Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1992-12-28
1995-10-24
Vargot, Mathieu D.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
264 8, 264 9, 264 12, 264349, 264443, 264489, 264491, 427600, 427195, 524599, 524904, B29B 900, C08J 312
Patent
active
054610893
DESCRIPTION:
BRIEF SUMMARY
This invention relates to powder coating compositions and to their preparation.
BACKGROUND OF THE INVENTION
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. The 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 nonpolluting.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments. 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, 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.
Whilst existing processes for the manufacture of powder coating compositions are satisfactory in some respects, there is nevertheless room for improvement and it is a general objective of the present invention to simplify the production of such compositions and also to make such production more economic.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic cross-section of an apparatus for performing a process according to the invention;
FIG. 2 is a cross-section of one form of rotary cup atomizer suitable for use in the apparatus of FIG. 1;
FIG. 3 is a cross-section of an alternative form of rotary cup atomizer suitable for use in the apparatus of FIG. 1;
FIG. 4 is a cross-section, partly in a diagrammatic form, of an apparatus including a modified form of atomizer provided with an additional hot air inlet;
FIG. 5 is an electron micrograph of a sample of the product of the invention; and
FIG. 6 is an electron micrograph of a sample of the product obtained by a conventional comminution process.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a process for the manufacture of a thermosetting powder coating composition, which comprises forming a molten mixture of a film-forming polymer containing reactive functional groups, a curing agent for the said polymer and optionally one or more colouring agents, atomising the resulting melt into droplet form, and causing or allowing the atomised droplets to cool to form solid powder particles, the conditions being controlled to avoid significant thermosetting of the composition during the process, whereby the product composition is suitable for application as a powder coating.
It is a specific objective of the process of the invention to produce, without comminution, a composition which is suitable for application as a powder coating, and it will be appreciated that the attainment of that objective depends on the prevention of any significant thermosetting of the composition during the process. On the basis of the present state of knowledge, it could not have been expected that the necessary performance criteria for a powder coating composition could be achieved by any process involving atomisation of a molten thermosetting material and no such process would have come into consideration on the basis of previous knowledge or proposals. Thus, it would on the
REFERENCES:
patent: 2439772 (1948-04-01), Gow
patent: 3272893 (1966-09-01), Mogensen
patent: 3962491 (1976-06-01), Sato et al.
patent: 4599294 (1986-07-01), Matsumoto et al.
Handyside Timothy M.
Morgan Andrew R.
Courtaulds Coatings (Holdings) Limited
Vargot Mathieu D.
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