Abstract:
A two component polyurea coating composition that exhibits a dual cure phenomena. The coating composition comprises a polyaspartic ester that is combined together with a polyisocyanate in such a manner that the polyisocyanate is present is an amount that is greater than a normal stoichiometric amount for the polyaspartic ester. By over indexing the polyaspartic ester with the polyisocyanate advantages of moisture curing and or “fast curing” can be combined together in the final finish.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a polyurea coating composition that can be applied as a wet finish on any substrate. More particularly, the present invention relates to a two component polyurea coating composition that exhibits a dual cure phenomena.  
           [0002]    Two component coating compositions containing a polyisocyanate component in combination with an isocyanate-reactive such as a polyhydroxyl component or a polyamine are known. These coating compositions are suitable for the formation of high quality coatings and can be adjusted to produce coatings which are hard, elastic, abrasion resistant, solvent resistant and weather resistant.  
           [0003]    Generally, there are two mechanisms by which the curing of polyurea coating compositions takes place—moisture cure or plural component “fast cure” which involves cross-linking the polyisocyanate component with an amine. Aliphatic coating compositions which rely upon moisture cure demonstrate very slow curing times which can limit their use in some applications. Coating compositions which rely upon plural “fast cure” are susceptible to adhesion problems when the curing proceeds too quickly.  
           [0004]    In accordance with the present invention, polyurea coating compositions based on a two component system of a polyisocyanate component and a polyaspartic ester isocyanate-reactive component are produced which demonstrate a dual cure phenomena which results in improved film properties and curing times.  
         SUMMARY OF THE INVENTION  
         [0005]    According to other features, characteristics, embodiments and alternatives of the present invention which will become apparent as the description thereof proceeds below, the present invention provides a polyurea coating composition that exhibits a dual cure phenomena, the polyurea coating composition including:  
           [0006]    a polyaspartic ester; and  
           [0007]    a polyisocyanate,  
           [0008]    wherein the polyisocyanate is present in an amount that is greater than a normal stoichiometric amount for the polyaspartic ester.  
           [0009]    The present invention further provides a method of preparing a polyurea coating composition which involves:  
           [0010]    providing a polyaspartic ester;  
           [0011]    providing a polyisocyanate; and  
           [0012]    mixing the polyaspartic ester and the polyisocyanate together so that the polyisocyanate is present in an amount that is greater than a normal stoichiometric amount for the polyaspartic ester.  
           [0013]    The present invention also provides a surface finish which comprises a cured composition that includes a polyaspartic ester and a polyisocyanate, wherein the polyisocyanate is present in an amount that is greater than a normal stoichiometric amount for the polyaspartic ester prior to curing.  
           [0014]    The present invention still further provides a method for forming a surface finish which involves:  
           [0015]    providing a polyaspartic ester;  
           [0016]    providing a polyisocyanate;  
           [0017]    mixing the polyaspartic ester and the polyisocyanate together so that the polyisocyanate is present in an amount that is greater than a normal stoichiometric amount for the polyaspartic ester;  
           [0018]    applying the mixed composition to a surface to form a surface coating; and  
           [0019]    allowing the applied surface coating to cure. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]    The polyurea coating compositions of the present invention provide a hybrid curing system that combines the “fast cure” of a polyaspartic ester polyurea reaction with the enhanced adhesion and superior film properties of a slower curing moisture cure polyurea. The polyurea coating compositions of the present invention demonstrate enhanced adhesion, rapid cure rates and light stability, and can be used to produce bubble free, low to high film builds with thicknesses that range from less than 1 mil to greater than 20 mil.  
         [0021]    The coating compositions of the present invention comprise two component polyureas that have exceptional direct-to-substrate adhesion and are based the use of a polyaspartic ester that is over indexed with a polyisocyanate. One component is a polyaspartic ester based component that can be pigmented or clear and incorporated with or without solvents. The other component is a polyisocyanate that can be incorporated with or with out solvents.  
         [0022]    Suitable polyisocyanates for use in accordance with the present invention include aliphatic polyisocyanates such as hexamethylenediisocyanate (HDI) and lysine diisocyanate; alicyclic polyisocyanates such as dicyclohexylene diisocyanate, isophorone diisocyanate (IPDI), cyclohexane diisocyanate (CHDI); aromatic polyisocyanates such as tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), naphthylene diisocyanate (NDI), xylylene diisocyanate (XDI) and tetramethylxylylene diisocyanate (TMXDI); and mixtures thereof. Higher functional Biruet polyisocyanates are usually preferred over trimers, dimers, and hexamethylenediisocyanate (HDI) was found to be particularly useful for purposes of the present invention.  
         [0023]    In formulating the coating compositions of the present invention, the polyaspartic ester is over indexed with an aliphatic polyisocyanate. That is, the polyisocyanate was used in an amount that is greater than the stoichiometric amount one would normally calculate for a specific amount of a polyaspartic ester. During the course of the invention, the applicant determined that measurable improvements in the film properties of a coating produced from the polyurea coating compositions of the present invention were obtained at an over indexing of the polyaspartic ester to a polyisocyanate at above about 1.5 NCO to NH. Optimum film properties were obtained without the use of a secondary catalyst at an over indexing of the polyaspartic ester to a polyisocyanate at above about 2.5±0.5 NCO to NH. When over indexing the polyaspartic ester with a polyisocyanate above about 3.0 NCO to NH, without the use of a secondary catalyst, the air dry cure times increase unfavorably.  
         [0024]    Although not intending to be bound by any particular theory, and understanding that an applicant need not comprehend the scientific principles on which the practical effectiveness of his invention rests, applicant theorizes that by selectively over indexing the polyaspartic ester to the polyisocyanate, it is possible to reach an optimum balance between moisture curing and “fast curing” which involves cross-linking the polyisocyanate component with an aliphatic amine. When the mechanism of moisture curing predominates, surface adhesion is optimized; however, the curing times are very long and the film is susceptible to the formation of CO 2  bubbles when the applied dried film thickness exceeds 5 mil, or under high humidity conditions. When the cross-linking mechanism associated with fast curing predominates, surface adhesion is reduced in favor of quicker curing times. Applicant has determined that within an over indexing range of from about 1.5 up to about 3.0 of NCO to NH, the polyurea coating compositions of the present invention demonstrate a dual cure property in which the cross-linking mechanism associated with fast curing causes the surface of a coating to dry relatively fast, while the mechanism of moisture curing at the interface between the coating layer and substrate allows the coating composition to cure more slowly and thereby develop good adhesive properties.  
         [0025]    The polyurea coating compositions of the present invention can be applied to virtually any surface as a wet coating which can be applied in any conventional manner such as spraying, dipping, brushing, etc. Once applied, if desired, the coatings can be air dried or forced dried according to conventional methods. The coating compositions can be suitably applied over a temperature range of about 40° F. to about 95° F. and relative humidity levels of about 40% to about 95%.  
         [0026]    The polyurea coating compositions of the present invention have been found to produce finishes that have strong adhesion properties, high tensile strengths, chemical resistance to solvents and other chemical agents, resistance to ultraviolet light, and excellent color and gloss retention. The finishes are hard as well as impact and chip resistant, and can be recoated as desired.  
         [0027]    It is noted that the polyurea coating compositions can include single polyaspartic esters or blends of polyaspartic with or without additional catalytic agents. In addition, the compositions can include other conventional components such as pigments, dyes, fillers, carriers, solvents, surface texturing agents, etc. For convenience of field use, the two components of the compositions can be formulated to be mixed in a 1:1 ratio. Such a mixing ratio eliminates the need for measuring different amounts of the two components. The coating compositions have been determined to be particularly useful as an alternative to conventional coatings that require baking, when the parts or articles to be coated are too large or otherwise unsuitable for baking.  
         [0028]    The following non-limiting examples were conducted to evaluate performance characteristics of the polyurea coating compositions of the present invention. The polyurea coating compositions tested in the following examples were non-pigmented clear coats that were applied at a dry film thickness (DFT) of 2 mil. The coating compositions were batch mixed and air spray applied.  
         [0029]    Performance characteristics in the examples were evaluated using the following evaluation scale (ranging from 0 to 5):  
         [0030]    0=Total Failure  
         [0031]    1=Near Total Failure  
         [0032]    2=Partial Failure  
         [0033]    3=Marginal  
         [0034]    4=Acceptable  
         [0035]    5=Excellent 
     
    
     EXAMPLE 1  
     Crosshatch Adhesion  
       [0036]    In this Example, non-pigmented coatings were tested according to the procedure set forth in ASTM 1-3359-95, Test Method B. The test results are presented in Table 1 below:  
                           TABLE 1                               Bonderite 1000 Pre-           Stoichiometric   Untreated Cold   Treated Cold Rolled   Grit Blasted       Index   Rolled Steel   Steel   Steel                   1.00   0   4   2       1.25   0   4   2       1.50   1   5   3       1.75   2   5   4       2.00   3   5   4       2.25   4   5   5       2.50   5   5   5       2.75   5   5   5       3.00   5   5   5                  
 
       EXAMPLE 2  
     Conical Mandrel  
       [0037]    In this Example, non-pigmented coatings were tested according to the procedure set forth in ASTM D 522-93, Test Method A. The test results are presented in Table 2 below.  
                           TABLE 2                               Bonderite 1000 Pre-           Stoichiometric   Untreated Cold   Treated Cold Rolled   Grit Blasted       Index   Rolled Steel   Steel   Steel                   1.00   0   3   N/A       1.25   0   4   N/A       1.50   0   4   N/A       1.75   1   5   N/A       2.00   2   5   N/A       2.25   4   5   N/A       2.50   5   5   N/A       2.75   5   5   N/A       3.00   5   5   N/A                  
 
       EXAMPLE 3  
     Recoat, Chemical Resistance and Drying  
       [0038]    In this Example, non-pigmented coatings were tested according to the procedures set forth in ASTM D 3359-95, Test Method B (for recoat) and ASTM D 1308-87 (for Chemical resistance using n-methyl N-methylpyrrolidinone (NMP),37%HCl, 20% HCl, 100% acetic acid and 50% acetic acid). In addition, drying characteristics were tested as indicated. The test results are presented in Table 3 below  
                           TABLE 3                           Recoat After   Chemical   Dry to Handle           48 Hour Cure:   Resistance   Air Dry @ 72° F.       Stoichiometric   Intercoat   After 30   and 40% Relative       Index   Adhesion   Day Cure   Humidity @ 2 mils DFT                   1.00   0   2   &lt;30 Minutes       1.25   1   3   &lt;45 Minutes       1.50   2   3   &lt;45 Minutes       1.75   3   3   &lt;45 Minutes       2.00   4   4   &lt;45 Minutes       2.25   5   4   &lt;60 Minutes       2.50   5   5   &lt;60 Minutes       2.75   5   5   &lt;75 Minutes       3.00   5   5   &lt;120 Minutes                   
 
       EXAMPLE 4  
     Exposure to UV Light  
       [0039]    In this Example, Gloss White coatings were tested according to the procedure set forth in ASTM D 4587-91, Procedure A (8 hour UV/70° C. followed by 4 hour CON/50° C.). The test results are presented in Table 4 below  
                       TABLE 4                           42 Day   42 Day       Stoichiometric Index   QUV-B 60 Gloss   QUV-B Delta E                   1.00-89.9 Gloss White   52.1   0.609       2.50-96.2 Gloss White   79.7   0.411                  
 
         [0040]    From the above Examples, it can be seen that the properties of the polyurea coating compositions of the present invention begin improving as the polyaspartic ester is over indexed with polyisocyanate at above a 1.00 and continues to improve up to a stoichiometric index of about 2.25, after which the properties maintain the level of improvement.  
         [0041]    Although the present invention has been described with reference to particular means, materials and embodiments, from the foregoing description, one skilled in the art can easily ascertain the essential characteristics of the present invention and various changes and modifications can be made to adapt the various uses and characteristics without departing from the spirit and scope of the present invention as described above and set forth in the following claims.