สำนักราชบัณฑิตยสภา

The Journal of the Royal Institute of Thailand √–∫∫∑ÿ ππ‘ ¬¡¿“¬„µâ √–∫Õ∫ª√–™“∏‘ ª‰µ¬·∫∫‰∑¬ Properties of Industrial Thai Silks Reeled by Hand and by Machine Vol. 32 No. 1 Jan.-Mar. 2007 144 filaments and 151-155 º C for the degummed filaments). The reasons for these results could be due to the higher birefringence and higher crystallinity of the machine reeled filaments (see the birefringence and crystallinity sections). Burning Test All the silk filaments showed very similar burning behavior as follows. Both the raw and degummed filaments shrunk on heat when they were placed near a flame. Upon placing them in the flame, they burned immediately. The degummed filaments gave a strong smell similar to hair burn- ing while the raw filaments did not produce a strong smell during burning. After burning the raw fila- ments, black, grey and hard solid ashes resulted, but while burning the degummed filaments, black and soft ashes developed. Fiber Strength The raw and degummed silk filaments were tested for their strength in terms of fiber tenacity in a unit of gram per denier (g/d). The test results indicated that the raw filaments had higher tenacity (2.4-3.6 g/d) than the degummed filaments (1.2-2.5 g/d) due to the presence of sericin. Sericin on the raw filaments could help in in- creasing the fiber strength or tena- city resulting from its elastic amor- phous structure. The Chinese raw filaments and the degummed fila- ments had approximately the same fiber tenacity (2.5 g/d for the raw filament and 2.4 g/d for the degummed filament). In the other words, the Chinese silk filament could maintain its strength after degumming (only 4% loss) while the local silk filaments lost 20-58% of fiber tenacity after degumming because the degumming agents in- troduced more fiber damage to the local filaments than to the Chinese filament. In terms of reeling methods, it was found that the local raw filaments reeled by machine had a slightly lower tenacity (2.4 -3.1 g/d) than those reeled by hand (2.6- 3.6 g/d) while the local degummed filaments showed the inconsistent tenacity of the machine and hand reeled filaments. The previous test results on birefringence, crystallin- ity and the softening point of the local raw and local degummed filaments indicated that the fila- ments reeled by machine had a higher birefringence, crystallinity and softening point than those reeled by hand. Thus it should be expected that the former filaments have a higher tenacity than the latter; unfortunately this was not the case. This is caused by the unevenness of the natural silk fiber from silk genetics and raising condition of the silk worms. Resistance to Sunlight Generally, silk fiber cannot resist sunlight because it tends to absorb UV radiation from sunlight through the peptide bonds and a small amount of the disulfide bonds on polypeptide chains and their natural pigments within the fiber. The high energy of UV radiation can later break these covalent bonds, degrade the fiber and decrease the fiber strength. This study revealed that after a 25-hour exposure to the imita- tion sunlight, both the raw and degummed silk filaments showed a significant strength loss in terms of %tenacity loss (38-53% for the raw filaments and 44-54% for the degummed filaments). The raw filaments lost less strength than their degummed filaments because the sericin on the raw filaments helped protect the silk from UV destruction. It was also found that the Chinese raw filament showed the lowest tenacity loss (38% loss) while its degummed filament showed the highest tenacity loss (54% loss) after a sunlight expo- sure of 25 hours, compared with the local raw and local degummed filaments. The Chinese degummed filament or silk fibroin could be damaged more by sunlight or have less resistance to the sunlight than the local silk fibroin. On the other hand, the test results showed that the local raw filaments lost 50-53% and their degummed filaments lost 44-50% after the exposure. Silk resistance to sunlight can be influenced by the parental ge- netic of each silk variety including many factors such as the number of peptide bonds and the various amino acid contents on polypeptide chains, natural pigment content within silk fibroin and silk sericin, sericin content, etc. It has been