Process for recovering and devulcanizing crosslinked rubber

Abstract

The process for recovering and devulcanizing vulcanized rubber (10) is carried out in a plant comprising: a mill (12) for grinding the vulcanized rubber into particles; a devulcanizing twin-screw extruder (20) equipped with a device (18) for forcibly feeding the vulcanized rubber particles and a thermostatic device; a single-screw extruder (22) arranged downstream of the twin-screw extruder (20) equipped with a thermostatic device, a filter (26) for the devulcanized rubber and a slot-shaped extrusion die (28) from which the devulcanized rubber emerges in the form of strips or sheets; and a cooling device for the devulcanized rubber strips or sheets. The twin-screw extruder (20) is configured to maintain a substantially constant shear rate throughout its entire length in the longitudinal direction, with a temperature of 35 to 450°C, a screw rotation speed of 15 to 600 rpm, and a shear rate of 11 to 18 Nm / cm. 3 The twin screw extruder (20) is operated at a torque density of

Description

[Technical field]

[0001] The present invention relates to a process for recovering and devulcanizing crosslinked rubber. [Background technology]

[0002] Recovered rubber typically consists of industrial process waste and / or end-of-life items and originates from specialised recovery centres and / or landfills.

[0003] The molecular chains of vulcanized rubber (or vulcanized rubber) are typically linked by chemical bonds that are extended (or extended or developed or grown) by crosslinking sulfur atoms that laterally connect the molecular chains. Thus, vulcanized rubber (or vulcanized rubber) has a structure similar to that of thermosetting polymers, but cannot be recycled (or reused or reused) in its manufacturing cycle like thermosetting polymers.

[0004] Plants are known which apply an amount of energy to the crosslinked rubber that breaks the chemical bonds formed by the sulfur, thereby returning the elastomeric material to its original physicochemical state before vulcanization (or vulcanization), and preparing it for reuse (or re-use or re-use) in the manufacturing cycle. Summary of the Invention [Problem to be solved by the invention]

[0005] However, the rubber recovered to be subjected to devulcanization usually comprises particles of other substances, despite the fact that selective and sophisticated waste sorting processes are becoming more and more widespread, which can contaminate the final devulcanized rubber and compromise (or impair) its properties.

[0006] The object of the present invention is therefore to provide a process (or method) for eliminating the drawbacks caused by such impurities, which are inevitably present in the vulcanized rubber (or vulcanized rubber) being processed. [Means for solving the problem]

[0007] This object is achieved by a process (or method) for the recovery and devulcanization of crosslinked rubber having the features set forth in the following claim 1. Preferred features of the process of the invention are as set forth in the claims dependent thereon.

[0008] The present invention meets the current market need for purity of devulcanized rubber. It allows the implementation of a non-degrading recovery process, which requires less energy input and gives a final plastic material that is devulcanized and substantially free of impurities and contaminants.

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

[0010] [Figure 1] FIG. 1 is a schematic front view of a plant for carrying out the process of the invention (divided into two parts for a clearer view). [Diagram 2] FIG. 2 is a schematic plan view of the plant of FIG. [Diagram 3] FIG. 3 is an enlarged view of a detail of FIG. [Figure 4] FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. [Diagram 5] FIG. 5 is a cross-sectional view taken along line VV in FIG. [Figure 6] FIG. 6 is an enlarged view of further detail of FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The plant for recovering and devulcanizing vulcanized rubber 10 comprises a mill 12 for grinding and devulcanizing the rubber 10 (FIGS. 1, 2). Downstream of the mill 12 in the plant are arranged a homogenizer 14 and a gravimetric / volumetric metering unit 16. The metering unit 16 feeds the rubber particles produced (or formed) in the mill 12 to a device 18, which forcibly feeds said particles into a twin-screw devulcanization extruder 20.

[0012] Downstream of the twin-screw extruder 20 is a single-screw extruder 22. The single-screw extruder 22 includes a gear pump 24, a filter 26 for the devulcanized rubber, and an extrusion die 28 (having a slot-like shape). From the extrusion die 28, the devulcanized rubber comes out in the shape of a strip (or a narrow piece or a band) or in the shape of a sheet (or a plate).

[0013] Downstream of the extrusion die 28 there is a tank 30 (comprising cooling water), a tunnel 32 (tunnel for dewatering and drying) and a device 33 (device for recovering strips or sheets of devulcanized rubber).

[0014] The forced-feed device 18 is known per se and allows the easy introduction of vulcanized rubber particles (particles of various shapes, grain sizes and apparent densities) into the twin-screw extruder 20. This device (18) is typically formed by a hopper 34 and a cylinder 36 (FIG. 3). Inside the cylinder 36, one or more screws 38 rotate. This device (18) has a conical or cylindrical shape (or geometry). This device (18) is arranged vertically (or transversely in some embodiments (not shown)) with respect to the extruder. This device (18) may comprise a mechanical, electrical or pneumatic device. In this way, the vulcanized rubber particles can be forced into the twin-screw extruder 20. The use of such a device 18 increases the filling rate of the free volume of the screw of the extruder 20, and increases the filling rate of the expanded vulcanized rubber (typically 50 to 350 kg / m 3 It has been proven to be particularly effective for the processing of rubbers with a density of 1000 mm or less. In this case, the increased screw loading can significantly increase productivity, i.e., by more than 100% compared to the case without the forced feeding device 18. Furthermore, the degradation of the rubber due to excessive shear stress can be dramatically reduced.

[0015] The shear rate (or geometric shear rate) is kept constant in the extruder 20 throughout its length, thereby breaking the cross-bonds between the rubber molecular chains and avoiding possible degradation in the devulcanized rubber. The high fill rate of the extruder 20 (which is determined by the force-feed device 18) ensures a constant presence of material in each zone, so that it moves forward and the applied stress is uniform. In contrast, without the force-feed device 18, the processed material may experience different shear rates in the extruder 20 due to its shape and density, which may degrade the material. Similarly, in the transition from vulcanized rubber to devulcanized rubber, the high fill rate promotes cooling, which can also counteract degradation.

[0016] Furthermore, the forced feeding device 18 increases the operational flexibility of the plant by allowing the processing of particles with various particle sizes (having a size between 0.2 and 15 mm) in powder or granular form.

[0017] The twin screw extruder 20 is advantageously composed of a plurality of cylindrical modules, which usually have a length equal to four times the outer diameter of the screws and are mounted (or arranged) in series, so as to form a continuous cylinder. Typically, the twin screw extruder 20 has a length equal to at least 64 times the outer diameter of the screws. Preferably, the length is no more than 80 times the outer diameter of the screws.

[0018] The extruder 20 is equipped with a thermostatic device, which on the one hand comprises a number of electrical resistors attached to the outer surface of the cylinder, and on the other hand comprises a water cooling circuit (FIG. 4). The water cooling circuit is formed of a number of tubular holes 40, which are obtained as labyrinths extending parallel to the longitudinal axis 42 of the extruder (inside the wall 44 of the cylinder relative to the periphery of the central cavity of the extruder). By regulating the alternating heating and cooling phases in the desired manner, the energy imparted to the elastomeric material is controlled and maintained, thereby preventing its degradation after the destruction of the cross-linked chemical bonds between the molecular chains.

[0019] The ratio of the external diameter to the internal diameter of the screw is between 1.22 and 1.78, preferably between 1.55 and 1.78. In a manner known per se (not shown), the conveying and mixing trains of the components are respectively formed on the multi-row shafts of the screw, in particular with a geometric profile which ensures uniformity of filling (as described in EP-1 136 228 B1), so that the shear stress imparted to the rubber (i.e. the specific energy absorbed by said rubber) can be kept uniform and controlled.

[0020] Typically, torque densities are between 11 and 18 Nm / cm 3This ensures a high screw filling rate with a screw speed of 15-600 rpm. The shear rate is then kept low and constant over the entire length of the extruder 20. The end result is a higher productivity, with top values ​​of up to 40-60% and lower operating temperatures (provided that the same screw speed and an extruder with the same geometric parameters (but with a lower torque density) are used).

[0021] The single screw extruder 22 comprises, in a manner known per se, a cylinder and a main motor, which is mechanically connected to a speed reducer. This motor is then connected to a plasticizing screw, which rotates in the cylinder. The length of the single screw extruder 22 is typically equal to 8 to 20 times the diameter of each screw.

[0022] The extruder 22 is equipped with a thermostatic device, which on the one hand comprises a number of electrical resistors fixed to the outer surface of the cylinder, and on the other hand comprises a water cooling circuit (FIG. 5). The water cooling circuit is formed of a number of tubular holes 46 obtained as labyrinths extending parallel to the longitudinal axis 48 of the extruder 22 (inside the wall 50 of the cylinder in the form of a circular crown around the central cavity of the extruder).

[0023] The screw that conveys the material inside the cylinder may be of the type with forced cooling by internal water circulation.

[0024] The extruder 22 is equipped with a filter 26, which includes a conventional grid. It may be of the semi-automatic or automatic type. Both types allow a continuous flow of molten material in two different channels. It goes without saying that the automatic type, unlike the semi-automatic type, can ensure continuous production without the technical supervision of an operator.

[0025] The presence of filter 26 increases the resistance encountered by the rubber moving along extruder 22, causing an increase in pressure and, as a result, an increase in temperature, which can cause the rubber to degrade (or deteriorate).

[0026] However, such effects are compensated for by a thermostatic device, together with the stirring action exerted by the screw, and in particular, preferably by a double water cooling circuit, which equalizes the temperature by gradually bringing the cooled (or cooled) surfaces of the screw and cylinder into contact with the new rubber. Thus, the combination of such measures succeeds in significantly lowering the temperature of the devulcanized rubber.

[0027] Advantageously, the die 28 at the exit of the extruder 22 has a variety of shapes (or geometries) so that the dimensions (or dimensions or dimensions) of the extruded devulcanized rubber strip or sheet can be determined in a desired manner (or method or manner).

[0028] The plant further comprises means by which the devulcanized rubber can be passed from the twin-screw extruder 20 to the single-screw extruder 22. In particular, in Fig. 6 a connecting element 52 is shown, which is intended to directly connect the twin-screw extruder 20 to the single-screw extruder 22.

[0029] In an alternative embodiment of the present invention (not shown), the means by which the devulcanized rubber can be passed from the twin screw extruder 20 to the single screw extruder 22 may include the following: A hopper for recovering the devulcanized rubber extruded from the twin-screw extruder 20 and supplying the devulcanized rubber to a single-screw extruder (or single-screw extruder) 22, or Pressure rollers (one or more pairs facing each other) that transfer the devulcanized rubber strip coming out of the twin-screw extruder 20 to a single-screw extruder (or single-screw extruder) 22.

[0030] The above-mentioned plant can be used for the treatment of vulcanized elastomers (or vulcanized elastomers), which can be of virtually any chemical nature and are conventionally used to construct any kind of article, such as tires for automobiles, heavy vehicles and aircraft. Examples of elastomers that may be devulcanized include natural rubber (NR), butadiene rubber (BR), ethylene propylene rubber (EPR), styrene butadiene rubber (SBR), nitrile rubber (NBR), ethylene propylene diene monomer rubber (EPDM), isoprene rubber (IR), chloroprene rubber (CR), acrylic rubber, silicone rubber, as well as polyurethanes and chlorosulfonated polyethylenes.

[0031] It should be noted that no other substances should be added to the elastomers undergoing treatment (such as thermoplastic materials or process additives), thus reducing the production costs as well as the environmental impact associated with their use.

[0032] Specifically, vulcanized rubber is fed to a mill 12 where it is reduced to particles of the desired size and then fed to a homogenizer 14. The rubber particles thus obtained are introduced through a dispenser 16 into a forced feed device 18 which feeds the rubber particles to a twin screw extruder 20 where the rubber is devulcanized.

[0033] The extruder 20 has a temperature of 35 to 450°C, a screw rotation speed of 15 to 600 rpm, and a feed rate of 11 to 18 Nm / cm 3 The result is a high screw filling rate and a substantially constant shear rate over the entire longitudinal length of the twin screw extruder.

[0034] The cooling of the material may be promoted by injecting water at one or more points 54 of the twin screw extruder 20. The points 54 are located at a distance equal to at least 32 times the external diameter of the screw from the initial section, i.e. only after devulcanization has taken place. The amount of water injected may be between 1 and 30% by weight, relative to the amount of rubber to be treated. The injected and subsequently vaporized water, as well as other gaseous components produced by devulcanization, can be removed in a manner known per se, by means of degassing means 56, for example an extraction screw or a suction pump, located in one or more discharge ducts branching off laterally from the extruder cylinder 20. The distance between the water injection point 54 and the degassing point 56 behind it is preferably between 4 and 24 times the external diameter of the screw, and even more preferably between 8 and 16 times the external diameter of the screw.

[0035] By injecting water, the temperature of the rubber can be lowered by about 20 to 30 degrees Celsius compared to operation without water. This prevents the phenomenon of decomposition (or deterioration) induced by temperature, and reduces the odor of the devulcanized rubber by a stripping effect (the effect caused by exposing the vapor to a substance that generates this odor). In addition, the injection of water to rapidly cool the rubber immediately after devulcanization has the effect of increasing the Mooney viscosity of the rubber by about 6%.

[0036] The devulcanized rubber leaving the twin-screw extruder 20 is then guided via a connecting element 52 to the single-screw extruder 22, where the temperature of the devulcanized rubber is reduced by about 15-30°C.

[0037] Passing through the filter 26 prevents (or prevents) the formation of agglomerates (or lumps) that are normally naturally present in the rubber undergoing treatment, thereby refining the rubber and bringing it to a quality level equivalent to that of new (or virgin) products.

[0038] The rubber is finally extruded through a die 28 in the form of a strip or sheet, which is then immersed and passed through a tank 30 where it is further cooled by water contained in the tank 30. Advantageously, the water may contain an anti-packing additive.

[0039] The strip or sheet emerging from the tank 30 then passes through a drying tunnel 32 and finally to a collection device 33.

[0040] Naturally, without prejudice to the principles of the invention, the details of implementation and embodiments may vary widely from those described purely by way of example, without departing from the scope of the invention as defined in the appended claims. The disclosure of this specification may include the following aspects. (Aspect 1) A process for recovering and devulcanizing vulcanized rubber (10) by using a plant comprising: a mill (12) for grinding the vulcanized rubber into particles; a twin-screw extruder (20) for devulcanization, comprising a device (18) for forcibly feeding the vulcanized rubber particles and a thermostatic device; a single-screw extruder (22) arranged downstream of the twin-screw extruder (20), the single-screw extruder (22) comprising a thermostatic device, a filter (26) for the devulcanized rubber, and an extrusion die (28) shaped like a slot, from which the devulcanized rubber emerges in the form of a strip or sheet; and Cooling device for the devulcanized rubber strips or sheets In the process, the twin-screw extruder (20) is controlled to maintain a substantially constant shear rate over the entire length in the longitudinal direction thereof at a temperature of 35 to 450°C, a screw rotation speed of 15 to 600 rpm, and a shear rate of 11 to 18 Nm / cm. 3 and operating the twin screw extruder (20) at a torque density of (Aspect 2) The process according to embodiment 1, wherein the ratio of the outer diameter to the inner diameter of the screws of the twin-screw extruder (20) is from 1.22 to 1.78, preferably from 1.55 to 1.78. (Aspect 3) 3. The process according to any one of the preceding claims, wherein the twin-screw extruder (20) has a length equal to at least 64 times the outer diameter of the screws, preferably no greater than 80 times the outer diameter of the screws. (Aspect 4) In order to transfer the devulcanized rubber from the twin screw extruder (20) to the single screw extruder (22), the plant further comprises: a connecting element (52) for directly connecting the twin screw extruder (20) to the single screw extruder (22); or a hopper for recovering the devulcanized rubber extruded from the twin-screw extruder (20) and supplying the devulcanized rubber to the single-screw extruder (22); or At least one pair of opposing pressure rollers that transfer the devulcanized rubber strip coming out of the twin screw extruder (20) to the single screw extruder (22). The process according to any one of embodiments 1 to 3, comprising: (Aspect 5) 5. The process according to any one of the preceding claims, wherein the extrusion die (28) has a variety of shapes. (Aspect 6) 6. The process according to any one of the preceding aspects, wherein the cooling device consists of a tank (30) filled with water, and the devulcanized rubber strips or sheets are immersed in the tank (30). (Aspect 7) The process according to any one of the preceding aspects, wherein water is injected into at least one point in the twin-screw extruder (20), said point being located at a distance from the inlet equal to at least 32 times the outer diameter of the screw. (Aspect 8) 8. The process of embodiment 7, wherein the twin-screw extruder (20) is equipped with a degassing means for removing gases and / or vapors formed therein. (Aspect 9) 9. The process according to embodiment 8, wherein the distance between the water injection point and the subsequent point at which the degassing means is located is between 4 and 24 times the outer diameter of the screw. (Aspect 10) 10. The process according to embodiment 9, wherein the distance between the water injection point and the subsequent point at which the degassing means is located is between 8 and 16 times the outer diameter of the screw.

Claims

1. A process for recovering and devulcanizing vulcanized rubber (10) by using a plant comprising: a mill (12) for grinding the vulcanized rubber into particles; a twin-screw extruder (20) for devulcanization, comprising a device (18) for forcibly feeding the particles of said vulcanized rubber and a thermostatic device; a single screw extruder (22) arranged downstream of the twin screw extruder (20), comprising a thermostatic device, a filter (26) for the devulcanized rubber obtained after the vulcanized rubber is devulcanized in the twin screw extruder (20), and an extrusion die (28) shaped like a slot, from which the devulcanized rubber emerges in the form of a strip or sheet; and A cooling device for said devulcanized rubber strips or sheets. In this process, the twin screw extruder (20) is operated at a temperature of 35 to 450°C, a screw rotation speed of 15 to 600 rpm, and a shear rate of 11 to 18 Nm / cm to maintain a constant shear rate over the entire length of the twin screw extruder (20). ３ and operating said twin screw extruder (20) at a torque density of

2. 2. The process of claim 1, wherein the ratio of the outer diameter to the inner diameter of the screws of the twin screw extruder (20) is from 1.22 to 1.

78.

3. The process described in claim 2, wherein the ratio of the outer diameter to the inner diameter of the screws of the twin-screw extruder (20) is 1.55 to 1.

78.

4. The process of any one of claims 1 to 3, wherein the twin screw extruder (20) has a length equal to at least 64 times the outer diameter of the screws.

5. The process of claim 4, wherein the twin-screw extruder (20) has a length less than 80 times the outer diameter of the screw.

6. In order to transfer the devulcanized rubber from the twin screw extruder (20) to the single screw extruder (22), the plant further comprises: a connecting element (52) for directly connecting the twin screw extruder (20) to the single screw extruder (22), or a hopper for recovering the devulcanized rubber extruded from the twin-screw extruder (20) and supplying the devulcanized rubber to the single-screw extruder (22); or At least one pair of opposing pressure rollers for transferring the strip of devulcanized rubber emerging from the twin screw extruder (20) to the single screw extruder (22). The process according to any one of claims 1 to 5, comprising:

7. The process according to any one of claims 1 to 6, wherein the extrusion die (28) has various shapes so that the dimensions of the extruded strips or sheets of devulcanized rubber can be determined in a desired manner.

8. The process according to any one of the preceding claims, wherein the cooling device consists of a tank (30) filled with water, in which the strips or sheets of devulcanised rubber are immersed.

9. 9. The process of any one of claims 1 to 8, wherein water is injected into at least one point in the twin screw extruder (20), said point being located at a distance from the inlet equal to at least 32 times the outer diameter of the screw.

10. 10. The process of claim 9, wherein the twin screw extruder (20) is equipped with degassing means for removing gases and / or vapors formed therein.

11. 11. The process of claim 10, wherein the distance between the water injection point and the subsequent point at which the degassing means is located is between 4 and 24 times the outer diameter of the screw.

12. 12. The process of claim 11, wherein the distance between the water injection point and the subsequent point at which the degassing means is located is between 8 and 16 times the outer diameter of the screw.

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