Humidity barrier properties

The web materials used to form dmms and cans can be customized to each packaging apphcation. The materials are selected based on the package properties of moisture and humidity resistance, nonstick resistance, or barrier properties required. The adhesives used to bond the phes together include sihcates, poly(vinyl alcohol), and poly(vinyl acetate) (10). Sihcate adhesives are most commonly used in the manufacture of dmm packages. 

Gas-Barrier Properties. The oxygen-barrier properties of PVA at low humidity ate the best of any synthetic resin. However, barrier performance deteriorates above 60% th (Fig. 9). No additives or chemical modifiers are known that can effectively reduce moisture sensitivity. The gas-barrier performance is affected by the degree of hydrolysis and rapidly diminishes as the hydrolysis is decreased below 98%. 

Several physical factors can affect the barrier properties of a polymer. These include temperature, humidity, orientation, and cross-linking. 

Measuring the barrier properties of polymers is important for several reasons. The effects of formulation or process changes need to be known, new polymers need to be evaluated, data are needed for a new apphcation before a large investment has been made, and fabricated products need to have performance verified. For some apphcations a full range of data is necessary, including P, Z9, and S plus the effects of temperature and humidity. 

Some evidence indicates that long-term use of topical antimicrobial agents may alter skin flora. Water content, humidity, pH, intracellular lipids, and rates of shedding help retain the protective barrier properties of the skin. When the barrier is compromised (e.g., by hand hygiene practices such as scrubbing), skin dryness, irritation, cracking, and other problems may result. Although the palmar surface of the hand has twice as many cell layers and the cells are >30 times thicker than on the rest of the skin, palms are quite permeable to water. 

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

The moisture barrier properties and humidity resistance make COCs suitable for container and film applications. The low moisture absorption prevents swelling. 

Humidity - [SIMULTANEOUS HEAT AND MASS TRANSFER] (Vol 22) -ceramic sensors for [CERAMICS - ELECTRONIC PROPERTIES AND MATERIAL STRUCTURE] (Vol 5) -effect on barrier properties [BARRIER POLYMERS] (Vol 3) -sensors for [SENSORS] (Vol 21)... 

The oxygen-barrier properties of PVA at low humidity are the best of any synthetic resin. However, barrier performance deteriorates above 60% rh,... 

The permeation of simple gases in polar polymers is also dependent on relative humidity because of the water s strong interaction with the polymer. Particularly large is the effect of humidity on the oxygen permeability of barrier layers made from EVOH. At 20 °C and 100 % relative humidity the permeability is 300 times higher than at 0 % relative humidity at the same temperature. By orientation and thermal treatments the barrier properties at high relative humidity can be significantly improved (up to a factor of 10). 

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. 

Oxygen barrier data for pure H40 systems and H40 network systems measured at RT and 0 and 50%RH are summarized in Table 1 which also reports Tg s and densities in the dry state. Due to the high concentration of hydroxyl functional groups in the periphery, excellent gas barrier characteristics for pure H40 are predicted. As seen in Table 1, the pure H40 at 0%RH displayed considerably better oxygen barrier characteristics than PET and comparable to those for EVOH with 48% (mol/mol) of ethylene. EVOH copolymers with low and moderate ethylene content are considered benchmark materials for packaging applications. When exposed to ambient humidity (50% RH) the barrier properties were reduced, but still better than PET. Figure 6 (a), (b), and... 

Barrier Property—Water Vapor Permeability (WVP) Protein hlms are excellent gas barriers but poor moisture barriers because of their hydrophilic nature. Mechanical properhes and gas permeability depend on the relative humidity (1). 

Before edible films can be applied to foods or pharmaceutical products, it is necessary top obtain knowledge of water sorption, permeability, as well as mechanical properties of edible films. Films with good water vapor barrier properties (low or no water permeation and diffusion through film) should not change or change very little with relative humidity (Lawton, 1996). Films should last under mechanical strain and stress to such an extent that they do not break easily under a reasonable mechanical force. Film-forming techniques and composition of films affect such properties of biomaterial... 

The ability of wax to prevent the transfer of moisture vapor is of primary concern in the food packaging industry. To maintain the freshness of dry foods, moisture must be kept out of the product, but to maintain the quahty of frozen foods and baked goods the moisture must be kept in the product. This results in two criteria for barrier properties moisture vapor transmission rates (A) at elevated temperatures and high relative humidity and (B) at low temperatures and low relative humidity, for frozen foods. 

The use of other materials as the central layer of the barrier laminate is, of course, feasible. Obvious candidates for this application include the high barrier amorphous polyamides (Chapter 5 (50)) and the liquid crystalline polyesters (Chapter 3(57)) which either develop slightly improved barriers under elevated relative humidity conditions or at least do not lose barrier properties. No reports are yet available concerning the performance of such structures. 

Selar PA and Selar PA blends have benefits in meat and cheese packages, replacing the nylon 6, PVDC, or EVOH barrier layer.Selar PA 2072 can be blended with EVOH (up to 40 wt% addition) without compromising the oxygen barrier properties of EVOH, especially at high humidity,... 

The barrier properties of nylon 6 blends fall between the performance of Selar PA 3426 and nylon 6. However, as the humidity increases, adding even small amounts of Selar PA 3426 improves the barrier more than would be predicted by a straight-line correlation. 

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