Tiny particles, huge effect

Transcription

1 Tiny particles, huge effect Radiation curable silica nanocomposites for scratch and abrasion resistant coatings. Christof Roscher. Nanotechnological reinforced composites consisting of monodisperse, non-agglomerated SiO 2 -particles and radiation-curing acrylic resins show a great enhancement of scratch and abrasion resistance and hardness without possessing the disadvantages known from other inorganic additives. Even with loads of silica up to 50 %, water-clear products with low viscosities and no sediment-formation are realized, making these nanocomposites a useful and versatile raw-material source suitable for all kinds of high performance applications. In polymer industries as well as material science the use of inorganic fillers becomes more and more important. Since many decades low-price fillers (based on e.g. CaCO 3, clay, quartz, carbon black) are used to reduce material costs of monomers or polymers. Additional to this cost reducing effect, suitable inorganic fillers can provide an improvement of material properties like tensile strength, hardness, abrasion resistance, thermostability, coefficient of expansion and gas-barrier effect. However, the applicability of such fillers, particularly in coatings, is restricted because of some major disadvantages (e.g. loss of transparency, high viscosity build-up, sedimentation). A solution for this problem can be the use of monodisperse nanoscaled SiO 2 -particles which do not inhere these drawbacks. Currently silica nanoparticles suitable for industrial use are mainly made from silicon tetrachloride by a flame hydrolysis process, followed in many cases by a silane treatment of the SiO 2 -surface [1]. The size of the primary particles of this fumed silica is in the range of 7 to 40 nm, but during the flame hydrolysis much larger agglomerates are formed. These agglomerates are only partially redispersable to the original primary particles, which is the reason for the dramatic viscosity increase and clarity loss of most dispersions containing fumed silica in amounts of more than a few percent. Therefore, the achievable filler load is usually limited below the level desirable from a performance standpoint. Surface modification of the silica with organofunctional alkoxysilanes together with mechanical aftertreatment may help up to a certain level and shows promising results [2,3] but at the expense of significantly increased cost and certain processing inconveniences. An alternative route to nanoscaled particles is the preparation of colloidal silica sols after the sol-gel-method[4]. Because of the expensive starting materials and the complicated processing the utilization of this procedure is limited and of low practical use in the industry. The most inexpensive SiO 2 -nanoparticle sources are silica sols prepared form aqueous sodium silicate using ion exchange techniques. Sodium silicate solutions are commodities and the sols produced thereof provide narrow particle size distributions within a range of 7 to 70 nm. UV-curable silica nanocomposites made from these sols have shown to highly improve material properties [5-7]. However, to get the silica particles from the aqueous into the hydrophobic organic environment without causing agglomeration is a major challenge. Therefore until recently marketed products were only available in a very limited number of solvents and acrylic monomers which show numerous incompatibilities when formulated. A new and highly efficient methodology of particle synthesis and subsequent organic surface-modification has been developed which delivers versatile silica nanocomposites (SNC) which contain colloidal silica with particle-diameters of about 20 nm. Previous investigations revealed, that Epoxy-SNC highly enhance the material properties of thermally or UV-cured epoxy resins [8]. A new product group of SNC in a very broad range of acrylic monomers and oligomers (polyester-, epoxy-urethane- and melamine-acrylates) now offers the possibility to enhance the material performance of radiation curing acrylate coatings. With silica concentrations of up to 60%, these products are as clear as the starting resins. Small Angle Neutron Scattering (SANS) examinations show very narrow particle size distributions being characteristic for these products (Figure 1) which proves that they are monodisperse and non agglomerated. This is additionally confirmed by TEM photography (Figure 2). Comparison of rheological properties of fumed silica and nanoscaled silica In order to verify that colloidal non-agglomerated SiO 2 -nanoparticles inhere the assumed benefits, first investigations were made to show the reduction in viscosity build-up and improvement of scratch and abrasion resistance compared to fumed silica. For this purpose the 50% silica containing Product A was diluted with the base resin (SR 494) to give samples containing different amounts of SiO 2. Comparative examples were generated by incorporating commercially available methacrylate-modified fumed silica into the pure binder using a dissolver. The mixture was then dispersed on a pearl mill. With all silica concentrations the obtained formulations were as clear as the starting resins. Figure 3 shows the dependence of the viscosity on the SiO 2 -content. It is clearly revealed, that in comparison with fumed silica the fluidity of SNC containing equal amounts of filler is drastically improved (30 % fumed silica cause a viscosity increase equivalent to 50 % colloidal silica). Manual scratch resistance Thin films (50 µm) of the above formulations were prepared on melamine impregnated paper and polycarbonate by using a doctor-blade coater. Passing these samples (containing 2 % of photoinitiator "Lucirin TPO-L") at velocities of about 6 m/min through two consecutive medium pressure Hg lamps (120 W/cm; residual O2 < 200 ppm) proved sufficient to achieve equivalent UV curing. In all cases excellent adhesion was attained and the presence of silica apparently had no impeding effect on the cure rate. Already a simple manual scratch test disclosed a distinctly improved scratch resistance. After 60 double rubs with a steel wool the unfilled coating was totally destroyed. Two samples containing 28% fumed and colloidal silica respectively to some extent already showed an improvement of scratch resistance. However, the incorporation of 50% colloidal silica leads to an improvement otherwise unachievable and the coating remains virtually untouched (Figure 4). S-42 Taber abrasion resistance In order to obtain more qualified data, S-42 Taber abrasion resistance of the test panels was determined. The weight loss was recorded after 50 cycles under a 500 gram load using CS-0 calibrase wheels covered with S-42 sandpaper. Additionally the diamond scratch hardness was determined according to DIN The results demonstrate a significant performance improvement of the SNC-samples

2 compared not only with the unmodified SR 494 but also the fumed silica containing formulation (Table 1). The latter result is the more remarkable considering the significantly lower viscosity and the much more convenient processing of the SNC. It is also revealed, that the improvement increases parallel to the silica concentration and doesn't reach a maximum. At a level of 50% silica the weight loss decreases from 51 mg to as low as 6 mg which is equivalent to a reduction of about 88%. CS-10 Taber abrasion resistance The very aggressive sand paper method suits well for demanding applications like parquet flooring. In order to obtain suitable data for less demanding applications a less severe test was performed, this time using the more gentle CS-10 rubber wheels. After 100 and 500 abrading cycles under a 500g load the resulting haze (difference of gloss before and after the test) was measured using a reflection densitometer (60 ). The results of both measurements again show a significant performance improvement of the SNC-samples (Table 2). At a level of 28 % colloidal silica the haze reduction is equivalent to about 83 % (after 100 cycles) respectively 88 % (after 500 cycles). Surprisingly this time the improvement reaches a maximum at this filler concentration and isn't further increased by incorporating higher amounts of silica. Universal suitability of nanoscaled silica Coating properties are primarily determined by the type of binder present and a reinforcement strategy must therefore harmonize with the widest possible variety of systems. To demonstrate the versatility and universal suitability of nanoscaled silica, SNC from a selection of different UV-resins were tested to comprise a wide parameter range concerning functionality, reactivity, viscosity as well as, with regard to the unmodified samples, network density, hardness and flexibility. The various varnishes (containing 4 % of photoinitiator "Darocur 1173") were applied to black glass plates to a dry-film thickness of 90 µm and cured under a 160 W/cm mercury lamp at a speed of 10 m/min. For determination of the scratch resistance of the different samples, this time a scrub resistance tester for emulsion paints (DIN ) was used. The standard brush was replaced with a "Scotch-Brite" pad (type 07442) which was loaded with a 350 g weight and pulled for 200 times across the coated test panels and once more the resulting haze was measured. Elasticity and hardness examinations at the coatings were also carried out. Table 3 shows the results with 5 different acrylate systems. It is clearly revealed that the colloidal silica substantially improves the scratch resistance regardless of the chemical nature of the binder and its intrinsic elasticity and hardness. The microhardness (DIN ) is also significantly increased while the pendulum hardness (DIN ) surprisingly remains unaffected. Macromol. Chem. Phys. (2000) 201, p.2765 [3] S. Frahn, V. Valter, G. Leder, Conference Proceedings 6th Nürnberg Congress 2001, p.145 [4] R. Corriu, D. Leclercq, Angew. Chem. 108 (1996), p.1524 [5] L. N. Lewis, D. Katsamberis, J. Appl. Polym. Sci. (1991) 42, p.1551 [6] M. Misra, G. Guest, M. Tilley, Surf. Coat. Inter. (1998) 12, p. 594 [7] C. Vu, O. LaFerté, A. Eranian, European Coatings Journal (2002) 1-2, p. 64 [8] T. Adebahr, C. Roscher, J. Adam, European Coatings Journal (2001) 4, p. 144 Details on examined materials Product A: 50% SiO 2 in polyether tetraacrylate "SR 494" (Cray Valley) Product B: 50% SiO 2 in polyether triacrylate "Laromer LR 8863" (BASF) Product C: 50% SiO 2 in polyether triacrylate "Laromer PO 33F"(BASF) Product D: 40% SiO 2 in polyester acrylate "Laromer PE 44F" (BASF) Product E: 40% SiO 2 modified epoxy acrylate "Laromer 8986" (BASF) Product F: 50% SiO 2 in amine modified polyether acrylate "Laromer LR 8996" (BASF) Results at a glance - A new method of particle synthesis and subsequent organic surface-modification delivers silica nanocomposites (SNC) consisting of monodisperse, non agglomerated SiO 2 -nanoparticles and radiation curing resins. - The manufacturing process is very flexible and allows the production of nanocomposites based on an extensive range of acrylic monomers and oligomers as well as cationic curing epoxies. - The resulting products are stable, highly transparent and exhibit a remarkably low viscosity despite silica loads of up to 60%. - On the basis of the results presented it has been shown that the scratch and abrasion resistance of UV-cured SNC are substantially improved, making these nanocomposites a useful and versatile raw-material source suited for all kinds of high performance applications. LIFELINE -> Dr. Christof Roscher, hanse chemie AG, born in 1968, studied Chemistry at the University of Würzburg. He received his Ph.D. for his work on NIR-transparent inorganic-organic copolymers (ORMOCERs) at the Fraunhofer Institut für Silikatforschung ISC. In 1998 he joined hanse chemie where he is a group leader of R&D and technical service of nanocomposites. Acknowledgement The author thanks Dr. H.-J. Gläsel from the IOM Leipzig and M. Biehler and K. Menzel from BASF for their efficient work on the coatings. We gratefully acknowledge the assistance of GKSS Geesthacht for SANS analysis an ISC Würzburg for TEM. REFERENCES [1] S. Frahn, M. Ettlinger, T. Ladwig, W. Mendel, European Coatings Journal (2001), 3, p.98 [2] H.-J. Gläsel, F. Bauer, H. Ernst, M. Findeisen, E. Hartmann, H. Langguth, R. Mehnert, R. Schubert,

3 Figure 1: Particle size distribution of a SNC derived from SANS.

4 Figure 2: TEM of an ultra thin film of cured silica nanocomposites.

5 Figure 3: Viscosity h of silica filled SR 494 as function of SiO2-content.

6 Figure 4a: Microscopic images of UV-cured coatings after steel wool scratching containing 0% silica.

7 Figure 4b: Microscopic images of UV-cured coatings after steel wool scratching containing 28 % fumed silica.

8 Figure 4c: Microscopic images of UV-cured coatings after steel wool scratching containing 28 % colloidal silica.

9 Figure 4d: Microscopic images of UV-cured coatings after steel wool scratching containing 50 % colloidal silica.

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