BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to the field of rubber polymers, and particularly to a method of devulcanizing a rubber vulcanizate, such as obtained from waste tire rubber. Description of the Related Art

The recycling of vulcanized rubber products presents a challenging problem because rubber vulcanizate is typically cross-linked with sulfur or peroxides and therefore forms a thermosetting network that cannot be reformed into other products without degrading the network.

Tires, hoses, belts and other rubber products are therefore conveniently discarded after they have been worn out during their service lifetime. There has however been a continuous interest in recycling vulcanized rubber products since dumping rubber products is undesirable from an environmental point of view. A major amount of rubber vulcanizate from tires and other rubber products is shredded or ground to obtain rubber crumb with relatively small particle size. This crumb is incorporated into various products as a filler, and can for instance be used in rubber formulations for new tires, or in other rubber products. Since the thermosetting network in the filler is substantially unaffected by shredding or grinding, the filler does not co-cure to an appreciable extent with the surrounding material.

Although shredding or grinding rubber vulcanizate to obtain a filler is certainly valuable, some value of the rubber is lost in the process. Indeed, the properties of the filled product may not be optimal.

Various techniques for devulcanizing rubber vulcanizate have been proposed in the hope of rendering the rubber suitable for reformulation and recurrence into new rubber articles without degradation of the rubber. In other words, the rubber vulcanizate is 'broken down' to its constituents and used again for its original intended purpose.

However, none of the devulcanization techniques previously developed has proven to be commercially viable, one of the main problems being the inability to obtain good properties. The poor properties of de-vulcanized rubber, caused by the structural changes of the polymer molecules is mainly due to the intensive physical forces, i.e., shearing and high temperature, applied in known devulcanization methods. Under severe shear and high temperature, various reactions occur during the devulcanization process causing main chain breakage next to the desired crosslink scission.

Another problem is that the known devulcanization methods are generally less controllable. In particular, it has been observed that the ratio of main-chain scission to crosslink scission, which should be as low as possible, is not easily controlled.

Recombination of radical moieties of broken rubber chains for instance may occur, leading to de-vulcanized rubber products with poor properties.

One aim of the present invention is to provide a method of devulcanizing a rubber vulcanizate that yields de-vulcanized rubber products with better properties than those obtained from known methods.

SUMMARY OF THE INVENTION

The invention thereto provides a method of devulcanizing a rubber vulcanizate of main chains and sulfur-containing cross-links, the method comprising mixing the rubber vulcanizate with a devulcanizing agent to a temperature sufficient to cause the devulcanizing agent to react with the sulfur of the sulfur-containing cross-links and to substantially prevent abstraction of hydrogen atoms from the rubber vulcanizate main chains by contacting the rubber vulcanizate with an oxidation stabilizer.

By utilizing the method of this invention, cured rubber is effectively de-vulcanized. The temperature at which abstraction of hydrogen atoms from the rubber vulcanizate main chains is substantially prevented is defined as the threshold temperature above which an increase in cross-link density is observed in the (partly) de-vulcanized rubber, compared to the cross-link density obtained at a lower devulcanization temperature, as will be explained in more detail below.

The term "substantially" when used herein is intended to mean to an extent of at least 90% of the stated values.

The devulcanizing agent used in the method according to the invention may be chosen from a wide range of devulcanizing agents, and the choice will for instance depend on the type of rubber vulcanizate to be de-vulcanized. Suitable devulcanizing agents comprise sulfur containing compounds such as disulfides and thiols, amine and amide compounds, phenolic compounds and alcohols. An organosulfur compound is preferably used and in a particular embodiment of the method according to the invention, the organosulfur compound comprises a polysulfide. Yet another embodiment of the invention provides a method, wherein the polysulfide is a disulfide, preferably diphenyldisulfide (DPDS)

In another embodiment of the method according to the invention, substantially preventing abstraction of hydrogen atoms from the rubber vulcanizate main chains is effected by substantially eliminating oxygen during the devulcanization step. An embodiment of the method according to the invention thereto performs the

devulcanization in a nitrogen-atmosphere.

Another embodiment of the invention provides a method, wherein the oxidation stabilizer comprises a phenolic compound, a phosphorous compound, or an aromatic amine compound, or a combination of both.

In a preferred embodiment of the method in accordance with the invention, the phenolic compound comprises tetrakis[methylene-3-(3,5-ditertbutyl-4- hydroxyphenyl)propionate]methane (Irganox 1010) and/or n-octadecyl-[beta]-(4- hydroxy-3,5-ditertbutyl phenyl)propionate (Irganox 1076). Hindered phenolic stabilizers act as hydrogen donor. The stabilizer reacts with peroxy radicals to form hydroperoxides and prevents the abstraction of the hydrogen from polymer molecules. Yet another embodiment of the invention is directed to a method, wherein the phosphorous compound comprises a phosphorous acid and its salts (phosphites). In a preferred embodiment of the method in accordance with the invention, the phosphorous compound comprises tris(2,4-ditert-butylphenyl)phosphite (Irgafos 168). Phosphite compounds function as hydroperoxide decomposers, the stabilizer prevents the split of hydroperoxide into extremely active radicals: alkoxy and hydroxyl.

In yet another preferred embodiment of the method in accordance with the invention, the aromatic amine compound comprises an arylene-amine compound, more preferably a p-phenylene diamine compound.

A particularly preferred embodiment of the invention provides a method wherein a combination of an organosulfur compound and an oxidation stabilizer is used. Another aspect of the invention relates to a method, wherein the temperature during devulcanization is between 180°C and 320°C, and more preferred between 220°C and 280°C. Raising the temperature may increase the speed of devulcanization but preferably measures have to be taken to substantially prevent abstraction of hydrogen atoms from the rubber vulcanizate main chains. The relatively high temperatures of the indicated range help in decreasing shear stresses acting on the rubber vulcanizate during devulcanization, which lowers the risk for main chain scission.

Still another aspect of the invention relates to a method, wherein the amount of devulcanizing agent ranges from 0.01 to 10 wt%, relative to the total weight of the rubber vulcanizate, and more preferred from 0.1 to 5 wt%, relative to the total weight of the rubber vulcanizate.

Yet another aspect of the invention relates to a method, wherein the amount of oxidation stabilizer ranges from 0.1 to 5 wt%, relative to the total weight of the rubber vulcanizate, and more preferred from 0.5 to 2 wt%, relative to the total weight of the rubber vulcanizate.

In an embodiment of the method according to the invention, the devulcanization is performed in an internal mixer or an extruder. The extruder is preferably of the co- rotating twin screw extruder type. The screw configuration of the extruder may among others comprise transporting elements, kneading elements and pressuring elements.

Another aspect of the invention is directed to a method, further comprising mixing the rubber vulcanizate with a swelling agent and/or solvent. It is known that energies of carbon-sulfur and sulfur-sulfur bonds in a rubber vulcanizate network are lower than that of carbon-carbon bonds and that carbon-sulfur and sulfur-sulfur bonds are more easily broken up by shear stress during mixing in for instance an extruder. Adding a swelling agent increases mobility of the devulcanizing agent and other aids and allows higher shear stresses, leading to an increased efficiency of devulcanization of the rubber.

In an embodiment of the method according to the invention, the solvent or swelling agent is selected from the group consisting of one or more of toluene, naphtha, terpenes, benzene, cyclohexane, diethyl carbonate, ethyl acetate, ethyl benzene, isophorone, isopropyl acetate, methyl ethyl ketone and derivatives thereof.

The method of the invention may be used for devulcanizing any known rubber vulcanizate. In suitable embodiments of the method according to the invention, the rubber vulcanizate comprises natural rubber ( R), butadiene rubber (BR), isoprene rubber (IR), butyl rubber (IIR, CIIR or BIIR), ethylene-propylene rubber (EPM), styrene-butadiene rubber (SBR), chloroprene rubber (CR), nitrile rubber ( BR), acrylic rubber (ACM), ethylene-propylene-diene rubber (EPDM), or mixtures thereof. Another embodiment of the invention is directed to a method, wherein the rubber vulcanizate is provided in a finely divided form with an average particle size ranging from 150 to 2000 microns. With larger particle sizes above about 2 mm, mechanical processing difficulties may arise due to poor mixing, while the use of particles significantly smaller than about 150 microns does not lead to the desired level of efficient devulcanization.

The method of the invention can be applied to unfilled as well as to filled rubbers. Filled rubbers may comprise fillers such as carbon black, silica, organic and inorganic fibers, and others. BRIEF DESCRIPTION OF THE FIGURES AND TABLES

The invention will now be described in more detail with reference to the accompanying figures and tables, without however being limited thereto. In the figures:

Table I shows the chemical names and structures of thermal stabilizers used in embodiments of the method in accordance with the invention;

Table II shows the formulation of a styrene-butadiene-rubber compound (SBR), used in the Examples and Comparative Experiments;

Table III shows the devulcanization conditions, as used in the Examples and

Comparative Experiments;

Figure 1 shows a schematic diagram of the sol fraction obtained as a function of the devulcanization temperature for de-vulcanized SBR obtained by several embodiments of the invention compared to untreated vulcanized SBR;

Figure 2 shows a schematic diagram of the crosslink density obtained as a function of devulcanization temperature for de-vulcanized SBR obtained by several embodiments of the invention compared to untreated vulcanized SBR;

Figure 3 shows a schematic diagram of the sol fraction generated during

devulcanization at different temperatures in function of the relative decrease in crosslink density of de-vulcanized SBR obtained by several embodiments of the invention;

Figure 4 shows a schematic diagram of the sol fraction as a function of the

devulcanization temperature for de-vulcanized SBR obtained by several embodiments of the invention compared to untreated vulcanized SBR;

Figure 5 shows a schematic diagram of the crosslink density as a function of devulcanization temperature for de-vulcanized SBR obtained by several embodiments of the invention compared to untreated vulcanized SBR;

Figure 6 shows a schematic diagram of the sol fraction generated during

devulcanization at different temperatures in function of the relative decrease in crosslink density of de-vulcanized SBR obtained by several embodiments of the invention; and Figure 7 schematically shows a simplified reaction scheme proposed for rubber devulcanization using diphenyldisulfide as devulcanization aid. DETAILED DESCRIPTION OF THE INVENTION

While the above description provides ample information to enable one skilled in the art to carry out the invention, examples of preferred methods will be described in detail without limitation of the scope of the invention.

EXAMPLES ACCORDING TO THE INVENTION AND COMPARATIVE EXPERIMENTS MATERIALS

In the Examples and Comparative Experiments, a styrene-butadiene-rubber (SBR) was used, in particular SBR 1723, an oil extended emulsion-polymerized SBR containing 37.5 phr of Treated Distillate Aromatic Extract (TDAE) oil, obtained from Dow Chemical, Germany. The polymer contained 23.5 wt% styrene and 76.5 wt% butadiene, and its Mooney viscosity ML(l+4) measured at 100°C was 40 MU. Zinc oxide (ZnO) and stearic acid were obtained from Flexsys, the Netherlands. The curatives: sulfur and N-tert-butyl-2-benzothiazylsulfenamide (TBBS) were obtained from Merck. The solvents, acetone and tetrahydrofuran (THF), which were used for extractions, and toluene, which was used for equilibrium swelling measurements, were obtained from Biosolve. TDAE oil used as devulcanizing processing oil was supplied by Hansen&Rosenthal, Germany. Diphenyldisulfide (DPDS) used as devulcanization aid was obtained from Sigma- Aldrich, Germany. The three types of oxidation stabilizers used were obtained from Ciba Specialty Chemicals Inc., Switzerland. Chemical names and structures of the stabilizers are given in Table I.

PREPARATION OF DE- VULCANIZED SBR

Mixing and vulcanization - The SBR was first compounded using a Brabender

Plasticorder 350S mixer with a mixing chamber volume of 350 cm ³ . The compounding formulation was as shown in Table II. The mixer was operated at a rotor speed of 60 rpm; a fill factor of 0.75 and an initial temperature of 50°C were used. The final compound temperature before dumping was in the range of 70-90 °C. The compound was tested for its cure characteristics using a RPA 2000 dynamic mechanical curemeter from Alpha Technologies at 170 °C, 0.833 Hz and 0.2 degree strain, according to ISO 6502. The compounds were then vulcanized for t _C;90 + 5 minutes in a Wickert WLP1600 laboratory compression molding press at 170 °C and 100 bar, into 2 mm thick sheets.

Grinding.- The vulcanized SBR sheets were subsequently ground in a Universal Cutting Mill Pulverisette 19 (Fritsch, Germany) with a 2 mm screen. The particle size of the ground rubber was in the range of 0.85-2.00 mm.

Devulcanization. - The thermo-chemical devulcanization was performed in a batch process in an internal mixer Brabender Plasticorder PL-2000, having a chamber volume of 50 ml and a cam-type rotor. A fill factor of 0.7 and a constant rotor speed of 50 rpm were used. The devulcanization temperature was varied from 180 to 300 °C and the devulcanization time was 5 minutes. The variations of the experimental conditions used are given in Table III.

CHARACTERIZATION OF THE DE VULC ANIZ ATE S

Rubber soluble fraction. - The soluble (Sol) and insoluble (Gel) fractions of the reclaimed materials were determined by extraction in a Soxhlet apparatus. The vulcanized and de- vulcanized SBR samples were extracted initially for 48 hrs in acetone in order to remove low molecular polar substances like remains of accelerators and curatives, followed by an extraction for 72 hrs in tetrahydrofuran (THF) to remove the apolar components: oil and non-cross-linked polymer residues or soluble polymer released from the network by the devulcanization process. The extraction was followed by drying the samples in a vacuum oven at 40°C and determining the weight loss until constant weight. The sol fraction was defined as the sum of the soluble fractions in acetone and THF. Correction for the oil contained in the original SBR was made. The gel fraction was calculated by the following equation: weight of rubber dissolved in solvents

Gel fraction 1 - (1) weight of pure rubber in the compound

Crosslink density.- The extracted SBR samples were subsequently swollen in toluene for 72 hrs at room temperature. The weight of the swollen vulcanizates was measured after removal of surface liquid with absorption paper. The crosslink density was calculated according to the Flory-Rehner equations (2) and (3):

Vr + %V _r ² + ln(l- Vr)

V _e = (2) with

m _r

Vr =

where: v _e = crosslink density per unit volume;

v _r = polymer volume fraction of the swollen sample;

V _s = solvent molar volume;

m _r = mass of the rubber network;

m _s = weight of solvent in the sample at equilibrium swelling;

p _r = density of the rubber;

p _s = density of the solvent;

χ = Flory-Huggins polymer-solvent interaction parameter.

RESULTS AND DISCUSSION The sol fractions and crosslink densities of the remaining gel as a function of the devulcanization temperature of SBR devulcanizates in presence of DPDS are depicted in figures 1 and 2, respectively. Basically, the increase of the rubber sol fraction and decrease of crosslink density indicate the extent to which the rubber network is broken. Thermo-chemical devulcanization of sulfur-cured SBR using DPDS as devulcanization aid shows an increase of rubber soluble fraction with increasing devulcanization temperature up to 220°C; above this temperature, the sol fractions decrease again. Furthermore, it can be seen in Figure 2 that above this temperature of 220°C a significant increase in crosslink density is observed again. Basically, the DPDS devulcanization agent appears to scavenge radicals formed during the reclaiming process. However, at high devulcanization temperature, i.e. above 220°C, a more extensive generation of reactive radicals occurs. These lead to formation of new inter- and intramolecular bonds resulting in a decrease of the rubber sol fraction and renewed increase in crosslink density. The experimentally determined sol fractions of DPDS de-vulcanized SBR at various devulcanization temperatures as a function of the relative decrease in crosslink density are shown in figure 3. Also shown are theoretical curves (solid and dashed line), which have been generated by the method developed by M.M. Horikx, J.Polym.Sci., 19, 1956, 335, in which the rubber sol fraction of the devulcanizates and the crosslink density of the rubber gel fractions are correlated. Horikx in particular derived a theoretical relationship between the soluble fraction generated after degradation of a polymer network and the relative decrease in crosslink density, as a result of either main-chain scission or crosslink breakage. This treatment of polymer degradation can equally well be applied to rubber reclaiming, where also a mix of main-chain scission and crosslink breakage takes place. When main-chain scission takes place, the relative decrease in crosslink density is given by:

1 - si

= 1 - (4)

1 - v where Si is the soluble fraction of the rubber network before degradation or reclaiming, Sf is the soluble fraction of the reclaimed vulcanizate, Vi is the crosslink density of the network prior to treatment and Vf is the crosslink density of the reclaimed vulcanizate. For pure crosslink scission, the soluble fraction is related to the relative decrease in crosslink density by:

where the parameters f and γι are the average number of cross-links per chain in the insoluble network after and before reclamation, respectively. The values for jf and ji have been determined as described in "M. A.L. Verbruggen, L. van der Does, J.W.M., Noordermeer, M. van Duin and H.J. Manuel, Rubber Chem. Technol, 72, 1999, 731." The solid curve in figure 3 corresponds to the theoretical situation where only random scission occurs while the dashed curve corresponds to the theoretical situation where only cross-links are broken. In the case of crosslink scission only, almost no sol is generated until most of the cross-links are broken; only then the long chains can be removed from the network. In the case of main-chain scission, sol is produced at a much earlier stage, because random scission of the polymers in the network results in small loose chains, which can easily be removed.

As further shown in figure 1, an increase of the devulcanization temperature to 220 °C results in a shift of the data point to the right hand side of the graph, which indicates a small increase in sol fraction and a large decrease of crosslink density. Nevertheless, a further increase of the devulcanization temperature to 260°C results in a shift of the experimental data points to the left hand side of the graph, which is contrary to the expected decrease of cross-link density. This reversion phenomenon is even more pronounced for devulcanization temperatures up to 300°C and beyond; for this temperature the data point is even found at the utmost left hand side of the graph. This may indicate a less efficient devulcanization above temperatures of 220°C, in which the crosslink density of the de-vulcanized rubber is increased rather than decreased with increasing treatment temperature.

Replacing DPDS solely with oxidation stabilizers in an attempt to improve the devulcanization efficiency at high devulcanization temperatures above 220°C does not significantly improve the efficiency of devulcanization, as can be derived from figures 2 and 3. On the contrary, replacement of DPDS by the indicated oxidation stabilizers, shows lower soluble fractions and higher crosslink densities than obtained by using only DPDS. This indicates a more inefficient devulcanization, as can also be seen in figure 1 where the corresponding data points are shifted to the left hand side of the graph with respect to the data points of the DPDS system.

Particularly preferred embodiments of the method of the invention use combinations of DPDS and oxidation stabilizers as devulcanization aids. A more efficient

devulcanization is obtained for these embodiments. As is apparent from figure 4, addition of the indicated stabilizers results in a smaller decrease of rubber sol fraction with increasing the devulcanization temperature above 220°C compared to the devulcanizates in which only DPDS is used. Furthermore, the increase of crosslink density with increasing devulcanization temperature above 220°C is absent with addition of oxidation stabilizers, as can be seen in figure 5. The positive effect of using the combination of these two chemical species as devulcanization aid is further clearly seen in figure 6. The experimental data for treatment above 220°C are situated at even higher position than the data for the devulcanizate treated at 220°C with only DPDS. The reversion phenomenon is noticeably absent in this case as no shift of the data points to the left hand side of the graph with increase of devulcanization temperature up to 300°C and beyond is noted. Therefore, utilizing the developed synergism of

devulcanization aid with oxidation stabilizers results in a more efficient and controlled devulcanization.

The DPDS used in accordance with the invention proves to be an effective

devulcanization aid, at least for devulcanization temperatures up to 220°C. A

mechanism proposed for the reaction of a radical scavenger like DPDS with a sulfur vulcanizate is the opening of cross-links or the scission of polymer chains by heat and shearing forces during mixing at the devulcanization temperature, and the reaction of fragments with disulfide based radicals, which prevent recombination. A simplified reaction scheme proposed for the rubber devulcanization with DPDS is given in figure 7. Based on the results, it is to be concluded that DPDS is an effective devulcanization aid, at least up to a devulcanization temperature of 220°C. Above this threshold temperature, devulcanization seems less efficient. This may tentatively be attributed to an excessive and uncontrolled generation of reactive radicals such that degradation of the rubber main chains is more likely to occur at the higher devulcanization

temperatures.

The devulcanization efficiency of known methods of devulcanizing a rubber vulcanizate is affected by uncontrolled degradation and oxidation. The method according to the invention in which a rubber vulcanizate is treated with an organosulfur compound to a temperature sufficient to cause the organosulfur compound to react with the sulfur of the sulfur-containing cross-links and to substantially prevent abstraction of hydrogen atoms from the rubber vulcanizate main chains offers improved devulcanizing efficiency. Embodiments wherein an organosulfur compound such as DPDS is used as devulcanization aid show a decrease of the crosslink density in first instance but an increase of the devulcanization temperature results in an increase of crosslink density again above a temperature threshold which depends on the specific rubber vulcanizate used, but for SBR is 220°C. Embodiments using a combination of an organosulfur compound and one or more oxidation stabilizers results in a more efficient devulcanization, especially at high devulcanization temperatures. A decrease in crosslink density without creating more sol fraction is provided. It appears that the organosulfur compound scavenges formed reactive radicals, preventing recombination of the rubber network, while the stabilizers appear to suppress the reaction of oxygen, which accelerates the degradation of the polymers and recombination into new cross-links. A synergistic effect is also observed which results in an increasingly efficient devulcanization of a rubber vulcanizate.