The requirements for polyester quality and chip drying in high-speed spinning
1. Requirements for Polyester Quality
Polyester spinning can be categorized into two types: melt direct spinning and chip spinning. The different polyester melts and chips significantly affect the quality of spinning and finished products. The spinning conditions and the quality of the POY (Partially Oriented Yarn) are not only related to the relative molecular weight of the polyester and its distribution, the rheological properties of the melt, and the heat capacity of the chips, but also related to the content of aggregated particles in the chips, the residue from the catalysts added during polymerization, the ash content, other mechanical impurities, and the characteristics of the added TiO2. Different spinning processes lead to different spinning situations and thus have different requirements for the raw materials. High-speed spinning has the following requirements for polyester quality:
The content of mechanical impurities and aggregated particles in the polyester should be as low as possible. The fluctuation value of the melt's characteristic viscosity should ideally be less than 0.01, with a central value between 0.63 and 0.68, with a slightly higher preference. A higher characteristic viscosity is beneficial for producing good POY, but excessive viscosity can lead to spinning difficulties and increased hairiness.
The relative molecular weight distribution of the polyester should be narrow, with a distribution index α being small (α < 2.2), and the average relative molecular weight should be moderate. A larger α results in poor spinning formation, leading to hairiness and fusion, many defects, and significant fluctuations in non-oil yarn viscosity and fiber strength. A higher relative molecular weight allows the polymer to withstand high tension during spinning, which is favorable. However, if the relative molecular weight is too high, the long molecular chains may not unfold and straighten easily, requiring greater force for molecular orientation, potentially resulting in incomplete orientation. Conversely, if the relative molecular weight is too low, shorter molecular chains may break under tension when being extruded from the spinneret and drawn for orientation. Therefore, the average relative molecular weight should be moderate. The relative molecular weight of polyester largely determines fiber performance and has a significant impact on spinning process conditions. The optimal range for relative molecular weight should be chosen in a region where it is least sensitive to spinning process conditions and product quality.
The filtration performance of the polyester melt should be good. The filtration performance of the polyester melt can be described and determined using the average pressure drop ΔP over a set time G (in minutes) on a filtration area S (in m²). Its value A is referred to as the filtration coefficient, which is expressed as follows:
If the value of A is small, it indicates good filtration performance. Chips with good filtration performance exhibit a relatively stable initial pressure phase at the outlet of the pre-filter, which then gradually decreases. In contrast, poor filtration performance shows no stable phase, and the pressure drops rapidly, often in a linear manner.
Low Content of Dust in Chips A high content of dust in the chips can cause severe sticking on the spinneret, with new spinnerets showing sticking phenomena just 8 to 12 hours after use. This leads to deterioration in spinning formation, and can even result in broken filaments or block defects, shortening the lifespan of spinning components. Dust accumulation can fill the side blowing windows, affecting the speed and uniformity of the cooling airflow, thereby resulting in poor quality POY. The melting point of dust is 10 to 15°C higher than that of regular chips, making it difficult to melt at typical spinning temperatures. Moreover, dust contains a considerable amount of unmelted material and aggregated particles, which worsens its spinnability. Therefore, the dust content in the chips should be less than 0.1%.
Minimize Gel Content in Chips The gel content in the chips should be as low as possible, especially eliminating old gels. Gels are three-dimensionally cross-linked polyesters formed by the thermal cracking of the polyester and have no distinct melting point. The thermal degradation of polyester is influenced by factors such as temperature, residence time, and the presence of oxygen during production. Therefore, the residence time of the melt during polymerization and spinning should be minimized, maintaining a low temperature when possible, while minimizing the use of containers and pipes that may lead to polymer degradation.
The presence of gel significantly increases spinning breakage, resulting in dark long filaments that quickly cause blockage in pre-filters and components. Gels can exist in three forms in spinning components:
Tender Gel: This type of gel resembles the melt under typical processing conditions and has good fluidity. It is a polyester with a short generation period and not severely cross-linked. It appears as a micro yellow fluorescent entity mixed in the polyester chips, difficult to distinguish under white light but visible under UV light. Its presence leads to spinning breakage, increased fiber dye uptake, and poorer filtration performance. Fine filtration media cannot remove it, adversely affecting the spinning process.
Mature Gel: This gel has a longer growth period and is tougher. Under standard processing conditions, it remains a deformed semi-solid, appearing yellowish and sometimes brown under white light. Its presence causes serious breakage and increased dye uptake in the fibers. It can typically be filtered out using fine filtration media, but quickly leads to blockage.
Old Gel: This type has a long growth period and significant cross-linking, making it a harder solid without brittleness. It appears as dark brown to black particles under white light, resembling carbonized material. These so-called black core particles, while not frequently occurring, can severely disrupt spinning stability and product quality. They can lead to rapid blockage of pre-filters, clogging of spinneret holes, and increased defects in the product, thus their presence is unacceptable.
Minimize High-Crystal Polymers High-crystal polymers are portions of polyester that have a melting point above 280°C and a crystallinity greater than 45% (in dry chips). They can appear as white cores in wet chips and exhibit fluorescence under UV light. These form when localized polyester remains near melting temperature (260°C) for extended periods during production and spinning. Due to their high melting point, they are difficult to melt under typical spinning temperatures, potentially forming tender gels or mature gels in the spinning components, leading to breakage and quick blockages. If these high-crystal components enter the yarn, they can cause uneven dyeing, uneven tensile strength, and uneven elongation, resulting in weak and low-strength filaments.
Minimize Catalyst Residues The esterification and polycondensation catalysts added during the polymerization process remain in the polyester melt (chips), affecting spinnability (filtration performance). Therefore, it is crucial to select metal catalysts with minimal impact on spinnability during polymerization and to use them sparingly to reduce their influence on spinning performance. Currently, antimony oxides (Sb2O3) are commonly used as polymerization catalysts in polyester production in China, with retention levels ranging from 2.0 to 4.5 ppm. The presence of antimony can affect the color of the polymer; higher levels can reduce the "L" value (increase grayness), while increasing the fouling inside spinneret capillaries, leading to more spinning breakage and reduced spinnability. When spinning ultra-fine fibers, it is crucial to use polyester chips with low catalyst content like antimony.
Reducing the amount of catalysts like antimony during polymerization depends on the process and equipment, as well as the quality and purity of the catalysts, particularly Sb2O3. Since antimony itself and its incomplete oxides (Sb2O5) have no catalytic effect, low purity of Sb2O3 containing significant amounts of metallic antimony and its oxides would require increased usage for the same catalytic effect, raising the final polymer's antimony content. Similar issues are observed with other catalysts like manganese and cobalt, especially if calcium metal catalysts are involved, leading to more significant deposits and greater impact on spinnability.
Minimum TiO2 Content to Meet Opacity Requirements TiO2 negatively affects spinning performance, especially when large TiO2 particles are present. Previously, the addition amount in China was 0.5%, now revised to 0.3%. Internationally, semi-matte polyester chips typically have TiO2 addition rates of 0.15% to 0.3%. TiO2 also has two adverse effects: it serves as a degrading catalyst for polyester, promoting its degradation during spinning, and its aggregates are insoluble in triethylene glycol, making it challenging to clean the melt filter. Additionally, the particle size of the TiO2 used and its emulsification and dispersion characteristics in glycol and polyester oligomers are crucial. If the TiO2 particle size exceeds 0.3μm or easily aggregates in glycol suspension, it results in TiO2 aggregated particles larger than 0.3μm, significantly impacting the spinnability of polyester.
Diethylene Glycol Content The content generally ranges from 0.7% to 1.5%, with a preference for higher amounts. Diethylene glycol (DEG) is a byproduct of the binary alcohol reaction during polymerization, arising from the excess of ethylene glycol, along with intentional addition during the condensation process. Although the generation of DEG during polymerization is unavoidable, proper control of processing conditions can adjust its levels. The intentional addition of DEG is thought to improve spinnability and the quality of the final fibers.
The amount of DEG in polyester effectively refers to the ether bond content. The ether bonds in DEG can alter the ethylene glycol segments in polyester macromolecules, thereby increasing the number of ether bonds. Since ether bonds are dye-absorbing groups, they can enhance the dye uptake of polyester fibers (which naturally contain very few dye-absorbing groups). Meanwhile, the presence of ether bonds disrupts the orderly arrangement of macromolecules. Ether bonds also have good polarity, high entropy, and thus can lower the melting point and decrease crystallinity, correspondingly reducing fiber strength. However, higher DEG content increases the b value (yellow hue) of polyester chips, so this content must be controlled. Importantly, uniformity of DEG content is crucial. If the amount is high but lacks uniformity, it can still impair spinnability and dyeing consistency in fibers. Ideally, the fluctuation range should be between 0.05% and 0.1%.
Additionally, an increase in ether bonds reduces the melting point of polyester chips and lowers heat oxidation stability; however, it does not affect thermal stability under anoxic conditions. Polyesters with high DEG content show poorer crystallization, leading to slower crystallization during spinning, which is beneficial for producing low-crystallinity and high-orientation POY, improving the quality of the final DTY.
Melting Crystallization Peaks The spinning process of polyester chips encompasses all changes occurring during melting to cooling and shaping. The crystallization ability of polyester influences not only the crystallinity and orientation of fibers, but also is affected by spinning conditions. The melting crystallization temperature and peak height on the DSC curve are significant indicators of a polyester's crystallization capability. The thermal analysis results of six different samples are shown in Table 10-2.
From the table, it is evident that the spinnability of polyester chips is closely related to their melting crystallization temperature and peak shape. Chips with lower melting crystallization temperatures and wider, flatter peak distributions exhibit better spinnability; conversely, chips with higher melting temperatures and sharply defined peak shapes show poorer spinnability. Generally, polyester chips with melting crystallization temperatures around 170-180°C are considered to have good spinnability. If characterizing spinnability by peak value, chips with values from 0.5 to 1.0 are deemed better, while those below 0.5 show poor spinnability. Polyester melts that crystallize too quickly upon exiting the spinneret rapidly form crystalline structures, complicating the parallel orientation of macromolecules and resulting in inferior fiber quality. In subsequent stretching processes, spinning must occur at higher temperatures, increasing deformation difficulty. During further heat-setting, rapid crystallization often produces large crystalline blocks with uneven crystalline structures. All of these factors can lead to poor quality in the final product. Therefore, a polyester with a lower melting crystallization temperature and slower crystallization rate is preferred.
2. Requirements for Chip Drying
The spinning temperature for high-speed spinning is generally 5 to 15°C higher than for conventional spinning. Thus, the moisture content of dry chips for high-speed spinning must be lower to reduce melt hydrolysis. Moreover, during high-speed spinning, if even minute amounts of moisture are present in the melt, the resulting bubbles can get trapped in the fine melt stream expelled from the spinneret, leading to flying filaments or hidden defects within single filaments, causing hairiness or breakage during subsequent stretching. Therefore, the moisture content of dry chips should be less than 50 ppm, ideally under 30 ppm. Higher moisture content negatively affects the characteristic viscosity of the melt during spinning, worsening the spinning conditions. To maintain good spinning state, not only must the moisture content of dry chips meet requirements, but it should also be uniform.
During chip drying, the drying temperature affects both the efficiency and quality of the dry chips. The drying temperature must ensure the complete and rapid evaporation of moisture while preventing reductions in the characteristic viscosity of the chips or yellowing of the hue at elevated temperatures. During drying, the actual temperature of the chips should ideally not exceed 160°C, and the temperature of the drying hot air should not exceed 185°C. Increasing the volume of drying air and lowering the humidity of the drying air can help improve drying efficiency. The choice of pre-crystallization temperature and time should also be tailored to different equipment and chip materials. For chips that crystallize quickly, lower temperatures and shorter pre-crystallization times should be employed.
