Frequently, our team in Applications Engineering receives a question along these lines: “Is your compound E0740 peroxide cured?” Usually, these questions are asked because an end customer specifies cure system as a requirement on the drawing for the part. Parker considers the ingredients used in our manufacturing process as a trade secret and are not at liberty to directly disclose what cure system is used for any given compound. However, there is a valid and scientific basis behind why customers are concerned with the cure systems of the O-ring material, but is it as critical as it seems?
What are cure systems?
When uncured rubber is subjected to high temperature and pressure, the polymer chains that make up the rubber material are being locked into place due to cross-linking. Cross-linking is when chemical bonds are formed between individual polymer chains and is what allows rubber to go from a sticky, soft playdough-like material to a solid, sturdy seal. To facilitate this reaction, cure agents are added into the raw compound. These substances form the cross-links while the parts are being molded. The key result of curing the rubber is that curing drastically affects the material properties for a given compound. Two compounds could theoretically be the same up until the point of curing, but once the rubber is cured, these two compounds would have very different material properties.
What difference do cure systems make?
Depending on the base polymer, there are different options for what cure system can be used during manufacturing. In general, each polymer family has at least two options for cure, each providing a different set of finished material properties. In addition to final material properties, cure systems can also differ in cost, with the costlier cure system typically resulting in a more desirable set of final properties for seal applications. Often when customers are specifying a cure system, their intended end-result is to obtain a material with a particular set of properties. A common example of this was laid out above, when a customer may ask whether “E0740 is peroxide cured.” In practicality, what they likely want to know is “Does E0740 have a high resistance to compression set, high tensile strength, and better-than-average high temperature resistance?” These are the properties that result from an EPDM material being peroxide cured. If, in some crazy advancement of science, we discovered that barbecue sauce provided EPDMs with material properties superior to that of peroxide while being less expensive, specifying a “peroxide cure” would no longer be beneficial, as the finished material properties of a peroxide-cured-compound would be inferior and more expensive than those obtained through barbecue-sauce-curing. While that is certainly a silly example, one day in the future, a new technology will be developed and rubber materials will be cured with different substances than what is currently considered to be “mainstream” technology.
How can we use cure systems to serve our customers?
When someone asks about the cure systems of a compound, our protocol is to understand more about the customer’s application. Each option for cure system of a rubber family has its advantages and disadvantages. Understanding each customer’s specific application tells us what properties are going to be critical in the material selected for their application. For nitrile compounds, many customers prefer peroxide curing, which typically provides increased compression set resistance, higher temperature performance, higher ultimate tensile strength, and increased chemical resistance. However, there are other customers who, maybe in the case of dynamic sealing, would prefer sulfur cure, which would provide better wear resistance, is more cost effective, provides higher ultimate elongation, and improves the ability to withstand repetitive bending. When it comes to sealing, it’s not always about how you got there, but whether or not you get the right material that gets the job done!