On its most basic level, the strength of a material, natural or synthetic, is derived from its molecular bonds. With the plastics and foams of the world, it is chains of repeating molecular units known as polymers that bond together to form material. This bond is known as a cross-link, and the strength of that link is reflected in the strength of the physical material, impacting its flexibility, weight bearing characteristics and compression resiliency. Polymer is also where the name for many foam products is derived, such as polyethylene or the polyurethane used in convoluted foam.
In polymer foams, there are two link types that impact the characteristics and qualities of a material; physical and chemical. A chemical cross-linking is a molecular bond that uses an external catalyst like heat, pressure or chemical additives to force bonds together. These forced links can result in the bonding of molecules that would not do so naturally, creating materials with greater strength, due to these locked-in connections. The connections in chemically cross-linked materials make for very strong products that are difficult to break down in a productive manner.
The other type of link, physical cross-links are weaker than the chemical variety because of the lack of compounds and additives included to create an artificially generated bond. Because they happen more naturally though, a physical-cross link is more versatile and can be used in a broader set of applications. The lack of added externalities or influences on the bonding of these materials makes for a more consistent material because it is easier to control formation with fewer steps going into their creation. This consistency results in fewer batch-to-batch irregularities, which aren’t problematic enough to stop creating chemically-linked foams, but they do still occur. Without catalyzed bonds, physically-linked products also have a greater reusability because their bonds are more easily broken, allowing the materials to be repurposed. types of reagents
Because of their molecular differences, these two product structures are best used in different applications. With very fine-celled physical structures in combination with their strengthened molecular structure, chemically cross-linked foams are very durable, long-lasting foam materials. Some examples of these materials are polyethylene, foam rubber, and the foam skin found on the outer surface of many products. These skins are more difficult to damage than the softer interior but are considered by some to lack aesthetic appeal. Chemically-linked foam’s strength also makes it resistant to water, sunlight and air, suiting it for marine and outdoor uses. Minicel, Volara and cross-link polyethylene are all chemically-linked materials. While also fine-celled, physically-linked products don’t quite have the minute structure of their chemically-linked relatives. They do feature smoother outer surfaces however, and can be cleanly cut into incredibly thin foam sheets for unique uses that require flexible foam. Because their bonds are more easily broken, physically-linked materials are more recyclable.