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

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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-DEpVBP) 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-DEpVBP 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-DEpVBP 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.

Introduction

The use of phosphorus compounds as components of flame-retardant additives in polymers is well established [1]. Such additives, which are often a mixture of a phosphorus compound with a carbonific such as pentaerythritol and a spumific such as melamine, invariably operate predominantly through a condensed-phase mechanism, i.e. a mechanism in which combustion of the outer layers of the polymeric article containing the flame-retardant mixture leads to the production of an intumescent carbonaceous char which acts as a physical and thermal barrier to further combustion by impeding heat transfer to the underlying layers of virgin polymer and thus the release of further flammable volatiles.

The use of additives as flame retardants, however, has disadvantages. Such additives often have to be used in relatively high concentrations (typically 10–40 wt%) to be effective, leading to concomitant undesirable changes in physical and mechanical properties. Also, additives may be leached or otherwise lost from the polymer during service, posing a potential environmental hazard.

To address some of the problems associated with the additive route to flame retardance, we have turned our attention to flame retardant strategies involving the chemical attachment of flame-retardant moieties directly to polymer backgrounds, i.e. a reactive strategy. Such strategies are readily adaptable to step-reaction polymers, namely those made by condensation and rearrangement reactions such as aromatic polyesters [2], polyurethanes [3] and epoxy resins [4]. However, they have been relatively little exploited for chain-reaction polymers such as polyethylene, polystyrene, and the acrylics, even on a laboratory scale, although the work of Allen et al. on copolymerizations of vinyl and acrylic monomers with alkenylphosphazene derivatives is notable in this respect [5].

In previous work we have studied the influence of post-polymerization phosphorylation and phosphonylation on the flame retardance of poly(vinyl alcohol), ethylene-vinyl alcohol copolymers and polyethylene [6], [7], and the effect of the incorporation of diethylvinylphosphonate comonomer units on the flame retardance of poly(methyl methacrylate), polystyrene, polyacrylamide and polyacrylonitrile [7], [8]. In all cases, significant increases in limiting oxygen index (LOI) were observed, together with increases in char yields on combustion.

Encouraged by these findings, we have embarked on a more systematic exploration of the effects of incorporating phosphorus-containing comonomer units on the flame retardance particularly of polystyrene and of poly(methyl methacrylate). Obviously, the reactive route to flame retardance also can lead to undesirable changes in physical and chemical properties, but we believe that in many cases it may be possible to keep such changes to an acceptable minimum since the amount of chemical modification required to achieve an acceptable level of flame retardance may be quite small.

This paper reports a screening for flame retardance of some phosphorus-containing polystyrenes and poly(methyl methacrylates) made by copolymerising styrene (ST) and methyl methacrylate (MMA) with various phosphorus-containing monomers, preliminary to exploring a selection of the systems in more detail. Three of the MMA copolymers have then been subjected to laser pyrolysis/time-of-flight mass spectrometry (LP/TOFMS) and their behaviours compared with that of PMMA. Laser pyrolysis is used to model the pyrolysis reactions which occur in the interface region just behind the flame front of a polymer fire thus providing an insight into the potential behaviour of a polymer in a real fire [9]. Complementary studies have also been carried out using a mass spectrometric thermal analysis technique [10].

Section snippets

Materials

The phosphorus-containing comonomers used in this work are: vinylphosphonic acid (VPA) (I), diethylvinylphosphonate (DEVP) (II), diphenylvinylphosphonate (DPVP) (III), di(2-phenylethyl)vinylphosphonate (PEVP) (IV), “pyrocatechol’’vinylphosphonate (PCVP) (V), α-phenylvinylphosphonic acid (αPVPA) (VI), diethyl-α-phenylvinylphosphonate (DEαPVP) (VII), diethyl-p-vinylbenzylphosphonate (DEpVBP) (VIII), diethyl(methacryloyloxymethyl)phosphonate (DEMMP) (IX), diphenylvinylphosphineoxide (DPVPO) (X),

General comments on flame retardance

Phosphorus contents of copolymers, TGA data, yields and phosphorus contents of chars produced by burning, and LOI values are given in Table 1 (for ST copolymers) and Table 2 (for MMA copolymers).

It can be seen from the tables that the introduction of phosphorus, irrespective of its environment within the comonomer unit, increases LOIs by significant amounts and leads also to the production of chars in both burning (LOI) and TGA experiments. Moreover, there is some correlation between the amount

Conclusions

Various phosphorus-containing monomers are readily synthesized and can be copolymerized free radically with both styrene and methyl methacrylate. The resulting copolymers in general have LOIs higher than those of polystyrene and poly(methyl methacrylate) and produce chars on burning, thus warranting detailed further investigation as potential flame retardant materials.

The laser pyrolysis experiments indicate the following.

  • Copolymerization of MMA with PCVP, DEpVBP and PEVP does not greatly

Acknowledgements

P.J. thanks the Commonwealth Scholarship Commission in the UK and the British Council for the provision of a Commonwealth Universities Scholarship. F.G. thanks ICI for the provision of a postdoctoral fellowship through its Strategic Research Fund. We all thank the Engineering and Physical Sciences Research Council (EPSRC) for financial support (Grant Nos. GR/L85879 and GR/L85886).

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    1

    Second corresponding author.

    2

    Present address: Department of Mechanical and Manufacturing Engineering, Nottingham Trent University, Burton Street, Nottingham NG1 4BU, UK.

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