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Rubber Manufacturing Blog / Materials / Are you putting too much Calcium Oxide ("CaO") in your rubber mix?
Posted:  27 Aug 2014 17:30
Are you putting too much Calcium Oxide ("CaO") in your rubber mix?

Any rubber compound formulator understands that Calcium Oxide ("CaO") is used in EPDM, EPDM Sponge, and several other elastomer recipes to act as a desiccant (meaning it absorbs and breaks-down H20), and to consequently reduce the risk of porosity in the cured, end product.   But unfortunately, that general "rule of thumb," when applied in real-life, is not as simple as it sounds.

While CaO does a great job as a drying agent, if it's put in the recipe in slightly excessive, disproportionate amounts, it can cause some serious problems; such as (1) lowering / reducing the "state of cure," and thus weakening the rubber product, and (2) significantly increasing the compression set percentage, meaning the rubber weakens and loses memory (also known as "rebound") when applying compression force; either over time, or during product testing.

So at the end of the day, this makes for an interesting balancing act when trying to dry-out a compound versus weakening its structural integrity.  Oftentimes, a chemist will be forced to reduce the state of cure or increase compression set, because he has no choice but to use CaO in, relatively speaking, large proportions in order to avoid porosity.  So this begs the question: is there a way to reduce the amount of CaO used in a recipe, while still achieving that desiccant or drying effect?  The answer is yes, and it is sometimes as easy as looking at the quality of CaO-in lieu of adding one or more additional additives to offset the negative impact of CaO.

How do you analyze the "quality" of a metal oxide like CaO?  Oftentimes, it simply comes down to a laser diffraction particle size analysis.  So if you're seeing reduced compression set or still looking for that right state of cure in a recipe, you may want to begin by doing some testing yourself, or having your vendor provide you information on your CaO's particle size.  Almost all CaO producers, for various reasons, either due to the end-products they manufacture, such as CaO pastes, or due to technical restraints in processing the raw calcium / lime, end-up with a somewhat large CaO particle size with reduced surface area (e.g. 40 microns is a common particle size).

ChemSpec however supplies a European-based material, which is mined and packaged on-site at one location, to reduce environmental impacts, such as humidity infiltrating the product.  This material is also, on average, less than 5 microns.  This is important, because it's finer material with typically a larger surface area, so you can reduce the amount of CaO you actually put in the recipe while maintaining its desiccant effect, and conversely, improving the performance of the vulcanized rubber. Additionally, Although this is a finer grade CaO, it generally disperses much better in the overall mix than many common CaO products, which also reduces the risk of blistering in the rubber.

Therefore, by doing something as easy as looking at CaO particle size, you may be able to increase the overall strength of the rubber product.   

Chris Wagner, Marketing Manager

Chris Wagner