WO2022223491A2

WO2022223491A2 - Improvements in or relating to plastic recycling

Info

Publication number: WO2022223491A2
Application number: PCT/EP2022/060199
Authority: WO
Inventors: Andrew Burns; Peter Malley; Leslie John ROSE; Thomas Rose; Steven Andrew BURNS
Original assignee: Reventas Limited
Priority date: 2021-04-18
Filing date: 2022-04-17
Publication date: 2022-10-27
Also published as: GB2605846A; WO2022223491A3; GB202105505D0

Abstract

A method of recycling plastics, using a polyolefin feedstock from reclaimed plastic, in which a solution is created of at least one polymer of the feedstock in at least one solvent by dissolution in a reactor vessel, one or more contaminants are removed from the solution, the solvent is separated from the polymer and the polymer extruded for re-use, wherein a chain extender is introduced to the solution to modify the properties of the polymer in the solvent and increase its use in secondary applications. Embodiments are described using a peroxide in a solution of PE and butylal or xylene, colour pigments are filtered out and the solution is fractionated by polymer molecular weight selectively before and after the peroxide is added. The method lends itself to commercial scale processing with reactor vessels of 1,000 litres and greater by use of a fast dissolution reactor.

Description

IMPROVEMENTS IN OR RELATING TO PLASTIC RECYCLING

The present invention relates to plastic recycling and more particularly, though not exclusively, to a method for recycling plastics in which chain extenders are introduced to modify the properties of polymers in solvents 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 plastic waste for resale and reuse. Currently recyclers extract value by simply separating the post-consumer use or post-industrial use plastics into individual materials, such as high-density polyethylene (HDPE) or poly(ethylene terephthalate) (PET), or mixed streams of other common plastics, such as polypropylene (PP), low-density polyethylene (LDPE), poly(vinyl chloride) (PVC), polystyrene (PS), polycarbonate (PC), and polyamides (PA).

In recycling of the plastics there is a degradation which occurs in the polymer as it undergoes chemical, mechanical, thermal and oxidative processes which reduces its molecular weight. This affects the viscosity, melt strength and mechanical properties of the recycled polymer and thus limits its usefulness for many applications. To compensate for polymer degradation, chain extenders have been proposed, which effectively increase the molecular weight of the polymer and thus increase its value on resale. US 4,857,603 discloses use of chain extenders on PET (polyethylene terephthalate). The use of chain extenders during melt processing of PET with reactions occurring in mixers, extruders and injection moulding machines is now known. Organic phosphites, bis-oxazolines, bis- anhydrites and diiocyanates are among the chain extenders used. For PE (polyethylene) peroxides have been investigated with the reaction occurring in the melt state using low concentrations of peroxide. Reactive extrusion is becoming the favoured method being an extruder-conducted process that uses a very low level, typically < 0.5% peroxide mixed in the melt within a twin-screw extruder. In the recycled PE melt, via crosslinking and end linking reactions, the molecular weight increases as long chain branches are produced. In this way, the properties of the PE can be improved.

However, a disadvantage of the reactive extrusion process is that the mixing of polymer and peroxide is very inefficient due to the low concentrations of peroxide and the nature of operation of an extruder which moves the melt through in a continuous process, limiting the dwell time and thus the mixing time. This means control of chain extension is very difficult and can only be done in a very haphazard way, risking cross linking of chains which causes gels and reducing the process-ability of the resulting polymer.

A further limit on the secondary application for recycled plastics is in the additives and contaminants that are present in the reclaimed plastics. Colour presents a particular difficulty as the vast majority of recyclable plastic feedstock, recyclate, is a mixed colour granulate giving moulders and manufacturers no choice in colour selection so any products are black, low value and out of sight. This results in a low resale value and recyclers can only extract value from a small percentage of material economically.

US 9,803,035 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.

A disadvantage of such processes is in the quantity of solvent required with a typical minimum polymer solvent ratio being 1:100. A further disadvantage is in the length of time it takes to dissolve polymers (hours). While it is known that undertaking the dissolution at elevated temperatures and pressures will allow for some decrease in the time, this is not significantly compensated for by the additional equipment and procedures required to control the pressure and temperature during dissolution which would be required to operate on an economically viable commercial scale i.e. reactor vessel volumes of 1,000 litres and greater.

It is therefore an object of the present invention to provide a method of recycling plastics which obviates or mitigates one or more disadvantages in the prior art.

According to the present invention there is provided a method of recycling plastics, comprising the steps:

(a) providing a polyolefin feedstock from reclaimed plastic;

(b) creating a solution of at least one polymer of the feedstock in at least one solvent by dissolution in a reactor vessel;

(c) removing one or more contaminants from the solution;

(d) separating the solvent from the polymer; and then

(e) using an extruder to extrude the polymer; characterised in that: a chain extender is introduced to the solution before step (d). In this way, small concentrations of chain extender can be added to the solution and efficient mixing of the chain extender and polymer can be undertaken before the polymer is extruded. It is critical to add the chain extender to the polymer solution rather than the polymer melt (after step (d)) to ensure intimate mixing as that then gives the user accurate control over the chain extension reaction. Chain extension can then take place either at this stage by heating the polymer solution above the activation temperature of the chain extender or chain extension can take place once the polymer is precipitated with the chain extender in the melt stage (during extrusion) as per the prior art.

As the least one polymer is one or more polyolefins, this allows the method to be used on a thermoplastic feedstock. Preferably, the at least one polymer is polyethylene (PE). More preferably, the at least one polymer is mixed PE/PP recyclate. Such recyclates are a typical product created for use in recycling which contain additives such as colour pigments.

Preferably, the chain extender is a peroxide. In this way, modification of the polymer is known by the results of peroxide use with PE and PP in reaction extruder methods. The peroxide may be dilauroyl peroxide, Di (4- tert-butylcyclohexyl) peroxy dicarbonate, Dicetyl peroxydicarbonate and benzoyl peroxide being powders in their pure form and activating at medium temperatures and dicup, dicumyl peroxide which is a crystalline solid in its pure form and 2,5-Dimethyl-2.5-di(tert-butylperoxy) hexane, but any peroxide may be selected which will dissolve in the solution so as to mix with the polymer. Preferably the chain extender is added to the reactor vessel. Alternatively, the chain extender may be introduced to an in-line pipe after the reactor vessel. Optionally, the chain extender may be added to a further reactor vessel positioned downstream of the reactor vessel. Chain extension may take place while the polymer is still in solution or after it is precipitated during the extrusion phase. Preferably, the reactor vessel comprises a tank having at least one input and at least one output, the tank including a mixing device comprising: a plurality of discs aligned parallel to each other in a vertical stack; each disc extending over a majority of the cross-sectional area of the tank and including a plurality of perforations to allow the solvent and the at least one polymer to flow vertically in the tank through the discs; one or more supports being affixed to and holding the discs in relative to each other; and the one or more supports being connected to a linear motion generator so that the discs oscillate vertically at a first frequency and first amplitude, without rotating, the linear motion dissolving at least a portion of the polymer in the solvent.

It has been surprisingly discovered that by use of such a reactor vessel the time duration for dissolution is greatly reduced to minutes rather than hours and that the polymer to solvent ratio can also be increased. The reactor vessel is of a commercial scale. Preferably, the reactor vessel is greater than 1000 litres in capacity. Those skilled in the art will recognise that a 2,000 to 3,000 litre vessel is practical, or a series of vessels. However, scaling to a 10,000 litre vessel can be done and even up to 30,000 litres.

Preferably, step (b) has a dissolution time of less than one hour. More preferably the time duration is selected from a group comprising: less than 30 minutes; less than 15 minutes; and less than 10 minutes. In this way, the dissolution time, being the time for the at least one polymer to dissolve in the solvent, is at a practical level for the commercial use. Thus the method may be considered as a batch-fed continuous process. The at least one polymer may be in the range of 0.1% to 100% wt addition. Preferably, the at least one polymer is in the range of 0.1% to 20% wt addition. More preferably, the at least one polymer is in the range of 0.3% to 10% wt addition. By being able to reduce the polymer solvent ratio to 1:10, commercial use becomes economical. Preferably the first frequency is in the range 1 to 15 Hz. Preferably the first amplitude is in the range 40 to 1000mm. Tuning the frequency and amplitude can further reduce the dissolution time.

Preferably, the method includes purging an inert gas into the reactor vessel to displace oxygen with an inert atmosphere. Preferably the inert gas is nitrogen.

Preferably, the method includes heating the reactor vessel to a first temperature. Preferably the temperature is between room temperature and the at least one solvent boiling point. Temperatures in the range of 80°C to 120°C may be used. Preferably the temperature is above 120°C. By raising the temperature of the mixture, the dissolution time can be reduced. The temperatures required are suitable for commercial application.

The method may include a first step in which a mixed plastic feedstock is mechanically separated to remove contaminants. The contaminants may be considered as, but not limited to, polyvinyl chloride (PVC), polyethylene terephthalate (PET) and acrylonitrile butadiene styrene (ABS). Examples of plastic feedstocks may be: single source end of life thermoplastics i.e. wheelie bins all colour, containers, pipe, bottle caps, bottles and tanks; post-consumer and post-industrial recycled plastics; mixed PE/PP recyclate; films i.e. multilayer films, laminate films, PE or PP films; and other mixed thermoplastics: ABS, Polystyrene, PVC. The polyolefin feedstock is preferably polyethylene and/or polypropylene which may be in flakes referred to as recyclates in the industry.

Preferably the at least one solvent is butylal or xylene. This has found to be most effective but any solvent which operates on a polymer can be used. Other typical solvents may be toluene and benzene, for example. The method may include the step of selecting the at least one solvent for the polarity of the polymer as the solute and the solvent. It has been found that this factor affects the solubility of the polymer in the solvent. Preferably at step (c) filtration of the solution is undertaken to remove the additives. Preferably at least one of the additives is a colour pigment. In this way the method provides for the removal of colour pigments from the recycled plastic as well as modifying the polymer to provide a more valuable recyclate.

Preferably at step (d) precipitation of the solution is undertaken so that the at least one polymer precipitates out of the solvent. The step may further include de-watering to mechanically separate the solvent and the at least one polymer. Preferably, recovered solvent is reused in step (b). Preferably at step (b) fractionation of the solution is undertaken to separate polymer of different molecular weights. Fractionation may occur after the peroxide has been added. Alternatively, the peroxide is added to one or more of separated streams of polymer with different molecular weights. In this way, the type and dosage of peroxide can be more finely tuned to the molecular weight of polymer in the solution.

Alternatively at step (d) fractionation of the solution is undertaken to separate polymer of different molecular weights. In this way, fractionation is completed together with precipitation. Fractionation may be controlled by varying the temperature of the solution. Precipitation may be controlled by varying the temperature. Alternatively, fractionation and/or precipitation may be controlled by use of an antisolvent.

In the description that follows, the drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is to be fully recognized that individual features the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Furthermore, the terminology and phraseology used herein is solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including," "comprising," "having," "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes.

All numerical values in this disclosure are understood as being modified by "about". All singular forms of elements, or any other components described herein including (without limitations) components of the apparatus are understood to include plural forms thereof.

Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings of which:

Figure 1 is flow chart of a method of recycling plastics according to a first embodiment of the present invention;

Figure 2(a) is an illustration of a reactor vessel including a mixing device according to an embodiment of the present invention and Figure 2(b) is an illustration of disc for use in the mixing device of Figure 2(a), according to an embodiment of the present invention;

Figure 3 is flow chart of a method of recycling plastics according to a second embodiment of the present invention; and Figure 4 is flow chart of a method of recycling plastics according to a third embodiment of the present invention.

Reference is initially made to Figure 1 of the drawings which shows a flow chart to illustrate a method, generally indicated by reference numeral 10, of recycling plastics 12 according to an embodiment of the present invention. A reclaimed plastic feedstock 14 made up of polyolefins such as PE, PP or a mix as flakes of these is fed into a reaction vessel 16 (see Figure 2(a)). Also into the reaction vessel is a solvent 17. The mixture is heated and in a dissolution step 18, a polymer/solvent solution 20 is created. The solution 20 is filtered in a filtration step 22 to remove pigments or other additives 24. A precipitation step 26 is undertaken so that the polymer 14 precipitates out of the solvent 17 providing a two-phase mixture. The two- phase mixture is separated in a dewatering and extruding step 30 to provide recovered solvent 32 with the recovered polymer 34 being passed through an extruder. Each of these steps will be recognised and reproduceable to those skilled in the art. In the present invention, there is a modification to one of the steps. In the dissolution step 18, there is the addition of a chain extender 36. The chain extender 36 is added to the reaction vessel 16. This is preferably done after the polymer and solvent are in solution. While it is known from the prior art to add a chain extender 36 at the extrusion step 30, mixing a small concentration of the chain extender 36 in a polymer passing through an extruder is not an effective mixing process. By mixing the chain extender 36 into the solution 20 the mixing can be done more effectively in a reaction vessel 16, were the volume of solution 20 is known so that a more accurate measurement of chain extender 36 can be added and intimate mixing can take place unlike when chain extenders are mixed in at the melt stage. The reaction vessel 16 can also be closed or sealed so that mixing in the reaction vessel 16 can be timed to provide a sufficient residence time in the vessel for the chain extender 36 to react with the polymer 14 in the solution 20. Reacting the chain extender in the polymer solution phase gives the user far more control over reaction conditions and hence control over the chain extension process. The chain extender 36 reacts with the polymer 14 to create a recovered polymer 34 with a higher molecular weight which has more uses as a recycled plastic. The method can be used in a process for the removal of additives in plastics such as colour pigments and odours, primarily for the re-use of consumer and industrial recyclable plastics. Initial sorting can be undertaken to provide an ideal pure polyolefin feedstock for the polymer 14 input. Plastic feedstocks may be: single source end of life thermoplastics i.e. wheelie bins all colour, containers, pipe, bottle caps, bottles and tanks; post-consumer and post-industrial recycled plastics; mixed PE/PP recyclate; films i.e. multilayer films, laminate films, PE or PP films; and other mixed thermoplastics: ABS, Polystyrene, PVC. Plastics can be sorted into individual materials, such as high-density polyethylene (HDPE) or poly(ethylene terephthalate) (PET), or mixed streams of other common plastics, such as polypropylene (PP), low-density polyethylene (LDPE), poly(vinyl chloride) (PVC), polystyrene (PS), polycarbonate (PC), and polyamides (PA). A mechanical separation step can then remove the ABS, polystyrene, PET and PVC. The preferred polymer 14 is polyethylene (PE) and/or polypropylene (PP). Current recycling methods can produce a mixed PE/PP recyclate which provides the polyolefin in the form of flakes which can be easily weight and introduced to the reactor vessel 16 in measured quantities. In the examples given herein PE is the polymer 14 in the forms of HDPE Roto moulding grade (green tank); HDPE Blow moulding grade (Blue Drum); HDPE Extrusion grade (PE100 - Pipe); LDPE; and LLDPE as would be recognised by those skilled in the art.

There are a large number of known solvents for use on polymers. A selected list from reference sources may include: O-Dichloro benzene; 1, 2,3,4- Tetrahydronaphthalene; 1,2-Dichloroethane; 1,4-Dioxane; 1,4-dioxane, acetone; Acetic acid; Acetic anhydride; Acetone; Acetonitrile; benzene; Benzyl alcohol; Bromobenzene; Butanone; butylglycol; Chloro benzene; Chloroform; Cyclohexane; Cyclohexanol; Cyclohexanone; Decahydronaphthalene; Dibutoxymethane; dibutyl ether; Dichlorobenzene; Dichloromethane; Diethyl ether; Diethylene glycol; Diisopropyl ether; Diisopropyl ketone; dimethyl formamide; dimethyl sulfoxide; Dimethyl sulfoxide; Dimethylformamide; Diphenyl ether; Ethanol; Ethyl acetate; Ethylbenzene; Ethylene carbonate; Ethylene glycol; Ethylg lycol ; Formamide; Formic acid; Glycerol; iso- Propanol; iso-Butanol; isopropanol; Kerosene; m-Cresol; Methanol; Methyl acetate; methyl ethyl ketone; Methyl isobutyl ketone; Methylcyclohexane; Methylene Chloride; Methyl-n-amyl ketone; N,N-Dimethyl acetamide; N,N-Dimethyl formamide; n-Butanol; n-Butyl acetate; n-Butyl chloride; n-Butyl ether; n- Decane; n-Fleptane; n-hexane; n-Flexane; Nitro benzene; Nitroethane; Nitromethane; N-methylpyrrollidone; n-Propanol; Phenol; o, m, p-Xylene and mixtures; Pyridine; 1,1,2,2-Tetrachloroethane; Tetrachloroethane; and tetrahydrofuran. In the example given, the solvent 17 is butylal. While a single solvent 17 is used for the example, it will be recognised that for a mixture of applicable solvents can be used instead of a singular solvent for dissolution. Additionally, a two-phase system of solvents can be used to preferentially dissolve different polymer materials.

The chain extender 36 is preferably a peroxide. These are known to provide chain extension with PE in prior art reaction extruder methods. The chain extender 36 is chosen so that it does not react with the solvent 17 while also being soluble to mix in the reaction vessel 16. Suitable peroxides may be, but are not limited to dilauroyl peroxide, Di (4-tert-butylcyclohexyl) peroxy dicarbonate, Dicetyl peroxydicarbonate and benzoyl peroxide being powders in their pure form and activating at medium temperatures and dicup, dicumyl peroxide which is a crystalline solid in its pure form and 2,5- Dimethyl-2.5-di(tert-butylperoxy) hexane, being liquid at room temperature in its pure form and activating at high temperatures. These peroxides are soluble in organic solvents. Those skilled in the art will recognise that the high temperatures may only occur within the extruder and as such the place of the reaction between the chain extender and the polymer may be at any step within the method.

It is recognised that in the prior art, the polymer solvent ratio must be high i.e. typically 1:100 and that the dissolution step 18 can take hours. Increasing temperature and pressure can decrease the time, but this still makes the process expensive to operate limiting its use at commercial quantities. The inventors have developed a mixing system for use in a reactor vessel which can reduce the polymer solvent ratio to 1:10 and the time duration for dissolution to a few minutes for 1,000 litre capacities.

Reference is now made to Figures 2(a) and 2(b) which illustrates a reactor vessel 16 for dissolving at least one polymer 14 in at least one solvent 17 in a process for recycling plastics. The reactor vessel 16 is a substantially cylindrical tank 38 having a height greater than its diameter. The capacity of the vessel 16 will be equal to or greater than 1,000 litres for commercial use. Those skilled in the art will recognise that a 2,000 to 3,000 litre vessel is practical, or a series of vessels. However, scaling to a 10,000 litre vessel can be done and even up to 30,000 litres. In this embodiment, there is a first input port 40 through which the solvent 17 or solvents are introduced, a second input port 42 for the reclaimed plastic feedstock 14 (which may be in the form of a melt), and a third input 43 for the chain extender 36. It will be realised that a reduced number of inputs may be used by introducing more than one substance through an input. An output 44, shown towards the bottom of the tank 38, is used to remove the dissolved polymer/solvent solution 20. The output 44 could be arranged at any position as the solution could be extracted throughout the height of the reactor vessel 16.

Within the tank 38, there is a mixing device, generally indicated by reference numeral 46, vertically arranged over the entire height in the centre of the tank 38. Mixing device 46 has a central shaft 48 upon which is attached a stack 52 of spaced apart circular discs 50a-g in parallel horizontal alignment to each other and perpendicular to the shaft 48. For illustrative purposes only seven discs 50 are shown. The discs 50, see Figure 2(b) are circular and may gave a diameter close to that of the tank 38. A series of apertures 65 are arranged through each disc 50 so that vessel contents can move throughout the tank 38 to create mixing and the dissolving of the polymer 14 in the solvent 17. The shaft 48 is attached to an actuator linear movement motor 54, so that it moves up and down, moving longitudinally on its own central axis, vertically with respect to the vessel 16. The discs 50 move up and down, longitudinally when the shaft 48 moves and thus they can be set to oscillate at a desired frequency and amplitude. This oscillation of the stack 52 can operate over a fixed time, it may be for short repeated pulses or can be stopped and started between checks to determine the dissolution of the vessel contents. Note that the stack 52, shaft 48 and discs 50 do not rotate so there is no stirring action. Oscillation of the stack 52 is controlled by circuitry 58 which operates the motor 54. Additionally, the tank 38 can be heated via heater bands 60a-b to heat the mixture 20. Sensors (not shown) can be used to determine the temperature of the mixture 20 and vessel 16, so that the temperature of the heater bands 60a-b can be adjusted to control the overall heating temperature and the temperature gradient across the tank 38 via a temperature control unit 62.

There is also a gas line 64, used to introduce an inert gas such as nitrogen into the vessel 16. The nitrogen or other inert gas is purged into the system to displace oxygen with an inert atmosphere. The vertical oscillation without rotation of the discs has been found to speed up the dissolution step. Fleating of the vessel also increases the speed of dissolution. The temperature selected is below the solvent boiling point. Additionally, the temperature selected is below the polymer melting point, though an embodiment of the invention may be used wherein the polymer is introduced as a melt stream with the use of an amorphous polymer decreasing the dissolution time. In the example, the temperature is increased to above 120°C.

Table 1. Dissolution Times in maximum polyethylene wt% addition in Butylal

Table 1 provides scaled predictions for a 10,000 litre reactor vessel, based on experimental results from a 500ml reactor vessel with the worked example described hereinbefore. Thus the time duration for 100% dissolution is shown in all cases to be under 12 minutes. When compared to the prior art processes, which take hours, this reduction in dissolution time is a distinct advantage in commercial recycling plants. The process is also achieved without requiring the vessel to be placed under elevated pressures as for the prior art. However, it would be recognised by those skilled in the art that pressure could be applied to the vessel, say up to 5 atm, which will increase the internal temperature and therefore increase the solubility of the solvent and consequently further reduce the time duration.

While the chain extruder 36 can be added to the reaction vessel 16 in the dissolution step 18, it may alternatively be added at any part of the process before the solvent is removed.

Reference is now made to Figure 3 of the drawings which illustrates a method, generally indicated by reference 110, according to a further embodiment of the present invention. Like parts to those of Figure 1 have been given the same reference numeral to aid clarity. In Figure 3, the chain extender 36 is not added to the reaction vessel in the dissolution step, but instead is added to pipework between the filtration step 22 and the precipitation step 26. At this stage the solution 20 remains as a solution but unwanted additives such as colour pigments 24 have been removed by filtering. While the chain extender 36 is shown being added to pipe work, an additional vessel could be added here to mix the chain extender 36 in the solution 20, if desired.

A further modification to the method is also shown in Figure 3. During precipitation, in the precipitation step 26, the polymer 14 may be selectively fractionated to separate out polymers by their molecular weight Thus, a combined step of fractionation 66 with precipitation 26 occurs in the method 110. For illustrative purposes, three resulting polymer/solution streams are shown, a high molecular weight polymer/solution stream 68a, a medium molecular weight polymer/solution stream 68b and a low molecular weight polymer/solution stream 68c. Those skilled in the art will recognise that the fractionation step 66 can be controlled by temperature so that different molecular weights precipitate and can be separated out in order. Each stream 68a-c can then be dewatered and extruded to provide the recovered solvent 32 and three recovered polymers 34a-c, graded by their molecular weights. These provide different selected polymers for onward use.

Figure 4 provides a further embodiment of the method, generally indicated by reference numeral 210. Like parts to those of Figure 1 and 3 have been given the same reference numeral to aid clarity. In the method 210, the fractionation step 66 is now combined with the dissolution step 18. In this way, the polymer/solution streams 68a-c with differing molecular weights of polymer are separated out as an initial step (iii) This is achieved by controlling the temperature/temperature gradient of the reactor vessel 16 and mixing speed/time of oscillation. In this way, different molecular weights of polymer are dissolved into solution at different times and at different locations within the tank 38. These streams 68a-b can be drawn off and processed separately. The chain extender 36 can now be added to one or more streams 68a-d. In this way, the chain extender 36 and its concentration can be selected to suit the molecular weight of polymer in a stream 68a-c, to provide improved productivity. Figure 4 shows the chain extender 36 being added before precipitation 26, but could be added before filtration 22, to the high molecular weight stream 68a exclusively. A complete reaction may occur in the pipe work or an intermediate reaction tank. Alternatively, the chain enhancer 36 may disperse in the solution and then off the solvent in step 26, leaves the chain enhancer 36 in the polymer 14 which then reacts in during the extrusion phase, however the chain enhancer 36 will be well dispersed by this point compared to the prior art methods. The remaining steps are carried out in the same manner to provide recovered polymers 34a-c which are modified to provide the greatest value for onward sale and use. Figure 4 also illustrates an initial step of sorting 70 the reclaimed recycled plastic 12 which may be in the form of mixed recyclate flakes as are known in the industry into a polyolefin feedstock 14 of preferably PE/PP flake. A similar reactor vessel can be used to that used for dissolution wherein the flakes are mixed with water and the plastics are separated out in layers within the vessel by their densities. The non-polyolefins 72 are removed and disposed of or recycled in a suitable process. Such a sorting process is disclosed in GB2522599.

The present invention provides the following benefits over prior art use of peroxides as chain extenders during melt processing of plastics. 1. Peroxides are easier to add in the solvent phase and will be more homogeneous in solution rather than a molten polymer.

2. Addition of the peroxide in the solution makes the reaction in either the solution or melt phase more homogeneous providing a more uniform product. 3. Any residual products from the decomposition of the peroxide will stay in the solvent after reaction in the solution phase making a cleaner final product.

4. In solution there will be less gel formation of the polymer. The principle advantage of the present invention is that it provides a method of recycling plastics in which a chain extender can be added in solution before the polymer is separated and extruded.

A further advantage of the present invention is that it provides a method of recycling plastics in which a chain extender can be added to a selected high molecular weight polymer/solvent stream before the polymer is separated and extruded.

A yet further advantage of the present invention is that it provides a method of recycling plastics which allows a chain extender to be used to increase the value of the recovered polymer and can be used in commercial processes with reactor vessel volumes in excess of 1,000 litres.

Claims

1. A method of recycling plastics, comprising the steps:

(a) providing a polyolefin feedstock from reclaimed plastic;

(c) removing one or more contaminants from the solution;

(d) separating the solvent from the polymer; and then

(e) using an extruder to extrude the polymer; characterised in that: a chain extender is introduced to the solution before step (d).

2. A method of recycling plastics according to claim 1 wherein the chain extender is a peroxide.

3. A method of recycling plastics according to claim 1 wherein the peroxide is one selected from a group consisting of: dilauroyl peroxide; Di (4-tert-butylcyclohexyl) peroxy decarbonate; Dicetyl peroxydicarbonate; benzoyl peroxide; dicup, dicumyl peroxide; and, 2,5-Dimethyl-2.5-di(tert-butylperoxy) hexane.

4. A method of recycling plastics according to any preceding claim wherein the chain extender is added to the reactor vessel.

5. A method of recycling plastics according to any one of claims 1 to 3 wherein the chain extender is introduced to an in-line pipe after the reactor vessel.

6. A method of recycling plastics according to any one of claims 1 to 3 wherein the chain extender is added to a further reactor vessel positioned downstream of the reactor vessel.

7. A method of recycling plastics according to any preceding claim wherein the reactor vessel comprises a tank having at least one input and at least one output, the tank including a mixing device comprising: a plurality of discs aligned parallel to each other in a vertical stack; each disc extending over a majority of the cross- sectional area of the tank and including a plurality of perforations to allow the solvent and the at least one polymer to flow vertically in the tank through the discs; one or more supports being affixed to and holding the discs in relative to each other; and the one or more supports being connected to a linear motion generator so that the discs oscillate vertically at a first frequency and first amplitude, without rotating, the linear motion dissolving at least a portion of the polymer in the solvent.

8. A method of recycling plastics according to any preceding claim wherein the reactor vessel is greater than 1000 litres in capacity.

9. A method of recycling plastics according to any preceding claim wherein step (b) has a dissolution time of less than one hour.

10. A method of recycling plastics according to claim 9 wherein the time duration is selected from a group comprising: less than 30 minutes; less than 15 minutes; and less than 10 minutes.

11. A method of recycling plastics according to any preceding claim wherein the at least one polymer is in the range of 0.1% to 100% wt addition.

12. A method of recycling plastics according to claim 11 wherein the at least one polymer is in the range of 0.3% to 10% wt addition.

13. A method of recycling plastics according to any one of claims 7 to 12 wherein the first frequency is in the range 1 to 15 Hz and the first amplitude is in the range 40 to 1000mm.

14. A method of recycling plastics according to any preceding claim wherein the method includes heating the reactor vessel.

15. A method of recycling plastics according to any preceding claim wherein the method includes a first step in which a mixed plastic feedstock is mechanically separated to remove contaminants.

16. A method of recycling plastics according to any preceding claim wherein polyolefins are polyethylene.

17. A method of recycling plastics according to any preceding claim wherein the at least one solvent is one selected from a group consisting of: butylal, xylene, toluene and benzene.

18. A method of recycling plastics according to any preceding claim wherein at step (c) filtration of the solution is undertaken to remove the additives.

19. A method of recycling plastics according to claim 18 wherein at least one of the additives is a colour pigment.

20. A method of recycling plastics according to any preceding claim wherein at step (d) precipitation of the solution is undertaken so that the at least one polymer precipitates out of the solvent.

21. A method of recycling plastics according to claim 20 wherein the step further includes de-watering to mechanically separate the solvent and the at least one polymer.

22. A method of recycling plastics according to claim 21 wherein recovered solvent is reused in step (b).

23. A method of recycling plastics according to any preceding claim wherein at step (b) fractionation of the solution is undertaken to separate polymer of different molecular weights.

24. A method of recycling plastics according to any claim 23 wherein fractionation occurs after the peroxide has been added.

25. A method of recycling plastics according to any claim 23 wherein the peroxide is added to one or more of the separated streams of polymer with different molecular weights.