COMPOSITE FOR CONDUCTIVE LEAF MOLDING BACKGROUND OF THE INVENTION 1. - Field of the Invention The present invention relates generally to compositions of electrically conductive sheet molding compounds and methods for forming the compositions and more particularly to a molding compound composition. of electrically conductive sheet including conductive carbon black adapted for electrostatic painting. 2. - Discussion of the Related Art Electrostatic painting of various automotive parts, including doors and chests, is commonly used today in the automotive industry. The electrostatic painting of substrates for sheet molding compounds (SMC = Sheet Molding Compound) for example is convenient because it reduces paint waste and emissions, compared to non-electrostatic painting techniques. Electrostatic painting techniques require that the substrate be electrically conductive or have an applied primer or coating that is identically conductive in order to exhibit increased paint transfer efficiency. Currently, an electrically conductive primer must be applied to a sheet molding compound composition to be coated before electrostatically painting the article, unlike steel, the sheet molding composition is not conductive. When an electrically conductive primer is used, the ground path is achieved by the conducting primer. An alternate technique is to use a grounding fastener. This undesirably causes higher film build-ups near the fastener to ground with decreasing film buildup as the distance from the grounding fastener increases. In addition, after several steps through the paint booth, significant earth resistance can be found, due to multiple layers of paint in the structure itself. Thus, there is a need in the art for providing a material for compound composition for sheet molding with increased electrical conductivity. This will overcome problems associated with electrostatic paint articles whose electrical conductivity is provided only through a conductive primer or precoat. These problems include excessive paint waste as a result of the need for excessive spraying, increased contamination and the inability to uniformize the coating of the substrate. Although the general idea of the conductive sheet molding compound has been suggested previously, there are problems in obtaining uniform conductivity within a molded part, resulting in poor paint adhesion. The present invention overcomes these problems by incorporating conductive carbon black in molded composite compositions, such as sheet molding compound compositions, to increase the electrical conductivity of the compositions for use with electrostatic painting. SUMMARY OF THE INVENTION The present invention provides an electrically conductive sheet molding compound composition adapted for electrostatic painting, comprising a resin blend including from about 20 to about 60% by weight of thermoplastic resin based on the total weight of the resin. the resin mixture, the thermoplastic resins are selected from the group consisting of ethylene-butadiene rubber, polystyrene, saturated polyester resin, and mixtures thereof; fibrous reinforcement material; and conductive material. The material. The conductor is present in an amount sufficient to make an article molded with the composition, with structural integrity and sufficient electrically conductive to be painted electrostatically. The present invention further provides an electrically conductive sheet molding compound composition comprising a resin mixture including a thermoset resin and about 20 to about 60% by weight of a styrene-butadiene: a catalyst; an inhibitor; a release agent or mold release; fibrous reinforcement material; conductive carbon black; Group IIA metal oxide or hydroxide; and isoxynate-terminated urethane pre-polymer dual function additive. The present invention is also directed to a method for producing an electrically conductive sheet molding compound composition. The method includes adding together a resin mixture including a thermoset resin of from about 20 to about 60% by weight of thermoplastic resins based on the total weight of the resin blend, the thermoplastic resin is selected from the group consisting of rubber of styrene-butadiene, polystyrene, saturated polyester resin and their mixtures; catalyst, inhibitor, mold release agent and conductive carbon black, to produce a mixture for sheet release compound composition. Black conductive carbon is present in an amount sufficient to make an article molded with the composition with structural integrity and electrically conductive enough to be painted electrostatically. The present method also comprises adding to the mixture a metal oxide or hydroxide thickening agent of the HA group, a dual functional additive of urethane-terminated urethane pre-polymer and fibrous reinforcing material. DETAILED DESCRIPTION OF THE OR THE PREFERRED MODALITIES
The following description of preferred embodiments is primarily exemplary in nature and is in no way intended to limit the invention or its application or uses. The present invention provides an electrically conductive sheet (SMC) molding compound composition that can be molded into an article having a conductive surface. The article can then be painted electrostatically without the use of an electrically conductive primer layer, since the sprayed paint will adhere directly to the surface of the electrically conductive article. The present invention provides an electrically conductive sheet molding compound composition, which comprises a resin mixture including a thermoset resin and approximately 20% by weight thermoplastic resin, based on the total weight of the resin mixture, the thermoplastic resin is selected from the group consisting of ethylene-butadiene rubber, polystyrene, saturated polyester resin and mixtures thereof;
fibrous reinforcement material; and conductive carbon black. The conductive carbon black is preferably present in an amount sufficient to make an article molded with the composition, with sufficient structural integrity and electrically conductive to be painted electrostatically. The molding compound composition may also include an isocyanate-terminated urethane prepolymer additive, catalyst, inhibitor, molding-release agent, metal oxide or hydroxide of the HA group, filler or inert filler, ethylenically unsaturated monomer and a spray catalyst. by free radicals and phase update agent. The present invention is predicated on the discovery that the method for mixing together a thermoplastic resin and a specified amount and a conductive carbon black as filler or filler, allows the production of an improved electrically conductive molded article that can be painted electrostatically. The conductive carbon black is preferably in powder form, but is currently granular in nature, resulting in a consistent conductivity even when the filler is added to the resin mixture. In this way, simple addition of the conductive carbon black to the sheet molding compound composition in combination with a specific thermoplastic resin, provides an article with a homogeneous black surface, indicating consistent conductivity and subsequently improved conductivity through the part. In addition, the use of the thermoplastic resin in the composition of the present invention results in improved paint adhesion. Conductive carbon black is preferably present in the sheet molding composition in an amount of at least about 3% by weight and more preferably about .3 to about 2.0% by weight, based on the total weight of the composition of molding. The thermoset resin employed in the sheet molding composition composition can be selected from a wide variety of these resins, preferably, but not limited to, resins such as polyesters, methyl esters, epoxides and mixtures thereof. The thermoset resin is more preferably an unsaturated polyester resin having a ratio of hydroxyl groups to carboxyl groups of about 5.7 to 0.8, acid monomer of at least 14 and an average molecular weight of about 800 to 5000. The thermoplastic resin employed herein invention can be selected from polystyrenes, saturated polyester resins, and more preferably styrene-butadiene rubber. In order to achieve improved conductivity and adhesion, the thermoplastic is preferably present in the present composition in the amount of from about 20 to about 60% by weight based on the total weight of the resin blend. The composition of the present invention may also comprise a catalyst, such as but not limited to, tert-butylperbezoate, in order to cure the composition. The composition of the sheet molding compound can also include any inhibitor commonly known in the art such as parabenzoquinone and hydroquinone. The inhibitor is used to slow the curing process in order to avoid the formation of a pre-gel of the composition. The mold release agent useful in the present invention can be any of those employed in the prior art, such as zinc stearate, calcium stearate, magnesium stearate, organic phosphate esters and other liquid organic internal mold release agents. The agents can be employed at their level described in the art. The fibrous reinforcing material or reinforcing fibers are usually present in an amount of about 10 to 80% by weight for composite sheet molding compositions and preferably are of glass fibers. The preferred range for this reinforcing fibers is approximately 15 to 65% by weight for use in thermosetting polyester resin applications such as sheet molding compound. Although not required, the sheet molding composition composition of the present invention may further comprise a dual thickening system consisting of a urethane prepolymer terminated in isocyanate, in an amount sufficient to react with at least 10%, but not more than 100% of the hydroxyl groups present, and a metal oxide or hydroxide selected from the HA group of the periodic table, in an amount to react with at least 20%, but not more than 100% of the carboxyl groups present. Useful isocyanate-terminated urethane prepolymers are based on a polyester or polyester polyol, or a mixture thereof, and preferably a polyether polyol and a diisocyanate or polyisocyanate. The polyol employed in the prepolymer is preferably a diol or triol having a molecular weight of about 600 to about 4000 and preferably about 2000 as exemplified by Pluracol P-2010 from BASF, and an approximate functionality of from 2 to 6. , preferably 2 to 3 and in particular 2. The double functional additive is formed in a one-step addition reaction between one weight equivalent of the polyol as described above and two equivalent weights of the polyisocyanate, in the presence of about 0 to 1% of a conventional urethane catalyst such as stannous octoate, dibutyltin dilaurate and the like and the amount of catalyst is determined according to the total weight of the urethane. The isocyanate-terminated urethane prepolymer is prepared by first reacting an organic polyisocyanate and preferably a diisocyanate with a polyol, using standard procedures to produce an isocyanate-terminated prepolymer of controlled molecular weight and having an NCO / OH ratio of about 1.2 / 1 to about 5/1 and preferably NCO / OH between 1.8 to 3 and in particular about 2. The polyisocyanates employed in the formation of the present invention include toluene di-isocyanate, such as the mixture of 80:20 or 65 isomers: 35 of the 2,4- and 2-, 6- isomeric, ethylene-di-isocyanate, propylene-diisocyanate, meta- and para-phenyl-di-isocyanate, 4,4'-diphenylmethane-diisocyanate (MDI) forms or a mixture of MDI and its trifunctional cyclic adduct products containing carbodiimide, 1,5-naphthalene di-isocyanate, para- and meta-xylene-di-isocyanate, alkylene diisocyanate such as tet ra -met ilen di-isocyanate and hexamethylene linkages -isoci anato, 2,4- and 2,6-di-isocyanate methylcyclohexane, dicyclohexylmethane diisocyanate, and polymeric MDI containing an average of two isocyanate groups per molecule. Other polyisocyanates that can be used include toluene diisocyanate polyisocyanate, polyisocyanate prepolymers of the aromatic type, adducts based on toluene diisocyanate, aromatic / aliphatic polyisocyanates and polyfunctional aliphatic isocyanate. The exact polyisocyanate used is not critical, but diisocyanates are preferred and of these 4,4'-diphenyl methanedi-isocyanate (MDI) or a mixture of MDI and its trifunctional cyclic adducts products containing carbodiimide bonds, are preferred. It will be noted that different results with respect to low shrinkage additives will be obtained by the use of different polyisocyanates and it is emphasized that di-isocyanates are preferred. The polyol reagent employed in the dual functional additive is selected from either a polyester polyol or polyether polyol, preferably polyester polyols and mixtures of two or more of these polyether polyol compounds. The polyol reagent, or its mixtures used, has an average equivalent weight between 600 to 4000 and a functional one already between 2 and 6, preferably 2 to 3 and in particular 2. Among suitable polyether polyols, it is contemplated that polyoxyalkylene polyols and their mixtures These may be prepared according to well-known methods such as by condensing an alkylene oxide or alkylene oxide mixture, using random or stepwise addition, with a polyhydric initiator or mixture of polyhydric initiators. The alkylene oxides contemplated for use the prepolymer include ethylene oxides, propylene oxide, butylene oxides, amilen oxide, aralkylene oxide such as triclorobutilen oxide and the like and the most preferred alkylene oxide is propylene oxide or a mixture thereof, with ethylene oxide that uses random or staged oxyalkylation. Polyhydric initiators used in preparing the prepolymer polyether polyol reactant include (a) aliphatic diols such as ethylene glycol, 1, 3-propylene glycol, 1,2-propylene glycol, butylene glycols, butane diols, pentane diols and the like, (b) aliphatic triols such as glycerol, trimethylolpropane, triethylolpropane, trimethylolhexane and the like, (c) the polyamines such as tetraethylene diamine and (d) the alkanolamines such as diethanolamine, triethanolamine and the like. Preferably, the polyhydric screening initiators for use in preparing the polyether polyol reagent comprise an aliphatic diol or triol such as ethylene glycol, propylene glycol, glycerol, trimethylolpropane, and the like. If a polyester polyol is chosen to be used as the polyol reactant of the dual functional additive, such a polyol is usually formed by reacting a polycarboxylic acid with a polyhydric initiator such as diol or triol. Polycarboxylic acids include oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic and the like acids. Illustrative polyhydric alcohols include various diols and triols and higher functionality alcohols such as ethylene glycol, 1, 3 -propilen glycol, 1, 2-propylene glycol, butylene glycols, butane diols, pentane diols, glycerol, trimethylolpropane, trimethylolhexane, hexane 1, 2,6-triol and the like. When a polyether polyol reagent is to be created by the alkylene oxide polyhydric initiator reaction, usually a catalyst such as the KOH catalyst, described in the art, is added to accelerate the reaction. The resulting polyether polyol should have an approximate average molecular weight of 600 to 4000. After reaction, the catalyst is preferably removed, leaving a suitable polyether polyol for reaction with the polyisocyanate reagents as discussed above to form the finished urethane prepolymer. isocyanate of the present invention. To form the pre-polymer isocyanate terminated urethane, an equivalent weight of the polyol reactant as defined above is reacted with 1.2 to 5 and preferably 2 equivalent weights of a polyisocyanate as defined previously in the presence of any catalysts conventional urethane such as stannous octoate, dibutyltin dilaurate and the like, whereby the isocyanate groups are placed at the terminal ends of the prepolymer, thus producing the isocyanate-terminated urethane prepolymer. It will be noted that the prepolymer can be made in the presence of a monomer or a monomer can be added to a solvent in the monomer after it has been made, without adversely affecting its function as a low profile additive and as a viscosity index modifier to impart the desired benefits. The isocyanate-terminated urethane prepolymer additive may optionally be employed with any of the conventional low shrinkage additives of the prior art, such as styrene-butadiene copolymer, polystyrene or a mixture thereof, or any other linear oligomer having a weight molecular in the range from about 400 to about 200,000 and preferably about 1000 to about 100,000. Furthermore, regardless of whether or not the prepolymer is employed - with a conventional low shrinkage additive, the ratio of the total amount of prepolymer when employed in the present invention to polyester resin optimally will be within the approximate range of 10. parts by weight of prepolymer to 100 parts by weight of polyester resin, to about 40 parts by weight of prepolymer to 60 parts by weight of polyester resin. When used in a sheet molding compound composition, the finished urethane prepolymer and isocyanate can be dissolved in styrene and then used as any other low shrinkage additive and used in an amount sufficient to react with at least 10% but not more than 105% of the hydroxyl groups present in the reaction. The metal oxide or hydroxide employed in the dual thickening system is preferably a metal oxide or hydroxide of the HA group of the Periodic Table, preferably calcium or magnesium. Although calcium can be used in its various oxides and hydroxides, magnesium is preferred, since superior results are achieved by the use of magnesium. Although the pre-polymer can be employed alone it can also be used with a monomer of the styrene, vinyl toluene and vinyl acetate group and any other ethylenically unsaturated monomer and when so used, it is ordinarily present in an amount to give 0.5 to 3.5 moles of unsaturated monomer per mole of unsaturation for example in an unsaturated polyester resin. Styrene and vinyl toluene are preferred monomers, although others may be employed. A free radical polymerization catalyst can also be employed in the present invention. The catalyst is preferably present in an amount of 1.0 to 5.0 parts per 100 parts of the total resin and monomer, the parts are measured by weight. The free radical polymerization catalyst is added to the uncured composition, such that upon heating to the activation temperature, the additive type interlacing polymerization reaction will begin between the polymerizable monomer and the unsaturated polyester resin, to form the matrix previously described. The catalyst is usually employed in an amount of about 1.0 to 3.0 parts per 100 parts of the total monomer and resin and is usually a peroxide. An additive or phase stabilizing agent for use in thermosetting resin products can also be employed in the composition of the present invention. The agent may comprise a fatty acid, dimer acid or a polyol polymer. The use of these compounds, despite the resulting increase in the viscosity of the total system (or in some cases the negligible effect on the viscosity) results in a system that retains its unsaturated, can join in the middle (unsaturated) positions. Mixtures of these materials can also be used. Particular preference is given to dimer acids or trimers prepared from the monomeric materials described above, ie trimer acids prepared by the attachment of at least two (and in the case of dimers-three acids) portions of acid selected from lauric, palmitic, stearic acids, oleic, linoleic, linolenic, capric, caprylic, capric, myristic and palmitoleic. Even more preferred are dimers having about 36 carbon atoms, that is, prepared by the union of two or more fatty acids with 18 carbon atoms and trimer acids having approximately 54 carbon atoms. The third class of phase stabilizing agents or additives useful in the practice of the present invention are polyester and polyols, mixtures of polyester polyols can also be employed. Preferred polyols are mixtures of polyols having an average molecular weight in the range of about 200 to about 6500. More preferably, the polyol employed has an average molecular weight of from about 300 to about 5000 and even more preferably from about 400 to about 4,500, and in particular approximately 600 unsaturated, they can be united in the middle (unsaturated) positions. Mixtures of these materials can also be used. Particular preference is given to dimer acids or trimers prepared from the monomeric materials described above, ie trimer acids prepared by the attachment of at least two (and in the case of dimers-three acids) portions of acid selected from lauric, palmitic, stearic acids, oleic, linoleic, linolenic, capric, caprylic, capric, myristic and palmitoleic. Even more preferred are dimer acids having about 36 carbon atoms, ie prepared by the attachment of two or more fatty acids with 18 carbon atoms and trimer acids having about 54 carbon atoms. The third class of phase stabilizing agents or additives useful in the practice of the present invention are polyester and polyols, mixtures of polyester polyols can also be employed. Preferred polyols are mixtures of polyols having an average molecular weight in the range of about 200 to about 6500. More preferably, the polyol employed has an average molecular weight of from about 300 to about 5000 and even more preferably from about 400 to about 4,500, and in particular about 600 to about 4,000. In a highly preferred embodiment, the polyol employed has an average molecular weight of from about 1,000 to about 3,000. Preferred polyols for use in the practice of the present invention possess an average functionality of about 2 to about 4 and preferably about 2 to about 3. Any number of non-reinforcing fillers, such as clay, carbon fibers and carbonate of calcium, can be added to the composition to reduce total material costs without sacrificing a significant degree of desirable physical properties in the final product, or can be added to impart specific properties to the uncured composition. The fillers can be employed in an amount in the range of about 20 parts to 1,000 parts by weight per 1,000 parts of the pure polyester resin in thermoset polyester resin applications, such as a composition of the sheet molding compound. The sheet molding compound compositions in the present invention may also contain common additives or fillers known in the art. In general, the sheet molding compound compositions of the present invention are prepared by mixing, adding or otherwise contacting at least two sub-mixtures or their parts. The first sub-mixture or part, generally contains the thermoset and thermoplastic resins, inhibitor, conductive carbon black, filler, catalyst and mold release agent. The sub-blend may additionally include phase stabilizing additives or agents, an inert filler and an ethylenically unsaturated monomer with a free radical polymerization catalyst. Alternatively, the composition of the sheet molding compound may include an additional sub-mixture comprising a pre-dispersion of conductive carbon black that is added separately from the first sub-mixture. A second sub-mixture or part generally contains the metal oxide (or hydroxide) of the HA group. A third possible sub-mix or part, contains the dual functional additive or prepolymer as set forth herein and described in U.S. Pat. Nos. 4,535,110 and 4,622,354 incorporated herein by reference. A fibrous reinforcing material can also be added to the composition. In a preferred method of the present invention the conductive carbon black is simply added as a filler to the resin mixture, avoiding the need to produce a pre-dispersion solution. In an alternate embodiment, however, the carbon black can be dispersed within the thermoset and thermoplastic resins or a pre-dispersion of conductive carbon black can be added from the resin mixture to the sheet molding compound composition. In addition, conductive carbon black can be extruded with the thermoplastic resin employed in the present invention. The compositions and methods of the present invention can be used to mold various articles or parts, including but not limited to, automotive parts such as chests or doors that may require Class A finishing, fenders and supports, ie a pit-free finish comparable to the panels against part of laminar metals. The invention is further described in the following examples. The examples are merely illustrative and in no way limit the scope of the invention as described and claimed. EXAMPLES
MATERIAL COMPOSITION 1 COMPOSITION 2 Parts in Weight Parts in Weight Polyester resin 60.0 60.0 Rubber solution 40.0 30.0 styrene-butadiene (32%) Ter- 1.0 1.0 butylperbenzoate MATERIAL "" .., COMPOSITION 1 COMPOSITION 2 Parts in Weight Parts in Weight Stearate of zinc 3.0 1.0 Dimer acid 2.0 Carbonate of 100.0 100.0 calcium Black carbon 2.0 2.0 conductor Magnesium oxide 1.0 0.5 Pre-polymer of 10 Urethane Fiberglass 140.0 140.0 (2.54 cm (1")) Capacity 160.0 165.0 Dew Ramsburg
Each of the laminar molding compound compositions of the outer example when molded into an article exhibits improved paint adhesion and consistent conductivity throughout the article. The sheet molding compound is typically molded under heat and pressure with compression molding techniques, such as those described in the following U.S. Patents, which are incorporated by reference herein: 4,488,862; 4,612,149; 4,855,097; 4,551,085; 5,130,071. Other molding techniques however can be used.