Views:74 Author:Site Editor Publish Time: 2021-05-17 Origin:Site
PET is a high crystallinity material with a high processing temperature. Most industrial PET resins with low molecular weight and narrow molecular weight distribution have low melt strength after reaching the crystallization melting temperature. In the foaming process, especially under the high shear rate of the extrusion process, as the bubbles grow, the surface tension and melt strength of the PET are insufficient, and the bubble walls become thinner, which easily leads to the rupture of the cells. Therefore, improving the melt strength to prevent cell rupture is a prerequisite for PET foam molding.
Using a chemical chain extender, it undergoes a chain extension reaction with the PET molecular end groups in the molten state to increase the molecular weight of the PET, thereby achieving a method of increasing the intrinsic viscosity of the PET. The chain-extended PET has a low melt flow rate, high extrusion expansion ratio, high viscosity, high shear sensitivity, remarkable non-Newtonian behavior, and high storage modulus, and its performance is more conducive to foaming.
The extrusion foaming method is to add the plastic raw material containing the foaming agent into the extruder. After the screw is rotated and the barrel is heated, the material is sheared, melted, and mixed. When the molten material is extruded from the die of the machine head, from high pressure to normal pressure, the gas dissolved in the material is expanded to complete foaming. The most prominent feature of this molding method is continuous production. Different types of products can be produced by changing the die head.
Nanjing Chuangbo Machinery Equipment Co., Ltd. cooperated with Beijing University of Chemical Technology to develop a PET drying-free reaction extruding supercritical carbon dioxide foam molding line, which mainly includes a raw material dosing system, a twin-screw extruder, an ultra-vacuum system, a single screw extruder, and the sheet forming system to produce PET foam sheets with a width of 1.1m and thickness of 60mm. It can produce foamed boards with a density of 80kg, 100kg, 130kg, 150kg, 200kg, 300kg per cubic meter.
PET raw material is easy to absorb water, and the viscosity of the PET raw material after absorbing water is reduced during the melting process. After the viscosity is reduced, the cell diameter and cell density of the product will be greatly affected, thereby reducing the mechanical properties of the PET product. Currently, the technologies used include drying-free online and chain extension reaction online. These two technologies are different from foreign technologies. In foreign technology, the raw materials needed must be crystallized and dried in advance. This crystal drying needs a large area and a lot of investment. The most important thing is that it consumes a lot of energy. Drying-free technology can save a lot of space and energy. The foaming agent used is completely environmentally friendly carbon dioxide. The density of the foam reaches 80kg per cubic meter, and 60kg per cubic meter of products is currently being developed. The original intention of product development is to solve the problem of lightweight while ensuring the performance of the product.