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KERATIN AND KERATINIZATION 



steps (Fig. 70) which according to the theory of Astbury and Woods (1933) 

 mark the successive extension of " phases " differing in the force required 

 to stretch them (see p. 174). American workers (Burte and Halsey, 1947), 

 with the complex elastic behaviour of polymers in mind, have been more 

 inclined to regard the curve as being that of a single, uniform cross-linked 

 polymer. No entirely satisfactory quantitative account of the whole range 

 of elastic behaviour has been given, but some success has been obtained in 



Fig. 103. Influence of relative humidity on the stress-strain curve of 



wool at 22° D with constant rate of loading of 18g/min (Peters and 



Woods, 1956). 



the simpler problem, the description of the elastic behaviour of fibres in 

 which most of the internal molecular restraints have been removed 

 (Burte and Halsey, 1947; Peters and Speakman, 1949). 



To assess numerically the effect of a chemical treatment on a keratin 

 fibre, Speakman (1934 and 1947) introduced what may be called the " 30% 

 work index", which is defined as the ratio of the work required to stretch 

 the treated fibre 30% to the work required to stretch it 30% before 

 treatment. The choice of 30% (or in some instances 20%) is based on 

 the experimental fact that the stretching of wool fibres in the range of 



