Filler Mixed Into Adhesives Does Not Necessarily Improve Their Mechanical Properties
To investigate the influence of filler type/loading on the micro-tensile fracture strength (μTFS) of adhesive resins, as measured ‘immediately’ upon preparation and after 1-week water storage (‘water-stored’). The morphology and particle-size distribution of three filler particles, referred to as ‘Glass-S’ (Esschem Europe), ‘BioUnion’ (GC), and ‘CPC_Mont’, were correlatively characterized by SEM, TEM, and particle-size analysis. These filler particles were incorporated into an unfilled adhesive resin (‘BZF-29unfilled’, GC) in different concentrations to measure the ‘immediate’ μTFS. After 1-week water storage, the ‘water-stored’ μTFS of the experimental particle-filled adhesive resins with the most optimum filler loading, specific for each filler type, was measured. In addition, the immediate and water-stored μTFS of the adhesive resins of three experimental two-step universal adhesives based on the same resin matrix but varying for filler type/loading, coded as ‘BZF-21’ (containing silica and bioglass), ‘BZF-29’ (containing solely silica), and ‘BZF-29_hv’ (highly viscous with a higher silica loading than BZF-29), and of the adhesive resins of the gold-standard adhesives OptiBond FL (‘Opti-FL’, Kerr) and Clearfil SE Bond 2 (‘C-SE2’, Kuraray Noritake) was measured along with that of BZF-29unfilled (GC) serving as control/reference. Statistics involved one-way and two-way ANOVA followed by post-hoc multiple comparisons (α<0.05). Glass-S, BioUnion, and CPC_Mont represent irregular fillers with an average particle size of 8.5-9.9 μm. Adding filler to BZF-29unfilled decreased μTFS regardless of filler type/loading. One-week water storage reduced μTFS of all adhesive resins except BZF-21, with the largest reduction in μTFS recorded for BZF-29unfilled. Among the three filler types, the μTFS of the 30 wt% Glass-S and 20 wt% BioUnion filled adhesive resin was not significantly different from the μTFS of BZF-29unfilled upon water storage. Adding filler particles into adhesive resin did not enhance its micro-tensile fracture strength but appeared to render it less sensitive to water storage as compared to the unfilled adhesive resin investigated.SUMMARY
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(A) Microstructure of Glass-S (Esschem Europe), (B) BioUnion (GC), and (C) CPC_Mont19 characterized using SEM.
(A) Microstructure of Glass-S (Esschem Europe), (B) BioUnion (GC), and (C) CPC_Mont19 when mixed into unfilled adhesive resin (BZF-29unfilled (GC)), as characterized using TEM and regarding BioUnion (GC) also chemically mapped by (D) STEM/EDS mapping.
Particle size distribution of Glass-S (Esschem Europe), BioUnion (GC), and CPC_Mont19.
Bar graph presenting the immediate μTFS of the filler-mixed adhesive resins with different weight percentages of (A) Glass-S (Esschem Europe), (B) BioUnion (GC), and (C) CPC_Mont19. Groups marked with the same lowercase letters indicate no significant difference in μTFS between the different adhesive resins.
Bar graph presenting the immediate and water-stored μTFS of the different adhesive resins investigated. Groups with the same uppercase or lowercase letters indicated no significant difference in μTFS between the immediate or water-stored μTFS of the different adhesives. Asterisks indicate significant difference in μTFS between the immediate and water-stored μTFS of the same adhesive.
SEM photomicrographs of the fractured specimens of (A) BZF-29unfilled (GC), (B) Glass-S (Esschem Europe), (C) BioUnion (GC), and (D) CPC_Mont19. A fractured surface of (A1) BZF-29unfilled (GC) is presented, with (A2 and A3) magnified views showing ruptured resin-matrix layers without filler. A fractured surface of (B1) Glass-S (Esschem Europe) is presented, with magnified views showing the (B2 and B3) filler distribution. Particle agglomerations were observed. A fractured surface of (C1) BioUnion (GC) is presented, with (C2 and C3) magnified views showing rough surfaces. Voids formed by filler detachment were observed. (D1) A fractured surface of CPC_Mont19 is presented, with (D2 and D3) magnified views. The particle size of CPC_Mont19 ranged from 1 to 25 μm.
(A) SEM photomicrographs of the fractured specimens of BZF-21 (GC), (B) BZF-29 (GC), (C) BZF-29_hv (GC), (D) C-SE2 (Kuraray Noritake) and (E) Opti-FL (Kerr). (A1) A fractured surface of BZF-21 (GC) is presented, (A2 and A3) with magnified view showing adhesives filled with silica and bioglass filler. Silica-particle aggregations were observed. (B1) A fractured surface of BZF-29 (GC) is presented, (B2 and B3) with magnified views showing the homogenous distribution of spherical silica filler. (C1) A fractured surface of BZF-29_hv (GC) is presented, with (C2 and C3) magnified views showing the adhesive filled with spherical silica filler. (D1) A fractured surface of C-SE2 (Kuraray Noritake) is presented, with (D2 and D3) magnified views showing the adhesive containing nanoscale silica filler. (E1) A fractured surface of Opti-FL (Kerr) is presented, with (E2 and E3) magnified views showing the adhesive.
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