Silk is usually dyed at the fabric or yarn stage. It is difficult to dye silk fibers at the cocoon stage because (1) cocoon loses its original shape and (2) dye cannot penetrate easily into the cocoon shell. This study is the first attempt at dyeing cocoons adequately while retaining their shapes. Cocoons were dyed with a reactive dye under various conditions. When the dyeing process was carried out for several hours without a surfactant, the cocoon shells were not dyed adequately from 293 K to 333 K, furthermore at 333 K the silk filaments on the surfaces of the cocoons came loose and were entangled. In contrast, the cocoon shells were almost completely dyed when a surfactant, namely, sodium dodecyl sulfate, was used; in this case, the concentration of the surfactant was greater than the critical micelle concentration. The results suggest that the use of a surfactant and an appropriate temperature are useful for dyeing cocoons.

Silkworm cocoons have been dyed using acid dyes (C. I. Acid Orange 7 and C. I. Acid Orange 63) while retaining their shape under various conditions, and the dyeing properties have been investigated for developing a new fiber technology. Whereas the corner of the cocoon shell was thin and could be dyed in shorter time, the center of the cocoon shell was thick and needed more time. The penetration depth of the dye molecule into the cocoon increased with time, but the penetration speed was not constant: the penetration rapidly increased within 5 minutes, but afterwards gradually increased. This result is due to the structural differences among the shell layer portions. In addition, when the dye bath was at a lower pH, the penetration speed was slightly slower. Moreover, the C. I. Acid Orange 63 molecule penetrated more slowly than the C. I. Acid Orange 7 molecule. These results are probably attributed to the difference in the adsorption state of the dye molecule on the fiber.