Sergei V. Levchik

Thermal decomposition of aliphatic nylons

An overview of the literature together with selected authors data on thermal and thermo-oxidative decomposition of commercial aliphatic nylons (nylon 6, nylon 7, nylon 11, nylon 12, nylon 6.6, nylon 6.10, nylon 6.12) is presented. Despite the high level of research activity and the large number of publications in the field, there is no generally accepted mechanism for the thermal decomposition of aliphatic nylons. Polylactams (nylon 6, nylon 11 and nylon 12) tend to re-equilibrate to monomeric or oligomeric cyclic products. Diacid–diamine type nylons (nylon 6.6, nylon 6.10 and nylon 6.12) produce mostly linear or cyclic oligomeric fragments and monomeric units. Because of the tendency of adipic acid to fragment with elimination of CO and H2O and to undergo cyclization, significant amounts of secondary products from nylon 6.6 are reported in some papers. n nxa0Many authors have shown that the primary polyamide chain scission occurs either at the peptide C(O)NH or at adjacent bonds, most probably at the alkyl–amide NHCH2 bond which is relatively the weakest in the aliphatic chain. Hydrolysis, homolytic scission, intramolecular CH transfer and cis-elimination (a particular case of CH transfer) are all suggested as possible primary chain-scission mechanisms. There are no convincing results reported which tend to generally support one of these mechanisms relative to the others; rather, it seems that the contribution of each mechanism depends on experimental conditions. This conclusion is also supported by the wide spread of kinetic parameters measured under the different experimental conditions. n nxa0More uniform results are observed in the literature regarding the mechanism of thermo-oxidative decomposition of aliphatic nylons. Most authors agree that oxygen first attacks the N-vicinal methylene group, which is followed by the scission of alkyl–amide NC or vicinal CC bond. Alternatively, it is suggested that any methylene group which is β-positioned to the amide group methylene can be initially oxidized. There are few mechanisms in the literature which explain discoloration (yellowing) of nylons. UV/visible active chromophores are attributed either to pyrrole type structures, to conjugated acylamides or to conjugated azomethines. n nxa0Some secondary reactions occurring during the thermal or thermo-oxidative decomposition lead to crosslinking of nylons. Nylon 6.6 crosslinks relatively easily, especially in the presence of air, whereas nylon 11 and nylon 12 crosslink very little. Strong mineral acids, strong bases, and some oxides or salts of transition metals catalyse the thermal decomposition of nylons, but minimize crosslinking. In contrast, many fire retardant additives promote secondary reactions, crosslinking and charring of aliphatic nylons. n n© 1999 Society of Chemical Industry

Combustion and fire retardancy of aliphatic nylons

The overview of the literature for a 25-year period on the combustion and fire-retardant performance of major commercial aliphatic nylons is presented. It is shown that aliphatic nylons are relatively easily ignitable materials which support combustion and are therefore required to be fire retarded for some applications. Nylons do not produce vision-obscuring smoke during combustion, and the toxicity of their combustion products is similar to or lower than that of many other man-made materials. Patent literature and technical publications on the fire retardancy of aliphatic nylons with halogenated products, phosphorus-containing compounds, nitrogen-rich compounds, sulfur-containing, boron-containing and silicon-containing products are discussed. Miscellaneous inorganic additives and char-forming organic products are also discussed. It is concluded that, in spite of significant attempts, no commercial solution of fire retardancy of aliphatic nylons without loss of mechanical properties is available.

A Review of Current Flame Retardant Systems for Epoxy Resins

The review covers materials currently available or which appear to be in serious development, with emphasis on electrical laminates and encapsulation, and brief coverage of other applications. The dominant technology for FR-4 printed wiring boards uses tetrabromobisphenol-A reacted into the epoxy resin. Nonhalogen systems include additives such as alumina trihydrate, alumina trihydrate plus red phosphorus, and aromatic phosphates. Reactives include a dihydrooxaphosphaphenanthrene oxide and various adducts thereof, and hydroxyl-terminated oligomeric phosphorus-containing esters. A further approach is the modification of the epoxy resin by placement of aromatic groups between the glycidyloxyphenyl groups, and/or by use of a triazine-modified novolac as crosslinker. Flame retardant epoxy coatings continue to make use of ammonium polyphosphate plus char-forming additives.

Current Practice and Recent Commercial Developments in Flame Retardancy of Polyamides

This review presents the currently used or promising flame retardant systems for the aliphatic polyamides. The largest applications are in electrical parts, with smaller usage in automotive and textiles. A polycyclic chlorohydrocarbon, DECHLORANE PLUS, is employed in low smoke formulations. Decabromodiphenyl ether has major use in polyamide 6. Alternative brominated additives include decabromodiphenylethane, polybrominated phenylindane, polymeric dibromophenylene oxide, polybrominated polystyrene and oligomeric glycidyl ethers of tetrabromobisphenol A, and polypentabromobenzyl acrylate. In non-reinforced polyamide 6, melamine cyanurate is effective. With glass reinforcements, some melamine pyro-or polyphosphates are useful. Polyamide 6 can be flame retarded with high loadings of magnesium hydroxide. Stabilized and coated red phosphorus is used in Europe and the Far East. A recent development is the use of aluminum dialkylphosphinate. In textile fiber, there has been some development of a built-in phosphinate. Polyamide fabric can be flame retarded with a thioureaformaldehyde resin finish.

Mechanistic study of combustion performance and thermal decomposition behaviour of nylon 6 with added halogen-free fire retardants

Mechanistic studies in nylon 6 with added halogen-free fire retardants, e.g. ammonium polyphosphate (APP), ammonium pentaborate (APB), red phosphorus, potassium nitrate or melamine and its salts, were carried out using combustion and thermal decomposition approaches. It was shown that APP interacts with nylon 6 producing alkylpolyphosphoric ester which is a precursor of the intumescent char. On the surface of burning polymer, APB forms an inorganic glass protecting the char from oxidation and hindering the diffusion of combustible gases. Red phosphorus reacts with decomposing nylon 6, probably through radical mechanisms producing phosphorus ester species. The condensed phase fire retardant action is proved for red phosphorus. Potassium nitrate, which is a strong oxidizer, reacts with the polymer in condensed phase increasing the char yield. Melamine and its salts mostly provoke flow of nylon 6 leading to extinction of the flame. Furthermore, they induce scission of H 2 C-C(O) bonds in nylon 6, which leads to increased cross-linking and charring of the polymer.

Phosphorus-nitrogen containing fire retardants for poly(butylene terephthalate)

Hexakis(phenylamino)cyclotriphosphazene, hexakis(phenoxy)cyclotriphosphazene, tris(o-phenylenediamino)cyclotriphosphazene, tris(phenylene-1,2-dioxy)cyclotriphosphazene and tris (phenylene-1-amino-2-oxy)cyclotriphosphazene have been prepared and characterized by IR and NMR spectroscopy. Phospham, a crosslinked phosphazene imide (PN 2 H) n , was prepared by heating hexaminotricyclophosphazene under vacuum. Phosphorus oxynitride (PON) m which is likely to be a crosslinked oxyphosphazene was prepared by intense heating of urea, melamine and phosphoric acid. These phosphorus-nitrogen containing compounds added to poly(butylene terephthalate) (PBT) at 10-20wt% provided increase of oxygen index (OI) from 22 to 29. In spite of relatively high OI only the V-2 rating was observed in the UL94 test because of the flaming drip phenomenon. Phosphorus oxynitride was found to be an efficient char promoter for PBT.

Fire-retardant action of resorcinol bis(diphenyl phosphate) in PC–ABS blend. II. Reactions in the condensed phase

The thermal decomposition of polycarbonate (PC), PC containing resorcinol bis(diphenyl phosphate) (RDP), and PC—acrylonitrile–butadiene–styrene (PC–ABS) blend containing RDP was studied by thermogravimetry. Volatile and solid products of thermal decomposition were collected at different steps of thermal decomposition and characterized either by gas chromatography–mass spectrometry or infrared and chemical analysis. It was found that phosphorus accumulates in the condensed phase. Upon combustion of the fire-retardant mixture PC–ABS + RDP, accumulation of phosphorus is observed in the charred layer, at the surface of the burning specimens. It is suggested that PC undergoes a Fries-type rearrangement upon thermal decomposition, and RDP reacts with the formed phenolic groups through a trans-esterification mechanism. Kinetic analysis of the thermal decomposition of PC containing RDP supports the proposed mechanism.

Commercial Flame Retardancy of Polyurethanes

The review covers those flame retardants (combustion modifiers) which are in commercial use or which have had active development leading to potential commercial use in polyurethanes and isocyanurates, with emphasis on foams but brief coverage of elastomers and coatings. The review also covers factors such as polyol choice, catalyst, surfactant, and blowing agent which impact on performance of flame-retardant polyurethanes and isocyanurates. Performance factors such as scorch and fogging are discussed in relation to the choice of flame retardant for foam.

Mechanistic study of thermal behaviour and combustion performance of carbon fibre-epoxy resin composites fire retarded with a phosphorus-based curing system

Abstract The fire retardant action in epoxy resins based on tetraglycidyl 4,4-diaminodiphenylmethane (TGDDM) either alone or combined with the diglycidylether of bisphenol A (DGEBA), cured with bis(m-aminophenyl)methylphosphine oxide (BAMPO) was studied. Comparison of oxygen index (OI) and nitrous oxide index suggests a condensed phase fire retardant mechanism. The yield of the intumescent char continuously increases providing polymer shielding upon increasing the concentration of the fire retardants. In the composite the carbon fibres play the role of inert fillers in combustion in the oxygen index test with, however, fibre orientation having an important effect. The carbon fibres suppress the intumescence behaviour of the char. The chemical mechanism of thermal degradation and charring of the resins is studied using thermogravimetry, thermal volatilisation analysis, char characterisation by FTIR, solid state NMR, X-ray, EPR and elemental analysis.

Synergistic action between aryl phosphates and phenolic resin in PBT

Abstract Fire retardant efficiency of aryl phosphates in combination with phenol-formaldehyde novolac type resin was studied in poly(butylene terephthalate) (PBT). It was shown that resorcinol bis(diphenyl phosphate) or bisphenol A bis(diphenyl phosphate) in combination with novolac is an efficient fire retardant in non-glass reinforced PBT. Co-addition of triphenyl phosphate was required for glass-reinforced PBT to reach similar fire retardant performance. Apparently aryl phosphates/novolac prevent exudation of aryl phosphates to the surface and facilitate crystallization of PBT. Although the aryl phosphates/novolac are compatible with PBT and do not decompose polymer during compounding, PBT exhibited some reduction of Izod impact strength and heat distortion temperature. These phenomena were attributed to changes in the crystalline morphology, and probably also in the amorphous regions of the polymer.