Irradiation device with excimer emitters as UV source

Abstract

An irradiation device with excimer emitters (13) as a UV radiation source, wherein the irradiation device formed from an emitter head (1), cooling (6 to 9) of the excimer emitter (13) with deionized water, nitrogen flushing (10-11) and a reflector (12) are the basic elements of the irradiation device, wherein the emitter head (1) is designed as a shaped body which accommodates the inner electrode (2) and outer electrode (3) of the cylindrical emitter and guides said electrodes with a form fit to a high-voltage socket (4) and / or to the earth connection (5) and, via bores, ensures the supply of cooling water to the inner cooling duct (6) and outer cooling duct (7), wherein the nitrogen flushing is carried out via distributor elements made from porous sintered metal (10) and a chamber (11) as a buffer volume, and wherein a reflector (12) with a parabolic cross section is arranged at the sides of the housing.

Description

[0001] Irradiation device with excimer lamps as UV source

[0002] Noble gas halogen excimer lamps with emission wavelengths of e.g. 207, 222, 253, 282 and 308 nm can be used for the UV crosslinking of radiation-curing coatings, printing inks and adhesives that contain acrylates as oligomers and monomers.

[0003] The prerequisite for efficient UV crosslinking is that these lamps have irradiances > 50 mW / cm 2 suitable photoinitiators are used to initiate UV crosslinking and the irradiation takes place under an inert atmosphere at oxygen concentrations of < 500 ppm.

[0004] The degree and speed of crosslinking then correspond to values ​​as with irradiation with medium-pressure mercury lamps.

[0005] Photons with wavelengths of 207 or 222 nm are absorbed in acrylates directly by exciting electrons of the acrylate double bond.

[0006] Here the penetration depth of the photons is given as 1.5 or 2.5 pm.

[0007] As a result of the resulting high radical concentration in a layer close to the surface, a high degree of cross-linking is achieved at the surface.

[0008] However, by appropriate selection of photoinitiators, the effect of preferential surface crosslinking can also be achieved for all other noble gas halogen excimer lamps mentioned above.

[0009] The basic geometric shape of both lamp types, namely medium-pressure mercury lamps or excimer lamps, is a quartz tube cylinder with an outer diameter between 9 and 40 mm and lengths of up to several meters. The electrically excited discharge takes place in the sealed cylinder, generating the UV radiation required for the chemical crosslinking process. An excimer lamp is described in EP 0 254 111 B1 and in B. Eliasson, U. Kogelschatz: Appl. Phys. B 46, p. 299 (1988).

[0010] In a dielectric barrier discharge, which is generated in a noble gas-halogen mixture by applying a sinusoidal alternating voltage with typical frequencies of 10 kHz to 1 MHz and amplitudes up to 20,000 V or corresponding high-frequency high-voltage pulses, excited noble gas atoms and ions are formed by electron impact. These ions recombine via short-lived noble gas cations and halogen anions to form excited noble gas-halogen excimers such as KrBr* (207 nm), KrCl* (222 nm), XeI* (253 nm), XeBr* (282 nm), or XeCl* (308 nm). The lifetime of the excited excimers is a few hundred nanoseconds. During decay, they release their excitation energy as radiation and subsequently decay into their ground-state atoms. The emission wavelengths for some technically usable excimers are given in parentheses in the text above.

[0011] A typical design of an excimer lamp is given in DE 41 40 497 C2.

[0012] An inner tube is arranged coaxially within an outer quartz tube with an outer diameter of 30 to 40 mm and wall thicknesses of 0.5 to 1.5 mm. The outer and inner tubes are joined and fused together at their ends. This creates a closed cylindrical cavity that serves as the discharge chamber, which is filled with a suitable noble gas-halogen mixture.

[0013] This discharge space is located between an inner and an outer electrode, which can be designed as a metal coil or mesh.

[0014] The electrodes are connected to the two poles of an alternating voltage or pulse generator. Depending on the gas filling, adjustable voltage amplitudes between 1000 and 20,000 volts at frequencies up to 1000 kHz are required to generate the discharge.

[0015] The advantage of this solution is that the discharge chamber can be cooled externally with water. Unlike medium-pressure mercury lamps, this does not impair the gas discharge. Due to the high voltage involved, deionized water is used for cooling, which is circulated through the channel of the inner electrode over the outer electrode. The outer electrode is surrounded by a cylindrical cladding tube through which the cooling water flows.

[0016] The surface temperature of the cladding tube then corresponds to the cooling water temperature. Compared to medium-pressure mercury lamps, noble gas halogen excimer lamps offer the following advantages: low heat input into the substrate due to water cooling of the inner and outer surfaces of the discharge tube; high energy efficiency and service life due to the elimination of internal electrodes; no warm-up time, switch-on and switch-off times in the millisecond range; no mechanical shutter required to switch the radiation on and off; no use of mercury.

[0017] It is also known that inhibition of crosslinking by oxygen can be eliminated by purging the irradiation chamber in which crosslinking takes place with nitrogen. A suitable device for purging the irradiation zone with nitrogen is proposed in EP 2 786 807 B1. Nozzles equipped with perforated or porous distribution elements are described there.

[0018] The invention is based on the object of creating a device which can be used as a technically applicable irradiation device with excimer lamps as UV source, preferably for the crosslinking of acrylate-based radiation-curable printing inks, varnishes and adhesives.

[0019] The irradiation device according to the invention is suitable for high voltages up to 20,000 V, has a cooling circuit with deionized water at flow rates of 1 to 10 l / min and generates a photon stream directed onto the irradiation plane by means of reflectors and inertizes the irradiation chamber 14 with nitrogen at flow rates of 1 to 100. The irradiation device according to the invention will be explained in more detail below using an exemplary embodiment and Figures 1 and 2.

[0020] Figure 1 shows a longitudinal section and Figure 2 a cross-section through the irradiation device according to the invention.

[0021] According to the invention, the radiator head 1 is designed as a molded body, preferably made of Teflon, and accommodates the inner 2 and outer electrode 3 of the cylindrical radiator and leads them to a high-voltage socket 4 and to the ground connection 5, respectively.

[0022] The receptacle for the inner electrode in the high-voltage electrode in the radiator head 1 is designed with a form-fitting design. This ensures that no air, which would be detrimental to the high-voltage strength, can be present between the cylindrical inner electrode 2 and the radiator head 1. A high-voltage socket 4 for connecting the irradiation device to a high-voltage source is also inserted in the radiator head 1 in a suction-type manner. Due to the design of the excimer radiator 13 as a hollow quartz cylinder, the cooling water is guided from the inner cooling channel 6 into the outer cooling channel 7, which is formed by the outer casing of the radiator cylinder and a cylindrical quartz cladding tube. Bores are provided in the radiator head 1 for the inflow 8 and outflow 9 of the cooling water, which lead to the inner cooling channel 6 and the outer cooling channel 7 respectively.Since the inner electrode located in the inner cooling channel 6 is at high voltage potential during operation, deionized water with an electrical conductivity < 10 pS is used for cooling.

[0023] Cooling channels 6 and 7 are designed so that the pressure drop in the cooling area of ​​the excimer lamp is < 0.5 bar. This largely prevents mechanical stress on the quartz hollow body caused by pressure surges in the cooling water.

[0024] The irradiation device according to the invention is intended for radiation crosslinking of paints, printing inks, and adhesives. To prevent inhibition of crosslinking by oxygen, the irradiation chamber 14, in which crosslinking takes place, is purged with nitrogen.

[0025] In the irradiation device according to the invention, distribution elements 10 made of porous sintered metal are arranged directly above the cylindrical excimer radiator 13. The nitrogen is fed into a chamber 11, which acts as a buffer volume.

[0026] From there, it can flow out through the porous distribution elements 10 arranged along the excimer emitter 13. The outflowing nitrogen reaches flow velocities of 0.4 to 5 m / s at flows of 0.5 to 20 Nm 3 / h. The pressure in chamber 11 is adjusted so that the nitrogen flow exiting the sintered metal is distributed as homogeneously as possible over the length of the excimer lamp.

[0027] With a sufficiently dimensioned volume of chamber 11 and a nitrogen pre-pressure of > 2 bar, a homogeneity of the outflow velocity of < 10% is achieved. The nitrogen flows around a significant portion of the surface of the excimer lamp and simultaneously purges the volume of the irradiation chamber 14. This reduces the oxygen concentration in the irradiation chamber 14 to < 500 ppm.

[0028] In this way, both the inerting of the irradiation room and the protection of the surface of the excimer lamp from contamination, e.g. by the condensation of volatile organic components from varnishes, printing inks or adhesives, are achieved.

[0029] The irradiation device according to the invention is used for crosslinking radiation-curable layers which pass through the irradiation zone in the inertized irradiation chamber.

[0030] To increase the irradiance in the layer to be crosslinked, reflectors 12 are mounted in the irradiation device so that a portion of the coaxially emitted excimer radiation is focused toward the layer to be crosslinked. Coated aluminum surfaces with a reflection of > 90% in the UV range are used as reflectors 12. Two reflector surfaces are arranged on the sides of the irradiation device housing so that the least possible reflection occurs on the surface of the excimer emitter 13. The cross-section of the reflector surfaces preferably has a parabolic shape and leads to a beam distribution that increases the illuminance in the irradiation plane by 30%.

Claims

Patent claims Claim 1 Irradiation device with excimer emitters (13) as a UV radiation source, characterized in that the irradiation device consists of an emitter head (1), cooling (6 to 9) of the excimer emitter (13) with deionized water, nitrogen purge (10-11) and reflector (12) as basic elements of the irradiation device, wherein the emitter head (1) is designed as a molded body that receives the inner (2) and outer electrode (3) of the cylindrical emitter and guides these positively to a high-voltage socket (4) and to the ground connection (5) respectively, and ensures the supply of cooling water to the inner (6) and outer cooling channel (7) via bores, that the nitrogen purge is carried out via distributor elements made of porous sintered metal (10) and a chamber (11) as a buffer volume, and that a reflector 12 with a parabolic cross-section is arranged on the sides of the housing. Claim 2 Irradiation device according to claim 1, characterized in that the emitter head (1) is designed as a molded body made preferably of Teflon and which accommodates the inner (2) and outer electrode (3) of the cylindrical emitter and leads these to a high-voltage socket (4) or to the ground connection (5). Claim 3 Irradiation device according to claim 1, characterized in that the cooling of the excimer emitter (13) is formed by an inner cooling channel (6) which is led into the outer cooling channel (7) which is formed by the outer shell of the emitter cylinder and a cylindrical quartz sheathing tube and is connected to the inlet (8) and outlet (9) of the cooling water arranged in the emitter head (1) and deionized water with an electrical conductivity <10 pS is used as the coolant. Claim 4 Irradiation device according to claim 1, characterized in that the excimer lamp is surrounded by nitrogen via a buffer volume (11) and porous distribution elements made of sintered metal (10) and this nitrogen is subsequently to inertize the irradiation zone (14) and is used in such a way that the oxygen concentration in the irradiation zone (14) is < 500 ppm. Claim 5 Irradiation device according to claim 1, characterized in that the cross-section of the surfaces of the reflectors (12) has a parabolic profile. Claim 6 Use of the irradiation device according to claim 1 for UV crosslinking of acrylate-based printing inks, varnishes and adhesives.

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