NL2010984C2

NL2010984C2 - Method of applying paint to a surface and resulting paint structure.

Info

Publication number: NL2010984C2
Application number: NL2010984A
Authority: NL
Inventors: Jacobus Gerardus Maria Steur
Original assignee: Steur Ind Coating B V
Priority date: 2013-06-14
Filing date: 2013-06-14
Publication date: 2014-12-18
Also published as: NL1041100A; NL1041100B1

Abstract

The method provides paint on the surface of a material, which is either a pre-coated material or an aluminium material. The method comprises the steps of blasting the surface with dry ice to obtain a surface that is clean on a molecular level; providing a primer paint composition which, on application, is wetting the surface and comprises a primer paint material for adsorption to and/or chemical reaction with said layer, and, applying said primer paint composition onto the surface. The process is based on matching surface energies.

Description

Method of applying paint to a surface and resulting paint structure

FIELD OF THE INVENTION

The invention relates to a method of applying paint to a surface .

The invention also relates to a paint structure provided on a surface comprising a primer paint material and a further paint material.

BACKGROUND OF THE INVENTION

The painting of objects is a well-known process for protection of these objects, which process however still is not perfect. Issues arise when using painted objects outdoor, particularly in the sun. These often turn out caused by an insufficient adhesion of a primer paint material onto an underlying surface. This primer paint material is typically based on an epoxy material. The underlying surface is for instance metal, but could alternatively be an initial coating onto which the paint is applied. Such a initial coating is typically a decorative coating. Chemically, such coating typically comprises acrylic and/or acrylate groups, and furthermore often a polyurethane or alkyd chemistry. In many situations, such initial coating is present on top of a metal surface. Aluminium is used increasingly. Examples hereof are vehicles and machines for outdoor use, such as cranes, trucks (and related objects) and the like. Such objects turn out to constitute a very hard class, due to the combination of use in extreme conditions (use in full sun during longer periods, relatively large surface area, which is typically quite flat, the use in the presence of dust, oils and other contaminants, the application of labels onto the paint). Particularly when used in the sun, the paint may be damaged. This is understood to be either the result of UV radiation that initiates chemical reactions within the paint, or the effect of differences in thermal expansion or both. Such UV radiation may for instance initiate a further polymerisation which results in higher molecular weight and therewith changes to the chemical properties. Differences in thermal expansion may result in delamination. Here dust particles or other contamination at the interface between the initial surface and the primer paint material may expand differently on heating up and/or cooling down than surrounding layers, leading to local delamination that may subsequently progress. Such a progress is easily accelerated by chemical and/ or mechanical cleaning processes, in which etching means may get into contact with the partly delaminated interface .

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an improved method of applying paint, resulting in a paint structure with improved stability, also under quite extreme conditions.

It is a further object to provide such a resulting paint structure .

This object is achieved in a method of providing a surface of a layer with paint, comprising the steps of:

Blasting the surface with dry ice to obtain a surface that is clean on a molecular level;

Providing a primer paint composition which, on application, is wetting the surface and comprises a primer paint material for adsorption to and/or chemical reaction with said layer, and;

Applying said primer paint composition onto the surface .

It has been understood, on the basis of investigations leading to the invention, that three conditions are to be met in order to arrive at a good adhesion: a cleaning procedure wherein not merely dust but also apolar compounds are removed from the surface; a good spreading of the applied primer paint composition over the surface, more particularly wetting of the surface; and an adequate chemical matching of the primer paint material with the underlying layer.

The cleaning is carried out by means of blasting with dry ice, i.e. carbon dioxide in solid form. Most suitably, use is made of carbon dioxide pellets. It was surprisingly found that this cleaning method not merely removes dust and other particles, but also apolar compounds, such as paraffine. The removal of those apolar compounds is believed to be achieved in that the carbon dioxide will be in a transient (supercritical) liquid state, and then dissolve the apolar compounds. As a result, the surface will be clean up to a molecular level. Moreover, a roughening likely occurs, due to the force of the dry ice onto the surface. The extent of roughening will depend on a duration of the blasting.

The wetting is achieved by a proper combination of the primer paint composition with the surface. The phenomenon of wetting is governed by Young's law, as known per se: if the contact angle is smaller than 90 degrees, wetting may occur. Wetting is evidently important to ensure that the primer paint composition extends on the full surface. The matching here is needed on the basis of the cleaned surface, which will have properties different from a dirty surface. Most suitably, the wetting is expressed in terms of surface tensions or surface free energy. In relation thereto, it was found that preferably the difference between the surface tensions of the applied paint composition and the cleaned surface is small. This difference is also known as the interfacial tension.

An adequate chemical matching is required so as to arrive at a high density of adsorption or binding sites between the underlying layer and the primer paint material. The higher the density, the better the interaction. Most suitably, the primer paint material comprises a regular chain, which allows to obtain a layered structure. This strongly increases the stability of the primer paint material and thus the resulting paint structure.

In one embodiment, the primer paint material comprises an epoxy vinyl resin material or blend. These materials may contain both vinyl acrylate groups and epoxy groups. The epoxy groups may be phosphated. The groups may be integrated into a single resin material known as an epoxy vinyl ester resin, as known per se, or alternatively be a blend of materials such as known from US5,344,858 which is included herein by reference. It is understood that this blend of materials may form an epoxy vinyl ester resin after deposition. Epoxy vinyl ester resins are known per se, and sold as dispersions in an organic solvent such as xylene.

The dispersion typically further comprises an initiator, such a cobalt-complex, a secondary typically unsaturated compound, such as styrene, with which the vinyl ester resin may react in a radical polymerisation, and any further additives. The epoxy vinyl ester resin is for instance based on epoxy resins that have reacted with acrylic acids. Examples of suitable epoxy vinyl ester resins are diglycidyl ether of bisphenol A, epoxy novolac, cycloaliphatic epoxy, diepoxidized hydrogenated bisphenol A and brominated versions thereof. An example is a brominated bisphenol A, wherein the phenyl rings have been substituted with Bromide groups, for instance two per phenyl ring. Such vinyl ester resin structure may comprise several bisphenol A units, as known per se. The acrylic acid is for instance chosen from the group of acrylic acid, methacrylic acid, crotonic acid and cinnamic acid. Acrylic acid appears suitable.

The underlying layer with which the primer paint material matches, is in one embodiment a topcoat coating layer, suitably comprising acrylic or acrylate groups. This matches very well with the use of an epoxy vinyl resin material as primer paint material. Under the term 'top coat coating layer' is understood a coating that is suitably for use as an uppermost coating layer. Example materials hereof are for instance aliphatic-acrylic polyurethanes (in use as thin film decorative coatings), acrylic coatings, epoxies, silicones, vinyl coatings modified with acrylic resins, or alkyd-based coatings. This use is important, because herewith it becomes possible to do recoating of vehicles and other objects without the need to remove the previous coating completely. Recoating may be applied for instance in case of selling vehicles but also in case of renewing a coating after intensive use.

It is believed by the inventor, as a provisional hypothesis, that acrylate groups of the epoxy vinyl resin may enter into chemical bonding with acrylate or similar groups at the surface. However, it is not excluded that there would merely be adsorption, and that the acrylate groups of the epoxy vinyl resin would form weaker intermolecular bonds, based on dipolar forces and/or on hydrogen bonding.

The use of epoxy vinyl resins may further have the advantage that an ordered structure is formed. Nanoscale ordered structures are based on intermolecular attraction between neighbouring molecules, which forms the basis for the formation of monolayers and often forms the background of crystalline areas within a polymer network. Such an ordered structure has the advantage of stability. Moreover, it is considered, as a matter of speculative explanation - that the epoxy vinyl resins would adsorb with their chains extending away from the surface. This is believed due to the preferred interaction of the acrylate groups with the surface and the difference in material structure from the etherified chain coupled thereto. This adsorption -primarily or substantially normal to the surface - allows from a very dense layer that is absorbed, with many sites of interaction between the surface and the primer paint material. Such a dense layer effectively results in high strength and stability. Coating thickness may be as limited as 10 microns or lower.

In an alternative embodiment, the underlying layer is an aluminium layer, onto which a native aluminium oxide is inevitably formed. Here again, an epoxy vinyl resin primer material is suitable, but alternative primer paint compositions such as epoxies seem appropriate. The primer paint composition is herein applied in sufficient thickness to cover the aluminium layer. The primer paint composition may also be applied as a sequence of layers, for instance a first layer and a second layer, wherein the first layer is dried before the second layer is applied. The primer paint composition applied as a first layer and a second layer may be identical or may be different. It is believed surprising that good adhesion on aluminium is achieved with epoxy vinyl material, since acrylate materials are known to have a bad adhesion on aluminium. Likely, the repeated propylene oxide chain with hydroxyl side groups (- O-CH2-CHOH-CH2-O-) interacts with the aluminium oxide surface. The material also meets requirements for edges and ridges between plates, which are known to be weak spots.

In order to obtain perfect wetting, and therewith the ability to build up a uniform layer on top of the surface, tests have been carried out, in which the surface free energy - also known as surface tension- have been measured. It has then been found that the surface free energy of the primer paint composition is preferably lower than that of the surface. Preferably, the surface free energy of the primer paint composition is at least half of that of the surface, and more preferably it is at most one third lower, by further preferably at most a quarter (relative to the surface energy of the surface).

Suitably, the surface energy of the surface of an existing paint layer is brought to a value higher than 30 mN/m (or dyne/cm) and preferably higher than 35 mN/m. Typically, such surface energy will be lower than 45 mN/m, or even be in the range of 35- 40 mN/m. It appears that the surface energy is not merely due to the cleanness of the surface, but also to the surface topology.

The progress of the surface energy can be followed by test fluids for testing surface energy, complying with ASTM Standard D-2578. Suitably, such test fluids are based on a mixture of 2-ethoxyethanol and formamide. In this manner, an endpoint of the blasting step may be determined, so that the primer paint material will wet the surface.

Preferably, the primer paint composition and any further paint composition are applied by spraying. Preferably, use is made of electrostatic spraying. This type of spraying results in smaller droplets, leading to a better wetting and adhesion to the substrate surface. Due to the better adhesion, such droplets will moreover get into any grooves, cavities and the like to ensure full coverage of the substrate surface.

In an even further embodiment, use is made of air mixed electrostatic spraying. This air mixed electrostatic spraying occurs at a relatively low pressure of less than 10 bar, preferably less than 5 bar. Settings for the voltage may be between 50 and 100 kV. Use is suitably made of an electrostatic spray gun including an air turbine, as for example disclosed in more detail in US7757973. More preferably, use is made of an gun with an internal voltage generator to obtain the high voltage needed for electrostatic spraying. It has been found, when applying the process of the invention and subsequently a top coat by means of air mixed electrostatic spraying, that an unexpectedly smooth coating is obtained, having a high gloss of for instance more than 60%. Particularly a coating was obtained which did not have the so-called orange peel effect.

According to a further aspect of the invention, a paint layer or structure on a surface is provided as obtainable with the method as claimed in any of the claims 1-14.

In again a further aspect, the paint structure comprises a stack of an initial coating layer comprising acrylic or acrylate groups, a primer paint material comprising an epoxy vinyl ester resin and a further paint material.

Peel strength tests indicate that the paint structure has an improved adhesion in comparison to previously applied coating processes, which resulted in delamination, particularly after some period of use in environments with intensive sun. Such peel strength tests are carried out in accordance with IS04624. It turned out that the adhesion was beyond an upper limit of the test apparatus. This was also the situation 1 month after coating.

The paint structure preferably comprises a primer paint material with an ordered sublayer based on epoxy vinyl ester resin molecules. Such an ordered sublayer is most suitably a monolayer .

The paint structure of the invention is suitably provided on and thereafter part of an article. The article is most suitably a vehicle or a machine for outdoor use. Herein, the initial coating layer is present on top of a metal surface.

According to a further aspect, the invention relates to the use of primer paint composition comprising an epoxy vinyl resin material or blend for wetting on and adhesion to a top coating layer that has been cleaned to a molecular level. Particularly, the applied primer paint composition has a surface free energy that is lower than the surface free energy of the cleaned top coating layer.

DEFINITIONS AND EXPLANATIONS

The surface free energy of a condensed phase material - i.e liquid or solid - is based thereon that the atoms at the surface are in a very different environment compared to atoms in the interior of the material. This difference arises from the asymmetrical environment: in the bulk material, each atom is surrounded by similar ones and they experience no net forces. However, those at the surface see this only on one side of the interface. Moreover, the atoms at the surface undergo forces from the environment.

As a result, there is an energy excess at the surface compared to the bulk. This is the surface free energy or the surface tension. For a solid material, one preferably refers to the surface free energy. For a liquid material, both terms are used both. Thermodynamically, the surface tension can be interpreted as the work required to extend a surface or to bring atoms from the interior to the surface region.

EXAMPLES

Example 1

The surface free energy of a surface was determined with reference fluids as supplied from Diversified enterprises Inc, 101 Mulberry street, suite 2N, Claremont NH 03743, USA. These test fluids comprise a mixture of 2-ethoxyethanol, formamide, distilled water, methyl violet 2B. The concentration ratio between 2-ethoxyethanol and formamide defines the surface energy value, as specified in dyne/cm. Each of the reference fluids indicates a surface free energy level. The surface free energy corresponds to an indicated value of a reference fluid, if such reference fluid just wets the surface. The test fluid provide a test in accordance with ASTM standard D-2578.

Tests were carried out with a sample of having a coating of 55 microns and a sample of bare extruded aluminium, directly obtained from an extrusion manufacturing site.

The test were carried out at room temperature of 22°C, with a relative humidity (RH) of 50%. The dry ice blasting was carried out at a pressure of 5 bar, with dry ice pellets of a 3 mm diameter on average and a dry ice flow of 50 kg/hr, with a duration of 10 seconds.

Table 1 - results of a dry ice treatment on selected substrates

It follows that the dry ice treatment increases the surface energy of the samples. The surface roughness does not change during the short duration, implying that the underlying material is not significantly roughened any further.

It is observed that many aluminium sheets applied for work products such as vehicles, trucks and other industrial machines are made of rolled aluminium. The use of (paraffine-based oil) during rolling tends to lower the surface energy of aluminium, and therefore require an appropriate cleaning. A surface of extruded aluminium appears cleaner, and the surface energy of the aluminium before cleaning confirms this.

Example 2 A primer material and a subsequent coating were applied onto the cleaned surface. Use was made of an epoxy vinyl resin material supplied by PPG Industries as Selemix™ 7-414, wherein the epoxy resin had a molecular weight between 700 and 1100 g/mol and was present in a solvent mixture of 30-40wt% methyl isobutyl ketone and 5-7wt% monopropylene glycol methyl ether, 3-5 wt% toluene, 2-3 wt% 2-methoxy-l-methylethylacetate, l-2wt% n-butyl acetate and l-2wt% xylene. This material was mixed - prior to deposition - with an epoxy hardener comprising triethylenetetramine and 2,4,6-tris (dimethylaminomethyl) phenol (PPG product number 1.959.4010 ) and with an epoxy thinner (PPG product number 1.911.4410) in line with prescribed ratio to end up at a suitable viscosity. The coated layer was in this example up to 30 microns thick. Subsequently, a top coat as supplied by PPG was applied, traded as Delfleet™ premium high solid top coats. Electrospraying was used for deposition of the materials. Drying and curing was carried out in line with specifications

The resulting coatings were reviewed. Results are shown in Table 2. For the adhesion, use was made of a TQC positest adhesion tester, operating in accordance with IS04624.

Table 2

Example 3

Use was made of a Krüss tensiometer K6 as commercially available for identifying the surface free energy of a fluid. The Krüss tensiometer K6 is a so-called Lecomte du Noüy Tensiometer and is provided with direct scale reading of lmN/m on the basis of a platinum-iridium ring, with a circumference of 6cm. The fluid to be measured is herein provided in a sample vessel of 50 mm. The vessel is placed on the apparatus, such that the ring touches the surface.

According to this Du Noüy or ring method, the surface tension relates to the force required to pull a wire ring off the surface. The radius of the wire ranges from 1/30 to 1/60 of that of the ring. As disclosed in the Encyclopedia of Surface and Colloid Science (2002), page 3154, the surface free energy corresponds to the ratio of the experimentally measured force and twice the circumference of the ring, as amended by a correction factor. Current equipment does not require separate calculation of the correction factor, since its calculation is incorporated in the software.

Results indicated that the epoxy vinyl resin material as used in accordance with Example 2 had a surface free energy of approximately 30 dyne/cm. This is lower than that of the surfaces used.

Comparative tests

Tests were done before wherein the blasting step was exchanged for an conventional cleaning and roughening step. The adhesion turned out insufficient, particularly after several weeks of use under heavy conditions, i.e. in full sunlight, for instance in Arabia.

Further tests were done wherein the primer material was an epoxy material. The surface energy was higher than 40 dyne/cm. No good adhesion was achieved.

Claims

A method for applying paint to the surface of a layer, comprising the steps of: Blasting the surface with dry ice to obtain a surface that is clean to the molecular level; Providing a paint primer composition which, upon application, moisten the surface and contains a paint primer material for adhering to and / or chemically reacting with the layer, and applying the paint primer composition to the surface.

The method of claim 1, wherein the paint primer material comprises an epoxy vinyl resin.

Method according to claim 1 or 2, wherein the layer is a topcoat paint layer that is provided with acrylic or acrylate groups.

The method of claim 1 or 2, wherein the layer is an aluminum layer with an aluminum oxide surface.

Method according to one of the preceding claims, wherein the blasting is carried out to obtain a surface free energy of more than 35 dyne / cm (mN / m)

The method of claim 1, wherein the surface blasting comprises dissolving apolar hydrocarbon compounds in liquefied dry ice.

The method according to any of the preceding claims, wherein the applied paint primer composition has a surface free energy of less than 40 dyne / cm (mN / m).

A method according to any one of the preceding claims, wherein the paint primer composition is applied with electrostatic spraying.

The method of claim 8, wherein use is made of air-mixed electrostatic spraying.

The method of any one of the preceding claims, wherein at least a portion of the paint primer material forms a textured layer on the surface.

A method according to any one of the preceding claims, wherein the applied paint primer composition has a surface free energy that is lower than the surface free energy of the surface

The method of claim 11, wherein the surface free energy of the paint primer composition is at least half of the surface free energy of the surface.

The method of claim 12, wherein the surface tension of the surface is at least 38 dyne / cm.

The method of claims 12-13, wherein the paint primer composition has a surface tension in the range of 25-35 dyne / cm.

The method of claim 1, further comprising applying a further paint composition to said paint primer material.

A paint layer or structure on a surface obtainable with the method according to any one of claims 1-14.

A paint structure comprising a stack of an initial coating containing acrylic or acrylate groups, a paint primer material comprising an epoxy vinyl resin, and a further paint material.

The paint structure of claim 17, wherein the paint primer material comprises an ordered partial layer.

The paint structure of claim 18, wherein the ordered sublayer is a monolayer.

A paint structure on an aluminum layer comprising a paint primer material comprising an epoxy vinyl resin and a further paint material.

An article provided with the paint structure according to any of claims 16 to 20.

The article of claim 21, wherein the initial coating is present on top of a metal surface.

The article of claim 21 or 22, wherein the article is a vehicle or machine for outside.

Use of a paint primer composition comprising an epoxy vinyl resin for wetting an adhesion to a coating that has been cleaned to the molecular level.

The use of claim 24, wherein the applied paint primer composition has a surface free energy that is lower than the surface free energy of the cleaned coating.