Modification of natural rubber by graft copolymerisation of non-ionogenic hydrophilic monomers in latex
This work is an investigation of the modification of natural rubber (NR) by graft copolymerisation of hydroxyethyl acrylate (HEA), hydroxypropyl acrylate (HPA), hydroxyethyl methacrylate (HEMA) and hydroxypropyl methacrylate (HPMA) in NR latex. Initial studies showed that, at low monomer concentrations, the latex stability increased substantially. This was attributed to the rapid adsorption of the monomers on to the surface of particles in NR latex. However, at high monomer concentrations, the latex stability decreased substantially. This is thought to be brought about by a dehydration process and interaction between the monomers and indigenous soaps. In the presence of the monomers, the stability of the latex decreased rapidly during maturation as a consequence of the hydrolysis of the monomers, dehydration processes, and interaction between the monomers and indigenous soaps. Dilution of the latex, together with subsequent polymerisation of the added monomers, improved the stability of the latex. The kinetics of the polymerisation of the monomers in the latex was studied. The rates of polymerisation were found to be in the following order: HEA > HPA > HPMA > HEMA. Kinetically the reactions for the four monomers were found to be first-order with respect to initial monomer concentration. However, the orders were found to be zero-order for HEA and HPA, first-order for HEMA, and second-order for HPMA when fitting the various order curves to data for individual polymerisations. No satisfactory explanation was offered to explain the apparent contradictions in the orders of reaction. However, it might be attributed to the locus of polymerisation being both at the surface of rubber particles and in the aqueous phase as a consequence of the heterogeneous nature of NR latex. Generally, the reactions were found to be half-order with respect to initiator concentration, with the exception of HEMA, for which the order was found to be 0.20. The orders of reactions were found to be first-order with respect to dry rubber content (ORC) indicating that the presence of rubber in the latex would accelerate rather than retard the polymerisations. In a subsequent detailed investigation, crosslinking was shown not to take place during polymerisation in the latex, despite the monomers containing diester impurities. The efficiency of grafting was determined by separation of the homopolymers. However, the degree of grafting could not be determined, as it was not possible to separate the free NR from the mixture. The mechanism of grafting is believed to be dominated by transfer reactions between the growing polymer radicals and NR. However, the grafting reactions might be also via addition reactions between growing radicals and the double bonds of rubber molecules leading to very high grafting efficiency. It is believed that the grafting reactions are temperature-dependent. It was found that the grafting efficiency was much higher when using a dissociative initiator which does not attack NR directly (4-4'-azobis(4-cyanovaleric acid)) at ca. 63°C than when using a redox initiator (potassium persulphate-sodium metabisulphite) at 30°C. The crude graft copolymer latices were found to produce cream. Generally, the vulcanised films from the crude graft copolymer latices were found have reduced water and oil resistance. It was also observed that the latices examined proved unsuitable for dipping application because the deposits ran down the formers.
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