Page 5 - Combined_4_OCR

*
its sister isocyanate polymethylene poly­ solvents such as methylene chloride, The volume of air exhausted must be
phenylisocyanate (PDI). These isocyan­ 1,1,1-trichloroethane and acetone are such that hood capture velocities are
ates are generally considered a lesser used for this purpose. The MAC values adequate to control the vapors released.
problem than TDI, based primarily on for these solvents are 500 ppm, 350 ppm The type of hood used depends upon the
their comparatively lower vapor pres­ and 1,000 ppm, respectively. To pre­ particular application. The ventilation
sure (See Table 1). vent overexposure, adequate local ven­ system should be designed by experi­
Figure 5 is a summary of isocyanate tilation must be provided. Even trace enced engineers and the following de­
samples taken during environmental concentrations of the chlorinated sol­ sign principles should be followed:
studies since 1959. It is apparent that vents can cause difficulties when allowed (a) The process should be enclosed
despite engineering control methods and to pyrolize in open flames or upon hot as much as possible. Where en­
substitution of the less-volatile isocyan­ surfaces. The resultant acids not only closure is not practical, the ex­
ates, excessive concentrations can still cause distress to workers but severe haust hood should be located as
be found at foaming operations. corrosion damage to equipment can re­ close as possible to the source
sult (Michigan’s Occupational Health, of the vapor. Downdraft or side
Table Is Volume 13, Number 2). Acetone is a draft ventilation is preferred to
flammable solvent and rigid fire safety
Relative Vapor Pressure of TDI, MDI avoid drawing vapors through
and PDI precautions must be taken. the workers’ breathing zone.
Baffles and side shields should be
Temp °F TDI MDI PDI ENGINEERING CONTROL used to the fullest extent pos­
sible.
Manufacturing Operations — Proper
50 898 1.5 1 (b) Adequate capture velocity must
77 834 1.5 1 exhaust ventilation can maintain con­ be maintained at the point of
100 612 1.5 1 centrations of isocyanate and solvent vapor dispersal.
150 478 1.5 1 vapors at a safe level. Operations re­
200 358 l.S 1 quiring ventilation are the pre-polymer (c) Curing ovens must be of tight
250 319 1.5 1 preparation kettles, mixing equipment, construction to avoid leakage
300 277 1.5 I mold filling stations, curing ovens, and external hoods are required
crush rolls, and waste disposal points. at doors or conveyor openings.
TDI vapors have been detected in (d) Exhaust ducts should discharge
At normal room temperatures, MDI chopped-up bits of uncured urethane vertically above the buildings at
and PDI are practically without hazard foam as long as eight days after its an adequate height to insure that
because of their low volatility. Many manufacture. the exhausted fumes do not en­
producers of foam now substitute these
less volatile isocyanates for TDI as a
practical measure to minimize isocyan­
ate release.
The atmospheric contamination as­
sociated with the pouring-in-place ap­
plication as opposed to the spray appli­ /
cation is significantly different as /
indicated in Table 2.

Table 2°
Contamination of the Atmosphere in a
Sealed Room Resulting from 10-Minute
Spray or Pour Periods of TDI and MDI.

Atmospheric
Concentra-
Isocyanate Application tion,* PPM
TDI Internal Mix Spray 3.9
MDI Internal Mix Spray 0.4
TDI Internal Mix Pour 0.3
MDI Internal Mix Pour 0.004

* Represents both vapor and particulate
isocyanate compound.
TDI, MDI and PDI may cause der­
matitis of varying degrees. TDI may
cause skin sensitization in certain in­
dividuals while PDI can develop der­
matitis only in the highly sensitive or
allergic individual. MDI generally falls
between these two levels.
Solvents used for foam machine and
mold cleaning are also toxic. Generally,
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