What’s the biggest complaint that seal manufacturers fear most? It’s definitely leakage.
When you remove a leaking O-ring, you’ll often discover a heartbreaking sight: it’s no longer the smooth, plump O-ring it once was; its cross-section has turned square or flattened into a D-shape. If you squeeze it with your fingers, it feels rock-hard and completely lacks elasticity.
In the rubber industry, this phenomenon has a technical term: compression set.
When faced with this issue, many technicians’ first instinct is to say, “Use a higher-quality raw rubber!” or “Add more carbon black!” The result is often higher costs with little improvement. Today, Dr. will take you into the molecular world of rubber to see exactly how your seals “die.”
Part 1: Understanding the Basics—What Is Compression Set?
In simple terms, compression set refers to the percentage of height that rubber cannot recover after being compressed under a specific temperature for a period of time and then released.
In laboratory testing, we use a precise scientific formula to calculate it:
- C: Compression set value(The lower the value, the better the resilience and the longer the service life.)
- h₀: Original height of the test specimen
- h₁: Height of the specimen after recovery
- h₈:Height of the spacer (limiter)
For seals, CS is a critical indicator. When CS reaches 80% or even 100%, the rubber completely loses its elastic memory. Even the slightest vibration will cause fluids — oil or water — to leak through the gaps.
Part 2: The Four Main Culprits — Who Killed Rubber’s Elasticity?
Culprit 1: “Genetic Defects” in the Vulcanization System
This is the most critical factor determining compression set! The common sulfur vulcanization system (CV) we typically use primarily produces polysulfide bonds (-S_x-).
Fatal flaw: Although polysulfide bonds offer good tear resistance, their bond energy is extremely low. Under high temperatures and compression, these bonds will break. After breaking, the molecular chains slide into new, flattened positions and then re-crosslink (forming new chemical bonds).
Result: When the pressure is removed, the newly formed chemical bonds tightly hold the molecular chains in place, preventing them from bouncing back. Your O-ring is thus “locked” in a flattened state.
Culprit 2: Under-Curing & Lack of Post-Curing
Phenomenon: To maximize production output, many factories push curing time to the limit (often not even reaching t90).
Consequence: A large number of unreacted crosslinking agents and active sites remain inside the rubber compound. When the seal is compressed under high-temperature operating conditions, these unreacted substances undergo secondary crosslinking. Crosslinking while in a compressed state is like permanently “fixing” the flat shape into its structure.
This is especially true for FKM and VMQ silicone: Without standard post-curing (typically oven curing at around 200°C for several hours) to remove volatile components and perfect the crosslinking network, their compression set values will be extremely poor.
Culprit 3: Stress Relaxation and Molecular Chain Breakage at High Temperatures
High temperatures are the arch-enemy of rubber. When subjected to prolonged pressure at 100°C or even 150°C:
Physical relaxation: The thermal motion of the rubber’s polymer chains intensifies, causing irreversible slippage between chain segments.
Chemical degradation: The main chain breaks down under the combined effects of heat and oxygen. Once the spring breaks, it naturally cannot spring back.
Culprit 4: Escape of Plasticizers (Oil)
If your formula contains large amounts of processing oil to reduce hardness or cost, these plasticizers will be extracted or evaporated when the seal is exposed to hot oil or chemical media.
Volume shrinkage combined with stress loss causes the seal to degrade and collapse rapidly.
Part 3: Doctor’s Prescription — How to Save Your Seals?
Now that we’ve identified the culprits, we can target solutions effectively. If you want to produce high-end seals with ultra-low compression set, here is your practical prescription:
1. Overhaul the Vulcanization System (Top Priority)
Abandon conventional sulfur systems:
Switch to EV (Efficient Vulcanization) or SEV (Semi-Efficient Vulcanization) systems. By increasing accelerator dosage and reducing sulfur content, you form more stable monosulfide and disulfide bonds.
Ultimate solution: Peroxide curing system (e.g., DCP, BIPB)
Peroxide crosslinking creates carbon-carbon bonds (CC), which have extremely high bond energy and excellent heat resistance. These bonds rarely break under compression.
For EPDM or NBR seals, whenever the customer requires low compression set, choose a peroxide system without hesitation.
2.Choose the Right Base Rubber
For applications above 150°C, NBR will fail no matter how you adjust the formula.
Upgrade directly to:
– HNBR (Hydrogenated Nitrile Rubber)
– EPDM (for water resistance, not oil resistance)
– ACM (Acrylate Rubber)
– FKM (Fluoroelastomer)
3.Strictly Implement Post-Curing
For high-end FKM and silicone seals, never save on oven time!
Post-curing is mandatory.
It not only reduces compression set but also completely removes toxic or corrosive byproducts from the vulcanization process.
4. Optimize Fillers and Plasticizers
Use carbon black with low structure and moderate particle size (such as N550, N774), or highly active silica (with coupling agents). High-structure carbon black tends to form a rigid network that restricts the recovery of molecular chains.
Control the amount of liquid plasticizers, and choose low-volatility, extraction-resistant, eco-friendly oils or ester plasticizers.
A compression set is essentially a microscopic war between destruction and reconstruction.
The “failure” of a seal is not sudden death. It is the gradual compromise and rearrangement of the internal crosslinking network under high temperature and compression.
As formulators and process engineers, our mission is to give rubber the strength to resist deformation —using the most stable chemical bonds (CC bonds, monosulfide bonds) and the most compact vulcanized network.
Next time you face leakage issues, don’t blindly add more carbon black.
Ask yourself: Is your vulcanization system correct?
