c H A P T E R - 5

STUDIES ON

BUTADIENE—_ACRYLONITRILE RUBBER/POLY(VINYL CHLORIDE) BLENDS

119

STUDIES ON gBUTADgIENE ACRYLONITRILE_ RUBBEB/POLY

(_'\_/INYL CHLORIDE) BLENDS

The range of application of traditional polymerscan be considerably broadened by their physical modification,that is, by blending the polymer being modified with otherlow or high - molecular weight compounds. Such multicomponent

blends often possess interesting properties which is thereason for the observed increasing interest in their investi­gation and application. Because of its low cost and versati­lity, poly(vinyl chloride) (PVC) has received great attentionin further improving its processability, heat distortiontemperature, impact strength, and service life etc. by blend­ing with other polymersl. The polymeric modifiers added toPVC may be divided into four categories: solid plasticizers,impact modifiers, heat distortion improvers and processingaids.

The property modification is highly dependent uponthe miscibility of the modifying polymer with PVC2. Ingeneral, blends of PVC with polymers that show high misci­bility will be microscopically homogeneous (single phase)

and display a single glass transition temperature (Tg) of

120

121

intermediate value between the Tés of the pure componentpolymers. In cases where the blend Tg is lower than thatof PVC, plasticization is said to occur. When the blend

Tg is increased then heat distortion temperature improvementis obtained. An additional benefit that can be imparted bymiscible polymers is an improvement in processability.These polymers can increase the rate of fusion of the PVCpowder leading to a more rapid formation of a homogeneousmelt having higher melt strength.

By far the widest range of polymeric additivesfor PVC fall into a broad category that can be classifiedas partially miscible3’4. A broad range of properties maybe obtained depending on the degree of miscibility. Parti­ally miscible blends display some phase separation (micro­scopically heterogeneous) but a significant degree of polymersegment mixing on a molecular scale occurs producing mechani­cally compatible phases (high interfacial addition) with

useful properties. Each phase will display a distinct Tgbut because of the partial intermixing, the Tés will bemoved closer together and become progressively suppressed in

intensity reflecting the ability of the high Tg component toraise the Tg of the low Tg component and vice versa. On theother hand, immiscible polymer blends will be microscopicallyheterogeneous (multiphase) and will display a distinct and

122

unshifted Tg for each phase. In practice, truly incompatiblepolymer-PVC blends are of little interest since they yieldpoor properties especially if the interfacial adhesion betweenthe phases is poor.

Butadisne-a¢r:Q<>9itri1s r<NBR>/P<>ly_<vir=y1r¢hl $>rr_-1518) ._ (PE/'9)blends

Miscibility of polymer blends was first observed withNBR/PVC systems. These blends probably have been the most

widely studied, having significant commercial interests. Theyhave been described as miscible, partially miscible and evenimmiscible based on the different experimental techniques andcopolymer compositions. Miscible blends are widely used asflexible compositions with the NBR acting as a permanent plasti­cizer. In general the acrylonitrile content sufficient toyield maximum miscibility appears to fall in the range of25-40% with gradual immiscibility occurring with either decrea­sing or increasing acrylonitrile levels6.

Low levels of NBR added to PVC yield impact modified

rigid PVC with optimum properties. Moderate levels of NBR

can act as a permanent plasticizer for PVC in critical appli­cations, where the migration of low molecular weight plasti­cizer is a severe problem. On the other hand, PVC added to

123

NBR improves ozone, thermal, ageing and chemical resistanceof NBR. PVC also vastly improves abrasion resistance, tearresistance and tensile properties7-9. It also adds gloss andimproves finish of the extruded stock and imparts flame retarant character. NBR/PVC blends can be conveniently milled,

extruded, and compression moulded using traditional processinequipments for natural and synthetic rubberslo.

QPTIMlZBTIQNlQF.BLFNPING;P5RAMETER5 F@P_THE PREPARATION”?!NBR/PYP BLENDS

Mixing in the molten state is often the method ofchoice for the preparation of polymer blendsll. The primarydisadvantage of melt mixing, however, is that the componentsmust be in the molten state, which can mean that temperaturesmay be high enough to cause degradation. Hence, care shouldbe exercised to do the blending at the minimum temperaturespossible. In the case of NBR/PVC blends, NBR acts as apermanent plasticizer for PVC and hence depending upon theamount of NBR, there might be a minimum temperature at which

the PVC phase fully homogenizes. If blending is done at thistemperature, the degradations occurring in the polymer phasescould be kept to a minimum. This means that there exists anoptimum temperature of preparing a blend of a particular compo­sition provided, the other blending parameters are fixed..Anhas been made to estimate this optimum temperature by studies

d­

Q

attempt

124

conducted on a Brabender Plasticorder. Mg0/ZnO/Stearic acidcombination was used as the stabilizer for the PVC phase inthis study.

Experimental

1. Studies on the Brabender Plasticorder

The study was conducted on a Brabender Plasticorder

Model PL3S using a roller type mixing head (rotor) at a speedof 30 rpm. A total weight of 34 gms was selected as the sampleweight for each run on the plasticorder after carefully observ­ing the torque curves for different sample weightslz. Sixcombinations of blends were made by taking PVC 10, 20, 30, 40,50 and 60% of the total weight. 4.0 parts per hundred PVC (phr)

Mg0, 4.0 phr ZnO and 3.0 phr stearic acid were taken togetherwith PVC as stabilizer. The rest of the weight constitutedNBR and its curatives - 1.5 phr sulphur and accelerators(1.5 phr MBTS and 0.5 phr TMTD) and activators (2.0 phr ZnO

and 1.0 phr stearic acid). The recipe for the blend contain­ing 20% PVC is shown in Table 5.1 as an example. For all thetest runs on the plasticorder, NBR was added initially, afterthe mixer attained the required temperature.- After giving1 minute for the rubber to heat up and homogenize in the mixer,PVC and its stabilizer were added. The polymers were blendedfor 5.5 minutes and then the curatives were added. Once themass broke down to crumbs it was removed. The study was

125

repeated for the six combinations of blends at differenttemperatures.

Table 5.1

Recipe for the NBR/PVC blend with

PVC and stabilizerPVC ..Mg0 ..ZnO ..Stearic acid ..

§§BR and curatiyesNBR ..ZnO ..Stearic acid ..MBTS ..TMTD ..Sulphur ..

6.80 gm (20%

0.27 gm (4.0

0.27 gm (4.0

0.20 gm (3.0

24.85 gm

0.50

0.25

0.37

0.12

0.37

34.00

(2.0

(1.0

1.5

0.5

(1.5

2. Evaluation of mechanical properties

For measuring the mechanical properties the blend

20% PVC

of total wt.)phr

phr

phr

phr

phr

phr

phr

phr

Pvc)

PVC)

Pvc)

NBR)

NBR)

NBR)

NBR)

NBR)

containing 20% PVC was made at 135, 145 and 155°C and the

blend containing 50% PVC at 145, 155 and 165°C. These blends

126

were prepared on the Brabender Plasticorder using the samerecipe as that used for Brabender studies. However, a

blending time of 10 minutes was employed13 and the curativesin each case were added later on a two roll laboratorymixing mill at about 50°C. The curatives were not added inthe plasticorder, since the torque involved in handling the

polymers at this low temperature was very high. The curecurves of the various compounds were taken at 150°C on theMonsanto Rheometer. Test samples for evaluating the physicalproperties were vulcanized upto the respective cure timesat 150°C. The tensile properties were determined on a ZwickUniversal Testing Machine as per ASTM D412 (1980). Shore-A

type durometer was used for determining the hardness of thevulcanizates as per ASTM D2240 (1981),

Results and discussion

Fig.5.1 shows the Brabender torque curves of theNBR/PVC blend containing 20% PVC and that of NBR alone at

155°C. The curve for the blend may be explained as follows.NBR is charged into the mixing chamber in the region A to B,and hence the torque rises. Once the addition of NBR is over,the rubber homogenizes and the torque begins to decrease atB mainly due to degradation in the rubber, especially mechani­cal. At C, PVC is added. As the PVC melts, the torque risesagain upto D and thereafter remains almost steady upto E.

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128

It may be observed that the tendency for degradation inthe blend is not so obvious as before, and this might bedue to the protection offered by the PVC phase. Curativesare added at E. The torque decreases to a minimum valueF due to the lubricant action of these additives. Oncecrosslinking starts, the torque rises again. At G, thecuring is completed and the material breaks down, whichaccounts for the sudden decrease in torque upto H. Thetorque curve for NBR alone shows that the decrease intorque due to degradation continues from the point B uptothe point E. However, the rate of decrease in torquefalls off from B to E due to the decrease in shear as aresult of the degradation. Table 5.2 shows the torqueat the points B, C, D, E and G of the blends containingdifferent percentages of PVC at 155°C. In the case of theblend containing 10% PVC there is some decrease in torquefrom D to E. This shows that this amount of PVC is not

enough to protect the rubber phase under the conditionsexisting in the mixer. The 20% PVC blend shows only aslight decrease in torque from D to E. For the blendscontaining 30, 40, and 50% PVC, the torque rises againafter the point D, and then stabilizes to the torque valueat E, which suggests that the melting of PVC is not completeat point D, but continues in the mixing time. In the case

Table 5.2

129

Brabender torque values of NBR/PVC blends at 155°C*

Blend Torques at various points (metergrams)composition% of PVC B C D E G

10%

20%

30%

40%

50%

60%

2500

2200

1900

1600

1200

1100

* Ref.Fig.5.1

1500

1400

1100

800

500

300

1800

1650

1750

1750

1700

1400

1550

1600

1750

1850

1950

1850

2000

2000

2200

2200

2300

2200

130

of the blend containing 60% PVC, the torque continues torise till the end of the mixing time, indicating insufficientmixing time and/or mixing temperature. The torque observedat point E increases with the percentage of PVC as expected,and this torque may be assumed to reflect the mechanicalstrength of blends. However, the 60% blend shows a lessertorque at E than that of the 50% blend. Clearly, the PVChas not melted properly to show the full torque. The torqueshown by the blends after curing (torque at G) seems to beonly a rough measure of the mechanical strength. This maybe due to the tendency of the mass to break down once curingstarts.

Fig.5.2 shows the Brabender torque curves of theblend containing 20% PVC at various temperatures (curingpart is not shown for clarity). The torque curve at 115°Cdrops down slightly after D, and then stabilizes to thetorque value at E. This delay in stabilizing may be due tothe presence of a good amount of unfused PVC. The torquecurves at 125 and 135°C also behave similarly but stabilizeto the torque at E after a shorter time. At 145°C, the torquecurve stabilizes to the torque at E immediately after D. At155°C, the melting of PVC seems to be almost complete at D

itself, since there is no initial unstable period. Thetorque curves at 165°C and 175°C do not stabilize to a parti­cular torque, but show a trend to fall of continuously after D

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132

which means that some degradative processes become active

at these temperatures. This might suggest that there is anoptimum temperature at which a particular blend is to bemade under identical rotor speed and blending time. Acomparison of the torque values at E of the 20% blend atvarious temperatures (Table 5.3) shows that when the tempera­ture goes up from 115 to 125°C, the torque goes down by150 metergrams. Since the PVC might not be fully melted atthis temperature even at the point E, this difference intorque might have occurred predominantly from two factors;an overall tendency for the viscosity to come down due tothe rise in temperature and the tendency of the PVC phaseto enhance the viscosity by melting more of that phase.From 125 to 135°C and from 135 to 145°C the difference in

torque remains the same, indicating that these two factorscontinue to exert almost the same influence. Between 145

and 155°C the difference in torque rises to 250 metergrams,which might suggest that the effect of_more PVC melting isalmost negligible in this case. Since some degradativeprocesses might be under operation at higher temperatures asobserved earlier, 145°C might be the best temperature_formaking 20% blends. Table 5.3 also shows the torque at thepoint E of the 50% blend at various temperatures. Again,comparison of the torques indicates that 155°C might be theminimum temperature at which this blend fully homogenizes

Table 5.3

133

Brabender torque values of 20% PVC and 50% PVC blendsat different temperatures*

Torque at the Torque at theTempera- point E of 20% point E of 50%ture (°C) PVC blend PVC blend(metergrams) (metergrams)

115

125

135

145

155

165

175

2300

2150

2000

1850

1600

1400

1225

* Ref. Figs.5.1 and 5.2.

2050

2250

2200

2100

1950

1625

1450

134

and hence this may be the optimum temperature for making50% blend.

Table 5.4 gives the scorch time, cure time, tensilestrength, elongation at break, and hardness of the 20 and50% PVC blends made at different temperatures. The 20%blend made at 145°C and the 50% blend made at 155°C show

the maximum scorch safety in agreement with the Brabender

evaluation. But for this slight change in the scorch times,the cure curves of the blends look very similar (Fig.5.3).The tensile strengths observed for the blends is much higherthan that observed for NBR alone (1.81 MPa). For the 20%

blend, maximum tensile strength is observed for the blendtaken at 145°C and for the 50% blend maximum is observed for

the blend taken at 155°C, again in agreement with the obser­vation made on the Brabender Plasticorder. The elongationat break of the blends is higher than that of NBR alone(285%). But this property seems to fall off after a parti­cular percentage of PVC since the 50% blend shows a lowervalue than the 20% blend. In this case also the highestvalue is observed for the blend made at 145°C for the 20%blend and for the blend made at 155°C for the 50% blend.

The hardness of the blends increases with the percentageof PVC, as expected. Although the change in hardness of ablend made at different temperatures is small, the 20%

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137

PVC blend made at 135°C shows a higher hardness than that

made at 145°C which might indicate inferior homogenity.Further, the blend made at 155°C shows a lesser value, whichindicates that some degradations have occurred.

l‘_49aDIFI ¢AT,I_ON~ OE NBR/FY? BLEl‘1D_aP5IN9a NATPR?\Pa RU5PERu STYRENEF

BU?’API:‘3NE RUBBER {W9 P9L}'?PTP:DlENa3 BUB!3§B

Blending of two or more rubbers is carried out forthree main reasons: improvement in technical properties,better processing and/or lower compound costl4. Many productsin the rubber industry are based on blends in all or part oftheir construction. General purpose rubbers, natural rubber(NR), styrene-butadiene rubber (SBR) and polybutadiene rubber(BR) have been used to replace NBR partly in a 70/30 NBR/PVC

blend in this study mainly to lower the compound cost. Sincethese general purpose rubbers are not likely to be compatiblewith either NBR or PVCl5, the physical properties of the result­ing ternary blends cannot be easily predicted13. However, theproducts are likely to show some improvement in technicalproperties since NR has higher tensile strength and tearstrength, SBR has higher hardness and modulus and BR has

higher abrasion resistance, compared to NBR16. MgQ/ZnO/Stearic acid combination was used as the stabilizer for thePVC phase.

138

Experimesfisol

1. Studies on the Brabender Plasticorder

An attempt was made to determine the relative effectof substituting part of NBR in a 70/30 NBR/PVC blend with NR,SBR and BR on a Brabender Plasticorder model PL3S (rotor

speed: 30 rpm, sensitivity 0-5000 metergrams, mixer tempera­ture 160°C). The total weight of polymers taken for each runwas 40 gms. In addition to the polymers, 4 parts of Mg0 perhundred parts of PVC resin (phr), 4 phr Zn0 and 3 phr stearicacid were also added as stabilizer for PVC. Initially, themixing characteristics of an NBR/PVC blend which contained

70% NBR (of the total polymer weight) was studied. NBR wasadded initially and homogenized for two minutes. Then PVCtogether with its stabilizer was added. The blending wascontinued for ten minutes after the torque passed the maximumvalue. The blending characteristics of the terpolymer blends,made by replacing NBR partly (30% of the total weight ofpolymers) by NR, SBR and BR, were investigated next. NR, SBRor BR was added initially and homogenized for two minutes.Then NBR was added and mixed for another two minutes. Then

PVC along with its stabilizer was added. After the torquepassed the maximum value, the blending was continued for

another ten minutes as before. The Brabender torque curvesalong with the amounts of polymers added are shown inFig.5.4.

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140

2- Determinationof mscbanical Properties

For measuring the mechanical properties also, anNBR/PVC blend containing 7O%.NBR (of the total polymer

weight) was chosen as the standard. Effect of modifyingthis blend by NR, SBR and BR on the mechanical propertieswas determined by replacing parts of NBR (15% and 30% of

the total polymer weight) by each of these rubbers and thenmeasuring the change in the properties. However, the poly­mers were not melt mixed for preparing the test samples,since the Brabender studies have shown that this leads to

some degradations particularly in the NR, SBR and BR phases.Hence NBR/PVC blend alone was made on the Brabender Plasti­

corder initially at the optimum temperature17, together withthe stabilizer for PVC (4 phr MgO, 4 phr ZnO and 3 phr stearicacid). The terpolymer blends were then made by adding NR,

SBR and BR to the NBR/PVC blend on a warm laboratory mixing

mill. The vulcanizing agents for the rubber(s) were alsoadded on the mill. The formulations employed for the prepara­tion of the various compounds are shown in Table 5.5.

The cure characteristics of the compounds were deter­mined at 150°C on the Monsanto Rheometer (Table 5.6). Test

samples were vulcanized upto the respective optimum cure timesat 150°C. The tensile properties were determined on a ZwickUniversal Testing Machine as per ASTM D412 (1980). Tear

Modification of NBR/PVC blends using NR, SBR and BR-­

Table 5.5

141

formulations used for preparing test samples

Polymers (weight percentage of total polymers)

PVC

NR, SBR or BR

NBR

30.00

0.0015.00

30.00

70.00

40.00

(for(forSBR

(forSBR

(for(forSBR

(forSBR

NBR/PVC blend)blends containing 15% of NR,or BR)blends containing 30% o ,or BR)

NBR/PVC blend)blends containing 15% of NR,or BR)blends containing 30% of NR,or BR)

Stabilizers for_PY§ (parts per hundred parts of PVC)MgO 4.00ZnO

Stearic

Curatives

acid4.003.00

for the rubber(s)ZnO

StearicCBS

TMTD

Sulphur

acid2.501.501.250.202.25

(parts per hundred parts oftotal rubber(s))

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143

strength of the samples was determined as per ASTM D624 (1981)The hardness of the samples was measured and expressed inShore-A units. The swelling characteristics were determinedin chloroform, toluene and an aromatic oil as per ASTM 471(1983). The properties of the blends are shown in Table 5.6.

Results and dis¢Ps§i@9

The Brabender torque curve for the NBR/PVC blend in

Fig.5.4 may be explained as follows. NBR is added in theregion C to D and hence the torque rises. Once the additionis over, the rubber homogenizes and the torque begins todecrease at D due to mechano-chemical degradation. PVC isadded at E and due to its melting the torque rises to F.Again there is a tendency for the torque to drop slightlyupto G. However, the rate of decrease in torque from F toG is much less than that from D to E indicating that PVCoffers good protection to NBR against the degradation,obviously due to the compatible nature of the polymers. Thereis also no visual evidence for any degradation of the blendduring blending. A comparison of the four torque curves inFig.5.4 indicates that maximum torque rise from E to F occursin the case of the NBR/PVC blend. This shows that even thoughthe same amount of PVC (12 gms) was added in each case the

maximum amount melted in this case showing again the good

compatibility of NBR and PVC. These observations might

144

indicate that the relative compatibility of PVC with therubber(s) may be approximately estimated from the followingfactors:

(a) Amount of PVC melting (torque rise from E to F).

(b) Extent of protection offered by the PVC phase againstthe degradation of the rubber(s)(Rate of decrease in torque from F to G compared tothe rate of decrease from D to E and visual examination

of the blend for any colour development during the testrun on the plasticorder).

If similar tests are conducted at a lower temperature theformer is likely to give more useful information and thelatter if the tests are conducted at a higher temperature.In the case of NR/NBR/PVC blend, the addition of a mere

12 gms of NR initially produces a fast rate of degradation,as seen from the torque curve from B to C. This is due tothe high molecular weight of NR. when NBR is also added, thenet rate of decrease in torque from D to E increases due tothe larger amount of rubber. with the addition of PVC, thetorque rises from E to F. However, this difference in torqueis less than that observed in the case of the NBR/PVC blend.Further, the protection offered by PVC against the degradationof the rubbers seems to be low, judging from the rate of

145

decrease in torque from F to G, indicating poor compatibility.The degradation of the blend inside the plasticorder couldalso be observed visually from the colour development. Inthe case of SBR/NBR/PVC blend, the initial rate of decreasein torque from B to C is very little, due to the lower mole­cular weight of SBR compared to that of NR. However, with theaddition of NBR, the degradation becomes obvious (D to E) dueto the greater amount of rubber. But with the addition ofPVC, the rate of degradation is reduced (F to G) indicatingbetter adhesion between the phases in this case than theNR/NBR/PVC blend. However, the rise in torque from E to Fis similar to that of NR, and there is a slight colour develop­ment indicating lesser compatibility than the blend of NBR andPVC alone. The behaviour of the BR/NBR/PVC blend is similar

to that of the SBR/NBR/PVC blend but indicates lower level ofblending.

An examination of the tensile properties of the blends(Table 5.6) shows that when NBR/PVC blend is modified with NR,

SBR and BR at 15% level, the properties are improved. It may beinferred that at this level the incompatibility of the polymersdoes not seriously affect the mechanical properties. NR/NBR/PVCblend shows the maximum tensile strength and elongation as

146

expected. However, when NR or BR is present at 30% level,the tensile properties deteriorated severely, obviouslyindicating the incompatible nature of the polymers. Thisshows that the optimum amount of NR or BR which could be

added to NBR/PVC blends is much less than this quantity.SBR which has similar tensile properties as NBR exhibits

more or less steady behaviour even when added at 30% level.This again shows that SBR blends better with NBR/PVC blendsthan NR or BR and that SBR could be added to the blend in

larger amounts than NR or BR. The tear strength of theblends also shows similar trend as the tensile properties.SBR and BR blends show a higher hardness than the NR blend

as expected. The percentage increase in weight of the blendsin solvents show that the swelling of NBR/PVC blends in polarsolvents like chloroform could be reduced by the addition ofNR, SBR and BR. Considering the enormous swelling of NBR

in polar solventsla this could be an advantage in some appli­cations. However, there is some increase in the swelling ofNBR/PVC blends in non-polar solvents and oils with the addi­tion of NR, SBR and BR as shown by the swelling of the blendsin toluene and aromatic oil.

1fl°BIFI¢F§'1f1QN-s°F NBR/PVB BBBBNB B$lNB PQBYBUBPHIBBs RUBBER

Polysulphide rubber is a costly speciality rubberwhich is well-known for its outstanding resistance to solventssuch as ketones, alcohols, acids, hydrocarbon solvents,

I

147

water etc. But processing of polysulphide rubber is diffi­cult, its mechanical properties are poor and it has anunpleasant odour19_21. Hence, it would be worthwhile to

attempt improving the processing and mechanical propertiesof polysulphide rubber without sacrificing much of itsexcellent solvent resistance for use in critical applications.Attempts have already been made in this direction by blendingpolysulphide latices with other synthetic laticeszz.Compared to hydrocarbon rubbers, polysulphide rubber is more

polar in nature and hence is likely to form successful blendswith other polar polymers or polymer blends. Hence in thisstudy, polysulphide rubber upto 50% of its weight was replacedwith 50/50 NBR/PVC blend and the properties were compared.

MgQ/ZnO/stearic acid system was used as the stabilizer forPVC.

Qxperimental

1 - Preparation 9f~P91Y§‘1LPhidE/NBB/PY9. V111 canizatss

50/50 NBR/PVC blend was made on the Brabender

Plasticorder employing a rotor speed of 30 rpm at the optimumtemperature17. In addition to the equal weights of NBR andPVC, stabilizers for PVC (MgO - 4 parts per hundred parts ofPVC resin (phr). ZnO - 4 phr and stearic acid 3 phr) wereadded at this stage. Blending of the polysulphide rubberand the NBR/PVC blend along with the additives for the

148

rubbers was done on a tight, warm (about 50°C) laboratorymixing mill. The formulations employed for the variousmixes are shown in Table 5.7. The cure characteristics ofthe mixes were determined on the Monsanto Rheometer at

150°C. The cure characteristics are shown in Table 5.8.The mixes were vulcanized upto the respective optimum cure

times at 150°C on a laboratory press in a specially designedmould which could be cooled immediately after moulding,

keeping the sample still under compression.

2. Determination of technical;propertie§

The tensile properties of the vulcanizates weredetermined as per ASTM D412 (1980) test method on a Zwick

Universal Testing Machine. The ageing characteristics ofthe samples were determined by keeping them at 100°C for24 hours in an air oven and then measuring the retentionin the tensile properties. The hardness of the vulcanizateswas determined according to ASTM 2240 (1981) and expressed

in shore-A units. The compression set was determined accord­ing to ASTM D395 (1982) under constant deflection. Thesolvent resistance of the vulcanizates in various solventswas determined as per ASTM 471 (1983).

Besulics 9251 §iSs=&1ssi<>n

Mill mixing of the polysulphide rubber was foundto be difficult even when a tight, warm mill was used.

mP_OOO.HOO.HOO.mN)P@_O@N%_omF@_OmN_OOm.OOO_mN@N.O@O_OmN_OOO_mOO_OmO©_OO®_OO®_OOO_ONm_OH_Om.ON_O¢_OOQ_ONOm_O®°_OOm.OOO_®OO_O@@¢_OO@_OO©.OOO.@H@NN_Om§0.0@NN.O@H.O@_OoO_mH@m_OFO_O@m_ODO_POO_OFOm.OO¢_OO¢_OOO_OH@H_O@0.0mH_OH_ON_OOO_O%O¢.O®O_OO¢_OOO‘®OO_O®@H_OON_OON_OOO.@mPO_OmN0.0mP0.0@0.0H_OOO_mm¢_O@O_Om¢_OOO.©OO_O@M M Q U mQ°_OO@_OOH_OO@_OOO_OHUfiom UnammflmO1NOUZU>@mm>HPHU©m Ucm U>&MSSQHDWDEEPWBMZUfiom UflumwflmOCNmmzmmglfl mam ZEZUfium UHMmmPmUQDWBMZOCNOO_OOH Hmnggh m©H£QHgm>HO&41 i \w®>'_HD_ Hmvmum __UC®hwnnsh mU%£mHDm%H0mxfizP_m wHnmBm®XfiE wO mCOfip®H5EHOh'|©cwHQ U>@\MmZ £PH3 umnnsh m©H£QH:m%HOQ mo cOHPmUHMHUoZ

150

Table 5.8

Cure characteristics of polysulphide/NBR/PVC mixes

Mix

Optimum curetime (mins.)

Scorch time(mins.)

Reversion(units droppedin 5 mins.)

A B C D E F16.5 8.5 11.5 22.5 23.5 28.5

3.5 2.5 2.5 3.0 3.5 4.0

1.0 1.0 1.0 Nil Nil Nil

151

However, with the addition of increasing amounts of NBR/PVC

mill mixing became progressively easier which might indicatethat the processing characteristics of the polysulphiderubber could be improved with the addition of NBR/PVC.

Further, the conventional method of curing and opening themould while hot was found to be unsuitable especially for theblends with low concentrations of NBR/PVC. This might be

due to the thermoplastic nature of the polysulphide rubber,particularly in the presence of the zinc salt of MBTS21.

The tensile properties of the blends are shown inFigs.5.5, 5.6 and 5.7. The tensile strength and modulus ofpolysulphide rubber are found to improve with the additionof NBR/PVC, while elongation at break decreases slightly asexpected. Other physical properties are also found toimprove with the addition of NBR/PVC as evidenced by theincrease in the hardness values and the decrease in thecompression set values (Fig.5.8). The ageing resistanceof the blends (Figs.5.5, 5.6 and 5.7) is interesting. Poly­sulphide rubber shows reasonably good resistance to heatageing. The ageing resistance of the blends is found toimprove with the concentration of NBR/PVC and the tensilestrengths after ageing of the blends with higher concentra­tion of NBR/PVC are found to be higher than their originalvalues. Since the tensile strengths of polysulphide rubber

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156

and NBR both deteriorate with ageing, this abnormal beha­viour might be due to the small amounts of crosslinksinduced in the PVC phase during ageing in presence of ZnOand TMTD23. The increase in the modulus values with ageing

is expected, since there is a considerable reduction inelongation with ageinga

The solvent resistance of the blends (Table 5.9)is not so outstanding as that of pure polysulphide rubber.However, the values are within reasonable limits for practi­cal applications. The resistance of the polysulphide rubberagainst ketones seems to be the most affected by blendingwith NBR/PVC. This may be due to the severe swelling ofNBR in polar solvents such as ketones and estersle.

§ummary

The following conclusions could be drawn from thisstudy on NBR/PVC blends.

1. An optimum value exists for a blending parameter suchas temperature (when other parameters are fixed) forthe preparation of an NBR/PVC blend of a particularcomposition and this optimum value could be determinedusing a Brabender Plasticorder.

Nw_fiO®_O@@m%.@¢¢@_P@N_@H%NF‘?w@_@wNNm_V¢@®_PNm_NHHw©_v@%_mNN¢®_@¢N@_®H@.H%H©¢_¢@o_@@HN€.m¢®¢_@®@.Fo%@@.VNN_@w¢@_N¢Nm_¢@m_F@@H.¢¢m_®N wcopwx fi@m_@m¢©_H©®.PPM N Q U m 4Amwmw om CH pgmfiwx CH MWMQHUCH XV@_m wfipmam©CmHQ U>&\%mZ\®UH£QHgm%HOQ MO wUCmPmHmwH Pcm>HoWHwpmghgpw H%£p®zUHUM UHPQU4HOCm£p®zOCQUHOEMmmmflmwmpmNHCmUH:>

158

NBR/PVC blends could be extended using general purpose

rubbers like NR, SBR or BR by replacing part of NBR.The resulting ternary blends also possess some improvedmechanical properties when the modification is limitedto small quantities. SBR could be added in largeramounts than NR or BR without much deterioration in the

mechanical properties of NBR/PVC blends. The compati­

bility of PVC with rubbers could be estimated using aBrabender Plasticorder.

Polysulphide rubber could be modified with NBR/PVC

blends to generate high performance ternery blendswith good solvent resistance and mechanical properties.

MgO/ZnO/Stearic acid system could be used to stabilize

the PVC phase in blends of PVC with rubber(s).,,MgO, ZnO and stearic acid are conventional ingredients

used in rubber compounding and hence they may not

adversely affect the rubber(s). Further this stabilizersystem does not impair colour, is non-sulphide stainingand nontoxic. Hence PVC blends stabilized with this

system are found to possess good mechanical propertiesand colour flexibility and they could be used in foodcontact applications.

159

Refere“¢e§

1. J.A.Manson in "Encyclopedia of PVC“, L.I.Nass andC.A.Heiberger (Eds.). Vol.1, Chap.10, Marcel Dekker,New York (1986).

2. G.H.Hofmann in "Polymer Blends and Mixtures", D.J.Wa1sh,J.S.Higgins and A.Maconnachie (Eds.), Chap.7, MartinusNijhoff Publishers, Dordrecht (1985).

3. J.A.Manson, Pure.Appl.Chem., QQ, 471 (1981).

4. R.A.Emnatt, Ind.Eng.Chem., Q6, 730 (1944).

5. 0.0labisi, L.Robeson and M.T.Shaw, "Polymer-PolymerMiscibility", Academic Press, New York (1979).

6. M.Matsuo et al, Polym.Eng.Sci., 2,197 (1969).

7. J.E.Pittenger and G.F.Cohan, Mod.Plast., ZQ, 81 (1947).

8. J.R.Dunn, Rubber Chem.Technol., 42, 978 (1976).

9. P.J.Corish and B.D.W.Powell, Rubber Chem.Technol.,41, 481 (1974).

10. A.H.Mazumdar and M.S.Majmudar(Ehemaprene Rubber Handbook",Chap.7, Synthetics & Chemicals Ltd., Bombay (1983).

11. M.T.Shaw in "Polymer Blends and Mixtures", D.J.Walsh,J.S.Higgins and A.Maconnachie (Eds.), Chap.3, MartinusNijhoff Publishers, Dordrecht (1985).

160

C- Roja Rao and K.Ramamurthy, Popular Plast., 12, 15(1977).

L.A.Utracki, Polym.Plast.Tech.Eng., gg, 49 (1984).

P.J.Cornish in "Polymer Blends and Mixtures", D.J.walsh,J.S.Higgins and A.Maconnachie, (Eds.), Chap.12, MartinusNijhoff Publishers, Dordrecht (1985).

S.Krause in "Polymer Blends", D.R.Paul and S.Newmann (Eds.),Vol.1, Chap.2, Academic Press, New York (1978).

R.J.Crawford, "Plastics and Rubbers Engineering Design andApplications", Mechanical Engineering Publishers Ltd.,London (1985).

K.E.George, R.Joseph and D.J.Francis, J.Appl.Polym.Sci.,32, 2867 (1986).

H.H.Bertram in "Development in Rubber Technology-2",A.Whelan and K.S.Lee (Eds.), Chap.3, Applied SciencePublishers Ltd., London (1981).

w.Cooper and G.M.Doyle, in "Rubber Technology and Manu­facture", C.M.Blow and C.Hepburn (Eds.), p.163, ButterworthsLondon (1985).

D.C.Blackly, "Synthetic Rubbers", p.300, Applied SciencePublishers, London (1983).

J.R.Panek in "Rubber Technology”, M.MortOn (Ed.). pp.349-367Van Nostrand Reinhold Company, New York (1973).

161

22. R.A.Garipova et al, Int.Polym.Sci.Technol., lg, No.1,4 (1983).

23. V.Duchacek and A.Kuta, J.Appl.Polym.Sci., Z1, 1549 (1982)