Fire and Polymers IV - American Chemical Society

Chapter 20

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on March 18, 2016 | http://pubs.acs.org Publication Date: November 24, 2005 | doi: 10.1021/bk-2006-0922.ch020

Mechanisms of the Flame Retardant Behavior of Covalently Bonded Phosphorus in Poly(methyl methacrylates) 1,

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Dennis Price *, L. K. Cunliffe , K. J. Bullett (formally Pyrah) , T. R. Hull , G. J. Milnes , J. R. Ebdon , B . J. Hunt , and P. Joseph 1

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Centre for Materials Research and Innovation, Bolton Institute, Deane Road, Bolton BL3 5AB, England Institute for Materials Research, Cockcroft Building, University of Salford, Salford M5 4 W T , England Chemistry Department, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, England 2

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Flammability studies concerned with the fire retardance of an additive fire retardant system of poly(methyl methacrylate), P M M A , containing triethyl phosphate (TEP) and two reactive systems of methyl methacrylate and diethyl 2(methacryloyloxy)ethylphosphate (DEMEP) and diethyl 2-(acryloyloxy)ethylphosphate (DEAEP) copolymers have already been reported. These studies showed improvements in the fire retardancy when the phosphate group was incorporated into the polymer. The purpose of this work is to establish the different modes of action of the additive and reactive fire retardants and to identify the causes of the different behaviours. A combination of T G with E G A , DSC, laser and microfurnace pyrolysis mass spectrometry and isothermal pyrolysis G C - M S were used for these studies. The greater extent of the condensed phase interactions shown by the M M A / D E A E P case explains why that system has superior flame retarding ability than does the M M A / D E M E P system.

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© 2006 American Chemical Society

Wilkie and Nelson; Fire and Polymers IV ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction We have previously reported our extensive flammability studies of a range of poly(methyl methacrylate) based copolymers in which phosphorus is covalently bonded to the polymer chain (1,2). The advantages of this 'reactive' system over the more common 'additive' approach have been discussed elsewhere (3). This can be seen from the decrease in peak rate of heat release for the PMMA polymer, 633 kWm' , to 500 kW m" for the additive system PMMA+TEP, to 360 kWm" for the copolymer MMA-DEMEP and to 360 kWm' for the copolymer MMA-DEAEP. The limiting oxygen indices increase in the same order, i.e. 17.2% for PMMA, 22.7% for PMMA+TEP, to 25.0 % for MMA-DEMEP and to 28.1% for MMA-DEAEP. Current studies are concerned with gaining an insight into the significance of the chemical nature of the phosphorus-containing comonomer on the flame retardant mechanism of the resultant copolymer. This paper reports a comparison of the behaviours of acrylate (DEAEP) and methacrylate phosphate (DEMEP) comonomers. In particular, why does the MMA-DEAEP show superior flame retardant behaviour compared to that of MMA-DEMEP? A variety of techniques have been used for these studies. The laser pyrolysis technique (4) provides information as to the initial breakdown of a polymer over the first few milliseconds after its surface is exposed to a very rapid temperature rise as would be the effect of radiation from a fire. The micro-furnace experiment (5) identifies the species evolved as the polymer temperature is raised in a controlled manner. Pyrolysis/GC-MS (6) data can provide insight into the thermal breakdown in these systems and hence the mechanisms of the flame retarding processes. Table 1 provides a comparison of the different reaction conditions generated by these three techniques. Nonisothermal thermogravimetric (TG) studies identify the temperature ranges of the various polymer decomposition steps and the extent of char formation whilst addition of a suitable evolved gas analysis (EGA) technique enables the gas phase products to be monitored. Differential scanning calorimetry (DSC) alongside TG data, can be used to identify any changes occurring within the condensed phase. Information from our current studies will illustrate the value of this combination of techniques.


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Experimental The laser pyrolysis- and microfurnace- mass spectrometry techniques have been described previously (4,5). Pyrolysis in air experiments were conducted at 400°C for 60s in a Wilks pyrolyser (6) with the evolved products subsequently being analysed using a VG Trio-1 GC/MS. A recent innovation is the addition of an infrared analyser, utilising characteristic wavelength specific filters, to monitor the exhaust gas line from a Polymer Laboratories TG 750. This provides continuous profiles of the CO, C 0 and hydrocarbons (measured as propane + hexane) evolved during a TG experiment. 2



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Table 1: Reaction time-scale of techniques Reaction Period Initial 2 ms

Technique LP/TOFMS

Heating Rate lk°C in 0.5 ms

MicrofurnaceTOFMS

10°C min"

40 min

Py/GC-MS

25—400 °C in about Is

6 0 s ® 400 °C

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Comments Dynamic Vacuum Primary reactions monitored Dynamic Vacuum Temperature dependence of primary products Air End product analysis


255 Materials. The methyl methacrylate/diethyl 2-(acryloyloxy)ethyl phosphate (MMA/DEAEP) and methyl methacrylate/diethyl 2-(methacryloyloxy)ethyl phosphate (MMA/DEMEP) copolymers were synthesised as previously described (1,2). Their structures and that of the additive triethyl phosphate are shown in figure 1. All of the systems contained 3.5wt% of phosphorus.


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diethyl-2-(acryloyloxy)ethylphosphate (DEAEP)

Diethyl 2- (methacryloyloxy)ethyl phosphate (DEMEP)

Figure 1. Structures of comonomers and additive used in this work

Results and Discussion MMA/DEAEP. Two typical time-of-flight mass spectrometric scans from a laser pyrolysis experiment are shown in figure 2. The peaks due to the MM A and DEAEP portions of the copolymer are indicated on the figure. It can be seen that peaks due to fragments from the reactive flame retardant, DEAEP, and for the MMA monomer appear shortly after the laser has been fired. The probable breakdown patterns of the DEAEP component, shown in figure 3, indicate that in a real fire situation, phosphorus-containing fragments are evolved concurrently with the MMA, i.e. the 'fuel', from the copolymer. This would optimise the efficiency of delivery of the flame retardant species into the flame region.


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MMA/DEAEP - scans after laser firing 9

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m/z Figure 2. MMA/DEAEP: mass spectra taken 300 and 750 ps after firing of the laser.

MMA/DEAEP 9«3

Fire and Polymers IV - American Chemical Society

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