Page 423 - Book Hosokawa Nanoparticle Technology Handbook

7.2 NANOPARTICLES AND ENVIRONMENT FUNDAMENTALS
7.2.6.1 General industrial processes particles has a distinct bimodality, one with the finer
The sources of emission of unwanted nanoparticles peak at around 10 nm and the coarser one at around
in general workplaces are categorized as fumes 1 m. The former results from within the grinder
from hot processes (e.g., smelting, refining, and motor and the volatilization or combustion of
welding) and from (incomplete) combustion amenable ground substrate and/or grinding materials,
processes. Favorable conditions required for the the latter from the mechanical abrasion and attrition.
generation of nanoparticles are found in workplaces However, the resultant total concentration on the
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where there is (1) presence of vaporizable material, order of 10 particles/cm is not so high.
(2) sufficiently high temperature to produce enough
vapor, followed by condensation to form an inde- 7.2.6.2 Industrial processes with cleanrooms
pendent aerosol, and (3) rapid cooling and a large Cleanrooms and associated controlled environments
temperature gradient. (e.g., in the case of an ISO Class 3 cleanroom, the
There have been so many studies on occupational maximum permissible airborne particle concentration
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exposure to fine particles in the field of public health. is less than 10 particles/m for particles with the size
In general, high spikes of nanoparticle concentration of 0.1 m or larger, while the airborne particle con-
are observed during active operations, followed by a centration in ordinary indoor environments is on the
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gradual decay after the operation, primarily because order of 10 particles/m or higher) are usually
of coagulation, evaporation, dilution, and/or deposi- adopted to avoid particle contamination in industrial
tion. The fraction of the total number of nanoparticles processes where precision products such as engi-
generally decreases, whereas that of the number of neered nanoparticles, semiconductors, and other elec-
submicrometer particles increases with time and dis- tronic or optical devices are fabricated because the
tance from the point of emission. In order to accu- deposition of particles onto product surfaces causes
rately estimate exposure, the effects of spatial and their yield reduction and quality deterioration. The
temporal changes will need to be evaluated. emission sources in cleanroom environments are tab-
Therefore, it is important to identify the time required ulated in Table 7.2.2. Since some of the listed emis-
for the concentration to decline to the normal or sion sources emit trace amounts of nanoparticles,
background levels. these nanoparticles are not regarded as particulate
As an example of reports on grinding processes, contaminant but as chemical or molecular one. In this
Fig. 7.2.11 shows the case where a steel substrate was section, these nanometer-sized solid substances
ground upon using a high-speed grinder [1]. From the formed on solid surfaces by chemical reaction are
figure the distribution of concentration of generated also included.

Figure 7.2.11
Size distribution of nanoparticles generated when a steel plate was ground with a high-speed grinder.

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