Results and discussion
SEM images of the separate layers
that compose 1863+ and 9101E respirators are presented in Fig. 3 and
Fig. 4, respectively.
Filtration material used in both tested respirators was composed of pure
polypropylene (PP), however investigation of the 1863+ respirator
filtration fabric revealed that the mask consist of four layers, each of
them is composed of the PP fibres of different diameters woven in
different density (Fig. 3). Filtration media that composes 9101E
respirator consists of three layers, inner and outer fabric seems to be
built of the fibres of similar diameter and density, while middle layer
is composed of the fibres of significantly smaller diameters (Fig. 4).
Any holes or cracks in the fibres structure are not visible therefore
one can conclude that applied irradiation doses do not effect morphology
of the fibres used for the respirators production.
The investigation of the thermal properties of respirators irradiated
with different EB doses was carried out in order to determine influence
of radiation on thermal stability. Filtration materials can undergo
degradation or cross-linking under
irradiation. In case of the 1863+
respirator small degradation of the material was confirmed. The higher
irradiation dose was the higher drop of onset temperature and
temperature of the maximum in the loss weight rate was observed (Fig. 5
and Fig. 6). However, observed decrease of the characteristic
temperatures is not big therefore degradation of the filtration material
under EB irradiation is minimal. Thermal decomposition of the material
is single step process with one peak in temperature
~445-460 oC which is characteristic
for PP degradation (Fig. 6).
Completely deferent phenomena in the thermal degradation of the 9101E
filtration material is observed. Onset temperature determined for the
filtration materials increase with irradiation dose (Fig. 7).
Thermograms obtained for control sample and the sample irradiated with
12 kGy are characteristics for non-oxidized PP, while peak present in
the thermogram of the sample irradiated with 25 kGy is typical for
oxidized PP (Fig. 8) [15]. Therefore, one can conclude that
oxidation of the PP took place in the irradiation process and the higher
dose was applied the level of PP oxidation was higher. PP oxidation lead
to increase of the material thermal stability and can be connected with
reduction of the third type hydrogen and formation of carboxylate salt
(COO-N-Me3Ph) in oxidized PP [16].
It was observed decrease in filtration efficiency for irradiated
respirators in comparison to the control samples. Decrease in filtration
efficiency observed for respirators irradiated with both doses was
similar. Moreover, for both respirators it was observed that filtration
efficiency decreased with the increase of the particle diameter (Fig.
9). The main mechanical mechanism of deposition for nanoparticles is
diffusion (Brownian motion). When the particle diameter increases, the
Brownian motion are less intense, thus and diffusional mechanism becomes
less important in process of particle deposition, which explain the
observed phenomenon.
Decrease in filtration efficiency may result from
elimination of the electric charge
from the PP fibers in the irradiation process. To support this theory
conditioning of the non-irradiated samples of respirators in isopropanol
(IPA) vapours was applied to remove the electric charge from the surface
of studied filtrating materials. Obtained results confirmed that drop in
filtration efficiency for the irradiated filters is connected with the
elimination of the electric charge from the fiber surface (Fig. 10).
Baseline filtration efficiency was very high for both respirators:
99.7% for 1863+ respirator and 90.2% for 9101E respirator, whereas
after irradiation with both doses filtration efficiency dropped to 62%
(average value for droplets and particles filtration) for respirator
1863+ irradiated with 12 kGy and similar value 66% was obtained for
this respirator irradiated with 25 kGy. Even more significant decrease
in filtration efficiency to 42% for respirators irradiated with 12 kGy
and 44% for masks irradiated with 25 kGy was observed for
9101E respirator (Fig. 11). The
decrease of filtration efficiency observed for control samples
conditioned in IPA was similar to the drop in filtration efficiency
determined for irradiated samples what supports theory that irradiation
eliminate electric charge from the surface of PP fibres. Moreover,
filtration efficiency observed for irradiated samples conditioned
additionally in IPA remained almost at the same level.
Additionally, pressure drop across the filtrating materials was
determined for each respirators do investigate influence of the
irradiation on the integrity and stability of the filtration materials
(Fig. 13). Small decrease of the pressure drop across the filtrating
materials after irradiation was observed for both respirators. However,
observed differences in pressure drop for control and irradiated samples
were so small that can not be connected with the changes of the
filtrating materials structure.