NO861861L - PROCEDURE FOR LASER HARDENING OF COATING AND COLORING MATERIAL. - Google Patents

Abstract

A method of curing polymer coatings by laser photochemistry in which UV laser irradiation of the coating containing a photoinitiator sensitive to a selected wavelength of the laser light causes a low energy curing of the coating. The total energy required to effect curing is markedly reduced by applying a series of pulses. for the laser light at low single pulse flux. A reduction of the single pulse flux at a constant pulse repetition rate or a reduction of the pulse repetition rate at a constant single pulse flux tends to further reduce the total energy required to cause hardening of the coating.

Description

The present invention relates to hardening of coatings and blacks and more particularly to laser hardening of such coatings and blacks.

Clear and pigmented coatings and blacks that used e.g. in the printing and painting industry and which can be subjected to ultraviolet curing, are increasingly used because they can be quickly cured, require little space for curing and minimize or eliminate solvents that are emitted to the environment. Ultraviolet coatings typically consist of a liquid or solid polymer and/or oligomer and/or a monomer, and a photoinitiator. A hydrogen donor such as amines or thiols and their analogues including polymer analogues is optionally included with the photoinitiator to increase the production of radicals which are used to start the polymer chains. When a photoinitiator molecule absorbs a photon of light, a radical can be formed which binds with a monomer or polymer and starts the growth of long-chain polymers and thereby causes the hardening of the coating or black. Typical UV lamp systems require a flux of Joules/cm^ or more to ensure cured coatings or films of practical thickness. While the expense of such energy leads to practical cure rates, the main benefits achieved by the flux can be reduced to allow more film to be cured with the same light energy, resulting in higher yields. The freedom to choose the best wavelength and best wavelength bandwidth is also beneficial. Furthermore, when a thick coating is produced, the surface often hardens before the body of the coating, which leads to wrinkling of the coating and poor surface adhesion.

Today, ovens are used to remove solvents and thus dry and/or polymerize conventional solvent or water-based coatings. Such ovens take up considerable space and require relatively large amounts of energy to maintain the temperature, and ventilation is also required to remove the solvent vapours. The desirability of a more uniform cure rate throughout the depth of the coating is obvious as are the benefits gained from reduced flux to effect photopolymerization. Today, UV lamps are used for clear coatings and blacks, while ozone, heat removal and unusable wavelengths for these can reduce efficiency. So little light from these lamps can penetrate a typical thick pigmented coating that it is not used commercially to any significant extent for this purpose.

The present invention proposes to improve the problems associated with known systems for curing coatings and inks by using ultraviolet laser energy specifically selected to effect curing of predetermined forms of coatings and inks.

In one aspect, the present invention consists in a method for curing a coating comprising applying the coating to a surface, the coating comprising a liquid or solid polymer and/or oligomer and/or a monomer and at least one photoinitiator, and irradiating the coating with ultraviolet laser light of at least one predetermined wavelength, where the at least one predetermined wavelength is selected so that it is preferentially absorbed by at least one photoinitiator instead of the other components of the coating. Where the coating is in the form of a pigmented coating or a black, the coating further comprises a pigment and the at least one predetermined wavelength of the laser light is selected so that it is minimally absorbed by the pigment.

A particularly preferred method according to the invention is that the laser light which is applied to the coating is pulsed. In a further embodiment, at least 25 pulses of laser light are required to effect curing. In another embodiment, at least 50 pulses are required to effect curing. Preferably, the individual pulse flux for this applied laser light is kept low and preferably < 3mJ/cm<2>. It has been found that it is advantageous that the total energy to effect curing is very significantly reduced where the single pulse flux is reduced and where the total number of pulses applied to the coating or black is increased while working at a predetermined pulse repetition rate.

A further embodiment uses two photoinitiators such that each of the two photoinitiators preferentially absorbs a different predetermined wavelength of the laser light so that one photoinitiator is selected to effect a cure through the coating and the other photoinitiator is selected to effect a cure on the surface of the coating.

Characteristically, pigmented coatings have a thickness of > 0.5 pm; clear coatings have a thickness of > than 2 pm and printing ink has a thickness of < 20 pm.

The laser curing process according to the invention requires little or no solvent in the black or the coating and thus requires a minimum of space and provides an environmentally acceptable process. Furthermore, the costs of laser curing by the method of the invention are a fraction of the costs of existing thermal processes and avoid problems in connection with air drying. Characteristic oven drying of pigmented coatings requires significant space and energy to maintain curing temperatures and ventilation options to remove solvent vapors.

By means of the present invention, pulses of laser light with fluxes as low as 0.0001 millijoules/cm^/pulse can be used to harden clear polymer coatings. A preferred flux is 0.002 millijoule/cm 2 /pulse and using 180 pulses. With this pulse, a 100 watt UV laser can cure a 1 m wide film with a thickness of 7 pm moving at a speed of 750 m^/min. This can be compared with characteristic speeds of 80 m^/min. for simple UV lamp curing (200-300 watt/tom-me lamp intensity over 1 m width) and typical speeds of 400 m2/min. for simple electron beam curing. With the help of low single pulse flux and multiple pulses, an extremely energy-efficient through-hardening of clear polymer coatings can be achieved. For low-flux curing, Beer's law can be used to select the optimum concentration of photoinitiator for the coating thickness required to achieve a cure through the thickness of the coating. With the help of the invention, one does not need to use more light than is required by the photoinitiator, as preferably no other component in the coating absorbs to a significant extent at the selected wavelength. This process leads to a markedly lower flux than is characteristically used when curing coatings and although the costs per photon is higher with a laser with a lamp, the net costs of carrying out a cure will be much lower because so many fewer photons are needed for the same purpose. Furthermore, the lower flux which is necessary to effect curing leads to a significantly higher yield of coated material to be cured, than previous ones.

In addition, the laser source can be placed further from the coating location because outgoing light is easily transmitted over long distances. Techniques to broaden the beam and make it sufficiently uniform are readily available.

As far as pigmented coatings are concerned, the present invention is used when curing commercial coatings of 20 µm and thicker with 50% by weight rutile TiC>2. In this example, laser light is used to achieve a thorough hardening of the coating by penetrating the coating at a wavelength with minimum absorption of the pigment. A photoinitiator is used which absorbs light at this wavelength and a certain concentration can be adjusted for maximum efficiency.

Advantageously, a nitrogen atom sphere is used to counteract oxygen inhibition during the curing process and very short light pulses, characteristically in the order of 20 - 40 nanoseconds, to initiate polymer growth in a particular area of the coating. The use of consecutive pulses of short duration promotes curing to completeness combined with low luminous flux. It has been found that the use of laser light in terms of the invention at 50% by volume TiC>2 concentration has led to a comparable curing of 3 mj/cm^ compared to 16 J/cm^ using a mercury H lamp and the use of laser allows room temperature curing. This means that a 100 W laser can operate a one meter wide belt moving at 50 m/min. Other pigments with absorption at shorter wavelengths in the UV range of the spectrum or in thinner coatings, such as blackners, require less energy per cm^.

Examples of components used in typical coatings which are suitable for curing according to the invention are the following.

Clear coating.

Typical UV laser-curable coatings that can be used according to the invention may consist of, but are not limited to, the following substances: 1. Reactive oligomers - a low molecular weight polymer (typically over 500 in weight average molecular weight) in one or more reactive functionalities including unsaturated reaction sites, such as a but not limited to an acrylates or acrylamides or polyesters, polyethers, alkyds, cellulosics, acrylics, epoxies, silicones, polyesters, urethanes or any combination thereof. A polyene/polythiol type system or a system such as cationic initiated types can also be used. Anionic initiated types can also be used. 2. Reactive diluents - this component can be mono-, di-, tri- or polyfunctional and will characteristically also be a low molecular weight product (typical diluents molecular weight 50-500). 3. A photoinitiator system - this component will use the incident light to induce radical or ion formation;

4. Optionally an amine or a derivative including amino acrylates, and

5. Various additives - these are usually used in conventional paint production, e.g. flow modifiers,

6. Any solvent or water.

A typical photoinitiator system could preferably include two of the following families of groups: 1. Alkyl benzoin ethers, as an example commercial Benzoin butyl ether of the type "EB3", 2. ot - Acyl oxime ester-l-phenyl-l,2-propanedione-2- (O-Ethoxycarboxy-syl)oxime, as an example commercial "Quantacure PDO",

3. Benzimidazoles such as azine-2-chloromethyl benzimidazoles,

4. Benzophenones and derivatives - Benzophenone. p-phenylbenzophenone, for example commercial "Trigonal 12", 5. Acetophenone and derivatives (cx.cx-diethoxyacetophenone) such as commercial "DEAP", 2,2-dimethyl-2-phenylacetophenone such as "Irgacure 651" or 2,2 -dimethyl-2-hydroacetophenone, such as "D1173", 6. Hydrogen donors such as amines or thiols and their analogues including polymeric and unsaturated analogues, e.g. methyl diethanolamine, 7. Ketones - Michler's Ketone, not usually used because possible carcinogenic properties, and "Darocure 1664",

8. Benzyl Ketal - Benzyl dimethyl ketal, e.g. "KB1" and "KB60",

9. Others that do not fit into the above categories - 2-methyl-1-(4-methylthiophenyl)-2-morpholinipropanone-1 and 1-benzoyl

cyclohexanol; acylphosphine oxide and analogues,

10. Thioxanthones - these would include the following: 2- and 4-chlorothioxanthones, e.g. "CTX", 2,4-diethylthioxanthone, e.g. "DETX", isopropyl thioxanthone, e.g. "ITX", 2,4-dimethylthioxanthone, e.g. "RTX"

CTX/MDEOA, DETX/MDEOA, ITX/MDEOA and RTX/MDEOA.

MDEOA can be replaced by ethyl p-dimethylaminobenzoate, EPB, and also p-dimethylaminoethylbenzoate. Also relevant i.a. are DETX analogues, related polymers and amines and aminobenzoates.

Pigmented coatings.

Typical UV-curable coatings for use according to the invention can characteristically consist of the above stated substances for clear coating but together with colored pigments such as TiC^. The photoinitiator for titanium dioxide pigment systems is characteristically CTX/MDEOA, DETX/MDEOA, ITX/- MDEOA and RTX/MDEOA. MDEOA can be replaced by ethyl-p-dimethylaminobenzoate, EPB, and also p-diemthylaminoethylbenzoate. Also relevant i.a. are DETX analogues and acylphosphine oxide and analogues, related polymers and amines and aminobenzoates.

In the case of pigmented coatings, the photoinitiator concentration can be adjusted to minimize the laser energy per cm^ at a given concentration of pigment which is necessary to completely harden the coating. Lasers used in carrying out the invention can characteristically have a bandwidth of approx. 5 nanometers or less centered on the treatment wavelength. There is a narrow optimum in terms of wavelength for each combination of pigment and photoinitiator. Doubling the wavelength - the spread or shift in wavelength of 5 nanometers to either side of the optimum will increase the energy requirement.

It has been found that the use of acylphosphine oxide as a photoinitiator is particularly advantageous and results in a hard and well-cured coating with a good and pure white color. Wavelengths for the UV laser between 400 and 450 nano meters have been successfully used as rutile Ti02 does not absorb so strongly in this range. As previously indicated, other pigments with absorption at shorter wavelengths in the UV range of the spectrum or thinner coatings such as inks will require less energy per cm^ on curing. The corollary is that photoinitiators are preferred where they absorb at these wavelengths where the pigments used have the least absorption.

Advantages of the invention shall be described in more detail with reference to the accompanying drawings in which: Fig. 1 is a diagram showing the comparison between a laser curing according to the invention and what is possible with a conventional mercury lamp for a special photoinitiator; Fig. 2 is a diagram of the energy for curing a clear coating versus changes in photoinitiator concentration for two different coating thicknesses; Fig. 3 is an energy diagram for curing a clear coating compared to a single pulse flux to achieve the same cure with pulses applied at a rate of 250/sec.; Fig. 4 is an energy diagram for curing a clear coating compared to single pulse flux at two different pulse repetition rates; Fig. 5 is a diagram of the transmission spectra of TiO 2 in a pigmented coating for two different concentrations of TiC 2 ; Fig. 6 is a diagram of the energy to cure a pigmented coating versus the laser wavelength to effect the cure; Fig. 7 is an energy diagram for curing a pigmented coating compared to laser wavelengths where the photoinitiator differs from that of Fig. 6; Fig. 8 is a diagram showing how the changes in concentration in two solvents in a dye laser can cause changes in the laser wavelength; Fig. 9 is an energy diagram for curing a pigmented coating compared to the thickness of the coating for two different pigment concentrations; Fig. 10 is an energy diagram for curing a pigmented coating compared to single pulse flux with pulses applied at a rate of 250/sec.; Fig. 11 is an energy diagram for curing a pigmented coating versus laser pulse repetition rate at constant wavelength; and Fig. 12 is a diagram of optimal concentration of a photoinitiator compared to the illumination intensity of the laser to effect a curing of a pigmented coating.

Lasers that are characteristically suitable in carrying out the invention are noble gas halide (excimer) lasers that have achieved marked improvements in reliability and mean power since their development in 1976. Lasers of interest include those mentioned in "Solvent Dependent Characteristics of XeCl Pumped UV Dye Lasers" by Cassard, Corkum and Alcook, Appl. Phys. 25, 17-22 (1981).

The basic principle for laser curing of coatings and blacks according to the invention is based on the fact that the laser light is essentially monochromatic so that all the light can be used for useful work. The photoinitiator is chosen to have its maximum activity at the laser wavelength. In Fig. 1, the absorption curve for the photoinitiator trigonal 12 is plotted with the line showing 100% of the laser light at 308 nm. In contrast, the mercury lamps in use today for curing clear coatings have many lines of which only a few do useful work. For example in Fig. 1, emission at wavelengths above 350 nm is poorly absorbed by the photoinitiator and emissions below 270 do not penetrate most of the coating. The remaining emission around 300 nm comprises only 10% of the lamp's output light in the ultraviolet range.

Fig. 2 shows that very thick (50 - 150 um) clear coatings can be hardened by the laser method. Total laser energy required for curing reaches an optimum for a given concentration of the photoinitiator, DETX. Here and in the following figures, the coatings were illuminated by an expanded laser beam in a nitrogen atmosphere until an excellent (commercially acceptable) cure was achieved.

The diagram in Fig. 3 shows that a lower flux (a more expanded laser beam) leads to a lower energy requirement for the curing. The lowest permissible flux is given for the largest practical lighting area. This in turn determines the lowest possible total flux and thus the highest "belt speed" at which the material can be hardened. The slope of these data is well explained by the radical recombination theory. Fig. 4 shows that for a given single pulse flux it is found that a higher pulse repetition rate reduces the performance. By going to a lower single pulse flux, some of this loss can be recovered. For maximum tape speed, the laser operates at its maximum repetition rate. Fig. 5 shows that ultraviolet curing of coatings containing TiC>2 (i.e. opaque paints) must take place at longer wavelengths than the absorption threshold for Ti02 (here shown at 400 - 410 nm). At shorter wavelengths, all light is absorbed before it reaches the bottom of the coating. The theoretical cure rate can be considered proportional to the intensity of the light reaching the bottom of the coating.

The experimental curve in Fig. 6 for energy for curing versus wavelength shows a very sharp optimum for the photoinitiator DETX. The illumination wavelength should preferably be 422 ± 5 nm. There was 50% rutile in a 20 µm coating, giving a sufficiently opaque paint for commercial use.

Fig. 7 shows that for another photoinitiator, acylphosphine oxide, there is a correspondingly narrow curing range, 413 ± 3 nanometers. At present, only one color laser has this narrowband tunable irradiation needed to cure these pigmented coatings. The color laser output was measured for Popop color in cyclohexane/dioxane.

Using different solvents, a Popop dye can be made to laser at many wavelengths (see Fig. 8) including those optimal for DETX pigmented and acylphosphine oxide (APO) pigmented coatings with Ti02.

By using different dyes, the dye laser can be brought to position from 330 to 1000 nm with approx. 25 watts of energy (assuming an input beam of 100 watts at 308 nm).

Fig. 9 shows that the dye laser method is able to harden thick, up to 60

pm, pigmented coatings with different fillings of TiC^.

In the same way as with clear coatings, if you go for lower intensity lighting of pigmented coatings, you achieve a higher curing speed (less total energy to cure and more efficient use of light). In the diagram i

Fig. 10 two laser wavelengths were used, 308 and 415 nm. The 308 nm light increases the surface curing rate.

As shown in Fig. 11, higher repetition rates of the laser lead to less efficient curing for pigmented coatings (as with the clear ones).

The optimal photoinitiator, e.g. acylphosphine oxide, concentration changes with illumination intensity as shown in the diagram in Fig. 12. Fortunately, low flux is associated with both higher cure rate and lower photoinitiator concentration, which means lower costs.

Examples of hardening achieved by working according to the invention will now be described in connection with blacks, clear coatings and pigmented coatings.

Blacks.

Blacks used in these examples are commercially available under the name "Viodri", their exact composition is unknown. However, the manufacturer's literature says that they contain polyfunctional acrylic esters and photoinitiators such as benzophenone.

The inks were spread out with a rubber pressure roller on cardboard to a thickness estimated at 3-5 µm. They were cured quickly to reduce penetration into the cardboard. Curing took place under a nitrogen atom sphere and illumination with excimer laser light at 308 nm, 250 pulses/sec. for red and yellow blacks and 415 nm for black and blue blacks, and a single pulse flux of 1 mj/cm^. A scratch test was used to determine successful curing and the minimum total energy for curing was noted.

Examples:

(Coat-type ink - total energy for curing in mj/cm^)

1) LUV 308 red - 18

2) LUV 307 yellow - 25

3) LUV 309 blue - 160

4) LUV 311 black - 350

Clear coatings and varnishes.

Clear varnishes in these examples consisted of epoxy acrylate and HDDA (hexanediol diacrylate) in the ratio 60:40. Into this base, a photoinitiator was mixed at a level of 0.5 - 3% together with an amine (N-methyldiethanolamine) in the same concentration as the photoinitiator. The coatings were spread on cardboard with a wire-wound rod to a thickness of less than 50 µm or between layers of tape for the 150 µm thickness. The coatings were cured under a nitrogen atmosphere with low flux illumination directly from an excimer laser operating at 308 nm (Lambda Physik 150 EMG 203) or 351 nm (Lambda Physik 150 ETS). A scrap test was used to determine successful curing and the minimum total energy to cure was noted.

1. Very thick layer

Photoinitiator - DETX in a concentration of 0.5%. Thickness 150 pm. Single pulse flux 1.3 mj/cm^. Laser wavelength 351 nm. Repetition rate 1 pulse/second. Minimum total energy for curing 20 mj/cm^.

2. Thick layer

Photoinitiator - Irgacure 651 in a concentration of 0.5%. Thickness 35 pm. Single pulse flux 0.008 mj/cm^. Laser wavelength 308 nm. Repetition rate 250 pulses/second. Minimum total energy for curing 4 mj/cm^.

3. Thin layer

Photoinitiator - Trignoal 12 in a concentration of 3%. Thickness

35 p.m. Single pulse flux 0.0023 mj/cm^. Laser wavelength 308 nm. Repetition rate 250 pulses/second. Minimum total energy for curing 0.41 mj/cm^. For a 100 watt laser this can be extrapolated to a belt speed of 1460 m^/minute for a thin layer coating.

4. Thick layer

Photoinitiator - Trigonal 12 in a concentration of 0.75%. Thickness 35 um. Single pulse flux 0.0008 mj/cm^. Laser wavelength 308 nm. Repetition rate 250 pulses/second. Minimum total energy for curing 2 mj/cm^. For this 100 watt laser, this can be extrapolated to a belt speed of 300 m^/minute for a thick layer coating.

White paints.

The paints in these examples consisted of the same formulations as the clear varnishes above with the addition of a pigment or a masking agent (always rutile Ti02 particles ground into the varnish). These paints were spread in the same way as the clear varnishes above on cardboard or primed zinc aluminum metal sheets and cured under nitrogen. To minimize absorption into the TiO 2 , the laser illumination was from a dye laser pumped by the excimer laser at 380 nm and emitting at 410 to 430 nm. A scratch test was used to determine successful curing and the minimum total energy to cure was noted.

1) Very thick layer on paper

Photoinitiator - acylphosphine oxide in a concentration of 3%. Thickness 60 um. Pigment filling 32% by weight TiC^. Single pulse flux 0.023 mj/cm^. Laser wavelength 414.5 nm. Repetition rate 250 pulses/second. Minimum total energy for curing 35 mj/cm^.

2) Thick layer on paper

Photoinitiator - DETX in a concentration aqv 2%. Thickness 20 pm. Pigment filling 50% by weight TiC^. Single pulse flux 0.05 mj/cm^. Laser wavelength 422 nm. Repetition rate 250 pulses/second. Minimum total energy for curing 27 mj/cm^.

3) Thick layer on paper

Photoinitiator - acylphosphine oxide in a concentration of 4% and

Trigonal 12 in a concentration of 3%. Thickness 35 um. Pigment filling 32% by weight Ti02. Dual wavelength curing: 308 nm at 0.0015 mj/cm^ and 415 nm at 0.003 mj/cm^. Repetition rate 250 pulse/second. Minimum total laser energy for curing 2.6 mj/cm^. When using the present 25% efficient laser, this corresponds to a belt speed of 50 m^/min. for a thick pigmented coating.

4) Thick layer on metal

Photoinitiator - acylphosphine oxide in a concentration of 2% and Trigonal 12 in a concentration of 2%. Thickness 28 um. Pigment filling 32% by weight Ti02. Dual wavelength curing: 308 nm at 1.39 mj/cm^ and 414.5 nm at 0.03 mj/cm^. Repetition rate 250 pulses/second. Minimum total laser energy for curing 30 mj/cm^. This is a belt speed of 50 m^/min. for a thick pigmented coating.

Examples of coating formulations and their application method and curing shall be described below.

Example 1.

A typical mixture of components for a clear coating in terms of the invention comprises the following in % by weight: 56.4% epoxy acrylate oligomer (51-650 of A.C. Hatrick Chemicals Pty. Limited (A.C.H.)); 37.6% H.D.O.D.A. hexane diol diacrylate (A.C.H.); 3% Trigonal 12 phenyl benzophenone (AKZO Chemicals); and 3% MDEOA methyl diethanol amine (Union Carbide).

This mixture was applied wet to a thickness of 7 µm by draw-down squeegees and cured under a blanket of nitrogen with a single pulse flux of 0.0065 mj/m 2 . The total energy required for curing was 0.9 mj/m^ and the state of cure was determined by measuring the hardness of the coating via a scratch test.

The substrate on which the coating was applied was high quality low porosity white paper. The wet test coating had dimensions of 10 cm x 5 cm and was irradiated with a laser beam of dimensions 15 cm x 15 cm. The result of this static test, extrapolated to a dynamic environment, showed a belt moving at 667 m/minute where the belt was 1 m wide and the illumination area was 1x3 m^.

Example II.

This example also relates to a clear coating formed from the following components in % by weight of the total mixture: 56.4% Celrad 3700 epoxy acrylate oligomer (Celanese Corporation); 37.6% HD ODA (A.C.H.); 3% Trigonal 12 (AKZO Chemicals); and 3% MDEOA (Union Carbide). The wet coating was applied to a thickness of 5 µm with a pull-down squeegee and the same substrate was used as in Ex. In but single pulse the flux applied was 0.00015 millijoules/cm^ and the total energy for curing was 0.29 mj/cm^. The equivalent belt speed in this case is 2069 m/minute for the same illumination area as in Ex. IN.

Example III.

A pigmented coating was produced by curing a mixture of the following ingredients combined in % by weight as follows: 61% of a 60/40 mixture of epoxy acrylate oligomer (ACH 51-650) and H.D.O.D.A.

(A.C.H.); 32% TiO 2 RHD 2 (I.C.I); 3% Trigonal 12; and 4% acyl phosphine oxide

(BASF).

The same application conditions were used as in examples I and II to a wet thickness for the coating of 35 µm. Two radiation wavelengths were used, the first was 308 nm at a single pulse flux of 0.0015 mj/cm 2 and the second was 415 nm at a single pulse flux of 0.0025 mj/cm 2 . The total energy for curing was 1.4 mj/cm 2 for the 308 nm irradiation and 2.6 mj/cm 2 for the 415 nm irradiation.

The equivalent belt speed in this case using an illuminated area of 1 x 3 cm^ is 50 m^/minute.

The advantages of the present invention are utilized in particular in connection with the application of coatings to paper, timber, furniture, metal boxes, flat metal sheets, steel and aluminum windings, interiors and exteriors of cars, component parts, plastic parts, etc.

Claims (12)

1. Method for curing a coating comprising applying the coating to a surface in which the coating comprises a liquid or solid polymer and/or oligomer and/or a monomer and at least one photoinitiator, and irradiating the coating with ultraviolet laser light with at least one prior determined wavelength, characterized in that the light energy for the at least one predetermined wavelength is selected so that it is preferentially absorbed by the at least one photoinitiator instead of the other components in the coating. 2. Method according to claim 1, characterized in that the at least one predetermined wavelength is a peak wavelength. 3. Method according to claim 1 or 2, characterized in that the laser light at the at least one predetermined wavelength is applied to the coating by pulsing. 4. Method according to claim 3, characterized in that at least 25 pulses are required to effect hardening. 5. Method according to claim 3, characterized in that at least 50 pulses are required to effect hardening. 6. Method according to any one of claims 3 to 5, characterized in that the single pulse flux is less than 3 mj/cm^. 7. Method according to claim 3, characterized in that the pulse repetition rate is 250 Hz or less. 8. Method according to any one of claims 3-7, characterized in that a reduction of the single pulse flux at a constant pulse repetition rate reduces the total energy required to harden the coating. 9. Method according to any one of claims 3-7, characterized in that a reduction of the pulse repetition rate at a constant single pulse flux reduces the total energy required to harden the coating. 10. Method according to any one of the preceding claims where the coating comprises a pigment, characterized in that at least one predetermined wavelength of the laser light is selected so that it is minimally absorbed by the pigment. 11. Method according to any one of the preceding claims, characterized in that the coating comprises at least two photoinitiators. 12. Method according to claim 11, characterized in that the first photoinitiator preferably absorbs the first wavelength of the laser light and that the second photoinitiator preferably absorbs a second wavelength of the laser light so that said one photoinitiator is selected to effect hardening through the coating and the other is selected to effect a hardening on the surface of the coating.