2. Materials and methods
To compare influence of EB irradiation on different respirators two
different polypropylene masks: 3M 1863+ conforming to the high FFP3
standard and 3M 9101E conforming to the FFP1 standard were chosen for
the investigation. Masks were sealed in plastic bags and irradiated with
doses 12 kGy and 25 kGy in air atmosphere, at ambient temperature.
Electron beam irradiation of samples was carried out using 10 MeV, 10 kW
linear electron accelerator “Elektronika”. Delivered doses were
confirmed using B3 radiochromic foil dosimeter measured with a flat bed
scanner and RisoScan software, with uncertainties evaluated at 8%. The
masks were irradiated in a single layer to minimise dose gradient and
ensure uniformity of delivered doses. For both tested samples the dose
increase inside the samples was below 1 %.
Doses were selected taking into account assumption the possible
variability in viral load in used masks and its random distribution
among products. On the base of microbial contamination found in surgical
masks one can realise that the standard deviation of the bioburden is
higher than the mean N: 47 ± 56 cfu/ml/piece for inside mask area and
166 ± 199 cfu/ml/piece from mask outside area [14]. This results
from the variability of environments where masks are used and
differences in the level of the bioburden. On the base of the maximum of
1000 microorganisms should be present in the product, decontamination
dose (a dose required for 5 or 6 order of magnitude reduction of
bioburden) was calculated as 12 kGy [14]. Moreover, masks were also
irradiated with standard sterilization dose 25 kGy which is defined as
sterilization dose according to VDmax method given in
ISO 11137-2 standard “Sterilization of health care products —
Radiation — Part 2: Establishing the sterilization dose”.. Masks that
were not irradiated were used as control samples.
SEM images of the masks layers were obtained, using a Hitachi TM-100
scanning electron microscope with an accelerating voltage of 15.0kV.
Samples for the SEM examination were prepared according to a standard
procedure, fixed with conductive glue, and coated with a thin layer of
gold. The samples were examined at a magnification of 500×.
Thermogravimetric analysis (TGA) of masks samples was used to determine
the thermal stability and possible degradation of respirators materials
was conducted with a Q500 TGA (TA Instruments) thermogravimetric
analyser in the temperature range 37–700oC at a heat
ingrate of 10oC per minute, under a constant flow (60
mL/min) of nitrogen gas.
To
determine the initial separation efficiency of the tested respirators
samples before and after irradiation the high quality test bench MFP
NanoPlus (PALAS GmbH) was used. The main elements of this test-rig are:
UGF2000 generator which is able to generated nanoobject from liquid
solution, cascade of impactors which were used to cut-off the largest
particles (which were not object of this research), the bipolar
neutralizer Kr-85, used to ensure equilibrium charge distribution on
particles, DEMC classification column, pneumatic filtration chamber and
universal fluid condensation particle counter UF-CPC. The UF-CPC
together with the DEMC classification column form a system called
U-SMPS. It allows to classify and count particles in size range from 20
to 200 nm. Scheme of MFP nanoPlus is presented in Fig.1.
Fractional and overall filtration effectiveness was determined for solid
particles (KCl nanocrystals) as well as oil nanodroplets of
Di-Ethyl-Hexyl-Sebacat (DEHS). For tests circular samples with diameter
of 60 mm were punched from tested respirators. All filtration test were
performed at the air flow rate 95 L/min and the air face velocity 0.559
m/s. For this experiments It was required to performed experiments in a
sequence of measurements without (upstream) and with (downstream) a
filtrating material in the tested chamber. One series consisted of two
upstream measurements and two downstream measurements. There were
carried out two series of measurements for each masks material. Next,
the average value of the filtration effectiveness was calculated and
presented on diagrams below. The time of a single measurement of
upstream and downstream was always 380 sec. For this time, the interval
between measurements was included and it was 60 sec. Such a long time of
a single measurement was necessary to correctly classify and count
nanoparticles. Moreover, during the tests there were also determined the
pressure drops across the tested materials and their initial overall
filtration efficiency. Size distribution of the generated aerosols used
in the experiments is presented in Fig. 2.