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 UnionCarbide Company in 1970s, which represented a great 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.
(A), LLDPE application field
The main application fields of LLDPE are agricultural films, packaging films, wires and cables, pipes and coated products.
Linear low density polyethylene (LLDPE) is mainly used for making films because of its high tensile strength, good puncture resistance and tear resistance. In 2005, the world LLDPE consumption was 65,438+0,665,438+0.7 million tons, up 6.4% year-on-year. In the consumption structure, film products still account for the largest proportion, with the consumption of 1 1.9 million tons, accounting for 73.6% of the total consumption, followed by injection molding, with the consumption of 1 1.48 million tons, accounting for about 7 1% of the total consumption of LLDPE.
In 2005, the total consumption of LLDPE and LDPE in China was 5.98 million tons, of which the consumption of LLDPE was 3.55 million tons, up 25.4% year-on-year, accounting for 59.4% of the total consumption of LLDPE/ LDPE. The consumption of low-density polyethylene was 2.43 million tons, up 0.7% year-on-year, accounting for 40.6% of the total consumption of LLDPE/ low-density polyethylene.
According to the consumption structure of LLDPE/LDPE, film is still the most consumed variety, with a consumption of 4.85 million tons, accounting for 77.5% of the total consumption of LLDPE/LDPE, of which 310.3 million tons of packaging film, accounting for 50% of the total consumption; 654.38+345,000 tons of agricultural film, accounting for 22.5% of the total consumption; 376,000 tons of special packaging films, accounting for 6% of the total consumption. Followed by injection molding products, the consumption is 557,000 tons, accounting for 8.9% of the total consumption. Followed by coated products, pipes and wires and cables, the consumption is 310.3 million tons,10.88 million tons and10.57 million tons respectively, accounting for 5%, 3% and 2.5% of the total consumption; Other consumption is 6.5438+0.88 million tons, accounting for 3% of the total consumption.
Judging from the consumption of LLDPE/LDPE from 2003 to 2005, the consumption ratio of film has been kept at around 77%, and the consumption ratio of the second largest variety of injection molding products has been hovering at around 9%. It is predicted that in the next 2 ~ 3 years, although the absolute consumption of various varieties will continue to increase, the consumption ratio will basically maintain the status quo; Due to the rapid growth of packaging film demand, the consumption proportion of agricultural film will drop to about 20%. Due to the continuous improvement of LLDPE performance, its application field is also expanding, and the market demand growth rate of LLDPE in the future will be much higher than that of LDPE and HDPE.
(ii) Classification of ㈡LLDPE
According to the monomer type of * * *, LLDPE is mainly divided into three kinds of * * * polymers: C4 (butene-1), C6 (hexene-1) and C8 (octene-1). Among them, butene polymer is the largest LLDPE resin in the world, and hexene polymer is the fastest growing LLDPE variety at present. In LLDPE resin, the typical dosage of * * monomer is 5% ~ 10% by weight, and the average dosage is about 7%. The average monomer content of metallocene-based LLDPE plastomer (mLLDPE) is more than 3 times that of traditional LLDPE. Chart 1 refers to the global output of three LLDPE monomers quoted from foreign journals 10.
At the end of 1984, the then United Carbon Company introduced the production of hexene * * * poly LLDPE, followed by Exxon, Mobil and other companies. Octene is almost always used as monomer in the low-pressure dissolution process of DowChemical Company, and octene is also used in the medium-pressure dissolution process of NOVA Chemical Company in Canada. The strength, tear resistance and processability of octene LLDPE resin are slightly better, but there is little difference between hexene * * * poly resin and octene * * * poly resin. At present, the main producers of hexene LLDPE resin are ExxonMobil Chemical Company, EastmanChemical Company, Equistar Company and ChevronPhillips Chemical Company. In addition, DowChemical, Basell, Innovene and SamsungTotal also produce hexene LLDPE.
Compared with common butene monomers, LLDPE produced with hexene and octene as monomers has better properties. The biggest use of LLDPE resin is to produce films. Films and products made of LLDPE resin with long-chain α -olefins (such as hexene and octene) as monomers are superior to LLDPE resin with butene as monomers in many aspects, such as tensile strength, impact strength, tear strength, puncture resistance and environmental stress cracking resistance. Since 1990s, foreign polyethylene manufacturers and users tend to replace butene with hexene and octene. It is reported that when octene is used as the monomer of * * *, the resin performance may not be further improved than that of hexene * * *, and the price is more expensive. Therefore, at present, the trend of using hexene instead of butene by major LLDPE producers abroad is more obvious.
At present, because there is no large-scale production of hexene and octene in China, and the import price is relatively expensive, LLDPE resin produced in China mainly uses butene as monomer. Although some domestic enterprises used hexene as the brand of LLDPE monomer when introducing LLDPE production equipment, they had to give up because there was no hexene production in China and only imported a small amount of hexene when they started the examination. Most of the high-grade LLDPE imported from China are of this kind. It is predicted that the demand for LLDPE with 1- hexene as monomer will increase greatly in the future.