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Valve Oring Technology
Created on 06.07
In the sealed polymer physical world, what often determines long-term sealing performance is not how hard the material is, nor how much it can stretch. What truly plays a decisive role is an often underestimated metric—Compression Set (CS)
01 Sealing – What exactly are we sealing against?
Before discussing compression set, let’s correct a common mechanical intuition: sealing does not rely on “blocking” – it relies on “pushing back.” When you install a rubber O-ring into a metal groove and tighten the flange, the rubber is compressed (typically at a compression ratio of 15%–30%). Rubber is a highly elastic material. When you compress it, its molecular chains are forcibly distorted, generating a strong restoring force (contact stress) – the rubber “wants to return to its original shape.” This restoring force presses firmly against the metal wall. Theoretically, as long as this restoring force is greater than the internal fluid pressure, the medium cannot leak out. Therefore, the essence of sealing is that the rubber molecular chains continuously provide a restoring force – “pushing back” against the metal wall – while under compression.
02 Why does sealing fail?
If sealing relies on restoring force, what happens when that force disappears? What is compression set?
Suppose we compress rubber by 25%, place it in a 120°C oven for 70 hours, then remove it, release the compression, and allow it to cool and recover. If it fully returns to its original thickness, CS = 0% (a perfect elastomer – does not exist in reality). If it remains completely flattened and does not recover at all, CS = 100% (it has become a dead plastic).
Why can’t the molecular chains “bounce back”?
The problem lies at the microscopic level:
Physical aspect – stress relaxation and chain rearrangement
Rubber consists of long molecular chains entangled with one another. When it is held under compression at high temperature over time, the chains slowly slip and rearrange to reduce internal stress – just like a fluffy ball of yarn pressed under a box for a year: when taken out, it has adapted to a flat shape and can no longer fluff up easily. This physical “compromise” causes the restoring force to gradually decay.
Chemical aspect – cross-link breakage and reformation
This is the most critical factor. Rubber’s elasticity depends on cross‑linking bonds between molecular chains (e.g., sulfur‑sulfur bonds). Under high temperature and long‑term compression, these bonds break, losing the restoring traction. Even worse, the free radicals generated by bond breakage can re‑form new cross‑links while the material is in the flattened state – effectively locking the molecular chains in the compressed shape. Even after the external force is removed, the new chemical bonds prevent recovery.
Physical slippage + chemical recombination = compression set.
The larger the CS value, the flatter the rubber becomes, and the weaker the contact stress. Eventually, the restoring force drops below the fluid pressure, and leakage occurs.
03 Engineering measures to address compression set
Now that we understand the underlying mechanisms, how can we avoid this risk in sealing design and material selection?
1. Always question the test conditions when interpreting CS data
Suppliers’ technical data sheets often state “compression set: 15%.” But at what temperature, compression ratio, and test duration was that value measured?
If 15% is measured at 100°C, but your actual operating temperature is 150°C, the actual CS may skyrocket above 50% due to accelerated thermal‑oxidative aging.
Key point: when evaluating CS, the test temperature should be at least the maximum operating temperature of the product, and the test duration should be at least 70 or 168 hours. Only data obtained under such conditions are meaningful for material selection.
2. Optimize the cross-linking system – do not blindly increase hardness
If the CS value does not meet requirements, do not try to compensate by increasing rubber hardness – hardness cannot fix compression set. You need to intervene in the chemical cross-linking system.
Take NBR or EPDM as examples:
- Traditional sulfur curing produces sulfur‑sulfur bonds with relatively low bond energy, prone to breakage and reformation at high temperatures → poor CS.
- Switching to peroxide curing produces carbon‑carbon single bonds with high bond energy, which resist breakage and reformation under high‑temperature compression → significantly improved compression set.
The trade‑off is that peroxide‑cured materials have lower tear strength. This is a classic balance: choose between “resistance to tearing” and “maintaining resilience.”
3. Use structural design to compensate for CS decay
If the CS of a material has been optimized to its physical limit (e.g., 30%), how can you ensure a long‑term seal?
Knowing that some restoring force will be lost, design the groove dimensions to incorporate this decay into the compression allowance in advance.
However, compression cannot be increased indefinitely. When the compression ratio exceeds roughly 40%, internal stress in the polymer rises exponentially, causing premature mechanical chain breakage and actually accelerating CS deterioration.
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