There are numerous factors that affect the elasticity of spandex yarn. Oxidants, reducing agents, caustic soda, high temperature and humidity, organic solvents, etc., all exert a certain impact on the elasticity of spandex yarn.

The elasticity of spandex yarn will gradually decrease over time. Spandex yarn produced by regular manufacturers has a long shelf life with minimal elasticity loss, which will not affect the wearing performance of the fabric.

Spandex yarn is not resistant to sunlight exposure. Particularly, low-quality spandex yarn may suffer severe breakage after being exposed to direct sunlight for about 2 hours, and the breakage of polyester textured network yarn is even more serious. In contrast, covered elastic yarn has relatively better light resistance. These phenomena are attributed to the morphological characteristics and low melting point of spandex yarn.

a. Setting temperature of spandex yarn: 134–200℃;

b. Softening temperature of spandex yarn: 175–200℃;

c. Melting temperature of spandex yarn: 230–250℃. When the temperature rises to 270℃, spandex starts thermal decomposition.

a. Cotton-spandex core-spun yarn: Since the spandex yarn is wrapped with a layer of cotton fibers, it takes a longer time for heat to transfer to the spandex yarn during singeing, thus naturally enhancing the heat resistance of cotton-spandex core-spun yarn.

b. Chemical fiber core-spun yarn: The spandex yarn is wrapped with a layer of chemical fibers, which slightly prolongs the heat transfer time to the spandex yarn during singeing. However, chemical fibers have higher thermal conductivity than cotton fibers. Therefore, chemical fiber core-spun yarn with the same fineness has slightly lower heat resistance than cotton-spandex core-spun yarn.

c. Chemical fiber network yarn: The spandex yarn is not covered, and the network points are relatively sparse, leaving much of the spandex yarn exposed on the fabric surface. This increases the probability of direct contact between spandex yarn and flame during singeing, making chemical fiber network yarn the least temperature-resistant among the three.

d. Heavy cotton-polyester-spandex elastic fabric: Try to avoid singeing or adopt one-pass quick singeing on the face side followed by immediate steam quenching. This method can quickly burn off the surface fuzz while minimizing elasticity damage to the spandex. For thin cotton-polyester-spandex elastic fabric, singeing is strictly prohibited, as it will cause severe elasticity loss of the spandex yarn.

Fabric samples were immersed or steamed at a liquor ratio of 1:20 with different caustic soda concentrations and temperatures for 45 minutes, after which the changes in elasticity were measured. The results showed that elasticity loss increases with higher caustic soda concentrations, presenting an approximately proportional relationship; elasticity loss also rises with increasing temperature. Moreover, the higher the temperature, the more significant the increment in elasticity loss as caustic soda concentration increases. Atmospheric steaming leads to a sharp increase in fabric elasticity loss.

During the scouring process of pre-treatment, elasticity loss increases with higher caustic soda concentrations, but the effects of scouring temperature and method are particularly critical. Steaming scouring causes much greater elasticity loss than scouring at 100℃. In other words, while caustic soda concentration affects fabric elasticity, the impact of treatment temperature and method cannot be ignored. Alternatively, treating elastic fabrics with dilute alkali at low temperatures is a safe practice.

Fabric samples were dipped in hydrogen peroxide solutions of varying concentrations (with 6 g/L stabilizer and pH value of 11), then steamed under atmospheric pressure. Elasticity loss was measured at different time intervals. The results indicated that elasticity loss increases slightly with higher hydrogen peroxide concentrations. Within 40 minutes of steaming, elasticity loss has little correlation with steaming time and only increases with rising hydrogen peroxide concentration. However, when steaming time is extended from 40 minutes to 50 minutes, elasticity loss increases drastically. Therefore, based on actual oxygen bleaching conditions in production, steaming time should be controlled within 40 minutes; otherwise, significant elasticity loss will occur. For hydrogen peroxide steaming bleaching, minimizing hydrogen peroxide concentration and shortening steaming time are conducive to reducing fabric elasticity loss.

Fabric samples were immersed in sodium hypochlorite solutions of different concentrations, and elasticity loss was measured at various time points. Sodium hypochlorite concentration and treatment time have relatively minor effects on the elasticity loss of elastic fabrics. Although elasticity loss increases slightly with higher sodium hypochlorite concentrations and longer treatment times, it is much lower compared to hydrogen peroxide bleaching. Thus, at the same dosage, hydrogen peroxide causes more severe damage to spandex elasticity than sodium hypochlorite.

Fabric samples were stretched to the specified length under appropriate tension, and their retraction length and elastic properties were measured under different temperature and time conditions. The fabric elongation ratio was 1:1.3, meaning the stretched length under tension was 1.3 times the original length. The results showed that elasticity loss increases with higher setting temperatures and longer setting times. High-temperature setting helps elastic fabrics achieve dimensional stability, but it also causes varying degrees of elasticity loss, which is highly correlated with setting temperature and time. If the temperature is too low, elasticity loss is minimal but the setting effect is unsatisfactory; if the temperature is too high and the time is too long, elasticity loss will be significant.

To balance the relationship among temperature, time, and elasticity, the setting process should be formulated based on various physical indicators of the fabric, fabric style, and customer requirements for the elasticity of finished garments. When fabrics are set below 160℃, elasticity loss is low but the setting effect is poor, and prolonging the setting time will not lead to a noticeable increase in elasticity loss. When setting at 190℃, the setting time must be strictly controlled; a setting time of 60 seconds will result in substantial elasticity loss. The ideal and safe setting temperature should be controlled at around 180℃.

Samples were padded with caustic soda solutions of different concentrations, and their elongation rate was measured. After standing for 2 minutes, the samples were washed to remove alkali, and then elasticity loss was tested.

After alkali padding, elastic fabrics exhibit elongation in the elastic direction, and the elongation rate increases with higher alkali concentrations. When the mercerizing alkali concentration reaches 200 g/L, the elongation stops increasing. This is mainly due to the swelling effect of caustic soda on spandex yarn, and the maximum swelling is achieved when the alkali concentration reaches 200 g/L.

High-concentration alkali has a significant impact on the elasticity of elastic fabrics. At an alkali concentration of 100 g/L, the elasticity loss can be as high as 16.5%. With the increase of alkali concentration, the elasticity loss of elastic fabrics increases, but the growth rate is relatively slow.

Samples were immersed in sodium hydrosulfite solutions of different concentrations and steamed at 140℃ in plastic films to simulate the dyeing process of vat dyes, after which elasticity loss was measured. The results showed that elasticity loss of samples increases slightly with higher sodium hydrosulfite concentrations. According to the test results, the impact of sodium hydrosulfite concentration on the elasticity of elastic fabrics during vat dyeing is not very significant. The slight increase in fabric elasticity loss with higher sodium hydrosulfite concentrations can be attributed to the dominant effect of steaming on fabric elasticity under such conditions.