Polyethylene is one of the five major synthetic resins. At present, my country is the largest importer and second largest consumer of polyethylene in the world. Polyethylene is mainly divided into three categories: high-density polyethylene (HDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE).
Comparison of HDPE, LDPE and LLDPE material properties
Plastic name |
HDPE |
LDPE |
LLDPE |
Performance comparison |
Low pressure |
High pressure |
|
Odor, toxicity |
Non-toxic, tasteless and odorless |
Non-toxic, tasteless and odorless |
Non-toxic, tasteless and odorless |
Density |
0.940～0.976g／cm3 |
0.910～0.940g/cm3 |
0.915～0.935g/cm3 |
Crystallinity |
85-65% |
45-65% |
55-65% |
Molecular Structure |
Contains only carbon-carbon and carbon-hydrogen bonds, which require more energy to break |
Polymers have smaller molecular weight and require less energy to break |
Linear structure, fewer branches and short chains, requiring less energy to break |
Softening temperature |
125-135℃ |
90-100℃ |
94-108℃ |
Physical properties |
High strength, good toughness, strong rigidity |
Less strong |
High strength, good toughness, strong rigidity |
Ductility |
Low |
Low |
Much high |
Elongation at break |
Much high |
Low |
Low |
Impact resistance |
Much high |
Low |
Low |
Moisture-proof and waterproof performance |
Good permeability to water, water vapor and air, low water absorption, and good anti-permeability |
Poor moisture and air barrier properties |
Good permeability to water, water vapor and air, low water absorption, and good anti-permeability |
Acid, alkali, corrosion, organic solvent resistance |
Resistant to corrosion by strong oxidants; resistant to acid, alkali and various salts; insoluble in any organic solvents, etc. |
Resistant to acid, alkali and salt solution corrosion, but poor solvent resistance |
Resistant to acids, alkalis, and organic solvents |
Heat/cold resistant |
It has good heat resistance and cold resistance, even at normal temperature and even at low temperatures of -40F. It has excellent impact resistance and low temperature embrittlement temperature < -90℃. |
Low heat resistance, low temperature embrittlement temperature <-70℃ |
Good heat and cold resistance, low temperature embrittlement temperature < -90℃ |
Resistant to environmental stress cracking |
Great |
Much great |
Great |
1. High density polyethylene
HDPE is non-toxic, tasteless and odorless, with a density of 0.940~0.976g/cm3. It is a product of polymerization under low-pressure conditions catalyzed by Ziegler catalysts, so high-density polyethylene is also called low-pressure polyethylene.
Advantage:
HDPE is a highly crystalline, non-polar thermoplastic resin produced by the copolymerization of ethylene. The appearance of original HDPE is milky white, and it is translucent to a certain extent in thin sections. It has excellent resistance to most domestic and industrial chemicals. It can resist the 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 waterproof vapor properties, and can be used for moisture-proof and seepage-proof purposes.
Shortcoming:
The disadvantage is that its aging resistance and environmental stress cracking resistance are not as good as those of LDPE. In particular, thermal oxidation will reduce its performance. Therefore, antioxidants and ultraviolet absorbers are added to high-density polyethylene when making plastic coils to improve its performance. shortcomings.
HDPE Spill Pallet
2. Low density polyethylene
LDPE is non-toxic, tasteless and odorless, with a density of 0.910~0.940g/cm3. It is polymerized under high pressure of 100~300MPa using oxygen or organic peroxide as a catalyst. It is also called high-pressure polyethylene. LDPE is generally referred to as PE pipe in the irrigation industry. For example, Huawei PE pipe for irrigation has the advantages of quick installation, no scaling, and easy maintenance, and is widely used in irrigation systems.
Advantage:
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 softness, extensibility, transparency, cold resistance and processability; its chemical It has good stability and can resist acid, alkali and salt aqueous solutions; it has good electrical insulation and breathability; it has low water absorption; it is easy to burn. It is soft in nature and has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance (can withstand -70℃).
Shortcoming:
The disadvantage is that its mechanical strength, moisture barrier, gas barrier and solvent resistance are poor. The molecular structure is not regular enough, the crystallinity (55%-65%) is low, and the crystallization melting point (108-126°C) is also low. Its mechanical strength is lower than that of high-density polyethylene, and its anti-seepage coefficient, heat resistance and anti-sunlight aging resistance are poor. It is easy to age and decompose and discolor under sunlight or high temperature, resulting in a decrease in performance. Therefore, low-density polyethylene is used when making plastic rolls. Antioxidants and UV absorbers are added to improve its deficiencies.
3. Linear low density polyethylene
LLDPE is non-toxic, tasteless and odorless, with a density between 0.915 and 0.935g/cm3. It is made of ethylene and a small amount of higher α-olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene -1, etc.) A copolymer formed by high- or low-pressure polymerization under the action of a catalyst. The molecular structure of conventional LLDPE is characterized by its linear backbone with few or no long-chain branches, but contains some short-chain branches. The absence of long chain 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, tear strength, and resistance to acids, alkalis, organic solvents, etc.
LLDPE IBC Spill Pallet
4. Distinguishing method
LDPE:
Sensory identification: soft to the touch; white and transparent, but with average transparency.
Combustion identification: The upper part of the burning flame is yellow and the lower part is blue; it is smokeless when burning, with the smell of paraffin, melting and dripping, and easy to draw.
LLDPE:
LLDPE can swell when exposed to benzene for a long time, and becomes brittle when exposed to HCL for a long time.
HDPE:
The processing temperature of LDPE is lower, about 160 degrees, and the density is 0.918-0.932 g/cubic centimeter. The processing temperature of HDPE is higher, about 180 degrees, and the density is also higher.
5. Summary
To sum up, the above three materials play important tasks in different types of anti-seepage projects. HDPE, LDPE and LLDPE all have good insulation, moisture-proof and anti-seepage properties. Their non-toxic, tasteless and odorless properties make them widely used in agriculture, aquaculture, artificial lakes, reservoirs and rivers. , and has been vigorously promoted and popularized by the Fisheries Bureau of the Ministry of Agriculture of China, the Shanghai Academy of Fishery Sciences, and the Fishery Machinery and Instrument Research Institute.
In the medium environment of strong acid, strong alkali, strong oxidant and organic solvent, the material properties of HDPE and LLDPE can be well exerted and utilized, especially the properties of HDPE in resistance to strong acid, strong alkali, strong oxidation and resistance to organic solvents. In terms of performance, it is much higher than the other two materials, so HDPE anti-seepage and anti-corrosion membranes have been fully utilized in the chemical industry and environmental protection industries.
LDPE also has good resistance to acid, alkali and salt solutions, and has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance, so it is used in agriculture, aquaculture, packaging, especially It is widely used in low temperature packaging and cable materials.
