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ASSESMENT OF NON-WOVEN MATERIALS BY CAPILLARY FLOW POROMETRY
The term non-woven refers to products made by processes that do not include weaving nor knitting
(ASTM D1117-80). The fibers in these materials are bonded together by chemical, mechanical,
heat or solvent treatment. Non-woven materials are used in applications in different sectors
(filtration, automotive, construction, aeronautical, food and medical, amongst other). In 2013 the
US nonwoven fabric industry market accounted for $5.4 billion and the demand for nonwovens is
forecast to rise 5.7% per year to $7.1 billion in 2016.
As for every material, an in depth characterisation of the structure of non-wovens is essential in
order to understand better and improve their performance for a given application.
Capillary Flow Porometry (CFP) measures pore size and pore size distribution of through pores
with good accuracy and reproducibility in one individual and fast measurement. The technique is
based on the displacement of an inert and nontoxic wetting liquid embedded in the porous network
of a material by applying an inert pressurised gas (e.g. nitrogen). The larger pores become empty
first and, as the applied pressure increases, so do the smaller ones until all through pores are
empty. The gas flow through the sample against the pressure applied is measured. The pressure
required to empty pores of a certain diameter is used to calculate the pore size accordingly to the
Young–Laplace formula P=4*cos (θ)*γ/D, where (P) is the pressure required to displace the liquid
from the pore, θ is the contact angle of the wetting fluid with the material, (γ) the surface tension
of the wetting liquid and (D) is the pore diameter.
APPLICATION TO NON-WOVEN MATERIALS
CFP measurements of melt blown and spun bound materials,
were carried out by using a POROLUXTM 100NW porometer from
POROMETER nv (right pictured). The POROLUXTM 100NW is
specially designed for non-woven characterisation and it is based
on the pressure scan method (the increasing pressure and the
resulting gas flow are measured continuously during a test). The
method is fast and typically generates very reproducible results,
which makes the POROLUX™ 100NW very suited for quality
control.
The maximum operating pressure of the device is 1.5 bars (22 psi), which permits measring pore
sizes between 0.427 µm and 500 µm. The determination of the first bubble point (FBP), or
maximum pore size, is based on the test method described in the standard ASTM F 316-03. The
FBP is defined “at the pressure required to blow the first continuous bubbles detectable by their
rise through a layer of liquid covering the filter”. Since the concept of the first continuous bubbles
is subjective. In consequence, a unique universal criteria to calculate the bubble point does not
exist.
The POROLUXTM 100NW permits selecting settings accordingly to different criteria traditionally used
for FBP calculations. Options include the calculation the FBP at the pressure that corresponds to a
flow rate of 30, 50, 100 ml/min, or taking the pressure at which the first measureable flow is
detected by the equipment, amongst other. The choice of one method or another is always the
ultimate decision of the user.
The criteria selected for calculating the FBP was the pressure at which the first flow was measured.
For very porous samples, like non-woven materials, it is recommended choosing “first flow" or "FBP
above a certain flow value" because it is not always easy to calculate the FPB at one specific flow
value (e.g. 30ml/min). In each measurement, a total of 30 data points were taken in the pressure
range from 0- 100 mbar. Three replicates for each sample were measured. The FBP, mean flow
pore (MFP) and smallest pore size (SP) of sample “A” (spunbond type material) and “B” (meltblown
type material) are given below:
Sample A
Average (µm)
Std (µm)
RSD (%)
Sample B
Average (µm)
Std (µm)
RSD (%)
FBP
375.8
2
0.6
FBP
70.5
0.1
0.2
MPF
82.8
6
7.5
MPF
19
1
6
SP
28.3
2
6
SP
7.3
0.3
3
The gas flow against the applied pressure on the dry sample is also carried out (dry curve). The
half-dry curve is obtained by dividing the flow values of the dry curve by 2. The MFP is calculated
at the pressure where the wet and the half-dry curves meet. It corresponds to the size at which 50
% of the total gas flow can be accounted. The SP corresponds to the pressure where the wet and
the dry curves meet. Due to the nature of the calculation of the MFP and the SP these have
normally a higher standard deviation than FBP values. As example, the wet, dry and half dry
curves for sample “A” are shown below. The rounded shape of the dry curves (dotted lines) is
typical of non-woven materials.
The gas flow through the sample is used to calculate the cumulative filter flow distribution against
the pore size (percentage of the total flow which corresponds to pores of a certain size and larger)
and the differential filter flow (increase in flow rate per unit increase in pore diameter, commonly
defined as pore size distribution). The resullts for sample B are shown in the following graphic.
Overall, CFP has proven to be a very useful tool for the non-wovens characterisation industry. If
you wish to know more about how CFP can help your non-woven business, contact us at
[email protected]
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