Branching in HDPE

Another source of long-chain branches in any nominally linear polyethylene is the cross-linking that can occur whenever it is heated above its melting point, particularly in the presence of air. This is a potential source of uncertainty in laboratory measurements. 

Because even a small amount of long-chain branching has an important effect on the flow behavior of polyethylene melt, it has been of interest to study this effect quantitatively. One 

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The number of branches in HDPE resins is low, at most 5 to 10 branches per 1000 carbon atoms in the chain. Even ethylene homopolymers produced with some transition-metal based catalysts are slightly branched they contain 0.5—3 branches per 1000 carbon atoms. Most of these branches are short, methyl, ethyl, and -butyl (6—8), and their presence is often related to traces of a-olefins in ethylene. The branching degree is one of the important stmctural features of HDPE. Along with molecular weight, it influences most physical and mechanical properties of HDPE resins. 

By NMR spectroscopy, and by comparison with copolymers containing 1-propene and 1-butene comonomer units (E/P and E/B copolymers), typical HDPE samples were shown to contain on the order of 0.5 2.5 methyl branches per thousand backbone carbon atoms longer branches were not detected and are assumed to be absent [47]. Combined NMR and IR evidence indicates that the number of methyl branches in HDPE samples does not correlate with attainable crystallinity values, which implies that the methyl branches can be largely accommodated in the crystalline domains of polyethylene [47, 48]. 

Crystallinity and Density. Crystallinity and density of HDPE resins are derivative parameters both depend primarily on the extent of short-chain branching in polymer chains and, to a lesser degree, on molecular weight. The density range for HDPE resins is between 0.960 and 0.941 g/cm. In spite of the fact that UHMWPE is a completely nonbranched ethylene homopolymer, due to its very high molecular weight, it crystallines poorly and has a density of 0.93 g/cm. ... 

Physical Properties. LLDPE is a sernicrystaUine plastic whose chains contain long blocks of ethylene units that crystallize in the same fashion as paraffin waxes or HDPE. The degree of LLDPE crystallinity depends primarily on the a-olefin content in the copolymer (the branching degree of a resin) and is usually below 40—45%. The principal crystalline form of LLDPE is orthorhombic (the same as in HDPE) the cell parameters of nonbranched PE are a = 0.740 nm, b = 0.493 nm, and c (the direction of polymer chains) = 0.2534 nm. Introduction of branching into PE molecules expands the cell slightly thus a increases to 0.77 nm and b to around 0.50 nm. 

See also van der Waals forces Long-chain aliphatic acids, 20 97 Long chain amphiphiles, 24 123 Long-chain branching, 19 840 extent of, 19 839 in HDPE, 20 160-162 in LDPE, 20 220, 232-234 in LDPE resins, 20 215 quantifying, 20 228-229 Long-chain polyphosphates,... 

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. 

The formation of oxyl radicals in reaction of alkyl radicals with oxygen is important for the possible occurrence of branching of oxidation reaction due to oxygen atoms. This aspect is not, however, obvious from the experiments performed. It seems that oxyl radicals in HDPE are not formed in propagation reaction of alkyl radicals and oxygen but as the product of termination of two peroxyl radicals. 

There are two forms of polyethylene low-density polyethylene (LDPE) and high-density polyethylene (HDPE). The chains in LDPE contain many branches and thus do not pack as tightly as those in HDPE, which consist of mostly straight-chain molecules. 

A third kind of polyethylene introduced in the late 1970s is called linear low-density polyethylene (LLDPE). It is made by the same metal-catalyzed reactions as HDPE, but it is a deliberate copolymer with other 1-alkenes such as 1-butene. It has some side groups (which reduce the crystallinity and density), but they have a controlled short length instead of the irregular, long side branches in LDPE. LLDPE is stronger and more rigid than LDPE it is also less expensive because lower pressures and temperatures are used in its manufacture. 

A major example of the second branched polymer type is the polyethylene that is made by free radical polymerization at temperatures of 100-300°C and pressures of 1,000-3,000 atm. The extent of branching varies considerably depending on reaction conditions and may reach as high as 30 branches per 500 monomer units. Branches in polyethylene are mainly short branches (ethyl and butyl) and are believed to result from intramolecular chain transfer during polymerization (described later in Chapter 5). This branched polyethylene, also called low-density polyethylene (LDPE), differs from linear polyethylene (high-density polyethylene, HDPE) of a low-pressure process so much so that the two materials are generally not used for the same application. 

The property differences cited in Table II are attributed primarily to differences in the degree and kind of branching in the polymer chains. LLDPE is reported to have fewer but longer branches than LDPE. As with HDPE, the branching is further modified by the introduction of higher a-olefins as comonomers (typically, about 8% of 1-butene, 1-hexene, or 1-octene). 

Generally, the density of a 100% amorphous PE sample is considered to be 0.85 g/cm whereas that of a 100% crystalline PE sample is 1.0 g/cm. The degree of crystallinity in HDPE is typically in the range of 60-80%, and 40-50% for LDPE. 50% crystallinity (in LDPE) corresponds to about two branches per hundred carbon atoms in the chain, and 60-90% crystallinity (in HDPE) corresponds to about 0.5 to practically zero branches per hundred carbon atoms. However, in polyethylenes usually the density rather than the crystallinity is referred to because both are connected by a certain linear relationship [4], and density can be faster and more precisely determined experimentally. 

From the results discussed above, it is concluded that the crystallization behavior of HDPE/LLDPE blends depends on the number, length, and distribution of branches in the LLDPE component. In general, it was found that increases in the number and degree of branch distribution in the LLDPE component reduce the tendency of cocrystallization in the LLDPE blend with HDPE. 

The miscibility and crystallization behavior of the three binary PE blends, HDPE/ LLDPE, HDPE/LDPE, and LLDPE/LDPE, were reviewed. In general, differences in the number, length, and distribution of branches in the PE blend components are the major factors governing their miscibility and crystallization phenomenon. In particular, the content and distribution of branches significantly affect both the miscibility and crystallization. Moreover, the presence of a few long chain branches, as well as... 

As has been indicated, the catalyst particle sites control the structural arrangement of the polymer. Unfortunately, not all of the sites on a ZN catalyst behave the same way. There are always some sites that produce a low molecular weight, highly branched material. In HDPE, this waxy material must be removed to produce a polymer with the desired characteristics. 

The property gap that exists between HDPE and LDPE has been filled by LLDPE. This polymer can be prepared by solution- or gas-phase polymerization, and is actually a copolymer of ethylene with 8 to 10% of an a-olefin, such as but-1-ene, pent-l-ene, hex-l-ene, or oct-l-ene. This produces a chain with a controlled number of short-chain branches and densities intermediate between HDPE and LDPE, thereby allowing it to be prepared in various grades by controlling the type of the comonomer. Thus, the use of oct-l-ene gives a lower-density product than that obtained when but-l-ene is incorporated in the chain because the longer (hexyl) branch in the former pushes the chains further apart than the ethyl branch of the latter, hence lowering the packaging efficiency of the chains. 

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