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In recent years, the development of foam plastics has been rapid. In the United States, Japan, Europe and other countries and regions, both the output and the varieties are rapidly increasing. The foams that have achieved industrial production include polystyrene (PS), polyvinyl chloride (PVC), polyurethane (PUR), ABS, phenolic, urea formaldehyde, epoxy resin, and polycarbonate (PC). Polypropylene (PP) has good performance and low price. PP foam can be obtained through physical or chemical foaming. Since JSP company first developed PP foam in 1982, it has been worldwide JSP, BASF, GEFINEX. Companies such as the company's factories are producing PP foam, the annual sales have reached several hundred thousand tons.
1 Forming of Polypropylene Foam According to the source of foaming power in foam plastic foam molding, it can be generally divided into three types: mechanical foaming, physical foaming and chemical foaming. Mechanical foaming is the use of mechanical strong stirring, the gas evenly mixed into the resin to form bubbles. Physical foaming is based on the change of the physical state of the foaming agent in the resin to form a large number of bubbles. Chemical foaming relies on the gas generated when the blowing agent chemically changes to foam the resin. Most of the PP foams are chemically foamed, and the foaming agents used are azodicarbonamide and diisopropyl azodicarboxylate; physical foaming can also be performed using nitrogen as a physical foaming agent.
The foaming process of foam can generally be divided into two stages. Firstly, a large number of uniform and fine bubble nucleus is formed in the plastic melt or liquid, and then it is expanded into the required foam structure, and finally the foam is fixed and shaped. Fix it down to get the foam.
PP foam commonly used molding methods include extrusion, injection molding and compression molding.
Extrusion molding is one of the main methods for molding foams. Extrusion molding is used for foamed products such as general profile, plate, pipe, diaphragm, and cable insulation. Two physical foaming or chemical foaming methods can be used in the extrusion molding process. The physical foaming mainly uses some fluorocarbons and their mixtures, as well as other low-boiling liquids. In addition, the physical foaming method of directly injecting gas is also used in the extrusion molding of PP foams. The process parameters affecting extrusion foaming include: extrusion pressure, extrusion temperature, residence time of the material in the extruder, and axial pressure of the die. In general, as the extrusion pressure increases, the cell size decreases and the number of cells increases; high quality foams are only obtainable over a narrow temperature range. The higher the melt temperature, the lower the melt strength of the polymer, and the foaming pressure in the bubble may exceed the surface tension of the foam to rupture the foam. It must be optimized according to the specific polymer system to determine a suitable foaming temperature. Extending the residence time of the material in the extruder will increase the number of bubbles.
The injection foam molding method is a one-step molding method, which can simplify the manufacturing process of foam plastic products. Its characteristics are high yield and good quality, and is particularly suitable for products with relatively complex shapes and high dimensional accuracy requirements. The main factors affecting the quality of injection-molded foam plastics are material formulation, molding equipment and process conditions. After the material formulation and molding equipment have been determined, the main factor that determines the product performance is the process conditions. Pressure, temperature and time are the most important process conditions. Whether the dissolved gas in the polymer melt can be released into bubbles and the bubbles formed in the melt can be stabilized, these two points are closely related to the melt pressure, because the pressure of the melt directly affects the gas in the melt. Solubility. When the external pressure of the melt increases, the solubility of the gas in the melt increases, the amount of supersaturated gas in the melt decreases, and the radius of the bubbles decreases. The change in pressure is very sensitive to the effect of the growth or collapse of bubbles in the melt. It is very effective to regulate the foaming process by controlling the pressure. The physical state of the polymer is a function of temperature. Determining the appropriate temperature is very important for foam molding. The main process parameters related to temperature are barrel temperature and mold temperature. - In general, increasing the melt outlet temperature favors the growth of bubbles, but if the melt temperature is too high, not only will the polymer degrade, but it will also cause the melt viscosity to drop. In general, the melt temperature is high, the bubble diameter increases, and the number of bubbles decreases. The foaming ratio of the product generally decreases with the decrease of the mold temperature. Increasing the mold temperature can improve the flow conditions of the melt in the mold, and can also improve the surface quality of the product and increase the foaming ratio of the product. However, the time for cooling and setting will be prolonged, which is not conducive to improving labor productivity. The injection rate has a great influence on the foam cell uniformity, foaming ratio and surface quality. The polymer melt can be filled at high speed to obtain even-sized cells. Increasing the injection rate can significantly increase the foaming ratio of the product.
PP foam can also be molded by a molding process. The foam molding process can be divided into one-step and two-step processes. The one-step foaming process is completed at one time. After the materials are mixed by the ingredients, the formed billets are molded and heated and pressurized to obtain foamed plastic products. The basic procedure of the two-step method is similar to the one-step method. After a single foaming process, a part of the foaming agent decomposes to partially foam the material, followed by cooling, and the second foaming is carried out under normal pressure while still hot. Since the expansion rate of the material during the two-step foaming process is greatly reduced, the foaming factor can be increased. There are many specific molding methods, depending on the formation of raw materials and the conditions of the products. A typical method is the compression molding of PP foamed particles. The closed-cell foaming PP is first introduced into a mold, gas is injected, and then steam is blown into the helium gas to melt the particles to form a product.
The typical molding process of BASP, the company's Neopolen P Series PP foams consists of five steps:
(1) Mold Fill The porous foam PP particles are introduced into the mold cavity, and the porous particles are compressed by pressurization.
(2) Foam PP particles in the molding cavity are heated by steam and swell and melt. According to the bubble PPW, it is necessary to consult with the government. It is within the range of 25-0.42MPa.
(3) Cooling setting In order to ensure that the product does not swell or crack after demolding, it should be cooled to an appropriate temperature to open the mold. The demolding time depends on the density of the PP foam and the wall thickness of the product.
(4) The release product is demolded by mechanical means or compressed air.
(5) The post-treated product was left in an environment of 80° C. for more than 6 hours, and heat treatment was performed to remove the absorbed water.
2 Properties of Polypropylene Foam BASF's Neopolen P series foamed plastic particles are spherical, with non-crosslinked closed cells as the main structure. Neopolen P's products are light in weight, strong in absorbing impact loads, have a high recovery rate after deformation, have no resistance to deformation and recovery after deformation, have low water absorption, good corrosion resistance, and have excellent heat resistance and heat insulation.
Neopolen P series products are generally supplied in the form of particles with a density of 20-85 kg/m3 and do not contain harmful blowing agents such as freon. They can be used in food contact applications and can be reused to a certain extent. Clean recycling materials Can be reused for injection molding or extrusion.
JSP's PP foam also does not contain harmful blowing agents such as Freon. It is recyclable, light in weight, and capable of absorbing impact energy. It has good heat insulation, flame retardancy, dimensional stability, and corrosion resistance.
VESTOCELL series PP foams from GEFINEX Chemical Co., Ltd. also do not contain Freon foaming agent, and are easy to be molded. They have excellent impact properties and good shape recovery over a wide temperature range, low water absorption, heat insulation and aging resistance. Good performance, excellent wear resistance and corrosion resistance, materials can be recycled.
3 Application of Polypropylene Foam PP foam can be used as the bumper core, side guards, door inner panel energy-absorbing protection pads, cushions, head combs, sun visors, and tool boxes in the automotive industry. As a packaging material, it can be used for the packaging of fragile items and precision instruments that need to be protected from earthquakes, so as to protect them from damage and damage during bumpy transportation. As a safety protection material can be used as a variety of safety helmets. In addition, it can also be used as construction materials with good insulation and sound insulation effects in the construction industry.
According to incomplete statistics, the consumption of PP foam in the United States in 1999 was 35,000 tons, 17,000 tons in Europe, 10,000 tons in Japan, and 0.7 million tons in other Asian countries. JSP company has built a number of factories in the world to produce PP foam products. Its PP foam products have a large market share in the world, occupying 70% of the market in the United States, and occupying 40% of the market in Europe. Japan occupies 60% of the market and holds 50% of the market in other Asian countries. In the United States, the JSP company's PP foams are mainly used to make automotive parts. In 1996, 63 types (including 29 universal, 13 Chrysler, and 8 Ford) PP foam automotive parts entered the auto parts market. In Europe, 60% of JSP company's PP foam is used in the manufacture of automotive parts, and the remaining 40% is mainly used in the packaging industry. In 1996, there were 25 types of auto parts made of PP foam, of which 3 were Mercedes, 5 were Volkswagen, and 6 were Opel. In Japan, 57 kinds of auto parts manufactured by JSP company's PP foam are used, including 17 kinds of Nissan, 15 types of Toyota, and 16 types of Honda. In other Asian countries, 70% of JSP company's PP foam is used in the manufacture of automotive parts, and about 30% is used for packaging products.
With plastic as the basic ingredient, a large amount of bubbles can be filled by physical or chemical methods to obtain a foam. Compared with pure plastic, it has a series of features such as small density, high specific strength, strong energy absorption capability, good sound insulation and heat insulation performance, etc. It has been widely used in transportation, military industry, aerospace, daily necessities and other fields, such as production. Various seat cushions, cushions, mattresses, comb cores, thermal insulation materials, packaging materials, collision and shockproof materials, decorative materials and building materials. As people deepen their understanding of foam plastics, new varieties will continue to be developed and their scope of application will be further expanded.