*2.2.5 Acid-based delamination*

This technique has been developed by researchers in China and focuses on the separation of LDPE and aluminum by means of a separation reagent, mostly aqueous solutions of organic acids or even mixtures of acids. The procedure works by breaking the mechanical bonds holding the laminate together and as such allows for recovery of the products.

The yield of the process is highly dependent on the conditions of the reaction. In the process some of the aluminum is dissolved by the acid—which is also

**Figure 9.** *The SDP process. Image inspired by Georgiopoulou et al. [33].*

#### **Figure 10.**

*The acid-based delamination process. Image inspired by Zhang Ji-fei et al. [37].*

consumed—and thus losses are to be expected. However, this depends on many factors such as acid used, temperature, etc. Product purity is also correlated with those factors.

After trial and error, it has been found that methanoic acid is the best separation reagent for Tetra Pak. Lastly, there seems to be a high correlation between the separation rate, the temperature the reaction is taking place at, and the concentration of the reagent. More specifically, reaction time decreases with the rise of reagent concentration and temperature (**Figure 10**) [37].

#### *2.2.6 Heat recovery and material recycling*

Thanks to the high heating value of the Al-LDPE laminate (40 MJ/Kg), it can be used as a sufficient fuel source. This has taken precedent especially in Europe. Although the laminate can be used directly after the hydropulping process, it is most usually used in conjunction with other fuel sources. This recycling route can be considered environmentally friendly as the LDPE of Tetra Pak burns cleanly without producing fumes containing elements such as sulfur, nitrogen, or halogens.

Also, the Al2O3 produced during pyrolysis, by the reaction between Al and moisture in high heat conditions, is in big part exploited by the cement industry, which uses it as a desired component of cement production [36]. Lastly there is the choice of forming finished products directly by using the laminates in roof tile production, injection and rotational molding, and PE-Al agglomeration and pulverization [42, 43].

In these times that society demands a more environmental way of thinking from the industry, recycling of multilayer packaging becomes a priority for many scientists. They have developed a plethora of ways to recycle such packaging, from using it as a fuel to using its pyrolysis products as a mercury absorbent. It is most likely that this field will keep on expanding with ever more innovative and cost-effective ways to fully exploit, reuse, and transform the Tetra Pak multilayer packaging as human development is going into the future.

#### **2.3 Brominated flame-retarded plastics originating in WEEE**

The rapid technological advances along with people's need for better living conditions resulted in a global rise in the consumption of EEE over the last years and so in huge amounts of WEEE [44]. Plastics in WEEE account for ~30% of WEEE and in most cases contain BFR that necessitates careful handling [9], since BFR's presence in plastics leads to the formation of various, toxic brominated substances in the liquid fraction obtained after pyrolysis, inhibiting its further use. In such cases a pretreatment step before or during the recycling is necessary in order to receive bromine-free products. So, due to the fact that *brominated plastics from WEEE* are increasing more and more and the BFR enhances the difficulties in their recycling, this unit focuses on pretreatment methods that can be applied either before or during their recycling.

One very common pretreatment method for the removal of BFR applied before pyrolysis is *solvent extraction*. "Traditionally" it takes place using a *soxhlet* extraction apparatus. It is a very popular method until now, due to its low cost and simplicity,

#### *Current Topics in Plastic Recycling DOI: http://dx.doi.org/10.5772/intechopen.101575*

although large amounts of solvents and much time are usually required [45]. For instance, Evangelopoulos et al. applied *solvent extraction* as a pretreatment before pyrolysis, via a soxhlet extraction apparatus, with the aim of reducing tetrabromobisphenol A (TBBPA) from real WEEE samples. They tried two different solvents, isopropanol and toluene, due to their different properties; and they found that isopropanol was more efficient in removing bromine from the solid fraction, whereas toluene was more efficient in removing TBBPA from the liquid fraction [46].

Apart from the typical soxhlet extraction, many advanced solvent extraction techniques have been explored over the years, including supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), ultrasonic-assisted extraction (UAE), and microwave-assisted extraction (MAE). These techniques require less time and volumes of solvents than those during soxhlet extraction [47]. Vilaplana et al. applied MAE for the removal of TBBPA and decabromodiphenyl ether (Deca-BDE) from virgin high-impact polystyrene (HIPS) and standard samples from real WEEE. They found that complete extraction of TBBPA took place when they used a combination of polar/nonpolar solvent system (isopropanol/n-hexane) and high extraction temperatures (130°C). On the other hand, in case of Deca-BDE, there were obtained lower extraction yields due to its high molecular weight and its nonpolar nature [47].

In another study [48], UAE and MAE were investigated for the recovery of TBBPA from real WEEE samples that consisted of ABS, polypropylene (PP), polycarbonate (PC), and blends of ABS/PC. From the results obtained it was proved that MAE was more efficient in extracting TBBPA than UAE, especially in case of ABS polymers. The optimal solvent media was isopropanol: n-hexane (1:1), which is a binary mixture of a polar –nonpolar solvent, whereas pure isopropanol, as a solvent, could not result in complete extraction of TBBPA [48].

As mentioned previously, SFE has also attracted a lot of attention as regards the degradation of brominated flame-retarded plastics from WEEE, because of the supercritical fluids' unique properties, such as high density, low viscosity, varied permittivity related to pressure, and high mass transfer, as well as the fact that their viscosity, density, and diffusion coefficient are very sensitive to changes in temperature and pressure. Supercritical fluids appear at temperature and pressure higher than their critical state. Supercritical CO2 is the most widely used fluid in SFE, since it presents remarkable advantages, including: low critical point, low cost, ease of availability, nontoxicity, recyclability, and simplicity as regards its operation. Water is also, a cheap, nontoxic, and easily available fluid, but it has a relatively high supercritical point [49].

Onwudili and Williams [50] studied supercritical water (T > 374°C and P > 22.1 MPa) due to the fact that it presents different characteristics in comparison with organic solvents. They focused on ABS and HIPS, since they are some of the most representative brominated plastics in WEEE and degraded them in supercritical water (up to 450°C and 31 MPa) in a batch reactor. Furthermore, they investigated the effect of alkaline additives, NaOH and Ca(OH)2, by treating the plastics both in the absence and in the presence of them. They noticed that oils, which were the main reaction products, had almost zero bromine and antimony content in the presence of NaOH additive [50]. In another work, [51] there was used subcritical water for the debromination of printed circuit boards (PCB) that contained BFR in a high-pressure batch reactor. They applied three different temperatures, 225, 250, and 275°C, and noticed that debromination increased with increase in temperature. After the debromination of the samples, they applied recycling methods, such as pyrolysis.

Apart from water, organic solvents such as acetone, methanol, and ethanol can also be used as supercritical fluids in chemical recycling of plastics from WEEE [52]. For instance, Wang and Zhang [52] used various supercritical fluids: acetone, methanol, isopropanol, and water with a view to studying the degradation of waste computer housing plastics that contained BFR. They came to the conclusion that

supercritical fluid process was efficient for the debromination and decomposition of brominated flame-retarded plastics enabling the recycling of bromine-free oil. As for solvent's efficiency in debromination, the order was the following: water > methanol > isopropanol > acetone.

It should be highlighted here that although SFE technology is considered as a green choice for resource recovery, it has some important drawbacks as well. One of the main obstacles in such technology is the fact that only equipment able to withstand high pressures and temperatures and very resistant to corrosion can be used. These demands, however, increase the cost a lot, and along with the large amount of energy that is required, prevent its industrial implementation [49].

To avoid the latter difficulties, there are other approaches that can be applied in case of flame-retarded plastics. One such approach is that of *two-step pyrolysis.* In this case pyrolysis steps affect the obtained products and by controlling the pyrolysis parameters, the formation of brominated products can be suppressed, without requiring resistant equipment. For instance, according to Ma et al. [53], who applied single- and two-step pyrolysis of waste computer casing plastics, two-step pyrolysis led to the transfer of the biggest part of brominated compounds into the liquid fraction of the first step, in comparison with that of the second step. This observation showed that high-quality oils with low bromine content can be obtained when applying two-step pyrolysis [53].

*Co-pyrolysis* is another worth mentioning process, in which two or more materials are pyrolyzed together, with the aim of improving the quality and quantity of the liquid fraction, without the need of a pretreatment step prior to pyrolysis. Co-pyrolysis is based on the synergistic effect of different materials that can react together during pyrolysis and leads to a reduction of the total volume of waste, since more waste (e.g., polymers) is consumed as feedstock. The mechanisms of co-pyrolysis and pyrolysis are almost the same, and it is performed at moderate operating temperatures and in the absence of oxygen [54]. Ma et al. [55] examined co-pyrolysis of HIPS, which contained decabromodiphenyl oxide (DDO) as the BFR and antimony trioxide (Sb2O3) as a synergist, in the presence of PP (at three different mass ratios) in order to investigate PP's effect on the bromine reduction. From the results obtained it was proved that PP's presence not only increased the yield of various, valuable products, such as toluene, styrene, etc., in the pyrolysis oil, but also led to a reduction of the bromine content [55].

As described above, during co-pyrolysis, the end-of-life brominated plastics along with other (plastic) waste are pyrolyzed together and result in bromine reduction in the derived pyrolysis oil, without any kind of pretreatment before the pyrolysis process. Another idea, in order to reduce bromine while avoiding the extra pretreatment step, is that of the *use of additives or catalysts* during pyrolysis. According to current literature data, many types of *additives*, such as NaOH, Ca(OH)2, CaO, scallop shell, and others, have been investigated for their effect on the reduction of bromine [56, 57]; but of course the degree of debromination depends on the types of the polymers and additives used.

During *catalytic pyrolysis*, as mentioned in the introduction, catalysts influence products' distribution. This has to do not only with favoring the formation of valuable products but also with reducing the formation of the undesirable ones, such as the brominated compounds. Here there are given some representative examples of catalysts that were examined for their debromination effect. In a recent work of Ma et al. [58], there were examined three zeolite catalysts: HY, Hβ, and HZSM-5 along with two mesoporous catalysts: all-silica MCM-41 and active Al2O3, for their influence on products distribution. They carried out catalytic pyrolysis of brominated flame-retarded HIPS and observed that catalysts enhanced the formation of

#### *Current Topics in Plastic Recycling DOI: http://dx.doi.org/10.5772/intechopen.101575*

valuable, aromatic compounds, such as toluene, styrene, etc., and, in the meantime, enhanced the debromination of the liquid fraction [58].

In another study [59], there was investigated activated Al2O3 for catalytic pyrolysis of waste PCB examining three different temperatures: 400, 500, and 600°C, as well as different ratios of PCB: Al2O3. They noticed that higher temperatures improved the oil production; and the optimal results as regards the production of light oil and the debromination were obtained at 600°C. The catalyst's presence increased the formation of light hydrocarbons and in the meantime the debromination. Wu et al. [60] carried out catalytic pyrolysis of brominated HIPS that also contained Sb2O3, in the presence of red mud, limestone, and natural zeolite, with a view to eliminating bromine and antimony from the pyrolysis oil. They found that in their presence, the total amount of bromine (and antimony) in the oil was reduced. Nevertheless, red mud was the most efficient catalyst in reducing bromine, since Fe2O3 present in red mud reacted with HBr that was formed during the degradation of the BFR and hindered the formation of the volatile SbBr3; in the meanwhile, its zeolite property catalytically destroyed the organobromine compounds [60].

Co-pyrolysis can also take place in the presence of catalysts, known as *catalytic co-pyrolysis*. For instance, in [61], they applied catalytic co-pyrolysis of PCB in the presence of (more waste) high-density polyethylene (HDPE) and PP, with a view to reducing the brominated compounds formed. Apart from using other waste polymers as co-feeding for catalytic pyrolysis, there have been reported studies (e.g., [62]) where additives such as CaCO3 and Fe3O4 were investigated along with the catalysts for their debromination efficiency in the pyrolysis oil. A two-step process (pyrolysis and catalytic upgrading) can also occur when catalysts are used and enable the conversion of e-waste plastics into high-value materials. In such cases the first step involves the pyrolysis of brominated plastics so as to decompose them; and the second one involves the catalytic upgrading of their products into valuable and bromine-free products. This two-step process is very useful when dealing with real WEEE plastics that contain impurities, etc., that may result in the catalysts' deactivation if direct catalytic pyrolysis occurs. It can be divided into two categories, based on which pyrolysis products are used as raw material; the first category includes pyrolysis vapors as raw material, and the second one includes pyrolysis oil [9].

An example that belongs in the first category is [63], in which they examined a small-scale two-stage pyrolysis and catalytic reforming of brominated flameretarded HIPS at 500°C using four zeolites: natural zeolite (NZ), iron oxide–loaded natural zeolite (Fe-NZ), HY zeolite (YZ), and iron oxide–loaded HY zeolite (Fe-YZ). They observed that the bromine content in the oil was reduced in the presence of catalysts; however, Fe-NZ and Fe-YZ showed better debromination results, due to the reactions between the iron oxide that was loaded and the derived HBr. Compared with Fe-YZ, Fe-NZ did not greatly change the pyrolysis products and so preserved the valuable single-ring aromatic compounds. As a result, Fe-NZ was more effective and feasible for the feedstock recycling of brominated HIPS via the pyrolysis process.

Areeprasert and Khaobang [64] studied pyrolysis and catalytic reforming of a polymer blend (ABS/PC) and PCB, at 500°C, using some conventional catalysts: Y-zeolite (YZ), ZSM-5, iron oxide–loaded Y-zeolite (Fe/YZ), and iron oxide– loaded ZSM-5 (Fe/ZSM-5), as well as some alternative, green catalysts: biochar (BC), electronic waste char (EWC), iron oxide–loaded biochar (Fe/BC), and iron oxide–loaded electronic waste char (Fe/EWC). They found that all catalysts increased the single-ring hydrocarbon products of the liquid fraction. As for the debromination, it was noticed that in case of ABS/PC, the most effective catalyst

was Fe/BC, whereas in case of PCB, it was Fe/EWC. Also, they concluded that the green-renewable catalysts could be a promising choice for removing bromine from the liquid fraction [64]. Ma et al. [65] investigated pyrolysis-catalytic upgrading of brominated flame-retarded ABS. The process took place in a two-stage fixed bed reactor; and the second stage included the catalytic upgrading of the vapor intermediates that were obtained from pyrolysis (first stage). The examined catalysts were: HZSM-5 and Fe/ZSM-5. Both catalysts had high catalytic cracking activities that led to an increased yield of oil and to a reduction of the bromine in the liquid fraction.
