Pentaerythritol Phosphate (PEPA), CAS 5301-78-0: Properties, Production and Applications

Pentaerythritol phosphate, almost always shortened to PEPA, is the phosphorus ester of pentaerythritol and the workhorse char former of halogen-free intumescent flame-retardant systems. Its commercial importance comes from one structural fact: a single molecule carries both the acid-generating phosphorus group and part of the carbon skeleton that becomes the protective char. That lets a formulator drop part of the classical ammonium polyphosphate / pentaerythritol / melamine package, cut water solubility, and keep UL-94 V-0 within reach at lower total loading.

This guide covers what PEPA is (CAS 5301-78-0), how it is produced, the specification window we supply (phosphorus 16-17.2%, melting range 195-210 deg C, loss on drying max 0.5%), how it behaves in PP, PE, cable compounds and fire-retardant coatings, and how to compare it with dipentaerythritol, pentaerythritol and ammonium polyphosphate when you are choosing a char former.

What Is Pentaerythritol Phosphate (PEPA)?

PEPA is a phosphate ester built on a pentaerythritol backbone. In practice the commercial product is not a single pure compound: it is a mixture of pentaerythritol mono-, di- and oligo-phosphates whose average composition is controlled by the phosphorus content, typically 16-17.2% P for the flame-retardant grade. That is why buyers should specify PEPA by phosphorus content and melting range rather than by a molecular formula – two suppliers can both sell “PEPA” with different mono/di ratios and quite different behaviour in a compound.

Functionally, PEPA belongs to the “char former” or “carbon source” class of intumescent chemistry, but with a twist: because it also delivers phosphorus, it works as a partial acid source at the same time. In a three-source intumescent system the roles are normally acid source (ammonium polyphosphate), gas source (melamine) and carbon source (pentaerythritol or dipentaerythritol). PEPA blurs the boundary between the first and third roles, which is exactly why it improves char yield per unit of additive.

Chemical Identity: CAS 5301-78-0

Item	Detail
Common name	Pentaerythritol phosphate
Abbreviation	PEPA
CAS Registry Number	5301-78-0
Chemical class	Organophosphate ester of a polyol (phosphate ester of pentaerythritol)
Typical commercial form	Mixture of pentaerythritol phosphate esters, characterised by phosphorus content
Appearance	White powder
Phosphorus content	16 – 17.2% (as P)
Melting range	195 – 210 deg C
Loss on drying	Max 0.5%
Halogen	None – suitable for halogen-free systems

Two specification points deserve attention when you write a purchase order. First, phosphorus content is the real quality lever: below roughly 16% P the material contains more unreacted or lightly esterified polyol, which raises water sensitivity and lowers char efficiency; above about 17.2% P the product tends to become more acidic and more hygroscopic. Second, loss on drying is a handling specification, not a purity claim – phosphate esters hydrolyse, and a powder that arrives at 0.5% moisture and sits in an open bag will lose acid value and char performance before it ever reaches the extruder.

How PEPA Is Produced

The industrial route is an esterification between pentaerythritol and a phosphorus source – phosphoric acid, polyphosphoric acid or phosphorus pentoxide – run until the target phosphorus content and melting range are reached, then cooled, milled and dried. Because the reaction can go to mono-, di- or higher esters, the process is controlled by stoichiometry, temperature programme and reaction time rather than by a single “pure” endpoint.

Three process consequences show up in the delivered product:

Phosphorus content distribution. A tighter P window means a tighter char-forming behaviour, which is why compounders ask for a per-batch COA rather than a generic datasheet.
Acidity and pH of the aqueous extract. Higher esterification degrees leave more free acid; this affects dispersion in water-borne coatings and interaction with basic co-additives such as melamine.
Particle size and caking. Milled PEPA is hygroscopic; the dryer and packaging step matters as much as the reactor for a powder that flows well in a loss-in-weight feeder.

Specifications We Supply

Property	Specification	Why it matters
Appearance	White powder	Indicator of thermal history; discoloured lots often carry more free acid
Phosphorus content (%, as P)	16.0 – 17.2	Drives char yield and the acid-source contribution
Melting point (deg C)	195 – 210	Proxy for the mono/di ester ratio and for the safe compounding window
Loss on drying (%)	Max 0.5	Moisture hydrolyses the ester and degrades char quality
Packaging	25 kg bag, inner PE liner; jumbo bags on request	Moisture barrier for a hygroscopic powder

We do not publish a single molecular formula or molecular weight for this product, because the commercial material is an ester mixture defined by its phosphorus content. Buyers who need a fixed stoichiometric compound should specify it as such at enquiry stage.

Key Properties and Why They Matter

Property	Consequence in use
High phosphorus content (16-17.2%)	Acid catalysis available from the same additive that supplies carbon, so less total additive is needed to reach a given char yield
Low water solubility compared with pentaerythritol	Char performance survives humid ageing, water immersion and water-borne coating formulations
Melting range 195-210 deg C	Below typical PP/PE processing temperatures, so the additive softens and disperses before decomposition begins
White colour	No colour contribution in natural or light-tinted compounds and in white coatings
Powder form	Straightforward to pre-blend, masterbatch or feed by loss-in-weight; requires dust control
Hygroscopicity	Must be dried and kept sealed; the main practical failure mode in storage

How PEPA Works in Intumescent Flame Retardancy

An intumescent system protects by building a char layer faster than the flame can consume the polymer. Four steps happen in sequence, and PEPA participates in the first three:

Acid release. Above roughly 200 deg C the phosphate ester decomposes and releases phosphoric and polyphosphoric acid species.
Dehydration and carbonisation. The acid dehydrates the polyol and the polymer into a carbonaceous network; PEPA’s own pentaerythritol skeleton contributes carbon to that network instead of being lost as volatiles.
Expansion. The gas source – typically melamine – releases nitrogen species that blow the softening char into a cellular foam.
Barrier formation. The expanded char insulates the substrate, restricts oxygen and fuel transfer, and suppresses dripping.

The practical result is a shift in the fire profile rather than a single number: peak heat release rate falls, time to ignition rises, and dripping-related UL-94 failures disappear. Because the char is phosphorus-rich, smoke and acid-gas emission stay far below what a brominated system delivers, which is why PEPA is standard in LSZH cable compounds and in halogen-free transport interiors.

Applications

Halogen-free intumescent compounds for PP, PE and TPO

The classic package is ammonium polyphosphate as the acid source, melamine as the gas source and a polyol as the carbon source. Substituting PEPA for part of the polyol raises char yield and reduces the water sensitivity that limits polyol-based systems in outdoor or humid service. Typical intumescent packages run 20-30 phr in polyolefins, with PEPA contributing roughly 3-10 phr of that total – the exact split depends on the polymer, the APP grade and the target UL-94 rating.

LSZH cable compounds

Cable sheathing and insulation must meet both flame spread and smoke/acid-gas limits. PEPA-based intumescent packages are used where halogenated flame retardants are excluded, often in combination with metal hydroxides at reduced loading, which protects elongation and processability that a high-load hydroxide system would destroy.

Intumescent fire-retardant coatings

In coatings for structural steel, wood and cable trays, PEPA works as the acid/char component alongside ammonium polyphosphate, melamine and a binder. Its low water solubility matters most here: an exterior or water-borne coating that leaches its polyol during weathering stops intumescing when it is finally needed. See the dedicated guide on PEPA in intumescent fire-retardant coatings for formulation detail.

Polyurethane foams

In rigid and flexible foams, phosphorus char formers reduce halogen demand and improve char cohesion. PEPA’s reactivity with isocyanate-bearing systems has to be checked in trials, since phosphate esters can influence the foam’s rising profile.

Coatings, adhesives and metal treatment

Phosphate ester groups are excellent adhesion promoters on steel and aluminium and act as cure catalysts in stoving systems. PEPA therefore also appears outside flame retardancy, in primers, coil coatings and conversion coatings, where the phosphorus content is specified for reactivity rather than for char.

Formulation Guidance

Parameter	Typical starting point	Notes
Total intumescent package	20 – 30 phr in PP/PE	Balance flame retardancy against impact and elongation
PEPA share of the package	3 – 10 phr	Higher share improves char strength but can reduce dispersion
Ammonium polyphosphate	10 – 18 phr	Re-balance downward when PEPA load increases
Melamine (gas source)	3 – 8 phr	Excess melamine lowers char density
Synergists	0.5 – 3 phr zinc borate, talc or zeolite	Used to tune smoke, char strength and drip behaviour
Coupling / dispersion	0.3 – 1 phr maleic anhydride grafted PO or silane	Improves filler wetting and mechanical retention

These are starting points, not a recipe. Intumescent performance is strongly system-specific: the same package that gives V-0 in a 1.6 mm PP specimen can fail in a glass-filled grade or in a thin-wall part, because char expansion depends on melt viscosity and on the specimen geometry. Any change in APP grade, melamine particle size or PEPA phosphorus content should be re-validated.

PEPA vs Other Char Formers

Char former	Phosphorus in molecule	Water solubility	Typical role	Practical comment
PEPA (CAS 5301-78-0)	Yes, 16-17.2%	Low	Acid source plus char former	Best balance of char yield and humidity resistance
Pentaerythritol	No	Moderate	Carbon source only	Cheapest, but blooms and leaches in humid service
Dipentaerythritol	No	Low-moderate	Carbon source	Higher char yield than pentaerythritol; still needs separate acid source
Sorbitol / starch	No	High	Carbon source (bio-based)	Attractive on paper; poor water resistance and thermal stability
Ammonium polyphosphate	Yes, high	Low (coated grades)	Acid source plus some char	Complementary to PEPA, not a substitute

The comparison that matters most in practice is PEPA versus dipentaerythritol versus ammonium polyphosphate, because those three cover most halogen-free formulations in PP and PE. Short version: APP supplies acid, a polyol supplies carbon, and PEPA supplies both – which is why it lets you reduce total loading but rarely lets you remove APP entirely.

Processing Notes

Stage	Practice
Incoming inspection	Check loss on drying, phosphorus content and appearance per batch; retain a sample for the life of the batch
Drying	Dry to below the 0.5% LOD limit before compounding if bags have been open or stored humid
Feeding	Loss-in-weight feeding or pre-blend; avoid pneumatic conveying over long distances where segregation of fine PEPA from coarse APP can occur
Compounding	Standard twin-screw profiles for filled polyolefins; keep melt temperature within the polymer’s normal window and short residence time at peak temperature
Dispersion check	Inspect extrudate for white specks; undispersed PEPA appears as bright points and predicts poor char uniformity
Storage of compound	Keep dry – residual moisture in the compound degrades both flame performance and electrical properties in cable grades

Test Methods and Compliance Evidence

Because PEPA is sold on function rather than on a single assay, buyers should ask for application evidence, not only a COA. The four tests that decide acceptance in most projects are:

UL-94 vertical burn on the final compound at the thinnest wall thickness the part will use – the rating is thickness-specific.
Cone calorimetry (ISO 5660 / ASTM E1354) for peak heat release rate, total heat release and time to ignition; this is where a PEPA substitution usually shows its benefit.
Limiting oxygen index as a fast screen; useful for ranking, weak as an acceptance criterion on its own.
Smoke and acid gas (e.g. smoke density and conductivity tests) for cable and rolling-stock applications, where halogen-free means more than “no bromine”.

Ageing evidence is equally important: a water-immersion or humid-ageing cycle followed by a repeat burn test is the single best way to see the difference between PEPA and a soluble polyol char former.

Safety, Handling and Regulatory Status

PEPA is an industrial powder. The relevant controls are dust-related: local exhaust ventilation at weighing and feeding points, dust masks and eye protection when handling open bags, and earthing where fine organic-inorganic powder is conveyed. Store away from strong bases, which neutralise the acid function, and away from moisture. Thermal hazard is low relative to the decomposition temperature being well above normal processing windows, but overheating in a blocked extruder zone still produces acidic fumes.

Regulatory documentation – SDS in English, batch COA, and country-specific statements where applicable – is provided per order; specific registration or compliance claims are confirmed for your destination market at enquiry stage rather than assumed from a generic datasheet.

Packaging, Storage and Supply

Item	Detail
Standard pack	25 kg net, multi-layer bag with inner PE liner
Bulk pack	500-1000 kg jumbo bag on request
Palletisation	Standard export pallets, shrink-wrapped, 40 bags per pallet typical
Shelf life	Use within the period stated on the COA; reseal opened bags and keep dry
Documents	COA per batch, SDS, packing list, and shipping documents as required

Frequently Asked Questions

Is PEPA the same as pentaerythritol? No. Pentaerythritol is a polyol used as a carbon source; PEPA is its phosphate ester and supplies phosphorus as well as carbon.

Can I run a halogen-free V-0 with PEPA alone? Practically, no. PEPA is normally part of an intumescent package with ammonium polyphosphate and a nitrogen source; it improves char yield and humidity resistance rather than replacing the whole system.

Why does the melting range matter? It is a practical proxy for the ester distribution. A range of 195-210 deg C indicates a consistent product that softens and disperses before decomposition, which helps dispersion in polyolefin compounding.

Does PEPA work in water-borne coatings? Yes – its low water solubility compared with polyol char formers is one of the main reasons it is chosen for water-borne intumescent coatings.

How much should I add? As a starting point, 3-10 phr within a 20-30 phr intumescent package in polyolefins. Always validate by UL-94 and cone calorimetry in your own compound.

Can you supply small samples? Yes, samples with COA are available for formulation trials.