Paint & Coatings Industry - March 2009 - (Page 30) Understanding and Use of Glass Flake left exposed to atmosphere. The moisture uptake will vary with surface area and the number of hydroxyl groups on the surface so this weight gain can be used to give some measure of surface reactivity. Evaluations show that the thinner glass flake has substantially greater reactivity than the thicker glass flake independent of surface area. Although not considered as a thixo- tropic agent, it was thought that there may be some synergistic effect capable of being used to benefit thixotropy e.g., in reducing the amount of fumed silica or other thixotrope within a coating film. A standard formulation containing 5 µm flake was used as a reference material, and the viscosity, thixotropic index and hold up on a vertical surface measured. Various for::: Intelligence in Rheometry Rheometry Focusing on Solutions mulations were produced with the 1.3 µm thick glass flake. The viscosity and thixotropic index were measured and a reduction in the fumed silica thixotrope made to compensate. The formulation which produced the nearest viscosity and spray characteristics was then used for further evaluation. Viscosity, thixotropic index and hold-up thicknesses are shown in Table 5. The glass flake level in the second formulation was reduced by 4% over that of the one containing 5 µm glass but has a surface area more than twice that of the original formulation. The thixotropic agent fumed silica was reduced by 50% for the second formulation, demonstrating the thixotropic properties of the thin flake and giving value to it. Processing/Mixing Time One of the areas that greatly affects glass flake performance within the carrier is the mixing time. Mixing time affects both wet out and distribution of the flake having significant impact upon not only MVT rate but also the mechanical properties of the resultant coating. Tests were carried out to evaluate the parameters using a polyester resin and different grades of flake (Table 6). Compounding the glass flake into the resin was done using a Z-blade mixer. The mixture was then moulded into a plaque using a steel compression mould with 33% by weight of glass incorporated into the resin; the test results are shown in Table 6. The smaller diameter flake B/1 required a longer mixing time than the other flakes with larger aspect ratios. Electron microscopy shows that the samples contained poorly dispersed flakes after 45 minutes mixing but even distribution of the flakes after 60 minutes, and subsequent micrographs taken of specimens that had been mixed for 75 and 90 minutes show breakdown of the flakes occurring. Optimum mechanical results are seen from specimens that have been mixed for 60 minutes with glass flake A/1 and A/2 and 75 minutes for the B/1. Overall, the higher the flakes’ apparent volume content in the specimen, the better the mechanical properties that specimen provides, given that satisfactory mixing is achieved. A/2 has the best mechanical properties probably due to the formulation having the greatest surface area for weight of glass flake, hence more reinforcement of the resin, resulting in a higher flexural modulus etc. Impact and flexural strength for A/2 is almost double the value for A/1 and B/1 values. This Fully automated, robotically operated: The HTR High Throughput Rheometer from Anton Paar. Automatic sample filling and cleaning of measuring systems Processes up to 96 samples in a single run up to 24 hours Modular setup, flexible to individual applications All standard MCR 301 test types, configurations and accessories apply 800-722-7556 info.us@anton-paar.com www.anton-paar.com Anton Paar® USA Visit ads.pcimag.com 30 MARCH 2009 | W W W . P C I M A G . C O M http://www.anton-paar.com http://www.anton-paar.com http://ads.pcimag.com http://WWW.PCIMAG.COM
For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.