Standard Practice for Preparation of Plastics Materials for coating.<br />Preparation of Plastic Substrates:<br />1. Alkaline Cleaning:<br />1.1. Cleaning in alkaline solutions is optional. If the parts are carefully handled and kept clean after molding, alkaline cleaning can usually be avoided.<br />1.2. Fingerprints, grease, and other shop soil should be removed by soaking plastic-molded parts in mild alkaline solutions that are commercially available. A suitable solution may contain 25 g/L of sodium carbonate and 25 g/L of trisodium phosphate operated at 55 to 65°C. Parts are immersed in the solution for 2 to 5 min.<br />2. Conditioning:<br /> Conditioning is an optional step that precedes the etching step. Conditioning can eliminate adhesion problems associated with inadequate etching. The conditioner may be a solution of chromic and sulfuric acids, or it may contain an organic solvent.<br />2.1 Chromic/Sulfuric Acid Type—This type of conditioner may contain 30 g/L of chromic acid and 300 mL/L of sulfuric acid (93 mass %; density 1.83 g/mL) dissolved in water and is maintained at a temperature of 60°C 63°C. Parts are immersed in the solution for 1 to 2 min. Because of the relatively large amount of sulfuric acid in the solution.<br />2.2 Organic Solvent Type—This type of conditioner is a solution of an organic solvent in deionized water. The organic solvent may be acetone or other ketone; The solution may contain 100 to 125 mL/L of the appropriate organic solvent and is maintained at a temperature of 40 to 45°C. Treatment is by immersion of the plastic parts for 2.0 to 2.5 min<br />3 Etching:<br />3. Etchants are strong oxidizing solutions that microroughen and chemically alter the surface of molded plastic parts. The etching step is the most important step in achieving serviceable adhesion of metals to plastics. Commercially used etchants are either chromic acid types, chromic/sulfuric acid types, or chromic-sulfuric-phosphoric acid types.<br />3.1 Chromic Acid Type-Concentrated etchants usually contain greater than 850 g/L of chromic acid and as much as 1200 g/L. The temperature of the solution is maintained at 50 °C and treatment is by immersion for 8 to 10 min. Concentrated solutions of chromic acid tend to oxidize butadiene rubber particles in the case of Acrylonitrile-butadiene-styrene (ABS).<br />3.2. Chromic/Sulfuric Acid Type-This type of etchant may contain 250 to 350 g/L of chromic acid and 200 to 250 mL/L of sulfuric acid (93 mass %, density 1.83 mL/L) dissolved in water. Immersion times of 5 to 10 min at a solution temperature of 65 °C are commonly used. Several proprietary baths are available.<br />3.3. Chromic-Sulfuric-Phosphoric Acid Type-This type of etchant solution normally consists of 3 % by mass chromic acid, 56 % by mass sulfuric acid (density 1.83 g/mL), 10.5 % by mass phosphoric acid (density 1.87 g/mL), and the balance water. An immersion time of about 3 min at 74 to 77°C is commonly used.<br />4. Neutralizing (Sensitizing):<br />4.1 After thorough rinsing, all residual chromic acid must be chemically removed from the surface of the molded-plastic parts. Neutralizers are used and are typically mild acid or alkaline solutions containing complexing or reducing agents. In the case of ABS, it is common to use a solution containing a mixture of an acidsalt and a reducing agent, such as sodium bisulfite, to eliminate all traces of chromic acid. Typical processing conditions are 1 to 2 min immersion at 40°C.<br />4.2 Neutralizers may also contain ionic surfactants to increase the adsorption of catalyst.<br />5. Activating:<br /> Small amounts of palladium are chemically deposited on the surface at this stage of processing. Palladium functions as a catalyst for autocatalytic deposition of copper or nickel to follow. Palladium is deposited either by the older, two-step procedure or by the more reliable one-step procedure.<br />5.1. Two-Step Procedure-The molded-plastic parts are first immersed in a solution of stannous chloride, 6 to 10 g/L, operated at pH 1.8 to 2.4 (pH adjusted with hydrochloric acid), and at 20 to 25°C for 1 to 3 min. The parts are then rinsed thoroughly to remove excess hydrochloric acid and transferred to a solution containing 0.1 to 1.0 g/L of palladium chloride dissolved in water at a pH of 1.6 to 2.0 adjusted with sulfuric acid. The palladious ions adsorbed on the surface react with stannous ions to form palladium metal and stannic chloride in the interstices of the etched plastic components. After thorough rinsing, the parts can be coated with nickel or copper by autocatalytic deposition.<br />5.2. One-Step Procedure-The molded-plastic parts are immersed in a solution of colloidal stannous chloride and palladium chloride containing excess hydrochloric acid. The stannous chloride concentration is 120 to 140 g/L, the palladium metal concentration is 0.05 to 0.15 g/L, and the hydrochloric acid concentration is approximately 3.0 N. The solution is maintained at 20 to 30°C and the parts are immersed in the solution for 1 to 3 min. Rinsing in water leads to the formation of metallic palladium nuclei on the surface surrounded by stannic hydroxide. The stannic hydroxide is removed in the acceleration step prior to autocatalytic deposition.<br />6. Acceleration:<br /> Stannous hydroxide is removed by treatment in a dilute solution of hydrochloric acid, or a solution of an acid salt. Fluoride or fluorinated compounds are often added to increase the effectiveness of the acceleration process. In most cases, a solution containing 1.0 N hydrochloric acid maintained at a temperature of 50°C, agitated with air, in which parts are immersed for 30 to 60 s adequately removes excess stannous chloride and stannic/stannous hydroxide remaining on the surface after the one-step palladium activation procedure.<br />