Method for removing ink layers from waste plastic packaging

Abstract

A method for removing an ink layer from a first plastic material to obtain ink-less first plastic material, the method comprising the steps of: (a) providing raw material comprising at least a layer of the first plastic material and an ink layer applied to the said layer of first plastic material; (b) contacting the raw material with a metal oxide in a solution in the presence of a base to provide a reaction mixture; (c) stirring the reaction mixture during appropriate time under UV light while degrading the ink layer, wherein the first plastic material remains intact; and (d) recovering the first plastic material from the reaction mixture.

Description

[0001] Method for removing ink layers from waste plastic packaging

[0002] Technical Field

[0003] The invention relates to a method for removing ink layers from waste plastic material, in particular waste plastic packaging.

[0004] Technical Background

[0005] Creating a sustainable circular economy for plastics must be based on the replacement of virgin plastics by high quality recycled materials. However, printing inks on the packaging plastics and their thermal degradation products may contaminate the recycled material. Replacement of the commonly used inks is not an option, because companies are reluctant to use less intense inks due to the marketing impact of well-established brand trademarks and colours. Therefore, the removal of inks, which can be described as “deinking”, is an important factor when recycling plastics.

[0006] Crosslinked UV inks are particularly difficult to remove without specific primers, since they are designed for optimal chemical, mechanical and thermal resistance. Due to their favourable characteristics and energy efficient processing, the use of UV inks is constantly growing in food packaging. Therefore, it is highly beneficial to develop a process which can deink all types of inks irrespective of their type or exact formulation.

[0007] WO2023091639 describes a method to recover constituent polymers in multilayer plastic films or mixed plastic wastes. The method comprises selectively dissolving a polymer in a solvent. Thereby most inks are separated from the dissolved polymer. Alternatively, WO2023091639 describes the removal of ink by treating the multilayer plastic film or mixed plastic waste in a solvent comprising tetra hydrofuran and N,N-dimethylformamide. The method is carried out a high temperature above 90°C, which leads to considerable energy consumption. A further disadvantage are the environmental impact of the used solvents for dissolving the different plastics.

[0008] Summary of the Invention

[0009] It is an objective of the invention to provide a method for the removal of printed inks from plastic waste material, which is effective, inexpensive and can be easily combined with further recycling steps. At least one of the objectives of the present invention is achieved by a method according to claim 1. The method for removing an ink layer from a first plastic material to obtain ink-less first plastic material comprises the steps of: (a) providing raw material comprising at least a layer of the first plastic material and an ink layer applied to the said layer of first plastic material; (b) contacting the raw material with a metal oxide in a solution in the presence of a base to provide a reaction mixture; (c) stirring the reaction mixture during appropriate time under UV light while degrading the ink layer, wherein the first plastic material remains intact; and (d) recovering the first plastic material from the reaction mixture.

[0010] With the method printed inks on the packaging plastics could be efficiently removed to obtain high quality recycled materials. Even UV-based inks could be broken down at ambient temperature or at slightly elevated temperatures (up to 50°C) and at atmospheric pressure (i.e. no pressurizing of the reaction mixture is required). Thus, the advantage of the method is that it may be effectively performed below 60°C or even around ambient temperature (20-25°C) and under approx, normal atmospheric pressure, i.e. about 1013.25 mbar. The broken-down ink can then be washed off from the plastic material, which remained intact.

[0011] During deinking step c), ester bonds in the ink layers are prone to be broken by alkaline hydrolysis thanks to the reaction environment with a basic solution, UV light, and photocatalyst.

[0012] The raw material may be obtained by shredding plastic waste, in particular plastic packaging. The plastic waste may be sorted to obtain single-type plastic material e.g. polypropylene or polyethylene, including printed ink layers, or may be a mixture of such plastics.

[0013] In some embodiments the first plastic material may be a polyolefin, preferably polypropylene (PP) or polyethylene (PE), such as low-density PE (LDPE) and high-density PE (HDPE), or a polyvinyl chloride (PVC) or a polystyrene (PS). These plastics do not contain ester bonds, which would otherwise be broken by the reaction.

[0014] In some embodiments the ink layer may be a water-based ink, solvent-based ink or UV ink. Water-based inks are composed of pigment and resin diluted in water. Solvent-based inks include pigment and resin (binder), together constituting the solid component of the ink, diluted using a solvent different from water. UV inks dry by UV rays. Their immense advantage is fast curing and high level of adhesion. Such ink layers typically contain ester bonds.

[0015] In some embodiments the appropriate time may be as low as 10 minutes and up to 5 hours. In some embodiments step c) may be performed at a temperature between 20°C and 60°C, and under approx, normal atmospheric pressure. No control of pressure and / or temperature is needed. The reaction may be performed at ambient conditions.

[0016] In some embodiments the metal oxide is selected from the group comprising TiO2, V2O5, Cr2O3, CrO3, Mn2O3, FeO, Fe2O3, Fe3O4, Co2O3, NiO, CuO, Cu2O, ZnO, ZrO2, Nb2 05, Mo2 03, RuO, RuO2, RuO4, RhO2, Rh2 03, PdO, Ag2 O, Ag2 02, CdO, In2 03, AI2 03, La2 03, CeO2, Ce2 03, HfO2, Ta2 05, W03, ReO2, ReO3, Re2 03, OsO2, OsO4, lrO2, PtO2, Au2O3, Li20, Na20, K20, MgO, CaO, SrO, BaO, or combinations thereof. In some preferred embodiments of the method of the present invention, the metal oxide is selected from the group comprising TiO2, ZnO, ZrO2, Nb2O5, Ta2O5, RuO, Fe2O3, W03. Most preferably, the metal oxide is TiO2 or P25.

[0017] In some embodiments the solution may be a solution containing an alcohol and / or water, or the solution may be an aqueous alcoholic solution. Alcohol and water can be present at different ratios, such as alcohol : water from 100:0 to 0:100, or 90:10 to 10:90, or 80:20 to 20:80, or 50:50 to 90:10, preferably 50:50 or 80:20. The alcohol comprises 1 to 5 carbon atoms and / or the alcohol is selected from the group comprising methanol, ethanol, propanol, butanol, pentanol or combinations thereof. Preferably, the alcohol is ethanol. More preferably the aqueous alcoholic solution is 80:20 ethanol : water solution. Most preferably the solution is ethanol or 90:10 to 10:90 ethanol : water solution.

[0018] In some embodiments of the method of the present invention, the base may be selected from the group comprising NaOH, NaO‘Bu, KOH.

[0019] In some embodiments, the alkaline hydrolysis may be carried out at pH 7 to 14, or 8 to 13, or 9 to 12, or 7 to 12, or 7 to 10, or 7 to 9, or 8 to 14, or 8 to 12, or 8 to 10, or 9 to 14, or 9 to 12, or 9 to 10.

[0020] In some embodiments of the method of the present invention, the ratio plastic polymer : base may be from 1 :1 to 1 :20 or 1 :1 to 1 :10. In preferred embodiments, the ratio plastic polymer : base is 1 :20, 1 :7.5, 1 :1 or 1 :3. In other preferred embodiments, the ration plastic polymer : base is 2:1 to 3:1 , preferably 2:1 or 3:1. In some embodiments of the method of the present invention, the ratio plastic polymer : metal oxide may be from 1 :0.0375 to 1 :0.00125. In preferred embodiments, the ratio plastic polymer : metal oxide may be 1 :0.0375, 1 :0.015, 1 :0.0075, or 1:0.00125.

[0021] In some embodiments of the method of the present invention, the UV light (ultraviolet light) may have a wavelength in the range from 100 to 400 nm, preferably in the range of from 315 to 400 nm. In other embodiments of the method of the present invention, the intensity of the light may be in the range 1 to 150 mW / cm2, such as 10 to 150 mW / cm2, such as 50 to 150 mW / cm2, such as 90 to 150 mW / cm2, such as 130 to 145 mW / cm2. The light intensity may be around 100 mW / cm2.

[0022] In some embodiments the reaction mixture may further comprise a surfactant, preferably a cationic surfactant selected from the group of cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), and alkyldimethylbenzylammonium chloride (ADBAC). The preferred surfactant is CTAB.

[0023] In some embodiments the recovering step d) may comprise washing the first plastic with water optionally followed by subsequent washing with alcohol, preferably ethanol or methanol. The presence of the surfactant in the reaction mixture may be used to improve the removal of the degraded ink molecules from the surface of the first plastic material.

[0024] The inventors found out that their previously developed method for depolymerization of PET as described in W02020173961 may also be used to efficiently remove ink layers from plastic packaging waste comprising PP and / or PE. The disclosure of W02020173961 , in particular the examples, are incorporated herewith by reference.

[0025] In some embodiments the method may be carried out wherein the raw material is multilayered comprising an additional layer of second plastic material, wherein the second plastic material is polyethylene terephthalate (PET).

[0026] In some embodiments the layer of polyethylene terephthalate (PET) is depolymerized into terephthalic acid (TPA) and ethylene glycol (EG) during step c).

[0027] Thus, the method may be used to recycle multi-layered waste plastic material comprising PE / PET or PP / PET including ink layers. During the same reaction, the PET layer is degraded I depolymerized, and the ink layer is removed from the remaining and intact PP or PE layers.

[0028] The degradation of the plastic material depends on the type of plastic material as starting material. A plastic material layer of PET (such as ID5 in this work) is depolymerized simultaneously with the deinking (sometimes called "delamination"). The deinking or depolymerization reactions targets similar types of bonds, mainly ester bonds, both in the PET layer and the ink layer. Other plastic material without these type of bonds (e.g. PP and PE) are not degraded during the reaction and therefore remain intact.

[0029] In some embodiments, terephthalic acid (TPA) and ethylene glycol (EG) may be recovered from the reaction mixture after step c) or d). Recovering terephthalic acid from the reaction mixture can be carried out by any suitable method, such as: - adding water into the reaction mixture until the reaction mixture is clear;

[0030] - separating the reaction mixture into a first solid phase and a first liquid phase;

[0031] - acidifying the first liquid phase (with for example concentrated HCI or H2SO4) until a terephthalic acid precipitate is formed;

[0032] - filtering the terephthalic acid precipitate and washing the terephthalic acid precipitate with water and alcohol (for example EtOH).

[0033] In some embodiments, degraded ink particles may be filtered out from the clear reaction mixture. Remaining ink molecules that cannot be filtered may be removed using the purification method described below before acidifying the first liquid phase.

[0034] In some embodiments, separating the reaction mixture into the first solid phase and the first liquid phase may include filtering the reaction mixture. Recovering ethylene glycol from the reaction mixture can be carried out by any suitable method, such as:

[0035] - collecting the dissolved ethylene glycol in the liquid phase;

[0036] - distilling the liquid phase until ethylene glycol is collected.

[0037] In some embodiments, the method of the present invention may further include recovering the metal oxide from the reaction mixture. The recovering of the metal oxide from the reaction mixture can be carried out by any suitable method, such as:

[0038] - adding water into the reaction mixture until the reaction mixture is clear;

[0039] - separating the reaction mixture into a first solid phase and a first liquid phase;

[0040] - separating the metal oxide from the first liquid phase (for example by filtration), or separating the metal oxide through washing the terephthalic acid with water and alcohol.

[0041] In some embodiments the TPA, which is obtained from depolymerization of the PET layers, may be further purified using graphite, activated carbon and molecular sieve to remove organic and inorganic contaminants and ink molecules if present. The purification may be performed with the method as described in U.S. provisional application No. 63425771 by the same applicant and incorporated herein by reference in its entirety.

[0042] The purification method may be performed before the above step of acidifying the first liquid phase during the recovery of TPA and EG and may include contacting unpurified terephthalic acid with graphite, activated carbon, and molecular sieve to provide a reaction mixture, stirring the reaction mixture for a first certain period of time, filtering the reaction mixture, to provide a reaction mixture filtrate, providing graphite, activated carbon, and molecular sieve to the reaction mixture filtrate, stirring the reaction mixture filtrate for a second certain period of time, filtering the reaction mixture filtrate, to provide a reaction output solution and precipitating purified terephthalic acid from the reaction output solution.

[0043] In connection with the purification method, the following features can be realised individually or in any combination with the aforementioned features:

[0044] - The reaction mixture may be stirred at a pH of 14.

[0045] - The reaction mixture filtrate may be stirred at a pH of 7.

[0046] - Graphite, activated carbon, and molecular sieve may be provided for contacting with the unpurified terephthalic acid at a 1 :6:2 ratio.

[0047] - Graphite, activated carbon, and molecular sieve may be provided to the reaction mixture filtrate at a 1 :6:2 ratio.

[0048] - Purified terephthalic acid may be precipitated from the reaction output solution using an acid, preferably hydrochloric acid or sulfuric acid.

[0049] - The first period of time may be between 10 and 120 minutes, preferably 30 minutes.

[0050] - The second period of time may be between 10 and 120 minutes, preferably 30 minutes.

[0051] - The method may be performed at room temperature.

[0052] - The graphite may comprise amorphous, crystalline or flake graphite, of a purity of 99% and a particle size range of 5-30 microns.

[0053] - The activated carbon may comprise particulate form activated carbon with a surface area of 500-1500 m2 per gram, and a particle size less than 1mm.

[0054] - The molecular sieve may comprise an aluminosilicate crystal molecular sieve, such as Zeolite 13X molecular sieve. Zeolite 13X comprises average pores measuring 9 angstrom, and may absorb molecules with a kinetic diameter smaller than 9 angstrom.

[0055] The purification method may be conducted as a batch operation, and performed in a reactor vessel, preferably equipped with an agitator, and a plug drain filter. In some examples, the method described herein may be conducted as a batch operation, across multiple reaction vessels, in a series configuration. In some examples, the method described herein may be modified to be operated as a continuous process.

[0056] The purification method may reliably output a purified terephthalic acid (PT A) which - when dried - is up to 1% purer and 5% brighter white in color than virgin terephthalic acid.

[0057] The output is typically a metalated salt of terephthalic acid (M-PTA, with “M” denoting a metal such as Na* or K* that may originate from a hydrolysis process) dissolved in water, at close to the maximum solubility of M-PTA in solution at 7 to 13% by weight. The output may then be subjected to a crystallization method.

[0058] Trials of the deinking method showed that even UV-based inks (next to the water and solvent based inks) from different formulations can be broken down at ambient temperature and pressure, or at slightly elevated temperature (50 °C) and room pressure. After deinking process, the broken-down ink could be washed off from the intact layers of plastic material.

[0059] In addition, the method can also be applied for the selective removal of polyethylene terephthalate (PET) layers from multilayer packaging material, separating the layers, and resulting in the recovery of the monomers terephthalic acid (TPA) and ethylene glycol (EG) in addition to the deinked intact plastic layer such as polypropylene (PP) or polyethylene (PE). This is an important step for deinking as well because the ink in the multilayer packaging materials is usually located between the layers and therefore it is hard to access it without delamination.

[0060] Brief Explanation of the Figures

[0061] The invention is described in greater detail below with reference to embodiments that are illustrated in the figures. The figures show:

[0062] Fig. 1 photographs (greyscale) of multi-layered raw material (PE / PET) before applying the deinking process and the first plastic material after the deinking process with I without surfactant;

[0063] Fig. 2 photographs (greyscale) of multi-layered raw material (PE / PET) before applying the deinking process and the first plastic material after the deinking process;

[0064] Figs. 3 to 13 FTIR spectrum measurements of several samples.

[0065] Embodiments of the Invention

[0066] Materials and Characterization Methods. Reagents and solvents were purchased from Sigma-Aldrich, TCI, and Carl Roth and used without further purification.

[0067] General considerations. Model samples and samples waste consumer packaging products were used to test the deinking process. The model samples were packaging materials that were produced to test ink quality. In the experiments, the samples were investigated for deinking and in some cases delamination purposes. Experimental conditions. Examples of the de-inking method are given in Table 1 showing the type of sample, i.e. the raw material and the experimental conditions. In the experiments, i) TiC>2 was used as the catalyst, ii) NaOH was used as the base, iii) 80 : 20 ethanol : water aqueous alcoholic solution was used as reaction solution, and iv) the UV light has a wavelength in the range from 100 to 400 nm, preferably in the range of from 315 to 400 nm.

[0068] The pH of the reaction mixture was about 14. Column "Ink type" describes the basis of the ink applied e.g. by printing to the layer of first plastic material (column "Plastic layer 1"). The ink may be applied as several layers of different colours (column "Ink layer(s)"). Sample ID5 has a further layer of second plastic material. The ink is then typically located between the layers of different plastic material.

[0069] Table 1

[0070] The samples were subject to the method for removing the ink layer(s) from the first plastic material to obtain ink-less first plastic material. Therefore, raw material with the specifications of Table 1 comprising at least a layer of the first plastic material and the applied ink layer(s) was contacted with a metal oxide in a solution in the presence of a base to provide a reaction mixture as explained above. The reaction mixture was stirred during appropriate time (columns "conditions") at ambient conditions (room temperature, room pressure) or at elevated temperature of 50°C (room pressure) under UV light to degrade the ink layer. Samples of first plastic material were recovered at different times during the reaction and washed. In sample ID5-2 and ID5-3 a cationic surfactant was added to the reaction mixture. Good results were achieved with cetyltrimethylammonium bromide (CTAB). The results are summarized in Table 2.

[0071] Table 2

[0072] Fig. 1 shows photographs (greyscale) of multi-layered raw material with PE and PET layer

[0073] (a) of samples ID5 (b), ID5-2 (c) and ID5-3 (d). Fig. 1 a) shows the raw material before applying the deinking process. Fig. 1 b) - c) show the layer of first plastic material (here PE) after the deinking process with conditions of samples ID5 (Fig. 1 b)), ID5-2 (Fig. 1 c)) and ID5-3 (Fig. 1 d)). For photographing, foils of raw material or the intact PE layers after the deinking were attached to a surface using transparent sticky tapes. The deinking in sample ID5 was only partial. Addition of surfactant lead to efficient deinking as shown in samples ID5-2 and ID5-3.

[0074] Five samples of waste consumer packaging products with unknown ink type were tested. The raw material was a multi-layered plastic with PE and PET layers and an ink layer in between (Table 3). The raw material was contacted with a metal oxide in a solution in the presence of a base and a cationic surfactant, such as CTAB, to provide a reaction mixture as explained above. The reaction mixture was stirred during appropriate time (columns "conditions") at elevated temperature of 50°C and room pressure under UV light to delaminated and depolymerize the PET layer and degrade the ink layer. Samples of first plastic material were recovered after 120 minutes and washed. The results are summarized in Table 3.

[0075] Table 3 Fig. 2 shows photographs (greyscale) of samples BID1 , BID2, BID3, BID6 and BID7 of multi-layered raw material with PE and PET layer before deinking and intact layer of first plastic material (PE) after deinking. For photographing, the samples of raw material and the resulting intact PE layers after the deinking were attached to a surface using transparent sticky tapes. All samples show efficient deinking.

[0076] FTIR Analysis. FTIR analysis was performed on each sample to detect the presence of ink on the surface of the first plastic material. The results are shown in Figs. 3 to 13: under (a) showing an FTIR spectra of the surfaces with ink before and after the deinking process; under (b) a comparison of the measured FTIR spectra of the surface of first plastic material (PP or PE) with a reference spectrum of pure and clean first plastic material (PP or PE). A summary of the FTIR spectrum measurements is given in Table 4.

[0077] Table 4 In all samples, after deinking the first plastic material showed a measured spectrum that matched well with a reference spectrum based on data retrieved from FTIR software. This proves that the deinking was complete even in presence of PET, which was successfully delaminated. Summary. The removal of different types of inks; such as solvent-based, oil-based and UV- based, from the plastic surfaces such as PET, PE or PP could be achieved. Delamination I depolymerization of PET in combination with the removal of inks could be achieved in the same process step. This is particularly efficient to recycle packaging material consisting of different layers of plastics, such as PET and PE. The PET layer can be degraded into its monomers, terephthalic acid and ethylene glycol, and be recovered with good purity and quality. At the same time, ink is removed not only from PET or its monomer fraction, but also from the intact PE material, which after washing may be reused with virgin quality.

[0078] The method is energy efficient as it works either at ambient conditions, or at room pressure with the temperature of as low as 50 °C.

Claims

Claims1 . A method for removing an ink layer from a first plastic material to obtain ink-less first plastic material, the method comprising the steps of: a) providing raw material comprising at least a layer of the first plastic material and an ink layer applied to the said layer of first plastic material; b) contacting the raw material with a metal oxide in a solution in the presence of a base to provide a reaction mixture; c) stirring the reaction mixture during appropriate time under UV light while degrading the ink layer, wherein the first plastic material remains intact; and d) recovering the first plastic material from the reaction mixture.

2. Method according to claim 1 , wherein the first plastic material is selected from the group of polyolefins preferably polypropylene (PP) or polyethylene (PE), such as low- density PE (LDPE) and high-density PE (HDPE), polyvinyl chloride (PVC) or a polystyrene (PS).

3. Method according to one of the preceding claims, wherein the ink layer is a waterbased ink, solvent-based ink or UV ink.

4. Method according to one of the preceding claims, wherein the appropriate time is 10 minutes to 5 hours.

5. Method according to one of the preceding claims, wherein step c) is performed at a temperature between 20°C and 60°C, and under approx, normal atmospheric pressure.

6. Method according to one of the preceding claims, wherein the metal oxide is selected from the group comprising TiO2, ZnO, ZrO2, Nb2O5, Ta2O5, RuO, Fe2O3, WO3.

7. Method according to one of the preceding claims, wherein the solution is ethanol or 90:10 to 10:90 ethanol : water solution.

8. Method according to one of the preceding claims, wherein the base is selected from the group comprising NaOH, NaO‘Bu, KOH.

9. Method according to one of the preceding claims, wherein the reaction mixture further comprises a surfactant, preferably a cationic surfactant selected from the group comprising cetyltrimethylammonium bromide (CTAB), cetyltrimethylammonium chloride (CTAC), and alkyldimethylbenzylammonium chloride (ADBAC).

10. Method according to one of the preceding claims, wherein the recovering step d) comprises washing the first plastic with water optionally followed by subsequent washing with alcohol, preferably ethanol or methanol.

11. Method according to one of the preceding claims, wherein the raw material is multilayered comprising a layer of second plastic material, wherein the second plastic material is polyethylene terephthalate (PET).

12. Method according to claim 11 , wherein the layer of polyethylene terephthalate (PET) is depolymerized into terephthalic acid (TPA) and ethylene glycol (EG) during step c).

13. Method according to claim 12, wherein terephthalic acid (TPA) and ethylene glycol (EG) are recovered from the reaction mixture after step c) or d).

14. Method according to claim 13, wherein the recovered terephthalic acid (TPA) is purified using graphite, activated carbon and molecular sieve to remove organic and inorganic contaminants.

15. Method according to one of the preceding claims, wherein the raw material comprises one type of first plastic material or a mixture of different types of first plastic material.

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