Click Image to Enlarge
The development of linear low-density polyethylene—originally referred to as “low-pressure” LDPE—was an offshoot of the invention of HDPE. The “linear low” resins were actually copolymers of ethylene and other monomers such as butene, hexane, and octene. The comonomer side branch kept the linear polymer chains from packing too closely together, thereby resulting in the lower density.
It was sufficiently different from standard “high-pressure” LDPE of similar density that LLDPE was termed by some observers “a third type of polyethylene.” It was stiffer, more heat resistant, and as tough or tougher than conventional LDPE. It also had greater ESCR. However, it processed somewhat differently, especially in blown film, which required design modifications in screws and dies over a period of years to take full advantage of these new resins. It also was some years before LLDPE grades were available for certain processes, such as cast film and extrusion coating.
Apart from attractive resin properties, momentum behind LLDPE came from resin suppliers who foresaw lower production costs from reactor pressures of only 100 to 300 psi instead of the usual 30,000 to 50,000 psi.
DuPont Canada is credited with operating the world’s first commercial LLDPE line, built in Corunna, Ont., in 1960. This 275 million-lb/yr “Sclair” resin unit was later acquired by Nova Chemicals, which shut it down in 2004. DuPont Canada produced Sclair resins for film, injection, and rotational molding but did not introduce them to the U.S. market until about 1976, and then initially only for rotomolding.
Several other companies launched LLDPEs at about the same time in 1977 and ’78. Dow Chemical began sampling a number of grades in 1977, using technology derived from its low-pressure HDPE process.
Both DuPont and Dow used solution processes to make their early LLDPE resins, most of which were considered to have “medium” density ranges of 0.925 to 0.940 g/cc. As early as 1970, Phillips test marketed some linear LDPE made by its low-pressure particle-form slurry process but chose to concentrate instead on its HDPE resins.
By that time, Union Carbide had been working for over two years to make LLDPE on a commercial scale via its proprietary Unipol gas-phase fluidized-bed process. It had produced one wire/cable grade, another for injection and rotational molding, and some film resins. At NPE ’79, Union Carbide officially introduced its LLDPE to the market. It also immediately offered to license the Unipol process to others—a decision that led to rapid proliferation of LLDPE. The first major licensee in the U.S. was Exxon Chemical.
Soon after Carbide, BP Chemicals commercialized a similar gas-phase process. By 1988, more than 70% of the world LLDPE capacity was based on gas-phase polymerization. By 1995, LLDPE’s penetration into the low-density market exceeded 50% in the U.S. though it was slower to catch on in Europe.
Blown film was the fastest growing market for LLDPE, with trash bags in the lead in the U.S., followed by pallet stretch wrap. Next in size was LLDPE pipe and conduit. By the late 1980s, film grades in both the U.S. and Europe accounted for 75% of LLDPE poundage.
The American Plastics Council (APC) figures North American consumption of LLDPE at 10.186 billion lb in 2004, compared with 6.438 billion lb for LDPE—a 61% share of the low-density market.
Very few readers of this issue can remember, or even imagine, what it was like when an injection mol...