Nature of the Adherends

Durability studies on bonded aluminum joints have established that a definite relationship exists between bond permanence and the corrosion resistance of the aluminum alloy. The differences in performance are most apparent when the test specimens have been exposed to a corrosive environment. 

bonds made with 6061-T6 alloy will normally last about four times longer than those prepared with the less corrosion-resistant 2024-T3 alloy, when exposed to a marine environment. Similarly, after three weeks exposure to 5% salt spray at 95°F, joints prepared with 2026-T4 alloy had failed, those prepared with 6151-T4 alloy had weakened considerably, but had not fallen apart, and those prepared with X5085-H-111 alloy had retained about 30% of their initial strength. The effects described are for bonds formed with only minimal preparation of the adherend surface, under which conditions bondline corrosion becomes the dominant factor in determining durability. The same trends may not be apparent when anodized surfaces, or corrosion-inhibiting primers, are used. Also the same order of relative durabilities may not apply to noncorrosive exposure conditions. For example, long-term tropical exposure indicates that more durable bonds are formed with 2024-T3 clad alloy than with 6061-T6 alloy.  

One difficulty in making comparisons of joint strengths, using the overlap shear configuration, is that the measured values depend on both the strength and thickness of the adherends. Care should be taken to allow for this, so that differences observed reflect real changes in joint strength and are not merely artifacts of the test method. 

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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. 

A variation of the t-peel test is a 180° stripping test, described in ASTM D 903. This method is commonly used when one adherend is flexible enough to permit 180° turn near the point of loading. This test offers more reproducible results than the t-peel test because the angle of peel is kept constant, although it is dependent on the nature of the adherend. 

In the practice of adhesive bonding for applications in construction, surface pretreatment is likely to be the most difficult process to control. The choice of treatments must be tempered by the scale of operations, the nature of the adherends, the required durability, the adhesive to be used, and the cost. The performance of joints constructed with cold-cure epoxies is likely to be critically dependent upon surface preparation, as exemplified by the experience of the Scottish Irvine Development Corporation. In 1978 they elected to use vertical externally-bonded steel plate reinforcement to strengthen the abutment walls of three pedestrian underpasses. A year later, the plates were reported to be falling off, accompanied by extensive interfacial corrosion the steel surfaces had been abraded by hand, and the concrete surfaces chemically etched. 

In recent years, the range of adhesive materials used in automotive manufacture has expanded to include polyurethanes, plastisols, phenolics, hot melts, anaerobics, cyanoacrylates, toughened acrylics and epoxies (see Structural and Hot melt adhesives). Selection criteria are based principally upon the nature of the adherends, the mechanical properties required under service conditions and application and curing characteristics. 

Thus the strength of a joint cannot be improved simply by increasing the overlap length. This can only be done by changing parameters such as the width of the joint, the thickness or nature of the adherend or substituting a more appropriate adhesive. 

May be critical and depends to a large degree on the nature of the adherend and the individual adhesive. 

In most cases, the ability of the adhesive to support the design loads under service conditions for the planned life of the structure is considered first. Thus the mechanical properties, durability, and environmental resistance of the bonded structure are of obvious importance. But of equal concern are the nature of the adherends, the application technique, the cure conditions, the handling requirements, and, perhaps, the cost. A list of some of the factors considered in the selection process, together with the properties of the major generic classes of adhesives, are given in Table VI." It is important to remember that, even within a given class of adhesives, the performance may vary considerably, so that it is essential to consider the properties of each individual adhesive. It is apparent from this brief discussion that adhesive selection is far from simple. A computer-based selection program has become available recently." ... 

The principles involved in producing bonds of optimum strength and durability are the same, irrespective of the nature of the adherends. The surface must be free of contamination, must be receptive to the adhesive (or primer, if used), and the adhesive/adherend interface formed must be stable to environmental exposure. It is the last mentioned factor that is of most concern in a discussion of the durability of adhesive bonds, particularly for metallic adherends. The prebond treatment of the metal surface is of paramount importance in determining joint performance. With wood and plastic adherends, long-term performance is usually determined by other factors, provided that good initial joints are formed. 

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions Effectively, we would need to assess intermolecular forces and this is not really possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding. 

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