What is HDPE?
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; its wear resistance, electrical insulation, toughness and cold resistance are better; its chemical stability is good, and it 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; its aging resistance is poor, 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.
Material Properties
[Basic properties] High-density polyethylene is an opaque white waxy material with a specific gravity of 0.941~0.960, which is lighter than water. 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 leaving the fire. The upper end of the flame is yellow and the lower end is blue. It will melt when burning, and there will be liquid dripping. There is no black smoke. At the same time, it emits the smell of burning paraffin.
[Main advantages] It is acid and alkali resistant, resistant to organic solvents, has 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, close to PP, 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, lower brittleness than PP, easy to stress crack, low surface hardness, and easy to scratch. Difficult to print. When printing, surface discharge treatment is required. Electroplating is not allowed, 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 process】HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 1/5 of the domestically produced HDPE. Injection molding grades generally have a melt index of 5 to 10, with tough lower fluidity grades and processable higher fluidity grades. Uses include daily necessities and thin-walled food packaging; tough, durable food and paint cans; high environmental stress cracking resistance applications, such as small engine fuel tanks and 90-gal garbage cans.
The general melting point of HDPE is 142°C, and the decomposition temperature is 300°C; the adjustable range of injection molding temperature is large. During injection molding, the general 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 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.
Dielectric properties and environmental stress cracking resistance are also good. Hardness, tensile strength and creep are better than low-density polyethylene; wear resistance, electrical insulation, toughness and cold resistance are all good, but slightly worse than low-density insulation; chemical stability is good, insoluble in any organic solvent at room temperature, resistant to acid, alkali and various salts; film has low permeability to water vapor and air and low water absorption; poor aging resistance, environmental cracking resistance is not as good as low-density polyethylene, especially thermal oxidation will reduce its performance, so the resin needs to add antioxidants and ultraviolet absorbers to improve this aspect.
Production Process
PE is most commonly produced by slurry or gas phase processing, and a few are produced by solution phase processing. All of these processes are exothermic reactions involving ethylene monomer, α-olefin monomer, catalyst system (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 a more commonly used large-scale ring reactor in which the slurry can be circulated and stirred.
When ethylene and comonomer (as needed) and catalyst come into contact, polyethylene particles are formed. After removing the diluent, the polyethylene particles or powders are dried and additives are added according to the dosage to produce pellets. Modern production lines with large reactors with twin-screw extruders can produce more than 40,000 pounds of PE per hour.
Processing Methods
1.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 wire, cable, hose, pipe 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 storm water drains and other sewer lines.
Sheets and thermoforming: The thermoformed linings of many large picnic-type coolers are made of PE, which is tough, light and durable. Other sheet and thermoformed products include fenders, tank linings, pan shields, shipping boxes and tanks. A large and rapidly growing sheet application is ground film or pool bottom village, which is based on the toughness, chemical resistance and impermeability of MDPE.
2.Roto-molding: Materials used in this process are generally crushed into powders that melt and flow during thermal cycles. Roto-molding uses two types of PE: general-purpose and cross-linkable. General-purpose MDPE/HDPE typically has a density range of 0.935 to 0.945 g/CC, with a narrow MWD that gives 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.
3.Film: PE film processing is generally done by ordinary blown film processing or flat extrusion processing. Most PE is used for film, and general-purpose 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.
Main uses
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, wire and cable sheathing, etc.
In addition, it can also be used to form architectural decorative panels, blinds, synthetic wood, synthetic paper, synthetic films and molded calcium plastic products.
Recycling
HDPE is the fastest growing part of the plastic raw material 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% of recycled materials, such as post-consumer recyclate (PCR), with pure HDPE to make bottles that do not come into contact with food.
