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
<[ 2
'('Y'
/
HARD
RUBBER....-_--r"i--
• • •-'
0
ALUMINUM
I
I
I
5
10
15
NUMBER
OF COMB
STROKES
N
Figure 6. ChargeQ generatedby combinghair tresseswith N manualcombstrokes,with variouscombmaterials,at 50 per cent RH (
) untreatedhair; (......
) hair treated with cremerinse A
with an eventual saturation level. When an aluminum comb is used, however, there is
very little increasein chargeon the hair with successive
combstrokes.
We hypothesizethat the conductivecombactsasa sink for charges.On the firstcomb
pass,the combbecomesnegativelycharged;the hair positivelycharged.As the combis
passedagainthroughsuccessive
incrementsof the chargedhair, mobile chargeson the
conductivecomb neutralizethe chargeon the hair, whichthen rechargesto its original
level when the comb leaves each increment
of the hair. This does not occur with non-
conductingcombsbecauseof their low chargemobility;in thiscase,chargeson the hair
are only partially neutralizedon additionalcombingstrokes,so that the total charge
increaseswith eachpassof the comb. Saturationis reachedwhen the chargedensityon
the comb reaches a certain level.
The magnitudeof chargegeneratedwith variouscomb materialswasfound to increase
in the order
aluminum•< hard rubber < nylon
at all humidity levels examined(20 to 50 per cent RH) and with all hair treatments
tested.
560
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
m
SHAMPOO
UNTREATED
A
3-
o
CREME
R
_
2-
CREME
RI
1-
0
10
20
RELATIVE
30
HUMIDITY,
40
50
60
PERCENT
Figure 7. Variation of chargegeneratedon hair tresseswith relative humidity. Five comb strokes,hard rubber comb
Data for hair treated with a commercialcreme rinse formulation containinga quaternary ammonium compound are shown in Fig. 6, as well as data for cleanuntreated
hair. The formulation is particularly effective in reducing the value of Q when hard
rubber and aluminum
combs are used.
The magnitudeof chargegeneratedwasunaffectedby groundingthe comb, even with
the metal comb.This is, in fact, to be expected,sincethe chargearisesfrom separation
of the two objects (hair and comb). Grounding the comb merely dissipatesthe charge
on the comb (if the combis a conductor);the chargeon the hair is unaffected.Grounding the hair, on the other hand, would causethe chargeon the hair to dissipateat a rate
related to its chargemobility half-life, so that the chargemeasuredwould vary with the
elapsedtime betweencombingand measurementof the charge.It is for thisreasonthat
the hair wasalwaysinsulatedfrom groundin thesetests.
The charge generated varies greatly with the relative humidity. It was found to
decreaselinearly over the range 20 to 50 per cent RH, as shownin Fig. 7. A similar
finding was reported by Barber and Posner (6) over the range 30 to 70 per cent RH.
ELECTROSTATIC
i
i
PROPERTIES
OF HAIR
I
I
30
40
561
RINSED
SBAC
UNRINSED
10
20
RELATIVE
HUMIDITY,
50
60
PERCENT
Figure 8. Variation of chargegeneratedon hair tresseswith relative humidity. Two comb strokes,hard rubber comb.SBAC: stearyldimethylbenzylammoniumchloride
We may write the empiricalrelationshipvalid over theserangesof humidity
Q(RH) = Q(0) - o•. RH
(3)
where RH is the relative humidity, Q(0) is the chargegenerated at zero per cent RH,
and -o• is the slopeof the plot of Q(RH) versusrelative humidity. In routine testing,
therefore, it wasonly necessaryto make measurementsat 2 humidity levels;30 and 50
per cent RH were selectedasconvenient for this purpose.
It is essentialto evaluatethe effect of relative humidity in order to obtain an accurate
measureof antistatperformance. Although treatments which reduce the staticcharge
generatedon hair generallygive resultswhich are superiorto untreatedcontrolsat all
humiditylevels,somematerialswere foundto havean antistaticeffecton hair at high
humidities,but gaveresultsworsethanuntreatedhair at low humidities.An exampleis
theionerie
polymer
[N+(CH3)2
ßCH2'CHOH'CH2]nn CI-, asshown
in Fig.8. This
material hasquaternary ammonium ions in the main chain of the polymer.
The amount of antistaticagent on the hair affectsthe magnitudeof chargegenerated.
Hair wastreated with stearyldimethyl benzyl ammoniumchloride (SBAC) by immersion in a 3 per cent aqueoussolution.When allowed to dry without being rinsed, so
that a relatively large quantity of material remained on the hair, the chargegenerated
562
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
I
I
I
I
I
I
|
I
I
I
I
22% RH
29% RH
43% RH
51% RH
•
•
20
•
•
40
I
ß
NEGATIVE
CHARGING
O
NEGATIVE
DISCHARGING
ß
POSTIVE
CHARGING
A
POSTIVE
DISCHARGING
I
60
I
I
I
80
TIME,
I
100
I
I
120
I
I
140
I
160
SEC,
Figure 9. Charge on untreated hair during chargingand discharging,as shownby relative voltageon the detector electrode.Chargingpotential:2100 V
was essentiallyzero (Fig. 8). When rinsed under a running tap for 20 or 300 seconds
before drying, however, the chargegeneratedby combingincreasedto finite but still
small values (Fig. 8).
CHARGE
MOBILITY
When a tressof hair, which is insulatedfrom electricalground, is chargedby combing,
the chargewill remain on the hair indefinitely. This wasconfirmedby an experimentin
which a combed tress was suspendedin a Faraday cageby a PTFE thread, with care
taken to ensure that the tress did not touch the Faradaycage.The chargeon the hair
was found to remain constantwith time. (Partial dischargeby dielectricbreakdownof
the atmospheremay occur, if the chargedensityon the hair is suchthat the dielectric
strengthof air is exceeded.The lossof chargewould occurinstantaneouslyupon combing, and would not be detectedby this experiment.)
When a charged tress is connected to electrical ground, the charge will decay. In the
caseof hair on the head, the scalpand body effectively act asa ground, becauseof their
large capacitancerelative to that of the hair. The rate of decaydependson the mobility
of chargeson the hair.
Charge mobility was measured by the Faraday shell apparatusdescribedabove, in
which the hair tress was chargedto and dischargedfrom a potential of 2100 V. The
chargeQ(t) on the hairwasmonitoredby the voltageV(t) on the detectorelectrode.
In Fig. 9, the detector electrode voltage is shown as a function of time, for untreated
hair at varioushumidity levels. Chargingand dischargingdata, for both positiveand
ELECTROSTATIC
PROPERTIES
OF HAIR
563
negativeapplied potentials,are plotted.
The linear relationshipbetween log V(t) and time in Fig. 9 confirmsthat the charge
builds up and decaysexponentially,as found by Wilson (8), Shashoua(9), and Ballou
(10). We cancharacterizethe processby the equations
Q(t)=Qo[1{-•77}
tin2
exp
]
(4)
during charging,and
Q(t)=Q0'exp{
- tIn21
(5)
during discharging,where Q(t) is the chargeon the hair at time t, Q0 is the equilibrium
charge,r c isthe half-lifeof chargeinduction,andr Dis the half-lifeof chargedecay.
In this work, no systematicdifferencewasfound betweenvaluesof r for positiveand
negativecharges,nor betweenvaluesfor chargingand discharging.This is shownby
the superpositionof the four curvesat eachhumidity level in Fig. 9, and a similar result
wasobtainedfor all hair samplesexamined,at all humiditylevels,whethertreatedwith
antistatic agents or not. We, therefore, computed the root mean square half-life as
follows
TI•,M
S= 1/2[(r+c)2+ (T+D)
2+ (T_C)
2-• (T_D)2]
1/2
(6)
Our findingscontrastwith those of Shashoua(9), who found that r+ was generally not
the sameasr_ in textile fabricsand films,and that r c and r Dwere alsounequalwhen the
voltageexceeded2000 V. Shashouaattributed the inequalityoff c andr Dto ionization
of the atmosphere.As discussedabove,we found no sucheffect in this work.
The logarithm of raMSvaries linearly with relative humidity, over the range 20 to 50
per cent RH (Fig. 10). Shashoua(9) obtained a similar result for textile fibers and
plasticfilms, over the range 15 to 65 per cent RH. Followingthis,we may write within
these ranges
TI•MS= T0exp [-fi ßRH]
(7)
where -fi is the slope of the line, and r0 is the extrapolated half-life at zero per cent
RH. Becauseof this linearity, experimentswere routinely conductedat only two humidity levels(30 and 50 per cent) in this test also.
Conventional shampoosand some creme rinses do not changer from its value for
untreated hair; other creme rinsesreduce r significantly(Fig. 10). The level of agent
remaining on the hair has a large effect on the half-life. The quaternary ammonium
compound SBAC, when not rinsed off the hair before drying, reducesr to very low
values. When it is rinsed off before drying, however, the half-life of the hair is little different from that of untreatedcontrols(Fig. 11). This is consistentwith the findingsof
Sprokel (11) and Steiger (18), who showedthat the surfaceconductivityof textile
fabricsandyarnsincreases
with the quantityof antistaticagentson the fibers.When the
quaternary compound SBAC is rinsed off hair before drying, only small quantities
remain on the fiber, so that the resistivityand hence the half-life are high.
564
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
10000
1000
100
UNTREATED
SHAMPOO
10
•
A
CREME
A
CREME
0
RINSE
B
I
I
I
i
I
10
20
30
40
50
RELATIVE HUMIDITY,
60
PERCENT
Figure 10. Variation of charge mobility half-life TitMSof hair tresseswith relative humidity. Charging
potential: 2100 V
ELECTROSTATIC
PROPERTIES
OF HAIR
565
100
SBAC:
10
300 SEC. RINSE
SBAC:
20 SEC. RINSE
m
SBAC:
0
10
20
30
RELATIVE
HUMIDITY,
UNRINSED
40
50
6O
PERCENT
Figure 11. Chargemobility half-life rR•lsof hair tresses.Chargingpotential:21 O0 V
CHARGE
DISTRIBUTION
ALONG
THE
HAIR
FIBERS
The variationof chargedensityq(x) alongthe lengthx of a dry hair tressasit is combed
was measuredwith the apparatusshownin Fig. 5. Some resultsare shownin Fig. 12
(upper graph). A clean untreated tressdevelopssome chargeall along its length, but
there is a substantialincreasein chargegeneratedas the comb passesthrough the final
few centimeters of the hair fibers. Hair treated with a commercial creme rinse formula-
tion which was rinsed 20 secbefore drying showsa similarcurve, but the end peak in
q(x) is much lower in this case(note the different ordinate scalefor this curve). When
the cremerinse is not rinsedbefore drying, the chargegeneratedis essentiallyzero.
The load p(x) required to pull the hard rubber comb through the dry tress showsa
pronouncedend peak force, which is likewisereducedsubstantiallyby the creme rinse
566
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
2.5
i
I
2.0
I
!I
I
I
.25
I
UNTREATED
CREME
RINSE
A:20 SEC. li -.15
RINSED
_ -- ---uJ
ß
ß
•
-
ß
,,••//•I
1
o
o
3.0
6.0
9.0
12.o
ooo!, ,UNTREATED
, ,
,00%
__.__
15.o
•-•--•
CREME RINSE A:
300
.....
RINSED
20SEC.
UNRINSED
/
0
.x
3.0
6.0
9.0
12.0
15.0
COMB TRAVEL ALONG TRESS, CM.
Figure12. Variation
of charge
density
q(x)(upper
graph)
andofcombing
forcep(x)(Dower
graph)
along
the
length
x ofa hairtress
asit iscombed.
Relative
humidity
50percent.Hardrubber
comb.
Combing
velocity
1.7 cm/sec
treatments
(Fig.12,lowergraph).The end-peak
forcehasbeendescribed
byTolgyesi
(19) andhe hasattributedthisto anaccumulation
of tanglingbetweenthefibersnear
thefreefiberends.Theeffectisnoteliminated
byprecombing
or byparallelizing
the
fibers.
Theexplanation
fortheriseingenerated
charge
in thefinalportionofthecombpass
is
not hardto find.It hasbeenshownby manyworkersthatthe amountof charge
generatedwhen two bodiesare rubbed togetherincreaseswith the normalcontact
forcebetweenthem(4,11,20,21,22).The effectarisesfrom the increasein realareaof
ELECTROSTATIC
PROPERTIES
OF HAIR
567
contactwith normal force; the chargegenerated dependson the real area of contact.
Now the combing force increasesin the last few centimeters of the hair tress, because
of tangling.The normal contactforce between hair and comb is, therefore, increased,
and so the amount of chargegeneratedin this region of the hair fibers is increased.
THE
MECHANISM
OF ANTISTATIC
AGENTS
By an examination of the available experimental evidence for the 3 properties
measured--the magnitudeof chargegeneratedby combing,the mobility of chargeon
the fibers,and the distributionof chargealongtheir length--we cannow considerthe
mechanismsof staticelectrificationof hair, and of the actionof antistaticagents.
The chargegeneratedby combingand the half-life of chargemobility both decrease
with increasingrelative humidity (Figs. 7, 8, 10, 11). The increasedchargemobility is
clearly a consequenceof the greater water content at higher humidities,althoughthe
exact relationship is not well understood and other factors are also involved (23).
Increasedmobility of chargeson the fiber leadsto a decreasein the chargegenerated
by combing, becauseof chargeconductionalong the fibers asthey are rubbed, and this
mechanismhas been postulated to explain the decreaseof generated charge with
increasingrelative humidity (3).
When a large concentrationof a quaternary ammonium compound is present on the
hair fiber, the surfaceconductivityis substantiallyincreased,asevidencedby the very
low half-life of charge (Fig. 11, SBAC, unrinsed). The negligible charge generated
under such circumstances(Fig. 8) can be explained by this high conductivity,by the
mechanismof charge conduction along the fibers. However, when the quaternary is
rinsed off with water before drying, so that only smallquantitiesremain on the fiber,
the charge generated by combing remains relatively low (Fig. 8), even though the
chargehalf-life increasessubstantiallyand is comparableto that of untreated fibers
(Fig. 11). The commercial creme rinse A also has relatively low generated charge(Fig.
7) in spite of a high half-life (Fig. 10). The reduced chargegeneratedwith these materials, therefore, cannot be explained by a mechanismof enhancedsurfaceconductivity. An alternativemechanismmust be sought.
We hypothesizethat the reduction of chargegenerated by combing,when hair is
treated with quaternaryammoniumcompounds,is primarily due to the lubricating
propertiesof these compoundson the dry hair, rather than to enhancedconductivity.
The quaternaryactsas a lubricant and reducestangling, so that the force required to
pull a comb through the hair is substantiallyreduced, especiallythe end peak force
(work in these laboratoriesnot reported here). A creme rinse containing the quaternary SBAC and other ingredients has a similar effect (Fig. 12, lower graph). The
reduced normal contact force between hair and comb leads to a reduced chargeon the
hair (Fig. 12, upper graph)becauseof a smallertrue areaof contactbetweencomband
hair.
Medley (20) haspostulatedthat an antistaticagentneed not be presentasa continuous
film in order to be effective. A discontinuousfilm would not give long-rangeconductivity and, therefore, the half-life of charge mobility would remain high. Medley
proposed a mechanismrequiring only localized conductivity at the contact site. This
568
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
mechanismcould be acting as a secondaryeffect in the antistaticmaterialsdiscussed
here. Another secondaryeffect could be a changein the chemicalnature of the fiber
surface,which would alter the magnitudeof chargegenerated.We believe, however,
that the reductionof combingforce by lubricationis the primary mechanisminvolved,
asevidencedby the substantialeffectson end-peakforceandchargeshownin Fig. 12.
The lossof chargeby conductionto ground (the scalp)is not generallya significant
contributor to reduced static in hair. Clean hair, and hair treated with most agents
examined,has a chargehalf-life in excessof 10 sec,except at higher humidities(Figs.
10 and 11). This is too long for there to be a significantdissipationof chargein the few
secondsbetween combing and the observationof troublesomestaticeffectsby the
consumer.At higher humidities(above60 per cent RH) the half-life dropsto 1 secor
less.Here the dissipationof chargeto electricalground(the scalp)is rapid enoughto
be effective,but at thesehighhumiditiesthe chargegenerated(Figs.7 and8) is smaller
in any case.The high chargemobility will alsoenable the concentrationof chargesnear
the fiber tips, produced by combing (Fig. 12), to be reduced by a redistributionof
chargealong the length of the fiber.
It is sometimesthought that difficulty in combing(high combing forces)is due to the
generationof staticchargeson the hair. This is erroneous.The end peak force is observedeven at high humiditiesor in the presenceof staticeliminators(19). Rather,the
reverseis true; it is the normalforce betweencomband hair fiberswhichgivesrise to
staticcharges.A possiblesourceof confusionis that there can be a secondaryinteraction betweencombingforceandstaticcharge,whichcanincreasethe combingforceon
further combing.Experimentsin our laboratoryhave shownthat the end peak force
increaseswith successivepassesof the comb. Charge is generated on the first comb
pass,and the hair fiberswill separatefrom one anotherand tangleso that an increased
combingforce is required on the next combpass.When a staticeliminatoris usedas
the hair is combed,this increasein end peak force from passto passis eliminated.
CONCLUSIONS
Instrumentationhasbeen developedto measurethe magnitudeof chargegeneratedby
combing,the mobility of chargeand the distributionof chargealongthe lengthof hair
fibers.Conventionalantistaticagentsfor hair reducethe chargegenerated;the half:life
of chargemobility varieswith the quantityof antistaton the hair. The densityof charge
on a combed hair tressis shown to be at a maximum near the end of the fibers, and cor-
respondsto the regionwhere thereis a peakin the combingforce.
A theory of the mechanismof action of quaternary ammonium antistaticagentsis
proposed. These agents do not normally achieve their effect by mechanismsof
increasedconductivityor of chargedissipation.Their primaryeffect is a lubricatingaction, which reducessubstantiallythe force required to comb hair, especiallythe end
peak force. The reduced normal contact force between hair fibers and comb leads to a
reductionof staticchargegeneratedon the hair.
ACKNOWLEDGMENTS
We wish to thank Mr. W. C. Wikstrand,who painstakinglyperformedmuchof the experimental work reported herein. We are also grateful to Mr. P. L. Faganfor his
contributionsto the assemblyof the electronicinstrumentation.
ELECTROSTATICPROPERTIESOF HAIR
569
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