Ask the Doctor
Please explain the difference between these terms: pot life, working life and gel time.
These terms are used to inform customers of the time that is available as a mixed thermoset system (resin and hardener or resin and catalyst) is starting the curing process. Thermoset systems start their curing when:
- The resin component and the hardener component are mixed together
- The mixed resin and hardener are thawed out after having been stored in a frozen state. These frozen systems have all the components necessary for curing except the temperature in the frozen state is so low that the reaction cannot start. When the system is removed and allowed to thaw to room temperature, the curing reaction can start.
- The resin and hardener are heated to start the curing reaction.
Customers need to know how long they have to work with the curing resin system and they also need to know when the system has gelled, which is the first state in the transformation from a liquid to a solid. Customers must not disturb a material after it has gelled because the final polymer will have a poorly cross-linked structure and inadequate physical properties.
Pot life is a defined timed during which the viscosity of the mixed system doubles. Example: if a mixed resin system has an initial viscosity of 10,000 cps and the viscosity increases to 20,000 cps after 30 minutes, then the pot life is 30 minutes. In some instances, if the starting viscosity is very low, e.g. 500 cps, then the pot life can be defined as the time it takes for the viscosity to increase by a factor of 4, or 5, or some other defined number.
The time factor for pot life does give an indication of how fast the system is curing (progressing from a liquid state to a solid state), however it does not necessarily define the amount of time the customer has to work with the resin, in terms of being able to use it in their process to make acceptable parts. In the example, even if the viscosity is 20,000 cps the customer may still be able to cast the material or use it as an adhesive, sealant, or encapsulant. What the pot life is best used for is to compare systems. If one system has a 30 minute pot life and one has a 100 minute pot life, the customer has an indication of how fast or how slow the two systems are curing.
Working life or working time is defined by the resin supplier to advise the customer how much time they have to work with the material before it reaches such a high state of viscosity that it cannot be properly worked with to produce an acceptable part. When working life is defined it does not have a defined value such as “viscosity doubles”. Working life is determined after running cure tests on the system to determine what is the exact point when the system gels and how fast the viscosity increases to this gel state in the final stage of curing. If a test shows a gel time of 30 minutes, but the viscosity increases rapidly between 25 and 30 minutes, the working life may be set at 20 minutes. If a test shows a gel time of 30 minutes but the viscosity increases significantly between 15 minutes and 30 minutes, then the working time may be set at 10 minutes.
The resin supplier will define the working life with a reasonable safety factor to advise the customer what time limit they have to properly work with and handle the material. The customer will use the working life to design his process so that there is some assurance that the material will be cast, applied, poured, or spread in a time frame that allows the full properties of the cross-linked polymer to develop.
Gel time is the time it takes for a mixed resin system to gel or become so highly viscous that it can no longer be considered workable or able to be handled. A thermoset resin system converts from a liquid mixture of chemicals to a solid material that has a highly cross-linked polymer as the major structural material. The gel time is the time when the polymer formation is in its early stages of cross-linking to the point that if the polymer gel state is disturbed then the final polymer will have properties that are not well established.
The best example is an adhesive material. If the curing adhesive is moved or manipulated after it has gelled, then the adhesive will never have adequate bonding strength or adhesion. It is customary for a thermoset system with a gel time of say 60 minutes, to advise the customer that the material must not be disturbed, moved, handled after it has cured for 50 minutes.
Gel time can be measured by following the viscosity until there is a point when the viscosity is so high it is no longer liquid. Gel timers measure this time by having a spindle rotate in the liquid until the time when rotation stops because the viscosity is too high, i.e. the material has gelled. The polymer will continue its cure from a gel to a solid, which is the final state of the thermoset system
The curing profile for a thermoset resin is very often dependent on the amount of material that is curing. More material will often cure faster than less material. This means the pot life, working life, and gel time are dependent on the volume of material. While they may be measured or defined, they are based on tests run with a certain amount, e.g. 100 g or 200 g. The customer will have a different pot life or working life or gel time if their process uses 1 g or 1000 g.
It is also important to mention that the pot life, working life, and gel time are only useful to the customer if the temperature is defined. Room temperature systems would have these properties defined at 25°C. If the system is a heat-cured system then the temperature could be 65°C or 100°C or another temperature that the supplier considers applicable. Viscosity increase could be monitored at 100°C, the gel time could be measured at 100°C and the working life would be specified after the gel time at 100°C is measured.