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Polyethylene plastic is one of the five most common plastic. It is a long-chain polymer composed of ethylene molecules connected to each other, and each chain link is composed of ethylene molecules. The main component of polyethylene plastic is polyethylene, and its raw material ethylene mainly comes from petroleum cracking and is a petrochemical product.
Polyethylene is divided into three categories: linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE) and high-density polyethylene (HDPE) according to the polymerization method, molecular weight and molecular chain structure. Polyethylene is cheap, durable, and does not interact with food. It is widely used in the manufacture of films, tube sheets, fibers, food packaging, grocery store shopping bags, etc.
The output and consumption of polyethylene plastic rank first among various synthetic resins. It is widely used in industry, agriculture and daily life, but polyethylene has poor high temperature resistance, and its mechanical properties and chemical resistance sometimes cannot meet the requirements of actual use. Polyethylene plastic are very stable, but difficult to degrade naturally. Normally, people dispose of discarded polyethylene plastic by incineration or landfilling, but the incineration process will produce a large amount of carbon dioxide and toxic gases, polluting the atmospheric environment; landfilling takes thousands of years to degrade, and in the process, microplastic will be released to pollute the soil and groundwater. In June 2023, the research team of Professor Zeng Jie of the University of Science and Technology of China designed a "hydrogen breathing" strategy to convert high-density polyethylene plastic into high-value-added cyclic hydrocarbons without the need for additional hydrogen or solvents, providing a new method for the "artificial carbon cycle" of discarded plastic.
Polyethylene (PE) is one of the five major synthetic resins and is the variety with the largest production capacity and the largest import volume among China's synthetic resins. Polyethylene is mainly divided into three categories: linear low-density polyethylene (LLDPE), low-density polyethylene (LDPE), and high-density polyethylene (HDPE).
Molding temperature: 140-220¡æ.
Corrosion resistance, electrical insulation (especially high-frequency insulation) are excellent, can be chlorinated, irradiated and modified, and can be reinforced with glass fiber. Low-pressure polyethylene has high melting point, rigidity, hardness and strength, low water absorption, good electrical properties and radiation resistance; high-pressure polyethylene has good softness, elongation, impact strength and permeability; ultra-high molecular weight polyethylene has high impact strength, fatigue resistance and wear resistance. Low-pressure polyethylene is suitable for making corrosion-resistant parts and insulating parts; high-pressure polyethylene is suitable for making films, etc.; ultra-high molecular weight polyethylene is suitable for making shock-absorbing, wear-resistant and transmission parts.
1. Crystalline material, low moisture absorption, no need to be fully dried, excellent fluidity, fluidity is sensitive to pressure, high-pressure injection is suitable for molding, uniform material temperature, fast filling speed, and sufficient pressure holding. It is not suitable to use direct gates to prevent uneven shrinkage and increased internal stress. Pay attention to the selection of gate position to prevent shrinkage and deformation.
2. The shrinkage range and shrinkage value are large, the directionality is obvious, and it is easy to deform and warp. The cooling speed should be slow, the mold should be equipped with a cold material hole, and there should be a cooling system.
3. The heating time should not be too long, otherwise it will decompose and burn.
4. When the soft plastic parts have shallow side grooves, they can be demolded forcibly.
5. Melt rupture may occur, and it is not suitable to contact with organic solvents to prevent cracking.
That is, high-pressure low-density polyethylene (LDPE), high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). Film is its main processed product, followed by sheets and coatings, hollow containers such as bottles, cans, barrels and other various injection and blow molding products, pipes and wires, insulation and sheathing of cables, etc. It is mainly used in packaging, agriculture and transportation departments.
Commercially available high-density polyethylene (HDPE), density 0.945~0.96 g/cm3, melting point 125~137¡æ;
Linear low-density PE (LLDPE), density 0.925 g/cm3, melting point 120~125¡æ;
High-pressure low-density PE (HP-LDPE), density 0.918 g/cm3, melting point 105~115¡æ.
Cling film, vest-type plastic bags, plastic food bags, baby bottles, buckets, kettles, etc.
PE is relatively soft and feels waxy. Compared with the same plastic, it is relatively light and has a certain degree of transparency. The flame is blue when burning.
Non-toxic, harmless to the human body.
The English name is "High Density Polyethylene", abbreviated as "HDPE". HDPE is a highly crystalline, non-polar thermoplastic resin. The appearance of the original HDPE is milky white.
HDPE is a thermoplastic polyolefin generated by ethylene copolymerization. Although HDPE was launched in 1956, this plastic has not yet reached a mature level. This general material is still developing new uses and markets.
Main characteristics
HDPE is a highly crystalline, non-polar thermoplastic resin. The appearance of the original HDPE is milky white, and it is translucent to a certain extent in thin sections. PE has excellent resistance to most domestic and industrial chemicals. Certain types of chemicals can cause chemical corrosion, such as corrosive oxidizers (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 very good electrical properties, especially high dielectric strength, making it very suitable for wire and cable. Medium to high molecular weight grades have excellent impact resistance, both at room temperature and 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 customized special performance polymers. These variables are combined to produce HDPE grades for different purposes; achieving the best balance in performance.
Density
This is the main variable that determines the characteristics of HDPE, although the four variables mentioned do have an impact on each other. Ethylene is the main raw material for polyethylene, and a small number of other comonomers, such as 1-butene, 1-hexene or 1-octene, are also often used to improve the properties of the polymer. For HDPE, the content of the above few monomers generally does not exceed 1%-2%. The addition of comonomers slightly reduces the crystallinity of the polymer. This change is generally measured by density, which is linearly related to crystallinity. The general classification in the United States is specified by ASTM D1248, and the density of HDPE is 0.940g/. C or above; medium density polyethylene (MDPE) density ranges from 0.926 to 0.940 g/CC. Other classifications sometimes classify MDPE as HDPE or LLDPE. Homopolymers have the highest density, the greatest stiffness, good permeability resistance and the highest melting point, but generally have poor environmental stress cracking resistance (ESCR). ESCR is the ability of PE to resist cracking caused by mechanical or chemical stress. Higher density generally improves mechanical strength, such as tensile strength, stiffness and hardness; thermal properties such as softening point temperature and heat deformation temperature; permeability resistance, such as air permeability or water vapor permeability. Lower density improves its impact strength and E-SCR. Polymer density is mainly affected by the addition of comonomers, but to a lesser extent by molecular weight. High molecular weight percentages reduce density slightly. For example, homopolymers have different densities within a wider molecular weight range.
Production and Catalysts
The most common production method for PE is through slurry or gas phase processing, and a few are produced by solution phase processing. All of these processes involve exothermic reactions of ethylene monomer, ¦Á-olefin monomer, catalyst system (which may be 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, more commonly, a large annular reactor in which the slurry is circulated and stirred. Polyethylene particles are formed as soon as the ethylene and comonomer (if necessary) and catalyst come into contact. After the diluent is removed, the polyethylene particles or powder are dried and additives are dosed to produce pellets. Modern production lines with large reactors with twin-screw extruders can produce more than 40,000 pounds of PE per hour. The development of new catalysts has contributed to the improvement of the properties of new grades of HDPE. The two most commonly used catalyst types are Phillips chromium oxide-based catalysts and titanium compound-alkylaluminum catalysts. Phillips-type catalysts produce HDPE with a medium-broad molecular weight distribution; titanium-alkylaluminum catalysts produce narrow molecular weight distributions. Catalysts used to produce narrow MDW polymers in compound reactors can also be used to produce wide MDW grades. For example, two reactors in series producing products of significantly different molecular weights can produce a bimodal molecular weight polymer with a full-width molecular weight distribution.
Molecular Weight
Higher molecular weights result in higher polymer viscosities, but viscosity is also a function of the temperature and shear rate used in the test. Molecular weight of a material is characterized by rheological or molecular weight measurements. HDPE grades generally have molecular weights ranging from 40,000 to 300,000, with the weight average molecular weight roughly corresponding to the melt index range, from 100 to 0.029/10min. In general, higher MW (lower melt index MI) increases melt strength, better toughness, and ESCR, but higher MW makes processing more difficult or requires higher pressures or temperatures.
Molecular Weight Distribution (MWD): The WD of PE varies from narrow to broad depending on the catalyst and processing used.
The most commonly used index for measuring MWD is the heterogeneity index (HI), which is equal to the weight average molecular weight (MW) divided by the number average molecular weight (Mn). This index ranges from 4 to 30 for all HDPE grades. Narrow MWD provides low warpage and high impact during molding. Medium to wide MWD provides processability for most extrusion processes. Wide MWD can also improve melt strength and creep resistance.
Additives
The addition of antioxidants prevents polymer degradation during processing and prevents oxidation of the finished product during use. Antistatic additives are used in many packaging grades to reduce the adhesion of dust and dirt to bottles or packaging. Specific uses require special additive formulations, such as copper inhibitors associated with wire and cable uses. Excellent weather resistance and resistance to ultraviolet light (or sunlight) can be achieved by adding anti-UV additives. PE without added UV resistance or carbon black is not recommended for continuous outdoor use. High-grade carbon black pigments provide excellent UV resistance and can often be used in outdoor applications such as wire, cable, tank village layer or pipe.
Processing methods
PE can be manufactured using a wide range of different processing methods. With ethylene as the main raw material, propylene, 1-butene and hexene as copolymers, slurry polymerization or gas phase polymerization process is adopted under the action of catalyst. The obtained polymer is subjected to flash evaporation, separation, drying, granulation and other processes to obtain a finished product with uniform particles. Including such as sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding and rotational molding.
Extrusion: The grades used for extrusion production generally have a melt index of less than 1 and a medium-wide to wide MWD. During the processing, low MI can obtain suitable melt strength. Wider MWD grades are more suitable for extrusion because they have higher production speeds, lower die pressures and reduced melt fracture tendency.
PE has many extrusion uses, such as wires, cables, hoses, pipes and profiles. The application range of pipes ranges from small-section yellow pipes for natural gas to thick-walled black pipes with a diameter of 48in for industrial and urban pipelines. Large-diameter hollow wall pipes are rapidly growing as substitutes for rainwater drainage pipes and other sewer pipelines made of concrete.
Sheets and Thermoforming: The thermoformed liners of many large picnic-type coolers are made of PE, which is tough, lightweight and durable. Other sheet and thermoformed products include fenders, tank liners, pan covers, shipping boxes and cans. A large and rapidly growing sheet application is ground film or pool bottom lining, which is based on the toughness, chemical resistance and impermeability of MDPE.
Blow Molding: More than 1/3 of HDPE sold in the United States is used for blow molding purposes. These range from bottles for bleach, motor oil, detergent, milk and distilled water to large refrigerators, automobile fuel tanks and cans. The properties of blow molding grades, 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 often used to make smaller containers (less than 16oz) for packaging medicines, shampoos and cosmetics. One advantage of this process is that the bottles are automatically deburred, eliminating the need for post-finishing steps as in conventional blow molding. Medium to wide MWD grades are generally used, although some narrow MWD grades are available for improved surface finish.
Injection molding: HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming one-fifth of the HDPE produced in China. Injection molding grades generally have a melt index of 5 to 10, with lower flow grades for toughness and higher flow grades for processability. Uses include thin-wall packaging for daily necessities and food; tough, durable food and paint cans; and high environmental stress cracking resistance applications, such as small engine fuel tanks and 90-gal trash cans.
Rotational molding: Materials used in this process are generally crushed into powders that are melted and flowed during thermal cycles. Two types of PE are used in rotational molding: general-purpose and cross-linkable. General-purpose MDPE/HDPE usually has a density range of 0.935 to 0.945 g/cc, with a narrow MWD, giving the product high impact and minimal warpage, and a melt index range of 3-8. Higher MI grades are generally not suitable because they do not have the impact and environmental stress cracking resistance desired for roto-molded products.
High-performance roto-molding applications utilize the unique properties of its chemically cross-linkable grades. These grades have good fluidity in the first stage of the molding cycle, and then cross-link to form their excellent environmental stress cracking resistance, toughness. Abrasion resistance and weather resistance. Cross-linkable PE is uniquely suitable for large containers, ranging from 500-gal tanks for transporting various chemicals to 20,000-gal agricultural storage tanks.
Film: PE film processing is generally processed by ordinary blown film processing or flat extrusion processing. Most PE is used for film, and general low-density PE (LDPE) or linear low-density PE (LLDPE) are available. HDPE film grades are generally used where superior stretchability and excellent barrier properties are required. For example, HDPE film is often used in commodity bags, grocery bags and food packaging.
Product Performance
High-density polyethylene is a non-toxic, tasteless, odorless white granule with a melting point of about 130¡ãC and a relative density of 0.941~0.960. It has good heat resistance and cold resistance, good chemical stability, high rigidity and toughness, and good mechanical strength. Dielectric properties and environmental stress cracking resistance are also good.
Packaging and Storage
When storing, it should be kept away from fire sources and insulated. The warehouse should be kept dry and tidy. It is strictly forbidden to mix any impurities, and it is strictly forbidden to be exposed to the sun or rain. Transportation should be stored in a clean, dry and roofed carriage or cabin, and no sharp objects such as nails are allowed. It is strictly forbidden to mix with flammable aromatic hydrocarbons, halogenated hydrocarbons and other organic solvents.
Recycling
HDPE is the fastest growing part of the plastic recycling market. This is mainly due to its easy reprocessing, minimal degradation characteristics and its large application in packaging purposes. The main recycling is to reprocess 25% recycled materials, such as post-consumer recyclate (PCR), with pure HDPE for the manufacture of bottles that do not come into contact with food.
Low density polyethylene (LDPE)
A plastic material that is suitable for various molding processes of thermoplastic molding. It has good molding processability, such as injection molding, extrusion, blow molding, rotational molding, coating, foaming process, thermoforming, hot air welding, thermal welding, etc.
Main uses
LDPE is a film product, suitable for making films, heavy packaging films, cable insulation materials, blow molding and foam products.
Such as agricultural films, ground covering films, agricultural films, vegetable greenhouse films, etc.; packaging films such as candy, vegetables, frozen food packaging; blown films for liquid packaging (milk, soy sauce, juice. Tofu, soy milk); heavy packaging bags, shrink packaging films, elastic films, lining films; building films, general industrial packaging films and food bags, etc. LDPE is also used for injection molding products, such as small containers, lids, daily products, plastic flowers, injection-stretch-blow molding containers. Medical devices, pharmaceutical and food packaging materials, extruded pipes, sheets, wire and cable coatings, profiles, thermoforming and other products; blow molding hollow molding products, such as food containers for dairy products and jams, drugs, cosmetics, chemical product containers, tanks, etc.
Production method
Low-density polyethylene can be divided into high-pressure method and low-pressure method according to the polymerization method. According to the type of reactor, it can be divided into kettle method and tubular method. Ethylene is used as raw material and sent into the reactor. Under the action of the initiator, the polymerization reaction is carried out under high pressure compression. After the material coming out of the reactor is removed from the unreacted ethylene by the separator, it is melt-extruded into granules, dried, blended, and sent for packaging.
Product Performance
Low-density polyethylene is milky white round beads. It is non-toxic, tasteless, odorless, and has a matte surface. The density is 0.916~0.930g/cm3. It is relatively soft, with good elongation, electrical insulation, chemical stability, processing performance and low temperature resistance (can withstand -70¡æ), but poor mechanical strength, moisture barrier, gas barrier and solvent resistance. The molecular structure is not regular, the crystallinity (55%~65%) is low, and the crystalline melting point (108~126¡æ) is also low.
Packaging and Storage
The product is packed in polyethylene heavy packaging film bags, and polypropylene woven bags can be added as outer packaging according to user needs. The product should be stored in a clean and dry warehouse and can be transported by train, car, ship, etc. During storage and transportation, attention should be paid to fire prevention, waterproofing, sun protection, dust prevention and pollution prevention. The means of transportation should be kept clean and dry, without sharp objects such as nails, and must have a shed or tarpaulin. Linear low-density polyethylene (LLDPE) is a copolymer formed by high-pressure or low-pressure polymerization of ethylene and a small amount of high-grade ¦Á-olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene-1, etc.) under the action of a catalyst, with a density between 0.915 and 0.940 g/cm3. However, according to ASTM D-1248-84, the density range of 0.926 to 0.940 g/cm3 belongs to medium-density polyethylene (MDPE). The new generation of LLDPE expands its density to plastomers (0.890 to 0.915 g/cm3) and elastomers (<0.890 g/cm3). However, the American Plastic Industry Association (SPI) and the American Plastic Industry Council (APC) only expand the scope of LLDPE to plastomers, excluding elastomers. In the 1980s, Union Carbide and Dow Chemical called their early plastomers and elastomers very low density polyethylene (VLDPE) and ultra low density polyethylene (ULDPE) resins.
The molecular structure of conventional LLDPE is characterized by its linear main chain, with only a small amount or no long chain branches, but contains some short chain branches. The absence of long chain branches makes the polymer more crystalline.
Usually, LLDPE resins are characterized by density and melt index. Density is determined by the concentration of comonomers in the polymer chain. The concentration of comonomers determines the amount of short chain branches in the polymer. The length of the short chain branches depends on the type of comonomer. The higher the comonomer concentration, the lower the density of the resin. In addition, the melt index is a reflection of the average molecular weight of the resin, which is mainly determined by the reaction temperature (solution method) and the addition of chain transfer agents (gas phase method). The average molecular weight has nothing to do with the molecular weight distribution, which is mainly affected by the type of catalyst.
LLDPE was industrialized by Union Carbide in the 1970s. It represents a major change in polyethylene catalysts and process technology, which significantly expands the product range of polyethylene. LLDPE replaces free radical initiators with coordination catalysts, and replaces high-cost high-pressure reactors with lower-cost low-pressure gas phase polymerization. In a relatively short period of time, it has replaced LDPE in many fields with its excellent performance and low cost. At present, LLDPE has penetrated almost all traditional polyethylene markets, including films, moldings, pipes and wires and cables.
LLDPE products are non-toxic, tasteless, odorless, and are milky white particles. Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance, cold resistance, etc. It also has good resistance to environmental stress cracking, tear strength and other properties, and is resistant to acids, alkalis, organic solvents, etc.
In 2005, China's LLDPE production was 1.88 million tons, accounting for about 35.5% of the total PE production; consumption was 3.55 million tons, accounting for about 33.8% of the total PE consumption. It is expected that in the next 2 to 3 years, LLDPE consumption will continue to grow at a rate of about 8%. According to the current market price of 12,000 yuan/ton, the market size of LLDPE in China has exceeded 40 billion yuan.
Application areas of LLDPE
The main application areas of LLDPE are agricultural film, packaging film, wire and cable, pipe, coating products, etc.
Linear low-density polyethylene is mainly used to make films due to its high tensile strength, good puncture resistance and tear resistance. In 2005, the world's LLDPE consumption was 16.17 million tons, a year-on-year increase of 6.4%. In the consumption structure, film products still account for the largest proportion, with a consumption of 11.9 million tons, accounting for 73.6% of the total consumption, followed by injection molding, with a consumption of 1.148 million tons, accounting for about 7.1% of the total LLDPE consumption.
In 2005, China's total consumption of LLDPE and LDPE was 5.98 million tons, of which LLDPE consumption was 3.55 million tons, up 25.4% year-on-year, accounting for 59.4% of the total LLDPE/LDPE consumption; LDPE consumption was 2.43 million tons, up 0.7% year-on-year, accounting for 40.6% of the total LLDPE/LDPE consumption.
From the consumption structure of LLDPE/LDPE, film is still the largest variety of consumption, with a consumption of 4.85 million tons, accounting for 77.5% of the total LLDPE/LDPE consumption, of which packaging film was 3.13 million tons, accounting for 50% of the total consumption; agricultural film was 1.345 million tons, accounting for 22.5% of the total consumption; special packaging film was 376,000 tons, accounting for 6% of the total consumption. The second largest category is injection molding products, with a consumption of 557,000 tons, accounting for 8.9% of the total consumption. The third largest category is coating products, pipes and wires and cables, with consumption of 313,000 tons, 188,000 tons and 157,000 tons, accounting for 5%, 3% and 2.5% of the total consumption respectively; the other categories are 188,000 tons, accounting for 3% of the total consumption.
From the consumption of LLDPE/LDPE from 2003 to 2005, the consumption proportion of film has remained at around 77%, and the consumption proportion of the second largest category, injection molding products, has also been hovering around 9%. It is expected that in the next 2 to 3 years, although the absolute consumption of various categories will continue to grow, their consumption proportions will basically maintain the current trend; due to the relatively rapid growth in demand for packaging films, the consumption proportion of agricultural films will drop to around 20%. As the performance of LLDPE continues to improve and its application areas continue to expand, the future market demand for LLDPE will grow much faster than LDPE and HDPE.
Classification of LLDPE
According to the type of comonomer, LLDPE is mainly divided into three types of copolymers: C4 (butene-1), C6 (hexene-1) and C8 (octene-1). Among them, butene copolymer is the LLDPE resin with the largest production volume in the world, while hexene copolymer is the fastest growing LLDPE variety. In LLDPE resin, the typical dosage of comonomer is 5% to 10% by weight, and the average dosage is about 7%. Metallocene-based LLDPE plastomers (mLLDPE) have an average comonomer content that is more than 3 times that of traditional LLDPE. Figure 1 shows the world's production of three types of comonomer LLDPE in the past 10 years, quoted from foreign publications.
Chart 1: World production of three comonomer LLDPE in the 10 years from 1997 to 2007
(Chart description: Butene: butene; Hexene: hexene; Octene: octene)
At the end of 1984, Union Carbide introduced the production of hexene copolymer LLDPE, followed by Exxon, Mobil and other companies. Dow Chemical (Dow Chemical Company) almost entirely uses octene as a comonomer in its low-pressure solution process, and Canada's NOVA (Nova Chemical) also mostly uses octene in its medium-pressure solution process. Octene copolymer LLDPE resin has slightly better strength, tear resistance and processing properties, while the performance of hexene copolymer and octene copolymer resins is not much different. At present, the main manufacturers of hexene LLDPE resins are ExxonMobil Chemical (ExxonMobil Chemical Company), Eastman Chemical (Eastman Chemical Company), Equistar (Equistar Company) and Chevron Phillips (Chevron Phillips Chemical Company). In addition, Dow Chemical, Basell, Innovene, Samsung Total and others also produce hexene LLDPE.
Compared with the commonly used butene comonomer, LLDPE produced with hexene and octene as comonomers has better performance. The biggest use of LLDPE resin is in the production of films. Films and products made of LLDPE resin produced with long-chain ¦Á-olefins (such as hexene and octene) as comonomers are superior to LLDPE resin produced with butene as comonomer in many aspects such as tensile strength, impact strength, tear strength, puncture resistance, and environmental stress cracking resistance. Since the 1990s, foreign PE manufacturers and users have tended to replace butene with hexene and octene. It is reported that using octene as a comonomer may not necessarily improve the resin performance more than hexene copolymerization, and the price is more expensive. Therefore, the current trend of major foreign LLDPE manufacturers using hexene to replace butene is more obvious.
At present, since China has not yet produced hexene and octene on a large scale and the import price is relatively expensive, the LLDPE resin produced in China today mainly uses butene as a comonomer. Although some Chinese companies have brands that use hexene as a comonomer when introducing LLDPE production equipment, they have to give up in the end because there is no hexene production in China, and only import a small amount of hexene during the start-up assessment. Most of the high-end LLDPE imported by China are of this type. It is expected that the demand for LLDPE with 1-hexene as a monomer will increase significantly in the future.
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