Moisture, absorption

Agro-based fiber-reinforced composites can absorb a significant amount of moisture, as the water retains in the inter-fibrillar spaces of these fibers [10]. The content of voids and the non-crystalline parts determine moisture absorption [3]. When these fibers are used in composites, the moisture can occupy the spaces in the flaws of the interface between the fiber and the matrix and the micro voids in the composites in addition to the inter-fibrillar spaces of the fibers [10]. The moisture absorption behavior of plant fibers is one of the main concerns related to the utilization in composite applications [24]. The moisture absorption that takes place as a result of the hydrophilic character of the fiber affects the performance of the composite negatively. Moisture uptake results in fiber swelling and this consequently changes the dimensional stability of the 

Fiber Type Tensile Strength (MPa) Specific Strength (MPa.cmVg) Elastic Modulus (GPa) Specific modulus (GPa. cmVg) Failure Strain (%) Reference 

Note Data obtained from Drzal et al. [69] and Aziz and Ansell [62], 

Panthapulakkal and Sain [10] reported that the water absorption of wheat straw-filled high-density polyethylene (HDPE) composite to be lower than corncob-filled HDPE and greater than corn straw-filled composite. The greater amount of water absorption recorded by corncob-filled composites was attributed to the greater amount of hemicellulose present in corncob. When used in composites, incorporation of com-patibilizer may decrease water uptake this may consequently enhance the mechanical properties of the composites upon being subjected to water. 

When determining water uptake behavior of composites, short time as well as long time absorption should be recorded [10]. De Rosa eta/. [15] reported varying moisture absorption ratios of untreated and treated okra bast fibers upon immersion in water for different durations. 

The impact, friction and spark sensitivities of pyrotechnic formulations are assessed by the methods given in Chapter 3. The outlines of methods for the determination of burning rate, luminous intensity, IR intensity, and total obscuring power of smoke are given in this section. 

10 mg of the sample of a formulation is taken for determination and the ignition temperature is determined on DTA instrument at a heating rate of 40 °C per min. The outline of the method is given in Chapter 3. 

The mechanical properties (compression strength and % compression) are determined by Instron Universal Testing Machine using a pellet of 20mm x 20mm pressed at a pressure of 1000kgcnf2 similar to the method used for plastics [68]. 

It is determined by exposing the formulation (2g) at 52%, 63%, 75% and 84% RH for a period of six hours. Moisture absorption is calculated from the increase in the mass which is due to moisture absorption. 

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Nylon 6 and 6/6 possess the maximum stiffness, strength, and heat resistance of all the types of nylon. Type 6/6 has a higher melt temperature, whereas type 6 has a higher impact resistance and better processibility. At a sacrifice in stiffness and heat resistance, the higher analogs of nylon are useful primarily for improved chemical resistance in certain environments (acids, bases, and zinc chloride solutions) and for lower moisture absorption. 

Cork [61789-98-8] is an effective low cost charring ablator. In order to reduce moisture absorption and related poor performance, cork particles are often blended in a silicone or phenoHc resin. The result is a uniform ablative material in a sheet form that is easy to apply. 

Com, wheat, and rice are the most desirable common grains and are used extensively ia pet foods. Oats and barley often tend to have excess fiber, which can be objectionable. However, barley is a preferred grain for moisture absorption and form ia caimed foods because the turgid white form is desired ia some canned dog foods. Milo has enormous variations ia tannin content which can influence digestibiUty and acceptabiUty, thus limiting its use ia pet foods (see Wheat and other cereal grains). 

Physical Properties. Table 1 (2) shows that olefin fibers differ from other synthetic fibers in two important respects (/) olefin fibers have very low moisture absorption and thus excellent stain resistance and almost equal wet and dry properties, and (2) the low density of olefin fibers allows a much lighter weight product at a specified size or coverage. Thus one kilogram of polypropylene fiber can produce a fabric, carpet, etc, with much more fiber per unit area than a kilogram of most other fibers. 

Olefin fibers are used for a variety of purposes from home furnishings to industrial appHcations. These include carpets, upholstery, drapery, rope, geotextiles (qv), and both disposable and nondisposable nonwovens. Fiber mechanical properties, relative chemical inertness, low moisture absorption, and low density contribute to desirable product properties. Table 7 gives a breakdown of olefin fiber consumption by use (73—75). Olefin fiber use in apparel... 

Polypropylene fibers are used in every aspect of carpet constmetion from face fiber to primary and secondary backings. Polypropylene s advantages over jute as carpet backing are dimensional stabiUty and minimal moisture absorption. Drawbacks include difficulty in dyeing and higher cost. Bulked-continuous-filament (BCF) carpet yams provide face fiber with improved crimp and elasticity. BCF carpet yams are especially important in contract carpets, characterized by low dense loops, where easy cleaning is an advantage. 

Physically or chemically modifying the surface of PET fiber is another route to diversified products. Hydrophilicity, moisture absorption, moisture transport, soil release, color depth, tactile aesthetics, and comfort all can be affected by surface modification. Examples iaclude coatiag the surface with multiple hydroxyl groups (40), creatiag surface pores and cavities by adding a gas or gas-forming additive to the polymer melt (41), roughening the surface... 

Decafluorobiphenyl [434-90-2] C F C F (mol wt, 334.1 mp, 68°C bp, 206°C), can be prepared by I Jllmann coupling of bromo- [344-04-7] chloro- [344-07-0] or iodopentafluorobenzene [827-15-6] with copper. This product shows good thermal stabiHty decafluorobiphenyl was recovered unchanged after 1 h below 575°C (270). Decafluorobiphenyl-based derivatives exhibit greater oxidative stabiHty than similar hydrocarbon compounds (271). Therm ally stable poly(fluorinated aryl ether) oligomers prepared from decafluorobiphenyl and bisphenols show low dielectric constant and moisture absorption which are attractive for electronic appHcations (272). 

A combination of excellent chemical and mechanical properties at elevated temperatures results in rehable, high performance service to the chemical processing and related industries. Chemical inertness, heat resistance, toughness and flexibiUty, stress-crack resistance, excellent flex life, antistick characteristics, Htfle moisture absorption, nonflammability, and exceptional dielectric properties are among the characteristics of these resins. 

PCTFE exhibits very good electrical properties ia terms of high iasulation resistance, minimal trackiag, corona formation, and surface flashover due to the polymer s nonwettable surface and ultralow moisture absorption (Table 3). 

Cereal Products. Breakfast cereals are susceptible to moisture absorption and require good water-vapor- and fat-barrier packagiag that retains dehcate flavors. Breakfast cereals are packaged in polyolefin coextmsion films in the form of pouches or bags within paperboard carton outer sheUs. 

Although thermal performance is a principal property of thermal insulation (13—15), suitabiHty for temperature and environmental conditions compressive, flexure, shear, and tensile strengths resistance to moisture absorption dimensional stabiHty shock and vibration resistance chemical, environmental, and erosion resistance space limitations fire resistance health effects availabiHty and ease of appHcation and economics are also considerations. 

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. 

In addition to time-related effects, the soUd-state physical properties are also affected by adsorbed water, which functions as a plasticizer. Water pickup is affected by the nature of the cation, with sodium ionomers absorbing about 10 times the level of the zinc equivalent (6) under the same conditions. Drying must be carried out at temperatures below 100°C and is therefore a slow process. In commercial practice, ionomers are suppUed dry, and techniques have been developed to minimize moisture absorption during processing. 

Plastic film Melting point, °C Moisture absorption, % Tensile strength, MPa " Ultimate elongation, % Coefficient of thermal expansion, 10-%K Dissipation factor at 1 kH2 Dielectric constant at 1 kHz Dielectric strength, MV/m Cost factor... 

The maximum recommended film thickness is 25 p.m. At greater thicknesses, volatiles from the curing reaction, mainly water and some formaldehyde and phenol, can cause defects. These coatings have excellent electrical insulation properties, ie, up to 20 V/p.m, because of low moisture absorption and low conductance. The coatings are hard with low flexibiUty, depending on curing conditions and film thickness. 

Nylon-12,12. Nylon-12,12 [36497-34-4] [36348-71-7] was introduced into the marketplace by Du Pont in the late 1980s (174). This polymer possesses very low moisture absorption, high dimensional stabihty, and excellent chemical resistance, with a moderately high melt point (T = 185° C)... 

A number of high melting poiat semiaromatic nylons, iatroduced ia the 1990s, have lower moisture absorption and iacreased stiffness and strength. Apart from nylon-6 /6,T (copolymer of 6 and 6,T), the exact stmcture of these is usually proprietary and they are identified by trade names. Examples iaclude Zytel HTN (Du Pont) Amodel, referred to as polyphthalamide or PPA (Amoco) and Aden (Mitsui Petrochemical). Properties for polyphthalamide are given ia Table 2. A polyphthalamide has been defined by ASTM as "a polyamide ia which the residues of terephthaUc acid or isophthahc acid or a combination of the two comprise at least 60 molar percent of the dicarboxyhc acid portion of the repeating stmctural units ia the polymer chain" (18). 

The hygroscopic and humectant properties of com symps are of great importance in many apphcations. Depending on the type of symp and on the specific conditions of temperature and humidity, the products may either resist or facihtate moisture loss or moisture absorption. The abihty to attract moisture or retard its loss increases with increasing DE value. Prevention of moisture pickup is more characteristic of symps having low DE values. 

The outer packaging must protect the tea from light and moisture absorption. Polypropylene or coated ceUophane outer wraps for paper board tea packages provide a barrier to loss of tea aroma and retard permeation of oxygen and foreign flavors. Low temperature improves storage stabiHty. Properly packaged and stored teas retain acceptable flavor for about a year. 

Comfort. In the past, the evaluation of fabric or garment comfort has been a subjective process influenced by such variables as temperature, insulating efficiency, moisture absorption, drying speed, softness, bulk, fabric constmction, and air permeability. Human factors must also be considered. 

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