Plastic and nonmetallic article shaping or treating: processes – Forming articles by uniting randomly associated particles – Agitating to form larger particles
Reexamination Certificate
2000-08-21
2003-10-21
Staicovici, Stefan (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Forming articles by uniting randomly associated particles
Agitating to form larger particles
C264S122000, C264S310000, C264S319000, C264S911000, C023S3130FB
Reexamination Certificate
active
06635207
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the recycling of powder coating waste.
BACKGROUND OF THE INVENTION
The preparation of powder coatings is mostly carried out by extruding the powder coating ready formulated by dry mixing of all the required components (powder coating fresh material) in the form of a paste-like melt, cooling off of the melt, coarse comminution into so-called powder coating chips, fine milling (grinding) and subsequent sieving and classifying to desired grain fineness (powder coating classification). Powder coating material that is too coarse-grained (high grain) on sieving can again be fine-milled. Powder coating material that is too fine-grained (low grain) represents waste.
Powder coating waste also arises in the form of powder coating overspray during the powder coating application.
Powder coating waste should if possible be recovered as powder coating to be employed for the original use, not only because of environmental considerations, but also because powder coatings are precious materials.
Various methods which allow recycling of powder coating waste to be used for the original purpose are described in the patent literature.
DE-A-40 28 567 thus describes direct recycling of powder coating overspray by addition to the powder coating fresh material or during extrusion. The process requires considerable expenditure during extrusion of the finely divided material.
In WO 96/15891 pressing of powder coating waste into friable tablets, which are then extruded together with powder coating fresh material, is described.
It is known from EP-A-0-683 199 to form a layer of powder coating waste, to expose the latter to heat, wherein the powder coating particles coalesce without degradation or cross-linking of the powder coating. IR-radiators can serve as the heat source. The coalesced powder coating particles are then again extruded alone or together with powder coating fresh material.
The processes known from DE-A-40 28 567, WO 96/15891 and EP-A-0-683 199 include a re-extrusion of the powder coating waste. This is linked to the danger of a too high thermal load.
WO 98/33848 describes the agglomeration of finely divided powder coating residues under the influence of pressure. The agglomerates can be returned to the powder coating preparation process.
WO 99/23068 and WO 99/23176 describe compacting of powder coating waste by applying pressure. The compacted powder coating waste can be ground together with fresh powder coating material that is to be ground.
The processes known from WO 98/33848, WO 99/23068 and WO 99/23176 require the application of pressure by suitable pressing devices.
WO 00/15705 describes the heating of powder coating fine grain without melting or cross-linking it completely. The powder coating fine grain thereby becomes sufficiently sticky and forms an agglomerated mass, which is treated further after cooling off in order to prepare powder coating particles therefrom having the desired particle size. The agglomerated mass is a powder coating material which is not convenient in operation.
EP-A-0 982 380 describes the preparation of powder coating particles having a particle size suited for application by agglomeration of powder coating fine grain by heat and preferably with the use of auxiliary products promoting the agglomeration. The process is extremely difficult to control due to the high requirements concerning the particle size distribution of the powder coating suited for application.
An object of the invention is to provide a simple process for recycling powder coating waste. It should be possible to carry out the process without pressure, i. e. without application of pressing forces onto the powder coating waste. The powder coating material to be recycled should be subjected as little as possible to heat. There should be no obligation to add auxiliary products, as these would necessarily bring about a change in the composition of the powder coating or would have to be removed.
SUMMARY OF THE INVENTION
The invention solves the problem in that powder coating waste is subjected to reduced heat while moving in a rotating vessel.
The invention therefore consists in a process for recycling of powder coating waste comprising the steps of placing powder coating waste into a vessel; rotating the vessel about a rotational axis inclined at ah angle of 20 to 70 degrees relative to the horizontal; and heating the powder coating waste until particulates with a size of 1 mm to 3 cm are formed.
DETAILED DESCRIPTION OF THE EMBODIMENTS
In the process according to the invention, powder coating waste is recycled. Powder coating waste is coating powder having particle sizes or particle size distributions which are not suitable for powder coating application or which deviate from the theoretical value. The powder coating wastes have average particle sizes (d
50
), in the range of 1 to 100 &mgr;m for example. Examples of powder coating waste are powder coating-overspray having for example an average particle size (d
50
) in the range of 10 to 100 &mgr;m, low grind according to the powder classification, for example having an average particle size (d
50
) in the range of 1 to 20 &mgr;m, faulty powder coating batches having particle size distributions which are not within the theoretical values or powder coatings which do not conform to the theoretical values of particle size distribution, for example because of a faulty or over-long storage. The powder coating waste to be recycled in the process according to the invention can be thermoplastic or in particular duroplastic powder coating materials, which are for example curable by radiation or in particular thermally curable. The powder coating waste can be of pigmented powder coating or of powder clear coat.
The process according to the invention is suitable in particular for recycling high-quality powder coating waste, such as arises for example in the preparation or application of powder coatings used in the automotive industry, for example automotive powder clear coats.
Powder coatings can be described according to the characteristic temperature behaviour of each powder coating, i. e. several characteristic temperatures or discrete temperature ranges exist for each powder coating. Position and breadth of these temperatures or temperature ranges depend on the composition of each powder coating and can be determined by suitable physical methods. One commonly used determination method is for example DSC (differential scanning calorimetry). Powder coatings have a glass transition temperature, below which they are stored. The glass transition temperature of powder coatings is usually not a precisely defined glass transition temperature but for example a 20 to 40° C. wide glass transition temperature range. At temperatures above the glass transition temperature range, for example, powder coatings change into a rubber-elastic state, before the melting temperature range of the powder coatings is reached with a further rise of temperature. The breadth of the melting temperature range of powder coatings is for example from 20 to 150° C. and the lower limit of the melting temperature range is for example from 20 to 50° C. above the upper limit of the glass transition temperature range. The chemical cross-linking of a thermally curable powder coating starts to be noticeable within or above the melting temperature range. Particular importance is given for example therefore to a rapid passage of the melted powder coating in the extruder in the preparation of thermally curable powder coatings. In the case of thermally curable powder coatings, the baking temperature chosen for curing is above the melting temperature range. The flow of a thermally curable powder coating can be controlled by the value of the baking temperature. On one hand, the viscosity of a powder coating melt falls with rising temperature, while on the other hand the speed of the chemical cross-linking of a thermally curable powder coating increases and therewith the viscosity.
For example, using DSC at a heating rate of 10° C./min, a temperature
Benjamin Steven C.
E. I. du Pont de Nemours and Company
Staicovici Stefan
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