The number of powder coatingsystems capable of UV cure hasbeen increased by the simpleexpedient of combiningmethacrylated polyesters withacrylated epoxy resins. Anabstracted paper from the PowderCoating 2004 conferencedescribes development workcarried out at Surface SpecialtiesUCB, adding UV curable versionsof the widely acceptedpolyester/epoxy binders that areclaimed to produce films whichare very smooth, and exhibit goodadhesion and chemical resistancewhen applied to wooden furniture,metal, and plastic or tonerapplications. The presentation isuseful for its comprehensivecoverage of the variables that canbe adjusted to optimize the hybridcombination of these systems.

Imidazoles have beenemployed for many years ascuring agents for epoxy resinsand as catalysts in promoting thereaction of the epoxy group withother polar groups. The extensivepaper by David Green, however,does little more than confirm whatformulators have long knownabout the severe limitations ofthese materials. It is a pity thatthe potential of cyclo-aliphatic ringstructures has not beenresearched more rigorously in the

quest for novel powderchemistries, for there are manystructures with greater potentialthan the imidazoles.

The paper by Chuck Danickprovides good coverage of thedo’s and the don’ts that must beobserved at all stages offormulation, pre-treatment, andapplication of powder coatings tooptimize the mechanicalproperties of the cured film. Hedoes not, however, make anydistinction between the effect ofunder curing and over curing. Inpractice, it is found that undercuring can have a disastrous andirreversible effect on filmperformance while most powdercoatings are generally quitetolerant of stoving above orbeyond the cure temperature andcure time of the coating. Correctobservance of the manufacturer’sstoving recommendations is thekey factor in achieving consistentperformance, but his catalogue ofother influences on mechanicalproperties is a useful check list.

One of the major problemsfacing the formulator of exteriordurable powder coatings is thetime consuming wait for durabilitycharacteristics even whenaccelerated tests are applied, andin many cases the results areoften inconsistent. In the October

A MONTHLY REPORT FROMSID HARRIS

JANUARY 2005

In this issue

TECHNICAL 2-6Performance of polyester/epoxy

UV powder coatingsImidazole curing agentsImproving flexibility and impact

resistancePredicting performance by

analytical techniques

INDUSTRY NEWS 6-8Return to profits for DuPont in

3Q 2004Akzo Nobel 3Q 2004: Coatings –

clearly up due to growth andacquisitions

China polyester powder: DSM toexpand coating resins

Eco-paint

NEW PRODUCTS 8New curing agent for powder

coatings

MARKETS 8Rosy outlook for paints

P O W D E R C O A T I N G SFOCUS ON

POWDER COATINGS POWDER COATINGS POWDER COATINGS POWDER COATINGS

AN INTERNATIONAL NEWSLETTER MONITORING TECHNICAL ANDCOMMERCIAL DEVELOPMENTS IN POWDER COATINGS ISSN 1364–5439

PROFITING FROM TECHNICAL EXPERTISE

issue, a method of predictingdurability and life expectancy ofpowder coating films by thermaldegradation kinetics wasdescribed, but it seemed to be arather elaborate technique forselecting promising recipes priorto natural ageing exposure. EricDumain describes an alternativemethod that could be the answerto speeding developmentprogrammes by a more logicalanalytical procedure thatcompares the relative responsesof polyester resins to UV visiblespectrometry. If the effect of longterm exposure to UV could becorrelated to an initial response toUV light in the visible range thenthis would drastically reduce thetime spent in the selection ofpromising powder systems. It is alogical method for sifting andfinger printing the most likelyformulations if the results quotedby Dumain are as reliable as theywould seem from the testsoutlined in the paper.

Many years ago, I visited theheadquarters of Asian Paints inMumbai, and during technicaldiscussions I was impressed bytheir competence and capabilities.The widespread influence ofAsian Paints in the Asian markethas not been diminished by theinflux of international groups intothis lucrative market zone. Whilethe other major companies havesought to strengthen their hold onthe coatings market by focusingheavy capital investment inChina, Asian Paints have wiselydecided to abandon growth byacquisition and to cut back inthose areas that are notcommercially viable, concentratingon technical excellence in acompetitive global climate. Theyhave decided to licence theirbrand names and technology togenerate additional revenue, andit is a commendable alternative tobattling against increasedcompetition in traditional areasand the current rapid increase inraw material costs. Anotherinteresting decision is to withdrawfrom the Chinese market and

concentrate on other growthareas in Asia.

Sid Harris

TECHNICALPerformance of polyester/epoxy UVpowder coatings

Many resin systems are nowcommercially available for theformulation of UV powders curedby a free-radical polymerization.These include acrylated ormethacrylated polyesters orpolyacrylates, fumaric acid basedpolyesters alone or incombination with solidvinylethers, acrylated ormethacrylated epoxy resins andunsaturated urethane basedsystems. Solid grade epoxyresins that are UV cured using aphoto cationic mechanism arealso available. Combining solidgrade UV curable polyester andepoxy chemistries it is possibleto achieve film performance formarkets using substrates suchas wood, wood composites,plastic and metal.

Hybrid UV cured powdercoatings based on methacrylatedpolyester with acrylated epoxyresins give an interesting blend ofproperties when cured. Inclusionof a polyester backbone improveslight resistance of the coatings inweathering tests, while the epoxybackbone gives outstandingchemical resistance, improvedadhesion and smoothness.

Manufacture of a UV curable,hybrid polyester/epoxy blend canbe achieved in four stages. Thefirst stage is the formation of theUV curable polyester componentby polycondensation of adicarboxylated acid derivative witha glycol at 240°C in the presenceof an esterification catalyst suchas butyl stannoic acid, to producea carboxy-terminated polyester.Next, glycidyl methacrylate (GMA)is added to the molten carboxyterminated polyester while it iskept below 200°C, and

methacrylate groups are graftedto the end of polyester chains bya fast addition reaction. Inhibitorsmust be used to prevent gelation.The third stage comprises theaddition of acrylic acid to amolten diepoxy resin to give anepoxy diacrylate polymer. Finally,the methacrylated polyester andthe acrylated epoxy resin arehomogeneously blended byextrusion.

The following variables havebeen evaluated to optimize thehybrid combination: selection ofaromatic diacid from ortho-, meta-and tere-phthalic acid; partialsubstitution of the selectedaromatic diacid by an aliphaticdiacid such as adipic acid or 1,4-cyclohexanedicarboxylic acid;partial substitution of NPG byother glycols; and branching thepolyester chains by incorporatingpolyfunctional monomers such asTME or TMA. Percentages weredetermined by Flory’s equation toprevent gelation.

The physical characteristics ofa simple methacrylated polyesterresin with two polyester/epoxywere compared in the firsttabulated results. The trials usedthe same polyester blend mixedwith an acrylated type 3 epoxyresin based on bisphenol A andepichlorohydrin in different ratios.It is apparent that there isdifference in melt viscosities whenthe polyester/epoxy materialsdescribed above are added to themethacrylated polyester. Thesecond of these blends showed areduction of more than 60% inmelt viscosity with a reduction of5°C in Tg. This had the highestfilm smoothness whilst retaininggood storage stability andhandling properties.

A formulation is given forpigmented products based onthese hybrid materials. Theseproducts were applied and meltedusing a combination of mediumIR radiation followed byconvection heating and curing byUV irradiation. The whole cycle iscompleted in approximately threeminutes and the temperature is

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F O C U S O N P O W D E R C O A T I N G S