Abstract:
Substituted polycarboxylic acids or substituted anhydrides as acidifying agents stabilize a waterborne polyester from saponification, provided: (a) the acidification is carried out to an extent sufficient to provide an acid number of at least 40 milligrams KOH per gram of polyester resin; and (b) the substitution on the polycarboxylic acid or anhydride is either an alkenyl group having at least four carbon atoms or a cyclic group in which the ring is not aromatic. The relatively high acid number renders the polyester dispersible in water without the need to rely on surfactants.

Description:
BACKGROUND OF THE INVENTION 
     The invention is directed to water dispersible or water soluble polyesters that may be used as binders in coating compositions and the like. In particular, the invention improves the stability of polyesters in aqueous media. 
     It is known to render polyesters dispersible in water by reacting hydroxyl functionality on a polyester with a polycarboxylic acid or an anhydride, thereby producing half-esters that provide carboxyl sites on the polyester. These carboxyl sites are then neutralized with a base such an amine, hydroxide, or carbonate to render the polyester compatible with water. Such an approach is dlsclosed in U.S. Pat. No. 4,576,990 (Mazaki et al.). 
     A known difficulty with such waterborne polyesters is that the presence of the base, which is necessary for solubilization, exposes the polyester to saponification reactions, whereby the half-esters may be cleaved from the polyester. As the saponification progresses over a period of time, a polyester that was initially stable in an aqueous medium may gradually lose its stability, and the product may thereby become unsuitable for its intended purpose. Therefore, the prior art has attempted to find ways to stabilize waterborne polyesters against saponification. 
     Blum, Hohlein, and Meixner, in an article titled &#34;Water Based Coatings with Excellent Saponification Stability,&#34; Advanced Organic Coating Science Technology Series, Vol. 11, pages 45-56, hypothesized that saponification of half-ester groups was internally catalyzed by the proximity of the carboxyl group, an effect termed the &#34;anchimeric effect.&#34; The authors report several attempts to prevent this anchimeric effect in polyesters, one of which involved the use of substituted succinic acids as the acidifying agent. Although a favorable suppression of the anchimeric effect was found by using substituted succinic acids, it was concluded that &#34;the saponification has also not been improved to such an extent that storage-stable lacquers in aqueous form could be manufactured on an acceptable basis.&#34; It may be noted that the initial acid number of the polyesters acidified with substituted succinic acid was 20.7 or 20.8 milligrams KOH per gram of resin. 
     Instead of acidifying the polyester, another approach taken in the prior art to make waterborne polyester coatings is to employ surfactants that permit the polyester to be dispersed in water. Such an approach is generally less desirable because the presence of the surfactant in the cured coating tends to reduce the water resistance of the coating. 
     It would be highly desirable to provide a coating that includes polyester as the primary binder resin, that does not require substantial amounts of surfactant, and which is resistant to saponification. 
     SUMMARY OF THE INVENTION 
     It has now been found that the use of substituted polycarboxylic acids or substituted anhydrides as an acidifying agent can successfully stabilize a waterborne polyester from saponification, provided that: (a) the acidification is carried out to an extent sufficient to provide an acid number of at least 40 milligrams KOH per gram of polyester resin; and (b) the substitution on the polycarboxylic acid or anhydride is an alkenyl group having at least four carbon atoms. The relatively high acid number renders the polyester dispersible in water without the need to rely on surfactants, thereby providing the cured coating with good water resistants. In preferred embodiments the acid number is sufficiently high (e.g., greater than 50 milligrams KOH per gram of resin) to render the polyester fully soluble in water with no surfactant. The high acid number permits substantial amounts of surfactant to be avoided. Surprisingly, it has been found that at these high acid numbers, saponification inhibiting action of the substitution on the polycarboxylic acid or anhydride is effective. 
    
    
     DETAILED DESCRIPTION 
     The waterborne polyesters of the present invention are particularly useful as the binder constituent in coating compositions. The reaction of polycarboxylic acids (or anhydrides) and polyols to make polyesters suitable for coatings is well known in the art. The reaction may be carried out in organic solvent, and the polyester may initially be hydrophobic. A sufficient excess of polyol is provided to yield a polyester having hydroxyl groups with which an acidifying agent may be subsequently reacted so as to render the polyester dispersible or soluble in water upon neutralization. 
     The solvent-based polyesters are prepared from the reaction of a polyol with a polycarboxylic acid, a polycarboxylic anhydride, or a lower alkyl ester. Both cyclic or acyclic polyols or mixtures thereof may be used. Examples of suitable polyols include 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, propylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, trimethylpentanediol and dipropylene glycol, or &#34;Dimer Diol,&#34; a C 36  diol available from Henkel Emery Company. Trimethylol propane or glycerol may be used in limited quantities. Epoxy compounds may also be used, and particularly useful for making low molecular weight polyesters is propylene oxide. 
     In addition, one or more polycarboxylic acids (or the corresponding anhydrides, if available) are used in the preparation of the polyester. These polycarboxylic acids may be cyclic, acyclic or a mixture thereof. Esters, particularly methyl esters of these acids may also be used as reactants from which the polyesters may be formed by transesterification. Suitable cyclic polycarboxylic acids include orthophthalic acid, isophthalic acid, terephthalic acid, hexahydrophthalic acid, methylhexahydrophthalic acid, and cyclohexyldicarboxylic acid (or dimethylcyclohexyldicarboxylate). The acyclic polycarboxylic acids with a carbon number range between 2 and 36 may also be employed, for example, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, undecanedioic acid, dodecanedioic acid, fumaric acid, maleic acid, and &#34;Dimer Acid, a C 36  1 diacid available from Henkel Emery Company. 
     In the condensation polymerization reaction of the polyol with the polycarboxylic acid to produce the initial polyester, the equivalent ratio of hydroxyl groups to carboxyllc groups is maintained greater than 1.0, preferably 6:1 to 1.5:1, most preferably 4.5:1 to 2.5:1, in order to provide hydroxyl functionality on the polyester. This hydroxyl functionality is subsequently partially reacted with an acidifying agent. Some hydroxyl functionality remains after acidification in order to provide reaction sites for curing with a crosslinking agent such as melamine after the coating composition has been applied onto a substrate. Alternatively, all of the hydroxyl functionality may be reacted, in which case a different curing mechanism may be employed, such as oxidative cure, radiation cure, or crosslinking carboxyl groups with hydroxyalkylamides. 
     The acidifying agent may be one or more substituted polycarboxylic acid or, preferably, substituted anhydride, wherein the substitution is selected from the group consisting of: alkenyl groups having at least four carbon atoms, and cyclic groups in which the ring is not aromatic. A suitable category of acidifying agents for use in the present invention are the alkenyl substituted succinic acid anhydrides, for example, isobutenyl succinic acid and octenyl succinic acid anhydride. Examples of cyclic substituted acidifying agents are hexahydrophthalic anhydride and tetrahydrophthalic anhydride. The anhydrides are preferred because at this stage they can react with the hydroxyl groups at lower temperatures to selectively yield the acid ester. 
     Although the principles of the present invention are not limited to polyesters having particular molecular weights, the preferred embodiments intended for use in waterborne coating compositions are generally characterized by relatively low molecular weight, for example, about 3000 (number average molecular weight, measured by gel permeation, polystyrene standard). 
     Where the polyester is of the crosslinking type, the coating composition contains a crosslinking agent such as the condensate of an amine, amide, urea, melamine or benzoguanamine reacted with formaldehyde. These may be further reacted with alcohol to yield an alkyl ether in which the alkyl groups contain from 1 to 4 carbon atoms. 
     Coating compositions containing the polyesters of the present invention may contain from 0 to 50 percent by weight of pigments. These pigments can be selected from the class of inorganic or organic pigments, or mixtures thereof. Suitable pigments might typically include titanium dioxide, zinc oxide, cadmium sulfide, iron oxide, chromium oxide, carbon black, and the like. Fillers and extenders such as silica, clays, talc, barium sulfate, calcium carbonate, and the like may also be included. 
     In addition to water, the polyester composition may be thinned with organic solvents, if desired. Acid catalysts, for example, dinonylnaphthalene sulfonic acid and dinonylnaphthalenedisulfonic acid, may also be used to aid in curing the coating composition. Defoamers and surfactants may also be present as needed to release air and achieve desired wetting during application. 
     The coating composition of the present invention may be applied onto a variety of substrates using conventional application techniques, including spray, disc, roll-coating, dip, and brush application. 
     The present invention is further illustrated by the following non-limiting examples. All measurements are reported in weight percent unless otherwise noted. 
     
         ______________________________________Example 1Charge Reactants     Wt. (grams)                          Equivalents                                  Moles______________________________________A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Octenyl succinic               250.0      1.20    1.20 anhydride______________________________________ 
    
     Charges (A), (B), (C), (D) and (E) were charged into a three-liter, four-necked, round bottom flask equipped with a motor-driven, stainless steel paddle agitator, a thermocouple to record batch temperature, a Dean-Stark water trap connected with a condenser to collect distillate evolved, and an inert gas sparge tube. Heat was applied to a Glas-Col heating mantle and the temperature was gradually increased to 220° C. and held until an acid value of less than four was obtained. The contents of the flask were cooled to 85° C. and material (F) was charged to the flask. Heating was resumed and temperature was increased to 110° C. and held until IR indicated disappearance of the anhydride band, yielding an acid value of approximately sixty. The polymer was thinned to 85% solids in butyl Cellosolve. The same procedure was used for the syntheses of Examples 2 through 10. 
     
         ______________________________________Charge Reactants     Wt. (grams)                          Equivalents                                  Moles______________________________________Example 2A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Hexahydrophthalic               196.7      1.28    1.28 anhydrideExample 3A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Phthalic anhydride               177.6      1.20    1.20EXAMPLE 4A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Trimellitic   147.4      1.49    0.745 anhydrideEXAMPLE 5A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Succinic anhydride               126.2      1.25    1.25EXAMPLE 6A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Tetrahydrophthalic               158.1      0.99    0.99 anhydrideEXAMPLE 7A     Phthalic anhydride               444.0      6.00    3.00B     Diethylene glycol               254.4      4.80    2.40C     Trimethylol pro-               214.6      4.80    1.60 paneD     Triphenyl phosphite               2.30E     Dibutyl tin oxide               1.60F     Methyl succinic               135.0      0.92    0.92 anhydrideEXAMPLE 8A     Phthalic anhydride               296.0      4.00    2.00B     Diethylene glycol               169.6      3.20    1.60C     Trimethylol pro-               214.6      3.20    1.07 paneD     Triphenyl phosphite               1.50E     Dibutyl tin oxide               1.10F     4-(4-Methyl-3-               174.0      0.56    0.56 pentyl)-4-cyclo- hexene-1,2-di- carboxylic anhydrideEXAMPLE 9A     Phthalic anhydride               1491.8     20.2    10.1B     Diethylene glycol               1669.5     31.6    15.8C     Triphenyl phosphite               7.9D     Dibutyl tin oxide               5.7F     Hexahydrophthalic               490.8      3.19    3.19 anhydrideEXAMPLE 10A     Tall oil fatty acid               560.0      2.00    2.00B     Isophthalic acid               332.0      4.00    2.00C     Trimethylol pro-               469.4      10.5    3.50 paneD     Triphenyl phosphite               3.40E     Dibutyl tin oxide               2.50F     Hexahydrophthalic               206.7      1.34    1.34 anhydride______________________________________ 
    
     In Examples 11 and 12, charges (A), (B), (C), (D), (E) and (F) were charged into a three-liter, four-necked, round bottom flask equipped with a motor-driven, stainless steel paddle agitator, a thermocouple to record batch temperature, a Dean-Stark water trap connected with a condenser to collect distillate evolved, and an inert gas sparge tube. Heat was applied to a Glas-Col heating mantle and the temperature was gradually increased to 220° C. and held until an acid value of less than four was obtained. The contents of the flask were cooled to 85° C. and material (G) was charged to the flask. Heating was resumed and temperature was increased to 110° C. and held until IR indicated disappearance of the anhydride band, yielding an acid value of approximately sixty. 
     
         ______________________________________Charge Reactants     Wt. (grams)                          Equivalents                                  Moles______________________________________Example 11A     Cyclohexane   352.6      4.10    2.05 dicarboxylic acidB     Isostearic acid               312.4      1.10    1.10C     Trimethylol pro-               411.7      9.21    3.07 paneD     &#34;Dimer Diol&#34;* 268.5      1.00    0.50E     Triphenyl phosphite               3.50F     Dibutyl tin oxide               2.40G     Octenyl succinic               379.8      1.81    1.81 anhydrideEXAMPLE 12A     Cyclohexane   352.6      4.10    2.05 dicarboxylic acidB     Isostearic acid               312.4      1.10    1.10C     Trimethylol pro-               411.7      9.21    3.07 paneD     &#34;Dimer Diol&#34;* 268.5      1.00    0.50E     Triphenyl phosphite               3.50F     Dibutyl tin oxide               2.40G     Hexahydrophthalic               206.4      1.34    1.34 anhydride______________________________________ *&#34;Dimer Diol&#34; is a C.sub.36 diol available from HenkelEmery. 
    
     In Example 13 charges (A), (B), (C), (D), (E) were charged into a three-liter, four-necked round bottom flask equipped with a motor driven stainless steel paddle agitator, a thermocouple to record batch temperature, a Dean Stark water trap connected with a condenser to collect distillate evolved, and an inert gas sparge tube. Heat was applied to a Glas-Col heating mantle and the temperature was gradually increased to 220° C. and held until an acid value of approximately fifty was obtained. 
     
         ______________________________________EXAMPLE 13Charge Reactants     Wt. (grams)                          Equivalents                                  Moles______________________________________A     Adipic acid   730.0      10.00   5.00B     Diethylene glycol               424.0      8.00    4.00C     Trimethylol pro-               201.2      4.50    1.50 paneD     Triphenyl phosphite               3.4E     Dibutyl tin oxide               2.4______________________________________ 
    
     TESTS FOR SAPONIFICATION RESISTANCE 
     Base master batch clear coating compositions incorporating the resins from each of the synthesis examples above were made using the following formulation: 
     140 grams of resin solids 
     60 grams of Cymel 303 
     1.8 grams of a 55% solution of dinonylnaphthalene disulfonic acid (&#34;Nacure 155&#34; from King Industries) 
     Dimethyl ethanolamine (DMEA) to pH of 7.80 (+/- 0.20) 
     Deionized water 
     Each formulation was divided into two groups, Group A and Group B. Group A was tested for pH drop with the initial neutralization indicated above. Group B was provided with a higher initial pH by adding more DMEA to bring the pH to 8.45 (+/-0.20). 
     Table I presents data for amounts of DMEA and water for the initial master batch and initial pH&#39;s and pH drops after one week at 120° F. of both splits of the master batch. 
     The table shows the greater pH stability (decrease in magnitude of pH drop) by using compositions of the current invention. 
     
                       TABLE I______________________________________Group A                  Group B  DMEA     Water    Initial                          pH    Initial                                      pHExample  (grams)  (grams)  pH    Drop  pH    Drop______________________________________1      14.15    116.3    7.78  0.40  8.26  0.682      15.6     116.3    7.85  0.45  8.34  0.693      13.0     116.3    7.98  1.27  8.60  1.634      18.0     116.3    7.95  1.38  8.47  1.795      11.55    116.3    7.96  1.12  8.63  1.736      14.0     116.3    7.81  0.37  8.30  0.667      14.95    116.3    7.77  0.69  8.45  1.268      13.0     90.0     7.89  0.78  8.53  1.229      12.8     90.0     7.67  0.33  8.40  0.8210     10.0     116.3    7.90  0.21  8.48  0.5011     9.5      116.3    7.75  0.07  8.43  0.3412     9.5      116.3    7.76  0.33  8.30  0.3213     7.0      90.0     7.69  0.72  8.30  1.35______________________________________ 
    
     It can be seen in Table I that the polyesters made with substitutions on the acidifying agent in accordance with the present invention (Examples 1, 2, 6, and 9-12) provided substantially better stability relative to similar polyesters acidified with compounds that were unsubstituted or with substitutions outside the scope of the present invention. A drop in pH is indicative of saponification, and therefore the significant lowering of the pH with the examples of the present invention is an indication of greater stability. Under the conditions of Group A, the pH drop of the examples of the present invention was less than 0.5, and in Group B less than 1.0. A general, long term objective is to maintain the pH above about 7.0. 
     The present invention has been described in connection with particular embodiments, but it should be understood that the invention as defined by the claims encompasses variations and modifications that are known to those of skill in the art.