Bonding agent for vulcanizable polymers and metallic reinforcements

The invention relates to an adhesion promoter substance for better adhesion between a vulcanizable polymer and a metallic reinforcement.

Many parameters have to be taken into account in order to ensure sufficient and long-lasting adhesion of rubber to metal. For example, the surface of the metallic reinforcement must be modified so that the rubber can adhere to it. To this end, metallic reinforcements (generally steel cord), which should be embedded in rubber mixtures vulcanizable with sulfur, have been coated with a brass layer. Further developments were to replace brass with zinc-cobalt or zinc-nickel alloys (EP 536 545). The brass layer or zinc alloy serves to produce a mechanical interlocking with the rubber mixture during the vulcanization by forming an intermediate layer. A disadvantage, however, is that the intermediate layer does not cover the metallic reinforcement (steel cord) at all points, so that these points represent weak points of adhesion. In addition, sulfur is required for the intermediate layer formation, which must also be added to the rubber mixture. It is to create a permanent bond between rubber and metal still required that the rubber compound is very resistant to aging. Mixtures containing cobalt salts have been proposed. These rubber compounds in turn had the disadvantage of being poor in durability. It is also known for steel cord rubber coatings that the addition of resorcinol-formaldehyde condensation products is said to increase the aging resistance of the rubber mixture. However, these condensation products are toxicologically unsafe, which makes their processing difficult. So the various attempts have already been made to optimize the adhesion between rubber and metal. However, as a rule it is still necessary to coordinate the reinforcement and the rubber mixture of the adhesive mixture within narrow limits. This in turn has the disadvantage that when a further adjacent rubber layer is combined, the composition of the latter has to be adapted to that of the adhesive mixture and can therefore only be varied within narrow limits. A variation of the rubber composition affects the properties of the rubber product, so that they cannot be optimally adjusted. The need to achieve permanent and stable adhesion to the steel cord therefore limits the freedom to optimize other desirable mixing properties.

The invention has for its object to produce an improved adhesion between rubber and metallic reinforcements, which can be achieved favorably in terms of process technology.

According to the invention, this object is achieved in that the adhesion promoter substance has the following structure:         <b>X- (R)</b>_<b>n</b><b>- (Ar)</b>_<b>i</b><b>- (R)</b>_<b>m</b><b>-Y,</b><b>in which</b><ul id="ul0001" list-style="none"><li>X: -SH; -Si (Cl)₃; -Si [R₁(Cl)₂]; -Si (OR₁)₃; -Si [R₁(OR₁)₂]; -COOH; -COCl    with R₁ : Alkyl radical (e.g. methyl, ethyl or propyl radical) (branched or unbranched) <b>and</b></li><li>R: -CH₂-; -CF₂-; -CH₂-CO-NH-CH₂-; -CF₂-CO-NH-CF₂-; -CH₂-CO-NH-CF₂-; -CF₂-CO-NH-CH₂-    with 0≤n, m≤16 <b>and</b></li><li>Ar: aromatic and / or heteroaromatic system with or without a substituent    with 0≤i≤6 <b>and</b></li><li>Y: unsaturated residue, where <maths id="math0001" num=""><math display="inline"><mrow><mtext>n = m = i ≠ 0</mtext></mrow></math><img file="EP0802217A2_D0001.tif" /></maths>.</li></ul>

The invention is based on the knowledge that the above-mentioned class of compounds has a great adsorption power to metallic surfaces due to the functionality X and at the same time a covalent bond to the rubber is produced by the functionality Y during the vulcanization of the rubber. This bifunctional connection class as an adhesion promoter enables the rubber to adhere strongly to the metallic reinforcement.

The above-mentioned X functionalities result in a strong bond to the metallic or metal oxide surface through adsorption. Conventional pure metals such as aluminum, in particular also the elements of the subgroups of the periodic table (eg platinum, titanium, silver, copper, nickel, zinc, iron), alloys such as, for example, can be used as metallic substrates. B. steel, brass, as well as metal oxides such. B. alumina, iron oxide can be used. The strong adsorptive power of the X functionality to the metallic surface makes it possible to apply a monomolecular covering of adhesion promoter substance to the metallic surface. As a result, little bonding agent substance is required, which is inexpensive. It has proven to be particularly advantageous that the adhesion promoter substance has a thiol group (-SH) as X to adhere to the metallic substrate, which in the most favorable case forms a covalent bond to the metal surface. It has been found that, especially with these compounds, a largely spontaneous adsorption on the metallic surface occurs, which leads to a monomolecular covering of the metallic surface.

As a further structural component of the class of compounds according to the invention, R (alkyl radical, fluorinated alkyl radical, alkyl radical or fluorinated alkyl radical with or without acid amide grouping in between) serves as a "spacer". The chain length of R can be varied depending on the size of n and m. This affects the mechanical stability (chain mobility) of the binding, so that an optimal adaptation to the type of metallic surface or can be done by varying m to the vulcanizable polymer. A -CH is preferred₂-or -CF₂-Grouping with n, m ≤ 12, whereby an optimal adhesion promoter effect is achieved.

Conventional compounds from the literature, in particular oligophenyls and oligothiophenes, can be used as aromatic and / or heteroaromatic systems. 0 to 6 aromatic units, preferably 0 to 3, can be present. There are preferably substituents on this aromatic and / or heteroaromatic system which are clearly electron-depressing, e.g. B. NH₂Grouping, OCH grouping or else electron-withdrawing substituents, such as, for example, CN grouping or NO₂-Grouping. Depending on the nature of the substituent, these in turn can have an influence on the bond to the metallic surface or to the vulcanizable polymer. Examples of Ar at this point include benzene, aniline or pyrrole.

An aromatic and / or heteroaromatic system (Ar) which contains a thiophene has also proven to be favorable. As already mentioned generally for the aromatic and / or heteroaromatic system, the thiophene component of the adhesion promoter substance can also be provided with suitable substituents. The thiophene ring can be bound directly to R. However, it is also possible to bind the thiophene ring to an already existing aromatic system with or without suitable substituents. Advantageously, the method of electrode deposition has proven itself particularly for the thiophene as a “connection method” to an already existing aromatic system. In this electrochemical process it is assumed that electrons are removed from the thiophene under the influence of an electrical voltage. The resulting thiophene radical is able to combine with the existing aromatic system by splitting off a hydrogen proton. Since the voltage required to extract an electron from the thiophene is relatively low, the thiophene is particularly suitable for connection to an already existing aromatic system by means of electrode deposition ("grafting"). In principle, other systems, e.g. also other monomeric or oligomeric building blocks can be bound using this method.

As already mentioned, the adhesion promoter substance according to the invention has a functionality Y which makes it possible to form a covalent bond with the vulcanizable polymer. Y can e.g. B. an unsaturated radical such as styrene or isoprene grouping; -CH = CH₂ ; -CH = CHCl; -CH = CH-D, with D branched or unbranched alkyl radical (z. B. methyl, ethyl, propyl radical).

Depending on the type of polymer to be vulcanized, the functionality Y can be selected such that there is a firm bond between the polymer and the adhesion promoter substance during the vulcanization and thus also the reinforcement. Both elastomers and thermoplastic elastomers can be used as vulcanizable polymers. As elastomers z. B. Natural rubber, styrene-butadiene rubber, butadiene rubber, butyl rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene copolymer or ethylene-propylene copolymer are used. Polymers known from the prior art can be used as the thermoplastic elastomer. By applying the adhesion promoter according to the invention to the metallic reinforcement, it is possible to produce an excellent bond, at least between the elastomer block of the thermoplastic elastomer and the adhesion promoter via its functionality Y. Sufficient adhesion between the thermoplastic elastomer and the reinforcement is already achieved in this way. It is possible to activate vulcanization with sulfur or with peroxides. In the case of sulfur vulcanization in particular, the functionality Y of the adhesion promoter must have double or triple bonds in order to be able to form a bond with the polymer.

For the production of a composite of metallic reinforcement, adhesion promoter substance and vulcanizable polymer, it is advantageous if the adhesion promoter substance is dissolved in an organic solvent and the metallic reinforcement is drawn through this solution (dip process). The adhesion promoter substance can already have all the functionalities (X, Y) that are required for adhesion to both the reinforcement and the vulcanizable polymer. The metallic strength supports coated with the adhesion promoter substance can then be coated with a vulcanizable polymer and then subjected to the vulcanization. However, it is also possible to add an adhesion promoter substance to the dip solution which does not yet have the complete structure of the adhesion promoter substance according to the invention, so that the metallic reinforcement is first at least covered with the functionality (X) of the adhesion promoter substance assigned to it. Then z. B. by electrode deposition, the remaining structural part of the adhesion promoter substance is bound at least with the functionality (Y) to the part of the adhesion promoter substance that is already adhering to the metallic reinforcement. Then, as already mentioned, the vulcanizable polymer can be applied and vulcanized.

In principle, however, other conventional ways of applying the adhesion promoter to the metallic reinforcement are also possible, such as. B. the transfer of the adhesion promoter substance into a gas phase and subsequent deposition of the substance on the metallic reinforcement.

The use of the adhesion promoter substance according to the invention, with a monomolecular covering of the adhesion promoter substance on the metallic reinforcement generally being sufficient, means that only a little of this substance is required to completely cover the reinforcement, so that the use of the adhesion promoter according to the invention also a cheaper production of a metal-reinforced rubber product is possible. By varying the building blocks R and Ar as well as n and m, it is possible to act on the functionalities X and Y in such a way that a variation in the bond strengths of the adhesion promoter substance to the metallic reinforcement or to the vulcanizable polymer is ensured. As a result, various types of metallic reinforcement can be coated with different vulcanizable polymers that have excellent adhesion between them. In principle, however, it is not even necessary to monomolecularly cover the adhesion promoter substance on the metallic reinforcement (for example in the case of thermoplastic elastomers). The functionalities X and Y cause adsorption forces or covalent bonds to ensure a firm bond. The bond strength of a covalent bond is significantly greater than that of a mechanical toothing, which brings about adhesion in conventional brass coatings, so that, according to the invention, stronger adhesion is achieved. In addition, the disadvantage of mechanical interlocking, namely a lack of aging resistance of the composite, is eliminated by using the adhesion promoter substance according to the invention. According to the invention, rubber products which are reinforced with metallic reinforcing elements can now be produced, which are more corrosion-resistant and have better mechanical properties. In the case of polymers vulcanizable with sulfur, it is now no longer necessary to introduce an additional addition of sulfur into the rubber mixture, which was previously necessary for adequate formation of an adhesive layer for brass-coated reinforcement elements. By using the adhesion promoter according to the invention, a separate adhesive mixture can even be dispensed with.

The adhesion promoter according to the invention can be used for all rubber products that are reinforced with metallic reinforcement, such. B. for conveyor belts, air springs, belts, tubes and in particular for vehicle tires. In the latter, in which wire cores (generally solid rubber tires and pneumatic tires) and / or belt layers and / or carcass are made of steel, particular advantages have been achieved through the use of the adhesion promoter. By omitting the adhesive mixtures, the individual rubber mixtures can be optimally coordinated with one another in the manufacture of the vehicle tire, so that it is now also possible according to the invention, for. B. is possible to apply the tread compound directly to the belt. This means that the tire can be manufactured more cheaply and at the same time its running properties can be optimized. The tire also has a longer service life.

An exemplary embodiment is explained in more detail below with reference to a drawing.

It shows:<dl id="dl0001"><dt>Figure 1</dt><dd>schematic sequence of the production of a vulcanized composite of steel cord and a rubber mixture, in which the substance according to the invention is used as an adhesion promoter.</dd><dt>Figure 2</dt><dd>a radial cross section through a pneumatic vehicle tire</dd></dl>

The diagram in FIG. 1 shows that a cord of any steel type (for example 0.8% carbon, trace elements) is treated in step 1 with a solution, in the simplest case thiophenol (phenylmethanethiol, phenylethanethiol, etc.) in toluene. In principle, instead of a thiol, alkyl silicon chloride, such as. B. trichloromethylsilane, or alkylsiloxanes, such as. B. hexamethyldisiloxane or organic acid (or organic acid chlorides can be used. The concentration of the thiol in the solvent toluene is, for example, 0.1 molar. In step 2, excess thiol is washed off the steel cord with, for example, an alcoholic solution and in step 3 is subjected to a drying process at approx. 60 ° C. On the steel cord there is now a building block of the adhesion promoter substance according to the invention which adheres to the metal (steel cord) through a covalent sulfur bond. The part of the adhesion promoter substance (Y) is now bound to the previously located building block of the adhesion promoter substance on the strength member by means of electrode deposition (step 4). For this purpose, the treated steel cord is led into a three-electrode cell in which the thiophene (for example vinylthiophene in acetonitrile) is present as a 0.1 molar bath solution, the steel cord being poled as one of these. Another electrode is used as a reference (silver / silver nitrate electrode) and a platinum crystal as the working electrode. The voltage is increased from 0 to 0.75 V in 500 mV per second steps. An electron is torn from the thiophene by the applied voltage and the remaining thiophene radical is able to attach to the first building block of the adhesion promoter substance, which is already on the reinforcement (steel cord), with the abstraction of a hydrogen atom, whereby the mechanism of the attachment has not yet been clarified is.

Steps 1 and 4 are briefly summarized below:<chemistry id="chem0001" num="0001"><img file="EP0802217A2_D0002.tif" /></chemistry>

After the electrode position (step 4) there is a further washing process (step 5) with water, for example, and a drying process (step 6) at approx. 60 ° C. Steps 4 to 6 can also be omitted if a sufficiently reactive adhesion promoter substance can already be applied to the steel cord using the dip process (step 1). For this, the X and Y functionality must already be present on the adhesion promoter substance. The following connections are an example:<chemistry id="chem0002" num="0002"><img file="EP0802217A2_D0003.tif" /></chemistry>

After the steel cord has been coated with the adhesion promoter substance, step 7 follows, namely the application of the unvulcanized rubber layer. For this purpose, the steel cord covered with adhesion promoter runs into a calender and is covered with a calendered rubber plate (eg natural rubber mixture). This composite of steel cord, bonding agent and rubber plate can be processed in a next processing step 8 for the respective product manufacture. For the manufacture of a pneumatic vehicle tire e.g. B. The composite could represent the belt package that is placed on the carcass ply during tire building. During the further tire production, the unvulcanized tread is applied to the belt package. After completion of the green tire, it is placed in the vulcanizing press and vulcanized at elevated pressure and temperature.

FIG. 2 shows a pneumatic vehicle tire with a tread 1, a metallic bead core 2, which is located in the bead area 3, a bead reinforcement 4, a carcass 5, side walls 6 and a metallic belt 7. The bead core 2 and the belt 7 are generally made of metallic reinforcements. But the carcass 5 can also be made of steel cord. In particular, such tires are used for use in trucks. By using the adhesion promoter substance according to the invention, tires can be produced more cost-effectively, since an additional adhesive mixture for metallic reinforcements can be dispensed with. Furthermore, the individual rubber compounds can be better matched to one another so that the running properties of the tire can be optimized. Because of the good adhesion between metallic Strength members and rubber largely avoid corrosion problems, so that the tire can withstand higher mechanical loads and thus has a longer service life.