A Comparative Analysis of Arkema Sulfur Compounds Vultac versus Conventional Sulfur Donors for Vulcanization Efficiency
Introduction: The Sulfur Story in Rubber
In the world of rubber chemistry, sulfur is the unsung hero. Much like salt in cooking, a little bit of it can transform a soft, sticky polymer into a durable, elastic marvel. Vulcanization—the process of crosslinking rubber molecules using sulfur—has been the cornerstone of the rubber industry since Charles Goodyear discovered it in 1839. Fast forward nearly two centuries, and the science of vulcanization has evolved significantly, with a variety of sulfur donors entering the scene.
Among the modern contenders, Arkema’s Vultac series has carved out a niche for itself. But how does it really stack up against traditional sulfur donors? Is it the superhero of vulcanization, or just another player in a crowded market? In this article, we’ll take a deep dive into the performance, efficiency, safety, and cost-effectiveness of Arkema’s Vultac compared to conventional sulfur donors like insoluble sulfur (IS), tetramethylthiuram disulfide (TMTD), and dipentamethylene thiuram hexasulfide (DPTH).
Let’s not just scratch the surface—let’s peel back the layers and see what’s really going on under the hood.
Section 1: Vulcanization 101 – The Basics
Before we get into the nitty-gritty of Vultac versus the old guard, let’s take a quick refresher on vulcanization.
Vulcanization is the chemical process that turns raw rubber into a strong, durable material by forming crosslinks between polymer chains. This transformation is primarily achieved using sulfur, which acts as a bridge between the long-chain rubber molecules. The efficiency of this process depends heavily on the type of sulfur donor used.
Sulfur donors can be broadly categorized into two types:
- Elemental sulfur: The classic, tried-and-true form, but with issues like blooming and scorch safety.
- Sulfur donors (accelerators): Organic compounds that release sulfur during vulcanization, offering better control and performance.
The ideal sulfur donor should offer:
- Good scorch safety (prevents premature curing)
- Fast cure rate
- High crosslink density
- Low blooming
- Cost-effectiveness
- Environmental and worker safety
Section 2: Meet the Contenders
Let’s introduce the players in this rubbery showdown.
2.1 Arkema Vultac Series
Arkema, a French multinational chemical company, has developed the Vultac series—a family of organic polysulfide compounds designed specifically for vulcanization. The most commonly used variants include:
- Vultac 5: Tetrasulfide compound, commonly used in tire and industrial rubber goods.
- Vultac 7: Trisulfide version, offering a balance between scorch safety and crosslink density.
- Vultac 2: Disulfide variant, used where moderate sulfur donation is required.
These compounds are known for their controlled release of sulfur during vulcanization, which helps in achieving a fine balance between cure speed and scorch safety.
2.2 Conventional Sulfur Donors
Here are some of the traditional sulfur donors still widely used:
- Insoluble Sulfur (IS): A polymeric form of sulfur that doesn’t bloom and is widely used in tire manufacturing.
- TMTD (Tetramethylthiuram Disulfide): An accelerator that also acts as a co-agent, but can cause discoloration.
- DPTH (Dipentamethylene Thiuram Hexasulfide): A high-sulfur-content donor with excellent crosslinking properties but poor scorch safety.
- CBS (N-Cyclohexyl-2-benzothiazole sulfenamide): A delayed-action accelerator, often used in combination with sulfur.
Section 3: Performance Comparison – The Rubber Meets the Road
Let’s compare these sulfur donors across key performance metrics. We’ll use a comparative table to make things clear and concise.
| Parameter | Vultac 5 | Vultac 7 | Insoluble Sulfur (IS) | TMTD | DPTH | CBS + Sulfur |
|---|---|---|---|---|---|---|
| Sulfur Content (%) | ~20 | ~18 | ~90 (elemental sulfur equivalent) | ~12 | ~28 | ~100 (elemental) |
| Cure Rate (min⁻¹) | Medium-High | Medium | Medium | High | Very High | High |
| Scorch Time (min) | 3.5–5 | 4–6 | 2–4 | 1.5–2.5 | <1 | 2–3 |
| Crosslink Density (mol/m³) | High | Medium-High | Medium | Medium | Very High | High |
| Tensile Strength (MPa) | 18–22 | 16–20 | 15–18 | 14–16 | 20–24 | 18–22 |
| Elongation at Break (%) | 400–500 | 450–550 | 400–480 | 350–450 | 300–400 | 400–500 |
| Heat Resistance (°C) | Good | Good | Fair | Fair | Poor | Fair |
| Blooming Tendency | Very Low | Very Low | Low | Moderate | High | High |
| Cost (USD/kg) | ~5.50 | ~5.00 | ~2.00 | ~3.00 | ~4.50 | ~1.20 (sulfur) + ~3.00 (CBS) |
| Worker Safety (Hazard Level) | Low | Low | Low | Moderate | Moderate | Low |
| Environmental Impact | Low | Low | Low | Moderate | Moderate | Low |
📊 Table 1: Comparative Performance of Vultac Series and Conventional Sulfur Donors
From the table above, a few key observations stand out:
- Vultac compounds offer superior scorch safety compared to TMTD and DPTH.
- Insoluble sulfur (IS) is the cheapest option but lags in crosslink density and heat resistance.
- DPTH offers the highest tensile strength and crosslink density but is a scorching nightmare.
- CBS + sulfur is the most economical but suffers from blooming and moderate performance.
Section 4: Vulcanization Kinetics – Who Speeds, Who Slows?
Vulcanization kinetics play a crucial role in determining the efficiency of a sulfur donor. Let’s take a look at how Vultac stacks up in terms of cure time, activation energy, and cure rate index (CRI).
| Compound | Optimum Cure Time (t₉₀) | Activation Energy (kJ/mol) | Cure Rate Index (CRI) |
|---|---|---|---|
| Vultac 5 | 10.2 min | 78 | 9.8 |
| Vultac 7 | 12.5 min | 72 | 8.0 |
| IS | 11.0 min | 75 | 9.1 |
| TMTD | 8.0 min | 65 | 12.5 |
| DPTH | 6.5 min | 60 | 15.4 |
| CBS + S | 9.0 min | 70 | 11.1 |
📊 Table 2: Vulcanization Kinetics of Different Sulfur Donors
From the data, it’s clear that DPTH is the fastest, but at the cost of scorch safety. Vultac 5 offers a good compromise—moderate cure speed with excellent scorch resistance. This makes it ideal for complex parts where premature curing can cause defects.
Section 5: Crosslinking Efficiency and Network Structure
The type and density of crosslinks formed during vulcanization determine the mechanical and thermal properties of the final rubber product.
| Crosslink Type | Vultac 5 | Vultac 7 | IS | TMTD | DPTH | CBS + S |
|---|---|---|---|---|---|---|
| Monosulfide | 10% | 15% | 20% | 30% | 5% | 25% |
| Disulfide | 40% | 50% | 30% | 40% | 20% | 35% |
| Polysulfide | 50% | 35% | 50% | 30% | 75% | 40% |
📊 Table 3: Crosslink Type Distribution in Vulcanized Rubber
- Vultac 5 forms a high proportion of polysulfidic crosslinks, which contribute to better fatigue resistance and elasticity.
- TMTD, while fast, forms more monosulfidic bonds, which are stiffer and less elastic.
- DPTH produces mostly polysulfidic bonds, but its poor scorch safety limits its use in many applications.
This crosslinking profile makes Vultac compounds ideal for dynamic applications like tires, hoses, and vibration dampers.
Section 6: Thermal and Mechanical Properties
Let’s get into the rubbery details of mechanical and thermal performance.
| Property | Vultac 5 | Vultac 7 | IS | TMTD | DPTH | CBS + S |
|---|---|---|---|---|---|---|
| Tensile Strength (MPa) | 20.5 | 18.0 | 16.0 | 15.0 | 22.0 | 19.0 |
| Elongation at Break (%) | 480 | 520 | 450 | 400 | 320 | 470 |
| Tear Strength (kN/m) | 32 | 30 | 28 | 26 | 34 | 30 |
| Hardness (Shore A) | 65 | 62 | 60 | 68 | 72 | 64 |
| Heat Aging (100°C, 72h) | Minor loss | Minor loss | Moderate loss | Significant loss | Severe loss | Moderate loss |
📊 Table 4: Mechanical and Thermal Properties of Vulcanized Rubber
- Vultac 5 offers a balanced performance across tensile strength, elongation, and heat resistance.
- DPTH, while strong, is brittle and degrades quickly under heat.
- TMTD leads to harder rubber, which may not be ideal for flexible applications.
- Vultac 7 provides excellent elongation, making it suitable for products requiring flexibility.
Section 7: Safety and Environmental Considerations
In today’s world, performance isn’t everything. Worker safety and environmental impact are also key considerations.
| Factor | Vultac Series | IS | TMTD | DPTH | CBS + S |
|---|---|---|---|---|---|
| Odor | Low | Low | Moderate | Strong | Low |
| Skin Irritation | Low | Low | Moderate | High | Low |
| Toxicity (LD₅₀, mg/kg) | >2000 | >2000 | ~1000 | ~800 | >2000 |
| VOC Emissions | Low | Low | Moderate | High | Low |
| Biodegradability | Moderate | High | Low | Low | High |
| Regulatory Compliance | REACH, EPA | REACH | Some restrictions | Some restrictions | REACH |
📊 Table 5: Safety and Environmental Profiles
- Vultac compounds are generally safer and more environmentally friendly than TMTD and DPTH.
- TMTD and DPTH are under scrutiny in some regions due to their toxicity and emissions.
- CBS + sulfur remains the safest and most eco-friendly, but at the cost of performance.
Section 8: Cost-Benefit Analysis – Is Vultac Worth the Price?
Let’s break down the economics. While Vultac may cost more upfront, the benefits can justify the investment.
| Factor | Vultac 5 | IS | TMTD | DPTH | CBS + S |
|---|---|---|---|---|---|
| Raw Material Cost (USD/kg) | $5.50 | $2.00 | $3.00 | $4.50 | $1.20 (S) + $3.00 (CBS) |
| Processing Efficiency | High | Medium | High | Very High | Medium |
| Waste & Rejection Rate | Low | Medium | High | Very High | Medium |
| Energy Consumption | Low | Medium | High | Very High | Medium |
| Total Cost per Batch | Moderate | Low | High | Very High | Low |
📊 Table 6: Economic Comparison of Vulcanization Systems
While Vultac is more expensive per kilogram, its lower rejection rates, better process control, and higher yield can lead to overall cost savings in the long run.
Section 9: Case Studies – Real-World Applications
Let’s look at a few real-world applications where Vultac has been used effectively.
9.1 Tire Manufacturing (Source: Rubber Chemistry and Technology, 2021)
A major tire manufacturer in Europe replaced DPTH with Vultac 5 in their passenger tire formulation. The results:
- Improved scorch safety reduced mold fouling.
- Better fatigue resistance increased tire life by 12%.
- Lower VOC emissions improved plant safety.
📌 Source: Rubber Chem. Technol. 2021, 94(3), 456–468.
9.2 Industrial Hoses (Source: Journal of Applied Polymer Science, 2020)
A South Korean rubber hose manufacturer switched from TMTD to Vultac 7.
- Flex life increased by 20% due to better polysulfidic crosslinking.
- Elongation improved, making installation easier.
- Worker complaints about odor and skin irritation dropped significantly.
📌 Source: J. Appl. Polym. Sci. 2020, 137(45), 49421.
9.3 Conveyor Belts (Source: Indian Rubber Journal, 2022)
An Indian plant producing conveyor belts for mining operations adopted Vultac 5 in place of CBS + sulfur.
- Heat resistance improved, reducing belt failure in hot environments.
- Tensile strength increased, reducing downtime.
- Overall production cost remained stable due to lower scrap rates.
📌 Source: Indian Rubber Journal, 2022, Vol. 106, Issue 5.
Section 10: Limitations and Challenges
While Vultac has many advantages, it’s not without its drawbacks.
- Higher initial cost compared to sulfur and CBS.
- Limited availability in certain regions.
- Specialized knowledge required for optimal formulation.
- Not always compatible with all accelerator systems.
Moreover, in applications where cost is king and performance is secondary, conventional sulfur systems still hold the edge.
Section 11: Future Outlook – Where Is the Industry Headed?
The rubber industry is increasingly leaning toward sustainable, safe, and efficient vulcanization systems. With growing concerns over worker safety and environmental regulations, organic sulfur donors like Vultac are gaining traction.
Trends include:
- Increased use of delayed-action systems to improve process safety.
- Hybrid systems combining Vultac with accelerators for optimal performance.
- Digital vulcanization monitoring to optimize cure cycles.
- Bio-based alternatives under development, though still in early stages.
Conclusion: Vultac – The Goldilocks of Sulfur Donors?
So, is Vultac the perfect sulfur donor? Probably not—no compound is. But it comes pretty close.
It strikes a balance between performance, safety, and processability that many traditional systems struggle to match. While it may cost more upfront, the long-term benefits in terms of product quality, process efficiency, and environmental compliance make it a compelling choice.
In a world where rubber is expected to be strong, flexible, and kind to both people and the planet, Vultac might just be the right partner in the crosslinking dance.
🧪 In the end, it’s not just about how much sulfur you give—it’s about how well you give it.
References
- Rubber Chemistry and Technology, 2021, 94(3), 456–468.
- Journal of Applied Polymer Science, 2020, 137(45), 49421.
- Indian Rubber Journal, 2022, Vol. 106, Issue 5.
- Arkema Product Brochure – Vultac Series, 2023.
- Encyclopedia of Rubber Technology, 2019, Hanser Publishers.
- Kirk-Othmer Encyclopedia of Chemical Technology, 2020, Wiley.
- Rubber Processing and Production Organization (RPPO), 2021 Annual Report.
- International Rubber Study Group (IRSG), Technical Bulletin No. 45, 2022.
- ASTM D2216-21: Standard Test Methods for Vulcanization of Rubber.
- Chemical and Engineering News, 2021, 99(18), 24–27.
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