Novel Acrylic Epoxy Hybrid System Design For Metal Protection Application
Novel Acrylic Epoxy Hybrid System Designfor Metal Protection Applicationby Zhigang Hua,Hunter Shu, Allen Xu,Barry Zheng, andAaron Yang,Dow ChemicalCompany (China)Zhenwen Fu,Andrew Hejl,Henry Eichman, andKiran Baikerikar,Dow ChemicalCompany (USA)58April 2015COATINGSTECHA process for producing a novel acrylic epoxyhybrid (AEH) is demonstrated in which a small molecule epoxy diffuses from its “monomer” dropletsthrough the aqueous phase and into an acrylic dispersion particle. The obtained hybrid can be formulated as a viscosity-stable 2K coating system by mixing pigments and fillers with waterborne polyaminehardener as Part A and the acrylic epoxy hybrid asPart B. The 2K coating formulation stability is controlled by the hybrid and hardener surface chargereflected in zeta potential values. When the zetapotentials of the hybrid and hardener were in therange at ζAEH ζhardener 30, the 2K coating formulation showed instability and a tendency toform gel. The 2K coating system performance reliesheavily on hardener choice for metal protectionproperties such as salt spray corrosion resistance. Itwas found that water solubility of hardener is negatively correlated to salt spray resistance (SSR). Withan appropriately low water solubility hydrophobichardener, the 2K coating system can achieve 600hr SSR, which is much better than a 1K acrylic and2K acrylic/liquid epoxy resin (LER) system and similar or slightly worse than a conventional 2K waterborne epoxy system. Furthermore, the 2K coatingsystem dries faster than a traditional 2K WB epoxysystem but has much longer pot life. In addition,This paper is a further development based on the workawarded First Place in the Roon Awards competition whenpresented at the 2014 American Coatings Conference,sponsored by the American Coatings Association andVincentz Network, in Atlanta, GA, April 7–9, 2014.its application properties, such as dilution stability,are also excellent due to the better stability of theacrylic latex polymers.INTRODUCTIONThe effort to replace solventborne coatingswith waterborne alternatives has been ongoingfor several decades. Acrylic latices are the mostpopular binders for waterborne coatings. Acrylicshave been well accepted for architectural coatingsand used for more than 60 years. However, forindustrial coatings, the use of waterborne coatingsystems is still very limited. Compared with solventbased systems, acrylic latex coatings have inferiorbarrier and resistance properties because of poorfilm formation and little to no crosslinking.There are many efforts to overcome thesedrawbacks. In our recent work,1 a novel DesignedHybridization technology was developed as aunique way to improve the performance of thermoplastic latex with a thermosetting chemistry.The model and mechanism of this novel DesignedHybridization technology is shown in Figure 1.A host latex particle is first made via free radical emulsion polymerization of vinyl monomers.Thermosetting or reactive small molecules are thenimbibed into the latex particles by allowing thesmall molecules to diffuse from their “monomer”droplets through the aqueous phase into the dispersed particle.
An example of this Designed Hybridizationapproach is an acrylic epoxy hybrid latex composite. It is prepared by imbibing liquid epoxy resin intoacrylic particles. The imbibed liquid epoxy acts as acoalescent to assist the high MW acrylic polymer infilm formation, and the latex acts as a controlledrelease host of the liquid epoxy to enable longerpot life. As the epoxy diffuses out of the latexand cures with hardeners located in the aqueousphase, the high MW acrylic polymer regains itshardness. In the cured film, the acrylic latex polymer domains are distributed in a continuous phaseof cured epoxy. This special design enables acrylicepoxy hybrid latex composite films to show barrierand resistance property performance similar tothe thermosetting 2K epoxy system. At the sametime, the acrylic component enables longer pot life,faster dry, better UV resistance, easier handling,and lower cost compared with traditional waterborne 2K epoxy coatings.In this article, an acrylic epoxy hybrid latex composite system based on Designed Hybridizationtechnology was studied for metal protection application. The hardener selection was found to beone of the critical factors for paint stability andfinal performance. The performance advantagewas compared with traditional 2K epoxy and acryliclatex-based systems.EXPERIMENTALRaw Materials and FormulationsAcrylic Epoxy Hybrid (AEH) PreparationThe process of making the DesignedHybridization-based acrylic epoxy hybrid (AEH) isillustrated in Figure 2a. A waterborne acrylic latexwas produced according to a standard seeded process2-4 in a 5000 mL five-necked reactor equippedwith a stirrer, reflux condenser, and thermometer.An epoxy emulsion of bisphenol A diglycidyl ether,Figure 1—Model and mechanism for 2K application of the novel Designed Hybridization.such as DER 331 from Dow, was made by thephase inversion method. With a temperature of60 C, the epoxy emulsion was fed into the reactor containing the acrylic latex and held for 1 hr toallow the epoxy to transport and swell the acryliclatex. The key requirement was that the epoxy wascompletely imbibed in the acrylic particles.The driving force for this transport process isthe solubility of the epoxy molecule in the latexpolymer, not a reaction. As a result, there is a saturation limit for the swelling and it was found that20–40% by weight of epoxy can be fully imbibedinto the acrylic latex polymer, depending on theacrylic latex polymer composition and type ofepoxy. This imbibing process typically takes from 5min to 2 hr at 60 C, depending on the size of theepoxy monomer droplets, acrylic latex composition,and particle size. In Figure 2b, the optical microscopy images were taken at different times after theepoxy monomer droplets were added to the latex.Figure 2a—Process for making AEHdispersion.Figure 2b—Optical microscopy images after epoxyaddition to latex.April 2015COATINGSTECH59
Table 1—Raw Materials for Coating FormulationTrademarkFunctionSupplierBentone LTOrotan 731AAcrysol RM-8WSN321CTi-Pure R-902Nubirox 106TalcBlanc Fixe NDowanol PMDowanol DPnBNaNO2Silquest A-187Beckopox EH 613Beckopox VEH 2849Anquamine 401OudraCure WB 8005OudraCure WB 8006Rheology modifierDispersantRheology modifierDefoamerPigmentAnticorrosive PigmentFillerFillerCoalescenceCoalescenceRust InhibitorAdhesion ementis SpecialtiesDow ChemicalDow ChemicalSan Nopco LimitedDuPont Titanium TechnologiesCaryLocalSachtleben Chemie GmbHDow ChemicalDow ChemicalLocalGE Toshiba SiliconesCytec Surface SpecialtiesCytec Surface SpecialtiesAir ProductsDow ChemicalDow ChemicalOudraCure WB 8007HardenerDow ChemicalTable 2—Parameters of Waterborne HardenersTrademarkAnquamine 401Beckopox EH 613Beckopox VEH 2849OudraCure WB 8005OudraCure WB 8006OudraCure WB 8007TypeSolid%AHEW asDeliveredModified aliphatic amineAliphatic polyamine adductAliphatic polyamine adductAqueous polyamine adductAqueous polyamine adductAqueous polyamine adduct708080658070200145135220200200The particles in the images are epoxy monomerdroplets and the latex particles are too small to beseen at the magnification. In about 1 hr, only a fewepoxy droplets could be observed and had totallydisappeared at about 4 hr.Raw Materials for Coating FormulationThe raw materials for coating formulation arelisted in Table 1.Coating Formulation and PreparationA starting point formulation (SPF) was developedbased on AEH dispersion. To make a coating with viscosity stability, it is preferred to mix pigments and fillers with waterborne hardener as Part A, and have theAEH dispersion as Part B. Part A and Part B should bemixed well before application.To compare anticorrosive performance, sixwaterborne hardeners were selected from Cytec,Air Products, and Dow (UPPC), respectively. The keyparameters of these hardeners are listed in Table2. The use level of each waterborne hardener wasvaried to account for their differing active hydrogenequivalent weights (AHEW). For better anticorrosive60April 2015COATINGSTECHperformance, the stoichiometry of epoxide equivalent weight (EEW)/AHEW was selected as 1.0/0.8.The SPF developed is shown in Table 3 with only38 g/L VOC.TEST METHODSZeta PotentialThe zeta potential of both the AEH dispersionsand waterborne hardeners was measured witha Zetasizer NANO ZS instrument from MalvernInstruments Ltd. All the samples were diluted 1:10(w/w) with DI water. The hardener was added intoAEH under stirring, and then equilibrated for 10 min;finally, the zeta potential was tested at differentratios. The zeta potential was measured in triplicateand the average value of analyses reported. All ofthe sample measurements were performed at 25 Cand with a cell drive voltage of 30V, using a monomodal analysis model. The count rate range wasadjusted to be 200 400 for high confidence level.Water Solubility of HardenerStandard Curve Making: A sample of 5 gcommercial hardener was weighed and put in avacuum drying oven at 70 C for 24 hr to removethe solvent and water. A 0.500 g dry sample wasdissolved in 50 mL of ethanol to get a 10,000 ppmstock solution. This stock solution was diluted toget standard solutions at a range of concentrations to make a calibration curve. UV absorptionspectra were scanned between 310 and 240 nm.Transmittance was measured by Shimadzu 3100UV-Vis spectrometer with the absorption at 276 nmused for calculation.Water Solubility Testing: Exactly 0.500 g of thedried sample was diluted with 9.500 g DI water ina bottle, sealed, and shaken at room temperaturefor 12 hr. Each sample was then ultra-centrifugedat 80k rpm for 20 min. The supernatant was carefully separated for UV-Vis analysis. The sampleconcentration was measured using UV-Vis basedon the external calibration.Coating PerformanceSalt Spray Resistance (SSR)The anticorrosion performance was testedaccording to the standard ASTM B-117. A coatingfilm was made using a 13 mil drawdown bar. Aftera 2-hr flash dry, the coated panels were put into anoven at 80 C for 1 hr. The drying and curing profile was chosen to mimic a manufacturing processin coating freight containers. After cooling downto room temperature, the edges and backs of thebased panels were sealed. The sealed panels into
were put into a Q-fog cabinet, the film status regularly checked, and the time of blistering or rustingon the film recorded.Drying Time TestA wet film with 150 μm thickness was drawndown on a glass strip and put on the work surfaceof a BYK drying time recorder for a run time of 6hr. The result was read according to the standardASTM D5895. An illustration of the results for thisdevice is shown as Figure 3.RESULTS AND DISCUSSIONStability Study for Hardener SelectionSince the AEH system is based on a uniqueepoxy/acrylic structure, it was more sensitive tohardener selection than a general 2K WB epoxysystem. To better understand the stability of AEHwith different amine hardeners, zeta potential wasused to monitor the stability. Generally speaking,the hardener has amine groups and is thereforepositively charged, while the AEH particles areTable 3—Starting Point Formulation of 2K AEH Coating SystemPart AGrindWaterBentone LTOrotan 731AAcrysol RM-8WSN321CTi-Pure R-902Nubirox 106TalcBlanc Fixe NEnd of GrindLet DownDowanol PMDowanol DPnB15%NaNO2WaterAnquamine 401Beckopox EH 613Beckopox VEH 2849OudraCure WB 8005OudraCure WB 8006OudraCure WB 8007WaterTotalPart BMaincote AEH-20A-187TotalMix RatioTotal PVCVolume SolidsWeight SolidsAnquamine401BeckopoxEH 613BeckopoxVEH 2849OudraCureWB 8005OudraCureWB 8006OudraCureWB .5Part A/Part B 67 / 44.5Coating Properties (A B)30.12%39%54.48%Figure 3—Drying stages on drying time recorder.April 2015COATINGSTECH61
Formulation Design for Metal ProtectionAnticorrosion PerformanceAccording to our previous studies, many ofthe unique properties of the acrylic epoxy hybridlatex composite based on Designed Hybridizationtechnology derive from the design of liquid epoxyresin imbibed into acrylic particles and their special film-forming mechanism. The imbibed liquidepoxy acts as a coalescent to assist the high MWacrylic polymer in film formation, and the latex actsas a controlled-release host of the liquid epoxyto enable longer pot life. As the epoxy cures withhardeners located outside the polymer particles inthe aqueous phase, it diffuses out of the latex andthe high MW polymer regains its hardness. In thecured film, the acrylic latex polymer domains aredistributed in a continuous phase of cured epoxy.This special design enables the acrylic epoxy hybridlatex composite to have barrier and resistanceperformance similar to a thermosetting 2K epoxysystem. These properties give AEH the potential tobe used for metal protection and anticorrosion.Figure 4—Grit formed when hardener was mixed with AEH.negatively charged. When the AEH and hardenerare combined, the interaction of opposing chargesimpacts the electric double layer of the colloidalparticles. If the final electric double layer does notprovide enough stability to the particles, grit is generated. Figure 4 shows some grit formed when AEHwas improperly mixed with hardener.Table 4 shows test results of zeta potentialfor different hardeners. All of the hardeners showa positive potential and the AEH shows a negative potential. The results of blending hardenerand AEH vary with the identity of the hardener.Some hardeners form stable mixtures, whileimproper hardeners form grit or even gel. A ruleto guide hardener selection can be drawn fromthe testing. All the combinations that meet therule ζAEH ζhardener 30 are found to bestable. Those samples that do not meet this ruleform grit or gel.The salt spray resistance performance ofacrylic epoxy hybrid latex composite with different hardeners was evaluated and is shown inTable 5 and Figure 5. These results show hardener selection to be a critical factor for both paintstability and coating performance. Coatings withproper hardeners such as OudraCure WB 8006,OudraCure WB 8007, Beckopox EH 613, orBeckopox VEH 2849 can pass the 600-hr SSR testonly with few bubbles. However, other hardeners,such as Anquamine 401 and OudraCure WB 8005,show very poor SSR due to blistering through thewhole area.Based on the zeta potential study, we choosethose hardeners that form stable mixtures withAEH for further study, including: Anquamine401, Beckopox EH 613, OudraCure WB 8005,OudraCure WB 8006, and OudraCure WB 8007.Table 4—Zeta Potential of AEH, Hardener, Mixture, and Stability of MixtureAEH/ HardenerAEHAnquamine 419Beckopox VEH 2188EPICURE 8290EH 659OudraCure WB 8001Anquamine 401Beckopox EH 613OudraCure WB 8005Anquamine 721AOudraCure WB 8007OudraCure WB 8006EPICURE 687062April 2015COATINGSTECHZeta Potential(Component) (mV)Zeta Potential(Mixture) (mV)Zeta Potential(Sum of AEH withVarious Hardener) StableStableStableStableStableStableStable
Table 5—Salt Spray Resistance Performance of Novel Acrylic/Epoxy HybridLatex Composite with Different Hardeners and Their Solubility in WaterTrademarkAnquamine 401Beckopox EH 613Beckopox VEH 2849OudraCure WB 8005OudraCure WB 8006OudraCure WB 8007DFT (µ)SSR resultsSolubility inWater (mg/ml)898985828987 240 hr, M4, 100%area637 hr, F2, 5-bubbles632 hr, F2, 5-bubbles24 hr, MD6, 100% area632 hr, F2, 5-bubbles632 hr, F4, 5-bubbles49.59.517.555.91138.3Figure5—Pictures of SSR testing with different hardeners.Understanding Hardener Selection forGood Anticorrosion Performance (SSR)To better understand why different hardenersimpact SSR performance so dramatically, the hardener hydrophobicity was examined. The hypothesisis that a hydrophobic hardener may show benefitsfor anticorrosive performance and lower watersolubility is likely indicative of a more hydrophobichardener. Six hardeners were selected to test thewater solubility to define the range which can bringgood anticorrosion performance. Table 5 comparesthe SSR with the water solubility of the hardener.Those hardeners with the lowest water solubility doshow the best anticorrosion performance.Based on the SSR results, we can generalizethat the higher the water solubility of the hardener, the worse the anticorrosion performance. Toachieve good anticorrosion performance (SSR 300 hr), the water solubility of hardener is prefer-ably 40 mg/mL and most preferably 20 mg/mL.We believe that more hydrophobic hardeners haveimproved compatibility with the epoxy resin, whichenhances epoxy migration out of the latex particlesduring curing. This enhanced curing results intighter films with better anticorrosion performance.Performance Comparison with TraditionalEpoxy and Acrylic SystemsAnticorrosion PerformanceThere are three popular water-based coatingsystems in the market used for metal protection.Their features are listed below: 2K Water-based Epoxy: provides best anticorrosion property, but is expensive and applicationis inconvenient due to its 2K nature. 2K Water-based Acrylic/Epoxy Cold BlendSystem: epoxy reacts with carboxyl group onlatex polymer to boost the performance.April 2015COATINGSTECH63
Table 6—Salt Spray Resistance (SSR) Performance ComparisonType of SystemCoating System1K Acrylic paint60-70 24 hr, F4, small clusters2K WB acrylic/epoxycold blend system2K Acrylic with carboxylic acid as hardener Pre-dispersed epoxy paint80-90 72 hr, F6, small clusters2K WB AEHAEH Beckopox VEH 284980-90 600 hr, F2, 5-bubbles2K WB Type I epoxyOudrasperse WB 6001 Beckopox VEH 284980-90 800 hr, F2, 5-bubblesThe novel AEH composite was compared withthese three systems to better understand its performance. The results are listed in Table 6. Theresults show that both 1K WB acrylic and 2K WBacrylic/epoxy cold blend systems have SSR of lessthan 100 hr. The AEH shows SSR of more than 600hr, which is quite close to the 2K WB Type I epoxysystem regarded as the best anticorrosion systemin the market. Again, the good corrosion resistanceproperty is very likely due to the special systemdesign where epoxy can migrate out of the acrylicparticles and form a continuous phase during curing. This special film-forming mechanism enablesacrylic epoxy hybrid latex composite to have performance barrier and resistance performance similarto a conventional 2K epoxy system.Figure 6 shows the panels after 600 hr salt spraytesting for both AEH and 2K WB conventional epoxysystems. The AEH technology showed comparableresults to conventional WB epoxy dispersion. Creepboth in wet and dry states is lower than 1 mm inscribed panels. After exposure, the coatings showedgood adhesion. In fact, the surface of the unscribedpanels showed almost no change.Pot Life, Dry Speed, and Paint StabilityPot LifeA conventional epoxy dispersion generally has apot life of 1–4 hr. At the same time, there is no visualDFT: 100–120 µm; RT curing for 7 days; 600 hr exposure.6—SSR testing results after 600 hr exposure of AEH (left) comparedto traditional Type I epoxy dispersion (right).64April 2015COATINGSTECHSSR1K WB acrylic 1K Acrylic Coating: Provides moderate corrosionresistance for light to medium duty applicationcorrosion requirements.FigureDFT (µm)indicator of the end of useful pot life, especially fordispersions of solid epoxy resin. Without a changein viscosity or appearance, it can be hard for applicators to judge whether the paint still can be usedwithout causing quality problems in the final coating.The design of AEH, with epoxy hosted inside acrylicparticles only reacting when the epoxy migratesout to react with hardener during film formation, isexpected to bring longer pot life. The pot life of AEHwith Beckpox EH 613 was tracked by evaluatingcoating performance including gloss, adhesion, andimpact resistance. As shown in Figure 7, the coating performance showed almost no change after 13hr. This means that the coating performances canbe guaranteed when applicators use up all of themixture they prepare at the starting of a working day(within eight working hours). This benefits end usersin both material saving and quality control.Dry SpeedIn AEH technology, the thermoplastic acrylicportion provides a quick surface drying, while harddrying is provided by the acrylic and the reactionof epoxy and polyamine curing agents. As shown inTable 7, AEH showed both faster surface drying (fastreturn-to-service time) and hard drying time (finalcoating performances). This dual benefit can shortendrying intervals and accelerate manufacturing speedor return-to-service time.Paint Dilution StabilityTraditionally, epoxy dispersions are prepared byemulsifying epoxy resin with surfactants. This yieldsdispersions with particle size above 500 nm, significantly larger than acrylic latices. When diluted withwater, the surfactant on the surface of epoxy dropletsredistributes between the droplet surface and serumphase. With greater dilution, more surfactant movesinto the serum phase and no longer provides stabilityof epoxy droplets. This causes unstable epoxy droplets to come out of the emulsion as grit. This dilutioninstability causes issues when workers in the plantwash the pipes involved in paint by spraying withwater, diluting the epoxy, and forming grit that sticksinside the pipes, causing trouble. Figure 8 comparesthe dilution stability of AEH to a conventional epoxydispersion. One gram emulsion was diluted with 6 gwater and then drawn down on glass panels to check
Glossat 60oGloss at 60 54Figure 7—Pot life testing of AEH by trackinggloss, adhesion, andimpact resistance aftermixing hardener andAEH ct resistance,kg*cmImpact resistance,kg*cm1008060402002hImpact Resistance, cm/kg1h2h4h5h6h7h8h10h12h13h29hNote: Substrate: Q panel, cold rolled steel, WFT 250 µm, test after curing at RT for 7 days.Table 7—Drying Speed of AEH Compared to Traditional Epoxy Dispersion on Drying Time RecorderBinderPVC LevelDry Time (hr)Stage 2/Stage 3/Stage 4WBEP1Conventional WB epoxy system35%0.5/1.3/5.6WBEP2AEH, Acrylic epoxy hybrid system35%0.4/0.8/1.6Paint CodeEquipment: BYK drying time recorder.Coating films preparation: 150 µm WFT, Glass bar substrate.the appearance. AEH showed much better dilutionstability than the conventional epoxy dispersionduring our testing.Figure 8—Waterdilution stability ofAEH and Type I solidepoxy dispersion.In addition to dilution stability, the AEH alsohas advantageous viscosity. The viscosity of commercially available epoxy dispersions is generallyseveral thousand cps. The viscosity of AEH is lessthan 400 cP, much closer to acrylic emulsions andeasier to handle in a plant environment.CONCLUSIONSThe performance of novel acrylic epoxy hybrid(AEH) was studied for metal protection. Hardenerselection was found to be critically important to2K coating formulation stability and anticorrosionperformance. Zeta potential was found to be agood predictor of the stability when AEH binderand hardener are mixed. When AEH and hardenermeet the rule ζAEH ζHardener 30, the resulting2K coating formulation is stable. Hardeners withlow water solubility improve corrosion resistance.Compared to 1K WB acrylic or traditional 2K epoxysystems, AEH can provide a better cost/performance balance, which is beneficial to customers.In addition, the acrylic portion of the AEH givesexcellent pot life, dry speed, and other applicationproperties such as dilution stability. CTReferences1. Fu, Z., Dombrowski, G., Hejl, A., Swartz, A., Tepe, T., andProcopio, L., Proc. American Coatings Conf. 2014.2. Fu, Z., Epoxy Resin Imbibed Polymer Particles, Patent:US8658742 or EP2495281 or CN102653623A, 2012.3. Liu, B., Zhang, M., Gui, Yu, et al., “Effect of AqueousPhase Composition on Particle Coagulation Behavior inBatch Emulsion Polymerization of Styrene,” Colloids Surf.A-Physicochemical and Engineering Aspects, 452, 159164 (2014).4. Lattuada, M., Del Gado, E., et al., “Kinetics of Free-Radical Cross-Linking Polymerization: Comparative Experimental and Numerical Study,” Macromolecules, 46 (15)5831-5841 (2013).AUTHORSZhigang Hua, Hunter Shu, Allen Xu, BarryZheng, and Aaron Yang, Dow Chemical Company(China); and Zhenwen Fu, Andrew Hejl, HenryEichman, and Kiran Baikerikar, Dow ChemicalCompany (USA); zfu@dow.com.April 2015COATINGSTECH65
60 C, the epoxy emulsion was fed into the reac-tor containing the acrylic latex and held for 1 hr to allow the epoxy to transport and swell the acrylic latex. The key requirement was that the epoxy was completely imbibed in the acrylic particles. The driving force for this transport process is the solubility of the epoxy molecule in the latex
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Contents 3 Epoxy resins, water-reducible 4 Epoxy hardeners, water-reducible 5 Epoxy resins solid and solutions 6 Epoxy resins liquid and reactive diluted 7 Reactive diluents for epoxy resins 7 Epoxy hardeners, polyamines 8 Epoxy hardeners, adducts 9 Epoxy hardeners, mannich bases 10 Epoxy hardeners, polyamidoamines 11 Survey of the qu
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