Density Polyethylene Foams

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). 

Polyethylene foams are used extensively in buoyancy applications because of their excellent water-resistant properties. These basically closed-cell foams absorb less than 0.5% by voliune of water after being immersed for 24 hours. The low density of the foams also contribute to their buoyancy. The excellent dielectric characteristics of polyethylene are retained when it is expanded into foams. Polyethylene foam is a candidate for many electrical-material uses requiring good properties of dielectric strength, dielectric constant, dissipation factor, and volume resistivity (6). 

Polyethylene foam is chemically inert and contains no water-soluble constituents. It is resistant to most chemicals and solvents at room temperature. It is unaffected by contact with fuel oil and heavier 

Ease of fabrication is one of the many advantages of polyethylene foam. It can be skived to precise thickness, cut and shaped to form custom parts, and joined to itself or to other materials without major investment in complex equipment. It can also be vacuum formed. Expanded polyethylene will adhere to itself by the use of heat alone. Hot air, or a plate heated to approximately 350°F (177°C) can be used to simultaneously heat the surfaces of two sections of foam to be joined. Upon softening, the two pieces are quickly joined together under moderate pressure, and an excellent bond formed, with only a short cooling period required. Release of the melted foam is aided by a coating of fluorocarbon resin or silicone dispersion on the heating surface. The foam may also be bonded to itself and to other materials by the use of solvents or commercially available adhesives (6). 

---

Economics. Rigid foam systems are typically in the range of 32 kg/m (2 Ibs/fT) and, in 1992, had a foam price of about 3.63/kg ( 1.65 per lb) with hquid foam systems at about 2.75/kg. Unit prices for pour-ia-place polyurethane packaging systems fall between the competitive expandable polystyrene bead foam at 3.30/kg and low density polyethylene foams at 5.80/kg. 

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. 

Swell Ratio Parameter for Prediction of Chemically Crosslinked Low Density Polyethylene Foam Expansion Characteristics... 

EFFECT OF MOLECULAR WEIGHT ON MECHANICAL AND MORPHOLOGICAL PROPERTIES OF HIGH DENSITY POLYETHYLENE FOAMS... 

THE USE OF POLYFUNCTIONAL MONOMERS IN COMPRESSION-MOULDED LOW-DENSITY POLYETHYLENE FOAM FORMULATIONS... 

The thermal conductivity of a section of a commercially produced high density polyethylene foam channel was measured. The walls consisted of a 6.4 mm foam core with a skin of 1.6 mm thickness on either side. Sqnares were machined from the outer surface of the channel, so that heat flow throngh the entire thickness the core pins one skin layer and the complete section conld be... 

Patent Number US 5905098 A 19990518 PROCESS FOR PRODUCING LOW DENSITY POLYETHYLENIC FOAM WITH ATMOSPHERIC GASES AND POLYGLYCOLS OR POLYGLYCOL ETHERS Wilkes G R Dunbar H A Bly K A Uhl E R Tenneco Protective Packaging Inc. 

Crosslinked low-density polyethylene foams with a closedcell structure were investigated using differential scanning calorimetry, scanning electron microscopy, density, and thermal expansion measurements. At room temperature, the coefficient of thermal expansion decreased as the density increased. This was attributed to the influence of gas expansion within the cells. At a given material density, the expansion increased as the cell size became smaller. At higher temperatures, the relationship between thermal expansion and density was more complex, due to physical transitions in the matrix polymer. Materials with high density and thick cell walls were concluded to be the best for low expansion applications. 16 refs. 

MULTIPLE-IMPACT PERFORMANCE OF HIGH-DENSITY POLYETHYLENE FOAM... 

Polymers such as polyethylene, which do not have polar groups, are excellent insulators of heat and electricity. The thermal insulating properties may be improved by foaming or by the incorporation of hollow glass spheres (syntactic foams). A low-density polyethylene foam will have a thermal conductivity in the order of 0.3 BTU/ft2 h F in. 

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. 

High-Density Polyethylene Foams These foams are defined by the specific gravity (ca. 0.96 g/cc) and melt index of the basic resin. They have been used extensively in the U.S. and Europe, particularly for materials-handling applications, such as boxes, crates, and pallets because, imtil recently, they were much cheaper than polypropylene (13). 

Polypropylene Foams Polypropylene foams are comparatively recent entries into the structural foam field, supplanting hi -density polyethylene foams, but their use is increasing rapidly because of the extreme range of grades and properties available, plus a favorable price advantage, compared with other thermoplastic foams. Glass-reinforced (30% glass) polypropylene foams are commonly used (13). 

Skochdopole, R.E. and Rubens, L.C., "Physical Property Modifications of Low-Density Polyethylene Foams, Journal of Cellular Plastics, 1 91-96 (January 1965). 

Cell Flake, Pigment flakes, Eckart America LP Cellu-Cushion, High-density polyethylene foam. Sealed Air Corp. 

Low density polyethylene foam products (thin sheets, planks, rounds, tubes) in the range of 32-160 kg/m (2-10 Ib/ft ) have been prepared by an extrusion technique using various gaseous fluorocarbon blowing agents (97,98). The techniques are similar to those described earlier for producing extruded polystyrene foam planks and foam sheets. 

Lee JWS, Park CB. Use of nitrogen as a blowing agent for the production of fine-celled high-density polyethylene foams. Macromol Mater Eng 2006 291 1233-44. 

Zhang Y., Rodrigue D., Ait-Kadi A., High Density Polyethylene Foams. IV. Flexural and Tensile Moduli of Structural Foams, Journal of Applied Polymer Science, 90 (8) 2139-2149 (2003). 

---