Polyethylene, types

The mechanism of branch formation in LDPE, as described in detail in Chapter 3, is different from that in coordination polymerization in LDPE, LCBs are formed by transfer 

From a polymer reaction engineering point of view, polyolefins made with metallocene catalysts provide an excellent opportunity for model development because they have well-behaved microstructures. Later it will be shown that models developed for single-site catalysts can also be extended to describe the more complex microstructures of polyolefins made with multiple-site catalysts such as Ziegler-Natta and Phillips catalysts. 

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The process yields a random, completely soluble polymer that shows no evidence of crystallinity of the polyethylene type down to —60°C. The polymer backbone is fully saturated, making it highly resistant to ozone attack even in the absence of antiozonant additives. The fluid resistance and low temperature properties of ethylene—acryUc elastomers are largely a function of the methyl acrylate to ethylene ratio. At higher methyl acrylate levels, the increased polarity augments resistance to hydrocarbon oils. However, the decreased chain mobiUty associated with this change results in less fiexibihty at low temperatures. 

Due to the low level of branching, there is little to hinder the crystallization of high density polyethylene. We routinely observe crystallinity levels in excess of 60%, which translate into densities ranging from approximately 0.94 to 0.97 g/cm3. High density polyethylene is the stiffest of all the polyethylene types. In some cases we incorporate small amounts of a comonomer, such as 1 -hexene, which reduce the crystallinity level. This improves toughness at the expense of stiffness. 

Polyester is the Madison Avenue name for PET, polyethylene terephtha-late. Be careful, though, because PET is not a polyethylene-type chemical— its a xylene derivative. If you remember that esters usually end in -ate and are based on the acid from which they are derived, then you can see that the ester in polyester is ethylene terephthalate. 

The majority of physisorption isotherms (Fig. 1.14 Type I-VI) and hysteresis loops (Fig. 1.14 H1-H4) are classified by lUPAC [21]. Reversible Type 1 isotherms are given by microporous (see below) solids having relatively small external surface areas (e.g. activated carbon or zeolites). The sharp and steep initial rise is associated with capillary condensation in micropores which follow a different mechanism compared with mesopores. Reversible Type II isotherms are typical for non-porous or macroporous (see below) materials and represent unrestricted monolayer-multilayer adsorption. Point B indicates the stage at which multilayer adsorption starts and lies at the beginning of the almost linear middle section. Reversible Type III isotherms are not very common. They have an indistinct point B, since the adsorbent-adsorbate interactions are weak. An example for such a system is nitrogen on polyethylene. Type IV isotherms are very common and show characteristic hysteresis loops which arise from different adsorption and desorption mechanisms in mesopores (see below). Type V and Type VI isotherms are uncommon, and their interpretation is difficult. A Type VI isotherm can arise with stepwise multilayer adsorption on a uniform nonporous surface. 

Refractive index increments (dn/dc) were measured using the Chromatix KMX-16 differential refractometer and its heated cell at 145°C the values obtained are given in Table II. Refractive index Increments are essentially Identical for all the polyethylene types in a given solvent. 

A fourth mechanism has been suggested by several researchers in order to explain the formation of near-linear polyethylene type oligomers produced on modified[22] and unmodified1231 ZSM-5 zeolite from olefins such as propene, isobutene and 1-decene. They propose a mechanism which proceeds by losing methyl branches via protonated cyclopropyl intermediates as presented in Scheme 6.4. 

Effect of Substrate. Again, polyethylene and ethylene-propylene copolymers are better substrates for block formation than polypropylene (Table XI). Polyethylene is better than polypropylene, and a polyethylene-polypropylene-polyethylene type of block polymer is better than polyethylene. This agrees with what has been found for AFR polymers containing methylvinylpyridine and acrylonitrile. It also supports our belief that AFR polymers are formed by the growing of a free radical polymer onto active ends of anionic polymer chains. If it were a random grafting reaction, it would be hard to explain why a propylene polymer with a more vulnerable tertiary hydrogen should give a lower... 

Volume differences between the guest molecules containing naphthoxy and phenoxy and the hole free volumes of the polyethylenes do not appear to be the reason for these results. An indication of what may be responsible is found in the 2a/ 4a ratios from 2a regardless of the polyethylene type or its unstretched/stretched state, the ratios remain near unity. If the rotational motion of the coin-shaped phenoxy moiety of a radical pair is faster than (or comparable to) to the rate combination of phenylacetyl to phenoxy, the overall effect on the relative rates would be the same as if there were translational motions between the two radicals. 

Other polyethylene-type polymers are made from monomers containing chloro, methyl, cyano, and phenyl substituents, as summarized in Table 22.7. In each case the double carbon-carbon bond in the substituted ethylene monomer becomes a single bond in the polymer. The different substituents lead to a wide variety of properties. 

Similarly, 2-methylpropene (isobutene) is an important monomer. It only polymerizes by a cationic mechanism, and its copolymers with dienes are known as butyl rubber. Higher 1-alkenes (1-butene, 1-hexene, 1-octene) are important copolymerization components [4, 5] they produce tailored branching of some polyethylene types prepared by a coordination mechanism. Longer-chain alkenes (Cjq, C,2, Cj ) are also sometimes used as comonomers... 

In addition to determining the time scales for several local motions in polyformal, two different interpretational models for segmental motion will be employed. An older model by Jones and Stockmayer (7 ), based on the action of a three bond jump on a tetrahedral lattice is compared with a new model by Weber and Hel-fand (8), based on computer simulations of polyethylene type chains. These two models for segmental motion have been compared before (5 ) for two polycarbonates but somewhat different results are seen in the polyformal interpretation. 

Polyethylene, type (density, g/cm ) Marlex Index Flexural modulus (psi)... 

Figure 3.1 Density, crystallinity, and polyethylene type scale.

Figure 8. Effect of progressive halogen substitution on wettability of polyethylene-type surfaces [96]...

Fig. 9 Viscosity in its dependence tm shear rate in three polyethylene types, temperature 150°C. The processing ranges are indicated at the upper right (BASF, Ludwigshafen)...

Kelusky et al. [11] have also applied TREF for EVA copolymer analysis. Their interest was mainly in the use of TREF for blend analysis involving these copolymers. They did, however, evaluate a series of six EVA copolymers in the range of 8 to 24 wt.%, showing how the TREF separation was dependent on the total branching, polyethylene type as well as VA. The resins analysed are listed in Table 4 which includes the analysis of all branches by NMR. The TREF curves (Fig. 23) clearly show the sensitivity of the separation to the total SCB level. 

T. Kotani, T. Taka, Y. Saito, High density polyethylene type transparent film and process for production thereof, U.S. Patent 4,954,391 of 04 Sept 1990, priority 1986, to Showa Denim Kabushiki Kaisha... 

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