[8]
2 The results in Table 1 indicate that the mass loss is strongly influenced by temperature. Mass loss is highest for samples #5 and #6 obtained from extractions at 250°C. At 400 bar the mass loss is 41 % and at 1000 bar 42 %. The photo in Fig. 1 indicates that a mass loss of 42 % may be considered as complete removal of the epoxy resin. The pressure influence appears to be minor: for a temperature of 200°C the increase in pressure from 200 to 1000 bar results in an enhancement of the mass loss from 4.8 to 8.2 %, respectively. The moderate pressure impact may be explained by the variation of CO2 density, which is also rather weak in going from 200 to 1000 bar. The residual matrix material associated with samples #1 to #7 is shown in Fig. 1. The yellow color indicates a rather thick coating of the matrix material in #1 to #4 and #7, while #5 and #6 are rather light in color and the matrix material is easily visible. For comparison #8 was introduced, which refers to a single extraction at 400 bar and 250°C for 5 h. The mass loss was 31%, clearly indicating that the first extraction step removes the largest fraction of the epoxy resin. Figure 1: Residual reference circuit board material after the three step extraction detailed in Table 1. #8 refers to a single step extraction at 400 bar and 250°C for 5 h. The extracts were analyzed via size exclusion chromatography to investigate how the extraction conditions impact the molar mass distributions. The elution chromatograms depicted on the left and on the right side of Fig. 2 refer to for extractions performed at 200°C and at 250°C, respectively. Note, the mass of #2 was too low to be analyzed in SEC. The elugrams are given for elution volumes between 16 and 28.5 mL. At higher elution volumes the system peaks occur, which are not shown. Fig. 2 indicates that in all cases the elution curves show a number of distinct peaks occurring at the same elution volumes, which are not varied for the different pressures applied. Close inspection of Fig. 2 (left) indicates that the extract elutes at significantly lower volumes in case of an extraction pressure of 1000 bar. At 400 bar the peaks are shifted to higher volumes. Since in size-exclusion chromatography larger molecules elute first, extraction at 1000 bar yields higher molar mass extract than at 400 bar. One explanation may be seen in the generally better polymer solubility in CO2 or mixtures with CO2 due to higher density [[] F. Rindfleisch, T.P. DiNoia, M.A. McHugh, Solubility of Polymers and Copolymers in Supercritical CO2, J. Phys. Chem. 100 (1996) 15581-15587. ]. The elugram obtained for #4 shows some stronger peaks at higher elution volumes, being indicative of a larger fraction of lower molar mass material. This observation suggests that the material is more degraded at the lower pressure, which may be explained by a larger content of 2-propanol and water in the ternary mixture. In all extractions discussed so far the same amount of 2-propanol and water was charged to the high-pressure cell. In order to reach the desired pressure CO2 was added to the cell. To reach 400 bar the absolute amount of CO2 in the system is significantly lower than in case of extraction at 1000 bar. In cases where the CO2 content is higher the amount of water in the ternary mixture is lower and consequently the pH of the mixtures is suspected to be higher. This explanation is in line with previous findings that the extraction with binary mixtures in the absence of water yields only small amounts of extract. In the future it is a matter of priority to determine the pH at extraction conditions for ternary mixtures of CO2, 2-propanol and water. Figure 2: SEC elution chromatograms for samples #3 to #7. For further details the reader is referred to the text. The extracts obtained at 250°C lead to elugrams that show similar features as seen on the right hand side in Fig. 2. Again extraction at the higher pressures of 400 and 1000 bar leads to extracts with slightly higher molar masses (lower elution volumes). Further, the fraction of molecules with smaller molecular weight is most pronounced for the lowest pressure of 200 bar In a second set of experiments single step extractions were carried out at 200 bar for 3 hours. In addition to 2-propanol, acetone and 1-propanol were used as cosolvent. In all cases 10 mL of organic solvent and 1 ml of water were used. The extractions were carried out at 200 and 250°C. The details are listed in Table 2. Table2: One step extractions at 200 bar for 3 h with a ternary mixture containing CO2, the organic solvent indicated and water. *: extraction without CO2 sample organic solvent temperature [°C] mass loss [%] referenced to #6 #9 1-propanol 200.
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06 #12 1-propanol 250 24.
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56 #13 2-propanol 250 27.
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49 The data in Table 2 indicates that in general 1-propanol and acetone as cosolvents lead to a lower mass loss. As above-discussed the extraction at 200°C yields only a small mass loss ranging from 2.4 to 3.9 %. At 250°C 2-propanol leads to a loss of 27% whereas for 1-propanol 24% and for acetone 21% were found. The last column in Table 2 compares the mass loss to the best result from Table 1. It is clearly seen that the one step extraction for 3 h leads to a significantly lower amount of extracted material. Again the extraction with CO2, 2-propanol and water at 250°C yields the best results, which is around 62 % of the value obtained in the three step process. Figure 3: SEC elution chromatograms for samples #12 to #14 obtained from extractions at 200 bar and 250°C for 3 h. For further details the reader is referred to the text. The results indicate that removal of the epoxy coating from printed circuit boards works well in the presence of a ternary mixture consisting of CO2, 2-propanol, and water. Thus, the use of potentially harmful solvents such as dimethyl sulfoxide, dimethyl formamide or dimethyl acetamide may be omitted. At optimal conditions the clean matrix material is obtained. The elution chromatograms clearly indicate that the extract consists of material with regularly increasing molar masses. In addition, independent of the extraction pressure the peaks are seen at the same elution volumes, thus, the molar mass distributions show identical peaks, too. At a given pressure comparison of the elugrams indicates that at extraction temperatures of 250°C slightly higher elution volumes are seen compared to material extracted at 200°C. Thus, at the higher temperature the material is degraded more than at 200°C. The finding is suggested to be due to slightly higher pH at higher temperature [14]. The original epoxy resin is insoluble in THF used as solvent in SEC. After showing that a reference circuit board material may be delaminated from the epoxy resin it was interesting to test what happens if electronic waste material is used in the extraction. For this purpose small pieces of the motherboard of an old PC were cut. One piece was freed from all electronic parts, the second piece contained some electronic parts. Two separate extraction experiments of both parts with CO2, 2-propanol and water at 250°C and 1000 bar for 3 h were conducted. Fig. 4 shows the pieces before and after the extraction. On the left hand side all electronic parts were removed. Inside the high-pressure cell the copper sheets and clean matrix material were found. In addition, the green upper coating remained in the cell. The extracted epoxy resin was obtained in a mixture with 2-propanol and water upon expansion of the reaction mixture. On the right hand side in Fig. 4 the result of the extraction of the equipped board is seen. Again the copper sheets and the matrix material were separated. The electronic parts fell of the board. The separation is not as good as for the empty board. However, it suggests that rough disintegration may be sufficient. Figure 4: Used printed circuit boards after extraction with ternary mixtures of CO2, 2-propanol and water at 1000 bar and 250°C for 3 h. Summary The use of supercritical CO2 allows for complete extraction of epoxy resins from printed circuit board materials in the absence of any harmful solvent. Ternary mixtures of CO2, 2-propanol and water were applied at pressures as low as 200 bar. It is shown that a temperature of 250°C is more effective compared to 200°C. The extract consists of various molar mass fractions. The masses appear to be independent of pressure and slightly influenced by temperature. It is important to identify the actual molar masses in the future. Applying the same extraction conditions to used circuit board material – even with electronic parts – the coatings may be removed, too. References.
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