How to Produce High-Shrinkage Polyester?

Introduction of High-shrinkage Polyester

To meet consumers’ demands for diversification, novelty, popularity, and high-end textile products, chemical fiber products are developing toward high quality and high added value, with high-shrinkage fibers being one such innovation. Fibers with a boiling water shrinkage rate of about 20% are typically referred to as general shrinkage fibers, while fibers with a boiling water shrinkage rate of 35% to 45% are called high shrinkage fibers. By blending high-shrinkage fibers with regular chemical fibers, wool, cotton, etc., or interweaving them with other types of yarns, new fabric styles characterized by soft drape, soft luster, two-tone shimmer, and concave-convex textures can be developed, laying the foundation for the advancement of this type of fabric.

There are two different methods to produce high-shrinkage polyester: physical modification and chemical modification. The physical modification involves low-temperature stretching and low-temperature setting of conventional POY yarns to produce high-shrinkage polyester fibers with a significantly increased boiling water shrinkage rate. High-shrinkage polyester fibers can also be produced from traditional chips or directly from upstream raw materials like PTA. Chemical modification, on the other hand, involves adding a third or fourth monomer during polymerization to produce modified polyester chips, which are then used to make high-shrinkage polyester fibers. While physical modification costs are relatively low, chemical modification offers more stable high shrinkage rates.

Physical Modification

The production of high-shrinkage polyester filaments from conventional POY yarn requires controlling a lower stretching temperature, which is critical in inhibiting further crystallization. The stretching temperature should be above the glass transition temperature to prevent surface cracking, internal voids, and the formation of hairy fibers and broken ends. It has been proven that controlling the stretching temperature slightly above the glass transition temperature results in an ideal shrinkage rate. Using a low-ratio stretching method under general heating conditions can produce polyester fibers with high boiling water shrinkage rates, although the tensile strength of the finished fibers may be low. To ensure the quality of the finished fibers, the stretching ratio is generally controlled at around 1.8. As the setting temperature increases, the fiber shrinkage rate decreases. Controlling a lower setting temperature can reduce crystallization, but if the setting temperature is too low, internal stresses in the yarn are not relieved, resulting in poor structural stability. A setting temperature of 50℃ is generally used to maintain a high boiling water shrinkage rate.

For producing textured yarns, the original yarn needs to be overfed in the single-filament deformation zone to ensure that the fibers enter the deformation nozzle in a relaxed state. Under the action of compressed air, the fibers are opened, bent, and entangled to form loops, achieving deformation. Conventional POY spinning is suitable for producing high-shrinkage polyester filaments with a high number of single fibers and no twist. This is because the original yarn with low single-fiber fineness can be easily dispersed by compressed air after passing through the nozzle, resulting in soft, fluffy, and high-quality textured yarns. The number of single fibers is generally required to be more than 70, and the single fiber fineness should be between 1.65 dtex and 3.3 dtex.

Besides using POY as raw material, high-shrinkage polyester filaments can also be produced from conventional chips or directly from raw materials like PTA. To reduce costs, some domestic companies use conventional PTA (90%) and EPTA (10%) as raw materials for direct melt spinning. By lowering the hot roller temperature and the stretching ratio, it is possible to produce high-shrinkage polyester fibers with a boiling water shrinkage rate of up to 50%, while other physical properties can meet the requirements for further processing.

Chemical Modification

How to Produce High-Shrinkage Polyester Chips?

To improve the rigidity of polyester molecules, flexible molecules such as aromatic dibasic acids and their derivatives, as well as aliphatic diols, are introduced into the macromolecular chain as modifiers. This increases the amorphousness of the copolyester, reduces molecular rigidity, and improves the shrinkage properties of the fabric. Studies have shown that the amount of monomer added is directly proportional to the shrinkage rate. Generally, the amount of monomer added is controlled based on the desired shrinkage rate. However, as the degree of amorphousness increases, the melting point of the copolyester decreases accordingly. If the decrease is too significant, it can cause problems during spinning, and even make spinning impossible, so the amount of modifier must be controlled within a suitable range. Some domestic companies and research institutions add isophthalic acid (IPA) and neopentyl glycol (DTG) with side chain structures to polyester to produce high-shrinkage polyester chips and fibers. As the amounts of IPA and DTG increase, the crystallization performance of the copolyester decreases, while the shrinkage rate of the fiber increases regularly, reaching over 50% with good long-term stability.

The addition of IPA or DTG disrupts the regularity of the macromolecular chain, making polyester crystallization difficult. The orientation of the fibers increases after stretching, but the crystallinity is not high. When the fibers undergo heat treatment at temperatures higher than the stretching temperature, molecular disorientation occurs, causing the fibers to exhibit high shrinkage macroscopically. When the mass fraction of IPA is 8% and the mass fraction of DTG is 1-4%, high-quality high-shrinkage polyester chips can be produced. The modified polyester fibers have both boiling water and dry heat shrinkage rates of up to 50%, far exceeding those of pure PET fibers.

The boiling water and dry heat shrinkage rates of high-shrinkage polyester fibers do not change significantly with extended storage time, indicating that the thermal shrinkage properties of high-shrinkage polyester fibers have excellent long-term stability, with their shrinkage rate remaining virtually unchanged over time.

How to Produce High-Shrinkage Polyester Filament Yarn by Using Modified Polyester Chips?

In the production of high-shrinkage polyester filaments using modified polyester chips, common issues include wall adhesion in the drying drum, agglomeration, high spinning breakage rates, poor winding formation, fiber fuzziness, and mid-process breakage. The modified chips contain a third monomer, resulting in a lower melting point, slower crystallization speed, and challenging process temperature control. Any mishandling can lead to adhesion, causing interruptions in spinning and significantly reducing production efficiency while increasing chip consumption. Therefore, during drum drying, it is crucial to ensure qualified vacuum levels in the drum, increase the cold feed rate, avoid overfilling each batch, ensure sufficient pre-crystallization time, and strictly control the heating rate within 10 degrees per hour. The maximum drying temperature should be controlled within 130 degrees to prevent adhesion to the drum’s inner wall and ensure continuous, normal spinning.

Modified polyester chips have a low melting point, necessitating lower spinning temperatures to prevent severe thermal degradation. However, excessively low spinning temperatures result in decreased melt rheology, leading to an increase in fiber fuzziness. The recommended temperature settings for the screw zones are 270-285°C, with the box (diphenyl) temperature set at 285-293°C. It is also essential to maintain appropriate winding tension and stretching processes to ensure no fiber fuzziness. A higher stretching ratio of 3.8 to 3.9 is preferred.

Stretching temperature directly affects the degree of molecular alignment and stability during the stretching process. Selecting the appropriate stretching temperature is key to achieving the desired boiling water shrinkage rate of the finished fibers. Excessively high temperatures increase breakage and fuzziness, while too low temperatures result in noticeable fuzziness around the cooling plate. A stretching temperature between 60-85°C is recommended to ensure both the boiling water shrinkage rate and minimal fuzziness and breakage. Some domestic companies produce high-shrinkage polyester low-elasticity colored yarn by using high-shrinkage chips, adding color masterbatches under suitable post-processing conditions, spinning to produce POY, and then processing it into DTY. This method can achieve a dry heat shrinkage rate of 34.0% and a boiling water shrinkage rate of over 10.2%.

How to Produce High-Shrinkage Polyester Short Fibers by Using Modified Polyester Chips?

The addition of the third monomer increases the crystallization temperature, slows down the crystallization rate, reduces crystallinity, and lowers the melting point of modified polyester chips. This irregular chain structure and loose arrangement make the chain segments easier to move, reducing the glass transition temperature and complicating chip drying. To prevent adhesion and agglomeration during drying, a slow heating rate and low temperatures are required, with a reduced feeding rate. The melting point of these polyester chips is around 243°C, and the actual melting and plasticizing temperature used in production is 280-285°C, about 40°C higher than the melting point. This is because the measured melting point is only the average melting temperature of the chips, indicating plastic flow but not complete molecular disintegration. To fully dissolve the chips into an amorphous liquid, the temperature must exceed the equilibrium melting point; otherwise, some undissolved material will remain. Due to the Addition of the Third Monomer, the Content of Diethylene Glycol and Terminal Carboxyl Group is Higher Than Conventional PET, Resulting in Lower Thermal Stability Compared to Conventional PET. A Lower Spinning Temperature is Adopted to Prevent Further Thermal Degradation and Resultant Broken Filaments, but If the Spinning Temperature is Too Low, the Melt Viscosity Increases, Resulting in Poor Flowability, More Defective Filaments, Poor Fiber Uniformity, and Melt Fracture during Spinning Head Stretching. The Screw Outlet Temperature Should Generally Not Be Lower Than 268°C.

High spinning speeds lead to high initial fiber orientation, often resulting in lower drawing ratios and lower orientation of the amorphous regions in the drawn filaments, which leads to lower dry heat shrinkage. Conversely, lower spinning speeds result in low initial fiber orientation, allowing for greater stretching in post-processing, and producing drawn filaments with higher amorphous orientation. When heated, these highly oriented macromolecules in the amorphous regions relax, resulting in high dry heat shrinkage of the fiber. Some domestic companies control the spinning speed at 1250-1300 m/min, with a stretching temperature of 60-70°C and a stretching ratio of 2.5-2.8. They do not use a heat setting process but only dry the fiber at a specific temperature to produce high-shrinkage polyester staple fiber with a dry heat shrinkage rate of around 50%.

Hubei Decon company can provide pre-crystallized high-shrinkage PET chips. The quality is widely approved by the domestic markets. For more details, you can click?https://www.polyestermfg.com/high-shrinkage-polyester-chip/.

Applications of High-Shrinkage Polyester

Combined with DTY to Produce Bumpy Tweed

High-shrinkage polyester yarn and low-elasticity polyester yarn are combined and network-processed to form blended yarn. This blended network yarn has a boiling water shrinkage rate of about 25.5%, thus developing a bumpy tweed with a fluffy and wool-like feel. The network technology improves the cohesive performance of the yarn, reduces pilling and snagging in the fabric, and reduces breakage and defective filaments during warping and weaving processes, eliminating the need for twisting, sizing, and desizing in the weaving and post-processing stages. After boiling water treatment, the high-shrinkage polyester yarn shrinks while the low-elasticity polyester yarn forms loops, creating a fluffy texture that is not easily lost.

Bumpy textures are a popular fabric style. Bumpy tweed uses a float weave structure, which is constructed by locally adding and removing warp points on a plain weave base, forming a grid pattern using the photonegative method. Each weave cycle contains four small plain weave areas, with high-shrinkage mixed network yarn arranged at the float points around each plain weave. In addition to using high-shrinkage mixed network yarn at intervals in the warp and weft, the remaining warp and weft yarns are ordinary yarns, such as polyester-viscose yarn, woven into the plain weave. For an enhanced decorative effect, a small amount of fancy yarn with similar shrinkage to the polyester-viscose yarn is inlaid in the plain weave area. After boiling water treatment, the high-shrinkage mixed network yarn at the float points shrinks maximally, causing the tightly woven plain weave areas to protrude outward. Due to the different forces, a concave-convex three-dimensional effect is formed. During the hot setting stage, a relaxed treatment is applied with a setting temperature of 160-170°C. After setting, the fabric exhibits good thermal stability, wrinkle resistance, and elasticity.

Some domestic research institutions use high-shrinkage polyester yarn combined with Modal single yarn to develop bumpy fabrics. Modal fiber is a high wet modulus viscose fiber that significantly outperforms ordinary viscose fibers. By utilizing the different boiling water shrinkage rates and dyeing properties of high-shrinkage polyester filaments and Modal single yarn, they have developed new fabrics with unique bicolor effects and bumpy textures using a double-layer structure.

Blended with Wool-Polyester Yarn to Produce Woolen Fabrics

High-shrinkage polyester filaments are selected with a fineness of 300D and a dry heat shrinkage rate or boiling water shrinkage rate of 30%. Merely relying on the high-shrinkage polyester filaments to create a crepe effect might not always be very obvious. Combining it with the fabric structure design can achieve more ideal effects. Comparing the same yarn arrangement with plain and combined weaves, it is found that the crepe effect is more prominent in the combined weave. To ensure that the high-shrinkage polyester filaments can be woven with wool-polyester yarn as warp yarns, the polyester filaments are twisted (if only used as weft yarns, no twisting is needed). For example, a twisting factor of 36.5 and a twist of 200 twists/m is chosen for the high-shrinkage polyester filament; the wool-polyester yarn has a single yarn twisting factor of 101.6 and a plied yarn twisting factor of 169, with a yarn count of 14.28tex×2. This produces a distinct crepe effect in the woolen fabric. The high-shrinkage polyester filaments only need a twisting process but do not require steaming because they shrink when preheated. During warping, the yarn tension should be even, and the tension of the polyester filaments can be appropriately reduced to ensure the quality of the warp beam. Because the polyester filaments generate more fuzz due to repeated friction during weaving, comprehensive adjustments of the shedding height, back beam height, and weaving tension are needed to prevent unnecessary warp breaks, ensuring the quality of the greige fabric.

Since polyester filaments shrink when exposed to high temperatures, if scouring and milling processes are conducted first, the fabric width will become uncontrollable. Therefore, a heat setting process must be performed first to pre-shrink the fabric to a certain width. After heat setting, the fabric undergoes scouring and milling, during which the width remains stable. A gentler steaming process ensures the fabric retains its unique texture and provides a superior hand feel. The resulting product, with its unique style and excellent properties, is an ideal material for making women’s shirts, skirts, and other garments.