Polyethylene is the most widely used packaging raw material for plastic packaging products. Since it is polymerized from ethylene in the cracked gas produced during petroleum processing, the ethylene monomer is non-toxic, so various types of polyethylene raw materials Even if it contains 200 to 300 ppm of ethylene monomer, it is still a non-toxic polymer and can be used in packaging situations that are in direct contact with various foods and drugs.

There is a large difference between the melting temperature and thermal separation temperature (above 315°C) of various types of polyethylene; the melt fluidity is good, so the melt processability of various types of polyethylene is good, and extrusion for plastic molding can be used , injection, compression, blow molding and other methods to produce various packaging products.

All types of polyethylene are non-polar polymers. They have good compatibility with each other and can be combined into blends in any proportion to improve performance. Since all types of polyethylene have relatively low melting temperatures and high thermal adhesion, polyethylene is often used as the heat sealing material for packaging in soft plastic packaging.

There are many types of polyethylene, usually divided according to the era when industrialization occurred. There are the first-generation polyethylene industrialized in 1939, namely: high-pressure polyethylene (low-density polyethylene); the second-generation polyethylene industrialized in 1953, namely : Low-pressure polyethylene (high-density polyethylene); the third-generation polyethylene industrialized in 1977, namely: linear low-density polyethylene (LLDPE); the fourth-generation polyethylene industrialized in 1984, ultra-low-density polyethylene (VLDPE) ); ultra-high molecular weight polyethylene (UHMWPE) industrialized in 1958 and metallocene polyethylene (MPE) that appeared in the 1990s.

During the production process of the above-mentioned polyethylene, some add a small amount of 4- or 8-carbon alpha olefin as a comonomer. However, because the amount of alpha olefin used is very small, many of the characteristics of polyethylene are still maintained.

Low density polyethylene (LDPE) (high pressure polyethylene)

The characteristics of LDPE are:

(1) LDPE is a white waxy granular solid with a density of 0.91~0.925g/cm3, which is tasteless, odorless and non-toxic;

(2) LDPE is a typical crystalline polymer with a crystallinity of 55%~65% and a melting point of 105~126°C;

(3) LDPE is a non-polar material that is easily charged with static electricity and has low surface energy. Therefore, corona treatment should be performed before printing and lamination to increase the surface energy. During the processing, attention should be paid to anti-static to avoid accumulation of static electricity that affects product quality or Electric spark discharge, causing fire;

(4) LDPE has excellent transparency and heat sealability, and can be widely used in the production of transparent low-temperature frozen packaging products;

(5) LDPE has excellent moisture resistance and is a high-quality raw material for making dry food or packaging items that require good moisture resistance. However, LDPE has high gas barrier properties and can easily pass through various gases;

(6) Although LDPE has certain grease resistance, its grease resistance and organic solvent resistance are not as good as polypropylene. Therefore, when the thickness is small, it is not suitable for long-term storage of gasoline, alcohol, grease, etc. When using LDPE, the best thickness should be