Water uptake and strength characteristics of a nanofilled resin-based composite

To investigate the influence of short- and medium-term immersion on water uptake and mechanical properties of a so-called 'nanofilled' compared with a conventional resin-based composite (RBC).

For each material (a microhybrid, Filtek Z250, FZ and 'nanofilled' RBC, Filtek Supreme in body and translucent shades, FSB and FST; 3M ESPE, St. Paul, USA) five specimen groups (n=20) were tested. Mean bi-axial flexure strength (BFS) of each group was determined following 24h 'dry' and 24h, 3, 6 and 12 months in a water-bath maintained at 37+/-1 degrees C prior to testing. The extent of water uptake following each storage regime was determined using a near-infrared spectroscopy (NIRS) technique.

No significant difference in BFS was identified for each material stored dry or wet for 24h (F=2.7; P=0.07) whilst BFS decreased following 3, 6 and 12 months (F=6.6; P<0.001). A significant decrease in BFS of FZ was observed following 3 and 6 months (147+/-34 and 102+/-30 MPa; P<0.001) with no further reduction following 12 months (94+/-13 MPa; P>0.05). In contrast, no significant decrease in BFS of FSB (97+/-34 MPa) and FST (112+/-28 MPa) was recorded following 6 compared with 3 months immersion (P>0.05). Storage for 12 months resulted in a further significant strength reduction of FSB and FST (56+/-11, 82+/-12 MPa; P=0.004, P<0.001, respectively). Water uptake of FZ and FSB increased up to 3 months before equilibrating, whereas water content of FST increased following all storage periods.

Strength degradation occurred at different rates between material types. Water uptake and mechanical properties of test materials were influenced by the size and morphology of the reinforcing particulate phase. The use of nanoparticles and associated agglomerates in modern RBCs exhibit distinct mechanical and physical properties compared with a conventional RBC type.