Isocyanates

by Eric LeBreton

This article focuses on the physical properties, toxicity, reactivity, and emergency response information for isocyanates, in their various forms often found in commerce.

The information was gathered from available literature and is presented per subject category.

  • MDI  Diphenylmethane-4,4'-diisocyanate or Methylene bisphenyl isocyanate
  • TDI  Toluene-2,4-diisocyanate
  • HDI  Hexamethylene-1,6- diisocyanate
MDI TDI HDI
TLV 0.005 ppm 0.005 ppm 0.005 ppm
LD50(oral-rat) 31.6 g/kg 5.8 g/kg 738 mg/kg(pure) 10g/kg(polymer)
Ceiling/STEL 0.02 ppm 0.02 ppm 0.02 ppm
IDLH (CCOHS) 10 ppm 10 ppm  
Odour Threshold above TLV (musty) 0.4-2.4 ppm (sweet,pungent) 0.01 ppm (sharp,pungent)
Colour (solid) wte/yel clr/wte n/a
Colour (liquid) yel/dk bwn clr/wte clr /pale yel
Vapour pressure 0.00001 mm Hg (25°C) 0.01 mm Hg (20°C) 0.05 mm Hg (20°C)
Specific gravity 1.2 (25°C) 1.2 (25°C) 1.0 (25°C)
Vapour density 8.6 - 8.7 6.0 5.8
Vapour Saturation 0.13 ppm(25°C) ~33 ppm(25°C) n/a
Melting Point + 37-41°C + 20-22°C - 67°C
Boiling Point +172°C 1atm (OSHA) 194-200°C 5 mm Hg (CCOHS) +251°C +212.8°C(ACGIH) +255°C (Aldrich)
Flash point 196-202°C(cc) 130-135°C(oc) 130-140°C
Explosives Limits n/a 0.9-9.5% 0.9-9.5%
Auto-ignition Temp. 240°C ~227°C 454°C
Decomp. Temperature 230°C > 275°C n/a

MELTING POINT: Pure products have the melting point mentioned in the above table. Most of the isocyanates found commercially: Modified MDI, Polymeric MDI, PMDI or PMPPI, TDI isomer mixtures (2,4 & 2,6), TDI+MDI or HDI mixtures etc. have melting points which ranges in the + 10 to 20°C but are most often found in the + 10 to 12°C range. Therefore most commercial isocyanates will be found in a liquid state at ambient temperatures except in the winter months.

SHIPPING STATE AND USES:

  • MMDI(So called pure MDI): solid form, white to pale yellow
  • PMDI (polymeric MDI): liquid form, brown to dark brown.
  • Modified MDI: liquid form, usually pale yellow colour
  • TDI (2,4): liquid, melting point: 20 - 22°C
  • TDI ((2,4 & 2,6, 80/20%): liquid, melting point: 14°C
  • HDI: Clear to pale yellow liquid, melting point - 67°C
  • HDI-Biuret: Clear to pale yellow liquid
  • HDI-Isocyanurates: Clear to pale yellow liquid

MDI is in its pure form a solid but it is often shipped in large quantities in a molten state (above 38°C, melting point, CCOHS).

MMDI (Monomeric MDI) is a purified material distilled from a polymeric MDI mixture. MMDI consists of over 97% 4,4'-MDI with small amounts of 2,4'-MDI and traces of the 2,2'isomer. It is a solid with a melting point of about 38°C and it starts to decompose at 230°C. It is used in thermoplastic and cast elastomer applications, coatings, adhesives, sealants, synthetic fibers (BASF). Purified MDI is used for high performance polyurethane elastomers and spandex fibers (Kirk-Othmer).

PMDI ( Polymeric MDI or isocyanates) or PMPPI (polymethylene polyphenyl isocyanate) are crude products that vary in their exact composition. The main constituents are 40-60% 4,4'-MDI, the remainder being other isomers of MDI (2,4' & 2,'), trimeric species and higher molecular weight oligomers. These are always found commercially in a liquid state (brownish liquid). Their main use is in the manufacture of rigid polyurethane foam (construction, refrigeration),carpet backing, polyurethane coatings, thermoplastic resins, adhesives, sealants and foundry core binders. (Kirk-Othmer, CCOHS, BASF).

MODIFIED MDI (or Liquid MDI) is made by converting some of the isocyanate groups into carbodiimide groups which react with the excess isocyanate which at the end liquefies the low melting MDI. Liquid MDI (also called prepolymers) are also made by the reaction of the diisocyanate with small amounts of glycols. Liquid MDI is used in RIM ("Reaction Injection Molding") polyurethane elastomers.(Kirk-Othmer); "RIM" polyurethane automotive parts, nitrocellular elastomers, integral skin foams, flexible foams, adhesives, coatings and sealants (BASF).

TDI is often seen commercially in its pure form. Therefore as it can be seen by its melting point (+20-22°C), it often solidifies when exposed to low ambient temperatures. It is also widely sold as a mixture (80% 2,4-TDI / 20% 2,6-TDI and 65% 2,4-TDI / 35% 2,6-TDI) where its melting point may be in the 12°C range. It is used in the manufacture of flexible polyurethane foams (furniture, bedding) and rigid polyurethane foams (insulation),elastomers, polyurethane paint coatings, varnishes, wire enamels, adhesives and sealants.

HDI is the cheapest diisocyanate and in view of its low boiling point, is no longer used. HDI may be used as an ingredient in the preparation of dental materials, contact lenses, and medical adsorbents. There are two main types of polyisocyanates made from HDI: HDI-biuret and Isocyanurate type HDI. HDI-biuret, a homopolymer of HDI (or polymeric HDI)is obtained by treating HDI with water and is widely used in rigid-coating applications (Kirk-Othmer). HDI-biuret typically contains less than 0.7% HDI but the HDI level may increase up to 1.6% during storage (3 to 6 months according to Aquarius MSDS and CCOHS). HDI-Isocyanurates usually contain less than 0.3% HDI when they are first produced and this level does not increase during storage (CCOHS). HDI and its polymers may be mixed in solvents such as Xylene and Toluene.

HEALTH HAZARDS:

General: With a TLV of 0.005 ppm, isocyanates would normally be considered extremely toxic, but this is not the case due to the very low vapour pressures (<0.05 mm of Hg compared to 18 mm of Hg for water) and high vapour densities (i.e. 6 to 9 times heavier than air) of isocyanates. Hazardous vapour concentrations may be encountered if the material is spilled in a confined area (ceiling of 0.02 ppm which may be exceeded since the minimum vapour saturation is 0.13 ppm at 25°C) or is heated (involved in fire or a chemical reaction i.e neutralization procedure etc.). Allergy-prone people who have been sensitized to isocyanates or even have not been previously exposed to isocyanates may experience symptoms at concentrations as low as 0.0014 ppm. Asthma sufferers or people who easily get contact dermatitis should therefore not be exposed to isocyanates. The isocyanate odour does not provide sufficient warning of overexposure due to the high odour thresholds.

Inhalation exposure: Symptoms resembling asthma, wheezing, chest tightness, shortness of breath, difficulty in breathing, coughing, fever, chills, nausea, loss of appetite. The symptoms may occur immediately or several hours after exposure. Exposure to high concentrations may lead to chemical bronchitis and accumulation of fluid in the lungs (pulmonary edema) which may be fatal. Some people developed skin rashes from inhaling isocyanates eventhough there was no direct skin contact. Respiratory irritation may first be noticeable at 0.1 ppm.
Note: HDI and its polymers may be mixed in solvents such as Xylene, Toluene.

Treatment: Remove the victim to fresh air. If the person is short of breath, give him oxygen. If breathing has stopped, administer artificial respiration.

Eye exposure: Eye irritation starts at approximately 0.05 ppm. At 0.5 ppm, severe irritation and tearing occurs (lachrymator). Liquid contact may cause severe watering, formation of solid particles in the eye fluid, glaucoma, photophobia (sensitivity to light), blepharospasm (uncontrollable winking), conjunctivitis (inflammation of the mucous membranes of the eye lids with possible discharge), keratitis (inflammation of the cornea) and damage the cornea (opacity or clouding).

Treatment: Have the victim remove his contact lenses if he is wearing them. Rinse the affected eye(s) with lukewarm water for at least 20 minutes. Ask the victim to look up, down and to side to side as to better rinse all parts of the eye. If the person cannot tolerate light, protect the eye(s) with a clean loosely tied handkerchief or bandage.

Skin exposure: Prolonged skin contact may cause redness, swelling, blistering and possible skin sensitization (dermatitis). MDI compounds have a mild tanning action on the skin. Oil-base no-bake binding systems (foundry) containing MDI may cause dark stains on unprotected skin which cannot be removed by normal cleaning methods but will gradually wear off.

Treatment: Remove contaminated clothing. Contaminated shoes, leather goods, should be discarded. Product may be removed with soap and water or with a clean cloth and Isopropyl alcohol (rubbing alcohol) followed by a wash with soap and water. If the material has hardened and cannot be removed using the above method, it may be necessary to remove it by gently rubbing the affected parts with a sponge type sanding bloc (E.L.).

Ingestion: The effects of ingestion include the irritation and burning of the mouth, esophagus and stomach. There is no systemic toxicity. Pure HDI (not the polymer) with a LD50(orl-rat)of 738 mg/kg is more toxic than TDI (5.8 g/kg) or MDI (31.6 g/kg).

Treatment: Vomiting should not be induced. Never give water if the victim is unconscious or is having convulsions. Rinse mouth thoroughly with water. The affected person should immediately drink large amounts of water to effectively reduce the concentration of the chemical. If vomiting occurs, administer more water. The administration of activated charcoal (100 g in a cup of water) may be helpful.
Note: The HDI polymer may be mixed in solvents such as Xylene and Toluene, which fumes may be more harmful than the HDI alone.

REACTIVITY (polymerization):

HEAT:

  • TDI may polymerize if heated above 177°C for a short period or above 45°C for prolonged periods (CCOHS).
  • MDI may polymerize if exposed to temperatures above 175°C (SPI), 204°C (CCOHS).

WATER:

  • Isocyanates react with water to produce insoluble polyureas (relatively non-toxic and inert), Carbon dioxide gas (CO2) which may rupture sealed containers and large amounts of heat(exothermic reaction) which will increase the evolution of isocyanate vapours.
  • Ambient temperature Reaction time (non-violent): 15 hours if mixed and up to 48 hours if allowed to react on its own.
  • Above 50°C: Possible violent reaction.
  • If a drum containing isocyanates (i.e. MDI) is contaminated with 1 oz of water, the resulting CO2 release may increase the pressure inside the drum by 15 psi. Only the quantity of water added to the isocyanates will react i.e. not a full blown or uncontrollable reaction.

POLYOLS (polyesters, polyethers, polyhydric alcohols eg glycerol):

  • Polyols are the ingredients used commercially to mix with isocyanates in the manufacturing of polyurethane foams. The reaction will release heat and Carbon dioxide gas.
  • Once started it cannot be stopped. During the foaming or curing operation, the foam may reach temperatures above 140°C, a condition which may result in spontaneous combustion. Note: Amines or metallic salt catalysts, auxiliary blowing agents (CFCs, Acetone, Methylene chloride) and silicone surfactants are also added in the polyurethane manufacturing process.

OTHER CHEMICALS:

  • Isocyanates react with bases (sodium hydroxide, ammonia) primary & secondary amines (catalysts) acids, alcohols, strong oxidizers, metallic salts (such as organotin catalysts).
  • The reaction may be violent, generating heat which will increase the evolution of isocyanate vapours and the formation of CO2 gas.

ORGANIC MATERIAL:

  • Sawdust or other organic material should not be used to absorb isocyanate spills as spontaneous combustion has occurred. This may be due to the oily rag phenomena and the presence of moisture.

CORROSIVITY:

  • Slightly corrosive to copper and aluminum. Copper, zinc, iron and their alloys should be avoided as they cause product deterioration (i.e. affect product quality). Isocyanates will attack and embrittle many plastic and rubber materials.

CONTAINERS:

  • Mild steel or epoxy-phenolic lined steel and stainless steel is suggested for containers and stainless steel is suggested for pumps. Isocyanates will attack and embrittle many plastic and rubber materials. Tank trucks and tank cars normally have external heating coils, thermometer wells and are top unloading. Isocyanate storage tanks are kept dry with a blanket of dry air or nitrogen under slight pressure (1 mbar). Usual precautions should be taken for exposure to possible oxygen deficient atmosphere. It is important to avoid water contamination as the combination of insoluble polyurea solids formed (possible line or safety vent restriction) and the evolution of Carbon dioxide gas may overpressure and rupture the tank.

FIRES:

GENERAL:

  • Isocyanates are not considered a severe fire hazard.
  • Flash points + 130°C, auto-ignition > + 225°C, low lel (0.9%) but extremely low vapour pressures (<0.05 mm of Hg at 20°C).
  • Hazardous polymerization may occur if the drums are exposed to 50°C temperatures for an extended period of time.

IF THE CONTAINERS ARE NOT ON FIRE:

  • Remove the affected containers if without risk.
  • If this is not possible, a water curtain should be positioned between it and the fire (preferable option) or cool the containers with a water spray.

IF THE CONTAINERS ARE ON FIRE:

  • Extinguish with dry chemical powder, CO2 extinguishers, protein based foam or large quantities of water.
  • The reaction between water and hot isocyanates may be vigorous.
  • If overpressurization is detected, evacuate 100 metres for drums, 800 metres for large tanks.

AFTER THE FIRE IS EXTINGUISHED:

  • Inspect the containers for possible polymerization (bulging drum) from water contamination or exposure to excessive heat.
  • It is also possible that a drum was damaged (i.e. leaked) in the fire but has resealed itself due to the formation of solid polyurea in the polymerization process...pressure buildup.
  • If a large tank is involved, it may have to be vented.
  • If the tank damage is severe, the product may have to be transferred into another vented container for disposal.

HAZARDOUS DECOMPOSITION PRODUCTS:

  • Carbon monoxide
  • Oxides of nitrogen
  • Traces of hydrogen cyanide
  • Isocyanate vapours.

Hazardous decomposition products may evolve if the product is on fire or is heated above its decomposition temperature: 230C (MDI), 275C (TDI), 204C (HDI-biuret).

MATERIALS RESISTANT TO ISOCYANATES:

  • Butyl rubber
  • Nitrile rubber
  • PVA or Polyvinyl alcohol
  • Teflon® (Dupont)
  • Viton® (Dupont Dow Elastomers)
  • Saranex® (Dow Chemical) & Laminated Tyvek® (Dupont)

SPILLS:

If the spill occurs indoor, ventilate the area. Wear protective clothing to avoid skin contact and respiratory protection if the vapour concentrations are above TLV. Absorb the spill with non-combustible absorbent material such as kitty litter, dry sand, cement powder etc. Scoop the absorbed material into an un-sealed container. Move the un-sealed container to a safe isolated area where it may be neutralized with a decontaminating solution. To avoid excessive evolution of heat it is preferable to add the absorbed isocyanate to the decontaminating solution at a rate of 1 part or less isocyanate to 10 parts decontaminating solution. To ensure complete neutralization, leave the container outdoor (covered with loosely fitted plastic sheet or cover) or in a properly vented area for up to 48 hours (if no mixing). Respiratory protection may be required due to the presence of ammonia vapours.

DECONTAMINATION SOLUTION:

A typical decontaminating solution consist of 3-8% household ammonia, 1-7% dishwashing detergent and water.

CAUTION: HDI and its polymers may be found mixed in solvents such as Xylene, Toluene. In this case, an alcohol (ethanol, methanol, isopropanol) may be used as the decontaminating solution. After the isocyanate component is neutralized, the flammable solvent residue may then be incinerated. If the ammonia decontaminating solution was to be used, SCBA and not cartridge masks may have to be worn due to the presence of corrosive vapours (ammonia) and organic vapours (xylene, toluene).

NOTE: An excess of water alone will neutralize the isocyanates (completion in about 13 hours if mixed continuously). The addition of ammonia, detergent or alcohol only speeds up the reaction. If the incident occurs in an isolated area, windshield washer fluid (methanol) or radiator antifreeze (ethylene glycol) may be used.

PRESSURIZED DRUMS:

A bulging drum is most probably the result of product contamination (usually water). Product contamination evolves carbon dioxide which will pressurize the drum and possibly cause it to burst. One ounce of water will pressurize a drum to 15 psi. Since it is impossible to stop the reaction once started, it may be necessary to depressurize the drum. Do not move the drum. Evacuate the immediate area. Place a large tarpaulin over the drum. While wearing protective equipment, pierce the drum with a sharp tool (long spike etc.). Once depressurized, place the drum into an unsealed overpack drum and move it to a safe location. As to prevent a recurrence of overpressure in the punctured drum (forming polyurea solids may plug hole), remove the cover (or open enough to vent) if this can be done safely.

DANGEROUS GOODS REGULATIONS:

  • MDI or Diphenylmethane-4,4'-diisocyanate used to be regulated as UN2489 class 6.1 PG III. It has since been deregulated (SOR/95-547).
  • TDI or Toluene diisocyanate, UN2078, class 6.1 PG II
  • HDI or Hexamethylene diisocyanate,UN2281, class 6.1 PG II.

REFERENCES:

  • Aldrich Chemical MSDS
  • American Conference of Governmental Industrial Hygienist (ACGIH)
  • Aquarius MSDS
  • BASF, MSDS and Handbook
  • Canadian Centre for Occupational Health and Safety (CCOHS)
  • Chemical Hazard Response Information System (CHRIS)
  • Dangerous Properties of Industrial Materials (Sax & Lewis)
  • Emergency Action Guides (AAR)
  • EPA Chemcical Profiles
  • Kirk-Othmer Encyclopedia of Chemcical Technology, 3rd Ed.
  • Manual for Spills of Hazardous Materials (Env. Canada)
  • Occupational Safety and Health Administration (OSHA)
  • Registry of Toxic Effects of Chemical Substances(RTECS)
  • Répertoire des Produits Dangereux (Guide Orange des Sapeurs-Pompiers Genevois)
  • Safety Information on Isocyanates (Dow Chemical)
  • Toxicology of the Eye, 2nd Edition (W. Morton Grant)


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