Method for preparing low density polypropylene foam section or beads by two-step extrusion molding

Abstract

The invention discloses a method for a preparing low density polypropylene foam section or beads by two-step extrusion molding, which comprises the following steps that: expandable master batch is prepared from 30 to 80 portions of polypropylene, 10 to 40 portions of foaming agent, 0 to 15 portions of foaming accelerant, 0 to15 portions of dispersant and 0 to 10 portions of nucleating agent at a temperature below the decomposition temperature of the foaming agent and above a viscous flow temperature of the polypropylene; 20 to 99 portions of polypropylene with high melt strength, 0 to 70 portions of common polypropylene and 1 to 10 portions of expandable master batch are plasticizated and extruded together; supercritical gas is added to perform the blending of the plasticizated and extruded mixture; and the product obtained is made to generate foamcores in a foaming nucleation apparatus to form the section or beads finally. A chemical foaming agent is partially decomposed and foamedduring the plasticization and extrusion, so that the viscosity of fused mass is reduced, the dissolution and dispersion of the supercritical gas are facilitated, and a problem that a foam imploding islikely to happen in a secondary forming process so as to influence the quality and appearance of secondarily formed product is solved as the chemical foaming agent retained in molding beads can be decomposed and foamed again during the secondary processing.

Description

technical field [0001] The invention belongs to the technical field of preparation of low-density polypropylene foam profiles and beads, and in particular relates to a method for preparing low-density polypropylene foam profiles and beads by two-step extrusion molding in a physical and chemical foaming manner. Background technique [0002] Polypropylene (PP) material has excellent characteristics such as good mechanical properties, non-toxicity, low relative density, heat resistance, chemical resistance, easy processing and molding, and high performance-price ratio. It has become the fastest growing among the five general-purpose synthetic resins. The most active variety in new product development. Expanded polypropylene (EPP), as a new type of material that has only appeared in the last ten years, has the advantages of light weight, heat insulation, and good elasticity of foam materials. , has more incomparable advantages, such as excellent heat resistance (maximum service...

AI Technical Summary

Problems solved by technology

Although nitrogen and carbon dioxide blowing agents are safe and environmentally friendly gas blowing agents, they are widely used in production, but only nitrogen or carbon dioxide is used as blowing agent to prepare low-density EPP foam profiles and beads. The dissolution process in the polymer is diffusion dissolution, so the speed is slow and time-consuming. Although the diffusion speed can be accelerated by technical measures of increasing temperature and pressure, the process of gas dissolution and uniform dispersion in the high melt strength PP melt It is difficult to control, and has high requirements for the control of mixing equipment structure and process parameters; ②The production of low-density EPP has high requirements for the content control of gas blowing agent. When the amount of supercritical gas blowing agent exceeds 20% by mass fraction, the The metering, sealing, mixing, and pressure control requirements of the supercritical gas injection system are very high, and the uniformity of the cells is difficult to control; Foam collapse is easy to occur during secondary molding, which affects the quality and appearance of secondary molding products

However, this technology, as a one-step foaming method, still fails to solve the problem that low-density EPP beads are prone to foam collapse during secondary molding due to a small amount of gas foaming agent escaping during storage, which affects the quality of secondary molded products. Quality and Appearance Issues

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