Surface preparation processes

Water is used in this subcategory as solution makeup and for rinsing in the surface preparation process, as the vehicle for the coating in the application process (normally done by spray coating), and for cooling and cleanup in the ball milling operation. 

Often, one must consider the time, trouble, and expense that may be necessary to use an adhesive. For example, certain plastics may require expensive surface preparation processes to allow the adhesive to wet the surface. Applications requiring high-temperature service conditions may require an adhesive that necessitates an elevated-temperature cure over a prolonged period. 

Surface preparation processes that are available and practical... 

Primers can be used to protect both treated metal and nonmetal substrates after surface treatment. The use of a primer as a shop protectant may increase production costs, but it may also provide enhanced and more consistent adhesive strength. The use of a primer greatly increases production flexibility in bonding operations. Usually primer application can be incorporated as the final step in the surface preparation process. The primer is applied as soon as possible after surface preparation and usually no more than a few hours later. The actual application of the adhesive may then be delayed significantly. 

The surface preparation method must be carefully considered, especially if the completed weldbond is to have long-term durability to hostile environments. The surface preparation should provide an optimal surface for both adhesion and welding. Thus, the choice of surface treatment is crucial, and there can be a conflict of requirements. The spot welding process requires a low electrical surface resistance, and many adhesive surface preparation processes provide a high surface resistance because of oxide layer buildup. When it is impossible to harmonize on a surface treatment, current practice tends to favor treatments that yield good weld nuggets at the expense of the adhesive bond. 

Various substrate surface treatments suggested for use with a common epoxy-substrate joint and service environment combinations are discussed in this chapter. Surface preparation processes for a range of specific substrates and detailed process specifications are provided in App. F. The reader is also directed to several excellent texts that provide prebond surface treatment recipes and discuss the basics of surface preparation, the importance of contamination or weak boundary layers, and specific processes for adhesive systems other than epoxy.1,2,3... 

In high-volume industries such as automotive and appliances, there is a desire to minimize or eliminate any surface preparation process for steel. Special adhesive systems have... 

Titanium is widely used in aerospace applications that require high strength-to-weight ratios at elevated temperatures. As a result, a number of different prebonding surface preparation processes have been developed for titanium. These generally follow the same sequence as for steel and other major industrial metal substrates degrease, acid-etch or alkaline-clean, rinse and dry, chemical surface treatment, rinse and dry, and finally prime or bond. Mechanical abrasion is generally not recommended for titanium surfaces. 

A proprietary alkaline cleaner, Prebond 700, appears to be satisfactory for a number of metal adherends including titanium and is recommended as a versatile one-step surface preparation process.48 A proprietary alkaline etch solution, Turco 5578, is available from... 

Surface Treatment Generally, some sort of surface preparation is required for reliable adhesion. The extent of the actual surface preparation process varies according to the performance characteristics desired, the nature of the adherend, and the time and cost considerations. 

Table 1.28 Prepainting cleaning and surface preparation processes...

Rigid process controls insure that each batch of bonded joints receives proper processing during the surface preparation, fabrication, and cure processes. Surface preparation processes are controlled with respect to... 

All of the features of plasma processing discussed above combine to make plasma a manufacturing tool that gives more yield of better, or less costly, parts than alternative surface preparation processes. 

To make an economical and practical joint, the surface preparation methods must also meet several other requirements. They must be safe to handle and should not be flammable or toxic. They should be inexpensive and provide fast processing times. The prebond processes should be easy to monitor and control in a production situation. In addition, the process should not in itself leave a weak boundary layer. If chemical solutions are used, they should rinse off easily and not continue to react with the surface past the time when the bond is made. The surface preparation process should allow for practical working time between preparation and application of the adhesive or sealant. Finally, the surface provided by the treatment should not change once the assembled joint is made and placed into service. 

Metal surface preparation processes and paint evaluation... 

Modified cyanoacrylates are now available which overcome the surface acidity and cure well on these surfaces (see Section 10.4.1). Acidic deposits may also be present as a result of an incorrect cleaning or surface preparation process. 

Crum, S., Surface Preparation Process Improvements, Electronic Packaging and Production, July 1993, p. 24. 

Copper Circuitry. A large majority of panels produced have solder mask coated over bare copper (SMOBC). The goals of the surface preparation process are to remove all unwanted oxides and contaminants from the copper and to impart a microroughness or tooth to the surface, and to leave a clean, dry surface for coating. 

Incoming Copper. The quality of the surface of the copper is very important for achieving good solder mask adhesion. All traces of the etch resist metallization, t icaUy tin or solder, must be removed prior to the solder mask surface preparation process. Solder mask does not adhere well to tin or solder residues or tin-copper intermetallic surfaces. 

Mixed Metals. Two good examples of the use of mixed metals are for panels that have been selectively solder-stripped and for panels that have had fingers or contacts plated with gold before the solder mask process. The surface preparation process must be selected based upon the most delicate of the metals on the panel. 

With resistance wire welding, surface preparation steps are necessary only when one of the substrates cannot be melted (e.g., thermosets and metals). Standard adhesive joining surface preparation processes such as those suggested in the next chapter can be used with these substrates. 

Functional requirements of a part also influence the selection of surface-preparation processes. For example, grinding processes can introduce stresses that could have a negative impact on fatigue properties. Choosing an alternative process, such as chemical milling, or mitigating the stresses by shot peening can alleviate the problem. 

Development of an appropriate surface preparation process which includes cleaning and may involve changing the surface morphology or chemistry (surface modification). 

These spectra would then form a baseline with which to compare subsequent as-received material. These same procedures could be used to characterize the polymer surface after surface preparation processing such as an oxygen plasma treatment or the application of a basecoat. 

Gases can be included into the substrate surface during surface preparation processes such as acid cleaning or in situ sputter cleaning. After the film has been deposited these gases may accumulate at the interface, giving poor film adhesion. 

Cleaning is the removal of surface contaminants and is part of what can be termed surface preparation, which can also include surface treatments (surface modification) to change the properties of the surface in a desirable way (Sec. 2.6). Care must be taken to ensure that the surface preparation processes do not change the surface in an undesirable or uncontrolled manner. One objective of any surface preparation procedure is to produce a homogeneous... 

The number of carriers striking a surface in time interval At is proportional to the carrier density n at that location, the surface area A, and At, so for a given surface preparation process the number of carriers that recombine when they hit the surface in a given time interval will also be proportional to those parameters ... 

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