Adhesives and sealants are complex formulations used to bind substrates or seal joints or gaps. They come in different forms but are commonly dispersions containing polymeric materials or curing agents, surfactants and solvents. Adhesives can be reactive or non-reactive. For reactive adhesives, adhesion can be induced by mixing two or more reactive components together, such as an epoxy resin and hardener, or can be induced by external stimuli such as UV radiation, heat or moisture. For non-reactive adhesives, adhesion is induced by physical stimuli such as pressure or solvent evaporation, for example. In the case of a sealant, the primary function is to seal joints or gaps and prevent moisture, solvents or gases from entering or escaping from a system or component, although many sealants can have multiple functions.
Most adhesives and sealants are composed of polymeric materials, or contain monomers or oligomers that form a cross-linked polymeric network following reaction. Consequently the molecular weight and molecular structure of these components are critical to material properties both before and after adhesion. These properties can be measured using Gel Permeation Chromatography (GPC), which when combined with Static Light Scattering (SLS) can be used to determine absolute molecular weights ranging from below 500 Daltons to more than several million Daltons. Structural information such as degree of branching can also be provided.
Many adhesive and sealant formulations are two phase systems, which include emulsions containing dispersed polymer, or dispersed solids in the case of caulk. In both cases particle size and droplet size can be critical to product performance. Particle size and droplet size are most commonly measured using Laser Diffraction or Dynamic Light Scattering (DLS) depending on size range of interest.
Rheology and viscosity are some of the most important characteristics associated with sealants and adhesives which should display liquid-like properties (they should flow) in order to be applied but must also have sufficient ‘tack’ to stick to or bind substrates together – behavior which is influenced by viscoelastic properties. Once applied, most materials are then designed to undergo a liquid-solid transition in order form a robust seal or adhesive bond.
Viscosity, viscoelasticity, curing behavior (including UV curing) and tack can all be measured using a research grade rotational rheometer which is an extremely versatile tool for these types of materials. For high speed coating and extrusion processes capillary rheometry is also widely used as it can better replicate the shear rates, and other characteristics such as die swell and extensional behavior that are associated with processing of complex polymeric and filled viscoelastic materials.