TIP 0404-20 OLD NUMBER 017-47 ISSUED – 1976 REVISED – 1984 REVISED – 1990 REVISED – 1996 REVISED – 2002 REVISED – 2004 2004 TAPPI The information and data contained in this document were prepared by a technical committee of the Association. The committee and the Association assume no liability or responsibility in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade secret laws. The user is responsible for determining that this document is the most recent edition published.TIP Category: Automatically Periodically Reviewed (Ten-year review) TAPPIPhysical characterization of pr ess fabrics: basis weight, air permeability, void volume under loading and pressure uniformity Introduction This Technical Information Paper provides a means of measuring several fabric characteristics of new press fabrics. For on-machine monitoring of the press fabric, refer to TAPPI TIP 0404-19 “Press Section Monitoring.” The characteristic properties covered in this TIP are (a) the basis weight or unit area weight, (b) the air permeability, (c) the void volume and (d) the pressure uniformity of a press fabric. The supplier typically performs these measures. This revision combines information from earlier versions of TIP 0404-20 with material from withdrawn TIPs 0404- 32 and 0404-34. Basis weight of a press fabric Scope The purpose of this method is to note the conditions under which a press fabric is measured for size and basis weight. This procedure is intended for use either in the mill or in a laboratory. Use of these conditions will enable paper mills to compare fabrics fr om one company to those of another with respect to basis wei ght. This test is to be used for all fabrics regardless of position or product being made on the machine. Summary The weight and size of a press fabric are measured at standard textile conditions of 21°C (70°F) and 65% RH, under a tension of 2.1 kN/m (12 pli). Significance The basis weight is a measure of the mass of material, per unit area, contained in the structure of a press fabric. No direct correlation has been established between the basis weight and performance of the fabric across paper grades, however this characteristic may relate, to some extent, to other properties such as thickness, void volume, robustness, and compaction re sistance. In actual use, the uniformity of mass distribution is usua lly a more critical parameter than t he basis weight i tself.
Transcript
7/27/2019 TIP 0404-20( Physical Characterization of Press Fabrics
The information and data contained in this document were prepared by a technical committee of the Association. The
committee and the Association assume no liability or responsibility
in connection with the use of such information or data, including but not limited to any liability under patent, copyright, or trade
secret laws. The user is responsible for determining that this
document is the most recent edition published.
TIP Category: Automatically Periodically Reviewed (Ten-year review)
TAPPI
Physical characterization of press fabrics: basisweight, air permeability, void volume under loadingand pressure uniformity
Introduction
This Technical Information Paper provides a means of measuring several fabric characteristics of new press fabrics.
For on-machine monitoring of the press fabric, refer to TAPPI TIP 0404-19 “Press Section Monitoring.”
The characteristic properties covered in this TIP are (a) the basis weight or unit area weight, (b) the air permeability,
(c) the void volume and (d) the pressure uniformity of a press fabric. The supplier typically performs these
measures.
This revision combines information from earlier versions of TIP 0404-20 with material from withdrawn TIPs 0404-
32 and 0404-34.
Basis weight of a press fabric
Scope
The purpose of this method is to note the conditions under which a press fabric is measured for size and basis
weight. This procedure is intended for use either in the mill or in a laboratory. Use of these conditions will enable
paper mills to compare fabrics from one company to those of another with respect to basis weight. This test is to be
used for all fabrics regardless of position or product being made on the machine.
Summary
The weight and size of a press fabric are measured at standard textile conditions of 21°C (70°F) and 65% RH, under a tension of 2.1 kN/m (12 pli).
Significance
The basis weight is a measure of the mass of material, per unit area, contained in the structure of a press fabric. No
direct correlation has been established between the basis weight and performance of the fabric across paper grades,
however this characteristic may relate, to some extent, to other properties such as thickness, void volume,robustness, and compaction resistance. In actual use, the uniformity of mass distribution is usually a more critical
parameter than the basis weight itself.
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under loading and pressure uniformity
It may be worth noting that most press fabrics are sold by weight. Total weight is simply the basis weight per unit
area multiplied by the total surface area of the fabric. Standard press fabric material behaves like paper in that some
moisture is absorbed into the material at room conditions, thus measured weight of the press fabric will vary withstorage conditions. Rather than wait for a newly finished press fabric to acclimatize before weighing and packaging,
many fabric suppliers simply quote a normalized fabric weight based on a “bone-dry” (0% moisture content) weight
plus 2% (moisture adjustment), which approximates the weight anticipated at typical room conditions. This mayexplain any discrepancy that is found between actual and quoted weight.
Test specimen
Weight is measured in moisture equilibrium at 21°C (70°F) and 65% RH. Length and width are to be the projected
running size at a tension of 2.1 kN/m (12 pli).
Procedure
1. Measure the length to the nearest 10 cm (4 in) and width to the nearest 2.5 cm (1 in) for the fabric under a
tension of 2.1 kN/m (12 pli).2. Weigh the fabric to the nearest 230 g (0.5 lb).
Calculations
Calculate basis weight, BW, as follows:
BW = (M x 100000) / (L x W ), g/m2
BW = (M x 328) / (L x W ), oz/ft2
where
M = weight in kg,
L = length in m,
and W = width in cm.
Using Imperial units, the calculation would be as follows:
BW = ( M x 16 ) / ( L x ( W / 12) ), oz/ft2
where
M = weight in lbs,
L = length in feet,and W = width in inches.
Conversion of oz/ft2 to g/m2 would be as follows:
g/m2 = oz/ft2 x 305.15
Report
Report the width, length, weight and basis weight. Indicate if the test condition varied from 21°C (70°F) and 65%
RH.
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Air permeability of a press fabric
Scope
Permeability is a measure of ease with which a fluid (or a gas) flows through a porous material. Air permeability of a press fabric is expressed as flow rate of air through a basis of a complex textile structure; i.e., a flux under specific pressure gradient. It should be recognized that similar net results can be obtained on drastically different fabric
constructions. Air permeability test results should not be used without consideration of other performance
parameters.
Summary
The method consists of measuring the volume of air passing through a known area of a press fabric under a pressure
drop of 125 kPa (0.5 in. of water).
Significance
The correlation of press fabric air permeability to performance on the paper machine has not been established. Air
permeability is significantly affected by the density of the press fabric and can drop in value as the result of manufacturer pre-compaction and fabric compaction during use on the paper machine. When comparing air
permeability for fabrics of similar design, fabric density should be considered.
Apparatus
The apparatus1 shall consist essentially of a suction fan for drawing air through a known area of fabric, a circular
orifice over which the fabric to be tested can be clamped, a means for adjusting the pressure drop across the fabric to
a known amount, and a means of measuring the volume of air flowing through the fabric. The clamp shall
effectively eliminate edge leakage. The apparatus shall be capable of testing fabrics of different thicknesses and of
testing large pieces of fabric without cutting (1).
Test specimens
Fabrics should be tested at standard textile conditions of 21°C (70°F) and 65% RH. Five tests shall be taken at
separate locations on the sample so that the test results will be representative. Test specimens shall be as widely
separated as possible without cutting.
Procedure
1. Handle the test specimen carefully to avoid altering the natural state of the material.
2. Mount of the test specimen between the clamp and the circular orifice with sufficient tension to draw the fabric
smooth. It shall not be distorted in its own plane. Draw conditioned air through the calibrated flow meter.Adjust the pressure drop across the fabric to 125 kPa (0.5 in. of water) and then record the reading of the flow
meter. Express the air permeability of the fabric in m3 of air/min/m2 of fabric ( or ft3 of air/min/ft2 of fabric)
Report
Report the average of the test results obtained at five locations on the fabric as air permeability.Indicate if the test condition varied from 21°C (70°F) and 65% RH.
1 Suitable equipment is known to be manufactured by the Frazier Precision Instrument Company Inc. (http://www.frazierinstrument.com) and by
TexTest Instruments (http://www.textest.ch). Any other suppliers of this equipment may be listed in this TIP by contacting the TAPPI Quality
and Standards Department.
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Void volume of a press fabric under load
Scope
The void volume of a press fabric is a measure of the space available in which water can reside during dewatering.The void volume of a press fabric is reduced in compression and it decays during the life of the fabric due tocompaction and filling. The procedure detailed here covers the calculation of void volume based on a direct measure
of thickness in the laboratory. The void volume can be determined as a function of load or compression cycles as
long as an accurate caliper measure of the fabric can be made at those conditions. This procedure does not accountfor the effect of filling on void volume.
Summary
The thickness of a sample of press fabric is measured in a laboratory during high speed compression cycling.
Knowing the density of the polymer(s) in the fabric, a void volume can be calculated from the measured thickness.
Significance
The void volume capacity is one characteristic to be considered in the design of a press fabric. A strong correlation between void volume and dewatering has not been established across paper grades on a paper machine. However,
void volume is sometimes used as an indicator of water handling capacity for a fabric in a given pressingapplication.
Apparatus
A servo-hydraulic tester 2 is used to measure the thickness of a press fabric continuously during high speed
compression cycling to simulate use on a paper machine.
Test specimens
A sample of fabric is cut to a size such that a maximum compressive pressure of 7 MPa (1000 psi) can be achieved
in the servo-hydraulic tester. The sample is soaked in water at room temperature (70°F) for at least 1 hour and then blotted to the desired initial moisture ratio.
Procedure
A sample is placed between smooth, parallel platens and cycled between compressive pressures of 0.01 MPa and 7
MPa for the desired number of cycles. At chosen cycles, the load/thickness data can be captured. Usually a total of
10,000 cycles (about 6 minutes for a 30 Hz frequency) is run with data collected at 10, 100, 1000 and 10000 cycles.
A minimum of five samples from different areas of each fabric should be tested.
Calculations
The void volume, V , is calculated from the average fabric thickness and basis weight as follows:
V = t – (0.001 x BW/ D), mm3/mm2
V = 0.0394 x (t – (0.001 x BW/ D)), in3/in2
2 Suitable equipment is known to be manufactured by MTS Systems Corp. (http://www.mts.com/) and the Instron Corporation
(http://www.instron.com). Any other suppliers of this equipment may be listed in this TIP by contacting the TAPPI Quality and Standards
Department.
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where
t = average fabric thickness under load, mmBW = basis weight, g/m2
D = polymer density, g/cm3
Density varies with polymer type; see the following table. In the case where a fabric is a mix of polymer types, a
weighted average density should be used in the calculation.
Polymer Density (g/cm3)
Nylon 6 1.14
Nylon 6,6 1.14
Nylon 6,10 1.08
Nylon 6,12 1.07
Polyurethane 1.20
Assuming a saturated fabric, the moisture ratio, MR, and water capacity, WC, can be calculated from the void
volume, V, as follows:
MR sat = 1000 x V /BW
WCsat = 1000 x V , ml/m2
For example, a fabric with a basis weight of 1600 g/m2 made entirely from PA66 was compression tested for 10,000
cycles. The average minimum thickness (at maximum pressure of 7 MPa) for the last cycle was measured to be 1.95mm. The calculated minimum void volume for that fabric is
V = 1.95 – (0.001 x 1600/1.14) = 0.55 mm3/mm2 (0.021 in3/in2)
MR sat = 1000 x 0.55/1600 = 0.34WCsat = 550 mL/m2
Report
Report the calculated void volume, along the applied load and cycle to which the thickness corresponds. The
thickness should be an average of at least five samples.
Pressure uniformity of a press fabric
Summary
Carbon impressions are prepared in the laboratory of the press fabric surface under compressive load. The resulting
pattern is compared visually to a set of standard impressions and ranking accordingly. Images of these standard
impressions are included here.
Significance
The uniformity of a press fabric surface can influence the quality of the sheet. Marking can sometimes be caused in
the sheet as a result of the press fabric structure. Also, experimental evidence exists linking the pressure uniformityof a press fabric to the final dryness and/or surface smoothness of lightweight paper sheets. While some suppliers
have developed quantifiable methods, the method still in general use is the qualitative method described herein.
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Apparatus
A screw driven platen press or compression tester with a set of flat parallel plates can be used. Nip impression paper is most commonly used3.
Test specimens
A square sample of fabric is cut of a size that will allow a compressive pressure of 7 MPa (1000 psi) to be applied in
the platen press, being sure a cut side is aligned with the MD of the fabric. For example, a 75 mm (2.9 in) squarewould require a compressive load of 39.4 kN (8700 lb) to achieve a pressure of 7 MPa (1000 psi).
Procedure
The fabric should be dry and free from debris. Place the fabric, sheet side up on the lower platen. Place a layer of carbon impression paper onto the press fabric with the ink side facing the fabric. Lower the top platen and load the
system to a pressure of 3.5 MPa (500 psi). Hold at this position for 20 seconds. Unload and repeat with another
sample to a pressure of 7 MPa (1000 psi). It is recommended that at least three samples from different regions of thefabric be tested at each pressure.
Examine the impression for the presence of fabric features (base fabric marking, batt fiber clumping, needle tracks,
etc) as well as for the overall intensity variation across the area. Compare the impression to the set of standardimpressions reproduced here and assign a rank to each impression, from 1 (most uniform) to 5 (least uniform).
The carbon impressions will fade with exposure to light. It is recommended that the evaluation be done immediately
upon removal from the platen press. If this is not practical, the impressions should be stored in darkness or
immediately scanned and stored digitally for future evaluation.
Report
Report the average rank for the impressions for each fabric sample at each pressure level, along with any
observations. For example, “At a pressure of 3.5 MPa, Fabric A had an impression rank of 3.5 with a pronounced
MD base weave pattern.”
Examples of Impressions
Rank 1 (3.5 MPa) Rank 1 (7 MPa)
3 Suitable paper is known to be available from Metso Paper (www.metsopaper.com). Any other suppliers of this material may be listed in this
TIP by contacting the TAPPI Quality and Standards Department.
7/27/2019 TIP 0404-20( Physical Characterization of Press Fabrics
1. Position, the fabric’s location on the machine; i.e. first press, bottom fabric or third press, top fabric.2. Estimate, method of calculating a given value without the actual physical measurement of that value.
3. Direct measurement , a value that is the direct result of an actual physical measurement.4. Projected running size, the size the fabric will attain under a given tension and running condition.
5. Air permeability, a measure of ease with which a fluid (or a gas) flows through a porous material.
6. Press fabrics, all fabrics used in the mechanical removal of water by a paper machine press nip.
7. Fabric constructions, the materials making up the fabric (i.e. wool, nylon, polyester) and the manner in which
they are assembled (i.e. weave, percent batt, needling).
8. Void Volume – volume of voids in a unit volume of press fabric9. Moisture ratio – the ratio of the mass of water to the mass of the press fabric for the same unit area
Keywords
Press felts, Pressing, Air Permeability, Weight, Forming fabrics
Safety precautions
No special safety precautions are required for these tests.
References
TAPPI TIP 0404-29 “Porosity measurements of press fabrics on the paper machine (HSPT)”
TAPPI TIP 0404-43 “Water permeability of press fabrics”
Literature cited
1. Schieffer, H.F., and Boyland, P.M., “Improved Instrument for Measuring the Air Permeability of Fabrics,” Journal of Research, National Bureau of Standards, Research Paper RP 1471.
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