Melt density Polyolefins

Melting point alone cannot uniquely identify an OBC. For example, blends of high and low density polyolefins also exhibit an elevated melting point at equivalent density. Sample 3 in Fig. 17 (small circle) is a 70 30 physical blend of 0.86 and 0.94 g cm-3 ethylene-octene copolymers, and the melting point is similar to the OBCs. Physical blends of polymers of such disparate densities are not phase-continuous, however, and segregate into domains of the high and low density polymers. Figure 18 reveals differences in appearance of pressed plaques of the polymer samples... 

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. 

This is a blend of LDPE, which generally has a density between 0.91 and about 0.93 g/cu.cm. and a melt index greater than 1 and a silane-grafted single-site initiated polyolefin, which is generally a copolymer of ethylene and a C3 to C20 alpha-olefm having a density between about 0.86 and 0.96 g/cu.cm. and a MWD between about 1.5 and 3.5. 

When a block copolymer is blended with a homopolymer that differs in composition from either block the usual result is a three-phase structure. Miscibility of the various components is not necessarily desirable. Thus styrene-butadiene-styrene block copolymers are recommended for blending with high density polyethylene to produce mixtures that combine the relative high melting behavior of the polyolefin with the good low temperature properties of the elastomeric midsections of the block polymers. 

The polyolefins produced by transition metal catalysts are characterized by the absence of large amounts of long- or short-chain branching, which causes variability in density, crystallinity, and melting points. Most catalysts used are heterogeneous but some homogeneous systems are known. A two-step mechanism for catalysis is widely accepted (1) adsorption of the monomer, which may be activated by the configuration established in this step, and (2) insertion of the activated monomer into a metal-carbon bond. 

Two important types of elastomeric polyolefin blends are reactor-made iPP/ EPR blends and postreactor blend iPP/EPDM. The latter is called thermoplastic vulcanizates (TPVs), produced by dynamic vulcanization of blends containing a thermoplastic and an elastomer. To make iPP/EPDM TPV, the two polymers PP and EPDM are mixed with curatives, such as peroxides, phenolic resins, or sulfur with accelerators, and dynamically cured in an extmder resulting in a blend consisting of micrometer-sized elastomer particles dispersed in the PP matrix (20-24). Paraffinic oils are added in the melt mixing process for viscosity control and cost. In iPP/ EPDM TPV, the crystalline iPP resin is normally the minor phase. Recently, polyolefin plastomers have been added to the class of elastomeric polyolefin blends. Polyolefin plastomers are ultralow molecular weight linear low density polyethylenes (ULMW-LLDPE). Nonelastomeric polyolefin blends are blends of polyolefins with mostly nonpolyolefin (other thermoplastic) matrices as mentioned earlier. 

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