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Red P

The term Red-phosphorus (P-red) is used for describing one of the allotropic forms of Phosphorus, obtained by heating White Phosphorus (P-w) at a temperature close to 300°C in absence of oxygen.

The colour ranges from the orange to the dark violet depending on molecular weight, particle size and impurities. P-red is largely amorphous inorganic polymer, although X-rays have established the existence of several crystalline forms, normally present in a limited extend (<10%w).

It is well known that P-red is active as single additive in nitrogen and/or oxygen containing polymers such as:

ThermoplasticsThermosetsNatural fibers
polycarbonatesMelamine formaldehyde 
ethylene-vinyl acetatepolyisocyanates 

while it has to be applied with spumific and carbonific agents and/or with inorganic hydroxides in polyolefins, styrenics, rubbers, a.s.o.

P-red is the most concentrate source of phosphorus, therefore it is an effective flame retardant additive at a concentration ranging from 2 % to 10 %w based on polymer.

It is generally applied for meeting high demanding flammability requirements, no formation of great amount of toxic smokes, good electrical ( i.e: high CTI value) and mechanical characteristics.Today its application seems only excluded, for color reasons, from white or very light colored final articles, but is widely applied from black to medium gray.

At last, the high thermal stability allows the product to overcome drastic extrusion temperature (up to 320°C) without decomposing, releasing dangerous substances, producing carbonaceous residues, causing corrosion to the extrusion equipments, also through new highly stabilised and protected grades.

Mode of action

The mechanism of P-red as flame proofing agent is still under discussion however the most accepted one is based on the activity of the product in intumescent systems.

Following this mechanism P-red is regarded as an acid source which:

  • Is mainly active in solid phase;
  • Extracts oxygen and/or water from the polymers producing phosphorus acid derivatives which undergo the dehydration at high temperature;
  • Catalyses char formation.
This mechanism rises by the following facts:
  • P-red is especially active as sole additive in oxygen and/or nitrogen containing polymers,
  • it needs co-agents in all oxygen lacking polymers
  • no massive content of phosphorus moieties are generally detected in the smokes during pyrolysis,
  • the LOI index of polymer articles is not very much affected by the presence of P-red.

However it has been also suggested the formation of P radicals occurring during the pyrolysis and combustion of P-red containing polymer articles and it has been proven, by EPR measurements, in nylons.

These radicals are assumed to react either with oxygen, by producing phosphoric structures, or with polymers, by acting as prodegradant, so promoting the dripping.
In addition to the above mentioned mechanisms, showing the product is active in solid phase, it has been also suggested that P-red can operate in gas phase as flame poisoning likely to volatile phosphorus compounds. According to this mechanism, P-red could generate volatile phosphorus moieties (P2, PO, PO2, HPO) which are in position to scavenge H radicals.