44 F & S International Edition No. 16/2016

Highlights 2015

1. Introduction

Porous paper and sintered materials

have been used as fi lter materials for a long

time. Their manufacture and their typical

fi lter material properties are described in

/1/ and /2/. New types of porous sheet-like

materials that can be made from different

metals have been developed as a result of

the cooperation between the Fraunhofer

Institute for Manufacturing Technology,

the Applied Materials Research Centre

in Dresden and the Paper Technology

Foundation in Munich. Methods used

during the manufacture of papermaking

and sintering technologies are combined

for the production. The porous materials

can then be called porous metallic paper.

The fi rst trials involving different samples

used as fi lter materials were carried out

at the University of Kaiserslautern. The

typical properties determined during the

trials were then presented. Possible fi lter

material application fi elds were also pre-

sented. The production of the materials is

described in greater detail in the following.

2. Production of metallic

fi lter papers

A combination of paper and sintering

technologies are used during the pro-

duction of porous metallic paper. Metal

powder or metal fi bres as well as wood

and cellulose, which are both used during

the manufacturing of paper, are mixed

together in a liquid as the starting materi-

als. Afterwards they are processed with a

so-called sheet former or by using a paper

machine into a fl at material. The material

is formed on a sieve during the dewatering

process which results in a compaction

compaction. The defi ned fi nal thickness of

the paper packed with metal powder can

be realised through a further calendering

process.

The suspension properties (mixing

ratio, particle shape, particle size distri-

bution, interparticle friction) are of huge

importance as the compaction properties

depend on them. The typical metal powder

content lies between 75% and 85%. Metal

powder and metal fi bre mixtures can also

be used. With these it is possible to adjust

the overall porosity, the pore size distri-

bution and the resulting surface weight

within a wide range.

It became necessary to develop a suit-

able retention/binding system for paper

manufacturing due to the relatively high

density differences between the cellulose

and the metal powder or metal fi bres. In

principle, metal powder with particle sizes

in the diameter range between approx. 2

and 50 μm can be processed into paper. It

is also possible to create graduated struc-

tures, i.e. a structure with differently sized

metal powder particles formed through

additional coatings. This creates a packed

paper, which exhibits a functional coat-

ing with defi ned properties. Porous metal

papers with thicknesses ranging from 0.1

up to 1 mm were produced previously.

After the packed paper sheet has been

formed, a so-called cauterizing and sin-

tering step will be run to produce the

metallic structure. The cellulose and other

organic ingredients will be removed in a

thermal process during the cauterizing,

which typically occurs between 200° and

650°C. During the sintering, which occurs

between 1050°C and 1,300°C for stainless

steel, the diffusion process causes material

bonding between the powder particles or

the metal fi bres, which gives the porous

structure high stability.

The porosity and the pore size can be

adjusted within a wide range via the parti-

New porous metallic-paper and its use as a fi lter mediumL. Petersen, S. Ripperger*, C. Kostmann, P. Quadbeck, G. Stephani**, J. Strauß, S. Schramm***

New types of porous, sheet-like materials that can be made from different metals are presented in the following

contribution. Methods used during the manufacture of papermaking and sintering technologies are combined for

the production. The suitability of these materials for use as fi lter material was tested using different samples.

The results are presented here.

Fig. 1: Cross-section of sintered and porous metallic paper Fig. 2: Further processed metallic paper

* Dipl.-Wirtsch.- Ing. Lars Petersen, Prof. Dr.-Ing. Siegfried Ripperger

Technische Universität KaiserslauternGottlieb-Daimler-Str.67663 Kaiserslautern, GermanyTel: +49 (0) 631 -205 -2121www.uni-kl.de/MVT** Dipl.-Ing- Cris Kostmann,

Dr.-Ing. P. Quadbeck, Dr.-Ing. G. StephaniFraunhofer Institut für Fertigungstechnik und angewandte MaterialforschungWinterbergstr. 2801277 Dresden, GermanyTel: +49 (0) 351 -2537 -301www.ifam-dd.fraunhofer.de*** Dipl.-Ing. J. Strauß, Dipl.-Ing. S. SchrammPapiertechnische StiftungHess-Strasse 13480797 Munich, GermanyTel: + 49 (0) 89 -12146 36www.cepa.de

Highlights 2015

F & S International Edition No. 16/2016 45

cle size of the metal powder being used, the

type of cellulose (short / long fi bres) and

the retention/binder system. Previously,

paper was manufactured with an average

pore size in the 2 to 40 μm range and with

a specifi c weight of between 450 g/m2 and

2,000 g/m2. An example of the cross-sec-

tion of a sintered and highly-porous paper

made from 1.4401 (316L) stainless steel

with a thickness of 0.6 mm is shown in

Fig. 1.

This can be used to produce metallic

fi lter materials for a wide range of appli-

cations. The fl exible manufacturing pro-

cesses allow the product to be optimised to

meet the respective requirements.

One advantage of the new technology is

that it is easy to further process the paper

before it is thermally treated using conven-

tional paper technology. It can be rolled,

coiled, pleated, creped or corrugated and

these processes enable it to be optimised

to meet different requirements.

Some examples are shown in Fig. 2. In

addition to this, the sintered structures can

be bonded onto other metallic structures

through welding or soldering, which is of

great signifi cance with regard to further

processing into fi lter cartridges or fi lter discs.

3. Properties of the tested samples

A total of six different samples were

tested, which are referred to as A, B, C,

D, E and F in the following. Materials

A - C differ due to the fact that different

sintering temperatures were used during

the production process. The surface mass

varies with materials D - F.

3.1 Material propertiesThe bubble point was determined using

fl ow porometry in accordance with the

ASTM F 316 method /3/. The bubble point

gives the gas pressure at which the fi rst

bubbles in a completely wet sample can

be seen to form. The pressure corresponds

to the opening pressure of the largest pores

in the sample. If the surface tension of the

wetting liquid σ is known, it can be con-

verted into the pressure difference Δp in

an equivalent, maximum pore diameter d:

(1)

C is an empirical capillary constant

here. According to ASTM F 316, a value

of 2.86 is the optimum for determining

the maximum pore size of the majority of

technical materials. It should be taken into

consideration here that the pore cross-sec-

tion deviates from the shape of a circle.

A PSM 165 porometer made by Topas

GmbH was used for the measuring. Topor

was the liquid used in the tests. According

to the manufacturer’s details, this is a

perfl uorinated compound with a surface

tension of 0.0163 N/m, which completely

wetted the usual fi lter materials. The max-

imum pore diameters in the samples being

tested lie in the 8.8 μm up to 14.7 μm

range. It was to be expected that particles

that were bigger than the maximum pore

diameter in the respective materials would

be completely separated during fi ltration.

The porosity was determined using a

gas pycnometer. The material thicknesses

were calculated from the specifi c masses

and the porosities.

Tab. 1: Material properties of the tested samples

Sample Sintering temperature

Specifi c mass

Bubble

PointMaximum

pore diameterPorosityThick-

ness

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46 F & S International Edition No. 16/2016

Highlights 2015

3.2 Filtration propertiesSo that the fi ltration effi ciency of the

specifi c samples could be tested, the sam-

ples were fi rst subjected to distilled water

fl owing over them in order to test if it was

possible to release particles from the clean

sintered metal paper. This did not occur at

an signifi cant level.

Afterwards the samples were fed into

a funnel fi lter made by BHS Sonthofen

with a fl ow surface of 20 cm2 from an

aqueous ISO medium test dust-suspension

as per ISO 12103-1 /4/ with a mass con-

centration of 0.025g/L at 24° ± 2°C. The

fi ltrated suspension volume amount was

always 300 ml and the fi ltration pressure

was a constant 0.5 bar. The fi ltrate mass

was recorded by electronic scales and dis-

played by a computer connected up to it.

The fi ltrate fl ow rate during the fi ltration

time was virtually constant with all of the

samples. The narrowing of the pores in the

sintered metal paper by the separated par-

ticles was not so high during the tests so

that there was always an appreciable fl ow

with regard to the sample’s permeability.

The fi ltration rates realised at a pressure

difference of 0.5 bar are listed in Table 2

for the sintered material samples that were

tested. The values were basically deter-

mined from the specifi c mass, the porosity

and the pore size. Samples A - C showed

that an increase in the sintering tempera-

ture during production was accompanied

by a reduction in the maximum pore

diameter, a reduction in the porosity and

a drop in the fi ltration rate. The results for

samples D and E showed that with using

the same sintering temperature an increase

in specifi c mass or thickness causes a

decreasing fi ltration rate.

Both the suspension that was used as

well as the fi ltrate was analyzed using the

V4A Abacus mobile fl uid single particle

counter made by Klotz. This enables to

determine the fi tration effi ciency of the

samples. The fi ltration effi ciency that were

determined for material samples A - C as

well as D - F are shown graphically in

Fig. 3.

It can be seen that the separation curves

for all of the sintered metal papers have a

similar shape. Virtually all of the particles

from a particle size of approx. 8 μm were

separated in all of the samples. The slight

decrease in the separation curves for sam-

ples D - F for particles sizes of approx. 13

μm is explained by measurement fl uctua-

tions. With larger particle sizes the total

number of recorded particles is very low

at intervals, so that any mild fl uctuations

here will have a huge effect on the result.

It shows that a small maximum pore

size is linked with improved separation of

the fi ne proportion of the particle spectrum

in samples A to C. An increase in the

specifi c mass or thickness of the sintered

metal paper will result in an increase in

particle separation, which can be seen in

the results from samples D - F.

Several scanning electron microscope

images were made from the samples in

order to provide more detailed information

about the types of separation. SEM images

from samples A - C taken after fi ltration

at an enlargement of 700 x are shown in

Fig. 4.

The sintered material is shown in white

in the images. The separated particles

can be seen in the darker colours. It can

be seen that the structure of the sintered

metal changes as the sintering temperature

increases. At low temperatures the metal

exists in the form of many bonded spheres,

whilst the original structure increasingly

fuses as the temperature increases.

This shows that the fi ner, open-pored

structure of sample A resulting from a

low production temperature exhibits better

fi ltration properties than samples produced

at higher temperatures (samples B and

C). The pore system is more branched in

Fig. 6: SEM images from sample E, 2,000 x

Fig. 4: SEM images from A, B and C (from left to right), 700 x

Tab 2: Filtration rates of the materials used

Sample Filtration rate at 0.5-bar in m/s

Fig. 3: Fractional degree of separation for samples A - C (left) and D - F (right)

Particle size / μm Particle size / μm

Deg

ree

of

sep

arat

ion

/ %

 

Deg

ree

of

sep

arat

ion

/ %

 

Highlights 2015

F & S International Edition No. 16/2016 47

sample A and exhibits better particle separation and higher per-

meability. It can be seen that far more particles have already been

separated on the surface of sintered material A as opposed to the

other two materials. This applies especially to large particles from

approx. 10 μm. As the particles of this size are virtually completely

removed with sintered paper B and C, it can be assumed that sepa-

ration partially occurs within the fi lter medium. SEM images from

samples D - F are shown in Fig. 5.

As expected, there are no structural differences in the fi lter

media that can be seen in the images from samples D - F. An

enlarged supplementary sample E structure is shown in Fig. 6. The

deposited particles can also be clearly seen in the darker colours.

Sample D deviates from samples E and F especially with regard

to its thickness. A similar proportion of relatively large particles

were separated on the surface of each.

Overall, all of the sintered papers showed a good fi ltration

effi ciency. All of the particles from a particle size of approx. 8 μm

were completely separated. Smaller particles were only partially

separated. The sintered paper particles were separated on the sur-

face as well as within the material.

A low sintering temperature resulted in a structure with a fi ne,

open pore system, hence a low fl ow resistance and improved sep-

aration were achieved. An increase in the sintering temperature

causes increased fusing within this fi ne structure. The pore system

shows less than with larger pores, hence a higher fl ow resistance

and poorer separation. An increase in the specifi c mass or the

material thickness results in improved separation and increased

fl ow resistance.

4. Possible application areas

The high porosity and the thin and stable structure of the sin-

tered papers are the best conditions for use as fi lter media both with

gas as well as liquids. In particular, utilisation can be introduced

when the known polymer fi lter media can no longer be used. As

the choice of fi lter media is clearly restricted from approx. 250°C,

which means that the specifi c advantages of metallic fi lter papers

come to the forefront in this temperature range. In principle, the

sintered paper can be produced from different metals. Porous

metallic paper can be made from chrome / nickel / steel, FeCrAl,

nickel-based alloys as well as copper alloys. Stability, fracture

toughness, wear and thermal shock resistance all play a role as

well as porosity and pore size distribution regarding the applica-

tions. Previous results have shown that a good performance can be

demonstrated both with abrupt pressure loads as well as thermal

load changes. Soldering and welding are also possible. This is a

huge advantage when compared to other high temperature fi lter

media, such as porous ceramics.

Metallic fi lters are normally easy to regenerate. The fi lter is

burnt out in some applications, e.g. when used as a particle fi lter.

The fi lter paper can also be used in combination with a stable

and large-pored carrier material under extreme mechanical loads.

Combinations with fabrics are also conceivable – they could be

layered into mats, pressed and then bonded by sintering. This will

form a solid bond that can accept high pressures and can be used

for the fi ltration of highly viscous liquids.

Literature:

/1/ S. Ripperger: Poröse Sinterwerkstoffe und ihre Anwendung als Filtermittel. Filtrieren und Separieren 24 (2010), Nr. 3, S. 124-126

/2/ S. Ripperger: Eigenschaften und Anwendungen von Filterpapieren. Filtrieren und Separieren 24 (2010), Nr. 4, S. 178-180

/3/ ASTM (American Society for Testing and Materials) F 316 - 86 Standard Test Method for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test (1986)

/4/ ISO 12103-1: Road vehicles -Test dust for fi lter evaluation (1997)

Fig. 5: SEM images from D, E and F (from left to right), 700 x