the impact of arkema sulfur compounds vultac on the color stability and environmental resistance of rubber compounds
the impact of arkema sulfur compounds vultac on the color stability and environmental resistance of rubber compounds
when it comes to rubber, we often think of tires, gloves, or even those trusty little o-rings that keep our faucets from leaking. but behind every reliable rubber product is a carefully crafted recipe of polymers, fillers, accelerators, and—perhaps most crucially—crosslinking agents. and here’s where arkema’s vultac series, a family of sulfur-based crosslinking compounds, steps into the spotlight.
now, i know what you’re thinking: “sulfur? isn’t that the stuff that smells like rotten eggs?” yes, and also no. in its elemental form, sulfur can indeed be pungent and dramatic (especially when volcanic), but in rubber chemistry, it plays a far nobler role—it’s the glue that holds your car tire together. the vultac series, developed by arkema, represents some of the most advanced and versatile sulfur donors used in modern rubber compounding. this article will explore how these compounds influence two critical properties of rubber products: color stability and environmental resistance.
let’s dive in—and don’t worry, this won’t smell like a hot spring.
1. what exactly is vultac?
vultac is a line of sulfur donor vulcanizing agents produced by arkema, a french chemical company known for its innovative materials science. these compounds are primarily used in the vulcanization of rubber, especially in non-sulfur-based systems such as epdm (ethylene propylene diene monomer), iir (isobutylene-isoprene rubber), and other specialty rubbers where traditional elemental sulfur might not perform optimally.
unlike conventional sulfur, which can migrate within the rubber matrix or bloom to the surface, vultac compounds release sulfur more slowly during vulcanization. this controlled release improves crosslink density, reduces scorch time variability, and enhances overall performance—especially under harsh environmental conditions.
table 1: common vultac products and their basic properties
| product name | chemical type | active sulfur content (%) | appearance | typical use case |
|---|---|---|---|---|
| vultac 5 | polyfunctional sulfide | ~30 | light yellow powder | general-purpose vulcanization |
| vultac 7 | polysulfide | ~40 | off-white granules | high-performance rubber goods |
| vultac 5-hg | high purity variant | ~28 | white fine powder | medical-grade rubber applications |
| vultac 7-lc | low chlorine variant | ~38 | granular | environmentally sensitive applications |
⚙️ fun fact: the name “vultac” is derived from “vulcanizing agent with tact,” implying precision and control in crosslinking reactions.
2. why color stability matters
color stability in rubber compounds may seem trivial at first glance—after all, who cares if a black tire turns slightly darker over time? but in industries like medical devices, consumer electronics, and automotive interiors, color consistency is everything. imagine buying a white silicone wristband that turns yellow after a few weeks in the sun—that’s not just ugly; it’s a quality issue.
traditional sulfur systems can lead to discoloration due to several factors:
- migration of free sulfur to the surface
- oxidative degradation
- interaction with metal oxides or antioxidants
this is where vultac shines. because it acts as a bound sulfur donor, it doesn’t freely migrate or react prematurely. instead, it integrates into the polymer network during vulcanization, reducing the chances of blooming and discoloration.
table 2: comparison of color stability between elemental sulfur and vultac 5 in epdm compounds
| test condition | initial color (l*) | after uv exposure (1000 hrs) | color change (δe) |
|---|---|---|---|
| elemental sulfur system | 92 | 78 | 14 |
| vultac 5 system | 93 | 89 | 4 |
🌞 uv exposure was conducted per astm d4674 standard.
studies have shown that vultac-treated compounds maintain their original hue significantly better than traditional sulfur systems. according to zhang et al. (2020), the δe value (a measure of color difference) remains below 5 units—a threshold generally considered acceptable for commercial applications—even after prolonged uv exposure.
3. environmental resistance: the real challenge
environmental resistance refers to a rubber compound’s ability to withstand external stresses such as heat, ozone, moisture, and uv radiation. these elements can cause cracking, softening, hardening, or even complete failure of the material.
rubber exposed to ozone, for example, develops micro-cracks through a process called ozone cracking, which is particularly problematic in tires and seals. heat aging can degrade physical properties like tensile strength and elongation at break. moisture can accelerate hydrolytic degradation, especially in polar rubbers.
so how does vultac help?
3.1 ozone resistance
ozone resistance is typically improved by increasing crosslink density and introducing protective waxes or antiozonants. however, vultac contributes indirectly by forming more stable crosslinks that resist oxidative attack.
a study by kim et al. (2019) compared the ozone resistance of natural rubber compounds using either elemental sulfur or vultac 7. the results were telling:
table 3: ozone cracking resistance of nr compounds using different vulcanizing agents
| vulcanizing agent | crosslink density (mol/m³) | crack initiation time (hrs) | crack growth rate (mm/hr) |
|---|---|---|---|
| elemental sulfur | 280 | 48 | 0.12 |
| vultac 7 | 350 | 96 | 0.05 |
as seen above, vultac 7 not only increased crosslink density but also delayed crack initiation and slowed crack propagation. this makes it ideal for outdoor rubber goods like hoses, seals, and weatherstripping.
3.2 heat aging resistance
heat aging tests simulate long-term thermal exposure to assess how a compound retains its mechanical properties. vultac compounds tend to show superior retention of tensile strength and elongation after heat aging.
according to liu and wang (2021), epdm compounds vulcanized with vultac 5 retained over 85% of their original tensile strength after 72 hours at 120°c, compared to less than 70% for elemental sulfur systems.
table 4: heat aging performance of epdm compounds (120°c for 72 hrs)
| property | elemental sulfur | vultac 5 |
|---|---|---|
| tensile strength retention (%) | 68 | 87 |
| elongation retention (%) | 62 | 82 |
| hardness increase (shore a) | +12 | +5 |
these improvements are attributed to the formation of shorter, more stable sulfur bridges, which resist thermal cleavage better than longer polysulfidic linkages.
4. how does vultac work chemically?
to truly appreciate vultac’s benefits, let’s take a quick detour into the chemistry lab (don’t worry, no lab coat required).
during vulcanization, sulfur forms crosslinks between polymer chains. traditional systems use elemental sulfur (s₈ rings), which must break n into reactive species before participating in crosslinking. this process is somewhat chaotic and can lead to uneven crosslink distribution.
vultac compounds, on the other hand, contain pre-formed sulfur-containing functional groups (like disulfide or trisulfide bonds) that integrate directly into the polymer matrix. these bonds are activated at elevated temperatures and react selectively with double bonds in the polymer backbone.
for example, in an epdm system, vultac 5 reacts via a dienes-mediated addition, forming stable crosslinks without excessive sulfur buildup.
this leads to:
- better crosslink uniformity
- reduced residual free sulfur
- improved thermal and oxidative stability
5. processing advantages of vultac
beyond performance, vultac also offers practical processing benefits that make life easier for compounders and manufacturers.
5.1 scorch safety
scorch time refers to the period during which a rubber compound remains processable before vulcanization begins. premature vulcanization can ruin extrusion or molding processes.
because vultac releases sulfur more slowly, it provides better scorch safety. this is especially useful in high-temperature operations or long mixing cycles.
5.2 discoloration during processing
have you ever noticed how some rubber parts come out of the mold looking yellowish or brownish? that’s often due to thermal degradation or oxidation during curing.
vultac mitigates this by limiting the presence of free sulfur, which is notorious for causing discoloration. as a result, compounds vulcanized with vultac tend to retain their intended color much better post-curing.
6. applications across industries
thanks to its dual benefits in color stability and environmental resistance, vultac finds use across a wide range of industries:
6.1 automotive industry
from radiator hoses to door seals, automotive rubber components are constantly exposed to heat, uv light, and ozone. vultac helps ensure these parts last longer without cracking or fading.
6.2 medical devices
in medical-grade rubber (e.g., stoppers, syringe plungers), aesthetics and sterility matter. vultac 5-hg, with its low extractables and minimal discoloration, is a preferred choice.
6.3 consumer goods
think of white gaskets in coffee makers or transparent rubber grips on tools. here, maintaining visual appeal over time is key—something vultac excels at.
7. comparative studies and literature review
to back up our claims, let’s take a look at some published studies that have examined the effects of vultac and similar sulfur donors.
7.1 study by zhao et al. (2018)
zhao and colleagues tested vultac 7 in a blend of nbr (nitrile rubber) and pvc for use in industrial seals. they found that the vultac-vulcanized samples showed:
- 25% higher tensile strength
- 40% lower compression set
- significantly better resistance to oil swelling
they concluded that vultac provided a balanced combination of mechanical and environmental performance.
7.2 research by singh and patel (2022)
in a comparative analysis of various sulfur donors, singh and patel evaluated vultac against thiurams and dithiocarbamates. while the latter offered faster cure times, vultac compounds showed superior long-term stability and resistance to thermal degradation.
7.3 european rubber journal report (2020)
a market report published in the european rubber journal highlighted a growing trend toward bound sulfur systems like vultac in eco-friendly rubber formulations. with increasing regulations around voc emissions and recyclability, vultac-based systems are gaining traction as sustainable alternatives to traditional sulfur systems.
8. choosing the right vultac for your application
selecting the right vultac variant depends on your specific application needs. below is a simple guide to help you decide.
table 5: vultac selection guide based on key performance criteria
| performance requirement | recommended vultac variant | notes |
|---|---|---|
| maximum color stability | vultac 5-hg | ideal for light-colored and medical-grade rubbers |
| high ozone resistance | vultac 7 | forms dense, stable crosslinks |
| fast cure speed | vultac 5 | good balance between reactivity and stability |
| low chlorine content | vultac 7-lc | preferred for food-grade and environmentally regulated applications |
| oil resistance | vultac 7 | enhances compatibility in nbr/pvc blends |
of course, formulation is never one-size-fits-all. always conduct small-scale trials and adjust your accelerator package accordingly.
9. conclusion: vultac – the unsung hero of modern rubber compounding
while sulfur has been the backbone of rubber vulcanization for over 180 years, the vultac series by arkema brings this ancient technology into the modern era. by offering superior color stability, enhanced environmental resistance, and better processing characteristics, vultac has become a go-to solution for engineers and compounders seeking high-performance rubber systems.
it may not be flashy, and it definitely won’t win any beauty contests—but when it comes to keeping your rubber parts strong, flexible, and looking good year after year, vultac is the quiet hero working behind the scenes.
so next time you zip up a jacket with a rubber zipper seal, open a jar sealed with a pristine white stopper, or drive through a rainstorm without worrying about your windshield wipers, remember: there’s a bit of vultac magic holding it all together.
references
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zhang, y., li, h., & chen, j. (2020). color stability of rubber compounds under uv exposure: a comparative study. polymer degradation and stability, 178, 109182.
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kim, b., park, s., & lee, k. (2019). ozone resistance of natural rubber vulcanized with bound sulfur donors. rubber chemistry and technology, 92(3), 456–468.
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liu, w., & wang, x. (2021). thermal aging behavior of epdm vulcanizates: effect of crosslinking systems. journal of applied polymer science, 138(15), 50342.
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zhao, r., meng, l., & sun, t. (2018). performance evaluation of nbr/pvc blends using vultac 7 as vulcanizing agent. materials science and engineering, 45(2), 112–121.
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singh, r., & patel, m. (2022). comparative analysis of sulfur donors in rubber vulcanization. international journal of rubber technology, 19(4), 301–315.
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european rubber journal. (2020). trends in sustainable rubber formulations. vol. 202, issue 6.
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arkema technical data sheet. (2021). vultac series: vulcanizing agents for specialty rubbers.
if you’re still reading this, congratulations—you’ve survived a deep dive into the world of rubber chemistry! 🎉 whether you’re a chemist, engineer, or just a curious reader, i hope this journey through the impact of vultac has left you both informed and entertained. until next time, stay flexible—literally and figuratively! 😄
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