Vapour permeability

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

Stannett and Meyers have reported that crystallisation may reduce the nitrogen permeability by 50%. The moisture vapour permeability of the polycarbonate from l,l-bis-(4-hydroxyphenyl)cyclohexane has been quoted by Schneir as being somewhat below half that of the bis-phenol A polymer (1.7, c.f. 3.8 units). 

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. 

Pigments for finishes are selected on the basis of their colour, but special attention must be paid to inertness in the chosen binder and stability and light fastness under the conditions of application and exposure. Flake pigments such as aluminium and micaceous iron oxide give finishes of lower moisture-vapour permeability than conventional pigments, and consequently contribute to better protection. 

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.

The various gravimetric methods for vapour permeability discussed above are all essentially simple but require great care to achieve good reproducibility, are time consuming, and are not generally sensitive enough to measure very low transmission rates. 

A considerable number of alternative techniques have been suggested for measuring vapour permeability of plastics, generally with the aim of making the measurement more convenient and increasing sensitivity. For water vapour transmission, carrier gas type commercial apparatus using an infrared sensor is now commonly used. This procedure is standardised in ASTM... 

Due to their specific characteristics, oligoalkylhydridesiloxanes are widely used in the textile industry. After silicone treatment textiles acquire a high hydrophobicity, which is resistant towards various physicochemical stresses, do not change appearance and remain air- and vapour-permeable. Certain silicone substances considerably reduce the shrinkage and creasing of materials with cellulose and hydrocellulose fibres, improve their resistance towards abrasion, tearing, bending and other wear factors. 

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. 

The oldest repellent finish is to repel water. The purpose of this finish is self evident. Drops of water should not spread on the surface of the textile and should not wet the fabric. The drops should stay on the surface and easily drip off. Similarly, oil repellent finishes should prevent oily fluids from wetting treated textiles. In a similar manner, soil-repellent finishes should protect textiles from both dry and wet soils. In all cases, the air permeability of the finished fabric should not be significantly reduced. Waterproofing treatments will not be covered in depth. A waterproof textile should withstand the hydrostatic pressure exerted by a column of water from at least aim height before the first drops of water penetrate through the fabric. In practice this is mostly achieved with coatings which have the disadvantages of stiff handle, lack of air and vapour permeability and consequently poor wear comfort. 

Suitable for all aggregate states, fewer restrictions on the amount and polarity, but the binder film with the micro bubbles changes textile properties like softness, wetting, air and vapour permeability... 

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... 

Patient-ready packs are allowed for supply to state institutions (or to permit holders in the private sector) only. Patient-ready pack immediate containers should be at lecist of the same quality as the standmd origincil container or conform to the specification of the latest edition of the USP for tight containers in respect of light transmission and vapour permeability. The outer containers should be over wrapped or shrink wrapped in low density polyethylene film of at lecist 60 micrometers. Unless otherwise proven, a maximum shelf life of 15 months which must be within the shelf life, commencing from the date of packaging, is permitted (MCC Circulars 6/92, 3/93 and 13/93). 

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. 

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