Long term durability of epoxy adhesive compounds for bonding similar/dissimilar substrates and maintaining their stability when exposed to an assortment of environmental characteristics is of prime importance in diverse applications. Void-free bond lines can be achieved by preventing air entrapment that can have multiple causes such as poor wetting or improper dispensing. While small voids may have an impact on the mechanical strength properties, it is critical to understand that large contiguous voids could have a deleterious consequence that could result in product failure. This premature failure could result in delamination, crack propagation at various stress concentrations.
Avoiding cracks, air bubbles, voids are essential for effective bonding performance. These issues can occur from joint design, dispense patterns, adhesive shrinkage, moisture contamination of the bonding agent and gases generated during curing. To alleviate these concerns, it is recommended to use low moisture absorption adhesives, degassing prior to cure, prebaking components to absorb moisture, step curing at low temperatures to allow air/volatiles to escape before cure or use of non-corrosive low outgassing epoxies. Particularly noteworthy to enhance void-free capability of these compositions--attention to optimizing the surface preparation process, enhancing wetting of the adhesive, modification of the toughness of the formulation to prevent propagation of cracks, and surface roughness need to be carefully evaluated. It is essential that the adhesive has enough time to fill the voids and displace the air on the surfaces being mated. Slower curing, longer pot life epoxies will have more time to fill crevices than faster curing systems. To optimize adhesion, pressure should be applied on the entire bonding surface during curing.
Additionally epoxies widely used for potting/encapsulation can be adversely impacted by air voids. Employment of low viscosity free flowing compounds are extremely helpful in such applications and reduce the probability of moisture absorption and voids in the encapsulant.