Polyesters are widely used in plastic objects, films, and fabrics. A recurr
ing mechanism of polyester aging is attack by humidity (hydrolysis), whereb
y the long chain molecules are cleaved and the material softens. This artic
le offers an approach for modeling the hardness/aging behavior of commercia
l polyesters. The model is qualitatively consistent with exposure tests on
fresh polyester plastic material made from a commercially available resin a
nd hardening agent. It expresses the rate of polyester hydrolysis in terms
of the relative humidity, and then connects the degree of hydrolysis to har
dness using a relation that accounts for stabilizers that are present in co
mmercial polymers to counter the hydrolysis.