摘要 |
A conversion coating primer for Zn, Al and Fe is applied from an aq. emulsion contg an acrylic or alkyd resin in which the ratio resin: PO4: CrO3 is 0.1-5:2:0.1-5, pref. 1-3: 1:0.3-3. Curing temp. is 50-125 degrees C, pref. 65-95 degrees C, and coating thickness 11-2200 g.m-2. Suitable acrylic resins have mol. wt. >100000 and pref. >500000. e.g. 'Neocryl' (RTM) or 'Rhoplex' (RTM). Suitable alkyds are modified rosin and drying oils such as 'Synthemul' (RTM). Phosphate and chromate are added as salts of NH4, Zn, Mn, Ni, Co, Fe but pref. as ZnCr2O7, CrO3, Zn(H2PO4)2 and H3PO4. Pref. coating thickness is 55-350 mg.m-2 for Al, and 0.2-1 g.m-2 for Zn and steel. |