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Sampling and Analysis of Isocyanates Used in Paints
Robert P. Streicher
National Institute for Occupational Safety and Health
Publications of the U.S. Government are within the public domain and therefore are not copyrighted.
Background
• Isocyanate compounds are major components of spray paints used in the aerospace industry
• Paints contain primarily polyisocyanates with low levels of isocyanate monomer
• Primarily aliphatic isocyanates
Commonly Used Aliphatic Isocyanate Monomers
NN
CO
CO
CH3NCH3
CH2
N C O
CO
H3C
HDI
IPDI
Polyisocyanate Species Found in Paints
CN
C
NC
N
N
N
N
OO
O
C O
CO C O
HDI Isocyanurate
Polyisocyanate Species Found in Paints (cont.)
CN
C
NH HN
N
N
N
OO
C O
CO C O
HDI Biuret
Polyisocyanate Species Found in Paints (cont.)
NC
NC
NN C OCO
O
O
HDI Uretidinedione Dimer
Complications in Total Isocyanate Determination
• Vapor and Aerosol
• Very reactive (unstable)
• Potentially numerous species for which no analytical standards are available
Six Steps in Sampling and Analytical Methods
• Collection
• Derivatization
• Sample Preparation
• Separation
• Identification
• Quantification
Collection Efficiency Problems
• Aspiration efficiency
• Internal losses
• Transmission losses
Collection• Particles
– Inhalable Sampler (e.g., IOM): all particles
– 37-mm filter: < 20 m– Impinger: 2 m < x < 20 m
• Vapor – may depend on derivatization
Overspray AerosolParticle Size
• Rudzinski (1995): 4.1 +/- 1.7 m 6.9 +/- 2.5 m
• Myer (1993): 10 - 15 m
• D’Arcy (1990): 2.9 - 9.7 m
Derivatization
• Capture (vapors on filters)
• Stabilization
• Detectability
Factors Affecting Derivatization Efficiency
• Inherent reagent reactivity
• Reagent concentration
• Reagent-isocyanate mixing
Impinger / Filter Comparisons• Most field comparisons have found
impingers to give higher results - Maitre (1996), Myer (1993), Czarnecki (1992)
• Rosenberg (1984) found filters to be higher
• Bello (2000) found IOM = impingers
Sampling Recommendations• Be flexible• Filter
– Used for:
- All particles < 2 m- Slow-cure particles > 2 m
– Extract filters in the field– High-boiling solvent on filters (?)
• Impingers– Used for fast-cure particles > 2 m
Sample Preparation• Extraction
• Sonication
• Filtration
• Solvent exchange
• Concentration
• Solid-phase extraction
Analytical Separation:
Reversed-Phase HPLC
Analytical Strategies
• Bulk product for calibration (Bayer)
• Monomer for calibration (MAP Method, MDHS 25/2, NIOSH 5522)
Bulk Product for Calibration
• Less demanding HPLC and detection
• Requires that isocyanate composition of calibrant matches composition of exposure
Monomer Calibration
• Demanding HPLC and detection requirements
• Does not require bulk product calibrant with composition matching exposure
• Bulk product used qualitatively
HPLC:
Isocratic or Gradient Elution?
Advantages of Isocratic Elution
• Simple
• Compatible with EC detector
• Stable baseline
• Unvarying response factor
Advantages of Gradient Elution
• Powerful - wider range of compounds
• Faster
• Better peak shape
HDI/IPDI Oligomer Bulk
Identification of Derivatized Isocyanates
• Monomers - Retention time
• Oligomers– Retention time– Two detectors - response ratio is
diagnostic– Multi-dimensional detector (mass
spectrometer, photodiode array)
Quantification
• Monomer and oligomer from monomer calibration curve
Conclusions
• Determination of isocyanates is complex
• Filter sampling of paints may be OK with short sampling times and field extraction
• Bulk product calibration or monomer calibration - choice depends on product variability, availability of product calibrants