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.
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
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
uniquein themselves and haveled us to believethat permanenthair straighteningis governedby a differentmolecularprocess.
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.
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
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
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.
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
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
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
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
JOURNAL OF THE SOCIETY OF COSMETICCHEMISTS
Figure1. Changes in curvature andfiberdiameter of curlyhairfiberimmersed in 1 N sodiumhydroxide solution(grid circle is usedas reference). Table
Relationship of Hair SwellingandFiberSupercontraction to Permanent Hair Straightening Reagents NaOH (1 N) NaOH (0.1 N)
pH 14.0 13.0
Degreeof hair straightening
% Hair swelling
% Hair supercontraction
DTT (0.8 M)
THP (1 M)
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]
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
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.
ROLE OF CYSTINE
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.
TreatmentTime Requiredfor Fiber Supercontraction and for Hair Straightening Fiber
20 min 50 min Over
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table
Effectiveness of VariousReagentson Hair Straightening
Reductionin hair cysteine (%)
Lanthionine formed (%)
DTT (0.8 M)
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
Resorcinol(40%) Boiling water*
NaOH (1 N) NaOH (0. t N)
Degree of hair straightening Complete/permanent Partial/temporary
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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