Chapter 1

Introduction

1.1. Polymer Blends

T oday, polymer blends consti tute over 30% of polymer consumption,

and with an annual growth ra te o f over 9%, their role is certain t o

increase. There are a number o f good reasons for blending; however,

these tend to change with time and geographic location. In the 1960s,

the principal reason for blending w a s modification of a specific resin for

a specific type of behavior, in most cases, improvement of impact

strength. During the next decade, blending was used t o gain direct

economy by diluting expensive engineering resins with commodity ones.

During the 1980s, the importance o f the high temperature specialty

resins required improvement of processabilty. ' Currently, blending aims

at securing sets of specific propert ies required for an envisaged

application For instance, the blend formulated for use in automotive

body panels must be easy t o mold t o precise dimensions, must retain

shape at temperatures upto 85OC, be impact resistant down t o -40°c ,

resistant to gasoline, motor oil, and soap solution, must be paintable,

recyclable, economic, etc. The only way such a dream list of properties

can be met is by combining characteristic properties of several polymers

into a multicomponent system. Generally, the following material-related

reasons are cited for blending:'

Developing materials with a full set of desired properties;

Extending engineering resin's performance by diluting them with low

cost commodity polymers;

Improving a specific property, e .g . , impact strength, rigidity,

ductility, chemical-cum-solvent resistance, barrier properties,

abrasion resistance, flammability, gloss, etc. ;

Adjusting the material performance t o fit customers specifications at

the lowest price;

Recycling industrial/municipal plastics waste.

As far a s the producer i s concerned, t h e .following advantages o f the

blending technology have been identified:'

Bet ter processability, t hus improved product uniformity and scrap

reduction;

Product tailorability t o specific cus tomer needs, thus bet ter customer

satisfaction;

Quick formulation changes, thus plant flexibility and high

productivity;

Blending reduces the number o f grades that need t o be manufactured

and stored, thus savings in space and capital investment .

The most sought after propert ies for engineering polymer blends

are listed in Table I I

'Table I . 1 - Principal propert ies claimed in polymer blend patents

No Proper ty Frequency %

I High impact s trength 3 8

2 . Processability 18

3 . Tensile s trength I I

5. Heat deflection tempera ture 8

6 Flammability 4

7 . Solvent resistance 4

8 . Thermal stability 3

9 . Dimensional stability 3

10. Elongation 2

I I . Gloss 1

Source: L . A . Von der Groep, Proc. Cornpolloy Europe'91

The blend manufacturers and users have used different s t rategies for

development of polymer blends. The procedure is schematically il lustrated

in Fig. I . I . The decision t o whether use a particular blend is based o n

replacement calculations. These involve not only t h e simple material cost ,

but the total comparable cos t s o f the materials, cus tomer needs, service life-

spans, ease o f disposal o r recycling, e tc . In pract ice, not one property but a

set o f six t o ten d ic ta tes the choice o f t h e const i tuent material.

How to Develop a Polymer Blend ?

I10

End (10) u

I Select con~ponents Check ntisctbility or . with compensaling compatibilisation

Select conrpoun~ling Sc,lect rheologv method n/' blend

properties (1)

Fig. 1 . 1 : Schematic representation of the steps to be taken when developing polymer blends with a specified set of desired performance characteristics (reproduced from: 'Commercial l'olymer Rlendci', Ed. L.A. Utracki, Chapman and Hall, 1998).

methods (2)

A

no yes

v

( 'or~tpnre properrias ~ ~ ? t h .spr~i/ictrtions

(9) . 1)etermine

Select processing Ikf ine desired niethods ~~torphology

(8) tnorphologv

(7)

1.2. Polymer Blends: 'Miscibil ity' and 'Compatibility'

From a thermodynamic point o f view,2 a necessary requirement

for polymers t o exhibit miscibility is that the Gibb's free energy o f

mixing

A = AHmix - TASmix ...( 1. I )

be negative. Here A H and AS,;, a re the enthalpy and entropy o f

mixing, respectively, and T is t he temperature. However, this is not a

sufficient condition fo r binary mixtures. Stability considerat ions

require in addition, that

where $i is the volume fraction o f ei ther component

A simple set o f guidelines t o qualitatively predict t rends i n

polymer-polymer nliscibility is: t h e closer the match o f the two non-

hydrogen bonded solubility parameters and grea ter the relative s trength

of the potential intermolecular interact ions present between the

polymeric components o f the blends, the grea ter the probability o f

miscibility Hydrogen bond formation, n-n, complex formation, and a

variety o f other specific interactions play an important role in

determining polymer blend miscibility, a s shown in Table 1 . 2 . Other

than this, until today, a theoretical approach t o predict miscibility in

polymer blends still remains t o be achieved.

From the industrial standpoint, there is lot o f interest in both

miscible and immiscible polymer blends. Miscible polymer blends are

desirable when the end property sought lies between the corresponding

properties of the individual polymer c o m p o n e n t s . ~ o r example,

poly(2.6-dimethyl- l ,4-phenylene oxide) (PPO) is an amorphous polymer

that softens at very high temperatures and is very difficult t o process;

Table 1 .2 : Examples o f intermolecular interact ions in polymer blends of varying degrees of miscibility

- lntermolecuiar interact ions Polymer blend examples

~nvo lved

Dispersive forces only Polyisoprene-Poly(viny1 ethylene)

Dipole-dipole interact ions Poly(methy1 methacrylate)-Polyethylene oxide)

Weak hydrogen bonds Poly(viny1 chloride)-Polycaprolactone

Moderate hydrogen bonds Poly(viny1 phenol)-Poly(viny1 ace ta te )

S t rong hydrogen bonds Poly(viny1 phenol)-Poly(viny1 methyl e ther )

.Source: ,4 / .Al Co/eman, C . J . Sernran, D.E. Bhagwagar and P .C . Painrer Polvnrer. { I . 1 / 8 7 (1990).

however, i t is miscible with polystyrene (PS). T h e addition o f PS t o

PPO leads t o a drast ic reduct ion in melt viscosity, which makes the

compound easy to process; o n t h e o ther hand, PPO is an effective

modifier o f PS, leading t o an increased heat dis tor t ion tempera ture . In

the case o f immiscible polymer blend systems, synergistic behaviour

between the polymers making u p the different phases leads t o propert ies

that are superior t o those o f the components . The most familiar

examples a re rubber-modified thermoplast ics .

Compatibility is f requently defined as miscibility o n a molecular

scale . But this definition encompasses only those blends showing t rue

thermodynamic miscibility, thereby excluding a very large number o f

blends, both academically s tudied and commercialised, which many

workers would consider compatible . Another way of defining

compatibility is as polymer mixtures tha t d o not exhibit g ross symptoms

o f phase separat ion. It is certainly t r u e tha t most compatibilised blends

contain very finely dispersed phases. But t h e definition still excludes

some blends which have been modified t o facili tate t h e generat ion o f

some preferred physical propert ies , but not necessarily a very fine

morphology. A third definition is simply t o consider blends as

compatible when they possess a commercially desirable set o f

properties. This leaves unanswered the quest ion o f how this i s achieved,

and therefore allows the material developer free rein t o exploit any

avenue, which will lead t o a technologically useful product .

1.3. 'Compatibilisation' in blends

The factors contr ibut ing t o end-use propert ies during manufacture

o f a blend by melt compounding, and subsequent conversion processing

t o produce a finished article, a re il lustrated in Fig. 1.2. The mechanical

I'I1OCESS ('I 'YPE,KAl'I<,

TEMPERATTIRE)

I

I M E C I I A N I C A L P R O P E R T I E S O F B L E N D I

C O M P O N E N T MECHANICAI . I'KOPER'I'IIIS

Fig. 1.2: Surn~nary o f thc factors conlributing to end-use propcrtics in melt compounded blends, highlighling the rolc o f c o ~ i ~ p a t i b i l i s c r s (*). (Reproduced froni: 'Polymer Blends & Alloys', Blackie Academic &. Professional, Glasgow ,1993).

K H E O L O G Y OF

C O M P O N E N T S

INTERFACIAL * T E N S I O N IN M E L T

P H A S E M O R P H O L O G Y

r

INTERFACIAI .* AI IHESION

propert ies o f a blend will be determined not only by the propert ies o f i ts

components, but also by the phase morphology and the interphase

adhesion The phase morphology will normally be determined by the

processing history to which the blend has been subjected, in which such

factors a s the process (mixer type, ra te o f mixing and tempera ture

history), the rheology of the blend components and interfacial

tension are important The phase morphology i s unlikely t o be in

thermodynamic equilibrium, but generally will have been stabilised

against de-mixing by some method o r the o ther This is usually

achieved by quenching the blend t o a tempera ture which i s below the

glass transition temperature o f one o r both the phases, o r by inducing

crystallinity in one o r both the phases, o r occasionally by cross-linking

In any case, it is readily understood tha t compatibilisation can in

principle interact in complex ways t o influence the final blend

propert ies . T o achieve reproducibility o f performance in immiscible

blends, compatibilisation is mandatory. There a re three funct ions o f the

process: ( I ) t o reduce the interfacial tension, thus obtaining finer

dispersion, ( 2 ) t o make certain that the morphology generated during

the blending stage will not be destroyed during high s t ress and strain

forming; and (3) t o enhance adhesion between the phases in the solid

state, facilitating the s t ress transfer, hence improving the mechanical

properties o f products . This complexity can be il lustrated by the

findings o f experiments aimed a t selecting compatibilisers for blends o f

HDPE with o ther commercial polymers.4 A Brabender Plast icorder was

used t o prepare melt mixtures o f HDPE with nylon 6, nylon 6-6,

nylon 6-3T (an amorphous polyamide) and polyethylene terephalate

(PET) , with and without low levels o f various proprietary

compatibilising agents . The maximum phase s izes and the tensile

elongation at break results, a re summarised a s a function of

compatibiliser addition level in Table 1 .3 . I t was clear that in many

cases9 compatibiliser addit ion had a remarkable effect on phase

dispersion and could cause substantial improvements in tensile

elongation behaviour. However, achievement of the finest phase

dispersion did not in itself guarantee the highest values of ultimate

elongation, confirming the complex nature of the compatibilising effect.

Table 1 .3 : Comparison of maximum phase size (d,.,) and average tensile elongation at break ( ~ b ) results for HDPE blends containing 15% by weight of various polymers and low levels of proprietary compatibilisers.

Compatibiliser level Nylon 6 Nylon 6-6 Nylon 6-3T PET

Sourcr:.-f.J. L ~ s r r r and P . S . Hope. European Symposiuni on Polymer R l e ~ i i l . ~ . S l r n s h < ~ u r g . \lo.v 2 5 - 2 7 ( 1 9 8 7 ) .

A comprehensive investigation on the emulsifying effect of

compatibilisers was undertaken by Noolandi and on^^.^ for immiscible

blends and by ~ e i b l e r ' for semi-compatible blends Noolandi found that

the reduction of interfacial tension of copolymer in an immiscible blend

is the result of the surfactant activity of the block copolymer chains

The situation was compared to the soap molecules at an oil-water

interface. This reduction of interfacial tension increases with increasing

copolymer concentration and molecular weight, causing a decrease in

the interaction energy of the copolymer at the interface. They also

reported that beyond a critical copolymer concentration, the surface

tension remained constant.

Compatibilisation can be regarded as a modification of the

interfacial properties. Helfand and Tagami's t h e ~ r ~ ~ ' ~ predicts that the

Fig.l.3: Density profile across the interface, defining the thickness of the interface. (Reproducedfrom: E. Hel/and and Y. Tagon!;, J Poljm. Sci. Polyn~. Letrr . , 9. 7 4 1 , 1971).

density profile across the interface follows an exponential decay

function (Fig. 1 . 3 ) . The intercept o f the steepest tangential line with

the horizontal lines defining the volume fraction of either one of

the two polymer ingredients , I$ = 0 and 1, defines the thickness of

the interface, dl. The interface thickness of some common polymer

system is given in Table 1.4. The equilibrium interphase thickness of

compatibilised blends ranged from 10 t o SO nm, which is larger than the

radius of gyration of component polymers.'.'u

Reactive compatibilisation 1 30-60

Table 1.4: Interphase thickness

I

Radius of gyration 15-35

Type of blend

Immiscible

Block copolymer

Polymer / copolymer

I I I Source: 'Polvrnrr Blrnak & ANuys 1 cdr. MJ. Folks & P.S Hopr,

Bluck~e Academic & Prujsssiunal, GI-w (1993).

Thickness (nm)

2

4-6

3 0

According t o Helfand and ~ a ~ a m i , ' . ~ the interfacial t h i c k n e s q A I,,,) and

the interfacial tension coefficient (v,,,) can be expressed a s

or, A I,V, = kspb2 I 3 .. . (1.5)

where b is a lattice parameter, k~ is the Boltzman constant , and T is the

absolute temperature. The Helfand and Tagami lat t ice theoryL9 was

based on the following assumptions: ( 1 ) t he t w o homopolymers were

assumed t o have the same degree o f polymerization; (2) the isothermal

compressibility was assumed t o be negligibly low; and (3) there was no

volume change upon blending. The theory predicts that ( I ) t he product

v is independent o f the thermodynamic binary interaction parameter

X ; ( 2 ) the surface free energy is proport ional t o ( 3 ) the chain ends

o f both the polymers concentrate a t the inerface; (4) any low molecular

weight third component is repulsed t o the interface; (5) the interfacial

tension coefficient increases with molecular weight .

Recently it was shown that the interfacial tension coefficient can

be calculated from the molecular s t ruc ture o f t w o polymers . ' ' When

A-B block copolymer is added t o an immiscible blend o f homopolymers

A and B, the reduction o f the interfacial tension follows the relation' '

where ZCA and Z ~ B are respectively the number o f A and B structural

units in the copolymer (having ZC = ZcA + ZCB total number o f

segments), ZA and Ze a re the degree o f polymerization for the

homopolymers A and B, respectively, a is the monomer length, Z the

~nter fac ia l area per copolymer chain that c rosses the interface. Eq 1 6

predicts that when the adsorption density z / a Z i s high, the interfacial

tension is low For the same z / a 2 value, higher molecular weight

copolymer is more efficient." The theory also predicts that the more

asymmetric is the copolymer composition, the less efficient a s an

interfacial agent it becomes. Since there is no distinction between

polymers A and B, the most efficient diblock copolymer is the one with

an equal amount of both components. When the interface becomes

saturated with copolymer, v reaches its lower plateau, v = VCMC, and the

copolymer macromolecules start forming micelles, i .e . the critical

micelle concentration CMC has been reached, 4 = &MC.

Concentration dependence o f the interfacial tension coefficient is

described by two semi-empirical relationships. The first was obtained

assuming an analogy between addition of a block copolymer t o a

polymer blend, and titration of an emulsion with a surfactant:I3

v = ( ~ V V M C $,w~o ) I (4 + $mean) . . . . .. (1.7) where

V C M ~ ' v (4 = CMC ), = (@cM<: + b0) 2

The second dependence, derived by Tang and ~ u a n g l ~ a n d modified by

Ajji and u t r a c k i i 5 is:

v = VCMV + (v, - VCMC) exp (-axZ4) . . . . . . ( I . 8)

d = ~ V M C + (do - ~ C M C ) exp {-a~xZc@) . . . . . . (1.9)

where a and a , a re adjustable parameters, and Zc is the copolymer's

degree of polymerization. As predicted theoretically, i t was

experimentally found that the diameter o f the dispersed drop (d) follows

the same titration curve as v .

Addition of X-b-Y block copolymer t o blends o f polymers A -

and B may also result in reduction of the interfacial tension coefficient,

provided that the binary interaction parameters x,, are appropriately

balanced. The reduction of v was expressed as16

for XI = XBY = bx = W ~ Y = XAB ...... (1.11)

The theoryI6 suggests possibilities of designing a universal compatibiliser

operating on the principle of competitive repulsive interactions.

Assuming that all the compatibiliser molecules cross the interface

once, the amount required to saturate the interface can be expressed as' .l6 a

function of the copolymer molecular weight M, the total surface area of the

interface, and the specific cross-sectional area of the 2 . 1 . 1 6 copolymer, a 2 5 nm .

where '$' is the volume fraction of the dispersed phase, 'R' is the radius of

the dispersed drop, NAY is the Avogadro number and 'a' is the area

occupied by a copolymer molecule. However, for randomly oriented diblock

copolymer macromolecules at the interface, the following dependence was

derived:"

where <r2> = KM is the square of the end-to-end distance of the copolymer

and K is the characteristic parameter of the polymeric chain. Eq.(1.13) and

(1.14) both predict that the amount of copolymer required to saturate the

interface is proportional to the total interfacial area,

expressed as +/R. The assumption that all copolymer molecules cross

the interface only once results in proportionality between $/R and M,

whereas the assumption that copolymer macromolecules are coiled i n

the interphase removes this proportionality'. The reality is somewhere in

between these two ideal cases.

The compatibiliser should be designed t o migrate t o the interface,

thereby broadening the segmental concentration profile, experimentally

expressed as Al ' Addition of a suitable compatibiliser reduces the

interfacial tension and alters the molecular s tructure at the interface.

The reduction in the dispersed particles size and the interfacial

tension coefficient upon compatibilisation o f binary polymer blends has

been widely reported. 18-2 1 In several publications, the effect o f

copolymer addition on particle size was studied. 22-24 The effects of the

hydrogenated butadiene-b-isoprene-b-styrene (SEB) copolymers on the

interfacial activity in PEIPS blends were i n v e ~ t i g a t e d . ~ ~ ~ ~ " ~ ~ It was

demonstrated that the copolymer was located at the interface, forming a

continuous layer around the dispersed particles, either of PE in PS or

PS i n P E Its thickness was similar t o the radius of gyration of the

copolymer, A1 -= 10-12 nm. A competition between microdomain

formation and migration to the interface was observed. It was observed

that 2-5 wt .% of SEB was sufficient to reduce the interfacial tension

coefficient, the particle's size and coalescence, a s well a s t o enhance

the mechanical propert ies.26 Perin and ~ r u d ' h o m m e ~ ' have reported that

upon addition of a PS-b-PMMA copolymer in PSIPMMA blends, the

interface thickness A1 was found t o increase from 2 t o 6 nm.

Block copolymers a re considered a s bet ter interfacial agents than

graft copolymers because o f less effective penetration o f the branches o f

the latter into homopolymer phases. For the same reason, diblock I copolymers are more effective than triblocks. But it is very difficult to

synthesize a proper compatibiliser containing the proper blocks.

Molecular weight o f t h e compatibi l iser p lays an important role in

the compatibilisation o f blends." A t o o shor t block in t h e copolymer is

detrimental to i t s anchoring in t h e domain and matrix homopolymers,

thus weakening the 'bridge' effect a t the interface and hence t h e

mechanical propert ies . 28

I t has been shown tha t the blending sequence in a compatibilising

system affects the morphology o f the resul tant blends. During the

I-eactive compatibilisation o f PBTILDPE and PBTIEPDM blends by

using bismaleimide,'" it was found tha t an early addit ion o f the

compatibiliser and a later addi t ion o f LDPE gave favorable increase in

viscosity o f LDPE, which improved t h e dispersion o f LDPE in the

poly(buty1ene terephthlate) matrix.

. One of the disadvantages o f addi t ion o f a copolymer a s

compatibiliser is the tendency o f t h e copolymer t o form micelles in the

bulk phases, thus on the one hand reducing i t s effect iveness as a

nloditier o f the interfacial tens ion and, o n the o the r hand, increasing t h e

blend viscosity, thus reducing i t s processabili ty. Qin e t al." have

reported that the optimum mechanical proper t ies were achieved only at

a proper amount of the compatibiliser, an excess o f which only did harm

t o the mechanical propert ies .

For these reasons, t h e copolymer must be designed in such a way

a s t o (1 ) maximize miscibility o f t h e appropr ia te part o f i t s

macromoiecule with the specific polymeric component o f the blends;

(2 ) minimize i t s molecular weight t o just about the entanglement

molecular weight for each interact ing segment; and ( 3 ) minimize its

concentrat ion i n the blend. Addition o f just 0.5-2 wt.% o f a properly

designed copolymer has been found to be capable of significant

penetrat ion into the homopolymer phases. 3 1 , 3 2

A brief review o f the various techniques encompassing

compatibilisation technology is discussed in this sect ion.

Compatibilisers may be added separately o r formed during

compounding, mastication o r polymerisation o f a monomer in t h e

presence o f another polymer. The copolymer compatibilisers often

contain segments which a re ei ther chemically similar t o those o f the

blend components (non-reactive compatibiliser) or capable of chemically

bonding with one o f the components in the blend (react ive

compatibiliser). In the case o f reactive copolymer compatibilisers, t he

segments o f the copolymer a re capable o f forming s t rong bonds

(covalent o r ionic) with at least o n e of the components in the blend. In

non-reactive compatibilisers, t he segments o f the copolymers are

miscible with each of the blend components .

i i ) Additiot~ of Block cr)id Graft C:opolymrr us <'ompatihili.ser.c.

The addition o f block o r graft copolymers represents the most

extensively researched approach t o compatibilisation o f blends. I t is

perhaps not surprising that block and graft copolymers containing

segments identical t o the blend components a re obvious choices a s

compatibilisers, given that miscibility between the copolymer segments

and the corresponding blend component is assured, provided that the

copolymer meets certain s tructural and molecular weight requirements,

and that the copolymer locates preferentially a t the blend interfaces.

Table 1 .5 il lustrates the u s e o f block and graft copolymers in the

physical compatibilisation o f blends.

The classical view o f how such copolymers loca te at interfaces i s

shown in Fig. 1 . 4 . This has been experimentally verified by Barentsen

et aI.'\nd Fayt et al., 24,26 both fo r blends o f low density

polyethylene(LDPE) with polystyrene (PS), and Yang e t al." for

HDPEipolypropylene (PP) blends.

'Sable 1.5: Copolymers as compatibilisers in blends (i.e., physical compatibilisation)

Minor component

I 1

PP S-EB-S

Compatibiliser

I I I

I - - -- I

PETIHDPE I HDPE 1 PET 1 S-EB-S

PEPS PE or PS

-

- 1 I I I

Sorirce: M Xmithos, I'olym. Et~g. Sci., 28, 1392 (1988).

PVDFPS I I I

G A A F l BLOCK

-.

PS or PE

SAVISBR

Fig. 1.4: Schematic illustration of graft and block copolymers at blend interface (Reproducedfrottr: 'Polytrrer Blends &Alloys ', eds. M.J. Folkes & P.S. Hope. Blocfile Acndetrric & Projessionnl, Glosgorv, 1993).

S-EB-S, S-B-S, S-I-S

PVDF PSPMhlA block copolymer

SBR SAN BRPLMMA block copolymer

The effect of different copolymer types on the compatibility o f

PEIPS blends has been studied extensively by Fayt e t a ~ . ~ ~ , ~ ~ - ~ ~ . Using

ultimate tensile propert ies a s yardstick, they concluded that :

a ) Block copolymers were more effective than graft copolymers.

b ) Diblock copolymers were more effective than triblock o r s tar-

shaped copolymers.

c ) 'Tapered ' diblock copolymers were more effective than pure

diblock copolymers.

Paul et a13"ound that the solubilisation o f a discretely

dispersed homopolymer into i t s corresponding domain of a block

copolymer compatibiliser only occurs when the homopolymer molecular

weight is equal t o o r less than that o f the corresponding block.

However. stabilisation o f a matrix homopolymer into its corresponding

domain of a block copolymer compatibiliser will occur even if the

molecular weights are mismatched.

Ciaylord3" offers the pragmatic view that a balanced molecular

weight is needed for copolymer compatibilisers; the segment needs t o be

long enough t o anchor t o the homopolymer ( i :e . , t o solubilise) but short

enough to minimise the amount of compatibiliser needed, and hence t o

be cost-effective. The requirement that the copolymer should locate

preferentially at the blend interface has implications for the molecular

weight o f the compatibiliser. Both the thermodynamic 'driving force ' t o

the interface and the kinetic 'resistive force ' t o diffusion increase with

molecular weight . This suggests that high molecular weight copolymers

may be used if sufficiently long times a re available during the process,

and lower molecular weight copolymers must be used if available

diffusion times are short . While copolymer with blocks o f chc~ii ical

con~posi t ion identical t o those o f the two homopolyn~crs pl-csent the

best technical opt ion fo r compatibilisation, such materials suffer a

number of disadvantages; they a re often not commercially available, o r

only available at high cost , and may limit flexibility, needing t o be

tailor-made for a particular blend. For these reasons, commercially used

compatibilisers are often multi-component materials and frequently rely

on miscibility or the presence o f some other interactions t o achieve a

compatibilising effect with one o r more blend components. For example,

ethylene-propylene copolymers, which contain a large number o f

randomly polymerised units, in addition t o ethylene and propylene

blocks, have found some success a s compatibilisers for blends o f PE and P P . ~ ~ - ~ ~ Another example is the use o f commercially available styrene-

ethylene-butadiene-styrene (SEBS) triblock copolymers, which have

proved effective in compatibilising blends o f HDPE with PET. The

research in this system received particular interest due t o the prospect

o f recycling scrap from PET beverage bottles. The add i t ion o f 20% by

weight o f SEBS copolymer t o a 50150 HDPEIPET blend in an extruder

at 300°C caused a dramatic increase in ductility, which increased from

3 % elongation t o greater than 200% elongationJ' (Fig. I ) The

researchers found that the use o f SEBS copolymer caused the blend to

form an interpenetrating network o f the two immiscible components,

and the copolymer increased the degree of interfacial adhcsion

3 C C . 200 f

I C O z- - - - = 50 -

1652 -

C - t- < - 5 Y. z C J l o w - B I N A R Y B L E N D - s - - -

- - ELENDS W I T H 20% K R A T O N G 1652 - - -

D I D NOT B R E A K I I I I 0 2 5 5 0 7 5 100

P E T Y. H D P E

Fig. 1.5: Effect of addition of S-EB triblock copolynler on ductility of HDPEJPET blends. (Rrprodvcedfron~: T.D. Tro~~gorr , D.R. Palrl and J . D . Barlow. J . Appl. Polyn~. S c i . , 28 . 2947. 1983).

For a system in which an A-B block copolymer is blended with a

homopolymer C (and, C is miscible with ei ther A or B) an interest ing

behaviour was observed. For instance, in blends o f s tyrene containing

block copolymers with poly(l ,4-phenylene oxide) (PPO) the molecular

weight o f the PPO and polystyrene (PS) blocks did not restrict t he 4 4 miscibility In this system miscibility may occur if the molecular

weight o f PPO is much higher than the molecular weight o f the P S

block

Tucker and ~ a u 1 ~ ~ found that because o f enthalpic effects the

molecular weight rat io becomes a relatively unimportant parameter .

Block copolymers o r reactive functionalised polymer usually require a

separate preparation step, and certain copolymers are difficult t o obtain.

The price of the blends are, hence, much higher than the sum of the

parent polymers due t o the high cost o f a compatibiliser. Therefore it

would be a great advantage simply t o use a commercially available

copolymer or else a small molecule compound as a compatibiliser

( l ; ) A d d i ~ r o n (~f ' funct ional polymers

The addition o f functional polymers as compatibilisers has been

reported widely in l i terature. Usually a polymer chemically identical t o

one of the blend components is modified t o contain functional (o r ,

reactive) units, which have some affinity for the second blend

component; this affinity i s usually the ability t o react with the second

blend component , but o ther types o f interact ions ( e .g . ionic) a re

possible The functional modification may be achieved in a reac tor o r

via an extrusion-modification process. Examples include the grafting of

maleic anhydride o r similar compounds t o polyolefins. 45.46 The maleic-

modified polyethylene and phenolic-modified polyethylene has been

reported t o be effective compatibilisers fo r HDPEINBR b ~ e n d s * ~

( i i i ) Heac l i ve blending

A comparatively new method o f producing compatible thermoplast ic

blends is via reactive blending, which relies on the in situ formation of

copolymers o r interacting polymers. This differs from other

compatibilisation routes in that the blend components themselves a re

either chosen o r modified so that react ion occurs during melt blending,

with no need for addition o f a separa te compatibiliser. The route has

found commercial application, a s il lustrated in Table 1 .6 . A number o f

reactive blending mechanisms may be exploited:46

a ) Formation in situ of graft o r block copolymer by chemical

bonding reaction between reactive groups on component

polymers

h ) Formation o f a block copolymer by an interchange reaction in

the backbone bonds o f the components; this is most likely in

condensation polymers.

c ) Mechanical scission and recombination o f component polymers

t o form graft o r block copolymers. High shear levels generally

induce this during processing.

d ) Promotion o f reaction by catalysis

Table 1.6: Reactive copolymers as compatibilisers in blends.

)stem I Minor component / Minor component 1 Compatibiliser wfl ABs or PA-6 I I

PA-6 or ABS

1- PP or PA-6 I I

SANIMA copolymer

PA-6 or PP I I I

EPMIMA copolymer

PETfPP or PETPE

PEPA-6

PA-6lAcrylate rubber

PE PA-6

PET

Ionomers, carboxyl functional PE's

Source: M. .Yanthos. Polym. Eng. M.. 28. 1392 (1988).

Acrylate rubber

PP or PE PP-g-AA, carboxyl functional PE's

PA-6 EPM-g-MA

'Table 1.7: Low molecular weight compounds as compatibilisers in blends

PBT I Polyfunctional epoxies

p,..,,,,.,o>p;nent v ~ w i 4 - 6

I / NR I Polyfbnctional monomers

PPE~PA-~ ,~ - 1 ~ A - 6 6 I I

PPE I Amino silane

Major component

Compatibiliser

i rubber Source: I1 .\nn/hos. I'~11vtrr. b:ng. Sci, 22, 1392 (1988).

NBRIPP

.-

NBRIFluoro

Low rnolecular weight compounds can also ac t a s compatibilisers

in many polymer blends. Co-crosslinking, crosslinking and graf t ing

teact ion may involve such systems and may lead t o the formation o f

certain copolymers. Table 1.7 dea ls with blend systems involving low

NBR

Fluoro ~ b b e r

molecular weight compounds as compatibilisers

NBR curatives and intercharn copolymer Triazine dithiol complex

Compat ib i l i sa~ion o f blend components is a major considerat ion

when designing blends and is of ten t h e primary cr i ter ion for commercial

success. Hence much o f t h e compatibilisation technology which exists is 46 proprietary information. This s i tuat ion is unlikely t o change in today's

increasingly competi t ive marketplace. The major fu ture challenges and

developments for compatibilisation technology lie in three main areas o f

blending:46 engineering polymer blends, superior performance

commodity polymers and polymer recycling. In t h e first t w o a reas t h e

primary objective is t o produce a material where some specific property

is enhanced. This blending opt ion is very of ten a significantly more

cost-effect ive route in producing an improved material than developing

a completely new polymer. Development cos t s for a blend can be

relatively low and capital investment minimal, particularly if

compounding equipment already exists.

1.4. Method of polymer blending

The process o f blending a l lows the intensive t ransfer o f polymer

chains occurr ing a t t he polymer-polymer interfaces t o achieve a

homogeneous blend. The level o f homogeneity obtained depends on the

nature o f the components t o b e blended and the blending technique

employed.

i i ) M e c h a ~ l i c u l blending

Mechanical blending o f polymers produces a very coarse

dispersion in blends. The propert ies o f the blends are strongly

influenced by the speed and temperature o f mixing. Homogeneous

mixing o f blends is only achieved after the melt processing s tage .

(11) , S ~ / I I / I ~ I I ~ I C I I ~ I I I ~

Solution casting technique as a method for the preparation o f

blends requires that the polymer components can be dissolved in a

common solvent(s) . Good molecular level o f mixing can be obtained

with cos t , depending on the solvent(s) and its recovery.

( I ; ; ) I 'o lymer isut~ot~

Emulsion polymerisation is employed for the preparation o f

rubber-toughened plastic blends. The polymers a re required i n the latex

o r emulsion form The mixing process o f these micro-size latexes and

the subsequent removal o f water produce excellent dispersion and

distribution of discrete phase.

( I V ) Keacrive h l e ~ r d i ~ i g

This innovative method facilitates the development o f novel

blends from highly incompatible pairs . A more homogeneous blending

with high productivity can be obtained with this method but with the

penalty o f involving a more stringent production cont ro l .

(1)) /~~dtr .v ir i t t l f~roce.ssitrg

It is difficult to generalize blend extrusion. Blends are generally

more sensitive t o processing variables and requires more sophisticated

line control than the single-phase polymers. 4 6 2 4 7 The key is stability o f

morphology, or in other words, a degree of dispersion and stabilization

achieved during the compounding stage. All blends can be grouped into

two types: well-stabilised and not . In the first type, it is the intention o f

the blend manufacturer to generate stable morphology and

processability not very different than that o f a single-phase polymer.

Within this type one can identify three categories defined by the volume

concentration of the second phase: (a) 0-12%, (b) 12-35% and

(c) 35-50%. In (a) the dispersed drops exist in the form of drops,

usually quite uniform in size, with drop diameter of the order of one

micron. This category includes most of the toughened engineering

polymers, although they are not identified as blends. In (b) the

toughening is accomplished by the addition o f a multifunctional

copolymer. At 35% loading the morphology is complex, requiring

intensive mixing during the compounding stage. In (c) the co-continuity

of two polymers is usually achieved. The interlocking generates

relatively stable morphology. Thus within the type of stabilized

morphology blends, category (b) is most sensitive t o processing

conditions. The second, non-stabilized morphology type includes blends

designed to create particularly desirable structures during the final

processing step This include blends used t o manufacture blow-molded

containers with superior barrier properties, where the concentration of

the dispersed phase is below IS%, i.e. , in the compounded blends there

are spherical drops dispersed in a matrix.

1.5. Thermoplast ic E las tomers

A large number of thermoplastics exhibit strong limitations in

their end-use when both toughness and high impact resistance are

required. These shortcomings may be overcome by blending these

materials with suitable rubbery components. The enhancement of

toughness obtained is due t o the presence. o f finely dispersed rubber

particles which are thought t o induce energy dissipation by both crazing

and shear yielding o f the matrix. PS, l ike some other polymers, is a

britt le polymer with poor notched impact strength, especially at

temperature below its glass transition temperature and in the dry s ta te

However, the impact performance o f this thermoplastic may b e

improved by incorporating an appropriate rubber into the matrix. In

recent years, elastomeric rubber-plastic blends have become

technologically interesting fo r use as thermoplastic elastomers. 48-50

Thermoplastic elastomers (TPEs) a re multiphase compositions in

which the rubber and plastic phases a re intimately dispersed (Fig. 1.6).

Commonly, the phases a re chemically bonded by block o r graft

copolymerisation. In others , a fine dispersion i s apparently sufficient. In

TPEs, at least one o f the phase consists o f a material that is hard at

room temperature but fluid upon heating. The o ther phase consists o f a

Fig. 1.6: Digrammatic morphology of a thermoplastic elastomer (polystyrene-elastomer-polystyrene block copolymer). (Reproducedfronr: 'Handbook of Elas~omers: New Developments and Technology', eds. A . K. Bho~vnrick R. H . L. S~ephens, rZ.Iorcel Dekker. New York, 1998).

softer material that is rubber-like at room temperature. For instance in

the TPEs of our interest, i .e . comprising styrenic plastics and nitrile

rubber, the PS segments would form separate spherical regions, i .e . ,

domains, dispersed i n a continuous elastomer phase. Thus, most polymer

molecules would have end polystyrene segments in different domains.

At room temperature, these polystyrene domains are hard and act a s

physical cross-links, tying the elastomer chains together in a three-

dimension network. Although the network formed in TPEs may appear

to be similar to vulcanized rubbers, the domains lose their strength

when the material is heated o r dissolved in a solvent. But when the

material is cooled down o r the solvent is evaporated, the domains

harden and the network regains its original integrity.

The properties of TPEs depend o n the following material properties

of the rubber and plastic components:50

I ) Dynamic shear modulus o r Young's modulus: This property is a

measure of stiffness of the polymer.

2 ) Tensile strength o f the hard-phase material: This property represents

a limit for the strength of the rubber-plastic blend.

. Crystallinity i'he ultimate strength of rubber-plastic blends shows a

definite dependence on the crystallinity of the hard phase. An

increase i n the crystallinity o f the plastic material component

improves both mechanical integrity and elastic recovery.

4) Critical surface tension for wetting: The difference between the

critical surface tension for wetting (for the rubber and the plastic) is

a rough estimate of the interfacial tension between the rubber and

plastics during melt mixing. The interfacial tension is an important

factor that determines the extent of phase heterogeneity. It has been

found that blends in which the surface energies o f the rubber and

plastic phases are closely matched are s t rong and extensible.

5 ) Critical entanglement spacing: This property is measured as the

number of polymer chain a toms that corresponds t o the molecular

weight sufficiently large for entanglements to occur between

molecules o f undiluted polymer. I t has been observed that when

polymers are blended with one another , f ibrous s t ruc tures appear

which then break up into droplets . The polymer molecules which

tend t o mutually entangle was found t o be drawn into finer "fibers"

during the early phase o f mixing, t o give emulsions o f drople ts o f

smaller size.

6 ) Melt v iscos i ty I t has been found that melt blending is most efficient

( i . e capable of giving the smallest particles o f dispersed phase) when

the viscosities of the phases a re the same.

The o ther factors determining the propert ies o f TPEs a re ( 1 ) the

rubberlplastic proport ions, (3 ) t he phase morphology, ( 3 ) the

interactions and propert ies a t the interfaces. 50

T P E s can be mainly classified into the following categories: 4 8 - 5 0

I ) TPEs from rubberlplastic blends

2 ) Polystyrene-elastomer block copolymers

3 ) Polyurethane-elastomer block copolymers

4 ) Polyamide-elastomer block copolymers

5 ) Polyether-elastomer block copolymers

Among these, T P E s from blends o f rubber and plastic have certain

advantages over the o ther TPEs. In these cases, t he required propert ies

can be easily obtained by a proper selection o f the rubber and plastic

component and by proper control o f their propor t ion . Moreover, the

overall performance o f these TPEs can be further improved by

cornpatibilisation technique, changing the crytallinity o f the plastic

component , and by proper incorporat ion o f suitable fillers and

crosslinkers.

The subject o f thermoplast ic elastomers has been extensively

reviewed in the l i terature. 48-50 Thermoplast ic elastomers from blends o f

N R I P M M A , ~ ' N R I P S , ' ~ N B R I P P , ~ ~ NBRJHDPE,~ ' and ~ B ~ l n ~ l o n ~ ~ have

been reported by Thomas and co-workers .

TPEs have many processing advantages over the conventional

vulcanised rubbers . 1 8 - 5 0 Conventional rubbers must be vulcanized t o

g iveqe fu l propert ies . Vulcanization is a irreversible and slow process,

and takes place only on heating. However, in the case o f TPEs, the

transition from a processable melt t o a solid rubber-like object is rapid,

reversible and takes place on cooling. The short processing cycle

involved consunies only very low amount o f energy. Processing

techniques like blow moulding, heat welding e tc . used in thermoplast ics

are not suitable for conventional rubbers. But these techniques can be

successfully applied t o TPEs. During the processing o f conventional

[rubbers, the scrap is considered a s was te a s it can not be reprocessed.

However, the mold flash and parts o f the TPEs can be simply ground

and reused. The scraps from TPEs can be recycled several t imes without

any major deter iorat ion in i ts propert ies . Apart from these advantages,

TPEs have certain disadvantages. They show high creep and set on

extended use and will melt only a t elevated tempera tures .

1.6. Dynamic Vulcanizat ion

The dynamic vulcanization process has been applied t o many

rubber-plastic combinations. I t can be described a s follows: Rubber and

plastic are first melt mixed in an internal mixer. After sufficient melt

mixing in the internal mixer, vulcanizing agents a re added.

Vulcanization then occurs while mixing continues. It is convenient t o

follow the progress o f vulcanization by monitoring the mixing torque .

After the mixing torque goes through a maximum, mixing can be

continued somewhat longer t o improve the fabricabil i ty After

discharge from the mixer, the thermoplast ic vulcanizate (TPV) is

handled in much the same way a s TPE. Thus, small rubber droplets

are vulcanized t o give a particulate vulcanized rubber phase o f stable

domain morphology during melt processing. The schematic mechanism

occurring during dynamic vulcanization is shown in Fig. 1.7.

dynarmc vulcanization

Fig. 1.7: Illustrative idealization of phase mrphologies of TPE Mends before and after dynamic vulcanization. (Reproduced eom: S. T b m s d A Oxnge, Em l'(im J , 28, 1451,1992)

The purpose of the dynamic vulcanization of elastomeric

lhermoplastic rubber-plastic blends is t o produce compositions that have

the following improvements (in comparison to similar compositions that

have not been vulcanized):

a . Reduced permanent set

b. lmproved ultimate mechanical properties

c . lmproved fatigue resistance

d. Greater resistance t o attack by aggressive fluids, e .g . , hot oils

e. lmproved high-temperature utility

f. Greater melt strength

g . Greater stability of phase morphology

Thus, dynamic vulcanization can provide composit ions that are

more rubber-like in their end-use performance character is t ics . 'The

other at t ract ive feature o f thermoplastic vulcanizates is that these can

also be processed a s easily and rapidly a s thermoplastics.

Krulis et a l S S described the dynamic crosslinking a s a route t o

improve the mechanical propert ies o f PPIEPDM blends. Blends with

vastly superior impact propert ies were obtained by the slow curing o f

E P D M with sulphur

Dynamic crosslinking a s a means t o improve the mechanical

propert ies o f polypropylene/elastomer blends has been reported in

l i terature. i h . 5 7 Compatibilisation along with dynamic vulcanization

technique have been employed in poly(buty1ene terephthalate) /EPDM

blends by Moffett and ~ e k k e r s . ' ~

Choudhary et aL5" have employed dynamic vulcanization a s an

effective technique t o improve the mechanical propert ies o f NRIPE

blends. The D C P cured blends showed bet ter propert ies than the

corresponding unvulcanized samples. Various TPVs consisting o f fully

vulcanized rubbers in different plastics were developed by Coran and 61)-65 co-workers

The potential and proven applications o f TPVs a re as follow^:^" a . Mechanical rubber goods applications, e . g Caster wheels,

gaskets, seals, suct ion cups , vibration isolators , oil-well injection

lines.

b . Under-the-hood automotive applications: Air-conditioning hose

cover, vacuum tubing, shock isolators, air ducts , and fuel line

hose.

c . Industrial hose applications: agricultural and paint spray.

d . Electrical applications: cable insulation and jacketing.

1.7. Re levance of s t y r e n i c p las t ics -n i t r i le r u b b e r T P E ' s \ .;

Styrene polymers have some unique propert ies , which make them

useful in a wide range o f products . The single most important

character is t ics o f general purpose PS a re that it is a glass like solid

below 100°C. Above this temperature, the polymer chain has rotat ional

freedom, which al lows for large segmental mobility. The polymer is thus

Fluid enough t o be easily shaped in to useful forms. Below the Tg, the PS

possess considerable mechanical s t rength . However , t h e l o w impact

s t rength, poor chemical resis tance and environmental s t ress resis tance

limit i ts appl icat ions.

SAN is one o f the largest volume copolymer o f s tyrene. It possess

higher tensile s t rength, be t te r toughness, and be t te r solvent resis tance

than PS, yet retains a similar processabi l i ty . The acrylonitrile impar ts

chemical resistance, be t te r heat dis tor t ion, and improved mechanical

propert ies 6 6 . 6 7

General purpose SAN copolymers have improved proper t ies

when compared with PS, but still fail in a br i t t le fashion when subjected

lo sudden, high-speed impact . Rubber reinforcement o f SAN increases

~ t s ability to withstand high speed impact, and t h e resultant materials

a re called acrylonitrile-butadiene-styrene resin. They have the

processabili ty o f s tyrene based resins and excellent toughness . The ABS

resins a re character ized by excellent processabili ty, good mechanical

properties, rigidity, impact s t rength, scratch resistance, dimensional

stability, paintability and plateability, modera te solvent , chemical and

heat resistance The most important commercial blends o f ABS a re

those with PVC, polycarbonate, polyamide, and polyurethane.68

Nitrile rubber assumes commercial interest d u e t o i t s excellent

oil-resistance, abrasion resis tance and elast ic propert ies . However , it

shows poor ozone resis tance. T h e technique o f blending, which offers , > , /

an at t ract ive alternative t o lessen the shortcomings o f the individual

components while retaining their usefulness can be used t o develop

nitrile rubberlstyrenic plastic thermoplas t ice las tomers (TPEs) .

Thus, thc blends o f acrylonitrile-butadiene rubber and styrcnic

plastics would result in a new and interesting class of thermoplastic

elastomers, which would combine the excellent oil-resistant propert ies

o f NBR, and the superior mechanical and processing character is t ics o f

the respective plastic.

1.8. Scope of the work

The present work systematically investigates the mechanical,

morphological, rheological and thermal propert ies o f the three binary

blends, viz. PSINBR, SANINBR and ABSINBR. Detailed investigations

have revealed that PSINBR blends a re characterized by a narrow

interface, coarse morphology and poor ultimate propert ies . Hence the

compatibilisation o f these blends is essential t o alleviate these problems.

Until now, no study has been reported on the compatibilisation o f

PSINBR blends. Thus, it i s a lso the goal o f this research t o examine the

effect o f S A N and ABS random copolymers a s compatibilisers on the

thermal, morphological and mechanical behaviour o f immiscible PSINBR

blends. These data may provide a useful guide t o obtain PSINBR blends

with improved propert ies from virgin o r recycled resins. Lastly, the

possibility of dynamic crosslinking o f the elastomer phase in the blends

t o enhance the ultimate propert ies , and reprocessabilty of the blends

were comprehensively investigated, which stand o u t a s t w o o f the

at t ract ive features o f thermoplast ic elatomer blends.

The following is a concise account o f the subject matter o f the

thesis discussed in the following chapters .

1.8.1. Computibility Studies of PS/NBR, SAN/NUR A! AHS/NI IR

ninury Systems

A systematic investigation into the compatibility o f the three

binary polymer hlends o f NBR/PS, N B R / S A N and N B R I A B S , i n the

solid as well a s solution s ta te was undertaken. The heat o f mixing

(AH,,,) and polymer-polymer interaction parameter o f all t he three

Illend systems were evaluated. T h e at t ract ive interacting nature, and

hence the resulting compatible nature o f t h e blends were also analysed

using FTIR spectroscopy, which showed a distinct relation with their

s tate of compatibili ty. Unlike in solid s tate , the s ta te o f miscibility o f

the blends differs in solut ion. The s ta tes o f compatibility o f the blends

in solution were studied using viscometric techniques.

I . 8.2. Polystyrene / Acrylonitrile-Huludiene Rubber Ijinury Hlen Js

The effects o f the variables in the melt-mixing operat ion, viz.

rnixing temperature, time o f mixing and speed o f mixing on the

n~echanical properties o f P S / N B R blends were investigated. This was

done in order t o obtain the best balance o f propert ies o f the blend at the

optimum processing conditions. The processability, morphology,

mechanical characteristics, viz , tensile and tear propert ies , and impact

toughness were studied for the whole range o f blend composit ion T h e

thermal character is t ics o f t h e blend were s tudied using differential

scanning calorimetry and thermogravimetr ic analysis.

1.8.3. Poly (styrene-co-acrylonitrile) / Acrylonitrile-Butudiene Rubber

Binary Blends

The toughening o f S A N has been obtained by adding t o the britt le

S A N matrix, a component having T, much lower than that o f the parent

matrix ( i . e . , by the addition o f NBR). Various mechanical models were

applied to the mechanical property-composition data. The proposed

theoretical models were correlated with the microstructure o f these

blends, as obtalned from SEM studies. The stress-strain curves were

correlated with the observed phase-change morphology. Melt flow

characteristics of the blends were studied. The immiscibility was

confirmed using glass transition temperature measurements.

1.8.4. Poly(ucrylonitri1e-co-butudiene-cu-styrene) / Acrylonitri le-

Hutudiene Rubher Rinury Blends

The mechanical, morphological, thermal and the melt-

rheological properties o f ABSINBR binary blends were evaluated.

The mechanical property of the blend was correlated with the s ta te

of compatibility in it . The effect o f blend composition o n the

thermal properties was studied.

1.8.5. Computibilisution of heterogeneous PS/NUR blends by the

addition of S A N and ABS copolymers

The effects of styrene-acrylonitrile copolymer (SAN) and acrylonitrile-

butadiene-styrene terpolymer (ABS) as potential compatibilisers for

PSINBR blends were investigated. The influence of compatibilisation

on the morphology, processing characteristics and mechanical propert ies

has been systematically analysed. Glass transition temperatures o f the

blend in the presence and absence of t h e copolymers were determined by

DSC to test the miscibility o f the system. The thermal stability o f the

blends containing the copolymers has been analysed by

thermogravimetry. A suitable mechanism was proposed t o explain the

compatibilising effect of the copolymers in PSINBR system.

1.8.6. Dynamic vulcanization of the blends

Cross-linking, or vulcanization, is the procedure in which a

polymer passes from the elastic state t o the plastic state, and it is an

irreversible transformation. In this reaction, linear macromolecules are

joined together by inter-molecular bridges, thus forming a three-

dimensional network. In dynamic cross-linking, the rubber and

thermoplastic are pre-mixed with the curative, and other additives are

then added. The rubber undergoes crosslinking in-situ t o give typically a

form of semi-interpenetrating polymer network, which is subsequently

capable of being molded or extruded. The effect of different cross-

linking systems on the mechanical, morphological and thermal properties

of styrenic plasticslnitrile rubber was comprehensively investigated.

1.8.7. Reprocessability of the blends

Since styrenic plastics/nitrile rubber TPEs are a viable

proposition, a thorough understanding of their reprocessed scraps is

Important. Usually degradation effects in melt-reprocessed polymers are

due to either a change in the molecular weight, a change in the chemical

structure, or both The first possibility was tested by gel permeation

chromatography and capillary rheometry, while the second by FTlR

spectroscopy. Thus, a systematic study was carried out on the effects

that successive molding cycles have on chemical structure, molecular

weight, mechanical properties and thermal properties of these blends.

The same processing temperature and only reprocessed TPE was

employed, instead of mixing it with virgin material.

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