Surface properties moisture effects

Effect of Relative Humidity on the Surface Area of Paper. Virtually all the physical and mechanical properties of fibres and paper vary strongly with moisture content, and it would be expected that surface properties should also be sensitive to relative humidity. Little work has been published on the effect of moisture on the surface properties of fibres and paper. As a starting point, it may be noted that the basic property of any surface is its surface area. How does the surface area of a fibre or paper sheet change with relative humidity ... 

Slow cure is normally seen in high moisture papers and boards and the effect will be greater for AKD than for ASA, because of the slower reactivity of AKD and the difference in melting points, i.e. a liquid versus a low melting point wax. It is possible to improve the curing of AKD (and ASA) by running to a lower moisture <4% prior to a size press/film sizer and subsequently drying less after the size press/film sizer, but this may impact on the surface properties. 

The effects of temperature on filter performance are complex in so far as physical properties which influence all parts of the cycle are affected. Temperature affects the rates of liquid flow through fluid viscosities and moisture contents during deUquoring through surface tensions (the effects of fluid densities being relatively insignificant). The important fluid property... 

Moisture. Absorbed and retained moisture, especially as ice, has a significant effect on the stmctural and thermal properties of insulation materials. Most closed-ceU plastic foams have low permeance properties most notably where natural or bonded low permeance surface skins exist (29,30). Design, building, and constmction practices requite adequate vapor retarders, skins, coatings, sealants, etc, in order to prevent the presence of moisture. However, moisture vapor cannot be completely excluded, thus the possibiUty of moisture absorption and retention is always present. The freezing of moisture and mpturing of cells result in permanent reduction of thermal and stmctural performance. 

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

The guarded hot-plate method can be modified to perform dry and wet heat transfer testing (sweating skin model). Some plates contain simulated sweat glands and use a pumping mechanism to deUver water to the plate surface. Thermal comfort properties that can be deterrnined from this test are do, permeabihty index (/ ), and comfort limits. PermeabiUty index indicates moisture—heat permeabiUty through the fabric on a scale of 0 (completely impermeable) to 1 (completely permeable). This parameter indicates the effect of skin moisture on heat loss. Comfort limits are the predicted metaboHc activity levels that may be sustained while maintaining body thermal comfort in the test environment. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

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