Introduction to High Density Polyethylene (HDPE) Knowledge

Introduction to High Density Polyethylene (HDPE)
HDPE is a thermoplastic polyolefin produced by copolymerization of ethylene.
. Although HDPE was introduced in 1956, this plastic has not yet reached a mature level. This universal material is constantly developing its new uses and markets. The most common production methods for PE and catalyst are through slurry or gas-phase processing, with a few also using solution phase processing for production. All of these processing steps involve exothermic reactions involving ethylene monomers, alpha 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. A slurry reactor is generally a stirred tank or a more commonly used large annular reactor, in which the slurry can be circulated and stirred. When ethylene and comonomers (as needed) come into contact with the catalyst, polyethylene particles are formed. After removing the diluent, polyethylene particles or powder particles are dried and additives are added according to the dosage to produce pellets. A modern production line with a large reactor equipped with a twin-screw extruder can produce over 40000 pounds of PE per hour. The development of new catalysts contributes to improving the performance of new grades of HDPE. The two most commonly used types of catalysts are Phillips' chromium oxide based catalyst and titanium compound monoalkyl aluminum catalyst. The HDPE produced by Phillips type catalyst has a medium width molecular weight distribution; The molecular weight distribution of titanium alkyl aluminum catalyst production is narrow. The catalyst used for producing narrow MDW polymers using a composite 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 with a wide range of molecular weight distributions. Main characteristics: HDPE is a highly crystalline and non-polar thermoplastic resin. The original appearance of HDPE is milky white, with a certain degree of semi transparency in the thin section. PE has excellent resistance to most household 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). This polymer is non hygroscopic and has good water vapor resistance, making it suitable for packaging purposes. HDPE has excellent electrical properties, especially high insulation dielectric strength, making it very suitable for wires and cables. Medium to high molecular weight grades have excellent impact resistance, even at room temperature and 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; Achieve the optimal balance in performance. Density: This is the main variable that determines the properties of HDPE, although the four variables mentioned do have a mutual influence. Ethylene is the main raw material for polyethylene, and a few other comonomers such as 1-butene, l-monoene, or 1-octene are often used to improve polymer properties. For HDPE, the content of these 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 has a linear relationship with crystallinity. The general classification in the United States is based on ASTM D1248, and the density of HDPE is 0.940g/. C or above; The density range of medium density polyethylene (MDPE) is 0.926 to 0.940g/CC. Other classification methods sometimes classify MDPE as HDPE or LLDPE. Homopolymers have the highest density, maximum stiffness, good impermeability, and 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 stress. Higher density generally improves mechanical strength performance, such as tensile strength, stiffness, and hardness; Thermal properties such as softening point temperature and thermal deformation temperature; Impermeability, such as breathability or water vapor permeability. Lower density improves its impact strength and E-SCR. The density of polymers is mainly influenced by the addition of comonomers, but to a lesser extent, it is also affected by molecular weight. The percentage of high molecular weight slightly reduces the density. For example, homopolymers have different densities within a wide range of molecular weights. Molecular weight (MW): Higher molecular weight results in higher polymer viscosity, but viscosity is also dependent on the temperature and shear rate used in the test. Characterize the molecular weight of materials using rheological or molecular weight measurements. The grade of HDPE generally has a molecular weight range of 40000 to 300000, and the weight average molecular weight roughly corresponds to the melt index range, which is from 100 to 0.029 per 10 minutes. Generally, higher MW (lower melt index MI) enhances melt strength, better toughness, and ESCR, but higher MW makes the machining process more difficult or requires higher pressure or temperature. Molecular Weight Distribution (MWD): The WD of PE varies from narrow to wide depending on the catalyst used and the processing procedure. The most commonly used MWD measurement index is the non-uniformity index (HI), which is equal to the weight average molecular weight (MW) divided by the number average molecular weight (Mn). The index range for all HDPE grades is 4-30. Narrow MWD provides low warpage and high impact during the molding process. 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 can prevent the degradation of polymers during processing and prevent oxidation of finished products during use. Antistatic additives are used in many packaging grades to reduce the adhesion of bottles or packaging materials to dust and dirt. Specific applications require special additive formulations, such as copper inhibitors related to wire and cable applications. Excellent weather resistance and UV (or sunlight) resistance can be achieved by adding UV resistant 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 be frequently used outdoors, such as in wires, cables, cable trays, or pipes. Processing and application: PE can be manufactured using a wide range of different processing methods. This includes processes such as sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding, and roll molding. Extrusion: The grades used for extrusion production generally have a melt index of less than 1 and a medium to wide MWD. During the processing, a low MI can achieve suitable melt strength. Wider MWD grades are more suitable for extrusion molding because they have higher production speeds, lower die pressures, and a reduced tendency for melt fracture. PE has many extrusion applications, such as wires, electricity, hoses, pipes, and profiles. The application range of pipe materials ranges from small section yellow pipes for natural gas to thick walled black pipes with a diameter of 48 inches for industrial and urban pipelines. The use of large-diameter hollow wall pipes as substitutes for concrete rainwater drainage pipes and other sewer pipelines is growing rapidly. Sheet metal and thermoforming: Many large picnic refrigerators have thermoformed liners made of PE, which is tough, lightweight, and durable. Other sheet materials and thermoformed products include mudguards, tank liners, basin guards, transport boxes, and tanks. A rapidly growing application of sheet materials is in plastic film or pond bottom villages, 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 purposes. These range from bottles containing bleach, engine oil, detergent, milk, and distilled water to large refrigerators, car fuel tanks, and cans. The characteristic indicators 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 manufacture smaller containers (less than 16oz) for packaging drugs, shampoo, and cosmetics. One advantage of this processing is that the production of bottles automatically removes the edges and corners, without the need for post-processing steps like typical blow molding. Although some narrow MWD grades are used to improve surface finish, medium to wide MWD grades are generally used.

Injection molding: HDPE has countless applications, ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 1/5 of domestically produced HDPE. The injection molding grade generally has a melt index of 5-10, with lower toughness and higher workability grades. Uses include thin-walled packaging for daily necessities and food; Resilient and durable food and paint cans; High resistance to environmental stress cracking applications, such as small engine fuel tanks and 90 gal garbage cans. Rolling molding: Materials processed using this method are generally crushed into powder and melted and flowed during thermal cycling. Roll molding uses two types of PE: universal and crosslinkable. General grade MDPE/HDPE typically has a density range of 0.935 to 0.945g/CC, with a narrow MWD that gives the product high impact and minimal warpage. Its melt index typically ranges from 3 to 8. Higher MI grades are usually not suitable as they do not possess the expected impact and environmental stress cracking resistance of rotational molded products. The high-performance rotational molding application utilizes the unique properties of its chemically crosslinkable grades. These grades have good flowability in the first stage of the molding cycle, and then crosslink to form their excellent resistance to environmental stress cracking and toughness. Wear resistance and weather resistance. Cross linked PE is only suitable for large containers, ranging from 500 gal storage tanks for transporting various chemicals to 20000 gal agricultural storage tanks. Film: PE film processing generally uses ordinary blown film processing or flat extrusion processing method. Most PE is used for thin films, and general-purpose low-density PE (LDPE) or linear low-density PE (LLDPE) can be used. HDPE film grade is generally used in places that require superior stretchability and excellent impermeability. For example, HDPE film is commonly used in commodity bags, grocery bags, and food packaging. Recycling HDPE is the fastest-growing part of the plastic recycling market. This is mainly because it is easy to reprocess, has minimal degradation characteristics, and has a wide range of applications in packaging. The main recycling process involves reprocessing 25% of recycled materials, such as post consumer recycled products (PCR), with pure HDPE to produce bottles that do not come into contact with food.