What is High-Density Polyethylene?

High density polyethylene (HDPE) is a white powder or granular product. Non-toxic, tasteless, crystallinity is 80% 90%, softening point is 125 l 35 , working temperature can reach 100 ; hardness, tensile strength and creep are better than low density polyethylene; Good electrical insulation, toughness and cold resistance; good chemical stability, insoluble in any organic solvents at room temperature, resistant to corrosion by acids, alkalis and various salts; thin films have low permeability to water vapor and air, and absorb water Low resistance; aging resistance is poor, resistance to environmental stress cracking is not as good as low-density polyethylene, especially thermal oxidation will reduce its performance, so antioxidants and ultraviolet absorbers must be added to the resin to improve this deficiency. High-density polyethylene film has a low thermal deformation temperature under stress, so pay attention to it during application.

In this century, a revolutionary progress has taken place in the field of pipelines, namely "replacing steel with plastic". With the rapid progress of polymer materials science and technology, the deepening of the development and utilization of plastic pipes, the continuous improvement of production processes, plastic pipes show their outstanding performance to the fullest. Today, plastic pipes are no longer mistaken for "cheap substitutes" for metal pipes. In this revolution, polyethylene pipes have been favored and increasingly dazzling. They are widely used in gas transportation, water supply, sewage, agricultural irrigation, fine particle solids transportation in mines, and in the fields of oil fields, chemicals, and post and telecommunications. Gas transmission is widely used.

HDPE is a kind of

[Full name in English] High Density Polyethylene

High density polyethylene has good heat resistance and cold resistance, good chemical stability, high rigidity and toughness, and good mechanical strength. Dielectric properties and resistance to environmental stress cracking are also good. Hardness, tensile strength and creep are better than low density polyethylene; wear resistance, electrical insulation, toughness and cold resistance are better, but slightly worse than low density insulation; good chemical stability at room temperature Under the conditions, it is insoluble in any organic solvents, resistant to acid, alkali and various salts; the film has low permeability to water vapor and air, and low water absorption; poor aging resistance, and environmental cracking resistance is not as good as low density polyethylene, In particular, thermal oxidation will reduce its performance, so resins need to be added with antioxidants and UV absorbers to improve and improve this deficiency. High-density polyethylene film has a low thermal deformation temperature under stress, which should be paid attention to during application ^[1]

The most common production method of PE is through slurry or gas phase processing, and a few are produced by solution phase processing. All these processes are exothermic reactions involving ethylene monomers, a-olefin monomers, catalyst systems (possibly more than one compound), and various types of hydrocarbon diluents. Hydrogen and some catalysts are used to control molecular weight. The slurry reactor is generally a stirred tank or a more commonly used large-scale annular reactor in which the slurry can be circulated and stirred. As soon as ethylene and comonomers (as needed) come into contact with the catalyst, polyethylene particles are formed. After removing the diluent, the polyethylene granules or powder granules are dried and the additives are added in a dosage to produce pellets. With

Higher molecular weight leads to higher polymers

PE can be manufactured in a wide range of different processes. With ethylene as the main raw material,

High-density polyethylene resin can be molded into plastic products by injection, extrusion, blow molding, and rotational molding. Various types of containers, industrial accessories, medical supplies, toys, shells, stoppers and shields can be formed by injection molding. Various types of hollow containers and ultra-thin films can be formed by blow molding. Extrusion can be used to form pipes, stretched tapes, strappings, monofilaments, wire and cable jackets, etc.

In addition, it can also form decorative panels, shutters, synthetic wood, synthetic paper, synthetic membranes, and molded calcium-plastic products, etc. ^[1]

Store away from fire and heat insulation. Keep the warehouse dry and tidy. It is strictly forbidden to mix in any impurities. Transportation should be stored in clean, dry covered cars or cabins, and there should be no sharp objects such as nails. Prohibition against flammable aromatic hydrocarbons, halogenated hydrocarbons, etc.

HDPE is the fastest growing part of the recycling market for plastic raw materials libraries. This is mainly due to its ease of reprocessing, its minimal degradation properties and its numerous applications in packaging applications. The main recycling is to reprocess 25% of recycled materials, such as post-consumer recycled materials (PCR), with pure HDPE to make bottles that are not in contact with food.
In this process, the polymerization solvent is n-hexane, and the catalyst is a highly active z-N catalyst. After mixing ethylene and hydrogen, it enters the first reactor and is mixed with the catalyst for polymerization reaction. In hexane, the polymerization temperature is about 80 ° C and the polymerization pressure is less than 10 bar. This process can produce products with a density range of 0.942 to 0.965 g / cm3, a melt index range of 0.2 to 80, and comonomer Acrylic and butene-1, which produces traditional HDPE and bimodal HDPE, have excellent performance in high-density pipes and are suitable for making pressure pipes to reach PE100 +. The continuous polymerization process of slurry tank reactor is characterized by: low operating pressure and operating temperature; double tank reactor can produce single-peak and double-peak products by using parallel and serial different forms; high process operation flexibility, product brand conversion Fast, the raw material purity requirements are not high; comonomer uses propylene, 1-butene; using hexane as a solvent, the recovery unit is simple. The process is characterized by the polymerization in an inert hydrocarbon diluent. The process flow is as follows: the polymerized ethylene (99.9% ethylene, 0.1% ethane) is sent to the dryer, and then enters the kettle reactor together with a circulating diluent composed of n-hexane, and the catalyst uses a carrier containing titanium and manganese, triethyl Aluminium-based catalyst grades. Add a small amount of hydrogen to control the molecular weight. Polyethylene particles are formed during the polymerization reaction. The reaction temperature is 90 ° C and the pressure is 1.8 MPa. The reaction can be performed in two polymerization reactors in two steps. The concentration is 34% (mass fraction). Of slurry solids, the monomer conversion rate can reach 97%. The polymer flowed out of the second reactor to a pressure of 0.14 MPa, and the unreacted ethylene, ethane in the effluent, and 2% cyclohexane diluent, which were flashed off, were cooled to 2.5 MPa by two compressions, and degassed. The tower recovers ethane for recycling. Most of the diluent is recovered by centrifugal separation of the slurry after flash evaporation. The solid filter cake is sent to a dryer to reduce the volatile component content to about 5% (mass fraction). The dryer is operated in a closed cycle with nitrogen protection. The dried polymer powder is sent to a fluidized bed for drying to remove all hydrocarbon diluents. After drying, the polymer particles are sent to the mixing section to add various additives, and then granulated. (2) Loop tube reactor process The typical representatives of loop tube reactor process are Phillips process of Phillips company and Innovene S process of INEOS company. The Phillips process uses isobutane as a diluent and uses a chromium-based catalyst. The catalyst is activated before use. The activated catalyst powder forms a catalyst slurry with high-purity isobutane under the protection of nitrogen, and then enters the loop reactor. After the raw material ethylene monomer is refined, it is premixed with hydrogen and comonomer hexene-1 and then injected into a loop reactor, and ethylene is produced into polyethylene under the action of a catalyst. The axial flow pump keeps the material in the reactor flowing at a high speed and mixes very uniformly. The reaction heat is evenly withdrawn from the jacket cooling water. The production range of this process is from 0.1 5 to 100 and the density is from 0.936 to 0.972 g / cm3. The characteristics of the loop reactor process are: less equipment, short process, low investment costs; no wax and oligomers, no sticky walls; good shape of the powder, easy to transport; reaction heat is taken out by the reactor jacket cooling water , Easy to remove heat, easy to adjust; high raw material requirements, need to be purified; comonomer using hexene; using isobutane as solvent, easy to remove residual solvents. The process is as follows: fresh polymer grade ethylene is prepared by mixing molecular weight regulator hydrogen, antifreeze and circulating diluent isobutane into a multi-loop continuous flow reactor, and the catalyst is supplemented with isobutane. Into the reactor. The reaction temperature was 106.7 degrees Celsius and the pressure was 3.9 MPa. The polymer and diluent residual slurry was passed through a loop reactor by an axial flow pump at a speed of 6 m / s. The water cooling in the reactor jacket controls the reaction temperature, and the polymer solids are discharged from the vertical sedimentation port in the loop reactor. So that the slurry concentration can reach 55% and the conversion rate is 98% -99%. After the polymer is discharged, de-flashing is performed to discharge isobutane and residual monomers into the diluent recovery device. Other solid polymers are mixed with additives and granulated. 2. Gas-phase polymerization The gas-phase polymerization (gas-phase fluidized-bed) process is typically represented by DOW Chemical's univation technology and INNOS's Innovene technology. The univation technology uses a low-pressure gas-phase fluidized-bed reactor with z / n catalyst and chromium. Based catalyst, the purified raw materials are injected into the reactor, and a polymerization reaction occurs under the action of catalyst storage, and the reaction is in the range of 85 to 110. [=. It is carried out at a pressure of 2.41 MPa, and the single-way conversion of ethylene is about 1% to 2%. The removal of the reaction heat is mainly through the cooling of the circulating stream. 0.970 g / cm3. The characteristics of the gas-phase fluidized bed polymerization process are: low operating pressure and low temperature; full-density polyethylene can be produced; catalyst systems include titanium-based and chromium-based; metallocene catalysts; Solvent-free, low energy consumption, low maintenance and operating costs. The production process is: dry monomers are added to the reactor system together with hydrogen, the raw materials are added to a large circulating steam flow loop, and the gas is distributed to the bottom of the large fluidized bed reactor. % Ethylene (ethylene content is 99.9%, 0.1% is ethane), 10.43% hydrogen, 7.56% ethane, and 12.44% nitrogen. This raw material gas composition produces a product with a melt index of 8g / 10min and a density of 0.964g / cm3. The catalyst is a mixture of magnesium dioxide with titanium trichloride and tetrahydrofuran as promoters, and the co-catalyst is triethylaluminum. The catalyst enters the reactor in a solid form together with nitrogen from the painless reactor site. The operating temperature is 105 ° C, and the specific temperature is determined according to the product brand. The operating pressure of the reactor was 2.0 MPa, and the reaction gas came out of the top of the reaction, was separated by a cyclone separator, and the catalyst containing solids was separated and sent back to the reactor. The gas from the cyclone separator is then circulated to the bottom of the reactor through compression and circulation of the cooler. The reactor discharge intermittently feeds product particles to the tank through an airlock system. Part of the gas entering the discharge tank enters the compressor circulation system through the upper buffer tank, filter, gas cooler, and separation tank. The polymer comes out of the lower part of the discharge tank into the purge tank and aftertreatment system. Post-treatment systems include adding various additives to the polymer, melting, pelletizing, and packaging.