Water sorption/desorption in polyacid-modified composite resins for dentistry

The water sorption and desorption behaviour of three commercial polyacid-modified composite resins used in clinical dentistry have been studied in detail. Cured specimens of each material were subjected to two successive water uptake cycles in an atmosphere of 93% relative humidity, with one intervening desorption cycle in a desiccating atmosphere over concentrated sulfuric acid. Specimens were found to absorb and desorb water according Fick's law until Mt/M(infinity) values of approximately 0.5. Diffusion rates for uptake varied between cycles, ranging from 2.37-4.53 x 10(-9 )cm(2) s(-1) for 1st cycle to 0.85-2.72 x 10(-8 )cm(2 )s(-1) for 2nd cycle. Desorption rates were similar to those for 2nd cycle sorption, and ranged from 0.86 to 5.47 x 10(-8 )cm(2 )s(-1). Equilibration times for 1st cycle water uptake were greater than for 2nd cycle sorption and for desorption and overall the behaviour of polyacid-modified composites in a high humidity atmosphere was similar to that of conventional composites in water. It is concluded that the hydrophilic components of the former do not bring about an enhanced rate of water transport.

Water transport in zinc oxy-chloride cements

The water uptake and water loss behaviour in three different formulations of zinc oxy-chloride cement have been studied in detail. Specimens of each material were subjected to a high humidity atmosphere (93% RH) over saturated aqueous sodium sulfate, and a low humidity desiccating atmosphere over concentrated sulfuric acid. In high humidity, the cement formulated from the nominal 75% ZnCl2 solutions gained mass, eventually becoming too sticky to weigh further. The specimens at 25% and 50% ZnCl2 by contrast lost mass by a diffusion process, though by 1 week the 50% cement had stated to gain mass and was also too sticky to weigh. In low humidity, all three cements lost mass, again by a diffusion process. Both water gain and water loss followed Fick's law for a considerable time. In the case of water loss under desiccating conditions, this corresponded to values of Mt/MĄ well above 0.5. However, plots did not go through the origin, showing that there was an induction period before true diffusion began. Diffusion coefficients varied from 1.56 x 10-5 (75% ZnCl2) to 2.75 x 10-5 cm2/s (50% ZnCl2), and appeared to be influenced not simply by composition. The drying of the 25% and 50% ZnCl2 cements in high humidity conditions occurred at a much lower rate, with a value of D of 2.5 x 10-8 cm2/s for the 25% ZnCl2 cement. This cement was found to equilibrate slowly, but total water loss did not differ significantly from that of the cements stored under desiccating conditions. Equilibration times for water loss in desiccating conditions were of the order of 2-4 hours, depending on ZnCl2 content; equilibrium water losses were respectively 28.8 [25% ZnCl2], 16.2 [50%] and 12.4 [75%] which followed the order of ZnCl2 content. It is concluded that the water transport processes are strongly influenced by the ZnCl2 content of the cement.