Processing of Polyethylene

Compression moulding is the process in which powder or granules of semi-finished product are put directly between tool faces, which are brought together under pressnre to shape the material. 

The injection moulding process involves the rapid pressnrised filling of a specific mould cavity with a fluid material, followed by solidification into a product. A high holding or packing pressure 

The cooling of the material in the mould is the limiting factor in injection moulding, because of the low thermal conductivity of polymers. After the cooling stage the monld is opened and the solid ejected. 

There are two types of injection moulding equipment plunger-type (also called ram or piston) and screw-type machines. Linear low-density PE (LLDPE) requires more heat to be moulded than LDPE. These materials are more easily moulded in a screw machine in which the mechanical work of the screw adds considerably to the heat delivered by external electric heaters. 

The projected area of a mould includes cavities and feed systems. The injection pressure is a function of part thickness, melt temperature, and mould temperature. It can vary within the mould. The three-zone reciprocating screw should achieve good plasticisation of the material. The estimation of the clamping force needed to keep the mould shut is a complex function of the projected area and the injection pressure. A rule of thumb for the clamping force required for PE is 0.2-0.5 tons/cm of projected area. A large safety factor of 25-50% is advisable. Computer simulation of flow will provide a more accurate estimation of the injection pressure and mould opening force. 

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Japon, S., Boogh, L., Leterrier, Y. and Manson J. A. E., Reactive processing of polyethylene terephthalate) modified with multifunctional epoxy-based additives, Polymer, 41, 5809 (2000). 

STUDY OF THE FOAMING PROCESS OF POLYETHYLENE WITH HIGH PRESSURE C02 IN A MODIFIED EXTRUSION SYSTEM... 

The different available high pressure polymerisation processes of polyethylene (PE) yield LDPE (low density PE), LLDPE (linear low density PE) and copolymer features of the same. The various process variations have been developed during recent decades and introduced a number of well developed steps and devices to achieve safe and economical operating conditions at the very high reaction pressures of 1500 to 3000 bar. 

Yasuniwa, M. and Takemura, T. Microscopic observation of the crystallization process of polyethylene under high pressure. Polymer 15, 661 (1974)... 

Fig. 20 a Change in adhesion forces measured by SFM using chemically modified tip during the time-dependent etching process of polyethylene films, b Adhesion titration curves of PE films unmodified ( ) and etched with chromic acid for 45 min ( )... 

Oxidation during processing of polypropylene is principally accompanied by chain scission made evident by a reduction in melt viscosity. The oxidation during processing of polyethylene on the other hand is accompanied mainly by crosslinking. The following two tests are, therefore, used ... 

Kollicoat IR is a unique polymer for pharmaceutical applications prepared by a graft polymerization process of polyethylene glycol (25%) with polyvinyl alcohol (75%). Kollicoat IR dissolves quickly in water and aqueous solutions of acid and alkali and reduces the surface tension of aqueous solutions to allow the solutions to have high spray rates. The polymer film is very flexible, not tacky, and easily colored. The polymer can be used as instant release coating, pore former, binder, protective colloid, etc. 

De Gennes scaling concepts for polymer solutions and melts and the concept of reptation movement of polymer chains in melts Pennings discovery of chain extension and shish-kebab formation in stirred solutions of very high MW polyethylene this eventually led to the ultra-high modulus gel-spinning process of polyethylene... 

Cabana, A., Ait-Kadi, A., and Juhasz, J. (1997), Study of the gelation process of polyethylene oxide copolymer (poloxamer 407) aqueous solutions,/. Colloid Interface Sci., 190, 307-312. 

Sanchez-Solis, A. Estrada, M.R. Cruz, J. Manero, O. On the properties and processing of polyethylene terephthalate/styrene-butadiene rubber blend. Polym. Eng. Sci. 2000,40 (5), 1216-1225. Luzinov, I. Xi, K. Pagnoulle, C. Huynh-Ba, G. Jerome, R. Composition effect on the core-shell morphology and mechanical properties of ternary polystyrene/styrene butadiene rubber polyethylene blends. Polymer 1999, 40 (10), 2511-2520. 

Yasuniva M, Nakafiiku C, Takemnra T (1974) Melting and crystallization process of polyethylene under hi pressure. Priymer J. 4 526... 

Jackson JF, Hsu TS, Brasch JW (1972) Direct microscope observatioiK of tte crystaDization process of polyethylene at high pressure, acicular bladed crystals. J. Pdymer ScL, Polymer Letters 10 207 see also Nature (London) Phys. Sci. 239 45 (1972)... 

Kinetics of Formation of Main Chemical Products in Process of Polyethylene Treatment in Pulsed RF Discharges... 

Microwave Curing Process of Polyethylene, Akzo Chemie, GmbH Diiren 1983... 

The chemical ionisation mass spectral data indicate that intramolecular exchange reactions predominate in the primary thermal fragmentation process of polyethylene oxalate resulting in the formation of cyclic oligomers. These products are not stable in the electron ionisation (El) mode and are therefore not directly observed in the El mass spectrum. 

The process of polyethylene oxidation is characterized by struc-turii - the formation of cross-linked structures [12]. Structuring is usually manifested at increased temperatures. Thus, the oxidation of a fusion of polyethylene at 180°C leads to the accumulation of insoluble products after 30 hr, about 50% of them accumulate after 61 hr, 90% after 150 hr, 100% by weight. Under the conditions of structuring, the viscosity of the polymer first drops rapidly during the oxidation process, and then rises as insoluble structures accumulate. 

Harper J, Price D and Zhang J (2007) Use of fillers to enable the microwave processing of polyethylene, J Microw Power Electromagn Energ 40 219-227. 

Westerberg LM, Pfaffli P, Sundholm F. Detection of lice radicals during processing of polyethylene and polystyrene plastics. Am Ind Hygiene Assoc J. 1982 43 544-6. 

Molecular weight distribution (MWD) is most important factor to determine the mechanical property and processability of polyethylene (PE). The control of MWD is one of the main targets for development of new generation polymerisation catalyst. In this work, a novel Si02-supported Ziegler-Natta catalyst designed in a molecular level on the silica gel surface modified with linear long alkene chains was developed for ethylene polymerisation. The coordinative nature of alkyl-Al co-catalyst should be expected to vary with its structure. The catalysts modified by various alkyl-Al co-catalysts were studied by Al MAS method, and the relationship between the coordinative states of A1 species and the MW and MWD of PE produced was clarified. ... 

The Bakelite Division of Union Carbide received US. Patent 2,462,331 on February 22, 1949, for the use of carboxylic acid esters or metal salts of carboxylic acids as additives to polyethylene, that improved the processing of polyethylene into film and sheets. The addition of 0.2-0.5 wt% of these compormds to polyethylene was reported to cause the ready release of the polyethylene film or sheet from hot milling and calendaring rolls at elevated temperatures (115-150°C) to provide films and sheets with good surface appearance. Examples of these releasing agents were calcium stearate, aluminum laurate and aluminum tristearate. 

Crystallization process is an important consideration in selection of an antiblocking agent for polyethylene. Silica antiblocking agents do not interfere with the crystallization process, but talcs may affect the crystallization process of polyethylene to a large extent. " ... 

Yasuniwa M, Nakafuku C, Takemura T. Melting and crystallization process of polyethylene under high-pressure. Polym J 1973 4(5) 526-33. 

Ghosh, R, Dev, D., Chakrabarti, A. Reactive melt processing of polyethylene Effect of peroxide action on polymer structure, melt rheology and relaxation behavior. Polymer (1997) 38, pp. 6175-6180... 

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