Adhesion of RFL-coated aramid fibres to sulphur- and peroxide-cured elastomers

The performance of fibre-reinforced composites is strongly dependent on the nature and the strength of the fibre–matrix interface. Good interfacial bonding is required to ensure load transfer from matrix to reinforcing fibres. For rubber-reinforced composites, resorcinol formaldehyde latex (RFL) is known as a fibre surface coating which is able to provide good adhesion between rubber and fibres. But the performance of this substance in many cases can be largely affected due to exposure of the coated fibres to air and light. Moreover, most data available in the literature concern sulphur-cured elastomers only. In the present study, aramid fibres are investigated, because of their significantly higher modulus and strength compared to other commercial fibres. The adhesion of these fibres in compounds based on sulphur-cured natural rubber and peroxide-cured ethylene propylene diene rubber is investigated after being coated with RFL which is the most common adhesive coating for various sort of fibres, including aramid. The effect of physical interaction between fibres and rubbers is shown to be minor, and the effect of ageing of RFL on its ability to bond with rubbers using peroxide and sulphur curing systems are shown. As a result of ageing, ozone is able to decrease the double bonds of the latex part of the RFL, which negatively affects the RFL–rubber adhesion in sulphur-cured systems, while it has almost no effect in peroxide-cured systems. It is also discussed that, unlike in sulphur vulcanization in which bonding happens just between the latex in the RFL and rubber, peroxide is able to generate bonds between elastomer and the resin structure of the RFL-coatin

Rubber vulcanizates degradation and stabilization

Degradation of rubber vulcanizates in the presence and absence of air as well as in presence of ozone is reviewed in this paper. The paper also outlines the means to overcome this undesirable phenomenon. Under anaerobic aging conditions, which is termed as reversion, the vulcanizates are exposed to elevated temperature in the absence of oxygen. The consequence of this process is reflected in a decline in physical properties and performance characteristics. These changes are directly related to modifications of the original crosslink structure. Decomposition reactions tend to predominate and thus leading to a reduction in crosslink density and physical properties as observed during extended cure or when using higher curing temperatures. The decrease in network density is common when vulcanizates are subject to an anaerobic aging process. However, in the presence of oxygen, the network density is increased with the main chain modifications playing a vital role. Over the years the rubber industry has developed several compounding approaches to address the changes in crosslink structure during thermal aging. This paper gives a review of these compounding approaches. As with many formulation changes in rubber compounding, there is a compromise that must be made when attempting to improve one performance characteristic. For example, improving the thermal stability of vulcanized natural rubber compounds by reducing the sulfur content of the crosslink through the use of the more efficient vulcanization systems will reduce dynamic performance properties such as fatigue resistance. The challenge is to define a way to improve thermal stability while maintaining dynamic performance characteristics. In the second part, the protection against aerobic ageing as well as in ozone environment is reviewed. The anti-degradant effects are summarized and means to counteract are outlined. The most commonly used antidegradants are N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD) and N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD). Although conventional antidegradants such as IPPD and 6PPD are still the most widely used antidegradants in rubber, there is a trend and demand for longer-lasting and non-staining products. The relatively low molecular weight (MW) antioxidants have undergone an evolutionary change towards higher molecular weight products with the objective to achieve permanence in the rubber polymer, without loss of antioxidant activity. In the last two decades, several approaches have been evaluated in order to achieve this objective: attachment of hydrocarbon chains to conventional antioxidants in order to increase the MW and compatibility with the rubber matrix; oligomeric or polymeric antioxidants; and polymer bound or covulcanizable antioxidants. The disadvantage of polymer bound antioxidants was tackled by grafting antioxidants onto low MW polysiloxanes, which are compatible with many polymers. New developments on antiozonants have focused on non-staining and slow migrating products, which last longer in rubber compounds. Several new types of non-staining antiozonants have been developed, but none of them appeared to be as efficient as the chemically substituted p-phenylenediamines. The most prevalent approach to achieve non-staining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The disadvantage of this approach however, is the complicated mixing procedure needed to ensure that the required small polymer domain size is obtained

Maleic-anhydride grafted EPM as compatibilising agent in NR/BR/EPDM blends

Incorporation of approximately 30 phr Ethylene–Propylene–Diene rubber (EPDM) into natural rubber (NR)/butadiene rubber (BR) is a means to achieve non-staining ozone resistance for tire sidewall applications. However, due to incompatibility of the elastomers and heterogeneous filler distribution in each of the rubber phases, the mechanical properties deteriorate. In the present work, maleic-anhydride modified EPM (MAH-EPM) is added as a compatibilising agent between NR/BR and EPDM. The addition of 5 phr of MAH-EPM results in significantly improved tensile and tear strength when compared to a straight NR/BR/EPDM blend. These improvements can mainly be attributed to a compatibilising effect of MAH-EPM, resulting in a more homogeneous phase distribution, but in particular a much better homogeneous carbon black distribution over the different rubber phases. In addition, ionic crosslinks are introduced into the blends by interaction of MAH-EPM with zinc oxide

Chemistry of various accelerators in an e-SBR model compound

Sulfur vulcanization was carried out with 5-phenyl hex-2-ene serving as a model of e-SBR. Various accelerators have been used to study and compare the reactivity in a system containing sulfur and activators. Both HPLC and GC-MS analytical tools were used to identify the reaction products. It has been observed that the vulcanization in the presence of N-cyclohexyl-2-benzothiazole sulfenamide (CBS) generates a large amount of 2-marcaptobenzothiazole (MBT), which continuously increases and finally decreases suggesting further participation in vulcanization generating new crosslinks. The sulfenamide, N-cyclohexyl-4,6 dimethyl-2-pyrimidine sulfenamide (CDMPS) behaves different. Although it generates considerable amount of corresponding thiol, (4,6-dimethyl pyrimidine-2-thiol, DMMP) at the beginning of the reaction, no decrease has been observed during the course of further reaction suggesting that the accelerator, DMMP, somehow remains deactivated and therefore no changes in network is feasible. Identical differences exist between bis(2,2′benzothiazyl) disulfide (MBTS) and corresponding bis (4-methyl-2,2′benzothiazyl)disulfide (M-MBTS) in the reaction kinetics

Solubility study of curatives in various rubbers

The previous works on solubility of curatives in rubbers were mainly carried out in natural rubber. Not too much information available on dissimilar rubbers and this is important because most of the compounds today are blends of dissimilar rubbers. Although solubility can be expected to certain level by the previous studies, the current work provides a much precise view in the solubility behavior of curatives. Solubility of sulphur and several accelerators N-cyclohexylbenzothiazole-2-sulphenamide (CBS), N-dicyclohexylbenzothiazole-2-sulphenamide (DCBS), and 2-mercaptobenzothiazole (MBT) is measured in dicumyl peroxide vulcanised Styrene-Butadiene rubber (SBR), Acrylonitrile-Butadiene rubber (NBR) and Ethylene-Propylene-Diene rubber (EPDM) rubber at room temperature and at 60 °C. The experimental results can be correlated with the calculated solubility parameters δ, as determined using the method of Hoftijzer and Van Krevelen. Results of are used to judge the solubility of curatives in a specific rubber in blends

Influence of fiber type and coating on the composite properties of EPDM compounds reinforced with short aramid fibers

There is a renewed interest in the application of short aramid fibers in elastomers because of the considerable improvement in mechanical and dynamic properties of the corresponding rubber composites. Possible applications of short aramid fiber–reinforced elastomers are tires, dynamically loaded rubber seals, diaphragms, engine mounts, transmission belts, conveyer belts, and hoses. Our studies are related to the investigation of dispersion, length distribution, and the fiber–matrix interaction of two types of short aramid fibers, standard coated and resorcinol formaldehyde latex (RFL) coated, in ethylene–propylene–diene rubber (EPDM). Because the detection of the polymer fiber morphology in rubber compounds is hampered in the presence of carbon black, which is typically used in industrial elastomer compounds, fiber length, fiber length distribution, and dispersion are investigated in corresponding carbon black–free model compounds. Optical methods, scanning electron microscopy, and tensile testing are employed to explore the short aramid fiber–reinforced elastomer composites. The effects of morphology and fiber–matrix interaction on the mechanical properties of composites are discussed. Regarding fiber type, it is shown that co-poly-(paraphenylene/3,4′-oxydiphenylene terephthalamide) (PP/ODPTA) fibers end up with a higher final length than does poly(para-phenylene terephtalamide) (PPTA), which results in considerably higher mechanical properties of corresponding rubber compounds. For each fiber type, the higher final length as a result of RFL coating and the interaction with the rubber matrix are the key factors that overcome even the negative effect of poorer dispersion of RFL-coated fibers. The differences between the short aramid fibers and aramid cords regarding the RFL coating are also discussed