J. Soc. Cosmet. Chem. gl 441-457 (1970) (•) 1970 Society of Cosmetic Chemists of Great Britain New reactions of alginate propylene glycol R. H. McDOWELL* Presented at the Symposium on "Gums and Thickeners", organised by the Society of Cosmetic Chemists of Great Britain, at Oxford, on 14th October 1969. Synop$i$--In mildly alkaline aqueous solutions, PROPYLENE GLYCOL ALGINATE can react with compounds containing amino or hydroxyl groups. If the compounds concerned have a suitable chain length, RHEOLOGICAL changes take place which suggest that CROSS LINKING has taken place. The reactions with GELATINE, POLYVINYL ALCOHOL and STARCH have potential uses in COSMETICS and PHARMACEUTICAL products. INTRODUCTION Propylene glycol alginate (referred to hereafter as PGA) is a water soluble polymer used for thickening and stabilising acidic products. It is prepared by the reaction of partially neutralized alginic acid with propylene oxide {1). Esterification of carboxyl groups takes place and in the course of the reaction the free acidity drops. It is not practicable to bring about complete esterification and in commercial products up to 10% of the carboxyl groups remain as free acid, while between 10% and 30% are neutralized with sodium or calcium. On treatment of the dilute aqueous solution with sodium hydroxide the expected ester saponification takes place, propylene glycol being split off. The alginate can be recovered in part as sodium alginate, but a side reaction involving [I-elimination brings about some degradation of the alginate chain. Some alginate is lost, and the part that can be recovered is of a lower degree of polymerization, so *Alginate Industries Ltd., London, W.C.2. 441

442 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS that the action of alkali on a PGA solution brings about a very marked drop in viscosity. The degraded product contains some non reducing end groups having a 4-5 double bond as can be demonstrated by means of the thiobarbiturate reaction (2). Some saponification and degradation will take place even if a PGA is made alkaline for only a short time and for that reason users of the product are advised to exercise great care in any adjust- ment of the free acidity of PGA solutions. The product is sold with a pH in the range $ to 5, and if for any reason this has to be adjusted to a slightly higher level very careful addition of dilute alkali with extremely good stirring is necessary to avoid any local alkaline conditions in the solution. Adjustment to a pH of above 6 is not advised. Some years ago a patent for the preparation of alginic acid amide by the reaction of anhydrous ammonia with PGA was published (3). It is stated therein that the moisture content of the system should not exceed 50% of the dry solids, and should preferably be much lower. The description in two later patents of the reaction of PGA with amino compounds in aqueous alkaline conditions, to give useful products was therefore rather surprising (4, 5). The important reaction to the inventor companies was that of the PGA with gelatine to give a photographic emulsion that could be processed at temperatures at which an untreated gelatine emulsion would melt, but one of them {4) included claims to cover the preparation of water insoluble film forming compounds from alginic acid esters and compounds having at least two primary or secondary amino groups. The general method described in these patents is to form on a suitable support a mixed film of the PGA and gelatine or a neutralised amino com- pound, at a pH of below 7, and to immerse it in an alkaline bath to bring about the insolubilization. In photographic work the normal alkaline developers have this effect. The insolubilization can also be shown by holding a mixed solution of PGA and gelatine at a temperature above the melting point of gelatine (say at 40 øC) and adding sufficient sodium carbon- ate to bring the pH to about 10. The mixture then rapidly stiffens and cannot be melted by holding in a boiling water bath, nor will it dissolve when mixed with more hot water. In the course of some work to study the conditions for reaction with various amines some control tests were made in which no amine was included. It was then found that if sufficiently concentrated solutions of PGA were made slightly alkaline there was a marked increase in viscosity instead of the expected decrease. Further experiments showed that increases in viscos- ity and gel formation could take place in mixed solutions of PGA and a