Closed cell sponge rubber can be formulated into several different elastomeric materials in a variety of densities. The most common materials are Neoprene/EPDM/SBR blend, Neoprene, EPDM, and Silicone. Sponge rubber can be specified in low, medium, or high density firmnesses. It is a highly flexible material that can adapt to seal almost any surface application when used with pressure sensitive adhesives or with Heat Activated Tape Systems (HATS). Sponge rubber is a closed cell material that delivers:
- Controlled compression and recovery characteristics
- Shock absorption attributes
- Highly effective sealing capabilities
Closed cell sponge rubber, or expanded rubber, is composed of individual cells that are a lot like balloons; they hold in gas and do not allow moisture to penetrate their walls so long as the cell walls do not burst. The cells in closed cell sponge rubbers are not interconnecting, meaning they do not allow air or water to wick through the sponge at low pressures.
The cell walls of sponge rubber can be compromised in the following ways.
- Constant deflection can cause gas to permeate through the cell walls
- If compressed beyond 50-70% deflection, the closed sponge cells may burst
In sponge rubber gaskets, over compression may cause the gasket to rebound very slowly, or not at all — compromising the overall integrity of the sponge gasket — because of this, most sponge rubber gaskets should be deflected 20-35% to insure optimal sealing performance.
Raw compounds of sponge rubber parts are mixed on the same type of equipment as solid rubber compounds with the addition of a chemical blowing agent. As a chemical additive, the blowing agent decomposes and mixes with the rubber compound to generate nitrogen gas under the heat and pressure of the pre-cure and pre-forming process. As this reaction takes place, the mixed batch of sponge rubber is placed into an oven where it then expands into a bun. Following this, the bun is aged and split into individual sheets.
Sponge rubber parts can be formulated from a variety of materials. The following are examples of materials that can be adapted according to needs:
Closed Cell Silicone Sponge Rubber
Silicone sponge is made from a thin sheet of uncured silicone gum that expands during the cure process inside a high temperature press or a rotocure. Silicone sponge rubber parts generally exhibit superior mechanical properties, providing for better weather proofing. Silicone sponge rubber products are less flame resistant, however, and are usually unable to achieve flame ratings beyond UL94HB.
Silicone rubber can be fabricated into sponge gaskets and cushioning pads. Some of the fabricating processes available to Timco include
- Adhesive lamination
- Slitting to width
- Die cutting
The silicone sponge material exhibits many properties that make it attractive as a gasketing material. It shows strong levels of resilience, high temperature stability and general inertness that are not available in any other closed cell sponge materials. Generally, silicones are unaffected by extreme temperature exposure in levels ranging from -100°F up to 500°F. Furthermore, silicones are usually unaffected by degradation and aging due to sunlight and ozone exposure.
Silicone sponge is often confused with silicon sponge or silicon material. However, silicone sponge continues to be the accepted terminology within the rubber industry. Silicon is a naturally occurring chemical element that is primarily a major component of common sand. On the other hand, silicone is the man made substance derived from silicon and a variety of other chemicals to form the polymer that used by rubber manufacturers.
Closed Cell EPDM Sponge Rubber
EPDM sponge rubber will exhibit many of the positive weathering properties of regular EPDM rubber, allowing it to withstand adverse outdoor conditions such as water, indirect sunlight, and ozone better than other materials such as neoprene. However, sponge rubber parts made with EPDM will not meet UL flame ratings without the addition of a proper additive, and they will also be vulnerable to oils & fuels.
Closed Cell Neoprene Sponge Rubber
Most closed cell neoprene sponge rubber parts will meet UL94HF-1 flame rating requirements. Generally, these parts are serviceable from temperatures ranging -40°F to 150°F of continuous exposure and can withstand up to 200°F intermittently. Premium closed cell neoprene sponge products will resist a variety of combatants including:
- light oil contact
- acids and alkalis
These physical characteristics make neoprene sponge rubber a good choice in general enclosure door gasket applications.
Blended Closed Cell Sponge Rubber
Produced from a combination of neoprene, EPDM, and SBR, these grades of sponge rubber are the most commonly supplied material for general purpose sponge gaskets. Most commercial closed cell neoprene gaskets are of the blended variety. These products will normally meet UL94HF-1 flame rating and can withstand temperatures ranging as low as -70°F without cracking from cold embrittlement. While the oil resistance on blended sponge rubber parts will not be as good as 100% neoprene sponge rubber, the cost effectiveness of these blends makes sponge rubber parts of this variety an attractive solution.