Comfortable clothing is important to us all. However, it is particularly critical to certain sports, leisure and occupational activities where the characteristics of the clothing can seriously overburden the wearer, impairing performance in carrying out their activities.
Many items of specialist clothing used as personal protective equipment (PPE) are intended to provide a physical barrier to hazards. Foul weather clothing and firefighters’ suits for instance, need to exhibit good permeability and water penetration resistance to allow the wearer to continue to operate effectively. If these properties are absent or at low levels, the wearer is likely to become prematurely fatigued and only able to operate in hostile environments for limited periods. In addition, firefighters’ clothing with low water vapour permeability can lead to an increase in steam burns.
SATRA has specialist laboratory facilities for physiological assessments and associated test methods that enable a number of wearer comfort parameters to be quantified.
Climatic chambers
Ambient conditions have a major influence on a wearer’s perception of comfort of products such as clothing and footwear. Low ambient humidity aids the evaporation of moisture, which helps to keep us cool and clothes to dry out quickly; people experience far less discomfort and can endure higher temperatures at low humidity than in high humidity environments.
Ambient temperature and humidity also can have a significant effect on the strength and performance properties of many materials from which consumer products are made. This applies particularly to natural materials (for example cotton – which is stronger when wet – leather and wood) but also to man-made materials (hydrolysis of polyurethane, heat softening of plastics, weakening of adhesive bonds).
SATRA has a range of climatic chambers that enable us to simulate a wide range of moderate and extreme environments. Our most versatile chamber operates between -40°C (-40°F) and +40°C (+104°F) with humidity levels up to 99% relative humidity. The room is big enough to comfortably accommodate several people or house a treadmill or large items of test equipment. The chamber’s internal temperature and humidity is continuously controlled and monitored. Thermography can be used to record temperature distribution over large areas at a glance.
Assessing permeability of water-resistant breathable materials
User comfort and reduced physiological stress have been significantly improved with the introduction of garments incorporating permeable membrane and other technologies. This means people can operate longer and more efficiently in adverse conditions.
There are many tests for assessing water vapour permeability but there are no generally agreed guidelines to allow realistic comparison between materials. This has meant it has been difficult for buyers to make comparisons when selecting garments. SATRA has carried out work to study the behaviour of various membranes, and has developed a treadmill evaluation protocol, where assessments are made of user comfort whilst monitoring the temperature and humidity within a standard undergarment during the trial. Comfort questionnaires can also be administered to evaluate other design aspects of the garments.
The treadmill studies led to the development of the SATRA TM287 laboratory test method. In this test, a clothing fabric assembly (outer material, membrane and lining) is sealed across the top of a pot containing a measured amount of water. The pot sits in a warm water bath so that water vapour is generated below the fabric inside the pot. After a defined time the amount of moisture that has both permeated through the materials, and been absorbed into them, is determined. The results of this test were found to correlate very well with the treadmill studies – higher permeability results being obtained for materials found to be more comfortable on the treadmill. This test is now in routine use at SATRA and is proving to be a reliable means of classifying the performance of water resistant, permeable materials for clothing and can also be applied to gloves.
Sweating guarded hotplate
The sweating guarded hotplate is used for measuring the thermal and water vapour resistance of sheet material. It is currently the approved method for assessing the performance of breathable and insulating materials for protective clothing in standards such as EN 343, EN 471 and EN 469.
The device consists of a measuring unit and thermal guard ring, which are fitted with heaters and temperature sensors. The top surfaces of the measuring unit and thermal guard ring are flat and level. The surface of the measuring unit is porous to allow water, at the same temperature as the measuring unit, to evaporate from the surface. The measuring unit incorporates a water reservoir. In operation, the plate and guard ring are mounted in an environmental chamber where the temperature and humidity are tightly controlled.
The temperatures of the measuring unit and thermal guard ring are controlled at 35°C ± 0.1°C in order to mimic the temperature of human skin and the test specimen is laid over the measuring unit and thermal guard ring so that it overlaps the thermal guard. The environmental chamber in which the instrument operates is set to 35°C for water vapour resistance measurements and to 20°C for thermal resistance measurements.
The device operates in two modes: i) for measuring water vapour resistance (or 'Ret'), and ii) for measuring thermal resistance (or 'Rct'). In the thermal test, no water is used.
The operation of the device is, in principle very simple. Initially we run the device without specimens and the bare plate constants for water vapour resistance or thermal resistance are calculated. With the test specimen in place, the plate and guard ring are controlled to the appropriate temperature. The device is allowed to settle down and achieve steady state in which the heat lost from the upper surface of the test specimen is equal to the heat input from the measuring unit. Once steady state conditions occur, the power (watts) required to maintain the steady state conditions over a period of time, is measured. The result for a particular material is the value obtained with the test specimen in place minus the bare plate constant.
SATRA has installed two sweating guarded hotplates. One is operated full-time in the Ret mode and the other operates in the Rct mode, which enables SATRA to turn this work around quickly and meet manufacturers’ deadlines.
