Performance and testing of waterproof and breathable fabrics
Performance and testing of waterproof and breathable fabrics
Waterproof and breathable fabrics mean that water does not penetrate into the fabric under a certain pressure, but the sweat emitted by the human body can be transmitted to the outside world in the form of water vapor through the fabric, thereby preventing sweat from accumulating and condensing between the body surface and the fabric to maintain the comfort of the garment. It is a high-tech, unique functional fabric. Waterproofing is not a problem for ordinary fabric workers. The key is how to achieve moisture permeability. Next, let’s take a deeper look at the types of waterproof and breathable fabrics.
1. Moisture permeability through fiber
1. Ventel fabric. The earliest waterproof and breathable fabric is the famous Ventile fabric. It was designed by the British Shirley Research Institute in the 1940s. It uses high-count, low-twist pure cotton yarn from Egyptian long-staple cotton and high-density heavy-weight flat weave fabric. It was initially mainly used for cold protection for British Air Force pilots during the Second World War. Soak. When the fabric is dry, the gaps between the warp and weft yarns are larger, about 10 microns, which can provide a highly moisture-permeable structure; when rain or water hits the fabric, the cotton yarns expand, reducing the gaps between the yarns to 3 to 4 microns. This closed-cell mechanism is combined with a special water-repellent finish to ensure that the fabric is not further penetrated by rainwater. At present, this type of fabric has long been replaced by other waterproof and breathable fabrics.
2. Coolmax fabrics. Major international companies such as DuPont and Japan’s Toray have researched the method of creating pores inside the fiber to discharge sweat from the body, which is the moisture-absorbing and sweat-wicking fabric on the market. The production technology of this type of fiber is concentrated in the hands of such large international companies, and the price is relatively high, making it difficult to become the mainstream of the market.
2. Achieve moisture permeability through coating
Using dry direct coating, transfer coating, foam coating, phase inversion or wet coating (solidified coating) and other process technologies, various coating agents with waterproof and moisture-permeable functions are applied to the fabric. On the surface of the fabric, the pores on the surface of the fabric are closed or reduced to a certain extent by the coating agent, thereby obtaining waterproofness. The moisture permeability of fabrics is determined by the microporous structure formed by a special method on the coating or the interaction between the hydrophilic groups in the coating agent and water molecules. With the help of hydrogen bonds and other intermolecular forces, water molecules are adsorbed on the high-humidity side, and then Pass to the low temperature side to obtain the analytical action. Coated fabrics are widely used because they are low in price and achieve a certain degree of moisture permeability. However, due to its poor waterproof and breathable performance and unsatisfactory feel, its market share is gradually decreasing.
The wet transfer coating fabrics developed now bring new opportunities to coated fabrics. Not only do they have high physical properties such as waterproofing and moisture permeability, the surface fabric can be treated with 100% Teflon, and its washing fastness can reach 25 times. Above, the hand feel is also very good.
3. Achieve moisture permeability by laminating waterproof and moisture-permeable membrane
1. PTFE film
The diameter of water vapor molecules is 0.0004 microns, while the diameter of small mist in rainwater is 20 microns, and the diameter of drizzle is already as high as 400 microns. If you can create a film with a pore diameter between water vapor and rainwater, it will be waterproof Isn’t it possible to achieve moisture permeability? The American GORE company became the first company to produce this membrane using polytetrafluoroethylene (PTFE). It was laminated with fabric and named GORE-TEX. However, because PTFE is very chemically inert, there are few materials that can laminate it well with other fabrics, resulting in very poor fabric fastness. Later, after continuous efforts, it was combined with other hydrophilic film layers to form a composite film, and special treatment was performed on the film, so that the fastness was greatly improved. It is generally believed that the water pressure of Gore-Tex fabric can reach 10,000mm, and the water pressure will drop significantly after washing 6-7 times; the moisture permeability can reach 10,000g/sqm*24hrs, but this is not the case with just made fabrics. To reach this value, it takes several washings to remove part of the glue, increase the available pores, and increase the moisture permeability.
PTFE fabrics are now mainly represented by Gore and Donaldson in the United States. Gore produces and laminates its own films and does not sell films separately. It designates better clothing manufacturers to make clothing and has separate sales staff to cooperate with them. Donaldson only produces films and laminates them at Japanese laminating manufacturers. The competition between these two companies in the market is also very fierce. Domestic PTFE manufacturers are also gradually emerging, but they mainly use single-component PTFE films without compounding with hydrophilic films, and the washing fastness is generally only about five times. Last time I met an Indonesian manufacturer at the Shanghai Industrial Fabrics Exhibition, it was said that the products were washed about five times.
Although PTFE fabric has better waterproof and breathable properties than other fabrics, due to its chemical inertness, the film is difficult to be degraded by nature, and its combustion temperature is as high as 405°C. Large-scale application has gradually made GORE-TEX an environmental killer. Faced with such a problem, GORE established a waste clothing recycling mechanism called Balance Project in order to reduce its impact on the environment. I hope that GORE can continue to move forward firmly as environmentalism becomes more and more popular.
2. TPU film
TPU is the abbreviation of thermoplastic polyurethane film, which is a non-porous hydrophilic film. Since the film itself has no pores, the waterproof effect is naturally very good, and it also makes the fabric windproof and warm. Moisture permeability mainly through its hydrophilicityOne can test evaporative thermal resistance under dynamic conditions such as while walking.
Currently, there are no design standards and testing procedures for sweat dummies. And because sweating dummies are more complex and expensive, dummy testing is more expensive than the hot plate method.
4. Other methods
A, Watkins method
B, Mernander method
C, Farnworth method
D, VanBeest method
E, Ruchman method
F, Gibson method
There are really too many, so let’s take a closer look at a few relatively useful methods.
1. Commonly used methods: ASTME96BW, American Society for Testing and Materials standards, water vapor cup method.
Since the United States has always been in the leading position in this industry, and the country’s market is also very large, the data tested by this method is also relatively high, and the ASTM standard has naturally become a commonly used method. Most customers now use this method for testing.
The test principle is as follows: Cover the fabric to be tested on a moisture-permeable cup filled with distilled water, fix it and turn it upside down, weigh it using a top-loading balance with an accuracy of 0.001g, and place it in the position of the wind tunnel row. Before the cup enters an environmental chamber with a temperature of 23°C, a relative humidity of 50%, and a wind speed of 2.5m/s for weighing, record the temperature and relative humidity of the environment. The weight of each cup and the time it took to weigh it were recorded. Record the weight at 3h, 6h, 9h, 13h, 23h, 26h, and 30h, and use the formula WVT=24*△m/(s*t) to calculate the water vapor transfer rate. The average value of 6 samples is used as the test result. The average value is multiplied by 24 and converted to the result in g/m2*24h.
What needs to be pointed out in particular is that the ASTME96BW method has 1995 and 2000 versions. The test conditions of the 95 version are immature and the relative test data is higher. Now the 2000 version is basically used.
2. Methods with high test results: JISL-1099B1, B2 Japanese Industrial Standards
The test principle is as follows: Use potassium acetate as a desiccant. Hoop the PTFE film around the plastic cup with a rubber ring to create a water vapor-permeable cup. Before wrapping the film on the cup, put enough potassium acetate solution into the water vapor permeable cup to fill it about 2/3 of the cup. Take 3 samples of 20cm*30cm square from each fabric. Each sample is placed on a test support frame. All coated or laminated fabrics are secured to the support frame with rubber rings, with the coated or laminated side facing outward. The installation of the sample support system shall be subject to being able to float in a water tank with a water temperature of 23°C. After testing the quality of the test cup (including sample, desiccant and film) with the film facing upwards, quickly turn the test cup upside down and place it into the sample holder. This assembly method is placed in a constant temperature device of 30℃±2℃. After 15 minutes, take the test cup out of the thermostatic device, turn it upside down and measure its weight. Water vapor permeability is calculated according to the formula WVT= 24*△m/(s*t).
In the moisture permeability test, Japan’s desiccant pouring cup method seems to be more favored by manufacturers because it can be quickly tested on smaller samples, does not require control of the relative humidity in the environment, and the test device and Consumables are relatively cheap. This method gives high moisture vapor transmission values, which is preferred by manufacturers, and correlates well with the perspiration hot plate method (used by many researchers).
3. Testing method that attracts attention: ISO11092 sweating hot plate method
The hot plate test method is a device used to test and simulate the heat and mass transfer process that occurs close to the skin. From the testing principle, the sweating hot plate test method belongs to the evaporative heat transfer impedance method, which is used to measure the resistance of different types of fabrics to water vapor (water vapor impedance refers to the vapor pressure difference on both sides of the fabric divided by the direction of the pressure gradient. Total evaporation heat flow per unit area). The higher the evaporation resistance, the worse the breathability of the fabric; the lower the evaporation resistance, the better the water vapor permeability, or the sweat hot plate method tests the vapor heat transfer resistance. The evaporation resistance test value (Ret value) range is generally 148.7~3.9m2Pa/w.
When the Ret value is less than 6, it is considered extremely breathable and comfortable to wear at high exercise levels; when it is between 6 and 13, it is very breathable, with average wearing comfort at high exercise levels, but comfortable to wear at ordinary exercise levels; Between 13 and 20, it is breathable, which is uncomfortable to wear at high exercise levels, but comfortable to wear at normal exercise levels; between 20 and 30, it is low breathability, very uncomfortable to wear at high exercise levels, but comfortable to wear at normal exercise levels. Generally comfortable at low activity levels; above 30 it is non-breathable and uncomfortable at all activity levels.
The sweating hot plate method is being adopted by more and more businesses because it is more reasonable. However, the big problem facing China is that there is currently no testing agency that can do this test, and Hong Kong can’t do it either. Taiwan’s Textile Association can do it, but it doesn’t have the qualifications for certification. Because of this, domestic fabric manufacturers do not know enough about this test and are unable to determine how to choose the correct film and process. This year, a brother company suffered heavy losses because it could not reach a Ret value of 13.
There are many different methods for testing fabric moisture permeability, and the results of various testing methods are not relevant or comparable. Therefore, determining the testing method is the prerequisite for all indicators. We often see how high the waterproof and moisture permeability indicators are on some brands, but they do not indicate which testing method to use.
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Disclaimer:
Disclaimer: Some of the texts, pictures, audios, and videos of some articles published on this site are from the Internet and do not represent the views of this site. The copyrights belong to the original authors. If you find that the information reproduced on this website infringes upon your rights, please contact us and we will change or delete it as soon as possible.
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