Polyester Resin for Textile Sizing

Currently, the four main types of Textile Sizing Agent are modified starch, polyvinyl alcohol (PVA), acrylic binder, and water-soluble polyester resin (WSP Resin). However, the viscosity of starch fluctuates greatly, and its gelatinization performance is poor. PVA is not biodegradable and causes environmental pollution. Traditional acrylic pastes have serious re-adhesion properties, making it difficult for leasing. Therefore, inexpensive and biodegradable polyester resin for sizing is becoming increasingly popular.

According to the theory of “more similarity and better solubility”, we have developed water-soluble polyester resin with a structure similar to that of polyester yarn or fibre. For more information about WSP resin, you can refer to https://www.polyestermfg.com/portfolio-items/water-soluble-polyester-resin/. Water-soluble or water-dispersible polyester resin (hereafter referred to as “polyester resin”) is a new type of textile slurry containing an ester group (-COO-) and a water-soluble group in its macromolecular structure. It is synthesised by a polycondensation reaction involving dibasic acid (e.g., terephthalic acid) and dibasic alcohol (e.g, ethylene glycol). In the macromolecular structure, the molecular chain is dominated by rigid benzene rings and flexible aliphatic hydrocarbon groups, which are connected by -COO-.

Polyester paste has excellent adhesion properties to polyester yarn or fiber. The ester group is easily hydrolysed and has excellent desizing performance. The viscosity and surface tension are lower and the wettability and permeability of the yarn are better. It is also easy to decompose by microbes and has good environmental performance.

1. How to Produce Water-Soluble Polyester Resin?

1.1 The Raw Material

Common water-soluble polyester resin contains terephthalate, flexible groups, and water-soluble groups. Terephthalate provides adhesion and toughness to the film. The amount must be at least 20% to ensure adequate film strength and adhesion. If the amount exceeds 94%, it is not possible to produce a water-dispersible or water-soluble polyester dispersion. Flexible groups are used to disrupt the regularity of the molecules, thereby reducing the glass transition temperature (Tg) to the desired level. Commonly used softener monomers are long-chain fatty dicarboxylic acids or diols, such as butanedioic acid, adipic acid (AA), polyethylene glycol (PEG), and diethylene glycol monocarbonate (DEG). Water-soluble groups, as hydrophilic units, are important modifiers for preparing WSP resin. Water-soluble groups often have an isophthalic acid sulfonate structure (such as isophthalic acid-5-sodium sulfonate or dimethyl isophthalate-5-sodium sulfonate). The sulfonic acid group is polar and hydrophilic, and has a strong electron-absorbing effect. This is conducive to the infiltration of water and reduces the degree of regularity of the polyester molecular chain segments. This increases the size of the non-crystalline zone and facilitates the infiltration of water molecules.

1.2 Synthetic Method

The synthetic process route of WSP Resin is: Ester Exchange Reaction→Direct Esterification→ Polycondensation

The ester exchange reaction involves the interaction between a lower alkyl ester of a dibasic carboxylic acid and a diol.

(Ester Exchange Reaction)

Esterification is the direct interaction between a dicarboxylic acid and a diol.

(Esterification)

Polycondensation reaction is the process of co-polymerization of dicarboxylic acid diol esters to produce macromolecular polyesters.

Polycondensation Reaction

2. How to Test WSP Resin Solution?

2.1  Structural Characters

Infrared spectroscopy tests were performed to determine the composition of the polymers, i.e., whether they were copolymers of various monomers. Nuclear magnetic resonance (NMR) tests were performed to determine the molecular structure of the polymers. And X-ray diffraction tests were performed to evaluate the crystalline properties of WSP resin.

2.2 Water Solubility and Solution Stability

A sample of the water-soluble polyester slurry was dried to a constant weight, and a 25% dispersion was prepared by adding water to a three-necked flask equipped with a reflux condenser tube. The solution was heated to a boil, stirred for 0.5 hours until completely dissolved, cooled down to room temperature, filtered to remove trace mechanical impurities, and its water solubility observed. The solution was left at room temperature for one week to observe any precipitation and evaluate stability.

2.3 Viscosity and Thermal Viscosity Stability

The viscosity of the water-soluble polyester slurry was tested using an NDJ-79 rotational viscometer. The viscosity of the slurry was determined at a solution mass fraction of 5% and 90 °C after holding for 30, 60, 90, 120, 150, and 180 minutes. The thermal stability of the slurry was evaluated according to the change in viscosity over time. If the viscosity does not change much, this indicates good thermal viscosity stability.

2.4 Surface Tension

It is now generally believed that low solution viscosity is good for yarn permeability. However, the permeability of the solution to the yarn is also closely related to its surface tension. Under the same viscosity conditions, the lower the surface tension, the better the permeability. Therefore, surface tension is also an important factor in evaluating the performance of the size. The surface tension of water-soluble polyester solutions can be determined using either the maximum bubble pressure method or the ring method.

2.5 Film-forming Properties

Dry the aqueous polyester slurry samples until they reach a constant weight. Add water to form a 6% slurry in a three-necked flask equipped with a reflux condenser tube. Spread the film onto a plastic film, dry it, and evaluate its film-forming properties. If the film can be peeled off, it is considered good.

2.6 Moisture Absorption

A 5% aqueous solution of waterborne polyester dispersion was used to spread the film. The film was then dried in an oven until it reached a constant weight, after which the moisture absorption rate was measured at 75% relative humidity and room temperature.

2.7 Sizing Performance

Use 5% water-soluble polyester paste to size the warp yarn. Then measure the raw yarn and sizing hairiness. The raw and sized yarns were subjected to a friction test and an analysis of their tensile properties. Finally, breaking strength, breaking elongation, reinforcement, elongation reduction, hairiness of 2 mm and below, hairiness reduction, and friction number were obtained, as well as the abrasion increase rate.

2.8 Biodegradability

The biodegradability of polyester size is measured by its BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand) values. A high BOD value indicates that the substance is easily decomposed by microorganisms, while a high BOD/COD ratio indicates biodegradability and reduced environmental pollution. According to the literature, current polyester pastes are highly biodegradable, ranking second only to modified starch and outperforming CMC and PVA. This is important for reducing environmental pollution. The COD and BOD values of several sizing binders are shown in the Table below.