Preparation Before Bonding

For more information on the issues of substrates preparation before bonding, please refer to the chapters ... 

In Volume 1, we have provided general information about adhesives and sealants with the chapters Technical Characteristics of Adhesives and Sealants , Surface Preparation before Bonding and Polyurethane Adhesives and Sealants . 

In order to eomplement their knowledge, the engineers who work with structural materials in automotive, aerospace, bonding of metals, plastics, and composites should also, of course, read the chapters Design and calculation of bonded parts , Physics and chemistry of adhesion , Surface preparation before bonding , Metal bonding , Bonding composites , and also Epoxy adhesives , Application equipment , etc. 

Extremely versatile, these adhesives will readily bond almost any substrate with the exception of some rubber-based materials and the difficult thermoplastics such as polyethylene. Even the latter may respond well if the surface is correctly prepared before bonding. The toughened acrylics are very robust and will cope with demanding environments. 

As mentioned previously, additive treatments involve the application of a polymer to the fibre. This is usually prepared before application and contains reactive groups. However, it is also possible to form the polymer in situ within the fibres. The traditional approach is to apply the polymer after a subtractive oxidation treatment but environmental concern over A OX problems is increasing demand for additive treatments that can stand alone. There is no denying that the oxidative step can facilitate subsequent treatment with a polymer, since the scission of cystine disulphide bonds to yield cysteic acid residues provides useful reactive sites for crosslinking or anchoring the polymer. 

The acetal surface is generally hard, smooth, glossy, and not very easy to bond with adhesives. The nonstick and solvent-resistant nature of acetal requires that the surfaces be specially prepared before adhesive bonding can occur. Once prepared, the surface can then adhere to like substrates or to others. 

One surface preparation method that is unique for composites employs a peel or tear ply.77 Utilization of the peel ply is illustrated in Fig. 16.5. With this technique, a closely woven nylon or polyester cloth is incorporated as the outer layer of the composite during its production layup. This outer ply is then torn or peeled away just before bonding. The tearing or peeling process fractures the resin matrix coating and exposes a clean, fresh, roughened surface for the adhesive. This method is fast and eliminates the need for solvent cleaning and mechanical abrasion. 

Differences of the type observed in Table 6-1 exist between columns for several reasons (1) the type of silica used (2) the preparation of the silica before bonding and (3) the type of chemical reaction used to bond the nonpolar alkyl (C g) group to the silica surface. 

The feature of the reaction with potassium alkoxide not to cleave inner Si-Si bonds made us optimistic that with larger silanes multiply metalated products could be obtained. Such compounds have been prepared before by cleavage of strained cyclosilanes with alkali metals (Eq. 12). ... 

To prepare the bonded connection Wmj bd the specimen was fabricated to the specifications in Figures 2 and 3. M16 steel bolts, tightened to a torque of 23.8 N m, were used to compress the mating surfaces together while the adhesive fully cured. Before testing commenced the bolts were removed. 

If acceptable to the component manufacturer, a peel ply may be bonded to a surface, which is peeled off just before bonding to another part to leave a clean, prepared, surface. Release agents should not be applied to the peel ply (Release) As peel plies may leave a little residual contamination, it is usually beneficial to abrade lightly after peeling and before bonding. 

All matrix resins and adhesives absorb some moisture, and a dry surface is necessary before bonding if strong and durable joints are to be made. The surface should then be dried with radiant lamps or hot air blowers for about 3 h at the intended bonding temperature or to the standard specified in the SRM for the component. The prepared surfaces should not be touched by hand or anything else from the time of the water break test to the time of application of the adhesive. Bonding should take place as soon as possible after drying or a suitable primer should be applied within about 4 h. Once the primer is dry, the part can be stored in a sealed plastic bag for several months prior to bonding. 

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