Aromatic polyamide short fibres-reinforced elastomers: adhesion mechanisms and the composite's performance properties

The current thesis consists of a chapter as literature review in which different scientific sources and the results of investigation of various researchers are reviewed, followed by 5 chapters (2 to 6) and an appendix covering the results of the experimental work performed. The results can be divided in 3 main sections: Section 1: Short-fibre rubber composites. In the first section rubber composites reinforced with short aramid fibres are focused upon; the reinforcement mechanisms and the mechanical and viscoelastic properties of the composites are explored. • In the second chapter, short fibre reinforced NR and EPDM rubbers are the subjects of investigation. In the different short fibre-rubber systems examined, clear chemical adhesion only happened in the case of peroxide-cured EPDM with RFL-treated fibres. This phenomenon was most clearly reflected in the tensile curves for this system, showing a large reinforcement factor particularly at tensile elongations below 100%. Next to potential chemical bonding, mechanical interaction between fibres and rubber matrix plays an important role. Surface phenomena on the fibres, as bending/buckling, dog-bone shaped fibre ends and surface roughness due to the RFL-coating cause mechanical interlocking. So, even in the absence of chemical adhesion, adding short aramid fibres improves the mechanical properties of rubber compounds because of the mechanical interaction. • Chapter three describes the results of the investigation of the composite morphology and interaction of two different types of short aramid fibres in an EPDM radiator hose compound and its corresponding model system. PPTA fibres with RFL-dip could not be dispersed completely to filament level due to the gluing effect of the dip. Irrespective of the mixing time, PP/ODPTA fibres could better withstand the applied shear stresses during mixing and maintain a high level of fibre length, which resulted in better mechanical properties in composites reinforced with these fibres compared to those reinforced with PPTA fibres. On the other side, the RFL-dip acted as protective layer and prevented the fibres from excessive breaking, which was especially the case for PPTA. It was shown that for RFL-coated fibres, chemical bonding only happens between a part of RFL-treated filaments and EPDM rubber. Still the composites containing these fibres showed better mechanical properties compared to those reinforced with standard finish. The reason for this superiority is not only the partial chemical bonds, but two more factors are involved: better mechanical interlocking of the RFL-treated fibres and the surrounding matrix (due to the roughness of the coating), and the higher final length of these fibres after mixing. • Due to the importance of viscoelastic properties in performance of a lot of rubber parts, especially tires, the dynamic properties of the short fibre-reinforced compounds are the subject of the fourth chapter. It was shown that mechanical and chemical interactions between short fibres and a rubber matrix, which are the reasons for an increase in storage modulus, can affect loss properties in different ways, depending on the type of matrix, temperature, dynamic strain and the possible application of a static pre-strain. In this respect two main influencing factors needed to be considered: 1) Reinforcement as a result of interaction between fibres and rubber which results in an increase in storage modulus of the composite; 2) If there is no perfect interphase and fibres and rubbers are not fully bonded, sliding of the rubber matrix along the fibre surface causes additional losses due to friction. There is no general rule that adding fibre increases or decreases the loss angle and it totally depends on the balance of these two factors at different strain, frequency and temperature regimes. The importance of mechanical interaction in short fibre reinforcement was emphasized again to be an important parameter also in determining the viscoelastic properties of the composites. Section 2: RFL-rubber interaction. This section consists of two chapters dedicated to the subject of the adhesion mechanism between rubbers and the RFL-layer itself. The effect of RFL-aging, the role of rubber curing systems and inter-diffusion of the latex part of RFL and elastomer are focused upon. • In chapter five the effect of physical interaction between fibres and rubbers was shown to be minor. Un-aged RFL is able to generate good adhesion to both sulphur and peroxide cured rubbers due to the generation of chemical bonds. As a result of the aging of RFL, ozone is able to decrease the number of 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 was also discussed that, unlike in sulphur vulcanization in which bonding happens only between the latex in the RFL and rubber, peroxide is able to generate bonds also between elastomer and the resin structure of the RFL-coating. It was also shown that mechanical interlocking increases the total interaction between (roughened) fibres and rubbers when the adhesion tests are done in the shear mode, while this effect is not reflected in peeling tests. • In the sixth chapter the adhesion of aramid fibres after being coated with RFL, in compounds based on NR and in NR blended with a small amount of SBR is investigated. It was shown that though having very similar tensile properties, the latter compound has much better adhesion to RFL which is also less sensitive to RFL aging, compared to the pure NR compound. It is argued that an interphase region is formed between RFL and the elastomer, which is stronger for the compound containing SBR due to its enhanced compatibility with the latex part of the RFL. Even if the RFL-treated fibres are aged, better compatibility causes better mutual diffusion between the latex of the RFL and the elastomer chains and increases the chance of co-vulcanization between rubber and intact latex chains in greater depth from the initial contact zone. This can partly compensate the negative effect of RFL aging. Section 3: Enhancement of rubber-fibre interaction. The last section based on the results of the two previous sections, deals with two different approaches for enhancement of the interaction between the aramid fibres and rubber matrices. • In the appendix, two different methods were employed to enhance the interaction between fibres and rubbers based on the two mentioned mechanisms. Increasing the roughness of the surface of aramid fibres with a laser, and functionalizing the surface of the fibres with chloroformates and isocyanates. Laser irradiation is performed on uncoated fibres (standard fibres) and the SEM pictures showed that roughening was successfully achieved. For chemical modification, fibres with an epoxy sub-coating only, were chosen because of the existence of hydroxyl/amine groups on the surface which provide reactive sites for the reaction with the chemicals mentioned. The presence of bound reactants on the fibre surface was demonstrated using X-ray photoelectron spectroscopy (XPS). Furthermore, the tensile curves showed enhanced properties of the composites reinforced with these modified fibres.