Thermal Degradation and Decomposition Products of Electronic

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Thermal Degradation and Decomposition Products of Electronic Boards Containing BFRs Federica Barontini,† Katia Marsanich,‡ Luigi Petarca,‡ and Valerio Cozzani*,§ Gruppo Nazionale per la Difesa dai Rischi Chimico-Industriali ed Ecologici, Consiglio Nazionale delle Ricerche, via Diotisalvi n.2, 56126 Pisa, Italy, Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universita` di Pisa, via Diotisalvi n.2, 56126 Pisa, Italy, and Dipartimento di Ingegneria Chimica, Mineraria e delle Tecnologie Ambientali, Universita` di Bologna, viale Risorgimento n.2, 40136 Bologna, Italy

Production of electronic boards containing brominated flame retardants is constantly increasing, posing important problems with disposal of products containing these materials. The present study investigated the thermal degradation behavior of electronic boards manufactured using tetrabromobisphenol A and diglycidyl ether of bisphenol A epoxy resins. Qualitative and quantitative information was obtained on the products formed in the thermal degradation process, and the bromine distribution in the different product fractions was determined. The more important decomposition products included hydrogen bromide, phenol, polybrominated phenols, and polybrominated bisphenol A species. The formation of considerable amounts of hydrogen bromide and high-molecular-weight organobrominated compounds, as well as the potential formation of limited quantities of polybrominated dibenzo-p-dioxins and dibenzofurans, is an important element of concern in the safety and environmental assessment of the thermal degradation processes of electronic boards containing brominated flame retardants. Introduction Epoxy resins based on bisphenol A (BA) and diglycidyl ether of bisphenol A (DGEBA) are high quality materials, and are widely used in advanced technological applications. To limit the possible hazards deriving from the scarce fire resistance and the inherent flammability of these materials, flame retardants are usually added in their formulation. In particular, tetrabromobisphenol A (TBBA), a widely used brominated flame retardant (BFR), is substituted to bisphenol A in the epoxy resins to obtain high-fire-resistance materials suitable for use in the electronics industry. Recent estimates show that the production of electronic boards based on TBBA/ DGEBA epoxy resins was about 3.3‚105 t/year (232‚106 m2/year) in 1999, and is growing constantly. The increasing use of electronic boards in a number of household products, and the wide spread of shortlife electronic products, such as PCs and mobile phones, are responsible for the increase in the production, but also in the disposal, of electronic boards, which may contain up to 15 wt % of bromine. A prudent estimate, based on several sources,1-6 indicates that presently, worldwide, more than 105 t/year of electronic boards should be disposed of. A 5-10% increase by year1-3 of these figures is expected. The constant increase of electronic scrap containing relevant quantities of BFRs causes growing concerns with respect to the possible environmental impact originated by the disposal of these products using conventional incineration technologies or by the effects of accidental fires. As a matter of fact, several studies evidenced the possible formation * To whom correspondence should be addressed. Tel. (+39)051-2093141. Fax (+39)-051-581200. E-mail: valerio.cozzani@ mail.ing.unibo.it. † Consiglio Nazionale delle Ricerche. ‡ Universita` di Pisa. § Universita` di Bologna.

of high-molecular-weight brominated products in the combustion of BFRs, as brominated phenols,7-12 and limited amounts of polybrominated dibenzo-p-dioxins (PBDD) and dibenzofurans (PBDF).8,13-29 Many investigations have been devoted to analyzing the pyrolysis and combustion products of electronic scrap,11,30-35 as well as of cured epoxy resins.35-37 Although a wide agreement exists in the literature concerning the formation of hydrogen bromide and several brominated organic products in the thermal degradation of these materials,11,30-37 a general understanding of the thermal degradation process of TBBA/DGEBA epoxy resins is still lacking. Data are present in the literature on the products formed in the thermal decomposition of electronic scrap mixtures38-41 or of waste mixtures containing additional other plastic wastes.41 Scarce information is reported on the fundamental pathways of thermal decomposition or on the distribution of bromine during the thermal degradation process of these products. The present study focused on the thermal degradation patterns of electronic boards based on TBBA/ DGEBA epoxy resins. The thermal decomposition of these materials at moderate heating rates (10 °C/min) was studied using thermogravimetric analyzers and a laboratory-scale fixed-bed reactor both in inert and oxidizing atmospheres. The results were compared to those of uncured TBBA/DGEBA polymers and the monomers, TBBA and DGEBA, and to literature data reported for the cured resins. Qualitative and quantitative information was obtained on the products formed in the thermal degradation process, and the bromine distribution among the different product fractions was determined. Experimental Section Materials. Printed circuit board base materials were obtained from local manufacturers. Three different

10.1021/ie048766l CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4187 Table 1. Results of the Proximate and Ultimate Analysis Performed on the Electronic Boards Investigated in the Present Study (Ultimate Analysis Carried Out on the Metal-Free Samples) Board A

Board B

Board C

Proximate Analysis (wt %) Organic Fraction 29.0 36.2 3.1 8.4 32.1 44.6

volatiles fixed carbon total metallic inerts nonmetallic inerts total C H N Br

Inert Fraction 16.0 51.9 67.9

19.8 3.5 23.3

0 55.4 55.4

44.6 32.1 76.7

Ultimate Analysis (wt %) 22.1 27.4 2.4 2.3 0.8 1.7 6.0 6.9

22.1 2.0 0.6 6.7

electronic boards were selected for the experimental investigations performed in the present work. These materials, which will be denoted as boards A, B, and C in the following discussions, constitute a cross-linked organic matrix (brominated epoxy resins) on a support of glass fibers, covered by thin metal layers. Table 1 reports the results of the proximate analysis of the three boards. The metallic layers were removed by treatment with a dilute aqueous solution of hydrochloric acid and hydrogen peroxide, followed by washing with deionized water and drying at 110 °C. The metal-free materials were milled in liquid nitrogen. All the experimental analyses performed in the present study were carried out on the milled metal-free samples. Table 1 reports the results of the CHN elemental analysis performed on the samples. These samples were analyzed to determine their bromine content as well, and the results obtained are also included in Table 1. Linear brominated epoxy resins are widely used to impregnate the fiber layer in the manufacture of printed circuit boards. During this step, in some formulations the linear brominated resins are blended with limited amounts of phenolic resins. The linear resins then undergo a cross-linking process, in which amines are used as cross-linking agents. To better understand the thermal degradation process of printed circuit boards, several experimental runs were carried out also on linear brominated epoxy resins. These were prepared by reaction of diglycidyl ether of bisphenol A (DGEBA, supplied by Shell) with tetrabromobisphenol A (TBBA, supplied by Aldrich), using the procedures described in detail elsewhere.42 In the following discussions, the brominated epoxy resins will be identified by the DGEBA/TBBA molar ratio. Techniques and Procedures. Simultaneous thermogravimetric (TG) and differential scanning calorimetry (DSC) data were obtained using a Netzsch STA 409/C thermoanalyzer. A constant heating rate of 10 °C/ min from ambient temperature to 800 °C was used in experimental runs. Typical sample weights of 15-50 mg were employed. Runs were carried out using a purge gas flow (60 mL/min) of pure nitrogen or air. Isothermal runs at temperatures between 250 and 400 °C were performed using a Mettler TG-50 thermobalance. A pure nitrogen purge flow of 200 mL/min was used in experimental runs. The samples were positioned on the pan of the TG balance and were inserted in the

TG furnace preheated at the programmed temperature. Temperature transients due to sample heating were calculated accounting for convective and conductive heat transfer inside the cylindrical furnace, and resulted less than 30 s at temperatures between 200 and 400 °C.43 FTIR measurements were carried out using a Bruker Equinox 55 spectrometer equipped with DTGS and MCT detectors. TG-FTIR simultaneous measurements for the on-line analysis of volatile compounds formed during TG runs were carried out coupling the FTIR spectrometer to the Netzsch TG by a 4-mm i.d. Teflon tube. The 800mm long transfer line and the head of the TG balance were heated at a constant temperature of 200 °C to limit the condensation of volatile decomposition products. FTIR measurements were carried out with a MCT detector in a specifically developed low volume gas cell (8.7 mL) with a 123-mm path length, heated at a constant temperature of 250 °C. The gas flow from the TG outlet to the IR gas cell was 60 mL and a residence time of 30 s in the transfer line could be evaluated for the evolved gases. This value was assumed as the time delay correction to be used for the comparison of TG and IR results. During TG-FTIR runs, spectra were collected at 4 cm-1 resolution, co-adding 16 scans per spectrum. This resulted in a temporal resolution of 9.5 s, more than sufficient to follow the gas evolution rates characteristic of TG runs at heating rates of 10 °C/min. Qualitative gas evolution profiles and quantities of gaseous decomposition products formed in TG-FTIR runs were obtained from the experimental data following the procedures described in previous publications.44,45 A laboratory-scale fixed-bed tubular batch reactor (BR) was used to carry out thermal decomposition runs. The experiments were mainly aimed at the recovery and characterization of the different fractions of decomposition products. Constant heating rates of 10 °C/min were used. Typical sample weights in the experimental runs were between 150 and 400 mg. Experimental runs were performed using a purge gas flow (80 mL/min) of pure nitrogen or air to control the reaction environment and to limit the extension of secondary gas-phase reactions. Volatile products evolved during thermal degradation were transferred by the nitrogen flow in a series of cold traps, maintained at -20 °C by a sodium chloride brine/ ice bath. Condensable products were recovered at the end of the run from the cold traps for chromatographic analysis. The traps were followed by a gas sampling cell for on-line FTIR gas analysis. Further details and a scheme of the experimental apparatus are reported elsewhere.46 A Fisons MD 800 quadrupole mass spectrometer interfaced to a Fisons GC 8060 gas chromatograph was used for gas chromatography/mass spectrometry (GC/ MS) analysis. A Mega SE30 fused-silica capillary column (25-m length, 0.32-mm i. d., crossbonded, 0.25-µm film thickness) was employed for the chromatographic separation, with helium as carrier gas. The column temperature program was the following: 5 min isothermal at 40 °C, heating to 250 °C (6 °C/min), then 20 min isothermal. Splitless injection with the injector at 250 °C was used. Mass spectrometric detection was performed in full scan conditions (scan range m/z 10-819) in electron impact ionization mode. Quantitative GC analysis was carried out using a ThermoQuest Trace GC 2000 gas chromatograph equipped with a flame ionization detector (FID). The

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Figure 1. Results of TG-DSC runs performed on electronic boards in 100% nitrogen (10 °C/min): (a) TG data, (b) dTG data, and (c) DSC data.

Figure 2. Results of TG-DSC runs performed on electronic boards in air (10 °C/min): (a) TG data, (b) dTG data, and (c) DSC data.

capillary column and the experimental conditions were identical to those used for GC/MS analysis; the detector temperature was fixed at 280 °C. More details on GC quantitative determinations are reported in previous studies.12,46

Figure 1b, which reports the weight loss rate data, shows that almost negligible differences are present in the temperature range of the thermal degradation process of the different samples. These data confirm that a common cross-linked organic matrix is present in the electronic boards obtained from different manufacturers. The results obtained are in good agreement with the data reported in previous studies, which suggest that the thermal degradation process of the organic components of electronic scrap at low heating rates takes place at temperatures between 280 and 380 °C.30,33,34 To investigate the influence of oxidizing conditions on the thermal stability of the samples, TG/DSC constant heating rate runs were performed using air as the purge gas. The results obtained are reported in Figure 2. In the presence of air, after the first decomposition step, a second weight loss step is detected at higher temperatures, probably due to a combustion process. To better compare the results obtained in inert and oxidizing atmospheres, the weight loss data normalized with respect to the organic fraction present in the samples (evaluated as the difference between the initial weight and the residual weight at 800 °C in air), are reported

Results and Discussion Thermal Stability of Electronic Boards The results of constant heating rate runs performed on the three electronic board samples in nitrogen atmosphere using the simultaneous TG/DSC analyzer are shown in Figure 1. The data reported in this and in the following figures were calculated as the mean of at least 3 experimental runs. Differences in the weight loss or in the heat flow with respect to temperature in the different runs were less than 2%. Figure 1a shows that at a heating rate of 10 °C/min the thermal degradation of electronic boards takes place mainly between 280 and 350 °C. A residue of about 65 wt % was obtained at 800 °C. The figure also provides evidence that limited differences are present in the extent of weight loss between the different materials.

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Figure 3. Comparison of the results of TG runs carried out on electronic boards in inert and oxidizing atmospheres: weight loss data normalized with respect to the organic fraction present in the samples.

in Figure 3. These data confirm the similar behavior of the organic fraction of the three samples analyzed. Furthermore, the weight loss process starts at similar temperatures in inert and oxidizing environments. However, in the presence of oxygen a higher residue results from the first decomposition step. A higher thermal stability in air at temperatures between 250 and 400 °C was reported in the literature for several materials (e.g., diisocyanates, polyurethanes, etc.), and was attributed to the occurrence of cross-linking and condensation radical reactions during the thermal decomposition.47,48 The organic fraction of the electronic boards is usually obtained from the cross-linking of brominated epoxy resins. Thus, the thermal stability of these materials was also compared to that of the linear brominated resins used for the manufacture of electronic boards. Figure 4a shows the results of constant heating rate runs performed in nitrogen atmosphere on brominated epoxy resins having different reactant molar ratios. As reported in the Experimental Section, the resins are identified by the DGEBA/TBBA ratio. The results in Figure 4a evidence a one-step degradation process for resin 1, while two steps may be detected in the degradation of resins having a molar excess of DGEBA. The first of these steps is possibly ascribed to the degradation of the unconverted DGEBA present in the material. The second step (the only step present in the degradation of resin 1) is possibly caused by the degradation of the linear brominated polymer. This assumption will be confirmed by the results of the simultaneous FTIR analysis of evolved gases, reported in the following. The temperature range of the second degradation step (320400 °C at a heating rate of 10 °C/min) is slightly higher than that of the cross-linked resin, as evident from the comparison of Figures 1 and 4. This is possibly due to the catalytic role of the inorganic components of the electronic boards, present during the decomposition of the cross-linked resin, as well as to the blending of the cross-linked resin with less stable materials. As a matter of fact, higher decomposition temperatures than those obtained in the present study were reported in the literature for cured brominated epoxy resins, namely

Figure 4. Results of TG runs performed on linear brominated DGEBA/TBBA epoxy resins (10 °C/min): (a) comparison of runs carried out in 100% nitrogen on epoxy resins having different reactant molar ratios, and (b) comparison of runs carried out on the DGEBA/TBBA 1.85 epoxy resin in inert and oxidizing atmospheres.

320-350 °C at a heating rate of 10 °C/min.37,49 Other sources report lower decomposition temperature ranges, but they were obtained using significantly lower heating rates: 300-350 °C at a heating rate of 3.7 °C/min,50 and 260-320 °C at a heating rate of 1 °C/min.51 TG/DSC constant heating rate runs were performed in oxidizing atmosphere as well. The TG data recorded in nitrogen and in air for the resin 1.85 are compared in Figure 4b. The main decomposition stage is similar for both materials, and very similar values were obtained for the decomposition temperatures and for the extent of volatile loss in the different reaction environments up to 450 °C, where the sample experienced a weight loss around 70% both in nitrogen and in air. However, in the presence of oxygen a further weight loss step occurred at higher temperatures (500-600 °C) due to the oxidation of the solid residue formed in the first decomposition stage. On the other hand, only a slight weight loss was experienced at the correspondent temperatures in nitrogen atmosphere, and at the end of the

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Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 Table 2. Kinetic Parameters Obtained for the Thermal Decomposition Process of Electronic Boards and Linear Brominated Epoxy Resins sample

A (s-1)

ξ0

electronic board B 1.2 × 10-2 3.4 × 1011 brominated epoxy resin 6.0 × 10-4 3.3 × 1012 DGEBA/TBBA 1.85 brominated epoxy resin 5.0 × 10-3 1.1 × 1011 DGEBA/TBBA 2.75

Ea T24 (kJ/mol) (°C) 146.3 166.2

204 247

156.1

254

The following kinetic expression was derived for the conversion rate:

dξ ) K(T)‚ξ‚(1 - ξ) ) A‚e(-Ea/RT)‚ξ‚(1 - ξ) dt

(1)

where t is time, T is the temperature, K is an apparent kinetic constant, A is the preexponential factor, Ea is the activation energy, and R is the gas constant. The sample conversion, ξ, was defined on the basis of experimental TG data:

ξ)

Figure 5. Results of isothermal TG runs performed on electronic board B (100% nitrogen): (a) weight loss data, and (b) conversion. Dots, experimental data; lines, kinetic model predictions.

run a black solid residue was obtained that was not observed at the end of runs performed in air. Apparent Kinetics of the Decomposition Process. To better characterize the thermal stability and the decomposition pattern at low heating rates of the organic fraction of the electronic boards, a simplified kinetic analysis was undertaken. The results of isothermal TG runs performed on sample B at temperatures ranging between 260 and 300 °C are reported in Figure 5. Similar behaviors were observed for samples A and C. The results in Figure 5 evidence that the degradation process has an autoaccelerating behavior, in accordance with the results obtained by Bremmer for the thermal decomposition of cured brominated epoxy resins.50 A slight decrease in the residual weight at the end of the isothermal runs at temperatures higher than 290 °C (less than 4%) possibly indicates the limited occurrence of condensation and aromatization reactions that contribute to the charring process. However, up to about 300 °C the decomposition process may be reasonably approximated by a single-step first-order autocatalytic kinetic model, lumping the complex decomposition pattern of the cross-linked resins to a single-step reaction:

resin f volatiles + char Although this lumping procedure results in an oversimplified kinetic model, the results are useful at least for a quantitative comparison of the thermal stability of these materials at the low heating rates used in experimental runs, since similar models were used in the literature to describe the thermal decomposition process of similar materials.12,41,44

W0 - W W0 - Wf

(2)

where W is the sample weight, W0 is the sample initial weight, and Wf is the weight at the end of the TG run, except for temperatures lower than 270 °C, for which the residual fraction of the 270 °C run was assumed as Wf. Integration of eq 1 yields the following expression for the conversion as a function of time at constant temperature:

ξ(t) )

ξ0‚eKt 1 - ξ0(1 - eKt)

(3)

where ξ0 represents the initial conversion. The value of the kinetic constant was obtained from the experimental data using the following expression, derived from the integration of eq 1:

( )

ξ 1-ξ 1 K(T) ) ‚ln t ξ0 1 - ξ0

(4)

where t is the experimental value of time at which sample conversion ξ is accomplished. The values of K calculated from experimental data at ξ ) 0.5 were used for kinetic evaluations. The apparent kinetic parameters and ξ0 were thus estimated by a best-fit procedure. Table 2 reports the values of the kinetic parameters calculated for sample B, while in Figure 5b model predictions are compared with experimental conversion values obtained from TG data. The good accordance shown in Figure 5b by the experimental data and the model results confirm that the simplified kinetic model used is adequate to represent the main step of the actual decomposition process in the experimental conditions of the present investigation. Similar values of the kinetic parameters ((2%) were obtained for the other electronic board samples. The value calculated for the activation energy in the present study, 146.3 kJ/mol, is quite similar to the apparent activation energies reported by Bockhorn et al.41 for the evolution of bisphenol A and styrene during


Figure 6. Results of isothermal TG runs performed on the DGEBA/TBBA 2.75 epoxy resin (100% nitrogen): (a) weight loss data, and (b) conversion during the autoaccelerating decomposition step. Dots, experimental data; lines, kinetic model predictions.

the thermal decomposition of electronic scrap, which ranged around 140 kJ/mol, and is lower than that reported for the thermal degradation of TBBA in similar experimental conditions (179 kJ/mol).12 It is interesting to compare these values with those obtained for the degradation of the linear brominated resin before the cross-linking process. Figure 6 shows the results of isothermal runs in pure nitrogen obtained for the 2.75 epoxy resin. As in the case of constant heating rate runs, these results show the presence of a first weight loss step, responsible for a 25% weight loss, possibly due to the evaporation and thermal degradation of unconverted DGEBA. The main weight loss process, responsible for a 55% weight loss, starts immediately at the end of the first weight loss step, and evidences an autoaccelerating behavior. Qualitatively similar results were achieved for resins 1.85 and 4, while the first weight loss step was absent in the experimental runs carried out on resin 1. Thus, the autoaccelerating step should be linked to the decomposition of the brominated fraction. The same kinetic analysis discussed above for the decomposition process of the crosslinked resins was applied to the autoaccelerating main weight loss step of linear epoxy resins in order to calculate the values of the kinetic parameters. In this case W0 in eq 2 represents the weight at the beginning of the second step. Table 2 reports the results obtained for the 1.85 and 2.75 resins, while Figure 6b shows the model predictions compared with experimental conversion values for the 2.75 resin. The results reported in

Table 2 confirm the higher thermal stability of brominated epoxy resins compared with the cross-linked resins, also evident from the analysis of Figures 1 and 4. However, it must be remarked that the results shown in Figure 6 were obtained for the pure linear resins, while the thermal degradation experiments in Figures 1 and 5 were carried out on electronic boards. Thus, the presence of inorganic components as well as the final formulation of electronic boards may play a role in the overall thermal stability of these materials. As a matter of fact, literature data reveal that higher decomposition temperatures are experienced for cured brominated epoxy resins37,49,50 with respect to electronic scrap materials.30,33,34 Thermal Effects of the Decomposition Process. The DSC data reported in Figure 1c show that the thermal degradation of the electronic boards is an exothermic process. The estimation of the heat of decomposition was possible by baseline subtraction and integration of the heat flow curves. The results obtained for the three samples are reported in Table 3. Also, in the case of the linear brominated resins the thermal degradation process resulted exothermic. The values of the overall heat developed in the decomposition process, estimated from the analysis of the DSC curves, are shown in Table 3. The table evidences that for all these materials the overall decomposition heat is greatly influenced by the extent of the first weight loss step: the overall decomposition heat increases from 30 to 145 kJ/kg as the weight loss in the first step increases from 0 (resin 1) to 35% (resin 4). The results obtained in the present investigation allowed the estimation of the time to maximum rate (TMR), that is the time required to attain the maximum degradation rate in adiabatic conditions starting from a reference temperature.52 The temperature at which a time to maximum rate value of 24 h is obtained (defined as T24 in the following) is a reference value for the safe processing temperatures of a material. The T24 values obtained for the electronic boards range around 200 °C, as shown in Table 2, while higher values (about 250 °C) were found for the linear resins, even if these materials evidence higher values of the overall heat generated in the decomposition process. Low-Molecular-Weight Volatile Thermal Decomposition Products. TG-FTIR simultaneous measurements allowed for the characterization of low-molecularweight volatile products generated during TG/DSC runs. Figure 7 shows the typical results of the FTIR on-line analysis of the volatiles evolved during a TG run performed in pure nitrogen on sample B. The FTIR spectra collected are reported as a function of the sample temperature in the TG furnace. Similar results were obtained for samples A and C. In accordance with the TG data, the formation of volatile products mainly takes place between 280 and 350 °C. Only the formation of N,N-dimethylformamide was detected at lower temperatures (150-200 °C) for the B and C samples. The emission of dimethylformamide from printed circuit boards during low-temperature tests (100 °C) was also reported by Wolf et al.,40 and might be caused by the use of this compound as a solvent in the formulation of the resins. In the 280-350 °C temperature range, the FTIR spectra evidenced the evolution of a complex mixture of products during electronic board degradation in inert atmosphere. The presence of low-molecular-weight com-

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Table 3. Decomposition Temperature Range and Heat Formed in the Decomposition of Electronic Board and Brominated Epoxy Resin Samples Evaluated from TG-DSC Data (10 °C/min, 100% nitrogen)

sample electronic board A electronic board B electronic board C brominated epoxy resin DGEBA/TBBA 1 brominated epoxy resin DGEBA/TBBA 1.85 brominated epoxy resin DGEBA/TBBA 2.75 brominated epoxy resin DGEBA/TBBA 4

decomposition temperature range (°C)

heat generated in the decomposition (J/g)

heat generated in the decomposition (J/g organic fraction)

280-350 280-350 280-350 310-370

44 58 41 30

130 162 133 30

260-400

90

90

260-400

125

125

260-400

145

145

pounds such as water, carbon monoxide, carbon dioxide, ammonia, and hydrogen bromide was detected. Qualitative emission profiles as a function of temperature were obtained for the gaseous compounds evolved using the FTIR data analysis procedure described in earlier works.44,45 Figure 8 shows the emission profiles of carbon monoxide, carbon dioxide, ammonia, and hydrogen bromide during the thermal degradation in pure nitrogen of sample B. Hydrogen bromide was formed from all the samples analyzed, and in the same temperature range. The hydrogen bromide emission is due to the presence of brominated flame retardants in the electronic boards investigated. TG-FTIR analyses were performed in oxidizing environment as well, using air as a purge gas during the experimental runs. No relevant differences were observed in the volatile products evolved during the main degradation step (280-350 °C) with respect to experimental runs performed in inert atmosphere. Figure 9 shows a comparison between the qualitative concentration profile of hydrogen bromide and the differential thermogravimetric (dTG) data recorded for sample A in nitrogen and in air. In both runs, hydrogen bromide concentration in the gas outflow from the TG analyzer showed a single peak, corresponding with the maximum of the sample weight loss rate. Similar results were obtained for samples B and C. On the other hand, as shown in Figure 10, the evolution of carbon monoxide and carbon dioxide was mainly detected during the second step of the thermal degradation of electronic boards in air (350-550 °C). This confirms that the second weight loss step during the thermal decomposition in air is mainly due to the oxidation of the residual char formed at lower temperatures (during the main thermal degradation process identified by the first weight loss step).

The FTIR data also evidenced the formation of highermolecular-weight organic compounds during the first weight loss step, both in inert and oxidizing environments. However, the simultaneous evolution of several volatile compounds having similar chemical structure did not allow the identification of the single species by the analysis of the FTIR spectra. Detailed information about the organic substances formed was thus obtained from GC/MS analysis of the degradation product fraction recovered at the end of runs performed using the fixed-bed tubular batch reactor (BR), and will be discussed in the following section. However, the BR runs also provided information on low-molecular-weight volatile products formed during the thermal degradation process. Constant heating rate (10 °C/min) BR runs up to a final temperature of 350 °C were performed. Experiments were carried out either in inert or oxidizing atmosphere. The FTIR on-line analysis of the gaseous products evolved confirmed the emission of ammonia, carbon monoxide, carbon dioxide, and hydrogen bromide during the decomposition process, both in inert and oxidizing environment. Following the same methodology used for the analysis of TG-FTIR data, qualitative emission profiles for the gaseous products identified were obtained as a function of sample temperature in BR runs. The emission temperatures were found to agree with those recorded in TG-FTIR runs. Methane was also detected in the gaseous fraction of decomposition products generated during BR experimental runs, although it could not be clearly identified in TG-FTIR runs due to the overlapping absorption bands of higher molecular weight organic compounds, which complicated the interpretation of the spectra recorded in the experimental runs. It must be recalled that in the BR runs, the gas flows through surface condensers at -20 °C before entering the FTIR measurement cell, thus

Figure 7. IR spectra collected during a TG-FTIR run carried out on electronic board B (10 °C/min, 100% nitrogen).


Figure 9. Results of TG-FTIR runs performed on electronic board A in inert and oxidizing atmospheres (10 °C/min): (a) weight loss rate data, (b) hydrogen bromide emission profiles. Continuous lines, 100% nitrogen; dotted lines, air.

Figure 8. Results of a TG-FTIR run performed on electronic board B (10 °C/min, 100% nitrogen): (a) weight loss, (b) weight loss rate, and (c) specific gas profiles of selected gaseous compounds formed in the decomposition process.

only high volatility compounds enter the gas sampling cell. It is interesting to compare the results obtained with the data collected during the TG-FTIR runs performed on the linear brominated resins using the same experimental conditions. The low-molecular-weight compounds identified from TG-FTIR analysis were hydrogen bromide, carbon dioxide, carbon monoxide, and methane. Ammonia was not detected among the decomposition products, thus suggesting that its formation in the thermal degradation of the cross-linked resin is due to the decomposition of the cross-linking agent (as previously mentioned, amines are often used in the crosslinking process of epoxy resins). Figure 11 shows the results obtained for the emission profiles of carbon monoxide, carbon dioxide, and hydrogen bromide during a TG-FTIR run carried out on the 1.85 resin in 100% nitrogen. Two emission peaks could be clearly detected from the analysis of FTIR results, corresponding to the two decomposition steps evidenced by the TG analysis. Carbon monoxide, carbon dioxide, and methane were clearly identified among the gaseous products evolved in both stages. On the other hand, hydrogen bromide emission was detected only in the second peak. Qualitatively similar results were obtained

from the TG-FTIR runs carried out on resins 2.75 and 4, while in the thermal decomposition of resin 1 a single emission peak was observed, and the presence of carbon oxides and hydrogen bromide was detected. This confirms that the decomposition of the linear brominated resin takes place only during the second main weight loss step. The first weight loss step is due to the degradation of nonbrominated compounds, and most likely of the excess of DGEBA present in the linear resin formulation. No significant differences were detected in the volatile products evolved in experimental runs carried out in oxidizing atmosphere. The further weight loss step in the 500-600 °C temperature range resulted in the emission of carbon monoxide and carbon dioxide, thus indicating an oxidation process of the primary residue. The results of BR runs confirmed the nature of the gaseous products formed in the degradation process and the relative emission temperatures, both in inert and oxidizing environment. A complex mixture of organic compounds was also generated in the thermal degradation of epoxy resins, however also in this case FTIR spectra analysis did not allow us to identify the single chemical species present in the evolved gases. Higher-Molecular-Weight Condensable Decomposition Products. The identification of highmolecular-weight products formed in the thermal decomposition of electronic boards was carried out by GC/ MS analysis of the condensate recovered at the end of runs performed using the fixed-bed tubular batch reactor (BR). The GC/MS analyses revealed the formation of a large number of products. Not all the detected species could be identified, but a structural assignment was possible for most of the compounds and for all the main chromatographic peaks. The identification was achieved by the analysis of mass spectra, by comparison

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Figure 11. Results of a TG-FTIR run performed on the DGEBA/ TBBA 1.85 resin (10 °C/min, 100% nitrogen): (a) weight loss and weight loss rate data, and (b) specific gas profiles of selected gaseous compounds evolved.

Figure 10. Results of a TG-DSC-FTIR run performed on electronic board A (10 °C/min, air): (a) dTG data, (b) DSC data, and (c) carbon oxides emission profiles.

with the best fits found in the NIST spectral library, and by comparison with published MS data,33,46 or by the use of standards. The molecular weight and the number of bromine atoms present in the molecule could be identified for all the products from the analysis of mass spectra. Table 4 lists the compounds identified in the pyrolysis of sample B. Similar results were obtained for the analysis of condensable products formed in BR runs performed on samples A and C. Moreover, almost negligible differences were found for the condensable products formed in BR experimental runs performed in oxidizing atmosphere, thus confirming that the main thermal degradation step is not greatly affected by the presence of air, as found for other brominated organic materials.7,9,12,44 The data in Table 4 indicate the formation of several phenolic species and of bisphenol A derivatives, either brominated or not: e.g., phenol, isopropylphenol, and other alkyl-substituted phenols, bisphenol A, bromo- and dibromophenols, and bromo-, dibromo-, and tribromobisphenol A. These products may well originate from the bisphenol A based epoxy resin. The source of brominated compounds should be the brominated bisphenol A component of the resin (TBBA). For the sake of comparison, Table 4 reports as well the compounds identified in the pyrolysis of resin 1.85 performed in the BR using the same experimental conditions. Constant heating rate (10 °C/min) BR runs up to a final temperature of 450 °C were performed. Similar results were obtained for all the brominated

resins investigated in the present work. Also in this case, no relevant differences were detected in experimental runs performed in oxidizing atmosphere. The analysis of data reported in Table 4 evidences that a relevant correspondence is present between the decomposition products of electronic boards and linear epoxy resins. To get some insights on the formation of decomposition products, thermal degradation runs were performed in the same experimental conditions on pure DGEBA. Main decomposition products resulted in alkylbenzenes, phenol, alkylphenols (methylphenols, dimethylphenols, ethylphenols, propylphenol, isopropylphenols, and isopropylmethylphenol), 2-methyl-2,3-dihydrobenzofuran, 4-(1-methyl-1-phenylethyl)phenol, and bisphenol A. Thus, the analysis of Table 4 confirms that most of the nonbrominated compounds identified in the pyrolysis of electronic board and epoxy resin samples seem to be originated from the decomposition of the DGEBA present in the resin. On the other hand, previous investigations carried out on TBBA in similar experimental conditions had shown that the pyrolysis of pure TBBA yields phenol, bromophenols, dibromophenols, tribromophenol, bisphenol A, and brominated bisphenol A species.12 Thus, the results reported in Table 4 suggest that most of the brominated products formed in the decomposition of the materials investigated in the present study originate from the TBBA component linked in the polymer backbone. However, tribromophenol was not detected among the decomposition products of electronic boards and linear brominated epoxy resins. On the other hand, brominated compounds such as 1-bromo-3-phenoxy-2propanol and 1,3-dibromo-2-propanol were identified in

Ind. Eng. Chem. Res., Vol. 44, No. 12, 2005 4195 Table 4. Products Identified by GC/MS Analysis in the Condensable Fraction Recovered from BR Thermal Degradation Runs Carried Out on Electronic Board B and on Brominated Epoxy Resin 1.85 Samples in 100% Nitrogena compound

MW

board B

resin 1.85

Nonbrominated styrene dimethylpyrazine phenol methylphenol (2 isomers) dimethylphenol (2 isomers) ethylphenol (2 isomers) propylphenol (1-methylethyl)phenol (2 isomers) methyl-(1-methylethyl)phenol (1-methylpropyl)phenol bis(1-methylethyl)phenol propoxybenzene 2-methylbenzofuran 2-methyl-2,3-dihydrobenzofuran 3,4-dihydro-2H-1-benzopyran 3,4-dihydro-2H-1-benzopyran-3-ol hydroxyacetophenone 1-phenoxy-2-propanone dibenzofuran p-hydroxybiphenyl methylenbis(phenol) 4-(1-methyl-1-phenylethyl)phenol bisphenol A DGEBA

104 108 94 108 122 122 136 136 150 150 178 136 132 134 134 150 136 150 168 170 200 212 228 340

id. id. id. id. id. id. id. id. id. id. id. nd id. id. id. id. nd id. nd id. id. nd id. nd

id. nd id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id. id.

Brominated 2-bromophenol 4-bromophenol 2-bromo-4-(1-methylethenyl)phenol 2-bromo-4-(1-methylethyl)phenol 5-bromo-2-methylbenzofuran 1-bromo-3-phenoxy-2-propanol 1,3-dibromo-2-propanol 2,4-dibromophenol 2,6-dibromophenol 2,6-dibromo-4-(1-methylethenyl)phenol 2,6-dibromo-4-(1-methylethyl)phenol bromobisphenol A dibromobisphenol A (2 isomers) tribromobisphenol A TBBA

173 173 213 215 211 231 218 252 252 292 294 307 386 465 544

id. id. nd id. nd id. id. id. id. nd id. id. id. id. id.

id. id. id. id. id. id. id. id. id. id. id. id. id. id. id.

a

id. ) identified. nd ) not detected.

the present work, within the products obtained in BR runs carried out on both electronic boards and linear epoxy resins. It must be noted that a likely pathway for the formation of tribromophenol involves secondary reactions of bromine radicals generated in the process with phenoxy radicals.12,46 The formation of 1-bromo-3phenoxy-2-propanol and 1,3-dibromo-2-propanol may well derive from the interaction of bromine radicals or hydrogen bromide formed from the TBBA unit in the degradation process with the aliphatic chains of the epoxy resin. Finally, it must be remarked that the formation of nitrogen compounds in the condensable decomposition product fraction of electronic board samples (e.g., dimethylpyrazine) is a further confirmation that amines were used as curing agents in the cross-linking process. The products identified in the present study well agree with those reported in previous investigations.11,30,33-35,37,39 In the study carried out by Luda et al., the pyrolysis of cured brominated epoxy resins was reported to yield 1,3-dibromo-2-propanol, phenol, 2bromophenol, 4-bromophenol, ethylphenol, propylphenol, 4-isopropylphenol, 2-bromo-4-isopropylphenol,

2,4-dibromophenol, 2,6-dibromophenol, dibromoisopropylphenol, dibromoisopropenylphenol, aromatic/aliphatic ethers, p-hydroxybiphenyl, bisphenol A, bromobisphenol A, and dibromobisphenol A.37 Phenol, various alkylphenols such as isopropylphenol, bromophenol, bromoisopropylphenol, dibromophenol, bisphenol A, and different brominated bisphenol A species were detected in several pyrolysis experiments performed on electronic scrap.11,33,34 Phenol, alkylphenols, bromophenols, and dibromophenols were identified as well among the products of the thermal oxidation of printed circuit boards.30 Quantitative Analysis of Decomposition Products. The use of specific calibrations for the quantitative analysis of TG-FTIR data, extensively described in previous publications,44,45 allowed the estimation of the quantities of gaseous compounds formed in the decomposition process. Table 5 summarizes the results obtained for CO, CO2, HBr, and NH3 generation in the main degradation process in inert atmosphere. These values were obtained as the mean of at least four experimental runs. The FTIR quantitative analysis of BR data performed for CO and CO2 by the use of “concentration-based” calibration procedures45 confirmed the results obtained in TG-FTIR runs. The amount of methane formed could also be evaluated, and was included in Table 5. The table also reports the results obtained from the analysis of TG-FTIR runs performed on the linear brominated epoxy resins using the same experimental conditions. The data in Table 5 clearly indicate that hydrogen bromide is the most important gaseous decomposition product formed in the degradation of electronic boards. Even though hydrogen bromide generation was reported in several thermal degradation studies concerning printed circuit boards,31-35 few data are present in the literature about the quantities formed. Mazzocchia et al. detected the generation of about 0.3 wt % of hydrogen bromide in the 500 °C pyrolysis of hardware components.34 In the investigation of Chien et al. on the fate of bromine in the pyrolysis of printed circuit board wastes, approximately 72.3% of total bromine in the sample was found in the product gas, mainly as HBr and bromobenzene.32 Table 5 shows that, in the present study, more than 50% of the bromine present in the samples was emitted as hydrogen bromide during the thermal degradation process. Table 5 also allows a comparison with the quantities of the gaseous products formed in the thermal degradation of the brominated linear resins. Wide differences are present in the quantities of carbon oxides produced. Lower amounts of carbon dioxide (about 1 order of magnitude), and higher quantities of carbon monoxide (approximately 1 order of magnitude), were generated in the thermal degradation of the linear resins. On the other hand, also in this case relevant quantities of hydrogen bromide were found to be formed in the thermal degradation process (up to 21 g per 100 g of sample). However, Table 5 evidences that, if the HBr yield is related to the initial bromine content, very similar results are obtained for the electronic boards and the 1 and 1.85 resin, while lower conversions to HBr are experienced for the resins having higher DGEBA content. Quantitative data on the condensable products formed in the thermal degradation of the electronic boards were obtained by GC/FID analysis. Specific data on the

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Table 5. Quantities of Gaseous Decomposition Products Evolved in TG-FTIR Runs Performed on Electronic Board and Brominated Epoxy Resin Samples (10 °C/min, 100% nitrogen)a sample electronic board A electronic board B electronic board C brominated epoxy resin DGEBA/TBBA 1 brominated epoxy resin DGEBA/TBBA 1.85 brominated epoxy resin DGEBA/TBBA 2.75 brominated epoxy resin DGEBA/TBBA 4 a

Br content (wt %) 6.0 6.9 6.7 34.9

gaseous Products (g/100 g sample) CO2 CO CH4

NH3 0.050 0.036 0.020

HBr

bromine released as HBr (%) N2 100% air

0.62 0.70 0.66 0.088

0.033 0.042 0.014 0.30

n.d. 0.006b n.d. 0.04

3.86 4.09 3.69 19.6

61.1 55.4 60.0 55.5

77.4 65.5 75.0 59.6

26.0

0.061

0.68

0.17

14.1

53.6

57.4

20.4

0.048

0.62

0.13

7.71

37.2

37.4

15.8

0.051

0.80

0.22

4.85

30.3

31.8

n.d. ) not determined. b Evaluated from BR data.

Table 6. Yields of the Main Decomposition Products in the Condensable Product Fraction Recovered from BR Runs Carried out on Electronic Board B and on Brominated Epoxy Resin 1.85 Inert Environment mol %

Oxidizing Environment Wt.%

mol %

Wt.%

compound

board B

resin 1.85

board B

resin 1.85

board B

board B

phenol 2-bromophenol 4-(1-methylethyl)phenol 2-bromo-4-(1-methylethyl)phenol 1-bromo-3-phenoxy-2-propanol 2,6-dibromophenol 2,6-dibromo-4-(1-methylethyl)phenol p-hydroxybiphenyl bisphenol A bromobisphenol A dibromobisphenol A (2 isomers) tribromobisphenol A others

58.3 1.7 12.3 1.2 0.4 3.3 0.4 4.1 6.4 3.0 2.3 0.9 5.7

43.0 4.7 6.0 2.4 1.2 6.0 1.1 1.0 5.2 6.2 10.1 3.5 9.6

38.3 2.1 11.7 1.7 0.7 5.8 0.8 4.9 10.2 6.4 6.3 2.8 8.3

20.6 4.2 4.2 2.6 1.4 7.8 1.7 0.9 6.0 9.8 20.0 8.4 12.4

56.7 1.4 9.9 1.2 0.5 4.0 0.3 4.6 8.1 3.8 3.3 1.5 4.6

35.1 1.6 8.9 1.7 0.9 6.7 0.6 5.1 12.1 7.6 8.5 4.6 6.6

quantities formed in the degradation of sample B are reported in Table 6. Only the products formed in quantities higher than 1% were reported in the table, accounting for more than 90% of the condensable products formed. Negligible differences were found also in the quantitative data obtained for the condensable products formed from the thermal degradation of samples A and C, both in inert and in oxidizing environment. For the sake of comparison, Table 6 also reports the corresponding data obtained for the 1.85 linear brominated resin. A strong correspondence was observed between the data obtained for electronic boards and linear brominated resins. Phenolic and bisphenol A species, either brominated or nonbrominated, were the main products generated in the decomposition of both electronic boards and linear epoxy resins. In the pyrolysis experiments performed on electronic boards, phenol was the main product recovered in the condensable product fraction; a phenol yield of about 38 wt % was evaluated. Relevant quantities of isopropylphenol (12 wt %), brominated phenols (10 wt %), bisphenol A (10 wt %), and brominated bisphenol A species (16 wt %) were also detected. Higher yields of brominated products were obtained in the pyrolysis of the linear resin. This may be simply due to the higher bromine content of the resin (the data reported in Table 6 were obtained for the DGEBA/TBBA 1.85 resin). Specifically, lower yields of phenol (21 wt %) and bisphenol A (6 wt %), and higher yields of brominated phenols (16 wt %) and brominated bisphenol A species (38 wt %), were found in the experiments performed on the linear resin.

Quantitative data on condensable products formed in the thermal degradation of electronic boards are scarce in the literature, and no data are reported for brominated epoxy resins. Only Mazzocchia et al.34 report the quantitative composition of the liquid phase obtained in the 500 °C pyrolysis of hardware component wastes originated from the circuit part of a hard disk controller. The liquid phase, the yield of which ranged from 19.9 to 22.7% of sample weight, was found to be composed mainly of phenol (36-61 wt %) and isopropylphenol (15-18 wt %), while the total amount of brominated products ranged from 7 to 15 wt % depending on the type of hardware component pyrolyzed. These data seem to confirm the results obtained in the present investigation. The quantitative data on the decomposition products of electronic boards and brominated epoxy resins were also compared with the corresponding results obtained for the decomposition of the pure TBBA.12 Figure 12 summarizes the yields, calculated as weight % with respect to sample organic fraction, of the main volatile decomposition products formed in the pyrolysis of electronic board B, epoxy resin 1.85, and TBBA. Higher quantities of hydrogen bromide and brominated organic compounds were generated in the degradation of TBBA with respect to both linear resins and electronic boards, possibly due to the higher sample bromine content. While phenol and alkylphenols resulted within the main products formed in the pyrolysis of the materials investigated in the present study, low amounts are originated in the TBBA decomposition. As far as brominated phenols are concerned, similar yields of 2bromophenol and 2,6-dibromophenol were obtained. On


bromide in the decomposition of all the materials considered. No relevant differences were found between the bromine distribution in the decomposition products of the electronic boards A and B, and of the 1.85 resin. The results obtained for the 2.75 resin evidenced that for this material a higher quantity of bromine was present in the condensable fraction of the decomposition products. Nevertheless, it must be remarked that for all the samples the high-molecular-weight organobrominated compounds evolved in the thermal degradation account for about 50% of the bromine initially present in the samples. These results are in agreement with the data obtained for the thermal degradation of TBBA,12 which is an important component in brominated epoxy resins formulation. Conclusions

Figure 12. Yields (weight % with respect to sample organic fraction) of main volatile decomposition products formed in the pyrolysis of electronic boards, linear brominated epoxy resins, and TBBA.

Figure 13. Bromine distribution in the different product fractions obtained from electronic boards, linear brominated epoxy resins, and TBBA thermal degradation (100% nitrogen).

the other hand, while significant quantities of 4-bromophenol, 2,4-dibromophenol, and 2,4,6-tribromophenol were formed during TBBA pyrolysis, almost negligible quantities of these compounds were detected in experimental runs carried out on linear brominated epoxy resins and electronic board samples. A likely pathway for the formation of such compounds involves secondary reactions of bromine radicals generated in the thermal degradation process with phenoxy radicals. If the TBBA unit is embedded in a polymer matrix, such as in linear and cured epoxy resins, the bromine radicals formed may well interact preferably with the polymeric chains, thus limiting the yields of the brominated phenols. No brominated dibenzo-p-dioxins or dibenzofurans were detected in the present study. This confirms the results of previous investigations17,19,21-29 that showed that TBBA and its flame-retarded polymers yield PBDD/ PBDF only at a ppm level in several pyrolysis and combustion conditions. Nevertheless, direct precursors of PBDD/PBDF are generated during the primary decomposition process, as dibromophenols.11,53 Bromine Distribution in Decomposition Products. Figure 13 reports the bromine distribution among the different fractions of thermal decomposition products obtained from experimental data for some of the materials considered in the present study. The figure shows that bromine is mainly evolved as hydrogen

The thermal degradation behavior of electronic boards was explored and compared to that of uncured brominated resins and to that of the pure components of the resins. The thermal stability of the organic components of electronic boards was somewhat lower than that of the uncured resins used for their production, possibly due to catalytic effects of the inorganic matrix and to blending with components having lower thermal stability. On the other hand, the yield in the main decomposition products was only slightly affected by the crosslinking process. The quantitative assessment of the products formed in the thermal degradation of electronic boards at moderate heating rates evidenced that the yield in the main decomposition products is quite similar to that obtained from the decomposition of the uncured resins. Hydrogen bromide, phenol, isopropylphenol, brominated phenols, bisphenol A, and brominated bisphenol A species were the main decomposition products. An important correlation was found also between the products formed in the thermal degradation process of these materials and those obtained in the thermal degradation of pure TBBA and DGEBA, the starting materials used in the manufacture of brominated epoxy resins. However, the negligible formation of tribromophenol and, in general, of para bromo-substituted phenols (e.g., 2,4-dibromophenol) in the pyrolysis of electronic boards and uncured epoxy resins resulted in an important difference with respect to TBBA thermal degradation behavior. It must be remembered that these compounds are specific precursors of PBDD/ PBDF. The experimental results obtained also allowed the investigation of bromine distribution between the different product fractions formed in the thermal degradation process. As in the case of other brominated polymers, bromine is mainly evolved as hydrogen bromide. However, both in the case of the electronic boards and of the uncured resins, an important fraction of bromine (40-60%) contributes to the formation of highmolecular-weight organobrominated compounds. The formation of considerable amounts of hydrogen bromide and of high-molecular-weight organobrominated compounds, as well as the potential formation of limited quantities of PBDD and PBDF, is an important element of concern in the safety and environmental assessment of thermal degradation processes of electronic boards. In particular, the possible formation of these compounds should be considered in fire accidents or in the disposal of electronic scrap by combustion processes.

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Received for review December 22, 2004 Revised manuscript received March 15, 2005 Accepted April 14, 2005 IE048766L

Thermal Degradation and Decomposition Products of Electronic

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