This study evaluates the effect of incorporation of an acrylate polyhedral oligomeric silsesquioxane nanostructure (APOSS) on the physical and mechanical properties and hydrolytic stability of octyl cyanoacrylate (CA) adhesives.
Materials and methods
CA was photopolymerized under irradiation of visible light using 1-phenyle-1,2-propandione (PPD), and 2,3-botanedione (BD) as photoinitiators. Following the polymerization shrinkage kinetics of the adhesives, the initiator concentration was optimized. Mechanical properties of the bulk CA–APOSS nanocomposites, including flexural strength and modulus, were investigated. Miroshear bond strength of dental composite bonded to human dentin applying the CA–APOSS adhesives was also determined. The effect of APOSS on the stability of CA adhesive against hydrolysis studied performing solubility, water uptake, and aging tests.
The results revealed higher efficiency of BD in comparison to PPD. A 3% (mol/mol) of BD was obtained as the optimum photoinitiator concentration. The incorporation of APOSS increased the polymerization shrinkage rate of the CA adhesives. The flexural strength of CA adhesive was significantly improved incorporating less than 20 wt.% APOSS while an increasing trend was observed in the flexural modulus with the nanostructures loading. The microshear bond strength to dentin was also enhanced using 10 wt.% APOSS as reinforcing/crosslinking nanofillers. A decrease in the solubility and water sorption was the result of incorporation of APOSS in CA adhesives. Degradation due to the hydrolysis in water was diminished in the specimens containing APOSS nanostructures, revealed after aging in water at 37 °C.
CA adhesives are good soft tissue adhesives which their low mechanical properties and lack of hydrolytic stability has made them less interesting in the applications deal with hard tissues. The study shows that the incorporation of POSS nanostructures into CA could reduce the drawbacks.
Cyanoacrylates (CAs) are synthetic glues that have recently been used in many bioengineering applications due to several advantages such as their good adhesion to tissues especially to wet substrates . The CAs have been applied as alternative to suturing to fix the zygomatic bone fracture , and reported to be bacteriostatic . Although the CAs are used in periodontics and oral surgery , their application in restorative dentistry is limited to cyanoacrylate modified glass ionomer cements . n -Butyl 2-cyanoacrylate has also been used to temporarily splint traumatized teeth . Among the cyanoacrylate monomers, 2-octyl cyanoacrylate is considered as a medical grade adhesive which was developed to be non-toxic and less irritating to human tissue. The CAs rapidly polymerize in contact with water and blood through a fast anionic polymerization. The stabilization of the propagating anions by two strongly electron-withdrawing groups, CN and COOR, facilitates the anionic polymerization of the CAs . On the other hand, the electron withdrawing side groups make the polymer backbone hydrolytically sensitive resulting in fast degradation times depending on the length of the alkyl side group . The weak stability in contact with water restricts their application in wet conditions . Although the cyanoacrylate adhesives have shown adequate bond strength in bonding of orthodontic brackets to enamel , the adhesives dramatically lose their bond strength when subjected to thermocycling . The rapid and dramatic decrease in the molecular weight deteriorates their mechanical and adhesion strength. Therefore, the CAs adhesives are not good candidates where a long term durable adhesion is needed.
Polyhedral oligomeric silsesquioxanes (POSS) are nano building blocks which have recently been introduced and applied in the preparation of nanostructured materials with improved physical and mechanical properties . The nanostructures have soon found their way to dental materials . It has also been reported that the incorporation of POSS as a crosslinking agent, affects shrinkage behavior and may considerably improve some mechanical properties of cyanoacrylate bioadhesives .
In order to find a solution to overcome the low mechanical properties and lack of hydrolytic stability of the cyanoacrylate adhesive, this study evaluates the effect of the incorporation of POSS nanostructures, as reinforcing agents, on the properties of the adhesive. The adhesive was photopolymerized using different photoinitiator systems after suppressing the anionic polymerization of the cyanoacrylate monomer. The shrinkage behavior, physical, and mechanical properties of the adhesive were also investigated.
2-Octyl cyanoacrylate (2-OCA) was obtained from Tong Shen Enterprise (Taiwan). Acrylo-POSS (APOSS, FW = 1321.75) was purchased from Hybrid Plastics (USA) which is used as crosslinking agent. 1-Phenyle-1,2-propanedione (PPD), and 2,3-botanedione (BD) were supplied by Sigma–Aldrich (Germany). Methane sulfunic acid (MSA) was obtained from Merck (Germany). Chemical structure of APOSS and 2-OCA are schematically represented in Fig. 1 .
Shrinkage strain measurement
The bonded-disk technique of Watts and Cash was utilized to measure the shrinkage behavior of the specimens using a 3 mm thick glass base plate. 2-OCA containing APOSS nanostructure as a crosslinking agent and 2000 ppm MSA as an inhibitor for premature ionic polymerization was polymerized using PPD and BD as photoinitiator systems. The adhesive was irradiated for 200 s, using a visible light curing unit with intensity of ca. 550 mW cm −2 (Optilux 501, Kerr, USA). The shrinkage was measured continuously and total shrinkage strain of the samples was measured 1000 s after starting the light radiation at which time the contraction has plateaued out. Numerical derivatives of the shrinkage strain data were calculated with respect to the time to calculate the shrinkage strain rates .
Flexural strength : The flexural properties of specimens were determined according to the 3 point bending method suggested in ISO 4049. Adhesives with 3 mol% BD and different APOSS contents (0, 10, 20, 30, 40, 50, 60, 80 and 100 wt.%) were inserted into 25 mm × 2 mm × 2 mm stainless steel rectangular molds and cured using the visible light curing unit. An overlapping regime was applied to irradiate the whole specimens in 3 different points (150 s for each irradiation). All specimens were then post-cured in a visible light cabinet (Traylux™, Taiwan) for 1 h. The specimens were stored in distilled water at 37 °C for 24 h prior to the test. The flexural properties of the specimens were determined using a universal testing machine (STM-20, Santam, Iran) at a constant cross-head speed of 1 mm min −1 . The flexural strength of the specimens was calculated from the fallowing equation:
σ = 3 P l 2 b h 2
where P is the maximum load (N), l is the span length (20 mm), and b and h are the width and thickness of the specimens, respectively.
Microshear bond strength : The microshear bond strength of the experimental adhesives containing 0, 10, 20 and 80 wt.% APOSS and Adper SingleBond ® (3M ESPE, USA), as control group, were measured according to the previous works and compared. The experimental adhesives were applied on the dentin and cured for 200 s and SingleBond ® was applied following the manufacturer’s instruction. The silicone tubes were filled with a resin composite (Premise, Kerr, USA) cured for 40 s. The test was carried out using the same universal testing machine at a cross-head speed 1 mm min −1 .
Water uptake and solubility
Disc-shaped specimens (1 mm thick and 10 mm diameter) were prepared in silicon rubber molds. The specimens were stored in desiccators to reach constant weight, m 0 . They were then stored in distilled water at 37 °C for 1 week to remove soluble materials and then removed from water and kept at 37 °C to reach constant weight, m 1 . The specimens were transferred to desiccators 2 h prior to weighing to reach the room temperature. The solubility in water in percent was then calculated as:
Solubility ( % ) = m 0 − m 1 m 0 × 100
The specimens were then immersed in distilled water at 37 °C and their weights measured in different time periods, m t . Water uptake in percent was calculated as:
To evaluate the degradation of the CA–APOSS adhesives due to the hydrolysis in water, disked-shaped specimens were stored in distilled water at 37 °C for 3 months and their weight loss were measured. To eliminate the effect of soluble matters, prior to aging, the specimens were stored in distilled water at 37 °C for 1 week, removed from water and kept at 37 °C to reach constant weight, m 1 . The aged specimens were then dried at 37 °C to reach constant weight, m d . The weight loss in percent was then calculated as:
Weight loss ( % ) = m 1 − m d m 1 × 100