Linear low density polyethylene (LLDPE) is a copolymer of ethylene and a small amount of advanced alpha olefins (such as butene-1, hexene-1, octene-1, tetramethylpentene-1, etc.), which is polymerized under high pressure or low pressure under the action of catalyst, with a density between 0.915 and 0.940 g/cm3. However, according to ASTM D-1248-84, the density range of 0.926~0.940 g/cm3 belongs to medium density polyethylene (MDPE). The new generation LLDPE expands its density to plastic body (0.890~0.915 g/cm3) and elastomer (<0.890 g/cm3). But the United States Plastic The Industrial Association (SPI) and the American Plastics Industry Council (APC) only extend the scope of LLDPE to plastic bodies, excluding elastomers. In the 1980s, Union Carbide and Dow Chemical called their early plastic and elastomer as very low density polyethylene (VLDPE) and ultra low density polyethylene (ULDPE) resins.
The molecular structure of conventional LLDPE is characterized by its linear main chain, with few or no long branches, but some short branches. The absence of long branched chains makes the polymer more crystalline.
In general, LLDPE resins are characterized by density and melt index. The density is determined by the concentration of comonomer in the polymer chain. The concentration of comonomer determines the amount of short branched chains in the polymer. The length of the short branched chain depends on the type of comonomer. The higher the comonomer concentration, the lower the resin density. 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 of chain transfer agent (gas phase method). The average molecular weight is independent of the molecular weight distribution, which is mainly affected by the type of catalyst.
LLDPE was industrialized by Union Carbide in the 1970s. It represents a major change in polyethylene catalyst and process technology, which significantly expanded the scope of polyethylene products. LLDPE replaces free radical initiator with coordination catalyst, and replaces high pressure reactor with low cost low-pressure gas-phase polymerization. In a relatively short time, it has replaced LDPE in many fields with its excellent performance and low cost. At present, LLDPE has penetrated almost all traditional polyethylene markets, including films, molding, pipes, wires and cables.
LLDPE products are non-toxic, tasteless, odorless, and milky white particles. Compared with LDPE, it has the advantages of high strength, good toughness, strong rigidity, heat resistance, cold resistance, good resistance to environmental stress cracking, tear resistance, acid, alkali, organic solvents, etc.
In 2005, China's LLDPE output was 1.88 million tons, accounting for 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 estimated that LLDPE consumption will continue to grow at a rate of about 8% in the next 2-3 years. Based on the current market price of 12000 yuan/ton, the market size of LLDPE in China has exceeded 40 billion yuan.
（1） Application fields of LLDPE
The main application fields of LLDPE are agricultural films, packaging films, wires and cables, pipes, coating products, etc.
Linear low density polyethylene (LLDPE) is mainly used to make films due to its high tensile strength, good puncture resistance and tear resistance. In 2005, the world LLDPE consumption was 16.17 million tons, up 6.4% year on year. In the consumption structure, film products still account for the largest proportion, with the consumption of 11.9 million tons, accounting for 73.6% of the total consumption, followed by injection molding, with the consumption of 1.148 million tons, accounting for 7.1% of the total consumption of LLDPE.
In 2005, the total consumption of LLDPE and LDPE in China was 5.98 million tons, including 3.55 million tons of LLDPE, a year-on-year increase of 25.4%, accounting for 59.4% of the total consumption of LLDPE/LDPE; LDPE consumption was 2.43 million tons, up 0.7% year on year, accounting for 40.6% of the total consumption of LLDPE/LDPE.
From the perspective of LLDPE/LDPE consumption structure, film is still the largest type of consumption, with consumption of 4.85 million tons, accounting for 77.5% of the total consumption of LLDPE/LDPE, including 3.13 million tons of packaging film, accounting for 50% of the total consumption; 1.345 million tons of agricultural film, accounting for 22.5% of the total consumption; 376000 tons of special packaging film, accounting for 6% of the total consumption. The second is injection molding products, with a consumption of 557000 tons, accounting for 8.9% of the total consumption. Then followed by coating products, pipes, wires and cables, with consumption of 313000 tons, 188000 tons and 157000 tons respectively, accounting for 5%, 3% and 2.5% of the total consumption; Other consumption was 188000 tons, accounting for 3% of the total consumption.
According to the consumption of LLDPE/LDPE from 2003 to 2005, the consumption proportion of film has always been around 77%, and the consumption proportion of the second largest variety of injection molding products has also been around 9%. It is predicted that in the next 2-3 years, although the absolute consumption of various varieties will continue to grow, their consumption proportion will basically maintain the current trend; Due to the relatively rapid growth of demand for packaging film, the consumption proportion of agricultural film will be reduced to about 20%. As the performance of LLDPE continues to improve and its application field continues to expand, the future market demand for LLDPE will grow much faster than LDPE and HDPE.
（2） Classification of LLDPE
According to the type of comonomer, LLDPE is mainly divided into three kinds of copolymers: C4 (butene-1), C6 (hexene-1) and C8 (octene-1). Among them, butylene copolymer is the LLDPE resin with the largest production in the world, while hexene copolymer is the LLDPE variety with the fastest growth at present. In LLDPE resin, the typical amount of comonomer is 5%~10% by weight, and the average amount is about 7%. The metallocene based LLDPE plastomer (mLLDPE) has more than three times the average comonomer content of traditional LLDPE. Chart 1 shows the output of three comonomers LLDPE in the world in the past 10 years quoted from the Foreign Journal.
At the end of 1984, the then United Carbon Company introduced the production of LLDPE for hexene copolymerization, followed by Exxon, Mobil and other companies. Dow Chemical almost uses octene as the comonomer in its low-pressure solution process, and NOVA Canada also uses octene in its pressurized solution process. Octene copolymer LLDPE resin has slightly better strength, tear resistance and processing properties, while hexene copolymer resin and octene copolymer resin have little difference in performance. At present, the main producers of hexene LLDPE resin are ExxonMobil Chemical, Eastman Chemical, Equistar and Chevron Phillips. In addition, Dow Chemical, Basell, Innovene, Samsung Total (Weibo) Total) also produces LLDPE of hexene.
Compared with commonly used butene comonomers, LLDPE produced with hexene and octene as comonomers has better performance. The greatest use of LLDPE resin lies in the production of films. The films and products made of LLDPE resin with long-chain alpha olefins (such as hexene and octene) as comonomers are superior to LLDPE resin produced with butylene as comonomers in many aspects, such as tensile strength, impact strength, tear strength, puncture resistance, environmental stress cracking resistance, etc. Since the 1990s, foreign PE manufacturers and users have tended to replace butene with hexene and octene. It is reported that with octene as the comonomer, the resin performance may not be further improved than that of hexene copolymerization, and the price is more expensive. Therefore, at present, the trend of major LLDPE manufacturers abroad to use hexene instead of butene is more obvious.
At present, because there is no large-scale production of hexene and octene in China, and the import price is expensive, the LLDPE resin produced in China today mainly uses butylene as the comonomer. Although some domestic enterprises used hexene as the brand of comonomer when introducing LLDPE production units, they had to give up because there was no hexene production in China, and only imported a small amount of hexene during start-up assessment. Most of the high-grade LLDPE products imported into China are of this kind. It is expected that the demand for LLDPE with 1-hexene as monomer will increase significantly in the future.
