Conventional Low Density Polyethylene

Distribution of short-chain branching in firactions of differing molecular weight. —) observed (---------) theoretical [1] 

The results obtained by Bergstrom and Avela [7] by methyl content analysis of the TREF fractions from these four resins are illustrated in Fig. 21. It was noted that only the reference resin and to a lesser extent grade 1 gave the normal relatively linear relationship between methyl content and separation temperature whereas many fractions from grades 2 and 3 exhibit significantly higher separation temperatures than expected from methyl content data. It was concluded that this was due to the presence of composite molecules and reflected effects of polymerization conditions. 

TREF was also used as one of the main characterization tools in the lUPAC, Working Party on Molecular Characterization of Commercial Polymers [23]. The study, Long and Short Chain Branching in LDPE , was a collaborative study by five laboratories. The study concluded that indeed TREF did fractionate according to SCB rather than LCB or molecidar weight and the technique was able to provide a good measure of SCB distribution for the LDPE sample. Good reproducibility between laboratories was also noted. 

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It is interesting to note that so-called linear low-density polyethylenes are said to be less pseudoplastic than conventional low-density polyethylenes. Thus on... 

CPE Conventional (Low Density) Polyethylene LPE Linear (High Density) Polyethylene PP Polypropylene PMP Polymethylpentene... 

LLDPE is generally intermediate in properties between conventional low-density polyethylene (LDPE) made by high-pressure processes and linear, high-density polyethylene (HOPE) of the Ziegler or Phillips type. A qualitative comparison of the more significant physical properties is given in Table II (some quantitative data are given in Table IV),... 

It is claimed that the branching of polyethylene can be controlled to the extent that the product can even be more branched than conventional low-density polyethylene (1,2—300 branches/1,000 atoms) [18, 19]. The cationic Ni-diimine catalyst shown above (R = H, CH3), with the methylaluminoxane analog, has been found to polymerize ethylene in toluene at room temperature at the rate of 110,000 kg/Ni/h. This is comparable to the metallocene rates. The Pd-based catalysts are less active than their Ni analogs [19]. 

It is interesting to note that the development of TREF as a routine polymer analysis tool has until recently taken place almost exclusively within industrial research laboratories. Further, the more sophisticated versions of the TREF have emerged from laboratories associated with companies engaged in the production of linear low density polyethylenes (LLDPE). Clearly this has been driven by the need to understand the nature of LLDPE which exhibits behavior indicative of considerable structural heterogeneity. This is in spite of the fact that, compared to conventional low density polyethylene (LDPE), it is narrow in MWD and contains little or no long-chain branching. 

The properties of polyethylene are strongly infiuenced by its molecular weight, molecular weight distribution, and branching content. Three major classes of polyethylene are commercially available. Conventional low density polyethylene (LDPE) is produced at high polymerization temperatures and pressures. As a... 

Kyu, T, Hu, S.R., and Stein, R.S. (1987) Characterization and properties of polyethylene blends 11. Linear low-density with conventional low-density polyethylene. J. Polym. Sci. Part B Polym. Phys., 25, 89-103. 

Ethylene and a few other monomers are in the gas phase at atmospheric conditions. Thus, conventional low-density polyethylene (LDPE) is produced at pressures in the range of 15,000-50,000 psi (1000-3400 atm), and temperatures of about 150-300 °C [5]. These pressures are well above the critical pressure of ethylene (731 psi), so the monomer can be considered to be in the fluid phase. Depending on the temperature, pressure, and polymer molecular weight, one or two phases may be present as the polymer forms. Peroxide and azo free-radical initiators are used, as are small amounts of oxygen. Oxygen reacts with monomer to form peroxide initiators in situ. 

Metallocene-catalysed very low density polyethylene (m-VLDPE) has become available with densities of as low as 0.903. This is of use for sealing layers of multi-layer films since sealing can commence at lower temperatures than with conventional materials such as LLDPE and EVA (see Section 11.6) with the polymer seal exhibiting both cold strength and hot tack strength. 

Fluidised-bed techniques, pioneered with low-density polyethylene, have been applied to PVC powders. These powders can be produced by grinding of conventional granules, either at ambient or sub-zero temperatures or by the use of dry blends (plasticised powders). The fluidised bed process is somewhat competitive with some well-established paste techniques, and has the advantage of a considerable flexibility in compound design. 

We evaluated the EGAs of Bionolle and starch compound in comparison with conventional resins like low-density polyethylene (LDPE) and polystyrene, because... 

Another important packaging development area is plastic. Various plastics have been and continue to be used high- and low-density polyethylene (HDPE, LDPE), polyvinyl chloride (PVC), polystyrene (PS) and various barrier plastics. These can be formed into bottles of conventional shape or fed into machines producing form-fill-seal packages, typically cups. 

A conventional approach to fhe controlled formation of short-chain branches is ethene copolymerization wifh co-monomers such as propene, butene(l), 4-mefhyl-pentene(l), hexene(l) or octene(l). In the ethene/propene copolymerization example given below an increased number of methyl groups compared with vinyl end groups is consistent wifh a propene incorporation of approximately 6 mol% [Eq. (13)], fhe observed lower DSC melt temperatures and lower densities are typical for medium density (MDPE) and hnear low density polyethylene (LLDPE). 

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