Coatings provided by such compositions possess good impact strength, flexibility, and ultra-violet (UV) stability.

DESCRIPTION OF THE RELATED ART Plastic materials used in the manufacture of powder coatings are classified broadly as either thermosetting or thermoplastic. In the application of thermoplastic powder coatings, heat is applied to the coating on the substrate to melt the particles of the powder coating and thereby permit the particles to flow together and form a smooth coating.

Thermosetting coatings, when compared to coatings derived from thermoplastic compositions, generally are tougher, more resistant to solvents and detergents, have better adhesion to metal substrates and do not soften when exposed to elevated temperatures. However, the curing of thermosetting coatings has created problems in obtaining coatings which have, in addition to such desirable characteristics, good smoothness and flexibility. Coatings prepared from thermosetting powder compositions, upon the application of heat, may cure or set prior to forming a smooth coating, resulting in a relatively rough finish referred to as an"orange peel"surface. Such a coating surface or finish lacks the gloss and luster of coatings typically obtained from thermoplastic compositions. The"orange peel" surface problem has led to thermosetting coatings to be applied from organic solvent systems which are inherently undesirable because of the environmental and safety problems that may be occasioned by the evaporation of the solvent system.

Solvent-based coating compositions also suffer from the disadvantage of relatively poor percent utilization. For example, in some modes of application, only 60 percent or less of the solvent-based coating composition being applied contacts the

article or substrate being coated. Thus, a substantial portion of solvent-based coatings is often wasted since that portion which does not contact the article or substrate being coated cannot be reclaimed.

In addition to exhibiting good gloss, impact strength and resistance to solvents and chemicals, coatings derived from thermosetting coating compositions must possess good to excellent flexibility. For example, good flexibility is essential for powder coating compositions used to coat sheet (coil) steel which is destined to be formed or shaped into articles used in the manufacture of various household appliances and automobiles wherein the sheet metal is flexed or bent at various angles.

Coating compositions based on fluoropolymer resins are known to exhibit superior weatherability over coatings based on widely used polyester resins. JP 06299092 describes a thermosetting powder coating composition containing a fluororesin having a crosslinking reaction group, a curing agent and a coupling agent. The coating composition has good adhesion on stainless steel. However, performance properties of fluoropolymer coatings, such as impact strength and flexibility are poor compared to the excellent performance properties of polyester coatings.

EP 371 599 describes a powder coating composition containing a thermosetting polymeric binder containing a hydroxyl functional fluorocarbon copolymer of a hydroxy alkyl vinyl ether and a fluoroolefin, a blocked isocyanate and a hydroxyl functional polyester polymer adapted to co-react with the blocked isocyanate. The hydroxyl function polyester polymer is prepared from glycols and aromatic dicarboxylic and tricarboxylic acids or acid anhydrides, and linear saturated dicarboxylic acid. However the use of an aromatic polyester resulted in a powder coating composition exhibiting poor UV stability and consequently a loss in gloss.

Accordingly, there still exists a need in the art for a fluoropolymer based powder coating exhibiting the characteristics of good impact strength, flexibility, UV stability and gloss.

SUMMARY OF THE INVENTION It has now been found that the physical properties of fluoropolymer based powder coatings can be improved by blending a solid or crystalline hydroxylated aliphatic polyester based on cycloaliphatic diacids and a diol such as, respectively, trans-1,4-cyclohexanedicarboxylic acid and 1,4-butanediol, with a fluoropolymer and cross-linking with a blocked polyisocyanate.

The invention provides a powder coating composition based on a polymer blend of a hydroxyl functional fluoropolymer, a solid or crystalline hydroxylated aliphatic polyester of a cycloaliphatic diacid and a diol, and a cross-linking agent.

The invention also provides a method for preparing a powder coating composition of the invention. The method dry-blends and melt-blends a polymer blend of a hydroxyl functional fluoropolymer and a solid or crystalline hydroxylated aliphatic polyester of a cycloaliphatic diacid and a diol, and a cross-linking agent.

The invention further provides a surface protection coating based on a powder coating composition of the invention. Still further, a method is provided for preparing a surface protection coating. The method applies a powder coating composition of the invention to a substrate to form a coated substrate and then heats the coated substrate.

DETAILED DESCRIPTION OF THE INVENTION A powder coating composition of the invention contains a hydroxyl functional fluoropolymer, a solid or crystalline hydroxylated aliphatic polyester of a cycloaliphatic diacid and a diol, and a cross-linking agent. A powder coating composition according to the invention containing a combination of a hydroxyl functional fluoropolymer and a solid or crystalline hydroxylated aliphatic polyester based on a cycloaliphatic diacid and a diol, exhibits excellent UV stability and gloss in addition to good impact strength and flexibility. In general, the relative amounts of the fluoropolymer and the polyester can be varied depending on a number of factors such as, for example, the hydroxyl number of the polyester and the properties desired or required of the powder coating compositions.

Preferably, the powder coating composition contains about 30 to about 70 wt% of a hydroxyl functional fluoropolymer based on the total weight of the composition, about 30 to about 70 wt% of a solid or crystalline hydroxylated aliphatic polyester based on a cycloaliphatic diacid and a diol based on the total weight of the composition, and a cross-linking agent present in an amount to effect cross-linking of the fluoropolymer and the polyester.

Hvdroxyl Functional Fluoropolymer According to the invention, the hydroxyl functional fluoropolymer may be any linear or branched hydroxyl functional fluoropolymer resin known in the art having sufficient hydroxyl functionality to undergo crosslinking and preferably containing terminal hydroxyl groups. A mixture of linear and branched hydroxyl functional fluoropolymers may also be used. The branched portions of the fluoropolymer may contain additional terminal hydroxyl groups. Preferably, a hydroxyl functional fluoropolymer or mixture thereof contains on average at least 2, preferably 2.5, hydroxyl functionality.

In a preferred embodiment, the hydroxyl functional fluoropolymer is a copolymer of a hydroxyalkyl vinyl ether, i. e. an alkyl vinyl ether containing at least one hydroxyl group on the alkyl portion of the ether, and a fluorolefin such as tetra or trifluoroethylene. Examples of suitable fluoropolymers include, for example, the LUMIFLON fluoropolymer resins available from ICI Resins US of Wilmington, MA, and LUMIFLON 71 OF, available from ZENECA Resins of Wilmington, MA.

Preferably, the fluoropolymer is LUMIFLON 710F. Preferably, the fluoropolymers are solid at ambient temperatures with Tg above about 40°C, a number average molecular weight of between about 8,000 and about 16,000, and a hydroxyl number of about 20 to about 100.

Hydroxylated Aliphatic Polyester The polyester of the invention may be any solid or crystalline hydroxylated aliphatic polyester of a cycloaliphatic diacid or an ester thereof and a diol. Such a polyester may be prepared by techniques well known in the art. For example, the polyester may be prepared by the polycondensation of a trans-1,4-cyclohexane

dicarboxylic acid or a diester thereof, such as, for example, dimethyl trans-1,4- cyclohexanedicarboxylate and a diol. Straight chain diols such as 1,4-butanediol are preferred. When polymerizing the diester of the cycloaliphatic diacid, excess diol may be used to achieve transesterification. Upon completion of the transesterification, the unreacted diol may be removed under reduced pressure until the desired viscosity is obtained. The Tg of the polyester may range between about 50°C to about 80°C such that the composition exhibits good flow. Preferably, the Tg is low enough to allow for good flow but not too low that the polyester is no longer a solid. More preferably, the polyester is a solid having a Tg of about >50 °C.

In another preferred embodiment, the resulting polyester is crystalline at or greater than about 50°C and has a Tg of about <50°C.

Preferably, the polyester is a hydroxylated aliphatic polyester such as, for example, poly (tetramethylene trans-1,4-cyclohexanedicarboxylate). In a preferred embodiment, poly (tetramethylene trans-1,4-cyclohexanedicarboxylate) is prepared from the polycondensation of trans-1,4-cyclohexanedicarboxylic acid or its diester and 1,4-butanediol. Trans-1,4-cyclohexanedicarboxylic acid as referred throughout contains at least 70% trans-isomer. The preferred aliphatic poly (tetramethylene trans-1,4-cyclohexanedicarboxylate) polyester of this invention has a melting point in the range of about 110° to about 160°C, a hydroxyl number in the range of about 25 to about 100, and a inherent viscosity of about 0.1 to about 0.5.

The aliphatic polyester component may also contain a branching agent, such as for example, trimethylolpropane, to increase the crosslink density of the coating.

Up to about 10 mole percent of the diol (1,4-butanediol) may be replaced with a glycol having 2 to 12 carbon atoms such as, for example, ethylene glycol, neopentyl glycol, 1,6-hexanediol, and the like.

Cross-linking Agent Suitable curing or cross-linking agents for use with hydroxyl functional fluoropolymer and polyester polymers are well known in the art. Preferred cross- linking agents include blocked polyisocyanates known in the art and may be obtained from commercial sources or may be prepared according to known

procedures. A"blocked polyisocyanate"as used throughout refers to compounds which contain at least two isocyanate groups which are blocked, i. e. reacted with another compound. The reaction of the isocyanate groups with the blocking compound is reversible at elevated temperatures of about 150°C and above.

Upon heating and thus curing the coating compositions of the invention, the isocyanate groups of the blocked cross-linking agents are unblocked. The isocyanate groups are then able to react with hydroxyl groups present on the fluoropolymer and the polyester, each as described above, to cross-link the polymer chains and to form urethane linkages. Upon curing the compositions form tough coatings. Examples of suitable cross-linking agents include those which are based on isophorone diisocyanate blocked with epsilon-caprolactam, commercially available as HULS 1530 and CARGILL 2400, or toluene 2,4-diisocyanate blocked with epsilon-caprolactam, commercially available as CARGILL 2450, and phenol-blocked hexamethylene diisocyanate.

The most readily-available, and thus the most preferred, blocked polyisocyanate cross-linking agents or compounds are those commonly referred to as epsilon-caprolactam-blocked isophorone diisocyanate, including, for example, those described in U. S. Pat. Nos. 3,822,240,4,150,211 and 4,212,962. However, the products marketed as epsilon-caprolactam-blocked isophorone diisocyanate may consist primarily of the blocked, difunctional, monomeric isophorone diisocyanate, i. e., a mixture of the cis and trans isomers of 3-isocyanatomethyl-3,5,5-trimethyl- cyclohexyl-isocyanate, the blocked, difunctional dimer thereof, the blocked, trifunctional trimer thereof or a mixture of the monomeric, dimeric and/or trimeric forms. For example, the blocked polyisocyanate compound used as the cross-linking agent may be a mixture consisting primarily of the epsilon-caprolactam-blocked, difunctional, monomeric isophorone diisocyanate and the epsilon-caprolactam-blocked, trifunctional trimer of isophorone diisocyanate.

The amount of cross-linking agent present in the composition of the invention may be varied depending on several factors as is known in the art relative to the amount of fluoropolymer and polyester utilized. Typically, the amount of

cross-linking agent which will effectively cross-link the hydroxyl-containing polymers to produce coatings having a good combination of properties is in the range of about 5 to about 30 wt%, preferably about 15 to about 25 wt%, based on the total weight of the powder coating composition.

Powder Coating Composition A powder coating composition of the invention may be prepared by dry-mixing and then melt-blending a hydroxyl functional fluoropolymer, a solid or crystalline hydroxylated aliphatic polyester of a cycloaliphatic diacid and a diol, and a cross-linking agent, each as described above, to form a solidified blend. Melt blending should be performed at a sufficiently low temperature to prevent the unblocking of the cross-linking agent and thus avoiding premature cross-linking.

The solidified blend may then be ground to a particle size suitable for producing powder coatings. In general, an average particle size ranges between about 10 to about 300 microns. For example, the ingredients of the powder coating composition may be dry blended and then melt blended in a Brabender extruder at about 90° to about 130°C, granulated and finally ground.

The components of a powder coating composition of the invention may also be dry blended in a Henschel mixer, followed by extruding in a ZSK-30 Extruder (Werner & Pfleiderer) at about 110-130°C, grinding, and screening to obtain a powder with average particle size of about 35 microns.

A powder coating composition of the invention may be deposited on various metallic and non-metallic (e. g., thermoplastic or thermoset composite) substrates by known techniques for powder deposition such as by means of a powder gun, by electrostatic deposition or by deposition from a fluidized bed. In fluidized bed sintering, a preheated substrate is immersed into a suspension of a powder coating composition in air. The powder coating composition is maintained in suspension by passing air through a porous bottom of the fluidized bed chamber. The particle size of the powder coating composition generally ranges between about 60 to about 300 microns. The substrates to be coated are preheated to about 250° to about 400°F, i. e. preferably about 121 ° to 205°C, and then brought into contact with the fluidized

bed of the powder coating composition. The contact time depends on the thickness of the coating desired but typically lasts from about 1 to about 12 seconds. The temperature of the substrate being coated causes the powder to flow and thus fuse together to form a smooth, uniform, continuous, uncratered coating. The temperature of the preheated substrate also effects cross-linking of the coating composition and results in the formation of a tough coating having a good combination of properties. Coatings having a thickness of between about 200 and about 500 microns may be produced by this method.

The compositions also may be applied using an electrostatic process wherein a powder coating composition of the invention having a particle size of less than about 100 microns, preferably about 15 to about 50 microns, is blown by means of compressed air into an applicator in which it is charged with a voltage of 30 to 100 kV by high-voltage direct current. The charged particles are then sprayed onto a grounded substrate to be coated. The charged particles adhere to the grounded substrate due to their electrical charge. The coated substrate may then be heated to melt and cure the powder coating composition particles. Coatings having a thickness of about 25 to about 120 microns may be obtained.

Another method of applying a powder coating composition of the invention is by means of an electrostatic fluidized bed process which is a combination of the two methods described above. For example, annular or partially annular electrodes are mounted in the air feed to a fluidized bed so as to produce an electrostatic charge of, for example, 50 to 100 kV. The powder coating composition is then introduced into the electrostatic fluidized bed. The substrate to be coated may be either heated (e. g. 250°-400° F), at ambient temperature or cold (e. g. 0°C). The substrate is then briefly exposed to the fluidized powder. The resulting coated substrate may then be heated to effect cross-linking if the substrate was not preheated to a temperature sufficient to cure the powder coating upon contact with the substrate.

Yet another method by which a powder coating composition of the invention may be applied to a substrate is the electrostatic deposition of the powder coating composition on the substrate by use of a powder gun. After deposition, the powder

coating composition may be heated to a temperature sufficient to cause its particles to flow and fuse together to form a smooth, uniform surface.

A powder coating composition of the invention may be used to coat substrates of various shapes and sizes constructed of heat-resistance materials such as glass, ceramic and various metal materials. A composition of the invention is especially useful for producing coatings on substrates constructed of metals and metal alloys, particularly steel substrates. For example, coatings may be prepared on 3 inch by 9 inch panels of 20-gauge, polished, cold roll steel, the surface of which has been zinc phosphated such as, for example, Bonderite 37, available from The Parker Company.

Additives A powder coating composition may further contain at least one additive well known in the art. Such additives include, for example, benzoin, flow aids or flow control agents which aid the formation of a smooth, glossy surface, catalysts, stabilizers, pigments and dyes. Examples of such additives can be found in U. S.

Patent No. 4,346,144, incorporated herein by reference.

A powder coating composition preferably contains a flow aid, also referred to as a flow control or leveling agent, to enhance the surface appearance of cured coatings of the powder coating composition. Examples of suitable flow aids typically contain acrylic polymers and include, but are not limited to, MODAFLOW from Monsanto Company and ACRONAL from BASF. Other flow control agents which may be used include, for example, MODAREZ MFP available from Synthron, EX 486 available from Troy Chemical, BYK 360P available from BYK Mallinkrodt and PERENOL F-30-P available from Henkel. An example of a preferred flow aid is an acrylic polymer having a molecular weight of about 17,000 and containing about 60 mole percent 2-ethylhexyl methacrylate residues and about 40 mole percent ethyl acrylate residues. The amount of flow aid present in the composition may preferably be in the range of about 0.5 to about 4.0 wt%, based on the total weight of the powder coating composition.

Conventional ultraviolet light stabilizers and hindered amine light stabilizers may also be used. The function of such additives is to prevent or at least minimize degradation of the resultant finish by ultraviolet light. Preferably, both an ultraviolet light screen and a hindered amine light stabilizer are used. Examples of ultraviolet light screens include, but are not limited to, 2- (o-hydroxylphenyl) benzotriazoles, nickel chelates, o-hydroxybenzophenones, and phenyl salicylates. Preferably, the ultraviolet screen is TINUVIN 234 available from Ciba Geigy. A preferred hindered amine light stabilizer is TINUVIN 144 also available from Ciba-Geigy. When the ultraviolet light screen and/or hindered amine light stabilizer is present, it is preferably present in a concentration of about 0.3 to about 4 wt% based on the total weight of the powder coating composition.

Catalysts, such as dibutyltin dilaurate, may be added to a powder coating composition of the invention to aid the cross-linking reaction of the composition.

Also, conventional dyes or pigments such as R960 titanium dioxide pigment available from Du Pont may be used.

Further examples of formulation methods, additives, and methods of powder coating applications may be found in User's Guide to Powder Coating, 2nd Ed., Emery Miller, editor, Society of Manufacturing Engineers, Dearborn, (1987).

The following examples are given to illustrate the invention. It should be understood, however, that the invention is not to be limited to the specific conditions or details set forth in these examples.

Examples GENERAL METHODS Viscosities All inherent viscosities are determined at 25 °C in a (60/40 by weight) mixture of phenol/tetrachloroethane at a concentration of 0.5 g/100 mL.

Acid and Hydroxyl Numbers Acid and hydroxyl numbers are determined by titration and are reported herein as mg of KOH consumed for each gram of polymer.

Melting Temperature The melting temperature (Tm) was determined by differential scanning calorimetry (DSC) on the second heating cycle at a scanning rate of 20° per minute after the sample had been heated to melt and quenched to below the grass transition temperature of the polymer.

Molecular Weight Determination The molecular weights were determined by gel-permeation chromatography (GPC) on a Perkin-Elmer instrument with tetrahydrofuran as a mobile phase and solvent. Values are reported in polystyrene equivalents.

Weatherabilitv The artificial weatherability of the coatings was determined by exposure of the coated panels in a Cyclic Ultraviolet Weathering Tester (QUV) with 313 nm fluorescent tubes. The test condition was 8 hours of light at 70°C and 4 hours of condensation at 45 °C.

Impact Strength Impact strength was determined using a Gardner Laboratory, Inc., Impact Tester. A weight was dropped within a slide tube from a specified height to hit a punch having a 5/8 inch diameter hemispherical nose which is driven into the front (coated face) or back of the panel. The highest impact which does not crack the coating was recorded in inch-pounds, front and reverse.

Gloss The 20 degree and 60 degree gloss were measured using a gloss meter (Gardner Laboratory, Inc., Model GC-9095) according to ASTM D-523.

Pencil Hardness The pencil hardness of a coating was determined to be that of the hardest pencil that would not cut into the coating according to ASTM 3363-74 (reapproved 1980). The results are expressed according to the following scale: (softest) 6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H (hardest).

Coating Flexibility Coatings were prepared on 3 inch by 9 inch panels of 24-gauge, polished, cold roll steel, the surface of which has been zinc phosphated (Bonderite 37, The Parker Company). The flexibility of the coatings was determined in accordance with ASTM 4145-83 at ambient temperature by bending or folding a coated panel back against itself, using a hydraulic jack pressurized at 8,000 pounds per square inch (psi), until the apex of the bend was as flat as could be reasonably achieved.

This initial bend was referred to as OT (zero thickness) meaning that there was nothing between the bent portions of the panel. The bend was examined using a 10X magnifying glass and, if fractures on the coating were observed, the panel was bent a second time (1T) to form a three-layer sandwich. The second bend was inspected for coating fracture and this procedure was repeated, forming 4-, 5-, 6-, etc. layer sandwiches, until a bend exhibited no fracture of the coating. The result of each bend test was the minimum thickness (minimum T-bend) of the bend which would not give any fractures of the coating. Therefore a T-bend result of zero (0) indicates no cracking or fracture of the coating after one bend (OT=zero thickness).

A T-bend test of greater than 1 OT indicated that no cracking or fracture of the coated panel was achieved after eleven or more bends and was considered a failure. A T- bend result of less than 10T was acceptable. Although the bend test used was excessively severe for most purposes for which coated articles would be used, it provides a means to compare the flexibility of different powder coating compositions.

EXAMPLE 1: General Procedure for Preparation of Aliphatic Polyester A 3000 mL, 3-neck, round bottom flask equipped with a stirrer, a short distillation column, and an inlet for nitrogen, was charged with dimethyl trans-1,4- cyclohexanedicarboxylate (1280.8 g, 6.40 mol), 1,4-butanediol (692.9g, 7.683 mol, 10% excess), and 100 ppm of titanium as titanium tetraisopropoxide in 2-propanol.

The flask and contents were then heated under a nitrogen atmosphere to a temperature of 170°C at which point methanol began to distill rapidly from the

flask. After the reaction mixture was heated with stirring at this temperature for about 1 hour, the temperature was then increased to 200°C for 2 hours, raised to 215 °C for 4 hours, and then to 235 °C for 3 hours. Afterwards, a vacuum of 10 mm of mercury was applied over a period of 12 minutes. Stirring was continued under 10 mm of mercury at 235 °C for about 3 hours to produce a low melt viscosity, colorless polymer resin. The resulting resin had an inherent viscosity of 0.21, a melting point of 150°C, a hydroxyl number of 45, an acid number of 2, and a molecular weight by GPC of 2500 polystyrene equivalents.

EXAMPLE 2: Preparation of a Powder Coating Composition Using an Aliphatic Polyester The polyester (216 g), prepared according to Example 1,264 g LUMIFLON 710F, 120 g HULS 1530,6 g dibutyltin dilaurate, 6 g benzoin, 10 g flow aid MODAFLOW, 6 g TINUVIN 144, and 6 g TINUVIN 234 were dry-blended in a Henschel mixer and then melt-blended in a ZSK twin screw extruder at 120°C, ground in a Bantam mill to which a stream of liquid nitrogen was fed, and classified through a 170 mesh screen on a KEK centrifugal sifter. The resulting finely- divided, powder coating composition had an average particle size of about 50 microns.

The powder coating composition was then electrostatically deposited using a powder gun on 3 inch by 9 inch panels of 20-gauge, polished, cold roll steel, the surface of which has been iron phosphated available from The Q-Panel Company.

The coating was cured (cross-linked) by heating the coated panels at 177°C in an oven for 20 minutes. The cured coatings were about 50 microns thick.

The coating on the panel had a pencil hardness of H, both front and reverse impact strength of 160 inch-pounds, and a T-bend flexibility value of 0.

COMPARATIVE EXAMPLE 1: Preparation of Powdered Coating Composition in the Absence of a Polyester Using the procedure described in Example 2, a powder coating composition was prepared from the following materials: 476 g LUMIFLON 710F, 124 g HULS 1530,6 g Dibutyltin Dilaurate, 6 g Benzoin, 6 g flow aid MODAFLOW, 6 g TINUVIN 144, and 6 g TINUVIN 234. Again following the procedure of Example 2, panels were coated with this powder coating composition. Then the coatings were cured and evaluated.

The coatings had a pencil hardness of 2H, a front impact strength of 30 inch- pounds and less than 10 inch-pounds reverse impact strength. The coated panels failed the T-bend test.

COMPARATIVE EXAMPLE 2: Preparation of Powder Coating Composition Using an Aromatic Polyester Using the procedure described in Example 2, a powder coating composition was prepared from the following materials: 216 g LUMIFLON 710F, 264 g RACED 107 (a polyester based primarily on terephthalic acid and 2,2-dimethyl-1,3- propane diol), 120 g HULS 1530,6 g Dibutyltin Dilaurate, 6 g Benzoin, 6 g flow aid MODAFLOW, 6 g TINUVIN 144; and 6 g TINUVIN 234. Using the procedure of Example 2, panels were coated with this powder coating composition and the coatings are cured and evaluated.

The coatings have a pencil hardness of 2H, a front impact strength of 30 inch-pounds and less than 10 inch-pounds reverse impact strength. The coated panels failed the T-bend test.

EXAMPLE 3: Impact Strength and T-Bend Results The impact strength and flexibility of the powder coating compositions of Example 2, Comparative Example 1 and Comparative Example 2 were measured and are summarized in Table 1. The fluoropolymer-aliphatic polyester blend powder coating composition of Example 2 exhibits better impact strength and

flexibility than the fluoropolymer powder coating composition of Comparative Example 1 and the fluoropolymer-aromatic polyester blend powder coating composition of Comparative Example 2.

Table 1. Impact Strength and T-Bend Test Results

Example Impact Strength T-Bend Test Front/Reverse Example 2 160/160 0 Comparative Example 1 30/10 Fail Comparative Example 2 30/10 Fail i

It should be understood that the foregoing discussion and examples merely present a detailed description of certain preferred embodiments. It will be apparent to those of ordinary skill in the art that various modifications and equivalents can be made without departing from the spirit and scope of the invention. All the patents, journal articles and other documents discussed or cited are herein incorporated by reference.