Ultra-fine silk powder preparation through rotary and ball milling

Three commercial silk varieties, namely mulberry, muga and eri, were used to prepare ultra-fine silk particles. Degummed silk fibres were chopped into short snippets and then pulverised using rotary and planetary ball milling. The effects of degree of degumming, size of milling media, water and lubricant on particle refinement were studied. Before milling, single fibre strength tests were conducted on silk fibres degummed under different conditions. The results indicate that while reducing fibre strength via harsh degumming could cut milling time drastically, too severe a reduction in fibre strength is actually detrimental to achievable minimum particle size due to increased particle aggregation. Water played an important role in affecting the performance of ball milling. With the milling processes used in this study, a volume based median particle size (d(0.5)) of around 200 nm was achieved, which is much smaller than previously reported results. To achieve a similar particle size, mulberry silk required more milling time, even though the overall milling behaviour was quite similar for the three silk varieties examined. SEM observations revealed axial spitting and fragmentation of micro and nanofibrillar architecture of silk fibres due to milling. Unlike ball milling, which produced particles with variable shapes, rotary milled particles remained fibrous through the size reduction process.