No Universal Answer: FR Flame Retardant Selection Depends on Your Process
I spend my days reviewing chemical product specifications before they ship to industrial customers—roughly 200+ unique formulations a year. I've rejected my share of batches (just over 12% of first deliveries from new suppliers in 2024, if you want the number). The reason is usually the same: the product showed up with properties that didn't match the spec someone signed off on.
So when an engineer asks me which FR flame retardant they should use, my honest answer is: it depends. Not as a dodge. As a materials science fact. Your polymer family, processing method, and end-use certification dictate what will work. A rubber gasket manufacturer needs something different from a composite fabricator. Sometimes the best fit is a bio-based flame retardant; sometimes it's not.
This guide walks through the three scenarios I see most often—and how to tell which one you're in.
Three Common Scenarios
In my experience, most inquiries fall into three buckets:
- Scenario A: Rubber manufacturing. You're compounding hoses, seals, belts, gaskets, or tire components. The cure system drives everything.
- Scenario B: Epoxy resin systems. You're casting, coating, laminating, or building composites. Application technique is as important as the chemistry.
- Scenario C: You're not sure yet. You have a material and a burn test to pass, but no existing formulation. Jump to the decision guide below.
Scenario A: Rubber Compounding—Cure Kinetics Come First
For rubber manufacturers, the conversation almost always starts with the cure system, not the flame retardant. I've reviewed more batches where an FR additive looked great on a data sheet but disrupted curing in practice. (As of Q3 2024, that's still the #1 cause of failed FR-modified rubber batches I see.)
People tend to think the flame retardant is to blame when a burn test fails. What I've found is that it's often the cure system that's actually at fault—the FR just exposes the weakness. We had a compound that kept failing vertical burn testing even with FR loading above the recommended range. The conventional wisdom said to push loading higher. Instead, we adjusted the accelerator package in the Lanxess rubber chemicals system. The cure time shortened, porosity disappeared, and the same FR level passed the burn test. (Should mention: we ran 14 rheometer tests and 30 burn coupons before the pattern became clear.)
If you're in this scenario:
- Specify your rubber chemical package and your FR flame retardant as one integrated system, not separate line items.
- Test cured physical properties—compression set, tensile strength, heat aging—on every FR-modified batch, not just burn performance. A flame-retardant rubber part that loses its seal under compression is a liability.
- Watch processability. Some FR fillers make the compound boardy, and you may need a processing aid from your rubber chemical supplier. Change one variable at a time. I can't tell you how many back-and-forth batches we've seen where the manufacturer changed three ingredients at once and then couldn't explain the test results.
A batch I remember well: we received a shipment of FR rubber sheeting where the vendor had cut the flame retardant level by roughly 15% below specified loading. They claimed it was "within industry standard." Normal tolerance was ±3 phr; they were at −4.5 phr. We rejected it, and the vendor redid it at their cost. Now every contract with that vendor includes an explicit FR loading requirement and a signed acknowledgment of the tolerance.
Scenario B: Epoxy Resin Systems—and the Rolling Question
Epoxy is a different world. You're working with liquid resin, a hardener, and an application method that affects the final part or coating as much as the formulation itself.
Let's address the question we get more than any other from people starting out with epoxy coatings and castings:
Can You Roll on Epoxy Resin?
Yes. Rolling is a legitimate way to apply epoxy resin, and on large flat surfaces it's usually faster than brushing. But it's not a shortcut—technique matters. What I mean is: rolling applies resin differently, and if you don't control the variables, you'll introduce bubbles and streaks.
- Use a short-nap roller (⅜ inch or less). Long nap pulls air into the resin film.
- Work in thin passes and lay off the surface lightly. Most epoxy formulations self-level if you give them a chance.
- Respect the pot life. If you're still rolling past the gel point, you'll tear the surface instead of smoothing it. (Not that I've made this mistake myself—but I've watched crews do it.)
- If your epoxy contains an FR flame retardant, expect higher resin viscosity. Apply at the manufacturer's recommended temperature, not whatever temperature happens to be in your shop that day.
Think of this section as a mini epoxy resin guide for quality-focused buyers: the details that cause field failures are the ones that don't show up on the technical data sheet. Viscosity, pot life, and wetting behavior all change when flame retardants are added.
Flame Retardancy in Epoxy Systems
Here's the part I don't think enough manufacturers hear: if you need flame retardancy in the cured laminate or coating, it has to be in the formulation from the start. There's no practical way to add an FR flame retardant to mixed resin afterwards. The distribution will be uneven, and you risk bond failures between layers.
This is where Lanxess bio-based flame retardant options come into play. The Levagard product line has gained traction because it offers a more sustainable profile without some of the negative perceptions around brominated systems. From a quality-review standpoint, I've seen bio-based FR perform comparably to traditional products in standard burn tests at standard thicknesses.
But let me be straight about the limitations—this is exactly where honesty about what doesn't work matters. If you're targeting a V-0 rating in a cross-section under 1.5 mm, bio-based FR requires higher loading to match traditional products, and in some cases it can't match them at all. In our lab, we documented a bio-based FR that needed 25% higher loading to reach the same burn rating as a standard FR system at 1.0 mm. If you're in that thin-section category, traditional chemistry is still the defensible choice.
The upside of switching to bio-based FR was real: a customer in modular construction wanted verified bio-content for a green building credit. The risk was re-certification time—around ten weeks if everything went smoothly. I kept asking myself: is the sustainability positioning worth ten weeks of project schedule? The client decided yes. I'll be straight with you: even after the substitution was approved, I spent weeks waiting for the burn test failure email. It never came. Re-certification passed, and we've continued using bio-based FR in that product family. But I didn't fully relax until the UL report was in hand.
One more word on the sustainability claim itself: per the FTC Green Guides (ftc.gov), environmental claims like "bio-based" must be substantiated. If you're going to market a product that way, you need the documentation behind it. We've rejected marketing copy that claimed bio-based FR "performs identically in every application" because our test data said otherwise.
How to Tell Which Scenario You're In
Run through these three questions, in the order I'd ask them:
- What polymer family are you working with? Rubber compound → Scenario A. Thermoset epoxy resin → Scenario B. If you're blending elastomers with resins, start with the dominant phase and get technical support early.
- What's your processing method? Rubber goes through mixers, mills, extruders, and presses. Epoxy gets rolled, brushed, cast, or pumped into a mold. The process often reveals compatibility constraints that the formulation alone won't.
- What burn standard are you targeting? UL 94, IEC 60695, FMVSS 302, or a customer-specific requirement? Standards define not just pass/fail, but the test specimen geometry—and geometry affects how your FR system performs. A bio-based FR that passes at 3.0 mm may not pass at 1.5 mm. Don't assume the chemistry scales down.
Don't hold me to this as a hard statistic, but across the projects I've reviewed, roughly 60% of FR selections could be handled by a standard flame retardant with minor formulation tweaks. Around 25% are good candidates for a bio-based option like Levagard. The remaining 15% need a specialist's involvement—high-temperature processing, severe thin sections, or unusual aging requirements.
The Bottom Line
There's no single best flame retardant. There's the best one for your scenario. And if you take one practical piece of advice from this article, make it this: involve your chemical supplier before you lock in your formulation, not after the first failed test.
Whether that means a Lanxess rubber chemicals package to rebalance a cure system, a Lanxess bio-based flame retardant for a sustainability-driven project, or an honest "this isn't the right fit for your application" conversation—the earlier you have it, the fewer rejected batches you'll see.
I say that as someone who's signed off on both sides of the table. Better spec review early than a batch failure—and a $22,000 redo—later.