SYNTHETIC HYDROCOLLOIDS AND DENTIFRICES 469 BIBLIOGRAPHY Morss, N.M. and Ogg, ]. Water Soluble Gums and Colloids, 1966. (S.C.I. Conference). Some special characteristics of cellulose ethers. Desmarais, A. J. and Esser, H. O. Relationship of chemical structure of water soluble cellulose ethers to physical properties. Nearn, M. R. Rheology of the ternary system carboxy methyl cellulose/glycerol/water. DISCUSSION MR. R. J. CHUDZ•KOWS•C•: Do you have any experience in the use of Pluronics in dentifrices? There is one particular material which at 20% concentration forms strong gels with a fairly high yield value, and a short recovery time. It is compatible with many materials and is a good suspending agent. THE LECTURET•: We have not done any work with it in dentifrices. We are looking for low cost materials which will give quite thick gels at 2% level in 50% aqueous glycerine, so Pluronics have not even been considered in this context. DR. B. S. N•uMa•: Do you think that inorganic gelling agents have any future in dentifrices? The requirements of forming a gel at 2% level in 50% glycerine would be fulfilled with certain inorganic gelling agents. TH• L•CTUR•R: I think we are talking about Laponite. We have done some work with this very interesting material, but so far we have had difficulty in preparing stable gels in the presence of glycerine. Work continues, and we would not rule out Laponite as a toothpaste thickener. MR. N.M. MoRss: You mentioned that a hydroxyl ethyl cellulose toothpaste did not go thin when treated with cellulose enzymes. It is our experience, but I think I am referring only to solutions of hydroxy ethyl cellulose and SCMC, that the rate of degradation with a cellulose enzyme is roughly the same. I wonder if there is something different about the behaviour in actual toothpaste. Why should HEC degrade in solution but not in toothpaste? TieE L•ZCTURER: I do not really know, but I can only report what we found. We did a considerable amount of work using complete toothpastes and storing them for a period of some months. We obtained some cellulase and found that just 2 ppm in an SCMC toothpaste would cause it to liquefy in a very short space of time. In the very rare event of stock spoilage from this source, it took a month or so for the product to soften, and only minute traces of cellulase would be present. Under these conditions, our test products with SCMC became thin, and those with HEC did not. MR. N. M. MORSS: It may be that HEC has partial rather than complete resistance to the cellulase enzyme. With unpreserved emulsion paint, SCMC will degenerate quickly, HEC less quickly, and methyl cellulose least quickly. THn L•ZCTUR•R: I am sure you are right, it is a question of degree. DR. J. J. M^us•ER: You have indicated three formulations in your paper and their pH ranges from 4.5 to 10.0. I was struck by the absence of preservative. I wonder how important this is from the formulation point of view, quite apart from any question of enzymatic activity but purely from the bacteriological aspect. TH• LECTURER: in our experience, in climatic conditions all round the world preservatives are not necessary in formulations like this, which have a high concen- tration of glycerine in the aqueous phase. The osmotic pressure of the aqueous phase

470 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS is too high to permit the gro•vth of micro-organisms. The flavour also has a strong germicidal action. \¾e have experienced gassing and tube blowing due to bacterial contamination of dentifrices having lower humectant levels, and our fairly extensive work in this field shows that formalin is the only effective preservative in this situa- tion. The enzymatic thinning of toothpastes can occur in sterile products where the organisms have been killed, but the enzyme remains. This is a speculative area, little is known about the sources of trace enzymes, and anyway the trouble is very rare. MR. S. J. STmANSE: In connection with the distribution coefficient of your hydro- colloids, I was wondering whether you could not tailor-make the right product, taking advantage of the many hydroxyl and carboxyl groups that you have. Could you not ethoxylate them or perhaps combine ethoxylation and propoxylation to get the distribution coefficient that you want? TI•E L•cxtm•R: Perhaps the hydrocolloid manufacturer should take note of that question. We only use the materials, and have no opportunity to vary their properties. MR. R. H. McDowm•x•: I was rather surprised that no mention was made of Irish moss because I gather that a few years ago it was a very popular thickening agent. What is the reason for its being dropped? Ti• LECXtJR•R: I restricted my paper to materials •vhich I consider to come under the term 'Synthetic hydrocolloids'. Irish moss and its derivatives are of course widely, and successfully, used in toothpastes. MR. A. Fosx•m If glycerine does impose certain limitations in the use of hydro- colloids, have alternatives been looked at to widen the scope for other thickening agents or are the advantages of glycerine such that there is no incentive to look for an alternative? T•F• L•cxtmFm: Sorbitol is widely used as an alternative to glycerine, and it may or may not be more compatible with the hydrocolloids depending upon the hydro- colloid and a host of other formulation considerations. Outside the three formulations which I have given, there are an almost infinite variety of possibilities, and each must be considered separately.