Thermosetting powders are known, which comprise a thermosetting resin, either alone or in admixture with one ormore inert additives. Known thermosetting resins include polyesters, epoxy resins and polyester/epoxy copolymers.

It is known to mix these resins with chemically and thermally inert additives to impart to the finished material a desired texture, colour or other special effect, for example a metallic effect.

The additives can be mixed with the resin as apure mechanical mixture, and this is acceptable where only a low density of additive is required and the overspraypowder is not recovered. Where a high density of additive is required and/or it is important to ensure that the mixture is homogeneous, it has been found advantageous to use aprocess in which the thermosetting resin is heated to its glass transition temperature, and the resin is bonded to the additive or additives, enabling the additives to adhere. Alternatively, the resin maybe heated to a temperature at which it melts and the additive or additives inpowder form maybe combined with the molten resin at elevated temperature. In either case, the mixture is then heated to the extrusion temperature, extruded, cooled, ground and then granulated to the required powder size.

Thermosetting powders ofthis type are used in a wide range of applications, including for example electrostatic powder deposition processes. For many of these applications, eitherbecauseoffurther processing considerations or because of the intended final use, it is essential that the powder, when cured, is stable at a particular elevated temperature. For example, where the cured powder is to be used as a substrate for dye sublimation printing, it is required that the cured resinbe stable above 180°C.

This process has heretofore only been used with inert additives, inparticularwith additives which are not affected by the changes in temperature experienced during the process and which do not, in the amounts in which they are added to the thermosetting resin, affect the thermal profile of the thermosetting resin. The process has not been used for the manufacture of powders containing a high concentration of additive or for the manufacture of powders containing additives which are themselves affected by the changes in temperature experienced during the process.

It is an obj ect of the present invention to provide a thermosetting powder comprising an additive which is temperature sensitive, which additive maybe present in a concentration which affects the thermalprofile ofthebasic thennosettingresin, whichpowderis suitable for use in a sublimation process, and a process for the production of this powder.

The present invention provides a thermosetting powder comprising a polyester, epoxy resin or a polyester/epoxy copolymer and an additive, characterised in that the additive is a thermosensitive material, present in an amount sufficient to impart thermosensitive properties to the powder.

In apreferred embodiment of the powder according to the invention, the thermosensitive material is a thermochromic material or a photochromic material.

In preferred embodiment of the powder according to the invention, the thermosetting powder has, in the absence of the additive, a curing temperature higher than the temperature at which the thermosetting powder is to be cured.

The invention further provides a process for the production of a thermosetting powder, in which apolyester, epoxyresin or apolyester/epoxy copolymer is heated at least to its glass transition temperature, an additive in powder form is mixed with the heated resin, the mixture is heated to the extrusion temperature, extruded, cooled, ground and then granulated to the required powder size, characterised in that the additive is a thermosensitive material.

Where the thermosetting powder according to the inventionis to bepreparedby aprocess inwhich

the resin is melted prior to mixing with the additive, the resin may comprise apolyester, epoxyresin or a polyester/epoxy copolymer which, in the absence of the additive, has a curing temperature higher than the temperature at which the thermosetting powder is to be cured, more preferably 10 to20'C higher. Suitable thennosetting resins for use in this process include polyesters, epoxyresins or polyester/epoxy copolymers, which have been modified by increasing the polymer chain length and/or degree of cross-linking, to increase the curing temperature from 180'C to 220 C.

Suitable thermosensitive materials include thermochromic materials such as the thermochromically active ingredients of the commercially available liquid thermochromic coating materials described for example in EP-A-0 677 564.

The thermochromic thermosetting powder according to the invention is particularly suitable for use as the coating powder in electrostatic powder deposition processes, in particular for the electrostatic powder deposition process described and claimed in our co-pending UK Patent Application No 0226916.5 The thermochromic thermosetting powder according to the invention preferably comprises less than 3 % by weight, more preferably from 3 to 25 % by weight, most preferably from 3 to 10 % by weight, based on the total weight of the powder, of the thermochromic material.

The thermochromic thermosetting powder according to the invention and the process for its production will now be described with reference to the following specific Example.

Example 5 g of a thermochromic powder having a particle size of less than 50 microns was dry mixed with 95 g of a powdered thermosetting polyester resin. The mixture was then heated to its glass transition temperature of 60° C, ensuring that the free thermochromic powder adhered to the surface of the thermosetting powder. The resultant mixture was then subjected to a conventional powder manufacturing technique, resulting in a graded powder suitable for deposition by an electrostatic process.