New routes to sol-gel systems - European Coatings Journal · coatings, materials and paint finishes...
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Quelle/Publication: Ausgabe/Issue: Seite/Page: European Coatings Journal 07-08/2006 40 New routes to sol-gel systems Polymer-based process extends scope of organic-inorganic coatings. Uwe Wienhold, Ulrich Westerwelle. Hybrid organic-inorganic coatings can be readily prepared by the sol-gel method and offer some excellent technical properties. However, they can be expensive and difficult to apply. A new method of preparing these coatings by incorporating organic polymeric binders at the synthesis stage offers improved application properties and reduces costs. Over the past few years, there has been a rapidly growing interest in industry in multi-functional coatings such as scratch-resistant barrier layers or scratch-resistant, dirt-repellent protective coatings. Through the use of such coatings, materials and paint finishes can be provided with greatly improved protective properties. These materials represent a novel technology with a high growth potential. Nano-scale hybrid (inorganic-organic) sol-gel-based coating materials have been found to be a valuable development in the field of scratch- and abrasion-resistant coating materials. These materials are characterised by an extraordinarily wide range of properties, with their composition and homogeneity being controllable at the molecular level. They result from the consistent application of well-known chemical synthesis principles in the development of new materials. However, the introduction of these materials on an industrial scale is still in the very early stages of development. How hybrid sol-gel materials are produced Different types of metal alkoxides such as silicic acid esters are usually used as inorganic components in the synthesis of hybrid sol-gel materials, while organically substituted silanes are generally used as the organic component. Patent applications for such hybrid sol-gel materials have been filed, for example, by INM Saarbrücken, by the Fraunhofer-Gesellschaft and Feinchemie GmbH Sebnitz [1-5]. In the sol-gel process, nano-scale hybrid composite materials are manufactured in most cases via hydrolysis and condensation reactions of liquid monomers in solution [6]. Two processes are usually applied during the production of such materials: - The silane linking principle - Complex formation The complex formation principle represents a new route for the manufacture of hybrid sol-gel materials. In this process, alkoxides are complexed and caused to react with ligands bearing functional groups, such as zirconium alcoholate with methacrylic acid. In case of the silane linking principle, silicic acid esters or condensates of silicic acid or silicones are linked together with modified silanes. Alkyl silanes, amino silanes, acrylic silanes and epoxy silanes are used as organically modified silanes. In the first step of the reaction, a colloidal solution, also referred to as a sol, is obtained as an intermediate product via hydrolysis reactions. The transparent gel is formed in the subsequent condensation reactions, as shown in Figure 1. Properties of sol-gel coatings can readily be varied The most important molecular structural units and the effects they have on the properties of the resulting material are shown schematically in Figure 2. It is possible, for example, by partly substituting the Si atoms with other metal atoms such as Ti, Al or Zr, to increase hardness or vary the optical properties [7-10]. The organic groups anchored to the inorganic network may be used to obtain specific functional properties, e.g. by making the composite material hydrophilic or hydrophobic. If the sol-gel matrix carries a hydrophobe, it is possible, for example, to obtain low-energy surfaces which are dirt- and water-repellent. If the anchored functional groups carry reactive species, an additional organic network can be created which makes it possible to control the flexibility, adhesion and mechanical properties of materials. Sol-gel technology offers the great advantage of incorporating in a controlled manner a wide variety of functionalities by deliberately varying the organic and inorganic structural elements and, through the use of different types of structural elements, producing materials with a wide range of properties [11]. Because this wide range of properties cannot be achieved by any other synthesis methods, nano-scale hybrid composite materials may be considered as ideal coating materials. The adaptation of different properties of the coating material and substrate, which is required for refining the properties of surfaces, can easily be carried out by the controlled variation of the structural elements of sol-gel materials [12-17]. Material surfaces coated with hybrid sol-gel materials may exhibit excellent properties in respect of adhesion, resistance to wear, UV radiation and climatic influences, with properties clearly superior to those of the coating materials currently used [18]. In particular, scratch resistance tests have shown that such surfaces are capable of withstanding loads which are 3 to 4 times higher [19]. Alternative sol-gel route uses organic binders Although these hybrid sol-gel materials exhibit excellent properties[20, 21, 22], their use is limited to very specific applications such as easy-clean ceramic coatings in sanitary facilities. On the one hand, they are very expensive due to the use of organically substituted silanes which have been specifically synthesised for this application. On the other hand, the coating process imposes demanding requirements on the handling of these materials and on the substrate, so that processing is often associated with considerable difficulties. One way to avoid these problems is to use organic polymers as the organic modification component. Apart from their considerably lower manufacturing costs, organic polymers are characterised by excellent processing properties. In addition, the use of conventional organic polymers makes sol-gel materials much more readily accepted by the manufacturers of paint raw materials as well as manufacturers and users of paints. Thanks to the use of organic polymers as an organic modification component, it is also possible to improve application properties, in particular the sprayability of the coating material. Also, the elasticity of the coatings can be increased without affecting scratch resistance. Thus, the use of organic polymers in the manufacture of hybrid sol-gel coating materials has to be regarded as a consistent further development of the new innovative sol-gel technology which is characterised by the combination of novel and conventional methods for the manufacture of coating materials. The utilisation of organic polymers for making hybrid coating materials via the sol-gel process has been rarely and only very unsystematically described in a few texts [23-27]. Vincentz Network +++ Schiffgraben 43 +++ D-30175 Hannover +++ Tel.:+49(511)9910-000
Transcript of New routes to sol-gel systems - European Coatings Journal · coatings, materials and paint finishes...
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New routes to sol-gel systems
Polymer-based process extends scope oforganic-inorganic coatings.Uwe Wienhold, Ulrich Westerwelle.Hybrid organic-inorganic coatings can be readily preparedby the sol-gel method and offer some excellent technicalproperties. However, they can be expensive and difficult toapply. A new method of preparing these coatings byincorporating organic polymeric binders at the synthesisstage offers improved application properties and reducescosts.Over the past few years, there has been a rapidly growinginterest in industry in multi-functional coatings such asscratch-resistant barrier layers or scratch-resistant,dirt-repellent protective coatings. Through the use of suchcoatings, materials and paint finishes can be provided withgreatly improved protective properties. These materialsrepresent a novel technology with a high growth potential.Nano-scale hybrid (inorganic-organic) sol-gel-based coatingmaterials have been found to be a valuable development inthe field of scratch- and abrasion-resistant coating materials.These materials are characterised by an extraordinarily widerange of properties, with their composition and homogeneitybeing controllable at the molecular level. They result fromthe consistent application of well-known chemical synthesisprinciples in the development of new materials. However,the introduction of these materials on an industrial scale isstill in the very early stages of development.
How hybrid sol-gel materials are producedDifferent types of metal alkoxides such as silicic acid estersare usually used as inorganic components in the synthesisof hybrid sol-gel materials, while organically substitutedsilanes are generally used as the organic component.Patent applications for such hybrid sol-gel materials havebeen filed, for example, by INM Saarbrücken, by theFraunhofer-Gesellschaft and Feinchemie GmbH Sebnitz[1-5].In the sol-gel process, nano-scale hybrid compositematerials are manufactured in most cases via hydrolysis andcondensation reactions of liquid monomers in solution [6].Two processes are usually applied during the production ofsuch materials:- The silane linking principle- Complex formationThe complex formation principle represents a new route forthe manufacture of hybrid sol-gel materials. In this process,alkoxides are complexed and caused to react with ligandsbearing functional groups, such as zirconium alcoholate withmethacrylic acid.In case of the silane linking principle, silicic acid esters orcondensates of silicic acid or silicones are linked togetherwith modified silanes. Alkyl silanes, amino silanes, acrylicsilanes and epoxy silanes are used as organically modifiedsilanes.In the first step of the reaction, a colloidal solution, alsoreferred to as a sol, is obtained as an intermediate productvia hydrolysis reactions. The transparent gel is formed in thesubsequent condensation reactions, as shown in Figure 1.
Properties of sol-gel coatings can readily be variedThe most important molecular structural units and theeffects they have on the properties of the resulting materialare shown schematically in Figure 2. It is possible, forexample, by partly substituting the Si atoms with other metalatoms such as Ti, Al or Zr, to increase hardness or vary the
optical properties [7-10].The organic groups anchored to the inorganic network maybe used to obtain specific functional properties, e.g. bymaking the composite material hydrophilic or hydrophobic. Ifthe sol-gel matrix carries a hydrophobe, it is possible, forexample, to obtain low-energy surfaces which are dirt- andwater-repellent.If the anchored functional groups carry reactive species, anadditional organic network can be created which makes itpossible to control the flexibility, adhesion and mechanicalproperties of materials.Sol-gel technology offers the great advantage ofincorporating in a controlled manner a wide variety offunctionalities by deliberately varying the organic andinorganic structural elements and, through the use ofdifferent types of structural elements, producing materialswith a wide range of properties [11]. Because this widerange of properties cannot be achieved by any othersynthesis methods, nano-scale hybrid composite materialsmay be considered as ideal coating materials. Theadaptation of different properties of the coating material andsubstrate, which is required for refining the properties ofsurfaces, can easily be carried out by the controlledvariation of the structural elements of sol-gel materials[12-17].Material surfaces coated with hybrid sol-gel materials mayexhibit excellent properties in respect of adhesion,resistance to wear, UV radiation and climatic influences,with properties clearly superior to those of the coatingmaterials currently used [18]. In particular, scratchresistance tests have shown that such surfaces are capableof withstanding loads which are 3 to 4 times higher [19].
Alternative sol-gel route uses organic bindersAlthough these hybrid sol-gel materials exhibit excellentproperties[20, 21, 22], their use is limited to very specificapplications such as easy-clean ceramic coatings in sanitaryfacilities. On the one hand, they are very expensive due tothe use of organically substituted silanes which have beenspecifically synthesised for this application. On the otherhand, the coating process imposes demanding requirementson the handling of these materials and on the substrate, sothat processing is often associated with considerabledifficulties.One way to avoid these problems is to use organic polymersas the organic modification component. Apart from theirconsiderably lower manufacturing costs, organic polymersare characterised by excellent processing properties. Inaddition, the use of conventional organic polymers makessol-gel materials much more readily accepted by themanufacturers of paint raw materials as well asmanufacturers and users of paints.Thanks to the use of organic polymers as an organicmodification component, it is also possible to improveapplication properties, in particular the sprayability of thecoating material. Also, the elasticity of the coatings can beincreased without affecting scratch resistance.Thus, the use of organic polymers in the manufacture ofhybrid sol-gel coating materials has to be regarded as aconsistent further development of the new innovative sol-geltechnology which is characterised by the combination ofnovel and conventional methods for the manufacture ofcoating materials. The utilisation of organic polymers formaking hybrid coating materials via the sol-gel process hasbeen rarely and only very unsystematically described in afew texts [23-27].
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Manufacture requires a two-stage processTo create hybrid systems from organic polymer structures,the polymers are reactively bonded to the inorganic solcomponent in a two-stage process using selectively reactingbifunctional coupling substances.In the first reaction step, the polymers carrying functionalgroups are converted with selectively reacting bifunctionalcoupling substances. Commercially available polymers suchas polyesters, polyisocyanates, polyacrylates and epoxyresins may be used as organic polymers carrying thefunctional groups, while adhesion promoters and varioussubstituted silanes may be used as the coupling materials.In the second reaction step, the inorganic sol component islinked to the remaining reactive group of the couplingsubstance on the polymer. Various commercially availablesilicic acid esters (e.g. TEOS, tetraethyl orthosilicate) maybe used as the inorganic component. Since thecondensation reaction of the sol component takes placemuch faster than the reaction of the modified polymer withthe sol component, the inorganic structural units are createdin situ via a sol-gel process. The chemical bond createdbetween the organic polymer structures and the inorganicunits very effectively prevents phase separation in amulticomplex system as shown in Figure 3.It is envisaged that, by developing such coating materials, itwill be possible to combine the positive properties ofinorganic materials such as hardness, thermal stability,resistance to weathering and chemicals with those oforganic polymers such as the elasticity, adhesive strength,functionality and good processability and thus achievesynergistic effects so that the final properties achieved willjustify the use of these materials as scratch-resistantfunctional layers.This work was carried out to determine whether, if organicpolymers are used in the sol-gel process, the properties ofcoating materials can be influenced in ways similar to thosein which organically substituted silanes are used.With this aim in mind, epoxy resins converted in a firstreaction step with amino silanes were used as a modelpolymer. These modified resins were subsequently reactedwith TEOS. In the investigations, the following parameterswere varied:- the coating thickness applied- the molar mass of the epoxy resin used (high- andlow-molecular mass)- the spacer length of the alkoxy group of the amino silane(methoxy and ethoxy)- the organic/inorganic ratio.
Useful and controllable film propertiesThe results obtained in term of the mechanical properties(König pendulum hardness DIN 53157, and microhardness(universal or Martens hardness and plasticity) to DIN 50359are summarised in Tables 1-3. Based on theseinvestigations, the following principal conclusions can bedrawn:- Increasing the wet coating thickness leads to a drasticdeterioration of the mechanical properties and of the filmformation. To make such coating systems suitable forindustrial purposes, the dry coating thicknesses should notexceed 5µm.- Higher molecular mass polymer structural units lead to anincrease of the universal hardness and of the plasticity, withthe pendulum hardness remaining almost the same.- Using methoxy-aminosilanes in place ofethoxy-aminosilane also leads to an increase of theuniversal hardness and of the plasticity, with the pendulumhardness remaining almost the same.
- By varying the ratio of inorganic to organic structuralelements, it is possible to influence the universal hardnessand plasticity to a greater extent than by varying the molarmass of the binding agent and spacer length of the couplingsubstance.The specimens produced in the laboratory werecomprehensively tested with respect to their properties intechnical applications. The results obtained are shown inTable 4.
Economic and versatileIt has been shown that when organic polymers are used inthe sol-gel process, the properties of coating materials canbe influenced in a similar and deliberate way as is the casewhere organically substituted silanes are used. It was foundin these investigations that:- It is possible to vary the mechanical properties as afunction of the organic and inorganic structural elementswithout significantly affecting the main technical propertiesof the coating;- The functionality of the coating materials can bedeliberately affected by varying the inorganic sol componentand organic polymer component.The studies were carried out with the aim of developing alow-cost product. By optimising the use of the raw materials,it may be possible to reduce the price of the materials toabout 25% - 50% of the current price of sol-gel coatingmaterials. It has to be pointed out with certain reservations,however, that the use of organic polymers in the sol-gelprocess does not in all cases yield the same excellentproperties that can be achieved with the 'classical' sol-gelmaterials.The use of organic polymers for the manufacture of hybridsol-gel coating materials thus represents a consistent furtherdevelopment of the new innovative sol-gel technology, atechnology characterised by the combination of new andconventional methods for the production of coatingmaterials.
A wide range of applicationsThe successful development of nano-scale hybrid sol-gelcoating materials suited for the scratch-resistant coating ofsurfaces opens up a wide range of applications for thesecomposite materials. Examples of the excellent technicalproperties obtainable with these coatings are shown inFigure 4. These coating materials may be used in future as,for example:- barrier layers with a high barrier effect against watervapour, oxygen and aromatics;- corrosion-protection layers on metals;- priming and top coats for metallisation layers;- scratch- and wear-resistant top coats in the motor vehicleindustry;- in the automotive industry: ~ window coating, reflector coatings; ~ wheel coatings, finish paint;- in sanitary equipment (water taps, covers);- for refining the properties of plastics (eg, polymethylmethacrylate, polycarbonate).Summing up, the following statements can be made:With respect to both the 'classical' hybrid sol-gel materialsand the hybrid sol-gel coating materials produced with thehelp of binding agents, a great potential for innovationexists.Further possible application areas include:- water-based systems;- UV-cured systems;- high solids coatings;- powder coatings.
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ACKNOWLEDGEMENTThese studies have been supported by funds from theGerman Federal Ministry of Economics (promotion projectReg. No. 1128/99 and 13371 BR).1
REFERENCES[1] DE 4 338 360[2] DE 4 411 862[3] DE 4 336 694[4] DE 4 417 405[5] DE 196 20 668[6] DE 3 925 901[6] D. Levy, L. Esquivias, Adv. Mater. 7 (1995) 120[8] J. D. Mackenzie, J. of the Soc. of Japan, Int. Ed. 101(1992) 2[9] A. Slama-Schwonk, D. Avnir, M. Ottolenghi, Ph. chem.Ph.bio., 54 (1991) 525[10] R. Kasemann, H. Krug, H. Schmidt, VDI-Berichte 933(1991) 303[11] K. H. Haas, S. Amberg-Schwab, K. Rose, G. Schottner,Gummi Fasern Kunststoffe 50 (1997) 102[12] K. Greiwe, JB Oberflächentechnik 49 (1993) 243[13] M. D. Rahn, Appl. Opt. 34 (1995) 8260[14] B. C. Dave, B. Dunn, J. S. Valentine, J. I. Zink, J. Anal.Chem. 66 (1994) 1120[15] H. Schmidt, Am. Cer. Soc. (1995) 253[16] M. Aslan, R. Naß, R. Nonninger, R. Rein, H. Schmidt,Am.Cer.Soc. (1995) 757[17] S. Schwab-Amberg, M. Hoffmann, H. Bader,Kunststoffe 86 (1996) 5[18] R. Kasemann, T. Burkhart, H. Schmidt, Ceram. Trans.55 (1995) 307[19] B. Wang, G. L. Wilkes, JMS-Pure Appl. Chem. 31/2(1994) 249[20 U. Wienhold, G. Wagner, Farbe + Lack 109 (6/2003) 29[21] K. Greiwe, Farbe und Lack 11 (1991) 968[22] R. Kasemann, T. Burkhart, H. Schmidt, Ceram. Trans.55 (1995) 307[23] C. V. Avadhani, Y. Chujo, K. Kuraoka, T. Yazawa,Polym. Bull. 38 (1997) 501[24] F. Surivet, T. M. Lam, J. P.Pascault, Q. T Pham,Macromol. 25 (1992) 4309[25] H. B.Sunaka, J. M. Jethmalaani, W. T. Ford, Chem.Mater. 6 (1994) 362[26] W. J Van Ooij, T. Child, Chemtech 2 (1998) 26[27] D. W. Mc Carthy, J. E. Mark, D. W. Schaefer, J. Pol.Sci. Part B 36 (1998) 1167
Results at a glance- Organic-inorganic hybrid coatings produced by the sol-gelprocess have high performance but are expensive and maypresent difficulties in application.- A novel route to sol-gel coatings is described, in which theorganic content is increased (and therefore the cost isreduced) and the application properties are improved byincorporating organic polymeric binders.- The initial tests confirm that it is possible to control thehardness and plasticity of the coatings by varying theamount and type of raw materials.- These coatings have potential applications in many areasincluding barrier coatings, corrosion protection, scratch andwear resistant coatings and toughening the surface ofplastics.
The authors:-> Since March 2006 Dr. Uwe Wienhold has been scientificfreelancer in the area of coating materials (WienholdConsult). Before that he worked as project manager and
scientific co-worker at the Institut für Lacke und FarbenMagdeburg e.V. and as head of the coating laboratory(Aerosil) at Degussa AG. His main focus is the formulationof functional coatings and the modification of binders.-> Ulrich Westerwelle studied Chemistry at the University ofBielefeld, Germany. In 1994 he received his Ph.D. in organicchemistry. During different positions in the coatings industryhe gathered a profound knowledge in the development ofwater-based coating materials for paper and plasticsubstrates. In 2005 he joined the Institut für Lacke undFarben in Magdeburg, Germany, where he is currently headof the coating materials laboratory.
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Figure 1: Sol-gel formation process with silicon alkoxide.
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Figure 2: Organic and inorganic structural elements and their influence (Source:Fraunhofer Institute for Silicate Research ISC, Würzburg).
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Figure 3: Formation of sol-gel coatings using organic polymers.
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Figure 4: Examples of the benefits of sol-gel coatings: an easy-clean surface fromwhich pure water readily removes the dirt (top), and a paint which has much better
scratch resistance than a polyurethane coating (bottom)..
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