The Theory of Shrinkproofing of Wool: Part V: Electron and Light Microscopy of Wool Fibers after Chemical Treatments

Abstract

Lincoln 36's top, subjected to ten different chemical treatments as described in previous publications of this series [4, 5], was examined by light and electron microscopy using replica and thin-sectioning techniques. The sectioning and staining technique proved useful in showing the extent of penetration of the fiber by the reagent. The more drastic treatments such as 5% sodium hydroxide in water, hydrogen peroxide (10 vol for 16 hr), and severe treatment with chlorine (16% on weight of wool) caused almost complete obliteration of cellular detail throughout the fiber. Milder oxidation treatments may not have destroyed cellular detail, but may have oxidized sufficient disulfide bonds to prevent the TGA-OsO4 staining procedure from staining the matrix, with the result that the fine structure of microfibrils and matrix in the cortex were not visible. Alternative section staining procedures utilizing permanganate or lead hydroxide were also useful. Modification of the cuticle was observed after every treatment which effected shrinkproofing; namely, acid chlorination, treatment with permonosulfuric acid, hydrogen peroxide, sodium hydroxide, and treatment with permanganate in saturated salt. Some treatments (ethanol extraction, for example) produced a marginal degree of shrinkproofing and showed a small amount of cuticle modification. Other treatments (with peracetic acid, for instance) did not modify the cuticle and did not effect shrinkproofing. Thus, the primary requirement for shrinkproofing by degradative chemical treatments is sufficient modification of the fiber cuticle. The chemical reaction by which cuticle modification (and thereby shrinkproofing) is brought about generally involves cystine, but with the use of ethanolic sodium hydroxide the major process is probably the removal of a wool wax constituent from the cell membrane complex beneath the cuticle. Although modification of the cortex by chemical reaction is undesirable from a practical point of view, it has only a secondary effect on the degree of shrinkproofing. Various methods of confining the reaction to the cuticle are discussed. In particular, it was found that sodium hydroxide in water penetrates throughout the fiber, whereas the reaction was limited to the cuticle by the use of sodium hydroxide in ethanol or in an aqueous solution saturated with salt. Similarly, reaction of the cortex occurred with permanganate in water (with no subsequent shrinkproofing), whereas the presence of salt limited the reaction to the cuticle, thus producing shrinkproofing. The correlation between cuticle modification (hence degree of shrinkproofing) and alteration of frictional coefficients was good in most cases, but the lack of correlation in a few cases emphasized the importance of a new approach to the measurement of frictional coefficients.