Grafting of vinyl monomers onto silk using redox systems. Yellowing of silk

摘要

The aim of this study is to investigate the influence of temperature, initiator concentration, and vinyl monomer on silk yellowing during grafting. A series of silk fabrics was treated at four different temperatures (70, 75, 80, and 88 degrees C), with different concentrations of initiator in the range 1-4% owf, with and without methacrylamide (MAA) or 2-hydroxyethyl methacrylate (HEMA). By treating silk with ammonium persulphate (APS), in the absence of a monomer, the degree of silk yellowing increased linearly with increasing both initiator concentration and treatment temperature, indicating that the initiator plays a specific role in enhancing silk yellowing through macroradical formation. The graft copolymerization of MAA (with APS as the initiator) caused only slight changes in the intensity of silk yellowing compared to blank-treated fabrics. On the other hand, the use of HEMA resulted in a deeper yellowing of silk fabrics, especially in the 70-80 degrees C temperature range, due to its higher reactivity and to the tendency to form a homopolymer, leaving unreacted macroradicals on silk fibroin backbone. Compared to APS, other initiators, such as 2,2'-azobis (isobutyronitrile) (AIBN) and 2-2'-azobis-(2-amidino propane) dihydrochloride (ADC), caused a significantly lower degree of silk yellowing when tested in the absebce of a monomer. The yellowness of silk fabrics tended to increase by grafting with HEMA, while decreased by grafting with MAA. The use of variable amounts of monomer (25-150% owf) did not influence the degree of yellowing with ADC as the initiator. The results reported in this study show that the extent of yellowing induced on silk fabrics by grafting MAA and HEMA with APS as the initiator can be limited by a suitable selection of the processing parameters (initiator concentration, temperature). Moreover, the use of both AIBN and ADC appears promising, owing to their negligible effect on silk yellowing. (C) 1998 John Wiley & Sons, Inc. [References: 17]