Plastics water barriers

Many polymers are used in barrier applications, either to keep contents in or contaminants out. Food packaging is an excellent example of such usage. Plastic films and containers of many types are used to package food. Blow moldedbottles often contain numerous layers, each of which provides specific benefits. Polyethylene layers are excellent water barriers, polyvinyl alcohol is a good oxygen barrier, and polyethylene terephthalate impedes the diffusion of carbon dioxide from carbonated drinks. Other barrier applications include toothpaste tubes, diaper backsheets, tarpaulins, and geomembranes, which are used to line containment ponds and landfill pits. 

The most important feature of a growing room is the ability to maintain a constant temperature. In this respect, insulation is critical. The walls should be insulated with R = 11 or R = 19 and the ceiling with R = 30 insulating materials. Fiberglass or styrofoam work well but should be protected from the high humidities of the growing room to prevent water from saturating them. For This purpose, a 2-4 mil. plastic vapor barrier is placed between The insulation and the interior wall. 

If water barrier or water resistance properties are reqttired hydrophobic materials are needed and these are listed towards the end of the hst. Still, care is taken that plasticizers are environmentally friendly to make entire product compatible with environment. 

Lagaron, J. M. and Fendler, A. (2009). High water barrier nanobiocomposites of methyl cellulose and chitosan for film and coating applications. Journal of Plastic Film and Sheeting 25, 47-59. 

Uses Defoamer for nonaq, systems, distillation, resin mfg., asphalt, oil refining, gas-oil separation water barrier internal/external lubricant/release agent for plastics and elastomers improves slip, mar resist, in coatings antiblocking lubricant, gloss aid, water repellent, softener for hair care prods. emollient, lubricant in anti-perspirants/deodorants aerosol prods. emollient for skin lotions and creams skin protectant aids spreading and rub-in fiber/yarn/thread treatment... 

Uses Release agent, defoamerfor nonaq. processes, esp. in the petrol., foods, and printing inks industries internal lubricant for plastics, rubber, and metal in furniture and auto-wax polishes, household and personal care prods. elegant, non-greasy emollient for skin care props. provides nontacky water barrier props, to eyeliners and eye shadow... 

Sugared cereals are often packaged in aluminum foil or barrier plastic, eg, ethylene vinyl alcohol, laminations to retard water vapor and flavor transmission (see Wheat and other cereal grains). 

To enhance water-vapor- or gas-barrier properties, layers of different plastics may be injected together or sequentially. Multilayer injection-molded pieces may be prepared as packaging or for blowing into bottle or jar shapes. 

Thermoform able sheet may be mono- or multilayer with the latter produced by lamination or coextmsion. Multilayers are employed to incorporate high oxygen-barrier materials between stmctural or high water-vapor barrier plastics. Both ethylene vinyl alcohol copolymers and poly(vinyhdene chloride) (less often) are used as high oxygen-barrier interior layers with polystyrene or polypropylene as the stmctural layers, and polyolefin on the exterior for sealing. 

More advanced insulations are also under development. These insulations, sometimes called superinsulations, have R that exceed 20 fthh-°F/Btu-m. This can be accomplished with encapsulated fine powders in an evacuated space. Superinsulations have been used commercially in the walls of refrigerators and freezers. The encapsulating film, which is usually plastic film, metallized film, or a combination, provides a barrier to the inward diffusion of air and water that would result in loss of the vacuum. The effective life of such insulations depends on the effectiveness of the encapsulating material. A number of powders, including silica, milled perlite, and calcium silicate powder, have been used as filler in evacuated superinsulations. In general, the smaller the particle size, the more effective and durable the insulation packet. Evacuated multilayer reflective insulations have been used in space applications in past years. 

Vapour corrosion The best way to pack articles made of metals susceptible to vapour corrosion is in boxes made of metal or of those plastics which do not themselves emit corrosive vapours. If wood cannot be avoided, then the less corrosive kinds should be chosen. Dryness, good ventilation and the inclusion of water-vapour barriers should be sought. Other obvious measures are the avoidance of susceptible metals and the use of protective treatments and paints. 

Pipework and other components should be insulated after the safety pressure test, but usually before prolonged running of the plant, since it is very difficult to remove water and frost once it has formed. Only the low-pressure piping is insulated, where it does not form part of the evaporator, i.e. after the expansion valve, where this may be outside the cooled space, and from the evaporator back to the compressor. Basic materials are cork and the expanded plastics. These are sufficiently rigid to be moulded to the correct shape, remain firmly in place, and support the external vapour barrier which is essential to prevent the ingress of water vapour (see also Chapter 15). 

---