(48f) Polymer-Silica Composite Nanoparticles for Electroplating Applications

The low cost and large recyclability of steel makes this material most effective for product manufacturing. The major limitation for its use is the low corrosion resistance which requires the use of protective layers like zinc coatings and solvent-born paints. The protective performance of the zinc coatings depends on their quality as well as on the level of paint adhesion thus requiring the use of adhesion promoting intermediate layers (e.g., phosphating) and primers. On the other hand, elimination of the phosphating treatment as a prerequisite for achieving good adhesion of paint on zinc coated steel is higly desirable since it is responsible for the largest part of the solid waste produced during the production process of electrogalvanized steel. Due to the above, the field of composite electrolytic coatings has received a lot of attention, since it is expected that with a minor change in the industrial process (e.g., addition of polymeric, inorganic and polymer-silica composite nanoparticles in the electrolytic bath), coatings with improved properties (e.g., lower friction, mechanical strength, better paint adhesion, improved corrosion resistance, etc.) will be produced. In addition, the tendency to use waterborne primers and paints makes necessary the synthesis of composite zinc coatings by electrolytic codeposition of waterborne polymeric or composite nanoparticles from an acid zinc plating bath. The polymeric or composite particles employed in a codeposition process should have a fairly narrow particle size distribution within a specific size range depending on the thickness of the coating (e.g., Dmean < 1£gm). In addition, the material should be carefully selected in order to be compatible with the ingredients of the electroplating bath. In the present study, polymer-silica composite nanoparticles (PS-Si) were synthesized by precipitation polymerization of styrene (St) in an alcohol (e.g., methanol, isopropanol, ethylene glycol) in the presence of an ultrafine alcoholic silica sol (NYACOL DP5820, COVENTYA) (20-45 % w/w based on St) employing azobisisobutyronitrile (AIBN) as initiator. Polymer-silica composite nanoparticles (P(St/4-VP)-Si) were also synthesized by precipitation copolymerization of St and 4-vinylpyridine (4-VP) (St/4-VP weight ratio : 4/1) in a methanol/water mixture in the presence of an ultrafine aqueous silica sol (Ludox AM, ~20nm, 30% SiO2, COVENTYA) (20-45 % w/w based on total monomers). The polymerization experiments were carried out in borosilicate glass vessels equipped with screw caps. The reaction mixture was thermostated by immersing the vessels in a constant temperature water bath. The resulting colloidal dispersions were purified by successive centrifugation-redispersion cycles. The surface morphology of the composite nanoparticles was assessed by scanning electron microscopy (SEM) and their size distribution was determined by laser diffraction. The weight percent of silica in the composite particles was measured by thermogravimetric analysis (TGA). Aqueous electrophoresis measurements were also performed. Both types of nanoparticles (i.e., PS-Si, P(St/4-VP)-Si), exhibited a rough surface morphology that can be attributed to the presence of silica nanoparticles on the particle surface, acting as stabilizers. The average diameter of the PS-Si particles varied between 1 and 3 £gm while the silica content was rather low (e.g., 0.3-0.95 % wt). Moreover, the silica content increased with an increase of the silica sol concentration. Regarding the effect of the solvent type on the size and morphology of the PS/Si particles, it was found that when the polarity of the solvent increased the particle size decreased. On the other hand, P(St/4-VP)-Si nanoparticles, prepared in the presence of an aqueous silica sol, had a smaller particle size (e.g., 0.2 - 1 £gm) and a higher silica content (e.g., 10-15.5 % wt). The silica content was found to increase when the monomer(s) concentration(s) was decreased. However, PS-Si particles synthesized, in the presence of the aqueous silica sol, had a rather low silica content (e.g., 3.4 %wt), thus proving the crucial role of 4-VP in the synthesis of polymer-silica composite nanoparticles. The latter observation was also confirmed by the aqueous electrophoresis measurements. Colloidal dispersions of polymer-silica composite nanoparticles were also synthesized by an emulsifier-free emulsion polymerization process of 4-VP as well as of 4-VP with St (St/4-VP weight ratio: 1/1) in the presence of an ultrafine aqueous silica sol Ludox AM (200 % w/w based on total monomers), employing ammonium persulfate (APS) as initiator. The polymerization experiments were carried out in a laboratory scale, water-jacketed, glass reactor of 100 ml equipped with a six-blade impeller, an overhead condenser and a nitrogen purge line. The reaction mixture was thermostated to within ?b0.05 oC with the aid of a constant temperature bath. The resulting colloidal dispersions were purified by successive centrifugation-redispersion cycles for the removal of the free silica nanoparticles. Spherical composite nanoparticles (Dmean: 102-116 nm) with increased silica content (e.g., ~50 % wt) were obtained. The examination of the composite nanoparticles morphology by transmission electron microscopy (TEM) revealed the presence of silica nanoparticles both inside the composite particles and on their surface. The stability of the composite nanoparticles was examined in an electroplating bath solution consisting of ZnSO4, 7H2O; KAl(SO4)2, 12H2O; H3BO3. A small quantity of purified particles were dispersed in the solution and kept under mild magnetic stirring for 24 hours. All types of particles were found to be perfectly stable in the electroplating bath and, thus, suitable for the production of composite zinc coatings.