Elastomeric compositions and compounds comprising a new secondary cross-linking system and tyres comprising them

Abstract

The present invention relates to elastomeric compositions and compounds for tyres, comprising a novel secondary cross-linking system comprising at least one methylene donor agent of formula (I) and at least one methylene acceptor agent, and tyres for vehicle wheels comprising them. The new secondary cross-linking system of the invention allows imparting the materials optimal stiffness, breaking strength and adhesion to the reinforcement elements, improving the cross-linking kinetics and the processability of the compound, while at the same time reducing or maintaining the hysteresis. These materials are advantageously used in those tyre components that require a certain rigidity and resistance to breakage and / or high adhesion to textile or metal reinforcement elements together with a low rolling resistance.

Description

[0001] ELASTOMERIC COMPOSITIONS AND COMPOUNDS COMPRISING A NEW SECONDARY CROSS-LINKING SYSTEM AND TYRES COMPRISING THEM

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates to a new secondary cross-linking system for elastomeric compounds comprising, in addition to a conventional methylene acceptor agent, at least one methylene donor agent with a bis-oxazolidine structure. The present invention also relates to elastomeric compositions for tyres comprising said secondary cross-linking system, to elastomeric compounds obtainable by cross-linking said compositions, to tyre components and to tyres for vehicle wheels comprising said elastomeric compounds.

[0005] Prior art

[0006] In the tyre sector, sulphur cross-linking (vulcanisation or primary cross-linking) is a process commonly used to improve the mechanical properties of rubbers.

[0007] In particular, sulphur cross-linking takes place following heating in the so-called vulcanisation step with the formation of sulphur-based bonds. Through vulcanisation, an elastic and not very swellable material is obtained.

[0008] Sulphur cross-linking influences the hardness, elasticity, hysteresis of the elastomeric material and, consequently, the properties and behaviour of the tyre that incorporates it.

[0009] Typically, with traditional sulphur vulcanisation systems, the vulcaniser and the vulcanisation additives are incorporated into the compound downstream of the production process, in controlled temperature steps, generally not exceeding 130 °C, and mixing for limited times.

[0010] However, the poor solubility of sulphur in elastomeric compounds together with the mild mixing conditions adopted for its incorporation mean that its dispersion is not always ideal. It follows that the final material, due to an uneven distribution of sulphur, may not have the desired properties, for example it may be characterised by a marked hysteresis, showing an increase in the heat dissipated under dynamic conditions. Increased hysteresis in tyre elastomer materials may be disadvantageous as it is associated with increased rolling resistance of the tyre in road use and, consequently, increased vehicle fuel consumption, in stark contrast to the current trend in the automotive industry to minimise fuel consumption as much as possible.

[0011] Over the years, various additives have been proposed to improve the cross-linking process, such as, for example, vulcanisation activators, accelerators and retarders. However, the primary cross-linking systems based on sulphur, despite the use of additives, do not always give satisfactory results. In fact, the primary lattice based on the sulphur bonds formed during the vulcanisation step is not always able to provide adequate reinforcement to the compound as these bonds are reversible and subject to breakage at high temperatures (reversion). As a consequence of this thermal instability, the mechanical properties of the tyre, particularly under stress, may deteriorate.

[0012] This problem has been tackled by introducing in the elastomeric compounds some thermally more stable secondary cross-linking systems, capable of compensating for the breaking of the sulphur bonds of the primary lattice and of imparting rigidity and resistance to materials. Increased stiffness and tear resistance may be particularly beneficial in certain tyre components that are typically subjected to load, such as beads or sidewall inserts of self-supporting tyres.

[0013] However, the use of secondary cross-linking systems may lead to processability problems of the compound due to an excessive increase in viscosity. Too high a viscosity may make it difficult to mix the components of the compound, as well as to process the compound itself; furthermore, it may an impair the uniform dispersion of the additives and hinder complete cross-linking, leading to defects in the final material, for example in terms of mechanical properties, and to increased costs and production times.

[0014] One of the secondary cross-linking systems conventionally used for this purpose includes phenols, such as resorcinol, and methylene donor agents, such as formaldehyde, which are able to react with the phenols to form a cross-link.

[0015] A cross-linking system, based on resorcinol and formaldehyde in latex, called the RFL system, is also widely used as an adhesive to make the reinforcement elements present in the reinforced structural elements of the tyres adhere to the rubber.

[0016] In tyres for vehicle wheels, the reinforced structural elements, comprising reinforcement elements, perform different functions which may be structural, containment, protection. One of the features to be checked to allow the integrity of the reinforced structural element is that the reinforcement elements adhere tenaciously to the elastomeric material that incorporates them to avoid tearing and disintegration of the composite.

[0017] The materials for the reinforcement elements of the layers of the carcass structure, belt structure, protection (chafer) or reinforcement (flipper) layers of the bead, may generally be, according to the positioning, the type of tyre and the use, metallic materials or non-metallic textile materials.

[0018] Typically, metal reinforcement elements are made using one or more carbon steel wires. Typically, the wires and / or cords are coated with a layer of brass to increase adhesion to the elastomeric compound and to protect the string from corrosion.

[0019] Depending on the carbon content and breaking strength, the following types of metal wire may be distinguished:

[0020] - NT (Normal Tensile Steel) steel wire having a breaking tensile strength of 2800 ± 200 MPa, for example having a breaking tensile strength of at least 2700 MPa for a wire diameter of 0.28 mm;

[0021] - HT (High Tensile Steel) steel wire having a breaking tensile strength of 3200 ± 200 MPa, for example a breaking tensile strength of at least 3100 MPa for a wire diameter of 0.28 mm;

[0022] - ST (Super Tensile Steel) steel wire having a breaking tensile strength of 3500 ± 200 MPa, for example a breaking tensile strength of at least 3400 MPa for a wire diameter of 0.28 mm;

[0023] - UT (Ultra Tensile Steel) steel wire having a breaking tensile strength of 3900 ± 200 MPa, for example a breaking tensile strength of at least 3800 MPa for a wire diameter of 0.28 mm.

[0024] The non-metallic materials most commonly used as constituents of reinforcement elements for tyres may be naturally derived polymeric fibres such as Rayon and Lyocell, or synthetic fibres such as aliphatic polyamides (nylon), polyesters and aromatic polyamides (generally known as aramids), these materials being selected according to the component in which they will be incorporated and the type of tyre (for two or four-wheeled vehicles, for heavy vehicles) and according to the required performance such as HP (high performance), UHP (very high performance), racing, on or off the road.

[0025] RFL-based adhesive compositions are applied to textile cords, generally by immersion. The cords thus treated can be incorporated into an elastomeric matrix to be subsequently assembled together with other semi-finished products in the building of a green tyre, which is subsequently shaped, subjected to moulding and vulcanisation. Typically, to further strengthen the adhesion of the reinforcement elements to the rubber, adhesion promoting additives may be introduced into the rubberising elastomeric compounds, comprising substantially the same or similar methylene donor and acceptor agents, such as hexamethylenetetramine and resorcinol. During the vulcanisation step, the cross-linking of these additives allows a higher adhesion to be obtained, more or less high according for example to the type of compound, promoter, fibre and its treatments.

[0026] The resorcinol - formaldehyde (RF) system is a very widespread and effective secondary cross-linking system and adhesive. However, at an industrial level, it is desirable to reduce the use of both resorcinol and formaldehyde to make the tyre component compounds that include them more sustainable.

[0027] In order to substantially reduce the use of free resorcinol at every step of tyre manufacturing, phenolic resins such as Alnovol have been used as methylene acceptors, which are generally more harmless since the phenolic component is at least partially already cross-linked.

[0028] In the case where the secondary cross-linking system includes these phenolic resins, the cross-linking that typically occurs during the vulcanisation step of the compound still requires the incorporation of at least one methylene donor, such as formaldehyde or, preferably, a more harmless donor selected from conventional formaldehyde precursors such as hexamethoxymethylmelamine (HMMM), hexamethylenetetramine (HMT) and the like.

[0029] However, the use of these phenolic resins in secondary cross-linking systems instead of resorcinol may result in slower cross-linking kinetics and, more importantly, higher material hysteresis.

[0030] Possible alternative methylene donors are already known from the literature, such as oxazolidine agents, used alone or together with formaldehyde or conventional formaldehyde precursor agents such as HMMM or HMT.

[0031] In this regard, some documents, for example EP2316881A1 , JP5448445B2, JP5317477B2 and JP2010150502A, generically mention among other methylene donors oxazolidines to be used in combination with phenolic resins in secondary cross-linking systems in elastomeric compounds for bead or sidewall inserts of self-supporting tyres. However, these documents generally do not show any specific oxazolidines nor provide experimental examples of them, preferring instead conventional formaldehyde precursor agents such as HMT and HMMM.

[0032] Patent US4361677 describes an elastomeric compound for tyres, in particular for bead filling, comprising, in addition to conventional elastomers, a particular thermosetting and vulcanising sulphurated phenolic resin, a thermosetting phenolic resin and a hardener for hardening such resins. The hardener may comprise, among others, a bis-oxazolidine agent of the formula (l-A) in mixture with another suitable methylene donor, preferably selected from HMT, polyfunctional derivatives of methylmelamine, oxazolidine and its derivatives, bis(1 ,3-oxazolidine) and its derivatives.

[0033] The document does not suggest the use of those compounds as rubberising compounds for metal or textile reinforcement elements, nor does it show any data on the hysteresis of the cross-linked elastomeric materials, on the trend of the cross-linking kinetics or on the thermal stability of the cross-linked compound. In the exemplified compounds, a conventional methylene donor (HMMT) is always present, alone or optionally mixed with the compound (l-A).

[0034] Other documents also mention bis-oxazolidine methylene donors, highlighting in particular the same bis-oxazolidine methylene donor mentioned above (l-A), substituted with CH2OH (methylol).

[0035] For example, US3256137A is directed to elastomeric compounds with improved adhesion to textile materials comprising a secondary cross-linking system consisting of at least one bis-oxazolidine methylene donor and at least one methylene acceptor. In the experimental part, cross-linking systems comprising mixtures of methylene donors are not exemplified. The document reports the performance of some compounds after cross-linking in the adhesion test on rayon and nylon cords but does not provide any indication on the cross-linking kinetics, processability or network stability (reversion) of the same, nor does it evaluate their static and dynamic mechanical properties.

[0036] Document GB1112007A deals with the adhesion of polyester textile materials to rubber and generally describes elastomeric compounds comprising a secondary cross-linking system consisting of at least one methylene donor, including a bis- oxazolidine structure, and at least one methylene acceptor. In the experimental part, cross-linking systems comprising mixtures of methylene donors are not exemplified. The document reports the performance of some compounds after cross-linking in the adhesion test on polyester ropes but does not provide data on cross-linking kinetics, processability, network stability (reversion) or hysteresis of the same.

[0037] Document US3969568A claims reinforced elastomeric compositions and compounds comprising a bis-oxazolidine methylene donor, a methylene acceptor, and discontinuous aramid fibres. In the experimental part, cross-linking systems comprising mixtures of methylene donors are not exemplified. The document reports the mechanical performances of some compounds comprising aramid, polyester, nylon, rayon or glass flakes after cross-linking but does not provide any indication on the cross-linking kinetics, processability, network stability (reversion) or their hysteresis.

[0038] Summary of the invention

[0039] In light of the prior art, the Applicant felt the need to provide more harmless secondary cross-linking systems, i.e. with a low content or preferably totally free of both resorcinol and formaldehyde, which, when incorporated into elastomeric compounds, would present a sufficiently fast cross-linking kinetics and would give a thermally stable network with little reversion, would be easily processable and able to give suitable rigidity and high breaking resistance to the materials, for use in particularly stressed tyre components, and at the same time would not increase or possibly reduce their hysteresis, in order to contain rolling resistance and therefore consumption. Furthermore, such secondary cross-linking systems ideally should also provide high adhesion of the elastomeric compounds to the reinforcement elements incorporated therein. Meeting all these needs and reconciling sometimes contrasting properties in a single material seemed very difficult.

[0040] The Applicant has undertaken studies to identify thermally stable, more sustainable secondary cross-linking systems, capable of imparting stiffness to elastomeric compounds of tyres, good processability and high adhesion to the reinforcement elements, with the aim of producing more performing tyres, structurally resistant even under stress conditions and possibly containing consumption.

[0041] The Applicant has surprisingly found that the above objects are achievable with a novel secondary cross-linking system comprising a particular class of methylene donor agents, which may partially or completely replace formaldehyde and conventional formaldehyde precursor agents (HMMM, HMT and the like), together with conventional methylene acceptor agents, such as resorcinol or, preferably, phenolic resins. In its studies the Applicant has found that this novel system is particularly effective as a secondary cross-linking agent for elastomers. The crosslinked elastomeric compounds may be used advantageously as constituent compounds of tyre components requiring stiffness and fatigue resistance (e.g. bead, sidewall insert, sidewall and the like) and / or high adhesion to reinforcement elements (e.g. carcass, belt, rubberising compounds of annular bead anchoring elements, bead protection layers - chafer - or reinforcement layers - flipper). The elastomeric compounds of the invention comprising the new cross-linking system are also better processable, have advantageous cross-linking kinetics, with a higher initial cross-linking speed and comparable or reduced reversion and, finally, have comparable or even lower hysteresis compared to known cross-linking systems.

[0042] A first aspect of the present invention therefore is a cross-linking composition for elastomeric compounds which comprises at least one methylene donor agent of formula (I). wherein R1represents H, linear or branched C1-C20alkyl, linear or branched C1- C20 alkenyl, said alkyl and alkenyl being optionally substituted in the chain with one or more oxygens, and at least one methylene acceptor agent.

[0043] A further aspect of the invention is an elastomeric composition comprising at least

[0044] - 100 phr of at least one diene polymer,

[0045] - at least 0.1 phr of at least one reinforcing filler,

[0046] - 0.1 to 20 phr of a vulcanising agent, and

[0047] - at least 0.05 phr of the cross-linking composition according to the invention. A further aspect of the present invention is a vulcanised elastomeric compound for tyre for vehicle wheel obtained by mixing and vulcanising the elastomeric composition according to the invention.

[0048] A further aspect of the present invention is a tyre for vehicle wheel comprising at least one tyre component comprising a vulcanised elastomeric compound according to the invention.

[0049] DEFINITIONS

[0050] The term “cross-linking system or composition” indicates a composition capable of transforming natural or synthetic rubber into elastic and resistant material due to the formation of a three-dimensional network of inter- and intra-molecular bonds.

[0051] The term “vulcanising agent” means a cross-linking agent preferably selected from sulphur-based cross-linking agents such as elemental sulphur, polymeric sulphur, sulphur donor agents such as bis[(trialkoxysilyl)propyl]polysulphides, thiurams, dithiodimorpholines and caprolactam-disulphide, peroxides, such as dialkyl peroxides R - 0 - 0 - R, wherein R is an alkyl group, alkyl-aryl peroxides R - 0 - 0 - R', wherein R is an alkyl group and R' aryl, diaryl peroxides R'- 0 - 0 - R', wherein R’ is an aryl group, diacylperoxides R-C(O)-O-O-(O)C-R', wherein R and R’ are aryl and / or alkyl groups, peroxyketal R-O-O(R)C(R’)-O-O-R’, wherein R and R’ are aryl and / or alkyl groups, peroxyesters R-C(O)-O-O-R’, wherein R and R' are aryl and / or alkyl groups, metal oxides, such as zinc oxide, quinones, resins and organic bases. The vulcanising agent is responsible for the primary crosslinking or vulcanisation of the elastomeric compound.

[0052] The term “methylene donor agent” refers to formaldehyde or an organic compound precursor of formaldehyde which, under usual vulcanisation conditions, may decompose at least partially, releasing formaldehyde in situ. The methylene donor agent is able to react with the methylene acceptor agent typically forming a lattice, remaining wholly or partially incorporated therein.

[0053] The term “conventional formaldehyde or methylene donor or conventional formaldehyde precursor agent” and similar agents are herein intended to indicate an organic compound capable of releasing formaldehyde in situ when heated, such as paraformaldehyde, hexamethylenetetramine (HMT), hexamethoxymethylmelamine (HMMM), hexamethylol melamine, N,N’- dimethylolurea, N-methylol dicyanamide, N-allyl dioxazine, N-phenyl dioxazine, N- methylol acetamide, N-methylol butyramide, N-methylol acrylamide, N-methylol succinimide, lauryloxymethylpyridinium chloride, ethoxymethylpyridinium chloride, trioxane hexamethoxymethylmelamine, hexamethylolmelamine pentamethyl ether (HMPE) and known oxazolidine derivatives other than those of formula (I).

[0054] The term “methylene acceptor agent” refers to an aromatic organic compound capable of reacting with a methylene donor agent through an aromatic electrophilic substitution reaction and formation of a lattice. Typical methylene acceptor agents are phenols and phenolic resins.

[0055] The term “elastomeric composition for tyre compound” means a composition comprising at least one diene polymer and one or more additives, which by mixing and typically heating provides an elastomeric compound suitable for use in a vehicle wheel tyre and their components.

[0056] The ingredients of said composition are not generally introduced simultaneously into the mixer but added in sequence. In particular, the vulcanisation additives, such as the vulcanising agent and optionally the accelerator and retarders, are usually added in a downstream step with respect to the incorporation and processing of all the other components.

[0057] In the elastomeric compound, the individual components of the composition may be altered or no longer individually traceable as modified, completely or in part, due to the interaction with the other components, of heat and / or mechanical processing.

[0058] The term “elastomeric compound” indicates the compound obtainable by mixing at least one diene polymer with at least one of the additives commonly used in the preparation of tyre compounds.

[0059] The term “vulcanisable elastomeric compound” refers to the compound obtainable by mixing at least one diene polymer with at least one vulcanising agent.

[0060] The term “vulcanised elastomeric compound” means the material obtainable by vulcanisation of a vulcanisable elastomeric compound.

[0061] The term “green” indicates a material, a compound, a composition, a component or a tyre not yet vulcanised.

[0062] The term “cross-linking” means the reaction of forming a three-dimensional lattice of inter- and intra-molecular bonds in a natural or synthetic rubber.

[0063] The term “vulcanisation” refers to the cross-linking reaction in a natural or synthetic rubber induced by a typically sulphur-based vulcanising agent. The term “vulcanisation accelerator” means a compound capable of decreasing the duration of the vulcanisation process and / or the operating temperature, such as TBBS, sulphenamides in general, thiazoles, dithiophosphates, dithiocarbamates, guanidines, as well as sulphur donors such as thiurams.

[0064] The term “vulcanisation activator” indicates a product capable of further facilitating the vulcanisation, making it happen in shorter times and possibly at lower temperatures. An example of activator is the stearic acid-zinc oxide system.

[0065] The term “vulcanisation retarder” means a product capable of delaying the onset of the vulcanisation reaction and / or suppressing undesired secondary reactions, for example N-(cyclohexylthio)phthalimide (CTP).

[0066] The term “vulcanisation package” is meant to indicate the vulcanising agent and one or more vulcanisation additives selected from among vulcanisation activators, accelerators and retarders.

[0067] The term “primary cross-linking system” or vulcanisation system refers to a crosslinking system in which the vulcanising agent is typically sulphur-based.

[0068] The term “secondary cross-linking system” is meant to indicate a cross-linking system, comprising at least one methylene donor agent and at least one methylene acceptor agent, additional to the primary cross-linking system.

[0069] The term “elastomeric polymer” indicates a natural or synthetic polymer which, after cross-linking, can be stretched repeatedly at room temperature to at least twice its original length and after removal of the tensile load substantially immediately returns with force to approximately its original length (according to the definitions of the ASTM D1566-11 Standard terminology relating to Rubber).

[0070] The term “diene polymer” indicates a polymer derived from the polymerisation of one or more monomers, of which at least one is a conjugated diene. The diene polymer may become an elastomeric polymer and acquire the characteristic properties following vulcanisation.

[0071] The term “reinforcing filler” is meant to refer to a reinforcing material typically used to improve the mechanical properties of tyre rubbers, preferably selected from among carbon black, conventional silica, such as silica from sand precipitated with strong acids, preferably amorphous, diatomaceous earth, calcium carbonate, titanium dioxide, talc, alumina, aluminosilicates, kaolin, silicate fibres, derivatives thereof and mixtures thereof. The term “white filler” is meant to refer to a reinforcing material selected from among conventional silica and silicates, such as sepiolite, paligorskite also known as attapulgite, montmorillonite, alloisite and the like, optionally modified by acid treatment and / or derivatised. Typically, white fillers have surface hydroxyl groups. The term “mixing step (1 )” indicates the step of the preparation process of the elastomeric compound in which one or more additives may be incorporated by mixing and optional heating, except for the vulcanising agent which is fed in step (2). The mixing step (1 ) is also referred to as “non-productive step”. In the preparation of a compound there may be several “non-productive” mixing steps which may be indicated with 1a, 1b, etc.

[0072] The term “mixing step (2)” indicates the next step of the preparation process of the elastomeric compound in which the vulcanising agent and, optionally, the other additives of the vulcanisation package are introduced into the elastomeric compound obtained from step (1 ), and mixed in the material, at controlled temperature, generally at a temperature of compound than 120 °C, so as to provide the vulcanisable elastomeric compound. The mixing step (2) is also referred to as “productive step”.

[0073] For the purposes of the present description and the following claims, the term “phr” (acronym for parts per hundreds of rubber) indicates the parts by weight of a given elastomeric compound component per 100 parts by weight of the polymer, net of any plasticising extension oils. Unless otherwise indicated, all the percentages are expressed as percentages by weight.

[0074] Brief description of the figures

[0075] With reference to the accompanying figures:

[0076] - Figure 1 shows a tyre for vehicle wheels comprising at least one component comprising the elastomeric compound according to the invention.

[0077] - Figure 2 shows the trend of the extraction force (Newton, on the ordinate) as the deformation (mm, on the abscissa) increases for reference carcass rubberising compounds (Ex. 1 and Ex. 2), according to the invention (Ex. 3) and comparative (Ex. 4).

[0078] - Figure 3 shows the trend of the cross-linking kinetics of compounds comprising conventional resorcinol + HMMM systems (sample 1 , Ex. 1 ) or phenolic resin + HMMM (sample 2, Ex. 2) compared to the phenolic resin + oxazolidine (I) system according to the invention (sample 3, Ex. 3) (MDR analysis). Detailed description of the invention

[0079] The cross-linking composition according to the present invention is characterised by one or more of the following preferred aspects taken alone or in combination with one another.

[0080] The cross-linking composition according to the invention comprises at least one methylene donor agent of formula (I) wherein R1 preferably represents linear or branched C1-C10alkyl, linear or branched C1-C10alkenyl, more preferably linear or branched Ci-C 5 alkyl, linear or branched C1-C5alkenyl, even more preferably R1 represents ethyl.

[0081] Said alkyl and alkenyl may optionally be substituted in the chain with one or more oxygens, to give alkoxy-alkyl or alkoxy alkenyl chains, including alkoxy-alkyl or alkoxy alkenyl chains comprising one or more repeating units such as -O-CH2- CH2- (PEG), -O-CH2-CHOH-CH2- (PPG) and the like.

[0082] The methylene donor agent of formula (!) may be a commercial product such as, for example, the bicyclic oxazolidine 5-ethyl-1-aza-3,7-dioxa-bicyclo[3.3.0]octane from Aldrich or it may be synthesised starting from known reagents according to methods described in the literature, for example in US3256137 and in the documents cited therein.

[0083] In one embodiment, the cross-linking composition of the invention may comprise, in addition to at least one methylene donor agent of formula (I), at least one other conventional methylene donor agent, selected from, but not limited to, formaldehyde, paraformaldehyde, preferably from hexamethoxymethylmelamine (HMMM), hexamethylenetetramine (HMT), hexamethylol melamine, N,N’- dimethylolurea, N-methylol dicyanamide, N-allyl dioxazine, N-phenyl dioxazine, N- methylol acetamide, N-methylol butyramide, N-methylol acrylamide, N-methylol succinimide, lauryloxymethylpyridinium chloride, ethoxymethylpyridinium chloride, trioxane hexamethoxymethylmelamine, such as described in US3751331 , hexamethylolmelamine pentamethyl ether (HMPE), oxazolidine derivatives other than those of formula (I) and mixtures thereof, more preferably selected from hexamethylenetetramine (HMT), hexamethoxymethylmelamine (HMMM) and mixtures thereof. In one embodiment, the cross-linking composition of the invention comprises as a methylene donor agent at least one agent of formula (I), preferably the agent of formula (I) wherein R1 is ethyl, in mixture with hexamethylenetetramine (HMT) and / or hexamethoxymethylmelamine (HMMM).

[0084] Preferably, in case of a methylene donor agent comprising mixtures of donors, the weight ratio between the at least one agent of formula (I) and the at least one other conventional methylene donor agent of formula different from formula (I) is generally between 1 :1 and 1 :6, preferably between 1 :1 and 1 :3.

[0085] In a preferred embodiment, the cross-linking composition of the invention comprises, as methylene donor agent, only one or more agents of formula (I), preferably it comprises as methylene donor agent only one agent of formula (I), even more preferably only the agent of formula (I), wherein R1 represents linear or branched C1-C10alkyl, saturated or unsaturated, preferably linear or branched Ci- C 5 alkyl, saturated or unsaturated, more preferably ethyl.

[0086] As highlighted in the present experimental part, the methylene donor agent of formula (I) of the cross-linking composition of the invention provides unexpected improvements compared to the oxazolidine compound of formula (l-A) described in US4361677 when incorporated into the compound, for example in terms of better static properties, lower brittleness, higher tear resistance and better comfort in tyre applications.

[0087] The cross-linking composition of the invention comprises at least one methylene acceptor agent.

[0088] In the present composition, said at least one methylene acceptor agent may be any agent capable of reacting during vulcanisation with the methylene donor.

[0089] In particular, the methylene acceptor agent may be a phenol, a substituted phenol, a phenolic resin obtained by partial cross-linking of phenol and / or at least one phenol substituted with formaldehyde or other methylene donors, and mixtures thereof, preferably a phenolic resin. Phenol may be, in addition to phenol itself, a dihydroxyphenol or a polyhydroxyphenol, for example it may be ortho-cresol, paracresol, meta-cresol, resorcinol, pyrocatechol, pyrogallol, fluoroglucinol, and the like and mixtures thereof, but preferably resorcinol and other less sustainable dihydroxyphenols are not used.

[0090] The phenolic resin of the cross-linking composition of the invention is a non-self- cross-linking (non-thermosetting) phenolic resin. Unlike thermosetting phenolic resin, which cross-links simply by heating, non-self-cross-linking phenolic resin requires the presence of at least one methylene donor in addition to heating to cross-link.

[0091] The term “phenolic resin” refers to the family of polymers obtained by reaction between phenols and formaldehyde or precursors thereof, which, depending on the ratio between the two reagents and the reaction conditions, are distinguished into novolacs and resoles.

[0092] Typically, novolacs are prepared with an aldehyde:phenol ratio less than 1 and acid catalysis, while resoles with an aldehyde:phenol ratio greater than 1 and basic catalysis.

[0093] In addition to formaldehyde, other aldehydes may be used as a substitute for or in mixture with formaldehyde; among them, acetaldehyde and furfural.

[0094] A suitable phenolic resin as a methylene acceptor agent is for example a novolac- type phenolic resin, a novolac-type cresol resin, a novolac-type xylenol resin, a novolac-type resorcinol resin or a resin obtained by modifying these resins with an oil. Preferably, the modified resin is modified with rosin oil, tall oil, cashew oil, linoleic acid, oleic acid and / or linolenic acid.

[0095] Preferably, in the present invention, phenolic resins prepared by polymerisation of phenol and formaldehyde, more preferably novolacs, even more preferably novolacs with a low content of free monomer (phenol), are used as methylene acceptors.

[0096] Preferred methylene acceptors are, for example, resins marketed under the names Alnovol PN760, Durez 12686, Elastobond A250 and the like.

[0097] Furthermore, natural products with a polyphenolic structure, such as lignin and its derivatives, may also be suitable as methylene acceptors.

[0098] All other things being equal, more sustainable and harmless methylene donors and acceptors are particularly preferred.

[0099] Depending on the use of the present cross-linking composition, the most suitable phenolic resin may be selected also on the basis of its molecular weight.

[0100] For example, higher molecular weight phenolic resins are preferred for use in elastomeric compositions intended to form the structure of those tyre components that require greater stiffness and tear resistance, such as bead fillers or sidewall inserts. Typically, these resins have a softening point of 90 °C or higher. Instead, lower molecular weight phenolic resins are preferred for use in elastomeric compositions intended to cover textile or metal reinforcement elements, such as elastomeric compositions of the carcass, belt, chafer or flipper structure. Typically, these resins have a softening point not exceeding 95 °C, preferably lower.

[0101] The methylene acceptor phenolic resin of the present cross-linking composition preferably does not include the sulphurated thermosetting and vulcanising phenolic resin of formula claimed and described in US4361677.

[0102] Preferably the methylene acceptor agent does not comprise free phenol, in particular it does not comprise free resorcinol, in a significant amount, preferably if it does comprise it, it is in the lowest possible amount, for example less than 2%, more preferably it consists only of one or more phenolic resins, advantageously more harmless and which show better processability, lower reversion (see MH and RET% in Table 7) and higher tear resistance (Table 15) in the compound.

[0103] In a more sustainable preferred embodiment, the cross-linking composition according to the invention comprises neither free phenols (resorcinol and the like) nor formaldehyde.

[0104] In a preferred embodiment, the cross-linking composition according to the invention comprises as the sole methylene donor agent one or more agents of formula (I) and as the sole methylene acceptor agent one or more phenolic resins.

[0105] In this embodiment, the weight ratio of the methylene donor agent of formula (I) to the total of the one or more phenolic resins is preferably between 1 :1 and 1 :6, more preferably between 1 :1 .2 and 1 :5.

[0106] In another embodiment, the cross-linking composition according to the invention comprises as a methylene donor agent the agent of formula (I) in mixture with one or more conventional formaldehyde donors (HMMM, HMT and the like) and as the sole methylene acceptor agent one or more phenolic resins. In this further embodiment, the weight ratio of the total methylene donor agents to the total of the one or more phenolic resins is preferably between 1 :1.5 and 1 :5, more preferably between 1 :2 and 1 :4.

[0107] In the secondary cross-linking composition according to the invention, the at least one methylene donor agent of formula (I) is preferably present in an amount of at least 2% by weight, more preferably at least 3% by weight, based on the total weight of the cross-linking composition.

[0108] In the secondary cross-linking composition according to the invention, the at least one methylene donor agent of formula (I) is preferably present in an amount not exceeding 70% by weight, more preferably not exceeding 60% by weight, based on the total weight of the cross-linking composition.

[0109] In the secondary cross-linking composition according to the invention the methylene donor agent preferably comprises at least 15 wt. %, more preferably at least 20 wt. % of at least one methylene donor agent of formula (I).

[0110] A further aspect of the present invention is represented by an elastomeric composition comprising the cross-linking composition according to the invention described above.

[0111] The preferences expressed above for the cross-linking composition of the invention apply mutatis mutandis to the elastomeric composition comprising it and to all subsequent aspects of the invention.

[0112] The elastomeric composition according to the present invention is characterised by one or more of the following preferred aspects taken alone or in combination with one another.

[0113] The elastomeric composition according to the invention comprises at least 100 phr of at least one diene polymer.

[0114] The diene polymer (A) may be selected from those commonly used in sulphur- vulcanisable elastomeric compositions, which are particularly suitable for producing tyres, i.e. from among solid elastomeric polymers or copolymers with an unsaturated chain having a glass transition temperature (Tg) generally lower than 20 °C, preferably in the range from 0 °C to -110 °C.

[0115] These polymers or copolymers may be of natural origin or may be obtained by solution polymerization, emulsion polymerization or gas-phase polymerization of one or more conjugated dienes, optionally mixed with at least one comonomer selected from monoolefins, monovinylarenes and / or polar comonomers in an amount not exceeding 60% by weight.

[0116] The conjugated dienes generally contain from 4 to 12, preferably from 4 to 8 carbon atoms and may be selected, for example, from the group comprising: 1 ,3- butadiene, isoprene, 2,3-dimethyl-1 ,3-butadiene, 1 ,3-pentadiene, 1 ,3-hexadiene, 3-butyl-1 ,3-octadiene, 2-phenyl-1 ,3-butadiene and mixtures thereof. 1 ,3-butadiene and isoprene are particularly preferred.

[0117] The monoolefins may be selected from ethylene and a-olefins generally containing from 3 to 12 carbon atoms, such as for example propylene, 1 -butene, 1-pentene, 1 -hexene, 1 -octene or mixtures thereof.

[0118] Monovinylarenes, which may optionally be used as comonomers, generally contain from 8 to 20, preferably from 8 to 12 carbon atoms and may be selected, for example, from: styrene; 1 -vinylnaphthalene; 2-vinylnaphthalene; various alkyl, cycloalkyl, aryl, alkylaryl or arylalkyl derivatives of styrene, such as, for example, a-methylstyrene, 3-methylstyrene, 4-propylstyrene, 4-cyclohexylstyrene, 4- dodecylstyrene, 2-ethyl-4-benzylstyrene, 4-p-tolyl-styrene, 4-(4- phenylbutyl)styrene, and mixtures thereof. Styrene is particularly preferred.

[0119] Polar comonomers that may optionally be used, may be selected, for example, from: vinylpyridine, vinylquinoline, acrylic acid and alkylacrylic acid esters, acrylonitriles, or mixtures thereof, such as, for example, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, acrylonitrile and mixtures thereof.

[0120] Preferably, the diene polymer (A) may be selected, for example, from among: cis- 1 ,4-polyisoprene (natural or synthetic, preferably natural rubber), 3,4-polyisoprene, polybutadiene (in particular polybutadiene with a high content of 1 ,4-cis), optionally halogenated isoprene / isobutene copolymers, 1 ,3-butadiene / acrylonitrile copolymers, styrene / 1 ,3-butadiene copolymers, styrene / isoprene / 1 ,3-butadiene copolymers, styrene / 1 ,3-butadiene / acrylonitrile copolymers, and mixtures thereof. The composition according to the invention may optionally comprise at least one polymer of one or more monoolefins with an olefinic comonomer or derivatives thereof. The monoolefins may be selected from: ethylene and a-olefins generally containing from 3 to 12 carbon atoms, such as for example propylene, 1 -butene, 1-pentene, 1 -hexene, 1 -octene or mixtures thereof. The following are preferred: copolymers selected from ethylene and an a-olefin, optionally with a diene; isobutene homopolymers or copolymers thereof with small amounts of a diene, which are optionally at least partially halogenated. The diene optionally present generally contains from 4 to 20 carbon atoms and is preferably selected from: 1 ,3- butadiene, isoprene, 1 ,4-hexadiene, 1 ,4-cyclohexadiene, 5-ethylidene-2- norbornene, 5-methylene-2-norbornene, vinylnorbornene or mixtures thereof. Among them, the following are particularly preferred: ethylene / propylene (EPR) copolymers or ethylene / propylene / diene (EPDM) copolymers; polyisobutene; butyl rubber; halobutyl rubbers, in particular chlorobutyl or bromobutyl rubbers; and mixtures thereof.

[0121] The elastomeric composition according to the present invention comprises at least 0.1 phr of at least one reinforcing filler (B), preferably at least 1 phr.

[0122] The present composition may comprise from 1 phr to 170 phr, from 5 phr to 150 phr or from 10 phr to 120 phr of at least one reinforcing filler.

[0123] Preferably, the reinforcing filler is selected from carbon black, white fillers, silicate fibres, derivatives thereof and mixtures thereof.

[0124] In one embodiment, said reinforcing filler comprises carbon black.

[0125] Preferably, carbon black as reinforcing filler (B) is present in the elastomeric composition according to the invention in an amount of between 1 phr and 120 phr, preferably between 5 phr and 100 phr.

[0126] Preferably, the carbon black is selected from those having a surface area not smaller than 20 m2 / g, preferably of at least about 40 - 50 m2 / g (as determined by STSA - statistical thickness surface area according to ISO 18852:2005).

[0127] The carbon black may be for example N375, N326, N339, N550 or N660 marketed by Birla Group (India) or Cabot Corporation.

[0128] In an embodiment, said reinforcing filler is a white filler selected from among hydroxides, oxides and hydrated oxides, salts and hydrated salts of metals, silica, silicates fibres, derivatives thereof and mixtures thereof.

[0129] In one embodiment, said reinforcing filler may comprise silica, for example selected from pyrogenic silica, precipitated amorphous silica, wet silica (hydrated silicic acid), anhydrous silica (anhydrous silicic acid), or mixtures thereof.

[0130] Preferably, silica as a reinforcing filler is present in the elastomeric composition according to the invention in an amount of 1 phr to 100 phr, more preferably 5 phr to 80 phr or 7 phr to 50 phr. The silica that may be used in the present invention may have a BET surface area (measured according to ISO standard 5794 / 1 ) in the range from 10 m2 / g to 300 m2 / g, preferably from 30 m2 / g to 250 m2 / g, more preferably from 40 m2 / g to 190 m2 / g.

[0131] Commercial examples of suitable silicas are Zeosil 1165 MP, Zeosil 1115 MP, Zeosil 185 GR, Efficium from Solvay, Newsil HD90 and Newsil HD200 from Wuxi, K160 and K195 from Wilmar, H160AT and H180 AT from IQE, Zeopol 8755 and 8745 from Huber, Perkasil TF100 from Grace, Hi-Sil EZ 120 G, EZ 160G, EZ 200G from PPG, Ultrasil 7000 GR and Ultrasil 9100 GR from Evonik. Another example of suitable silica is rice husk silica described in WO2019229692A1 .

[0132] In one embodiment, said reinforcing filler comprises silica mixed with carbon black and / or silicate fibres.

[0133] The elastomeric composition according to the invention comprises from 0.1 to 20 phr of at least one vulcanising agent.

[0134] The vulcanising agent is preferably selected from sulphur-based agents such as elemental sulphur, polymeric sulphur, sulphur donor agents such as bis[(trialkoxysilyl)propyl]polysulphides, thiurams, dithiodimorpholines and caprolactam-disulphide, peroxides, such as dialkyl peroxides R - O - O - R, wherein R is an alkyl group, alkyl-aryl peroxides R - O - O - R', wherein R is an alkyl group and R' aryl, diaryl peroxides R'- O - O - R', wherein R’ is an aryl group, diacylperoxides R-C(O)-O-O-(O)C-R', wherein R and R’ are aryl and / or alkyl groups, peroxyketal R-O-O(R)C(R’)-O-O-R’, wherein R and R’ are aryl and / or alkyl groups, peroxyesters R-C(O)-O-O-R’, wherein R and R' are aryl and / or alkyl groups.

[0135] Said at least one vulcanising agent is preferably a sulphur-based agent selected from sulphur, or alternatively, sulphur-containing molecules (sulphur donors), such as for example bis(trialcoxysilyl)propyl]polysulphides and mixtures thereof. Preferably, the vulcanising agent is sulphur, even more preferably selected from soluble sulphur (crystalline sulphur), insoluble sulphur (polymeric sulphur), (iii) oil- dispersed sulphur and mixtures thereof Commercial example of a vulcanising agent suitable for use in the composition of the invention is the Redball Superfine sulphur from Flexsys. Preferably, the elastomeric composition according to the invention comprises at least 0.5 phr, 0.8 phr or 1 phr of at least one vulcanising agent, preferably sulphurbased, selected from those indicated above.

[0136] Even more preferably, the composition comprises from 0.1 to 15 phr, 0.2 to 10 phr, 1 to 10 phr or 1.5 to 7 phr of at least one vulcanising agent, preferably a sulphurbased agent selected from those indicated above.

[0137] The elastomeric composition according to the invention comprises at least 0.05 phr of the cross-linking composition according to the invention.

[0138] Preferably, the elastomeric composition according to the invention comprises from 0.1 phr to 30 phr, more preferably from 3 phr to 20 phr of the cross-linking composition according to the invention.

[0139] The methylene donor agent of formula (I) may be present in the elastomeric composition in amount of from 0.5 to 15 phr, preferably from 0.5 to 8 phr.

[0140] The methylene donor agent, if in addition to agent (I) it includes at least one other methylene donor agent, may be present in the elastomeric composition in a total amount preferably of from 0.5 to 15 phr or from 0.5 to 8 phr.

[0141] The methylene acceptor agent is preferably present in the elastomeric composition in amounts of 0.5 to 30 phr, more preferably 0.5 to 20 phr or 0.5 to 10 phr.

[0142] In a preferred embodiment, the elastomeric composition comprises one or more agents of formula (I), as the sole methylene donor agents, and one or more phenolic resins as defined above, as the sole methylene acceptor agents.

[0143] In a preferred embodiment, the elastomeric composition does not comprise phenols, in particular resorcinol, nor formaldehyde or conventional formaldehyde precursor agents such as HMMM, HMT and the like, having a formula other than formula (I).

[0144] The elastomeric composition according to the invention may further comprise adjuvants known to those skilled in the art such as vulcanisation activators, accelerators and / or retarders.

[0145] The vulcanisation activators which may be included in the present composition are zinc compounds, in particular ZnO, ZnCOs, zinc salts of saturated or unsaturated fatty acids containing from 8 to 18 carbon atoms, which are preferably formed in situ in the composition by reaction of ZnO and of the fatty acid, as well as BisOs, PbO, Pb3O4, PbO2, or mixtures thereof. For example, zinc stearate is used, preferably formed in situ in the composition, by ZnO and fatty acid, or magnesium stearate, formed by MgO, or mixtures thereof. Preferred activators derive from the reaction of zinc oxide and stearic acid. An example of activator is the product Aktiplast ST marketed by Rheinchemie.

[0146] In particular, the vulcanisation activators indicated above may be present in the composition of the invention in amounts preferably from 0.2 phr to 15 phr, more preferably from 1 phr to 5 phr.

[0147] The elastomeric composition according to the invention may further comprise at least one vulcanisation accelerator.

[0148] Vulcanisation accelerators commonly used may be for example selected from dithiocarbamates, guanidines, thioureas, thiazoles, sulphenamides, sulphenimides, thiurams, amines, xanthates, or mixtures thereof. Preferably, the accelerator is selected from mercaptobenzothiazole (MBT), N-cyclohexyl-2- benzothiazol-sulphenamide (CBS), N-tert-butyl-2-benzothiazol-sulphenamide (TBBS) and mixtures thereof.

[0149] Commercial examples of accelerators suitable for use in the present composition are N-cyclohexyl-2-benzothiazyl-sulphenamide Vulkacit® (CBS or CZ), and N- terbutyl 2-benzothiazil sulphenamide, Vulkacit® NZ / EGC marketed by Lanxess.

[0150] In particular, the vulcanisation accelerators indicated above may be used in the present composition in an amount preferably from 0.05 phr to 10 phr, preferably from 0.1 phr to 7 phr, more preferably from 0.5 phr to 5 phr.

[0151] The elastomeric composition according to the invention may further comprise at least one vulcanisation retarder.

[0152] The vulcanisation retarder suitable for use in the present composition is preferably selected from urea, phthalic anhydride, N-nitrosodiphenylamine N- cyclohexylthiophthalimide (CTP or PVI) and mixtures thereof. A commercial example of a suitable retarder is N-cyclohexylthiophthalimide VULKALENT G of Lanxess. The vulcanisation retarder may be present in the present composition in an amount of preferably from 0.05 phr to 2 phr.

[0153] In one embodiment, typically when silica is present as a reinforcing filler, the elastomeric composition according to the invention may further comprise at least 0.05 phr, preferably at least 0.1 phr or 0.5 phr, more preferably at least 1 phr or 2 phr of at least one silane coupling agent. Preferably, the elastomeric composition according to the invention comprises from 0.5 phr to 10.0 phr, more preferably or from 1.0 phr to 8.0 phr, even more preferably from 3.0 to 8.0 phr of at least one silane coupling agent.

[0154] Preferably, said silane coupling agent is selected from those having at least one hydrolysable silane group which can be identified, for example, by the following general formula (II):

[0155] (R”)3Si-CnH2n-X (II) wherein the groups R”, equal or different from each other, are selected from: alkyl, alkoxy or aryloxy groups or from halogen atoms, provided that at least one of the groups R” is an alkoxy or an aryloxy group; n is an integer of from 1 to 6; X is a group selected from: nitrose, mercapto, amino, epoxide, vinyl, imide, chloro, - (S)mCnH2n-Si-(R”)3and -S-COR”, wherein m and n are integers of from 1 to 6 and the groups R” are as defined above.

[0156] Particularly preferred silane coupling agents are bis(3- triethoxysilylpropyl)tetrasulphide and bis(3-triethoxysilylpropyl)disulphide. Said coupling agents may be added as such or in mixture with an inert filler (such as carbon black) so as to facilitate their incorporation into the elastomeric composition.

[0157] An example of the silane coupling agent is TESPT: bis(3- triethoxysilylpropyl)tetrasulphide Si69 marketed by Evonik.

[0158] The elastomeric composition according to the invention preferably does not comprise the thermosetting and vulcanising sulphurated phenolic resin of formula as claimed and described in US4361677.

[0159] A further aspect of the present invention is a vulcanised elastomeric compound for tyre for vehicle wheel obtained by mixing and vulcanising the elastomeric composition described above.

[0160] The preferences expressed above for the cross-linking composition and the elastomeric composition of the invention apply mutatis mutandis to the vulcanised elastomeric compound and to all subsequent aspects of the invention. The elastomeric compound according to the invention may typically be consolidated by reacting the secondary cross-linking composition according to the invention, which comprises at least one methylene donor agent of formula (I) and at least one methylene acceptor agent as defined above, during vulcanisation, i.e. simultaneously with the conventional primary cross-linking system, comprising the sulphur-based vulcanising agent.

[0161] Advantageously, the cross-linking of the compound according to the invention occurs according to kinetics such as that illustrated in Figure 3, in which a fast initial step is followed by a plateau, with stabilisation of the network and minimal reversion.

[0162] The present elastomeric compound may be prepared according to a process which typically comprises one or more mixing steps in at least one suitable mixer, in particular at least one mixing step 1 (non-productive) and a mixing step 2 (productive) as defined above.

[0163] Each mixing step may comprise several intermediate processing steps or substeps, characterised by the momentary interruption of the mixing to allow the addition of one or more ingredients but without intermediate discharge of the compound.

[0164] The mixing may be carried out, for example, using an open mixer of the open-mill type or an internal mixer of the type with tangential rotors (Banbury®) or with interpenetrating rotors (Intermix), or in continuous mixers of the Ko-Kneader™ type (Buss®) or of the twin-screw or multi-screw type.

[0165] For this purpose, after one or more thermomechanical treatment steps (nonproductive step 1 ), the rubber is typically processed with some of the additives, including methylene acceptor agents, with the exception of vulcanisers, vulcanisation accelerators and retarders and methylene donor agents, which are incorporated into the compound in the next step. In the final treatment step (production step 2), the temperature is generally kept below 120 °C and preferably below 100 °C, so as to prevent any undesired pre-vulcanisation phenomena. Thereafter, the compound is incorporated in one or more components of the tyre and subjected to vulcanisation, according to known techniques. Advantageously, the elastomeric composition of the invention is even more processable than similar known compositions. The vehicle wheel tyre component comprising, or preferably consisting essentially of the vulcanised elastomeric compound according to the invention, is selected from tread band, underlayer, anti-abrasion layer, sidewall, sidewall insert, minisidewall, liner, underliner, carcass structure rubberising layers and / or belt and / or zero-degree belt, annular bead anchoring structure, bead filler, bead reinforcement layers (flipper), bead protection layers (chafer) and sheet.

[0166] In one embodiment, the elastomeric compound according to the invention is a compound reinforced by incorporating reinforcement elements of various nature, metallic or textile, also called rubberising compound.

[0167] Preferably, the elastomeric compound of the invention finds application in reinforced tyre components such as belt structure, carcass structure, rubber layers, annular bead anchoring structure, bead reinforcement layers (flipper), bead protection layers (chafer).

[0168] Preferably, said reinforcement elements are made of one or more textile materials. Said reinforcement elements may consist of aliphatic polyamide fibres (e.g. Nylon 6, Nylon 6.6, Nylon 4.6, Nylon 4.10, Nylon 10.10, Nylon 11 , Nylon 12, Nylon 6.10, Nylon 6.12), or aromatic polyamides (e.g. aramid), polyester fibres (e.g. polybutylene terephthalate, polyethylene terephthalate, polyethylene isophthalate), polyaryletherketone fibres (e.g. polyetheretherketone), or mixtures thereof. More preferably, the aforementioned polyester fibres are polyethylene terephthalate (PET) fibres. Said reinforcement elements may be made of cellulose derivatives such as Rayon or Lyocell,

[0169] In a preferred embodiment, said reinforcement elements consist of aliphatic polyamides, PET or mixtures thereof.

[0170] Advantageously, the present cross-linking composition gives the elastomeric compound of the invention an excellent adhesion to the reinforcement elements, as highlighted in the present Tables 15 to 18.

[0171] In another embodiment, the elastomeric compound according to the invention is used as a constituent compound of tyre components typically subjected to load, which require a certain stiffness and above all fatigue resistance, for example as a filler compound for bead or sidewall inserts of self-supporting tyres.

[0172] Advantageously, the elastomeric compound according to the invention has a faster vulcanisation kinetics than compounds comprising only conventional formaldehyde precursor agents such as HMT or HMMM, associated with better processability and comparable or superior thermal stability, as evidenced by the T90, MH and %RET values (see Tables 6 to 9) and the graph in Figure 3.

[0173] Furthermore, with equal static properties, the elastomeric compound of the invention shows a comparable or lower hysteresis (see for example the Tan Delta values in Tables 10 and 12) predictive of an equal or lower rolling resistance in the tyre and therefore of similar or lower consumption.

[0174] A further aspect of the present invention is represented by a tyre for vehicle wheels comprising at least one tyre component comprising or, preferably, essentially consisting of the elastomeric compound according to the invention.

[0175] The term “essentially consisting of the elastomeric compound” is meant to indicate that the tyre component, in addition to the elastomeric compound according to the invention, may comprise other elements such as textile or metallic reinforcement elements but no other elastomeric compounds other than that according to the invention.

[0176] In one embodiment, the present tyre comprises at least one reinforced tyre component comprising or, preferably, essentially consisting of the elastomeric compound according to the invention preferably selected from rubberising compounds for carcass structure, belt structure, zero-degree belt structure, bead protection (chafer) or reinforcement (flipper) layers.

[0177] In one embodiment, the present tyre comprises at least one tyre component, unreinforced (i.e. without reinforcement elements) and typically subject to fatigue, comprising or, preferably, essentially consisting of the elastomeric compound according to the invention, preferably selected from bead filler and sidewall insert.

[0178] The tyre according to the invention may comprise more components among those indicated above, comprising or, preferably, essentially consisting of the elastomeric compound according to the invention, for example it may comprise a combination of one or more components selected from rubberising compounds for carcass structure, belt structure, zero-degree belt structure, protection layers (chafer) or reinforcement layers (flipper), bead filler and / or sidewall insert.

[0179] Based on the predictive data shown in the experimental part, the tyre according to the invention may exhibit lower rolling resistance, better comfort and fatigue resistance and, overall, longer life.

[0180] The tyre according to the invention may be a tyre for vehicles with two, three or four wheels and may be for summer or winter use or for all seasons. In one embodiment, the tyre according to the invention is a tyre for motorcycle wheels, wherein at least one component comprises, or essentially consists of, the elastomeric compound according to the invention. Typically, a tyre for motorcycle wheels is a tyre that has a straight section characterised by a high transverse curvature.

[0181] In a preferred embodiment, the tyre according to the invention is a tyre for wheels of sports or racing motorcycles.

[0182] In one embodiment, the tyre according to the invention is a tyre for car wheels.

[0183] In one embodiment, the tyre according to the invention is a tyre for high performance cars (HP, SUV and UHP), wherein at least one component comprises, or essentially consists of, the elastomeric compound according to the invention.

[0184] In one embodiment, the tyre according to the invention is a tyre for bicycle wheels. A tyre for bicycle wheels typically comprises a carcass structure turned around a pair of bead cores at the beads and a tread band arranged in a radially outer position with respect to the carcass structure.

[0185] The tyre according to the present invention may be produced according to a process which comprises:

[0186] - building components of a green tyre on at least one forming drum;

[0187] - shaping, moulding and vulcanising the tyre; wherein building at least one of the components of a green tyre comprises:

[0188] - manufacturing at least one green component comprising, or essentially consisting of, the elastomeric compound according to the invention.

[0189] Description of a tyre according to the invention

[0190] A tyre for vehicle wheels according to the invention, comprising at least one component comprising the present elastomeric compound, is illustrated in radial half-section in Figure 1 .

[0191] In Figure 1 , “a” indicates an axial direction and “X” indicates a radial direction, in particular X-X indicates the outline of the equatorial plane. For simplicity, Figure 1 shows only a portion of the tyre, the remaining portion not shown being identical and arranged symmetrically with respect to the equatorial plane “X-X”.

[0192] The tyre (100) for four-wheeled vehicles comprises at least one carcass structure, comprising at least one carcass layer (101) having respectively opposite end flaps engaged with respective annular anchoring structures (102), referred to as bead cores, optionally associated to a bead filler (104).

[0193] The tyre area comprising the bead core (102) and the filler (104) forms a bead structure (103) intended for anchoring the tyre onto a corresponding mounting rim, not shown.

[0194] The carcass structure is usually of radial type, i.e. the reinforcement elements of the at least one carcass layer 101 lie on planes comprising the rotational axis of the tyre and substantially perpendicular to the equatorial plane of the tyre. Said reinforcement elements generally consist of textile cords, for example rayon, nylon, polyester (for example polyethylene terephthalate PET or polyethylene naphthalate PEN). Each bead structure is associated to the carcass structure by folding back of the opposite lateral edges of the at least one carcass layer (101) around the annular anchoring structure (102) so as to form the so-called carcass flaps (101a) as shown in Figure 1.

[0195] In one embodiment, the coupling between the carcass structure and the bead structure can be provided by a second carcass layer, not shown in Figure 1 , applied in an axially outer position with respect to the first carcass layer.

[0196] An anti-abrasive strip (105) optionally made with elastomeric material is arranged in an outer position of each bead structure (103).

[0197] The carcass structure is associated to a belt structure (106) comprising one or more belt layers (106a), (106b) placed in radial superposition with respect to one another and with respect to the carcass layer, having typically textile and / or metallic reinforcing cords incorporated within a layer of elastomeric material.

[0198] Such reinforcing cords may have crossed orientation with respect to a direction of circumferential development of the tyre (100). By “circumferential” direction it is meant a direction generally facing in the direction of rotation of the tyre.

[0199] At least one zero-degree reinforcement layer (106c), commonly known as a "0° belt”, may be applied in a radially outermost position to the belt layers (106a), (106b), which generally incorporates a plurality of elongated reinforcement elements, typically metallic or textile cords, oriented in a substantially circumferential direction, thus forming an angle of a few degrees (such as an angle of between about 0° and 6°) with respect to a direction parallel to the equatorial plane of the tyre, and coated with an elastomeric material.

[0200] A tread band (109) is applied in a radially outer position to the belt structure (106). Moreover, respective sidewalls (108) of elastomeric material are applied in an axially outer position on the lateral surfaces of the carcass structure, each extending from one of the lateral edges of tread (109) at the respective bead structure (103).

[0201] In a radially outer position, the tread band (109) has a rolling surface (109a) intended to come in contact with the ground. Circumferential grooves, which are connected by transverse notches (not shown in Figure 1) so as to define a plurality of blocks of various shapes and sizes distributed over the rolling surface (109a), are generally made on this surface (109a), which for simplicity is represented smooth in Figure 1 .

[0202] An under-layer (111) made of elastomeric material may be arranged between the belt structure (106) and the tread band (109), said under-layer preferably extending over a surface substantially corresponding to the extension surface of said belt structure.

[0203] A strip consisting of elastomeric material (110), commonly known as “minisidewall”, may optionally be provided in the connecting zone between the sidewalls (108) and the tread band (109), this mini-sidewall being generally obtained by co-extrusion with the tread band (109) and allowing an improvement of the mechanical interaction between the tread band (109) and the sidewalls (108). Preferably, the end portion of the sidewall (108) directly covers the lateral edge of the tread band (109).

[0204] In the case of tubeless tyres, a rubber layer 112, generally known as “liner”, which provides the necessary impermeability to the inflation air of the tyre, may also be provided in a radially inner position with respect to the carcass layer 101.

[0205] The rigidity of the tyre sidewall 108 may be improved by providing the bead structure 103 with a reinforcement layer 120 generally known as “flipper” or additional strip-like insert.

[0206] The flipper 120 is a reinforcement layer which is wound around the respective bead core 102 and the bead filler 104 so as to at least partially surround them, said reinforcement layer being arranged between the at least one carcass layer 101 and the bead structure 103. Usually, the flipper is in contact with said at least one carcass layer (101) and said bead structure (103).

[0207] The flipper 120 typically comprises a plurality of textile cords incorporated within a layer of elastomeric material. The reinforcing annular structure or bead (103) of the tyre may comprise a further protective layer which is generally known by the term of “chafer” (121) or protective strip and which has the function of increasing the rigidity and integrity of the bead structure (103).

[0208] The chafer (121 ) usually comprises a plurality of cords incorporated within a rubber layer of elastomeric material. Such cords are generally made of textile materials (such as aramide or rayon) or metal materials (such as steel cords).

[0209] A layer or sheet of elastomeric material (not shown) may be arranged between the belt structure and the carcass structure. The layer may have a uniform thickness. Alternatively, the layer may have a variable thickness in the axial direction. For example, the layer may have a greater thickness close to its axially outer edges with respect to the central (crown) zone.

[0210] Advantageously, the layer or sheet may extend on a surface substantially corresponding to the extension surface of said belt structure.

[0211] The elastomeric compound according to the present invention may be advantageously incorporated into one or more of the above-mentioned tyre components, preferably selected from the carcass (101), bead core (102), bead (104), belt (106), flipper (120) and chafer (121).

[0212] The building of the tyre according to the invention described above, may be carried out by assembling respective semi-finished products adapted to form the components of the tyre, on a forming drum, by at least one assembling device.

[0213] At least a part of the components intended to form the carcass structure of the tyre may be built and / or assembled on the forming drum. More particularly, the forming drum is intended to first receive the possible liner, and then the carcass structure. Thereafter, suitable devices coaxially engage one of the annular anchoring structures around each of the end flaps, position an outer sleeve comprising the belt structure and the tread band in a coaxially centred position around the cylindrical carcass sleeve and shape the carcass sleeve according to a toroidal configuration through a radial expansion of the carcass structure, so as to cause the application thereof against a radially inner surface of the outer sleeve.

[0214] After building the green tyre, a moulding and vulcanisation treatment is generally carried out in order to determine the structural stabilisation of the tyre through cross-linking of the vulcanisable elastomeric compositions, as well as to impart a desired tread pattern on the tread band and to impart any distinguishing graphic signs at sidewalls.

[0215] Experimental part

[0216] Methods of analysis

[0217] Rheometric analysis MDRfaccordinq to ISO 6502)

[0218] An Alpha Technologies type MDR2000 rheometer was used for this analysis. The tests were carried out at 170 °C for 30 minutes at an oscillation frequency of 1.66 Hz (100 oscillations per minute) and an oscillation amplitude of ± 0.5°, measuring the time necessary to achieve an increase of one or two rheometric units (TS1 , TS2) and the time necessary to achieve 5%, 30%, 60%, 90%, 95% and 100% (T05, T30, T60, T90, T95 and T100), respectively, of the final torque value (Mf). The maximum torque value MH, the minimum torque value ML and the reversion (RET%) were also measured.

[0219] Vulcanisation curves of elastomeric compounds comprising different secondary cross-linking systems are reported in Figure 3.

[0220] Static mechanical properties (CA05 load at 50% elongation, CA1 load at 100% elongation, CA3 load at 300% elongation, CR load at break, AR % elongation at break, Energy at break) according to the UN! 6065:2001 standard were measured at 23 °C on samples of the ring-shaped elastomeric materials (average on 3 rings), vulcanised at 170 °C for 10 minutes.

[0221] Compressive dynamic mechanical properties E’, E” and Tan delta were measured using an Instron model 1341 dynamic device in the tension-compression mode as described herein. A specimen of vulcanised material (170 °C for 10 minutes) having a cylindrical shape (length = 25 mm; diameter = 14 mm), pre-loaded in compression up to a longitudinal deformation of 10% with respect to the initial length and maintained for the entire duration of the test at the pre-set temperature of 23 °C, 70 °C or 100 °C, respectively, was subjected to a dynamic sinusoidal stress having an amplitude of ± 3.33% with respect to the length under pre-load, with a frequency of 10 Hz. The dynamic mechanical properties are expressed in terms of dynamic elastic modulus (E’), dynamic viscous modulus (E”) and Tan delta (loss factor). The Tan delta value was calculated as the ratio between the viscous dynamic modulus (E”) and the dynamic elastic modulus (E’).

[0222] Peeling test: The peeling test was performed by measuring the force (N) required to detach two elastomeric compound samples having the same composition co-vulcanised to create an interface area. This test is predictive of the tear resistance of the finished product. The test was also carried out after subjecting the samples to a thermo- oxidative ageing process in an oven at 70 °C for 48 hours.

[0223] Tables 15 to 17 show the mean detachment force values relating to the tear test expressed in Newton (3 samples for each material).

[0224] Adhesion test (H-test, ASTM D4776): The test measures the force required to pull the cords off the block of elastomeric material of the sample after vulcanisation and evaluates the coverage remaining on the cords after traction.

[0225] The samples vulcanised for 30 minutes at 150 °C comprised PET textile cords. The textile cords were previously treated by immersion in an adhesive RFL composition, comprising a latex of a styrene-butadiene-vinyl pyridine polymer, resorcinol and formaldehyde, and subsequent heating to about 200 -250 °C for fixing.

[0226] The cords thus treated were rubberised with reference or inventive compounds, the detailed compositions of which are reported in Table 3 to give representative samples of reinforced structural elements of tyres. The samples thus prepared were subjected to adhesion evaluation, as described herein.

[0227] Table 18 shows the values of the average pull-out force in N and the coverage evaluation (on 3 samples per compound).

[0228] Examples

[0229] Elastomeric compounds for carcass, belt or bead filler rubberising according to the invention, for reference or comparison, were prepared starting from the elastomeric compositions reported in the following Tables 1 to 5. These compounds were studied in terms of reactivity and properties by varying the secondary cross-linking system. In particular, the properties of elastomeric compounds comprising the cross-linking composition according to the invention (Inv.), characterised by the presence of a methylene donor agent of formula (I), alone or in mixture with conventional formaldehyde donors, were compared with known cross-linking systems corresponding to those used in commercial tyres or reported in literature, comprising resorcinol or phenolic resin together with conventional formaldehyde donors (HMMM, HMT and the like) and, finally, with cross-linking systems comprising resorcinol together with the methylene donor agent of formula (I), wherein R1 = ethyl (as shown in US3256137) or a phenolic resin together with the oxazolidine methylene donor agent of formula (l-A) wherein R1 =CH2OH described in US4361677 (comparative).

[0230] Table 1 : Elastomeric compositions for carcass rubberising compounds wherein

[0231] - Resor indicated resorcinol, Resin a phenolic resin, oxaz-inv a bis-oxazolidine according to the invention (formula (I), wherein R1 = CH2CH3) and oxaz-comp the bis-oxazolidine according to US4361677 (formula (l-A) wherein R1 = CH2OH);

[0232] - Natural rubber was NRP91 (cis 1 ,4-polyisoprene) type SIR 20 (Indonesia);

[0233] - CB N326 was carbon black (300% modulus ASTM D3192 IRB8 - ASTM D412 method B) of -3.9 + / -1 .8 MPa; CTAB: 83m2 / g) from Birla Carbon;

[0234] - Zinc oxide was a zinc oxide obtained from metallic zinc by an indirect process from Zincoloxides;

[0235] - Stearic acid was supplied by Oleon;

[0236] - 6PPD was Santoflex N-(1 ,3-Dimethylbutyl)-N’-phenyl-p-phenylenediamine, antiozonant, antioxidant from Flexsys;

[0237] - ALNOVOL PN 760 was a modified phenolic novolac resin, containing 1 % free phenol and 0.1 % free formaldehyde, with a softening point between 80 - 95 °C, supplied by Allnex (Germany);

[0238] - RESORCINOL 80 was 80% resorcinol and 20% binder polymer and dispersing agents supplied by RDC;

[0239] - HMMM was 65% hexamethoxymethylmelamine on amorphous silica support supplied by Brenntag; - Oxaz-inv was 5-ethyl-1-aza-3,7-dioxabicyclo[3.3.0]octane (CAS No. 7747-35-5) (formula (I), R1 = CH2CH3) supplied by Sigma Aldrich;

[0240] - Oxaz-comp (R1 = CH2OH) formula (l-A) was prepared as described in US8466294B2.

[0241] - TBBS 80 was N-tert-butyl-benzothiazyl sulphenamide (accelerator) from RDC;

[0242] - SULPHUR was Redball® Superfine amorphous sulphur insoluble in carbon disulphide and toluene, treated at 33% with hydrotreated heavy naphthenic distillate (petroleum) from Flexsys (Germany).

[0243] The composition of Example 1 provided information on the effects of the combination of HMMM and resorcinol, while the composition of Example 2 provided information on the combination of HMMM and Alnovol phenolic resin. These compositions corresponded to compositions used in commercial tyres. The composition of Ex. 2 represented the reference composition for the compositions of the Ex. 3 (Inv.) and Ex. 4 (Comp.) having the same components, in the same amounts except the methylene donor agent (HMMM vs bis-oxazolidines).

[0244] The composition of Example 3, according to the invention, instead concerned the combination of the bis-oxazolidine of formula (I) wherein R1 = ethyl with the phenolic resin Alnovol.

[0245] The composition of Example 4 was a comparative composition combining bis- oxazolidine of formula (l-A) where R1 = CH2OH, shown in US4361677, and the phenolic resin Alnovol.

[0246] Table 2: Elastomeric compositions for bead filler compounds wherein

[0247] - CB N375 was 300% modulus carbon black (ASTM D3192 IRB8 - ASTM D412 method B) 0.4+ / -1.8 MPa from Birla Carbon;

[0248] - Zinc salt of a fatty acid mixture (CAS No. 67701 -12-6) from Eigemann and Veronelli

[0249] - MES oil was a mineral base oil, refined by solvent and / or hydrotreating (CAS No. 64742-65-0 or 64742-54-7);

[0250] - t-butyl phenolic resin was OFF APM SL1410 resin, a resin obtained by condensation of pt-butylphenol with formaldehyde, from Sino Legend Company;

[0251] - novolac phenolic resin was DUREZ 12686 phenolic resin, modified with cashew nut shell oil, from Sumitomo Bakelite Europe, with a softening point of between 90 and 105 °C;

[0252] - HMT 80 was hexamethylenetetramine, 80% methylene donor agent with 20% polymer binder and RDC dispersing agents;

[0253] - silanising agent (silane coupling agent) was a 1 :1 mixture of bis[3- (triethoxysilyl)propyl]tetrasulphide (CAS No. 40372-72-3) and carbon black N330 (CAS No. 1333-86-4) from EVONIK;

[0254] - silica was ZEOSIL 1115 MP, precipitated amorphous silica, from Solvay Rhodia operations;

[0255] - TBBS was Nt-butyl-2-benzothiazylsulphenamide (CAS No. 95-31-8) from Lanxess;

[0256] - PVI was N-cyclohexylthiophthalimide (CAS No. 17796-82-6) from Shandong Derek New Materials; and natural rubber, stearic acid, zinc oxide, 6PPD, HMMM, Sulphur were the same as used in Table 1.

[0257] The composition of Example 5 provided information on the effects of combining the phenolic resin with HMMM and HMT.

[0258] The elastomeric composition according to the invention of Example 6, instead had the bis-oxazolidine of formula (I) wherein R1 = ethyl instead of HMMM, in combination with HMT and phenolic resin. The comparative composition of Example 7 was the same as that of the invention of Ex. 6 except in the bis-oxazolidine of formula (l-A) where R1 = CH2OH, shown in document US4361677, instead of that of the invention of formula (I) where R1 = ethyl. Table 3: Elastomeric compositions for carcass rubberising compounds in which the ingredients were the same as in Table 1 .

[0259] In these examples, in addition to the reference compositions (Ex. 8 and Ex. 10), where HMMM was in combination with resorcinol or Alnovol phenolic resin, respectively, two compositions according to the invention were prepared and compared (Ex. 9 and Ex. 11 ), wherein HMMM was replaced with Bis-oxazolidine according to the invention (formula (I), R1 = CH2CH3) in combination with resorcinol or with the phenolic resin Alnovol, respectively.

[0260] Table 4: Elastomeric compositions for belt rubberising compounds wherein

[0261] - natural rubber was a natural rubber STR 20 (Thailand);

[0262] - MANOBOND 680 C rubber to metal adhesion promoter, cobalt / boron salt from Shepherd Ltd; - DCBS was N,N-Dicyclohexyl-2-benzothiazolsulphenamide from Huatai

[0263] Chemicals; and the other ingredients were the same as those already reported in Table 1 , 2 or 3.

[0264] In these examples, the reference composition of Ex. 12 instead of resorcinol included the phenolic resin Alnovol and HMMM. The elastomeric composition according to the invention of Ex. 13 included Bis-oxazolidine of formula (I) (R1 = CH2CH3) as the only methylene donor in combination with the phenolic resin Alnovol.

[0265] Table 5: Elastomeric compositions for bead filler compounds in which the ingredients were those reported in Table 2. In these examples, the reference composition of Ex. 14 was the same as the previous Table 2 as the composition of Ex. 15 according to the invention comprising Bis-oxazolidine of formula (I) (R1 = CH2CH3) instead of HMMM, in combination with HMT and phenolic resin.

[0266] The composition of Ex. 16 was an elastomeric composition according to the invention comprising Bis-oxazolidine of formula (I) (R1 = CH2CH3) instead of HMT, in combination with HMMM and phenolic resin.

[0267] The compositions of Ex. 17 and Ex. 18 were compositions according to the invention in which the conventional cross-linkers HMMM and HMT (present in Ex. 14) were completely replaced by increasing amounts of Bis-oxazolidine of formula (I) (R1 = CH2CH3), in combination with the phenolic resin.

[0268] Preparation of the elastomeric compounds

[0269] Starting from the elastomeric compositions shown in Tables 1 -5, the corresponding elastomeric compounds were prepared according to the following process.

[0270] The mixing of the components was carried out in two steps using an internal mixer (Banbury, Intermix or Brabender)

[0271] In the first step (1) all the ingredients were introduced with the exception of the vulcanisers, accelerators and the methylene donor agent. The mixing was continued for a maximum time of 5 minutes, reaching a temperature of approximately 145 °C. Subsequently, in the second step (2), again carried out using an internal mixer, the vulcanisers, accelerators and the methylene donor agent were added, and the mixing was continued for about 4 minutes, maintaining the temperature below 100 °C. The raw compounds were then discharged. After cooling and at least 12 hours from preparation, some samples of the compounds were vulcanised in a press at 170 °C for 10 min to give the specimens useful for mechanical characterisations.

[0272] Characterisation of the compounds

[0273] MDR rheometric analysis

[0274] Some of the raw elastomeric compounds obtained from the elastomeric compositions reported in Tables 1 to 5, were subjected to MDR rheometric analysis as described above.

[0275] Figure 3 shows the trend of the cross-linking kinetics of compounds comprising conventional resorcinol + HMMM systems (sample 1 , Ex. 1 ) or phenolic resin + HMMM (sample 2, Ex. 2) compared to the phenolic resin + oxazolidine (I) system according to the invention (sample 3, Ex. 3). As can be appreciated from the graph, the cross-linking of sample 3 according to the invention included a very steep initial phase like sample 1 followed by a plateau, with stabilisation of the network and minimal reversion comparable to that of sample 2.

[0276] The following Tables 6 to 9 contain the results of the rheometric analysis carried out on the various samples:

[0277] Table 6: MDR analysis

[0278] Elastomeric compositions for carcass rubberising compounds (per Table 1 ) As evidenced by the data reported in Table 6, the compound of the invention of Ex. 3 showed faster cross-linking kinetics both compared to the reference compound of Ex. 2 and compared to the comparative compounds of Ex. 4, with a maximum MH torque lower than both, predictive of better processability, together with a reduction in reversion compared to the conventional production compound of Ex. 2.

[0279] Table 7: MDR analysis

[0280] Elastomeric compositions for carcass rubberising compounds (per Table 3)

[0281] As evidenced by the data reported in Table 7, the compound of the invention of Ex. 9 showed faster cross-linking kinetics compared to the reference compound of Ex. 8 (about -20% on T90) with a lower maximum MH torque (about -10%), predictive of better processability of the compound, together with a minimal increase in reversion (about +10%) within completely acceptable values.

[0282] Similar results (faster kinetics about -20% on T90, lower maximum torque about - 5% on MH), with a comparable reversion of about 3%, were obtained for the compound of the invention of Ex. 11 compared to the reference one of Ex. 10, both compounds comprising a phenolic resin instead of resorcinol. Table 8: MDR analysis

[0283] Elastomeric compositions for belt rubberising compounds (per Table 4)

[0284] As evidenced by the data reported in Table 8, the compound of the invention of

[0285] Ex. 13 showed faster cross-linking kinetics compared to the reference compound of Ex. 12 at all T points (approximately -20% on T30, -30% on T60 and -40% at T90) with a lower maximum MH torque (approximately -20%), predictive of better processability of the compound.

[0286] Table 9: MDR analysis

[0287] Elastomeric compositions for bead filler compounds (per Table 5)

[0288] As evidenced by the data reported in Table 9, the compound of the invention of Ex. 15 comprising phenolic resin, HMT and oxaz-inv in combination, showed cross-linking kinetics similar to the reference compound of Ex. 14 at the lowest values of T30 and T60 while an increase in speed was noted for values above T60 (about +5% at T90). Furthermore, a slightly higher maximum MH torque was observed (approximately +7%) for the invention compound, which was still acceptable.

[0289] In conclusion, from the MDR analysis data reported in Tables 6 to 9 it was highlighted that the cross-linking kinetics of the compounds in which the agent of formula (I) of the invention was used was generally faster than the reference compounds, the compound was more easily processable and with a lower or comparable reversion.

[0290] Static and dynamic mechanical properties of the compounds

[0291] The following Tables 10 to 14 show the results of the analyses of the mechanical properties, conducted according to the methods described above, of the elastomeric compounds prepared starting from the compositions shown in Tables 1 to 5:

[0292] Table 10: Static and dynamic mechanical properties

[0293] Elastomeric compositions for carcass rubberising compounds (per Table 1 ) to the reference of Ex. 2, the elastomeric composition according to the invention of Ex. 3 had static properties that were substantially in line or only slightly worse. Instead, the comparative composition of Ex. 4, containing the bis-oxazolidine described in US4361677, showed higher static stiffness with poorer breaking properties than the composition of the invention of Ex. 3. In particular, from the lower values of CR, AR% and energy and higher values of CA3, it resulted that the comparative composition of Ex. 4 was worse, more fragile, with a greater tendency to tear associated with a lower resistance to fatigue.

[0294] With regard to the dynamic properties, it was observed that the elastomeric composition according to the invention of Ex. 3 had lower dynamic moduli E’ than both Ex. 2 and Ex. 4, indicative of greater comfort, in tyre applications. Furthermore, the elastomeric composition according to the invention of Ex. 3 was characterised by hysteresis at all temperatures below or substantially comparable to those of reference Ex. 2 and the comparative of Ex. 4.

[0295] The comparative composition of Ex. 4 with respect to the composition of the invention of Ex. 3 and also to the references of Ex. 2 and Ex. 1 instead showed higher dynamic moduli at all temperatures, predictive of lower comfort in the specific carcass application due to excessive stiffness and poor damping capacity.

[0296] Table 11 : Static and dynamic mechanical properties

[0297] Elastomeric compositions for bead filler compounds (per Table 2) desired, however, it was important to avoid a worsening of the breaking properties and the possible propagation of cracks in the bead.

[0298] From the static property data reported in Table 11 , it was observed that, taking the composition of Ex. 5 as a reference, the elastomeric composition according to the invention of Ex. 6 had higher CR, AR and Energy, indicative of high tear resistance.

[0299] Instead, the comparative composition of Ex. 7 showed with respect to the elastomeric composition according to the invention of Ex. 6 significantly worse breaking properties, indicative of poor fatigue resistance.

[0300] With regard to the dynamic properties, it was evident that the comparative composition of Ex. 7 showed with respect to the elastomeric composition according to the invention of Ex. 6 higher E' values at all temperatures, predictive of lower tyre comfort. Table 12: Static and dynamic mechanical properties

[0301] Elastomeric compositions for carcass rubberising compounds (Table 3)

[0302] From the static property data given in Table 12, it was observed that the composition of the invention of Ex. 9 containing resorcinol compared to the reference one of Ex. 8, showed similar static stiffness with slightly lower fracture properties. Instead, the elastomeric composition according to the invention of Ex. 11 when compared to the reference composition of Ex. 10, both containing phenolic resin, exhibited similar static stiffness and comparable fracture properties.

[0303] With regard to the dynamic properties, the composition of the invention according to Ex. 9, compared to the reference composition of Ex. 8, had a slightly higher dynamic stiffness at all temperatures (about +5%) together with a significantly decreased hysteresis (about -20% at 23° and 70 °C, about -13% at 100 °C).

[0304] Instead, the elastomeric composition according to the invention of Ex. 11 compared with the reference composition of Ex. 10 showed similar dynamic stiffness at all temperatures together with low hysteresis (about -12% at 23° and 70 °C, about -9% at 100 °C).

[0305] Table 13: Static and dynamic mechanical properties

[0306] Elastomeric compositions for belt rubberising compounds (per Table 4)

[0307] From the static property data given in Table 13, it was observed that the composition of the invention of Ex. 13 compared to the reference one of Ex. 12, showed lower static stiffness (Ca3 about - 20%) with similar fracture properties. With regard to the dynamic properties, the composition of the invention according to Ex. 13, compared to the reference composition of Ex. 12, showed lower dynamic moduli with significantly reduced hysteresis, resulting in improved fatigue resistance and reduced rolling resistance.

[0308] Table 14: Static and dynamic mechanical properties

[0309] Elastomeric compositions for bead filler compounds (per Table 5)

[0310] From the static property data given in Table 14, it was observed that the composition according to the invention of Ex. 15 compared to the reference one of Ex. 14, showed similar static stiffness with comparable fracture properties.

[0311] Also with regard to the dynamic properties, the composition of the invention according to Ex. 15, compared to the reference composition of Ex. 14, showed similar dynamic modules and hysteresis.

[0312] Peeling test

[0313] Some of the elastomeric compounds reported above were subjected to the peeling test described above with the results shown in the following Tables 15 to 17:

[0314] Table 15: Peeling test at 100 °C carcass rubberising compounds (per Table 1 )

[0315] From the data reported in Table 15, it was observed that the rubberising compound according to the invention of Ex. 3 containing phenolic resin and oxaz- inv showed peeling force values only slightly lower than the excellent values of the reference of Ex. 2, much higher than those of the reference composition of Ex. 1 in production, predictive of high material performance in terms of tear resistance and reduced crack propagation.

[0316] Instead, the comparative composition of Ex. 4 containing bis-oxazolidine (l-A) described in US4361677, showed significantly lower peeling force values than all the other samples, effectively cancelling out the improvement in adhesion obtained with the use of the phenolic resin Alnovol instead of resorcinol (see the increase in the peeling force by switching from the composition of Ex. 1 with resorcinol to the composition of Ex. 2 with Alnovol). A peeling force as low as that of Ex. 4 was predictive of a greater tendency for the compound to tear and a lower resistance to crack propagation.

[0317] As may be observed from the graph in Figure 2, which shows the trend of the pulling force (N on the ordinate) as the deformation increases (mm on the abscissa) for the various samples, the curve of the compound according to the invention (Ex. 3) was in line with that of the reference composition of Ex. 2, containing in addition to the phenolic resin a conventional methylene donor, while that of the composition of Ex. 4 containing oxazolidine (l-A) as donor had a similar trend to that of the reference in Ex. 1 , with significantly poorer adhesion performance.

[0318] Table 16: Peeling test at 100 °C carcass rubberising compounds (per Table 3)

[0319] From the data reported in Table 16, it appeared that it was possible to successfully replace both resorcinol and formaldehyde in the secondary cross-linking composition with more sustainable products, i.e. while maintaining performance. In fact, the values of the peeling force of the compound according to the invention of Ex. 11 , although lower than those of the composition of Ex.10, were still acceptable being in line with the performance of the reference production compound of Ex. 8.

[0320] Table 17: Peeling test at 100 °C

[0321] Belt rubberising compounds (per Table 4)

[0322] From the data reported in Table 17 it was observed that the compound according to the invention of Ex. 13 unexpectedly showed an increase in the peeling force compared to the reference compound of Ex. 12, comprising phenolic resin and HMMM.

[0323] In conclusion, the comparative experimental data between the secondary crosslinking system of the invention comprising the oxazolidine agent of formula (I) and the comparative cross-linking system suggested by document US4361677, comprising instead the oxazolidine agent of formula (l-A) reported above, demonstrated clear and unexpected advantages of the materials according to the invention for example in terms of better static properties and lower fragility (lower CA3 Table 10, higher CR and AR - Table 10 and Table 11 ), higher tear resistance (Table 15), higher comfort (lower dynamic moduli - Table 10) and lower hysteresis (Table 10).

[0324] Adhesion tests to textile reinforcement materials

[0325] Inventive and reference carcass rubberising compounds, prepared from the compositions shown in Table 3, were subjected to the adhesion test described above with PET fibre textile materials.

[0326] The following Table 18 shows the results of this test.

[0327] Table 18: Adhesion test (T= 23 °C) carcass rubberising compounds (per Table 3) wherein

[0328] PET were PET 1672 F105 polyethylene terephthalate fibres from HYOSUNG VN;

[0329] Coverage refers to the visual assessment of the amount of compound that remains covering the strings after pulling.

[0330] From the data reported in Table 18 it was observed that the compound according to the invention of Ex. 11 , compared to the reference compound of Ex. 10, showed better adhesion to PET cords.

[0331] In conclusion, the experimental data reported above highlighted that the crosslinking composition according to the invention, comprising at least one methylene acceptor agent such as resorcinol or, preferably, at least one phenolic resin and at least one methylene donor agent of formula (I), when incorporated into elastomeric compounds for tyres, was able to impart to the materials optimal stiffness, breaking strength and adhesion to the reinforcement elements, to improve the cross-linking kinetics and the processability, at the same time maintaining or even reducing the hysteresis, compared to known secondary cross-linking systems.

[0332] The elastomeric materials according to the invention, comprising the present cross-linking system, more harmless than conventional ones and characterised by the above-mentioned properties, are advantageously used in those tyre components which require a certain rigidity and resistance to breakage (e.g. bead filling, sidewall insert) and / or a high adhesion to textile or reinforcement elements (e.g. carcass, belt, bead protection (chafer) or reinforcement (flipper) layers).

Claims

CLAIMS1. Cross-linking composition for elastomeric compounds comprising at least one methylene donor agent of formula (I)wherein R1 represents H, linear or branched C1-C20alkyl, linear or branched C1- C20alkenyl, said alkyl and alkenyl being optionally substituted in the chain with one or more oxygens, and at least one methylene acceptor agent.

2. The cross-linking composition according to claim 1 , wherein R1 represents linear or branched C1-C10alkyl, linear or branched C1-C10alkenyl, preferably linear or branched C1-C5alkyl, linear or branched C1-C5alkenyl, more preferably R1 represents ethyl.

3. The cross-linking composition according to claim 1 or 2, further comprising at least one other methylene donor agent selected from hexamethoxymethylmelamine (HMM) hexamethylenetetramine (HMT), hexamethylol melamine, N,N’-dimethylolurea, N-methylol dicyanamide, N-allyl dioxazine, N-phenyl dioxazine, N-methylol-acetamide, N-methylol-butyramide, N- methylol-acrylamide, N-methylol-succinimide, lauryloxymethylpyridinium chloride, ethoxymethylpyridinium chloride, trioxan hexamethoxymethylmelamine, hexamethylolmelamine pentamethyl ether (HMPE), oxazolidin derivatives other than those of formula (!) and mixtures thereof.

4. The cross-linking composition according to any one of the preceding claims, wherein the methylene acceptor agent is selected from phenol, a substituted phenol, a phenolic resin obtained by partial cross-linking of phenol and / or at least one substituted phenol with formaldehyde and / or other methylene donor agents, and mixtures thereof.

5. The cross-linking composition according to claim 1 or 2, comprising as sole methylene donor agent one or more agents of formula (I) and as sole methylene acceptor agent one or more phenolic resins.

6. Elastomeric composition comprising at least- 100 phr of at least one diene polymer,- at least 0.1 phr of at least one reinforcing filler,- 0.1 to 20 phr of a vulcanising agent, and- at least 0.05 phr of the cross-linking composition according to any one of claims 1 to 5.

7. The elastomeric composition according to claim 6, wherein:- said reinforcing filler is present in amount of at least 1 phr,- said vulcanising agent is a sulphur-based agent and is present in an amount of at least 0.5 phr,- said cross-linking composition is present in an amount from 0.1 phr to 30 phr.

8. Vulcanised elastomeric compound for vehicle wheel tyre obtained by mixing and vulcanising the elastomeric composition according to claim 6 or 7.

9. A vehicle wheel tyre comprising at least one tyre component comprising a vulcanised elastomeric compound according to claim 8.

10. The tyre according to claim 9, wherein said tyre component is selected from tread band, under-layer, anti-abrasive layer, sidewall, sidewall insert, minisidewall, liner, underliner, carcass structure rubberising layers, belt, zero-degree belt, annular bead anchoring structure, bead filler, bead reinforcement layers (flipper), bead protection layers (chafer) and sheet.

11. The tyre according to claim 9 or 10, wherein said tyre component comprises reinforcement elements, preferably made of aliphatic polyamides, polyethylene terephthalate or mixtures thereof.

12. The tyre according to claim 9 of self-supporting type comprising said vulcanised elastomeric compound at least in the sidewall inserts.

13. The tyre according to any one of claims 9 to 12 for high-performance passenger cars (HP, SUV and UHP).

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