Process for recycling at least one polyolefin containing impurities

Abstract

The invention relates to a process for recycling at least one polyolefin containing impurities, in which polymer-containing waste containing the at least one polyolefin is provided and the at least one polyolefin is selectively dissolved in at least one solvent. Subsequently, at least one precipitant is added to the polyolefin solution, wherein before, during or after this addition, a temperature of the mixture is adjusted which leads to the melting of a portion of the at least one polyolefin and to the formation of a melt phase and a liquid phase with a higher proportion of polyolefin than the melt phase. This is followed by mechanical separation of the melt phase from the liquid phase and drying of the liquid phase and / or the melt phase to obtain a low-impurity polyolefin fraction and an impurity-enriched polyolefin fraction as solids.

Description

[0001] The invention relates to a process for recycling at least one polyolefin containing impurities, in which polymer-containing waste containing the at least one polyolefin is provided and the at least one polyolefin is selectively dissolved in at least one solvent. Subsequently, at least one precipitant is added to the polyolefin solution, wherein before, during or after this addition, a temperature of the mixture is adjusted which leads to the melting of a portion of the at least one polyolefin and to the formation of a melt phase and a liquid phase with a higher proportion of polyolefin than the melt phase. This is followed by mechanical separation of the melt phase from the liquid phase and drying of the liquid phase and / or the melt phase to obtain a low-impurity polyolefin fraction and an impurity-enriched polyolefin fraction as solids.

[0002] Plastic waste often contains contaminants that have migrated to the plastic products during production (e.g., printing inks, processing aids), during use (e.g., contact materials, the environment), or during disposal (e.g., cross-contamination from other waste). These contaminants can usually only be inadequately removed through pretreatment or filtration processes, even in solvent-based recycling, and thus remain in the recyclate. This leads to undesirable discoloration and odors in the material and can also negatively impact processability and mechanical properties.

[0003] To date, solvent-based recycling processes for separating coarser contaminants after solid-liquid separation have used extraction processes (WO 2022 / 029318 A1) or liquid filtration processes (WO 2018 / 068973 A1) using an additional solvent immiscible with the solvent used for further purification of dyes, pigments, or degradation products. Nevertheless, very fine particles, such as soot, often remain in the recyclate. These cannot be separated by these processes and impair the color of the recyclate, resulting in an undesirable gray tone.

[0004] Another option for purifying polymer solutions is to precipitate the polymer from the solution by cooling or adding a precipitant. In one case, the target polymer can be completely precipitated, while the impurities remain in the solvent and are removed from the polymer by solid-liquid separation (EP 1 888 677 B1). However, this approach also has limited effectiveness, as fine contaminant particles often remain trapped in the precipitating polymer and are thus not separated.

[0005] In another case (WO 03 / 018679), only a portion of the polymer is precipitated, which encloses the fine dirt particles and is subsequently removed from the remaining precipitation solution by solid-liquid separation. The remaining dissolved portion of the polymer can then be recovered from the purified precipitation solution. However, this variant results in a high loss of target polymer and involves a technically complex solid-liquid separation of the polymer and the precipitation solution.

[0006] In addition, polyolefin recyclates with a residual solvent content of less than 100 ppm are already known from DE 197 44 436 A1. In this process, the starting material is brought into contact with a solvent which dissolves its polyolefinic plastic components, the dissolved polyolefins are separated from the other components of the starting material and processed into a polyolefin mixture, the viscosity of the polyolefin mixture and / or the polyolefin contents in the mixture are measured, polyolefins are added to the solution in order to adjust the desired viscosity and / or mixing ratio, and the polyolefin mixture is further processed into a processable polyolefinic plastic. Based on this, the object of the present invention was to provide a process for recycling polyolefins which enables an improved separation of impurities and at the same time makes the process as economical and ecological as possible, e.g.with the lowest possible use of solvents.

[0007] This object is achieved by the method having the features of claim 1. The further dependent claims show advantageous developments of the invention.

[0008] According to the invention, a process for recycling at least one polyolefin containing impurities is provided, comprising the following steps: a) providing polymer-containing waste containing the at least one polyolefin, b) Selectively dissolving the at least one polyolefin in at least one solvent, c) adding at least one precipitant to the polyolefin solution from step b), wherein before, during and / or after the addition in step c), a temperature of the mixture is set which leads to the melting of a part of the at least one polyolefin and to the formation of a melt phase and a liquid phase, d) Mechanical separation of the melt phase from the liquid phase and e) drying the liquid phase and / or the melt phase to obtain a low-impurity polyolefin fraction and an impurity-enriched polyolefin fraction as solids.

[0009] The process according to the invention allows impurities to be removed from a polymer solution by partial precipitation using a precipitant. The process is particularly suitable for polymer solutions made of polyethylene and polypropylene. These can, for example, originate from the solvent-based recycling of mixed waste fractions.

[0010] After the solid-liquid separation of the solution and the undissolved residue, and possibly a further purification step, a dark gray, highly viscous solution is obtained. Suitable solvents and precipitants can be identified using Hansen's solubility parameters.

[0011] Preferably, the at least one solvent has a value for the hydrogen bond strength δ H of the Hansen solubility parameter from 0 to 10.0 MPa 0.5 , preferably from 0.1 to 6.0 Mpa 0.5 , particularly preferably from 0.2 to 3.0 MPa 0.5 and / or preferably a value for the polar bond strength δ P of the Hansen solubility parameter from 0 to 8.0 MPa 0.5 , preferably from 0.1 to 5.0 MPa 0.5 , particularly preferably from 0.2 to 3.0 MPa 0.5 on.

[0012] Preferably, the at least one solvent is selected from the group consisting of hydrocarbon compounds, preferably aliphatic hydrocarbon compounds, particularly preferably cycloaliphatic, linear or branched hydrocarbon compounds, in particular cycloaliphatic, linear or branched hydrocarbon compounds having 5 to 18 carbon atoms and mixtures thereof.

[0013] Preferably, the at least one precipitant has a value for the hydrogen bond strength δ H the Hansen solubility parameter of at least 5.0 MPa 0.5 , preferably in the range of 8.0 to 15.0 MPa 0.5 , particularly preferably from 12.0 to 22.0 MPa 0.5 and / or preferably a value for the polar bond strength δ P of the Hansen solubility parameter from 4.0 to 20.0 MPa 0.5 , preferably from 4.0 to 17.0 MPa 0.5 , particularly preferably from 6.0 to 15.0 MPa 0.5 on.

[0014] The at least one precipitant is preferably an alkyl ester, preferably a dicarboxylic acid dialkyl ester, particularly preferably dimethyl ester or diethyl ester of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid and mixtures thereof.

[0015] The polymer content in the polymer solution can preferably be from 1 to 20 wt.%, preferably between 4 and 10 wt.%.

[0016] The temperature during precipitation in step c) essentially corresponds to the dissolution temperature or is above the dissolution temperature. Preferably, the precipitation temperature and the dissolution temperature are in the range from 130°C to 190°C, particularly preferably in the range from 130°C to 160°C.

[0017] The ratio between the amount of solvent and the amount of precipitant in order to achieve partial precipitation of the polymer depends on the temperature of the solution and is preferably in the range from 3:1 to 1:5, more preferably in the range from 3:1 to 1:4, particularly preferably in the range from 2.5:1 to 1:3.

[0018] It is preferred that the mixture of solvent and precipitant has a density greater than the density of the melt phase. Preferably, the mixture of solvent and precipitant has a density ≤ 1 g / cm 3 preferably ≤ 0.9 g / cm 3 , and particularly preferably ≤ 0.75 g / cm 3 .

[0019] The precipitation process is initiated by adding the precipitant at the required process temperature. An alternative is to add the precipitant at a higher or lower temperature and then heat or cool the mixture to the process temperature. Under these process conditions, part of the polymer precipitates as a polymer-rich liquid melt phase, while the remaining part remains as a second liquid phase in a polymer-poor precipitation solution. Less than 50% of the polymer precipitates in the liquid melt phase.

[0020] The fine particulate impurities in the solution, i.e., dyes, pigments, and degradation products such as carbon black and titanium dioxide, are essentially bound in the melt phase, thereby purifying the precipitation solution. Due to the concentration of the impurities, the melt phase has a dark gray to black color, while the remaining precipitation solution is whitish to light green.

[0021] The process conditions are chosen so that as little polymer as possible goes into the melt phase, but as many impurities as possible are bound in it.

[0022] Due to its lower density, the melt phase collects at the surface of the precipitation solution, where it can be easily separated by process technology. This can be achieved preferably by filtration or decanting the precipitation solution. This removes impurities such as dyes, pigments, fine particles, or cross-linked regions in the polymer. Since the described process also involves a size separation of the polymer molecules into the two phases, partial precipitation can also be used preferentially to adjust the molecular weight.

[0023] It is preferred that the drying in step e) is carried out at a temperature preferably from 25 to 275°C, preferably from 100 to 265°C, particularly preferably from 150 to 250°C.

[0024] A preferred embodiment provides that drying in step e) is carried out by evaporation of the solvent or precipitation of the polyolefin by adding a precipitant or lowering the temperature, followed by mechanical separation. The dried product is colorless to slightly greenish.

[0025] The process according to the invention offers the advantage that even the smallest impurities that cannot be separated from the polymer by other processes collect there through the precipitation of a partial fraction and can thus be removed from polymer solutions in a targeted and effective manner. In contrast to prior art processes, precipitation in the form of a melt achieves significantly higher purification efficiency with less material loss. At the same time, the form of a coherent melt phase and its floating due to its lower density enable a significantly easier separation of the precipitated partial fraction.

[0026] A further advantage of the process according to the invention is that precipitation results in a significantly lower proportion of solvent bound in the polymer fractions. This significantly reduces the energy required to dry these fractions. Furthermore, the precipitation of a partial fraction allows for separation by molecular weight. The precipitated polymer contains significantly longer polymer chains than the remaining solution. This also allows for the separation of different types of the same plastic from a mixed solution.

[0027] A polyolefin recyclate is also provided, which can be produced according to the process described above. The polyolefin recyclate has a maximum content of at least one solvent in the range of 1000 ppm and a maximum content of at least one precipitant in the range of 1000 ppm.

[0028] It is preferred that the proportion in the polyolefin recyclate of the at least one solvent is in the range of 50 to 500 ppm and / or of the at least one precipitant is in the range of 50 to 500 ppm.

[0029] The method according to the invention will be described in more detail with reference to the following examples and figures, without wishing to restrict it to the specific embodiments described here. Fig. 1 shows a photograph of recyclate samples according to the invention compared to non-inventive ones

[0030] In Fig. Figure 1 shows photographs of 500 µm thick press films. Shown are: • a LDPE virgin sample (1), • a polymer recycled according to the state of the art from the comparative example without precipitation (2), • a low-impurity fraction of the polymer (3) recycled according to the invention, • a fraction of the polymer (4) recycled according to the invention enriched with impurities, • a polymer purified according to the state of the art from comparative example (5) and • a recycled polymer (6) precipitated according to the state of the art from the comparative example. Example

[0031] A waste fraction containing polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) is processed at 115 °C in the solvent mixture CreaSolv ® PO63 HI dissolved. After separating the undissolved components, a dark gray solution (sample 2) is obtained, which is rich in PE and contains fine particulate impurities in addition to the plastic. By adding the precipitant CreaSolv ®FR1 in a 1:1 ratio to the solvent and increasing the temperature to 148°C, a small portion of the polymer precipitates from the solution, while the majority of the polymer remains dissolved. The precipitated polymer appears as a highly viscous, polymer-rich melt, has a dark gray-black color, and collects on the surface of the solution. The solution undergoes a distinct brightening to a light green color. By skimming off the floating melt or draining the solution, the two phases are separated. The polymer remaining in the solution (sample 3) is then dried, has a light green color after drying, and is largely free of particles ( Fig. 1). By drying the polymer in the melt phase (sample 4), a black-colored product with a high proportion of fine particles is obtained.

[0032] To evaluate the color, a color value determination can be performed in the LAB color space. To ensure comparability between different recyclates, the recyclate samples are pressed into round pressed films with a thickness of 500 µm using a hydraulic hot press. Using a DigiEye-700 mm Cube color measurement system from VeriVedi with the DigiEye software version 2.8.0.3, the LAB color value is determined according to DIN EN ISO / CIE 11664-4. To compare LAB color values, the CIEDE2000 color difference can be calculated according to DIN EN ISO / CIE 11664-6. Table 1 shows the LAB color values ​​of the dried samples and their CIEDE2000 color difference to a reference sample of natural-colored (transparent) virgin LDPE of the type Lupolen 1800H (sample 1). Table 1 Probe L a b CIEDE2000 1 95 -1 5 0 2 51 -2 7 33 3 83 -7 4 11 4 42 -1 -2 43 5 53 -2 7 31 6 46 -3 -1 38

[0033] Table 1 shows the high purification of the solution through precipitation, resulting in a color difference of 11 between the dried solution with precipitation (sample 3) and the virgin material (sample 1). Without precipitation (sample 2), a significantly higher color difference of 33 to the virgin material is evident. The enrichment of the removed contaminants in the filtered precipitated polymer (sample 4) is reflected in the high color difference of 43 to the virgin material (sample 1). Comparison example

[0034] For comparison, a reference sample was produced according to the method described in WO03 / 018679.

[0035] A waste fraction containing PE, PP and PET was processed at 115 °C in the solvent mixture CreaSolv ®PO63 HI dissolved. After separating the undissolved components, a dark gray PE solution (sample 2) is obtained, which is rich in PE and contains fine particulate impurities in addition to the plastic. Lowering the temperature to 97 °C leads to partial precipitation of the solution. The precipitated polymer appears as dark gray-black flakes in the solution. The two phases are separated by filtration. The polymer remaining in the solution (sample 5) is then dried and has a dark green-gray color after drying ( Fig.1). By drying the filtered-off polymer (sample 6), a dark gray-colored product is also obtained. The ratio of polymer remaining in solution to precipitated polymer is 1:5. To evaluate the color, a color value determination in the LAB color space and a calculation of the CIEDE2000 color difference between the dried samples 5 and 6 and the virgin material can be determined. Table 1 shows that only a slight purification is achieved by precipitation, resulting in a high color difference of 31 between the dried solution with precipitation (sample 5) and the virgin material (sample 1), only slightly below the color difference of 33 of the reference sample without precipitation (sample 2). A slight enrichment of the removed contaminants in the filtered-off precipitated polymer (sample 6) is indicated by the color difference of 38 compared to the virgin material (sample 1).

Claims

[1] Process for recycling at least one polyolefin containing impurities, comprising the following steps: a) providing polymer-containing waste containing at least one polyolefin, b) Selectively dissolving the at least one polyolefin in at least one solvent, c) adding at least one precipitant to the polyolefin solution from step b), wherein before, during and / or after the addition in step c), a temperature of the mixture is set which leads to the melting of a part of the at least one polyolefin and to the formation of a melt phase and a liquid phase, d) Mechanical separation of the melt phase from the liquid phase and e) drying the liquid phase and / or the melt phase to obtain a low-impurity polyolefin fraction and an impurity-enriched polyolefin fraction as solids. [2] Method according to claim 1, characterized bythat the at least one solvent has a value for the hydrogen bond strength δ H of the Hansen solubility parameter from 0 to 10.0 MPa 0.5 , preferably from 0.1 to 6.0 MPa 0.5 , particularly preferably from 0.2 to 3.0 MPa 0.5 and / or a value for the polar bond strength δ P of the Hansen solubility parameter from 0.0 to 8.0 MPa 0.5 , preferably from 0.1 to 5.0 MPa 0.5 , particularly preferably from 0.2 to 3.0 MPa 0.5 has. [3] Method according to one of claims 1 or 2, characterized by that the at least one solvent is selected from the group consisting of hydrocarbon compounds, preferably aliphatic hydrocarbon compounds, particularly preferably cycloaliphatic, linear or branched hydrocarbon compounds, in particular cycloaliphatic, linear or branched hydrocarbon compounds having 5 to 18 carbon atoms and mixtures thereof. [4] Method according to one of claims 1 to 3, characterized by that the at least one precipitant has a value for the hydrogen bond strength δ H the Hansen solubility parameter of at least 5.0 MPa 0.5 , preferably in the range of 8.0 to 15 MPa 0.5 , particularly preferably from 12 to 22 MPa 0.5 and / or a value for the polar bond strength δ P of the Hansen solubility parameter from 4.0 to 20.0 MPa 0.5 , preferably from 4.0 to 17.0 MPa 0.5 , particularly preferably from 6.0 to 15.0 MPa 0.5 has. [5] Method according to one of the preceding claims, characterized by that the at least one precipitant is an alkyl ester, preferably a dicarboxylic acid dialkyl ester, particularly preferably dimethyl ester or diethyl ester of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid and mixtures thereof. [6] Method according to one of the preceding claims, characterized bythat the polymer content in the polymer solution 1 to 20 wt.% and preferably between 4 and 10 wt.%. [7] Method according to one of the preceding claims, characterized by that the temperature during the precipitation in step c) substantially corresponds to the dissolution temperature or is greater than the dissolution temperature, wherein it is preferred that the precipitation temperature and the dissolution temperature are in the range from 130 °C to 190 °C, particularly preferably in the range from 130 °C to 160 °C. [8] Method according to one of the preceding claims, characterized by that the ratio of solvent quantity to precipitant quantity is in the range 3:1 to 1:5, preferably 2.5:1 to 1:4, particularly preferably 2:1 to 1:

3. [9] Method according to one of the preceding claims, characterized bythat the mixture of solvent and precipitant has a density which is greater than the density of the melt phase, wherein it is preferred that the mixture of solvent and precipitant has a density ≤ 1 g / cm 3 , preferably ≤ 0.9 g / cm 3 , and particularly preferably ≤ 0.75 g / cm 3 has. [10] Method according to one of the preceding claims, characterized by that the mechanical separation in step d) is carried out by density separation, in particular sedimentation, decantation, centrifugation, filtration or combinations thereof. [11] Method according to one of the preceding claims, characterized by that the drying in step e) is carried out by evaporation of the solvent and / or by the addition of a precipitant and mechanical separation of the polyolefin.

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