452 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Only "single point" determinations can be •nade with the apparatus in this form. To study non-Newtonian flow behaviour the constant speed motor is replaced by a balance pan on which different weights can be placed. The time (t) taken for rupture over a wide range of pan loadings (mg) is determined. A plot of mg against 1/t gives a curve characteristic of non-Newtonian flow (Fig. 2). Under these conditions the viscosity equations for the linear portions of pseudoplastic (%), and plastic flow (•p,), curves are (T + c)' mg 4.6 t % = r:R4 (X) and (T + c)2 (1rig - i) 4.6 t qp, = r:R 4 (XI) where T is the sample thickness, c is a constant with a value which is determined by trial and error, R is the radius of the metal plate, and i is the intercept on the 1rig axis in the mg - 1/t plot. The consistency of semi-solid materials may be examined also by shear- ing a thin layer between two parallel vertical plates (17), one of which is stationary while movement of the other is activated by a loading device. PARALLEL PLATE VISCOELASTOMETER (18) A rectangular block is cut with the sampling device shown in Fig. 5a. It is fitted with a plunger for easy removal of the sample, which is then cut into two equal halves. The pieces S• and S2 are placed on either side of a thin ribbed metal plate D (Fig. 5b) and then introduced between two ribbed plates A and B. Plate A is stationary, but B can be brought into contact with the upper surface of S• by carefully tightening with the screw device. One end of D is connected to a balance pan W by a cord which passes over two low-friction pulleys P and P,. When W is loaded S, and S• are subjected to shear, the magnitude depending on the weight used. The surfaces of S, and S• in contact with A and B remain stationary, while the maximum shear is experienced by the surfaces in contact with the two sides of D. Shear in this latter region depends on the rate at which D moves this is followed by a travelling microscope focussed on the knife edge K. To ensure minimal breakdown of sample structure, the maximum movement of D should not exceed a few mm. If temperature control is desired the apparatus can be mounted in a box made of polystyrene foam 1{--2 • thick, which contains a heating or cooling unit.

TECHNIQUES FOR ASSESSING RHEOLOGICAL PROPERTIES 453 I I Figure 5a Creep Apparatus A,B,D, Ribbed plates. C.C•. Constant compression devise. S.S•. Samples. P.P•. Pulleys (Low friction). •r. Weight pan. K. Knife edge. ( Figure 5b Sample Cutting Device. Solid materials which are essentially disperse systems give a creep compliance- time curve indicative of viscoelastic behaviour (Fig. 6), i.e. they behave like solids in the initial stages of shear, and subsequently they exhibit fluid behaviour "in the sense that work of shearing deforma- tion is not completely conserved, as in solids, nor is it completely dissipated as in fluids" (19).