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aAdvanced Composite Centre, Advanced S

Hyderabad-500 058, IndiabPropellant and Polymer Divsion, Director

Development Laboratory (DRDL), Kanchanb

kmohan_s@yahoo.com; scientistdrdohyd@g

Cite this: RSC Adv., 2014, 4, 12526

Received 9th October 2013Accepted 3rd January 2014

DOI: 10.1039/c3ra45684f

www.rsc.org/advances

12526 | RSC Adv., 2014, 4, 12526–1253

A new approach with prepregs for reinforcingnitrile rubber with phenolic and benzoxazine resins

S. Rajesh Kumar,a P. M. Asseref,a J. Dhanasekarana and S. Krishna Mohan*b

Acrylonitrile-co-butadiene rubbers (NBR) reinforced with phenolic and benzoxazine resins (NBR–Ph and

NBR–Bz respectively) are prepared by a novel method using co-curing. The relative effect of these resin

reinforcements with NBR in comparison to the non-reinforced NBR rubber compound NBR–S under the

same conditions has been investigated. It was found that these resins exist in the form of a localized,

interpenetrating network structure in the NBR matrix. For both NBR–Ph and NBR–Bz composites, the

tensile strength, tear strength and elongation at break increases compared to NBR–S. The tensile

strength in particular was increased by about 91% for NBR–Ph and by about 109% for NBR–Bz.

Thermogravimetric analysis (TGA) provides evidence for the superior thermal stability for NBR

composites over NBR–S. A decrease in the swelling values are observed in the NBR composites and the

retained tensile strength, elongation at break and modulus from thermal aging studies are found to be

superior than NBR–S. These results have shown that both the phenolic and benzoxazine resins are

effective reinforcements for NBR materials.

1. Introduction

Acrylonitrile-co-butadiene rubber (NBR) is used in aircras,automobiles, tanks, oil drilling industries and other militaryapplications as seals, gaskets, O-rings etc. due to its excellent oiland fuel resistances and low gas permeability. However, rubbercompositions based on NBR have limited high temperaturestability, chemical resistance and service life because of thepresence of an unsaturated backbone. The incorporation ofsmall particle size llers to the elastomer matrix results insubstantially improved mechanical properties.1,2 Results ofsmall and well-dispersed silica generated by an in situ sol–gelprocess demonstrated the improvement of the rubber–llerinteractions and better ller dispersion3,4 using a silanecoupling agent.5–7 The modication of elastomers with carbonnanobres or nanotubes8–11 of high aspect ratio and low densityhas also been studied.

Recently, polymer reinforced elastomeric composites haveattracted signicant attention due to their outstandingmechanical properties when compared to conventional elasto-mers. The polymeric resins are recognized as potential mate-rials for improving the physical–mechanical properties ofelastomers with retention of the aging properties. Studies onnatural llers such as soy protein aggregates and modiedstarch for reinforcement with styrene butadiene rubber

ystems Laboratory (ASL), Kanchanbagh,

ate of Propulsion, Defence Research &

agh, Hyderabad-500 058, India. E-mail:

mail.com

3

(SBR)12–14 and lignin with NBR showed good compatibility withthe rubber matrix, and their reinforcement effects were superiorto those of carbon black.15 The reinforcement of syntheticpolymers such as cardinol–formaldehyde, ultra high molecularweight polyethylene (UHMWPE), polyaniline, resorcinol–form-aldehyde and epoxy resins in various rubbers has been studiedto improve the mechanical properties.16–22 The reinforcement ofphenolic resins with chloroprene, ethylene propylene dienemonomer (EPDM) and NBR, and epoxy resin with SBR throughin situ polymerisation has also been discussed.23–25 The resinreinforcements for NBR are prepared by most commonmethods such as in situ polymerization, solution, melt mixing,blending, coating and latex blending. However, detailed studiesare required due to difficulties arising in the uniform dispersionof the resins while incorporating them into the elastomers,which plays a critical role in the enhancement of the properties.

Benzoxazine resins are a new class of phenolic polymers beingdeveloped as an alternative to traditional high-performancethermosetting resins for aerospace applications.26 Benzoxazineshave the design exibility for various applications, and they canbe tailored to display a range of properties which cover advancedepoxy and phenolic resins to bismaleimides and polyimides.Their distinct advantages are re resistance and superior proc-essability compared with the majority of thermosetting resinsknown today. In addition, these materials have high glass tran-sition temperatures (Tg), high moduli, low water absorption,good electrical properties and no shrinkage upon curing.27 Eventhough the applications of benzoxazines are increasing rapidly,to the best of our knowledge the reinforcement of benzoxazineresins with nitrile rubbers has not been attempted so far.

This journal is © The Royal Society of Chemistry 2014

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In this work, we have explored a novel method for thepreparation of NBR/resin composites for the rst time by usinga co-curing process at high temperature (i.e. 200 �C) from resinprepregs. The co-curing process has many advantageous due toits simplicity, lower capital investment and eco-friendly nature.This method is faster, more versatile and produces moreuniform cross linking when compared with the conventionaltechniques. Themechanism of reinforcement of the resin to thenitrile rubber by this method may proceed in two stages. In therst stage during co-curing, the resin present in the prepregows in to the rubber compound layer through porous releasingfabric, and in the second stage this resin forms the reinforce-ment with the nitrile rubber compound at high temperature.This study attempts the reinforcement of phenolic and ben-zoxazine resins with nitrile rubber for the rst time using a co-curing process. The relative effect of these resin reinforcementsto NBR in comparison to the non-reinforced NBR compoundhas been also investigated under the same conditions.

Fig. 1 Chemical structures of NBR and the thermosetting resins.

2. Experimental2.1. Raw materials

The thermosetting resins used for the reinforcement with NBRare phenolic (resol type, solid content: 61.69%, PR-100) andbisphenol-F-benzoxazine resins (mp: 65 �C–85 �C; Araldite MT35700) provided by ABR Organics, India and Huntsman Ltd,Switzerland respectively. The carbon (rayon-based) and release(D200 TFP) fabrics were obtained from Aerospace Ltd., Coim-batore, India and De-comp Composites Inc., USA respectively.Nitrile rubber compound sheet procured from RAP Vijayawada,India was used as received. Both the breather fabric (AB 1060V)and vacuum bagging lm (50 micron thick) were obtained fromAerovac, UK. Methyl ethyl ketone (MEK) solvent was procuredfrom Fluka, Switzerland. The characteristics of the nitrilerubber compound sheet, carbon fabric, release and breatherfabrics, vacuum bagging lm are given in Table 1. The chemicalstructures of NBR and the thermosetting resins are representedin Fig. 1.

Fig. 2 Cure cycle followed in the co-curing process.

2.2. Preparation of NBR–S

Two plies of release fabric (200 � 200 mm each) are placedabove and below the 200 � 200 mm sized 2 mm thick NBRrubber compound sheet. Four carbon fabric layers of 200 � 200mm were plied over the release fabric on both sides of thelayers. The complete setup of plies was placed on a metallic

Table 1 Details of raw materials used

Materials

Nitrile rubber compound sheet (rocasine)

Carbon fabric (rayon-based carbon fabric)Release fabric (D200 TFP)Breather fabric AB 1060VVacuum bagging lm 2.0 M, 518280

This journal is © The Royal Society of Chemistry 2014

mould, and a metallic caul plate was placed over the layers. Thesetup was enclosed in breather cloth and a vacuum bag, andcuring was carried out in an oven as per the cure cycle repre-sented in Fig. 2 to obtain non-reinforced NBR–S. This material

Characteristics

Composition in PPhr: NBR: 100, sulphur: 1.5, ZnO: 5.0, stearic acid: 1.0,DEG: 2.0, DOP: 10.0, silica: 50.0, pet. resin: 10.0, TTD: 0.2, 2-MD: 1.5Carbon content: 94%, sp. gravity: 1.8 � 0.1, thickness: 0.3–0.4 mmPorous Teon coated plain wave thickness: 0.003 inchesHeavy weight synthetic 330 g m�2 polyester non-woven breather fabricCapran 518, heat-stabilised nylon 6 tubular bagging lm, 50 micronsthick

RSC Adv., 2014, 4, 12526–12533 | 12527

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will act as a reference material in studying the reinforcementeffects of both phenolic and benzoxazine resins with NBR.

Fig. 3 Schematic representation of the co-curing process. (a) NBRcompound layer, (b) layering of NBR with releasing fabric and fabric/

2.3. Preparation of carbon–phenolic/carbon–benzoxazineprepregs

Carbon–phenolic prepregs were prepared by spreadingphenolic resin (55 g) uniformly on the carbon fabric manuallyby hand. Prior to the preparation of the carbon–benzoxazineprepregs, benzoxazine resin (100 g) was added to a vessel andMEK solvent (30 g) was added, and the mixture was heated at70 �C to obtain a clear solution of benzoxazine resin. From theprepared resin, 55 g was taken and was uniformly spread on thecarbon fabric. The prepared prepregs were dried at roomtemperature until they became tacky.

prepreg layers, (c) layering with metallic and coul plates, (d) final setupwith breather and vacuum bag, (e) final setup in oven, (f) separatedlayers including reinforced NBR, (g) reinforced NBR.

2.4. Preparation of NBR–Ph and NBR–Bz by a co-curingprocess

The above prepared prepregs (carbon–phenolic/carbon–ben-zoxazine) were used for the preparation of the NBR compositesNBR–Ph and NBR–Bz, respectively. They were prepared by asimilar procedure as used for the preparation of NBR–S, butinstead of carbon fabric, carbon–phenolic and carbon–benzox-azine prepregs were used for NBR–Ph and NBR–Bz respectively.The co-curing was carried out using a similar cure cycle (Fig. 2).

The sample codes and formulations of NBR and the NBRcomposites are given in Table 2. A schematic representation ofthe co-curing process is given in Fig. 3, and the ow chart isrepresented in Fig. 4.

2.5. Instrumentation

ATR-FTIR spectra were recorded using an Agilent 640 seriesFTIR equipped with the Ge-ATR accessory. All samples wereexamined with a spectral resolution of 4 cm�1 and scannedfrom 400 to 4000 cm�1 in the transmission mode.

The tensile strength, modulus and elongation at break werecarried out on dumbbell-shaped samples using a DAK SystemsINC, model no: 9052 Universal testing machine (UTM) operatedat room temperature with a gauge length of 25 mm and acrosshead speed of 500 mm min�1 as per the ASTM D 412. Thetensile values reported herein are the average of the results oftests run on at least six specimens. The tear strength (type C) ofthe samples was also determined with the DAK Systems INCUTM by using no-notched 90 degree angled tear test pieces asper ASTM D 624-48. Tensile set was determined as per ASTM D412. The hardness of the reference and reinforced rubber

Table 2 Sample codes and formulations of NBR–S and reinforcements

Sample code NBR (area, mm) Phenolic resina (content, %)

NBR–S 200 � 200 —NBR–Ph 200 � 200 40NBR–Bz 200 � 200 —

a Resin content is determined as per ASTM D 3171-09.

12528 | RSC Adv., 2014, 4, 12526–12533

samples was determined with a Wallace model 1 HT 16A digitalShore-A durometer as per ASTM D 2240.

Thermogravimetric analysis curves were recorded with a TAInstruments, model no. SDT 2960 instrument as per ASTM E2550. The TGAmeasurements were conducted with a heating rateof 10 �C min�1 under nitrogen gas ow between 30 and 600 �C.

Morphological characterization was performed using ScanningElectron Microscopy (SEM, JEOL JSM 5800 Digital). Fracturedsurfaces of the tensile test specimens were cut carefully withouttouching the surface, and the fracture topography was studied.

2.6. Swelling studies

Samples of 25 mm � 25 mm of weight approx. 1 g (w1) were cutfrom the central portion of the moulded sheet and allowed toswell in various solvents and turbine fuel (ATF) at ambientconditions. Aer 72 h, the swollen samples were removed,wiped dry and weighed again (w2). The degree of swelling iscalculated using the following equation:

Swelling (%) ¼ (W2 � W1)/W1 � 100.

2.7. Aging studies

The accelerated aging test was carried out at 100 � 2 �C for 72 hin a heat oven as per ASTM 537-67. The samples were allowed torest at room temperature for 24 hours before determining thepercentage retention in the mechanical properties.

Benzoxazine resina (content, %)Carbon fabric(area, mm) MEK (mL)

— 200 � 200 —— 200 � 200 —40 200 � 200 100

This journal is © The Royal Society of Chemistry 2014

Fig. 4 Flow chart of the co-curing process.

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3. Results and discussion

In this study, low molecular weight phenolic and benzoxazineresins were incorporated in the NBR compound. Reinforcementof the NBR matrix with these resins occurs during the co-curingprocess. The porous Teon-coated release fabric placed betweenthe resin prepregs and the NBR compound sheet not only allowsthe resins to ow from the prepregs to the NBR during heating,but also separates the reinforced NBR compound from theprepregs once the co-curing process is completed. The reinforcedNBR–Ph and NBR–Bz materials were inspected with reference toNBR–S in terms of their mechanical, thermal, swelling and agingproperties. The physico-mechanical and swelling properties ofNBR–S, NBR–Ph and NBR–Bz are depicted in Table 3.

Fig. 5 Infrared spectra of NBR–S and the reinforced NBR samples(NBR–Ph & NBR–Bz).

3.1. FT-IR

ATR-FTIR analysis was carried out before studying the bulkproperties of the reinforced composites in order to verify thepresence of interactions between the rubber matrix and thereinforced resins. The infrared spectra of NBR–S and the rein-forced NBR composites are shown in Fig. 5. The characteristicinfrared absorption peaks of NBR–S are found at 967 cm�1 and2230 cm�1 due to the trans double bond of butadiene and thenitrile group of NBR respectively. The absence of any change tothese absorption peaks in NBR–Ph and NBR–Bz compared toNBR–S indicates the absence of the in situ polymerisationreaction between the NBR matrix and reinforced resins during

Table 3 Physico-mechanical and swelling properties of NBR–S, NBR–P

Property NB

Tensile strength (MPa) 9.Modulus at 100% elongation (MPa) 2Modulus at 200% elongation (MPa) 4.Modulus at 300% elongation (MPa) 8.Elongation at break (%) 403Tear strength (kg cm�1) 44Hardness (Shore A) 74Tensile set (%) 10Cross linking density (mmol cm�3) (benzene) 0.

Volume swelling in % aer 72 ha. Toluene 1.4b. MEK 2.2c. Ethanol 0.0d. ATF 1.8Density, g cm�3 1.1

This journal is © The Royal Society of Chemistry 2014

curing. This further supports the formation of a localisedinterpenetrating network structure between the NBRmatrix andthe phenolic25 or benzoxazine resins. The presence of sharperand higher absorption peaks (increased concentration of peaks)in NBR–Ph and NBR–Bz than NBR–S in the region between 1800and 800 cm�1, and also the absence of negative peaks, showsthe enhancement of the resin–matrix interactions. The absenceof negative peaks further indicates that the reinforcement didnot cause any chain scission in both the matrix and the resin.The schematic representation of NBR (without an inter-penetrating network, IPN) and the interpenetrating network ofNBR with benzoxazine (NBR–Bz) and phenolic resin (NBR–Ph)are shown in Scheme 1–3 respectively.

3.2. Mechanical properties

The phenolic and benzoxazine resins were transferred andabsorbed with NBR, resulting in cross linking in the presence of

h and NBR–Bz

R–S NBR–BZ NBR–Ph

68 21.14 17.72.3 2.58 2.2546 4.65 3.2808 8.41 5.31.47 573.63 668.38.51 46.15 56.84.5 74.5 72.8 13.2 16.640 0.50 0.41

5 1.16 1.132 2.07 1.8643 0.029 0.0232 1.61 1.6097 1.191 1.191

RSC Adv., 2014, 4, 12526–12533 | 12529

Scheme 1 NBR–S.

Scheme 2 NBR–BZ.

Scheme 3 NBR–Ph.

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heating during the curing process. Aer reinforcement,enhancement of the tensile strength, tear strength, tensile setand elongation at break were observed. The mechanical

12530 | RSC Adv., 2014, 4, 12526–12533

properties of NBR–S, NBR–Ph and NBR–Bz are shown in Fig. 6,and the stress–strain curves are shown in Fig. 7. Fig. 6a displaysthe tensile strength, elongation at break and tear strength ofNBR–Ph and NBR–Bz when compared with NBR–S, and Fig. 6bcompares the modulus and hardness. Due to reinforcementwith phenolic resin, the tensile strength, elongation at break,tear strength and tensile set of NBR–Ph increased by about 91,83, 64 and 77% respectively in comparison with NBR–S, whereasthe modulus (100%) remains almost the same. Similarly, thereinforcement with benzoxazine resin increases the tensilestrength, elongation at break, tear strength, tensile set andmodulus (100%) of NBR–Bz to about 109, 71, 52, 63 and 56%respectively in comparison with NBR–S. The Shore A hardnessof NBR–Ph decreased slightly, but no change was observed forNBR–Bz when compared to NBR–S.

It was observed that the resin-reinforced composites showeda remarkable enhancement in the ultimate tensile strength.Resins being polar and chemically reactive causes good wettingwith improved rubber–resin interactions. The rise in tensilestrength might also be related to the ability of these resins tocontribute to the stress transfer process of the reinforcementphase due to sufficient interaction with NBR, and also becauseof the improved compatibility. Hence, the applied load duringthe tensile testing of NBR was therefore partially shared by theresin matrix, which shows better strength properties.28

3.3. SEM analysis

The exceptional reinforcement behaviour of the phenolic andbenzoxazine resins on NBR was further studied by microstruc-ture analysis. Fig. 8a shows a SEM image of the tensile fracturedsurface of NBR–S which shows that the vulcanised NBR withoutresin modication displayed a smooth fracture surface withoutpores. Well dispersed ne silica particles could be easily

This journal is © The Royal Society of Chemistry 2014

Fig. 6 (a) Tensile, elongation and tear of NBR–S, NBR–Ph and NBR–Bz; (b) 100%, 200%, 300% modulus and hardness of NBR–S, NBR–Phand NBR–Bz.

Fig. 7 Stress–strain curves of NBR–S, NBR–Ph and NBR–Bz.

Fig. 8 SEM images of (a) NBR–S, (b) NBR–Bz and (c) NBR–Ph.

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identied in the NBR matrix, with many bright spots indicatingthe presence of rubber chemicals such as zinc oxide, stearic acidetc. Fig. 8b and c represent SEM images of NBR–Ph and NBR–Bzrespectively. The phenolic25 and benzoxazine resins formed alocalised interpenetrating network structure within the NBR.Also, these resins have higher strength than that of rubber,therefore the long discontinuous bres formed within therubber matrix could behave as strengthening materials, and the

This journal is © The Royal Society of Chemistry 2014

mechanical properties of NBR were signicantly enhanced, asshown in Table 3. The brittleness of the NBR composites andfailures in separate planes are clearly visible (Fig. 8). Thisjusties the formation of a separate resin phase which is closelyadhered to the rubber matrix and takes part in sharing theapplied stress.15

3.4. Thermogravimetry

The results of thermogravimetric analysis of the NBR–S andNBR composites are shown in Fig. 9. The nature of the curve issimilar for all of the components. The major weight lossobserved at the temperature range of 370 to 510 �C is due to thedegradation of the NBR component. The next weight loss at thetemperature range of 520 to 585 �C is due to the decompositionof carbonaceous residue.29 The onset temperature of majordegradation starts at a higher temperature for NBR–Ph andNBR–Bz in comparison to NBR–S. The values of onset temper-ature and thermal stability (Tmax) obtained from the DTG curvesare shown in Table 4. It was observed that the Tmax of NBR–Phand NBR–Bz in the major degradation step occurs at an elevatedtemperature when compared with NBR–S, which indicates thegreater thermal stability of NBR composites over that of NBR–S.The value of Tmax is slightly higher for NBR–Bz than NBR–Ph.

3.5. Aging resistance

The thermal aging resistance of NBR is limited because of thepresence of an unsaturated backbone. The effect of thermalaging was investigated at 100 �C for 72 h for NBR–S and the NBRcomposites. The thermal aging of NBR will be faster at thebeginning of the aging due to the higher level of oxygen uptakefor oxidation followed by cross-linking.30–32 The oxidationresults in partial degradation of the polymer chain, which leadsto a reduction of the tensile strength. Fig. 10 compares the

RSC Adv., 2014, 4, 12526–12533 | 12531

Fig. 9 DTG curves of NBR–S, NBR–Ph and NBR–Bz.

Table 4 Thermogravimetric analysis of the rubber composites

Sample Onset temperature (�C) Tmax (�C)

NBR–S 379 439NBR–Ph 390 466NBR–Bz 424 469

Fig. 10 Comparison of the mechanical properties of unaged and agedsamples of NBR–S, NBR–Ph and NBR–Bz (UA: unaged, A: aged).

Table 5 Percentage of retention of the mechanical properties afteraging at 100 �C for 72 h

Property NBR–S (%) NBR–Bz (%) NBR–Ph (%)

Tensile strength 74.38 88.12 85.55Elongation at break 77.28 80.56 83.28Modulus at 100% 84.34 86.43 94.54

Fig. 11 Swelling ratios of NBR–S, NBR–Bz and NBR–Ph in solventsand ATF.

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percentage retention of the tensile strength, elongation at breakand modulus of the NBR–Ph and NBR–Bz materials with NBR–Swhich were aged at 100 �C for 72 h, and the retention values aregiven in Table 5. From the gure it is clearly evident that theretained tensile strength, elongation at break and modulus of

12532 | RSC Adv., 2014, 4, 12526–12533

NBR–Ph and NBR–Bz are superior when compared to NBR–S.The more than 85% retained tensile strength of NBR–Ph andNBR–Bz shows that the detrimental inuence of heat was morethan equalized by the protective inuence of the benzoxazineand phenolic resins. The cured resin layers may have acted asbarriers for the diffusion of degradation products into theinterior of the rubber matrix. However, the loss in the elonga-tion at break aer aging may be due to the increase in cross-linking density, which leads to restriction for chain extensionand decreases the chain length between cross-linking points.32

3.6. Swelling properties

The reinforcement effect of the phenolic and benzoxazineresins with the NBR matrix on the cross-link density is esti-mated from the swelling experiment data given in Table 3. Theswelling ratios of the NBR–S and NBR composites in solventssuch as MEK, toluene, ethanol and turbine fuel (ATF) are shownin Fig. 11. Due to the higher degree of cross linking, a decreasein the swelling values of the reinforced samples NBR–Ph andNBR–Bz was observed when compared with NBR–S. Thepercentage of solvent uptake was considerably decreased forNBR–Ph and NBR–Bz. The reinforcement of these phenolic andbenzoxazine resins in the NBR matrix restricts the extensibilityof the rubber chains induced by swelling, which makes itdifficult for these solvents and ATF to penetrate into the gapsbetween rubber molecules, and thus the swelling percentagewas decreased.33,34 Hence, the reinforced compounds NBR–Phand NBR–Bz possessed higher barrier properties compared toNBR–S. The volume swelling percentages for the phenolic andbenzoxazine resins were more or less the same.

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4. Conclusions

NBR was reinforced with phenolic and benzoxazine resins toform NBR–Ph and NBR–Bz composites by a co-curing process. Itwas found that both these resins existed in the form of alocalized interpenetrating network structure in the NBR matrix.For the NBR–Ph composite, the tensile strength, tear strength,tensile set and elongation at break were improved, whereas themodulus remained almost the same when compared with NBR–S. Similar results were observed for NBR–Bz, except for a slightimprovement in the modulus. In particular the tensile strengthof NBR–Ph and NBR–Bz has increased nearly twofold whencompared to NBR–S. The Shore A hardness of NBR–Ph is slightlyaffected, but no change was observed for NBR–Bz. The occur-rence of Tmax of NBR–Ph and NBR–Bz in the major degradationstep at elevated temperature indicates their greater thermalstability over NBR–S. The aging studies at 100 �C for 72 hshowed that the retained tensile strength, elongation at breakand modulus of NBR–Ph and NBR–Bz are superior to NBR–S.Further, due to the higher degree of cross linking, a decrease inthe swelling values is observed in the reinforced samples NBR–Ph and NBR–Bz when compared with NBR–S. The investigationhas clearly demonstrated for the rst time that benzoxazineresin, similar to phenolic resin, could be an effective additivefor strengthening various rubber materials. The study has alsodemonstrated a practical co-curing process to reinforce ther-mosetting resins with NBR which can be possibly extended toother resins and rubber matrices.

Acknowledgements

The authors wish to thank Director, DRDL and Director, ASL forsupporting this study.

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