Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1995-06-12
1997-06-03
Szekely, Peter A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
427475, 428336, 428413, 428480, 428512, 524403, 524413, 524423, 524425, 524427, 524430, 524432, 524433, 524437, 524445, 524447, 524449, 524451, 524414, 524405, 524904, C09J 320, C09C 310, C09D 503, C08K 320
Patent
active
056355483
DESCRIPTION:
BRIEF SUMMARY
This invention relates to powder coating compositions and to their use.
Powder coatings form a rapidly growing sector of the coatings market. Powder coatings are solid compositions which are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate is earthed or oppositely charged. Charging of the powder in the spray gun is effected by means of an applied voltage or by the use of friction (tribo-charging). The applied composition is then heated to melt and fuse the particles and to cure the coating. The powder coating particles which do not adhere to the substrate can be recovered for re-use so that powder coatings are economical in use of ingredients. Also, powder coating compositions are generally free of added solvents and, in particular, do not use organic solvents and are accordingly non-polluting.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments, and optionally also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer). Powder coating compositions are generally prepared by intimately mixing the ingredients (including colouring agents and performance additives) for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding, to the desired particle size. The particle size distribution required for most commercial electrostatic spray apparatus is between 10 and 120 microns, with a mean particle size within the range of 15 to 75 microns, preferably 25-50 microns. Such powders are normally applied at film thicknesses of 40-50 microns and upwards.
Using powder coatings of such conventional particle size distribution it is difficult to apply thin film coatings of, say, 30 microns or less, for which there is an increasing demand in certain sectors of the powder coatings market, to achieve a uniform opacity and an aesthetically pleasing appearance, particularly in a gloss white coating. The achievement of such results is possible only within a restricted range of powder chemistries, with the best performance generally being obtained with polyurethane powders using blocked isocyanates. To achieve coatings of less than 20 microns with a uniform opacity and an aesthetically pleasing appearance is very difficult if not impossible. The problems encountered ("orange-peel" imperfections, etc.) are considered to be attributable to the relatively large size of the majority of the particles in powder coating compositions of conventional particle size distribution.
In addition to the increasing demands within the powder coatings market itself, it is also recognised that the inability of powder coatings reliably and routinely to achieve film thicknesses of 30 microns or less with aesthetically pleasing appearance is one of the factors preventing further substitution of solvent-based "wet" paints by powder coatings.
It is believed that the problems of achieving satisfactory thin film powder coatings could in principle be alleviated with the use of powder coating compositions of a finer particle size. There are problems, however, in the fluidisation, handling and application of relatively small particles, especially particles which are 10 microns in diameter or less. Such problems become more pronounced as the proportion of fine particles increases, and powder coating compositions have hitherto generally been manufactured so as to comprise not more than 10% by volume of particles which are 10 microns in diameter or less.
It is an object of the present invention to alleviate the problems of fluidisation, handling and application of fine particles, especially particles which are 10 microns in diam
REFERENCES:
patent: 5319001 (1994-06-01), Morgan et al.
patent: 5470893 (1995-11-01), Sinclair-Day et al.
Kittle Kevin J.
Rushman Paul F.
Courtaulds Coatings (Holdings) Limited
Szekely Peter A.
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