Moisture Absorption Behavior

The siloxane incorporation into the epoxy system reduced the moisture absorption behavior. For example, the percentage of moisture uptake for unmodified epoxy system is 0.1251, whereas for 10% siloxane-modified systems, it becomes 0.1125. This may be attributed to the presence of free rotation and partial ionic polarization of the Si-O bond, which accordingly results in hydrophobic properties [42]. The inclusion of PBMI into the epoxy and siliconized epoxy systems reduced the moisture uptake due to the rigid aromatic hydrophobic structure. The moisture resistance property increased with increasing PBMI content, and it was observed that the PBMI incorporation plays an important role in improving the resistance to moisture absorption of siliconized epoxy matrix systems. This may be due to the combined effect of PBMI and siloxane, since both have a negligible tendency to absorb moisture. 

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Fig. 31. Moisture absorption behavior of fully crosslinked Fiberite 934 resins as influenced by physical aging...

Figure 22. Moisture absorption behavior of fully-cross-linked Fiberite 934 epoxy as influenced by 140 C sub-T aging.

Z. Leman, S.M. Sapuan, A.M. Saifol, M.A. Maleque, and M.M. H.M. Ahmad, Moisture absorption behavior of sugar palm fiber reinforced epoxy composites. Mater. Des. 29(8),1666-1670 (2008). 

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... 

However, after chemical treatment of C. indica fibers, dielectric constant values of finally fabricated biocomposites have been found to be lowered, which could be due to the decrease in orientation polarization of polymer composites containing surface-modified fiber. We have reported earlier that benzoylation reduces the moisture absorption behavior of fibers because of blockage of —OH groups on cel-lulosic fiber backbone. Thus resultant decrease in hydrophUicity of the polymer composites leads to lowering of orientation polarization and ultimately dielectric constant value [22]. It has also been observed that mercerization is one of the most effective methods in the reduction of dielectric constant values of the obtained biocomposites. 

The possible interaction between MCNF and PMMA molecules during free radical polymerization resulted in an improved mechanical, thermal and moisture resistance property. They also prepared equivalent PMMA/cellulose nanocomposites by conventional ex-situ solution dispersion technique (MMEPC) which showed much lower properties. This observation clearly showed the benefit of in-situ polymerization technique in terms of mechanical property enhancement. It is well known that cellulose-reinforced composites show extensive moisture absorption behavior which is considered to be their major drawback when practical application possibilities are considered in the fields like packaging, coating, etc. The % moisture uptake of MMIPC was reported to be 52% lower than that observed in the equivalent ex-situ composites (MMEPC) although both contained same amount of MCNF fillers (Figure 5.14). 

The moisture content of a plastic affects such conditions as electrical insulation resistance, dielectric losses, mechanical properties, dimensions, and appearances. The effect on the properties due to moisture content depends largely on the type of exposure (by immersion in water or by exposure to high humidity), the shape of the product, and the inherent behavior properties of the plastic material. The ultimate proof for tolerance of moisture in a product has to be a product test under extreme conditions of usage in which critical dimensions and needed properties are verified. Plastics with very low water-moisture absorption rates tend to have better dimensional stability. 

The effect of the type and level of initiator on polymerization time, monomer conversion, and polymer molecular weight, the effect of polymerization temperature on the crystallization behavior of the polyamide generated, and the role of a higher lactam like laurolactam on the moisture absorption characteristics of the copolymers are discussed in our previous publication [23]. 

It will reversibly take up moisture without any effect on the appearance of the material at humidities up to 60% RH. Equilibration at RH values above 60% will result in deliquescence. Once in this state, the material can be dried, but will give a glasslike product. This water absorption behavior is typical of amorphous hygroscopic materials. 

The 6FDA/BDAF polyimide was modified using PPD in an effort to "stiffen" the polymer backbone and improve thermal performance (Tg). A better overall property balance was achieved in several of these 6FDA/BDAF/PPD copolyimides. A series of random copolymers was prepared in which the level of PPD was varied from 0% to 100% based on the total moles of diamine. The incorporation of PPD had little effect on the dielectric constant but did result in improved thermal performance and was accompanied by increased moisture uptake (Figures 1,2, and 3). This behavior is consistent with the overall reduction in the amount of bound fluorine in the polymer backbone however, additional work is required to establish a direct correlation. A reasonable property balance was realized over a range of 40 to 60 mole% PPD which displayed dielectric constants from 2.85 to 2.90, moisture absorption from 1.5% to 2.0%, and Tg from 280°C to 290°C. In addition, the 6FDA/BDAF/PPD copolyimides displayed somewhat less solvent sensitivity than the 6FDA/BDAF homopolymer as described above. 

Data obtained from typical CRH determinations uti-li2ing the method described above are shown in Hgurc 18.11. The data obtained for prilled urea are typical of relatively pure materials, in that the critical point is quite definite. There was no moisture absorption in 3 hours at 70% relative humidity, but there was continuous absorption at 75%. The data shown for the fertilizer grade diammonium phosphate, on the other hand, illustrate typical behavior of fertilizers that contain small amounts of soluble impurities. It is obvious that the CRH of the main constituent is between 70% and 75% for practical purposes, however, it would have to be recognized that exposed product would pick up some moisture at relative humidities between 60% and 70%. At 70% relative humidity, a maximum of 2% would be absorbed. For most diammonium phosphate products, this is insufficient to cause intergranule transfer of moisture thus, only the surface granules would be affected in a bulk... 

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. 

Decrease in water absorption behavior is reported to occur with alkalizing of okra bast and banana stem and bxmch fibers [5,26]. Ganan et al. [26] reported an increase in contact angle and decrease in surface free energy. Yilmaz [11] also found a decrease in moisture content of corn husk fibers with the increase in concentration and duration of alkalization. The decrease in moisture content might be due to the reduction in hydroxyl groups. 

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