This invention is directed to radiation-curable liquid compositions that include an associative reactive blend. These compositions are useful in optical fabrication processes.

Optical fabrication techniques, as disclosed in U.S. Patent No. 4,575,330 to Hull, are utilized to form three-dimensional objects of complex shape using ultraviolet light to solidify superimposed layers of liquid ultraviolet-curable ethylenically unsaturated material at or near the surface of a liquid reservoir of such material. The ultraviolet dosage utilized is limited to speed the process and to ensure that only the irradiated areas at or near the surface of the liquid unsaturated material will be solidified. This, however, results in the objects being incompletely cured, becoming distorted and having inadequate strength and durability. Furthermore, when the objects are completely cured they tend to be very brittle and fragile. Free radical chain polymerization can be characterized as composed of three interdependent phases; initiation, propagation, and termination. It is generally desirable that the propagation phase procedes with a high velocity of monomer conversion and to a high degree of monomer conversion in order to achieve maximum physical properties of the polymer in question.

Associative monomer blends of an electron donating monomer and an electron accepting monomer display enhanced polymer propagation characteristics and form higher molecular weight alternating copolymers compared to non-associative monomer blends. These associative monomer blends are also referred to as "charge transfer complexes" and "electron releasing-withdrawing pairs". See, Plochocka,

K., Effect of the Reaction Medium on Radical Copolymerization, J. Macromol. Sci.-Rev. Macromol. Chem., C20 (1), 67-148 (1981). In, Odian, G., Principles of Polymerization, 2nd Ed., pp. 460-466 a supposedly quantitative mathematical relationship is given that can be used to predict relative monomer reactivities in copolymerization reactions, called the Alfred-Price Q-e scheme. Alfrey-Price Q-e scheme describes the electron donating monomer and electron accepting monomer as possessing lower and higher polarity values, respectively. However, these associated monomer blends, when used in prior art compositions, are unsuitable for use in optical fabrication techniques such as investment casting because the cured compositions expand upon heating and thus crack or distort the mold prior to removal therefrom.

Polymeric ionomers are high molecular weight polymers having pendant ionic functional groups, e.g., neutralized carboxylic acid groups, attached to a non-crosslinked polymeric backbone chain. Ionomers are produced by copolymerization of a nonionic backbone component and an ionizable monomer having a pendant acid group followed by neutralization of the acid group to form the iono er. See, Bazuin et al.. Modification of Polymer Properties Through Ion Incorporation, Ind. Eng. Chem. Prod. Res. Dev., 1981, Vol. 20, No. 2, pp 271-286 at p 272, Lundberg, Encyclopedia of Polymer Science and Engineering, 1987, Vol. 8, "Ionic Polymers", pp 393-423 at pp 395-400, and Reese, Encyclopedia of Polymer Science and Engineering, 1987, Vol. 4, "Cross-linking, Reversible", pp 395-417, at p 396. Neutralization is conducted after copolymerization because the unneutralized ionizable monomer copolymerizes much more readily than the neutralized ionizable monomer. See, Lundberg, Jji. at p 397.

The ions of the ionic functional groups aggregate with each other to form strong ionic bonds that result in the ionomers having characteristics normally associated with cross-linked thermoset polymers. The ionic bonds decompose

upon exposure to solvents and/or heat causing the ionomers to have characteristics normally associated with uncross-linked thermoplastic polymers. Bazuin et al.,

Lundberg and Reese, Id., describe ionomers in more detail.

Conventional ionomers are solids and cannot be utilized in applications, such as optical fabrication and coatings, wherein free-flowing liquids capable of rapid polymerization upon exposure to radiation are required and neutralization cannot be performed after copolymerization.

Thus, there is a need to provide compositions that permit relatively rapid and inexpensive production of accurate parts by optical fabrication techniques. The present invention overcomes the aforementioned shortcomings of the prior art and provides new and improved compositions for use in optical fabrication processes.

The present invention is directed to a radiation-curable liquid composition that includes an associative ethylenically unsaturated reactive blend and a resinous poly(meth)aerylate oligomer. The associative reactive blend is an admixture of monomers, oligomers, or at least one monomer and at least one oligomer that provides a ratio of electron donating groups to electron accepting groups in the range of about 5:1 to about 1:5.

The associative reactive blend is present in the liquid composition in an amount in the range of about 10 to about 90, preferably about 30 to about 80, weight percent. The resinous poly(meth)acrylate oligomer is present in the liquid composition in an amount in the range of about 10 to about 90, preferably about 20 to about 70, weight percent. The above weight percents are based upon the total weight of the liquid composition.

In a preferred embodiment of the present invention, the viscosity of the radiation curable liquid composition is less than about 1000 mPas (1 mPas=l cps), preferably less than about 800 mPas, and more preferably in the range of about 100 to about 800 mPas measured by a Brookfield Viscometer, Model RV, from Brookfield Engineering

Laboratories, Inc., Stoughton, Maine.

The radiation utilized to cure the liquid composition is preferably actinic energy having a wavelength in the ultraviolet (UV) and/or visible range. Curing of the liquid composition results in the production of a cross- linked polymeric matrix.

The radiation-curable liquid composition can be utilized to produce objects and as a coating for substrates such as glass (especially optical glass fibers), plastic, wood, concrete and the like.

The liquid composition can be utilized in a variety of optical fabrication processes, as for example investment casting or in the process set forth in the aforementioned Hull patent. When used to produce a pattern for investment casting, a pattern produced from the liquid composition is invested in a refractory material utilizing a conventional investment procedure. The refractory material and invested pattern are then heated to an elevated temperature to cause removal of the pattern and produce the mold.

The term "(meth)aerylate", and various grammatical forms thereof, identifies esters that are the reaction product of an acrylic or methacrylic acid with mono- or poly-hydroxy compounds, such as ethanol, butanol, ethylene glycol, trimethylol propane and the like.

The electron donating monomers and oligomers suitable for use in the associative reactive blend can be specified as having a negative polarity value according to the Alfrey Price Q-e scheme referred to hereinabove.

Alternatively, the electron donating monomers and oligomers can be specified as having an electron releasing substituent at the ethylenic carbon. Representative substituents that provide an electron releasing group include alkyl, alkoxy, amino, alkenyl and phenyl groups, cyclic moieties containing nitrogen or phosphorous and the like.

Representative electron donating monomers and oligomers include alkyl styrenes, vinyl pyrrolidone, styrene, vinyl caprolactam, vinyl imidazole, vinyl

pyrridine, dialkylaminoalkyl acrylates, polystyrene, dialkylaminoalkyly polyacrylates, the like and mixtures thereof.

The above alkyl groups preferably contain 1 to about 4 carbon atoms, e.g., methyl, ethyl, propyl, and butyl groups.

The electron accepting monomers and oligomers of the associative reactive blend have a positive polarity value according to the Alfrey Price Q-e scheme referred to hereinabove. Alternatively, the electron accepting monomers and oligomers can be specified as having an electron withdrawing substituent at the ethylenic carbon. Representative substituents that provide an electron withdrawing group include carboxyl, carbonyl, and cyano groups, halogens and the like.

Representative electron accepting monomers and oligomers include acrylic acid, carboxyalkyl acrylates, adducts of a hydroxyalkyl acrylate and an anhydride, e.g., an adduct of hydroxyethyl acrylate and succinic anhydride, and an adduct of hydroxyethyl acrylate and phthalic anhydride, C, to about C, ₂ alkyl acrylate esters, alkyl acrylamidoglycolate alkyl ethers, isobutoxyalkyl acrylamides, N,N-dialkyl acrylamides, tetrahydrofurfuryl acrylate, diesters of an epoxy function resin that is a diglycidyl ether of bisphenol A, e.g., Ebecryl 3700 commercially available from Radcure Specialties, the like and mixtures thereof. The above alkyl groups wherein the chain length is not specified preferably contain 1 to about 4 carbon atoms.

Preferred associative reactive blends include vinyl pyrrolidone/carboxyalkyl acrylates, vinyl pyrrolidone/metal carboxylic acid partial salts, vinyl pyrrolidone/alkyl acrylamidoglycolate alkyl ethers and vinyl caprolactam/- carboxyalkyl acrylates.

The most preferred blend comprises vinyl capro- lactam/carboxyalkylacrylates.

The associative reactive blend preferably has a

ratio of electron donating groups to electron accepting groups in the range of about 2:1 to about 1:2. Most preferably the ratio is about 1:1.

The compositions of the present invention can also contain minor amounts of metal carboxylic acid partial salts of an ethylenically unsaturated acid having at least one pendant carboxylic acid group per acid molecule. The term "partial salt", as used in its various grammatical forms, indicates that some, but not all, of the carboxylic acid groups are neutralized to produce the salt. The metal carboxylic acid partial salt is produced by partial neutralization of an ethylenically unsaturated acid having at least one pendant carboxylic acid group per acid molecule with a metal-ion producing component. Representative acids include ethylenically unsaturated carboxylic acids, saturated polycarboxylic acids partially esterified with an ethylenically unsaturated alcohol, carboxyalkyl(meth)acrylates, the reaction product of a hydroxyalkyl(meth)acrylate and an anhydride, the like and mixtures thereof.

The ethylenically unsaturated carboxylic acids can be monocarboxylic or polycarboxylic acids. Representatives of these acids are (meth)acrylic acid, crotonic acid, oleic acid, fumaric acid, maleic acid, itaconic acid, elaidic acid, and the like.

Representative saturated polycarboxylic acids that are esterified with an ethylenically unsaturated alcohol include oxalic acid, succinic acid, adipic acid and the like. Suitable ethylenically unsaturated alcohols include allyl alcohol, crotyl alcohol, and the like.

The carboxyalkyl(meth)acrylates preferably have C, to C ₄ alkyl groups and are represented by carboxymethyl- acrylate, carboxyethylacrylate, carboxybutylmethacrylate, and the like.

The hydroxy alkyl(meth)acrylates that are reacted with an anhydride preferably have C, to C. alkyl groups and are represented by hydroxymethylmethacrylate, hydroxypropyl-

acrylate, hydroxybutylacrylate and the like. Representative anhydrides are succinic anhydrides, maleic anhydrides, phthalic anhydrides, and the like.

The number average molecular weight of the acid is preferably about 50 to 600, more preferably about 70 to about 300 daltons.

The term "dalton", as used in its various grammatical terms, identifies a unit of mass that is l/12th the mass of carbon-12.

The metal ion-producing component provides at least one metal ion that is preferably monovalent, divalent or trivalent and is capable of neutralizing the acid groups of the ethylenically unsaturated acid. Representative of the metal ions are those of the Group la, lb, Ila, lib. Ilia, IVa, Vb, and VIII elements of the Periodic Table of Elements. Preferred metal ions are sodium, lithium, barium, magnesium, calcium, zinc and aluminum. Preferred metal ion-producing components are metal hydroxides, metal carbonates, metal acetates, the like, and mixtures thereof. Representative of these metal ion-producing components are magnesium hydroxide, zinc hydroxide, calcium carbonate, sodium acetate and barium acetate.

The metal ion-producing component and the amount utilized are selected to achieve the desired percent neutralization of the acid groups of the ethylenically unsaturated acid. When the percent neutralization of acid groups is too low, the ions of the ionic functional groups are unable to form strong ionic bonds and therefore do not form aggregates. While not desiring to be bound by a theory, it is presently believed that the formation of aggregates enhances the polymerization of the ethylenically unsaturated metal carboxylate partial salt. When the percent neutralization is too high, the cured material becomes brittle. Therefore, the metal ion-producing component is present in an amount sufficient to neutralize a percentage of the acid groups and the percent neutralization does not

prevent the formation of aggregates.

Preferably, about 2 to about 65, more preferably about 10 to about 50, percent of the acid groups are neutralized. Most preferably, about 30 to about 40 percent of the acid groups are neutralized.

Neutralization can be accomplished by admixing the acid and the metal ion-producing component at ambient o temperature and pressure, i.e., a temperature of about 20 C o to about 30 C. and a pressure of about 1 atmosphere. A solvent can be utilized to facilitate admixing. Preferably, the solvent is reactive with the other materials of the composition. Representative solvents include vinyl pyrrolidone, vinyl caprolactam and the like.

The resinous poly(meth)acrylate oligomers suitable for use in the present invention are subject to considerable variation. The poly(meth)acrylate oligomers contain an average of at least about 1.2, and more preferably at least about 2.0, (meth)acrylate groups per molecule. The (meth)- acrylate oligomers should have a flowable viscosity and be stable at the operating conditions and are selected to achieve these ends.

The resinous poly(meth)acrylate oligomers can be a diacrylate of an epoxy functional resin. These diacrylates are exemplified by the commercial product Ebecryl 3700 available from Radcure Specialties, which is the diester of Epon 828 and acrylic acid. Epon 828 is an epoxy functional resin that is a diglycidyl ether of bisphenol A and is commercially available from Shell Chemicals, New York, NY. The number average molecular weight of Ebecryl 3700 is about 500 daltons and of Epon 828 is about 390 daltons.

Further representative of the diacrylates is Ebecryl 3703 commercially available from Radcure Specialties which is an amine modified Ebecryl 3700.

RDX 26936 is an epoxy dimethacrylate resin having a number average molecular weight of about 550 daltons which is commercially available from Interey, Inc., Louisville, KY that is a representative dimethacrylate.

Poly(meth)acrylate-modified polyurethanes are also useful as the resinous poly(meth)acrylate oligomer, especially those that have a polyester base. Particularly preferred are polyacrylate-terminated polyurethanes that are the urethane reaction products of a hydroxy-functional polyester, especially those having an average of about 2 to about 5 hydroxy groups per molecule, with a monoacrylate monoisocyanate.

These poly(meth)acrylate-modified polyurethanes can e.g. be obtained from a polyester made by reacting trimethylol propane with caprolactone to a number average molecular weight of about 600 daltons followed by reaction of one mole of the polyester with three moles of the reaction product of 1 mole of 2-hydroxyethyl acrylate with 1 mole of isophorone diisocyanate. The end product is a polyurethane triacrylate. The urethane-forming reaction is conventionally performed at about 60°C in the presence of about 1% by weight of dibutyltin dilaurate.

A commercial, polyester-based, polyacrylate- modified polyurethane that is useful herein is ϋvithane 893, available from Morton Thiokol Inc., Chicago, IL. The polyester in the ϋvithane 893 product is the reaction product of adipic acid with about 1.2 molar proportions of ethylene glycol polyesterified to an acid number of less than about 5. This polyester is converted as described above to a polyacrylate-modified polyurethane that is a semi-solid at ambient temperature and that has an average of about 0.15 to about 0.175 ethylenically unsaturated groups per 100 grams of resin. The number average molecular weight of Uvithane 893 is about 620 daltons.

In polyester processing, the acid number, defined as the number of milligrams of KOH base required to neutralize one gram of polyester, is used to monitor the progress of the reaction. The lower the acid number, the further the reaction has progressed.

An additional polyacrylate-modified polyurethane that is suitable as the poly(meth)acrylate oligomer is the

reaction product of a diisocyanate, a hydroxyalkyl acrylate and a catalyst reacted at a temperature of about 40°C for a time period of 4 hours followed by reacting therewith a commercial hydroxy end-functional caprolactone polyester at a temperature of about 60°C for a time period of about 2 hours. An illustrative polyacrylate-modified polyurethane can be prepared from 1 mole of isophorone diisocyanate, 1 mole of 2-hydroxyethyl acrylate, about 1 weight percent, based on the weight of the diisocyanate, acrylate and catalyst, dibutyltin dilaurate (a catalyst) and 1 mole of the caprolactone polyester. A suitable caprolactone polyester is the reaction product of caprolactone and an alkylene glycol reacted at a temperature of about 60°C for a time period of 4 hours. An illustrative caprolactone polyester can be prepared from about a 2:1 mole ratio of caprolactone: ethylene glycol. A commercial caprolactone polyester is available from Union Carbide Corp., New York, NY, under the trade designation Tone M-100 which has a number average molecular weight of about 345 daltons.

Another representative poly(meth)acrylate oligomer suitable for use in the present invention is Potting Compound 363, a modified acrylate, commercially available from Locktite Corporation, Newington, CT.

A process for making poly(meth) crylate oligomers suitable for use in the present invention is described in U.S. Patent No. 4,100,141 to 0'Sullivan.

The resinous poly(meth)acrylate oligomer preferably includes both acrylate- and methacrylate- functional materials to further minimize distortion in the optical fabrication process. Most preferably, the poly(meth)acrylate oligomer includes at least about 40 weight percent, based on the weight of the ethylenically unsaturated material utilized, of acrylate-functional material (including vinyl monomers having a radiation polymerization mechanism similar to acrylates) and at least about 5 weight percent of methacrylate-functional material.

Optionally, the radiation-curable liquid

composition can include an inert thermoplastic material that can be a monomer, oligomer, compound or mixture thereof. Upon cure of the thermoplastic material-containing composition, a cross-linked three-dimensional matrix having intersticial spaces that contain thermoplastic material is produced. Thus, the thermoplastic material is dispersed throughout the matrix. Objects produced by curing the thermoplastic material-containing composition can be utilized in an investment casting method as disclosed in U.S. Patent No. 4,844,144 to Murphy, et al.

Investment casting is a conventional industrial process that employs a disposable pattern that is used to produce a ceramic mold in which a part can be cast.

Attempts to utilize optically fabricated objects as patterns for investment casting have been unsuccessful because the pattern is made of a cross-linked, rigid, thermoset polymer produced by curing the liquid ethylenically unsaturated material. Thus, the patterns do not melt when heated, but instead expand. This thermal expansion of the rigid object causes the mold material in which the pattern is invested to crack or distort before the pattern can be heated sufficiently to cause it to be removed. Upon exposure to the elevated temperature, the thermoplastic material is removed from the interstices as by flowing therefrom or decomposition which prevents thermal expansion of the matrix from destroying the mold. Upon exposure to the elevated temperature, the thermoplastic material is removed from the interstices as by flowing therefrom or decomposition which prevents thermal expansion of the matrix from destroying the mold.

The inert thermoplastic material is present in the liquid composition in an amount in the range of about 5 to about 40, preferably about 10 to about 25, weight percent. The thermoplastic material can be a monomer, oligomer, or mixture thereof and can also be referred to as thermoplastic compound.

The thermoplastic material is substantially

chemically inert, i.e., non-reactive, with the remainder of the composition. Thus, the thermoplastic material cannot contain any reactive ethylenic functionality, e.g., an acrylate group. Reactive groups, such as hydroxyl groups or carboxyl groups, can be present in the thermoplastic material provided the ethylenically unsaturated materials do not contain groups that are reactive therewith. The thermoplastic material also should not adversely effect the radiation cure of the composition from the liquid to the solid state. Thus, amine groups that can adversely effect cure, and cause the thermoplastic material to chemically bond with the polymeric matrix that is formed, are preferably excluded.

The thermoplastic material is sufficiently soluble in the remainder of the radiation-curable composition to provide uniform distribution of the thermoplastic material in the cross-linked, thermoset polymer matrix that is produced. A non-soluble thermoplastic material can cause scattering of the radiation used to cure the composition thus resulting in loss of dimensional accuracy of the pattern.

The thermoplastic material suitable for use in the present application must flow (flow may result from depolymerization as well as softening) at a temperature less than the temperature at which, in the absence of the thermoplast, the degree of thermal expansion of the pattern would crack or deform the mold. The temperature at which, the pattern would destroy the mold is partially dependent upon the size, thickness and composition of the mold, the thickness of the pattern, and the like. The presence of the thermoplastic material reduces the softening temperature of the pattern. The term "depolymerize", as used in its various grammatical forms, means a reduction in molecular weight. Such reduction can cause the thermoplastic material to flow by making the material softer, by lowering its melting point, or even by vaporizing a portion of the thermoplastic

material. The objective is to weaken the polymeric matrix of the pattern so that it yields instead of destroying the mold.

The thermoplastic material should not significantly add to the viscosity of the overall radiation-curable composition.

The thermoplastic oligomers can be a liquid at room temperature, i.e., a temperature of about 20° to about 30°C. However, patterns (objects) formed by the present composition are solid at about room temperature, the liquid oligomer being held within the cross-linked polymeric matrix that is formed upon curing. The thermoplastic oligomer desirably has a number average molecular weight in the range of about 200 to about 5000, preferably 250 to 1500 daltons, and preferably is a liquid or waxy solid at room temperature.

The thermoplastic oligomer preferably has a melting point at a temperature below about 100°C, most preferably about 10°C to about 40°C, since this permits adequate weakening of the polymeric matrix on heating while retaining maximum strength (as measured by tensile modulus) at room temperature. The thermoplastic oligomer typically has a relatively higher molecular weight (as compared to the thermoplastic compounds) or melts over a relatively broad temperature range of greater than about + 10°C. Such thermoplastic oligomers desirably have a melting point below about 100°C to inhibit expansion of the pattern.

Illustrative thermoplastic oligomers suitable for use in the present composition include natural waxes, e.g., animal waxes (beeswax), vegetable waxes (carnauba), mineral waxes (ozecerite, paraffin, and microcrystalline petroleum), synthetic waxes (ethylenic polymers, ethylenic polyol ether- esters, and chlorinated naphthalenes), plasticizers (phthalate, adipate and sebacate esters of alcohols containing about 4 to about 22 carbon atoms and of polyols such as ethylene glycol, glycerol, and pentaerythritol).

Low molecular weight polyesters formed by reacting a large excess of a diol with a polycarboxylic acid, such as adipic acid or trimellitic acid are also useful. Combinations of the foregoing are also useful.

Preferred thermoplastic oligomers are low molecular weight polyesters, e.g., epsilon caprolactone polyester polyols. These are made by polyesterifying a polyol, such as ethylene glycol, propylene glycol or butylene glycol, with the lactone. Polyols with more than two hydroxy groups are also useful, such as trimethylol propane and pentaerythritol. Control of the proportion of lactone and the selection of the polyol permits selection of a polyester having the desired number average molecular weight. Triols, such as trimethylol propane, are particularly useful in this process and are preferred.

Two preferred thermoplastic oligomers that are epsilon caprolactone polyesters of a polyhydric alcohol and that are useful herein are the commercial products Tone 0301 and Tone 0310. These are available from Union Carbide Corp. of New York, NY. Tone 0301 is a polyester formed by esterifying ethylene glycol with the caprolactone to provide a number average molecular weight of about 300 daltons. This product is a liquid at room temperature. Tone 0310 is a polyester formed by .esterifying trimethylol propane with the caprolactone to provide a number average molecular weight of about 900 daltons. This product is a waxy solid at room temperature, melting at about 32°C. The thermoplastic compounds are generally solid at room temperature, and are easily heat softenable. The melting point of the thermoplastic compound is at a temperature less than about 150°C, preferably less than about 125°C. The compound has a sharp melting point and preferably goes from a solid state to a liquid state over a temperature range of preferably + about 5°C, more preferably + about 3°C, of the melting point. Typically, these compounds are relatively pure, i.e., commercial technical grade purity. Patterns formed from the present composition

are generally solid at about room temperature.

The preferred thermoplastic compounds have a number average molecular weight of less than about 250 daltons, preferably about 120 to about 210 daltons.

The compounds can be aliphatic or aromatic in nature, and linear, branched or cyclic in structure. Provided they meet the requirements of being substantially monomeric, solid at ambient temperature, soluble in the ethylenically unsaturated liquid composition, nonreactive with respect to the free radical reaction of the unsaturated liquid material, and possess a sharp melting point less than about 150°C. Suitable thermoplastic compounds are selected from the group consisting of caprolactam, 2,2 dimethyl-3-hydroxy propyl propionate (which is commercially available from Union Carbide Corp., New York, NY, under the designation Esterdiol 204), dimethyl terephthalate, dimethyl cyclo- hexanol, dimethyl dioxane dione, the like and mixtures thereof. The thermoplastic material can contribute to the flexibility of the cured object.

A conventional photoinitiator effective to initiate radiation-polymerization upon exposure to actinic energy is utilized. The radiation-curable liquid composition can be supplied without the photoinitiator which can be added prior to cure. Representative photoinitiators include Darocur 1173 which is a benzyl ketal-based photoinitiator commercially available from EM Chemicals that contains 2-hydroxy- 2-methyl-l-phenyl-propane-l-one as the active ingredient and Irgacure 184 which is an aryl ketone photoinitiator commercially available from Ciba Geigy Corp. that contains hydroxycyclohexyl phenyl ketone as the active ingredient. Suitable photoinitiators for use with visible light are disclosed in European Patent Application No. 0 097 012 to Patel.

The photoinitiator can be present in an amount in the range of about 1 to about 10 weight percent based upon the total weight of the associative reactive blend and the

poly(meth)acrylate oligomer.

Preferably the associative reactive blend is present in an amount in the range of about 30 to about 60 weight percent, the resinous poly(meth)acrylate oligomer is present in an amount in the range of about 20 to about 40 weight percent and the inert thermoplastic material is preferably present in an amount in the range of 10 to about 25 percent.

The viscosity of the radiation-curable composition is preferably less than about 10,000 mPas. More preferably, the viscosity of the radiation-curable composition is in the range of about 200 to about 2000 mPas. Most preferably, the viscosity is in the range of about 300 to about 800 mPas. The viscosity is measured at a temperature of 25°C using a conventional Brookfield viscometer operated in accordance with the instructions provided therewith. Low viscosity helps in the formation of thin layers in the optical fabrication process, and it also helps in draining away excess liquid composition when the specimen is removed from the bath of liquid composition in which it was formed.

The radiation-curable liquid composition is preferably cured utilizing radiation in the form of actinic energy preferably having a wavelength in the range of about 200 to about 550, more preferably about 250 to about 450, nanometers (nm).

The radiation is provided in an amount effective to convert the radiation-curable liquid composition to the cross-linked three-dimensional matrix having the inert thermoplastic material present in the intersticial spaces of the matrix.

The radiation-curable liquid composition can be utilized to produce objects in a conventional optical fabrication process such as objects that are suitable as patterns in an investment casting process.

A preferred method of making the object is by optical fabrication wherein the radiation-curable liquid composition is utilized as the liquid in a reservoir. A thin

liquid layer is formed upon a supporting platform of the optical fabrication device. This thin layer is solidifed, by at least partial cure, by exposure to radiation to form a cross-section of the object. A thin layer of radiation- curable liquid composition is applied over the solidified layer. This process of applying a thin layer of liquid radiation-curable liquid composition and solidifying it is repeated to superimpose one layer upon another to produce the three-dimensional object of partially cross-linked polymer within the liquid reservoir. Solidification of a layer causes it to adhere at least to the previously solidified layer. The term "solidify" as used in its various grammatical forms defines an at least partially cured yet self-supporting condition.

The following examples are presented by way of illustration, and not limitation, of the present invention.

EXAMPLE 1

Radiation-curable liquid composition 1 to 5 that contained an associative reactive blend were prepared by admixing the components of TABLE I, below, in a suitable vessel.

Comparative composition Cl that did not contain any associate reactive blend was prepared by admixing the components of TABLE I, below, in a suitable vessel. Aliquots of the compositions were placed in reservoirs and cured utilizing a helium-cadmium laser having output of 11 milliwatts at a wavelength of 325 nm. The laser was traced at a speed to achieve a radiation dose of 0.17

2 joules per square centimeters (j/cm ). A greater specimen thickness indicates a faster cure. The percent conversion was determined by removing the specimen from the reservoir, removing uncured composition therefrom and obtaining an initial weight of the specimen. The specimen was then placed in methylethyl ketone

(MEK) both at room temperature for a time period of 24

hours. After this time period, the specimen was removed from the MEK, dried at a temperature of about 65°C for a time period of two hours and weighed to obtain the final weight. The final weight was divided by the initial weight and the result was multiplied by 100 to calculate the percent conversion.

The green strength was qualitatively determined by making a square specimen, removing the specimen from the reservoir and holding it in a set of tweezers. If the specimen was self supporting, it was given an excellent rating. If the specimen could not support its own weight at all, it was given a poor rating. Ratings of good and fair were given to specimens whose self-supporting ability was between excellent and poor.

TABLE I COMPOSITIONS AND PHYSICAL PROPERTIES

COMPOSITIONS (parts by weight)

COMPONENT 1 2 3 4 Cl Ebecryl 3700 ¹ 60 — — 30 30

Uvithane 892 ² ~ 60

RDX 26936 ³ — — 60

Tone 0301 ⁴ — — — 16 16

Tetrahydrofurfuryl acrylate — — — 10 10 ς Isobornyl acrylate — — — 22 22

5 Isobutoxymethyl acrylamide — — — — 22

₅ Phenoxy ethyl acrylate 20 20 20

Vinyl caprolactam 20 20 20 22

Darocur 1173 ⁷ 4 4 4 4 4

TABLE I (continued)

COMPOSITIONS AND PHYSICAL PROPERTIES

COMPOSITIONS (parts by weight)

Cl

Physical Properties Results

Specimen thickness, mils 21 24 18 24 23

% conversion 78.3 76.1 71.5 61.1 53.4 o

Green strength exc. exc.exc. exc. fair

A polyacrylate oligomer commercially available from

Radcure Specialities 2

A polyester-based, polyacrylate-modified polyurethane oligomer that is commercially available from Morton Thiokol, Inc. 3

An epoxy dimethacrylate oligomer commpercially available from Interey, Inc.

An inert thermoplastic material commercially available from Union Carbide Corp. 5 An electron accepting monomer

6 An electron donating monomer 7 A photoinitiator commercially available from E.M.

Chemicals

Excellent

The radiation-curable liquid compositions of the present invention (1-4) all exhibit good specimen thickness and percent conversion and excellent green strength. In contrast, comparative example Cl has a significantly lower percent conversion and only fair green strength.

EXAMPLE 2

Preparation of a Metal Carboxylic Acid Partial Salt

A metal carboxylic acid partial salt was prepared by admixing 92 parts by weight of carboxyethyl acrylate commercially available from Radcure Specialties and 8 parts by weight of magnesium hydroxide commercially available from Aldrich Chemical, Milwaukee, Wisconsin at ambient conditions. Admixing was continued until substantial homogeneity was achieved. Approximately, 40% of the acid groups of the carboxyethyl acrylate were neutralized and a metal (magnesium) carboxylic acid partial salt was produced.

EXAMPLE 3

Effect of Metal Carboxylic

Acid Partial Salt Upon Initiation

The effect of utilizing a metal carboxylic acid partial salt was studied using a resin premix that was an admixture of 40 parts by weight of Ebecryl 3700, commercially available from Radcure Specialties, 20 parts by weight of trimethylpropane triacrylate and 20 parts by weight of phenoxyethyl acrylate. Aliquots of the resin premix were admixed with 20 parts by weight of a control additive neopentylglycol propoxy diacrylate (control 1), the electron accepting monomer carboxyethyl acrylate (Cl) or the electron accepting metal (magnesium) carboxylic acid salt of carboxyethyl acrylate prepared in accordance with EXAMPLE 2 (C3). Aliquots of the resin premix were also admixed with 10 parts by weight of either neopentylglycol propoxy diacrylate (control 2), carboxyethyl acrylate (composition 5) or the metal (magnesium) carboxylic acid partial salt of carboxyethyl acrylate (composition 6) and 10 parts by weight of the electron donating monomer vinyl pyrrolidone.

Control 1 and Cl and C2 are not according to the invention in that they contain only a control component NPGPDA respectively only the electron accepting part of the

associative blend. Control 2 only contains the electron donating vinyl pyrrolidone together with the NPGPDA.

Compositions 5 and 6 both contain the associative blend according to the invention.

The initiator utilized for all compositions was an admixture of benzophenone and ethyl diethanol araine.

2 A dose of 0.5 j/cm was applied to the surface of the reservoir to determine solidificationdepth verses dose.

Bulk polymerization characteristics were evaluated by exposing a 10 gram aliquot of each composition to a dose

2 of 1.0 j/cm from a medium pressure mercury-arc lamp followed by MEK extraction to determine the relative percent conversion. All percent conversions were normalized so that the control 1 composition, i.e., the neopentylglycol propoxy diacrylate-containing composition that lacked vinyl pyrrolidone, had a normalized percent conversion of 100.

The test results are presented in Table II, below.

TABLE II EFFECT OF METAL CARBOXYLIC ACID PARTIAL SALT

VARIABLE

Normaliz Vinyl Solidification Percent

Composition Additive Pyrrolidone De th mils Conversi

Control 1 NPGPDA No

2

C2 CEA No

C3 MCA Salt' No

Control 2 NPGPDA Yes

5 CEA ² Yes

6 MCA Salt- Yes

1 2 Neopentylglycol propoxy diacrylate 3 Carboxyethyl acrylate

Metal (magnesium) carboxylic acid partial salt of carboxyethyl acrylate

Not measured because the structure was too weak

Comparative compositions C2 and C3 which did not utilize the associative reactive blend exhibited poor test results when compared to the control 1 composition.

Radiation-curable liquid compositions 5 and 6 utilized the associative reactive blend and exhibited improved test results as compared to comparative compositions C2 and C3. The controls 1 and 2 do not contain the associative blend and are therefor not usefull for applications such as optical fabrication, or investment casting, although their curing behaviour is relatively good.

This experiment shows that the detoriation of the curing behaviour due to the addition of the metal carboxylic acid partial salty (C3) can be ameliorated by adding the other part of the associative blend (composition 6).

EXAMPLE 4 A radiation-curable liquid composition 7 that contained an associative reactive blend was prepared by admixing the components of TABLE III, below, in a suitable vessel. A comparative composition C4 that did not contain an associative reactive blend was prepared by admixing the components of TABLE III, below, in a suitable vessel. The post cured flexibility was qualitatively determined by postcuring under a medium pressure mercury/arc lamp, and then bending the specimen.

TABLE III

COMPOSITIONS AND PHYSICAL PROPERTIES Composition (Parts By Weight)

Component

Ebecryl 3703 ³

Tone 0301

Tetrahydrofurfuryl acrylate Carboxyethyl acrylate

Phenoxy ethyl acrylate

Vinyl caprolactam

₇ Darocur 1173

Physical Properties

Viscosity, cp Specimen thickness, mils % Conversion Green strength

Postcured flexibility

A radiation-curable liquid composition of the present invention that contained an associative reactive blend.

A comparative composition that did not contain an associative reactive blend.

A polyacrylate oligomer commercially available from

Radcure Specialties.

A thermoplastic material commercially available from

Union Carbide Corporation.

An electron accepting monomer.

An electron donating monomer.

A photoinitiator commercially available from EM

Chemicals.

Both of the compositions exhibit the desired low viscosity. However, the radiation-curable liquid composition 7 of the present invention exhibited a faster cure, as indicated by a greater specimen thickness, a higher percent conversion and superior green strength as compared to the comparative composition C4 that did not contain the associative reactive blend.

EXAMPLE 5

Compositions were prepared utilizing a resin premix that was an admixture of 54 parts by weight of Ebecryl 3700 commercially available from Radcure Specialties, 28 parts by weight of Tone 0301 commercially available from Union

Carbide Corporation and 18 parts by weight of tetrahydro- furyl acrylate. Aliquots of the resin premix were admixed in a suitable vessel with the other components listed in TABLE IV, below, to produce compositions. Compositions 8 to 14, represent the radiation-curable liquid compositions of the present invention that contain an associative reactive blend. Comparative compositions C5 to C7 do not contain an associative reactive blend. The compositions were cured and tested in accordance with the methods described in EXAMPLES 1 and 3 above.

TABLE IV COMPOSITIONS AND PHYSICAL PROPERTIES

COMPOSITION (Parts by_ Weight)

COMPONENT 8 £ i M 12 I3 .1i C5 C6 C7

Resin premix ¹ 56 56 56 56 56 56 56 56 56 56

Darocur 1173 ² 4 4 4 4 4 4 4 4 4 4

Carboxyethyl acrylate ³ 22 — — — 22 — ~ — 22 Acryloxyethyl acid phthalate ³ — — 22 22

2-Ethylhexyl acrylate ³ — 22 — — — — — — 22 22

Methyl acryl- amido-glyco- late methyl eetthheerr ³ — — — ~ — 22 22 — — 22 Vinyl

4 ppyyrrrroolidone — — — — — 22 — 22

Vinyl ccaapprroolactam 22 22 — — — — — 22

Vinyl imidazole ⁴ — — 22

Diethylamino- ethyl acrylate — — — 22 22 — 22

TABLE IV (continued) COMPOSITIONS AND PHYSICAL PROPERTIES

PHYSICAL PROPERTIES RESULTS

COMPONENT 8. £ 10 H 12 13 1.4 C5 C6 C7

Viscosity, cp 400 200 300 400 600 200 400 200 300 30

Specimen thickness, mils 24 24 18 25 26 26 22 27 21 2

Green

5 strength exc. good good good exc. exc. good fair poor p

% Conversion 55.6 53.8 NR ⁶ NR 48.2 56.2 46.1 46.8 43.2 38

Resin premix that is an admixture of 54 parts by weight of Ebercyl 3700, 28 parts by weight of Tone

0301 and 18 parts by weight of tetrahydrofurfurol acrylate 2

A photoiniator commercially available from EM

Chemicals. 3 An electron accepting monomer

4 An electron donating monomer 5 Excellent 6 Not reported

All of the present compositions (2 to 8) possessed the desired characteristics for the reported test results. Comparative composition C2 only utilized electron donating monomers and exhibited only fair green strength.

This result would have been worse except that the Ebecryl 3700 and tetrahydrafurfurl acrylate possess ethylenic bonds that act as electron acceptors.

Comparative compositions C3 and C4 only utilized electron accepting monomers and exhibited poor green strength and the lowest percent conversion.