JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Alkaline assistants such as borax, carbonates, phosphates, ammonia or ammonium salts, in the concentra- tions used for high temperature waving were not satisfactory at lower temperatures unless used for long periods ranging from 4 to 24 hours. Attempts to use solutions of greater concentration resulted in excessive damage to the hair. Obviously then, ß a waving mechanism not depending' on initial disulphide linkage hydroly- sis is required for cold waving, and an indication of the correct approach was obtained when ammonium hy- drogen sulphide, which brings about keratolysis by reduction of cystine linkages, proved to be a more speedy reagent. The malodorousness of this material, however, resulting from liberation of hydrogen sulph!de by hydrolysis proved a serious dis- advantage. Moreover, sorption of sulphide by hair gave rise to a dis- agreeable odour whenever the hair was re-wet ted, while continued reduc- tion of cystine linkages caused relax- ation of the hair fibres so that the wave became less tight with each re-wetting. As already indi- cated, work in the biochemical and textile fields had demonstrated the use of mercaptans for reduction of the disulphide linkage in scleropro- teins, and in 1940 cysteine hydro- chloride at a pH not less than 10 formed the basis of a patent speci- fication. *• While of considerable theoretical interest this process does not appeai- to have received any wide application, probably owing to the fact that the material is costly, the solution is unstable, leaves a dusty deposit on the hair, and although the hair is well waved it is very dull. z, McDonough, ø-'• in a U.S.P. specifica- tion filed in 1941, but not granted until 10 years later, then showed that satisfactory cold waving could be obtained using mercaptans of general formula R.SH, where R is an organic group such as ethyl or benzyl, in solutions of 1-10, per cent concentration and pH7-9'5. More- over, substitution in the alkyl or aryl radical gave products with less objectionable odours. Such substi- tuent groups may be polar (acidic or basic) or non-•polar. For example' (i) Carboxyl or sulphonic acids, nitro, chloro or halogenated groups. (ii) Primary, secondary or tertiary..: amino groups. (iii) Alcohol, aidehyde, keto, este? or ether groups. '• ß •: In general, the polar •ompounds containing a carboxyl group give .? excellent waving results when used in the form of the ammonium salts. It has been found, however, that the':i:i! corresponding sodium and potassium.i! • salts, even in ammoniacal solution !'•)i are much less satisfactory, and the:.?:• same applies to ethanolamine salts?: i•i As will be considered in detail alkaline earth salts of certain of thio-acids possess depilatory perties, e.g., calcium thio In the case of mercaptans water solubilising non-polar such as. glyceryl mono-mercaptan I it is interesting, in view of presen! developments in one solution waving methods, 'to note that Donough states that the removal' mercaptan is so complete by 176

DISULPHIDES AND MERCAPTANS IN HAIR CHEMISTRY one that excellent non-relaxing ,aves can be obtained without need additional fixing or setting •ses. materials are used in concen- of 3 to 8 per cent, the speed waving increasing with concentra- tion up to 15 per cent. By varying concentration of mercaptan the y practical adjustments of 'waving speeds Car/be made to suit .liferent systems of hot or cold waving as well as different types of air. Treatment can be carried out in aqueous solutions of pH7 up to but not including 10, but in prac- it is found desirable not to pH9'5, otherwise the time .:in which it is possible to wave the is too short. Optimum alka- •:•:,•.• . is pH9'2-9'5. Ifi cold waving ::/•'•/•!i?.i::..,careful pH control is essential, speed ::½ji!( :!øf waving increasing with pH of the •i?.:!?:'thiol solution. 'So far as pH regula- •'?•i•111':tion is concerned McDonough states :'i:i?i:'•'that while all alkaline materials are •,•: :' satisfactory, bases with a dissocia- tion constant less than 5 x 10-' and- ?!?Preferably about 10 -• are much less i?,:}:.destructive in action. For cold i?i'::.'waving, volatile bases such as am- }.?.!•: monia, methylamine or ethylamine :i•!i}:-•:i are preferable to non-volatile alkalis :-:: :• since the former give tighter waves. {?::!Ammonia, in particular, is outstand- ?•!(.ing in its advantages since its odour blends pleasantly with that of ii:'.:i:' mercaptans, the ionisation constant :•:i.. is satisfactory, and between pH 7-10 ß : i::: it is relatively non-irritating to the !::!½: skin. These basic U.S. patents, }i': together with 22 similar patents and : ( .:' patent applications in other coun- ß tries, cover the use of mercaptans in cold waving and have recently resulted in the issue of world-wide licensing agreements. Thioglycollates are the mercap- tans most widely used, and formu- lated with resin dispersions produce cloudy or opaque cold wave prepara- tions. Recently, '* it was suggested that 0'3 to 0'5 per cent lanette wax SX or cetyl alcohol, distilled water and wetting agent (secondary sodium alkyl sulphate or alkyl aryl sodium sulphonate) be emulsified in the usual way and aftei' cooling to room temperature, thioglycollic acid and ammonia be added. Since early in 1949 thioglycerol has been widely used for home waving in France. Prepared as a 20 per cent solution from glycerol monochlorhydrin and sodium hydro- sulphide, it is sold to the public as 5 per cent thioglycerol and 15 per cent ammonia. •' This solution of pH 10'5 gives excellent waves, the setting time being at most 2 hours and even with difficult hair the waves last 6 months? For saloon u•e, however, the speed of waving is too low, and while good results are obtained at high temperatures, liber- ation of h•drogen sulphide makes the process impracticable. Instabil- ity of an. ammoniacal solution of thioglycerol necessitates addition of ammonia to the mercaptan solution immediately before use, which is a further manipulative disadvantage. Morelle TM has, therefore, developed the use of thiolactic acid, formed by reaction of •-chloropropionic acid with sodium hydrosulphide, followed 177