Polyethylene melting point

In the poly(alkylene arylate) series, Tm decreases with increasing length of flexible — (CH2) — moieties and, as in the aliphatic series, approaches the limiting value of polyethylene melting point for large n values (Table 2.6). Aromatic -aliphatic polyesters with even numbers of methylene groups melt at higher... 

Various transition metal compounds were tested (cf. Table 1). In such a large number of examples a wide variety of experimental conditions is reported the solid polymers obtained contain, after purification, not more than 20 p.p m. of dissolved metals. For polyethylene, melting points between 125 and 134, and molecular weights between 6500 and 23000 were reported. 

Figure 10. Functionalized polyethylene melting point as a function of the group, R. Reproduced with permission from Macromolecules 2000,33, 8963-8970. Copyright 2000 Am. Chem. Soc.

When ethylene is polymerized in conventional media at temperatures well below the polyethylene melting point, very long-lived pol3mier radicals form ) ) ). In silver salt solution, however, such "living polymers" are not observed — exhaustion of initiator causes immediate cessation of polymerization. Among the termination reactions which may account for this effect is the oxidation ) of growing radicals by hyper-oxidized silver ion as in equation [l] ... 

Wunderlich [23] reviewed expressions generally similar to Eq. (28), and proposed Eq. (29) for correlating polyethylene melting points with molecular weights ranging from 3200 to 1300,000 (earbons from 228 to 92,678) [23, 60]... 

For illustration we may look on Fig. 5 (19) In this figure the growth of the thickness xc of the layer (between the center of the moving zone and the wall) is demonstrated as a function of time t for a high density polyethylene (melting point 14.3 C). Initial temperature of the melt v/as always 170°C. Several temperatures of the quenched vessel wall are indicated near the curves. 

Polyethylene Melting point (°C) Density (g/cm ) Tensile strength (MPa) Number/type of branches... 

Many polymers, including polyethylene, polypropylene, and nylons, do not dissolve in suitable casting solvents. In the laboratory, membranes can be made from such polymers by melt pressing, in which the polymer is sandwiched at high pressure between two heated plates. A pressure of 13.8—34.5 MPa (2000—5000 psi) is appHed for 0.5 to 5 minutes, at a plate temperature just above the melting point of the polymer. Melt forming is commonly used to make dense films for packaging appHcations, either by extmsion as a sheet from a die or as blown film. 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

DistHlation is then used to separate the hydrocarbons into different products, including Hquid fuels and waxes with melting points ranging from about 45—106°C. Currently the waxes are produced in large volumes in South Africa and Malaysia, with an estimated 12,000—14,000 t consumed in the United States in 1994. Uses are similar to those for polyethylene waxes, including hot-melt adhesives and additives for inks and coatings. 

HDPE melts at about 135°C, is over 90% crystalline, and is quite linear, with more than 100 ethylene units per side chain. It is harder and more rigid than low density polyethylene and has a higher melting point, tensile strength, and heat-defiection temperature. The molecular weight distribution can be varied considerably with consequent changes in properties. Typically, polymers of high density polyethylene are more difficult to process than those of low density polyethylene. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

In the case of crystalline polymers better results are obtained using an amorphous density which can be extrapolated from data above the melting point, or from other sources. In the case of polyethylene the apparent amorphous density is in the range 0.84-0.86 at 25°C. This gives a calculated value of about 8.1 for the solubility parameter which is still slightly higher than observed values obtained by swelling experiments. 

Since polyethylene is a crystalline hydrocarbon polymer incapable of specific interaction and with a melting point of about 100°C, there are no solvents at room temperature. Low-density polymers will dissolve in benzene at about 60°C but the more crystalline high-density polymers only dissolve at temperatures some 20-30°C higher. Materials of similar solubility parameter and low molecular weight will, however, cause swelling, the more so in low-density polymers Table 10.5). 

This polymer is typical of the aliphatic polyolefins in its good electrical insulation and chemical resistance. It has a melting point and stiffness intermediate between high-density and low-density polyethylene and a thermal stability intermediate between polyethylene and polypropylene. 

The acetal polymer moleeules have a shorter backbone (—C—O)—bond and they pack more closely together than those of polyethylene. The resultant polymer is thus harder and has a higher melting point (175°C for the homopolymer). The position of the glass transition is a subjeet of debate since at least two transitions in addition to the melting point are discernible. The true glass transition is usually associated with the temperature at which movement of segments of about 50-150 baekbone atoms becomes relatively easy, in the... 

With all six series of polyester illustrated in Figure 25.14, as the number of methylene groups in the repeating unit increases so the polymer becomes more like a linear polyethylene (polymethylene). Thus the melting points for five of the six classes are seen to converge towards that of the melting point of polymethylene. In the ca.se of the sixth class, the poly(alkylene adipates), there would appear no reason to believe that additional data on other specific members of the class would not lead to a similar conclusion. 

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