Which flame retardant additive should a protective case use?

Answer: Follow the market: EU electronics, rail and marine buyers need halogen-free at 28%–30%, while industrial storage in Asia suits brominated material at 20%–22% with a wider process window.

Does halogen-free always need 30% loading?

Answer: Roughly, for phosphorus: 28% is the floor and 30% is stable. Magnesium hydroxide needs over 55% and aluminium hydroxide over 60%, so loading alone cannot be compared without density and strength loss.

Is halogen banned by RoHS?

Answer: No. RoHS restricts polybrominated biphenyls and diphenyl ethers at 1000 ppm each; other brominated additives stay legal, but tenders apply halogen-free limits.

How much harder is 5VA than V-0?

Answer: Both specimen and flame change. V-0 uses a 13 mm bar and a 50 W flame; 5VA uses a 150 mm plaque with 500 W applied five times and no burn-through.

Why does halogen-free need twice the drying time?

Answer: Ammonium polyphosphate and hydroxides pick up 2–3 times more moisture. After 2 h at 80 °C moisture is still 0.15%; 4 h brings it to 0.05%, and short drying leaves silver streaks.

How much weight does the additive add?

Answer: At equal volume a brominated case gains 1.1 kg and a halogen-free one 1.3 kg, up 35%–42% on neat resin. By air at 12 CNY/kg that is 13–16 CNY per case; by sea it barely registers.

Flame Retardant Additive: Halogen vs Halogen-Free

Quick answer: Flame retardant additive choice starts with loading: a halogen-free system needs about 8 points more than brominated material to reach V-0.

Two flame retardant additive systems
Same top rating, 8 points more additive
What the extra loading costs in strength
V-0 versus 5VA: how the tests differ
Smoke density and toxic gas numbers
Cost split and regulatory gates
FAQ

1. Two flame retardant additive systems

Halogenated additives work in the gas phase: they release HBr or HCl, which scavenges the H and OH radicals that carry the flame, and bromine needs antimony trioxide at a 3:1 mass ratio to be carried into the flame zone.

Halogen-free work two ways. Metal hydroxides decompose endothermically, aluminium hydroxide giving off water at 180–200 °C and absorbing about 1.0 kJ/g; phosphorus systems build char in the condensed phase, ammonium polyphosphate crosslinking at 250 °C into a 1–2 mm layer.

Decomposition must sit above processing temperature: PP runs at a 200–230 °C barrel and cannot use aluminium hydroxide, which already releases water in the screw; ABS at 220–240 °C accepts both.

2. Same top rating, 8 points more additive

On one homo-polymer PP and one mould, V-0 takes 20%–22% of a brominated system, 16% brominated polystyrene plus 5% antimony trioxide, against 28%–30% of the phosphorus route, 22% ammonium polyphosphate plus 6% char former. That is 8 points, or 8 kg per 100 kg.

Flame retardant additive trial lot: black cases stacked by the inspection station, worker in pale blue shirt steadying the top one

The hydroxide route needs 55%–62% magnesium hydroxide for V-0, doubling melt viscosity and dropping a threaded boss from 900 N to 480 N.

More is not safer: above 32% the char layer turns brittle, cracks on the second application and pushes afterflame from 4 s to 9 s.

System	Active ingredient	Loading	Synergist or char	Decomposition
Bromine with antimony	Brominated polystyrene	20%–22%	Antimony trioxide 5%	Above 300 °C
Intumescent phosphorus	Ammonium polyphosphate	28%–30%	Char former 6%	250 °C
Magnesium hydroxide	MDH	55%–62%	Silane 0.8%	330 °C
Aluminium hydroxide	ATH	60%–65%	Silane 0.8%	180–200 °C

3. What the extra loading costs in strength

On a 3.0 mm lid, neat resin tests 28 MPa; the brominated compound lands at 22 MPa, 21% down, and the phosphorus one at 19 MPa, 32% down. Notched impact falls from 9 kJ/m² to 5.5 kJ/m² and elongation from 120% to 45%.

Density is the second loss: 0.91 g/cm³ neat against 1.22 and 1.28, so a 12 L case goes from 3.1 kg to 4.2 and 4.4 kg, or 13 CNY more by air at 12 CNY/kg.

Holding the drop-test result needs 0.2 mm more wall, another 0.18 kg, so flame retardant additive choices have to be costed with the structure attached.

4. V-0 versus 5VA: how the tests differ

V-0 uses a 125 × 13 × 3.2 mm bar, a 20 mm flame applied twice for 10 s, afterflame under 10 s each and 50 s across five specimens, with no drip igniting cotton 300 mm below.

5VA uses a 150 × 150 mm plaque, a 500 W flame 125 mm high applied five times for 5 s, afterflame under 60 s and no burn-through; 5VB allows burn-through. A bar tests one edge, a plaque a whole face.

Flame retardant additive compound feeding a blue injection press, white material sacks beside it, black moulded shells stacked on the yellow floor

A case wall is a whole face, so compounds passing V-0 on bars often sit at 5VB on a plaque.

Item	V-0	5VA	5VB
Flame power	50 W, 20 mm	500 W, 125 mm	500 W, 125 mm
Afterflame	10 s each, 50 s total	60 s	60 s
Burn-through	Not applicable	Not allowed	Allowed

5. Smoke density and toxic gas numbers

Smoke density follows ASTM E662 as Ds at 4 min: 300–450 for brominated systems with antimony, 150–250 for phosphorus, under 120 for hydroxides, whose water vapour carries particles away.

Toxic gas reverses the ranking. A brominated compound releases about 180 mg/g of HBr at 700 °C while halogen-free stays under 10 mg/g, but carbon monoxide climbs to 0.08 g/g from 0.05 g/g, a 60% rise from less complete combustion.

Enclosed cabins and rail cars weigh smoke and acid gas first; open storage weighs heat release, where the gap narrows. Which flame retardant additive fits depends on where the case burns.

6. Cost split and regulatory gates

At 2026 prices, with PP at 8 CNY/kg, brominated polystyrene 40, antimony trioxide 80, ammonium polyphosphate 13 and char former 20, the brominated compound costs 19.4 CNY/kg and the phosphorus one 12.6.

Per case the gap shrinks: 4.2 kg at 1.22 g/cm³ is 81 CNY, 4.4 kg at 1.28 g/cm³ is 55 CNY. Add 0.18 kg of wall, drying from 2 h to 4 h and scrap of 3.0% against 1.5%, and both land within 1–2 CNY.

Item	Brominated	Halogen-free	Gap
Compound price	19.4 CNY/kg	12.6 CNY/kg	6.8 lower
Density	1.22 g/cm³	1.28 g/cm³	5% higher
Case weight	4.2 kg	4.4 kg	0.2 kg heavier
Material per case	81 CNY	55 CNY	26 lower
Wall and drying cost	0	9 CNY	9 higher
Scrap rate	1.5%	3.0%	1.5 points

The real gap sits at the market gate: a brominated grade loses the EU electronics order outright.

RoHS bans only polybrominated biphenyls and diphenyl ethers at 1000 ppm each, not halogen as a class. REACH is sharper: short-chain chlorinated paraffins and hexabromocyclododecane sit on the SVHC list, with notification above 0.1%.

Halogen-free in a tender usually means IEC 61249-2-21: chlorine under 900 ppm, bromine under 900 ppm and total halogen under 1500 ppm. Without those three numbers a supplier ships low-halogen material and still passes the paperwork check.

Rail and marine add EN 45545-2 R22, where brominated systems rarely reach HL3, so the decision runs market first, rating second, price last.

Conclusion

Order the decision as market, rating, then cost. EU electronics and rail take halogen-free, with chlorine and bromine each under 900 ppm and total halogen under 1500 ppm; industrial storage can stay brominated at 20%–22% and still hold the 10 s afterflame line. Set the rating by the burning surface next: a case wall is a whole face, so 5VA fits better than a bar result. Only then compare price, where 19.4 CNY/kg against 12.6 CNY/kg narrows to 1–2 CNY per case once density, wall and scrap are counted. The difference that decides the programme is whether the order can be placed at all. Order it wrong and the programme pays twice: for extra loading first, then for a tool that has to be re-cut.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.