Polyethylene and other polyolefins

Three types of radical can be detected in irradiated polyethylene [4 — 65]. The methylene radical, —CH2—CH—CH2 — (I), possesses a sextet electron spin resonance spectrum. It is formed exclusively during irradiation at liquid nitrogen temperature. At or near room temperature, the ESR spectrum is the superposition of this sextet, which progressively disappears, and a more stable septet assigned to the allyl radical —CH—CH=CH2 — (II), Fig. 11. At very high doses, a singlet assigned to the polyenyl radical is observed. 

Free radicals have been detected in irradiated polyethylene by ultraviolet and infrared absorption [60—64]. The Am ax for allyl and polyenyl free radicals are given in Table 6 and compared with the corresponding 

When irradiation of polypropene with 7-rays and ESR measurements are both performed at 77°K, an octet is observed (Fig. 13(a)). For irradiation at room temperature, nine lines are observed at 77°K (Fig. 

13(b)) and seventeen at room temperature [66,67]. Early workers assigned the octet (spectrum L) to the radical 

The absence of any orientation effect supports this interpretation. Dipolar anisotropic coupling would result from the presence of a hydrogen atom on the carbon carrying the unpaired electron. The nine- and seventeen-lines spectra were previously assigned [68, 71, 72] to the allyl radical 

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There are two great families of synthetic polymers, those made by addition methods (notably, polyethylene and other polyolefines), in which successive monomers simply become attached to a long chain, and those made by condensation reactions (polyesters, polyamides, etc.) in which a monomer becomes attached to the end of a chain with the generation of a small by-product molecule, such as water. The first sustained programme of research directed specifically to finding new synthetic macromolecules involved mostly condensation reactions and was master-... 

In the 1990s, a new family of polyethylenes based on metallocene catalysts emerged. These catalysts offered significant new ability to tailor the properties of linear polyethylenes and other polyolefins. In particular, they have the ability to provide more uniform incorporation of comonomers. 

Cross-linking can also be achieved by compounding a polymer with a peroxide such as dicumyl peroxide or di-/-butyl praoxide, followed by heating. This technique, although economically disadvantageous compared to sulfur vulcanization, is particularly useful for saturated polymers that cannot be easily cross-hnked, particularly polyethylene and other polyolefins. Similar results can be obtained through radiation cross-linking, and this method is employed commercially in the production of electrical wires and cable insulation made of polyolefins and poly(vinyl chloride). 

T. McKee, Laser Marking of Polyethylene and Other Polyolefins with Additives, Plastics Formulating <6 Compounding. 

The next major step, almost two decades after the discovery by ICI of then-high-pressure free radical polymerization of ethylene, was the discovery, from 1951, of (metal oxide and organo-metallic) catalysts that produced essentially linear high molecular weight polyethylene (and other polyolefins) under much lower pressures. These catalyst discoveries occurred almost simultaneously and independently in several laboratories in the USA and Europe [12]. 

Polyethylene and other Polyolefines Ed. by V. P. Kozlov andN. A. PlateA/ir, Moscow, 302... 

It was originally developed for polyethylene and other polyolefins, and is still used to some extent in those applications, but its use in polyamide 6,6 has become more important. 

Packham, D.E., Mechanics of Failure of Adhesive Bonds Between Metals and Polyethylene and other Polyolefins. In Developments in Adhesives 2 Kinlock, A.J. (Ed), Appl. Sci. Publ. (1981). 

There are three basic types of polyethylene foams of importance (/) extmded foams from low density polyethylene (LPDE) (2) foam products from high density polyethylene (HDPE) and (J) cross-linked polyethylene foams. Other polyolefin foams have an insignificant volume as compared to polyethylene foams and most of their uses are as resia extenders. 

The list of examples of successful Th-FFF separations of lipophilic polymers is extensive and includes polystyrene [29,34,76,118,144,164,165,168,196,200,345-350], polyisoprene [55,110,144,196,349,350], polytetrahydrofuran [144,196,349,350] and poly(methyl methacrylate) [55,110,144,196,349,350], polybutadiene [349], poly(ethyl methacrylate), poly(n-butyl methacrylate), polyfoctadecyl methacrylate), poly(a-methylstyrene), poly(dimethylsiloxane), poly (vinyl acetate), po-ly(vinyl chloride) and poly(vinyl carbazole) [144],polyethylene [351] and other polyolefins [221]. The polyolefin separations were achieved in a special high temperature channel [15,351]. Asphaltenes have also been separated with Th-FFF [352]. 

The improved control over the polymer structure offered by these catalysts offers the polymer producer a significantly greater ability to tailor the polymer to the end-user requirements. Polymer research with metallocene catalysts continues, so more advances can be expected for polyethylene, polypropylene, and other polyolefins. 

Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Flame treatment and corona discharge have also been used. Table 7.20 shows the relative joint strength of bonded polyethylene and other plastic substrates pretreated by these various methods. 

Lubricants can be fatty acids such as stearic acid or hydroxy stearic acid, esters such as stearates, amides or alcohols. Copolymers of styrene and an acrylate can be used. Paraffin and the montan waxes, ethylene bis-stearamide (EBS, or amide wax ), the oxidised polyethylene waxes, and other polyolefin waxes are also employed. 

Synthetic polymers are an integral part of everyday life. Synthetic polymers in the form of polyolefins such as polypropylene and polyethylene and other synthetic polymers such as poly(acrylate)s and poly(styrene)s (PSs) are predominantly used in packaging materials. Condensation polymers are produced in smaller amounts than addition polymers. Nevertheless, some condensation polymers such as polyesters with specific reference to poly(ethylene terephthalate) (PET) and aliphatic polyethers, aliphatic polyamides (i.e., nylons), and polysiloxanes are quite important. 

In addition to PTFE in reducing friction using solid materials as well as films and coatings there are other materials with excellent properties for surface sliding. Polyethylene and the polyolefins in general have low surface friction, especially against metallic surfaces. UHMWPE (ultra... 

It was shown that for most crystalline polymers, including polypropylene and other polyolefins, the tensile drawing proceeds at a much lower stress than kinematically similar channel die compression [10,17]. Lower stress in tension was always associated with cavitation of the material. Usually a cavitating polymer is characterized by larger and more perfect lamellar crystals and cavities are formed in the amorphous phase before plastic yielding of crystals. If the lamellar crystals are thin and defected then the critical shear stress for crystal plastic deformation is resolved at a stress lower than the stress needed for cavitation. Then voiding is not activated. An example of such behavior is low density polyethylene [10]. 

Polyethylene (PE) and other polyolefines are discussed from various angles. La Mantia and Curto propose methods of recycling of photooxidized polyethylene in blend with Nylon 6. It is shown that the recycled PE behaves like a functionalized PE, having compatibilizing attributes due to which blends exhibit improved mechanical properties. 

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers... 

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