Characterization of Pore Structure in Textiles

Last Updated on 19/03/2021

Characterization of Pore Structure in Textiles

Arpita Kothari
M. Tech. Scholar
Department of Textile Technology,
NIT Jalandhar, India
Email: geniousarpita@gmail.com

Abstract:
Pore structure is very complex and its distribution varies according to application area. The filtration properties can indirectly analyze by pore size measurements and pore size distribution. There are so many characteristics which define the pore structure such as pore volume, pore size, pore surface area, permeability, throat diameter etc. Characterization of pore sizes is helpful in predicting the performance of fabric in use. In this study different characterization techniques of pore structure used for textiles has been studied. One of them, Porometry has the unique ability to measure all the important pore structure characteristics. The technique is being constantly updated and has undergone many changes during the recent years.

Keywords: Characteristics, Pore distribution, Porometry, Permeability.

1. Introduction:
Pore structure of a textile material can be varied depending upon the application of fabric. Pore size and its distribution is one of the most important physical parameters.[2] Achieving a particular level of pore size is simpler in case of woven and knitted fabric as compared to nonwoven fabric. Again, pore geometry is quite complicated and difficult to predict in nonwoven fabric. Pore size distribution in nonwoven fabrics is usually unimodel and relatively broad, whereas woven fabric has bimodal distribution because of inter-fibre and inter-yarn pores. The performance of nonwoven is governed by its properties such as fibre diameter, orientating, pore size and packing density etc. In this study pore size and its distribution has been studied, how these affects the performance of nonwoven such as liquid and gas permeability, barrier property, filtration efficiency etc.

Table 1: Importance of pore structure in different fields. [7]

2. Characteristics of pore:
As already discussed that defining the characteristics of pore is very complex job but it can be characterized by following parameters. [5]

2.1. Porosity:
Porosity or void fraction is a measure of empty space in material, and is a fraction of volume of voids over the total volume. [9]

P = Vv / [Vv +Vs]
P = porosity
Vv = volume of pores
Vs = volume of solid in the material

2.2. Pore path:
Tortuosity is a property of curve being tortuous (twisted; having many turns). There have been several attempts to quantify this property. Tortuosity is commonly used to describe diffusion in porous media, such as soils and snow. [9]

Ʈ = ( lc / l )
Ʈ = tortuosity
l = thickness of porous medium
lc= length of pore path

(Flow Rate) ∞ [(D4 ∆p) / lc]

Flow through tortuous path is less.

2.3. Pore size:

2.4. Pore size distribution:

• May be very narro
• May be broad
• May be multi-modal

2.5. Pore surface area:

2.6. Pore volume:

Total pore volume = through pore volume + blind pore volume + closed pore volume [9]

3. Pore characteristics and their application in various industries:
Different pore characteristics have their own utility in different applications which are listed below in table no. 2: [7]

Table 2: Importance of pore characteristics in various fields

4. Characterization techniques:
The techniques used for characterizing the pore structure are described below in table no. 3: [5]

Table 3: Techniques used for measurement of characterization of pore

4.1. Brief study of different techniques:
Different techniques used in textiles for characterization of pore are listed below in table no.4. [5]

Table 4: different techniques with their capabilities:

4.2. Capabilities and limitations of techniques:
There are two techniques which are most widely accepted for measurement of pores; one is Porosimetry which can measure only porosity (pore volume and pore size distribution), another is porometry which can measure largest pore, smallest pore as well as pore size distribution. [6]

Characterization technique in table 5 and 6 showing different capabilities and limitations of different pore with their pore diameter range.

Table 5: Capabilities of different technique [7]

Table 6: Limitations and pore diameter range of techniques [7]

5. Analysis of experimental data measured from capillary flow porometer:
Porometry has the unique ability to measure all the important pore structure characteristics. [4] The differential pressure on dry and wet sample has gradually increased and then gas flow rate measured as a function of differential pressure. [1]

For dry sample:
Gas flow rate through the dry sample increases with increase in differential pressure because all the pores in the dry sample are free from the wetting liquid and are open for gas flow. The manner in which the gas flow rate through the dry sample increases with differential pressure can be predicted. [1]

Flow rate depends on:

• Pore diameter
• Number of pores
• The inlet pressure, that changes

For wet sample:
Initially pores are blocked by liquid, as pressure increase largest pore emptied, further increment causes smallest pore emptied and gas flow started. [1]

Pore Structure Characteristics Measured:

5.1. Throat pore diameter:
The differential pressure required for displacement of liquid at the most constricted part of the pore (pore throat) is the maximum. When this maximum pressure is reached, the gas removes liquid completely from the constricted part of the pore as well as from the rest of the pore beyond the most constricted part and begins to flow through the pore. The porometer detects the increase in flow. Therefore, the differential pressure of the gas at which gas flow through a pore occurs yields the diameter at the most constricted part of that pore. [1].

Pore diameter, D= equivalent cylindrical pore diameter

[dS/dV] pore = [dS/dV] cylindrical pore = 4/D
p = 4 γ cos Ɵ / D

Where,

p = differential gas pressure on wetting liquid in pore
γ = surface tension of wetting liquid
Ɵ = contact angle of the liquid
dV = displaced volume of liquid in the pore
dS = increase of solid/gas surface area due to displacement of liquid.

5.2. The Largest Through Pore Throat Diameter (The Bubble Point Pore Diameter):
The largest pore throat diameter is the largest of the most constricted pore diameters of all pores. The differential pressure required to start gas flow through a wet sample is known as the bubble point pressure or simply the bubble point because at this pressure the first air bubbles start forming on the sample. The differential pressure that can empty the largest of the throat diameters of all through pores, initiates gas flow through a wet sample and is the bubble point pressure. [1]

Gas flow through a wet sample is zero at the beginning of the wet test, but starts at certain differential pressure, which is the bubble point pressure. The pore diameter computed from bubble point pressure is the bubble point pore diameter.

Calculation of bubble point pore diameter:
From experimental data (Figure 15):

5.3. Mean Flow Through Pore Throat Diameter:
In order to determine the mean flow through pore diameter, half-dry flow needs to be computed from measured dry flow (gas flow through dry sample). Half-dry flow is half of the dry flow at a given differential pressure. The differential pressure at which the wet flow and the half-dry flow are the same is known as the mean flow pressure. The pore diameter calculated from the mean flow pressure is known as the mean flow pore diameter. Mean flow pore diameter is such that half of the flow through a dry sample is through pores having diameter greater than the mean flow pore diameter, and the other half of the flow is through pores having diameter smaller than the mean flow pore diameter. [1]

5.4. Range of Through Pore Throat Diameters:
The bubble point pressure gives the largest through pore throat diameter, and the differential pressure at which the wet and dry curves meet gives the smallest pore throat diameter in the sample. These two pore diameters give the pore size range in the sample. [1]

5.5 The Relation between the Measured Pore Diameter and the Actual Pore Size:
Many pore cross-sections may be considered to be elliptical with minor axis, d, and major axis, nd. Simply by assigning different numbers to the axial ratio, n, a variety of pore cross-sections may be represented. For a pore having elliptical cross-section: [1]

Measured pore diameter, D

The largest particle that can pass through the elliptical pore is d. The ratio of the diameter, d, of the largest particle that can pass through and the measured pore diameter, D, is the pore shape factor. It is given by:

The pore shape factors, [d/D], for a few typical cross-sections are listed in Table no. 5.

Table 7: Relation between measured and actual pore sizes 

Filtration media                     Pore cross-section                          Shape factor

The predictions in Table 5 agree very well with the results obtained with fabrics. For example, the average sizes of openings in polyamide fabrics computed from fiber diameters and mesh counts are in good agreement with the largest particle that can pass through computed from the pore diameters measured by the porometer.

Measured pore diameters may be made comparable with d by including a multiplying factor,

Table 8. Comparison of the maximum diameter of particle that can pass through the pores obtained from fiber diameter and mesh count of the fabric and from the pore diameter measured by Porometer.

5.5. Pore Distribution:
Let Fw,j and Fd,j be the flow rates through wet and dry samples respectively at the differential pressure pj corresponding to pore diameter, Dj, and Fw,j+1 and Fd,j+1 be the flow rates through wet and dry samples respectively at the next higher differential pressure pj+1 corresponding to pore diameter, Dj+1 as shown in figure 9: [1]

Pore distribution can be represented in three ways:

5.5.1. Pore size frequency:
Pore size frequency defined by the percentage flow through pores in a given size range.

5.5.2. Cumulative Filter Flow:
The quantity [(Fw, j / F d, j) × 100] plotted as a function of pore size is designated as cumulative filter flow by PMI in its report program. Figure 11 shows cumulative filter flow as a function of pore diameter. For example, this figure shows that 45 % flow is through pores having diameter greater than 15 microns. [1]

5.5.3. Flow Distribution:
The pore size distribution function, fF. It is defined in the following manner:

Where the leading negative sign incorporates the fact that decrease in pore size increases the flow rate. The function is equal to the increase in percentage flow rate per unit increase in pore diameter. Integration of above Equation suggests that area under the distribution function in any pore diameter range (Figure 20) gives percentage flow in that diameter range. The distribution function gives the flow distribution over pore diameter. Because gas flow rate through a pore is proportional to the fourth power of its diameter and number of such pores, the pore distribution is expected to be similar to the flow distribution, but shifted to lower pore diameters. Therefore, the flow distribution is designated as pore distribution. [1]

Since the increase in the percentage flow is determined essentially by the increase in the number of pores and the pore diameter, a sharp increase in pore size distribution suggests that the number of pores of that diameter is large.

The three quantities describing flow distribution are related to each other in a simple manner. For a given change in pore diameters, the pore size frequency (%) is equal to the consequent change in the cumulative filter flow (%), which is also equal to the area under the flow distribution function.

5.6. Gas Permeability:
Gas flow rates measured through the dry sample is used to compute gas permeability using Darcy’s law. According to this law, flow of fluids through porous media is proportional to the pressure gradient causing flow.[1]

Equation suggests that the slope of the plot in Figure 13 is the product of pressure and permeability. If the permeability is constant, the slope should increase with increase in pressure. The linear behavior observed in Figure 13 suggests that the permeability has a tendency to decrease with increasing pressure. This is expected to be due to the effect of pressure on pore structure.

5.7. Envelope Surface Area:
The surface area of through pores, which allow gas to pass through, is considered the envelope surface area. flow rate of gas through a porous media can be expressed by the following equation:[1]

Where,

This technique measures the envelope surface area, which cannot be measured by any other technique. The technique has the added advantage of experimental simplicity.

Errors are introduced in the measurement can be reduced if the flow is primarily viscous. When the volumes of closed pores and blind pores are negligible, the porosity, which enters in to calculation, is primarily due to through pores. However, if the volume of closed and blind pores is appreciable, the porosity value would be greater than that due to only the through pores and would introduce errors in the envelope surface area calculation. Errors are also introduced when the largest pore diameter is much larger than the average pore diameter, because large diameter pores make excessively low contribution to surface area, while their contribution to flow is much in excess of their number.

6. Development in characterization technique:

6.1. Flowmetry with extended capability:
The technique is being constantly updated and has undergone many changes during the recent years. [4]

• Advanced Flow Porometry[3] (Fully Automated & Computer Controlled Device)
• Microflow Porometry (Measures very low permeability samples)
• In-Plane Porometry (Directional Porometry) (Measures pore structure in any orientation & of layers)
• Compression Porometry (Measures pore structure of sample under stress)
• Cyclic Compression Porometry (Measures pore structure of sample under cyclic stress)
• Clamp-On Porometry (no need of sample cutting from bulk material)
• Nanopore Porometry (Measures pore diameter down to O.005mm)

6.2. Digital Image Analysis:

• A new image-based PSD determination method has been developed for nonwoven geotextiles.
• The method uses planar and cross-sectional views to capture the 3D structure of a nonwoven geotextile using an optical light microscope [8]
• The method consists of three steps: specimen preparation, image analysis, and pore opening size determination.
• The image analysis method was developed using various mathematical morphology algorithms to determine geotextile pore opening sizes, O95 and O50 Finally, the measured values were checked against the manufacturer’s reported AOS values.

7. Summary and conclusions:

1. Pore size and its distribution is one of the very important physical parameters, which can be defined by many characteristics.
2. Each technique used for characterizing the pore structure has their own capabilities and limitations.
3. Each of the characteristic may be critical in certain end use application.
4. Flow porometry is a highly versatile technique capable of measuring many pore characteristics.
5. The image analysis method is unique and can measure fiber thickness and pore opening sizes in a given image.
6. It is clear that adequate characterization of textiles is essential for development of novel textiles and deciding suitability of textiles for applications.

8. References:

1. “Wetting Liquid Extrusion Techniques: Part I- Capillary Flow Porometry”, Dr. Akshaya Jena, Porous Materials, Inc., USA
2. “Pore Structure of Textile Material”, V. K. Kothari & A. Mukhopadhyay, Indian Textile J., 109, No. 11 (1999) pp. 8-23.
3. “Capillary flow porometry with Extended Capability”, Dr. Krishna Gupta, Porous Materials Inc. USA.
4. Presentation on “Advances in pore structure evaluation by porometry”, Dr. Akshaya Jena, Dr. Krishna Gupta, Porous Materials, Inc., USA.
5. Brochure of Porous Materials, Inc., USA.
6. Presentation on “Methods for characterization of porous materials”, Centre of excellence POEMES, Bulgarian Academy of Sciences.
7. Presentation on “Pore Structure of Advanced Textiles”, Dr. Akshaya Jena & Dr. Krishna Gupta, Porous Materials, Inc., New York, USA.
8. “Digital Image Analysis to Determine Pore Opening Size Distribution of Nonwoven Geotextiles”, Ahmet H. Aydilek, Seyfullah and Tuncer, Journal of Computing in Civil Engineering, October 2002, pp.280- 290.
9. N.C.Ray, Presentation on, “Pore size measurement and its analysis”

Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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