Concept · Conditions and application

Pot life and induction time

The difference between pot life and induction (sweat-in) time for two-component coatings, and why mixing ratios matter.

Two-pack coatings cure by a chemical reaction that starts when base and hardener are mixed.

Why the clock starts at mixing

The base and the hardener sit in separate cans, so they cannot react. Mixing brings their reactive groups together. In an epoxy, the amine groups of the hardener react with the epoxy groups of the base. In a polyurethane, the isocyanate groups react with the hydroxyl groups of the polyol. The molecules link into long chains and then into a network.

As the network grows, the viscosity of the mixed paint rises. At the end the paint gels and nobody can apply it. Pot life is the time in which the mixed paint is still fit to use. The maker measures it at a stated temperature and batch size.

The reaction gives off heat. In a full pot the heat stays in the batch, the paint warms and the reaction speeds up. Large batches therefore have a shorter pot life than small ones.

Temperature has a strong effect. As a rule of thumb, a reaction doubles its rate for each 10 °C rise, so the pot life halves. The real factor depends on the hardener. Data sheets list pot life at two or three temperatures, and those values govern. The rule is only an estimate.

Paint past its pot life can still look liquid, because the reaction progress and not the look sets the pot life. The network has started to form. The film cannot reach its full properties. Do not thin it to keep it going.

Induction time

Some products ask for a waiting time after mixing, the induction or sweat-in time. The data sheet gives it, if it applies.

The wait lets the free hardener start to react with the resin before application. Research on amine-cured epoxy shows what this achieves. Larger molecules form and the paint becomes more compatible. Less free amine is left to reach the surface. Less amine means less amine blush (amine blush). In the study, a longer induction time reduced the surface layer clearly at 80 % RH. It had no measurable effect at 40 % RH and below.

Induction time is part of the pot life. The pot life does not start again after the wait. Applying the paint before the end of the induction time can give poor film properties, also with the correct ratio.

Why the ratio matters

The mix ratio gives the right balance of reactive groups. Too much hardener leaves free amine in the film. That can cause blush. Too little hardener leaves resin without a partner, and the film stays soft.

• Mix the components in the stated ratio, by volume or by weight as the data sheet says.
• Mix full kits where possible. Split kits only by measured quantity.
• Do not change the ratio to speed up or slow down the cure.
• Fast-reacting products such as pure polyurea are mixed at the gun with plural-component equipment.

The mixing ratio calculator helps with part kits.

In the field

1. Read the data sheet. Note the ratio, the induction time, the pot life at the site temperature and the thinner limit.
2. Check that both components are in the kit, with matching batch numbers.
3. Write down the time the mixing starts. This starts the pot life.
4. Make sure the induction time has passed before the paint goes on.
5. Measure the temperature during the work. Compare it with the pot life table of the data sheet.
6. Stop spraying at the end of the pot life and discard the rest. Do not mix fresh paint into old paint.
7. After a long stop, flush the hoses and the gun. Do not leave mixed paint in the equipment.

A worked example

A data sheet gives a pot life of 6 hours at 15 °C. The rule of thumb gives about 3 hours at 25 °C and about 1.5 hours at 35 °C. On a hot day the paint is at 35 °C. The crew mixes at 09:20, and the induction time is 30 minutes.

Spraying may start at 09:50. The pot life ends at 09:20 + 1.5 hours = 10:50. The usable spraying time is 1 hour.

Check: 10:50 − 09:50 = 1 hour. On a cool day at 15 °C the same product would give 6 hours. Use the table on the data sheet where it exists, because the rule is only an estimate.

What the inspector records

• The batch numbers and the mix ratio method.
• The mixing time, the end of the induction time and the end of the pot life.
• The paint and air temperature.
• The time of the last application from each batch.
• Any paint discarded, and any equipment flushing.

Exam traps

• Trap: counting the pot life from the end of the induction time. It starts at mixing.
• Trap: judging by the look of the paint. Fluid paint can be past its pot life.
• Trap: adding thinner to rescue paint that is near the end of its pot life.
• Trap: using the pot life at 20 °C on a hot day. Heat and large batches shorten it.
• Trap: mixing half a kit by eye.
• Trap: mixing induction time up with pot life. Induction is the wait. Pot life is the usable time.
• Trap: mixing pot life up with shelf life. Shelf life is the storage time of the unmixed cans.

How the exams treat it

The FROSIO book covers paint composition and data sheets. The data sheet gives the ratio, the induction time and the pot life, so read it first. For the first hours after the paint goes on, see drying, curing and recoat windows.

Use it: Mixing ratio calculator

46 practice items in the members area mention this topic (CIP 1: 22 · CIP 2: 12 · FROSIO written: 12).

Related

• Standards: SSPC-PA 1
• Defects: Amine blush · Undercure
• Conditions and application: Dew point and the 3 °C margin · Volume solids, wet film and dry film · Drying, curing and recoat windows · Stripe coats