Surface Pretreatment

Following the preliminary surface preparation it may be necessary to pretreat the surface using one of variety of mechanical, chemical, or physical methods.

Mechanical Methods

Mechanical methods involve the use of handheld sandpaper or hand cleaning tools like wire brushes and scrapers. Such instruments are convenient to remove rust, scale, paint, and weld splatter. But they are too slow to use on large areas.

One drawback of the abrasion process is that it causes particles of debris to accumulate on the abraded surface. These particles come from the abrasive, the surface contaminants, and the surface material itself. All such particles must be removed before adhesive is applied. This may be accomplished with a clean cloth or brush, or with filtered compressed air.

After the abrasion debris has been removed, it is usual to give another solvent clean before bonding. A solvent-moistened cloth is convenient for this, but as the cloth will become contaminated during this operation, it should be renewed frequently.

Mechanical cleaning also includes a number of much faster abrading methods such as sandblasting, tumbling, and abrading with power tools.

Chemical Methods

In chemical treatments that alter the surface of the adherend, the part is dipped into a chemically active solution. This solution either dissolves part of the surface or transforms it, making it more chemically active and thus more receptive to adhesive bonding.

Acid etching involves immersing a metal substrate in an aqueous acid solution to remove a loose layer of oxide from its surface. The particular acid used depends upon the metal and type of oxide being treated. In many cases acid etching may provide enough surface preparation for bonding - depending, of course, upon the degree of adhesion desired. Acid etching can also be effectively used with certain plastics, for example chromic acid is used to treat polyolefins.

Anodization involves the electrochemical modification of the surface. The process deposits a porous and stable oxide layer on top of the oxide layer formed after etching of the substrate.

Physical Methods

These are techniques where the surface is cleaned and chemically modified by exposure to highly energetic charges or other ionic species. The most common methods are flame treatment, corona discharge and plasma. These pre-treatment methods have been applied to metals and, in particular, composites and plastics.

Flame treatment of the substrate surface for just a few seconds with an oxygen-containing (blue) propane or acetylene gas flame leads to the incorporation of oxygen-containing groups at the surface. This improves the wetting properties and hence the adhesion. Flame treatment is used almost exclusively for polyethylene and polypropylene substrates. The effect of the pretreatment subsides within a short time so that flame-treated substrates must be bonded immediately.

A corona discharge is essentially a plasma generated in air at atmospheric pressure by applying a high frequency and high voltage between two electrodes. It contains a number of energetic species that can clean and introduce polar groups, mostly oxidation products, at the substrate surface. The corona discharge may also lead to crosslinking of the polymer surface. The pretreatment effects are short lived so bonding should be carried out immediately. It is used mainly for polyolefin films and is capable of high processing speeds.

A plasma is usually generated in a low pressure chamber and so is best suited to batch processing. Commercial units of various sizes are available. The advantage of this method is that it allows treatment of substrates by plasmas of gasses other than oxygen, for example argon, ammonia, or nitrogen. Plasmas created from inert gases are generally used to clean substrate surfaces.