Coating processes – With post-treatment of coating or coating material – Heating or drying
Patent
1994-11-03
1996-08-20
Marquis, Melvyn I.
Coating processes
With post-treatment of coating or coating material
Heating or drying
427393, 4273936, 428500, 523410, 523411, 523412, 525123, 525175, 525176, B05D 302
Patent
active
055477100
DESCRIPTION:
BRIEF SUMMARY
This invention relates to crosslinkable aqueous coating dispersions in which the basis of the crosslinkability is provided by the reaction in an applied coating between epoxy groups and certain polymer-bound epoxy-reactive groups.
Polymeric film coatings are well established in the technology of protection of surfaces, and are frequently required to become cross-linked for certain applications. Thermal curing (often referred to as storing) is widely used, especially in the case of melamine-formaldehyde cured coating systems. However, such systems suffer the disadvantage of release of formaldehyde on curing. They also require a high storing temperature (typically about 150.degree. C.) which can be somewhat reduced by use of strong acid catalysts but at the expense of impairment of the coating performance. Such disadvantages are overcome by the present invention.
Epoxy-cured aqueous coating systems in which epoxy groups and epoxy-reactive groups are present in separate particles of an aqueous dispersion are known from PCT Application No. W091/14715 and U.S. Pat. No. 4,367,298. However, these are expensive to produce and present difficulties in the maintenance of storage stability.
When high temperature applications are envisaged for coating dispersions, as for example in the coating of heated substrates, a relatively high minimum film forming temperature (MFT) for the dispersion can be tolerated, provided that such MFT is the same as, or less than, the temperature of the substrate. This is quite a common situation for storing applications on metallic substrates. However, in low temperature applications, for example room temperature, it is necessary to depress the MFT of the coating composition, for example by the addition of a co-solvent (acting as a coalescing solvent). However, care is required in the choice of such MFT depressants. A cosolvent which partitions strongly in the polymer phase of the aqueous polymer dispersion can itself depress the glass transition temperature (Tg) of the polymer in the dispersed particles and which therefore, in certain instances (notably when the depressed polymer glass transition temperature is lower than the storage temperature to which the dispersion is exposed), may lead to premature cross-linking in the polymer system.
We have discovered that certain water-soluble or water-dispersible polymers are particularly effective in suppressing the MFT of crosslinkable compositions without significantly affecting the storage stability thereof.
According to the present invention there is provided an aqueous curable polymer dispersion which comprises a polyester and/or polyurethane and a polymeric product having a Tg within the range 25.degree. C. to 100.degree. C. obtainable or obtained by subjecting to a free-radical-initiated polymerisation process a mixture comprising the components: not have an epoxy-functional group or an epoxy-reactive group or a group which is subsequently converted to an epoxy-reactive group; least one epoxy-functional group and/or at least one multi-functional epoxy compound having an average of 2 or more epoxy groups per molecule and which is not derived from the polymerisation of an olefinically unsaturated monomer system: and following: are provided directly in the resulting polymeric product and/or such that epoxy-reactive amine groups are provided in the resulting polymeric product by conversion subsequent to polymerisation of at least some of the carboxyl groups to epoxy-reactive amine groups; are provided directly in the resulting polymeric product; and polymersiation to provide epoxy-reactive amine groups in the resulting polymeric product: and wherein (c), is from 2 to 48 weight %, preferably from 5 to 30 weight % and more preferably from 10 to 25 weight %; (c) is from 50 to 96 weight %, preferably from 60 to 80 weight %; and (c) is from 2 to 40 weight % preferably from 5 to 25 weight %.
The aqueous curable polymer dispersion preferably comprises from 5% to 70% by weight (relative to the weight of polymeric product) of the polyurethane and/or p
REFERENCES:
patent: 4933213 (1990-06-01), Fourquier et al.
patent: 5281655 (1994-01-01), Mitsuji et al.
Abstracts of Japanese Patent Nos. 70-028999, Jun. 1967, 52-146453, Dec. 1977, 60-156769, Aug. 1985.
Chemical Abstracts, vol. 100, No. 8, abstract No. 53325, "Primer coatings for polyester magnetic tapes", p. 96 see abstract JP,A, 58 124 561, Jul. 25, 1983.
Chemical Abstracts, vol. 104, No. 22, abstract No. 187718, "Electrophoretic coatings in manufacture of insulated wires", p. 60, see abstract & JP,A,60 189 119, Sep. 26, 1985.
Carey John G.
Hinde David C.
Padget John C.
Satgurunathan Rajasingham
Aylward D.
Marquis Melvyn I.
Zeneca Limited
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