Here's a study that I helped author in 1995 regarding the physical properties of African American hair and how it differs from Caucasian hair. Enjoy.
Scientific innovations in chemical treatments that alter the texture of African-American hair, as well as a plethora of style trends, have spurred phenomenal growth in this
segment of the hair-care market throughout the past three decades. The level of research & development by manufacturers in the area of product formulation has steadily elevated; yet, a great deal remains to be learned about the unique physical properties of African American hair. Compared to the vast body of research regarding Caucasian hair, the study of African-American hair is at best limited.
A Review of Research
In published studies, J. Menkart et al, Epps et al and Kamath et al reveal that some important research has indeed been conducted on the physical properties of highly curly hair. The efforts of these researchers and the results of their studies serve as a starting point for the further study of African-American hair. Other general observations about hair, such as those pertaining to static charge (Jachowicz and C.R. Robbins) and theories about moisture content, serve as a springboard for advancing the study of African American hair.
Shape: Menkart and Wolfram report that African-American hair has a physical shape resembling a twisted oval rod, whereas Caucasian hair is more cylindrical. They found evidence of this when they made elliptic comparisons or hair cross-sections. Using a formula in which the minor axis is divided by the major axis, Menkart and Wolfram
determined that African-American hair has a ellipticity index of 0.56 and Caucasian hair has an index or 0.7.1. The tensile-strength data gathered during this research shows that the breaking stress of African·American hair (1.24 g/denier, a unit of fineness equal to the fineness of a yarn weighing 0.05g for each 450m of length or 19 for each 9000m.) is less than that of Caucasian hair (1.41 g/denier). The yield stress for African-American hair, however, is slightly higher (0.46 g/denier VS 0.42 g/denier).
Kamath and Hornby studied the fractographic behavior of African-American hair to view both major and minor rods at low levels of extension. They examined the ellipticity of
hair fibers and reported ellipticity indices of 1.89±0.083 and 1.0 to 1.4 for African-American and Caucasian hair, respectively. In this study, the ellipticity index was calculated by dividing the major axis by the minor axis, which is the reverse of the formula used by Menkart and Wolfram.
Combability: Epps and Wolfram conducted combing comparisons between African-American and Caucasian hair using combability techniques used by Garcia and Diaz. Garcia and Diaz report that African-American hair, due to its curliness, is much more difficult to comb than Caucasian hair. It was also determined that African-American hair is easier to comb wet than dry.
Using a scanning electron microscope at a magnification of about 300X, Kamath and Hornby also observed that African American hair display frequent twists with random reversals in direction and pronounced flattening. During fractographic study, the break stress of African-American hair was reported to be (0.123 ± 0.016) 10^9 N/m^2 when dry (65% relative humidity [RH]) and (0.119 ± 0.010) 10^9 N/m^2 when wet. The breaking elongation for wet and dry (65% RH) fibers was found to be 44% ±3% and 27% ± 5%, respectively.
Jachowicz et al and C.R. Robbins discuss static-charge determination of hair fibers and the fibers positive or negative sign in detail. An examination of African-American hair,
however, was not involved in the study.
Chemists and observers in the ethnic hair-care industry generally theorize that African-American hair has less moisture (water) content than does Caucasian hair. Very little
scientific data, however, is available to substantiate this cIaim.
To expand our knowledge of the differences between African-American and Caucasian hair, we will review the findings of scientific studies we conducted. Some of our investigations pertained to properties that have already received attention from researchers, namely fiber diameter, tensile strength, and ease of wet and dry combing. To extend the body of research specific to African-American hair, we also analyzed static charge and moisture content. Comparative studies like these may help hair-care manufacturers more succinctly identify ethnic consumer needs, paving the way for refinement of existing product formulations and the conceptualization of more effective ones.
All of the African-American and Caucasian hair fibers studied in our current project were purchased from DeMeo Brothers of New York. Except where noted, all of the hair tested is untreated, virgin hair. Bulk samples of each hair type purchased may represent a blend of fibers from more than one individual.
Hair diameter via image analysis: To determine African-American hair-fiber diameter, we selected 10 hair fibers at random from the blend. Each hair fiber was singly mounted between 2 glass slides and placed under a compound microscope. The microscope was connected to a video camera and used image analysis software. We took diameter measurements of each fiber at 1 mm intervals from its root to end. We
noted the maximum and minimum diameters and divided the maximum diameter by the minimum diameter to determine an ellipticity measurement (Table 1). The ellipticity
measurements for the 10 fibers were averaged for purposes of comparison against Caucasian fibers. The same procedure was used for Caucasian hair (Table 2).
Results: Fibers of African-American hair exhibit considerable
variations in diameter. The ellipticity index averages 1.8297 ± 0.1456 for the 10 hairs studied, indicating a high degree of irregularity in the diameter of African-American hair along a single strand. This measurement is very close to Kamath's and Hornby's findings of 1.89 ± 0.083. The slight difference in the indices could be due to the fact that Kamath
and Hornby studied fibers from one individual head, whereas we used hair from more than one individual.
Caucasian hair is more regular compared with African American hair. The ellipticity index of Caucasian hair is 1.2887 ± 0.1182, demonstrating little variation between the maximum and minimum hair-shalt diameters. These findings substantiate those of Kamath and Hornby, which reflect all ellipticity index of 1.0-1.4. Figure 1 graphically illustrates the highly irregular diameter or African-American hair vs. the more consistent diameter of Caucasian hair.
Comparison of Tensile Strength Properties
To find the tensile strength of wet African-American hair, fibers were first cleansed with a 1.25% solution of sodium lauryl sulfate, rinsed for 30 minutes, allowed to dry and then
equilibrated overnight at 65% RH and 21º C. Fibers of visually similar diameters were selected and crimped at a 30.00 mm length for mounting on a Dia-Stron MTT tensile strength determination device. Each crimped fiber was individually scanned at 1.0 mm intervals along the hairshaft. The maximum and minimum diameters were determined in
the manner described above.
Since African-American hair is highly elliptical (Table 1), the area of a cross-section cannot be determined from the fiber diameter as it could if the hair fiber was cylindrical.
Therefore, we used the following formula:
A = D x d x π/4
Where A= Area of cross-section; D = Major axis;
and d = Minor axis.
We studied the breaking stress and breaking elongation of the fibers using the Dia-Stron MTT. Fibers similar in cross-sectional area were selected for stress-strain determinations
under wet conditions. While keeping them immersed in water, the fibers were mounted singly between the "jaws" of the device. The stress and elongation at break point were determined using the following formulas (Table 3):
Stress = Load/A
Stress at break point = [(Breaking load in Kg) x (g)]/ A
= [(Breaking load in Kg) x(9.81m/s^2)]/A in m^2
= Breaking stress in N/m^2
= Breaking stress in Pascals
The parameters of the device are shown in Table 4
Dry African-American hair: The same procedure was used to find the tensile strength of dry African-American hair, except that the stress-strain determination was made under dry conditions at 65% RH and 21°C (Table 3).
Again, we used the same procedure to test wet Caucasian hair except the fiber was twisted once at 180º while being mounted between the jaws of the Dia-Stron MTT to view both major and minor axes. The average stress and elongation at break point is shown in Table 5.
The same procedure was used for wet Caucasian hair as for dry Caucasian hair except the stress-strain determination was made under dry conditions (Table 5).
Results: The break stress of wet, untreated African-American hair is 0.089 10^9 N/m^2 ± 0.025 10^9 N/m^2, considerably lower than that of the Kamath and Hornby Study. Break elongation is 42.04% ± 4.9%.
The break stress of wet Caucasian hair is 0.165 ± 0.025 N/m^2, higher than that of wet African-American hair. The break elongation of wet African-American hair is 42.04% ± 4.90%, and break elongation of wet: Caucasian hair is 61.61 ± 4.1%. When compared, these values suggest that wet African-American hair does not extend as much before
breaking as does wet Caucasian hair (Tables 3 and 5).
The break stress of dry African-American hair is (0.153 ± 0.015) 10^9 N/m^2 as compared to the break stress of 0.189 10^9 N/m^2 ± 0.019 10^9 N/m^2 for dry Caucasian hair (Tables 3 and 5). Upon comparison of these values, it is clear that dry African-American hair breaks more readily than does dry Caucasian hair.
The break elongation of dry African-American hair fibers is 39.35% ± 4.9% as compared to the break elongation of 49.57% ± 4.4% for dry Caucasian hair (Tables 3 and 5). Comparing these values, it is clear that dry African-American hair does not extend as much before breaking as dry Caucasian hair. Therefore, African-American hair is more
fragile in wet and dry conditions than Caucasian hair.
Comparison of Wet and Dry Combing Properties
Wet combing: African-American hair was assembled into a 1.5 g tress and cut to 12 cm in length to test wet combing.The tress was then soaked in tap water at room temperature for 15 minutes and mounted on a Dia-Stron MTT combing device using the parameters in Table 6.
The teeth of the hard-rubber comb were spaced 2.0 mm apart. The work done (in joules) to pass the comb through the hair tress reflects an average or three combings.
The same procedure and parameters used above were used for wet combing of Caucasian hair (Figure 2 and Table 7).
Dry combing: The same procedure was used for dry combing both African-American and Caucasian hair, except that the hair tress was equilibrated and combed at 65% RH (Figure 3 and Table 7).
Results: The work of combing wet black hair is almost 5 times that of combing wet Caucasian hair (Table 5). Moreover, the work of combing is almost 50 times greater for African-American hair than Caucasian hair when dry. African-American hair is almost 8 times more difficult to comb wet than dry, The relatively high degree of work required to comb the assembly of dry African American hair is due to its extremely curly configuration and consequential entanglement. Figures 2 and 3 depict the combing curves of wet and dry hair of both types.
Comparison of Static·Charge Properties
African-American hair fibers were assembled into a 1.0 g tress cut to 20.0 cm in length. The background static charge of the tress was measured before combing. All measurements were conducted at 21º C and 38% RH. The tress was combed 10 times with a hard-rubber comb with 2.0 mm between the teeth. The tress was then placed at a predetermined distance in Trek ESD 425 Model 1 and static charge was measured. An average static charge of three tresses is shown in Table 8 and Figure 4.
We used the same procedure to determine the static charge on Caucasian hair. An average static charge of three tresses is shown in Table 8 and Figure 4.
Results: African-American hair develops a highly negative electrostatic charge (-25.4 KV/m) (Table 8). In contrast, Caucasian hair develops a very low positive electrostatic charge (+6.6 KV/m). The relatively high negative charge acquired by dry African-American hair during combing may be due to the extraordinarily high degree of pulling force required to pass the comb through the entangled hair fibers.
The combing of chemically straightened black hair results in a positive electrostatic charge (+25.9 KV/m). This reversal is thought to be attributable to a greater ease of
combing and minimal hair-fiber entanglement.
Whether untreated or chemically straightened, African-American hair develops a significantly higher electrostatic charge than does Caucasian hair. The relatively high electrostatic charges could produce a "balloon effect" and contribute to the higher degree of unmanageability.
Comparison of Moisture Contents
To determine moisture content of African-American hair, African-American hair fibers were cleansed and equilibrated to 65% RH at room temperature as in the above procedure for determining tensile strength. The root end of each hair fiber was cut into small pieces and placed in a pre-weighted aluminum pan using a Cahn Microbalance. The aluminum pan containing the hair sample was then placed in an analyzer (DSC) and a moisture isotherm was obtained using the following parameters in Table 9.
After the run is complete, the aluminum pan containing the hair sample was transferred to a desiccator for 2 hours and then weighed again using a balance. The moisture content was determined as follows:
% Moisture content = [(weight of hair sample before healing) - (weight of hair sample after heating)] / [(weight of hair before heating) x 100]
Again, the same procedure was used to determine the moisture content of Caucasian hair.
Results: The moisture content of African-American hair tends to be less than that of Caucasian hair. African-American hair contains 17.69% moisture by weight vs. 18.66% for Caucasian hair. The African-American hair has 5.20% less moisture content than Caucasian hair (Table 10).
African-American hair fibers, when compared with Caucasian hair fibers, exhibit the following properties:
• Irregular diameter along the hair shaft. This phenomenon may well be a major contributor to the comparative weakness of African-American hair.
• A relative low break stress and break elongation in both wet and dry states. Thus, African-American hair generally has less tensile strength and breaks more easily than Caucasian hair does. Thus, African-American hair tends to be the more delicate of the two hair types.
• A relatively high resistance to combing in the wet state and even more so when dry. The likelihood of effecting mechanical damage by simple grooming is, therefore, much greater for African-American hair because of its highly curly configuration.
• A high static charge when combed in a dry state. This finding indicates that normal styling and grooming can make African American hair considerably less manageable
since static causes fly-away fibers.
• A lower level of moisture. The lower moisture content of African-American hair may be a significant contributor or to its relative fragility.
The results of this research have particular relevance for chemists and new-product development specialists who work in the ethnic hair-care market. It is clear that African-American hair differs from Caucasian hair in a number of important ways, suggesting that the products formulated for Caucasian hair and sold in the general market may not
adequately address the special hair-care needs of the African-American consumer. It may well be that some of the products and practices now used in the ethnic market are inappropriate and ineffective, given that African-American hair tends to be relatively fragile and difficult to comb and as a high static charge and lower moisture content.
This research along with other studies conducted on African-American hair, make it painfully apparent that there are many more questions to be answered before we have an
adequate knowledge of this hair type, Those of us who are professionally and/or academically involved with ethnic hair are beckoned to delve even further into its unique
characteristics and how it reacts with various products, treatments, temperatures. environmental conditions and styling/grooming practices. As the body of scientific
knowledge broadens, the consumer is sure to be better served and the entire ethnic hair-care industry will be enormously enriched.