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

Abstract

Objectives

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).

Method

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 24 h ‘dry’ and 24 h, 3, 6 and 12 months in a water-bath maintained at 37 ± 1 °C prior to testing. The extent of water uptake following each storage regime was determined using a near-infrared spectroscopy (NIRS) technique.

Results

No significant difference in BFS was identified for each material stored dry or wet for 24 h (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.

Conclusions

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.

Introduction

The routine use of amalgam in restorative dentistry is gradually decreasing due to patient demand for “white fillings” which mimic the appearance of natural dentition, concern over alleged toxicity and environmental considerations arising from disposal.¹ Whilst the use of amalgam declines, although at different rates internationally,2, 3 a range of tooth coloured restorative materials, such as resin-based composites (RBCs) have been developed and continue to be refined for use in both anterior and posterior regions of the oral cavity. However, whilst the failure rates of RBCs and amalgams may be similar when placed correctly,³ RBCs frequently exhibit a lower clinical survival rate as a consequence of inherent technique sensitivity and material property limitations.⁴

The majority of conventional RBC-type materials in use today are based on dimethacrylate resins introduced by Bowen in 1962.⁵ Whilst resin chemistries have not been significantly modified, the size of filler particles incorporated in the resin matrix of commercial RBCs has generally decreased (Fig. 1). The modification of filler morphology has improved mechanical properties and aesthetics compared with earlier RBC restoratives. However, the reduction of filler size and subsequent increase in surface area to volume ratio has limited the achievable filler loading resulting in decreased working characteristics and mechanical properties.⁶ To increase filler loading prepolymer particles containing a high volume fraction of silanated colloidal silica were incorporated into the resin with 10–20 volume percent (vol%) of discrete sub-micron particles. These materials were classified as ‘heterogeneous microfills’ and possessed overall filler loadings of ∼60–70 vol%. The development of hybrid RBCs sought to further improve both the aesthetic and mechanical properties. ‘Hybrid’ materials contain two distinct graded filler size distributions allowing for efficient filler packing. The production of nanosized fillers for conventional microfill and modern ‘nanohybrid’ materials has required a shift from traditional milling techniques to a bottom-up synthetic chemical sol–gel process. One so-called ‘nanofilled’ RBC, Filtek Supreme (3M ESPE, St. Paul, MN, US) claims to “possess polish retention similar to that of microfills and mechanical and physical properties comparable with hybrid composites”.⁷ This nanohybrid RBC contains a combination of individually dispersed filler particles (5–75 nm) and partially calcined porous clusters (∼1.3 μm) of agglomerated nanosized particles infiltrated with silane prior to incorporation into the resin matrix.⁷ Some dental material manufacturers have marketed modern resin composites as a development in the field of nanotechnology. By definition, a ‘nanomaterial’ possesses components and/or structural features, such as fibres or particles, with at least one dimension in the range of 1–100 nm and, as a result, will demonstrate novel and distinct properties.8, 9 Notwithstanding the unremitting hype and often aggressive marketing, it is interesting to note that the size of fillers present in microfilled RBCs do not differ vastly from that of modern ‘nanohybrid’ composites (Fig. 1). Consequently, speculation exists amongst researchers⁸ and manufacturers¹⁰ alike as to whether ‘nano’composites exhibit any improved mechano-physical properties compared with pre-existing RBCs. The aim of the present investigation was to test the hypothesis that filler size and morphology would influence the water uptake and resultant mechanical properties of a so-called ‘nanofilled’ compared with a conventional RBC.