The Art of Rubber Compounding
The majority of design engineers have little experience with rubber and aren’t sure how to select the rubber best suited for their applications, especially since there are so many options. This is why many designers turn to companies that specialize in rubber compounding for guidance. Many people consider rubber compounding both an art and a science because the base polymer is highly customizable. Here is a simple, high-level overview of compounding:
There are 5 main ingredients in a rubber compound formula and each are measured in parts per hundred (phr):
- Processing Aid
- Curing System
To start a rubber compound, you must select an elastomer. When selecting an elastomer, it’s important to know what substances the finished rubber component may come into contact with (e.g., oils, ozone, etc.) because some elastomers have properties better suited for certain applications than others. It is also important to understand the overall intended application and desired physical properties. For example, if you need a rubber component that has excellent recovery, you might select a natural or synthetic polyisoprene as your base elastomer.
Next, fillers need to be chosen and, again, this will vary depending on what the desired properties of the finished rubber component are. The most common fillers are carbon black, amorphous silica and clay. So, if a designer identified that their end product needed to have high abrasion resistance, there would be specific fillers added into the formula to help achieve that property. It’s also important to note that there is no rubber compound formula that will give you all of the ‘ideal’ properties at the same time. One property will have to be compromised in order for another to be made prominent.
The third part of a rubber compound is a protectant, which can be antioxidants, antiozonants, or waxes that help create a physical barrier to improve its performance. Sometimes the protectant can also be mixtures of other elastomers that have some of the physical properties you’re seeking in your component. For example, tires tend to contain a number of different elastomers blended together to achieve the properties that are desired and needed.
The fourth component in a rubber compound is some sort of processing aid, normally an oil, that adds some moisture to the otherwise dry compound to help with mixing and processing. The last part of the formula, arguably the most important, is the curing system. There are different types of curing systems, such as sulfuric curing and peroxide curing, which also influence the final properties of the rubber compound. It’s important to also note that the smallest differences in the amount or ratio of curing agents can drastically change the properties of the rubber compound. Accelerators are usually part of the curing system as well and are used to shorten the time to cure. Without an accelerator, the rubber compound would cure with the curing ingredients but it would take exponentially longer.
As if compounding wasn’t already difficult enough, compounding rubber can be even more difficult because it’s a natural product. Even if the compound formula is “perfect,” there can still be differences in the properties of the final compound due to natural variances in the rubber itself. For instance, natural rubber harvested during a rainy season versus natural rubber harvested during a dry season can vary in their properties and compound formulas may need adjusted accordingly.
Due to the complexity and seemingly infinite number of rubber formulas, it’s important to have custom rubber compounders and manufacturers involved in the early stages of the design process so they can create a formula that is truly suited for its intended application.
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