Assembly Considerations

Even though they are fundamentally different, adhesive and solvent bonding share some similarities, such as surface preparation and joint design. They also share some other attributes, such as uniform stress distribution in the joint area, low overall assembly weight, and the ability to seal as well as bond. However, their bonding mechanisms are different. Adhesives hold components together by surface attachment, whereas solvents work by chemically softening the surfaces and then escaping the joint area by evaporation as the bonding process progresses.

An adhesive is spread, as a liquid, (known as "surface wetting" in adhesive terminology) over one or both of the surfaces being bonded. Bonding takes place when the adhesive solidifies. Surface wetting is an important performance indicator for an adhesive, which relates to surface adhesion. When an adhesive separates from the bonding surface, it fails in adhesion. If the failure occurs in the body of the adhesive itself, (e.g. when it separates into two layers each sticking to one of the bonding surfaces), it fails cohesively. Adhesive forces hold two materials together at their surfaces. Cohesive forces hold adjacent molecules of a single material together (see figure).

In general, 100% cohesive failure is considered ideal as it indicates that adhesion is satisfactory. However, this may indicate that the adhesive is not strong enough (higher molecular weight adhesives are generally stronger) or the coverage area is not large enough. It is possible that the adhesive performs so well that it does not fail adhesively or cohesively but the parent material of the components fails cohesively. In solvent bonds, all failures are cohesive as there is no adhesion. The joint interface in a solvent bond is made up of diffused molecules of the parent material from both the bonding surfaces.

Some notable characteristics of these technologies are:

Mechanical fasteners produce stress concentrations at the fastening sites, whereas stress is distributed evenly over the entire joint surface in an adhesively bonded joint
As compared to mechanical fasteners, adhesives do not add much weight to the assembly as they are spread thin and have low specific weights. Solvents do not add any weight to the assembly
Adhesives and solvents, when spread over the entire joint surface, have the ability to bond as well as seal the interior of the assembly from the environment
Dissimilar materials can be bonded with adhesives. Flexible adhesives can be used to bond materials with somewhat different thermal expansions
Solvent bonding requires clamping for a short term. Clamping and fixturing for adhesives bonding may be required for long duration depending on the type of adhesive system used, thus adding to cycle time and overall cost
Most solvents are flammable. They also pose health and environmental concerns and may necessitate the use of personal ventilation equipment and emission control devices. Some solvent-based adhesives have similar concerns
Solvents must be allowed to escape to the atmosphere. Problems can be caused when the evaporated solvents are trapped inside a part with no way to escape.
Long-term strength of an adhesive joint can't be easily predicted because the mechanical properties of adhesives are influenced by environmental factors such as time, temperature, humidity, etc. They are sometimes used in combination with mechanical fasteners to improve joint reliability
Adhesive joints are generally irreversible and the quality of joints is difficult to ascertain
Surface cleaning is an important step for both adhesives and solvent bonding. Surface treatments are required for some thermoplastics, which have low surface energy, to improve their wetting ability and to promote adhesion. Frequently used treatments include chemical, corona, plasma and flame treatments

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