Melamine and urea resins: properties and applications / Articles

Melamine and urea (urea-formaldehyde) resins represent a critically important class of thermosetting synthetic polymers, collectively known as aminoplasts. These resins are produced by the polycondensation of formaldehyde with compounds containing amino groups — melamine and urea, respectively. Thanks to a unique combination of valuable properties, these materials have found widespread application in the woodworking, construction, textile and other industries.

Chemical Structure and Synthesis

Urea-formaldehyde resins (UF) are products of the polycondensation of urea with formaldehyde. The production technology typically consists of three stages and is carried out in a reactor with a stirring device. The first stage is conducted in an alkaline medium (pH = 7.5-8) at a urea : formaldehyde molar ratio of approximately 1:1.6-1.9. The second stage takes place in an acidic medium (pH = 4.5-4.8) and is characterised by a sharp increase in viscosity due to the growth in molecular weight. After the acid condensation stage, an additional portion of urea is introduced into the resin. In industry, UF resins are produced mainly as liquid suspensions, but dry powder forms (UF concentrate) are becoming increasingly common.

Melamine-formaldehyde resins (MF) are produced by the polycondensation reaction of melamine (1,3,5-triazine-2,4,6-triamine) with formaldehyde. The reaction is carried out at 40–60 °C in a medium with a pH of no less than 7. Melamine resins are part of the broader class of amino resins and are characterised by a more complex structure that includes a three-dimensional crosslinked network. There are also urea-melamine-formaldehyde resins (UMF) , which are products of the co-polycondensation of urea and melamine with formaldehyde. Such resins combine the cost-effectiveness of urea components with the improved properties provided by melamine.

Properties and Comparative Characteristics

Urea and melamine resins, while similar in chemical nature, differ significantly in their performance characteristics.

Urea resins possess the following key properties:

Lightfastness and odourlessness.
Hardness and good dielectric properties.
Resistance to solvents such as alcohols, acetone and chloroform.
Relative low cost and availability of raw materials.
Insufficient moisture resistance and limited durability — not suitable for exterior applications.
Emission of free formaldehyde during service, requiring special measures.

Melamine resins significantly outperform urea resins in a number of parameters:

Increased moisture resistance and hardness.
Higher mechanical strength and wear resistance.
Excellent chemical resistance, including to acids, bases and organic solvents.
Good heat and fire resistance.
High electrical insulation properties.
Transparency in the cured state, allowing materials with various colours and finishes to be obtained.
The main disadvantages are higher cost and potential brittleness.

Applications

Urea resins find their main application in:

Production of particleboard (PB), medium-density fibreboard (MDF, HDF) and oriented strand board (OSB).
Bonding of plywood, furniture and joinery products.
Production of moulding compounds (aminoplasts) with various fillers.
Production of urea-formaldehyde foam.
Manufacture of varnishes, paints and enamels.
Soil stabilisation in road construction and well drilling.

Melamine resins are used in more demanding and high-tech applications:

Production of high-pressure decorative laminates (HPL) and countertops.
Manufacture of impact-resistant and water-resistant tableware, including dishwasher-safe items.
Impregnation of decorative paper and textiles.
Production of water-resistant plywood and glued timber structures (including load-bearing ones).
Fire-retardant coatings and heat-resistant materials.
Automotive coatings and electrical insulation materials.
As crosslinking agents in polymer compositions and to improve the water resistance of paper towels.

Production and Market

The global aminoplast resin market is highly concentrated. The largest producers of melamine resins are BASF (Germany), Allnex (Belgium), Ineos Melamines (Switzerland), Hexion (USA), and Borealis Agrolinz Melamine (Austria). The melamine market as a whole is also controlled by giants such as China Haohua Chemical Group, Mitsui Chemicals and Nissan Chemical.

In Russia, the production of melamine-formaldehyde resins is mainly concentrated in the Volga Federal District (over 67% of production). Until recently, the key suppliers were foreign concerns from Norway, Sweden and Germany. However, under sanctions restrictions, domestic developments are actively progressing, such as the ProtoMin adhesive system, certified according to GOST 33122–2014 for the production of load-bearing glued timber structures.

The main market trend is the tightening of requirements for free formaldehyde content in finished products. Resins with low formaldehyde emission, corresponding to E-1 and E-0 classes, are becoming increasingly common. Active work is also underway to create modified and bio-based formulations that reduce environmental impact.

Conclusion

Melamine and urea resins are among the most widely used thermosetting polymers in the world. Urea resins, due to their cost-effectiveness and good basic properties, remain the foundation for the production of a wide range of wood-based panels and adhesives. Melamine resins, possessing significantly higher strength, moisture resistance and hardness, are indispensable in the production of laminates, high-quality tableware and critical structures. The further development of this class of materials will be determined by environmental safety requirements and the need to create more advanced, durable and safe polymer systems.