WO1997034709A1

WO1997034709A1 - Process for the preparation of electrostatically coated, thermoset articles containing fillers

Info

Publication number: WO1997034709A1
Application number: PCT/US1997/004148
Authority: WO
Inventors: Richard A. Lundgard
Original assignee: The Dow Chemical Company
Priority date: 1996-03-21
Filing date: 1997-03-18
Publication date: 1997-09-25
Also published as: AU2328797A

Abstract

A process for electromotively coating an article molded from a composition containing a mixture of (a) a thermosetting resin, (b) a filler, a reinforcing material, or a combination thereof, and (c) a carbon black having a primary particle size of less than 125nm, a nitrogen surface area of at least 50 m2/g, and a dibutyl phthalate absorption of at least 50cc/100 g; wherein (i) component (b) is present in an amount, based on the volume of the composition, of at least 30 percent; (ii) component (c) is present in an amount, based on the weight of the composition, in the range of from 0.15 percent to 3 percent; and (iii) the article has a conductivity of at least 10-14 Siemens/cm (S/cm).

Description

PROCESS FOR THE PREPARATION OF ELECTROSTATICALLY COATED, THERMOSET ARTICLES CONTAINING FILLERS

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. Patent 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.

In one aspect, this invention is a process comprising electromotively coating an article molded from a composition comprising a mixture of (a) a thermosetting resin, (b) a filler, a reinforcing material, or a combination thereof, and (c) a carbon black having a primary particle size of less than 125 nm, a nitrogen surface area of at least 50 m²/g, and a dibutyl phthalate absoφtion of at least 50 cc/100 g; wherein (i) component (b) is present in an amount, based on the volume of the composition, of at least 30 percent; (ii) component (c) is present in an amount, based on the weight of the composition, in the range of from 0.15 percent to 3 percent; and (iii) the article has a conductivity of at least 10 '⁴ Siemens/cm (S/cm).

It has been discovered that electromotively coatable articles based on sheet molding compound or bulk molding compound (BMC) may be prepared utilizing certain carbon blacks in minimal amounts. It has also been discovered that compositions which employ carbon blacks in such amounts do not result in a loss of physical properties or distinctness of image (DOI) properties which 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 which follows.

Suitable thermosetting resins, reinforcing materials, and fillers which may be employed in the composition of the invention include materials which are employed in the preparation of SMC and BMC. 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. Patent 5,491 ,184. Examples of vinyl ester resins are described in U.S. Patent 5,034,437. Examples of epoxy resins and thermosetting phenolic resins are described in the Encyclopedia of Polvmer Science and Engineering. Vol. 6, pp. 322-382 (1988) and Vol. 11 , pp. 45-93 (1988), respectively. The thermosetting resin is preferably employed in an amount, based on the weight of the composition, of at least 5 percent, more preferably at least 8 percent, but is preferably no greater than 50 percent, more preferably no greater than 35 percent.

The term "filler" as used herein refers to particulate matenals having a size in the range of from 0.1 μm to 50 μm and an aspect ratio of less than 5, which are wholly

5 inorganic particulate matenals, particles of inorganic materials which have been surface- treated with an organic material which increases its wettabihty or dispersibility, carbon blacks (other than the carbon blacks referred as component (c) above), and mixtures thereof. Examples of inorganic particulate matenals include glass particles and minerals such as calcium carbonate, dolomite, clays, talc, zinc borate, perlite, vermiculite, alumina trihydrate, l o and solid or hollow glass microspheres.

The term "reinforcing material" as used herein refers to glass fibers, boron fibers, and fibers of extruded polymers, having an aspect ratio greater than 5 and a length in the range of from 0 1 μm to 3 cm Suitable polymer-based fibers should comprise polymers which are solid at 25°C. Examples of polymer-based fibers include nylon, polyester, 15 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 reinforcing material is present in an amount, based on the total volume of the composition, of at least 10 percent by volume, and more preferably at least 15 percent by volume.

20 The combined volume percentage of fillers and reinforcing agents 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 25 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 30 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 125 m7g, more preferably at least 200 m²/g, and most preferably at least 750 m7g. 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 75 cc/100 g, more preferably at least 150 cc/100 g, most preferably at least 250 cc 100 g, and may be measured according to ASTM Method No. D 2414-93. The carbon black is preferably employed in an amount, based on the weight of the composition of at least 0.25 percent, but preferably no greater than 2.5 percent, more preferably no greater than 2.0 percent.

Other ingredients which may also be present in the composition include crosslinking compounds; viscosity-modifying agents such as magnesium oxide; initiators; mold release agents; free-radical inhibitors such as benzoquinone or hydroquinone; catalysts such as organic peroxides or hydroperoxides; colorants; and "low profile" thermoplastic additives such as polyvinyl acetate, saturated polyesters, polystyrene, polyacrylates or polymethacrylates, and saturated polyester urethanes. 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 " 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 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 reinforcing materials, 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. Sheets of the batches are extruded separately. The reinforcing material is then deposited between the sheets and compressed, forming a composite of the sheets and the material. The sheets are stored for several days to permit the composition to thicken, and then cut into a suitable shape and heated and/or molded under conditions sufficient to cure the thermosetting resin. In processes for the preparation of BMC, all ingredients except for the reinforcing material are first combined, and then mixed with the reinforcing material under conditions sufficient to wet the material. The resulting mixture is then compression molded and heated under conditions sufficient to cure the thermosetting resin. The composition preferably has a conductivity of at least 10⁹ S/cm, and more preferably at least 10⁷ S/cm, but is preferably no greater than 10 ' 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 (Ε-Coaf ) 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 distinctness-of-image 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 (Fiexural Stress, Flexural Strain, Flexural Modulus); D 3763 (Dynatup Impact); and D256 (Notched Izod).

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 0.5 weight percent carbon black was prepared by mixing 180 g of an unsaturated polyester resin styrene mixture (Aropol Q-6585, available from Ashland Chemical, which contained 70 percent by weight polyester resin), 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, OH)), 14.4 g of a mold release compound (S-1058, zinc stearate, available from Synpro (Cleveland, OH)), 441.3 g of calcium carbonate (Atomite, available from E.C.C. International (Atlanta, GA)), and 6.0 g of a conducting carbon black (BP-2000, available from Cabot

Corp.). After mixing for several minutes, the resulting paste was mixed in a planetary mixer with 327.6 g of glass fiber (Type 5509 Roving, available from PPG Industries) until the fiber was well coated. The mixture was then calendered between two sheets of polyethylene film to form a sheet of the mixture with a thickness of 6 mm. The sheet is stored for 1 to 3 days, and 15-cm-by-15-cm test plaques were cut therefrom and molded in a compression molder at 300°F (149°C) for 3 minutes under 10,000 lbs (4545 kg) of pressure. Approximately 160 g of material was used for each plaque. The plaques were removed from the mold and trimmed for testing. The conductivity of the plaques was tested according to the following procedure: portions of each side of the plaque which are located directly opposite to each other were painted with a 1-cm-by-1-cm square of silver paint (available from SPI Supplies

(Westchester, PA)). Conductive graphite paper (Grafoil™ , available from UCAR Carbon Co. (Cleveland, OH)) was pressed against the painted area and resistance is measured with a 9-volt digital ohm-meter with its leads connected to the graphite paper. The conductivity of the plaque (S/cm) was calculated by dividing the thickness of the plaque, in cm, by the measured resistance. The physical properties of the plaques were tested in accordance with the ASTM test methods referred to above.

Examples 2-10

Example 1 was repeated using the amounts of carbon black and the particular carbon blacks shown in Table I. In Table I, "XE 2" refers to Printex XE-2, a conducting carbon black available from Degussa Corp., and "XC 72" refers to a conducting carbon black available from Cabot Corp. In Examples 2-8, the amount of calcium carbonate was 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 were prepared and tested in accordance with the procedure described in Example 1 , and the results are shown in Table I. Examples 11-14

Example 1 was repeated four times using the same carbon black, except that the carbon black was utilized in the amounts of 0.54, 0.81 , 1.1 , and 1.4 percent 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 were prepared and their conductivity tested in accordance with the procedure described in Example 1. The conductivities of the plaques are 8.1 by 10\ 1.9 by 10^"4, 3.2 by 10"\ and 1.2 by 10^'3, respectively.

Claims

CLAIMS:

I . A process comprising electromotively coating an article molded from a composition comprising a mixture of (a) a thermosetting resin, (b) a filler, a reinforcing material, or a combination thereof, and (c) a carbon black having a primary particle size of less than 125 nm, a nitrogen surface area of at least 50 m²/g and a dibutyl phthalate absorption of at least 50 cc/100 g; wherein (i) component (b) is present in an amount, based on the volume of the composition, of at least 30 percent; (ii) component (c) is present in an amount, based on the weight of the composition, of less than 3 percent; and (iii) the article has a conductivity of at least 10 '⁴ S/cm.

2. The process of Claim 1 wherein the carbon black has a nitrogen surface area of at least 125 mVg.

3. The process of Claim 1 wherein the carbon black has a nitrogen surface area of at least 200 m7g.

4. The process of Claim 1 wherein the carbon black has a nitrogen surface area of at least 750 m7g.

5. The process of Claim 1 wherein the dibutyl phthalate absorption of the carbon black is at least 75 cc/100 g.

6. The process of Claim 1 wherein the dibutyl phthalate absoφtion of the carbon black is at least 150 cc/100 g.

7. The process of Claim 1 wherein the dibutyl phthalate absoφtion of the carbon black is at least 250 cc/100 g.

8. The process of Claim 7 wherein the carbon black has a nitrogen surface area of at least 750 mVg.

9. The process of Claim 1 wherein component (b) is present in an amount, based on the volume of the composition, of at least 40 percent.

10. The process of Claim 1 wherein the composition comprises at least 10 percent by volume of glass fibers.

I I . The process of Claim 1 wherein the composition comprises at least 15 percent by volume of glass fibers.

12. The process of Claim 1 wherein the article has a conductivity of at least

10 S/cm.

13. The process of Claim 1 wherein the article has a conductivity of at least 10^'7 S/cm.

14. The process of Claim 1 wherein component (c) is present in an amount, based on the weight of the composition, of less than 2.5 percent.

15. A process comprising electromotively coating an article molded from a composition comprising a mixture of (a) a thermosetting resin, (b) a filler, a reinforcing material, or a combination thereof, and (c) a carbon black having a primary particle size of less than 125 nm, a nitrogen surface area of at least 200 m7g and a dibutyl phthalate absorption of at least 75 cc/100 g; wherein (i) component (b) is present in an amount, based on the volume of the composition, of at least 30 percent; (ii) component (c) is present in an amount, based on the weight of the composition, of less than 3 percent; and (iii) the article has a conductivity in the range of 10 ^M S/cm to 10^'1 S/cm.

16. The process of Claim 15 wherein the composition comprises at least 10 percent by volume of glass fibers.

17. The process of Claim 15 wherein the composition comprises at least 15 percent by volume of glass fibers.

18. A process comprising electromotively coating an article molded from a composition comprising a mixture of (a) a thermosetting resin, (b) a filler, a reinforcing material, or a combination thereof, and (c) a carbon black having a primary particle size of less than 125 nm, a nitrogen surface area of at least 750 mVg and a dibutyl phthalate absorption of at least 250 cc/100 g; wherein (i) component (b) is present in an amount, based on the volume of the composition, of at least 30 percent; (ii) component (c) is present in an amount, based on the weight of the composition, of less than 3 percent; and (iii) the article has a conductivity in the range of 10 ^M S/cm to 10^"' S/cm.

19. The process of Claim 18 wherein the composition comprises at least 10 percent by volume of glass fibers.

20. The process of Claim 18 wherein the composition comprises at least 15 percent by volume of glass fibers.