Process for the preparation of electromotively coated filled thermoset articles

A process comprising electromotively coating an article molded from a sheet molding compound comprising a mixture of: (a) 10-40 percent by weight of a thermosetting resin, (b) 20-60 percent by weight of calcium carbonate particles, (c) 10-40 percent by weight of glass fibers having an aspect ratio of at least 5 and a length of at least 2 cm, and (d) 0.4-3 percent by weight of a carbon black having a primary particle size of less than 125 nm, a nitrogen surface area of at least 275 m.sup.2 /g, and a dibutyl phthalate absorption of at least 180 cc/100 g; wherein the article has a conductivity of at least 10-7 Siemens/cm (S/cm), a tensile strength of at least 8,500 psi, and a flexural strength of at least 21,000 psi.

BACKGROUND OF THE INVENTION
 This invention relates to electrostatically coatable compositions based on
 thermosetting polymers and, more particularly, relates to such
 compositions based on sheet molding compound or bulk molding compound.
 It is known to prepare coated articles by electrostatic painting methods.
 In such methods, a paint or coating is charged or ionized and sprayed on a
 grounded, conductive article, and the electrostatic attraction between the
 paint or coating and the grounded article results in a more efficient
 painting process with less wasted paint material, and thicker and more
 consistent paint coverage, particularly when the article has a complex
 shape. When articles fabricated from metals are painted, the metal, which
 is inherently conductive, is easily grounded and efficiently painted. In
 recent years, there has been an emphasis on the use of polymeric materials
 in the manufacture of articles, particularly in applications requiring
 reductions in weight and improved corrosion resistance, such as automotive
 applications. However, polymers typically used in such processes are
 insufficiently conductive to efficiently obtain satisfactory paint
 thickness and coverage when the article is electrostatically painted.
 Methods are known for the incorporation of conductive fillers into polymers
 in order to improve their conductivity for use in electrostatic coating
 applications. However, the conductivity of articles made therefrom, as
 well as the physical, and/or surface appearance properties of the coated
 articles, may be less than desirable for certain applications. The use of
 conductive primer compositions to prime the article in order to increase
 its conductivity is also known. However, depending on the particular
 primer employed, the cured primer may have adhesion, surface smoothness,
 hydrolytic stability, and durability characteristics, which are less than
 desirable for a particular application. In addition, such primers
 typically contain volatile organic solvents, the emission of which during
 the priming process may be undesirable.
 U.S. Pat. No. 5,490,893 illustrates a method for making a laminate of a
 thermoformable conductive material and an article of sheet molding
 compound (SMC) to provide an SMC-based article having good surface
 conductivity for use in electrostatic coating applications. However, the
 use of such laminates, or the use of conductive primers, represents extra
 steps in the forming of the article, the addition of which is less than
 desirable in a commercial process. European Patent Application No. 623782
 describes a method for making vehicle headlight reflectors by
 injection-molding a bulk-molding compound that contains conductive carbon
 black. However, bulk-molding compound typically contains a relatively
 lower proportion of glass fibers, and fibers having a shorter length
 (which may result in relatively lower physical properties), than SMC,
 which makes its use less than desirable for certain applications.
 SMC is a moldable thermosetting material that is used to prepare structural
 parts for a variety of applications, including automotive. The material is
 typically prepared in a continuous process by depositing glass fibers
 between two or more sheets of a high-viscosity composition comprised of
 thermosetting polyester resin, calcium carbonate, and alkaline earth metal
 oxide- or alkali-metal hydroxide- based thickeners between two carrier
 films of polyethylene. This "sandwich" type of composite is then rolled up
 and allowed to "mature" and thicken for a period of time to increase the
 viscosity and handleability of cut portions of the material. The matured
 rolls of material are then cut into desired shapes for molding, the
 carrier film is removed, and the SMC is then compression molded into the
 desired three-dimensional shape of a part having good surface
 characteristics and physical properties.
 Adjusting the formulation of SMC requires a balancing of the desired
 characteristics of handleability of the uncured SMC material, relatively
 low viscosity of the resin composition used to make the material (since
 the resin composition is pumped using conventional pumping equipment
 instead of high-shear extruders), low cost (achieved by minimizing the
 amount of resin and maximizing the amount of calcium carbonate), as well
 as the surface gloss and physical characteristics of the final part. The
 resin demand characteristics of the components to be added to the
 formulation and the cost of the proportionate amount of resin needed to
 achieve the desired low viscosity are several factors that must be taken
 into account. Carbon black has a high resin demand, relative to glass
 fiber, calcium carbonate, and the thickening agents typically employed in
 the preparation of SMC.
 SUMMARY OF THE INVENTION
 In one aspect, this invention is a process comprising electromotively
 coating an article molded from a sheet molding compound comprising a
 mixture of: (a) 10-40 percent by weight of a thermosetting resin, (b)
 20-60 percent by weight of calcium carbonate particles, (c) 10-40 percent
 by weight of glass fibers having an aspect ratio of at least 5 and a
 length of at least 2 cm, and (d) 0.4-3 percent by weight of a carbon black
 having a primary particle size of less than 125 nm, a nitrogen surface
 area of at least 275 m.sup.2 /g, and a dibutyl phthalate absorption of at
 least 180 cc/100 g; wherein the article has a conductivity of at least
 10.sup.-7 Siemens/cm (S/cm), a tensile strength of at least 8,500 psi, and
 a flexural strength of at least 21,000 psi.
 It has been discovered that electromotively coatable articles based on SMC
 may be prepared utilizing certain carbon blacks in minimal amounts. It has
 also been discovered that compositions that employ carbon blacks in such
 amounts do not result in a loss of physical properties or
 distinctness-of-image (DOI) properties that would be undesirable for many
 applications for which such compositions may be employed. These and other
 aspects of the invention will be apparent from the description that
 follows.
 DETAILED DESCRIPTION OF THE INVENTION
 Suitable thermosetting resins, glass fibers, and fillers that may be
 employed in the composition of the invention include materials that are
 employed in the preparation of SMC. Examples of thermosetting resins
 include oligomers or polymers having a molecular weight of greater than
 1000 and having pendant functional groups which will react with a
 crosslinking compound to provide a crosslinked polymer. Further, an
 article consisting of the crosslinked compound will have a tensile
 strength of at least 13 MPa (2000 psi). Examples of thermosetting resins
 include unsaturated polyesters, epoxy resins, vinyl ester resins, and
 thermosetting phenolic resins. Preferred crosslinking compounds include
 styrene (for polyester resins), amines (for epoxy resins), styrene or
 vinyl toluene (for vinyl ester resins), and hexamethylenetetraamine (for
 phenolic resins). Examples of unsaturated polyester resins are described
 in U.S. Pat. No. 5,491,184. Examples of vinyl ester resins are described
 in U.S. Pat. No. 5,034,437. Examples of epoxy resins and thermosetting
 phenolic resins are described in the Encyclopedia of Polymer Science and
 Engineering, Vol. 6, pp. 322-382 (1988) and Vol. 11, pp. 45-93 (1988),
 respectively. Further, the term "thermosetting resin" as used herein
 includes resins containing thermosetting components which, in addition,
 also contain at least one thermoplastic polymer component in a minor
 amount, such as a "low profile additive" thermoplastic polymer that is
 commonly employed in the formulation of SMC compositions. Examples of such
 include polyvinyl acetate, saturated polyesters, polystyrene,
 polyacrylates or polymethacrylates, and saturated polyester urethanes. The
 thermosetting resin is preferably present in an amount, based on the
 weight of the composition of at least 15 percent, more preferably at least
 20 percent; but preferably no greater than 36 percent, more preferably no
 greater than 32 percent.
 The calcium carbonate particles employed preferably have a size in the
 range of from about 0.1 .mu.m to about 50 .mu.m and an aspect ratio of
 less than 5. Such particles are preferably employed in an amount by
 weight, based on 100 parts by weight of the thermosetting resin, of at
 least 60 parts, more preferably at least 80 parts, and most preferably at
 least 100 parts; but preferably no greater than 250 parts, more preferably
 no greater than 220 parts. Based on the weight of the total composition,
 such particles are preferably present in an amount of at least 22 percent,
 more preferably at least 27 percent; but preferably no greater than 54
 percent, more preferably no greater than 51 percent.
 Other fillers or particulate materials which may be used in the SMC include
 those having a size in the range of from about 0.1 .mu.m to about 50 .mu.m
 and an aspect ratio of less than 5, which are wholly inorganic particulate
 materials, particles of inorganic materials which have been
 surface-treated with an organic material which increases its wettability
 or dispersibility, carbon blacks (other than the carbon blacks referred as
 component (d) above), and mixtures thereof. Examples of inorganic
 particulate materials include glass particles and minerals such as calcium
 carbonate, dolomite, clays, talc, zinc borate, perlite, vermiculite,
 alumina trihydrate, and solid or hollow glass microspheres.
 Glass fibers used herein have an aspect ratio of at least 5 and a length of
 at least 2 cm. Suitable polymer-based fibers should comprise polymers that
 are solid at 25.degree. C. Examples of polymer-based fibers that may also
 be used include nylon, polyester, polybenzoxazole, and aramid fibers.
 Fibers may be woven or nonwoven, chopped (if desired), or may be used in
 the form of fiber bundles coated with a sizing agent. Preferably, the
 glass fibers are used in an amount by weight, based on 100 parts by weight
 of resin, of at least 60 parts, more preferably at least 80 parts, and
 most preferably at least 90 parts; but preferably no greater than 150
 parts, more preferably no greater than 130 parts. Based on the weight of
 the total composition, such fibers are preferably present in an amount of
 at least 15 percent, more preferably at least 20 percent; but preferably
 no greater than 38 percent, more preferably no greater than 35 percent. If
 desired, boron fibers and fibers of extruded resins may also be used to
 prepare the SMC.
 The combined volume percentage of fillers and glass fibers present in the
 composition, based on the total volume of the composition, is preferably
 at least 30 percent, more preferably at least 40 percent, but is
 preferably no greater than 80 percent, more preferably no greater than 70
 percent.
 Examples of suitable carbon blacks include particles of carbon having an
 average primary particle diameter of less than 125 nm, more preferably
 less than 60 nm. The carbon black is preferably utilized as an aggregate
 or agglomerate of primary particles, the aggregate or agglomerate having a
 size of 5 to 10 times the primary particle size. The carbon black
 preferably does not comprise a graphite form of carbon. Larger
 agglomerates, beads, or pellets of carbon particles may also be utilized
 as a starting material in the preparation of the composition, so long as
 they disperse during the preparation or processing of the composition
 sufficiently to reach an average size in the cured composition of less
 than 10 microns, more preferably less than 5 microns, and most preferably
 less than 1.25 microns. The carbon black preferably has a nitrogen surface
 area of at least 275 m.sup.2 /g, more preferably at least 500 m.sup.2 /g,
 and most preferably at least 750 m.sup.2 /g. The nitrogen surface area of
 the carbon black may be determined using ASTM Method No. D 3037-93. The
 dibutyl phthalate absorption of the carbon is preferably at least 180
 cc/100 g, more preferably at least 250 cc/100 g, and most preferably at
 least 300 cc/100 g, and may be measured according to ASTM Method No. D
 2414-93. The carbon black is preferably used in an amount by weight, based
 on 100 parts by weight of resin, of at least 0.5 parts, more preferably at
 least 1 part, and most preferably at least 1.5 parts; but preferably no
 greater than 9 parts, more preferably no greater than 7 parts. Based on
 the total weight of the composition, the carbon black is employed in an
 amount of less than 2.5 weight percent, more preferably less than 2 weight
 percent, more preferably less than 1 weight percent, and most preferably
 less than 0.9 weight percent.
 Suitable thickeners include alkaline earth metal oxides and hydroxides.
 Preferably, the thickener is magnesium oxide, magnesium hydroxide, calcium
 oxide, or calcium hydroxide, a mixture thereof, and is most preferably
 magnesium oxide, magnesium hydroxide, or a mixture thereof. The thickener
 is preferably used in an amount by weight, based on 100 parts by weight of
 resin, of no greater than 10 parts, more preferably no greater than 4
 parts.
 Other ingredients which may also be present in the composition include
 crosslinking compounds, initiators, mold release agents, free-radical
 inhibitors such as benzoquinone or hydroquinone, catalysts such as organic
 peroxides or hydroperoxides, and colorants. In addition, electronically
 conductive additives other than carbon black may also be utilized in the
 preparation of the compositions. The carbon black and other conductive
 additives are employed in an amount sufficient to provide a conductivity
 of at least 10.sup.-14 S/cm, but the combined weight percentage of the
 carbon black and other conductive additives may not exceed 3 percent,
 based on the weight of the composition. Examples of such additives include
 conductive salts, carbon fibers, graphite fibers, and particles of
 conductive polyaniline. The viscosity of the resin-containing composition
 prior to combining it with the glass fibers is preferably at least 4,000
 cps, more preferably at least 8,000 cps, most preferably at least 10,000
 cps; but preferably no greater than 80,000 cps, more preferably no greater
 than 70,000 cps, and most preferably no greater than 60,000 cps, as
 measured by rotational viscometry (such as, for example, a Brookfield
 viscometer) at a temperature of about 25.degree. C.
 The composition may be prepared by any method suitable for mixing the
 components and curing the thermosetting resin. Preferably, in processes
 for the preparation of SMC, the resin, fillers, carbon black, curing
 agents, and all other components except for the glass fibers, are
 thoroughly mixed together in two separate batches having similar volumes,
 one containing the thermosetting agent and the other containing any
 initiators or crosslinking compounds. The batches are then combined and
 deposited in sheet form onto carrier sheets of an inert plastic material
 such as polyethylene. The glass fibers are then deposited between two
 sheets of the resin-containing material and compressed, forming a
 composite of the sheets and the material. The composition is then stored
 for several days to permit it to thicken, and then cut into a suitable
 shape and heated and/or molded under conditions sufficient to cure the
 thermosetting resin.
 The composition preferably has a conductivity of at least 5.times.10.sup.-7
 S/cm, and more preferably at least 1.times.10.sup.-6 S/cm. The
 conductivity of the composition may be measured according to the procedure
 set forth below.
 Once fabricated, the electronically conductive article can be painted or
 coated on at least one of its surfaces using any suitable electromotive
 coating process. The term "electromotive coating process" as used herein
 refers to any coating process wherein an electrical potential exists
 between the substrate being coated and the coating material. Examples of
 electromotive coating processes include electrostatic coating of liquids
 or powders, electrodeposition ("E-Coat") processes, electromotive vapor
 deposition, and electroplating processes. The article may be painted or
 coated with any suitable water-based or organic-based composition (or
 water/organic mixture), including conductive primer compositions which
 further enhance the electronic conductivity of the article, or with a
 solventless organic composition by a powder coating or vapor deposition
 method.
 The DOI gloss characteristics of the coated article may be measured by ASTM
 Test Method No. E 430-91. The physical properties of the coated or
 uncoated article may be determined by ASTM Test Method Nos. D 638 (Tensile
 Strength, Tensile Elongation, and Tensile Modulus), D 790 (Flexural
 Stress, Flexural Strain, Flexural Modulus), D 3763 (Dynatup Impact), and D
 256 (Notched Izod). The tensile strength of the molded and cured
 composition is preferably at least 9,000 psi, more preferably at least
 9,500 psi. Its tensile modulus is preferably at least 0.8 million psi,
 more preferably at least 1,000,000 psi, and most preferably at least 1.2
 million psi. Its flexural strength is preferably at least 22,000 psi, more
 preferably at least 23,000 psi; its flexural modulus is preferably at
 least 0.8 million psi, more preferably at least 1 million psi, most
 preferably at least 1.2 million psi, and its Notched Izod impact strength
 is preferably at least 6 ft-lb/inch, more preferably at least 8
 ft-lb/inch, most preferably at least 10 ft-lb/inch.
 The coated articles prepared by the process of the invention are useful in
 any application for coated plastic articles, but are particularly useful
 as components in applications where the use of a lightweight non-corrosive
 material is desirable, such as automotive and other transportation
 applications, as well as static-dissipation and shielding applications.

Illustrative Embodiments
 The following examples are not intended to limit the scope of the invention
 in any way. Unless stated otherwise, all parts and percentages are given
 by weight.
 EXAMPLE 1
 A mixture containing carbon black is prepared by mixing 180 g of an
 unsaturated polyester resin/styrene mixture (Aropol Q-6585, available from
 Ashland Chemical), 120 g of low profile additive (LP40A, a solution of
 polyvinyl acetate containing styrene, available from Union Carbide), 3.0 g
 of a peroxybenzoate initiator (Trigonox C, available from Akzo Nobel),
 14.1 g of a 38 weight percent dispersion of MgO (PG 9033, available from
 Plasticolors (Ashtabula, Ohio)), 14.4 g of a mold release compound
 (S-1058, zinc stearate, available from Synpro (Cleveland, Ohio)), 441.3 g
 of calcium carbonate (Atomite, available from E.C.C. International
 (Atlanta, Ga.)), and 6.0 g of conducting carbon black (Black Pearls 2000,
 available from Cabot Corp.). After mixing for several minutes, 327.6 g of
 glass fiber (Type 5509 Roving, available from PPG Industries) is deposited
 between two layers of the resulting paste. The resulting composite is then
 calendered between two sheets of polyethylene film to form a composite
 with a thickness of about 6 mm. The sheet is stored for 1 to 3 days, and
 15-cm-by-15-cm test plaques are cut therefrom and molded in a compression
 molder at 300.degree. F. (149.degree. C.) for 3 minutes under 10,000 lbs
 (4545 kg) of pressure. Approximately 160 g of material is used for each
 plaque. The plaques are removed from the mold and trimmed for testing. The
 conductivity of the plaques is tested according to the following
 procedure: portions of each side of the plaque which are located directly
 opposite to each other are painted with a 1-cm-by-1-cm square of silver
 paint (available from SPI Supplies (Westchester, Pa.)). Conductive
 graphite paper (Grafoil.TM., available from UCAR Carbon Co. (Cleveland,
 Ohio)) is pressed against the painted area and resistance is measured with
 a 9-volt digital ohmmeter with its leads connected to the graphite paper.
 The conductivity of the plaque (S/cm) is calculated by dividing the
 thickness of the plaque, in cm, by the measured resistance. The physical
 properties of the plaques are tested in accordance with the ASTM test
 methods referred to above. The plaque has a conductivity of at least
 1.times.10.sup.-7 S/cm.
 EXAMPLES 2-10
 Example 1 is repeated using the amounts of carbon black and the particular
 carbon blacks shown in Table 1. In Table 1, "XE 2" refers to Printex XE 2,
 a conducting carbon black available from Degussa Corp., "XC 72" refers to
 Vulcan XC-72, a conducting carbon black available from Cabot Corp., and
 "BP 2000" refers to Black Pearls 2000, a conducting carbon black available
 from Cabot Corp. In Examples 2-8, the amount of calcium carbonate is
 adjusted in order to keep the total volume of carbon black and calcium
 carbonate the same as in Example 1. In Examples 9 and 10, the amount of
 calcium carbonate is 150 parts by weight per 100 parts of polyester resin
 (which is a 60/40 weight percent mixture of Aropol Q-6585 and LP40A,
 respectively). The plaques are prepared and tested in accordance with the
 procedure described in Example 1. All plaques have a conductivity of at
 least 1.times.10.sup.-7 S/cm.
 TABLE 1
 Carbon Black
 Example Loading and Type
 1 0.54% BP 2000
 2 0.68% BP 2000
 3 0.81% BP 2000
 4 1.1% BP 2000
 5 0.54% XE 2
 6 0.81% XE 2
 7 1.1% XE 2
 8 1.36% XC 72
 9 1.9% XC 72
 10 2.4% XC 72
 EXAMPLE 11-14
 Example 1 is repeated four times using the same carbon black, except that
 the carbon black is utilized in the amounts of 0.54, 0.81, 1.1, and 1.4
 percent by weight of the total composition, respectively, for each
 example, and the amount of calcium carbonate utilized in each example is
 150 parts by weight per 100 parts of polyester resin (which is a 60/40
 weight percent mixture of Aropol Q-6585 and LP40A, respectively). Plaques
 of the compositions are prepared and their conductivities tested in
 accordance with the procedure described in Example 1. The conductivities
 of the plaques are at least 1.times.10.sup.-7 S/cm.