Sulfuric acid-dichromate etch

Table 3.3 Composition and Process Variables for Sulfuric Acid-Dichromate Etching " ...

Sulfuric acid-dichromate etch Acrylonitrile-butadiene-styrene (ABS), polyacetal, melamine or urea polymer, polyolefin, polyethylene oxide, polystyrene, polysulfone and styrene-acrylonitrile (SAN) Most common method... 

Chemical treatments are usually most effective with aluminum alloys, especially when long-term environmental exposure is required. The sulfuric acid—dichromate etch FPL etch, named after Forest Products Laboratory, US Department of Agriculture www.fpl.fs.fed.us) has been used successfully for many decades. Techniques more recently developed are often modifications of the FPL procedure. Other important methods include chromate conversion coating and anodizing. Corrosion-resistant adhesive primer, as well as anodic and chromate conversion coatings, helps to prevent corrosive failure of adhesion. ... 

Use 320-grit emery cloth for foil For maximum bond strength. Suitable for brass and bronze. ASTM D 2651 sulfuric acid-dichromate-ferric sulfate etch... 

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. "... 

From Tables 2 and 3, an epoxide adhesive (surface tension 45 dynes/cm) would not be expected to wet and effectively bond a low energy surface such as polyethylene (critical surface tension 31 dynes/cm). When the polyethylene surface was etched for increasing times in a sulfuric acid-dichromate solution, bond strengths markedly increased and the surface s contact angle with water (increasing polarity) similarly decreased (Fig. 4). ... 

Substrate is aluminum treated with a sulfuric acid—sodium dichromate etch. 

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

The substrate should then be washed in tap water and rinsed with distilled water. The final step in the process is an oven-dry of 10 min at 121 to 177°C.26 Several other surface preparation procedures for beryllium have been reported to have merit. Epoxy and epoxy hybrid adhesives have been found to provide high strengths on sulfuric acid-sodium dichromate etched beryllium.27... 

The sodium dichromate-sulfuric acid process has been found by some to be superior to other ferric chloride methods for the prebond treatment of copper. This dichromate-sulfuric acid method is also defined by ASTM D 2651. Nitric acid and ferric chloride etching processes have also been found to be useful for copper, brass, and bronze substrates in certain applications. 

Generally only solvent cleaning and abrasion are necessary for surface preparation of polystyrene parts. Methanol and isopropanol are acceptable solvents for solvent cleaning of polystyrene. For maximum bond strength the substrates can be etched with sodium dichromate-sulfuric acid solution at elevated temperature. Table 16.14 shows the results of a study on the durability of joints formed between polystyrene and aluminum with different types of adhesives exposed to different environments. 

Acetal (copolymer) Acetone 1. Abrasion. Grit or vapor blast, or medium-grit emery cloth followed by solvent degreasing. 2. Etch in the following acid solution for 10 s at 25°C Potassium dichromate 75 Distilled water 120 Sulfuric acid (96%) 1500 Rinse in distilled water, dry in air at RT. For general-purpose bonding For maximum bond strength. ASTM D 2093... 

Chemical surface treatments vary with the type of plastic being bonded. These processes can involve the use of corrosive and hazardous materials. The most common processes are sulfuric acid-sodium dichromate etch (polyolefins) and sodium-naphthalene etch (fluorocarbons). Both of these processes are described in ASTM D-2093. 

The oldest treatment is an etching process in highly concentrated chromic-sulfuric acid with a temperature near 80°C. Highest adhesional strength between PP and, for example, a two-component epoxy (EP) adhesive is reached after etching times between 120 and 240 s. The recipe of the etching solution is 5078 parts of sulfuric add (density 1.82g/ml), 120 parts demineralized water and 75 parts potassium dichromate. 

Based, in part, on the results of the previous section, we decided to investigate the use of several organic hydration inhibitors on PAA and Forest Products Laboratory (FPL)(92) sodium dichromate-sulfuric acid etched surfaces.(3.5,i3,9i,93) One of the inhibitors that is very successful is nitrilotris methylene phosphonic acid NTMP, N[CH2P(0)(OH)2]3. It provides hydration resistance for FPL surfaces in a manner similar to anodization in phosphoric acid, i.e., it forms a protective molecular layer that does not allow the aluminum oxide substrate to hydrate until the inhibitor complex dissolves. 

Samples of 0.030 in 2024 T3 bare aluminum were pre-cut, to suitable size for mounting in the spectrometer, with a tab to facilitate handling. The metal was subjected to 10 minute etch at 62-63 C in a sodium dichromate-sulfuric acid solution (per Section One, Handbook of Adhesives, Bloomingdale Aerospace Products, Havre de Grace, Maryland 21078), rinsed ten minutes in running, deionized water, and dried one hour at 63 C. One sample was then additionally subjected to the phosphoric acid anodization as outlined in BAG 5555. ... 

Fortunately, a great deal of work has been accomplished in a short time, and notably by aircraft manufacturers as well as adhesives suppliers. There are several important contributions in this area. First, in the area of FPL etch, the important consideration is what kind of bonding surface is provided by the preparation method. The chromic acid/sulfuric acid not only removes air oxide and leaves base metal it also has a chemical potential which produces a very thin anodic type oxide layer of the surface. This oxide layer is porous, due to the dissolving action of the strong acid mixture, and thus the surface produced may be characterized as a thin, porous anodic oxide. (A. W. Smith compared it to a 3V chromic acid anodize based on impedance measurements.) The optimum conditions for this etch as to time, temperature, and composition have been studied at Fokker and by Smith and generally a somewhat higher concentration of sodium dichromate or chromic acid was recommended than was commonly used. 

Aluminum and aluminum alloys. The effects of various aluminum surface treatments have been studied extensively. The most widely used process for high-strength, environment-resistant adhesive joints is the sodium dichromate-sulfuric acid etch, developed by Forest Product Laboratories and known as the FPL etch process. Abrasion or solvent degreasing treatments result in lower bond strengths, but these simpler processes are more easily placed into production. Table 7.14 quahtatively lists the bond strengths that can be realized with various aluminum treatments. 

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