Urea-formaldehyde resin

The technology for producing resins of this class consists, as a rule, of three stages. The synthesis is carried out in an apparatus with a mixing device equipped with a jacket. At the first stage, the polycondensation process is carried out in an alkaline medium at a pH value of 7.5-8 (alkaline condensation) and a urea : formaldehyde molar ratio of 1: 1.6-1.9, respectively. The second stage is carried out in an acidic medium at pH = 4.5 - 4.8 (acid condensation). This stage is characterized by a sharp increase in viscosity due to an increase in the molecular weight of the product. After the end of the acid condensation stage, an additional portion of urea is introduced into the resin to bring the residual molar ratio of urea: formaldehyde to 1: 1.3 - 1.4, respectively, and the resulting binder is dried to the desired conditional viscosity (drying stage). After the resin is cooled and its modification is carried out, aimed at improving its toxic (lowering the level of free formaldehyde to below 0.3%) and physical and mechanical properties.

The synthesis temperature should be in the range 88–94 ° С, for which, after loading formalin and dissolving urea in it, steam is supplied to the reactor jacket until a temperature of about 80 ° С is reached. The heating rate should be 1-1.5 ° C / min. Upon reaching a temperature of 80 ° C, the supply of steam to the jacket is stopped, and a further rise in temperature is carried out due to the heat of the exothermic reaction . When the temperature in the reactor rises above 94 ° C, recycled water is supplied to the jacket. The modification stage is usually carried out at a temperature of 40-50 ° C.

As raw materials for the production of urea-formaldehyde resin can be used:

• urea
• urea-formaldehyde concentrate
• formalin

Urea-formaldehyde resins are widely used in various fields of production and construction. They are used in the production of urea-formaldehyde polystyrene , chipboard and fiberboard, as well as plywood. In addition, they are used in the manufacture of special moisture-resistant grades of paper and cardboard.

The main brands of urea-formaldehyde resins are KF-MT-15 (TU 6-06-12-88), KFZhm (GOST 14231-88), KF-KhT-P (TU 2223-001-51119346-2003).

Currently, in Russia, as a rule, resin in liquid form (suspension) is sold and used, which has a number of disadvantages, including: shelf life of about 2-3 months, inconvenience in transportation, and some others.

At the same time, with the development of technologies in the chemical industry, urea-formaldehyde resins in the form of a white powder are also already successfully used.

The advantages of urea-formaldehyde resin powder are:

long shelf life (for example, T800 / T820 TRF resins are stored without loss of properties for one year), ease of transportation of resins (packaging of 20-25 kg per bag is accepted), ease of operation. Urea-formaldehyde powder is diluted in water in compliance with the technology (the proportion is from 50 to 150 parts of water per 100 parts of powder).

In appearance, it is a suspension of a uniform white color, although often the color can vary from white to light brown, depending on additional inclusions and its shelf life.

Urea-formaldehyde resins of grades VPS-G and KF-KhT-P , and in rare cases KF-MT-15 are used for the manufacture of foam by air foaming. KFZhm and KF-MT-15 are also used as a binder in the production of chipboards and fiberboards .

Approximate properties of liquid resins:

• Mass fraction of solids%, not less than - 51.0
• Density , kg / m³, not less than 1200
• Mass fraction of free formaldehyde%, no more than 0.3
• Conventional viscosity, s, 20-35
• The concentration of hydrogen ions (pH) 7.5-8.5
• The maximum miscibility of the resin with water at which coagulation is observed, by volume 1: 1-1: 10
• Gelatinization time at 100 ° C, s, no more than 110
• Dielectric constant Er at 20 ° C-2.35

The shelf life of the resin with proper storage should be at least 45 days from the date of manufacture.

The shelf life of the urea-formaldehyde resin in the powder is about one year.