Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Patent
1997-02-10
1999-11-09
Teskin, Fred
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
528483, 528490, 528498, 528502E, 523331, 523342, 264 5, 264 12, 4273722, 4273855, 427421, C09D 503, C08F 606
Patent
active
059816961
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for producing coating media in powder form, which can also be carried out directly during the production of coatings from such coating media within the scope of a spray coating process. According to one embodiment, the invention also relates to a process for the recycling of powder coating materials.
There is a multiplicity of methods of producing duroplastic powder coatings which have hitherto become known. The process which is currently by far the most widely employed in practice is the production of powder coatings by the extrusion, in the form of a paste-like melt, of the powder coating which is ready-formulated by mixing all the requisite components, followed by cooling the melt, coarse comminution, fine grinding and a downstream sieving stage to produce the desired fineness of grain (see Ullmans Enzyklopadie der technischen Chemie, Volume 15, page 680, 4th Edition, 1978, Verlag Chemie Weinheim; and H. Kittel, Lehrbuch der Lacke und Beschichtungen, Volume 4, page 355, Verlag W. A. Colomb, Berlin). Coating powders produced in this manner possess a series of disadvantages.
If highly reactive binder vehicle mixtures are processed, the extrusion operation often proceeds too slowly, so that partial gel formation is observed. These powders can then no longer be used for coatings.
A very broad particle size spectrum is normally formed on comminution, namely in the range from less than 0.1 .mu.m to what is usually up to 500 .mu.m. A subsequent sieving stage to produce particle size fractions suitable for the specific purpose of application may become necessary.
The oversize grain then has to be comminuted again in a downstream comminution stage. The fine grain fraction, which comprises particle sizes smaller than 5 .mu.m, is particularly problematical as regards workplace hygiene, due to its capacity of entering the lungs and due also to the increased risk of dust explosions. Furthermore, fine-grained powders have process technology disadvantages.
In addition, processes have become known which are suitable for producing powder coatings and for coating with powder coatings such as these, which employ the spraying of powder coating melts.
Thus DE-A-22 33 138 describes the production of duroplastic and thermoplastic powder coatings by the spraying of molten binder vehicles or binder vehicle mixtures. The base resin and hardener are melted and mixed together in a heated mixing vessel. When duroplastic powder coatings are produced, only those hardeners are employed which do not yet exhibit their crosslinking effect in the temperature range which is typical for the process. This process is particularly unsuitable for highly reactive powder coatings.
A process for producing powder coatings is known from WO-92 00 342 in which a molten mixture of a film-forming binder vehicle and a hardener for this binder vehicle is atomised, i.e. sprayed. The droplets formed in the course of this procedure are cooled to below their softening point in a short time, so that no significant crosslinking occurs within the particles. Solid lacquer particles of spherical form and narrow particle size distribution are formed. After cooling, the particles are separated from the carrier gas stream by suitable measures. Their chemical composition does not differ from that of customary powder coatings produced by the extrusion and comminution of a resin/hardener melt, and they can be applied using typical methods of application for powder coatings.
The processes described in DE-A-22 33 138 and WO-92 00 342 have the common disadvantage that the base resin and hardener components of the duroplastic powder coating are melted before spraying and have to be mixed as melts. Relatively high temperatures have to be used for melting, which results in thermal loading of the powder coating composition. This is problematical, particularly in the case of highly reactive powder coatings, since in order to prevent gelling or partially gelling a considerable effort has to be made to minimise the time of contact of the base r
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Ullmans Enzyklopedia der Technischen Chemie, v. 15, p. 680, 4th ed. 1978, Verlag chemie, Weinheim (English translation attached).
H. Kittel, Lehrbuch der Lacke und Beschichtungen, v. 14, Verlag W.A. Colomb in der H. Heenemann GmbH, Berlin (English translation attached), 1976.
Derwent abstracts, JP 53005239, Jan. 1994.
Matson et al, "Rapid Expansion of Supercritical Fluid Solutions: Formation of Powders, Thin Films, and Fibers", Ind. Eng. Chem. Res., 1987, 26(11), pp. 2298-2306.
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Berger Thomas
Diener Wolfgang
Klostermann Peter
Maag Karin
Saatweber Dietrich
Herberts GmbH
Teskin Fred
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