ES2784730T3 - Procedure for adjusting the amplitude and frequency of micro-folding in the photochemical matte finish of radiation curable coatings - Google Patents

Abstract

Procedure for adjusting the amplitude and frequency of micro-folding in the photochemical matte finish of radiation-curable coatings, in which, based on acrylate / methacrylate formulations, the coating is irradiated with monochromatic short-wave radiation from a mercury radiator pressure with emission lines at 185 and 254 nm and doses of 15 to 600 mJ / cm2 or, based on photoinitiator-free acrylate / methacrylate formulations, with monochromatic radiation of a wavelength of 185 nm with doses of 100 to 1500 mJ / cm2, after which, it is microstructured by means of an excimer radiator with emission lines in the range of 172 to 222 nm by irradiation with doses of 0.5 to 10 mJ / cm2 and then the Microstructured coating is cured by UV or electron irradiation.

Description

DESCRIPTION

Procedure for adjusting the amplitude and frequency of micro-folding in the photochemical matte finish of radiation curable coatings

The invention relates to a method for establishing fold amplitudes and frequencies in the photochemical matte finish of radiation-curable surfaces.

The effect of photochemical micro-folding of radiation curable coatings by absorbing high energy UV radiation, as generated in the wavelength range 160 to 230 nm by, for example, xenon excimer radiators is known. or KrCl and is described in detail in the literature (SCHUBERT ET AL., FARBE LACK, vol. 117/5, 2011, pages 21 FF, BAUER ET AL., PROGRESS IN ORG. COATINGS, vol. 64, 2009, pages 474 - 481, SCHUBERT ET AL., SURFACE & COATINGS TECHNOLOGY, vol.

203, 2009, pages 3734-3740) and is used for matte finish of radiation curable coatings.

In this way, deep matte finish surfaces can be generated without the use of chemical matte finishing agents.

The micro-folding itself is characterized topographically by measurement quantities such as folding amplitudes and folding frequencies. Both quantities determine the degree of gloss and the haptic appearance of the microstructured surface.

This relationship is shown, for example, in German patent application publication DE 102006042063 A1. The topographic structure of the micro-fold is determined by the type of excimer radiator used, the radiation dose, the coating formulation, the time that elapses between the preparation of the micro-fold and the final cure, and by the coating properties such as viscosity and temperature.

The viscosity of the coating is particularly important in establishing the structure of the micro-fold. This is determined by the formulation and generally conforms to the coating procedure used. For this reason, it cannot be adapted, or only to a limited extent, to the requirements of photochemical micro-folding.

EP 2794 126 A1 discloses a process for preparing matte finish surfaces that are generated by micro-folding of acrylate / methacrylate formulations by irradiation. In such a case, the applied formulation is first previously irradiated by means of a medium pressure Hg lamp, doped with Ga, which emits a wavelength between 200 nm and 420 nm. The coating obtained is then irradiated with an excimer lamp with a wavelength of 172 nm, which generates a micro-folding. The coating is then cured with UV radiation. It is disclosed that homogeneous matt finish coatings can be prepared if the radiation-curable coating is irradiated in an earlier stage of micro-folding with long-wave polychromatic UV radiation by means of, for example, gas-doped Hg medium pressure lamps. This step in the procedure is also known as "pre-gelation."

Physically, this pre-gelling is incomplete UV curing of the coating. Here only a part, typically less than 50%, of the originally existing acrylate double bonds is converted. Long-wave polychromatic UV radiation penetrates the entire layer and generates incomplete polymerization and crosslinking throughout the thickness of the layer, even at radiation doses of 20-100 mJ / cm2. The coating still remains liquid, but has a higher viscosity than the non-irradiated coating.

However, it is known that with such incomplete UV curing, crosslinking and radical polymerization processes continue to occur in time and space, even after irradiation has been discontinued.

These operations are called post-curing (R. MEHNERT ET AL., EUROPEAN COATING SHOW 2011, CONGRESS PROCEEDINGS).

As a result of UV post-cure, the viscosity increases throughout the layer depth, but is not stable in terms of time and space. In this way, the structure of the posterior micro-fold is negatively influenced.

The temporal and spatial instability of incomplete polymerization and crosslinking throughout the thickness of the layer proves to be a technical disadvantage.

Despite the constant dose rate of long-wave polychromatic UV radiators, fluctuations occur in micro-fold topography, leading to differences in surface brightness and haptics.

As a result of this instability, the measured brightness fluctuates at 85 ° incidence of light.

This technically undesirable effect occurs in particular if, by increasing the dose of the preliminary irradiation, for example, the degree of gloss should be reduced at 60 ° of incidence of light.

In the case of grazing light, the matt surface appears partially shiny and stained.

In this way, a predetermined and specific setting of the amplitude and frequency of the micro-folding and, therefore, of the haptic of the surface is not possible.

The object of the present invention is to provide a method that remedies this disadvantage. This objective is achieved by a method according to claim 1.

Surprisingly, it has been found that a pre-determinable and uniform micro-folding structure can be generated over the entire coated surface if incomplete UV curing is carried out only on the surface of the coating in a layer zone up to a few microns deep.

Monochromatic shortwave UV radiators, which are available, for example, as low pressure Hg radiators, are particularly suitable for activating incomplete UV curing in the surface area of a few microns deep.

At a pressure of a few millibars, a mercury discharge emits mainly Hg resonance lines at 185 and 254 nm.

In coating formulations containing acrylates or methacrylates and photoinitiators, photons with a wavelength of 185 nm are absorbed by both the acrylate / methacrylate molecules as well as the photoinitiator.

Deviating from the above, the wavelength of 254 nm is in most cases above the acrylate / methacrylate absorption zone and is absorbed exclusively by the photoinitiator.

At typical photoinitiator concentrations of 10-1 mol / l and extinction coefficients of the photoinitiator at 254 nm of £ = 1 - 4 x 104 l / mol cm, a penetration depth l of 2.5 to 10 pm is calculated at an extinction (EXT) of 1, corresponding to 90% absorption of the photons, according to EXT = cx £ x l.

The penetration depth of 185 nm photons in acrylates / methacrylates is even less and is between 0.5 and 1.5 pm.

When monochromatic short-wave radiation from low-pressure Hg radiators is absorbed into acrylate / methacrylate formulations, a depth profile of initial polymerization radicals is produced in the surface region of a few microns.

In coating formulations that contain acrylates or methacrylates, but not photoinitiators, only photons with a wavelength of 185 nm are absorbed.

The depth profile of the initial radicals for polymerization is then limited to a maximum of 1.5 µm and an increase in viscosity is limited to this depth zone.

In this way, it is also possible to adjust the amplitude and frequency of the micro-folding for formulations whose final cure is carried out with electrons and do not contain photoinitiators.

However, a relatively high irradiation dose is required for this adjustment of the amplitude and frequency of the micro-folding, since the intensity of the effective line of 185 nm constitutes only 20% of the total intensity of the emission.

In the case of acrylate / methacrylate formulations containing photoinitiators, the penetration depth of the photons is adjusted to 185 and 254 nm and thus the resulting increase in the viscosity of the coating on the surface by choosing the concentration. proper photoinitiator.

In the case of acrylate / methacrylate formulations that do not contain photoinitiators, the viscosity is increased in a surface layer to a depth of 1.5 pm by absorption of the photons of 185 nm.

However, even the addition of photoinitiators with a fraction of a few tenths of a% by weight causes a significant reduction in the irradiation dose required to set a predetermined folding frequency and amplitude. With a constant irradiation dose, the amplitude and frequency of folding can also be adjusted by varying the concentration of the photoinitiator.

In a first step, according to the method according to the invention, the coating is irradiated with short-wave monochromatic radiation of Hg resonance lines at 185 and 254 nm with doses of 60 to 1000 mJ / cm2, preferably from 100 to 600 mJ / cm2 to increase the viscosity in the surface zone to depths of 10 pm, preferably up to 5 pm, before, in a second step, the coating provided in this way, which has an inhomogeneous depth profile of viscosity, be irradiated by photons from a xenon or KrCl excimer radiator to activate the photochemical microstructure.

In a special configuration of the first step of the procedure, the folding amplitude and frequency can be adjusted by irradiating a coating without photoinitiator with short-wave monochromatic radiation at 185 nm with doses of 100 to 1500 mJ / cm2

It is also possible to achieve the adjustment of the amplitude and the folding frequency with short-wave monochromatic radiation of Hg resonance lines at 185 and 254 nm at constant irradiation dose by selecting the concentration of the photoinitiator in the range of 0.1 to 0. , 5% by weight.

Acrylate / methacrylate formulations generally consist of higher viscosity oligomeric binders such as urethane, epoxy, and polyester acrylates, as well as reactive diluents such as acrylate monomers with one to four acrylate double bonds per molecule.

Photoinitiators are added to cause polymerization by UV radiation. If electrons are used to cure the coatings, photoinitiators can be dispensed with. Nanocomposites are a special type of radiation curable formulations. Here, the nanoscale silicon dioxide particles are incorporated into the acrylate / methacrylate matrix (R: MEHNERT, F. BAUER, UV-CURABLE ACRYLATE NANOCOMPOSITES [UV-curable acrylate nanocomposites], J.POLYM.RES.vol. 12 (2005), pages 483-491).

Low-pressure mercury radiators are gas discharge lamps made of synthetic or natural quartz, which emit at wavelengths of 254 and 185 nm and have an optical efficiency of approximately 40%.

The spectral irradiance that can be achieved with photons at 185 nm is approximately 20% of that at 254 nm.

Amalgam lamps are an embodiment of low pressure mercury lamps that is up to an order of magnitude more powerful. Due to its low power compared to medium pressure Hg radiators, low pressure Hg radiators are generally not used for UV curing. However, this property is advantageous if incomplete surface curing is to be achieved, as used in the process according to the invention.

Excimer lamps are gas discharge lamps with a lamp body made of synthetic quartz and a filler made of xenon (xenon excimer radiators with an emission at 172 nm) or krypton with a chlorine donor (KrCl radiators with an emission at 222 nm). They emit quasi-monochromatic VUV / UV radiation, which causes excited dimeric molecules such as Ar2 *, Xe2 * or KrCl * in case of decomposition (B. ELIASON, U. KOGELSCHATZ, APPL.PHYS. B, vol. 46 (1988) , pages 299-303).

A number of other dimer molecules are also suitable as excimer radiation sources, but so far they have found almost no technical application.

The penetration depth of 126 nm photons in acrylate / methacrylate coatings is about 10 nm, less than 500 nm for 172 nm, and 2 pm for 222 nm.

As a result of the absorption of excimer irradiation, initiation radicals are created in the coating, leading to polymerization and crosslinking in the surface region. This increases the surface area by up to 20% and forms pleat structures. This procedure is called photochemical micro-folding.

A crosslinked solid surface layer is created by photochemical micro-folding, but the coating remains unreticulated in depth.

Therefore, at a later stage, the coating is completely cured by UV or electron beam curing.

The white light interferometry (WLI) procedure is used to examine the surface topography created by photochemical micro-folding.

This procedure uses interference from broadband light. The beam from a light source is divided by a beam splitter. One part is led to a reference mirror, the other perpendicular to the object to be measured. One beam is reflected in the reference mirror and the other in the sample to be measured. Both reflected beams overlap on the way back and are recorded by a camera. Interferences occur if the difference in length between the reference mirror and the sample is less than the coherence length of the light. The sample moves vertically through the interference zone.

As a result, interference images are created that reflect the surface topography in the analysis (scan) area.

The solutions according to the invention are described in more detail by means of the following examples.

Example 1

A radiation curable acrylic nanocomposite coating from Cetelon Nanotechnik Eilenburg (Germany) with type designation 830/800 103 is applied to a melamine substrate using a laboratory varnish machine from Barberan Castelldefels (SP). The thickness of the layer is 90 pm. After waiting a few seconds for a smooth surface to establish, the coating is irradiated with low pressure Hg lamps. Ten low-pressure Hg lamps of the type UVN 804C 15/1000 from UV Meyer Ilmenau (Germany) are arranged geometrically in parallel and are located in an irradiation chamber, which, among other things, also serves for optical radiation shielding.

The coated samples are moved on a conveyor belt through the irradiation chamber. In case of constant performance of the Hg low pressure lamp, the irradiation dose is adjusted by the speed and the number of cycles. The dose is measured with a dosimeter from Epigap Berlin (Germany). In the case of a cycle at a speed of 10 m / min, a dose of 85 mJ / cm2 is achieved. Irradiation is always carried out in air.

The sample is then moved through the beam region of a 172 nm excimer radiator at a speed of 20 m / min. The irradiation dose is adjusted to 2 mJ / cm2 and is maintained in all experiments. In the same cycle, the final UV curing is carried out using a Hg medium pressure radiator with a specific electrical power of 150 W / cm. Both excimer irradiation, as well as UV curing, takes place under nitrogen at oxygen levels <50 ppm.

Since the cured coating does not adhere sufficiently to the melamine surface, the parts can be removed from the substrate and analyzed. The surface topography of the samples is determined in this way with white light interferometry. The analysis area is about 600 * 600 pm. Since a profile can be derived at each position on the surface, the folding amplitude and frequency can be measured.

Figure 1 shows a series of folding structures that were recorded in the dose range of 0 to 510 mJ / cm2. The very rough fold can be seen clearly without prior irradiation with low pressure Hg lamps. The fold becomes finer even with previous irradiation at 85 mJ / cm2.

When the dose is increased, the folding amplitude decreases while the folding frequency increases. The corresponding measured values are compiled in Table 1 (last column). The amplitudes and frequencies of folding of the surfaces generated by the method according to the invention can be established within wide limits by means of the irradiation dose with low-pressure mercury radiators.

Figure 1 shows this relationship. According to the invention, the folding amplitude decreases and the folding frequency increases with increasing dose of irradiation. By touching the surface flush with the hand, it will be observed that the surface becomes increasingly smooth as the pre-irradiation dose increases. According to the invention, the haptic of the surface can also be adjusted.

Table 1 shows the gloss values for the different samples. A uniform matte finish is not possible without prior irradiation. Haptic is insufficient. However, the sample already has a deep and haptically attractive mat surface from a previous irradiation dose of 85 mJ / cm2. This trend increases with higher doses. The brightness measured at 60 ° decreases slightly with increasing dose, while the brightness value determined at 85 ° increases. The coating verified in case of incidence of grazing light has a uniform matt finish. This assessment is supported by the haptic test. The gloss values of the matt coatings produced by the process according to the invention can be adjusted within certain limits by choosing the dose of irradiation with low pressure Hg lamps.

Example 2 (not part of the present invention)

An acrylate nanocomposite coating from the company Cetelon Nanotechnik Eilenburg (Germany) with the type designation 830/800 103 is applied to a melamine substrate using a laboratory varnish application machine from the company Barberan Castelldefels, Barcelona (Spain) .

The thickness of the layer is 90 | jm. After waiting a few seconds until a smooth surface is established, the coating is pre-irradiated in a manner corresponding to the state of the art with a Ga-doped Hg medium pressure lamp.

The Ga lamp is located in an irradiation chamber through which a conveyor belt is guided. The irradiation dose is measured with a UV radiometer from Epigap Berlin (Germany).

To achieve doses in the range of up to 100 mJ / cm2, the electrical power of the Ga lamp is set to the technical minimum of 30% and the cycle speed varies between 10 and 50 m / min. The irradiation is carried out in the air.

The sample is then moved through the beam region of a 172 nm excimer radiator at a speed of 20 m / min. The irradiation dose is adjusted to 2 mJ / cm2 and is maintained in all experiments.

In the same cycle, the final UV curing is carried out using a Hg medium pressure lamp with a specific electrical power of 150 W / cm. Both excimer irradiation and UV curing take place under nitrogen at oxygen levels <50 ppm.

Since the cured coating does not adhere sufficiently to the melamine surface, parts can be peeled off and analyzed. The surface topography of the samples is determined in this way with white light interferometry.

The analysis area is about 600 * 600 jm. Since a profile can be derived at each position on the surface, the folding amplitude and frequency can be measured. Figure 2 shows a series of folding structures that were recorded in the dose range of 0 to 110 mJ / cm2.

Again the fold is clearly very rough without prior irradiation. However, already at 23 mJ / cm2 the folding becomes very fine and its structure hardly changes until 43 mJ / cm2. At 110 mJ / cm2, in the case of grazing incidence of light, the matte finish of the sample appears inhomogeneous until stained. The folding amplitude is in the range of 1 µm. The folding frequency increases sharply and can no longer be evaluated.

] Table 2 shows the gloss values of the samples. The gloss values are comparable for doses between 23 and 43 mJ / cm2. The gloss value at 60 ° is slightly higher than in Example 1.

At 85 °, the gloss value is already between 23 and 28. At 110 mJ / cm2, a uniform matte finish is no longer possible. All brightness values fluctuate depending on the measurement location,

Although pre-irradiation with Ga-doped Hg medium pressure lamps leads to a homogeneous matte finish with satisfactory haptics in the dose range <100 mJ / cm2, this cannot be controlled by dose.

Another disadvantage is the high brightness in the event of grazing light. Even with doses in the 100 mJ / cm2 range, the matte finish becomes inhomogeneous and technically unacceptable.

Neither the amplitude and the folding frequency nor the brightness values can be adjusted by the dose of the previous irradiation according to the method corresponding to the state of the art.

Example 3

An acrylate varnish from the Lott Lacke Herford company (Germany) with the type designation LM 4446, which does not contain a photoinitiator, is applied to a varnish test card using a 30 µm wire scraper. After waiting a few seconds for a smooth surface to establish, the coating is irradiated with low pressure Hg lamps. The irradiation arrangement and dose measurement correspond to the data in Example 1. As in Example 1, the irradiation is carried out in air.

The sample is then moved through the beam region of a 172 nm excimer radiator at a speed of 20 m / min. The irradiation dose is adjusted to 3 mJ / cm2 and is maintained in all experiments. In the same cycle, the final UV curing is carried out using a Hg medium pressure radiator with a specific electrical power of 150 W / cm.

Both excimer irradiation and UV curing take place under nitrogen at oxygen contents <50 ppm.

The structure of the surface is measured and recorded with an optical microscope. As shown in Figure 3, the folding structure of the non-previously irradiated sample is thicker than that of the previously irradiated sample according to the invention. By pre-irradiating the coating with photons of wavelengths 185 and 254 nm, according to the invention the folding amplitude is reduced and the folding frequency is increased.

Example 4

To an acrylate lacquer from Lott Lacke Herford (Germany) with the type designation LM 4446, which originally does not contain a photoinitiator, 0.125 to 0.5% by weight of a photoinitiator mixture consisting of 50% is added by weight of Darocure 1183 and 50% by weight of Irgacure 184 (BASF Ludwigshafen (Germany)). The varnish is applied to a varnish test card with a 30 pm wire scraper. After waiting a few seconds for a smooth surface to establish, the coating is irradiated with low pressure Hg lamps. Formulation LM 4446 is called "soft varnish" because it forms particularly fine folding structures.

The irradiation arrangement and dose measurement correspond to the data in Example 1. As in Example 1, the irradiation is carried out in air.

The sample is then moved through the beam region of a 172 nm excimer radiator at a speed of 20 m / min. The irradiation dose is adjusted to 3 mJ / cm2 and is maintained in all experiments. In the same cycle, the final UV curing is carried out using a Hg medium pressure lamp with a specific electrical power of 150 W / cm. Both the excimer irradiation and UV curing take place under nitrogen at oxygen contents <50 ppm.

The surface topography of the samples is determined using white light interferometry. The analysis area is about 600 * 600 pm. Since a profile can be derived at any position on the surface, it is possible to measure the amplitude and frequency of folding.

The folding amplitudes and frequencies of the surfaces produced according to the process according to the invention can be adjusted in the event of a constant irradiation dose by choosing the concentrations of photoinitiator of the coating with low-pressure Hg lamps.

Figure 4 shows this relationship.

Table 4 indicates the folding amplitudes and frequencies for the different samples. According to the invention, at a constant irradiation dose, with an increasing concentration of the photoinitiator, the folding amplitude decreases and the folding frequency increases.

Claims (2)

1. Procedure for adjusting the amplitude and frequency of micro-folding in the photochemical matte finish of radiation-curable coatings, in which, based on acrylate / methacrylate formulations, the coating is irradiated with monochromatic short-wave radiation from a radiator of low pressure mercury with emission lines at 185 and 254 nm and doses of 15 to 600 mJ / cm2 or, based on photoinitiator-free acrylate / methacrylate formulations, with monochromatic radiation of a wavelength of 185 nm with doses of 100 to 1500 mJ / cm2, after which, it is microstructured by means of an excimer radiator with emission lines in the range of 172 to 222 nm by irradiation with doses of 0.5 to 10 mJ / cm2, and then , the microstructured coating is cured by UV or electron irradiation. Method according to claim 1, wherein the amplitude and frequency of the micro-folding in the photochemical matte finish of radiation-curable coatings can be adjusted by irradiating with short-wave monochromatic radiation from a low-pressure mercury radiator to a constant irradiation dose choosing the concentration of the photoinitiator in the range of 0.1 to 0.5% by weight.