Characteristics of high density polyethylene HDPE pipe
High density polyethylene, the English name is "High Density Polyethylene", referred to as "HDPE". HDPE is a highly crystalline, non-polar thermoplastic resin. The original HDPE has a milky white appearance and a semi-transparent shape in a thin section. . Certain types of chemicals can cause chemical corrosion, such as corrosive oxidants (concentrated nitric acid), aromatic hydrocarbons (xylene), and halogenated hydrocarbons (carbon tetrachloride). The polymer is non-hygroscopic and has good water vapor resistance and can be used for packaging purposes. HDPE has excellent electrical properties, especially high dielectric strength, making it suitable for wire and cable. Medium to high molecular weight grades have excellent impact resistance, both at ambient temperatures and even at low temperatures of -40F.
HDPE is a thermoplastic polyolefin formed by copolymerization of ethylene. Although pe pipes have been introduced in 1956, the plastics have not yet reached maturity. This versatile material is constantly developing its new uses and markets.
HDPE is a highly crystalline, non-polar thermoplastic resin. The original HDPE has a milky white appearance and a semi-transparent shape in a thin section. PE has excellent resistance to most living and industrial chemicals. Certain types of chemicals can cause chemical corrosion, such as corrosive oxidants (concentrated nitric acid), aromatic hydrocarbons (xylene), and halogenated hydrocarbons (carbon tetrachloride). The polymer is non-hygroscopic and has good water vapor resistance and can be used for packaging purposes. HDPE has excellent electrical properties, especially high dielectric strength, making it suitable for wire and cable. Medium to high molecular weight grades have excellent impact resistance, both at ambient temperatures and even at low temperatures of -40F. The unique characteristics of various grades of HDPE are the appropriate combination of four basic variables: density, molecular weight, molecular weight distribution, and additives. Different catalysts are used to produce custom specialty polymer. These variables combine to produce HDPE grades for different applications; achieving the best balance in performance.
This is the main variable that determines the characteristics of HDPE, although the four variables mentioned do interact. Ethylene is the main raw material of polyethylene. A few other comonomers, such as 1-butene, 1-hexene or 1-octene, are also often used to improve polymer properties. For HDPE, the content of the above monomers is generally not more than 1% - 2%. The addition of comonomer slightly reduces the crystallinity of the polymer. This change is generally measured by density, which is linear with crystallization. The US general classification is in accordance with ASTM D1248, and the density of HDPE is 0.940 g/. Above C; medium density polyethylene (MDPE) has a density ranging from 0.926 to 0.940 g/cc. Other taxonomies sometimes classify MDPE as HDPE or LLDPE. Homopolymers have the highest density, maximum stiffness, good barrier properties and the highest melting point, but generally have poor resistance to environmental stress cracking (ESCR). ESCR is the ability of PE to resist cracking caused by mechanical or chemical stresses. Higher densities generally improve mechanical strength, such as tensile strength, stiffness and hardness; thermal properties such as softening point temperature and heat distortion temperature; and barrier properties such as gas permeability or water vapor transmission. The lower density improves its impact strength and E-SCR. The polymer density is primarily affected by the addition of comonomers, but to a lesser extent by molecular weight. A high molecular weight percentage results in a slight decrease in density. The pe tubing, for example, has a different density of homopolymers over a wide range of molecular weights.
The most common production method for PE is by slurry or gas phase processing, and a few are produced by solution phase processing. All of 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 the molecular weight. The slurry reactor is typically a stirred tank or a more commonly used large loop reactor in which the slurry can be agitated. When ethylene and comonomer (as needed) are contacted with the catalyst, polyethylene particles are formed. After the diluent is removed, the polyethylene granules or granules are dried and the additives are added in a dose to produce pellets. A modern production line for large reactors with twin-screw extruders that can produce more than 40,000 pounds of PE per hour. The development of new catalysts contributes to improving the performance of new grade HDPE. The two most common types of catalysts are Phillips' chromium oxide based catalysts and titanium compound monoalkyl aluminum catalysts. The HDPE produced by the Phillips type catalyst has a medium width molecular weight distribution; the titanium monoalkyl aluminum catalyst produces a narrow molecular weight distribution. Catalysts used to produce narrow MDW polymers using a duplex reactor can also be used to produce wide MDW grades. For example, two series reactors producing significantly different molecular weight products can produce bimodal molecular weight polymers having a full broad molecular weight distribution.
PE pipe fittings
A higher molecular weight results in a higher polymer viscosity, although the viscosity is also related to the temperature and shear rate used in the test. The molecular weight of the material is characterized by rheological or molecular weight measurements. The grade of HDPE generally has a molecular weight range of 40 000 to 300 000, and the weight average molecular weight generally corresponds to the range of the melt index, that is, from 100 to 0. 029/10 min. Generally, a higher MW (lower melt index MI) enhances melt strength, better toughness, and ESCR, but higher MW makes the process more difficult or requires higher pressures or temperatures.
Molecular Weight Distribution (MWD): The WD of PE varies from narrow to wide depending on the catalyst used and the processing.
The most commonly used MWD measurement index is the Unevenness Index (HI), which is equal to the weight average molecular weight (MW) divided by the number average molecular weight (Mn). This index range for all HDPE grades is 4-30. The narrow MWD provides low warpage and high impact during the molding process. Medium to wide MWD provides processability for most extrusion processes. Wide MWD also improves melt strength and creep resistance.
The addition of an antioxidant prevents degradation of the polymer during processing and prevents the finished product from oxidizing during use. Antistatic additives are used in many package grades to reduce the adhesion of bottles or packages to dust and dirt. Specific additives require special additive formulations such as copper inhibitors associated with wire and cable applications. Excellent weatherability and UV resistance (or daylight) can be achieved by adding anti-UV additives. PE without UV or carbon black is not recommended and should not be used outdoors. High grade carbon black pigments provide excellent UV resistance and are often used outdoors, such as wires, cables, tanks or pipes.
PE can be manufactured in a wide variety of different processing methods. Using ethylene as the main raw material, propylene, 1-butene and hexene are copolymers. Under the action of the catalyst, the obtained polymer is flashed, separated, dried, granulated, etc. by slurry polymerization or gas phase polymerization. In the process, a finished product having uniform particles is obtained. These include, for example, sheet extrusion, film extrusion, tube or profile extrusion, blow molding, injection molding, and rotational molding.
Extrusion: Grades used for extrusion production typically have a melt index of less than 1 and a medium to wide MWD. A low MI gives a suitable melt strength during processing. The wider MWD grade is more suitable for extrusion because of their higher production speed, lower die pressure and reduced melt fracture tendency.
PE has many extrusion applications such as wires, cables, hoses, tubes and profiles. Pipe applications range from small-section yellow tubes for natural gas to thick-walled black tubes for industrial and urban pipelines up to 48 in. diameter. Large diameter hollow wall tubes are used as an alternative to rainwater drains and other sewer lines made of concrete to grow rapidly.
Sheets and thermoforming: The thermoformed linings of many large picnic-type reefers are made of PE for toughness, light weight and durability. Other sheet and thermoformed products include fenders, tank liners, tray guards, shipping boxes and cans. A large number of rapidly growing sheet applications are in the mulch or pool bottoms, which are based on MDPE's toughness, chemical resistance and impermeability.
Blow molding: HDPE 1/3 or more sold in the United States is used for blow molding. These range from bottles containing bleach, motor oil, detergents, milk and distilled water to large refrigerators, car fuel tanks and canisters. Blow molding grade characteristics such as melt strength, ES-CR and toughness are similar to those used for sheet and thermoforming applications, so similar grades can be used.
Injection-blow molding is commonly used to make smaller containers (less than 16 oz) for packaging medicines, shampoos and cosmetics. One advantage of this process is the automatic cornering of the bottles, without the need for post-finishing steps like normal blow molding. Although some narrow MWD grades are used to improve surface finish, medium to wide MWD grades are typically used.
Injection molding: HDPE has countless applications ranging from reusable thin-walled beverage cups to 5-gsl cans, which consume 1/5 of domestically produced HDPE. Injection molding grades generally have a melt index of 5 to 10, and have a higher flowability grade with a lower toughness grade and processability. Uses include daily necessities and food thin wall packaging; tough, durable food and paint cans; high environmental stress cracking applications such as small engine fuel tanks and 90-gal trash cans.
Rotational molding: Materials using this processing method are generally pulverized into a powder to be melted and flowed in a thermal cycle. Rotational molding uses two types of PE: general purpose and crosslinkable classes. General purpose MDPE/HDPE typically has a density ranging from 0.935 to 0.945 g/cc and has a narrow MWD for high impact and minimal warpage, with a melt index typically ranging from 3-8. Higher MI grades are generally not applicable because they do not have the desired impact and environmental stress crack resistance of rotomoulded articles.
High performance rotational molding applications utilize the unique properties of their chemically crosslinkable grades. These grades are fluid in the first stage of the molding cycle and then cross-linked to form their excellent resistance to environmental stress cracking and toughness. Abrasion resistance and weather resistance. The cross-linkable PE is uniquely suited for use in large containers ranging from 500-gal to various chemical storage tanks to 20,000-gal agricultural storage tanks.
Film: PE film processing is generally performed by ordinary blown film processing or flat extrusion processing. Most PEs are used for film, general low density PE (LDPE) or linear low density PE (LLDPE). HDPE film grades are typically used where superior stretchability and excellent barrier properties are required. For example, HDPE films are commonly used in merchandise bags, grocery bags, and food packaging.
High density polyethylene is a non-toxic, odorless, odorless white granule with a melting point of about 130 ° C and a relative density of 0.941 to 0.960. It has good heat resistance and cold resistance, good chemical stability, high rigidity and toughness, and good mechanical strength. Dielectric properties and environmental stress crack resistance are also good.
HDPE is the fastest growing segment of the plastics recycling market. This is mainly due to its ease of rework, minimal degradation characteristics and its extensive use in packaging applications. Keep away from fire and heat during storage. Keep clean and tidy in the warehouse. Do not mix any impurities. It is strictly prohibited to be exposed to sunlight or rain. Transportation should be stored in a clean, dry, covered compartment or cabin, with no sharp objects such as nails. It is strictly forbidden to mix with organic solvents such as flammable aromatic hydrocarbons and halogenated hydrocarbons. The main recycling is to reprocess 25% of the recycled materials, such as post-consumer recycled (PCR), with pure HDPE to make bottles that are not in contact with food.
PE pipe for water supply is a replacement for traditional steel pipe and polychloroic acid ethylene drinking water pipe.
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