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Home > Tech Tips > FAQs about Material Properties and Application Notes

Frequently Asked Questions about Fabricating Rubber Materials

Question:
Why do we normally discourage laminating pressure sensitive adhesives onto solid EPDM rubber, low durometer (10 to 40 Shore A) Neoprene and Buna-N?

Answer:
EPDM compounds often contain ingredients, such as paraffins, that bloom or migrate to the surface over time. When these additives migrate to the surface, an adhesive backing will likely be shed from the surface. Although this bloom effect often benefits the resistance of EPDM to sunlight, acids and other conditions - long term bonding is made difficult. Low durometer neoprene and Buna-N compounds often contain oils and plasticizers that make these surfaces difficult to bond to. An adhesive backing may appear to have acceptable anchoring onto the gasket surface in the immediate days following the lamination, only to have the adhesive separate due to the migration of oils and plasticizers. If your specification calls for one of these materials with an adhesive backing, the adhesive should only be counted on as a temporary assembly aide. Such gaskets supplied with adhesive backings must be applied immediately, and the design must include bolt holes such that mechanical fastening is the prime method of maintaining the position of the gasket in final assembly.

Question:
Why do open cell foams and sponges have better compression set resistance than closed cell sponges?

Answer:
The structure of closed cell sponge rubber has non-interconnecting, or individual cells that contain air - typically nitrogen gas. When deflected, the force applied to these cells causes the gas to be gradually squeezed through the thin cell membranes. When the deflecting force is released, the cells attempt to return to the original shape, drawing air back into the individual cells. Some sponge materials rebound quickly and some rebound very slowly or take a permanent deformation. Materials such as closed cell silicone sponge are more resilient and are better suited to rebound to close to the original sponge thickness. Deflection under high temperatures and severe deflection over 50% may damage the cell structure and impact the ability of the closed cell sponge to resist taking a permanent compression set. Open cell foams and sponges have cells that interconnect, allowing for air to pass through them quickly under deflection - and air is drawn back into open cell material faster when the deflection force is removed. Open cell foams and sponges are known for fast rebound, and generally return to original thickness. PORON® cellular urethane is an excellent example of open cell foam having excellent compression set resistance.

Question:
If PORON® cellular urethane is open cell, how can it be specified as a gasketing material for an enclosure gasket that might see occasional water contact?

Answer:
From our experience, when PORON is deflected 35% or more, it can perform as a good enclosure gasket material for NEMA 4 wash downs and similar water spray applications. The edge of the gasket should be protected from direct high pressure spray, such as with an edge shield or lip on a door closure. We would not recommend PORON for submerged enclosures, or for applications where the gasket might be flexed under water, as the squeeze / release action will cause water absorption. The microcellular structure of PORON, having very small cell openings, helps to resist the wicking effect when gasket edges are in contact with water in static (versus dynamic) applications.

Question:
Why should we be sure to request post-cured silicone rubber, when specifying silicone rubber for high temperature gasket applications?

Answer:
Many silicone rubber compounds utilize curing agents that require a secondary post-cure to finish the curing process. Although most silicones can pass the physical parameters of silicone specifications with a press cure at 300° to 340°F, the mechanical properties such as compression set resistance, can be enhanced using a 4 hour post-cure at 400°F in a hot air circulating oven. Most sheet silicone manufacturers post-cure to some extent. Further, if your application requires high temperature resistance, such as a door gasket on a high temperature chamber or oven, or high temperature testing instrument, an un-post-cured gasket may appear to smoke as it releases these trapped curing agents. These by-products are non-toxic; however they may be annoying to your customers!

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