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Hidden strength

Hydrophilic pyrogenic silicas improve mechanicalproperties of coatings.The mechanical properties of coatings can be adjusted bychanging the binder and additives, though high additivelevels may make the coating more brittle. Hydrophilicpyrogenic silicas are shown to be capable of improving thestrength of coatings without loss of elasticity. Examplesbased on epoxy and polyurethane coating systems arepresented.Michael Dreyer,* Torsten Gottschalk-Gaudig, HerbertBarthel.Modern high performance coatings, apart from having goodoptical properties such as gloss, colour or transparency,must also satisfy mechanical requirements includingadhesion, elasticity and hardness [1]. Pyrogenic silicas arewidely used in paint formulations to adjust thixotropicproperties. But because of their high specific surface areaand their excellent adsorption capacity for polar film-formingagents (see Figure 1), they are also capable of improvingthe mechanical properties of coatings [2, 3, 4].However, before pyrogenic silicas can be effectively used asactive fillers, the extent to which they increase viscositymust be limited. It is shown below that this can be achievedvia steric stabilisation, even at high loadings of particulatefillers.

Sintered structure creates thixotropic effectsPyrogenic silicas consist of chain-like sintered aggregateswhich are highly branched in three dimensions. Theseaggregates are produced by partial sintering of primaryparticles during high-temperature hydrolysis in the flameprocess (Figure 2).The typical particle size of the sintered aggregates is of theorder of 150-300 nm (determined, for example, as ahydrodynamic diameter by photon-correlationspectroscopy). No isolated primary particles are detectableeven under intense shear [5, 6]. The transparency ofclearcoats containing pyrogenic silicas also indicates that itis not the aggregates but their individual branches that actas light scattering centres [3].Pyrogenic silicas are used as effective thixotropic agents insolvent-based, low-solvent, solventless and water-basedcoatings. The thixotropic effect is produced by theimmobilisation of the liquid matrix within the aggregates andthe formation of three-dimensional networks. Thesenetworks are formed by van der Waals forces and polymerbridges between adjacent silica aggregates [7, 8, 9]. Thepotential for particle-particle interactions is stronglydependent on the high specific surface area of the pyrogenicsilicas (Table 1).

Coating binders can stabilise silica dispersionsFor pyrogenic silicas to be effective as reinforcing fillers, thesilica aggregates must be adequately colloidally dispersed inthe system matrix. In organic media, this is mainly achievedthrough steric stabilisation. There are references in theliterature to adsorption layer thicknesses of 0.5 to 10 nm andmolar masses of about 1000 g/mol, which result in stericstabilisation of particles in colloidal form [10, 11]. Largermolar masses, for example on adsorbed polymer chains,can create particle-particle bridges which createunacceptably high viscosities.The preferred steric stabilisers for hydrophilic pyrogenicsilicas are, for example, polymers containing oxygen atomssuch as polyols (polyacrylate polyols, polyester polyols and

polyalkylene oxide polyols). Low molecular weight polyolsare ideal, such as those typically used in one- andtwo-component polyurethane (PUR) high-solids coatingsystems or combined with melamine resin for stovedsystems. Liquid epoxy resins and their oligomeric orpolymeric curing agents with primary and secondary aminestructures are also suitable [9, 10].As much as 20 wt% of "HDK" grade silica based on totalsolids (polyol plus silica) or 12 wt% silica based on theweight of the cured film can be used in a two-componentPUR clearcoat based on polyacrylate polyols plushexamethylene diisocyanate (HDI). These relatively highloadings of pyrogenic silica do not significantly increase theshear viscosity and do not cause thixotropic flow behaviourat low shear rates (see Table 2). This steric stabilisation wasachieved despite the high specific surface areas of thepyrogenic silicas.In addition to polyols and other suitable film-formersthemselves, steric stabilisers that can be used withpyrogenic silicas typically include polymeric dispersingadditives. The polar structures of, for example, tertiaryamines, act as strongly adsorbent anchor segments. Theside chains of the additives can be chosen to suit thepolarity of the ambient medium.

Coatings must be elastic to resist damageSubstrate protection is an important function of coatings,which should if possible be retained despite dimensionalchanges. In practice, coatings are subject to short-termloads, such as impacts. Permanent deformations such asthose occurring in coil coatings also must not compromisethe integrity of the coating. Thermally induced volumechanges that the substrate and coating undergo must beaccommodated without damage.These requirements are met by coatings with sufficientelasticity. They exploit the deformability of polymers, whichdepends on the mobility of the molecular chains, and istherefore temperature-dependent. In elastic film-formingmaterials, such as those used for coating plastics or in coilcoatings, the glass transition temperature (Tg), at which theviscoelastic properties change significantly, is typicallybelow 50°C.Continuous abrasive loads, such as those which occur inparquet lacquers, are typically counteracted by increasingthe resistance of the coating system. Intrinsically hardcoating systems are formulated using film-forming agentsthat have a significantly higher glass transition temperature,typically above 60°C [11].

Silicas are evaluated in hard and elastic epoxiesThe influence of pyrogenic silica on mechanical coatingproperties was tested in both hard and elastic solvent-freeepoxy resin formulations. Both types were formulated onBADGE (bisphenol A diglycidyl ether, Mn= ca 360) theelastic coating having a Tg of 27°C and the hard glassy onea Tg of 161°C. In addition, elastic coatings were formulatedon bis-(2-aminopropyl) polypropylene glycol (Mn = ca 2000)and hard, glassy ones on4,4'-methylene-bis-(2,6-diethylaniline).As Table 3 shows, in the case of the elastic epoxy resincoatings, the failure stress (failure energy, W) could beincreased by a factor of 4 to 5 by the addition of pyrogenicsilica, while at the same time the elasticity (in terms of themaximum elongation, εmax) is typically increased by about50%.The glass transition temperature (Tg) of the model systems

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remained virtually unchanged at silica loadings of up to 16wt%. As expected, the modulus of elasticity (E) increased asthe filler loading rose.With glassy coatings, the use of pyrogenic silica onlyachieved minor improvements in the mechanical propertiesinvestigated here (Table 4). This can be attributed to the factthat the network density is already high in the model system.

Wear resistance of polyurethanes is improvedAs shown in Figure 3, trials with the two-componentpolyurethane systems were aimed at investigating theinfluence of pyrogenic silica on the resistance of the coatingto scratching and abrasion. Solvent-based two-componentpolyurethane clearcoat systems were tested, based on apolyacrylate polyol with 4.7% OH content and an acidnumber of 13 mg KOH/g plus an HDI trimer with 22% NCOcontent.The addition of pyrogenic silicas greatly improved glossretention. Silica grades with specific surface areas of 50 to130 m2/g proved particularly advantageous in these dryscratch tests, as shown in the figure. If, in addition to glossretention, the coating is also required to show hightransparency, "HDK S13" with a surface area ofapproximately 125 m2/g is recommended.Transmission electron microscopy (TEM) images showedexcellent stabilisation of the aggregates in the coating matrix(Figure 4). The TEM sections of the coatings clearlyillustrate the uniform distribution of the pyrogenic silicasthroughout the entire coating thickness (Figure 5). Thisresult might be predicted from the improved resistance tointensive abrasion, such as occurs with parquet lacquers orcoil coatings [12].

Silica can also speed up curing of 2K coatingsIt is also notable that hydrophilic pyrogenic silica cansignificantly catalyse the curing reaction.Significant catalytic effects of the silanol groups on thepyrogenic silicas were also observed in the reactionbetween the epoxy resin and the amine curing agent. Thiseffect can be used to significantly increase the early strengthof coatings or greatly reduce the curing temperaturerequired.Finally, it should also be pointed out that the work describedhere was performed throughout with the type of dispersiontechnology that is conventional in the coating industry, suchas a dissolver together with a pearl mill or three-roll mill. Theeffect of the disperser was determined by means of agrindometer. The milling fineness was below the 0-5 µmresolution limit of the instrument used.

Pyrogenic silicas show potential in many applicationsHydrophilic pyrogenic silicas are capable of increasing thestrength of coatings without reducing elasticity. They canalso increase resistance to continuous abrasion even inhard, glassy coatings. The catalytic effect aided by their highspecific surface area often leads to higher early strength andfaster curing. An undesirable viscosity increase in thecoating formulations can be prevented through controlledsteric stabilisation.Hydrophilic pyrogenic silicas have already begun to be usedas active fillers in industry. They have the potential topreserve the appearance of furniture, parquet, plastics andcoated coils. Another interesting potential application is toimprove long-term stress-cracking resistance in thick filmcorrosion protection coatings.In general, for opaque topcoats, pyrogenic silicas withspecific surface areas of up to 125 m2/g can berecommended, while for clearcoats specific surface areas ofgreater than 125 m2/g or equal may be preferable.

REFERENCES[1] J. V. Koleske, Mechanical properties of solid coatings,Encyclopedia of analytical Chemistry, John Wiley & SonsLtd, Chichester, ISBN 0471 97670 9[2] S. Zhou, L. Wu, The change of the properties ofacrylic-based polyurethane via addition of nano silica,Elsevier Science B.V. 2002.[3] S. Zhou, L. Wu, Dispersion and UV-VIS properties ofnanoparticles in coatings, Journal of Dispersion Science andTechnology Vol. 25, No 4, 417-433, 2004, Marcel DekkerInc. 2004.[4] Institut für Verbundwerkstoffe Kaiserslautern,Nanocomposites verbessern Eigenschaften -Werkstoffkennwerte lassen sich gezielter steuern, KunststoffMagazin 1/2006[5] H. Barthel, M. Heinemann, M. Stinz, Particle sizes offumed silica, Chem. Eng. Technol. 21 (1998) 9, Wiley VCHVerlag GmbH, Weinheim 1998[6] H. J.Walls, S. A. Khan, Rheology of silica dispersions inorganic liquids: new evidence for solvation forces dictatedby hydrogen bonds, Langmuir 2000, 16, 7920-7930[7] H. Barthel, Surface interactions of dimethylsiloxy groupmodified fumed silica, Colloids and Surfaces A:physicochemical and engineering Aspects 101 (1995)217-226, Elsevier Science B.V., Berlin 1995[8] H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, Fumedsilica - rheological additive for adhesives, resins and paints,presented at XXVI FATIPEC Congress 2002 in Dresden,Macromolecular Symposium: Quo Vadis - Coatings, WILEY-VCH Verlag GmbH , Weinheim 2002[9] S. R. Raghavan, J. Hou / G. L. Baker, S. A. Khan,Colloidal interactions between particles with tetherednonpolar chains dispersed in polar media: direct correlationbetween dynamic rheology and interaction parameters,Langmuir 2000, 16, 1066-1077[10] J. Bieleman, Additives for Coatings WILEY-VCH VerlagGmbH , Weinheim 2000[11] A. Goldschmidt, H.-J. Streitberger, BASF - HandbuchLackiertechnik, Vincent Verlag Hannover 2002[12] S. Zhou, L. Wu, Study of the morphology andtribological properties of acrylic based polyurethane / fumedsilica composite coatings, Journal of Materials Science 39(2004) 1593-1600

ACKNOWLEDGEMENTSThe authors are especially grateful to Dr. Elodie Bugnicourtfor her work as part of her doctoral thesis "Development ofsub-micro structured composites on an epoxy matrix andpyrogenic silica - Mechanical behavior related to theinteractions and morphology at multi scale".

Results at a glance- Hydrophilic pyrogenic silicas can be used to improve themechanical properties of coatings in addition to their normaluse as thixotropic additives. They are most effective inelastic (low Tg) coating formulations. Both scratch andabrasion resistance can be improved.- Pyrogenic silicas with low specific surface areas (50 to 130m2/g) have proved to be particularly advantageous.However, materials with a higher specific surface area maybe desirable in clearcoats.- Practically useful viscosities can be achieved with relativelyhigh loadings of pyrogenic silica through steric stabilisation.Polar oligomeric or polymeric coating components arethemselves effective stabilisers.- In addition, hydrophilic pyrogenic silica can significantlycatalyse the curing reactions of some coating materials.

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The authors:-> Michael Dreyer studied industrial chemistry at theNiederrhein University of Applied Sciences and joinedWacker Chemie AG in 1996. He is now laboratory managerfor pyrogenic silicas, responsible for technical marketing inthe coatings, inks and composites markets.-> Dr. Torsten Gottschalk-Gaudig studied chemistry atFriedrich-Alexander-Universität Erlangen and gained hisdoctorate in 1997. He joined Wacker Chemie AG aslaboratory manager for pyrogenic silica R&D. In 2003, healso became platform manager for pyrogenic silicas.-> Dr. Herbert Barthel studied chemistry at the University ofUlm, the École Supérieur de Chemie de Strasbourg andWilhelms University Münster/Westphalia, gaining hisdoctorate in 1986. He then joined Wacker Chemie AG andsince 2005 has been responsible for innovationmanagement for pyrogenic silicas.* Corresponding Author. Contact: Dipl.-Ing. Michael Dreyer,Wacker Chemie AG, Johannes-Hess-Strasse 24, 84489Burghausen, Germany, michael.dreyer@wacker.com

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Figure 1: Schematic diagram of pyrogenic silica surfaces bearing polar silanol groups.

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Figure 2: Schematic view of the production of pyrogenic silicas by the flame process .

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Figure 3: Improvement of gloss retention in two-component PUR clearcoats by theaddition of hydrophilic pyrogenic silicasm (Dry scratch test according to ISO 105-X12

at a scratch loading of 500 g. Curing conditions : 30 min at 70°C plus 30 min at 130 °C).

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Figure 4: TEM images of hydrophilic pyrogenic silica in a hard epoxy resin modelsystem.

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Figure 5: TEM cross-section of distribution of hydrophilic pyrogenic silica (specificsurface area 125 m2/g) in the two-component polyurethane clearcoat model system.

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