Views: 69 Author: Site Editor Publish Time: 2022-04-15 Origin: Site
Foamed plastics are aggregates formed by dispersing gases in solid polymers. They have the advantages of light weight, heat insulation, buffering, insulation, anti-corrosion, and low price. Therefore, they are widely used in daily necessities, packaging, industry, agriculture, transportation, military industry, and aerospace industry. For example, the demand for foam plastics in the US market has continued to grow at an annual rate of 6.2% in recent years. Since the 1990s, the development of foamed plastics in China has been very rapid, and the main varieties are polyurethane (PU) soft and rigid foamed plastics, polystyrene (PS) foamed plastics and polyethylene (PE) foamed plastics. However, these three types of foam materials have certain limitations:
The use temperature of polystyrene (PS) materials is relatively low, and a slightly higher temperature will cause pollution by volatilization of residual monomers. Foaming agents such as chlorofluorocarbons or butane are usually used in the PS foaming process, which is harmful to the environment and safety. Product degradation is difficult and it is easy to form "white pollution", so developed countries and China have successively formulated relevant laws to stop the use of PS foam products in the food field. According to the "National Act on the Phase-out of Ozone-Depleting Substances in China", China has pledged to completely stop the use of HCFCs in foam production before December 31, 2026, and replace them with more environmentally friendly foaming agents.
Polyurethane foam is a thermosetting system, which is usually not recyclable after use, and there are isocyanate residues in the foam that are harmful to the human body.
Polyethylene foam plastic has low modulus and poor heat resistance. XPE prepared by cross-linking process can improve heat resistance to a certain extent, but the maximum use temperature is also lower than 90 °C, and there are problems such as difficulty in recycling.
Polypropylene resin has the characteristics of light weight, abundant raw material sources, superior cost performance, excellent heat resistance, chemical resistance, and easy recycling. It is the fastest growing general- purpose thermoplastic resin in the world. The research and development of the foam has always been a technical hotspot and difficulty in the industry. At present, some developed countries are vigorously developing it as the next- generation green foam material to replace the above three kinds of foams. Among them, polypropylene expanded beads (EPP), foamed sheets and plates are the most typical and the most used varieties.
Polypropylene foam has many advantages:
The rigidity of polypropylene (PP) is significantly better than that of polyethylene (PE). The flexural modulus of PP is about 1.52Gpa, and that of PE is only 207Mpa. Its compressive strength is higher than soft PUR and PE.
The glass transition temperature of polypropylene is lower than room temperature, the impact resistance is better than that of PS, and compared with the recyclability of PS foam, polypropylene foam is an environmentally friendly material.
Polypropylene has a high heat distortion temperature and can be used in some high temperature fields. Foamed polypropylene can usually withstand high temperatures of 120 ° C, which is much higher than polyethylene foam, and is widely used in many fields such as food and automobiles.
Good low temperature properties: The product exhibits good performance even at -30°C.
Energy absorption: PP foam products have good energy absorption properties, excellent compression resistance and energy absorption performance, and are widely used in automobile bumper energy treatment systems and other anti-collision energy-absorbing components, and also have good sound absorption and shock absorption.
Dimensional shape recovery stability: PP foamed products will quickly return to their original shape without permanent deformation after being subjected to multiple consecutive impacts and flexural deformations.
Light weight and can be used repeatedly: The density of PP foam products can reach a very low level, so the weight can be greatly reduced. At the same time, PP foam products have good flexibility and can be used repeatedly. They are easy to recycle and reuse, easy to decompose. The product does not contain toxic and harmful ingredients to the human body. Burning does not produce toxic substances.
Good surface protection and sound insulation performance: PP foam products are semi-rigid forming, with suitable hardness and softness, and will not scratch or bruise objects in contact with them, and have good surface protection.
At present, there are more than 70 PP resin manufacturers in China, with more than 100 sets of various devices, and the production capacity exceeds 5 million tons/year. It is predicted that by the end of 2020, the total production capacity of PP resin China will exceed 6.5 million tons/year. However, the application of PP resin in China is mainly woven products and films. The PP resin industry faces the problems of a single consumption structure, low demand and low product quality. All resin manufacturers are actively adjusting their production ideas, developing and producing marketable, high-quality, high-value-added special materials, so that PP has greater application value and economic benefits.
It can be seen that polypropylene foam products have good performance and application prospects, but the technical difficulty of continuous extrusion of polypropylene foam materials is very difficult. The main reasons are: polypropylene is a crystalline polymer, and it hardly flows below the crystalline melting point, but above the crystalline melting point, the melt viscosity decreases sharply, and the melt strength is very low. In addition, polypropylene changes from the molten state to the crystalline state, a lot of heat will be released, and it takes a long time to change from melt to solid. Also, the diffusion coefficient of polypropylene melt is high, and the foaming gas is easy to escape. At the same time, the molding temperature range suitable for polypropylene foaming is very narrow, and the foaming process is difficult to control. Therefore, in order to obtain a product with uniform and fine cells and a high foaming ratio, it is usually necessary to cross-link the PP resin before foaming, so as to slow down the reduction of its melt viscosity, and to have suitable properties of foaming viscosity and melt strength in a wide temperature range. However, the recyclability of the material will be reduced after polypropylene cross-linking. Therefore, from the perspective of the trend of technological development, it is the next development direction to obtain physical foamed non-cross-linked polypropylene foam products through the modification of commercial grades.