The good bonding strength of two part polyurethane is relative with many conditions, among which the changes of working environment is a key affecting factor. That’s to say, with the changing of weather, it is necessary to make some small adjustments for the using technology of polyurethane.
To be simple, the main weather reasons to impact on the bonding strength include environmental moisture and temperature. Generally, in spring and summer, especially in the early summer rains, the air humidity is great or even reaches the maximum value. However, it is dry with low humidity in the autumn and winter. As for temperature, it is much higher in summer than winter, and the maximum difference value is around 30～40℃.
Polyurethane often cannot cure fully in the humid condition, and remains great residual tack. Seriously, there is a wire drawing phenomenon in a peeling test for adhesive film.
In addition, polyurethane becomes condensate in the humid air. As time goes on, polyurethane gradually becomes muddy and white, and even loses bonding strength.
The bonding strength and reactivity of polyurethane are related closely to temperature. The standard temperature for polyurethane is 25 ℃。Its viscosity is lower with good flowability in the high temperature. Otherwise, the viscosity is high with poor flowability.
Some adjustments must be adopted:
1. If the air is humid with a little high temperature, increase the amount of curing agent by 10%-20%;
2. Use dry cotton yarn to absorb the water drops on the adhesive machine so as to prevent drops falling into adhesive plate.
3. Add small amount of solvent in the rest adhesives to dilute it, and seal and keep it. If it is possible, keep it in the refrigerator. Unfreeze it in the sealing package when using it next time, and mix it with new polyurethane.
4. When it is cold in winter, add some solvent to reduce the viscosity when mixing the polyurethane, and improve the flowability.