Polyethylene low pressure

Figure 2 Light permeability of polyolefins after quenching (1-4) and of nonquenched samples (l -4 ) l,l -poly-propylene (PP) 2,2 -high-pressure polyethylene (HPPE) 3,3 -low-pressure polyethylene (LPPE) 4,4 -medium-pres-sure polyethylene (MPPE). Film thickness-150 fic moulding time-10 minutes, moulding pressure HPPE-160°C LPPE, MPPE, PP-190-200X.

Thermal destruction of low-pressure polyethylene with molecular weight of 34,800 and of high-pressure polyethylene is completely retarded by potassium hydroxide. The molecular weight of high-molecular polyethylene decreases by a factor of 1.8, and without an... 

Boric acid esters provide for thermal stabilization of low-pressure polyethylene to a variable degree (Table 7). The difference in efficiency derives from the nature of polyester. Boric acid esters of aliphatic diols and triols are less efficient than the aromatic ones. Among polyesters of aromatic diols and triols, polyesters of boric acid and pyrocatechol exhibit the highest efficiency. Boric acid polyesters provide inhibition of polyethylene thermal destruction following the radical-chain mechanism, are unsuitable for inhibition of polystyrene depolymerization following the molecular pattern and have little effect as inhibitors of polypropylene thermal destruction following the hydrogen-transfer mechanism. 

The polyethylene produced by radical polymerization is referred to as low-density polyethylene (LDPE) or high-pressure polyethylene to distinguish it from the polyethylene synthesized using coordination catalysts (Sec. 8-1 lb). The latter polyethylene is referred to as high-density polyethylene (HDPE) or low-pressure polyethylene. Low-density polyethylene is more highly branched (both short and long branches) than high-density polyethylene and is therefore lower in crystallinity (40-60% vs. 70-90%) and density (0.91-0.93 g cm 3 vs. 0.94-0.96 g cm-3). 

In 1953 K. Ziegler and coworkers discovered a class of heterogeneous catalysts that allowed ethylene to be polymerized at low pressures and low temperatures (low-pressure polyethylene=high-density polyethylene=PEHD). 

The influence of the polymer structure on the irradiation grafting has been examined in the case of styrene to high pressure and low pressure polyethylene films (114). The most important factors which determine the efficiency of grafting are the degree of crystallinity, the thickness of the films and the dose rate. 

T 8 — Molecular weight-intrinsic viscosity relationship and molecular weight distribution of low pressure polyethylenes. J. Polymer Sci. 24,333 (1957). 

T9 —-A light-scattering study of low pressure polyethylene fractions. J. Polymer Sci. 36, 287 (1959). 

The reason for the observation of different kinetic laws of radical decay in the above mentioned papers is probably due to a difference of poyethylene samples. Thus Charlesby et al. (19) have found that the rate of alkyl radicals decay in samples of low pressure polyethylene is much higher than in high pressure polyethylene. Loy (22) has investigated radical decay in irradiated cellulose and found that at low irradia-tiori doses (up to 10 Mrad) the radical decay kinetics can be represented by superposition of two processes. At high radical concentrations the kinetics corresponds to a monomolecular law and then a transition to... 

Aluminum alkyls used in making organometallic catalysts and as initiators for processes such as ethylene-propylene rubber, polybutadiene, low-pressure polyethylene, and ethylene oligomerization to make alpha-olefins and C6-C18 alcohols... 

Mr. Wellington E. Walker died suddenly on May 8, 1980. His death is a deep loss to his friends at Union Carbide Corporation. He was honest in his private and professional lives and never surrendered his principles he was a good friend always ready to help he worked every day honestly with excellence and dedication. He was an inventor of commercialized reactions for low pressure polyethylene, the synthesis of n-octanol and the rhodium catalyzed homogeneous conversion of C0 H2 into polyols. He was a coauthor of ca. forty-six patents and twenty-seven papers. 

Decay of ionic and coordination centres always leads to the formation of some end groups and centre residues. The centres usually lose their polymerizing activity on contact with atmospheric humidity. A residue of very active centres, which are rare, is usually not removed from the polymer (e.g. of the order of one ppm of the transition metal in low-pressure polyethylene). Larger residues have to be washed out (some types of polypropylene are still washed at the present time). 

As is showm in Figure 2, methane is formed over nickel and ruthenium catalysts, especially at low pressures (atmospheric up to 10 bar) and elevated temperatures. Paraffins and olefins are produced over nickel and cobalt catalysis at mild temperatures (< 200 0) and pressures of I -10 bar. With iron catalysts, olefins,parafllns.and minor amounts of alcohols are formed at medium pressures (10 100 bar) and temperatures of 210--340 C. Ruthenium catalysts give, at elevated pressures (150-1000 bar) and low temperatures (100-180 C), poly methylene with a molecular weiglii of up to I 000000. This polymer has similar properties as Ziegler-type low pressure polyethylene. 

Barriers to entry into the pseudocommodity business include all the barriers for commodities plus the all-important customer know-how. Lack of technical expertise and patent protection can be formidable barriers to potential producers of a pseudocommodity. Du Pont was the sole producer of nylon from 1939 to 1951. Barriers to entry were removed under threat of government antitrust action in 1951, with the licensing of Chemstrand, which later became part of Monsanto. Since then a number of companies have entered the nylon business. Technical know-how and patent protection played a major role in both high- and low-pressure polyethylene manufacture in the years shortly after World War II. ICI developed polyethylene, but Union Carbide had a superior high-pressure process. Ziegler, Du Pont, and Phillips Petroleum all developed low-pressure processes, which they subsequently licensed to other manufacturers. Many pseudocommodities eventually become commodities by the diffusion of technology, standardization of the product, and the entry of many firms into the business. 

Fig. 20. Differential thermal analysis curves for low-pressure polyethylene Hizex 5000 [41], Upper curve, in air lower curve, in a nitrogen atmosphere.

Ziegler s timing was also fortunate in bringing out a new product at a time when the chemical industry was in an aggressive, expansionist mood. Had he been a few years earlier, that wave would not yet have crested had he been even a year or two later, he might have lost out to the other linear, low-pressure polyethylene processes that were developed independently and contemporaneously. 

Zletz was attempting to use the supported cobalt catalyst for alkylation with ethylene and found, to his surprise, that considerable solid polymer was formed. Like Hogan, he was not a trained polymer chemist, but had enough curiosity, initiative, and freedom to pursue the interesting bypath. Other metals and other supports were tested by Zletz and his coworkers and led eventually to improved catalysts such as molybdenum on alumina that formed the basis for development of a practical low-pressure polyethylene process. 

In 1955, Ziegler proposed a synthesis at atmospheric pressure, with the help of organo-metallic catalysts the low pressure polyethylene thus obtained is a better product, than the high pressure polyethylene its density is larger, its branching rate is lower, and its crystallinity ratio is higher. The characteristics are indicated in Table 1.2. 

The identification of polymer blends is illustrated by the DTA curve in Figure 7.48. Chiu (154) studied a physical mixture of seven commercial polymers high-pressure polyethylene (HPEE), low-pressure polyethylene (LPPE), polypropylene (PP), polyoxymethylene (POM), Nylon 6, Nylon 66, and polytetrafluoroethylene (PTFE). Each component shows its own characteristic melting endothermic peak, at 108,127,165,174,220,257, and 340°C, respectively. Polytetrafluoroethylene also has a low-temperature crystalline transition at about 20°C. The unique ability of DTA to identify this polymer mixture is exceeded by the fact that only 8 mg of sample was employed in the determination. 

Fro. 15-34. Low-pressure polyethylene jMrocess (Ziegler). [Petroleum Refiner 34, 179 (1966).]... 

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