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Indian Journal of Textile Research

Vol. 8, June 1983, 29-36

Particulate Soiling and Soil Release Behaviour of Silk Fabric

RAJ KUMAR & JA YSHREE K DAVE

Ahmedabad Textile Industry's Research Association, Ahmedabad 380015

Received 18 December 1982; accepted 7 May 1983

The soiling and soil release behaviour of silk fabric has been investigated using a model particulate soil (ferric oxide). The /

results obtained have been compared with those for cotton and polyester fabrics of nearly the same.construction. It ispbserv,ed -'that the soiling offabrics of the three different fibre types increases in the order: silk polyester. The location and distribution of soil particles on the fibre surface ha.vebeen examined using the scanning electron microscope. The kinetics of particulate soil release from fabrics has been found to

follow the first order rate process. The effect of washing period and washing temperature on the rate coefficient and the orderof the soil release process has also been studied.

One of the important criteria for determining theclothing value of a textile material is the accumulation

of foreign matter on the fabric surface during its use.The accumulated foreign matter is called spot when itis localized, stain when it is localized and difficult to

remove, and soil when it is evenly distributed over alarge surface. Soils occur as particulate matter (dust,carbon black), fluids (oil, grease) or a mixture of thetwo. Soiling with particulate matter involves(I) entrapment of particles in the inter- and intra-yarnspacings (macro-occlusion) and in the irregularities ofthe fibre surface (micro-occlusion), and (2) depositionof solid matter by van der Waal or electrostatic forcesat the surface or within the pores and cavities(adhesion), whereas soiling with fluid occurs by inter-yarn, intra-yarn and intra-fibre wicking mechanisms.The soiling characteristics of composite soils arepredominantly governed by their liquid components.

The soiling and soil release behaviour of textilematerials has attracted considerable attention.

Numerous experimental and theoretical studies 1-8have been carried out in the past several years to see theeffect of various factors like the nature and structure

of fabric, type of finish applied, nature andcomposition of soil, particle size and particle sizedistribution, method of soiling and the atmosphericconditions, on the soiling and soil-release behaviour oftextile materials. However, most of the publishedwork9 -12 is devoted to substrates obtained from

cotton, polyester, wool and their blends. Studies withother types of textile materials are relatively few13•14and, in some cases, conflicting views have beenexpressed regarding their relative soiling behaviour.Since different types of textile fibres offer surface ofvarying physical and chemical nature to the soiling

materials, affinity of soils for them is expected to be

different. Therefore, studies on the relative soiling andsoil release behaviours would be of considerable

interest. In the present work, the soiling and soil releasebehaviour of silk fabrics has been investigated using amodel particulate soil (ferric oxide) under the standardsoiling and washing conditions. The results obtainedhave been compared with those for polyester andcotton fabrics of nearly the same construction.

Materials and Methods

Fabrics-Scoured, bleached and mercerized cottonpoplin was used. Polyester fabric was obtained by thecarbonization of polyester/cotton blend fabric (warp,67/33 polyester-cotton spun yarn; and weft, polyestermultifilament) with 70% (wt/wt) aqueous sulphuricacid. Silk fabric was of mulberry origin. It wasdegummed before use. The physical characteristics ofthe fabrics used are given in Table 1.

Table I-Physical Characteristics of Fabrics Used

Characteristic

SilkCotton Polyester

Weight, g/m2

59.468.169.8

Construction, ends/picks

123/103 123/94107/90

Air permeability, cm3/cm2/sec

95140114

Hydrophilicity Moisture regain, %

9.27.60.5

Vertical wickirig height, cm

l.l4.60.5

Static electricity Charge build-up, m V

11.58.531.0

Decay time, sec

2010240

Elastic recovery24, %

525265

Fibre/water zeta-potential 1.25, mV

-24-24-74

Surface energy2.26 Fibre/water contact angle, deg

424775

Fibre/water work of adhesion, erg/ cm2

12612191

29

INDIAN J TEXT RES, VOL. 8, JUNE 1983

Soil-t-Out of various model particulate soils 1, ferricoxide was selected because of its availability indifferent average particle sizes, particle sizedistribution, similarity with natural soils in the soilingbehavidur, and sufficient colour to enable itsestimation by spectrophotometric measurements 15.Ferric oxide used was red co loured powder and

represehted the size function between 0.5 and 1.6,Lt.Surfactant-Sodium dodecylsulphate (SDS), ob-

tained from Koch-Light Laboratories Ltd, England,was used as such without further purification. It was oflaboratory reagent grade.

Soiling-Soiling of the fabric was carried out by theAcceler.otor method7,16. Two 10 x 10cm fabricswatches were cut with a scissors containing serratedblades to minimize fibre loss during soiling, and

placed, one on each of the two rotor blades, in anAccelerotor(Atlas Electric Device Co., Chicago, TJSA)

soiling chamber whose abrasive liner was previouslyremoved. Soiling levels of fabrics, a measure of thequantity of soil coming in contact with the fabricsurfacei during soiling, were calculated using thefollowing relationship:

Soiling level (Sp)

mass of soil used in soiling x 100mass of fabric used in soiling

... (1)

possibilities of redeposition of soil on the fabric duringwashing. The effect of temperature on the soil releasecharacteristics of silk fabrics was studied by

maintaining different constant levels of temperature,ranging from 30 to 90°C, in the thermostatic bath ofthe Terg-o-tometer with an accuracy of ± 1°C.

Estimation of soil-The soiled fabrics, before andafter washing, were evaluated spectrophotometrically

by reflectance measurements with a green tristimulusfilter using a reflec'tion meter model CG-I06(Canadian Research Institute, Ontario, Canada). There:lative soiling values of different types of fabrics werecalculated by converting the reflectance readings intothe corresponding Kubelka-Munk functionsI7.18.

Results and Discussion

The apparent soiling values of the three types offabrics, soiled with ferric oxide at various soiling levelsin the chamber, are shown in Fig. I. It is observed thatthe apparent soiling value (C.) of all the fabricsincreases non-linearly with increase in the quantity ofsoil present in the chamber and the dependence issimilar to that observed for adsorption on solidsubstrates from a solution. The following empirical

equation has been found to be adequate for thedescription of soiling data:

Cs=CoS~or log Cs= log Co + 0: log Sp .. , (2)

!~1.

',.-I

,..... 3-{)III

U

Fig. I-Soiling behaviour offabrics with Fe203 [0, silk; x, cottonand .1, polyester]

j\

~Io 5 10 15Soiling lrzvel(Sp)

C:l

~ 2.0I\l>(Jlc'0(f) 1-0+'CC:l...~a.«

where Co and 0: are the constants dependent upon

the nature of soil, fabric and soiling conditions; Sp, thesoiling level representing the quantity of soil coming incontact with the fabrics during soiling; and Co, thesoiling value of the fabric when Sp = I and 0: has valuesless than or equal to unity, indicating that the soilingvalue increases less' rapidly than the quantity of soilcoming in contact with the fabrics. The relative

Different levels of soil ranging from 2 to 15 wereintroduced into the chamber. The fabric swatches were

then rotated together with the soil in the chamber at apreselected speed for a known period, maintainingconstant mechanical energy during soiling.Preliminary experiments indicated that a rotor speedof 1,600 rpm for I min was adequate for uniformsoiling.The loose soil from the fabric was blown offbypressurized air from a jet at a constant flow rate for 60sec.

Washing of soiled samples-The soiled fabricswatches were washed in I g/litre SDS at 50°C and 50oscillatory rotations per min (stroke, 315°) for 10 minusing Ii Terg-o-tometer (Osaka Tsuruga ElectricWorks Ltd, Japan). The fabric swatches were thenrepeatedly washed with running water followed bydistilled water, air dried and analyzed for their soilingvalues. For studying the effect of washing period on

detergency, fabric samples obtained after the first washwere again introduced in a fresh surfactant solution ofthe same strength, washed under similar conditions ofmechanical action for 10 min, and analyzed for their

soiling values. This process was repeated for a totalperiod of 50 min, i.e. five such 10 min washing cycles.Fresh surfactant solution was employed in each of thefive successive washing cycles to minimize the

30

----- .- ----- ------ -.-

RAJ KUMAR & DA VE: PARTICULATE SOILING & SOIL RELEASE BEHAVIOUR OF SILK FABRIC

Fig. 2-Soiling value versus soiling level: Test of validity of Eq. (2)for soiling of fabrics [0, silk; x, cotton; and D., polyester]

*Values of constants were determined using the method of leastsquares.

Table 2-Soiling and Soil Release Parameters* of theExperimental Fabrics

Soiling Soil release-- Co exk(sec-') nxlO0.43

0.676.521.08

0.54

0.6\5.611.03

0.88

0.463.461.08

2 L, 6 810 20

Soiling level (Sp) (log scal~)

2-0

c 0·2~f1lc-c..4: 0

~ 4-0

~ 3-0'mo

If)u "0-- ~0'8~ ~ .'.

:J .. ~~0-6· ~:/

>rn 0·4

c::;0If)

Fabric

Silk

Cotton

Polyester

surface of cotton fibre occurs in its macro- and micro-

surface rugosities (Fig. 3h). Silk and polyester fibres,which appear to have smooth surfaces at low

magnification, also show micro-surface irregularitiesacting as sites for soil accumulation20,21 (Fig. 3g, i).

Soiling by the Accelerotor method involves highspeed rotation of fabric samples in an atmosphere ofair-borne soil, causing filtration of the soil particlesfrom air passing through the fabric under the relativemotion of the two phases with respect to each other.The loosely held soil particles, particularly thosemechanically entrapped in the inter- and intra-yarnspaces, are generally removed by blowing air across thefabric or by vacuum cleaning. On the other hand, thesoil particles transferred on the fibre surface by directimpingement are held by energy bonds, such aselectrostatic forces, hydrogen bonds and van derWaals forces, and, in certain cases, may also beoccluded in the microscopic rugosities on the fibresurface. These soil particles are, therefore, difficult to

soilability of different types of fabrics, at any level ofsoil in the soiling chamber below the saturation limit, isgoverned by both the constants, Co and rx, and forfibre-soil systems having almost the same value of theexponent, it is mainly determined by Co.

The applicability of Eq. (2) is obvious from Fig. 2,wherein the soiling values have been plotted againstthe quantities of soil coming in contact with the fabricduring soiling, on a logarithmic scale. All the plots arestraight lines as required by Eq. (2). The values ofconstants Co and rx estimated from the intercepts andslopes respectively of the straight lines using themethod of least square analysis are given in Table 2 forall the three types of fabrics. An equation similar to Eq.(2) has been used earlier by other investigators for thedescription of their soiling data 19. It can be inferredfrom Table 2 that the initial soiling value of fabricsincreases in the order: silk < cotton < polyester,whereas the rate of increase in soiling value withincrease in soiling level, determined by the exponent rx,follows the opposite trend, i.e. silk> cotton> polyester. However, the overall relative soila bility ofthe three fabrics, at any level of soil in the chamberbelow the saturation limit, increases in the order: silk

d

INDIAN J TEXT RES, VOL. 8, JUNE 1983

b

e

c

f

i.

hFig. 3'-jScanning el~ctr~~ micrographs of soiled and unsoiled fabrics [(a) soiled silk, x 1820;(b) soiled cotton x 1820;(c) soiled polyesterx 1820;(d) soiled silk x 4550; (e) soiled cotton x 4550;(I) soiled polyester x 4550; (g) unsoiled silk x 4550;(h) unsoiled cotton x 4550;and

(i) unsoiled polyester, x 4550]

32

RAJ KUMAR & DAVE: PARTICULATE SOILING & SOIL RELEASE BEHAVIOUR OF SILK FABRIC

remove by the mechanical forces only and require theinvolvement of chemical energy. The low soiling valueof silk fabric is expected on the basis of its relativelysmooth surface and high elastic recovery compared tothat of cotton. The high soiling value and more soil

aggregation of polyester fabric, which possessesmacroscopically smooth fibre surface and high elasticrecovery, are probably due to the generation ofelectrostatic charge during the high speed revolution

which can attract the soil. The high apparent soilingvalue may be partly due to the lack of macro-surface

irregularities, which hide the soil from exposure to theincident light.

Soil release-Soil release from fabrics duringwashing involves interaction of fabric-soil complexwith surfactant solution, and mechanical and thermal

energies. As washing proceeds, more and more soilr.eleased from the fabric gets accumulated in thesolution up to a particular level after which the soil

starts redepositing on the fabric. At any instant,therefore, there is a competition between soil releaseand soil redeposition processes and the overall soilrelease is governed by a combination of the two.

According to Kissa22, the r'ate of release of a

particulate soil from a fabric in a detergency processcan be represented by the general kinetic expression

where C [=(Cs + Cw)/2] is the average soiling value ofthe fabric, Cs and Cw being the apparent soiling valuesof the fabric before and after washing respectively;dC [ = Cs - Cw], the change in the apparent soilingvalue of the fabric during washing for a period dt; k,the average kinetic rate coefficient over the washingperiod; and n, the order of the soil release process.Since the rate coefficient determines the rate of soil

release from a fabric having an average soiling value ofunity, it can be used as a parameter for comparing thesoil release characteristics of fabrics of different fibre

types following the kinetic rate process of the sameorder. Under similar operating conditions, the higherthe value of k, the greater will be the release of soil fromthe fabric.

Data on the release of particulate soil from fabrics ofall the three different fibre types are presented in Fig. 4wherein the rate of soil removal is plotted against theaverage apparent soiling value on a logarithmic scale.

A constant washing period of 10 min is taken ~s onewashing cycle. Different values of LlC and C wereobtained by taking soiled fabrics of different initialsoiling values. It is observed from Fig. 4 that all theplots are straight lines as required by Eq. (3). The ratecoefficient (k) and the order of the process (n), obtainedfrom the intercept at C= I and· slope of the straight

i

; 1

dC =kC"dt ... (3)

lines respectively, for all the three fabrics of differentfibre types, are given in Table 2. It is observed that theprocess of particulate soil release from fabrics of all thethree fibre types follows the first order rate kinetics

with the average rate coefficient decreasing in theorder: silk> cotton> polyester; this is reverse of thetrend observed in the process of soiling.

The removal of uniformly distributed small soilparticles of the range under investigation from thefibre surface occurs in two steps 1. In the first step, thesurfactant solution interacts with the fibre-soil

complex, releases soil from the fibre surface byovercoming adhesive bonding between them andstabilizes by the formation of a solvation/adsorptionlayer of the surfactant molecules on the surface of both

the fibres and the soil. In the second step, the stabilized, detached soil particles are taken away into the bulk of

the wash liquor where the forces of fibre-soil

interaction are ineffective. The first step, therefore,mainly requires the use of chemical and/or thermalenergy, whereas in the second soil transporting step,mechanical energy, much smaller than that needed forbreaking the adhesive bonds, is largely used. Under theadequate mechanical action of washing, the first stepdetermines the rate of soil release with the rate

coefficient depending upon the strength of fabric-soil,fabric-surfactant and soil-surfactant interactions.Since the concentration of the fibre substrate is

practically constan t, as a very small portion of the fibresurface is occupied by the soil particles, the rate processis pseudo-first order, depending on a single variable,i.e. the concentration of the soil. The relatively lowvalue of the rate coefficient for polyester fabric ascompared to those for cotton and silk is expected onthe basis of its surface energetics and electro-kineticpotential at the fibre-surfactant solution interface. Thelower soil release in the case of cotton fabric comparedto that in silk is probably due to the presence of folds

2-0

~ 0-6

-- 0-4u

INDIAN J TEXT RES, VOL. 8, JUNE 1983

and macro-surface irregularities on the surface of thecotton fibre which can lodge soils.

Effea,t of washing cycle-Data on the release ofparticulate soil from silk fabrics of different initialapparent soiling values after five successive washingcycles are plotted against the corresponding washingperiods~ expressed as washing cycles, in Fig. 5. It isobserved that the release of soil, relatively rapid during

the eafily stages of w.ashing, becomes progressivelyslower and finally levels off at a maximum removal

figure corresponding to ~ 65% on the basis of theinitial soiling value, indicating that complete removalof soil from the fabrics becomes progressively difficult.

The kinetics of soil removal during multiple washing

cycles is examined in Fig. 6 wherein ~9values have beenplotted. against the average soiling value (C) on alogarithmic scale for all the five successive washingcycles. All the plots are straight lines nearly parallel toeach other (slope ::,:1). This indicates that the kineticorder of the particulate soil detergency from silkfabrics is unity, irrespective of the washing period. Onthe other hand, the rate coefficient, calculated from the

intercept of the straight lines with an ordinate at C= I,shows a decreasing trend with increase in washingperiod; this is in conformity with the findings of otherinvestigators22•23.

Textile fibres have numerous active sites and surface

irregularities for soil accumulation of varyinginteraction energy which may further vary with thenature and size of the soil particles. Since themagnitude of the rate coefficient depends upon thestrength of the fibre-soil interaction, it appears that theweakest fibre-soil complexes are broken first with thehighest rate coefficient. As the washing periodincreases, the relatively strong complexes start

i

2-00'4 0'60·8 1'0

C (109 scale)

"""0·40ell

~0-30uVl

010'20o

.•...•..

~ 0·10uO-08~"""0-06

0·2

15 3.0 45 60 75 90Temperature, °C

11

10

9

8

o 7~X 6u

RAJ KUMAR & DAVE: PARTICULATE SOILING & SOIL RELEASE BEHAVIOUR OF SILK FABRIC

2-0 '

(\J 1-0(1\

0-8u V1 0-6OJ

0 0-4~ --u

INDIAN J TEXT RES, VOL. 8, JUNE 1983

8 Raj K)umar, ATIRA Technical Digest, 15 (1981) 12.9 BerchlJ & Peper H, Text Res J, 33 (1963) 137.

I 0 Bowe~s C A & Chantrey G, Text Res J, 39 (1969) I.I I Fort T (Jr), Billica H R & Sloan C K, Text Res J, 36 (1966) 7.12 Reeveii W A, Beninate J V, Perkins R M & Drake G L (Jr), Am

Dyestuff Reptr, 57 (1968) 35.13 Hart W J & Compton J, Text 'Res J, 23 (1953) 164.14 Weat~urburn A S & Bayley C H, Text Res J, 25 (1955) 549.15 Kissa E, Text Chem Color, 5 (1973) 249.16 Dave AM, Das T K & Madan G L, Indian J Text Res, 3 (1978)

100.

17 Kubelka P & Munk F, Z tech Phys, 12 (1931) 593.

36

18 Schappel J W, Text Res J, 26 (1956) 214.19 Kissa E, Text Res J, 43 (1973) 86.20 Salsbury J M, Cooks T F, Pierce E S & Roth P B, Am Dyestuff

Reptr, 45 (1956) 190.21 Tripp VW, Moore A T, PorterB R & Rollins M L, Text ResJ, 28

(J 958) 447.

22 Kissa E, Text Res J, 48 (1978) 395.23 Schott H, Text Res J, 46 (J 976) 459.24 Morton W E & Hearle J W S, Physical properties oftextilefibres

(The Textile Institute, London) 1975, Chap. 15.25 Kanamaru K, Z Kolloid, 168 (J 960) 115.26 Stewart J C & Whewell C S, Text Res J, 30 (1960) 903.