Adhesives, critical flow properties

The flow properties of adhesives and sealants depend on their composition. Critical flow properties are listed in Table 2. 

Of critical importance in the development of DPI products is the evaluation, optimization, and control of flow and dispersion (deaggregation) characteristics of the formulation. These typically consist of drug blended with a carrier (e.g., lactose). The properties of these blends are a function of the principal adhesive forces that exist between particles, including van der Waals forces, electrostatic forces, and the surface tension of adsorbed liquid layers [7], These forces are influenced by several fundamental physicochemical properties, including particle density and size distribution, particle morphology (shape, habit, surface texture), and surface composition (including adsorbed moisture) [8]. In addition,... 

The rheological properties of adhesives and sealants are important in many applications. When these products must be pumped or applied through automated equipment, the flow characteristics at pertinent shear rates are critical. Sophisticated rheological measurements can be performed to predict performance. The rheology of silicone adhesives and sealants can be tailored through adjustment of polymer viscosity, filler loading, and incorporation of various additives. 

For a flow velocity exceeding a certain value (first critical velocity) the particles will rebound. The value of the first critical velocity depends on the elastic properties of the particle and obstacle surfaces and is inversely proportional to the particle size [see formula (V.ll)l. The coefficient of deposition increases when the possibility of particle rebound is eliminated. This may be achieved by making the surface tacky or increasing the adhesive force, particularly by virtue of the triboeffect and the cooling of the molten zone of contact. 

Two critical parameters of an aqueous adhesive formulation are its viscosity and rheological properties. Different types of conventional coating equipment will require different viscosities, and the rheological properties will determine the flow and leveling characteristics of the formulation. 

There are clearly very significant differences between these products, in terms of stiffness, strength, and ductility. Each is formulated for a specific type of assembly. As noted above, the mechanical behavior is only one element to be considered in material choice. For a boat-builder, gap filling or sufficient viscosity to allow bonding onto vertical surfaces may be as or more important, while for an offshore flow line thermal properties may be critical. In addition to a range of standard commercial products, adhesive suppliers will therefore formulate special products designed specifically for an end user s requirements. 

Multilayer polymeric composites combine the desirable properties of two or more polymers in a layered structure, often produced by a coextrusion process. Interlayer adhesion is critical to the performance of such composites. During coextrusion, this Interface is established as the layers flow together for a finite time through the die. 

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