Fire-retardant melamine salts

Mechanism of Thermal Degradation of Fire-Retardant Melamine Salts... 

A thorough study of the mechanism of degradation of several fire retardant melamine salts is carried out using thermogravimetry, evolved gas analysis and spectroscopic characterisation of the products of degradation. 

Costa, L. Camino, G. Luda di Cortemiglia, M.P. Mechanism of thermal degradation of fire-retardant melamine salts, in G.L. Nelson, Ed., Fire and Polymers Hazard Identification and Prevention. ACS Symposium Series, Vol. 425. American Chemical Society, Washington, DC, 1990, pp. 211-238. 

Uses. Dicyandiamide is used as a raw material for the manufacture of several chemicals, such as guanamines, biguanide and guanidine salts, and various resias. Siace 1975, it has also been used ia the manufacture of potassium or sodium dicyanamide which are used as iasecticides and ia chemotherapy. Melamine has extensive appHcations ia the resia and plastic iadustry guanamines are used as copolymers (qv) ia many resia compositions. Guanidine phosphate [1763-07-1] is employed as a fire retardant ia appHcations where water solubiHty is not a drawback. 

The salts can be classified in three main classes salts which undergo thermal dissociation to acid and melamine salts of strong acids which catalyse melamine condensation salts of acids which react with melamine condensation products. Implications of the thermal behaviour of the salts in the mechanism of fire retardance is briefly discussed. 

Melamine and its salts are widely used in formulations of fire retardant additives, particularly of the intumescent type (4-71. The role played by melamine structures in these additives is however not yet understood. The thermal behaviour is of paramount importance in studies of the fire retardance mechanism. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named "melam", "melem", "melon" (8.91. The following schematic reaction is reported in the literature (10-121 ... 

The experimental weight loss in the first and second step (4.0 and 4.5% respectively) is in agreement with that corresponding to condensation to pyrophosphate (4.0%) and polyphosphate (4.2X, n 1). Furthermore, the IR spectra of melamine phosphate and of the residues at 300 and 330 C (Figure 14 spectra A, B and C respectively) show that besides the typical bands of phosphate salts (950-1300 cm-1) which are present in the three spectra, a new absorption due to P-O-P bonds (ca. 890 cm-1) appears in the spectra of the residues. The absorptions due to melamine salt structures (e.g. 780-790 and 1450-1750 cm ) are closely similar in the three spectra of Figure 14. Fire retardants based on melamine pyrophosphate and polyphosphate are reported in the literature 51 as well as methods for preparation of these salts (25-... 

The use of polyols such as pentaerythritol, mannitol, or sorbitol as classical char formers in intumescent formulations for thermoplastics is associated with migration and water solubility problems. Moreover, these additives are often not compatible with the polymeric matrix and the mechanical properties of the formulations are then very poor. Those problems can be solved (at least partially) by the synthesis of additives that concentrate the three intumescent FR elements in one material, as suggested by the pioneering work of Halpern.29 b-MAP (4) (melamine salt of 3,9-dihydroxy-2,4,8,10-tetraoxa-3,9-diphosphaspiro[5,5]-undecane-3,9-dioxide) and Melabis (5) (melamine salt of bis(l-oxo-2,6,7-trioxa-l-phosphabicyclo[2.2.2]octan-4-ylmethanol)phosphate) were synthesized from pentaerythritol (2), melamine (3), and phosphoryl trichloride (1) (Figure 6.4). They were found to be more effective to fire retard PP than standard halogen-antimony FR. 

Because these fire-retardant salts are water-soluble and subject to leaching, several new methods have been developed which provide a water-resistant or permanent treatment for cotton fabrics and cellu-losic materials. Perkin developed a process involving successive treatment with sodium stannate and ammonium sulfate, which precipitates stannic oxide in the cellulose libers. Antimony oxide in combination with vinyl chloride or other chlorinated polymers has also been found effective. A more recent approach to this problem involves application of fire-retardant resin-forming or cross-linking compounds. " These include materials and methods based on the copolymerization of tetrakis(hydroxymethyl)phosphonium chloride and methylol — melamine, the reaction of bromoform and triallyl phosphate to form a cross-linked polymer, and the cross-linking reaction of tris(l-aziridinyl)phosphine oxide with cellulose, or its copolymerization with tetrakis(hydroxymethyl)phosphonium chloride and other materials. ... 

The combustion behavior of melamine pyrophosphate and dimelamine phosphate are different from those of melamine and the other melamine salts (Table 3.4.1). The former are ineffective at low concentrations (> 15%) and become effective at a loading of 20-30% because the intumescent char is formed on the surface of burning specimens. The mechanism of fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP since, by analogy with ammonia melamine volatilizes, whereas the remaining phosphoric acids produce esters with nylon-6, which are precursors of the char [146]. Some part of the freed melamine condenses probably forming melem and melon [147]. 

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