Different applications and related technical requirements require a specific viscosity grade. Viscosity is different, the molecular weight is also different. The intrinsic viscosity corresponds to the weight of the molecule. According to the intrinsic viscosity of different PET is divided into homopolymers and copolymers. In short, homopolymers and copolymers have the following differences: homopolymer, single repeat unit. The copolymer typically has two different repeating units. Copolymer is slower than homopolymer crystals and can be used in certain special aspects such as hot filling bottles and multiple bottles. Copolymer PET produces less acetaldehyde, so it is suitable for bottled mineral water. 2) PET biaxial orientation Overview Molecular orientation is the result of ordered rearrangement of macromolecules, usually under the influence of tensile strength at the appropriate temperature, molecular rearrangement is conducive to improve the physical properties of the material. Pull and blow at low temperature (TG or so), the vesicles are stretched in the radial and axial directions, and the molecules are aligned in both directions (ie, biaxially oriented). PET semi-crystalline orientation (semi-crystalline polymer) under natural conditions, PET semi-crystalline polymer from the crystallization zone or spherulite region randomly distributed amorphous phase composition. When the temperature is slightly higher than the glass transition temperature (Tg), PET is a high elastic state, due to stretching, the formation of molecular orientation. In PET different aggregation, due to uneven stretching and produce different effects; different effects of the mixture determines the final performance of the material. ◎ amorphous phase stretching to macromolecules or crystallization of macromolecules on the secondary link points on the part of the orientation, the material becomes more orderly.
◎ crystal area in the direction of tension, the tensile rate to promote the formation of crystallization. This crystal has a different structural characteristic from the spherulites, which is static or thermally stable and crystallizes when the adjacent molecular chains are pulled together. And for PET, the benzene rings of adjacent molecular chains are attracted to each other. This crystal is called induced crystal, the tension caused by structural changes. The mixing effect of the disordered phase and the crystalline phase produces a regular final structure that directly affects the amount of crystallites formed in the semi-crystalline polymer due to the orientation of the molecule. This requires that the degree of crystallization be stretched as long as possible before being stretched for orientation So that the PET preform must be solidified in an amorphous state after injection molding. The biaxial orientation of PET preforms ◎ The change of macro state Biaxial orientation fundamentally changes the performance of the material.