WO2008155453A1

WO2008155453A1 - Coating and method for producing same

Info

Publication number: WO2008155453A1
Application number: PCT/FI2008/050348
Authority: WO
Inventors: Marke Kallio; Juha Mannila; Anne Pahkala; Mika Kolari
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2007-06-20
Filing date: 2008-06-11
Publication date: 2008-12-24
Also published as: FI20070497A0

Abstract

The invention relates to a coating for coating a metallic surface. According to the invention, the coating comprises a first and a second layer that are arranged onto the surface of a metallic surface in such manner that the first layer is against the metallic surface and the second layer is arranged on top of the first layer, and the second layer is a sol-gel layer. Furthermore, the invention relates to a method for coating a metallic surface.

Description

COATING AND METHOD FOR PRODUCING SAME

FIELD OF THE INVENTION

The invention relates to the coating defined in the preamble of claim 1 for coating a metallic surface and to the method defined in the preamble of claim 8 for coating a metallic surface.

BACKGROUND OF THE INVENTION Known from prior art are many different kinds of coatings for use as a coating in metal products.

Also known from prior art are the sol-gel coating technique and different methods for forming a sol-gel coating. The sol-gel coating can be used e.g. on a metallic, ceramic, wooden, wood composite or concrete surface in order to protect the surface against dirt and/or abrasion. The sol-gel coating normally forms a thin film on the protected surface.

The production of sol-gel coatings starts from the use of liquid source materials, and the organic and inorganic parts are combined in a molecular- scale, so that there does not exist any macroscopic phase boundaries that are typical for the traditional composite coatings. The liquid source materials are crosslinked and solidified by hydrolysis and polycon- densation reactions. Removal of the solvents and final hardening of the coating can be performed e.g. by applying heat.

OBJECTIVE OF THE INVENTION

The objective of the invention is to disclose a new type of extremely abrasion-resistant coating for metallic surfaces. Another objective of the invention is to provide a new method for coating a metallic sur- face. SUMMARY OF THE INVENTION

The coating and the method according to the invention are characterized by what has been presented in the claims .

The invention is based on a coating for a metallic surface. According to the invention, the coating comprises a first and a second layer that are arranged onto the surface of a metallic surface in such manner that the first layer is against the metallic surface and the second layer is arranged on top of the first layer, and the second layer is a sol-gel layer.

Furthermore, the invention is based on a method for coating a metallic surface. According to the invention, a first layer is arranged onto the surface of a metallic surface, and a second layer is arranged on top of the first layer in such manner that a sol- gel layer is used as the second layer.

The invention is specifically based on improv- ing the surface resistance and reducing the abrasion of the coating of a metallic surface.

In one embodiment of the invention, a polyester-based first layer is arranged onto the surface of a metallic surface. The metallic surface is treated with a polyester-based compound e.g. by covering, coating, painting or in a similar manner. The polyester-based compound may be in the form of powder, solution, paint, primer or the like. In one embodiment, the polyester- based layer may be arranged onto the surface of the metal by powder-painting.

In one embodiment of the invention, the metallic surface is a steel surface.

In one embodiment of the invention, the sol- gel layer comprises a ceramic compound, acrylic-type hydrocarbons and/or alkoxy silane. In one embodiment of the invention, the sol-gel layer is organically modified.

In one embodiment of the invention, the sol- gel layer comprises silicon, carbon, oxygen, hydrogen and/or an organic substance. In one embodiment of the invention, the organic portion in the sol-gel layer is 30 to 50% by weight.

In one embodiment, the organic part of the coating is mainly an acrylic-type partly grafted straight-chain hydrocarbon chain and the inorganic part is mainly formed by a silicon-oxygen network.

In one embodiment, the thin film coatings, preferably the films of about 1 to lOμm, are mainly alkoxy-silane-based organically modified coatings. In one embodiment of the invention, the first and the second layer are hardened by heat treatment. In one embodiment, the heat treatment is performed at a temperature of about 80 to 14O⁰C, preferably at a temperature of 80 to 120⁰C. In one embodiment, harden- ing of the coating and removal of the solvents are performed at a temperature of about 110⁰C, e.g. by a 60- minute heat treatment. The heat treatment period may be shortened or extended if necessary according to the coating material. In one embodiment of the invention, the first and the second layer are hardened by UV radiation.

The coating can be produced and the coating action performed in manners known per se in the art. The coating action can be performed using a wet coat- ing method, e.g. as cast and spray coating, but in principle all wet coating methods are possible.

In one embodiment, the polyester treatment, the sol-gel-coating and/or the hardening treatment may be performed in the same production line and even in the same step. Alternatively, the different production steps may be performed as separate processes. A suitable metallic surface to be coated, the first layer to be used, e.g. the polyester compound to be used, and the composition of the sol-gel coating can be selected for each application. The invention provides the advantage that good abrasive resistance of the metallic surface, gloss and low soiling of the surface are achieved with the coating according to the invention. Furthermore, the coating according to the invention has good adhe- sion of the sol-gel layer to the first layer of the coating.

Thanks to the invention, it is possible to use the coated products according to the invention in sites that require strong abrasive resistance.

DETAILED DESCRIPTION OF THE INVENTION

In the following section, the invention will be described with the aid of a detailed example of an embodiment .

Example 1

The test series comprised examining the abrasive/scratch resistance of steel materials and the effect of the coating according to the invention upon them by a method developed for a paint washer (DIN 53 788) .

First, an organically modified ceramic sol- gel coating was prepared. The composition of the sol- gel coating that was formed is presented in Table 1. The table shows the amount of the organic portion in the total mass. The rest is inorganic network. The table specifies the portions of the different elements in the coating by the principal components. In addition, small amounts of e.g. processing agents or simi- lar additives may be used in the coating. Table 1

Hardening of the coating and removal of the solvents was performed in a heating chamber at a temperature of 110⁰C by a 60-minute heat treatment.

The examined surface was abraded with an abrasive brush mounted to the paint washer in such man- ner that the load level with respect to the abraded surface was adjusted to 8g/cm². The abrasive brush moves back and forth on the abraded surface. The abrasions were continued up to 700 abrasive rounds, and changes in the properties of the abraded surface were observed by contact angle measurements at intervals of every 50 to 100 rounds. In addition to the contact angle measurements, abrasion of the coating was observed by gloss measurements (Micro-Tri-Gloss glossmeter, measurement at an angle of 60°) . Also, scratch resis- tance of the coating was determined by a pin abrasion method.

A first test series comprised examining stainless and galvanized steel material. The steel materials (stainless and galvanized) were coated with the sol-gel coating presented in Table 1. It was discovered that the sol-gel coating was abraded when treated with the abrasive brush. The coating on a galvanized steel base was the most resistant to abrasion with the abrasive brush. The results showed that the base material to be treated has great effect on the adhesive and other properties of the coating. A second test series comprised examining the abrasive resistance of a polyester-painted (red paint) steel surface. The polyester-painted steel surface was coated with the sol-gel coating presented in Table 1. It was discovered that the abrasive resistance of a polyester-painted steel surface can be improved with the sol-gel coating. It was discovered that also the mere porous red polyester paint without the sol-gel layer was strongly abraded during abrasion with the abrasive brush, and in some parts a bare steel surface was found in the sample. The sol-gel thin film coating coated on top of the polyester film evened out the porosity and topography of the surface. The sol-gel coating also improved the abrasive resistance of the samples to a considerable degree.

A third test series comprised examining the abrasion of sol-gel-coated steel samples painted with another polyester paint (white paint) when abraded with the abrasive brush. A ceramic-containing sol-gel coating improved the abrasive resistance of the surface also in this test series.

A fourth test series comprised examining steel surfaces treated with a polyester primer (PET). A soft surface, a steel surface treated with a polyes- ter primer is vulnerable to abrasion with an abrasive brush. During the abrasion, the polyester primer was in some parts entirely detached from the steel surface, exposing bare areas. It was discovered that it was possible to improve the abrasive resistance of the polyester-treated steel surface with the sol-gel coating arranged on top of the polyester layer.

The effect of abrasion with the abrasive brush upon the surface properties of sol-gel-coated steel substrates was also examined in terms of the gloss properties of the surface. Surface gloss before and after abrasion was evaluated by glosses measured with a Micro-Tri-Gloss glossmeter (60°). The measurements showed that the sol-gel coatings reduce the gloss of untreated steel (RT) , galvanized steel (ST) and chrome-plated galvanized steel (KST) . However, in surfaces treated with the polyester primer (PET) and polyester paint (VMT and PMT) , the sol-gel coatings increased the surface gloss. After abrasion with the abrasive brush, gloss properties of the surface were slightly attenuated, which was observed in all exam- ined steel substrates. The changes in gloss can be explained by the abrasion of the polymeric component of the coating and by the scratches formed onto the coating. However, the ceramic-containing sol-gel coating clearly improved the abrasive resistance of the poly- ester-primered and polyester-painted steel surfaces.

Abrasive resistance of the sol-gel coatings was also examined by pin abrasion tests. The pin abrasion tests comprised rubbing coated and uncoated reference samples for 5 minutes with a POM (polyoxy- methylene) pin at a weight of 100Og. The results suggested that the sol-gel coatings improved the abrasive resistance of the steel plates.

Five-minute abrasion with a pin abrasion device at a weight of lOOOg abrades clear scratches on the uncoated steel sample while the coated sample does not yet show any signs of abrasion. The uncoated steel plates were the most abraded in the samples. In the uncoated ones, a polyester-primered RR3 steel plate was the least abraded. The sol-gel-coated samples were the least abraded if at all.

Effect of the sol-gel coatings upon the surface hardness of the steel materials was examined by Vickers hardness measurements. The measuring device was Shimadzu Micro Hardness Tester (Type M) . The Vick- ers hardness measurement comprised pressing the surface of the sample with a pyramid-shaped diamond in- sert at a specific force for a specific time. Weights of 25 to lOOOg and testing time of 10s were applied in the measurements. The diagonals of the indentation created to the surface of the sample during measure- ments were measured and used for calculating the HV (Vickers Hardness) values. All hardness measurement results from the examined steel materials showed that the surface hardness of steel can be improved with the sol-gel coating according to the test. Also, the meas- uring method was found applicable for measuring the surface hardness of the samples because the standard deviations of the results were very small. Furthermore, all results, irrespective of the weight used, gave similar results. In one test, the sol-gel coating was hardened by UV treatment (2min) . The coatings were prepared on clear PMMA plastic. The abrasive resistance was examined with a Taber abrasion test and Haze measurements. Microhardness and elastic modulus were determined by nanointender measurements. Adhesion was measured with a Tape test and found very good. The results are presented in Table 2.

Table 2

Adhesion of the sol-gel coating to the poly- ester-painted steel samples was very good. This was also discovered in a T-bend test in which a coated plate was bent. Adhesion of the sol-gel thin film coating to the polyester paint was so strong that, in the most extreme bendings, both the coated paint and the priming paint were torn up to the zinc layer, but the coating still remained adhered to the paint.

Dirt resistance of the surface was improved together with the abrasive resistance of the coating. As a number of deep scratches were produced onto the surface, the dirt became easily accumulated in these grooves. Good results were achieved with the polyester-treated and sol-gel-coated samples in terms of hy- dro- and oleophobic properties.

One can conclude from the tests that coating a metallic surface with the coating according to the invention comprising a first and a second layer provides excellent abrasive resistance to the metallic surface.

The coating and the method according to the invention are applicable in different embodiments for coating different kinds of metallic materials. With the method according to the invention, it is possible to produce any kinds of coatings for applications that require abrasive resistance.

The embodiments of the invention are not limited merely to the examples referred to above; instead many variations are possible within the scope of the accompanying claims.

Claims

1. A coating for coating a metallic surface, charac teri z ed in that the coating comprises a first and a second layer that are arranged onto the surface of the metallic surface in such manner that the first layer is against the metallic surface and the second layer is arranged on top of the first layer, and the second layer is a sol-gel layer.

2. The coating according to claim 1, char- ac teri z ed in that a polyester-based first layer is arranged onto the surface of the metallic surface.

3. The coating according to claim 1, charac teri z ed in that the metallic surface is a steel surface.

4. The coating according to any one of claims

1 to 3, charac teri z ed in that the sol-gel layer comprises a ceramic compound, acrylic-type hydrocarbons and/or alkoxy silane.

5. The coating according to any one of claims 1 to 4, charac teri z ed in that the sol-gel layer is organically modified.

6. The coating according to any one of claims 1 to 5, charac teri z ed in that the sol-gel layer comprises silicon, carbon, oxygen, hydrogen and/or an organic substance.

7. The coating according to any one of claims 1 to 6, charac teri z ed in that the organic portion of the sol-gel layer is 30 to 50% by weight.

8. A method for coating a metallic surface, charac teri z ed in that a first layer is arranged onto the surface of the metallic surface and a second layer is arranged on top of the first layer in such manner that a sol-gel layer is used as the second layer .

9. The method according to claim 8, charac teri z ed in that a polyester-based first layer is arranged onto the surface of the metallic surface.

10. The method according to claim 8 or 9 , charac teri z ed in that the metallic surface is a steel surface.

11. The method according to any one of claims 8 to 10, charac teri zed in that the first and the second layer are hardened by heat treatment.

12. The method according to claim 11, charac teri z ed in that the heat treatment is performed at a temperature of 80 to 14O⁰C.

13. The method according to any one of claims 8 to 12, charac teri z ed in that the first and the second layer are hardened by UV radiation.