In the process of textile dyeing and finishing, fabrics are subjected to various complex interactions, including physical-mechanical and chemical effects. This leads to changes in the external form and structural dimensions of the product, and in some cases, the fabric loses the desired form, appearance, and style, severely impacting its performance. Therefore, ensuring the stability of the external form and dimensions of the fabric is an important criterion for assessing product quality.

01 Heat Finishing

Heat setting refers to the processing of maintaining the fabric at a certain size under appropriate tension, heating it at a certain temperature for a certain period, and then rapidly cooling it. Heat setting can eliminate existing creases on the fabric, improve the dimensional stability of the fabric, making it less prone to difficult-to-remove creases. It also enhances the fabric's resistance to pilling and its surface smoothness, with certain effects on the fabric's strength, feel, and dyeing performance.

The entire heat finishing process can be divided into four stages:

(1) Heating Stage: Dry or wet fabrics enter the heat setting machine, and the fabric surface is heated to the setting temperature.

(2) Thermal Balance Stage: Heat energy penetrates into the fibers, ensuring that the fiber surface and interior reach the same setting temperature.

(3) Transformation and Molecular Adjustment Stage: Under stress, once the setting temperature is reached, weaker secondary bonds in the fiber structure are broken, and the fiber molecules are rearranged and reoriented.

(4) Cooling Stage: The fabric undergoes rapid cooling before leaving the stretching heat setting machine, fixing the fabric's shape according to the new arrangement of fiber molecules.

02 Heat Finishing Mechanism

Synthetic fibers are thermoplastic, but when the temperature is below the glass transition temperature, the polymer chains in the fibers are in a frozen state, and under stress, they can only undergo common elastic deformation. When the temperature exceeds the glass transition temperature, the molecular chains begin to move, and the fibers enter a high-elastic state, where they exhibit high elastic deformation under stress. Since synthetic fibers have both crystalline and non-crystalline regions, the polymer chains are in a viscous flow state only when the temperature is higher than the melting point and above the flow temperature. In this state, the polymer chains can undergo plastic deformation. Otherwise, they remain in a high-elastic state.

When synthetic fibers are in a high-elastic state, applying tension to the fibers causes the polymer chains to creep and rearrange along the direction of the external force. New intermolecular forces are established in new positions. By maintaining tension and cooling, this new state is fixed, achieving the purpose of heat setting.

03 Factors Affecting Heat Finishing

When synthetic fibers undergo heat finishing, they are primarily influenced by factors such as temperature, time, and tension. The reasonable control of these factors is of great significance in achieving good heat finishing results.

Temperature

The heat setting temperature has a close relationship with the post-setting fabric's heat shrinkage, mechanical properties, dyeing properties, and whiteness.

(1) The Impact of Heat Finishing Temperature on Fabric Dyeing Performance

Acrylic fibers have a distinctive feature when dyed with cationic dyes. When the dyeing temperature exceeds the glass transition temperature, the dye uptake rapidly increases. For example, the dye uptake at 95°C for 1 minute surpasses that at 75°C for 3 hours. Furthermore, when the temperature surpasses the glass transition temperature, the fibers are in a wet heat setting process. Thus, as the wet heat setting temperature of acrylic fabrics increases, the dye uptake also increases.

The dye uptake of polyester and its blends with cotton or viscose fabrics is also related to the pre-setting temperature. In high-temperature, high-pressure dyeing, there is an inverse U-shaped relationship between the setting temperature and dye uptake. When the setting temperature is below 190°C, the increased crystallinity of fibers leads to reduced dye uptake. However, at temperatures above 190°C, the increased crystallinity and larger crystal grain size, along with increased space between grains, lead to higher dye absorption. This effect is more pronounced in high-temperature, high-pressure dyeing where dye diffusion is extensive. Certain specific dyes may exhibit a linear relationship between setting temperature and dye uptake, with higher setting temperatures resulting in lower dye uptake. The effect of setting temperature on dye uptake also exists in thermal melt dyeing.

The dyeing process of disperse dyes on polyester involves the continuous diffusion of dye molecules from the dye liquor to the fiber's surface and further into the fiber's interior. As the fiber's heating conditions change and the internal molecular structure is altered, this dynamic equilibrium state transforms into a new one. With higher setting temperatures, the dye uptake of disperse dyes on polyester decreases. This is different from the situation in high-temperature, high-pressure dyeing, where dye uptake increases with higher setting temperatures.

(2) The Impact of Setting Temperature on Fabric Shrinkage Stability

The heat shrinkage of acrylic and its blended fabrics differs from that of polyester. This is because the heat setting temperature of acrylic is limited by its molecular structure and is generally in the range of 140-160°C. The post-setting crystallinity of acrylic only increases by 3%, but the integrity of the crystalline regions significantly improves. Therefore, when acrylic fabrics are set under tension at 140-160°C, the fiber's modulus decreases, elongation increases, and fabric shrinkage in boiling water decreases. This results from the fact that acrylic must be set within a relatively small temperature range; if the temperature exceeds 160°C, the fiber's strength decreases due to yellowing. Thus, the relationship between temperature and shrinkage is more important in comparing the non-set and set fabrics. The difference between the two is substantial. The post-setting fabric exhibits significant improvement in size stability. After heat setting, blended fabrics with polyester exhibit increased size stability. The improvement in fabric shrinkage stability is due to changes in the polyester molecular structure and increased density.

(3) The Impact of Setting Temperature on Fabric Elasticity

The wrinkle resistance and anti-wrinkle properties of synthetic fibers and their blended fabrics are significantly influenced by the setting temperature.

For polyester and blended fabrics, within a certain range, the wrinkle recovery angle increases with higher setting temperatures, and the fabric's wrinkle recovery is better than that of non-set fabrics. However, when the setting temperature exceeds 200°C, the wrinkle recovery angle decreases with higher setting temperatures, and the fabric becomes stiffer. From the perspective of wrinkle resistance, it is advisable to set the temperature below 200°C.

The wet rebound performance of nylon fabrics improves significantly with increasing setting temperature and time. Additionally, at the same setting temperature, the wrinkle recovery angle sharply increases within 30 seconds, and the amplitude of change is substantial. After setting for more than 30 seconds, the curve becomes flatter, and the change in the rebound angle decreases.

(4) The Impact of Setting Temperature on Fabric Whiteness 

The whiteness of the fabric during setting is also affected by various factors. One prominent factor is the pH value of the fabric surface before setting. Fabrics with an alkaline surface, with a pH value above 8, tend to yellow after heat setting. The degree of yellowing is related to the level of alkalinity on the fabric surface. Fabrics with higher alkalinity tend to exhibit more severe yellowing, and if the alkalinity is uneven, the yellowing will also be uneven. Therefore, in addition to requiring uniform whiteness on the fabric, it's necessary to minimize the alkalinity, with a general standard of a fabric surface pH value below 8.

Aside from this, the temperature during setting also affects whiteness. For all types of fabrics, the whiteness decreases with higher setting temperatures. Pure viscose fabrics show a greater decrease than polyester-viscose blends and pure polyester fabrics. This is because during heat setting, viscose fibers lose moisture, leading to some fibers dehydrating and carbonizing, causing yellowing. The same situation occurs in polyester-cotton blends. Furthermore, even at lower temperatures, if the setting time is long, cellulose fibers can partially dehydrate and yellow.

(5) The Impact of Cooling Temperature on Setting Results

The cooling and temperature reduction conditions after heat setting have a significant impact on the physical and mechanical properties of the set fabric. The higher the cooling temperature, the greater the Young's modulus. In general, Young's modulus is directly related to the fabric's wrinkle recovery, and higher modulus fibers make fabrics have better wrinkle recovery. Therefore, higher cooling temperatures contribute to improved wrinkle recovery of set fabrics. At the same setting temperature, higher cooling temperatures correspond to larger wrinkle recovery angles. The slower the cooling, the better the fabric's wrinkle recovery.

(6) The Relationship between the Finishing Machine Oven Temperature and the Fabric Surface Temperature

The specified temperature in the heat finishing process typically refers to the actual temperature reached by the fabric substrate, and it is the most crucial factor in ensuring setting quality. However, the heat finishing of synthetic fibers takes place inside the finishing machine. The temperature displayed on the machine's instruments only represents the temperature inside the finishing machine and does not indicate the actual temperature reached by the fabric's main body. The temperature of the machine's oven can usually be controlled and kept constant during the heat finishing process, but the temperature of the fabric's main body varies depending on factors such as fiber type, fabric structure, running speed, and more. Since it is challenging to test and display the actual temperature on the fabric's surface in practical production, the process's setting temperature is often replaced by the machine's oven temperature. The difference between the fixed oven temperature and the varying fabric surface temperature can affect the quality of heat finishing

To address this issue effectively, several measures are typically taken:

• Reduce Fabric Moisture Content: Practical experience has shown that it is advisable to control the fabric's moisture content to below 10% before setting.
• Infrared Pre-Drying: Perform infrared pre-drying before the setting process.
• Increase Oven Temperature: The oven temperature is generally maintained at around 200°C.
• Properly Control the Temperature Gradient in the Setting Machine Oven: For the reasonable distribution of temperatures in the heating zones before and after the setting machine, a temperature gradient is formed. There are generally three temperature distribution methods: front low, rear high; front high, rear low; or uniform temperature distribution. From the perspective of increasing the fabric's main body surface temperature and reducing the warm-up time, a uniform temperature distribution is more reasonable.
• Control the Running Speed: Properly control the speed of the fabric.

Time

In the process of heat setting fabric, the entire processing time can be divided into the following parts:

• Heating Time: The time required for the fabric's surface to reach the setting temperature after entering the setting machine.
• Heat Penetration Time: The time it takes for heat to penetrate from the fabric's surface to the inside of the fabric fibers, ensuring that all parts of the fabric reach the same setting temperature.
• Fiber Macromolecular Rearrangement and Adjustment Time: After the fabric's main body reaches the setting temperature, the time required for the macromolecules of the fibers to adjust their structure under the setting conditions.
• Cooling Time: The time required for the fabric to cool and lower in temperature after leaving the setting machine's oven, fixing its structural shape. Typically, the cooling time is not included in the defined setting time, with the heating time being considered as the preparation time for setting. Therefore, the control of setting time generally focuses on the heat penetration time and macromolecular rearrangement and adjustment time.

The fabric's heat penetration time (including heating time) is closely related to factors such as the heating method of the setting machine, type of heat source, thermal conductivity of the fibers, fabric structure, and moisture content. Setting machines that use direct combustion of gas for heating have higher heat transfer efficiency, faster temperature rise, and shorter setting times compared to indirect heating hot-air setting machines. For a given fabric on specified equipment, thicker and denser fabrics with higher moisture content require longer setting times. Taking all factors into account, practice has shown that heating and penetration times typically range from 2 to 15 seconds.

The time required for macromolecular rearrangement and adjustment is a very quick process, typically completed within 1 to 2 seconds. Therefore, it is sufficient to ensure that the fabric is uniformly heated to the required setting temperature, as the subsequent macromolecular rearrangement and adjustment process is extremely fast and can be negligible.

Excessive setting time has been shown to have no significant effect on improving fabric dimensional stability and can lead to decreased whiteness, stiff handfeel, and reduced strength. Under the same setting temperature, as the setting time increases, the dry heat shrinkage rate of the fabric decreases. In particular, the warp direction shrinkage rate decreases significantly, while the weft direction reaches a certain point, and the shrinkage rate decrease is not significant or remains unchanged. Generally, fabric setting time is controlled within the range of 20 to 30 seconds, which is sufficient to achieve stable dimensions and reduce heat shrinkage.

The rate of cooling and solidifying the fabric after heat treatment should be moderate. If the cooling time is too short or insufficient, it can cause further deformation of the fabric. Rapid cooling and temperature reduction can create internal stress, making the fabric prone to wrinkling and lacking resilience. If the cooling rate is too slow, it reduces production efficiency.

Tension

Tension has a certain degree of influence on the heat setting quality and product performance indicators (such as heat shrinkage rate, strength, and breaking elongation) in the heat setting process. For thermoplastic fibers like synthetic fibers, when the fabric is heat-treated under a relaxed state, both warp and weft shrinkage rates can reach over 5%. However, when the fabric is heat-treated under a certain tension, the macromolecular chains elongate, move, and rearrange along the direction of the applied force, making the fibers denser and with higher orientation. Once this state is fixed by cooling, the fabric's shrinkage rate can be significantly reduced, even to zero, and its dimensional stability is fundamentally improved. Therefore, applying a certain tension to the fabric during the setting process helps improve the setting effect.

When applying tension for heat setting, different tensions are applied in the warp and weft directions. The magnitude of tension depends on the quality requirements of the product. Typically, in the heat setting process, warp tension is represented by the overfeed rate, and weft tension is represented by the fabric's width extension. It has been observed that increasing the overfeed rate in the warp direction during setting leads to a reduction in dry heat shrinkage rate and increased dimensional stability, while the weft dry heat shrinkage rate increases with the increase in the width extension. The dimensional stability decreases with higher weft tension.

After heat finishing, the changes in the breaking elongation of the fabric in the warp and weft directions are different. The weft breaking elongation decreases with an increase in the width extension, while the warp breaking strength increases with a higher overfeed rate. Therefore, to enhance the fabric's performance and dimensional stability, it is essential to reasonably control the warp overfeed rate and weft width extension, ensuring that the tensions applied to the warp and weft directions are coordinated within an appropriate range.