Once you have decided that rubber is the material of choice for your profile, the next big decision is whether the profile will function best with a dense or sponge compound. Dense and sponge rubber share many of the same properties that are common to all rubber compounds such as their elastic properties, ability to be extruded and then vulcanized to set their shape. The main difference in the two types of rubber is that sponge has an additional ingredient called a blowing agent that reacts to the heat in the oven to create gas that generates bubbles in the cellular structure.
You can think of it like adding yeast to bread. The yeast expands the structure of the bread creating a lattice of air and dough which creates a light, airy texture inside. If no yeast is added, then you will have a dough that is dense, heavy and has no open cellular structure inside.
The hardness of both dense and sponge rubbers is measured by their durometer. Even though sponge rubber has a lighter and airier internal structure, it doesn’t necessarily mean that dense is harder (or has a higher durometer) than sponge rubbers.
What are the advantages of each and how does it affect the end application?
Dense rubber is a good choice for industrial applications where abrasion and wear resistance are most needed. Applications that will experience strong forces and need maximum impact absorption are good uses for a dense rubber profile. In the case of a seal between two surfaces, a dense rubber with a mechanical fastening device will provide maximum sealing capabilities to ensure no moisture or dust enters the area.
Dense compounds are also good choices for profiles that need to maintain a rigid shape, when compression is not required, as when a profile needs to slide onto an aluminum extrusion channel.
Most sponge rubber profiles are softer than dense rubber because of the air inside the profile. These profiles will typically compress easier than dense rubber and will bounce back to their original shape without getting indented. Because of the lower compression force required they are used regularly in compartment and door seals where there is no mechanical fastening involved. The uses for sponge rubber profiles are too many to mention, but they can be commonly found in applications involving weather-stripping, insulation, and vibration dampening.
Sponge rubber has two main classifications of cellular structure: closed-cell and open-cell. We almost exclusively use a closed-cell structure sponge rubber at Uni-Grip because of its resistance to being permeated by liquids and gases. A closed-cell structure has individual pockets of air which are separated from each other inside the lattice structure. This structure makes it more difficult for air, fluids and gases to permeate into the air pockets. This reduces the time it takes for the profile to recover after being compressed and protects the profile from waterlog when exposed to rain.
An open-cell structure is just the opposite of a closed-cell structure. The cell structure in the rubber is not a group of separate walled off pockets, but rather a group of interconnected structures that allow air to flow throughout the structure freely. This allows for a quick recovery of the profile when compressed, but also allows for liquid and gas to enter the structure and saturate it quickly. Not a great characteristic for keeping these elements out of your house, truck, toolbox or anything else.
Rubber is a very versatile material with a lot of characteristics such as hardness, tensile strength, elongation, specific gravity and more. Each of these characteristics can be modified by the compounder and made to be a perfect fit for the application. Uni-Grip also makes dual-durometer profiles which have a dense rubber carrier and a sponge rubber bulb, combining the gripping power and strength of dense rubber along with the compression and sealing characteristics of sponge. Don’t worry if you aren’t sure exactly what you need, just give us a call and we can help you find the seal and compound that’s right for you.