VAPOUR TRANSMISSION

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. 

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]. 

Water resistance. Coatings must show a negligible absorption of water and must be highly impermeable to water or water-vapour transmission. 

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)... 

Laminated tapes In more general use now than pressure sensitive tapes are tapes consisting of polyvinyl chloride or polyethylene films in conjunction with butyl rubber. These tapes are applied with an adhesive butyl rubber primer. Thicknesses of up to 0-75 mm are in use and loose protective outer wraps of p.v.c. or polyethylene sheet are commonly applied. Tape quality control is exercised with reference to ASTM standard test methods and may include water vapour transmission rate and elongation. 

ISO 2528, Sheet materials - Determination of water vapour transmission rate -Gravimetric (dish) method, 1995. 

ISO 1663 1999 Rigid cellular plastics - Determination of water vapour transmission properties ISO 2556 1974 Plastics - Determination of the gas transmission rate of films and thin sheets under atmospheric pressure - Manometric method ISO 6179 1998 Rubber, vulcanized or thermoplastic - Rubber sheets and rubber-coated fabrics - Determination of transmission rate of volatile liquids (gravimetric technique)... 

ISO 15106-1 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 1 Humidity detection sensor method ISO 15106-2 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 2 Infrared detection sensor method ISO 15106-3 2003 Plastics - Film and sheeting - Determination of water vapour transmission rate - Part 3 Electrolytic detection sensor method... 

Table 2.9 Oxygen vapour transmission rates (OVTR) for some common packaging containers...

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... 

ASTM D814, 1995 (2000). Standard test method for rubber property - Vapour transmission of volatile liquids. 

Note. For gas permeability the partial gas pressure gradient across the film is 1 atm. For water vapour transmission the humidity gradient is 90%. a Increases with ethylene content. b Decreases with ethylene content. 

Fromder, A. and Lippold, B.C., Water vapour transmission and occlusivity in vivo of lipophilic excipients used in ointments, Int. J. Cosmet. Sci., 15, 113, 1993. 

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. 

For flexible film applications such as carrier and trash bags PLA has the potential to replace LDPE and HDPE bags when a compostable solution is desired. Furthermore, the high water vapour transmission rate of PLA is beneficial for fresh food applications where it is important that the water vapour escapes quickly from the packaging. PLA also reduces fogging on the lid of the packaging. 

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... 

Several studies reviewed formulations, barrier properties and possible application of edible protein-based films (Table 23.3) (Gennadios et al. 1994 Krochta and Me Hugh 1997 Torres 1994). Overall, similarly to polysaccharide films, proteins exhibit relatively low moisture barrier properties, two to four times lower than conventional polymeric packaging materials (McHugh and Krochta 1994d). The limited resistance of protein films to water vapour transmission is attributed to their substantial hydro-philicity and to the amounts of plasticizers, such as glycerol and sorbitol, incorpo-... 

As compared with their low water absorption, silicone elastomers show an extremely high rate of water vapour transmission. The positive aspect of this property permits one to apply sihcone systems over concrete masonry and other substrates upon which other polymeric coatings blister, crack and peel (Cash, 1970). The negative aspect is that silicone rubber cannot be used as a vapour barrier. The permeabihty to liquids is less compared with that to vapours. 

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. 

Banker, G.S. Gore, A.Y. Swarbrick, J. Water vapour transmission properties of applied polymer films. J. Pharm. Pharmacol. 1966,18, 205S-211S. 

Udeala, O.K. Aly, S.A.S. Degradation kinetics of thiamine hydrochloride in directly compressed tablets. III. Water vapour transmission through free and applied eudragit films. Drug Dev. Ind. Pharm. 1989, 15 (11), 1797-1825. 

Lindberg NO. Water vapour transmission through free films of hydroxypropyl cellulose. Acta Pharm Suec 1971 8 541-548. Banker G, Peck G, Williams E, et al. Evaluation of hydroxypro-pylcellulose and hydroxypropylmethylcellulose as aqueous based film coatings. Drug Dev Ind Pharm 1981 7 693-716. 

ISO 1663-81 Cellular Plastics — Determination of Water Vapour Transmission Rate of Rigid Materials, 4 pp... 

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