In several applications 1-component, thermosetting coatings are being used exclusively like in coil coatings, can coatings, or at least predominantly like in automotive coatings. Polyisocyanates as such can`t be used for 1-component systems due to the high reactivity towards the polyols (“2-pack systems”). Therefore, polyisocyanates are thermally reversibly “blocked” with H-acidic components. Reaction of such blocked polyisocyanates does not occur in presence of a polyol at ambient temperature and therefore a stable 1-component system is obtained. A proper selection of the “blocking agent” determines the reactivity, i.e. the splitting temperature of the blocked polyisocyanate and therefore the curing conditions of such an aliphatic, 1-component thermosetting polyurethane system.
Actually only a few blocking agents are of commercial and technological interest: more important are 2-Butanone-oxime (methyl ethyl ketoxime,MEKO), ε-caprolactam and to a limited extend 3,5-dimethylpyrazole.
In specific areas, especially in powder coatings, the uretdion/dimer structure is used to deactivate isocyanate groups thermally reversible without using blocking agents.
Since a significant portion of the blocking agent is released to the air during the curing process, the toxicological and environmental aspects of these substances have to be considered, too.
Blocked Polyisocyanates are mainly based on HDI or IPDI. Representative products are
- HDI trimer, MEKO (2-butanone oxime) blocked
- IPDI trimer, MEKO (2-butanone oxime) blocked
Other blocking agents commonly used are ε-caprolactam and 3,5-dimethylpyrazole. Blocked Polyisocyanates are supplied as 100 % solids and/or dissolved in a variety of solvents. Solvents and blocking agents, that are released under stoving conditions, have to be taken into account for an overall assessment of individual products. A manifold of products with differing properties are offered by ALIPA members. In the following you find a compilation of aspects which apply to most Blocked Polyisocyanates:
Due to the higher molecular weight and the fact that virutally no free NCO-groups are present these compounds pose a lower health hazard compared to the corresponding Monomeric Di- and Polyisocyanates.
They should nevertheless also be handled under controlled conditions only. They are not or only slightly irritating to the skin, eyes and the respiratory tract (nose, throat, lung). Some Blocked Polyisocyanates might act as skin sensitizers. Available information does not provide evidence that Blocked Polyisocyanates might be mutagenic. Details on toxicological properties should be taken from product specific safety data sheets.
Blocked Polyisocyanates are not readily biodegradable. Within the limits of water solubility, Blocked Polyisocyanates have a low toxicity for aquatic organisms. Details on ecological properties should be taken from product specific safety data sheets.
Respiratory protection is required in insufficiently ventilated work areas and during spraying. An air-fed mask, or for short periods of work, a combination of charcoal filter and particulate filter is recommended. Body protection should be chosen based on activity and possible exposure, e.g. apron, protecting boots, chemical-protection suit. Face protection/protective goggles as well as chemical resistant protective gloves should be worn. Details on suitable glove material should be taken from product specific safety data sheets.
Occupational Exposure Limits (OELs) for individual Blocked Polyisocyanates can be found under OELs. For REACH registered Blocked Polyisocyanates DNEL’s (Derived no effect level) can be found in the corresponding safety data sheets.
Upon skin contact, remove any contaminated clothing immediately. Wipe off mechanically and wash affected areas thoroughly with soap and water. Contaminated clothing should be discarded or washed thoroughly before reuse. For severe exposures, the affected person should get under a safety shower, using the flushing action of the water to remove the bulk of the chemical, then remove contaminated clothing and wash skin with soap and water. Seek medical attention. For lesser exposures, the individual should seek medical attention if irritation develops or persists after the area is washed.
Following uptake by inhalation, move person to an area free from risk of further exposure. Oxygen or artificial respiration should be administered as needed. Treatment is essentially symptomatic. A physician should be consulted.
Upon eye contact, flush with large amounts of lukewarm water for at least 10 minutes, holding eyelids open all the time. Refer the affected individual to an eye specialist or other physician for immediate follow-up.
Following ingestion, the person should drink 1 to 2 cups of water. Vomiting should not be induced and nothing should be given orally to an unconscious or convulsing person. A physician should be consulted
More detailed information can be taken from the suppliers’ Material Safety Data Sheets.