DE102022105040A1 - PROCESSES FOR THE RECOVERY OF HYDROGENATED, VULCANIZED NITRILE RUBBER WASTE MATERIALS AND THEIR USE IN TECHNICAL COMPOUNDS - Google Patents

Abstract

A process is described for the devulcanization of hydrogenated nitrile rubbers (H-NBR) by their thermomechanical treatment at a temperature of 240°C to 360°C, preferably using a co-rotating twin-screw extruder, whereby a H-NBR rubber which is partially devulcanized is obtained and can be reused in technical mixes.

Description

The present invention relates to a process for the recovery and subsequent use of processing scraps or surplus or off-spec end products consisting of vulcanizates based on hydrogenated nitrile rubbers (hereinafter referred to as H-NBR) and their subsequent recycling in technical compounds; the recovery after previous grinding is carried out via a devulcanization process in a co-rotating twin-screw extruder, choosing the appropriate temperature, power and residence time conditions and choosing the combination of the parameters mentioned with the aim of obtaining a material whose chemical and chemical-physical characteristics are such as to confer the best fluidity characteristics on the technical mixture it contains and the best technological characteristics on the final product.

For the definition of "hydrogenated nitrile rubbers" and their different typologies and structures, see also, for example, Robert Keller in Practical Guide to Hydrogenated Nitrile Butadiene Rubber Technology, 2012, Smithers Rapra Technology, ISBN:978-1-84735-522-5 (E- books).

H-NBR rubbers, together with acrylic rubbers (ACM) and fluorinated elastomers (FKM, FFKM), belong to the class of high-performance elastomers and are characterized by excellent performance at temperatures unattainable by traditional elastomers, as well as a high cost that at least an order of magnitude higher than that of conventional rubbers using alkenic, dienic and vinyl aromatic monomers. It is therefore of interest from an economic point of view to find a technology capable of recovering and consequently using processing waste, which represents on average 25-30% of the transformation phase with peak values up to 50%.

Depending on the amount of acrylonitrile (ACN) of the NBR gum used in the hydrogenation process, there are several different grades of HNBR gums where the ACN content can vary between 17 and 50% (by moles), in addition to unsaturation levels, which are between 4 and 9% (grades with degrees of hydrogenation <1% are not missing) and with different Mooney viscosities.

HNBR rubbers are valued for their resistance to heat and lubricating oils, with technological properties that are generally inferior to those of fluorinated elastomers, but are usually cheaper because of their lower density.

The term "hydrogenated vulcanized nitrile rubber" means HNBR having crosslinks between the polymer chains, the above crosslinks being obtained by reacting the unvulcanized rubber with peroxides or with sulfur-based systems and accelerators.

For the most part, commercially available HNBR contain residual unsaturations capable of interacting with the curing systems of choice and forming the cross-links mentioned above. In all cases, the HNBR rubbers require significantly higher amounts of peroxides and co-agents (6-10 phr peroxide and 2-6 phr co-agent) or sulfur and accelerators due to the lower amount compared to an NBR rubber of duplicate residual connections (4-8% on average).

The prior art does not provide any relevant documentation or teachings that would make it possible to devulcanize vulcanized H-NBR rubber quickly and with a high transformation rate and thereby obtain devulcanized polymer materials that can be recycled in whole or in part for the production of compounds, suitable for the creation of finished products with technological characteristics that meet the specifications required for the various fields of application. For other types of vulcanized elastomers, usually the most common, such as styrene-butadiene copolymers (SBR), ethylene-propylene-diene copolymers (EPDM), ethylene-propylene copolymers (EPR), acrylonitrile-butadiene Copolymers (NBR) and the vulcanized silicone rubbers, several devulcanization processes have been tried; we want to mention here, among others, the thermomechanical, chemical and hydrolytic treatments, bacterial digestion, omitting technologies that envisage the degradation of vulcanizates to non-polymer products (such as pyrolysis and incineration). For example, the use of ultrasonic generators at the head of an extruder, through which a vulcanized elastomer is pressed will, by Isayev in U.S. 5,284,625 and U.S. 5,258,413 described as a system capable of devulcanizing the material so treated, when added in a minor amount not exceeding 20-30% by weight with respect to its raw equivalent, are used for the manufacture of high quality articles can.

In all the cases cited in the technical, scientific and patent literature, the working methods and conditions chosen for the devulcanization process aim to restore the original molecular weight distribution of the raw rubber.

In order to achieve this goal, it is therefore necessary to identify conditions such as to favor the breaking of crosslinks over the activation of reactions involving carbon atoms which are part of the molecular chain of the polymer, such as the oxidation reactions, which are known to lead to the formation of Causing peroxy radicals on tertiary carbon atoms, in the case of saturated polymers such as EPR rubbers, or on carbon atoms in the allyl position, as in the case of polymers or copolymers based on diene.

The process of the present invention thus intends to provide a simple and efficient process for the transformation of vulcanized H-NBR rubbers into polymeric materials that can be reused for the manufacture of technical compounds and their corresponding products.

Brief description of the invention

1. 1) die Einspeisung der Zusammensetzung, welche ein oder mehrere hydrierte Nitrilgummis (H-NBR) umfasst, in einen gleichläufigen Doppelschneckenextruder, der mit einer Drehzahl von 200 bis 600 U/Min. funktioniert, wobei der o.g. gleichläufige Doppelschneckenextruder jeweils eine Vielzahl von Heizelementen umfasst, die auf vorbestimmte Temperaturen eingestellt sind.
2. 2) die Beibehaltung der Zusammensetzung von Punkt 1) im Innern des gleichläufigen Doppelschneckenextruders auf den oben genannten voreingestellten Temperaturen, das dadurch gekennzeichnet ist, dass die Höchsttemperatur von einem oder mehreren Heizelementen von 240 bis 360° C beträgt, und zwar vorzugsweise von 260 bis 330 °C.

Accordingly, the present invention relates to a process for devulcanizing a composition containing one or more hydrogenated nitrile rubbers I(H-NBR), comprising the following stages:

1. 1) feeding the composition comprising one or more hydrogenated nitrile rubbers (H-NBR) into a co-rotating twin screw extruder operating at a speed of 200 to 600 rpm. works, wherein the above-mentioned co-rotating twin-screw extruder each comprises a plurality of heating elements which are set at predetermined temperatures.
2. 2) the maintenance of the composition of point 1) inside the co-rotating twin-screw extruder at the above preset temperatures, characterized in that the maximum temperature of one or more heating elements is from 240 to 360 ° C, preferably from 260 to 330 °C

The vulcanized H-NBR rubbers used in the process of the present invention may be derived from H-NBR processing scraps or end-of-life H-NBR based end products. Preferably, before devulcanization, the above vulcanized H-NBR rubbers are subjected to a preliminary crushing to obtain particles with an average diameter of 1 to 5 mm, preferably 1 to 3 mm, and appropriate drying if necessary. In addition, the process of the present invention is applied to vulcanized H-NBR rubbers having an acrylonitrile content of 17 to 50 mol%. Finally, the residence time of the vulcanized H-NBR rubber inside the co-rotating twin screw extruder (in the equipment and size described) is from 45 to 140 seconds, preferably from 70 to 120 seconds.

1. (i) eine Mooney Viskosität von 20 bis 80 Mooney Einheiten (MU) aufweist, vorzugsweise von 35 bis 70 MU, und dass sie
2. (ii) aus einer in Tetrahydrofuran (THF) unlöslichen Fraktion und einer in THF löslichen Fraktion besteht, wobei das Gewichtsverhältnis zwischen der unlöslichen Fraktion und der löslichen Fraktion von 90:10 bis 60:40, vorzugsweise von 80:20 bis 70:30 beträgt.

A second subject of the present invention relates to a H-NBR rubber composition obtainable by the process of the present invention, characterized in that it:

1. (i) has a Mooney viscosity of from 20 to 80 Mooney Units (MU), preferably from 35 to 70 MU, and that it
2. (ii) consists of a tetrahydrofuran (THF) insoluble fraction and a THF soluble fraction, the weight ratio between the insoluble fraction and the soluble fraction being from 90:10 to 60:40, preferably from 80:20 to 70:30 .

In a preferred embodiment, the value α of the devulcanized material, which is derived from stress relaxation measurements, is from -0.700 to -0.300, preferably from -0.400 to -0.320.

A third subject of the present invention relates to the use of the devulcanized H-NBR rubber obtained according to the process of the present invention for the manufacture of technical mixtures suitable for the manufacture of vulcanized products, or objects made by injection, Compression or extrusion were made. The above technical mixtures include both devulcanized H-NBR rubber and raw H-NBR rubber, the weight ratio between devulcanized rubber and raw rubber being between 60:40 and 5:95. A final object of the present invention relates to products made from the mixtures mentioned above, such as gaskets, O-rings, plates, technical articles.

Detailed Description of the Invention

The process of the present invention allows the elimination or reduction of the crosslinks present in the vulcanized H-NBR rubber, causing the destruction of the elastic network and restoring the fluidity of the polymer chains still partially connected, which are consequently reused for the manufacture of appropriately formulated compounds be able.

This process of the present invention, which is denoted by the term devulcanization, provides a material which, however, differs from the raw polymer in terms of macrostructure, since the thermomechanical treatment cannot avoid reactions leading to fractures anywhere in the elastic network and consequently at some points of the mesh of the network consisting of a segment of the polymer chain and therefore to the subsequent random recombination. The main difference between the devulcanized material (which we will refer to as TT-HNBR in the following) and the raw HNBR is that it has a minority fraction that is soluble in polar organic solvents such as THF, alongside an insoluble fraction that we will subsequently be called “carbon gel”.

This insoluble fraction, when re-inflated by the solvent used to extract the soluble fraction, presents a gelatinous appearance, with the typical behavior of materials in which there are compounds that bind the macromolecules together or attach them to a filler bind, typically soot. These macromolecule-macromolecule linkages and macromolecule fillers create a three-dimensional network with a low degree of linkage, which can accommodate the solvent molecules, causing a large increase in volume at the macroscopic level. The insoluble fraction has a leather-like consistency after it has been separated from the soluble fraction and dried.

In the case of TT-HNBR, the three-dimensional structure is therefore composed of a slightly reticular fraction of polymer bound to the particles of reinforcing filler, said fraction being capable of absorbing both the unbound portions of the filler and the remainder of the polymer not adsorbed on the filler to take over, which consequently turns out to be soluble in the solvent. More specifically, when speaking of carbon gel, the scientific literature refers to a rubber fraction present in an unvulcanized compound, but which turns out to be insoluble in a suitable solution. The phenomenon has been known since 1925 (D.F. Twiss, J. Chem. Soc. Ind. 44 , 1067 (1925)) and represents the visible result of the interactions between the reinforcing filler and the elastomer, an interaction that occurs during the compound preparation phase arises.

For the modalities by which the measurement of the amount of carbon gel is made, see, for example, E.M. Dannemberg in Bound Rubber and Carbon Black Reinforcement, Rubber Chemistry and Technology, July 1986, 59, No. 3, pages 512-524.

In current practice, the term carbon gel or bonded rubber refers more precisely to a polymer fraction that is adsorbed on the surface of the reinforcing filler during the mixture preparation phase, where it creates a kind of bond strong enough to withstand the impact to be able to withstand the solvent; in our case, said carbon gel, which may be present in a small amount (10-15%) in the mixture before vulcanization, during the heat treatment phase, with the modalities and under the conditions that form the subject of the present patent, for the most part.

The method of the present invention therefore consists of a thermomechanical process leading to the elimination of a part of the cross-linkages connecting the molecular chains of the HNBR, with consequent formation of complex structured molecular structures (the carbon gel) but which are not interconnected and free to flow when subjected to shear stress in the presence of a minority fraction of soluble polymer. Said structured fraction, mixed with raw rubber in the appropriate proportions depending on the desired technological properties of the final technical product, gives the final vulcanizate properties of higher mechanical resistance compared to similar vulcanizates made exclusively with raw rubber. In addition, it guarantees Presence of a minority fraction of soluble polymer good flowability of the mixture with correspondingly good processability.

In all the cases mentioned in the technical, scientific and patent literature, the working methods and conditions chosen in the devulcanization process aim to restore the original molecular weight distribution of the raw rubber.

In the case of oxidation or proton extraction resistant polymers, such as FKM-based compounds, devulcanization can successfully focus on the crosslinks to obtain devulcanized materials with molecular weight distributions close to the original products, suitable for the manufacture of technical articles. Like the patent US2014/378562 , in which the rupture of the ethereal-type cross-connections is favored by the presence of protogenic varieties. In the patent application IT201800010348 In the absence of protogenic substances and with a different thermal profile, the formation of a soluble fraction goes hand in hand with the presence of an insoluble fraction which, in the right proportions, results in an improvement in the mechanical properties when the vulcanization is incomplete, such as in the vulcanization processes im Continuous operation, especially with rotocuring technology.

The object of the present invention is the fact that working conditions have been found with which, starting from vulcanized H-NBR, it is possible to obtain materials consisting of majority fractions of reticular structures (carbon gel) together with soluble HNBR fractions, which are consequently not bound to the reticular structures . The latter have a reticular structure sufficient to confer mechanical properties, but with a sufficiently low degree of crosslinking to allow their mixing with raw rubber and consequently formulated to obtain vulcanized end products with the desired properties.

In fact, the identification of the optimal conditions to be able to maximize the performance of the cross-link cleavage reactions is complex and non-derivable; in the case of a machine that works in continuous operation, such as a twin-screw extruder, it is necessary to take into account the contrasting effect of numerous variables, such as mainly the geometry of the machine, the combination of rotating elements and their positioning depending on the temperature profile that the material encounters when passing through it of the extruder is exposed.

In addition to the temperature, important operating parameters are the average residence time and the distribution of the residence times, the first being primarily influenced by the power and speed of the screws, while the second, in addition to the two parameters mentioned above, depends heavily on the geometry of the screws.

Here and hereinafter we use the term "devulcanized" in a different sense than the usual meaning, namely as "partially devulcanized" material, where there exists alongside a soluble minority fraction an insoluble majority fraction which is more or less reticular but flowable, even when processed at room temperature.

In all cases, the distribution of molecular weights of the starting H-NBR polymer is not restored, but a completely different material is obtained, with properties of its own, which can otherwise only be obtained with the modalities described in the present invention and is therefore classified as a "new material". ' must be viewed.

According to the above definition of "partially vulcanized" material, the subject of the present invention is a process for the devulcanization of compounds based on vulcanized HNBR having an ACN content ranging from 17 to 50 mol%, and both with the aid of systems with peroxide such as with sulfur, are vulcanized with said co-rotating twin-screw extruder.

It emphasizes the importance of controlling the distribution of residence times and average residence times together with the temperature profile used to control the degree of crosslinking of the insoluble fraction; it proves particularly useful to use snails in such a configuration that the phenomenon called "back streaming" is minimized in order to reduce the amount of material that takes longer than average residence times. This effect, obtained by introducing inverted augers and necessary to guarantee the right pressure in the more critical areas, must be carefully measured and evaluated; it is necessary to determine the correct assembly of the screws and the operating conditions in terms of power and speed of the screws by measuring the residence time distribution (RTD). This measurement envisages the use of a tracer injected into the input flow in such a way as to ensure an injection time that is significantly shorter than the scope of the response time distribution.

The concentration of the tracer was measured by introducing BaSO ₄ into the loading funnel of the extruder, the concentration of which was determined in the strip of TT-HNBR by means of fluorescence spectroscopy X (XRF).

aus der sich die durchschnittliche Verweilzeit τ ergibt, die so definiert wird: $$\tau =\frac{{\mathrm{\Sigma }}_t\text{ E}\left(t\right)\Delta t}{{\mathrm{\Sigma }}_t\text{ E}\left(t\right)\Delta t}$$

The concentration-time curve (C(t)) obtained by measuring the amount of tracer in the extruded material is normalized by applying the following relation: $$E\left(t\right)=\frac{C\left(t\right)}{{\mathrm{\Sigma }}_{t}C\left(t\right)}$$

which results in the average residence time τ, which is defined as: $$\tau =\frac{{\mathrm{\Sigma }}_t\text{E}\left(t\right)\Delta t}{{\mathrm{\Sigma }}_t\text{E}\left(t\right)\Delta t}$$

In the adjacent shows the function E(t) resulting from the conditions of experiments presented in Examples 3, 4, 5, 6 and 7, from which an average residence time of 91 seconds can be observed.

The curve reveals the presence of a fraction of material to the right of the mode that has a longer residence time than that of the mode due to the reversed transport elements, and a minor fraction of material that has a residence time shorter than that of the mode due to the phenomenon known as "channeling" caused by the distance of the tip of the screw in relation to the inner wall of the extruder; while this second effect is specific to the machine, the part to the right of the curve can be modified by acting on the power (while keeping the internal geometry fixed).

For further depth reference is made to Residence time distribution in a commercial Twin-Screw Extruder, D.Wolf, N. Holin and H. White, Polymer Engineering and Science, 1986, Vol. 26, 9, 640-646 and Residence Time Distribution, V . Sans, N. Karbass et al., RSC Advanced, 2012, 2, 8721-8728) and the literature cited there.

The co-rotating twin screw extruder is used with speeds ranging between 200 and 600 rpm and with a length/diameter ratio between 50 and 150, at temperatures ranging between 240 and 360°C. The configuration of the extruder screws includes the elements that form the feeding zone, the compression zone and the dosing zone, each zone being kept at a preset temperature by heating with electrical resistances. This is followed by a degassing area and a final conveyance where the material is partially cooled. In all cases, the optimal speed, after setting the temperature or temperatures along the extruder axis, depends on the geometry of the extruder, its dimensions, especially its length, the shape of the screws and the geometric structure, as well as the power used, which is related to the amount of material related, which is processed in the unit of time, are known in the prior art; see eg Reactive Extrusion Principles and Practice, Ed. Marino Xanthos, Polymer processing Institute at Steven Institute of Technology, 1992, (ISBN 3-446-15677 Carl Hanser Verlag, Munich, ISBN 0-19-520951-6 Oxford University Press NY, and corresponding bibliographical references. The method of the present invention allows obtaining a devulcanized material suitable for technical mixing ments to be used in amounts between 5% and 95% by weight of the elastomeric filler, the make-up to 100 being crude H-NBR, with a Mooney viscosity between 20 and 80, depending on the properties required for the final product. In the preferred embodiment, the soluble portion of the devulcanizate obtainable by the process of the present invention has a soluble fraction H-NBR in an amount of about 20%, an amount that is practically constant for the preferred temperature ranges. Increasing the temperature beyond the limit of 350° (sample A of the experimental part) results in a higher amount of soluble fraction, but with a deterioration in the mechanical properties of the vulcanized product.

As for the distribution of the molecular weights of the soluble fractions of the devulcanizates obtained by the process of the present invention, is given in the appended as an example, the result of exclusion chromatographies on sample A (high temperature) and on sample F (low temperature) are shown.

It can be observed that the devulcanizate that can be obtained with the process of the present invention, with respect to the soluble part, has a substantial equivalence of the distributions of molecular weights for the range of molecular weights, which is below the distribution mode, while with respect to the range of molecular weights , which are above the mode, the low temperature treatment shows a limited but significantly larger amount of molecular weights > 200 kD (10.6 vs. 4.8%) for sample F.

wobei

1. 1. M, das Drehmoment, in Mooney Einheiten ausgedrückt wird
2. 2. K, eine Konstante ist, die dem Wert des Drehmoments eine Sekunde nach dem Stoppen des Rotors entspricht
3. 3. t, die Dauer des Experiments in Sekunden ist
4. 4. α, der Exponent im Zusammenhang mit der Geschwindigkeit der Kraftabnahme ist.

With regard to the properties of the heat-treated material, the processing conditions therefore make it possible to obtain different Mooney viscosities, with a quantity of soluble fraction that remains practically quantitatively constant and similar in terms of molecular weight distribution. As stated, the materials have different Mooney viscosities versus substantial equivalence in the soluble fraction; this difference is due to the different behaviors that can be observed when taking Stress Relaxation (ASTM D1646 - 19a) measurements. This measurement, also known as Mooney decay, measures the decay rate of torque when, after completing a measurement of "Mooney Viscosity", the rotor is stopped moving. The presence of reticular structures inside the mass results in a higher resistance to viscous flow and the decay of the effort is correspondingly slower. According to the ASTM D1646 method mentioned, the torque versus time data is represented by a distribution according to the following power law: $$\text{M}={\text{k t}}^{\mathrm{a}}$$

whereby

1. 1. M, the torque, expressed in Mooney units
2. 2. K, is a constant corresponding to the value of the torque one second after the rotor has stopped
3. 3. t, is the duration of the experiment in seconds
4. 4. α, which is an exponent related to the rate of depletion.

Distribution according to the linearized power law allows to determine the value of parameter α $$\text{log M}=\mathrm{a}\text{log t}+\text{log k}$$

Since the value of α is always negative (the force decreases), higher values of α correspond to more bound structures that oppose viscous flow, the opposite is true for lower values of α.

In our case, it has been found that the stress relaxation measurement applied to TT/HNBR samples produced under different experimental conditions gives values of the parameter α that are very different from each other and related both to the temperature of the process and to the processing conditions, and usually with a value α varying between -0.700 and -0.300, preferably between -0.400 and -0.320; as a result, materials with α values below -0.500 are weakly structured and have Mooney viscosity values < 30, while materials that have α values between -0.300 and -0.500 are more structured and have Mooney viscosity values >45.

The use of TT-HNBR with values of α < -0.500 proves beneficial when fluidity properties are required, while the use of TT-HNBR with α > -0.500, better still > -0.400, provides blends showing significant improvements in mechanical properties entail properties.

The determination of the parameter α therefore allows the evaluation of the residual degree of crosslinking of the materials in a simple and efficient manner and thus represents a useful tool for process control.

As indicated above, the elastomeric blend which can be obtained by the process of the present invention can be used for the preparation of blends.

This property is appreciated in the transformation industry of H-NBR, whose difficulties encountered in the typical processing phases, such as the incorporation of fillers and poor fluidity, are well known, problems that can usually and partially be solved by the addition of appropriate processing oils and here were of course resolved.

E is therefore evident that only an optimal balance of the individual fractions provides heat-treated materials useful to be used in mixtures used for technical articles: the presence of a low molecular weight fraction constitutes the rheological balance for the high molecular weight fraction Molecular weight , while the insoluble fraction provides a preformed network with varying degrees of crosslinking depending on the process conditions, whose properties are additive to the elastic network that forms during the vulcanization phase.

Since this balance of the individual fractions depends both on the process conditions (temperature and residence time) and on the internal geometry of the twin-screw extruder (geometry of the screw elements and their combination) as well as on the length/diameter ratio, it follows that achieving the properties mentioned is also possible for an experienced technician is neither self-evident nor foreseeable. Another object of the present invention is the use of the elastomeric composition of the present invention for the manufacture of mixtures suitable for the manufacture of vulcanized products such as plates, O-rings or gaskets of any type, or any other H -NBR obtained by injection, compression or extrusion, said final products being made starting from a devulcanized H-NBR mixture according to the process of the present invention and with raw H-NBR, and the ratio between the two each of different formulated ones 80:20 to 5:95 is sufficient, depending on the desired mechanical properties.

1. a. Die Herstellung von TT-HBNR bei unterschiedlichen Versuchsbedingungen ausgehend von vulkanisiertem HNBR
2. b. Die Herstellung und Evaluierung von Mischungen, die bei Verwendung von Roh-HNBR hergestellt wurden, im Vergleich zu Mischungen, die die Elastomerzusammensetzung der vorliegenden Erfindung enthalten und entsprechenden Mischungen.

The following experimental examples describe:

1. a. The production of TT-HBNR under different test conditions starting from vulcanized HNBR
2. b. The Preparation and Evaluation of Blends Made Using Crude HNBR Compared to Blends Containing the Elastomeric Composition of the Present Invention and Corresponding Blends.

The examples below are cited to provide a better overview of the present invention.

EXAMPLES

Analytical Methods

1. 1) Insoluble fraction
   • The fraction of insoluble material is determined as the insoluble residue after extraction. A portion of about 50 g of material heat treated and cut into small pieces, weighed on the analytical balance ( _P1 ) and placed in a Soxhlet thimble, whose weight (P2) is determined, and then extracted with 250 cc THF for 6 hours. The sleeve that holds the extraction back contains stand is then dried and weighed (P ₃ ); all weights show an approximation of 0.001 g. The amount of material that cannot be extracted is given by the term "% insoluble" and is quantified using the following relation: $$\%\text{unl}\ddot{\text{O}}\text{itself}=\left({\text{P}}_3-{\text{P}}_2\right)\times 100/{\text{<figure-callout id="1" label="P" filenames="DE102022105040A1\_0013.png" state="{{state}}">P</figure-callout>}}_1$$
     
2. 2) Molecular mass distribution (MWD) of the soluble fraction
   • To determine the distribution of molecular masses, number average (Mn), weight average (Mw) and degree of polydispersion (Mw/Mn), the technique of elution chromatography on gel (GPC), also called size exclusion chromatography (SEC), is used. Analysis of the sample dissolved in THF was performed using a Waters Model 590 pump, a series of Ultrastyragel columns 10 ³ , 10 ⁴ , 10 ⁵ and 10 ⁶ Angstroms, a refractive index detector RI HP 1047 and, as eluant, THF at 30° C The molecular masses are determined by the universal calibration method using the following values of the Mark-Houwink equation parameters k = 5.6 10 ^-4 and α = 0.68.
3. 3) Measurement of Mooney Viscosity and Stress Relaxation Test according to ASTM D1646-19a
4. 4) DSC according to ASTM E 1356
5. 5) TGA per ASTM D6370
6. 6) Rheometry according to ASTM D2084
7. 7) Tensile properties according to ASTM D412
8. 8) Hardness according to ASTM D2240
9. 9) Density according to ASTM D297

Method of the Invention

In the preferred embodiment of the invention, the vulcanized HNBR scrap rubber is subjected to a coarse grinding step, preferably to obtain particles between 1 and 5 mm in diameter, more preferably between 1 and 3 mm, before being fed into a co-rotating twin screw extruder. In the case of the extruder used, the part with a diameter of less than 1 mm must be sorted out by sieving, because during processing, smaller grain sizes would allow their exit from the extruder with residence times shorter than the mode of distribution of residence times, due to " channeling”, which is caused by an increase in the distance between the tip of the screw and the inner wall of the extruder. This fraction undergoes partial processing, turns out to be insufficiently devulcanized and results in irregularities on the surface of the vulcanizate, in addition to reduced mechanical properties.

The increase in the distance between the tip of the screw and the inner wall of the extruder can vary depending on the type of extruder or be caused by a progressive loss of the initial tolerances; this aspect must be taken into account in order to remove, by sieving, from the material fed, the fraction with sizes that could be eliminated at a rate greater than the average.

The use of a co-rotating twin screw extruder allows to carefully adjust the intensity of the shearing stresses and strains necessary to partially devulcanize the vulcanized HNBR rubber, utilizing the modularity of the screw, consisting of a very specific series of direct and inverse Conveyor elements (transports) consists, as well as mixing elements (kneading, hammers). All of these elements have types of geometries that can vary, such as the screw threads and the corresponding dimensions.

1. 1. Einen Anfangsabschnitt für die Einspeisung
2. 2. Einen Bereich, in dem die Wärmebehandlung (Aufweichen und teilweise Degradierung des Materials) erfolgt
3. 3. Einen Bereich, in dem die Mischung und Devulkanisierung durch Ausübung einer intensiven mechanischen Einwirkung auf das Material erfolgen
4. 4. Einen vakuumierten Kühl- und Entgasungsbereich vor dem Austritt des Materials aus dem Extruder.

A typical configuration of the screw includes the following:

1. 1. An initial section for the feed
2. 2. An area where the heat treatment (softening and partial degradation of the material) takes place
3. 3. An area where mixing and devulcanization are carried out by exerting an intense mechanical action on the material
4. 4. A vacuumed cooling and degassing area before the material exits the extruder.

In all cases, after establishing the temperature profile along the axis of the extruder, the optimal speed depends on the geometry of the extruder, its dimensions, especially its length, the diameter and shape of the screws and the geometric structure and the power used in relation to the amount of material processed per unit time is known; see for example K. Kohlgruber, Co-rotating Twin-Screw Extruders, Hanser Gardner Pubns., October, 2007 and Reactive Extrusion Principles and Practice, Ed. Marino Xanthos, Polymer processing Institute at Steven Institute of Technology, 1992, Carl Hanser Verlag, Munich, ISBN 0-19-520951-6 Oxford University Press N.Y.

1. 1. Seitenansicht ( und Ansicht von oben ( des Extruderschemas
2. 2. Die Konfiguration der Schnecken in den einzelnen Elementen (TABELLE 1)

TABELLE 1: Zusammensetzung der Schnecken in den verschiedenen Bereichen Bereich 1° 2° 3° 4° 5° 6° 7° 8° Hammeranzahl/insges. 0/6 4/6 6/6 0/7 0/8 0/10 0/7 0/8 Transportanzahl/insges. 6/6 2/6 0/6 7/7 6/8 8/10 7/7 8/8 Transportanzahl/ umgekehrt/insges. 0/6 0/6 0/6 0/7 2/8 2/10 0/7 0/8 In the enclosed are shown:

1. 1. Side view ( and top view ( of the extruder scheme
2. 2. The configuration of the screws in the individual elements (TABLE 1)

TABLE 1: Composition of the snails in the different areas area 1° 2° 3° 4° 5° 6° 7° 8° number of hammers/total 0/6 4/6 6/6 0/7 0/8 0/10 0/7 0/8 number of transports/total 6/6 2/6 0/6 7/7 6/8 8/10 7/7 8/8 Number of transports/ vice versa/total 0/6 0/6 0/6 0/7 2/8 2/10 0/7 0/8

All the above elements have the purpose of better describing the process and must be considered as non-limiting examples. It is effectively known in the prior art that the optimal speed, after setting the temperature or temperatures along the extruder axis, depends on the geometry of the extruder, its dimensions, in particular its length, the shape of the screws and the geometric structure and the power used , which is related to the amount of material processed in the unit of time, can be varied with a high degree of discretion.

Mandatory and part of the invention, however, is the fact that it has been found that the typical process parameters such as residence time and temperatures, used in an appropriate combination that depends on the geometry of the extruder, provide an a priori unpredictable experimental interval in which a partially devulcanized material can be produced, but which is flowable and therefore comparable to a highly viscous liquid containing a structured fraction characterized by exhibiting stress relaxation properties falling below a certain interval.

The preferred conditions for carrying out the process of the present invention are those indicated above. They make it possible to obtain a pasty mass, fluid under the process conditions, but at the same time consistent and elastic when exiting the extruder, capable of delivering a strip made up of an insoluble fraction, in an amount that varies depending on each varies with the process conditions. In particular, it follows that, under the preferred operating conditions, the higher the temperature of the central elements (4, 5 and 6), together with an increase in the residence time, the insoluble fraction has a lower degree of crosslinking, while maintaining the amount of the soluble fraction practically constant values.

The controlled devulcanization process is extremely sensitive to the operating conditions, in particular the temperature of the elements that make up the extruder and the relative residence times; Changes of as little as 5-10°C cause significant changes in the viscoelastic properties of the material, as evidenced by the variability of the coefficient α, which was previously determined.

As for the sensitivity of the properties of the treated material to limited changes in the process parameters, in particular temperature and residence times, it becomes clear when examining the loss in weight of the vulcanized reference mixture V-RIF, resulting from a thermogravimetric curve (TGA), those with a temperature increase of 20°C/min (see enclosed ) was recorded.

As can be seen, the weight loss of a polymeric material, which is the last step in a degradation process, is very rapid. In an extruder, this degradation process, visible in TGA, is triggered by subtle changes in temperature and residence times; therefore, the operating parameters must be precisely defined in order to obtain materials with degrees of crosslinking that fall below the acceptance intervals dictated by the final properties that are desirable in vulcanizates. It goes without saying that the change in the geometry of the extruder, length and diameter, the geometry of the screws and the power are all variables that are dependent on each other and that changing them consequently changes the values; for this reason it is imperative that the product properties are correlated with the value of the coefficient a derived from stress relaxation measurements. In addition, the internal geometry of the screws, in particular the number and position of the inverted transport elements, necessary to ensure sufficient pressure in the critical areas, must be carefully chosen, taking into account the distribution of the residence times, in order to avoid excessive residence times of the material inside the to avoid extruders.

In any case, the thermogravimetric data make it possible to estimate the temperatures of the elements of the extruder in which the controlled degradation reaction is to be carried out.

preparation of materials

Example 1 (preparation of the reference mixture)

Crude H-NBR (grade H-NBR ZETPOL 2020L ^® ) is formulated as indicated in Table 2 TABLE 2: Composition of the reference mixture M-RIF ingredients phr H-NBR ZETPOL 2020L ^® 100 Carbon black N990 60 Carbon Black N762 5 LUPEROX F40 T-SP 11 TAIC 70 3.5 Vanox CDPA 1.5 Vanox ZMTI 1 stearic acid 1
H-NBR ZETPOL 2020L®, hydrogenated nitrile rubber, manufactured by Nippon Zeon, ACN 36% by weight, Mooney viscosity 50-65, iodine number 28 g/100g
LUPEROX F40 T-SP, peroxide, 1,3 1,4-bis(tert-butyl-peroxy-isopropyl)benzene
Taic 70, Vulcanization Aid, 1,3,5, Triallil Isocyanurate
Vanox CDPA, Antioxidant, 4,4'-Bis(alpha,alpha-dimethylbenzyl)diphenylamine
Vanox ZMTI, Antioxidant, Zinc Methyl Mercapto Benzimidazole

A reference mixture (M-RIF) is thus obtained, which is used as a basis for the addition of TT-HBNR in the amounts and recipes given below.

A portion of the reference mixture is vulcanized in the form of 200x200x2 mm slabs at a temperature of 180°C for 10' and then subjected to a four-hour standard post-curing at 160° to evaluate the mechanical properties, which is indicated by V-RIF. A post curing of less than 2 hours was also considered.

Example 2 (Production of TT-HNBR at different temperatures)

Processing scraps of peroxide vulcanized H-NBR with a rubber content of about 60% are ground to obtain a material with grain sizes between 1 and 5 mm, which is fed into the extruder by means of a stepless weighing system with screw conveyor.

Using a screw arrangement such as that indicated in Table 1 and with a temperature profile indicated in Table 3, at 400 rpm and 20 kg/h, a very low viscosity material is obtained which is collected at the exit of the extruder as a very liquid paste and was unable to solidify and form a continuous strip. TABLE 3: Temperatures of the individual components of the extruder used in the production of TT-HNBR A. example 2 TT-HNBR A Areas# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 350 350 350 350 330 250 130 110

Examples 3-4-5-6-7

TT-HNBR B (Example 2), C (Example 3), D (Example 4) and F (Example 5) are obtained with different temperature profiles but the same machine and process setup as in Example 1. TABLE 4: Temperatures of the individual Extruder components used to manufacture TT-HNBR B, C, D, E, F Example 3: TT-HNBR B Areas# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 340 330 330 330 310 230 130 110 example 4 TT-HNBR C Areas# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 300 310 310 310 290 230 110 90 Example 5 TT-HNBR D Series# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 285 285 285 285 270 230 100 90 Example 6 TT-HNBR E Areas# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 250 275 275 275 260 220 90 90 Example 7 TT-HNBR F Areas# 1 2 3 4 5 6 7 8th 9 Temp (°C) 200 240 260 260 260 260 210 90 90

The products made with temperature profiles D, E, F have sufficient viscosity to be extruded in the form of a continuous strip that is sufficiently tough at the extrusion temperature (150-170°C).

The materials prepared under various operating conditions have the Mooney viscosities {ML1+5+1@100°C} reported in Table 5. In the same table the amount of soluble material and the value of the coefficient α taken from the stress relaxation measurements are given. TABLE 5: Mooney viscosity, parameter a and molecular properties of the soluble fraction of samples TT HNBR, A, B, C, D, E, F. Unit of measurement TT-HNBR-A TT-HNBR-B TT-HNBR-C TT-HNBR-D TT-HNBR-E TT-HNBR-F Mooney viscosity MU 21:4 26.7 27.3 44.5 64.8 67.9 a dimensionless -0.625 -0.605 -0.596 -0.395 -0.350 -0.322 solvent Portion % Weight 20.8 20.7 21:1 20.5 21.2 21:1 Mn kD 29.2 29.1 28.8 29.3 29.1 28.9 VAT kD 81.2 81.5 83.5 93.1 96.0 99.5 Mw/Mn dimensionless 2.8 2.8 2.9 3.2 3.3 3.4

• 1. Die Menge der löslichen Fraktion praktisch konstant für alle untersuchten Temperaturprofile ist
• 2. Die Verteilungen der Molekulargewichte des löslichen Anteils sehr ähnlich sind, da sie sich durch eine geringfügig höhere Fraktion als hohe Molekulargewichte unterscheiden, die bei Sinken der Verfahrenstemperatur zunimmt
• 3. Der Wert des Koeffizienten α, der den abnehmenden Verlauf des Drehmoments in einem Stressrelaxationsversuch beschreibt, sich bei Sinken der Höchsttemperatur des Transformationsverfahrens erhöht: dies wird durch eine höhere Residualvernetzung des Materials verursacht, das mit den Temperaturprofilen E und F gewonnen wurde.

The data of Table 5 make it clear that

• 1. The amount of the soluble fraction is practically constant for all temperature profiles studied
• 2. The molecular weight distributions of the soluble portion are very similar, differing by a slightly higher fraction than high molecular weights, which increases as the process temperature decreases
• 3. The value of the coefficient α, which describes the decreasing course of the torque in a stress relaxation test, increases as the maximum temperature of the transformation process decreases: this is caused by a higher residual crosslinking of the material obtained with the temperature profiles E and F.

Table 6 below shows the data obtained from the TGA and DSC analyzes of the materials: data are given relating to the reference mixture (M-RIF) and the materials TT-HNBR-A and B, which are typical for low Mooney viscosity materials, and TT-HNBR-D and F, which are typical for medium and high Mooney viscosity materials. TABLE 6: Results of the DSC analyzes (columns 2 and 3) and TGA of the samples M-RIF and TT HNBR A, B, C, D, E, F. material day crystallinity Fugitive Faction rubber fraction Oxidizable Fraction ashes Rubber Degradation * oxidation soot* °C y/g %w %w %w %w °C °C M-RIF -31.1 1.9 4.3 60.9 28.6 6.2 475 730 A -31.6 0 6.4 58.9 28.4 6.3 468 748 C -31.5 1.1 5.3 59.3 28.1 6.3 471 750 D -31.8 0.9 6.1 60.1 27.9 6.6 476 738 f -32.6 1.6 5.5 59.8 28.3 6.4 468 740
* Determined as the temperature at the inflection point of the curve

Table 6 shows how the materials made with different modalities all have practically the same thermal properties while compared to Table 5 differentiate according to rheological properties. If one considers that both the amount of soluble fraction and the molecular weight distribution of the individual fractions (A, B, D, F) are very similar, the different Mooney viscosity can result from a different degree of crosslinking of the insoluble fraction, which is caused by the high Variability and temperature dependence of the transformation process of the coefficient α is illustrated.

We then show how the physical properties of the vulcanizates are affected by the applied temperature profile, which can be practically adjusted to a higher or lower average temperature to which the material is exposed; the transformation conditions must therefore be chosen in such a way that a material is produced in which the balance between rheological properties and mechanical properties is correspondingly optimized for the respective special application.

If you intend to obtain mixtures that have a high degree of fluidity in order to be able to fill molds with complex geometries without problems and at the same time a medium-low hardness, it is advisable to focus on production on TT-HNBR, with a Mooney viscosity between 20 and 40, if on the contrary one wishes to obtain compounds whose vulcanizates have high mechanical properties, preference should be given to TT-HNBR with Mooney viscosity between 40 and 80, preferably 50-70.

The presence of a pre-crosslinked fraction guarantees in all cases better tear values in the final vulcanizate, especially when hot, making it easier, for example, to extract vulcanized products from complex shapes and with critical undercuts.

Examples 7a and 7b Production of TT-HNBR with different performances

To study the effects of each power on the exiting material properties, two different operations were performed with the same heat treatment shown in example 7, changing the speed and bringing it to 300 (sample G) and 500 rpm (Sample H); the feed rate is maintained at 20 kg/h, using a feeding technique called "starve feeding"; in this case the hopper is always empty and the amount of product fed is regulated by the loading auger.

Under the conditions described, increasing the number of revolutions leads to a reduction in the residence time of the material at high temperature; the opposite is the case when the speed is reduced.

The determination of the residence time distributions, carried out in the cases cited according to the explanations described above, allows the evaluation of the average residence time for the three samples.

This allows the evaluation of the effect of the residence time of the material in the extruder at the same temperatures. TABLE 7: Change in properties depending on the average residence time Unit of measurement TT-HNBR-F TT-HNBR-G TT-HNBR-H example no 7 (reference) 7a 7b rotation speed rpm 400 300 500 avg dwell time s 91 112 74 Mooney viscosity MU 73.3 45.5 78.2 soluble portion % Weight 23.9 24.1 22.5 Fraction with molecular weight <20 kD % Weight <20% < 20% < 20% particle diameter <0.5 mm in the extruded product When observed missing missing numerous a dimensionless -0.322 -0.375 -0.295

1. 1. Die Erhöhung des Wertes des Koeffizienten α
2. 2. Die Verringerung der Mooney Viskosität
3. 3. Die Beibehaltung des Wertes der löslichen Fraktion auf sehr ähnlichem Level
4. 4. Das Erscheinen einer nicht quantifizierbaren, aber sichtbaren Fraktion an Partikeln eines nicht ausreichend devulkanisierten Materials.

Compared to the production conditions of the reference fraction F, the increase in speed (TT-HNBR-H), in addition to reducing the residence times, causes the following:

1. 1. Increasing the value of the coefficient α
2. 2. The reduction in Mooney viscosity
3. 3. Maintaining the value of the soluble fraction at a very similar level
4. 4. The appearance of an unquantifiable but visible fraction of particles of insufficiently devulcanized material.

It is emphasized that the presence of insufficiently devulcanized particles has a negative impact on both the mechanical and aesthetic properties of the final product; in fact, in addition to reduced breaking loads caused by the presence of foreign materials unbound inside the elastomeric matrix, an irregular and rough-to-the-touch surface can be observed.

1. 1. Die Verringerung des Wertes des Koeffizienten α
2. 2. Die Verringerung der Mooney Viskosität
3. 3. Die Beibehaltung des Wertes der löslichen Fraktion auf sehr ähnlichem Level

Compared to the manufacturing conditions of the reference fraction F, the reduction in speeds (TT-HBNR-G) causes:

1. 1. Decreasing the value of the coefficient α
2. 2. The reduction in Mooney viscosity
3. 3. Maintaining the value of the soluble fraction at a very similar level

In this case, the main effect of increasing the residence times is the reduction in the degree of crosslinking of the insoluble fraction, with a consequent progressive prevalence of the viscosity over the elastic properties of the heat-treated material.

Examples 8-18

The mixtures indicated as M2, M3, M4, M5, M6, M7, M8, M9, M10, M11, M12 are obtained by mixing TT-HNBR-F with the mixture M-RIF in the proportions indicated in Table 8; in Table 9 the typical parameters of the cure curves are shown.

Table 9 gives the parameters of the vulcanization curves, the values of the density and hardness.

Examples 19-30

Starting from the mixtures M-RIF, M2, M3, M4, M5, vulcanized sheets measuring 200×200×2 mm are produced in a press at 180° C. for 10'; the panels are then post-cured at 160°C for 4 hours. The vulcanizates are given the abbreviations V-RIF, Vn with n from 2 to 12.

Examples 31 - 35

The sheets of the vulcanizates V-RIF, V5, V8, V9 and V10 and M5 are subjected to a different post-treatment of two hours at 160° C. after vulcanization at 180° C. for 10 minutes. They are designated M-RIF-2, V5-2, V8-2, V9-2, V10-2. Table 10 gives the respective mechanical properties. TABLE 11: Mechanical properties after vulcanization {10' @180°C} and subsequent post-vulcanization {2 h @ 160°C} examples 31 32 33 34 35 Ref.abbr. V-RIF-2 V5-2 V8-2 V9-2 V10-2 hardness ShA 69.4 70.00 70.1 70.3 70.4 specific weight g/cm ³ 1.24 1.23 1.23 1.23 1.23 Module 100% MPa 3.94 4.39 4.05 4.32 4.33 breaking load MPa 20.6 20.2 20.4 20.1 20.2 elongation at break % 342.1 304.1 365.2 338.8 339.3

1. 1. Die Verwendung von Mischungen, die TT-HBNR bis zu Gehalten von 27 Gew.% enthalten, ändert die technologischen Eigenschaften des vulkanisierten Materials nicht wesentlich, wenn man die Rezepturen aus Tabelle 8 anwendet.
2. 2. Materialien mit Gehalten über 27% (bis 50%) zeigen einen progressiven Verlust der technologischen Eigenschaften, was teilweise durch eine Änderung der Rezeptur ausgeglichen wird; dieser Verlust ist allerdings sehr gering und akzeptabel bei zahlreichen Anwendungen, zudem bei zusätzlichen wirtschaftlichen Vorteilen.
3. 3. Überraschenderweise sind die Eigenschaften des Vulkanisats, das einem kürzeren Post curing (2 St @ 160°) unterzogen wurde, besser bezw. entsprechen zumindest denen des Materials mit einem längeren Post curing Zyklus, bei entsprechenden Vorteilen in produktiver Hinsicht.

Remarks

1. 1. The use of mixtures containing TT-HBNR up to levels of 27% by weight does not significantly change the technological properties of the vulcanized material when using the recipes from Table 8.
2. 2. Materials with levels above 27% (up to 50%) show a progressive loss of technological properties, which is partially offset by a change in formulation; however, this loss is very small and acceptable in many applications, also with additional economic advantages.
3. 3. Surprisingly, the properties of the vulcanizate that was subjected to a shorter post-curing (2 h @ 160°) are better or at least correspond to those of the material with a longer post-curing cycle, with corresponding advantages in terms of production.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• US5284625 [0008]
• US5258413 [0008]
• U.S. 2014/378562 [0025]
• IT 201800010348 [0025]

Claims (11)

A process for devulcanizing a composition containing one or more hydrogenated vulcanized nitrile rubbers (H-NBR) comprising the steps of: 1) feeding the composition containing one or more hydrogenated vulcanized nitrile rubbers (H-NBR) into a co-flow Twin-screw extruder operating at a speed of 200 to 600 rpm, said co-rotating twin-screw extruder comprising a plurality of heating elements controlled at predetermined temperatures, 2) maintaining the composition of point 1) at the above predetermined temperatures in the co-rotating twin-screw extruder, characterized in that the maximum temperature of one or more heating elements of the twin-screw extruder ranges from 240°C to 360°C. The procedure according to claim 1 , with the maximum temperature of one or more heating elements ranging from 260°C to 330°C. The process according to claims 1 to 2, wherein the vulcanized H-NBR rubbers are selected from the processing scraps and the articles that have reached the end of their useful life. The process according to claims 1 to 3, wherein the composition containing one or more H-NBR rubbers consists of particles, possibly dried and sieved, having a diameter comprised within an interval between 1 and 5 mm, preferably between 1 and 3 mm. The process according to claims 1 to 4, wherein the acrylonitrile content of the vulcanized H-NBR rubber is from 17 to 50 mol%. The process according to claims 1-5, wherein the residence time is from 45 to 140 seconds, preferably from 70 to 120 seconds, due to the geometry of the extruder used. Composition of H-NBR rubber obtainable according to one or more of claims 1 to 6, characterized in that: (i) it has a Mooney viscosity of 20 to 80 MU, preferably 35 to 70 MU and (ii) consists of a tetrahydrofuran (THF) insoluble fraction and a THF soluble fraction, the weight ratio between the insoluble fraction and the soluble fraction being from 90:10 to 40:60, preferably from 80:20 to 70:30. Composition of H-NBR rubber according to claim 7 , where the value of the coefficient α derived from stress relaxation measurements is from -0.700 to -0.300, preferably from -0.400 to -0.320. The application of the composition of H-NBR rubber prepared according to the method of Claims 1 - 8th for the manufacture of technical mixtures suitable for the manufacture of vulcanized products or objects intended for injection, compression or extrusion. Mixtures for the manufacture of products according to claim 9 are suitable which comprise devulcanized H-NBR rubber produced according to one or more of claims 1-8 together with raw H-NBR rubber, the ratio between the two ranging from 80:20 to 5:95 respectively. Vulcanized products, which starting from mixtures according to claim 10 are manufactured, in particular gaskets, O-rings and plates produced by injection, compression or extrusion.

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