Silicone, neoprene, EPDM and other polymers can be made in solid form (e.g. tires and tubing), but they can also be expanded with a blowing agent. A good analogy is bread – flat bread is unleavened (no yeast) which would represent solid rubber, adding yeast expands the dough adding pockets of air, this would represent expanded rubber. Adjusting the amount of expansion effectively adjusts the material density and level of firmness. Expanded rubber is usually referred to as sponge rubber or foam rubber.
Sponge or Foam?
|Expanded Silicone, Neoprene and Urethane (PORON)|
There is some debate on which term to use and they are often interchanged. Stockwell Elastomerics refers to sponges as product being made from gum base and foams from liquid bases. Example: Silicone sponge would be made from High Consistency Rubber (HCR) which somewhat resembles potters clay in raw form. Foams on the other hand are made from Part “A” and Part “B” liquids that are then cast; some foams are also made via mechanical frothing. Note: there are differences between cast foams and expanded sponge. Stockwell Elastomerics’ Application engineers are available to help select a material suited for your project.
Why Use Expanded Rubber?
One of the main benefits of expanded rubber is its softness – a very soft solid rubber would be a 10 durometer on the Shore A scale, foams and sponges are usually even softer. These soft rubber materials can be used to seal anything from car doors to hand-held electronics. Expanded silicone and other expanded polymers have a wide compression range – this means more tolerance is allowed between sealing faces. Also, since there is less actual material due to the air cells it generally costs less per unit volume.
Example: Large NEMA enclosures may have significant flatness tolerances between the door and enclosure; a soft closed cell sponge gasket can be used to take up the variation without putting too much stress on the door (0.5″ thick gasket compressed 50% = 0.25″ gap filling).
Hardness of Expanded Rubber
Hardness of expanded rubber isn’t typically measured on a durometer scale the way solid rubber materials are, they are measured per ASTM D1056 which compresses a 1in² disc 25% and reports a value. This is referred to as Compression Force Deflection (@25%) or CFD. Most expanded rubber manufacturers report the CFD to allow side by side comparison of materials. Note: CFD is not to be confused with compression set (a materials inability to rebound, view more info on C-set).
Closed Cell Rubber
Typically when the cell structure is mentioned, it is referring to a foam or sponge as opposed to a solid rubber. Expanded rubber can be either open cell or closed cell, it will depend on the specific product. Expanded sponge rubber tends to be closed cell but there are products that are then crushed to make them ultra-soft. The crushing process pops the closed cells, making them open. As mentioned above, Stockwell Elastomerics’ Applications Engineers are available to help navigate material options.
Common expanded rubber materials offered by Stockwell Elastomerics:
- Silicone Foam
- Silicone Sponge
- PORON Foam (microcellular urethane)
- EPDM Sponge
- Neoprene Sponge
- ECH Sponge