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Silicone Resin Networks: the structuredetermines the effect

Silicone resins are eminently suitable for use in masonryprotection due to their inorganic/organic hybrid nature andtheir excellent functionality. Unique for their spreadingproperties, silicone resins coat mineral substrates withamazing speed and remain irreversibly anchored there.Examination of the microstructure reveals somethingastonishing: the silicone resin network also completelyenvelops the calcite filler and is also exceptionally durable.Anette Lork, Ingeborg König-Lumer, Hans MayerThe microstructure of a silicone resin emulsion paint("SREP") formulated above the critical PVC dependscrucially on the distribution and development of the bindermatrix [4], which consists of a combination of a silicone resinand a polymer dispersion. Knowledge of the microstructurecan be used to make inferences about the characteristicproperties of silicone resin emulsion paints. The good waterrepellency and extreme durability of silicone resin emulsionpaint derive from the fact that the silicone resin network,which imparts hydrophobicity to pores, completely envelopsthe filler and reinforces the pores down to the nanometerlevel without clogging them, i.e. the open- pore structure isretained.The goal of this article is to take a closer look at themechanism behind network-forming silicones in siliconeresin emulsion paints with emphasis on the high affinity ofthe silicone resin for the mineral substrate.

Basic silicone chemistryThe principles underlying silicones may be greatly simplifiedas follows:- Silicones possess silicon-oxygen bonds (Si-O-Si) knownas siloxane or polysiloxane linkages. (Si-O) bonds areexceptionally stable, have high bond energies and areresponsible for the inorganic nature of silicones.- Silicones also have organic groups (R), which conferhydrophobic properties and are chemically bonded to theinorganic backbone via silicon-carbon bonds (Si-C).

Silane, siloxane, silicone resin, silicone resin networkThe baisc elements of silicone chemistry are describedsufficiently in the literature [7, 5, 8] and need not beexplained here.The term silicone resins covers all products that are basedon trifunctional units ("T-units"). These T-units, which bearjust one organic group and have a high degree offunctionality, can crosslink in space via three oxygen atoms.The simplest product contains a single T-unit, and is areadily volatile, monomeric alkyltrialkoxy silane. It bears oneorganic group (R) and three alcohol groups attached viatheir oxygen atoms. When 3 to 6 T-units of this monomericsilane condense in the presence of water, thereby formallyreleasing alcohol, the result is a relatively short-chainoligomer, which is also called a siloxane (contains theSi-O-Si linkage). Further condensation ultimately yields ahigh silicone resin polymer.

Predominantly inorganic: Methyl silicone resinsSilicone resin binders for facade paints are almostexclusively methyl silicone resins, i.e. they contain organicmethyl (CH3) groups. Methyl silicone resins arepredominantly inorganic: only 11wt% is thermallydegradable organic content; the rest is pure silicon dioxide.Methyl silicone resins consist of 30 to 80 T-units and have

average molecular weights of 2,000 to 6,000g/mol. Thedensity of the resins varies from 1.1 to 1.2g/cm3 as afunction of the resin blend. The resins' high crosslinkingdensity permits them to cure to form hard, brittle siliconeresin networks. By virtue of their special molecular structure,the methyl silicone resin chains coil up into randommolecular configurations as the temperature increases.These coils show strongly pronounced molecularinteractions [9], a phenomenon that results in little or noviscosity reduction, and explains inter alia the lack ofthermoplasticity of the silicone resin network in the SREP[4].Methyl silicone resins contain up to 4% bound ethanolgroups, which via the formation of silanol will enter intocondensation reactions both with OH groups present onmineral substrates (chemisorption), and among themselves,thereby undergoing molecular enlargement to yield thesilicone resin network.

The theory: attachment of the network to a mineralsubstrateFigure 1 is a simplified schematic diagram of networkattachment to calcium carbonate that reveals the unusualfeature of the silicone resin network, namely the polarbonding nature conferred by the Si-O linkage and thenonpolar nature bestowed by its methyl groups.There is evidence to suggest that silicones on the surfacesof mineral building materials align themselves with the poresand capillary walls and, in paint layers, with the mineralsubstrates (fillers/pigments) [5, 9]. Whereas the inorganic,polar siloxane backbone (Si-O-Si) of the silicone resin isattracted by polar mineral substrates, the nonpolar CH3groups are repelled by them (Figure 1).The high affinity of silicone resin for the structurally relatedquartz [4] and its ability to form chemical bonds with quartzsurfaces is generally recognized [6, 5, 10, 11]. However, thecrucial issue of attachment of the resin network to the mostcommon type of filler substrate, carbonate, is still disputed.Opinion is divided as to how silicones interact with calcite(Cc) [10, 11].

Attachment mechanisms and working principleThere is no longer any doubt that the special characteristicsof network-forming silicone products depend on theirmolecular structure [9]. Any explanation of the extremedurability and long-lasting water repellency of the siliconeresin network in building-material or paint microstructuresneeds to consider further properties of silicones, such ashigh spreading power and strong physical and chemicalinteractions with the mineral substrate [5, 1, 9].This raises the question as to whether simple tests could beused to demonstrate the characteristics of the propertyprofile of silicone resins that are key to masonry protection?- How strong is the interaction between the silicone resinand the mineral substrate and can the attached siliconeresin be quantitatively removed from mineral surfaces?- Does silicone resin vary in its affinity for TiO2, calcite andcristobalite and what are the key attachment mechanisms?- What interaction occurs between the polymer dispersionand the silicone resin binder in the SREP and can it bevisualized? Are the two binders compatible at a molecularlevel?

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Sample preparation and analytical methodsNetwork attachment to pigments (titanium dioxide) and fillers(calcium carbonate, cristobalite) was studied by means ofsimple manual tests. Pigment/filler pastes were preparedwith water and silicone resin emulsion (50%) in the ratio 1 to1 and subjected to high shear. After immediate washing withcopious amounts of water under mechanical agitation(decanted twenty times), the pigment and filler powder wasdried.Binder blends afford a good means of elucidating thesuspected incompatibility of the polymer dispersion and thesilicone resin. To this end, 5% titanium dioxide was added to1:1 blends of polymer binder (styrene-acrylate) and siliconeresin emulsion (both 50% solids content). The blends weredrawn down in thin layers and dried for a week.1. The best way to prepare for transmission electronmicroscopic (TEM) examination of aqueous emulsions is toemploy the drying-in method on special copper sampleholders, which involves a prediluting negative contrastingstep [3] with uranyl acetate solution followed by vacuuminfiltration.2. The fillers/pigments coated with silicone resin that hadbeen made for the manual test were strewn onto a sampleholder and then vacuum coated with ultra-thin layers ofcarbon. Field emission scanning electron microscopy(FE-SEM) performed at maximum resolution is the onlymethod suitable for visualizing the nanoscalar network.3 For the quantitative determination of silicone resin inhardened SREP, the paint was ground and then extracted ina soxhlet with organic solvents (tetrahydrofuran, toluene,acetone, white spirit (K30) for several days. An IR controlanalysis was performed on the mineral residue to assessthe quality of the extraction.4. Cryo-ultramicrotomy was used to slice the dried films ofthe binder blends perpendicularly to their surfaces inpreparation for the TEM studies. This slicing techniquepresupposes the use of a very fine-grained, mineralsubstrate: TiO2.

Aqueous silicone resin emulsions for the SREPSilicone resin emulsions are disperse systems [2] consistingof at least two insoluble liquids. The silicone resin is thedispersed, inner phase (Figure 2), which is kept stable bysurfactants at the interface with the water or continuousphase.TEM is a precise method for determining the particle sizeand distribution of the silicone resin phase [2]. Samplepreparation can give rise to artefacts, such as particledeformation/fractionation [3, 2] and, in special cases,particle enlargement. Figure 2 shows the various sizes ofsilicone resin particles present in a commercial silicone resinemulsion. The component particles of internally structuredparticle aggregates may be in the submicron size range(Figure 2b). Nanoscalar particle distribution of the siliconeresin phase (Figure 2c) is not a prerequisite for goodattachment capability to a mineral substrate.

Structure-effect relationship and affinity for the mineralsubstrateKey factors found to govern the proven durability of siliconesin paint applied to reference objects are an absence ofreactive groups, high interfacial surface tension in contactwith water and, in particular, strong adhesion betweensilicone resin and substrate. This durability necessitates ahigh bond strength in addition to the affinity of thepolysiloxane for the mineral substrate. Simple manual testswere used to demonstrate the extensive irreversibility of thenetwork attachment.

A simple manual test yields surprising resultsFillers/white pigments brought into just brief contact withsilicone resin produced surprising results (Figure 3): waterdroplets applied to the dried mineral residue suggest a highdegree of water repellency. The high contact angle of thewater droplets on all samples indicates similarly goodcovering with silicone resin of the nanoscalar titaniumdioxide, the silicon dioxide and even the calcium carbonate,irrespective of whether or not dispersing/wetting agentswere added to the filler batch (Figure 3, sample 3). Theimplication for paint production is that the silicone resinimmediately coats polar substrates (fillers/TiO2) and can nolonger be removed.

Proof is in the microstructure: total envelopmentThe "visible" water repellency of the fillers, as suggested byFigure 3, can be proved beyond all doubt by microscopicanalysis of the filler surfaces. Figure 4 juxtaposes thesilicone resin coating on the calcium carbonate filler (Figure4a-4c) with that on the silicate filler (cristobalite) (Figure4d-4f). The intact, complete envelopment of all shown fillersurfaces by silicone resin network is astonishing in view ofboth the sample preparation method described and theimmediate washing of the two filler batches. Even thecalcite, deemed difficult to coat with silicone [6, 11], is foundexperimentally to have the same even coating as the silicatematerial. On a comparative basis, the network coating onthe calcite looks slightly thicker. High resolution reveals thatthe extremely thin network coating on the cristobalite fills afew of the nanometer-wide cracks (see arrow in Figure 4e),seemingly rounds off fracture surfaces and forms menisci oncompletely enveloped ultrafine particles (Figure 4f).

Can silicone resin be quantitatively determined?Nor was it possible to remove silicone resin from a mineralsubstrate with a solvent. Quantitative removal of attachedsilicone resin from mineral filler components in a hardenedpaint by extraction with organic solvents is not possible at all(Table 1). An IR control analysis of the mineral residueconfirms the presence of residual silicone resin network onfiller surfaces, even after several days of soxhlet extractionwith tetrahydrofuran, toluene, acetone or white spirit (K30).

Key factor in attachment: structure-effect relationshipWhen silicone resin in aqueous solution is brought intocontact with the polar fillers/pigments (Figure 1), it veryquickly and irreversibly attaches itself to their surfaces. Thesilicone resin initially competes against temporary coating ofthe fillers by water and surfactants. The high spreadingpressure exerted by the silicones rapidly causes othersurfactants to be displaced. This assertion is also borne outby the consistently good hydrophobic effect of filler mixtureswith added wetting agent (Figure 3, sample 3).

Crucial processes1. Rapid attachment of the silicone resin molecule to a polarsubstrate is linked to its molecular structure, the highmobility of the inorganic siloxane backbone conferred by thelow barrier to rotation of the Si-O linkage [9] and the lowsurface energy of the methyl groups.2. The irreversibility of the attachment indicates both strongadhesion to the substrate that has its origin in strong polarinteractions with the mineral substrate (van der Waalsforces) and chemical bonding (Si-O-Si linkage). The highspreading pressure causes the silicone resin to completelyenvelop all fillers (Figure 4); the particle size of the siliconeresin phase in the emulsion may be ignored (Figure 2).3. The mechanisms by which the silicone resin attachesitself to calcium carbonate are the subject of much

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controversy [10, 11]. While it is acknowledged that copiousamounts of reactive silanol groups (Si-OH) are present onsilicate mineral surfaces (e.g. quartz, layer silicates) [6, 10,11], this is not generally accepted in the case of calciumcarbonate [6, 11]:The method described in [10], which seeks to explainnetwork attachment in terms of the crystal structure of thecalcite, agrees with the perfect network attachmentdemonstrated in this article. The attempt by Ca2+ to achievehexavalent coordination on the crystal surface promotes theattachment of its own hydroxyl groups, as evidenced by therise in pH in the perturbed crystal surfaces. It only takes acombination of physical adhesion (surface roughness) and afew attachment sites on the silicone resin polymer moleculefor surface coating to occur via widely spaced OH groups.

Blends of silicone resin emulsion, polymer dispersionand titanium dioxideGood silicone resin emulsion paints always consist of twooptimally combined binders: a preferably hardstyrene-acrylate dispersion and a silicone resin emulsion.The phase distribution in a dried film of a binder blend istherefore shown in Figure 5. Small proportions of titaniumdioxide represent mineral material.Silicone resin emulsion and polymer dispersion are notmiscible with each other. The clear phase separationrevealed by the experiment is likely to have an influence onthe distribution of the polymer binder in the silicone resinemulsion paint. In accordance with its surface energy andbecause there is a polarity gradient, the nanoscalar TiO2 ismostly to be found at the interface of the nonpolar polymerdispersion and the polar silicone resin phase, which derivesits polarity from the siloxane linkage (Figure 5b). Thisconfirms the high affinity of the silicone resin for polar,mineral substrates.

References[1] R.M. Hill, "Superspreading and the dynamics ofsurfactant enhanced spreading.- Silicones in coatings"(2000) PRA Paint Res. Assoc., Conference Papers, March1998, p. 1-11[2] G. Lagaly, O. Schulz, R. Zimehl, Dispersionen undEmulsionen, Steinkopff Verlag, Darmstadt, (1997)[3] K.G. Lickfeld, Elektronenmikroskopie, eine Einführung indie Grundlagen der Durchstrahlungs-Elektronenmikroskopieund ihrer Präparationstechniken, Stuttgart (1979)[4] A. Lork, I. König-Lumer, H. Mayer, "Fine network showsstrength of character", European Coatings Journal, 12(2002), p. 14-21[5] H. Mayer, Siliconharze, Chemismus und Eigenschaften,in: W. Schulze (ed.), WässrigeSiliconharz-Beschichtungssysteme für Fassaden -Grundlagen - Formulierungen - Anwendung in der Praxis -Problemlösungen, Vol. 522, expert verlag,Renningen-Malmsheim (1997), p. 63-80[6] J.E. Moreland, Silica Fillers, extenders, andreinforcements, in: H.S. Katz, J.V. Milewski (eds.),Handbook of fillers and reinforcements for plastics, NewYork (1978) p. 136-159[7] W. Noll, Chemie und Technologie der Silicone, 2nd edn,Verlag Chemie-GmbH, Weinheim/Bergstr (1968)[8] J. Pfeiffer, J. Weis, Silicone - Multitalente aus Sand -Eigenschaften und Anwendungen, CLB Chem. in Lab. undBiotech., Silicon-Chemie (1), Vol. 53, 3/2002, p. 84-89[9] J. Pfeiffer, J. Weis, Silicone - Multitalente aus Sand -Eigenschaften und Anwendungen, CLB Chem. in Lab. undBiotech., Silicon-Chemie (2), Vol. 53, 4/2002, p. 128-135[10] R. Snethlage, Steinkonservierung 1979 - 1983,Arbeitshefte des Bayerischen Landesamtes für

Denkmalpflege, Arbh. 22: 9 - 143, München (1984)[11] G. Wypych, Handbook of fillers, 2nd Edition, PlasticsDesign Library, Toronto (1999)

AcknowledgementsThe authors are grateful to Dr. Heckmann,BASF/Ludwigshafen, for providing micrograph 5a.

Results at a glance- Complete, irreversible attachment of the silicone resin tothe solids under investigation (calcium carbonate, silicondioxide, titanium dioxide) was proved beyond all doubt.·- The similarly good affinity of the silicone resin for thecalcium carbonate substrate (calcite) is reflected inimmaculate envelopment by the network.- The structure determines the effect: the mobility of siliconeresin molecules is such that they can readily realignthemselves in the presence of polar substrates and attachthemselves to mineral substrates at an astonishing rate. Theoutwardly pointing nonpolar methyl groups are highly waterrepellent.- From the mechanism by which the silicone resin attachesitself to a mineral substrate and the proven phaseseparation of the binders, the following conclusions may bedrawn concerning production of SREP: - The largely shear-stable silicone resin emulsion shouldalways be added as the last element of the grinding batch ordirectly to the ground batch - Polymer dispersion added at the end of the productionprocess brings the advantage of good attachment tocompatibilized filler surfaces because the nonpolar groupsof the silicone resin coating are oriented.The strength of the composite composed of filler and rigidsilicone resin network explains the pronounced durability ofthe resultant property profile of the SREP, especially thewater repellency. Silicone resin thus supports the bindingfunction of the polymer dispersion in a unique manner.

LIFELINE-> Anette Lork studied geology - paleontology at MünsterUniversity, where she became a research assistant in 1984and then at the DFG's geochronological lab in 1985. Afterone year on a field trip in the Andeas, Argentina, sheconducted DFG projects and lectured at the MineralogicalInstitute. In 1993 she joined MPA Bremen, and continuedher BMBF research in 1996 at the IAAC, HamburgUniversity. In 1999, she headed R&D at a cement plant and,in 2000, ran the lime plant's lab additionally. She joined theBuilding & Coatings BT of Wacker-Chemie GmbH inSeptember 2000.-> Ingeborg König-Lumer was engaged in the field ofscientific research at Hoechst Corporation from 1966 to1982. Much of her work was patented. In 1985, she came toWacker-Chemie, where she runs the Silicone ResinCoatings lab and is responsible for development work andtechnical marketing of Decorative Coatings all around theworld.-> Dr. Hans Mayer gained his doctorate in organic chemistryat Regensburg University in 1983. He then joinedWacker-Chemie GmbH and was made head of appliedtechnology in silicone masonry protection in 1993.Appointed BT manager in 1996, he had global responsibilityfor marketing, technology and sales. He was appointeddirector of BT Building & Coatings in 2002 after integrationof the Industrial Coatings BT.

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Theory of silicone resin network attachment to a substrate. Figure 1: Simplifiedscheme of the structure-effect relationship between silicone resin and mineralsubstrate as exemplified by calcium carbonate: Primarily helical silicone resin

molecules quickly reorient themselves on contact with a polar substrate. While theionic nature of the polar siloxane backbone causes it to face the calcium carbonatesubstrate, all nonpolar methyl groups (CH3) are pointing away from the substrate..

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Figure 2: TEM images of a contrasted commercial silicone resin emulsion with arepresentative particle distribution: (a) Silicone resin particles loosely dispersed as a

result of dilution; drying of the waterphase and the contrasting solution producesslight particle deformation. (b) Typical internal structure of particles, whose

aggregation cannot be detected by other measuring methods. (c) Size distribution ofsubmicron particles..

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Figure 3: Visualization of the hydrophobic effect of fillers and pigments coated withsilicone resin: Overview of silicone-resin-coated fillers and titanium dioxide strewn

evenly on glass plates. The perfect beading behavior of the water droplet is illustratedby the attachment - induced in the simple manual test - of the silicone resin (SRE) to

coated titanium dioxide (rutile, TiO2), calcium carbonate (calcite, Cc) and silicate(cristobalite, SiO2). The detailed view of water droplets provides a definitive

explanation: due to their comparably high contact angle, all substrates demonstrateequally good water repellency..

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Figure 4: FE-SEM secondary electron (SE) micrographs of silicone-resin-coated, calciteand silicate filler surfaces: (a-c): Silicone resin coatings on calcium carbonate, sample2 in Figure 3: (a) The overview reveals a slight aggregation of mineral grains as a result

of filled grain voids. (b) Detail from Figure 4a (see arrow): Crystal edge roundingthrough envelopment by silicone resin (c) Fracture surface rounding as sign of

network coating. (d-f) Silicone resin coatings on cristobalite, sample 4 in Figure 3: (d)Overview with typical, calcination-induced cracks. (e) Detail of Figure 4d showing thepartially crack-filling network that perfectly envelops the surface (see arrow). (f) Under

maximum resolution, the network appears as an extremely thin veil that also coversthe ultrafine particles; menisci (see arrow) confirm its existence..

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Figure 5: TEM images of non-contrasted dry film of a binder blend of silicone resinemulsion, styrene-acrylate + 5% titanium dioxide (see sample preparation) ultrathin

section: (a) The overview reveals the clear phase separation of a commercialstyrene-acrylate dispersion (bright) and silicone resin (gray). (b) The detail view showsthat nanoscalar TiO2 (black), due to its polarity, prefers to attach itself to the silicone

resin. .

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