Quick answer: Glass fibre makes a plastic harder to flame retard because of the candlewick effect — the fibres wick molten polymer toward the flame and hold it there, exactly like the wick in a candle. Mechanisms that rely on the polymer dripping away (melamine cyanurate in unfilled nylon is the classic) stop working entirely. What does work is condensed-phase char chemistry: aluminium phosphinate paired with melamine polyphosphate, typically 15–20% total in glass-filled PA66 and PBT. Expect to raise the loading as the glass content rises, not lower it.
Every connector engineer eventually runs into this. A flame retardant package that gives clean V-0 in unfilled nylon is dropped into a 30% glass-filled grade for a part that needs stiffness, and the burn test falls apart. The instinct is to add more. That instinct is wrong roughly as often as it is right, because the problem is usually not how much flame retardant is present but which mechanism it uses.
The Candlewick Effect
Q: Why does glass fibre make plastics burn worse? Because the fibres form a wick that draws molten polymer up to the flame and holds it in the burning zone.
Picture a candle. Wax alone is difficult to ignite — hold a match to a block of paraffin and very little happens. Add a wick and the same wax burns steadily for hours, because the wick draws molten fuel by capillary action into the flame, where it vaporises and burns.
A glass-fibre-reinforced thermoplastic contains millions of tiny wicks. When the surface melts, capillary forces along the fibres pull molten polymer toward the heat instead of letting it fall away. Two consequences follow, and both are bad for a UL94 result:
- Fuel keeps arriving at the flame. Combustion is sustained rather than starved, so afterflame times stretch out past the limits.
- The polymer cannot escape. In unfilled resin, dripping physically removes burning material from the specimen — unhelpful for V-0, which forbids flaming drips, but genuinely effective for V-2. Glass fibre removes that escape route entirely.
There is a third, subtler effect: the fibres conduct heat into the specimen and the char that does form is riddled with fibre ends, making it mechanically weaker and less continuous as a barrier.
Why Melamine Cyanurate Fails Here
This is the most instructive case in the whole topic. Melamine cyanurate gives excellent, cheap V-0 in unfilled PA6 and PA66 at 8–15%. Its mechanism is partly gas dilution and partly a deliberate reduction in melt viscosity that makes the burning polymer drip away from the flame front — it uses dripping as a tool.
Add 30% glass fibre and that tool is gone. The polymer cannot drip because the fibres hold it, so MC’s contribution collapses to the gas-dilution fraction alone, which is not enough. The additive did not become weaker; the escape route it depended on was closed. This is why MC is described across our site as an unfilled-nylon solution that gets demoted to a supporting role once fibres enter — the boundary is a mechanism boundary, not a quality one.
What Actually Works: Condensed-Phase Char
Q: Which flame retardant works in glass-filled nylon? Aluminium phosphinate with a melamine polyphosphate synergist, at roughly 2:1 and 15–20% total.
If the polymer cannot escape the flame, the answer is to protect it where it stands. Condensed-phase systems build a phosphate-rich char layer over the surface, insulating the polymer underneath and cutting off fuel release — a barrier strategy rather than an escape strategy. The industry-standard pairing:
- Aluminium diethylphosphinate (DEPAL) supplies efficient phosphorus, acting in both the condensed phase and, to a degree, the gas phase.
- Melamine polyphosphate (MPP) adds nitrogen gas dilution plus additional phosphate char, and survives the 280–300 °C processing that glass-filled PA66 demands.
Together they reach V-0 at wall thicknesses down to 0.4–0.8 mm in glass-filled polyamide, while holding the glow-wire and tracking requirements that electrical parts stack on top of the flame rating. The underlying mechanism logic — why condensed-phase and gas-phase systems suit different problems — is in our article on flame retardant mechanisms.
Loading Scales With Glass Content
| Glass content | Wicking severity | Typical FR approach |
|---|---|---|
| Unfilled | None | MC at 8–15% is the economical answer |
| 15% GF | Moderate | Phosphinate-based package; MC alone no longer reliable |
| 30% GF (most common) | High | Phosphinate + MPP, ~2:1, 15–20% total |
| 50% GF | Severe | Upper end of loading; every percent competes with fibre for volume |
The 50% row contains a squeeze worth understanding. At half glass and a fifth flame retardant, roughly 70% of your compound is not polymer — and the remaining 30% has to wet out the fibres, carry the additive and still flow into a thin-walled part. This is where formulations stop being a matter of choosing chemistry and start being a matter of what physically fits, and where a real trade against mechanical performance usually has to be accepted.
The Other Problems Glass Brings
- Surface fibre and appearance. High additive loadings on top of high fibre loadings worsen fibre read-through on the surface. For visible parts this is a genuine constraint, sometimes solved by a lower-fibre skin in a two-shot part rather than by formulation.
- Thin-wall flow. Fibres and additives both raise viscosity while thin walls demand flow. Expect to need a higher-flow base grade than the unfilled equivalent, and expect gate and runner design to matter more.
- Tracking and glow wire. Electrical parts rarely need only UL94. CTI and GWIT/GWFI stack on top, and some flame retardant chemistries help one while hurting another — which is why zinc borate turns up at 1–5 phr in these formulations as an afterglow and tracking fixer. The synergist family is mapped in our guide to flame retardant synergists.
- Anisotropy in the burn test. Fibre orientation varies with flow direction, so specimens cut or moulded differently can genuinely give different results. Test bars moulded the way the part is moulded, not in whatever orientation is convenient.
Practical Sequence for a Glass-Filled Project
- Fix the glass content first — it is driven by mechanical requirements and it sets the flame retardant problem. Deciding it late means redoing the flame work.
- Choose a condensed-phase package, not a drip-dependent one. This single decision resolves most failures we are asked to diagnose.
- Qualify at the real wall thickness, in the real moulding orientation. Glass-filled results transfer between thicknesses even less reliably than unfilled ones.
- Test the whole stack together — UL94, glow wire, CTI, plus mechanical retention. Optimising the flame rating alone frequently breaks something else in the list.
The resin-specific routes are covered on our flame retardant nylon page and in our articles on flame retardant PBT and flame retardant nylon 66. For a package sized to your specific grade, send the resin, glass content, wall thickness and full test stack — we ladder the phosphinate-to-MPP ratio on your actual material and burn-test it before quoting a number.
FAQ
What is the candlewick effect in flame retardant plastics?
The phenomenon where glass or other reinforcing fibres draw molten polymer toward a flame by capillary action, sustaining combustion in the same way a candle wick feeds wax to a flame. It is the main reason reinforced grades are harder to flame retard than unfilled ones.
Why does my flame retardant work in unfilled nylon but not glass-filled?
Almost certainly because the package depends on dripping to remove burning material, which fibres prevent. Melamine cyanurate is the usual culprit. Switch to a condensed-phase char system — a phosphinate with an MPP synergist — rather than increasing the existing additive.
Does more glass fibre always mean more flame retardant?
As a rule yes, over the practical range: wicking severity rises with fibre content, so loadings that pass at 15% glass often fail at 30%. The complication at very high fibre levels is that there is progressively less polymer available to carry the additive and fill the part.
Can mineral fillers replace glass fibre to make flame retardancy easier?
Particulate mineral fillers do not wick the way fibres do, so they avoid the candlewick problem — and mineral hydroxides contribute flame retardancy themselves. But they do not deliver fibre’s stiffness and strength. If the part’s mechanical requirements permit mineral filling, it is a genuinely easier flame retardant problem; if it needs fibre stiffness, the wicking has to be engineered around instead.
Worth noting where reinforced parts actually end up: overwhelmingly in electrical service — connectors, switches, breaker bodies. That brings requirements a burn rating never covers, which is the subject of our flame retardant for electronics page.
Final Thoughts
Glass fibre changes the problem rather than adding to it: the wicking effect feeds melt to the flame and it disables dripping-based chemistry entirely. Expect higher loading than the unfilled datasheet suggests, and do not raise MCA dosage hoping it will work. Phosphinate with MPP is the standard answer for reinforced grades – send your glass content and we will start from there.
—— Rectivas Materials 团队 老陈