Polyacid-modified composite resins water uptake

Polyacid-modified composite resins have undergone considerable development since they first appeared. The very limited nature of the acid-base reaction means that they have had to have the fluoride-releasing capability augmented, for example, through the inclusion of extra ytterbium fluoride in the formulation [38]. There has also been concern that the abihty to draw in water from the environment might also lead to staining and softening, and reformulation has partly been driven by the need to minimize any such moisture uptake, so as to preserve the physical properties of the composite. 

As already mentioned, the distinctive property of polyacid-modified composite resins is that, once the polymerization reaction has occurred, the set material is able to take up traces of moisture. This activates the acidic character of the carboxylic functional monomer and triggers an acid-base reaction with the glass [1,2]. The water uptake behaviour of these materials has been studied in detail [15], along with the corresponding water desorption processes. Three commercial polyacid-modified composite resins were used in a study of water uptake and loss, and cured samples were prepared as small discs of size 6 mm diameter x 2 mm thickness. Water uptake was allowed to take place in a controlled humidity environment at 93% relative humidity. Following the initial water uptake, there was an intervening desorption cycle in which specimens were stored in a dry atmosphere over concentrated sulfuric acid. 

In fact, studies of water uptake with a direct comparison of water uptake in com-pomers and conventional composite resins do not show particularly large differences between the two different types of composite material. For example, when the polyacid-modified composite resin brands Dyract and Compoglass were compared with the conventional composite resin Pekafill , there were only minor differences in equilibrium water uptake in both pure water and in 0.9% saline solution (Table 4.1) [18]. Pekafill showed lowest equilibrium water uptakes in both storage media, but only by a very small amount, and one that was not statistically significant in the case of pure water. 

The study showed that water affected the properties of the conventional composite resin to slight but statistically significant extents, a finding which is consistent with them taking up small amounts of water under these conditions [31], Despite the possibility of the water uptake in polyacid-modified composite resins triggering the secondary add-base reaction, they showed similar behaviour to the conventional composites, with a reduction in both properties on taking up water. This was a greater problem for these materials, as they had lower values to start with. 

Re-formulated polyacid-modified composite resins have also been found to have properties that decline following water uptake and secondary acid-base reaction (Table 4.6). In one study that demonstrated this, Dyract AP was shown to have lower compressive and biaxial flexure strength values following soaking in water for 4 weeks than they did at 24 h [32] (Table 4.6). By contrast, when stored in dry conditions, strengths were not significantly different between 24h and 4 weeks. 

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