POLYESTER Resins

This article describes the prepa-ration and use of high-perfor-mance, yet inexpensive, polyestersfor durable coatings. Starting for-mulations based on these polyestersand coating properties are alsocompared to commercial high-per-formance systems.

MPDiol Glycol (2-methyl-1,3-propanediol) is a relatively newperformance diol with a unique

combination of properties. Itsbranched structure imparts flexibili-ty in polyesters and lowers their vis-cosity. These features make MPDiolparticularly useful for the productionof polyesters for can, coil, high-solids, gel coats and powder where acombination of flexibility, toughnessand weatherability are required. Italso allows greater use of terephthal-ic acid and phthalic anhydride, two

inexpensive raw materials. After acrylics, polyesters are the

most common type of resin used inhigh-performance coatings. This isdue to their low cost, good exteriordurability, excellent mechanicalproperties and versatility. In 1997,the estimated annual U.S. demandwas more than 900 MM pounds (1).

Alkyd resins, or oil-modifiedpolyesters, are the largest type of

New polyester resins for durable coatings allowformulators to obtain high performance withoutbreaking the bank.

Bang for

The Buck

POLYESTER Resins

By Daniel B. Pourreau, coatings development manager, Performance Coatings Division, Stephen H. Harris,senior research advisor, Shanti Patel, senior research chemist, L. Joseph Junker, chemist, and Tek ChongSoon, chemist, of Lyondell Chemical Co., and Carl J. Sullivan, technical director, Ruco Polymers.

over condensation with diacids.However, the choice of inexpensiveanhydrides is limited to phthalic andmaleic anhydrides.

Several other factors affect thecost of polyesters, such as rawmaterial costs, polymerizationkinetics, energy requirements andbatch yields. Proper selection ofraw materials is the most importantfactor because it affects not only thecost of the polyester but also itsperformance. Both diols and diacids

can affect the polymerization kinet-ics, cycle times and, consequently,cost.

Diols with primary hydroxylgroups and high boiling points, such

as MPDiol and 1,6-hexanediol, reactfaster than those with secondaryhydroxyl groups or low boilingpoints.

The melting point of the diolaffects both its handling and the rateof reaction, especially with relativelyinsoluble diacids such as terephthalicacid. Liquid diols can be pumped intothe reactor and help solvate solidacids and anhydrides. This generallyresults in faster esterification kineticsand lower production costs.

RAW MATERIAL SELECTIONTypical U.S. prices for

polyester raw materials areon page 36.

Maleic anhydride is unsat-urated and is used predomi-nantly in the synthesis ofunsaturated polyesters for gelcoats and composites.Phthalic anhydride is mostlyused in lower-performancecoatings because it is per-ceived to produce polyestersless resistant to UV light andwater than the more expensiveisophthalic or adipic acids.Terephthalic acid is usedextensively in powder coat-ings because of its low costand hardness. Its lower UVstability and esterification ratelimits its use in other coatingsystems.

Higher-priced diols suchas neopentyl glycol are stillpredominantly used forweatherable polyesters. Someneopentyl glycol producershave also advanced the theo-ry that polyesters degrade byfree-radical abstraction of thediol beta-hydrogens, followedby carbon-carbon bond cleav-age.

This has led to the misper-ception that only the moreexpensive diols and diacids

can be used to produce weatherableor corrosion-resistant polyesters fordemanding applications. In reality,P D i o l / i s o p h t h a l a t e /adipates are known (2) to give

polyester and have been decliningat a rate of 2 to 3 percent per yearmainly because of VOC regulations.Other polyesters, on the otherhand, have been growing at anaverage annual growth rate of 6percent, spurred by the growth ofpowder and extrusion coatings, andunsaturated polyesters for gel coats.Lately, saturated polyesters for coilcoatings have also experiencedstrong growth in the United Statesdue to strong cyclical demand forconstruction materials.

Aside from VOC restric-tions, several new regulations,raw materials and health con-cerns are driving growth orreformulations in the polyesterresin market. In the compos-ites area, resin producers arelooking for lower-viscosityresins to reduce their styreneemissions and are reevaluatingUV-curable systems. Recentconcerns over the long-termhealth effects of bisphenol-A-based epoxies are also prompt-ing can-coating manufacturersto evaluate polyesters for theinterior of can coatings.

New raw materials, such as2-methyl-1,3-propanediol(MPDiol), are also generatingnew opportunities to develophigh performance polyestersat a lower cost. This articledescribes several types ofpreparations, formulationsand performances of poly-esters for coil, exterior can,high solids and gel coats.

POLYESTER SYNTHESISPolyesters are prepared by

condensation of diacids oranhydrides and diols. Thebyproduct water must beremoved from the reactor bydistillation. This makes poly-ester synthesis more energyintensive than, say, acrylate polymer-ization, which produces no waterand is exothermic. Condensation ofdiols with anhydrides produces lesswater and is, therefore, preferable

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weatherable coatings suitable forexterior applications, despite the factthat MPDiol has a beta-hydrogen.Frontier molecular orbital calcula-tions also predict that other sites onthe polyester are far more susceptibleto free-radical attack (3). Morerecently, we have found that a similarpolyester is suitable for exterior canapplications and that low-costMPDiol/anhydride polyesters canproduce weatherable high-solidscoatings and gel coats. Thisinformation is reviewedbelow.

COIL COATINGSPerformance require-

ments for coil and othertypes of coatings apply toboth the formulation andfilm properties.

Formulations must bestorage stable, have low-VOCcontents and be inexpensive.Coil coatings are applied toflat metal stock, heat curedand the coated metal is thenformed into various shapes.Typical applications includealuminum siding, gutters,truck trailers and appliances.

Because the parts arecoated before they areformed, flexibility and adhe-sion are critical performancerequirements for coil coat-ings. Typical requirementsinclude zero T-bend flexibili-ty, excellent adhesion, hard-ness and abrasion resistance,excellent exterior weather-ability, stain, detergent andcorrosion resistance

Because of the extremeweatherability requirements,polyesters for coil coatingapplications had traditional-ly been prepared with neo-pentyl glycol. However, thesuperior flexibility andexcellent weatherability ofpolyesters made from MPDiol (2-methyl-1,3-propanediol) has ledseveral coil coating producers toconvert to this new diol. A represen-tative resin composition, coating

formulation, and performance dataare listed in this article. (This poly-ester coating outlasted a commer-cial neopentyl glycol-based coatingunder Florida exposure condi-tions.)

CAN COATINGSThe use of polyesters for can coat-

ings is limited to beer and beverage(B&B) exterior basecoats. However,recent concerns over the health

effects of bisphenol-A are promptingsuppliers to re-evaluate their use ininterior applications as well. The useof polyesters in exterior basecoats isinteresting in that it demonstrates

the utility of polyesters in low-VOCwaterborne formulations for a fairlydemanding coating application.

Typical formulations blend water-reducible acrylic and polyesterresins in a melamine-crosslinkedbaking enamel. The following exam-ple, however, demonstrates that anall-polyester enamel can also meetthe performance requirements forthis application and may also besuitable for interior applications.

The performance datamentioned in this articledemonstrates that an all-polyester enamel can meetthe demanding performancerequirements for B&B andeven food cans. Boilingwater resistance is requiredin beer and beverage cansbecause of the pasteurizationstep that follows the coatingand filling operation.

Food cans are subjectedto even higher heat condi-tions for sterilization andmust retain gloss and lowcolor under retort condi-tions. This suggests thatproperly formulated poly-ester enamels may be suit-able for interior can applica-tions as alternatives tobisphenol-A-based epoxy-acrylates.

HIGH-SOLIDS COATINGSHigh-solids, thermoset-

ting, solventborne polyestercoatings are used in a varietyof applications, includingprimers for automobiles andtopcoats for metal furnitureand appliances. Low-viscositypolyesters are obtained bymaintaining a 15 to 20 per-cent excess of diol overdiacid. This limits the poly-ester molecular weight (Mn)to less than 2,000, which low-ers the viscosity but also typi-

cally reduces the coating’s weather-ability. Popular diols are neopentylglycol and 2,2,4-trimethyl-1,3-pen-tanediol. Small amounts of multi-functional polyols, such as trimethy-

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Polyester Raw Material Prices

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Effect of Diol on Polyester Production Rates

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Polyester Resins Are Big BusinessUS Market: 420 KT in 1997

Alkyd

Saturated

Powder

Extrusion �coated

Unsaturated

Skeist Coatings VI, 1998

lolpropane, are also used to increasethe crosslink density and improvefilm properties. On the acid side,phthalic anhydride is often used tolower cost and adipic acid is used toimprove flexibility and UV-resistance.A typical resin composition is 45 per-cent neopentyl glycol, 40 percentphthalic anhydride and 15 percentadipic acid (4).

Using MPDiol instead ofneopentyl glycol increases polymer-ization rates and improves the flexi-bility of the polyester, requiring lessadipic acid to be used. This lowersraw material and production costs,gives formulations superior storagestability and provides coatings withgood mechanical properties andexcellent weatherability. This isillustrated by the following example.

The film properties of this low-cost, high-solids coating were compa-rable to those of a commercial con-trol, except for lower impact resis-tance and higher hardness (100/80and 2H for control). These propertiescan be easily adjusted by loweringthe melamine/polyester ratio to 20/80or increasing the adipic acid content.On the other hand, the Floridaweatherability of the low-cost poly-ester was significantly better than thecommercial control.

GEL COATSConsumption of unsaturated

polyesters for gel coats in the UnitedStates was estimated to be over 60MM pounds in 1996 (5). Gel coatsare the exterior coatings for fiber-glass-reinforced composite productssuch as recreational boat hulls, bathtubs and shower stalls and exterior

truck parts. They are considered sol-ventless systems because styrene isused to dissolve the resin andbecomes part of the coating duringthe curing process. However, gelcoats and composites fabricationemits significant amounts of styrene,which has recently become an issuein the U.S. As a result, gel coat man-ufacturers are looking for lower-vis-cosity polyesters to reduce thestyrene content of their coatings.

Proper selection of raw materialsis critical in this application as well.Cost and performance requirementsvary significantly in this market, butgel coats are considered to be high-end products and have stringent per-formance requirements. In additionto maleic anhydride, the standardraw materials for gel coat polyestershave been neopentyl glycol andisophthalic acid. This is because theresulting gel coats have superiormechanical, UV and corrosion resis-tance compared to resins made frompropylene glycol and phthalic anhy-dride (“ortho” resins). However, ourresearch shows that properlydesigned, MPDiol-based ortho resinscan match the performance ofneopentyl glycol-based “ISO” resinsand provide the polyester manufac-turer with significant cost-savingopportunities. Based on typical rawmaterial costs in the United States,the MPD/PAN resin mentioned earli-er is estimated to cost 20 cents perpound less than an NPG/ISO resin.This is very significant, consideringtheir performance is comparable.

Aside from lower cost, the MPDortho resin had lower solution vis-cosity, improved storage stability and

yellowed less under UV-A exposure.The mechanical properties of clearcastings and gloss retention underUV-A exposure were comparable tothe neopentyl glycol/iso resin.

CONCLUSIONA number of factors are creating

opportunities to develop new high-performance polyesters at lowercost. Strong growth in powder coat-ings, composites and coil areincreasing the market need forpolyesters. Health concerns overbisphenol-A are also opening thedoor for the use of polyesters in tra-ditional epoxy applications, such asinterior can coatings.

Finally, new raw materials, suchas MPDiol, are allowing for greateruse of inexpensive diacids, such asphthalic anhydride and terephthalicacid, for the manufacture of high-performance, yet lower-cost, poly-esters.

REFERENCES1. Coatings VI, Skeist Inc.,

October 1998.

2. Carl Sullivan, High SolidsCoatings, Vol. 15, 1994.

3. Carl Sullivan and Charles F.Cooper, Journal of Coatings Techno-logy, Vol. 67, No. 847, August 1995.

4. Coatings VI, Skeist Inc.,October 1998, p. 838.

5. Reinforced Plastics II, SkeistInc., December 1992, p. 568.

This article was published in the October 1999 issue of Modern Paint & Coatings magazine and appears

here with permission.LCC-PC149-0108

For more information, please call Lyondell at 1-888-777-0232.