Chemical recycling of post-consumer compact discs towards novel polymers for powder coating applications

Abstract

New processes for the chemical recycling of bisphenol A polycarbonate (PC), directly from post-consumer compact discs, have been developed. The first recycling method is based on the use of molecules that can be derived from renewable resources (isosorbide, ethylene carbonate and succinic acid). A second procedure involves a one-pot reaction of post consumer PC with ethylene carbonate, without complete degradation to low molecular weight products. The final polymers feature properties that make them suitable for powder coating applications.

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Introduction

The desire to reduce the dependence on fossil resources shifted the attention of plastic manufacturers toward the use of biomass as a source of energy and commodity chemicals.^1–3 Moreover, from an industrial point of view, due to the increased and fluctuating prices of most chemicals obtained from crude oil, both renewable resources and the recycling of plastic waste has become much more attractive to produce polymers.⁴ The recycling of polymers also provides an important route for the reduction of the carbon footprint of materials since it reduces the use of non-renewable resources. Recycled plastics can be considered nowadays a very promising feedstock for the production of value-added materials.⁴ Almost 25 million tons of wastes were collected in Europe in 2009. About 90% of such amount consists of thermoplastic materials such as poly(ethylene terephthalate), high and low density polyethylene, polypropylene, polyvinyl chloride, polystyrene and polycarbonate.⁵ The constant increase in the recovery of post-consumer waste all over the world is actually driven by modern governmental initiatives and a more widespread social awareness toward environmental issues. The synthesis of new sustainable materials should therefore conjugate recycling with the use of bio-based feedstocks. The recycling of plastics can be performed not only by mechanical processes but also by a chemical approach through a depolymerization process.⁴ Indeed, chemical recycling allows to obtain low molecular weight materials that are easier to be purified as compared to the viscous polymer melts processed in the mechanical approach.⁶ Chemical recycling thus appears as the best choice for products made of different materials that are difficult or impossible to separate. Also, the use of chemical recycling makes it possible to obtain monomers that can be used to produce not only the original polymer but also other types of materials with more specific and tailored properties. For example, bis(hydroxyethyl ether) of bisphenol A (BHEEB), that is obtained by chemical recycling of bisphenol A polycarbonate (PC),^7–9 can be inserted in terephthalate polyesters and used to increase their glass transition temperature and thermal stability. It was also recently demonstrated that the addition of bisphenol A (BPA) units in polyesters increase their flame-retardant performances.¹⁰ BHEEB can be prepared by reaction of PC with ethylene carbonate, that can be obtained from renewable resources.¹¹ Indeed, chemical recycling can also be performed by using reagents obtained from renewable resources, thus making this approach even more interesting from an environmental point of view. For example, renewable resources such as isosorbide and succinic acid have been used to chemically recycle poly(ethylene terephthalate).¹²

Bisphenol A polycarbonate is one of the most widely used engineering thermoplastic materials due to its outstanding properties in terms of excellent optical quality (high transparency and low birefringence) and high impact resistance.¹³ For these reasons, it is used in several applications such as, for example, optical media storage (CDs and DVDs), building and construction, motorcycle helmets and eyewear lenses and frames. The annual production of PC was around 3000 kt per year in 2006 with a consistent market increase in the last decades.¹⁴ In the last years there has been a significant concern about the toxicity of BPA.¹⁵ For this reason bisphenol A polycarbonate cannot be used for containers that are in contact with food and drinks for infants. Nevertheless, polymers containing BPA are widely used for food packaging applications. Moreover, monomers containing ethoxylated BPA are extensively used in restorative dentistry (BIS-GMA).¹⁶ Therefore, the recycle of PC through the formation of ethoxylated BPA compounds is, in our opinion, of interest from an environmental and industrial point of view.

PC, however, is not easily mechanically recycled due to its high processing temperature (exceeding 300 °C) and to its high sensibility to degradative processes caused by impurities.¹⁷ Moreover, in several cases (e.g. in optical storage systems) PC is combined with metallic materials that prevent the direct mechanical recycling without the separation of the different components. Therefore, there clearly arises the need for a chemical recycling method that can also be used for multi-phase materials (e.g. CDs and DVDs) performed with a process that does not use toxic solvents and employs renewable materials for the depolymerization process.

Powder coating is a very interesting class of eco-friendly coatings since its production process is solvent-free and therefore the problems linked to the emission of volatile organic compounds are completely absent.¹⁸ We recently reported the possibility to chemically recycle poly(ethylene terephthalate) using isosorbide and other naturally derived molecules (i.e. succinic acid and glycerol) for the production of polyesters for powder coating applications.^12,19 The method makes it possible to obtain coatings with properties comparable – and in some cases superior – to commercial ones, that are obtained using petrol derived materials.

To the best of our knowledge there is no scientific study that reports the chemical recycle of a polycarbonate-based product using renewable reagents. In this paper a method for the chemical recycling of PC scrap derived from optical storage media through the use of bio-based compounds is reported. The synthesis of monomers and oligomers suitable for powder coating applications via two different procedures (one going through the formation of BHEEB as intermediate and one yielding a recycled polymer in a one-pot process) that do not employ toxic solvents and reagents has been developed. The present methods can also be applied to the recycling of PC derived from other sources (e.g. from building and construction) and for the production of other types of polymers (e.g. polyurethanes).

Experimental

Materials

Ethylene carbonate (EC), sodium hydroxide, methanol, potassium hydroxide, tetrahydrofuran, hydrochloric acid, isosorbide (IS), succinic acid (SA), phenolphthalein, acetic anhydride, monobutyltin oxide (MBTO), and potassium carbonate were all purchased from Aldrich Chemicals and not purified prior to use. Verbatim metal AzoTM, Verbatim CD-R and Emtec CD-R were used as postconsumer optical media storages.

Syntheses

Treatment of post-consumer CDs to remove the metallic layer and dyes. Stripes of adhesive tape (Scotch™ gold tape) were positioned on the aluminium film of the optical media storage. The peeling of the tape resulted in the complete removal of the aluminum layer. The extraction of the dye was performed in a Soxhlet apparatus using methanol at reflux for 30 minutes. The disk was then cut into small parts (0.2–1 cm²).

Synthesis of BHEEB from post-consumer PC. A post-consumer CD (28.6 g, 112.6 mmol) was ground to produce a powder containing mainly polycarbonate and aluminium fragments. The mixture was introduced into a 500 ml flask under nitrogen flux, equipped with mechanical stirring and reflux column with ethylene carbonate (20.3 g, 231 mmol) and K₂CO₃ (60 mg, 0.43 mmol). The heterogeneous batch was left to react at 200 °C under nitrogen atmosphere and reflux. The reaction proceeded vigorously and the particles started to melt into a homogeneous phase. After 1 hour, the temperature was lowered down to 100 °C and 150 ml of a 1 M suspension NaOH (6.30 g, 158 mmol) in methanol was added. After 2 hours at reflux the suspension was cooled to 25 °C, and HCl (15 ml 37% in 150 ml of methanol) was introduced. The suspension was then filtered in order to remove NaCl and aluminum particles. The solid fraction was washed with 3 portions of 50 ml of MeOH. The solvent was removed under reduced pressure and the material obtained was washed in water (300 ml) and filtered to obtain BHEEB (33.5 g, 94.1% yield).

Preparation of polyesters from BHEEB. In a round bottom flask equipped with mechanical stirring, nitrogen atmosphere and condenser, recycled BHEEB (20 g, 63 mmol), isosorbide (9.2 g, 63 mmol), succinic acid (14.6 g, 123 mmol), MBTO (21.6 mg, 300 ppm) were introduced. The mixture was heated at 200 °C for 1 hour under nitrogen atmosphere. Subsequently reduced pressure was applied for 1 hour at 60 mbar followed by a decreasing pressure gradient down to 0.1 mbar in 1 hour. The molten polymer was then recovered from the reactor.

Synthesis of oligomers for powder coating from PC in one-pot. In a round bottom flask equipped with mechanical stirring, nitrogen flux and reflux column, the PC flakes (15.05 g, 59.25 mmol of repeating units) obtained after removing the aluminium layer, ethylene carbonate (11.50 g, 130.2 mmol) and K₂CO₃ (30 mg, 0.22 mmol) were introduced. The mixture was heated at 200 °C for 1 hour. A strong effervescence was observed within few minutes. Isosorbide (864 mg, 5.92 mmol) was introduced after 1 hour and the mixture was left to react for further 30 minutes. The molten polymer was then recovered from the reactor.

Instrumental

NMR spectroscopy. ¹H NMR analyses were performed at room temperature on samples dissolved in CDCl₃ with TMS as the internal reference using a Varian Mercury 400 spectrometer operating at 400 MHz.

Gel permeation chromatography. Molecular weight data were obtained by gel permeation chromatography at 30 °C using a 1100 Agilent Series system with an UV spectrophotometer (at 254 nm wavelength) as detector, equipped with Agilent PLgel 5 μm MiniMIX-C column. Chloroform was used as eluent with a 0.3 ml min⁻¹ flow with a sample concentration of about 1 mg ml⁻¹. A molecular weight calibration curve was obtained using monodisperse polystyrene standards.

Thermal analysis. Calorimetric measurements were carried out using a Perkin Elmer DSC6 instrument equipped with a liquid sub-ambient accessory and calibrated with high purity standards. Weighted samples of approximately 10 mg were encapsulated in aluminum pans and heated from 40 °C to 220 °C at a rate of 20 °C min⁻¹ (1^st scan), held at this temperature for 1 minute, cooled to −10 °C at a rate of 20 °C min⁻¹, held at this temperature for 5 minutes and then heated to 220 °C at a rate of 10 °C min⁻¹ (2^nd scan).

A first scan was carried out in order to erase the sample thermal history. During the second scan the glass transition temperature (T_g), the midpoint of the heat capacity increment (ΔC_p) associated with the glass-to-rubber transition, the exothermic peak of crystallization temperature (T_cc) with the relative enthalpy (ΔH_cc), if present, and the melting temperature (T_m) with the relative fusion enthalpy (ΔH_m) were measured.

Thermogravimetric analysis. The thermogravimetric analysis (TGA) was performed using a Perkin-Elmer TGA4000 thermobalance under nitrogen atmosphere (gas flow 20 ml min⁻¹) at 20 °C min⁻¹ from 50 °C to 600 °C. The temperature of the maximum degradation rate (T_D), corresponding to the maximum of the differential thermogravimetric curve, was determined.

Determination of the carboxylic and hydroxyl end-groups. The determination of the carboxylic end groups was made by titration, according to DIN 53402, using a standard potassium hydroxide 0.1 N solution in methanol. The sample was dissolved in tetrahydrofuran at room temperature, then the solution was titrated using phenolphthalein as an indicator until a persistent colour change was observed.

The determination of the hydroxyl end-groups was performed, according to DIN 5324, by titration of the acetic acid formed after acetylation reaction of the hydroxyl groups using a standard KOH 0.5 N methanolic solution. The acetylation reaction was performed adding acetic anhydride to the sample dissolved in tetrahydrofuran at room temperature. The solution was then titrated using phenolphthalein as an indicator until a persistent colour change was obtained.

Results and discussion

BHEEB synthesis from PC

The strategy here developed to recycle PC is based on the possibility to depolymerize it and produce BHEEB that is then exploited for polyester synthesis. Indeed, it is also possible to recover bisphenol A (BPA) from PC wastes through depolymerization. However, BPA will not react with carboxylic acids to obtain polyesters, since it features aromatic hydroxylic groups that are thermodynamically too stable to react with carboxylic acids. For this reason, aromatic OH groups of BPA must be converted into aliphatic hydroxylic groups, generally by using molecules containing ethylene units. Therefore, BPA is converted in BHEEB, which is the most widely used dihydroxy aliphatic compound containing BPA.^7–10 It can be obtained by reaction of BPA with ethylene oxide or with ethylene carbonate (EC).⁷ The latter process is generally preferred since ether linkages can be formed when using ethylene oxide, as caused by the reaction of ethylene oxide with OH groups of BHEEB. Moreover, EC can be prepared using renewable resources²⁰ and is an intermediate of the phosgene free route for the synthesis of PC.²¹

On the other hand, in the present study BHEEB is directly obtained from PC by insertion, catalysed by bases,⁷ of ethylene carbonate into the polymeric chain of PC. The subsequent reaction of the aliphatic carbonates formed with sodium hydroxide results in the complete depolymerization of the macromolecular structure, affording BHEEB in high yields. Although such a method has been previously reported in the literature^7–9 here we report the first attempt to use a multicomponent feedstock (i.e. PC from a postconsumer commercial CD without performing any separation of the metallic layer). Furthermore, the procedure (Fig. 1) was optimized in order to use an environmentally friendly catalysts (K₂CO₃) and to minimize the use of solvents.

Fig. 1 BHEEB synthesis by chemical recycling of PC.

Moreover, no purification steps are involved in the procedure except for the filtration of the aluminium that constitutes the reflecting layer of the CD. With this method pure BHEEB can be obtained, as confirmed by the NMR spectrum shown in Fig. 2, in high yields (94.1%). No discoloration or presence of particles was observed.

Fig. 2 ¹H NMR analysis of BHEEB obtained from PC.

Synthesis of polyesters from BHEEB and renewable resources

BHEEB from recycled PC was subsequently polymerized with bio-based succinic acid (SA) using a tin based catalyst (MTBO) to obtain, by melt polycondensation, a new polyester, called BHEEB/SA, with a T_g of 32 °C. Moreover, in order to improve the characteristics of the final polymer, more specifically to achieve an increment of the T_g value, in view of coating applications, some further chemical modifications in the final structure were carried out. A second rigid diol, isosorbide (IS), which is also obtained from renewable resources, has been used to improve the rigidity of the structure. The synthetic pathway is described in Fig. 3. The resulting material is a copolyester, called BHEEB/SA/IS, where BHEEB/SA and BHEEB/IS comonomeric units are randomly distributed along the macromolecular chain.

Fig. 3 Co-polyester from BHEEB, SA and IS.

The polyesters thus obtained present low molecular weight suitable for coating applications (Table 1). Their composition was determined by ¹H NMR analysis considering representative signals of each component (Fig. 4) and is reported in Table 1. The chosen signals were respectively 1.63 ppm for BHEEB (signal e, 6H), 2.60–2.73 ppm for succinic acid (signal f, 4H) and 5.09–5.25 ppm for isosorbide (signals h–l, 2H) (Fig. 4).

Table 1 Molar composition of the different species, BHEEB (N_BHEEB) and isosorbide (N_IS), respect to the amount of succinic acid (N_SA), molecular weight of BHEEB based polyesters

Sample	Feed compositions		Final compositions		Molecular weight
	NBHEEB/NSA	NIS/NSA	NBHEEB/NSA^a	NIS/NSA^a	Mn × 10^−3^b	Mw × 10^−3^b
a Determined by ^1H NMR.b Determined by GPC.
BHEEB/SA	1.00	—	1.13	—	2.8	9.2
BHEEB/SA/IS	0.500	0.500	0.635	0.385	3.7	12

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Fig. 4 ¹H NMR analysis of BHEEB/SA/IS polyester.

Moreover, the NMR analysis of the polymer obtained shows that no side-reactions occur and only the peaks due to the expected co-polymer are present. Finally, a considerable reduction of the molar fraction of succinic acid and isosorbide was observed as a consequence of isosorbide and succinic acid higher volatility with respect to BHEEB (Table 1). The thermal behavior of the samples is described in Table 2. BHEEB/SA is characterized by a T_g equal to 32 °C, significantly high thanks to the rigid structure of the BHEEB units. In comparison, the polyester derived from succinic acid and an aliphatic diol, for example 1,4-butandiol for poly(butylene succinate), is characterized by a high chain flexibility and a T_g value of −30 °C. Moreover, due to its chemical structure, BEEHB/SA does not have the characteristics to crystallize and the final material is fully amorphous.

Table 2 T_g, degradation temperature (T_D), carboxylic end-group number (AN) and hydroxyl end-group number (OHN) of BHEEB based polyesters

Sample	Tg^a (°C)	TD^b (°C)	OHN^c (mgKOH g^−1)	AN^c (mgKOH g^−1)
a Determined by DSC at 10 °C min^−1.b Determined by TGA in nitrogen at 20 °C min^−1.c Determined by titration.
BHEEB/SA	32	453	40.4	5.15
BHEEB/SA/IS	44	459	17.6	6.78

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The BHEEB/SA/IS copolymer is also amorphous and the fused bicyclic structure of isosorbide confers additional rigidity to the macromolecular backbone. Indeed, such polymer presents an increased T_g (44 °C) if compared with the polyester based just on BHEEB and succinic acid (Table 1). In order to obtain polymers with mainly OH end-groups, i.e. a polymeric structure suitable to coating applications, an excess of diol (15%) has been used. The titration of end-groups indicates that both BHEEB-based polymers are mainly OH terminated. Thermal degradation of the materials was evaluated by TGA analysis. As reported in Table 2, the different composition of the materials does not deeply affect the stability of the polyesters that present maximum degradation at temperatures above 450 °C.

It is noteworthy that the method here described is just an example of the possibility to use BHEEB obtained from chemical recycle. Indeed, BHEEB obtained from PC can be used to modify poly(ethylene terephthalate) and increase its glass transition temperature, its thermal stability and flame retardancy.^7–10

One-pot synthesis of ether–carbonate oligomers

A second recycling route of PC from CD wastes has been also developed. In this second method oligomers are formed avoiding the complete depolymerization of PC into monomers. Since this procedure does not involve the complete depolymerization and the use of solvents, the optical device needs to be treated to remove the aluminum layer and the dyes contained in the CD. This process must be performed before the chemical recycling process since the viscous nature of the final molten oligomers would not allow a proper filtration of the aluminum particles. Moreover, if the dyes are not removed, they could produce a colored coating.

Fig. 5 reports the stages of the removal of aluminum and dyes. In the first step the aluminum film is physically removed by means of a commercial adhesive tape. The inorganic foil, in fact, detaches easily from the CD leaving the surface completely metal-free. Then the dye is extracted from the surface of the CD by means of a Soxhlet extractor in methanol for 30 minutes. Water extraction of the dye was successful for Verbatim metal-azo devices but proved to be less efficient for other devices.

Fig. 5 Treatment of the optical device: (1) post-consumer CD, (2) after removal of aluminium layer, (3) after extraction of the dye.

After this treatment PC was used for the preparation of coatings without any further purification step. The synthesis of oligomers deriving from PC is performed by insertion of two equivalents of ethylene carbonate for BPA unit, producing a poly(ether-carbonate) (Fig. 6, step 1).

Fig. 6 Synthesis of PC oligomers by reaction with isosorbide.

The mechanism of this reaction has been previously reported in the literature and involves a selective decarboxylation reaction that produces aromatic–aliphatic ether moieties and aliphatic–aliphatic carbonate moieties.²² The insertion of aliphatic groups allows to increase the polymer chain mobility thus reducing the T_g of commercial PC (145 °C) to the desired range of value (45–60 °C) for coating applications. In Table 3 are reported the physico-chemical data of the obtained poly(ether-carbonate): this polymer presents an amorphous character and a T_g of 54 °C and the weight average molecular weight (M_w) of the polymer is reduced from 35 000 of the starting PC to 19 300.

Table 3 Molecular weight, T_g, degradation temperature (T_D) and hydroxyl end-group number (OHN)

Sample	Mn × 10^−3^a	Mw × 10^−3^a	Tg^b (°C)	TD^c (°C)	OHN^d (mgKOH g^−1)
a Determined by GPC.b Determined by DSC at 10 °C min^−1.c Determined by TGA in nitrogen at 20 °C min^−1.d Determined by titration.
PC	11.0	35.0	145	494	0
Step 1	7.7	19.3	54	341	15.0
Step 2	2.4	6.2	44	344	48.1

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In order to obtain oligomers suitable for powder coating applications, the molecular weight should be further decreased while the hydroxyl number should be increased. For this purpose, a partial depolymerization of the poly(ether-carbonate) using a diol has been further performed (step 2). The chosen diol was the isosorbide, because of its rigid structure and bio-based nature (Fig. 6). It is noteworthy that the procedure entirely avoids the use of catalysts, solvents and purification steps since it is performed in one pot.

The alcoholysis of the isosorbide on the polymer obtained in step 1 further decreases the molecular weight of the oligomers. However, their glass transition temperature does not have a consistent lowering since the rigid structure of the isosorbide units partially compensates the effect of molecular weight decrease. In this way a final T_g of 44 °C is obtained, which, together with a high content of OH end-groups, makes the oligomers appropriate for powder coating applications.

The degradation temperature as well is affected during the process. As expected, the introduction of aliphatic carbonates in the polymer structure reduces the temperature of thermal degradation from 494 °C to 341 °C in the first step due to the low thermal stability of aliphatic–aliphatic carbonates groups.²² The presence of isosorbide in the second step does not particularly affect the thermal stability of the material. Nevertheless, the polymer obtained does not present a weight loss below 300 °C and therefore features a sufficient thermal stability to be used for powder coating applications.

The NMR analysis (Fig. 7) shows that isosorbide is incorporated into the polymer backbone and that no side reaction occurs. In particular, signals at 5.2–5.3 ppm (i + m) indicate that isosorbide is directly connected to a carbonate group while signals at 3.6 ppm (q¹) and 4.7 ppm (p) refer to isosorbide terminal groups.²³ The final amount of isosorbide is 0.07 equivalents with respect to the repeating unit although 0.1 equivalents were introduced in the feed. According to these results we can estimate that 84.3% of the isosorbide has a terminal position while 15.7% is incorporated into the macromolecular chain. Signals at 3.9 and 4.1 ppm (f and g, respectively) are related to BHEEB terminal groups. Such functionalities represent 67% of the total terminal hydroxyl groups. In particular ¹H NMR analysis demonstrates that 21% of the total final terminal groups are generated during the first step because of the reduction of molecular weight. During the second step a larger number of BHEEB terminal groups are produced because of the reactions of the isosorbide.

Fig. 7 ¹H NMR analysis of final oligomers.

Conclusions

A new process for the chemical recycling of bisphenol A polycarbonate (PC), directly performed on post consumers compact discs, has been developed. The method uses molecules that can be derived from renewable resources (isosorbide, ethylene carbonate, and succinic acid). The polymers obtained present properties that make them suitable for powder coating applications. The pure BHEEB that have been obtained in the first step could be in principle used for other applications, such as for example for the synthesis of polyesters with increased glass transition temperature and improved flame retardancy. A second method that involves a one-pot reaction of post-consumer PC with ethylene carbonate, without complete degradation to low molecular weight molecules, has also been developed and, in this case as well, polymers were obtained that can be used for coating applications. Also, in this case no solvents were used except for methanol that has been employed to extract the dyes present on certain types of CDs.

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