sampling and analysis of isocyanates used in paints robert p. streicher national institute for...

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