What is Viscosity and Why Does it Matter in Glue and Adhesives?

The viscosity of an uncured fluid tells a chemical designer a great deal about its qualities. Fundamentally, this gelatinous quality effects everything about the fixative agent, everything from the way it flows to the time it takes to cure. In truth, no sticky stuff worth its name should ever be considered fully formulated unless this important parameter is fully evaluated, so let’s get down to the basics by opening up a dictionary.

Viscosity: Definition

The dictionary explanation talks in terms of flow resistance. At the lowest end of this resistance scale, fluids run like water. The higher end covers substances that have become thick and gelatinous. For adhesives, water-like products are popular because they run to cover large surfaces in thin films. Meanwhile, way at the distant end of this thickness scale, pasty substances don’t flow well, so surface area coverage is minimal. Poor fluid distribution issues are correctable, though, with a little manual effort.

Effects on Application

Cohesion properties rank highly on the back labels of branded chemical fixatives, but how good is the best glue when it doesn’t incorporate the right coverage factor? High flow glues, for example, are thin and easy to control. They apply rapidly to form a thin coating, be it on a thin-walled cup or a wide picture frame. On the other hand, super-glutinous adhesives don’t spread as well, and they certainly don’t create a thin film, but the syrupy seam formed by the curing agent does tend to deliver more cementing strength. Already we can see this important property makes a big difference during both the application and curing stages.

Why Does It Matter?

There are times when a liquid-like glue creates the best possible product, but there are also occasions when this fine-line format doesn’t work well. That thin stuff runs everywhere, given half a chance, which is why caution is advised during its application. It can get on clothing, in eyes, and between fingers. Pasty glues don’t create this kind of trouble, not when they’re mixed or applied directly from a tube. Instead, the viscosity factor exerts chemical resistance, a syrupy property that facilitates a slow and methodical coating methodology.

Viscosity matters during the coating stage because uncured adhesives react differently when they “run” on a surface. Finally, viscosity also varies with the temperature of the application surface and the environment, so the fluid will become runnier as it gets hotter. If that’s a problem, consider keeping heat-sensitive glues stored in cool rooms so that they retain their pasty qualities.