Nano-functionality withcalcium carbonate

Transcription

1 Nano-functionality withcalcium carbonate A new development in architectural paints is the use of nanotechnology. The innovation lies in the nano-functionality of calcium carbonate micro particles. One of the advantages is the stabilisation of a nanostructure on coarse particles. Thus the nano effect is fully retained. This is in contrast to the reduction that normally occurs due to agglomeration. The benefits of such a product have been studied in various matt architectural paints. Nano-structured micro particles boost light scattering and opacity The filler industry was on of the first to be active in the development and subsequent production of nano-materials. Therefore "nano" is a pervasive buzzword in the sector. Before discussing nano-products and their process possibilities, some clear definitions are needed. After all, not every product that bears a nano prefix really merits the term. In theory, an ultra fine ground natural filler with a D 50% median particle size of 0.8 µm, or 800 nm, might be described as a nano-filler. Yet such a product does not match the consensus view of nano-products. What are these materials and how are they defined? Regrettably, there is still a lack of consistent terminology. From discussions and publications, it is reasonable to conclude that materials with a particle size below 100 nm may safely be termed nano-products [1].The filler industry was an early user of nano-products [2]. Currently available nano-fillers produce a variety of technical effects in coating material applications such as rheology, reinforcement, matting, adhesion strength, etc. In addition to technical progress, environmental and safety aspects are important. The particle size of nano-products not only makes their surfaces very reactive, it also allows them to penetrate cell membranes. Depending on the size of the particles and their chemical composition, it is possible that nano-products could have a negative effect on health and the environment [3,4]. This in particular, is a reason for pushing a safe nanotechnology. Different particle morphology Nano-functional calcium carbonate is produced from the ground, natural product (GCC). Chemical modification transforms the solid particles into amorphous particles, which have completely different particle morphology: a porous structure. Therefore, this group of mineral fillers are called modified calcium carbonate (MCC). Differences in particle structure of the calcium carbonate treated to grinding, precipitation or modification technologies have been investigated by means of scanning electron microscopy (SEM) [5-7]. The modification process enables the structure of inorganic filler particles based on calcium carbonate to be designed for the first time. To date, this degree of freedom was available only in organic chemistry, e.g. the design of copolymers used as binders in paints. Now, physical parameters such as particle morphology, particle porosity, specific surface area, absorption characteristics and others can be defined. In Figure 1 a series of "golf ball" structures is seen from two different viewpoints. In the overview, the very similar particle size is recognizable. This means that the GCC typical particle size distribution no longer exists. The close-up view partially explains why these relatively coarse particles with a particle diameter of around 15 µm have such a high specific surface area of between 31 and 56 m²/g. The second part of the explanation is revealed by particle porosity measurements. Comparison of MCC and GCC structures For a fundamental understanding of the unusually high specific surface area, MCC and GCC structures have been compared using a mercury porosimetry. The mercury intrusion curves obtained, seen in Figure 2, show a bimodal pore structure for the MCC structures of eggs and roses. The porosity of MCC is much higher, whereas samples of GCC show only slight porosity, due to the interstices between the particles in a film. These findings are illustrated by a cross-section of such an MCC particle. The internal structure of a golf ball particle viewed with SEM explains the high specific surface area [7]. The coarse MCC particles provide, at their surface, plate-like particles with a thickness of less than 100 nm. Thus a nano-structured surface has been established and stabilised on a coarse particle. For maximum light scattering, or opacity, the surface structure was further modified leading to a structure with a highly closed surface, which was still very amorphous [8]. This is essential for the mechanical properties in the application of architectural paints. Use in emulsion paints The performance of nano-functional calcium carbonate in emulsion paints has been compared to ultra-fine, ground calcium carbonate (UFGCC) and titanium dioxide [9]. The most important physical properties of the materials under test are summarised in Table 1. In order to see the pure filler performance in emulsion paints, a test formulation without titanium dioxide at a medium pigment volume concentration (PVC) of 50 % has been developed (Table 2). The fillers and pigments were replaced on a volume basis i.e. at identical PVC. The paints were tested for dry and wet opacity (ISO 2814), whiteness Ry (DIN 53145) and sheen (DIN 67530) (gloss at 85 ). Changing the fillers and pigment led to big differences in dry opacity. As expected, it was greatly increased with nano-functional calcium carbonate compared to UFGCC, but was very similar to pure titanium dioxide, see Figure 3. However, the MCC technology does not convert the filler to a pigment as can be seen in Figure 4. Nano-functional calcium carbonate has still a refractive index close to common fillers, but can slightly increase the wet opacity of emulsion paints. At 45 % wet opacity, it is only half that of titanium dioxide, but 30 % more than UFGCC. The resulting whiteness Ry of 87 % in the emulsion test paint is identical for both fillers, although nano-functional calcium carbonate is based on chalk, whereas the UFGCC is produced from white marble. The pigment titanium dioxide simply reached a significant higher level in whiteness Ry of 93 %. Ultrafine ground calcium carbonate

2 fillers also lead to high reflection at an angle of 85. This is called sheen. UFGCC shows this clearly in Figure 5. In contrast, the MCC technology has a very low sheen level and thus is suitable for matt emulsion paints. As anticipated, the very fine titanium dioxide helps the reflectivity. Better scrub resistance To study the influence of MCC on wet scrub resistance, nano-functional calcium carbonate was added in differing amounts to emulsion paint with a PVC of 77 %. The addition of nano-functional calcium carbonate was compensated by the reduction in the level of fillers with much lower oil absorption than MCC. Normally, the high oil absorption of MCC filler should worsen the wet scrub resistance. Surprisingly, the addition of nano-functional calcium carbonate led to an improvement. See Figure 6.At first sight, this result does not seem logic. However, nano-functional calcium carbonate has a porous, amorphous particle structure with pores and capillaries which give 18 times more total intruded volume than GCC. These small pores and capillaries enable liquid access and therefore wetting of an emulsion binder. However, this occurs only on the particle surface and at the entrance of the pores and capillaries. It leads to a reduction in the amount of binder needed and improved adherence of the emulsion to the capillaries and pores close to the particle surface of the MCC filler. Thus, an interpenetrating network with strong cohesive forces is created between the polymer chains and MCC filler particles. In contrast, normal GCC particles, without a porous, amorphous surface, are just wetted on the homogeneous surface without forming a network of sterically integrated filler particles. Acknowledgements I would like to thank all of my colleagues at Omya for their assistance in this publication. I would also like to thank Pierre Berger, Olav Bergset, Dr. Patrick A. C. Gane, Dr. Thoralf Gliese, Andreas Kumm, Fritz Lehner, Dr. Cathy Ridgway and S. Zioerjen for providing copious results and data. European Strategy for Nanotechnology", COM 388 final, 2004 [2] D. Gysau, "Fillers for Paints", p , Vincentz Network, Hannover 2006 [3] Nanotechnologies: A preliminary risk analysis on the basis of a workshop organized in Brussels on 1-2 March 2004 by the Health and Consumer Protection Directorate General of the European Commission [4] UK Royal Society and the Royal Academy of Engineering, "Nanoscience and nanotechnologies: opportunities and uncertainties", Final Report, 2004 [5] D. Gysau, "Rosen und Golfbälle", vol. 107, Farbe und Lack, Vincentz Network, Hannover, 09/2001 [6] F. Lehner, D. Gysau, "Modified Calcium Carbonate A New Generation of Functional Filler", 38th International Conference on Coatings Technology, Sec/Czech Republic, May 21-23, 2007 [7] D. Gysau, "The Time is Ripe for Revolution Modified Calcium Carbonate (MCC)", 29th FATIPEC Congress, Ghent/Belgium, June 9-11, 2008 [8] Omya Product Information "Omyabrite 1100OM", Omya International AG, Oftringen, 05/2007 [9] Omya Product Flyer "Omyabrite 1100OM Functional Filler for High Quality Emulsion Paints", Omya International AG, Oftringen, 2007 * Corresponding Author. Contact: Detlef GysauOmya AGTel [email protected] THE AUTHOR? Detlef Gysau obtained his Master of Science in paint chemistry at the University of Applied Science in Stuttgart. Since 1996, he has worked at the headquarters of the Omya group at Oftringen, Switzerland. Currently he is Director of Applied Technology Services with global responsibility for the segment paints, coatings and adhesives. Results at a glance - Matt emulsion paints have been used to illustrate the good performance of nano-functional calcium carbonate. - Pigment-like dry opacity is obtained with MCC. - The nano-functional filler provides high opacity in combination with low sheen as well as good wet scrub resistance for emulsion paints. References [1] EC Communication of 12 May 2004: "Towards a

3 Figure 4: Wet opacity of fillers and pigments in emulsion test paint

4 Figure 5: Sheen of fillers and pigments in emulsion test paint

5 Bild zu Nano-functionality withcalcium carbonate

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8 Figure 1: Golf ball structures with different specific surface area

9 Figure 2: Mercury intrusion curves for MCC (modified calcium carbonate) and GCC (ground calcium carbonate)

10 Figure 3: Dry opacity of fillers and pigments in emulsion test paint