Use of solid epoxy resins for powder coating composition for metal reinforcing bars and a method for coating metal reinforcing bars therewith

Abstract: A powder coating composition for metal reinforcing bars which comprises (A) an advanced epoxy resin resulting from reacting (1) a mixture of (a) an aromatic based epoxy resin and (b) an aliphatic based epoxy resin with (2) a dihydric phenol; (B) a curing agent for component (A); (C) an accelerator for component (C) and (D) one or more fillers, pigments or dyes. A metal reinforcing bar coated with the powder coating composition is also disclosed.

Description: The present invention concerns the use of an epoxy resins for powder coating compositions for metal reinforcing bars. In WO 87/102043 a powder coating composition containing a solid epoxy resin is described which has an average epoxide functionallity per molecule of greater than 2 and a solid curing agent for the epoxy resin. The solid epoxy resin is a multifunctional polyglycidyl ether of a bisphenol compound or a multifunctional polyglycidyl ether of a polyglycol advanced in molecular weight with bisphenol compounds. These coating compositions are particularly useful for coating metallic substrates. From GB-A-2 167 071 a solvent based advanced epoxy resin prepared by reacting in the presence of a suitable catalyst (A) a composition comprising (1) at least one aliphatic diepoxide and (2) at least one aromatic diepoxide, with (B) at least one compound leaving two aromatic hydroxyl groups per molecule wherein components (A-1) and (A-2) employed in quantities such that from 10 to 50 percent of the epoxide equivalents contained in component (A) are contributed by component (A-1), from 50 to 90 percent of such epoxide equivalents are contributed by component (A-2) and wherein components (A) and (B) are employed in quantities such that the resultant advanced epoxy resin has an average epoxide equivalent weight of from 350 to 30.000 is known. GB-A-1 527 258 refers to a process for the preparation of a solid polyepoxide having high stability and transformable into a mobile liquid epoxy resin ester by reacting with a fatty acid, characterized by bringing into contact: A) a diglycidyl ether of bisphenol-A B) a diglycidyl ether of polyoxyethylene-glycol. C) bisphenol-A; the amount of the B component being of from 5 to 20 parts by weight for every 100 parts by weight of the sum of the A and B components and the amount of the C component being of from 10 to 30 parts by weight for every 100 parts by weight of the said sum, and the reaction being carried out at a temperature of from 140° to 200°C, until constant, or approximately constant, epoxy equivalent values are obtained in the resulting polyepoxide. Metal reinforcing bars are coated so as to prevent corrosion in a normal service environment, particularly in exterior structures such as highways and bridges. Current compositions involve a curing agent based on methylene dianiline which is becoming more highly regulated by federal agencies. The coating industry is therefore seeking alternative coating compositions for reinforcing bars which do not contain methylene dianiline. The replacement formulation must meet certain property criteria such as excellent low temperature flexibility and impact resistance, preferably at high filler loadings. The present invention pertains the use of a solid epoxy resin having an epoxy equivalent weight from 1200 to 2400 and being prepared by reacting: (1) a mixture of diepoxides containing (a) at least one aromatic diepoxide which is a diglycidyl ether of any of resorcinol, catechol, hydrochinone, bisphenol A, bisphenol F, bisphenol K, and bisphenol S, and (b) at least one aliphatic diepoxide, wherein from 70 to 95 percent of the epoxide groups in the mixture are contributed by the aromatic diepoxide and from 30 to 5 percent of the epoxide groups are contributed by the aliphatic diepoxide; with (2) a compound having two aromatic hydroxyl groups per molecule which is any of hydroquinone, catechol, hydroquinone, bisphenol A, bisphenol F or bisphenol S for corrosion-resistant powder coating compositions that obtain good low temperature flexibility and impact resistance without methylene dianiline. The use is particularly for powder coating compositions further comprising (i) as a curing agent for the solid epoxy resin a curing amount of a compound having an average more than one phenolic hydroxyl group per molecule; (ii) as an accelerator for the curing agent, an accelerating amount of a tertiary amine or an adduct of a secondary amine with a diglycidyl ether of a compound having two phenolic hydroxyl groups per molecule; (iii) one or more fillers, pigments or dyes; and (iv) one or more flow modifiers. Another aspect of the present invention pertains to a process for coating metal reinforcing bars with the aforementioned powder coating. Suitable aromatic diepoxides include those described by P. H. Martin in U S-A-3,391,109. Particularly Suitable aromatic diepoxides include the diglycidyl ethers of resorcinol, catechol, hydroquinone, bisphenol A, bisphenol F, bisphenol K, and bisphenol S. Mixtures of aromatic diepoxides can also be employed if desired. Suitable aliphatic diepoxides which can be employed herein include those described by Hickner et al in U S-A-4,608,313. Particularly suitable aliphatic diepoxides include the diglycidyl ethers of propylene glycol, dipropylene glycol, tripropylene glycol and polyoxypropylene diols having an EEW up to 400. Mixtures of aliphatic diepoxides can also be employed if desired. The aromatic diepoxides and aliphatic diepoxides are employed in quantities such that in the mixture thereof suitably from 70 to 95, more suitably from 75 to 95, most suitably from 80 to 95, percent of the epoxide groups are contributed by the aromatic diepoxide and suitably from 30 to 5, more suitably from 25 to 5, most suitably from 20 to 5, percent of the epoxide groups are contributed by the aliphatic diepoxide. Suitable compounds containing two aromatic hydroxyl groups per molecule which can be employed herein include those described by the aforementioned Hickner et al in U S-A-4,608,313. Particularly suitable compounds containing two aromatic hydroxyl groups per molecule include, for example, catechol, hydroquinone, bisphenol A, bisphenol F, bisphenol K, and bisphenol S. Mixtures of these aromatic hydroxyl containing compounds can also be employed if desired. The compounds containing two aromatic hydroxyl groups per molecule are employed in quantities which provide the resultant advanced epoxy resin with an epoxide equivalent weight (EEW) from 1200 to 2400, more suitably from 1600 to 2000. Suitable solid epoxy resin curing agents which can be employed herein include, for example polyhydric phenols such as those described in in U S-A-4,322,456. Particularly suitable curing agents include, for example, that prepared by reacting an excess of bisphenol A with a diglycidyl ether of bisphenol A. The resultant product is terminated in phenolic hydroxyl groups. The curing agent is employed in a functionally equivalent amount which is that amount which is sufficient to cure the epoxy resin at the curing conditions employed. Particularly suitable amounts of curing agent are suitably from 5 to 25, more suitably from 5 to 20, most suitably from 7 to 14 percent by weight based upon the weight of the epoxy resin. Suitable accelerators for the curing agents which can be employed herein include, for example, tertiary amines and adducts of secondary amines and diglycidyl ethers of compounds having two phenolic hydroxyl groups per molecule. Suitable secondary amines include, for example, imidazole, 2-methylimidazole, 2-methylimidazoline, 2-ethylimidazole, 2-ethylimidazoline and combinations thereof. Suitable diglycidyl ethers of compounds having two phenolic hydroxyl groups per molecule include the diglycidyl ethers of resorcinol, catechol, hydroquinone, bisphenol A, bisphenol F, bisphenol K, dihydroxy biphenyl, and combinations thereof. Particularly suitable accelerators include, for example, an adduct of a secondary amine and an epoxy resin commercially available from Shell Chemical Company as EPON™ P-101. The accelerator is employed in a functionally equivalent amount which is that amount which is sufficient to accelerate the cure of the epoxy resin at the curing conditions employed. Particularly suitable amounts of accelerator are from 1 to 20, more suitably from 1 to 10, and most suitably from 1 to 8 percent by weight based upon the weight of the epoxy resin. Suitable fillers which can be employed herein include, for example, metal oxides, metal sulfates, metal carbonates, metal silicates, and silica. Particularly suitable fillers include, for example, barium sulfate, calcium carbonate, titanium dioxide, calcium meta silicate, iron oxide, mica, and any combination thereof. Suitable dyes or pigments which can be employed herein include, for example, metal oxides, organo metallic compounds and any of those described in the National Paint and Coatings Association's Raw Materials Index for Pigments. Particularly suitable dyes or pigments include, for example, chrome oxide, titanium dioxide, iron oxide, phthalocyanine green and blue, and any combination thereof. The combined amount of fillers, pigment or dyes which can be employed herein is suitably from 0.5 to 30, more suitably from 0.5 to 25, most suitably from 0.5 to 20 percent by weight based upon the weight of the total powder coating formulation. The powder coating compositions of the present invention can be applied to the reinforcing bars by any conventional means such as spraying the powder coating onto the preheated metal, or passing the preheated reinforcing bar through a fluidized bed of the coating powder. The coatings are cured by the residual heat of the reinforcing bar at temperatures suitably from 130°C to 300°C, more suitably from 180°C to 250°C, most suitably from 220°C to 250°C for a time sufficient to cure the coating, suitably from 5 to 60, more suitably from 5 to 30, most suitably from 5 to 15 seconds. Suitable reinforcing bar materials include steel. The present invention will further be described with reference to the following examples. The following materials were employed in the Examples. Aromatic Epoxy Resin A is a diglycidyl ether of bisphenol A having an EEW of 1822. Aromatic Epoxy Resin B is the epoxy resin resulting from reacting 788.9 g (4.173 epoxide equiv.) of a mixture containing 10 weight percent of a diglycidyl ether of polypropylene glycol having an EEW of 315 and 90 weight percent of a diglycidyl ether of bisphenol A having an EEW of 181 with 406.9 g (3.569 aromatic hydroxyl equiv.) of bisphenol A. The resultant advanced epoxy resin has an EEW of 1792. Aromatic Epoxy Resin C is the epoxy resin resulting from reacting 893.9 g (4.518 epoxide equiv.) of a mixture containing 20 weight percent of a diglycidyl ether of polypropylene glycol having an EEW of 315 and 80 weight percent of a diglycidyl ether of bisphenol A having an EEW of 181 with 438.2 g (3.844 aromatic hydroxyl equiv.) of bisphenol A. The resultant advanced epoxy resin has an EEW of 1827. Aromatic Epoxy Resin D is the epoxy resin resulting from reacting 934.1 g (4.612 epoxide equiv.) of a mixture containing 25 weight percent of a diglycidyl ether of polypropylene glycol having an EEW of 315 and 75 weight percent of a diglycidyl ether of bisphenol A having an EEW of 181 with 446 g (3.912 aromatic hydroxyl equiv.) of bisphenol A. The resultant advanced epoxy resin has an EEW of 1947. Aromatic Epoxy Resin E is a blend of 25 weight percent of an epoxy novolac modified diglycidyl ether of bisphenol A epoxy resin having an EEW of 804 and 75 weight percent of Aromatic Epoxy Resin A. The blend has an EEW of 1384. PREPARATION OF COATING FORMULATIONS The coating formulations were prepared by dry blending all of the components followed by melt mixing in a Buss Condux PKL-46 extruder at a temperature of 90°C and a screw speed of 120 rpm resulting in a residence time in the extruder of about 20 seconds. The extrudate was ground and sieved through an 80 mesh screen having openings of 0.1778 mm (0.0070 inch). PREPARATION OF COATED BARS The powder coating formulation was applied to 25.4 mm x 25.4 mm x 3.175 mm (1 in. x 8 in. x 1/8 in.) mild steel bars containing a 2 mil NACE near white finish. The bars were preheated to 232°C for 30 minutes, dipped into an Armstrong Model B fluidized bed containing the powder to achieve a 6-8 mil coating thickness, post baked at 232°C for 5 minutes, and subsequently water quenched. The following tests were performed on the coating compositions or coated bars. GEL TIMES The stroke cure gel time indicates the relative reactivity of a powder coating formulation at a specific temperature and is usually expressed as the number of seconds to reach gelation. Determinations were made using a Thermo-Cure (Thermo Electric Co.) hot plate. Gel points were indicated when a spatula no longer provided a polymer thread. IMPACT The 3.175 mm (1/8 in.) thick coated bars were forward impacted at ambient temperature with a Gardner Impact Tester and checked for cracking at the rims of the indentations. The reported values were the highest impacts at which the coatings did not contain cracks. The maximum reading the tester reached was 18.08 J (160 in-lbs). FLEXIBILITY Three bars for each formulation were chilled and stabilized at the desired temperature. The bars were removed from the cold box one at a time, placed in a vice, and the vice jaws closed onto their top and bottom edges. The bars were bent to a 180 degree angle in 30 seconds by closing the vice. The bent specimens were visually checked for cracks. Unless otherwise noted, the cracks were small (less than 3.175 mm (1/8 in.,)) in length and the substrate could not be seen through the coating. The coating formulations are given in Table I and the test results are given in Table II. Formulation Designation A B C D E Resin, Type pbw A B C D E 69.2 69.2 69.2 69.2 66.5 Hardener , pbw 4.9 4.9 4.9 4.9 6.25 Accelerator , pbw 4.9 4.9 4.9 4.9 6.25 Flow Modifier 1.0 1.0 1.0 1.0 1.0 Barium Sulfate, pbw 10.0 10.0 10.0 10.0 10.0 Calcium Carbonate, pbw 7.0 7.0 7.0 7.0 7.0 Titanium Dioxide, pbw 3.0 3.0 3.0 3.0 3.0 Formulation Designation A B C D E Gel Time @ 232°C,sec. 9 8 11 11 7 Impact, In-lbs J 60 100 160 160 80 6.78 11.3 18.08 18.08 9.04 Flexibility, No.of Cracks @ -18°C, 3 1 0 0 4 @ -30°C, 5 1 0 0 6

Claims: Use of a solid epoxy resin having an epoxy equivalent weight from 1200 to 2400 and being prepared by reacting: (1) a mixture of diepoxides containing (a) at least one aromatic diepoxide which is a diglycidyl ether of any of resorcinol, catechol, hydrochinone, bisphenol A, bisphenol F, bisphenol K, and bisphenol S, and (b) at least one aliphatic diepoxide, wherein from 70 to 95 percent of the epoxide groups in the mixture are contributed by the aromatic diepoxide and from 30 to 5 percent of the epoxide groups are contributed by the aliphatic diepoxide; with (2) a compound having two aromatic hydroxyl groups per molecule which is any of hydroquinone, catechol, bisphenol A, bisphenol F or bisphenol S for corrosion-resistant powder coating compositions. Use as described in Claim 1, wherein from 80 to 95 percent of the epoxide groups in the mixture are contributed by the aromatic diepoxide and from 20 to 5 percent of the epoxide groups are contributed by the aliphatic diepoxide. Use as described in any of the preceding Claims, wherein the solid epoxy resin has an epoxy equivalent weight form 1600 to 2000. Use as described in any of the preceding Claims, wherein the aliphatic diepoxide is a diglycidyl ether of any of: propylene glycol, dipropylene glycol, tripropylene glycol, or polyoxypropylene diols whose epoxy equivalent weight of the aliphatic diepoxide is no more than 400. Use as described in any of the preceding Claims, wherein the powder coating composition further comprising (i) as a curing agent for the solid epoxy resin a curing amount of a compound having an average more than one phenolic hydroxyl group per molecule; (ii) as an accelerator for the curing agent, an accelerating amount of a tertiary amine or an adduct of a secondary amine with a diglycidyl ether of a compound having two phenolic hydroxyl groups per molecule; (iii) one or more fillers, pigments or dyes; and (iv) one or more flow modifiers. Use as described in Claim 5 wherein: (1) the weight of curing agent in the composition is from 5 to 25 percent of the weight of solid epoxy resin in the composition; and (2) the weight of accelerator in the composition is from 1 to 20 weight percent of the weight of epoxy resin in the composition. Use as decribed in any of Claims 5 or 6, wherein the combined amount of fillers, pgiments or dyes in the powder coating composition is 0,5 to 30 weight percent of the entire composition. A method for coating a metal reinforcing bar, wherein a powder coating composition as decribed in any of claims 5 to 7 is applied to a preheated metal reinforcing bar and for a time suitable to provide a cured coating. A method as described in Claim 8, wherein the temperature is 130°C to 300°C, and the bar contains steel.

Inventors: LAMPTON ROBERT D JR, TYLER MANUEL C JR, LAMPTON, ROBERT D., JR., TYLER, MANUEL C., JR.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1989
Main classification: C08G59
IPC codes: C08G59, C09D163, C09D5