Elastomers, commonly known as rubbers, are materials valued for their ability to stretch significantly and return to their original shape, a property called resilience. This elasticity makes them indispensable in applications requiring flexible sealing or dampening. Under prolonged stress, however, these materials can fail to fully recover, a phenomenon quantified as compression set. Understanding this permanent deformation is fundamental to predicting an elastomer’s long-term performance and reliability.
Defining Compression Set
Compression set is the measure of permanent deformation remaining in an elastomeric material after a constant compressive force has been applied for a specific time and then released. When a rubber sample is squeezed and held, it will not fully spring back to its initial thickness once the pressure is removed. The portion of the original deformation that the material fails to recover is its compression set.
This measurement is expressed as a percentage of the initial deflection. Zero percent indicates perfect recovery, while 100 percent signifies a complete lack of recovery, meaning the material has become permanently flat. Standardized tests, such as ASTM D395, provide a quantifiable metric, usually by compressing a sample by a fixed percentage for a set duration and temperature. A lower percentage value suggests the material retains more of its elastic properties over time.
The Molecular Basis of Material Failure
The ability of an elastomer to recover is rooted in its unique molecular architecture. This structure consists of long, highly coiled polymer chains connected by occasional chemical bridges called cross-links. When compressed, these chains temporarily uncoil and slide past each other, storing potential energy. The cross-links act like anchors, ensuring the chains snap back to their original configuration once the external force is removed.
Compression set occurs when the material is held under stress for an extended time, especially when heat is involved. Heat increases molecular movement, allowing strained chains to break temporary cross-links and form new, permanent bonds in the compressed state. This rearrangement of the polymer network into a new, lower-energy state causes the permanent deformation. This process effectively reduces the material’s internal reaction force.
Environmental and Material Factors Influencing Set
The extent of compression set is heavily influenced by the conditions of use, with temperature being the most significant variable. Elevated temperatures dramatically accelerate the molecular reformation process, speeding up the rate at which new, permanent cross-links form. A material performing well at room temperature may exhibit a major compression set when exposed to slightly higher temperatures.
The duration of the compressive force is also a determining factor, allowing more time for permanent bond reformation. The base chemistry of the elastomer plays a defining role in its inherent resistance to set. For example, polymers like silicone and fluoroelastomers have a naturally low compression set due to their stable chemical structures. The specific compounding and curing process, known as the cure state, can also be adjusted to optimize resistance to permanent deformation.
Practical Impact on Gaskets and Seals
The phenomenon of compression set has its most significant real-world consequence in static sealing applications, such such as gaskets and O-rings. These components rely entirely on their elastic recovery—their ability to exert a continuous reaction force—to prevent leakage between mating surfaces. When an installed seal experiences a high compression set, it loses its original thickness and can no longer press against the hardware with sufficient force. This reduction in sealing force creates a pathway for fluid or gas to escape, leading to system failure or the ingress of contaminants. Selecting materials with a low compression set is a primary engineering consideration for any application requiring long-term, leak-proof performance.