Quick answer: PET burns readily (LOI ~21) and its worst habit is flaming drip, so flame retardant PET film and molding grades rely on three routes: phosphinate salts (DEPAL-type) at 8–15% for engineering PET, brominated systems with antimony synergist where cost-per-part rules, and reactive phosphorus comonomers built into the polyester chain for thin film where additive particles would wreck optics. Thin gauge changes everything — below 100 µm, dispersion quality decides the outcome as much as chemistry does.
PET sits in an odd spot in our RFQ inbox. It is the world’s highest-volume polyester, yet most of it — bottles, fibers — never needs flame retardancy. The PET that does come to us is film and engineering grades: electrical insulation film, adhesive tape backing, photovoltaic backsheet, appliance parts, and lately recycled-PET sheet chasing a UL rating. Each has a different right answer.
Why PET Is Both Easy and Hard to Flame Retard
Q: Is PET flammable? Yes — standard PET ignites and, more dangerously, drips flaming molten polymer that spreads fire downward.
The polyester backbone gives PET a split personality in a fire:
- The easy part: like its cousin PBT, PET responds well to phosphorus chemistry. The ester groups participate in char formation once a phosphorus acid source shows up, so condensed-phase systems work at civilised loadings.
- The hard part: PET’s melt viscosity collapses near the flame, so it drips — and a UL94 vertical test treats flaming drips that ignite the cotton as an instant downgrade. Anti-dripping control matters as much as ignition control.
- The film-specific part: biaxially oriented PET film runs 12–250 µm thick. At that gauge, an additive with 5 µm particles is a defect, not a flame retardant. Particle size, thermal stability at 280–300°C processing, and haze all become gating specs before flame performance is even measured.
The Three Commercial Routes
| Route | Typical loading | Best for | Watch out for |
|---|---|---|---|
| Phosphinate salt (DEPAL-type), halogen-free | 8–15% | Glass-filled engineering PET, connectors, appliance parts targeting V-0; RoHS/REACH-sensitive customers | Cost per kg; needs nitrogen synergist for tougher specs |
| Brominated FR + Sb₂O₃ | 10–18% + 3–5% Sb₂O₃ | Cost-driven molded parts and sheet where halogen is acceptable | Smoke; brominated system must survive 280°C+ PET processing |
| Reactive phosphorus comonomer (in-chain) | Built into resin (~0.5–1% P) | Thin optical film, fiber — no particles, no haze, no blooming | You buy it as a special resin grade, not an additive; limited sourcing |
The chemistry logic here is nearly identical to what we covered for PBT — the two are sibling polyesters and share flame retardant families almost one-for-one, so if your part is a connector or bobbin, read our flame retardant PBT route map alongside this one. The phosphinate route runs through aluminum diethylphosphinate, the same DEPAL family used in PA and PBT; the halogen route through our brominated flame retardant additives line with antimony synergist.
PET Film: Where the Masterbatch Route Earns Its Keep
Q: Can you add flame retardant to PET film on the film line? Yes — but only via a well-dispersed masterbatch, and only if the additive survives orientation.
Film producers almost never dose powder. A BOPET line hates dust, and dispersion demands are brutal: one agglomerate becomes a die line or a breakage during stretching. The workable pattern is a PET-carrier flame retardant masterbatch, let down at the extruder, with three specs agreed up front:
- Particle size: top-cut comfortably below the film gauge — for a 50 µm insulation film we spec d97 under 10 µm.
- Thermal stability: the additive must sit at 280–300°C melt temperature without yellowing or gassing; this alone eliminates several chemistries that work fine in molded PET.
- Moisture discipline: PET hydrolyses at melt temperature. The masterbatch has to arrive dry and stay dry, or IV drop shows up as brittle film long before flame testing.
A case from this spring: a customer laminating PET tape backing for cable wrap needed a flame retardant grade to pair with their LSZH cable program. Their first attempt — a molded-grade brominated compound reground into the film extruder — gave gels and a hazy web within an hour. We rebuilt it as a fine-cut phosphinate masterbatch on a matched-IV PET carrier at 4:1 letdown; the web ran clean and the finished wrap passed their customer’s vertical flame requirement. Wrong format, not wrong chemistry — a distinction that saves a lot of dead trials.
Recycled PET: The Growing Question
rPET sheet and strapping makers increasingly ask for UL-rated recycled content. Two honest cautions from our bench work. First, rPET’s IV is already degraded, so the anti-dripping problem is worse than in virgin resin — budget for PTFE anti-drip from the start. Second, contamination scatter between bales means your flame test results scatter too; we recommend qualifying the FR package against the worst incoming IV you accept, not the average. A chain extender plus phosphinate package is the combination we screen most often for this segment.
Where Rectivas Fits — and Where We Don’t
Straight answer: FR-PET is not one of our standard compound codes the way FR-PP or FR-PBT are. What we supply for PET programs is the flame retardant layer of the stack — phosphinate and brominated additive systems qualified for polyester processing temperatures, and custom masterbatch on PET-compatible carriers built to film-grade dispersion specs. Reactive in-chain comonomer resin is a polymerisation product and comes from resin producers, not from us — if your spec points that way, we will tell you so in the first email. For everything additive-based: send the gauge, processing temperature and the exact flame spec, and request a sample — PET trials start with a dispersion screen on our lab line before anything ships.
FAQ
What UL94 rating can flame retardant PET achieve?
Glass-filled engineering PET with a phosphinate or brominated package routinely reaches V-0 at 1.6 mm. Unfilled PET is harder because of dripping — V-2 comes easily (drips extinguish), but V-0 needs anti-drip control on top of the flame retardant.
Is PET film naturally flame retardant?
No. Standard BOPET film burns and shrinks away from flame, which sometimes looks like self-extinguishing in casual tests but fails formal vertical burn standards. Rated film uses either an FR additive masterbatch or an inherently flame retardant copolyester resin.
Flame retardant PET vs PBT — same additives?
Largely yes: phosphinates, brominated systems and PTFE anti-drip serve both. The practical differences are PET’s higher processing temperature (tighter thermal stability requirement) and its stronger tendency to hydrolyse, which puts stricter drying demands on any masterbatch.
Does flame retardant affect PET clarity?
Particle-based additives always cost some clarity — acceptable in tapes and backsheet, unacceptable in optical film. For genuinely clear FR PET film the industry uses reactive phosphorus copolyesters; for everything else, fine particle grades and dispersion quality keep haze within spec.
—— Rectivas Materials 团队 老陈