Aliphatic diisocyanates monomers are key raw material for aliphatic polyurethanes. 
Due to their specific toxicological profilethe aliphatic diisocyanate monomers are typically not used as such in the final application. In fact the diisocyanates are converted by the chemical industry into so-called polyisocyanates , which contain only very small amounts of free diisocyanate monomer, or into aliphatic polyurethane resins, in which often all isocyanate groups are used up to impart other functionalities into such resins. Examples for the latter of which are waterborne polyurethane dispersions, radiation curable urethane acrylates or thermoplastic polyurethanes. Polyisocyanates are crosslinking compounds in so-called 2-pack or 2-component polyurethane coatings. Further derivatization of polyisocyanates leads to blocked polyisocyanates which are relevant in certain stoving applications of coatings.

The most relevant products in this sector are:

  • Hexamethylene-diisocyanate-1,6 (HDI)

    CAS-No. 822-06-0

    HDI
  • Isophorone Diisocyanate (IPDI)

    (3-Isocyanatomethyl- 3,5,5- trimethyl- cycohexylisocyanate),
    mixture of cis-and trans isomers CAS-No. 4098-71-9

    IPDI
  • 4,4'- Dicyclohexylmethane-diisocyanate

    (H12-MDI), main component, mixture of isomers
    CAS-No. 5124-30-1

    Dicyclohexylmethane.

More details can be found in the following safety information sheets:

  • file pdf HDI (Hexamethylene Diisocyanate)
  • file pdf H12MDI (4,4'-Methylenebis (cyclohexyl isocyanate)
  • file pdf IPDI (Isophorone diisocyanate)

The most important product group within the family of aliphatic di- and polyisocyanates are the aliphatic polyisocyanates. These derivatives are pre-polymerized diisocyanate monomers of medium molecular weight still bearing unreacted isocyanate-(NCO-) groups. The prepolymerization is performed to overcome the hazardous potential of the diisocyanate monomers, to achieve a low vapour pressure of the isocyanate compound and also to have benefits in the final application, i.e. to reduce the time for complete formation of the highly crosslinked polyurethane network.

Polyisocyanates are either based on HDI or IPDI. The most important products are

  • HDI trimer
  • HDI biuret
  • HDI uretdione
  • IPDI trimer,

available as 100% solids and/or dissolved in a variety of solvents.

Polyisocyanates still contain small amounts of Monomeric Diisocyanate (typically <0.5 parts per weight) and both – the Polyisocyanate and the monomer – have toxicological importance. In addition, solvents also contribute to the overall toxicity of these products. A manifold of products with a wide range of different properties are offered by ALIPA members. In the following you find a compilation of general aspects which apply to most Polyisocyanates:

Typical reactions to generate polyisocyanates – mainly for the application in coatings - are homooligomerisation reactions like trimerisation (isocyanurate formation), dimerisation (uretdione formation) and biuret formation (reaction of urea of a diisocyanate with excessive diisocyanate). These reactions are quenched at certain conversion rates and the excess of diisocyanate monomer is removed by a distillation process. Therefore, such products typically contain less than 0.5 % of diisocyanate monomer. Due to the physical nature of the products and the requirements in the coatings applications, the polyisocyanates are often supplied as a solution in organic solvents.
The basic structures of the most important polyisocyanates are shown below. Commercial products always contains significant amounts of higher oligomers.

  • Isocyanurate Trimer

    isocyanurate / trimer

  • Biuret

    biuret

  • Uretdione Dimer

    uretdione /dimer

The market of those polyisocyanates for coating applications is dominated by the biuret- and trimer-polyisocyanates of HDI. To some extend the trimer of IPDI is used as well. These materials are the standard crosslinkers in light-stable 2-component or 2-pack polyurethane coatings.

Dimers are thermally not stable, splitting back into the original NCO-groups. Starting temperature and rate of the cleaving reaction depends mainly on the structure of the basic diisocyanate.

The adduct formation with oligo- and polymeric di- and triols is another oligomerisation technique. The resulting polyisocyanates are used, besides the coatings area, in the field of industrial adhesives, sealants and elastomers. Depending on the molecular weight of the polyol as well as the ratio of diisocyanate and polyol a content of lower than 2 pbw. of residual aliphatic diisocyanate monomer in the final product can be achieved without applying a distillation step. With distillation step the free monomer content can be reduced to below 0.5 %.

Toxicological Information

Due to the higher molecular weight and the much lower vapor pressure the Polyisocyanates exhibit a significantly reduced health hazard as compared to the corresponding monomers. Nevertheless they should only be handled under controlled conditions. They are not or only slightly irritating to the skin and eyes, but might be irritating to the respiratory tract (nose, throat, lung). Polyisocyanates might act as skin sensitisers. From animal models, however, there is no evidence that Polyisocyanates are sensitising to the respiratory tract. Results from animal tests with repeated aerosol exposures indicate that under these conditions the respiratory tract is the primary target of aliphatic Polyisocyanates, other organs are not significantly affected. Available information does not provide evidence that Polyisocyanates might either be mutagenic, carcinogenic or toxic to reproduction.
Details on toxicological properties should be taken from product specific safety data sheets.

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Classification regarding sensitisation

The ALIPA member companies have conducted toxicological studies required to evaluate the properties of their products as sound basis for classification and labelling. On that basis there is clear evidence from sensitive animal models that HDI-based as well as IPDI-based polyisocyanates and prepolymers may cause skin sensitisation. Consequently, ALIPA member companies have decided to classify all HDI-based and IPDI-based polyisocyanates and prepolymers as  skin sensitizers (Hazard Statement H317 - "May cause an allergic skin reaction").  In contrast available evidence from animal experiments suggest that HDI-based and IPDI-based polyisocyanates do not act as respiratory sensitisers. Such products are therefore not classified as respiratory sensitizers (Hazard statement H334 - "May cause allergy or asthma symptons or breathing difficulties if inhaled), unless monomer content or other ingredients require this risk phrase.

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Ecological Information

All Polyisocyanates are not readily biodegradable. However, due to other elimination mechanisms (hydrolysis, adsorption), long retention times in water are not to be expected. The resulting polyrea is more or less inert and, due to its molecular size, not bioavailable. Within the limits of water solubility, Polyisocyanates have a low to moderate toxicity for aquatic organisms.

Details on ecological properties should be taken from product specific safety data sheets.

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Exposure Controls / PPE

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 Polyisocyanates can be found under OELs. For REACH registered Blocked Polyisocyanates DNEL's (Deried no effect level) can be found in the corresponding safety data sheets.

Personnel who work with Polyisocyanates should have a pre-placement medical examination and periodic examinations thereafter, including a pulmonary function test. Anyone with a medical history of chronic respiratory disease, asthmatic or bronchial attacks, indications of allergic responses, recurrent eczema or sensitization conditions of the skin should not handle or work with Polyisocyanates. Anyone who develops chronic respiratory distress when working with Polyisocyanates should be removed from exposure and examined by a physician. Further exposure must be avoided if a sensitivity to Polyisocyanates or the corresponding monomeric diisocyanates has developed.

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First Aid Measures

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. Asthmatic-type symptoms may develop and may be immediate or delayed up to several hours. 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.

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.

  • 2 Butanone oxime

    2-butanone-oxime

  • ε caprolactam

    ε-caprolactam

  • 35 dimethylpyrazole

    3,5-dimethylpyrazole

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:

Toxicological Information

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.

Ecological Information

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.

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Exposure Controls / PPE

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.

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First Aid Measures

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.