GB2168623A - Coating apparatus - Google Patents

Description

GB2168623A 1 I

SPECIFICATION

Coating apparatus 5 The invention relates to the drying of coat- 70 ings, films and the like as described and claimed in GB Application 8416447 which is based on International Application PCT/AU83/00196 filed 30th December, 10 1983.

The invention provides an apparatus for forming a dried coating upon a suitable substrate comprising coating a vehicle upon said substrate, and subjecting the coated vehicle to 15 treatment with a drying agent, the said agent being electrostatically deposited upon the coated vehicle.

The invention finds application in the drying of paints, lacquers, varnishes, printing vehicles 20 and printing inks, liquid adhesives, surface coatings, caulking compounds and the like. In the above definition, the following are to be understood:

1. In respect of coating, film or the like, 25 the term "drying" is to understood as (i) including within its ambit "curing" and as (ii) indicated that the coating is either free from "tack", insoluble in solvent, possessed of an advanced degree of integrity, or able to withstand reasonable abrasion or pressure without damage. It will also be appreciated that, in some circumstances, a dry coating may evidence all of the foregoing qualities. The expression "coating, when used as a noun, is, 35 for the purpose of this invention, to be under-' stood as synonymous with "film" (or the like).

2. The expression "substrate" is to be construed in the widest possible sense, any 40 surface to which the vehicle can be adheringly applied, and upon which it will be retained while treatment with a agent is being effected. Thus, such wide-ranging materials as paperboard, metal foil, steel plate, plastics material, 45 thermally sensitive material and so on may (depending upon other circumstances) be employed.

3. The expression "vehicle" includes within its ambit the paints et al as particularised 50 herebefore.

4. The expression "agent" connotes the at least one chemical compound which effect(s) the curing or drying of the vehicle. It may be sometimes alternatively referred to, in this 55 text, as a catalytic agent, or simply as a cata lyst.

The vehicle may be of the type which contains free isocyanate groups. The expression "free isocyanate groups" includes within its 60 ambit potentially free such groups, the meaning to be conveyed being that the pre-polymer has isocyanate groups which are releasable, or available, for reaction with any other compound possessing active hydrogen sites (for 65 the purpose of polymer propagation and/or film formation). Compounds containing free insocyanate groups are to be understood as embracing all such compounds. Accordingly, comprehended thereby are, not only isocyanate with urethane structure and polyisocyanates, but also those with polyisocyanate, biuret, and allophanate structure.

The drying (or catalytic) agent, which may effect its treatment is vapourphase, may be ammonia, or an amine, or any other compound, such as organo metals, capable of accelerating the desired reaction pathway. The expression "amine" includes within its ambit not only those of simple primary aliphatic mo- 80 nofunctional structure, but also amines characterised yb (i) polyfunctionality and (ii) a more advanced degree of hydrogen substitution. The expression "vapour-phase" denotes that the agent-namely ammonia, an amine et al-- 85 is in gaseous, vapour, or any other entrained air-borne form (e.g. dispersion, fog or aerosol) in which it is available for reaction.

The amine itself may be widely exemplified. Thus, typical examples are mono compounds 90 such as methylamine, ethylamine, propylamine, isopropylamine and the numerous isomers of butylamine, and polyfunctional amines such as hydrazine, ethylene-diamine, proplylene diamine and diethylene triamine. Further examples are 95 diethylamine, triethylamine and dimethylethanolamine (DMEA) the latter being an alkanolamine, and ditertiary amines such as N, N, N', N'tetra- methylethylenediamine (TMEDA) and N, N, N', N', 2-pentamethyl 1-1, 2- propanediam- 100 ine (PMT)-and, indeed, any combination of such amines, proportioned as required, whereby advantage may be taken of the synergistic effect of such a combination.

The organo metals may also be widely ex- 105 emplified. Thus, typical examples are dibutyl tin dilaurate, lead tetraethyl, titanium acetyl acetonate, dimetyl tin dichloride, and stannous and zinc octoates. Likewise, advantage can be taken of the synergistic effect of these com110 pounds in association both with one another and with the above-mentioned amine(s).

The vehicle may be one- or two- component paint et al which contains free isocyanate groups (as defined above). A typical such 115 paint-which is able to be electrostatically or otherwise deposited upon a substrate to be coated, and speedily dried by a vapour-phase drying agent as further demonstrated hereafter-is a two-component preparation formu- 120 lated from a hydroxyl-bearing synthetic resinous first component, and an isocyanate terminated pre-polymer second component. It will be appreciated that these components are themselves capable of wide exemplification.

One suitable such paint is a two-component white polyurethane preparation in which pigment dispersion has been carried out using a coconut alkyd based resin which is subsequently mixed with an isocyanate terminated 130 pre-polymer based on XDI (xylene diisocyan- GB 2 168 623A 2 ate). In alternative formulations, the XDI-based isocyanate terminated pre-polymer can be replaced by one or more pre-polymer(s) based on (using the standard abbreviations) IVIDI, 5 TDI, HDI, H,,MDI, IPDI, and H,,XDI-or the reaction products of these diisocyanate monomers with appropriate polyols, polycarboxy or polyamine intermediates. Likewise, the first component can be alternatively chosen from 10 inter alia (using generic designations) acrylic, epoxy, polyether, polyester and polysiloxane resins.

A further exemplary vehicle, which is able to be electrostatically or otherwise coated upon 15 the substrate, and speedily dried using the apparatus of the invention, is a two-part vehicle in which the first part comprises a polyepoxide resin containing hydroxyl groups, and the second part comprises a resin con- 20 taining free polyamide groups.

According to the invention there is provided apparatus for forming a dried coating upon a suitable substrate employing a vehicle to be coated upon the substrate and a drying agent 25 for drying the vehicle upon the substrate, comprising means including atomizing nozzles for vapourising a liquid drying agent; control means for controllably delivering the vapourised agent to an electrostatic spraying means; 30 and sensing means provided in the delivery path and operative to ensure that the concentration of vapourised agent delivered to the spraying means is maintained within predetermined limits.

35 The electrostatic spraying means may be an 100 electrostatic paint gun.

The apparatus may carry out the co-deposi tion of vehicle and drying agent by the action of an electrostatic field. The apparatus may

40 comprise means for directing an electrostati- 105 cally charged vehicle, such as a paint, upon the substrate to be coated; and means for simultaneously directing an electrostatically charged drying agent in vapour-phase, upon 45 the substrate, the first and second-mentioned 110 means being concentrically arranged in relation to one another. This apparatus, which can also function to direct drying agent only upon the substrate, will be described in greater de 50 tail hereafter.

An embodiment of the invention will now be described, by way of example, with refer ence to the accompanying drawings, in which:- Figure 1 is a perspective-type view of appa- 120 ratus of the invention for providing a drying agent in vapour-phase, and Figure 2 is a perspective-type view of the apparatus for effecting deposition of the dry- 60 ing agent -or co-deposition of vehicle and drying agent The apparatus of Fig. 1 (generally designated by the numeral 1) is_ comprised of a box-like outer structure 2 containing a tank 3 65 for the liquid catalyst. The catalyst is atomized 130 within an inner chamber 4, situated beneath tank 3, by means of atomizing nozzles 5 which receive catalyst from the tank 3 under gravity. Air is admitted to the chamber 4 via 70 an air inlet filter 7 in a side 8 of the structure. Within the chamber 4, a turbulent air flow is created in order to facilitate mixing and atomization therein. This is achieved by a turbulence-creating fan 6 in the base of the cham- 75 ber.

the atomizing nozzles 5, whilst being fed with catalyst from the tank 3, receive compressed air via a hose 9 which is used to produce a fine atomizing spray at the nozzle 80 5.

To deliver the vapour-phase catalyst from the chamber 4, a variable-speed turbulence fan 10, the operation of which is controlled from mechanism 16, is located in a side 11 of the 85 structure opposite the side 8 containing the air filter 7. The fan 10 directs vapour-phase catalyst to a required location-in a particular instance to the co-deposition apparatus as defined above and described hereafter with ref- 90 erence to Fig. 2-via a flexible conduit 12. Located in a cowling 13 disposed around the fan 10 is a catalyst sensor 14. This sensor measures the concentration of vapour-phase catalyst passing along the conduit and pro- 95 vided a concentration reading which is fed back to, and recorded on, a dial 15.

By adjusting the atomization and vapourization of the liquid catalyst stored in the tank 3, which can be controlled within pre-determined limits from the dial 15, the concentration of the vapour-phase catalyst being delivered from the apparatus can be monitored as required. Also, as indicated heretofore, the rate of delivery of the catalyst is controlled by operation of the variable speed fan 10. In this manner, preset concentrations of catalyst can be maintained with accuracy.

In the case of molecular solution of the vapour-phase catalyst in air, the concentrations will of course vary between zero and the saturation. concentration for the particular catalyst used at the temperature under consideration. In the case of aerosol fogs this restriction does not exist.

As indicated above, the apparatus of Fig. 1 can use flexible conduit 12 to feed vapourphase catalyst to the apparatus illustrated in Fig. 2 (and generally designated by the numeral 20). This apparatus can be used for electrostatic deposition of catalyst only, or for the electrostatic codeposition of catalyst and a vehicle -typically paint.

The apparatus 20 comprises a standard electrostatic gun 21 having a barrel 22 from 125 whence the electrostatic charge is emitted. Feeding into the rear of the gun 21 is a supply conduit 23 which delivers the paint to the barrel 22 of the gun, from which it is likewise emitted with an electrostatic charge. Power lead 24 provides the power for the production GB 2 168 623A 3 of the electrostatic charge in the gun 21. Thus far, the gun is conventional -and well known in the art.

As indicated above, the flexible conduit 12 5 can connect the apparatus of Fig. 1 to the gun 21. The vapour-phase catalyst is projected to the barrel of the electrostatic gun by means of the pressure difference created by the fan 10 of Fig. 1. In the apparatus of Fig.

10 2, a barrel shroud 25, concentrically deposed around the barrel 22, ensures that the vapourphase catalyst is not emitted from the gun 21 before the tip 26 of the barrel is reached. In this way, the catalyst is also charged by the 15 electrostatic field produced at the tip of the barrel 22-and is charged sufficiently to allow the vapourised, and now charged, catalyst to deposit itself on an earthed substrate at which the gun is being pointed.

If the gun is to be used, in one mode, for electrostatic deposition of drying agent only (following previous painting of the substrate), actuation of trigger mechanism 27 will achieve this end. If, in another mode, co-deposition is 25 required, the gun 21, controlled by operation of trigger mechanism 27, will be simultaneously supplied with paint via conduit 23, vapour-phase catalyst via conduit 12 (and electrostatic charge via lead 24). When the 30 trigger 27 is depressed in this mode, paint from the barrel 22 and catalyst from the shroud 25 will be simultaneously electrostatically charged. Both the flow of paint and the flow and concentration of catalyst can be con- 35 trolled to achieve a desired ratio in the codeposition of paint and catalyst. It will of course be readily appreciated that, when the components of Fig. 1 deliver the drying agent in vapour-phase to the components of Fig. 2, 40 via flexible conduit 12, the so associated com ponents function as a single apparatus.

The process by which electrostatically charged catalyst is applied to prepainted articles (to all surface thereof) or where co-de- 45 position of electrostatically charged catalyst and paint is effected (to all such surfaces), results in a significant acceleration of the curing of the paint film, to give drying times of obvious commercial significance. The manner 50 in which deposition or co-deposition is ef- fected at all surfaces will be understood from Fig. 2-which depicts diverging path of drying agent and vehicle as the said agent and vehicle leave the apparatus.

55 The invention will now be described with 120 reference to five numerial examples. In relation thereto, the following should be noted.

The paint employed in these examples is a two-component white polyurethane preparation 60 as mentioned hereinbefore.

In so far as it is of known structure, the electrostatic gun is of three basic types-which, for convenience, are referred to hereafter as types 1, 11 and III respectively.

65 Breifly, the type I gun performs the electrosta- 130 tic deposition of paint (or other vehicle) by means of a rotating disc which atomizes the paint within an electrostatic field generated at the tip of a wire filmament disposed for that

70 purpose ( the thus charged paint being delivered to the substrate being coated). The type 11 gun generates an electrostatic field per medium of a filament disposed at the end of a barrel, through which is delivered the paint to

75 be carried by the electrostatic charge (the paint being delivered to and through the barrel by means of air assistance). The type III gun operates in substantially the same manner as the type 11 gun; however, the paint to be 80 electrostatically charged is delivered to the barrel hydraulically.

EXAMPLE 1 (sequential deposition) A metal panel, earthed properly, is coated 85 on both faces with a paint as previously described, using an electrostatic hand type I gun. Air containing approximately 2,000 parts per million of dimethylethanolamine in vapour form which has been generated from the 90 apparatus of Fig. 1, is passed at right angles to the painted plate- and within two minutes of elasped time, the vapour-phase catalyst is charged using an electrostatic type III gun, with no paint being supplied to the gun, and 95 only a charge being generated. The gun is arranged opposite the plate thus allowing the charged field to intersect the catalyst flow. The passage of vapour-phase catalyst and the electronic field from the type III gun is run

100 continuously for approximately two minutes after which time both catalyst and electrostatic charge are discontinued. After a further eight minutes post-curing time in slightly turbulent air, the film is found to have been ac- 105 celerated in drying on both sides of the plate to give a satifactory dried film.

EXAMPLE 2 (sequential deposition) Vapourized dimenthylethanolamine (DMEA 110 hereafter) from the apparatus of Fig. 1 is passed through flexible tube 12 to plastic barrel shroud 25 around the barrel 22 of an electrostatic gun. This is the apparatus of Fig. 2 with the gun, in this case, being an electro- 115 static type 11 gun. Painted panels prepared in a manner as described in Example 1 above, are exposed to a flow of the vapour-phase catalyst from the gun as illustrated in Fig. 2. The vapour-phase catalyst is applied at a concentration of approximately 2,500 parts per million, for approximately sixty seconds after painting. This follows of catalyst is maintained, for approximately two minutes, after which it is cut out and the plate exposed to 125 gently turbulent air approximately eight minutes. After this post- curing period, drying of the painted film is found to have been markedly accelerated on both sides of the plate.

EXAMPLE 3 (co-deposition) 4 GB2168623A 4 A deposition gun, as described in Example 2 and illustrated in Fig. 2, is set up and connected to the apparatus of Fig. 1. Paint, as previously described, is then passed through 5 the gun-and simultaneously, the vapour-phase catalyst (DMEA) was introduced in a concentration of approximately 4,000 parts per million of DMEA as a molecular solution in air. Properly earthed panels were painted by the 10 simultaneous application of paint and catalyst. Under these conditions, the painted plates showed equally rapid attainment of touch-dry condition and overall commercial dryness.

15 EXAMPLE 4

This example is similar to that described in Example 3 above, i.e. the same procedures were repeated but in this instance, the vapour-phase catalyst employed was PMT (all 20 other parameters were the same). The degree of accelerated drying was even more marked than with DMEA and an even more rapid commercial dryness was effected.

25 EXAMPLE 5

In this example, the vapour-phase catalyst was lead tetraethyl. All other conditions as detailed in Example 3 above were identical. Once again an equally marked acceleration in 30 the curing of the paint was achieved.

Claims (2)

1. Apparatus for forming a dried coating upon a suitable substrate employing a vehicle 35 to be coated upon the substrate and a drying agent for drying the vehicle uporn the substrate, comprising means including atomizing nozzles for vapourising a liquid drying agent; control means for controllably delivering the 40 vapourised agent to an electrostatic spraying means; and sensing means provided in the delivery path and operative to ensure that the concentration of vapourised agent delivered to the spraying means is maintained within pre- 45 determined limits.

2. Apparatus as claimed in Claim 1, substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1986, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.