Technical Notes: Temperature

Low Temperatures

Moisture curing polyurethanes cure by a combination of solvent loss and reaction with water. Both of these processes are temperature dependent; the lower the temperature, the lower the evaporation rate and the cure rate. The ideal temperature range for polyurethane curing is between 10 and 25�C, with temperature ranges below 10�C leading to a slower cure time, while high temperatures above 25�C may lead to complications with curing.

EFFECTS OF LOW TEMPERATURES
Prolongs curing time, risk of condensation forming on surface of coating if the substrate is at or below dew point. Refer to the technical notes on humidity for more details.

SOLUTIONS

Increase airflow

The solvents in the Duracoat MCR, MCL and Marine Clearcoat Range have a relatively high flash point (and low volatility) and low toxicity for health and safety reasons. They are also relatively denser than traditional moisture cure solvents, and the vapours tend to stay low in the area. This is one reason why there is also much less smell with our coatings.

Actively managing the ventilation of the heavy vapours may be required if the coatings are applied in a confined spaces or for flooring. The solvent vapour layer can be encouraged to flow outside if a suitable exit is arranged. For example, if there is a breeze, opening doors to the sheltered side of the house, (not high windows), will let the vapours flow to the low-pressure area created by the wind. Please note that except for the last coat, when dust-free conditions are essential, a little dust is not critical.

If opening windows for air is not possible, arranging for a fan to blow past a doorway in the direction of the outside ventilating can help shorten the curing time.

If window and doors are unable to be left open after coating, the rate of solvent loss and the cure time can still be markedly improved by creating plenty of low level ventilation while coating, and for the half hour or so while cleaning up. Good ventilation, especially in the first hour after application, is a very effective way to ensure a fast and thorough cure, as well as minimising smells.

Catalyst addition

Another device to improve the cure rate is to add a catalyst. Adding catalysts can be a tricky area, as excessive addition rates can cause problems in intercoat adhesion, exterior durability and pot life. Uroxsys Through Cure Catalyst solution uses a catalyst type that is very effective at increasing the moisture curing reaction, without markedly affecting the pot life. It is also formulated to improve the ability of moisture to move through the films and complete the curing reaction right through.

It can be used together with Duracoat MCR Gloss and MCL Gloss very effectively, to a maximum additive level of 5% (1 litre to 20 litres, or 50 mls per litre) in MCR Gloss, and a maximum level of 1.5% (15 mls per litre in MCL Gloss. Using the 5% maximum additive rate and good ventilation, MCR Gloss is tack free in 4 to 6 hours.

The catalyst is not recommended for use with the Uroxsys Marine Clearcoar Range, or with any Satins or Matts.

Additional Notes

Please observe the following recommendations when over-coating any Gloss coatings with a Satin or a Matt topcoat in colder conditions, as the Gloss base coats may 'fry' if they are over-coated before they are properly cured.

Speed up the cure rate of the Gloss coat by adding Through Cure Catalyst at the rate of 1.5% by volume (15 mls per litre).
Improve the cure on site by ensuring good ventilation.
If either of the above presents difficulty, allow an extra day for the Gloss coat to cure before over-coating with Satin or Matt topcoat.
If in doubt that the Gloss coat has sufficiently cured, do a test patch in an unobtrusive area.

High Temperatures

EFFECTS OF HIGH TEMPERATURES

Carbon dioxide bubbles can be formed.
Poor flow and levelling
Air release bubbles and pock marks
Decrease in viscosity - affects hold up on vertical surfaces
Intercoat adhesion may be reduced.

SOLUTIONS

The most direct way to minimize high temperature effects is to adapt your work plan and apply coatings before the day gets too hot, or after the heat of the day. This often means early starts or coating late in the day.
Stop direct sunlight from heating the work area by masking out windows or erecting screening.
If there is air conditioning, bring the temperature down before coating. Leaving the air conditioning on during cure may be impractical, especially if the system re-circulates air (and solvent fumes) throughout the work area, but exterior extracting domestic units may be able to be left on to keep the temperatures tolerable and to help get rid of the solvents.
Do not leave or store your polyurethane in the sun or in the back of your vehicle. At temperatures over 50�C, the contents can start to react and thicken, and the satins can begin to settle out.

EXCESSIVE VENTILATION

Finally, a note about ventilation. Excessive ventilation, ie a breeze across the surface of the work area, can also speed up the solvent evaporation rate and can magnify the effect of heat promoted solvent loss. The net effect of the surface curing before the solvents in the body of the film have escaped, is a drop in gloss as the surface wrinkles on the shrinking film.

For Satins or Matts, premature surface curing can result in glossy patches.