Ethafoam and Other Polyethylene Foams in Conservation

A. Brands and Types of Polyethylene Foam

1. Ethafoam is a Trademark of The Dow Chemical Company (not DuPont) for a range of closed cell expanded regular (uncrosslinked) polyethylene foam products, manufactured by Dow <http://www.dow.com/cgi-bin/frameup.cgi?/ethafoam>. No other company makes Ethafoam Brand Polyethylene Foam Products. Ethafoam products most commonly encountered in conservation applications are white Ethafoam 220 Brand Polyethylene Foam Plank ranging in thickness from 2" to 4", and white Ethafoam 221 Brand Polyethylene Foam Sheet ranging in thickness from 1/8" to 1/2". Other colors of Ethafoam 220 Plank, include black, blue and pink. As far as I know, the pigments will not extract or rub off or in any other way be removed from the foam, unless the foam is dissolved in hot toluene or similar hot aromatic solvent. Pink Ethafoam is an anti-static grade and has anti-static agents which create a electrically conductive layer on the surface of the foam to dissipate electrostatic charge. It may be possible to remove some of the anti-static agent from the surface of the pink foam, but not to remove the pink colorant.

   Dow also makes Strandfoam polypropylene foam.

2. Many other companies make similar regular closed-cell polyethylene foam products. None of these are Ethafoam Brand Polyethylene Foam, although in the loose vernacular of conservation they are often incorrectly referred to as ethafoam (in the same way that facial tissues are incorrectly called Kleenex, and photocopies are called Xeroxes).

   Some other manufacturers and the products they make include:

   AVI (Astro-Valcour Inc., part of Tenneco Pkg--AVI , TAVI?) <http://www.avipkg.com/products.html>, makers of PolyPlank plank, Astro-foam sheet, Microfoam polypropylene sheet, and the air bubble cushioning products Astro-Cell, Astro-SupraBubble, and Astro-Cell Green.

   Sealed Air Corp <http://www.sealedair.com/products/default.htm>, makers of Cell-Aire foam sheets, CelluPlank, Cellu-Cushion, and Stratocell foam planks, Stratocell laminated planks, plus air cellular cushioning Barrier Bubble products, namely AirCap, PolyCap, Bubblebag Pouches, and Bubble Wrap.

   I could find no reference in the Thomas Register of American Companies to the companies Sealdare, Tavi, and Polylamb (or Polylam) mentioned by Ann Frisina. Perhaps she is referring to Sealed Air and Tenneco-AVI for the first two.

   Also, I could find no reference to Ethafoam or other foam products on the DuPont web site, http://www.dupont.com. As explained above, Ethafoam is not a DuPont product.

3. The chemical composition of the various brands of polyethylene foam, although similar, varies slightly between the different manufacturers in terms of original starting polymer, type of additives (particularly blowing agents and cell formation control agents) and conditions of manufacture. This means that the long term stability of the products of different manufacturers is not the same.

4. Another type of polyethylene foam used in conservation is crosslinked polyethylene foam. Manufacturers and their products include:

   Sentinel Products Corp., <http://www.sentinel-products.com/product_specs.htm> and <http://www.sentinel-products.com/product_index.asp>, makers of crosslinked silane-grafted metallocene polyolefin foams including Microcell (bun form), Opcell (open-celled bun form), F-Cell (continuous roll), T-Cell (rolls), H-Cell (rolls), GHNE (bun), MDL (laminate), SSP (bun), and T-Board (sheets).

   Voltek, Division of Sekisui America, Corp., http://www.voltek.com, makers of Volara sheet (equivalent to Alveolit in Europe and Softlon in Japan), Minicell bun (equivalent to Alveolux in Eurpoe and Softlon Board in Japan), and Volextra extrusion coated Volara (equivalent to Hardlon in Japan).

   Zotefoams Inc. (formerly BXL Plastics?), http://www.zotefoams.com makers of Plastazote, Evazote (ethylene vinyl acetate, EVA), Suprazote, and Propozote (polypropylene).

   Nalgene Clean Sheets Bench/Drawer Liner, closed-cell polyethylene foam (crosslinked?), smooth, resilient, nonabsorbent, available from Fisher Scientific, http://www.fisher1.com, manufacturer unknown (Nalgene?).

5. Crosslinked polyethylene foams have different chemical and physical properties from the uncrosslinked foams described above, and from each other. Of particular note is that crosslinked polyethylene foams generally have much smaller cell sizes (less that 1 mm diameter) than uncrosslinked polyethylene foams (about 1-3 mm diameter), although some of the Sentinel crosslinked foams have the same cell size as Ethafoam.

6. 6. Other types of foam which are occasionally confused with polyethylene foam are polystyrene and polyurethane foam. These foams are totally unlike polyethylene foam in all their chemical and physical characteristics, and these must not be substituted for each other without careful consideration. Polystyrene foam is found in such products as insulation panels (extruded plank) or coffee cups and packaging cushions (expanded bead board). Polyurethane foams are commonly found in cushioning applications like upholstery and mattresses.

B. Polyethylene foam degradation

I have observed samples of uncrosslinked polyethylene foams that have degraded by becoming yellow, brittle, and strongly scented with an acrid rancid smell (which differs from the more sweet or solvent-like smell of the residual blowing agents). The development of brittleness is evident in plank products when a finger is pushed into the plank to create a permanent dent which does not elastically rebound. Sometimes during this test a crackling feeling or sound is evident, cracked cells are produced, and friable fragments are broken off. In sheet products, the brittleness can be so extreme that the sheet cannot be folded or crumpled without breaking into tiny fragments like crystalline snow or sawdust.

I have only observed yellowing in a few thick plank products. The yellow discoloration is uniform throughout the thickness of the foam, therefore I surmise that this is due to thermal degradation at room temperature, not to exposure to light, which most likely would result in a greater discoloration at the exposed surface. Most manufacturers caution against exposure of foams to sunlight, which the say causes surface yellowing and surface chalking (development of powdery surface of degraded material), but no yellowing in the interior of the foam (presumably because damaging radiation does not penetrate below the foam surface). This known surface yellowing and chalking is different from the yellowing and embrittlement I have observed throughout the entire thickness of some foams.

Brittleness and yellowing may be related, but I have noted brittleness in plank and sheets that do not appear to be yellowed. I have not seen or heard of any examples of this type of degradation in crosslinked polyethylene foams. This is in accordance with the expectation (and the manufacturers' claims) that crosslinked foams have greater resistance to UV degradation.

I believe that the susceptibility of a particular polyethylene foam product is related to the grade or type of polyethylene used in the manufacture of the foam, and is therefore dependent on the brand of the foam (i.e., some brands are more susceptible to degradation than others). At this time I cannot say which brands of foam are more susceptible. I have undertaken a research project to investigate this problem.

A difficulty with this project has been getting examples of degraded foam, that are of a known brand. The indifferent reference to all polyethylene foam as ethafoam by conservators, curators, conservation material suppliers, and commercial/industrial foam suppliers has made it very difficult to find accurately provenanced degraded foam. Examination of purchase records has been of little use due to the imprecise use of the term "ethafoam".

I would be most grateful to receive information about, and samples of, degraded polyethylene foam, especially those foams that have unambiguous brand information associated with them (such as the printing of the label directly on the edge of the foam plank). If you have information that you wish to share, please contact me personally at the phone, fax, postal, or email addresses listed below. I would be happy to hear anecdotal information as well.

I will close on a final positive note. Although some polyethylene foams appear to be degrading, this does not appear to be a widespread occurrence. Furthermore, it appears that the degradation is manifested by a mechanical change in the properties of the foam (brittleness, less cushioning, etc.) but not by evolution of chemical products (volatile or not) that can cause chemical damage to adjacent objects. Thus the effect on objects in a collection should be restricted to only the specific objects in contact with the foam (support, cushioning, gasketing). Other adjacent objects that are not in direct contact with the degrading foam should not be affected.

I am still convinced that polyethylene foams are the most stable for conservation/museum applications.

I would be happy to attempt to answer further questions that are posted about polyethylene foams in the conservation/museum context.