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Cotton's long journey to chemical processing

Time : 2021-10-08 Hits : 146

Cotton is a very old textile processing material, but from the moment it is picked from the cotton tree, it has to undergo a long journey of chemical processing.

Sizing material:

Cotton is just picked as a ball of cotton and cannot be made directly into clothes. This is because clothes are made by cutting and sewing together cloth, which is made up of a single strand of yarn, which in turn is made up of a single strand of fibre.

We need to comb the cotton balls into a single cotton fibre by means of a textile machine, but the cotton fibres are so fine that they break very easily during machine traction and there is no way to weave them into cloth. Soa layer of sizing (modified starch sizing + PVA polyvinyl alcohol, CMC carboxymethyl cellulose, PA polyacrylate) needs to be applied to the yarn to give it a protective film to prevent breakage, and also to keep the hairiness of the yarn close to the fibres, reducing friction and improving yarn quality.

Once the countless cotton yarns have been formed into a single piece of cotton fabric, it has to be sent to a printing and dyeing factory for processing.

A piece of cloth mostly goes through 3 steps: pre-treatment - dyeing - finishing

Pre-treatment:

Paste is beneficial when weaving, but harmful when dyeing. When the surface of the fibre is tightly covered by a film of pulp, the dye cannot get inside the fibre to dye it and it is then necessary to treat the pulp before subsequent dyeing.

So we have to use hotcaustic soda NaOH to dissolve the slurry into a good water-soluble sodium salt, and someJFC permeate (fatty alcohol polyoxyethylene ether) to help dissolve it better. This step is called desizing.

Cotton is naturally grown, so it contains many, many impurities, including pectin, waxes, cottonseed hulls, inorganic salts, pigments, ash, nitrogenous substances and so on. These impurities cause the cotton germ to be yellow and covered in black cotton seed husks.

image 

It is then necessary to add the oxidising agenthydrogen peroxide H2O2 to bleach the pigment, and at the same time to remove the blackened cottonseed hulls,caustic soda NaOH andsodium bisulphite NaHSO3 are added to react with the phenolic hydroxyl groups in the lignin to produce derivatives that are readily soluble in alkali. This step is called refining bleaching.

Amount of caustic soda adsorbed or consumed by 100 g of cotton fibre 

Amount of caustic soda adsorbed or consumed by 100 g of cotton fibre

Amount of caustic soda absorbed or consumed/g

Pectin

0.2-0.3

Nitrogenous substances

1.0

Waxy substances (fatty acids)

0.1

Carboxyl groups in fibres

0.2-0.3

100g fibre repair soda

1.0-2.0

Total

2.5-3.7

Because hydrogen peroxide can easily be catalyzed by the metal ions of iron and copper in the water to occur ineffective decomposition, so that the cotton embryo cloth is not only not bleached but also because of the violent reaction and make holes in the cloth, so also addsodium silicate Na2SiO3, EDTA, sodium hexametaphosphate to adsorb these metal ions to prevent the occurrence of metal ion catalytic phenomenon.

Dyeing.

Dyes are completely different frompigments, and it is the strong bond between thedye and thegarment that ensures that the colour does not fall out in everyday washing and that the colour on thegarment does not stain the person.

The combination of pigment and fibre for painting and writing is not so demanding; it is enough to have colour.

There are three types of dyes commonly used for cotton fibres: direct dyes, reactive dyes and reduction dyes. They all bond to the cotton fibre in different ways.

Direct dyes: negatively charged, while cotton fibres are also negatively charged in aqueous solutions because they contain many hydroxyl-OH and carboxylic acid groups - COOH, resulting in mutual repulsion. So it is necessary to add theSodium sulfate Na2SO4, which has a small ionic radius and is positively charged, to neutralise the electrical properties and reduce the mutual charge repulsion, and then the direct dyestuff relies on its own van der Waals forces and hydrogen bonds to bond closely with the cotton fibres.

纤维素电荷图 

Reactive dyes: also known as reactive dyes, these dyes have both vinylsulfone and homotriazine groups on them, both of which can be substituted with the hydroxyl-OH on the cellulose to produce a stable covalent bond. However, the substitution reaction has to be at about pH: 11 to fully react, sosodium carbonate Na2CO3 needs to be added to adjust to an alkaline pH.

Reduction dyes: Normally they are solid, so there is no way to dye them directly onto the cotton fibres, so we have to addSodium dithionite (sodium hydrosulfite) andRongalit powder (sodium formaldehyde sulfoxylate) to react the reduction dyes into soluble cryptic sodium salts, which can then be dissolved in water and dyed onto the fibres, and then placed in the air to use oxygen or add hydrogen peroxide H202 to re-oxidise the cryptic sodium salts The dyeing is then completed by reoxidising the recessive dyestuff to insoluble reductive dyestuff in air or by adding hydrogen peroxide H202.

Reducing dyes do not dissolve under normal circumstances, so it is very difficult to lose colour.

After dyeing, there is still a large amount of floating colour on the surface of the cotton fabric, so a large number ofsurfactant compounds are needed to wash this layer of floating colour away and to prevent the washed out colour from re-staining the fabric, so a variety of different surfactants need to be used to compound the wash again.

After washing, it is necessary to use anactive colour fixing agent (quaternary ammonium salt compound), which can react and combine with the dye, making it less soluble or covering the surface of the garment directly with a film, making it difficult for the dye to come off.

With the addition of a colour fixing agent, the improvement in fastness compared to a blank colour is obvious.

Post finishing:

Fabrics are all woven from yarns, butthe properties of the fabric ≠ the properties of the fibres themselves.

Printers and dyers can impregnate fabrics in afluorocarbon polymer solution and, through a certain baking process, can make cotton fabrics water repellent, as I describe in this article (what are the two organic solvents that are not mutually soluble and both insoluble in water?)

This improves the performance of fabrics and is known in our industry as aprinting and dyeing auxiliaries.

The feel of the garment will deteriorate with more washes simply because the softener in the garment will decrease with more washes.

Common softeners include cationic softeners and silicone softeners. Cationic surfactants are also the main ingredients in hair conditioners, relying mainly on hydrophobic agents to attach to the fibres and hydrophilic groups with a cationic charge that repel each other to reduce friction.

The effect of the silicone oil softener will be even more pronounced, as the energy required to rotate the Si-O silicone oxygen bond in the silicone oil is almost zero, and the two methyl groups on the dimethyl silicone oil also occupy a larger spatial position, allowing the fibres to increase their distance from each other, thus improving softness.

image 

Silicone oil structural formula

Waterproof, soft, colour changing, antibacterial, aromatic, flame retardant, anti-wrinkle, whitening, blackening, weight gain, fluorescent, mosquito repellent, etc., only you can't think of, no you can't do, and this can all be achieved by adding different printing and dyeing additives.

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