Plastics Technology: Oct, 2005
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HDPE

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The invention of the low-pressure process to make HDPE took place in both the U.S. and Europe at nearly the same time and centered around the development of types of catalysts that promoted ethylene polymerization at milder temperatures and pressures than were (and still are) used for LDPE.

The first of these new catalysts was discovered in 1951 by Robert Banks and John Hogan at Phillips Petroleum (who were also pioneers in PP polymerization). In 1953, Prof. Karl Ziegler at Germany's Max Plank Institute for Coal Chemistry developed another low-pressure/low-temperature catalyst system. By the late 1950s, both methods were being used for HDPE production. In Europe, the first hill-scale low pressure HDPE plant was erected by Farbwerke Hoechst AG in late 1955. PLASTICS TECHNOLOGY reported in September 1955 that Hoechst's Hostalen resin, with a density of 0.94 g/cc, was the talk of the Hanover Industrial Fair in Germany, where it was shown for the first time in applications such as film, pipe, tubing, and molded household articles.

In the U.S., Phillips produced the first commercial HDPE (0.963 density) under the Marlex tradename in the summer of 1956, although it had provided pilot plant quantities as early as 1955. At NPE 1958, commercial HDPE resins were featured by Celanese, Phillips, Spencer Chemical (marketing for Exxon), and W.R. Grace. These suppliers highlighted the potential of blow molded rigid HDPE for replacing glass containers. Other early suppliers to follow included Hercules, Koppers, and Dow.

Among the earliest products were extrusion blow molded bottles for bleach and detergents, baby bottles, and injection molded housewares, such as the famous Tupperware containers.

One of the largest volume applications emerged in an unexpected way in 1957, when the Hula Hoop, an extruded tube bent into a circle, became a fad among teenagers throughout the U.S. and abroad. Wham-O, which trademarked the Hula Hoop name and was its most successful manufacturer, produced the toy using Phillip's Marlex HDPE. It was this fad that led to large-volume manufacturing of extruded HDPE pipe for high-performance applications such as natural-gas distribution, handling mine railings, and sewer lines.

Use of blow molded HDPE bottles grew far beyond detergents and bleach to include shampoos, motor oil, drug and cosmetic products, water, and milk. Milk bottles, in fact, are the single biggest volume HDPE package. Other large-volume applications are ice chests, beverage coolers, jerry cans, trash cans, storage drums, auto fuel tanks, injection molded food containers (e.g., for margarine and yogurt), and wire/cable coatings--including transoceanic cables.

Meanwhile, high-molecular-weight (HMW) HDPE made its impact in the film market a bit later. In 1979, Sonoco tested the shopping/grocery bag potential of HMW-HDPE. At NPE '88, HDPE blown film lines were a highlight.

The American Plastics Council (APC) figures North American HDPE consumption in 2004 at 15.264 billion lb, making it the second-largest volume commodity thermoplastic after PP.

LLDPE
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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.

PP
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The invention of PP was nearly simultaneous in the U.S. and Europe. Today, PP is the largest volume commodity thermoplastic. The American Plastics Council (APC) pegs North American PP consumption last year at 16.446 billion lb. This compares with 6.438 billion lb for LDPE, the only polyolefin that existed prior to 1955.

In 1953, Prof Giulio Natta at Milan Polytechnic in Italy further developed the breakthrough of that same year by Prof. Karl Ziegler at the Max Planck Institute for Coal Chemistry in Germany. Prof. Ziegler discovered that titanium tetrachloride could catalyze stereo-specific polymerization of polyethylene. With financial aid from Montecatini (later Montedison), a large Italian chemical company, Prof Natta extended Prof. Zieglefs discovery to the development of isotactic PP, which Montecatini was the first to produce on an industrial scale in 1957 in its Ferrara plant. At NPE 1956, Montecatini exhibited PP film and monofilaments.

In the U.S., meanwhile, Paul Hogan and Robert Banks of Phillips Petroleum (now Chevron Phillips) were experimenting with high-octane gasoline when they discovered high-melting crystalline PP in 1952. Phillips did not start commercial production of PP until the early '60s. The first commercial producer in the U.S. was Hercules Powder Co. (bought by Montedison and ultimately part of Basell) in 1958. Others quickly followed.

By NPE 1958, PP homopolymers were available from Hercules, Chemore, Spencer Chemical (marketing for Exxon), and Capac Plastics. Besides Montecatini, Hoechst in Germany and ICI in England were bringing on commercial capacity. Avisun Corp. soon after was producing PP homopolymer and random copolymer at New Castle, Pa. Although Montecatini was first to make both random and impact copolymers, Avisun introduced the first impact copolymers in the U.S. at NPE '61, with Hercules and others soon to follow. Avisun was acquired by Amoco in 1967.

Among the very first commercial PP applications were monofilaments for fabrics, carpets, and ropes, as well as clear, biaxially oriented (BOPP) films used as replacements for cellophane in cigarette wrap, bread wrap, and retail packaging for shirts and other garments. Closely following in the late 1950s and early '60s were extruded pipe, strapping, drinking straws, and wire/cable coatings. One of the first injection molded products was a two-piece bed pan. This was followed by a variety of electrical parts, washing-machine tubs and impellers, hypodermic syringe barrels, medicine bottles, and food containers. A big breakthrough was the screw cap with a living hinge for Seagram's liquor bottles. This essentially knocked polystyrene out of the closure business and led the way for PP into all types of consumer bottle caps.

By the early 1960s, PP and its copolymers were being used in several automotive applications. This included the use of impact copolymers for battery cases and fender liners, random copolymers for washer-fluid reservoirs, and filled homopolymers for air-intake and heater ducting. Impact copolymers were soon used for auto door interior trim and liners, as well as dashboards. There is currently an average of more than 50 lb of PP used in an automobile.

In the late '60s, blow molded containers, such as bottles for detergents, food, shampoos, and other liquids, made a splash. The first was for Procter & Gamble's Dawn dish soap in 1967, which initially was made by the Phillips Orbet extrusion stretch-blow molding process, made obsolete a decade later by PET injection stretch-blow molding. PFs impact resistance and ease of blow molding soon spawned applications such as ice chests, water coolers, housewares, toys, furniture, appliance housings, dairy containers, and snack-food packaging (metalized BOPP film).