Reinforced dental glass ionomer cements: from conventional to nanocomposites

Fareed, Muhammad Amber (2010). Reinforced dental glass ionomer cements: from conventional to nanocomposites. University of Birmingham. Ph.D.

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Abstract

The use of nanoclays as reinforcement to create “polymer-clay nanocomposites” with properties greater than the parent constituents has been well established over the past few decades. In this study a systematic investigation of the reinforcement possibility of poly(acrylic acid) (PAA) with polymer-grade nanoclays (PGV and PGN nanoclays) to develop glass-ionomer cements (GICs) is presented. Chemical characterisations in order to understand the dispersion mechanism of nanoclays and PAA-nanoclay interactions were performed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. Gel permeation chromatography (GPC) was conducted in order to measure the molecular weight of PAA used in cements. Several mechanical properties including, compressive strength (CS), diametral-tensile strength (DTS), flexural strength (FS), flexural modulus (E\(_f\)), and Vickers hardness (HV) of control groups (Fuji-IX and HiFi cement) and cements formed after the reinforcement of nanoclays were measured. The rheological studies were conducted with a Wilson’s oscillating rheometre to establish the working and setting times of cements after the reinforcement with nanoclays. Additionally, the wear resistance of cements was also measured by two different wear-test methodologies namely, reciprocating wear test (ball-on-flat) and Oregon Health & Sciences University (OH&SU) wear-simulator. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), environmental SEM (ESEM) and cryo-SEM studies were also performed to study the microstructure.

XRD demonstrated that an interlayer space in nanoclays formed by a regular stacking of silicate layers along the [001] direction (\(d_{001}\)). The peaks in the diffraction pattern of PGN nanoclays at 2θ ~ 7.13° and PGV nanoclays at 2θ ~ 6.95° were attributed to the interlayer \(d\)-spacing. The interlayer space of PGN was expanded from 12.83 Å to 16.03 Å in PAA-PGN indicating that intercalation occurred whereas exfoliation occurred in PAA-PGV indicated by the lack of peak at \(d_{001}\). XPS scans of PGV and PGN nanoclays depicted the main peak of O 1s photoelectron which was attributed to the various oxygen containing species such as Si-O-M (M: Mg, Al, Fe, etc) within the two layers of the silicate plate, while, Si-O-Al linkages were identified by Si 2p or Si 2s and Al 2p or Al 2s peaks. The disappearance of the Na peak in PAA-nanoclays in wide-angle XPS scans confirmed that PAA molecules exchanged sodium ions on the surface of silicate layers of nanoclays. It is suggested, that the removal of the cations from the interlayer galleries of nanoclays by the PAA chains reduced significantly the electrostatic Van-der-Waals forces between the silicate plates resulting in intercalation or exfoliation. FTIR spectra demonstrated the presence of a new peak at 1019 cm\(^{−1}\) in PAA-nanoclay suspensions and the absorbance intensity of the peak at 1019 cm\(^{−1}\) associated with Si-O- stretching vibrations increased with increasing nanoclays loading. The shift of the peak at 973 cm\(^{−1}\), associated with Si-O- stretching vibrations in nanoclays, to 1019 cm\(^{−1}\) in PAA-nanoclay and to 1041 cm\(^{−1}\) in HiFi-nanoclay suspensions may be associated to the change of the chemical environment in the PAA- nanoclay suspensions. The setting reaction of GICs involves the neutralisation of PAA by the glass powder, which was linked with the formation of calcium and aluminium salt-complexes. The real-time study of the setting reaction of GICs over a period of one hour before and after nanoclays dispersion in Fuji-IX system and HiFi system presented slight differences in the FTIR spectra.

The working and setting times of GICs determined by the Wilson’s rheometre exhibited a small decrease both in working and setting times of GICs prepared with the polymer liquids formed after the nanoclays dispersion, when compared to control groups (CPA-HP and CF-IX). Moreover, the mechanical properties (CS, DTS, FS, E\(_f\) and HV) of GICs were determined at various aging times 1 hour, 1 day, 1 week and 1 month of storage in distilled water. Generally, GICs formed with modified PAA liquid that contained less than 2 wt % nanoclays, exhibited slightly higher CS (range 124.0– 142.0 MPa), similar DTS (range 12.0–20.0 MPa) and higher biaxial flexural strength (range 37.0–42.0 MPa) compared to the control cement groups (Fuji-IX and HiFi). E\(_f\) of GICs ranged between 8.0 to 14.5 GPa and the highest values of modulus were obtained for 1 wt % nanoclay reinforcement in the HiFi GIC system. HV of cements was calculated between 62-89 HV. However, the effect of nanoclays reinforcement on various mechanical properties was not statistically significant. The ball-on-flat wear test and the OH&SU wear simulation studies suggested that the determination of the wear volume is more reliable than the wear depth. A comparison between Fuji-IX and HiFi reflected that the HiFi system always showed better wear-resistance than the Fuji- IX. It is proposed, that the improved wear resistance of the HiFi system after the dispersion of nanoclays may be due to a better methodology employed for nanoclay dispersion. However, no significant difference was observed in wear volume for different wt % nanoclays loading. TEM micrographs indicated the microstructure of PAA matrix and glass particles as well as their interaction with dispersed nanoclays. The micro-structure of porous glass particles and the presence of siliceous hydrogel phases surrounding the remnant glass core were identified. In PAA matrix, the association of porous glass particles with the nanoclays was also observed.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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