NO843426L - PROCEDURE FOR PREPARING A DRY COAT ON A SUBSTRATE - Google Patents

Description

The present invention relates to the drying of coatings, films and the like. With the help of the invention, an improved method (and resulting product) is obtained, as the drying is carried out more efficiently than before.

Viewed from a broad angle, the invention provides a method for forming a dried coating on a suitable substrate, comprising applying an agent to the substrate and subjecting the applied agent to a treatment with a desiccant, the desiccant being deposited electrostatically on the applied agent. The invention also includes other pages that will appear in it

the following.

The invention finds application in connection with

drying of paints, varnishes, varnishes, paints and printing inks, liquid adhesives, surface coatings, sealants and the like. With the definitions given above, the following is to be understood: 1. With regard to coating, film or the like - which is to be or has been exposed to the method according to the invention - the term "drying" is to be understood as (i)

to include within its scope "hardening" and (ii) to indicate that the coating is either free from "sticking", insoluble in a solvent, has an increased degree of integrity, or is capable of withstanding appropriate wear and tear or pressure without destruction. It should also be understood that under some conditions a dry coating may include all of the preceding qualities. The term "coating", when used as a noun, shall in connection with the present invention be understood as synonymous with

"film" (or similar).

2. The term "substrate" is to be understood in the broadest possible sense, as any surface to which the agent can adhere when applied, and on which it can be retained when the treatment with the desiccant is carried out, will lie within the scope of the invention. Thus, a wide range of materials can be used, e.g. cardboard, metal foil, steel sheets, plastic materials, thermally sensitive materials, etc. (depending on other circumstances). 3. The term "agent" includes paints

etc. as specified above.

4. The term "drying agent" denotes the at least one chemical composition which causes the hardening or drying of the agent. It will occasionally be alternately referred to in this text as a catalyzing agent or simply a catalyst.

In one form of the invention, the agent can be of the type that contains free isocyanate groups. The term "free isocyanate groups" includes within its scope potential free such groups, the meaning to be conveyed being that the pre-polymer has isocyanate groups which can be released or be available for reaction with any composition comprising active hydrogen sites (with polymer dispersion and/or film formation as purpose). Compositions containing free isocyanate groups shall be understood to include all such compositions. Consequently, those covered herein will not only be isocyanates with a urethane structure and polyisocyanates, but also those with a polyisocyanurate, biuret and allophanate structure.

The drying (or catalyzing) agent which can carry out its treatment in the vapor phase# may be ammonia or amine or any other composition, e.g. organometals or inorganic metal salts that enable acceleration of the desired reaction pathway. The term "amine" includes in its term not only those of simple, primary, aliphatic, monofunctional structure, but also amines which are characterized by (i) polyfunctionality and (ii) a more advanced degree of hydrogen substitution. The term "vapour phase" indicates that the agent - namely ammonia, an amine and more - is in gas-vapour- or any other entrained airborne form (e.g. dispersion,

mist or aerosol) in which it is available for reaction.

The amine itself can be exemplified to a high degree. Thus, typical examples involve monocompositions, e.g. methylamine, ethylamine, propylamine, isopropylamine and a number of isomers of butylamine and polyfunctional amines, e.g. hydrazine, ethylenediamine, propylenediamine and diethylenetriamine. Further examples are diethylamine,; triethylamine and dimethylethanolamine (DMEA), and ditertiary amines, e.g. N,N,N<1>,N'-tetramethylethylenediamine (TMEDA) and N,N,N',N',2-pentamethyl-1,2-propanediamine (PMT) - and, of course, any any combination of such amines, proportioned as needed, taking advantage of the synergistic effect of such a combination.

The organometallics can also include many examples. Thus, typical examples are dibutyl tin dilaurate, lead tetraethyl, titanium acetyl acetonate, dimethyl tin dichloride, and stannous and zinc octoates. Among the inorganic metal salts that have proven effective, mention should be made of bismuth nitrate and ferric chloride. In the same way, one can take advantage of the synergistic effect of these compositions in conjunction both with each other and with the above-mentioned amines.

The agent can be a one- or two-component paint, etc. containing free isocyanate groups (as defined above).

A typical such paint - which has the properties of being electrostatically or otherwise deposited on the substrate to be coated, and rapidly drying by means of a vapor phase desiccant as will be demonstrated below - is a two-component preparation which is composed of a hydroxyl-bearing synthetic resinous first component, and a second component comprising isocyanate-terminated prepolymer. It should be understood that these components in themselves enable broad exemplification.

A suitable such mixture is a two-component white polyurethane preparation, in which the pigment dispersion has been carried out using a coconut alkyd-based resin which is later mixed with an isocyanate-terminated prepolymer based on XDI (xylene diisocyanate). In alternative compositions, the XDI-based isocyanate-terminated prepolymer can be replaced by one or more prepolymers, based on (using standard abbreviations) MDI, TDI, HDI, H^MDI, IPDI and H^XDI - or the reaction products of these diisocyanate monomers with suitable polyols , polycarboxy or polyamine intermediates. In the same way, the first component can alternatively be selected from among others (generic designating is used here.) resins from acrylic, epoxy, polyether, polyester and polysiloxane.

Another example of a remedy capable of

electrostatically or otherwise be coated on the substrate,

and quickly be dried according to the invention, is an agent comprising two parts, the first part comprising a poly-epoxide resin containing hydroxyl groups, and the second part comprising a resin containing free polyamide groups.

The electrostatic deposition of the drying or catalyst agent can be carried out by exposing the evaporation agent to a generated electrostatic field. In one form of the invention, the electrostatic field can be created in the usual way, typically by means of an electrostatic gun of the type usually known for this purpose.

In one embodiment of the invention, the electrostatic deposition of the drying agent (catalyst) can be filled by a preceding step which comprises coating the agent on the substrate. However, simultaneous deposition will also be within the scope of the invention, as this includes electrostatic co-deposition - that is, simultaneous application of both the coating agent and the drying agent through the action of an electrostatic field. Thus, in connection with a further process aspect, the invention provides a method, as broadly defined above and further characterized in that the steps relating to the coating of the agent on the substrate and electrostatic deposition of the desiccant are carried out simultaneously. In connection with a related aspect, the invention provides an apparatus for carrying out this method, as will appear in the following.

As an introduction to the definition of the device just mentioned, the way in which the agent is coated on the substrate shall be mentioned. In this connection, the agent, typically a paint, will be applied by hand painting (using a broom), dipping, spraying - or by using the device, where the application of the paint to the substrate is itself carried out electrostatically. Such electrostatic deposition of paint can be carried out using an electrostatic paint gun.

The apparatus for effecting the simultaneous deposition of the agent and desiccant may do so (as indicated above) by the action of an electrostatic field. In this related aspect of the invention, the invention provides an apparatus which in combination comprises means for directing the electrostatically charged agent, e.g. a paint on the substrate to be coated, as well as means for simultaneously directing an electrostatically charged desiccant, typically in vapor phase, onto the substrate, the former and latter means being concentrically arranged in relation to each other. This apparatus, which can also function for directing only the desiccant onto the substrate, will be described in detail below.

The desiccant in the vapor phase can be obtained in a number of ways, including evaporation or injection techniques. An apparatus with which a vapor form of the desiccant is efficiently produced forms another aspect of the invention. This side of the invention provides an apparatus which in combination includes means for evaporating a drying agent in liquid form, means for controllably delivering the evaporated agent to a required position - e.g. to the latter means in the apparatus as defined above, as well as sensing means arranged in the delivery path, which act to ensure that the concentration of the vaporized agent delivered is maintained within predetermined limits. When the vaporized agent is delivered to the latter means, the vapor phase generation components and co-deposition components will be combined to function as a single apparatus.

The invention will now be sequentially described with reference to the attached drawing and some special numerical examples. It should be understood that such a subsequent description is intended to illustrate the performance features of the invention and thus should not be perceived as limiting. Fig. 1 is a perspective view of the apparatus which provides (generates) the desiccant in vapor phase. Fig. 2 is a perspective view of the apparatus for carrying out the deposition of the desiccant - or simultaneous deposition of the agent to be deposited and the desiccant.

The apparatus of fig. 1 (which is generally denoted by the reference number 1) is composed of a box-like outer construction 2 which contains a tank 3 for the liquid catalyst. The catalyst is atomized in an inner chamber 4, which is arranged below the tank 3, by means of atomizing nozzles 5 which receive catalyst from the tank 3 under the influence of gravity.

Air has access to the chamber 4 via an inlet filter 7 in the side

8 of the construction. Inside the chamber 4, a turbulent air flow is provided with the intention of simplifying the mixing and atomization therein. This is achieved by means of a turbulence-producing fan 6 at the bottom of the chamber.

The atomizing nozzles 5 will, during the feeding with catalyst from the tank 3, receive compressed air via a hose 9 which is used to provide a finely atomized spray at the nozzles 5.

For delivery of the vapor phase catalyst from chamber 4,

a turbulence fan 10 with variable speed is arranged on a side 11 of the structure opposite the side 8 containing the air filter 7, the operation of which is controlled from a mechanism 16. The fan 10 carries the vapor phase catalyst to a necessary place - in a special case to the co-deposition apparatus, as defined above and described in the following with reference to fig. 2 - via a flexible line 12. A catalyst sensor 14 is arranged in a cover 13 placed around the fan 10. The sensor measures the concentration of vapor phase catalyst that is passed through the line and provides a concentration reading that is fed back and recorded on a meter 15.

By adjusting the atomization and evaporation of the liquid catalyst stored in the tank 3, which can be controlled within predetermined limits from the meter 15, the concentration of the vapor phase catalyst delivered from the apparatus can be monitored as needed. Also, as indicated above, the rate of delivery of catalyst can be controlled by operating the fan 10 at variable speed. In this way, preset concentrations of the catalyst can be maintained with accuracy.

In the case of molecular dissolution of the water vapor phase catalyst in air, the concentration will of course vary between zero and saturation concentration for the particular catalyst used at the current temperature. In the case of aerosol mists, this limitation does not exist.

As stated above, the apparatus according to fig. 1 use the flexible line 12 for feeding the vapor phase catalyst to the apparatus shown in fig. 2 (and generally denoted by reference number 20). This apparatus can only be used for electrostatic deposition of catalyst, or for electrostatic co-deposition of catalyst and a coating agent - typically paint.

The apparatus 20 comprises a standard electrostatic gun 21 with a barrel 22 from which the electrostatic charge comes out. At the rear end of the gun 21 there is arranged a supply line 23 which supplies and delivers the paint to the barrel 22 in the gun, the paint being carried out from here with an electrostatic charge. Power lines 24 provide the power required for the production of the electric charge in the gun 21. So far, the gun is conventional and known within the state of the art.

As indicated above, the flexible cord 12 can connect the apparatus of fig. 1 with the gun 21. The vapor phase catalyst is fed to the barrel in the electrostatic gun by means of the pressure difference which is produced by means of the fan 10 in fig.

1. In the case of the device in fig. 2, a barrel cover 25 concentrically arranged around the barrel 22 will ensure that the vapor phase catalyst is not emitted from the gun 21 before it reaches the tip 26

of the barrel. In this way, the catalyst is also charged by the electrostatic field provided at the tip of the barrel 22, and is sufficiently charged to allow the vaporized, and now charged, catalyst to deposit itself on

a grounded surface at which the gun is aimed.

If the gun is to be used in a first way - for electrostatic deposition of only the desiccant (which follows previous painting of the substrate) - the influence of the release mechanism 27 will contribute to this. If, in connection with another method of use, co-disposal is required, the gun will

a 20, which is controlled by the action of the release mechanism 27, simultaneously have paint supplied via the line 23 and vapor phase catalyst via the line 12 (and electrostatic charge via the line 24). When the trigger 27 is actuated in this operating mode, paint from the barrel 20 and catalyst from the cover 25 will be charged electrostatically at the same time. Both the paint flow and the flow and concentration of catalyst can be controlled to achieve a desired ratio in the co-deposition of paint and catalyst. It will of course be easily understood that when compo-

the nents in fig. 1 supplies the desiccant in vapor phase to the components in fig. 2, via the flexible line 12, the thus connected components will function as a single device.

The method by means of which the electrostatically charged catalyst is applied to pre-painted objects (on all surfaces of the objects) or where co-deposition of electrostatically charged catalyst and paint is carried out (on all such surfaces), results in a significant acceleration of the hardening of the paint film, which implies drying times of obvious commercial importance. The manner in which the deposition or co-deposition is carried out on all surfaces will be understood from fig. 2, which illustrates the divergent path of desiccant and application agent when said desiccant and application agent leave the apparatus.

The invention will now be described with reference to five numerical examples. In this context, the following should be noted.

The paint used in these examples is a two-component paint with white polyurethane preparation as stated earlier.

As far as the electrostatic gun is of known construction, three basic types are used, which for the sake of simplicity are hereafter indicated as I, II, and III respectively. Briefly, gun type I will perform the electrostatic deposition of paint (or other application medium) by means of a rotating disc which atomizes the paint in an electrostatic field, which is provided at the tip of a wire filament provided for this purpose (the thus charged paint is delivered to the substrate to be coated). Gun Type II provides an electrostatic field by means of a filament placed at the end of the barrel through which the paint is delivered to be carried by the electrostatic charge (the paint being delivered to and through the barrel by air assistance). Gun type III functions in essentially the same way as gun type II, but the paint to be charged electrostatically is delivered to the barrel hydraulically.

EXAMPLE 1 (sequential deposition)

A metal plate properly grounded is coated on both sides with a paint as previously described, using a manual electrostatic gun type I. Air containing approximately 2000 parts per million of diraethylethanolamine in vapor form which has been obtained from the apparatus of fig. 1, is carried at right angles to the painted plate and before two minutes have passed the vapor phase catalyst is charged using an electrostatic gun type III, no paint being supplied to the gun, while only one charge is provided. The gun is arranged opposite the plate and thus allows the charged field to cross the catalyst stream. The passage of the vapor phase catalyst and the electrostatic field from the type III gun is made continuously for approximately two minutes, after which both the catalyst and the electrostatic charge are stopped. After a further eight minutes of post-curing in slightly turbulent air, it is found that the film has come so

far with regard to drying on both sides of the plate, that a satisfactorily dried film has been obtained.

EXAMPLE 2 (sequential deposition)

Evaporated dimethylethanolamine (hereinafter referred to as DMEA) from the apparatus of fig. 1, is passed through the flexible tube 12 to the plastic barrel cover 25 around the barrel 22 of an electrostatic gun. It is constituted by the device in fig. 2, the gun in this case being an electrostatic gun type II. Painted sheets prepared in a manner as described in Example 1 above are subjected to a stream of vapor phase catalyst from the gun, as shown in fig. 2. The vapor phase catalyst is applied at a concentration of approximately 2,500 parts per million,

for approximately sixty seconds after painting. This catalyst flow is maintained for approximately two minutes, after which it is cut off and the plate is exposed to weak turbulent air for approximately eight minutes. After this post-cure period, it was found that the drying of the painted film had been markedly advanced on both sides of the plate.

EXAMPLE 3 (joint deposition)

A deposition gun as described in Example 2 and shown

on fig. 2 was set up and connected to the apparatus in fig. 1. Paint, as previously discussed, was then fed through the gun, at the same time as the vapor phase catalyst (DMEA) was fed in at a concentration of approximately 4000 parts per litre. million of DMEA

as a molecular solution in air. Properly grounded panels were painted by simultaneous application of paint and catalyst.

Under these conditions, the painted boards showed a similarly rapid attainment of touch-dry condition and total commercial dryness.

EXAMPLE 4

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

EXAMPLE 5

In this example, the vapor phase catalyst was tetraethyl. All other conditions stated in Example 3 were the same. Again, an equally marked acceleration was achieved during the hardening of the paint.

In conclusion, it must be repeated that the above detailed description is only intended to be illustrative of the invention. As long as the main criteria are observed, realities that lie within these and are not critical in themselves can be varied according to situational requirements.

Claims (15)

1. Method for forming a dried coating on a suitable substrate, comprising applying a coating agent to the substrate and subjecting the applied coating agent to treatment with a drying agent, the drying agent being electrostatically applied to the applied coating agent. 2. Method as stated in claim 1, characterized in that the steps of applying the coating agent to the substrate and electrostatically depositing the desiccant are performed either sequentially or simultaneously. 3. Method as stated in claim 1 or 2, characterized in that the desiccant carries out its drying treatment in the vapor phase. 4. Method as stated in one of claims 1-3, characterized in that the coating agent is an agent consisting of one or two parts containing free isocyanate groups. 5. Method as stated in claim 3 or 4, characterized in that the drying agent is selected from ammonium, an amine, an organometallic catalyst or any combination of the aforementioned. 6. Method as stated in claim 5, characterized in that the drying agent is an alkanolamine. 7. Method as specified in claim 3 or one of claims 4-6 in connection therewith, characterized in that the desiccant is prepared in the vapor phase by controlled atomization of a precursor liquid desiccant. 8. Method for forming a dried coating on a suitable substrate, characterized in that it comprises the steps of applying a coating agent to the substrate and exposing the coating agent to treatment with a drying agent in the vapor phase, the drying agent being electrostatically deposited on the coating agent and i) the step of applying the coating agent to the substrate precedes the step of electrostatically depositing the desiccant in the vapor phase, or ii) the step of applying the coating agent to the substrate and the step of electrostatically depositing the drying agent in the vapor phase are carried out simultaneously. 9. Method as stated in claim 8, characterized in that the coating agent is an agent comprising one or two parts containing free isocyanate groups. 10. Method as stated in claim 8 or 9, characterized in that the coating agent is electrostatically applied to the substrate. 11. Method as stated in claim 1 or 8, substantially as described here, with reference to any of the preceding examples and/or attached drawing. 12. Apparatus for use in carrying out the method as set forth in claim 1 or 8, comprising in combination means for directing an electrostatically charged coating agent, e.g. a paint, on a substrate to be coated, as well as means for simultaneously directing an electrostatically charged desiccant in vapor phase onto the substrate, the former and latter means being arranged concentrically in relation to each other. 13. Apparatus for use when carrying out the method according to claim 1 or 8, comprising in combination, means for evaporating a liquid desiccant, means for controllable delivery of the evaporated desiccant to a specific location, and sensor means which are arranged in the delivery path and can be operated to ensure that the concentration of delivered evaporated desiccant is maintained within predetermined limits. 14. Apparatus as set forth in claim 13, characterized in that the specific location is constituted by the apparatus according to claim 12, the vaporized desiccant being delivered controlled to the latter organ in the latter apparatus. 15. Apparatus as stated in one of claims 12-14, mainly as described here, with reference to figures 1 and 2 in the attached drawing.