Chemistry:Expanded polyethylene
Expanded polyethylene (aka EPE foam) refers to foams made from polyethylene. Typically it is made from expanded pellets ('EPE bead') made with use of a blowing agent, followed by expansion into a mold in a steam chest - the process is similar to that used to make expanded polystyrene foam.
Properties
EPE foams are low density, semi-rigid, closed cell foam that are generally somewhere in stiffness/compliance between Expanded polystyrene and Polyurethane. Production of EPE foams is similar to that of expanded polystyrene, but starting with PE beads.[1] Typical densities are 29 to 120 kg/m3 (49 to 202 lb/cu yd) with the lower figure being common.[1] Densities as low as 14 kg/m3 (24 lb/cu yd) can be produced.[2]
Base polymer for EPE foams range from Low-density polyethylene (LDPE) to High-density polyethylene (HDPE).[2]
Co-polymers
Expanded polyethylene copolymers (EPC) are also known - such as 50:50 (weight) materials with polystyrene. Though other properties are intermediate between the two bases, toughness for the copolymer exceeds either, with good tensile and puncture resistance. It is particularly applicable for re-usable products.[1]
Production
EPE foams were first manufactured in the 1970s.[2]
Production of the PE beads is usually by extrusion, followed by chopping, producing a 'pellet'.[3] Autoclave expansion is the most common route the bead foam.[4] Butane or pentane is often used as a blowing agent (before 1992 CFCs may have been used). Depending on the specific process uses the beads may be cross-linked either by electron beam irradiation (see Electron beam processing), or by the addition of a chemical agent such as dicumyl peroxide.[3]
An alternate route (JSP Process) to the beads uses carbon dioxide as a blowing agent which is impregnated into the pellets in an autoclave at a temperature close to the plastic's crystalline melting point. The pellets are foamed by "flashing" into the (lower pressure) atmosphere to expand.[5]
Finally molding is done by steam chest compression molding; usually the low pressure variant of the process is used, though the high pressure variant may be used for HDPE based EPE foams.[6]
Uses
Polyethylene bead foams (including) EPE can be used to replace both polystyrene foam, and both rigid and flexible polyurethane. Uses include cushioning applications, and impact absorption applications including packaging.[4]
Consumption of polyethylene for PE foam was estimated at 114x106 kg in 2001. The majority was used for non-crosslinked foams, but crosslinked PE foams represented a significant (~ one third) fraction of demand. Use in protective packaging represented the largest use sector for such foams.[7]
References
- ↑ 1.0 1.1 1.2 Rosato, Rosato & Rosato 2004, p. 359.
- ↑ 2.0 2.1 2.2 Spalding & Chatterjee 2017, p. 639.
- ↑ 3.0 3.1 Spalding & Chatterjee 2017, Fig. 21.2 p.643; pp.650-652.
- ↑ 4.0 4.1 Spalding & Chatterjee 2017, p. 640.
- ↑ Spalding & Chatterjee 2017, pp. 651-2.
- ↑ Spalding & Chatterjee 2017, pp. 645-6.
- ↑ Mills 2003, p. 34.
Sources
- Mills, N.J. (2003), "(Report 167) Polyolefin Foams", Rapra Review Reports (iSmithers Rapra Publishing) 14 (11), ISSN 0889-3144
- Rosato, Dominick V.; Rosato, Donald V.; Rosato, Matthew V. (2004), Plastic Product Material and Process Selection Handbook, Elsevier
- Spalding, Mark A., ed. (2017), Handbook of Industrial Polyethylene and Technology: Definitive Guide to Manufacturing, Properties, Processing, Applications and Markets Set, Scrivener Publishing (Wiley)
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