CHAPTER 111 - A. MORPHOLOGY

8. KINETICS OF VULCANISATION AND MECHANICAL

PROPERTIES OF NR-EVA BLENDS.

Part o f the results d e s c r i b e d in this Chapter has been p u b l i s h e d in

the Indian Journal o f Natural Rubber R e s e a r c h . 3(2): 77-81. 1990.

Elastomer blends have the advantage that a wide range of

propert ies can ea s i l y be obtained by varying the composition of

the blends. Proper t ies of elastomer blends depend not only on the

s i ze of t he d i spersed phase but a l so on the difference in the r a t e s

of vulcanisation of the components and on the extent of cure of each

phase. To select a blend r a t i o and a crosslinking system su i tab le

for a par t icu la r appl icat ion, a c l ea r understanding of the change

in phys ica l p roper t ies with blend r a t i o and crosslinking systems

is essent ia l . While N R can be vulcanised using sulphur . EVA is not

vulcanisable b y su lphur due to i t s saturated backbone s t ructure .

But both these po lyn~e r s can be cured by using peroxides . However

in a blend of t he two, t he extent of cure undergone by each phase

sha l l depend on t h e d i s t r ibu t ion of t he curat ive in each phase and

also on the r a t e of cure of the individual polymers. The morphology

of t he blend w i l l a l so exe r t a strong influence on the d i s t r ibu t ion

of t he curat ive and on the phys ica l p roper t ies of t he vulcanisates.

This chapter of the thes i s consists of the resu l t s of a sys te -

matic s tudy conducted on the morphology of uncrosslinked NR-EVA

blends and on the effects of t h r ee crosslinking systems namely.

su lphur , peroxide and a mixed cure system consisting of sulphur

and peroxide on the vulcanisation kinet ics and mechanical p roper t ies

of the blends. This chapter ha s been d iv ided into two pa r t s .

In Par t A , the change in morphology of the uncrosslinked blends

with the change in blend r a t i o of t he cornponents is discussed. Par t

tl contains the results of the studies on the effects of three cross-

linking systems on vulcanisation kinetics and mechanical properties

of the blends.

A. MORPHOLOGY

1II.A Morphology of t h e Blends

Figure I I I . A . l is the photomicrograph of the blend G , the

composition of which is 60:40 EVA:NR. The holes seen on the surface

are formed by extraction of the N R phase. At th is blend ratio N R

remained as dispersed particles in the EVA matrix. The size of

the dispersed part icles is also small compared with its size in

those blends which contained a higher proportion of N R . A s the

proportion of NR in the blend is increased to 50 per cent (blend F ) ,

the size of the holes has increased several fold (Figure III.A.21.

The boundary of EVA separating the holes has narrowed down to

a thin layer. When the EVA:NR rat io is 40:60 (blend E ) th is narrow

boundary layer is seen broken, since i t is unable to withstand the

forces due to the swelling of the NR phase (Figure I I I .A .3 ) . It is

possible that in blends E and F, both N R and EVA formed continuous

phases because of the higher proportion of the N R phase and the

lower melt viscosity of the EVA phase. The photomicrograph of the

30:70 EVA:NR blend showed particles of EVA remaining on the benzene

extracted surface (Figure I I I .A.4) which indicated that EVA formed

the dispersed phase when i t s proportion is below 40 per cent and

N R formed the d i spersed phase when i t s proportion was 40 pe r cent

o r below. In blend r a t i o s 40:60 and 50:50 EVA:NR both t he polymers

remained a s continuous phases leading to a sponge-like interpenetrating

s t ructure for t h e blend.

6. KINETICS OF VULCANISATION AND MECHANICAL PROPERTIES

A general equation for the kinet ics of a f i r s t o rde r chemical

reaction can be written a s

In (a -x) = -kt + In a

where a = in i t i a l reactant concentration,

x = reacted quantity a t time ' t ' , and

k = f i r s t o rde r reaction r a t e constant.

The r a t e of crosslink formation is usually monitored b y

measuring the torque developed during the course of vulcanisation

by using a curometer and the torque values thus obtained a r e

proportional to the modulus of the rubbe r . Hence i f a physical

p roper ty such a s modulus is being measured r a t h e r than the change

in reactant concentration, then the following subst i tut ions can be

made:

(a -x) = (I&-M)

where Q = maximum modulus,

Mo = minimum modulus, and

M = modulus a t time ' t '

Substituting torque values for modulus. we get

(a -x) = (Mh-Mt I

where M h = t he maximum torque developed, and

M t = the torque a t time ' t '

If the reaction is of f i r s t o r d e r , a plot of In ( M h - M t ) against

' t ' should give a s t ra igh t l ine whose s lope w i l l be t he specif ic

reaction r a t e constant k t . Typical plots of In ( M h - M t ) V s ' t ' of

the blends El. E2 and E3 a r e shown in Figure I I I . B . l . A s t ra igh t

l ine graph in each case indicates that the reaction is of f i r s t o rde r .

To find out the activation energy of the vulcanisation reaction.

the modified Arrhenius equation is used

tgg = Ae E/RT

log tgo = log A + - E 2.303 RT

where IS = activation energy,

tgO = time in millutes tu atlo111 9Ut ol I ~ I ~ X ~ I I I U I I I torque, and

T = absolute temperature O K .

A plot of log tyO v s 1/T gives a s t r a igh t l ine , from the s lope

of which E is calculated. Typical plots of log tgO v s l/'l' for the

blends E l . E2 and E3 a r e shown in Figure I I I . B . 2 .

111. B. 1 Kinetic factors and cure characteristics

The activation energy ( E l , the f i r s t o r d e r specif ic r a t e

constant (k) and the optimum cure tinlo It ) of t he blends a t 160% 90

a r e given in Table 111 .1 . For a l l the th ree types of cure systems

s tud ied , the activation energy for vulcanisation increased and the

f i r s t o r d e r r a t e constant decreased with increase in t he proportion

of EVA in the blend. The h igher E and lower K values for the

compounds J2 and J 3 compared with those of compounds A and A3 2

a r e due to t he fact that EVA h a s a saturated backbone s t ruc ture

and is less reac t ive than N R . The increase in E and decrease in

k of the sulphur cured blends with over 3 0 pe r cent EVA a r e

expected, a s EVA cannot be vulcanised by sulphur and the sulphur

which is d ispersed in the EVA phase migrates to the N R phase a s 4 r

t he reaction progresses . Between DCP and the mixed cure system.

the l a t t e r showed lower E and h igher k values. While the differences

in E were prominent in blends having higher proportions of EVA.

the differences between the k values were more evident in blends

containing h igher proportions of N R . Sulphur is known to ac t a s

a co-agent in peroxide vulcanisation of elastomers. I t is a l so possible

that most of the su lphur remained in t he N R phase because of i t s

h igher solubility101. The h igher k values of the mixed cure system

compared with those of the DCP cure system can be due to these

reasons. These observations a r e fur ther supported by the optimum

cure time of the blends a t 160°C. The effect of sulphur in activating

peroxide is evident from the lower cure time of the pure EVA

compound having the mixed cure system ( J 3 ) compared with that

of the DCP cured compound ( J 1. While the change in cure time 2

with increase in the proportion of EVA in t he blend was only

marginal in the DCP cured b lends , i t was qui te significant in the

case of blends which contained su lphur and the mixed cure system.

The mixed cure system has sho r t e r cure time a t a l l blend ra t ios

of N R and EVA compared with t he DCP cure system. Moreover, the

cure times of the blends having the mixed cure system were more

s imilar to those of the blends containing the su lphur system,

cspccial ly a t h igher proportions of N R in tho blonds.

111.8.2 Physical properties

111. B. 2.1 Tensile strength. modulus and elongation

The changes in tensile s t rength , 300% modulus and elongation

a t break with increase in EVA content in the blend and the effect

of the th ree different cure sys tems on these proper t ies a r e shown

in Figures 1II.B. 3 , 1II.B. 4 and 111.8.5, respec t ive ly . Tensile s t rength

of those blends which contained su lphur and mixed cure systems

was maximum when the proportion of N R in the blend was in t he

range of 70 to 80 p e r cent, whereas t he tensile s t rength of the D C P

cured blends increased s t ead i ly with increase in EVA content in

the blend. When the proportion of t he minor component in the blend

is in the range of 20 to 30 p e r cent, i t remains a s d i spersed

par t ic les and the bulk of the curat ive ge ts d i spersed in the

continuous phase. In the present case , t h i s is more pronounced

because sulphur is h ighly reac t ive with N R and not a t a l l effect ive

in curing EVA. This w i l l f ac i l i t a te migration of su lphur which is

d ispersed in t he EVA phase to the N R phase during vulcanisation.

Thus a higher extent of crosslinking of the N R phase of the blends

B to D may be the reason for the observed h igher tensile values

of these blends in the case of sulphur and mixed cure systems.

At h igher proportions of N R in the blend, the tensile s t rength

values were in the o rde r : sulphur cure ) n~ ixed cure) , DCP cure.

ThiS observat ion can be explained based on the type of crossl inks

normally obtained when such systems a r e used. In sulphur cure with '

conventional dosage, the c ross l inks formed a r e mainly polysulphidic

in nature whereas. DCP cure g ives carbon-carbon type crossl inks.

The more f lex ib le polysulphidic l inkages faci l i ta te higher extensions

and h igher tensi le s t rength during s t re tching by reforming the

ruptured c ross l inks in prefe r red configurations whereas the less

102 f lexible C-C type linkages provide only lower tensile s t rength . In t he case of mixed cure system the re is a poss ib i l i ty that both

the types of c ross l inks a r e formed and the blends at ta in a h igher

crosslink density compared to the o ther cure systems. When mixed

crossl inks a r e presen t , the tensile s t rength w i l l be lower because

of the unequal d i s t r ibu t ion of load during s t re tchinglU3. This explains

the lower tensile s t rength of the blends having mixed cure system

compared with that of the su lphur cured blends. The higher

f lex ib i l i ty of t he polysulphidic linkages is a l so evident from the

higher elongation a t break of the blends cured with sulphur and

mixed systems compared with those containing DCP (Figure III .B.5) .

In the case of blends such a s those of N R and EVA, crossl ink

densi ty measurement b y the normal swelling method is very difficult

because of the wide difference i n so lubi l i t i es and polymer-solvent

interaction parameters of the components. However, indirect evidence

of a higher crossl ink densi ty can be obtained from the modulus

values. Figure III.B.4 shows h igher modulus values for those blends

vulcanised with the mixed cure system compared with those of the

sulphur and DCP cured blends a t higher proportions of N R . This

observation is fu r the r supported by the h igher rebound resi l ience

values of the blends having mixed cure system ( F i g u r e I I I . B . 6 )

compared with those of the blends having the o the r two cure

systems. For blends having a h igher proportion of EVA a l so , the

mixed cure system gave h igher tensile s t rength than DCP and

comparable modulus and resi l ience values.

Hardness of the blends having different cure systems is shown

in . Figure 111. B . 7. Hardness increased with increase in the proportion

of EVA in t he blend. The change in hardness is more s h a r p in blends

E and F compared with the o ther blends. This is 'because in these

blends EVA a lso tends to form a continuous phase since i t s melt

viscosi ty is lower than that of NR. In hardness measurement, the

deformation involved is only a t the surface and EVA can form an

outer l aye r during proccsslng. This is the reason fo r the 111ore o r

less s imi la r hardness of the blends from F to J . The mixed cure

system gave h igher hardness in the case of the blends having higher

proportions of N R because of h igher extent of crosslinking.

111. B. 2.3 Compression set

Compression s e t of the blends is found to increase a s the

proportion of EVA in t he blend increased (Figure I 1 1 . B . 8 ) which

is due to the thermoplastic nature of EVA. For blends A to G t he

sulphur cure system caused higher s e t compared with the other two,

because of the predominance of polysulphidic c ross l inks and also

due to the presence of t he uncrosslinked EVA phase. I t is seen that

a t the leve l of 40 p e r cent EVA in the blend, i t a l so forms a

continuous phase because of i t s lower melt v i scos i ty . Between mixed

and DCP cured blends, a change in the compression s e t pat tern was

observed a t 60:40 NR:EVA (blend E). The mixed cure system which

caused higher compression se t for blends A to E , showed lower

se t for blends G to J compared with the DCS' cured blends. Blends

A to E contained polysulphidic a s well as carbon-carbon type cross- 3 3

l inks and hence showed higher set values compared with blends

A2 to E 2 which contained only C-C crosslinks. In blends G to J .

NR remained a s the dispersed phase. It is possible that the

continuous EVA phase of the blends G3 to J3 attained higher levels

of crossiinking than blends G2 to J2 as i t is well known that in

peroxide vulcanisation of EPDM and EVA. sulphur can activate the

reaction to produce higher extents of c r o s ~ l i n k i n ~ ~ ~ ~ . The dispersed

N R phase also can have a higher extent of crosslinking due to higher

solubility of sulphur in this rubber.

111. B. 2.4 Tear and abrasion resistance

Tear resistance of the blends followed similar trend a s that

of tensile strength (Figure lII.B.9). A s the proportion of EVA

increased, the tear resistance also increased for the blends with

DCP and mixed cure systems. The mixed cure system showed better

tear strength compared with DCP cure system at almost a l l blend

ratios. In the case of abrasion resistance also. a s the proportion

of EVA increased, the blends showed better resistance to abrasion

(Figure 111.8.10). The behaviour of the mixed cure system was

different in blends having higher proportions of N R and EVA. When

the proportion of N R was higher (blends A to D ) th is system caused

bcller t.csislance to abrasion t11a11 the UCl' cure system. llowever

this trend was reversed in blends E to J, in which the DCP cure

system gave better abrasion resistance. A similar trend was seen

in compression set which was already explained on the basis of

the type of crosslinks and extent of crosslinking attained by using

the mixed cure system.

Table 111.1. Activation energy, ra te constant and cure time of NR-EVA blends.

Blend Blend ratio Activation energy. E Reaction rate constant Cure time. tgO at 1 6 0 ° C number N R : EVA

(kcal mole-') k ( min I Sulphur DCP Mixed Sulphur DCP Mixed Sulphur D C P Mixed

cure cure cure cure cure cure cure cure cure

20KU Xl000 3032 1 0 3 ~ RRLSM [

figure I I I .A . l . } SEM Photomicrograph of blend 8.

1 NR remained as dispersed phase. I

I 20KU Xi000 3034 lux RRLSM (

Figure III.A.2 i SBY Photomicrograph of blend F. I both ph- continuow. - . - - - - --

Fi@re III.A.3 1

SBM Photomfmgraph of blend B. both phases continuous.

P i g ~ ~ r - c 1 1 1 - A e 4 !)l*,M I'hotr~mic;rograph O C hlcnti I ) , p;~r'ticlrs 0 1 1:V.I rcsl trrg orr Ihr: ~ ~ x l rncicd sur'far:c:. 7' - .. .

-

SULPHUR

MIXED

4 EVA C O N T E N T . '/.-

Figure III.B.4

Effect of blend ratio and cure system on 300% modulus.

SULPHUR

MIXED

- EVA CONTENT,%.-

Figure III.B.%

Effect of blend ratio an3 cure system on compression set.

SULPHUR

MIXED