j. Soco Cosmet. Chem., 37, 111-124 (May/June 1986) Measurement of combing forces Y. K. KAMATH and HANS-DIETRICH WEIGMANN, Textile Research Institute, Princeton, NJ 08542. Received April 22, 1985. Dedication This paper is dedicated to Proj•ssor Helmut Zahn on the occasion of his 70th birthday. Synopsis A double comb method has been developed for the determination of combing forces for hair tresses, and the effects of environmental humidity have been studied. The origin of the midlength and end-peak forces is discussed, and the contribution of comb-hair friction to the midlength force has been shown to be small as compared to fiber-fiber interactions. No significant effect of environmental humidity on midlength and end-peak combing forces has been observed. On the other hand, wetting the tress results in a drastic increase in the midlength force and a concurrent decrease in the end-peak force. It appears that surface tension forces involving the liquid film between fibers are largely responsible for this behavior. INTRODUCTION Combing is an important method of grooming human hair, basically involving paral- lelization of hair fibers. The use of liquids (such as hair oils), which form films on the hair surface, as grooming aids facilitates the process of combing. These liquids play the dual role of holding the fibers together by surface tension forces and lubricating their surface, thus reducing interfiber and comb-fiber friction. One way of characterizing a hair assembly in terms of the ease with which a comb can traverse through it is to measure the force and the energy required to pull a comb through the assembly. Such combing parameters would permit the evaluation of hair care products that are intended to improve combability. One of the early attempts to measure combing forces for a hair assembly was made by Newman et al. (1). The method basically involved the measurement of the force re- quired to pass a comb through a hair assembly and was principally used to compare the conditioning effects imparted by different commercial formulations. Although factors such as comb-hair and hair-hair friction which contribute to the combing force were discussed, the nature and significance of the combing forces obtained in these measure- ments were not explored in any detail. An indirect method was developed by Waggoner and Scott (2) in which the sound generated during combing was amplified and used as a measure of conditioning and ease of combing. First attempts to explain the nature of the combing 'force curves obtained during the passage of a comb through a hair as- 111

112 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS sembly were made by Tolgyesi and co-workers (3), who used a method similar in prin- ciple to that of Newman et al. (1). During the passage of the comb through the hair assembly, the force remained at a low steady level for the most part but suddenly increased to a peak value as the comb approached the free end of the hair assembly. Tolgyesi referred to these forces as midlength (ML) and end-peak (EP) forces, respec- tively. The fundamental processes that contribute to the midlength force are hair-hair and comb-hair friction and the force required to compress small fiber bundles into the spaces between the teeth of the comb. The end-peak force is produced as the result of entanglements of the fiber ends with one another. Tolgyesi and his co-workers clearly demonstrated the association of free fiber ends with the EP force by combing a hair tress made of two different fiber lengths, in which case two force peaks were obtained: one related to the short fibers and the other to the long ones. More recently Garcia and Diaz (4) used this technique to characterize the effectiveness of various conditioning treat- ments in improving combability. Although a considerable amount of work has been done in this area from the point of view of product development, no detailed study of the method dealing with the effects of environmental conditions on the combing forces is available in the literature. There- fore, an attempt has been made to study the effect of various environmental factors. Furthermore, the method has been modified to minimize the effect of entanglements and to maximize the effect of surface treatments on combing forces. EXPERIMENTAL MATERIALS All measurements were made on European dark brown hair supplied by De Meo Brothers of New York. Hair samples were cleaned either by Soxhlet extraction with methanol/chloroform/methanol, in that order, or by washing with a 2% solution of sodium dodecyl sulfate. In both cases, the samples were subsequently rinsed thoroughly in distilled water, dried, and conditioned at 65% RH and 21øC. Tresses were built from these samples by gluing the fibers to a plastic tab. Care was taken to preserve the natural curl of the hair, and fibers were aligned prior to gluing to minimize the forma- tion of severe tangles. Unless mentioned otherwise, the fiber ends were anvil cut with a razor blade. The tresses were about 25 mm wide and 200 mm long, containing about 2- 2.5 g of hair. METHOD I Two different methods have been used to determine combing forces in this investiga- tion. In the first method the hair tress was mounted on the crosshead of an Instron tensile tester as shown in Figure 1. A hard rubber comb with 16 teeth/25 mm was mounted on the load cell. Only 25 mm of the center part of the comb was used for insertion of the hair tress and this area was marked with adhesive tape. A smooth retaining bar was used to prevent fibers from slipping out during the measurement. The tress was combed to remove severe tangles and was then introduced into the teeth of the comb. The load cell was preloaded, and the reduction in load was measured as the tress was pulled through the comb at a rate of 8.3 mm/s. An environmental chamber was used to control temperature and humidity during the measurements.