. Pure ABS has a Young's modulus of z 361 MPa. The

moduli of the blends decrease with the increase in NBR content. The

decrease in modulus with NBR content is due to the fact that NBR is a very

low modulus material.

Fig. 6.4 shows the variation of stress-at-break as a function of wt.% of

ARS. Pure ABS has the highest tensile strength. With the increase in ARS

conlcnt, the tensile strength increases. The observcd increasc in tcnsile

strength with ABS is due to the higher proportion of the hard plastic phase

and thc cxtcnt of ~ntcrfacial adhesion. The tensile strength increases abruptly

from 5 0 wt.% of ABS onwards, corresponding to the change in phase

morphology of the blends.

It can be observed that the tensile modulus and stress-at-break

composition curves show negative deviation, i.e., blend propertics lie bclow

the additivity line upto 50wt.% ABS content. The observed negative

deviation is due to the poor interfacial adhesion between the homopolymer

phases, which causes poor stress transfer between the matrix and the

dispersed phase. An abrupt change in the slope of the tensile properties-

composition curve is seen between the composition range (50150) and (30170)

ABS/NBR. This change in slope can be explained in terms of the changc in

morphology of the blcnds. From 50 wt.% ADS, the NCR is dispersed as

domains in the continuous AL3S phase. 'l'heretbre the observcd change in

slope is attributed to phase inversion of ABS from dispersed phase to

continuous phase.

0 20 40 60 80 100 W t % of ABS in the blend

Fig 6 3. Young's modulus as a function of ABS coricentrat~on in ABSINBR blends

0 20 40 60 80 100 W t % of ABS in the blend

Fig 6 4. Stress-at-break as a function of ABS concentration in ABSINBR blends.

The variation of break strain % with blend composition is shown in

Fig. 6.5. The break-strain % of ABSINBR blend is found to decrease

monotonically with the addition of ABS. The break strain % also shows

negative deviation. The poor interfacial adhesion between the homopolymers

is reflected in the poor elongation property of the blends.

The effect of blend ratio on tear strength is shown in Fig. 6.6. l'he tear

strength increases as the ABS content increases. The tear strength values also

exhibit a n e g a t i ~ e deviation. 'l'he blends with high proportions of Al3S

(greater than 50 wt.%) tear at higher force, and it can be understood that in

these blends ABS forms the continuous phase.

Materials of superior impact toughness have been obtained in the past

by blending ABS with e~astomers.~. ' Among the rubber modified tough matrix

polymer blends, PCIABS blends and polysulfone of bisphenol (PSU)/ABS'

has been commercialized. PVCIABS blends have been developed with

significant increase in impact strength and heat distortion temperature.' The

impact properties of ABS/NBR blends are presented in Table 6.2. It is seen

that the notched as well as the unnotched Izod impact strength increases with

the addition of NBK upto 50 wt.%. After 50 wt.% NBR, the impact strength

is found to decrease sharply. 'I'his sharp increase is due to ihe continuous

rable 6.2: lmpaet properties o f ABS/NBR blends

Notch sensitivity#

-- 1.3 1

1.05

1.03

1.07

- -- - -- Unnotched Izod impact strength

# (ratio of impact strength unnotched / notched).

Notched Izod impact strength

- (J/m)

ABSINBR (30170)

ABSNBR (50/50)

ABSNBR (70/30)

ABSNBR (100/0)

516.7

2203.0

7560.5

3010.0

393.8

2084.8

7331.5

2796.5

-+-experimental

- - 0 - -theoretical

2 . , . .

0 20 40 60 80 100

Wt.% of ABS in the blend

Fig. 6.5: Break strain % as a function of ABS concentration in ABSlNBR blends.

140

120 4 experimental E 0 ~ -theoretical g 100 r 6 80 C

P t; 60 L

m

0 20 40 60 80 100

Wt.% of ABS in the blend Fig. 6.6: Tear strength as a function of ABS concentration

in ABSlNBR blends.

nature of the NBR phase. For making rubber- toughened thermoplastics, the

greatest toughness is achieved when the interparticle distance is smaller than

a critical valuey depending on how the fracture energy is dissipated. It is seen

that materials of superior impact toughness can be obtained by blending NBR

with ABS.

r

-experimental .- .-- - - -0 . . . theoretical

10 1 4 0 20 40 60 80 100

Wt.% of ABS in the blend

Fig. 6.7:Hardness as a function of ABS concentration in ABSINBR blends.

The effect of blend ratio on Shore hardness of AUSINBR blends is

shown in Fig. 6.7. I'he increase in hardness and the abrupt increase at higher

proportion of ABS can be explained by the phase inversion of ABS from

dispersed to continuous phase when its concentration in the blend was

increased from 50 to 70%.

The applicability of the various composite modelslO," described in

chapter 4 has been used to predict the modulus o f these blends. The parallel

model holds for blends in which the components are arranged parallel to one

another so that an applied stress elongates each component by the same

amount. The lowest series bound model holds good for blends in which the

components are arranged in series with the applied stress. The Halpin-Tsai

model predicts the modulus on the basis of the phase morphology in a rubber-

plastic blend systern. 'The Coran's model also takes into account the phase

morphology of thc blends, and predicts the modulus as well as the volume

fraction of the plastic phase corresponding to phase inversion. The Kerncr's

equation predicts modulus of blend systems in which monodisperse spherical ~ - --. -

0

0 0.2 0.4 0.6 0.8 1

Volume fraction of ABS in the blend

210

- Ser~es Eq. - Parallel Eq. - Halpin-Tsai Eq. - Coran's Eq., n = 1.97 - Kerner's Eq.

- - * - - Exper~mental

Fig.6.8: Experimental and theoretical values of Young's modulus as a function of volume fraction of ABS in ABSINBR blends.

domains are dispersed in the continuous phase. Fig. 6.8 shows the

experimental and theoretical curves of Young's modulus as a fuaction of the

hard phase volume fraction. It is seen that the experimental data are higher

than most of the theoretically predicted values. The data is close to the

Coran's model, in which the value of n= 1.97. The value of n = 1.97,

corresponds to Vk, = 0.49 as the hard phase volume fraction that corresponds

to a phase invers~on lrom dispersed phase to continuous phase.

'I'he average particle size versus blend composition is shown in

Fig. 6.9. The average particle sizes have been evaluated by preparing

ABSINBR blends of six different blend compositions, by solution-casting

technique. and measuring the size of about 4.00 particles using the image

analyser. The average particle size is lower in NBR-rich blends. A co-

continuous morphology for blends with 40 - 60 wt.% ABS is observed.

Addition of ABS beyond 60 wt.% increases the dispersed phase s i x .

The torque-time and temperature-time profiles obtained during melt-

mixing of ABSINBK blends were similar to those shown in Fig.5.1 and 5.2

for SANINBR blends. The final Brabender torque values of the blends

are given in Table 6.3. The final torque values decrease with increase in

Fig. 6.9: Effect of blend compositionon the dispersed particle size in ABSINBR blends.

I

CO-

continuous

2 5

5 2 %- .- In

Dispersed PBS phase

15

2

: : A a, 0 P a, 4 0 5

0 0 20 40 60 80

Wt.% of ABS in the blend

' morphology

*

-

D~spersed NBR phase

concentration of ABS in the blend. his' decrease in torque values from

ABS,@ to ABS7o can be attributed to the decrease in melt viscosity o f NBR on

addition of ABS

Table 6.3: Steady torque values (Nm) of ABSINBR blends.

AHSo ABSio ABSso ABS7o ABSloo

6.2. Rheological properties

Thc shear stress versus shear rate curves for A13SlNt3R blends are

shown in Fig. 6 10. 'The shear stress values increase linearly with shear rate

for the blends. The flow index values (n) were calculated from these plots

using the power law relationship (Eq. 4.14) and are given in Table 6.4. All

the samples show flow index values lower than unity due to their

pseudoplastic nature.

I'he effects o f blend ratio and shear rate on the shcar viscosity of

ABSINBR blends at an extrusion temperature of 190°C are shown in Fig 6.1 1.

As the percentage of ABS in the blend increases, viscosity decreases. The

differences are more prominent at low and intermediate shear rate region.

The viscosity of the blends shows deviation from the additivity line. The

experimental shear viscosities have been compared with the log-additive

values calculated using Utracki's equationi2 (Eq. 4.19) in Table 6.5. The

experimental viscosities o f (70130) and (50150) ABSINBR blends are

higher than the log-additive based viscosity values. In these blends,

NBR forms the dispersed phase and strong interactions among the domains

can be expected. This can lead to a reversible structural build-up leading to

an increase in the bulk viscosity.

. In the case of polymer blends, the shear rates at the phase interface of

the dispersed phase and the continuous phase may be different because of the

Fig 6 10 Var~atlon of shear stress w~th shear rate for ABSINBR

blends, at 1 90°C.

Table 6 4: Value of power law exponent (from Eq 4 14) for

ABSINBR blends at 190°C. Blends n' values

ABSINBR 01100 0.364 ABSINBR 30170 0.342 ABSINBR 50150 0.351 ABSINBR 70130 0.312 ABSINBR 10010 0.293

shear rate

0 20 40 60 80 100 Wt.% of ABS in the blend

Fig. 6 11 : Variation of shear viscosity with ABS concentration and shear rate for ABSINBR blends, at 1 90°C.

Table 6.5: Comparison of the theoretical (Utracki's Eq.4.19) and experimental melt viscosity* of ABS/NBR blends.

-~ ~~ - ~ ~~ . -- ~~ ~~~ ~ ~ ~ .

Wt.%of ABS 1 Shear rate = 691s / Shear rate - 2781s ' 1

. - . 1 . I I -1 *un i t : poise

difference in particle size and its distribution, interfacial adhesion and

difference in the viscoelastic properties of the two fluids. For this reason,

shear viscosity of polymer blends obtained from capillary rheometer is always

correlated with shear stress instead of shear rate. The effect of blend ratio

and shear stress on the viscosity of SANINBR blends at 190, 200, and 210°C

is shown i n Fig 6 12 i t is seen that the viscosity of all the systems decreases

with increase in shear stress at a fixed temperature. It is clear from the figure

that the viscosity of NBR decreases rapidly on addition of ABS and the

decrease is more than the expected intermediate viscosity values of the

blends

In order to further understand the effect of temperature on viscosity,

Arrhenius plots at a constant shear stress were made (log rl vs. 1IT). The

activation energies calculated from these plots are given in Table 6.6. It is

seen that the blends have lower activation energies and so are less

temperature sensitive than pure ABS.

Table 6.6. Flow activation energyu of ABSJNBR melts at a shear rate of 69.4 s- ' .

F1g 6 12a

ABSO I 9 ~ f c A ABS30 o ABS50

ABS70 ABSIOO

Shear stress, dyneslcrn2

Fig 6 12b 100000 --- -

a, !? o ABSO a X - A ABS30 - UI g 10000

o ABS50

!2 i I ABS70 > L

ro ABSIOO a, r 0

Shear stress, dyneslcm7

Shear stress, dyneslcm2

a, UI .- 0 a s - .- LO 0 10000 :: .- > L

m a,

6

1000

Fig. 6.12: variation of shear viscosity with shear stress at 190, 200, and 210°C for ABSlNBR blends.

ABSO A ABS30 0 ABS50

ABS70 ABSIOO

-

100000 1000000 10000000

6.3. Melt elasticity of blends

he elastic properties of SAN/NBR melts in shear flow are given in

Table 5 8 The addition of NBR in ABS resulted in higher extrudate swell,

first normal stress difference, and recoverable shear strain. The ABSlNBR

blends show higher melt elasticity than ABS.

Table 5.8: Melt elastic properties' of ABS/NI3R blends

-

Blend composition Die swell, Principal normal Recoverable shear

@el&) stress difference, strain, (7,)

(TI 1-TZ), dynes/cm2

6.4. Thermal properties

1 - fmslw 1 1.09

The thermal properties of ABS, NBR, and their blends were analysed

by DSC The Tg's of pure ABS and NBR are found to be at - 25.3 and

+1 10°C, respectively The DSC traces of the blends are shown in Fig. 6.13.

The transitions corresponding to NBR and ABS phases indicate that the

blends are incompatible and phase separated.

Table 6.8: Glass transition temperature# of ABS/NBR blends

# measured at shear rate = 691s and 190°C.

3.83 x 10' 1.20

-

T, of NBR-rich phase (OC) T, of ABS-rich phase ("C) # Based on DSC measurements.

ABSD

-25.3

--

ABSo

-25.3

107.0

ABSo

-25.6

104.5

ABSo

-27.8

103.3

ABSo

--

1 1 0 0

; j c i a . . l . . , . , , t l . , , 0ilr. l 3 3 1 - C " < d " t 2 5 6 .P I * . , I " , ~ < , 5,,L i,,,,",. . , 2 :<,.

I ', " I

i t . i r l i i n ~ $ t . o n

U , i r l 9 1 6 * C i l l * i l l i d , ,"L ", , , .L ,"I 3 . I I , I " L I l l I <

I ". : 1 1

. . . . , . . , / . . . , . . . 5 ,\

t . . 5" , ,> 8, 8 :,<, ,,,,

! I , I , , ! I , , I , . , ! , , , , 8

Fig. 6.13: 1)SC' ~ ~ l o l i l e s of AUSlNI1I< blends. (a) 30170; (h) 50150, (c) 70130 blcnds.

TGA O,"

I'lg. 6.4 f( i plots showing mass loss vs. tetnperatllre, ('or AHS/NLIK blends

1 : . 6.1s: 1l ' I 'Ci plots ~howing mass loss rillc vs. Iclnpcl-aiclrc, li,l- AI3SINI%I< I,lcnds

Therrnograv~n~ctric analysis o f the blends in air was carried out. The

TG and DTG curves of the blends are shown in Fig. 6.14 and 6 . 1 5 . The mass

loss at the end of the heating o f the sample is highest for A B S ~ O and lowest for

ABSW l'he results obtained from the TG studies are summarized in

'Table 6.9. The tempcraturc onset of rapid degradation (T,,,,,,,) and temperature

at which 50% mass loss occurs (T~ov,) increases with NBR concentration in the

blend Blend~ng o f N B K with ABS results in blends with improved thermal

stdblllty

l 'able 6.9: TGA data of ABS/NBR blends in air

1- Sample I Weight loss at 500°C. I To.sL% Tsos; / DTG peak 1

I,,,,,,, : Temperature at which onset of degradation occurs.

T511.,, Temperature at which 50% weight loss occurs

Comparison ol' the rate o f thermo-oxidat ion o f ADS with

polybutadiene (Pt3) and SAN copolymer indicate the fol lowing rank in

order o f decreasing oxidat ion rate: 1 4 , l S PB > ABS > SAN. I t was found

that the rubber phase in ABS oxidizes more rapidly than the rigid

component . Oxidat ion o f P B under these condi t ions results in

embri t t lement of the rubber owing t o crossl inking. Such embri t t lement

o f the elastomer phase in ABS would reduce the impact resistance. I K

examination o f thermo-oxidat ively degraded ARS indicated the

disappearance o f double-bond content associated with the

corresponding development o f carbonyl and hydroxy funct ions. 16

Kolawole and Agboola have reported the thermogravimeric

analysis o f PSIABS blends in air." The thermal resistance o f both

ABS and PS was improved upon blending. The explanation of this

behaviour has been associated with the degradation o f ADS into PS

phase, acting as inhibitors o f the PS macroradicals.

Similarly, i n case o f ABSINBK blends, the NHR degradation

products (in particular, 4-vinyl cyclohexene) diffuse into the ABS

phase, acting as inhibitors o f the macroradicals formed during thermo-

oxidation. Thus. an improvement i n the thermo-oxidative stability o f

ABSINBR blends is achieved compared t o ABS.

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