PROCESS FOR THE PRODUCTION OF THERMOFORMED ARTICLES

Abstract: Optical articles are obtained by polymerizing, in the form of a flat plate, by means of a free-radical initiator, a polymerizable liquid composition containing diethylene glycol bis(allyl carbonate), tris-hydroxyeth­ ylisocyanate tris(allyl-carbonate) and vinyl acetate and/or methyl methacrylate and then submitting the so-­ obtained plate to thermoforming.

Description: The present invention relates to the production of thermoformed optical articles and to a polymerizable liquid composition useful to that purpose. In the field of optical articles the products are interesting which derive from the polymerization of glycol bis(allylcarbonates), e.g., those disclosed in European Patent Application/Publn. No., 35304, and among these, outstandingly interesting from the industrial viewpoint are the prod­ ucts of polymerization of diethylene glycol bis(allyl carbonate), and this in view of the optical and mechanical characteristics of these particular polymerizates. More precisely, said optical articles can be obtained by sub­ mitting to polymerization a liquid composition of dieth­ ylene glycol bis (allylcarbonate), and of one or more polymerization initiators, by means of the casting tech­ nique, inside glass moulds of shape corresponding to that of the article the production of which is desired. However, for the preparation of some optical arti­ cles it results advantageous replacing the technique of casting inside single moulds with the technique providing the production of flat plates, which are subsequently bent by thermoforming. In particular, the product out­ coming from the polymerization, as a flat plate, is heated to a temperature higher than glass transition temperature and is subsequently formed inside the mould, by suitably applying a pressure or a vacuum, in corres­ pondence of one of the major surfaces of the plate. This thermoforming technique is presently used for the produc­ tion of lenses, curved screens, visors, and the like. However, as the products of polymerization of dieth­ ylene glycol bis(allylcarbonate) are poorly thermoforma­ ble, due to their considerable crosslinking degree, in the industrial practice such diethylene glycol bis(allyl­ carbonate) is copolymerized with one or more monofunc­ tional monomers, with plastifying characteristics, such as methyl methacrylate, vinyl acetate, and so forth, so as to produce polymerizates with improved thermoforming characteristics. It has been observed however that the articles ob­ tained by the thermoforming of said plastified polymeri­ zates are able to maintain their shape over long time periods only if they are stored at temperatures equal to room temperatures, or slightly higher. When on the contrary they are kept at relatively slightly increased temperatures for even short time pe­ riods (e.g., one hour at 60°C), they sensibly loose the curvature imposed by the thermoforming, by tending to reassume their inital flat shape, and this is greatly disadavantageous. It has been found now that this draw­ back can be overcome, or at least considerably alle­ viated, when to the thermoforming polymerizates obtained starting from a particular polymerizable liquid composi­ tion are submitted. Accordingly, the present invention relates to a process for the preparation of thermoformed optical arti­ cles, by means of the polymerization, in the form of flat plate, of a polymerizable liquid composition, followed by the thermoforming of said flat plate to give the desired optical article, said process being characterized in that the polymerizable liquid composition contains: (A) from 30 to 80% by weight of diethylene glycol bis(al­ lylcarbonate); (B) from 10 to 40% by weight of tris-hydroxyethylisocyanurate tris(allylcarbonate); and (C) from 10 to 40% by weight of vinyl acetate or of meth­ yl methacrylate; the composition containing moreover at least one free­ radical polymerization initiator in an amount of from 1 to 5% by weight relatively to the sum of the weights of components (A), (B) and (C). The component (A) of the polymerizable liquid compo­ sition can be defined by the formula: wherein R is the radical of diethylene glycol. This component can be prepared by reaction between diallylcar­ bonate and diethylene glycol, in a molar ratio to each other equal to or greater than 6/1, and preferably of the order of 12/1, operating in the presence of a basic catalyst, e.g., according to what disclosed in European Patent Application Publn.N°353004. Under these conditions, a reaction product is ob­ tained which is constituted by at least about 70% by weight by compound (I), the residual percentage being constituted by oligomer diethylene glycol bis(allylcar­ bonates). Thus, the component (A) of the polymerizable liquid composition can be constituted by the compound (I) only, or by a mixture containing at least about 70% by weight of compound (I), the residual percentage being constituted by oligomers thereof. In the preferred form of practical embodiment, the component (A) is present in the polymerizable liquid composition in amounts of from 45 to 65% by weight. The component (B) of the polymerizable liquid compo­ sition can be defined by the general formula: wherein R I is the radical of tris-hydroxyethylisocyanur­ ate. This component can be prepared by means of the reaction between diallylcarbonate and tris-hydroxyethyli­ socyanurate, in a molar ratio to each other equal to or higher than about 6/1 and preferably of the order of 12/1, in the presence of basic catalysts, similarly to the foregoing component (A). Under these conditions a reaction product is ob­ tained, which is constituted by about 60% by compound (II), the residual percentage being constituted by oligo­ mer tris-hydroxyethylisocyanurate tris(allylcarbonates). Thus, the component (B) of the polymerizable liquid com­ position can be constituted by the compound (II) alone, or by a mixture containing at least about 60% by weight of the compound (II), the residual percentage being con­ stituted by oligomers thereof. In the preferred form of practical embodiment, the component (B) is present in the polymerizable liquid composition in an amount comprised within the range of from 10 to 25% by weight. The component (C) of the polymerizable liquid compo­ sition is constituted by vinyl acetate and/or methyl methacrylate which, in the preferred practical embodi­ ment, are present in amounts of from 15 to 30% by weight. The polymerizable liquid composition according to the present invention contains moreover one of more po­ lymerization initiators, soluble in the same composition, and able to generate free radicals within a temperature range of from about 30 to about 120°C. Non-limitative examples of said initiators are diisopropylperoxydicar­ bonate, dicyclohexylperoxydicarbonate, di-sec.butylper­ oxydicarbonate, dibenzoyl-peroxide and tert.butylperben­ zoate. In the preferred form of practical embodiment, the polymerization initiator is present in amounts of from 2 to 4.5% by weight, relatively to the total of the weights of components (A), (B) and (C). The polymerizable liquid composition of the present invention can additionally contain one or more conven­ tional additives, such as stabilizers, mould-release agents, dyes, pigments, U.V. light and I.R. light ab­ sorbers or the like, however in an overall amount not higher than 1% by weight relatively to the total of the weights of components (A), (B) and (C). The liquid composition of the present invention is converted into the related polymerizate by heating at a temperature comprised within the range of from 30 to 120°C. The corresponding polymerization times are of the order of 3-100 hours. The polymerizates are prepared directly as flat plates, with thickness generally ranging from 1 to 4 mm, operating inside suitable moulds. These flat plates are finally submitted to thermo­ forming by heating at a temperature higher than the glass transition temperature, and generally comprised within the range of from 140 to 200°C and subsequent curvature in a mould, not heated, with the application of vacuum between the plate and the same mould. In this way, thermoformed optical articles are ob­ tained, such as lenses (for sunglasses or for goggles), protecting visors, visors for motorcyclist helmets and the like, provided with high thermal resistance. Moreo­ ver, said articles display a whole of optical and mechan­ ica1 characteristics similar to those of the commercial articles formed by the polymerizates of diethylene glycol bis(allylcarbonate), but relatively to these latter show considerably improved characteristics of thermal resis­ tance. In particular, the polymerizates obtained from com­ positions containing components (A) and (B) only are not transformable, and those obtained from compositions con­ taining components (B) and (C) only show a whole of optical and mechanical characteristics considerably lower than those of diethylene glycol bis(allylcarbonate) poly­ merizate. By means of the liquid and polymerizable composi­ tions of the present invention, polymerizates and related thermoformed optical articles are achieved, the whole of characteristics of which results unexpectedly good in view of the characteristics achievable when single com­ ponents of the composition, or couples of the same com­ ponents are used. In the following experimental Examples, the follow­ ing components have been used for the polymerizable liq­ uid composition: (A) Diethylene glycol bis(allylcarbonate): product from the reaction between diallylcarbonate and diethylene glycol, in the molar ratio of 12/1 to each other, operating according to what disclosed in European Patent Application publ. N. 35304; This reaction product contains 85-90% of diethylene glycol bis(allylcarbonate), the residual percentage being constituted by oligomers of the same. (B) Tris-hydroxyethylisocyanurate tris(allylcarbonate): product from the reaction between diallylcarbonate and tris-hydroxyethylisocyanurate in the molar ratio of 12/1 to each other, obtained by operating accord­ ing to the general lines of said European patent application. This reaction product contains 70-80% of tris-hydroxyethylisocyanurate tris(allylcarbonate), the residual percentage being constituted by oligo­ mers of the same; (C) The components (A), (B) and (C) are mixed in the suitable ratios, the polymerization initiator is then added and the so-obtained liquid compositions are poured into moulds constituted by two flat plates, with dimensions 20x20 cm, provided with a gasket of plastified polyvinyl chloride of 3.2 mm in thickness. The polymerization is carried out in forced-air cir­ culation oven, at the temperature of 48°C for 72 hours. At the end of this time period the plates are kept two hours at 110°C, to the purpose of destroying the possible unreacted catalyst and of eliminating inner stresses. On so-obtained polymerizates, the following characteris­ tics are determined: a) Optical characteristics - refractive index n, by Abbe refractometer (ASTM D-­ 1003); - Haze (%) and visible transmittance (%), measured by Gardner Hazegard XL-211 (ASTM D-1003); - Yellow index (YI), defined as determined by means of Gardner XL-805 colorimeter (ASTM D-1925). b) Physical and mechanical characteristics - Density in g/ml, determined by specific-gravity balance at the temperature of 20°C (ASTM D-792); - Rockwell (M) hardness, measured by Rockwell durometer (ASTM D-785); - Abrasion resistance, Taber method (ASTM D-1004 modified), executed with a pair of VS-10F grinding wheels and under a load of 500 g on both wheels. The results are expressed as the ratio of the number of passages necessary to produce a Haze increase of 10% for the sample under test, to that for a refer­ ence sample (polymethylmethacrylate, PMMA); - Notch-free IZOD impact resistance (ASTM D-256 modified). c) Thermal characteristics - Distorsion temperature under load (HDT) (°C/1.82 MPa), (ASTM D-648). The thermoforming is carried out on flat specimens of 75x65 mm, of 1.4 mm in thickness, prepared from the corresponding flat plates, obtained by polymerization of the liquid compositions under the previously described conditions. The specimens are softened at 150°C for 2 minutes and are then bent on an aluminium mould, coated with a film of silicone rubber, with base curvature 6. This step of 2.5 minutes of duration is carried out at room tempe­ rature, by applying a vacuum between the specimen and the mould. After this time period, the bent speciment (useful, e.g., as a neutral lens for sunglasses), now cold, is extracted from the mould, and its curvature is measured by means of a spherometer, obtaining constant values in the nearby of base 6. The heat resistance of heat-bent specimens is eva­ luated by determining the decrease of their curvature after heat treatment of 1 hour at 60°C and of 1 hour at 60°C + 1 hour at 70°C, in forced-circulation oven. The following experimental Examples are illustrative and not limitative of the scope of the invention. Example 1 A liquid composition (Composition I) formed by: To comparison purposes, a liquid composition (Composition II) formed by: The compositions I and II are submitted to polymeri­ zation, operating at the temperature of 48°C, over a time of 72 hours. The polymerizate obtained from composition I shows optical and mechanical characteristics similar to those of the polymer obtained from composition II, but has a HDT value higher by about 15°C. Such characteristics of polymerizates are reported in Table 1. With such polymerizates , neutral lenses heat-bent to base 6 have been prepared, on which the heat resis­ tance is evaluated, by determining their capability to maintain the initial curvature, submitting them to the previously described heat treatments. The results, reported in Table 2, demonstrate that the heat-bent lenses obtained by starting from the poly­ merizate of composition I display a heat resistance con­ siderably higher then the heat-bent lenses obtained starting from the polymerizate of composition II. Example 2 The liquid compositions III and IV having the fol­ lowing composition, expressed as % by weight are pre­ pared: The compositions III and IV are submitted to polym­ erization, operating as described in Example 1, and the characteristics of the polymerizates are shown in Table 3. With said polymerizates, lenses heat-bent to base 6 are prepared, on which the heat resistance is determined by determining their capability to maintain the initial curvature, submitting them to the heat treatments as previously described. The results are reported in Table 4. Example 3 Four compositions (from V to VIII) are prepared, respectively containing 15%, 20%, 25% and 30% by weight of vinyl acetate, and 4% by weight of dicyclohexylperoxy­ dicarbonate, the residual percentage being constituted by diethylene glycol bis(allylcarbonate) and by tris-hydr­ oxyethylisocyanurate tris(allylcarbonate) in weight ratio to each other kept constant and equal to 75/25. In a similar way, further four compositions (from IX to XII) are prepared, respectively containing 15%, 20%, 25% and 30% by weight of vinyl acetate and 4% by weight of dicyclohexylperoxydicarbonate, the residual percentage being constituted by diethylene glycol bis(allylcarbon­ ate) and by tris-hydroxyethylisocyanurate tris(allylcar­ bonate) in weight ratio to each other kept constant and equal to 70/30. The compositions from V to XII are submitted to polymerization, by operating as described in Example 1. With said polymerizates, neutral lenses heat-bent to base 6 are prepared, on which the heat resistance is evaluated, by determining their capability to maintain the initial curvature, submitting them to the heat treat­ ments as previously described. The results are reported in Table 5. In these Examples, the compositions VI and X are respectively equal to compositions I and IV, but they are reported in the present Example, to the purpose of show­ ing more clearly the influence of the concentration of vinyl acetate on the characteristics of eventually ob­ tained articles. Example 4 Three compositions (from XIII to XV) are prepared, respectively containing 15%, 20% and 25% by weight of methyl methacrylate, and 3% by weight of dicyclohexylper­ oxydicarbonate, the residual percentage being consti­ tuted by diethylene glycol bis(allylcarbonate) and by tris-hydroxyethylisocyanurate tris(allylcarbonate) in weight ratio to each other kept constant and equal to 75/25. Said compositions are submitted to polymerization, by operating as described in Example 1. With said polymerizates, neutral lenses heat-bent to base 6 are prepared, on which the heat resistance is evaluated, by determining their capability to maintain the initial curvature, submitting them to the heat treat­ ments as previously described. The results are reported in Table 6.

Claims: 1. Process for the preparation of thermoformed optical articles, by means of the polymerization, in the form of flat plate, of a polymerizable liquid composi­ tion, followed by the thermoforming of said flat plate to give the desired optical article, said process being characterized in that the polymerizable liquid composi­ tion contains: (A) from 30 to 80% by weight of diethylene glycol bis(allylcarbonate); (B) from 10 to 40% by weight of tris-hydroxyethylisocyan­ urate tris(allylcarbonate); (C) from 10 to 40% by weight of vinyl acetate or of meth­ yl methacrylate; the composition containing moreover at least one free-­ radical polymerization initiator in an amount of from 1 to 5% by weight relatively to the sum of the weights of components (A), (B) and (C). 2. Process according to claim 1, characterized in that the component (A) is the product of the reaction between diallylcarbonate and ethylene glycol, in a molar ratio to each other equal to or greater than 6/1, and that it contains at least about 70% by weight of diethylene glycol bis(allylcarbonate), the residual percentage being constituted by oligomers thereof. 3. Process according to claim 1, characterized in that the component (B) is the product of the reaction between diallylcarbonate and tris-hydroxyethylisocyanur­ ate in a molar ratio to each other equal to or higher than about 6/1 and that it contains about 60% by weight of tris-hydroxyethylisocyanurate tris(allylcarbonate), the residual percentage being constituted by oligomers thereof. 4. Process according to claim 1, characterized in that the polymerization initiator is selected among di­ isopropylperoxydicarbonate, dicyclohexylperoxydicarbon­ ate, di-sec.butylperoxydicarbonate, dibenzoyl-peroxide and tert.butylperbenzoate. 5. Process according to claim 1, characterized in that the polymerizable liquid composition contains from 45 to 65% by weight of component (A), from 10 to 25% by weight of component (B), and from 15 to 30% by weight of component (C), the polymerization initiator being present in an amount of from 2 to 4.5% by weight relatively to the sum of the weights of components (A), (B) and (C). 6. Process according to claim 1, characterized in that the polymerizable liquid composition additionally contains at least one from stabilizers, mould-release agents, dyes, pigments, U.V. light and I.R. light ab­ sorbers or the like, in an overall amount not higher than 1% by weight relatively to the total of the weights of components (A), (B) and (C). 7. Process according to claim 1, characterized in that the polymerization of the polymerizable liquid com­ position is carried out at temperatures comprised within the range of from 30 to 120°C, for times of from 3 to 100 hours, to yield plates with thickness of from 1 to 4 mm, and that said plates are thermoformed, after heating at a temperature of from 140 to 200°C, inside not-heated moulds. 8. Polymerizable liquid composition suitable to yield thermoformable polymerizates, characterized in that it contains: (A) from 30 to 80% by weight of diethylene glycol bis(al­ lylcarbonate); (B) from 10 to 40% by weight of tris-hydroxyethylisocyan­ urate tris(allylcarbonate); (C) from 10 to 40% by weight of vinyl acetate or of meth­ yl methacrylate; said composition containing moreover from 1 to 5% by weight, relatively to the sum of the weights of compo­ nents (A), (B) and (C), of one free-radical polymeri­ zation initiator. 9. Thermoformed optical articles obtained from the polymerizable liquid compositions and by the process of preceding claims.

Inventors: RENZI, FIORENZO, RIVETTI, FRANCO, ROMANO, UGO
Applicants: ENICHEM SYNTHESIS S.P.A.
Application year: 1986
Main classification: C08F218
IPC codes: C08F218, C08F220, C08L31, B29L11, C08F18, B29C51, G02B1