Compression set- Permanent deformation that leaves an elastomer when the latter has been pressed during a long time and especially at high temperatures. In silicone rubber, it is termed as percent: the lower it is the higher is the recovery to the actual thickness upon load release.
In the case of gaskets and seals, compression set is a direct determinant of whether a silicone gasket will retain sealing pressure with time – and the risk of leakage, durability, and maintenance cycles. With a high compression set the material is permitted to take away the pressure on the mating surfaces and finally allow the escape of fluid or gas. The temperature is the dominating factor: at high temperatures, the chain relaxation in the polymer network is more rapid, and when the temperature reaches high levels, the risk of permanent deformation increases nearly many times in comparison to ambient temperature.
This is because many engineers believe that an increase in Shore hardness will result in an increase in sealing performance. In practice the material which had a great initial hardness may fail in case the compression set is excessively high as the material cannot be resistant to compression at constant loading.
Low compression set is necessary to ensure long term sealing force especially in the high-temperature or stationary gasket where the parts may take up to years to be dismantled.
What Is Compression Set? Basic Definition and Testing
Compression set is used to measure the failure of an elastomer to resume its initial dimension following withdrawal of compressive force placed on it over a period of time.
The test standard adopted by the industry is ASTM D395 (Method B with the most common used being 22 hours with 175body temperature) wherein a cylindrical specimen is compressed(usually to 25% deflection) at a given temperature and time (e.g., 22 hours at 175body temperature) then left to recover (usually 30 minutes or 24 hours at room temperature). The set is calculated as:
Compression Set (%) = [(Original Thickness -Recovered Thickness)/(Original Thickness-Spacer Thickness) ×100
This indicates better memory elasticity because of the lower percentages. The outcomes are dependent on compounds, system of cure, and test conditions.
| Compression Set % | Meaning |
| 0–20% | Excellent recovery |
| 20–40% | Good |
| 40–60% | Moderate |
| 60%+ | Poor sealing reliability |
The highest percentage is always desirable in case of demanding seal applications since it is associated with retention of loads.
Why Compression Set Matters in Gaskets and Seals
High compression set impairs the capacity of a gasket or seal to exert a constant closing force under continued compression which directly increases the likelihood of leakage in service life.
With a static application the gasket needs to be stiff enough to resist the compressive stress to the internal pressure or expansion. Stress relaxation during polymer chain rearrangement with crosslink modification over time due to their changes lead to reduced bolt load efficiency and permits micro-gaps. This is made worse by thermo-cycling, which results in repeated expansion / contraction, increased set speed. The outcome: the time of maintenance is going to be unrealistic, since the leak will not take place as late as supposed.
| Application Type | Compression Set Importance |
| Static gasket | Very High |
| O-ring | High |
| Dynamic seal | Moderate |
| Cushion pad | Low |
In the case of critical static seals like engine covers or lids on enclosures, moderate set may cause an early failure, excessive downtimes and cause safety issues.
How Temperature Affects Silicone Compression Set
The high temperature has a pronounced effect on compression setting up silicone by enhancing the mobility of molecules, disentangling chains and the relaxation of residual stress in the crosslinked network.
Set is weak at room temperature since the movement of chains is restricted. With increase in temperature, the rate of viscous flows is hiked, resulting to an augmented permanent deformation. Constant high-temperature exposure (e.g. engine bays) results in more set compared with intermittent exposure to heat (e.g. sterilization cycles every now and then). Silicone continues to excel against most organic rubbers in the heat with lower set values up to 200 o C along with optimized grades.
| Condition | Effect on Compression Set |
| Room Temperature | Low deformation |
| 100–150°C | Moderate increase |
| 200°C+ | Higher deformation risk |
Traditional high performance silicones exhibit 10-30% set during 22h/175 o C testing which is much better than other materials that decay quickly at temperatures above 150 o C.
Silicone vs Other Rubbers: Compression Set Comparison
Silicone has good compression set resistance to high temperatures and hence it is good in thermal cycling condition where other elastomers do not perform well.
Though it is not the lowest-set material (specialty fluorocarbons perform better here), silicone is a well-balanced material in terms of heat stability and flexibility, as well as recovery.
| Material | High-Temp Compression Set | Oil Resistance | UV Stability |
| Silicone | Good | Moderate | Excellent |
| NBR | Moderate | Excellent | Moderate |
| EPDM | Good | Poor | Excellent |
| Viton (FKM) | Very Good | Excellent | Good |
Compromises are application-dependent: Use silicone and high temperature and UV/ozone protection; use Viton and harsh oils/chemicals; use EPDM and out-of-door weathering but no hydrocarbons.
Factors That Influence Silicone Compression Set
Compression set in silicone rubber depends on several formulation and environmental variables although crosslink density and curing technique has the most severe effect.
The increased crosslink density inhibits mobility of the chain which enhances recovery. The ionic exchange systems that are platinum cured tend to perform better than the peroxide cured in set resistance because of more stable clean networks. The volatiles are removed by post-curing, and set is also greatly reduced.
| Factor | Influence |
| Higher Hardness | Better shape retention |
| Higher Temperature | Increases compression set |
| Platinum Cure | Often more stable |
| Post-Cure | Reduces residual deformation |
The type of filler (e.g. silica reinforcement), and gasket thickness also have an effect: thicker portions can exhibit slightly higher apparent set because of heat transfer gradients.
Design Considerations for Minimizing Compression Set Failure
Seal design is a wise way of reducing compression set effects even with commercial silicone grades.
Get to an ideal of 15-30 nominal compression to strive to balance sealing stress without undue strain that encourages set. Compression exceeding 40 to 50 compresses permanently and causes decreased recovery. Maximize flange tolerances on thickness, uniform loading on bolts and smooth surface finishes to reduce stress concentrations in localized situations.
| Design Step | Purpose |
| Control Compression % | Prevent permanent deformation |
| Select Proper Hardness | Balance flexibility & retention |
| Thermal Evaluation | Predict aging |
| Material Testing | Validate real conditions |
Before it starts producing, the performance is proven by finiteness element analysis or prototype testing at application temperatures.
Signs of Compression Set Failure in Silicone Gaskets
Excessive pressure compression failures appear at a slow rate and show up through functional and visual signs in a field.
Examine the gasket surface, visible permanent flattening or memory grooves, loss of rebound on probe, leakage after long service or quantifiable shape distortion on disassembly. The hardening of the material in other instances is slight as a result of additional crosslinking during heat aging.
Routine examinations in the course of maintenance (e.g., a decrease in retention of torque or other signs of seepage caught at an early stage) can reveal any problem.
When Silicone Is the Right Choice Despite Compression Set Concerns
Silicone is still the material of choice in most situations where the total profile of its properties would vastly override the mean compressive set restrictions.
It is also effective in high temperature (up to 200 or 250 o C intermittently) unsteady sealing, in open air enclosures that are vulnerable to UV radiation, electronic enclosures that must have an elastic range of thermal-related requirements, and in low-oil applications. Where wide range of temperatures, electrical resistivity and weather resistance are important in an application, rather than the extreme contact with oil/chemicals, silicone can be used to achieve good performance with the appropriate formulation and design.
Conclusion — Compression Set Defines Long-Term Seal Reliability
Compression set affects directly on the capability of silicone gasket to withstand sealing force with time. Although silicone generally is a good-to-excellent material at a broad temperature range, good recovery can occur even when the material has been subjected to significant high temperature; nonetheless, appropriate formulation of the material (e.g. platinum cure, post-cure), and intelligent seal design are necessary; to reduce permanent deformation to the lowest as well as to make the silicone dependable over time.
The engineers have to weigh data of compression set in the conditions that are going to be used in the application, weigh it against other conditions such as hardness and chemical compatibility, and prove it through testing. With low-set silicone involved as priorities and compression ratios being optimized, silicone sealing systems obtain longer service life without the possibility of leakage or early replacement.