PROCEDURE FOR MANUFACTURE OF A COATED RUBBER ARTICLE AND RUBBER ARTICLE

Abstract

A method for manufacturing a coated rubber article comprising the steps of: (i) obtaining a semi-finished product having the basic shape of the rubber article, this semi-finished product being made from a rubber composition comprising at least one rubber and at least one crosslinking agent, (ii) coating the semi-finished product with a solvent-free coating composition containing 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 radically polymerizable monomer has a boiling point ≤ 150°C and the crosslinking of the rubber composition and the polymerization of the coating composition are carried out simultaneously.

Description

1. Field of the Invention The present invention relates to a solvent-free coating method for rubber articles and to a rubber article obtained by implementing this method. The invention also relates to a surface coating that reduces friction. Prior Art The present invention relates to a method for manufacturing a rubber article equipped with a coating that gives the surface of this rubber article advantageous properties, in particular, a reduced coefficient of friction. Such a rubber article is, by way of example, known from document WO 03 / 080717 A1. This document describes a method for manufacturing a wiping blade, this wiping blade being coated with an anti-friction varnish to improve its sliding properties, which is hardened under the action of ultraviolet radiation.This antifriction varnish, in one embodiment, contains graphite as a lubricant, an acrylate as an irradiation-curable binder, and a solvent, in particular, water. Polymerization is initiated using a photoinitiator and ultraviolet radiation. The described process comprises an extrusion step of the rubber profile followed by salt bath vulcanization and then coating with the antifriction varnish. This multi-step curing process is time-consuming, as it requires first manufacturing a vulcanized rubber article, then coating it with the antifriction varnish, and finally curing it. The vulcanization and curing times thus required are unacceptable for in-line manufacturing, as is the norm for high-volume consumer goods such as wiper blade profiles.Another drawback of the resulting rubber article is that the coating tends to peel off because the bond between the surface of the rubber article and the coating layer is insufficient. These problems have been resolved by the manufacturing process described below and the coated rubber articles thus produced.Summary of the invention 5 The invention relates to a method for manufacturing a coated rubber article comprising steps of: (i) preparing a semi-finished product having the basic shape of the rubber article, this semi-finished product being made of a rubber composition containing at least one rubber and at least one crosslinking agent, (ii) coating the semi-finished product with a coating composition containing at least one radical polymerizable monomer, (iii) crosslinking the rubber composition and polymerizing the coating composition to obtain a coated rubber article, characterized in that the radical polymerizable monomer has a boiling point of 150°C and the crosslinking of the rubber composition and the polymerization of the coating composition are carried out simultaneously.The semi-finished product according to the invention is made from a rubber composition containing at least one rubber and at least one crosslinking agent. The rubber represents 80 to 99.99% by weight of the rubber composition, and the crosslinking agent represents 0.01 to 3% by weight of the rubber composition. Suitable rubbers are, in principle, all vulcanizable compositions of natural or synthetic polymers or copolymers that can be transformed, in the presence of a crosslinking agent that gives rise to radicals, into elastomers (i.e., vulcanized rubber). Suitable rubbers include, for example, compounds 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 of these compounds.3046604 3 The rubber composition preferably consists of a composition containing EPDM rubber and, particularly preferably, a composition in which the proportion of EPDM rubber represents at least 50% by weight and, in particular, at least 70% by weight of the rubber composition. According to one embodiment of the invention, EPDM rubber is used exclusively as a rubber component, such that the proportion of EPDM rubber represents 80 to 99.99% by weight of the rubber composition. 10 As a crosslinking agent, in principle, any compounds known to specialists that are suitable for generating radicals within the manufacturing process can be used, thereby causing the crosslinking of the rubber molecules, i.e., the vulcanization of the rubber, and obtaining an elastomer.Preferred crosslinking agents are thermally unstable compounds that decompose at high temperatures (e.g., above 100°C, particularly above 140°C), forming radicals. Inorganic or organic peroxide compounds, and especially organic peroxide compounds, are particularly preferred. Examples of such compounds include peroxides such as potassium persurbate (K25208), N,N-azobisisobutyro nitrile (A1BN), dibenzoyl peroxide (BPO), dicumyl peroxide, di-tert-butyl peroxide, or mixtures of these compounds.25 According to a preferred embodiment, the rubber composition produced according to this process contains, as rubber, EPDM in an amount of at least 50% by weight, in particular at least 70% by weight of the rubber composition, and a peroxide compound selected from dicumyl peroxide 30 and di-tert-butyl peroxide as a crosslinking agent. The rubber composition may further contain common additives. Examples of such additives are fillers such as carbon black, silicic acid, or chalks; plasticizers such as mineral oils; masticating agents; manufacturing auxiliaries; adhesives; anti-aging preservatives; and crosslinking auxiliaries such as vulcanizing catalysts or crosslinking comonomers. These additives can represent 0 to 19.99 `) / 0 by weight of the rubber composition.5. In one embodiment, the rubber composition contains at least one crosslinking comonomer for improving the crosslinking of the rubber and the bonding of the coating layer to the base body of the rubber article. The crosslinking comonomer is, for example, selected from an acrylate or methacrylate monomer that can be polymerized by radical polymerization. The amount of crosslinking comonomer represents, for example, 1 to 10% by weight of the rubber composition. A particularly preferred rubber composition contains, for example, EPDM as the rubber and ethylene glycol dimethacrylate (EGDMA) as the crosslinking comonomer. The rubber composition is obtained by mixing its components in the desired quantities. This mixing can be carried out continuously using internal mixers or extruders.To this end, it is necessary to ensure that the temperature of the compound is kept as low as possible, preferably 140°C and in particular 120°C, to prevent premature vulcanization of the rubber compound. The resulting rubber compound is shaped as desired, corresponding to the shape of the subsequent rubber article. The intermediate product thus obtained is designated as a semi-finished product. Shaping can be carried out by any process known to specialists. Examples include extrusion, rolling, and compression molding. Shaping is preferably achieved by an extrusion process. This process of shaping rubber compounds using a suitable extruder is known to specialists. The rubber article obtained by implementing this process is advantageously a wiper blade profile.The semi-finished product is thus extruded in the form of an endless strand, for example, by a suitable forming process, in particular an extrusion process. In the next step, the surface of the semi-finished product is coated, in whole or in part, with a coating composition. This coating composition is characterized by the fact that it does not contain a solvent. For the purposes of this description, a solvent is defined as inorganic or volatile organic compounds with a boiling point below 200°C, and in particular below 150°C, which do not participate as a reactive agent (e.g., reducer, initiator, catalyst) in any of the reactions characterizing the process. The coating composition contains at least one polymerizable monomer.This radical polymerizable monomer has a boiling point of 150°C, preferably 180°C, and particularly preferably 210°C. The coating composition can thus remain stable in subsequent curing steps, even at high temperatures, and is not extracted by vaporization from the surface to be coated. Furthermore, the radical polymerizable monomer preferably has a melting point above 60°C, particularly above 80°C. The radical polymerizable monomer is further characterized in that the homopolymer of the monomer, or the copolymer of a mixture of monomers, has a glass transition temperature (GT) measured by dynamic differential scanning calorimetry (DSC) below 20°C. Preferably, the glass transition temperature TG is located below 0°C, particularly preferentially below -20°C and especially below -30°C.30 Suitable root-polymerizable monomers include, for example, monomers containing ethylenically unsaturated functional groups (i.e., carbon-carbon double bonds at the ends of chains). Preferred examples of such root-polymerizable monomers 35 are monomers containing at least one functional group selected from an acrylate, methacrylate, acrylamide, methacrylamide, styryl, or vinyl group. Root-polymerizable monomers may also be mono- or poly-substituted.Suitable substitutes include, for example, entities selected from halogen atoms such as fluorine, chlorine, bromine, or iodine atoms; linear or branched, unsaturated or (preferably) saturated alkyl groups having 1 to 6 carbon atoms such as methyl, ethyl, propyl, or isopropyl groups; and linear or branched, unsaturated or (preferably) saturated alkoxy groups having 1 to 6 carbon atoms such as methoxy, ethoxy, or isopropoxy groups. The monomer polymerizable by root polymerization is preferably an acrylate monomer or a methacrylate monomer, particularly a methacrylate monomer.15 Particularly suitable examples are radical polymerizable monomers corresponding to formula (1) below: 20 in which R represents H or CH3 and R' represents a linear or branched, saturated or unsaturated alkyl group having 8 to 30 carbon atoms, a linear or branched, saturated or unsaturated ether group having 4 to 30 carbon atoms, or 25 a linear or branched, saturated or unsaturated ester group having 4 to 30 carbon atoms. Particularly preferred radical polymerizable monomers corresponding to formula (1) are monomers in which R represents CH3 and R' represents a saturated linear alkyl group having 10 to 14 carbon atoms or 30 an unsaturated linear or branched ester group having 5 to 16 carbon atoms.Preferred monomers include, for example, lauryl methacrylate, N-decyl methacrylate, ethyltriglycol methacrylate (ETMA), ethylene glycol dimethacrylate (EGDMA), and trimethylol propane trimethacrylate (TMPTMA). Radical polymerizable monomers can be used alone or in combination (i.e., as a monomer mixture). Radical polymerizable monomers preferably constitute 27 to 89.99% by weight, preferably 37 to 79.99% by weight, and particularly 47 to 69.99% by weight of the coating composition. The coating composition also preferably contains at least one anti-friction additive which has the function of improving the sliding properties of the surface coating by reducing friction resistance.This antifriction additive can be, for example, selected from graphite, polytetrafluoroethylene (PTFE), MoS2, and / or a polyamide. The antifriction additive preferably contains graphite particles. The particle size of the antifriction additive is, for example, in the range of 1 µm to 10 µm, particularly from 1 µm to 10 µm. The quantity of antifriction additive, particularly graphite particles, preferably represents 10 to 70% by weight, more preferably 20 to 60% by weight, and particularly 30 to 50% by weight of the coating composition. According to one embodiment of the invention, the solvent-free coating composition contains at least one thermal initiator. This is a thermally unstable compound, 25 that is to say, decomposing into radicals at high temperatures (for example above 100°C and in particular above 140°C).Suitable thermal initiators include potassium persulfate (K25208), N,N-azobisisobutyronitrile (AIBN), dibenzoyl peroxide (BPO), dicumyl peroxide, di-tert-butyl peroxide, or mixtures of these compounds. This thermal initiator preferably represents 0.01 to 3% by weight, and more preferably 0.1 to 2% by weight, in particular 0.5 to 1.5% by weight of the coating composition. The coating composition may, where appropriate, contain other additives to enhance its properties. 35 These are, in particular, wetting agents, such as alkoxylated acetylene diols, especially ethoxylated acetylene diols (for example 2,5,8,11-tetramethyl1-6-dodecyn-5,8-diol ethoxylate), polysiloxanes modified by polyethers, especially polyether-modified dimethyl polysiloxanes, or polyacrylates modified by fluorinated hydrocarbons.These compounds may be added in quantities of up to 3% by weight to the coating composition. The coating composition may, in principle, be applied to at least part of the surface of the semi-finished rubber product by any coating process known to specialists, such as, for example, dip coating, extrusion coating, wand coating, brush coating, spray coating, squeegee coating, sprinkler coating, rotary coating, or slot coating. Preferred processes are spray coating (inkjet printing, valve-jet printing), dip coating, and curtain coating. The coating is preferably carried out 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 semi-finished rubber product is also within this range. The thickness of the resulting coating layer is preferably 0.1 to 50 µm, particularly 1 to 25 µm, and especially 15 µm. After the semi-finished rubber 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 rubber composition and the coating composition described allow for the thermal induction of the semi-finished product (i.e., the vulcanization of the rubber) and the hardening of the coating composition (i.e., the polymerization of the coating composition).Due to the application of thermal energy, the crosslinking agent contained in the composition of the rubber is transformed into radicals that cause the crosslinking (vulcanization) of the rubber, as the double bonds within the rubber are linked by radical reaction. This process can, if necessary, be improved by the addition of suitable vulcanization catalysts or accelerators. Radicals present or formed on the surface of the semi-finished product thus come into contact with the coating layer and the radical polymerizable monomers contained therein. These surface radicals induce polymerization of the coating layer. The polymer chains of the coating layer, formed from the radical polymerizable monomers, are thus grafted onto the surface of the rubber or elastomer / rubber (i.e., the crosslinked rubber).This results in a homogeneous and intimate bond between the surface coating and the surface of the cross-linked rubber or rubber article, as a large number of covalent bonds are formed between the two components: the rubber and the coating layer. In one embodiment, the coating composition may contain thermal initiators that generate radicals, thereby enhancing the polymerization of the coating layer. The hardening of the coating composition is thus initiated by the decomposition of the contained thermal initiators. This decomposition produces radicals that initiate the radical polymerization of the radically polymerizable monomers. The coating layer is thus transformed into a polymer layer in which the antifriction additives and any other adjuvants contained therein are bonded.Moreover, the various reactions occurring via a radical mechanism can propagate across the boundary between the semi-finished rubber product and the coating layer, such that radicals from the rubber initiate the polymerization of monomers in the coating layer or react with radicals from the coating layer. Similarly, radicals from the coating layer can initiate the vulcanization of the rubber or react with suitable double bonds in the rubber. This results in improved adhesion between the two layers (rubber and coating layer) since the two layers are bonded to each other by a large number of covalent bonds. In a preferred embodiment, both the rubber composition and the coating composition contain a thermally unstable peroxide compound.According to 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 include, for example, dicumyl peroxide or di-tert-butyl peroxide. This ensures that the thermally initiated reactions in the rubber composition and the coating layer occur at the same temperature and as simultaneously as possible. To achieve vulcanization of the rubber composition and polymerization of the coating layer, the coated semi-finished rubber product is heated to a temperature above the temperature at which the thermal decomposition of the crosslinking agent or thermal initiator occurs in at least one of the compositions (i.e., the rubber composition or the coating composition).This temperature is preferably 150°C, more preferably 175°C, and in particular 200°C, for example, within a range of 200 to 250°C, and in particular within a range of 210 to 230°C. The temperature increase can be carried out in any manner known to specialists, for example, by a hot air process, a salt bath process, or a microwave process. From a process technology standpoint, 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. This salt bath is preferably a mixture of 50% by weight of KNO3, 40% by weight of NaNO2, and 10% by weight of NaNO3 and has a melting point of 150°C. Such a bath is inert with respect to rubbers and thus allows vulcanization and polynerization to be carried out in the absence of oxygen from the air.3046604 11 The coated rubber article can then be freed of impurities and, where applicable, excess monomer by suitable washing processes and, where applicable, cut to the desired dimensions. 5 The present invention also relates to a coated rubber article obtained by implementing the process described above. This rubber article is characterized, as already indicated, in that the coating layer and the base body of the rubber article, made of cross-linked rubber, are 10 bonded by a large number of covalent bonds. The coating layer is thus applied particularly effectively, and detachment or peeling of this layer is prevented.The present invention also relates to a coated rubber article 15 characterized in that it comprises a cross-linked rubber body onto which is grafted a polymer layer obtained by radical polymerization of ethylenically unsaturated monomers having a boiling point of 150°C. This rubber article is particularly characterized in that the polymer layer or coating layer 20 is covalently bonded to the cross-linked rubber base body. According to a preferred embodiment, the coated rubber article has a composition gradient such that—from the surface of the coating of the coated rubber article—the proportion of elastomer in the composition increases continuously.Such a continuously increasing gradient is obtained in particular when a crosslinking comonomer, specifically a crosslinking comonomer that is also contained in the coating composition, has been added by mixing to the rubber composition. According to one embodiment, the rubber composition contains EPDM rubber and ethylene glycol dimethacrylate (EGDMA) as a crosslinking comonomer, and the coating composition contains ethylene glycol dimethacrylate (EGDMA) as a radically polymerizable monomer. The coated rubber article obtained by implementing the described process is characterized by a particularly continuous compositional gradient and an intimate bond between the coating layer and the base body of the rubber article.5 The present invention further relates to an antifriction surface coating obtainable by: (a) applying a coating composition containing at least one radically polymerizable monomer having a boiling point above 150°C, at least one antifriction additive, and at least one thermal initiator to a surface to be coated, and (b) heating the coated surface to a temperature above the decomposition temperature of the thermal initiator. The constituents of the coating composition forming the surface coating correspond, in their entirety, to the components of the coating compositions described above. The antifriction surface coating is preferably applied to the surface of a rubber article as well as to the surface of an object on which the coated rubber article is to slide.This combination of surface coating, which reduces friction on the two surfaces that must slide against each other, allows for a particularly effective reduction in frictional resistance. In a preferred embodiment, the rubber article is, for example, a wiper blade profile of a windshield wiper 25, and the object on which the rubber article must slide is a window, for example, a vehicle windshield. The windshield can, for example, be made of glass or a synthetic material. The windshield is preferably made of a synthetic material. Advantages of the invention 30 The process described above makes it possible to significantly reduce the time required to manufacture a coated rubber article.This process allows for the vulcanization of the rubber compound and the polymerization of the coating compound in a single step, thus reducing manufacturing time and costs for coated rubber articles. Furthermore, due to the absence of solvents, drying the coating layer before polymerization is unnecessary. The coated rubber article obtained by the process according to the invention is further characterized, compared to conventionally produced rubber articles, by improved properties. This manufacturing process described allows for the application of surface coatings to rubber articles in which the surface coating is bonded to the surface of the rubber article by a large number of covalent bonds. The product is further characterized by the fact that the surface coating is highly resistant to separation or peeling.In combination with another surface equipped with the same coating layer, improved sliding characteristics and particularly low friction values ​​can thus be obtained. Description of an example embodiment The characteristics of the process and of the rubber article which are the subject of the invention will be described in more detail with reference to the attached non-limiting figure which represents a semi-finished rubber product consisting of a wiping blade profile.The wiper blade profile 10 shown in the figure comprises a wiper lip 12 having a wiper edge 11 made of a rubber composition consisting of 100 parts by weight 25 of EPDM rubber and 2 parts by weight of di-tert-butylperoxide, as well as a head part 13 made of this rubber composition which is only partially shown and a flexible bar 14 also made of this rubber composition which is located between the head part 13 and the wiper lip 12. 30 The wiper lip 12, the head part 13 and the flexible bar 14 have a surface which is made of a coating layer 15 which, for clarity, is shown enlarged in relation to the wiper lip 12, the head part 13 and the flexible bar 14.The coating layer 15 has a thickness of approximately 35 µm and is, for example, composed of 54% by weight of ethylene glycol methacrylate (ETMA), 45% by weight of graphite particles with a particle size of 30 to 50 µm, and 1% by weight of di-tert-butyl peroxide. According to the example shown, the coating layer 15 is present only in areas that, as a rule, may come into contact with the window of a vehicle during the wiping process. According to other embodiments of the process according to the invention, the coating layer 15 may also be present over the entire surface.10. The manufacturing of the wiping profile 10 shown in the figure is carried out by extruding, first, a basic shape consisting of the head portion 13, the flexible bar 14, and the wiping lip 12, then covering this basic shape with the coating composition described above, by a spraying process. 15. The resulting coated semi-finished product is vulcanized in a salt bath (50% by weight of KNO3, 40% by weight of NaNO2, 10% by weight of NaNO3) at 220°C. The polymerization of the coating layer 15 occurs simultaneously. If necessary, pretreatment of the surface of the rubber body 20, consisting of the wiping lip 12, the flexible bar 14, and the head portion 13, may be required to obtain a homogeneous surface.After the vulcanization of the semi-finished rubber product and the polymerization of the coating layer 15, the wiping blade profile 10 and in particular the wiping lip 12 are 25 subjected to a washing process and then cut to the desired dimensions. 30.

Claims

CLAIMS 1°) Method for manufacturing a coated rubber article comprising steps consisting of: (i) procure a semi-finished product having the basic shape of Article 5 made of rubber, this semi-finished product being made of a composition rubber comprising at least one rubber and at least one crosslinking agent, (ii) coating the semi-finished product with a solvent-free coating composition containing at least one monomer polymerizable by radical polymerization, (iii) crosslinking the rubber composition and polymerizing the coating composition to thereby obtain a coated rubber article, characterized in that the radically polymerizable monomer has a boiling point of 150°C or higher and the crosslinking of the rubber composition and the polymerization of the coating composition are carried out simultaneously. 2°) Method according to claim 1, 20 characterized in that the crosslinking agent is a peroxide compound. 3°) Method according to claim 1 or 2, characterized in that 25 the coating composition contains an antifriction additive and, where appropriate, at least one thermal initiator. 4°) Method according to one of claims 1 to 3, characterized in that 30 the radically polymerizable monomer comprises at least one functional group selected from an acrylate group, a methacrylate group, an acrylamide group, a methacrylate group, a styryl group or a vinyl group. 5°) Method according to one of claims 1 to 4, characterized in that the monomer of the monomer or the copolymer of the mixture of monomers has a glass transition temperature Tg measured by dynamic differential scanning calorimetry (DSC) of less than 20°C. 6°) Method according to one of claims 1 to 5, characterized in that the monomer polymerizable by radical polymerization has a melting point above 60°C. 7°) Method according to one of claims 1 to 6, characterized in that the step of vulcanizing the rubber composition and polymerizing the coating composition (step (iii)) is carried out in a salt bath. 8°) Coated rubber article obtained by implementing the method according to one of claims 1 to 7.

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