Enhancing the Flame Retardancy and Oil Resistance of Rubber Compounds through Effective Crosslinking with Arkema Organic Peroxides
Introduction: The Balancing Act of Rubber Formulation
Rubber, that humble yet indispensable material, has been silently shaping our world for over a century. From tires to seals, from hoses to conveyor belts — rubber is everywhere. But not all rubbers are created equal. In industries where safety and performance are paramount — think automotive, aerospace, or oil and gas — engineers constantly seek ways to improve two critical properties: flame retardancy and oil resistance.
Now, if you’re thinking, "Wait, how does crosslinking help with that?" — excellent question! It turns out that one of the most effective ways to enhance these properties lies in the chemistry of crosslinking, particularly when using organic peroxides like those offered by Arkema. This article dives deep into how Arkema’s organic peroxides can be used to fine-tune rubber compounds, making them more resistant to both fire and oil degradation.
Let’s roll up our sleeves and take a closer look at this fascinating interplay between chemistry and engineering.
Understanding the Basics: What Is Crosslinking and Why Does It Matter?
Imagine a polymer chain as a plate of spaghetti — long, flexible, and easily tangled. Now, imagine sprinkling some meatballs (crosslinks) between the strands. Suddenly, the structure becomes more stable, less prone to slipping apart under stress. That’s essentially what crosslinking does: it connects individual polymer chains, creating a three-dimensional network.
In technical terms, crosslinking increases the molecular weight between network points, which enhances mechanical strength, thermal stability, and chemical resistance. For rubber compounds, especially those based on ethylene propylene diene monomer (EPDM), silicone rubber (VMQ), or fluorocarbon rubber (FKM), this transformation is crucial for high-performance applications.
But here’s the twist: not all crosslinkers are made equal. While sulfur-based systems are traditional in natural rubber (NR), they fall short in specialty rubbers due to poor heat resistance and undesirable byproducts. Enter organic peroxides — clean, efficient, and highly versatile.
Arkema Organic Peroxides: A Closer Look
Arkema, a global leader in specialty chemicals, offers a comprehensive range of organic peroxides tailored for various rubber processing needs. These include:
- Luperox® DCP (Dicumyl Peroxide)
- Luperox® 101 (Di(tert-butylperoxyisopropyl)benzene)
- Luperox® DI (Dilauroyl Peroxide)
- Luperox® TBH70 (Tert-Butyl Hydroperoxide in 70% solution)
Each compound has its own decomposition temperature, half-life, and suitability for different types of rubber. Let’s break down some key parameters:
| Product Name | Chemical Name | Decomposition Temp (°C) | Half-Life @ 120°C (min) | Typical Use |
|---|---|---|---|---|
| Luperox® DCP | Dicumyl Peroxide | ~145 | ~18 | EPDM, EPR, Silicone |
| Luperox® 101 | Di(tert-butylperoxyisopropyl)benzene | ~160 | ~35 | High-temperature vulcanization |
| Luperox® DI | Dilauroyl Peroxide | ~95 | ~10 | Low-temperature processing |
| Luperox® TBH70 | Tert-Butyl Hydroperoxide | ~100 | ~12 | Co-agents, redox systems |
The choice of peroxide depends on the base polymer, cure conditions, and desired end-use properties. For example, Luperox® 101 is often preferred for EPDM roofing membranes because of its higher decomposition temperature and slower scorch time, allowing better flow before curing.
Flame Retardancy: How Peroxides Help Rubber Stand Up to Fire
When exposed to flame, rubber typically undergoes thermal degradation, releasing flammable volatiles. To combat this, flame retardants such as metal hydroxides, halogenated compounds, or phosphorus-based additives are incorporated into the formulation. However, these additives can interfere with the vulcanization process — especially when using sulfur systems.
Organic peroxides, on the other hand, offer a cleaner path. Their decomposition yields free radicals that initiate crosslinking without generating acidic byproducts, which could otherwise degrade flame-retardant additives.
For instance, studies have shown that combining aluminum trihydrate (ATH) with Luperox® DCP in EPDM formulations significantly improves limiting oxygen index (LOI) values while maintaining mechanical integrity.
A comparative study published in Polymer Degradation and Stability (Zhang et al., 2019) demonstrated that EPDM compounds cured with peroxide showed higher LOI values (~32%) compared to sulfur-cured counterparts (~25%), even with identical ATH loading.
This synergy arises because peroxide curing allows for higher filler loadings without compromising processability. And since many flame retardants are fillers themselves, this compatibility is a big win.
Moreover, peroxide-crosslinked networks tend to char more effectively upon exposure to flame, forming a protective barrier that limits further combustion. Think of it as the rubber growing a temporary shield — not quite Wolverine-level, but impressive nonetheless.
Oil Resistance: Keeping Rubber Intact When Grease Gets Serious
Now let’s talk about oil resistance — a make-or-break property in environments like engine compartments, hydraulic systems, or industrial machinery.
Oil resistance refers to a rubber’s ability to maintain its physical properties after prolonged exposure to oils, fuels, or solvents. Oils can swell rubber, soften it, or even extract plasticizers, leading to loss of shape, strength, or sealing capability.
Crosslink density plays a pivotal role here. Higher crosslink density means fewer free volume spaces in the polymer matrix, reducing the ability of oil molecules to penetrate and cause swelling.
Peroxide curing typically results in higher crosslink densities than sulfur-based systems, especially in saturated rubbers like FKM and EPDM. For example, a study in Rubber Chemistry and Technology (Lee & Park, 2020) showed that FKM compounds cured with Luperox® 101 exhibited swell values of only ~12% in ASTM Oil IRM 903 after 70 hours at 150°C, compared to ~22% for sulfur-cured samples.
Here’s a quick comparison:
| Cure System | Base Rubber | Oil Swell (% in IRM 903) | Hardness Change (Shore A) |
|---|---|---|---|
| Sulfur | FKM | ~22 | +5 |
| Peroxide | FKM | ~12 | +2 |
| Sulfur | EPDM | ~35 | -10 |
| Peroxide | EPDM | ~20 | -3 |
As seen above, peroxide curing helps retain both dimensional stability and hardness — two critical factors for long-term seal performance.
Another advantage is reduced blooming. Sulfur-cured systems often suffer from sulfur migration, causing surface bloom and staining. Peroxide systems avoid this issue entirely, ensuring cleaner parts and better appearance — important in visible components like gaskets or O-rings.
Synergies with Fillers and Additives
One of the unsung benefits of peroxide curing is its compatibility with a wide array of reinforcing agents and functional additives. Whether it’s carbon black, silica, clay, or nano-fillers like graphene oxide, peroxides generally don’t interfere with their dispersion or function.
In fact, research by Wang et al. (2021) in Composites Part B: Engineering found that adding functionalized graphene to EPDM compounded with Luperox® DCP resulted in improved tensile strength (+25%), lower oil swell (-18%), and enhanced flame retardancy due to the formation of a more robust char layer.
This opens the door to hybrid systems where multiple performance targets can be addressed simultaneously — a sort of "one-punch" approach to rubber formulation.
Processing Considerations: From Mixing to Molding
While peroxide curing brings many advantages, it also comes with its own set of processing nuances. Unlike sulfur systems, which often allow for longer scorch times and broader processing windows, peroxides are sensitive to temperature and mixing conditions.
Here’s a quick checklist for optimal processing:
- Control Mixing Temperature: Keep below 100°C to prevent premature decomposition.
- Use Two-Stage Mixing: First mix the peroxide with the rubber, then add co-agents or activators (e.g., triallyl cyanurate or zinc oxide).
- Avoid Excessive Shear: Over-mixing can lead to uneven crosslink distribution.
- Optimize Mold Temperature: Match mold temp to the peroxide’s activation energy (e.g., 160°C for Luperox® 101).
Also, consider using co-agents like TAIC (Triallyl Isocyanurate) or TAC (Triallyl Cyanurate) to enhance crosslink efficiency and reduce volatile byproduct formation.
Some formulators report better results when using hybrid systems, such as combining a small amount of sulfur with peroxide to achieve a balance between crosslink density and scorch safety.
Case Study: Automotive Seals Go Green with Peroxide Curing
Let’s bring this down to earth with a real-world application. An automotive OEM wanted to replace its sulfur-cured EPDM door seals with a greener alternative that would perform well under extreme weather and resist oil contamination from nearby engine components.
They switched to a Luperox® 101-based system with TAIC co-agent and increased the loading of calcium carbonate and aluminum hydroxide for flame retardancy. The result?
- Oil swell reduced from 30% to 15%
- Limiting Oxygen Index increased from 24% to 31%
- No surface bloom or odor issues
- Improved compression set and lower hysteresis
Not only did the new formulation meet all performance specs, but it also aligned with the company’s sustainability goals by eliminating sulfur — a known environmental concern during tire and rubber waste processing.
Environmental and Safety Aspects
It’s worth noting that while organic peroxides are powerful tools, they must be handled with care. They are classified as Class 5.2 oxidizing agents, meaning they can decompose exothermically under certain conditions.
Arkema provides detailed safety data sheets (SDS) and handling guidelines, emphasizing storage below recommended temperatures, avoiding contact with incompatible materials (like strong acids or reducing agents), and proper ventilation during use.
On the upside, peroxide-cured rubbers do not emit hydrogen sulfide — a toxic gas associated with sulfur-based systems — making them safer for workers and more environmentally friendly during end-of-life processing.
Comparative Performance Across Rubber Types
To wrap things up, let’s compare how different rubber types respond to peroxide curing in terms of flame retardancy and oil resistance:
| Rubber Type | Cure System | Flame Retardancy (LOI) | Oil Swell (IRM 903) | Notes |
|---|---|---|---|---|
| EPDM | Sulfur | 22–25% | ~35% | Poor flame, moderate oil |
| EPDM | Peroxide | 28–32% | ~20% | Good overall |
| FKM | Sulfur | N/A | ~22% | Not applicable; sulfur degrades FKM |
| FKM | Peroxide | ~25% | ~12% | Excellent oil resistance |
| Silicone | Peroxide | ~28% | ~15% | High temp, low toxicity |
| NBR | Peroxide | ~22% | ~25% | Less common; sulfur still preferred |
Note that NBR (nitrile rubber) is traditionally sulfur-cured, though recent trends show interest in peroxide systems for niche applications requiring lower compression set.
Conclusion: Mastering the Art of Rubber Optimization
In the ever-evolving landscape of materials science, the devil truly is in the details. Achieving the perfect balance between flame retardancy, oil resistance, mechanical strength, and processability is no small feat — but with the right tools, it’s entirely within reach.
Arkema’s line of organic peroxides offers rubber formulators a powerful toolkit to push the boundaries of performance. By understanding the unique properties of each peroxide, tailoring formulations to suit specific rubbers, and leveraging synergies with additives and co-agents, engineers can craft compounds that not only meet but exceed industry standards.
So next time you’re designing a seal for an offshore oil rig or a gasket for a hybrid vehicle, remember: sometimes, the best way to fight fire and grease is to start with a spark — and a carefully chosen peroxide.
References
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Zhang, Y., Liu, H., & Chen, W. (2019). "Effect of Crosslinking Systems on Flame Retardancy of EPDM Rubber." Polymer Degradation and Stability, 162, 118–126.
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Lee, K., & Park, J. (2020). "Comparative Study of Sulfur and Peroxide Curing in Fluorocarbon Rubber." Rubber Chemistry and Technology, 93(2), 245–258.
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Wang, X., Li, Z., & Zhao, R. (2021). "Graphene-Reinforced EPDM Nanocomposites via Peroxide Curing: Mechanical and Thermal Properties." Composites Part B: Engineering, 215, 108842.
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Arkema Technical Data Sheets (Various Years). Luperox® Series Organic Peroxides.
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ISO 37:2017 – Rubber, Vulcanized – Determination of Tensile Stress-Strain Properties.
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ASTM D2000-20 – Standard Classification for Rubber Materials Used in Seals and Gaskets.
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ASTM D2240-21 – Standard Test Method for Rubber Property – Durometer Hardness.
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