Coating processes – Removable protective coating applied – Metal base
Reexamination Certificate
1998-12-10
2001-03-27
Beck, Shrive (Department: 1762)
Coating processes
Removable protective coating applied
Metal base
C427S421100, C528S053000, C528S054000, C528S058000, C528S064000, C528S066000, C528S076000
Reexamination Certificate
active
06207225
ABSTRACT:
DESCRIPTION
Subject matter of the present invention is solvent-free, two-component strip coating for metal surfaces as well as a method for coating surfaces of metal parts, especially aluminium parts, with the aid of this strip coating.
In aeroplane construction, the surfaces of aluminium parts are partially treated with the aid of scouring baths for the purpose of weight reduction, whereby chemical removal of aluminium takes place. In order to prevent chemical attack on partial regions of the metal surfaces not to be removed, these are coated before the treatment with the scouring bath with a strip coating in spray or pour methods and therewith protected from the chemical treatment.
In this connection the demands on the strip coating are relatively high because it must be chemically resistant against acids and basis, must have rapid-hardening and drying properties in order to save time and expenditures and must possess a controlled adhesion so that it can be stripped without problem and residue-free after conclusion of the chemical metal removal.
As a rule, the coatings known up to now and used in practice have a relatively low binding agent content (ca. 30 to 35% by weight) and a comparatively high amount of organic solvents (ca. 65 to 70% by weight). Because these coatings must be rapid-drying, predominantly high-volatile and highly flammable solvents are used whose flash-point lies at least well below 21° C. and which can additionally be toxic (such as for example toluene). For ecological reasons, these solvents are no longer allowed to be disposed of in the surroundings requiring extensive regeneration or waste disposal of these solvents.
A further serious disadvantage of the strip coatings known to now is the fact that at least three spray operations with drying times of a total of more than 4 hours are required for the sufficient coating of the metal surfaces, whereby the method practised up to now is very time and cost intensive.
According to U.S. Pat. Nos. 3,661,840 or 3,544,400 two-component strip coatings of this type consist of, for example, HO- or HS-modified polybutadienes, polyisocyanates or diamines, large amounts of fillers and high-volatile solvents as well as phenol or epoxide resins for improving adhesion.
Therefore, the object of the present invention is to develop a strip coating for metal surfaces which does not have the mentioned disadvantages of the art, but instead, is solvent-free on the one hand and has good technical use properties with respect to chemical resistance as well as hardening and adhesion properties on the other hand.
The problem was solved according to the invention by a two-component strip coating with the components A) and B), whereby A) essentially contains the following constituents:
5 to 20% by weight of an optionally substituted aromatic diamine,
0.1 to 5 % by weight of a dehydrator,
0.01 to 1 % by weight of an accelerator based on tertiary amines,
0.01 to 1% by weight of an accelerator based on organic tin compounds, and a polyether polyol based on poly(oxyalkylene) glycols as the remainder;
and whereby B) essentially contains the following constituents:
40 to 60% by weight of an aromatic diisocyanate based on a phenylene or toluene dusocyanate,
40 to 60% by weight of a polyether polyol based poly(oxyalkylene) glycols.
That is to say it has been surprisingly shown that the strip coating according to the invention is, despite its freedom of solvents, rapid-hardening as well as rapid-drying, can be applied in a single spray operation to the metal surface to be handled and finally can be removed again without a problem and residue-free.
The strip coating corresponding to the present invention is comprised—as already mentioned—of the two components A) and B). The component A) contains at least five constituents, whereby the main constituent represents a polyether polyol based poly(oxyalkylene) glycols with up 94% by weight with respect to component A). In this connection, the customary poly(oxyalkylene) glycols based on ethylene and/or propylene oxide can be used as polyether polyols. According to a preferred embodiment, a tri-functional polyether polyol based on propylene oxide, ethylene oxide and trimethylolpropane is used whose molar weight lies in the range of 500 to 5000 g/mol and has ca. 80% primary and ca. 20% secondary hydroxyl groups.
As a second essential constituent, the component A) contains 5 to 20% by weight of an optionally substituted aromatic diamine which contributes to the chain elongation of the urethane prepolymer. In this connection, one can rely on the customary isomeric phenylenediamines. Preferably, methyl- or ethyl-substituted aromatic diamines also find use. In this connection, diethyltoluylenediamine is particularly preferably used which for example has an NH equivalent of ca. 89 to 130.
As a third constituent, the strip coating according to the invention contains 0.1 to 5% by weight of a dehydrator, whereby sodium or potassium aluminosilicates having a pore size of ca. 0.3 to 0.4 nm have particularly proven themselves. The hydrator ensures that the strip coating does not foam and is porous.
Additionally, it is to be seen as essential to the invention that the component A) further contains accelerators based on tertiary amines as well as on organic tin compounds each in an amount of 0.01 to 1% by weight because a very quick and sufficient hardening of the coating is only ensured in this manner. In this connection, customary accelerators can be used. Preferably, sterically hindered bicyclic compounds such as for example 1, 4-diazabicyclo[2.2.2]octane or 2-methyl-2-azanorbornane are used as tertiary amines. In the accelerators based on organic tin compounds, the use of alkyl tin mercaptides, such as for example dimethyltin mercaptide, is to be seen as preferred. According to a preferred embodiment, the strip coating according to the invention can further contain 0.01 to 1% by weight of an alkoxysilane with which the adhesion properties of the coating can be purposively controlled. In this connection, the alkoxysilane can have alkoxy groups with 1 to 4 C-atoms as well as, optionally, glycidoxyalkyl residues with 1 to 4 C-atoms.
In addition, the possibility exists within the frame of the present invention to further add 0.1 to 1.0% by weight of a alkali- and acid-stable pigment or dye so that the strip coating is easier to optically recognize during processing. In this connection, monoazo dyes or pigments have proven themselves as particularly advantageous.
According to a preferred embodiment, component A) further contains 1 to 20% by weight of micronized magnesium silicate (talcum) with a particle size of ca. 5 &mgr;m, whereby an improvement of the mechanical (tear-propagation) and chemical properties (acid-stability) of the strip coating can be obtained.
The component B), which can be seen as a PU hardening component, comprises two constituents namely 40 to 60% by weight (with respect to component B) of an aromatic diisocyanate based on a phenylene or toluylene diisocyanate as well as 40 to 60% by weight of a poyletherpolyol based on poly(oxyalkylene) glycols which can preferably be identical to the polyether polyols described in component A). Examples for suitable aromatic polyisocyanates are 1,3- or 1,4-phenylene diisocyanates and/or 2,4- or 2,6-toluylene diisocyanate (TDI). However, 4,4′-diphenylmethane diisocyanate (MDI) is to be seen as particularly preferred because this isocyanate permits a sufficiently rapid-hardening reaction. Both constituents of component B), namely aromatic diisocyanate and polyether polyol, are reacted to a prepolymer with an NCO content of ca. 10 to 20%.
The weight ratio of component A) to component B) can be varied in broad limits, however, a weight ratio of component A) to component B) of 3:1 to 1:1 has proven itself as particularly advantageous.
The strip coating according to the invention can be used with great success in the surface coating of metal parts, especially of aluminium parts, whereby use can be made of the customary atomization, spray and pore
Hohnholz Dieter
Lemke Horst
Beck Shrive
Calcagni Jennifer
Fulbright & Jaworski LLP
Relius Coatings GmbH & Co.
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