Storage Stable One Component Urethane Compounds and Method for Making and Using Same

(A) Field of the Invention

This invention relates to polyurethane coating compositions and more particularly relates to one-component storage stable polyurethane compositions.

(B) History of the Prior Art

In the prior art, it has been known that polyurethanes can be formed by the reaction of isocyanates with active hydrogen compounds such as polyols (polyfunctional alcohols). Since isocyanates and active hydrogen compounds usually react very quickly even at room temperature, the two components to form the polyurethane compositions usually must be stored separately and mixed together only at the time when the coating is to be used.

Several approaches have been investigated to provide a one-component composition in which the reacting materials are combined before using and then later activated. For example, prepolymer isocyanates have been converted into derivatives of isocyanates which regenerate the isocyanate only upon heating. This heating process has several disadvantages especially when the polyurethane is to be used as a coating. It is, for example, very difficult to heat a very large surface or to heat a surface which is temperature sensitive.

Another technique for the manufacture of one-component polyurethane systems is the formation of moisture cure systems. This process involves the formation of an isocyanate prepolymer which reacts with moisture from the atmosphere to form a polymeric coating. Although this type of system has met with large success, it nevertheless suffers from certain disadvantages. For example, heavy coatings are difficult to make because of the requirement to release carbon dioxide formed by the reaction of the isocyanate with water. Furthermore, since the chain extender is water, polymer characteristics are not as good as when polyols or polyamines are used as the chain extenders in the two part urethane coating compositions. It is therefore desirable to provide polymer forming compositions which are only one-component in nature, which are stable on storage and which can be cured in the absence of any excessive heating and without the specific addition of other materials.

It is disclosed in U.S. Pat. Nos. 3,743,626; 4,024,117 and 4,032,686 that certain oxazolidine compositions and polyfunctional aliphatic or aromatic isocyanates can be cured in the presence of moisture to give polymeric materials which are suitable for coatings. It is further disclosed that some such compositions can be stored in the absence of moisture without significant reaction between the oxazolidine and the isocyanate. Unfortunately such compositions, as disclosed in the foregoing U.S. Patents, have serious disadvantages. In particular, such prior art coatings in general have very poor color. In addition, abrasion resistance is not as high as desired. Furthermore, the oxazolidine compounds used in these prior art compositions were difficult to prepare due to long, complex or difficult synthesis routes. High solids in the coating compositions and good strength in the coatings obtained by good crosslinking were also difficult to obtain. Additional disadvantages of many of such prior art compounds were that they were solids or waxes which were difficult to dissolve or disperse, they were unstable during storage or required polymer chains of specific and limited chemical structure, e.g. only those formed from certain lactones.

Additionally, except for certain compositions described in U.S. Pat. No. 4,381,388, such coatings are usually not high performance coatings, i.e., they do not have the hardness, strength, or weather resistance suitable for high performance applications such as those encountered in exterior environments.

In accordance with the present invention, there is provided a storage stable one-component air curable urethane coating composition having improved color and abrasion resistance. In addition, the composition is more easily prepared than prior art oxazolidine compositions and can optionally use inexpensive polyether polyol isocyanate prepolymers or in high performance compositions can use polyester or polycarbonate polyol isocyanate prepolymers. Additionally, high solids in the coating compositions and good crosslinking can easily be obtained.

The composition comprises at least twenty percent by weight of resin solids of a compound comprising at least one oxazolidine ring chemically combined from the nitrogen atom of the ring through an alkylene-urethane linking group to an isophorone diisocyanate prepolymer radical, which prepolymer radical contains at least one isocyanate group. Twenty percent by weight of resin solids are used herein means twenty percent by weight of the compounds in the composition which will react together to form a solid urethane resin. The oxazolidine ring in the compound will react with moisture in the air which is believed to form hydroxy and amine groups which are in turn believed to react with the isocyanate radicals to form a cured urethane-urea compound. The compound may be a liquid and is very storage stable.

The invention also comprises a method for preparing the compound and composition and a method for coating a surface by applying the composition of the invention to the surface and exposing the applied composition to air having a relative humidity of at least 10% and includes an article coated with the cured composition.

The oxazolidine-isocyanate compound is usually a compound of the formula: ##STR1## wherein R.sub.1 and R.sub.2 are independently at each occurrence hydrogen, lower alkyl, substituted lower alkyl of 1 to 12 carbon atoms, phenyl or substituted phenyl. R.sub.3 is a substituted or unsubstituted alkylene group of two or three carbon atoms excluding substituents. A is an isophorone diisocyanate prepolymer radical containing a urethane-alkylene group which is connected to the nitrogen atom of the ring. Suitable substituents in the R.sub.1, R.sub.2 and R.sub.3 groups are substituents such as halogen, alkyl, isocyanate, and ester groups which do not contain an active hydrogen atom, i.e., a hydrogen atom which will react with an isocyanate group. Substituents which are unsuitable therefore include aliphatic or aromatic hydroxy groups, acid groups and primary or secondary amine groups. "Lower alkyl" usually means from 1 to 6 carbon atoms.

A particularly desirable oxazolidine-isocyanate compound is represented by the formula: ##STR2## where (OCN).sub.2 R.sub.x -- is the residue of a polycarbonate, polyether or polyester isocyanate prepolymer, and R.sub.1 is independently at each occurrence hydrogen, lower alkyl, substituted lower alkyl, phenyl or substituted phenyl. Most preferably R.sub.1 is hydrogen.

The method for the manufacture of the composition of the invention comprises reacting an oxazolidine ring compound containing a hydroxy group connected through a substituted or unsubstituted alkylene group to the nitrogen atom of the ring, with a polycarbonate, polyester or polyether isophorone isocyanate prepolymer containing at least two isocyanate groups. The equivalent weight ratio of oxazolidine to prepolymer is usually between about 1:6 and about 4:6 and most commonly between about 1:3 and about 1:5. The equivalent weight of prepolymer is the weight of prepolymer required to give one mole of reactive isocyanate groups and the equivalent weight of oxazolidine is the weight required to give one mole of reactive hydroxy groups when the oxazolidine ring is unopened.

The prepolymer used in accordance with the present invention is a prepolymer formed from a polyester, polycarbonate or polyether polyol, a low molecular weight triol and isophorone diisocyanate. The prepolymer is prepared by reaction of the polyol and triol with an excess of isocyanate to yield an isocyanate terminated prepolymer. Usually the equivalent weight ratio of isophorone diisocyanate to combined polyester polyol and low molecular weight polyol is from 1.4:1 to 2.5:1. Most desirably the ratio is about 2:1. Suitable reaction temperatures are usually between about 75.degree. and 150.degree. C. and the reaction time is usually between about 1 hour and 8 hous. Desirably, a catalyst such as dibutyltin dilaurate or the like is present in the reaction mixture to reduce the time for preparing the prepolymer.

The polyol can have a hydroxy functionality of between 2 and 3 and may be prepared by methods known in the art. Polyester polyols, for example, may be prepared by known means of reacting a dibasic acid with a diol to form a difunctional polyester polyol. A trifunctional alcohol such as trimethylol-propane can be incorporated therein to increase the functionality towards a preferred maximum of 3. Polyester polyols can also be made by the opening and subsequent polymerization of lactone rings such as caprolactone, butyrolactone or valerolactone. Such a polyester polyol that can be used in carrying out the invention is based on E-caprolactone. A polycaprolactone-diol with a functionality of 2 and made by polymerization of the monomer in the presence of a difunctional initiator can be used as well as a polycaprolactonetriol which is made by polymerization in the presence of a trifunctional initiator. Still another polyester polyol useful in this invention is a polycarbonatediol or triol, an ester of carbonic acid. These ester diols or triols are made by transesterification of monomeric carbonate esters with the proper glycols. The polyol usually has a molecular weight of between about 300 and about 2,000 and preferably from about 450 to about 1,500. Polycaprolactone esters have been reported to impart improved weather resistance and hydrolytic stability to polyurethanes containing such polyesters.

In addition to the polyester, polycarbonate, polyether or other polyol, the reaction mixture to prepare the prepolymer must contain a low molecular weight polyol, i.e., a polyol having a molecular weight below 500 and preferably below 400. The low molecular weight polyol is usually a triol such as glycerine, trimethylolethane, trimethylolpropane, hexanetriol or tris(2-hydroxyethyl) isocyanurate. The low molecular weight polyol could, however, be a tetrol such as pentaerythritol. The most preferred polyol is tris(2-hydroxyethyl) isocyanurate. The low molecular weight polyol is present in the prepolymer reaction mixture at an equivalent weight ratio (based upon a comparison of hydroxy groups) of polyester polyol to low molecular weight polyol of from 0.25:1 to 5:1 and preferably 0.5:1 to 3:1.

The reaction mixture to prepare the prepolymer for use in the compound of the invention must also contain isophorone diisocyanate. Sufficient diisocyanate is used to react with essentially all of the hydroxy groups of the polyester polyol and the low molecular weight polyol to form a prepolymer terminated with isocyanate groups. The equivalent weight ratio of isophorone diisocyanate to combined polyol and low molecular weight polyol is from 1.4:1 to 2.5:1 and is preferably about 2:1.

The isophorone diisocyanate used in the reaction to form the prepolymer of the composition is a diisocyanate well known in the art and is also known as 3-isocyanatomethyl-3',5,5-trimethyl cyclohexylisocyanate and is often abbreviated as IPDI.

The hydroxy oxazolidine which is reacted with the prepolymer usually has the formula: ##STR3## wherein R.sub.1 and R.sub.2 are independently at each occurrence hydrogen, lower alkyl, substituted lower alkyl of one to 12 carbon atoms, phenyl or substituted phenyl. R.sub.3 is a substituted or unsubstituted alkylene group of two or three carbon atoms excluding substituents and R.sub.4 is lower alkyl.

The hydroxy oxazolidine is most commonly ##STR4## where R.sub.1 is independently at each occurrence hydrogen, lower alkyl, substituted lower alkyl, phenyl or substituted phenyl.

Many of such hydroxy oxazolidines can be easily made by condensing an appropriate aldehyde or ketone with an appropriate dihydroxyl alkyl or substituted alkyl amine, such as diethanol amine.

A particularly desirable curing agent is hydroxyethyl oxazolidine wherein R.sub.1 is hydrogen. Hydroxyethyl oxazolidine (HEOX) can be made by the reaction between formaldehyde and diethanolamine. Other hydroxy oxazolidines which are useful in carrying out the objects of this invention include hydroxyethyl dimethyloxazolidine made by reacting acetone with diethanolamine and hydroxyethyl-methylethyl oxazolidine made by condensing methylethyl ketone with diethanolamine.

The quantity of hydroxyethyl oxazolidine curing agent used in combination with the isocyanate prepolymer is that amount necessary for reacting the hydroxy group with only some of the three available isocyanate groups, leaving the others for reaction with the amine and hydroxy group formed after exposure of the oxazolidine ring to atmospheric moisture. The equivalent weight ratio of oxazolidine to prepolymer in the reaction is usually between about 1:6 and about 4:6 and is most commonly between 1:3 and 1:5.

The reaction temperature to form the oxazolidine-isocyanate compound from the prepolymer and the hydroxyoxazolidine is usually between about 75.degree. and 100.degree. C. and the reaction time is usually between about 15 minutes to one hour. It is to be understood that in reacting the hydroxyoxazolidine with the prepolymer, it is believed that in most cases only one hydroxyoxazolidine will react with a prepolymer molecule. This is especially true when there is a low equivalent weight ratio of oxazolidine to prepolymer. However, it is expected that more than one hydroxyoxazolidine will sometimes react with a single prepolymer molecule. This is especially true when a high equivalent weight ratio of oxazolidine to prepolymer is used.

The composition disclosed in this invention differs from those described in U.S. Pat. Nos. 4,381,388 and 3,743,626 in that the prepolymer is partially reacted with the oxazolidine curing agent; whereas, in the aforementioned patents, initial reaction takes place only when the product is exposed to atmospheric moisture. While not wishing to be bound by any particular theory, it is believed that the polymeric urethane formed from the composition of the invention results from a series of reactions involving atmospheric moisture, i.e. ##STR5## where the long lines to the NCO groups represent prepolymer chains.

The resulting amino alcohol produced contains active hydrogens which rapidly react with isocyanate groups from a different prepolymer or possibly from the same prepolymers. In essence, a highly crosslinked network is obtained upon exposure to atmospheric moisture. This coating thus exhibits better abrasion resistance and strength. In addition, when appropriate prepolymers are used, hardness and weather resistance are improved.

In making coating compositions in accordance with the present invention, the compound of the invention is generally incorporated into a carrier which is usually a solvent for the compound. The coating composition may contain small or large percentages of the compound, e.g. from a few percent to in excess of 50% by weight. The only maximum limitation upon the percentage of compound permitted is determined by the viscosity of the coating composition, i.e. whether the composition can practically be applied. In addition, the percentage may depend upon the particular carrier used and its ability to dissolve or disperse the compound. In general, the solvent or other carrier should not contain active hydrogens which will react with the compound. For this reason, water, alcohols, and phenols are undesirable. In addition, other carriers should be essentially anhydrous. Examples of suitable solvents are xylene and ethoxyethyl acetate.

When the composition is coated upon a surface, atmospheric moisture initiates polymerization of the composition to form a urethane coating. A trace amount of moisture is usually all that is necessary to initiate the reaction and cure the composition.

The relative humidity of the atmosphere to which the composition is exposed is, however, desirably at least 10% and preferably 40% or higher.

The composition of the invention can be used in a variety of applications including high performance applications to form articles having weather resistant top coats for substrates such as metal, plastic and wood and as adhesives and tie coats for both natural and synthetic materials such as paper, textiles, glass, wood, plastics, metal or leather.

The composition of the invention is storage stable, i.e., the one part composition can be stored at high solids at relatively low viscosity for months and by using an appropriate prepolymer can be made weather resistant, i.e., can be exposed to sunlight and water for months without a significant change in strength or surface appearance. In addition, as previously discussed, the composition results in coatings having improved abrasion resistance and superior color.

The following examples serve to illustrate and not limit the present invention.

Into a reactor equipped with an agitator, a Dean Stark water removal apparatus, a thermometer and reflux condensor were charged 338 grams (1.13 equiv.) of TONE.RTM.-0310 polycaprolactone triol polyester, molecular weight of 900, available from Union Carbide, 98 grams (1.13 equiv.) of tris(2-hydroxyethyl)isocyanurate, 347 grams of xylene and 347 grams of ethoxyethyl acetate. The mixture was heated to reflux and held there until no more water was removed in the Dean Stark trap. The temperature was then increased to 125.degree.-130.degree. C. and 502 grams (4.52 equiv.) of isophorone diisocyanate were added as rapidly as possible, while maintaining the temperature between 125.degree. and 130.degree. C. After a one-hour hold, 0.9 grams of dibutyltin dilaurate was added and the solution heated at 125.degree.-130.degree. C. for another hour. The prepolymer (containing 2.26 equiv. of NCO) was then cooled to 70.degree. C. and 88 grams (0.75 equiv.) of 2-hydroxyethyl oxazolidine (HEOX) was added all at once. The solution was then heated to 90.degree. C. and held there for one more hour. At the end of this time, a product which exhibited the following properties was obtained: Non-Volatiles, 60%, Viscosity 9 Stokes, NCO Content 3.7%, Gardner Color 1-. As a control, the same product was made wherein Acryloid AU-568, a Rohm & Haas acrylic modified oxazolidine was used as the curing agent. The physical properties of the coatings are summarized below.

The data indicate the improvement in hardness and gloss retention as well as abrasion resistance and color.

The procedure of Example I was followed except that 50.9 grams (1.13 equiv.) of trimethylolpropane (TMP) was substituted for the tris(2-hydroxyethyl) isocyanurate (THEIC) in making the prepolymer. The amount of HEOX curing agent used was based on an equivalency of NCO/OH of 3/1. A product exhibiting a solids content of 60.5%, a viscosity of 6 Stokes, an NCO content of 2.7%, and a Gardener Color of 1- was obtained. Properties of the coatings were as follows:

The data show the improvement in hardness and color along with better weathering characteristics, as indicated by retention of gloss.

Into a reactor equipped with an agitator, thermometer, reflux condensor and nitrogen inlet tube are charged 536 grams (0.563 equiv.) Permanol KM10-1122 (a 2000 molecular weight polycarbonatediol available from the Permuthane Division of Beatrice Foods), 72.3 grams (1.607 equiv.) of trimethylolpropane and 769 grams of propylene glycol monomethylether acetate solvent. The mixture is heated to effect solution after which 475 grams (4.279 equiv.) of isophorone diisocyanate and 0.5 grams of dibutyltin dilaurate are added all at once. The temperature is slowly heated to 90.degree. C. and held there until an NCO value of 4.8% is obtained. The prepolymer is next cooled to 70.degree. C. and 82.3 grams of HEOX (0.703 equiv.) are added. The temperature is increased to 85.degree. C. and held there for another hour. At the end of this time, a product with the following characteristics is obtained: Non-Volatiles 60%, Viscosity 10 Stokes, NCO Content 3.05%, Gardner Color 1-. As a control, the product was made using Acryloid AU-568. It exhibited the following properties: Non-Volatiles 66%, Viscosity 8 Stokes, NCO Content 3.0% and Gardner Color 4+. The physical properties are summarized below.

The results indicate the improvement in gloss retention, abrasion resistance, and color with the HEOX based products.