Water, absorption

During application, plastics are exposed to ambient conditions where temperature and humidity vary. This combined effect, known as the hygrothermal effect, may influence the mechanical properties of plastics. The properties of glass fiber-reinforced plastics are more affected by this combined influence than by one of the two factors alone. Moisture absorption in reinforced materials is mainly characterized by diffusion. The capillary effect and transport along microcracks is another way to absorb moisture. Moisture absorption by the latter two mechanisms is often a direct consequence of moisture absorption caused by hot and humid ambient conditions. 

The effect of water absorption must be taken into consideration when designing with water-absorbent plastics. In many cases, water absorption in plastics follows Pick s law (Eq. 5.2), so that the mass of absorbed water increases linearly with the root of time until equilibrium is reached. If the material does not follow Pick s law, surface damage or material degradation has taken place [596]. 

Chemical resistance of plastics depends on their molecular structure, the type and aggressiveness of the chemicals (e.g., acids, bases, polar or non-polar solvents), temperature, and duration of contact. The higher the temperature, the stronger chemicals attack the plastic material. The longer the duration of contact, the more pronounced the chemicals influence on the material. 

In addition, plastics behave differently under simultaneous mechanical load [243]. The attack mechanisms function entirely differently in plastics than they do in metals. The intramolecular secondary valence bonds (van der Waals forces) are several orders of magnitude smaller (1/100 to 1/1000) in polymers than in metals. Therefore, the free volume between bulky and entangled molecular chains is so large that the comparatively small gas and liquid molecules can easily diffuse into the intermediate spaces and become embedded there. Thus, the influence on the plastic is not limited to its surface, but takes place virtually throughout its volume. In glass fiber-reinforced plastics with their heterogeneous structure, interfacial problems also develop [243]. 

The service life of a plastic material often depends less on the polymer itself than on the stabilizers used, therefore it is important to always consider the polymer-stabilizer combination. Stabilizer distribution is a further important influencing factor here, see Section 5.5.6.6 [63], [191]. 

The data should indicate the temperature and time of immersion and the percentage of weight gain of a test specimen. The same applies to data at the saturation point of 73.4°F (23°C), and, if the material is usable at 212°F (100°C), also to saturation at this temperature. 

Moisture or water absorption is an important design property. It is particularly significant for a product that is used in conjunction with other materials that call for fits and clearances along with other close tolerance dimensions. 

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. 

---

Other methods for analyzing combustion products can be substituted for chromatography. Gravimetry can be used, for example, after a series of absorption on different beds, as in the case of water absorption in magnesium perchlorate or CO2 in soda lime infra-red spectrometry can be used for the detection of CO2 and water. 

When in position, the side-arm k of the carbon dioxide absorption tube is ttached to the T side-arm of the water absorption tube R (Fig. 85) by impreg-... 

Key properties of cellulose nitrate are good dimensional stability, low water absorption, and toughness. Its disadvantages are its flammability and lack of stability to heat and sunlight. 

This thermoplastic shows good tensile strength, toughness, low water absorption, and good frictional properties, plus good chemical resistance and electrical properties. 

Melting temperature, °C Crystalline Amorphous Specific gravity Water absorption (24 h), % Dielectric strength, kV mm ... 

The properties of SAN are significantly altered by water absorption (16). The equiUbrium water content increases with temperature while the time requited decreases. A large decrease in T can result. Strong aqueous bases can degrade SAN by hydrolysis of the nittile groups (17). 

Acrylonitrile has been grafted onto many polymeric systems. In particular, acrylonitrile grafting has been used to impart hydrophilic behavior to starch (143—145) and polymer fibers (146). Exceptional water absorption capabiUty results from the grafting of acrylonitrile to starch, and the use of 2-acrylamido-2-methylpropanesulfonic acid [15214-89-8] along with acrylonitrile for grafting results in copolymers that can absorb over 5000 times their weight of deionized water (147). 

Aloisture Absorbent Synthetic Paper. Processes for making a water absorbent synthetic paper with dimensional stabihty have been developed by several companies. In a process developed by Mitsubishi Rayon, acrylic fiber is insolubilized by hydra2ine and then hydroly2ed with sodium hydroxide. The paper, formed from 100 parts fiber and 200 parts pulp, has a water absorption 28 times its own weight (96). Processes for making hygroscopic fibers have also been reported in the patent Hterature. These fibers are used in moisture absorbing nonwovens for sanitary napkins, filters, and diapers. 

---