Method for recycling thermoplastic matrix composite materials and industrial facility for implementing same

Abstract

The invention relates to a method for recycling polymer-based synthetic sealing membranes composed of two or more layers laminated to one another and between which a glass or polyester fibre weft is inserted, wherein the membranes undergo the following steps: - mechanical grinding and densimetric separation; - pulsed-power treatment; - drying and densimetric separation. The invention also provides an industrial facility.

Description

[0001] Process for recycling thermoplastic matrix composite materials and industrial installation for its implementation

[0002] General technical field and prior art

[0003] The present invention relates to a new technology for recycling composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers.

[0004] A typical use of thermoplastic matrix composites is flexible membranes for waterproofing roofs, terraces, or even swimming pools or basins, truck tarpaulins, garden hoses, etc. They can have a single layer or several layers of reinforcements.

[0005] Reinforcements are generally in the form of woven or non-woven fabrics based on fiberglass, synthetic fibers such as polyester fiber, polyolefins, etc. or natural fibers such as linen fiber, cotton, etc. Thermoplastic polymers are synthetic polymers such as ethylene-propylene-diene (EPDM), poly(vinyl chloride) (PVC) and thermoplastic polyolefins (TPO)...

[0006] The interest in recycling these thermoplastic composite materials is to be part of a circular and sustainable economy aimed at reusing the raw material and reducing the use of virgin raw material.

[0007] Another interest is to reduce the carbon footprint of the manufacturing processes of thermoplastic composite materials by reincorporating recycled material instead of virgin material and thus greatly reduce (up to 400%) the carbon impact of said composite materials.

[0008] And given the weight that the purchase of raw materials represents in the cost price, the use of recycled material instead of virgin material and without altering the mechanical properties of the product makes it possible to reduce the manufacturing cost.

[0009] There are conventional recycling techniques for thermoplastic matrix composite materials based on very fine mechanical grinding (in the order of a millimeter) which, after successive mechanical grinding stages, separate the granular particles of different weights and densities by air flow. After this grinding and separation phase, the ground plastic material is transmitted to an extruder which allows it to be remelted and transformed into granules. Depending on the applications, these recycled granules can replace all or part of virgin polymers in new manufacturing. Reinforcing fibers can be thermally recycled.

[0010] However, the recycling techniques known to date are not entirely satisfactory. Fine mechanical grinding requires a significant level of energy. It also often requires the use of cryogenics to achieve the desired particle sizes, making it expensive.

[0011] On the other hand, in the case of thermoplastic matrix composite materials, the grinding and separation steps do not allow 100% of the fibers to be extracted. In fact, it is estimated that, with a mechanical recycling process, up to 20% of the residual fiber quantity remains:

[0012] - either encapsulated between the layers of polymers

[0013] - either fixed to the surface of the polymers by electrostatic charge.

[0014] The presence of 20% of residual fibers in these so-called "semi-finished" products quickly clogs / blocks the filters of the granulating extruder and makes the mechanical recycling operation difficult to industrialize.

[0015] These fibers further contribute to the poor quality of the recycled polymer raw material.

[0016] Finally, thermal recycling is not feasible for all types of fibers. For example, polyester fibers have a melting point > 230°C, which means that up to this temperature, the fibers remain in the form of fibers and can generate complexity in the processes (clogging, complex filtration, deterioration of the mechanical properties of the membranes produced from these products, etc.).

[0017] There is therefore a significant need for a specific technology for recycling thermoplastic matrix composite materials which, for example, can come from production offcuts, construction site waste and building renovation.

[0018] Furthermore, techniques for recovering materials and / or products using pulsed power are already known. In FR 3.117.404 in particular, it is provided that mechanical grinding is not required to achieve separation.

[0019] For this purpose, it is proposed to implement a pulsed power treatment and to combine this treatment with dissolution in a two-phase medium. This dissolution makes it possible to increase yields.

[0020] If grinding is considered, it does not allow extraction. It is only a conditioning grinding, as part of a pre-cutting step to prepare the materials before they enter the reactor.

[0021] After this pre-cutting, the materials have metric or centimeter sizes.

[0022] Patent application WO2010 / 092136, which may be referred to with advantage, also describes in detail pulsed power separation treatments.

[0023] General presentation of the invention

[0024] One aim of the invention is to propose an industrially viable process for recycling thermoplastic matrix composite materials.

[0025] Another aim of the invention is to propose a process allowing excellent polymer / reinforcement separation without degradation of the polymer, in order to have a high-performance recycled thermoplastic raw material for new applications.

[0026] Another aim of the invention is to propose a recycling process for recovering the fibers from the extracted reinforcements.

[0027] Thus, the invention proposes a method for recycling composite materials based on thermoplastic polymers composed of at least two layers, including at least one reinforcement based on mineral or synthetic fibers, in which the following steps are implemented:

[0028] Mechanical grinding to obtain a polymer ground material of intermediate granulation level, the granulometry of the material resulting from the mechanical grinding step being between 2 mm and 6 mm, more than 75% of the initial mass of fibers being extracted by this mechanical grinding, preferably more than 78%, even more preferably 80% or more,

[0029] Separation of polymers and fibers released during this mechanical grinding,

[0030] Treatment by pulsed electric power to obtain a finer polymer grind and extract the remaining fibers, more than 99% of the initial mass of fibers having been extracted at the end of this step, preferably more than 99.9%,

[0031] Drying and separation of the polymers from the fibers thus released,

[0032] Possibly extrusion and granulation of polymers.

[0033] Thus, the proposed process combines mechanical grinding and complementary treatment by pulsed electric power.

[0034] This combination allows for relaxation of the requirements on the fineness of the particle size to be obtained at the end of the mechanical grinding stage. Typically, as a purely illustrative example, the particle size at the end of the grinding stage can be 6 mm instead of 1 mm. With a higher particle size, cryogenics is then not necessary. The cost in grinding power is lower.

[0035] Pulsed electric power treatment complements mechanical grinding and allows residual fibers to be extracted sufficiently to ensure the expected separation quality.

[0036] It also prevents the reinforcing fibres from sticking to the surface of the polymer material, the fibres and the material being instead repelled from each other by electrostatic effect during treatment with pulsed electrical power.

[0037] Such a process thus makes it possible to extract almost all of the reinforcements (typically, greater than 99% of the initial mass of the fibers) facilitating the potential subsequent granulation stages within an extruder.

[0038] The polymer entering the extruder and the recycled raw material in the form of granules leaving it are therefore of excellent quality.

[0039] The method is for example supplemented by the following different characteristics taken alone or in combination: mechanical grinding is implemented with a power of between 0.2 kWh / kg and 55 kWh / kg; pulsed power treatment is implemented with a power inversely proportional to that of the mechanical grinding of between 0.05 kWh / kg and 3.00 kWh / kg; pulsed power treatment implements a number of shots per kg of material of between 500 shots and 6000 shots at a discharge frequency of between 1 and 40 Hz, a stored energy of between 100 joules and 10,000 joules and a circuit frequency of between 100 and 900 kHz.

[0040] The invention further relates to an industrial installation suitable for implementing a recycling method as proposed, comprising a grinding unit, a reactor for pulsed power treatment and a centrifugation or fluidized bed drying station.

[0041] The installation can also include an extrusion and granulation line.

[0042] The separation and drying station may include a density separator.

[0043] Such an industrial installation is for example used for the recycling of composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers, in particular for the recycling of PVC / PET membranes, TPO / PET membranes, reinforced PVC garden pipes, reinforced plasticized PVC truck tarpaulins, etc.

[0044] Brief description of the drawings

[0045] Other characteristics and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and must be read in conjunction with the appended figures in which: Figure 1 schematically illustrates different stages of a method in accordance with a possible mode of implementation for the invention; Figure 2 is a schematic representation of an industrial installation for implementing the method of Figure 1.

[0046] Description of one or more methods of implementation and production

[0047] In general, the proposed process allows the recycling of composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers.

[0048] As an illustration, the following will be taken as a case of a synthetic membrane, although other applications are of course possible.

[0049] A synthetic membrane to be recycled comes, for example, in the form of a roll of two layers of PVC laminated to each other and between which is inserted a woven matrix based on PET.

[0050] The proposed recycling process involves several successive stages.

[0051] In a preparatory treatment, the material is shredded to form waste strips (step A in Figure 1).

[0052] In a first step, the solid waste obtained is subjected to mechanical grinding (step B) to reduce the size of the polymer and the fibers.

[0053] After sieving, this mechanical grinding is repeated until the desired particle size level is obtained. Typically, for shredded membranes with a thickness between 1.2 and 2.0 mm, the average particle size expected at the end of mechanical grinding step B is between 6 mm and 2 mm, preferably between 3 mm and 2 mm).

[0054] The resulting ground material is then passed through a separator (by air flow or any other densimetric separation technique) (step C).

[0055] Such a separator allows to separate several bulk materials according to their density, shape and size. The lightest material (the fiber) is extracted from the top, the one with the highest density (Polymer) from the bottom. This allows to extract a large majority of the fibers (about 80% of the initial mass (more than 75% of the initial mass of fibers, preferably more than 78%, even more preferably 80% or more).

[0056] However, the higher the average particle size of the ground materials from stage B and stage C (greater than 5 mm), the greater the probability of finding a significant quantity of encapsulated fibers in these ground materials, as initially, between the two layers of polymer.

[0057] This prevents the direct reuse of the polymer shreds obtained in a possible extrusion-granulation circuit, and therefore requires additional treatment.

[0058] Steps A, B and C are implemented in the same shredding, grinding and separation unit 1 as illustrated in Figure 2. For shredding, depending on the type and size of the material, this unit 1 incorporates guillotines or shredders.

[0059] It also includes, for example, a knife mill which provides grinding and a separator.

[0060] In a subsequent step of the process (step D), the polymer ground materials thus obtained are subjected to a grinding treatment by electric pulsed power (energy discharges). This step D is carried out in a separate reactor compared to unit 1 - reactor 2 in Figure 2).

[0061] The polymer mixture containing encapsulated fibers and / or fibers attached to the surface by electrostatic force is for this purpose conveyed to the high voltage zone of said reactor 2, for example by a conveyor.

[0062] Reactor 2 comprises one or more sets of tip electrodes, as well as a power unit with storage capacitor(s) and / or inductor(s) which, with a power switch, make it possible to discharge a succession of very high power electrical pulses onto products and materials previously immersed in a liquid ambient medium.

[0063] The resistivity of the liquid, the switching time and the pulses cause the passage of energy-charged electric arc channels inside the immersed material between the two electrodes and passing through said immersed material. The passage of the electric arc through said material separates the constituents of the materials.

[0064] The discharge circuits enable ultra-fast switching with a switching time between 50 ns and 150 microseconds.

[0065] The voltages between electrodes are in the order of ten to several hundred kilovolts. The energy used for each reactor is in the order of 50 joules to 10,000 joules, which can be in the order of 100 joules to 10,000 joules. The discharge frequency is 1 to 40 Hz at a frequency and the circuit frequency between 100 and 900 kHz.

[0066] Thus, for example, starting from an electrical energy source delivering 1 kW, we can store 10 kJ of energy in capacitors or storage inductors for 10 s. The energy is restored in 10 ps, ​​which makes it possible to deliver 1 GW of power.

[0067] A pulsed power treatment uses a succession of discharges, typically a number of discharges between 500 and 6000 shots per kg of polymer material to be treated, preferably less than 3000 shots per kg and even more preferably less than 1500 shots per kg of material for example.

[0068] The time and energy required to extract almost all of the residual fibers by pulsed electric power treatment are related to the quantity of residual fibers present and the form of their adhesion to the polymers after the mechanical grinding operation of step B and separation step C.

[0069] At the end of stage D, we obtain a ground material of the same diameter as at the end of the mechanical grinding of stage B, but almost free of its encapsulated fibers by the effect of sonic and subsonic shock waves and electronic avalanche.

[0070] Furthermore, with this pulsed electrical power treatment step, and contrary to what would have been the effect of mechanical grinding alone (micronization), the residual fibers and the surfaces of the polymer grinds are electrically charged and move away from each other.

[0071] The ground material thus obtained is transmitted to a third unit (unit 3 in figure 2) where it is treated and dried (step E). If necessary in combination with screening, it is also subjected in this unit 3 to treatment by a densimetric separator (air flow or other technique) (step F) which separates the reinforcement fibers and the polymer ground material.

[0072] The polymer material thus isolated is of optimized quality: the treatment by pulsed electric power makes it possible to separate during this stage the fibers and the polymer shreds, which would otherwise have been stuck together by surface effect and electrostatic charge. It should be noted that the micronization of polymers is not a relevant solution for the recycling of synthetic membranes based on polymers. Indeed, it becomes difficult, if not impossible, to separate the fiber from the polymers by air flow or by density, just as the increase in the specific surface also increases the fraction of fibers stuck electrostatically to the surface of the aggregates. Furthermore, the micronization of a thermoplastic matrix composite requires cryogenic grinding and cooling of the materials below their glass transition temperature, which increases the cost of the treatment.

[0073] At the end of step F, more than 99% of the initial mass of fibers has been removed, preferably more than 99.9%. This produces a polymer free of any reusable fibers with or without regranulation. The mechanical properties of the recycled polymer thus obtained (breaking load (N / mm2) and elongation at break (%)) are equivalent to the virgin polymer.

[0074] It should be noted that for a given average granulometry expected at the end of step F, the energy consumed during step D of treatment by pulsed electrical power is a function of the energy consumed during mechanical step B.

[0075] The lower this energy, the larger the diameter of the intermediate ground material obtained at the end of stage B, which will require significant energy at the stage of grinding by pulsed electric power.

[0076] Conversely, the higher the mechanical grinding energy in stage B, the finer the intermediate grind and the more the pulsed electrical power processing stage can be limited.

[0077] Typically, in the case of membrane recycling given here for illustration purposes, for an average particle size objective of the order of 1 mm at the outlet of step F, the mechanical grinding can be implemented with a power of between 0.2 kWh / kg and 1.22 kWh / kg, while the treatment by pulsed power is implemented with a power depending on that of the mechanical grinding of between 0.083 kWh / kg and 0.67 kWh / kg, preferably between 0.17 kWh / kg and 0.33 kWh / kg.

[0078] The ground polymer material thus obtained and separated can then be regranulated in an extruder included in unit 3 (step G).

[0079] For an extruder with a flow rate of 28 kg / h, an outlet filter with openings of 200 pm and a pressure lower than 165 bars at the level of said filter, it is considered necessary to change the filter when the injection pressure reaches between 80 and 100 bars. For membranes of 1.5 mm thickness in plasticized PVC, with a particle size between 2 and 3 mm and a number of shots equal to 1000 shots or less allows an operating time without clogging of the filter to 20 minutes.

[0080] By increasing the number of shots (1500 shots or more), times greater than 40 minutes of continuous operation could be observed.

[0081] The resulting recycled plasticized PVC is of good quality. It does not show any deterioration. The proposed process allows processing rates of 700 kg per hour up to 1000 kg per hour. The recycling rate and efficiency are uniform regardless of the input for use in industrial conditions.

[0082] The separated fibers can be reused by being baled directly, re-extruded or even chemically recycled by glycolysis, for example.

[0083] It should also be noted that the proposed process is environmentally friendly since it does not require any chemical additives or heat treatment.

[0084] The proposed process applies generally to the recycling of composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers.

[0085] For example, it is advantageously used for recycling waterproofing membranes made of plasticized PVC reinforced with polyester grids, waterproofing membranes made of plasticized TPO reinforced with polyester grids, reinforced PVC garden hoses, reinforced plasticized PVC truck tarpaulins, and other applications are of course possible.

Claims

Claims 1. Process for recycling composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers, in which the following steps are carried out: Mechanical grinding to obtain a polymer ground material of intermediate granulation level, the granulometry of the material resulting from the mechanical grinding step being between 2 mm and 6 mm, more than 75% of the initial mass of fibers being extracted by this mechanical grinding, preferably more than 78%, even more preferably 80% or more. Separation of polymers and fibers released during this mechanical grinding, Treatment by pulsed electric power to obtain a finer polymer grind and extract the remaining fibers, more than 99% of the initial mass of fibers having been extracted at the end of this step, preferably more than 99.9%, Drying and separation of the polymers from the fibers thus released, Possibly Extrusion and granulation of polymers.

2. Method according to claim 1, in which the mechanical grinding is carried out with a power of between 0.2 kWh / kg and 55 kWh / kg.

3. Method according to claims 1 to 2, in which the pulsed power treatment is carried out with a power inversely proportional to that of the mechanical grinding of between 0.05 kWh / kg and 3.00 kWh / kg.

4. Method according to claims 1 to 3, in which a pulsed power treatment implements a number of shots per kg of material of between 500 shots and 6000 shots at a discharge frequency of between 1 and 40 Hz, a stored energy of between 100 joules and 10,000 joules and a circuit frequency of between 100 and 900 kHz.

5. Industrial installation adapted to implement a recycling process according to one of the preceding claims, comprising a grinding unit, a reactor for treatment by pulsed power and a drying station by centrifugation or by fluidized bed.

6. Industrial installation according to claim 5, further comprising an extrusion and granulation line.

7. Industrial installation according to claim 5, in which the drying station comprises a density separator.

8. Use of an industrial installation according to one of claims 5 to 7 for the recycling of composite materials based on thermoplastic polymers composed of at least two layers including at least one reinforcement based on mineral or synthetic fibers.

9. Use according to claim 8 of PVC / PET, TPO / PET membranes.

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