High-pressure, low-density

PE produced by a high-pressure polymerization process (pressure 1000-3000 atm) using a free radical initiator is a highly branched material that contains both LCBs and SCBs. The polymer so produced is a low-density material (density up to about 0.925 g/cc) and is known as high-pressure low-density PE (HP LDPE). The LCBs are formed via intermolecular hydrogen transfer [19], whereas SCBs are formed by intramolecular hydrogen abstraction [16]. 

The method outlined above for characterizing branched polymers will hereafter be referred to as the molecular weight and branching distribution (MWBD) method. In the following sections, its application to the long chain branching in polyvinyl acetate and high pressure low density polyethylene will be demonstrated. 

In order to determine the branching structure factor e, Foster ( ) studied a large qroup of high pressure low density polyethylene resins (HP-LDPE). Using the MWBD method, he calculated the whole polymer number average number of branch points per JOOO carbon atoms from SEC data as a function of e. Then the Xfj values were compared with those obtained by nMR. 

PROPERTY SUMMARY FOR HIGH PRESSURE, LOW DENSITY POLYETHYLENES... 

The samples investigated initially were commercial high pressure low density, linear low density and high density polyethylenes and had properties given in Table 1. Solutions of these polymers were prepared in concentrations of 0.8 to 3.5 g/1 by dissolving the pol5rmer over a time period of two hours in an oven maintained at 145°C. To avoid degradation 0.05% 4,4 -thiobis(3-methyl-6-tert-butyl phenol) was used as an antioxidant in the solutions. 

Experimental data given in Fig. 12 are interesting both from the practical and theoretical point of view. The figure gives tentative flow curves of high-pressure (low-density) polyethylene with MFI = 2.0 g/10 min at a temperature of 170 °C in chan-... 

High pressure low density polyethylene resins are produced in two types of reactors. One is a continuous-flow mechanically stirred autoclave (origi-... 

Kinetics of High Pressure (Low Density) Ethylene Polymerization... 

Theonly important current application of tubular reactors in polymer syntheses is in the production of high pressure, low density polyethylene. In tubular processes, the newer reactors typically have inside diameters about 2.5 cm and lengths of the order of I km. Ethylene, a free-radical initiator, and a chain transfer agent are injected at the tube inlet and sometimes downstream as well. The high heat of polymerization causes nonisothermal conditions with the temperature increasing towards the tube center and away from the inlet. A typical axial temperature profile peaks some distance down the tube where the bulk of the initiator has been consumed. The reactors are operated at 200-300°C and 2000-3000 atm pressure. 

Ethylene/carbon monoxide copolymers containing 2 or 3 wt. o carbon monoxide are photo degradable polymers with the same general processing properties as high pressure, low density polyethylene. Exposure to UV radiation causes decomposition. The polymer is essentially a low density polyethylene with an environmental feature. Commercialization of these materials took place in the late 1960s. 

Example 10.8 The production of high-pressure low-density polyethylene is carried out in tubular reactors of typical dimensions 2.5 cm diameter and 1 km long at 250°C and 2500 atm. The conversion per pass is 30% and the flow rate is 40,000 kg/h. Assuming that the polymerization reaction is first order in ethylene concentration, estimate the value of the polymerization rate constant. 

From this brief survey of contemporaneous discoveries on polyolefin polymerization, it is evident that catalysis became a dominant technology factor to consider within 20 years of the discovery of high-pressure (low-density) polyethylene. These various catalysts enabled linear polyethylene of higher density and higher... 

High-pressure, low-density polyethylene, LDPE, was blended with cyclOTubber, to give compounds useful for bonding polyethylene to metal... 

DuPont in USA disclosed a process for the preparation of expanded PE in 1942, using nitrogen as a blowing agent. In 1945 carbon dioxide was used instead of nitrogen. Commercial production of expanded PE as an electric cable insulation started in 1950s. In 1958 chlorofluorocarbons (CFC) were introduced, and foamed PE insulation was based on high pressure, low density PE (LDPE) [24]. 

LDPE is also known as high pressure, low density or HPLD, because it is exclusively made by the high pressure process. LDPE is produced imder high pressures (81-276 MPa) and high temperatnres (130-330°C) with a free-radical initiator, such as peroxide or oxygen. The polsnnerization mechanism is a free-radical... 

The occurrence of impurities and structural deviations, such as branching and double-bond formation, depend on the method of polymerization. The degree of branching is related to the crystallinity and consequently influences the density of the polyethylene. The more branching the PE has, the more amorphous (i.e., the lower the density vide infra) and the more transparent the PE becomes. Three types of polyethylene are differentiated high-pressure, low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). All three of these PE types are manufactured commercially [30]. 

The first step in the characterization of polyethylene is to determine the class or morphology of the material HDPE, LLDPE, or LDPE (also high-pressure, low density PE, HPLDPE). When the IR spectra of polyethylene are thoroughly examined, subtle but definite differences are observed among with these different forms of polyethylene [31-33]. Conversely, the Raman spectra of these materials do not show any significant variation, except for differences in line width [34]. For this reason, the infrared spectra of the polyethylenes will be the main focus of this section, and only a brief review of the Raman spectra will be provided later. 

Kiparissides, C., Krallis, A., and Meimaroglou, D. (2010) From molecular to plant-scale modeling of polymerization processes A digital high-pressure low-density polyethylene production paradigm. Chem. Eng. ej Techn., 33, 1754-1766. 

E/CO is produced commercially by the high-pressure copolymerization of ethylene and carbon monoxide using techniques similar to those used to make high-pressure, low-density polyethylene homopolymer (LDPE). The monomers undergo random copolymerization under well-controlled temperatures and pressures in either tubular or stirred autoclave reactors ... 

Usami, T., Takayama, S. Fine-branching structure in high-pressure, low-density polyethylenes by 50.10-MHz C NMR analysis. Macromol. (1984), 17, pp. 1756-1761... 

The effect of increased crystaUinity on the mechanical properties of a sample is, e.g., reflected in a higher density. High-pressure (low-density)... 

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