A study of the compressibility of wool, with special reference to South African merino wool

Abstract

1. A study has been made of the resistance offered by wool samples to compression at 65 per cent. relative humidity and 70°F. (21.1°C.) temperature. The study has been based mainly on results obtained with the "Pendultex" instrument, designed by Henning (1934), but some additional determinations were made by means of a static cylinder and piston method. 2. A relation has been derived whereby the work done in compressing a wool sample in the "Pendultex" apparatus may be calculated from the number of swings during which the amplitude is reduced from one fixed value to another. 3. During the final constant cycle of compression by the static method, the pressure bears to the inverse cube of the volume a linear relation. With the dynamic method, the law is obeyed by the first compression. 4. The pressure-volume relation is discussed from a theoretical point of view, and it shown that the inverse cube law may be derived on the basis of certain assumptions. An approximate value of Young's modulus by bending can be calculated. 5. An empirical exponential relation between pressure and volume is considered. 6. It is concluded that since the density of packing is not uniform at low degrees of compression, results obtained at low pressures should not be considered together with those obtained at higher pressures, where the density of packing is more uniform and the pressure-volume relation follows the inverse cube law. 7. The method of expressing compressibility and resilience by means of the work done during compression and release is discussed. It is concluded that in the comparison of different wools the work done should be evaluated between volume limits. 8. A marked reduction in resistance to compression with the adsorption of water has been found. 9. Fibre length has no influence on the resistance to compression down to staple lengths of approximately one inch. 10. No correlation has been found between resistance to compression and fibre thickness. Although this result agrees with theoretical expectation, a highly significant partial correlation coefficient of +0.4330 is obtained when the effect of crimping is allowed for. It has been concluded, either that the fibre thickness has a positive influence which is masked by the crimping, or that fibre thickness is correlated with other factors, besides the crimping, which influence resistance to compression. 11. A highly significant positive correlation coefficient has been found between the resistance to compression and the number of crimps per inch. Possible ways in which the crimping can influence the resistance to compression are discussed. 12. For wools whose fineness and crimping agree with Duerden's standards, the resistance to compression increases with the quality number. Wools which are coarser than the crimps indicate have a higher resistance to compression than wools which are finer than the crimps indicate. 13. A significant partial correlation between resistance to compression and variability in fibre thickness has been found, but the coefficient is probably too small to be an important factor in breeding. 14. No correlation exists between the resistance to compression of a sample and the surface friction of its component fibres. It is concluded that the crimping is a more important factor in controlling fibre slippage during compression. 15. No correlation has been found between the resistance to compression of a sample and the tensile strength of the fibres. There are, however, factors which may influence one of these attributes and not the other, thus masking a possible correlation. 16. Samples presumed to have been selected for specific gravity by a sheep and wool expert were found to have been selected for resistance to compression. It is recommended that the term specific gravity should not be employed in wool practice. 17. Fibre thickness was the main factor to determine the harshness of two sets of samples as subjectively estimated. Resistance to compression and the non-wool fleece constituents were less important, though definite, factors. Harshness is, therefore, determined by the resistance to bending of individual fibres, rather than by the resistance to compression of the mass as a whole. An increase in tl1e surface friction of the fibres is responsible for the increased harshness of alkali treated wool. 18. Dipping wool in a lime-sulphur dip has no effect on the resistance to compression. 19. The variation in resistance to compression over the fleece has been studied and the major part of the variation found to be associated with the variation in fibre thickness and crimping. The results are discussed in relation to sampling in experimental work. 20. There is a highly significant negative correlation between the resistance to compression and the percentage clean yield of the fleece, and a highly significant negative correlation between percentage yield and number of crimps per inch, and no correlation between percentage yield and fibre thickness. 21. No difference in the average resistance to compression of fleece of rams and ewes could be found. It is concluded that differences observed in practice are duo to selection of stud rams for the ''substance'' of their wool. 22. On the average, the resistance to compression of the wool increases with the age of the sheep for the first four years, and the increase can be associated almost entirely with the increase in fibre thickness. 23. In a feeding experiment, the plane of nutrition had no effect on the resistance to compression of the wool in spite of a marked effect on the fibre thickness. 24. The distribution of resistance to compression is considered, and it is shown that South African Merino wool covers a range of at least 3:1 in this attribute. 25. The bearing of the correlations found on wool practice, with special reference to breeding, is discussed. 26. Possible results of breeding for "substance" are considered, and the desirability of breeding for this attribute is regarded with some doubt. 27. The importance of breeding for uniformity is stressed. 28. Support is given to a scheme of fleece recording in stud breeding, and emphasis is laid on the necessity of employing exact methods of measuring wool characteristics. 29. The establishment of a wool testing house in South Africa is recommended.