Have you ever mixed up a batch of epoxy and of course unexpectedly turned the mixture into solid concrete just before you were done working? When you've done this, you've experienced the effects of gel time. This term is a common measure of products that thicken, harden, gel, or harden when mixed. Products such as epoxies, adhesives, resins, and coatings all use gel time measurements.
Gel time explained
Gel time is defined as the point at which a product becomes too viscous to work with. Knowing the gel time for a product gives an indication of the maximum working time after mixing.
Gel time is calculated by following the change in viscosity of the product over time. Most instruments that measure gel time work in a similar way to rotational viscometers. These gel timers rotate a spindle in a sample of liquid product and monitor the resistance to rotation of the spindle in the liquid. When the resistance becomes high enough to prevent the spindle from turning, the test stops. The time it takes to reach this test break point, or for the product to become a gel, is measured and is known as the gel time.
Other commonly used terms: working time and pot life
In addition to gel time, there are two other terms that are commonly used to quantify this curing process: working time and pot life. All three terms are often used interchangeably, which can lead to confusion.
The working time is determined empirically based on the rate of viscosity increase during the final stages of cure. The indication of useful life is usually set by the product suppliers to determine the theoretical limit of the actual useful life of the product. For example, suppose a product with a gel time of 15 minutes shows a rapid increase in viscosity starting at 12 minutes. The supplier can then set the lifetime to this 12 minute point or, to allow for a better margin of error, the specification can be set to 10 minutes instead.
Pot life a specification determined by the time it takes for the viscosity to double. To illustrate, a product with an initial viscosity of 1000 cP is timed until it doubles in viscosity to 2000 cP. If this takes 20 minutes, the pot life would be adjusted to 20 minutes. An exception to this rule are some products that have a very low initial viscosity. For these low viscosity products, pot life could be defined as the time it takes for the viscosity to increase by 4 times or more.
Pot life is not a true indicator of the time a product can be processed. It is most commonly used as a standard for comparing and evaluating different products.
Using gel timers to predict workability
Gel Timer Diffraction Modern gel timers record detailed viscosity readings during the test, often recording data points as close as every second. These measured values are recorded in special software and displayed graphically. Plotting these data points shows when the curing process begins. It can also track the rate of change of viscosity. Using vectors developed from this plotted data, predictions of realistic working hours are extrapolated.
These gel timer systems create data-based, real-time, objective predictive curves to determine the optimal time a product is workable.
Summary
At first glance, the concept of gel time seems clear and simple: how long does it take for a two-part epoxy to turn to stone?
However, related terms such as working time and pot life are sometimes used interchangeably with gel time and can be confusing if you don't understand the terminology. I hope this article helped break the haze a bit.
If this is the case and you have found these notes useful, please share them with colleagues who deal with products that thicken, harden, gel or harden when mixed.
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