Method and apparatus for continuous hardening of visible light-curing resins

Abstract: A method and an apparatus for continuously hardening visible-light-curing resins are disclaimed. An object (10), such as an orthodontic bracket, made of visible-light-curing resins, is carried on a conveyor (2) past a row of halogen lamps (1), and sensors (6) detect the temperature in zones of the row whereby controllers (7) activate cooling fans (5) to maintain the temperature in each zone. The objects are moved continuously, either in a non-stop or intermittent manner, and deep uniform hardening is achieved.

Description: The present invention relates to a hardening apparatus and a method for visible light-curable resins, and in particular to apparatus for continuously hardening of objects made of visible light-curable resins and a method of continuously hardening the objects whilst irradiated. Conventionally, the hardening of visible light-curable resins has been performed by a hardening apparatus, comprising an exposure stage to place an object made of visible light-curable resin in an enclosure and a visible light irradiation equipment to generate the visible light. It is used for the hardening of dental materials using composite resins in the field of dentistry. For example, the Japanese Provisional Patent Publication Sho 62-38149 describes a hardening apparatus comprising fluorescent lamps emitting visible light of wavelength of 400 - 500 nm and a stand, on which the object to be irradiated is rotatably placed, and the Japanese Provisional Patent Publication Sho 62-47354 discloses a hardening apparatus having a halogen lamp as the light source of the irradiation light and a turntable to place the object to be irradiated. These conventional hardening methods and apparatuses for the visible light-curing resins have the following advantageous features: 1) Because the irradiation light is visible light, there is no possibility to adversely affect the clothing or the skin of the users as often seen in case of ultraviolet ray or infrared ray; 2) Because the irradiation light is visible light, there is little loss of light due to scattering in the object to be irradiated as in the case of ultraviolet ray, and the irradiated object can be hardened deep into the core. Namely, the object with larger thickness can be easily hardened; 3) Because the stand to place the irradiated object is rotatable, the illuminance on the object is distributed evenly, and the entire object can be evenly hardened. In order to further develop the above advantageous features of the hardening method and apparatus of the visible light-curing resins, the Japanese Provisional Patent Publication Hei 1-130920 discloses a method by which a light-curable monomer and/or prepolymer is placed in a molded frame consisting partly of the object with light transmission property to produce the molded objects for optical application such as contact lenses or lenses for eyeglasses or the molded objects for electrical and mechanical applications such as compact disk, display plate, micro-screws, gears, etc. However, when a number of objects of visible light-curing resins are to be produced by the conventional technique as described above, the following problems arise: 1) The conventional hardening apparatus for visible light-curing resins is of batch type to harden one or more objects at one time, and it is necessary to repeatedly carry and take the objects into and from the hardening apparatus and to irradiate the objects by many times. Thus, complicated procedure and much time are required for hardening. 2) Halogen lamp, xenon lamp or fluorescent lamp used as visible light source in the conventional technique have wide variations in product quality (particularly, in luminous intensity), and the variations usually range between 10 - 50%. Therefore, when the halogen lamps of the same standards are used, for instance, there are the variation of 10 - 50% in the illuminance of the irradiation light on the object, and this leads to the wide variations in the quality of the hardened products. Also, there is the problem of deviation in the positions of the light source in the hardening apparatus between the product batches, and the illuminance of the irradiation light on a number of objects produced in two or more batches widely differs from each other even when the batch type hardening apparatuses of the same standards are used. 3) Even when a large quantity of the hardened products are to be produced by repeatedly using the batch type hardening apparatus for visible light-curing resins based on the conventional technique, it is impossible to obtain the hardened products from the visible light-curing resins having a constant property because the thermal history differs according to each batch. Specifically, in the batch type hardening apparatus, the atmosphere temperature in the hardening apparatus is increased from normal temperature to high temperature due to the heat generated from the light source of the irradiation light or to the heat generated by polymerization reaction of visible light-curing resins. Also, when the hardened objects are taken out, the temperature is decreased to normal temperature, and the atmosphere temperature during the irradiation of visible light changes from normal temperature to high temperature and then to normal temperature. Thus, it is difficult to have the same thermal history for each batch, and this means that the hardened products produced from the batches widely differ in their physical property (such as surface hardness, mechanical strength, etc.). When a number of the objects are distributed to two or more batch type hardening apparatuses of the same standards, the physical property of the hardened products thus produced widely differs, and the complicated hardening procedure and much labor and time are required. In the method and apparatus to perform the hardening of light-curing resins by irradiation light, the method and the apparatus using ultraviolet light as the irradiation light has been widely known. Also widely known are the method and the apparatus to irradiate the ultraviolet light using high pressure mercury vapor lamp or arc discharge lamp as light source and to irradiate the light by continuously moving the objects made of ultraviolet light-curing resins. However, because ultraviolet light is used as the irradiation light in this case, it is disadvantageous because of the loss of light due to the scattering of light within the objects and to the difficulty to perform polymerization hardening to the core of the thick objects because of the ultraviolet absorption of the ultraviolet light-curing resins itself. Accordingly, when ultraviolet light is used, the thickness of light-curable monomer and/or prepolymer is limited to the ink, film, coating film, etc. having thickness of several µm to several hundreds of µm. Namely, when ultraviolet light is used as the irradiation light, the interior of the hardened objects remains unpolymerized and satisfactory bracket cannot be obtained even if the composite resin containing inorganic filler is filled in the mold with light transmission property and the orthodontic bracket is to be molded and hardened. Even when ultraviolet light-curing resins containing coloring agents are used, hardening is performed only insufficiently, and even a color plate with thickness of about 2 mm cannot be molded. To avoid the above problems when ultraviolet light is used as the irradiation light, the illuminance may be increased to such degree that the hardening to the deep core can be accomplished, whereas the surface of the irradiated object is exposed to extremely strong ultraviolet light in such case. As the result, the hardened products may be thermally deformed due to extreme temperature rise, or coloring or discoloring or even ignition may occur in some cases. US 3,985,100 discloses that complexly-shaped articles of EPDM material are treated with benzophenone by cleaning the article, coating it with benzophenone and irradiating the benzophenone-coated article with ultraviolet light. The complexly-shaped articles are uniformly treated by exposure within a reflective enclosure that bathes the article in light by transporting it while rotating around an ultraviolet source and between the source and the reflective enclosure. Chemical Abstracts 76(12), No 60342n discloses a photopolymerisation apparatus comprising a continuous moving belt passing through a tunnel with ultraviolet lights, and with air ducts above the lamps connected to a ventilator and with cold water piping for cooling alongside; this is used for polymerisation of polymethacrylate. According to the present invention we provide a method and an apparatus to continuously harden a shaped article, as defined in the appended claims. The "continuous movement" as used herein means that the objects to be irradiated are introduced one after another to the irradiation position within the hardening apparatus and are moved out of there. Accordingly, it includes not only the smooth movement but also intermittent movement of the objects to be irradiated. According to this invention, visible light from the visible light irradiation equipment disposed at a certain position is irradiated on the objects made of visible light-curing resins continuously in movement. Accordingly, a number of the objects coming through the hardening apparatus have the same exposure history although the illuminance normally differs at a certain point within the hardening apparatus and at another point (e.g. near the inlet or at the center of the apparatus). As the result, there is no variation of physical property among a number of the hardened products produced by light-curing. Because visible light is used as the irradiation light, the influence of the scattering and absorption of light within the irradiated objects are reduced to a lower level, and the photo-polymerization hardening can be accomplished to the deep core of the molded products of visible light-curing resins having thickness of 10 mm or more. The time from the introduction of the objects into the hardening apparatus to their leaving it is preferably within the range of 1 second to 24 hours, and more preferably within the range of 10 seconds to 3 hours. The moving mechanism may include means to move the objects continuously at all times such as a belt conveyor, or moving means to move the objects stepwise according to a stroke, such as a walking beam conveyor or pneumatic or hydraulic cylinder. Or, a pulse motor may be used, which moves the objects by numerical control. The movement of the irradiated objects may be a linear or curved movement on the same plane or upward or downward movement. As the light sources to generate the irradiation light, light source having a main wavelength of 400 - 700 nm such as a halogen lamp, xenon lamp or fluorescent lamp may be used. The light source may be furnished with a known type of reflection mirror or diffusion plate to provide a desired distribution of the illuminance, or with a heat-cut filter to give the effect of reducing the temperature rise of the irradiated objects. The illuminance of the visible light to be irradiated is in the range of 100 luxes to 10,000 kiloluxes, and preferably from 1,000 luxes to 1,000 kiloluxes. The visible light is irradiated under the controlled atmosphere temperature onto the objects made of visible light-curable resins, and this can be achieved by furnishing a fan, cooling water, cooler, heater and/or infrared generator at the desired positions in the apparatus. In addition, temperature control may be accomplished by selecting a halogen lamp (high temperature range) or fluorescent lamp (low temperature range) as the light source for visible light irradiation equipment, considering the types of lamp and the desired illuminance of the irradiation light. During irradiation of the visible light according to the present invention, it is preferable to control the temperature of the atmosphere to within the range of 4 - 95°C, and more preferably to 20 - 60°C. The visible light-curable resins suitable for the purpose of this invention include one or more mixtures of the compounds having at least one unsaturated carbon-carbon double bond in the molecule, including monofunctional (meth)acrylic acid derivatives, such as methyl (meth)acrylate, ethyl(meth) acrylate or cyclohexyl(meth)acrylate, or multifunctional acrylates such as ethylene-glycol-di(meth)acrylate, diethylene-glycol-di(meth)acrylate, or bisphenol-A-di(meth)acrylate, with the addition of a small quantity of photoinitiator activated by the visible light. The term "(meth)acrylic" herein means "methacrylic" and "acrylic". Further, a photo-sensitizer or reducing agent such as tertiary amine may be used when necessary. Also, the above components may be filled with organic or inorganic fillers or there may be added an anti-oxidant, antistatic agent, coloring agent or thermal polymerization inhibitor. In the accompanying drawings: Fig. 1 is a schematic view of an Embodiment 1 according to the present invention; Fig. 2 is a schematic view of an Embodiment 2; and Fig. 3 is a schematic view of an equipment for comparison. In the following, the invention will be described in detail with reference to the embodiments shown in the drawings. The compound having at least one unsaturated carbon-carbon double bond in the molecule of the visible ray polymerization type of resin used in the present invention includes: monofunctional (meth)acrylic acid derivative such as methyl(meth)acrylate, ethyl(meth) acrylate, propyl(meth)acrylate, cyclohexyl(meth) acrylate, benzyl(meth)acrylate, (meth)acrylic acid, hydroxyethyl(meth)acrylate and hydroxypropyl(meth) acrylate, and polyfunctional (meth)acrylate such as ethyleneglycol-di(meth)acrylate, diethylene-glycol-di(meth)acrylate, triethyleneglycol-di(meth)acrylate, bisphenol-A-di(meth)acrylate, bisphenol-A-ethyleneglycol modified di(meth)acrylate, bisphenol-A-propyleneglycol modified di(meth)acrylate, bisphenol-A-glycerol modified di(meth)acrylate and trimethylolpropane-tri(meth)acrylate. As a photo-initiator activated by the visible light, there are: Dicarbonyl compounds such as camphorquinone, benzil, anthraquinone and di-α-naphthyldiketone, or thioxanthone derivatives such as 2-chlorothioxanthone, 2-methylthioxanthone and 2,4-dimethyl-thioxanthone. As inorganic and organic fillers, there are: glass beads, silica fine powder, titanium oxide fine powder, glass fiber, whiskers, carbon fibers and various types of polymer powder. Embodiment 1: Fig. 1 is a schematic view of an apparatus showing Embodiment 1 according to this invention. This apparatus is furnished with fifteen halogen lamps of 150W power each as the light source for the visible light irradiation equipment, and the halogen lamps 1, ..... are linearly arranged above the conveyor 2 driven by the chain 3, which transmits the rotation of the motor 4. In this apparatus, the atmosphere temperature (ambient temperature) of the irradiated objects is detected for each of three zones by three temperature sensors 6, and the temperature is controlled to the desired value by three cooling fans 5, which are stopped by three temperature controllers 7 for each zone. The operating speed of the conveyor 2 in this apparatus can be set as desired by means of the conveyor speed regulator 9. The power for the fifteen halogen lamps 1 is supplied by the power source 8. In front of the halogen lamp 1, which acts as a light source for the visible light irradiation equipment, a heat-cut filter 13 is furnished to eliminate heat rays, and this prevents an unnecessary temperature rise of the irradiated objects when visible light is irradiated. Next, description will be given of the method to harden the objects consisting of visible light-curable resins by the apparatus of this invention. In this apparatus, the object 10 consisting of visible light-curable resin is placed at the object inlet 11 of the apparatus and the conveyor 2 is driven. By the action of the conveyor 2, the object 10 is continuously moved below the halogen lamps 1, irradiating visible light under the controlled ambient temperature, and reaches soon the object outlet 12, terminating the hardening process. In case a large number of the objects are to be hardened by this apparatus, the objects may be introduced into the apparatus from the object inlet 11 one after another according to the movement of the conveyor 2. As the result, the hardened objects continuously come out of the object outlet 12 after undergoing the same exposure of the visible light and the heating. Next, description will be given of the case where orthodontic brackets are hardened and molded using the visible light-curing resins by the apparatus of this invention. Into a light transmission type mold made of poly-4-methyl-1-pentene for the molding of a bracket, the visible light-curing resins were injected, which was composed of 52 weight parts of bisphenol-A-ethyleneglycol modified dimethacrylate, 39 weight parts of methyl methacrylate, 9 weight parts of finely powdered silica with hydrophobic treatment (Nippon Aerosil Co.; R-972), 0.7 weight part of camphorquinone, and 0.7 weight part of dibenzoyl peroxide. Then, the light transmission type molds filled with visible light-curing resins were supplied one after another to the object inlet 11 and were introduced into the apparatus. The conveyor of this apparatus is 210 cm long, and the moving speed of the conveyor 2 was 15 cm/min. The illuminance of visible light was 140 ± 30 kiloluxes above the conveyor 2, and the atmosphere temperature in the apparatus was controlled to 45 ± 2°C. The objects thus irradiated were obtained one after another from the object outlet 12, and the molds were disassembled and the polymerized and hardened brackets were taken out. The brackets were hardened to the deep core. For 30 brackets of visible light-curing resins thus obtained, the base of the bracket was cemented on the stand for the tensile test, and piano wire was attached on its wing. Thus, the tensile test on the wing was performed using a universal testing machine of Instron Co. The rupture strength of the bracket wing was 3.7 ± 0.5 kgf with very small variations, showing the stable property of the products. This apparatus has high productivity, producing 250 brackets per hour. Embodiment 2: Fig. 2 is a schematic view of the Embodiment 2 according to this invention. This apparatus is furnished with twelve fluorescent lamps 20 of 20W power each, emitting light of main wavelength of 450 nm. Similarly to the Embodiment 1, a conveyor 21 is provided, which is driven by the chain 22, transmitting the rotation of the motor 23. In this apparatus, the atmospheric temperature is detected by the temperature sensors 26 at three points. In order to maintain the temperature set by the temperature controller 27, three heaters 25 are furnished under the conveyor 21 and the exhaust speeds of the cooling fans 24 are controlled. Because the fluorescent lamps 20 are used as the light source for the visible light irradiation equipment, it is possible to maintain the atmospheric temperature in the apparatus approximately to room temperature. The speed of movement of the conveyor 21 of this apparatus is set by the conveyor speed regulator 31 as desired. Next, description will be given of the case where paperweights are produced by embedding coins in visible light-curable resins. Into a polypropylene container with an inner diameter of 50 mm and 25 mm deep, was placed to the depth of 5 mm, a visible light-curable resin which was composed by weight:- Parts bisphenol-A-glycerol modified dimethacrylate 68 methyl methacrylate 15 laurylacrylate 7 triethyleneglycol-dimethacrylate 10 camphor quinone 0.05 dibenzoyl peroxide 0.02 hydroquinone-monomethylether 0.02 The objects were introduced into the apparatus one after another from the object inlet 29, and a number of the support stands made of resin were produced, on which coins are to be placed in the subsequent step. The conveyor 21 was 120 cm long, and the speed of movement of the conveyor 21 was 30 cm/min. The illuminance of visible light was 40 ± 5 kiloluxes above the conveyor 21, and the atmospheric temperature in the apparatus was set to 30 ± 1°C. Thus, a number of the containers having the resin support stands made of hardened visible light-curable resins at the bottom of the container were brought out. The brass coin with diameter of 30 mm and thickness of 1.5 mm was placed on the upper surface of the resin stand at the bottom of the container. After the visible light-curing resins were poured into the container, the containers were introduced again into the apparatus one after another. In this case, the moving speed of the conveyor 21 was 3 cm/min. and the other conditions were the same. The hardened irradiated objects were continuously carried out of the object outlet 30, and hard transparent paperweights made of the hardened visible light-curing resins embedded with coins were obtained. When the surface hardness of 20 paperweights thus obtained was measured by micro-Brinnel hardness tester, the hardness was 13 ± 1 Hb, and the variations of surface hardness were small, showing the stable finish of the products. This apparatus has high productivity, producing 50 molded products per hour. Comparative example 1: Fig. 3 is a schematical view of a comparative example. The apparatus in this example is furnished with 5 halogen lamps 40 having the same standards as the lamps of the Embodiment 1, and the lamps are installed above the rotary turntable 41 with the irradiated objects on it as if the table 41 is surrounded by the lamps. Using 3 apparatuses of the same standards as this apparatus, orthodontic brackets made of visible light-curing resins were produced as in the case of the Embodiment 1. The visible light-curing resins having the same composition as that of the Embodiment 1 were injected into the light transmission type molds, and 5 such molds were placed on the turntable of the apparatus and the visible light was irradiated for 14 minutes for hardening. The illuminance of the visible ray of this hardening equipment on the turntable was 150 ± 50 kiloluxes. The brackets thus obtained had good external appearance as the brackets of the Embodiment 1, but when the tensile test of the wing was performed on 30 brackets, the rupture strength was 2.7 ± 1.5 kgf. The mechanical strength varied extremely according to each item, and a number of defective products were found, which may not be suitable for practical application. In this example, only 60 brackets could be produced per hour, and the productivity was lower than the Embodiment 1. Comparative example 2: As a comparative example, 4 units of the batch type hardening apparatus for visible light-curing resins were used to produce the paper weights by hardening the visible light-curing resins as in Embodiment 2. Each apparatus was in rectangular parallele piped shape having the dimension of 300 × 250 × 220 mm (W × D × H), and was furnished with 3 fluorescent lamps of the same standards as the Embodiment 2 and also with a turntable to place the objects under the fluorescent lamps. The illuminance of visible light above the table of these hardening apparatuses was 45 ± 7 kiloluxes. The irradiation time of visible light was set to 40 minutes as in Embodiment 2. The micro-Brinnel hardness of 20 paperweights produced by hardening the visible light-curing resins was 12 ± 4 Hb, and the surface hardness showed extreme variations. The apparatuses of this example could produce only 10 paperweights per hour. The operation was complicated, and the productivity was low. As described for the embodiments of this invention, a number of hardened resin molded products can be easily produced evenly by the method and the apparatus to continuously harden the visible light-curing resins according to this invention. There are not very much variations in the physical properties of the molded products, and it is possible to obtain the hardened products consisting of visible light-curing resins with very stable properties. Also, it is possible to produce a large quantity of objects having thickness of 10 mm or more. The present invention thus offers: (1) a method to continuously harden the visible light-curing resins, characterized in that the visible rays from visible light irradiation equipment disposed at a certain position are irradiated onto the objects made of visible light-curing resins continuously in movement, and (2) and apparatus to continuously harden the visible light-curing resins, comprising equipment to continuously move the objects made of visible light-curing resins and visible light irradiation equipment disposed at a certain position. The "continuous movement" as described herein is defined as the status in which the objects to be irradiated are introduced to the irradiation position in the hardening apparatus one after another and are carried out of the apparatus, including smoothly moving the objects and also moving them intermittently. Because visible light is irradiated from the visible light irradiation equipment disposed at a certain position, onto the objects made of visible light-curing resins continuously in movement according to this invention, the illuminance normally differs at a certain point in the hardening apparatus and at another point (e.g. near the inlet or at the center of the apparatus), whereas a large number of the objects to be irradiated passing through the hardening apparatus have the same exposure history of visible light and heating. As a result, there is no variation in the physical properties of these photo-polymerized products. Because visible light is used as the irradiation light, there is very little influence from the scattering or absorption of light in the objects to be irradiated, and the photo-polymerization hardening can be achieved to the deep core of the products of the visible light-curing resins.

Claims: Apparatus for continuously hardening an object (10,31) made of a synthetic resin which is curable by exposure to visible light, which comprises: an enclosure containing a plurality of sources (1, 20) of visible light arranged along a linear or curved path, means (2, 21) for continuously at all times or intermittently conveying the objects (10, 31) to be cured along a path illuminated by said light sources (1, 20) so that the objects pass from an inlet (11, 29) to an outlet (12, 30 of the apparatus, and means (6, 26) for detecting the temperature of the atmosphere around the light sources (1, 20) and the conveying means (2, 21), and means (5, 7; 25, 24, 27) for adjusting the temperature of that atmosphere to a value within a desired range . Apparatus as claimed in Claim 1, wherein the means for controlling said temperature comprises a plurality of temperature sensors (6, 26) arranged adjacent to the light sources (1, 20). Apparatus as claimed in Claim 2, wherein respective sensors (6, 26) are arranged adjacent to groups of lamps (1, 20) as the light sources. Apparatus as claimed in Claim 3, wherein the means for controlling the temperature includes cooling means (5, 24) controlled by said temperature sensors (6, 26). Apparatus as claimed in Claim 3 or 4, wherein the means for controlling the temperature includes heating means (25) controlled by said sensors (26). Apparatus as claimed in any preceding claim, which includes a filter (13) which filters out heat rays, arranged between the light sources (1) and the conveying means (2). A method of hardening a plurality of objects (10, 31) made of a synthetic resin which is curable by exposure to visible light, which comprises placing said objects in turn on the conveying means (2, 21) of apparatus as claimed in any preceding claim, conveying them through the apparatus whilst illuminated with visible light from said light sources (1, 20) and sensing the temperature of the atmosphere around the light sources and the objects and controlling the sensed temperature to a desired value by means of cooling or cooling and heating the atmosphere. A method as claimed in Claim 7, wherein the objects to be hardened are within the apparatus for a period of 10 seconds to 3 hours. A method as claimed in Claim 7 or 8, wherein the temperature of the atmosphere is controlled within the range of 20 to 60°C. A method as claimed in Claim 7, 8 or 9, wherein the objects are made of one or more compound having at least one unsaturated ethylenic bond in the molecule, together with a small quantity of a photoinitator, and optionally other additives.

Inventors: KIMURA SHUSUKE C O KABUSHIKI K, KOMIYA SHIGEO, MASUHARA EIICHI, NAKAJIMA KENSUKE C O KABUSHIKI, NISHIYAMA NOBORU C O KABUSHIKI, OZEKI KOJI C O KABUSHIKI KAISH, SANBONMATSU KIYOMI C O KABUSHI, SAWAMOTO TAKEYUKI, KIMURA, SHUSUKE C/O KABUSHIKI KAISHA MORITA, KOMIYA, SHIGEO, MASUHARA, EIICHI, NAKAJIMA, KENSUKE C/O KABUSHIKI KAISHA, NISHIYAMA, NOBORU C/O KABUSHIKI KAISHA, OZEKI, KOJI C/O KABUSHIKI KAISHA, SANBONMATSU, KIYOMI C/O KABUSHIKI KAISHA, SAWAMOTO, TAKEYUKI
Applicants: JAPAN INST ADVANCED DENTISTRY, JAPAN INSTITUTE OF ADVANCED DENTISTRY
Application year: 1990
Main classification: C08F2
IPC codes: B29C35, C08F2, A61K6, A61C13, B01J19, B29L31