A new approach with prepregs for reinforcing nitrile rubber with phenolic and benzoxazine resins

Abstract

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.

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1. Introduction

Acrylonitrile-co-butadiene rubber (NBR) is used in aircrafts, automobiles, tanks, oil drilling industries and other military applications as seals, gaskets, O-rings etc. due to its excellent oil and fuel resistances and low gas permeability. However, rubber compositions based on NBR have limited high temperature stability, chemical resistance and service life because of the presence of an unsaturated backbone. The incorporation of small particle size fillers to the elastomer matrix results in substantially improved mechanical properties.^1,2 Results of small and well-dispersed silica generated by an in situ sol–gel process demonstrated the improvement of the rubber–filler interactions and better filler dispersion^3,4 using a silane coupling agent.^5–7 The modification of elastomers with carbon nanofibres or nanotubes^8–11 of high aspect ratio and low density has also been studied.

Recently, polymer reinforced elastomeric composites have attracted significant attention due to their outstanding mechanical properties when compared to conventional elastomers. The polymeric resins are recognized as potential materials for improving the physical–mechanical properties of elastomers with retention of the aging properties. Studies on natural fillers such as soy protein aggregates and modified starch for reinforcement with styrene butadiene rubber (SBR)^12–14 and lignin with NBR showed good compatibility with the rubber matrix, and their reinforcement effects were superior to those of carbon black.¹⁵ The reinforcement of synthetic polymers such as cardinol–formaldehyde, ultra high molecular weight polyethylene (UHMWPE), polyaniline, resorcinol–formaldehyde and epoxy resins in various rubbers has been studied to improve the mechanical properties.^16–22 The reinforcement of phenolic resins with chloroprene, ethylene propylene diene monomer (EPDM) and NBR, and epoxy resin with SBR through in situ polymerisation has also been discussed.^23–25 The resin reinforcements for NBR are prepared by most common methods such as in situ polymerization, solution, melt mixing, blending, coating and latex blending. However, detailed studies are required due to difficulties arising in the uniform dispersion of 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 being developed as an alternative to traditional high-performance thermosetting resins for aerospace applications.²⁶ Benzoxazines have the design flexibility for various applications, and they can be tailored to display a range of properties which cover advanced epoxy and phenolic resins to bismaleimides and polyimides. Their distinct advantages are fire resistance and superior processability compared with the majority of thermosetting resins known today. In addition, these materials have high glass transition temperatures (T_g), high moduli, low water absorption, good electrical properties and no shrinkage upon curing.²⁷ Even though the applications of benzoxazines are increasing rapidly, to the best of our knowledge the reinforcement of benzoxazine resins with nitrile rubbers has not been attempted so far.

In this work, we have explored a novel method for the preparation of NBR/resin composites for the first time by using a co-curing process at high temperature (i.e. 200 °C) from resin prepregs. The co-curing process has many advantageous due to its simplicity, lower capital investment and eco-friendly nature. This method is faster, more versatile and produces more uniform cross linking when compared with the conventional techniques. The mechanism of reinforcement of the resin to the nitrile rubber by this method may proceed in two stages. In the first stage during co-curing, the resin present in the prepreg flows in to the rubber compound layer through porous releasing fabric, and in the second stage this resin forms the reinforcement with the nitrile rubber compound at high temperature. This study attempts the reinforcement of phenolic and benzoxazine resins with nitrile rubber for the first time using a co-curing process. The relative effect of these resin reinforcements to NBR in comparison to the non-reinforced NBR compound has been also investigated under the same conditions.

2. Experimental

2.1. Raw materials

The thermosetting resins used for the reinforcement with NBR are phenolic (resol type, solid content: 61.69%, PR-100) and bisphenol-F-benzoxazine resins (mp: 65 °C–85 °C; Araldite MT 35700) provided by ABR Organics, India and Huntsman Ltd, Switzerland respectively. The carbon (rayon-based) and release (D200 TFP) fabrics were obtained from Aerospace Ltd., Coimbatore, 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 film (50 micron thick) were obtained from Aerovac, UK. Methyl ethyl ketone (MEK) solvent was procured from Fluka, Switzerland. The characteristics of the nitrile rubber compound sheet, carbon fabric, release and breather fabrics, vacuum bagging film are given in Table 1. The chemical structures of NBR and the thermosetting resins are represented in Fig. 1.

Table 1 Details of raw materials used

Materials	Characteristics
Nitrile rubber compound sheet (rocasine)	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: 1.5
Carbon fabric (rayon-based carbon fabric)	Carbon content: 94%, sp. gravity: 1.8 ± 0.1, thickness: 0.3–0.4 mm
Release fabric (D200 TFP)	Porous Teflon coated plain wave thickness: 0.003 inches
Breather fabric AB 1060V	Heavy weight synthetic 330 g m^−2 polyester non-woven breather fabric
Vacuum bagging film 2.0 M, 518280	Capran 518, heat-stabilised nylon 6 tubular bagging film, 50 microns thick

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

2.2. Preparation of NBR–S

Two plies of release fabric (200 × 200 mm each) are placed above and below the 200 × 200 mm sized 2 mm thick NBR rubber compound sheet. Four carbon fabric layers of 200 × 200 mm were plied over the release fabric on both sides of the layers. The complete setup of plies was placed on a metallic mould, and a metallic caul plate was placed over the layers. The setup was enclosed in breather cloth and a vacuum bag, and curing was carried out in an oven as per the cure cycle represented in Fig. 2 to obtain non-reinforced NBR–S. This material will act as a reference material in studying the reinforcement effects of both phenolic and benzoxazine resins with NBR.

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

2.3. Preparation of carbon–phenolic/carbon–benzoxazine prepregs

Carbon–phenolic prepregs were prepared by spreading phenolic resin (55 g) uniformly on the carbon fabric manually by hand. Prior to the preparation of the carbon–benzoxazine prepregs, benzoxazine resin (100 g) was added to a vessel and MEK solvent (30 g) was added, and the mixture was heated at 70 °C to obtain a clear solution of benzoxazine resin. From the prepared resin, 55 g was taken and was uniformly spread on the carbon fabric. The prepared prepregs were dried at room temperature until they became tacky.

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

The above prepared prepregs (carbon–phenolic/carbon–benzoxazine) were used for the preparation of the NBR composites NBR–Ph and NBR–Bz, respectively. They were prepared by a similar procedure as used for the preparation of NBR–S, but instead of carbon fabric, carbon–phenolic and carbon–benzoxazine 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 NBR composites are given in Table 2. A schematic representation of the co-curing process is given in Fig. 3, and the flow chart is represented in Fig. 4.

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

Sample code	NBR (area, mm)	Phenolic resin^a (content, %)	Benzoxazine resin^a (content, %)	Carbon fabric (area, mm)	MEK (mL)
a Resin content is determined as per ASTM D 3171-09.
NBR–S	200 × 200	—	—	200 × 200	—
NBR–Ph	200 × 200	40	—	200 × 200	—
NBR–Bz	200 × 200	—	40	200 × 200	100

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Fig. 3 Schematic representation of the co-curing process. (a) NBR compound layer, (b) layering of NBR with releasing fabric and fabric/prepreg layers, (c) layering with metallic and coul plates, (d) final setup with breather and vacuum bag, (e) final setup in oven, (f) separated layers including reinforced NBR, (g) reinforced NBR.

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

2.5. Instrumentation

ATR-FTIR spectra were recorded using an Agilent 640 series FTIR equipped with the Ge-ATR accessory. All samples were examined with a spectral resolution of 4 cm⁻¹ and scanned from 400 to 4000 cm⁻¹ in the transmission mode.

The tensile strength, modulus and elongation at break were carried out on dumbbell-shaped samples using a DAK Systems INC, model no: 9052 Universal testing machine (UTM) operated at room temperature with a gauge length of 25 mm and a crosshead speed of 500 mm min⁻¹ as per the ASTM D 412. The tensile values reported herein are the average of the results of tests run on at least six specimens. The tear strength (type C) of the samples was also determined with the DAK Systems INC UTM by using no-notched 90 degree angled tear test pieces as per ASTM D 624-48. Tensile set was determined as per ASTM D 412. The hardness of the reference and reinforced rubber samples was determined with a Wallace model 1 HT 16A digital Shore-A durometer as per ASTM D 2240.

Thermogravimetric analysis curves were recorded with a TA Instruments, model no. SDT 2960 instrument as per ASTM E 2550. The TGA measurements were conducted with a heating rate of 10 °C min⁻¹ under nitrogen gas flow between 30 and 600 °C.

Morphological characterization was performed using Scanning Electron Microscopy (SEM, JEOL JSM 5800 Digital). Fractured surfaces of the tensile test specimens were cut carefully without touching 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 cut from the central portion of the moulded sheet and allowed to swell in various solvents and turbine fuel (ATF) at ambient conditions. After 72 h, the swollen samples were removed, wiped dry and weighed again (w₂). The degree of swelling is calculated using the following equation:

Swelling (%) = (W₂ − W₁)/W₁ × 100.

2.7. Aging studies

The accelerated aging test was carried out at 100 ± 2 °C for 72 h in a heat oven as per ASTM 537-67. The samples were allowed to rest at room temperature for 24 hours before determining the percentage retention in the mechanical properties.

3. Results and discussion

In this study, low molecular weight phenolic and benzoxazine resins were incorporated in the NBR compound. Reinforcement of the NBR matrix with these resins occurs during the co-curing process. The porous Teflon-coated release fabric placed between the resin prepregs and the NBR compound sheet not only allows the resins to flow from the prepregs to the NBR during heating, but also separates the reinforced NBR compound from the prepregs once the co-curing process is completed. The reinforced NBR–Ph and NBR–Bz materials were inspected with reference to NBR–S in terms of their mechanical, thermal, swelling and aging properties. The physico-mechanical and swelling properties of NBR–S, NBR–Ph and NBR–Bz are depicted in Table 3.

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

Property	NBR–S	NBR–BZ	NBR–Ph
Tensile strength (MPa)	9.68	21.14	17.72
Modulus at 100% elongation (MPa)	2.3	2.58	2.25
Modulus at 200% elongation (MPa)	4.46	4.65	3.28
Modulus at 300% elongation (MPa)	8.08	8.41	5.31
Elongation at break (%)	403.47	573.63	668.38
Tear strength (kg cm^−1)	44.51	46.15	56.84
Hardness (Shore A)	74.5	74.5	72
Tensile set (%)	10.8	13.2	16.6
Cross linking density (mmol cm^−3) (benzene)	0.40	0.50	0.41
Volume swelling in % after 72 h
a. Toluene	1.45	1.16	1.13
b. MEK	2.22	2.07	1.86
c. Ethanol	0.043	0.029	0.023
d. ATF	1.82	1.61	1.60
Density, g cm^−3	1.197	1.191	1.191

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3.1. FT-IR

ATR-FTIR analysis was carried out before studying the bulk properties of the reinforced composites in order to verify the presence of interactions between the rubber matrix and the reinforced resins. The infrared spectra of NBR–S and the reinforced NBR composites are shown in Fig. 5. The characteristic infrared absorption peaks of NBR–S are found at 967 cm⁻¹ and 2230 cm⁻¹ due to the trans double bond of butadiene and the nitrile group of NBR respectively. The absence of any change to these absorption peaks in NBR–Ph and NBR–Bz compared to NBR–S indicates the absence of the in situ polymerisation reaction between the NBR matrix and reinforced resins during curing. This further supports the formation of a localised interpenetrating network structure between the NBR matrix and the phenolic²⁵ or benzoxazine resins. The presence of sharper and higher absorption peaks (increased concentration of peaks) in NBR–Ph and NBR–Bz than NBR–S in the region between 1800 and 800 cm⁻¹, and also the absence of negative peaks, shows the enhancement of the resin–matrix interactions. The absence of negative peaks further indicates that the reinforcement did not cause any chain scission in both the matrix and the resin. The schematic representation of NBR (without an interpenetrating network, IPN) and the interpenetrating network of NBR with benzoxazine (NBR–Bz) and phenolic resin (NBR–Ph) are shown in Scheme 1–3 respectively.

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

Scheme 1 NBR–S.

Scheme 2 NBR–BZ.

Scheme 3 NBR–Ph.

3.2. Mechanical properties

The phenolic and benzoxazine resins were transferred and absorbed with NBR, resulting in cross linking in the presence of heating during the curing process. After reinforcement, enhancement of the tensile strength, tear strength, tensile set and elongation at break were observed. The mechanical 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 displays the tensile strength, elongation at break and tear strength of NBR–Ph and NBR–Bz when compared with NBR–S, and Fig. 6b compares the modulus and hardness. Due to reinforcement with 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, whereas the modulus (100%) remains almost the same. Similarly, the reinforcement with benzoxazine resin increases the tensile strength, elongation at break, tear strength, tensile set and modulus (100%) of NBR–Bz to about 109, 71, 52, 63 and 56% respectively in comparison with NBR–S. The Shore A hardness of NBR–Ph decreased slightly, but no change was observed for NBR–Bz when compared to NBR–S.

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–Ph and NBR–Bz.

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

It was observed that the resin-reinforced composites showed a remarkable enhancement in the ultimate tensile strength. Resins being polar and chemically reactive causes good wetting with improved rubber–resin interactions. The rise in tensile strength might also be related to the ability of these resins to contribute to the stress transfer process of the reinforcement phase due to sufficient interaction with NBR, and also because of the improved compatibility. Hence, the applied load during the tensile testing of NBR was therefore partially shared by the resin matrix, which shows better strength properties.²⁸

3.3. SEM analysis

The exceptional reinforcement behaviour of the phenolic and benzoxazine resins on NBR was further studied by microstructure analysis. Fig. 8a shows a SEM image of the tensile fractured surface of NBR–S which shows that the vulcanised NBR without resin modification displayed a smooth fracture surface without pores. Well dispersed fine silica particles could be easily identified in the NBR matrix, with many bright spots indicating the presence of rubber chemicals such as zinc oxide, stearic acid etc. Fig. 8b and c represent SEM images of NBR–Ph and NBR–Bz respectively. The phenolic²⁵ and benzoxazine resins formed a localised interpenetrating network structure within the NBR. Also, these resins have higher strength than that of rubber, therefore the long discontinuous fibres formed within the rubber matrix could behave as strengthening materials, and the mechanical properties of NBR were significantly enhanced, as shown in Table 3. The brittleness of the NBR composites and failures in separate planes are clearly visible (Fig. 8). This justifies the formation of a separate resin phase which is closely adhered to the rubber matrix and takes part in sharing the applied stress.¹⁵

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

3.4. Thermogravimetry

The results of thermogravimetric analysis of the NBR–S and NBR composites are shown in Fig. 9. The nature of the curve is similar for all of the components. The major weight loss observed at the temperature range of 370 to 510 °C is due to the degradation of the NBR component. The next weight loss at the temperature range of 520 to 585 °C is due to the decomposition of carbonaceous residue.²⁹ The onset temperature of major degradation starts at a higher temperature for NBR–Ph and NBR–Bz in comparison to NBR–S. The values of onset temperature and thermal stability (T_max) obtained from the DTG curves are shown in Table 4. It was observed that the T_max of NBR–Ph and NBR–Bz in the major degradation step occurs at an elevated temperature when compared with NBR–S, which indicates the greater thermal stability of NBR composites over that of NBR–S. The value of T_max is slightly higher for NBR–Bz than NBR–Ph.

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	439
NBR–Ph	390	466
NBR–Bz	424	469

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3.5. Aging resistance

The thermal aging resistance of NBR is limited because of the presence of an unsaturated backbone. The effect of thermal aging was investigated at 100 °C for 72 h for NBR–S and the NBR composites. The thermal aging of NBR will be faster at the beginning of the aging due to the higher level of oxygen uptake for oxidation followed by cross-linking.^30–32 The oxidation results in partial degradation of the polymer chain, which leads to a reduction of the tensile strength. Fig. 10 compares the percentage retention of the tensile strength, elongation at break and modulus of the NBR–Ph and NBR–Bz materials with NBR–S which were aged at 100 °C for 72 h, and the retention values are given in Table 5. From the figure it is clearly evident that the retained tensile strength, elongation at break and modulus of NBR–Ph and NBR–Bz are superior when compared to NBR–S. The more than 85% retained tensile strength of NBR–Ph and NBR–Bz shows that the detrimental influence of heat was more than equalized by the protective influence of the benzoxazine and phenolic resins. The cured resin layers may have acted as barriers for the diffusion of degradation products into the interior of the rubber matrix. However, the loss in the elongation at break after aging may be due to the increase in cross-linking density, which leads to restriction for chain extension and decreases the chain length between cross-linking points.³²

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

Table 5 Percentage of retention of the mechanical properties after aging at 100 °C for 72 h

Property	NBR–S (%)	NBR–Bz (%)	NBR–Ph (%)
Tensile strength	74.38	88.12	85.55
Elongation at break	77.28	80.56	83.28
Modulus at 100%	84.34	86.43	94.54

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3.6. Swelling properties

The reinforcement effect of the phenolic and benzoxazine resins with the NBR matrix on the cross-link density is estimated from the swelling experiment data given in Table 3. The swelling ratios of the NBR–S and NBR composites in solvents such as MEK, toluene, ethanol and turbine fuel (ATF) are shown in Fig. 11. Due to the higher degree of cross linking, a decrease in the swelling values of the reinforced samples NBR–Ph and NBR–Bz was observed when compared with NBR–S. The percentage of solvent uptake was considerably decreased for NBR–Ph and NBR–Bz. The reinforcement of these phenolic and benzoxazine resins in the NBR matrix restricts the extensibility of the rubber chains induced by swelling, which makes it difficult for these solvents and ATF to penetrate into the gaps between rubber molecules, and thus the swelling percentage was decreased.^33,34 Hence, the reinforced compounds NBR–Ph and NBR–Bz possessed higher barrier properties compared to NBR–S. The volume swelling percentages for the phenolic and benzoxazine resins were more or less the same.

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

4. Conclusions

NBR was reinforced with phenolic and benzoxazine resins to form NBR–Ph and NBR–Bz composites by a co-curing process. It was found that both these resins existed in the form of a localized 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 the modulus remained almost the same when compared with NBR–S. Similar results were observed for NBR–Bz, except for a slight improvement in the modulus. In particular the tensile strength of NBR–Ph and NBR–Bz has increased nearly twofold when compared to NBR–S. The Shore A hardness of NBR–Ph is slightly affected, but no change was observed for NBR–Bz. The occurrence of T_max of NBR–Ph and NBR–Bz in the major degradation step at elevated temperature indicates their greater thermal stability over NBR–S. The aging studies at 100 °C for 72 h showed that the retained tensile strength, elongation at break and modulus of NBR–Ph and NBR–Bz are superior to NBR–S. Further, due to the higher degree of cross linking, a decrease in the swelling values is observed in the reinforced samples NBR–Ph and NBR–Bz when compared with NBR–S. The investigation has clearly demonstrated for the first time that benzoxazine resin, similar to phenolic resin, could be an effective additive for strengthening various rubber materials. The study has also demonstrated a practical co-curing process to reinforce thermosetting resins with NBR which can be possibly extended to other resins and rubber matrices.

Acknowledgements

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

Notes and references

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