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The level of moisture in seed-cotton atharvest through to the level in baledlint can have signicant effects on the

quality of bre sold to the spinning mill.There are optimum moisture levels forseed-cotton and lint for each harvest and

ginning process that allow good and ef-cient harvesting, cleaning, ginning, balingand safe storage.

To optimise processing and bre qualitythe amount of moisture taken up or lostby cotton needs to be balanced with anappropriate amount of drying or moistureapplication during the pre-cleaning, gin-ning and baling processes.

It is well known that cotton with an ac-ceptable moisture level has improved -bre tensile properties resulting in greaterstrength, extensibility and work-to-breakvalues. These effects make cotton bremore resilient during ginning and lintcleaning, and less prone to damage.

Dry cotton is easier to clean but will bedamaged during ginning and lint cleaning.

On the other hand, cotton with excessivemoisture is difcult to gin and clean, andwill degrade during storage.

This article is the rst in a series of re-views on the management of moistureand bre quality in cotton from harvestingthrough to bale storage in warehouses.Its aim is to provide growers, ginners andmerchants with an up-to-date and concise

collection of information on the subject of

measuring and managing bre quality dur-ing early stage processing and shipment.

In this rst article the fundamental re-lationships that determine the extent and

rate of moisture absorption (or more cor-rectly moisture adsorption) and drying incotton are discussed. The interaction ofwater at a molecular level and the effectson bre properties are also discussed andwill be used in subsequent reviews on theaddition of moisture or humidication ofcotton during ginning and baling.

Cotton bre cellulose and water

As living plant cells, cotton bres within

a growing boll are literally full of water. Wa-ter is critical for bre growth and the avail-

ability of water during early boll growth isrelated to the nal length achieved by thebre.

When the boll matures and opens, breslose water and equilibrate with the ambienthumidity. Both constituents of seed-cotton bre and seed are hygroscopic, for ex-

ample sensitive to moisture, but to differ-ent degrees. For example, if seed-cotton isplaced in air of 50 per cent relative humid-ity (rh) and 21C: The bres will tend to reach a moisture

content of approximately six per cent;

The seed will tend to reach a moisturecontent of about nine per cent; and,

The combined moisture content will be

around eight per cent.In its mature dried form, nearly 90 per

cent by weight of the cotton bre is cel-

lulose. In fact the cellulose found in cottonbres is the purest form of cellulose foundin all plants. The cellulose in cotton bres

is mostly (8896.5 per cent) -cellulose1.

The non-cellulose components (412 percent) are located either on the outer lay-ers of the cotton bre in the cuticle andprimary cell wall or inside the residual pro-toplasm called the lumen. The secondarywall of mature bres is primarily cellulosein its most highly crystalline and orientedform (see Figure 1).

Figure 2 shows the structure of the cel-lulose molecules in cotton. From a physi-cal viewpoint the molecule is a ribbon-likestructure of linked six-membered ringseach with three hydroxyl groups (OH) onthe C2, C3 and C6 atoms projecting outof the plane of the ribbon.

As well as providing structural stabilitythe hydroxyl groups allow extensive inter-

molecular hydrogen bonding with manymolecules, including water. The acces-sibility of water to these hydroxyl groupsdepends on the spacing between crystallattice planes. From a completely dry state,water molecules will form hydrogen bondswith hydroxyl groups that are not alreadylinked within crystalline regions.

32 THE AUSTRALIAN COTTONGROWER DECEMBER 2009JANUARY 2010

Moisture in cotton the fundamentals

By Stuart Gordon1, Susan Horne1 and Marinus van der Sluijs1

AT A ANCEThe level of moisture in seed-cotton

at harvest through to the level in baledlint can have signicant effects on the

quality of bre sold to the spinning mill.Dry cotton is easier to clean but willbe damaged during ginning and lintcleaning. On the other hand, cotton withexcessive moisture is difcult to gin andclean, and will degrade during storage.Both constituents of seed-cotton breand seed are hygroscopic, for examplesensitive to moisture, but to differentdegrees. In this article the fundamentalrelationships that determine the extent andrate of moisture absorption and drying incotton are discussed.

FIGURE 1: Representation ofthe structure of a cotton bre

FIGURE 2: Assembly ofcellulose molecules in asheet. Hydrogen bondsare shown by dotted lines.Circled carbon atoms; C2,C3 and C6, show location ofhydroxyl (-OH) groups.

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As humidity is increased, water willbe attracted to these accessible hydroxylgroups. In this respect, available hydroxylgroups will ordinarily be located on thesurface of crystalline brils, a unit of thecellulose crystal structure. The rst water

molecules to be adsorbed will be directlyattached to hydroxyl groups. Later adsorp-tion may occur on the remaining availablehydroxyl groups or form secondary layersattached to already adsorbed water mol-ecules.

Figure 3 is interesting because it showsthe change in density of cotton withchanges in moisture content. From the

dry state the density of cotton increases asempty space (in proximity of available hy-droxyl groups) within the cellulose structurells with water. Direct adsorption of watermolecules onto these hydroxyl groups re-sults in more efcient molecular packingand gives an initial increase in density.

The density then decreases as moisturecontent increases past four per cent, and

layers of water molecules in effect dilutethe density of the cotton cellulose struc-

ture. The relationship in Figure 3 suggeststhat at moisture contents in excess of veper cent, cotton cellulose is as full of boundwater as it can be, and that beyond thislevel cotton contains more loosely boundwater, or unbound water held by surfacetension effects such as capillary action.

The shape of the drying and wettingcurves (called isotherms, see Figure 4)for cotton are determined by availabilityof adsorption sites for water molecules inthe cotton cellulose structure under dif-ferent relative humidity (rh) conditions. In

particular, the shapes of these curves are

determined by the multi-layer adsorptionof water molecules within the cellulosestructure. The salient point is that for every

combination of ambient air temperatureand rh, there are corresponding moistureequilibriums for the seed-cotton, bre andseed. The equilibrium moisture content ata given rh is also a function of barometricpressure.

The isotherms in Figure 4 show typicalrh-moisture content curves for cotton brecoming from the dry side at three differ-

ent ambient temperatures. In a standardatmosphere of 65 per cent rh and 20C,the absorption moisture content of cottonis nominally six to seven per cent.

The rh and temperature traces in Fig-ure 5 show the cycle of transport airtemperature and relative humidity in gin

ducting throughout a single day in winter.The traces illustrate the changes in mois-ture equilibrium during ginning and theincrease in moisture overnight. Note thatconditions for this particular period werereasonably moist and cool compared withconditions in other years, when rh values

typically fall to less than 20 per cent rhand temperatures exceed 30C during theafternoon.

Effect of moisture on cotton breproperties

Moisture has important effects on thephysical properties of cotton particularlytensile properties and other property de-scriptions normalised for weight. The in-

crease in strength with increased moisturecontent is attributed to the release of inter-nal stresses as hydrogen-bonding is weak-ened and to the ability of the structure tobe pulled into a more oriented form.

In one study, at 55 per cent rh, tenacity

was 25.8 g/tex and this increased to 29.1g/tex after conditioning at 75 per cent rh.Fibre crimp, compress-ability and torsional

rotation properties are also affected byhigh humidity.

These properties affect the resilienceand toughness of cotton bres and textilessubject to normal wear conditions. Mois-ture content also increases with trash con-tent and bre yellowness.

The glass transition temperature

These physical changes in cotton prop-erties with changes in moisture content

reect changes in the glass transition tem-perature (Tg), which describes the temper-ature (and rh) where a material changesfrom being hard and glassy to one that is

DECEMBER 2009JANUARY 2010 THE AUSTRALIAN COTTONGROWER 33

FIGURE 3: Change in cotton cellulose density (g/cm3) withmoisture regain (per cent)3

FIGURE 4: Cotton moisture absorption isotherms at threetemperatures

34s

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soft and rubbery. The Tg can be estimatedin advance by means of the Fox equation.

Whilst the Tgs of synthetic bres are

well known and easily measured, the Tgs

of natural (polymer) bres are less wellknown. The glass transition tempera-

ture of wool has been measured and rh

and temperature are known to affect the

amount of damage during processing and

handling properties of yarns, such as snarl-

ing and entangling during knitting.

Little has been reported of cottons Tg.But recent work at CSIRO suggests thereare sound reasons to expect a Tg in cot-

ton and that the implications for ginning

cotton could be quite profound. As per the

Fox equation, the Tg in natural materials

varies signicantly with the amount of wa-

ter in the bre and this is determined by

the external humidity.

It is proposed that keeping seed-cotton

or lint near the Tg of cotton could improvethe effectiveness of ginning and that these

conditions may improve the mechanical

properties of the bre and reduce damage.

To gain some insight into the potential

interactions between the ginning process

and the bre properties, some theoretical

calculations have been made using the Fox

equation and plotted in Figure 6.The implications of this are that cotton

with a moisture content at temperaturesless than the Tg will be more prone todamage than bres with the same mois-ture content but processed at temperaturesgreater than the Tg. Practically, this means

cotton will be more resilient if it is proc-

essed under warmer, more humid condi-tions, as dened by the Tg line in Figure 6.

Rate of drying

Dry cotton placed in damp air for long

periods will gain moisture, and wet cottonplaced in dry air will lose moisture. Therate of adsorption and drying of waterwith change of humidity is very fast for anisolated bre or small bundle of bres. Fordenser assemblies of bre, the changes aremuch slower and involve complex interac-tions between the rate of diffusion of water

molecules and the evolution and transmis-sion of heat of sorption for example, theaction of water vaporising and detachingfrom the surface of cotton cellulose pro-duces a temperature rise.

The diffusion gradient between the bre

assembly and the surrounding environ-ment (rh, temperature and air movement)is also important.

Experimental and theoretical stud-ies suggest that bre assemblies with thedensity of a commercial high density (HD)bale, for example around 0.4 g/cm3, havea standard half-change period of around10 hours. That is, a bale of cotton with

seven per cent moisture and stored at atemperature of 18C, will take around 10hours to reach a half-way point in its equi-librium to new ambient conditions.

Ninety-nine per cent of the total changeis completed in around 11 times the half-

change value (about ve days). Bale weightlosses experienced in Australia reect thisrelationship. For example, HD bales withregains replenished to 7.5 per cent will losetwo to three kg of weight within half a dayof storage in open, dry conditions, and willlose another two to three kg if left in these

conditions for periods longer than a week.Studies by the USDA ARS show that

after cotton bre is baled, moisture trans-fer occurs very slowly, especially at highdensities. Bales at densities of 0.2 g/cm3required over 60 days to equilibrate withthe environment while bales at 0.47 g/cm3 required over 110 days. Equilibrationtime is also a function of the starting mois-ture content as well as the humidity andtemperature of the environment duringstorage and bale covering.

CONCLUSIONIn this article we review current knowl-edge of the interactions between water,

cotton cellulose and cotton bres. Thisknowledge is important for understanding

34 THE AUSTRALIAN COTTONGROWER DECEMBER 2009JANUARY 2010

FIGURE 5: Temperature and relative humidity of transport airin gin ducting throughout a single day in the 2007 Australian

winter (Southern NS)

FIGURE 6: The theoretical Tg of cotton with moisture content asper the Fox Equation

w33MOISTUREINCOTTON

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the affects of moisture on processing, es-pecially through the gin, on bre testingand on bale storage. In future articles weshall review the inuence of cotton mois-ture content on these different processes.

The authors grateully acknowledge thefnancial support o the Australian CottonResearch and Development Corporation (CRDC)and CSIRO in preparing this work. The authorsalso grateully acknowledge the commentsmade by Drs Keith Millington and Mike Bangeon the drat o this paper.1CSIRO Materials Science and Engineering,Henry Street, Belmont Vic 3216.

FIGURE 7: The relationship between bre packing density andthe half-change period for cotton cellulose reaching moistureequilibrium

Ginning & Fibre Quality Series

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