Ethylene-1-hexene copolymer density

The second polymer (sample E) was an ethylene-hexene copolymer which Is slightly branched, and had an My of 211,000 and a nominal density of 0.955 g/cm. Specimens approximately one millimeter thick were prepared from both polymers by molding flat sheets from which dumbbell-shaped bars were cut with a die. The sheets were molded by heating the polymer for ten minutes at 440K In a press at which time the heat was turned off and pressure applied. The polymer was then cooled In the press over... 

Lower activation temperatures are usually preferred in commercial operations, despite the lower activity. The corresponding redistribution of sites results in improved polymer physical properties. An example is shown in Table 12, which is a list of the ESCR values, the resistance to chemical attack, of several ethylene-butene and ethylene-hexene copolymers. These copolymers were made at constant MI and density with Cr/ silica catalysts activated at various temperatures. The measured ESCR value decreases as the activation temperature is raised. This change results in part because the MW distribution is narrowed, but more so because the longest chains contain less branching.13... 

Figure 7.4 Dependence between density of ethylene-hexene copolymers and hexane content [10].

Figure 13.32 Environmental stress cracking resistance (ESCR) against natural draw ratio (NDR). Cri and Cr2 are high density polyethylenes with different catalysation regimes, and ZN is an ethylene-hexene copolymer. (Reproduced from Cazenave, j., Seguela, R., Sixou, B. et al. (2006) Short-term mechanical and structural approaches for the evaluation of polyethylene stress crack resistance. Polymer, 47, 3904. Copyright (2006) Elsevier Ltd.)...

Polyethylene (PE) is the most widely used plastic throughout the world, and high density PE (HDPE) is the most widely used type of PE. HDPE has generally been taken to mean the product of ethylene polymerization having density greater than about 0.935 (or 0.94). It includes ethylene homopolymers and also copolymers of ethylene and alpha-olefins such as 1-butene, 1-hexene, 1-octene, or 4-methyl-1-pentene. Other types of PE include low density PE (LDPE), made through a free-radical process, and linear low density PE (LLDPE). 

This patent teaches the importance of preparing the same catalysts discussed in patent (VIII) above on silica calcined at a temperature at or above 800°C in order to manufacture ethylene/1-hexene copolymers with a very narrow molecular weight distribution. The catalysts shown in Table 2.6 were evaluated in a continuous gas-phase fluidized-bed reactor with triethylaluminum as cocatalyst to produce LLDPE polyethylene samples with a density of 0.918 g/cc. 

In an experiment to determine if this type of catalyst may have properties required for industrial applications, Jolly et cd. carried out a polymerization at 80°C with catalyst 3. The results showed that this particular catalyst exhibited an activity of 50,750 Kg PE/mol Cr/hr with a constant polymerization rate for an extended period, thus indicating very beneficial polymerization properties for commercial applications. In addition, catalyst 6 was evaluated in ethylene/1-hexene copolymerization experiments. In neat 1-hexene an elastomeric material was isolated with an activity of 40,000 Kg PE/mol Cr/ hr under polymerization conditions described in Table 3.12, while in toluene containing 20 vol% 1-hexene, an ethylene/1-hexene copolymer was isolated with a DSC melting point of 112°C, suggesting a LLDPE material (i.e., density of ca. 0.91-0.92 g/cc) was produced with a homogeneous branching distribution, which is expected based on the polydispersity value of 1.58 for the same catalyst in an ethylene homopolymerization experiment. This copolymerization data also shows that this particular catalyst has necessary copolymerization characteristics for industrial applications. 

The introduction of single-site catalyst technology has expanded the polyethylene product space to densities as low as about 0.86 g/cc with traditional comonomers such as 1-butene, 1-hexene and 1-octene. In addition, single-site catalysts have provided new ethylene-based copolymers with comonomers based on styrene and cyclic olefins such as norbomene. 

Figure 4.12 Density and 1-hexene content of ethylene/1-hexene copolymers with a heterogeneous branching distribution (—) and homogeneous (solid line) branching distribution [41].

Production of ethylene/1-hexene copolymers was limited by the reactivity of the catalyst with 1 -hexene. Ethylene partial pressure was therefore limited in order to remain in the dry mode and adjust the 1-hexene/ethylene molar ratio at the desired level to produce a LLDPE resin with a density of about 0.916 g/cc. 

Homo- and copolymerlzation of ethylene are receiving considerable attention In the present decade, due to different reasons polyethylene Is the world s most used polymer low linear density polyethylene (LLDPE) produced by copolymerization of ethylene with high a-olefins, such as 1-butene, 1-hexene, 1-octene, etc.. Is one of the most rapidly growing polymers possibilities of producing different types of ethylene-propylene copolymers, such as random and block copolymers, etc. 

Ethylene-propylene copolymers are important materials and are discussed in sections 2.7 and 2.8. Copolymers containing small amounts of a-olefins such as 1-butene, 1-hexene and 1-octene are normally referred to as linear low density polyethylene and are described in section 2.3. 

Crystallinity and Density. These two parameters, which are closely related, depend mosdy on the amount of a-olefin in the copolymer. Both density and crystallinity of ethylene copolymers are also influenced by their compositional uniformity. Eor example, for LLDPE resias with different a-olefin (1-hexene) content, the density (g/cm ) is as follows ... 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

The monomers used to make an addition polymer need not be identical. When two or more different monomers are polymerized into the same chain, the product is a copolymer. For instance, we routinely copolymerize ethylene with small percentages of other monomers such as a-olefins (e.g., 1-butene and 1-hexene) and vinyl acetate. We call the products of these reactions linear low density polyethylenes and ethylene-vinyl acetate copolymer, respectively. We encounter these copolymers in such diverse applications as cling film, food storage containers, natural gas distribution pipes, and shoe insoles. 

We can incorporate short chain branches into polymers by copolymerizing two or more comonomers. When we apply this method to addition copolymers, the branch is derived from a monomer that contains a terminal vinyl group that can be incorporated into the growing chain. The most common family of this type is the linear low density polyethylenes, which incorporate 1-butene, 1-hexene, or 1-octene to yield ethyl, butyl, or hexyl branches, respectively. Other common examples include ethylene-vinyl acetate and ethylene-acrylic acid copolymers. Figure 5.10 shows examples of these branches. 

The general structure of linear low density polyethylene is shown in Fig. 18.2 c). Linear low density resins are copolymers of ethylene and 1-alkenes principally 1-butene, 1-hexene, and 1-octene. Comonomer levels range from approximately 2 to 8 mole %. This family of polyethylene is widely known as LLDPE. Linear low density polyethylenes are polydisperse with regard to molecular weight and branch distribution. 

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