The cracking of human hair cuticles by cyclical thermal stresses

Abstract

Cycles of wetting and blow-drying were applied to hair fibers and resulted in the formation of multiple cracks on the hair cuticles. The peculiarity of these cracks was that they always appeared aligned parallel to the longitudinal axis of the hair fiber. The cracks appeared to be initiated at the end of the cuticles close to the cortex and propagated invariably towards the outer cuticle edges. The maximum growth length of each crack was seen to be limited to the size of one cuticle. Crack formation did not only occur at the outer edges of the cuticles but also took place in the second and third overlaid hidden cuticle sections. The results show that these cracks form when the external portions of the cuticles undergo drastic reduction in their hydration water. Under these conditions the outer cuticle portions become rigid and brittle and crack by the action of circumferential tension stresses arising from the swelling pressure of both the cuticle layers underneath and the cortex itself. Hair cuticle analysis from a panel of 100 individuals showed that these cracks are present in the hair of people who commonly blow-dry their hair and appear to a much lesser extent in the hair of subjects who do not practice this type of grooming process. The combing of hair fibers presenting this type of cracking was seen to result in the breakage of large portions of cuticle. The effect of some cosmetic actives on the formation of these cracks is also discussed.

Full text

j. Cosmet. Sci., 49, 141-153 (May/June 1998)
The cracking of human hair cuticles by cyclical
thermal stresses
MANUEL GAMEZ-GARCIA, Croda North American Technical
Center, 180 Northfield Avenue, Edison, NJ 08832.
Accepted for publication May 15, 1998.
Synopsis
Cycles of wetting and blow-drying were applied to hair fibers and resulted in the formation of multiple
cracks on the hair cuticles. The peculiarity of these cracks was that they always appeared aligned parallel to
the longitudinal axis of the hair fiber. The cracks appeared to be initiated at the end of the cuticles close
to the cortex and propagated invariably towards the outer cuticle edges. The maximum growth length of
each crack was seen to be limited to the size of one cuticle. Crack formation did not only occur at the outer
edges of the cuticles but also took place in the second and third overlaid hidden cuticle sections. The results
show that these cracks form when the external portions of the cuticles undergo drastic reduction in their
hydration water. Under these conditions the outer cuticle portions become rigid and brittle and crack by the
action of circumferential tension stresses arising from the swelling pressure of both the cuticle layers
underneath and the cortex itself. Hair cuticle analysis from a panel of 100 individuals showed that these
cracks are present in the hair of people who commonly blow-dry their hair and appear to a much lesser extent
in the hair of subjects who do not practice this type of grooming process. The combing of hair fibers
presenting this type of cracking was seen to result in the breakage of large portions of cuticle. The effect of
some cosmetic actives on the formation of these cracks is also discussed.
INTRODUCTION
Hair cuticles represent the hair structure components most exposed to various grooming
and environmental stresses during the life of a hair fiber. The outer cuticles at the hair
surface are frequently subjected to harsh stresses such as abrasion, UV light, bleaching,
and blow-drying (1-4). It is well known that once the cuticles are one or two centimeters
away from the hair root, patterns of breakage and abrasion start to appear at the cuticle
edges (5). By the time the hair is about 14 or 20 cm long, if not enough care is taken,
the cuticles might be totally absent at the tips, giving rise to the earlier formation of
split ends (6). It is mostly because of these reasons that the effects of combing abrasion
on hair cuticles have been thoroughly studied in the past. There are, however, other
grooming practices, such as hair blow-drying, whose effects on the cuticle degradation
process are still poorly understood.
As it is well known, hair blow-drying is nowadays a common practice by many people.
When keratin fibers are heated, not only water adsorbed to hair by capilarity is rapidly
evaporated but there is also a rapid loss in the free and bound water of the hair (7,8). The
rapid loss of hydration water may cause morphological changes within the cortex due to
141

142 JOURNAL OF COSMETIC SCIENCE
destabilization of the keratin crystalline regions (9). The consequences of this type of
process in hair cuticles after cyclical thermal stresses have not yet been analyzed. This
paper represents part of a study whose aim is to reproduce patterns of cuticle damage
found in a panel of 100 individuals (10). This article reports the production of small
longitudinal cracks by cyclical thermal stresses. The cracks were mainly found in the hair
cuticles of subjects who frequently blow-dry their hair.
EXPERIMENTAL METHODOLOGY
The panel of 100 individuals participating in this study was mainly composed of women
with Caucasian brown hair never treated chemically. A total of ten fibers per each
individual was analyzed. The different patterns of cuticle damage found in these indi-
viduals such as cuticle decementation, abrasion, craters, and cracks were classified and
quantified. Cycles of mechanical tension, torsion, bending, and thermal stresses were
then applied to single hair fibers in order to reproduce such patterns (10). Only the
cyclical thermal stresses were seen to reproduce the types of cracks mentioned above, and
therefore, only the experimental conditions related to this type of stress will be described
here. The other types of damage and experimental conditions are described elsewhere
(10-12). Each thermal cycle consisted of immersing single hair fibers for a period of ten
seconds into de-ionized water followed by ten seconds of blow-drying. The temperatures
used during blow-drying and measured at the level of the wet hair surface varied
between 30 ø and 120øC. The number of applied wetting/blow-drying cycles varied
between five and 100 as required. A total of ten fibers per each set of thermal cycles was
analyzed for short longitudinal cuticle cracks. The number of these cracks per millimeter
of hair was then counted. From this data, means and standard deviations were calculated.
Prior to the thermal cycles the fibers were thoroughly washed with SLS and rinsed with
tap water.
The hair used in the single-fiber experiments was from a subject whose hair was washed
only with a 10% SLS aqueous solution for a period of one year. Sections of hair fibers
three inches long and cut close to the root were used in the experiments. The hair fibers
were subsequently cut into two snippets 1.5 inches long each; one snippet was used as
a control while the other one was subjected to thermal cycling. All selected hair fibers
presented an average diameter of 82 + ll l•m. Caucasian virgin brown hair from
International Hair Importers in the form of tresses was also used to study effects of
combing abrasion on wet/blow-dry cycled hair. Aqueous solutions of glycerin, propylene
glycol, polyquaternium ll, cetrimonium chloride, steralkonium chloride, and hydro-
lyzed wheat protein polysiloxane copolymer (13) at a 2% w/w were also used as wetting
solutions during thermal cycling. These solutions were prepared in order to test the
effect of some cosmetic actives on cuticle cracking. After thermal cycling, the fibers were
prepared for SEM analysis.
RESULTS AND DISCUSSION
SHORT LONGITUDINAL CUTICLE CRACKS FOUND IN A HAIR ANALYSIS FROM A PANEL OF
100 INDIVIDUALS
SEM analysis of hair fibers pertaining to subjects in the panel showed that most of them
presented different numbers of short longitudinal cracks in their cuticles. The popula-

CRACKING OF HUMAN HAIR CUTICLES 143
tion of these individuals could, however, be divided into two main groups, namely one
with a high number of cuticle cracks and the other with a very low number. The first
group, representing about 40% of the panel, showed an average of 184 + 15.5 cracks per
mm of hair (cpmh), while the other 60% showed an average of about 7 + 1.3 cpmh.
Figures la and lb show typical images of these cracks as found in the panel analysis.
Incidentally, the higher number of cuticle cracks corresponds to the hair of people who
manifested to blow-dry their hair on a daily basis. It was precisely this observation that
suggested the possibility of reproducing the cracks in the laboratory by cycles of wetting
and blow-drying. In Figure l it can be seen that the particularity of these cracks is their
position on the cuticles, i.e., they always appear aligned parallel to the longitudinal axis
of the hair fiber, and their length is no longer than a single cuticle size. The cracks were
found to occur more frequently at the discontinuities formed by the cuticle edges. They
seemed to be initiated near the middle section of the cuticle at that end close to the
cortex. Their direction of propagation also seemed to be towards the outer edges of the
cuticles. Figure lb shows, for instance, a long crack whose width is larger at the cuticle
end close to the cortex and very sharp at its tip towards the outer edge of the cuticle; this
crack has not yet been able to propagate all the way throughout the outer cuticle edge.
CRACK REPRODUCTION IN THE LABORATORY
After observing that the short longitudinal cracks were consistently found in a large
portion of people from the panel, a way to reproduce them in the laboratory was
researched. First, single hair fibers were subjected systematically to cyclical tension,
torsion, and bending stresses, and it was found that none of these conditions could
reproduce the cracks. The cuticle patterns of damage resulting from these stresses have
been reported elsewhere (10-12). The next step was to take fibers three inches long and
' x2.4k x4.0k
la ='" lb
Figure 1. Typical cuticle cracks found in hair from individuals who blow-dry their hair. la, x2.4k; lb,
x4.0k.

144 JOURNAL OF COSMETIC SCIENCE
I . ! litk x 13k v 2?3
-- --
7.
I .3tlkx 8kv 3•'9
2b
Figure 2. Cuticle cracks reproduced in the laboratory as follows: 2a and 2b after 20 thermal cycles; 2c after
60 thermal cycles.

CRACKING OF HUMAN HAIR CUTICLES 145
2o
Figure 2. Continued.
cut them in half. One half was used as a control, and the other half was subjected to
cycles of blow-drying and wetting at different temperatures and for different intervals of
time. The number of cracks already present in the control were counted and compared
with those appearing in the thermally cycled fibers. A total of ten fibers per each trial
were analyzed. The average number of cracks per millimeter found in the control samples
was 6 + 2.1.
After several trials it was found that short thermal cycles of ten seconds of blow-drying
at 75øC combined with ten seconds of wetting at 25øC were able to reproduce and
increase dramatically the number of short longitudinal cuticle cracks. In Figures 2a, 2b,
and 2c are shown typical images of hair fibers subjected to 20 and 60 of these cycles. A
total of 21 cracks can be counted in the 76 x 52 micrometer section of hair represented
in Figure 2b. In Figure 2c it can be seen that the total number of visible cracks reaches
a value of 26; the surface hair dimensions captured by this micrograph are approximately
67 x 42 microns. The number of cracks shown in Figures 2b and 2c correspond to an
average of 472 and 562 cracks per millimeter of hair, respectively. Such high concen-
trations of cuticle cracks are rarely found in people's hair and can only be produced in
the laboratory. Figures 3a, 3b, and 3c show higher magnifications of these cracks, while
Figure 4 shows the average number of cuticle cracks found in hair samples as a function
of thermal cycles in relation to the control. In this last figure it can be seen that the
number of cracks per unit area on the hair surface increases up to a limit and then levels
off with the number of thermal cycles.
A close examination of Figures 1, 2, and 3 reveals that, in all cases, the applied thermal
cycles resulted in the production of cracks similar to those already observed in the panel

146 JOURNAL OF COSMETIC SCIENCE
x2 4k "
3a 3b 3c.
x2.4k
Figure 3. Magnified views of cracks shown in Figure 2 as follows: 3a after 20 thermal cycles; 3b and 3c
after 60 thermal cycles. Each cycle consisted of ten seconds of blow-drying at 75øC followed by ten seconds
of wetting at 25øC.
Number of short longitudinal cuticle cracks per mm of hair
700
(a) Average number of • . -- ,
600 - cracksinha, I I L '
500 -
4OO
3O0
200 - / ' (b) Average number .
/•' ofcra•s in controls •
'100 - f (SD=2.1) '•
0 5 10 15 20 25 30 35 40 45 50 55 60
Number of applied thermal cycles
Figure 4. Variations in the average number of cuticle cracks found in the laboratory as follows: (a) in
thermally cycled fibers as the number of thermal cycles increases, and (b) in their corresponding half snippet
used as a control (non-exposed to thermal cycles). Each point represents average of cracks found in ten hair
samples. Each cycle consisted of ten seconds of blow-drying at 75øC followed by ten seconds of water
immersion at 25øC. Error bars represent one standard deviation about the mean.
analysis. This observation clearly shows that the cuticle cracks found in hair subjects
from the panel arise mainly as a consequence of subjecting hair to thermal stresses during
blow-drying. An analysis of the cracks shown in Figures 1, 2, and 3 indicates that their

CRACKING OF HUMAN HAIR CUTICLES 147
formation is mainly limited to the outer part of the cuticles. This observation suggests
that the stresses involved in crack formation are more intense at those cuticle portions
near the outer hair surface. Since blow-drying and wetting involves swelling and
deswelling of the hair fiber, the following experiment was carried out in order to test
whether the phenomenon of swelling per se plays a role in crack formation. Several
solvents with limited swelling capacity were used in the thermal cycling experiments
instead of water. The solvents were ethanol, iso-propanol, and methanol; these solvents
have already been reported in the literature as poor keratin swelling solvents (14-16).
The results showed that thermal cycling experiments with these solvents do not lead to
crack production at all, indicating that cuticle swelling is a necessary phenomenon for
cracks to occur.
Increasing both the water-swelling and blow-drying times to periods longer than ten
seconds did not have any effect on the number of produced cracks. Also, it was observed
that non-swollen hair fibers, which were thermally cycled with water immersion time
periods as short as five seconds, underwent cuticle cracking. In such short time periods
of water immersion, only the cuticular system and a small portion of the cortex can be
expected to swell. These observations indicate that cuticle cracking is not due to a
thermal shock arising from rapid changes in cuticle temperature. It seems rather that
cuticle cracking occurs because those outer cuticles sections lack elasticity to comply
with the dimensional changes either of the swelling cuticle layers underneath or of the
swelling cortex.
In the case of blow-drying, the lack of elasticity in the outer cuticle sections will
originate from rapid cuticle dehydration at high temperatures. Thus, it would appear
that when hair is wet or dried at room temperature, all cuticle portions and the cortex
contract in a synchronous manner. However, if during a water evaporation process only
the outer cuticle sections contract more rapidly than those cuticle layers underneath or
than the cortex itself, cracking will occur. Cracking during blow-drying takes place,
thus, as a consequence of circumferential extension stresses set up on dry portions of
cuticle by the swollen pressure of both the cuticle layers underneath and the cortex itself.
It should be mentioned here that circumferential or "hoop" stresses are known to occur
in cyclindrical pipes subjected to internal positive high pressures (17). Cracks at lower
hair-swelling pressures may also occur if the cuticles lose their natural elasticity due to
weathering. This might explain why cuticle vertical cracks are found at lower hair-
surface concentrations in people who do not blow-dry their hair.
The repetitive action of cuticle "rigidization" and water swelling set up on the outer
cuticle sections by the absorption and desorption of water during thermal cycling did
not lead immediately to crack formation. For instance, it was found that before the
cracks became fully developed, they appeared first as sharp white lines on the cuticle
surfaces (see Figure 5a); then, upon further thermal cycling, the white lines turned into
full cracks. The appearance of these white lines indicates that before the cuticles crack,
the mechanical energy accumulated by the circumferential tension stresses or "hoop
stresses," is first dissipated by the formation of localized shear yield regions. This form
of mechanical energy dissipation is a very common phenomenon that takes place in
polymeric materials before they fracture (18).
Increasing the water temperature during thermal cycling to about 50øC resulted in more
diffuse and wider shear yield regions that did not turn into cracks even after the

148 JOURNAL OF COSMETIC SCIENCE
.... x2.4k x2.4k
5a 5b
Figure 5. Cuticles with shear yield regions formed before cracking takes place (5a) and with shear yield
regions produced during thermal cycling with water at 50øC (5b).
application of a high number of thermal cycles (see Figure 5b). Below this temperature,
shear yield regions and cracks were always produced. Thus, increasing the water-swelling
temperature during thermal cycling softens the cuticle proteins, preventing the shear
yield regions from becoming full cracks. It should be mentioned here that those cuticles
cracked thermally were seen to be easily broken during hair combing. For instance,
Figure 6 shows a hair fiber from a tress that has been subjected to thermal cycles
followed by combing. This micrograph shows that the removal of cuticles by abrasion
occurs mainly at the cracked sites.
EFFECTS OF BLOW-DRYING AND WATER TEMPERATURE
The temperature at which air from the blow-dryer reaches the hair surface seemed to be
crucial in the incubation and propagation of thermal cracks. In the trial experiments it
was observed that the average number of cracks produced for a particular number of
cycles was maximum when the hair surface temperature was maintained for about ten
seconds between 75 ø and 95øC. In Figure 7 is shown the average number of cracks as a
function of air temperature at the wet hair surface. In this figure it can be seen that
temperatures lower than 50øC do not increase the average number of cracks already
present in unexposed hair, while temperatures higher than 95øC lead rather to hair
surface and bulk distortion. It is quite plausible, thus, that temperatures lower than
65øC do not produce the critical rate of water evaporation needed for the top part of the
cuticles to contract and become rigid, while temperatures higher than 85øC might
soften the cuticle proteins, releasing, thereby, the mechanical stresses by viscous flow.
The temperature rate used during thermal cycling was found also to be an important
parameter. For instance, if the hair surface temperature was increased at a very slow rate,

CRACKING OF HUMAN HAIR CUTICLES 149
2.kx {}81
Figure 6. Typical combing abrasion pattern of hair with cracked cuticles.
i.e., 10øC per minute up to 75øC, crack formation was almost nil. In contrast, if hair
samples at 25øC were suddenly exposed to a temperature of 75øC, they immediately
showed an increase in the number of new thermal cracks. Thus, high temperature rates
cause cracks because they produce a rapid contraction of the top part of the cuticles while
maintaining in a swollen state both the cuticle layers underneath and the cortex itself.
Conversely, low temperature rates do not cause cracking because cuticles and cortex
contract in a more synchronous manner.
It is worth mentioning here that cracks similar to those produced thermally were also
formed in fibers swollen with mixtures of strong swelling solvents and strong dehy-
drating solutions. As is well known, formic acid is a strong swelling solvent because it
is able to break hydrogen bonds and salt bridges not accessible to water and to cause
higher levels of swelling in keratin fibers (19,20). In contrast, solutions of saturated
NaCI are known to dehydrate the fibers (21). Experiments with mixtures of formic acid
and saturated solutions of NaCI (50/50%), or mixtures of formic acid and glycerin
(20/80%), were seen to produce similar cracks on the cuticles (see Figure 8). The cuticle
cracks were observed to appear after ten hours of fiber immersion in these solutions.
Also, it was observed that if a thin film of gold is deposited onto a clean hair fiber and
then allowed to swell in water, that part of the cuticle covered with gold shows also
vertical cracks somewhat similar to those produced thermally (see Figure 9). The ex-
planation to these phenomena is straightforward, i.e., in both cases, with the swelling
experiment and with the gold film, only a portion of the cuticle is restrained to expand
during the swelling process of both the cuticle layers underneath and the cortex, and
cracking occurs by the already advanced mechanism.

150 JOURNAL OF COSMETIC SCIENCE
Average number of short longitudinal cuticle cracks per mm of hair
600
5OO
4OO
300
2OO
lOO
20 30 40 50 60 70 80 90 lOO 11o 12o
Air temperature (C)
Figure 7. Plot of average number of cuticle cracks reproduced in the laboratory vs blow-drying tempera-
ture. The number of applied thermal cycles was kept constant at 20 cycles, and the air temperature was
measured at the level of the hair surface. Error bars represent one standard deviation about the mean.
Figure 8. Cuticle cracks produced after immersing a hair fiber in a 50/50% solution of saturated NaCI and
formic acid for ten hours.

CRACKING OF HUMAN HAIR CUTICLES ! 5 !
9b
• x2.4k
Figure 9. Fiber with a thin gold film deposited on its surface before water immersion (9a) and after water
immersion (9b).
EFFECT OF SOME COSMETIC ACTIVES
When 2% w/w aqueous solutions of glycerin and propylene glycol were used during
thermal cycling instead of water, the hair cuticles did not show any increase in the
average number of cracks characteristic of virgin hair fibers. This observation suggests
that during thermal cycling these actives retard water evaporation and also are able to
plasticize the cuticles, thereby preventing crack formation. Glycerin and propylene
glycol could, however, be easily removed from the hair fiber by simple water rinsing, and
under these conditions the cuticles cracked again. The use of 2% aqueous solutions of
various quarternaries instead of water during thermal cycling did not indicate any cuticle
crack prevention at all. The quaternaries analyzed were as follows: polyquaternium 11,
cetrimonium chloride, and steralkonium chloride.
The deposition of four alternating layers of a positive polymer (polyethylenimine) and a
negative polymer (polyacrylate) on the hair surface was not capable of preventing crack
formation. For instance, in Figure 10 it can be seen that cracks still formed, both on the
hair cuticles and also on the deposited polymer layers. Other substances that did not
prevent crack formation when deposited onto the hair surface were oils such as triglyc-
erides, silicon oils, mineral oil, and petrolatum. In contrast, an aqueous solution of
hydrolyzed wheat protein polysiloxane copolymer at 2%, used instead of water during
thermal cycling, prevented cuticle cracking. The crack prevention effect was seen to take
place even after the hair was water rinsed. This protein copolymer, which crosslinks upon
heat application, is believed to retard water evaporation and also to give a strong
cohesiveness to the cuticles, thereby preventing thermal crack formation.
CONCLUSIONS
Hair blow-drying produces cuticle cracks that can be reproduced in the laboratory by the
application of alternating cycles of hair wetting and blow-drying. The cracks were seen
to result as a consequence of circumferential tension stresses imposed on the dried
portion of the cuticles at the top by the swollen cortex. The temperature range at which

152 JOURNAL OF COSMETIC SCIENCE
I . ::-:kx ;kv
Figure 10. Cracks formed on cuticles of hair treated with four alternating layers of polyethylenimine and
polyacrylate.
these cracks seem to take place is between 75 ø and 95øC. It was also shown that the
combing of hair with cracked cuticles results in the removal of big portions of cuticle.
The prevention of crack formation by the use of some cosmetic actives was shown to be
possible.
ACKNOWLEDGMENTS
The author wishes to thank Ann Harder for her valuable technical assistance and Herb
Eldestein for his helpful discussions.
REFERENCES
(1) R. Robbins, Chemical and Physical Behavior of Human Hair, 3rd ed. (Springer-Verlag, New York, 1994),
pp. 211-226.
(2) V. N. E. Robinson, A study of damaged hair, J. Soc., Cosmet. Chem., 27, 155-161 (1976).
(3) M. L. Garcia, J. A. Epps, R. S. Yane, and L. D. Hunter, Normal cuticle wear patterns in human hair,
J. Soc. Cosmet. Chem., 29, 155-178 (1978).
(4) E. Hoting, M. Simmermann, and S. Hiltherhaus-Bong, Photochemical alterations in human hair. I.
Artificial irradiation and investigations of hair proteins, J. Soc. Cosmet. Chem., 46, 85-99 (1995).
(5) J. A. Swift, The critical determination of fine changes in the surface architecture of human hair due to
cosmetic treatment, J. Soc. Cosmet. Chem., 23, 695-702 (1972).
(6) J. A. Swift, Fine details on the surface of human hair, Int. J. Cosmet. Sci., 13, 143-159 (1991).
(7) P. Milczarek, M. Zielinski, and M. Garcia, The mechanism of stability of thermal transitions in hair
keratin, Colloid Polym. Sci., 270, 1106-1115 (1992).

CRACKING OF HUMAN HAIR CUTICLES 153
(8) H. Ito, T. Miyamoto, and H. Inagaki, States of water sorbed on wool as studied by differential scanning
calorimetry, Text. Res. J., 57, 66-72 (1987).
(9) A.R. Haly and J. W. Snaith, Specific heat studies of various wool-water systems, Biopolymers, 6,
1355-1377 (1968).
(10) M. Gamez-Garcia, Morphological Changes in Human Hair Cuticles upon the Simultaneous Action of Cyclical
Mechanical and Thermal Stresses: Their Relevance to Grooming Practices, presented at the Annual Conference
of the Society of Cosmetic Chemists, New York, December 1997.
(11) M. Gamez-Garcia, Cuticle de-cementation and cuticle buckling produced by circumferential com-
pression stresses on the cuticular envelope of human hair (in preparation).
(12) M. Gamez-Garcia, Plastic yielding and fracture of human hair cuticles by cyclical torsion stresses (in
preparation).
(13) Hydrolyzed wheat polysiloxane copolymer, a Croda product with the name of Crodasone W.
(14) E. I. Valko and G. Barnett, A study of the swelling of hair in mixed aqueous solvents,J. Soc. Cosmet.
Chem., 3, 108-117 (1952).
(15) L. A. Holt, Wool treatments from non-swelling solvents, Proc. 7th Int. Wool Text. Res. Conf Tokyo, Vol.
IV, 1985.
(16) J. H. Bradbury and J. D. Leeder, Sorption of liquids by wool. Part I. Determination of the sorbate
content by a new technique, J. Appl. Polym. Sci., 7, 533 (1963).
(17) W. A. Nash, Strength of Materials, Schaum's Outline Series (McGraw-Hill, New York, Chap. 3), pp.
40-53; also see F. Ziegler, Mechanics of Solids and Fluids (Springer-Verlag, New York, 1981), pp.
88-91.
(18) C. B. Bucknail, "Deformation Mechanics in Glassy Polymers," in Toughened Polymers, C. B. Bucknail,
Ed. (Applied Science Publishers Ltd, London, 1977), p. 136.
(19) N.J. Abbott, S.C. Temin, and Chungi Park, The stress-strain behavior of wool in various swelling
media, Text. Res. J., 38(10), 1026-1039 (1968).
(20) J. D. Leeder, The cell membrane complex and its influence on the properties of the wool fibre, Wool
Science Review 63 (Pub. Int. Wool Secretariat Dev. Center, October 1986).
(21) P. Alexander and R. F. Hudson, in Wool, Its Chemistry and Physics, P. Alexander and R. F. Hudson, Eds.
(Reinhold New York, 1954), pp. 218-219.