NL193722C - Method of coating a substrate. - Google Patents

Description

1 193722

Method of coating a substrate

The invention relates to a method of coating a substrate by applying to the substrate a film of a liquid coating composition comprising a compound containing aromatic hydroxyl groups and a cross-linking agent containing mainly or completely aliphatic polyisocyanate by applying the liquid coating composition. atomizing with a gas stream comprising an inert carrier gas and directing this gas stream onto the substrate and curing the applied film by action of a catalytic amount of a vaporous tertiary amine.

Such a method is known from British patent application 2.099.723 A. More specifically, this patent application relates to a method for coating substrates with a pore-rich or imperfect surface, such as conventionally formed plate materials, etc., using liquid coating materials based on an aromatic polyhydroxyl compound and a polyvalent isocyanate compound. After applying such a coating material as a film to the substrate using conventional air spray techniques, this film is exposed to a stream of a carrier gas and a vaporous tertiary amine as a catalyst and thereby cured. In the general list, polyisocyanate compounds include both aromatic and aliphatic polyisocyanates. However, it is pointed out in this connection that it appears from the passage on page 3, line 21, as well as the examples, that the use of aromatic polyisocyanates is necessary to effect the desired rapid reaction, while aliphatic polyisocyanates are merely for the improvement of the properties of the coating such as initial color, resistance to sunlight discoloration, etc. are used. It can be deduced from the context of this British patent application 2,099,723 A that a substrate is first coated with a first coating film ("primer") based on an aromatic polyhydroxyl compound and an aromatic polyisocyanate, after which it may optionally be provided in the form of a top layer ( "top coat"), a film of an aromatic polyhydroxyl compound and a (largely or completely) aliphatic poly-isocyanate is applied, the two layers each being cured with a vaporous tertiary amine as a catalyst. Furthermore, such a coating process requires a curing chamber in order to maintain and handle the vaporous tertiary amine catalyst. Such curing chambers, when transporting the coated substrates, for example, via a conveyor belt, should be large enough to ensure adequate contact time between the film to be cured on the substrate and the tertiary amine-30 catalyst.

A process has now been found which is not faced with the above-mentioned drawback, in which the film of aromatic polyhydroxyl compound and substantially or completely aliphatic polyisocyanate compound can be cured at room temperature, no heat has to be added during curing and the concentration of the vaporous tertiary amine in the carrier gas.

The invention therefore relates to a method mentioned in the opening paragraph, which is characterized in that a. An atomizing gas stream is formed, which comprises an intimate mixture of the carrier gas and the catalytic amount of the tertiary amine in the vapor form, by using a amine vapor generator, if desired, by mixing the amine vapor with the carrier gas, 40 b. the liquid coating composition sprays with the vaporous tertiary amine-containing sputtering carrier gas stream of steps (a) and c. directs the atomized material from step (b) onto the substrate to form the applied film.

Admittedly, it is known from U.S. Pat. No. 2,823,143 published in 1958 to use an amine vapor generator to intimately mix a catalytic amount of a vaporous tertiary amine 45 in a sputtering gas stream and then the sputtering gas stream - after mixing with a coating composition - on a substrate and perform the curing at room temperature, but this patent nowhere describes the use of a coating mixture based on an aromatic polyhydroxyl compound and a polyisocyanate compound, let alone a substantially or completely aliphatic polyisocyanate compound.

Furthermore, U.S. Pat. No. 4,051,886 has a vapor generator resp. a process for vaporizing a liquid crosslinking agent under normal conditions, which is converted under pressure to a saturated vapor by means of a carrier gas under equilibrium and subsequently discharged.

With reference to the above, it is pointed out that it is not possible to use the process of the invention polyisocyanates which are substantially or completely aliphatic, which classes have hitherto not been recommended for use in vapor permeation curable coatings.

193722 2

A further advantage of the new process of the invention is the lack of significant investments required to perform the curing chamber process, that is, the equipment required for the new vapor amine catalyst spraying process has an apparatus for supplying amine, a conventional component spray gun, a conventional spray booth or cap and a conventional amine scrubber. However, the spray gun and spray booth are common and are normally found in factories that have conventional coating lines. The formulations of the coatings need not be changed except perhaps in terms of viscosity adjustment for use in the inventive method of vaporizing amine catalyst spray as described herein. Therefore, the invention can be easily adapted and carried out on paint spraying lines which are carried out in a manner customary in the art. As will become more apparent from the following description and examples, the coated parts can be handled easily after a short period following the coating, e.g. 5-15 minutes, which means that shorter lines are possible in the factory. In addition, as will be seen from the examples, when mild heating by means of propelled air on the coated substrates is used, the removal of solvent from the films will be accelerated and the curing times will be drastically reduced.

As for the liquid coating compositions that can be used in the process of the invention for spraying the vaporous amine catalyst, substantially all of the vapor permeable coating formulations can be cured according to the novel process of the invention. Vapor-curable coating formulations typically include at least one aromatic hydroxyl group-containing functional polymer or resin, a various isocyanate group curing agent of the invention, and optionally a volatile organic solvent therefor.

Several isocyanate groups-containing cross-linking agents cross-link with the aromatic hydroxyl groups of the formed adduct-masked polymer under the influence of a vaporous tertiary amine, thereby forming urethane linkages and curing the coating. For coatings with high quality properties, an initial color as well as the coloration due to sunlight can be minimized by including at least a moderate amount of aliphatic isocyanate in the curing agent. Of course, polymeric isocyanates are used to reduce toxic fumes from isocyanate monomers. Furthermore, according to the invention alcohol-modified and otherwise modified isocyanate compositions are used. Several compounds containing isocyanate groups will preferably contain about 2-4 isocyanate groups per molecule for use in the coating compositions of the invention. Suitable compounds containing various isocyanate groups which are used according to the invention are, for example, hexamethylene diisocyanate, diisocyanate (MDI), cyclohexane diisocyanate (CHDI), bis- (isocyanatomethyl) -cyclohexane (H6XDI), dicyclohexylmethane-diisethylanate (His) diisocyanate) diisocyanate (DD), dicyclohexylmethane diisocyanate and dimethyl derivatives thereof, trimethylhexamethylene diisocyanate, lysine diisocyanate and its methyl ester, isophorone diisocyanate, methylcyclohexane diisocyanate. Aliphatic polyisocyanate dimers, trimers, oligomers, polymers (including biuret and isocyanurate derivatives) and functional isocyanate-containing prepolymers are often available as preformed packages, and such packages are also suitable for the use of the present invention.

The ratio of the aromatic hydroxyl equivalents of the functional phenol-containing compound to the isocyanate equivalents of the various isocyanate-containing cross-linking agent should preferably be greater than 1: 1 and may be up to about 1: 2. The exact intended use of the coating composition will often determine this ratio or isocyanate index. At high crosslinking densities or isocyanate equivalents, harder but relatively rigid films are obtained, while at lower crosslinking densities or isocyanate equivalents, the flexibility of the films increases. The optimization of the respective property or combination of desired properties 50 can be determined by a person skilled in the art.

The solvent or carrier for the coating composition is a mixture of volatile organic solvents, which preferably contains ketones and esters to keep the viscosity of the composition as low as possible. Some aromatic solvents may be required, and in particular they are part of the volatiles in the commercially available isocyanate polymers. Usually, enough solvent is added to reduce the non-volatile content of the coating composition to about 50-80% by weight to achieve a spray suitable viscosity depending on pigmentation. It should be noted that the effective non-volatile solids content of the coating composition can be increased by incorporating a relatively low or non-volatile (high temperature boiling) ester plasticizer, which is mostly in the hardened film is retained. Suitable ester plasticizers include, for example, dibutyl phthalate, di- (2-ethylhexyl) phthalate (DOP) and the like. The amount of the ester-5 plasticizer should not exceed 5-10% by weight, otherwise loss of decay resistance may occur.

As for the quality requirements met by the coating composition, it should be noted that the coating composition, the polyol resin and the isocyanate cross-linking agent have a minimum shelf life of at least 4 hours in an open vessel and generally the shelf life is more than 8 hours and can even be 18 hours or more. Such long shelf lives mean that refilling of the vessel during use is generally not required during exchanges. In addition, the shelf life of the coating composition in a closed container is generally over a month. After storage of the coating composition, the stored composition can be cut with a suitable solvent to the viscosity suitable for the application and such a composition retains all the excellent quality properties it initially had.

Additional ingredients that can be suitably incorporated into the coating composition of the invention include color pigments, plasticizers, flatting agents, flow control agents, and a wide variety of conventional paint additives.

A coating composition (eg polyol, cross-linking agent containing various isocyanate groups and optional solvent) is suitable for the use of the present invention if it can be transported or moved through lines to the spray nozzle and atomized therefrom with a vaporizing amine catalyst-containing gas stream. Usually this is transferred into the coating composition, which is liquid. For the purposes of the invention, a liquid coating composition includes a coating composition which is liquid at room temperature, can be liquefied by heating to be sprayed, or liquefied by dispersing in a solvent to be sprayed. Any way in which the coating composition can be liquefied or liquefied for spray spraying is suitable for the use of the present invention, provided that the curing chemistry using vapor permeation is maintained.

The vaporous amine catalyst will be a tertiary amine such as, for example, triethylamine, dimethyl ethylamine, cyclohexyldimethylamine, methyl diethylamine and the like. The amount of vaporous amine catalyst can be from less than 1 wt% to more than 6 wt%. Larger amounts of amine catalyst are not recommended when air or other molecular oxygen sources are present, since explosive mixtures can be formed. The tertiary amine catalyst is in the vapor form in a carrier gas, which may be inert, such as nitrogen or carbon dioxide, or may be in air or mixtures thereof. Depending on the carrier gas and the particular tertamine catalyst selected, certain minimum temperatures and pressures of the atomizing gas stream must be maintained so that the amine catalyst remains vaporous and does not condense in any line. However, maintaining the tertamine catalyst in the vapor phase can be carried out by a skilled person.

As to the type of device required to generate the vaporous amine and release the vaporous amine into carrier gas, a large number of different amine vapor generators are manufactured in the art and most commonly used in the "cold box" process in the 45 casting industry. A representative amine vapor generator which is used in the coring industry is shown in U.S. Pat. No. 4,051,886 described above.

From the amine generator or collection vessel, the atomizing gas stream containing catalytic vaporous tertiary amine will preferably be transported through a steam or otherwise heated pipeline to the spray gun. Virtually all conventional or unusual spray guns for spraying liquid coating or paint compositions can be used in accordance with the principles of the present invention. The atomizing gas stream containing the vaporous tertiary amine will be the gas stream which will atomize the liquid coating composition through the spray gun in the usual manner. Typically, the atomizing gas stream will be heated to a temperature sufficient to ensure that the vaporous tertiary amine remains in the vapor phase. It is also possible to preheat the liquid coating composition to ensure a suitable viscosity for spraying and / or for achieving special effects. Since tertiary amine is discharged from the spray gun, a safety and environmental precaution is to operate 193722 4 the spray gun with vapor amine catalyst in a conventional paint spray canopy or paint spray booth. Such paint spraying booths are so common that a further detailed description thereof is unnecessary here. It should be noted that the spray booth material may be vented into the atmosphere or the amine may be subjected to a conventional waxing treatment, particularly using an acid such as sulfuric or phosphoric acid or other conventional means. drained.

Due to the unique intimate contact of the vaporous tertiary amine from the atomizing gas stream and the atomized liquid coating composition, the thickness of the coating composition on substrates can obtain a substantial thickness and yet be fully cured. This is in contrast to the conventional curing technique using vapor permeation using a vapor containing curing chamber in which very thin films must be cured to ensure complete diffusion of the vapor amine through the film. However, films with a thickness of about 0.25-0.38 mm or more (dry) can be successfully adjusted and cured according to the method of spraying the vaporous amine catalyst of the present invention.

The coated portion can be air-dried at the prevailing temperature and rapid curing will take place. Typically, shorter lines will be required at the factory because the coating becomes tack free in such a short time. In addition, conventional curing ovens are no longer required. However, the cure speed can be increased even more by increasing the amount of solvent that is expelled from the applied film. Such solvent expulsion may be enhanced or enhanced by a treatment which is most suitably thermal. This means that the coating applied to the substrate by spraying using a vaporous amine catalyst can be subjected to a heat treatment at low or medium temperature (e.g. about 50-150 ° C, preferably for a short period of time, e.g. about 1- 5 minutes). Of course, an increased treatment temperature means a shorter treatment time and vice versa. Such thermal treatment is carried out under conditions that are more favorable (e.g., in time and temperature) than the conditions required for heat curing an isocyanate / polyol coating, especially since no catalyst becomes catalyst during such thermal treatment added.

The invention is further illustrated by the following examples.

30

Examples

In the examples, the new vapor curing spray method utilized a DeVilbiss model MBC 510-36EX siphon spray gun (orifice 1.788 mm, rated flow rate 10-12 cm3 / min, gas consumption 3.07 l / sec at a pressure of 2 , 1 kg / cm2, fan-shaped spray pattern, DeVilbiss 35 Company, Toledo, Ohio 43692). The air supply from the spray gun was connected to a heated collection vessel, which was kept at a temperature of about 38 ° C. The collection vessel contained nitrogen with 2.7% triethylamine (TEA) catalyst vapor, and was maintained at a total pressure of about 4.2 kg / cm2.

The TEA containing nitrogen stream was generated by an amine generator consisting of a 190 L container containing 114 L of liquid TEA (38 ° C and 1.4 kg / cm2). The container was connected to a filled column (152.5 cm Koch Sulzer compact filling) with a diameter of 7.62 cm, which column was connected to a spray nozzle and a conventional mist removal device. Liquid TEA was pumped at a rate of about 3.8 l / min to the spray nozzle, which sprayed the liquid TEA down onto the fill. Nitrogen was passed through the column to a saturation of more than 95% 45 and then passed to the collection vessel. The amine generator is further described in "attorney's docket ASH 4469" by Maher L. Mansour.

Comparative spray tests were also conducted, in which the liquid coating composition was mixed with liquid triethylamine catalyst in the mixing head of a DeVilbiss model MBC 510-AV601-FX siphon spray gun with an MMBC 444 FX liquid needle (orifice 1.067 mm, rated flow-50 speed 10- 30 cm3 / min, air consumption 3.07 l / sec at a pressure of 2.1 kg / cm2). Air was supplied to the spray gun at a pressure of 2.1 kg / cm 2 and 3 wt% triethyl amine catalyst in MEK solvent was supplied at a pressure of 1.4 kg / cm 2. Using a ball valve, precise control of the supply of a catalyst solution to be investigated into the mixing head of a spray gun was possible. The mixture of liquid coating composition and catalyst solution gelled 55 in the mixing head so quickly that extreme care was required. For example, only two panels could be sprayed at a time, followed by immediate solvent flushing. Also, a blue liquid was added to the catalyst solution so that the catalyst release through the 193722 ball valve could be visually confirmed. Both spray guns were found to deliver equal consumption of coating composition used based on the visually assessed appearance of the fan-shaped spray pattern produced by each gun. Also, the amount of solvent in "pack" sprayed formulations was virtually the same.

All studies were performed on Bonderite 37 steel panels and all sprays were performed under a laboratory syringe hood with a drain. During all spray tests according to the method according to the invention, no amine odor was observed outside the spray hood by the operating personnel.

Example I

The liquid coating composition was formulated from 500 parts by weight of the aromatic hydroxyl-terminated polyester of Example 1 of U.S. Pat. Nos. 4,374,167, 4,343,839 or 4,365,039 and 350 parts by weight of isocyanate No. 1004, which is a mixture of equal weights of Mondur HC isocyanate (tetrafunctional reaction product of hexamethylene diisocyanate and 15 toluene diisocyanate, NCO content 11.5 wt%, equivalent weight 365, 60 wt% solids in Cellosolve acetate / xylene, Mobay Chemical Company, Pittsburgh, PA.) and Desmodur L-2291A isocyanate (aliphatic polyfunctional isocyanate of the hexamethylene diisocyanate biuret type, Mobay Chemical Company). The resinous mixture was cut with additional MIBK (methyl isobutyl ketone) solvent to obtain a spray viscosity of 20 sec. in a Ford cup Φ 4 (this viscosity was maintained in all 20 examples). In an open vessel, this coating composition has been found to have a shelf life of more than 48 hours.

Two panels were each coated according to the new spraying method using vaporous catalyst and the conventional spraying method using liquid catalyst. The panels were allowed to air dry at ambient temperature and then the following results were measured (see Table A).

TABLE A

Panel no Time (min) Film thickness Double 30 (10'3cm) rubs with MEK, fixed after 1 hour with print-free2) touch1) 35 Vaporous catalyst spray 1 2 6 1.3 80 2 2 5 1.8 11Ό

Liquid Catalyst Spray 40 3 4 15 1.3 22 4 3 12 1.0 13 1) Coating, removed by finger, applied to coated panel with light to medium pressure.

2) Fingerprint on cover, applied to panel with light to medium pressure.

From the results listed above it appears that the spraying method according to the invention using a vaporous catalyst yields a coating which hardens faster than the coating obtained according to the usual spraying method with a liquid catalyst. Coating lines can be shortened in companies because the coated panels can be handled shortly after coating. In addition, thermal curing is not required. After 24 hours, all coatings were able to withstand more than 500 double 50 rubs with MEK. The final properties therefore appear to be comparable.

Example II

In this example, the vapor spray catalyst coated panels were subjected to a light heat treatment after curing to increase the curing of solvent from the film. The coating composition of Example 1 (isocyanate index of 1.1: 1) was sprayed on with the following results (see Table B).

193 722 6

TABLE B

Panel No. Film thickness Post-heating Double rubs (10'3cm) with MEK after 1 hour 5 --- 1 1.3 none 68 2 1.3 1 min at 65 ° C 77 3 1.3 2 min at 66 ° C 120 4 1.3 5 min. At 66 ° C 442 10 -

The post-heating conditions are certainly insufficient in time and temperature to cure the coatings, yet these results demonstrate that the degree of heating curing is improved in such a manner. It is believed that larger amounts of solvent in the film 15 are expelled by the post-cure heat treatment. That is, improving the properties of the film. These results mean that the coating lines can be further shortened by performing the post-cure thermal treatment. After 5 minutes, the properties of the film reach their maximum value. It is noted that all panels could be handled after the heat treatment and that the air-dried (without heat) panel was print-free in the course of 5-6 minutes after coating.

Example III

The following liquid coating compositions were formulated: Formulation 1 (Comparison)

Polyol 14151) 500 parts of adipic acid 7 moles 1,4-butanediol 6 moles trimethylolpropane 2 moles 30 diphenol acetic acid 2 moles

Mondur CB-60 Isocyanate 21 445 parts by weight MIBK 90 parts by weight 1) Resin 514 in Example 1 of U.S. Patent 4,368,222.

2) Aromatic isocyanate (NCO equivalent of 10.0-11.0) compound, Mobay Chemical Company.

Wording 2

Polyol 51400-9A31 760 parts by weight dimethyl terephthalate 1 mole 40 1,4-butanediol 8 moles azelaic acid 6 moles diphenolic acid 2 moles isocyanate 1004 350 parts MIBK 180 parts 45 ---- 3) resin 120 in Example 1 of the U.S. Patent 4,374,181 with dimethyl terephthalate in place of terephthalalic acid. Wording 3

Polyol 51400-12 (4) 760 parts by weight 50 2-hydroxyethyl methacrylate 2 moles styrene 2 moles butylacrylate 4 moles 2-ethylhexyl acrylate 2 moles butyl methacrylate 4 moles 55 diphenolic acid 2 moles isocyanate 1004 350 parts 7 193722 MIBK 200 parts 4) diphenolic acid, reacted in a second stage after all other ingredients were reacted in the first stage.

5

Formulation 4 polyol 51400-12 760 parts by weight of isocyanate KL5-2444 (S) 231 parts by weight 10 MIBK 150 parts by weight 5) Isocyanate KL5-244 is an aliphatic isocyanate of hexamethylene diisocyanate (NCO content 20% by weight, 90% by weight) .% solids in Cellosolve acetate, equivalent weight of 210), Mobay Chemical Company.

15

Each of the formulations was used with the following results according to the new vapor catalyst catalyst spray method and liquid catalyst spray method (see Table C).

TABLE C

20 -—----------

Formulation No. Film thickness Fixed at Print-free Double friction with

(10'3cm) touch (min) MEK

(min) 25 1 hour 24 hours

Vaporous catalyst spray 1 1.3 9 15 500+> 1000 2 1.3 10 27 150> 500 30 3 1.0 4 6 10 55 4 1.0 20 70 6 175

Liquid Catalyst Spray 1 1.3 10 15 285> 1000 35 2 1.0 12 30 12> 500 3 1.0 5 12 25 55 4 1.0 25 90 3 40 40 Using the data in Table C, several important comments are made. In general, in the new spraying process, the coatings were more liquid to the touch with the catalyst and free of imprint, except in formulation 3 (which results are inconsistent with all other runs). The MEK rub resistance was also greater after an hour after coating the vapor catalyst catalyst spray method.

45 The most notable results, however, are those of formulation 4, which contained only aliphatic isocyanate as a cross-linking agent. General notions in the vapor permeation curing technique are that aliphatic isocyanates will not fully cure or cure so slowly in the presence of vaporous tertiary amines that their use is undesirable. However, according to the spray method of the invention with a vaporous catalyst, a remarkable hardening was achieved, as evidenced by the 50 175 rubs with MEK, 24 hours after application of the coatings. For the first time, the use of only aliphatic or substantially aliphatic multiisocyanate as cross-linking agents in vapor permeable curable coatings appears to be practically applicable. The significant differences between vaporous amine and liquid amine catalysts are evident.

55 Example IV

To demonstrate that the spray method of the invention can provide very thick cured coatings, the polyol polyester of Example I (cut to an amount of solid material of

Claims (2)

193722 8 70 wt.% In MIBK, which is used rather than Cellosolve acetate) and isocyanate 1004 crosslinker cut with MIBK until the viscosity required for spraying was obtained. The first panel was sprayed to form a film with a dry thickness of about 0.2 mm and the second panel was sprayed to form a film with a dry thickness of about 0.4 mm. The coating on both panels was solid to touch within 3 minutes and print-free within 5 minutes. (In these tests, the room was connected to the outside air and it was a dry, warm day. The relatively warm weather may have caused faster drying times compared to the thinner films in the other examples.) Each film proved manageable within 20-30 minutes and do not show contact adhesion. Within 10 hours of application, each film was fully cured and tightly adhered to the substrate. Therefore, due to the greatly anticipated skin formation of the applied film, curing throughout the thickness of the film was not suppressed and solvent extrusion from the film was not disturbed. That such thick films can be fully cured by vapor permeation curing is yet another unique advantage of the present invention. 15 1. A method of coating a substrate by applying to the substrate a film of a liquid coating composition comprising a compound containing aromatic hydroxyl groups and a crosslinking agent containing substantially or completely aliphatic polyisocyanate, by spraying the liquid coating composition with a gas stream comprising an inert carrier gas and directing this gas stream onto the substrate and curing the applied film by the action of a catalytic amount of a vaporous tertiary amine, characterized in that a sputtering gas stream is formed which intimately mixture of the carrier gas and the catalytic amount of the tertiary amine in the vapor form, using an amine vapor generator, if desired, by mixing the amine vapor with the carrier gas, b. the liquid coating composition sprays with the vaporization tertiary amine-containing sputtering carrier gas stream of steps (a) and 30c. directs the atomized material from step (b) onto the substrate to form the applied film. A method according to claim 1, characterized in that the atomized material from step (b) is directed onto the substrate to form a cured film with a thickness of 0.4 mm.