Viscosity is a concept which describes how a liquid material flows. Adhesive viscosity is an important factor to consider when setting up your bonding process, regardless of whether it’s manual in a small scale or an automatic high-speed production.
If something has a low viscosity, it flows easily, and if something has a high viscosity, it flows with more difficulty – it is a viscous fluid.
Values of so called dynamic viscosity (which is the type normally used within the adhesives industry) is usually stated in the technical data sheet (TDS) of the adhesive. The unit can be centiPoise (cP or cPs) or mPa·s where 1 cP = 1 mPa·s.
Examples of dynamic viscosity values:
Regarding thicker adhesives,such as certain silicones or SMP-polymers, they can be difficult to measure using dynamic viscosity testing. Another type of measurement for flow can be used instead, extrusion rate. This value is usually listed in units such as g/min or ml/s. What is measured is the amount of material extruded from a certain apparatus during a specific time and pressure.
Apart from viscosity values, you will often also find a mention of the consistency of of the adhesive in its specification. Some terms commonly used are flowing/free-flowing, paste/non-flowing and thixotropic.
Please note that it can be quite difficult to compare viscosities of different adhesives due to the fact that any values will depend on the method or standard used when measuring, such as the rpm. The viscosity is also dependent on outer factors including temperature.
Which viscosity do I need?
When choosing an adhesive, it is important to consider that the adhesive itself as well as its viscosity is suitable for both the final product and the gluing process.
Some examples of questions to ask yourself:
- Does the adhesive need to flow out to a bigger surface area than the string of glue I dispense? Or du I want it to stay put?
- Do I need to build up a certain thickness of the adhesive bond? (Tip: look in the technical data sheet where the recommended maximum gap fill is usually stated).
- How will I dispense the adhesive? Is any type of equipment already determined?
What does it mean if an adhesive is thixotropic?
A fluid is thixotropic when its viscosity is affected by movement (shear stress) and also is dependent of the time during which this movement takes place. In reality, this means that the viscosity is lowered more when the fluid is stirred for longer.
It also means that an adhesive that is thixotropic will not flow. Instead, it will maintain the shape it had when you dispensed it.
When the adhesive is not exposed to shear stress, it will stop flowing. But as you stir it, or dispense it for that matter, it will start to get more thin until you stop (with a delay). Ketchup is an example of a thixotropic fluid. A thixotropic adhesive does not really tell you anything about the adhesive viscosity. A fluid can be thixotropic and still have a low viscosity, as viscosity is measured under shear stress.
A thixotropic adhesive is favorable to choose when assembly is to be performed vertically or when it is important that adhesive material does not flow where it is not supposed to.
A non-thixotropic adhesive can sometimes also be made into a thixotropic one by adding a special kind of filler. This way you can get the same chemical and adhesive properties but with a different type of flow. Compare for example Epo-Tek OG198-54 and Epo-Tek OG198-55.