Patent Application
20220305697
This invention refers to a process for the recovery and devulcanization of cross-linked rubber.
Typically, the rubber to be recovered is made up of industrial processing waste and/or articles that have reached the end of their useful life, coming from specialized collection centers and/or landfills.
The molecular chains of vulcanized rubber are typically joined by chemical bonds developing through bridges of sulfur atoms that join them transversely. Vulcanized rubber thus has a structure similar to that of a thermosetting polymer and may not be reused as such in the production cycle.
Plants are known that administer to the cross-linked rubber a quantity of energy such as to break the chemical bonds created by the sulfur, so that the elastomeric material returns to its original physical-chemical state prior to the vulcanization treatment and is ready to be reused in the production cycle.
However, despite increasingly selective and high-performance waste sorting processes becoming more widespread, the rubber collected to undergo devulcanization treatments usually contains particles of other substances that pollute the devulcanized rubber finally obtained, compromising the properties thereof.
An object of this invention is therefore to provide a process to overcome this drawback due to the impurities inevitably present in the vulcanized rubber to be processed.
This object is achieved through a process for the recovery and devulcanization of cross-linked rubber having the features indicated in claim 1 below. Preferred features of the process of the invention are described in the claims dependent on claim 1.
This invention satisfies the needs currently felt on the market regarding the purity of devulcanized rubbers, making it possible to carry out a non-degrading recovery process which requires a reduced energy input and produces a final plastics material, devulcanized and substantially free from impurities and pollutants.
Further advantages and features of this invention will be evident from the detailed description below, provided by way of non-limiting example with reference to the accompanying drawings, wherein:
A plant for the recovery and devulcanization of vulcanized rubber 10 comprises (
Downstream of the twin-screw extruder 20 there 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 devulcanized rubber emerges in the form of a strip or sheet.
Downstream of the extrusion die 28 there are a tank 30 containing cooling water, a water removal and drying tunnel 32 and a device 33 for collecting the strip or sheet of devulcanized rubber.
The forced feeding device 18 is known per se and allows an easy introduction into the twin-screw extruder 20 of vulcanized rubber particles having different shape, particle size and apparent density. The device is typically formed (
The shear rate (or geometric shear rate) is kept constant within the extruder 20 for its entire length, allowing the cross bonds between the molecular chains of the rubber to break and the possible degradation of the devulcanized rubber to be avoided. The high filling rate of the extruder 20, determined by the forced feeding device 18, ensures that in each zone there is a constant presence of material so as to make the advancement, as well as the stresses imparted, uniform. Conversely, in the absence of the forced feeding device 18, the processed material would be subjected, due to its shape and density, to different shear rates within the extruder 20, which could degrade it. Likewise, in the transition from vulcanized rubber to devulcanized rubber, the high filling rate facilitates cooling which is also capable of counteracting degradation.
Moreover, the forced feeding device 18 increases the operating flexibility of the plant by allowing particles with different particle size in the form of powders or granules with a size between 0.2 and 15 mm to be processed.
The twin-screw extruder 20 is advantageously comprised of cylindrical modules normally having a length equal to 4 times the outer diameter of the screws and mounted in series so as to form a continuous cylinder. Typically, the twin-screw extruder 20 has a length at least equal to 64 times the outer diameter of the screws, and preferably not greater than 80 times the outer diameter of the screws.
The extruder 20 is provided with a thermostatting device which on one hand comprises a plurality of electrical resistors attached to the outer surface of the cylinder, and on the other (
The ratio between the outer and inner diameter of the screws is between 1.22 and 1.78 and preferably between 1.55 and 1.78. In a manner known per se and not illustrated in the figures, sequences of conveying and mixing elements are individually configured on the multi-row shafts of the screws, in particular having a geometric profile so as to make the filling uniform, as described in EP-1 136 228 B1, which allow the shear stresses imparted to the rubber, and therefore the specific energy absorbed by said rubber, to be kept uniform and under control.
Typically the torque density is between 11 and 18 Nm/cm3 and ensures, together with a rotation speed of the screws between 15 and 600 rpm, a high filling rate of the screws with a shear rate remaining low and constant for the entire length of the extruder 20. The final result is greater productivity, with top values up to 40-60% and a lower operating temperature with respect to the case wherein an extruder having similar geometric parameters with the same rotation speed of the screws but with a lower torque density is used.
The single-screw extruder 22 comprises in a manner known per se a cylinder and a main motor mechanically connected to a speed reducer in turn connected to a plasticizing screw which rotates inside the cylinder, the length of which is typically equal to 8 to 20 times the diameter of the respective screw.
The extruder 22 is provided with a thermostatting device which on one hand comprises a plurality of electrical resistors fixed on the outer surface of the cylinder, and on the other a water cooling circuit formed (
The screw for conveying the material inside the cylinder may be of the type with forced cooling by internal water circulation.
The filter 26 with which the extruder 22 is provided includes a conventional grid and may be of the semiautomatic or automatic type, both types allowing a continuous flow of the molten material on two distinct channels. It goes without saying that, unlike the semi-automatic type, the automatic type may ensure a continuous production without the operators' technical supervision.
The presence of the filter 26 increases the resistance that the rubber encounters to proceed along the extruder 22 causing an increase in pressure and consequently in temperature, which could cause a degradation thereof.
This effect is however compensated by the thermostatting device, in particular by the preferably double water cooling circuit, together with the mixing action exerted by the screw, which makes the temperature uniform by gradually bringing new portions of rubber into contact with the cooled surfaces of the screw and cylinder. The combination of these measures thus succeeds in significantly lowering the temperature of the devulcanized rubber.
Advantageously, the die 28 at the outlet from the extruder 22 has a variable geometry, so as to allow the dimensions of the extruded devulcanized rubber strip or sheet to be determined in a desired manner.
The plant further comprises means for allowing the devulcanized rubber to pass from the twin-screw extruder 20 to the single-screw extruder 22. In particular,
In alternative embodiments of the invention not illustrated in the figures, the aforesaid means for allowing the devulcanized rubber to pass from the twin-screw extruder 20 to the single-screw extruder 22 may include:
The plant described above may be used to process vulcanized elastomers having substantially any chemical nature and previously used to make up any type of articles, such as in particular tires for cars, heavy vehicles and airplanes. Examples of elastomers that may be devulcanized are 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.
It should be noted that other substances must not be added to the elastomers to be processed, such as thermoplastic materials or process additives, which consequently reduces process costs as well as the environmental impact associated with the use thereof.
Specifically, the vulcanized rubber is sent to the mill 12, which reduces it into particles of the desired size, and then to the homogenizer 14. The rubber particles thus obtained are then introduced through the dispenser 16 into the forced feeding device 18 which feeds them into the twin-screw extruder 20 which carries out the devulcanization of the rubber.
The extruder 20 operates at a temperature between 35 and 450° C., with a rotation speed of the screws between 15 and 600 rpm, and a torque density between 11 and 18 Nm/cm3, resulting in a high filling rate of the screws and a substantially constant shear rate for the entire longitudinal development of the twin-screw extruder.
The cooling of the material may be facilitated by the injection of water at one or more points 54 of the twin-screw extruder 20 located at a distance equal to at least 32 times the outer diameter of the screws from the initial section, i.e. only after the devulcanization has taken place. The quantity of injected water may be between 1 and 30% by weight with respect to the quantity of rubber processed. The injected and subsequently vaporized water, as well as other gaseous components produced by devulcanization, may be removed in a manner known per se by degassing means 56 such as extraction screws or suction pumps located in one or more discharge ducts which branch transversely from the extruder cylinder 20. The distance between a water injection point 54 and the subsequent degassing point 56 is preferably between 4 and 24 times the outer diameter of the screws, and even more preferably between 8 and 16 times the outer diameter of the screws.
The injection of water allows the temperature of the rubber to be lowered by about 20-30° C. relative to operating in anhydrous conditions, so as to prevent degradation phenomena induced by temperature and to reduce the odor of the devulcanized rubber produced due to the stripping action exerted by the vapor on the substances that generate this odor. The injection of water which suddenly cools the rubber immediately after devulcanization also has the effect of increasing its Mooney viscosity by about 6%.
The devulcanized rubber exiting the twin-screw extruder 20 is then guided through the connecting element 52 into the single-screw extruder 22 where its temperature is reduced by about 15-30° C.
Passing through the filter 26, the lumps of foreign substances normally originally present in the rubber to be processed are stopped, so that said rubber is purified, reaching a qualitative level comparable to that of the virgin product.
The rubber is finally extruded through the die 28 in the form of a strip or sheet, which is passed by immersion into the tank 30 to be further cooled by the water contained therein. Advantageously, the water may contain anti-packing additives.
The strip or sheet coming out of the tank 30 is then made to pass into the drying tunnel 32 and finally sent to the collection device 33.
Naturally, without prejudice to the principle of the invention, the details of implementation and the embodiments may vary widely relative to that which has been described purely by way of example, without thereby departing from the scope of the invention as defined in the accompanying claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102019000016061 | Sep 2019 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2020/058373 | 9/9/2020 | WO |
