MOISTURE BARRIER

Almost aU ceUophane is coated with either a moisture barrier or heat-sealable coating. This is generaUy done in tower coaters after film production. 

In extmsion coating a polymer is extmded from a slot die into the nip of two roUs where it is bonded to a substrate under pressure (Fig. 6). A corona discharge may be appUed to the substrate just prior to the nip to enhance adhesion. Polyethylene or ionomer are the most common resins used in extmsion coatings. They provide improved moisture barrier (on paper), or sealabUity (on foU, polypropylene, or polyester). When a second substrate is introduced to the nip, laminated stmctures may be produced. 

Aryl Phosphates. Aryl phosphates were introduced into commercial use early in the twentieth century for flammable plastics such as cellulose nitrate and later for cellulose acetate. CeUulosics are a significant area of use but are exceeded now by plastici2ed vinyls (93—95). Principal appHcations are in wire and cable insulation, coimectors, automotive interiors, vinyl moisture barriers, plastic greenhouses (Japan), furniture upholstery, conveyer belts (especially in mining), and vinyl foams. 

Nylon. Nylon is the designation for a family of thermoplastic polyamide materials which in film form are moderate-oxygen barriers. The gas-barrier properties are equal to odor and flavor barrier properties important in food appHcations. Nylon films are usually tough and thermoform able, but are only fain moisture barriers (see Polyamides). 

Moisture Barrier. When impregnated with an asphaltic emulsion, geotextiles become impermeable and can then be used as moisture barriers. The primary appHcation for this type of geotextile is in pavement rehabiHtation (Fig. 8). 

One of the primary factors in the deterioration of a pavement stmcture is the intmsion of surface water into the support stmcture of the pavement. When rehabihtating a pavement, the installation of a moisture barrier between the old, existing pavement surface and the new overlain surface acts to retard moisture intmsion, thus prolonging the life of the overlay. 

Plastic. A plastic bag usuaUy consists of a single heavy waU of plastic film, woven sheets of plastic tape, or laminates. Principal materials of constmction are polyethylene and polypropylene (see Fibers, olefin). Both transparent and opaque sheeting are used, and printabUity usuaUy is exceUent. Plastic bags can be fiUed and closed with conventional equipment beat-sealing is essential for open-mouthed bags to effect a moisture barrier. 

Poly(vinyl alcohol) is an innocuous material having unlimited storage stabiUty. It is most commonly suppHed in 20-kg, 22.7-kg (50-lb), and 25-kg bags equipped with a moisture barrier to prevent caking. PVA is also available in bulk or in super sacks. The FDA regulations governing the use of PVA are shown in Table 8. Poly(vinyl alcohol) maintains an exemption for tolerance from the EPA. 

Copper(II) sulfate monohydrate [10257-54-2] CuS04-H2 0, which is almost white in color, is hygroscopic and packaging must contain moisture barriers. This product is produced by dehydration of the pentahydrate at 120—150°C. Trituration of stoichiometric quantities of copper(II) oxide and sulfuric acid can be used to prepare a material of limited purity. The advantages of the monohydrate as opposed to the pentahydrate are lowered freight cost and quickness of solubilization. However, these advantages are offset by the dustiness of the product and probably less than one percent of copper sulfate is used in the monohydrate form. 

To assess unknown incompatibilities it is customary to make a small mix of drug substance with an excipient [54,55], place it in a vial, place a rubber stopper in the vial, and dip the stopper in molten carnauba wax (to render it hermetically sealed). The wax will harden and form a moisture barrier up to 70°C. A list of common excipients characteristic of this type of test is shown in Table 1. At times it is possible to obtain quantitative relationships of... 

In the past, the initial layers of coating (the sealing coat) were achieved by applying one or two coats of shellac. However, due to the variability between batches of this material, PVP-stabilized types of shellac or other polymeric materials, such as cellulose acetate phthalate (CAP) and poly(vinyl acetate phthalate) (PVAP), are now more popular. It should be appreciated that a fine balance must exist between minimizing the thickness of the sealing coat and providing an adequate moisture barrier. 

Due to difficulties often encountered in sealing subfloors and insulating pipes in crawlspace houses, which rarely have a poured floor slab, another radon-resistant alternative that can be applied after construction should be considered. This mitigation technique is a variation of the successful SSD methods used in basements. Polyethylene sheeting is often used as a moisture barrier applied directly over the soil in crawlspaces. The polyethylene sheeting can be used as a gastight barrier that forms a small-volume plenum above the soil where radon collects. A fan can be installed to pull the collected soil gas from under the sheeting and exhaust it outside the house. 

In summary, it is worthwhile to continue the installation of a vapor barrier that serves as the added valid function of moisture barrier. More comprehensive installation measures and more expensive materials may be merited in areas where the radon source is strong because of either high radon concentrations or high soil gas how rates. 

This type of cake is often stored for a while after baking because it is to be covered with royal icing. The cake not only needs to cool but the surface needs to equilibrate with the bulk. There is a paradox that if the cake is just wrapped in greaseproof paper and kept in a tin that is not air tight there will be no problem. However, if the cake is wrapped in aluminium foil and kept in an air tight tin there is a possibility of mould formation because with the better moisture barrier any condensation or sweating can not evaporate. 

Providing good moisture barrier thus preventing dehydration of food items. 

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