MECHANICAL PROPERTIES OF HAIR 71 tored by several 1% straining cycles at 40 mm/min and stored in the computer memory as force-deformation data. Moisture hysteresis cycling measurements of F(1) and its decay, as well as fiber length variations, were carried out throughout the following equilibrium relative humidities: 10%, 40%, 60%, 79%, and 87% RH. CALCULATION OF THE RELAXATION DATA The method used to follow the short-term relaxation behavior of hair while at moisture equilibrium or in the process of drying consisted in imposing on it a constant strain of 1% during a period of five minutes. The forces opposed by hair both at the beginning, F o, and at the end, F 5, of the experiment were then registered. Previously, in a separate experiment, the same fiber was immersed in de-ionized water and measurements of the force needed to produce 1% extension, F w, were made. The percentage of relaxation undergone by the fiber during the five-minute period, IR, or index of relaxation was then calculated as follows: IR = 100X(F o - F5)/(F o - Fw). The recovery period before starting another experiment with the same fiber was 30 minutes. It should be pointed out that five minutes of stress relaxation were chosen because the force decay in this period was found to be sensitive to variations in the moisture content of hair produced either by environmental humidity changes in steps of 8% or by the loss of water as the fiber was drying after de-immersion. The effects of each immersion system on F(1) and IR were carried out in at least five fibers presenting the same diameter. These fibers were mechanically selected from a group of 20 by discarding those whose F(1) and IR values at 10% and 87% RH equilibrium conditions differed in more than one standard deviation i.e., 21.04 +- 0.13 and 9.20 +- 0.22 for F(1) in gm force, and 11.03 +-- 0.16 and 19.3 +- 0.34 in percentages for IR, at 10% and 87% RH, respectively. Measurements on a single fiber were performed five times their standard deviation was found to be no higher than 0.3% and 2% for those carried out under moisture equilibrium and non-equilibrium conditions, respectively. RESULTS AND DISCUSSION IMMERSION OF HAIR IN OILS Figure 1 shows the variation of F(1) immediately after the hair was immersed in four different oils, namely, cetyl palmitate, wheat germ oil triglycerides, and, 200 cSt and 350 cSt polydimethylsiloxane oils. In this figure it can be seen that the variation in F(1) was dependent upon the conditioning history of both hairs and oils. For instance, if the fiber was equilibrated at 87% RH and the oils at 10% RH, the hair showed length contractions and increments in F(1) that later decayed slowly as time elapsed. However, if the fiber was equilibrated at 10% RH and the oil at 87% RH, then upon immersion, the fiber initially presented increments in length and decrements in F(1). In both cases if enough time was allowed to elapse with the fiber submerged in the oils, this tendency eventually reversed and the length and F(1) returned to their initial values before immersion. When hair fibers and oils were equilibrated at the same moisture

72 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ERCENTAGE OF Fill VARIATION -t00 0 i I i I i I ! I I i I I I I I I I I I õ to 20 25 to 40 45 50 TIME IMINUIES) Figure 1. Percentage of F(1) variation as a function of time as follows: of hair conditioned at 87% RH and immersed in cetyl palmirate, (1) wheat germ oil, (2) polydimethylsiloxane 200 cSt, (3) and 350 cSt, (4), all conditioned at 10% RH of hair conditioned at 10% RH and immersed in wheat germ oil, (5) polydimethylsiloxane 350 cSt, (6), both conditioned at 87% RH. level, no variations were detected in the force and length values either upon immersion or after de-immersion. Hysteresis cycle measurements after de-immersion showed that the oils have no signif- icant influences on the equilibrium values of F(1), IR, and length dimensions. An unexpected observation was that fibers being submerged in the oils showed similar hysteresis values upon variation of the environmental moisture surrounding the immer- sion vessel. These observations indicate that hair fibers sense moisture changes while immersed in the oils. The equilibrium values of F(1)s, relaxation indices, and length dimensions took, however, about 70 hours to be attained. IMMERSION OF HAIR FIBERS IN DIFFERENT AQUEOUS SOLUTIONS The F(1)s, IRs, and changes in length at equilibrium were measured for hair fibers and immersed and de-immersed in the following aqueous solutions at different concentra- tions: de-ionized water, LiC1, CaCI2, NaC1, MgCI2, urea, citric acid, lysine, silk amino acids, 1 and hydrolyzed wheat proteins and wheat oligosaccharides. 2 Silk amino acids is a Croda product under the trade name of Crosilk liquid. Hydrolyzed wheat protein and wheat oligosaccharides is a Croda product under the trade name of Cropeptide W.