Three names dominate halogen-free flame retardancy in polyolefins: ammonium polyphosphate (APP), dipentaerythritol (DPE) and pentaerythritol phosphate (PEPA, CAS 5301-78-0). They are usually presented as alternatives, but they are not the same kind of additive, and picking between them on price per kilogram is the most common formulation mistake in intumescent work.
This guide gives a decision framework: what each material actually does in the three-source system, where each one wins, what happens to water resistance and ageing, and how to re-balance a package when you swap one for another.
What each additive actually does
| Additive | Primary function | Supplies phosphorus | Supplies carbon | Supplies gas |
|---|---|---|---|---|
| Ammonium polyphosphate (APP) | Acid source | Yes, high | No | Ammonia on decomposition |
| Dipentaerythritol (DPE) | Carbon source | No | Yes | No |
| PEPA | Carbon source plus acid source | Yes, 16-17.2% | Yes | No |
| Melamine | Gas source | No | Some | Yes, nitrogen |
So the real question is not “which is better” but “how much acid and how much carbon do I need, and which raw material delivers it most robustly”. APP and PEPA overlap on acidity; DPE and PEPA overlap on carbon. Melamine is orthogonal to all three.
Water resistance: the property that decides long-term performance
Polyol char formers are water-soluble to a degree that surprises formulators. Pentaerythritol dissolves at around 6 g per 100 mL at room temperature; dipentaerythritol is less soluble but still extractable; a phosphate ester such as PEPA is far less soluble again. In a dry interior application this barely matters. In humid, wet or exterior service it decides whether the part still passes a burn test after two years.
| Char former | Water solubility | Behaviour after humid ageing |
|---|---|---|
| Pentaerythritol | High | Blooms to the surface as a white film and is extracted; char yield drops sharply |
| Dipentaerythritol | Moderate | Better than pentaerythritol; still migrates in water immersion |
| PEPA | Low | Char performance largely retained; the powder’s own hygroscopicity is the handling issue, not leaching |
| Coated APP | Low (surface treated) | Depends entirely on coating quality – uncoated APP hydrolyses |
Char yield and loading efficiency
Char yield per unit of additive is what determines total loading, and total loading is what determines whether your compound still passes impact and elongation. Because PEPA carries phosphorus in the same particle as its carbon skeleton, the dehydration and the carbonisation happen in close proximity, which typically raises the mass of char retained at a given total additive level.
| Package style | Typical total loading in PP | Practical consequence |
|---|---|---|
| APP + pentaerythritol + melamine (classic) | 25 – 30 phr | Cheapest on raw material; worst humidity resistance; high bloom risk |
| APP + DPE + melamine | 22 – 28 phr | Better char than pentaerythritol; still needs high total loading |
| APP + PEPA + melamine | 20 – 26 phr | Lower total loading for the same rating, better aged performance |
Thermal behaviour and processing
All three additives constrain processing, but for different reasons. APP starts to release ammonia and water from roughly 250 deg C upward, so drying and compounding temperatures must stay below its decomposition onset – moisture in the feed accelerates that decomposition. DPE melts well below polyolefin processing temperatures and, if over-dried and overheated, can volatilise or react prematurely with the acid source in the extruder, causing screw slip and black specks. PEPA melts at 195-210 deg C, which sits inside but below the typical PP/PE window, so it disperses readily; its practical enemy is moisture, which hydrolyses the phosphate ester.
| Risk | APP | DPE | PEPA |
|---|---|---|---|
| Moisture sensitivity | High (hydrolysis of the polyphosphate) | Low-moderate | Moderate (phosphate ester hydrolysis) |
| Premature reaction in extruder | Possible above 250 deg C | Possible if APP and DPE are both over-dried and hot | Low below the melting range |
| Feeding and dust | Fine coated powder; dust control needed | Granular; flows well | Fine white powder; hygroscopic, caking risk |
| Dispersion | Needs good wetting, otherwise white specks | Melts and disperses easily | Softens inside the processing window; good dispersion |
How to re-balance a package when you switch
- Write down the current phosphorus-to-carbon ratio. Calculate grams of phosphorus delivered by APP plus any phosphorus in PEPA, and grams of carbon available from the polyol.
- Substitute on function, not on weight. Replacing 5 phr of DPE with 5 phr of PEPA adds phosphorus; reduce APP accordingly, typically by 1-3 phr for a 5 phr PEPA addition.
- Keep melamine ratio first. Set the gas source against total intumescent solids, then adjust acid and carbon.
- Re-test at the thinnest wall. A V-0 at 3.2 mm tells you nothing about a 1.6 mm part.
- Add an ageing step. Water immersion for 7 days or 70 deg C / 90% RH for 14 days, then repeat the burn test: this is where the substitution earns or loses its case.
Cost-in-use, not cost per kilogram
PEPA is normally priced above pentaerythritol and above DPE. Used at 3-10 phr inside a 20-26 phr package it can still be the cheaper option in use, because it allows a lower total loading, reduces the APP share, and avoids the reformulation work that a bloom failure forces on you. The comparison that matters is cost per kilogram of finished compound that passes the required rating after ageing – not the invoice price of the additive.
Decision table
| If your priority is… | Choose | Why |
|---|---|---|
| Lowest raw material cost, dry indoor use | Pentaerythritol + APP + melamine | Cheapest package; humidity tolerance not needed |
| Humid, wet or exterior service | PEPA + APP + melamine | Low solubility keeps the carbon source in the part |
| Maximum char strength at moderate loading | DPE with a PEPA partial substitution | Carbon-rich DPE plus phosphorus from PEPA |
| Thin-wall parts, tight mechanical limits | PEPA-based package at minimised total loading | Lower total filler for the same rating |
| Water-borne coating | PEPA as the carbon source | Soluble polyols extract into the aqueous phase |
Frequently asked questions
Is PEPA a drop-in replacement for APP? No. APP is primarily an acid source; PEPA is a char former that also supplies acid. Replacing APP with PEPA is possible only partially, and the gas source and carbon balance must be reworked.
Which is more effective, DPE or PEPA? Per unit of carbon, DPE is at least as good a char former. Per unit of total package, PEPA usually wins because it adds acid function and survives ageing.
Does PEPA work with coated APP? Yes, and that combination is the most common LSZH and PP intumescent package in use.
What specification should I buy on? Phosphorus content 16-17.2% and loss on drying max 0.5%, with a per-batch COA. Phosphorus content is the real quality lever for PEPA.
Contact PolyBlueChem for PEPA samples, COA and formulation guidance.