What is Warp Sizing?

Warp sizing is the process of applying the sizing agent to the warp yarn to improve its weavability. Weaving ability refers to the performance of warp yarns that can withstand repeated friction, stretching, and bending of warp stoppers, healds, reeds, etc. on the loom without causing a lot of fluffing or even breaking. The single yarn fibers without sizing are not tightly entangled with each other, and the surface hairiness is more, and it is difficult to weave. After sizing, a part of the sizing liquid penetrates between the fibers, and the other part adheres to the surface of the warp yarn. The sizing mainly composed of the sizing liquid penetrating between the fibers is called the impregnable sizing. And the sizing mainly composed of the sizing liquid adhering to the surface of the warp yarn is called the covering sizing. The sizing operation is the link between yarn making, weaving, and finishing. It has a significant effect on quality and processing costs in the sizing process.

What is Sizing Chemical?

The main process of sizing is conducted by applying sizing liquid to the inside and outside of the yarn. The sizing liquid is formed by a certain proportion of various sizing agents. The material that conforms to the size, in simple terms, should conform to the principle of “sizing primely, weaving well, and desizing easily”.
There are two types: main agents and auxiliary agents. The former is also called adhesive, and the latter is also called auxiliary.

Main Adhensive

It is a polymer compound. Their molecular structure must have the same or similar functional groups as the fibers and should have good adhesion to the fibers, so they are also called adhesives. There must be a certain degree of self-adhesion between the macromolecules of the main slurry, and the solution can form a film with certain mechanical strength and elongation after drying. In addition, the main slurry should also contain a certain amount of hydrophilic groups in order to prepare a slurry. Commonly used main slurries include starch, animal glue, carboxymethyl cellulose, polyvinyl alcohol and acrylic copolymers, water-soluble polyesters, etc.

Starch is one of the traditional main sizing agents. Commonly used starches are edible, including wheat, potato, maize, sweet potato, indica rice, and tapioca. Among them, maize and wheat starch have better sizing performance and can be used for warp sizing of fine-meter high-density cotton. Non-edible starches such as acorn starch can also be used as sizing materials. Starch swells in water as the temperature increases gelatinizes into a slurry and forms a gel after the temperature decreases. The starch slurry is only suitable for sizing after gelatinization and higher than the gel temperature. The fluidity of starch slurry is poor, the viscosity is not stable enough, the serosal film is brittle and hard, and it is easy to be corroded and corrupted by microorganisms. Therefore, it must be equipped with auxiliary slurries such as softeners, surfactants, starch decomposition agents, and preservatives. Starch contains a large number of hydroxyl groups and has good adhesion to cotton, hemp, viscose, and other fibers that also have a large number of hydroxyl groups. The starch slurry has good mechanical properties after drying to form a film. Various starch derivatives, such as oxidized starch-containing carboxyl groups, water-soluble starch ethers (such as carboxymethyl starch), starch esters (such as starch acetate), cationic starch, grafted starch, etc., are widely used in various yarns Sizing.

Animal glue is extracted from animal bones, tendons, skin, or other connective tissues. It belongs to the hard prion protein and is composed of various amino acids. Commonly used are bone glue and gelatin, which are suitable for sizing natural silk and viscose yarn. The animal glue slurry has poor permeability, the serosal film is brittle and hard, and it is easy to spoil. It must be equipped with auxiliary slurries such as grease, surfactants, and preservatives.

Algin widely exists in various algae. It is a natural polymer formed by the condensation of β-D-mannuronic acid. Alginic acid produced by chemical processing is insoluble in water, and its sodium (or ammonium) salt is soluble in water, called sodium alginate (ammonium). Its aqueous solution has the characteristics of low concentration and high viscosity and can be used as the main slurry alone for sizing cotton yarn and viscose yarn. Now it is rarely used due to its inferior cost and performance.

Cellulose derivatives are mainly water-soluble cellulose ethers. A commonly used one is carboxymethyl cellulose (CMC). CMC is formed by the etherification of alkali cellulose with monochloroacetic acid. When the degree of substitution is greater than 0.4, it is water-soluble. Industrial CMC is generally the sodium salt of carboxymethyl cellulose, which is suitable for sizing cotton yarn and viscose yarn. CMC aqueous solution has the characteristics of low concentration, high viscosity, poor permeability, good hygroscopicity, strong emulsifying power, and fine miscibility with other main slurries. Crude products have high salt content and are highly corrosive to machine parts, so they are not suitable for sizing. Due to its high viscosity, re-viscosity, and cost reasons, it cannot be used as the main slurry. Another commonly used cellulose derivative is hydroxyethyl cellulose (HEC), which has similar sizing properties to CMC.

Polyvinyl alcohol is a water-soluble polymer obtained by alcoholysis of polyvinyl acetate. Commonly used polyvinyl alcohol (PVA) can be divided into high polymerization and low polymerization according to different polymerization degrees. The former is suitable for spun yarns, and the latter is suitable for synthetic filaments. According to the different degrees of alcoholysis, PVA can be divided into two types, complete alcoholysis, and partial alcoholysis. The former is suitable for hydrophilic fibers, and the latter is suitable for hydrophobic fibers. PVA serious film is tough and elastic, and it is the best one among the existing sizing agents. It can also be miscible with other main slurries in a wide range of mixing ratios to obtain a good sizing effect. Its main disadvantages are slurry foaming, skinning, and low adhesion to hydrophobic synthetic fibers. Especially the non-degradability of its secondary hairiness and desizing waste liquid during skewing have made it a trend to advocate “less and no PVA”. Europe and the United States have clearly proposed not to import fabrics that use PVA sizing. However, due to the excellent serosity characteristics of PVA and the immaturity and cost of the alternative slurry, it cannot be completely replaced in China.

Acrylic copolymer mainly refers to the copolymer and polyacrylamide with acrylate as the main body. The components are complex and changeable and can be produced according to the application. Most of them are viscous liquids with a solid content of 10-40%. Polyacrylic acid and its salt sizing agents were initially used for sizing nylon filaments, and later they were often mixed with starch for sizing synthetic fiber blended yarns. Polyacrylate sizing agent has good adhesion to polyester fibers and can be used for sizing polyester filaments. If the hydrogen on the α-carbon atom is replaced by a methyl group, it becomes a polymethacrylic slurry. The carboxyl group is replaced by the amide group (-CONH2) to form polyacrylamide (PAAm), which is a non-ionic polar size, suitable for warp sizing of fine-gauge high-density cotton fabrics. The common characteristics of this type of agent are good adhesion to synthetic fibers, low size film strength, and large breaking elongation (except for polyacrylamide, which has high size film strength and low breaking elongation), moisture absorption, and strong re-adhesiveness. It can not be used alone for sizing spun yarn. It should be mixed with PVA or starch. Appropriate selection of the composition ratio can help to control its hygroscopicity and re-adhesiveness, which is effective for sizing synthetic fiber filaments. What can be used as the main slurry are polyoxyethylene, polyvinylpyrrolidone, polyvinyl methyl ether, and other types of copolymers.

Water-soluble polyester is a polymer obtained by copolymerizing polyester monomers (terephthalic acid and ethylene glycol) as raw materials and adding water-based third monomers (such as phthalic acid sulfonate). Because of its affinity for polyester, it has been used on the polyester filament and staple fiber. The cost of water-soluble polyester resin produced by Decon has been sharply decreased, which has successfully broken the monopoly of resin from foreign markets. Our product has been exported to India, Pakistan, Bangladesh, and other countries.

The following table is Technical Comparison among Maize Starch, PVA, and WSP Resin. These conclusion is provided by one customer who has 25 years of experience in textile sizing industry.

Parameters Maize Starch PVA WSP Resin
Flexibility Poor So-so Good
Penetration Poor So-so Good
Viscosity Size High High Low
Abrasion Resistance Low High Very High
Hairiness Control Low High Very High
Desizing Difficult Not Easy Easy
Recyclability So-so So-so Good
Residual Elongation Low High Very High
Solubility in Size Mix Poor So-so Good
Effluent COD&BOD Low High Low
Surface Smoothness & Sofetness of Size film poor So-so Good

From the table, we can see that Water-soluble polyester sizing is the best option for the textile.

Auxiliary Agents

It Mainly includes softeners, surfactants, starch decomposition agents, preservatives, and waxes.

A softening agent can improve the flexibility of the serosa, so as to withstand the dynamic load during weaving. Commonly used are beef tallow, mutton fat, cottonseed oil, etc., which are all glycerides of higher fatty acids. Among them, tallow is most suitable for sizing.

Surfactant is a substance that can significantly reduce the surface tension of a liquid. Mainly used surfactants are anionic and non-ionic, which can improve the permeability of the slurry, and also have the effects of eliminating static electricity and emulsification.

Starch decomposition agents include Acid, alkali, oxidant, and enzyme, which can be used as decomposition agents to moderately hydrolyze glycoside bonds in starch macromolecules to improve the fluidity of the slurry. Sodium silicate is a commonly used alkaline decomposer, and oxidative decomposers such as chloramine T and sodium hypochlorite can also be used. Acids and enzymes are used less frequently.

The preservative can inhibit the reproduction of microorganisms in the sizing liquid or on the gauze to prevent the sizing liquid from deteriorating, the sizing, or the fabric from becoming moldy. Natural slurries such as starch, alginate, and animal glue often require preservatives. Commonly used preservatives are α-naphthol, which is a light reddish-brown scaly solid, which can be dissolved in lye but cannot be added to the slurry with the chlorine-containing oxidant at the same time, so as to avoid the addition reaction of available chlorine and α-naphthol and weaken the oxidative decomposition effect.

Wax is divided into auxiliary slurry wax (for size mixing) and post waxing. The wax used as an auxiliary should be able to be melt at 50-60°C and solidify at room temperature and be water-soluble. Post-waxing generally needs to be emulsified and applied to the sizing surface after leaving the drying room and before splitting to improve its smoothness and reduce static electricity accumulation. There is mainly paraffin wax (a mixture of higher alkanes, difficult to emulsify, difficult to desizing), beeswax (ester composed of higher fatty acid and higher monoalcohol, also difficult to desizing), and synthetic wax (ether or ester copolymer, such as poly Oxyethylene ether, good affinity with fiber and easy to desizing). Paraffin wax and beeswax are not suitable for fabrics that require printing and dyeing.