Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
2000-03-28
2002-12-10
Short, Patricia A. (Department: 1712)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S386000, C525S109000, C525S110000, C525S118000, C525S119000, C525S438000, C525S934000
Reexamination Certificate
active
06491973
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to powder coating compositions produced using a particular process for coating metal components. The metal components may be not only individual. components and sheets, but also finished manufactured products.
BACKGROUND OF THE INVENTION
Metal surfaces, for example of aluminium and other non-ferrous metals and of steel, are provided with coatings, increasingly with powder coating compositions, for corrosion protection purposes. The metal surface must be finished for coating in such a manner that it ensures adequate adhesion of the powder coating. This is achieved, for example, by pretreating the metal surface with agents which cause the deposition of a microscopic layer of crystals, so passivating the surface with regard to corrosion and providing keying points for the powder coating material. Such pretreatment methods comprise, for example, chromating and phosphating, in particular zinc or iron phosphating.
In addition to aluminium and the alloys thereof as well as bright steel, galvanised steel is increasingly being used with a coating of corrosion- and weathering-resistant powder coatings. The galvanised steel may be produced by both the hot-dip and the electrogalvanisation processes. The particular advantage of using galvanised steel is that the zinc coating alone continues to provide protection from corrosion if the powder coating film is damaged. A higher level of corrosion protection may be achieved by such zinc coatings than is achieved by the above-stated passivation pretreatment of ungalvanised metal surfaces.
However, powder coating films do not adhere adequately to galvanised metal surfaces. In order to provide adhesion, galvanised metal surfaces must, for example, be pretreated with phosphating solutions, for example according to DE-A-21 53 809, FR-A-26 81 33, WO 92/15727, and are then coatable using a powder coating, for example based on polyester, epoxy, acrylate and alkyd resins.
It is furthermore possible initially to coat the galvanised steel surface with a primer, before subsequent coating operations are performed, c.f. for example the use of a powder primer based on an epoxy resin and an aromatic diamine according to JP-A 61 250 067.
The above-stated pretreatment methods require the additional processing steps of priming and phosphating and/or chromating and thus associated additional steps such as rinsing, drying, coating, stoving. Such processes are accordingly time-consuming and cost-intensive and, in the case of chromating, environmentally hazardous.
Steel or zinc surfaces may furthermore, for example, be coated with an epoxy powder coating substantially consisting of a bisphenol A epoxy resin and a phenolic crosslinking agent for subsequent coating with a lacquer based on a fluorocarbon resin according to JP-A-01 040 329. Potential applications include, for example, coloured roofing.
Powder coating compositions are known, the use of which does not require pretreatment, for example phosphating pretreatment. According to U.S. Pat. No. 3,882,064, rapid-curing powder coating compositions containing as binder an epoxy resin mixture together with polyethylene wax and dicyandiamide may be used on galvanised steel. Such coating compositions are suitable as protective coatings for containers for foodstuffs and beverages and are not intended for weathering-resistant coatings.
EP-A-0 057 334 describes coating galvanised metal with a powder coating, in which the metal is cleaned with acid and then rinsed before coating. During cleaning, for example using low concentration solutions of phosphorous acid, under certain temperature conditions the zinc surface undergoes only very slight attack accompanied by the formation of very small quantities of zinc phosphate crystals. Powder coatings based on epoxy, polyester and acrylic resins and mixtures thereof may be used as the coatings. Hybrids which are mentioned are, for example, mixtures of polyester and epoxy resins. The objective is to ensure the slightest possible attack on the zinc coating which provides corrosion protection while still ensuring good adhesion of the powder coating by rapid coating after drying. However, when used in exterior applications, these compositions are susceptible to weathering, especially on long-term exposure to the action of the weather.
Known hybrid powders, for example mixtures of polyester and epoxy resins in a 50:50 ratio, are produced using conventional powder coating technology by mixing all the components of a powder formulation, such as binder, curing agent, additives and pigments and fillers, in a dry mixer for a mixing time optimised for the type of mixer. The resultant mixed material (premix) is homogenised, for example extruded, by means of an extruder under temperature and rotational speed conditions optimised for the particular formulation, cooled using appropriate cooling means and pre-comminuted using a crusher. This pre-comminuted intermediate product is then finely ground in appropriate classifying mills to the grain size distribution adapted to the intended application. If such hybrid powders are applied onto metal substrates which have not been chemically pretreated, for example galvanised steel, corrosion protection problems may arise due to detachment of the powder film. Known hybrid powders of the above-stated composition moreover cannot be used as weather resistant coating systems.
SUMMARY OF THE INVENTION
The object of the present invention is accordingly to provide a specific coating material for metals and a specific process for the production of this coating material, such that good corrosion protection combined with good adhesion of the coating material on the metal surface may be achieved by application of a single layer and moreover such that, depending upon requirements, very good weather resistance is achievable, so combining several important lacquer properties in only one layer to be applied.
It has been found that this object may be achieved by a powder coating composition which is obtained by a selected processing method.
The present invention accordingly provides a powder coating composition obtainable by homogeneous mixing of a separately produced particulate component
A prepared from one or more epoxy resins having an epoxy equivalent weight of 250 to 2500, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers and
a separately produced particulate component
B prepared from one or more resins differing from A having an acid value of 20 to 70 and/or an OH value of 10 to 100, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers,
in a mixing ratio of component A to component B of 1:99 to 50:50, relative to weight.
The present invention also provides a process, in which a powder coating composition which contains
A one or more epoxy resins having an epoxy equivalent weight of 250 to 2500, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers and
B one or more resins differing from A having an acid value of 20 to 70 and/or an OH value of 10 to 100, one or more curing components together with conventional lacquer auxiliary substances and additives and optionally pigments and/or fillers
in a mixing ratio of component A to component B of 1:99 to 50:50, is produced in such a manner that component A and component B are initially produced separately using conventional powder coating production processes and the two components A and B in the stated mixing ratio are then subjected to a further operation, for example an extrusion operation, to ensure homogeneous mixing of the two components.
The content of epoxy resin in component A may, for example, be in a range between 10 wt. % and 95 wt. %.
The curing component is present in conventional quantities, generally in a quantity suitable for reaction with functional groups of the resin component in order to achieve th
Davydov Evgueni
Zimmermann Frank Dieter
Dupont Performance Coatings GmbH & Co. KG
Pillsbury & Winthrop LLP
Short Patricia A.
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