Behind the numbers of common textile breathability tests
By Martin Vilaboy
Once discussed largely in terms of its relationship with waterproofness, the performance attribute commonly known as “breathability” has grown to become an often-repeated element of maintaining overall comfort in both wet and dry conditions. Historically associated with outerwear and water-friendly footwear, the breathability buzzword is attached to just about every layer on the wearer’s body – from baselayer to insulation and socks to backpack straps and panels.
Yet despite the proliferation of breathable fabrics, breathability claims remain somewhat esoteric to consumers reading specifications on a sales floor or attempting to compare the capabilities of certain garments. After all, can we expect Josephine Q. Public to know if 10,000 g/m²/24h is adequate vapor permeability for a warmhike in a dry climate?Indeed, it’s not that adequate testing and quantification, and the corresponding international standards, are unavailable. Rather, at issue are concerns that the most common tests fail to mimic conditions found in the changing environment of real-life use. These tests also differ in procedure, requirements, set-up and the ambient conditions in which each test is carried out. The various tests also use different units to express the results, meaning it can be difficult to compare the results of the different test methods.
According to scientists at testing lab and research center the Hohenstein Institute, the currently established and standards-defined methods for measuring breathability – described more appropriately as the “water vapor permeability” of a textile – can be divided into two groups. At one end are the basic gravimetric methods, commonly known as “cup methods.” The second group is considered as “complex methods.” Most notably, these include the “sweating guarded-plate” methods, which are based on using a skin model for the measurement process.
Defined in numerous standards and requiring minimal equipment, cup methods can be broken into two groups: upright cup tests (ASTM E96 and ISO 2528) and inverted cup tests (ASTM E 96 BW). Both are based on determining the amount of water vapor that passes through a textile by gravimetric analysis (i.e. measured by weight or gravity).
For an upright cup test, the textile being tested is sealed to the open mouth of a test dish containing either water or a desiccant, a substance with a great affinity for water. The covered dish is then placed into a controlled atmosphere (constant temperature and humidity). Periodic weighing then determines the rate of water vapor movement through the test fabric and into the desiccant, or, in the case of the water method, the rate of water vapor movement out through the test fabric and into the controlled atmosphere.
The inverted cup method is somewhat identical to the upright cup method except that the apparatus is inverted, which removes the air gap between the test fabric and the water or desiccant. Once again, periodic weighing measures the amount of water vapor that is either “pulled” into the desiccant or “pushed” through the fabric into the controlled area.
Requiring little or no specialized equipment or expertise, cup methods can be simple to set up and execute and take relatively little time. In other words, they are low-cost and can be performed at different types of locations, such as for quality control purposes at an actual production site. They likewise are cost-effective for use during product development.
On the other hand, cup method tests are performed under constant temperatures and humidity, so they are not exactly representative of the changing environments of real-life conditions, according to the Textile Center for Excellence. And due to the multiple number of cup methods, it can be difficult to choose the right method for the textile that requires testing, say Hohenstein researchers, “because not all cup methods are suitable for all textiles.”
Generally, the upright cup model tests described here (ASTM E96 A – D) are most suitable for products in which only small quantities of water vapor need to be transported through the textile, say Hohenstein researchers, while the inverted cup method is mainly suitable for textiles with waterproof coatings, monolithic hydrophilic films or PTFE film, because it describes water vapor permeability best when the sweating rate is high.
Choosing the wrong method, therefore, “can result in incorrect results and consequently to defects in quality,” says Hohenstein. “It is therefore necessary to indicate the measuring method that was used when quoting the result and to take this into account.”
Likewise, results are expressed in similar yet non-corresponding figures. For example, performance measurements of the upright cup model range from about 4,000 gr/m2/24 (grams per meter squared per 24 hours) on the low end to as high as 10,000 to 15,000 gr/m2/24 on the high end. The inverted cup results, meanwhile, range from under 10,000 gr/m2/24 to as high as 30,000 gr/m2/24 on the uppermost range. So expressing these moisture vapor transfer rate (MVTR) figures without indication of which test was employed can be confusing, if not misleading.
Another way to determine water vapor permeability of a textile is the “sweating guarded-hotplate” method, also known as the “skin model” or “Ret test.” In this case, measurements are obtained using a porous sintered metal plate, heated to 35C, which simulates the heat and sweat given off by the skin. The skin model test is typically performed in a climate-controlled cabinet under standardized conditions, according to Hohenstein, which developed a skin model in the 1980s and performs the Ret test as part of its laboratory services.
The textile being tested is placed on the hotplate and water vapor is passed through the pores of the hotplate, simulating the evaporated sweat with which a textile comes into contact, explain sources at the lab and research institute. To prevent the hotplate from being cooled down by the evaporative sweat, energy is supplied to keep the hotplate temperature constant. The water vapor that is transported away by the textile is replaced by new vapor passing through the pores.
The skin model or Ret test conditions and set-up are consistently defined, which means that the results from different laboratories can be easily reproduced and compared. The procedure is normally suitable for any textiles worn by people, including water-repellent textiles, as well as textiles intended to transport large or small volumes of water vapor, says Hohenstein. “The test results can even be used to compare differently structured textile materials and show where there is potential for improvement,” said Dr. Edith Classen, senior scientist in the Clothing Physiology, Chemistry and PSA department at the Hohenstein Institute.
Results are expressed in simple numbers ranging from 0 to 20-plus. The lower the rating number, the better the breathability of a textile. In the case of a non-laminated membrane, for instance, 0 to 6 would be considered very good water vapor permeability.
Due to the complexity of the test set-up and specialized apparatus, however, the Ret test entails higher test costs and requires trained laboratory staff. The complexity of the test also means that it takes longer than most cup methods. Because of its complexity, the test is mainly carried out in laboratories, but it is also possible to carry out testing with a skin model at a production site or for product development and optimization.
According to Dr. Classen, numerous wearing tests have been carried out with volunteers in order to analyze how people experience breathability in relation to the Ret value, and these wear tests are still being conducted so that the data pool is constantly being expanded.
This correlation between ratings and human perception, along with the use of skin replication, leads
Hohenstein to argue that its skin model testing is a better indication of the actual “wearing experience” of a garment than cup methods are. Either way, sales staffers and marketing executives would be wise to arm themselves with quantified test results and an understanding of what tests were employed before throwing out performance claims to customers.
After all, many things can be called “breathable,” but only a verified statement of a textile’s ratings indicates a garment’s true “breathability.
