EP4622786A1

EP4622786A1 - Solvent-based plastic recycling using membranes

Info

Publication number: EP4622786A1
Application number: EP23841590.5A
Authority: EP
Inventors: Zahra BOZORGMEHR; Sareh REZAEI HOSSEINABADI; Laurens RUTGEERTS; Sutapa Roy SWARNA; Ivo Vankelecom
Original assignee: Katholieke Universiteit Leuven
Current assignee: Katholieke Universiteit Leuven
Priority date: 2022-11-21
Filing date: 2023-11-21
Publication date: 2025-10-01
Also published as: WO2024110485A1; CN120435367A

Abstract

Method of removing one or more solvent soluble additives from a polymer comprising less than 40 wt % polyvinylchloride or from a mixture of polymers comprising less than 40 wt % polyvinylchloride, the method comprising the steps of: a) dissolving said polymer or mixture of polymers comprising said one or more solvent soluble additives in a solvent or a solvent mixture that dissolves said polymer or polymer mixture, and that dissolves as well said one of more additives, b) filtrating the solution comprising the dissolved polymer or polymer mixture and the one or more dissolved additives over a solvent resistant membrane, whereby the dissolved polymer is retained by the membrane and the one or more solvent soluble additives permeate through the membrane.

Description

SOLVENT-BASED PLASTIC RECYCLING USING MEMBRANES

Field of the invention

The invention relates to membrane filtration and plastic recycling.

Background of the invention

The use of plastic creates challenges in waste management, which resulted in an accumulation crisis. Global demand for polymer production has been dramatically increasing, approximately 350 million metric tons per year. Only a few commercial and economical methods are well known for reusing polymers, while 55% of polymer waste is disposed in the environment or landfills, 25% is burned to recover energy, and only 25% is recycled.

Recycling process can be described to recover and reuse of waste materials even in different process, which makes a cleaner environment and improve the circular economy models. There are a number of recycling methods such as chemical, mechanical, and thermal recycling.

Chemical recycling needs different reactive reagents and solvents. Moreover, this method is time and energy consuming and costly to implement. Which makes, that this method is only under investigation just in a few number of companies due to the need for experts and big investments. In thermal recycling process, composites are heated at the temperature range of 350-800 °C. However, this method introduces impurities and creates toxic substances in the recycling process. 98% of plastics has been recycling by mechanical recycling method in Europe, which is so problematic due to the variety of plastics and additives and inability to remove and recovery of additives.

Solvent-based purification (dissolution precipitation) is an interesting way to recycle plastics, because of selective solubility of polymers in a mixture of polymers and dissolution process to separate additives including organic additives from polymers.

The Creasolv® process is typically based on (1) the dissolution of the waste polymeric material (containing the additives) in a solvent, and (2) recovering the purified polymer by the addition of an anti-solvent for the polymer, thus inducing its precipitation while keeping the unwanted additives in solution.

EP4 067 421 discloses methods wherein halogenated polymers are dissolved and contaminants such as heavy metals reside is a discontinuous phase. Separation of the discontinuous phase from the continuous phase with the dissolved polymer by e.g. centrifugation or filtration removes the contaminants. Summary of the invention

The invention relates to the removal of additives from waste plastics by dissolution and solvent-resistant membrane filtration.

The invention is summarized in the following statements:

1. A method of removing one or more solvent soluble additives from a polymer comprising less than 40 wt % polyvinylchloride or from a mixture of polymers comprising less than 40 wt % polyvinylchloride, the method comprising the steps of: a) dissolving said polymer or mixture of polymers comprising said one or more solvent soluble additives in a solvent or a solvent mixture that dissolves said polymer or polymer mixture, and that dissolves as well said one of more additives, b) filtrating the solution comprising the dissolved polymer or polymer mixture and the one or more dissolved additives over a membrane resistant against solvent, whereby the dissolved polymer is retained by the membrane and the one or more solvent soluble additives permeate through the membrane.

2. The method according to statement 1, wherein said polymer comprises less than 20 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 20 wt % polyvinylchloride.

3. The method according to statement 1 or 2, wherein said polymer comprises less than 10 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 10 wt % polyvinylchloride.

4. The method according to any one of statements 1 to 3, wherein said polymer comprises less than 5 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 5 wt % polyvinylchloride.

5. The method according to any one of statements 1 to 4, wherein said polymer comprises less than 1 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 1 wt % polyvinylchloride.

6. The method according to any one of statements 1 to 5, where the membrane has a molecular weight cut off below 20 000 Da.

7. The method according to any one of statements 1 to 6, wherein said membrane is a cross-linked polyimide membrane.

8. The method according to any one of statements 1 to 6, wherein said membrane is a cross-linked PVDF membrane.

9. The method according to any one of statements 1 to 8, to wherein said polymer is selected from the group consisting of polystyrene, polyethylene, and polypropylene. 10. The method according to any one of statements 1 to 9, wherein said solvent soluble additive has a Mr below 1000 Da, below 750 Da or below 500 Da.

11. The method according to any one of statements 1 to 10, wherein said membrane has in a reference measurement with a 0.1% (w/v) solution of polystyrene with a Mw between 250 and 300 kDa in n-BiOAc at a temperature of 25 °C and a pressure of 5 bar, a permeance of more than 3, 4, 5, 6, por 7 L.m-2.h-l. bar-1 and a retention of polystyrene of more than 90 %.

12. The method according to any one of statements 1 to 11, wherein said membrane has in a reference measurement with a 0.1% (w/v) solution of polystyrene with Mw between 250 and 300 kDa in n-BiOAc at a temperature of 100 °C and a pressure of 25 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m-2.h-l. bar-1 and a retention of polystyrene of more than 90 %.

13. The method according to any one of statements 1 to 12, wherein said solvent soluble additive is dye, a plasticizer or a UV-scavenger, a compound providing thermal or a compound providing thermal radiation stability.

14. The method according to any one of statements 1 to 13, wherein the solvent is selected from the group consisting of NMP, THF, DMF, DMAc, butylacetate, methylethylketone, chloroform and dichloromethane.

15. The method according to any one of statements 1 to 14, wherein the dissolved polymer, after step a) and prior step b) is present at a concentration of between 0.1 up to 5, 10, 15 or 20 wt %.

16. The method according to any one of statements 1 to 15, wherein the filtration is performed at a temperature of between 10 °C and 100 °C.

17. The method according to any one of statements 1 to 16, wherein the filtration is filtration is performed at a pressure of between 1 and 25 bar.

18. The method according to any one of statements 1 to 17, wherein the filtration in step b) is a dead end filtration.

19. The method according to any one of statements 1 to 17, wherein the filtration in step b) is a cross-flow filtration.

20. The method according to any one of statements 1 to 19, wherein after step a) and prior to step b) non-solubilized material is removed.

21. The method according to any one of statements 1 to 19, wherein after step b) the polymer is recovered by evaporation of the solvent. The method according to any one of statements 1 to 19, wherein after step b) said additives are recovered by evaporation of the solvent. A method of removing one or more solvent soluble additives from a polymer other than polyvinylchloride or from a mixture of polymers other than a mixture comprising polyvinylchloride, the method comprising the steps of: a) dissolving said polymer or mixture of polymer comprising said one or more solvent soluble additives in a solvent or a solvent mixture that dissolves said polymer as well as said one of more additives, b) filtrating the solution comprising the dissolved polymer and the one or more dissolved additives over a solvent resistant membrane, whereby the dissolved polymer is retained by the membrane and the one or more solvent soluble additives permeate through the membrane. The method according to statement 23, where the membrane has a molecular weight cut off (MWCO) below 100 000 Da, below 50 000 Da, below 20 000 Da, below 10 000 Da, below 5000 Da, below 2000 Da, below 1000 Da. ["below" refers to an upper limit of a range, the lower range can be 100, 200 or 500], Higher upper limits of MWCO may lead at the one hand to the loss of a part of the dissolved polymer as it passes the membrane. Polymer chains can coil or extend, depending on the solvation by the liquid and the flexibility of the polymer backbone. A polymer chain with high MW can thus be retained less than one with low MW if the former is more extended in that solvent and the latter is more coiled, hence larger in size. On the other hand the higher the MWCO, the less additive is retained and the higher the flux will be. A higher flux allows the use of a smaller surface of membrane.

It is up to the skilled person to balance loss of polymer versus enhanced flux and removal of additives. The method according to statement 23 or 24, wherein the solvent resistant membrane has in a reference measurement with a 0.1% (w/v) polystyrene (Mw 250-300 kDa) solution in n-BiOAc at a temperature of 25 °C and a pressure of 5 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m-2.h-l. bar-1 and a retention of polystyrene of more than 90 %. The method according to statement 23 or 24, wherein the solvent resistant membrane has in a reference measurement with a 0.1% (w/v) polystyrene (Mw 250-300 kDa) solution in n-BiOAc at a temperature of 100 °C and a pressure of 25 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m-2.h-l. bar-1 and a retention of polystyrene of more than 90 %.

27. The method according to any one of statements 23 to 26, wherein the membrane is a cross-linked polymer membrane, such as a cross-linked polyimide membrane, a cross-linked polysulfone membrane, a cross-linked polyvinylidene fluoride membrane or a cross-linked polyvinylchloride membrane.

28. The method according to any one of statements 23 to 27, wherein the membrane is a cross-linked polyimide membrane or cross-linked PVDF membrane.

29. The method according to any one of statements 23 to 28 to wherein the polymer is selected from the group consisting of polystyrene, polyethylene, and polypropylene The invention is equally application to heteropolymers.

30. The method according to any one of statements 23 to 29, wherein the solvent soluble additive has a Mr below 1000 Da, below 750 Da or below 500 Da.

31. The method according to any one of statements 23 to 30, wherein the additive is dye, a plasticizer or a UV-scavenger, a compound providing thermal or radiation stability.

32. The method according to any one of statements 23 to 31, wherein the additive is present in the dissolved polymer at a concentration of between 0,5 and 50 wt %.

33. The method according to any one of statements 23 to 32, wherein the solvent is selected from the group consisting of NMP, THF, DMF, DMAc, butylacetate, methylethylketone, chloroform, dichloromethane.

34. The method according to any one of statements 23 to 33, wherein the dissolved polymer, after step a) and prior step b) is present at a concentration of between 0.1 up to 5, 10, 15 or 20 wt %.

A typical starting polymer concentration is generally as high as possible, typically 5-20wt %, depending on the viscosity of the solution. If possible, this may be even higher. During filtration, viscosity will increase and higher temperatures or pressure may be applies to facilitate the process. If a high concentration of additive is to be removed from the polymer, a filtration is applied in which the concentrated retentate is diluted first and then again filtered. Such dilution-filtration sequence can be repeated several times, and both the dilution factor and the concentration factor can be chosen.

35. The method according to any one of statements 23 to 34, wherein the filtration is performed at a temperature of between 10 and 100 C°.

36. The method according to any one of statements 23 to 35, wherein the filtration is filtration is performed at a pressure of between 1 and 25 bar. 37. The method according to any one of statements 23 to 36, wherein the filtration in step b) is a dead end filtration or a cross-flow filtration.

In industrial processes, membrane filtrations are typically run in cross-flow mode. When permeances decrease below 1 Lmh/bar, the process will typically be stopped and post-treated (e.g. via distillation, non-solvent addition, cooling,...) or the retentate rediluted when operated in diafiltration mode.

38. The method according to any one of statements 23 to 37, wherein the solution is step b) contains less than 10, 8, 6, 4, 2, 1, 0.5, 0,% water.

39. The method according to any one of statements 23 to 38, wherein after step a) and prior to step b) non-solubilized material is removed (by filtration, centrifugation, precipitation, decantation)

40. The method according to any one of statements 23 to 39, wherein during step b) the solution is stirred.

41. The method according to any one of statements 23 to 40, wherein after step b) the polymer is recovered by evaporation of the solvent and/or wherein said additives are recovered by evaporation of the solvent.

Figure legends

Figure 1. Graphical depiction of the results of filtration experiments performed with XL- PI (crosslinked polyimide) membranes made form PI dope solutions with different concentrations of Matrimid (15, 16, 17 and 18 wt.%), and 0.1 w/v% feed solutions consisting of three different types of polystyrene (PS) (A,B,C). The average retention of the polystyrene is depicted by the gray bars and the average permeance is shown as the red dots, both obtained from a set of 4 data points. Error bars depict the standard deviation.

Figure 2. Graphical depiction of a) the effect of operating temperature (°C) on PI-XL membrane and pure n-BuOAc as a feed solution at a constant pressure of 5 bar, b) the effect of operating pressure (bar) on PI-XL membrane and pure n-BuOAc as a feed solution at room temperature (25 °C), and c) the effect of operating pressure (bar) on PI-XL membrane flux and pure n-BuOAc as a feed solution at RT.

Figure 3. Graphical depiction of a) the effect of increasing the concentration of the three different PS (A, B, C) solutions (w/v%) in n-BuOAc on membrane permeance and the viscosity of the feed solution, and b) the average retention of PS (A,B,C) solutions from 1 w/v% to 8 w/v%.

Figure 4. Membrane permeance of three different polystyrene solutions (A, B, C) as a function of temperature (°C) on the left side (green line), and viscosity (mPa.s) on the right side (red line). Thermal conditions were investigated by two polystyrene solution concentrations, 1 and 5 w/v%.

Figure 5. Arrhenius plots of the effect of temperature (K) for three different polystyrene (A, B, C) solutions flux by two polystyrene solution concentrations, 1 w/v%, and 5 w/v%.

Figure 6. The permeance values of three different polystyrene solutions (A, B, C), which is affected under operating pressure (bar). Pressure condition was studied by two polystyrene solution concentrations, 1 and 5 w/v%, that is shown in two columns, a: PS A, 1 w/v%, b: PS A, 5 w/v%, c: PS B, 1 w/v%, d: PS B, 5 w/v%, e: PS C, 1 w/v%, f: PS C, 5 w/v%.

Figure 7. Dead-end filtration process under operating pressure was investigated on two polymer solution concentrations (1 w/v% and 5 w/v%, two columns) of three different polystyrenes (A, B, C) at RT and 40 °C (on two rows), a: 1 w/v% PS(A, B, C) solution at RT, b: 5 w/v% PS(A, B, C) solution at RT, c: 1 w/v% PS(A, B, C) solution at 40 °C, d: 5w/v% PS(A, B, C) solution at 40°C.

Figure 8. P-Value is interpreted as the level of significance of all inputs (temperature, pressure, concentration, and a combination of these) on output dependency (permeance), since that is considered less than 0.05 (vertical line indicated with arrow) to show the maximum probability of each variable.

Detailed description

Throughout the present invention, wherever "polymer" is mentioned as a general term it is limiting or excluding halogenated vinyl polymers, more particularly limiting or excluding polyvinylchloride (PVC).

Polymer accordingly means a polymer comprising less than 40 wt % halogenated vinyl polymers or PVC, comprising less than 30 wt % halogenated vinyl polymers or PVC, comprising less than 20 wt % halogenated vinyl polymers or PVC, comprising less than 10 wt % halogenated vinyl polymers or PVC, comprising less than 5 wt % halogenated vinyl polymers or PVC, or comprising less than 1 wt % halogenated vinyl polymers or PVC.

"mixture of polymers" means that within the mixture of dissolved polymers, the concentration of PVC is less than 40 wt %, less than 30 wt %, less than 20 wt %, less than 10 wt %, less than 5 wt % or less than 1 wt %.

The invention relates to the use of ultrafiltration with for example cross-linked polyimide membranes to recycle polymers under different operating conditions, such as operating pressure, temperature, concentration and viscosity of feed solution on membrane permeance of polymer to find an optimal condition to separate the dissolved polymers (such as polystyrene) from the solvent, (e.g. n-Butyl acetate (n-BuOAc). The pure solvent permeance was investigated under operating increasing pressure and temperature, and then decreasing them step by step in a continuous process.

Operating pressure, temperature, concentration and viscosity of feed solution have a significant impact on membrane separation performance, where pressure, concentration and viscosity of feed solution have been increased, membrane permeance decreased because of concentration polarization and accumulation of solute on the membrane surface. Membrane permeance has been increasing due to the increase of temperature and activation energy. A hysteresis line can be observed in pure solvent filtration during operating pressure and temperature in the process returning, which related to the compaction of feed solution with the membrane and changing the membrane polymer chain in high temperature, respectively.

The present invention discloses methods to recycle polymers and obtain a high quality of virgin-alike plastics. Membrane technology is a reliable and repeatable separation process that shows a variety of applications in chemical, environmental, and water treatment. Solvent-resistant nanofiltration-ultrafiltration (SRNF-SRUF), is an excellent method for separation performance even with long-term stability in harsh solvents. Compared to the other methods, polymeric membranes have shown many advantages, being able to tune the separation properties, selectivity, greener, faster production, lower cost and energy, easier processing, and mechanical stability. PI membrane recently is applied in different applications because of thermal and chemical resistance, easy to prepare and commercial expansion [Vanherck et al. (2008) J. Membrane Sei. 320, 468-476],

The invention relates to methods of removing one or more solvent soluble additives from a polymer or mixture of polymers other than a polyvinylchloride. These methods comprise a step (a) of dissolving polymer and additives and a step (b) of filtrating polymer.

In step a) a polymer or mixture of polymers comprising one or more solvent soluble additives are dissolved in a solvent or a solvent mixture that the polymer or mixture of polymer of as well as the one of more additives that one want to remove from the polymer.

In step b) the solution comprising the dissolved polymer of mixtures thereof and the one or more dissolved additives are filtered over a solvent resistant membrane.

In this filtration step the dissolved polymer is retained by the membrane and the one or more solvent solubilized additives permeate through the membrane. A non-limiting list of polymers which come into account for the methods of the present invention are listed below, with exemplary solvents to dissolve the polymer. Table 1. Solvents for Representative Homopolymers. The left column mentions the repeating unit of a polymer. [info taken from Aldrich] Solvent resistance of a membrane can be tested by comparing the behaviour of the membrane under aqueous conditions and in a solvent of choice e.g. with respect physical stability, flow, retention of large compound and passage of small compounds.

Depending of the size of the polymer, the size of the additive and the desired flux, there is a wide choice of MWCO of membrane that come into account.

In embodiments of the methods of the present invention the membrane has a molecular weight below 100 000 Da, below 50 000 Da, below 20 000 Da, below 10 000 Da below 5000 Da, below 2000 Da, below 1000 Da. ["below" refers to an upper limit of a range, the lower range can be 100, 200 or 500]. Higher upper limits of MWCO may lead at the one hand to the loss of a part of the dissolved polymer as it passes the membrane. Polymer chains can coil or extend, depending on the solvation by the liquid and the flexibility of the polymer backbone. A polymer chain with high MW can thus be retained less than one with low MW if the former is more extended in that solvent and the latter is more coiled, hence larger in size. On the other hand the higher the MWCO, the less additive is retained and the higher the flux will be. A higher flux allows the use of a smaller surface of membrane.

It is up to the skilled person to balance loss of polymer versus enhanced flux and removal of additives.

To further guide the skilled person to the use of a membrane for the filtration of additives from a polymer, the examples section provides are reference test to identify such membranes.

In a less stringent test, the solvent resistant membrane has in a reference measurement with a 0.1% (w/v) polystyrene (Mw 250-300 kDa) solution in n-BiOAc at a temperature of 25 °C and a pressure of 5 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m ².h Tbar¹ and a retention of polystyrene of more than 90 %.

In a more stringent test, the solvent resistant membrane has in a reference measurement with a 0.1% (w/v) polystyrene (Mw 250-300 kDa) solution in n-BiOAc at a temperature of 100 °C and a pressure of 25 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m^-2.h Lbar¹ and a retention of polystyrene of more than 90 %.

With respect to the physicochemical composition, wherein the membrane is typically a cross-linked (XL) polymer membrane. Examples hereof are polyimide XL, polysulfone XL, polyvinylidene fluoride and polyvinylchloride XL membranes.

Specific membranes for the methods of the present invention are a cross-linked polyimide membrane or a cross-linked PVDF membrane. Specific polymers for use in the methods of the present invention are polystyrene, polyethylene and polypropylene.

Dependent on the MWCO of the membranes, a wide variety of additives can be removed. In specific embodiments, a solvent soluble additive has a Mr below 1000 Da, below 750 Da or below 500 Da.

Examples of additives are dyes, plasticizers, UV-sca ven gers, or compounds providing thermal or radiation stability.

Other non-limiting examples are blowing agents, epoxy compounds, 1,3-dicarbonyl compounds, polyols, lubricants, antioxidants, fillers, impact modifiers, processing aids, antistatics, biocides, metal deactivators, optical brighteners, flame retardants and antifogging agents [such compounds are disclosed in more detail in EP4067421].

Additives may be present in polymers in concentrations between 0,5 and 50 wt %.

To remove additives to the maximal extent, the dissolved polymer which is concentrated at the end of a filtration cycle, is diluted and subject to a further filtration step.

Solvents for dissolving polymers are known to the skilled person. Such solvents are equally tested for dissolving the additive to be removed.

Examples of such solvent are NMP (N-methylpyrrolidon), THF (tetrahydrofuran), DMF (Dimethylformamide), DMAc (Dimethylaceetamide), butylacetate, methylethylketone, chloroform, dichloromethane, dioxane, acetone, ethyl acetate, halogenated organic solvents, methylethylketone, methylpropylketone, methylisopropylketone methylbutylketone and methylisobutylketone.

In embodiments of the methods of the invention the dissolved polymer, after step a) and prior to step b) is present at a concentration of between 0.1 up to 5, 10, 15 or 20 wt %.

A typical starting polymer concentration is generally as high as possible, typically 5-20 wt %, depending on the viscosity of the solution. If possible, this may be even higher. During filtration, viscosity will increase and higher temperatures or pressure may be applies to facilitate the process. If a high concentration of additive is to be removed from the polymer, a filtration is applied in which the concentrated retentate is diluted first and then again filtered. Such dilution-filtration sequence can be repeated several times, and both the dilution factor and the concentration factor can be chosen.

In embodiments of the invention the filtration is performed at a temperature of between 10, 20, 30, 40 up to 60, 70, 80, 90, 100 °C. All ranges of lower values and higher temperature values are herewith explicitly disclosed. The optimal choice of temperature can be influenced by factors such as viscosity of the polymer solution, thermal stability of the additives or boiling point of the additive. In embodiments of the invention, the filtration is filtration is performed at a pressure of between 1, 5, 10 up to 10, 15, 20 and 25 bar. All ranges of lower and higher pressure values are herewith explicitly disclosed.

The filtration in step b) can be a dead end filtration or a cross-flow filtration.

In industrial processes, membrane filtrations are typically run in cross-flow mode. When permeances decrease below 1 Lmh/bar, the process will typically be stopped and posttreated (e.g. via distillation, non-solvent addition, cooling,...) or the retentate rediluted when operated in diafiltration mode.

In embodiments of the invention the solution with dissolved polymer applied in step b) contains less than 10, 8, 6, 4, 2, 1, 0.5 or contains 0 % water.

In specific embodiments, after step a) and prior to step b) non-solubilized material is removed (by filtration, centrifugation, precipitation, decantation). The selection of solvent can be chosen such that a maximal amount of additives is not soluble and can be removed prior to the filtration method of b).

To increase the flux of the process, during step b) the solution is stirred. Alternatively or in addition the solution in step b) can be diluted and subjected to a further filtration. After the removal of an additive the polymer can be recovered by evaporation of the solvent

Equally the additives that passed through the membraned can be recovered by evaporation of the solvent, or rendering the additive insoluble.

One aspect of the invention relates to an energy-efficient and waste-free membrane technology to purify additives from dissolved polymers such as PCV. This potentially avoids using non-solvents, thus ending up with a much higher CO₂ saving per ton recycled polymer. As non-solvents can be excluded, also a wider choice of solvents is possible to be used for polymer recycling.

The present invention allows the production of a near-virgin quality polymer, without the energy need of cracking the polymer. Moreover, it allows to keep the additives intact, avoiding a range of by-products.

In an embodiment of the present invention, the stability of a polymeric membrane under high temperature and pressure was evaluated by the pure solvent permeance (n- BuOAc). Further, the impact of operating pressure, temperature, concentration and viscosity of the feed solution (Polystyrene (PS) in n-BuOAc) on the cross-linked polyimide membrane performance was determined. Decreasing viscosity is observed by the increase of the temperature, resulting the higher permeance compared to the room temperature (RT) filtration process. The polymer solution viscosity at RT is higher than solution viscosity at higher temperatures. Membrane permeance improves and accelerates at low viscosity. In general, membrane flux increases by rising the temperature, which is correlated by an increase in activation energy, solvent diffusion coefficient, and polymer chain mobility. This can be calculated by Arrhenius equation to predict temperature dependent process. Molecule energy can overcome the activation energy in higher temperature. Increasing the filtration process temperature induces fouling resistance, which is more favourable for filtration performance.

Solute characterizations such as molecular weight, viscosity, shape, and size play a role in filtration process permeance due to the concentration polarization (CP) and fouling. The viscosity of polymers in an ideal solvent with high solubility also attributed to the molecular weight of the polymers.

By increasing the feed solution concentration, osmotic pressure is induced and CP may happen due to the accumulating the solute (Polystyrene) on the surface of membrane and a drop of permeance is expected. The effect of polystyrene solutions concentration on viscosity is increased by the rise of the concentration. High viscosity of feed solution can result in the generation of a layer on the top of membrane and which acts as a barrier for membrane flux.

Separation process and permeance is also affected by pressure. Making a cake layer of macromolecules on the top of membrane and fouling are tend to increase while the pressure increases. Feed solution containing organic macromolecules (polymers, proteins) led to inevitable membrane fouling. Membrane pores are blocked due to the fouling, which leads to the form a cake layer at the membrane surface and a drop in permeance.

The present invention provides highly stable membranes for use with dissolved polymers. Molecular weight cut-off values typically range from 200-1000 Da in nanofiltration or above 1000 Da in UF. Chemical cross linking of the membranes makes them suitable for use in solvents that dissolve polymers.

The examples section provides a reference test to determine which type of membranes have the desired permeance and retention of dissolved polymer. The below listed membranes allow separations with a MWCO-value between 100-1000 Da. Most of these commercial membranes that are currently available have been designed to prevent excessive swelling in organic media. This is mostly realized by crosslinking the membrane material through creation of covalent bonds, typically leading to amide- bonds, such as in Duramem™ membranes. These amide bonds however restrict use in e.g. presence of acids and bases. Excessive swelling and thermal expansion also strongly decreases membrane selectivity at higher temperatures.

Table 2: Overview of widely used commercial NF membranes for aqueous and solvent applications. Data taken from manufacturer specification sheets.

n.s. = not specified by manufacturer

EXAMPLES

Example 1 Material and methods for Polystyrene (PS) and High-Impact Polystyrene (HIPS)

Materials n-Butyl acetate (n-BuOAc) 99% and l-methyl-2-pyrrolidone (NMP) 99% were purchased from Fisher Chemical and Acros Organics, respectively. Matrimid polymide powder (5218) Huntsman (Switzerland) was dried for 24 hours at 100 °C before use. Three different commercial polystyrenes were obtained by Ineos company (see Table 3). The supporting polypylene/polyethylene (PP/PE) fabric (Viledon® Novatexx 2471) was obtained from reudenberg Vliesstoffe (Germany). 1,6-Hexanediamine (HDA) 99.5% was obtained frfrom Alfa Aesar.

Feed solution preparation

15 g solutions (0.1 w/v%) were prepared at room temperature by dissolving polystyrene in n-BuOAc while stirring in dosed vials.

Molecular weight measurement

The molecular weight and dispersity of the different types of polystyrene in THF was measured using gel permeation chromatography (GPC), (SHIMADZU, LC-lOADvp). The results of these experiments are shown in Table 1.

Table 3: molecular weight and dispersity of different types of polystyrene in THF

Viscosity measurement

Viscosities of the different PS solutions (concentration 1 - 8 w/v%) were determined at different operating temperatures ( 25 - 100 °C) using a SVM 3001 (Anton Paar).

Membrane preparation

Matrimid was dissolved in NMP, overnight at room temperature, resulting in dope solutions with concentrations of 15 w/v% to 18 w/v%. Dope solutions were degassed by placing the open vials containing the dope solution in a vacuum oven (Dwards, Sheldon manufacturing, INC) for 15 minutes at room temperature under 160 mBar. The homogeneous PI solution was cast at room temperature on a porous PE/PP non-woven on an automatic casting device (Promoter, Belgium) using a casting knife thickness of 250 pm at a casting speed of 0.02 m.s ¹. The temperature and humidity in the casting setup were controlled to be 20 ± 2 °C and 50 ± 10% relative humidity, respectively. Immediately after casting, the PI films were immersed for 20 minutes in non-solvent (deionized water) bath containing 3 w/v% HDA for the solidification and simultaneous cross-linking [Vanherck et al. (2008) cited above]. The cross-linked PI membranes were stored in deionized water for 24 hours to remove all residual solvents before the filtration experiment.

Filtration experiments

Dead-end filtrations were performed under controlled temperature and pressure on the above described cross-linked PI membranes with an active filtration area of 0.003 m² and volume capacity of 250 ml. The membrane was placed on a sintered plate and sealed with EPDM O-ring. The cell was exposed to nitrogen to generate the desired pressure and the feed solution was stirred at 300 rpm. For each of the experiments the average of the values of 2 samples is reported.

Pure solvent permeance

A pure n-BuOAc, without dissolved polymer, dead-end filtration was performed. Experiments were done by stepwise increasing the operating pressure with 5 bar every one hour (5, 10, 15, 20, 25 bar) and inversely decreasing every one hour [at 25 °], Similar experiments were done by increasing the temperature stepwise every one and half hours (whereby it takes about 30 minutes to reach the desired temperature) from (25 °C to 100 °C, and vice versa in 5 bar). After every step of changing pressure or temperature, the membrane permeance was measured.

Filtration experiments using PS feed solutions

To study the effect of operating temperature on the membrane performance, the filtration process (different polystyrene concentration in n-BuOAc) was done by increasing the temperature stepwise, from 25 °C (RT) to 100 °C at a constant pressure of 5 bar. For each step fresh feed solution and a new cross-linked PI membrane as described above were used. Samples were collected after allowing the membrane to stabilize for one hour after the center plate reached the required T. In order to study the effect of operating pressure, the pressure under which the filtration process occurred was increased step by step from 5 bar to 25 bar, while the temperature was kept constant at 25 °C. The increase was either done in a continuous way or as a set of separate experiments. For the latter, new membranes and a fresh feed solution were used for testing each different pressure. The membrane was allowed to stabilize for 1 h at the desired pressure, after which the samples were collected.

To determine the maximum workable polystyrene concentration of the feed solution, filtration experiments were performed at RT under 5 bar. The limit was set at a of 0.05 L/m².h.bar.

Membrane performance

Membrane permeance (J) was calculated using equation 1.

Where V is the permeate volume (L), A is the membrane effective area (0.003 m²) and t is the time (h) necessary for the permeate volume to be collected, and Ap is the operation pressure (bar).

The Rejection (R) of polystyrene is defined by equation 2. loo (2)

Where C_r is the concentration of polystyrene in the retentate and C_pis the concentration of polystyrene in the permeate [Verbeke et al. (2020) J. Membrane Sei. 612, 118438]. The concentrations were determined using UV-Vis spectroscopy (Shimadzu, UV-1800) around 270-280 nm.

Data analysis

Quantitative analysis of data was performed using SPSS (developed by IBM) and JMP (SAS institute) statistical software for data management.

The p-value, calculated by SPSS, is interpreted as the level of significance of all inputs (temperature, pressure, concentration, and a combination of them) on the output dependency (permeance). The maximum probability of each variable is considered to have a p-value less than 0.05, which is depicted by the blue line in Figure 8. The more "extreme" outcome by these experiments shows the smallest of p-value [Gibbons & Pratt (1975) Am. Stat. 29(1), 20-25],

The prediction profiler, calculated by JMP, is used to show the relationship between multiple factors and responses in an experiment. This simulation study is used to evaluate the performance of new conditions on methodology, design of experiments, and explore new variances. It is a combination of a matrix of plots, including a prediction variance of plots. Independent and dependent variables show on x and y axis on each plot, respectively.

Example 2. Selection of membranes

To identify suitable membranes for the filtration of additives from polymer solutions, dead-end filtrations were performed on several crosslinked-polyimide membranes with concentrations varying from 15 wt.% to 18 wt.% polyimide in casting agent. The feed solutions used in these filtrations consisted of three different types of polystyrene (properties of the polystyrene depicted in table 3) which were dissolved in n-BiOAc at a concentration of 0.1 w/v%. The results of these filtrations are shown in Figure 1. Herein, retention increased, and the permeance decreased, with increasing PI concentration. A retention of more than 95% of the polymer with a high permeance are preferred for the filtration of additives from polymer solutions. Therefore, cross linked PI membranes as prepared above, with a 18 wt.% PI concentration (retention more than 97% and permeance of 7 Lm^.h^.bar ¹) were used for further experiments.

Example 3. Membrane stability at high temperatures and pressures

The influence of temperature and pressure on the performance of the prepared membrane was evaluated by measuring the pure solvent (n-BuOAc) permeance in different operating conditions. In this way the membranes membranes are tested for their behavior in the solvent under temperature and pressure conditions, without influence of polymer or additives.

Figure 2. a shows the effect of operating temperature at constant pressure (5 bar), and Figure 2.b shows the effect of operating pressure at constant temperature (RT, 20-25 °C). Both parameters were first increased stepwise until an arbitrarily chosen value. For temperature the value of the permeance increased from 25 °C to 100 °C.

For pressure the value of the permeance decreased form 5 bar to 25 bar.

Hereafter, the respective parameters were then decreased in the same step-by-step manner. It was observed that both temperature and pressure had an effect on the membrane performance after each respective cycle. It can be seen that the permeance slightly increased after the temperature cycle (from 44.66 to 48.64 L/m^z.h.bar), while it decreased significantly after the pressure cycle (from 44.66 to 34.36 L/m².h.bar).

A possible explanation is that the polymer chains in the membrane rearrange at higher temperature, resulting in an increase in permeance. The decrease in permeance after the pressure cycle is probably due to compaction of the polymer. Since, the values of the flux increase with increasing pressure (see Figure 2c), increasing pressure remains a valid option for enhancing the filtration for more viscous solutions. These experiments illustrates that the membranes are stable at pressures up to 25 bar, which provides a workable window for the subsequent experiment.

Example 4. Membrane permeance in different operating conditions

Effect of feed solution concentration on filtration process

A significant decrease in permeance is observed when increasing the polystyrene concentration in the feed solution (from 1 w/v% to 8 w/v%), which is shown in Figure 3a. Apparently increasing the polystyrene concentration leads to the formation of a solute layer on the top of membrane, which in combination with concentration polarization leads to a significant decrease in permeability. Also, an increased polymer solution viscosity, due to the increasing polymer concentration, plays a role in the permeance decrease (see Figure 3).

The type of polymer also has an effect on permeance. The polystyrene A solution, wih the smallest Mw has the lowest viscosity, resulting in the highest permeance [ Zhao, S. & Zou (2011) J. Membrane Sei. 379, 459-467].

The intrinsic viscosity of polystyrene in an ideal solvent with high solubility is measured by equation 3:

[q] = KM^ (3)

Where K is a constant, independence of molecular weight, for polystyrene at a given temperature, and M is the molecular weight [Krigbaum and Flory, (1953) J. Pol. Sci. 11(1), 37-51],

The dead-end filtration experiments at room temperature were limited to polystyrene concentrations of 8 w/v% due to the extreme decrease in permeance values. The change in polystyrene retention was negligible for all concentrations (above 95 %). An average retention of all polystyrene solutions, from 1 w/v% to 8 w/v%, is presented in Figure 3b.

Example 5. Effect of operating temperature on membrane permeance

Figure 4 shows membrane permeance [18 % PI crosslinked membranee] of the three different polystyrene polystyrenes (A, B, C) at two concentrations (1 wt/v% and 5wt/v% in n-BuOAc) and as a function of temperature (°C) on the [eft side (green line), and viscosity (mPa.s) on the right side (red line).

Membrane permeance increases with increase of temperature.

At higher temperature the mass transfer resistance decreases, which can result in higher permeance, even for feed solutions with higher concentrations of polymer (5 w/v%), see Figure 4 right column. As the polystyrene solution viscosity is influenced by temperature, a higher permeance can be observed due to the reduction of the solution viscosity.

Filtration process permeance in lower concentration (lw/v%) of polystyrene solutions was more effected by temperature compared to the 5w/v% of polystyrene solutions in the range of 25°C to 100°C. [from 3.32 to 22.16L/m².h.bar in polystyrene A, from 3.57 to 21.98 L/m².h.bar in polystyrene B, and from 2.3 to 16.04 L/m².h.bar in polystyrene C], while this was less at the higher concentration (5w/v%) [from 2.1 to 7.47 L/m².h.bar in polystyrene A, from 2.58 to 10.68 L/m².h.bar in polystyrene B, and from 1.7 to 8.56 L/m².h.bar in polystyrene C].

The membrane permeance at each temperature (from 25 °C to 100 °C), pressure (from 5 bar to 25 bar), and feed solution concentration (1 w/v% to 8 w/v%) was determined. Permeation results are affected by the activity of solute (aj) which is temperature and concentration dependent. Activity is defined by concentration and activity coefficient. Equation 4.

Activation energy was calculated by Arrhenius equation, Equation 5. -Ej

J = Aje RT (5)

Where A is an exponential term that depends on the activation energy (Ej, J is permeation activation energy, R is the gas constant, and T is absolute temperature (kelvin) [Machado et al. (1999) J. Membrane Sci. 163, 93-102].

As viscosity is influenced by temperature and results in an increase of the membrane permeance, viscosity activation energy is calculated by Eq 6:

Ee.

H = A^eRT (6)

Figure 5 shows that Arrhenius plots of the effect of temperature (K) for three different PS (A, B, C) solutions, each in two different concentrations, 1 w/v%, and 5 w/v%. Activation energy values measured for 6 different polystyrene solutions (PS A, PS B, PS C, by 1 and 5 w/v%) and that is summarized in Table 4. Activation energy value depends on temperature and concentration. Increasing temperature led to the higher movement of feed solution molecules and lower viscosity, resulting to an increase of permeance. Activation energy and flux are higher in with increasing polymer solution concentration at the same temperature as illustrated in Figure 5.

Table 4: Activation energy values measured for 6 different polystyrene solutions (PS A, PS B, PS C, at concentration of 1 and 5 w/v%).

Example 6. Effect of operating pressure

The permeance value of three different polystyrene solutions (A, B, C), at different operating pressure (bar) is shown in Figure 6. The influence of pressure was studied at two polystyrene solution concentrations, 1 and 5 w/v% polystyrene, which is shown in two columns, a: PS A, 1 w/v%, b: PS A, 5 w/v%, c: PS B, 1 w/v%, d: PS B, 5 w/v%, e: PS C, 1 w/v%, f: PS C, 5 w/v%.

Permeability decreased with increasing operating pressure, due to high compaction of feed solution on the membrane. Therefore, high permeances are hindered by the increase of pressure.

Such increase in pressure can be avoided or reduced by cross-flow filtration to avoid formation of a solute layer on the top of membrane. By increasing operating pressure, permeance declined rapidly because of forming a solute layer on the membrane surface, fouling and CP (concentration polarization).

At higher concentrations, viscosity of the feed solution is higher, with accumulating of more polymers on the membrane surface, resulting to higher membrane fouling and reducing the membrane permeance. Figure 6, describes the non-linear behavior observed in operating pressure on three different types of polystyrene feed solution in filtration process. Example 7. Simultaneous effect of operating pressure, temperature, and feed solution concentration on the filtration process

A dead-end filtration process under different operating pressures (5, 10, 15, 20 and 25 bar) was investigated for two polymer solution concentrations (1 w/v% and 5 w/v%) of three different types of polystyrene (A, B, C) at two different temperatures (RT and 40 °C), a: 1 w/v% PS (A, B, C) solution at RT, b: 5 w/v% PS (A, B, C) solution at RT, c: 1 w/v% PS (A, B, C) solution at 40°C, d: 5 w/v% PS (A, B, C) solution at 40 °C in figure 7.

By increasing feed solution concentration and pressure, permeance decreased due to the accumulation of solute on the membrane surface, but membrane permeance can be increased by the increasing the temperature because of rising the activation energy and decreasing the viscosity. Figure 7 shows that an increase of 15 °C (from 25 °C to 40 °C, c and d), doubles permeance. Temperature reduces the limiting role of pressure and feed solution concentration and permeance can be improved by increasing temperature. The R² value of 0.94 shows a high correlation of independent inputs (temperature, pressure, and concentration) on dependent output (permeance).

Claims

2. The method according to claim 1, wherein said polymer comprises less than 20 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 20 wt % polyvinylchloride.

3. The method according to claim 1 or 2, wherein said polymer comprises less than 10 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 10 wt % polyvinylchloride.

4. The method according to any one of claims 1 to 3, wherein said polymer comprises less than 5 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 5 wt % polyvinylchloride.

5. The method according to any one of claims 1 to 4, wherein said polymer comprises less than 1 wt % polyvinylchloride or wherein said mixture of polymers comprises less than 1 wt % polyvinylchloride.

6. The method according to any one of claims 1 to 5, where the membrane has a molecular weight cut off below 20 000 Da.

7. The method according to any one of claims 1 to 6, wherein said membrane is a cross-linked polyimide membrane.

8. The method according to any one of claims 1 to 6, wherein said membrane is a cross-linked PVDF membrane.

9. The method according to any one of claims 1 to 8, to wherein said polymer is selected from the group consisting of polystyrene, polyethylene, and polypropylene.

10. The method according to any one of claims 1 to 9, wherein said solvent soluble additive has a Mr below 1000 Da, below 750 Da or below 500 Da.

11. The method according to any one of claims 1 to 10, wherein said membrane has in a reference measurement with a 0.1% (w/v) solution of polystyrene with a Mw between 250 and 300 kDa in n-BiOAc at a temperature of 25 °C and a pressure of 5 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m‘^z.h Tbar ¹ and a retention of polystyrene of more than 90 %.

12. The method according to any one of claims 1 to 11, wherein said membrane has in a reference measurement with a 0.1% (w/v) solution of polystyrene with Mw between 250 and 300 kDa in n-BiOAc at a temperature of 100 °C and a pressure of 25 bar, a permeance of more than 3, 4, 5, 6, or 7 L.m ^z.h ^.bar ¹ and a retention of polystyrene of more than 90 %.

13. The method according to any one of claims 1 to 12, wherein said solvent soluble additive is dye, a plasticizer or a UV-scavenger, a compound providing thermal or a compound providing thermal radiation stability.

14. The method according to any one of claims 1 to 13, wherein the solvent is selected from the group consisting of NMP, THF, DMF, DMAc, butylacetate, methylethylketone, chloroform and dichloromethane.

15. The method according to any one of claims 1 to 14, wherein the dissolved polymer, after step a) and prior step b) is present at a concentration of between 0.1 up to 5, 10, 15 or 20 wt %.

16. The method according to any one of claims 1 to 15, wherein the filtration is performed at a temperature of between 10 °C and 100 °C.

17. The method according to any one of claims 1 to 16, wherein the filtration is filtration is performed at a pressure of between 1 and 25 bar.

18. The method according to any one of claims 1 to 17, wherein the filtration in step b) is a dead end filtration.

19. The method according to any one of claims 1 to 17, wherein the filtration in step b) is a cross-flow filtration.

20. The method according to any one of claims 1 to 19, wherein after step a) and prior to step b) non-solubilized material is removed.

21. The method according to any one of claims 1 to 20, wherein after step b) the polymer is recovered by evaporation of the solvent.

22. The method according to any one of claims 1 to 20, wherein after step b) said additives are recovered by evaporation of the solvent.