PHOTOCURABLE COMPOSITIONS AND METHOD OF INVESTMENT CASTING

Abstract: A method of investment casting that utilizes a pattern produced by stereolithography is disclosed. The pattern has a polymeric matrix that has intersticial spaces dispersed throughout. The spaces have therein a thermoplastic material that flows from the matrix upon heating during the investment casting process, softening the pattern to prevent the thermal expansion of the pattern from cracking or deforming the mold.

Description: PHOTOCURABLE COMPOSITIONS AND METHOD OF INVESTMENT CASTING DESCRIPTION Technical Field This invention relates to photocurable liquid compositions and a method of investment casting utilizing stereolithography to produce a three-dimensional object from these photocurable compositions. The compositions include a thermoplastic material. Background of the Invention Investment casting is a conventional industrial process which employs a disposable pattern that is used to produce a ceramic mold in which a part can be cast. The pattern is conventionally made by injecting a liquid pattern material, e.g., wax, a heat-softenable plastic and the like, into a pattern die. The pattern die is typically manufactured from a durable material, e-g-, aluminum, steel, and the like, by conventional machining processes. The pattern is removed from the die after the pattern material solidifies. A refractory material, e.g., an aqueous ceramic slurry, is then built up around the pattern to invest the pattern therein. A mold is produced by heating the refractory material having the pattern invested therein to remove the pattern and fuse the refractory material. The details of investment casting process vary depending upon the type of metal to be cast in the mold. The casting of ferrous alloys will be used to illustrate a conventional investment casting process. The pattern is coated with successive layers of refractory material. Each layer is coated with fine ceramic sand and dried before the next layer is applied. Usually about 10 to 20 layers are utilized to invest the pattern in the refractory material. The invested pattern is then placed in an open ended metal container which is filled with a coarse slurry of ceramic back-up material which hardens. The container is then placed into a furnace or autoclave. The temperature of the furnace or autoclave is elevated to cause the refractory material to dry and then fuse. The pattern is removed, as by melting or burning out the material constituting the same, during the heating step. The resulting fused ceramic structure is the desired mold. Removal of the pattern leaves a cavity in the container corresponding in shape and dimension to the final part. The cavity (and therefore the pattern) can be slightly larger than the final part to compensate for the shrinkage which takes place in the subsequent casting operation, or to allow for machining when desired. The mold is sometimes fired to burn out the last traces of pattern material and to fuse the refractory material before the cavity is filled with molten metal. This firing process proceeds slowly in a controlled cycle which can be in a time range of 12 to 18 hours to avoid cracking the mold. The molten metal is introduced into the cavity of the mold and solidified by cooling to form a casting. After solidification, the mold is broken away to release the part. The process described above is relatively slow and expensive because of the time and cost of the machining required to make the dies used in forming the pattern. Consequently, the prior art process is impractical for use when only a few parts are desired. Even when a larger number of parts is desired, the prior art process is expensive because it may be necessary to prepare several sets of pattern dies having varying dimensions. This is because the size of the pattern (and thus of the pattern die) that is necessary to compensate for shrinkage of the pattern or shrinkage and machining of the cast part must be determined empirically. A number of patterns of varying sizes are typically produced until the proper size is determined to achieve the desired dimensions of the cast part. The problem of shrinkage is particularly severe in connection with casting materials such as powdered metal where shrinkage may amount to 35 volume percent or more of the cast part. 'Elimination of expensively machined pattern dies would be beneficial for economic and production reasons. It is known, as illustrated in U.S. Patent No. 4,575,330 to Hull, to form three-dimensional objects of complex shape using computer guided ultraviolet light to solidify superposed layers of a liquid ultraviolet-curable ethylenically unsaturated material at the surface of a liquid reservoir of such material. The partially polymerized objects formed in this manner are removed from the reservoir and cured to strengthen the objects. Dimensionally accurate objects, that are usually thin-walled, are formed in this manner. These thin-walled objects usually have a wall thickness of about 0.05 inches. Thicker walls are obtained by forming thin walls in a honeycomb-like structure which results in hollow-walled objects. This process for making objects is known as "stereolithography". Attempts to utilize these stereolithographically produced objects as patterns for investment casting have been unsuccessful heretofore. This is because the so-formed pattern is made of a cross-linked, rigid, thermoset polymer produced by radiation-curing an ethylenically unsaturated material. Thus, the patterns do not melt when heated, but instead expand. This thermal expansion of the rigid object causes the refractory material in which the pattern is invested to crack or distort before the pattern can be heated sufficiently to cause it to be removed. The problem of thermal expansion is even greater when stereolithographically produced solid objects or objects having relatively thick, solid walls are utilized. Thick, solid walls can be produced by filling the honeycomb structure of the hollow-walled object with ethylenically unsaturated material and curing it therein. Other methods can also produce objects having a wall thickness up to 1/8 of an inch or even up to 1/4 of an inch. These thick-walled objects retain their strength better as the heat expands them. Thus, solid objects and objects having thick, solid walls are more likely to cause the mold to crack or distort than are the thin-walled object. If the mold is cracked or distorted it is useless. The combination of stereolithography with investment casting in this invention provides a powerful production system because it enables the relatively rapid and inexpensive production of accurate patterns for use in the investment casting process. To accomplish this, the present invention has overcome the aforementioned shortcomings of both conventional investment casting and stereolithography. More particularly, the burden and expense of forming the patterns are reduced by forming them using stereolithography, and the resulting patterns do not destroy the mold when used in an investment casting process. Summary of the Invention In accordance with this invention, a method of investment casting is disclosed which utilizes a stereolithographically formed pattern that loses structural rigidity when exposed to elevated temperatures. Radiation-polymerizable (photopolymerizable) liquid compositions from which the above-described pattern can be produced are also disclosed. The liquid composition comprises an ethylenically unsaturated material and a thermoplastic, low molecular weight material that is an oligomer or compound and which (1) is inert with respect to, and soluble in, the ethylenically unsaturated material, and (2) has a melting point less than about 150 * C. When an oligomer is used, it desirably has a melting point at a temperature below about 100° C. When a compound is used, it is a solid at ambient temperature, i.e., about 20°C. to about 35°C. , and has a sharp melting point at a temperature less than about 150°C. Mixtures of oligomers and compounds can also be utilized. The present method, which is suitable for producing a mold from a stereolithographically produced pattern, comprises the steps of: (a) investing a pattern in a refractory material, the pattern being a cross-linked polymeric matrix of a radiation-cured ethylenically unsaturated material having distributed throughout the above-described thermoplastic material, the thermoplastic material being present in an amount effective to prevent the pattern from cracking or distorting a mold during a heating step; and (b) heating the refractory material and the pattern to produce the mold. A method of investing the pattern .has been previously discussed in more detail. The pattern is a cross-linked, thermoset polymer matrix of the radiation-cured ethylenically unsaturated material. The thermoplastic material, which is easily heat-softenable, is distributed throughout the matrix. The thermoplastic material flows from the matrix when the pattern is heated thus weakening the matrix and inhibiting thermal expansion thereof. This weakening of the matrix also causes the pattern to have a softening point at a temperature that is sufficiently low to prohibit destruction of the mold as by cracking or distortion. Further heating can cause the pattern to decompose and be burned out of the mold. As previously discussed, solid thick-walled patterns having a thickness of up to about 1/4 of an inch and manufactured from conventional, non-thermoplastic material-containing compositions maintain a high degree of rigidity at the elevated temperature encountered in the investment casting process, destroying the mold. However, solid, thin- and thick-walled patterns manufactured from the present liquid composition and having the thermoplastic material distributed throughout can be utilized in investment casting because the thermoplastic materials flow from the matrix of the pattern lessening the rigidity of the pattern at elevated temperatures. The thermoplastic compounds which are preferred, are ambient temperature solids having a sharp melting temperature of less than about 150°C. and further enhance the efficacy of stereolithographically produced objects in investment casting as compared to oligomers. These compounds have a sharp melting point and thus go from a solid state to a liquid state within a relatively small temperature range. The flowability of the compound in the liquid state results in expulsion of the compound from the pattern more readily and hence loss of structural rigidity. Detailed Disclosure of the Invention Although this invention is susceptible to embodiment in many different forms, preferred embodiments of the invention are shown. It should be understood, however, that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated. The present invention includes a method suitable for producing a mold from a stereolithographically produced patten comprising the steps of investing a pattern in a refractory material, the pattern comprising a polymeric matrix having a thermoplastic material distributed throughout, and heating the refractory material and the pattern to remove the pattern and produce a mold. The thermoplastic material is present in an amount effective to prevent the pattern from cracking or distorting the mold during heating and has a low molecular weight and is an oligomer or a compound having a meting temperature less than about 150 β C. When an oligomer is utilized it desirably has a melting point at a temperature below about 100°C. When a compound is utilized it is an ambient temperature, i.e., about 20°C. to about 35°C. , solid having a sharp melting point less than about 150°C. Mixtures of oligomers and compounds are also suitable for use in this invention. The term "sharp melting point", in its various grammatical forms, indicates that the compound goes from a solid state to a liquid state over a relatively narrow temperature range. Radiation-polymerizable liquid compositions suitable for producing the pattern are also disclosed. The composition comprises an ethylenically unsaturated material and the thermoplastic material. The thermoplastic material is dissolved in the liquid ethylenically unsaturated material. The pattern is produced by curing the composition to produce a polymeric matrix of the cured ethylenically unsaturated material having intersticial spaces containing the thermoplastic material. A conventional stereolithographic process, as disclosed in the aforementioned Hull Patent, can be utilized to produce the pattern. The present radiation-polymerizable (photopolymerizable) liquid composition is used in the reservoir. The object constituting the pattern can be utilized in a conventional investment casting process as previously discussed. The thermoplastic material is substantially chemically inert, i.e., non-reactive, with the other materials, constituents and components of the composition. Thus, the thermoplastic material cannot contain any reactive ethylenic functionality, e.g., an acrylate group. Reactive groups, such as hydroxy groups or carboxy 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 ethylenically unsaturated material 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 the degree of thermal expansion of the pattern cracks or deforms the mold. The temperature at which the pattern destroys 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 herein 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, or by lowering its melting point, or even by vaporizing a portion of it. 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 composition which preferably is less than about 10,000 centipoise (cp) . More preferably, the viscosity is in the range of about 200 to about 2000 cp. Most preferably, the viscosity is in the range of about 300 to about 800 cp. 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 stereolithographic 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 thermoplastic oligomers can be a liquid at ambient temperature. However, patterns (specimens) formed by the present composition are solid at about ambient temperature, the liquid oligomer being held within the cross-linked polymeric matrix that is formed. 27 10 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 which is soluble in the ethylenically unsaturated liquid. The term "dalton", as used in its various grammatical forms, defines a unit of mass that is l/12th the mass of carbon-12. The 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 oligomers typically have a relatively higher molecular weight (as compared to the thermoplastic compounds) or melt over a relatively broad temperature range of greater than about + 10°C. Such oligomers desirably have a melting point below about 100°C. to inhibit expansion of the pattern. Illustrative 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 oligomers. Two 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 solid at ambient 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 727 12 the present composition are solid at about ambient 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 onomeric, solid at ambient temperature, soluble in the ethylenically unsaturated liquid composition, nonreaσtive 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 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 cyclohexanol, dimethyl dioxane dione, the like and mixtures thereof. The compounds are presently the preferred thermoplastic material. The ethylenically unsaturated materials used in the stereolithographic process can vary considerably and are conventional. It is preferred that the ethylenically unsaturated material not include reactive groups other than the ethylenic unsaturation to ensure avoidance of reaction with the thermoplastic compound, Reactive groups other than the ethylenic unsaturation can be tolerated only if these groups do not react with the thermoplastic compound. An exemplary radiation-polymerizable liquid composition that is suitable for use in the present invention can include a liquid ethylenically unsaturated material that polymerizes by a free-radical mechanism. The ethylenically unsaturated material comprises a resinous (meth)acrylate copolymerizable and cross-linkable component [ (meth)acrylate component] dissolved in a liquid reactive diluent, preferably an ethylenically unsaturated liquid that can comprise a liquid mono(meth)acrylate, a liquid poly(meth)acrylate, or a mixture of these liquids, and a photoinitiator. The term "(meth)acrylate", and various grammatical forms thereof, identifies esters that are the reaction product of an acrylic or a methacrylic acid with mono- or poly-hydroxy compounds, such as ethanol, butanol, ethylene glycol, trimethylol propane and the like. The terms "(meth)acrylate copolymerizable and cross-linkable component" and "(meth)acrylate component", and various grammatical forms thereof, identify (meth)acrylates and poly(meth)acrylates. These terms also identify monomers and polymers that have a * radiation-polymerization mechanism similar to (meth)acrylates. The term "reactive diluent", and various grammatical forms thereof, identifies a diluent capable of dissolving, and copolymerizing with, the (meth)acrylate component. The resinous (meth)acrylate copolymerizable and cross-linkable component suitable for use in the present invention can contain monomers and polymers and is subject to considerable variation. The (meth)acrylate component contains an average of at least about 1.2, and more preferably at least about 2.0, (meth)acrylate groups per molecule. The (meth)acrylate component should have a flowable viscosity and be stable at the operating conditions and the monomers and polymers are selected to achieve these ends. The resinous (meth)acrylate component can also be a poly(meth)acrylate containing an average of at least about two (meth)acrylate groups per molecule, e.g., a diacrylate of an epoxy functional resin. These diacrylates are exemplified by the commercial product Novacure 3700 available from Interez, Inc., Louisville, KY, 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 Novacure 3700 is about 500 daltons and of Epon 828 is about 390 daltons. Poly(meth)acrylate-modified polyurethanes are also useful as the resinous (meth)acrylate component, 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 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 Uvithane 893, available from Thiokol Chemical Corp., Trenton, NJ. The polyester in the Uvithane 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. In polyester processing, the acid number, defined as the number of milligrams of 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 (meth)acrylate component 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 illustrative ethylenically unsaturated material 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 ethylenically unsaturated material suitable for use as the present invention is described in U.S. Patent No. 4,100,141 to O 1 Sullivan. The resinous (meth)acrylate component preferably includes both acrylate- and methacrylate- functional materials to minimize distortion in the stereolithographic process. Most preferably, the (meth)acrylate component includes at least about 40 weight percent, based on the weight of the ethylenically unsaturated constituent, 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. The resinous (meth)acrylate component is dissolved in a liquid reactive diluent that preferably includes a polyethylenically unsaturated liquid material such as a poly(meth)acrylate. Liquid tri(meth)acrylates, e.g., trimethylol propane triacrylate, and di(meth)acrylates, e.g., 1,6-hexanediol di(meth)acrylate, are suitable. Liquid tetra(meth)acrylates, e.g., pentaerythritol tetraacrylate, are also useful. Preferred liquid poly(meth)acrylates include Sartomer C 9003, a polypropoxylate-modified diacrylate of neopentyl glycol with an average of two propylene oxide units per molecule and having a number average molecular weight of about 330 daltons and SR 339, a 2-phenoxyethyl acrylate, both commercially available from Sartomer, West Chester, PA. A triacrylate that is suitable as the liquid reactive diluent is Photomer 4094, a propoxylated glyceryl triacrylate having a number average molecular weight of 430 daltons which is commercially available from Henkel Corp., Ambler, PA. The liquid reactive diluent also preferably includes a monoethylenically unsaturated liquid material such as a mono(meth)acrylate. Suitable materials include phenoxyethyl(meth)acrylate, hydroxyethyl- (meth)acrylate, hydroxypropyl(meth)acrylate, N-vinyl pyrrolidone, and the like. Mixtures of these monoethylenically unsaturated materials are also suitable. The liquid reactive diluent is preferably a mixture of monoethylenically and polyethylenically unsaturated materials in a weight ratio of about 4:1 to about 1:4, respectively. An optional, reactive diluent can be a liquid N-vinyl monomer that is regarded to be embraced by the term (meth)acrylate because its radiation-polymerization mechanism is similar to that of (meth)acrylates. Illustrative N-vinyl monomers include N-vinyl pyrrolidone and N-vinyl caprolactam, with N-vinyl pyrrolidone being preferred. A photoinitiator effective to initiate radiation-polymerization upon exposure to actinic energy such as light in or near the ultraviolet and visible ranges, e.g., light having a wavelength of about 200 to about 500 nanometers, is utilized. The radiation-polymerizable liquid composition can be supplied without the photoinitiator, the photoinitiator being added prior to cure. These photoinitiators are themselves well known and in common use. They are usually ketonic, and frequently aromatic, such as the benzophenones. Darocur 1173 is an illustrative, commercially available benzyl ketal-based photoinitiator from EM Chemicals that contains 2-hydroxy-2-methyl-l-phenyl-propane-l-one as the active ingredient. A commercially available aryl ketone photoinitiator, Irgacure 184, from Ciba Geigy Corp. that contains hydroxycyclohexyl phenyl ketone as the active ingredient is also suitable. The term "actinic energy", as used herein in its various grammatical forms, defines a type of light radiation capable of producing chemical change in the radiation polymerizable liquid composition. The resinous (meth)acrylate component is preferably present in an amount of about 15 to about 80, more preferably about 40 to about 70, weight percent of the weight of the ethylenically unsaturated material. The liquid reactive diluent is preferably • present in an amount of about 20 to about 80, more preferably about 30 to about 60, weight percent of the weight of the ethylenically unsaturated material. The photoinitiator is present in an amount of about 1 to about 10 weight percent of the weight of the ethylenically unsaturated material. The optional reactive diluent, when utilized, is present in an amount of about 10 to about 40 weight percent of the weight of the ethylenically unsaturated material and replaces an equal amount of the (meth)acrylate component. A stabilizer and polymerization inhibitor can also be present in an amount less than about 1 weight percent of the ethylenically unsaturated material. Illustrative is methyl ether of hydroquinone. The viscosity of the radiation-polymerizable liquid composition can be adjusted utilizing a diluent that is non-reactive and inert with respect to the ethylenically unsaturated material. The diluent can be present in an amount up to about 5 weight percent based on the ethylenically unsaturated material. When the diluent is present, the amount of the ethylenically unsaturated material present is reduced. An illustrative diluent is n-hexanol. When the thermoplastic material is a thermoplastic oligomer, the oligomer is present in an amount in the range of about 5 to about 50, preferably about 15 to about 35 weight percent, based on the total weight of the radiation-polymerizable liquid composition. Thus, the ethylenically unsaturated liquid material is present in an amount in the range of about 50 to about 95, preferably about 65 to about 85, weight percent based on the total weight of the radiation-polymerizable liquid composition. When the thermoplastic material is a thermoplastic compound, the compound is present in an amount in the range of about 5 to about 40, preferably about 15 to about 25 weight percent, based on the total weight of the radiation-polymerizable liquid composition. Thus, the ethylenically unsaturated liquid material is present in an amount in the range of about 60 to about 95, preferably about 75 to about 85, weight percent based on the total weight of the radiation-polymerizable liquid composition. Preferably, patterns produced from the present compositions have softening temperatures of about 180°C. or less. The term "softening temperature", as used herein in its various grammatical forms, indicates the temperature at which the cured composition and pattern produced therefrom, loses its structural rigidity. Typically, the object removed from the reservoir is somewhat gelatinous. The cure of this object to a rigid solid can be completed by conventional means such as further exposure to actinic energy after the object has been removed from the reservoir in which it was formed. Alternatively, the object can be cured by heat. A free-radical polymerization catalyst can be utilized to make the thermal cure more rapid or effective at lower temperature. Regardless of how the cure is completed, the cured object has the thermoplastic material dispersed within the interstices of the polymeric matrix and is suitable for use as a pattern. Actinic energy suitable for curing the radiation-curable liquid composition can be produced by a Liconix Model 4240 N, helium-cadmium laser having an output of 15 milliwatts at 325 nanometers focused to 350 micron diameter. The usual dosage is about 3.0 Joules per square centimeter of surface area which results in a polymerized layer about 20 mils thick. The wavelength of the actinic energy and dosage will vary depending upon the radiation-curable liquid composition utilized. The following example is provided as an illustration, and not as a limitation, of the present invention. EXAMPLE 1: Comparison of Percent Weiσht Loss for Various Compositions Comparative tests were conducted utilizing control compositions containing no thermoplastic materials (A and B) , compositions of the present invention each containing a different thermoplastic oligomer (C, D, E, and F) , and compositions of the present invention each containing a different thermoplastic compound (G and H) . The compositions were prepared by admixing the respective components of each composition in the proportions of TABLE I, below, at ambient temperature to a substantial homogeneous condition. TABLE I COMPOSITIONS FOR COMPARATIVE TESTING Composition (Parts by weight) Component B D E Novacure 3700 1 42.5 Uvithane 893 2 30 Epon 828 diacrylate 3 30 30 30 Photomer 4094 4 18.5 Trimethylol propane triacrylate 20 N-vinyl pyrrolidone 20 Tet ahydrofuran acrylate 20 20 20 Trimethylol propane ethoxy triacrylate 5 10 10 10 Trimethylol propane ethoxy trimethacrylate 6 10 10 10 SR 339 7 35.0 25.0 25.0 25.0 Polyethylene glycol 8 30 Tone 0310 9 30 30 20.0 Caprolactam 10 20.0 Esterdiol 204 11 20 Irgacure 184 12 4.0 4.0 4.0 4.0 1 Novacure 3700, commercially available from Interez Corp., Jefferson, NY, a bisphenol A based epoxy diacrylate having a number average molecular weight of 500 daltons. 2 Uvithane 893, commercially available from Thiokol Chemical Corp. , Trenton, NJ, a polyester-based acrylated polyurethane. 3 Epon 828, commercially available from Shell Chemicals, New York, NY, a diglycidyl ether of bisphenol A which is reacted with two moles of acrylic acid to produce the Epon 828 diacrylate. 4 Photomer 4094, commercially available from Henkel Corp. , Ambler, PA, a propoxylated glyceryl triacrylate having a number average molecular weight of 430 daltons. 5 Ethoxylated trimethylol propane reacted with three molar proportions of acrylic acid and having a number average molecular weight of 428 daltons. 6 Ethoxylated trimethylol propane reacted with three molar proportions of methacrylic acid and having a number average molecular weight of 475 daltons. 7 SR 339 is 2-phenoxyethyl acrylate, commercially available from Sartomer Corp., West Chester, PA. 8 Polyethylene glycol having a number average molecular weight of about 400 daltons. Tone 0310, commercially available from Union Carbide Corp., New York, NY, a thermoplastic oligomeric polycaprolactone polyester polyol having a number average molecular weight of 900 daltons and a melting temperature of about 32°C. 10 Caprolactam (2-oxohexamethyleneimine) , a thermoplastic compound commercially available from Aldrich Chemical Co. , Milwaukee, WI having a number molecular weight of 113 daltons and a melting point of 71°C. 11 Esterdiol 204 (2,2 dimethy1-3-hydroxypropyl 2,2 dimethyl-3-hydroxypropionate) , a thermoplastic compound commercially available from Union Carbide Corp., New York, NY having a number molecular weight of 204 daltons and melting point of 50°C. 12 Irgacure 184, commercially available from Ciba-Geigy Corp., Oak Brook, IL, is an aryl ketone photoinitiator. The viscosity of Compositions B, D and E were 600, 520 and 620 cp, respectively, as measured at 25°C. using a Brookfield viscometer. Reservoirs of these radiation-polymerizable liquids were exposed to ultraviolet light using a Liconix Model 4240 N, helium-cadmium light having an output of 15 milliwatts at 325 nanometers focused to a 350 micron diameter. The dosage was about 3.0 Joules per square centimeter of surface area which resulted in polymerized layers about 20 mils thick. The layer was formed in accordance with the previously described stereolithographic method and used as the object. The object was removed from the reservoir and unreacted radiation-polymerizable liquid was drained therefrom. This object was then conventionally cured to a rigid solid in a sealed chamber utilizing ultraviolet light. The cured object having a thickness of about 20 mils was then utilized as a pattern in the tests described hereinbelow. Patterns produced from Composition C exhibited about 20 to about 25 weight percent extractables in methyl ethyl ketone (MEK) . Weight percent extractables was determined by placing a pattern of known weight in a MEK bath having a temperature of about 25°C. for a time period of 2 hours. The pattern was then removed from "' the bath, dried and reweighed and the percent weight loss calculated. In contrast, specimens obtained from a composition similar to that of Composition C but without the thermoplastic oligomer (the Tone 0301) exhibited less than 1 weight percent extractable under the same conditions. This demonstrates that the oligomer did not react to become a part of the cross-linked polymeric matrix constituting the cured specimen, so as to be able to act independently of that matrix. Patterns produced from Composition C also exhibited significant softening and loss of structural rigidity and dimensional integrity at a temperature of about 250°C. In contrast, specimens obtained from a composition similar to that of Composition C but without the thermoplastic oligomer retained structural rigidity and dimensional integrity at this temperature. Patterns produced from Composition C were invested in a refractory slurry. The slurry was dried for a time period sufficient to cause hardening. The pattern and slurry were then heated to a temperature sufficient to remove the pattern and fuse the slurry. A mold was thereby produced. Patterns were subjected to thermographic analysis in which each pattern was heated at a controlled rate increase (10°C./minute) and the resulting weight loss was graphed as a function of temperature. The analysis was performed utilizing a thermogravimetric analyzer Model 651 commercially available from DuPont Co., Wilmington, Delaware operated in accordance with the manual of operation. The results of the thermographic analysis are presented in TABLE II. NA = Not available. The lower the temperature at which a 20 weight percent loss is achieved, the better adapted the pattern is for investment casting. The weight loss indicates reduced structural rigidity of the patterns produced therefrom.during the heating step of the investment casting process. Compositions D, G and H achieve this 20 weight percent loss at a temperature less than 300°C. This indicates that these compositions, which each utilize a thermoplastic material, are better suited for use in investment casting than the control Compositions A and B which did not utilize a thermoplastic material. Also, Compositions D, G and H achieved this weight percent loss at a lower temperature than Compositions E and F. However, patterns produced from Compositions E and F are suitable for use in investment casting, experiencing a significant weight loss, i.e., 56 weight percent, at 400°C. , which indicates the 20 weight percent loss was achieved at a much lower temperature. Patterns produced from Compositions A and B do not experience sufficient weight loss at a low enough temperature and can crack or distort the mold. The ambient temperature strength was measured conventionally for some of the compositions of TABLE I, above, and is reported as the tensile modulus [pounds per square inch (psi) ] in TABLE III. It is preferable to have adequate strength at ambient temperature with softening occurring at an elevated temperature, As shown in TABLE III, above. Composition B had a relatively high modulus. However, patterns produced from Composition B do not exhibit sufficient softening (see TABLE II and accompanying discussion) and are not suitable for investment casting. However, both Compositions D and E are suitable and Composition E is most suitable because it has a higher tensile modulus than Composition D. Physical properties, i.e., coefficient of thermal expansion and softening temperature, of patterns produced in accordance with this Example from some of the compositions of TABLE I, above, were determined. The results are provided in TABLE IV. TABLE IV PHYSICAL PROPERTIES Softening Composition temperature (°C . ) A 250°C . F 200°C. G 146°C. H 168°C. The determinations of the coefficient of thermal expansion and softening temperature were made using a Model 943 ther omechanical analyzer from DuPont Co., Wilmington, Delaware, according to the procedure specified by DuPont in the published manual of operation. Patterns produced from Composition A would thermally expand without softening and therefore crack or distort a mold. Patterns produced from Compositions F, G or H would not crack or distort a mold because of the relatively low coefficients of thermal expansion and softening temperatures. Furthermore, patterns produced from Compositions G and H, which each utilize a thermoplastic compound, exhibit lower softening temperatures than that of Composition F, which uses a thermoplastic oligomer, making these two compositions even more suitable for use in investment casting. This invention has been described in terms of specific embodiments set forth in detail, but it should be understood that these are by way of illustration only and that the invention is not necessarily limited thereto. Modifications and variations will be apparent from the disclosure and may be resorted to without departing from the spirit of the invention, as those skilled in the art will readily understand. Accordingly, such variations and modifications of the disclosed products are considered to be within the purview and scope of the invention and the following claims.

Claims: WE CLAIM: 1. A method of investment casting comprising the steps of: (a) investing a three-dimensional pattern in a refractory material, said pattern being a cross-linked polymeric matrix of a radiation-cured ethylenically unsaturated material having dispersed therein a thermoplastic material having a melting point of less than about 150"C, said thermoplastic material being non-reactive with said ethylenically unsaturated material and being present in an amount effective to prevent the pattern from cracking or distorting a mold during a heating step; and (b) heating said refractory material with said pattern within it to fuse the refractory material and to remove said pattern to produce the mold. 2. The method of investment casting as recited in claim 1 in which said pattern is produced from a radiation-polymerizable liquid composition comprising (1) an oligomer having a melting point below about 100°C. and a number average molecular weight in the range of about 200 to about 5000 daltons and (2) said ethylenically unsaturated material, said oligomer being present in an amount in the range of about 5 to about 50 weight percent based on the total weight of said radiation-polymerizable composition and said ethylenically unsaturated material being present in a range of about 50 to about 95 weight percent based on the total weight of said radiation-polymerizable composition. 3. The method of investment casting as recited in claim 1 in which the pattern is produced from a radiation-polymerized liquid composition comprising (1) an oligomer having a melting point within the temperature range of about 10° to about 40°C. and a number average molecular weight in the range of about 250 to about 1500 daltons and (2) said ethylenically unsaturated material, said oligomer being present in an amount in the range of about 15 to about 35 weight percent based on the total weight of said radiation-polymerizable composition and said ethylenically unsaturated material being present in a range of about 65 to about 85 weight percent based on the total weight of said radiation-polymerizable composition. 4. The method of investment casting as recited in claim 1 wherein said pattern is produced from a radiation polymerizable composition comprising (1) a thermoplastic compound having a number average weight less than about 250 daltons and having a sharp melting point and (2) said ethylenically unsaturated material, said theromplastic compound being present in an amount in the range of about 5 to about 40 weight percent of said radiation-polymerizable composition and said ethylenically unsaturated material being present in an amount in the range of about 60 to about 95 weight percent of said radiation-polymerizable composition. 5. The method of investment casting as recited in claim 1 wherein said pattern is produced from a radiation-polymerizable liquid composition comprising (1) a thermoplastic compound having a number average molecular weight of about 120 to about 210 daltons and having a sharp melting point at a temperature less than about 125 β C. and (2) said ethylenically unsaturated material, said thermoplastic compound being present in an amount in the range of about 15 to about 25 weight percent of said radiation-polymerizable composition and said ethylenically unsaturated material being present in an amount in the range of about 75 to about 85 percent weight percent of said radiation-polymerizable composition. 6. The method in accordance with claim 1 wherein said thermoplastic material is a thermoplastic compound selected from the group consisting of caprolacta , 2,2 dimethyl-3-hydroxy propyl 2,2 dimethyl-3-hydroxy propyl propionate, dimethyl terephthalate, dimethyl cyclohexanol , dimethyl dioxane dione and mixtures thereof. 7. The method of investment casting as recited in claim 1 wherein said pattern has a softening temperature of about less than about 180°C. 8. A radiation-polymerizable liquid composition adapted to produce patterns suitable for use in investment casting comprising (1) a thermoplastic material having a sharp melting point at a temperature less than 150"C. and a number average molecular weight less than about 250 daltons and (2) an ethylenically unsaturated material wherein said thermoplastic compound is non-reactive with said ethylenically unsaturated material and being present in an amount in the range of about 5 to about 40 weight percent of said radiation-polymerizable composition and said ethylenically unsaturated material being present in an amount in the range of about 60 to about 95 weight percent of said radiation-polymerizable composition. 9. The composition in accordance with claim 8 wherein said thermoplastic compound has a melting point at a temperature less than about 125 β C, a number average molecular weight of about 120 to about 210 daltons and being present in an amount in the range of about 15 to 25 weight percent of said radiation-polymerizable composition and said ethylenically unsaturated material is present in an amount in the range of about 75 to about 85 weight percent radiation-polymerization composition. 10. The composition in accordance with claim 9 wherein said thermoplastic compound is selected from the group consisting of caprolactam, 2,2 dimethyl-3-hydroxy propyl 2,2 dimethyl-3-hydroxy propyl propionate, dimethyl terephthalate, dimethyl cyclohexanol, dimethyl dioxane dione and mixtures thereof.

Inventors: ANSEL ROBERT E, KRAJEWSKI JOHN J, MURPHY EDWARD J, ANSEL, ROBERT, E., KRAJEWSKI, JOHN, J., MURPHY, EDWARD, J.
Applicants: DESOTO INC, DESOTO, INC.
Application year: 1989
Main classification: G03C1
IPC codes: B22C7, B22C9, B29C67, G03F7