Water Resistance of Polymeric Materials
Water Resistance of Polymeric Materials
Bringing a polymer into contact with water usually results in swelling of the polymer, so that the shape of an object may change, and its strength, dielectric properties, and so on may decrease. Water absorption may also lead to the breaking of chemical bonds in the polymer molecule; this process proceeds with appreciable speed only at high temperatures and with polymers obtained by polycondensation.
The water resistance of polymeric materials is evaluated by the change in the dielectric indexes, degree of swelling (water absorption), or one of the physicomechanical indexes of the material upon contact with water. In most cases this resistance is characterized by the water absorption, which is the amount of water absorbed by the material after immersion for 24 hours in water at room temperature. The water absorption is stated as a percentage of the weight of the specimen or as the weight of water absorbed per unit surface area of the specimen in grams per square decimeter (g/dm2) or kilograms per sq m (kg/m2) respectively (see Table 1).
| Table 1. Water resistance of some polymeric materials | |
|---|---|
| Water absorption | |
| High-density polyethylene............... | 0.2 g/dm2 |
| Expanded polystyrene............... | 3 g/dm2 |
| Bulk-polymerized polystyrene............... | no absorption |
| Vinyl plastic............... | 0.4-0.6% |
| Fiberglass laminate............... | 0.3-1.0% |
| Amino plastics............... | 0.45-0.7% |
The water resistance of polymeric materials depends on the chemical nature of the polymer, its structure, the degree of hardening (vulcanization), the method of processing, the thickness and porosity of the object, and the composition of the material. Finished goods are heat-treated or given a water-resistant coating to raise the water resistance of polymeric materials. The water resistance of laminated plastics is increased by using treated fillers.
REFERENCES
Pik, I. Sh. Pressovochnye, lit’evye i podelochnye plasticheskiehexkie massy. Moscow, 1964.64.Molchanov, lu. M. Fizicheskie i mekhanicheskie svoistva polieti-polieti-lena, polipropilena i poliizobutilena: Spravochnik. Riga, 1966., 1966.