j. Soc. Cosmet. Chem., 44, 69-87 (January/February 1993) Effects of some oils, emulsions, and other aqueous systems on the mechanical properties of hair at small deformations M. GAMEZ-GARCIA, Crod• North American Technical Center, 180 Northfield Avenue, Edison, NJ 08837-3873. Received August 17, 1992. Synopsis The changes on the forces to 1% deformation, F(1), and short-term stress relaxation, IR, of single hair fibers during and after their immersion in some oils, emulsions, and other aqueous systems are studied. Upon immersion, F(1) is seen to increase or decrease with time, depending on the available water partitioning characteristics of the immersion system. The recovery behavior of F(1) after de-immersion is also typical of the immersion system and in most cases takes place in a two-stage time-dependent fashion. It is also shown that the soft feel, which for several hours characterizes wet hair, results from a water de-immersion recovery pattern with the following features: a first stage where 85% of F(1) is recovered rapidly in about 5 or 10 minutes, and a second stage of slow recovery where the hair's water-enhanced plasticity decays in about 10 to 12 hours, Furthermore, it is shown that this behavior and the short-term relaxation characteristics of the fibers in general can be modified if some of the solutes remain on the hair shaft. The results suggest that these modifications can be accounted for by three different plasticizing mechanisms: swelling, chaotropicity, and film-water inclusion/exclusion plasticity. INTRODUCTION It is customarily accepted by most cosmetic scientists and technicians that a good conditioning of hair is obtained when its optical reflectivity, charge neutralization, and mechanical stiffness are somehow improved (1-3). Mechanical conditioning, the topic of this paper, involves strains within the so called "Hookean region" and at deformations not greater than 1%, where the fiber has been reported to behave viscoelastically (4-7). Because of the hair's viscoelasticity at small deformations, one would anticipate that its mechanical conditioning as defined by Robbins (8) will depend mainly on the combi- nation of two properties, namely: 1) its response to rapidly varying strains, and 2) its response to semipermanent or very slowly imposed strains. A thicker fiber will, for instance, oppose a stronger elastic resistance to the rapid strains of combing or natural movement than will a thin one, even when the fibers have the same elastic modulus. However, if the thick fiber relaxes too rapidly to strains of longer duration, such as those imposed during styling, or to the strains of its own weight, it 69

70 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS will give the sensation of limpness after it has been styled. Conversely, if the fiber stress relaxes rather too slowly, its behavior upon styling will be that of a rubber fiber with the sensation of difficulty in manageability. Measurement of hair stress relaxation should, thus, be helpful in complementing other parameters such as interfiber friction and bending modulus on the evaluation of con- ditioning agents. The relevance of stress relaxation in conditioning studies becomes evident when it is considered that the compression and extension stresses that compose bending will vary with time unavoidably as the hair's matrix phase relaxes viscoelasti- cally after styling. Information on hair stress relaxation at small deformations and chemical deposition, solvent swelling, or salt penetration is, however, incomplete or limited to wool shrink- age studies (9-12). For instance, most of the studies on plasticization of keratinous fibers at small deformations have been carried out using water vapor as the only plasticizer (13-18). Also, the abundant data on fiber swelling by water (19-25), solvents (26-28), and other salt solutions does not include stress relaxation information (26). Furthermore, studies of integral and interval water absorption, and free and bound water, have given some attention to the occurrence of stress relaxation but only as a diffusion-assisting mechanism with no external mechanical stresses applied to the fiber (18,29-38). Before initiating the experiments, it was considered that hair stress relaxation studies can only be valuable for cosmetic purposes if they at least take into account the following phenomena: 1) after washing or conditioning the hair undergoes a short drying period where its diameter, length, stiffness, and stress relaxation are out of equilibrium for several hours 2) any study of hair mechanical conditioning involving stress relaxation has to take into account long-term equilibrium hysteresis effects and 3) since the mechanical constraints to which hair is subjected change on a daily basis, the choice of experimental observation time periods for the fiber to relax has to be of practical value. In this part of the research the effects of simple aqueous systems on the 1% extension forces are analyzed during and after fiber immersion. The after-treatment effects on the short-term relaxation of hair as it dries and goes through an equilibrium moisture hysteresis cycle are also analyzed. EXPERIMENTAL CONDITIONS Single-hair relaxation-recovery experiments were carried out at room temperature using a Dia-Stron© rheometer. The hair length used was 50 mm, and the deformation was set at 1%. The force required to extend the fiber by 1% at a rate of 40 mm/min is called F(1). Prior to any experimentation the fibers were washed with a 5% active TEALS water solution and rinsed thoroughly with de-ionized water. Hair tresses were virgin brown De Meo quality from these a careful selection of fibers having a diameter of 96 --- 3 •m was made. The fiber diameters were measured only at 60% RH at equilibrium condi- tions by microscopy (25). The chamber housing the Dia-Stron was equipped with polyethylene gloves that per- mitted the immersion of fibers into hermetically closed reservoirs containing the liquids under study. The rapid changes in F(1) and fiber length upon immersion were moni-