Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Process of treating scrap or waste product containing solid...
Reexamination Certificate
2000-04-19
2002-08-13
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Process of treating scrap or waste product containing solid...
C521S048000
Reexamination Certificate
active
06433030
ABSTRACT:
The invention provides a process for working up and re-using milled powder lacquer wastes, in which these wastes may be secondary products obtained during the preparation of powder lacquers or the overspray obtained when applying powder lacquers, to prepare powder lacquers without loss of quality.
Crosslinkable powder lacquers are an environmentally friendly alternative to solvent-containing liquid lacquer systems because no solvent emissions occur during application. For this reason, the use of powder lacquers has increased greatly in recent years.
Crosslinkable powder lacquers conventionally consist of one or more polymeric binders, hardeners, pigments and fillers as well as additives. The preparation of a powder lacquer is divided into the following process steps:
1. The constituents of the powder lacquer are intensively premixed dry, as solids, in the ratios by weight required.
2. The mixture is melted in an extruder at the lowest possible temperature, to avoid premature crosslinking, and intensively and thoroughly mixed. The binder and hardener are thereby plasticised and wet the pigments and fillers.
3. The optionally coloured extrudate obtained is rolled out into a thin layer, cooled and crushed to give coarse granules.
4. The granules are milled in a mill to give the final powder lacquer.
During the milling process, an unavoidable fines fraction with particle sizes <10 &mgr;m is generally removed in a subsequent screening process. The powder obtained typically has an average particle size of 40 to 70 &mgr;m.
Powder lacquers are generally prepared in a batch process. Conventional compositions and the classical method for preparing powder lacquers are described, for example, in the monographs “The Science of Powder lacquers,” volumes 1 and 2 (publisher, D. A. Bate, London 1990).
The objective of the milling process is to prepare a powder with a narrow particle size distribution and the smallest possible proportion of fine fraction of <10 &mgr;m since a high proportion of fine fraction acts negatively on the processability of the powder lacquer in an electrostatic spray process. Generally, therefore, a powder with an average particle size of 40 to 70 &mgr;m is prepared and the proportion of fine fraction of less than 10 &mgr;m is separated using a suitable process, e.g. with the aid of a cyclone. This fine fraction generally amounts to about 2 to 5 wt. %. If the average particle size of the powder is reduced, which is often preferable from a technical point of view, then the proportion of fine fraction with particle sizes <10 &mgr;m increases greatly because a relatively wide particle size distribution is always obtained in the milling process.
Powder lacquers with a low average particle size and a narrow distribution of particle sizes are desirable, for example, for the preparation of thin layers of lacquer (B. Fawer, Powder lacquer, October 1996, p. 56).
The fine fraction from the milling process thus represents a secondary product of powder lacquer preparation which in many cases has to be disposed of as industrial waste. In addition, there are further residues or wastes from powder lacquer production or processing which have to be destroyed or recycled. Examples of such residues are the proportion with too large a particle size, residues from spray application, faulty batches and also residues of unsold quantities.
DE 4028567 A1 describes the recycling of overspray obtained during the application of powder lacquers and blending this with the starting materials for a fresh powder lacquer batch. There, this mixture is then further processed via conventional extruders. The constituents, in particular also the overspray, are thereby thermally melted and stressed. This process has the disadvantage that homogenous blending is difficult due to the small particle size of the overspray, there are problems with feeding the mixtures into the extruder and sometimes proportions of the powder lacquer produced have been thermally stressed several times.
WO 96/15891 describes a process which gets round the problem of metering and feeding fines by compacting the fine powder in a tablet press. The tablets obtained are then blended with the other starting materials for a fresh powder lacquer batch and introduced into the extruder. As an essential condition for compacting, it is stated that the original particles must be “substantially still recognisable” in the tablets.
EP 0 683 199 A2 describes a process for recycling fine fraction to the starting mixture during powder lacquer preparation before the extrusion step by thermal sintering of the powder, that is by distributing the fines onto a surface, agglomerating them there and then being able to add them as coarser particles to the starting mixture in a suitable amount.
A common factor in all these known processes is that recycling of the fine fraction or powder residues takes place in the first step of powder lacquer preparation. The material must therefore again pass through the entire powder lacquer production process. This procedure has the main disadvantage that the material is again melted and thermally stressed in the extruder. Since powder lacquers in many cases are thermally crosslinking systems, repeated extrusion leads to losses of quality in the powder lacquer, in particular when a high proportion of fine fraction is used. In addition, admixture to the starting materials for a fresh batch is a logistical problem because the fine fracture can only be incorporated into materials with the same formulation. In many cases the same formulation is prepared only at irregular intervals so the fine fraction has to be stored separately. For those formulations which are prepared only very rarely, recycling using the processes described hitherto is therefore not economically viable.
According to DE-C 19703376, a process for the re-use of fine powder lacquer residues obtained during preparation by milling or as overspray is described in which the powder lacquer residues are agglomerated and returned to the preparation process for the same powder lacquer. The extrusion step may then optionally be omitted and direct milling of powder lacquers which can be re-used takes place.
The object of the invention is therefore the provision of a simplified process for working up milled powder lacquer wastes which facilitates their re-use. The process is intended to lead to a homogeneous powder lacquer of undiminished quality. It is intended to provide the opportunity to produce colour mixing with the production of desired shades of colour.
It was found that this object can be achieved by the process representing one object of the invention for working up milled powder lacquer wastes by compacting the powder lacquer wastes, without fully melting them, to give a sintered product, wherein different types of powder lacquer residues or different powder lacquer residues and standard powder lacquers are milled to give powder lacquers without an extrusion treatment.
Another object of the invention is the powder lacquers obtainable by the process according to the invention.
In the process according to the invention, different types of powder lacquer residues are used. Different types of powder lacquer residues may be in particular powder lacquer residues with different colours and/or powder lacquer residues with different chemical compositions.
In a first step, the powder lacquer wastes are separated, homogeneously blended and compacted.
Then, in a second step, the sintered product obtained, e.g. in the form of coarse granules, is milled. The compacted material does not then disintegrate again into the originally present fine particles.
The coatings obtained from the powders according to the invention do not differ in quality from commercially available powder lacquers.
When preparing powder lacquers, the coloured pigments are generally added as powders prior to extrusion or are added as so-called materbatches. Subsequent submixing of pigments has proven difficult because homogeneity has to be ensured and also no subsequent separation must occur.
Su
Blatter Karsten
Bolm Helene
Montgomery David
Deshmukh Sudhir G.
E. I. du Pont de Nemours and Company
Michl Paul R.
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