12 - Rheology for adhesion science and technology

Abstract

The basic building blocks of rheology are dynamics and kinematics, and the relation between them, for given classes of materials, is the mechanical constitutive equation. Constitutive equations contain material functions which can be obtained from controlled testing—rheometry. Under suitable conditions, a certain class of materials may behave like a viscous liquid, an elastic solid, or they may exhibit characteristics of both. Such viscoelastic materials, if subjected to small enough mechanical stimuli, are seen to have moduli and compliances that are independent of the magnitude of the applied stimulus. This chapter illustrates two rheology problems that have direct relevance to adhesion. When pressure is applied on a pressure-sensitive adhesive, the material will flow between the backing and the substrate. This is an example of the so-called “squeeze flow”. Likewise, if we have two plates that are placed at an angle to each other, with adhesive in between them at the joint, and proceed to press the plates together, then it is an example of “wedge flow”. The Linear viscoelasticity thus serves two important purposes: (1) it offers an array of analytical tests that probe the molecular structure and build up a mechanical “fingerprint” and (2) it provides a limiting form to which the myriad constitutive relations must reduce under sufficiently small deformations.