Foam plastics have the advantages of light weight, heat insulation, cushioning, insulation, corrosion resistance, and low price. Therefore, they are widely used in daily necessities, packaging, industry, agriculture, transportation, military industry, and aerospace industry. Since the 1990s, China has developed foam plastics very rapidly. The main varieties are polyurethane (PU) soft and hard foam plastics, polystyrene (PS) foam plastics and polyethylene (PE) foam plastics.

Product Hazards

Polyurethane foam contains isocyanate residues that are harmful to the human body during the foaming process, and the foaming material cannot be recycled. However, chlorofluorocarbons or butane are usually used in the foaming process of polystyrene (PS), which has an adverse effect on the environment. The product is difficult to degrade and easily forms "white pollution". The United Nations Environmental Protection Organization has decided to stop using PS foam products. Cross-linked polyethylene foam plastics have low rigidity and the maximum operating temperature is 80буC.

Material Advantages

Compared with the above three foam plastics, polypropylene foam material has many advantages:

1) Polypropylene (PP) is more rigid than polyethylene (PE), the flexural modulus of PP is about 1.52Gpa, while PE is only 207Mpa, and its chemical resistance is similar to PE.

2) The glass transition temperature of polypropylene is lower than room temperature, and its impact resistance is better than PS. Moreover, compared with the difficulty of recycling PS foam, polypropylene foam is an environmentally friendly material.

3) Polypropylene has a higher heat deformation temperature and can be used in some high-temperature fields. Foamed polypropylene can usually withstand high temperatures of 130буC, which is much higher than the maximum operating temperature of polyethylene foam of 80буC. The closed cell structure makes its thermal conductivity unaffected by moisture, so it can be used as a thermal insulation material.

4) Good low-temperature properties: the product shows good performance even at -30буC.

5) Energy absorption: Since PP foam products have good energy absorption characteristics, they have excellent compressive energy absorption performance. Widely used in automobile bumper energy processing systems and other anti-collision energy-absorbing components.

6) Dimensional shape recovery stability: PP foam products will quickly recover their original shape after multiple continuous impacts and flexural deformations without permanent deformation.

7) Lightweight and reusable: 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, can be used repeatedly, and are not easy to break; they are easy to recycle and reuse, easy to decompose, and the products do not contain toxic and harmful ingredients to the human body, and combustion does not produce toxic substances.

8) Good surface protection and sound insulation performance: PP foam products are semi-rigid forming, with moderate hardness and softness, will not scratch or bump objects in contact with them, and have good surface protection.

Polypropylene resin has the characteristics of light weight, rich raw material sources, superior performance-price ratio, excellent heat resistance, chemical corrosion resistance, and easy recycling. It is the general thermoplastic resin with the fastest production growth in the world. At present, China's PP resin has become the largest resin variety. However, the main applications are woven products and agricultural films. The PP resin industry faces the problems of a single consumption structure, weak demand and low product quality. All resin manufacturers are actively adjusting their production ideas and developing and producing marketable, high-quality and high-value-added special materials to make PP have greater application value and economic benefits. Among them, the research and development of polypropylene foam has also become a hot topic in recent years. Although polypropylene foam products have good performance and application prospects, the development of polypropylene foam is very difficult. At present, China does not have its relevant and industrializable core technology. At present, some developed countries are vigorously developing and using it as a green packaging material to replace expanded polystyrene.

At present, EPP foam particles are mostly achieved through intermittent foaming. Due to the complex production process and high cost, the selling price is also high. The price of EPP parts in the Chinese market is about 120,000 yuan/ton, while the price of EPP parts in the European market is about 20,000 euros/ton. The molding process of EPP parts is similar to EPS (the process is mature). Despite such a high price, the market for EPP is gradually expanding. With the continuous expansion of the demand for EPP in China, JSP plans to build a new EPP production base in Dongguan, Guangdong Province.

Preparation method

Raw materials

Polypropylene PP resin, foaming agent, antioxidant, diluent (such as acetone), cross-linking agent, auxiliary cross-linking agent

Equipment

Extruder, forming die, cooling and shaping device, traction device, cutting device, winding device

Steps

Method 1

1. Before mixing, put polypropylene resin, foaming agent, antioxidant, cross-linking agent, auxiliary cross-linking agent in diluent (such as acetone), stir until slurry, dry, remove acetone, and obtain a uniform dispersion. 2. Put the dispersion into a two-roll plastic mill and mix it with the elastomer. 3. Before the compression molding and foaming process, put the sheet into the mold frame of the press, preheat for 4 minutes, and then heat and pressurize (about 4Mpa) to preform the sheet. 4. Place the preformed sheet into the plunger mold (preheated to 190буC), and immediately place the preheated plunger mold into the press heated to 220-230буC. 5. Apply pressure to the mold to ensure that the bottom plate and the mold frame are closed. When heated until the foaming agent decomposes, the mixture foams in the plunger mold and pushes the plunger upward. Maintain this temperature and pressure for 2 minutes to complete the decomposition reaction of the foaming agent and the cross-linking reaction of the peroxide.

Method 2

Seal the preformed sheet between two chrome-plated steel plates and directly place it in the press (heated to 216буC) to foam it. Heat and pressurize for about a few minutes to decompose the foaming agent and complete the partial cross-linking of the resin. Then quickly open the press to release the pressure. The foamed sheet pops out of the mold and completes the three-way foaming at the same time.

Key

Key to polypropylene foaming:

1. The polymer/foaming agent must form a homogeneous solution, and the extruder head pressure must be high enough to inhibit the foaming system from foaming prematurely near the extrusion die;

2. The foaming system must undergo rapid nucleation and growth through the molding head. During this process, it is necessary to induce a greater supersaturation pressure and a faster supersaturation pressure drop to form as many bubble nuclei as possible at the same time;

3. During growth, the stability of the growing bubbles and the control of the foaming ratio of the final product are key to the appropriate head temperature;

4. During the shaping and curing process, the cooling rate greatly affects the density and performance of the final product, so a reasonable cooling rate is required.

5. If a physical foaming agent is selected, the injection and metering of the physical foaming agent and the process parameters (temperature and pressure, etc.) will have a significant impact on the bubble structure and density, mechanical properties and thermal properties of the final foamed polypropylene material.

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