Process for producing a vulcanizable rubber composition with silica-based reinforcing filler

IS 2 149 392 T3

DESCRIPTION

Process for producing a vulcanizable rubber composition with a silica-based reinforcing filler.

In its most general aspect, the present invention relates to a vulcanizable rubber composition of the type comprising a low carbon black content and incorporating a solid-based reinforcing filler - partially or fully replacing carbon black. - on a polymeric base, unsaturated chain, crosslinkable.

More particularly, this invention relates to a process for producing a composition of the aforementioned type, as well as to a rim obtainable from said composition.

Throughout the description that follows and the subsequent claims, the term "crosslinkable unsaturated chain polymeric base" is intended to indicate any non-crosslinked polymer, either natural or synthetic, capable of assuming all physico-chemical and mechaonic characteristics. topical elastomers, as a result of crosslinking (vulcanization) with sulfur-based systems. As is known, in the pneumaotics manufacturing sector, the main object of an increasing part of the research is to reduce the rolling resistance of tires as much as possible.

For this reason, the use of rubber compositions having a low content of carbon black in the manufacture of pneumatic tires has been proposed in the state of the art.

In these rubber compositions, the carbon black is partially or totally replaced by inorganic reinforcing fillers, called "light", such as gypsum, talc, kaolin, bentonite, titanium dioxide, various silicates and, especially, solid.

However, since the solid has a low affinity for the polymeric matrix of the composition, a suitable coupling agent is usually incorporated in said matrix, capable of chemically binding the solid to the polymeric matrix.

Thus, for example, European Patent Application No. EP 0 191 929 discloses adhesion promoters for the production of vulcanizates obtained from thermically vulcanizable compositions, which do not give rise to unpleasant odors during handling, which limit the scorching phenomena in the subsequent processing of the vulcanizable composition and that make it possible to produce vulcanizates with filler / elastomer bonds, without producing a significant reduction in vulcanization speed.

According to this European Patent Application, such adhesion promoters are oligosulfonates having the following general formula:

(RO) 3Si-Y-Sx-Y-Si (OR) 3 (I) or by means of a mixture of oligosulfonates of formula (I) and a structurally related, substantially dimeric condensation product having the formula

OR OR (RO) 3Si-Y-Sx-Y-Si-O-Si-Y-Sx-Y-Si (OR) 3 (II)

OR OR

Additionally, United States Patent No. 5,227,425 describes rubber compositions for tires, with low rolling resistance, obtained by mixing a polymeric base, which includes a copolymer of a conjugated diene, with a vinolic aromotic compound that has a content in vinyl groups from 5 to 50%, with large amounts of solid as filler and a solid-based coupling agent.

Similarly, European Patent Application EP-A-0 447 066 describes compositions for tires with low rolling resistance, in which an essential component for chemically binding the solid to the polymeric base is a coupling agent based on silane.

IS 2 149 392 T3

In accordance with the constant teachings of the prior art and the indications constantly provided by the manufacturers of said coupling agents (see, for example, the publication: "SILICA BASED TREAD COMPOUNDS: BACKGROUND AND PERFORMANCE" SILICE: BACKGROUND AND PERFORMANCE], page 14, tab. IV, published by Degussa in connection with the TYRETECH '93 conference that took place in Basel on 28-29 October 1993), the silica and the coupling agent would have to be added to, and incorporated into, the composition therein. time, in order to achieve the required effects of chemically bonding the solid to the coupling agent, during the preparation of the composition, and to the polymeric matrix during subsequent vulcanization operations.

However, the need to simultaneously incorporate the solid and the coupling agent into the composition has severely limited the maximum temperature achievable during mixing and mechanical working of the composition.

In fact, during the preparation of the composition the temperature of the same does not have to exceed 160 ° C - 170 ° C, if an irreversible tormic degradation of the coupling agent is to be avoided.

But the observance of the above temperature restriction implies a notable reduction in the mixing activity itself, which is essential for an optimal dispersion of the solid throughout the polymeric matrix.

The resulting insufficient dispersion of the solid in the composition causes, in turn, several drawbacks, substantially due to the extreme variability and the lack of homogeneity of the physico-mechaonic characteristics of the rubber composition from one area of ​​the same to another.

More particularly, in the various attempts to produce tires from the above compositions, a notable difficulty in drawing and a variability in size of the tire obtained from the rubber compositions also produced has been observed.

To all these drawbacks must be added another not negligible, that is, the reduced production capacity of the drafting devices used for the production of rims.

Actually, with rubber compositions, of the known state of the art, which comprise a high solid content, the drawing speed usually used for conventional rubber compositions cannot be maintained, but must be lowered to such a value as to ensure that the composition coming out of the row has a temperature of approximately 120^°C.

This is achieved with a draw speed equal to approximately 50% of normal speeds.

In fact, at higher speeds, porous areas have been found in the rim, which significantly compromises the stability of its dimensional characteristics.

It is believed that the origin of such zones is essentially due to the rapid evaporation of the water bound to the solid, which is contained in the rubber composition, as a consequence of the high temperature caused by the high drawing speed.

Obviously, the higher the drawing speed - and, consequently, the temperature reached by the composition - the more evident this negative phenomenon will be.

As a consequence, there is a drastic fall in the plant's hourly production capacity, which significantly affects production times and costs.

The technical problem underlying the present invention is to provide a process for producing a vulcanizable rubber composition incorporating at least one solid-based reinforcing filler, possessing both improved solid dispersion and phosphoric characteristics. improved mechanics and handling.

Consequently, according to a first aspect, the invention relates to a process for producing a vulcanizable rubber composition having a solid-based reinforcing filler, characterized in that it comprises the following steps:

IS 2 149 392 T3

- intimately mixing, at a temperature between 165 ° C and 180 ° C, and in the substantial absence of silica coupling agents, an unsaturated chain crosslinkable polymeric base, at least one reinforcing filler based on silica and, optionally, one or more non-crosslinking ingredients;

- adding to the rubber composition thus obtained at least one silica coupling agent based on a silane and, optionally, a second portion of said reinforcing filler based on silica;

- subjecting the resulting rubber composition to an intimate mixing process, at a temperature in the range of 110^°C to 160^°C;

- adding to said rubber composition, homogeneously dispersing, a suitable vulcanizing agent, at a temperature lower than the vulcanizing temperature.

In the following description and in the appended claims, the term "silica-based reinforcing filler" is intended to indicate a reinforcing filler based on silicon dioxide (silica), silicates, and mixtures thereof, which has a surface area, measured according to the BET method, between 100 and 300 m²/ g.

For the sole purpose of simplifying the present description, the fillers based on silica, object of the invention, would be indicated hereinafter by the term "silica".

According to the invention, it is particularly advantageous to incorporate into the rubber composition from 10 to 80 parts by weight of silica per 100 parts by weight of polymeric base.

In fact, when the rubber composition incorporates such an amount of silica, optimal results have been obtained for the tires obtained from it, in terms of its mechanical characteristics, the reduction of rolling resistance, to stability while running, in particular to adherence to a wet floor.

As would be readily apparent from the following, in fact, it has been found that when the amount of silica is less than 10 parts by weight per 100 parts by weight of polymeric base, its reinforcing activity on the rubber composition and on the products obtainable from it is reduced; while between 60 and 80 parts by weight, the handling characteristics of the rubber composition begin to drop drastically and notably, to cease to be, finally, unacceptable above 80 parts.

Among the useful polymeric bases for the purposes of the invention, the unsaturated chain polymers or copolymers obtained by polymerization of conjugated dienes and / or aliphatic or aromatic vinyl monoimers are preferred.

More particularly, the polymeric bases of the invention can be selected from the group comprising: natural rubber, 1,4-cis-polybutadiene, polychloroprene, 1,4-cis-polyisoprene, optionally halogenated isoprene-isobutene copolymers, butadiene-acrylonitrile , styrene-butadiene and styrene-butadiene-isoprene terpolymers, either prepared in solution or in emulsion, ethylenepropylene-diene terpolymers, and mixtures thereof.

According to the invention, such polymeric bases can be used either individually or mixed together, according to the desired characteristics of the finished product.

According to a preferred embodiment of the invention, the non-crosslinking ingredients, necessary to confer the necessary mechanical and workability characteristics on the rubber composition, are added to, and incorporated into, the polymeric base during the first mixing stage.

Such ingredients, known per se, are selected from the group comprising reinforcing fillers, such as, for example, carbon black, plasticizers, processing aids, antioxidants, aging retarding agents, etc.

Each of these ingredients is also chosen in amounts and proportions that can be easily determined by those skilled in the art, in order to obtain optimal values ​​of the above characteristics.

In addition, depending on the amount of silica to be incorporated into the rubber composition, the silica is

ES 2 149 392 T3 can be added to the polymeric base in two or more portions, the last of which can be dosed simultaneously with the coupling agent, as will be seen more clearly in the description that follows.

In fact, it has been noted that when the amount of silica to be incorporated into the rubber composition exceeds 50 parts by weight per 100 parts by weight of polymeric base, the addition and mixing of the salice in two or more separate stages they notably improve its dispersion in the polymeric base.

In accordance with the invention, it has also been surprisingly found that the required effects of the chemical bonding of the salt to the coupling agent during preparation of the rubber composition, and to the polymeric base during vulcanization, can be achieved by incorporating these compounds separately. in the rubber composition.

In fact, contrary to the constant teaching of the state of the prior art, it has been observed that the incorporation, at different times, in the rubber composition of the coupling agent and of all the silica, or at least a substantial part thereof, does not harm neither to the possibility that the salyx and the coupling agent react, nor to the stability of the bonds that form between the salyx, the coupling agent and the polymeric matrix.

On the contrary, the process that is the object of the present invention makes it possible to achieve important and unexpected advantages compared to what the state of the prior art may have allowed to anticipate.

According to a preferred embodiment of the invention, all of the salt, or at least a substantial part thereof, is incorporated into the rubber composition without any coupling agent, and is intimately mixed with the other ingredients of said composition of the rubber. rubber, at a temperature comprised between 165 ° C and 180 ° C, preferably approximately 175 ° C, much higher than that permitted up to now by the processes of the prior art.

Advantageously, the aforementioned high mixing temperature can be easily and quickly achieved by mixing the polymeric base, the salt and the optional non-crosslinking ingredients in conventional mixing apparatus known to those skilled in the art under the name "Banbury ”.

In fact, in such apparatuses, the rubber composition being prepared is subjected to a mechanical work which, because of its remarkable viscoelastic characteristics, causes its temperature to rise rapidly within the range of the above values.

According to the invention, the possibility of increasing the mechanical work experienced by the rubber composition makes it possible, on the one hand, to homogeneously disperse the salt throughout the rubber composition and, on the other hand, to eliminate a substantial part of the water from the latter. attached to the salice.

As regards the moisture content of the rubber composition, it has been observed, in particular, that said moisture can easily be reduced to a value equal to or less than about 80% of the value found in rubber compositions prepared from in accordance with the state of the prior art, in order to substantially reduce the creation of porous structures in the subsequent stages of processing the rubber compositions.

During the course of repeated tests carried out on the rubber compositions object of the invention, it has also been noticed that the salice is dispersed throughout the rubber composition and in the semi-finished products (for example, rims) or in the finished products (for example pneumatic) obtainable therefrom, in an extremely homogeneous, definable way, as will be better seen later by means of a specific dispersion index.

With the mixing apparatus of the above type, one has the added advantage of achieving the above advantageous characteristics in a time interval ranging from 1.5 to 5 minutes, preferably in about 3 minutes.

Clearly, the time required to complete this first mixing stage may vary within the range of the above values ​​depending on the actual formulation of the rubber composition and the structural and functional characteristics of the chosen mixing apparatus.

IS 2 149 392 T3

According to the invention, after having obtained a homogeneous dispersion of the solid throughout the rubber composition, accompanied by a suitable reduction of its water content, an appropriate coupling agent, capable of reacting chemically with the solid and to join the latter to the polymeric base in the subsequent vulcanization stage of the same.

For this purpose, suitable coupling agents contain at least one functional group capable of reacting with the reactive groups (hydroxyl, silyl, etc.) present on the external surface of the solid, at the pressure and temperature conditions existing during the tests. stages of preparation of the rubber composition; and at least one functional group capable of reacting with the polymeric base at the pressure and temperature conditions existing during the vulcanization of the rubber composition.

For the purposes of this invention, a particularly suitable coupling agent is a silane-based agent, having the following structural formula:

(R) 3-Si-CnH2nX (I) in which:

R is an alkyl or alkoxy group comprising 1 to 4 carbon atoms or one chlorine atom, n is an integer number from 1 to 6, and X is -Si_m-C_nH_2n-Sir)₃, a nitroso group, a mercapto group, an amino group, an epoxy group, a vinyl group, an imido group, a chlorine atom, one or more sulfur atoms or an SMY group in which Y is selected from the following functional groups:

<img file="ES2149392T3_D0001.tif" />

-CN- (CH3) 2 -CC = CH2

S OCH3 where m and n are an integer number from 1 to 6 and R is an alkyl or alkoxy group containing 1 to 4 carbon atoms or one chlorine atom.

Particularly suitable coupling agents having the above formula (I) are:

- bis (3-triethoxysilylpropyl) tetrasulfide,

- bis (2-triethoxysilyl ethyl) tetrasulfide,

- bis (3-trimethoxysilylpropyl) tetrasulfide,

- bis (2-trimethoxysilyl-ethyl) tetrasulfide,

- 3-mercaptopropyl-trimethoxysilane,

- 3-mercaptopropyl-triethoxysilane,

- 2-mercaptoethyl-trimethoxysilane,

- 2-mercaptoethyl-triethoxysilane,

- 3-nitropropyl-trimethoxysilane,

- 3-nitropropyl-triethoxysilane,

- 3-chloropropyl-trimethoxysilane,

- 3-chloropropyl-triethoxysilane,

- 3-chloroethyl-trimethoxysilane,

- 2-chloroethyl-triethoxysilane,

- 3-trimethoxysilylpropyl-N, N-dimethyl-thiocarbamool tetrasulfide,

IS 2 149 392 T3

- 3-triethoxysilylpropyl-N, N-dimethylthio-carbamoyl tetrasulfide,

- 2-triethoxysilyl-N, N-dimethylthiocarbamoyl tetrasulfide,

- 3-trimethoxysilylpropylbenzothiazole tetrasulfide,

- 3-triethoxysilylpropylbenzothiazole tetrasulfide,

- 3-triethoxysilylpropyl methacrylate monosulfide,

- 3-trimethoxysilylpropyl methacrylate monosulfide and analogs.

Particularly preferred among them is bis (3-triethoxysilylpropyl) tetrasulfide, marketed by DEGUSSA under the trade name Si69.

The silica coupling agent is incorporated into the rubber composition by subjecting the latter to intimate mixing according to the method described above, for the purpose of the preceding silica incorporation step, but at a temperature ranging from a value minimum of 110^°C up to a maximum value between 140 ° C and 160 ° C, depending on the amount of silica added simultaneously together with the coupling agent to the rubber composition.

In fact, as highlighted above, the silica can either be fully incorporated into the rubber composition separately from the coupling agent, in one or two different stages, or partially added with said coupling agent during the stage. mixing of the latter.

In this regard, it has been observed that the above advantageous effects of optimal dispersion and surface activation of the silica, as well as the removal of a substantial part of the water bound to it, can also be achieved by simultaneously adding silica to the composition. rubber the coupling agent together with a portion of the silica equal to not more than 1/3 of the total weight of the silica to be incorporated into the polymeric base.

In this case, the temperature of the rubber composition is maintained during the mixing stage between a minimum value close to 140^°C (in the substantial absence of silica) and a maximum value close to 160^°C, when the above maximum amount of silica is used (not less than 1/3 of the total weight).

According to the invention, it has surprisingly been found that the silica and the coupling agent, in addition to reacting with each other to a substantially complete degree, even when added to the rubber composition at different times, are also capable of chemically bonding with each other. yes at temperatures below those considered optimal (160^°C -165^°C).

Although this phenomenon has not been fully clarified, it is believed that the previous stage of mixing the silica, the polymeric base and other optional non-crosslinking additives is capable of somehow "activating" the external surface of the silica and / or making that a greater number of hydroxyl or silyl groups become, in some way, accessible to react with the coupling agent.

Thanks to this higher reactivity of silica, it has also been observed that a mixing time of 0.5 to 1.5 minutes is sufficient to allow both the incorporation of the coupling agent into the rubber composition and the reaction between the latter. and silica.

For the purposes of this invention, and regardless of the method chosen to add the silica (separately, in one or more stages, or at least partially with the coupling agent), the coupling agent based on a silane is added to the composition. of rubber in an amount of 4 to 15 parts by weight per 100 parts by weight of silica and preferably 8 to 10 parts by weight per 100 parts by weight of silica.

In fact, it has been observed that below 4 parts by weight, the coupling agent is not capable of reacting with all the functional groups accessible on the external surface of the silica, while above 15 parts by weight (for the type of silica indicated), there is no significant improvement in binding activity.

According to a preferred embodiment of the invention, the step of incorporating and homogeneously dispersing the silica and the step of incorporating and reacting the silica and the coupling agent are carried out at different times and / or in different apparatuses, after have downloaded the composition

ES 2 149 392 T3 made of rubber and have allowed it to cool once each of the stages has been completed.

As an alternative, the aforementioned mixing steps can also be carried out in the same mixing apparatus (Banbury or twin screw drawing bench), taking care, however, to lower the temperature of the rubber composition being worked. within the range of the aforementioned values ​​(110 ° C - 160 ° C), before starting the step of incorporation of the coupling agent.

Once the coupling agent has been homogeneously incorporated into the rubber composition, the latter is made vulcanizable by adding and incorporating thereto a suitable vulcanizing agent, optionally and preferably accompanied by suitable vulcanization activators and accelerators.

If the polymeric base is chosen from the preferred unsaturated chain crosslinkable polymers, the most advantageous vulcanizing agent to use is sulfur, or sulfur-containing molecules (sulfur donors), with accelerators and activators known to those skilled in the art. matter.

Among the vulcanization activators, zinc stearate, formed directly in the rubber composition by the addition of zinc oxide and steaoric acid, is preferred.

The vulcanizing agent, with the possible accelerators and activators, is incorporated into the rubber composition by subjecting the latter to an intimate mixing process according to the method previously specified for the purpose of the preceding steps of incorporation of the solid and its agent. coupling.

In order to avoid premature and undesired vulcanization of the rubber composition, its temperature was maintained at values ​​lower than the vulcanization temperature and, preferably, within a range of values ​​comprised between 100^°C and 110^°C.

For better temperature control, it is preferable, in this case, before adding the vulcanizing agent, discharge the rubber composition from the apparatus after incorporation of the coupling agent and allow it to cool, if necessary.

At this stage, it has been found that a mixing time of between 2.5 and 4 minutes is sufficient to achieve a homogeneous dispersion of the vulcanization system (vulcanization agent, accelerators and activators) throughout the rubber composition.

In a further embodiment of the process object of the present invention, vulcanization activators, for example zinc oxide and steaoric acid, can be incorporated into the rubber composition during the first of the above mixing stages, together with the solid , the polymeric base and the other non-crosslinking agents.

According to a further advantageous embodiment of the invention, it is also possible to reduce the overall number of process steps by simultaneously adding the above coupling agent, vulcanizing agents and accelerators to the rubber composition.

According to this invention, a vulcanizable rubber composition is provided by means of the aforementioned process, with a solid-based reinforcing filler and with a low content of carbon black, a composition that is particularly suitable for the manufacture of rubber tires. pneumaotics, in particular with low rolling resistance.

The above rubber composition comprises at least one crosslinkable, unsaturated chain polymeric base and at least one solid-based reinforcing filler homogeneously dispersed throughout the composition.

The rubber composition, object of the invention, preferably contains from 10 to 80 parts by weight of solid for every 100 parts by weight of the polymeric base.

As a non-limiting example only, a topical composition (expressed in parts by weight) of a rubber composition according to the invention is provided below:

IS 2 149 392 T3

<td>- polymeric base</td><td> 100</td>

<td>- carbon black</td><td> 0 - 80</td>

<td>- silica</td><td> 10 - 80</td>

<td>- coupling agent</td><td>4 - 15% of silica</td>

<td>- ZnO</td><td> 1 - 3</td>

<td>- steoric acid</td><td> 0 - 3</td>

<td>- antioxidants</td><td> 1 - 3</td>

<td>- anti-fatigue agents</td><td> 0,5 - 3</td>

<td>- sulfur or sulfur donors</td><td> 0,5 - 3</td>

<td>- accelerators</td><td> 0,5 - 3</td>

<td>- plasticizers</td><td> 3 - 30</td>

Thanks to the aforementioned high degree of dispersion, the rubber composition obtainable by the process of the invention is not only achieved with homogeneous characteristics, but also makes it possible to obtain pneumatic tires having the desired low value of rolling resistance.

Thanks to its constant characteristics and its low moisture content, the rubber composition obtainable by the process of the invention also has improved workability characteristics, which makes it possible to obtain semi-finished products by drawing, molding or calendering, and in particular pneumatic , of stable dimensions and with a porosity onyx of less than 2%, totally acceptable for subsequent uses.

According to a further aspect of the present invention, there is provided a low rolling resistance tire for high performance vehicle pneumaotics as defined in appended claim 13.

The rim object of this invention is preferably obtained by drawing, molding or calendering, at a temperature between 80 ° C and 120 ° C.

Thanks to the above-mentioned advantageous characteristic features of homogeneity and low moisture content, the vulcanizable rubber composition of the invention can be stretched at a speed much higher than the maximum speed permissible for solid-containing rubber compositions. of the state of the previous technique.

The extruded rim obtainable in this way advantageously possesses, in addition to the low rolling resistance required, a degree of porosity - defined as

[(dm - dt) / dm] x 100 where:

dm = density of a sample of the rubber composition as such and subjected to a compression treatment to eliminate any porosity thereof;

dt = density of a sample of the rubber composition, taken at the exit of the drawing bench from 0 to 2%.

Such a value is fully compatible with the characteristics required for a tire when the latter is mounted on a tire structure to be vulcanized.

Furthermore, the rim according to the invention has a solid dispersion onyx such that the difference between said ondx and that of a substantially solid-free rim, of the same composition (that is, in which all the solid has been replaced per carbon black), is less than 20,000.

In the description that follows and in the appended claims, the term "dispersion onyx" is intended to indicate a dimensionless numerical value related to the roughness of the external surface of the semi-finished product (for example, a rim) or finished ( for example, a pneumaotic) considered.

IS 2 149 392 T3

The value of the above index can be measured by means of commercially available devices, such as, for example, the dispersion analyzer from FEDERAL (an ESTERLINE company), which comprises a punch that is rolled on the surface of the sample to be analyzed and a device capable of registering and processing the movements of the punch at a right angle with respect to said surface.

These displacements can be graphically shown by representing the profile of the displacement of the punch, a profile that takes the appearance of a broken line characterized by a succession of peaks, in comparison with a reference line with a different amplitude and frequency.

Obviously, a perfectly smooth surface would produce a straight line, while a "rough" surface would produce a broken line; The more irregular and jagged the line is, the more numerous and different will be the obstacles encountered by the punzoín during its trajectory.

It is well known that a low dispersion throughout the rubber composition produces, due to vulcanization, a finished product with a surface full of micro-roughness, that the more abundant and noticeable they are, the worse the dispersion will be.

The analyzer detects the number of peaks found by the punch during a trajectory of a predetermined length on the surface of the sample and transforms them into a numerical value - defined here as the "dispersion index" - related to the product of the square of the Peak frequency times mean peak height, where peak frequency is measured in peaks per cm and mean peak height in micrometers.

According to a further aspect of the invention, there is provided a process for the manufacture of tires for vehicle wheels, particularly suitable for the manufacture of high performance tires, as defined in appended claim 14.

According to a further aspect, this invention also relates to a tire for vehicle wheels whose rim, manufactured according to the process of the invention, shows an optimal degree of dispersion of the silica throughout the polymeric base, so that the difference between said index and the dispersion index of a substantially silica-free tire of the same composition is less than 20,000.

Very surprisingly, tires manufactured according to the procedure of the invention, subjected to rolling tests and compared with totally identical tires but incorporating a rim manufactured according to the prior state of the art, have given much better results, as you will see mine ahead.

Although the phenomena involved have not been fully clarified so far, it is possible to think that the behavior of the tires running could be strongly affected not only by the presence of silica in the rim, but also by the way in which the silica has been incorporated. on the polymeric base.

Thus, according to a further aspect, this invention relates to a method for improving the rolling behavior of tires, as defined in appended claim 16.

More characteristics and advantages will become more easily evident by means of the description, which is included below, of some embodiments of the process for preparing vulcanizable rubber compositions, according to the invention, as well as the process for the manufacture of tires. and tires equipped with such tires.

The examples given below are given by way of non-limiting indication only, with reference to the accompanying drawing which shows a partially interrupted cross section of a pneumatic tire according to the invention.

With reference to the previous figure, a pneumatic tire 1 conventionally comprises at least one protective cover 2 whose opposite lateral edges are externally bent around a taloin wires 3, each of them incorporated in a taloin 4, defined along a inner circumferential edge of the tire where the tire itself engages on a wheel crown 5.

Along the circumferential development of the protective cover 2, one or more strap strips 6 are applied, made of textile or metallic cords, incorporated in a line of the rubber composition.

IS 2 149 392 T3

Externally to the protective cover 2, on the respective opposite side parts thereof, a pair of side walls 7 are applied, each of which extends from the heel 4 to a so-called "shoulder" area, 8, of the pneumaotic , defined in correspondence of the opposite ends of the strap strips 6.

On the strap strips 6 a rim 9 is applied circumferentially whose side edges end in correspondence of the shoulders 8 that join the side walls 7. The rim 9 has an external rolling surface 9a, designed to come into contact with the ground, in which circumferential grooves 10 can be created, interspersed by transverse grooves, not shown in the attached figure, which define a plurality of blocks. of rim 11 variously distributed along said tread surface 9a.

The pneumaotic 1 described above can be manufactured by means of a process that includes a plurality of production steps, which are conventional per se, and known in the state of the art.

More particularly, such a process comprises the steps of preparing, preliminarily and independently, various semi-finished products corresponding to the different parts of the tire (protective covers, strap tapes, bead wires, fillers, sidewalls and tires), which are successively mounted to each other by means of a suitable mounting machine.

The subsequent vulcanization step then welds the above semi-finished products to form a monolotic block, that is, a pneumaotic.

Clearly, the stage of preparing the above semi-finished products is preceded by a stage of preparation and formation of the corresponding rubber compositions.

In the pneumaotics of the invention, for example those with low rolling resistance, the rim 9 - in which the carbon black usually used as reinforcing filler is partially or totally replaced by solid or some other suitable component to improve the hysterotic characteristics of the rim - it is produced by forming a vulcanizable rubber composition, obtainable by the procedure described above.

Such a procedure has made it possible to avoid the process difficulties already described, as will be clarified in greater detail by means of the examples that are given below only by way of an indicative label and in no way limiting.

Example 1

In a closed rotor mixer (Banbury) model F270 from POMINI, which has been rotated at a speed of approximately 40 rpm, the following ingredients were successively loaded: 10 7 kg of E-SBR polymeric base, 37 kg of carbon black, 32 kg of solid, 2 kg of zinc oxide, 1 kg of stearic acid, 2.5 kg of antioxidant, 1 kg of an anti-fatigue agent and 16 kg of an aromatic oil as a plasticizer, as well as other ingredients commonly used in smaller quantities.

The characteristics of the ingredients used were the following:

E-SBR = butadiene-styrene copolymer prepared in emulsion, having a styrene content of 23%;

carbon black = type N115 (Cabot Corporation);

solid = BET 175 m²/ g, VN type (Degussa);

aging retardant agent = 6PPD, also known as SANTOFLEX 13 (Monsanto);

anti-fatigue agent = TMQ, also known as VULCANOX 4020 (Bayer).

More particularly, the various ingredients were loaded according to the following sequence of times (the times were measured from the moment the rotors were started):

IS 2 149 392 T3

- polymeric base, carbon black and solid 15 ”

- 1'30 ”additives

- 2 'plasticizer

The ingredients of the rubber composition loaded in this way were then intimately mixed for an additional period of about 1.45 ", for an overall time span of this first stage equal to about 3.45".

During mixing operations, the mechanical work imparted to the rubber composition was controlled so as to maintain its temperature within the range of approximately 165 ° C.^°C170^°C.

In this way, 200 kg of a rubber composition were obtained in which the solid turned out to be homogeneously dispersed.

After cooling to room temperature, the rubber composition obtained was charged to a mixer with closed rotors, type 11D, from the company POMINI, together with 2.35 kg of the coupling agent, of the silane type, Si69 [tetrasulfide of bis (triethoxysilylpropyl) (DEGUSSA).

In this regard, it should be noted that when the silaonic coupling agent is in liquid form, such as, for example, Degussa's Si69 agent, the incorporation of the latter into the rubber composition can lead to process difficulties that - in some cases - they can worsen the quality of the finished product; In such a case, and in a known manner, it is preferable to charge a small amount of an inert filler (such as, for example, carbon black or solid) together with the coupling agent, in order to facilitate the incorporation of the latter in the rubber composition.

As an alternative, coupling agents supplied in solid form can be used, such as, for example, X50S from Degussa (50% carbon black, 50% silane), in an amount that respects the values ​​for each one. of the ingredients listed above.

Also in this case, the rotors of the mixer were rotated at approximately 40 rpm, in order to subject the rubber composition and the coupling agent to intimate mixing, keeping the temperature of the composition at a value equal to approximately 135^°C.

After approximately 2'30 ”180 kg of a rubber composition were discharged, which were then reloaded again, after cooling to room temperature, along with 3.2 kg of a vulcanization system comprising: sulfur 1 kg, diphenylguanidine (DPG) (Monsanto) 0.6 kg, SANTOCURE NS 1.6 kg and vulcanization accelerators well known in the state of the art, in the same closed rotor mixer (Banbury) model 11D of the POMINI company, which was spun at approximately 20 rpm

Next, the rubber composition was subjected to intimate mixing in order to disperse the vulcanization system, taking care to maintain the temperature of the composition at a value equal to approximately 100^°C.

After 3 ', 170 kg of a vulcanizable rubber composition was discharged comprising, in parts by weight:

IS 2 149 392 T3

<td>- polymeric base</td><td> 100</td>

<td>- carbon black</td><td> 35</td>

<td>- silica</td><td> 30</td>

<td>- coupling agent</td><td>8% silica</td>

<td>- ZnO</td><td> 2</td>

<td>- stearic acid</td><td> 1</td>

<td>- antioxidants</td><td> 2,5</td>

<td>- anti-fatigue agents</td><td> 1,0</td>

<td>- plasticizers</td><td> 15</td>

<td>- sulfur</td><td> 1,2</td>

<td>- accelerators</td><td> 2,5</td>

Examples 2 - 5

According to the procedure described in Example 1 above, rubber compositions having the composition shown in Table I below were prepared.

TABLE I

<td>Ingredients</td><td>Ex. 2</td><td>Ex 3</td><td>Ex. 4</td><td>Ex. 5</td>

<td>S-SBR</td><td> 100</td><td></td><td></td><td> 75</td>

<td>E-SBR</td><td></td><td> 75</td><td> 75</td><td></td>

<td>NR</td><td> -</td><td> 25</td><td> -</td><td> -</td>

<td>BR</td><td> -</td><td> -</td><td> 25</td><td> 25</td>

<td>lampblack</td><td> 20</td><td> 50</td><td> 40</td><td> 20</td>

<td>silica</td><td> 50</td><td> 15</td><td> 20</td><td> 50</td>

<td>coupling agent</td><td> 5</td><td> 1,5</td><td> 2</td><td> 5</td>

<td>ZnO</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td>

<td>stearic acid</td><td> 1</td><td> 1</td><td> 1</td><td> 1</td>

<td>antioxidant</td><td> 2,5</td><td> 2,5</td><td> 2,5</td><td> 2,5</td>

<td>sulfur</td><td> 1,5</td><td> 1,5</td><td> 1,5</td><td> 1,5</td>

<td>accelerators</td><td> 2</td><td> 2</td><td> 2</td><td> 2</td>

S-SBR = butadiene-styrene copolymer prepared in solution, having a styrene content of 26%, a content of 1,4-trans bonds of 22% and a content of vinyl groups of 60%;

NR = natural rubber;

BR = polybutadiene having a 1,4-cis bond content equal to 93%;

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Other ingredients: as in Example 1.

Example 6

A rubber composition having the same composition as that of Example 1 above was prepared by simultaneously carrying out the steps of adding and mixing the coupling agent and the vulcanizing system into the rubber composition incorporating the silica.

In this case, the temperature of the final mixing stage was kept at a value equal to about 110 ° C, in order to avoid premature vulcanization of the rubber composition.

Example 7

A rubber composition having the same composition as that of Example 1 above was prepared by carrying out the mixing of the coupling agent just at the end of the first mixing step, without discharging the rubber composition from the Banbury mixer.

However, the temperature of the rubber composition was lowered to about 140 C at the end of the first stage gradually reducing the mechanical work to which the rubber composition was subjected by the rotors of the Banbury mixer.

In this case, the overall time span of the two stages was equal to approximately 6 '.

The subsequent step of incorporating the vulcanization system was carried out according to the method of Example 1.

Example 8 (comparative)

With the same amounts and the same ingredients used in Example 1, a first rubber composition was prepared, for comparison, by means of a preparation procedure according to the state of the prior art.

More particularly, all the ingredients, except sulfur and vulcanization accelerators, were loaded into a Banbury model F270 mixer from POMINI, which was rotated at approximately 40 rpm.

Given the simultaneous presence of the silica and the coupling agent, the temperature of the composition was maintained at a value equal to approximately 160 ° C during the mixing operations, which lasted approximately 3 '.

Next, the vulcanization system was added to the obtained rubber composition, and the whole was subjected to a mixing process in a Banbury model 11D mixer from the POMINI company, which was rotated at approximately 20 rpm.

Also in this case, the temperature of the composition was kept at a value of approximately 100^°C, lower than the vulcanization temperature, during mixing operations that lasted approximately 2 '.

Example 9 (comparative)

According to traditional methods known per se, a second rubber composition was prepared, for comparison, which included - as a reinforcing filler - only carbon black, and which had the following composition, expressed in parts by weight:

IS 2 149 392 T3

<td>- polymer base (E-SBR)</td><td> 100</td>

<td>- carbon black</td><td> 68,0</td>

<td>- ZnO</td><td> 2,0</td>

<td>- steoric acid</td><td> 1,0</td>

<td>- antioxidants</td><td> 2,5</td>

<td>- anti-fatigue agents</td><td> 1,9</td>

<td>- plasticizers</td><td> 2,4</td>

<td>- sulfur</td><td> 1,2</td>

<td>- accelerators</td><td> 1,8</td>

Example 10

Various batches of rubber compositions according to each of the above Examples 1, 8 and 9 were produced at subsequent times, each batch being made up of 15/20 fillers.

A sample was taken from each of the loads and then it was subjected to a vulcanization process using a method and apparatus known in the state of the art, and, later, it was subjected to various tests to determine, within each batch and for all the lots, the coefficients of variation of some parameters of particular significance.

Such parameters were:

- volumetric mass (mv): measured according to ISO 2751;

- modulus of elasticity at 100% elongation (CA1): measured according to ISO 37 standard (annular specimen);

- modulus of elasticity at 300% elongation (CA3): measured according to ISO 37 (annular test piece);

- breaking strength: measured according to ISO 37 (annular test piece);

- hardness: measured according to ISO 48 standard.

Table II below shows the results, expressed in the form of coefficients of variation of the tests carried out, coefficients that were both smaller the lower their variability, measured within each production batch.

TABLE II

Average values ​​of the coefficients of variation

<td>Composition</td><td>Lots Nos.</td><td>mv</td><td>CA1</td><td>CA3</td><td>CR</td><td>Hardness</td>

<td>Ex. 1</td><td> 6</td><td> 0,10</td><td> 2,33</td><td> 2,52</td><td> 5,50</td><td> 0,70</td>

<td>Ex. 8</td><td> 10</td><td> 0,29</td><td> 6,21</td><td> 5,24</td><td> 6,13</td><td> 1,86</td>

<td>Ex. 9</td><td> 17</td><td> 0,23</td><td> 4,47</td><td> 4,94</td><td> 4,83</td><td> 2,00</td>

Example 11 (Evaluation of solid dispersion onyx)

The rubber compositions obtained according to Examples 1, 6, 8 and 9, above, were subjected to a drawing to prepare tires for pneumotics of vehicles, using a method and apparatus known in the state of the prior art.

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Samples were taken from the tires of the tires (vulcanized) thus obtained and subjected to various tests in order to determine the dispersion index of the silica.

In the tests, carried out by means of the aforementioned dispersion analyzer from the company FEDERAL, the reference values ​​taken were those measured on rim samples obtained starting with the silica-free rubber composition of Example 9 above.

The analyzer used for the test detected the number of peaks found by the punch during a trajectory with a length equal to 14.25 mm.

Table III below shows the dispersion indices of tire samples, obtained from the rubber compositions of Examples 1 and 6 (according to the invention), from the rubber composition 8 (produced with the procedure of the state of the prior art) and from the rubber composition 9 (silica-free). These last samples were taken as references.

In order to indicate the dispersion of the ingredients, and that of the silica in particular, the first column shows the value of the dispersion index as recorded by the analyzer, the second column shows said value once the “noise of background ”caused by the roughness of the silica-free samples, thus highlighting, by difference of the two values, the degree of dispersion of the silica in the compared rubber compositions.

It can be easily observed that the difference between the dispersion index of the rubber composition containing silica and that of the same rubber composition in which all the silica has been replaced by carbon black is always less than 20,000.

TABLE III

Silica dispersion index

<td>Rubber composition</td><td>as such</td><td>only silica *</td>

<td>Example 1</td><td> 28.869,3</td><td> 11.688,0</td>

<td>Example 6</td><td> 33.283,7</td><td> 16.082,3</td>

<td>Example 8</td><td> 38.259,0</td><td> 21.057,7</td>

<td>Example 9</td><td> 17.201,3</td><td> -</td>

* = numerical data referred to values ​​measured on silica-free samples (rubber composition of Example 9).

Since the dispersion index of the silica is the higher the worse the dispersion of the silica throughout the tested rubber composition, the drastic improvements achieved by the invention, in comparison with the state of the prior art, are readily apparent to from the above data. Example 12

Shooting behavior

With the rubber compositions obtained according to Examples 1, 6 and 8, above, various tires were produced by drawing on conventional apparatuses, which tires were then mounted on 235 / 40-18 size pneumatics.

According to the invention, the rubber compositions of Examples 1 and 6 were stretched at a speed of approximately 21 m / min, producing tires having a porosity equal to approximately 2%, whereas with the rubber composition of Example 8 (state of the prior art) it was necessary to adopt drawing speeds lower by 30% (14.7 m / min).

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Despite the speed reduction, approximately 20% of the tires had to be scrapped due to lack of compliance with the geomometric requirements set.

The pneumaotics thus obtained were then subjected to standard tests in order to evaluate their performance while running.

In all the tests carried out, the tires incorporating a rim made with the rubber compositions of this invention were evaluated taking as reference the pneumaotics made with the rubber compositions of Example 8.

At the end of each of the tests, the test driver assigned a score from 0 to 10 to each of the examined evaluation parameters.

Tables IV and V show the results thus obtained, expressed as average values ​​of the evaluations made by the two different test drivers.

TABLE IV Tests on dry floor

<td>Parameters</td><td>Ex. 1</td><td>Ex. 6</td><td>Ex. 8</td>

<td>response to the wheel</td><td> 6</td><td> 5,5</td><td> 4.5</td>

<td>speed change speed</td><td> 5</td><td> 5</td><td> 4</td>

<td>transversal benefits</td><td> 6,5</td><td> 6</td><td> 4</td>

<td>behavior in curves</td><td> 6,5</td><td> 6</td><td> 3,5</td>

<td>straight line stability</td><td> 6,5</td><td> 6</td><td> 4</td>

TABLE V

Wet floor tests

<td>Parameters</td><td>Ex. 1</td><td>Ex. 6</td><td>Ex. 8</td>

<td>cornering stability (oversteer)</td><td> 7,5</td><td> 6</td><td> 5</td>

<td>cornering stability (understeer)</td><td> 7,5</td><td> 6</td><td> 4,5</td>

<td>straight line stability</td><td> 7</td><td> 6</td><td> 4,5</td>

<td>slip on the floor</td><td> 7,5</td><td> 6</td><td> 5</td>

<td>grip ability when cornering</td><td> 7</td><td> 6</td><td> 5</td>

As can be seen from the above tables, the pneumaotics of the invention give notably better performance than the pneumatics manufactured according to the state of the prior art.

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In particular, a notable improvement was noted in terms of transversal performance, the characteristics of grip capacity and behavior in curves, and in straight-line stability, both on wet and dry surfaces.

From what has been described and illustrated above, the various advantages achieved by the present invention can be easily appreciated, such as:

a) an improved dispersion of silica throughout the rubber composition and in the products that can be obtained from it;

b) a low moisture content of the rubber composition, with the resulting possibility of dramatically increasing its drawing speed, as well as improving the production capacity of the plants;

c) highly reproducible and constant values ​​of the physico-mechanical characteristics of the rubber compositions and of the products obtainable from them;

d) an improved performance of pneumaitics when taxiing, in particular on wet ground.

Obviously, those skilled in the art can introduce variants and modifications to the invention described above in order to satisfy specific and eventual requirements, variants and modifications that fall, in any case, within the scope of protection as defined in the claims that are attached below.