Modification of the hardener of urea-formaldehyde resins with a low molar ratio of formaldehyde to urea

To reduce the toxicity of particle boards, urea-formaldehyde resins with a low molar ratio of formaldehyde to urea are used, – 1.0 or less. Plates based on these resins have reduced strength and water resistance and increased pressing time. These shortcomings are explained by the features of the new resins. In the oligomer molecules, formaldehyde binds to urea by predominantly methylene bonds, and contains a small number of hydroxymethyl groups necessary for curing the resin. Since there is a little amount of free formaldehyde in the resin, – 0.05...0.10%, the hardener is forced to react with hydroxymethyl groups. Oligomer molecules without hydroxymethyl groups become unstable in an aqueous medium and precipitate. The oligomer, which is notsoluble in water, slows down significantly the curing process of UFR. In addition, it is not involved in the formation of a spatially cross-linked polymer and the formation of adhesive bonds. New resins contain a lot of unbound urea. An aqueous solution of urea has a weakly alkaline character and neutralizes part of the hardener. To eliminate these shortcomings, it is proposed to use the addition of formaldehyde in the form of its aqueous solution – formalin. It must protect hydroxymethyl groups and increases the amount of acid formed. The curing time of industrial resin with a formaldehyde to urea ratio of 0.85:1.0 with a modifier content of 1.0% is reduced from 63.2 to 38.0 s or by 39.9%. Particle boards on this resin with 1.0% formaldehyde in the hardener have increased strength and water resistance compared to plates without additives. The content of formaldehyde remains at the same level, – 5.68 against 5.70 mg/100 g. Tensile strength perpendicular to the plate layer increases by 30%. This is due to the acceleration of the heating of the inner layer of the plate, which is carried out as a result of the mass transfer of steam and gases. In aqueous solutions of acids, free urea is unstable and begins to decompose when heated above 50 ° C into ammonia and carbon dioxide. Provided that the content of the binder acid increases, this process is accelerated. When the plates are pressed at a temperature of 210...220 C, water vapor is heated to a maximum of 130 ° C, and gases – to the pressing temperature. Moreover, the steam moves into the carpet of the plate step by step, condensing on cold chips. Heating with gases is more effective. The invariability of the toxicity index of the plates is explained by the fact that the residues of free formaldehyde are bound to urotropin by ammonia formed from urea. A formulation and technology for the synthesis of an innovative urea-formaldehyde resin with a mole resin has been developed. The ratio of formaldehyde to urea is 0.9:1.0. Plates on this resin with 1% formaldehyde in the hardener have a formaldehyde content of 3.7 mg/100 g. This meets the requirements of emission class E0.5.

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