Reasons for the Apparent Universal Applications of Polyurethanes as Adhesives

REASONS FOR THE APPARENT UNIVERSAL APPLICATIONS OF POLYURETHANES AS ADHESIVES 

On theoretical grounds it is generally accepted (de Bruyne s rule) that with pure or simple substances as adhesives strong bonds can never be made by polar adherends with non-polar adhesives or to non-polar adherends with polar adhesives. This observation is based on the fact that a liquid polar adhesive has a higher critical surface tension than the critical surface tension of wetting of a non-polar adherend hence poor wetting is considered to result between these two materials no or reduced penetration of the gas pocket formations, always present on the surface of an adherend, results if only poor wetting between two surfaces occurs with the consequence of poor adhesion. 

However, many non-polar adhesives (e.g. natural rubber) are very hydrophobic materials, whereas many polar adherends are mostly hydrophilic and so can even absorb some water. Hence in a normal humid atmosphere, poor wetting and spreading of the non-polar adhesive would occur on contact with the slightly moist surface of the polar adherend. Many polar liquid adhesives will be able to either absorb the atmospheric moisture or displace through surface chemical action the film of water adsorbed on the surface of the polar adherend, hence adequate wetting and spreading of the adhesive may occur under ordinary atmospheric conditions. 

As polyurethanes are both strongly polar and hydrophilic in nature it can be concluded, by applying the above deduction, that one of the principal reasons for their almost universal applicability as adhesives is in this ability to combine excellent surface contact or wetting properties with 

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