Method for manufacturing a coated rubber article and rubber article

Abstract

Method for manufacturing a coated rubber article, comprising the steps: (ii) Providing a semi-finished product in the basic form of the rubber article, wherein the semi-finished product consists of a rubber composition comprising at least one rubber and at least one crosslinking agent; (iii) Coating the semi-finished product with a solvent-free coating composition comprising at least one radically polymerizable monomer, (iv) Crosslinking the rubber composition and polymerizing the coating composition to obtain a coated rubber article; characterized in that the at least one radically polymerizable monomer has a boiling point of ≥150°C and the crosslinking of the rubber composition and the polymerization of the coating composition take place simultaneously.

Description

[0001] The invention relates to a method for solvent-free coating of rubber articles, as well as to the coated rubber articles produced by this method. The invention also relates to a friction-reducing surface coating. State of the art

[0002] The invention relates to a method for manufacturing a rubber article which is provided with a coating that gives the surface of the rubber article advantageous properties, in particular a reduced coefficient of friction.

[0003] Such a rubber article is known, for example, from WO 03 / 080717 A1. This document discloses a method for manufacturing a wiper blade, wherein the wiper blade is coated with a lubricant to improve its gliding properties, which is cured under the influence of ultraviolet radiation. According to one embodiment, the lubricant comprises graphite as a lubricant, an acrylate as a radiation-crosslinkable binder, and a solvent, in particular water. Polymerization is initiated using a photoinitiator and ultraviolet radiation. The described method includes extruding the rubber profile followed by vulcanization in a salt bath and subsequent coating with the lubricant. This multi-stage curing process is time-consuming, as a vulcanized rubber article must first be produced, which then has to be coated with a lubricant and subsequently cured.The vulcanization and curing times required for this process are unacceptable for inline production, which is the norm for mass-produced items like wiper blade profiles. Furthermore, it is disadvantageous that the resulting rubber product is prone to coating delamination, as only an insufficient bond is achieved between the surface of the rubber product and the coating layer.

[0004] The aforementioned problems are solved by the manufacturing process described below and the resulting coated rubber article.

[0005] In this regard, it is also known from DE 10 2010 003 139 A1 to manufacture wiper blades by providing an unvulcanized base body with a heat-curing coating and finally vulcanizing the molded body and hardening the coating.

[0006] Furthermore, it is known from GB 1 120 803 A that the coefficient of friction of elastomeric products can be reduced by applying a coating based, among other things, on acrylate monomers and a thermal initiator.

[0007] Furthermore, a method for protecting the surface of a double-joint cuff is known from DE 195 24 297 B4, in which acrylates with friction-reducing additives are heated to a temperature above the decomposition temperature of an initiator. Disclosure of the invention

[0008] A method for manufacturing a coated rubber article is proposed, comprising the following steps: (i) Providing a semi-finished product in the basic form of the rubber article, wherein the semi-finished product consists of a rubber composition comprising at least one rubber and at least one crosslinking agent; (ii) Coating the semi-finished product with a coating composition comprising at least one radically polymerizable monomer, (iii) Crosslinking the rubber composition and polymerizing the coating composition to obtain a coated rubber article, characterized in that the at least one radically polymerizable monomer has a boiling point of ≥150°C and the crosslinking of the rubber composition and the polymerization of the coating composition take place simultaneously.

[0009] The semi-finished product provided according to the invention consists of a rubber composition comprising at least one rubber and at least one crosslinking agent. The at least one rubber constitutes 80 to 99.99 wt.% of the rubber composition, and the at least one crosslinking agent constitutes 0.01 to 3 wt.% of the rubber composition.

[0010] In principle, all vulcanizable compositions of natural or synthetic polymers or copolymers are suitable as rubber, provided they can be converted to elastomers (i.e., cross-linked rubber) in the presence of radical-generating cross-linking agents. Suitable rubbers include, for example, materials such as isoprene rubber (natural rubber, NR), styrene-butadiene rubber (SBR), butadiene rubber (BR), acrylonitrile butadiene rubber (NBR), chloroprene rubber (CR), ethylene propylene rubber (EPDM), or mixtures thereof.

[0011] Preferably, the rubber composition comprises EPDM rubber, and more preferably, the proportion of EPDM rubber constitutes at least 50% by weight, and in particular at least 70% by weight, of the rubber composition. In one embodiment of the invention, only EPDM rubber is used as the rubber component, such that the proportion of EPDM rubber constitutes 80 to 99.99% by weight of the rubber composition.

[0012] In principle, any compound known to those skilled in the art can be used as a crosslinking agent, provided it is suitable for supplying radicals during the manufacturing process to effect the crosslinking of the rubber molecules, i.e., the vulcanization of the rubber, and to produce an elastomeric polymer. Preferred crosslinking agents are thermally labile compounds that decompose at elevated temperatures (e.g., >100°C, particularly >140°C) by forming radicals. Inorganic and organic peroxide compounds, especially organic peroxide compounds, are particularly preferred. Examples include peroxide compounds such as potassium persulfate (K₂S₂O₈), N,N-azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO), dicumyl peroxide, di-tert-butyl peroxide, or mixtures thereof.

[0013] In a preferred embodiment, the rubber composition in the present process comprises EPDM rubber in an amount of at least 50 wt.%, in particular at least 70 wt.% of the rubber composition, and a peroxide compound selected from dicumyl peroxide or di-tert-butyl peroxide as a crosslinking agent.

[0014] Furthermore, the rubber composition can include common additives. Examples include fillers such as carbon black, silica, or chalk; plasticizers such as mineral oils; masticating agents; processing aids; adhesives; antioxidants; and crosslinking agents such as vulcanization catalysts or crosslinking comonomers. These additives can constitute between 0 and 19.99% by weight of the rubber composition.

[0015] In one embodiment, the rubber composition comprises at least one crosslinking comonomer, which serves to improve the crosslinking of the rubber and the bonding of the coating layer to the base body of the rubber article. The crosslinking comonomer is selected, for example, from a radically polymerizable acrylate or methacrylate monomer. The amount of the crosslinking comonomer constitutes, for example, 1 to 10 wt.% of the rubber composition. A particularly preferred rubber composition comprises, for example, EPDM as the rubber and ethylene glycol dimethacrylate (EGDMA) as the crosslinking comonomer.

[0016] The rubber composition is obtained by mixing the components in the desired quantities. This can be done discontinuously using roller mills or internal mixers, or continuously in mixing extruders. It is important to keep the temperature of the composition as low as possible, preferably ≤140°C, and particularly ≤120°C, to prevent premature vulcanization of the rubber composition.

[0017] The resulting rubber composition is formed into the desired shape, which corresponds to the shape of the final rubber article. The intermediate product thus obtained is called a semi-finished product. Shaping can be carried out by any method known to those skilled in the art. Examples include extrusion, calendering, and compression molding. Preferably, shaping is achieved by extrusion. This method of shaping rubber compositions using suitable extruders is known to those skilled in the art.

[0018] Preferably, the rubber article produced by the method is a wiper blade profile for a windshield wiper. The semi-finished product is accordingly extruded as a continuous profile, for example, by a suitable forming process, in particular an extrusion process.

[0019] In the next step, the surface of the semi-finished product is completely or partially coated with a coating composition. This coating composition is characterized by the fact that it does not contain any solvent. In this context, solvents are understood to be volatile inorganic or organic compounds with a boiling point of less than 200°C, particularly less than 150°C, which do not participate as reactants (e.g., starting material, initiator, catalyst) in any of the reactions that characterize the process.

[0020] The coating composition comprises at least one radically polymerizable monomer. This at least one radically polymerizable monomer has a boiling point of ≥ 150°C, preferably ≥ 180°C, and particularly ≥ 210°C. This ensures that the coating composition remains stable even at elevated temperatures during the subsequent curing step and does not evaporate from the surface to be coated.

[0021] Furthermore, the radically polymerizable monomer preferably has a melting point of more than 60°C, in particular more than 80°C.

[0022] Furthermore, the at least one radically polymerizable monomer is characterized by the fact that the homopolymer of the monomer or the copolymer of a monomer mixture has a glass transition temperature T G, measured by differential scanning calorimetry (DSC), of less than 20°C. Preferably, the glass transition temperature T G at less than 0°C, particularly preferably at less than -20°C, especially at less than -30°C.

[0023] Suitable radically polymerizable monomers are, for example, monomers possessing ethylene-unsaturated functional groups (i.e., terminal carbon-carbon double bonds). Preferred examples of such radically polymerizable monomers are those containing at least one functional group selected from an acrylate, methacrylate, acrylamide, methacrylamide, styryl, or vinyl group. Furthermore, the radically polymerizable monomers may be singly or multiply substituted.Suitable substituents include, for example, residues selected from halogen atoms such as fluorine, chlorine, bromine or iodine atoms, linear or branched, unsaturated or (preferably) saturated alkyl residues with 1 to 6 carbon atoms, such as methyl, ethyl, propyl or isopropyl groups, as well as linear or branched, unsaturated or (preferably) saturated alkoxy residues with 1 to 6 carbon atoms, such as methoxy, ethoxy or isopropoxy groups.

[0024] Preferably, the radically polymerizable monomer is an acrylate monomer or a methacrylate monomer, in particular a methacrylate monomer.

[0025] Particularly suitable examples are radically polymerizable monomers of the following formula (I): where R represents H or CH3 and R' is a linear or branched, saturated or unsaturated alkyl group with 8 to 30 carbon atoms, a linear or branched, saturated or unsaturated ether group with 4 to 30 carbon atoms, or a linear or branched, saturated or unsaturated ester group with 4 to 30 carbon atoms. Radically polymerizable monomers of formula (I) are particularly preferred, where R represents CH3 and R' is a linear, saturated alkyl group with 10 to 14 carbon atoms or a linear or branched unsaturated ester group with 5 to 16 carbon atoms. Preferred monomers include, for example, lauryl methacrylate, N-decyl methacrylate, ethyl triglycol methacrylate (ETMA), ethylene glycol dimethacrylate (EGDMA) and trimethylolpropane trimethacrylate (TMPTMA).

[0026] The radically polymerizable monomers can be used individually or in combination with each other (i.e., as a monomer mixture). The radically polymerizable monomers preferably constitute 27 to 89.99 wt.%, preferably 37 to 79.99 wt.%, and particularly 47 to 69.99 wt.% of the coating composition.

[0027] Furthermore, the coating composition preferably comprises at least one friction-reducing additive, which serves to improve the sliding properties of the surface coating by reducing frictional resistance. The friction-reducing additive can, for example, be selected from graphite, polytetrafluoroethylene (PTFE), MoS₂, and / or polyamide. Preferably, the friction-reducing additive comprises graphite particles.

[0028] The particle size of the at least one friction-reducing additive is, for example, in the range of ≥1 µm to ≤10 µm, in particular ≥1 µm to ≤5 µm. The amount of friction-reducing additive, in particular graphite particles, preferably constitutes 10 to 70 wt.%, more preferably 20 to 60 wt.%, in particular 30 to 50 wt.% of the coating composition.

[0029] In one embodiment, the solvent-free coating composition comprises at least one thermal initiator. This is a thermally unstable compound, meaning it decomposes into radicals at elevated temperatures (e.g., >100°C, particularly >140°C). Suitable thermal initiators are potassium persulfate (K₂S₂O₈), N,N-azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO), dicumyl peroxide, di-tert-butyl peroxide, or mixtures thereof. The at least one thermal initiator preferably comprises 0.01 to 3 wt.%, more preferably 0.1 to 2 wt.%, and particularly 0.5 to 1.5 wt.% of the coating composition.

[0030] Optionally, the coating composition may include further additives to improve its properties. This applies in particular to wetting agents such as alkoxylated acetylenediols, especially ethoxylated acetylenediols (e.g., 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol ethoxylate), polyether-modified polysiloxanes, especially polyether-modified dimethylpolysiloxanes, or fluorocarbon-modified polyacrylates. These may be added in an amount of up to 3% by weight of the coating composition.

[0031] The coating composition can, in principle, be applied to at least a portion of the surface of the rubber semi-finished product using any coating process known to those skilled in the art, such as dip coating, roller coating, rod coating, brush coating, spray coating, doctor blade coating, flow coating, rotary coating, or slot coating. Spray coating (including inkjet and valvejet printing), dip coating, and curtain coating are preferred. The coating is preferably applied at a temperature at which the coating composition is in a liquid state. For example, the coating is applied at a temperature above 60°C, particularly within a temperature range of 60°C to 140°C.Preferably, the surface temperature of the rubber semi-finished product is also in the range of 60°C to 140°C. The thickness of the coating layer thus obtained is preferably ≥0.1 to ≤50 µm, particularly preferably ≥1 to ≤25 µm, and especially ≥5 to ≤15 µm.

[0032] After the rubber semi-finished product has been coated with the coating composition on at least part of its surface, the vulcanization of the rubber is initiated by the application of thermal energy. The described composition of the rubber and coating compositions allows the crosslinking of the semi-finished product (i.e., the vulcanization of the rubber) and the curing of the coating composition (i.e., the polymerization of the coating composition) to be thermally induced. The application of thermal energy converts the crosslinking agents contained in the rubber composition into radicals, which cause the crosslinking (vulcanization) of the rubber by radically linking the double bonds contained in the rubber. This process can be enhanced, if necessary, by the addition of suitable vulcanization catalysts or accelerators. [The last sentence appears to be incomplete and requires context to be translated accurately.]Radicals generated there come into contact with the coating layer and the radically polymerizable monomers it contains. These surface radicals thus induce polymerization of the coating layer. The polymer chains of the coating layer formed from the radically polymerizable monomers are therefore grafted onto the surface of the rubber or elastomer (i.e., the cross-linked rubber). This results in a uniform and intimate bond between the surface coating and the surface of the cross-linked rubber or rubber article, as a multitude of covalent bonds are formed between the two components, rubber and coating layer.

[0033] In one embodiment, the coating composition can include radical-forming thermal initiators, which thus improve the polymerization of the coating layer. The hardening of the coating composition is initiated by the decomposition of the thermal initiators contained therein. This decomposition generates radicals that initiate the radical polymerization of the radically polymerizable monomers. In this way, the coating layer is transformed into a polymer layer in which any friction-reducing additives and auxiliary substances are incorporated.

[0034] Furthermore, the reactions, each proceeding via a radical reaction mechanism, can bypass the interface between the rubber semi-finished product and the coating layer. This occurs because radicals from the rubber initiate the polymerization of monomers from the coating layer or react with radicals from the coating layer. Similarly, radicals from the coating layer can initiate the vulcanization of the rubber semi-finished product or react with suitable double bonds from the rubber. This results in improved adhesion between the two layers (rubber and coating layer) because both layers are linked together by a multitude of covalent bonds.

[0035] In a preferred embodiment, both the rubber composition and the coating composition comprise at least one thermally labile peroxide compound. In a particularly preferred embodiment, the same compound is used as the peroxide compound in both the rubber composition and the coating composition. Suitable peroxide compounds are, for example, dicumyl peroxide or di-tert-butyl peroxide. This ensures that the thermally induced reactions in the rubber composition and the coating layer begin at the same temperature and thus as simultaneously as possible.

[0036] To induce vulcanization of the rubber compound and polymerization of the coating layer, the coated rubber semi-finished product is heated to a temperature above the temperature at which the thermal decomposition of the crosslinking agent or the thermal initiator begins in at least one of the compositions (i.e., the rubber compound or the coating compound). The temperature is preferably ≥150°C, more preferably ≥175°C, and particularly ≥200°C, for example, in a range of ≥200°C to ≤250°C, and particularly in a range of ≥210°C to ≤230°C.

[0037] The temperature increase can be achieved in any way known to a person skilled in the art, e.g. by a hot air process, a salt bath process or a microwave process.

[0038] From a process engineering perspective, it is particularly advantageous to carry out the vulcanization of the semi-finished product and the polymerization of the coating composition in a salt bath. The salt bath is, for example, a mixture of 50 wt% KNO3, 40 wt% NaNO2, and 10 wt% NaNO3 and has a melting point of approximately 150°C. It is inert to rubbers and thus enables vulcanization and polymerization in the absence of atmospheric oxygen.

[0039] The coated rubber article can then be cleaned of impurities and any excess monomer by suitable washing processes and, if necessary, cut to the desired dimensions.

[0040] The invention also relates to a coated rubber article obtained according to the described method. As already described, this article is characterized by the fact that the coating layer and the base body of the rubber article, consisting of cross-linked rubber, are connected to each other by a multitude of covalent bonds. This results in a particularly effective application of the coating layer and prevents delamination or peeling of the layer.

[0041] The disclosure also relates to a coated rubber article (not claimed), which is characterized in that the rubber article comprises a body made of cross-linked rubber onto which a polymer layer is grafted, the polymer layer being obtained by radical polymerization of ethylene unsaturated monomers with a boiling point of ≥150°C. This rubber article is particularly characterized in that the polymer or coating layer is covalently bonded to the base body made of cross-linked rubber.

[0042] In a preferred embodiment, the coated rubber article has a gradient in its material composition such that—starting from the coating surface of the coated rubber article—the proportion of elastomer in the material composition increases continuously. A continuously increasing gradient is obtained, in particular, if a crosslinking comonomer has been added to the rubber composition, especially a crosslinking comonomer that is also contained in the coating composition. In one embodiment, the rubber composition comprises EPDM rubber and ethylene glycol dimethacrylate (EGDMA) as a crosslinking comonomer, and the coating composition comprises ethylene glycol dimethacrylate (EGDMA) as a radically polymerizable monomer.The coated rubber article obtained according to the described process is characterized by a particularly continuous material composition gradient and an intimate bond between the coating layer and the base body of the rubber article.

[0043] Furthermore, the invention relates to a friction-reducing surface coating on a rubber article, which is obtainable by (a) Applying a coating composition comprising at least one radically polymerizable monomer with a boiling point above 150°C, at least one friction-reducing additive and at least one thermal initiator to a surface to be coated of a semi-finished product in the basic form of a rubber article, and (b) Heating the coated surface to a temperature above the decomposition temperature of the initiator.

[0044] The components of the coating composition constituting the surface coating correspond in all details of execution to the components of the coating composition described above. The friction-reducing surface coating is preferably applied to the surface of a rubber article and to the surface of an object over which such a coated rubber article is to slide. This combination of the friction-reducing surface coating on both surfaces that are to slide past each other results in a particularly effective reduction of frictional resistance. In a preferred embodiment, the rubber article is, for example, a wiper blade of a windshield wiper, and the object over which the rubber article is to slide is a glass pane, for example, a windshield of a motor vehicle. The windshield can be made of, for example, glass or plastic.Preferably, the windshield is made of plastic. Advantages of the invention

[0045] The presented method makes it possible to significantly reduce the time required for the production of a coated rubber article.

[0046] This process enables the vulcanization of the rubber compound and the polymerization of the coating compound to be carried out in a single step, thus reducing the production time and manufacturing costs of coated rubber articles. Additionally, the absence of solvents eliminates the need to dry the coating layer prior to polymerization.

[0047] Furthermore, the coated rubber article produced using this method exhibits improved properties compared to conventionally manufactured rubber articles. The described manufacturing process yields surface coatings on rubber articles where the coating is bonded to the surface of the article by a multitude of covalent bonds. The product is therefore characterized by a surface coating that is particularly resistant to peeling or delamination.

[0048] In combination with another surface that has the same coating layer, improved sliding properties and particularly low coefficients of friction are achieved. Description of an exemplary embodiment

[0049] The drawing shows a rubber semi-finished product in the form of a wiper rubber profile 10, comprising a wiper lip 12 having a wiping edge 11 made of a rubber composition consisting of 100 parts by weight of EPDM rubber and 2 parts by weight of di-tert-butyl peroxide, a head part 13 made of the rubber composition (only partially shown here), and a so-called rib 14 made of the rubber composition, which is arranged between the head part 13 and the wiper lip 12.

[0050] The wiper lip 12, the head 13, and the rib 14 are provided with a surface formed by a coating layer 15, which, for clarity, is shown enlarged in comparison to the wiper lip 12, the head 13, and the rib 14. The coating layer 15 has a thickness of approximately 10 µm and consists, for example, of 54 wt.% ethyl triglycol methacrylate (ETMA), 45 wt.% graphite particles with a particle diameter of 30 to 50 µm, and 1 wt.% di-tert-butyl peroxide.

[0051] In the present case, the coating layer 15 is only formed in those areas that typically come into contact with a vehicle windshield during the wiping process. However, in further embodiments of the inventive method, it is also possible to form the coating layer 15 over the entire surface.

[0052] The wiper rubber profile 10 shown in the drawing is manufactured by first extruding a basic shape consisting of the head part 13, the rib 14, and the wiper lip 12. This basic shape is then coated with the coating composition described above using a spraying process. The resulting coated rubber semi-finished product is vulcanized in a salt bath (50 wt.% KNO3, 40 wt.% NaNO2, 10 wt.% NaNO3) at 220°C. Simultaneously, the coating layer 15 is polymerized.

[0053] If necessary, pretreatment of the surface of the rubber body, consisting of the wiper lip 12, the rib 14, and the head 13, may be required to create a homogeneous surface. After vulcanization of the rubber semi-finished product and polymerization of the coating layer 15, the wiper rubber profile 10, in particular the wiper lip 12, is subjected to a washing process and cut to the desired dimensions.

Claims

[1] Method for manufacturing a coated rubber article, comprising the steps: (ii) Providing a semi-finished product in the basic form of the rubber article, wherein the semi-finished product consists of a rubber composition comprising at least one rubber and at least one crosslinking agent; (iii) Coating the semi-finished product with a solvent-free coating composition comprising at least one radically polymerizable monomer, (iv) Crosslinking the rubber composition and polymerizing the coating composition to obtain a coated rubber article; characterized by , that the at least one radically polymerizable monomer has a boiling point of ≥150°C and that the crosslinking of the rubber composition and the polymerization of the coating composition occur simultaneously. [2] Method according to claim 1, characterized bythat the crosslinking agent is a peroxide compound. [3] Method according to claim 1 or 2, characterized by that the coating composition includes a friction-reducing additive and, if necessary, at least one thermal initiator. [4] Method according to any one of claims 1 to 3, characterized by , that the at least one radically polymerizable monomer has at least one functional group selected from an acrylate, a methacrylate, an acrylamide, a methacrylamide, a styryl or a vinyl group. [5] Method according to any one of claims 1 to 4, characterized by that the homopolymer of the monomer or the copolymer of the monomer mixture has a glass transition temperature T G , measured using differential scanning calorimetry (DSC), of less than 20°C. [6] Method according to any one of claims 1 to 5, characterized bythat at least one radically polymerizable monomer has a melting point of more than 60°C. [7] Method according to any one of claims 1 to 6, characterized by , that the step of vulcanizing the rubber composition and polymerizing the coating composition (step (iii) of the process) is carried out in a salt bath. [8] Coated rubber article obtained according to a method according to any one of claims 1 to 7. [9] Friction-reducing coating on a rubber article, produced according to a method according to any one of claims 1 to 7, further obtainable by c) applying a coating composition comprising the at least one radically polymerizable monomer having a boiling point of ≥150°C, at least one friction-reducing additive and at least one thermal initiator to a surface to be coated of a semi-finished product in the basic form of a rubber article, and d) heating the coated surface to a temperature above the decomposition temperature of the initiator. [10] Surface of a rubber article which is provided with the friction-reducing coating according to claim 9. [11] Surface according to claim 10, characterized by , that the rubber article is a wiper rubber profile (10) of a windscreen wiper.

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