RHEOLOGICAL MEASUREMENTS IN THE COSMETIC INDUSTRY 93 moment the stress is released, it immediately "stays put." i:".:". "'.:':' '" :"•:.::v?•'.:::' ß ....... :.-•,•. '3 ..:..4• •:,• ":..'z, • Figure 1.•Simple rotating cup viscometer. It consists of a cup which can be rotated at constant speed and a metal disc or a b• suspended by a torsion wire. The torsional de- flection is measured by means of a graduated circular protractor. The cup is filled with the mamrial under test and the disc lowered so that it is fully immersed. After the lapse of a suitable peri• of time, the motor is started and the maximum angul• deflection is noted and the time observed. The angular deflection is noted after 1, 2, 3, 4, 5 and 10 minutes. The curve should be asymptotic at the end of this pefi• if the speed of the rotation• is suitable for that particular preparation. (Normally the apparatus is enclosed in a cabinet with temperature conwok The temperature of the emulsion at the commencement and at the end of the run should be deter- mined.) the dispersion becomes rigid in the sense that Others, like many cosmetic products, re- quire a considerable time in order to set up. This is notably the case in emulsions of oil-in-water made by means of beeswax (and various beeswax substitutes) as the emulsi- fier. The very act of charging a rotating cup viscometer breaks down the internal structure so that the exact yield point can- not be measured. The only way properly to do this, would be to charge the viscom- eter with the material under examination and to allow it to rest at some precise tem- perature for a specified length of time. If then the viscometer be started, a true yield value can be obtained. It is accepted that by using a variety of speeds one can then ex- trapolate the curves back and get a yield value on which some reliance could be placed. But it has to be appreciated that in doing this, the emulsion itself can be put under stress which can bring about an alteration in the internal structure of the emulsion, while multiplication of determinations takes time even though one may have a recording attachment to the viscometer. Of the viscometers in which this disability is least apparent,is the Brookfield which I gather is extremely popular in the United States. Especially is this true when the bar plunger is employed because this can be in- serted into the product with little disturbing of the texture of the cream. But even in this case, unless the material can be set up quickly the time factor arises. Because of its general design, repeat determination can be made on the same sample if the container is of significant area and the bar plunger can be placed at different locations. On the other hand, with any given product, one can by-pass all this by inserting a disc suspended by a torsion wire into a cylindrical container filled with the emulsion and then rotating the cup at a slow, constant speed and measuring the torsional deflec-

94 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tion at equilibrium. It has been found that with each emulsion there is a minimum speed of rotation which will produce in a given time (ten minutes) a torsional deflection which is reproducible and characteristic of that emulsion when it has been made correctly and from good quality materials (9). For one particular preparation, it was found that when the cup is rotated at four r.p.m., the torsional deflection will become constant within a rota- tional time of ten minutes. Because it is known that when the preparation has been standing undisturbed for forty-eight hours at 20øC., its yield point is significantly high, this one reading can be used to measure the degree of uniformity in the product. In another emulsion of a similar type but which has much higher body, all that is needed is to use a smaller disc and here again, the characteristics have shown that the "ten minute read- ing" is a very good and true measurement of what the customer finds when the product is used, especially if, in addition, a photometer is employed to measure the transmission of light through a film 0.25 min. thick. The percentage transmission of light can be used to define the average particle size of the dispersed phase. It must, however, be pointed out that with new or unknown emulsions, it is necessary to check the yield point before reliance can be placed on the "ten minute" reading as a measure of quality. In addition, of course, steps must be taken to measure or control the temperature. Viscosity is very sensitive to temperature but interestingly enough, the temperature/vis- cosity gradient can be changed in the instances of two phase systems. By carrying out the test at 15 ø, 20 ø and 25øC., the gradient can be determined and a measure obtained of the stability of the preparation under conditions of storage involving heat and cold. It might well be mentioned that the slow rate of rotation is more akin to the speed of movement to which the product is subjected by the consumer when he or she puts a finger into the jar to withdraw some material and to smooth it over the skin. To that extent, therefore, there is a similarity between the laboratory experiment and the test of the consumer. SPREADABILITY This is a very difficult rheological feature properly to measure. One of the difficulties lies in the fact that, by and large, the user spreads the material with the fingers over the surface of the skin which is heated as it were. Just what the temperature of the surface of the skin is, is a matter for conjecture. Nevertheless, there is a heat source immediately under- neath so that with time, the preparation becomes a little warmer. It is true to say that, certainly in one or two firms, the spreading of lipstick is measured by individual operators smearing it on the back of the hand and noticing how far a uniform shiny surface can be obtained and, indeed, more or less the extent to which it can be easily spread.