Blocked isocyanates
The terms " blocked isocyanates " or " Capped isocyanates " describe the fact that the addition compounds of highly reactive isocyanates with alcohols ( urethanes ) and amines ( urea ) can release these isocyanates again at higher temperatures. This reversibility is industrially in the manufacture of polyurethane - baking coatings exploited.
properties
Blocked isocyanates result from the reaction of isocyanates with H-acidic compounds. They are thermally unstable and decompose ( deblock ) above about 120 ° C again to isocyanates. They only continue to react when heated, which means that the time of the reaction can be better controlled and that the paints can be stored for a long time. Blocked isocyanates are used not only as crosslinkers for paints and coatings but also as flexibilizers in epoxy systems. These products are based on flexible polyurethane prepolymers that are blocked with phenol. The deblocking takes place by reacting with the amine hardener , so that a urea is formed, which is built into the epoxy network and functions as a soft segment.
Blocking agents with different unblocking temperatures are used. However, the temperature is also dependent on the chemical structure of the isocyanate and the volatility of the blocking agent. Tin and bismuth compounds lower the deblocking temperature by a few degrees Celsius.
| Blocking agents | Deblocking temperature [° C] | Residual amount of blocking agent after 30 min stoving time |
|---|---|---|
| Acetoacetic acid , malonic ester (e.g. diethyl malonate ) | 120-130 | |
| 3,5-dimethylpyrazole | 140 | 14% |
| Butanone oxime | 150 | 27% |
| secondary amines | 120-160 | |
| Caprolactam , phenols | 160-180 | |
| Alcohols | > 180 |
KTL coating
In the automotive industry, for example, aqueous mixtures of epoxyamine adducts (as their formates ) and bisisocyanate- butylglycol- urethanes are used for KTL coating of metal bodies. Typically, the bisisocyanates "TDI" or "MDI" are used for bisisocyanate-butylglycol-urethanes ; a small proportion of bisisocyanate- trimethylolpropane- urethane ensures subsequent cross-linking.
After the cataphoretic deposition of this mixture on the conductive metal surface at around 200 volts, the second step is “burn-in” at around 180 ° C. The “blocked bisisocyanate” thermally splits off butyl glycol (boiling point 171 ° C) and when it cools, the reactive diisocyanate adds the hydroxyl functions of the epoxyamine adducts and trimethylolpropane . The result is an impact-resistant and mirror-smooth polyurethane initial coating of the metal body with a layer thickness of around 20 µm, which is then given further synthetic resin coatings. Traces of substances containing silicone have an extremely negative impact on the surface quality.
Allophanate formation
Urethanes can initiate a gradual exothermic polycondensation from approx. 80–90 ° C. Here the “blocked isocyanate” adds to the amide nitrogen of a second urethane molecule. The mechanism corresponds to that of biuret formation.
urea
Also urea acts as a blocked isocyanate. Even before its melting point is reached, ammonia is split off from 130 ° C. The resulting highly reactive isocyanic acid adds to another urea molecule and biuret (carbamoylurea) and traces of triuret are formed .
Individual evidence
- ↑ E. Delebecq, JP Pascault, B. Boutevin, F. Ganachaud: On the versatility of urethane / urea bonds: reversibility, blocked isocyanates, and non-isocyanate polyurethane. In: Chemical Reviews . Volume 113, Number 1, January 2013, pp. 80-118. doi : 10.1021 / cr300195n . PMID 23082894 .
- ↑ Entry on blocked isocyanates. In: Römpp Online . Georg Thieme Verlag, accessed on January 29, 2014.
- ↑ Mainly from bisphenol A bisglycidyl ether [ MW approx. 400] and diethanolamine
- ↑ KTL dip painting, with a picture of a combustion chamber for car bodies