GB1603191A - High stress-strain thermosetting polyesters - Google Patents

Description

(54) HIGH STRESS-STRAIN THERMOSETTING POLYESTERS (71) We, OWENS-CORNING FIBERGLAS CORPORATION of Fiberglas Tower, Toledo, Ohio, United States of America, a corporation organised and existing under the laws of the State of Ohio, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in andby the following statement: The present invention relates to a process for producing a branched thermosetting polyester polymer.

Unsaturated polyester polymers can be blended with ethylenic monomers such as vinyl and/or allyl monomers such as styrene, vinyl toluene or diallyl phthalate that can be cured at room temperature or under heat and/or pressure to form a thermoset molded structural part. These molding resins often are combined with inert fillers, glass fibers, glass flakes, or talcs, for the purpose of obtaining improved impact strength, flexural stength, and rigidity in the molded parts.

Conventional thermosetting polyester resins are often either rigid polvmers of flexible polymers. Blended thereof merely provide a blend of physical characteristics which are merely averated and do not provide both high stress and high strain properties. Although resiliency can be improved by esterifying isophthalic acid with conventional polyols, propylene or ethylene glycol esterified with isophthalic acid still produces high stress and low strain whereas diethylene glycol-isophthalic polyesters can produce low stress-high strain materials, but have low heat distortion temperature.

In accordance with this invention there is provided a method for producing a branched thermosetting polyester polymer comprising: (a) esterifying dipropylene glycol and a minor amount of a higher primary polyol selected from trimethylolpropane, trimethylolethane and pentaerythritol with isophthalic acid, up to 35 molar percent of which can optionally be replaced -by terephthalic acid, sufficient in amount to provide carboxylic equivalents in excess of the primary hydroxyl equivalents, but less than the total hydroxyl equivalents, but less than the total hydroxyl equivalents of said higher primary polyol and said dipropyl glycol, to form a branched prepolymer having unreacted secondary hydroxyl groups, and (b) further esterifying said branched prepolymer with an a,-ethylenically unsaturated dicarboxylic acid or anhydride.

Preferably the carboxyl equivalents provided by said isophthalic acid are at least equal to the sum of the primary hydroxyl equivalents of said higher primary polyol and 25/ó of the total hydroxyl equivalents of said dipropylene glycol.

Preferably the esterification of step (a) is continued to an acid number less than 10.

Preferably the esterification of step (b) is continued to an acid number between about 15 and 40.

Preferably the relative molar proportions of reactants are: Dipropylene glycol Higher primary polyrol 0.04--0.375 Isophthalic acid 0.1330.875 Ethylenically unsaturated dicarboxylic acid or anhydride 0.87-2.25 Preferably step (b) further comprises esterifying excess carboxyls with an alkylene glycol selected from propylene glycol, ethylene glycol and neopentyl glycol.

Preferably said ethylenically unsaturated discarboxylic acid or an hydride comprises maleic acid or anhydride and the alkylene glycol is added to the esterifying mixture in step (b) only after the initial exotherm from the esterification of the ethylenically unsaturated dicarboxylic acid or anhydride with the branched prepolymer has started to subside.

The invention includes a thermosetting polyester resin mixture comprising a polyester polymer produced by the method described above and between 20% and 65% by weight styrene.

Embodiments of the invention are hereafter described with reference to the accompanying drawings, in which: Figure 1 is a stress-strain diagram showing curves of relative strength of materials relating the cured tough polyester (b) of this invention to steel (a); and Figure 2 is a stress-strain diagram similar to Figure I relating the cured tough polyester (b) of this invention to various similar cured polyesters in addition to listing the respective Heat Distortions.

Referring first to the drawings shown in Figure 1 is a strength of materials stress-strain diagram comparing a cured thermoset polyester (b) made in accordance with this invention to steel (a). The steel curve A in Figure I was taken from test data for conventional stress and strain data set forth in Strength of Materials, Ferdinand L. Swinger, Table B-l, 0.20/, carbon hot rolled steel. The actual stress data on the steel was converted to a relative stress by multiplying the actual test data for steel by 72/490 wherein 72 pound/cubic foot is the density of cured polyester and 490 pounds/cubic foot is the density of steel in accordance with strength of materials procedures for comparing dissimilar materials. Curve B in Figure 1 is the actual stress-strain test data of the cured polyester of this invention and measured on a curved polyester test sample in accordance with ASTM Test No. D-638, and more particularly described in Example 1.

In Figure 2, curve B is identical to curve B in Figure 1 and produced by testing a cured polyester resin synthesised by step-wise esterification of isophthalic acid with dipropylene glycol and minor amounts of trimethylol propane followed by second stage esterification of maleic anhyride as more particularly set forth in Example 1. The polyester polymer in curve B was produced by first stage esterification of 0.295 moles of isophthalic acid with 0.635 moles of dipropylene glycol and 0.05 moles of trimethylol propane at temperatures between 320"F and 420"F until substantially all of the isophthalic acid was reacted to produce a branched prepolymer having an Acid No. less than 10. The branched propolymer was further esterified with the addition of 0.705 moles of maleic anhydride and 0.302 moles of propylene glycol at temperatures between 3300 F and 4300 F until an Acid No. of about 20 was reached. The polyester polymer was then diluted with styrene and tested as set forth in more detail in Example 1.

Figure 2 further indicates comparative prior art polyesters produced and tested comparatively to curve B. Curve C represents a semi-rigid polyester comprising 1.05 moles propylene glycol esterified with 0.5 moles of maleic anhydride and 0.5 moles of isophthalic acid; curve D represents a flexible polyester comprising 1.05 moles diethylene glycol esterified with 0.7 moles of maleic anhydride and 0.3 moles of phthalic anhydride; curve E represents a resilient polyester comprising 0.70 moles of diethylene glycol and 0.35 moles propylene glycol esterified with 0.45 moles of maleic anhydride and 0.55 moles of isophthalic acid; and curve F represents a rigid polyester comprising 1.0 moles of propylene glycol and 0.2 moles of dicyclopentadiene esterified with 1.0 moles maleic anhydride. The polyester of this invention represented in curve B produces a cured solid having substantially increased toughness requiring a substantial increase in work to break a cured test sample compared to the other cured polyester materials with the exception of curve B. The polyester materials are further compared by listing the heat distortion of the respective materials in Figure 2 in accordance with ASTM D-648. Polyester B of this invention has a substantially improved heat distortion in addition to the improved toughness thereby providing a substantially improved tough polyester.

In accordance with this invention, the ethylenically unsaturated polyester polymer is produced by first reacting dipropylene glycol and very minor amounts of higher primary polyols such as triols with excess molar equivalents of isophthalic acid, relative to primary hydroxyls, to produce an ordered, highly branched, hydroxyl terminated polyester prepolymer.

The first stage prepolymer contains polymer branching introduced into the polymer chain by the inclusion of very minor amounts of higher primary polyol such as trimethylolpropane (TMP), trimethylolethane (TME), or pentaerythritol (PE). The primary polyols and dipropylene glycol are esterified with lesser molar equivalents of isophthalic acid to provide a hydroxyl terminated branched prepolymer having substantially unreacted secondary hydroxyls on the prepolymer. The primary hydroxyls on the primary polyols like the primary hydroxyls on any dipropylene glycol isomer esterify much quicker than the secondary hydroxyls on dipropylene glycol and accordingly provide the necessary polymer branching in the first stage prepolymer development to achieve the desired polymer chain sterer spacing between the ethylenic double bond unsaturation subsequently introduced into the polymer chain in the second stage processing as will become more apparent hereinafter. In the first step there may be used e.g. 0.3 to 1.5 moles of trimethylolpropane, 4.0 to 7.5 moles of dipropylene glycol and 1.0 to 3.5 moles isophthalic acid. Commerical dipropylene glycol may contain mixed isomers up to approximately a 50/50 mixture of di-secondary hydroxyl and primary-secondary hydroxyl isomers of dipropylene glycol along with insignificant amounts of other isomers. Thus, primary hydroxyls in commercial dipropylene glycol ordinarily comprise only about up to 25% of the available hydroxyls. Primary hydroxyls are much more reactive than the preponderance of secondary hydroxyls available in conventional dipropylene glycol and accordingly, preferential esterification of the primary hydroxyls in the dipropylene glycol isomer mixture with isophthalic acid is preferred in the first step to complete esterification of the primary hydroxyls.

In the first stage processing, sufficient carboxyl equivalents of e.g. isophthalic are charged to at least esterify the primary hydroxyl on said primary polyols (TMP, TME, PE) as well as theoretically at least 25% of the dipropylene glycol equivalents or the primary hydroxyls which may be present in the dipropylene glycol isomer mixture. Preferably, at least about 10% excess carboxyl equivalents of isophthalic acid are available in the first stage to exceed the theoretical primary hydroxyl groups and desirably between from 30% to 60% excess isophthalic carboxyl groups should be available in the first stage for esterification of the primary hydroxyls of the triol and dipropylene glycol. The isophthalic carboxyl equivalents are completely esterified with total available hydroxyls in both the dipropylene glycol and primary polyol in the first stage esterification step. Although isopththalic acid is preferred, up to 35 molar percent of the isophthalic acid can be replaced with terephthalic acid. The first stage processing can be a fusion or solvent cook and efficient esterification ordinarily can be effected at a temperature of 390"F to 420"F in a conventional manner until the unreacted carboxyl groups of the highly branched prepolymer approach zero (Acid No. less than 10) indicating esterification is complete. Accordingly, primary hydroxyls are first esterified by isophthalic acid in the first stage branched prepolymer development due to the primary hydroxyls esterifying kinetically much faster than secondary hydroxyls.

Forming the branched prepolymer in the first stage in accordance with this invention is important to particularly space the double bonds of the unsaturated acid in the final branched polymer. The prepolymer first stage processing ensured.

branched polymer development in addition to esterifying substantially all of the primary hydroxyls. Secondary hydroxyls esterify slower and at temperatures well above the isomerisation temperatures thus avoiding undesirable di-esterification of e.g. maleic anhydride carboxyls prior to isomerisation of the maleic anhydride from cis-maleate to the trans-fumarate configuration. Accordingly, maleic anhydride is preferentially added to secondary hydroxyls of the hydroxyl terminated prepolymer and preferably reacted therewith prior to the addition of any subsequent additions of glycol, if additional glycols are desired. Fumaric acid on the other hand can be charged directly with second stage glycols to complete development of the branched prepolymer and produce a branched ethylenically unsaturated polyester polymer. The a,:-unsaturated dicarboxylic acids include for example maleic acid (anhydride), fumaric acid, and itaconic acid although the preferred unsaturated dicarboxylic acids are maleic and fumaric. The second stage esterification can be effected at temperature of 390"F to 4200F and can include additional glycol which can be charged directly to the second stage in conjunction with fumaric acid or subsequent to the trans-fumarate isomerisation of maleic acid or maleic anhvdride is utilised as the ethylenically unsaturated dicarboxylic acid.

The preferred branched prepolymer is produced in the first stage from raw materials comprising on a relative molar basis in the first stage of I mole of dipropylene glycol, 0.133 to 0.875 moles isophthalic acid, and 0.04 to 0.375 moles primary polyol. The isophthalic acid can be partially replaced with minor amoknts of up to about 35 molar percent of terephthalic acid. The primary polyols can be selected from trimethylolpropane, trimethylolethane and pentaerythritol.

Preferably the first stage glycol is substantially all dipropylene glycol and preferably the primary triols are selected from trimethylol propane and trimethylol ethane. The first stage polymer can be then reacted with 0.87 to 2.25 moles of an a,b-unsaturated dicarboxylic acid.

Preferably in the second stage at least 1% excess equivalent hydroxyls are maintained. The second stage processing continues the highly branched polymer first developed in the first stage whereby the final polymer is highly branched containing properly spaced double bonds that permit several units of ethylenically unsaturated monomer to cross-link between the polyester polymer double bonds.

The finished polyester polymer ordinarily contains up to about 10% excess hydroxyl equivalents and may be esterified to an Acid No. of between about 15 and 40 in a conventional manner. Excess carboxyls can be esterified with propylene glycol, ethylene glycol or neopentyl glycol.

The final branched unsaturated polyester polymers may be dispersed in ethylenically unsaturated monomers such as vinyl and/or allyl monomers to dissolve the polyester polymer and form a resin mixture of polymer and monomer.

Such ethylenically unsaturated monomers are copolymerisable with the ethylenic unsaturated double bonds in the polyester polymers and include vinyl monomers such as styrene, methylstyrene, chlorostyrene, vinyl toluene, divinyl benzene, vinyl acetate, acrylic and methacrylic acid, as well as lower alkyl esters of acrylic and methacrylic acid, and divinyl phthalate. For reasons of efficiency and economy the preferred vinyl mononers are styrene and vinyl styrene. The polyester resin mixture contains between 20% and 65% by weight vinyl monomer.

Catalyst and promoters are often incorporated in small amounts into the thermosetting polyester resins containing vinyl unsaturated monomer for curing or cross-linking the ethylenically unsaturated polyester polymer to form a thermosetting polyester resin. Such catalyst and promoters are well known and can be similarly utilised in this invention. Typical catalysts include for example organic peroxides and peracids such as tertiary butyl perbenzoate, tertiary butyl peroxide and benzoyl peroxide. The examples of conventional promoters include cobalt octoate, cobalt naphthenate, and amines such as dimethyl amine. The amounts of catalyst and promoters can be varied with the molding process or with the level and type of inhibitors utilised and ordinarily at the level of between 1% and 3 by weight based on the polyester resin mixture.

Fibers, fillers and pigments normally added to polyester molding resin compositions can be likewise utilised in formulating the molding composition of this invention. An example of additives that can be added include for example glass fibers, chopper fibers, clays, chalk, asbestos, calcium carbonate, talc, ceramic, and quartz. Examples of pigments include for example cadmium yellow, carbon black, iron oxide, titanium oxide, as well as organic pigments such as phthaloorganamine blues and greens.

A surprising result of the polyester polymer produced in accordance with this invention is the energy required to break the cured polyester polymer. The energy required can be referred to as toughness indicated by the area under a stress-strain curve in conventional terminology of a strength of materials. In Figure 1, curve A for steel has a steep stress-strain curve until the yield point is reached whereupon the curve sharply breaks off as is conventional for steel material. As noted hereinbefore, curve A for steel is converted to specific strengths relating steel strength to cured polyesters on the same volume basis. Curve B indicates the cured polyester of this invention wherein the highly branched polyester produces a substantial increase in energy required to break the cured polyester structure as indicated by the slope and length of the curve B wherein both stress and strain are mutually high at the yield point. Polymer B further exhibits a high heat distortion temperature of greater than 240"F. The energy required to break the cured polyester structure is measured as the area under curve B and is less than steel but substantially greater than the conventional polyester polymers. Figure 2 similarly compares the strength of the cured polyester B of this invention with various similar cured polyesters contrasting the high energy required to reach the yield point as represented by the total area under curve B. Figure 2 indicates a semi-rigid polyester C represented by a steep curve C indicating high stress properties but very low strain properties at the yield point. A flexible polyester D is represented by relatively flat curve D indicating high strain but low stress at the yield point.

Polyester curve E has stress and strain properties approximately equal to B with respect to energy required to break, but has a low heat distortion of only 1460F which compares unfavourably to greater than 240"F heat distortion for curve B.

Figure 2 further states the heat distortions of the various polyester materials as measured by ASTM 648 indicating a substantially higher heat distortion for polyester B in addition to the substantially increased energy required for polyester B to reach the yield point of the cured polyester of this invention.

Although not intended to be bound by theory, the first step processing in accordance with this invention is deemed necessary to provide the necessary polymer branching due to preferential esterification of the primary triols to properly form polymer branching in the first stage. Preferential esterification of isophthalic acid with triols as well as the primary hydroxyls in the dipropylene glycol isomer mixture in the first stage effectively eliminates availability of primary hydroxyls for esterification of the unsaturated dicarboxylic acid in the second stage processing. Maleic anhydride undergoes transisomerisation to the fumarate prior to esterification in the second stage due to less reactivity of the second hydroxyls thus assuring proper spacing of the fumarate double bonds in the branched polymer. Proper spacing of the fumarate double bonds has the surprising result of permitting substantially increased amounts of ethylenically unsaturated monomer such as styrene to cross-link between adjacent polymer chains without diminishing the toughness characteristics of the thermoset solid thus indicating that a low molecular weight polystyrene chain develops between the spaced polyester double bonds. Prior to this invention, optimum heat distortion was thought to be achieved when the equivalents of monomer styrene are approximately equal in number for cross-linking double bonds in the polyester polymer wherein one styrene unit crosslinks two proximately disposed double bonds in separate polymer chains. The polyester of this invention further has surprisingly high heat distortion above about 240"F. High heat distortion is maintained even with higher styrene levels indicating polystyrene chain ends cross-link the branched polyester chains and consequently free polystyrene chains do not develop as could be the case in prior art polyesters.

In this invention, the branched prepolymer effectively spaces the polyester polymer chains wherein about six equivalents of monomer such as styrene crosslinks with two double bonds in adjacent polymer chains. Accordingly, the polyester resin (polymer+monomer) contains at least about two and preferably about three monomer units per double bond in the polymer wherein at least about four and preferably about six monomer units cross-link two adjacent double bonds in the polymer. On a weight basis, the preferred polyester resin contains about 60 parts styrene and about 40 parts branched polyester polymer. The dimensional stability of polyesters of this invention are maintained relatively constant over wide temperature ranges between 30"F to +200"F which makes polyester particularly suitable for automotive, appliance and electrical components where drastic heat changes are commonly encountered. The polyester polymer of this invention can have molecular weight per double bond greater than 200 and as high as about 300 per polymer double bond. The high reactivity of the fumarate double bonds in the polymer is further evidenced by a surprisingly high peak exotherm indicated by the standard SPI gel test to produce exotherms of about 450"F in a 1800 bath.

The optimum styrene level can be measured by peak exotherm and is achieved at the 50% to 60% level of styrene in the resin mixture wherein the equivalent double bond ratio of styrene per furmarate double bond in the polymer is about 3 to 1 or about a 50% to 100% increase over conventional polyesters. The branched prepolymer development is believed to be substantially enhanced by the aromatic ring of isophthalic acid wherein the branched polymer chains cannot coil but rather are predeposed in noncoiled, extended polymer chains whereupon the fumarate double bonds introduced into the polymer in the second stage necessarily extends the polymer chains and consequently the average distance between the fumarate double bonds in the polymer increases as the polymer chains' length increases. The distance between developing polymer chains likewise are theorised to be increasing due to the volume contribution of the extended polymer chains which permits increased styrene-to-styrene (homostyrene) formation between the polymer chains when cross-linked into a thermoset solid and believed to cause the superior properties achieved in accordance with this invention.

The foregoing will become more apparent and better understood by referring to the following examples.

Example 1 A conventional reactor containing a reflux column, an agitator and heat control was charged with the following ingredients under inert gas (Nz) in the manner described hereinbelow.

isophthalic acid 1409 Ibs.

dipropylene glycol 2449 Ibs.

trimethylol propane 193 Ibs.

dibutyl tin oxide 1.47 Ibs.

hydroquinone 4.11 Ibs.

The foregoing were heated up to 4000F to 4200 F and held until Acid No. reached less than 10. The batch was then cooled to about 310"F whereupon 1988 pounds of the maleic anhydride was added. The batch was heated and held at 340 F for 9 hour whereupon an exotherm occurred at 335 F which subsided whereupon 660 pounds of propylene glycol was added and the batch was heated to 3850 F.

Processing was terminated with the Acid No. 25-30 and the viscosity at Gardner Holt of about U at 60% NVM (non-volatile matter) in styrene. The relative molar ratios of the foregoing polymer are as follows: moles isophthalic acid 2.95 dipropylene glycol 6.35 trimethylol propane 0.50 maleic anhydride 7.05 propylene glycol 3.02 The polymer was thinned with 5790 pounds of styrene containing 3.54 pounds ditertiray butyl cresol (Ionol Trade Mark) to provide approximately a NVM of 51%. Resin samples were gelled with 1% BPO (benzoyl peroxide) at 140 F and then further cured at 2000F for 1 hour, 2500F for 1 hour, and 300 F for 1 hour. The cured samples gave the following physical properties.

Tensile Test No.

Tensile strength 10.78xl03psi ASTM D638 Modulus 4.46x 105 psi ASTM D-638 % Elongation 4.18 ASTM D-638 Toughness 299 Flexural Flexural Strength 21.24x 103 psi ASTM D-790 Modulus 4.97x 105 psi ASTM D-790 % Strain 4.58 Toughness 490 Example 2 (Comparison) A series of polyester polymers were produced by charging the raw materials indicated below to esterify in one-stage processing the glycol with the dicarboxylic acids at temperatures between 185"C and 2000C to an Acid No of between 25 and 35. The polymers were thinned with styrene to provide 50% by weight polymerstyrene resin solution. The stress-strain curves in Figure 2 were produced from these polyester resins.

Resin C, rigid polyester 2.2 moles propylene glycol 1.0 moles isophthalic acid 1.0 moles maleic anhydride Resin D, flexible polyester: 1.13 moles diethylene glycol 0.70 moles maleic anhydride 0.30 moles Dhthalic anhydride Resin E, resilient polyester 0.55 moles propylene glycol 0.55 moles diethylene glycol 0.45 moles maleic anhydride 0.55 moles isophthalic acid Resin F, rigid polyester 1.0 moles propylene glycol 0.2 moles dicyclopentadiene 1.0 moles maleic anhydride The polymers were diluted with styrene to provide resin mixtures containing 55% by weight polymer. As an example, the physical properties for rigid polyester C were obtained by curing and testing in the manner set forth in Example 1 as follows: Cured Resin C Barcol Hardness 45 Inst. 934-1 Flexural Strength 14.5x 103 psi ASTM D-790 Flexural Modulus 5.8 x 105 psi ASTM D-790 Tensile Strength 7.4x 103 psi ASTM D-638 Tensile Modulus 5.6x 105 psi ASTM D-638 Tensile Elongation 1.5% ASTM D-638 Heat Distortion 216"F ASTM D-638 102"C.

Example 3 The polyester of Example 1 was mixed with a polyethylene powder having a particle size less than 30 microns as well as other compounding components, as indicated hereinbelow, to produce a low-shrink polyester system.

Composition pounds polyester Example 1 63 polyethylene powder 9 styrene 7 TBPB (tributyl perbenzoate) 1.25 zinc stearate 1.0 Calwhite 260 Maglite K (Trade Mark) 1.0 OCF-832 1/4" glass fibers 59.0 (15%) The composition was pigmentable and was suitable for use for injection, transfer, or compression molding where toughness is required of the molded article. The physical properties of the cured composition are as follows: Linear shrinkage I mil/inch (cold mold/cold part) Anti-sink Fair Profile Good Gloss Good Pigmentable Yes, uniform colour Falling Ball 1--1/2 inch-lbs.

Impact Resistance Flexural 15,000 psi Strength Flexural 1.60x 105 psi Modulus Flexural 1.77% Elongation Example 4 Similar to Example 3, the polyester of Example I was mixed with a thermoplastic branched fatty acid alkyd having an equivalent hydroxy functionality of 2.08 and produced from the following components: pounds adipic acid 32.16 propylene glycol 23.84 trimethylol propane 4.18 stearic acid 2.46 dibutyl tin oxide 0.626 The propylene glycol, trimethylol propane, adipic acid and dibutyl tin oxide were charge to a reactor, upheated under N2 blanket to about 3 F whereupon water of reaction was drawn off. The reactant batch was increased in temperature gradually to 440"F and held at 4400F until an Acid No. of about 50 was reached indicated that about 90% of the dicarboxylic groups were esterified. The stearic acid was then charged lowering the batch temperature to about 420"F which was held until a Gardner-Holt viscosity of W-X was obtained at 60 /,1 NVM in styrene. The final Acid No. was 17. The polymer solid was thinned in inhibited styrene to provide a solution consisting of by weight about 60% branched alkyd polymer and 40% styrene which was mixed with the

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Example 4 Similar to Example 3, the polyester of Example I was mixed with a thermoplastic branched fatty acid alkyd having an equivalent hydroxy functionality of 2.08 and produced from the following components: pounds adipic acid 32.16 propylene glycol 23.84 trimethylol propane 4.18 stearic acid 2.46 dibutyl tin oxide 0.626 The propylene glycol, trimethylol propane, adipic acid and dibutyl tin oxide were charge to a reactor, upheated under N2 blanket to about 3 F whereupon water of reaction was drawn off. The reactant batch was increased in temperature gradually to 440"F and held at 4400F until an Acid No. of about 50 was reached indicated that about 90% of the dicarboxylic groups were esterified. The stearic acid was then charged lowering the batch temperature to about 420"F which was held until a Gardner-Holt viscosity of W-X was obtained at 60 /,1 NVM in styrene. The final Acid No. was 17. The polymer solid was thinned in inhibited styrene to provide a solution consisting of by weight about 60% branched alkyd polymer and 40% styrene which was mixed with the polyester of Example 1 to produce a low-shrink resin for molding low-profile parts in the following ratios: pounds polyester Example 1 62 alkyd 25 styrene 13 TBPB 1.25 zinc stearate 10.0 Calwhite 270 OCF-832 1N glass fibers 65 (15%) The foregoing low-shrink composition cured to zero shrink to provide nonautomotive low profile. Molded parts are extremely flexible with good flexural strength and toughness. Reverse fall balling ball impact is 2 in.-lb. Dimensional stability and linear shrinkage properties were excellent. The cured physical properties are as follows: Linear shrinkage 0 mils/inch (cold mold/cold part) Anti-sink Fair Profile Fair Gloss Fair Falling Ball Impact 2 inch-lbs Resistance Flexural Strength 15,000 psi Flexural Modulus 1.50xl06 psi Flexural Elongation 1.50% The foregoing Examples illustrate the merits of the tough polyester of this invention providing improved flexural and tensile strengths as well as improved impact resistance and low-shrink properties. WHAT WE CLAIM IS:-

1. A method for producing a branched thermosetting polyester polymer comprising: (a) esterifying dipropylene glycol and a minor amount of a higher primary polyol selected from trimethylolpropane, trimethylolethane and pentaerythritol with isophthalic acid, up to 35 molar percent of which can optionally be replaced by terephthalic acid, sufficient in amount to provide carboxylic equivalents in excess of the primary hydroxyl equivalents, but less than the total hydroxyl equivalents of said higher primary polyol and said dipropylene glycol, to form a branched prepolymer having unreacted secondary hydroxyl groups, and (b) further esterifying said branched prepolymer with an cY,P-ethylenically unsaturated dicarboxylic acid or anhydride.

2. A method according to claim 1 wherein the carboxyl equivalents provided

by said isophthalic acid are at least equal to the sum of the primary hydroxyl equivalents of said higher primary polyol and 25% of the total hydroxyl equivalents of said dipropylene glycol.

3. A method according to claim I or claim 2 wherein the esterification of step (a) is continued to an acid number less than 10.

4. A method according to claim 3 wherein the esterification of step (b) is continued to an acid number between about 15 to 40.

5. A method according to any preceding claim wherein the relative molar proportions of reactants are: Dipropylene glycol I Higher primary polyol 0.04-0.375 Isophthalic acid 0.133-0.875 Ethylenically unsaturated dicarboxylic acid or anhydride 0.87-2.25

6. A method according to any preceding claim wherein step (B) further comprises esterifying excess carboxyls with an alkylene glycol selected from propylene glycol, ethylene glycol and neopentyl glycol.

7. A method according to any preceding claim wherein said ethylenically unsaturated dicarboxylic acid or anhydride comprises maleic acid or anhydride and the alkylene glycol is added to the esterifying mixture in step (b) only after the initial exotherm from the esterification of the ethylenically unsaturated dicarboxyic acid or anhydride with the branched prepolymer has started to subside.

8. A thermosetting polyester resin mixture comprising a polyester polymer produced by the method of any preceding claim and between 20% and 65% by weight styrene.

9. A thermosetting polyester resin mixture according to claim 8 substantially as described herein with reference to any one of Examples 1, 3 and 4.