INTERNATIONAL JOURNAL OF
OCCUPATIONAL SAFETY AND ERGONOMICS 1997, VOL. 3, NO. 3-4, 141-149
Investigation of Blended Fibre Filtering Materials
Izabella Kruciriska
Stawomir Zakrzewski1
Irena Kowalczyk
Jadwiga Wisniewska-Konecka
Technical University of Lodz, Poland
1 Central Institute for Labour Protection, Poland
Five variants of mixtures of different synthetic fibres at different area ratios were manufactured
into needled nonwovens intended to be used as a filtering material for respiratory protection.
Two variants were produced according to an earlier patent, and the contents of the other three
was completely new. Samples of the nonwovens were tested for sodium chloride particles
penetration and for breathing resistance. The results showed that one variant of a nonwoven,
designated PP/PPFM, had very valuable filtering properties and that those properties were
stable in time.
filtering material
respiratory protection
nonwoven
triboelectrical exchange
aerosol penetration
breathing resistance
1. INTRODUCTION
Electrically charged filter m aterial has had a history of several decades, since the
Hansen filter had been invented (Brown, 1993; Gradori, 1984; Hansen, 1931; W arych,
1994). The advantage o f this type of m aterial results from the fact that the charge on
the fibres considerably augments the filtration efficiency w ithout increasing the
resistance to air flow. These features, that is, high filtration efficiency and low resist­
ance to air flow play a key role in the designing of filtering m aterial m anufacturing
process. Various types of electrically charged m aterials are available (Brown, 1993;
W arych, 1994) on the m arket at present. They can be classified into the following
groups:
materials charged by corona
- split-flbre m aterial,
- m aterial charged as a whole,
materials charged by induction
- fine-fibre m aterial provided from a solution,
- fine-fibre m aterial provided from a melt,
Correspondence and request for reprints should be sent to Slawomir Zakrzewski, Central Institute for
Labour Protection, ul. Wierzbowa 48, 90-143 Lodz, Poland.
142
I. KRUCINSKA, S. ZAKRZEWSKI, I. KOWALCZYK, AND J. WISNIEWSKA-KONECKA
materials charged by triboelectrical exchange
- resin wool m aterial,
- mixed-fibre m aterial.
A corona discharge is a common means of applying electric charges to polymers
(Ando, T akahashi, Togashi, & Okum ara, 1990). The first type of m aterial from this
group is m ade from a corona-charged sheet of polypropylene shredded into fibres.
This m aterial was described in detail by van T urnhout, van Bochove, and van
Veldhuizen (1976). The second type of electret material is also made from polypropylene
fibres but is charged by a corona discharge in felted form (Baumgartner, Loeffler,
& U m hauer, 1985).
Induction o f charge is the second form of an electric charging mechanism used in
filtering m aterial technology. Fine-fibre m aterials provided from a solution or a melt
belong to the group of m aterials that are charged during electrostatic extrusion.
M ore inform ation concerning the fine-fibre filtering m aterial produced by electro­
static extrusion of a solution of polycarbonate is presented in the literature (Brown,
1993; Schmidt, 1980). A description of m aterials produced by electrostatic extrusion
of a m elt of polypropylene has also been published (Trouilhet, 1981).
The last group consists of a m aterial charged by triboelectrical effect. The oldest
filter m aterial, charged by triboelectrical charge exchange, was m ade from a m ixture
o f wool noils and particles of natural resin. This m aterial is called H ansen’s filter
(Hansen, 1931) and has been described in detail (Felthman, 1979). D uring carding,
wool fibres develop a positive charge and resin particles develop a negative one. In
the 1980s, Brown and coworkers in the United Kingdom and G radon in Poland
elaborated a new class of filtering m aterials called blended fibre filter m aterials
(Brown, 1984; Brown, 1986; G radon, 1984; Smith, East, Brown, & W ake, 1988),
which are related to the concept of the Hansen filter. These m aterials consist of
a m ixture of two synthetic fibres charged by triboelectrical charge exchange during
carding. One com ponent of the m ixture develops a positive charge and the other
develops a negative one. The m ixture invented by Brown (Brown, 1986) comprises
a blend of clean fibres of a substituted addition polymer containing chloride and
cyanide groups. A nother m ixture developed by Carl Freudenberg Com pany (Brown,
1993) consists of bicom ponent polypropylene-polyethylene and polyacrylonitryle
fibres (PP-PE/PAN ). In Poland, research on blended fibre mixtures, initiated by the
Central Institute for L abour Protection, has been carried out for the last 2 years.
The first positive results of the experiments on blended fibre filtering m aterial
were obtained and published 2 years ago (Krucinska, Zakrzewski, K ot, & Brochocka,
1995). According to those results, the best efficiency of filtration was achieved for the
filtering m aterial composed of polyvinyl chloride fibres and polypropylene fibres
blended together in a 50/50 area ratio. On the basis of those results the following
research was undertaken to clarify the influence of fibre m ixture content on the
efficiency o f filtration. The second series of investigations showed that there was
a problem with the repeatability of aerosol penetration results characterizing the
investigated filtering materials. Therefore, this research was aimed at finding the best
type of m ixture of fibres, which would guarantee the stabilization o f charge on
m aterials m anufactured at various ambient conditions and preserve the same level of
charge after a certain period o f time.
BLENDED FIBRE FILTERING MATERIALS
143
2. MATERIALS
M aterials consisted o f two variants of filtering m aterial produced from fibrous
m ixtures, according to Brown and Carl Freudeberg Com pany’s patent specification.
Additionally, three new blends of fibres were used for filtering m aterial preparation.
The characteristics of the fibres used are presented in Table 1 and the characteristics
of nonwovens m anufactured according to the procedure described further are listed
in Table 2.
TABLE 1.
Characteristics of Fibres
Symbol
of Fibres
Type of Fibres
Polypropylene
Shape of
Cross-Section
Linear Density
(dtex)
Length
(mm)
40
PP
round
2 .8
Polyvinyl chloride
PVC
bilobal
4.2
60
Polyacrylonitryle
PAN
round
3.3
60
5.0
60
Polypropylene/
Polyethylene
PP/PE
round
Component PPFM
PPFM
round
Note. PPFM— polycondensational polymeric fibrous material.
3. MANUFACTURING PROCESS
Commercially produced synthetic fibres are normally covered with “spin finish,”
which is a m ixture of a lubricant and an antistatic agent. To produce triboelectrical
effect, this spin finish should be removed before carding. This can be done before or
after the opening process. Any of the usual processes of textile scouring could be
used, for example, detergent, alkali, or solvent scouring. In our research, all fibres
were scoured using a non-ionic detergent and they were well-rinsed afterwards. A fter
washing, fibre flocks were opened by a rag machine. The prepared m ixtures of fibres
were carded into fleeces and then the fleeces were needled into felts. As a result of
the m anufacturing process, a set of blended fibre filtering m aterials were obtained.
TABLE 2.
Characteristics of Blended Filtering M aterials
Type of Mixture
Area Content
of Fibres
Number
Mass Density
(g/m2)
PP/PVC
PP/PVC
30/70
series I
494
30/70
PP/PPFM
PP/PPFM
30/70
series II
series I
477
533
30/70
50/50
series II
series I
PP/PPFM
523
PP/PPFM
50/50
series II
534
554
PP-PE/PAN
30/70
series I
546
Note. PP— polypropylene, PVC— polyvinyl chloride, PP-PE/PAN— polypropylene polyethylene
polyacrylonitryle, PPFM— polycondensational polymeric fibrous material.
144
I. KRUCINSKA, S. ZAKRZEWSKI, I. KOWALCZYK, AND J. WISNIEWSKA-KONECKA
4. EXPERIMENTS AND RESULTS
The m ain requirem ent for the new type of particle filter m aterial to be approved as
a raw m aterial for antidust respiratory protection, is that it should not exceed the
defined value of standard aerosol particles penetration. The filtering m aterial should
satisfy the requirements in the EN 143 standard (Comite Europeen de N orm alisation
[CEN], 1990). This standard describes the methods of evaluating the initial penetration
of standard aerosols, breathing resistance as well as other properties im portant for
the user. Values o f penetration and breathing resistance are used as a basis for filter
classification. According to the EN 143 standard, one of the standard aerosols is the
sodium chloride aerosol, which is produced as a polydisperse aerosol with a mass
m edian particle diam eter of about 0.6 pxn. Tests are carried out at the specified
volume air rate of 1.6 dm 3/s or 0.8 dm 3/s for filters intended to be used with
a twin-filter facepiece, taking care that the whole working surface of the filter takes
part in the filtration process. Penetration is calculated according to the form ula
p = — x 100%
Ci
where,
Ci is the sodium chloride concentration before the filter,
C2 is the sodium chloride concentration after the filter.
To assess the m anufactured filtering m aterial, samples were tested for sodium
chloride particles penetration and for resistance to air flow. In each case, three
m easurem ents were m ade and the results were averaged. To com pare different
variants o f the m anufactured m aterial, tests of penetration were carried out at the air
linear velocity of the aerosol passing through the sample equal to 0.1 m /s. The
diagram of the rig for testing penetration is presented in Figure 1. Sodium chloride
w ater solution dispersion was produced in a Collisson atomiser at conditions
8
Figure 1.
Diagram of measuring rig for sodium chloride particles penetration.
Note. 1— co m p re sse d air, 2— filter, 3— ae roso l g e n e ra to r, 4— m e a s u rin g ch a m b e r, 5
sam p le ,
6—
m a n o m e te r, 7— valve,
8
— flow m eters, 9— flam e p h o to m e te r.
tested
BLENDED FIBRE FILTERING MATERIALS
145
allowing to obtain an aerosol of a mass m edian of particle diam eter 0.6 /an, after
w ater evaporation. The geometrical average value of the particle diam eter was equal
to dg = 0.18 /an, with natural logarithm o f standard deviation In <Sg = 0.65. The tested
samples— circles— of 0.1 m in diam eter allowed to obtain a volume air rate equal to
0.8 dm 3/s, the same as for twin filters according to CEN (1990). The m easurem ents
of resistance to air flow were carried out at the same linear velocity of air flow
0.1 m /s. To investigate the influence of preconditioning on filter efficiency, before
testing, the samples were exposed for 24 hrs to atmospheres of three relative
humidities: 20, 65, and 100%. To investigate the stabilization of the filtering m aterial
effectiveness in time, all tests were repeated after 5 m onths. D uring the period of
storage, ambient room conditions were m aintained.
The results of aerosol penetration and resistance to air flow of the tested filtering
m aterials obtained in both series of measurements are shown in Table 3. Table 4
presents the results of sodium chloride penetration and resistance to air flow for
filtering materials m anufactured according to Brown and Carl Freudenberg Com pany’s
invention, as well as for other tested blended nonwovens.
TABLE 3. Sodium Chloride Penetration, Breathing Resistance, Mass Density, and Humidity
of Fleeces for Two Variants of Fibre Blends Manufactured at Specified Ambient
Conditions
Type of
mixture
PP/PVC
Penetration
at 0.1 m/s (%)
4.87
Breathing
resistance
at 0.1 m/s (Pa)
Mass density
(g/m2)
Humidity of
fleeces (%)
Ambient
conditions
temperature
and relative
humidity
30.0
494
5.66
21 °C
30/70
series
25%
1
PP/PVC
1 1 .2 0
43.0
477
6.23
30/70
25 °C
23%
series II
PP/PPFM
1 .2 0
37.7
523
1.28
30/70
series
21 °C
17%
1
PP/PPFM
30/70
1.90
35.0
533
2.23
23.5 °C
17%
series II
Note. PP— polypropylene, PVC— polyvinyl chloride, PPFM— polycondensational polymeric fibrous material.
146
I. KRUCINSKA, S. ZAKRZEWSKI, I. KOWALCZYK, AND J. WISNIEWSKA-KONECKA
TABLE 4.
Sodium Chloride Penetration and Breathing Resistance of Tested Filtering
M aterials
Breathing resistance at 0.1 m/s (Pa)
Penetration at 0.1 m/s (%)
Type of
mixture
dry
65%
after 5 months
new
after 5 months
new
wet
dry
65%
wet
dry
65%
wet
dry
65%
wet
PP/PVC
30/70
5.18
4.87
6.24
4.16
4.49
5.60
32
30
28
33
34
30
1.19
1 .2 0
1.67
0.79
0.81
1.90
38
38
34
38
39
35
1.81
2.04
2 .2 2
1.87
1.48
2.35
45
42
47
48
51
43
32.80
29.50
36.70
—
—
—
36
40
39
—
—
—
494 g/m 2
PP/PPFM
30/70
523 g/m 2
PP/PPFM
50/50
554 g/m 2
PP-PE/PAN
30/70
546 g/m 2
Note. PP— polypropylene, PVC— polyvinyl chloride, PP-PE/PAN— polypropylene polyethylene polyacrylonitryle,
PPFM— polycondensational polymeric fibrous material.
6. DISCUSSION
The results presented in Table 3 point to the lack of m easurem ent repeatability of
sodium chloride penetration, particularly in the case of a m ixture of polyvinyl
chloride and polypropylene fibres. To find the reason, further investigations were
undertaken and the relative air humidity during production and the hum idity of
fleeces were controlled. It turned out that the penetration of filtering m aterials
composed o f PP/PV C fibres, m anufactured at similar ambient conditions but from
fleeces of different humidity, changes. This might be a consequence of the greater
discharging caused by the great am ount of m oisture in fleece. This would m ean that
the final result is very sensitive to the humidity of raw m aterial and the whole
process of m anufacturing should be carried out in conditions of very low relative air
hum idity. Therefore, further investigations were undertaken to find out another
m ixture, which would give better results of penetration, less dependent on the
hum idity of fibres. The new m ixture composed of polycondensational polymeric
fibres (PPFM ) and polypropylene fibres (PP) was m anufactured to produce filtering
m aterial and was tested for sodium chloride penetration and breathing resistance.
The results of the tests of this m ixture are presented in Table 3 and illustrated in
Figure 2. The new m aterial is characterized by better filtration efficiency, which is
m uch less sensitive to the changes of m oisture content in it.
BLENDED FIBRE FILTERING MATERIALS
147
s? 10-
c
--------------------1------ — ......
1-28
—
i------ r —
2.23
,------------------------------------
5.66
6.23
Humidity of fleeces (%)
□ PP/PPFM-series I
□ PP/PPFM-series II
■ PP/PVC-series I
□ PP/PVC-series II
Figure 2. Influence of humidity of fleeces composed of various mixtures of fibres
on aerosol penetration.
A nother param eter characterizing the stability of the filtering m aterial properties
is the penetration of aerosol measured after a certain time. The results presented in
Table 4 and in Figure 3 indicate that in the case of two mixtures, that is, PP/PVC
and PP/PPFM , sodium chloride penetration does not increase after 5 months. On the
contrary, a slight decrease of sodium chloride penetration, especially for samples
exposed, before testing, to atmosphere of relative humidity of 65%, was observed.
New
After 5
months
■ PP/PPFM-50/50 □ PP/PPFM-30/70
Figure 3. Sodium chloride particles penetration of blended fibre filtering
m aterials measured directly after fabrication and after five months later.
148
I. KRUCINSKA, S. ZAKRZEWSKI, I. KOWALCZYK, AND J. WISNIEWSKA-KONECKA
The com parative analysis o f penetration of sodium chloride particles through the
filter m aterials composed of various type of mixtures indicates that the best results
were obtained for filtering m aterials m ade of a mixture of PPFM and PP fibres, as
illustrated in Figure 4. The good filtering properties developed during the carding of
this m ixture are probably less sensitive to the humidity of the m aterial than the
filtering properties of the PP/PVC mixture. However, the relative air hum idity during
the production of this m ixture was lower than the relative air humidity during the
production of the PP/PV C m ixture and this fact, as well as some variability of mass
density, m ight also contribute to the final result. Penetration of the filtering m aterial
m ade of the PP/PPFM m ixture depends on the content of the m ore conductive fibres
in the m ixture. The higher the content of the m ore conductive fibres (PPFM ), the
lower the value of aerosol penetration. In the case of the m ixture of PPFM and PP
blended in a 70/30 ratio, penetration of aerosol takes the relatively low value of
1.20% for samples exposed to atmosphere of relative humidity equal to 65% before
testing. Decrease of content of the m ore conductive PPFM fibres in a m ixture to the
value of 50% , causes an increase of sodium chloride penetration to 2.04%.
i
PP/PVC
30/70
i -------
PP/PPFM
30/70
PP-PE/PAN
30/70
Figure 4. Comparison of aerosol penetration
of various blended fibres filtering m aterials.
The aim of the last part of the research was to find out if the relative hum idity of
atm osphere to which the samples are exposed before testing, influences the filtering
properties of the m aterial. In all cases, an increase of sodium chloride penetration for
the samples exposed to saturated air (100% r.h.) was observed. This increase depended
on the type of m ixture and changed from 8.8% for a m ixture of PP/PPFM blended
in a 50/50 ratio to 39.1% for a m ixture of PP/PPFM blended in a 30/70 ratio.
Despite the fact th at the change of penetration for samples exposed to saturated air
was so distinctive, the filtering m aterial composed of PP/PPFM fibres was still
relatively low. This could not be said about the samples constituted of biocom ponent
polypropylene-polyethylene and polyacrylonitryle (PP-PE/PAN) fibres and samples
m ade from the m ixture of PP/PVC fibres exposed before examination to saturated
air. The change of resistance was negligible in every case.
BLENDED FIBRE FILTERING MATERIALS
149
7. CONCLUSIONS
M anufacturing nonwovens using certain fibre blends considerably augments the
filtration efficiency w ithout changing the resistance to air flow of investigated
m aterials in comparison to the filtration properties of one-component nonwovens or
other blends of fibres. The improvement of particle capture efficiency results from
the charge development on the surface of fibres by triboelectrical effect. The filtering
efficiency o f the m ixture of polypropylene and polyvinyl chloride fibres is probably
m uch m ore sensitive to the humidity of raw m aterial and the relative hum idity of
ambient air than the filtering efficiency of the mixture of polycondensational polymeric
fibrous m aterial and polypropylene fibres (PPFM /PP). The valuable filtering proper­
ties of this m aterial are stable in time. All this reflects that filtering m aterials m ade
from such a m ixture can be considered a raw m aterial for m anufacturing filters for
respiratory protection.
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