Chromic acid etching plastics

One is to use catalyzed engineering plastic for the first molding and noncatalyzed plastic for the second molding. Adhesion promotion consists of a swell-and-etch step followed by chromic acid etch. The other alternative is to mold the first part with noncatalyzed plastic materials and expose the molded part to an adhesion promotion process and catalyze. The second part is then molded, as in the first case. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

The solutions accumulate gradually dissolved material from the plastics and from time to time have to be discarded and replaced disposal presents environmental difficulties and for this reason efforts have been made to find alkaline rather than acidic liquids which are capable of etching satisfactorily but do not contain chromium. At the time of writing, unfortunately, development has not reached the stage at which the process based on chromic acid can be superseded for the entire range of substrates. 

Etching - In adhesive and solvent bonding, a process used to prepare plastic surfaces for bonding. Exposure of the plastic parts to a reactive chemical, such as chromic acid, or to an electrical discharge results in oxidation of the surface and an increase in surface roughness by removal of surface material. 

AMJ designs and sells the AMJ Electro Cell System for plastic preplate etch. This is an electrodialysis operation that requires the reoxidation of the reduced chromium in solutions that contain pure chromic acid. 

Sulfuric acid-dichromate is the most commonly applied chemical etching treatment for many plastics. There are few hard rules to determine the composition of this treatment media, although ranges can be found (Table 3.3). The exact formulation should be determined by testing the adhesive bond strength for any given plastic. Combinations of chromic acid, sulfuric acid, and phosphoric acid have been successfully used. "... 

Chromic acid removes amorphous regions of the polymer morphology, leading to the formation of complex (labyrinthine) cavities on the plastic surface. Some surface oxidation takes place. Improved wettability and bond strength have been ascribed to the topography of etched plastic surfaces rather than the polarity of the surface. ... 

Before electroless plating plastic parts, the surfaces have to be treated to ensure good adhesion. The etching process usually involves the use of a chromic acid solution to provide a microscopically roughened surface to the plastic part. The catalytic process is sometimes referred to as seeding. Here very small particles of an inactive noble metal catalyst, normally palladium, are deposited into the micro-cracks created during the etching process. The palladium will act as active catalyst sites for chemical reduction of the electroless... 

Brewis. Abrasion and solvent wiping have already been mentioned, but other techniques involve treating plastics in a gas flame, corona discharge for the treatment of both plastics and metals, and the use of acid-etch baths (chromic or phosphoric acid) for metals and plastics or anodising in acid baths for metals. 

Chemical treatment, or etching, oxidizes the plastic surface similarly to corona treatment. For instance, chromic acid is used to etch the surface of polyethylene and polypropylene (PP). An increase in etching time and temperature intensifies the surfaee treatment by inereasing the degree and depth of oxidation. 

Among the various plastic materials used for chrome-plated parts, ABS (copolymer of acrylonitrile-butadiene-styrene) and PC/ABS (blends of ABS with polycarbonate) are most common. Nearly 85% of the total plated plastic surface area is based on ABS or PC/ABS. The technology for etching out the rubber particles from the ABS phase using an oxidative mixture of sulfuric and chromic acid has been widely used in the industry for nearly 40 years. The etched surface is subsequently activated using precious metal species, which is then followed by an electroless deposition of a thin layer of nickel or copper. In this marmer, the smface of the otherwise non-conductive plastic is rendered electrically conductive. Subsequently, multiple layers of different... 

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