Chromic acid etching

While polymeric surfaces with relatively high surface energies (e.g. polyimides, ABS, polycarbonate, polyamides) can be adhered to readily without surface treatment, low surface energy polymers such as olefins, silicones, and fluoropolymers require surface treatments to increase the surface energy. Various oxidation techniques (such as flame, corona, plasma treatment, or chromic acid etching) allow strong bonds to be obtained to such polymers. 

Use the rinse bath water again in a different area. For example, if there is a line with a chromic acid etch bath followed by counter-flow rinse baths and a neutralizer bath followed by counter-flow rinse baths, use the dirtiest rinse after the neutralizer bath and pipe it to the rinse baths after the chromic acid tank. This saves water and reduces sludge. 

Gluing is difficult, needing pre-treatments such as, for example, chemical etching (sulfo-chromic acid etching), flame oxidation or hot-air (500°C) treatment, corona discharge, plasma or UV treatments. The exposure must be brief and superficial and the original and aged properties must be tested. 

Chromic or sulfo-chromic acid etching, for polyolefins, polystyrene, ABS, polyacetal, polyphenylene ether. .. These treatments have two effects ... 

Aqua regia/chromic acid etch 10 secs, in solution of "1", after normal aqua regia etch. 

The completely featureless chromic acid etched-face is not shown). The progressively increasing surface area (from A-C) is evident. 

Figure 5. Scanning electron microscope pictures of single-crystal CdSe after sev-eral surface treatments. (0001) face, Cd-side, shown. (1120) and (1010) faces behave similarly. (A) After aqua regia/chromic acid etch (B) after aqua regia etch (C) after aqua regia/photoetch.

Chromic acid etch (sodium dichromate-sulfuric acid) Maximum-strength... 

For low- to medium-strength bonds, simple mechanical abrasion is a suitable surface preparation if the substrates are cleaned first. This surface preparation has found success in most ABS applications. A silane primer such as Dow Coming A-4094 or General Electric SS-4101 may be used for higher strength.57 For maximum joint strength, a warm chromic acid etch of the ABS substrate is suggested.58... 

Because of the solvent and chemical resistance of acetal copolymer, special etching treatments have been developed for surface preparation prior to adhesive bonding. A chromic acid etch and a hydrochloric acid etch have been suggested. Acetal parts that have been formed by heat treatment or machining should be stress-relieved before etching. 

Various adhesives can be used to bond polyphenylene oxide to itself or to other substrates. Parts must be prepared by sanding or by chromic acid etching at elevated temperature. Methyl alcohol is a suitable solvent for surface cleaning. The prime adhesive candidates are epoxies, modified epoxies, nitrile phenolics, and polyurethanes. Epoxy adhesive will provide tensile shear strength on abraded polyphenylene oxide substrates of 600 to 1300 psi and 1300 to 2200 psi on etched (chromic acid) substrates.71... 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

Rinse in water and dry below 60°C. 3. Vapor-degrease. Immerse for 10 min at 71-88°C in Water 4 Chromic acid 1 High bond strength. ASTM D 2651 chromic acid etch... 

ASTM D 2651 sodium hydroxide-chromic acid etch... 

P. Blais, D.J. Carlsson, G.W. CsuUog, D.M. Wiles, Chromic acid etching of polyolefin surfaces and adhesive bonding. J. Colloid Interface Sci. 47, 36-649 (1974)... 

D. Briggs, D.M. Brewis, M.B. Konieezo, X-ray photoelectron spectroscopy smdies of polymer surfaces I chromic acid etching of polyolefins. J. Mater. Sci. 11, 1270-1277 (1976)... 

Chromic acid-etched A1 adherends. Lap-shear, 90° peel and honeycomb joint geometries. Two years exposure in temperate, hot-dry, and hot-wet regions. 

Epoxy, polyester, polyurethane, and thermosetting acrylic have been used to bond modified PPO to itself and other materials. Bond strengths are approximately 600 to 1500 psi on sanded surfaces and 1000 to 2200 psi on chromic acid etched surfaces. 

There are many references in the literature to this method of surface preparation of aluminium. The name refers to the US Forest Products Laboratory, which published a specification based on etching the metal at c. 65 °C in a solution of sodium dichromate and sulphuric acid, sometimes loosely referred to as chromic acid . In the United Kingdom, a similar Ministry of Supply Aircraft Process Specification known as DTD 915 has been widely used. Over the years, a number of slightly different solutions ( chromic acid etches ) and procedures have been developed. Clearfield gives recipes for six such solutions. ... 

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