The water resistance of a fabric, also known as water proof, is mainly reflected in two aspects: water penetration and surface wetting. The permeability of a fabric is the property of liquid water penetrating from one side of the fabric to the other, the opposite property is called resistance to water penetration. In addition, the property of a fabric to be easily wetted by water is known as surface wetting of fabric. For the water resistance of fabrics, there are 3 main factors in general.
1 Wettability of the fibre surface: contact angle of fiber surface
As shown in the diagram: When the contact angle of the fibres θ < 90°, the fibre aggregate material is a water-conducting material and the tighter structure only leads to more capillary cores to absorb and conduct water. When the contact angle of the fibres θ > 90°, the fibres are water repellent and the more compact the fabric structure (i.e. the smaller the pores), the better the water repellency. Therefore, only when the contact angle of the fibres is known that the water resistance of the fabric can be better discussed.
2 Coating of fabrics
The water resistance of a fabric can be increased by applying a layer of impermeable, water-insoluble film to the surface of the fabric. This method is less suitable for clothing, but can be used for tarpaulins or ponchos etc. The use of a waterproof, microporous coating membrane results in a coated fabric with excellent water resistance and good water permeability and breathability.
3 Environment of use
Waterproof fabrics or coated fabrics are mostly made of non-hygroscopic fibres or coated materials, so changes in relative humidity do not affect their waterproofing properties. Water-conducting fabrics, on the other hand, are mostly made of moisture-absorbing fibres. Relative humidity increases, the fibres absorb moisture to enhance, the fibres swell and the capillary effect is enhanced, so the water conductivity of the fabric is enhanced. The influence of temperature is the same as that of humidity. Therefore, the environment has a great influence on the water-conducting fabric.
In summary, the water resistance of fabric consists of two main properties: preventing liquid water from passing through the fabric and wetting the fabric. There are two main methods of testing the water resistance of fabrics according to the various scenarios in which they are used: the hydrostatic pressure test and the spray test.
1 Hydrostatic pressure test method for testing the water resistance of fabrics
When water passes through a fabric it encounters resistance and this resistance to water penetration is the waterproof og fabric. The hydrostatic pressure test is an important test method for waterproof and permeable fabrics and is applicable to all types of fabrics, including those with a waterproof finish. The hydrostatic pressure test is a simple, rapid testing technique that allows accurate quantification and good reproducibility.
Under standard atmospheric pressure conditions, water pressure is applied to one side of the fabric until water droplets seep out from the back of the fabric. The hydrostatic pressure measured by this method is not only related to the water resistance of the fabric, but also to the nature of the fabric itself. The greater the hydrostatic pressure that the fabric can withstand, the better the waterproofness.
The water resistance of a fabric is related to the structure of the fibres, yarns and fabrics. The water resistance of fabrics is measured by hydrostatic pressure methods, both hydrostatic and dynamic. Under laboratory conditions, the hydrostatic pressure test has the following test methods.
1 Increasing the water pressure on one side of the fabric and determining the size of the hydrostatic pressure that the fabric can withstand until a specified number of water drops appear on the other side of the fabric. ISO 811 uses this test method for testing the hydrostatic pressure of compact fabrics such as canvas, tarpaulins, tent fabrics etc.
2 Under the specified conditions, make one side of the specimen subject to a continuous rise of water pressure, until it reaches the specified water pressure value. Observe whether penetration occurs within a specified time period. If no water penetration point appears on the surface of the specimen and there is no sign of wetting, the specimen passes the test. ISO 1420 adopts this test method for testing whether the coated fabric meets the requirements.
3 Maintain a certain water pressure on one side of the fabric and determine the time taken for the first drop of water to seep out from one side of the fabric.
4 Maintain a certain water pressure on one side of the fabric and measure the amount of water per unit time that passes through the fabric.
The common hydrostatic pressures testing standards
- ISO 811: textile fabrics – determination of resistance to water penetration – hydrostatic pressure test
- ISO 1420: rubber or plastics coated fabrics – determination of resistance to penetration by water
- AATCC 127: water resistance – hydrostatic pressure test
- BS EN 20811: textile – determination of resistance to water penetration – hydrostatic pressure test
- ASTM D751: standard test method for coated fabrics
- ASTM D3393: standard specification for coated fabrics – waterproofness
- JIS L1092: testing method for water resistance of textiles
- GB/T 4744: determination of resistance to water penetration of textile – hydrostatic pressure test
The more widely used international standards for hydrostatic pressure testing methods are ISO 811 and ISO 1420. Most of the standards in various countries are based on these two. The test principles of ISO 811 and ISO 1420 are basically the same, both apply a water pressure on one side of the specimen and observe whether the specimen appears to be permeable, the difference is that ISO 811 is mainly for general fabrics and ISO 1420 is for coated fabrics.
Using ISO 811 as an example, the ratings are as follows
|Level||Hydrostatic pressure value
|Water resistance assessment|
|0||P < 4||Poor resistance to hydrostatic pressure|
|1||4 ≤ P < 13||Some resistance to hydrostatic pressure|
|2||13 ≤ P < 20||Some resistance to hydrostatic pressure|
|3||20 ≤ P < 35||Good resistance to hydrostatic pressure|
|4||35 ≤ P < 50||Good resistance to hydrostatic pressure|
|5||50 ≤ P||Excellent hydrostatic resistance|
In addition, there are some differences in the expression of test results between the two standards. ISO 811 is to continuously increase the water pressure in the test until the specimen shows water seepage in three places, and record the pressure value at this point as the test result. ISO 1420, on the other hand, is maintained under the specified water pressure conditions for a period of time, and if the specimen does not show water penetration, the specimen passes the test.
In addition to the above two method standards, there are also EN, AATCC, JIS, GB and other standards, but these standards are all developed on the basis of equivalent adoption or reference to ISO standards. BS EN 20811 is equivalent to ISO 811, JIS L1092 Method A and GB/T 4744 are based on ISO 811, with the same test parameters and expression of results as ISO 811. AATCC 127 has fewer specimens than ISO 811, with a minimum of three specimens, and there is only one option for the rate of water pressure rise in AATCC 127, whereas ISO 811 has two options for the rate of water pressure rise.
Note on testing the water penetration of fabrics by the hydrostatic pressure method
When placing the fabric, the air should be discharged from the fixture first, so that the front side of the fabric is attached to the water, to avoid the impact of air pressure on the water pressure resulting in inaccurate data.
In some composite fabric testing, the reverse side of the fabric is not waterproof, so you need to cut the test sample larger than the instrument sink to avoid water penetration into the fabric to affect the assessment of the hydrostatic pressure phenomenon.
Factors affecting the water penetration of fabrics
(1) The distance between the yarns affects the level of water resistance. Generally speaking, the tighter the fabric structure, the better its resistance to water penetration.
(2) Thickness of the fabric. The thicker the fabric, the greater the wet resistance and the greater the water resistance.
(3) The thickness of the yarn. For moisture-absorbing fibres woven into compact fabrics, due to the existence of the capillary effect, reducing the yarn radius can improve the water resistance of the fabric.
(4) The quality of the composite membrane. The larger the pore size of the membrane, the worse the hydrostatic pressure resistance of the coated fabric.
(5) Coating thickness. With a thick coating, the fabric’s resistance to water pressure will increase.
(6) The uniformity of the coating. The better the quality of the coating, the better the resistance to water penetration performance. The coating is not uniform, the lower the hydrostatic pressure value.
2 Spraying method for testing surface wetting performance of the fabric
Spraying method: By continuously spraying or dripping water onto the specimen and observing the characteristics of the wetting on the surface of the specimen after a certain period of time. Then assess the water resistance of the fabric by comparison with various sample photographs.
The spray method simulates the degree of wetting of clothing when exposed to rain. This method is applicable to all water repellent treated and non-water repellent treated fabrics and the test results are highly dependent on the fibres, the yarn, the treatment of the fabric and the fabric construction.
The common spray testing standards
- ISO 4920: Textile fabric – determination of resistance to surface wetting – spray test
- AATCC 22: Water repellency – spray test
- JIS L1092: Testing method for water resistance of textiles
- GB/T 4745: determination of resistance to surface wetting – spray test
There are five levels of evaluation. Take the standard ISO 4920: Textile fabric – determination of resistance to surface wetting – spray test as an example, the evaluation criteria for waterproof performance are as follows.
|ISO level||AATCC level||surface wetting performance|
|0||0||The entire surface of the specimen is completely wetted and has no resistance to spraying.|
|1||50||The surface is wetted and has no resistance to spraying.|
|2||70||Partial wetting of the specimen face beyond the spray points.|
|3||80||Wetting of specimen face at spray points.|
|4||90||Slight random sticking or wetting of the specimen face.|
|5||100||No sticking or wetting of the specimen face.|
This is usually determined using a spray rating tester. In the AATCC 22 test method, hold the test sample in place with a 152.4mm diameter iron ring, the sample is in tension and the surface is flat without wrinkles.
Spray 250mL of distilled water from a standard nozzle at 45° to a specimen 150mm below the nozzle for 25~30s. After the spraying, gently tap the fabric with the front side down in a horizontal position and then assess the water repellency properties of the fabric by observing the state of the water stains on the surface of the specimen.
Note on testing the water penetration of fabrics by the spray testing method
For fabrics with greater elasticity, the fabric should not be pulled hard when clamping the fixture, otherwise it will easily lead to a larger gap in the fabric and water droplets will penetrate the wetted fabric, resulting in inaccurate test results.
Hold the fixture for knocking after the spraying is over, the strength should be even and moderate, the strength is too light will make the water droplets remain, the strength is too large will easily lead to the fabric from the fixture, all will affect the assessment of the water staining phenomenon.
To regularly calibrate the instrument, when the spray water flow is found to have blockage phenomenon or spray time has abnormal, to promptly carry out cleaning spray mouth or replacement, so as not to affect the accuracy of the test.