2.1. Characterization of the CFRP Waste
The residual CFRP used in this work is an expired pre-impregnated form from the aeronautical industry composed of a poly(benzoxazine) resin and reinforced with Toray T300/3k carbon fibers of 7 µm diameter. The proximate and elemental analyses, together with the fiber content of the waste CFRP sample, can be seen in Table 1
. Table 1
also includes the results of the same analyses applied to the carbon fiber of this CFRP, with the aim of comparison. This CF was obtained by applying the D3171 method of the American Society for Testing and Materials (ASTM), which consists of the dissolution of the resin in a H2
mixture. As can be seen, the fiber content of the sample is 61 weight percent (wt.%), which is in line with some published fiber content values for pre-impregnated poly(benzoxazine)–CF composites [14
]. As far as the proximate analysis of the CFRP waste is concerned, the moisture and ash contents are very low, so the sample is mainly composed of volatiles and fixed carbon. If the results of the proximate analysis of the CF are examined, it can be seen that the carbon fibers are almost entirely fixed carbon, which can give an idea of their thermal stability, and in addition, they have neither moisture nor ashes. Therefore, it can be concluded that the volatiles in the CFRP sample come mainly from the resin, while the fixed carbon corresponds largely to the carbon fibers. Furthermore, it can be said that moisture and ash derive from the resin; the moisture is probably water adsorbed from the environment, while the ashes could be inorganic fillers of the resin composition.
The carbon fiber elemental analysis serves to discuss the origin of the different elements found in the CFRP sample. Table 1
shows that the carbon fiber is composed mainly of carbon and small quantities of nitrogen and “others”. Nitrogen is probably a remnant of the fiber production process, since in most cases, poly(acrylonitrile) is used as a precursor of CF [2
]. On the other hand, taking into account that the surface treatments of carbon fibers are usually oxidative, “others” are expected to be mainly oxygen [16
]. The elemental analysis of the CFRP sample clearly shows that resin provides the hydrogen, sulfur, and some more nitrogen and “others” content. Hydrogen and nitrogen derive from the resin itself, since benzoxazine monomers are usually synthesized from phenols, amines and formaldehyde [17
]. For the same reason, it can be stated that the “others” group is mainly oxygen. On the contrary, the presence of sulfur cannot be directly related to the composition of the resin monomers, but probably to sulphur-containing crosslinking agents [19
]. In any case, the elemental analysis of this poly(benzoxazine) CFRP sample is very similar to other poly(benzoxazine) CFRP compositions in literature [14
shows the dynamic thermogravimetric analysis of the CFRP waste. As it can be seen, the sample shows a main decomposition process in the temperature range between 330 °C and 465 °C, with the maximum decomposition rate at 391 °C. In this temperature range, the sample loses approximately 23% of its initial weight and then shows a light constant rate weight loss up to 1000 °C. By this temperature, the sample weight loss is 29.4% of the initial mass. These weight loss values are in agreement with the results reported by other authors concerning the decomposition of phenolic resins containing carbon fiber reinforcement [12
]. Such authors attributed the steady weight loss of the sample to volatilization of carbonaceous products formed due to carbonization reactions that occur during polymer decomposition. Carbon fibers are highly stable in a nitrogen atmosphere up to 1000 °C and only undergo a small weight loss (≈1 wt.%) between 300 °C and 400 °C due to the decomposition of the sizing agent, so they do not lose weight appreciably in such heating process [20
The carbonization reactions explain the fact that the weight loss at 1000 °C (29.4 wt.%) is lower than the amount of resin present in the CFRP (from Table 1
, amount of resin = 100 − 61.0 = 39.0 wt.%). It is a well-known fact that the majority of the thermosetting resins (especially the benzoxazine-based ones) yield a carbonaceous product called char when decomposing in a non-oxidizing atmosphere [17
]. The char remains in the surface of the carbon fibers and cannot be eliminated by temperature in absense of oxygen. That is, the poly(benzoxazine) resin is not completely removed only by the presence of heat and this fact explains why the remaining mass after the analysis (70.6 wt.%) does not correspond to the fiber content of the sample (61.0 wt.%).
2.2. Pyrolysis Experiments and Gas/Liquid Composition
A series of pyrolysis experiments at 500 °C were carried out at two different heating rates and by applying different treatments to the pyrolysis vapors and gases. In such treatments, two catalysts were used: a commercial reforming catalyst composed of Ni on alumina and a ZSM-5 zeolite doped with Ni. Table 2
shows the pyrolysis yields obtained in the experiments carried out, together with the composition of the gases and liquids obtained in such experiments.
The E1 experiment represents a typical pyrolysis run, with an intermediate heating rate and no treatment of vapors and gases. The results of this experiment show that the main product of CFRP waste pyrolysis is the solid fraction, where the carbon fibers are, followed by the condensed products and finally a small amount of gas. These results are in agreement with those obtained by other authors in the pyrolysis of benzoxazine and epoxy-based CFRP [5
]. The solids yield is greater than the carbon fiber content of the sample, which is to be expected from the thermogravimetric analysis shown in Figure 1
. Indeed, the solids yield is very similar to the remnant weight in that analysis at 500 °C. Then, around 20 wt.% of liquid is produced, which is composed of two phases. The aqueous phase is mainly composed of water, while the organic phase is dominated by nitrogenous and oxygenated aromatic compounds, in which aniline and phenol stand out (these are also part of the aqueous phase). The presence of water in the liquids of poly(benzoxazine) pyrolysis has also been reported by other authors [14
]. These authors attributed the formation of water to the oxygen containing functional groups, such as –OH and –COO–, which are present in the matrix chemical composition. The formation of water is also very common in the pyrolysis of oxygenated samples such as biomass [25
]. The dominant presence of aniline and phenol in pyrolysis liquids of benzoxazine type CFRP has been previously reported by other authors and can be explained by the fact that it is very common to employ aniline and phenol derivatives as feedstock to produce this type of polymeric resin [11
]. Finally, 5% of gas is also produced, mainly composed of CH4
and CO and with relatively high heating values (HHV).
The high generation of liquids, whose industrial utility is very complicated, is the main cause of burning the vapors generated in the existing CFRP recycling processes. The reason is that in the absence of utility, the composition of these liquids makes them classified as hazardous waste, with the environmental and economic implications that this entails. Therefore, the following experiments were designed with the aim of reducing the quantity of pyrolysis liquids, and at the same time, enhancing the production of gas that can be used in different industrial aplications. The first step was to decrease the heating rate to 3 °C min−1
to achieve a slow pyrolysis process (E2 experiment). This decision was taken based on the previous experience of the authors in the investigation of pyrolysis with other type of waste [27
As shown in Table 2
, slow pyrolysis of CFRP (E2) produces a four percentage point reduction in condensates generation, and more importantly, a reduction in more than half of the collected liquids, which are more representative of changes in operating conditions than the condensates that may remain in the pipelines, which are more dependent on plant design. The decrease in condensates yield is evenly distributed, increasing the solid and gas yields. The solid yield increases due to carbonization reactions that can take place at a greater extent when polymeric waste is slowly heated up [27
]. The explanation concerning the change between liquid and gas yields could be that slow heating rates make the release of chemical compounds from the decomposition of the resin slower. This has a very important influence on the secondary reactions that can occur between these compounds when they are already in the gas phase. High heating rates generate more compounds in short time intervals, which multiplies the possibilities of the reactions between them. When slow heating rates are used, gas phase interactions occur between fewer compounds [27
]. Therefore, the interactions between multiple compounds could favor the formation of condensable products, while the generation of gases is greater when the interactions are smaller. Looking at the composition of liquids, there are no important changes in terms of applicability; that is, the two phases are in significant proportions and have a similar composition. The gas composition is simplified under these slow pyrolysis conditions. In this case, it appears that CH4
may be primary decomposition compounds (slow heating rate), which are capable of generating H2
, CO and other hydrocarbons when in contact with more chemical compounds (rapid heating rate). In any case, the gas fraction retains its potential industrial utility.
The results of the E3 experiment were obtained after adding a thermal treatment of pyrolysis vapors and gases before condensation. Details about this treatment can be found in Section 3.2
of the paper. As Table 2
shows, there is an important change in the yield ratio of liquids and gases after treatment. The total condensates are reduced by half compared to the E1 test, and most of them are condensates in pipelines, as the collected liquids decrease to a negligible by-product. As a consequence, the gases suffer a corresponding increase in yield, becoming the second product of the process. The mentioned behavior is the consequence of cracking reactions of organic molecules that take place at high temperatures during the thermal treatment [28
]. This is a very positive result, as there is a significant reduction in the liquids produced. This fact is even more interesting looking at the composition of the gases, since H2
becomes the main compound in the mixture (more than 40 volume percent (vol.%)), followed by CH4
(almost 25 vol.%), both of which are widely employed in industry. In addition, the amount of CO2
is reduced considerably, causing a relatively high HHV. With regard to the liquids composition, no major changes are observed except for the apparent reactivity of the aniline and the oxygenated compounds of the organic fraction under these cracking conditions, as the proportion of all of them decrease appreciably. Aniline may be converted to other nitrogenous aromatic compounds, while oxygen may pass into the gas phase of the process (including unquantified water vapor). Also noteworthy is the appearance of PAH in the organic fraction, something common in cracking reactions of organic molecules at high temperature [29
In order to improve the properties of the produced gases and liquids, two catalysts were tested in the E4 and E5 experiments. In E4, a commercial reforming catalyst, Ni on alumina, was employed, and in E5, a homemade Ni on ZSM-5 zeolite catalyst (prepared in the laboratories from a commercial zeolite) was used. Both have a similar amount of Ni (≈12 wt.%) and an approximately equal Ni crystal size (≈5 nm). The main difference between them is that zeolite is a more acid support, with greater surface development and significantly smaller pores than the reforming commercial catalyst (see Section 3.1
). If attention is paid to the yields obtained in these experiments, it can be observed that they are very similar to those obtained in the non-catalytic thermal treatment experiment (E3). Condensates and collected liquids appear in the same quantities, while there is a small decrease in the gas yields, which is due to the fact that the yield in solids is slightly higher than that obtained in the E3 experiment. These variations are not very significant, as they fall within the dispersion of the results usually obtained in these experiments. What is worthy of note is the significant decrease in the organic phase of liquids with respect to the thermal treatment experiment (from 27.6 wt.% to 4.0/5.8 wt.%). This is another very important result, because the reduction of the amount of the organic phase considerably increases the options of using these liquids, since the aqueous phase consists of a mixture of water and aniline. There is no significant difference between the composition of the organic phases of the E3, E4 and E5 experiments, except that in the organic phases of the thermo-catalytic experiments, there are fewer compounds (a lower percentage of “others” is observed). However, the organic phase yield values of the E4 and E5 experiments correspond to small spots of liquid supernatant in the aqueous phase and on the walls of the condensers, i.e., they are totally negligible quantities in comparison with that of E3 experiment, and of course, in comparison with E1 and E2 experiments.
The composition of the gases also shows the action of the catalysts. In both cases there is a clear reduction in the hydrocarbon content (mainly CH4, C2 and C6) in favor of a higher production of H2 and CO, which indicates that both catalysts have promoted reforming reactions. The composition of the gases in the E4 and E5 experiments is practically the same, but a greater amount of H2 can be seen when the zeolite-based catalyst is used. Bearing in mind that the amount of Ni is very similar in the two catalysts, one of the reasons could be that the zeolite-based catalyst also contributes to the generation of hydrogen through the cracking of organic substances due to its high acidity, producing hydrogen through two different routes (reforming and cracking). In any case, the fact that the amount of hydrogen in the gases is greater than 50% by volume is not a trivial matter, as it is from this value that the separation of this compound from other gaseous compounds begins to be profitable. In other words, the action of the catalysts allows these gases to be considered as a source of H2, which can be used in any of its multiple industrial applications. These results clearly open the door to the possibility of recovering chemicals from the CFRP polymer resin.
2.3. Pyrolysis Solids
shows the proximate and elemental analyses of the pyrolysis solids. In this case, only the analysis of the solids obtained in the E1 and E2 experiments has been included, since the operating conditions in the pyrolysis reactor are the same in the E2, E3, E4 and E5 experiments.
As far as the comparison between the two solids is concerned, neither the proximate analysis nor the elemental analysis reveal important differences caused by the change in the heating rate of the residual CFRP, as they present practically identical results. More interesting is the comparison of these results with those of the proximate and elemental analysis of the carbon fiber in Table 1
, as in principle, the solid remaining after the pyrolysis should be recovered carbon fibers. In relation to the proximate analysis, the first difference is that the solids obtained have some moisture in contrast to the carbon fiber, which does not absorb moisture significantly. In addition, carbon fiber has a slightly higher amount of fixed carbon (and therefore a lower amount of volatiles) than the solids obtained after pyrolysis. Similarly, elemental analysis shows that carbon fiber is mainly composed of carbon and nitrogen, while pyrolysis solids have higher amounts of hydrogen, nitrogen and “others”, making their percentage of carbon lower (91 wt.% vs. 96 wt.%). All of this indicates that, in addition to the carbon fibers, there is probably some other substances in these pyrolysis solids. These substances could be undecomposed resin and/or char, as discussed in Section 2.1
, which is in agreement with all authors who have investigated the recovery of carbon fibers through pyrolysis [5
The images obtained by scanning electron microscopy (SEM) of the solids obtained in the E1 and E2 experiments is shown in Figure 2
. The figure clearly shows that undecomposed resin remains partially covering the fibers in the solid obtained after pyrolysis when the heating rate was 15 °C min−1
(E1, a figure). However, better elimination of the polymer can be observed in the solid pyrolyzed at 3 °C min−1
heating rate (E2, b figure). This fact reinforces the utilization of slow pyrolysis in the recycling of CFRP, given that the liquids and vapors obtained are better than those of the conventional pyrolysis, especially when thermo-catalytic treatment is used. Small particles, which are most probably char, can be seen in both SEM images, coming from the carbonization of the resin, as explained above.
shows the surface topography of the pyrolysis solids obtained by means of atomic force microscopy (AFM). These images confirm what has been observed in Figure 2
. The surface of the pyrolysis solid obtained at 15 °C min−1
(a) presents a mostly homogeneous area, with low roughness, together with a small section of high roughness. The homogeneous area can indicate the presence of undecomposed resin, whereas the rough section could be big char particle or thicker undecomposed resin. In the solid obtained at 3 °C min−1
(b), the roughness is lower, but very homogeneously distributed. This may indicate that we are observing the inherent roughness of a cleaner fiber, without resin.
The main parameters of surface roughness are collected in Table 4
. The roughness parameters of the initial CFRP have also been included in the table, with the intention of using them as a reference. The arithmetical mean deviation (Ra) is the most used amplitude parameter and shows the vertical deviations of the roughness profile from the mean line. Based on this parameter and looking at the table, it can be said that the initial CFRP has the lowest surface roughness, followed by the E1 pyrolysis solid and then by the E2 solid. These results confirm the fact observed in the images above. The initial CFRP has a low roughness because it is a finished product covered by the resin; then, the surface roughness increases as the polymer resin is being removed due to defects that are created on the surface and the apparition of char particles. This is the reason why E2 solids present higher surface roughness than E1 solids. This tendency is also observed in the values of the maximum valley depth (Rv); a higher value indicates a less rough surface. The third roughness parameter, the maximum peak height (Rp), maintains the tendency between E1 and E2 solids, but in this case, the highest value corresponds to the initial CFRP. However, this value may be affected by the great deviation in the measure ( ±3.83 µm).
By the data and images collected in Table 3
and Table 4
, and Figure 2
and Figure 3
, it can clearly be stated that the carbon fibers coming from the pyrolysis step still contain undecomposed resin and char particles. The char and the undecomposed resin must be completely eliminated in order to avoid problems in the adhesion between the recovered fibers and the new resin when reusing the fibers in new composite manufacturing. Normally, a controlled oxidation stage is used for this purpose [5
]. In any case, the good performance of the fibers recovered by pyrolysis–oxidation in new composites has been demonstrated by several authors and there are even commercial products available on the market [10