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Some adhesive suppliers state their products produce “chemical” versus “mechanical” bonds, however, there are many factors involved and some are related more to the substrate properties than the adhesive.
Adhesive mechanisms are referred to, as theoretical, since calculated bond strength is seldom equal to actual bond strength. Although application of the theory will generally produce the desired results, testing the finished product is the only sure way to determine the quality of the adhesive.
Adhesive Mechanisms that Effect Assemblies
There are more, but these are the major forces:
Mechanical Interlocking (sometimes called Mechanical Bonding)
The adhesive locking into pores and irregularities on the substrate surface generates a bond.
Surface condition and preparation are very important as well as the flow properties and the cohesive strength of the adhesive. Fabricators can improve or optimize the strength of a joint by ensuring that the surface has been properly prepared. The surface should be cleaned and abraded with sandpaper prior to assembly. This is particularly important on surfaces that may contain residual release agents left over from the production process.
Adsorption
A bond believed to be generated by attractive forces between the atoms of the substrate and the adhesive. This type of bond requires a high degree of intimate contact between the two materials. The flow properties, and surface tension are important factors.
Fabrication techniques have little effect on this mechanism unless the part is bonded under pressure or vacuum.
Chemisorption (sometimes called Chemical Bonding)
A chemical reaction or molecular cross-link occurs between the substrate and the adhesive polymer. This mechanism requires un-reacted molecular sites for the polymer to link with and is generally associated with bonds to substrates that are not fully cured or highly cross-linked crystalline structures.
Chemical bonding can also be affected somewhat through the addition of multifunctional coupling agents, acids and adhesion promoters.
Diffusion
A bond is formed when a liquid adhesive dissolves the surface of a substrate prior to curing. The resultant bond line is a commingling of the two polymers.
The temperature, molecular weight and open time of the adhesive as well as the solubility of the substrate have a major effect on this type of bond.
The best adhesive or adhesive joint system will take advantage of as many of these mechanisms as possible. There are many trade off’s to consider for each system.
Some adhesives are designed for very rapid cure; this may reduce the diffusion and chemisorption bond. Additives can be incorporated to offset the loss in strength however color or shelf life etc. may be compromised.
Highly volatile compounds can be added to increase the diffusion of the adhesive into the substrate surface, however the rapid evaporation of these materials may cause the adhesive to skin over and not wet out the substrate prior to assembly thereby defeating the purpose.
Princeton Chemical’s commitment to the surfacing industry, our practical knowledge of fabrication techniques and our in house testing capability, allows us to formulate to take advantage of these adhesive mechanisms while carefully considering the end use and performance requirements of the fabricator.
We also custom formulate products to our customer requests, and provide feedback on the cause and effect of any alterations to ensure the long-term stability of assemblies.
Feel free to give us a call if you would like more information.
This information is for general interest only and is accurate to the best of our knowledge. The information is based on generally accepted principals, derived from personal experience, experimentation and industry trade journals.