Process For Hardening Coatings With Lasers Emitting Infra-red Radiation

Abstract

This invention is directed to a process for hardening or curing conventional protective coatings applied to a suitable substrate utilizing lasers emitting infra-red radiation with a wave length of from 1-100.mu. and preferably 3-25.mu.. The coatings are hardened within a few minutes of treatment without discoloration normally associated with short curing durations.

Description

FIELD OF THE INVENTION

The present invention is concerned with the curing of protective coatings. More particularly, it is concerned with a process for hardening coatings with lasers emitting infra-red radiation. Hardening or curing as used herein embraces the physical and/or chemical formation of films of protective coatings on substrates from a solution or dispersion of a paint, varnish or the like. Laser (light amplification by stimulated emission of radiation) is a source of light or radiation emitting monofrequency radiation (monocolored light) in optimally parallel form.

The heat sources utilized in hardening coatings can be convection ovens or thermical infra-red radiators. The heat transport by convection is effected in most cases by the air which is heated with heating rods or the like, and which will transfer the heat to the object by natural or artificial flow. Infra-red radiators are advantageous over convection ovens in that a part of the heat energy is directly absorbed by the coating and not by the air. From the start of the curing,the coating will harden not only on the surface -- as is the case in convection ovens -- but throughout the coating. Solvents and/or eventual decomposition products are released completely, i.e. evaporate or volatilize. Deficiencies of the surface of the cured film are,therefore,minimized.

There are two groups of thermal radiators, light infra-red radiators and dark infra-red radiators. The light infra-red radiators emit radiation of a wave length of from 0.5- 4.0.mu. with a maximum emission at a wave length of 1-2.mu.. Such radiators have disadvantages in that the solvents and decomposition products released during the hardening are highly absorptive of the short infra-red wave lengths of the spectrum, and thus the major portion of the energy is lost in the heating of the ambient, and the advantages of hardening by infra-red radiation -- uniform hardening of the entire coat without deficiencies of the surface, etc., -- are realized only to a minor extent. Furthermore, the absorption of the coating in the near infra-red region is dependent upon the binding agent and the pigments, so that different color shades will not harden uniformly. The dark infra-red radiators emit infra-red radiation of longer wave length with maximum wave lengths between 3-5.mu.. Solvents and decomposition products absorb these longer waves to a minor extent. The absorption of the longer waves by the coating is substantially independent from the type of the binding agent and the pigments. However, the power per surface unit of the dark radiator is much lower on account of the lower temperature and thus radiators with greater surfaces are required. Since the radiation power emitted by a plane radiator cannot be concentrated at random, the radiation flux density obtainable with dark infra-red radiators is relatively small. The hardening times are therefore considerably longer than with light infra-red radiators.

It has now been found that with a laser radiator emitting infra-red radiation, the hardening time required with dark infra-red radiators can be shortened considerably. The essence of the invention is the fact that coatings are hardened with laser emitting infra-red monofrequency and parallel radiation with a small beam diameter. Thus irradiation with great density and high local temperatures in the irradiated coating and the substrate, respectively, are possible, which results in very short hardening times. The hardened films have no deficiencies. Despite the high temperatures the films are not discolored due to the short hardening times. Thus, the invention is characterized in that coatings based on condensation resins or polymerization resins are hardened with lasers emitting infra-red radiation with a wave length of from 1-100.mu., preferably 3-25.mu..

The radiation emitted by lasers within the range selected according to this invention will be absorbed by the coating, independent from the type of binding agent, or pigments, etc. Thus, laser hardening combines the advantage of high density (higher than that of light infra-red radiators) with the advantage of an infra-red emission frequency which is, in comparison to dark infra-red radiators, still more distant from the visible range. The hardening times required with this source of infra-red radiation lie between 30 seconds and a few minutes. All chemical and/or physical film-forming coating materials can be hardened with lasers, e.g. polycondensation resins, polymerization resins, sinterable synthetic resin powders, polymers, etc., which are optionally present as solutions and can form films by releasing the solvents. The coatings can be hardened on metals, stone materials, glass, wood, rubber, synthetic materials, etc. However, the hardening process of the invention is particularly suitable for painting or coating small parts, e.g. parts of precision instruments, and also for the refinishing of coatings, since the heating by radiation can be localized very exactly.

The laser beam can be diverged by optical elements such as mirrors, lenses, etc. With relatively simple means, e.g. a motile mirror, it is possible to periodically pass the undiverged beam with a determined speed over the surface to be irradiated ("scanning"). In this way, coatings on large objects can be hardened. The high power CO.sub.2 lasers available up to now, however, have a far field pattern with multimode structure, i.e. the distribution of the radiation intensity is not uniform within the beam diameter. Furthermore, the distribution varies during the action. With the above-mentioned optical measures alone, such as beam diversion, beam division, combination, etc., it is not possible to obtain an optimal uniformity of the irradiation of an area. An optimally uniform hardening of coatings is obtained, if the laser beam is diverged by optical elements and the diverged beam is passed over the area to be scanned, directly or indirectly over an element in the beam path. Instead of the periodic movement of the beam, the object or objects and beam may be moved. The higher the frequency of the movement periods, the more similar is the effect to that of a constant irradiation. It is important that in the average of time each point of the areas receives the same amount of irradiation.

The most suitable method of moving the beam relative to the object or vice versa, is dependent upon the requirements of a particular application. However, the following methods are representative:

a. The laser beam is diverged in two dimensions with a concave mirror or with a focusing lens. The direction of the beam is deflected by suitable movement of the optical elements. The movement can be an eccentric rotation or a periodic swinging. In the latter case, in order to cause a uniform irradiation, the periodic swinging must be carried out with constant velocity (with the exception of the turning points) and the deflection angle should be a linear function of time. The deflection of the beam can also be effected by a moving plane mirror.

b. The laser beam is diverged in one dimension by a cylindrical concave mirror or by a cylindrical focusing or cylindrical diverging lens (diversion to a strip). Through a swinging movement (linear function of time as above) of the diverging elements, the beam is periodically deflected vertically. Also in this case the deflection may be effected with an additional plane mirror.

c. The laser beam is not diverged but is periodically deflected in two dimensions by means of one or two plane mirrors. The duration of the periods may be equal or different. In an extreme case, the movement in one of the two dimensions can be without return, i.e. the area is scanned periodically only in one direction, and only once in the other direction (linear scanning).

With method (a) the inhomogeneous intensity of the laser beam is compensated in restricted areas only. The decreasing intensity towards the edges of the beam cannot be compensated and will result in a relatively broad marginal area getting reduced irradiation intensity. Methods (b) and (c) have an advantage over (a) in that the "exposure" of the area is dependent with method (b) partly and with method (c) exclusively upon the relative movement between laser beam and object and can thus be controlled better. It is evident that in all cases the movements of beam and/or object can be combined. For hardening coatings on objects of complicated shape and/or with other than plane surfaces, the elements and movements are varied in known manner so that a uniform irradiation is obtained. The following Examples are being set forth to illustrate the invention. However, they are not to be construed as limiting its scope.

EXAMPLE 1

a. A paint is applied to a steel panel 0.9 mm thick with a film applying blade to give a wet film thickness of 0.150 mm. The film is allowed to dry for 5 minutes at room temperature and is then hardened with a CO.sub.2 laser at a wave length of 10.6.mu., a maximum power of 100 watt, and a beam diameter of 1,5 cm.

The Paint Composition is as follows:

binding agent:

80 parts by weight (p.b.w.) of commonly available stoving alkyd resin based on dehydrated castor oil fatty acid, about 34 percent fatty acid content, 60 percent in xylol/butanol

20 p.b.w. available melamine-formaldehyde resin, non-plasticized, partially etherified with butanol, 60 percent in butanol

pigmentation: total binding agent (solid resin) :

Ti O.sub.2 (Rutile type) = 1 : 1

The same paint was applied and hardened in an analogous manner, to:

wood, 1 cm thickness,

glass, 0.5 cm

asbestos cement, 0.7 cm

polypropylene, 0.3 cm

Irradiation time and obtained film hardness is shown in Table I.

EXAMPLES 2-6

Analogous to Example 1 five different paints were applied to a steel panel and hardened. The results are shown in Table II and are compared with the stoving schedules required in convection ovens to obtain equal film hardness.

Paint (a) comprises:

binding agent:

80 p.b.w. available saturated polyester resin based on dicarboxylic acids and polyhydric alcohols (so-called oil-free alkyd resin), 60 percent in xylol/butanol

20 p.b.w. available non-plasticized melamine-formaldehyde resin, partially etherified with butanol, 60 percent in butanol

pigmentation: total binding agent (solid resin): Ti O.sub.2 (Rutile type) = 1 : 1

Paint (b) comprises:

binding agent:

80 p.b.w. available heat-hardenable acrylic resin,

50 percent in xylol/butanol

20 p.b.w. available melamine-formaldehyde resin, partially etherified with butanol, 50 percent butanol

pigmentation: total binding agent (solid resin): Ti O.sub.2 (Rutile type) = 1 : 1

Paint (c) comprises:

binding agent: available heat cross-linking acrylic resin, 50 percent xylol/butanol

pigmentation: total binding agent (solid resin): Ti O.sub.2 (Rutile type) = 1 : 1

Paint (d) comprises:

clear varnish

70 p.b.w. available, relatively low molecular epoxide resin, 50 percent isobutylketone

30 p.b.w. available non-plasticized etherified phenolic resin, 50 percent xylol/butanol/isobutylketone

Paint (e) comprises:

dipping primer, dark-grey, 67 percent in water and alcohols

binding agent: available water soluble, plasticized phenolic type stoving resin

pigmentation: binding agent (solid resin): pigments and fillers, 1 : 1.8.

TABLE I

Substrate Laser Radiation Pencil Stoving Schedule Power/watt Time Hardness Convection Oven* Paint according to Example 1 on: steel 30 45 sec 2 H C wood 10 180 sec H glass 30 40 sec 2 H C asbestos cement 30 60 sec 2 H polypropylene 10 150 sec H *to provide equivalent hardness

TABLE II

Substrate Laser Radiation Pencil Stoving Schedule Power/watt Time Hardness Convection Oven* Example 2, paint a) on steel 30 90 sec 4 H C Example 3, paint b) on steel 30 75 sec 4 H C Example 4, paint c) on steel 30 180 sec 3 H C Example 5, paint d) on steel 30 150 sec 3 H C Example 6, paint e) on steel 30 240 sec 2 H C *to provide equivalent hardness

EXAMPLE 7

A pigmented paint is applied to a steel panel of 0.9 mm with a rotospinner, type 334/II of Messrs. Erichsen, to give a wet film thickness of 0.09 mm, and is allowed to dry at room temperature for 15 minutes and is then hardened with a laser beam. The laser beam is diverged by means of a convex mirror of 11.8 cm radius. The diverged beam is deflected to the predried film over a periodically swinging plane mirror. If the plane mirror is not moved, the beam will reach the panel such that an elliptic area with only the semi-axis of 2 cm and 4 cm being hardened. By swinging the plane mirror an area of 8 .times. 10 cm is hardened. In this arrangement this means a deflection of the axis of the diverged beam by about 6.degree.. (The deflection angle should be a linear function of time.) The swinging has a frequency of about 1 Hertz. The pigmented paint consists of 80 parts by weight of a commonly available stoving alkyd resin based on dehydrated castor oil fatty acid, about 34 percent fatty acid content, 60 percent in xylol/butanol, and 20 p.b.w. of an available non-plasticized melamine-formaldehyde-resin, partially etherified with butanol, as binding agent. Pigmentation: total binding agent (solid resin) : Ti O.sub.2 (Rutile type) 1 : 1. Laser: CO.sub.2 laser, type LG 106, Siemens A.G., Munich, power 100 watt, beam diameter 1.5 cm.

Laser Powermeter: Model 201 Broad Band CW Laser Powermeter of Coherent Radiation Laboratories, Palo Alto, Calif., USA.

A comparison of Radiation Time and Stoving Conditions in Convection Ovens to obtain Equal Film Hardness is as follows:

sec Watt/cm.sup.2 Radiation Pendulum Convection oven *) Time sec hardness 100.degree.C Minutes Persoz**) 120.degree.C 135.degree.C 2.2 - 2.4 120 210 50 20 18 2.1 - 2.7 150 230 60 25 23 2.1 - 2.3 180 236 60 25 23 2.0 - 2.3 195 241 70 30 25 2.2 - 2.7 210 242 70 30 25 2.1 - 2.4 240 255 90 35 30 *) measured power density W/cm.sup.2 of the diverged laser beam under the conditions of hardening **) 0.4 .mu. dry film thickness

All films cured in the oven were yellowed to a relatively great extent, whereas the films cured with lasers were free of yellowing.

EXAMPLE 8

A paint coating according to Example 1 is hardened with the following laser arrangement: The laser beam is diverged by means of a cylindrical mirror with a radius of 14.7 cm. The beam is directed to the paint film. The cylindrical mirror is swung periodically. If the mirror is not moved, the beam will harden an area of 5 .times. 1 cm. This area is increased to 5 .times. 9.5 cm by swinging the mirror. The axis of the beam in this arrangement is deflected by about 12.degree.. The swinging has a frequency of about 0.25 Hertz. The deflection angle should be a linear function of time. The results are as follows:

measured power density of the diverged laser beam irradiation time pendulum hardness Watt/cm.sup.2 sec Persoz, sec 10 - 11 60 111 10 - 11 70 155 10 - 11 80 194 10 - 11 90 211

EXAMPLE 9

The pigmented paint of Example 1 is hardened with the following laser arrangement. The coherent laser beam is directed to the dried film over a periodically swinging plane mirror. The coated object is moved forward vertically to the plane of the scanning laser beam. The scanning breadth is 9 cm, which corresponds to a deflection of the axis of the laser beam of about 16.degree.. The deflection angle should be a linear function of time. The swinging mirror has a frequency of about 0.25 Hertz. The feed to the scanning beam is 0.3 - 0.5 mm/sec. The results are as follows:

measured power irradiation pendulum density of the time hardness feed laser beam Persoz Watt/cm.sup.2 sec sec 39 -- 43 30 115 0.5 mm/sec 38 -- 43 40 187 0.37 mm/sec 38 -- 43 50 201 0.3 mm/sec

The power of the laser was measured with the laser powermeter during the hardening of the coatings.

It should be appreciated that the present invention is not to be construed as being limited by the illustrative embodiments. It is possible to produce still other embodiments without departing from the inventive concept herein disclosed. Such embodiments are within the ability of one skilled in the art.

Claims

It is claimed:

1. The method of forming a protective coating on a substrate comprising the steps of

1. providing a substrate coated with a coating composition in the unhardened state but which is capable of hardening to a thermoset state when exposed to infra-red radiation;

2. periodically and controllably scanning the entire surface of said coated substrate with an infra-red laser beam in a dosage effective to harden said unhardened coating composition by thermic action, the wavelengths of the radiation being in the range of from about 1 to 100 .mu.; and

3. discontinuing said scanning when said coating on said substrate is hardened to the thermoset state.

2. The method according to claim 1 wherein said scanning is with a diverged infra-red laser beam, said diverging being performed by directing said beam through an optical element.

3. The method according to claim 1 wherein said scanning is with an undiverged laser beam.

4. The method according to claim 3 wherein said scanning is accomplished by moving the laser.

5. The method of claim 3 wherein said scanning is accomplished by moving optical elements to deflect the laser beam.

6. The method according to claim 1 wherein the scanning is accomplished by periodically moving the protective coating being irradiated relative to the laser beam.

7. The method according to claim 1 wherein said scanning is accomplished by moving both said beam and protective coating being irradiated.

8. The method according to claim 1 wherein said laser beam is provided from a CO.sub.2 laser.