Water vapour permeability tests

Galbraith et al [20] compared cotton, water repellent cotton, and acrylic garments through wearing tests and concluded that the major factor causing discomfort was the excess amount of sweat remaining on the skin surface. Niwa [21] stated that the ability of fabrics to absorb liquid water (sweat) is more important than water vapour permeability in determining the comfort factor of fabrics. 

To complete this chapter a final section is required, i.e. consideration of tests and test schedules which may be used for the evaluation of materials and completed packs. This represents the area where information tends to change the most, and the subject is so diverse that it would only be effectively covered by a full textbook. It is therefore not practical to cover all tests and methods in detail but only to provide broad reference to possibilities. Tests can be applied to materials (e.g. water vapour permeability), on components or completed packs. Tests generally fall into three categories, i.e. those required ... 

In order to offer an optimal protection against cold, sportswear basically has to be waterproof, windproof and breathable. The test requirements in standards such as the European Standard for protection against cold EN 342 (EN 342, 2004) are therefore thermal insulation, air permeability, resistance to water penetration and water vapour permeability. In addition, the mechanical stability of the fabrics (tear resistance) is assessed. The thermal insulation of the complete garment is measured with an anatomically formed manikin (as shown in Figure 9.1) placed in a climatic chamber at defined climatic ccmditions (ISO 15831, 2003). The surface of the manikin is heated to skin temperature and the thermal insulation is calculated by measuring the heating power needed to maintain its temperature for a defined temperature gradient. 

From the water vapour permeability point of view, textile materials behave in two different ways first, materials in which moisture vapour transfer takes place predominately by diffusion through air spaces betweeu yams and fibres, following Pick s law. The measured water vapour permeability values are independent of the measuring conditions. Woven, nonwoven, and semipermeable membrane laminates fall into this category. Second are textile material composites that contain a layer of hydrophilic membranes and which behave quite differently. In particular the rate of diffusion through the hydrophilic manbrane is dependent on the test conditions, such as concentration of water vapour in the layer or relative humidity. ... 

There are several methods available for testing the water vapour permeability of textiles. However, the most commonly used methods are described below. 

The water vapour permeability index can also be calculated by expressing the water vapour transmission (WVT) of the fabric as a percentage of the WVT of a reference fabric which is tested alongside the test specimen. The main drawback of this method is that the still air layer inside the cup between the water surface and the fabric offers higher water vapour resistance than the fabric itself. 

Desmelle and Schmid developed a set of tools to study the effects of exposure to hot water steam atmospheres on human physiology and to evaluate the protective capacities of fabrics under steam stress. However, their work involved lengthy time scales, wherein the skin would likely be destroyed, and unrealistically held the skin surface at constant temperatnre. Rossi et aV° and Reiser et al. found that steam protection depended on water vapour permeability, thermal insulation, and thickness of the samples tested. Like Lawson et al., - they also found that the presence of moisture in the fabric could have either a positive or negative effect on protection. 

Thermo-physiological and climate-controlling properties of the spacer-fabrics will also be discussed in this paper. These tests directly relate to the functional comfort characteristics of the spacer-fabric structures which include thermal resistance, thermal absorpitivity, water vapour permeability, and evaporative heat loss. In all of the tests undertaken, comparisons are made to results obtained for traditional compression bandages and padding bandage materials. 

European Committee for Standardization, EN 1015-19, Methods of Test for Mortar for Masomy. Determination of Water Vapour Permeability of Hardened Rendering and Plastering Mortars, 1998. 

Water vapour polyethylene has a low permeability, evaluated from 0.9 up to 2.5 compared to the full range of 0.05 up to 400 for all tested plastics. 

The barrier properties of PLA have not been extensively studied. The first articles treating the permeability of PLA film have been published in 1997 [120, 121], when PLA started to be considered for packaging applications. PLA films with various L/D ratios, different crystallinity degree and blends with numerous additives and polymers have been tested in recent years with gases, water vapour and organic compounds. 

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