Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1999-03-18
2001-04-17
Beck, Shrive (Department: 1762)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C427S409000, C427S333000, C427S336000, C427S142000, C427S156000
Reexamination Certificate
active
06217945
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the application to a substrate of a coating system in which a cured primer coating is overcoated with a top coat which is intended to be strippable, that is to say removable from the primed substrate by a stripping solvent without removal of the primer from the substrate. Such coating systems are widely used on aircraft and may also be used on other substrates, for example commercial vehicles, cars, yachts or railway vehicles.
Aircraft are usually painted with a primer which is based on an epoxy resin and a curing agent, particularly an amino-functional curing agent, although the primer can be an alternative cured coating, for example polyurethane. The top coat is usually based on polyurethane, particularly a polyester-urethane. The coatings must resist water and certain organic solvents such as aviation fuel and preferably also resist hydraulic fluids based on phosphate esters such as tricresyl phosphate or tri-n-butyl phosphate, e.g. that sold under the trade mark “Skydrol”. Each coating is generally cured by the reaction of components which have been packaged separately and are mixed at or shortly before application. When an aircraft is to be repainted it is usual to remove at least the top coat using a paint stripper before repainting. The preferred stripping solvents used nowadays are based on aqueous benzyl alcohol with acidic (formic acid) or alkaline additives although some halogenated solvents such as methylene chloride with phenol are still used. In general, the top coat adheres to the primer so strongly that the top coat cannot be stripped without removing or at least damaging the primer. Aircraft owners and operators would prefer a paint system in which the primer remained on the aircraft to prevent the stripping solvent contacting any resins, sealants or adhesives used in the aircraft construction or the metallic substrate and to avoid repeating the pre-treatment and primer coating of the aircraft.
BACKGROUND ART
Existing strippable coating systems generally use an intermediate coating between the primer and the top coat. One type of intermediate coating, described in GB-A-1511935, is a non-crosslinked polyamide. This at least partially resists the solvent used to remove the top coat and can itself easily be removed by alcohol-based solvents to reveal the primer. Similarly, DE-A-2528943 describes a linear non-crosslinked elastic polyurethane coating used between crosslinked polyurethane primer and top coat layers. EP-A-147984 describes a barrier intermediate coating comprising a film-forming addition polymer comprising units of a vinyl aromatic monomer and units of a monomer containing a hydroxy alkyl group in an amount to provide a hydroxyl content of the addition polymer in the range 0.5 to 5 per cent by weight reacted with a polyisocyanate containing at least 2 isocyanate groups per molecule.
All these prior art intermediate coatings are based on solutions of synthetic resins in organic solvents. There has been a demand for an intermediate coating which releases less, preferably no, volatile organic solvent.
DISCLOSURE OF THE INVENTION
In a process according to the invention for overcoating a substrate having a cured primer coating with a curable top coat which when cured is swellable by a stripping solvent, a non-volatile polar material is applied to the primed substrate from an aqueous liquid vehicle before coating with the top coat, the said polar material having sufficiently high affinity for the primer surface that it is not washed off the primer surface by water rinsing or by application of the top coat paint, whereby the said polar material facilitates the removal of the top coat from the primer by the stripping solvent.
The invention also provides a process for overcoating a substrate having a cured primer coating to facilitate the removal by a stripping solvent of a subsequently applied top coat, characterised in that a non-volatile acidic aromatic polar organic compound is applied to the primed substrate from aqueous solution, the said acidic aromatic compound having sufficiently high affinity for the primer surface that it is not washed off the primer surface by water rinsing.
By a cured primer coating we mean a coating which is curable by chemical reaction when applied and which has cured sufficiently to be at least touch dry at the time when the polar organic compound (polar material) is applied. The polar organic compound is thus usually applied at least 2 or 3 hours after the substrate has been primer coated and may be applied at any later time, including several days or more later when the primer is fully cured.
The polar material is not an intermediate coating as known from the prior art and is generally not a film-forming material. It is non-volatile, that is to say it does not evaporate to any significant extent at 20° C. In some instances it is preferably not polymeric, although polymers or oligomers having a plurality of anionic groups can be used and in other instances may be preferred. The polar material, particularly if it is not polymeric, may be applied at such a low thickness that it is present on the primer surface as a molecular monolayer or partial monolayer, for example a monolayer covering 10 to 60 or 80% of the surface area. Unexpectedly, the polar material allows good adhesion between the top coat and the primer on application of the top coat and in service but greatly weakens the interlayer adhesion between the primer and the top coat when the top coat is swollen by stripping solvent. The suitability of a candidate polar material can be readily assessed by applying it to the primed substrate and rinsing with water to see if it is completely removed.
The primer coating is preferably based on an epoxy resin cured with an amino-functional curing agent such as an amino-functional polyamide and/or an araliphatic diamine. A crosslinked polyurethane primer is an alternative. The primer generally contains an anticorrosive pigment such as a chromate, phosphate, phosphonate or molybdate and may contain one or more other pigments and/or fillers. The primer coating can be applied from solution or dispersion in an organic solvent or can be a high solids or solventless composition or may be applied as an aqueous composition.
Many of the polar materials which have been found to be effective, particularly over amine-cured epoxy primers, contain at least one acidic group such as a carboxylic acid group or sulphonic acid or phosphonic acid group present in free acid or salt form. In general, carboxylic acid groups if present are preferably in free acid form whereas sulphonic or phosphonic acid groups if present are preferably fully or partially neutralised e.g. in alkali metal salt form. The polar material is usually a polar organic compound, preferably a polar organic aromatic compound. The acidic group is preferably attached directly to an aromatic ring. Many of the effective compounds additionally contain at least one hydroxy group and/or amino group. The polar material (polar organic compound) preferably has a molecular weight in the range 120 or 150 up to about 1000 or 1500. Many of the effective compounds are known as dyes, although one of the most preferred compounds is gallic acid (3,4,5-trihydroxy-benzoic acid), which is not generally regarded as a dye. 2,5-Dihydroxybenzoic acid is also effective, although 3,5-dihydroxybenzoic acid is much less effective in giving stripping. 4,5-Dihydroxy-1,3-benzenedisulphonic acid (generally used in disodium salt form) is also effective in giving selective stripping. Examples of acidic dyes which have been found effective in giving selective stripping are Catechol Violet, Fast Green, Lissamine Green SF, Orange G, Amido Black and Methyl Orange. The chemical structures of these dyes are shown below.
Mixtures of polar materials can be used, for example mixtures of gallic acid with a dye such as Past Green. Further examples of acidic dyes which can give selective stripping are sulphonyl porphyrins, for example 5, 10, 15, 20-tetrakis (4-sulphonatophenyl)-
Beck Shrive
Cannoni Ann Marie
Kolb Jennifer
PRC-DeSoto International, Inc.
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