THE RIGIDITY OF RAW COTTON HAIRS AT 20° C. ON THE MODULUS OF RIGIDITY AT 20° C

Abstract

The influence of humidity on the elastic properties of cotton is involved in many specific problems of testing and technology which cannot be satisfactorily attacked without a systematic investigation. In the present paper the variation of the rigidity of raw cotton hairs from dryness to saturation at 20°C. (68°F.) is described as a matter of general and intrinsic interest, particular applications being temporarily ignored. The experiments show that the effect is so considerable that no testing or research which involves it can lead to sound conclusions without taking account of the influence of moisture. Furthermore, the quantitative relations obtained give interesting evidence on the swelling of cotton by water. I.—The Rigidity of Raw Cotton Hairs at 20°C.—The rigidity of the hair, or its resistance to twist, is measured by the method described in a previous paper³ but under conditions of closely controlled humidity at 20°C. Each hair is examined at a number of different humidities, the test being non-destructive, and the results on all the seventy specimens are compared by expressing the observations as ratios to the rigidity at 50% R.H. (relative humidity). The relation between rigidity and humidity is then given by an S-shaped curve, the value at saturation (100% R.H.) being less than one-sixth of that in dry air (0% R.H.). The decrease is rapid in the first and last 10% regions, whilst the middle portion shows an approximately regular decrease with increasing humidity. Over the range 3–83% R.H. the relation is sufficiently well expressed by a linear equation (2), p. T507. The water vapour in the air affects the elastic properties of the cotton only indirectly, the immediate cause being the absorbed moisture. When the logarithm of the rigidity is plotted against the moisture regain, given by the figures of Urquhart and Williams⁶, the points lie on a straight line. The slope of this, averaged over all the specimens, shows that the rigidity is halved by the addition of 10% of moisture and may be expressed by the simple exponential formula (5), p. T511. Increase of temperature decreases the rigidity at constant moisture regain, and this is only partly counteracted by the decrease of absorbed moisture when the relative humidity remains constant. At constant regain, the decrease is about 0.25% for 1°C. in dry cotton, increasing to about 0.6% at 7% regain. Around 60% R.H. the effect of 10°C. (18°F.) change of temperature, at constant humidity, is about the same as that of a change of 5% R.H. at constant temperature (p. T511), that is the effect of temperature is of a lower order than that of humidity in the limits of variation normally encountered. In saturated air, the rigidity appears to increase with change of temperature above or below 20°C. II.—Modulus of Rigidity at 20°C.—The variations of individual hairs are analysed and it is concluded that the differences are mainly due to different hygroscopicity. The individual slopes, or the empirical exponential co-efficients, are then an indication of the varying absorption by the single hairs. At intermediate humidities, the differences are much greater than those between the mean values of different varieties obtained by weighing bulk samples. The absorption is not a characteristic of the variety, for one Sea Island cotton gives values covering the whole range found on all kinds down to a coarse Indian. Generally speaking, the more fully thickened hairs absorb less moisture and the differences may be ascribed to differences in porosity and minor chemical constituents due more to environment and development than to hereditary nature. Using previous results³ and an approximate correction for swelling, the variation of the rigidity modulus is evaluated; see equation (12), p. T515. The empirical relations are interpreted quantitatively by a simple theory, according to which water molecules are absorbed, first in a definite relation to individual cellulose groups (C₆H₁₀O₅), later molecules being absorbed in a less intimate manner. The former produce all the alteration in elastic properties but little vapour pressure, whilst the latter have little effect on the cohesion but determine the vapour pressure and the progress of chemical reactions.