The molecular structure of conventional LLDPE is characterized by its linear main chain, few or no long branches, but some short branches. The absence of long branches makes the polymer more crystalline.
Generally speaking, LLDPE resins are characterized by density and melt index. The density is determined by the concentration of monomers in the polymer chain. The concentration of * * * monomer determines the number of short branches in the polymer. The length of the short chain depends on the type of monomer. * * * The higher the monomer concentration, the lower the density of the resin. In addition, the melt index is a reflection of the average molecular weight of the resin, which is mainly determined by the reaction temperature (solution method) and the addition amount of chain transfer agent (gas phase method). The average molecular weight has nothing to do with the molecular weight distribution, but is mainly influenced by the type of catalyst.
LLDPE was industrially produced by Union Carbide Company of the United States in 1970s, which represented a major change in the catalyst and processing technology of polyethylene and greatly expanded the product range of polyethylene. With its excellent performance and low cost, LLDPE has replaced LDPE in many fields in a short time by replacing free radical initiator with coordination catalyst and high-pressure reactor with low-cost gas phase polymerization. At present, LLDPE has penetrated into almost all traditional polyethylene markets, including films, molds, pipes and wires and cables.
LLDPE products are non-toxic, tasteless and odorless, showing milky white particles. Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance, cold resistance, good environmental stress cracking resistance, tear resistance, acid resistance, alkali resistance and organic solvent resistance.
In 2005, the LLDPE output in China was 6.5438+0.88 million tons, accounting for about 35.5% of the total PE output. The consumption is 3.55 million tons, accounting for 33.8% of the total consumption of PE. It is predicted that LLDPE consumption will continue to grow at a rate of about 8% in the next 2-3 years. According to the current market price of 12000 yuan/ton, the market scale of LLDPE in China has exceeded 40 billion yuan.
Application field of LLDPE
The main application fields of LLDPE are agricultural films, packaging films, wires and cables, pipes and coated products.
PTA is the abbreviation of purified terephthalic acid. White powdery crystal at room temperature, non-toxic, flammable. If mixed with air, it will burn in case of fire within a certain limit.
(A) PTA production process
PTA is a low-end product of petroleum. Petroleum produces naphtha (alias light gasoline) through a certain process, extracts MX (mixed xylene) from naphtha, and then extracts PX (p-xylene). PTA uses PX (65%-67%) as raw material, acetic acid as solvent, and air oxidation (35%-33% oxygen) under the action of catalyst to produce crude terephthalic acid. Then the crude terephthalic acid is hydrofined to remove impurities, and then the refined terephthalic acid product, namely PTA finished product, is obtained through crystallization, separation and drying.
There are manufacturers of crude terephthalic acid at home and abroad, such as EPTA of Sanxin Petrochemical and QTA of South Korea. There is no later refining process in the production process. Low cost and obvious price advantage, which can meet the needs of different polyester enterprises.
(B) the use of PTA
PTA is one of the important bulk organic raw materials, which is widely used in chemical fiber, light industry, electronics, construction and other aspects of the national economy. At the same time, the application of PTA is relatively concentrated. More than 90% of PTA in the world is used to produce polyethylene terephthalate (PET). The production of 1 ton PET requires 0.85-0.86 tons of PTA and 0.33-0.34 tons of MEG. Polyester includes fiber chips, polyester fibers, bottle chips and film chips. In the domestic market, 75% PTA is used to produce polyester fiber; 20% is used to produce bottle-grade polyester, which is mainly used for the packaging of various beverages, especially carbonated beverages; 5% is used in film-grade polyester, mainly used in packaging materials, films and adhesive tapes. It can be seen that the downstream extension products of PTA are mainly polyester fibers.
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