COMPOSITE MATERIAL, PROCESS FOR PREPARING THE COMPOSITE MATERIAL, AND USE THEREOF

The invention relates to composite materials comprising rubber and a sulfur- containing vulcanizing agent.

Rubber compositions generally contain activators which speed up sulfur cure. Zinc oxide, optionally in combination with stearic acid, is the most widely used activator. Lead and magnesium oxides are also used, but less often. However, there is a concern about the release of eco-toxic zinc species into the environment. Release of zinc into the environment from rubber occurs during production, during disposal and recycling of rubber products, and through wear of tires. Hence, there is an interest in potential substitutes. However, up to now, no viable alternative has been found to eliminate ZnO completely from rubber compounds, without significantly jeopardizing processing and performance characteristics.

It is therefore an object of the present invention to provide a substitute for ZnO. It is a further object of the present invention to provide a composite material comprising rubber with improved physical properties, reduced air permeability, greater flexibility, and a reduced amount of conventional activators, such as ZnO. It is also an object of the present invention to provide a composite that is free of Zn.

This object is achieved with a composite material according to the present invention, which comprises rubber, a sulfur-containing vulcanizing agent, a layered double hydroxide comprising hydroxyl and/or an organic anion as charge-balancing anion(s), and less than 1.5 phr (per hundred rubber) ZnO.

It has been found that said layered double hydroxide (LHD) can be a suitable substitute of ZnO in sulfur-curable rubber composites.

And although US 2003/0158319 already disclosed rubber composites comprising layered double hydroxides, the sulfur-curable composites that it discloses all contain a calcined form of layered double hydroxide, i.e. a so- called solid solution which is not layered any longer, and a significant amount of ZnO (1.82 phr). The recognition that the ZnO content could be significantly reduced when using an LDH in sulfur-curable compositions is not disclosed or recognized in this document.

The composite material of the present invention can be converted into cured composite materials by vulcanization. These cured composite materials have good heat stability, dimensional stability, tear strength, scratch resistance, flame resistance, and/or strength-to-weight ratios. The material furthermore reveals a low permeability towards gases and/or liquids, such as nitrogen, carbon dioxide, oxygen, water vapour, and hydrocarbons. The LDH present in the composite material of the invention may further adsorb and/or absorb additives or byproducts of initiators used in the polymerization of the polymer. Additionally, the cured composite material of the invention exhibits improved elongation at break and strength at break compared to neat rubber material that does not comprise an LDH . Furthermore, the composite material exhibits good dynamic properties (e.g. a low tan delta) during deformation at constant force, thus showing good viscoelastic properties, allowing the production of tyres with a low heat build-up and rolling resistance. The term "tan delta" is known to a skilled person, and is defined as the ratio of the loss modulus (G') to the storage modulus (G").

The layered double hydroxide may partly or completely replace conventionally used activators, such as zinc oxide, in rubber compositions. Hence, it allows the preparation of rubber composite materials containing less than 1.5 phr ZnO, preferably less than 1.0 phr ZnO, more preferably less than 0.5 phr ZnO, and most preferably no ZnO at all. In an even more preferred embodiment, the composite material according to the invention does not contain any Zn species.

Further, the composite material may contain less than 1.5 phr of any activator, or, more preferably, contain no activator at all. Examples of conventional activators are - apart from zinc oxide - zinc stearate, zinc laurate, zinc 2- ethylhexoate, lead oxides such as PbO and Pb ₃ O ₄ , cadmium oxides, and mixtures thereof. Further details can be found in Chapter 3, "Activators and Retarders" of Rubber Chemicals, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., online posting date: December 4, 2000.

Use of the layered double hydroxides of the invention may further enable a reduction of the amount of accelerators conventionally used in vulcanization processes of rubber.

The sulfur-containing vulcanization agent used in the composite material of the invention can be any sulfur-containing vulcanization agent known in the art, such as the ones described in Chapter 1 , "Accelerators of Vulcanization" and in Chapter 2, "Cross-linking agents" of Rubber Chemicals, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., online posting date: December 4, 2000. Suitable sulfur-containing vulcanization agents include both accelerators and cross-linking agents. Examples of suitable sulfur- containing accelerators are thiazoles including benzothiazoles, sulfenamides, carbamates including dithiocarbamates, thiuram sulfides, guanidines, thioureas, xanthates, thiazolidine thiols, thazinthiones, dithiophosphonates, and metal derivatives thereof. Also combinations of sulfur-free accelerators with sulfur- containing accelerators and/or sulfur-containing cross-linking agents are envisaged. Examples of sulfur-free accelerators are amines including hexamethylene tetraamine, mono- and dibenzyl amine, N-ethylcyclohexyl amine, dehydrogenated tallow amine, amine isophthalate, and anilines and combinations of aldehyde and amine such as butyraldehyde-aniline, butyraldehyde-monobutylamine, and heptaldehyde-aniline, and quaternary ammonium compounds including dimethylammonium hydrogen isophthalate. Further details of these accelerators can be found in Chapter 1 , "Accelerators of

Vulcanization" of Rubber Chemicals, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., online posting date: December 4, 2000. Examples of suitable sulfur-containing cross-linking agents are sulfur, insoluble sulfur, sulfur donors including 4,4'-dithiodimorpholine and N,N'-caprolactam disulfide, multifunctional sulfur-containing compounds including alkylphenol disulfides, 2,5-dimercapto-1 ,3,4-thiadiazole derivatives, and hexamethylene- 1 ,6-bisthiosulfate. Also combinations of sulfur-free cross-linking agents with sulfur-containing cross-linking agents and/or sulfur-containing accelerators are envisaged. Examples of sulfur-free cross-linking agents are peroxides, quinone dioximes, phenol-formaldehyde reaction products, diamines, methylene dianiline, triethylene tetraamine, and 1 ,3-bis(citraconimidomethyl) benzene. Further details of these cross-linking agents can be found in Chapter 2, "Cross- linking agents" of Rubber Chemicals, Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., online posting date: December 4, 2000.

The layered double hydroxide present in the composite according to the present invention has a layered structure corresponding to the general formula:

wherein M ²⁺ is a divalent metal ion such as Zn ²⁺ , Mn ²⁺ , Ni ²⁺ , Co ²⁺ , Fe ²⁺ , Cu ²⁺ , Sn ²⁺ , Ba ²⁺ , Ca ²⁺ , and Mg ²⁺ , M ³⁺ is a trivalent metal ion such as Al ³⁺ , Cr ³⁺ , Fe ³⁺ , Co ³⁺ , Mn ³⁺ , Ni ³⁺ , Ce ³⁺ , and Ga ³⁺ , m and n have a value such that m/n = 1 to 10, and b has a value in the range of from 0 to 10. X ^" is a charge-balancing anion. This charge-balancing anion is selected from hydroxyl, organic anions, and combinations of (i) two or more of these anions or (ii) one or more of these anions with other anions such as carbonate.

As the LDH typically has a layered structure, the charge-balancing ions may be situated in the interlayer, on the edge or on the outer surface of the stacked LDH layers. Such ions situated in the interlayer of stacked LDH layers are

referred to as intercalating ions. LDHs containing charge-balancing organic ions are rendered organophilic and are also referred to as "organoclays".

Stacked LDH or organoclay may also be delaminated or exfoliated, e.g. in a polymeric matrix. Within the context of the present specification the term "delamination" is defined as reduction of the mean stacking degree of the LDH particles by at least partial de-layering of the LDH structure, thereby yielding a material containing significantly more individual LDH sheets per volume. The term "exfoliation" is defined as complete delamination, i.e. disappearance of periodicity in the direction perpendicular to the LDH sheets, leading to a random dispersion of individual layers in a medium, thereby leaving no stacking order at all.

Swelling or expansion of the LDHs, also called intercalation of the LDHs, can be observed with X-ray diffraction (XRD), because the position of the basal reflections - i.e. the 6(001) reflections - is indicative of the distance between the layers, which distance increases upon intercalation. Reduction of the mean stacking degree can be observed as a broadening, up to disappearance, of the XRD reflections or by an increasing asymmetry of the basal reflections (001). Characterization of complete delamination, i.e. exfoliation, remains an analytical challenge, but may in general be concluded from the complete disappearance of non-(hkθ) reflections from the original LDH. The ordering of the layers and, hence, the extent of delamination, can further be visualized with transmission electron microscopy (TEM).

In one embodiment of the invention, the LDH comprises hydroxyl as charge- balancing anion. This LDH can have only hydroxyl as charge-balancing anion, or a combination of hydroxyl with at least one inorganic and/or organic anion as indicated above. The presence of hydroxyl as charge-balancing anion allows for improved vulcanization properties of the LDH compared to other LDHs. Generally, at least 10 mol% of the total amount of charge-balancing ions in the LDH present in the composite material according to the invention is hydroxyl, preferably at least 30 mol%, more preferably at least 60 mol%, and most

preferably at least 90 mol% of the total amount of charge-balancing ions is hydroxyl. In a further embodiment, the LDH comprises a mixture of hydroxyl and carbonate, or a mixture of hydroxyl and an organic anion. The amount of charge-balancing hydroxyl anions in an LDH containing both hydroxyl and carbonate anions can be determined by treating the LDH in boiling water under nitrogen flow, adsorbing the expelled carbon dioxide in a solution of acetone and barium chloride, titrating the resulting solution with sodium hydroxide, and using the titration result to calculate the carbonate contribution - and, therewith, the hydroxyl contribution - to the total amount of charge-balancing anions in the LDH.

The amount of charge-balancing hydroxyl anions in an LDH containing also organic charge-balancing anions can be calculated from the hydroxyl concentration of the starting LDH and the amount of organic anions added thereto.

An LDH comprising both hydroxyl and carbonate as charge-balancing anion can be prepared, for instance, by drying of an aqueous slurry of an LDH comprising hydroxyl in the presence of air or carbon dioxide.

The LDH may be a hydrotalcite-like material, meixnerite, manasseite, pyroaurite, sjόgrenite, stichtite, barberonite, takovite, reevesite, or desautelsite. A preferred LDH has a layered structure corresponding to the general formula:

wherein m and n have a value such that m/n = 1 to 10, preferably 1 to 6, and b has a value in the range of from 0 to 10, generally a value of 2 to 6. X ^" is a charge-balancing ion as defined above. It is preferred that m/n should have a value of 2 to 4, more particularly a value close to 3.

The LDH may be in any crystal form known in the art, such as described by Cavani et al. (Catalysis Today, 11 (1991 ), pp. 173-301 ) or by Bookin et al. {LDHs and LDH Minerals, (1993), Vol. 41 (5), pp. 558-564), and any polytype, such as 3Hi, 3H ₂ , 3Ri or 3R ₂ stacking.

In another embodiment of the invention, the LDH comprises one or more organic anions as charge-balancing anion and thus are organoclays. This/these charge-balancing organic anion(s) can be any organic anion known in the art. The organic anion generally has at least 2 carbon atoms, preferably at least 6 carbon atoms, even more preferably at least 8 carbon atoms, and most preferably at least 10 carbon atoms, and generally at most 1 ,000 carbon atoms, preferably at most 500 carbon atoms, and most preferably at most 100 carbon atoms. The distance between the individual LDH layers in such an organoclay is generally larger than the distance between the layers of a conventional LDH that does not contain organic anions. Preferably, the distance between the layers in the LDH present in the composite according to the invention is at least 1.0 nm, more preferably at least 1.5 nm, and most preferably at least 2 nm. The distance between the individual layers can be determined using X-ray diffraction.

Suitable organic anions include mono-, di- or polycarboxylates, sulfonates, phosphonates, nitrates, borates, phosphates, thiols, malonates, 1 ,3-diketones, and sulfates. It is envisaged to use two or more organic anions. Preferred examples of organic anions of the invention are monocarboxylic acids such as fatty acids and rosin-based ions. Particularly preferred organic anions are fatty acids having from 8 to 22 carbon atoms. Suitable examples of such fatty acids are caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, decenoic acid, palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and mixtures thereof. Another preferred organic anion is rosin. Rosin is derived from natural sources, is readily available, and is relatively cheap compared to synthetic organic anions. Typical examples of natural sources of rosin are gum rosin, wood rosin,

and tall oil rosins. Rosin commonly is a mixture of a wide variety of different isomers of monocarboxylic tricyclic rosin acids usually containing about 20 carbon atoms. The tricyclic structures of the various rosin acids differ mainly in the position of the double bonds. Typically, rosin is a mixture of substances comprising levopimaric acid, neoabietic acid, palustric acid, abietic acid, dehydroabietic acid, seco-dehydroabietic acid, tetrahydroabietic acid, dihydroabietic acid, pimaric acid, and isopimahc acid. Rosin derived from natural sources also includes rosins, i.e. rosin mixtures, modified notably by polymerization, isomerization, disproportionation, hydrogenation, and Diels- Alder reactions with acrylic acid, anhydrides, and acrylic acid esters. The products obtained by these processes are referred to as modified rosins. Natural rosin may also be chemically altered by any process known in the art, such as for example reaction of the carboxyl group on the rosin with metal oxides, metal hydroxides or salts to form rosin soaps or salts (so-called resinates). Such chemically altered rosins are referred to as rosin derivatives. Such rosin can be modified or chemically altered by introducing an organic group, an anionic group or a cationic group. The organic group may be a substituted or unsubstituted aliphatic or aromatic hydrocarbon having 1 to 40 carbon atoms. The anionic group may be any anionic group known to the man skilled in the art, such as a carboxylate or a sulfonate.

Further details of these rosin-based materials can be gleaned from D. F. Zinkel and J. Russell (in Naval Stores, production-chemistry-utilization, 1989, New York, Section II, Chapter 9), and J. B. Class ("Resins, Natural," Chapter 1 : "Rosin and Modified Rosins," Kirk-Othmer Encyclopedia of Chemical Technology, online posting date: December 4, 2000).

In one embodiment of the invention, the charge-balancing organic ion comprises a first functional group and a second functional group. The first functional group is an anionic group capable of interacting with the LDH. Examples of such first functional groups are carboxylate, sulfate, sulfonate, nitrate, borate, phosphate, and phosphonate. The second functional group is capable of forming a chemical link with the rubber, optionally in conjunction with

the sulfur-containing vulcanization agent of the invention. Examples of such second functional groups are acrylate, methacrylate, hydroxyl, chloride, bromide, amine, epoxy, thiol, vinyl, di- and polysulfides, carbamate, ammonium, sulfonic, sulfinic, sulfonium, phosphonium, phosphinic, isocyanate, hydride, imide, nitrosobenzyl, dinitrosobenzyl, phenol, acetoxy, and anhydride. Suitable examples of organic anions in accordance with this embodiment include 8- amino octanoic acid, 12-amino dodecanoic acid, 3-(acryloyloxy) propanoic acid, 4-vinyl benzoic acid, 8-(3-octyl-2-axiranyl) octanoic acid, and unsaturated fatty acids such as oleic acid and unsaturated tallow acid.

The composite material can further comprise a silane coupling agent. In a preferred embodiment, the LDH is modified with this coupling agent. The silane coupling agent preferably has at least one alkoxysilane group and at least one reactive group, the alkoxysilane group being chemically linked to the layered double hydroxide, the reactive group being chemically linked to the rubber. The LDH can be modified during or after the synthesis of the LDH, either in the presence of or without the rubber. The reactive group may be the same group as the second functional groups defined above. Examples of such silane coupling agents are bis(3-triethoxysilylpropyl) tetrasulfide (Si69 ^® ex Degussa), bis(3-triethoxysilylpropyl) disulfide, gamma-mercaptopropyl trimethoxysilane (SiSiB ^® PC2300 ex PCC), and 3-octanoylthio-i -propyltriethoxysilane (NXT™ ex GE).

An advantage of the presence of this silane coupling agent is that during vulcanization a chemical link may be formed between the layered double hydroxide and the rubber, which results in an improvement in dynamical and mechanical properties.

The amount of silane coupling agent is such that at least part of the LDH is chemically linked to the rubber in the composite material. If the rubber is a rubber other than a silicone rubber, the amount of silane coupling agent generally is at least 0.5 wt%, preferably at least 1 wt%, and most preferably at least 5 wt%, based on the total weight of the LDH, and the amount of silane

coupling agent generally is at most 50 wt%, preferably at most 40 wt%, and most preferably at most 30 wt%, based on the total weight of the LDH. If the rubber is a silicone rubber, the amount of silane coupling agent generally is at least 10 wt%, preferably at least 20 wt%, and most preferably at least 30 wt%, based on the total weight of the LDH, and the amount of silane coupling agent generally is at most 99 wt%, preferably at most 90 wt%, and most preferably at most 80 wt%, based on the total weight of the LDH. In one embodiment of the invention, the LDH comprises both a silane coupling agent and one or more charge-balancing organic anions, in particular organic anions comprising a first and a second functional group as defined above.

The amount of LDH in the composite material of the invention is preferably 0.01 -75 wt%, more preferably 0.05-60 wt%, and most preferably 0.1 -50 wt%, based on the total weight of the composite material. It is envisaged to use a combination of two or more LDHs in these composite materials.

Examples of rubbers to be present in the composite material according to the invention include natural rubber (NR), styrene-butadiene rubber (SBR) polyisoprene (IR), polybutadiene or butyl rubber (BR), polyisobutylene (MR), halogenated polybutadiene rubber, halogenated polyisobutylene rubber, nitrile butadiene rubber (NBR), hydrogenated nitrile butadiene rubber, styrene- isoprene-styrene (SIS) and similar (hydrogenated) styrenic block copolymers (SBS, hydrogenated SIS, hydrogenated SBS), poly(epichlorohydrin) rubbers (CO, ECO, GPO), silicone rubbers (Q), chloroprene rubber (CR), ethylene propylene rubber (EPM), ethylene propylene diene rubber (EPDM), fluorine rubbers (FKM), ethylene-vinylacetate rubber (EVA), vinyl butadiene rubber, halogenated butyl rubber, polyacrylic rubbers (ACM), polynorbornene (PNR), polyurethanes, and polyester/ether thermoplastic elastomers. Preferred rubbers are natural rubber, SBR, EPDM, halogenated butyl rubber, and butadiene rubber.

The composite material according to the present invention may further comprise additives commonly used in the art. Examples of such additives are pigments, dyes, anti-reversion agents, peptizers, UV-stabilizers, heat-stabilizers, antioxidants, fillers (such as hydroxyapatite, silica, silane coupling agents, compatibilizers, oil, waxes, carbon black, glass fibres, polymer fibres, non- intercalated clays, and other inorganic materials), flame retardants, plasticizers, rheology modifiers, cross-linking agents, and degassing agents. A further commonly used additive is extender oil. It is also contemplated to mix the modified LDH with the extender oil before adding this mixture to the rubber. This has the advantage that the LDH is dispersed in the oil and can be easily and more uniformly mixed into the rubber. These optional addenda and their corresponding amounts can be chosen according to need.

The composite material according to the present invention may also comprise a vulcanization rate reducer or retarder in order to tune the vulcanization rate. Examples of such vulcanization rate reducers are anhydrides such as maleic anhydride, alkenyl succinic anhydride, and phthalic anhydride, acids such as benzoic acid and salicylic acid, tetra-iso-butyl thiuram monosulfide, and imides such as N-(cyclohexylthio) phthalimide. The required amount of these rate reducers generally depends on the desired vulcanization rate. Typically, the weight ratio of vulcanization rate reducer to layered double hydroxide is between 50:1 and 1 :50, preferably between 30:1 and 1 :30, and most preferably between 20:1 and 1 :20.

The composite material according to the present invention can be a so-called "nanocomposite material", i.e. a composite material wherein at least one component is inorganic in nature and has at least one dimension in the 0.1 to 100 nanometer range.

The invention further pertains to a masterbatch, i.e. a highly concentrated additive premix, comprising, based on the total weight of the masterbatch: (i) 30 and 90 wt%, preferably 25 and 80 wt% of a rubber or cured rubber,

(ii) 9 and 69 wt%, preferably 15 and 60 wt% of a layered double hydroxide comprising hydroxyl and/or an organic anion as charge-balancing anion(s), and (iii) 1 and 20 wt%, preferably 5 and 15 wt%of a sulfur-containing vulcanizing agent. This masterbatch preferably does not contain ZnO. More preferably, it does not contain any Zn at all, nor any other conventional activator. These masterbatches may comprise LDHs that are delaminated or exfoliated. However, if the LDH in such masterbatches is not completely delaminated, further delamination may be reached at a later stage, if so desired, when blending the masterbatch with rubber to obtain rubber-based composite materials. If the LDH comprises an organic anion having a first and a second functional group, at least part of the organic anions of the LDHs may be chemically linked to the rubber or rubber precursor through the second functional group.

The invention further pertains to a rubber-free mixture of layered double hydroxide defined above and a sulfur-containing vulcanizing agent. This mixture can be suitably used in processes for preparing rubber composite materials. In this way, the LDH and the sulfur-containing vulcanizing agent can be added to the rubber simultaneously.

Generally, the amount of LDH is at least 0.5 wt%, preferably at least 1 wt%, and most preferably at least 5 wt%, based on the total weight of the mixture, and the amount of LDH generally is at most 99 wt%, preferably at most 90 wt%, and most preferably at most 80 wt%, based on the total weight of the mixture.

The invention further pertains to a process for preparing the composite material according to the invention comprising the steps of: a1 ) contacting the layered double hydroxide, optionally mixed with a first solvent, with a rubber comprising one or more polymers and optionally a second solvent; or a2) contacting the layered double hydroxide, optionally mixed with a first solvent, with a composition comprising one or more monomers of a rubber

and optionally a second solvent, and polymerizing the monomers to form the rubber; b) contacting the sulfur-containing vulcanizing agent with the rubber or the monomers prior to, during, or after step a1 ) or step a2); c) optionally removing the first and/or second solvent during or after any one of steps a1 ), a2), or b).

The process of the invention comprises two alternative steps a1 ) and a2). In step a1 ) the LDH, optionally mixed with a first solvent, can be added to the rubber without a reaction taking place between the particulate material and the rubber. Alternatively, the addition of the LDH is carried out under such conditions that part of the anions reacts with the rubber, for instance through second functional groups. Upon curing of the composition resulting from step a1 ), the remaining anions that have not reacted with the rubber may become chemically linked to the rubber.

In step a2) the LDH, optionally mixed with the first solvent, is added to one or more monomers of the rubber, which monomers are subsequently polymerized. Depending on the polymerization conditions and the anion chosen, part of the organic anions may react with the monomers during polymerization thereof, for instance via second functional groups, causing the LDH to be chemically linked to the rubber. Alternatively, at least part of the anions may react with the rubber upon curing of the precursor in step b), causing the LDH to be chemically linked to the rubber. It is noted that exfoliation and/or delamination of the LDH may occur in any one of steps a1 ), a2), b), and c).

In one embodiment of the process of the invention, the LDH is added to the rubber while the rubber is kept at a temperature at which it is fluid. In this way, it is ensured that the LDH is easily mixed in the rubber, enabling a uniform distribution of LDH particles throughout the rubber within an even shorter time, rendering the process more attractive economically. The mixing and/or compounding steps can be performed in a batch process, e.g. in a Banburry

mixer, or in a two-roll mill, or in a continuous mode, e.g. in tube reactors, extruders such as (co-rotating) twin- or single-screw extruders or a Buss- kneader (reciprocating single screw extruder), and plow mixers. In the context of the present application the term "compounding" refers to the action of mixing together; in the case of an LDH containing an organic anion and/or a silane coupling agent the mixing may be performed under application of sufficient shear stress on the polymer-based mixture so as to convert at least part of the LDH particles of micrometer size to nanometer-sized particles. This shear stress can be applied by mixing the polymer-based mixture in, e.g., a Banburry mixer or in an extruder.

Instead of mixing the individual components (LDH and sulfur-conaining vulcalizing agent) with the rubber or its monomers, it is also possible to add these compounds to the rubber in combination; either by way of the rubber-free mixture or the masterbatch described above.

The LDH used in the process of the invention may have been reduced in size prior to step a1 ) or a2). The LDH may have a d50 value of less than 20 μm and a d90 value of less than 50 μm. Preferably, the d50 value is less than 15 μm and the d90 value is less than 40 μm, more preferably the d50 value is less than 10 μm and the d90 value is less than 30 μm, even more preferably the d50 value is less than 8 μm and the d90 value is less than 20 μm, and most preferably the d50 value is less than 6 μm and the d90 value is less than 10 μm. The particle size distribution can be determined using methods known to the man skilled in the art, e.g. using laser diffraction in accordance with DIN 13320. The use of LDHs having such a smaller particle size distribution enables good mixing of the LDH throughout the rubber as well as an easier exfoliation and/or delamination of the LDH. The desired particle size distribution can be obtained by any method known in the art for reducing the particle size of inorganic materials such as LDHs.

Examples of such methods are wet milling and dry milling. Alternatively, such LDHs can be produced during preparation, as is exemplified by WO 02/085787.

The first and the second solvent used in the process of the invention can be any solvents suitable for use in this process and are known to the man skilled in the art. Such first and/or second solvents may be the same or different and are preferably solvents compatible with the LDH as well as with the rubber, its monomer and/or the resulting (cured) rubber composite material. The first and/or second solvents include alcohols such as methanol, ethanol, isopropanol, and n-butanol; ketones such as methyl amyl ketone, methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; unsaturated acrylic esters such as butyl acrylate, methyl methacrylate, hexamethylene diacrylate, and thmethylol propane triacrylate; aromatic and non-aromatic hydrocarbons such as hexane, petroleum ether, toluene, and xylene; and ethers such as dibutyl ether, tetrahydrofuran (THF), and methyl tert-butyl ether (MTBE).

The composite material according to the invention can be used to prepare cured, i.e. vulcanized, rubber by a vulcanization step. This vulcanization can be performed before or after step c) of the process of the invention.

The composite material of the present invention and/or its cured form can be suitably applied in tyre manufacture, such as in car tyres, truck tyres, tractor tyres, off-the-road tyres, and aircraft tyres, in winter tyres, in latex products including gloves, condoms, balloons, catheters, latex thread, foam, carpet backings, and rubberized coir and hair, in footwear, in civil engineering products such as bridge bearings, rubber-metal-laminated bearings, in belting and hoses, in non-tyre automotive applications including engine mounts, rubber bearings, seals, grommets, washers, and boots, in wires and cables, and in pipe seals, medical closures, rollers, small solid tyres, mountings for domestic and commercial appliances, rubber balls and tubing, milking inflations and other agricultural-based applications.

If the rubber present in the composite material is a silicone rubber, the rubber composite matehalcan be suitably applied in coating compositions, pressure- sensitive adhesive compositions, plastic hardcoats, and paper release coating compositions, in fibre finishing applications including textile and hair care applications, sealants, adhesives, encapsulants, and solar cell units.

The composite material according to the invention can be used in any part of the tyre where an inorganic filler, such as carbon black or precipitated silica, is conventionally used. In particular, the rubber composition can be used in the undertread or tread base, the tread, the sidewall, the rim cushion, the inner layer, the carcass, the apex, the bead, and the belt layer. It is also envisaged to use a combination of the composite material of the invention and a conventional inorganic filler such as carbon black or precipitated silica. The use of the layered double hydroxide enables a reduction of the total amount of inorganic filler in the rubber composition, while maintaining similar or improved mechanical properties. The use of the composite material of the present invention in tyres may improve the mechanical and dynamical properties of the tyre; it may further enhance the bonding or adhesion between different rubbers, e.g. in different parts of the tyre, or between rubber and metal (e.g. in metal cords), or between rubber and fibres. The rubber used in the tread - usually solution SBR rubber - can be replaced by a cheaper rubber, e.g. emulsion SBR rubber, without loss of mechanical or dynamical properties of the tread. The LDH also causes the rubber to have an improved puncture resistance.

In a preferred embodiment, the layered double hydroxide is modified with a coupling agent comprising a vulcanizable group, or with an organic anion having a vulcanizable group. Such a coupling agent can be a silane coupling agent like bis(3-triethoxysilylpropyl) tetrasulfide (Si69 ^® ex Degussa), bis(3- triethoxysilylpropyl) disulfide, gamma-mercaptopropyl thmethoxysilane (SiSiB ^® PC2300 ex PCC), and 3-octanoylthio-1-propyltriethoxysilane (NXT™ ex GE). Examples of vulcanizable organic anions are 12-hydroxysteahc acid, 12-chloro-

stearic acid, 12-aminododecanoic acid, expoxidized fatty acids, mercapto- propionic acid, oleic acid, conjugated unsaturated fatty acids, dithiodipropionic acid, p-hydroxybenzoic acid, and maleimidopropionic acid. The advantage of these modified layered double hydroxides is that the time needed to produce a tyre, in particular a green tyre, can be reduced. Moreover, the dimensional stability of the uncured tyre as well as the final tyre will improve. In conventional processes the precipitated silica is added to the rubber together with a coupling agent such as bis(3-triethoxysilylpropyl) tetrasulfide, the rubber composition is allowed to react at elevated temperatures, the ethanol produced is removed, and an uncured tyre is obtained, which is then cured at a higher temperature to start vulcanization and to form the tyre. The use of these modified layered double hydroxides in the production of tyres has the advantage that the coupling agent is already attached to the particulate material and no ethanol is formed, rendering a reduction in processing time which may enhance the production rate of (green) tyres.

If a combination of the layered double hydroxide and a conventional filler such as precipitated silica is used, a coupling agent may be added to the mixture separately, so that it can react with the precipitated silica. The layered double hydroxide may be added to the rubber in the form of a (colloidal) suspension in a suitable solvent (containing no or hardly any water), or it may be added in an extender oil, or as solids. In case of an extender oil or solids, no solvent has to be removed, leading to a further reduction in processing time and to an improved process safety.

The composite material in accordance with the invention can also be usedin solar cell units. For this use it is preferred that the rubber in the composite material is transparent to visible light. Examples of such transparent rubbers are polyurethane, ethylene vinyl-acetate rubber and silicone rubber. Preferably, the transparent rubber is a silicone rubber. The solar cell unit can be any solar cell unit known in the art. Examples of such solar cell units are crystalline Si solar cells, amorphous silicon solar cells, crystalline silicon thin film solar cells, and compound semiconductor solar cells based on e.g. CdTe, CulnSe2, Cu(In,

Ga)(Se, S)2 (so called CIGS), and Gratzel cells. Further details can be gleaned from F. Pfisterer ("Photovoltaic Cells", Chapter 4: "Types of Photovoltaic Cells," Ullmann's Encyclopedia of Industrial Technology, online posting date: June 15, 2000). The composite materials used in solar cell units may serve to connect two juxtaposed layers in the unit. The advantage of the rubber composite and/or cured rubber composite of the present invention is its transparency to visible light, which enables application at a position where light travels through the rubber composition before it reaches the part of the cell where the light is converted into electrical energy. The rubber composition may also serve to connect the solar cell unit to a substrate, e.g. a plate or a roof tile. In such cases the rubber composition does not have to be transparent. Generally, the rubber composition exhibits improved mechanical properties over conventional rubber compositions. In one embodiment, the solar cell unit comprises a back electrode, a photovoltaic layer, a front electrode, and a transparent top layer wherein a layer of the composite material of the invention is present in between the front electrode and the transparent top layer. As indicated above, the rubber of the composite material preferably is a transparent rubber, and most preferably the rubber is a silicone rubber. The rubber composition serves as an adhesive or binding layer for the transparent top layer and the front electrode. Due to the aforementioned improved mechanical properties, the adhesive power and the tear strength of the composite material are increased and the solar cell unit (in use) is capable of better withstanding the weather effects or other mechanical forces to which it is to be exposed. Consequently, the lifetime of the solar cell unit is increased. Moreover, if the composite material of the invention is transparent to visible light, it brings about an improved light yield and solar energy recovery as compared to solar cell units comprising a rubber composition with particles having sizes in the range of or exceeding the visible light wavelengths, i.e. between 400 and 800 nm.

Solar cell units comprising a back electrode, a photovoltaic layer, a front electrode, and a transparent top layer are known to the man skilled in the art.

Generally the back electrode, a photovoltaic layer, a front electrode, and a transparent top layer are provided in layers one on top of the other. A more detailed description of such solar cell units can be found in EP 1 397 837 and EP 1 290 736, which specific descriptions of the back electrode, the photo- voltaic layer, the front electrode, and the transparent top layer are incorporated herein by reference.

EXAMPLES

In the experiments described below, a commercially available fatty acid blend (Kortacid® PH05) was used. The material was used as received.

In addition to fatty acids, stabilized rosin was used. The stabilized rosin was produced in-house by melting Chinese gum rosin and heating it to 235°C. During melting 3.5% Vultac ^® -2 (Arkema Inc.) by weight on rosin was added. The molten rosin was stirred at 235°C for 15 hours, after which the resin was cooled and ready for use.

Preparation of the layered double hydroxide

68.2 grams of magnesium oxide (Zolitho 40, ex Martin Marietta Magnesia Specialties LLC) and 43.6 grams of aluminium hydroxide (Alumill F505) were mixed in 840 grams of demineralized water and ground to an average particle size (d ₅ o) of 2.5 μm. The slurry was fed to an oil-heated autoclave equipped with a high-speed stirrer and heated to 80 ⁰ C. Then 132 grams of a 50/50 mixture by weight of Kortacid ^® PH05 and stabilized rosin as prepared above were added to the autoclave over a period of 15 minutes. Before addition, the acid mixture was heated to 80 ⁰ C. After the acid addition, the autoclave was closed and heated to 170°C and kept at that temperature for 1 hour. Then the autoclave was cooled to about 40 ⁰ C and the resulting slurry was removed. The slurry was then centhfuged at 2,000 rpm for about 10 minutes. The liquid was poured off and the solids were dried under vacuum in an oven overnight at 80°C.

20 mol% of the charge-balancing anions in the resulting LDH were hydroxyl anions.

Examples A and B

First, the LDH was mixed with natural rubber (SMR CV) in a 50/50 masterbatch on a two-roll mill at 40 ⁰ C. The rest of the mixing procedure, which was done in a 1.6 I Banbury with a load factor of 70%, was as follows:

- Mastication of natural rubber for 1 minute at a starting temperature of 60 ⁰ C.

- Addition of the 50/50 masterbatch and mixing for 5 minutes.

- Sweeping, 1 minute.

- Dumping, the final temperature of the mixture was 140°C maximum.

After cooling, the vulcanization ingredients were added on a two-roll mill. The final compositions are depicted in Table 1.

Table 1

amounts of the ingredients are given in parts per hundred rubber (phr)

Example A is a comparative example, Example B is in accordance with the invention. Rheometer charts of these samples were recorded at 150 ⁰ C for 30 minutes, with a Rheocord MDR2000E (arc 0.5° and a torque axis of 0.5 Nm) according to ISO 6502:1999. The results of these measurements show that the scorch time for the composite of Example A is much longer than for composite B, which is evidence for the activation and/or acceleration of the vulcanization process.

Examples C-G

First, the LDH was mixed with natural rubber (SMR CV) in a 50/50 masterbatch. The rest of the mixing procedure, which was done in a 1.6 I Banbury with a load factor of 70%, was as follows: - Mastication of natural rubber for 1 minute at a starting temperature of

60 ⁰ C.

- Addition of the 50/50 masterbatch and mixing for 5 minutes.

- Addition of half of the final amount of carbon black and all retarders, mixing for 2 minutes. - Addition of the remaining amount of carbon black and all other ingredients except the vulcanization ingredients, mixing for 1 minute.

- Sweeping, 1 minute.

- Dumping, the final temperature of the mixture was 140 ⁰ C maximum.

After cooling, the vulcanization ingredients were added on a two-roll mill. The final compositions are depicted in Table 2. Examples C, E, and G are not in accordance with the invention. Examples D and F are in accordance with the present invention.

Table 2

amounts of the ingredients are given in parts per hundred rubber (phr)

Examples H-K

The LDH was mixed with bromobutyl rubber (Bromobutyl X2) in a 50/50 masterbatch according to the procedure described above from Examples C-G. The final compositions are depicted in Table 3. Examples H and I are not in accordance with the invention. Examples J and K are in accordance with the present invention.

Table 3

amounts of the ingredients are given in parts per hundred rubber (phr)

The air permeability of the above-mentioned composites C-K was determined according to Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using a Coulometric Sensor, ASTM D3985. The crosslink density was determined from the Rheometer charts measured in accordance with ISO 6502:1999.

The results indicated that when ZnO is replaced by LDH, the crosslink density is reduced, leading to greater flexibility. And in contrast to what would be expected from lower crosslink densities, the air permeability of the rubber was also reduced. In other words, replacing ZnO by LDH results in greater flexibility and reduced air permeability of rubber.