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Water absorption

Essential changes can be observed in polymeric materials under the influence of moisture and water vapor. They are exhibited more sharply in hydrophylic plastics than in hydrophobic ones. Diffusion of moisture into a polymer is followed by reduction of intermolecular interaction in it that makes negative impact to material strength properties.

At the same time the moisture sorbed by a polymer surface, promoting formation of new surfaces, can facilitate emergence of microcracks with the subsequent loss of material consumer properties. Such effect is observed at rigid amorphous thermoplastics like polycarbonate and bulk-polymerized polystyrene.

Polyamides refer to hydrophilic polymers. They are characterized by hygroscopicity and are capable to sorb moisture from air even in standard conditions of relative humidity (50 - 60 % at a temperature of 20 °C). Depending on the chemical structure and composition the moisture absorption of polyamides can be from 0.7 to 4 %.

The deformation and strength characteristics of polymers determined after samples drying up to moisture content about 0.1 % significantly exceed strength properties of materials being in standard conditions.

Significant influence of moisture on properties is exhibited at composite plastics with hydrophilic fillers (polyamide fiber, some kinds of carbon fiber). In relation to composites and reinforced plastics the process of moisture sorption happens along the border between the polymeric matrix and the filler and, therefore, it will significantly depend both on a ratio of physical and chemical properties of system components and on technological conditions of composite production.

In general at conducting assessment of a possibility of influence of external liquid environment on plastics it is also necessary to consider to what extent this liquid is solvent for this polymeric matrix. So, for example, for PMMA (polymethylmethacrylate) active solvents such as acetone, ethanol, and benzene sharply reduce durability, at the same time kerosene, oleic acid and water have much less weakening effect.

Comparison of polyamides by water absorption