Crystalline polyethylenes, LDPE

High Density Polyethylene (HDPE). This material has a density in the range 935-965 kg/m and is more crystalline than LDPE. It is also slightly more... 

Low-density polyethylene (LDPE) is produced under high pressure in the presence of a free radical initiator. As with many free radical chain addition polymerizations, the polymer is highly branched. It has a lower crystallinity compared to HDPE due to its lower capability of packing. 

E-plastomers, particularly the high- and medium-density materials, have found extensive use in films [17]. They are valued for their excellent seal character which allows the formation of mechanically strong seals at relatively low temperatures compared to traditional low-density polyethylene (LDPE). In addition, these E-plastomers can be obtained in a range of crystallinities and softness. These higher-density materials are typically made in the blown-film process and are used for protective film covers and disposable bags. 

Due to both kinds of branching leading to chain irregularities, the crystallisation of radical chain-polymerised polyethylene is strongly hindered. Its maximum degree of crystallinity is limited to about 50%, its melting temperature ranges from 80°C to 115°C and its density remains low ( 0.92). From this latter property, it received the name of low-density polyethylene (LDPE). 

The polyethylene obtained (the already cited HDPE) is more highly crystalline and more rigid and dense and has a much more regular structure than the one previously known and obtainable at very high temperature and pressure and which had been industrially produced in the previous 15 years (now called low-density polyethylene, LDPE). The latter shows macromolecules with both long and short branches and is consequently less crystalline than the almost completely linear HDPE. It is obvious that polyethylene does not present tertiary carbon atoms in its constitutional unit, hence it does not show problems of stereoisomerism. 

Free radical vinyl polymerization, the oldest process, leads to branched low density polyethylene (LDPE). Macromolecules have numerous short branches, which reduce the melting point, tensile strength and crystallinity. Polymers are relatively flexible because of the high volume of the branched molecule and the low crystallinity. 

Eoams were extruded from low density polyethylene (LDPE) and blends of LDPE with syndiotactic polypropylene (sPP), using isobutane as the blowing agent. The extruded materials were characterised by measurement of dimensional stability at room temperature, density, tensile properties, dynamic stiffness, and crystallinity determined by differential scanning calorimetry. The sPP, with a slow crystallisation rate, did not interfere with the expansion of the LDPE, and enhanced the temperature resistance by in-situ crystallisation. The blends were flexible, dimensionally... 

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). 

The crystallinity of a polymer such as polyethylene typically increases as the molecular weight and the structural regularity increase but decreases as the extent of irregular branching in the polymer molecule increases. Thus because of its regular structure, hdpe, like linear paraffins, readily forms crystals. In contrast, branched or low-density polyethylene (ldpe) is less crystalline because of its more irregular structure. 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

Crystalline polymers such as high-density polyethylene (hope), PP, PTFE, and polyoxymethylene (POM) exhibit somewhat higher X values than amorphous polymers such as low-density polyethylene (ldpe), atactic PS,... 

Ethylene Polymers. Depending on the polymerization conditions, three major types of polyethylene are manufactured low-density polyethylene (LDPE) by free-radical polymerization, linear low-density polyethylene (LLDPE) by copolymerization of ethylene with terminal olefins, and high-density polyethylene (HDPE) by coordination polymerization. The processes yield polymers with different characteristics (molecular weight, molecular weight distribution, melt index, strength, crystallinity, density, processability). 

ABA ABS ABS-PC ABS-PVC ACM ACS AES AMMA AN APET APP ASA BR BS CA CAB CAP CN CP CPE CPET CPP CPVC CR CTA DAM DAP DMT ECTFE EEA EMA EMAA EMAC EMPP EnBA EP EPM ESI EVA(C) EVOH FEP HDI HDPE HIPS HMDI IPI LDPE LLDPE MBS Acrylonitrile-butadiene-acrylate Acrylonitrile-butadiene-styrene copolymer Acrylonitrile-butadiene-styrene-polycarbonate alloy Acrylonitrile-butadiene-styrene-poly(vinyl chloride) alloy Acrylic acid ester rubber Acrylonitrile-chlorinated pe-styrene Acrylonitrile-ethylene-propylene-styrene Acrylonitrile-methyl methacrylate Acrylonitrile Amorphous polyethylene terephthalate Atactic polypropylene Acrylic-styrene-acrylonitrile Butadiene rubber Butadiene styrene rubber Cellulose acetate Cellulose acetate-butyrate Cellulose acetate-propionate Cellulose nitrate Cellulose propionate Chlorinated polyethylene Crystalline polyethylene terephthalate Cast polypropylene Chlorinated polyvinyl chloride Chloroprene rubber Cellulose triacetate Diallyl maleate Diallyl phthalate Terephthalic acid, dimethyl ester Ethylene-chlorotrifluoroethylene copolymer Ethylene-ethyl acrylate Ethylene-methyl acrylate Ethylene methacrylic acid Ethylene-methyl acrylate copolymer Elastomer modified polypropylene Ethylene normal butyl acrylate Epoxy resin, also ethylene-propylene Ethylene-propylene rubber Ethylene-styrene copolymers Polyethylene-vinyl acetate Polyethylene-vinyl alcohol copolymers Fluorinated ethylene-propylene copolymers Hexamethylene diisocyanate High-density polyethylene High-impact polystyrene Diisocyanato dicyclohexylmethane Isophorone diisocyanate Low-density polyethylene Linear low-density polyethylene Methacrylate-butadiene-styrene... 

Figure 1 Polymer enchainment patterns occurring in polyethylene (PE), ethylene-propylene copolymer (EP), and polypropylene (PP) chains (HDPE = crystalline high-density polyethylene, LLDPE — linear low-density polyethylene, LDPE = low-density polyethylene, EP rubber = elastomeric ethylene-propylene copolymer).

For example, irradiation of either 2-naphthyl acetate (2-NA) or 2-naphthyl myristate (2-NM) in the rubbery state of a low density polyethylene (LDPE) or a high density polyethylene (HDPE) yields a variety of in-cage rearrangement products and 2-naphthol (2-NOL), a cage-escape product (Table 13.2). Both LDPE and HDPE consist of amorphous and crystalline regions LDPE has a larger volume... 

For certain monomers such as vinyl acetate and ethylene, branching is much more significant. The free-radical (high-pressure) polymerization of low-density polyethylene (LDPE) includes a back-biting internal chain-transfer reaction that results in the formation of a short branch. It is this branching that results in an upper limit for the crystallinity of LDPE of about 60%-70% and a melt temperature of 110 C the backbiting reaction preferentially occurs with the formation of an intramolecular six-membered ring that results in preferential formation of a C4 short-chain branch as shown in Scheme 1.41. 

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