There is a need in the coatings industry for high strength, high modulus, chemically resistant coatings.  For high modulus, the coatings must exhibit exceptional hardness, while for high strength, they must have good resistance to impact.  As to chemical resistance, it is particularly desirable for automotive coatings to have excellent acid etch resistance.  This invention is directed toward the above objective.  Crosslinkable polyester coatings resins are synthesized and formulated into industrial baking enamels from which surface coatings can be obtained, the cured films exhibiting an unique combination of hardness (pencil hardness&gt;4H) and impact resistance (160 lb in) as well as exceptionally high acid etch resistance. 
It has been demonstrated that thermotropic liquid crystal polymers (LCPs) possess many advantages over conventional thermoplastics.  The self reinforcement of the ordered polymeric molecules in the liquid crystalline (LC) state generally yield thermoplastics with exceptionally high strength and high modulus.  In addition, LCPs have also been shown to have excellent solvent resistance, chemical resistance (e.g., acid, base, detergent), and weatherability.  As described above, these properties are highly desirable in surface coatings.  Recently, coating scientists have attempted to apply the LCP technology to the coatings art. 
U.S. Pat.  No.  5,043,192 (Jones) discloses the application of liquid crystalline polyesters to coatings which display high hardness and high impact resistance.  In one of the examples, linear oligoester diols were prepared and modified with p-hydroxybenzoic acid (PHBA) to yield LC oligoesters.  The PHBA/diol mole ratio varied from 2.1/1 to 13/1.  It was suggested that the excess of PHBA formed repeating p-oxybenzoyl LC segments in the oligoester chain ends.  The resulting resins, however, were highly colored (i.e., brownish).  The use of large quantities of the expensive raw material, PHBA, also made the resins commercially impractical. 
European Patent Application No.  419088 discloses non liquid crystalline esterphenol capped liquid polymer and polyol compositions in combination with an amino crosslinking agent which provided films having superior properties.  The resins were prepared by following a procedure similar to that of Jones at a lower reaction temperature (i.e., &lt;200.degree.  C.) A PHBA/aliphatic hydroxyl equivalent ratio of 1/1 was used mostly, although it was suggested the ratio could be ranged from about 0.05 to about 1.25.  This reference teaches that the coatings showed improved hardness and impact resistance.  However, by repeating their examples I found the coatings did not exhibit high acid resistance. 
Curable resin compositions modified with hydroxybenzoic acid had also been reported by others.  U.S. Pat.  No.  2,993,873 disclosed that drying times and coating properties of oil modified alkyd resins could be improved by replacing part of unsaturated fatty acids with hydroxybenzoic acid in the resin formulations.  The coatings were cured by air dry or baking without the presence of a crosslinking agent.  U.S. Pat.  Nos.  4,267,239 and 4,298,658 describe the modification of alkyd resins with PHBA.  The resulting resins could be rapidly cured at ambient temperatures with isocyanates in the presence of a tertiary amine vapor.  U.S. Pat.  Nos.  4,343,839 and 3,836,491 disclose a coating composition which is rapidly curable at room temperature in the presence of a tertiary amine catalyst vapor.  The coating compositions comprised phenolic terminated polyesters and multi-isocyanate curing agents.  U.S. Pat.  No.  4,331,782 discloses the improved synthesis of a phenol-functional polyester polymer which utilizes a preformed adduct of a hydroxybenzoic acid and an epoxy compound.  Japanese Patents No.  75 40,629, 76 56,839, 76 44,130, and 77 73,929 disclose powder coating compositions containing phenolic hydroxy end groups.  These resins had high softening points and were applied to the surface as powders. 
In an effort to raise the softening point of the melamine type crosslinking agent for powder coatings, U.S. Pat.  No.  4,189,421 taught the synthesis of solid addition products having a softening point above 100.degree.  F. by the reaction of a monohydroxy, single-ring aromatic compound and a hexakis(alkoxymethyl)amino-triazine ring (e.g., hexamethoxymethylmelamine, HMMM).  They found the phenol compound not only could react with HMMM to form ether linkages (O-alkylation), but it could also form methylene bridges at the phenol ortho- or para-position (C-alkylation).  The extent of both reactions was essentially equal.  Further ring closure of the resulting product could also have occurred. 
In the present invention, a smaller amount of PHBA was incorporated into the resins (i.e., about 15-16 mole %) to provide phenolic functionalities.  Unlike the procedures described in both Jones' and Yezrielev's examples, no organic solvent was used for the synthesis of the resin.  This feature allows the resins to be used in waterborne coating compositions.  Further, this process is suitable for economical industrial production. 
A branching agent, trimethylolpropane(TMP), was also incorporated into the resins which provided higher crosslink density.  Further, I have discovered that the presence of TMP, a trifunctional compound, is required in order to yield higher molecular weight resins containing PHBA.  Ostensibly, because the phenolic groups are not reactive under the conditions employed for resin synthesis, PHBA should be viewed as a monofunctional monomer which could terminate the polymer chain growth.  I found that a triol such as TMP helps to propagate the growth of the polymer chain and affords surprisingly higher molecular weight resins; the coatings made possible by this discovery have strikingly better properties.  This effect was found to be especially significant when PHBA was reacted with an excess of TMP only in the first stage of the resin synthesis as described below in the Examples.  In these examples below, the carboxyl group of PHBA was capped by TMP to form a diol adduct in the first stage.  The diol adduct was then polymerized with other diols and diacids in the second stage without causing the problem of decarboxylation of PHBA.  The resulting resin had a number average molecular weight of greater than 1600 which was higher than other resins prepared by different methods.  Finally, attempts at end capping the reins with PHBA in the final stage of the reaction were unsuccessful in providing resins with higher molecular weight--these attempts resulted in significant PHBA sublimation as well as degradation of the polymer chain due ostensibly to ester interchange reactions.