Barry J. Hunt

Flame retardance of poly(methyl methacrylate) modified with phosphorus-containing compounds

Abstract MMA has been copolymerised with pentavalent phosphorus-containing monomers and the flame retardance of the resulting copolymers has been assessed by limiting oxygen indicies (LOI) and cone calorimetry experiments. The thermal stability of the copolymers has also been assessed by conventional thermogravimetric analysis (TGA). Poly(methyl methacrylate) (PMMA) modified with phosphorus-containing additives have also been synthesised and the flame retardance assessed. All of the modified PMMA samples contain 3.5 wt.%, allowing a comparison of the relative merits of an additive and a reactive approach to flame retardance. The chemical environment of the phosphorus in terms of flame retardance achieved is also considered in this paper. The incorporation of 3.5 wt.% phosphorus in both reactive and additive approaches increases the limiting oxygen index of PMMA from 17.8 to over 21. However, cone calorimetry shows that the phosphorus-containing copolymers are inherently more flame retardant than PMMA and the PMMA modified with phosphorus-containing additives. The methyl methacrylate (MMA) copolymers have significantly reduced peak rates of heat release and leave substantial char residue during combustion, as compared to PMMA. Cone calorimetry has also shown that the phosphates are more effective flame-retardants for PMMA than are the phosphonates in both additive and reactive approaches. TGA of the polymers indicates that the copolymers are more thermally stable than PMMA whilst PMMA containing the additives are less thermally stable. A condensed phase mechanism in which diethyl(methacryloyloxymethyl)phosphonate reduces the flammability of PMMA has been identified.

Flame retardance in some polystyrenes and poly(methyl methacrylate)s with covalently bound phosphorus-containing groups: initial screening experiments and some laser pyrolysis mechanistic studies

Abstract Styrene (ST) and methyl methacrylate (MMA) have been copolymerized with a variety of comonomers containing covalently-bound phosphorus-containing groups, including vinyl phosphonic acid, several dialkyl vinyl phosphonates, and various vinyl and allyl phosphine oxides. The flame retardance of these polymers has been preliminarily assessed through thermogravimetric analysis and measurements of limiting oxygen index (LOI) and char yields. All the phosphorus-containing polymers produce char on burning (and also on heating in air or nitrogen) and have LOIs higher than those of the parent homopolymers, indicating significant flame retardance involving condensed-phase mechanisms. However, despite there being general correlations between LOI, char yield and phosphorus-content, some copolymers have higher than expected LOI and/or char yield, whilst others have lower, indicating that phosphorus environment is important. In order to explore mechanisms of flame retardance in more detail, laser pyrolysis/time-of-flight mass spectrometry and mass spectrometric thermal analysis have been applied to study the decomposition behaviour of three of the MMA copolymers: those containing pyrocatecholvinylphosphonate (MMA-PCVP), diethyl- p -vinylbenzylphosphonate (MMA-DE p VBP) and di(2-phenylethyl)vinylphosphonate (MMA-PEVP) as comonomers. The laser pyrolysis experiments provide an insight into probable polymer behaviour behind the flame front in a polymer fire and show that copolymerization of MMA with the comonomers does not greatly change the pyrolysis mechanism compared with that of poly(methyl methacrylate) (PMMA). However, the amounts of MMA monomer evolved during pyrolysis are much reduced for the copolymerized samples. Since MMA is the major fuel evolved during the combustion of PMMA and its copolymers, this effect must be a major contributing factor to the reduced flammability shown by the copolymers. MMA-DE p VBP underwent the most extensive decomposition following laser pyrolysis. In particular, significant amounts of highly flammable methane and ethene were detected. Such increased amounts would occur also if the copolymer were to be exposed to high temperature conditions when burnt. Hence, its seems reasonable that the MMA-DE p VBP has a lower LOI value than expected, despite it giving a relatively high yield of char. Mass spectrometric thermal analysis studies of the MMA-PEVP provide evidence that the PEVP unit decomposes around 200°C, eliminating styrene, with evolution of MMA reaching a maximum some 50°C higher. Possible mechanisms for these processes are suggested.

Flame retarding poly (methyl methacrylate) with phosphorus containing compounds: comparison of an additive with a reactive approach

Abstract The flame retardance and thermal stability of a methyl methacrylate (MMA) copolymer reactively modified by copolymerisation of the MMA with diethyl (methacryloyloxymethyl) phosphonate (DEMMP) have been compared with those of poly(methyl methacrylate) (PMMA) containing equivalent amounts of the additive diethyl ethyl phosphonate (DEEP). DEEP can be regarded as having a structure similar to that of a DEMMP comonomer unit and therefore the two compounds might be expected to confer about the same levels of flame retardance to PMMA when used at similar concentrations. The incorporation of 3.5 wt.% phosphorus in both cases raises the limiting oxygen index of PMMA from 17.2 to over 22. However, cone calorimetry shows that the MMA/DEMMP copolymer is inherently more flame retardant than PMMA containing DEEP: the former has a significantly lower peak rate of heat release than the latter (449 and 583 kW m −2 , respectively) and gives rise to a greater amount of char. Thermogravimetric analysis (TGA) of the polymers indicates also that the MMA/DEMMP copolymer is more thermally stable than PMMA whilst PMMA containing DEEP is less thermally stable. Dynamic mechanical thermal analysis (DMTA) shows that the MMA/DEMMP copolymer has physical and mechanical properties similar to those of PMMA, whilst the low molecular weight DEEP plasticises PMMA, resulting in a significantly reduced glass transition temperature, T g . A condensed phase mechanism of flame retardance in MMA/DEMMP has been identified.

Thermal degradation and flame retardance in copolymers of methyl methacrylate with diethyl(methacryloyloxymethyl)phosphonate

Methyl methacrylate (MMA) has been free radically copolymerized, both in bulk and in solution, with diethyl(methacryloyloxymethyl)phosphonate (DEMMP), to give polymers which are significantly flame retarded when compared with PMMA, as indicated by the results of limiting oxygen index (LOI) measurements, UL 94 tests, and the results of cone calorimetric experiments. The physical and mechanical properties of the copolymers are similar to those of PMMA, except that the bulk copolymers are slightly crosslinked, and are better than those of PMMA flame retarded to a similar extent by some phosphate and phosphonate additives. Examination of the some of the gaseous products of pyrolysis and combustion, and of chars produced on burning, show that flame retardation occurs in the copolymers by both a condensed-phase and a vapour-phase mechanism. The condensed-phase mechanism is shown to involve generation of phosphorus acid species followed by reaction of these with MMA units giving rise to methacrylic acid units. The methacrylic acid units subsequently form anhydride links, which probably impede depolymerization of the remaining MMA sequences, resulting in evolution of less MMA (the major fuel when MMA-based polymers burn). By undergoing decarboxylation, leading to interchain cyclisation and, eventually, to aromaticisation, the anhydride units are probably also the principal precursors to char.

Chemical modification of polymers to improve flame retardance—I. The influence of boron-containing groups

Polystyrene has been silylated by a two-stage process, involving reaction with n-butyl lithium in the presence of tetramethylethylenediamine, followed by reaction with trimethylchlorosilane, dichlorodimethylsilane or trichloromethylsilane. The degree of silylation can be increased if the polystyrene is initially brominated, in the presence of thallic acetate, prior to lithiation and silylation. The surfaces of poly(vinyl alcohol) films have also been modified with a number of chlorosilanes. Flame retardant effects of these modifications have been assessed by measurements of char yields, weight losses and limiting oxygen indices. The limiting oxygen indices of the silylated polymers are all significantly higher than those of the parent polymers and small char yields are obtained on combustion of them. Silicon is shown to have a greater effect on flame retardance in the presence of halogen for each of the modified polymers.

Ignition temperatures and pyrolysis of a flame‐retardant methyl methacrylate copolymer containing diethyl(methacryloyloxymethyl)‐phosphonate units

The ignition and pyrolysis of some copolymers of methyl methacrylate (MMA) with diethyl(methacryloyloxymethyl)phosphonate (DEMMP) have been studied by a simple tube furnace and by isothermal and non-isothermal TGA. The results indicate that copolymers containing DEMMP thermally degrade, under both air and nitrogen, by a mechanism which is more complex than that (simple depolymerization) for poly(methyl methacrylate) (PMMA). The copolymers containing 10 mol% or more of DEMMP are inherently flame retardant in that they fail to autoignite at 480 °C and take longer to autoignite at 490 °C than PMMA or MMA-DEMMP copolymers containing only 5 mol% DEMMP. The formation of a carbonaceous residue or char on combustion of the MMA-DEMMP copolymers suggests that flame retardance is due mainly to reactions in the condensed phase.

Thermal degradation and flammability characteristics of some polystyrenes and poly(methyl methacrylate)s chemically modified with silicon-containing groups

Several Si-containing methacrylates and acrylates have been free radically copolymerised with methyl methacrylate and with styrene. The copolymers have been examined by DSC to determine glass transition temperatures (Tg), by TGA to determine thermal stabilities, and by measurements of limiting oxygen index (LOI) to determine ignitability/flame retardance. The copolymers are found to have Tgs similar to those of the parent homopolymers suggesting that the mechanical properties are little affected by the incorporation of the Si-containing groups and to have slightly improved flame retardance. LOI and char yields suggest that the mechanism of flame retardance, i.e. whether vapour phase or condensed phase, depends upon the nature of the Si-containing substituent.

Time-resolved fluorescence anisotropy studies of the cononsolvency of poly(N-isopropyl acrylamide) in mixtures of methanol and water

Time-resolved anisotropy measurements (TRAMS) have provided invaluable information concerning the molecular interactions responsible for cononsolvency in the poly(N-isopropylacrylamide), PNIPAM, H2O, and methanol ternary system. TRAMS have successfully monitored the intramolecular segmental dynamics of an acenaphthylene labelled sample, ACE–PNIPAM, revealing details about the conformation adopted by the polymer constituent as a function of both alcohol composition and temperature of the system. In pure aqueous solution, ACE–PNIPAM undergoes a conformational transition from an expanded solvent swollen structure to a compact globule at the lower critical solution temperature (LCST). With increasing alcohol content up to 55% v/v of methanol there is both a reduction in the magnitude and an onset temperature of the LCST of ACE–PNIPAM. From 57.5–65% v/v methanol, ACE–PNIPAM forms an extended solvent swollen structure: observation of an LCST at higher polymer concentrations (e.g., 0.1 wt%) is a consequence of intermolecular aggregation between expanded chains.

Manipulating the thermoresponsive behaviour of poly(N-isopropylacrylamide) 3. On the conformational behaviour of N-isopropylacrylamide graft copolymers

The thermally triggered conformational change of poly(N-isopropyl acrylamide), PNIPAM, in aqueous media occurs at the lower critical solution temperature, LCST. Manipulation of the switch can be achieved via simple free radical copolymerisation, for example. However, the magnitude of the transition is reduced which has a detrimental effect on the solubilisation and controlled release properties of the polymer. In an attempt to over come these limitations the effect of architecture on the thermal response has been examined through syntheses of a range of fluorescently labelled graft copolymers. To examine the effect of topography, samples containing a NIPAM-based backbone and NIPAM branches have been prepared. Simultaneous variation of the entropic and enthalpic contribution to the thermal response has been achieved through syntheses of macromolecules containing a dimethylacrylamide-based backbone and NIPAM grafts. Time-resolved fluorescence anisotropy (TRFA) measurements have been successful in determining the onset, magnitude and dispersion of the LCST of these samples and, by selective labelling of sites, have provided information concerning the role of backbone and graft on the resultant thermorespsonive behaviour. TRFA measurements confirm that the temperature of the conformational switch can be varied through simultaneous manipulation of the entropic and enthalpic contribution to the thermal response of the graft copolymers, whilst maintaining the magnitude of the transition.

The effects of some transition-metal compounds on the flame retardance of poly(styrene-co-4-vinyl pyridine) and poly(methyl methacrylate-co-4-vinyl pyridine)

Abstract Several copolymers of both styrene and methyl methacrylate with 4-vinyl pyridine have been prepared and modified by coordination with the transition-metal compounds vanadium acetylacetonate (VO[acac] 2 ), vanadyl dichloride (VOCl 2 ) and ferric chloride. The flame-retardant effects of these modifications have been assessed by measurements of limiting oxygen indices, by thermogravimetric analysis, and by examination of chars by scanning electron microscopy. Effects on mechanical properties have been assessed by dynamic mechanical thermal analysis. The limiting oxygen indices of the modified polymers are significantly higher than those of the parent polymers, and the production of considerable amounts of rigid, intumescent chars suggests predominantly condensed-phase mechanisms of flame retardance.