Powder coating compositions

#This invention relates to powder coating compositions and to their use in coating substrates, especially substrates of complicated shape, with particular reference to articles having recessed portions.

#Powder coating compositions generally comprise a solid film-forming resin binder, usually with one or more colouring agents such as pigments, and optionally also contain one or more performance additives. They are to usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer), but thermoplastic systems (based, for example, on polyamides) can in principle be used instead. 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. Other homogenisation processes also come into consideration, including non-extruder-based processes such as, for example, processes involving mixing using supercritical fluids, especially carbon dioxide.

#Powder coating compositions are generally applied by an electrostatic spray process in which the powder coating particles are electrostatically charged by the spray gun and the substrate (normally metallic) is earthed. The charge on the powder coating particles is normally applied by interaction of the particles with ionised air (corona charging) or by friction (tribostatic or “Itribo” charging). The charged particles are transported in air towards the substrate and their final deposition is influenced inter alia by the electric field lines that are generated between the spray gun and the workpiece. A disadvantage of this process is that there are difficulties in coating articles having complicated shapes, and especially articles having recessed portions, as a result of restricted access of the electric field lines into recessed locations (the Faraday cage effect), especially in the case of the relatively strong electric fields generated in the corona-charging process. The Faraday cage effect is much less evident in the case of tribostatic charging processes, but those processes have other drawbacks.

#The compositions T1 to T6 inclusive of Example 1 were prepared as follows. The base powder coating formulations (presented in Appendix 1) were dry mixed in a blender and fed into a twin-screw extruder operating at 105° C. The extrudate was rolled flat on a chilled plate and broken into chip form.

#Milled powders T1 to T6 were prepared from the resulting chip, incorporating in each case by dry blending the dry flow additive (Acematt TS 100-silica) shown on the last line of each formulation presented in Appendix 1. Milling was carried out in an impact mill (Hosokawa ACM5) to produce in each case a powder coating having the following particle size distribution (Malvern Mastersizer X): 3 d(v)99 <100 microns d(v)50 40 microns 8.0% <10 microns

#The individual powder coatings were blended in pairs in 1:1 weight ratios to produce 250 grams of mixed powder coating in each case, as summarised in the following table. Each mixture was homogenised by tumble mixing (30 minutes at 1 Hz using a Turbula “T2” mixer with a 1 litre glass mixing vessel). The resulting homogenised mixtures were reserved for future use. 4 T1 T2 T3 T4 T5 T6 T1 * a b c d e T2 * f g h i T3 * j k l T4 * m n T5 * o T6 * * 250 grams of each single component powder was retained for comparative purposes.