Nicoat

Coating Rewetting

This Tech Note refers to two-sided sheet-fed jobs. Work-and-turn, work-and-flop, sheet-wise. Using water-based coatings, aqueous.

Re-Softening

When running 2-sided aqueous coating work in sheet-fed pressrooms, various sources of technical information (press, drier, ink and coating manufacturers) recommend different load temperatures.

They use the term "re-softening" when warning press operators not to run too high of a load temperature. It should always be considered that the coating formulation is still the most critical part of successfully running two-sided work.

Drying Variables

There are three variables that impact the drying of aqueous or water-based coatings: Hot air movement, air evacuation and infrared. Although load temperature is important, we believe it is only one part of the drying equation.

There are many configurations of drying systems, some with and some without extended deliveries. Hot air movement is still the most important criteria to dry water-based coatings. Yet the infrared which helps dry the ink and coating should not be forgotten.

Load Temperature

Load temperature is directly related to the amount or effectiveness of the infrared system and the thickness of the stock. If the sheet can be delivered dry at a minimal load temperature, that would be an ideal situation. However, if the sheet is not dry with the current drying system, we feel the first side load temperature should be 90°F +/- 5°.

The sheet, when delivered should be dry to the touch. If side one is dried sufficiently and it is a two-sided coating, it should pass a block test of 140°F/2psi/24hrs (coating to coating), provided the ink is dry.

If the load temperature for side one is 90° +/- 5°F, we recommend side two load temperature be maintained between 85°F and 90°F. If side one is not thoroughly dried, it will be susceptible to re-wetting by side two. The longer the time between coating the second side, the higher the load temperature can be.

Sticking or Picking

A backup 12 hours later might allow the second side load temperature to be the same as the first side. Coating side one, but not adequately drying side two, could allow side two to re-wet side one. This is not a resoftening of the first side coating, but sticking or picking side two to side one.

Watch Out For Re-Wetting

Most importantly, the second side needs to be dried as well as the first side. Why? To dry the aqueous coating and prevent re-wetting. Remember, if the coating is dry to touch in delivery at a lower load temperature than the first side, with the current drying system, then that should be the load temperature run.