At its core, a material’s strength is derived from the bonds that hold it together on a molecular level. In the world of plastics and foams, it’s polymers – chains of repeating molecular units – that are bonded together to form the material. The strength of these bonds, known as a cross-link, is reflected in the strength of the material itself, with its ability to stretch, bear loads or return to shape after pressure is applied. Polymer is also where the names of many foam types are derived from, such as polyurethane and polyethylene.
With polymer foams, there are two kinds of cross-links that impact the material and its characteristics: chemical cross-links and physical cross-links. Chemical cross-links are bonds that are initiated by externalities, such as the application of heat, pressure, or chemical additives that act as catalysts to impact molecular structure. This forced linking can cause molecules that would not normally bond to link together, adding additional strength to the material. The aided bonding of chemical cross-linking creates very strong materials that are incredibly difficult to break down.
Physical cross-links, while weaker, are more versatile because they happen more fluidly than the forced bonds of chemical cross-linking and can be used for broader, less specialized applications. With no compounds or other externalities added to the material to accomplish the linking, physically cross-linked products are more consistent in nature. This reduces the chance of batch-to-batch irregularities. Physically cross-linked products also have a better potential for reusability because their bonds can be more easily broken.
Because of their variation, chemically and physically cross-linked products can be suited for different applications. Chemically cross-linked foams feature very fine-celled structures that, in combination with their molecular bonds, make them very tough and long-lasting materials, such as cross-linked polyethylene or foam rubber. They may also have a tendency to have a rougher outside finish than physically cross-linked foams, making them more difficult to damage but not as aesthetically appealing. This is commonly called a foam skin. Its strength also makes it resistant to breakdown by water, air, and sunlight, making it a natural fit for outdoor or marine use. Physically cross-linked products are also fine-celled, but not always to the degree of very fine-celled chemically cross-linked foams. They generally have a smoother finish and can also be cleanly cut into very thin foam sheets for detail-oriented applications that need flexible foam. Physically cross-linked materials are also more easily recycled because their bonds aren’t as difficult to break down or restructure.
Both types of cross-links create very productive materials. They perform well in insulating capacities because of their low thermal transmission, and are also very buoyant materials. The fine cellular structures of these closed-cell foams make them stand out as shock absorbing products, excellent for packaging and storing delicate materials. Anti-Static compounds can also be added to make the packaging of sensitive electronics safe as well. Foam Factory’s cross-linked polyethylene (XLPE), polyethylene roll foam, and cross-link polyethylene are all chemically cross-linked foams.