Adhesive & Sealant Industry News

How the Rheology of Two Component Adhesives Impact Mixing Behavior in Static Mixers

Two-component adhesives are increasingly becoming the bonding solution of choice in a variety of markets. Static mixers are the most frequent choice for mixing the two-component material before application. Static mixer selection is affected by the rheological properties of the starting components and their mixing ratio play an important role. In particular, highly viscous materials with a high proportion of fillers often have a pronounced non-Newtonian behavior. 

For Newtonian fluids, viscosity is independent of load. This means that the viscosity depends on the temperature, but not on the shear stress. Only a small group of mostly low-viscous fluids (such as water, milk, salad oil) exhibit such constant viscosity. For most fluids, the viscosity changes under shear stress; these are called non-Newtonian fluids.

Non-Newtonian fluids can exhibit shear-thinning (structurally viscous) or shear-thickening (dilatant) flow behavior. Shear-thinning flow behavior is characterized by a decrease in viscosity with increasing shear rate. Typical materials exhibiting this behavior are coatings, adhesives, polymer solutions, and polymer melts. Shear thickening means that viscosity increases with increasing shear rate. Materials that typically exhibit such behavior include highly filled dispersions, such as ceramic suspensions, starch dispersions, sometimes dental fillings (dental composites), and special composites for protective clothing.

In addition to the rheological behavior, the viscosity of a material is also strongly dependent on temperature. Generally, in the case of liquids the viscosity decreases with increasing temperature. This can have a strong impact. For example, the viscosity of a typical engine oil decreases by a factor of three when the temperature is increased from 23 °C to 50 °C.

The optimal achievable mixing quality depends on the type of mixer, the number of mixing elements, the mixing ratio, and the viscosity ratio of the two components. A pre-defined mixer generally achieves the best mixing quality when materials with a mixing ratio of 1 to 1 and the same viscosity are mixed.

In many cases, however, this optimum condition does not exist and a rather high-viscous, shear-thinning resin is to be mixed with a rather low-viscous Newtonian curing agent. Since optimum mixing occurs at equal viscosity, it is advantageous to select a mixer that mixes at a shear rate that results in equal viscosities of the two components.


Source: medmix/Adhesives Magazine