Dear Zahra,

your question is almost unanswerable. The UF resins /oligomeric and lower molecular weight polymeric fractions/ as adhesives prior to their industrial applications are time of storing and temperature sensitive materials. With time of storing they interact slowly to form linear and branched moieties with increased molecular weight, and their viscosity is proportionally increasing. I am affraid, there is not a chemical to stop this process, without reduction of UF reactivity. Diminished reactivity of the resin prolongs time of hardening, or requests use of increased pressing temperature.

One of the chemical methods, despite the above text, might be reduction of free CH2=O in the resin to CH3OH. Disadvantage may be cost of e.g. NaBH4, slow reaction rate in a viscous anvironment, and boric acid formation as by-product - shortening the time of resin hardening. Potentional risk represents also formation of  free H2.

From physico-chemical point of view an addition of inert, low boiling point liquid might influence the rate of viscosity increase with time and temperature of UF resin storing. A smaller addition of methanol or ethanol - a few per-cent  might moderately /depending on the amount  added/ reduce increase in the resin viscosity. The mentioned additives are, however easily flammable and might represent a risk while UF use. Also time of pressing at the temperature might be moderately increased.

You might experiment also with a slight change in the pH of the UF resin.

Best egards,

R.S.

Dear Professor Rastislav,

Thanks for your comprehensive answer. 

Conversion of liquid resin into solid powder from is the ideal solution [freeze drying technique]. There by it is possible to the transportation in poly sacks, and no containers are needed.

As in fact there seems no chemical reagent(s) available for this purpose.

 Addition of some solvents such as di-ethanolamine or try-ethanolamine can be carried in small percentages.

Thanks for upvoting, Zahra.

Best regards,

R.S.

Hello, I almost agree fully with the above comments. But there are still some small tricks to stabilize UF adhesives. In most of the cases, two processes are running in parallel. Crosslinking and linear reaction of the molecules, catalysed by OH- or H+-Ions and changes in the surface of the dispersed particle. The outside layer of this dispersion is also stabilized by some ionic loadings. The best to stabilise a UF for both effects, is to work with a buffer substance. This will stabilize the pH and prevent the system from further condensation. At 30 °C you can maybe get a storage stability of 2 weeks maximum. There are internal buffer based on e.g. Sodium-formiate (Formic acid is made by Canninzarro out of formaldehyde, Sodium hydroxide is added during condensation. External buffer could be e.g. borax or phosphate. The right concentration you have to find out, because it is specific based on the UF recipe.

The UF resin is the function of temperature, if you constant temperature then then duration of the product is also variable factor.Some addititive like methanol/ethanol also function of viscocity. The reacting urea and formaldehyde first in alkaline medium and subsequently in mild acid medium at reflux (conventional two - stage alkaline - acid synthesis), by conducting the reaction only in purely acid medium at 50oC (modified one - stage acid synthesis).The manipulation of UF resin viscosity by adjusting the condensation reaction was much more efficient than by adding wheat flour in improving the adhesion performance. The addition of addtitive.These results indicated that a way of controlling the viscosity of UF resin adhesives has a great influence to their adhesion. Urea-formaldehyde resin solutions are shown to be dominated by physical associations rather than primary chemical bonding. These physical associations, or colloidal dispersions, are directly related to the thermodynamic balance of secondary bond formation between resin and solvent systems.