Cross-linked polyethylene foams

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. 

The Hst price of 35 kg/m polyethylene foam boards is about 0.58/BE ( 6.90/kg) and that of cross-linked polyethylene foam is about 0.33 to 0.88/kg more expensive than uncross-linked polyethylene foam depending on the density and thickness of the foam. 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

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. 

Danaei, M., N. Sheikh, and F. A. Taromi. 2005. Radiation cross-linked polyethylene foam Preparation and properties. J Cell Plast 41 551-62. 

In the Toray process, the foamable sheet is expanded as it floats on the surface of molten salts and is heated from the top by infrared lamps. The mol-fen salt mixture consists of potassium nitrate, sodium nitrate, and sodium nitrite. The salt residues from the surface of the foam are blown off by hot air and stripped in water. The Toray process is suitable to produce cross-linked PP foam sheet as well as polyethylene foam sheet. In fact, Toray was the first one to produce commercial PP foam. ... 

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). 

Flexible plastic foams may be found in the form of very soft cushioning materials used in upholstery, clothing interlayers, automobile seats, vibration absorbers, etc. The most common flexible foam resins are polyurethanes, foamed vinyls, cellular polyethylene, cross-linked polyethylene, and silicone foam. Semirigid foams are used for floatation devices, marine bumpers, special electrical insulation on television cables, packaging, and a host of other applications. Rigid foams are used in the production of airplane parts, boats, electronic encapsulation, and many furniture applications where wood was formerly used. 

Cross-linking of polyethylene can be accomphshed either chemically or by high energy radiation. Radiation cross-linking is usually accomphshed by x-rays (44) or electrons (45,46). Chemical cross-linking of polyethylene is accomphshed with dicumyl peroxide (47), d4-tert-huty peroxide (48), or other peroxides. Radiation cross-linking (49) is preferred for thin foams, and chemical cross-linking for the thicker foams. 

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. 

Molded foam for package cushioning, flotation devices, and sports equipment is made from foamed pellets. Polyethylene pellets are foamed with volatile organic liquid and cross-linked by peroxide or electron beam. The foamed pellets are poured into a... 

SAFETY PROFILE Moderately toxic by intraperitoneal route. Combustible when exposed to heat, flames, or reducing agents. To fight fire, use water spray, foam, dry chemical. When heated to decomposition it emits acrid smoke and irritating fumes. Used in the polymerization of styrene and in cross-linking of various grades of polyethylene. See also PEROXIDES, ORGANIC. 

Other important products include radiation cross-linked foamed polyethylene which is used for thermal insulation, floor mats, crash padding, floating jackets, and wood/plastic composites cured by gamma irradiation. These have been used successfully for flooring in places such as department stores, airports, hotels, and churches where their excellent abrasion resistance, the beauty of the natural grain, and low maintenance costs are important. This latter technique is also being used in the conservation of objects made of stone and wood of interest to our cultural heritage. 

A high-temperature cyclic peroxyketal peroxide cross-linking agent for polyethylene has been found to function as a blowing agent as well. This is another example of dual-function additives. Activated by thiodipropionate antioxidants, it evolves COj and should be useful in making crosslinked polyethylene foams. 

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