When designing sponge rubber compounds, it’s critical to use the correct blowing agent to create the proper expansion and physical properties for the end-use application.
A blowing agent is a chemical included in the compound formulation that produces a gas and generates a cellular structure inside the rubber during vulcanization.
As the rubber compound is heated during the curing process, it reaches a temperature that causes the blowing agent to decompose and form gas, creating the cellular structure. Activators can also be added to the formulation to help facilitate the expansion process.
Use of high quality blowing agents with consistent and uniform particle size is extremely important. The blowing agents must be weighed accurately each time. Quanex Custom Mixing uses computer-controlled scales and extremely tight scale tolerances to prepare the blowing agents for mixing. During mixing, proper dispersion of the blowing agent is absolutely critical to achieving a uniform cell structure and smooth surface finish.
The type of blowing agent selected is based on both the curing process being used and the end application of the final product. An application requiring quick recovery from compression may be best served by an open-cell sponge. Closed-cell sponge is more suitable for insulation and water-resisting applications.
Open-cell rubber is generally produced by selecting a blowing agent with low temperature or rapid decomposition rate like sodium bicarbonate, p-toluene sulfonyl hydrazide (TSH), or 4,4´-oxybis-benzenesulfonylhydrazide (OBSH). Use of any or a combination of these will typically produce an open-cell sponge with its characteristic interconnected cells.
Azodicarbonamide (AZO) is by far the most commonly used blowing agent used in producing closed cell rubber. Instead of using a strong activator, as is generally the case with open-cell sponge, adding a more active blowing agent like OBSH to the AZO system serves the role of an activator “kicker” in being able to dial in the proper closed-cell structure. Proper design of the blowing agent system is a key feature of developing discrete, individual cells in closed-cell rubber, which is ideal for preventing fluids from seeping in, making it the right choice where a watertight seal is required in both outdoor and automotive applications.
Quanex Custom Mixing has extensive experience in developing and mixing sponge extrusion compounds. Quanex’s technical team works to balance the compound formulation so the correct sponge cell structure for the end-use application is achieved. Proper dispersion of blowing agents and other recipe ingredients is achieved through weighment accuracy and computer-controlled equipment during the mixing process.
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