Intumescent Phosphorus Systems

Polyphosphoric acids and ammoniimi polyphosphate form a surface glassy layer on heating. The layer is then foamed to increase its thermal insulation, so that the combustible material becomes separated Ifom the heat source by a heat barrier, which also impedes the supply of 

The flame retardant mechanism operates in the solid state. Ammonium polyphosphate decomposes endothermically on heating to 275 °C, liberating polyphosphoric acid and 

Intumescence in a system of this kind occurs during strong heating in live stages  

Phosphorus compoimds have several advantages. Fairly low concentrations are usually sufficient, so there is not much adverse effect on the physical properties. They often have good UV stability and are easy to incorporate in the polymer. They are competitively priced, and products containing them are usually easy to incinerate safely. 

Other disadvantages of phosphorus-based FRs include the health hazards of some of the organic varieties (this affects those involved in processing rather than end users) and their tendency to release toxic combustion products during a fire. Inorganic phosphorus compounds absorb moisture, and can suffer from a lack of permanency. 

---

Intumescent systems based on ammonium polyphosphate can produce flame retardant polypropylene. Great Lakes manufactures Reogard 1000, an intumescent phosphorus and nitrogen based, melt blendable flame retardant for polypropylene homopolymers and low ethylene PP copolymers that need V-0 ratings. It is not particularly hygroscopic and gives good electrical properties with improved heat distortion temperature. 

Nitrogen-containing FRs (without any phosphorus compounds) hold only a small share of the global FR market at present, although their popularity is growing. The main active ingredients are usually melamine cyanurate and melamine hydrobromide, although melamine phosphate is used in intumescent FR systems in combination with pentaerythritol. Melamine based flame retardants employ several modes of flame retardant action. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Ammonium polyphosphates, on the other hand, are relatively water insoluble, nonmelting solids with very high phosphorus contents (up to about 30%). There are several crystalline forms and the commercial products differ in molecular weights, particle sizes, solubilities, and so on. They are also widely used as components of intumescent paints and mastics where they function as the acid catalyst (i.e., by producing phosphoric acid upon decomposition). They are used in paints with pentaerythritol (or with a derivative of pentaerythritol) as the carbonific component and melamine as the spumific compound.22 In addition, the intumescent formulations typically contain resinous binders, pigments, and other fillers. These systems are highly efficient in flame-retarding hydroxy-lated polymers. 

This chapter develops at first the more frequent combinations of nanoparticles that concern layered silicates associated with phosphorus compounds, as well as metallic hydroxides and halogen compounds. The association of natural layered silicates with intumescent FR (IFR) systems represents one of the main contributions of the combined use of nanoparticles and FRs. Moreover, combinations of layered silicates with other phosphorus compounds have been studied and have led to significant improvements for fire retardancy. 

The efficiency of intumescent fire retardants could be enhanced by interlayers that deliver the active components to the surface (shown by two examples). The fire-retardant additives, delivered to the surface at early stage of combustion, accelerate the formation of protecting surface layer that hinders the degradation of the underlying material. This coating structure could be reinforced by an interlayer of ceramizing capability (e.g., polyborosiloxane). Phosphorus-free intumescent fire-retardant system could be formed by using such additive. 

Less than 10% of the polyamide produced is made in a flame retardant version. The best system is composed of a combination of red phosphorus and zinc borate (see table above). The only drawback of this system is its color which is restricted to brick red or black. If other colors are required, ammonium polyphosphate is used either in combination with organic flame retardants or with antimony trioxide. It is possible to manufacture a very wide range of colors in the halogen free system. Some systems make use of the addition of novolac or melamine resins. For intumescent applications, ammonium polyphosphate, in combination with other components, is the most frequently used additive. Figure 13.6 shows that fillers such as calcium carbonate and talc (at certain range of concentrations) improve the effectiveness of ammonium polyphosphate. This is both unusual and important. It is unusual because, in most polymers, the addition of fillers has an opposite influence on the efficiency of ammonium polyphosphate and it is important because ammonium polyphosphate must be used in large concentrations (minimum 20%, typical 30%) in order to perform as a flame retardant. 

Dimelamine phosphate CAS 56974-60-8 Empihcal C6H15N12O4P Properties Wh. powd. sol. 2 g/l in water m.w. 350.24 dens. 1.66 kg/l dec. 300 C Uses Flame retardant for polymers, intumescent paints/coatings, plastics (polyesters, polymethyl methacrylate, polyolefins, PS, PU foams), textiles, paper intumescent paint/mastic ingred. catalyst in intumescent systems Manuf/Distrib. DSM Chem. N. Am. http //www. dsmna. com Trade Name Synonyms Amgard ND t[Rhodia/Phosphorus Perf. Derivs. 

Intumescent flame retardants (IFR) that contains phosphorus are also used in halogen-free flame-retardant systems. Most reported IFRs are mixtures of the three ingredients, an acid source, a polyol, and a nitrogen-containing compound (Halpem et al. 1984). Since processing of ABS resin requires that the additives withstand temperatures in excess of 200 °C, the commonly used intumescent system, ammonium polyphosphate, pentaerythritol, and melamine, which do not have sufficient thermal stability, cannot be incorporated into ABS resin under normal processing conditions they are usually used in polyolefins. 

Cosynergists may also be used. Addition of synergetic products such as pentaer-ythritol derivatives, carbohydrates, and spumific agents will significantly improve the flame-retardant performance of APP. Red phosphorus and APP are used in intumescent coatings and paints suitable for materials such as wood and steel as well as polymer systems. 

Materials/characteristic.s Some inorganic fillers. Nitrogen-donors melamine compounds. Antimony compounds with halogen donors. Halogenated containing chlorine, bromine. Halogen-free systems aluminium trihydroxide (ATH), magnesium hydroxide, zinc borate. Intumescent systems phosphorus compounds. 

Intumescent systems are a particular class of phosphorus-based flame retardant. They are characterized by the production of an expanded foamed char on exposure to heat which then protects the underlying polymer. Ideally the char volume should be large, with a thick continuous surface crust and an inner structure which should be similar to a closed-celled foam. The char structure is important in providing a good insula-tive effect and hence reducing thermal decomposition of the polymer to volatile flammable products. 

---