Abstract:
Solvent-borne alkyd resin compositions are disclosed comprising a 22-carbon, tall oil fatty acid derived aliphatic anhydride. Substitution of the trifunctional acid component of the alkyd resin composition with a 22-carbon anhydride permits a reduction in the viscosity of the resin without adversely affecting the physical properties of he cured film of the resin.

Description:
FIELD OF INVENTION 
     The invention relates to a novel composition of matter for reducing the volatile organic compound levels in solvent-borne coatings. Particularly, this invention relates to a novel solvent-borne composition that can be employed to reduce the viscosity in alkyd resin formulations without producing adverse effects on the physical properties of the cured film. The invention composition permits the use of reduced amounts of environmentally harmful solvents in coatings based on alkyd resins. 
     BACKGROUND OF THE INVENTION 
     One of the first applications of synthetic polymer synthesis in coatings technology was the production of the alkyd resin. It was successful in chemically combining oil or oil-derived fatty acids into a polyester polymer structure, thereby enhancing the properties of these coating vehicles over those of the oils alone. 
     A typical alkyd resin is a polyester made by reacting an unsaturated fatty acid, a dibasic carboxylic acid, and a polyol. Usually, the dibasic acid is phthalic anhydride or isophthalic acid, and the polyol is a trifunctional alcohol. The reaction of the dibasic carboxylic acid with the polyol produces the backbone of the polyester, thereby building its molecular weight. In turn, the fatty acid groups attach to this backbone and, in the presence of oxygen and a metal catalyst, serve to cure the resin into the final crosslinked film. 
     It is also known to produce alkyd resins by replacing the described dibasic acids with trifunctional acids, as shown by T. C. Patton, Alkyd Resin Technology (Interscience Publishers, 1962), pp. 70-1, which is hereby incorporated by reference. The use of trifunctional acids allows the introduction of additional branching points onto the backbone of the polymer. When these branching points are added a less linear polymer possessing different rheological properties is produced. This change in rheology results in a different viscosity for the branched polymer than for a linear polymer with the same molecular weight. The mechanics of this rheological behaviour are reviewed in &#34;An Overview of the Basic Rheological Behavior of Polymer Fluids with an Emphasis on Polymer Melts&#34;, G. L. Wilkes, Journal of Chemical Education, Vol. 58, November 1981, pp. 880-892. 
     Resins are typically classified by the amount of fatty acid that they contain, i.e. their &#34;oil length&#34;. The oil length of an alkyd is defined as the weight of any fatty acid present plus the weight of the polyol needed to completely esterify this fatty acid (minus the weight of evolved water from the esterification), expressed as a percentage of the total weight of the finished resin. Oil length (OL) is divided into three classes: short-, medium-, and long-oil. A medium-oil alkyd contains between 45 to 55% oil, with short-oil alkyds containing less and long-oil alkyds containing more oil. Long-oil and medium-oil lengths are commonly used in solvent-based decorative coatings, while short-oil alkyds are found in automotive and general industrial stoving compositions. 
     Traditionally, solvent-based alkyds have obtained their good film properties by a combination of high resin molecular weights coupled with drying from pendant, unsaturated fatty acids present in the alkyd. While contributing to desirable film properties, the high molecular weight of the resin also causes the resin to have an unsuitably high viscosity, thereby necessitating low-solids formulations for coating applications. Due to environmental concerns and increasing governmental regulations on the volatile organic content (VOC) of coatings, coating producers are attempting to find alternatives to these conventional low-solids level, solvent-based formulations. 
     One approach to address the VOC regulations is the development of water-based formulations. While the use of water as a solvent is attractive, it has proven difficult to obtain the properties and performance of a solvent-based coating via a water-based system. These problems are especially pronounced in the areas of high gloss and corrosion resistance. 
     The other popular approach to the VOC regulations has been to reduce the amount of volatile organic solvent necessary for a formulation by reducing the viscosity of the alkyd resin. This so-called high-solids approach involves developing resins that are soluble in organic solvents at levels as high as 60 to 100 weight percent solids. These high-solids levels must be attained while maintaining a low viscosity to allow for formulation and application of the coating. 
     Therefore, it is the object of this invention to have an alkyd resin composition that addresses environmental concerns by significantly reducing the viscosity of the resin, thereby reducing the amount of solvent necessary for the coating formulation. In addition, organic solvents are frequently expensive and any means of reducing their presence could lead to lower costs for coatings. 
     SUMMARY OF THE INVENTION 
     The object of the invention is met by the use of a 22-carbon aliphatic branched-chain anhydride in the solvent-based alkyds in lieu of the traditional aromatic triacids (e.g., trimellitic anhydride). This anhydride, when polymerized into the backbone of a alkyd resin, significantly reduces the viscosity of the resin compared to similar, unmodified resins. Thus, the substitution allows systems to be formulated with a smaller amount of solvents than their aromatic counterparts. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is well known in the art to react an unsaturated fatty acid, a dibasic carboxylic acid, and a polyol to produce an alkyd resin, as shown in &#34;Binders for Higher-Solids Coatings Part 1: Synthesis of Model Alkyd Resins&#34;, S. L. Kangas and F. N. Jones, Journal of Coatings Technology, Vol. 59, No. 744, January 1987, pp. 89-97, which is hereby incorporated by reference. Partial substitution of trifunctional acids for the above described dibasic acids is also known. Examples of the use of trifunctional acids in alkyd and polyester compositions are given in Amoco Technical Bulletins GSTR - 21 and GSTR - 36, which are hereby incorporated by reference. 
     Unsaturated fatty acids which are suitable for use in the reaction include, but are not limited to, the following: 
     tall oil 
     soybean oil 
     tung oil 
     linseed oil 
     linoleic acid 
     linolenic acid. 
     Polyols which are suitable for use in the reaction include, but are not limited to, the following: 
     1, 4-butanediol 
     ethylene glycol 
     propylene glycol 
     neopentyl glycol 
     glycerol 
     trimethylolethane 
     trimethylolpropane 
     pentaerythritol 
     hexanediol 
     isomeric cyclohexane diols. 
     Dibasic carboxylic acids which are suitable for use in the reaction include, but are not limited to, the following: 
     phthalic anhydride 
     orthophthalic acid 
     terephthalic acid 
     maleic anhydride 
     fumaric acid 
     azelaic acid 
     C-36 dimer acid. 
     Trifunctional acids which are suitable for use in the reaction include, but are not limited to, the following: 
     trimellitic anhydride 
     isomeric benzene tricarboxylic acids 
     isomeric napthalene tricarboxylic acid 
     citric acid. 
     These lists are intended to be representative and it will be obvious to those skilled in the art that a variety of other acids and polyols can be used. Therefore, other acids and polyols suitable for use in the reaction can be considered part of this invention when used with the described composition of matter below. 
     Most alkyd resin formulations utilizing triacids are based on trimellitic anhydride due to its commercial availability. While other aromatic or aliphatic triacids could be used they are not as reactive as trimellitic anhydride and can give gelation in the alkyd since they have similar reactivity in their carboxylic acid groups. For the purpose of describing this invention, trimellitic anhydride is considered as the comparative prior art trifunctional material since it is the most readily available commercial product (Amoco Chemicals) and also has the desired reactivity in the anhydride linkage. 
     It has been discovered that use of anhydrides derived from the reaction of maleic anhydride with either oleic, linoleic, or linolenic acids serve a similar function as trimellitic anhydride in regards to reacting with an alkyd system. Significantly, these materials, when used according to the descriptions of this invention, can substantially reduce the viscosity of resins in solvent media when substituted for aromatic moieties at the same solids level. Film properties are not seriously affected. An alternative embodiment is to formulate the resins to similar viscosities. The maleinized fatty acid derived resins require less solvent. 
     The invention that is useful in reducing the viscosity of a resin without adversely affecting the physical properties of its cured film is produced by replacing 0.5 to 50% of the standard dibasic carboxylic acid used to make the alkyd resin with either of two 22-carbon aliphatic branched-chain anhydrides, one of which is an acid anhydride while the other is an ene-anhydride. The anhydrides are prepared via known reactions which require suitable starting materials containing unsaturated C 18  fatty acids. Any of the basic oils containing a significant amount of such fatty acids is a suitable starting material, including tall oil, corn oil, cottonseed oil, lard, linseed oil, mustard seed oil, olive oil, palm oil, peanut oil, rapeseed oil, rice bran oil, safflower oil, sesame oil, sunflower oil, tallow, whale oil, and the like. A preferred starting material is a mixture of tall oil fatty acids containing about 40-50% linoleic acid and 40-50% oleic/elaidic acid. Oleic and elaidic acids are the cis and trans forms, respectively, of the mono-unsaturated C 18  fatty acid. Typically, the amount of functionalized fatty acid is between 85  and 95% in such mixtures. 
     To produce the 22-carbon aliphatic acid anhydride one performs a Diels-Alder adduction of maleic anhydride onto linoleic or linolenic acid. Here, a suitable amount of maleic anhydride is charged into a pressure reactor and the material is heated for approximately two hours at 240° C. in the presence of iodine catalyst. This crude material, composed primarily of the Diels-Alder adduct of linoleic or linolenic acid, is distilled under suitable conditions (i.e. via wiped film distillation) to remove any unreacted fatty acid which is still present. The Diels-Alder adduct formation is taught in March, Advanced Organic Chemistry, Reactions, Mechanism and Structure (2nd ed. 1977), pp. 761-766, which is hereby incorporated by reference. The chemical structure for the acid anhydride is shown as the first structure in FIG. 1 on page 9 below. 
     Alternatively, one may produce the 22-carbon aliphatic ene-anhydride via treating oleic/elaidic acid under more vigorous conditions with additional maleic anhydride to form an ene adduct. The ene reaction and its product are taught in the March, Advanced Organic Chemistry, Reactions, Mechanism and Structure (2nd ed. 1977), p. 726, which is hereby incorporated by reference. This reaction mixture is also put though a series of distillations under suitable conditions (i.e. via wiped film distillation) to yield a high-purity eneanhydride of sufficient quality to be utilized in the production of polymers. The chemical structure for the eneanhydride is shown as the second structure in FIG. I below. 
     Methods for producing the same C 22  anhydrides are taught in commonly assigned U.S. Pat. No. 4,927,669, which is hereby incorporated by reference. However, the anhydrides employed in the present invention are high-purity versions of the anhydrides taught in the above patent, and are of a sufficient quality to be utilized in the production of polymers. The chemical structures for the anhydrides are shown in FIG. I below: ##STR1## In the first structure, x and y are integers which when added together equal 13. This 22-carbon aliphatic branched-chain acid anhydride is hereafter referred to as TA-95. In the second structure, a and b are integers which when added together equal 12. This 22-carbon aliphatic branched-chain ene-anhydride is hereafter referred to as TE-95. (Westvaco TA-95 and TE-95 are highly purified, maleinized, tall-oil fatty acid derivatives, sold by Westvaco.) 
     The invention compositions are useful in reducing the viscosity of long-oil, medium-oil, and short-oil alkyd resins. As appreciated in the art, the exact components and properties of components desired for any coating application can vary; and, therefore, routine experimentation may be required to determine the optimal components and proportions of components necessary to achieve the desired properties for a given application. 
     The following examples are provided to illustrate further the present invention and are not to be construed as limiting the invention in any manner. 
     EXAMPLES 
     A standard medium-oil alkyd resin, made from PAMOLYN® 200 (a tall-oil derived, unsaturated fatty acid containing 78% linoleic acid, made by Hercules, phthalic anhydride (PA), and trimethylol propane (TMP) was produced based on a formulation given in the above referenced article by S. L. Kangas and F. N. Jones (Alkyd No. T-7-C on p. 91). A series of modified resins were created by substituting, on an equivalents basis, various trifunctional acids for a given percentage of the PA used in the standard resin. 
     All alkyds were prepared by a conventional solvent procedure, as illustrated by the following preparation of the 10% TE-95 modified resin. (It should be noted that the same procedure can be used employing TA-95 in place of TE-95). PAMOLYN® 200 (67.50 g, 0.24 mol), PA (38.60 g, 0.26 mol), TE-95 (7.23 g, 0.02 mol), TMP (39.90 g, 0.30 mol) and xylene (20.00 g) were charged into a 250-ml three neck round-bottom flask equipped with a Dean-Stark trap and cold water condenser, a heating mantle, a thermometer, a nitrogen inlet and a mechanical stirrer. Lithium ricinoleate, at 0.1% based on the fatty acid present, was used as a catalyst for all the resin preparations. The mixture was heated to 175° C., under nitrogen, and the temperature was increased 10° C. every 30  minutes until a top temperature of 240° C. was reached. This temperature was maintained until an acid number of approximately eleven was attained. The resins were cooled, poured into glass jars and stored under nitrogen. 
     
                       TABLE I______________________________________RESIN FORMULATIONSResin      Weight % Of ChargeDescription      Pamolyn 200 ®                  PA       TMP  Triacid______________________________________Std. TMP   45.0        28.5     26.5 0.010% TMA    45.1        25.8     26.7 2.4710% TE-95  44.1        25.2     26.0 4.7210% CA     45.1        25.8     26.7 2.4720% TMA    45.3        23.0     26.8 4.9520% TE-95  43.2        21.9     25.5 9.37______________________________________ CA Citric Acid (triacid) TMA Trimellitic Anhydride (triacid) PA Phthalic Anhydride TMP Trimethylol Propane Pamolyn 200 ® A talloil derived, unsaturated fatty acid containing 78 linoleic acid, made by Hercules. TE95 A tall oil fatty acid derived, C.sub.22 acid anhydride (triacid) mad by Westvaco. 
    
     
                       TABLE II______________________________________RESIN CONSTANTSResin   Constants       Rxn Time   FinalDescription   K      Fav    OL   R    (hrs)    Acid No.______________________________________Std. TMP   1.006  2.07   53   1.083                           8.0      1210% TMA 0.999  2.09   53   1.091                           7.5      1110% TE-95   0.999  2.09   51   1.091                           7.5      1110% CA  0.998  2.09   55   1.088                           11.0     1020% TMA 0.987  2.12   53   1.093                           10.0      820% TE-95   0.987  2.12   51   1.093                           8.0      11______________________________________ 
    
     The standard and modified resin formulations are given in Table I. Table II contains a series of parameters which can be used to help predict the final properties of the alkyd resins. These parameters were kept as constant as possible in each of the formulations so that the resins being compared would be similar. K is the alkyd constant, equal to the total moles of starting material divided by the total equivalents of acid present. To obtain a satisfactory resin this value should be 1.0 ±0.15. F av  is the average functionality of the alkyd and is equal to the total equivalents of starting material divided by the total moles of starting material. OL is the oil length of the resin as explained earlier in the application. R is the ratio of total OH groups from the polyol to total acid groups present in the starting material. The last two columns are the length of time (in hours) for the reaction and the final acid number of the resin, respectively. 
     Once prepared, the resins were reduced to 50% solids in mineral spirits so that films could be made and evaluated. The resin viscosity versus solids content, in mineral spirits and xylene, was also tested (see Table IV below). A combination of three metal driers (calcium, cobalt, and zirconium) was added to the diluted resins. The driers were added at levels of 0.05% Ca, 0.05% Co, and 0.3% Zr (based on the resin solids weights) to the 50% solids resins and allowed to sit overnight before the films were drawn down. No other additives were used. Films were cast on both steel and aluminum panels using a draw down bar at a wet film thickness of 2 mls. The films were dried for one week at ambient conditions before any evaluations were performed, except for the 24 hour hardness determinations. 
     The alkyd resin film evaluations ar shown in Table III below. Dry times were reported for a 300 g Zapon test. Hardness was determined using the standard pencil test after one day, and again after seven days. Impact resistance, both direct and reverse, was tested with a Gardner Impact Tester on steel panels. The Mandrel Flexibility and the cross-hatch adhesion tests were performed on aluminum panels. 
     
                                           TABLE III__________________________________________________________________________ALKYD RESIN PROPERTIES           PENCIL    DRY    HARD-    TIMES (min)           NESS*   IMPACT**DESCRIPTION    Zapon  1 Day               7 Day                   Direct                       Reverse                            FLEX**                                 ADHES***__________________________________________________________________________Std TMP  80     B    B  &gt;160                        100 &gt;32% 10010% TMA  85     H   2H  &gt;160                       &gt;160 &gt;32% 10010% TE-95    95     2H  3H  &gt;160                       &gt;160 &gt;32% 10010% CA   220    2B  HB  &gt;160                       &gt;160 &gt;32% 10020% TMA  135    H    H  &gt;160                       &gt;160 &gt;32% 10020% TE-95    190    H   2H  &gt;160                       &gt;160 &gt;32% 100__________________________________________________________________________ *Series in order of increasing hardness: 6B to 1B, HB, F, H to 6H. **160 lbs/in and 32% are the maximum limits of the impact resistance and Mandrel Flexibility texts, respectively. ***Crosshatch adhesion test with a maximum of 100 squares. 
    
     Overall, the dry times for the aromatic systems were quite good, which is to be expected for a conventional alkyd prepared with a high linoleic acid content fatty acid such as PAMOLYN® 200. As citric acid is such a strong complexing agent it is believed that trace amounts of non-reacted material complexed the drying agents in the system, thereby resulting in the extremely long dry time. The aliphatic TE-95 systems dry times were slightly longer than for the aromatic systems. However, at lower levels of substitution of TE-95 (10%), this difference in dry times is minimal. 
     The pencil hardness values show that substitution of the trifunctional materials gives harder films than the difunctional phthalic anhydride. Initial (one day) hardnesses are similar, but after seven days the trifunctional materials are several pencil values harder. The exception, due to incomplete curing, is the citric acid based system. 
     Other properties such as impact resistance, adhesion, and flexibility were generally equal. This indicates that there is little loss of film quality in substituting TE-95 for either the difunctional phthalic anhydride or the trifunctional trimellitic anhydride. 
     Two solvents, xylene and mineral spirits, were used to determine the viscosity of the resin as a function of the solids level. All viscosity data were collected at 25° C. using a Brookfield viscometer. Table IV gives the viscosity data for the evaluated resins. Of primary interest are the values in mineral spirits (as xylene is a photoactive solvent under rigorous environmental control). Data indicate that at lower substitution levels (10%), when TE-95 is substituted for either phthalic or trimellitic anhydride, relatively large viscosity reductions can be observed. At higher substitution levels (20%) the viscosity of the TE-9 based system is similar to the phthalic anhydride and 10% equivalent TMA resin. At higher levels (20%) the increase in viscosity from crosslinking overcomes the decreases from having an aliphatic substituent replacing the aromatic phthalic or trimellitic anhydrides. 
     
                       TABLE IV______________________________________VISCOSITY DATA FOR RESINSResin         Solids LevelDescription   Solvent   80%      70%    60%    50%______________________________________Std. TMP   MS        16,425   4,200  790    155   Xylene     3,950     325   90     2010% TMA MS        16,240   4,264  976    196   Xylene     2,444     488  112     3610% TE-95   MS         5,472   1,584  472    120   Xylene     1,436     304   84     2410% CA  MS        21,280   6,080  1,136  256   Xylene     3,792     628  132     3620% TMA MS        90,400   19,220 4,216  784   Xylene    12,620   1,816  376     9620% TE-95   MS        14,160   3,736  976    252   Xylene     3,832     792  184     60______________________________________ 
    
     These decreases in viscosity were achieved without adversely affecting the physical properties of the systems. 
     Many modifications and variations of the present invention will be apparent to one of ordinary skill in the art in light of the above teachings. It is therefore understood that the scope of the invention is not to be limited by the foregoing description, but rather is to be defined by the claims appended hereto.