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US-12617918-B2 - Or relating to plastic recycling

US12617918B2US 12617918 B2US12617918 B2US 12617918B2US-12617918-B2

Abstract

A method to purifying reclaimed polymers for improving the grade and consistency in the properties of mixed recycled plastic to increase their use in secondary applications. A desired property of the purified reclaimed polymer is determined with a first parameter value of a reclaimed polymer feedstock also determined to provide the desired property in the purified reclaimed polymer. Ratios of polymer feedstocks with differing parameter values are combined to give the first parameter value. These polymer feedstocks are then dissolved into a solution before the purer polymer with the desired property is separated out. Embodiments of reactor vessels for the dissolution step are described. Polyethylene feedstocks can be used with parameters of density and molecular weight to provide high grade polyethylene with desired physical and mechanical properties such as density, molecular weight, strength, stress, stiffness and impact resistance.

Inventors

  • Peter Malley
  • Andrew Burns

Assignees

  • Reventas Limited

Dates

Publication Date
20260505
Application Date
20210825
Priority Date
20200826

Claims (20)

  1. 1 . A method of purifying reclaimed polymers, comprising the steps: (a) selecting a desired value of a first property of the purified reclaimed polymer; (b) determining a first value of a second property of reclaimed polymer feedstock which will provide the desired value of the first property in the purified reclaimed polymer; (c) selecting at least two reclaimed polymer feedstocks, each with a different value of the second property and determining quantities of each of the reclaimed polymer feedstocks which, when combined, will provide a reclaimed polymer feedstock having the first value of the second property; (d) dissolving the quantities of the at least two reclaimed polymer feedstocks in at least one solvent to combine the at least two reclaimed polymer feedstocks and produce a polymer solution; and (e) separating a purer polymer from the polymer solution, the purer polymer being the purified reclaimed polymer with the first value of the second property and hence the desired value of the first property.
  2. 2 . A method of purifying reclaimed polymers according to claim 1 wherein the second property is selected from a group comprising: density and molecular weight.
  3. 3 . A method of purifying reclaimed polymers according to claim 1 wherein the first property is selected from a group comprising: density, molecular weight, environmental stress resistance, strength resistance, stiffness resistance and impact resistance.
  4. 4 . A method of purifying reclaimed polymers according to claim 1 wherein step (d) comprises the steps: (i) introducing the plurality of reclaimed polymer feedstocks to the at least one solvent in a reactor vessel to create a mixture; (ii) operating a mixing device within the reactor vessel for a first time duration; characterised in that, the mixing device comprises: a plurality of discs aligned parallel to each other in a stacked arrangement; each disc extending over a majority of the cross-sectional area of the reactor vessel and including a plurality of perforations to allow the mixture to flow from a first end of the reactor vessel to a second end of the reactor vessel, through the discs; one or more supports to hold the discs in position; and the one or more supports connected to a linear motion generator so that the discs are moved to oscillate at a first frequency and first amplitude, the linear motion dissolving at least a portion of polymer in the solvent in the first time duration.
  5. 5 . A method according to claim 4 wherein the first time duration is selected from a group consisting of time intervals of: less than 60 minutes, less than 30 minutes, and less than 15 minutes.
  6. 6 . A method of purifying reclaimed polymers according to claim 1 wherein the plurality of polymer feedstocks is in the range of 0.1% to 30% wt addition to the at least one solvent so that the weight of the polymer feedstocks is 0.1% to 30% of the weight of the solvents.
  7. 7 . A method according to claim 6 wherein the plurality of polymer feedstocks is in the range of 0.3% to 10% wt addition to the at least one solvent.
  8. 8 . A method of purifying reclaimed polymers according to claim 1 wherein the reactor vessel has a volume of 1,000 litres or greater.
  9. 9 . A method of purifying reclaimed polymers according to claim 1 wherein the first frequency is in the range 1 to 15 Hz.
  10. 10 . A method of purifying reclaimed polymers according to claim 1 wherein the first amplitude is in the range 40 to 1000 mm.
  11. 11 . A method of purifying reclaimed polymers according to claim 1 wherein the method includes heating the mixture to a first temperature, the first temperature being between room temperature and the at least one solvent boiling point.
  12. 12 . A method according to claim 11 wherein the mixture is heated by heating the reactor vessel.
  13. 13 . A method according to claim 11 wherein the first temperature is in the range of 800 C to 1200.
  14. 14 . A method according to claim 11 wherein the first temperature is above 1200 C.
  15. 15 . A method of purifying reclaimed polymers according to claim 1 wherein the plurality of polymer feedstocks is one or more polyolefins.
  16. 16 . A method according to claim 15 wherein the plurality of polymer feedstocks is polyethylene (PE).
  17. 17 . A method according to claim 15 wherein the plurality of polymer feedstocks is polypropylene (PP).
  18. 18 . A method of purifying reclaimed polymers according to claim 1 wherein the at least one solvent is selected from a group comprising: butylal, xylene, ethyl benzene and toluene.
  19. 19 . A method of purifying reclaimed polymers according to claim 1 wherein the plurality of discs are aligned parallel to each other in a vertical stack, with the mixture flowing vertically in the reactor vessel through the discs, and the discs are moved up and down at the first frequency and the first amplitude.
  20. 20 . A method according claim 1 wherein the plurality of discs are aligned parallel to each other in a horizontally stacked arrangement, with the mixture flowing along the reactor vessel through the discs, and the discs are moved back and forth at the first frequency and the first amplitude.

Description

The present invention relates to plastic recycling and more particularly, to a method of purifying reclaimed polymers for improving the grade and consistency in the properties of mixed recycled plastic to increase their use in secondary applications. Recycling of waste materials has now become a major environmental driver. In this regard the recycling of plastics is placed high on the agenda as these are non-biodegradable. Unfortunately, a key stumbling block to the ideal of a closed loop circular economy is recycler's ability to extract value from mixed plastic waste for resale and reuse. There are many feedstocks that can be used for supplying to a plastic recycling process. Examples of plastic feedstocks are: single source end of life thermoplastics i.e. wheelie bins all colours, containers, pipe, bottle caps, bottles and tanks; post-consumer and post-industrial recycled plastics; mixed PE/PP (polyethylene/polypropylene) recyclate; films i.e. multilayer films, laminate films, PE or PP films; and other mixed thermoplastics such as ABS (acrylonitrile butadiene styrene), Polystyrene, PVC (polyvinyl chloride). Currently recyclers extract value by simply separating the plastics into groups for resale with the vast majority of it being a mixed colour granulate. This colour limits the secondary application for recycled plastics as moulders and manufacturers have no choice in colour selection so any products are grey/black, low value and out of sight. This, combined with additives and contaminants that can be present in recyclate, result in low resale value and recyclers can only extract value from a small percentage of material economically. Much has been done to remove additives and contaminants to increase the value. U.S. Pat. No. 9,803,035, for example, discloses a method for purifying reclaimed polymers, such as polyethylene reclaimed from post-consumer use or post-industrial use to produce a colourless or clear, odour free, virgin-like polymer. The method involves obtaining the reclaimed polyethylene and contacting it at an elevated temperature and pressure with a fluid solvent to produce an extracted reclaimed polyethylene. The extracted reclaimed polyethylene is dissolved in a solvent at an elevated temperature and pressure to produce a polyethylene solution, which is purified at an elevated temperature and pressure by contacting the polyethylene solution with solid media to produce a purer polyethylene solution. A purer polyethylene is then separated from the purer polyethylene solution. The elevated temperatures are 90° C. to about 220° C. and elevated pressures from about 350 psig (2.41 MPa) to about 20,000 psig (137.90 MPa). While this process provided a purified polyethylene, which may be sourced from post-consumer waste streams, which is essentially contaminant-free, pigment-free, odour-free and homogenous, it could only be provided with mechanical and physical properties similar to virgin polymers. Such variation in the mechanical and physical properties, for example the density and melt index, of the purified polyethylene limit it's uses in secondary applications where manufacturers are seeking a particular grade of polyethylene in terms of density and molecular weight with specific mechanical properties such as environmental stress resistance, along with strength, stiffness and impact resistance. It is therefore an object of the present invention to provide a method of purifying reclaimed polymers which obviates or mitigates one or more disadvantages in the prior art. According to a first aspect of the present invention there is provided a method of purifying reclaimed polymers, comprising the steps: (a) determining a desired property of the purified reclaimed polymer;(b) determining a first parameter value of a reclaimed polymer feedstock to provide the desired property in the purified reclaimed polymer;(c) selecting at least two proportions of reclaimed polymer feedstock with differing parameter values, the percentage of the at least two proportions being equated to produce a combined parameter value equal to the first parameter value;(d) dissolving the percentages of the at least two proportions of reclaimed polymer feedstock in a solvent to produce a polymer solution; and(e) separating a purer polymer from the polymer solution, the purer polymer being the purified reclaimed polymer with the desired property. By monitoring the properties of the feedstocks, selecting the ones to be used and then mixing them in a desired ratio the resulting product at the end of the process can be designed to have the specific properties required. This produces a purified reclaimed polymer with a higher number of grades at a higher consistency, which are more useful and thus valuable for secondary applications. Preferably, the parameters are density and molecular weight. Such parameters are examples most preferable for polyethylene as the reclaimed polymer feedstock. These are selected as they can be determined