CHEMISTRY OF KERATIN* By MILTON HA•.•as Harris Research Laboratories, IFashington 11, D.C. PERHAPS NOTHING in research today is more fascinating than the dependence of extremely diverse industries on a given field of basic science. An excellent example of this is found in a branch of chemistry which has been rapidly developing during the last fifteen years and which has come to be known as the chemistry of organic high polymers. This branch of modern science has been the basis for many of the out- standing developments on rubber, resins, fibers, and leather, materials which have all found an extremely wide range of application in in- dustry. In addition and somewhat indirectly, it can be considered re- sponsible for some of the more recent developments in the cosmetic in- dustry. I refer p•articularly to the relationship of the chemistry of wool to that of human hair and to the applications of some of the prin- ciples of high polymer science to a number of present-day cosmetic de- velopments. As a background for the present discussion, I should like to consider a few of these funda- mental principles. * Presented at the December 8, 1948, Meeting of The Society of Cosmetic Chem- ists in New York City. The great industrial importance of the organic polymeric systems stems from their wide range of mechanical properties. Thus, rub- ber is important because of its long- range elasticity plastics can be molded into materials of consider- able strength or modified so as to yield systems possessing consider- able flexibility or elasticity textile fibers have a great variety of prop- erties which lend themselves well to the fabrication of materials rang- ingfromwomen's sheer hose to heavy industrial fabrics. A most interesting feature of' all of these systems is that, in many cases, a small'difference in chemical structure may determine whether a material is a rubber, plastic, or a fiber. An excellent example of this is found in the behavior of the polyam- ides of the type used for producing nylon. Polyamides such as those produced by the condensation of adipic acid and hexamethylene dia- mine make outstanding textile fibers. However, the substitution of an occasional isobutyl group for a hydrogen atom in the amide group results in a rubber-like material with an elastic extension which may 223

224 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS reach several hundred percent. In other words, the problems of those who are particularly interested in fibers are very similar to those of workers who are interested in other high polymer fields whether they deal with leather, human hair, skin, rubber, or plastics. From a chemi- cal point of view, there is an over-all similarity among these systems in that they are all polymers of high molecular weights,-most of which are linear although a few are branched or cross-linked. True, they differ in details of chemical structure which accounts for their different properties. And from an industrial point of view, all of these systems are important because of their diverse mechanical proper- ties. As a matter of fact one can go still farther in emphasizing these inter-relationships and put forth a set of qualitative rules for predict- ing the mechanical behavior of organic high polymers (1). For ex- ample, if the chemical structure of the molecules is such that they fit poorly into a lattice structure (in other words, an amorphous state is favored) and further if the intermolecular forces are weak, then the material should have rubber- like properties. In contrast, if the structure of the molecules is such as to provide for an easy fitting into a lattice (that is, a relatively high state of crystallinity is favored) and the intermolecular forces are strong, the material is a fiber. In inter- mediate cases, plastic systems are obtained. If we now pass from the broad field of high polymers to a narrower section dealing with fibers, the same considerations are found to be ap- plicable. In some respects, the classification just given is even more useful for classifying different fiber systems. For example, if the mole- cules fit readily into a lattice and the intermolecular forces are strong, a very strong fiber with low elonga- tion and at best very short range elasticity is obtained (cotton, linen, nylon). If the molecules fit poorly into a lattice and the intermolecular forces are weak, a fiber of low strength but greater extensibility results further, if the molecules are flexible, the fiber can exhibit rela- tively long range elasticity (wool, elastic, nylon). Again, for inter- mediate cases, fibers With inter- mediate properties may be obtained (silk). It is at once obvious that there will be a number of borderline cases but, nevertheless, this is a useful qualitative classification. If we now narrow the section dealing with fibers still further and consider one type of fiber, the keratin fibers, as exemplified for ex- ample by wool, we find the same considerations to be applicable, and this will be borne out in the subse- quent discussion. The distinguishing mechanical property of wool is its long-range elasticity, that is, the ability to re- cover from deformations of a magn i- tude considerably greater than that permitted by other types of fibers, regardless of wkether the deforma- tions have been produced by stretch-