High Density Polyethylene (HDPE)

High Density Polyethylene (HDPE)

August 19, 2024

wanplas

High-density polyethylene (HDPE) is a white powder or granular product. It is non-toxic, odorless, with a crystallinity of 80% to 90%, a softening point of 125 to 135°C, and an operating temperature of up to 100°C; its hardness, tensile strength and creep are better than those of low-density polyethylene; it has good wear resistance, electrical insulation, toughness and cold resistance; it has good chemical stability, is insoluble in any organic solvent at room temperature, and is resistant to corrosion by acids, alkalis and various salts; the film has low permeability to water vapor and air, and low water absorption; it has poor aging resistance, and its resistance to environmental stress cracking is not as good as that of low-density polyethylene, especially thermal oxidation will reduce its performance, so antioxidants and ultraviolet absorbers must be added to the resin to improve this aspect. The heat deformation temperature of high-density polyethylene film is low under stress, so you should pay attention when using it.

History

This century has seen a revolutionary progress in the field of pipelines, namely, “plastics replacing steel”. With the rapid progress of polymer material science and technology, the deepening of the development and utilization of plastic pipes, and the continuous improvement of production processes, plastic pipes have fully demonstrated their excellent performance. Today, plastic pipes are no longer mistaken as “cheap substitutes” for metal pipes. In this revolution, polyethylene pipes are highly favored and increasingly shining, and are widely used in gas transportation, water supply, sewage discharge, agricultural irrigation, mining fine particle solid transportation, as well as oil fields, chemicals, and post and telecommunications, especially in gas transportation.

HDPE is a thermoplastic polyolefin produced by copolymerization of ethylene. Although HDPE was introduced in 1956, this plastic has not yet reached maturity. This versatile material is still finding new uses and markets.

The chinese producers of high-density polyethylene (HDPE here does not include HDPE produced by full-density polyethylene units) are three major enterprises: CNPC, Sinopec and CNOOC. By the end of 2006, there were four HDPE units belonging to CNPC, namely, the Lanzhou Petrochemical HDPE unit, the Daqing Petrochemical HDPE unit, the Liaoyang Petrochemical HDPE unit and the Jilin Petrochemical HDPE unit.

High-density polyethylene is usually produced using the Ziegler-Natta polymerization process, which is characterized by the absence of branched chains on the molecular chain, so the molecular chain is arranged regularly and has a higher density. This process uses ethylene as the raw material and oxygen or organic peroxide as the initiator in a tubular or kettle low-pressure reactor to initiate the polymerization reaction.

High-density ethylene is an environmentally friendly material. It can be recycled and reused when heated to its melting point. It should be noted that plastic raw materials can be divided into two categories: “thermoplastic” and “thermosetting”. “Thermosetting plastic” becomes a solid state after being heated to a certain temperature, and its state cannot be changed even if it is heated further. Therefore, products with environmental problems are “thermosetting plastic” products (such as tires), not “thermoplastic plastic” products (such as plastic pallets Note: Pallets are called “plywood” in Hong Kong and Macau), so not all “plastics” are not environmentally friendly.

Material properties

Full English name: High Density Polyethylene

Abbreviation: HDPE

Common name: Low-pressure ethylene

Monomer: Ethylene

Basic Characteristics: High-density polyethylene is an opaque white waxy material, lighter than water, with a specific gravity of 0.941~0.960. It is soft and tough, but slightly harder than LDPE and slightly stretchable. It is non-toxic and odorless.

Combustion Characteristics: It is flammable and can continue to burn after being away from the fire. The upper end of the flame is yellow and the lower end is blue. It will melt when burning, and liquid will drip without emitting black smoke. At the same time, it will emit the smell of burning paraffin.

Main advantages: Acid and alkali resistance, organic solvent resistance, excellent electrical insulation, and can still maintain a certain toughness at low temperatures. The mechanical strengths such as surface hardness, tensile strength, and rigidity are higher than LDPE and close to PP, but tougher than PP, but the surface finish is not as good as PP.

Main Disadvantages: Poor mechanical properties, poor air permeability, easy to deform, easy to age, easy to become brittle, brittleness is lower than PP, easy to stress crack, low surface hardness, easy to scratch. Difficult to print, surface discharge treatment is required when printing, cannot be electroplated, and the surface is dull.

Applications: Used for extrusion of packaging films, ropes, woven bags, fishing nets, water pipes; injection molding of low-end daily necessities and shells, non-load-bearing components, plastic boxes, turnover boxes; extrusion blow molding containers, hollow products, bottles.

Injection Molding: HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 1/5 of the HDPE produced by china. Injection molding grades generally have a melt index of 5 to 10, with tough lower fluidity grades and processable higher fluidity grades. Uses include thin-walled 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.

The general melting point of HDPE is 142℃, and the decomposition temperature is 300℃; the adjustable range of injection molding temperature is relatively large. During injection molding, the general use temperature is 180℃-230℃; because it is an olefin plastic, it does not absorb water, and does not need to be dried during production, but for the sake of product quality, it can be dried at 60℃ for 1 hour to remove floating water; polyethylene has a high melt viscosity and a small flow length ratio, and thin-walled products may lack glue, so the gate and runner are relatively large; the product is prone to static electricity and the surface is prone to absorb dust. The shrinkage rate is 16‰, and the overflow value is 0.05mm.

Features

High-density polyethylene has good heat resistance and cold resistance, good chemical stability, high rigidity and toughness, and good mechanical strength. It also has good dielectric properties and resistance to environmental stress cracking. Its hardness, tensile strength and creep are better than those of low-density polyethylene; its wear resistance, electrical insulation, toughness and cold resistance are all good, but slightly worse than low-density polyethylene in insulation; it has good chemical stability and is insoluble in any organic solvent at room temperature. It is resistant to corrosion by acids, alkalis and various salts; the film has low permeability to water vapor and air and low water absorption; its aging resistance is poor and its resistance to environmental cracking is not as good as that of low-density polyethylene. In particular, thermal oxidation will reduce its performance. Therefore, the resin needs to be added with antioxidants and ultraviolet absorbers to improve this aspect. The heat deformation temperature of high-density polyethylene film under stress is low, so this should be noted when applying it.

Production process

PE is most commonly produced by slurry or gas phase processing, with a few being produced by solution phase processing. All of these processes involve exothermic reactions involving ethylene monomer, α-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. Slurry reactors are generally stirred tanks or, more commonly, large ring reactors in which the slurry can be circulated and stirred. Polyethylene particles are formed as soon as ethylene and comonomer (as needed) come into contact with the catalyst. After the diluent is removed, the polyethylene particles or powders are dried and additives are added in doses 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 Philips’ chromium oxide-based catalysts and titanium compound-alkyl aluminum catalysts. Philips-type catalysts produce HDPE with a medium-broad molecular weight distribution; titanium-alkyl aluminum catalysts produce narrow molecular weight distributions. Catalysts used to produce narrow MDW polymers in a duplex reactor can also be used to produce broad MDW grades. For example, two reactors in series producing significantly different molecular weight products 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. Rheological or molecular weight measurements are used to characterize the molecular weight of a material. HDPE grades generally have a molecular weight range of 40,000 to 300,000, with a weight average molecular weight that roughly corresponds to the melt index range of 100 to 0.029 g/10 min (230°C, 2.16 kg). In general, higher Mw (lower melt index MI) increases melt strength, better toughness and ESCR, but higher Mw makes processing more difficult.

The process is more difficult or requires higher pressure or temperature.

Molecular Weight Distribution (MWD): The WD of PE varies from narrow to broad depending on the catalyst used and the processing.

The most commonly used MWD measurement index 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.

Additive

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 wires, cables, tank layers or pipes.

Processing methods

PE can be manufactured by 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 is used under the action of a catalyst. The resulting polymer is flashed, separated, dried, granulated, and other processes to obtain a finished product with uniform particles. These include sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding, and rotational molding. HDPE is suitable for various molding processes of thermoplastic molding, and has good molding processability, such as injection molding, extrusion, blow molding, rotational molding, coating, foaming process, thermoforming, heat sealing welding, heat welding, etc.

Extrusion: Grades used for extrusion production generally have a melt index of less than 1 and a medium to wide MWD. During processing, low MI can achieve 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. Pipe applications range from small-section yellow pipes for natural gas to thick-walled black pipes for industrial and urban pipelines. Large-diameter hollow-wall pipes are growing rapidly as a substitute for concrete-made stormwater drains and other sewer lines.

Sheet 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 membrane or pool 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. 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 commonly used to make smaller containers (less than 16 oz) for packaging medicines, shampoos and cosmetics. One advantage of this process is that the bottles are automatically deburred, without the need for post-processing finishing steps like conventional blow molding. Although there are some narrow MWD grades for improved surface finish, medium to wide MWD grades are generally used.

Injection molding: See “Material properties” above.

Roto-molding: Materials for this process are usually crushed into powders that melt and flow in a heat cycle. Roto-molding uses two types of PE: 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 usually not suitable because they do not have the impact and environmental stress cracking resistance desired for roto-molded products.

Film: PE film processing generally uses 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) can be used. HDPE film grade is generally used in places requiring superior stretchability and excellent impermeability. For example, HDPE film is often used in commodity bags, grocery bags and food packaging.

Main Applications

High-density polyethylene resin can be used to form plastic products by injection, extrusion, blow molding and rotational molding. Injection molding can be used to form various types of containers, industrial accessories, medical products, toys, shells, bottle stoppers and shields. Blow molding can be used to form various hollow containers, ultra-thin films, etc. Extrusion molding can be used to form pipes, stretch strips, strapping tapes, monofilaments, wires and cable sheaths, etc.

In addition, it can also be used to form building decorative panels, blinds, synthetic wood, synthetic paper, synthetic films and formed calcium plastic products.

Packaging, storage and transportation

When storing, it should be kept away from fire sources, insulated, and kept dry and tidy in the warehouse. It is strictly forbidden to mix any impurities, and it is strictly forbidden to be exposed to the sun and rain. It should be stored in a clean, dry, roofed carriage or cabin during transportation, 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. For example, the four-liter barrels of Nongfu Spring’s mineral water are made of this material.

Recycling

HDPE is the fastest growing segment of the plastics recycling market. This is mainly due to its ease of reprocessing, minimal degradation and its wide application in packaging. The main recycling is the use of 25% recycled materials, such as post-consumer recyclate (PCR), with virgin HDPE for the manufacture of bottles that do not come into contact with food.

In this process, the polymerization solvent is n-hexane, the catalyst is a highly active zN catalyst, ethylene and hydrogen are mixed and then enter the first reactor, where they are mixed with the catalyst for polymerization reaction. The polymer in the reactor is suspended in hexane in the form of slurry, 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 0.942-0.965 g/cm3 and a melt index range of 0.2-80. The comonomers are propylene and 1-butene, and traditional HDPE and bimodal HDPE are produced. The high-density pipe has excellent performance and is suitable for making pressure pipes, reaching PE100+. The characteristics of the slurry method kettle reactor continuous polymerization process are: low operating pressure and operating temperature; the double kettle reactor can produce single-peak and bimodal products by adopting different forms of parallel and series connection; the process operation is highly flexible, the product brand conversion is fast, and the raw material purity requirement is not high; the comonomers are propylene and 1-butene; hexane is used as a solvent, and the recovery unit is simple. The characteristic of this process is that the polymerization is carried out in an inert hydrocarbon diluent. The process flow is as follows: polymerized ethylene (ethylene 99.9%, ethane 0.1%) is fed into a dryer and then fed into a kettle reactor together with a circulating diluent consisting of n-hexane. The catalyst is a catalyst containing titanium, manganese, triethylaluminum on a carrier. It is the brand of the co-catalyst. A small amount of hydrogen is added to control the molecular weight. The polymerization reaction forms polyethylene particles. The reaction temperature is 90℃ and the pressure is 1.8MPa. The reaction can be carried out in two steps in two polymerization kettles. The concentration of the produced slurry solid is 34% (mass fraction), and the monomer conversion rate can reach 97%. The polymer flows out of the second reactor and flashes to a pressure of 0.14MPa. The unreacted ethylene, ethane in the discharge and 2% of the cyclohexane diluent are compressed and cooled to 2.5MPa twice. The degassing tower recovers the ethane for recycling. The slurry left after the flash is centrifuged to recover most of the diluent, and the solid filter cake is sent to the 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 the fluidized bed to remove all hydrocarbon diluents. The dried polymer particles are sent to the mixing section to add various additives and then granulated. (2) Loop reactor process The typical representatives of the loop reactor process are the Phillips process of Phillips Company and the Innovene S process of INEOS Company. The Phillips process uses isobutane as a diluent and a chromium-based catalyst. The catalyst must be activated before use. The activated catalyst powder is mixed with high-purity isobutane under nitrogen protection to form a catalyst slurry, and then enters the loop reactor. The raw material ethylene monomer is refined, pre-mixed with hydrogen and comonomer hexene-1, and then injected into the loop reactor. Ethylene is converted into polyethylene under the action of the catalyst. The axial flow pump maintains the high-speed flow and very uniform mixing of the materials in the reactor, and the reaction heat is evenly withdrawn by the jacket cooling water. This process produces MI in the range of 0.15-100 and a density of 0.936-0.972g/cm3. The characteristics of the loop reactor process are: less equipment, short process, low investment cost; no wax and polymers are produced, no wall sticking; good powder shape, easy to transport; reaction heat is removed by cooling water in the reactor jacket, easy to remove heat, convenient adjustment; high requirements for raw materials, need to be purified; hexene is used as the comonomer; isobutane is used as a solvent, it is easy to remove residual solvent. The process flow is as follows: fresh polymerization-grade ethylene is dried, mixed with the molecular weight regulator hydrogen, antifreeze and circulating diluent isobutane, and then sent to a multi-loop continuous process reactor, and the catalyst supplement isobutane is filled into the reactor. The reaction temperature is 106.7℃ and the pressure is 3.9MPa. The polymer and diluent slurry pass through the loop reactor at a speed of 6m/s with the help of an axial flow pump. The water cooling in the reactor jacket controls the reaction temperature, and the polymer solid is discharged from the vertical sedimentation port in the loop reactor. As a result, the slurry concentration can reach 55% and the conversion rate is 98%-99%. After the polymer is discharged, it is flashed 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 typical representatives of gas phase polymerization (gas phase fluidized bed method) are the univation technology of DOW Chemical Company and the Innovene technology of INNOS Company. The univation technology adopts a low-pressure gas phase fluidized bed reactor, ZN catalyst and chromium catalyst, and the purified raw materials are injected into the reactor. The polymerization reaction occurs under the action of the catalyst storage. The reaction is carried out under the conditions of 85-110℃ and 2.41 MPa. The single-pass conversion rate of ethylene is about 1%-2%. The removal of reaction heat is mainly through the cooling of the circulating flow. The MI range of the production product is 0.01-150, and the density range is 0.915-0.970 g/cm3. The characteristics of the gas phase fluidized bed polymerization process are: low operating pressure and low temperature; can produce full-density polyethylene; catalyst system includes titanium and chromium; metallocene catalyst; high requirements for raw material purity, all raw materials must be refined; no solvent is required, low energy consumption, low maintenance and operation 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 enter the bottom of the large fluidized bed reactor through gas distribution. According to the design, the reactor raw materials are 69.57% ethylene (ethylene content is 99.9%, 0.1% is ethane), 10.43% hydrogen, 7.56% ethane and 12.44% nitrogen. The product produced by this raw gas composition has a melt index of 8g/10min and a viscosity of 0.964g/cm3.The density of 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 solid form together with nitrogen from the reactor part. The operating temperature is 105°C, and the specific temperature is determined according to the product brand. The operating pressure of the reactor is 2.0MPa. The reaction gas comes out from the top of the reaction and is separated from the catalyst containing solid matter by a cyclone separator and sent back to the reactor. Then the gas coming out of the cyclone separator is compressed and circulated to the bottom of the reactor after the circulating cooler. The reactor discharge intermittently sends the product particles to the material 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 from the lower part of the discharge tank and enters the purge tank and post-processing system. The post-processing system includes adding various additives to the polymer, melting, granulation and packaging.