DE19547088A1 - Elastomers with a friction-reducing coating - Google Patents

Abstract

A process is disclosed for producing hard elastic antifriction coating layers that adhere firmly to elastomer substrates. The substrates are coated with a sol obtained by hydrolysing and condensing silanes having the formula (I): R<1>Si(OR')3, in which R<1> is a glycidyloxy or mercapto-functional rest and R' is a lower alkyl rest, then hardening the coating layer into a gel. This process is particularly suitable for coating rubber surfaces, for example of windshield wiper blades.

Description

As is well known, elastomers are polymeric materials with rubber elastic behavior that is practically fully correct at room temperature versibly extended to a multiple of their normal length can be multiple and precisely because of this property Have found application in technology. In addition to the ela The behavior is often also a problem Resistance to exercise, which in some applications, e.g. B. at Motor vehicle tires and shoe soles, as high as possible, at their applications, however, e.g. B. as a wiper rubber, should be as low as possible.

On windshield wiper blades high, sometimes counter-rotating requirements. On the one hand, they have to be so hard and be stable so that they can withstand the mechanical stresses the conditions of practical operation as long as possible oh Endure damage. On the other hand, they have to be elastic enough to cover the entire swept area transfer the same possible contact pressure onto the pane In addition, the frictional resistance should be compared Glass with wet friction and especially with dry friction be as small as possible so that the wiper mecha niche is used as little as possible and with as much as possible low energy can be operated and on the other hand the Stick-slip effect, d. H. the rubbing when wiping in particular re the dry disc, is avoided. It is known, that the frictional resistance by post-treatment of the Can reduce rubbers, for example by dusting fe ster substances, such as talc or zinc stearate, or by a chemical aftertreatment, e.g. B. with a halogen, sulfur acid or potassium permanganate. Chlorination with elementa Rem chlorine is one of the most common aftertreatments  method. However, it is with health and safety risks, is not satisfactory and reproducible is also deteriorating with physical and che mixer properties connected, for example reduced Resistance to ozone, faster material fatigue and increased Wear.

It is also known that the frictional resistance reduce or improve the sliding properties leave the surfaces coated. This procedure ren is mainly used for elastomers where chlorination fails, for example in ethylene propy len-diene copolymers. The coating can be soft Positions of the copolymers mentioned exist, but based to a large extent on organofluorine compounds, e.g. B. Polytetrafluoroethylene, which is characterized by excellent sliding properties properties. Perfluorinated polymers adhere but bad on the substrates, so that possibly on Agile pretreatments are required to ensure good adhesion to guarantee. Perfluorinated polymers are also soft. Your surface is therefore easily damaged, causing the function of the component in question, e.g. B. a slices wiper blade or a seal, significantly impaired becomes.

Nass et al. (Synthesis and Properties of Transparent ZrO₂ Containing SiO₂ Polymethacrylate Polymers, SPIE Vol. 1328 Sol-Gel Optics [1990], p. 258ff) have a method wrote in which a sol consisting of 3-methacryloxysilane, Zirconium propylate and methacrylic acid are produced as coatings means is used. By radical polymerisa In addition to the inorganic structure, a stabilization is formed organic network. The zircon is in both bound. These inorganic-organic polymers, also called Ormocere (organically modified ceramics) however relatively brittle and show on rubber surface unsatisfactory liability. In addition, zirconium propylate a relatively expensive raw material.  

Advantages of the invention

The hard elastic coatings according to the invention ver reduce the frictional resistance of elastomers Glass with both dry and wet friction Significantly, they adhere well to rubber surfaces and are highly elastic aesthetic and sufficiently abrasion-resistant. They are suitable there excellent for coating wiper blades rubber. The coatings reduce friction resistance of non-chlorinated rubber to values similar to those of chlorinated rubber, and they set the ver track-low frictional resistance of chlorinated gum mi significantly further down. The materials for the Her position of the coating according to the invention who needs are cheap and often in technical quantities in the Available commercially. Che questionable for environmental reasons chemicals, such as chlorine and chlorinated hydrocarbons, who that not needed. The process is not complex and uses equipment and processes common in the art. It is reliable, d. H. it gives reproducible results.

Description of the invention

The advantages described are with the method according to the Claims 1 to 10 and through the wiper blade ter achieved according to claim 11. To the elastomer Substrates coated successfully according to the invention rubber in particular is what counts Product made by vulcanizing natural rubber, poly chloroprene etc. with relatively small amounts of sulfur is available in a known manner. It can be used immediately for the method according to the invention can be used or before one of the surface treatments mentioned, e.g. Legs Chlorination.

The generated by the method according to the invention  cured coatings, such as that of Nass et al. be written materials, inorganic-organic composites, So Orcomere, but do not show the ones described above Disadvantage. In the first stage of the process after Invention, a sol is made from a silane. The Silane is an alkoxysilane and corresponds to the formula R¹Si (OR ′) ₃ (I), in which R¹ is a glycidyloxy-functional or a mercapto-functional residue and R ′ a lower one Alkyl radical, preferably having 1 to 4 carbon atoms, and in particular means a methyl radical. The Glycidyloxyfunk tional radicals R 1 are usually glycidyloxy substituted Alkyl radicals with 1 to 8 carbon atoms. The preferred Silane with a glycidyloxy-functional radical R 1 is 3-glyci dyloxypropyl-1-trimethoxysilane (GPTS). Merkaptofunctional R¹ radicals are usually mercaptoalkyl radicals with 2 to 8 Carbon atoms. The preferred mercaptoalkyl group is 3-mercaptopropyl residue, the preferred silane with mercapto functional radical R¹ is 3-mercaptopropyl-1-trimethoxy silane.

The first step in the conversion of the silanes (I) into a sol is a hydrolysis reaction in which the alkoxy radicals give rise to hydroxyl groups, which in turn are capable of condensation reactions - with other hydroxyl groups with the leakage of water, with alkoxy groups with the leakage of alcohol. The condensation reactions lead to the formation of siloxane structures. The hydrolysis and condensation take place side by side. However, the condensation is only completed when the sol then hardens to form a gel with the formation of a highly cross-linked siloxane structure. Dilute aqueous acid, such as 0.1N hydrochloric acid, is expediently used for the hydrolysis. If one uses the stoichiometric amount of water ( 3 moles of water / mole of silane) or even an excess in the case of glycidyl oxy-functional silanes for the hydrolysis, part of the epoxy rings is acid-catalyzed to the diol and is thus used for the polyaddition in the following Hardening lost which leads to a loss of elastic properties. It is therefore advisable to work with a stoichiometric deficit, for example with half the stoichiometric amount. The condensation then predominantly takes place between hydroxyl and alkoxy groups with the formation of alcohol, and the hydrolysis of the epoxy ring is suppressed.

The sol reaction can be done without solvents or diluents be carried out. The addition of an inert solution or Diluent can be useful if the silane (I) or, as explained in the following, the mixture of the silane (I) and the silanes (II) and / or (III) and optionally other additives at the selected reaction temperature ren - generally room temperature or slightly increased or low temperatures, such as -10 to + 60 ° C - is so viscous, that the mixing of the water for hydrolysis by the Solvent or diluent is facilitated. As a sol ches is particularly suitable the alcohol R'OH, the alk corresponds to oxy residues of silane (I). However, it should only as much solvent and diluent are used that the sol after application to the substrate and the Evaporation of the solvent and diluent Coating with the desired layer thickness results.

The sol reaction takes place relatively quickly; she is generally finished after 15 minutes to 5 hours.

The use of a silane of the formula R² _x Si (OR ′) _4-x (II) promotes the wettability of the substrates by the sol and thus the formation of coherent, flat layers on the substrate. This effect should be related to the hydrophobic character of the substituent (s) R². In the formula, R² is in each case a halogenated higher alkyl radical or a lower alkyl radical, but at least one substituent R² is a halogenated, in particular a fluorinated, higher alkyl radical. R 'denotes a lower alkyl radical, and x stands for an integer from 1 to 3. The silane (II) is incorporated into the siloxane structure of the sol due to its alkoxy group (s) and improves through its (n) hydrophobic substituents R² the wetting of the hydrophobic surface of the substrate. Particularly suitable silanes (II) are those which have a halogenated, in particular fluorinated, alkyl radical having 5 to 18 carbon atoms, in which at least 50% of the hydrogen atoms are replaced by fluorine, and 4-x alkoxy radicals, preferably 1 to 4 carbon atoms, contain in particular methyl radicals. Examples of suitable silanes (II) are tridecafluoro-1,1,2,2-tetrahydro-octyl-1-triethoxysilane, tridecafluoro-1,1,2,2-tetrahydro-octyl-1-methyl-dimethoxysilane, and heptadecafluoro-1,1,2,2-tetrahydrodecyl-1-triethoxysilane. The silane (II) is expediently added in amounts of 0.1 to 10 mol%, in particular 0.5 to 2 mol%, based on the silane (I).

Another starting material that is optionally used in the manufacture of the sol is a silane of the formula R³ _x Si (OR ′) _4-x (III), which further reduces the frictional resistance, especially when using mercapto-functional silanes (I). In the formula, R³ is in each case an alkyl radical, preferably having 1 to 18 carbon atoms, R ′ is a lower alkyl radical, advantageously having 1 to 4 carbon atoms, and in particular the methyl radical, and x is an integer from 1 to 3. The silane (III) is optionally used in amounts which are generally in the range from 5 to 40 mol%, in particular from 10 to 20 mol%, and is also incorporated into the siloxane skeleton of the sol because of its alkoxy group (s).

If the silane (I) is glycidyloxy functional, the sol also contain an epoxy resin hardener. As such aromatic polyhydroxy compounds, e.g. B. dihydric phenols such as bisphenol A. The epoxy Harder contributes to the subsequent hardening of the sol further networking of the siloxane structure. It becomes purpose moderate in the stoichiometric deficit, based on the silane (I) added, e.g. B. up to 0.4 moles of bisphenol A per mole Silane (I).  

The sol can also be reinforcing, finely divided inorganic Contain materials such as boehmite with particles sizes in the nanometer range. With quantities of 1 to 20% by weight, based on the silane (I), one obtained with inorganic Material-enriched network of high stability, the is still sufficiently elastic.

As condensation catalysts, the hardening of the sol promote to gel, have tertiary amines with aromatic Structure especially proven. Suitable amines of this type are for example imidazole, 1-methylimidazole and, albeit less effective, pyridine and the isomeric methylpyridines. These catalysts are advantageously used in amounts of 0.1 to 15, in particular 2 to 10 mol%, based on the silane (I) and optionally the silanes (II) and (III) added.

To produce the sol, the silane (I) can be submitted and with stirring at room temperature gradually that for the Hydrolysis required water, conveniently in the form of dilute ter mineral acid, add. After that, if necessary, at the same time or in any order, the finely divided inorganic material, the epoxy hardener, the condensate tion catalyst and the silanes (II) and (III) added, and the latter are hydrolyzed.

The sol is optionally mixed with an inert solution and Diluent diluted to a solids content of the desired layer thickness from that produced from the sol corresponds to hardened coating. This is usually between 1 and 10 µm. The relationship between the salary a sol of solvent and diluent and the For a given system, layer thickness can be predefined try to determine easily. The sol is made according to the usual metho applied to the substrates. These can, like him thinks they have undergone a chemical surface treatment and are useful before coating with a  Solvents such as ethanol, isopropanol or acetone, upper surface cleaned and blown off with oil-free air. A Priming the substrates after cleaning and before Coating with the sol by treatment with a Merkap to-functional silane, for example a Merkapto-funk tional silane (I) is particularly recommended if that silane (I) used for the production of the sol is a gly is cidyloxy functional silane. The Merkapto-functional Silane promotes the adhesion of the sol during hardening standing gels on the substrate. As a coating process are z. B. the dip coating or the centrifuge layering (spin coating). Of course it is possible to use the sol to apply only to parts of an object, for example on the lips of wiper blades.

The applied sol becomes the solution and dilution medium evaporates and the sol is cured to a gel. At the can at elevated temperature, for example from 50 to 120 ° C happen in an air stream. The curing takes usually 10 minutes to 20 hours, advantageously 1 to 10 Hours, wherein the coatings are made of glycidyl cure oxy-functional silanes (I) faster than that Mercaptofunctional silanes (I).

The figure shows a top view of the equipment used to determine the coefficients of friction in the following examples:
A circular glass pane 1 is made to rotate by a motor. At one end of the arm 2 there is a holder for a 5 cm long section 3 of a wiper blade, the other end dips to dampen the vibrations in an oil bath 4th The pivot point of the arm is mounted on a cross strut 2 a freely rotatable and actuates a strain gauge 5th The arm 2 is weighted by the Ge weight 6 so that the wiper blade section 3 acts with a force of 16 N / cm on the glass sheet. By the frictional resistance between the ro tierendem glass and the wiper blade section 3 , the arm 2 is deflected horizontally and pressed against the strain gauges 5 . This generates an electrical signal, the intensity of which is read on a scale after amplification. This is the (dry measured) unitless coefficient of friction µ ^T. It represents a characteristic for the coating of the wiper blade section 3 for a certain sliding speed with a certain contact force. The sliding speed of the glass pane 1 at the center of the wiper blade section 3 is adjusted to 20 cm / sec. The measurements were carried out at room temperature and the values were read in each case after a waiting time of 1 min. After 5 measurements in each case, the surface of the glass pane 1 was cleaned with the aqueous solution of a customary washing-up liquid, rinsed with demineralized water, rubbed dry and blown off with oil-free air. Before the measurement, the wiper blade sections 3 were cleaned with isopropanol and blown off with oil-free compressed air. To determine the wet friction coefficient µ ^N , some water was applied to the disc.

The substrate was rubber made from vulcanized natural rubber, chlorinated vulcanized natural rubber, vulcanized Chloroprene rubber and chlorinated chloroprene rubber ver ends. The chlorination was by treatment with chlorwas performed in the usual way.

The abbreviations used in the examples have the fol meaning:

NKu = vulcanized natural rubber, unchlorinated,
CKu = vulcanized chloroprene rubber (neoprene), unchlorinated,
NKc = vulcanized natural rubber, chlorinated,
CKc = vulcanized chloroprene rubber (neoprene), chlorinated,
GPTS = 3-glycidyloxypropyl-1-trimethoxysilane,
BPA = bisphenol A,
FTS = tridecafluoro-1,1,2,2-tetrahydrooctyl-1-triethoxysilane,
MI = 1-methylimidazole,
MTMO = 3-mercaptopropyl-1-trimethoxysilane,
MTES = methyltriethoxysilane,
PTMS = propyltrimethoxysilane,
OTMS = n-octyltrimethoxysilane,
ODTMS = octadecyltrimethoxysilane.

Example 1 - Sol from GPTS / BPA / FTS / MI

9.44 g GPTS (0.04 mol) were placed in a 40 ml snap lid Glass submitted, and with stirring 1.08 g of 0.1 N salt acid (0.06 mol) was added dropwise. The mixture was let down 45 sec became clear, stir for 12 h at room temperature. Then was added 3.65 g (0.016 mol) of solid BPA. After dissolving the solid was 0.16 g MI (2 mmol) and 0.20 g FTS (0.04 mmol) was added and it was ge an additional hour stirs. The resulting sol was treated with 8 g of 2-isopropoxyethanol diluted.

Example 2 - Sol from GPTS / FTS / MI

The preparation followed the instructions of Example 1, wherein however, the step of adding the BPA was omitted.

Example 3 - Sol from GPTS / BPA / Böhmit / MI

9.44 g GPTS (0.04 mol) were placed in a 40 ml snap lid Glass submitted, and with stirring 3.24 g of 0.1 N salt acid (0.18 mol) was added dropwise. The mixture was stirred became clear after 45 sec, 12 h at room temperature. Then were 1.32 g (corresponding to 11% by weight); 1.00 g (corresponding to 8  % By weight) or 0.50 g (corresponding to 4% by weight) boehmite with a mean particle size of 15 nm added. They left that Stir mixture for 24 h, then add 3.65 g of solid BPA (0.016 mol), 0.16 g MI (2.0 mmol) and 14 g 1-isopropoxyethanol and allowed the mixture to stir for an additional hour. The clear Brine was then passed through a membrane filter (pore size 1.3 µm) filtered.

Example 4 - Sol from MTMO / MI

7.84 g of MTMO (0.04 mol) was placed in a 40 ml snap lid Glass submitted, and with stirring 1.08 g of 0.1 N salt acid (0.06 mol) was added dropwise. The mixture was let down became clear for about 1 min, stir at room temperature for 12 h. Then 0.16 g of MI (2.0 mmol) was added. The mixture was stirred for an additional hour and then with 4 g of 2-isopropoxy diluted ethanol.

Example 5 - Sol from MTMO / FTS / MI

7.84 g MTMO (0.04 mol) are placed in a 40 ml snap lid submitted glass, and while stirring 1.08 g of 0.1 N salt acid (0.06 mol) was added dropwise. The mixture was let down became clear for about 1 min, stir at room temperature for 10 min. Then 0.20 g FTS (0.04 mmol; 1 mol%), 0.40 g FTS (0.08 mmol; 2 mol%), 0.60 g FTS (0.12 mmol; 3 mol%) or 0.80 g FTS (0.16 mmol; 4 mol%) was added and the mixture became 12 h stirred at room temperature. Then 0.16 g of MI (2.0 mmol) added. The mixture was ge an additional hour stirred and then diluted with 4 g of 2-isopropoxyethanol.

Example 6 - Sol from MTMO / Boehmite

7.84 g of MTMO (0.04 mol) was placed in a 40 ml snap lid Glass submitted, and with stirring 3.24 g of 0.1 N salt acid (0.18 mol) was added dropwise. The mixture was let down 45 sec became clear, stir for 10 min at room temperature. Then 1.32 g, 1.00 g and 0.50 g boehmite (corresponding to mol  MTMO / Boehmite ratios of 1: 0.55, 1: 0.4 and 1: 0.2) with an average particle size of 15 nm was added. You let stir the mixture for 24 h and dilute it by adding 5 g 1-isopropoxyethanol. The brine was then replaced by a mem branfilter (pore size 1.2 µm) filtered.

Example 7 - Brine from MTMO / MI and MTES, PTMS, OTMS or ODTMS

6.27 g MTMO (0.032 mol) and 0.008 mol MTES, PTMS, OTMS or ODTMS were placed in a 40 ml snap-lid jar and 1.08 g of 0.1N hydrochloric acid (0.06 mol) added dropwise. After the mixture became clear, the 4 g ethanol (for MTES and PTMS) or became a mixture from 3 g of toluene and 1.5 g of ethanol (for OTMS and ODTMS) and the mixture was stirred for an additional 24 h. The Toluene was added to avoid solubility problems puts.

Coating of substrates and hardening of the brine to gels

There were wiper blades made of different gummity pen coated. These were washed with ethanol, Isopro Lint-free wiping paper soaked in panol or acetone wipes and then blown off with oil-free compressed air. All sub strate was primed by placing it in a freshly brewing shop immersed 10 wt.% solution of MTMO in ethanol, the solution After pulling out, drain briefly and then the substrate Allow to dry at 90 ° C for 1 h.

The wiper blades, of which the sections for the Determination of the friction values were cut, allowed their wiping lips into the diluted brine according to the examples Immerse 1 to 7 and after pulling out with constant Drain the speed briefly. The solution and dilution The solvent was evaporated and the brine became gels hardened by wiping the wiper blades in a forced air oven have been heated to 90 ° C. The curing time was the brine based on GPTS 4 h and the brine based  MTMO 8 hours. The brine described was so diluted that coatings with a thickness of about 5 microns are obtained were.

The measured friction values are shown in the following table forth.  

table

Coefficients of friction of various coatings

Coefficients of friction of various coatings

A comparison of the dry friction values for the coated Wiper blades with that for uncoated windows wiper blades shows significantly lower values for the be  layered wiper blades. This is an exception System GPTS / BPA / MI / Boehmite on chlorinated rubber from natural chem rubber (NKc), in which the coefficient of friction does not improve will. The wet friction values of the coated wipers sheets are consistently lower than those of the comparable ones uncoated wiper blades.

Claims (11)

1. Process for the production of firmly adhering, hartela static and friction-reducing layers on elastomers Substrates, in which the substrates with a sol be stratified by hydrolysis and condensation of silanes of the formula R¹Si (OR ′) ₃ (I), in which R¹ is a glyci dyloxy or Merkapto functional residue and R 'a low ren means alkyl radical, and the coating to a gel hardens. 2. The method according to claim 1, characterized in that a silane of the formula R² _x Si (OR ') _4-x (II) is used in the preparation of the sol, in which R² each represents a halogenated higher alkyl radical or a lower alkyl radical , with the proviso that at least one of the radicals R² is a halogenated higher alkyl radical, R 'each denotes a lower alkyl radical and x represents an integer from 1 to 3. 3. The method according to claim 2, characterized in that that in the silane (II) one of the substituents R² is high is fluorinated alkyl radical having 5 to 18 carbon atoms and the substituents R ′ are lower alkyl radicals, preferably 1 are up to 4 carbon atoms. 4. The method according to any one of claims 1 to 3, characterized in that a Si lan of the formula R³ _x Si (OR ') _4-x (III) is used in the preparation of the sol, in which R³ each has an alkyl radical and R' each represents a lower alkyl radical and x represents an integer from 1 to 3. 5. The method according to claim 4, characterized in that in the silane (III) one of the substituents R³ is an alkyl rest with 1 to 18 carbon atoms and the substituents ten R ′ lower alkyl radicals with preferably 1 to 4 carbons are atoms.   6. The method according to any one of claims 1 to 5, characterized characterized in that the substituent R¹ of silane (I) Glycidyloxy functional residue and the sol is an epoxy contains resin hardener. 7. The method according to any one of claims 1 to 6, characterized characterized in that the substituent R¹ of silane (I) Glycidyloxy functional residue is and the surface of the Substrate before applying the sol with a mercapto functional silane is primed. 8. The method according to any one of claims 1 to 7, characterized characterized in that the sol is a finely divided inorganic Contains filler. 9. The method according to any one of claims 1 to 8, characterized characterized in that the sol is a condensation catalyst contains. 10. The method according to any one of claims 1 to 9, characterized characterized in that the sol in such an amount and with such a solids content that the out hardened coating has a thickness of 1 to 10 microns. 11. Wiper blades made of rubber, coated according to the method of one of claims 1 to 10.