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84
Handbook of Filter Media
first step is to assemble several layers of carded fibre into a 'lofty' (i.e. bulky) web
or 'batt'; this is then compressed into a denser structure by needle punching with
a mass of special barbed needles reciprocating at speeds up to 2000 strokes/
minute, as illustrated in Figure 3.1.
With perhaps 100 or more needle penetrations per square centimetre, the
effect is to entangle the fibres and to reduce the thickness of the web
substantially, to a degree that is controlled as desired. Punching may be on both
sides of the web instead of just the one, as in Figure 3.1; this improves the
uniformity of the felt.
Before needling, the web of loose fibres is prepared with great care, using the
traditional carding methods of the textile industry: several layers of carded fibre
are stacked on top of one another, according to the desired thickness and density
of the final needlefelt. Carding aligns the fibres along the length of the machine,
so that a stack of layers in parallel produces a felt that is far stronger in the
machine direction then transversely. Cross laying of alternate layers can
eliminate this directional difference, or even reverse it, depending on the angle
between consecutive layers.
Most felts are mechanically strengthened by needling, but an alternative, and
more specialized, technique employs a set of high-pressure water jets to fix the
fibres in place- a technique known as hydroentanglement.
3.3.'!. 1 Needlefelt properties
The relatively low tensile strength of a plain felt is significantly improved by
needling. Even greater strength can be achieved by forming the needlefelt
around an inner scrim, which is a single layer of very open woven mesh, as in
Figure 3.2. The scrim layer is placed within the pile of individual webs that make
up the felt; some felts are asymmetric in structure, with the scrim located
accordingly for optimum abrasion resistance. Formation around a scrim is the more
common structure of needlefelts for filtration, although scrimless felts are also
sometimes used. A scrimmed needlefelt is essential in the case, say, of filter bags that
are cleaned by a reverse jet of air, to expand the bag. The frequent and regular
expansions and contractions would be more than a plain needlefelt could tolerate.
The shape of the cross-section of the fibre is a significant factor in determining
the strength of a needlefelt. This has accordingly received considerable attention
from manufacturers seeking to meet the demanding conditions imposed on their
Figure 3.1. Principles of the needle felting process using barbed needles.
Figure 7.2. A typical scrim around which a felt is formed by needh' punching.
86
Handbook of Filter Media
products in applications varying from carpets to clothing, with filtration by
comparison generally providing a relatively small-scale market. Figure 3.3
shows the highly profiled form of Lenzing's P84 fibres, while various fibre shapes
are illustrated in Figure 2.5 of Chapter 2.
The fineness of the fibres in a needlefelt has a significant impact on filtration
efficiency, notably in respect of the concentration of particles in the exhaust from
bag house fabric filters. A paper by Dilger ~5~ summarizes the results of a
development programme by Du Pont utilizing fine fibres of both Nomex and
Teflon. The emission level of a 520 g/m 2 needlefelt of standard 2.2 dtex Nomex
fibres was 1.19 g/m 3 higher than that of a 500 g/m 2 felt of 1.1 dtex fibres.
In many ways, needlefelts would appear to be ideal for filtration, combining
the possibility of greater flexibility and versatility in construction, including the
ability to produce asymmetric forms, by exploiting variations of fibre diameter
and shape, plus the final felt density. Thereby it should be possible to achieve a far
more uniformly open surface and controlled in-depth structure than with woven
fabrics. On both of these counts, however, the reality falls short of the ideal, but
still provides a rich source of media of great industrial value, especially in dry
filtration for the collection of dusts.
The possibility of optimizing construction of a needlefelt to suit a particular
application is, in practice, limited by the practical realities of the textile industry,
with its commitments to fields other than filtration. Two factors must be borne in
mind: first, felt manufacturers are generally dependent on outside suppliers for
the fibres they require, and can only buy grades that the suppliers find it
economic to produce on the large scale implicit in their own manufacturing
processes; secondly, felting itself is also essentially a large-scale operation, and is
therefore inevitably geared to large markets.
Figure 3.3. Microphotograph showing the cross section ofLenzing's P84 fibres.
Non-woven Fabric Media 87
As for the uniformity of the surface and control of the in-depth structure of a
needlefelt, close inspection under magnification reveals something of the
underlying technical difficulties. Figure 3.4 is a microphotograph of a section cut
through a felt that has been needle punched on both sides. In Figure 3.4 the
points of penetration by the needles are clearly visible, revealing the orientation
of fibres imposed at a point of penetration: it also shows the yarns of the scrim
along the centre line of the sample. It is very possible that the needle holes may be
significantly larger than the pores in the rest of the structure.
For much of the last 30 years, needlefelts have been the dominant material for
filter media, especially for gas cleaning. However, they are now steadily being
replaced by the thermally bonded spun polymeric media, discussed in Section
3.5, which are capable of much finer degrees of filtration.
Initial enthusiasm for the seemingly endless potential of these then new types
of filter media in the early 1970s stimulated Wrotnowski 16~ to propose a
theoretical model to relate the pore size of a needlefelt to the diameter of the fibres
and the density of the felt. For a time, this relationship was used as a guide to the
ranges of needlefelt available- as shown for polyester and polypropylene
needlefelts in Table 3.1, but subsequent experience and material development
led to its being largely discarded in favour of the empirical summary of the
available fabrics, as discussed later.
3.3.7.2 Surface coatings
The finishing processes applied to needlefelts are much the same as for woven
fabrics, as discussed in Section 2.3.1.2. These include calendering and singeing,
the latter being illustrated in Figure 3.5, as techniques to modify the surface
finish, rather than adjusting pore size.
The coating of needlefelt fabric surfaces is a little more complex, and
sometimes it is difficult to draw the line between coated fabrics and the bonded
media discussed in Section 3.4, or between coated fabrics and the membranes
Figure 3.,I. Cross section through a needle felt. showing ttle scrim and also the fibre re-orientation caused by
the needling, at • 62 magnification.
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Non-woven Fabric Media
89
9 Supaweb OR aids release of oily cakes:
9 Supaweb CR greatly improves resistance to chemical attack;
9 Supaweb FR resists the effects of incandescent particle carry over.
The corresponding treatment processes employed by Fratelli Testori, a long-
established maker of filtration fabrics, include:
9 Novates, a coating of polyurethane on polyester or acrylic felts, which is
hydrophobic and oleophobic: it resembles a membrane, although the
finished pore size is only stated as 'below 15 l~m':
9 Mantes, a chemical treatment of the felt with a resin containing PTFE, for
application to acrylics, and high-temperature fibres such as aramid,
polyimide and sulfar (PPS), giving good chemical resistance:
9 Kleentes, which involves steeping the fabric in a chemical solution
containing PTFE and fluoride resins at high concentrations, following
which the fabric is dried and heated to fix the fluorides on the fibres; used
Figure 3.5. The surface of a heavily singed needle felt.
Figure 3.6. 'Ravlex' coating on spunbonded polypropylene needle felt.
90 Handbook of Filter Media
on polyester or acrylic fibres to give good cake release and protect from
chemical activity;
Rhytes, which is made in a similar fashion to Kleentes but is applied to
higher temperature fibres, to improve the high-temperature performance,
and reduce chemical attack.
Madison has also developed an abrasion-resistant coating in its Tuf-tex
coatings for polypropylene, nylon and PET substrates (woven as well as non-
woven). These are thermosetting resins sprayed or knifed onto the surface,
giving not only abrasion resistance, but also improved dimensional stability.
3.3.7.3 Needlefelt fabrics
The type and range of needlefelt fabrics available are well illustrated by the
data of Table 3.2. This shows the main products of Andrew Textile, a long-
established needlefelt maker (and sister comapny of Webron Products), in the
company's standard range, for four different fibres: polyester, polypropylene,
homopolymer acrylic, and copolymer acrylic. A similar table, Table 3.3, shows
the corresponding data for a range of higher temperature polymers: aramid, PPS,
PTFE and polyimide. These materials have porosities between 72 and 8 70 and
Table 1.2 Standard needlefelts a
Product
Weight Thickness b Density Air Breaking Elongation c Lineal Shrinkage f.
(g/m e) (mm) (g/cm 3) permeability c strength d 1%) I%) C
Polyester
T350TFS 350 1.35 0.26 350 1000 3 3 170
T4OOTFS 400 1.40 0.2 7 260 1100 3 3 170
T450TFS 450 1.45 O. 31 220 1200 3 3 170
T5OOTFS 500 1.75 0.29 190 1200 3 3 170
T550TFS 550 1.80 0.31 165 1 300 3 3 170
T640TFS 640 2.15 0.30 140 1 300 3 3 170
Polypropylene
P4OOPFS 400 2.00 0.20 22 5 450 4 3 100
P450PFS 450 2.20 0.20 170 500 4 3 100
P5OOPFS 500 2.30 0.22 150 500 4 3 100
P550PFS 550 2.75 0.20 130 550 4 3 100
Acrylic HP
H4OOHSS 400 1.78 0.22 260 500 4 3 150
H5OOHSS 500 2.15 0.23 200 600 4 3 150
H550HSS 550 2.33 0.24 165 650 4 3 1 50
Acrylic CP
C500HSS 500 2.10 0.24 600 600 4 3 140
a
Andrew Textile Industries Ltd.
b Thickness at 2.2 kPa.
c Air permeability, dm3/dm2/min C~ 20 mmWG.
d Minimum breaking strength. N/5 cm.
e
Maximum elongation (~ 50N/5 cm.
f Maximum lineal shrinkage after 24 h exposure to dry heat.
Table 3.$ High-temperature needlefelts a
Non-woven Fabric Media 91
Product Weight Thickness b Density Air
(g/m 2) (mm) (g/cm3) permeabilityC
Breaking Elongation e Lineal Shrinkage f.
strength d i%) I%) C
Aramid
X407XSS 400
X489XSS 480
X500XSS 500
X550XSS 550
X559XSS 55O
X509XSS 5OO
PPS
R552RSH 550
R500RSH 500
PTFE
F702FFH 700
F750FFH 750
F840FFH 840
G800FFH 800
F700FFH 700
Polyimide
I55OISS 55O
2 O0
2 30
210
2 30
2 40
2 30
0.20 265
0.21 200
0.24 180
0.24 165
0.23 175
0.22 200
1.85 0.30 225
1.60 0.31 230
0.96 0.73 145
1.00 0.75 115
1.05 0.80 90
1.40 O. 57 110
1.05 0.67 135
2.65 0.21 170
50O 3 3 24O
65O 3 3 24O
650 3 3 240
70O 3 3 24O
750 3 3 240
650 3 3 240
500 3 2 200
600 3 2 200
500 6 3 2 50
600 6 3 250
600 6 3 250
650 6 3 2 50
600 6 3 2 50
600 4 3 2 50
a
Andrew Textile Industries Ltd.
b Thickness at 2.2 kPa.
c Air permeability, dm3/dm2/min (a 20mmWG.
d Minimum breaking strength, N/5 cm.
e Maximum elongation (g 50N/5 cm.
pore sizes between 3 5 and 66 l.tm. The tensile strengths range from 40 to 100
kgf/5 cm strip.
Two significant recent introductions by Andrew Textile concern scrims and
microfibres. The use of a scrim in needlefelt has been traditional in Europe, but it
has been less common in the USA. Andrew introduced its Fibre-Locked felts to
Europe, to overcome the problems of the weakening of a scrim in the needling
process. This material has lower tensile strengths than those of scrim supported
felts, but the general filtration performance is better.
Consequent upon the availability of finer, so-called 'microdenier' fibres,
Andrew has also introduced its Micro-felt, made 100% from fibres of less than 10
l.tm in effective diameter. This material is able to achieve much finer degrees of
filtration, with mean pore sizes of 12 to 2 5 l.tm (from fibres of O. 5 to 2.2 5 denier).
Figure 3.7 shows an interesting variant of the flat sheet form in which
needlefelts are generally produced, this being Webron's Circron circular
seamless tubes or sleeves. They are produced by continuously winding and
needling a web of fibres around a rotating mandrel, so as to apply a number of
layers to build up the required final thickness: the tube thus formed is drawn
continuously off one end of the mandrel, so that the length is virtually unlimited.
Circron tubes of various fibres are available with inside diameters from 68 to 3 50
mm, wall thicknesses from 8 to 18 mm, and in materials including
92 Handbook of Filter Media
Figure 3.7. 'Circron'seamless needle felt tubes.
polypropylene, polyester, acrylic, aramid and PPS; porosities are 65-90%, with
pore sizes up to 400/~m.
3.3.2 Electrostatic
effects
The filtration of solids from fluids can create electrostatic effects, or can benefit
from the existence of electrostatic charges on the filtration media. Especially in
the case of the filtration of dusts, the presence or absence of such charges can
make a great difference to the filtration performance.
3.3.2.1 Electrically charged non-wovens
Many particles in fluid suspension carry a small electric charge, and so will be
more effectively removed from suspension if the filter medium carries an opposite
charge. This effect is utilized in the media known as
'electrets',
and the phenomenon
of'zeta potential'. The effects are covered in detail in Chapters 4 and 5.
3.3.2.2 Anti-static media
A well-known problem in applying needlefelts to dust filtration is the hazard
that can arise from the build-up of electrostatic charges on the filter surface. To
guard against this, the system must be well earthed, which is only possible if the
fabric of the filter bags has a sufficiently high electrical conductivity. By contrast
with this requirement, the synthetic polymers from which needlefelts are made
have a high electrical resistance, and are therefore very susceptible to becoming
highly charged with static electricity.
The solution to this difficulty is to increase the conductivity of the fabric, either
by chemical treatment, so that the polymeric fibres become coated with metal
salts, or by incorporating into its structure a small quantity of other fibres that
are themselves highly conductive. It is worth emphasizing the importance of
ensuring that, in use, filter bags of anti-static cloth are properly earthed; if they
Non-woven Fabric Media 9 3
are not earthed, they will actually increase the static hazard since their much
higher capacitance will enable correspondingly high static charges to
accumulate.
Chemical treatment has the disadvantage that it is not durable, since the
coating is likely to abrade and disintegrate in use, especially if filter bags are
occasionally laundered. By contrast, the inclusion of conductive fibres provides
permanent protection. Examples of conductive fibres are DuPont's Epitropic and
Bekaert's Bekinox. The latter are of a special grade of stainless steel that is of
extremely high purity, so as to avoid the risk of inclusions within the very fine
6.5, 8, 12 or 22/am diameter fibres.
Epitropic fibres are primarily polyester, with an outer sheath of polyester
isophthalate copolymer, which is impregnated with particles of carbon black, as
illustrated in Figure 3.8. The sheath has a melting point 35~ lower than the
core; it can be softened by controlled heating so that the carbon particles
become embedded in it, to be an integral part of the fibre surface. The electrical
conductivity of these fibres is 50 times higher than that of stainless steel; this,
combined with their significantly lower density, is claimed to give them a
significant cost advantage for lower temperature applications suited to
polyester.
An alternative approach is to make the scrim conductive as well as having
stainless steel fibres among those of the felt. This feature is exemplified by
Andrew Textile's conductive felts, listed in Table 3.4.
3.4 Bonded Media
The fibres in felted media or needlefelts are held together by the natural
interlocking of the staple fibre, or by the additional entangling of needling or
hydroentanglement, coupled with a small effect of temperature at the points of
contact of the fibres. Another group of non-woven materials achieve their
Polyester core ,/1 Carbon particles
re@ oo ~ 9 @ 9 9 9 9
p 9 9 9 9 9
9 9 9 9 9 9
9
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t 9 9
9 9 ~ 9 ~, 9 9 9
9 9
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Polyester
isophthalate sheath
Figure 3.8. The structure of'Epitropic' fibres.