Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
2000-09-15
2002-08-20
Pianalto, Bernard (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S058000, C427S314000, C427S317000, C427S322000, C427S325000, C427S326000, C427S385500, C427S389900, C427S391000, C427S393000, C427S393500, C427S394000, C427S395000, C427S397000, C427S407100, C427S408000, C427S411000, C427S413000, C427S508000, C427S513000, C427S557000, C427S595000
Reexamination Certificate
active
06436485
ABSTRACT:
DESCRIPTION
The invention relates to a method of applying a powder coating to a substrate, in particular a temperature-sensitive substrate such as wood, wood-fibre material, plastic, rubber, cloth, paper or cardboard. The invention further relates to the use of a halogen bulb for the powder-coating process.
A crucial factor in the cross-linking and curing of a coating powder is that the powder be warmed to the curing temperature as homogeneously and rapidly as possible. It is only in this way that the molten powder can reach the viscosity minimum without being considerably hindered from spreading out by the early occurrence of cross-linking reactions. A non-optimal spreading of the powder would result in unevenness of the surface.
In a known method of cross-linking thermoreactive powder, the necessary curing temperature is reached by energy transfer that takes place in several steps. First, by infrared (IR) radiation or convection, the surface of the powder layer is warmed. Only thereafter is the interior of the powder layer heated, by thermal conduction processes, down to the substrate interface. There the energy, in particular in the case of metallic substrates, is dissipated into the substrate much more rapidly because of the higher thermal conductivity. Not until the substrate has been almost completely warmed throughout does the interface reach the necessary cross-linking temperature. In this known method the sole driving process for warming the interior of the powder layer is the temperature gradient between the surface of the layer and the substrate. To ensure a homogeneous cross-linking and perfect adhesion to the substrate, heating times of several minutes are required.
Frequently the cross-linking and curing temperatures of coating powders are between 120° C. and 300° C. Because these temperatures are so high it is impossible, or possible only with restrictions, to apply a powder coating to temperature-sensitive substrates by the known method.
There is another known method of cross-linking and curing a layer of thermoreactive powder on a substrate, in which prior to application of the thermoreactive powder a primer is applied to the surface of the substrate. The primer consists, for example, of water-based lacquer. In particular for substrates made of wood or wood-fibre materials, the primer serves to smooth out inhomogeneities in the surface structure, to form a barrier to moisture, and to enable adhesion of thermoreactive powder. The powder can subsequently be cross-linked and cured by exposure to electromagnetic radiation, in particular radiation at wavelengths in the middle of the infrared region. In this known method the primer also constitutes a barrier to thermal conduction, which hampers heat transfer to the substrate during the cross-linking reaction in the powder layer. In particular for temperature-sensitive substrates, this is the only means by which application of a powder coating becomes possible at all. However, the only thermoreactive powders with which this known method can be used are those having a cross-linking temperature only slightly higher than the temperature that would damage the substrate.
In the case of substrates that contain or absorb moisture, in particular wood or wood-fibre materials, another problem with the known methods is that although it is desirable for the substrate to have a certain minimal moisture content, the moisture hinders the application of a uniform powder coating. Moisture in the substrate on one hand enables an electrostatic charge to be established so that thermoreactive powder can be deposited on the charged surface. But on the other hand, during the subsequent cross-linking and curing reaction the moisture in the substrate vaporizes, because during the long reaction period at temperatures above the vaporization temperature, the substrate is heated to the vaporization temperature at least at its surface. Therefore at the surface, below the already cross-linked powder, bubbles form which cause irregularities in the coating layer. Even a primer layer is of no help here, because it is not effective as a thermal-conduction barrier in the long term, and the vaporization temperatures are usually considerably lower than the cross-linking and curing temperatures of the thermoreactive powder. Furthermore, for instance in the case of a water-based primer, it is necessary to wait until the primer has completely dried before a layer of coating powder can be applied to the primer.
The known methods also present the difficulty that because the heating of the powder coating penetrates only to slight depths, a relatively long heating period is needed before a fusion between the powder layer and the substrate surface or the primer can be completed.
The object of the invention is to disclose a method for the powder coating of a substrate, in particular a temperature-sensitive substrate such as wood, wood-fibre material, plastic, rubber, cloth, paper or cardboard, that enables the powder to be applied to the unprotected surface of the substrate without damaging the substrate, and that produces a uniform, completely cross-linked and tightly adhering coating.
An essential idea in the method of applying a powder coating in accordance with the invention is that the energy required for cross-linking is supplied to the powder in a targeted manner such that it penetrates the entire thickness of the powder layer, which is applied as a base layer to the substrate without any other underlying layer. The gelling or cross-linking energy is introduced into the base layer, at least, in the form of radiation energy and is absorbed there. The radiation used for this purpose comprises at least some components in the near and/or short-wave infrared region. Preferably the powder layer and the substrate surface are warmed homogeneously by near-infrared (NIR) radiation and in a matter of seconds are brought to the required gelling or cross-linking temperature. The term “near infrared” denotes the wavelength range between the visible region and 1.2 &mgr;m wavelength. The term “short-wave infrared” denotes the wavelength range between 1.2 &mgr;m and 2 &mgr;m.
In accordance with the invention the infrared radiation either heats the thermoreactive powder to the cross-linking temperature and cures it, or heats the powder to the gelling temperature, after which in a subsequent processing step the cross-linking is completed and the powder is cured. In the latter case the gelling step binds the powder together without causing complete cross-linking or curing to form a finished coating.
The targeted introduction of energy by means of infrared radiation, in particular NIR radiation, preferably such that the energy is homogeneously distributed over the thickness of the base layer, accelerates the process of binding or cross-linking of the powder particles so that it is considerably faster than in the known method, in which the introduction of energy into the depths of the base layer occurs substantially by thermal conduction. In addition, the present means of introducing energy provides excellent control of the binding or cross-linking process, in particular because precisely the desired rate of progress can be achieved by controlling the radiation flux density, the spectral distribution of the radiant energy and/or the duration of irradiation. It is advantageous for the above-mentioned process parameters to be adjusted to suit the absorption properties of the thermoreactive powder, the reflection properties of the substrate surface, and the thermal conductivity of the substrate.
Furthermore, the rapid heating throughout the base layer ensures good adhesion to the substrate surface.
Preferably after the base layer has been cured or undergone a preliminary gelling process, a second layer of a thermoreactive powder is applied and the entire, not yet completely cross-linked coating is cross-linked and cured by infrared radiation. In a further development of the method the base layer, after it has been cured or gelled, is cooled below the gelling or curing temperature
Advanced Photonics
Marshall Gerstein & Borun
Pianalto Bernard
LandOfFree
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