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J. oc. Cosmet. Chem. 29 441-446 (1978)

Evaluation of mechanical stresses set up inlipstick during application

R. G. DREW Beecham Products, Applied Research Laboratories, Randalls Road,Leatherhead, Surrey, KT22 7RX

Received 19 January 1978

Presented as part of a lecture or the Society of Cosmetic Chemists of Great Britain,London, January, 1977

Synopsis

A device is described which measures he stresses et up when lipstick is applied; this device s used toassess he application properties of two ranges of commercially available lipsticks. The values obtainedfacilitate definition of the strength requirements of a lipstick. Knowledge of the in-use stresses willenable formulators to avoid fracture problems during lipstick application. A trial of ten commerciallythe available ipsticks ndicates hat the application orce is related to the ease of colouring of the lips beapplied ipstick. Application orces were greater when the lipstick was applied by older subjects.

INTRODUCTION

A lipstick consists ssentially f a solid stick n a retractable applicator. n recent yearslipstick ormulations have altered considerably; ard sticks with high levels of stainingdye have been almost completely superseded y softer sticks n which soluble dye hasbeen argely eplaced by insoluble dye in lipsticks pigments). With the reduction n levelof staining dye in lipsticks much easier transference s required in order to achieveadequate colouration of the lips. Elliott (1) indicated hat this has been achieved byintroducing oilier bases. The increased iquid and semi-solid ontent of recent formu-lations has made the mechanical trength equirements f the stick critical.

A common method of lipstick strength ssessment escribed y Lauffer (2) is to holdthe lipstick horizontally n a socket itting over 1 cm of its base and to apply weights at ameasured distance rom the edge of the socket. The weight is increased by increments(suitably 0.1 N) every 30 min until the lipstick breaks. At least our readings are neededfor each batch of lipstick and broken surfaces must be examined o ensure hat noweakening laws educe he strength. Breaking point measurements ust be carried out ata known emperature suitably 5øC) on sticks tored at that temperature or at least 30min. This technique s unrealistic s the method of loading nvolves disproportionatelylarge shear component nd a small bending moment when measuring he total force tofracture. Lauffer (2) also described method of roughly measuring he force of appli-cation. A strip of smooth paper s drawn between wo flat lipstick ends at constant speed.The force required to pull the paper when a given total weight is applied to the upperlipstick s recorded.

0037-9832/78/0700-0441 02.00 ̧ 1978 Society of Cosmetic Chemists of Great Britain

441

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442 R. G. Drew

This paper describes n apparatus iving ealistic measurements f the force of (easeof) application and force to fracture.

EXPERIMENTAL

MATERIALS

Ten lipsticks manufactured or retail distribution were selected or examination rom twolipstick anges*. hey were chosen o cover a range of colours rom clear gloss, hroughshades f pink, red and light brown o dark brown. These ipsticks were hen arbitrarilycoded. The first range was coded a, b, c, d and e, and the second ange was coded , w,x, y and z. All lipsticks ested were 41 mm long and 11 mm in diameter at their base.

APPARATUS

The apparatus onsisted f a brass beam carrying strain gauges, pike and base ontowhich he lipstick was mpaled Fig. 1). A conventional olystyrene older o match hebase diameter of the lipstick was fixed o the beam with epoxy resin.

Figure 1. Brass beam, with semiconductor train gauges, used o hold the lipstick. Samplelipsticks were mpaled on the spike set n a cylindrical older on the left. Applied orces weretransmitted o two pairs of strain gauges itted on opposing ides of the beam.

A semiconductor train gauge** was ixed o each of the four sides f the square-sectional eam. The strain gauges ormed wo half Wheatstone esistance ridges. heother half of each Wheatstone ridge circuit was a balancing ariable esistor ncor-porated n the main recording quipment ituated way rom the device. n the recordingequipment he outputs rom the two bridge circuits were amplified y a factor of 1000and ed nto a vector sum computation nit.'• Hence, by carrying ut a vector sum reat-ment of the two messages he resolved oad on the lipstick at any instant could be com-puted electronically nd the output ed to the pen recorder.** he bridges were powered

* Margaret Astor Silver Frost and Ultra Soft lipstick from Cola GmbH, Cosmetik-Fabrikations-Gesellschaft bH 612 Michelstadt/Odenwald, undesrepublik eutschland.

** The strain gauges sed were encapsulated xiaMead ype supplied by Kulite Ltd, 20 Wote Street,Basingstoke, ampshire, U.K. (Code No. DCP-120 090).

t The vector sum circuit was designed by Analog Devices nc., Norwood, Massachusetts 2062,U.S.A., and based on an integrated circuit chip, ADS31 KD.

:• Phillips pen recorder PM 8221.

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444 R. G. Drew

Each application yielded a trace of either the resolved oad (via the vector summingdevice) or the separate components f each resistance ridge on the pen recorder. Atypical trace is shown n Fig. 3. From each trace the maximum resolved orce duringan application was recorded.

RESULTS AND DISCUSSION

Maximum application orces obtained rom 840 applications were plotted n the form oftwo histograms Fig. 4).

120 -

•,, Ioo

.s 80

o

• e0• 40

z 20

0'2 0'4 0'6 0'8 I-0 1'2 1.4 1.6 1.8 2.0 2'2

Maximum pplied orce (N)

l*igt•e 4. Histogram of the maximum applied forces rom each lipstick range during the trial.Most of the subjects pplied the lipsticks with a force ess han 1 N, but a 2 N force was exceededoccasionally. - Ultra Soft; .... Silver Frost.

Median forces of application were 0-68 N for the Silver Frost range 10th percentile0.46 N, 90th percentile .92N) and 0.73 N for the Ultra Soft range 10th percentile .50 N90th percentile '27 N). The subjects ended o apply the Ultra Soft range using higherapplication orces han the Silver Frost range. A Mann-Whitney U-test showed hat thisdifference was significant at the P <0.00001 level.

Each subject's pplications f the same ipstick often covered wide range of forces(Table ). Comparison f the application oads of two lipsticks, however, evealed hatwithin the twenty-eight subjects ested significant ifferences n maximum applicationforce were ound Table I). From hese ifferences ranking order was compiled ndicat-ing which lipsticks would be applied with a greater force: b > c > a> y, e > w> v> x>z>d.

Table I. The maximum application forces observed rom one subjectusing a single ipstick during the trial

1 0.604 0.512 0.443 0.660 0.7382 0.667 0.438 0.708 0.676 0.851

3 0.811 0-678 0.743 0.640 0.8084 0.397 0.604 0.682 0.996 0.7055 0.261 0.696 0.732 0.931 0.8476 0.642 0.774 0.942 0.782 0.829

Maximum application force (N)Application

number Day 1 Day 2 Day 3 Day 4 Day 5

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Mechanical stresses n lipstick 445

Table 1I. Subjects who exhibited significantdifferences Mann-Whitney U-test, P< 0-02) inmaximum application forces between the two

applied lipsticks

Subjects Ultra Soft v. Silver Frost3,4 c >w6 d>w8, 10, 11 b > z9 d>z13 c >x14 d<x

16, 17 c > v21 a >y22,24 c >y23 d >y25 b >y28 a >y

Four independent volunteers anked the applied lipsticks n order of colouringpower: b > a > c > e, z > v, w > y > x > d. The Kendall Rank Correlation Coefficient, x, forthe ranking of applied oad and subjectively ssessed overing ower was 0.683, ndicatingthat individual subjects would use a lower application orce to apply a lipstick with bettercolouring power.

The subjects ere divided nto wo age groups f approximately qual size 18-29 and30-60 years old). Subjects ver 30 years old used significantly igher application orces

(Mann-Whitney U-test, P <0.025).A similar analysis of their choice of colours ndicated at the P < 0.025 level that the18-29 years group were biased owards rown colours nd the 30-60 years group owardsthe paler brown, pink or red colours.

Subjects who applied ipstick requently daily or more often) used significantly igherapplication orces han those who applied lipstick less requently.

Other parameters possibly elated to lipstick selection and application were alsoexamined. The subjects' omments pon the applied ipstick was compared with the mostcommon application orce range. No overall rend was observed nd subjects requentlydescribed he same ipstick using antonyms and the reasons or lipstick selection seemed

to have been many and varied. The lipstick choice combinations f a lipstick rom eachrange revealed no bias towards particular colour combinations such as brown and darkbrown, or red/pink and pale brown etc. The selection f lipstick was finally comparedwith subject hair colour but no trend towards any particular choice of lipstick wasobserved.

DISCUSSION

The main purpose of the work reported here was to assess he capability of the lipstickapplication equipment; o achieve his an application rial was carried out. The equip-ment was shown to be capable of meaningful measurements nd is a useful tool forassessing he force of application of lipstick.

This trial was designed o assess he maximum application orces of two types oflipsticks. Subjects were expected o apply a lipstick of their own choice n a more typical

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446 R. G. Drew

manner han a lipstick selected y the nvestigator nd hence only one ipstick of each ypewas tested by the subject.

Although a difference was detected etween he maximum application orces used neach ipstick ange, an investigation f the data showed hat this may be due o either he

colouring ability of the lipstick, or the age of the subject. The colours applied with thegreater orce were predominantly ale brown or a clear gloss. The mainly dark colours,of better colouring ability, were applied with a smaller orce.

The application data implies that lipsticks with relatively low colouring power(usually ale colours) re more ikely o fracture during use f the strengths f all lipsticksare equal.

Differences n maximum application orce were detected between requent andoccasional ipstick users, but not between users and non-users f lipstick. This may beexplained by the selection f lipsticks, because on-users f lipstick more frequentlyselected lossy nd paler lipsticks.

REFERENCES

1 Elliott, T. J. Present and future trends n make-up. Cosmet. Perrum. 88 35 (1973).2 Lauffer, P. G.I. Lipsticks. n: Balsam, M. S. and Sagarin, E. Cosmetics, cience nd Technology ol.

1 388 (1972). Wiley-Interscience, ew York.