Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2000-08-16
2002-06-18
Cameron, Erma (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S518000, C427S521000, C427S195000
Reexamination Certificate
active
06406757
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of powder finishes for metallic and non-metallic substrates such as wood and plastic substrates, glass and ceramics.
BACKGROUND OF THE INVENTION
Using powder coatings to provide decorative or functional finishes has found widespread acceptance in metals coating due to the elevated economic viability of the process and its favourable environmental characteristics. Numerous powder coating formulations have been developed for different applications. The processes hitherto available for curing powder coatings require that the powder deposited on the substrate first be melted by heating to temperatures above the glass transition temperature or melting point of the powder coating formulation. Heat sources which are used are, for example, convection ovens, infra-red light sources or combinations of the two. In the case of thermally crosslinking systems, the powder coating is typically cured by heating to temperatures of between 140 and 200° C. for a period of approx. 10 to 30 minutes.
In the case of UV-curing powder coating formulations, the molten powder coating is cured within a few seconds by means of ultraviolet radiation. The powder coatings are generally crosslinked by polymerising double bonds or cyclic ethers using a free-radical or cationic reaction mechanism.
Both processes have disadvantages. Elevated temperatures are necessary for thermally curing powder coatings which, on the one hand, do not allow temperature-sensitive surfaces such as wood or plastic to be coated and, on the other hand, require an elevated energy input for metal components. Using UV-curing powder coatings entails two process steps as the powder must first be melted by heating and curing with UV radiation then proceeds in a second step. Moreover, curing thick films of pigmented powder coatings is problematic as the UV radiation is absorbed by the colouring components, such that achieving a complete cure of the coating is more difficult.
Apart from the above-stated conventional curing processes, another method is known by means of which powder coatings may be cured with near infra-red (NIR) radiation by using high intensity radiation (after Bar, Sedelmey,
Fokussierte NIR
-
Technologie,
annual conference,
Die EPS
-
Praxis
1997, Bad Neuheim 27-28.11.97). Using this method, it is possible to perform both melting and curing of powder coatings in a single process step, wherein elevated curing temperatures may be achieved without the coated substrate being substantially heated. However, the coatings obtained are not always satisfactory and reproducible with regard to properties such as coating quality and hardness.
GB-A 2 056 885 describes a process for coating cellulosic material such as wood and paper, in which a powder coating is applied and radiation cured. IR irradiation proceeds over a range from 1 to 5 &mgr;m in a period of 20 to 120 seconds and thus results with a relatively high energy input in obtaining the desired properties.
SUMMARY OF THE INVENTION
The object of the invention is accordingly to provide a process in which powder finishes are obtained by melting and curing powder coating compositions in a single process step with short curing times, wherein the finishes are distinguished by a uniform coating and improved mechanical properties as well as by improved solvent resistance.
This object is achieved by a process in which powder coating compositions are used which contain resins which may be crosslinked by means of functional groups capable of forming hydrogen bonds, wherein these functional groups are present at a concentration of greater than 100 mmol/kg of powder coating composition, the powder coating compositions are applied to the substrate and are melted and cured by NIR irradiation.
DETAILED DESCRIPTION OF THE INVENTION
Powder coating compositions which may, for example, be used are those based on polyester resins, epoxy resins, (meth)acrylic resins and optionally crosslinking resins. The resins may, for example, contain OH, COOH, RNH, NH
2
and/or SH as the functional groups capable of forming hydrogen bonds. Suitable crosslinking resins are, for example, di- and/or polyfunctional carboxylic acids, dicyandiamide, phenolic resins and/or amino resins. The functional groups may here be attached to the binder to be crosslinked and/or to the crosslinking resin (curing agent).
The quantity of the binder and curing agent functionalised according to the invention is selected such that the functional groups are present in the powder coating composition at a concentration of greater than 100 mmol/kg. The compositions may, for example, contain 15 to 95 wt. % of the resins functionalised according to the invention, such as for example polyesters, epoxy resins and/or (meth)acrylate resins and 0.1 to 50 wt. % of the curing agents functionalised according to the invention. The binder present in such cases may comprise 20 to 95 wt. % of a polyester containing carboxyl groups and/or 15 to 95 wt. % of a hydroxy-functionalised polyester. Curing agents according to the invention, which are, for example, used for curing epoxy binders in quantities of 1 to 30 wt. %, may be, for example, di- and/or polyfunctional carboxylic acids, dicyandiamide, phenolic resins and/or amino resins. The above stated values in “mmol/kg” and “wt. %” in each case relate to the complete powder coating composition (which optionally contains pigments and/or fillers and further additives).
The crosslinking reaction may be additionally accelerated by the presence in the powder coating composition according to the invention of catalysts known from thermal crosslinking. Such catalysts are, for example, tin salts, phosphides, amines and amides. They may be used, for example, in quantities of 0.02 to 3 wt. %. Such crosslinking catalysts are preferably used.
In accordance with the process according to the invention, the powder coating compositions, which may contain constituents conventional in powder coating technology, such as pigments and/or fillers as well as further lacquer additives, as further components, are applied to the substrate to be coated using conventional methods and then melted and cured by NIR radiation. Melting and curing generally amounts to less than 7 minutes, for example 1 second to 300 seconds, depending upon the particular composition of the powder coating.
The polyesters usable according to the invention may be produced in a conventional manner by reacting polycarboxylic acids, the anhydrides and/or esters thereof with polyalcohols, as is, for example, described in D. A. Bates,
The Science of Powder Coatings,
volumes 1 & 2, Gardiner House, London, 1990. Hydroxy- and/or carboxy-functionalised polyester resins are preferably used. The hydroxy and carboxy functions may be introduced by appropriate selection of the starting materials and/or the proportions thereof.
Mixtures of carboxyl and hydroxyl group containing polyesters may also be used. The carboxy-functionalised polyesters according to the invention conventionally have an acid value of 10 to 200 mg of KOH/g of resin and the hydroxy-functionalised polyesters have an OH value of 10 to 200 mg of KOH/g of resin.
Curing agents which may be used for the polyester resins are conventional curing agents such as, for example, cycloaliphatic, aliphatic or aromatic polyisocyanates, crosslinking agents containing epoxy groups, for example triglycidyl isocyanurate (TGIC), polyglycidyl ethers based on diethylene glycol, glycidyl-functionalised (meth)acrylic copolymers as well as crosslinking agents containing amino, amido or hydroxyl groups. Such carboxy-functionalised polyesters may, for example, be crosslinked using polyfunctional epoxides or polyfunctional hydroxyalkylamides. Hydroxy-functionalised polyesters crosslink, for example, with polyfunctional isocyanates which may, for example, be reversibly blocked by forming urethdione groups.
(Meth)acrylate resins bearing the functional groups according to the invention may furthermore be used. These may, for example, be produced from alkyl (meth)acryl
Blatter Karsten
Niggemann Frank
Zimmermann Frank
Benjamin Steven C.
Cameron Erma
E. I. du Pont de Nemours and Company
LandOfFree
Process for coating a surface with a powder coating composition does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for coating a surface with a powder coating composition, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for coating a surface with a powder coating composition will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2967272