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
O
C
N
N
C
O
HDI
O
C
N
CH3
CH3
N
C
H2
H3C
IPDI
C
O
Polyisocyanate Species
Found in Paints
N
O
C
N
O
O
C
C
N
N
C
O
O
C
N
N
HDI Isocyanurate
C
O
Polyisocyanate Species
Found in Paints (cont.)
N
O
N
C
NH
O
C
C
O
O
C
HN
N
N
HDI Biuret
C
O
Polyisocyanate Species
Found in Paints (cont.)
O
O
C
C
N
N
N
N
C
O
HDI Uretidinedione Dimer
C
O
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 Aerosol
Particle 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