1. Introduction
Polymethacrylate (PMMA) and polystyrene (PSt) are frequently used in the construction sector; the former as transparent
Perspex sheets and the latter as an insulation material. Transparency, ease of processing and relatively good resistance to weathering are the factors that make PMMA a suitable substitute for glass-based construction elements, whereas the inherent thermal properties of PSt, especially in the foamed state (e.g., expanded polystyrene: EPS), means it can function as a good insulating material. However, both materials are highly amenable to thermal degradation, resulting in the release of low-molecular-weight gaseous fragments (including the monomeric species), which in turn can form flammable mixtures with an ambient air. The flammable mixtures formed can easily undergo combustion in the presence of a suitable ignition source or spontaneously at temperatures greater than the ignition temperature, subsequently producing toxic vapours and gases [
1].
Fire safety regulations are adopted and strictly enforced in recent times with an aim to protect high-rise buildings especially, where many polymer-based construction components are used. The tendency of polymeric materials to melt and flow, forming a pool of flammable decomposition products, can also constitute a very serious secondary hazard as this can often lead to the further burning of surrounding fuel loads [
2]. Generally, this tendency strongly depends on the class of the polymeric material (e.g., thermoplastic, or thermoset) as well as on their chemical constitutions [
3]. In addition, common polymers exhibit a wide range of propensities for thermal degradation. Usually, they require a temperature range from approximately 270–470 °C to undergo decomposition to generate volatile fuel fragments which may lead to combustion. However, in the case of foamed products like polystyrene foams, these products are highly ignitable even in the presence of low-intensity sources for piloted ignition [
4,
5,
6,
7]. The main reason behind this relatively high ignitability/flammability can be attributed to ease of the monomer liberation at a low temperature [
1]. Polymethyl methacrylate (PMMA) is a classic example of a polymer which, upon thermal decomposition, produces a near quantitative yield of the monomer (MMA), through a chain ‘unzipping’ process [
8]. Furthermore, owing to its complete decomposition at higher temperatures, PMMA leaves no char residue after a fire. Several incidents of fire hazards are reported, where PMMA melts, and thus enhances the flame spread. During the burning process, polystyrene can also melt, flow and drip, which can lead to a distributed fuel load feeding into an enhanced flame spread [
2].
Attempts to fire retard both PMMA and PSt are well documented. For this purpose, it is often necessary to treat the parent polymeric matrix through a suitable methodology, in which an appropriate combustion inhibitory reagent (e.g., a flame retardant, FR) is incorporated into the final product. Generally, a large number of flame retardants were used for many years to protect polymeric materials from decomposition and subsequent combustion [
9]. These flame retardants may be mixed as additives in the polymer matrix by physical means. Another way of improving flame retardancy is to prepare inherently less flammable polymers through the copolymerization with compounds that can impart fire resistance [
10,
11]. Organohalogen compounds are primarily used for this purpose as they are excellent in reducing the flammability of polymers. However, the environmental toxicity of these materials caused a ban on their use for commercial purposes. Therefore, inorganic fillers such as magnesium hydroxide, graphene oxide and modified nanoparticles such as clays and silica, etc., and phosphorus-containing compounds are now more commonly used [
11]. The efficiency of phosphorus-based compounds generally depends on several factors: the chemical environment and oxidation state of the P atom, volatility, and the nature of the decomposition products formed upon thermolysis, etc. [
12]. The condensed-phase activity of phosphorus compounds predominantly involves char formation which is facilitated by the dehydration of the polymeric structure leading to cyclization, cross-linking and aromatization/graphitization [
13]. Cross-linking can be also induced by the decomposition by-products of the phosphorus compounds. For polymers with hydroxyl or amino, and groups in their monomeric units, such as in the case of cellulose or wool, phosphorus compounds work mainly in the condensed phase. In the case of olefin-based polymers, these compounds act mainly in the gaseous phase by scavenging radicals such as H*, HO* and preventing their oxidation. Phosphorus-based compounds, such as phosphine, phosphine oxides, phosphonates, and phsophorylamino esters are found to have similar effects to halogen-containing compounds when incorporated into PMMA and PSt [
12,
14,
15].
Deciphering the processes that are responsible for the flame retardant effects of some phosphorus-modified PMMA- and PSt-based bulk polymer samples is attempted by using a variety of analytical techniques. The syntheses of the control and phosphorus-modified PMMA and PSt systems, their spectral characterization, and thermal and calorimetric investigations are communicated separately [
16,
17]. The analytical investigations primarily included char analyses (using Inductively-coupled Plasma/Optical Emission Spectroscopy: ICP/OES; solid-state NMR spectroscopy:
13C and
31P; FT-IR, in Attenuated total reflectance mode-ATR), and investigation of gaseous-phase products using GC/MS and pyrolysis GC-MS measurements).
2. Materials and Methods
All the chemicals, reagents and solvents were purchased from Aldrich Chemical Company, except the following: 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and diethyl-1-propylphosphonate (Thermofisher Scientific). Generally, the solid compounds were used as received, whereas liquid reagents and solvents were, optionally, dried by keeping them over molecular sieves (4 Å). Furthermore, thermally labile initiators and monomers were stored under sub-ambient temperatures in a refrigerator, or in a freezer, as the case may be. The inhibitors (typically hindered phenolic compounds, such as hydroquinone monomethyl ether), were removed from methyl methacrylate by passing through proprietary inhibitor removal columns, purchased from Aldrich Chemical Company.
The synthetic procedures for the preparation of the precursor compounds, the additive (diethylbenzylphosphonate), and various functional monomers and polymeric products are given elsewhere [
16,
17]. These additives/reactives included: triphenylphosphine (TPP); triphenylphosphineoxide (TPPO); 9,10-Dihydro-9-oxa-10-phosphaphenenthrene-10-oxide (DOPO); diethylphosphite (DEHPi); triethylphosphite (TEPi); triethylphosphate(TEPa); diethylpropylphosphonate (DEPP); diethylbenzylphosphonate (DEBP); diethyl-1-(acryloyloxyethyl)phosphonate (DE-1-AEP); acrylic acid-2-[(diethoxyphosphoryl) methyl amino] ester (ADEPMAE); diethyl-2-(acryloyloxy)ethylphosphate (DEAEPa); diethyl-p-vinylbenzylphosphonate (DEpVBP) (see
Table 1 below).
All the polymeric products, including their controls, were synthesized through the bulk polymerization route. In this method, the required amount of monomer(s) and initiators were stirred thoroughly in a conical flask under a nitrogen atmosphere for the specified duration (ca. 1 h at 70–80 °C for MMA and ca. 5 h at 70 °C for St), until a visible increase in the viscosity was observed. The calculated amount of the additive/reactive was then added, and stirred for another 1 h, and the mixture was subsequently poured into an aluminium pan of ca. 50 mL volume and the pan was stoppered with an aluminium lid. The pan was placed in an air oven preheated at 40 °C and kept for curing for about 20 h. During the second stage of curing, the temperature of the oven was raised to 60 °C for 8 h. In the case of the PMMA-based polymers, another 20 h of curing at 80 °C was conducted, whereas for St-based polymers the corresponding stage involved 20 h of curing at 80 °C followed by a period of 3 h at 100 °C. The final products were extracted from the pans after cooling to room temperature.
The detailed procedures for sample preparation, instruments used, operating parameters, and data accusation and processing, relating the various analytical instrumentation and associated techniques were published previously [
18]. These involved the following: the phosphorus contents of aqueous extracts of the test samples were measured in triplicate by using Shimadzu ICPE-9000, and the average values were taken. The solid-state NMR (
31P with CP/MAS mode) spectra of the char residues was obtained by employing a 500 MHz Bruker machine at ambient probe conditions, typically at 10 kHz rotor speed, and the signals were calibrated against phosphoric acid as the external calibrant. The raw data were then processed by using a proprietary software from the manufacturer (TopSpin 4.0.6). A For FT-IR measurements, a Perkin-Elmer 1600 model instrument was used, in which infrared radiation in the range 4000 to 600 cm
−1 was absorbed by the test sample in the attenuated total reflectance (ATR) mode (typically 32 scans at a resolution of 4 cm
−1).
Pyrolysis-GC/MS was performed with the pyrolysator Pyroprobe 5000 (CDS Analytical, Inc., Oxford, PA, USA) with platinum filament coupled with gas chromatograph GC7890A (Agilent Technologies, Santa Clara, CA, USA), and with GC column HP-5MS (non-polar, length: 30 m; inner diameter: 250 μm; layer thickness: 0.25 μm), (Agilent Technologies, Santa Clara, CA, USA). The carrier gas was helium with a gas flow rate of 1 cm3 min−1. The GC was equipped with the mass-selective detector MSD 5975C inert XL EI/CI (Agilent Technologies, Santa Clara, CA, USA) with a mass scan range between 15–550 m/z and EI at 70 eV. The samples were pyrolyzed at the temperatures of maximum mass losses found in TGA. The inlet temperature of the GC was variable, and the oven temperature programme was fixed (2 min at 50 °C; heating with 12 K min−1 to 280 °C). Here, the temperature for the pyrolysis of the admixtures with PMMA and PSt with the solid additives (TTP, TPPO and DOPO) were selected from the first derivative of the corresponding TGA runs obtained at a heating rate of 10 °C min−1 in nitrogen (PMMA: 381 °C; PMMA + TPP: 246 °C; PMMA + TPPO: 371 °C; PMMA + DOPO: 388 °C; PSt: 412 °C; PSt + TPP: 418 °C; PSt + TPPO: 416 °C; PSt + DOPO: 431 °C).
4. Conclusions
In the case of the PMMA-based systems, there is evidence that P-bearing compounds/groups, except TPP, TPPO and DOPO, upon thermal cracking during the early stages of flaming combustion, produce ‘phosphorus’ acid species. These acidic species can subsequently initiate the chemical pathway used to produce char precursors. In the case of TPP, TPPO and DOPO, it is more likely that they produce phosphorus- and/or oxygenated phosphorus-containing volatiles that can act in the gaseous-phase [
18].
However, in the case of programmed heating under controlled environments, such as in TGA, DSC, PCFC and ‘bomb’ calorimetry, where a flaming mode of combustion is not attained, any such cooperative interaction between the parent polymer matrix and the modifying compound/groups can be assumed to be absent [
16]. On the other hand, with PSt-based systems, the modifying moieties seem to exert some degree of cooperative interactions even in the controlled decomposition tests (e.g., TGA and DSC), and certainly in measurements where combustion occurs (PCFC and ‘bomb’ calorimetry) [
17]. Here, a probable process involving the phosphorylation of the phenyl rings leading to crosslinking and char formation is proposed. Furthermore, the thermal degradation of polystyrene is strongly dependent on temperature, where the initial event is established to be the rapid evolution of the monomer [
22].