Permeability water vapour

A sufficiently low water vapour permeability for many packaging, building and agricultural applications. 

Compared with more common plastics used as packaging materials, the compound does have some disadvantages, such as a high water vapour permeability and limited heat resistance, losing dimensional stability at about 70°C. It is also substantially more expensive than the high-tonnage polyolefins. Last but not least its biodegradability means that it must be used in applications that will have completed their function within a few months of the manufacture of the polymer compound. 

Table I. Oxygen and water vapour permeabilities (P02 PH2O) with mean deviation (m.d.), water solubility (S1J20) and diffusion coefficient (Djj o) at 21°C.

Water vapour permeability. The most notable phenomenon overlooking the data presented In Table I Is that the water vapour permeabilities of the HMMM-based coatings are not widely different. The Isocyanate coatings show somewhat larger differences. GTG/lsocyanate and the coatings made from butanedlol and neopentyl glycol are more permeable. 

Figure 17-3. Water vapour permeability dish, (a) Waxing template (b) lid (c) shallow dish for materials of normal permeability (d) deep dish for materials of high permeability.

Comfort is felt if the heat insulation and the water vapour permeability agree with the key values in Table 27.3, which are based on experience. Since the ratios of the heat and water vapour permeability coefficients do not differ much, it suffices to assess one of them. 

They eUminate or reduce the absorbancy without clogging the pores They do not affect the water vapour permeability and drying characteristics of the biding material... 

PTFE has a remarkable range of properties. It possesses the simple monomer unit, -(CFj-CE )-, with a molecular weight of 100, an SG of 2.1-2.3 and a degree of polymerisation of 10 000. PTFE is extremely stable and inert, is not soluble in any known liquid solvent and is unaffected by concentrated acids and alkalis. It will not bum in air, is flexible down to -80 °C and dimensionally stable up to 4-250 °C. It has a low coefficient of friction, good abrasion resistance and has excellent liquid repellency properties. It is predominantly available as a thin membrane that is laminated to one or more textile fabrics. PTFE is thus well suited for use against chemicals and liquids in harsh environments. One of the special uses of the membrane is in a shock-expanded microporous form, which confers high liquid barrier properties with high water vapour permeability. In this form laminates are... 

The ability of fabric to allow water vapour to penetrate is commonly known as breathability. This should more scientifically be referred to as water vapour permeability. Although perspiration rates and water vapour permeability are usually quoted in units of grams per day and grams per square meter per day, respectively, the maximum work rate can only be endured for a very short time. During rest, most surplus body heat is lost by conduction and radiation, whereas during physical activity, the dominant means of losing excess body heat is by evaporation of perspiration. It has been found that the length of time the body can endure arduous work decreases linearly with decrease in fabric water vapour permeability. 

It has also been shown that the maximum performance of a subject wearing clothing with a vapour barrier is some 60% less than that of a subject wearing the same clothing but without a vapour barrier. Even with two sets of clothing that exhibit a small variation in water vapour permeability, the differences in the wearer s performance are significant [1],... 

Osczevski and Dolhan [16] and Farnworth et al [17] reported a strong dependency of water vapour resistance of hydrophilic membranes or coatings the higher the relative humidity at the membrane, the lower the water vapour resistance (i.e., the higher the water vapour permeability or breathability). 

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. 

Meanwhile, measurements of water vapour permeability of woven fabrics have indicated that in the lower ranges of fabric density, the main path of water vapour transfer is through the air spaces between fibers and yarns. This covers the densities characteristic of most apparel fabrics made from staple fibers, although filament yarn fabrics may be woven to higher densities in which the kind of fiber itself in the passage of water vapour, it is necessary to account for the water vapour passage through air spaces. 

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 ... 

Rubber meets most of these requirements, but so far no plastic material has been found as a substitute. The nearest alternative is a silicone rubber which is expensive, has a relatively high water vapour permeability and readily absorbs preservatives. 

Siparsky et al. [121] showed that the increase in the L-lactide content in PLA induced no significant modification of the water vapour permeability whatever the temperature or relative humidity, except a decrease in the permeability coefficient at 90% RH between 50 and 90% of L-lactide content. This was not confirmed by Tsuji et al. and Auras et al. who showed a constant permeability, around 1.8-1.9xl0 kg.m.m ls .Pa at 20°C and 40-90% RH, despite an increase in the L-lactide content [106,136]. 

Furthermore, the effect of the temperature on measured water vapour permeability changes in function of the study. Auras et al. showed a decrease of the permeability with the temperature, whereas Shogren highlighted an increase [5,120]. Siparsky et al. [121] showed that the diffusion coefficients increase and the solubility coefficients decrease with the temperature at 90% RH, which is contradicted by Holm et al. who showed higher values of moisture sorption at a higher temperature [139]. The various and contradictory results show that the mechanism of water transport in PLA is not completely understood yet [5]. In particular the presence of water clusters in the PLA matrix and their potential to diffuse in cluster form are still controversial. 

PL A fabrics exhibit the comfort and touch of natural fibres such as cotton, silk and wool while having the performance, cost, and easy care characteristics of synthetics. PLA fibres demonstrate excellent resiliency, outstanding crimp retention and improved wicking compared with natural fibres. Fabrics produced from PLA are being utilized for their silky feel, drape, durability and water vapour permeability used to create breathability suitable for sport clothing applications [195]. 

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