Physical properties of film-grade PET chips mainly include intrinsic viscosity (IV), melting point, crystallization performance and transformation temperature, b-value, DEG content, and the terminal carboxyl group (-COOH) content, etc.
Intrinsic Viscosity (IV)
Among them, IV mainly affects the strength of the film. When the viscosity reaches a certain value, the strength no longer increases. Generally, the IV value of film-grade chips should be (0.62-0.68) ±0.01 dl/g. When producing films with higher strength, PET with a higher IV value should be selected. To ensure good melt compatibility between substrate and masterbatch chips, the IV of the two types should not differ significantly. If the difference is too large, it will affect the uniformity of the film’s optical properties or even directly affect the production process. Intrinsic viscosity is also the characterization of the relative molecular weight of polyester chips. Only the molecular weight and its distribution can ensure the stability of BOPET production. The molecular weight distribution is preferably between 16,000 and 18,500.
The melting point indirectly reflects the DEG (diethylene glycol) content, molecular weight distribution, and oligomer content. The lower the melting point is, the poorer the heat resistance is. For thin films that require good heat resistance, such as insulating films, transfer films, and hot stamping films, it is advisable to choose chips with a higher melting point, and correspondingly, their plasticizing temperature should also be slightly higher.
Crystallization Performance And Transformation Temperature
The crystallization performance and transformation temperature are also important factors that affect the production of stretched films. Generally, the glass transition temperature of film-grade polyester chips is 68℃. The cold crystallization peak temperature (TC1) is at 124℃ or higher. And the melting crystallization peak temperature (TC2) is 225℃ or higher. A slightly lower melting crystallization peak temperature can better meet the needs of film stretching production. The requirements for the cold crystallization peak temperature and the melting crystallization peak temperature are exactly opposite because the smaller the difference between TC2 and TC1, the lower the crystallization rate. In the production process of polyester chips, TC2 minus TC1 can be reduced by adjusting the polymerization process conditions, adding third monomers (copolymer), etc., in order to reduce the crystallization rate of PET and better meet the requirements of stable film stretching production process. The decomposition temperature of PET is generally above 380℃.
The b-value of polyester chips directly affects the color of BOPET films. There are many factors that affect the color of the chips, mainly caused by the raw material quality, the type and content of additives, production processes, and process control. At present, a more direct way to control the b-value from the process is to change the amount of redness agent and blueness agent. The b-value of slices can also be improved by adjusting the polymerization process (such as optimizing the polymerization reaction temperature, reducing the liquid level of materials, etc.).
High content of DEG (diethylene glycol) will affect the heat resistance and light resistance of PET. But because the ether bond in DEG has a certain flexibility, the presence of ether bonds reduces the rigidity of PET molecules, lowers the melting point and TC2 of PET, and also reduces the crystallization rate of PET, which is beneficial for film stretching. The content of DEG is mainly determined by the production process. In production, it can be adjusted by changing process conditions (such as adjusting the EG/PTA molar ratio) or adding an appropriate amount of DEG.
The high or low content of carboxyl end groups (-COOH) not only affects the film stretching performance of the chips but also affects the properties of the film. If the carboxyl end group content of polyester chips is too high, it is easy to cause degradation of the melt, resulting in film breakage during stretching. And the increase of hydrogen ions will reduce the insulation performance of the film. Generally, it is required that the -COOH content of bright film-grade polyester chips be controlled between 20mol/t and 30mol/t, otherwise, it will affect the film stretching performance.
Excessive moisture content in the chips can cause melt degradation. The moisture content is generally required to be less than 0.2wt%. During production, the moisture content is required to be less than 30PPM.
High iron content will cause the yellowing of the film and deterioration of the color. Generally, the iron content is less than 3 ppm.
It refers to the content of inorganic metal salts without TiO2. Too much TiO2 indicates too many impurities, which will obviously affect the performance of the film. Generally, less than 300 ppm is required.
It refers to additive precipitates, polymer oxidation gels, high melting point materials generated on the reactor wall (the main cause of crystallization when pulling film), carbides, etc. Coalescing particles will increase with continuous production time and seriously affect the load and life of the melt filter and component filter layer. Generally, less than 10μm is required.