1、 High density polyethylene
HDPE is nontoxic, tasteless and odorless, with a density of 0.940-0.976g/cm3. It is a product of polymerization under low pressure catalyzed by Ziegler catalyst. Therefore, HDPE is also called low-pressure polyethylene.
HDPE is a kind of thermoplastic resin with high crystallinity and non-polar formed by ethylene copolymerization. The appearance of the original HDPE is milky white and translucent to a certain extent in the thin section. It has excellent resistance to most domestic and industrial chemicals. It can resist the corrosion and dissolution of strong oxidant (concentrated nitric acid), acid and alkali salt and organic solvent (carbon tetrachloride). The polymer is non hygroscopic and has good water vapor resistance, which can be used for moisture-proof and anti-seepage purposes.
The disadvantage is that the aging resistance and environmental stress cracking resistance of LDPE are not as good as LDPE, especially the thermal oxidation will reduce its performance. Therefore, antioxidant and ultraviolet absorber are added to HDPE to improve its shortcomings.
2、 Low density polyethylene
LDPE is non-toxic, tasteless and odorless, with a density of 0.910-0.940g/cm3. It is polymerized with oxygen or organic peroxide as catalyst under high pressure of 100-300mpa, also known as high-pressure polyethylene.
Low density polyethylene (LDPE) is the lightest type of polyethylene resin. Compared with HDPE, its crystallinity (55% ～ 65%) and softening point (90 ～ 100 ℃) are lower; it has good softness, extensibility, transparency, cold resistance and processability; it has good chemical stability and can resist acid, alkali and salt aqueous solutions; it has good electrical insulation and air permeability; it has low water absorption; it is easy to burn. It has soft properties, good extensibility, electrical insulation, chemical stability, processability and low temperature resistance (resistant to - 70 ℃).
The disadvantage is that the mechanical strength, moisture barrier, gas barrier and solvent resistance are poor. The molecular structure is not regular, the crystallinity (55% - 65%) is low, and the crystallization melting point (108-126 ℃) is also low. The mechanical strength of LDPE is lower than that of HDPE, and its anti-seepage coefficient, heat resistance and anti light aging property are poor. It is easy to decompose and change color under sunlight or high temperature, resulting in performance degradation. Therefore, antioxidants and ultraviolet absorbents are added to the low-density polyethylene to improve its shortcomings when it is made into plastic coil.
3、 Linear low density polyethylene
LLDPE is non-toxic, tasteless and odorless, and its density ranges from 0.915 g / cm3 to 0.935 g / cm3. LLDPE is a copolymer of ethylene and a small amount of higher α - olefins (such as butene-1, Hexene-1, octene-1, tetramethylpentene-1, etc.) 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 branched chains, but some short branched chains. No long branched chain makes the crystallinity of polymer higher.
Compared with LDPE, LLDPE has the advantages of high strength, good toughness, strong rigidity, heat resistance and cold resistance, and also has good environmental stress cracking resistance, tear resistance and other properties, and can resist acid, alkali, organic solvents, etc.
4、 Distinguishing method
LDPE: sensory identification: soft handle; white and transparent, but general transparency.
Combustion identification: the combustion flame is yellow and blue below; it is smokeless, has the smell of paraffin wax, melts and drips, and is easy to draw wire.
LLDPE: LLDPE will swell when contacting with benzene for a long time, and will become brittle when contacting with HCl for a long time.
HDPE: the processing temperature of LDPE is lower, about 160 ℃, and the density is 0.918-0.932 g / cm3. HDPE processing temperature is higher, about 180 degrees, density is also higher.
To sum up, the above three materials play an important role in different types of anti-seepage engineering. HDPE, LDPE and LLDPE 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 river channels, and have been vigorously promoted and widely used by the Fisheries Bureau of the Ministry of agriculture, Shanghai Academy of Fishery Sciences, and the Institute of fishery machinery and instruments.
In the medium environment of strong acid, strong alkali, strong oxidant and organic solvent, the material properties of HDPE and LLDPE can be well developed and utilized. Especially, HDPE is much better than the other two materials in terms of resistance to strong acid, strong alkali, strong oxidation and organic solvent. Therefore, HDPE anti-seepage and anti-corrosion coiled material has been fully used in chemical industry and environmental protection industry.
LDPE also has good acid, alkali, salt resistance characteristics, and has good extensibility, electrical insulation, chemical stability, processing performance and low temperature resistance, so it is widely used in agriculture, aquaculture, packaging, especially low temperature packaging and cable materials.