Chapter 22 Materials Selection and Design Considerations

W118 • Chapter 22 / Materials Selection and Design Considerations
Computation of
exposure rate of a
hazardous chemical
that is diffusing
through a chemical
protective glove
material
C A
C B
substitutions for
sion for re: and into Equation 22.28, we obtain the following expres-
r e � DAS A
/
(22.29)
Table 22.5 also includes, for these several glove materials, values of that were
determined using Equation 22.29.
At this point, a key question is: What is an acceptable and safe exposure rate?
Based on airborne exposure limits set by the Occupational Safety and Health Administration
(OSHA) of the US, the maximum allowable to methylene chloride
is approximately 1 g/h.
Now let us examine and compare computed breakthrough times and exposure
rates for the several glove materials, as listed in Table 22.5. First of all, with regard
to exposure rate, two of the seven materials meet or exceed the standard set by
OSHA (viz., 1 g/h)—viz. multilayer (Silver Shield TM ) and Viton TM re rubber (with re values of 0.43 and 0.35 g/h, respectively). Relative to breakthrough time, the multilayer
material has the longer tb (24 h versus about 1 h for the Viton rubber). Furthermore,
the multilayer gloves are considerably less inexpensive (at $US4.19 per
pair compared to $US72.00 for Viton rubber, Table 22.5).
Therefore, of these two glove materials, other relevant characteristics/properties
being equal, the one of choice for this application is multilayer Silver Shield. It
has a significantly longer breakthrough time and is much less costly than the Viton
rubber material, whereas there is very little difference between their exposure rates.
The photograph of Figure 22.27 shows a pair of Silver Shield gloves.
It should be noted that calculated breakthrough time values presented in Table
22.5 assumed the glove material had no previous exposure to methylene chloride.
For a second application, some of the methylene chloride that dissolved in the glove
during the first exposure probably remains; thus, the breakthrough time will be much
shorter than predicted for an unused glove. For this reason, CPC gloves are often
discarded after one usage.
One final comment is in order: one should always consult an industrial hygiene
specialist when selecting chemical protection clothing. These specialists are experts
as to what materials are suitable for exposure to specific toxic chemical substances,
Figure 22.27 Photograph of Silver Shield
multilayer chemical protective gloves.
(Photograph courtesy of North Safety
Products, Anjou, Quebec, Canada.)
r e