The English abbreviation is MDPE (Medium density polyethylene)
Medium density polyethylene (MDPE) is made by copolymerizing alpha olefins during the synthesis process to control the density.
R&D Experience
In 1970, the United States produced it by slurry method, and its production and synthesis process adopted the LLDPE method. Alpha olefins commonly used are propylene, 1-butene, 1-hexene, l-octene, etc., and the amount of olefins used affects the density. Generally, the amount of olefins used is about 5% (mass fraction).
Molecular structure
On average, 20 methyl branches or 13 ethyl branches are introduced into every 1000 carbon atoms in the main chain of MDPE molecules. Its performance varies depending on the number and length of branches. Copolymerization can increase the number of connecting chains between small crystals.
Performance characteristics and parameters
MDPE is characterized by environmental stress cracking resistance and long-term strength retention. MDPE has a relative density of 0.926-0.953, a crystallinity of 70%-80%, an average molecular weight of 200,000, a tensile strength of 8-24 MPa, an elongation at break of 50%-60%, a melting temperature of 126-135°C, a melt flow rate of 0.1-35 g/10 minutes, and a heat deformation temperature (0.46 MPa) of 49-74°C.
Uses
MDPE can be processed by extrusion, injection, blow molding, rotational molding, rotation, and powder molding. The production process parameters are similar to those of HDPE and LDPE. It is often used for pipes, films, hollow containers, etc. The product is used for high-speed molding of various bottles, high-speed automatic packaging films, various injection molding products, rotational molding products, wire and cable coatings, waterproof materials, water pipes, gas pipes, etc.
Processing methods
It is divided into three types: high-pressure method, low-pressure method and medium-pressure method. The high-pressure method is used to produce low-density polyethylene. This method was developed early, and the polyethylene produced by this method accounts for about 2/3 of the total polyethylene production. However, with the development of production technology and catalysts, its growth rate has lagged far behind the low-pressure method. In terms of its implementation method, the low-pressure method includes slurry method, solution method and gas phase method. The slurry method is mainly used to produce high-density polyethylene, while the solution method and gas phase method can not only produce high-density polyethylene, but also produce medium and low-density polyethylene by adding comonomers, also known as linear low-density polyethylene. In recent years, various low-pressure processes have developed rapidly. The medium-pressure method is still only used by Philips, and it mainly produces high-density polyethylene.
The high-pressure method uses oxygen or peroxide as an initiator to polymerize ethylene into low-density polyethylene. After two-stage compression, ethylene enters the reactor (Figure 3) and polymerizes into polyethylene under the action of an initiator at a pressure of 100-300 MPa and a temperature of 200-300°C. The reactants are separated by decompression, and the unreacted ethylene is recovered and recycled. The molten polyethylene is extruded and granulated after adding plastic additives. (See color picture)
The polymerization reactors used are tubular reactors (tube length can reach 2000m) and kettle reactors. The single-pass conversion rate of the tubular process is 20% to 34%, and the annual production capacity of a single line is 100kt. The single-pass conversion rate of the kettle process is 20% to 25%, and the annual production capacity of a single line is 180kt.
The low-pressure method is divided into three types: slurry method, solution method and gas phase method. Except for the solution method, the polymerization pressure is below 2MPa. The general steps include catalyst preparation, ethylene polymerization, polymer separation and granulation.
- The polyethylene produced by the slurry process is insoluble in the solvent and is in the form of a slurry. The polymerization conditions of the slurry process are mild and easy to operate. Alkyl aluminum is often used as an activator and hydrogen is used as a molecular weight regulator. A kettle reactor is often used. The polymer slurry coming out of the polymerization kettle passes through a flash kettle and a gas-liquid separator to a powder dryer, and then to granulation (Figure 4). The production process also includes steps such as solvent recovery and solvent refining. By using different combinations of polymerization kettles in series or in parallel, products with different molecular weight distributions can be obtained.
- Solution polymerization is carried out in a solvent, but ethylene and polyethylene are both dissolved in the solvent, and the reaction system is a homogeneous solution. The reaction temperature (≥140°C) and pressure (4-5MPa) are relatively high. The characteristics are short polymerization time, high production intensity, and the ability to produce high, medium and low density polyethylene, and better control of product properties; however, the polymer obtained by the solution method has a low molecular weight, a narrow molecular weight distribution, and a low solid content.
- Gas phase method: Ethylene is polymerized in gaseous state, and fluidized bed reactor is generally used. There are two types of catalysts: chromium and titanium. They are quantitatively added to the bed from the storage tank, and high-speed ethylene circulation is used to maintain the fluidization of the bed and remove the heat of polymerization. The generated polyethylene is discharged from the bottom of the reactor (Figure 5). The pressure of the reactor is about 2MPa and the temperature is 85-100℃. The gas phase method is a production line of low-density polyethylene.
The most important method, the gas phase method, eliminates the processes of solvent recovery and polymer drying, and saves 15% of investment and 10% of operating costs compared to the solution method. It is 30% of the investment of the traditional high-pressure method and 1/6 of the operating cost. Therefore, it has been rapidly developed. However, the gas phase method needs to be further improved in terms of product quality and variety.
The medium pressure method uses a chromium catalyst loaded on silica gel to polymerize ethylene under medium pressure in a loop reactor to produce high-density polyethylene. Processing and application can be processed by blow molding, extrusion, injection molding and other methods, and is widely used in the manufacture of films, hollow products, fibers and daily sundries. In actual production, in order to improve the stability of polyethylene to ultraviolet rays and oxidation, and improve processing and use performance, a small amount of plastic additives needs to be added. Commonly used ultraviolet absorbers are o-hydroxybenzophenone or its alkoxy derivatives, etc., and carbon black is an excellent ultraviolet shielding agent. In addition, antioxidants, lubricants, colorants, etc. are added to further expand the application range of polyethylene.
Chinese market
For the chinese market, medium-density polyethylene (MDPE) seems to be a new term, but because its performance is between high-density polyethylene (HDPE) and low-density polyethylene (LDPE), it not only maintains the rigidity of HDPE, but also has the flexibility and creep resistance of LDPE, and combines the advantages of both. It has an absolute advantage in the manufacture of gas distribution pipes, water distribution pipes, communications and cable sheaths, and is therefore increasingly attracting market attention. Unique performance establishes market position: MDPE has been rapidly developed in recent years due to its good resistance to environmental stress cracking, weldability and long service life. The amount of use has increased rapidly. MDPE is considered to be the most suitable sheath material for communication cables and optical fiber cables in the world.
Abroad, the development of MDPE is synchronized with LDPE plastic raw materials and HDPE plastic raw materials. Dow Chemical Co., Ltd. has successfully developed the enhanced PE resin DOW LEX2344 E using Insite technology. This resin is a very stable MDPE, an ethylene -octene copolymer resin, with a special linear ethylene main chain and octene side chains., with superior toughness and outstanding long-term water pressure resistance. Borealis has developed a variety of MDPE grades, which are widely used in the production of natural gas pipes, drinking water pipes, foamed coaxial cables, foamed insulated telephone boards, communication and power cable sheaths. Among them, the PE100 resin produced using North Star’s bimodal technology has successfully extruded the world’s largest pipe with a diameter of 1600mm. Samsung of South Korea has developed a series of rotational molding grade MDPE products, which are mainly used for chemical storage tanks, toys, oil tanks, and water tanks. SK of South Korea uses Mitsui and Sclairtech processes on HDPE devices, combining their advantages to develop rotational molding, pipe coating, injection molding, and wire drawing grade MDPE products. In Europe, MDPE has established a dominant position in the gas and water distribution pressure pipe market. In the field of large-diameter gas distribution pipes (over 100mm in outer diameter), gas transmission pipes (over 300mm in outer diameter) and water distribution pipes (over 200mm in outer diameter), MDPE has great potential to enter the metal and polyvinyl chloride territories. In terms of water distribution pipes alone, Europe’s annual MDPE consumption is 160,000 tons.
The pace of chinese research and development needs to be accelerated: the development of MDPE resin in the chinese market lags behind, and there are not many products on the market. Most cable sheath manufacturers can only use blending. The full-density polyethylene production facilities introduced by Maoming Petrochemical and Zhongyuan Ethylene Company have MDPE grades, such as pipe material DGDA-2401 and for rotational molding.DNDD-7152, but the output is very small. The 200,000 tons/year LDPE production unit newly introduced by Yanhua Company from EXXON Chemical Company has three MDPE grades (117, 151, 157), with density values ranging from 0.9280g/cm³ to 0.9355g/cm³. Two of the grades, 117 and 151, have produced 600 tons of products, but the product density is low and it is not a copolymer product. It has poor environmental stress cracking resistance and is not suitable for making pressure pipes such as gas pipes. The 250,000 tons/year polyethylene unit put into production by Shanghai Petrochemical Company in 2004 uses the bimodal polyethylene technology of Borealis Chemicals and can produce special materials for PE100 pipes, but the product grading work will not be completed until 2004, which means that the raw materials used in China still need to be imported. The market demand potential is considerable: the fields in which MDPE is used in China include wire and cable sheath materials, water transmission pipelines, buried gas pipelines, agricultural irrigation hoses (belts), etc., and the demand for usage is increasing.
Most manufacturers use HDPE plus LDPE to produce MDPE communication cables and optical cable sheath materials, with an annual output of about 30,000 tons; gas pipe materials are required to have good weldability, environmental stress cracking resistance and cracking rapid transmission resistance, and manufacturers all use imported MDPE raw materials; agricultural irrigation hoses (belts) are also one of the application areas of MDPE. In order to reduce prices and control production costs, only a few of these manufacturers use imported MDPE, and most use chinese LDPE and LLDPE. Some manufacturers add a small amount of HDPE to blend to increase rigidity; the world’s best hose reel automatic sprinkler is a sprinkler suitable for China’s national conditions. The PE hose wound on the machineIt not only has to withstand bending, tensile stress and internal water pressure, but also has to withstand ground friction, sun and rain, hot and cold alternation and other combined effects. MDPE pipes have been developed abroad to synchronize the service life of the hose with the sprinkler. China developed this type of sprinkler in the mid-to-late 1980s. However, there has been no report of the successful development of high-quality MDPE hoses that match it, and the market demand is about 20,000 to 30,000 tons/year. The development of the West-East Gas Transmission and West-East Power Transmission projects in the Western Development, as well as the construction of urban natural gas transmission pipelines, urban and rural water and gas pipelines, and China’s status as a major agricultural country have all painted a bright future for the new favorite of the PE family – MDPE and its products. However, for the chinese market, chinese users have a very insufficient understanding of MDPE both rationally and emotionally. However, by increasing the publicity and development of MDPE brands, it is believed that MDPE will gradually be recognized by people and will make more and more outstanding contributions to the construction and development of the country.
