What Is Glue Rheology?
Why are many glues so thick?
Well Rheology is the study of how materials deform and flow under the influences of external forces.
For all glues and coatings, it is an important property which must be controlled.
In the manufacture of adhesives, and within our manufacturing processes, we use rheological additives that effect the flow properties of our products.
Understand, or at least having a basic knowledge of both shear stress (the external stress upon the adhesive) and shear rate (given that stress, the resulting flow of the adhesive), give values which are essential for the correct use and application of an adhesive.
The Viscosity of a glue, is a measure of the resistance of that glue, to flow. There are four classes of flow:
- Newtonian Flow: An adhesive which maintains a constant viscosity regardless of how much it is stirred.
- Dilatant Flow: Commonly known as shear thickening, this refers to an adhesive which increases in viscosity as stirring increases.
- Pseudoplastic Flow: An adhesive which decreases in viscosity as stirring increases.
- Thixotropic Flow: An adhesive which decreases in viscosity over time and then takes a fixed time to return to a more viscous state
Many types of adhesive exhibit one of these flow properties and in all cases, it is desirable for the adhesive to flow sufficiently to form a uniform layer,then to resist further flow. In doing this, the glue will form a uniform layer onto the materials it has been applied to, and prevent sagging on a vertical surface. In addition, the use of rheology modifiers will reduce dripping and spatter in some roller applied adhesives.
From a production point of view, what role does rheology play in the application of glue?
The control of rheology is vital actually. For example, all of our degreasing agents and primers are Newtonian, that is to say, irrespective of the amount of stirring or shaking, they never increase or decrease in viscosity. This is an important attribute, since we want the degreaser or primer to easily flow over the surfaces onto which they have been applied.
All of our waterbased adhesives exhibit dilatant flow. This shear thickening property is important when adhesives applied vertically on high speed packaging machines. We don’t want the adhesive to drip!
Many of our adhesives have Pseudoplastic flow. When they are applied the shear created by the brush or roller will allow them to thin and wet out the surface evenly. Once applied the adhesives regain their higher viscosity which avoids drips and runs. This is particularly important when adhesives are applied vertically, for example, building adhesives which are used to glue insulated panels.
Our threadlocker adhesives are thixotropic. When the threadlocker is applied to the engagement area of the bolt, and as the nut is being placed, the threadlocker will decrease in viscosity, and flow around all of the threads and ensure a full coat of adhesive.
So there you have it, rheology plays a very important part in the selection of the correct glue for the job in hand.
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