low gloss powder coatings. Apaper presented at PowderCoating 2004 by Degussadescribes another technique forobtaining low gloss inpolyurethane powder coatingsusing a blend of amorphous andcrystalline polyester resins. Theabstract is longer than normaldue to the extent of the trialscarried out with this novel systemto assess the effect upon thereproducibility of gloss andmechanical properties, whendifferent resins, curatives andpigment blends are used in theformulation. This comprehensivework also covers processingdetails and consistency, the effectof formulation upon weatherability,and storage stability. A useful aidto formulators charged with thetask of formulating low glosspolyurethane powder coatingsystems.

The third abstract describesthe modification of pearlescentpigments to overcome theproblems associated with dryblending of the unmodifiedpigments. It is a valuable tool forapplicators who can adjust theappearance and lustre of powdercoatings, on site.

A modern trend in thepublication of conferenceproceedings is the inclusion of anexcessive number of slidepresentation items that may wellhave been relevant to theaudience, but are almost entirelyunintelligible to readers of theconference proceedings.Presentations based completelyupon repetitive slides are a sureindication of the speaker’s lack oftechnical knowledge of thesubject under discussion, andtheir publication in printed form isa shear waste of good qualitypaper! One conference that wouldnot have tolerated a slidepresentation was the annualInternational Conference inOrganic Coatings Science andTechnology held in Athens underthe guidance of the late ProfessorPatsis. This conference wascancelled in 2004 but it is

scheduled to be replaced byCoatings Science International(COSI) in the Netherlands at theend of June 2005. It will containcontributions from both industrialand academic researchers, andthe new location should attractmany more delegates.

Sid Harris

TECHNICALNew additives for powder coatings

Additives are employed in powdercoatings to improve specificcoating properties, and thetraditional materials are flowmodifiers, degassing agents andUV stabilizers. A paper by RonGuida of EMS Primid describesthree new, speciality additives.

Additives can be either reactiveor non-reactive. Reactiveadditives can take part in thechemical reaction during thecuring process of thermosettingpowder coatings, or interact withexternal environmental factors. Incontrast, non-reactive additivesinfluence the physical propertiesof the coating without influencingthe curing reaction. Commonreactive additives include:catalysts, antioxidants, UVabsorbers, hindered amine lightstabilizers, chemical brighteners,certain matting agents, adhesionpromoters and photo-initiators.Non-reactive additives includeflow modifiers, texturizing agentsand degassing agents.

The first two new additivesdescribed were designed toimprove specific deficiencies thatare common to all powder types,while the third was developed toproduce reduced gloss finishesand to enhance other criticalproperties of the coating.

Impact fading is the reductionor complete loss of mechanicalproperties over time, and isrelevant to all binder types. Thisdeterioration is due to thedevelopment of internal stresses

and the absorption anddesorption of water. The firstadditive discussed is an impactmodifier that can be used toextend the mechanicalperformance of cured films basedon standard durable polyesters,and also improves the initialimpact strength of super durablepolyesters. The product is a highmolecular weight co-polyester thatis supplied as a white, free-flowing powder. It is non-toxicwith a melting point of 70°C.

In the assessment of theimpact fading additive, thereverse impact test was used tomonitor the impact strength of thecoatings. Films were applied at 2-3 ml to AL-36 panels and aftercuring were subjected to 24 hourexposure cycles, consisting of a16 hour humidity period at 40°Cand 100% humidity followed byan 8 hour dry period at 40°C and10% humidity. Coatings weretested following each 24 hourcycle and continued until failurewas detected.

The time required for asignificant or complete loss ofimpact strength varies anddepends upon a number offactors. To minimize the influenceof external factors the studyfocused on a single bindersystem based on ahydroxyalkylamide hardener and acarboxyl functional polyester withan acid value of 33 mg KOH/g.Ratio of resin to curative was95:5.

It was found that powdercoatings without the impactmodifying additive failed after 100 to 200 hours exposure. Theimpact strength of formulationsthat contained 1% by weight ofthe additive was extended to 300-500 hours. Variations in thefailure development, dependedupon the pigmentation used. RAL8014 Sepia Brown modifiedcoatings maintained 85% oforiginal impact strength after 350hours testing. RAL 3016 CoralRed panels had a faster failurerate than the brown, although theadditive was effective in retarding

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the degradation of mechanicalstrength. Similar trends wereobserved with RAL 9010 PureWhite; RAL 5016 Sky Blue andRAL 6005 Moss Green.

In super durable systems it iswell known that the higherisophthalic acid concentration hasan adverse effect on cured filmflexibility. The results of the testsshow that this additive improvesthe initial impact strength ofpowder coatings that areformulated with super durablepolyesters. Degree ofimprovement in mechanicalproperties is dependent on theamount of the additive present inthe formulation. Addition of 1% byweight of additive can result in asuper durable coating with fullimpact strength usingconventional baking schedules.

This impact modifier can beused for exterior applicationswithout reducing the UVresistance of the cured coatingsand had no adverse effect ongloss retention and chalking.

Environmental stress crackingis a common problem for manyplastic materials includingtransparent powder coatings. Thisfault is defined as the acceleratedfailure of a polymeric material dueto the combined effect of stressand environmental exposure.Environmental factors may beresidual, assembly, or applicationinduced and may include adynamic component caused byvibrations and thermal orpressure fluctuations. Exposureto organic fluids such asindustrial oils or solvents andoutdoor weathering can alsocause cracking in the coating,which can seriously compromisemechanical integrity.

The additive discussedimproves the flexibility of thecoating and increases theresistance to environmentalfailures due to stress cracking.This additive has no influence onother critical properties. It is anon-reactive co-polyester, which issupplied as a white powder with amelting point of 80°C.

Incorporation at levels of 3% byweight results in powder clearcoats with higher and moreconsistent performance levels.

There are a number of ways inwhich stress cracking resistancecan be evaluated and the mostreliable method is to analyze itsperformance under actual end-useconditions. The internal testapplied in this evaluationcombines the application ofexternal stress with direct contactto an organic solvent. Thetransparent coating is applied atapproximately 6 ml to an AL-36panel and cured at the correctbake schedule. Testing isperformed within 24 hours ofcuring. The test panel is securedin a straining rig and undergoesinternal buckling at 2 centimetreincrements. Following eachincrement the substrate,remaining secured in the strainingapparatus, is immersed in pureethanol for 30 seconds. Thecoating is removed from thesolvent and visually inspected forfilm faults, such as cracking orcrazing. This test cycle iscontinued until film failure isdetected. Extensive testing of thestress cracking additive wasperformed in a hydroxyalkylamidebinder system at levels rangingfrom 1% to 5% by weight of thetotal formulation. Improvementswere noted at 1 and 2% levels,but addition levels of 3% showedthe optimum stress crackingresistance without sacrifice ofother critical properties.

Transparent powder coatingsare applied as a protectivecoating on brass products andalso as the final coating onbicycle frames, automotive wheelsand full body automotive frames,and the ability of the clear coatingto be resistant to discolourationupon extended baking cycles orelevated cure schedules is acritical requirement. Nodifferences in total colour changewere detected in films containingthe recommended usage levels ofthe stress cracking additive.Extended baking schedules

included 15 minutes at 180°C and15 minutes at 200°C, followingthe standard cure schedule.Resistance to weathering is alsoof importance and short durationweathering tests carried out onthe modified powder coatingsshow that the additive has noinfluence on the UV resistance ofthe cured coating. There were noapparent differences in glossretention at the 3% addition level.Products evaluated wereformulated with two differentindustrial clear coat polyestersand did not contain UV stabilizingadditives. Although the tests werecarried out with hydroxyalkylamidesystems, this non-reactive additiveis effective in other binder types.

The final product discussed isan additive that provides veryeffective matting for satin or semi-gloss powder coating finishes.The resulting coatings giveexcellent gloss stability, very goodflow, and outstanding yellowingresistance. It also acts as aprocessing aid during extrusion orfine grinding, and improves theslip resistance of the film. In awhite polyester hydroxyalkylamidesystem a gloss reading of 65 at60° angle is achieved with 2% ofthe additive, while the sameamount of commercialpolypropylene or polyethylenewaxes give gloss readings thatare 10 to 20% higher. Theconcentration of additive affectsthe gloss reading and glosslevels between 65 and 80 can beachieved. It is possible to reducethe level to 50 to 55 units byformulating with appropriatefillers.

The ability to maintainconsistent gloss levels over awide temperature range isimportant since temperaturevariations within a single oven orbetween different ovens can resultin gloss fluctuations on differentsections of a profile. The problemis often apparent on profileshaving sections with differentthicknesses of metal. Tests haveshown that the gloss levels withthis additive remain virtually

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unchanged over a temperaturerange from 165°C to 220°C.

Paper entitled “Performance EnhancingAdditives for Powder Coatings” by RonaldGuida of EMS-Primid presented at PowderCoating 2004, Formulator’s TechnologyConference in Charlotte NC, on 20-21 Sep2004. Bound copies of the Conferencepapers available from the Organizers, ThePowder Coating Institute, 2121 EisenhowerAvenue, Suite 401, Alexandria, VA 22314,USA

One-shot, low gloss, polyurethanepowder coatings

A paper by Corey King ofDegussa Corporation introduces anovel method of reducing thegloss of a powder coating by theuse of a crystalline polyesterresin. This crystalline resin has ahigh melting temperature ofaround 115°C and limitedcompatibility with the amorphouspolyester resins that are typicalfor powder coatings. Thecrystalline resin melts and mixeswith the amorphous resin duringextrusion, then crystallizes rapidlyupon cooling. In the curing cyclethe crystalline resin reacts withthe polyisocyanates, andrecrystallizes after cure. Thereare, however, many variables thataffect this method of glossreduction. The crystalline resinwill have different solubilities inthe amorphous resin dependingupon the structure of theamorphous resin, which will affectthe crystallization process. Thestoichiometric ratio ofpolyisocyanate to polyester resinis also likely to affect the glossreduction. Additionally, theamounts and types of fillers andpigments could affect the overallvolume of crystals present persurface area unit of the coating,thus affecting the gloss level. Ifthe crystals do not fully melt inthe extruder the coating may notfully achieve the level of glossassociated with a well or poorlydispersed crystalline resin. Thepaper is presented to promote abetter understanding of how thecoating formulation and theprocessing of the coating will

affect the coating gloss andperformance properties.

A number of variables wereinvestigated to determine theireffect on coating gloss, powderstability, coating physicalproperties, and acceleratedweathering characteristics. Theratio of crystalline resin wasexamined as a function of totalresin by weight, and variedbetween 0% and 50% (half thetotal resin is crystalline). In thesetests there was one crystallineresin, CR30 with an OH value of30. There were two super durablepolyester resins, SP40 and SP85, with OH values of 40 and 85respectively, and there were twostandard durable polyester resins,P40 and P50, with OH values of40 and 50. The NCO:OHstoichiometric ratio was variedbetween 0.8 and 1.0, though itwas kept at 1.0 unless statedotherwise. A polyisocyanateblocked with ε-caprolactam, B1,and two uretdiones, U1 and U2,were used in this study. Theaverage NCO contents for thesethree products were 15.3%,16.1% and 14.0%, respectively.

The pigmentations of thesesystems primarily consisted of30% titanium dioxide unlessstated otherwise. Some blackformulations were prepared, andin place of the titanium dioxideconsisted of 1% of carbon blackand 20% of micronized bariumsulfate. The amount of resin(s)and crosslinker was increased tomake up the difference. Additivesin all formulations consisted of1% flow and levelling agent 0.5%benzoin.

Coating components were pre-mixed using a high speed blademixer, then extruded with a singlepass through a twin screwextruder at zone temperaturesbetween 80-100°C, and typicallywere 85°C/100°C for zones 1 and2. The extrudate was crushedand ground, followed by sievingto less than 100µm. Powder wasthen applied to cold rolled steelat film thicknesses of 50-75µmand cured at 200°C for 10-12

minutes. Cured films were testedfor direct and reverse impactresistance and 60° gloss. Theywere also exposed to UV-A340nm radiation in accordancewith ASTM G154, using 6 houralternating UV and condensationcycles. Powder stability was alsodetermined for certain coatingsusing a 40°C water bath, with thepowder sprayed after a certainperiod of time, cured, and testedfor gloss, gel time and impactresistance.

Experiments were designed toshow the effect of the crystallineresin on coating gloss andperformance. Super durable resinSP85 was substituted by 30-50%of CR30 in formulationscrosslinked with differentpolyisocyanates. The amount ofpolyisocyanate was adjustedaccordingly to achieve astoichiometric ratio of 1:1. It wasfound that the gloss decreaseddramatically as the level of CR30was increased to 50% and thegloss value depends on theweight% of CR30 and thecrosslinker type that was used.Gloss values of the coatingsbased on U1 and U2 uretdioneswere virtually identical, whilethose based on B1, the ε-caprolactam blocked crosslinker,were much lower atcorresponding amounts of CR30.Gloss values in the range of 15-40 were easily obtained using thistechnique of mixing CR30 withthe polyester resin when usingpercentages of CR30 higher than30%. This is due to thecrystallization of the CR30 withinthe coating after cure and duringcool-down, and shows that thelevel of CR30 is critical to thematte formulation.

The most popular method ofproducing low gloss systems is togenerate multiple reaction rates,and therefore the gloss level issensitive to reaction stoichiometry.The NCO:OH ratio was variedfrom 0.8 to 1.0 using all threedifferent crosslinkers, and thesame trend was observed in allcases: the gloss decreased as

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