Lens sheet

Abstract: A lens sheet comprising a transparent substrate and a lens portion composed of a cured product of an ultra­ violet-curable resin composition, which is formed on at least one surface of the transparent substrate. The resin composition exhibits a polymerization shrinkage at curing of not larger than 20% as calculated based on the difference of the specific gravity at 25°C, and a modulus of elasticity at 25°C after curing of 10 to 1,000 kg/cm². The ultraviolet-curable resin composition is preferably comprised of a polyvalent (meth)acrylate, a mono(meth)acrylate and a photopolymerization initiator.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lens sheet such as a Fresnel lens or lenticular lens used for a screen of a video projection television, or a condensing Fresnel lens. 2. Description of the Related Art In the production of a lens sheet such as a Fresnel lens or lenticular lens, when the size is relatively small and mass production is carried out, an injection molding process for synthetic resins is generally adopted. When a relatively large lens, such as the above-mentioned screening lens or condensing lens, is prepared, a process is generally adopted in which a resin plate is fitted to a flat lens mold and is heated and pressed to transfer the lens surface profile to the resin plate. In the latter process, however, a long time is needed for heating and cooling, and the process is disadvantageous in that the productivity cannot be increased. As the means for eliminating this disadvantage, there has been proposed a process comprising interposing an ultraviolet-curable resin liquid between a lens mold and a transparent substrate and curing the resin under irradiation with ultraviolet rays (see Japanese Unexamined Patent Publication No. 61-177215), and a transmission type screen comprising a Fresnel lens or lenticular lens prepared by this process is known (see Japanese Unexamined Patent Publication No. 63-167301). When a lens sheet as mentioned above is prepared, an ultraviolet-curable resin used for this production must have such properties as adhesion to a transparent substrate, release from a lens mold, strength and shape-retaining of a lens portion after curing, and light transmission. The processes disclosed in the above-mentioned two patent publications will now be discussed with reference to these properties. In the former process, ultraviolet-curable resins are not specifically explained, and in the screen of the latter technique, it is taught that an ultraviolet-curable resin having a viscosity not larger than 500 cps before curing, a polymerization shrinkage not larger than 10% at curing, and a modulus of elasticity of 5,000 to 30,000 kg/cm² after curing is used. If this ultraviolet-curable resin is used, however, the Young's modulus is too high and removal from the lens mold is difficult, and if the molded lens is forcibly removed from the mold, the lens portion is sometimes chipped. Moreover, the specific ultraviolet-curable resin most preferably used in this technique is discolored after the curing, and thus it is obvious that this resin is not suitable for lens. EP-A-0 156 430 discloses a laminated optical component which comprises a supporting member manufactured from a transparent inorganic material and has on at least one side a layer of a radiation-cured lacquer, the free surface of which has a defined shaped, wherein the lacquer comprises at least 50% by weight of a compound of the formula: wherein R₁ is a hydrogen atom or a methyl group, R is a group of the formula -O-R₂- or -O-R₃-O-R₄-, wherein R₂ is an alkylene group having 2-8 carbon atoms which may be substituted with one or more hydroxyl groups, R₃ and R₄ are equal or different from each other and each represent an alkylene group having 1-4 carbon atoms, and m is 0 or 1. The lacquer may furthermore comprise an aromatic monoacrylate. SUMMARY OF THE INVENTION Under the above-mentioned background, the inventors carried out research, and a result, found that the use of the above-mentioned ultraviolet-curable resin, which becomes hard after curing, renders the release from a mold difficult, whereas better results are obtained when an ultraviolet-curable resin which is soft after curing is used. More specifically, in accordance with the present invention, there is provided a lens sheet comprising a transparent substrate and a lens portion composed of a cured product of an ultraviolet-curable resin composition, which is formed on at least one surface of the transparent substrate, wherein the polymerization shrinkage, calculated based on the difference of the specific gravity at 25°C, of the ultraviolet-curable resin, caused at curing, is not larger than 20% and the modulus of elasticity at 25°C after curing is 10 to 1,000 kg/cm², said ultraviolet-curable resin composition being formed on at least one surface of the transparent substrate, wherein the polymerization shrinkage, calculated on the basis of the difference of the specific gravity at 25°C, of the ultraviolet-curable resin composition, caused at curing, is not larger than 20%, and the modulus of elasticity at 25°C after curing is 10 to 1,000 kg/cm², said ultraviolet-curable resin composition comprising, as main components, (i) 30 to 60% by weight of a polyol dimethacrylate of the formula (a): wherein ℓ and n are positive integers and the sum of ℓ and n is 8 to 12, 5 to 25% by weight of a polyol diacrylate of the formula (b): wherein ℓ′ and n′ are positive integers and the sum of ℓ′ and n′ is 3 to 5, and 30 to 60% by weight of a tetrahydrofurfuryl acrylate of the formula (c): and (ii) a photopolymerization initiator. DESCRIPTION OF THE PREFERRED EMBODIMENTS The lens sheet of the present invention will now be described in detail. An ultraviolet-curable resin composition composed of compounds described hereinafter is used in the present invention. The polymerization shrinkage caused at curing must be not larger than 20%. If this polymerization shrinkage is 20% or larger because of the shrinkage of the lens sheet in the plane direction, strangling the lens mold occurs, and simultaneously, because of the shrinkage of the lens sheet in the thickness direction, rising from the lens mold occurs and a satisfactory transfer of a lens profile from the lens mold cannot be obtained. The polymerization shrinkage referred to herein is a polymerization shrinkage calculated based on the difference of the specific gravities of the resin composition at 25°C, which is defined by the following formula: Polymerization shrinkage S = [dp - dm)/dp] x 100 (%) wherein dm represents the specific gravity of the ultraviolet-curable resin composition (monomer mixture) at 25°C and dp represents the specific gravity of the cured product of the ultraviolet-curable resin composition at 25°C. Also, the modulus of elasticity at 25°C of the ultraviolet-cured resin after curing must be 10 to 1,000 kg/cm². If the modulus of elasticity is larger than 1,000 kg/cm², when the formed lens sheet is released from the lens mold, a large force is necessary, and thus release from the mold is difficult and chipping occurs at the top end of the lens if the formed lens sheet is forcibly released from the mold. If the modulus of elasticity is smaller than 10 kg/cm², the resin is too soft and the shape of the lens cannot be retained. If the modulus of elasticity is from 10 to 1,000 kg/cm², the release property is improved, and even if the formed lens comes to contact with a vessel or implement, a dented part is restored to the original shape by the rubber-like elasticity and the formed lens is not damaged. Furthermore, scratching is not caused by friction, and when at least two sheets are combined and used as a transmission type screen, especially good results can be obtained. Preferably, the viscosity at 25°C of the ultraviolet-curable resin composition used in the present invention before the curing is not larger than 300 mPa·s. If this viscosity is larger than 300 mPa·s, air bubbles are readily included in the resin composition when cast in the lens mold, and such bubbles are difficult to remove. Moreover, if the viscosity is thus increased, the flowability is lowered, casting in the lens mold becomes difficult, and thus a long time is needed for casting. Preferably the total luminous transmittance of the lens portion composed of the ultraviolet-cured resin used in the present invention, as calculated as the thickness of 1 mm is at least 85%. If the total luminous transmittance is not larger than 85%, the light transmission performance of the lens sheet is poor. Preferably, the lens sheet has no warp. In connection with the warping, preferably, when the lens sheet is horizontally placed, the rise of the lens sheet satisfies the requirement represented by the following formula: D ≦ R/50 wherein D represents the rise (mm) and R represents the distance (mm) in the horizontal direction from the center of the lens sheet. If the warp quantity of the lens sheet fails to satisfy the requirement represented by the above-mentioned formula, the designed optical performances cannot be obtained in the lens. In the present invention, the ultraviolet-curable resin composition comprises 30 to 60% by weight of a polyol dimethacrylate of the following formula (a), wherein ℓ and n are positive integers satisfying the requirement of ℓ + n = 8 to 12, 5 to 25% by weight of a polyol diacrylate of the following formula (b): wherein ℓ′ and n′ are positive integers satisfying the requirement of ℓ′ + n′ = 3 to 5, and 30 to 60% by weight of tetrahydrofurfuryl acrylate of the following formula (c), which is a reaction product between tetrahydrofurfuryl alcohol and acrylic acid: The reasons why the ultraviolet-curing resin composition is based on the above-mentioned three components (a), (b) and (c) are as follows. Namely, if the amount of the dimethacrylate component (a) is smaller than 30% by weight, violent heat generation or shrinkage often occurs upon irradiation with ultraviolet rays, and cracking or distortion of the cured resin often occurs. Furthermore, the transparent substrate is deteriorated, and when the formed lens sheet is removed from the mold, the transparent substrate is easily damaged. If the amount of the dimethacrylate component (a) is larger than 60% by weight, the rate of the curing reaction is low, and an insufficient curing often occurs at a part coming into contact with oxygen, resulting in contamination of the transparent substrate. Since the viscosity of the component (a) is about 400 mPa·s, i.e., relatively high, the viscosity of the ultraviolet-curable resin is increased, and therefore, when the resin composition is cast in the mold, air bubbles are readily included in the cast resin composition, and removal of these air bubbles is difficult. If the amount of the diacrylate component (b) is smaller than 5% by weight, the strength is reduced, the lens portion is easily chipped and sometimes is caused to collapse by compression. Furthermore, when the formed lens sheet is released from the lens mold, chipping and breaking often occur, and the handling is difficult. If the amount of the diacrylate component (b) is larger than 25% by weight, since the strength is raised and the elongation is lowered, an especially large force becomes necessary for removing the formed lens sheet from the mold, and upon release from the mold, breaking and chipping often occur. Furthermore, since the viscosity of the component (b) is high, i.e., about 900 mPa·s, air bubbles are readily included in the resin component when cast in the mold, and these bubbles are difficult to remove. If the amount of the acrylate component (c) is smaller than 30% by weight, since the high viscosities of the other components cannot be moderated, the viscosity of the resin composition is high, air bubbles are readily included in the resin composition at the casting step, and it is difficult to remove these bubbles. Moreover, when a polymethyl methacrylate plate is used as the transparent substrate, a sufficient adhesion to the substrate can be obtained. If the amount of the component (c) is larger than 60% by weight, since the strength is reduced, chipping of the lens portion or crushing by compression often occurs. Moreover, when the formed lens sheet is released from the lens mold, chipping or breaking easily occurs. If a urethane polyacrylate (d) is further incorporated in an amount of 5 to 25% by weight in addition to the above-mentioned dimethacrylate (a), diacrylate (b) and monoacrylate (c), the elongation of the resin after the curing is increased although the strength is little changed, and therefore, defects such as chipping can be eliminated more easily. Accordingly, this embodiment is especially preferable. If the amount of the urethane polyacrylate (d) is smaller than 5% by weight, the effect of the increased the elongation is not satisfactory. If the amount of the urethane polyacrylate is larger than 25% by weight, the viscosity is increased, air bubbles are readily included in the resin composition at the casting step, and it is difficult to remove these bubbles. As the urethane polyacrylate used in the present invention, there can be mentioned a polyether urethane polyacrylate, a polyester urethane polyacrylate, a polyamide urethane polyacrylate and a polyisocyanate urethane polyacrylate. As specific examples of the urethane polyacrylate, there can be mentioned urethane polyacrylates represented by the following formula (I): wherein R₉ , R₁₀ , R₁₁ , R₁₂ and R₁₃ represent an alkylene or arylene group containing an ether, ester or amide group. A small amount of a photopolymerization initiator is incorporated into the ultraviolet-curing resin composition used in the present invention, namely the composition comprising the above-mentioned dimethacrylate (a), diacrylate (b) and monoacrylate (c) or the composition comprising the urethane acrylate (d) in addition to these components (a), (b) and (c). Most preferably, at least one photopolymerization initiator selected from the group consisting of methylbenzoyl formate, 1-hydroxycyclohexyl phenyl ketone and 2-hydroxy-2-methyl-1-phenylpropan-1-one is incorporated in an amount of 0.1 to 5 parts by weight per 100 parts by weight of the resin components. If this photopolymerization initiator is used, discoloration of the resin composition is controlled before and after the curing reaction, and a lens sheet having a high light transmission and excellent optical characteristics can be obtained. If the amount of the photopolymerization initiator incorporated is smaller than 0.1 part by weight per 100 parts by weight of the resin components, the reactivity is reduced and sometimes curing is not sufficiently advanced under usual conditions. If the amount of the photopolymerization initiator incorporated is larger than 5.0 parts by weight, discoloration of the cured product occurs. Glass and plastics can be used for the transparent substrate constituting the lens sheet of the present invention. Plastic plates such as an acrylic resin plate and a polycarbonate plate, and plastic films such as a polyester film and an acrylic resin film, are preferably used. The present invention will now be described in detail with reference to the following examples. In the examples, the characteristics were determined by the following methods. (1) Polymerization shrinkage The polymerization shrinkage was determined by the above-mentioned method. (2) Viscosity The viscosity was measured by a B-type viscometer (Model BM supplied by Tokyo Keiki). (3) Warp (rise D at the horizontal posture) A square sample having a side of 300 mm was placed on a horizontal stand, rises at the four corners thereof were measured, and the mean value of the measured values was calculated. (4) Total luminous transmittance An ultraviolet-curing resin sheet having a thickness of 1 mm was prepared, and the total luminous transmittance was measured according to the method of ASTM D-1003. (5) Removal from mold A nickel stamper having a size of 900 mm x 1200 mm was fixed to a stand so that the total weight was 40 kg, an ultraviolet-curable resin was cast on the stamper, and a transparent acrylic resin plate having a size of 890 mm x 1190 mm was piled on the cast resin. Curing was effected by irradiation with ultraviolet rays. Then the formed lens sheet was peeled from the corner, the state thereof was observed, and the property was evaluated by the following standards. A: stamper or stand not moved at peeling B: stamper moved at peeling, and peeling was possible C: stamper moved and chipping or the like occurred (6) Adhesion A sample not having a lens profile was prepared by using a mirror-surface stamper, 100 1-mm square chequers were formed on the side of the ultraviolet-curable resin by cross cutting, peeling by using a cellophane adhesive tape was carried out three times, and the number of peeled chequers was counted. Class 5: all chequers remained Class 4: 75 to 99 of 100 chequers remained Class 3: 74 to 50 of 100 chequers remained Class 2: 49 to 25 of 100 chequers remained Class 1: number of remaining chequers was 24 or less among 100 chequers Examples 1 through 6 and Comparative Examples 1 through 5 An ultraviolet-curable resin composition shown in Table 1 was prepared, and 2-hydroxy-2-methyl-1-phenylpropan-1-one (Dalocure supplied by Merk) was added as the photopolymerization initiator in an amount of 2 parts by weight per 100 parts by weight of the resin composition. Then, 600 g of the resin composition was cast on a lens mold constructed by fixing a stamper having a size of 900 mm x 1200 mm to a stand so that the total weight was 40 kg, and a transparent acrylic resin plate (Acrylite #000 supplied by Mitsubishi Rayon) having a size of 890 mm x 1190 and a thickness of 3 mm, not containing an ultraviolet absorber and having a surface which was uniformly roughened by sand paper #800, was piled on the cast resin composition. The curing was effected by irradiation with an integrated dose of 760 mJ/cm² of ultraviolet rays by using an 80 W/cm high-pressure mercury lamp. The results of the evaluation of the respective characteristics are shown in Table 1. The abbreviations in Table 1 indicate the following compounds. FA-321M: A compound supplied by Hitachi Chemical, which is represented by the following formula: A-BPE-4: NK Ester supplied by Shin-Nakamura Kagaku, which is represented by the following formula: THFA: Tetrahydrofurfuryl acrylate HDDA: 1,6-Hexanediol diacrylate UK-6036: Urethane diacrylate supplied by Mitsubishi Rayon UV-3000B: Urethane diacrylate marketed under tradename of "Goselac" by Nippon Synthetic Chemical Ind. SNEX-5487: Sannopco represented by the following formula: M-315: Aronix supplied by Toagosei Chemical Ind. which is represented by the following formula: NVP: N-vinylpyrrolidone TAS: A compound represented by the following formula: As apparent from the results shown in Table 1, the lens sheet of the present invention has a good release property, a high total luminous transmittance, and little warpage. Examples 7 through 11 and Comparative Examples 6 through 8 An ultraviolet-curable resin composition shown in Table 2 was prepared, and a photopolymerization initiator shown in Table 2 was added. In the same manner as described in Example 1, the resin composition was cast in a lens mold, an acrylic resin plate was piled thereon, and the resin composition was cured by irradiation with ultraviolet rays. The results are shown in Table 2. The abbreviations in Table 2 indicate the following compounds (the abbreviations in Table 1 are omitted). HX: 1,6-Hexanediol dimethacrylate HEMA: 2-Hydroxyethyl methacrylate HMPP: 2-Hydroxy-2-methyl-1-phenylpropan-1-one MBF: Methylbenzoyl formate NCPK: 1-Hydroxycyclohexyl phenyl ketone Examples 12 through 16 and Comparative Examples 9 through 11 An ultraviolet-curable resin composition shown in Table 3 was prepared, and a photopolymerization initiator shown in Table 3 was added. In the same manner as described in Example 1, the resin composition was cast in a lens mold, an acrylic resin plate was piled on the cast resin composition, and curing was carried out by irradiation with ultraviolet rays. The results are shown in Table 3. In Table 3, M-1100 indicates an urethane acrylate marketed under the tradename of Aronix by Toagosei Chem. Ind. (other abbreviations indicate compounds described above with reference to Tables 1 and 2). Examples 17 and 18 and Comparative Examples 14 and 15 An ultraviolet-curable composition shown in Table 4 was prepared, and 2-hydroxy-2-methyl-1-phenylpropan-1-one (Dalocure supplied by Merk) was added as the photopolymerization initiator in an amount of 2 parts by weight per 100 parts by weight of the resin composition. Then, 600 g of the resin composition was cast in a lens mold constructed by fixing a nickel stamper having a size of 900 mm x 1200 mm to a stand so that the total weight was 40 kg, and a transparent polyester film (Diafoil #125 supplied by Diafoil) having a size of 920 mm x 1220 mm, the surface of which was roughened by sand paper #800, was piled on the cast resin composition. The assembly was pressed by a rubber roller to uniformalize the thickness, and curing was effected by an integrated dose of 760 mJ/cm² of ultraviolet rays by using an 80 W/cm high-pressure mercury lamp. The results of the evaluation of the respective characteristics are shown in Table 4. The abbreviations in Table 4 indicate the same compounds as in Table 1. In the lens sheet of the present invention, the ultraviolet-curable resin constituting the lens portion has an excellent adhesion to a transparent substrate, especially an acrylic resin and since the strength of the resin per se is relatively low, the formed lens sheet can be easily removed from the mold, and is little damaged upon removal from the mold. Moreover, since the resin is a crosslinked polymer, the lens portion has an excellent shape-retaining property. The lens sheet is not cracked, and chipping or breaking rarely occurs during transportation. Moreover, the respective monomers have a good compatibility and a resin having an excellent transparency is formed. Furthermore, since the resin composition has a low viscosity the resin composition can be easily cast in a mold, and very few air bubbles are included in the cast resin; if air bubbles are incorporated, they can be easily removed. Still further, the polymerization shrinkage at curing is small, the modulus of elasticity of the cured resin is low, and the cured resin has rubbery characteristics. Therefore, little warping due to the two-layer structure occurs in the lens sheet of the present invention. Moreover, when the transparent substrate of the lens sheet of the present invention is a film, the lens sheet can be prepared without cracking. The foregoing advantages are achieved when the composition comprising the above-mentioned dimethacrylate (a), diacrylate (b) and monoacrylate (c) or the composition comprising the urethane acrylate (d) in addition to the foregoing components (a), (b) and (c) is used.

Claims: A Fresnel or lenticular lens sheet comprising a transparent substrate and a lens portion composed of a cured product of an ultraviolet-curable resin composition being formed on at least one surface of the transparent substrate, wherein the polymerization shrinkage, calculated on the basis of the difference of the specific gravity at 25°C, of the ultraviolet-curable resin composition, caused at curing, is not larger than 20%, and the modulus of elasticity at 25°C after curing is 10 to 1,000 kg/cm², said ultraviolet-curable resin composition comprising, as main components, i) 30 to 60% by weight of a polyol dimethacrylate of the formula a): wherein ℓ and n are positive integers and the sum of ℓ and n is 8 to 12, 5 to 25% by weight of a polyol diacrylate of the formula b): wherein ℓ′ and n′ are positive integers and the sum of ℓ′ and n′ is 3 to 5, and 30 to 60% by weight of a tetrahydrofurfuryl acrylate of the formula c): and ii) a photopolymerization initiator. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the ultraviolet-curable resin composition further comprises 5 to 25% by weight of a urethane polyacrylate in addition to the three components a), b) and c). A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the ultraviolet-curable resin composition comprises at least one photopolymerization initiator selected from the group consisting of methylbenzoyl formate, 1-hydroxycyclohexyl phenyl ketone and 2-hydroxy-2-methyl-1-phenylpropane-1-one in an amount of 0.1 to 5 parts by weight per 100 parts by weight of the resin components. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the viscosity of the ultraviolet-curable resin composition at 25°C is not larger than 300 mPa·s. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the total luminous transmittance of the lens portion as calculated as the thickness of 1 mm is at least 85%. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein when the lens sheet is horizontally placed, the rise of the lens sheet satisfies the requirement represented by the following formula: D ≦ R/50 wherein D represents the rise in millimetres and R represents the distance in millimetres in the horizontal direction from the center of the lens sheet. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the transparent substrate is an acrylic resin plate. A Fresnel or lenticular lens sheet as set forth in claim 1, wherein the transparent substrate is a polyester film.

Inventors: AOSAI FUMITO C O PRODUCTS DEVE, INOUE MASAO C O OTAKE PLANTS, KOBAYASHI YUKIO C O PRODUCTS D, NAGAI SHOICHI C O PRODUCTS DEV, TAKESUE MASATOSHI C O PRODUCTS, AOSAI, FUMITO C/O PRODUCTS DEVELOPMENT LAB., INOUE, MASAO C/O OTAKE PLANTS, KOBAYASHI, YUKIO C/O PRODUCTS DEVELOPMENT LAB., NAGAI, SHOICHI C/O PRODUCTS DEVELOPMENT LAB., TAKESUE, MASATOSHI C/O PRODUCTS DEVELOPMENT LAB.
Applicants: MITSUBISHI RAYON CO, MITSUBISHI RAYON CO., LTD.
Application year: 1989
Main classification: G02B1
IPC codes: B29D11, G02B1, G02B3, B29C35