Understanding Adhesive Viscosity and Rheology
Most adhesive manufacturers list the viscosity of the uncured adhesive on their literature to help engineers determine which product is most suitable for a specific application and can be integrated into their production line most effectively. Temperature and shear (stirring or agitating) can affect adhesive viscosity and flow behavior, therefore it is important to take these factors into consideration when selecting an adhesive product.
Viscosity is the measure of resistance to gradual deformation by stress. For the purpose of liquid adhesives, viscosity corresponds to the thickness of the liquid. Water has a viscosity of 1 cP (or mPa.s) at 20°C. Liquids that have a viscosity less than water are mobile liquids and those with viscosity higher than water are viscous liquids.
Viscosity varies with temperature.
Following is a chart of the viscosity of water at different temperatures – as you can see the viscosity decreases (becomes “runnier”) as it gets hotter. In practical terms, an adhesive that has just been taken out of the fridge may be difficult to dispense but once it has warmed up to normal room temperature (around 22°C) it can be more easily dispensed – if using a handgun, less muscle power will be required to pump out the adhesive.
|Temperature °C||Temperature°F||Viscosity cP|
Following is a table of common fluids and with the viscosity at 25°C.
|Fluid||Viscosity cP @ 20°C|
Water is a Newtonian fluid – its viscosity will change with temperature but not with shear or agitation (put simply, water has the same viscosity if it remains still or is shaken).
Rheology (or Non-Newtonian fluid mechanics) is the study of the flow of matter primarily in the liquid state. Newton learned that when shear (stirring) or other work is applied, viscosity can change. Newtonian fluids maintain the same viscosity. Other fluids, called Dilatant fluids get thicker when work is applied, but the most interesting type of relating to adhesives is the Thixotropic fluids. These fluids reduce in viscosity as work is applied – then when the work (pressure from dispensing – or stirring stops) the fluid returns to its previous viscosity. Adhesive chemists use this knowledge to create formulations that are easily dispensed but do not flow once they have been dispensed allowing for better accuracy and less mess.
The thixotropic index or thixotropic ratio defines how much the fluid reduces and how quickly they return to the original state.
Some thixotropic fluids return to the original viscosity almost instantly (pseudoplastic fluids) other take longer but the time is fixed for each fluid.
In addition to the flow properties of the uncured adhesives, flow properties of heat cured epoxies during the cure are generally noted. These descriptions (often listed as free flowing, or non-sag) refer to the properties of the adhesive during the heat cure process. We noted above that viscosity thins when temperature rises. Heat causes the adhesive to cure – harden – but before cure occurs the heat can cause the adhesive to reduce in viscosity and start flowing. Adhesive formulators work with various ingredients to control the flow during the initial portion of the heat cure phase. Often it is desirable to have the adhesive flow and level before it begins to cure – in other applications it is vital that the adhesive does not flow out of the bond joint.
For further help and advice, please contact Permabond.