Surface Preparation1

Surface preparation of a material prior to bonding is the keystone upon which the adhesive bond is formed. Extensive field service experience with structural adhesive bonds has repeatedly demonstrated that adhesive durability and longevity depends on the stability and bondability of the adherend surface.

In general, high performance structural adhesive bonding requires that great care be exercised throughout the bonding process to insure the quality of the bonded product. Chemical composition control of the adhesive, strict control of surface preparation materials and process parameters, and control of the adhesive lay-up, part fit-up, tooling, and the curing process are all required to produce durable structural assemblies.

The first consideration for preparing a composite part for secondary adhesive bonding is moisture absorption of the laminate itself. Absorbed laminate moisture can diffuse to the surface of the laminate during elevated temperature cure cycles, resulting in weak bonds or porosity or voids in the adhesive bondline, and in extreme cases, where fast heat-up rates are used, actual delaminations within the composite laminate plies. If honeycomb is used in the structure, moisture can turn to steam resulting in node bond failures or blown core. Relatively thin composite laminates (0.125 in. or less in thickness) can by effectively dried in an air-circulating oven at 250° F for 4 h minimum. Drying cycles for thicker laminates should be developed empirically using the actual adherend thicknesses. After drying, the surface should be prepared for bonding and then the actual bonding operation conducted as soon as possible. It should be noted that prebond thermal cycles, such as those using encapsulated film adhesive to check for part fit-up prior to actual bonding, can also serve as effective drying cycles. In addition, storage of dried details in a temperature and humidity controlled lay-up room can extend the time between drying and curing.

Numerous surface preparation techniques are currently used prior to the adhesive bonding of composites. The success of any technique depends on establishing comprehensive material, process, and quality control specifications and adhering to them strictly. One method that has gained wide acceptance is the use of a peel ply. In this technique, a closely woven nylon or polyester cloth is used as the outer layer of the composite during lay-up; this ply is torn or peeled away just before bonding or painting. The theory is that the tearing, or peeling process, fractures the resin matrix coating and exposes a clean, virgin, roughened surface for the bonding process. The surface roughness attained can, to some extent, be determined by the weave characteristics of the peel ply. Some manufacturers advocate that this is sufficient, while others maintain that an additional hand sanding or light grit blasting is required to adequately prepare the surface. The abrasion increases the surface area of the surfaces to be bonded and may remove residual contamination, as well as removing fractured resin left behind from the peel ply. The abrading operation should be conducted with care, however, to avoid exposing or rupturing the reinforcing fibers near the surface.

The use of peel plies on composite surfaces that will be structurally bonded certainly deserves careful consideration. Factors that need to be considered include: the chemical makeup of the peel ply (e.g., nylon vs. polyester), as well as its compatibility with the composite matrix resin; the surface treatment used on the peel ply (e.g., silicone coatings that make the peel ply easier to remove can also leave residues that inhibit structural bonding); and the final surface preparation (e.g., hand sanding vs. light grit blasting) employed. The reader is referred to References6 7 for a more in-depth analysis of the potential pitfalls of using peel plies on surfaces to be bonded. The authors of References 6 and 7 maintain that the only truly effective method of surface preparation is a light grit blast after peel ply removal. Nevertheless, peel plies are very effective at preventing gross surface contamination that could occur between laminate fabrication and secondary bonding.

A typical cleaning sequence would be to remove the peel ply and then lightly abrade the surface with a dry grit blast at approximately 20psi. After grit blasting, any remaining residue on the surface can be removed by dry vacuuming or wiping with a clean dry chessecloth. Although hand sanding with 120-240 grit silicon carbide paper can be substituted for grit blasting, hand sanding is not as effective as grit blasting in reaching all of the surface impressions left by the weave of the peel ply. In addition, the potential for removing too much resin and exposing the carbon fibers is actually higher for hand sanding than it is for grit blasting.

If it is not possible to use a peel ply on a surface requiring adhesive bonding, the surface can be precleaned (prior to surface abrasion) with a solvent such as methyl ethyl ketone to remove any gross organic contaminants. In cases where a peel ply is not used, some type of light abrasion followed by a dry wipe (or vacuum) is then required to break the glazed finish on the matrix resin surface. The use of solvents to remove residue after hand sanding or grit blasting is discouraged, due to the potential of recontaminating the surface.

Another method can be used to avoid abrasion damage to fibers. When the carbon composite is first laid-up, a ply of adhesive film is placed on the surface where the secondary bond is to take place. This adhesive is then cured together with the laminate. To prepare for the secondary bond, the surface of this adhesive ply is abraded with minimal chance of fiber damage; however, this sacrificial adhesive ply adds weight to the structure.

All composite surface treatments should have the following principles in common: (1) the surface should be clean prior to abrasion to avoid smearing contamination into the surface; (2) the glaze on the matrix surface should be roughened without damaging the reinforcing fibers, or forming subsurface cracks in the resin matrix; (3) all residue should be removed from the abraded surface in a dry process (i.e., no solvent); and (4) the prepared surface should be bonded as soon as possible after preparation.

Aluminum and titanium are often bonded in composite assemblies. Although aluminum should not be bonded directly to carbon/epoxy because the large differences in the coefficients of thermal expansion will result in significant residual stresses, and because carbon fiber in contact with aluminum will form a galvanic cell that will corrode the aluminum. Although seemingly adequate bond strength can often be obtained with rather simple surface treatments (e.g., surface abrasion or sanding of aluminum adherends), long-term durable bonds under actual service environments can suffer significantly, if the metal adherend has not been processed using the proper chemical surface preparation.

Several different methods are used to prepare aluminum alloys for adhesive bonding. All have advantages and disadvantages that should be considered, including cost, cycle time, bond durability, performance, and environmental compliance. Aluminum alloys can be precleaned by vapor degreasing followed by alkaline cleaning. The main objective of aluminum etching, or anodizing procedures, is to create a clean surface that contains a porous oxide layer that the adhesive can flow into and become mechanically interlocked. The surface morphologies of the three most prevalent commercial processes are shown in Fig. 8.10. Forest Products Laboratory (FPL) etching is a chromic-sulfuric

5 nm

5 nm

Chromic-Sulfuric Acid Etch (Forest Products Laboratory - FPL)

5 nm

~40 nm Oxide film

Aluminium t

5 nm

~40 nm Oxide film

Aluminium

Chromic Acid Anodize (CAA)

-100 nm

Chromic Acid Anodize (CAA)

-100 nm

Phosphoric Acid Anodize (PAA)

Fig. 8.10. Surface Morphologies of Etched Aluminum Surfaces8

Chromic-Sulfuric Acid Etch (Forest Products Laboratory - FPL)

1500 nm

Oxide Aluminium

Phosphoric Acid Anodize (PAA)

Fig. 8.10. Surface Morphologies of Etched Aluminum Surfaces8

acid etch and is one of the earliest modern methods developed for preparing aluminum for bonding. Chromic acid anodizing is a later method and is perhaps more widely used than the FPL etch. Chromic acid etching produces a thicker, more robust oxide film than the FPL process. Different manufacturers use minor variations of this method, usually in the sealing steps after anodizing. Phosphoric acid anodize (PAA) is the most recent of the well-established procedures and has an excellent service record for environmental durability. It also has the advantage of being very forgiving of minor variations in procedure. The PAA process produces a more open oxide film and thinner oxide film than that produced by the CAA process. It also results in a bound phosphate that improves the durability of the bond.

Several methods are also used with titanium. Any method developed for titanium should undergo a thorough test program prior to production implementation, and then must be monitored closely during production usage. A typical process used in the aerospace industry involves:

• Solvent wiping to remove all grease and oils,

• Liquid honing at 40-50 psi pressure,

• Alkaline cleaning in an air agitated solution maintained at 200-212° F for 20-30 min,

• Thoroughly rinsing in tap water for 3-4 min,

• Etching for 15-20 min in a nitric-hydrofluoric acid solution maintained at a temperature below 100° F,

• Thoroughly rinsing in tap water for 3-4 min followed by rinsing in deion-ized water for 2-4 min,

• Inspecting for a water break free surface,

• Oven drying at 100-170° F for 30 min minimum, and

• Adhesive bonding, or applying primer, within 8 h of cleaning.

The combination of liquid honing, alkaline cleaning, and acid etching results in a complex chemically activated surface topography, which contains a large amount of surface area that the adhesive can penetrate and adhere to. The adhesive bond strength is a result of both mechanical interlocking and chemical bonding. Other methods, such as dry chromic acid anodizing are also used.

Because metallic cleaning is such a critical step, dedicated processing lines are normally constructed, and chemical controls, as well as periodic lap shear cleaning control specimens, are employed to insure in-process control. Automated overhead conveyances are used to transport the parts from tank-to-tank under computer controlled cycles to insure the proper processing time in each tank.

Due to the rapid formation of surface oxides on both titanium and aluminum, the surfaces should be bonded within 8h of cleaning, or primed with a thin protective coat (0.000 1-0.000 5 in.) of epoxy primer. Primer thickness is important. Actually, thinner coatings within this range give better long-term durability than thicker coatings. Color chips are often used in production to determine primer thickness. For parts that will undergo a severe service environment, priming is always recommended, because today's primers contain corrosion inhibiting compounds (strontium chromates) that enhance long-term durability. The two critical variables in corrosion of metal bonds are the metal surface preparation treatment and the chemistry of the primer. Some primers also contain phenolics, which have been found to produce outstanding bond durability.9 Once the primer has been cured (e.g., 250° F), the parts may be stored in an environmentally controlled clean room for quite long periods of time (e.g., up to 50 days or longer would not be unusual).

All cleaned and primed parts should be carefully protected during handling or storage to prevent surface contamination. Normally, clean white cotton gloves are used during handling and wax free Kraft paper may be used for wrapping and longer storage. Gloves, which are used to handle cleaned and/or primed adherends, should be tested to insure that they are not contaminated with sil-icones or hydrocarbons, which can contaminate the bondline, or sulfur which can inhibit the cure of the adhesive.

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