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].
Manufacture requires a two-stage process To create hybrid systems from organic polymer structures, the polymers are reactively bonded to the inorganic sol component in a two-stage process using selectively reacting bifunctional coupling substances. In the first reaction step, the polymers carrying functional groups are converted with selectively reacting bifunctional coupling substances. Commercially available polymers such as polyesters, polyisocyanates, polyacrylates and epoxy resins may be used as organic polymers carrying the functional groups, while adhesion promoters and various substituted silanes may be used as the coupling materials. In the second reaction step, the inorganic sol component is linked to the remaining reactive group of the coupling substance on the polymer. Various commercially available silicic acid esters (e.g. TEOS, tetraethyl orthosilicate) may be used as the inorganic component. Since the condensation reaction of the sol component takes place much faster than the reaction of the modified polymer with the sol component, the inorganic structural units are created in situ via a sol-gel process. The chemical bond created between the organic polymer structures and the inorganic units very effectively prevents phase separation in a multicomplex system as shown in Figure 3. It is envisaged that, by developing such coating materials, it will be possible to combine the positive properties of inorganic materials such as hardness, thermal stability, resistance to weathering and chemicals with those of organic polymers such as the elasticity, adhesive strength, functionality and good processability and thus achieve synergistic effects so that the final properties achieved will justify the use of these materials as scratch-resistant functional layers. This work was carried out to determine whether, if organic polymers are used in the sol-gel process, the properties of coating materials can be influenced in ways similar to those in which organically substituted silanes are used. With this aim in mind, epoxy resins converted in a first reaction step with amino silanes were used as a model polymer. These modified resins were subsequently reacted with TEOS. In the investigations, the following parameters were varied: - the coating thickness applied - the molar mass of the epoxy resin used (high- and low-molecular mass) - the spacer length of the alkoxy group of the amino silane (methoxy and ethoxy) - the organic/inorganic ratio. Useful and controllable film properties The results obtained in term of the mechanical properties (König pendulum hardness DIN 53157, and microhardness (universal or Martens hardness and plasticity) to DIN 50359 are summarised in Tables 1-3. Based on these investigations, the following principal conclusions can be drawn: - Increasing the wet coating thickness leads to a drastic deterioration of the mechanical properties and of the film formation. To make such coating systems suitable for industrial purposes, the dry coating thicknesses should not exceed 5µm. - Higher molecular mass polymer structural units lead to an increase of the universal hardness and of the plasticity, with the pendulum hardness remaining almost the same. - Using methoxy-aminosilanes in place of ethoxy-aminosilane also leads to an increase of the universal hardness and of the plasticity, with the pendulum hardness remaining almost the same. - By varying the ratio of inorganic to organic structural elements, it is possible to influence the universal hardness and plasticity to a greater extent than by varying the molar mass of the binding agent and spacer length of the coupling substance. The specimens produced in the laboratory were comprehensively tested with respect to their properties in technical applications. The results obtained are shown in Table 4. Economic and versatile It has been shown that when organic polymers are used in the sol-gel process, the properties of coating materials can be influenced in a similar and deliberate way as is the case where organically substituted silanes are used. It was found in these investigations that: - It is possible to vary the mechanical properties as a function of the organic and inorganic structural elements without significantly affecting the main technical properties of the coating; - The functionality of the coating materials can be deliberately affected by varying the inorganic sol component and organic polymer component. The studies were carried out with the aim of developing a low-cost product. By optimising the use of the raw materials, it may be possible to reduce the price of the materials to about 25% - 50% of the current price of sol-gel coating materials. It has to be pointed out with certain reservations, however, that the use of organic polymers in the sol-gel process does not in all cases yield the same excellent properties that can be achieved with the 'classical' sol-gel materials. The use of organic polymers for the manufacture of hybrid sol-gel coating materials thus represents a consistent further development of the new innovative sol-gel technology, a technology characterised by the combination of new and conventional methods for the production of coating materials. A wide range of applications The successful development of nano-scale hybrid sol-gel coating materials suited for the scratch-resistant coating of surfaces opens up a wide range of applications for these composite materials. Examples of the excellent technical properties obtainable with these coatings are shown in Figure 4. These coating materials may be used in future as, for example: - barrier layers with a high barrier effect against water vapour, oxygen and aromatics; - corrosion-protection layers on metals; - priming and top coats for metallisation layers; - scratch- and wear-resistant top coats in the motor vehicle industry; - 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, polymethyl methacrylate, polycarbonate). Summing up, the following statements can be made: With respect to both the 'classical' hybrid sol-gel materials and the hybrid sol-gel coating materials produced with the help of binding agents, a great potential for innovation exists. Further possible application areas include: - water-based systems; - UV-cured systems; - high solids coatings; - powder coatings.
ACKNOWLEDGEMENT These studies have been supported by funds from the German Federal Ministry of Economics (promotion project Reg. 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 scientific co-worker at the Institut für Lacke und Farben Magdeburg e.v. and as head of the coating laboratory (Aerosil) at Degussa AG. His main focus is the formulation of functional coatings and the modification of binders. -> Ulrich Westerwelle studied Chemistry at the University of Bielefeld, Germany. In 1994 he received his Ph.D. in organic chemistry. During different positions in the coatings industry he gathered a profound knowledge in the development of water-based coating materials for paper and plastic substrates. In 2005 he joined the Institut für Lacke und Farben in Magdeburg, Germany, where he is currently head of the coating materials laboratory. Results at a glance - Organic-inorganic hybrid coatings produced by the sol-gel process have high performance but are expensive and may present difficulties in application. - A novel route to sol-gel coatings is described, in which the organic content is increased (and therefore the cost is reduced) and the application properties are improved by incorporating organic polymeric binders. - The initial tests confirm that it is possible to control the hardness and plasticity of the coatings by varying the amount and type of raw materials. - These coatings have potential applications in many areas including barrier coatings, corrosion protection, scratch and wear resistant coatings and toughening the surface of plastics. The authors: -> Since March 2006 Dr. Uwe Wienhold has been scientific freelancer in the area of coating materials (Wienhold Consult). Before that he worked as project manager and
Figure 1: Sol-gel formation process with silicon alkoxide.
Figure 2: Organic and inorganic structural elements and their influence (Source: Fraunhofer Institute for Silicate Research ISC, Würzburg).
Figure 3: Formation of sol-gel coatings using organic polymers.
Quelle/Publication: European Coatings Journal 07-08/2006 Ausgabe/Issue: 40 Seite/Page: Figure 4: Examples of the benefits of sol-gel coatings: an easy-clean surface from which 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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