The Chemistry of Polyethylene

Cross-linking may be induced deliberately during the fabrication process to enhance certain properties that would otherwise be deficient. The principal aim of cross-linking is to improve high temperature structural integrity, i.e., to prevent viscous fiow when the crystalline melting temperature is exceeded. Cross-linking can be effected by chemical means or by treatment with radiation. Chemi- 

Surface modification of polyethylene is carried out principally to increase the surface energy of products that come into contact with liquids. Specifically, surface treatment aids the adhesion of paints, inks, and glues to polyethylene products. 

Reactive side groups may be grafted onto the backbone of polyethylene to endow it with specific chemical properties. Grafting can provide sites for subsequent reactions, to improve miscibility with other polymers or enhance adhesion to various inorganic fillers. 

Copolymerization of ethylene with polar comonomers results in such resins as ethylene-co-vinyl acetate, ethylene-co-vinyl alcohol, and ethylene-co-metha-crylic acid copolymers. The polar side groups so incorporated may interact with each other to endow the product with specific physical properties, or they may be used as sites for subsequent chemical reactions. A major family of polymers falling into this category are ionomers, which consist of ethylene-co-vinyl acid copolymers, the acid functions of which have been neutralized to form metal salts. 

Pyrolysis occurs when thermal degradation takes place in the absence of oxygen at high temperatures. Conditions suitable for pyrolysis are rarely encountered unless a deliberate effort is being made to depolymerize polyethylene. The use of pyrolysis as a tertiary recycling technique is discussed in Chapter 10. 

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A. Barlow, "The Chemistry of Polyethylene Insulation," IEEE Electrical Insulation Magacjne, 8—19 (1991). 

Polycondensation reactions were also carried out using a mixture of ethylene-diamine and adipic acid (55). IR techniques again were used to confirm the polymer composition. The results are summarized on Table 9. The chemistry of polyethylene terephthalate) mechanical polycondensation with diamines proceeds as follows ... 

Barlow A. The chemistry of polyethylene insulations. IFFF Electr Insul M 1991 7 8. 

AUoys of ceUulose with up to 50% of synthetic polymers (polyethylene, poly(vinyl chloride), polystyrene, polytetrafluoroethylene) have also been made, but have never found commercial appUcations. In fact, any material that can survive the chemistry of the viscose process and can be obtained in particle sizes of less than 5 p.m can be aUoyed with viscose. 

A carbon-carbon double bond is a reactive functional group because of its iz electrons. Remember from Chapter 10 that ethylene has a CDC bond made up of one a bond plus one itt bond. As shown in Figure 13-1. the electrons in the iTrbond are located off the bond axis, making them more readily available for chemical reactions. Moreover, 71 electrons are less tightly bound than a electrons. Consequently, the reactivity patterns of ethylene are dominated by the chemistry of its n electrons. Polyethylene is one familiar polymer whose monomer is ethylene. We describe the polymerization reaction of ethylene and other monomers containing CDC bonds in Section 13-1. 

Capillary electrophoretic separations are performed in small diameter tubes, made of Teflon, polyethylene, and other materials. The most frequently used material is fused silica. Fused silica capillaries are relatively inexpensive and are available in different internal and external diameters. An important advantage of a fused silica capillary is that the inner surface can be modified easily by either chemical or physical means. The chemistry of the silica surface is well established due to the popularity of silica surfaces in gas chromatography (GC) and liquid chromatography (LC). In capillary electrophoresis, the silica surface is responsible for the EOF. Using surface modification techniques, the zeta potential and correspondingly the EOF can be varied or eliminated. Column fabrication has been done on microchips.13... 

Andrew Peacock is a Development Associate with Tredegar Film Products, Richmond, Virginia. Previously he worked as a Senior Research Chemist with Exxon Chemical Company, Baytown, Texas. Publications include the Handbook of Polyethylene - Structures, Properties and Applications , nine patents in the field of polymer science, and numerous journal articles. Dr. Peacock received a B. Sc. in Chemistry from the University of London, England, an M. Sc. in Polymer Science and Technology from Lancaster University, England and a Ph. D. in Chemistry from the University of Southampton, England. 

Oxidation is the first step for producing molecules with a very wide range of functional groups because oxygenated compounds are precursors to many other products. For example, alcohols may be converted to ethers, esters, alkenes, and, via nucleophilic substitution, to halogenated or amine products. Ketones and aldehydes may be used in condensation reactions to form new C-C double bonds, epoxides may be ring opened to form diols and polymers, and, finally, carboxylic acids are routinely converted to esters, amides, acid chlorides and acid anhydrides. Oxidation reactions are some of the largest scale industrial processes in synthetic chemistry, and the production of alcohols, ketones, aldehydes, epoxides and carboxylic acids is performed on a mammoth scale. For example, world production of ethylene oxide is estimated at 58 million tonnes, 2 million tonnes of adipic acid are made, mainly as a precursor in the synthesis of nylons, and 8 million tonnes of terephthalic acid are produced each year, mainly for the production of polyethylene terephthalate) [1]. 

Lefkovitz, L. Crecelius, E. McElroy, N. 1996, The use of Polyethylene Alone to Predict Dissolved-phase Organics in the Columbia River. Presented at the 17th annual meeting Society of Environmental Toxicology Chemistry November 17-21, 1996 Washington, D.C. 

More importantly, understanding the chemistry of PP requires you to know the critical difference between PP and the polyethylenes—the asymmetry of the PP molecules. backbone. In polyethylene, every carbon looks like every other carbon in the chain. In PP, the polymer linkage is between succeeding double-bonded carbons, like polyethylene. But, the methyl group survives as a branch on every second carbon in the PP backbone chain. See Figure 23—7.) Furthermore, the orientation of that branch is crucial to the properties of the polymer. See Figure 23-8.)... 

Rochow, E.G. (1951). An Introduction to the Chemistry of Silane. 2nd, ed.. Chapman Hall. London. Rostami, H., Iskandarni, B. and Kamel, I. (1992). Surface modification of Spectra 900 polyethylene fibers using RE-plasma, Polym. Composites 13, 207-212. 

The types and reactions postulated for reactive intermediates in the radiation chemistry of polyethylene are reviewed. Ultraviolet spectroscopy is an important tool in complementing data obtained from electron spin resonance studies. Finally, the kinetics of growth and decay of the allyl and polyenyl free radicals as inferred from ultraviolet spectra are discussed. 

T wo aspects of the radiation chemistry of polyethylene terephthalate (PET) are reviewed here the dependence of product yields on radiation dose and on dose rate. The review is limited to work with thin films from which air and water were pumped prior to irradiation. Moreover, it is judged that in the experiments described postirradiation effects were negligible. 

Budzol,M., Dole,M. The radiation chemistry of polyethylene. XI. The molten state. J. Phys. Chem. 75,1671-1676 (1971). 

R.O. Gibson, The discovery of polyethylene, Lecture series, Vol 1., Royal Institute of Chemistry 1964. 

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