Cross-linked low-density polyethylene

There has been interest, particularly in Japan, in the production of cross-linked low-density polyethylene foam. Some processes, such as the Furukawa process and the Hitachi process, use chemical cross-linking techniques whilst others, such as the Sekisui process, involve radiation cross-linking. 

Cross-linked low-density polyethylene foam can be produced by batch and continuous processes. The cross-linking is accomplished by chemical or irradiation methods. Chemical cross-linking of PE foam is produced in a batch process, and because of production economics is limited to "plank products." The radiation cross-link process permits the continuous production of cross-linked PE foam. This process is limited, however, to production of relatively thin cross sections (up to 3/8 inch) or sheet products (6). 

Morshedian, J. Khonakdar, H.A. Mehrabzadeh, M. Eslami, H. Preparation and properties of heat-shrinkable cross-linked low-density polyethylene. Adv. Polym. Technol. 2003, 22 (2), 112-119. 

In power transmission and distribution cables rated 5 kv and above low density polyethylene (PE), cross-linked low density polyethylene (XLPE) and ethylene-propylene rubbers (EPR) are used in large quantities. 

LDPE—Low-Density Polyethylene HDPE—High-Density Polyethylene XLPE—Cross-linked High-Density Polyethylene PP—Polypropylene... 

There are three basic types of polyethylene foams of importance (/) extmded foams from low density polyethylene (LPDE) (2) foam products from high density polyethylene (HDPE) and (J) cross-linked polyethylene foams. Other polyolefin foams have an insignificant volume as compared to polyethylene foams and most of their uses are as resia extenders. 

Fig. 1. Schematic diagram of polymer stmctures (a) linear (b) cross-linked and (c) branched, where LDPE — low density polyethylene and...

PVC, another widely used polymer for wire and cable insulation, crosslinks under irradiation in an inert atmosphere. When irradiated in air, scission predominates.To make cross-linking dominant, multifunctional monomers, such as trifunctional acrylates and methacrylates, must be added. Fluoropolymers, such as copol5miers of ethylene and tetrafluoroethylene (ETFE), or polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), are widely used in wire and cable insulations. They are relatively easy to process and have excellent chemical and thermal resistance, but tend to creep, crack, and possess low mechanical stress at temperatures near their melting points. Radiation has been found to improve their mechanical properties and crack resistance. Ethylene propylene rubber (EPR) has also been used for wire and cable insulation. When blended with thermoplastic polyefins, such as low density polyethylene (LDPE), its processibility improves significantly. The typical addition of LDPE is 10%. Ethylene propylene copolymers and terpolymers with high PE content can be cross-linked by irradiation. ... 

Cross-linked polyethylene foams are produced by eilher radiation or chemical cross-linking of an extruded expandable sheet containing a chemical blowing agent. These products have finer texture and a softer, more resilient feel than extruded low density polyethylene foams and are used in comfort cushioning and cushion packaging applicutinns. 

Example 11.2 Investigation of the Effects of Interfacial Cross-linking, Diffusion, and Area Generation Rates on Multilayer Miscible (PE-8% GMA)/PE-4% MAH) Films on the Extensional Rheometry of Such Films. Saito and Macosko (54) prepared multilayer films of two low density polyethylene (LDPE) miscible copolymers a random copolymer of... 

Materials. Low density polyethylene (LDPE) foils (thickness 37 im) were purchased from a commercial supplier. Some LDPE samples were cross-linked using electron beam irradiation (dose 250 kGy) under inert gas conditions. S02 of > 99.98% purity as well as compressed air were received from Linde AG. 

Expanded low density polyethylene is found as either a cross-linked foam or a noncross-linked foam, with significant developments being initiated in Japan. It has been offered as a replacement moulding material for closure wadding either with or without an additional facing. When used as a wadding material, densities can be varied to give different levels of compression. 

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