in situ dynamic vulcanization of poly(vinyl chloride)/acrylonitrile-butadiene rubber blends

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In Situ Dynamic Vulcanization ofPoly(Vinyl Chloride)/Acrylonitrile-butadiene Rubber BlendsFábio Roberto Passador a , Antonio Rodolfo Jr. b & Luiz AntonioPessan aa Department of Materials Engineering , Federal University of SãoCarlos , Brazilb Braskem S.A. , BrazilPublished online: 02 Mar 2009.

To cite this article: Fábio Roberto Passador , Antonio Rodolfo Jr. & Luiz Antonio Pessan (2009) InSitu Dynamic Vulcanization of Poly(Vinyl Chloride)/Acrylonitrile-butadiene Rubber Blends, Journal ofMacromolecular Science, Part B: Physics, 48:2, 282-298

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Journal of Macromolecular Science R©, Part B: Physics, 48:282–298, 2009Copyright © Taylor & Francis Group, LLCISSN 0022-2348 print / 1525-609X onlineDOI: 10.1080/00222340802679607

In Situ Dynamic Vulcanization of Poly(VinylChloride)/Acrylonitrile-butadiene Rubber Blends

FABIO ROBERTO PASSADOR,1 ANTONIO RODOLFO JR.,2

AND LUIZ ANTONIO PESSAN1

1Department of Materials Engineering, Federal University of Sao Carlos, Brazil2Braskem S.A., Brazil

Poly(vinyl chloride) (PVC)/acrylonitrile-butadiene rubber (NBR) blends can be ob-tained through a dynamic vulcanization process as a melt-processible thermoplasticelastomer which produces parts that look, feel and perform like vulcanized rubberwith the advantage of being processible as a thermoplastic material. In this study, avulcanized thermoplastic was obtained by in situ dynamic vulcanization of PVC/NBRblends using a sulphur/ tetramethylthiuram disulphide (TMTD) and mercaptobenzoth-iazyl disulphide (MBTS) curative system during processing at the melt state. The blendswere melt-mixed using a Haake Rheomix 600. The curing behavior of NBR was theninvestigated by a Monsanto rheometer. The thermal analyses were performed and thecross-linking at different mixing times was calculated using DSC. FT-IR was also per-formed for characterization of the blends. The cross-link densities of the samples weremeasured by a swelling method. The degree of cure increases with the mixing time. Thecross-linking formation was verified through the formation of C S bonds in the blends.

Keywords dynamic vulcanization, cross-linking, poly(vinyl chloride), acrylonitrile-butadiene rubber, reactive processing, PVC/NBR blends

Introduction

Vulcanization is a chemical process performed on rubber to strengthen it by causing cross-linking of the polymer molecules. Static vulcanization, used commercially since the days ofCharles Goodyear, involves the heating of a rubber stock, fully compounded and mixed witha cure system, usually sulfur, at a temperature of 130◦C to 180◦C for a specified time, duringwhich chemical cross-links are formed between the macromolecules of the elastomer. Thisprocess transforms the rubber into a tough, elastic, durable thermoset material.[1−5]

Dynamic vulcanization, on the other hand, involves the curing of a rubber compositionduring its mixing or mastication, and one of the ingredients of this rubber composition mustbe a thermoplastic resin. This process results in an extremely useful elastomeric alloy mate-rial with the properties of a conventional thermoset rubber, but processible as a conventionalthermoplastic. The temperature reached during the mixing must be sufficiently high to meltthe thermoplastic resin and allow the cross-linking reaction to take place.[1−5] A largenumber of elastomers and thermoplastics have been combined to produce thermoplastic

Received 2 December 2008; accepted 11 October 2008.Address correspondence to Luiz Antonio Pessan, Department of Materials Engineering, Federal

University of Sao Carlos, Rodovia Washington Luıs, Km 235, P.O. Box 676, Sao Carlos, SP, Brazil,13565-905. E-mail: [email protected]

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Dynamic Vulcanization of PVC/NBR Blends 283

vulcanizates by dynamic vulcanization; these include NBR/Nylon, PP/EPDM, andNBR/PVC. PVC/NBR blends can be classified as melt-processible, thermoplastic elas-tomers producing parts that look, feel, and perform like vulcanized rubber with the advan-tage of being processible as thermoplastics.[6]

Blends of PVC and nitrile rubber have been commercialized since 1940. NBR hasgood oil resistance and low gas permeability; its use in automotive applications is re-markable. Its resistance to aging, however, is limited because of the butadiene-unsaturatedbackbone.[7−11] While PVC improves ozone and mechanical resistance in blends with NBR,the elastomeric component can act as a permanent plasticizer in such PVC applicationsas electrical wires and cable coatings, conveyor belts, and food containers. The presenceof PVC improves the ozone resistance of NBR, allowing the use of this blend in suchindustries as gaskets, wires and cables, artificial leather, and the like.[7−11]

Cross-linking of PVC and NBR blends was reported by Oravec et al.,[12] who con-trolled the interfacial bonds between PVC and NBR by using ammonium salts of triazinethiols and dithiodimorpholine. These additions resulted in a branching structure of tri-azine thiols in PVC, increasing the mechanical properties of the PVC/NBR blends. Moriand Nakamura [13,14] studied the modification of PVC with NBR, suggesting that triazinedithiols are effective functional additives for preparing composite materials. The products,however, did not give satisfactory properties to the blends because the interfacial reac-tion between PVC and NBR in the interface was not significantly controlled. Ghosh etal.[15] reported that cross-linking of NBR and PVC can be accomplished individually in aPVC/NBR blend by using an ethylenethiourea (ETU) /sulfur curing system. The ETU/Ssystem acts as a co-cross-linker and intercross-linker for NBR/PVC systems, producing agood balance between curing characteristics and vulcanizate physical properties. Manoj etal.[7,16] reported that poly(vinyl chloride)–nitrile rubber blends undergo a self-cross-linkingreaction (cross-linking without the aid of any external curing agents) during processing atelevated temperatures in the presence of a PVC stabilizer. The reaction proceeds throughthe hydrolysis of the nitrile groups in the presence of HCl liberated from PVC during itsdegradation.

The purpose of this study was to obtain and characterize in situ dynamic vulcanizedblends of poly(vinyl chloride)–nitrile rubber using a sulfur (S)/tetramethylthiuram disulfide(TMTD) and mercaptobenzothiazyl disulfide (MBTS) curative system during processingin the melt state.

Experimental

Materials

The poly(vinyl chloride) (PVC) used was a suspension-grade PVC resin (Norvic SP1300HP, Braskem, Brazil) characterized by a K value of 71 ± 1. The plasticizer added tothe system was an industrial-grade 2-ethylhexyl phthalate (DOP) and the thermal stabilizerwas an industrial grade of barium and zinc base. A commercial grade of partially cross-linked acrylonitrile-butadiene rubber (NBR) (Thoran NP-3351 P, containing 33 wt% ofacrylonitrile, Petroflex, Brazil), was used. Tetramethylthiuram disulfide (TMTD) and mer-captobenzothiazyl disulfide (MBTS), and sulfur (S), were used as the accelerator-sulfurcurative system, while a combination of zinc oxide (ZnO) and stearic acid was used as theactivator system for the vulcanization of the NBR.

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284 F. R. Passador et al.

Table 1Formulation used in preparation of NBR compound

Component phr∗

NBR 100.0Zinc oxide 5.0Stearic acid 0.5TMTD 1.0MBTS 2.0Sulfur 2.0

∗phr: parts per hundred rubber.

Compounding Procedure

The PVC was dry-blended with 60 phr of DOP and 3 phr of stabilizer following usualprocedures with an intensive mixer at 120◦C. The NBR compound was prepared using anintensive mixer at 90◦C–100◦C to avoid pre-cure during the mixing. The NBR was mixedfor 2 minutes; the other curing agents were then added every 2 minutes. Table 1 shows theformulation used in the preparation of the NBR compounds.

Determination of the Vulcanization Parameters

A Monsanto 100 oscillating disk rheometer operating at 160◦C and 1◦ arc was used todetermine the vulcanization parameters according to ASTM D 2084 [17].

Rheological Measurements

The viscosity at the shear rates range used during process was evaluated using an Instroncapillary rheometer (Model 4467), with L/D = 20 at 160◦C.

Blends Preparation

Two different kinds of PVC/NBR blends were prepared. The first system, called a dy-namically vulcanized blend (DVB), was prepared using the PVC and NBR compound;the second system, called conventional blend (CB), was prepared using PVC and NBRcompounds without the curing agents.

The dynamically vulcanized blends of PVC/NBR at the ratios of 90/10, 80/20 and70/30 wt% were produced by melt blending using a internal mixer (Haake rheometer,model Rheomix 600) operated at 160◦C with cam rotors running at 60 rpm for differentmixing times. Samples were removed after every 30 seconds of mixing and immersed inliquid nitrogen to interrupt the reaction.

The conventional blends of PVC/NBR (90/10, 80/20 and 70/30 wt%) were melt-mixedusing the Haake Rheomix 600 at 160◦C and cam rotor speed of 60 rpm for 5 minutes.

Thermal Analysis

The thermal behavior of the PVC, NBR, and PVC/NBR blends during heating was evaluatedby differential scanning calorimetry (DSC) (TA Instruments, model QS100). The tests were

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performed at a heating rate of 20◦C/min from −50◦C to 125◦C. All the measurements werecarried out under a nitrogen atmosphere.

Degree of Cure

The degree of cure was calculated using differential scanning calorimetry for differentmixing times. Measurements of reaction heat were performed using a TA Instrumentsmodel QS100 at a heating rate of 20◦C/min from room temperature to 300◦C. The totalamount of heat used in the thermal cross-linking reaction can be related to the exothermicpeak areas of the DSC curves.[18,19] The degrees of cure were calculated based on the valueof residual heat from thermal cross-linking of the cured samples. The reference heat valuefor the completely cured sample was considered to be the heat of the thermal cross-linkingof the uncured formulation.

Crosslink Density

The crosslink densities of the samples were measured by a swelling method [20] as follows:the PVC was first removed by a Soxleht extraction with acetone (C3H6O) for 20 h; thesamples were then soaked in methyl ethyl ketone (MEK) for 22 h, the weights of the swollensamples measured, and the crosslink density calculated. The χ (interaction parameterbetween the rubber network and the swelling agent) of 0.44, the bulk density of the rubberof 1.01 g/cm3, and the molar volume of MEK of 89.7 cm3.mol−1 were employed in thecalculation.

FT-IR

Infrared spectroscopic studies were done in a Thermo Fisher Scientific Inc. spectropho-tometer (model Nicolet 4700) using samples with a thickness of 2 mm on a KBr disc.

Results and Discussion

Vulcanization Parameters

Table 2 shows the vulcanization parameters obtained for the prepared NBR compound.Measurements on the oscillating disk rheometer can be related to important character-

istics of the materials, including minimum torque (Tm), associated with the viscosity of theuncured mix; maximum torque (TM), related to the amount of cross-links; the scorch time

Table 2Vulcanization parameters of NBR compound

Vulcanization parameters

Tm (N.m) TM (N.m) tS (min) t90 (min)17.0 53.8 2.0 3.5

Tm: minimum torque TM: maximum torque ts: scorch timet90: optimum vulcanization time

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(ts), which gives information on processing safety; and the time necessary to achieve 90%vulcanization ( t90), used as the optimum vulcanization time.[21]

The curative system used in this study promoted a decrease in optimum vulcanizationtime (t90) compared to the literature,[22] in which a sulfur/TMTD-MBTS curative systemwas used to promote cross-linking in the NBR phase in EPDM/NBR blends whose maincharacteristic was the formation of large-scale mono- and disulfide bonds only in the NBRphase.[22] Polysulfide bonds and the degree of reticulation increased with the amount ofsulfur and the number of allylic hydrogen molecules in the repetitive unit of the rubberused.[22,23] The curative system used in this study was considered efficient because thecontent of the accelerator was larger than the sulfur content.[3]

Torque Rheometer Curves

Figure 1 shows the variations of torque and temperature during mixing of the dynamicvulcanized blends of PVC/NBR at ratios of 90/10, 80/20 and 70/30 wt%.

The first torque peak corresponds to the addition of the compounds. It was observedthat the torque increases with increasing mixing time. Figure 1 (a) shows that a long mixingtime (700s) was necessary to observe an increase in the torque indicating the occurrence ofcross-linking in the NBR phase. Comparing the results with those of dynamically vulcanizedblends with more NBR as shown in Figures 1(b) and (c), the authors found that increasingthe rubber content allows cross-linking to occur more rapidly. In addition, an increase intorque during short mixing times was observed for the PVC/NBR blends at the ratios of80/20 and 70/30 wt% (370s for 80/20 wt% and 200s for 70/30 wt%). This increase wasfollowed by a drop and another improvement for the PVC/NBR blend at the ratio of 80/20wt%. In the cross-linked region of NBR it was observed that an increase in temperatureinfluences the formation of crosslinks during the vulcanization reaction.

PVC and NBR have different viscosities during mixing, as shown in Figure 2.An increase in friction between the particles occurs during mixing and causes a rise

in the temperature of the material, offering ideal conditions for NBR cross-linking witha consequent decrease in the total energy demand. The effectiveness of the curing agentdepends also on the temperature reached during mixing. As the temperature rise due tofriction is higher when the rubber content in the system is higher, an increase in the rubbercontent in the formulation leads to more cross-linking reactions due to higher temperaturesduring mixing.

Thermal Analysis

The DSC thermograms and the glass transition temperatures (Tg) of the pure polymers andboth types of polymer blends are shown in Figure 3 and Table 3.

The Tgs values were obtained from the derivative of the heat flow vs. temperature curveby determining the point of inflection characteristic of this transition; the values were takenin the peak of the derivative curve. Two glass transitions were observed for all blends, indi-cating their immiscible nature. Figures 3(c) and 3(d) include an enlargement of the regionwhere the first glass transition occurs. The first glass transition corresponds to the NBRphase and the second to the PVC phase. Many authors [13−16,24] have found a single broadtransition over the temperature range between the two Tgs of the unblended componentsfor blends reported as miscible. According to the literature the PVC/NBR blend can bedescribed as semi-compatible at 20% acrylonitrile (AN) content and almost homogeneous

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Figure 1. Variation of torque and temperature during mixing of dynamic vulcanized blends ofPVC/NBR, (a) 90/10 wt%, (b) 80/20 wt%, and (c) 70/30 wt%.

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Figure 2. Viscosity vs. steady shear rate of PVC compound and uncured NBR at 160◦C.

at 40%.[16] The miscibility of a PVC/NBR blend, however, depends on the blend ratio,the acrylonitrile (AN) content of the NBR, and the testing method. The PVC/NBR blendhas been described as miscible, partially miscible, and even immiscible depending on themethod of preparation and the acrylonitrile (AN) content of the NBR.[24−27]

The authors observed in this study that increasing the amount of nitrile rubber inall blends, both conventional and dynamically vulcanized, resulted in a shift in the Tgscloser together (Figure 3). The dynamically vulcanized blends showed a minor differencein the position of the Tgs values. This difference may be caused by the specific chemicalinteractions that influence crosslink formation during the vulcanization reaction.

Figure 3. DSC sample thermograms: (a) PVC; (b) cured and uncured NBR; (c) CB PVC/NBR(90/10 wt%), CB PVC/NBR (80/20 wt%), and CB PVC/NBR (70/30 wt%); and (d) DVB PVC/NBR(90/10 wt%), DVB PVC/NBR (80/20 wt%), and DVB PVC/NBR (70/30 wt%).

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Figure 3. Continued

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Figure 3. Continued

Table 3Glass transition temperature of blends obtained from DSC

Sample Tg NBR (◦C) Tg PVC(◦C)

PVC — 90NBR uncured −33 —NBR cured −24 —CB PVC/NBR (90/10 wt%) 4 60DVB PVC/NBR (90/10 wt%) 0 62CB PVC/NBR (80/20 wt%) 5 62DVB PVC/NBR (80/20 wt%) 4 61CB PVC/NBR (70/30 wt%) 5 62DVB PVC/NBR (70/30 wt%) 5 60

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Degree of Cure

The degree of cure was calculated by Eq. 1:

Degree of cure (%) =[

1 − �Hresidual cure

�Htotal cure

]× 100

where �Htotal cure corresponds to the value of residual heat from thermal cross-linking ofpartially cured samples and �Hresidual cure corresponds to the value for the completely curedsample. The �Htotal cure of the blends were 1.86, 1.23, and 2.10 J.g−1 for the dynamicallyvulcanized blends of PVC/NBR at the ratios of 90/10, 80/20, and 70/30 wt% respectively.DSC analyses of �Hresidual cure were performed on samples obtained every 30 seconds duringmixing. Figure 4 shows the degree of cure (%) versus mixing time (s) of the dynamicallyvulcanized blends.

All PVC/NBR blends studied in this work showed that an increase in mixing timepromoted in situ dynamic vulcanization of the nitrile rubber. The degree of cure values in-creased during long mixing times because there was sufficient time for crosslink formation.It has long been known that an increase in processing time for rubber compounds leads toan increase in the degree of vulcanization reaction and a simultaneous increase in crosslinkformation.

The results shown in Figure 4 suggest that by increasing the rubber content from 10to 30 wt%, the in situ dynamic vulcanization occurred during shorter mixing times. Thedegree of cure was 93% for 210s of mixing for the PVC/NBR blend at the ratio of 70/30(wt%) while the degree of cure was 86% for 660s of mixing for the PVC/NBR blend atthe ratio of 90/10 (wt%). The curative system used was considered efficient, thus enablingcrosslink formation during the reactive processing.

Figure 4. Degree of cure (%) vs. mixing time (s) of dynamically vulcanized blends of PVC/NBR atratios of 90/10, 80/20, and 70/30 wt%.

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Table 4Crosslink density of cured NBR and dynamically vulcanized blends

Sample Crosslink density (10−4 mol/cm3)

NBR cured 4.0DVB PVC/NBR (90/10 wt%) 1.3DVB PVC/NBR (80/20 wt%) 2.0DVB PVC/NBR (70/30 wt%) 2.4

Crosslink Density

Further evidence of cross-linking in the systems was obtained through the results of theswelling studies, shown in Table 4.

The curative system used in this study was designed to promote cross-linking only ofthe NBR phase; thus no reaction occurs between PVC and NBR. It was possible to extract85% of the PVC phase through the acetone extraction process. Swelling studies were donein methyl ethyl ketone (MEK), which is able to dissolve and swell only the NBR phase. ThePVC remainders after the extraction can limit the swelling of the NBR, decreasing the valuesof crosslink densities for the blends. The results obtained were normalized for the amountof residual PVC in the blends. An increase in crosslink density was observed for the blendswith higher amounts of rubber. According to these results, the vulcanization process for theblends was verified through the increase in the crosslink density values.

FT-IR Analysis

Figure 5 shows the infrared spectra of the PVC compound, NBR cured and uncured, andthe PVC/NBR blends.

The prominent IR bands are shown in Table 5.It is necessary to consider the chemical structure of DOP (Figure 6) for the interpretation

of the infrared spectra of the PVC compound.

Table 5Observed IR frequencies of PVC compound and nitrile rubber

Bands (cm−1) Assignment

2237 CN stretching vibration1723 C = O stretching vibration1600, 1580, 1462, 1435 aromatic C = C stretching vibration1465 CH3 stretching vibration1274 CH2-Cl stretching vibration1124, 1073 C-C stretching vibration968 cis - 1,4 - butadiene angular deformation960 C-Cl stretching vibration759 C-Cl stretching vibration743 ortho-disubstituted ring deformation718 C S stretching vibration

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In Figure 4a the band at 1723 cm−1 (C=O stretching vibration) corresponds to a smallquantity of double-bond structures from the DOP; the bands at about 1600, 1580, 1462,and 1435 cm−1 are assigned to the aromatic C=C stretching vibration; and the band atabout 743 cm−1 is assigned to the ortho-disubstituted ring deformation, also characteristicof DOP. The bands at 1124 and 1073 cm−1 correspond to the C-C stretching vibration ofPVC. The characteristic bands at 1274 cm−1 and 960 cm−1 are assigned to the CH2-Clstretching vibration and the C-Cl stretching vibration of PVC respectively.[28]

Figure 5. Infrared spectra of compounds: (a) PVC compound; (b) uncured and cured NBR; (c) CBPVC/NBR (90/10 wt%) and DVB PVC/NBR (90/10 wt%); (d) CB PVC/NBR (80/20 wt%) and DVBPVC/NBR (80/20 wt%); and (e) CB PVC/NBR (70/30 wt%) and DVB PVC/NBR (70/30 wt%).

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Figure 5. Continued

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Figure 6. DOP structure.

Figure 7 shows the chemical structure of the NBR, indicating the three possible isomericstructures for the butadiene segments.

The band at 2237 cm−1 is assigned to the –CN stretching vibration characteristicof NBR shown in Figure 5(b). The band at about 1465 cm−1 corresponds to the CH3

stretching vibration, and the band at 968 cm−1 is assigned to the cis - 1,4 - butadieneangular deformation, which is characteristic of NBR. In addition, a band due to C-Clstretching vibration is observed at 759 cm−1, corresponding to 12.5 wt% of PVC (used asa partitioning agent in NBR to avoid particle agglomeration during transport and storage).The main difference between the cured and uncured NBR is the band at 718 cm−1 assignedto the C S stretching vibration that resulted from crosslink formation.[28]

The acrylonitrile-butadiene rubber was vulcanized using an activation system fol-lowed by the addition of sulfur. Sulfur vulcanization occurs through radical substitutionin the forms of polysulfide bridges and sulfur containing intracyclization of the polymermolecules. Figure 8 shows the mechanism of NBR cross-linking using sulphur.

In the first stage a reaction between zinc oxide (ZnO) and sulfur (S8) takes place,resulting in the formation of a salt of the activator and sulfur (OS8ZnS8O). In the secondstage, the salt reacts with nitrile rubber, producing an intermediate compound. In the finalstage the intermediate compound reacts with other nitrile rubber molecules leading to thecross-linking of the NBR.

The bands observed in the infrared spectra of the blends, shown in Figures 5 (c), (d),and (e), correspond to the PVC phase and the NBR phases. The presence of the bandat 718 cm−1 assigned to the C S stretching vibration was observed for the dynamicallyvulcanized blends (DVB) as a result of crosslink formation during the reactive processing.This band was not observed for the conventional blends. Therefore, the curing system used

Figure 7. NBR structure.

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Figure 8. Mechanism of NBR cross-linking using sulfur and zinc oxide.

in this work to promote crosslink formation in the blends was efficient as verified throughthe formation of the C S bonds.

Conclusions

In situ dynamic vulcanization of PVC/NBR blends with a sulfur (S)/TMTD-MBTS curativesystem was evident from rheometric, thermal, and infrared spectroscopic studies. Thecurative system used was considered efficient because the accelerator content was higherthan the sulfur content. An increase in mixing time promoted in situ dynamic vulcanizationof nitrile rubber as verified through an increase of the degree of cure calculated by DSCanalysis. Overall, the degree of cure values increased for long mixing times because therewas enough time for crosslink formation. DSC analyses showed two Tgs intermediatebetween those of the individual components of the blend, indicating the semi-misciblenature of the composition. Crosslink formation was verified through the formation of C-Sbonds in the blends with curing agents.

Acknowledgments

The authors are grateful to Braskem S.A. for the donation of PVC; to the NEO-PVCProgram for technical support; and to CAPES for financial support.

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in situ dynamic vulcanization of poly(vinyl chloride)/acrylonitrile-butadiene rubber blends

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