Permeability
Electromagnetism or permeability is a measure of the resistance of a material against the formation of a magnetic field, otherwise known as distributed induction in the transmission line theory. Thus, it is the degree of magnetization that takes any element in response to any applied magnetic field. Magnetic permeability is usually represented by the Greek letter μ.
Different types of permeability are as below-
1. Air Permeability
2. Water Permeability
3. Metal permeability
4. Magnetic permeability
Air permeability
The air permeability of a fabric is the amount of air measured in cubic centimeters per second beyond 1 centimeter 2 of the fabric at a pressure of 1 centimeter of water. The air permeability of fabric is a measure of how well it allows air to pass through. Air permeability is important for a few fabric edges such as industrial filters, tents, sailcloths, parachutes, raincoat materials, shuttering, waterproof fabrics, and airbags.
It depends on-
i. Type of Yarn
ii. Fabric structure
iii. Fiber parameters
Air resistance: The air resistance of a fabric is the time in seconds for 1 cm3 of air to pass through 1 cm2 of the fabric under a pressure head of 1 cm of water.
Air Porosity: The porosity of a fabric is the ratio of air space to the total volume of the fabric expressed as a percentage.
Let,
S= A total volume of sample in cm3
F= A total volume of fabric in cm3 Air Porosity, P= (𝑺−𝑭)/𝑺 ×100
Air permeability and cloth cover
The openness of the weave and the rate at which air flows through it is closely related. The more open the structure the greater the air permeability. Air permeability is inversely proportional to the cover factor.
I.e. Air permeability ∞ 1/𝐾𝑐
Where, Kc is the cloth cover factor, calculated from the following formula, Kc = (k1+K2) - 𝑘1𝑘2/28 Where,
K1=Warp cover factor and
K2= Weft cover factor
Clayton used a permeability value P, calculated below-
P=100𝑣/𝐴𝑝
Where,
V= the volume of air in cm3 per second flowing through the sample
A= area of the test specimen in cm3 and
P= Pressure head in 1 cm of water
Shirley air permeability tester
a. At 20±2ºC and 65±2%, R.H. air is drawn from the laboratory through the test specimen ‘S’ using a suction pump ‘A’. The rate of flow is controlled using bypass valve ‘B’ and the series valve C.
b. The rate of flow is adjusted until the required pressure drop across the fabric is indicated on a draught gauge D, graduated from 0 to 25mm head of water.
c. For high-resistance fabric, the rate of airflow through the sample is insufficient for proper operation of the suction pump as it can be overcome by opening the bypass valve B.
d. For low-resistance fabric, the bypass valve ‘B’ is closed, and fine control is obtained by adjusting the series valve.
e. ‘E’ is a reservoir that soothes out any disturbance due to varying velocities of the streams of air drawn through the various paths by the pump.
f. When the required pressure drop, which is normally 1 cm of water, is attained and the indicator of the draft gauge is steady, the rate of airflow is read off one of the four Rota meters R selected according to the permeability of the test specimen.
g. The Rotameters are calibrated, at 20ºC and 760 mm of mercury to indicate airflow in cm3/sec. And they cover the following ranges: R1:0.005-0.5, R2:0.5-3.5, R3:3-35, R4:30 – 350.
h. The test is commenced with R4 open and the other meters closed if the flow is less than 30 cm3/sec. R3 is opened and R4 is closed.
i. This procedure is repeated until the most suitable range for the fabric under test has been selected.
j. To prevent damage to the draught gauge a safety valve F is provided.
k. The test area is 5. 07 cm2; since a 1 in diameter, the circle is exposed when the specimen is clamped in the holder. From the readings on the Rotameter, either the air permeability or the resistance can be calculated. The average rate of flow from five specimens is calculated.
The results can be obtained by using the following formula-
Air permeability = Average rate of airflow from 5 specimens/5.07 cm3 per second
Water permeability
Water permeability is one of the critical parameters used in the assessment of textile structures for vascular implants, which is a measure of the water flux through a fabric under a fixed pressure of 120 mmHg.
Water Permeability Influencing Factors of Fabric-
Fabric permeability is the ability of liquid water to enter from one side to the other. Three main factors affect the waterproofness and water permeability of the fabric.
i. Fiber surface shaking
ii. Fabric Coating: A multi-micropores coating can form a good waterproof, water permeability, and good air permeable coating fabric, mostly used on thin or rain skin, and much more.
iii. Environment: For water-conducting fabric, relative humidity increases, fiber absorption increases, and the water conductivity of the fabric increases.
Water Repellency
Water-repellent fabrics are those that resist being wetted by water, water drops will roll off the fabric. A fabric’s resistance to water will depend on-
a. the nature of the fiber surface,
b. the porosity of the fabric
c. the dynamic force behind the impacting water spray
The difference between water-repellent and water-proof fabrics is as below-
Water repellents are chemical finishes that-
a. Resist the penetration of water into the fabric
b. Permits the passage of moisture or air through the fabric
There are three types of water-repellent finishes-
a. non-durable finishes
b. semi-durable finishes
c. durable finishes
a. Non-durable finishes
i. Finishes contaminated with this type of water are usually based on paraffin wax-aluminum acetate emulsion.
ii. Non-durable finishes are easily removed in laundering or dry-cleaning.
iii. Non-durable finishes do not provide satisfactory resistance to liquids.
iv. Examples: Aquarol AX, Aridex WP, Dry On, Rephlex, etc.
b. Semi-durable finishes
i. These are wax or salt solutions.
ii. They have emulsified off the fabric in laundering.
iii. They are resistant to dry cleaning.
iv. Examples: Bishop, Dry on A-B, Nalan W, etc.
Water Repellency Rating-
Rating Description
1. 100 No sticking or wetting of the upper surface.
2. 90 Slight random sticking or wetting of the upper surface.
3. 80 Wetting of upper surface at spray points.
4. 70 Partial wetting of the whole of the upper surface.
5. 50 Complete wetting of the whole of the upper surface.
6. 0 Complete wetting of the whole of upper and lower surfaces.
Apparatus
1. Spray tester
2. Water
3. Fabric
Working Procedure
1. The sample fabric is mounted on the embroidery hoop and fixed on the instrument at 45 angles.
2. Now the beaker is filled with 250 cc water and poured into the funnel.
3. The water is showered through the spray nozzle on the fabric.
4. After spraying the sample holder will be removed and the excess water will be removed by tapping the bar frame with the sample face against the solid object.
5. Water repetition is assessed from the spray rating chart.
6. 5 tests should be made and the nearest rating assigned to each since no interpolation is allowed, i.e. a rating for a specimen cannot be 75.
7. The mean of the 5 ratings is taken as the result.
Table: Ratings found for different fabrics
Metal Permeability
Relative permeability refers to a material's ability to attract and conduct magnetic lines of flux. The more conductive material is to magnetic fields, the higher its permeability. Most materials have μR near 1, including copper, and gold. The exceptions are nickel, iron, etc. The permeability of most metals is 1.0. However, metals such as nickel are a special case and can have permeability as high as 100 or more.
Magnetic permeability
A permanent magnet is “attracted” to a ferrous workpiece because magnetism is “induced” in the workpiece. Magnetism is induced by the magnetic field emanating from the permanent magnet. A magnet is not attracted to a piece of wood because no internal field is induced in the wood. With no induced internal field in the wood, there is no field interaction and no attraction.
So, a magnetic field can be induced in a piece of steel. As stated above, an external magnetic field is needed to do this. That is why two pieces of steel do not attract each other. They do not induce fields in each other. No field, no interaction, no attraction.
Major points of magnetic permeability
a. Magnetic Permeability is the characteristic of a material that represents the establishment of an induced internal magnetic field by an external magnetic field. The magnetic permeability is the proportionality between the Induced Field (B) and the applied Field Strength (H).
b. A material’s permeability indicates how easily an external magnetic field can induce an internal field in the material. The higher the internal field, the higher the force of attraction.
c. A material’s permeability is not constant and changes based on number a of factors. The effective permeability of a material can change with the temperature, how it was processed, the intensity of the applied drive field, humidity, etc.
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