WO2001032790A1

WO2001032790A1 - Liquid coating compositions and coated substrates made therewith

Info

Publication number: WO2001032790A1
Application number: PCT/US2000/026517
Authority: WO
Inventors: Edward R. Millero; Truman F. Wilt; Robert A. Montague
Original assignee: Ppg Industries Ohio, Inc.
Priority date: 1999-11-02
Filing date: 2000-09-27
Publication date: 2001-05-10
Also published as: AU7834300A

Abstract

The present invention provides liquid coating compositions and substrates coated therewith. The liquid coating compositions include a film forming system. This film forming system includes a polymer component and a crosslinker component. The polymer component includes at least 70 weight percent of a polyester component. This weight percentage is based upon the total weight of the resulting coating's polymer component. This polyester component, in turn, includes at least 70 weight percent of a predominantly cyclolinear polyester compound. This weight percentage is based upon the total weight of the polyester component. When in their uncured state, the coating compositions of the present invention are extremely stable. On the other hand, when in their cured state, the coating compositions of the present invention produce flexible, durale, films which are stain and scratch resistant.

Description

LIQUID COATING COMPOSITIONS AND COATED SUBSTRATES MADE THEREWITH

Field of the Invention

This invention pertains to liquid coating compositions. While these liquid coating compositions have many uses, those skilled in the art ,after reading this specification, will appreciate that they are especially useful for application onto coil metal stock. Particularly, the invention pertains to stable, liquid coil coating compositions which form durable, high gloss, flexible films that are stain and scratch resistant.

Background of the Invention

It is well known in the coating industry that coil coating processes are cost effective and efficient methods of applying liquid coating compositions onto a number of different substrates, particularly coiled metal stock. As such, coil coating processes are often used for applying finishes to metal stock used to produce panels employed in the fabrication of metal cabinetry, major appliances (e.g., washers, dryers, refrigerators, freezers, etc.), and building panels.

Since coil-coated metal stock is often cut into blanks and, thereafter, post- formed into a desired shape, the liquid coating compositions used in these processes require many specific properties, both in their cured and uncured states. For example, when in their uncured state, the liquid coating compositions have to be stable. As used herein, the term "stable,^'' as it applies to a liquid coating composition, means that the liquid coating has a resin system which resists crystallization until subjected to a particular curing process. In addition to the above, when in their uncured state, stable liquid coating compositions should also have the ability to be easily applied by conventional coil coating processes. On the other hand, when in their cured state, the film resulting from liquid coating compositions should have sufficient flexibility which enables it to withstand conventional cutting, stamping and bending processes to which such coated metal stock is often subjected. Moreover, since the finished product often times needs to be scratch-resistant, the cured coating compositions must also be hard. Furthermore, since appearance is often important, the resulting finish often needs to be glossy and durable. As used herein, the term "durable" refers to the cured film's ability to retain a majority of its original gloss after being exposed to ultra violet light over an extended period of time. Finally, since coated coil metal stock is often used for the fabrication of many consumer goods (e.g., metal cabinets, major appliances, buildings, etc.), it is often very desirable that the resulting cured film possess a high level of stain resistance.

If liquid coating compositions have all of the aforementioned cured and uncured features, the coating would be extremely desirable to those in a number of different industries. However, notwithstanding the obvious desirability of having such a coating, skilled artisans know that the achievement of certain properties comes at the expense of others. Therefore, the coating industry continually strives to develop liquid coating compositions that possess most, if not all, of the aforementioned properties. Liquid coatings based on polyester resins have been well known in the art for many years. Such polymers are typically prepared from a blend of cyclic and acyclic (or open-chain) monomers in order to produce the desired properties in the subsequently cured films. This has been the historical strategy to obtain a good balance of hardness, flexibility, and chemical or stain resistance. Typical examples are shown in U.S. Patent Nos. 4,205,115; 4,238,583; 4,861,672; 5,262,494; 5,376,460 and in Japanese Kokai Patent No. Sho 61[1986]-236865. Likewise, polyesters which are encompassed by the term cyclolinear are also known in the art. For example, liquid crystalline polyesters are discussed by Polk et.al. ,Macromolecules 1981, 14 (6), 1626 and in Macromolecules 1984, 17 (2), 129. In addition, these polymers are often either semicrystalline solids or glassy solid materials that are useful in the formulation of shaped plastics, powders, fibers, sheet films, or adhesives. British patents 1,073,162 and 1,073,163 disclose copolyesters of this type as the basis for melt-spun fibers. U.S. Patent Nos. 4,554,343 and 4,578,453 disclose high molecular weight, high glass transition (> 50°C) cyclolinear polyesters useful for forming shaped articles. U.S. Patent No. 5,656,715 discloses a method of melt processing a solid copolyester with a high content of a cyclic diol. Moreover, WO 98/23661 gives some examples of acid-functional and epoxy-functional cyclolinear polyesters that are useful as solid powder coatings.

Summary of the Invention

Accordingly, one object of this invention is to provide liquid coating compositions which, when cured, form flexible, high gloss, durable films which exhibit excellent stain and scratch resistant properties.

Another object of this invention is to provide coated coil metal stock, wherein the coil coating composition applied thereover imparts at least the following film properties: stain and scratch resistance, flexibility, durability and a high gloss.

Still another object of this invention is to provide liquid coating compositions which, when uncured, resist crystallization until subjected to a particular curing process.

These and other objects, which will become apparent to those skilled in the art after reading the specification and appended claims, are achieved through the formulation of novel liquid coating compositions. The novel liquid coating compositions of the present invention include, among other things, a novel film forming system. The novel film forming system, in turn, includes a polymer component and a crosslinker component.

The novel film forming system's polymer component used when practicing this invention includes at least 70 weight percent of a polyester component. This weight percentage is based upon the total weight of the resulting coating's polymer component. This polyester component, in turn, includes at least 70 weight percent of a predominantly cyclolinear polyester compound. This weight percentage is based upon the total weight of the polyester component. As used herein, the term "predominantly cyclolinear polyester compound" refers to a polyester compound wherein at least 70 percent of its constituent monomers are cyclic.

When in their uncured state, the liquid coating compositions of the present invention are extremely stable. On the other hand, when in their cured state, the liquid coating compositions of the present invention produce high gloss films which not only are flexible and durable, but also are stain and scratch resistant.

Detailed Description of the Invention

The coating compositions of the present invention include, among other things, a novel film forming system. This novel film forming system includes, in turn, a polymer component and a crosslinker component. The novel film forming system can further include an optional catalyst component. If present, the catalyst component should at least aid in the initiation and/or promotion of the reaction between the novel film forming system's polymer and crosslinker components.

The novel film forming system's polymer component includes at least 70 weight percent of a polyester component. Preferably, the polymer component includes at least 80 weight percent of a polyester component; and more preferably, at least about 90 weight percent of a polyester component. These weight percentages are based upon the total weight of the film forming system's polymer component. The polymer component's polyester component, in turn, includes at least 70 weight percent of a predominantly cyclolinear polyester compound. Preferably, the polyester component includes at least 80 weight percent of a predominantly cyclolinear polyester compound; and more preferably, at least about 90 weight percent of a predominantly cyclolinear polyester compound. These weight percentages are based upon the total weight of the polymer component's polyester component.

The predominantly cyclolinear polyester compound includes a polymer wherein at least 70 percent of its constituent monomers are cyclic. Preferably, at least 80 percent of the cyclolinear polyester compound's constituent monomers are cyclic; and more preferably, at least 90 percent are cyclic. These percentages are based on the total number of constituent monomers in the polymer(s) making up the predominantly cyclolinear polyester compound.

Generally, the predominantly cyclolinear polyester compounds used when practicing this invention have unsubstituted or substituted alkylene and/or alkenylene groups that alternate with carboxylic acid ester groups. Preferred groups of such polyesters include those represented by the structures:

H-(- 0-R^;-0-C-R²-C -)„- O - R'- OH

O O or

H-<- 0-R^!-0-C-R²-C -)„- O - R³- OH

O O or

H-<- 0-R¹-0-C-R2-C -)„—(- O-R^l-O-C-R -C-O-Rl -)_m- OH O O O O In the above structures, R¹, R² and R³ denote divalent cycloaliphatic, cycloaliphatic- aliphatic, aromatic or heterocyclic-aliphatic moieties; and, n denotes a number from 1 to 30; and preferably, from 4 to 20. Moreover, in the above structures, at least one of R¹, R² and R³ contain a carbocyclic or heterocyclic ring or a ring system of this type, and at least one of R¹, R² or R³ is cycloaliphatic. If the desired end use of the liquid coating disclosed herein requires the resulting film to have not only excellent durability, but also excellent stain resistance, it is preferred that R¹ or R² includes an aromatic ring. Upon reading this specification, those skilled in the art would know of other suitable polyesters which can be employed.

The predominantly cyclolinear polyester compounds used when practicing this invention can be obtained by any suitable means known to those skilled in the art. In one preferred means, the polyesters are obtained by reacting a cyclic diacid or an acid anhydride component with a cyclic polyol component.

In embodiments of this invention where the predominantly cyclolinear polyester compound includes the reaction product of a cyclic polyol component and a cyclic diacid or an acid anhydride component, any suitable cyclic polyol can be used, provided that its use results in the formation of a predominantly cyclolinear polyester compound as defined above. Some examples of such suitable cyclic polyols which can be employed include: 1,2-benzenedimethanol; 1,3-benzenedimethanol; 1,4- benzeπedimethanol; isomers of cyclohexanedimethanol; and hydrogenated 4,4- isopropylidenediphenol (also known as hydrogenated Bisphenol A). In certain embodiments of this invention, one preferred example of such a suitable cyclic polyol comprises cyclohexanedimethanol (CHDM).

If the predominantly cyclolinear polyester compound used when practicing this invention includes the reaction product of a cyclic polyol component and a cyclic diacid or acid anhydride component, the amount of the cyclic polyol component employed is typically at least about 10 weight percent. More typically, the amount of the cyclic polyol component employed is at least about 20 weight percent; and even more typically, at least about 30 weight percent. On the other hand, in these embodiments of the invention, the amount of the cyclic polyol component employed is typically not greater than about 70 weight percent. More typically, the amount of the cyclic polyol component employed is not greater than about 60 weight percent; and even more typically, not greater than about 50 weight percent. All of the aforementioned weight percentages are based upon the total weight of the polymer component's polyester component.

The cyclic diacid or acid anhydride component that can be reacted with a cyclic polyol component to form the predominantly cyclolinear polyester compound employed when practicing this invention can include any suitable cyclic diacid or acid anhydride. Some examples of such suitable cyclic diacids or acid anhydrides include: phthalic anhydride; tetrahydrophthalic anhydride; hexahydrophthalic anhydride; 1,3- cyclohexanedicarboxylic acid; 1 ,4-cyclohexanedicarboxylic acid; isophthalic acid; terephthalic acid; and methylhexahydrophthahc acid. In certain embodiments of this invention, one preferred example of such a suitable diacid or anhydride is phthalic acid or anhydride.

If the predominantly cyclolinear polyester compound used when practicing this invention includes the reaction product of a cyclic polyol component and a cyclic diacid or acid anhydride component, the amount of the cyclic diacid or acid anliydride component employed is typically at least about 20 weight percent. More typically, the amount of the cyclic diacid or acid anhydride component employed is at least about 20 weight percent; and even more typically, at least about 40 weight percent. On the other hand, the amount of the cyclic diacid or acid anhydride component employed is typically not greater than about 70 weight percent. More typically, the amount of the cyclic diacid or acid anhydride component employed is not greater than about 60 weight percent; and even more typically, not greater than about 55 weight percent. All of the aforementioned weight percentages are based upon the total weight of the polymer component's polyester component. In addition to the above, the novel film forming system's polymer component can also include at least one of the following optional components: additional monomers such as neopentyl glycol, 1,6-hexanediol, 2-methyl propanediol, ethylene glycol, propylene glycol, diethylene glycol, butanediol, trimethyl pentanediol dipropylene glycol 2,2-dimethyl-3-hydroxypropyl-2,2,-dimethyl-3- hydroxypropionate, trimethylol propane, trimethylol ethane and other additives known to those skilled in the art. It should be noted, however, that the use of optional components in the film forming system's polymer component can affect the properties of the coating and/or any film resulting therefrom. Therefore, the type and/or amount of optional components employed, if any, depend in part on the coating's desired end use.

However, if any optional components are employed as part of the novel film forming system's polymer component, the total amount of these optional components should not exceed about 30 weight percent. Preferably, the amount of the optional component employed should not exceed about 20 weight percent; and more preferably, not more than about 10 weight percent. These weight percentages are based upon the total weight of the novel film forming system's polymer component.

In addition to a polymer component, the novel film forming system used when practicing this invention further includes a crosslinker component. The crosslinking component must include at least one curing agent which is capable of reacting with the active hydrogens in the polymer component's polyester component.

The crosslinker component's at least one curing agent can be any suitable curing agent(s) known to those skilled in the art. Examples of such suitable curing agents include: aminoplasts and isocyanates, including blocked isocyanates. Typically, the crosslinker component^'s at least one curing agent includes an aminoplast curing agent. Examples of suitable aminoplast curing agents which can be employed include: benzoguanamine-formaldehyde resins and melamine-formaldehyde resins. These types of aminoplast curing agents can be obtained from the reaction of formaldehyde with an amine or an amide. The most common and preferred amines or amides are melamine, urea or benzoguanamine. However, condensates with other amines and amides can be employed, for example, aldehyde condensates or diazines, triazoles, guanidines, guanamines and alkyl and aryl disubstituted derivatives of such compounds including alkyl and aryl-substituted ureas and alkyl and aryl-substituted melamines and benzo guanamines. Some examples of such compounds are N,N- dimethylurea, N-phenylurea, dicyandiamide, formoguanamine, acetoguanamine, 6- methyl-2,4-diamino-l,3,5-triazine, 3,5-diaminotriazole, triaminopyrimidine, 2,6- triethyltriamine-l,3,5-triazine and the like. While the aldehyde employed is most often formaldehyde, other aldehydes such as acetaldehyde, crotonaldehyde, benzaldehyde and furfural may be used.

In one preferred embodiment wherein the crosslinker component's at least one curing agent includes an aminoplast curing agent, the aminoplast curing agent comprises at least one melamine-formaldehyde resin. One specific example of a particularly useful curing agent is the melamine-formaldehyde resin commercially available from Cytec Industries, Inc. under the trade name of CYMEL™ 303 curing agent.

The crosslinker component's at least one curing agent can also include at least one isocyanate curing agent. Examples of suitable isocyanates curing agents include: polyisocyanates such as toluene diisocyanate and 4,4'-methylene-bis-(cyclohexyl isocyanate), isophorone diisocyanate and NCO-prepolymers such as the reaction products of monomeric diisocyanates such as those mentioned above with polyester or polyether polyols. If employed, particularly useful polyisocyanates include at least one of the following polyisocyanates: the isocyanurates from isophorone isocyanate or 1 ,6 hexamethylene diisocyanate and the biuret from 1 ,6-hexamethylene diisocyanate. If employed, the polyisocyanate may optionally be blocked. Examples of suitable blocking agents are those materials which would unblock at elevated temperatures such as diols, low aliphatic alcohols (e.g., methanol), oximes (e.g., methyl ethyl ketone oxime), lactams (e.g., caprolactam) and combinations thereof. Blocked isocyanates can be used to form stable one-package systems. On the other hand, polyfunctional isocyanates with free isocyanate groups can be used to form two-package room temperature curable systems. In these systems, the novel film forming system's polymer and crosslinker components are mixed just prior to their application onto the desired substrate. As stated above, it is also within the scope of this invention for the novel film forming system to further include an optional catalyst component. If present, the catalyst component is typically used to increase the reaction rate between the film forming system's polymer and crosslinker components.

When and if employed, a suitable catalyst component is one which is typically added to the film forming system at levels ranging from about 0.001% to about 15%. Typically, the amount of the catalyst component employed ranges from about 0.01% to about 10%; and more typically, from about 0.1% to about 5%. These percentages are based upon the total weight of the novel film forming system's polymer component. Any suitable catalyst(s) which can catalyze the reaction between the film forming system's polymer and crosslinker components can be employed when practicing this invention. Typically, the catalyst employed depends, in part, upon the type of crosslinker employed (e.g., an aminoplast-based curing agent or an isocyanate- based curing agent). Accordingly, examples of suitable catalysts for use with isocyanate-based curing agents include: dibutyl tin dilaurate, zinc compounds such as zinc acetate, and/or amines such as triethyl amine, imidazoles, cyclic amidine, alkyl/aryl ammonium halides, and zinc alkyl/aryl thiocarbamates. However, if employed, this type of catalyst should preferably include at least one metal salt or complex (e.g., dibutyltin dilaurate, acetyl acetonate), at least one quaternary compound (e.g., quaternary ammonium or phosphonium compounds), and/or the like.

On the other hand, catalysts which can be used with aminoplast-based curing agents are typically acid catalysts and metal salt catalysts. Specifically, a suitable catalyst will have a pk_a value of about 4 or less, preferably of about 3 or less. The acid catalyst for this type of a reaction may be an acid or a derivative which will generate the acid in situ during curing.

Accordingly, examples of suitable catalysts for use with aminoplast-based curing agents include all of the acids generally used for curing aminoplast/polyester compositions such as: sulfonic acids, mineral acids, and carboxylic acids. Moreover, suitable metal salts useable as catalysts for such reactions include: magnesium bromide, aluminum nitrate, and zinc nitrate.

In addition to a polymer component, a crosslinker component and an optional catalyst component, the novel film forming systems employed when practicing this invention can further include other additives known to those skilled in the art. Examples of such other additives include: adhesion promoters, waxes, slip and mar additives, plasticizers, anti-oxidants, UN. light absorbers and stabilizers, flow control agents, surfactants and other formulating additives. It is important to note, however, that the addition of such additives will probably affect the cured and/or uncured state of the resulting liquid coating composition. Accordingly, the types and amounl of such additives employed, if any, depend in part upon the desired use of the liquid coating composition.

When practicing this invention, the polymer component is typically present in the novel film forming system in an amount of at least about 50 weight percent.

Preferably, the polymer component is present in an amount of at least about 55 weight percent; and more preferably, of at least about 60 weight percent. On the other hand, when practicing this invention, the polymer component is typically present in the novel film forming system in an amount of not greater than about 99 weight percent. Preferably, the polymer component is present in an amount of not greater than about 97 weight percent; and more preferably, of not greater than about 95 weight percent. These weight percentages are based upon the total weight of the liquid coating composition's novel film forming system.

Also when practicing this invention, the crosslinker component is typically present in the novel film forming system in an amount of at least about 1 weight percent. Preferably, the crosslinker component is present in an amount of at least about 3 weight percent; and more preferably, of at least about 5 weight percent. On the other hand, when practicing this invention, the crosslinker component is typically present in the novel film forming system in an amount of not greater than about 45 weight percent. Preferably, the crosslinker component is present in an amount of not greater than about 35 weight percent; and more preferably, of not greater than about 25 weight percent. These weight percentages are based upon the total weight of the liquid coating composition's novel film forming system.

Another way of viewing the above is that the amounts of polymer component and crosslinking component employed when practicing this invention are typically adjusted so that the weight ratio of the polymer component's polyester component to the crosslinking component is within the range of about 6:1 to 0.5:1. Preferably, the amounts of polymer component's polyester component and crosslinking component employed when practicing this invention are typically adjusted so that the weight ratio of polyester component to the crosslinking component is within the range of about 3: 1 to 1.5:1; and more preferably, within the range of about 3:1 to 1.8:1. Ratios of polymer component's polyester component to the crosslinking component greater than 6:1 are not typically preferred because the resulting film's hardness and durability properties tend to suffer. On the other hand, ratios of polymer component's polyester component to the crosslinking component less than 0.5:1 are also not typically preferred because the resulting film's flexibility property tends to suffer. However, if these particular properties are not desired for the particular end use of the product, then these weight ratio limits can be exceeded.

In addition to the novel film forming systems described above, the liquid coating compositions encompassed by the present invention can also include a pigment system. If present, such a system can be used to impart a color to and/or affect the gloss of the resulting cured film. As used herein, the terms "pigment system" and "pigment" refer not only to color-producing pigments, but also to gloss- affecting agents, the latter of which may or may not alter the color of the cured film, and/or fillers. Accordingly, it is within the scope of this invention for the liquid coating compositions to produce either pigmented or clear films.

It should be noted that, it is also within the scope of this invention for the pigment system, if present, to include compositions which produce more than one of the aforementioned results. Moreover, it is within the scope of this invention for the pigment system, if present, to include only one of the aforementioned types of compositions, or any combination thereof.

If the pigment system is designed to alter the gloss of the finish among other things, any suitable gloss altering pigments can be employed. Examples of such suitable gloss altering pigments include: silicas, barytes, calcium carbonate, talcs, magnesium silicate and/or aluminum silicate. If the desired result is to lower the gloss of the resulting finish, the pigment component typically includes at least one silica.

If the pigment system is designed to alter the color of the finish among other things, any suitable color altering pigments can be employed. Examples of such suitable color altering pigments include: iron oxides, lead oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, color pigments such as cadmium yellow, cadmium red, chromium yellow, and metallic pigments such as aluminum flake. If the pigment system is designed to be used as a filler among other things, any suitable filler(s) can be employed. Examples of such suitable fillers include: silicas, barytes, calcium carbonate, talcs, magnesium silicate and/or aluminum silicate.

If present, the pigment system is usually expressed as a weight ratio of the weight of the pigment system to the weight of the film forming resin's polymer component. In the practice of this invention, when the liquid coating composition includes a pigment system, this weight ratio can be as high as 2: 1. However, for most liquid coating compositions made in accordance with the present invention which include an optional pigment system, this ratio is typically in the range of about 0.01 : 1 to about 1.5:1 ; and more typically, in the range of about 0.05 : 1 to about 1 :1.

The concentration of the film forming system in a liquid coating encompassed by this invention will vary depending on whether or not the liquid coating includes a pigment system. Typically, if a pigment system is not employed, the film forming system is typically present in an amount of at least about 70 weight percent; more typically, at least about 80 weight percent; and even more typically, at least about 90 weight percent. On the other hand, if a pigment system is employed, the film forming system is typically present in an amount of at least about 40 weight percent; more typically, at least about 50 weight percent; and even more typically, at least about 60 weight percent. The aforementioned weight percentages are based on the total weight of the liquid coating composition.

The liquid coating compositions of the present invention typically have a solids concentration which is greater than 50 weight percent. In many instances, these liquid coating compositions have a solids concentration which ranges from about 40 to about 95 weight percent. Preferably, the solids concentration in these liquid coating compositions ranges from about 50 to about 85 weight percent; and more preferably, from about 60 to about 75 weight percent. All of the aforementioned weight percentages are based upon the total weight of the liquid coating composition. Although the liquid coating compositions of the present invention can be formulated to be of the higher solids type, a diluent system can be present. As used herein, the term "diluent system" refers to solvents or non-solvents which can be removed after the liquid coating is applied to the substrate and can reduce viscosity sufficiently to enable forces available in simple coating techniques to spread the liquid coating to controllable, desired and uniform thicknesses. If employed, the diluent system can include any suitable diluent compound(s). Examples of suitable diluent system include at least one of the following compounds: aromatic hydrocarbons (e.g., toluene and xylene), ketones (e.g., methyl ethyl ketone and methyl isobutyl ketone), alcohols (e.g., isopropyl alcohol, normal-butyl alcohol), monoethers of glycols (e.g., the monoethers of ethylene glycol and diethylene glycol), monoether glycol acetates (e.g., 2-ethoxyethyl acetate), other esters, as well as compatible mixtures thereof.

If present in the liquid coatings of this invention, the diluent system is typically employed in an amount not greater than about 60 weight percent. More typically, if present in these liquid coatings, the diluent system is employed in an amount not greater than about 55 weight percent; and even more typically, in an amount not greater than about 50 weight percent. On the other hand, if present in the liquid coatings of this invention, the diluent system is typically employed in an amount not less than about 10 weight percent. More typically, if present in these liquid coatings, the diluent system is employed in an amount not less than about 15 weight percent; and even more typically, in an amount not less than about 20 weight percent. These weight percent are based on the total weight of the liquid coating composition.

In addition to the novel film forming system, optional pigment system, and optional diluent system, the liquid coatings of the present invention can further include other additives known to those skilled in the art. Examples of such additives include: adhesion promoters, waxes, slip and mar additives, plasticizers, anti- oxidants, UN. light absorbers and stabilizers, flow control agents, surfactants and other formulating additives. The use of these additives depends largely upon the desired end use of the resulting product.

If employed, these additives can be present in a total amount which is typically not greater than about 20 weight percent of the liquid coating composition. More typically, if employed, the total weight of such additives is not greater than about 15 weight percent; and even more typically, not greater than about 10 weight percent. These weight percentages are based on total solids in the resulting liquid coating composition. It is important to note, however, that the addition of such additives will probably affect the cured and/or uncured state of the resulting liquid coating composition. Accordingly, the types and amount of such additives employed, if any, depend in part upon the desired use of the liquid coating composition.

When preparing the liquid coatings of the present invention, the aforementioned components can be combined in any suitable manner known to those skilled in the art. For example, for liquid coating compositions which contain a pigment system, one possible means of formulating such a composition includes the preparation of the polyester component. Thereafter, mixing the pigment system with the polyester component in a weight ratio of ranging from about 0.5:1 to 1 :0.5; preferably, in a weight ratio ranging from about 0.7:1 to 1 :0.7; and more preferably, from about 0.9: 1 to 1 :0.9. Thereafter, the crosslinking component can be added to this mixture. Also a catalyst component, as well as other additives, can also be added to this mixture either before, during or after this point in time.

The liquid coating compositions of this invention can be applied to any suitable substrate. Examples of such substrates include: metal, paper, leather, cloth, plastics, and the like. Although the coating compositions prepared in accordance with this invention can be applied over any of the aforementioned substrates by any suitable means known to those skilled in the art, their properties make them especially useful for application over sheet metal stock by spraying, direct roll-coating, reverse roll-coating, electrodeposition, flow coating, and the like. In fact, the liquid coating compositions of the present invention are especially useful for coating coil metal stock such as aluminum, steel, tin plated steel, electrogalvanized steel, zinc-aluminum coated steel, zinc-nickel coated steel, and hot dipped galvanized steel. If applied over such coil metal stock, the metal substrates are usually cleaned and chemically treated to improve the adhesion of subsequent coating compositions prepared in accordance with the present invention.

Coating thickness will vary depending upon the application desired. Typically, the coatings of the present invention will be applied over substrates at coating thicknesses ranging from about 0.01 to about 40 mils; more typically, from about 0.05 to 30 mils; and even more typically, from about 0.1 to about 20 mils.

After application of the liquid coating composition onto the desired substrate, the coatings are cured. Curing is usually conducted by subjecting the coated substrate to elevated temperatures. Typically, the peak metal temperature for typical curing processes of the coatings encompassed by this invention ranges from about 50°C. to about 300°C; more typically, from about 100°C. to about 275°C; and even more typically, from about 150°C. to about 255°C. At these temperatures, the duration of the curing process is typically a period of time ranging from about 15 seconds to about 120 minutes; and more typically, a period of time ranging from about 20 seconds to about 90 minutes. Higher or lower temperatures with correspondingly shorten or lengthen curing times. The preferred curing process depends upon the nature of the substrate as well as the particular components used in formulating the coating compositions.

Once cured, and absent the use of any gloss-reducing agents or curing techniques, the coating compositions of the present invention typically produce films which have a very high gloss. Specifically, the cured coating compositions prepared in accordance with this invention can typically produce films which have a 60° gloss ranging from about 80%> to about 100% light reflectance; and more typically, ranging from about 90% to about 100% light reflectance. Unless otherwise stated, a film's 60° gloss recited herein is that determined by a process similar to that described in ASTM D 523-78. As stated above, the gloss level of the cured coating compositions encompassed by the present invention can be decreased by the addition of gloss- reducing pigments and/or by the use of gloss-reducing curing techniques for those instances where a high gloss film is not desired.

Moreover, in addition to having a very high gloss potential, the films resulting from cured liquid coating compositions encompassed by this invention have excellent stain resistant properties. Moreover, considering the high ring content of the polyester component, it was unexpected to discover that the films resulting form cured liquid coating compositions encompassed by this invention also have excellent flexibility.

The combination of outstanding properties of the coating compositions of the present invention make them useful for a wide variety of applications. For example, they are especially useful for coating coil metal stock used in the manufacture of automotive trim parts, architectural panels, metal office furniture, vending machines and appliances such as refrigerators, dishwashers, washing machines and dryers.

EXAMPLES

The following examples illustrate the invention and should not be construed at a limitation on the scope thereof. Example 1 illustrates the preparation of polyester resins used in the preparation of coating compositions and pigmented tint pastes.

Example 2 illustrates the preparation of a pigmented tint paste used in the preparation of coating compositions.

Example 3 illustrates the preparation of a base coat coating compositions overwhich is applied a clear, high gloss top coat coating composition.

Example 4 illustrates the preparation of pigmented and clear top coat coating compositions used to demonstrate the novelty of the present invention. Example 5 illustrates physical properties of multilayered film systems applied over a substrate, wherein the film systems incorporate the top coat coating compositions prepared in Example 4.

Example 6 illustrates the durability of multilayered film systems applied over a substrate, wherein the film systems incorporate the top coat coating compositions prepared in accordance with the present invention.

TESTING PARAMETERS USED IN THE EXAMPLES:

Percentages: Unless specifically indicated otherwise, all percentages and amounts are understood to be by weight.

Pencil Hardness: Unless specifically indicated otherwise, a cured film's pencil hardness was determined in accordance with ASTM D3363 -92a. For coatings used in the fabrication of metal cabinetry for metal appliances and office furniture from coil metal stock, the industry typically requires the resulting film to have a pencil hardness in the range from about H to about 3H, the latter of which is often preferred.

T-Bends: Unless specifically indicated otherwise, a cured film's flexibility was determined in accordance with ASTM D3794-94. For coatings used in the fabrication of metal cabinetry for metal appliances and office furniture from coil metal stock, the industry typically requires the resulting film to have a T-bend flexibility in the range from about 2T no pick to about 0T no pick, the latter of which is often preferred.

Stain Resistance: Unless specifically indicated otherwise, a cured film's stain resistance was determined by evaluating its resistance to staining by commercially- available yellow mustard (hereinafter a "Mustard Stain Test"). The Mustard Stain Test used herein was performed using the following procedure: FRENCH'S^® brand yellow mustard was applied onto a coated substrate and covered with a flat piece of glass. After 24 hours, the glass was removed; and, the mustard was wiped off. Thereafter, the site was washed with water. Within 15 minutes after cleaning, the color difference between the unstained and the stained area of the panel, if any (hereinafter referred to as "Delta E"), was measured using a MacBeth ColorEye™, Model No. 2025+. Theoretically, there is no staining when Delta Ε equals 0.0. However, when Delta Ε is less than 1.0, staining is not visible to the naked eye. Accordingly, a stain-resistant coating is one having a Delta Ε which is not greater than about 10.0; preferably, not greater than about 8.0; more preferably, not greater than 5.0; and even more preferably, not greater than about 3.0.

EXAMPLE 1

This Example illustrates the preparation of polyester resins used in the preparation of liquid coating compositions. The first three polyester resins, hereinafter referred to as POLYESTER 1 through POLYESTER 3 were all prepared using essentially the same procedure. The only significant difference was the charge of reactants.

The procedure used to formulate POLYESTERS 1 - 3 was as follows: The charge of reactants was added into a three-liter, four-necked round bottom flask equipped with a motor driven stainless steel paddle agitator, a thermocouple to record batch temperature, a Dean Stark water trap connected with a condenser to collect distillate evolved, and an inert gas sparge tube. Heat was applied to the flask and the temperature was gradually increased to 250 °C. and held at that temperature until the reaction mixture reached an acid value of between 10 to 20.

The table set out below shows the charge of reactants used to prepare POLYESTERS 1 - 3.

Next, a fourth polyester was prepared, hereinafter referred to as POLYESTER 4. POLYESTER 4 was prepared by mixing together the following monomers: neopentyl glycol (15.7 mole percent), propylene glycol (4.4 mole percent), trimethylolpropane (1.2 mole percent), dodecanedioic acid (12.7 mole percent), phthalic anhydride (23.7 mole percent), isophthalic acid (26.6 mole percent), 2-methylpropane diol (15.7 mole percent), butyl stannoic acid (0.1-0.2 mole percent), and triphenyl phosphite (0.1-0.2 mole percent). This mixture was esterified under a nitrogen atmosphere over a period of 8 to 15 hours at a temperature of 180° to 240°C. When the acid value of the mixture dropped to around 25, a small amount of Aromatic 100 solvent (i.e., an aromatic hydrocarbon solvent blend commercially available from Exxon) was incorporated for azeotropic distillation of the remaining water that evolved as a condensate by-product.

EXAMPLE 2

This Example illustrates the preparation of a pigmented tint paste used in the preparation of certain coating compositions. In this example a white tint paste was prepared, hereinafter referred to as WHITE PASTE by adding the materials set out in the following table into a 1 -quart stainless steel beaker, blending the materials to form a mixture, and dispersing the mixture until a Hegman reading of 7+ was obtained. MATERIALS AMOUNT - (grams)

POLYESTER 4 313.6

2-Butoxethanol 58.4

Aromatic solvent blend¹ 198.5

Pigment wetting additive² 5.8

Titanium dioxide³ 967.8

1 This solvent is commercially available from Exxon under the tradename of SOLVESSO™ 150.

2 This pigment wetting additive is commercially available from Avecia

Colors under the tradename of SOLVSPERSE™ 26000.

3 This pigment is commercially available from Du Pont under the tradename of TIPURE™ R-706

EXAMPLE 3

This Example illustrates the preparation of a base coat coating compositions overwhich will be applied a clear, high gloss top coat coating compositions. In this example a base coat coating composition was prepared, hereinafter referred to as BASE COAT, by mixing together the compositions set out in the following table:

MATERIALS AMOUNTS - (grams)

POLYESTER 4 100.8 WHITE PASTE 155.1

Methylated, ethylated benzoguanamine¹ 7.7

Methylated melamine aminoplast² 7.5

Epoxy resin³ 3.3

Polyethylene slip additive⁴ 0.4

Acrylic flow modifier⁵ 0.9

Silicone flow additive⁶ 0.6

Amine blocked p-toluene sulfonic acid⁷ 1.4

Dibasic ester solvent 16.8

Ethanol 1.4

1 The methylated, ethylated benzoguanamine resin is commercially available from Cytec under the tradename CYMEL™ 1123.

2 The methylated melamine aminoplast is commercially available from

Cytec under the tradename CYMEL™ 303.

3 The epoxy resin is commercially available from Shell under the tradename EPON™ 1001.

⁴ The polyethylene slip additive is commercially available from Shamrock under the tradename VERSAFLOW™ BASE.

5 The acrylic flow modifier is commercially available from DuPont under the tradename RK-5345ι

6 The silicone flow additive is commercially available from OSI under the tradename SILWET™ L7500.

1 The amine blocked p-toluene sulfonic acid is a catalyst commercially available from King Industries under the tradename NACURE™ 2500.

EXAMPLE 4

This Example illustrates the preparation of top coat coating compositions used to demonstrate the novelty of the present invention. These top coat coatings, hereinafter referred to as TOPCOAT 1 through TOPCOAT 7 were all prepared using essentially the same procedure. The only significant difference was the charge of materials.

In this example, the top coats were prepared by mixing together the compositions set out in TABLE 1 set out below by using the following procedure. First, all of the materials are added to a 1 -quart container. Thereafter, the materials are mixed under low agitation with a high lift blade until a homogeneous blend was formed.

TABLE 1

' POLY BD600 is a liquid hvdow l-terminated epoxidized homopolymer of butadiene that is commercially available from Elf Atochem

EXAMPLE 5

This Example illustrates the properties of TOPCOAT 1 through TOPCOAT 7. In this Example, TOPCOATS 1 - 6 are prepared in accordance with the present invention; and TOPCOAT 7 is provided as a control. The properties of

TOPCOATS 1-7 are determined after being applied to a coated and treated steel substrate.

TOPCOATS 1-4, 6 and 7 are pigmented coatings. They were each applied to separate steel panels in essentially the same manner as set out below. On the other hand, TOPCOAT 5 is a clear coating. The manner in which this was applied to a steel substrate is also set out below.

With regard to the application process of TOPCOATS 1-4, 6 and 7, a conventional urethane coil coating primer was applied at 0.2 mils dry film thickness with a wire wound bar and baked at 465 °F (241°C) peak metal temperature for about 20 seconds over individual panels of BONDERITE™ 902 treated cold rolled steel. Thereafter, TOPCOATS 1-4, 6 and 7 were each individually applied over the separate primed steel panels at 0.80 mils dry film thickness with a wire wound bar and baked at 450°F (232°C) peak metal temperature for about 20 seconds. The properties of the individual coating systems incorporating TOPCOATS 1-4, 6 and 7 are set out in TABLE 2.

With regard to the application process of TOPCOAT 5, the BASE COAT from Example 3 was applied at 0.80 mils dry film thickness with a wire wound bar and baked at 450°F (232°C) peak metal temperature for 30 seconds over BONDERITE™ 902 treated cold rolled steel. Thereafter, TOPCOAT 5 was applied at 0.45 mils dry film thickness to the coated substrate with a wire wound bar and baked at 450°F (232°C) peak metal temperature for 30 seconds. The properties of the individual coating systems incorporating TOPCOAT 5 is also set out in TABLE 2. TABLE 2

EXAMPLE 6

This Example illustrates the durability of the top coat coating compositions prepared in accordance with the present invention. In this Example, the coating systems which incorporate TOPCOATS 1 and 6 had their outdoor durability evaluated after 1 year in south Florida at an exposure angle of 45° to the horizon facing due south. The observations are set out below.

In many instances, when a film is subjected to such a "Florida exposure" weathering test, if the film retains more than 80% of its original gloss, it is often considered by many end users as durable

Both Topcoats 1 and 6 (i.e., Topcoats made in accordance with the present invention) retained more than 80% of their original gloss after the Florida exposure weathering test. On the other hand, Topcoat 7 (i.e., a Topcoat made outside of the present invention) retained less than 80% of its original gloss after the Florida exposure accelerated weathering test. It is evident from the foregoing that various modifications, which are apparent to those skilled in the art, can be made to the embodiments of this invention without departing from the spirit or scope thereof. Having thus described the invention, it is claimed as follows.

Claims

THAT WHICH IS CLAIMED IS:

1. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. at least 60 weight percent of a polymer component, said weight percentage being based upon the total weight of the liquid coating composition's film forming system, wherein: i. said polymer component comprises at least 90 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, and ii. said polyester component comprises at least 90 weight percent of a cyclolinear polyester compound wherein at least 90 percent of its constituent monomers are cyclic, this weight percentage being based on the total weight of the polyester component; b. at least 5 weight percent of a crosslinker component, said weight percentage being based on the total weight of the liquid coating composition's film forming system, and said crosslinker component comprising at least one curing agent selected from the group consisting of: a melamine-formaldehyde resin, benzoguanamine-formaldehyde resins, isocyanurates from isophorone diisocyanate, isocyanurates from 1 ,6 hexamethylene diisocyanate and the biuret from 1 ,6-hexamethylene diisocyanate; and

(c) at least 0.1 weight percent of a catalyst component, said weight percentage being based on the total weight of the film forming system, and said catalyst component comprising at least one compound selected from the group consisting of: dibutyltin dilaurate, acetyl acetonate, quaternary ammonium or phosphonium compounds, sulfonic acids, mineral acids, carboxylic acids, magnesium bromide, aluminum nitrate, and zinc nitrate.

2. A coating composition as recited in claim 1 further comprising a pigment system, wherein the weight ratio of the weight of the pigment system to the weight of the film forming resin's polymer component being in the range of about 0.05:1 to about 1 :1.

3. A coating composition as recited in claim 1 further comprising a diluent system present in an amount not greater than about 50 weight percent ,said weight percent being based on the total weight of the liquid coating composition.

4. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. a polymer component, wherein: i. said polymer component comprises at least 70 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, and

ii. said polyester component comprises at least 70 weight percent of a cyclolinear polyester compound wherein at least 70 percent of its constituent monomers are cyclic, this weight percentage being based on the total weight of the polyester component; and

b. a crosslinker component comprising at least one curing agent selected from the group consisting of: aminoplast curing agents and isocyanate curing agents.

5. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. a polymer component, wherein: i. said polymer component comprises at least 70 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, ii. said polyester component comprises at least 70 weight percent of a cyclolinear polyester compound, this weight percentage being based on the total weight of the polyester component, and iii. at least 70 percent of said cyclolinear polyester 's monomers are cyclic; and b. a crosslinker component.

6. A coating composition as recited in claim 5, wherein the cyclolinear polyester is comprises at least one of the following structures:

H-{- O-R¹ -O-C-R² -C -)_n- O - R¹- OH

O O

H-(- O-R¹ -O-C-R² -C -)_n- O - R3- OH II II o o

H-(- -R¹- -C-R²-0 -)„—<- O-R¹ -O-C-R* -C-O-R-¹ - -OH

II II II II o o o o wherein R¹, R² and R³ denote divalent cycloaliphatic, cycloaliphatic-aliphatic, aromatic or heterocyclic-aliphatic moieties, wherein at least one of R¹ , R² and R³ contain a carboxylic or heterocyclic ring, wherein at least one of R¹, R² and R³ is cycloaliphatic, and wherein n denotes a number from 1 to 30.

7. A coating composition as recited in claim 5, wherein the cyclolinear polyester is the reaction product of cyclic diacid or acid anhydride component with a cyclic polyol component.

8. A coating composition as recited in claim 7, wherein the cyclic polyol component comprises at least one polyol selected from the group consisting of: 1 ,2- benzenedimethanol; 1,3-benzenedimethanol; 1,4-benzenedimethanol; isomers of cyclohexanedimethanol; and hydrogenated 4,4-isopropylidenediphenol.

9. A coating composition as recited in claim 7, wherein the cyclic polyol component comprises cyclohexanedimethanol.

10. A coating composition as recited in claim 7, wherein the cyclic diacid or acid anhydride component comprises at least one cyclic diacid or acid anhydride selected from the group consisting of: phthalic anhydride; tetrahydrophthalic anhydride; hexahydrophthalic anhydride; 1,3-cyclohexanedicarboxylic acid; 1,4- cyclohexanedicarboxylic acid; isophthalic acid; terephthalic acid; and methylhexahydrophthahc acid.

11. A coating composition as recited in claim 7, wherein the cyclic diacid or acid anhydride component comprises at least one compound selected from the group consisting of: phthalic anhydride, and phthalic acid.

12. A coating composition as recited in claim 6, wherein at least one of R¹ or R² contain an aromatic group.

13. A coating composition as recited in claim 6, wherein neither R¹ nor R² contain an aromatic group.

14. A coating composition as recited in claim 6, wherein crosslinker component comprises at least one curing agent selected from the group consisting of: aminoplast curing agents, and isocyanate curing agents.

15. A coating composition as recited in claim 6, wherein crosslinker component comprises at least one aminoplast curing agent.

16. A coating composition as recited in claim 15, wherein the at least one aminoplast curing agent comprises at least one compound selected from the group consisting of: benzoguanamine-formaldehyde resins and melamine-formaldehyde resins.

17. A coating composition as recited in claim 15, wherein the at least one aminoplast curing agent comprises a melamine-formaldehyde resin.

18. A coating composition as recited in claim 6, wherein crosslinker component comprises at least one isocyanate curing agent.

19. A coating composition as recited in claim 18, wherein the at least one isocyanate curing agent comprises at least one compound selected from the group consisting of: polyisocyanates, isophorone diisocyanate and NCO-prepolymers.

20. A coating composition as recited in claim 18 wherein the at least one isocyanate curing agent comprises at least one compound selected from the group consisting of: toluene diisocyanate; 4,4'-methylene-bis-(cyclohexyl isocyanate); isophorone diisocyanate; isocyanurates from isophorone diisocyanate; and 1 ,6 hexamethylene diisocyanate; and the biuret from 1 ,6-hexamethylene diisocyanate.

21. A coating composition as recited in claim 6, wherein the polymer component is present in an amount of at least about 50 weight percent, and the crosslinker component is present in an amount of at least about 1 weight percent, said weight percentages being based on the total weight of the coating composition's film forming system.

22. A coating composition as recited in claim 6, wherein the weight ratio of polyester component to the crosslinking component is within the range of 6:1 to 0.5:1

23. A coating composition as recited in claim 6, wherein the film forming system further comprises a catalyst component.

24. A coating composition as recited in claim 6, further comprising at least one additional system selected from the group consisting of: a pigment system and a diluent system. AMENDED CLAIMS

[received by the International Bureau on 27 March 2001 (27.03.01); , original claims 1, 4 and 5 amended; original claims 10 and 1 1 cancelled; remaining claims unchanged (4 pages)]

1. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. at least 60 weight percent of a polymer component, said weight percentage being based upon the total weight of the liquid coating composition's film forming system, wherein: i. said polymer component comprises at least 90 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, and ii. said polyester component comprises at least 90 weight percent of a cyclolinear polyester compound wherein at least 90 percent of its constituent monomers are cyclic, this weight percentage being based on the total weight of the polyester component, at least one of the constituent monomers being a cyclic diacid or acid anhydride component comprising at least 20 percent by weight of the polyester component, the cyclic diacid or acid anhydride selected from the group consisting of phthalic acid and phthalic anhydride; b. at least 5 weight percent of a crosslinker component, said weight percentage being based on the total weight of the liquid coating composition's film forming system, and said crosslinker component comprising at least one curing agent selected from the group consisting of: a melamine-formaldehyde resin, benzoguanamine-formaldehyde resins, isocyanurates from isophorone diisocyanate, isocyanurates from 1,6 hexamethylene diisocyanate and the biuret from 1,6- hexamethylene diisocyanate; and c. at least 0.1 weight percent of a catalyst component, said weight percentage being based on the total weight of the film forming system, and said catalyst component comprising at least one compound selected from the group consisting of: dibutyltin dilaurate, acetyl acetonate, quaternary ammonium or phosphonium compounds, sulfonic acids, mineral acids, carboxylic acids, magnesium bromide, aluminum nitrate, and zinc nitrate. 2- A coating composition as recited in claim 1 further comprising a pigment system, wherein the weight ratio of the weight of the pigment system to the weight of the film forming resin's polymer component being in the range of about 0.05:1 to about 1 :1.

4. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. a polymer component, wherein: i. said polymer component comprises at least 70 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, and ii. said polyester component comprises at least 70 weight percent of a cyclolinear polyester compound wherein at least 70 percent of its constituent monomers are cyclic, this weight percentage being based on the total weight of the polyester component, at least one of the constituent monomers being a cyclic diacid or acid anhydride component comprising at least 20 percent by weight of the polyester component, the cyclic diacid or acid anhydride selected from the group consisting of phthalic acid and phthalic anhydride; and

5. A liquid coating composition which comprises a film forming system, said film forming system comprising: a. a polymer component, wherein: i. said polymer component comprises at least 70 weight percent of a polyester component, this weight percentage being based on the total weight of the coating composition's polymer component, ii. said polyester component comprises at least 70 weight percent of a cyclolinear polyester compound, this weight percentage being based on the total weight of the polyester component, at least one of the constituent monomers being a cyclic diacid or acid anhydride component comprising at least 20 percent by weight of the polyester component, the cyclic diacid or acid anhydride selected from the group consisting of phthalic acid and phthalic anhydride, and

Hi. at least 70 percent of said cyclolinear polyester' s monomers are cyclic; and b. a crosslinker component.

H-(- O - Λ-7 - O - C - *² - C -)„- O - .R-f- OH

II I i o o

H-<- O -R¹ - O - C - R² - C -)„- O -R³- OH

II I I o o

H-{- O -^ - O - C -Λ^ - C -)«—(- O -R¹ - O - C ~R3 - C- O -R¹ -)„r OH

II II II II

0 0 0 0 wherein R', R² and R³ denote divalent cycloaliphatic, cycloaliphatic-aliphatic, aromatic or heterocyclic-aliphatic moieties, wherein at least one of R', R² and R³ contain a carboxylic or heterocyclic ring, wherein at least one of R¹, R² and R³ is cycloaliphatic, and wherein n denotes a number from 1 to 30.

8. A coating composition as recited in claim 7, wherein the cyclic polyol component comprises at least one polyol selected from the group consisting of: 1,2- benzenedimethanol; 1,3-benzenedimethanol; 1,4-benzenedimethanol; isomers of cyclohexanedimethanol; and hydrogenated 4,4-isopropylidenediphenol.

10. canceled

11. canceled

12. A coating composition as recited in claim 6, wherein at least one of R' or R² contain an aromatic group.