FPL Etch

Fig. 6. Top Morphology of FPL surface following fluorine contamination. This is to be compared to the standard FTL morphology of Fig. 14. Middle Climbing drum peel (CDP) strength as a function of F surface concentration. Bottom F concentration on FPL surfaces following doping the FPL etch solution and the rinse water with NaF. The dotted line corresponds to the surface concentration with which the CDP strength began to decrease. Adapted from Ref. [37].

Fig. 14. Top High-resolution stereo micrograph of an FPL-etched 2024 aluminum surface. Bottom Schematic diagram of the oxide stmcture. Diagram is from Refs. [9,59].

Processing details for the optimized FPL etching procedure, other chromic-sulfuric acid etches and the P2 etch are summarized in Table 1 [2,62J. 

Fig. 36. Pari of a Morphology Catalog illustrating the unacceptable morphologies resulting from incorrect surface treatment (a) Turco alkaline clean with no FPL etch, and (b) Amchem deoxidizer with no FPL etch [169. ...

One might expect that the more developed porous layer produced by the PAA process would tend to provide a greater number of successful mechanical interlocking sites 54-86-135-136> and initiate a larger degree of plastic deformation in the resin upon failure than the FPL oxide. Test data i29) comparing PAA and FPL pretreated systems have supported this conclusion. Also, the oxides formed by the PAA pretreatment have exhibited better stability in wet environments 54). Hence, the PAA process has replaced the FPL etch as the method of choice for the pretreatment of aluminum for adhesion systems i32-133>. 

The spectra shown in Fig. 4 were obtained after the FPL-etched substrates coated with y-APS were rinsed with distilled water. When the take-off angle was 75°, two components were observed in the N(ls) spectrum near 399.6 and 401.6 eV. However, the relative intensities of the two components were considerably different from those for the unrinsed films and it was determined that 65% of the amino groups were protonated. When the take-off angle was changed from 75° to 15°, the positions of the components and their relative intensities showed little change. 

Figure 3. N(ls) high resolution spectra of FPL-etched 1100 aluminum coated with y-APS from a 1% aqueous solution at pH 10.4. The take-off angles were (A)—15° and (B)—75°.

The results obtained here also indicate that the acidity of the oxidized surface of a metal depends on the pretreatment of the surface. For a take-off angle of 75°, about 42% of the amino groups were protonated when y-APS films were applied to mechanically polished 1100 aluminum. That increased to about 57% when the silane films were rinsed, probably because of the dissolution of a small amount of silane from the free surface of the film on which the amino groups were mostly not protonated. However, for a take-off angle of 75°, about 48% of the amino groups were protonated when y-APS was applied to FPL-etched aluminum substrates. That increased to about 65% when the silane films were rinsed. These results imply that the FPL-etched surface is more acidic than the polished surface. 

TABLE 15.19 Effect of Adhesives on the Sustained Load Durability of FPL-Etched Aluminum Specimens46... 

Medium- to high-strength bonds, suitable for noncritical applications Optimum bond strength per FPL etch process. Specified in ASTM D 2651 and MIL-A-9067. Solvent degrease may replace alkaline cleaning. 

Figure 9 Wedge test results for FPL-etched aluminum adherends with FM-123 (moisture-wicking) adhesive, with FM-300 (moisture-resistant) adhesive, with an NTMP treatment and FM-123, and with BR-127 primer and FM-300. 

C Curing Film Adhesives. The metal is FPL etched 2024T-3 clad aluminum- (Shear numbers in parnethesis are in psi, peel numbers in parenthesis are in pounds/ inch width). 

Measured on EC-1945 B/A primed FPL etched 5052T-4 bare aluminum. 

Much work has been done concerning the surface preparation of aluminum for adhesive bonding in the aerospace industry. Bethune (11) described the evolution of surface treatments from the FPL-etch (H2S0 /Cr02) to the... 

The method used for testing durability of adhesive bonds was developed at 3M by W.D. Sell (16) and is called "sustained load stress durability". The metal substrates were 2024T-3 clad or bare aluminum alloy, 5052T-4 bare aluminum alloy or 1010 cold rolled steel. The surface preparation for the aluminum alloys was either the "optimized" FPL-etch" or the H PO -anodization process (12). The steel was solvent wiped. If a primer was used, it was cured before application of the adhesive. Film... 

---