HDPE, LDPE and LLDPE resin material performance comparison

HDPE, LDPE and LLDPE resin material performance comparison

Production materials, molecular structure, density, crystallinity, softening point, corrosion resistance, temperature range, mechanical properties, tensile strength, elongation at break, resistance to environmental stress cracking of three resin materials of HDPE, LDPE and LLDPE See Table 1) and industrial production principles, processes and additives to analyze the engineering application scope of the three materials and their respective application characteristics and applicable engineering scope.

Polyethylene (Polyethylene) is one of the five major synthetic resins. It is the variety with the largest production capacity and the largest import volume of synthetic resins in China. At present, China is already the world's largest importer of polyethylene and the second largest consumer. Polyethylene is polymerized by vinyl monomer. Polyethylene plastic is a plastic product made by adding polyethylene resin as a base material and adding a small amount of antioxidants, slip agents and other additives. Polyethylene is mainly divided into three categories: linear low density polyethylene (LLDPE), low density polyethylene (LDPE), and high density polyethylene (HDPE).

1. High-density polyethylene, the English name is "High Density Polyethylene", referred to as "HDPE". HDPE is non-toxic, odorless, and odorless, with a density of 0.940 to 0.976g / cm3. It is a product of polymerization under low-pressure conditions catalyzed by Ziegler catalyst, so high-density polyethylene also becomes low-pressure polyethylene. HDPE is a non-polar thermoplastic resin produced by copolymerization of ethylene. The appearance of the original HDPE is milky white, with a certain degree of translucency in the thin section. It has excellent resistance to most household and industrial chemicals. It can resist corrosion and dissolution of strong oxidants (concentrated nitric acid), acid and alkali salts, and organic solvents (carbon tetrachloride). The polymer is non-hygroscopic and has good water vapor resistance, and can be used for moisture and impermeability.

The disadvantage is that its aging resistance and environmental stress cracking are not as good as LDPE, especially thermal oxidation will reduce its performance, so high-density polyethylene is added with antioxidants and ultraviolet absorbers to improve its performance when it is made into plastic coils. Deficiencies.

2. Low density polyethylene, referred to as "LDPE" for short. LDPE is non-toxic, odorless, and odorless, and its density is 0.910 ～ 0.940g / cm3. It is polymerized by using oxygen or organic peroxide as a catalyst under high pressure of 100 ～ 300MPa. It also becomes high-pressure polyethylene. Low density polyethylene is the lightest variety among polyethylene resins. Compared with high-density polyethylene, its crystallinity (55% ～ 65%) and softening point (90 ～ 100 ℃) are lower; it has good flexibility, extensibility, transparency, cold resistance and processability; its chemistry Good stability, resistant to acid, alkali and salt aqueous solutions; good electrical insulation and breathability; low water absorption; easy to burn. The properties are relatively soft, with good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance (withstands -70 ℃).

The disadvantage is its poor mechanical strength, moisture barrier, gas barrier and solvent resistance. The molecular structure is not regular enough, the crystallinity (55% -65%) is low, and the crystal melting point (108-126 ° C) is also low. Its mechanical strength is lower than that of high-density polyethylene, and it has poor impermeability, heat resistance and sunlight aging resistance. It is easy to aging and decompose under sunlight or high temperature and change color, resulting in reduced performance. Therefore, when low-density polyethylene is made into plastic coils Antioxidants and UV absorbers were added to improve its deficiencies.

3. Linear low density polyethylene, referred to as "LLDPE" for short. LLDPE is non-toxic, tasteless and odorless, with a density between 0.915 and 0.935g / cm3. It is ethylene and a small amount of higher alpha-olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene) -1 etc.) Under the action of a catalyst, a copolymer made by high-pressure or low-pressure polymerization. The molecular structure of conventional LLDPE is characterized by its linear main chain, with few or no long branches, but containing some short branches. The absence of long branches makes the polymer more crystalline. Compared with LDPE, LLDPE has the advantages of high strength, good toughness, strong rigidity, heat resistance and cold resistance. It also has good resistance to environmental stress cracking and tear strength, and is resistant to acids, alkalis and organic solvents.

Based on the above analysis, the above three materials have their respective important tasks in different types of anti-seepage projects. The three materials HDPE, LDPE and LLDPE have good insulation, moisture-proof, anti-seepage performance, non-toxic, odorless, and odorless performance, which makes their application in agriculture, aquaculture, artificial lakes, reservoirs, rivers extremely It is widely used and promoted and applied widely by the Fishery Bureau of the Ministry of Agriculture of China, the Shanghai Academy of Fishery Sciences, and the Institute of Fishery Machinery and Instruments.

In the medium environment of strong acid, strong base, strong oxidant and organic solvent, the material properties of HDPE and LLDPE can be well exerted and utilized, especially the characteristics of HDPE in resistance to strong acid, strong alkali, strong oxidation and organic solvent The aspect is far higher than the other two materials, so HDPE anti-seepage and anti-corrosion coils are fully utilized in the chemical and environmental protection industries. The low-density polyethylene also has good acid, alkali, salt solution characteristics, and has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance. Therefore, it is widely used in agriculture, water storage culture, packaging, especially low temperature packaging and cable materials.