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    Compression set is a key indicator for assessing the ability of rubber materials to recover their original shape after being subjected to compression at a specified temperature and for a given period of time. It comprehensively reflects the viscoelastic properties of rubber, its vulcanization state, and its resistance to aging. This article delves into the mechanisms underlying the formation of compression set and the relevant testing standards. Focusing specifically on nitrile rubber, ethylene-propylene rubber, silicone rubber, and fluoroelastomer, it systematically proposes concrete formulation adjustments and process improvements aimed at reducing compression set, thereby providing practical guidance for rubber formulation engineers.

Keywords Compression set; rubber formulation; crosslink density; aging; improvement strategies

I. Mechanism of Compression Set

The essence of compression set is the irreversible deformation that occurs in rubber materials under stress. Its mechanism primarily stems from the following aspects:

Physical Relaxation and Chemical Relaxation ：

Physical relaxation Under pressure, rubber molecular chains slip and rearrange, leading to a decrease in entropy. Once the stress is removed, due to friction and entanglement among the molecular chains, the material cannot fully return to its initial state.

Chemical relaxation Under conditions such as heat, oxygen, and pressure, the molecular network structure of rubber undergoes chemical changes. The main changes include:

Cross-link bond rupture In particular, polysulfide bonds are prone to breakage under heat and pressure, leading to a weakening of the network structure.

Main-chain degradation The polymer backbone breaks, directly disrupting the three-dimensional network.

Oxidative aging Oxygen attacks the molecular chains, triggering chain scission and new cross-linking, thereby disrupting the original network equilibrium.

The influence of the vulcanization system The structure of a vulcanized network determines its stability. A high crosslink density and stable types of crosslinks—such as C-C bonds and single sulfur bonds help reduce permanent deformation.

The Influence of Fillers and Compounding Agents ：

Filler Active fillers (such as carbon black and white carbon black), by binding with rubber molecular chains to form a filler network, can effectively restrict the sliding of molecular chains; however, excessive filling can lead to a reduction in elasticity.

Plasticizer /Softener Under pressure, migration or volatilization may occur, causing the rubber compound to shrink and increasing permanent deformation.

Antioxidant It can effectively slow down the oxidation aging process and is key to reducing chemical relaxation.

II. Test Method for Compressive Permanent Deformation

Domestic and international standards primarily specify the dimensions of the test samples, the compression ratio, the test temperature, and the duration.

International standard ：

ASTM D395 ： Method B (commonly used): At the specified temperature and time, compress the cylindrical specimen to a predetermined height. After unloading, allow it to recover at room temperature for 30 minutes, then measure the deformation.

Chinese standard ：

GB/T 7759.1 : With Equivalent to ASTM D395 Method B.

Core test parameters ：

Compression ratio Usually for 25%.

Test temperature : Select based on the material and operating conditions, such as 70°C, 100°C, 125°C, 150°C, 200°C, etc.

Test time Usually for 24 hours, 70 hours, 168 hours (1 week), or longer.

III. Specific Solutions for Improving Compression Set in Various Types of Rubber

1. Nitrile rubber

Mechanism characteristics ： The NBR backbone contains unsaturated double bonds, making it susceptible to oxidative and thermal degradation, which can lead to the cleavage of cross-linking bonds and degradation of the backbone.

Improvement plan ：

Optimize the vulcanization system ：

Adopt effective /Partially Effective Vulcanization System Reduce the content of polysulfide bonds, increase the content of monosulfide and disulfide bonds, and enhance the thermal stability of cross-linking bonds.

Add a vulcanizing aid : Such as HVA-2 This can enhance cross-linking efficiency and form a more stable network.

Of course, it is possible to use a peroxide vulcanization system with secondary baking, which results in carbon-carbon bond crosslinking, improved compression set, and better heat resistance. However, this approach is not suitable for rubber products that require high dynamic fatigue resistance and high tear strength.

Select the appropriate raw rubber. ：

Select High acrylonitrile content of the NBR has high polarity, strong intermolecular forces, poor chain segment mobility, and inherently low permanent deformation.

Use Hydrogenated nitrile rubber Its main chain is saturated, exhibiting excellent resistance to thermal and oxidative aging, making it an ideal solution for achieving low compression set.

Filler system ：

Use High-structure carbon black ( N550) Coarse-particle carbon black, which forms a filler network that is stronger and more resilient.

Plasticizing system:

Select Low volatility, high molecular weight plasticizers, such as TP-95, TOTM Reduce mass loss and shrinkage caused by volatilization.

Minimize the amount of plasticizer used.

Anti-aging system ：

An efficient antioxidant composite system must be employed, such as: RD/MB or 445 To effectively inhibit thermo-oxidative aging.

2. Ethylene-propylene rubber

Mechanism characteristics ： The EPDM main chain is fully saturated, exhibiting excellent resistance to thermal and oxidative aging. Its compression set primarily depends on the vulcanization system.

Improvement plan ：

Vulcanization System ：

Peroxide vulcanization This is to obtain. Low compression set The preferred solution. Peroxide formation C-C cross-linking bonds have high bond energy and excellent thermal stability. Recommended for use. DCP or BIPB 。

Auxiliary crosslinking agent Add TAIC, TAC, HVA-2 It can enhance cross-linking efficiency, prevent polymer chain scission, and significantly reduce compression set. Adding a small amount of sulfur appropriately can improve the hot tear resistance and... “Hand feel” is highly elastic.

Avoid using sulfur vulcanization. The sulfur vulcanization system forms polysulfide bonds, which have poor thermal stability and exhibit significant permanent deformation.

Filler system ：

Use Fine-particle furnace carbon black (such as N550, N660) or Precipitated fumed silica and in combination with silane coupling agents (such as Si-69 or KH550 improves their interfacial adhesion with rubber.

Plasticizer : Select Paraffin oil , and EPDM has good compatibility and low volatility.

3. Silicone rubber

Mechanism characteristics The main chain of silicone rubber is: The Si-O bond has a high bond energy and excellent thermal stability. Its compression set is primarily influenced by the density of crosslinking points and the extent of scission reactions.

Improvement plan ：

Raw Rubber Selection ：

Use High molecular weight, high vinyl content silicone rubber raw rubber. The vinyl groups provide additional crosslinking sites, helping to form a denser and more uniform crosslinked network.

Vulcanization System ：

Peroxide vulcanization : Use 2,5-Dimethyl-2,5-di-tert-butyl peroxide hexane Its decomposition products volatilize quickly, have a mild odor, and can form a stable cross-linked structure.

Platinum vulcanization This is A relatively better solution The addition reaction produces no byproducts, and the crosslinked structure is uniform and stable, resulting in extremely low compression set and exceptionally high transparency. If a platinum-catalyzed addition reaction is chosen, care must be taken to prevent platinum contamination. “Poisoning.”

Filler system ：

Use High-surface-area gas-phase precipitated silica And ensure that it undergoes thorough surface treatment (using silane or hydroxyl silicone oil) to reduce surface silanol groups, thereby minimizing degradation and structural effects caused by water absorption.

Additive ：

Add Structural control agent And and Heat-resistant additive (such as Iron oxide ), which can further enhance thermal stability.

4. Fluororubber

Mechanism characteristics ： FKM exhibits excellent resistance to high temperatures and chemical media. Its compression set has historically been a technical challenge, primarily due to the vulcanization system used.

Improvement plan ：

Vulcanization System ：

Bisphenol vulcanization system Traditional system, generally with low compression set.

Peroxide vulcanization system This is Core technology for improving compression set It needs to be coordinated. Polyene co-crosslinking agent (such as TAIC ). This system can form The C-C cross-link bonds exhibit far superior thermal stability compared to the diene ether bonds formed by bisphenol AF, significantly reducing permanent compression deformation at high temperatures.

Select raw rubber with low Mooney viscosity. It helps improve the fluidity of the rubber compound, enabling more uniform dispersion of fillers and a more complete crosslinking network.

Filler system ：

Use Medium-particle thermal cracking carbon black or Barium sulfate Use neutral fillers and avoid using acidic fillers (such as fumed silica) that could interfere with peroxide vulcanization.

Processing technology ：

Ensure Sufficient vulcanization , especially Second-stage vulcanization It must be strictly carried out according to the process curve to thoroughly remove sulfurization byproducts and stabilize the crosslinking network. For the second-stage vulcanization, it is recommended to adopt a gradual temperature-raising approach to address this type of issue.

IV. Summary

Reducing the compression set of rubber products is a systematic undertaking that requires comprehensive consideration—from molecular structure design to formulation and processing. The core strategies can be summarized as follows:

Build a stable and robust cross-linked network. : Priority selection Peroxide vulcanization system (For EPDM, VMQ, FKM) or Effective vulcanization system (For NBR), and use it appropriately. Auxiliary crosslinking agent 。

Enhance the body's anti-aging capabilities. : Select raw rubber with high saturation or high bond energy (such as HNBR, EPDM, FKM, VMQ), and in conjunction with efficient and sufficient Antioxidant 。

Optimize the filling and plasticizing system Select fillers with good reinforcing properties and avoid overfilling; choose plasticizers with low volatility and high molecular weight.

Ensure sufficient vulcanization. Ensure that both the first and second stages of vulcanization are fully completed to form a perfect and stable three-dimensional network structure.

Five, References

1. ASTM D395-18, Standard Test Methods for Rubber Property—Compression Set.

2. Dick, J. S. (2014). How to Improve Rubber Compounds Hanser Publications.

3. Stevens, R. D., & Lamm, G. (2003). Reducing Compression Set in Peroxide-Cured Fluoroelastomers Rubber World.

4. Datta, R. N. (2002). The effect of co-agents on the properties of peroxide-cured EPDM Rubber, rubber products, plastics.

5. GB/T 7759.1-2015, Determination of Compression Set of Vulcanized Rubber or Thermoplastic Rubber—Part 1: At Ambient and Elevated Temperatures.

6. Handbook of the Rubber Industry (Volumes 1 and 2), Chemical Industry Press.

Yang Qingzhi Editor-in-chief Modern Rubber Technology. China Petrochemical Press.

7. Zhan Maosheng, Wang Kai. Study on the Compression Set Performance of Nitrile Rubber [J]. Special Rubber Products, 2019, 40(1): 12-15.

8. Liu Li, Zhang Liqun. Effect of Peroxide Vulcanization System on the Properties of Fluororubber [J]. Synthetic Rubber Industry, 2020, 43(4): 285-290.

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