Polyethylene terms Links

The overall crystallization kinetics of an unfractionated linear polyethylene, cross-linked by a peroxide reaction has also been studied. (95,96) A special feature of this work was the study of the separated sol and gel portions at different levels of cross-linking. The overall crystallization rates, in terms of the reciprocal of the half-time, l/ti/2, are plotted against the crystallization temperamre in Figs. 10.41 and 10.42 for a set of sol and gel fractions respectively. The gel fractions are characterized by the molecular weight between cross-links. Me, assuming ideal network formation. The sol portions are defined by their number average molecular... 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

However, the Lorentzian form of the dipolar broadening function, which has the advantage of mathematical simplicity, is not suitable for an interpretation in terms of second moments it is replaced with a Gaussian dipolar function S(oa, AG), where the parameters AG correspond to the appropriate fractions of the square root of the intra-group rigid lattice second moments. With appropriate values for AG, calculated and experimental line shapes I(oo) are found to be in a good agreement for cross-linked polyethylene oxide) swollen in chloroform 1U). 

We present here some aspects of the surface chemistry and some explanatory models for vrater-polymer and related Interfaces. The term "poljrmer" will be taken to mean an essentially organic material, of sufficiently high molecular weight and (or) sufficiently cross-linked that a stiff (as opposed to fluid) phase is Involved. The material Is Insoluble In water, so that the term "water-polymer" Interface refers to what Is macroscopIcally an ordinary phase boundary. Typical polymers In the present context will be polytetrafluorethylene (PTFE), and polyethylene (PE). Solutions of macromolecules are thus not considered, nor is the related topic of so-called hydrophobic bonding, although some of what Is discussed here is relevant to that subject. 

The above polyethylene was irradiated in 0.009-inch-thick sheet at 21° to 23°C. in a nitrogen atmosphere with 1-m.e.v. electrons to a total dose of 20 megarads to give the material which is termed irradiated or cross-linked polyethylene. 

However some non-rubber compounds are also called elastomers if they exhibit a nondeforming elastic property similar to rubber at room temperature, even if the compound is relatively hard. The two main groups of non-rubber elastomers are thermoplastics, e.g., polyvinyl chloride, polypropylene and thermosets, e.g., ethylene propylene rubber, cross-linked polyethylene. These two groups are also covered by the term plastic . 

Cross-linking polyethylene enhances its heat resistance (in terms of resistance to melt flow) since the network persists even about the crystalline melting point of the uncross-linked material. Cross-Knked polyethylene thus finds application in the cable industry as a dielectric and as a sheathing material. Three main approaches used for cross-linking polyethylene are (1) radiation cross-linking, (2) peroxide cross-linking, and (3) vinyl silane cross-linking. 

An estimate of the number of monomer units per equivalent random link can be obtained by dividing the value of Aa calculated from the stress-optical coefficient by the anisotropy of the polarisability of the monomer unit calculated from bond polarisabilities. This number can more interestingly be expressed in terms of the number of single bonds in the equivalent random link and is found to be about 5 for natural rubber, about 10 for gutta percha and about 18 for polyethylene. (For the last two the values are extrapolated from measurements at elevated temperature.) The number for polyethylene is considerably higher than the value of 3 suggested by the assumption of totally free rotation around the backbone bonds (see section 3.3.3 and problem 3.7). 

Polyethylene blends having toughness and elastic recovery comparable to those of plasticized PVC comprised >50 wt% of a copolymer of ethylene and either butene or hexene [LLDPE, p = 880-915 kg MI < 1 dg min , long-chain branching =0.5-1.5 long chalns/l.OOOC, M > 200 kg mol ] >5 wt% of a copolymer of ethylene and either vinyl acetate or ethyl acrylate, EVAc or EEA and 5-30 wt% liquid hydrocarbon oil. The blends showed essentially no yield point and behavior similar to that of cross-linked materials, although they were not cross-linked (strain recovery). They were found competitive with plasticized PVC in terms of both physical properties and economics... 

Because of some of the problems with FRP vessels, cross-linked polyethylene (XLPE) is sometimes specified for hypochlorite service. The long-term stability of this material, however, may be inadequate under certain conditions [78]. Rotationally molded... 

Low-density polyethylene (LDPE) This term is generally considered to include polyethy-lenes having densities between 0.915 and 0.925 g/cm. In LDPE, the ethylene molecules are linked in random fashion, with many side branches, mostly short ones. This branching prevents the formation of a closely knit pattern, resulting in material that is relatively soft, flexible, and tough, and which will withstand moderate heat. See also high-density polyethylene and polyethylene. 

The method of choice for the determination of these quantities on HDPE resin is gel permeation chromatography. In this chromatographic method a solvent with the dissolved polyethylene flows through a column filled with a porous material. It is usually a gel of cross-linked polymers. Depending upon the size of the polyethylene coils in the solution, they can fit to the pores of the gel, stick there and block them whereby the transport is retarded. Larger polyethylene chains are transported unhindered with the solvent flow. Thus a size-dependent retardation occurs which justifies the other term size exclusion chromatography (SEC). In the chromatogram, which shows polymer concentration (determined for example via optical measurements (refractive index)) as a function of time, the time coordinate corresponds to a polymer molecular mass coordinate and the concentration... 

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