Microdense Compounds

Efficiency, strength, and functionality are all important factors in designing today’s automobiles and tomorrow’s transportation solutions. Vehicles that utilize lighter and more sustainable engineered components are more energy efficient and reduce dependency on fossil fuels. EPDM rubber compounds have traditionally been used for automotive sealing systems because they offer exceptional weatherability and ozone resistance. LMI has developed a line of “Microdense” EPDM compounds that can be precisely tailored for a customer’s given part configuration, allowing designers to create sealing profiles that are lighter without sacrificing mechanical performance.

What is a microdense compound?

Microdense compounds are solid, cured rubber formulations in which a foaming agent has been introduced to create a very fine micro-cell structure during vulcanization. They are typically used in non-functional and static portions of the seal. Microdense compounds are firm, yet maintain a degree of flexibility. Technically, microdense compounds are considered hybrid compounds between dense and sponge, although some manufacturers do not classify them as such.

Compound Design Considerations

Polymers

The properties of the EPDM polymers used in creating a microdense compound are extremely important. Microdense compounds typically require the use of high viscosity polymers but may also be blended with medium or low viscosity polymers to promote the proper cell structure during the “expansion” phase. Higher ethylene grades are commonly used to promote mechanical properties and smoother surface quality. Medium to high ENB levels are used to promote fast cure times, high degrees of crosslink density, and better compression set.

Fillers

Carbon black and mineral fillers are used to extend the compound and to help minimize cost. Extrusion grade blacks are commonly used for this purpose and will promote both a smooth surface and fine cell structure. Small particle size carbon blacks are generally not used and mineral fillers can be used but at lower levels. Process aids should be kept to a minimum to avoid surface fouling of the final product.

Plasticizers

Paraffinic oils are commonly used as plasticizers due to their compatibility with EPDM. High viscosity oils are preferred for microdense as they are lower in volatility for high temperature extrusion processes and can exhibit better heat age properties. Naphthenic oils are generally not used as they are less compatible and can introduce the possibility of undesirable iridescence on the finished part.

Cell Generation

In a microdense formulation, both chemical and mechanical forming agents can be used. Chemical foaming agents that generate less gas are preferred. The selection of foaming agent will certainly impact the overall cell structure and density of the final part. If not designed properly, the cell structure can negatively impact surface cosmetics of the finished part. The foaming agent particle size should be very consistent, so the cell structure is consistent and uniform. And the decomposition temperature should match the compound’s cure rate, which together are aligned with the capabilities of the extrusion line.

Cure

Sulfur cure systems are generally favored as they offer more flexibility with the overall curing process. Peroxides are generally not used.

Why choose Quanex Custom Mixing?

Quanex Custom Mixing provides superior consistency and customizable solutions for many unique rubber compounds, including those used in microdense applications. Customers choose us because of our highly controlled mixing process which ultimately leads to a higher quality final product. Our microdense compounds offer a high degree of flexibility in the design of lightweight and function rubber parts.

We work closely with each customer to optimize a formulation for their specific extrusion or molding process. We also offer compounds designed for calendering processes for customers who specialize in the production of sheets or rolls.

For information on Quanex Custom Mixing non-black colored sponge, contact us.