Polyurethane Ester Foam

Material Name
Polyester-Based Urethane Polymer

Brand Name
N/A - Polyurethanes are produced by multiple manufacturers.

Nicknames
Polyurethane Ester
Polyester Urethane Foam
Polyurethane Polyester Foam
Ester-Based Polyurethane
Esterfoam
Ester
Museum Foam
Charcoal Foam

Description
Polyurethane ester foam is an open-cell foam with excellent memory and very soft compression characteristics. It has a fairly high tensile strength for a polyurethane, but not compared to standard polyethylenes. It can be torn by hand, for example, but most people would need to start a tear at a sharp corner where the material is thin. It can be punctured with a blunt object like a pen with only moderate pressure. Its open-cell structure also makes it permeable to air and water.

Ester has a uniform texture and density throughout, with no skin on either face. Sheet dimensions tend to be pretty consistent, but can vary in thickness by a fraction of an inch. Its natural color is charcoal-gray with a textured appearance - almost sparkling in strong light.

Ester carves complex contours easily with a sharp knife, but it is soft enough to stretch under the blade and cause faceting and flashing in cut edges. In contrast, ester machines very well with the right power tools. Reciprocating foam rubber cutters work especially well to produce smooth-edged blocks and strips.

Ester's gritty surface texture and chemical instability require the use of one or more intervening materials between the foam and any art object. Intervening material can be applied by lining the foam where it will make contact, wrapping the object, enclosing the object in a structural inner package, or any combination of the above.

Applications
Due to off-gassing, ester foam is recommended only for short periods of time. It is generally considered safe enough for transporting most objects between venues, including international shipments. However, it is not recommended for use in storage or any other long-term application.

Ester foam is most often used as one or two layers of interior protection inside shipping crates and boxes. It is an excellent thermal insulation material, and in many cases, an equally effective cushioning material for shock absorption and vibration dampening. It is typically used with itself in that combination, rather than mixed with another type of foam. In other words, crates with ester insulation and ester cushion pads are much more common than crates with extruded polystyrene insulation and ester cushion pads.

One reason for this is that a 2" layer of ester thermal insulation will compress a small amount, adding a second phase of firmer cushioning to the inner layer of smaller ester cushion pads. This introduces two levels of cushioning; the primary pads in contact with the package, and the more firm support of the insulation lining the crate walls. Because even broad planes of ester will provided cushioned support for certain objects, like paintings of moderate weight riding upright, sometimes ester insulation is used without the addition of small cushion pads. In such cases an extra layer is typically added to the bottom lining of the crate at least, under the riding edge of the object. 4" is a common minimum for the total thickness of ester foam under objects.

Another reason ester is usually used with itself is that it bonds well with itself when a full-coverage adhesive is used, such as spray glue. Another factor still might be the notion that once ester is chosen for either the insulation or the cushioning, it may as well be chosen for the other - because either way the issue of ester's off-gassing has been introduced to the crate.

Ester cushions need a larger surface area to match the support of closed-cell foams with equivalent density, which means more material is used. Ester is also typically more expensive per square foot than equivalent options in polystyrenes and polyethylenes. These two factors make it a relatively expensive option for crate insulation and cushioning among the common materials.

Cavity Packing
Ester's relatively low surface tension makes it an excellent material for fine contouring in response to the shapes of delicate objects. It is very useful for cavity packing, where custom form-fitting holes are carved into deep blocks of laminated foam, usually for small objects. Custom cavities carved by hand will not be aesthetically appealing, providing further incentive to line them with a thin protective material like Tyvek Softwrap. Note: For a more economical and chemically stable alternative in cavity packing, consider a very soft polyethylene such as PolyPlank LAM090.
Ether vs Ester
Ester is very similar in manufacture and appearance to polyurethane ether foam, another urethane polymer, and the lack of brand names combined with casual labeling can complicate the identification of polyurethane foam offered by some regional foam suppliers.

Ether was developed approximately a decade after ester to provide a softer and more flexible cushioning, a better reaction to wet environments, and somewhat lower manufacturing costs. It has a slightly larger cell structure than ester, and is less resistant to punctures or tears. Despite the larger cells, however, ether looks and feels smoother, with a more uniform surface texture. In contrast, ester tends to look more textured, with a little sparkle, and feel rougher, stiffer, and more dense to the touch.

The newer foam's qualities listed above can make it sound like a superior urethane for art handling as well as for industrial applications, but it is not generally approved in this context. 2 pcf ether tends to be soft to a fault, providing insufficient support in some cases. It is also significantly less durable than ester under repeated use, and much less chemically stable.

Although two distinct gray colors are shown in the photo below, ether and ester can be found with virtually identical charcoal coloration. In such cases, it takes an experienced eye to spot the difference, and sometimes a pinch as well.


See this link for more information on the differences and backgrounds of ester and ether, provided on The Foam Factory website. More technical information is available online in the DOE report prepared by J.P. Madden, G.K. Baker, and C.H. Smith, "A Study of Polyether-Polyol- and Polyester-Polyol-Based Rigid Urethane Foam Systems."

Common Fabrication Tools
Knife (deli knife, extendable box cutter, specialty foam knife)
Foam rubber cutter (electric) - For best results, mount cutter under saw table with an adjustable fence
Glue Gun - Limited facility with hot glue, due to the fragile matrix in small areas. Large globs of glue are typically needed to form an effective grip rather than long beads.
Spray Glue - Non-toxic, water-based spray glues, such as Simalfa, are highly effective adhesives for ester.

Thermal & RH Properties
Ester is not watertight, and offers little or no humidity protection. However, its thermal insulation is considered highly effective. The R-value rating for thermal resistance (h·ft²·°F/BTU) can vary depending on the information source, and the rating itself is sometimes expressed in a range. The Building Insulation Materials wiki has posted ratings of R-7 to R-8 per inch thickness for new pu ester foam (CFC-HCFC expanded), and a somewhat lower rating of R-6.25 per inch thickness for the same foam after 5-10 years of use. Based on these numbers, ester foam can be considered to have roughly 50% better thermal resistance rating than XPS, and roughly three times that of polyethylenes.

Stability
Ester is not chemically stable, and over time it will off-gass. This limits its viability in a sealed container to short-term use. It is generally considered suitable for use in closed shipping crates during transit, including multiple venues or repeated international shipments, but once a container with ester components reaches a climate-controlled storage destination it should be opened to allow air circulation. There should also be a barrier between ester and any art objects, such as poly sheeting or Tyvek Softwrap. This both protects objects from exposure to airborne pollutants in the crate, and eliminates abrasion against ester's slightly rough texture.

Safety
See this manufacturer's link to view a Materials Safety Data Sheet covering both ester and ether foams. Regional foam suppliers should be able to provide MSDS from their sources upon request.

Forms
In the context of art handling, ester is most commonly stocked in sheets of 1" or 2" thickness. 4" thick sheets are sometimes stocked for faster installation to the bottom walls of 2D painting crates. Sheets can be 76 x 54", 108 x 54", or other dimensions, depending on the regional foam supplier. The density used in fine art crating is typically 1.8 pcf - 2.2 pcf. The color is typically charcoal.

Available densities range between 1.5 pcf - 20 pcf. There is also a range of colors available.

Other Variations
There is an anti-static formulation of ester, tinted pink.

Primary Use & Background
Ester was developed in Germany between the wars, but not implemented until after World War II. It is still used in more specialized industrial applications, but was soon replaced by ether in many areas, most noticeably in furniture cushioning.

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Polyurethane Ether Foam