J. Soc.Cosmet. Chem.,28, 549-569 (September1977)
Theelectrostaticpropertiesof humanhair ANTHONY
C. LUNN
and ROBERT E. EVANS, American
CyanamidCompany,ChemicalResearch Division,Stamford,CT 06904.
Received October 12, 1976. Presented, Ninth IFSCC Congress, Boston, MA.,June 1976.
Synopsis
Three factorshave been studiedwhich are significantin the developmentof ELECTROSTATIC CHARGE on HAIR FIBERS: (1) the chargegeneratedby separationbetween hair fibers and brush or comb; (2) the mobility of chargeon the fibers;and (3) the distributionof chargealongthe fiber length. Instrumentationhas been developed to measureeach of these parameters,and the effect upon them of quaternaryammonium compoundsand other fiber treatments. Quaternary antistaticagentsare found to reducesubstantiallythe chargegeneratedon the fibers;the half-life of chargemobility varieswith the quantityof agenton the hair. The densityof chargeis greatestnear the fiber tips, correspondingto the regionof a peak in the combingforce. It is concludedthat the mechanismof actionof theseantistaticagentsis primarilyone of lubrication:a reductionin combingforceleadsto a reduction of staticchargegeneratedon the hair.
INTRODUCTION
While the phenomenonof staticelectrification,first recorded by the ancientGreeks, has intrigued physicistsover the centuries, our knowledge and understandingof electrostaticsas related to practicalproblems remains even today at an elementary level. Yet problemsassociated with the buildupof electrostatic chargeon a bodyare of commercialimportancein manyindustries.For example,staticelectrificationhasbeen of major concernto textile manufacturersand users,especiallysincethe development of syntheticpolymers,and, of course,to the plasticsindustryitself. In the hair-careindustry, problems arise from staticchargesin brushedor combed hair, particularly at low humidity levels. The fibersare mutually repelled by thesecharges,thereby showing the phenomenonof "flyaway"which is unattractiveand which makeshair hard to combor to keep in place. 549
550
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
Substantial effortshavebeenmadeto developmeansto ameliorateelectrostatic problems.On humanhair, cationicquaternaryammoniumcompounds are in commonuse for thispurpose.Little is knownof the mechanism of actionof suchantistatic agents, however,andin suchcircumstances it isdifficultto developimprovedmaterials.
Hypotheticalreasoning led us to believethat 3 principalfactorscontributeto the severityof the "flyaway"of humanhair.The firstis the magnitudeof chargewhichis generated bythecontact andsubsequent separation of hairandcomb.Thesecond factor is themobilityof chargeanditsrateof dissipation fromthefibers.Thethirdfactor is the distributionof chargealongthe lengthof the combedfibers.In principle,the desiredobjectiveof reducedelectrostatic effectscanbe approached by alteringeach one of these factors.Either a reductionin the magnitudeof chargegeneratedor an
increasein the mobilityof thatchargecanbe effective.Mutualrepulsionof fiberscan alsobe alteredby changing the distribution of chargedensityalongthe lengthof the fiber.
The generationof staticchargeswhen unlike objectsare rubbedtogetherarisesfrom an unequaltransferof chargesacrossthe interfacebetweentwo bodiesin contact. Whenthe bodiesareseparated, theyareeachleft with netcharges of oppositesignand of magnitudeequalto the differentialchargetransferred.Theoreticalaspects of this processare discussed by Vick (1), Arthur (2), and Hersh and Montgomery(3). The chargegeneratedby rubbingfilamentstogetherhasbeen studiedexperimentally by Hersh andMontgomery(4). Henry eta/. (5) measuredboth chargemagnitudeandthe rate of its decayfrom rubbed textile fabrics.Barber and Posner(6) measuredthe chargegeneratedby combinghumanhair.Mills etaL (7) alsoattemptedto measurethe chargegeneratedby combinghair, but the methodemployeddid not permit a distinction to be made betweengenerationand dissipationmechanisms.
The rateof dissipation of chargeto electrical grounddepends on theeaseof movement of chargeson the body,a propertywhichwe here call "chargemobility."A complementaryphenomenon,the rate at whichchargedevelopson the bodyin the presence of an electrostatic potential,is similarlydeterminedby the chargemobility.Charge mobilityis itself dependentprimarilyon the conductivityof the material(5,8). Shashoua(9) measured the ratesof buildup anddecayof chargefrom filmsandfabrics. Ballou(10)measured decayratesfromtextiles; healsoconsidered charge generated on movingyarns.Unfortunately,little informationis availableon the mobilityof charge on human hair.
The distributionof chargealongthe lengthof a fiber, althoughnotedby Ballou(10) as
important,hasreceivedvery little investigation. The only other discussion of such phenomenais by Sprokel(11), who studiedthe variationof chargealonga running textile yarn.
In a published work,the relativeimportance of chargegeneration, mobility,anddistributionto the incidenceand controlof electrostaticchargesis rarelyconsidered, and a cleardistinctionbetweenthem is not alwaysdrawn.Instrumentationhas,therefore, been developedat theselaboratoriesto studyeachof theseparametersseparately, on treated and untreatedhair, with the intention of evaluatingtheir relative importance andof elucidatingthe mechanism of actionof antistaticagentson humanhair.
ELECTROSTATIC
PROPERTIES
OF HAIR
551
EXPERIMENTAL HAIR
TRESSES:
TREATMENT
AND
CONDITIONING
For the work reported here, virgin brown hair*wasused.Test tresseswere cut perpendicularly to a length of 20 cm and glued at the root endsto a plastictab, on which the hair was spreadover a width of 3.8 cm. The weight of hair in each tresswas 1.3 +- 0. ! g. In order to get reproducibleresultsin chargemobility measurements,it wasimportant to spreadthe hair uniformly over the 3.8 cm width of the tab. A mounting jig containing a fixed fine-toothed comb was employed to facilitate samplepreparation; the hair fiberswere spreadevenly acrossthe comb before beingglued. Before use, the tresseswere cleaned with a solution of sodium lauryl sulfate, then rinsed thoroughly. When the effect of antistatic and other treatments was to be studied, these materials were typically applied as follows: 0.6 cc of the particular shampoo, creme rinse, or antistat agent was applied to the wet hair, worked in manuallyfor 40 sec,rinsedin runningtapwater for 20 sec,and then air dried. All treatments discussedbelow were rinsed in this manner before being dried, unlessotherwise specified. As is well known, relative humidity is a critical variable in electrostaticexperiments. All experiments were conducted in an environment controlled to + ! per cent RH at 23 + 0.5øC.To avoiderrorsarisingfrom the hysteresisin the water uptake of hair (12), tresseswere alwaysbrought to equilibrium at the test humidity from a higher humidity level. It was found necessaryto conditionthe hair for at least40 h at the test humidity before making measurements,in order to obtain consistentresults.
THE
MEASUREMENT
OF CHARGE
GENERATED
BY
COMBING
The generation of electrostaticcharge by the separationof 2 bodies is a notoriously variable procedure subject to considerableirreproducibility, and highly sensitiveto test conditionssuchas surfacecontamination(•[3). For this reason,many workers have eschewedmeasurementsof chargegeneration, preferring to determine electricalresistivity or chargemobility rates (8,9). Nevertheless, the processof chargegeneration is critically important, and it was consideredessentialthat it be studied. A method was developed to measurethe generation of chargeunder conditionssimulatingactualuse, i.e., the combing of hair, with a procedure designedto control the variablesas closely aspossible. The apparatusused is shown in Fig. •[. The hair tresses,comb, and Faradaycagewere all enclosedin a humidity controlled box which wasmaintainedat 23øC. Both tressand combwere carefullyinsulatedfrom electricalgroundduring the combingoperation,to ensure that no chargewould be lost by conductionto ground before measurement. The tresswas held in a polystyreneinsulatedgrip while being combed,and the comb was mounted in a polystyrenehandle. Some experimentswere alsoperformed with the combgrounded. Commerciallyavailablecombsof variousmaterialswere used.
*DeMeo Brothers, New York, N.Y.
552
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS RELEASE
CLAMP-
INSULATED
FROM
GROUND
HAND
HELD
INSULATING
HANDLE
% ß
FARADAY
CAGE
ELECTROMETER I
Figure1. Apparatus used tomeasure electrostatic charges generated onhairtresses bycombing
Residual charges onthetress werefirstremoved byexposure toaradioactive polonium deionizer.The tresswashandcombedfor the requisitenumberof strokeswith the
insulated comb.Charge wasthenmeasured byreleasing thetressfromthepolystyreneinsulatedgrip and depositingit in a Faradaycage,whichwasconnectedto an electrometer. • The capacitance of the Faradaycageandconnecting cableswas100 pF, whichwasnegligible compared to thecapacitance of theelectrometer. Thecharge Q onthetresscould,therefore,bereaddirectlyfromtheelectrometer scale. The principlesources of errorwerevariations in the relativehumidity,variations betweenreplicate tresses, andthe irreproducibility of handcombing. For accurate measurements,3 to 5 replicate tresseswere used, with 5 successivedeterminationson
eachtress.In thiswaythecharge Q couldbedetermined witha 95 percentconfidence intervalof -+ 15 per cent. THE MEASUREMENT OF CHARGE MOBILITY ON HAIR
Themobilityof electrostatic charge onabodycanbecharacterized bytherateatwhich charges builduponanddecayfromit. The half-lifeof charge induction, r c,isthetime *Model 610 BR, KeithleyInstruments, Cleveland,OH.
ELECTROSTATIC
PROPERTIES
OF HAIR
553
CONNECTOR COAX-CABLE
WIRE
UPPER SHELL :TOR •)DE
COMB
HIGH VOLTAGE CONNECTOR
LOWER SHELL
MOUNT
HIGH VOLTAGE EL ECTRODE (a)
VOLTAGE
HIGH
CABLE
VOLTAGE
ELECTRODE
(b)
Figure 2. Faradayshell apparatusfor measurementof chargemobility, with hair tressin position:(a) side view;(b) exposedtop view (uppershellremoved)
554
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
2.1 KV
• +
POWER
20MEG •
SUPPLY
TEST.i.•..•,,,•--I
I I i I_-
__ HUMIDITY
I
ELECTROMETER
ZERO (•) OSW. 0
I CABINET
RECORDER
Figure 3. Wiring diagramfor chargemobilitymeasurements
required for chargeto build up to one-half of its equilibrium value when the body is exposedto a high potential. The half-life of chargedecay,r I•, is the time taken for chargeto diminishto one-halfof its initial valuewhen the chargedbody is connectedto electrical ground. A perfect conductorchargesand dischargesinstantaneously,and, therefore, has a charge mobility half-life of zero. Charges on a perfect insulator, however, are immobile, and the half-life is infinite in such a material. Poor insulators suchas human hair havefinite half-liveswhich vary widely with surfaceconditionand with relative humidity. The experimentalprocedureusedin thiswork for the measurementof chargemobility is a modificationof an ASTM method for the determinationof chargemobility on flexible plasticfilms (14). The apparatusis shownschematicallyin Figs.2 and 3. The principle of operation is as follows. The fibers of the hair tress, connectedto electrical groundat eachend, are chargedby inductionfrom a highvoltageelectrode.The charge on the hair is monitoredby a detectorelectrode.The rate at which chargebuildsup is characterizedby the half-life of charge induction. When the high voltage sourceis removed, the chargeon the hair diminishesto zero at a rate characterizedby the halflife of chargedecay.The basicprinciple of the method is similarto that usedby Shashoua (9) with the exception that in his case the specimenwas chargeddirectly, whereas,in the presentprocedure,it is chargedby inductionand thereby,acquiresa chargeof polarity oppositeto that of the voltagesource.
ELECTROSTATIC
PROPERTIES
OF HAIR
555
The test fixture, which we call here a Faradayshell, was constructedaccordingto the
ASTM description.The apparatusconsistsof 2 cylindricalbrassshells, 10 cm in diameter. Each cell contains an electrode 5 cm in diameter which is insulated from
groundby a polytetrafluoroethylene (PTFE) spacer.The electrodesare recessed0.6 cm from the planeof the specimen.The wallsof the fixtureare groundedandare heavy to providegood electricalcontactwith the sample.The Faradayshell and the hair tressesundertestwere placedin a chamberof controlledrelativehumidityat 23øC.
Safetyinterlocks,which disconnectthe high voltagesource,were containedin a box which coveredthe test fixture. This ensuredthat the high voltagesupplycould not deliver a lethal shockto the operator. The hair tresswas spreaduniformly over a 3.8 cm width by insertingthe tressat the
plastictab end in one of the combsmountedadjacentto the shells.A hand-heldcomb was inserted behind the fixed comb and pulled acrossthe shell; the tress was then affixedin the combon the oppositeside.The finalpositionof the tressis shownin Fig. 2. The combswere usedsolelyasa guide for specimenmounting,and were not usedto
generatechargeon the tress.Any residualchargeson the specimenafter mounting were removed with a radioactivedeionizer before closingthe shellstogether.
In addition to the provisionof combsadjacentto the Faradayshell for mounting the specimen,the other importantmodificationof the ASTM procedurewasthe partial enclosure of the bottom shell; this shell was covered with a thin brasssheet with the exceptionof a 4 cm width in whichthe specimenwasmounted(Fig. 2). The purposeof this modification was to shield the detector electrode in the upper shell from stray fieldsof the chargingelectrodeleakingaroundthe specimen.
In operation,a potentialof--- 2100 V is appliedto the electrodein the lower shellby a high voltage sourceat time zero. The upper detector electrode is usedto monitor the electrostaticfield potential,andis connectedto an electrometerandchartrecorder(see Fig. 3). At time zero, the detectorelectrodeinstantaneously chargesby inductionto a potentialoppositeto that of the chargingelectrode.Sincehair is not a perfectconductor, the chargeinitiallyinducedon the hairis zero. Chargebuildsup on the hairby induction at a finite rate; this chargehasa polarity oppositeto that of the high voltage source.Sincethe total chargewithin the Faradayshellsis zero from Gauss'Law (15), the chargeon the detector electrodedecreasescorrespondingly.When the hair is fully charged,the chargeon the detectorelectrodefallsto zero. The output of the chartrecorderduring this processis shownin Fig. 4, togetherwith a representationof the stateof chargeon the hair and on the two electrodesasthe hair is chargedand discharged.The charge,Q(t), on the hair at time t is related to the voltage, V(t), on the detector electrode as follows: Q(t) Q0
- 1---
v(t) v0
during charging,and Q(t)
V(t)
Qo
Vo
(2)
556
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
i
(•) A ]D•'.I.'IOA
I
ELECTROSTATIC
PROPERTIES
OF HAIR
557
duringdischarging. Q0is the chargeon the hairwhen fully charged. The initial voltageV0 inducedon the detectorelectrode is not accuratelyrecordedby the recorderpen becauseof inadequateresponsetime. V0 is thereforedeterminedin a separateexperiment in which no specimenis present.The half-life for chargeinduction
r c is determinedfrom the recordertraceasshownin Fig.4. To measuredecayof chargefrom the fully chargedhair, the chargingelectrodeis disconnectedfrom the highvoltagesource,and connectedto ground.The chargeon the detectorelectrodethen becomesequal and oppositeto that on the hair, sincethe total charge in the enclosuremust remain zero. The half-life for charge decay r D is determined from the dischargecurveof the detectorelectrode(Fig. 4). For all measurements, the electrometer output must be correctedfor drift. Other experimental details
andprecautionsare describedin the ASTM procedure(14). Four separatedeterminationsof r were madeon eachhairtress:both charginganddischarging,each with both positive and negativechargesinducedon the hair. The halflife was calculatedas a root mean squarevalue, followinggeneralpractice(9). With 3 replicate tresses,raMSof the 4 determinationscould be obtained with a 95 per cent confidencelimit of -+ 25 per cent.
THE
MEASUREMENT
or
CHARGE
DISTRIBUTION
ALONG
THE
HAIR
FIBERS
An apparatuswas devised to measure the variation of charge generated along the length of hair fibers as they are combed.The systemis shownin Fig. 5. The hair tressis attachedat the tab end to the cross-headof an Instron testingmachine.• It is insertedin a lower test comb of hard rubber, and passesalsothrough an upper metal combwhich is grounded. A cylindrical brassdetector electrode, on the inside of a glasscylinder, surroundsthe specimenabovethe rubber comb. A brassshieldingelectrodewhich is connectedto ground surroundsthe outer surfaceof the glasscylinder. Grounded guard electrodesare placed adjacent to the inner detector electrode. The aluminum comb also acts as a guard electrode. The inner electrode is connectedto an electrometer and chart recorder.
When the hair tressis pulled through the apparatus,chargeis generatedon the fibers as they passthrough the lower comb. The charge on that part of the fibers which is immediately above this comb is sensedby the detector electrode. The guard electrodes and the upper metal comb serveto screenchargeson the rest of the hair tressfrom the detectorelectrode. A fiber length of 1.9 cm is sensedby the detectorelectrode.By recordingthe electrometeroutput asa function of time as the hair is pulled through the combs,the variationof chargealongthe length of the hair is obtained.
The force required to pull the tressthrough the comb can also be recorded on the Instron. Becauseof interference from the metal comb which is present for measurements of chargedistribution,however, force measurementswere made in a separate experiment in which the metal comb was removed from the apparatus.
*Model 1125, Instron Corp., Canton, MA.
558
JOURNAL
OF THE SOCIETY OF COSMETIC CHEMISTS INSTRON
LOAD CELL
INSTRON
RECORDER
SHIELDING
ELECTRODElhi J
METAL
COMB
(GUARD ELECTRODE)
DETECTOR ---L.J
ELECTRODE TEST COMB
(HARD RUBBER)
HAl R TRESS
CHART
RECORDER
Figure 5. Apparatususedto measuredistributionof chargedensityalonglengthof hair asit is combed
RESULTS CHARGE
GENERATED
BY
COMBING
The chargegenerated on the hair by combing was found to be of positive sign,for typical hair treatments and for all comb materialsexamined.This finding is consistent with two factors.First, keratin is at or near the positive end of the tribolectric series(9), meaningthat when it is rubbed againstother materialswhich are lower than keratin in the series,a positivechargeis developedon the keratin.(It is possibleby certaintreatments to alter the positionof keratin in the tribolectric series(16)). Second,when 2 bodies are rubbed together under conditions where the bodies contribute unequal areas to the rubbing surface, the body which contributes the larger area tends to develop a positive charge(17). When hair is combed, it is the hair which contributes the larger area of contact. The magnitude of charge generated Q varied with the comb material and with the number N of manualcombstrokesappliedto the tress,asshownin Fig. 6. The slopeof the curve with nylon and hard rubber combsis consistentwith the findingsof Barber and Posner(6) who useda polystyrenecomb.These combmaterialsare very poor conductors, and the results show an increaseof chargewith each successivecomb stroke,
ELECTROSTATIC
PROPERTIES
I
'""
OF HAIR
I
559 I
5
o o
x
0 3