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.