Polyester, scientifically known as polyethylene terephthalate (PET). It is a thermoplastic polymer widely utilized in various applications. It is a pivotal material in the production of non-woven fabrics through the spun-bond method. In China, the output accounts for about 6% of the total output of spun-bond non-woven fabrics.
1. Molecular Structure of PET
PET molecules are linked via ester groups, with most of their chemical properties associated with these bonds. Ester bonds undergo hydrolysis easily in high temperatures or a strong alkaline medium. It leads to chain scission and a decrease in polymerization degree. So, strict control of moisture content during the PET spinning process is crucial. In production, polyester chips typically require a moisture content of less than 50 mg/kg.
2. Physical Properties of PET
2.1 Chemical Properties
Apart from its poor alkali resistance, PET exhibits excellent resistance to other chemical reagents.
(1) Acid resistance: PET remains stable in acids, especially organic acids. It can endure immersion in a 5% hydrochloric acid solution at 100℃ for 24 hours or a 70% sulfuric acid solution at 40℃ for 72 hours without strength loss. However, it cannot withstand prolonged exposure to concentrated sulfuric or nitric acid at room temperature.
(2) Alkali resistance: The ester group on the PET molecule is prone to alkali hydrolysis. PET is unstable when encountering strong alkali at room temperature or dilute alkali at high temperature. It remains relatively stable only when exposed to dilute or weak bases at low temperatures.
(3) Solvent resistance: PET demonstrates strong resistance to general non-polar organic solvents. For instance, its fiber strength remains unaffected after 24 hours of immersion in various solvents at room temperature. Yet, when heated, PET is soluble in specific solvents.
(4) Antimicrobial resistance: PET shows resistance to microbial action, insect damage, and fungal attack. So it is suitable for applications like geotextiles and environmentally friendly products. These include PET nonwovens used in landfills.
2.2 Thermal Performance
Polyester’s thermal properties significantly influence fiber performance. Polyester exhibits good heat resistance. It has a softening point ranging from 238-240℃. Its melting point is usually between 255-260°C. PET maintains its excellent physical and mechanical properties over a wide temperature range. It is usable from 20-120°C for extended periods, with short-term usage possible up to 150°C.
The melting process of crystalline polymers entails a phase change process. It is distinct from low molecular weight substances. It encompasses a considerable temperature range, influenced by factors like crystal thickness, crystallization temperature, and molecular weight. Higher crystallization temperature and molecular weight result in higher melting points.
The glass transition temperature, related to the melting point, should not be excessively low. As it may render the product unusable when close to the use temperature.
3. PET Molding Process
PET’s molding characteristics depend on factors such as relative molecular weight, distribution, macromolecular aggregate structure, and impurity content. Typically, PET resin for spinning has a relative molecular weight between 15,000 and 22,000. Lower relative molecular weights lead to decreased melt viscosity. It makes spinning fibers prone to breakage and unsuitable for high-speed stretching. This also affects various properties such as thermal stability, photostability, and chemical stability.
When the relative molecular weight is below 8000-10000, spinning becomes nearly impossible. PET with the same average relative molecular weight but a wider distribution is prone to issues like yarn breakage and fluffing during spinning. It results in non-woven fabrics with inferior properties.
The spinning temperature of PET must be controlled between melting point and decomposition temperature. The melt viscosity is closely related to spinning formation. It is influenced by factors like temperature, pressure, polymerization degree, and shear rate.
4. Spinnability of Polyester Chips
The spinnability of polyester chips is evaluated based on:
4.1 Difficulty in sifting materials: Chips with fewer impurities facilitate smooth filtration. It prevents issues like clogging, wall sticking, and uneven melting during drying or spinning.
4.2 Screw spinning pressure: Chips with good spinnability maintain stable spinning pressure. It ensures a consistent pump supply.
4.3 Pressure rise rate before filtering: Excessive impurities in chips can clog filters. It accelerates the increase in melt pressure and shortens the service life of spinning components. Thereby it affects product quality.
