All mixed up? Consider these custom compounding factors

AKRON—The heart of custom mixing, with its unique proprietary formulas and chemical blends, is in imparting precise performance characteristics to the final product.

The values espoused by a particular custom mixer—integrity, innovation and flexibility in difficult market environments—are vital.

But what truly sets one company apart from another is the science behind the custom mixing.

From conversation to compound, custom mixing can involve a dizzying array of chemicals, cure systems and polymer chain crosslinks.

Scott Sarah, a lab and mixing technician with 30 years of industry experience, is in charge of the mixing and compounding work at Smithers, a rubber and polymer testing firm headquartered in Northeast Ohio.

"No matter what, I'm usually covered in carbon black by the end of the day if I am mixing," Sarah said.

The benefits of contracting with a custom compounder are many, he said.

A company might seek a custom compounder if it is striving to achieve a particular performance benchmark or if it is upgrading its own legacy product.

Other companies need a specifically engineered rubber compound that can withstand a particularly rigorous or harsh environment.

And without question, supply chain challenges play into the choice to seek a third-party for custom compounding, as a company facing supply shortages may look for alternative ways to "bake the cake."

"At Smithers, we see a wide range of factors that drive a company to pursue custom mixing," Sarah said. "For companies that generally produce compounds on an industrial scale, it can be cost-prohibitive to develop an experimental custom compound in house."

These companies often turn to independent rubber testing laboratories like Smithers, which boast on-site, small-batch mixing capabilities for support.

Experts can mix and test custom compounds on a much smaller scale, which allows for more efficient and more cost-effective development. Once the formulation is finalized, the company can then begin large-scale production.

"That's where Smithers and I come in," Sarah said. "Our small-batch mixers give our clients the flexibility to develop and evaluate multiple formulations so they can confidently identify the optimized compound for production.

"In other words, the work I do helps our clients produce a better product, and I feel great about that."

The initial step in the custom mixing process is a conversation, said Critt Ohlemacher, a senior materials scientist at Smithers, who, like his colleague Sarah, has about three decades of experience in the industry.

Each chemical ingredient, in powder form or as a liquid dispersion, affects final product performance, processing and cost.

"So each of these must be considered when making decisions," Ohlemacher told Rubber News.

Determining the polymer blend is critical to understanding the physical and material properties of the resulting compound—and whether those properties match the performance expectations of the final product.

Sarah noted that natural rubber has excellent tensile strength, tear resistance and fatigue resistance, making it "a resilient choice" for tires.

Polyisobutylene rubber, commonly called butyl rubber, has excellent damping properties and thus is selected for use in seismic absorption products—components that are crucial in helping people prepare for earthquakes.

And silicone rubber has a broad service temperature range, making it popular in medical, cookware and aerospace applications.

Once the blend has been chosen—essentially the scaffolding, or general outline for the final product—fillers, process aids and antidegradant packages also must be chosen, according to the Smithers experts.

With literally dozens of reinforcing and non-reinforcing fillers on the market, including carbon black and silane-treated silicas, Ohlemacher said important choices must be made in this realm as well.

While reinforcing fillers build and increase the gum and hysteresis properties of the compound—think modulus, tensile strength, durometer and abrasion-resistance—non-reinforcing fillers often are added to reduce the overall cost of the custom compound.

Common non-reinforcing fillers include talcs, clays and calcium carbonates, according to Sarah.

"Some fillers are also referred to as 'cost extenders,' " said Sarah. "It's important to consider the effect of the filler's non-specific gravity on overall costs."

There can be functional considerations for non-reinforcing fillers as well, as aluminum trihydroxide can improve flame resistance and certain clays can reduce permeability.

After a filler is selected, cure systems—vulcanization—form the foundation of the compound, establishing the connections, or crosslinks, between the polymer chains.

At that point, accelerators can be implemented into the process to "control the onset, speed and extent of the vulcanization process," Ohlemacher said.

"The density and structure of crosslinks in a vulcanized rubber can have a significant impact ... including heat aging resistance, fatigue resistance, compression set and more," he said. "These properties are also affected by the type of curing agent used."

To wit, Sarah said a sulfur-cured rubber maintains a higher tensile and tear strength, while peroxide-cured rubber has better resistance to aging, chemicals and oils.

A final step—not the last step, which can be field testing or achieving a certification—to consider when traveling the custom compounding route is the addition of an antidegradant to slow product decay.

How the product ultimately is used plays into which antidegradant is selected. Abrasion, ozone cracking, chemical decay and compression set at high temperatures all factor into the erosion of a compound.

Exposure to oxygen, heat and humidity in storage can lead to loss of elasticity and tensile strength.

"What properties are you trying to conserve?" Sarah said. "The right antidegradant can extend the shelf life of a compound."

So what advice would Ohlemacher and Sarah impart to make sure their testing program at Smithers is as valuable as possible?

"Triple-check to verify that you are referencing the correct and updated specification when requesting a testing program rather than an outdated one," Sarah said. "When we're testing to the right specification, we can ensure valuable, accurate data and help you get your product to market faster."