The Impact of Paint Polyurethane Flame Retardants on the Gloss, Hardness, and Scratch Resistance of the Final Coating.

Date：2025-08-07 Categories：News

The Impact of Paint Polyurethane Flame Retardants on the Gloss, Hardness, and Scratch Resistance of the Final Coating
By Dr. Lin Wei – Materials Chemist & Coating Enthusiast
(Yes, I actually enjoy staring at drying paint. Judge me.)

Let’s talk about fire. Not the kind that warms your marshmallows, but the kind that turns your fancy furniture into charcoal. Now, imagine a world where your sofa doesn’t just look good—it refuses to burn. Enter: polyurethane flame retardants. These sneaky little additives are like the bodyguards of the coating world—silent, invisible, but ready to jump in when things get too hot.

But here’s the catch: when you invite flame retardants into your polyurethane paint formula, they don’t always play nice with other properties. Think of it like adding garlic to chocolate cake. Sure, it might keep vampires away, but your dessert might not win any taste awards.

So, what happens to gloss, hardness, and scratch resistance when you spike your coating with flame retardants? Let’s dive in—no lab coat required (though I do recommend gloves).

🔥 Flame Retardants 101: Who Are These Mysterious Guests?

Flame retardants in polyurethane coatings are typically added to meet fire safety standards (like ASTM E84 or EN 13501-1). They work by either:

Cooling the system (endothermic decomposition),
Forming a protective char layer (physical barrier),
Or diluting flammable gases (releasing inert gases like CO₂ or H₂O).

Common types used in PU coatings include:

Flame Retardant Type	Chemical Class	Mode of Action	Typical Loading (%)
APP (Ammonium Polyphosphate)	Inorganic	Condensed phase (char formation)	15–30%
TPP (Triphenyl Phosphate)	Organophosphate	Vapor phase (radical quenching)	5–15%
DOPO-HQ (DOPO-based)	Reactive phosphorus	Both phases	2–8%
Melamine Cyanurate	Nitrogen-based	Endothermic + gas dilution	10–20%
ATH (Aluminum Trihydrate)	Inorganic filler	Endothermic + water release	30–60%

Source: Levchik & Weil (2006), "Thermal Decomposition and Flame Retardancy of Polyurethanes"; Zhou et al. (2020), "Recent Advances in Flame Retardant Coatings"

Note: APP and ATH are the heavy lifters—they get the job done, but they come with baggage. More on that later.

🌟 Gloss: When Your Paint Stops Shining (Literally)

Gloss is that je ne sais quoi of coatings—the reason your car looks like a mirror and your kitchen cabinets scream “I have my life together.”

But add flame retardants? Suddenly, your high-gloss finish looks like a matte-finish existential crisis.

Why? Two main culprits:

Particle dispersion issues – Inorganic fillers like APP or ATH don’t dissolve; they disperse. Poor dispersion = surface roughness = light scattering = dull finish.
Refractive index mismatch – If the flame retardant particles have a different refractive index than the PU matrix, light bounces off weirdly. Think of it like putting sand in your contact lens.

Let’s look at some real-world data:

Flame Retardant	Loading (%)	Gloss (60°) – Initial	Gloss (60°) – After Addition	% Drop
None (control)	0	92	92	0%
APP	20	92	58	37%
TPP	10	92	78	15%
DOPO-HQ (reactive)	5	92	85	8%
ATH	40	92	42	54%

Data adapted from Wang et al. (2018), "Effect of Flame Retardants on the Surface Properties of Polyurethane Coatings"

Ouch. ATH and APP are basically the fog machines of the coating world.

Pro Tip: If you need high gloss, go reactive. DOPO-based additives chemically bond into the PU network—less phase separation, less scattering. Or, grind your filler particles real fine (submicron size), but don’t complain when your disperser starts crying.

💪 Hardness: Is Your Coating Tough or Tofu?

Hardness tells you whether your coating can survive a key in your pocket or a clumsy elbow. We usually measure it with a pencil hardness test (yes, like school pencils—HB, 2H, etc.) or a Shore D durometer.

Now, here’s where flame retardants get interesting.

Inorganic fillers (APP, ATH): Act like tiny rocks in a soft matrix. They can increase hardness… up to a point. But too much, and they create stress points. It’s like reinforcing tofu with gravel—sounds strong, but one tap and it crumbles.
Organophosphates (TPP): These are plasticizers. They make the PU softer. Great for flexibility, bad if you want your desk to resist pen marks.

Check this out:

Flame Retardant	Loading (%)	Pencil Hardness (Initial)	Pencil Hardness (After)	Shore D (Before)	Shore D (After)
None	0	2H	2H	78	78
APP	20	2H	3H	78	82
TPP	10	2H	H	78	70
DOPO-HQ	5	2H	2H	78	77
ATH	40	2H	2H (but brittle)	78	85

Source: Li et al. (2019), "Mechanical and Thermal Properties of Flame Retardant Polyurethane Coatings"

So APP and ATH boost hardness, but often at the cost of flexibility. And brittle coatings? They crack under stress like a teenager during finals week.

TPP softens things—useful in flexible substrates (like car interiors), but a nightmare for flooring.

🔪 Scratch Resistance: The “Oops, I Dropped My Keys” Test

Scratch resistance is where coatings prove their worth. Will that nail leave a white line? Will sand on your shoe ruin the finish?

Flame retardants affect scratch resistance in two ways:

Abrasion from hard particles – Fillers like ATH are abrasive. They can actually increase resistance to light scratches (like fingernails), but they make the coating more prone to microcracking under repeated stress.
Reduced cohesion – If the flame retardant isn’t well bonded, it creates weak spots. Think of it like a brick wall with Styrofoam bricks—looks solid, but push and it caves.

Here’s how different additives stack up in Taber abrasion tests (lower weight loss = better resistance):

Flame Retardant	Loading (%)	Weight Loss (mg/100 cycles)	Scratch Visibility (1–5, 5=bad)
None	0	8.2	1.5
APP	20	10.1	3.0
TPP	10	15.6	4.2
DOPO-HQ	5	9.0	2.0
ATH	40	18.3	4.5

Data from Zhang et al. (2021), "Scratch and Abrasion Behavior of Flame Retardant Polymer Coatings"

TPP and ATH are the problem children here. TPP softens the film, making it easy to gouge. ATH, while hard, creates internal stress and poor adhesion at high loadings.

DOPO-HQ? The quiet overachiever. Minimal impact, maximum fire protection.

🎯 The Balancing Act: Performance vs. Safety

So, what’s the takeaway? You can’t have your cake and eat it too—unless you’re a chemist with a good formulation.

Additive	Fire Performance	Gloss	Hardness	Scratch Resistance	Best For
APP	⭐⭐⭐⭐☆	⭐⭐	⭐⭐⭐☆	⭐⭐	Industrial, low-gloss applications
ATH	⭐⭐⭐⭐	⭐	⭐⭐⭐☆	⭐	High-loading, cost-sensitive systems
TPP	⭐⭐⭐⭐☆	⭐⭐⭐	⭐⭐	⭐⭐	Flexible interiors, automotive
DOPO-HQ	⭐⭐⭐⭐⭐	⭐⭐⭐⭐	⭐⭐⭐	⭐⭐⭐⭐	High-performance, aesthetic-critical coatings

Rule of thumb: If you need aesthetics, go reactive or low-loading. If you need cost-effective fire protection, inorganic fillers work—but manage expectations on finish quality.

🧪 Final Thoughts (and a Few Lab Jokes)

Formulating flame-retardant polyurethane coatings is like being a chef who has to make a five-star meal… but can only use fire extinguisher powder as seasoning. You can do it, but someone’s probably going to complain about the aftertaste.

The key? Balance. Use synergistic systems (e.g., APP + melamine for char reinforcement), optimize dispersion, and consider reactive flame retardants when appearance matters.

And remember: a coating that passes the burn test but fails the “does it look nice?” test is like a superhero who saves the city but wears socks with sandals. Noble, but awkward.

So next time you see a shiny, fire-safe surface—give a silent nod to the chemists who made beauty and safety hold hands, even when they’d rather fight.

🔖 References

Levchik, S. V., & Weil, E. D. (2006). Thermal decomposition and flame retardancy of polyurethanes—a review of the recent literature. Polymer International, 55(6), 557–563.
Zhou, Y., et al. (2020). Recent advances in flame-retardant coatings based on polyurethane and its composites. Progress in Organic Coatings, 148, 105834.
Wang, H., et al. (2018). Effect of flame retardants on the surface properties of waterborne polyurethane coatings. Journal of Coatings Technology and Research, 15(3), 567–576.
Li, X., et al. (2019). Mechanical and thermal properties of flame-retardant polyurethane coatings containing ammonium polyphosphate. Polymer Degradation and Stability, 167, 1–9.
Zhang, L., et al. (2021). Scratch and abrasion behavior of flame-retardant polymer coatings: Role of filler dispersion and interfacial adhesion. Tribology International, 153, 106582.
Kiliaris, P., & Papaspyrides, C. D. (2010). Polymer/layered silicate nanocomposites: A review on flame retardant additives. Progress in Polymer Science, 35(8), 902–958.

—

Dr. Lin Wei is a materials chemist who once tried to make a fireproof birthday cake. It didn’t end well. (Spoiler: The cat wouldn’t go near it.) 🔥🍰🐱

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