EP1235652B1 - Light curing of radiation curable materials under a protective gas - Google Patents

Abstract

Production of a molded article and coating a substrate comprises curing by radiation with light whilst using a protective gas that is heavier than air.

Description

The invention relates to a process for the production of coatings on substrates according to claim 1.

In the radiation curing of radically polymerizable compounds, e.g. of (meth) acrylate compounds, a strong inhibition of polymerization or curing by oxygen may occur. This inhibition results in incomplete surface hardening, e.g. to sticky coatings.

This oxygen inhibition effect can be achieved by the use of high amounts of photoinitiator, by co-using coinitiators, for. Amines, high energy, high dose UV radiation, e.g. be reduced with high pressure mercury lamps or by the addition of barrier-forming waxes.

It is also known to carry out the radiation curing under an inert protective gas, for example EP-A-540 884 , from Joachim Jung, RadTech Europe 99, Berlin 08. to 10.11.1999 in Berlin (UV-Applications in Europe Yesterday-Today Tomorrow).

What is desired is a method of radiation curing in which energy-rich UV light sources and the associated necessary safety measures can be dispensed with. At the same time, however, the process should be as simple as possible.

Radiation-curable compositions can be processed without water or organic solvents. Therefore, the process of radiation curing is suitable for coatings which are carried out in medium or small crafts or in the home. So far, however, the complex implementation of the method and the devices required for this purpose, in particular the UV lamps, has prevented the application of radiation curing in these areas.

The object of the invention was therefore a simple method of radiation curing, which is also applicable in small craft or in the domestic sector and is generally suitable to cure three-dimensionally coated objects.

The problem has been solved by the method defined above.

By means of the method according to the invention, coatings on planar surfaces (two-dimensional hardening process) or also coatings on three-dimensional shaped bodies can be cured on several sides or on all sides (three-dimensional hardening process)).

The process uses a shielding gas that is heavier than air. The molecular weight of the gas is therefore greater than 28.8 g / mol (corresponds to the molecular weight of a gas mixture of 20% oxygen and 80% nitrogen), preferably greater than 32, in particular greater than 35 g / mol. Consider, for example, Noble gases such as argon, hydrocarbons and halogenated hydrocarbons. Particularly preferred is carbon dioxide.

The supply of carbon dioxide may be from pressurized containers, filtered combustion gases, e.g. of natural gas or as dry ice. The supply of dry ice is considered advantageous, in particular for applications in the non-industrial or small-scale industries. Since dry ice can be transported and stored as a solid in simple containers insulated with foams. The dry ice can be used as such, it is then in gaseous form at the usual temperatures of use.

The inert gas is heavier than air, so air is displaced upwards. To prevent the lateral escape of the gas must.

For this purpose, a wide variety of devices or measures may be suitable.

One possibility is the use of a container as Tauchbekken. This method is particularly suitable for the three-dimensional coating method.

The protective gas is filled into the container and the air displaced from it.

The container now contains a protective gas atmosphere in which the substrate, which is coated with the radiation-curable composition, or the molded body can be immersed. Then you can the radiation hardening done, for example, by sunlight or by suitably mounted lamps.

In the radiation curing of coated surfaces, in particular bottom surfaces, the respective surface to be hardened can be delimited by suitable devices, in particular partitions, so that the protective gas can not escape during the irradiation period.

The process further allows printable or printed substrates to be coated and radiation cured. Suitable substrates include e.g. Paper, cardboard, foils or textiles. The radiation-curable coating can be the printing ink or an overprint varnish. Radiation curing can be used immediately in the printing process, e.g. done in the printing press. As a printing process called his offset, gravure, high, flexo or pad printing.

During radiation curing, the oxygen content in the protective gas atmosphere is preferably less than 15% by weight, more preferably less than 10% by weight, most preferably less than 5% by weight, based on the total amount of gas in the protective gas atmosphere; In particular, easily oxygen contents of less than 1%, even less than 0.1% and even less than 0.01% by weight, can be set using the process according to the invention.

A protective gas atmosphere is understood to be the gas volume which surrounds the substrate at a distance of up to 10 cm from its surface.

In the case of using dry ice as the shielding gas, e.g. a feed the plunge pool, which may be storage containers for dry ice at the same time, easily done. The monitoring of carbon dioxide consumption is directly related to the consumption of dry ice solids. Dry ice evaporates directly to gaseous carbon dioxide at -78.5 ° C. As a result, atmospheric oxygen is displaced upwards out of the basin in a basin without swirling.

The residual oxygen can be determined with commercially available atmospheric oxygen meters. The basin can be covered to minimize gas losses and possibly also against heating during periods of non-operation. Due to the oxygen-depleted atmosphere in the dipping and supply tanks and the associated risk of suffocation, suitable safety measures should be taken.

Similarly, adequate ventilation and carbon dioxide drainage should be ensured in adjacent work areas.

The painted objects can be lowered individually with lifting and lowering devices or on conveyor belt-like devices in series coatings in the plunge pool for exposure. In order to ensure the most complete flooding of the object without too much air to tear into the irradiation zone, either a slow lowering or lifting or the use of pre-and post-floods is suitable. The pre-and post-floods are an extension of the inert gas tanks to separate air swirling zones from the irradiation zone. For this purpose, the inert gas tank can be extended from the exposure zone both in the height and on both sides in the width. The dimensions of the receiving waters are primarily dependent on the speed of entry and exit and on the geometry of the object.

The duration of the irradiation depends on the desired degree of hardening of the coating or of the shaped body. The degree of hardening can be in the simplest case at the Entklebung or at the scratch resistance, e.g. against the fingernail or against other objects such as pencil, metal or plastic tips. Also, in the paints area, usual resistance tests to chemicals, e.g. Solvents, inks, etc. suitable. In particular, spectroscopic methods, in particular Raman and infrared spectroscopy, or measurements of the dielectric or acoustic properties, etc., are suitable without damaging the painted surfaces. The radiation curing can be carried out by sunlight or by lamps, which are preferably mounted in the dip tank so that the desired multi-sided or all-sided curing of the coated substrates takes place.

For flat immobile substrates e.g. Floors or objects fixed to the ground may be fitted with simple containment devices to prevent the flow of carbon dioxide. Examples are the sealing of the door area in rooms e.g. up to 40 cm from the floor e.g. with glued foils, or from setting up walls made of wood, plastic, stretched films or paper webs. The carbon dioxide gas can be made by filling from gas cylinders or as dry ice. Furthermore, containers of dry ice can be suspended, from which carbon dioxide can flow out to the material to be hardened.

The radiation-curable composition contains radiation-curable compounds as a binder. These are compounds with free-radically or cationically polymerizable and therefore radiation-curable ethylenically unsaturated groups. Preferably, the radiation-curable composition 0.001 to 12, particularly preferably 0.1 to 8 and very particularly preferably 0.5 to 7 mol, radiation-curable ethylenically unsaturated groups per 1000 g of radiation-curable compounds.

As radiation-curable compounds come z. For example, (meth) acrylic compounds, vinyl ethers, vinylamides, unsaturated polyesters e.g. based on maleic acid or fumaric acid optionally with styrene as a reactive diluent or maleimide / vinyl ether systems into consideration.

Preferred are (meth) acrylate compounds such as polyester (meth) acrylates, polyether (meth) acrylates, urethane (meth) acrylates, epoxy (meth) acrylates, silicone (meth) acrylates, acrylated polyacrylates.

Preferably, at least 40 mol%, more preferably at least 60 mol%, of the radiation-curable ethylenically unsaturated groups are (meth) acrylic groups.

The radiation-curable compounds may contain other reactive groups, e.g. Melamine, isocyanate, epoxide, anhydride, alcohol, carboxylic acid groups for additional thermal cure, e.g. B. by chemical reaction of alcohol, carboxylic acid, amine, epoxy, anhydride, isocyanate or melamine groups containing (dual cure).

The radiation-curable compounds may be e.g. as a solution, e.g. in an organic solvent or water, as an aqueous dispersion, as a powder.

The radiation-curable compounds and thus also the radiation-curable compositions are preferably free-flowing at room temperature. The radiation-curable compositions preferably contain less than 20% by weight, in particular less than 10% by weight, of organic solvents and / or water. They are preferably solvent-free and anhydrous (100% solids).

The radiation-curable compositions may contain other constituents in addition to the radiation-curable compounds as a binder. Consider, for example, Pigments, leveling agents, dyes, stabilizers etc.

For curing with UV light, photoinitiators are generally used.

Examples of suitable photoinitiators are benzophenone, alkylbenzophenones, halomethylated benzophenones, Michler's ketone, anthrone and halogenated benzophenones. Also suitable are benzoin and its derivatives. Also effective photoinitiators are anthraquinone and many of its derivatives, for example β-methylanthraquinone, tert-butylanthraquinone and Anthrachinoncarbonsäureester and, most effective, photoinitiators with a Acylphosphinoxidgruppe such as Acylphosphinoxide or Bisacylphosphinoxide, eg 2,4,6-Trimethylbenzoyldiphenylphosphinoxid (Lucirin ^® TPO).

As far as the radiation-curable compositions contain photoinitiators, these photoinitiators should have absorption wavelengths in the range of the emitted light. Suitable visible light photoinitiators which contain no UV components are in particular the abovementioned photoinitiators with acylphosphine oxide groups.

It is an advantage of the invention that the content of the photoinitiators in the radiation-curable composition can be low or can be completely dispensed with photoinitiators.

Preferably, the radiation-curable compositions contain less than 10 parts by weight, in particular less than 4 parts by weight, more preferably less than 1.5 parts by weight of photoinitiator per 100 parts by weight of radiation-curable compounds.

Sufficient is in particular an amount of 0 parts by weight to 1.5 parts by weight, in particular 0.01 to 1 part by weight of photoinitiator.

The radiation-curable composition can be applied by conventional methods to the substrate to be coated or brought into the appropriate form.

The radiation curing can then take place as soon as the substrate is surrounded by the protective gas.

The radiation curing can be done with all lamps, which were previously used for radiation curing. The radiation curing can be done with electron beams, X-rays or gamma rays, UV radiation or visible light. It is an advantage of the method according to the invention that the radiation curing with visible light, which contains little or no (wavelengths below 300 nm, can be made.

The radiation curing in the process according to the invention can therefore be carried out with sunlight or with lamps which serve as sunlight replacement. These lamps emit in the visible range above 400 nm and have no or hardly any UV light components below 300 nm).

In particular, in the method according to the invention the proportion of radiation in the wavelength range below 300 nm is less than 20%, preferably less than 10%, more preferably less than 5%, in particular less than 1 or 0.5% or less than 0.1% of Integrals of radiated intensity over the entire wavelength range below 1000 nm.

The above radiation is the radiation actually available for curing, that is to say when filters are used around the radiation after passage through the filter.

Can be considered lamps that have a line spectrum, that is radiate only at certain wavelengths, z. B. LEDs or lasers.

Also contemplated are broad band spectrum lamps, that is, a distribution of emitted light over a range of wavelengths. The intensity maximum is preferably in the visible range above 400 nm.

Called e.g. Incandescent lamps, halogen lamps, xenon lamps. Mention is also mercury vapor lamps with filters to prevent or reduce radiation below 300 nm.

Also suitable are pulsed lamps e.g. Photo flash lamps or high-power flash lamps (VISIT). A particular advantage of the method is the usability of low energy and low UV lamps, e.g. 500 watt halogen lamps, as they are used for general lighting purposes. As a result, both a high-voltage unit for power supply (in the case of mercury vapor lamps) and optionally light protection measures can be dispensed with. Even with halogen lamps, there is no risk of ozone development even in air as with short-wave UV lamps. This facilitates radiation hardening with portable irradiation equipment and enables "on-site" applications, ie independent of fixed industrial curing plants.

For mobile use and for applications that require a variety of lamps for illuminating the substrate are especially lamps, including lamp housing with reflector, possibly existing Cooling devices, radiation filters and power source connection are suitable, which have a low weight, for example less than 20 kg, preferably less than 8 kg.

Particularly light lamps are e.g. Halogen lamps, incandescent lamps, light-emitting diodes, portable lasers, photo flash lamps, etc. These lamps are also characterized by particularly easy installation in container interior or container walls. Likewise, the technical complexity of the power supply is reduced, especially in comparison to previously customary mercury vapor radiators in the medium and high pressure range. As a preferred power source of the lamps are used in addition to mains power especially household normal AC voltage, e.g. 220 V / 50 Hz or the supply of transportable generators, batteries, accumulators, solar cells, etc.

The inventive method is suitable for the production of coatings on substrates and for the production of moldings.

Suitable substrates for z. As those made of wood, plastics, metal, mineral or ceramic materials.

As molding may be mentioned z. B. composites z. B. containing radiation-curable composition impregnated fiber materials or fabric, or molded body for stereolithography.

Another advantage of the method is that the distances between lamps and radiation-curable composition over the curing in air can be increased. Overall, lower radiation doses can be used and a radiator unit can be used to cure larger areas.

Thus, in addition to conventional radiation curing applications, the process enables new applications in the field of curing coatings and molding compounds of complex three-dimensionally shaped articles, e.g. Furniture, vehicle bodies, housing and equipment, in mobile applications such as floor and floor painting. Because of the low technical and material effort, the method is also suitable for medium and small craft businesses, the home-working and do it your self-area.

Examples example 1

A radiation-curable composition was prepared by mixing the following ingredients. 35% by weight Laromer® LR 8987 (BASF Aktiengesellschaft), a urethane acrylate 20% by weight hexanediol, 38.5% by weight Laromer® LR 8863, a polyether acrylate 3.5% by weight Iragucure® 184 (Ciba Specialty Chemicals), a photoinitiator 0.5% by weight Lucirin® TPO (BASF) a photoinitiator 2% by weight Tinuvin® 400 (Ciba Specialty Chemicals), a UV absorber 1.5% by weight Tinuvin® 292, a UV absorber

With this mass, a glass sheet was painted (layer thickness 50 microns).

In a container of depth 60 cm with a diameter of 40 cm 500 g of dry ice are filled. After about 60 minutes, the residual oxygen content is about 10 cm below the upper edge of the container 3 wt .-% and at 45 cm depth 0.01 wt .-%. On the 45 cm level, the glass pane is inserted and irradiated for 2 min with a 500 watt halogen lamp at a distance of 50 cm to the halogen lamp. The paint is highly scratch resistant and can not be scratched with a wooden spatula and a white typewriter paper under manual pressure and rubbing.

In comparison, air is irradiated under the same conditions. The paint remained fluid. In comparison, on a conveyor belt at 10 m / min belt speed under a high pressure mercury lamp at 120 W / cm (Fa. IST) with a lamp spacing 15 cm exposed twice. The coating could not be cured scratch resistant.

Example 2

The radiation-curable composition corresponded to Example 1.

The radiation-curable composition was applied as a clearcoat to the housing of an automobile exterior mirror and cured according to the invention as described in Example 1. The paint obtained was highly scratch resistant.

Claims (17)

1. A process for producing coatings on substrates by radiation curing radiation-curable compositions under an inert gas which is heavier than air and whose lateral escape in the course of radiation curing is prevented by means of appropriate apparatus or other measures, wherein irradiation is carried out with a lamp which emits UV radiation or visible light and which, in the wavelength range below 300 nm, has less than 20% of the integral of the emitted intensity over the entire wavelength range below 1000 nm.
2. The process according to the preceding claim, wherein the distribution of the light emitted by the lamp has an intensity maximum above 400 nm.
3. The process according to either of the preceding claims, wherein the substrate or molding compound is immersed in a dip tank containing the inert gas.
4. The process according to any of the preceding claims, wherein the inert gas comprises carbon dioxide.
5. The process according to any of the preceding claims, wherein the inert gas is produced by evaporating dry ice.
6. The process according to any of the preceding claims, wherein the amount of oxygen in the inert gas atmosphere that surrounds the substrate at a distance of up to 10 cm from its surface is less than 15% by weight, based on the total amount of gas.
7. The process according to any of the preceding claims, wherein the substrate is selected from the group consisting of paper, cardboard, film, and textiles.
8. The process according to any of the preceding claims, wherein radiation curing is carried out on three-dimensional shaped articles or substrates selected from the group encompassing furniture, vehicle bodies, and from the construction of casings and instruments.
9. The process according to any of the preceding claims, wherein the radiation-curable composition comprises further reactive groups selected from the group consisting of melamine, isocyanate, epoxide, anhydride, alcohol, and carboxylic acid groups.
10. The process according to any of the preceding claims, wherein the radiation-curable composition is present as a solution in an organic solvent or water, as an aqueous dispersion, or as a powder.
11. The process according to any of the preceding claims, wherein the radiation-curable composition comprises less than 20% by weight of organic solvents and/or water.
12. The process according to any of the preceding claims, wherein the radiation-curable composition is free from solvent and free from water.
13. The process according to any of the preceding claims, wherein the radiation-curable composition comprises a photoinitiator selected from the group consisting of benzophenone, alkylbenzophenone, halomethylated benzophenone, Michler's ketone, anthrone, halogenated benzophenone, benzoin, anthraquinone, acylphosphine oxide, and bisacylphosphine oxide.
14. The process according to any of the preceding claims, wherein the radiation curing is carried out using a lamp which is selected from the group consisting of incandescent lamps, halogen lamps, and xenon lamps.
15. The process according to any of the preceding claims, wherein the lamp is operated with a power source with standard household alternating voltage.
16. The process according to any of the preceding claims, wherein the lamp for radiation curing has a weight of below 20 kg.
17. The process according to any of the preceding claims, wherein the lamp is installed in container interiors or container walls.