Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-04-19
2002-12-31
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
C419S050000
Reexamination Certificate
active
06500385
ABSTRACT:
This invention relates to a process for the continuous working up and reuse of ground powder coating waste, which substantially comprises by-products from powder coating production, for the production of powder coatings without loss of quality.
Since no solvent emissions occur on application, crosslinkable powder coatings are an environmentally friendly alternative to liquid lacquer systems containing solvent. As a result, the use of powder coatings has risen sharply in recent years.
Crosslinkable powder coatings conventionally consist of one or more polymeric binders, curing agents, pigments and extenders together with additives. The powder coating production process may be divided into the following steps:
1. the constituents of the powder coating are vigorously premixed in the required quantity ratios as dry solids.
2. the mixture is melted in an extruder at the lowest possible temperature in order to avoid premature crosslinking and vigorously mixed. This mixing plasticises the binder and curing agent and wets the pigments and extenders.
3. the resultant, optionally coloured extrudate is rolled out into a thin layer, cooled and broken into coarse pellets.
4. the pellets are ground in a mill to form the finished powder coating.
In general, the fine fraction of a particle size of <10 &mgr;m, which are unavoidable in the grinding process, are removed in a subsequent screening process. The resultant powder typically has an average particle size of 40 to 70 &mgr;m.
Powder coatings are generally produced in a batch process. Conventional compositions and the classic production process for powder coatings are described, for example, in the monograph
The Science of Powder Coatings
vol. 1 and 2 (ed. D. A. Bate, London 1990).
The aim of the grinding process is to produce a powder having a narrow particle size distribution and the smallest possible content of fine fraction, as a large content of fine fraction has a negative impact on the processability of the powder coating by electrostatic spraying. In general, the fine fraction of a particle size of below 10 &mgr;m are separated by suitable processes, for example using a cyclone. These fine fractions conventionally amount to approx. 2 to 5 wt.%. If the average particle size of the powder is reduced, which is frequently preferred from a technical standpoint, there is a sharp increase in the content of fine fraction with a particle size of <10 &mgr;m, as grinding always gives rise to a relatively wide particle size distribution. Powder coatings having a low average particle size and a narrow distribution of the grain size range are desirable, for example, for producing thin lacquer layers (B. Fawer,
Powder Coating
, October 1996, p. 56).
The fine fraction from the grinding process is accordingly a by-product in the production of powder coatings and in many cases must be disposed of as industrial waste.
DE 4028567 A1 describes the recirculation of the overspray arising during use of powder coatings and the mixing thereof with the starting materials for a new batch of powder coating. This mixture is then further processed using conventional extruders. This processing involves fusing the constituents, in particular also the overspray, and exposing them to elevated temperatures. This process has the disadvantage that homogeneous mixing is difficult due to the small particle size of the overspray, problems occur with feeding the mixtures into the extruder and some fractions of the resultant powder coating are repeatedly exposed to elevated temperatures.
WO 96/15891 describes a process which avoids the problem of metering and feeding fines by compacting the fine powder in a tabletting press. The resultant tablets are then mixed with the remaining starting materials for a new batch of powder coating and introduced into the extruder. One essential condition which is stated for compacting is that the original particles must be “substantially still discernible” in the tablets.
EP 0 683 199 A2 describes a process for the recirculation of fine fraction into the starting mixture in powder coating production before the extrusion step by thermal sintering of the powder by distributing the fines on a surface, agglomerating them thereon, whereupon they may be apportioned to the starting mixture in a suitable quantity as relatively large particles.
A feature common to all these processes is that the fine fraction or powder residues are recirculated to the first stage of powder coating production. The material has to pass again through the entire powder coating production process. This approach has the disadvantage in principle that the material is again melted and exposed to elevated temperatures in the extruder. Since, in many cases, powder coatings are thermally crosslinking systems, repeated extrusion degrades the quality of the powder coating, especially if a large proportion of fines is used.
DE-C 19703376 describes a process for reusing finely divided powder coating residues, in which the powder coating residues are agglomerated and returned to the powder coating production process. The extrusion step may optionally be omitted in this process and grinding may be performed directly to yield reusable powder coatings.
The batchwise working up method described in DE-C 19703376 may result in known logistical problems relating to the necessary storage and transport conditions. The agglomerated powder coating, for example produced from overspray residues, is generally supplied to the powder coating manufacturer by the powder coating user in order to be reused.
The object of the present invention is accordingly to provide a simplified process for working up ground powder coating waste which facilitates the reutilisation thereof and avoids the stated logistical problems. The process is intended to give rise to a homogeneous powder coating of undiminished quality.
It has been found that this object may be achieved by the process provided by the present invention for working up powder coating waste by compacting the powder coating waste without complete melting to yield a sintered product, wherein the powder coating waste is continuously separated in the powder production process after the grinding step, continuously compacted, optionally broken up and continuously ground together with fresh material to be ground of the same batch of powder coating without extrusion to yield a powder coating.
The present invention also provides the powder coatings obtainable from the process according to the invention.
In the process according to the invention, powder coating waste, in particular the fine fraction arising in the grinding process, are continuously separated, for example using a cyclone or filter, directly compacted and the compacted product reintroduced into the mill, preferably together with the original coarse pellets from the same batch.
This method has the advantage that the fine fraction is directly reincorporated into the same batch. There are consequently no problems with colour deviations and the known logistical problems of a discontinuous method are avoided.
The continuous mode of operation moreover makes it possible to keep the powder coating waste in a sealed system. Especially when using the fine fraction, this is advantageous for reasons of occupational hygiene and quality assurance. The problems described in DE-C 19703376 relating to conveying the fine fraction, for example in pipework, are also avoided.
The process according to the invention is thus particularly advantageous for the continuous production of powder coatings having a small average particle size and narrow distribution, as large quantities of fine fraction are produced in such processes which may be directly reused.
For example, it is possible to produce a powder having an average particle size (d50) of 30 &mgr;m, which after the classifying operation has a content of fine fraction of <10 &mgr;m of less than 15%, in particular of less than 10%, without significant quantities (<2%) of fine powder being produced.
If such a particle size distribution is to be achiev
Blatter Karsten
Bolm Helene
Montgomery David
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Jenkins Daniel J.
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