PENETRATION OF OILS INTO HAIR FIBERS 285 fabricated 3.8-cm-long brass cylinder with a diameter of 0.8 cm by pulling the hair tress through the cylinder. The excess of hair fibers was removed. From the packed hair assembly a single hair fiber was partly pulled out in the root-to-tip direction from the middle of the cylinder, and then attached to a Kevlar monofilament. The Kevlar mono­ filament was subsequently attached to a recording electrobalance (TRI-Scan) via a stain­ less steel hook. The force of withdrawal (mg) was recorded while the TRI-Scan stage was moving downward at a rate of 0.02 mm/sec. From the curve of force versus distance moved, an average force was calculated. For each oil treatment, measurement was re­ peated on ten specimens, i.e., pulling out ten single fibers, one at a time, from each packed hair assembly. Interfiber adhesion measurements were repeated after keeping the packed cylinder at a constant temperature and humidity (21 ° C and 65% RH) for 24 hours. A third set of measurements was performed after additional heat treatment using a blow-dryer at medium heat for five minutes. Warm airflow was directed on both cylinder walls and its openings. Even though this procedure is not highly reproducible in terms of the temperature experienced by the hair assembly compared to a constant­ temperature oven, we used this procedure mainly because of its use in practice. MEASUREMENT OF ANGULAR REFLECTED LIGHT PROFILE The goniophotometer (GP) was used to record the scattered light intensity as a function of the angle. The light source was a He-Ne laser of 632 nm. The measurements were performed on single Indian hair fibers placed horizontally in the sample holder at an angle of incidence of 45°. The scale angle a was calculated from the GP curves of fiber in the root-to-tip (R-T) and tip-to-root (T-R) position, given by: a = (0TR - SRT)/4, where 0TR and 0RT stand for the angle of specular peak appearance for tip-to-root and root-to-tip positions, respectively (4). Measurements were performed on the same fiber as in the case of adhesion measurements, which were done on the same hair assembly, directly after oil application, 24 hours after oil application, and with short-term heat treatment using a blow-dryer. A thin oil layer was applied by moving the fiber through an oil drop at the tip of a needle. Additional scans were recorded after removing the remaining surface oil by dissolution, moving the fiber through a drop of acetone, as described previously. RES UL TS AND DISCUSSION EFFECT OF VARIOUS OILS ON INTERFIBER ADHESION Fiber withdrawal forces for hair treated with various oils are shown in Table I. Compared to untreated Indian hair, the withdrawal force is increased by a factor 3 to 9 for all oils used in this study. The withdrawal force of a fiber from a bundle is a function of the number of points of contact and the normal forces acting (laterally) at these points. Upon oil application, liquid bridges are formed between fibers and additional normal force arises from the negative Laplace pressures of these liquid bridges (capillary adhesion). The magnitude of the adhesive force is a product of the Laplace pressure of the liquid bridge and its area. Comparing the initial withdrawal forces (i.e., directly after oil application) with forces measured after 24 hours, we observe a decrease of about 20% for mustard and olive oil.
286 JOURNAL OF COSMETIC SCIENCE Table I Average Withdrawal Forces with 95% Confidence Levels for Various Oil Treatments Calculated from the Dynamic lnterfiber Adhesion Measurement Initial Force (mg) Force (mg) Treatment force (mg) after 24 hr after 24 hr w/heat Untreated 29.6 ± 8.1 Coconut oil 116.2 ± 7.7 103.7 ± 9.6 63.9 ± 10.4 Mineral oil 107.4 ± 2.8 110.5 ± 1.7 108.1 ± 10.9 Sunflower oil 184.6 ± 24.3 108.3 ± 13.2 83.9 ± 12.9 Ricebran oil 109.4 ± 2.0 109.8 ± 1.8 Mustard oil 287.3 ± 27.4 221.2 ± 19.3 Sesame oil 203 ± 58.7 192.2 ± 85.5 Olive oil 264.0 ± 45.0 192.0 ± 61.0 For these oils, results showed considerable fiber-to-fiber variation, giving a large stan­ dard deviation. The reason for this could be the change in the area and thickness of the oil films, leading to uneven oil distribution on the fiber. There is no significant change in withdrawal forces for mineral, sesame, and ricebran oil, whereas for coconut oil and sunflower oil, forces after 24 hours have decreased by 10% and 40%, respectively. Further discussion will mainly focus on mineral, coconut, and sunflower oils, as these oils are found to give highly reproducible results representing different penetration charac­ teristics. For these oils, results are summarized and presented graphically in Figure 1. For mineral oil, the withdrawal force remains unchanged even after heat treatment, sug­ gesting that the nature of the oil film and the associated capillary adhesion remain unchanged. The difference in the initial adhesion force between sunflower, coconut, and mineral oil-treated fibers could be due to the different cohesive properties. The with­ drawal forces for coconut oil and sunflower oil after heat treatment are further decreased by approximately 40% and 20%, respectively, compared to the force measured after 24 250 Initial 24 hrs 200 D 24hrs w/heat s 150 0 u. 100 l! "0 50 o Coconut Mineral Sunflower Figure 1. Average withdrawal forces for Indian hair tress treated with coconut, mineral, and sunflower oils, initially, after 24 hours, and with additional heat treatment.
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