Aminoplast

Aminoplasts (also: amino resins or amide resins ) are relatively low molecular weight, curable synthetic resins that are formed by polycondensation of carbonyl compounds (often formaldehyde ) and compounds with NH groups, such as urea ( urea resins , UF), melamine ( melamine resins , MF) or dicyandiamide (dicyandiamide resins , DD). The resins can be used as aqueous solutions (e.g. as glue resins) or as spray-dried powders as molding compounds. By adding hardeners, the low molecular weight resins are crosslinked and converted into insoluble, infusible thermosets . The term aminoplast is often used both for the preliminary product that has yet to be cured and for the fully cured end product.

Manufacturing

In general, aminoplasts are produced by the polycondensation of formaldehyde with compounds that contain two or more amino groups (e.g. urea , thiourea , melamine , cyanamide , diaminohexane , benzoguanamine ). As an example, here is the synthesis of urea-formaldehyde resin:

${\displaystyle \mathrm {n\ CH_{4}N_{2}O+n\ CH_{2}O\longrightarrow [-HN-CO-NH-CH_{2}-]_{n}+(n-1)\ H_{2}O} }$

n urea + n formaldehyde excerpt from aminoplast chain + n -1 water ${\displaystyle \longrightarrow }$

This reaction takes place in a first step via the addition of urea with the formation of N - hydroxymethyl groups and then chain growth via polycondensation with elimination of water. This reaction is usually carried out in a basic solution, since OH ^- ions are required as a catalyst and uncrosslinked precondensates are formed. The precondensates are stable for several months at room temperature.

When hardening under acidic conditions, further polycondensation of the linearly structured chain molecules of the resin also leads to links with one another. The more formaldehyde added, the more cross-connections there are.

use

The majority of aminoplasts are used as adhesives in the wood-based materials industry (e.g. in chipboard as "chipboard glue ") and as insulating material. By far the largest share is made up of urea-formaldehyde resins ( urea resin , UF resins), due to their widespread use in the wood-based materials industry. They are also used in household items, body parts and packaging material. Melamine resin is also used in the manufacture of table tennis tables. Weatherproof table tennis tables are now mostly made with melamine resin surfaces.

The property of hardening with gaseous formaldehyde also enables it to be used as a binding agent for molding sand in sand casting .

See also

• Phenoplast

Individual evidence

1. ↑ Entry on aminoplasts. In: Römpp Online . Georg Thieme Verlag, accessed on June 26, 2014.
2. ^ MD Lechner, K. Gehrke and EH Nordmeier: Makromolekulare Chemie , 4th edition, Birkhäuser Verlag, 2010, pp. 132-133, ISBN 978-3-7643-8890-4 .
3. ^ Wolfgang Kaiser : Synthetic chemistry for engineers. 3. Edition. Carl Hanser, Munich 2011, p. 419 ff.