- 1 Introduction
- 1.1 Methods of Measurement of Water Repellency,
- 1.2 Nomenclature
- 1.3 Characteristics of Fabric Surfaces
- 1.4 Mechanism of Wetting
- 1.5 The Concept of Angle Of Contact
- 1.6 Factors Affecting Wettability of The Surface
- 1.7 Water Repellent Properties of Textile Fibres and Structures
- 1.8 Effect of Yarn and Fabric Construction on Water Repellency
- 1.9 The requirement of Ideal Structure for Rain Resistance
- 1.10 Reference
- 1.11 Related
Water repellent and waterproofing factors play an important role in specific applications.
“Water Repellency (textile): The ability of a textile fibre, yarn, or fabric to resist wetting.”
“Water Resistance (fabric): Ability of a fabric to resist wetting and penetration of water.”
Methods of Measurement of Water Repellency,
- Resistance to hydrostatic pressure.
- Resistance to absorption of water during immersion.
- Resistance to penetration of water by the impact.
- Resistance to water spray.
- Resistance to rain test.
A fabric in which the pores, open spaces between the warp and weft, between the fibres, filled with substances which give continuous fabric surface and very very small air permeability.
a fabric in which like water proof fabric fibres, the pores are not filled during treatment, therefore, this type of fabrics are quite permeable to air and water vapor.
Characteristics of Fabric Surfaces
Mechanism of Wetting
Liquids are distinguished from gases by the attraction and close approach of neighboring molecules. One result of the molecules holding together is that the surface of liquid contracts to the minimum value consistent with other conditions it has to fulfill.
Thus water falling freely forms a spherical drop, because the sphere has the minimum surface required to contain the water in the drop.
The tendency of the surface to contrast is best measured by the energy required to form the surface. It is known as the surface energy or surface tension. It is usually measured in ergs/cm or [Dynes /cm].
Water has quite a high surface tension 72 Dynes /cm, while other liquids have 20 to 40 Dynes /cm
Example of water spreading on the glass and paraffin wax
Water spreads on the glass because glass molecules hold water molecules more firmly than water molecules do. There is less energy in the glass -water interface than the water-water interface and as all the system come in equilibrium with the minimum possible potential energy.
Paraffin wax, on the other hand, does not hold water molecules so powerfully as water does, because potential energy of the system increases. Therefore surface that behaves like paraffin wax, are known as water repellent surfaces.
The Concept of Angle Of Contact
The angle formed by the tangent to the drop at the point of contact with the surface (angle measured through the liquid) reveals that the size of the angle is related to the repellency of the surface.
S = solid.
Ɵ = angle of contact.
A = air.
L = liquid.
RLA = force acting between liquid and air.
If the angle is greater than 90º it depict water repellency and usually, it is shown by hydrophobic fabric whereas if the angle is less than 90º it depict wettability which is shown by hydrophilic fibres.
Factors Affecting Wettability of The Surface
Chemical Nature of the Solid Surface
When purified, the natural fiber, cotton, wool, silk, etc., are hydrophilic in character and hence the drops assume shapes similar to B. whereas if the surface is hydrophobic then water drops assumed to form a shape like A.
The Roughness of the Surface
It has a peculiar effect on the angle of contact by assuming the following equation
Where R is roughness factor and it is an angle of contact.
The Porosity of the Surface
Since textile fabric surface are not smooth continuous surface, but rather porous screen surface then, in that case, the angle of contact will be less than 90º.
Water Repellent Properties of Textile Fibres and Structures
Some rapid and reasonable conclusions concerning the inherent water repellent property of fibres may be obtained by referring following regain values
Standard Regain (%)
|Flax and hemp||12|
|Cellulose acetate rayon||6|
The natural fibres can absorb and transfer water, and of course are not water repellent; cellulose acetate and nylon have lower water absorption capacities while “vinyon”, “orlon”, “Dacron,” & “dynel” have zero regain.
Effect of Yarn and Fabric Construction on Water Repellency
Effect of yarn and fabric construction plays a greater part than the inherent fibre properties. Because by using hydrophilic fibres in the form of tight woven, thickest fabric, water repellency can be achieved.
When fabrics of various constructions made from various fibres are tested for three test (hydrostatic pressure, spray, and immersion) then it is observed that-
- Water repellent treatments enhances properties under all test conditions.
- Of the untreated samples, the cotton duck and wool Melton offers the best resistance to a hydrostatic head and spray test because these fabrics are thickest and most tightly woven.
The requirement of Ideal Structure for Rain Resistance
The stricture should have large and uniform separation of the component fibres as is possible, this is observed in case of animal fur and nature has solved the problem of a rain resistant greatly. e.g Animal’s skin, bird’s feather.
Because these feather acts exactly as water repellent finishes though are not applied this property helps to protect animals/birds skin in the rainy season.
Therefore Fibres shall be spaced uniformly and as far apart as possible. They should be held so as to prevent ends drawing together
The capacity of fabrics to absorb kinetic energy in the form of falling rain.
A falling raindrop has a certain amount of kinetic energy depending upon its mass and velocity. This kinetic energy of the drop must be converted into strain energy within the fabric.
The ability of the fabric to absorb this impact energy is related to the resilience characteristics because if fabric is soft and elastic then it lengthens the time of impact of falling drops and so some of the energy of the drop is absorbed in bending the fibres or moving the fabric instead of forcing the water through the available capillaries.
Therefore compressional resilience characteristics are required in the fabric. This is achieved by backing material or doubling the fabric.
Book Textile fibres, yarns and fabrics by Ernest R. Kaswell.