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

Dennis Pricea,*, Kelly Pyraha, T. Richard Hulla, G. John Milnesa,John R. Ebdonb, Barry J. Huntb, Paul Josephb

aInstitute of Materials Research, Cockroft Building, University of Salford, Salford M5 4WT, UKbThe Polymer Centre, Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK

Received 12 August 2001; accepted 15 September 2001

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 thecopolymers has also been assessed by conventional thermogravimetric analysis (TGA). Poly(methyl methacrylate) (PMMA) mod-ified 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 flameretardance. The chemical environment of the phosphorus in terms of flame retardance achieved is also considered in this paper. Theincorporation 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 retardantthan PMMA and the PMMA modified with phosphorus-containing additives. The methyl methacrylate (MMA) copolymers havesignificantly reduced peak rates of heat release and leave substantial char residue during combustion, as compared to PMMA. Conecalorimetry 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 whilstPMMA containing the additives are less thermally stable. A condensed phase mechanism in which diethyl(methacryloylox-ymethyl)phosphonate reduces the flammability of PMMA has been identified. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Phosphorus; Additive; Reactive; MMA copolymers; Combustion behaviour; Mechanism

1. Introduction

The use of phosphorus-containing additives to reducethe flammability of chain polymers such as poly(methylmethacrylate) (PMMA) is well-established [1]. Thisapproach to flame retardance however has several dis-advantages including high loadings required to achieve asufficient level of flame retardance, detrimental changes tothe polymer’s physical and mechanical properties andleaching of the flame retardant additive. We believe thatthrough strategic copolymerisations of methyl methacry-late (MMA) with phosphorus-containing monomers, theflammability of PMMA may be significantly reducedwithout the problems associated with an additive approachto flame retardance.

The likelihood of promoting a condensed phasemechanism of flame retardance may also be increasedwhen phosphorus is chemically incorporated into thepolymer. This route, particularly if the phosphorus isretained in the char (and, hence, not unnecessarilyincreasing the toxicity of the combustion gases) is highlydesirable.Phosphorus-containing components have already

been used in the synthesis of several flame-retardant,step-reaction polymers, e.g. polyesters [2–5], poly-urethanes [6–8], and epoxy resins [9–15]. Their use in thesynthesis of chain reaction polymers, however, is muchless well developed, although there has been work in thisarea by Allen and Bahadur [16] who have flame retar-ded several chain reaction polymers through incorpor-ating vinyl phosphazenes as comonomers.The work presented in this paper is part of an ongoing

project to determine the effectiveness of phosphorus-containing monomers as flame-retardant components in

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00038-1

Polymer Degradation and Stability 77 (2002) 227–233

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +44-161-295-4262; fax: +44-161-

295-5111.

E-mail address: d.price@salford.ac.uk (D. Price).

chain reaction polymers, especially styrenics and acryl-ics [17–19].We report here some studies on the thermal and

combustion behaviour of MMA copolymers containingdiethyl(methacryloyloxymethyl) phosphonate (DEMMP,I), diethyl(acryloyloxymethyl) phosphonate (DEAMP, II),diethyl(methacryloyloxyethyl) phosphate (DEMEP, III)and diethyl(acryloyloxyethy) phosphate (DEAEP, IV).DEMMP (I) has been previously described in the litera-ture as a potential flame retardant for plastics [20–22].However, to the best of our knowledge, no detailed exam-inations of the properties of acrylic polymers based uponDEMMP have been reported, although we have alreadypublished some data relating to the thermal stability andflame retardance of MMA–DEMMP copolymers [23–25].We also report some data on the thermal and combus-

tion behaviour of PMMA modified with relatively lowmolecular weight pentavalent phosphorus-containingadditives. Diethylphosphonate (DEEP, V) and triethyl-phosphate (TEP, VI) have structures that are analogousto the DEMMP and DEMEP monomers, respectively,allowing an assessment of the relative advantages of bothan additive and a reactive approach to flame retardance.Some data is also given on PMMAcontaining the additivetri-n-butylphosphine oxide (TnButPO, VII).

2. Experimental

2.1. Phosphorus-containing comonomers

DEMMP and DEAMP were synthesised following,with minor adaptation, the method of Liepins et al. [26]

DEMEP and DEAEP were synthesised following, withminor adaptation, the method of Clouet et al. [27]. Thesynthetic procedures have been published elsewhere [24],and are not repeated here.

2.2. Synthesis of polymer plaques

Plaques of PMMA suitable for cone calorimetricexperiments were prepared in the laboratory as follows.A mixture of methyl methacrylate (MMA) (ca. 350 ml),benzoyl peroxide (3 g l�1) and lauryl peroxide (1.5 g l�1)was purged with nitrogen for 10 min and then heatedunder nitrogen, with stirring to 70 �C. When the mix-ture became sufficiently viscous (typically 1 h) it waspoured into a cell made from two thick, borosilicateglass plates, separated around their edges by a siliconerubber gasket. The assemblies were placed in an air ovenand the plaques cured at 40 �C for 24 h, 60 �C for 8 h and80 �C for 24 h. Optimum curing of the plaques wasdetermined by trial and error. The resulting plaquesmeasured ca. 120�120�4 mm and were cut to size forcone calorimetry experiments.Plaques of PMMA containing the additives were pre-

pared as above with a minor adaptation. The additivewas added to the viscous prepolymer mixture (prior topouring between the glass plates) and left to stir for ca.5 min, allowing the mixture to become homogeneous.The amount of additive added was calculated to yield aplaque containing 3.5 wt.% phosphorus. Curing wascarried out in a manner identical to that for PMMA.The DEMEP, DEAEP and DEAMP copolymer pla-

ques were prepared by a method similar to that of thehomopolymer. The amount of phosphorus-containing

228 D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233

monomer was calculated so as to yield plaques contain-ing 3.5 wt.% phosphorus, which was added to MMA atthe start of the pre-polymerisation mixture. Curing wasidentical to that of the homopolymer.Plaques of MMA/DEMMP copolymer containing 10

mol% DEMMP (corresponding also to �3.5 wt.%phosphorus) were prepared in the laboratory by heatingde-oxygenated mixtures of MMA, DEMMP and azoiso-butyronitrile (2 g l�1) between glass plates separatedaround their edges with a silicone rubber gasket. Curingof the plaques was carried out in an air oven, first at 80 �Cfor 2 h, then at 100 �C for 2 h and finally at 120 �C for 2 h.

2.3. Limiting oxygen indices

Limiting oxygen indices (LOI-ASTM-D-2863) weremeasured on a Stanton–Redcroft FTA flammability uniton off-cuts of polymer plaque measuring 100�6�4 mm.

2.4. Cone calorimetry

The combustion behaviour under ventilated condi-tions was measured using a Fire Testing Technologycone calorimeter, in conformance with ISO DIS 5660[28]. Testing was performed on 100�100�4 mm plaquesat a heat flux of 35 kW m�2, which is the recommendedheat flux for exploratory testing [29]. The plaques werethen placed in a sample holder with retainer frame,resulting in 88 cm2 of the sample surface being exposedto the radiating cone heater. Each test was carried outtwice to ensure the results obtained were reproducible.Optimally, the test should be repeated at least three times,but limited quantities of the phosphorus-containingmonomers allowed for only two plaques of the copoly-mers to be prepared and tested.

2.5. Thermogravimetric analysis

Thermogravimetric analyses (TGA) were carried outon ca. 10 mg samples contained in silica crucibles on aMettler TG 50 thermogravimetric analyser under nitro-gen and in air at a heating rate of 10 �C min�1.

2.6. Dynamic mechanical thermal analysis (DMTA)

DMTA was carried out on a Polymer LaboratoriesDMTA Mark II instrument at a fixed frequency of 1 Hzand at a heating rate of 5 �C min�1 over the temperaturerange 35–180 �C on small pieces of polymer plaques heldin a single-point cantilever clamp. The principal datarecorded were the temperatures corresponding to themaximum values of tan d, which approximate to thezero frequency glass transition temperatures, Tg.

3. Results and discussion

3.1. Combustion behaviour

The flammability of the modified PMMA has beenassessed by the LOI test and the results are given in Table 1.The results show that the incorporation of just 3.5

wt.% phosphorus, irrespective of chemical environmentor the approach used, increases the LOI of PMMAfrom 17.8 to above 21. Although the improvement is inmost cases relatively modest, any improvement in theflammability of PMMA without detriment to thepolymers physical and mechanical properties is wel-come. The DEAEP and DEMEP copolymers werefound to have the most improved LOIs of 28.1 and25.0, respectively.

Table 1

LOI results

Sample LOI (%O2)

PMMA 17.8

MMA/DEMMP 22.8

MMA/DEAMP –

MMA/DEMEP 25.0

MMA/DEAEP 28.1

PMMA+TEP 22.7

PMMA+DEEP 22.4

PMMA+TnButPO 21.4

Table 2

Cone calorimetry results of mma copolymers

PMMA DEMMP DEAMP DEMEP DEAEP

Time to ignition (s) 53 58 82 75 88

Peak RHR (kW/m2) 640 460 414 294/383 383

Average (RHR) 174 171 166 84/121 139

Peak CO (kg/kg) 0.016 0.169 0.209 0.175 0.158

Peak CO2 (kg/kg) 3.23 2.39 2.02 1.66 1.75

Peak SEA (m2/kg) 156 755 987 944 749

Average EHC (MJ/kg) 23.2 19.3 17.5 13.7/19.9 19.7

Residue (%) 1.3 6.7 9.2 15.9/20.6 10.7

Table 3

Cone calorimetry data of PMMA with phosphorus-containing additives

PMMA + DEEP +TEP +TNButPO

Time to ignition (s) 53 63 52 52

Peak RHR (kW/m2) 640 583 501 711

Average (RHR) 174 193 147 229

Peak CO (kg/kg) 0.016 1.087 0.132 0.131

Peak CO2 (kg/kg) 3.23 2.74 2.98 3.02

Peak SEA (m2/kg) 156 379 470 630

Average EHC (MJ/kg) 23.2 22.6 19.1 27.2

Residue (%) 1.3 2.7 2.5 1.4

D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233 229

Whilst the LOI is a useful small-scale test for high-lighting and ranking flame retardant polymers, the conecalorimeter provides a wealth of information on thecombustion behaviour under ventilated conditions.Some of the important data determined during the cur-rent work is given in Tables 2 and 3. Some of the data isalso given as a function of time in Figs. 1–8.The results show that the copolymers have somewhat

improved times to sustained ignition, as compared toPMMA which ignited at 53 s. This is particularly so forthe acrylate DEAMP and DEAEP copolymers whichhad times to ignition of 82 and 88 s respectively.

Both the average and peak rates of heat release(RHR) of the copolymers are significantly lower thanthe homopolymer. The phosphate copolymers contain-ing DEMEP and DEAEP were found to be the mostflame retarded, although the peak RHR result obtainedfor MMA/DEMEP were not reproducible. Despite this,the highest peak RHR value obtained for MMA/DEMEP copolymer was 383 kW/m2, which is over a40% reduction compared to that for PMMA. The peakRHR for the MMA/DEAEP was also 383 kW/m2.The phosphonate copolymers also had reduced peak

RHR’s as compared to PMMA, which has a peak RHR

Fig. 4. [CO]/[CO2] ratio of MMA copolymers at 35 kW/m2.

Fig. 5. RHR of PMMA containing additives at 35 kW/m2.

Fig. 3. SEA of MMA copolymers at 35 kW/m2. Fig. 6. Mass loss of PMMA containing additives at 35 kW/m2.

Fig. 2. Mass loss curves of MMA copolymers at 35/m2.

Fig. 1. RHR of MMA copolymers at 35 kW/m2.

230 D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233

of 640 kW/m2. The MMA/DEAMP copolymer has apeak RHR of 414 kW/m2, which is a reduction of over35%. The MMA/DEMMP copolymer did not performas well as the other copolymers and the peak RHR wasreduced by just over 30% (peak RHR=460 kW/m2),which is still a substantial reduction.Fig. 1 shows that the RHR in the first two minutes

after ignition for the phosphate copolymers is sig-nificantly lower than that of PMMA, and also lowerthan that of the phosphonate copolymers. At 2 minafter ignition, the RHR for PMMA is 581 kW/m2. TheRHR120 for the MMA/DEMEP copolymer is sig-nificantly lower at 274 kW/m2, which is a reduction ofover 50%. The DEAEP copolymer also performed well,with a RHR120 of 293 kW/m

2, which is just less than a50% reduction.In contrast, the PMMA modified with phosphorus-

containing additives had peak RHRs which were com-parable with the unmodified PMMA, and the PMMAcontaining TnButPO actually had a higher peak RHRthan PMMA at 711 kW/m2. Of the additives tested, onlyTEP showed any improvement in the peak RHR (whichwas reduced by 22%) and the RHR120 (which was 29%lower than PMMA). In addition, the additives did notsignificantly increase the time to sustained ignition andonly DEEP showed any improvement at 63 seconds.

Figs. 2 and 6, which show the mass loss of the samplesagainst time, also highlight, a significant differencebetween the additives and reactives. Whilst the massloss of PMMA containing the additives is comparableto PMMA, the copolymers rate of degradation is sig-nificantly reduced and a substantial percentage of themass is retained as char. This is a clear indication thatthe comonomers affect the condensed phase of thePMMA degradation, whilst the additives do not. Thelikely condensed phase mechanism in which DEMMPimparts flame retardance to PMMA has already beenpublished elsewhere [24] and so a brief account only isgiven here. In summary, ethene is eliminated from theDEMMP comonomer units leaving bound phosphonicacid species which transesterify with MMA units to givemethacrylic acid (MA) units. The MA units then dehy-drate and probably react with other MMA units, viaalcoholysis, to form anhydride links, disrupting thechain unzipping of PMMA. The anhydride sequencessubsequently decarboxylate, resulting in unsaturatedchar precursors. This is in contrast to the cross-linkingmechanism leading to char formation proposed byLomakin et al. [31] from studies of the thermal degra-dation of two (trimethylolpropane triacrylate and tri-methylolpropane trimethacrylate) network copolymersof methyl methacrylate.The results show that the DEMEP copolymer pro-

duced the most char during combustion although theresults were not reproducible and char yields of 15.9and 20.6 wt.% were recorded.All the modified samples tested had significantly

increased CO and smoke production during combus-tion, and Figs. 4 and 8 show that the [CO]/[CO2] ratio ismuch higher, indicating incomplete combustion andhence, possible gas phase activity.

3.2. Thermal behaviour

3.2.1. Thermogravimetric analysis (TGA)Dynamic TGA and DTG curves of the MMA copo-

lymers under nitrogen are given in Fig. 9a and b.Clearly the comonomers significantly alter the ther-

mal degradation mechanism of PMMA. The degra-dation of PMMA is quite simply a reverse of thepolymersisation [30], and the PMMA chains depoly-merise (unzip) after chain scission to evolve 100%monomer.The unmodified PMMA begins to degrade after

145 �C. The rate of degradation is significantlyincreased after 300 �C, giving rise to a single peak on theDTG curve (Tmax=365

�C). By 390 �C, degradation ofthe PMMA is complete, resulting in 100% mass loss.Under nitrogen, MMA/DEMMP begins to degrade

after 150 �C, but at a slower rate than the homo-polymer, PMMA. After 300 �C, the rate of degradationincreases giving rise to the first peak on the DTG curve

Fig. 8. [CO]/[CO2] ratio of PMMA containing additives at 35 kW/m2.

Fig. 7. SEA of PMMA containing additives at 35 kW/m2.

D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233 231

(Tmax=369�C). The rate of degradation then decreases

until 369 �C. Above 345 �C, the rate increases giving riseto the second DTG peak (Tmax=406

�C). Degradationof MMA/DEMMP is not observed after 480 �C and achar yield of 5% at 500 �C was recorded.The acrylate copolymer, MMA/DEAMP is sig-

nificantly more thermally stable than the other copoly-mers and begins to degrade above 250 �C, at a muchslower rate than MMA/DEMMP. After 300 �C, the rateof degradation increases and like MMA/DEMMP, twopeaks are observed on the DTG curve (Tmax1=362,Tmax2=420

�C). Degradation is not observed after480 �C and a char yield of 7.7% at 500 �C was recorded.Degradation of the DEMEP copolymer begins after

180 �C. After 240 �C, the rate of degradation increasesand the rate becomes faster than the pyrolysis ofPMMA. A hump on the DTG curve is observed around320 �C, which is not observed in the other DTG curves.After 340 �C, the rate of degradation increases givingrise to the first DTG peak (Tmax=357

�C). The ratethen decreases until 400 �C. After 400 �C, the rateincreases and a second peak is observed on the DTGcurve (Tmax=438

�C). Degradation is not observed after480 �C and a char residue of 7% was recorded at 500 �C.Degradation of MMA/DEAEP begins after 200 �C

and at a slower rate than the methacrylate, MMA/DEMEP. The MMA/DEAEP differs from the othercopolymers in that only one peak is observed on the

DTG curve (Tmax=404�C). Also, the degradation is

complete by 480 �C and MMA/DEAEP is the onlycopolymer not to leave any char residue.In contrast, the PMMA modified with phosphorus

containing additives are significantly less thermally stable.The TG and DTG curves for the PMMA containingadditives under nitrogen are given in Fig. 10a and b.Both PMMA containing DEEP and TEP begin to

degrade after just 85 �C. Evidence from dynamic ther-mal probe-time of flight/mass spectrometry experiments[25] would indicate that this is due to the volatilisationof the additive. After 305 �C the rate of degradationincreases, but to a lesser extent than the homopolymer.One major peak is observed on the DTG curve at 357and 371 �C for PMMA+TEP and PMMA+DEEPrespectively. Degradation of both modified polymers iscomplete by 420 �C, resulting in 100% mass loss.

3.2.2. Dynamic mechanical thermal analysis (DMTA)The glass transition temperature of the PMMA,

MMA/DEMMP copolymer and PMMA containing theadditive DEEP has been determined using DMTA. Theresults are given in Table 4.The results show that whilst the DEMMP copolymer

has a glass transition temperature just slightly lowerthan the homopolymer, the PMMA containing DEEPhas a significantly reduced Tg. This implies that anadditive approach to the flame retardance of PMMA

Fig. 10. (a) TGA curve of PMMA containing additives under nitro-

gen. Heating rate=10 �C/min; (b) DTG curve of PMMA containing

additives under nitrogen. Heating rate=10 �C/min.

Fig. 9. (a). TGA curve of MMA copolymers under nitrogen. Heating

rate=10 �C/min; (b) DTG curve of MMA copolymers under nitrogen.

Heating rate=10 �C/min.

232 D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233

has a greater deterioration of the physical and mechanicalproperties of PMMA than does the reactive approach.

4. Conclusions

The results obtained indicate that the physical incor-poration of 3.5 wt.% phosphorus does not significantlyreduce the flammability of PMMA. Only the additiveTEP effectively reduced the rate of heat release duringcombustion, but this was to a much lesser extent thanthe copolymers.All the copolymers tested showed significantly

improved flame retardance as compared to the homo-polymer, PMMA. This implies that the chemical mod-ification of PMMA is a more effective method atreducing the flammability of PMMA than is the physi-cal incorporation of additives. However, the incorpora-tion of phosphorus results in polymers that producesignificantly more smoke and carbon monoxide duringcombustion than does the unmodified PMMA.The results have also shown that a reactive approach

to flame retardance results in copolymers that are morethermally stable than PMMA, unlike the low molecularweight additives that significantly reduce the thermalstability and glass transition temperature of PMMA.The results have also highlighted the importance of

the phosphorus chemical environment in terms of theflame retardance achieved. In both the reactive andadditive approaches to flame retardance, the phosphatespecies were more effective flame-retardants than thephosphonate species. The reason for this is not fullyunderstood at present, as further analysis of the com-bustion gases and chars is required.

Acknowledgements

We thank the Engineering and Physical SciencesResearch Council (EPSRC), the Ministry of Defence(MoD) and ICI for financial support (Grant Nos. GR/L85879 and GR/L85886) and Dr. N.R. Bosley of LGC

(North West), Runcorn, Cheshire, for supplying theTGA/DSC/MS data.

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Table 4

DMTA results

Sample Tg/�C

PMMA 124

MMA/DEMMP 117

PMMA+DEEP ca. 70

D. Price et al. / Polymer Degradation and Stability 77 (2002) 227–233 233