Tailor-made additives

Transcription

1 Tailor-made additives Controlled polymer structures enhance performance. Clemens Auschra, Ernst Eckstein, Ralf Knischka, Frank Pirrung, Peter Harbers. Novel polyacrylate-based coatings additives such as pigment dispersants and levelling agents have been developed utilising nitroxyl-mediated controlled free radical polymerisation. The ability to control and optimise the polymer structure offers unique performance advantages. Improvements in rheology, levelling and compatibility have been demonstrated. Controlled free radical polymerisation (CFRP) is a technology which allows well-defined functional polyacrylate copolymers to be synthesised. In recent years it has drawn increasing interest in industrial research [1]. It permits the design of complex polymer architectures such as defined block copolymers, which so far have not been obtainable through existing commercial polymerisation techniques. Among the various CFRP techniques, nitroxyl-mediated controlled free radical polymerisation (NOR technology) has proven to be especially suitable for the development of new coating raw materials and has recently found its first industrial use in the area of pigment dispersants [2-5]. The ability to create well-defined polymer structures is the key to enhancing effects in rheology and many other applications in which functional polymers are used to modify surfaces or interfaces. Application areas Dispersion technology is a typical area of use, as the "tailor-made" structure of controlled polymers can be translated into unique application properties. Controlled polymer pigment dispersants improve the rheology of pigment concentrates and their pigment stabilisation. This offers potential economic advantages as well as technical benefits such as enhanced colour properties in the final paints. Levelling agents are another area in which enhanced rheological effects and surface activity offer novel solutions, for example in powder coatings. The design criteria for these additives are outlined below with recent examples of practical developments. NOR offers close control of polymerisation The basic mechanism of NOR polymerisation is the reversible capping of a growing polymer chain radical, which is achieved through an equilibrium involving a stable nitroxyl radical (Figure 1). Compared with existing commercial polymerisation processes, the two essential features offered by CFRP are the potential to: - achieve narrow molecular weight distributions; - create block copolymer or similar sequential polymer structures. Special NOR polymerisation regulators have recently been developed for use with acrylic monomers [6, 7]. These are sterically hindered alkoxyamine compounds, for example those derived from 2,2,6,6-tetraalkylsubstituted piperidines. The same class of compounds is already well known in the form of hindered amine light stabilizers or HALS, which are frequently used in coatings and plastics applications. HALS effectively prevent radically induced degradation of binder resins under severe service conditions such as exterior weathering. A comparison between the structure of an NOR polymerisation regulator and HALS is shown in Figure 2, while further details of the chemistry and use of NOR polymerisation regulators have been described elsewhere [8]. NOR technology eliminates unwanted contaminants As shown in Figure 1, the NOR polymerisation regulator is incorporated as the chain end of the controlled polymer and does not need to be removed, because as a "built-in stabiliser", it does not cause incompatibility in any coatings application. This represents a very significant advantage compared to other controlled polymerisation methods such as ATRP (atom transfer radical polymerization) or RAFT (reversible addition-fragmentation chain transfer polymerisation), in which the catalysts or regulator compounds usually cannot be tolerated in end-use applications. In ATRP for example [9], soluble copper amine complexes have to be removed, not only because they cause unacceptable discoloration, but also because of potentially adverse effects on coating stability or interference with the crosslinking of binders. Experience with different CFRP techniques indicates that NOR is ideal for use with polyacrylate-based coating raw materials, as it combines the key advantages of a high level of control of the polymer structure, ease in processing and the perfect compatibility of the HALS-derived polymerisation regulator chemistry. Improving the design of pigment dispersants Block copolymers have been shown to be some of the most efficient types of pigment dispersants [10], especially when compared to conventional copolymers with a lower level of structural control such as random copolymers. The main reason for this is that a sequential arrangement of pigment anchoring groups allows stronger and more efficient adsorption on pigment surfaces to be achieved and, for that reason, much better stabilisation of the pigment particles. Particle-particle interactions are efficiently reduced, which translates into improved rheology and improved stabilisation against flocculation. AB-type block copolymer pigment dispersants have been developed based on NOR technology and have been demonstrated to be very useful in demanding high solids solvent-borne applications such as the dispersion of high surface area carbon black pigments or resin minimal pigment concentrates [8]. Amongst the various design parameters such as chemical type and chain length of the A and B blocks, the selection of the chemistry of the pigment anchoring block is most important for optimising dispersant performance for specific pigments [11]. Some non-polar organic pigments in particular still represent a challenge, because they do not allow good adsorption of standard dispersants based on common anchoring groups such as aliphatic amino groups or acid functionalities. In such cases, optimisation of dispersant performance requires the use of special anchoring groups with good absorption properties for the pigments concerned. Both rheology and optical properties can be enhanced In a recent development, pigment anchoring groups were optimised by using special aromatic groups in addition to the common amino groups. This results in block copolymer dispersants which are tailor-made for non-polar organic pigments. The AB-block copolymer AB-1 was compared in resin-free pigment concentrates to some state-of-the-art commercial dispersants. In a resin-free millbase, the performance requirements on dispersants are most demanding, as the pigment stabilisation depends solely on the dispersant without additional support from any resin. The key properties of the new dispersant are shown in

2 comparison with commercial products in Table 1. The controlled block copolymer dispersant AB-1 provides very good control of rheology with a range of organic red, yellow, violet and blue pigments. For example, Figure 3 shows the effects of different dispersants on the rheology of a resin-free millbase with a phthalocyanine blue pigment, PB 15:2. The good pigment particle stabilisation provided by AB-1 also produces very good optical quality in tinted paints. The corresponding masstone formulations give high transparency and high gloss as shown in Figures 4 and 5 using an amino anthraquinone pigment PR 177, a phthalo blue pigment PB 15:3 and a high surface area channel carbon black pigment as examples. Levelling the playing field for powder coatings Levelling agents are another area in which controlled polymers offer novel solutions by combining effects which so far have not been available with conventional polyacrylates. This applies especially in demanding applications such as powder coatings. Polyacrylate levelling agents are an indispensable component of every powder coating formulation, since the thermal curing process creates a difficult application window between the overlapping processes of melting, film formation, levelling and crosslinking. Conventional polyacrylate levelling agents are polymers with low to medium molecular weights, a relatively broad molecular weight distribution and glass transition temperatures well below 0 C, which allows them to be very fluid at the high stoving temperatures of powder paints. Therefore, at room temperature these materials are viscous oils or sticky resins which are difficult to handle and cannot be directly incorporated into powder coatings. They must first be transformed into a solid form which can be easily processed with the other solid components of a powder coating recipe. One solution is through adsorption onto solid silica particles, but this can lead to unacceptable turbidity, haze or gloss reduction, especially in clear coats. Another way is to produce masterbatches using a solid resin component, but aside from the disadvantage of the extra production step, this approach also lacks flexibility in formulation across different coatings. Narrow molecular weight distribution enhances performance In contrast to conventional polyacrylates, controlled polymer levelling agents can be produced directly as solid polymers, without the need for solid carriers and their associated disadvantages. Due to their narrow molecular weight distribution and the precise control of copolymer compositions, they can be made with a very narrow glass transition temperature range. This allows levelling agents to be designed which are solid at room temperature, but which also transform into the fluid state with very low viscosities at temperatures above approximately 100 C. Table 2 shows characteristic data of typical commercial levelling agents in comparison with controlled polymers which have been developed recently and used with excellent results in powder coatings. In Figure 6 the molecular weight distribution of one of these solid controlled polymer levelling agents is compared with that of a conventional levelling agent of similar chemistry and comparable molecular weight, which is a viscous oil at room temperature. In Figure 7 the softening behaviour of controlled polymer levelling agents is compared to typical commercial products in terms of DSC (differential scanning calorimetry). It can be seen that the solid controlled polymers exhibit a relatively narrow glass transition range at around C. Despite their unusually high softening point, these controlled polymers offer excellent performance as levelling agents in powder coatings. Table 3 shows performance data for a GMA-acrylic powder clear coat with a concentration of 1% by weight levelling agent. The controlled polymers provide a very smooth surface as measured with a "Wavescan" instrument and at the same time a high gloss without any adverse effects on haze or yellowing (measured as b*). The same trends can be observed under more severe testing conditions, for example by decreasing the film thickness of the powder clear coat or by reducing the addition rate of the levelling agent. NOR technology will continue to advance Controlled polymers have recently found their way into industrial use in the form of polyacrylate pigment dispersants. In powder coatings in particular, controlled polymer levelling agents have been shown to offer distinct advantages and can be expected to contribute to future advancements. NOR technology is particularly suited to coating applications, because the chemistry of the polymerisation regulator avoids the need for special post-treatment of the final polymer products. The development of tailor-made additives for various demanding applications is continuing through the use of these enhanced polymer design capabilities. Acknowledgements The authors would like to thank all their colleagues at Ciba SC and Efka Additives for their contributions and cooperation, especially: Peter Nesvadba, Andreas Muehlebach, Sevgi Zeren, Andreas Möck, Almut Staniek, Werner Steiner, Nicolas David, Tissa Rebmann, Matthias Graber, Piet van der Steeg, Marleen Suurmeijer and Harm-Jan Kok. REFERENCES [1] K. Matyaszewski, J. Xia, Chem. Rev., 2001, 101, [2] D.H. Solomon, G. Waverly, E. Rizzardo, P. Cacioli, US , 1986 [3] F.O.H. Pirrung, P.H. Quednau, C. Auschra, Chimia, 2002, 56, [4] C. Auschra, E. Eckstein, A. Mühlebach, M.O. Zink, F. Rime, Prog. Org. Coat., 2002, 45, [5 P. Harbers, product presentation EFKA-4300 and EFKA-4330, European Coatings Show Nuernberg, April 2003 [6] M.O. Zink, A. Kramer, P. Nesvadba, Macromolecules, 2000, 33, [7] P. Nesvadba, L. Bugnon, R. Sift, Polymer International, 2004, 53(8), [8] C. Auschra, E. Eckstein, R. Knischka, F. Pirrung, P. Harbers, ECJ, 06/2004, [9] Y. Shen, H. Tang, S. Ding, Prog. Polym. Sci., 2004, 29(10), [10] H.L. Jakubauskas, J. Coat. Techn. 1986, 58 (736), [11] H.J.W. van den Haak, J. Coat. Techn., 1997, 69 (873), Results at a glance - Controlled free radical polymerisation allows the formation of polymers with closely controlled properties and a narrow molecular weight distribution.

3 - The nitroxide-mediated controlled free radical polymerisation (NOR-technology) is particularly suited to the production of coatings additives because the polymerisation regulator does not adversely affect coating properties. - NOR technology has already been used to design novel polyacrylate coating additives such as pigment dispersants and levelling agents. - In powder coatings in particular, controlled polymer polyacrylate levelling agents offer distinct performance benefits. - Using the enhanced polymer design capabilities of NOR technology, developments continue towards tailor-made additives for various demanding applications. The authors: -> Dr Clemens Auschra received his Ph.D. in polymer chemistry from University of Mainz in In 1998 he joined Ciba Specialty Chemicals and is now research & development manager for Polymer Specialties, currently engaged in the development of coating additives. -> Ernst Eckstein holds a B.S. degree in chemical engineering from the Basle Institute of Technology. Since 1998 he has been in charge of Ciba Specialty Chemicals' application laboratory for Polymer Specialties. -> Ralf Knischka received his Ph.D. in Macromolecular Chemistry from the University of Freiburg in He joined Ciba Specialty Chemicals in He is engaged in research and development of coating additives for Polymer Specialties. -> Dr Frank Pirrung received his Ph.D. in organic chemistry from the University of Amsterdam in He joined EFKA Additives in the same year and since 2000, has been head of research and development. -> Peter Harbers joined EFKA Additives in 2000 as an area manager in technical service, and is now the company's head of application development.

4 Figure 1: Basic mechanism of nitroxyl-mediated controlled free radical polymerisation. Figure 2: Structure of NOR polymerisation regulators (left, with R = methyl & ethyl) compared with a standard HALS used in coatings (right, with R = methyl). Figure 3: Millbase viscosity of solvent-borne resin free blue pigment concentrates (30% Heliogen Blue L 6975 D, 24% dispersant on weight of pigment). Figure 4: Transparency of masstone tinted paints derived from resin free pigment concentrates. Binder:acrylate / CAB / melamine Visual rating: 0 = very good, 8 = very poor.

5 Figure 5: Gloss of masstone tinted paints derived from resin free pigment concentrates. Figure 6: Molecular weight distribution of polyacrylate levelling agents determined by gel permeation chromatography. For numerical values of molecular weight and polydispersity, see Table 2. Figure 7: DSC analysis of softening behaviour of polyacrylate levelling agents. Heating rate: 20 C/min. Curves are shown with arbitrary offset for clarity.

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