RAPID CURE THERMOSETTING FUNCTIONAL POWDER COATINGS

Abstract: Thermosetting powder coating compositions which are rapid curing at elevated temperatures and which exhibit extended shelf lives in the powder state are described. The powder coating compositions have utilily as functional protective coatings against heat, corrosion and moisture for materials such as steel pipelines, steel reinforcing bar, deep well petroleum drilling pipes and electrical cables. The powder coatings comprise 1,3-phenylenebis-2-oxazoline in combination with any two coreactants chosen from the group consisting of a nucleophile such as a phenolic compound, an electrophile such as a diacid dianhydride, and an epoxy resin.

Description: RAPID CORE THERMOSETTING FUNCTIONAL POWDER COATINGS Field of the Invention This invention provides thermosetting powder coatings for functional applications (i.e., protective rather than decorative) which are very rapidly cured while at the same time having extended shelf lives in the powder state, and which show excellent protective characteristics. Background of the Invention Thermosetting epoxy resin powders enjoy wide use as protective (i.e.. functional) coatings for a variety of materials such as steel pipe, metal sheets or bars, and other materials where flexibility, abrasion-resistance, and corrosion-resistance of the coatings are required. These coatings are described in "Epoxy Surface Coatings," by E. Linak, Chemical Economics Handbook. SRI International, October, 1991. Such coatings are advantageous because of these superior protective properties, and also because the powders are applied in the absence of any volatile solvents and curing of the powders does not release harmful vapors. In addition, when properly formulated, such powder resins exhibit very long shelf lives yet cure rapidly on application to a hot substrate. While powder coatings which generally fit these criteria are known, they usually suffer from two deficiencies. First, many powder coatings exhibit unsatisfactory shelf lives at ambient conditions, by clumping or fusing in the presence of moisture or slightly elevated temperatures. Typically, this is due to the presence of added curing catalyst in the powder formulation which promotes some cure even at ambient temperatures. Second, even with added catalyst, many powder coatings do not cure rapidly enough for the high volume production required in industry. A gel time on the order of ten seconds at application temperatures of 180'C or greater is often required for economical production. JP 62-104837, published May 15, 1987, by E. Hosokawa and M. Fukushima, assigned to Showa Densen Denzoku K.K. , describes a powder coating consisting of Scotchcast™ 5256 epoxy resin (available from 3M) , PBOX, trimellitic anhydride, and Modaflow™ leveling agent (available from Monsanto) . While a powder coating incorporating PBOX is described in JP , 837, it requires a monocarboxylic acid anhydride, which also bears a free carboxyl group, as the preferred embodiment. Such anhydrides are known to react differently from dianhydrides having no free carboxyl groups, in resin formulations. Several formulations of liquid epoxy resin coatings containing PBOX are known. U.S. Patent No. 4,652,620 describes a resin comprising PBOX, Epon™ 828 liquid epoxy resin, and Alnovol™ PN320 phenol resin. The presence of PBOX reportedly increases the T g of the product resin from 117 * C to 150 * C. The resultant resin is a liquid, not a powder. In addition, U.S. Patent No. 4,652,620 requires a minimum of 20% PBOX in any coating composition. European Patent Application No. 342,035, assigned to the assignee of the present application, describes powdered coating compositions for metal substrates comprising uncured epoxy resins and aromatic compounds having hydroxy groups in adjacent or available adjacent positions, including catechol novolak resins. The incorporation of PBOX is not disclosed therein. Epoxy-based protective powder coatings are well-known, having the desirable characteristics of high temperature stability, toughness, corrosion- resistance, and flexibility. However, uncatalyzed powder coatings suffer from slow gel times, and catalyzed compositions often have poor shelf-life. A need exists for a powder resin system with rapid gel time and cure time and which is also stable on the shelf for periods of six to twelve months. Brief Description of the Invention We have discovered such a protective powder coating. The present invention relates to powder coating compositions which are nonblocking or storage stable between about 15 * C and about 50 * C, preferably between about 20'C and about 45 * C, and most preferably between about 22 * C and about 40'C, comprising the reaction product of a composition comprising: (a) about 4 to about 40 percent by weight of 1,3-phenylenebis-2-oxazoline; (b) optionally about 10 to about 70 percent by weight of a nucleophilic material selected from the group consisting of (i) phenolic novolac compounds, (ii) bisphenols, (iii) bisphenol-terminated epoxy resins, and (iv) non-heat-reactive aromatic hydroxy-functional compounds, having an average of greater than one aromatic hydroxyl group per molecule, and (v) mixtures thereof, wherein, for the nucleophilic material, at least one of the following (1) and (2) is true: (1) the nucleophilic material has a ring-and-ball softening point above about 70 * C; (2) the nucleophilic material has a crystalline melting point above about 40°C; (c) optionally about 4 to about 70 percent by weight of an electrophilic material selected from the group consisting of polyanhydrides and mixtures thereof; (d) optionally about 20 to about 80 percent by weight of an epoxy resin wherein for the epoxy resin at least one of the following (i) and (ii) is true: (i) the epoxy resin has a ring-and-ball softening point above about 70 * C; (ii) the epoxy resin has a crystalline melting point above about 40 * C; wherein the composition comprises one of the following combinations of components: (a) (b) and (c) ; (a) (b) and (d) ; and (a) (c) and (d) ;and wherein the weight percentages are based upon the total weight of (a) plus any optional components selected from the group consisting of (b) , (c) and (d) . The term "non-heat-reactive" as used herein refers to materials which will not homopolymerize when subjected to a temperature of about 100 * C to about 300 * C. Another aspect of this invention relates to protective coatings prepared from the powder coating compositions of the invention. This invention demonstrates PBOX (1,3- phenylene bisoxazoline) as a latent curing agent in a variety of thermosetting resin systems. Three unique aspects of the present systems are emphasized, in contrast to known systems: 1) no catalysts are required to achieve the rapid and thorough cures seen with the present system; 2) the present invention demonstrates that PBOX is an effective curing agent even at concentrations much less than 20% by weight; and 3) the present system features a flexible combination of three active materials, allowing for improved tailoring of the properties of the coating. PBOX chemistry is unusual in that the oxazoline ring of PBOX is susceptible to attack by both electrophilic agents (at the ring nitrogen) and nucleophilic agents (at the unsubstituted carbon adjacent to the ring oxygen) . It is this duality that makes it an effective curing agent in a multi-component system such as the present invention. Electrophiles such as acidic hydrogen, an unsubstituted carbon of an epoxide ring, and an anhydride carbonyl carbon readily attack the ring nitrogen, highly activating the unsubstituted carbon adjacent to the ring oxygen to nucleophilic attack. Since the system is rich in nucleophiles such as phenolic oxygen, epoxide oxygen, amines, or the free oxygen of cleaved anhydrides, reactions in this system are quite rapid at coating temperatures. The thorough crosslinking which results contributes to higher T g and enhanced thermal stability of the coatings. The composition of the present invention is advantageous in that it is coatable as a powder. Powder coatings are advantageous due to their lack of solvents, and hence are environmentally superior. Powder coatings are also more efficient than solvent- based spray coatings, in that there is virtually no overspray or excess material usage. The composition of the present invention is also advantageous in that it does not require the use of catalyst. The present invention, using a three- component or four-component system, provides polymer gellation and curing at a rate equal to or greater than what is now available only with the use of a catalyst. Catalysts can be expensive and/or toxic and can contribute to decreased shelf life. The composition of the present invention is also advantageous in that PBOX is very versatile as a coreactant and curative. Very few three-part systems incorporating PBOX have been described. The composition which can comprise a variety of components is highly tailorable and versatile. PBOX has been found to be a very effective latent curing agent; one that is completely unreactive at room temperature, but which is very fast at coating temperatures typically used for functional powder coatings. Epoxy resins need not be present to afford powder coatings of the present invention, since one skilled in the art can envision a number of combinations of PBOX/electrophile/nucleophile, as outlined above, which do not include epoxy resins. The present invention provides for flexible, rapid curing thermosetting powder coatings for protection of metallic surfaces under adverse conditions of heat, moisture, and corrosive materials. Typical substrates include steel reinforcing bars for cement constructions ("rebar") , steel pipelines, especially those that are buried (both interior and exterior coatings) , deep well petroleum drilling pipes, and electrical cable. Detailed Description of the Invention The powder coating compositions are cured using 1,3-phenylene-bisoxazoline (PBOX) as a coreactant with a nucleophile/electrophile pair to form a coating. A wide variety of resin formulations are shown to accommodate the PBOX, all of which are characterized by rapid onset of gelation and rapid cure at working temperatures. All formulations of the present invention are three-component: PBOX, a suitable nucleophilic moiety capable of participating in polymerization reactions, such as a (poly)phenol or a (poly)epoxide, and a suitable electrophilic moiety also capable of participating in polymerizations, such as a multifunctional anhydride or a (poly)epoxide. The choice of starting materials is further limited to those which produce powders suitable for coating that are solid and are nonblocking (i.e.. non-fusing) at room temperatures. Surprisingly, the powder coating compositions described by this invention do not require a catalyst to effect the observed rapid curing. Preferred concentrations of each of the three components of the present powders vary widely as a function of which components are present and on the desired properties of the resulting coating, as will be seen from the enabling examples presented herein. PBOX PBOX is referred to by various names including the following: l,3-phenylenebis-2-oxazoline; meta-phenylenebis-2-oxazoline; 2,2 ' -(1,3- phenylene)bis(2-oxazoline) ; 2,2'-m-phenylenebis(2- oxazoline) ; l,3-bis(2-oxazolin-2-yl)benzene; and isophthaloyl bisoxazoline. The preferred Chemical Abstracts name for PBOX is 2,2'-(l,3-phenylene)bis[4,5- dihydro-]oxazole, CAS Registry No. 34052-90-9. As a percentage of the total weight of the powder coating composition, PBOX can be present in a concentration ranging from about 4% to about 40%. PBOX is commercially available from a number of sources including the Ashland Oil Co. PBOX is an essential component of the powder coatings of this invention. If it is left out, the resulting binary mixtures of nucleophiles and electrophiles are very slow to react, or are not reactive at all, especially in the absence of any catalyst. Nucleophilic Materials Nucleophilic materials of the invention are normally solid at standard temperature and pressure, by which is meant that they exhibit a ring-and-ball softening point of at least about 70 * C or exhibit a crystalline melting point of at least about 40'C. Examples of suitable nucleophilic materials include those selected from the group consisting of phenolic novolac compounds, bisphenols, bisphenol-terminated epoxy resins, and non-heat-reactive aromatic-hydroxy- functional compounds having an average of more than one aromatic hydroxyl group per molecule which may be ring- substituted or ring-unsubstituted, wherein the ring substituents include but are not limited to those selected from the group consisting of alkyl substituents having from about 1 to about 4 carbon atoms not including alkylene bridges between aromatic rings; halogen atoms such as fluorine, chlorine, bromine and iodine; amines; N-alkyl and N, N-di- alkylamines having from about 1 to about 8 carbon atoms in the alkyl group(s) ; nitro; amides; N-alkyl and N, N- dialkylamides having from about 1 to about 8 carbon atoms in the alkyl group(s) , and mixtures thereof. The choice of preferred nucleophilic material is made based on the end use and desired properties of the coating. When a high-T g coating is desired, ring-substituted or ring-unsubstituted phenolic novolac resins are the preferred nucleophile. When a flexible coating is required, aromatic-hydroxy-functional phenolic- terminated epoxy resins are the preferred nucleophile. When toughness and adhesiveness of the coating is desired, phenolic novolac resins bearing more than one aromatic hydroxyl group per aromatic ring are the preferred nucleophile. Reagents that donate an electron pair in chemical reactions are said to be nucleophilic ("nucleus loving"), according to Roberts and Caserio, Basic Principles of Organic Chemistry. 2nd Edition, W. A. Benjamin, Menlo Park, CA, 1977, p. 208. In the present invention, a nucleophilic material, if included, can react with PBOX by attacking the C-4 carbon atom of the oxazoline ring in a ring-opening reaction, which contributes to the polymer crosslinking and/or chain extension that is critical to the high T. values of the cured powder coating compositions of the invention. Among the phenolic compounds useful in the present invention are bisphenols such as Bisphenol A alone or in combination with an aromatic hydroxy- functional phenolic-terminated epoxy resin such as DEH™-85, from Dow Chemicals, or Bisphenol F, phenolic novolac resins and substituted and modified phenolic novolac resins. Bisphenol-A is an aromatic phenolic compound which has been extensively described in the literature. Chapter 2 of the Handbook of Epoxy Resins. H. Lee and K. Neville, McGraw Hill, New York, 1967, describes Bisphenol-A and its reactions with epoxy compounds. Phenolic novolac resins are obtained by the polycondensation of phenol (or a substituted phenol) with formaldehyde to form a resin of idealized structure C 6 H 5 OH-[CH 2 -C 6 H 5 OH] n , wherein n is an integer of about 1 to about 10. A special case, where n=l, is known in the art as Bisphenol F. An introduction to the preparation of such resins can be found in the Handbook of Epoxy Resins, (supra) . pages 2-10. In contrast to resole resins, in which the reactive functional groups are methylols (i.e., -CH 2 OH) , novolac reactive functional groups are phenols (i.e., -C 6 H 5 OH) . Representative novolac resins include Borden Durite™ SD series resins. The generic term "phenolic novolac resins" is meant to specifically include novolac resins obtained by reaction of aromatic rings bearing more than one hydroxyl group, examples of which include but are not limited to catechol, resorcinol, hydroquinone, pyrogallol and related naphthalenic compounds with formaldehyde, usually in the presence of acid and a stoichiometric excess of the phenolic reactant. In addition to more than one hydroxyl group, aromatic rings of phenolic novolac resins of the invention may bear substituents which include but are not limited to those selected from the group consisting of alkyl substituents having from about 1 to about 4 carbon atoms not including alkylene bridges between aromatic rings; halogen atoms such as fluorine, chlorine, bromine and iodine; amines; N-alkyl and N, N-di- alkylamines having from about 1 to about 8 carbon atoms in the alkyl group(s); nitro; amides; and N-alkyl and N, N-dialkylamides having from about 1 to about 8 carbon atoms in the alkyl group(s) . Nucleophiles useful in the present invention are selected from the group consisting of nucleophiles having a ring-and-ball softening point above about 70 * C, those having crystalline melting points above about 40 * C, and mixtures thereof. That is, they are generally friable solid materials at room temperature. Epoxides and epoxy resins, while certainly nucleophilic, are discussed below and are not considered in this section. Electrophilic Materials Reagents that acquire an electron pair in chemical reactions are said to be electrophilic ("electron loving")", according to Basic Principles of Organic Chemistry. (Supra), p. 208. Electrophilic reagents participate in reactions with PBOX by attacking the electron-rich nitrogen atom of the oxazoline ring, a process which apparently highly activates the C-4 carbon atom to nucleophilic attack. In the present invention, the use of polyfunctional electrophiles such as polyanhydrides and epoxy resins adds to the strength of the resulting coating through the many crosslinks thus formed. In addition, ring activation by electrophiles apparently contributes significantly to the observed high reaction rate. Examples of electrophiles of element (c) useful in the present invention include but are not limited to those selected from the group consisting of polyanhydrides such as aliphatic and aromatic dianhydrides, and mixtures thereof. Dianhydrides comprise a large group of acid anhydride materials which are suitable for these formulations. Dianhydrides have traditionally been employed as crosslinkers for hydroxy-containing polymers via esterification reactions of the -OH with anhydride carbonyl. However, in the present case, carbonyl carbons of the anhydride act as electrophilic agents in attacking the PBOX ring nitrogen. This apparently activates the PBOX ring carbon adjacent to the ring oxygen, rendering it susceptible to attack by oxygen of phenolic hydroxyls in a PBOX ring-opening reaction. Likewise, this ring carbon can attack an oxirane oxygen in a crosslinking step. Thus, both the dianhydride and PBOX act as crosslinking agents for the epoxy resin. The dianhydride appears to act as an accelerator for the PBOX reactions; in the absence of a dianhydride these curing reactions are noticeably slower. Examples of typical dianhydrides include but are not limited to those selected from the group consisting of benzophenone tetracarboxylic acid dianhydride (BTDA) , pyromellitic dianhydride, and mixtures thereof. BTDA is preferred. The known electrophilic nature of epoxies and epoxy resins is purposefully excluded from the above discussion and will be dealt with below. Epoxy Resins Depending on the nature of the other reactive species in the powder coating mixture, epoxy resins participate as either electrophilic or nucleophilic reactants with PBOX. Epoxy resins useful in the invention are well-known in the literature. Examples of such epoxy resins are disclosed in U.S. Patent No. 3,971,745, assigned to the assignee of the present case. Monomeric or polymeric polyepoxides suitable for use in the present invention comprise any of the conventional polyepoxides containing more than one 1,2- epoxide (i.e., oxirane) ring per molecule, the two carbon atoms of the epoxide ring being catenary atoms of an acyclic aliphatic chain which can be straight or branched. The epoxide rings of the polyepoxide may be in internal and/or terminal positions. The backbone structure connecting the epoxide rings may comprise aliphatic, cycloaliphatic, heterocyclic and/or aromatic constituents and may also contain hetero atoms such as oxygen, nitrogen or sulfur. For purposes of brevity, polyepoxide is often referred to herein as epoxy or epoxy resin. Polyepoxides having glycidyl ether groups are the preferred type of polyepoxides to be used in this invention because of the commercial availability thereof. One class of polyglycidyl ether polyepoxides can be prepared by the reaction of epichlorohydrin and a polyol or polyphenol such as 2,2-bis(4- hydroxyphenyl)propane (Bisphenol A) . Other common polyepoxide forming reactants useful in this invention are disclosed in the literature. See, for example, U.S. Patent NOS. 2,840,541; 2,892,809; 2,921,049; 2,921,923; 2,943,096; and 3,629,167. A wide variety of polyepoxide resins useful in this invention is commercially available with a wide range of epoxide equivalents, e.g., about 100 to about 1,500, such as those commercially available under the trademark Epon which are available from the Shell Chemicals Company, and Araldite, available from the Ciba-Geigy Company. A class of epoxy resins useful in the present invention are the "aromatic epoxy resins," which are herein defined as resins comprising at least aromatic or fused aromatic rings and epoxy groups. Typically, such aromatic epoxy resins arise from the reaction of epichlorohydrin and a compound having at least one aromatic hydroxy substituent. Another class of useful epoxy resins includes those resins that are solid aliphatic and aromatic copolymeric epoxy resins produced by the copolymerization of a mixture of aliphatic epoxy resins and aromatic epoxy resins with a bisphenol compound such as Bisphenol-A. An additional comprehensive description of typical epoxy resins can be found in U.S. Patent No. 5,013,791 (assigned to PPG Industries). Epoxy resins are also thoroughly described in the monograph Handbook of EPOXV Resins. H. Lee and K. Neville, McGraw-Hill, New York, 1967 and in Epoxy Resin Technology. P. F. Bruins, ed. , Interscience Publishers, New York, 1968. The proportion of epoxy resin, if used, in the final coating composition is determined by the nature of the other constituents and the use to which the powder coating will be put. Preferably the epoxy resin selected comprises from about 20% to about 80% by weight in order to assure good coating performance. Epoxy resins selected from the group consisting of epoxy resins and polyepoxide monomers having a ring- and-ball softening point above about 70 * C, those having crystalline melting points above about 40 * C, and mixtures thereof are useful herein. That is, epoxy resins useful in the present invention are generally solid friable materials at room temperatures, and may herein be described as "solid epoxy resins." Within the scope of the present invention, numerous combinations of PBOX, nucleophile, and electrophile can be envisioned. Preferred embodiments are dictated by the desired properties of the resultant coating. A preferred combination to produce a coating having a high T g value comprises PBOX, BTDA (electrophile) , and a phenolic resin (nucleophile) . A preferred combination to produce a coating showing good toughness and durability comprises PBOX, an epoxy resin (electrophile) and a phenolic resin (nucleophile) . To obtain a coating with excellent flexibility, one would advantageously combine PBOX, either of a novolac resin or a bisphenol-A endcapped epoxy resin, especially long-chain aliphatic polyepoxides (nucleophile) , and any of a number of solid epoxy resins (electrophile) . To obtain a powder coating which gels and cures at low temperatures, e.g.. about 190°C, a preferable combination would be PBOX, a difunctional phenolic resin such as Bisphenol A (nucleophile) and BTDA (electrophile) . A preferred combination for a coating with good hydrolytic stability comprises PBOX, a phenolic novolac resin (nucleophile) and an epoxy resin (electrophile) . Optional Components Typically, a flow control agent such as a Modaflow™ acrylate flow control agent material, available from Monsanto, is included in the final powder formulation. Examples of useful flow control agents which the coating composition may further comprise include but are not limited to those selected from the group consisting of acrylic polymers and/or copolymers such as polylauryl acrylate, polybutyl acrylate, poly(2-ethylhexyl)acrylate, poly(ethyl 2- ethylhexyl)acrylate, polylauryl methacrylate, polyisodecenyl methacrylate , and mixtures thereof. Flow control agents prevent cratering of the cured coating. Flow control agents, when used, are present in amounts of less than about 3 % by weight of the powder coating composition. Typically, flow control agents, if used, comprise from about 0.5% to about 2% percent by weight of the powder coating composition. Additional optional components which the powder coating composition may further comprise may be selected from the group consisting of reinforcing fillers, such as ground silica, talcs, clays, calcium carbonate, and the like; pigments; fumed silica; adhesion promoters or coupling agents well known in the art, such as silanes; and mixtures thereof. Method of Use A thermosetting powder coating comprising a mixture of 1,3-phenylenebisoxazoline, a reactive nucleophilic polymer-forming material and a reactive electrophilic polymer forming material is prepared and ground to a fine powder, then applied by conventional powder coating means to various substrates such as steel reinforcing rod for concrete structures (rebar) , petroleum pipelines or drills, or other materials in need of protection from heat, moisture and/or corrosive materials. The result is a smooth, pinhole-free, tough, corrosion-resistant, and yet flexible, protective coating. The three-part composition affords rapid cure without the need for added catalyst. Typically, the powder coating is prepared by dry grinding a mixture of PBOX and the other ingredients - nucleophile and electrophile as well as other additives as outlined above - to a mean grain size of about 44 microns. Optionally, the mixture of ingredients can be melt-blended, e.g., in a twin-screw extruder, quenched, then ground to a powder using, e.g., a hammer mill. The powder is sprayed onto a metal surface that has been heated to a temperature of about 180°C to about 230°C. Gel times for the coatings after application to the heated substrate from about 1 to about 15 seconds are preferred in order to maintain efficient production. Gel times can be controllably varied by varying the proportion of one or more of the essential components in the mixture. Storage Stability An exceptional feature of the resin powder coatings of the invention is their nonblocking, or storage stable, character. Blocking is defined as clumping or coalescing of the powder which renders it incapable of being effectively applied to the substrate which is to be coated. Blocking occurs in powder coatings when they begin to cure under the action of ambient moisture or by self-condensation. Many commercial products are not storage stable for useful lengths of time, such as months, even at moderate temperatures and humidities. Refrigerated storage is usually recommended. Often, however, powder coatings must be stored in situations where no refrigeration is available, and they subsequently have a rather limited shelf life. In contrast, the powder coatings of the invention exhibit excellent storage stability between about 15 * C and about 50°C, preferably between about 20 * C and about 45 * C, and most preferably between about 22"C and about 40"C, without refrigeration or other unusual precautions. The latent, higher temperature cure afforded by the use of PBOX as a curative accounts for the excellent storage stability of the powder coatings of the invention. While we have described powders that are stable up to at least 40 * C, that temperature is considered to be a minimum temperature requirement at which the powders are to be storage stable. Indeed, many of the powders described herein are stable at temperatures significantly in excess of 40 β C. Glossary The following trade names and abbreviations are used herein. BPA: Bisphenol A BTDA: Benzophenone 3,4,3' ,4'- tetracarboxylic acid dianhydride DSC/TGA: Differential Scanning Calorimetry / Thermal Gravimetric Analysis Durite™ SD-7280 Phenolic novolac resin available from Borden. EPON™2004 Solid DGEBA, from Shell Chemical Company PBOX: 1,3-Phenylenebis-2-oxazoline DGEBA: diglycidyl ether of Bisphenol A Physical Tests and Methods The test substrate is typically a 2.54 cm x 4.0 cm strip of 0.005 mil copper or steel. Gel Time: A standard surgical scalpel is dragged across the hot resin immediately after it is sprayed onto the test substrate, preheated to the specified temperature. The time at which the scalpel no longer makes a visible impression in the resin is measured and recorded, along with the temperature at which the test was performed. The method is also known as the "Scalpel Drag Test (SDT)". Flexibility: Flexibility of the coating is measured by observing the effect on the coating of bending the test substrate at a 90 * angle. Results are quantified as: 1 - Poor: significant delamination 2 - Fair: noticeable stress cracking with some delamination 3 - Good: minor cracking observed; no delamination 4 - Excellent: 90" bend exhibits no stress fractures Adherence: Adherence is measured by attempted scratching of the cured coating from the test substrate, using a standard surgical scalpel. Results are quantified as: 1 - Poor: delaminates in flakes when scratched 2 - Fair: significant delamination when scratched 3 - Good: slight delamination along leading edge when scratched 4 - Excellent: no delamination along leading edge when scratched. Differential Scanning Calorimetrv (DSC) : Industry-standard DSC equipment is used to determine the glass transition temperature (T g ) of a sample of the powder coating resin in powder form. Typically, the powder sample is heated to 300° C at a rate of 20° per minute, and the melting temperature is determined by the observed endotherm. A Peak Exotherm temperature is observed when the polymerization reaction reaches its maximum rate during the programmed DSC heating. The sample is cooled, then reheated to determine the glass transition temperature (T g ) . Results are obtained in graphic form as standard output from the test equipment. Thermal Gravimetric Analysis (TGA) TGA data is obtained using a Perkin-Elmer 7- Series Thermal Analysis System. A powder sample is heated at 20° C per minute and the temperatures at which 1% weight loss occurs and at which thermal degradation begins are recorded. Ring-and-Ball Softening Point The ring-and-ball softening point of a resin is determined according to ASTM test method E 28 - 67. The softening point is defined as the temperature at which a disk of the sample held within a horizontal ring is forced downward a distance of 2.54 cm under the weight of a steel ball as the sample is heated at a prescribed rate in a water bath or glycerin bath. In a typical .determination, a 25-50 g sample of resin is heated above its melting point and poured into a preheated brass ring (1.9 cm outside diameter x 1.6 cm inside diameter) until the ring is completely full. The resin-filled ring is allowed to cool until the sample solidifies, then is positioned 2.54 cm above a receiving plate in a stirred ethylene glycol bath which also contains an ASTM High Softening Point Thermometer. A 9.5 mm diameter steel ball weighing between 3.45 and 3.55 g is placed on the sample in the ring, and the ethylene glycol bath is heated at a rate of not more than 5 * C per minute. The softening point is determined as the temperature at which the sample touches the receiving plate. Storage Stability Storage stability of a powder coating resin is determined by placing a 5 g sample of the formulated powder, as described in Examples 1-5, in a sealed 8- dra vial in an air-circulating oven for two hours at 40'C in an upright position. The vial is removed and immediatly tipped to a horizontal position. To be considered "storage stable," or "nonblocking," the powder must freely flow within the vial with no evidence of lumps or of clumping together. Examples All parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight unless indicated otherwise. Example 1 PBOX / CATECHOL NOVOLAC / EPOXY RESIN A mixture of 12 parts 1,3-phenylenebisoxazoline (PBOX, Ashland Oil Co.), 24 parts catechol novolac resin (comprising approximately a 4:3 ratio of catechol to formaldehyde) and 64 parts Epon™2004 (Shell Chemicals) epoxy resin was ground into a fine powder and applied to a preheated copper strip at about 235'C. The coating gelled in 15 seconds then was cured at the application temperature for 1 minute, and finally removed from the heat source for cooling and testing. The coating exhibited a composite adherence and flexibility rating of 4. Analysis of the powder by DSC/TGA showed a glass transition temperature (T g ) of 102"C and a 1% weight loss at 205°C. Example 2 PBOX / PHENOLIC NOVOLAC / EPOXY A mixture of 25 parts PBOX, 25 parts Durite™SD- 7280 phenolic novolac resin (Borden Co.) and 50 parts Epon™2004 (Shell Chemicals) epoxy resin was ground into a fine powder and applied to a copper strip preheated to 235°C. The powder coating gelled in 20 seconds then was cured at the application temperature for 1 minute, and finally removed from the heat source for cooling and testing. On cooling, the coating exhibited a composite adherence and flexibility rating of 2.5. The powder showed a T g of 96"C and a 1% weight loss at 171°C by DSC/TGA. Example 3 PBOX / DIANHYDRIDE / EPOXY RESIN A mixture of 5 parts PBOX, 13 parts BTDA (3,3 ' ,4,4 '-benzophenone tetracarboxylic acid dianhydride, from Allco Chemical Co.) and 87 parts Epon™2004 (Shell Chemicals) epoxy resin was ground into a fine powder and applied to a copper strip preheated to 235°C. The coating gelled in 6 seconds then was cured at the application temperature for 1 minute, and finally removed from the heat source for cooling and testing. On cooling, the coating exhibited a composite adherence and flexibility rating of 2.5. DSC/TGA analysis of the powder showed T g s at 114 * C and 205'C and 1% weight loss at 263'C. Example 4 PBOX / PHENOLIC RESIN / DIANHYDRIDE A mixture of 42 parts BTDA and 29 parts PBOX was stirred and heated at 215 * C until all of the BTDA was dissolved, then cooled to room temperature. The solid mixture was mixed with 29 parts Bisphenol A and ground to a fine powder, then applied to a copper strip preheated to 190 * C. The coating gelled in 8 seconds then was cured at the application temperature for l minute, and finally removed from the heat source for cooling and testing. On cooling, the coating exhibited a composite adherence and flexibility rating of 3. -Analysis of the powder by DSC/TGA showed a T g of 136"C and a 1% weight loss at 224'C Example 5 PBOX / BISPHENOL A-TERMINATED EPOXY / DIANHYDRIDE A mixture of 26 parts BTDA, 37 parts PBOX and 37 parts DEH™-85 (aromatic hydroxy-functional phenolic- terminated epoxy resin containing an excess of Bisphenol A, from Dow Chemical) was ground to a fine powder and applied to a copper strip preheated to 205'C. The powder coating gelled in 10 seconds then was cured at the application temperature for 1 minute, and finally removed from the heat source for cooling and testing. On cooling, the coating exhibited a composite adherence and flexibility rating of 2. The powder showed a T g of 200 * C and a 1% weight loss at 178 * C by DSC/TGA. Reasonable modifications and variations are possible from the foregoing disclosure without departing from either the spirit or scope of the present invention as defined by the claims.

Claims: CLAIMS 1. A powder coating composition which is storage stable between about 15"C and about 50'C comprising: (a) about 4 to about 40 percent by weight of 1,3-phenylenebis-2-oxazoline; (b) optionally about 10 to about 70 percent by weight of a nucleophilic material selected from the group consisting of (i) phenolic novolac compounds selected from the group consisting of phenolic novolac resins, catechol novolac resins, cresol novolac resins, and mixtures thereof; (ii) bisphenol-terminated epoxy resins selected from the group consisting of aromatic hydroxy- functional phenolic-terminated epoxy resins and mixtures of such compounds having an average of more than one aromatic hydroxyl group per molecule; (iii) non-heat-reactive aromatic hydroxy-functional compounds selected from the group consisting of Bisphenol A and Bisphenol F and mixtures thereof; wherein, for the nucleophilic material, at least one of the following (1) and (2) is true: (1) the nucleophilic material has a ring-and-ball softening point above about 70'C; (2) the nucleophilic material has a crystalline melting point above about 40"C, and mixtures thereof; (c) optionally about 4 to about 70 percent by weight of an electrophilic material selected from the group consisting of benzophenone tetracarboxylic acid dianhydride, pyromellitic dianhydride, and mixtures thereof; and mixtures thereof; (d) optionally about 20 to about 80 percent by weight of an epoxy resin selected from the group consisting of solid aromatic epoxy resins, solid aliphatic and aromatic copolymeric epoxy resins, and mixtures thereof, wherein for the epoxy resin at least one of the following (i) and (ii) is true; (i) the epoxy resin has a ring-and-ball softening point above about 70 * C; (ii) the epoxy resin has a crystalline melting point above about 40 * C; wherein the composition comprises one of the following combinations of components: (a) (b) and (c) ; (a) (b) and (d) ; and (a) (c) and (d) ; and wherein the weight percentages are based upon the total weight of (a) plus any optional components selected from the group consisting of (b) , (c) , and (d) . 2. The powder coating composition of Claim 1 which further comprises about 0.5 to about 2 percent by weight of a flow control agent based upon the total weight of the powder coating composition, and which further comprises an additive selected from the group consisting of fillers, pigments, fumed silica, coupling agents, adhesion promoters, and mixtures thereof. 3. The powder coating composition of claim 1 which comprises l,3-phenylenebis-2-oxazoline, benzophenone tetracarboxylic acid dianhydride, and a non-heat reactive phenolic resin. 4. The powder coating composition of claim 1 which comprises l,3-phenylenebis-2-oxazoline; a nucleophilic material selected from the group consisting of bisphenol-A, novolac resin, bisphenol-A endcapped epoxy resin, and mixtures thereof; and an epoxy resin. 5. A coating comprising the reaction product of the powder coating composition of claim 1, wherein the coating is applied to a metallic substrate. 6. A protective coating comprising the reaction product of: (a) about 4 to about 40 percent by weight of l,3-phenylenebis-2-oxazoline; (b) optionally about 10 to about 70 percent by weight of a nucleophilic material selected from the group consisting of (i) phenolic novolac compounds selected from the group consisting of phenolic novolac resins, catechol novolac resins, cresol novolac resins, and mixtures thereof; (ii) bisphenol-terminated epoxy resins selected from the group consisting of aromatic hydroxy- functional phenolic-terminated epoxy resins and mixtures of such compounds having an average of more than one aromatic hydroxyl group per molecule; (iii) non-heat-reactive aromatic hydroxy-functional compounds selected from the group consisting of Bisphenol A and Bisphenol F and mixtures thereof; wherein, for the nucleophilic material, at least one of the following (1) and (2) is true: (1) the nucleophilic material has a ring-and-ball softening point above about 70°C; (2) the nucleophilic material has a crystalline melting point above about 40°C, and mixtures thereof; (c) optionally about 4 to about 70 percent by weight of an electrophilic material selected from the group consisting of benzophenone tetracarboxylie acid dianhydride, pyromellitic dianhydride, and mixtures thereof; and mixtures thereof; (d) optionally about 20 to about 80 percent by weight of an epoxy resin selected from the group consisting of solid aromatic epoxy resins, solid aliphatic and aromatic copolymeric epoxy resins, and mixtures thereof, wherein for the epoxy resin at least one of the following (i) and (ii) is true; (i) the epoxy resin has a ring-and-ball softening point above about 70 * C; (ii) the epoxy resin has a crystalline melting point above about 40 * C; wherein the composition comprises one of the following combinations of components: (a) (b) and (c) ; (a) (b) and (d) ; and (a) (c) and (d) ; and wherein the weight percentages are based upon the total weight of (a) plus any optional components selected from the group consisting of (b) , (c) , and (d) . 7. The protective coating of Claim 6 which further comprises about 0.5 to about 2 percent by weight of a flow control agent based upon the total weight of the powder coating composition, and which further comprises an additive selected from the group consisting of fillers, pigments, fumed silica, coupling agents, adhesion promoters, and mixtures thereof. 8. The protective coating of Claim 6 which comprises l,3-phenylenebis-2-oxazoline, benzophenone tetracarboxylic acid dianhydride, and a non-heat reactive phenolic resin. 9. The protective coating of Claim 6 which comprises l,3-phenylenebis-2-oxazoline; a nucleophilic material selected from the group consisting of bisphenol A, novolac resin, bisphenol-A endcapped epoxy resin, phenolic novolac resin, and mixtures thereof; and an epoxy resin. 10. A coating comprising the reaction product of the protective coating of claim 6, wherein the coating is applied to a metallic substrate.

Inventors: GRIGGS ALLEN L, MCKENZIE TAUN L, GRIGGS, ALLEN, L., MCKENZIE, TAUN, L.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1993
Main classification: C08G59
IPC codes: C08G59, C09D5