Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2001-07-20
2003-03-11
Barr, Michael (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S508000, C427S180000, C427S201000
Reexamination Certificate
active
06531189
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the curing of powder lacquers on metallic and non-metallic substrates by irradiation with radiation from the near infrared range (NIR radiation).
In the coating of metals the decorative or functional coating of surfaces with powder lacquers has acquired a wide field of application because of the high efficiency of the process and the favourable assessment from the point of view of environmental protection. Numerous powder lacquer formulations have been developed for the various fields of use. The powder lacquer curing processes available hitherto require the powder deposited on the substrate initially to be melted by heating to temperatures above the glass transition temperature and/or melting point of the powder lacquer formulation. Convection ovens, infrared radiators or combinations of both are used as heat sources, for example. For heat-curing systems the powder coating is typically cured by heating the entire object to be coated to temperatures between 140 and 200° C. for a period of approx. 10 to 30 minutes.
For UV-curing powder lacquer formulations, the powder coat melted in conventional manner is cured within a few seconds with the aid of ultraviolet radiation. The powder lacquers are usually cured via the polymerization of double bonds or cyclic ethers according to radical or cationic reaction mechanism.
Both processes have considerable disadvantages. High temperatures are required over a period of approx. 20 to 30 minutes for heat-curing powder lacquers; on the one hand these high temperatures do not permit coating of temperature-sensitive surfaces such as wood or plastic and on the other hand they require a considerable amount of energy for metal components. The use of UV-curing powder lacquers requires two process steps, since first of all the powder must be melted by heating and then be cured with UV radiation in a second step. Furthermore it is difficult to cure pigmented powder lacquers in larger coating thicknesses as the UV radiation is absorbed by the components which impart colour and this renders complete curing of the coating more difficult.
The recently developed technology for generating high-intensity radiation in the near infrared range (NIR), the wavelength range from 750 to 1200 nm, offers a further possibility for curing lacquer compositions. The NIR radiation intensity employed in the field of analytical chemistry or process control is not, however, sufficient to heat the substrate or start chemical reactions, such as the crosslinking of a lacquer.
The article “Sekundenschnelle Aushärtung von Pulverlack” (“Curing powder lacquer in seconds”) (Kai Bär, JOT 2/98) describes that powder lacquers may be cured with the aid of high-intensity NIR radiation without the substrate being substantially heated. NIR technology enables powder lacquers to be melted and cured in a single process step without the drawbacks of conventional thermal curing and/or UV curing described above. A uniform heating of the entire lacquer coat and a reflection of the radiation on metallic surfaces are achieved. Powder lacquers which are particularly suitable for this curing process and in particular the way in which tailor-made powder lacquers for curing with the aid of NIR radiation may be developed are not described.
German patent application 198 06 445.4 describes powder lacquer binders which are particularly suitable for curing powder lacquers by means of NIR radiation. The absorption behaviour of powder lacquers which are produced with these binders, and the influence of the absorption behaviour on the curing characteristics and the coating properties are not mentioned therein.
In particular, pigmented powder lacquers with the same binder system but different composition of the remaining constituents may exhibit considerable differences in absorption behaviour in NIR curing. Different rates of heating of the lacquer film and different curing speeds are observed in identical conditions as regards radiation intensity and irradiation time. For example, a conventional commercial white powder lacquer (Durotherm AL93-9010, Herberts Pulverlack GmbH) corresponding to the prior art is completely cured in 30 seconds when irradiated with a conventional commercial NIR radiation source (obtainable from Messrs Industrie SerVis) whereas a conventional commercial black powder lacquer with the same binder basis (Durotherm AL93-9017, Herberts Pulverlack GmbH) corresponding to the prior art achieves a curing time of 5 seconds in the same irradiation conditions.
SUMMARY OF THE INVENTION
The object of the invention is therefore to provide a process for curing powder lacquers which makes it possible to cure powder lacquers with the aid of NIR radiation irrespective of the colour or of the binder system used and further lacquer components with virtually identical curing times and surface temperatures of the substrates. Furthermore it should be possible to adjust the absorption behaviour of powder lacquers in the NIR range so as to be able to adapt the absorption characteristics to the desired application conditions such as the film thickness or the substrate to be coated for example.
In the context of the invention it has been shown that the absorption behaviour of a powder lacquer composition may be changed and/or adapted in order to achieve desired conditions by varying its barium sulfate and/or aluminium oxide and/or carbon black content.
The invention therefore provides the use of barium sulfate, aluminium oxide and/or carbon black to control the absorption behaviour of powder lacquers on curing with radiation in the near infrared range (NIR radiation), particularly the control of the curing times and/or the surface temperatures of substrates coated with the powder lacquers.
The invention further provides a process for curing powder lacquers by coating substrates with the powder lacquers and irradiating by radiation in the near infrared range (NIR radiation), which is characterized in that the curing times and/or the surface temperatures, obtainable by means of the radiation during certain curing times, of the substrates coated with the powder lacquers are controlled by setting the barium sulfate and/or aluminium oxide content of the powder lacquers to 1 to 50 wt. % and/or setting the carbon black content of the powder lacquers to 0.1 to 5 wt. %, the percentages by weight relating to the entire powder lacquer composition in each case.
The procedure according to the invention is that the total barium sulfate and aluminium oxide content in the powder lacquers does not exceed 50 wt. %, if carbon black is additionally present the total barium sulfate and/or aluminium oxide content together with carbon black not exceeding 55 wt. %.
DETAILED DESCRIPTION OF THE INVENTION
The barium sulfate and/or aluminium oxide and/or carbon black content crucially influences the absorption behaviour of the entire powder lacquer composition and enables the NIR curing characteristics to be set for a specific purpose. By means of an increased barium sulfate and/or aluminium oxide and/or carbon black content, for example, the absorption capacity of the powder lacquer composition for radiation in the NIR wavelength range may rise greatly, and in constant radiation conditions higher surface temperatures and shorter curing times may be achieved.
Furthermore the absorption behaviour of the powder lacquer compositions may be adapted to the particular application of the powder lacquer. Depending on the substrate it may, for example, be desirable to increase the absorption of the radiation in the lacquer film in order to minimise the heating of the substrate. In these cases formulations, the barium sulfate and/or aluminium oxide and/or carbon black content of which is increased, in the upper range of the quantities quoted above (wt. %) for example, are preferred.
For very thick powder lacquer coatings it may be necessary to reduce the NIR absorption so as to ensure that the deeper-lying powder coat melts and cures. If the NIR absorption is too high all the radiation will be
Blatter Karsten
Thiele Olaf
Barr Michael
Blanton Rebecca A.
E. I. du Pont de Nemours and Company
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