A cellular, or sponge rubber compound, is made up of raw materials found in most standard rubber compounds such as a base polymer, fillers and plasticizers as well as processing, antidegradant and curing additives. However, it also includes a blowing agent that creates a specific type of cellular structure within the rubber matrix during the curing process.
There are two primary types of cellular rubber: open-cell and closed-cell.
Open-cell sponge rubber has distinct cells that are interconnected to neighboring cells. There are no discreet and separated individual cell walls. Rather, this network of elastic rubber tunnels allows air to easily re-inflate the sponge article once a deforming force is removed, enabling the article to rapidly regain its original physical form and dimension.
Due to this resilience and the ability of the cell structure to rapidly return to its original uncompressed state, open-cell rubber can be an ideal choice for sound damping and cushioning applications.
Closed-cell sponge rubber consists of individual cells, each with its own elastomeric wall that separates it from the other. Closed-cell sponge often exhibits slower recovery when compressed compared to open cell sponge. This is because when compressed, some gases must slowly re-permeate the individual cell walls to re-inflate the individual cell.
The benefit of closed-cell sponge is that it is resistant to the flow of both gases and fluids into the rubber matrix. For this reason, it is the material of choice for use in sealing and weatherstripping applications.
Since fluids cannot easily penetrate, closed-cell rubber retains flexibility at low temperatures and prevents the passages of fluids at both high and low pressures.
When determining which type of sponge compound to use, careful consideration should be given to the end-use application and the properties needed for that application.
Material design is key in achieving the desired open- or closed-cell properties, many of which are determined by the base polymer material used in the formulation.
An EPDM sponge material, for example, will have excellent ozone, weathering and aging resistance due to the inherent properties of the base polymer.
Another important consideration for deciding between open-cell and closed-cell is the manufacturing process that will be used to cure the rubber compound.
Open-cell sponge rubber is usually manufactured in sheets or press-molded articles.
Closed-cell sponge can also be molded, but is most commonly produced into profiles via continuous extrusion processes. In special cases, it can be press molded and oven-expanded into large rubber “bunstock” and then later scythed into uniform sheets for die cutting.
LMI Custom Mixing's history and experience in developing and mixing both open-cell and closed-cell sponge compounds has made sponge one of our core rubber compounds.
Contact LMI Custom Mixing to discuss what our expertise can do for your application.