COMPOSITION ON THE BASIS OF FLUORINATED POLYMERS IN AQUEOUS DISPERSION, CONTAINING ALKOXYSILANES, AND ITS USE FOR COATING METAL SURFACES

Abstract: A composition on the basis of fluorinated polymers in aqueous dispersion, having a basic pH, containing alkoxysi­ lanes and magnesium and/or aluminum as cations complexed with amino- or hydroxy-carboxy acids, acting as bonding agents, which is suitable to form a highly-adhesive layer on metal surfaces, in particular as a primer.

Description: The present invention relates to a composition suitable to form a coating layer on metal articles, in particular of iron or of aluminum or of alloys thereof, said coating layer showing a very high adhesion to the metal substrate, and being suitable for use as a primer for the application of subsequent finishing coating layers. It is known to use aqueous dispersions of polytetrafluoroethylene (PTFE) for obtaining coatings endowed with a high resistance to heat and to the chemical agents, with good mechanical strength to wear and friction. It is also known to use PTFE modified with small amounts of a fluorinated comonomer, or PTFE blends with other fluorinated polymers, for the purpose of obtaining improved characteristics of impermeability and of flexibility of the coating layer. The essential technical problem in the accomplishment of these coatings is shown by the adhesion to the metal substrate, which must be very high also in view of the temperatures of use of the manufactured articles, which can sometimes be very high. A good adhesion obtained by incorporating chromic acid in the aqueous dispersion of the polymer, in particular in combination with phosphoric acid. This system is not allowable if the treated articles are used in the alimentary field, due to the high toxicity of chromic acid which is present, even if in trace amounts, in the obtained layer of polymeric coating. US-A-3 644 260 discloses a primer composition containing an alkylsilicate (e.g. ethyl orthosilicate), a particulate fluorocarbon polymer and a liquid carrier which may be water or an organic liquid. US-A-4 495 247 refers to a primer composition comprising a fluorocarbon polymer of vinylidene fluoride and hexafluoropropylene, a metal oxide, an aminoalkylalkoxy silane and a reinforcing pigment. The purpose of the present invention is to provide a coating layer performing the function of a primer with a very high adhesion to the substrate, which is free from highly toxic components and may easily and reliably be applied to the article to be coated. Providing a coating layer endowed with these characteristics is possible by using the composition according to the invention, which is constituted by an aqueous dispersion of fluorinated polymer, containing an alkoxysilane soluble in the aqueous phase at a pH value comprised within the range of from 8 to 10, and magnesium and/or aluminum in the form of cations complexed by a hydroxy- or an amino-carboxy acid. The amount of alkoxysilane is comprised within the range of from 0.05 to 0.4 parts by weight (pbw) per 1 pbw of fluorinated polymer, preferably of from 0.15 to 0.22 pbw. The amount of metal cation is comprised within the range of from 0.001 to 0.05 pbw per 1 pbw of fluorinated polymer, preferably of from 0.02 to 0.04 pbw. The amount of complexing agent for the metal cation is comprised within the range of from 1 to 4 mols per metal atom, preferably of from 1 to 2 mols per each metal atom. Alkoxysilanes useful for the composition according to the invention are those which contain 2-3 alkoxy groups and are water-soluble at pH 8-10. Owing to their good solubility, those which contain amino groups in their alkyl radical are particularly suitable. Water-insoluble alkoxysilanes can be used after a partial hydrolysis aimed at rendering them soluble. For exemplifying purposes, 3-aminopropyl-triethoxysilane and N-β-(aminoethyl)-γ-aminopropyl-trimethoxysilane can be mentioned, for the direct use, as well as γ-methacryloxypropyl-trimethoxysilane and γ-glycidyloxypropyl-trimethoxysilane, which must undergo a preliminary partial hydrolysis at pH 3-5 to be sufficiently soluble in water at pH 8-10. The fluorinated polymer can be one of those already used in the prior art, such as polytetrafluoroethylene (PTFE), polymers of C₂F₄ modified with small amounts of a,preferably fluorinated, comonomer, C₂F₄ polymers with other fluoroolefins, and the like. As the complexing acids for Mg and Al cations, tartaric acid, citric acid, glycolic acid, amino-acetic acid, and the like, can be mentioned. As already said, the aqueous-medium composition according to the invention must have a pH comprised within the range of from 8 to 10, preferably of from 9 to 9.5. The pH value can be adjusted by adding basic substances, such as NH₃, NaOH, KOH, LiOH. This formulation can be pigmented according to conventional methods known to those skilled in the art, diluted with deionized water and further stabilized by adding non-ionic surfactants, preferably belonging to the class of the polyethoxylated alkylphenols containing from 8 to 90 ethoxy molecules per phenol molecule, at a concentration comprised within the range of from 0.5 to 8% by weight, based on the dry polymer. The so-obtained paint can be applied onto previously sandblasted and degreased metal substrates, by using the most common application techniques (spray-painting, flow-painting, roller-coating, etc.), in order to have a film which, once sintered, has a thickness comprised within the range of from 1 to 20 microns, preferably of from 3 to 10 microns. The so-obtained coating has good adhesion to the metal support. The composition according to the invention can be advantageously used on surfaces of aluminum, iron, and of their alloys. The following Examples are supplied for the purpose of illustrating the present invention. Example 1 8 g of (MgCO₃)₄.Mg(OH)₂.5H₂O was dissolved in 40 g of deionized water, containing 10.5 g of citric acid. Then 12 g was added of dibasic ammonium citrate. The solution was filtered and its weight was adjuested at 75 g with deionized water. The primer paint was then prepared by blending, in the order: - 60%-water PTFE suspension, dispersed with 3.5% of Triton® X-100 20 g - deionized water 10 g - Mg solution 17.6 g - 3-aminopropyl-triethoxysilane 2.4 g The solution pH was adjusted at the value of 9.3 by adding LiOH. This paint was applied by means of a film-spreader on an aluminum support, which had been previously sandblasted and degreased with toluene. The film was sintered at 400°C for 9 minutes, thus a 7-micron thick coating being obtained. On said coating, an adhesion test was carried out as described by the ASTM D3359-78 Standard: by squaring the coated surface, using a special squaring instrument, a grid of 100 squares (each one of 1 mm of side) is prepared; the percentage of the squares of coating which are removed by an adhesive tape applied to the same grid and peeled away is determined. Also after dipping the specimen in water for 3 days an adhesion degree of 5, according to the scale supplied by the same ASTM Standard, is observed. Example 2 A solution is prepared, which has the following composition: - citric acid monohydrate 31.3 g - (MgCO₃)₄.Mg(OH)₂.5H₂O 11.3 g - 30% NH₄OH 16 g - deionized water to 100 g The pH value of this solution is 9. The primer paint is prepared by blending: - Aqueous PTFE suspension at 60% by weight, dispersed with 3.5% of Triton® X-100 45 g - aqueous dispersion at 20% of thermostable black F-6331 (Holland) pigment 20 g - 3-aminopropyl-triethoxysilane 5 g - Mg solution at pH 9 17.6 g This paint is sprayed onto an aluminum substrate, to yield a film which, after sintering at 400°C for 9 minutes, shows a thickness of 7 microns. The adhesion test gave, in this Example too, an adhesion degree of 5. Example 3 The following solution is prepared: - deionized water 50 g - Al(NO₃)₃.9H₂O 29.13 g - citric acid monohydrate 31.3 g - NH₄OH at 32% 23.5 g This solution is adjusted at pH 9 with NaOH, and then at 200 g with deionized water. This solution is used for preparing a primer paint having the following composition: - PTFE suspension at 60% by weight, dispersed with 3.5% of Triton® X-100 45 g - aqueous suspension at 20% by weight of black F-6331 pigment 20 g - 3-aminopropyl-triethoxysilane 5 g - Al solution at pH 9 20 g - deionized water 10 g This paint is sprayed onto an aluminum sheet (dimensions: 20 cm x 10 cm, thickness 1 mm) in order to obtain a film which, after sintering at 400°C for 9 minutes, yields a thickness of 6 microns. Also in this case, the adhesion degree is equal to 5. Example 4 A paint is prepared which has the following composition: - PTFE suspension at 60% by weight, dispersed with 3.5% of Triton® X-100 45 g - aqueous suspension at 20% by weight of black F-6331 pigment 20 g - 3-aminopropyl-triethoxysilane 5 g - Mg solution prepared for Example 2 15 g - deionized water 15 g This paint is used to coat an aluminum substrate by means of a spray gun. The film obtained after sintering at 400°C for 9 minutes, shows a thickness of 7 microns and a good adhesion to the support. Example 5 The same solution of Al used in Example 3 is used for preparing a primer paint which has the following composition: - PTFE suspension at 60% by weight, 45 g - aqueous suspension at 20% by weight of black F-6331 pigment 20 g - N-β-(aminoethyl)-γ-aminopropyl-trimethoxysilane 5 g - Al solution at pH 9 20 g - deionized water 10 g When sprayed on a sand-blasted aluminum substrate this paint forms a film which, after sintering according to the modalities as disclosed in the preceding examples, shows a good adhesion to the support. Example 6 The paint having the composition of Example 1 is prepared, wherein instead of 3-aminopropyl-triethoxysilane, the same amount of 4.8% of the compound γ-methacryloxypropyl-trimethoxysilane, not previously hydrolised (and hence insoluble under the application conditions) is used. The paint is applied by means of a film-spreader and is sintered at 400°C for 9 minutes. The resulting film, having a thickness of 6 microns, shows an adhesion level lower than that of Example 1 (value 2 of ASTM scale). Comparative Example 1A A primer paint is prepared as follows: - PTFE suspension at 60% by weight, dispersed with 3.5% of Triton® X-100 45 g - aqueous dispersion at 20% by weight, of thermostable black F-6331 pigment 20 g - aqueous solution of Na₂SiO₃.9H₂O at 23%, equivalent to 0.016 gram-atoms of Si 20 g - deionized H₂O 15 g This paint was applied on a sandblasted aluminum substrate by means of a film-spreader in such a way as to obtain, after sintering at 400°C for 7 minutes, a coating having a thickness of 6 microns. Such a film does not have a good adhesion, above all, after the specimen being kept dipped in water for some hours. Comparative Example 2A A primer paint is prepared with the following composition: - PTFE suspension at 60% 45 g - aqueous suspension at 20% by weight of black F-6331 pigment 20 g - 3-aminopropyl-triethoxysilane (M.W. = 221,37); Si = 0.0223 gram-atoms 5 g - deionized water 10 g When applied on a sandblasted aluminum substrate by means of a film-spreader, this paint produces a film which, after sintering at 390°C for 7 minutes, does not show a good adhesion to the support (coating thickness = 7 microns). Comparative Example 3A A primer paint is produced, which is analogous to that of Example 1, except for using sodium silicate (1.32 g of Na₂SiO₃) instead of 3-aminopropyl-triethoxysilane. The contemporaneous presence of Mg ions and silicate causes an immediate and intense precipitation. A dispersion is thus obtained, which is no longer spreadable on the aluminum support.

Claims: Composition suitable for forming a coating layer on a metal surface, comprising an aqueous dispersion at a pH comprised within the range of from 8 to 10, of a fluorinated polymer, containing an alkoxysilane soluble in the aqueous phase at pH 8-10, magnesium and/or aluminum in form of cations complexed with a hydroxy- or amino-carboxy acid, the amount of alkoxysilane being comprised within the range of from 0.05 to 0.4 parts by weight per part of fluorinated polymer, and the amount of metal cation being comprised within the range of from 0.001 to 0.005 parts by weight per part of fluorinated polymer. Composition according to claim 1, wherein the fluorinated polymer is polytetrafluoroethylene. Composition according to claim 1 or 2 wherein the complexing acid for Mg or Al cations is selected from tartaric acid, citric acid, aminoacetic acid, glycolic acid. Composition according to any one of claims 1 to 3 wherein the alkoxysilane is 3-aminopropyl-triethoxysilane or N-β-(aminoethyl-γ-aminopropyl-trimethoxysilane. Composition according to any one of claims 1 to 3, wherein the alkoxysilane is selected from γ-glycidoxypropyl-trimethoxysilane and γ-methacryloxypropyl-trimethoxysilane previously partially hydrolysed at pH 3-5. Use of the composition of any one of claims 1 to 5 for forming a coating layer performing the function of a primer. Use of the composition of any one of claims 1 to 5 for forming a coating layer on metal surfaces, preferably of iron, aluminum or their alloys.

Inventors: FONTE, ALBERTO, LENTI, DARIA, TENTORIO, ANGELO
Applicants: AUSIMONT S.P.A.
Application year: 1987
Main classification: C09D127
IPC codes: C09D127, C09D5