Process for the recovery and devulcanization of crosslinked rubber

Abstract

The process for the recovery and desulfurization of vulcanized rubber (10) is carried out in an apparatus comprising: a grinder (12) for grinding the vulcanized rubber into particles; a desulfurization twin-screw extruder (20) provided with a device (18) for forced feeding of the vulcanized rubber particles and a constant temperature device; a single-screw extruder (22) arranged downstream of the twin-screw extruder (20) provided with a constant temperature device, a filter (26) for desulfurized rubber and an extrusion die (28) shaped like a slot, from which the desulfurized rubber is extruded in the form of a strip or sheet; a cooling device for the desulfurized rubber strip or sheet. The twin-screw extruder (20) is operated at a temperature between 35 and 450°C, a screw speed between 15 and 600 rpm, and a torque density between 11 and 18 Nm / cm 3 The shear rate is kept substantially constant throughout the longitudinal extension of the twin-screw extruder (20).

Description

Technical Field

[0001] The present invention relates to a process for the recovery and devulcanization of crosslinked rubber. Background Art

[0002] Typically, the rubber to be recycled consists of industrial processing waste from specialized collection centers and / or landfills and / or articles that have reached the end of their useful life.

[0003] The molecular chains of vulcanized rubber are usually connected by chemical bonds formed by sulfur atom bridges that connect sulfur atoms laterally. Therefore, vulcanized rubber has a structure similar to that of thermosetting polymers and cannot be reused in production cycles.

[0004] It is known to apply a certain amount of energy to the crosslinked rubber, for example to break the chemical bonds produced by the sulphur, so that the elastomeric material returns to its initial physicochemical state before the vulcanization treatment and is ready for repeated use in the production cycle.

[0005] However, despite the increasing prevalence of more selective and high-performance waste sorting methods, the rubber collected for devulcanization often contains particles of other substances that contaminate the final devulcanized rubber, thereby impairing its properties. Summary of the invention

[0006] It is therefore an object of the present invention to provide a process which overcomes this disadvantage due to the impurities which are inevitably present in the vulcanized rubber to be treated.

[0007] This object is achieved by a process for the recovery and devulcanization of crosslinked rubber.

[0008] The present invention meets the current market demand for a purity of devulcanized rubber that allows for a non-degrading recycling process that requires reduced energy input and produces a final plastic material that is devulcanized and substantially free of impurities and contaminants. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Further advantages and features of the present invention will become apparent from the following detailed description, which is provided by way of non-limiting example with reference to the accompanying drawings, in which:

[0010] Figure 1 is a schematic elevational view of an apparatus for carrying out the method of the present invention, the apparatus being divided into two parts for the sake of greater clarity;

[0011] Figure 2 yes Figure 1 A plan view of the device in FIG.

[0012] Figure 3yes Figure 2 A magnified view of the details in;

[0013] Figure 4 is based on Figure 1 A cross-sectional view taken along line IV-IV in FIG.

[0014] Figure 5 is based on Figure 1 A cross-sectional view taken along line VV in FIG. 1 ; and

[0015] Figure 6 yes Figure 2 A zoomed in view of further details. DETAILED DESCRIPTION

[0016] A device for recovering and desulfurizing vulcanized rubber 10 comprises: Figure 1 and Figure 2 ) A grinder 12 for grinding and devulcanizing rubber 10, a homogenizer 14 and a weight / volume metering unit 16 are arranged downstream of the grinder 12, which feed the rubber particles produced in the grinder 12 to a forced feeding device 18, and the forced feeding device 18 force-feeds the particles into a twin-screw extruder 20.

[0017] Downstream of the twin-screw extruder 20 is a single-screw extruder 22 equipped with a gear pump 24, a filter 26 for the devulcanized rubber and an extrusion die 28 shaped like a slot from which the devulcanized rubber is extruded in the form of a strip or sheet.

[0018] Downstream of the extrusion die 28 there is a tank 30 containing cooling water, a dewatering and drying tunnel 32 and a device 33 for collecting the devulcanized rubber strip or sheet.

[0019] The forced feeding device 18 is known per se and allows vulcanized rubber particles of different shapes, particle sizes and apparent densities to be easily introduced into the twin-screw extruder 20. The device generally comprises a hopper 34 and a cylinder 36 ( Figure 3 ), one or more screws 38 with conical or cylindrical geometry rotate inside the cylinder 36; it is positioned vertically relative to the extruder (or transversely in an embodiment not shown) and can have a mechanical, electrical or pneumatic drive to force the vulcanized rubber particles into the twin-screw extruder 20. The use of such a forced feeding device 18 increases the filling rate of the free volume of the screws of the twin-screw extruder 20 and has proven to be particularly effective in the processing of expanded vulcanized rubber, which generally has a density of 50 to 350 kg / m 3In this case, the greater filling of the screw allows a significant increase in the productivity, ie an increase of more than 100% compared to the case without the forced feeding device 18, thereby further significantly reducing the degradation of the rubber due to excessive shear stresses.

[0020] The shear rate (shear rate) (or geometric shear rate) in the twin screw extruder 20 remains constant over its entire length, so that the crosslinking bonds between the molecular chains of the rubber are broken, thereby avoiding possible degradation of the vulcanized rubber. The high filling rate of the twin screw extruder 20 determined by the forced feeding device 18 ensures that there is constant material in each area, so that the advancement and the applied stress are uniform. On the contrary, in the absence of the forced feeding device 18, due to the shape and density of the processed material, the processed material will be subjected to different shear rates in the twin screw extruder 20, which will degrade it. Similarly, in the transition from vulcanized rubber to devulcanized rubber, the high filling rate is conducive to cooling, which can also resist degradation.

[0021] Furthermore, the forced feed device 18 increases the operating flexibility of the apparatus by allowing the processing of particles having different particle sizes, in the form of powders or granules, having a size between 0.2 mm and 15 mm.

[0022] The twin screw extruder 20 advantageously comprises a cylindrical module, which is generally of a length equal to 4 times the outer diameter of the screw and is mounted in series to form a continuous cylinder. Typically, the length of the twin screw extruder 20 is at least equal to 64 times the outer diameter of the screw, preferably not more than 80 times the outer diameter of the screw.

[0023] The twin-screw extruder 20 is provided with a thermostatic device which comprises, on the one hand, a plurality of resistors attached to the outer surface of the cylinder and, on the other hand ( Figure 4 ) comprises a water cooling circuit formed by a plurality of tubular holes 40, realized as a labyrinth, extending in the wall 44 of the associated cylinder around its central cavity parallel to the longitudinal axis 42 of the extruder. By regulating the alternation of heating and cooling phases in the desired manner, it is possible to control the energy applied to the elastomeric material, preventing it from degrading after the breaking of the cross-linking chemical bonds between the molecular chains.

[0024] The ratio between the outer diameter and the inner diameter of the screw is between 1.22 and 1.78, preferably between 1.55 and 1.78. In a manner known per se and not shown in the figures, the sequence of conveying and mixing elements is individually arranged on the multi-row shaft of the screw, in particular with a geometric profile to make the filling uniform, as described in EP-1 136 228 B1, which allows the shear stress applied to the rubber and therefore the specific energy absorbed by said rubber to remain uniform and controlled.

[0025] Typically, the torque density is 11Nm / cm 3 and 18Nm / cm 3 The screws are rotated at speeds between 15 rpm and 600 rpm, thereby ensuring a high fill rate of the screws and a low and constant shear rate over the entire length of the twin screw extruder 20. The end result is a higher productivity (up to 40-60%) and a lower operating temperature relative to an extruder with similar geometric parameters, the same screw speed but a lower torque density.

[0026] The single-screw extruder 22 comprises, in a known manner, a cylinder and a main motor mechanically connected to a reducer which in turn is connected to a plasticizing screw rotating inside the cylinder and having a length generally equal to 8 to 20 times the corresponding screw diameter.

[0027] The single screw extruder 22 is provided with a thermostat, which comprises, on the one hand, a plurality of resistors fixed on the outer surface of the cylinder, and, on the other hand, a water cooling circuit ( Figure 5 ), the water cooling circuit is formed with a plurality of tubular holes 46 formed as a labyrinth extending parallel to the longitudinal axis 48 of the single screw extruder 22 in the wall 50 of the cylinder in the form of a circular crown around the central cavity of the cylinder.

[0028] The screw used to convey the material inside the column may be of the type forced-cooled by internal water circulation.

[0029] The filter 26 provided with the single screw extruder 22 comprises a conventional grid and can be of a semi-automatic or automatic type, both of which allow the molten material to flow continuously on two different channels. It goes without saying that, unlike the semi-automatic type, the automatic type ensures continuous production without the technical supervision of an operator.

[0030] The presence of the filter 26 increases the resistance encountered by the rubber as it advances along the single screw extruder 22, resulting in an increase in pressure and, therefore, in temperature, which may result in degradation of the rubber.

[0031] However, this effect is compensated by the thermostatic device, in particular by the preferred double water cooling circuit, and the mixing action exerted by the screw, which evens out the temperature by gradually bringing the new rubber part into contact with the cooling surfaces of the cylinder and the screw. The combination of these measures thus succeeds in significantly reducing the temperature of the devulcanized rubber.

[0032] Advantageously, the die 28 at the outlet of the single-screw extruder 22 has a variable geometry, allowing the extruded devulcanized rubber strip or sheet to be dimensioned in the desired manner.

[0033] The apparatus further comprises means for allowing the devulcanized rubber to pass from the twin-screw extruder 20 to the single-screw extruder 22. In particular, Figure 6 A connecting element 52 for connecting the twin screw extruder 20 directly to the single screw extruder 22 is shown.

[0034] In an alternative embodiment of the present invention not shown in the drawings, the aforementioned means for allowing the devulcanized rubber to pass from the twin-screw extruder 20 to the single-screw extruder 22 may comprise:

[0035] a hopper which collects the devulcanized rubber extruded from the twin-screw extruder 20 and feeds it into the single-screw extruder 22; or

[0036] One or more pairs of facing pressure rollers which transfer the devulcanized rubber strip coming from the twin-screw extruder 20 into the single-screw extruder 22 .

[0037] The above-described apparatus can be used to treat vulcanized elastomers of substantially any chemical nature and previously used in the manufacture of any type of article, such as, in particular, tires for cars, heavy vehicles and aircraft. Examples of elastomers that can be devulcanized are natural rubber (NR), butadiene rubber (BR), ethylene-propylene rubber (EPR), styrene-butadiene rubber (SBR), nitrile rubber (NBR), ethylene-propylene-diene rubber (EPDM), isoprene rubber (IR), chloroprene rubber (CR), acrylic rubber, silicone rubber as well as polyurethane and chlorosulfonated polyethylene.

[0038] It should be noted that no other substances may be added to the elastomer to be processed, such as thermoplastic materials or process additives, thus reducing the processing costs and the environmental impact associated with its use.

[0039] Specifically, the vulcanized rubber is sent to a grinder 12. The grinder 12 reduces it to particles of the desired size, which are then sent to a homogenizer 14. The rubber particles thus obtained are then introduced through a weight / volume metering unit 16 into a forced feeding device 18, which feeds it into a twin-screw extruder 20. The twin-screw extruder 20 performs devulcanization of the rubber.

[0040] The twin screw extruder 20 is operated at a temperature between 35°C and 450°C, a screw speed between 15 rpm and 600 rpm, and a torque density of 11 Nm / cm 3 and 18Nm / cm 3 Thus, a high filling rate of the screws and a substantially constant shear rate over the entire longitudinal extension of the twin-screw extruder are achieved.

[0041] The cooling of the material can be promoted by injecting water at one or more points 54 of the twin-screw extruder 20. The one or more points 54 are located at a distance from the initial part (i.e., just after the desulfurization has taken place) equal to at least 32 times the outer diameter of the screw. The amount of water injected can be between 1 and 30% (by weight) relative to the amount of rubber treated. The injected water and the water that subsequently evaporates, as well as other gaseous components produced by the desulfurization, can be removed in a manner known per se by degassing means 56, such as an extraction screw or a suction pump located in one or more discharge ducts, which branch laterally from the extruder cylinder. The distance between the water injection point 54 and the subsequent degassing point 56 is preferably 4 to 24 times the outer diameter of the screw, even more preferably 8 to 16 times the outer diameter of the screw.

[0042] The injection of water reduces the temperature of the rubber by about 20-30°C relative to operation under water-free conditions, thereby preventing degradation phenomena caused by temperature and reducing the odor of the devulcanized rubber due to the stripping effect of steam on substances that produce the odor of the devulcanized rubber. The injection of water immediately after devulcanization to cause the rubber to cool suddenly also has the effect of increasing its Mooney viscosity by about 6%.

[0043] The devulcanized rubber leaving the twin-screw extruder 20 is then led through the connecting element 52 into the single-screw extruder 22, where its temperature is reduced by about 15-30°C.

[0044] When passing through the filter 26, the impurity masses which are usually initially present in the rubber to be treated are retained, so that the rubber is purified to a quality level comparable to that of the original product.

[0045] The rubber is finally extruded in the form of a strip or sheet through a die 28 and immersed in a tank 30 for further cooling by water contained therein. Advantageously, the water may contain an anti-buildup additive.

[0046] The strips or sheets coming out of the tank 30 are then fed into a drying tunnel 32 and finally sent to a collection device 33 .

[0047] Of course, without prejudice to the principle of the invention, the details of the implementation and the examples may vary widely with respect to what has been described purely by way of example, without thereby departing from the scope of the invention as defined in the appended claims.

Claims

1. A method for recovering and devulcanizing vulcanized rubber (10) by using an apparatus comprising: - a grinder (12) for grinding the vulcanized rubber into particles; a twin-screw extruder (20) for desulfurization, provided with a device for forced feeding of the vulcanized rubber particles (18) and a thermostatic device; a single-screw extruder (22) arranged downstream of the twin-screw extruder (20) and provided with a thermostatic device, a filter (26) for devulcanized rubber and an extrusion die (28) having a slot shape, from which the devulcanized rubber is extruded in the form of a strip or sheet; and - a cooling device for cooling the strip or sheet of devulcanized rubber, The twin-screw extruder (20) is operated at a temperature between 35°C and 450°C, a screw speed between 15 rpm and 600 rpm, and a speed of 11 Nm / cm 3 and 18Nm / cm 3 The twin screw extruder (20) is operated at a torque density between 100 and 1000 mm / s so that the shear rate remains constant over the entire longitudinal extension of the twin screw extruder (20).

2. The method according to claim 1, wherein The ratio of the outer diameter to the inner diameter of the screw of the twin-screw extruder (20) is between 1.22 and 1.

78.

3. The method according to claim 2, wherein: The ratio of the outer diameter to the inner diameter of the screw of the twin-screw extruder (20) is between 1.55 and 1.

78.

4. The method according to claim 1, wherein: The length of the twin-screw extruder (20) is at least equal to 64 times the outer diameter of the screw and does not exceed 80 times the outer diameter of the screw.

5. The method according to claim 1, wherein: In order to transfer the devulcanized rubber from the twin-screw extruder (20) to the single-screw extruder (22), the apparatus further comprises: - a connecting element (52) for connecting the twin-screw extruder (20) directly to a single-screw extruder (22); or - a hopper that collects the devulcanized rubber extruded from the twin-screw extruder (20) and feeds it into the single-screw extruder (22); or - At least one pair of facing pressure rollers which transfer the devulcanized rubber strip from the twin-screw extruder (20) into the single-screw extruder (22).

6. The method according to claim 1, wherein: The extrusion die (28) has a variable geometry.

7. The method according to claim 1, wherein: The cooling device comprises a tank (30) filled with water, wherein a strip or sheet of devulcanized rubber is immersed in the tank.

8. A method according to any one of the preceding claims, wherein: Water is injected into at least one point of the twin-screw extruder (20) located at a distance from the inlet equal to at least 32 times the outer diameter of the screw.

9. The method according to claim 8, wherein: The twin-screw extruder (20) is provided with a degassing device for removing the gas formed therein.

10. The method according to claim 9, wherein: The distance between the water injection point and the subsequent point where the degassing device is located is between 4 and 24 times the outer diameter of the screw.

11. The method according to claim 10, wherein: The distance between the water injection point and the subsequent point where the degassing device is located is between 8 and 16 times the outer diameter of the screw.

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