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Molecular defoamers

Resolving stability and compatibility problems.Wim Stout, Christine Louis.Though surfactants allow waterborne coatings to wet lowsurface energy substrates even in high-speed applications,they often stabilise foam. Traditional defoamers can causesurface defects or storage stability problems. Newsilicone-free defoamers eliminate foam and microfoam byattacking foam stabilisation on a molecular level, minimisingside-effects and reducing the total additive package needed.Conventional defoamers, which operate via anincompatibility with the water phase, provide gooddefoaming properties in waterborne coatings but oftengenerate unwanted side effects such as surface defects,poor recoatability and a loss of efficiency after relativelyshort storage periods. Using further additives to overcomethese problems may adversely affect foam knock down.Molecular defoamers are a new type of multifunctionaladditive, which have the potential to simplify the challengesof water-based formulations. Technical studies have beencarried out in various types of systems to illustrate themultifunctional behaviour of molecular defoamers. Theresults highlight enhanced wetting and defoamingcharacteristics, while reductions in minimum film formationtemperature (MFFT), enhancement of gloss and improvedflow and levelling were also observed.

Changing the nature of foam-breakingMolecular defoamers are a new class of defoamers whichcombine foam control and dynamic wetting. The namederives from the fact that the defoamers are surface-activeagents, which break foam at a molecular level rather thanthrough incompatibility. As described in foam stabilisationtheory [1], foam-stabilising components such as wettingagents, dispersants, emulsifiers and soluble resins stabilisefoam due to one or a combination of effects including ionicforces, hydrogen bonding and Van der Waals forces.Molecular defoamers act to destabilise foam by disruptingthese forces, and in consequence cause the foam tocollapse as shown in Figure 1.At the foam interface, molecular defoamers reduce the filmelasticity of bubbles to prevent their stabilisation. They alsoreduce the surface viscosity of the foam lamella andincrease the rate at which liquid drains out of the bubblefilms by interrupting the even packing of the foam stabilisingcomponents. These combined effects enhance theirdefoaming efficacy. In addition, molecular defoamers areeasily incorporated into aqueous systems, eliminating theneed for high shear forces, which can entrap air during theirincorporation.Conventional defoamers have an optimum particle sizewhich ensures best performance. However, over time theirparticle size may increase due to incompatibility (the worstcase will be complete separation) or may decrease as theybecome more compatible. In either case, they will lose theireffectiveness in controlling foam over time. By contrast,since molecular defoamers work at a molecular level and donot rely on particle size, they remain effective in the systemindefinitely.

Benefits from surface tension reductionBy the nature of their structure and chemistry, moleculardefoamers are free to migrate in the system and stronglyreduce many foam stabilisation mechanisms [2]. Moleculardefoamers are also capable of lowering the surface tension,which is required to enter and penetrate the foam lamella for

foam destabilisation. They are surface active, and are alsoeffective in reducing both the equilibrium surface tension(EST) and dynamic surface tension (DST), providing goodwetting properties, especially under dynamic conditions.Thus molecular defoamers can be regarded as foam controlagents with additional wetting capabilities. A series ofdifferent molecular defoamers has already been developedand tests on these products, which are referred to as AD01,MD20, DF110C, DF110D, AE01, AE02 and AE03, arereported here.It is important to note that molecular defoamers not onlyprevent foam formation but also provide very good surfacetension reduction of water both under equilibrium and moreespecially under dynamic conditions. Table 1 summarisesthe surface tension and Ross-Miles foam test data ofaqueous solutions containing several different moleculardefoamers. Surface tension was measured by the maximumbubble pressure method and the foam data was generatedaccording to the standard Ross-Miles method ASTM D1173. The combination of antifoaming, defoaming andwetting performance is a strong advantage with respect tolimiting surface defect formation, allowing coatingsformulators to simplify the additive package. The followingdiscussion illustrates the multifunctional performance of themolecular defoamers in several applications.

Waterbased UV wood lacquer: improved appearanceThe molecular defoamer AD01 was tested against a siliconedefoamer in a water-based UV-curable wood lacquer. Asshown in Table 1, the chosen molecular defoamer providesno foam stabilisation and the reduction in DST alsosuggests that this product may improve the aestheticappearance of the coating once applied on white oak. InFigure 2 it can be seen that this molecular defoamerdemonstrated defoaming control similar to that of theconventional silicone defoamer without creating any surfacedefects. In addition, it enhanced the aesthetic appearance ofthe wood coating due to the reduced dynamic surfacetension. It should be noted that the new product was used atonly half the concentration of the silicone defoamer in thisparticular formulation, thus helping to reduce costs.

Furniture coatings: Reducing coalescents and the MFFTThe emulsifiers present in the polymer dispersions used forwater-based wood coatings can stabilise foam. Theseformulations are often applied by brush, roller or spray,which may generate significant foam during application andthus require the use of defoamers. Unfortunately, defoamersare often difficult to dose, affect gloss, flow and levelling andcan even generate surface defects such as craters orfish-eyes due to their incompatibility.Different types of surfactants were tested in a clear furniturelacquer based on an acrylic dispersion in order to comparethe defoaming capabilities of molecular defoamers withdifferent surfactant chemistries. Figure 3 shows that theformulation containing the molecular defoamer AE03 hasthe most efficient foam control compared to the modelformulation and formulations containing other chemistriessuch as acetylenic diol (TMDD), silicone, fluoro orbranched-alcohol ethoxylate (BAE).The wetting performance was also studied. It was observedthat the molecular defoamer AE03 as well as AE01 andAE02 aid film coalescence. These products have a strongeffect on the minimum film formation temperature (MFFT) ofa variety of polymer dispersions including a urethane-acrylichybrid (A) and three acrylic dispersions (B, C, and D). Figure

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4 shows the effect of these molecular defoamers when usedat 2% by weight on the liquid polymer dispersions, withoutany additional coalescent.Thus in addition to the foam control and dynamic surfacetension reduction (as already shown in Table 1), the AE-typemolecular defoamers improve film coalescence, allowing forpartial or complete coalescing agent removal and thusreducing the total VOC level of the formulation. Theseproducts are suitable for a wide variety of water-based andother systems but stability at pH extremes may beformulation-specific.

PSA's: Defoaming without loss of adhesive propertiesMolecular defoamer MD20 was tested in an acrylic-basedpressure sensitive adhesive (PSA) formulation for labelapplication in order to eliminate foam stabilised by ananionic wetting agent present in the formulation. The foamarising from the anionic surfactant can be controlled by theuse of traditional defoamers such as silicone. However, theincompatibility of such defoamers often generates craters inthe adhesive film and the foam control deteriorates overtime.Figure 5 shows a picture taken after agitating three PSAformulations containing the anionic surfactant at 0.75% byweight on total formulation and compares the effect on foamcontrol of the molecular defoamer MD20 with a siliconebased defoamer. The silicone defoamer is more efficientthan the molecular defoamer in eliminating foam. However,as mentioned above, silicone defoamers generate craters inthe film because of their incompatibility and reduce adhesionto the substrate. They are therefore rarely used in PSAs. Itis also important that surfactants and defoamers do notadversely affect the adhesive properties of the formulationssuch as peel, tack and shear strength. Figure 6 shows thatthe molecular defoamer had little effect on these three keyproperties. It provides similar peel and tack strength andslightly superior shear strength (20% improvement) whencompared to the same formulation containing only dioctylsulfosuccinate (DOSS) as the surfactant.

Automotive refinish: Additive packages can besimplifiedThe shift from solvent-based to water-based automotiverefinish basecoats has forced a different approach informulating technology. Newly developed single or 2component resin systems may contain foam stabilisingingredients which increase the need for foam control. Theuse of traditional defoamers is known to generate surfacedefects especially in low film thickness applications.Flow and levelling agents, which are used to decrease theadverse effects of traditional defoamers, often seriouslyinfluence the recoatability of the base coat by creatingsurface defects when a solvent-based automotive repairclearcoat is applied over it (see Figure 7). Moleculardefoamers such as AE01 can replace both traditionaldefoamers and flow and levelling agents, and can providedefect-free foam control without influencing the recoatabilityof the basecoat, resulting in improved appearance. Suchmultifunctional problem solvers allow formulators both toenhance products and to simplify the additive package.

Molecular defoamers are truly multifunctionalMolecular defoamers provide multiple benefits in manyformulations and in many different applications such as UVcuring wood coatings, furniture coatings, adhesives andautomotive coatings. These products control foamgeneration during production and application, irrespective ofthe application method used, because their surface-activecharacter provides additional wetting effects.

Because these novel molecules defoam at the molecularlevel, they eliminate the longevity issues and accompanyingdefects commonly associated with traditional defoamers.The wetting properties, not present in traditional defoamers,enhance film appearance and good wetting performance ismaintained under dynamic conditions.These unique chemistries allow for reduced use levels andsimplification of the total additive package, helpingformulators to create water based alternatives for solventbased coatings, inks and adhesives. Due to their highcompatibility within the system, molecular defoamers canalso be combined with traditional defoamers to optimiseboth overall use level and performance.

AcknowledgementsThe authors wish to thank Roger Reinartz, Samir El Ajaji,Wilco Chaigneau and Yvonne Lavrijsen for theircontributions to the results reported in this paper

References[1] S. Y. Chan, C. Louis, New additives for water basedcoatings: a new class of defoamers is born, Eurocoat 2003,Lyon, France, September 23-25, 2003.[2] R. Reinartz, J. Reader, S. Sundaram, K. Lassila, Newgemini surfactants as paint additives, 7th NürembergCongress, Nüremberg, Germany, April 7, 2003.

Results at a glance- A range of new silicone-free molecular defoamerscombines dynamic wetting with strong defoaming properties.- The mechanism by which they attack foam isfundamentally different from that of traditional defoamers: inparticular, effectiveness is not dependent on particle size.- These products avoid the problems of surface defects orlimited storage stability often associated with traditionaldefoamers.- Molecular defoamers attack the foam stabilisationmechanisms provided by surfactants and other formulationcomponents, are easy to incorporate, retain their efficiencyfor long periods and improve surface appearance.- The new products can be combined with traditionalsurfactants or defoamers in order to enhance theirperformance, while reducing the total additive packageneeded.

The authors:-> Christine Louis obtained a masters degree in MaterialsSciences at the University of Nantes (France) in 1998. Shejoined Air Products and Chemicals, Inc. in March 1999 as aLaboratory Technician, since January 2001 she works as anApplication Development and Technical Service Chemist.-> Wim Stout has over 14 years of experience in developingcoatings and tinting systems as a paint chemist for Motip BVand Air Products and Chemicals, Inc. He works as a SeniorApplication Development and Technical Service Chemist atAir Products.

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Figure 1: Defoaming mechanisms compared. Conventional defoamer (left) utilisesincompatibility to break the surfactant-stabilised film that forms the surface of

bubbles; molecular defoamer (right) disrupts film through surface activity.

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Figure 2: Performance of molecular defoamer AD01 (right) compared with siliconedefoamer (left) in a UV-curable wood lacquer. Coatings defects appear as white marks

in the pictures.

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Figure 3: Comparison of the density after agitation of a wood lacquer formulation, withvarious additives present at 0.75% by weight. Key: AE03 = molecular defoamer; TMDD

= acetylenic diol; BAE = branched alcohol ethoxylate.

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Figure 4: MFFT (minimum film forming temperature) measurements on moleculardefoamers in various polymer dispersions.

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Figure 5: Foam control agitated PSA formulations containing different additives (DOSS= dioctyl sulfocuccinate).

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Figure 6: Comparison of the effect of additives on a PSA: molecular defoamer MD20gives adhesive properties similar to those with only a conventional surfactant (dioctyl

sulfosuccinate, DOSS) in the formulation.

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Figure 7: Spray applied water based automotive repair basecoat, showing difference inappearance between use of silicone-based defoamer (left) and molecular defoamer

(right).

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.

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