Patent Description:
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.

<CIT> discloses a method for preparing a regenerated rubber sheet by continually cooling and mixing desulfurized rubber powder.

<CIT> discloses a method for producing a rubber mixture, in which an input rubber mixture is fed into an outwardly sealed, pressure-tight and gas-tight unit at a pressure of at least <NUM> MPa and a temperature of at least <NUM>° C.

<CIT> discloses a process for producing a filled polymeric composite material by an apparatus comprising first and second extrusion devices.

<CIT> discloses a process for recycling plastic waste material, including shredding washing, drying and agglomerating such waste material.

<CIT> discloses a method for producing a pellet for a molding material using a laminated film of a hardly recyclable material which has an aluminum deposition layer.

<CIT> discloses a process for recycling and de-vulcanizing vulcanized rubber, by using a two-screw co-rotating extruder having a L/D ratio of <NUM> or more.

<NPL>, discloses a process for de-vulcanizing EPDM in a two-screw co-rotating extruder having a L/D ratio of <NUM>.

<NPL> discloses a process for de-vulcanizing rubber waste by using a twin-screw extruder.

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 <NUM> below. Preferred features of the process of the invention are described in the claims dependent on claim <NUM>.

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 <NUM> comprises (<FIG> and <FIG>) a mill <NUM> for the grinding and devulcanization of the rubber <NUM>, downstream of which mill <NUM> a homogenizer <NUM> and a gravimetric/volumetric dosing unit <NUM> is located, which feeds the rubber particles produced in the mill <NUM> to a device <NUM> which forcibly feeds said particles into a twin-screw devulcanization extruder <NUM>.

Downstream of the twin-screw extruder <NUM> there is a single-screw extruder <NUM>, equipped with a gear pump <NUM>, a filter <NUM> for the devulcanized rubber and an extrusion die <NUM> shaped like a slot, from which devulcanized rubber emerges in the form of a strip or sheet.

Downstream of the extrusion die <NUM> there are a tank <NUM> containing cooling water, a water removal and drying tunnel <NUM> and a device <NUM> for collecting the strip or sheet of devulcanized rubber.

The forced feeding device <NUM> is known per se and allows an easy introduction into the twin-screw extruder <NUM> of vulcanized rubber particles having different shape, particle size and apparent density. The device is typically formed (<FIG>) of a hopper <NUM> and a cylinder <NUM> within which one or more screws <NUM> with conical or cylindrical geometry rotate; it is positioned vertically relative to the extruder (or laterally in embodiments not shown) and may have mechanical, electrical or pneumatic drive, so as to force the vulcanized rubber particles into the twin-screw extruder <NUM>. The use of such a device <NUM> increases the filling rate of the free volumes of the screws of the extruder <NUM> and has proven to be particularly effective in the processing of expanded vulcanized rubbers, typically having a density between <NUM> and <NUM>/m<NUM>. In this case, a greater filling of the screws allows a considerable increase in productivity, i.e. by over <NUM>% compared to the case wherein the forced feeding device <NUM> is absent, further reducing drastically the degradation of the rubber due to an excess of shear stress.

The shear rate (or geometric shear rate) is kept constant within the extruder <NUM> 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 <NUM>, determined by the forced feeding device <NUM>, 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 <NUM>, the processed material would be subjected, due to its shape and density, to different shear rates within the extruder <NUM>, 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 <NUM> 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 <NUM> and <NUM> to be processed.

The twin-screw extruder <NUM> is advantageously comprised of cylindrical modules normally having a length equal to <NUM> times the outer diameter of the screws and mounted in series so as to form a continuous cylinder. Typically, the twin-screw extruder <NUM> has a length at least equal to <NUM> times the outer diameter of the screws, and preferably not greater than <NUM> times the outer diameter of the screws.

The extruder <NUM> 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 (<FIG>) a water cooling circuit formed of a plurality of tubular holes <NUM> obtained as a labyrinth that extend parallel to the longitudinal axis <NUM> of the extruder in the wall <NUM> of the relative cylinder about the central cavity thereof. By adjusting the alternation of heating and cooling phases in a desired manner, the energy administered to the elastomeric material is kept under control, preventing its degradation after the breakdown of the cross-linked chemical bonds between the molecular chains.

The ratio between the outer and inner diameter of the screws is between <NUM> and <NUM> and preferably between <NUM> and <NUM>. 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 <CIT>, 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.

The torque density is between <NUM> and <NUM>/cm<NUM> and ensures, together with a rotation speed of the screws between <NUM> and <NUM> rpm, a high filling rate of the screws with a shear rate remaining low and constant for the entire length of the extruder <NUM>. The final result is greater productivity, with top values up to <NUM>-<NUM>% 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 <NUM> 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 <NUM> to <NUM> times the diameter of the respective screw.

The extruder <NUM> 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 (<FIG>) with a plurality of tubular holes <NUM> obtained as a labyrinth that extends parallel to the longitudinal axis <NUM> of the extruder <NUM> in the wall <NUM> of the cylinder in the form of a circular crown about the central cavity thereof.

The screw for conveying the material inside the cylinder may be of the type with forced cooling by internal water circulation.

The filter <NUM> with which the extruder <NUM> 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 semiautomatic type, the automatic type may ensure a continuous production without the operators' technical supervision.

The presence of the filter <NUM> increases the resistance that the rubber encounters to proceed along the extruder <NUM> 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 <NUM> at the outlet from the extruder <NUM> 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 <NUM> to the single-screw extruder <NUM>. In particular, <FIG> illustrates a connecting element <NUM> for the direct connection of the twin-screw extruder <NUM> to the single-screw extruder <NUM>.

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 <NUM> to the single-screw extruder <NUM> 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 <NUM>, which reduces it into particles of the desired size, and then to the homogenizer <NUM>. The rubber particles thus obtained are then introduced through the dispenser <NUM> into the forced feeding device <NUM> which feeds them into the twin-screw extruder <NUM> which carries out the devulcanization of the rubber.

The extruder <NUM> operates at a temperature between <NUM> and <NUM>, with a rotation speed of the screws between <NUM> and <NUM> rpm, and a torque density between <NUM> and <NUM>/cm<NUM>, 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 <NUM> of the twin-screw extruder <NUM> located at a distance equal to at least <NUM> 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 <NUM> and <NUM>% 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 <NUM> such as extraction screws or suction pumps located in one or more discharge ducts which branch transversely from the extruder cylinder <NUM>. The distance between a water injection point <NUM> and the subsequent degassing point <NUM> is preferably between <NUM> and <NUM> times the outer diameter of the screws, and even more preferably between <NUM> and <NUM> times the outer diameter of the screws.

The injection of water allows the temperature of the rubber to be lowered by about <NUM>-<NUM> 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 <NUM>%.

The devulcanized rubber exiting the twin-screw extruder <NUM> is then guided through the connecting element <NUM> into the single-screw extruder <NUM> where its temperature is reduced by about <NUM>-<NUM>.

Passing through the filter <NUM>, 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 <NUM> in the form of a strip or sheet, which is passed by immersion into the tank <NUM> 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 <NUM> is then made to pass into the drying tunnel <NUM> and finally sent to the collection device <NUM>.

Claim 1:
Process for the recovery and devulcanization of vulcanized rubber (<NUM>) through the use of a plant comprising:
- a mill (<NUM>) for grinding said vulcanized rubber into particles,
- a twin-screw extruder (<NUM>) for devulcanization, which is provided with a device (<NUM>) for the forced feeding of said vulcanized rubber particles and a thermostatting device,
- a single-screw extruder (<NUM>) arranged downstream of said twin-screw extruder (<NUM>), and provided with a thermostatting device, a filter (<NUM>) for the devulcanized rubber and an extrusion die (<NUM>) shaped like a slot, from which devulcanized rubber comes out in the form of a strip or sheet, and
- a cooling device for the strip or sheet of de-vulcanized rubber,
wherein said twin-screw extruder (<NUM>) operates at a temperature between <NUM> and <NUM>° C, a screw rotation speed between <NUM> and <NUM> rpm, and a torque density between <NUM> and <NUM>/cm<NUM>, so that the shear rate remains substantially constant for the entire longitudinal extension of the twin-screw extruder (<NUM>).