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

Abstract

ABSTRACT: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 88ЊC), 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 70-80ЊC 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.

Grafting vinyl monomers onto silk (Bombyx mori) using different initiators: Properties of grafted silk

“…For this reason we were interested to undertake a study on the problem of silk yellowing associated with grafting. In a recent study13 we reported the results of a systematic investigation on the role of different initiators [an inorganic peroxide: ammonium persulfate; and two azo compounds: 2,2′‐azobisisobutyronitrile and 2,2′‐azobis(2‐methylpropionamidine) dihydrochloride] in inducing the yellowing of silk, as well as on the reaction conditions (initiator concentration, temperature, type of vinyl monomer), which are likely to enhance the degree of yellowing. The results emphasized the role of the initiator as the chemical agent mostly responsible for yellowing.…”

Physical properties of silk fibers grafted with vinyltrimethoxysilane

“…APS is used extensively as an initiator for grafting reactions on both the laboratory and industrial scales and many related basic information studies on APS and its reaction mechanisms are available. The use of AIBN and ADC for silk grafting was recently reported by Tsukada et al 7 ; they showed that these initiators were effective in inducing a lower degree of fiber yellowing than APS, although the grafting efficiency was lower.…”

Silk‐Based Biomaterials in Biomedical Textiles and Fiber‐Based Implants

“…Hence, substantial physical, chemical and even genetic treatments have been used for the modification of native silk fibers [66–71] . The silk fibers can be easily produced from aqueous protein solutions using a natural process in which the temperature and pressure in the spinning container are lower, and no noxious solvent are used in the spinning bath [72] .…”