The application of rotational molding technology in cross-linked polyethylene
in recent years, China's rotational molding industry has developed rapidly, and there are dozens of various rotational molding manufacturers all over the country. These manufacturers produce more and more types of rotational molding products, and their application fields are more and more extensive. However, compared with the advanced level of foreign developed countries, the gap is very obvious. In the early 1990s, more than 3000 kinds of rotational plastic products were registered with the American rotational plastic Association, while there are only 40 kinds in China at present. Foreign rotational plastic products have involved more than 30 industries, while China's rotational plastic products only involve more than 10 industries. There are many reasons for this situation, one of which is the single variety of rotational molding raw materials used by various manufacturers. At present, more than 99% of the raw materials of rotational molding products in China are PE, 95% of which are rotational molding grade LLDPE, which is very suitable for rotational molding. However, it is inferior to cross-linked polyethylene in terms of heat creep resistance, low-temperature impact resistance, environmental stress cracking resistance, and can not be wrongly identified and swelling resistance. At present, the proportion of various rotational polyethylene materials abroad is roughly: LLDPE is 65%, xhdpe (cross-linked polyethylene) is 2%, HDPE is 10%, LDPE is 5%. In China, there is no cross-linked polyethylene material and molding process for rotational molding. Accelerating the development of cross-linked polyethylene rotational molding technology is an urgent task for the rotational molding industry in China
1 development overview at home and abroad
cross linked polyethylene has a shape structure and is thermosetting. It has outstanding wear resistance, stress cracking resistance and weather resistance, good low temperature toughness, excellent heat resistance, softening point up to 200 ℃, and can be used for a long time at 140 ℃. The cross-linked polyethylene (xhdpe) used for rotational molding can be cross-linked during the molding process to form a shaped structure. The products have excellent impact strength, heat creep resistance, low temperature toughness, good impermeability, chemical resistance, etc. the cross-linked polyethylene rotational molding oil tank is the only rotational molding oil tank that can hold gasoline, and the only rotational molding polyethylene product that can hold gasoline. Some people have done the following tests: use a cross-linked polyethylene rotational plastic container with a capacity of 8L, fill it with 75% of its volume of gasoline, store it at 55 ℃, the internal pressure is as high as 50kPa, and it will not be damaged after more than 3 years of storage. However, using an uncrosslinked polyethylene container to do a comparative test under the same conditions, damage occurred in only 4 days. The cross-linked polyethylene roll molded products also have extremely excellent low-temperature impact resistance. The cross-linked polyethylene roll molded products with a volume of 14L are loaded with antifreeze rolls, frozen at -25 ℃ for 2h, and then fall from a height of 6.1m, leaving the products intact. In addition, cross-linked polyethylene products also have outstanding melt strength and special "memory effect". Because of its excellent melt strength, it will not stick when heated, and the memory effect makes it remember the shape obtained during rotational molding when undergoing "re molding". For example, after heating and softening, flattening, cooling and shaping, and then heating again to reach the transparent temperature, the product will recover its shape without any external force. This performance of cross-linked rotational polyethylene products is of great use value: use this product to transport liquid to a certain place, flatten the container on the return trip, save space, and then heat it after coming back, Use the "memory effect" to restore them. The above advantages of cross-linked rotational polyethylene products establish a good performance of macro-control and market conditioning mechanism, so that they can be used in many fields that ordinary rotational polyethylene products cannot be used and are not applicable, such as fuel tanks, containers for chemicals, garbage cans, industrial battery shells, etc
in foreign countries with developed roll forming industry, many cross-linked polyethylene raw materials for roll forming have been successfully used in production in the 1970s. It belongs to the third generation of roll forming raw materials. Although LLDPE and MPE account for a large proportion of roll forming raw materials in recent years, when products are required to have certain pressure resistance and heat creep resistance in some occasions, the most suitable is cross-linked roll forming PE raw materials. There are many famous brands of cross-linked rotational polyethylene in the market, such as Marlex CL-100 and CL-50 of Philips; Du Pont's sclairlink 800, etc. these raw materials are widely used in rotational plastic fuel tanks, chemical containers, temperature resistant storage tanks, small yachts, etc. in military industry, many hollow parts of some military facilities of military powers such as the United States, Germany, France, Italy, etc. are made of cross-linked rotational plastic polyethylene raw materials. Many manufacturers have also developed the forming process of cross-linked rotational molding polyethylene, such as the cross-linking forming and cooling process using McNeil rotational molding equipment (i.e. bidirectional rotational molding equipment with heating box), and the cross-linking forming process using swing rotational rotational molding equipment. These different molding processes can solve the problems of poor plasticization, premature crosslinking and warpage of cross-linked rotational polyethylene raw materials in the molding process
at present, most domestic rotational molding grade raw materials are LLDPE, while cross-linked polyethylene raw materials are blank in China. Domestic rotational molding LLDPE is produced by imported foreign equipment and technology. Practice has proved that domestic rotational molding LLDPE is a good rotational molding material. However, due to some problems in the polymerization process, its ESCR performance is generally poor after testing. Moreover, the heat creep resistance of rotational plastic LLDPE is also poor. LLDPE is mainly suitable for industrial and civil products at room temperature because the domestic rotational grade LLDPE raw materials have been classified into the medium density category in terms of density and its low temperature resistance is also limited. Many roto plastic manufacturers still have a great demand for cross-linked roto plastic polyethylene raw materials. On the one hand, they are based on the development demand of the motor fuel tank Market, on the other hand, they are based on the demand of temperature resistant containers, and some manufacturers are based on the demand of temperature resistant lining raw materials. Some manufacturers have developed special materials for cross-linked rotational polyethylene by themselves. At present, due to their weak technical strength and lack of scientific research means, they have not been developed successfully. If high-density raw materials are used to form temperature resistant containers, although the humidity resistance can be improved, the impact strength and environmental stress cracking resistance of products at low temperature will decrease. The best way to solve these problems is to crosslink HDPE. The rotational molding equipment of many domestic manufacturers can be roughly divided into two categories: one is the advanced equipment imported from abroad, including McNeil type and computer-controlled temperature swing rotational molding equipment; The other is self-made swing equipment, which has a slightly poor temperature control means. Both of these two types of equipment have the ability of measuring and controlling shaped cross-linked polyethylene with good long-term stability. For the former, it is very convenient to shape cross-linked rotational plastic products, and for the latter, it is also feasible to shape cross-linked rotational plastic products after appropriate transformation. With the development of automobile and chemical industry in China, there is an increasing demand for rotational molded fuel tanks, temperature resistant containers and temperature resistant linings for motor vehicles. It is of great significance to step up the development of cross-linked rotational molded polyethylene technology
2 introduction to cross-linked rotational molding polyethylene technology
cross-linked rotational molding is a kind of peroxide liquid-phase cross-linking, which is achieved through the C-C bond formed by alkyl. The peroxide (roor) forms an active free radical after heating. The cross-linking reaction formula is as follows:
roor → 2ro ·
ro · +r "H → roh+r"
R "· +r" → R "-r"
the cross-linking reaction occurs above the melting point of the material. The generation of cross-linked gel limits the movement of the molecular chain, The molecular chains cannot be fully, rapidly and regularly arranged to form crystals, and the perfection of the generated crystals is also poor, which has a great impact on the crystallization behavior of the material, followed by a decrease in the density of the material. The resin used for cross-linking rotational molding is generally high-density polyethylene, and its density can be reduced to 0 after cross-linking Between 940g/cm3, the product has ideal performance. The selection of crosslinking agent should take into account the crosslinking efficiency, decomposition temperature and the influence of decomposition products on the final products. In terms of molding, the rotational molding process has a long thermal process. The whole heating process can be divided into two stages: "heat conduction period" and "melting period". The heat conduction period is the time required for the mold and resin to be heated to the melting point of the resin, and the melting period is the time required for the powder to melt on the inner surface of the mold to form products. In this process, the powder is continuously adhered to the mold wall until all the resin is adhered to the mold wall. How to control the cross-linking reaction after the heat conduction period and the melting period, so as not to affect the performance of products in advance, is the key to the successful roll molding of cross-linked polyethylene. For McNeil type equipment, it is convenient to carry out cross-linking and rotational molding, and the temperature control of the heating box is relatively accurate. The temperature of the heating box can be controlled to control the temperature in the mold by controlling the temperature of the heating box. The temperature of the heating box is generally set between ℃, which is lower than 290 ℃, the heating time is too long, the production efficiency is low, the cross-linking reaction is insufficient, which is higher than 330 ℃, which is prone to early cross-linking, and the product surface is prone to chickenpox like defects, Affect product performance. For rocking and rotational molding equipment, because the heating method of this equipment is mostly open fire heating, the heating efficiency is low, and the temperature in the mold often cannot reach the temperature required for the crosslinking reaction. Foreign manufacturers have developed a process of filling the mold with 350 ℃ hot air after molding, so that the internal surface of the product is fully and evenly crosslinked. Another remarkable phenomenon of cross-linked rotational molding is the warpage of products. When forming curved structure products by ordinary rotational molding process, warpage will never occur due to the characteristics of curved structure. Experiments have proved that for cross-linked rotational molding products, curved structure products also have serious warpage, which will be more serious if it is a box structure. Many foreign rotational molding equipment can overcome this problem by installing a "mpg" device to cool the products by moving the air in the mold, so as to achieve uniform cooling inside and outside, and maintain a certain pressure. The grinding of crosslinkable materials is also very critical. The grinding of thermoplastic resin at room temperature is relatively difficult. If the material is rigid, it is easy to heat and bond in the grinding process. For cross-linked rotational plastic polyethylene materials, there is also the risk of early cross-linking in the grinding process. Only with appropriate grinding process and cheap cooling method can we successfully grind qualified rotational plastic cross-linked polyethylene powder. The necessary crosslinking degree is an essential condition for cross-linked rotational polyethylene products to obtain good performance. In the production process, the crosslinking degree of products must be controlled. The main method to test the crosslinking degree of cross-linked rotational polyethylene products is Naphthalene Extraction Method (ASTM D 2765), and another simple and fast method is to cut 100% from the products × For a 13mm long strip, bend it outward to make the outer surfaces of both ends contact. At this time, observe its inner surface. If there is no cracking, it indicates that the crosslinking is relatively sufficient. Otherwise, it is necessary to adjust the forming process
the application of cross-linked rotational molding polyethylene is still blank in China, and the market potential is huge. With the increase of polyethylene raw material brands in China, the development and development of high-performance products is the development direction of material application in the future. As a basic product development technology, rotational molding, with relatively low investment, started late in China, and the development potential needs to be further developed. The technical development of raw materials and molding process has a long way to go, More extensive plastic science and technology development
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