Dear engineers, procurement partners, and technical decision-makers:
In our daily technical consultations, the question we get asked most often isn't "What's the price?" but rather, "How long will this sealing strip actually last?"
Many customers have encountered the following issue: the sample exhibits proper sealing during testing, but begins leaking water or air after three months of operation; or, although the hardness meets the specified requirements, irreversible indentations are observed upon disassembly. The root cause of all these phenomena lies in the most fundamental physical property in the rubber industry – Compression Set (CS).
Today, we'll move beyond abstract concepts and, starting from molecular chains and pore structure, break down the underlying principles that determine sealing performance.
I. Revisiting "Compressive Permanent Deformation"
In the field of foamed silicone, the sealing performance is equivalent to hardness; the sealing performance is also equivalent to the ability to resist compressive deformation.
Compressive permanent deformation refers to the amount of deformation that a standard test specimen cannot recover after unloading, when the specimen is compressed at a specific rate (e.g., 25% or 50%) at a specified temperature (e.g., 100°C) for a specified duration (e.g., 22 h or 72 h). A smaller value for this parameter indicates a higher elastic retention rate of the material and a lower risk of leakage.
Once significant permanent deformation occurs, it indicates:
Interface stress relaxation: The clamping force at the flange connection decreases and falls below the internal medium pressure.
Microchannel formation: pore rupture or collapse, resulting in interconnected leakage channels.
Frequent maintenance: Bolts must be tightened regularly to compensate for stress losses.
II. The Four Major Dimensions That Determine Deformational Fate
The influence of compressive deformation is not due to a single factor, but rather represents a complex "systematic engineering" process. We have categorized this process into four distinct levels, spanning from the microscopic to the macroscopic scale:
1. Material Gene: The Game Dynamics of Formulation Systems
This is the intrinsic cause of deformation, determined by the silicone raw rubber and additive system.
Raw rubber (base polymer) selection: The vinyl content and molecular weight distribution directly influence the crosslinking density. Raw rubber with excessively high viscosity exhibits severe molecular chain entanglement, making it difficult for the chains to slide during compression; as a result, the permanent deformation is often excessive.
Reinforcement and crosslinking system: There is a "golden inflection point" in the dosage of vulcanizing agents. Although an excess of vulcanizing agent increases hardness, it can make the molecular chains too rigid and reduce their flexibility, thereby adversely affecting the compressive fatigue resistance.
Foaming agent matching: The foaming rate must be synchronized with the vulcanization rate. If foaming occurs too rapidly without sufficient cross-linking, the bubble walls will be too thin and will rupture immediately upon compression; if cross-linking occurs too rapidly while foaming is delayed, the bubbles will not be able to expand fully, resulting in an insufficient closed-cell rate.
2. Microstructure: Closed-pore volume fraction and pore size distribution
This is the key battleground where foamed silicone differs from solid silicone.
Ideal state: A uniform, fine-honeycomb structure with a high closed-cell rate. When subjected to compression, the gas within the closed cells obeys Boyle's law (P₁V₁ = P₂V₂), providing pneumatic support that facilitates the rapid rebound of the cells upon pressure release.
Defective pores: If the pore sizes are uneven (either closed or open pores are present), the compressive stress will be concentrated on the largest pores. Once these weak points undergo plastic deformation or rupture, the effective compression rate of the entire sealing cross-section will be permanently lost.
3. Operating conditions: Thermal and media-induced aging effects
This is an external factor and the primary cause behind many on-site failure incidents.
Thermal-oxidative aging: High temperatures accelerate the oxidative cleavage of the silicone polymer backbone. Over time, the material surface hardens, and the internal cross-linking density changes, leading to intensified compressive stress relaxation. At 150°C, the deformation rate of ordinary silicone may be 3–5 times that observed at room temperature.
Compression ratio setting: It is generally recommended that the compression ratio be between 15% and 25%. An excessively high compression ratio (>30%) will directly crush the cell pores, causing irreversible physical damage; an excessively low compression ratio will fail to establish an effective initial sealing ratio pressure.
4. Injury mechanism: Physical failure during the compression process
Under a microscope, compression failure typically involves three stages:
Phase I (Gas Escape): Sudden pressure increase forces the gas within the closed cavity to escape through the micropores in the bubble wall.
Phase II (Bubble Wall Folding): The bubble wall, oriented parallel to the compression direction, develops permanent creases under shear force.
Phase III (Bubble wall rupture): The bubble wall, oriented perpendicular to the compression direction, ruptures due to excessive stretching; adjacent bubble cavities then merge, forming a macroscopic crack.
III. A Guide to Avoiding Common Pitfalls When Selecting Products
As the operator, we do not recommend that customers focus solely on the "values" listed in the test report; instead, they should pay attention to the "values under actual operating conditions." Here are three practical recommendations:
Request "post-aging" data, rather than "initial inspection" data.
Please refer to the compression permanent deformation data obtained after aging at 150°C for 72 h or at 225°C for 24 h, as provided by the supplier. Good performance at room temperature does not guarantee stability at elevated temperatures.
Evaluate the "stress relaxation" curve
Compressive permanent deformation is a static phenomenon, whereas stress relaxation is a dynamic process. When conditions permit, the supplier should be requested to provide a curve showing how the applied force decays over time at a specified compression rate. A gentler slope indicates a longer sealing service life.
Deriving hardness and density from operating conditions：
Low-pressure, static sealing (e.g., dust protection for lighting fixtures): Low-hardness, high-rebound materials (density 0.35–0.45 g/cm³) may be selected.
High-pressure, dynamic sealing applications (e.g., power battery packs): It is recommended to use materials with medium-to-high hardness and high closed-cell rate (density: 0.5–0.7 g/cm³), and to incorporate appropriate support rib designs.
As a renowned Chinese supplier in the foam silicone industry, our value extends beyond simply delivering a roll of sealing strips; it lies in providing you with a "maintenance-free" sealing solution throughout the entire lifecycle – achieved through precise formulation optimization (optimizing cross-linking/foaming compatibility) and rigorous process control (ensuring uniform pore structure).
If you are looking for sealing materials suitable for specific operating conditions-such as ultra-low temperatures, intense ultraviolet radiation, or high-frequency vibration-please feel free to contact us at any time. We can provide customized deformation prediction data tailored to your actual compression ratio and temperature requirements, helping you mitigate design-related risks through data-driven insights.