Water repellent finishes:
The easy wetting of textiles is very important for its processing as well as in a large number of its end uses. Higher repellency or poor wetting with water and oil as well as resistance to staining are essential for various end uses in clothing, home and technical textiles. The textile materials are modified to have a wide range of resistance to fit the varying demands of the users for the intended purpose. The oldest repellent finish meant repellency to water. The purpose of this finish is that the drops of water should not spread on the surface of the textile and should not wet the fabric, as shown in Figure-1. The drops should stay on the surface and easily slide off.
Similarly, oil-repellent finishes should prevent oily fluids from wetting treated textiles. In a similar manner, soil-repellent finishes should protect textiles from both dry and wet soils. In all cases, the air permeability of the finished fabric, in general, should not be significantly reduced.
Water repellency is most difficult to define, as various static and dynamic tests based on varied criteria are used to measure it. In general, water repellent fabrics are those which resist being wetted by water; the water drops will simply roll off the fabric. A fabric's resistance to water will depend on the nature of the fibre surface, the porosity of the fabric and the dynamic force behind the impacting water spray. The conditions of the test must be stated when specifying water repellence.
There are various terms use to describe water refellency such as, showerproof; showerproofed; rainproof; waterproof; rain-resistant; shower-repellent; shower-resistant; rain-repellent; stainproof; siliconised etc. These are also called marketing terms of water repellency. Fabrics to be used as raincoats, umbrellas, and tarpaulins have to be treated with chemicals to give them a property which makes them water resistant.
Repellency of textile fabrics:
The repellency of a textile fabric depends upon the resistance to wetting and penetration by a liquid. Water and oil are the most important liquids for normal textile fabric end-uses.
The main parameters that determine the resistance of a fabric to wetting are:
The chemical nature of the fibre surfaces (for example, presence of polar or non-polar groups);
The geometry and roughness of the fibre surfaces (for example, longitudinal striations, fissures, crenulations and so on, and modified cross-sections that promote wicking);
The nature of the capillary spacings in the fabric (for example, inter-fibre and inter-yarn capillary spaces).
Oil and Water Repellency Test Method:
There are three main types of test methods available for assessing the water repellency of the specimen, which should be suitably preconditioned prior to testing under standardized conditions:
Class I spray tests for assessing rain impact
Class II hydrostatic pressure tests, which measure water penetration
Class III sorption of water due to immersion of specimen in water
The most widely used test methods are briefly discussed here.
Class I: Spray tests to simulate exposure to rain:
In the AATCC Test Method 22, Water-Repellency: Spray Test, water is showered on the fabric specimen, which has been preconditioned for 4 h prior to testing, producing a wetted pattern. A rating will be given by comparison of the wetted pattern to standard chart pictures. This is a rapid, simple method, which is technically equivalent to ISO 4920 and BS EN 24920.
The AATCC Test Method 35, Water Resistance: Rain Test assesses fabric performance when it is sprayed with rain water as well as the pressure due to the rain’s impact. This test is applicable to all type of fabrics whether treated with a water repellent chemical or not. The test specimen is conditioned at a relative humidity of 65% ± 2% and a temperature of 21°C ± 1°C for at least 4 h. The specimen is placed on a weighed blotter and water is showered on it when it is placed in the rain tester for 5 min. In this test method, rain impact can be varied by changing the height of the water from 60 to 240 cm. At the end, the blotter will be weighed again to assess the quantity of water that has leaked through the fabric. The fabric performance is assessed by various parameters by determining the maximum pressure where no penetration is observed, the effect of a change in pressure on fabric penetration, and the least pressure required for penetration of 5 g of water onto the tested specimen.
The AATCC Test Method 42, Water Resistance: Impact Penetration Test is also useful. The fabric resistance to impact by water is measured and used to predict the penetration of rain into the fabric. In this test, 500 mL of water is showered onto the sample at a height of 2 ft. The rest of the procedure is the same as that for Test Method 35.
Other standard test methods are ISO 9865, Textiles, Assessing the Repellency of Water by Bundesmann Shower Test and BS EN 29865; both determine the repellency of fabrics that are permeable to air. Water is filtered and de-ionized, which is then passed through jets of specific dimensions and sprayed onto the fabric surface. Four test specimens will be kept at a specific angle to the cups and are simultaneously exposed to a heavy rain shower of controlled intensity while the under-surface of each specimen is subjected to a rubbing action. Water that is passed through the fabric will be collected in the cup and later its volume will be measured. In addition, the amount of water that is retained by the test specimen will be measured by comparison of the weight of the fabric before and after the testing. Note that all rain simulation tests should, in theory, replicate rain conditions that occur in practice.
Class II: Hydrostatic pressure tests:
For many high-performance fabrics that are rendered waterproof, a hydrostatic pressure test may be conducted in one of two ways:
By applying a gradual hydrostatic pressure on the fabric and assessing the minimum pressure necessary for penetration
By subjecting the fabric to a constant hydrostatic pressure for a lengthy time duration and assessing any penetration
Both the International Standard and the British Standard tests subject the specimen to gradual hydrostatic pressure and measure the pressure necessary for penetration. Two test methods are ISO 811: 1981, Water Textile Fabrics, Determination of Resistance to Penetration, Hydrostatic Pressure Test and BS EN 20811: 1992, Resistance of Fabric to Penetration by Water—Hydrostatic Head Test. In the AATCC Test Method 127, Water Resistance: Hydrostatic Pressure Test (related to ISO 811) the pressure of the water applied to the recessed base where the specimen is placed will be gradually increased as per the standard rate; the fabric surface will then be observed for any signs of penetration by water. The end point will come when penetration occurs for the third time (that is, three points of leakage) and is determined by the penetration pressure, which is measured in centimeters in a water gauge.
Class III: Sorption of water by the fabric immersed in water:
The AATCC Test Method 70, Water-Repellency: Tumble Jar Dynamic Absorption Test assesses the absorption of water into the specimen under conditions similar to actual use.
Preconditioned and pre-weighed samples are kept in water for a specific time; extra water is eliminated by the wringer method and the sample is weighed again. The percentage weight increase of the specimen will reflect the sample’s absorption.
The AATCC Test Method 118, Oil-Repellency: Hydrocarbon Resistance Test (technically equivalent to ISO 14419) can be used to assess the sample capacity for repellency of oil under specific conditions. Drops of the standard test liquids are assessed on the test fabric. The rating reflects the highest numbered test oil that is not able to wet the fabric.
The visual evaluation of the liquid drop on the fabric surface is graded as;
A = Pass: clear well-rounded drop
B = Borderline pass: rounding drop with partial darkening
C = Fail: wicking apparent and/or complete wetting
D = Fail: complete wetting
The 3M test uses mixtures of Nujol Oil and n-heptane in various proportions numbered from 50 (100% Nujol) to 150 (100% n-heptane). It should be noted that the oil-repellency test is conducted under static conditions and depends completely upon the contact angle of the oil on the fibers.
The 3M test uses mixtures of Nujol Oil and n-heptane in various proportions numbered from 50 (100% Nujol) to 150 (100% n-heptane). It should be noted that the oil-repellency test is conducted under static conditions and depends completely upon the contact angle of the oil on the fibers.
References:
Advanced Textile Testing Techniques Edited by Sheraz Ahmad, Abher Rasheed, Ali Afzal, and Faheem Ahmad
Principles of Textile Finishing by A K Roy Choudhury
Textile Finishing Edited by Derek Heywood
