MOISTURE VAPOUR

Certain copolymers of this type have been found to have excellent gas barrier properties, with the dry polymer having an oxygen permeability only about 1/lOth that of polyvinylidene chloride. Unsurprisingly, the copolymer has a high moisture absorption and a high moisture vapour transmission rate. Where the material is swollen by water, gas permeability is also higher. 

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). 

Sealants for insulated glass windows are based on butyl polymers. These sealants have excellent weathering and ageing, low-moisture vapour transmission and are non-fogging [66]. 

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. 

Material Thickness of film plus adhesive (mm) Tensile strength (kg/cm width) Elongation at break (%) Moisture absorp- tion (%) Moisture- vapour transmission rate (g/m per 24 h Dielectric strength (V)... 

For dry dosage forms, since it is their very low A which is their protechon against microbial attack, the moisture vapour properties of packaging materials requires careful examination. 

Plastic Suppliers Inc., a US extruder of blown film for labels and envelopes, has produced the world s first blown film from NatureWorks PLA. It was hitherto thought that PLA was unsuitable for blown film extrusion. Plastic Supplies claims that its EarthFirst film is 100% compostable, has high gloss, optimum clarity and transparency, high moisture vapour transmission rate, flavour retention, odour barrier, is breathable and is US Food and Drug Administration (FDA) compliant. Areas of application for EarthFirst include window carton film for food packaging, label film, floral wrap film, shrink film and envelope film. 

Gas barrier properties - Metabolix PHA polymers have lower moisture vapour transmission rates than other biodegradable polymers. The oxygen transmission rates for unoriented PHA films are 25-30 cc-mil/(100 in2-day) at 77 °C, 0% relative humidity. 

Moisture Vapour Transmission The rate at which water vapour permeates through a plastic... 

The use of silicone sealant for insulating glass (double glazing) gives excellent adhesion to glass which is unaffected by UV radiation but which must be used with a waterproof sealant such as polyisobutylene to reduce moisture vapour transmission (MVT). The use of silicone sealant alone will result in water condensation inside the unit in a relatively short period of time and would permit water vapour to be absorbed into the unit and eventually exhaust the desiccant used in the space. The use of small beads of polyisobutylene as a primary seal (Figure 6.3) reduces the MVT to a very low number, thus correcting the one deficiency of silicone (Panek and Cook, 1984). Sealants for SSG systems may be applied on... 

Figure 6.3 The use first of a waterproof seal of polyisobutylene in double glazing to reduce the moisture vapour transmission (a) outer view (b) section...

Moisture vapour transmission (MVT) A measurement of the transmission of moisture vapour through a film, usually expressed in terms of grammes of water per square metre per 24 hours. 

ASTM D1653 Moisture vapour transmission of organic coating film. 

Relevant physical properties, e.g., leak test, tensile strength, moisture vapour transmission, closure removal torque... 

All plastics are to some degree permeable to moisture, vapour and gases, but are considerably superior to untreated or uncoated paper. The following material factors may require consideration in the selection of a plastic film, but it should be noted that only some of the factors listed below are subsequently considered under the general review on each plastic ... 

Metal containers are strong, relatively unbreakable, opaque, and impervious to moisture vapour, gases, odours and bacteria, providing they are pinhole-free. They are also resistant to both high and low temperatures. However, metals require the application of coatings and lacquers to prevent chemical reaction and corrosion from the inside or outside. Special coatings and coating techniques have therefore been developed for this purpose, e.g. tinplate is in fact a coated material. 

Moisture vapour, e.g. Mocon Permatran W or Dynamic water vapour tester measures moisture by a photoelectric sensor. 

Note that impact modified grades of PVC usually soften at a lower temperature hence may improve machine throughput. Vinyl acetate is widely used as an impact modifier. Impact modified grades generally show higher moisture vapour figures than UPVC. 

Henry solubility coefficient describes max concentration of a component i with partial pressure 1 bar at temperature 298 K and has the dimension atm-A,-ml". But it may be expressed also as atm 1 mole" or atm m mole". At partial pressure of the component i in solutions of 1 bar, in conditions of low pressure and with the exclusion of the effect of moisture vapour, Henry solubility coefficient is equal to the inverse concentration value of this component in water. In a more rigorous thermodynamical form Henry solubility coefficient is partial molar fugacity of component i at standard temperature ... 

The fabric is highly breathable and does not restrict the movement of moisture vapour. 

Heat is lost due to liquid moisture on the skin (i.e., sweat) undergoing a phase change from liquid to gas, thus consuming energy from the environment (skin and air by the skin). For effective evaporative heat transfer, sweat should be wicked and spread across a large surface at the skin and the outer garment should be permeable to moisture vapour and minimise condensation on the inside so that the gradient can be maintained. 

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