Cotton fiber is made of yarn hairs that cover the seeds located inside the capsule fruits of various species of “Gossypium”, the cotton plant cultivated in the tropical, subtropical and temperate areas. Because this protecting wool is practically composed of protecting hairs that are covering the fruit seeds, after harvesting, the fibers are separated from the seeds that are weighing about two-thirds of the weight of the harvested raw cotton.

During this operation known as shelling, the cuticle particles called “shells,” are detached from the seeds, that are only partially removed during the yarn spinning process.  If there is a significant number of brown-yellow shells still left in the staple, the dyer is obliged (fabric or garment) to apply them a preliminary bleaching, otherwise they would be too much evident in light-medium and pastel colors. Once the bale has been separated from the seeds, the cotton fibers are compressed into approx. 230 kg bales, wrapped in jute or polypropylene bags, tied with metal strapping and shipped to spinning mills. The cotton compacted in this way, is called hard cotton or raw cotton.

Seen in microscopic longitudinal view, the cotton fiber looks like a flattened, spiral ribbon with slightly raised edges and a central groove (lumen). The loops, sometimes very large and widely spaced and other times tightly close together, frequently change the twisting direction. In the cross-section microscopic view, the cotton fiber has an elliptical variable shape, often described as an oval kidney-shape, depending on the fiber section of the cut. In all cases, however, the contours of the central “lumen” cavity are clearly visible.

cotton fiber cross-section

 

 

 

 

 

cotton fiber longitudinal view

Colour resistance
Generally, the color fastness to water and sweat treatments is low, because the fiber is dyed with direct dyes that cost less compared to reactive dyes, that instead give a good color fastness through wet treatments, but with higher costs. All of these can cause some possible reactions: possible discharges especially for the dark colors, and therefore for dark solid-color garments it is advisable to avoid washing with light-colored garments; while for the garments in contrasting colors it is necessary to carry out preliminary tests of color resistance (passing the iron over a white cotton fabric tester, soaked in a water and sweat solution or checking possible colour discharges).

colour discharge of black zip on white fabric

colour discharge of black zip on white fabric

Laboratory testing
The most commonly used colourings are the direct dyes and the reactive dyes. In the table below are indicated the colour fastnesses and brightness for each type of dye.

Dimensional instability
Due to the low fibre stretching capability under stress (see diagram below), cotton fabrics cannot distort during all treatments undergone during the weaving cycle (spinning, weaving, dyeing, etc.), so they accumulate internal tensions with a magnitude depending on the stresses they have been subjected; when the fabric is damp/wet, it tends to relax and to return to its balanced position: as a result, it shrinks in a percentage depending on the previously accumulated stresses.

It is therefore necessary to treat the fabric before packaging, through:
– Copper plating
– Sanforization
– Mercerizing

according to the table, the cotton is not elastic with a high breaking load, as it has low stretching properties

 

Pilling
Pilling is related to the fibre length: the shorter the fibre is, the more easily the pilling.

 

 

Polymerisation degree (cotton is cellulose-based)
Cellulose is a polymer composed from hundreds of glucose molecules. The number of glucose molecules constitutes the polymerisation degree. Cellulose is composed from the following elements: carbon 44,4%, oxygen 49,4%, hydrogen 6,2%. These chemical elements are organised as an elementary molecule known as glucose with the formula C6 H12 O6. Cellulose is a macromolecule obtained through the polycondensation of a certain number of these elementary glucose molecules, called polymerisation degree. The fabric resistance and elasticity are increasing when the polymerisation degree is higher.

The use of bleaching agents reduces the polymerisation degree by transforming the cellulose into water-soluble hydrocellulose. The repeated industrial washing treatments carried out during the cotton lifecycle reduces the average polymerisation degree, and when the cotton value (2.600 when new) is dropping below 1.000, there is 50% more mechanical strength loss compared to the new one, and therefore the items are tending to tear. When arriving to a value below 500, the garment can be already considered as waste. The harsher is the washing process, moreover the concentration of bleaching agent used, the faster is the depolymerisation of the cellulose.

With each washing using hypochlorite, if well executed, without exceeding the timing, temperatures and concentrations, about 15-20 degrees of polymerisation are being lost, so if you start from 2.500 you can perform around 80 washes (approximately) before the fabric gets damaged; if we start with fabrics with a low polymerisation degree, the number of washes using bleaching agent is proportionally reduced. When the P.D. descends below 1.000, there is 50% more mechanical strength loss compared to the new one, and therefore the items are tending to tear.

When we are talking about the linen for the hospitals, in the specifications for bidding to the tenders, the polymerisation degree is stated among the priorities:
2.500-2.700 for the raw cotton
2.000-2.200 for the bleached cotton.
The polymerisation degree is determined by measuring the viscosity of the cellulose solution in a mix of copperammonium liquid.

Frequent problems
The items can shrink in both directions and they could also twist, (the side seams can turn into oblique position) or simply after water-washing.

holes caused by chemical attack

 

 

 

 

 

 

low colour fastness

items yellow colouring after exposure
to nitrogen oxides

 

 

 

 

 

items yellow colouring after light exposure
or the use of wrong bleaching agents

items grey colouring after degradation
with optical brighteners

 

 

 

 

colour shading after using
optical brightening agents

Cotton garments care
Cotton is a quite resistant fibre and can therefore be washed by hand or in the machine, using either fabric soap or detergent, without suffering any damage. When washing it in the washing machine, we must pay a special attention to shrinkage; if the fabric or the garments were not subjected to special treatments such as sanforization, mercerisation, finished items or raw fabric shrinkage baths, machine washing must be avoided or special precautions must be taken, especially by reducing mechanical stirring.

Washing up to a temperature of 50-60°C has no influence on colour fastness; on the contrary, as the temperature increases, colour resistance tends to improve. In addition, it easily absorbs optical brighteners present in the washing products, always maintaining a perfect whiteness degree. It has a good resistance to the action of alkaline products (up to PH 12). On the other hand, it is affected by acid solutions with a PH < 3. Hypochlorite bleaching in neutral baths causes an average of 5% strength loss; in acid or alkaline baths, the strength is reduced by up to 30%. On long term, even under neutral conditions, the hypochlorite damages the fibre: the most suitable bleacher is the perborate.

It is necessary to take special care when using bleach to avoid “catalytic” damage: too much bleach turns cellulose into watersoluble oxy-cellulose. Cotton is insoluble, has an excellent resistance to solvents and can be safely dry-cleaned; in case of presumed dimensional instability, when dry-cleaning it is advisable to reduce mechanical stirring and avoid adding water. When ironing, it is recommended to interpose a damp cloth, as the prolonged iron contact causes yellowing already at 120°C and browning at 150°C; in a humid environment it can resist to temperatures of up to 180°C.

It is quite stable in the light, but long-term exposure to the light should always be avoided, to prevent yellowing and resistance loss. If the fabric or garment is dimensionally stable, it is also possible to dry it in the tumbler dryer, as the mechanical properties of the cotton are strong enough to withstand the temperature action and mechanical stirring specific to this type of drying. It is not attacked by the insects, but if kept humid, it can easily be affected by bacteria and mould.

The most suitable care symbols are the following:

 

for dimensionally unstable fabric and all knitwear garments

 

for fabrics with medium/poor dimensional stability

 

 

for fabrics with excellent dimensional stability, like the fabrics that have undergone special stabilisation treatments like sanforization

Textile Research and Analysis Laboratory
Via Vasco de Gama 2 – 41012 Carpi (MO)
T. 059 645279
lart@lartessile.it – www.lartessile.it

By ING. VITTORIO CIANCI
Director LART – Textile Research and Analysis Laboratory
DETERGO Magazine # June 2023