j. Soc.Cosmet.
Chem.,45, 347-352 (November/December
1994)
Mechanismof hair straightening
MICHAEL WONG,
GABRIELA WIS-SUREL, and
JOSEPH EPPS, ClairolInc,, 2 Blachley
Road,Stamford,
CT 06922.
Received
November
22, 1993. Presented
at theAnnualMeetingof the
Society
of Cosmetic
Chemists,
New York,December
1993.
Synopsis
The practiceand theoryof permanenthair straighteningare discussed.
Hair straighteningas a cosmetic
process
is the reverseof hair waving.It removes
hair curliness
and makeswavyhair straight.While it is
generallyacceptedthat permanenthair straighteningcan be achievedonly at very high pH with strong
alkali, we haveobserved
that it canalsobe achieved
at neutralpH with lithium salt, or evenat acidicpH
with resorcinol.Also, while someof thesetreatmentsdo resultin considerable
lossof cystine,othersleave
the cystineentirelyintact. Lanthionineis producedin somecases,but is absentin others.It appearsthat
the prime requirementfor an effectivehair straighteningtreatmentis to be ableto inducesupercontraction
of the hair fiber.
Basedon theseobservations,
a modelis proposed
to explainpermanenthairstraightening
in termsof certain
moleculareventssuchaschainfoldingandalpha-betatransformation
of the polypeptidein the hair keratin.
INTRODUCTION
Severalprocesses
areapplicableto hair straightening,whichhasbecomeoneof the more
commonhair styling practices.There are the hot comband pressiron, which were
popular yearsago. There are the "Jeri-curl" products,and there are the chemical
relaxers.They all are able to straightenhair to someextent, but not with the same
degreeof permanency.
The hot combprocess,
for example,whichrelieson the actions
of heat and moisture,can only achievehair straighteningthat is temporary.The hair
will revert to its original curly stateevenon exposureto high humidity. The Jeri-curl
process,
whichisbasedonthe "thioglycolate
wavingsystem,"canproducehairstraighteningthat is moreresistantto humidity, but the hair will still reverton washing.The
chemicalrelaxers,whichare typicallyformulatedwith strongalkali at pH in excess
of
13, are the only onesthat are able to achievepermanenthair straighteningthat will
survivewashing.The generalbelief is that hair straighteningis analogousto hair
waving. When hair is treatedwith a chemicalrelaxer,hair cystineis cleaved,and a new
crosslink,the lanthionine,is subsequently
formedto help stabilizethe hair in the
straightconfiguration.The theoryis reasonable
in view of the fact that lanthionineis a
major reactionproductbetweenalkali and cystine(1-3). Also, publisheddata have
shown that hair treated with chemical relaxersindeed contain, a substantialamount of
lanthionine.However,we haveobservedseveralaspectsof hair straighteningthat are
347
348
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
uniquein themselves
and haveled us to believethat permanenthair straighteningis
governedby a differentmolecularprocess.
EXPERIMENTAL
HAIR
SAMPLES
Two typesof hair sampleswereusedin the experiments.
Curly hair samples,obtained
from Afro-Americans,wereusedto assess
the efficacyof hair straightening.Typically,
thesehair samplesconsisted
of coilsof about0.5 cm in diameter.To studythe fiber
swellingand supercontraction
characteristics,
Caucasian
darkbrownhair wasused.All
hair sampleswere cleanedwith a solutionof 12.5% sodiumlauryl sulfateprior to
treatments.
EVALUATION
OF HAIR
STRAIGHTENING
EFFICACY
Temporaryhair straightening
is typicallycharacterized
by a readyreversion
of the hair
fiber to its original curlinessafter evenone washing.A criterionthat we usedin this
study to determinethe permanencyof hair straighteningwashow well the hair fiber
remainedin a straight configurationafter the fiber was immersedin water (at room
temperature)
for five minutesanddried. The degreeof hair straightness
wasdetermined
visually:the hair fiber wasconsidered
straightwhentherewasno evidenceof curliness.
EVALUATION
OF HAIR
SWELLING
AND
HAIR
SUPERCONTRACTION
To determinethe degreeof hair swelling,the diameterof a wet hair fiber wasmeasured
prior to treatmentusing a stereoscopic
microscope
equippedwith an eyepiecethat has
a micrometerscale. The diameterof the samehair fiber was again measuredafter
exposureto a treatmentreagent.To determinethe extentof fiber supercontraction,
the
length of straight Caucasianhair fiber was immersedin water and measuredin a
microcapillarybeforeand after the treatment.
ANALYSIS
OF CYSTINE
AND
LANTHIONINE
CONTENT
Hair sampleswere first hydrolyzedby heatingwith 6 N HCI in a sealedtube for 24
hoursat 110øC.After removalof the acidin a rotaryevaporator
at 30øCanddilution
with 0.2 N sodiumcitratebuffer(pH 2.2), the aminoacidcontentwasdeterminedwith
a Beckman120-C amino acid analyzer.
RESULTS
HAIR
AND
DISCUSSION
STRAIGHTENING
WITH
THIOGLYCOLATE/HYDROGEN
PEROXIDE
SYSTEM
One of the unusualaspects
of hair straightening
is thefactthat conventional
hairwaving
productsare not particularlyeffectivein achievingpermanenthair straightening.When
a straighthair fiber is set in a curlyconfiguration
(woundon a rod, for example)and
HAIR
STRAIGHTENING
349
treatedwith a commercialthioglycolate
wavinglotion for 30-40 minutes,followedby
a neutralizationstep, the hair fiber will acquirea curly configurationthat will survive
subsequent
washing.On the otherhand, if a curly Afro hair fiber is set in a straight
configuration(the fiber is held straightundera smallstrain,for example),and treated
with a similar thioglycolatewaving lotion and neutralizationstep, the hair fiber will
acquirea temporarystraightconfiguration.However,the hair fiber will revertto most
of its originalcurliness
on evenonesubsequent
washing.The failureof this reduction/
re-oxidationschemeto impart a morepermanentstraightness
to the hair fiber suggests
that a molecularmodelcomparable
to hair wavingmay not be entirelyapplicable.
THE PHENOMENON
OF SPONTANEOUS
HAIR
UNCOILING
Anotherunique aspectof hair straighteningis the spontaneous
uncoilingof the hair
fiber. Placedin a solutionof appropriatereagent,a curly hair will uncoilitselfwithout
the helpof an externalforce.This phenomenon
canbe demonstrated
by the following
experiment:A curly hair fiber is immersedin a 1 N sodiumhydroxidesolution.The
curvatureof the hair is observed
andcanbe seento increase
slowly,and in about 10 to
15 minutes, the hair fiber becomespracticallystraight. In other words, unlike hair
waving, hair straighteningcanbe achievedwithout the applicationof an externalforce
to strain the hair fiber.
The questionarisesas to what causesthe curly hair fiber to uncoil and allow hair
straightening
to proceed.We thoughtof two possibilities:
One explanation
couldbe the
swellingactionof the reagent,sincesodiumhydroxideis an excellentswellingagentfor
keratinfiber. In fact, it hasbeenpointedout in the literature(5,6) that, basedentirely
on geometricalconsideration,
any radialswellingof a curvedsegmentof a fiber should
resultin an increasein the radiusof curvatureof the fiber. Anotherexplanationcould
be the resultof fiber supercontraction
(7). In fact, undercloserexamination,asshown
in Figure 1, it canbe seenthat in a 1 N sodiumhydroxidesolution,the uncoilingof
the hair fiber is accompanied
by somechanges
in fiber geometry.
HAIR
SWELLING
AND
HAIR
SUPERCONTRACTION
To assess
the relativeimportanceof thesetwo factorsto permanenthair straightening,
we proceeded
to examinethe swellingand supercontraction
behaviorof hair fibersin a
varietyof reagents
andto establish
howtheywouldimpactonhairstraightening.
In this
setof experiments,
curlyAfro hair fibersweretreatedwith variousreagents,someof
which are known to be goodkeratinswellingagents,while othersare alsoknown to
causesupercontraction
in keratin fibers.Thesereagentsincludedsodiumhydroxide,
resorcinol,lithium chloride,urea, dithiothreitol(DTT), thioglycolicacid (TGA), tris
(hydroxymethyl)phosphine
(THP), boilingwater, and cuprammonium
hydroxide.The
degreeof hair swellingand the amountof fiber supercontraction
were measuredand
correlatedwith the corresponding
degreeof permanenthair straightening.Resultsare
shown in Table I below.
It is interestingthat whileall the reagents
in the studyareeffectivehairswellingmedia,
permanenthair straighteningis achievedonly by thosethat are alsoableto inducehair
supercontraction.
From the data shownin Table I, it is obviousthat effectivehair
350
JOURNAL OF THE SOCIETY OF COSMETICCHEMISTS
Dry
After
5 Minutes
After
Fiber
in
Solution
After
2 Minute
16 Minutes
in
in
Solution
Solution
Figure1. Changes
in curvature
andfiberdiameter
of curlyhairfiberimmersed
in 1 N sodiumhydroxide
solution(grid circle is usedas reference).
Table
I
Relationship
of Hair SwellingandFiberSupercontraction
to Permanent
Hair Straightening
Reagents
NaOH (1 N)
NaOH (0.1 N)
pH
14.0
13.0
Degreeof hair
straightening
Complete/permanent
Partial/temporary
% Hair
swelling
% Hair
supercontraction
>40
40
5.7
0
DTT (0.8 M)
3.5
None
>50
0
THP (1 M)
8.5
Complete/permanent
>50
6
Urea (50%)
7.0
None
>30
TGA ( 1.2 M)
LiCl (40%)
Resorcinol
(40%)
Boiling water*
Cuprammonium
hydroxide
9.6
7.0
7.0
Partial/temporary
Complete/permanent
Complete/permanent
Complete/permanent
Complete/permanent
80 [ 11]
60 [ 12]
>50
> 15 [ 13]
> 50 [ 14]
12.0
0
2.0
11.5
10.0
6.0
9.4
* Under tension.
straightening
canonlybe achieved
whenthehairfiberhassupercontracted
5% or more
of its ownlength.Reagents
suchasurea,dithiothreitol,andthioglycolic
acidareall
excellentswellingagentsfor thehairfiber,andyet noneof thesereagents
wouldbeable
to impartanydegreeof permanenthair straightening.
In fact, sodiumhydroxideat a
concentration
ashigh as0.1 N is not a particularlyeffectivehair straightener,
sinceat
this concentration,
sodiumhydroxideis not producinga significantamountof fiber
supercontraction.
Boilingwater,ontheotherhand,will produce
effective
hairstraighteningif the hair fiber is strainedto about20%, sinceundertheseconditionshair fiber
isbelieved
to undergo
supercontraction
(8). It thusappears
thatfiberswellingalonewill
HAIR
STRAIGHTENING
351
not be sufficientwithout alsothe presence
of fiber supercontraction.
Indeed, hair fiber
supercontraction
is perhapsthe dominantrequirement.
The importanceof fiber supercontraction
is further demonstrated
by the observations
that the time requiredto achievepermanenthair straighteningtendsto coincidewith
the onsetof supercontraction,
asdatain Table II clearlyindicate.For example,in the
caseof 1 N sodiumhydroxide,it takesabout 15-20 minutesto achieveeffectivehair
straightening.On the other hand, it takesas long as 45 minutesfor THP. Closer
examinations
of the rate of supercontraction
showthat in sodiumhydroxidesolution,
hair fiber will start to supercontract
in 10 minutes, and supercontract
completelyin
about 20 minutes. For THP, it takesmuch longerfor fiber supercontraction
to start,
and it takesabout40-45 minutesfor completion.What this seemsto suggestis that
radial swellingof the hair fiber is only the initiation step that helpsto achievethe
necessary
uncoilingof the curlyfiberinto a straightconfiguration.
The primarydriving
forceto impart permanentstraighteningto the hair is the subsequent
supercontraction
of the fiber.
THE
ROLE OF CYSTINE
AND
LANTHIONINE
The formationof a newcrosslink
suchaslanthionineisgenerallybelievedto be required
in hair straighteningto stabilizethe fiber in the new straightconfiguration.However,
the observations
describedin Table I would suggestthat the formationof lanthionine
may not be ascritical. Lithium chloride,for example,is not expectedto produceany
significantamountof lanthioninein the hair, and yet it is able to achieveeffectivehair
straightening
underappropriate
conditions.On the otherhand,0.1 N sodiumhydroxideproduces
onlypartialhairstraightening,
but its actiononhairis expected
to produce
a substantialamountof lanthionine.On analyzingthe treatedhair samplesdescribedin
Table I for cystineand lanthioninecontents,we wereindeedable to showthat neither
the formationof lanthioninenor the reductionof cystinewill alwayscorrespond
to the
efficacyin hair straightening.In addition to lithium chloride, for example,other
reagentssuch as resorcinol,tris(hydroxymethyl)phosphine,
and boiling water are all
found to be able to achieveeffectivehair straighteningwithout any evidenceof lanthionineformation,asshownin Table III below. Also, a reagentsuchasdithiothreitol
is found to be totally ineffectiveeventhoughit would causesubstantialbreakdownof
cystinein hair. The assumptionthat permanenthair straighteningrequiresthe cleavage
of cystineand the subsequent
formationof lanthioninedoesnot appearto be entirely
valid.
Table
II
TreatmentTime Requiredfor Fiber Supercontraction
and for Hair Straightening
Fiber
Reagents
supercontraction
Sodiumhydroxide,1 N
15-20 min
THP
40-50 min
LiC1
120 min
Cuprammoniumhydroxide
70-80 min
Permanent hair
straightening
20 min
50 min
Over
120 min
90 min
352
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
Table
III
Effectiveness
of VariousReagentson Hair Straightening
pH
Reductionin
hair cysteine
(%)
Lanthionine
formed
(%)
14.0
13.0
35
30
24
17
DTT (0.8 M)
3.5
90
0
None
LiC1(40%)
THP (1 M)
TGA ( 1.2 M)
7.0
8.5
9.6
3
45
54
0
0
0
Complete/permanent
Complete/permanent
Partial/temporary
Urea (50%)
7.0
--
0
None
Resorcinol(40%)
Boiling water*
7.0
-5
0
0
Complete/permanent
Complete/permanent
Reagents
NaOH (1 N)
NaOH (0. t N)
* Under
A MODEL
Degree
of hair
straightening
Complete/permanent
Partial/temporary
tension.
FOR HAIR
STRAIGHTENING
Basedon our observations,
it appearsthat the first stepin the process
of hair straighteningis the uncoilingof the hair curl throughthe actionof radialswellingof the fiber.
But it is the subsequent
supercontraction
of the fiber that determinesthe success
of
permanentstraightening.If the supercontraction
is irreversible,and if the supercontractionis substantial(5% or more), permanentstraighteningcan indeedbe achieved.
The kineticsof supercontraction
is alsoimportantin determiningthe outcomeof hair
straightening.Permanenthair straighteningcanbe achievedrapidlyif fiber supercontractionis fast. Theseobservations
suggestthat the effectof the fiber supercontraction
is essentiallyto "lock" the fiber in the straight configuration,preventingit from
revertingto its nativecurly configuration.On the molecularlevel, fiber supercontraction is the resultof changesin the secondary
structure,involvingthe so-calledalphabetaphasetransitionin the organizedphaseof the keratin(9-11). It is believedthat it
is the irreversibleconsequence
of thesemolecularconformational
changesthat leadsto
permanenthair straightening.
Thus, the c.leavage
of cystineand the formationof lanthionine are merely by-productsrather than prime requirementsof permanenthair
straightening.
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