High-density polyethylene 1-hexene production

The LLDPE product represents the outcome of a method developed to produce a low-density polyethylene but by using the more moderate, and therefore less costly, conditions employed by the processes used to produce high-density polyethylene. It is not, strictly speaking, a polyethylene since it is not a homopolymer of ethylene. An LLDPE is actually a copolymer of ethylene, which includes traces of 1-octene (Dow and Du Pont), 1-hexene (Phillips), or 1-butene (Union Carbide). This results in a polymer that has entirely short branches, and these are more uniformly spaced along the backbone than in LDPE. The spacing of the branches obtained in these cases can be closely controlled by the proportion of a-olefin to ethylene used in the feed, and the lengths by the choice of the a-olefin comonomer. Properties intermediate to those of low- and high-density polyethylene are obtained for the product (Table 23.2). 

Coordination copolymerization of ethylene with small amounts of an a-olefin such as 1-butene, 1-hexene, or 1-octene results in the equivalent of the branched, low-density polyethylene produced by radical polymerization. The polyethylene, referred to as linear low-density polyethylene (LLDPE), has controlled amounts of ethyl, n-butyl, and n-hexyl branches, respectively. Copolymerization with propene, 4-methyl-1-pentene, and cycloalk-enes is also practiced. There was little effort to commercialize linear low-density polyethylene (LLDPE) until 1978, when gas-phase technology made the economics of the process very competitive with the high-pressure radical polymerization process [James, 1986]. The expansion of this technology was rapid. The utility of the LLDPE process Emits the need to build new high-pressure plants. New capacity for LDPE has usually involved new plants for the low-pressure gas-phase process, which allows the production of HDPE and LLDPE as well as polypropene. The production of LLDPE in the United States in 2001 was about 8 billion pounds, the same as the production of LDPE. Overall, HDPE and LLDPE, produced by coordination polymerization, comprise two-thirds of all polyethylenes. 

Polyethylene (PE) is the most widely used plastic throughout the world, and high density PE (HDPE) is the most widely used type of PE. HDPE has generally been taken to mean the product of ethylene polymerization having density greater than about 0.935 (or 0.94). It includes ethylene homopolymers and also copolymers of ethylene and alpha-olefins such as 1-butene, 1-hexene, 1-octene, or 4-methyl-1-pentene. Other types of PE include low density PE (LDPE), made through a free-radical process, and linear low density PE (LLDPE). 

The early Phillips catalysts only needed to be activated and reduced to generate active centers, but several patents since have described the use of co-catalysts to promote the production of LLDPE. For example, a typical catalyst that had been modified with titanium, activated in air and reduced in caibon monoxide was then further activated by addition of triethylboron prior to operation." This procedure led to the production of linear low density polyethylene, LLDPE, which had a density of 0.9726 g cm, directly, without the need for the addition of an a-olefin to the pure ethylene feed. Olefins were produced in situ and these were incorporated into the polyethylene. A second catdyst was made using silica with a very high pore volume, modified with titanium, activated in air, and finally reduced with caibon monoxide at 350°C. This catalyst was then treated with triethylboron before use. LLDPE polymers with densities in the range 0.890 to 0.915 were obtained from a feedstock of ethylene and hexene-1, but the addition of some hydrogen to the gas stream was required to limit the length of the polymer chain. 

Polymerization conditions may vary over a wide range, where polymerization temperatiues vary from 60-110 C and total reactor pressure from 100-350 psig. Comonomers are usually limited to 1 -butene or 1-hexene to control polymer density, but 1-octene may be used if a particular catalyst exhibits a relatively high reactivity with 1-octene such as metallocene catalyst systems. Resin production rates are controlled by primarily two variables catalyst feed rate and ethylene partial pressure. In a modern gas-phase process, operating in what Univation designates as supercondens-ing mode, production rates for many grades of polyethylene of at least 125,000 Ibs/hr are reached. 

Individual LLDPE manufacturers use different a-ole-fins to reduce the regularity and crystallinity of linear polyethylene. Even though their products have similar densities and melt flow indices, they are not identical, and not necessarily miscible with each other. LLDPEs made from 1-butene and 1-hexene were miscible at low molecular weight but not at high molecular weight [42]. 

---