Patent GB-912464-A (GB)
IPC: B05C19/02
Filed: 1959-09-23
Abstract: 912,464. Coating surfaces. INDEVCO Ltd. Sept. 23, 1959 [Sept. 29, 1958; Feb. 2, 1959; April 16, 1959; July 15, 1959; Sept. 3, 1959], No. 32427/59. Class 95 [Also in Group XV] A coating process comprises contacting the surface to be coated with a bed of particulate coating material in an adequately mobile state of even aeration for substantially uniform contacting of the surface to be coated with the particulate material intermediate between that of the truly fluidized material and that of the loosely settled material during at least a substantial part of the coating period while maintaining conditions suitable to cause the adherence of the particles to the surface to be coated either by rendering the said surface itself adhesive prior to the above mentioned contacting step or by heating the surface to be coated to a temperature at which the particles become at least plastiform enough to adhere to the surface. The particulate material may be a cellulose derivative a thermoplastic resin such as polyethylene, polypropylene, nylon or a vinyl resin, a thermosetting resin such as an epoxy or phenolic resin or a thermoplastic inorganic material such as glass; it may also comprise a mixture of a fusible material with a particulate material which is infusible at the temperatures employed such as a ceramic material. The specified condition of the coating bed may be brought about by first setting up a normal type of fluidized bed and then causing this to collapse in contact with the surface to be coated. The collapsing may be brought about either immediately before immersing the object to be coated or immediately after and may be caused by increasing the gravitational head on the fluidized bed or increasing the pressure above the fluidized bed without applying more gas pressure from underneath, or by reducing the feed pressure of the fluidizing medium, or by applying vibration to the fluidized bed so as to cause movement of the particles in a direction normal to the direction of flow of the fluidizing medium. The surface to be coated which is preheated may be kept in a confined space above the normal level of a fluidized bed under optimum fluidized conditions and the volume of the space occupied by the normally fluidized bed may then be decreased in such a way that the aerated material is displaced into contact with the hot surface and simultaneously the gravitational head on the bottom of the fluidized bed is increased to such an extent that the bed collapses. The process may be adapted to the internal coating of pipes and like hollow bodies in which case the pipe forms the confined space referred to. After the coating step the fluidization space proper may again be expanded so as to allow the particulate material to return to this space; the speed of this return is preferably controlled so that the particulate material passes the surface to be coated with a wiping action. More than one coat of the coating material may be applied; these coats may be of different materials. A first layer of a thermoplastic material may be deposited and while this layer is still tacky a further layer of a particulate material which is infusible at the temperature employed may be applied. The specified condition of the coating bed may also be brought about direct by applying a slow gas stream to the powder while agitating it carefully in a direction mainly from the bottom of the vessel to the top. As shown in Fig. 1, apparatus for carrying out the process consists of a fluidizing vessel 2 with the sides constructed in the form of bellows and an aperture in the cover 10 to which a pipe to be coated may be attached. In operation the bed is fluidized in a normal manner, the bellows are then contracted so that the level of the fluidized bed is raised into the pipe which is to be coated and the corresponding increase in the gravitational head of the fluidized material initiates the desired collapsing of the bed. In an alternative form of apparatus, Fig. 8, the top of the fluidizing container is open and the article 49 to be coated is suspended above the normal level of a normal fluidized bed. A piston 51 is then dropped into the fluidized bed with the result that the top level thereof rises to the position 39a, and the bed because of the increased gravitational head then begins to collapse to level 39b in contact with the object to be coated. Specification 759,214 is referred to.