WO1989011497A1

WO1989011497A1 - Thermoset drawable coating compositions

Info

Publication number: WO1989011497A1
Application number: PCT/US1989/002071
Authority: WO
Inventors: William Joseph Birkmeyer; Ronda Smart Bradford; Dennis Wayne Carson; David Thomas Mckeough; Robert Alan Montague; Jamal Steven Richardson; Carl Allen Seneker
Original assignee: Ppg Industries, Inc.
Priority date: 1988-05-20
Filing date: 1989-05-12
Publication date: 1989-11-30
Also published as: ES2018895A6; KR900701882A; AU3750889A

Abstract

A coating composition is provided containing as a film former a polyester polyol having a controlled amount of branching and having a weight average molecular weight of from about 4,000 to about 40,000, a hydroxyl value of from about 10 to about 100 and formed from the reaction of: (a) a diol component, (b) a dicarboxylic acid component or a functional equivalent thereof, (c) a polyfunctional component in which the functionality is selected from hydroxyl, carboxylic acid or functional equivalent thereof or a combination of the two functional groups, the amount of the polyfunctional component (c) being adjusted to incorporate up to about 0.25 gram-mole of polyfunctional component per 500 grams of the total reactants used to prepare the polyester polyol; and a polyisocyanate curing agent in an amount such that the isocyanate to hydroxyl equivalent ratio ranges from about 0.3/1 to about 1.5/1. The coating composition is subject to the proviso that the coating composition when applied direct to a 24 gauge cold rolled steel circular blank having a diameter of about 3.25 inches (82.6 millimeters) at a dry film thickness of about 0.8 mil (20 microns), cured at about 240°C peak metal temperature for about 50 seconds, and drawn into a square cup with rounded corners subjected to about 120 percent elongation and about 50 percent compression on the rounded corners of the base of the cup using a 90 ton (8 x 1010 dyne) press, about 40 pounds per square inch (2.7 x 106 dynes/cm2) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder, the square cup formed having a depth of about 26 millimeters, rounded corners each corner having an outer radius of about 6.5 millimeters, and a width of about 36 millimeters on one side, followed by five minutes immersion in boiling deionized water results in an essentially tack-free, essentially defect-free films.

Description

THERMOSET DRAWABLE COATING COMPOSITIONS

Background of the Invention The present invention relates to coating compositions containing a polyester polyol or polyester-urethane polyol as the film former which is crosslinked with a polyisocyanate curing agent.

In some automotive applications precoated metal coil stock is used to draw metal parts. For example, gas tanks, oil filters, air filter housings, valve covers and other similar under-the-hood parts can be prepared in this way. For such applications, the coating composition which is utilized to coat the metal coil to be drawn must be capable of withstanding the drawing operation without exhibiting forming failures such as cracking, wrinkling, and/or complete delamination. Moreover, even after the coating composition has withstood the drawing operation, it must be also capable of passing severe tests such as boiling water immersion and solvent resistance tests since the coated, drawn parts will be exposed to such conditions as temperature extremes and agents such as motor oil, gasoline, grease, brake fluid, antifreeze solution and a variety of other materials which are capable of aggressively attacking a coated film.

The drawing operation is particularly severe for the coating composition since the film must withstand two types of metal strain associated with drawing-stretching or elongation and compression. Many available coating compositions such as those disclosed in U.S.

4,205,115 are either deficient in one or more features or are uneconomical. For example, even though the coating composition is capable of withstanding the drawing operation, it will fail the physical properties testing by exhibiting delamination upon exposure to wet heat, corrosion upon salt spray exposure and also exhibit poor solvent resistance.

There is a need, therefore, for an economical coating composition which is capable not only of being drawn without failing the forming operation but also which exhibits excellent solvent resistance, excellent adhesion upon exposure to wet heat and other aggressive agents and also good corrosion resistance upon exposure to salt spray.

Summary of the Invention

In accordance with the present invention there is provided a coating composition comprising as a film former a polyester polyol having a controlled amount of branching and having a weight average molecular weight of from about 4,000 to about 40,000, a hydroxyl value of from about 10 to about 100 and formed from the reaction of: (a) a diol component;

(b) a dicarboxylic acid component or a functional equivalent thereof;

(c) a polyfunctional component in which the functionality is selected from hydroxyl, carboxylic acid or functional equivalent thereof or a combination of the two functional groups, the amount of the polyfunctional component (c) being adjusted to incorporate up to about 0.25 gram-mole of polyfunctional component per

500 grams of the total reactants used to prepare the polyester polyol; and a polyisocyanate curing agent in an amount such that the isocyanate to hydroxyl equivalent ratio ranges from about 0.3/1 to about 1.5/1. The coating composition is subject to the proviso that the coating composition when applied direct to a 24 gauge cold rolled steel circular blank having a diameter of about 3.25 inches (82.6 millimeters) at a dry film thickness of about 0.8 mil (20 microns), cured at about 240°C peak metal temperature for about 50 seconds, and drawn into a square cup with rounded corners subjected to about 120 percent elongation and about 50 percent compression on the rounded corners of the base of the cup using a 90 ton (8 X 10¹⁰ dyne) press, about 40 pounds per square inch (2.7 X 10⁶ dynes/cm ) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder, the square cup formed having a depth of about 26 millimeters, rounded corners each corner having an outer radius of about 6.5 millimeters, and a width of about 36 millimeters on one side, followed by 5 minutes immersion in boiling deionized water results in an essentially tack-free, essentially defect-free film.

Brief Description of the Drawing

FIGURE 1 illustrates a drawn square cup with rounded corners which is utilized in evaluating the claimed coating compositions for drawability.

Detailed Description of the Invention The coating composition of the present invention comprises as a film former a polyester polyol. The polyester polyol used as the film former is prepared such that it contains a controlled amount of branching. By "controlled amount of branching" is meant that the number of branch points in the polyester polyol is adapted so as to facilitate crosslinking and result in a film which is nontacky and has acceptable solvent resistance yet not so numerous that the crosslinking is too extensive and disallows drawing of the composition. The amount of branching can be controlled by adjusting the amount of polyfunctional component used in preparing the polyester polyol. The polyfunctional component can contain hydroxyl functionality, carboxyl functionality or a functional equivalent thereof, i.e., anhydride or alkyl esters, or a combination of the two functional groups. The amount is adjusted to incorporate up to about 0.25 gram-mole of polyfunctional component per 500 grams of the total reactants used to prepare the polyester polyol. Usually the amount ranges from about 0.01 to about 0.25, typically from about 0.01 to about 0.20 and preferably the amount ranges from about 0.01 to about 0.15 gram-mole of polyfunctional component per 500 grams of the total reactants. Examples of these components include materials having greater than two hydroxyl groups per molecule such as polyols described in detail below; polycarboxylic acids or functional equivalents thereof such as trimellitic anhydride; and also materials with both carboxylic acid and hydroxyl groups such as dimethylolpropionic acid. Preferably the polyfunctional component is a material having greater than two hydroxyl groups per molecule, more preferably a triol. These materials are discussed in detail below in the specification. The remaining components utilized to prepare the polyester polyol film former, that is the diol component and dicarboxylic acid component or functional equivalents thereof, are also discussed below.

The polyester polyol utilized as the film former in the claimed coating composition has a weight average molecular weight of from about 4,000 to about 40,000, usually from about 5,000 to about 40,000, typically from about 7,500 to about 40,000, preferably from about 10,000 to about 40,000, more preferably from about 15,000 to about 30,000 and most preferably from about 16,500 to about 25,000.

The polyester polyol film former has a hydroxyl value of from about 10 to about 100, preferably from about 10 to about 70, more preferably from about 10 to about 50 and most preferably from about 10 to about 30.

The weight average molecular weight is determined utilizing gel permeation chromatography using polystyrene as a standard. In measuring the weight average molecular weight using polystyrene as the standard, a Waters Associates gel permeation chromatograph was used.

Six micro-styragel columns were used. Each column has the dimensions of 30 centimeters long and 7.8 millimeters inside diameter. A differential refractometer was used as detector. Tetrahydrofuran was used as a solvent with a flow rate of 2.0 millimeters/minute. The quality of the columns is checked by their "theoretical plate number" determined from ortho-dichlorobenzene and have minimum theoretical plate numbers of 3,000/30 cm. To determine molecular weight by gel permeation chromatography (GPC), the instrument is first calibrated using polystyrene standards. Polystyrene standards used were purchased from Pressure Chemicals Company, Pittsburgh, Pennsylvania. The polystyrene standards have dispersities (dispersity = weight average molecular weight/number average molecular weight) ranging from 1.05 to 1.10. The weight average molecular weights of the polystyrene standards used were 900,000; 830,000; 233,000; 50,000; 34,500; 17,500 and 4,000. To obtain a calibration curve, a set of 0.10 percent (10 milligram polystyrene/10 ml tetrahydrofuran) polystyrene solutions in tetrahydrofuran were prepared and a 0.3 ml sample size was injected into the columns and a GPC chromatogram was obtained. The elution volume of each peak corresponding to a given molecular weight of the polystyrene standard was measured. A linear least squares plot of log₁₀ (molecular weight) versus elution time is used as a calibration curve. The lowest molecular weight of the polystyrene standard used was 4,000 and the calibration curve beyond that was extrapolated down to 100. The upper and lower exclusion limits of this set of columns are about 1,000,000 and 100, respectively, in terms of polystyrene molecular weight. The sample whose molecular weight averages are to be determined was prepared as a 1.0 percent solution in tetrahydrofuran. After filtration through a 0.2 micron GELMAN filter, available from Acrodisc CR, a 0.3 milliliter sample size was injected into the columns and a GPC chromatogram obtained under the same experimental conditions as the calibration. A computer acquires data corresponding to the height of the curve every two seconds. The retention times for each interval are converted to molecular weight

(Mi) using the calibration curve. The weight average molecular weight (M_w) is calculated according to the following equation: M_w = (∑H_i X

M_i)/∑H_i. In a preferred embodiment of the present invention the polyester polyol film former is formed from the reaction of a polyol component comprising a positive amount not exceeding 0.25 gram-mole per 500 grams of the total reactants used to prepare the polyester polyol of a material having greater than two hydroxyl groups per molecule. Preferably this material is a triol. In this preferred embodiment preferably the polyol component comprises a positive amount of triol not exceeding 0.20 gram-mole, more preferably a positive amount not exceeding 0.15 gram-mole and preferably the amount of triol is within the range of from about 0.03 gram-mole to about 0.20 gram-mole per 500 grams of reactants used to prepare the polyester polyol. Examples of suitable materials having greater than two hydroxyl groups per molecule include trimethylolpropane, trimethylolethane, 1,2,6-hexanetriol, glycerine and pentaerythritol. Preferably the triol is trimethylolpropane. The diol component comprises one or more diols. Examples of suitable materials include acyclic diols containing from 2 to 12 carbon atoms such as ethylene glycol, diethylene glycol, propylene glycol, 1,2-butanediol, 1,3-butanediol 1,4-butanediol, 2,2,4-trimethyl-1,3-pentane diol, neopentyl glycol and 1,6-hexanediol; and cyclic diols which contain from 6 to 20 carbon atoms including cycloaliphatic diols such as

1,2-cyclohexane diol, 1,4-cyclohexane diol, 1,2-cyclohexanedimethanol, 1,4-cyclohexanedimethanol and 1,3-cyclohexanedimethanol.

Although if desired aromatic diols can be used, preferably the polyol component is essentially free of such aromatic diols for considerations of convenient resin processing. The dicarboxylic acid component contains primarily monomeric dicarboxylic acids or functional equivalents thereof having from 2 to 18 carbon atoms per molecule. Among the acids which are useful are phthalic acid, isophthalic acid, terephthalic acid, tetrahydrophthalic acid, 1,4-cyclohexane dicarboxylic acid, hexahydrophthalic acid, adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, glutaric acid, chlorendic acid, tetrachlorophthalic acid, decanedioic acid, dodecanedioic acid, and other dicarboxylic acids of varying types. As has been mentioned above, functional equivalents of the aforesaid dicarboxylic acids can also be utilized. By this is meant that where acids are referred to above, it is understood that anhydrides of those acids which form anhydrides can be used in place of the acids and in addition lower alkyl esters of the acids can also be used. The polyol component can also include minor amounts of monobasic acids such as benzoic acid, stearic acid, acetic acid, hydroxystearic and oleic acid. In a preferred embodiment of the present invention, the ingredients utilized in preparing the polyester polyol contain at least 5 percent by weight of cycloaliphatic moieties, preferably from about 5 percent to about 40 percent, and more preferably from about 20 percent to about 35 percent. The cycloaliphatic moieties can be introduced either through the polyol component or the polycarboxylic acid component. In one embodiment the ingredients of the polyester polyol contain at least 5 percent of aromatic moieties, preferably from about 5 percent to about 40 percent and more preferably from about 20 percent to about 35 percent, which are preferably introduced through the polycarboxylic acid component. In a preferred embodiment of the present invention the ingredients utilized in preparing the polyester polyol contain both cycloaliphatic and aromatic moieties.

In this embodiment the amount of cycloaliphatic moieties and the amount of aromatic moieties is at least 5 percent. Of course, these percentages are based on the total weight of the ingredients used in preparing the polyester polyol.

For the purposes of determining the aforesaid percentages, the amount of cycloaliphatic moieties is based solely on the formula weight of the cycloaliphatic moiety

and the amount of aromatic moieties is based on the formula weight of the aromatic moiety

By way of example therefore, in order to determine the amount of cycloaliphatic moieties in one mole of dimethyl 1,4-cyclohexane dicarboxylate, the formula weight of the cyclohexane moiety, i.e., 82, is divided by the formula weight of dimethyl 1,4-cyclohexane dicarboxylate, i.e., 200. It is preferred that cycloaliphatic and aromatic moieties be present in the polyester polyol film former because they provide hardness and chemical resistance to the resultant coating compositions prepared from the film former. The acyclic moieties are preferably present because they provide flexibility.

The polyester polyol film former of the present invention is generally a saturated polyester and is preferred as such for preparing the coating compositions according to the present invention. However, if desired a small amount, i.e., less than 10 percent by weight, based on the total weight of the ingredients of the polyester, of alpha,beta-ethylenic unsaturation can be introduced from ingredients such as from maleic acid and fumaric acid which have been included above in the listing of suitable exemplary dicarboxylic acids.

Although preferably substantially free of alpha,beta-ethylenic unsaturation, the polyester polyol film former of the present invention may optionally be modified with an amount not exceeding 20 percent of a fatty acid or derivative thereof which is suitable for the production of alkyd resins. Examples of such fatty acids include tall oil fatty acid, linseed oil fatty acid, the fatty acids of soya oil, safflower oil, and dehydrated castor oil. The polyester polyol film forming component of the present invention can be prepared by mixing the polyol and polycarboxylic acid components and heating the mixture generally to a temperature of up to about 300°C. An esterification reaction catalyst such as a tin compound, for example, dibutyltin oxide and butyl stannoic acid can be employed. A solvent can be utilized such as xylene or toluene to distill azeotropically with the water of the reaction. The polyesterification reaction is preferably conducted without azeotroping agents as, for example, by means of a fusion process in which a nonreactive gas is blown through the reaction mixture in order to remove the water or transesterification by-products such as low molecular weight alcohols. In a preferred embodiment of the present invention, the film former utilized in preparing the claimed coating composition is a polyester-urethane polyol which has been prepared by chain extending the polyester polyol which has been described in detail above with a polyisocyanate. Generally the amount of polyisocyanate is a positive amount not exceeding 15 percent by weight, usually not exceeding 10 percent by weight, preferably not exceeding 8 percent and more preferably not exceeding 6 percent by weight, the percentage based on the total weight of all of the ingredients of the polyester-urethane polyol. Typically the amount of chain extending polyisocyanate ranges from about 1 percent by weight to about 15 percent by weight, preferably from about 2 percent to about 10 percent and more preferably from about 2 percent to about 8 percent. The polyisocyanate extension is conducted such that the resultant polyester-urethane polyol has a hydroxyl value as has been detailed above. It should be understood that the limitations which have been described above in connection with the polyester polyol, e.g., weight average molecular weight, hydroxyl value, reactants, amount of polyfunctional component, isocyanate to hydroxyl equivalent ratio in connection with the crosslinking agent and test for drawability, all apply to the chain extended polyester, i.e., polyester-urethane polyol. The polyisocyanate which is utilized to chain extend the polyester polyol to form a polyester-urethane polyol can be selected from a variety of polyisocyanates. Preferably the chain extending polyisocyanate is a diisocyanate. Examples of suitable diisocyanates which can be utilized herein include toluene diisocyanate,

4,4'-methylene-bis(cyclohexyl isocyanate), isophorone diisocyanate, an isomeric mixture of 2,2,4- and 2,4,4-trimethyl hexamethylene diisocyanate, 1,6-hexamethylene diisocyanate, tetramethyl xylylene diisocyanate and 4,4'-diphenylmethylene diisocyanate. Also suitable for use herein are isocyanurates such as the isocyanurate from isophorone diisocyanate commercially available from Veba Company as

T1890 and the isocyanurate of 1,6-hexamethylene diisocyanate which is available from Bayer as DESMODUR N 3390 and biurets such as the biuret from 1,6-hexamethylene diisocyanate commercially available from Bayer as DESMODUR N. In addition, isocyanate prepolymers of polyester polyols can be utilized. Preferably 4,4'-diphenylmethylene diisocyanate is utilized as the chain extender.

The coating composition of the present invention utilizes a polyisocyanate curing agent to cure the polyester polyol or polyester-urethane polyol to form a film. It is critical to the present invention that the crosslinking agent be a polyisocyanate. When the specifically defined polyester polyols or polyester-urethane polyols of the present invention are cured with aminoplast crosslinking agents, the resultant coating compositions are not consistently drawable in accordance with the requirements set forth herein. The specific proportion of the polyisocyanate curing agent is also very critical in achieving the claimed coating composition which is capable of being drawn according to the specifications set forth herein. Specifically the polyisocyanate curing agent is utilized in an amount such that the isocyanate to hydroxyl equivalent ratio ranges from about 0.3/1 to about 1.5/1. It should be understood that the specific amount of crosslinking agent within the claimed range can vary from polymer to polymer and depends upon the precise parameters of the particular polyol. Therefore, depending upon the particular polyester polyol or polyester-urethane polyol to be crosslinked one level of crosslinker within the range may be more acceptable than another level. In preferred embodiments of the present invention, the amounts of isocyanate crosslinking agent and hydroxyl from the triol component are tailored with the proviso that when the amount of triol incorporated in the preparation of the polyester polyol or polyester-urethane polyol is within the range of from about 0.03 gram-mole to about 0.07 gram-mole per 500 grams of the total reactants used to prepare the polyester or polyester-urethane polyol, the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.7/1 to about 1.5/1; and when the amount of triol is within the range of from about 0.10 gram-mole to about 0.25 gram-mole per 500 grams of the total reactants, the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.3/1 to about 0.7/1. In one specific embodiment, when the amount of triol is 0.25 gram-mole, the isocyanate to hydroxyl equivalent ratio is in the range of from about 0.3/1 to about 0.4/1. It should be understood that the isocyanate to hydroxyl equivalent ratio which is discussed herein relates to the amount of total hydroxyl from the polyol component and the amount of isocyanate from the polyisocyanate crosslinking agent. The polyisocyanate which is utilized to chain extend the polyester polyol to prepare a polyester-urethane polyol in a preferred embodiment of the present invention is not included when calculating the aforesaid ratio. The polyisocyanate crosslinking agent can be selected from a variety of available materials including, for example, any of the materials described above for use as chain extenders.

The coating compositions of the present invention can be prepared either as a one package composition or a two package composition. When it is desired to prepare a two package composition an unblocked or a free polyisocyanate crosslinking agent can be utilized. When a one package composition is desired, a blocked polyisocyanate crosslinking agent can be utilized. Examples of suitable blocking agents are those materials which would unblock at elevated temperatures such as lower aliphatic alcohols such as methanol, oximes such as methyl ethyl ketoxime and lactams such as caprolactam. A preferred crosslinking- agent for use in the present invention is a blocked polyisocyanate curing agent. Preferably the methyl ethyl ketoxime blocked isocyanurate of hexamethylene diisocyanate is utilized. It should be understood that a fundamental feature of two package systems is that the hydroxyl (and/or other active hydrogen containing material) and isocyanate be in separate packages since in these types of systems the reactive hydroxyl (active hydrogen) and isocyanate are mixed just prior to application. The other components which are utilized in formulating the completed composition can be placed in either package, the specific package chosen dependent upon the particular material selected. This is of course well appreciated by those skilled in the art and no additional discussion will be included herein. A very critical feature of the claimed coating compositions is the ability of the coating composition to be drawn or formed according to the test set forth herein. The test involves applying the coating composition direct to a 24 gauge cold rolled steel circular blank having a diameter of about 3.25 inches (82.6 millimeters) at a dry film thickness of about 0.8 mil (20 microns), cured at about 240°C peak metal temperature for about 50 seconds and drawn into a square cup with rounded corners subjected to about 120 percent elongation and about 50 percent compression on the rounded corners of the base of the cup. The drawing operation is performed using a 90 ton (8 X 10¹⁰ dyne) press (of the type obtainable from the

V&O Press Company, of Hudson, New York, Number 6 press) using about 40 pounds per square inch (2.7 X 10⁶ dynes/cm²) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder, the square cup formed having a depth of about 26 millimeters, rounded corners each corner having an outer radius of about 6.5 millimeters, and a width of about 36 millimeters on a side. A representative drawn square cup is shown in Figure 1. Referring to Figure 1, a drawn square cup is shown wherein R is the outer radius of a rounded corner which is about 6.5 millimeters, BC is a rounded corner of the base of the cup, A is the width on a side of the cup which is about 36 millimeters and E is the depth of the cup which is about 26 millimeters.

Subsequent to the drawing operation, the drawn cup is immersed for five minutes in boiling deionized water (wet heat exposure). It should be understood that depending upon the particular atmospheric conditions, the temperature at which water boils can vary slightly. These variations are contemplated and have been taken into account by use of the term "boiling". The cured coating film after having been subjected to the aforedescribed drawing operation and wet heat exposure must result in an essentially tack-free, essentially defect-free film. By essentially tack-free film is meant that the cured film must not show any blocking after the following test: two coated square blanks 2 inches X 2 inches (50.8 millimeters X 50.8 millimeters) are placed one on top of the other such that the coated surfaces are in contact under 1.0 pound per square inch (6.9 X 10⁴ dynes/cm²) pressure for 12 hours at 150°F (65.5°C). If the coating is tack-free, the two coated blanks can easily be disengaged after the blocking test. If the coating composition fails the blocking test, the two coated blanks will be blocked together after the pressure is removed and cannot be separated by reasonable force using fingernails as the only prying instrument. To summarize, in order for the coating composition to pass the test for drawability set forth herein, the coating must be nontacky and pass both the drawing operation and boiling water immersion (wet heat) with essentially no defects. If the coating fails any one of these tests, it fails the test for drawability.

It should be pointed out that the circular blanks which are utilized to perform the drawability test can be prepared in different ways. In one manner, flat coil stock (cold rolled steel) is used. The coating composition is applied in any conventional manner, but preferably applied with a wire bound bar and cured. The circular blanks are then cut from the coated metal. In another way, the circular blanks can be cut out of the coil stock first and then the coating composition applied and cured.

By "essentially defect-free film" is meant that the film which results after drawing and wet heat exposure is free of any coating defects as these are defined in Compilation of ASTM Standard Definitions, sixth edition, 1986, sponsored by the ASTM Committee of Terminology. These defects include but are not limited to pinholing, pock marking, cratering, cracking, blistering, wrinkling, mottling, delamination, flaking, peeling and swelling. The resultant film will have the appearance of a smooth continuous film. The claimed coating compositions can contain in addition to the resinous polyester polyol film forming binder a variety of other optional materials. The coating composition usually also contains pigment and a liquid diluent for the resinous binder. In addition, if desired, other resinous materials can be utilized in conjunction with the primary film forming binder so long as the resultant coating composition is capable of being drawn according to the specifications set forth herein and also results in an essentially tack-free, defect-free film after exposure to wet heat according to the test set forth herein. For example, in pigmented compositions a pigment dispersion vehicle is often utilized. In a preferred embodiment of the claimed invention a branched polyester polyol is used as the pigment dispersing vehicle.

A liquid diluent is generally an organic solvent or a non-solvent which is volatile, is removed after the coating is applied and is utilized to reduce viscosity sufficiently to enable application. Also diluents assist in substrate wetting and coalescence for film formation. Generally, a diluent is present in the composition in an amount of from about 40 to about 80 percent, based on the total weight of the ingredients of the coating composition. Examples of suitable liquid diluents include aromatic hydrocarbons such as toluene, xylene, and aromatic distillates such as AROMATIC 100 which is commercially available from Exxon Chemical, ketones such as methyl ethyl ketone and methyl isobutyl ketone, alcohols such as isopropyl alcohol, normal butyl alcohol, monoethyl ethers of glycols and mixtures thereof. In addition, the coating composition can contain solvents which have been introduced in the process of preparing the polyester polyol film former such as have been described above.

The pigments which can be utilized are of various types, for example, iron oxides, lead oxides, carbon black, titanium dioxide, talc, as well as a variety of color pigments and metallic pigments such as aluminum flake. The specific pigment to binder ratio can vary widely so long as it provides adequate hiding at the desired film thickness without detrimentally affecting the drawability of the coating composition.

The coating compositions of the present invention are particularly preferred for use in coil coating applications. For such applications, the coating compositions are usually cured at a peak metal temperature of from about 230°C to about 250°C, preferably from about 240ºC to about 250°C for a period of time ranging from about 40 to 60 seconds. However, other methods of applying the coating composition are not precluded and can be utilized such as brushing, dipping, flow coating and spraying. The compositions are preferably applied over metal, however, other substrates are not precluded and in addition, the coating compositions are particularly suitable for application over primed metal utilizing a variety of primer coating compositions. The claimed coating compositions are preferably applied over the primer coating compositions detailed in U.S. 4,692,382 and 4,692,383 which are incorporated by reference herein.

In general, the coating thickness will vary depending upon the application desired. For a coil coating application, coatings from about 0.2 (5 microns) to about 1.2 mils (30 microns) can be applied, preferably from about 0.8 (20 microns) to about 1 mil (25 microns). For other methods of application, coatings from about 0.5 (12 microns) to about 1.2 mils (30 microns) can typically be applied.

In the event that other methods of application are utilized, the coatings can be cured at temperatures ranging from about 120°C to about 180°C for a period of about 20 minutes to about 60 minutes. Generally, a catalyst is utilized to assist in cure. A catalyst can permit the use of lower cure temperatures and/or shorter cure schedules. Examples of preferred catalysts for polyisocyanate crosslinking agents include dibutyltin dilaurate, dibutyltin mercaptide, dibutyltin diacetate, dibutyltin dichloride and other organotin compounds. The coating compositions of the present invention are particularly advantageous in that they result in cured films which are not only capable of being drawn without exhibiting forming failures, but the cured films are also essentially tack-free and essentially defect-free after being subjected to wet heat. This is very unexpected and surprising since many conventionally available coating compositions will exhibit a variety of coating defects after either the drawing operation or after exposure to wet heat. Examples of such fabrication induced defects range from wrinkling to various degrees of delamination to complete delamination from the substrate. These defects have been discussed in detail above. The metal drawing operation is particularly aggressive to the cured film since it results in strain of two types, both stretching and compression which are particularly detrimental to the coated film. In addition, the claimed coating compositions have excellent solvent resistance. Solvent resistance can be determined by measuring the resistance of the cured coatings to a good solvent for the film former such as, for example, methyl ethyl ketone. In the solvent resistance test, the cured coating is wiped, using finger pressure with a solvent saturated cheesecloth. The number of double rubs required to dissolve and remove the coating from the substrate is recorded. The cured coatings of the present invention are typically capable of withstanding more than 50 double rubs without being dissolved and being removed from the substrate. In addition the cured films exhibit good gloss, impact resistance and corrosion resistance. The combination of surprising and outstanding properties of the coating compositions of the present invention make them extremely suitable for many applications. They are particularly desirable for coil coating applications wherein the precoated metal coil stock will be utilized to draw metal parts. This process is suitable in the preparation of under-the-hood automobile parts such as gas tanks, oil filters, air filter housings, valve covers and, in addition, for industrial parts.

The following examples are only intended to be illustrative of the invention and are not intended to limit it. EXAMPLE I

This example illustrates the preparation of a polyester polyol according to the present invention:

A reactor vessel was equipped with a mechanical stirrer, thermometer, nitrogen inlet tube and condenser which was set up to remove the methanol and water by-products by a fusion process. The vessel was charged with (A) and the mixture was heated to 180°C and held for 30 minutes to allow the reaction to proceed. The mixture was subsequently heated to 195 °C and held 30 minutes and then heated to

220°C at which temperature 295 grams of methanol were collected. The reaction mixture was then cooled to 180°C followed by the addition of

Charge (B). The mixture was then heated to 240°C and held until an acid value of 11.0 was obtained. The mixture was then cooled to

185 °C, and the condenser was set up for azeotropic reflux. Then Charge (C) was added, and the reaction was heated to 240°C to azeotropically distill the remaining water of condensation. When an acid value of 5.7 was reached, the reaction mixture was cooled to

180°C and Charge (D) was added. The mixture was heated to 240°C to azeotropically remove the water of condensation. When an acid value of 5.4 was reached and a total of 190 grams of water was collected, the reaction product was thinned with 1753 grams AROMATIC 100 and 584 grams of benzyl alcohol. The polyester polyol had a total solids content of 50.4 percent as determined at 110ºC for two hours, an acid value of 2.8 and a weight average molecular weight of 35,013 as determined by GPC. The hydroxyl value of the undiluted polyester polyol was 8.4

EXAMPLE II

This Example illustrates the preparation of a polyester-urethane polyol according to the present invention.

A reactor vessel equipped as described in Example I was charged with (A) and the mixture was heated to 190°C and held for 30 minutes. The reaction mixture was then heated to 200°C and held for

30 minutes to allow the reaction to proceed. The mixture was subsequently heated to 240°C and held at this temperature until 240 grams of methanol were collected. The reaction mixture was then cooled to 180°C and Charge (B) was added. The mixture was then heated to 240°C and held at this temperature until an acid value of 1.0 was reached. The reaction mixture was then cooled to 200°C and Charge (C) was added to the reaction vessel. The mixture was allowed to cool to 80°C. To this polyol solution, Charges (D) and (E) were added, and the temperature was raised to 95 °C. This temperature was maintained until all the isocyanate groups were reacted as determined by infrared spectroscopy. The resultant polyester polyurethane polyol had a total solids content of 49.4 percent as determined at 110ºC for two hours, an acid value of 0.4, a hydroxyl value of 11.9 and a weight average molecular weight of 16,556 as determined by GPC.

EXAMPLE III

This Example illustrates the preparation of a polyester polyol according to the present invention.

A reactor vessel was equipped as described in Example I and charged with (A). The mixture was heated to 180°C and held for 30 minutes to allow the reaction to proceed. The mixture was subsequently heated to 195°C for 30 minutes and then heated to 220°C at which temperature 237 grams of methanol were collected. The reaction mixture was then cooled to 180°C followed by the addition of

Charge (B). The mixture was then heated to 240°C and held until an acid value of 13.1 was obtained. The mixture was then cooled to 180°C and the condenser was set up for azeotropic reflux. Then 60 grams of xylene were added and the reaction was heated to 240°C to azeotropically distill the remaining water of condensation. When an acid value of 5.2 was reached the reaction product was thinned with

1406 grams of AROMATIC 100 (an aromatic distillate commercially available from Exxon Chemical) and 469 grams of D0WANOL PM acetate

(propylene glycol monomethyl ether acetate which is available from Dow Chemical Company). The polyester polyol had an acid value of 3.2, a hydroxyl value of 14.8, a weight average molecular weight of 17,461 as determined by GPC and a total solids content of 60.8 percent as determined at 110°C for two hours.

EXAMPLE IV In this example the polyester polyol of Example I and the polyester-urethane polyol of Example II were formulated into coating compositions and evaluated as detailed below.

(1) This crosslinking agent was an isocyanurate of 1,6-hexamethylene diisocyanate which is commercially available from Bayer as DESMODUR BL-3175. (2) The grind paste was prepared as follows:

(a) This polyester polyol was prepared from 46 percent neopentyl glycol; 10 percent adipic acid, 5 percent terephthalic acid, 30 percent isophthalic acid and 9 percent trimellitic anhydride. It had a hydroxyl number of 90.

The grind paste was prepared by first preblending Charge (A) with high speed Cowles agitation. Charge (B) was then sifted in slowly under low speed Cowles agitation and the mixture ground to a Hegman grind of 7.5⁺ using ceramic beads as the grind media.

Charge (C) was then added slowly under high speed Cowles agitation.

The coating composition was prepared as follows: Charge (A) was placed in a mixing vessel and shear applied using a Cowles blade to mix thoroughly. Charge (B) was added slowly under high speed Cowles agitation to disperse the pigment. Charge (C) was then added with high speed agitation. The resultant coating composition was reduced with toluene to an application viscosity of 34 seconds using a number 4 Zahn Cup. The coating composition was applied using a wire wound rod onto 24 gauge cold rolled steel panels at a dry film thickness of 0.8 mil (20 microns), cured at 240°C peak metal temperature for 50 seconds and then circular blanks were cut from the coated panels. Each circular blank had a diameter of 3.25 inches (82.6 millimeters). The blanks were drawn into square cups having rounded corners using a 90 ton (8 X 10¹⁰ dyne) press obtained from the V&O Press Company, Number

6 press, 40 pounds per square inch (1.7 X 10⁶ dynes/cm²) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder. The rounded corners on the base of each cup were subjected to about 120 percent elongation and about 50 percent compression. Each of the square cups formed had a depth of about 26 millimeters and a width of about 36 millimeters on one side. The cups were then immersed for five minutes in boiling deionized water (wet heat) and evaluated. The films were rated in the following manner. The films passed and were rated 6 if after they were drawn and exposed to wet heat they exhibited no defects. The films which passed and were rated 6 had appearances which were smooth, continuous and essentially defect-free. A rating of 5 and below indicated that the films failed because they exhibited some defect. The entire cup rating system is as follows:

6 - no defects (Pass)

5 - corner softening, wrinkling or stress fracturing (Fail)

4 - one corner delaminated (Fail) 3 - two corners delaminated (Fail)

2 - three corners delaminated (Fail) 1 - four corners delaminated (Fail)

A film passed the test for drawability and met the claims of the present invention only if it was rated a 6 for both draw and wet heat and was non-tacky. The blocking test to determine if the film was essentially tack-free involved placing in contact the coated surfaces of two square blanks (2 inches X 2 inches) (50.8 X 50.8 millimeters) under

1.0 pound per square inch (6.9 X 10⁴ dynes/cm²) pressure for 12 hours at 150°F (65.5°C). If the coating passed the test, the two surfaces could be easily disengaged. If the coating failed, i.e., the film was tacky, the two surfaces could not be separated by reasonable force using fingernails as the only prying instrument. This failure is indicated as a "T" following the rating for drawability and wet heat.

Only the films which failed show the "T" rating. The films were also evaluated for solvent resistance.

Solvent resistance testing like the blocking test was done prior to drawing and wet heat exposure. The cured coating was rubbed using finger pressure with a cheesecloth saturated with methyl ethyl ketone. The number of double rubs required to dissolve and remove the coating from the substrate was recorded. The data for the materials tested are recorded in Table I, below. It should be noted that the polyols were tested with various levels of crosslinking agent.

COMPARATIVE EXAMPLE I

This example illustrates the preparation of a polyester- urethane polyol according to U.S. 4,205,115 which does not meet the requirements of the present invention.

(b) This polyester solution was prepared as follows:

A reactor as described in Example I was charged with (A).

The mixture was heated to 210-220⁰C and held at this temperature range until an acid value of 9.1 was reached. The reaction product was cooled to 120⁰C and Charge (B) was added. The polyester polyol solution had a total solids content of 65.1 percent, an acid value of 4.8, a hydroxyl value of 43.1 and a weight average molecular weight of 14,267 as determined by GPC. (c) This was 4,4'-methylene-bis(cyclohexyl isocyanate) from Bayer. (d) This was an ultraviolet light absorber commercially available from Ciba-Geigy. To prepare the polyester-urethane polyol a reactor vessel was equipped with a mechanical stirrer, thermometer, nitrogen inlet tube and condenser. The vessel was charged with all the ingredients listed above. The mixture was heated under a nitrogen blanket to 60°C and subsequently to 90°C. This temperature was maintained until all of the isocyanate groups were reacted as determined by infrared spectroscopy. The resultant polyester-polyurethane had a total solids content of 55.7 percent as determined at 110°C for two hours, a hydroxyl value of 5.3 and a weight average molecular weight of 146,818 as determined by GPC.

COMPARATIVE EXAMPLE II This example illustrates the preparation of a polyester polyol according to U.S. 4,205,115 which does not meet the requirements of the present invention.

A reactor vessel as described in 1ϊxample I was cha

(A). The mixture was heated to 70°C to melt the neopentyl glycol. At 70°C Charges (B) and (C) were added. The temperature was increased to 135°C and held for 30 minutes to remove the water included in Charge (A). The reaction mixture was then heated to 160°C and held 30 minutes to allow the reaction to proceed. It was then heated to 180°C and held for 30 minutes, and subsequently heated to 220°C and held until an acid value of 4.5 was obtained. The mixture was then cooled to 120°C, and the condenser was set up for azeotropic reflux. Then Charge (D) was added. The reaction mass was heated to 170-175ºC under reflux conditions and held until the viscosity of a dilute solution of the polymer at 65 percent total solids in a 79/21 blend of methyl isobutyl ketone/xylene reached 107.6 seconds as determined by a Gardner Holdt viscosity tube. Then the polyester was thinned from theoretical 90 percent total solids to theoretical 55 percent total solids with 318 parts of methyl isobutyl ketone. The polyester polyol solution had a total solids content of 54.4 percent as determined at 110°C for two hours, an acid value of 5.7 and a weight average molecular weight of 99.503 as determined by GPC.

COMPARATIVE EXAMPLES III - IV The polyester polyol and polyester-urethane polyol of comparative examples II and I, respectively, were formulated into coating compositions as described in Example IV. Different levels of crosslinking agent were tested. Both the comparative polyester polyol and polyester-urethane polyol failed the requirements of the compositions of the present invention. Although one of experiments done with the comparative polyester-urethane was rated a 6 for drawability and wet heat resistance, the composition was tacky and therefore failed. The film was so tacky that the test panels became blocked together without the application of any pressure, and could not be disengaged.

The data for the comparative tests are shown in Table II, below.

COMPARATIVE EXAMPLE V

This example illustrates that the use of aminoplast crosslinking agents is not preferred for use in the present invention. A series of three coating compositions was prepared using the polyester polyol of Example I, above, and three different levels of aminoplast crosslinking agent (CYMEL 1123, a highly methylated/ethylated benzoguanamine commercially available from

American Cyanamid) 2 percent, 5 percent and 10 percent. The coating compositions were evaluated for drawability, wet heat resistance and solvent resistance as detailed in Example IV, above. As the data below shows, all of the coating compositions failed the test for drawability.

EXAMPLES V to XI The following examples in Table III illustrate various polyester polyols and polyester-urethane polyols according to the present invention. The polyols were formulated into coating compositions and evaluated as described in Example IV, above. It is believed that some of the coating compositions failed the test for drawability at certain levels of crosslinking agent within the range claimed by the present invention because the particular level of crosslinker was not preferred for the particular polymeric polyol. As has been stated above in the specification, the specific amount of crosslinking agent within the claimed range can vary from polymer to polymer and depends upon the precise parameters of the particular polyester polyol or polyester-urethane polyol to be crosslinked. The following abbreviations are used and have the meanings indicated:

TMP is trimethylolpropane

NPG is neopentyl glycol

EG is ethylene glycol

HD is 1,6-hexanediol

DMT is dimethyl terephthalate

IPA is isophthalic acid

C-M3 is a dibasic acid commercially available from

E. I. DuPont de Nemours as Corefree M3

PA is phthalic anhydride

AA is adipic acid

MDI is 4,4'-diphenyl methylene diisocyanate

DMCD is dimethyl 1,4-cyclohexane dicarboxylate T means tacky

Claims

WHAT IS CLAIMED IS:

1. A coating composition comprising as a film former a polyester polyol having a controlled amount of branching and having a weight average molecular weight of from about 4,000 to about 40,000, a hydroxyl value of from about 10 to about 100 and formed from the reaction of:

(a) a diol component, (b) a dicarboxylic acid component or a functional equivalent thereof, (c) a polyfunctional component in which the functionality is selected from hydroxyl, carboxylic acid or functional equivalent thereof or a combination of the two functional groups, the amount of the polyfunctional component (c) being adjusted to incorporate up to about 0.25 gram-mole of polyfunctional component per 500 grams of the total reactants used to prepare the polyester polyol; and a polyisocyanate curing agent in an amount such that the isocyanate to hydroxyl equivalent ratio ranges from about 0.3/1 to about 1.5/1 with the proviso that the coating composition when applied direct to a 24 gauge cold rolled steel circular blank having a diameter of about 3.25 inches (82.6 millimeters) at a dry film thickness of about 0.8 mil (20 microns), cured at about 240°C peak metal temperature for about 50 seconds, and drawn into a square cup with rounded corners subjected to about 120 percent elongation and about 50 percent compression on the rounded corners of the base of the cup using a 90 ton (8 X 10¹⁰ dyne) press, about 40 pounds per square inch (2.7 X 10⁶ dynes/cm²) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder, the square cup formed having a depth of about 26 millimeters, rounded corners each corner having an outer radius of about 6.5 millimeters, and a width of about 36 millimeters on a side, followed by five minutes immersion in boiling deionized water results in an essentially tack-free, essentially defect-free film.

2. The coating composition of claim 1 wherein the film former is said polyester polyol which has been chain extended with a polyisocyanate to form a polyester-urethane polyol.

3. A coating composition comprising as a film former a polyester polyol having a controlled amount of branching and having a weight average molecular weight of from about 4,000 to about 40,000, a hydroxyl value of from about 10 to about 100 and formed from the reaction of a polyol component comprising a positive amount not exceeding 0.25 gram-mole of a material having greater than two hydroxyl groups per molecule per 500 grams of the total reactants used to prepare the polyester polyol and a polycarboxylic acid component or functional equivalent thereof; and a polyisocyanate curing agent in an amount such that the isocyanate to hydroxyl equivalent ratio ranges from about 0.3/1 to about 1.5/1 with the proviso that the coating composition when applied direct to a 24 gauge cold rolled steel circular blank having a diameter of about 3.25 inches (82.6 millimeters) at a dry film thickness of about 0.8 mil (20 microns), cured at about 240°C peak metal temperature for about 50 seconds, and drawn into a square cup with rounded corners subjected to about 120 percent elongation and about 50 percent compression on the rounded corners of the base of the cup using a 90 ton (8 X 10¹⁰ dyne) press, about 40 pounds per square inch (1.7 X 10⁶ dynes/cm²) air pressure on an approximately 16 inch (406 millimeter) diameter cylinder, the square cup formed having a depth of about 26 millimeters, rounded corners each corner having an outer radius of about 6.5 millimeters, and a width of about 36 millimeters on one side, followed by five minutes immersion in boiling deionized water results in an essentially tack-free, essentially defect-free film.

4. The coating composition of claim 3 wherein the material having greater than two hydroxyl groups is a triol.

5. The coating composition of claim 4 wherein the film former is said polyester polyol which has been chain extended with a polyisocyanate to form a polyester-urethane polyol.

6. The coating composition of claim 4 wherein the weight average molecular weight of the polyester polyol ranges from about 5,000 to about 40,000.

7. The coating composition of claim 6 wherein the weight average molecular weight of the polyester polyol ranges from about 7,500 to about 40,000.

8. The coating composition of claim 7 wherein the weight average molecular weight of the polyester polyol ranges from about 10,000 to about 40,000.

9. The coating composition of claim 8 wherein the weight average molecular weight of the polyester polyol ranges from about 15,000 to about 30,000.

10. The coating composition of claim 9 wherein the weight average molecular weight of the polyester polyol ranges from about 16,500 to about 25,000.

11. The coating composition of claim 4 wherein the hydroxyl value ranges from about 10 to about 70.

12. The coating composition of claim 11 wherein the hydroxyl value ranges from about 10 to about 50.

13. The coating composition of claim 12 wherein the hydroxyl value ranges from about 10 to about 30.

14. The coating composition of claim 4 wherein the amount of triol is a positive amount not exceeding 0.20 gram-mole.

15. The coating composition of claim 14 wherein the amount of triol is a positive amount not exceeding 0.15 gram-mole.

16. The coating composition of claim 14 wherein the amount of triol ranges from about 0.03 gram-mole to about 0.20 gram-mole.

17. The coating composition of claim 4 wherein the coating composition is organic solvent based.

18. The coating composition of claim 4 wherein the ingredients of the polyester polyol contain at least 5 percent of aromatic moieties.

19. The coating composition of claim 18 wherein the proportions of ingredients of the polyester polyol are adjusted such that there is a weight percentage of aromatic moieties in the polyester which ranges from about 5 percent to about 40 percent.

20. The coating composition of claim 19 wherein the percentage ranges from about 20 percent to about 35 percent.

21. The coating composition of claim 4 wherein the ingredients of the polyester polyol contain at least 5 percent of cycloaliphatic moieties.

22. The coating composition of claim 21 wherein the proportions of ingredients of the polyester polyol are adjusted such that there is a weight percentage of cycloaliphatic moieties which ranges from about 5 to about 40 percent.

23. The coating composition of claim 22 wherein the proportions of ingredients of the polyester polyol are adjusted such that there is a weight percentage of cycloaliphatic moieties in the polyester which ranges from about 20 percent to about 35 percent.

24. The coating composition of claim 5 wherein the weight average molecular weight of the polyester-urethane polyol ranges from about 5,000 to about 40,000.

25. The coating composition of claim 24 wherein the weight average molecular weight of the polyester-urethane polyol ranges from about 7,500 to about 40,000.

26. The coating composition of claim 25 wherein the weight average molecular weight of the polyester-urethane polyol ranges from about 10,000 to about 40,000.

27. The coating composition of claim 26 wherein the weight average molecular weight of the polyester-urethane polyol ranges from about 15,000 to about 30,000.

28. The coating composition of claim 27 wherein the weight average molecular weight of the polyester-urethane polyol ranges from about 16,500 to about 25,000.

29. The coating composition of claim 5 wherein the hydroxyl value ranges from about 10 to about 70.

30. The coating composition of claim 29 wherein the hydroxyl value ranges from about 10 to about 50.

31. The coating composition of claim 30 wherein the hydroxyl value ranges from about 10 to about 30.

32. The coating composition of claim 5 wherein the amount of triol is a positive amount not exceeding 0.20 gram-mole.

33. The coating composition of claim 32 wherein the amount of triol is a positive amount not exceeding 0.15 gram-mole.

34. The coating composition of claim 32 wherein the amount of triol ranges from about 0.03 gram-mole to about 0.20 gram-mole.

35. The coating composition of claim 5 wherein the coating composition is organic solvent based.

36. The coating composition of claim 5 wherein the ingredients of the polyester-urethane polyol contain at least 5 percent of aromatic moieties.

37. The coating composition of claim 36 wherein the proportions of ingredients of the polyester-urethane polyol are adjusted such that there is a weight percentage of aromatic moieties in the polyester which ranges from about 5 percent to about 40 percent.

38. The coating composition of claim 37 wherein the percentage ranges from about 20 percent to about 35 percent.

39. The coating composition of claim 5 wherein the ingredients of the polyester-urethane polyol contain at least 5 percent of cycloaliphatic moieties.

40. The coating composition of claim 39 wherein the proportion of ingredients of the polyester-urethane polyol is adjusted such that there is a weight percentage of cycloaliphatic moieties which ranges from about 5 percent to about 40 percent.

41. The coating composition of claim 5 wherein the amount of chain extending polyisocyanate is a positive amount which does not exceed 15 percent by weight based on the total weight of all of the ingredients of the polyester-urethane polyol.

42. The coating composition of claim 41 wherein the amount of chain extending polyisocyanate does not exceed 10 percent.

43. The coating composition of claim 40 wherein the amount of chain extending polyisocyanate ranges from about 1 percent to about 15 percent by weight.

44. The coating composition of claim 4 with the further proviso that when the amount of triol is within the range of from about 0.03 gram-mole to about 0.07 gram-mole the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.7/1 to about 1.5/1; and when the amount of triol is within the range of from about 0.10 gram-mole to about 0.25 gram-mole the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.3/1 to about 0.7/1.

45. The coating composition of claim 4 wherein when the amount of triol is 0.25 gram-mole the isocyanate to hydroxyl equivalent ratio is in the range of from about 0.3/1 to about 0.4/1.

46. The coating composition of claim 5 with the further proviso that when the amount of triol is within the range of from about 0.03 gram-mole to about 0.07 gram-mole the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.7/1 to about 1.5/1; and when the amount of triol is within the range of from about 0.10 gram-mole to about 0.25 gram-mole the isocyanate to hydroxyl equivalent ratio is within the range of from about 0.3/1 to about 0.7/1.

47. The coating composition of claim 5 wherein when the amount of triol is 0.25 gram-mole the isocyanate to hydroxyl equivalent ratio is in the range of from about 0.3/1 to about 0.4/1.