j. Soc.Cosmet. Chem.,36, 39-52 (January/February 1985)

Effect of chemical and hurnectant treatments on the

mechanicaland fractographic behaviorof Negroidhair Y. K. KAMATH,

S. B. HORNBY,

and H.-D.

WEIGMANN,

TextileResearch Institute,P.O. Box 625, Princeton, NJ 08542.

Received October 31, 1984. Presented at theAnnualMeetingof the Society of Cosmetic Chemists, New York, December 6-7, 1984. Synopsis The fracturebehaviorof Negroid hair, both untreatedand treatedwith relaxersand humectants,was studiedto clarify the causesof fiber breakageat low levelsof extension.Microscopicobservation of the fiber revealsfrequenttwists, with randomreversals in directionalong the length, and high ellipticity. Measurements of the effectof tensileload on axial angleof untwistingof specimens with a singletwist indicate that failure at low extensionsis due to the initiation of cracksat flaws near the twists, which

relievestorsionalstresses in theseregions.Extensionat failureis higherin wet fibers,probablybecause plasticizationrelaxesthesestresses. Treatmentof fiberswith solutionsof humectants suchas polyacrylic acid or glycerinhasbeenfound to reduceprematurefailuresignificantly.However,pretreatmentwith relaxers andthioglycolicacideliminatesthe beneficialeffectsof the humectanttreatment.Scanning electron microscopy of fractureendsrevealsa predominance of stepfracturesand fibrillatedends,indicatinga large numberof flaws.Fatiguinghair fibersseemsto accentuate fiber damage,leadingto a predominance of fibrillated

fracture ends.

INTRODUCTION

Negroid hair presentssevereproblemsin manageabilitybecauseof its kinky structure. The highly entangledhair massis difficult to comb(1) and requiresmuchhigherforces in groomingproceduresthan straighthair, so that it is subjectedto higher degreesof mechanicaldamage.The widely usedpracticeof "hot combing"to detanglethe hair mass,especiallyin combinationwith the useof relaxers,seemsto introduceextensive damageto the hair fiber. This is reflectedin the tensile mechanicalproperties,a significantlyhigher fractionof fibersbreakingat low elongations(prematurefailures) than hair fibersof other ethnicorigin. In somecasesthe extentof damageis sosevere that even manual squeezingand rubbing resultsin the breakageof fibers in half wavelengthsections.This observationsuggeststhat combingand stretchingprobably damagesthe fibersin the regionsof twist.

Microscopicexaminationof the fiber showsfrequenttwists along its length with no preferreddirectionality (2). Subjectingsuch a fiber to tensile deformationinvolves developmentof torsionalstresses in the regionsof twist, the magnitudeof which will dependon the angle of twist and the appliedtensileforce.In earlierwork a method was developedto study torsionaleffectsin the tensileloadingof suchspecimens in39

40

JOURNALOF THE SOCIETYOF COSMETICCHEMISTS

volving a single region of twist (2). The extent of untwistingof the fiber under continuous loadingwasmeasuredin termsof the angleof rotation.In manyinstances fiber failureseemsto havebeenpreceded by the generation of a crackin the regionof twist, althoughthis couldnot be established with certainty.

Combinga hair massinvolvesthe applicationof low levelsof tensileforcesimilarto thosein a tensile fatigue experimentwith a fiber. Becauseof entanglements,these forcesare generallyhigher in the caseof Negroid hair than in Caucasian hair. To simulatethe effectof combing,an apparatuswasdesignedto subjectsinglefibersto tensilefatigueat variousload levels(2). It shouldbe notedthat the tensilefatigue proceduregivesrise to torsionalfatiguein the regionsof twist, which can enhance damagein theseregionsof the fiber.Thismayberesponsible for the breakingof fibers into half wavelengthsectionson manualsqueezing.

The mechanical andfractographic behaviorof untreatedNegroidhairhasbeenreported earlier (2). In this communicationthese studieshave been extendedto include the effects of chemical

treatments

and humectants.

EXPERIMENTAL MATERIALS

These studieswere carried out on hair obtainedfrom a 31-year-oldmale. The hair

samplewascleanedwith sodiumdodecylsulfate(12.5%), exhaustively washedwith distilledwater,andconditioned at appropriate relativehumidityat 2 iøC. EVALUATION

METHODS

Tensilemechanical properties weremeasured with 50-mm longspecimens extended at 50%/min (strainrate 0.0083 s-•). Wet measurements werecarriedout on a sample soakedin waterfor 2 hours.The apparatus andexperimental procedures for the study of torsionaleffectsin tensileloadingand tensilefatiguehavebeendescribed earlier(1). Fatigueexperiments extended to 11 kc, with appliedloadsrangingfrom 10 to 40 g. HAIR

TREATMENTS

Treatment withCreme Relaxer.Singlefibersweretapedto a plasticboardandan alkaline permanent cremerelaxer(RevlonRealistic ©, superstrengthformulation) wasapplied for 20 min with strokingmotionsof the fingersto straightenthe hair. During this procedure about8% of thefibersbroke.The remainingfiberswererinsedwith distilled water to removethe relaxerand were subsequently treatedwith a neutralizershampoo (Revlon Realistic © Herbal Deep Clean Shampoo).The fiberswere then thoroughly rinsedwith distilledwater,blotteddry, andconditioned at 65% RH and 21øC.This samplewill be referredto as "superrelaxed hair."

Treatment withThioglycolic Acid.Hair samples in theformof smalltresses wereimmersed in a 5.5% solutionof thioglycolicacid (pH = 9.3, adjustedwith ammoniumhydroxide)at 35øCfor 20 min. During this treatmentan attemptwasmadeto straighten the fibersby strokinggentlywith glovedhands.Subsequently, the tresses wererinsed

EFFECTS OF TREATMENTS

ON NEGROID HAIR

with distilledwater,blotteddry, andconditioned at 65% RH and2 iøC. No breakage wasobserved during the treatment. Humectant Treatments. Samplesof Negroid hair fibersweretreatedwith either distilled

waterfor 30 rain at 40øCor 5% monoethanolamine thioglycolate at pH 8.5 for 30 min at 40øC.The fiberswerenot straightened duringthesepretreatments. Samplesof fiberstreatedas abovewere then treatedwith the followinghumectant solutions:

1. 5% aqueoussolutionof polyacrylicacid (PAA) (MW = 1800) at pH 8.5 (adjusted with ethanolamine).

2. 30% aqueoussolutionof glycerin. 3. 30% aqueoussolutionof glycerinthat was 2M in urea at pH 8.5 (adjustedwith ethanolamine).

4. 10% aqueoussolutionof dimethylsulfoxide (DMSO). 5. 5% aqueoussolutionof polyacrylicacid that was5% in dimethylsulfoxideat pH 8.5 (adjustedwith ethanolamine). Humectant treatmentswere carriedout in two different ways. In the first, the hair

fiberswere treatedwith the appropriatehumectantsolutionfor 3 daysat 40øC and thenwereconditioned at 65% RH and21øCwithoutrinsing.In the second treatment method,the samples weretreatedfor 1 hourat 40øCeachday for a periodof three days,with no rinsingbetweentreatments;fiberswereconditionedat 65% RH at 2 IøC. MOISTURE

SORPTION

Moisturesorptionby the hair samples wasdeterminedby equilibratingmO.5 g quantities of dry hair (dried in vacuumover P205 for 3 days)for one week in desiccators containingappropriatesaturatedsalt solutionsto give the requiredrelativehumidities. RESULTS COMPARISON

AND

DISCUSSION

OF NEGROID

AND

CAUCASIAN

HAIR

The manageability problemsand damagearisingfrom the kinky natureof Negroid hairhavebeendiscussed earlierin thispaper.The relativelystraightnatureof Caucasian hair makesit moremanageable and lessproneto mechanical damage,sothat thereare significantdifferences in the mechanical andfractographic behaviorof thesetwo types of hair. In addition,thereseemsto be a significantdifferencein ellipticity, whichmay be of biogenericorigin. The ellipticity, extension and stressat break,and fractionof prematurefailures(arbitrarilydefinedasthe percentage of fibersbreakingbelow22% extension) are shown in Table I.

The datain Table!, thoughobtainedfor onlyonesampleof Negroidhair, arebelieved to showcharacteristic differencesand illustratethe problemsassociated with Negroid hair. Although thesemeasurements were made at different strain rates, the differences in propertiesaretypical. Somecaution,however,shouldbe exercised in interpretation of theseresultsasthey are at variancewith thoseobtainedby otherworkers(3). Scanningelectronmicroscope studiesof fractureendsof Negroid hair haveshownfive differentpatterns,i.e., smooth,step, angle,fibrillated,and split ends,diagrammed

42

JOURNAL OF THE SOCIETYOF COSMETICCHEMISTS Table

I

SomePropertiesof Negroid and CaucasianHair at 65% RH

Ellipticity Strain rate (%/min) Brk. ext. (%) Brk. stress(GN/m2) Premature failure (%)

Negroid (2)

Caucasian(4)

1.895 + 0.083 50 27 + 5 0.123 + 0.016 22

1.17 + 0.04 100 38 + 1 0.202 + 0.011 5.1 50

40

I

3O

/ / I I

20

I I

I

/ /

I0 / /

o

o

EXTENSION

Figure4. Stress-strain curves fortypicaluntreated andtreated Negroidhairfibersfatigued to 11kcunder 30-gload,showing highlevels ofcreep.Solidlines:measured forfatigued fibers.Dashed lines:calculated if fibershad not beenfatigued.

the cuticleandnearly4 timeshigherin the cortexof thioglycolic acid-treated hairthan in the corresponding regionsof alkali-relaxed hair. The swellingbehaviorof fibersin formicacid, shownin Table VI, clearlyreflectsthe differencein the crosslinkdensity in the treatedfibers.This is alsoprobablyresponsible fbr their fractographic behavior.

EFFECTS OF TREATMENTS ON NEGROID HAIR Table

49

VI

DiameterSwelling(%) of Untreated,Superrelaxed, and Thioglycolate-Treated Negroid Hair

PREMATURE

in Formic

Acid

Treatment

Formic Acid

Untreated

47.8

SR

76.8

+

13.3

TGA

118.6

+

18.2

FAILURE

AND

THE

EFFECT

_+ 10.6

OF HUMECTANTS

The mechanicaland fractographicbehaviorof keratin fibers is profoundlyaffectedby their moisturecontent.Distributionsof extensionto breakfor the Negroid hair sample usedin this work are shownin Figure 5. At 65% RH, nearly26% of the fibersshow prematurefailure (breakingbelow22% extension).Wet fibers,on the otherhand, do not showprematurefailuresto any significantlevel. This showsthat the presenceof moistureplays an important role in the fracturebehavior,possiblycausingstressrelaxationat the cracktip and thus reducingthe rate of crackgrowth. From a practicalpoint of view, a slight increasein moisturecontentcan be achieved by treating the fibers with humectants,which can be incorporatedin hair care formulations.Two suchhumectants,polyacrylicacid (PAA) and glycerin, along with the strongswellingagent dimethyl sulfoxide(DMSO), havebeeninvestigated.The effects of these treatments on breaking extensionand prematurefailure are shown in Table VII.

NUMBER FRACTION

65 %RH

0

8

16

24

32

40

.14

.12 .10

....

II

wet

, , ,

EXTENSION-TO- BREAK (%)

Figure 5. Changeof extension-to-break distributionfor untreatedNegroid hair fiberswith relativehumidity.

50

JOURNALOF THE SOCIETYOF COSMETICCHEMISTS Table

VII

BreakingExtensionsand PrematureFailuresfor Negroid Hair Treated in Various Humectantsfor Three Days Premature

Treatment

Failures

Brk. Ext. (%)*

(%)

Untreated

29 --- 3

26

PAA (5%)

38 --- 2

Glycerin (30%) Glycerin (30%) + 2 M urea DMSO (10%) DMSO (10%) + PAA (5%)

38 39 33 38

+ + --+

3 2 3 2

2

6 4 14 4

* 95% confidence,50 specimens.

The data in Table VII showthat humectanttreatmentsincrease the breakingextension significantly and alsoreducethe numberof prematurefailures.Polyacrylic acidseems to be the mosteffectivein reducingprematurefailures,followedby glycerin.Because DMSO is only partially effectiveand the additionto ureato glycerindoesnot havea significanteffect, DMSO and urea were eliminated from further studies. Sincemost relaxing formulationsinvolvereductionof the fiber, the effectof humectants

wasstudiedon fibersreducedby 5% thioglycolicacid(TGA) at pH 8.5 and 40øC for

30 min. Thesefibers,alongwith untreatedfibers,weresubjected to the two typesof humectanttreatmentdescribedearlier,i.e., 3 dayscontinuousand 3 1-hourtreatments on consecutive days. Specimenswere then subjectedto tensilestresson the Instron machine.Breakingextensions andprematurefailuresareshownin Table VIII.

Whereasthe data in Table VII showan approximatecorrelationbetweenbreaking extensionand prematurefailure, the datain TableVIII do not supportthis conclusion unequivocally. Breakingextensions of fiberstreatedfor 3 daysaregenerallylowerthan for thosetreatedfor 3 1-hourperiods.This must be due to somedegradation or loss of structuralelementsfrom the fiber structure.No significantdifferencein terms of breakingextensionwasobserved for the two typesof treatmentwith PAA on untreated fibers. However, on TGA-reducedfibers, a significantdifferenceis seen,suggesting that either low molecularweight componentsof PAA diffuse into the reducedfibers or some structuralelement is more easily extractedfrom reducedfibers than from untreated fibers. Overall, the data in Table VIII indicate that PAA is more effective

in eliminatingprematurefailuresthanglycerinandthat short-termtreatmentsaremore effectivethan long-term treatments. MOISTURE

SORPTION

The moistureregain of humectant-treatedNegroid hair was determinedto establish differencesbetweenthe humectantsusedin this investigation.Moisture regain measurements made at three different relative humidities

are summarized in Table IX.

Samplestreatedwith PAA showa trend toward higher moistureregainthan those treated with glycerin. However, the breaking extensionfor PAA-treatedfibers is slightly lower than that of the glycerin-treatedfibers. PAA treatmentis considerably

EFFECTS OF TREATMENTS

Table

ON NEGROID

HAIR

51

VIII

Effectof Humectantson BreakingExtensions and PrematureFailuresof Reduced(TGA) NegroidHair BreakingExt. (%)* Treatment Untreated PAA (5%)

Premature failures (%)

3 Days**

3 X 1 h**

3 Days**

3 X 1 h**

29 -+ 3 38 + 2

29 -+ 3 38 -+ 3

26 2

26 6

14

Glycerin (30%)

38 + 3

42 ___1

6

TGA + PAA (5%)

34 -+ 3

47 q- 3

10

4

TGA + Glycerin(30%)

43 - 4

52 -+ 5

10

16

* 95% confidence,50 specimens. **

Treatment

time.

more effectivein reducingprematurefailure in untreatedfibersthan glycerin.It is possiblethat in PAA-treated fibers more moistureis held near the surfacewhere the polymer is concentrated,thus increasingstressrelaxationnear the crack tip, which reducesthe numberof prematurefailures.This assumption is supportedby the trend towardlowerbreakingextensions for PAA-treatedfibersas comparedwith glycerintreated fiber. Glycerin is known to penetratethe fiber and is unlikely to localize moisturein the vicinity of the surface.

CONCLUSIONS

Kinky structure,fiber twist, and ellipticity of fiber crosssectionare responsible for damageproblemsassociated with Negroidhair. Because of severeentanglements, routine groomingproceduresinvolvehigh enoughloadsto damagethe fiber. It hasbeen shownexperimentallythat tensileloadingof thesefibersinvolvestorsionalstresses that can lead to initiation or enhancement of damagein the regionsof twist. Although untreatedfiberssubjectedto fatigueshowa large numberof fibrillatedfractureends, fiberssurvivingthe 11-kcfatiguetest do not showsignificantincreases in the number

of fibrillatedendsaftertensilefracture.This suggests that the largenumberof fibrillatedendsobservedin the fibersthat failedin the fatiguetestresultedfrom the effects Table

IX

Moisture Regain of Humectant-TreatedNegroid Hair at Various Relative Humidities Premature

Regain (%) Treatment

Breaking Ext. at 65% RH

33% RH

44% RH

Untreated

8.7

10.3

Glycerin (30%) PAA (5%) TGA + Glycerin

7.1 9.6 7.5

9.5 12.4 11.5

10.7

11.8

15.9

TGA

+ PAA

Treatment

time.

65% RH

Failures (%)

(%)

3 Day*

3 h*

15.3

29 q- 3

26

26

15.1 17.2 15.6

42 _+ 1 38 -+ 3 52 q- 5

6 2 10

14 6 16

47 q- 3

10

4

52

JOURNALOF THE SOCIETYOF COSMETICCHEMISTS

of fatiguein combinationwith somepreexistingdamage.Treatmentof the fiberswith alkali or thioglycolicacid reducesthe number of fibrillated ends, especiallyat high fatigue loads,and increases the numberof smoothfractures. Prematurefailuresresultfrom preexistingdamagein the fiber. Moisturecontentplays an important role in controllingthe number of prematurefailures,suggestingthat stressrelaxationat crack tips may be involved. Treatment with humectantssuch as polyacrylicacid and glycerinhavebeenfound to be beneficial,PAA appearingto be more effective.There is someevidencethat increasein moisturecontentmay be responsiblefor the reductionin prematurefailuresin humectant-treatedhair. Breaking extensiondata on humectant-treated fiberssuggestthat in PAA-treatedfibersa larger amountof moistureis likely to be presentnearthe peripheryof the fiber wherecracks exist.Thioglycolicacidpretreatments seemto enhance preexistingdamagein the fibers, thus eliminating the beneficialeffectsof humectants. ACKNOWLEDGEMENT

Thesestudiesformedone aspectof the work on the Textile ResearchInstitute project "Propertiesof Negroid Hair," supportedby NicholasLaboratories,Inc.

REFERENCES

(1) J. Eppsand L. J. Wolfram, Letter co the ediror,J. Soc.Cosmet. Chem.,34, 213 (1983). (2) Y. K. Kamarh, S. B. Hornby, and H.-D. Weigmann, Mechanicaland fracrographic behaviorof negroidhair, J. Soc.Cosmet. Chem.,35, 21-43 (1984). (3) J. Menkarr, L.J. Wolfram, and I. Mao, Caucasian hair, Negro hair and wool. Similaritiesand differences. J. Soc.Cosmet. Chem.,17, 769 (1966). (4) Y. K. KamarhandH.-D. Weigmann,Fracrography of humanhair,J. Appl.Po/ym.ScJ.,27, 38093833 (1982).

(5) J. Chao, E. Newsom, I. M. Wainwright, and R. A. Mathews, Comparisonof the effectsof some reactivechemicalson the proteinsof wholehair, cuticleand cortex,J. So;.Cosmet. Chem.,30, 401413 (1979).