5.3.1 Materials: Materials used for this study are listed below:
Synthetic zeolite: Synthetic zeolite was obtained from PQ Corporation, Malvern, PA, USA. This synthetic zeolite was designated as type A and contained only sodium cations for exchange. Average particle size of this zeolite was in the range of 2 to 3 micron.
Reagents: All chemicals used in this study were analytical grade reagents. DI water was used in all stages of the experiments. Test microorganism used for this experiment has been mentioned in Chapter 4.
Methods: A two-step method was employed fabricate the additives used in powder coating.
5.3.2 Functionalization of synthetic zeolite A:
Silver nitrate and copper nitrate were used to functionalize the zeolite samples. 25 gm zeolite in each batch was functionalized with 500ml of 0.05M AgNO3 and 0.1M Cu(NO3)2 by adjusting the pH at 4-4.5 with concentrated HNO3. All the zeolite samples were prepared by mixing with the aforementioned salt solutions at 60oC by stirring them continuously at 500 rpm for 24 to 48 hours. Different addition protocols were used during functionalization to maximize the cations in the zeolite after ion-exchange. Silver loading of the zeolite was executed in the dark environment to stay away from the photo- oxidation of silver [18]. Two different phases were separated by centrifuging at 3500 rpm for 25 min. The solid phase was washed several times with deionized water to remove excessive ions. Later a 0.1N NaCl solution was used to identify any ionic silver species in
the supernatant fluid after centrifuging and washing. All functionalized samples were then dried at 80oC for 5-6 hrs while maintaining the dark condition. After sufficient drying, the particles, were broken down into their initial respective particle sizes by a small grinder and kept in the dark container to avoid oxidation. Here after all the samples are referred to additive.
5.3.3 Encapsulation of additives: 2% and 6% PVA solutions were made by dissolving Poly (vinyl alcohol), [-CH2CHOH-]n, Mw 89,000-98,000, 99% hydrolyzed, Sigma 341584 in DI water at 80oC for 45 minutes. The prepared additive was then added to the solution and mixed until became a thick slurry to avoid phase separation. Later the same was dried at room temperature followed by grinding. Powder spraying and curing process are mentioned in the earlier chapter.
5.3.4 XPS (X-ray photoelectron spectroscopy): To assess the change of Ag+ with and without the presence of copper during curing of the sprayed powder, X-ray photoelectron spectroscopy (XPS) analysis was carried out using a Kratos AXIS Ultra Spectrometer equipped with Mg Kα X-ray source and the XPS spectra were recorded at -5 to 1000eV with 2eV step size. The existence of silver is commonly determined by using the binding energy but it is difficult to assess the oxidation state of silver from XPS peaks due to very small chemical shift. So, MNN Auger electron analysis was used to determine the different states of silver.
5.3.5 Transfer efficiency of additives: Additives, which were added into the resin powder may have changed in concentration during the electrostatic spraying due to the different chargeability of two materials. Additive concentration was determined using ASTM D5630-06 Standard Test Method [8]. Powder was collected from the surface after spraying, which is called the transferred powder. This powder was heated at 550°C in a ceramic crucible for 90 minutes to remove all combustible material from the powder, leaving behind the non-combustible material in the form of ash. The amount of ash was then used to calculate the actual additive concentration of the transferred powder.
5.3.6 Impact resistance analysis (ASTM D2794): Impact resistance of any powder coated surface is one of the most important parameters which gives a indication of the mechanical performance of the same. It is done by an impact tester (Elcometer 157014) which imposes an instant stress by an object of a weight from a certain height. When the impact is given on the coated side of the testing surface, it is called direct impact. On the contrary, if the impact is given on the uncoated side of the surface, then it is called indirect impact. The objective of this test is to find out the threshold of the bearing load of the coated surface. The procedure which was followed for the evaluation of the impact resistance is given in the instructions of the “ASTM D2794 Standard Test Method”. The apparatus consists of a vertical guide tube, 0.6m to 1.2m long through which a free falling cylindrical weight, which has a round impact head, can slide freely. The vertical guide has a length scale in millimetre attached next to its path of travel, which facilitates to keep record of the height of the weight from which it was dropped each time. The kinetic energy of the falling object was actually imparted on both side of the coated panel,
having a coating of 40-45 micron and the results of the load bearing test gave the impact resistance of the said surface.
5.3.7 Pencil scratch hardness test (ASTM D3363): The scratching resistance to any hard element was measured by this analysis following the “D3363 Standard Test Method for Film Hardness by Pencil Test” pencil with different hardness were used as the testing tool. The pencil that will not cut into or gouge the coated surface determines the hardness of the coated surface. A set of calibrated wood pencils with the following scale of hardness from the softest (6B) to the hardest (6H) were used (6B – 5B – 4B – 3B – 2B – B – HB – F – H – 2H – 3H – 4H – 5H – 6H). The coated surface was kept on a firm flat surface. The pencil was firmly held against the coated surface at an angle of 45°. Uniform pressure was applied forward and downwards and the stroke was around 6.5 mm long. In order to have consistent result, at least two strokes per lead were done on the coating.
5.3.8 MEK Test (Methyl Ethyl Ketone) ASTM D4752: This is a solvent rubbing testing for assessing the solvent resistance of the coating. To remove loose dirt, 6 inches long coated surface was cleaned with water and dried with cloth. A MEK saturated cotton bud was hold on the test surface at 45o angle and rubbed with moderate pressure, away from the operator and then back towards the operator. Each double rubbing, one forward and back motion, was completed in approximately 1 sec and was continued up to 50 times. The rubbing effect was recorded after the test was completed.
5.3.9 FTIR (Fourier transform infrared spectroscopy): Additives prepared with silver and copper ions individually and together were characterized by ATR-FTIR spectroscope (Nicolet 6700 FTIR) with a single reflection diamond ATR accessory. The spectra were scanned between 4000 and 500 cm-1.
5.3.10 Autoclave analysis: The antimicrobial coated surface including the control surface were autoclaved at 121°C for 15 minutes immediately after every exposure with microorganism. The pressure inside the autoclave was maintained at 15 psi and the same were repeated for four times. The color changes and antimicrobial efficiency of the coated surface were observed and recorded after every autoclave treatment.
All other analysis methods (XRF, ICP-OES, TGA, XRD, Antimicrobial analysis, Color analysis, Toxicity analysis) used in this study have reported earlier in Chapter 4.
The same coated surface was used repeatedly called as 'trial' to check the efficiency against microorganisms. It is to be noted that the aforesaid coated surface was cleaned with soap and water and dried before every use.