GRAFTING OF VINYL MONOMERS ONTO SILK USING REDOX SYSTEMS - YELLOWING OF SILK

The aim of this study is to investigate the influence of temperature, initiator concentration, and vinyl monomer on silk yellowing during gr afting. A series of silk fabrics was treated at four different tempera tures (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 ammoniu m persulphate (APS), in the absence of a monomer, the degree of silk y ellowing increased linearly with increasing both initiator concentrati on and treatment temperature, indicating that the initiator plays a sp ecific 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 bl ank-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 te mperature range, due to its higher reactivity and to the tendency to f orm a homopolymer, leaving unreacted macroradicals on silk fibroin bac kbone. Compared to APS, other initiators, such as 2,2'-azobis (isobuty ronitrile) (AIBN) and 2-2'-azobis-(2-amidino propane) dihydrochloride (ADC), caused a significantly lower degree of silk yellowing when test ed in the absebce of a monomer. The yellowness of silk fabrics tended to increase by grafting with HEMA, while decreased by grafting with MA A. The use of variable amounts of monomer (25-150% owf) did not influe nce the degree of yellowing with ADC as the initiator. The results rep orted 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 limi ted by a suitable selection of the processing parameters (initiator co ncentration, temperature). Moreover, the use of both AIBN and ADC appe ars promising, owing to their negligible effect on silk yellowing. (C) 1998 John Wiley & Sons, Inc.