Restricted free foaming extrusion process

The use of simple methods such as blending, filling, and compounding to improve the performance of general-purpose, large-scale plastics, improving their grades, developing new products, and increasing added value are of great significance. The author of this article, Professor Wu Qichen, is a doctoral tutor of the School of Polymer Science and Engineering of Qingdao University of Science and Technology, and has conducted research in this field for many years. He will publish plastic extrusion in Shanghai from May 11 to May 12 this year. The application of the construction industry in China presented his results. Due to space limitations, we can only extract Prof. Wu's speech for publication.

The high performance of GM large-scale plastics is a major strategic task in the field of plastics engineering in China and has achieved remarkable results in recent years. The five major varieties of general-purpose plastics have large output, low price, light weight, high strength, oil resistance, corrosion resistance, chemical resistance, and excellent electrical properties. They occupy an unquestionable important position in materials engineering. The use of simple and easy methods of blending, filling, alloying, compounding, etc. to improve its performance, increase its grade, increase new varieties, expand its use, and increase added value, to enhance the competitiveness of enterprises, expand economic and social benefits, and further enhance China's plastics The level of industrial strength plays an important role.

The restricted free foam extrusion molding process project is the National Science and Technology Commission's "Ninth Five-Year Plan" scientific and technological research project. The main achievements are: Research and development of successful absorption-extraction heat balance, stable and rapid release, self-nucleating compound chemical foaming agent--HCB series compound chemical foaming agent and composite foaming agent masterbatch. Successfully researched and developed a new type of low-expansion extrusion molding process for thermoplastics-restricted free-foam extrusion process, and applied for a patent. Research and design of foamed pipe and profile extrusion heads suitable for constrained free foaming extrusion molding processes. The use of new materials, new equipment, new processes, and new molds to produce low-foaming PVC building tubing with soft and hard outer surfaces, fine and dense cell structure at the core, and controlled apparent density, and low foaming PVC and PS decoration Profiles, performance and quality meet or exceed the requirements of the assessment indicators.

This achievement has been promoted by several domestic manufacturers and has achieved good economic and social benefits. In 2000, he won the second prize for technological invention of the China Petroleum and Chemical Industry Association, and won the second prize for scientific and technological progress in Qingdao in 1999. At the same time, it was supported by the National Natural Science Foundation and published more than 20 papers.

The main features of this project are: It is different from other domestic low-foam extrusion processes (such as free foaming, inward foaming, multi-layer co-extrusion foaming, etc.) and uses self-developed composite chemistry Foams, new processes, and new molds can be extruded directly from a single layer of material, a single extruder (single-screw extruder or twin-screw extruder), and the core layer can be directly foamed. PVC construction pipe and dozens of different cross-sectional shapes, realistic wood grain, used for interior decoration and decoration of PVC low-foaming profiles, plates, their performance meet and exceed the relevant national standards and corporate standards, for our low foam building materials Production has opened up new paths, which have enriched China's chemical building materials market.

The scientific significance of the results lies in the deep understanding of a series of basic chemical-physical issues in the extrusion process of gas-containing polymers and gas-bearing melts through in-depth study of the foaming kinetics and thermodynamic laws of various foaming systems. Not only has it developed a new type of HCB compound chemical foaming agent with superior performance to foreign products of the same kind, but it will also play a guiding role in the research of different foaming products and foaming processes in other fields. At present, we are conducting research on micro-foamed materials, a new material for the 21st century.

High quality and efficient foaming agents are the key to the development of rigid low foam extruded products. Conventional exothermic foaming agents (such as AC) have the drawbacks of rapid gas evolution, large heat release, and difficult to control. Endothermic foaming agents (such as NaHCO3) have a slower gas evolution rate and a smaller gas evolution. We have successfully developed a new type of suction-exothermic equilibrium, stable and rapid release, self-nucleating compound chemical foaming agent-HCB series compound chemical foaming agent with a short decomposition induction period, rapid initial decomposition rate, and stable and controllable decomposition process. The large amount of gas is emitted, and the heat exchange with the surrounding melt during the decomposition is basically balanced, so that the extrusion foaming process is stable and easy to control, and the cell structure is uniform and fine. The foaming agent is most suitable for the limited free foaming extrusion molding process pioneered by the research group, and can also be used for other various foaming processes (used as foaming agents or nucleating agents).

There are two main traditional processes of low foaming extrusion of thermoplastics: free foaming process, inward foaming process (also known as Celuka process, or skinning foaming process). Combining the characteristics of the two basic foaming processes, we have succeeded in developing a new type of thermoplastic low-expansion extrusion molding process - a constrained free foam extrusion process.

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Restricted Free-foaming Extrusion Process Schematic ( a extrusion die; b die)

Normal align=left> The principle of the restricted free foaming extrusion molding process is that, first, the gas-containing melt melts freely after being extruded from the die (the same as the free foaming process), and the expanded melt is quickly introduced into the size. Equal to or slightly larger than the die of the die (same as the inward foaming process), forced cooling to set. This process not only allows the free foaming of the melt, but also limits the free expansion of the cells without restriction. Limiting the free foaming results in an increase in the thickness of the non-foamed skin layer of the product, a decrease in the cell size of the core, and a uniform and fine cell structure, which evolved from low foaming to micro-foaming. The surface quality of the product was excellent, and the mechanical properties were improved. The surface finish and texture of the foamed article can be adjusted, and the density and wall thickness can be adjusted by adjusting the ratio of the pulling speed to the extrusion speed. The mold and the fixed mold structure are simple and compact, and the manufacturing cost is low. It fully embodies and combines the advantages of the free foaming and inward foaming processes, and has broad application fields and development prospects.

Normal align=left> This new process has been successfully applied to PVC low-foam constructions that use a single extruder (single-screw or twin-screw extruder) to directly extrude surface skins and micro-foam cores. Pipes (low-pressure pipe for drainage), and dozens of different cross-sectional shapes, realistic wood grain, used for interior decoration PVC, PS low-foaming profiles, plates. Its performance meets and exceeds the relevant national standards and corporate standards. This process has applied for national invention patents.

Normal align=left> Using new materials, new processes, and new molds, we produce low-foaming PVC building tubing with low surface smoothness and hardness, fine and dense cell structure in the core, and controlled density, low foaming PVC, and PS decoration. Decoration profiles, performance and quality meet or exceed the requirements of assessment indicators, ranking the international advanced level. Tables 1 and 2 show the test performance specifications of the low-foaming PVC construction pipe and the low-foaming PVC decoration profile produced by us.