Solution blends comprised of alkyd having high carboxylic acid (ACOOH) content and epoxidized natural rubber (ENR 50, having nominally 50% of the isoprene units converted to epoxide groups) are self-crosslinkable at 30 C in toluene solution. Two modified palm stearin alkyds with different amounts of ACOOH groups were synthesized by reacting phthalic anhydride with alkyd containing excess hydroxyl (AOH) groups under controlled conditions, leading to the formation of different amounts of ACOOH as side-chains. The crosslinking reactions between the alkyds and ENR 50 in toluene solutions were investigated using FTIR and DSC. The compatibility between ENR 50 and alkyd was indicated by the presence of a single glass transition temperature (T g ) in the blend. Intercomponent crosslinking reactions have occurred between the epoxide groups of ENR 50 and ACOOH groups of alkyd with the formation of ester linkages. The crosslink density is proportional to the amount of ACOOH groups in the alkyd. As the ACOOH content is increased, the blend produces higher percentage of gel insoluble in toluene.
In this work, the potential of palm stearin alkyd as a cross-linking agent for rubber compounding was investigated. Alkyd carrying pendent -COOH groups was blended with epoxidised natural rubber (ENR50) via solvent casting technique at ambient temperature. Spectroscopic characterisations show that ENR50 and alkyd have interacted with each other via chemical reaction involving the epoxide of the rubber and -COOH of alkyd. Consequently, notable increase in the glass transition temperature (T g ) and gel content of the blend was observed. In addition to the epoxide ring-opening reaction, the extent of cross-linking in the blend was further increased by means of ultraviolet (UV) curing. As a result, the UV-cured blends experienced lower percentage of swelling, produced higher percentage of gel, have higher cross-link density and higher T g . The cross-link density in the rubber/alkyd blend has increased from 3.0 Â 10 À6 mol cm À3 to 2.0 Â 10 À4 mol cm À3 after UV curing. This is clearly evidenced from the swelling test results, where ENR and alkyd are completely soluble in the solvent, but upon blending and subsequent UV irradiation, the blend produced >90% of gel and swelled by 318%. Findings from this work show that sustainable material such as vegetable oil could be utilised in development of cross-linking agent for rubber. Significant increase in the cross-link density of the UVcured blend suggests that environment friendly compound such as alkyd has great potential to serve as an alternative to conventional compounds in rubber vulcanisation.
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