Interest in the surface finishing or modification processes for

fabric filters began in the late 1960s, early 1970s. It coincided

with the introduction of synthetic media for use in industrial

filters because such media were easier to modify and lent

themselves to the finshing technologies.

The first modification techniques used were:

(i) Singeing - specifically for fabrics made from short stable fibres.

Such fabrics possess short protruding fibres that can impede cake

discharge because of mechanical adhesion of the fibre to the cake.

However, this can be overcome by quickly passing the fabric over a

gas flame or bringing it in rapid contact with a metal strip heated

to a very high temperature.

(ii) Calendering (Figure 1) - this process not only improves the

surface smoothness of the fabric, and therefore its cake release

capability, but also regulates the fabric’s permeability, and therefore

its filter efficiency. This is achieved by passing the fabric between

heated, pressurised rollers, with temperature, pressure and speed

through the machine suited to the particular fabric or particular

polymer type. However, known disadvantages of calendering are

that it may result in a reduced surface area and reduced permeability.

Fabric finishing/modification procedures are carried out for

three main reasons:

• To ensure the fabric quality

• To modify the surface characteristics

• To regulate the fabric's permeability

From the early to mid-1970s onwards other surface modifi-

cation techniques were explored. The interest was driven by

industry that was facing tightening environmental pollution

control, i.e. in both air/dust emissions to atmosphere and

discharge limits for liquid waste streams. Furthermore, end users

from a technical and an economic point of view were also

demanding higher filtration performance from their filter media,

as well as a longer operating life.

ePTFE membranesFrom 1973 W L Gore & Associates has pioneered the use of

Gore-Tex® expanded polytetrafluoroethylene (ePTFE)

membranes that are applied to a needlefelt support for air/dust

filtration applications (Figure 2).

Over the years Gore has developed several ePTFE membranes.

For example, its high-durability filter bag incorporates a new

generation of membranes laminated onto a polyester needlefelt.

They are designed to meet the process requirements of pulse jet

baghouses routinely used in the chemical industry. The company

also launched a series of filter bags that incorporate re-engineered

ePTFE membranes that specifically achieve a higher air flow and a

lower pressure drop. Aimed primarily at the cement market, Gore

also improved the strength of the new membrane, which was

demonstrated to have approximately four times the strength of its

predecessor. This higher strength translates into reduced cracking

and better resistance to damage from penetrating particles.

Once some of the Gore-Tex intelluctual property expired

companies, such as Tetratex (now part of Donaldson) and BHA

Technologies (now part of GE), entered the ePTFE market and

went on to further develop the technology.

Microporous coatingsOne of the first commercially available microporous coatings was

developed by P&S Filtration (now Madison Filter) for dust filtration

applications (Doran & Collinge, 1985). Here, a needlefelt is coated

with a polymer emulsion to create a filter medium with a thin

microporous coating that exhibits improved filtration properties,

including greater particle retention, (especially of small particles),

reduced particle penetration, improved cake release and a more

rapid equilibrium of the pressure drop.

Ravlex™The Ravlex membrane product line was developed by

Ravensworth Ltd (in the late 1980s) and comprises three main

product groups: Ravlex MX, Ravlex PPC and Ravlex YP.

Ravlex MX is a PTFE microporous embedded membrane

that is applied to the filtration surface of the carrier needlefelt.

The coating process causes the coating to sit within the surface

fibres of the needlefelt, which is then cured to help produce

the desired heat, chemical and abrasion resistance, and cake

release properties. Ravlex MX is generally used for the filtration

20 November 2004 ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved

Filter media surface modificationtechnology: state of the art

Over the last 30 years the demand from industry for filter media that exhibitan improved filtration performance and a longer operating life has grown.

These demands in turn have driven the area of surface modication for fabric filter materials. With the help of Professor Richard Lydon,

Filtration+Separation takes a look at the history of this technology andhighlights several of the currently available finishing methods that have

proven to be effective for both air and liquid filtration.

filtermediafocus

Figure 1: Monofilament fabric before andafter calendering

of fine particle in the chemical, pharmaceutical and food

industries.

The Ravlex PPC membrane is also formulated from PTFE and is

applied via a similar coating process. However, it possesses a larger

pore size, and therefore a higher permeability than the Ravlex MX,

making it suitable for filtering agglomerating or sticky dusts.

In contrast to the other two membrane coatings, Ravlex YP is

formulated from polyurethane (PU). However, it is applied using

the same coating technique and has a pore structure similar to

Ravlex PPC. It offers high abrasion resistance, effective cake

release and resistance to blinding. The membrane comes in two

colours: green for general industrial use, and white for dust

filtration in food and pharmaceutical applications.

Primapor™All of the surface modification techniques outlined above are

concerned with air/dust filtration. However, there was also a need

to develop modification techniques for liquid filtration appli-

cations. One such coating, developed by Scapa Filter Media (now

Madison Filter), was Primapor.

Primapor is a microporous low medium density PU coating of a

cell-like structure, with regulated pores in a 3-D format. It is applied

to a woven polyester substrate. It was primarily introduced for use

on process filters, as unlike many other polymers PU is able to

withstand relatively high liquid pressures. This com-posite material

has been shown to exhibit good product retention, improved filtrate

clarity, excellent cake release and extended media life.

During its development phase, it was discovered that vacuum and

pressure filters require different liquid permeability characteristics,

therefore two grades of Primapor were developed - Primapore Blue

(Figure 3) for pressure filters and Primapor Tan for vacuum filters.

Azurtex™Building on the success of Primapor, Madison Filter went on to

develop an advanced composite filter medium called Azurtex. It

incorporates a microporous network onto and into a polypropylene

(PP) or polyethylene terephthalate (PET) woven substrate and can

operate across a wide range of process conditions. For example in

fine TiO2 applications, Azurtex exhibits an excellent throughput

and cake release, and good particle retention, combined with good

abrasion resistance.

Other modification techniquesWebron introduced its MicroWeb 2000 and MicroWeb II media,

which comprise PTFE and acrylic coatings, respectively, on a

polyester needlefelt. Both are designed to offer relatively high

permeabilities. The company also offers chemical treatments,

under the brand name “Supaweb”, which are applied and

thermally bonded to the base substrate. Each treatment conveys

a particular additional property to the the felt, e.g. improved

cake release, hydrophobicity and improved chemical resistance.

Fratelli Testori also produces a number of treatment processes

for filter fabrics, including “Novates”, a coating of PU on polyester

or acrylic felts , that is hydrophobic and oleophobic, and

“Mantes”, a resin chemical treatment containing PTFE that is

applied to acrylics and high-temperature fibres to give a good

chemical resistance.

Madison Filter has developed Tuf-tex™ coatings for PP,

polyamide and PET substrates, which are thermosetting resins

sprayed or knifed onto the surface of the filter fabric to give it good

abrasion resistance, combined with improved dimensional stability.

ConclusionsAs can be seen from the above article, the field of filter media

surface modification has evolved significantly in 30 years. This

will continue as process requirements, such as higher

temperature and even finer filtration, become more demanding.

However, the enforcement of environmental control directives is

likely to be the major driver behind this technology, leading to a

whole host of new modificiation techniques coming to the

market for industrial filtration.

Reference sources1. Doran F & Collinge G. 1985. Micorporous finish an improved dust

collection filter medium, Filtration+Separation, Vol. 22, No. 1, p.44-45.

2. Hardman E. 1994. Some aspects of the design of filter fabrics for use in

solid/liquid separation processes, Filtration+Separation, Vol. 31, No.

10, p.813-818.

3. Lydon, R P. 2000. New Composite Filter Media. Filtration+Separation,

Vol 37, No. 5, p.26-28.

4. Purchas D B & Sutherland K. 2002. Handbook of filter media,

Elsevier Advanced Technology, UK.

Filtration+Separation November 2004 21

filtermediafocus

Figure 2: SEM of Gore-Tex ePTFE membrane (x 600 magnification).

Figure 3: Cross section SEM image of PrimaporBlue, showing the upper layer of microporous

coating combined with the woven support (x 400 magnification)