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J. Cosmet. Sci., 62, 265–282 (March/April 2011)
265
The effect of various cosmetic pretreatments on protecting
hair from thermal damage by hot fl at ironing
Y. ZHOU, R. RIGOLETTO, D. KOELMEL, G. ZHANG,
T.W. GILLECE, L. FOLTIS, D. J. MOORE, X. QU, and C. SUN,
International Specialty Products, Wayne NJ (Y.Z., R.R., D.K., G.Z.,
T.W.G., L.F.), and ISP Shanghai Global R&D, Shanghai, China
(X.Q., C.S.).
Synopsis
Hot fl at irons are used to create straight hair styles. As these devices operate at temperatures over 200 °C they
can cause signifi cant damage to hair keratin. In this study, hair thermal damage and the effect of various
polymeric pretreatments were investigated using FTIR imaging spectroscopy, DSC, dynamic vapor sorption
(DVS), AFM, SEM, and thermal image analysis. FTIR imaging spectroscopy of hair cross sections provides
spatially resolved molecular information such as protein distribution and structure. This approach was used
to monitor thermally induced modifi cation of hair protein, including the conversion of α-helix to β-sheet and
protein degradation. DSC measurements of thermally treated hair also demonstrated degradation of hair
keratin. DVS of thermally treated hair shows the reduced water regain and lower water retention, compared
to the non-thermally treated hair, which might be attributed to the protein conformation changes due to heat
damage. The protection of native protein structure associated with selected polymer pretreatments leads to
improved moisture restoration and water retention of hair. This contributes to heat control on repeated hot
fl at ironing. Thermally stressing hair led to signifi cantly increased hair breakage when subjected to combing.
These studies indicate that hair breakage can be reduced signifi cantly when hair is pretreated with selected
polymers such as VP/acrylates/lauryl methacrylate copolymer, polyquaternium-55, and a polyelectrolyte
complex of PVM/MA copolymer and polyquaternium-28. In addition, polymeric pretreatments provide
thermal protection against thermal degradation of keratin in the cortex as well as hair surface damage. The
morphological improvement in cuticle integrity and smoothness with the polymer pretreatment plays an
important role in their anti-breakage effect. Insights into structure-property relationships necessary to pro-
vide thermal protection to hair are presented.
INTRODUCTION
Hair damage from thermal treatment with styling appliances such as hot fl at irons, blow
dryers and curling irons has become an increasing concern in hair care. This is especially
true with hot fl at irons that can exceed temperatures of 200°C. Because of the growing
popularity of using high temperature thermal styling appliances, there is a need for ther-
mal protective ingredients/products and test methods to show their effi cacy. To meet this
challenge, understanding and assessing hair damage from thermal treatment is needed.
In recent decades, thermal damage of hair by curling irons has been discussed by several
publications (1–3) that have studied various effects on hair thermal damage, such as
JOURNAL OF COSMETIC SCIENCE266
moisture content, conditioners, polymers and heating modes. Changes in hair mechanical
properties, combing force and tryptophan by curling ironing treatment at 120–160°C
were demonstrated in the literature as well. High-temperature decomposition of hair
keratin has been studied by using DSC (4,5). On the other hand, the literature refl ects
limited amount of research on hair damage and protection from using hot fl at ironing at
a temperature over 200°C.
In this work, hair damage from thermal treatment was studied in different aspects by
several techniques towards understanding hair thermal damage and the protective effect
by cosmetic pretreatment. It is also our objective to understand the thermal protection
mechanism, such as the role of moisture regain of hair on controlling hair temperature
from repeated heating. Also, the alleviation of weakening of hair and the consequent re-
duction in hair breakage through combing using polymers with different functional
groups highlights the structure-property relationships important for thermal protection
effi cacy.
MATERIALS AND METHODS
POLYMERS
VP/acrylates/lauryl methacrylate copolymer, PEC (polyelectrolyte complex of methylvi-
nylether/maleic acid copolymer and polyquaternium-28 (6,7)), polyquaternium-55, co-
polymer of VP and DMAPA acrylates, and other polymers used in this study were
supplied by International Specialty Products (ISP). Hydroxyethylcellulose (HEC) was
supplied by Aqualon. These ingredients are used as supplied and not purifi ed and modi-
fi ed in any way.
HAIR SAMPLES
European dark brown hair was purchased from International Hair Importers. Each hair
tress was 1.5” wide, 3.5 g in weight and 6.5” in length of loose hair. Asian hair tresses
were supplied from a local commercial source in China made with the same specifi cations.
THERMAL TREATMENT OF HAIR
Hair tresses were hot fl at ironed by a controlled 12-minute treatment schedule. The tem-
perature of hot iron used in this work was 232°C unless specifi ed elsewhere. First, the hair
tresses were washed with 10% sodium lauryl ether sulfate (SLES) and dried with a hair
blow dryer set on hot. Then hair tresses were thermally exposed for a short (12 seconds)
intermittent heating cycle separated with SLES washing every 4 minutes for a total of 12
minutes thermal treatment. If a protective agent was tested, tresses were pretreated with
0.5 g of a 1% polymer solution for Asian hair or 0.5 g of a 1% polymer solution made
into 0.5% hydroxyethyl cellulose (HEC), after the SLES wash, then dried and followed
with hot fl at ironing. At the end of the 12-minute hot ironing, the tresses were washed
with 10% SLES again and dried for subsequent combing to quantify hair breakage. The
polyelectrolyte complex (PEC) was supplied and tested at 2%, unless specifi ed.
2010 TRI/PRINCETON CONFERENCE 267
ASSESSING HAIR DAMAGE BY PHYSICAL TOOLS
Differential scanning calorimetry (DSC). DSC was used to measure hair damage by assess-
ing hair keratin degradation and the effect of cosmetic pretreatments. DSC measure-
ments were performed on tresses after the 12-minute controlled hot ironing treatment
schedule. Two thermal parameters derived from the DSC peak were used to assess hair
damage: the denaturation temperature, Td, of the helical protein and the denaturation
enthalpy, ΔH. All hair samples were run on a Q2000 DSC (TA Instruments) at a heat-
ing rate of 2°C per minute. Between 8 to 13 milligrams of cut hair fi bres were used per
run in high volume stainless steel pans. Fifty microliters of water were added to each
pan prior to sealing. The sealed hair fi bers were hydrated in their pans overnight before
running.
FTIR spectroscopic image analysis of hair fi bers. Fourier transform infrared imaging spectros-
copy (FT-IRIS)was utilized to examine the molecular modifi cation of hair keratin from
thermal insult with and without protective treatment. This novel technique provides
signifi cant advantages of direct spatially resolved concentration and molecular structure
information for sample constituents. In this study, hair cross sections were imaged by a
Perkin Elmer Spotlight system which couples a FT-IR spectrometer to an optical micro-
scope. The system consists of a linear array mercury-cadmium-telluride (MCT) detector
and an automated high precision XY sample stage. In the FTIR images, each pixel size is
6.25μm and 16 scans were collected for each spectrum with 8μm−1 spectral resolution.
Five-micrometer-thick hair cross sections were prepared by slicing a short hair bundle
which is embedded into ice mounted on the top of a sample holder under -30°C using
a Leica CM 1850 Microtome. Hair cross sections were collected on CaF2 windows for
conducting FT-IR imaging analysis. Spectral Dimensions Isys 3.1 software was used for
data analysis and image construction. Spectral data were baseline-corrected before peak
heights and integrated area were measured.
Scanning electron microscopy (SEM). SEM was used to examine the morphological changes of
cuticle layers on the hair surface after thermal treatment with and without the protective
treatment. The Amray Model 1820 SEM was used to collect digital photomicrographs.
Four to fi ve fi bers were examined for each hair sample treatment.
Dynamic vapor sorption analysis (DVS). The sorption and desorption of water vapor on hair
were determined with a DVS Advantage-1 gravimetric vapor sorption analyzer (Surface
Measurement Systems Ltd., London, UK). The experimental temperature was 25.0 ±
0.1°C and the total N2 gas fl ow was 200 ml/min. Approximately, 40mg of hair samples
formed into a 20-30 strands of loop were loaded onto a tared quartz sample pan. The di-
ameter of the hair fi ber, which was chosen as the average of 30 fi bers (59 um), was deter-
mined using a Mitutoyo micrometer. The sorption sequence consisted of the following
steps:
1. The hair sample was initially wet at 95% RH for 1 hour.
2. The hair was dried at 25 °C and 0 %RH for 12 hours.
3. The hair samples were exposed to an isothermal humidity ramp from 0–90 % RH
followed by a 90–0% RH desorption in 10% RH steps. Each sorption-desorption step
was 4 hours in duration to approximate gravimetric equilibration.
4. At the end of each partial pressure, step points were averaged to produce an isotherm
plot, which showed the change in mass of hair samples as a function of relative
humidity.
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Atomic force microscopy (AFM).
•Specimen preparation. European dark brown hair fi bers were mounted onto a steel sample
disk using a nail polish liquid. A thin layer of the liquid was brushed on the surface of the
metal disk. When the liquid hardened into a tacky state, hair fi bers were carefully placed
on the metal disk. The liquid dries quickly to keep the hair fi bers fi rmly in place.
•Instrumentation. AFM was performed using a Mutimode Nanoscope V supplied by
Veeco Instruments, Inc (Santa Babara, CA) at ambient conditions (22°C, 50% humidity).
A sharp Nitride lever (SNL) probe combining a sharp silicon tip with a silicon nitride
cantilever was used for the topographic imaging acquisition. The nominal radius of the
tip was about 2 nm and the spring constant of the cantilever is 0.06 N/M. The scan was
fi rst carried out perpendicular to the longitudinal axis of the hair fi ber. After the tip was
centered over the cross section and located at the very top of the fi ber, the scan direction
was changed to parallel with the longitudinal axis of the hair fi ber. A scan rate of 1Hz was
used for all measurements. The data collection was set to defl ection channel and the error
signal images, which are very sensitive to the changes in height, were recorded at 15×15
and 5×5 μm2. The image data presented in this paper are raw and unfi ltered.
Hair temperature measurement during hot fl at ironing with thermal image analysis. In order to evalu-
ate the heat control effect of polymer pretreatment, hair temperature during hot fl at ironing
was measured. An infrared camera (Flir P series) was used to measure hair temperature after
hot fl at ironing with an IR beam aiming on hair. Hair tresses were hot fl at ironed from the top
of the tress to the bottom, with three 5-second strokes as one heating cycle, and the maximum
temperature was taken during the third stroke. Three hair tresses were tested for each treat-
ment and the average temperature of hair was taken from the three tresses.
ASSESSING HAIR THERMAL DAMAGE BY QUANTIFYING HAIR BREAKAGE FROM COMBING
Hair breakage is quantifi ed by combing the dried tresses that was exposed to the 12 minutes
thermal treatment and washed with 12% SLES. To do so, a translucent plastic is fi rst placed
under the tress. The tress is then combed vigorously 100 times with a fi ne-toothed comb. The
fragments of hair that are collected as a result of combing are secured by tape and numbered.
Five hair tresses were tested for each treatment and the average number of hair breakage was
taken from the tests of fi ve tresses. The % hair breakage reduction by a cosmetic pretreatment
is calculated as the number of hair pieces of control (untreated) minus the number of hair
pieces from the polymer pretreatment test divided by the number of hair pieces of the control:
CT
% Hair breakage reduction 100
C
−
=×
where C = the number of hair pieces collected for the control and T = the number of hair
pieces collected for the test.
RESULTS AND DISCUSSION
THERMAL DEGRADATION OF HAIR KERATIN FROM THERMAL TREATMENT
Hair is composed primarily of proteins. The cortex region contains the bulk of the hair
keratin fi bers. There are different types of protein components in human hair, with the
2010 TRI/PRINCETON CONFERENCE 269
organized α-helical protein accounting for about 40% of the fi ber's cross section (8) in the
fi brous cortex surrounded by the multicellular fl at cuticle sheath.
One way of showing the degradation of hair by thermal treatments is through DSC.
Figure 1 shows the DSC results of thermally treated hair at two temperatures, 205°C and
232°C. DSC yields two thermal parameters from protein thermal transition: protein de-
naturation temperature or the DSC peak temperature, Td and the denaturation enthalpy
or the area of the peak, ΔH. The results in Figure 1 show the reduction of Td and ΔH
after thermal treatment of European hair, indicating protein degradation. With the heat-
ing temperature increasing from 205°C to 232°C, Td is reduced by an additional 20 de-
gree. Also, ΔH is reduced by an additional 14.9J/g. Therefore, at higher heating
temperature, the protein degradation becomes more severe.
THERMAL PROTECTION OF HAIR KERATIN BY VARIOUS POLYMER TREATMENTS
AND THEIR ANTI-BREAKAGE EFFECT
A thermal protection route was developed aiming to putting polymer barrier on the hair
surface to reduce overheating spots, and to improve hair vapor retention/restoration
which can serve as a heat sink to reduce thermal damage from repeated heat treatment.
Polymers with different chemistries are evaluated for their effect on hair thermal protec-
tion. Figure 2 shows the structures of these polymers. From a structure-property point of
view, high molecular weight polymers having fi lm-modifying groups for a smooth and
fl exible fi lm formation and polymers having hydrophobic units were evaluated. The poly-
electrolyte complex (PEC) of a high molecular weight anionic polymer and cationic poly-
mer was included in the study as it forms a smooth fi lm on drying. All polymers studied
contain PVP (polyvinylpyrrolidone) in the repeated unit. A copolymer of VP and DMAPA
acrylates contains a fi lm modifying group, DMAPA (dimethylaminopropyl methacryl-
amide) for smooth and fl exible fi lm formation. Its analogue, polyquaternium-55 (PQ-55)
contains a quaternary group with a lauryl chain. Another VP copolymer, VP/acrylates/
lauryl methacrylate copolymer, is anionic with a lauryl chain.
Figure 1. DSC results of thermally treated hair at two temperatures, 205°C and 232°C. Dark brown Euro-
pean hair.
JOURNAL OF COSMETIC SCIENCE270
Table I summarizes the results of peak temperature and denaturation enthalpy from the
DSC analysis of thermally treated Asian hair at 205°C with and without cosmetic
pretreatment. The hair breakage results during the subsequent combing are listed in
Table I as well. The thermally treated hair shows reduction in both parameters, Td and
ΔH, indicating that thermal treatment causes hair protein damage. The shaded areas in
Table I is the hair fi bers pretreated with the polymer containing a fi lm modifying group
or a hydrophobic unit and made in 1% polymer solutions. The results demonstrate that
the polymer pretreatment provide signifi cant reduction in Td and ΔH loss. The percent-
age of protein thermal protection was calculated based on the difference in ΔH reduction
between the untreated hair sample and the polymer pretreated hair sample. The polymer
Table I
DSC Results of Peak Temperature and Denaturation Enthalpy from DSC Analysis of Thermally
Treated Asian Hair at 205°C with and without Polymer Pretreatment and Hair Breakage
Results During Subsequent Combing
Asian hair, 205°C
thermal treatment Td°C ΔH(J/g) Td loss ΔH loss
% Protein
protection
No. of breakage/
anti-breakage (%)
No thermal treatment 140.4 20.5
Thermal treated, no
protection
136.5 16.0 3.9 4.5 193
Polyquaternium-55 139.6 18.6 0.8 1.9 57.7 132/31%
VP/acrylates/lauryl
methacrylate
*copolymer
138.7 17.9 1.7 2.6 42.3 91/52.5%
VP/DMAPA acrylates
Copolymer
138.9 18.5 1.5 2.0 55.5 130/32.6%
PVP K-90 135.0 14.6 5.5 5.9 0.0 192/0%
Figure 2. The chemical structures of polymers tested for their thermal protective effects.
2010 TRI/PRINCETON CONFERENCE 271
pretreatments provide about 50% thermal protection to the hair protein in Asian hair
subjected to 205°C thermal treatment. In addition, these polymer pretreatments reduce
hair breakage from subsequent combing, i.e. by 52% with VP/acrylates/lauryl methacry-
late and 31% for PQ-55. However, the homopolymer, PVP which contains no fi lm mod-
ifying groups or hydrophobic units shows no protection against protein thermal
degradation and no anti-breakage effect.
The thermal protective effect of selected polymer pretreatments was also tested with dark
brown European hair. Table II summarizes the results of Td and ΔH for European hair
after thermal exposure at 232°C with and without the protective polymer pretreatment.
The DSC results show the thermal degradation of hair keratin, indicated by a 25°C re-
duction in denaturation temperature Td and a 17.2 J/g loss of enthalpy ΔH. The protein
denaturation enthalpy is associated with the energy required for the helical protein dena-
turation and, therefore, depends on the amount and structural integrity of the α-helical
material in the intermediate fi laments of human hair cortex (9). Therefore, the enthalpy
reduction after the current thermal treatment corresponds to approximately 90% loss of
helical protein compared with the enthalpy reduction of the untreated hair sample. The
helix content occupies about 40% of hair cross section, suggesting that the helix protein
degradation from the thermal treatment is responsible for at least 36% degradation of
overall hair protein. The DSC data in Table II also shows that the polymer pretreatments
signifi cantly reduce the protein degradation. The ΔH reduction is especially low for 1%
VP/acrylates/lauryl methacrylate copolymer and 2% PEC where it is observed that ΔH
losses are than 10% for these polymer pretreated hair. These polymers are made in 0.5%
hydroxyethylcellulose (HEC), a thickener to enhance distribution on hair. However, the
pretreatment with HEC alone shows only small protein protection (Table II).
Figure 3 shows the hair breakage results of thermally treated European hair with and
without polymer pretreatment before heating. Thermally stressing hair led to increased
hair breakage from 52 to 214 fragments when subjected to combing. The pretreatment
of hair samples with the polymers tested provides anti-breakage effect on the subsequent
combing after heating. Among them, 2% PEC and 1% VP/acrylates lauryl methacrylate
copolymer treatments show the highest anti-breakage effect, 76% and 55%, respectively.
Although three polymers were formulated with 0.5% HEC, the data clearly show that
Table II
DSC Results of Thermally Treated Hair at 232°C with and without Polymer Pretreatment
(dark brown European hair)
Dark brown European
hair, 232°C heating Td °C ΔH(J/g) Td Loss ΔH Loss % ΔH Loss % Td Loss
No thermal treatment 141.6 19.1
Thermal-treated 116.7 1.9 25 17.2 90.1 17.7
HEC and heat damage 123.6 4.9 18 14.2 74.3 12.7
Polyquaternium-55+
HEC
131.6 12.4 10 6.7 35.1 7.1
VP/DMAPA acrylates
copolymer+HEC
133.6 13.2 8 5.9 30.9 5.6
VP/acrylates/lauryl
methacrylate copolymer+HEC
141.2 18.6 0.4 0.5 2.6 0.3
2% PEC 140.2 17.2 1.36 1.85 9.7 1.0
JOURNAL OF COSMETIC SCIENCE272
the HEC pretreated hair does not have an anti-breakage benefi t. The error bars of VP/
DMAPA acrylates copolymer and polyquaternium-55 pretreated hair indicates that their
results are not statistically different, however, the trend of hair breakage numbers shows
that these two polymers provide anti-breakage effect, which is supported by the results of
DSC and FTIR imaging analysis. The hair breakage results of 2% PEC and 1% VP/acrylates
lauryl methacrylate copolymer pretreatment are statistically different. Robbins has studied
the pathways of hair breakage and suggests that extending and impacting or compressing
hairs with fl aws or cracks and/or chemically weakened hair during combing may be one of
the possible pathways for hair breakage (10). Alleviation of weakening of the thermally
insulted hair through polymer pretreatments allows the hair to withstand these combing
stresses and indicates thermal protection through a reduction in fi ber fragmentation.
PROTEIN STRUCTURE MODIFICATION FROM THERMAL TREATMENT—FTIR IMAGE ANALYSIS
OF HAIR CROSS SECTION
One type of protein denaturation is a change in protein conformation. The undamaged
hair has a α- helical coiled coil protein confi rmation, a well organized structure in the
cortex. Once the protein is damaged, it can unfold and convert into the extended protein
chain or beta sheet structure. The protein conformation changes will change the hydro-
gen bonding structure that stabilizes the helical structure and, therefore, may change the
water accessibility to hair.
Further, IR image analysis was conducted on thermally treated hair fi bers to examine the
hair keratin damage at the molecular level such as protein structural changes due to heat
treatment. FTIR image analysis provided the spatially resolved spectroscopic imaging of
chemical components over the cross section of hair. It consists of an array of detectors that
Figure 3. Hair breakage reduction of thermally treated hair at 232°C with polymer pretreatment, 1% poly-
mer solution + 0.5% HEC. European dark brown hair.
2010 TRI/PRINCETON CONFERENCE 273
collect IR spectra pixel by pixel. By sectioning hair, and collecting spatially resolved in-
frared spectra of hair samples, spatially resolved images of the changes in hair protein
structure as a result of thermal stresses to the hair were generated.
Figures 4a and 4b show the typical IR spectra and the second derivative analysis of a
random location in the cortex of undamaged European dark brown hair from 1480-1700
cm−1 (Amides I and II) and 3000- 3700 cm−1 (Amide A) spectral regions. Bands from 1480-
1700 cm−1 region are sensitive to changes in the protein secondary structural conforma-
tion. In order to get the resolutions of the IR bands, secondary derivative analysis was
used to locate the different protein peak positions under the curve. The second derivative
curve displays the minor component of β-sheet and a major α-helical structure under the
curve for undamaged hair. Amide II at 1548 cm−1 is assigned to α-helical structure and
Amide II at 1516 cm−1 is assigned to β-sheet conformation (11.). The radio of β-sheet
peak intensity to the α-helix band intensity was used to quantify the additional conver-
sion of α-helix to β-sheet conformation from thermal treatment. An increase in the ratio
indicates an increase in β-sheet composition or a decrease in α-helix content correspond-
ingly, and if the ratio remains the same as the undamaged hair, there will be no change in
the two components.
The ratio maps of β-sheet peak intensity to the α-helix band intensity of hair cross sec-
tions are shown in Figure 5a. The ratio bar at the right side with higher numbers and
corresponding colors indicates the relative β-sheet intensity. It can be seen that the outer
layer of hair has a higher β-sheet level than inside the hair as indicated by the brighter
color in the outer layer of the hair cross section. Moreover, the β-sheet content becomes
more pronounced in the outside layer of thermally treated hair due to the heat of the iron
affecting this part of the hair fi rst. Pretreatment with all three tested polymers tested
Figure 4. IR spectra and their second derivative curves of undamaged European dark brown hair. a. Amide
I & II region (1480–1700 cm-1), b. Amide A region (3000–3700 cm-1).
JOURNAL OF COSMETIC SCIENCE274
effectively prevented the conversion of α-helix structure to the β-sheet conformation.
HEC pretreatment provided slight protection to β-sheet conversion.
Protein helices are held together by hydrogen bonds between the carbonyl oxygen of
amide bonds in the main chains with the imido hydrogen of amides. The Amide A band
(N-H stretching) at ~3290 cm−1 is very sensitive to the disruption of hydrogen bonding.
When some of the helix unfolds and changes to the extended protein chain or β-sheet
conformation, the hydrogen bonds will break, leading to the shift of the Amide A band.
Figure 4b shows the IR spectrum of Amide A region and its second derivative curve. The
second derivative curve of the Amide A region shows bands at 3292 cm−1 and 3200 cm−1
which are assigned to the trans-bonded and cis-bonded N-H stretching bands, respec-
tively. The cis-bonded Amide A band is attributed to the interruption of hydrogen bond-
ing due to helix unfolding. To compare the changes in the trans-bonded structure to the
cis-bonded structure after thermal treatment, the ratio of the peak intensity at 3200 cm−1,
which is attributed to the cis-bonded structure, to the peak intensity at 3292 cm−1, which
is attributed to the trans-bonded structure, is used to quantify the additional conversion
of trans-bonded Amide A to cis-bonded Amide A structure due to thermal treatment. An
increase of the ratio will indicate the increase of cis-bonded component and a decrease of
trans-bonded structure correspondingly; and if the ratio remains the same as the undam-
aged hair, there will be no change in the two components. The ratio maps of cis-bonded
Amide A structure to trans-bonded Amide A structure over hair cross sections are shown
in Figure 5b. The ratio bar at the right side with higher numbers and corresponding
colors indicates the relative cis-bonded Amide A content. Consistently, the content of cis-
bonded amide A for thermally treated hair increases after heat exposure. The increase of
cis-bonded A content is consistent with the increase of β-sheet formation as stated above.
This results confi rms the disruption of the hydrogen bonding structure of helical protein
and suggests the unfolding of some helical structure. Pretreatment with all three poly-
mers tested effectively prevents the formation of cis-bonded amide A protein bands.
Therefore the IR image analysis results are consistent with the DSC results on the ther-
mal protection effect of polymers.
Figure 5. IR images of thermally treated hair cross section at 232°C with and without polymer pretreat-
ment. (a) The ratio maps of b-sheet peak intensity to the a-helix band intensity. (b) The ratio maps of cis-
bonded Amide A band intensity at 3200 cm-1 to that trans-bonded Amide A at 3292 cm-1. Dark brown
European hair.
2010 TRI/PRINCETON CONFERENCE 275
In addition to the protein conformation change, hair protein modifi cation from thermal treat-
ment was further assessed using a band at 2960 cm−1, a C-H asymmetric stretching mode of
the CH3 group. This CH3 band is mainly attributed to the terminal amino residues of hair
proteins with minor lipid contribution. Figure 6 shows the spatial IR images of the hair cross
section which depict the concentration profi le of hair protein with minor lipid contribution
obtained from the CH3 band area. The intensity color bar at the right side with higher num-
bers and corresponding colors indicates higher protein concentration. It is observed from the
fi ber cross sections that there is an overall protein and lipid loss for the thermally treated hair
as indicated by a reduction in the integrated area of the bond. Pretreatment with all three
polymers tested effectively prevents the overall protein and lipid loss.
FTIR results support the DSC analysis and provide additional insights to the total helical
protein degradation. As little is known about the molecular conformational state of other
protein components in hair (8), other protein components besides the β-sheet structure,
such as other uncoiled, random coil, or denatured cross linking structures that α-helix
can transform to but are undetected by this FTIR analysis, may exist.
THERMAL PROTECTION OF THE HAIR SURFACE BY COSMETIC PRETREATMENT
Figure 7 shows atomic force microscopy (AFM) images of the surface of the hair cuticle
with and without thermal treatment. The AFM images indicate that thermal treatment
at 232°C causes damage on the cuticle surface, including cracks, holes from over-heating,
and formation of micropores. These surface damages will increase the hair permeability
resulting in faster water loss during drying.
Figure 6. IR images of a thermally treated hair cross section at 232°C with and without polymer pretreat-
ment. Maps were developed from the peak area of 2960 cm-1 band, representing the relative protein concen-
tration in the hair cross section. European dark brown hair.
JOURNAL OF COSMETIC SCIENCE276
Figure 8 shows the scanning electron microscopy (SEM) images of European hair fi bers
with and without thermal treatment at 232°C and with the pretreatment of the tested
polymers. Four to fi ve fi bers were examined for each hair sample to ensure reproducibility.
Thermal treatment causes severe cuticle damage to the hair fi ber surface by showing cu-
ticle disintegration with missing cuticle pieces and jagged cuticle layers. The 0.5% HEC
(hydroxyethylcellulose) solution pretreated hair has damage on cuticle layers and shows
the fusion of some cuticle cells. Once the cuticle is damaged, hair breaks easily since there
is no protection for the cortex. The SEM images also show that polymer pretreatment
prevents signifi cant cuticle damage due to thermal treatment. Among them, VP/acry-
lates/lauryl methacrylate copolymer-treated hair fi bers have well defi ned cuticle layer.
This result is consistent with the polymer’s high anti-breakage effect, 55%. Therefore,
hair surface protection to ensure good cuticle integrity and surface smoothness also plays
an important role in their anti-breakage effect besides protecting cortex protein from
thermal damage.
WATER VAPOR SORPTION AND DESORPTION OF THERMALLY TREATED HAIR AND THE ROLE
OF WATER RESTORATION IN HEAT CONTROL
Water changes the properties of human keratin fi bers and, therefore, plays an important
role in cosmetic performance. Hot fl at irons that lack heat control can destroy the hair
Figure 7. AFM Images of the hair cuticle surface with and without thermal treatment. (a) Not thermally
treated. (b, c, d) Thermally treated at 232°C.
2010 TRI/PRINCETON CONFERENCE 277
protein structure resulting in changes in hair water absorption and desorption profi les. In
this work, water sorption/desorption and the kinetics of these processes on thermally
treated hair were studied. The effect of polymer pretreatment on the water sorption/de-
sorption performance of hair was evaluated.
Figure 9a shows the water sorption and desorption isotherms of hair fi bers with and with-
out thermal treatment and polymer protection. The thermally treated hair has a lower
maximum water regain than the unheated hair in each sorption step. The maximum
Figure 9. Water sorption and desorption isotherms and apparent diffusion coeffi cients of hair fi bers with and
without thermal treatment and polymer protection. Dark brown European hair.
Figure 8. SEM images of the hair fi ber surface with and without thermal treatment at 232°C and polymer
protection. Dark brown European hair.
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water regain for the unheated hair at 90% RH is 21.95% while the heated hair is
17.27%. To avoid the variation among different hair tresses, the unheated and the heated
hair are from the same hair tress split into two halves. One half is heated and the other
is not heated. The hysteresis (the difference in net moisture changes between the
desorption and sorption processes) is higher for heat-treated hair than unheated hair,
indicating a lower water retention of the heated hair on drying. The less water regain
and lower retention for thermally treated hair might be attributed to the helical pro-
tein conformation change to the beta sheet or other uncoiled denatured cross-linking
structure. The new protein conformation may have reduced water accessibility or bind-
ing sites. Figure 9a shows that polyquaternium-55 pretreatment increases the water
regain of heated hair compared with its untreated control possibly due to the protective
effect of the polymer on thermally induced hair protein damage. As shown in the FTIR
and DSC studies described previously, polymer pretreatment reduces protein degrada-
tion and denaturation, thereby protecting the protein structure and native hydrogen
bonding interactions. The data indicates that this has the effect of improving the water
sorption of hair, compared with the unprotected thermally damaged hair. The mecha-
nism of increased water restoration of hair via polymer protection of native protein
structure is further supported by studying the water sorption and desorption of virgin
hair without thermal treatment, both with and without 1% polyquaternium-55 treat-
ment. This is illustrated in Figure 10a. Both isotherms are identical, indicating that
the polymer treated and untreated unthermally-stressed hair fi bers have the same water
sorption and desorption performance. This supports a mechanism in which thermal
protection of the native protein structure is a major factor in moisture restoration and,
thus, thermal protection.
The apparent diffusion coeffi cients have been utilized to measure the kinetics of moisture
uptake and loss in hair fi bers (12,13). Diffusion rates for moisture into and out of the fi ber
at each relative humidity were calculated from the sorption and desorption data in each
Figure 10. Water sorption and desorption isotherms and apparent diffusion coeffi cients of virgin hair fi bers
with and without polymer treatment. Dark brown European hair.
2010 TRI/PRINCETON CONFERENCE 279
sorption or desorption step. The apparent diffusion coeffi cients (D) for hair are calculated
from Fick’s diffusion model applied to a cylindrical geometry:
Mt/Mf = 4(Dt/
π
r2)1/2
where D is the apparent diffusion coeffi cient, Mt is the vapor concentration at time t, Mf
is the vapor concentration at equilibrium, and r is the radius of the hair fi ber. If the frac-
tional absorbed or desorbed water, Mt/Mf, is plotted against the square root of the absorp-
tion or desorption time, the points should form a straight line: Mt/Mf = 4/π1/2 r((D)1/2
(t)1/2. The apparent diffusion coeffi cient of moisture for sorption or desorption can be cal-
culated from the slope as
222
(/16)( ) /s
Dr slopecm
=
π
In Figure 9b, the apparent diffusion coeffi cient plots calculated from the isotherm data
show that the thermally damaged hair has a much higher water diffusion coeffi cient on
desorption during drying than the non-thermally-treated hair, i.e. water comes out of the
damaged hair fi bers much faster than the unheated hair during drying. The difference is
more pronounced at the higher humidity at which water is multi-layer absorbed. There-
fore the heat damaged hair has increased permeability. On the sorption process, the ther-
mally treated hair or thermally damaged hair has a slower water uptake rate than the
unheated hair, though the difference is much smaller, compared with the desorption pro-
cess. This is because sorption takes place in the dry and un-swollen fi bers in which diffu-
sion is more diffi cult than desorption, which starts from wet and swollen hair fi bers
experienced from the lengthy sorption process (12). At low humidity less than 30% RH,
the water diffusion rate for both thermally treated and untreated hair fi bers are similar
because at low humidity (relative humidity less than 25%), water molecules are princi-
pally bonded water to hair (14).
Figure 9b shows that polymer pretreatment of hair by polyquaternium-55 reduces the
water diffusion coeffi cient on desorption compared with untreated and heated control
samples, indicating that the polymer pretreatment slows down the loss of moisture from
hair during drying. In Figure 10b, the diffusion coeffi cient plots of virgin hair with and
without PQ-55 treatment are almost identical, again, suggesting that the reduced water
diffusion coeffi cient on desorption by PQ-55 pretreatment for the thermally treated hair
in Figure 9b is due to the protective effect of the polymer on hair protein structure. Fig-
ure 11 shows the water sorption and desorption isotherm of thermally treated hair fi bers
pretreated with PEC versus untreated control sample. The PEC-treated hair and the un-
treated hair are the two split halves from the same tress to avoid variation among different
hair samples. The PEC-pretreated hair after heating has a much higher water regain than
the untreated control samples.
The increased water regain on sorption, faster vapor sorption rate and slower vapor de-
sorption of hair from the polymer pretreatment will, in turn, help to provide heat control
to hair during repeated hot fl at ironing. This will have the effect of reducing further ther-
mal damage.
In order to evaluate the heat control effect of polymer pretreatment, the hair tempera-
ture during hot fl at ironing was measured in three different heating schedules. Figure
12 shows the hair temperature of hair samples during hot fl at ironing at 232°C with
and without polymeric pretreatments. The lowest temperatures are seen after the fi rst
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heating cycle (each cycle is three 5-second strokes). Seven cycles of continuously re-
peated heating result in much higher measured temperatures. However, another seven
cycles of heating with an overnight interval between cycles at 60% RH, to allow the
hair samples to have a chance to rehydrate, show much lower temperatures as expected.
These results indicate that the water restoration of hair contributes to heat control on
hot fl at ironing. The thermal protective polymers tested in this study shown in the
shaded box reduce hair temperatures signifi cantly. The temperature reduction of hair
pretreated with PEC increases signifi cantly with the increasing level of PEC used from
1% to 4%, supporting the critical role of the polymer barrier in protecting the hair
from thermal damage.
Figure 12. Hair temperatures of hair samples during hot fl at ironing at 232°C with and without cosmetic
pretreatment.
Figure 11. Water sorption and desorption isotherms of thermally treated hair fi bers pretreated with PEC.
Dark brown European hair.
2010 TRI/PRINCETON CONFERENCE 281
CONCLUSIONS
This study has shown through the use of various instrumental techniques that the ther-
mal insult of hair from hot fl at ironing appliances causes damage to the hair surface and
the structural proteins in the cortex. One measure of this damage is the conversion of
proteins from the α helical to the β-sheet conformation, as well as a measurable loss of
protein. Also evident is damage to the hair cuticle including micropore formation and
cuticle cell disintegration. The internal and surface damage resulting from thermal treat-
ment increases hair breakage especially with the additional stress of hair combing. Dy-
namic vapor sorption (DVS) data indicate that thermally damaged hair has reduced water
regain and lower water retention possibly resulting from the thermally induced changes
in protein structure. Pretreatment of hair with selected high molecular weight polymers
containing fi lm-modifying groups or hydrophobic units such as VP/acrylates/lauryl
methacrylate copolymer, PEC, and polyquaternium-55 clearly provide thermal protec-
tion to the hair surface and cortex resulting in reduced hair breakage during combing.
The pretreatment of hair with selected polymers also improve moisture restoration and
water retention of thermally treated hair. The studies continue to improve our under-
standing of the many changes that occur on, and in, the hair fi ber with thermal stress and
provide insights into the mechanisms whereby polymer pretreatments can provide sig-
nifi cant protection to the hair fi ber as it is exposed to repeated thermal stress.
ACKNOWLEDGMENTS
The authors to thank William Thompson for providing SEM analysis of hair samples used
in this work, Grisel Tumalle for her assistance in measuring hair temperature, Jean Karolak
for her contribution to some of the anti-breakage data used in this work, Larry Senak for
his support in obtaining FTIR image analysis data, and Roger McMullen for his help in
the thermal imaging and AFM techniques.
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