Page 421 - Book Hosokawa Nanoparticle Technology Handbook

7.2 NANOPARTICLES AND ENVIRONMENT FUNDAMENTALS
The former does not entrain impurities in water when method (positive in this case) [7]. This method
it is fed into indoor spaces of interest. Meanwhile, the atomizes liquid fed to a tip of a capillary electrode to
latter has the drawback that NVRs are suspended in form fine droplets with large amounts of charge by
spaces to be humidified by feeding water via spray- applying high voltage between the tip and the down-
ing and sonication. The NVRs in tap water include stream counter electrode. When water in the gener-
colloidal particles and soluble fraction such as sili- ated droplets evaporates and their surface charge
cates, sulfates, carbonates, and chlorides. The size of density attains the charge limit called “Rayleigh
can be estimated from the fol- limit”, this phenomenon induces their self-fragmenta-
p
NVR particles, D _ r
lowing equation: tion followed by the formation of a high concentration
of cluster ions. In this figure, the high-mobility peak
C on the right side corresponds to the cluster ions. These
D pr D pm 3 m (7.2.1) ions are 2 nm in size assuming that they are singly
_
_

r
charged. Meanwhile, another peak on the figure
results from NVR in water and then its height
where Dp _ m is the droplet size, C the mass fraction of increases with the increase in the fraction of tap water
NVR in the droplets, the droplet density, and the in the fed liquid. The electrical-mobility-equivalent
m
r
NVR particle density, respectively. Assuming that size of NVR particles measured by differential
3
2 m-sized droplets ( 1,000 kg/m , 2,500 mobility analyzer (DMA) – condensation nucleus
r
m
3
kg/m ) are formed by an ultrasonic nebulizer, and the counter (CNC) method ranges from 10 to 20 nm and
4
mass fraction of NVR in city water is 100 ppm (10 ), their concentration is on the order of 10 4
the NVR particle size, D p_r is estimated to be 68 nm. particles/cm . Comparing the forementioned electri-
3
Recently, a wide variety of negative ion genera-
tors using the Lenard effect, corona discharge, cal mobility distribution with the particle size one, the
NVR nanoparticles are estimated to hold about 100
UV/photoelectron emission and electrospray have charges.
been commercialized and attention has been
focused on features such as air purification and 7.2.5.3 Laser printers and photocopiers
physiological activation [6]. Among them there are Accompanying the recent proliferation of comput-
the ion generators that atomize water based on the ers, the use of inkjet printers and electrophoto-
Lenard effect and electrospray form the NVR parti- graphic machines such as laser printer and
cles as by-product in addition to ion products if the photocopier is becoming common in homes as well
supplied water contains nonvolatile impurities. as offices. It has been reported that these devices
Fig. 7.2.9 shows an example of electrical mobility
distribution for ions generated by the electrospray emit various sorts of pollutants. The eco-friendliness-
oriented standards such as Blue Angle Standard [8]
regulate the maximum permissible limits of ben-
zene, styrene, total volatile organic compounds
(TVOC), ozone and particles. As the regulation of
particles is based on the emitted mass per hour,
mainly the relatively coarser particles such as toner
and dust adhering to paper have been targeted.
However, some reports have revealed that nanoparti-
cles are emitted from inkjet printers or laser printers
[9]. Fig. 7.2.10 depicts the size distribution of parti-
cles emitted from a laser printer measured by a scan-
ning mobility particle sizer (SMPS). As seen in the
figure, nanoparticles with a peak diameter of around
30 nm are generated in printing mode, whereas the
emission in the case of feeding paper without print-
ing is about one third of the normal printing mode.
Furthermore, these particles were dried by passing
them through a diffusion dryer because they are
thought to originate from the nucleation of water
vapor emitted from papers in the fixation process.
As a result, it was found that most particles formed
in the paper feed mode evaporated and then van-
ished, while particles in the printing mode contained
nonvolatile components as well as water. From these
Figure 7.2.9 results it is anticipated that the particles are derived
Electrical mobility distribution of cluster ions and nonvolatile from styrene remaining in the toner even though
residue (NVR) particles generated by the electrospray method. their composition has still not been identified.
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