Tensile shear strength of wood bonded with urea–formaldehyde with different amounts of microfibrillated cellulose

Urea–formaldehyde (UF) adhesive mixtures with a 5% suspension of microfibrillated cellulose (MFC) at 0.5, 1, 3, and 5 wt% loading levels based on the solid weight (62.4%) of the UF adhesive were prepared. Beech lamellas with dimensions of 5 mm×20 mm×150 mm were prepared from beech lumbers using a planer saw. The UF adhesive (E0 class) was mixed with the MFC using a magnetic stirrer to achieve a proper distribution of the MFC in the UF adhesive. The tensile shear strength of single lap-joint specimens bonded with UF adhesive containing MFC was determined in accordance with EN 205 (2003). The specimens bonded with UF adhesive containing the MFC showed better tensile shear strengths as compared to the control. As compared to the control specimens, the tensile shear strength of the specimens increased by 5.7% as 3 wt% of the MFC was incorporated into the UF adhesive. However, a further increment in the MFC content up to 5 wt% decreased the tensile shear strength of the specimens (−14.3% of control specimen). The MFCs were well dispersed in the UF resin and were cross-linked to form a network to reinforce the bond line, improving bonding performance.     

Effect of steam on bonding strength and dimensional stability of laminated veneer lumbers manufactured using different adhesives

Laminated veneer lumbers (LVLs) manufactured from wood with different adhesives are being increasingly used in the construction of furniture frames and buildings. Yet there is little information available concerning the dimensional stability and shear strength of LVLs after being exposed to steam. In this study, LVLs were manufactured from poplar and beech veneers with phenol/formaldehyde (PF), poly(vinyl acetate) (PVAc), Desmodur-VTKA (D-VTKA) and urea/formaldehyde (UF) adhesives. Dimensional stability of LVLs was measured after being exposed to steam for 2, 6, 12, 24, 48 and 96 h, according to the Turkish Standard (TS) 3639, and also shear strength was measured (according to BS EN 205). The highest initial density of 0.93 g/cm³ was for beech LVL with VTKA adhesive. After exposure to steam for 96 h, the highest weight increase of 65.7% was for poplar LVL with PVAc, the highest radial swelling of 5.7% was for beech LVL with UF, the highest tangential swelling of 7.9% was for beech LVL with PF and the highest longitudinal swelling of 0.5% was for beech LVL with VTKA. The highest shear strength value of 15.8 N/mm² among all samples without exposure to steam was obtained for beech LVL with PVAc adhesive and the lowest shear strength was obtained as 4.48 N/mm² for poplar LVL with UF adhesive.     

Thermoplastic polymers as modifiers for urea–formaldehyde wood adhesives. III. In situ thermoplastic‐modified wood composites

Acrylic monomers and free‐radical initiators were dispersed in an aqueous urea–formaldehyde (UF) suspension and polymerized in situ to afford a suspension containing 5 wt % thermoplastic (5 g of thermoplastic/100 mL of suspension). The viscosity of the thermoplastic‐modified UF suspension (65 wt % solids at 25°C) ranged from 240 to 437 cP versus 121 cP for the unmodified UF control. Wood‐flour composites (sugar maple and 50 wt % adhesive) were prepared with thermoplastic‐modified UF suspensions and cured with the same cycle used for the composites prepared with the unmodified UF adhesive (control). The effect of the thermoplastic‐modified UF adhesive was evaluated on the notched Izod impact strength and equilibrium moisture uptake of the wood‐flour composites. The notched Izod impact strength of the composites prepared with modified UF adhesives increased by as much as 94% above that of the control. The increase depended on the initiator and the monomer composition. The modification affected the equilibrium moisture uptake and rate of moisture uptake in the wood‐flour composites. Preliminary results for particleboard prepared with 10 wt % modified UF adhesive (5% thermoplastic in the UF resin) and unoptimized cure conditions confirmed a significant effect of the thermoplastic modification on both the internal‐bond strength and thickness swelling of the particleboard. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Which is effective on bond strength of resin-based sealers: incorporation of powdered dentin to primer or adhesive?

The aim of this study was to investigate the effect of adding powdered dentin to primer or adhesive in a self-etch system on the shear bond strength of three resin-based sealers. Seventy-two premolars were sectioned buccolingually, and 144 root halves were divided into three groups according to the sealer used: epoxy resin-based sealer (AH Plus), methacrylate resin-based (RealSeal, Hybrid Root SEAL) n = 48. The surfaces were irrigated with 5% NaOCl, 17% EDTA, distilled water for 5 min. Four subgroups were created (n = 12): control group; Clearfil Liner Bond 2 V treated group; powdered dentin added to the primer of Clearfil Liner Bond 2 V (40 wt.%); and powdered dentin added to the adhesive of Clearfil Liner Bond 2 V (20 wt.%). Dentin powder was prepared. Three mm high buildups with a constant surface area of 3.45 mm² were created using the sealers and allowed to set (37 ºC, 100% humid, 72 h). The samples were tested to failure for shear bond strength (1 mm/min). The data were calculated (MPa) and analyzed using two-way ANOVA, one-way ANOVA, and Tukey HSD tests. Adhesive use decreased the bonding performance of AH Plus (p = 0.00). Mean bond strength of the other sealers was found similar to control. Primer or adhesive resin with powdered dentin did not increase the adhesive performance of the self-etch system used. The shear bond strength of RealSeal was significantly increased when powdered dentin was added to primer or adhesive (p = 0.00). The effect of adding powdered-dentine to primer or adhesive in a self-etch system on the shear bond strength was sealer-dependent .     

Effects of adding nano-clay (montmorillonite) on performance of polyvinyl acetate (PVAc) and urea-formaldehyde (UF) adhesives in Carapa guianensis,a tropical species

The aim of this study was to improve the bond strength resistance of polyvinyl acetate (PVAc) and urea-formaldehyde (UF) adhesives modified with nano-clay (montmorillonite) with a tropical species of wood known to exhibit adhesion related problems. These adhesives were evaluated with 1.0 and 1.5 wt% nano-clay concentrations with lap shear strength (SS), and the percentage of wood failure (PWF) in dry and wet conditions being evaluated. An additional aim of this study was to observe the presence of nano-clay within both adhesive types using Atomic Force Microscopy (AFM) and the Transmission Electron Microscopy (TEM). Color, viscosity and the thermostability of these adhesives with nano-clay were also evaluated. First, AFM and TEM studies showed adequate dispersion and impregnation of nano-clay. The viscosity of PVAc adhesive was not affected by the incorporation of nano-clay, whereas the UF adhesive was. With both PVAc and UF adhesives, the presence of nano-clay increased the L^⁎ and b^⁎ color parameters, especially when 1.5 wt% nano-clay was used. The incorporation of the nano-clay improved thermostability, as determined by thermogravimetric analysis (TGA). Finally, it was shown that the nano-clay incorporation improved SS and PWF. The highest values of SS were obtained when nano-clay was added at 1.5 wt% concentration in the PVAc adhesive under dry conditions. SS was not affected by nano-clay addition in the UF adhesive under dry conditions. However, under wet conditions, both 1.0 and 1.5 wt% loadings of nano-clay increased SS with both adhesive types. The addition of nano-clay in both proportions increased PWF by approximately 15% and between 20–30% in dry and wet conditions, respectively, for the PVAc adhesive. For the UF adhesive, PWF increased by approximately 10% under dry conditions and 25–50% in wet conditions.     

Evaluation of properties of starch-based adhesives and particleboard manufactured from them

The objective of this study was to evaluate some of the mechanical and physical properties of experimental particleboard panels manufactured from rubberwood (Hevea brasiliensis) bonded using oil palm starch, wheat starch, and urea formaldehyde (UF) at a density of 0.60 g/cm³. Bending characteristics, internal bond strength, thickness swelling, and water absorptions of the samples were determined based on Japanese Industrial Standard (JIS). Overall mechanical properties for natural binder oil palm starch resulted in higher values than those made from wheat starch. The highest internal-bonding strength (IB) value of 0.41 N/mm² was determined for the samples made from oil palm starch. Dimensional stability in the form of thickness swelling of the samples made from oil palm starch had higher values, ranging from 4.24 to 22.84% than those manufactured from wheat starch. Natural adhesive showed comparable strength with panels manufactured with UF. Overall results meet the Japanese Industrial Standard (JIS) requirements except for water absorption and thickness swelling of the samples.     

Influence of adding multiwalled carbon nanotubes on the adhesive strength of composite epoxy/sol–gel materials

The tensile shear strength of a composite epoxy/sol–gel system modified with different ratios of multiwall carbon nanotubes (MWCNTs) was evaluated using a mechanical testing machine. The experimental results showed that the shear strength increased when lower than ~0.07 wt% of MWCNTs were added in the composite solution. The increase of the shear strength was attributed to both the mechanical load transfer from the matrix to the MWCNTs and the high specific surface area of this material that increased the degree of crosslinking with other inorganic fillers in the formulation. However, a decrease in the adhesive shear strength was observed after more than ~0.07 wt% MWCNTs were added to the composite. The reason for this may be related to the high concentration of MWCNTs within the matrix leading to excessively high viscosity, dewetting of the substrate surfaces, and reduced bonding of MWCNTs with the matrix, thereby limiting the strength. SEM observation of the fracture surfaces for composite epoxy/sol–gel adhesive materials with 0.01 wt% MWCNTs showed a mixed interfacial/cohesive fracture mode. This fracture mode indicated strong links at the adhesive/substrate interface, and interaction between CNTs and the matrix was achieved; therefore, adhesion performance of the composite epoxy/sol–gel material to the substrate was improved. An increase of a strong peak related to the C–O bond at ~1733 cm^?1 in the FTIR spectra was observed. This peak represented crosslinking between the CNT surface and the organosilica nanoparticles in the MWCNTs-doped composite adhesive. Raman spectroscopy was also used to identify MWCNTs within the adhesive material. The Raman spectra exhibit peaks at ~1275 cm^?1 and in the range of ~1549–1590 cm^?1. The former is the graphite G-band, while the latter is the diamond D-band. The D-band and G-band represent the C–C single bond and C=C double bond in carbon nanotubes, respectively.     

Adhesive joints for structural timber/glass applications: Experimental testing and evaluation methods

A study of three different adhesives, silicone, acrylate and polyurethane, intended for adhesive joints in structural timber/glass applications is presented in this paper. Intentionally, adhesives with a wide range of properties were chosen. The adhesive bonds between timber and glass were tested both in tension and in shear with a bond area of 800 mm². Special fixtures were designed both for gluing and testing the specimens. The results include strength and failure type of the adhesive bond as well as deformation of the bond lines, measured with LVDTs and a non-contact optical 3D-deformation measuring system used in combination with finite element modelling in order to obtain detailed information about the behaviour.Of the tested adhesives, the acrylate (SikaFast 5215) provided the largest strength, both in tension and shear. The mean strength obtained for this adhesive bond was 3.0 MPa in tension and 4.5 MPa in shear.Further, it is demonstrated how rotations in the specimen during the test can be detected with the optical measuring system and how finite element modelling can be used to study the stress distribution internally in the adhesive bond. One conclusion obtained from the combination of results from the optical measuring system and finite element modelling is that the behaviour of the silicone adhesive is highly influenced by its near incompressible behaviour.     

The shear strength of Calabrian pine (Pinus brutia Ten.) bonded with polyurethane and polyvinyl acetate adhesives

In this study, the change in shear strength on radial and tangential surfaces of Calabrian Pine (Pinus brutia Ten.) wood having different roughness values as the result of sawing with a circular ripsaw, planing and sanding, and bonded with polyurethane (PU) and polyvinyl acetate (PVAc) adhesives at the pressure levels of 3, 6, and 9 kgf/cm², was studied. Each of the 360 specimens prepared to determine the effect of the variables on bond performance were subjected to shear test in an universal test machine in accordance with the ASTM D 905–98 standards. The values obtained were analyzed statistically and the results were interpreted. The highest shear strength (11.83 N/mm²) for plane of cut was obtained on the tangential surface after sanding and applying PVAc adhesive with a pressing pressure of 9 kgf/cm². The lowest shear strength (6.01 N/mm²) was obtained in the joinings made on the planed surfaces by using PU adhesive and a pressing pressure of 3 kgf/cm². The highest shear strength (9.10 N/mm²) on the radial surface was obtained after sanding and applying PVAc adhesive and pressing with a pressure of 6 kgf/cm². The lowest shear strength (3.76 N/mm²) was obtained in the specimens whose surfaces were sanded and glued with PU adhesive with a pressing pressure of 3 kgf/cm². In general, in the radial surfaces, just like in the tangential surfaces, the specimens bonded with PVAc exhibited a higher shear strength compared with those glued with PU. According to these results, it is definitely necessary to sand the surfaces prior to the bonding process to have a higher shear strength. The bonding process should be made on the tangential surfaces with higher pressures. The PVAc adhesive should be preferred instead of the PU adhesive. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3050–3061, 2006     

Adhesive strength of steel–epoxy composite joints bonded with structural acrylic adhesives filled with silica nanoparticles

This paper reports a study on the effect of silica nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nano-silica in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with addition of the filler content up to 1.5 wt%, after which decreased with addition of more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. Differential scanning calorimeter analysis revealed that T_g values of the adhesives rose with increasing the nanofiller content. The equilibrium water contact angle was decreased for adhesives containing nanoparticles. Scanning electron microscope micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.     

Effect of active rubber powder on structural two-component epoxy resin and its mechanical properties

The aim of the research was to investigate a possibility to use active rubber powder (ARP) from a process of tyres recyclation in an area of a filler into a reactoplastics matrix applied in structural adhesive bonds. This study focuses on an analysis of a tensile strength, a shear impact strength and a hardness of the composite mixture itself and further on the interaction with an adhesive bonded material, i.e. it evaluates an influence of different ratio of ARP on the adhesive bond strength. Effects of the cyclic degradation environment combining the decreased temperature ?40 °C and the increased temperature 90 °C at the simultaneous acting of the increased moisture up 90% is a part of the research. The tensile strength and the hardness were decreased by adding ARP. The experiment results proved a positive effect of ARP in the area of adhesive bonds exposed to the cyclic degradation at increased and decreased temperatures. Elastic ARP is able to absorb an inner tension in the layer of the adhesive bond.     

Steel-Epoxy Composite Joints Bonded with Nano-TiO2 Reinforced Structural Acrylic Adhesive

This article reports a study on the effect of TiO₂ nanoparticles on the adhesion strength of steel–glass/epoxy composite joints bonded with two-part structural acrylic adhesives. The introduction of nano-TiO₂ in the two-part acrylic adhesive led to a remarkable enhancement in the shear and tensile strength of the composite joints. The shear and tensile strengths of the adhesive joints increased with adding the filler content up to 3 wt.%, after which it decreased with adding more filler content. Also, addition of nanoparticles caused a reduction in the peel strength of the joints. Differential scanning calorimeter analysis revealed that glass transition temperature (T_g) values of the adhesives rose with increasing the nano-filler content. The equilibrium water contact angle decreased for adhesives containing nanoparticles. Scanning electron microscope micrographs revealed that addition of nanoparticles altered the fracture morphology from smooth to rough fracture surfaces.     

Shear strength of calabrian pine (Pinus brutia Ten.) bonded with polyurethane and polyvinyl acetate adhesives

In this study, the change in shear strength on radial and tangential surfaces of Calabrian pine (Pinus brutia Ten.) wood having different roughness values as the result of sawing with a circular ripsaw, planning, and sanding, and bonded with polyurethane (PU) and polyvinyl acetate (PVAc) adhesives at the pressure levels of 3, 6, and 9 kg f/cm² were studied. A total of 360 specimens of each, prepared with the objective of being able to determine the effect of the variables on bond performance, were subjected to the shear test in the universal test machine in accordance with the ASTM D 905–98 standard. The values obtained were analyzed statistically and the results were interpreted. The highest shear strength (11.83 N/mm²) for plane of cut was obtained on the tangential surface after sanding and applying PVAc adhesive with a pressing pressure of 9 kg f/cm². The lowest shear strength (6.01 N/mm²) was obtained in the joinings made on the planed surfaces by using PU adhesive and a pressing pressure of 3 kg f/cm². The highest shear strength (9.10 N/mm²) on the radial surface was obtained after sanding and applying PVAc adhesive and pressing with a pressure of 6 kg f/cm². The lowest shear strength (3.76 N/mm²) was obtained in the specimens whose surfaces were sanded and by using PU adhesive and with a pressing pressure of 3 kg f/cm². In general, in the radial surfaces, just like in the tangential surfaces, the specimens bonded with PVAc produced a higher shear strength compared to those glued with PU. According to these results, it is definitely necessary to sand the surfaces prior to the bonding process to have a higher shear strength. The bonding process should be made on the tangential surfaces with higher pressures. The PVAc adhesive should be preferred instead of the PU adhesive. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4856–4867, 2006     

Glass/epoxy composites and aluminium alloy joint using Kevlar® intermediate layer and nanoclay-reinforced epoxy adhesive

Adhesive lap joint between glass fibre/epoxy composites and aluminium alloy (2014 T4) was prepared by an in situ moulding process using a matched die mould. The surface of aluminium alloy was treated with chromic acid before adhesive bonding. Lap shear strength and fatigue life were evaluated in tensile mode and tension–compression mode (at 40% of lap shear load of adhesive joint), respectively. Knurling on the surface of aluminium alloy improved the lap shear strength of the adhesive joint but did not influence the fatigue life of the same. Lap shear strength and fatigue life of adhesive joint made with neat epoxy adhesive and reinforcement of an intermediate layer of Kevlar^® between glass/epoxy composite and aluminium alloy were observed to be 0.44?kg/mm² and 3.6?×?10⁵ cycles, respectively. In another case, lap shear strength and fatigue life of similar type of adhesive joint made from nanoclay (Cloisite 30B)-reinforced epoxy adhesive and without reinforcement of an intermediate layer of Kevlar^® were observed to be 0.38?kg/mm² and 2.3?×?10⁵ cycles, respectively. Whereas, lap shear strength and fatigue life of adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® were 0.48?kg/mm² and 3.9?×?10⁵ cycles, respectively. Therefore, adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® was the best.     

Properties and thermal analysis study of modified polyvinyl acetate (PVA) adhesive

Polyvinyl acetate (PVA) adhesive is one of the most common types of adhesives has been used in the wood industry for decades. However, many drawbacks are still associated with this adhesive including low water resistance, poor bond strength, and low viscosity. In this reported study, two additives, sulfanilamide and N,N-dimethylethylenediamine, were used to modify a PVA adhesive to improve its comprehensive practical performance. The prepared adhesive samples were characterized by Fourier transform infrared spectroscopy (FT-IR). Furthermore, the thermal decomposition characteristics of the PVA adhesives were studied using Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry analysis (DSC) combined with the Kissinger method. The experimental results showed that when compared to the pure PVA adhesive, the solid content, viscosity, dry bond strength, and wet bond strength of the modified PVA adhesive (PVA + N,N-dimethylethylenediamine + sulfanilamide) were improved by 34.8, 41.4, 47.0 and 35.2%, respectively. FT-IR analysis indicate that these two additives altered the chemical bond ratio that resulted from the generation of new chemical bonds, which explained the improved performance of the modified PVA adhesive. The pure PVA adhesive possessed two thermal decomposition steps, while the modified PVA adhesive (PVA + N,N-dimethylethylenediamine + sulfanilamide) exhibited only one thermal decomposition step. The thermal decomposition process of the pure PVA adhesive is characterized by a quick thermal decomposition stage and a slow thermal decomposition stage. Since the ΔH > 0, ΔS < 0 and ΔG > 0 in the thermal decomposition process it can be concluded that the decomposition reactions of the PVA adhesive were non-spontaneously endothermic and the entropy decreased during the reaction.     

Bond Strength and Interfacial Ultramorphology of Current Adhesive Systems

This study evaluated the bond strength and ultramorphology of the resin-dentin interfaces produced by current dental adhesive systems. Nine dentin bonding agents were investigated. Restored teeth were vertically, serially sectioned to obtain bonded slices for interfacial TEM analysis or to produce bonded beams for the microtensile bond strength test. The one-step self-etching adhesives (Futurabond® NR and Hybrid Bond®) showed lower bond strength values than the three-step etch-&-rinse adhesive system All-Bond 3. Most bonding agents presented statistically similar mean bond strength values, which ranged from 41.3 ± 17.9 to 35.0 ± 5.3 MPa. The thickness of the hybrid layer varied according to the type of adhesive system used. While the etch and rinse adhesives with alcohol as organic solvent showed bond strength means higher than 40 MPa, the self-etching systems showed bond strength lower than 40 MPa. Resin-dentin interdifusion zone and resin tags were noted in all bonded interfaces.     

Improvement of the Molar Tube Bond Durability with Preheated Orthodontic Adhesive

This study aimed to evaluate the effect of preheated orthodontic adhesives and thermal cycling on the bond strength of molar tubes. One hundred sixty molar tubes were bonded to acid-etched bovine incisors using a conventional orthodontic adhesive (Transbond XT), two microhybrid (Wave and Permaflo), and a nanofilled (Filtek Z350) flowable composite resins, at room temperature or preheated at 60°C. Transbond XT primer and Single Bond 2 adhesive system were used in association with Transbond XT and the flowable composites, respectively. The specimens were stored in water (37°C) for 24 h, and half of the sample was subjected to thermal cycling for 6000 cycles. Ashear bond strength (SBS) test was performed, followed by the appraisal of the adhesive Remnant Index (ARI). Three-way analysis of variance (ANOVA) and the Tukey test were performed at a significance level of 95% (P < .05). Samples bonded with preheated adhesives showed higher SBS regardless of the aging method. Only samples bonded with preheated conventional orthodontic adhesive maintained their bond strength after thermal cycling for 6000 cycles. Preheating orthodontic adhesives improved the bond strength of molar tubes, but only the preheated conventional orthodontic adhesive was capable of maintaining bond strength after thermal cycling.     

Wheat gluten fractions as wood adhesives—glutenins versus gliadins

Plant proteins, such as wheat gluten, constitute attractive raw materials for sustainable wood adhesives. In this study, alkaline water dispersions of the protein classes of wheat gluten, glutenin, and gliadin were used as adhesives to bond together wood substrates of beech. The aim of the study is to measure the tensile shear strength of the wood substrates to compare the adhesive performance of glutenin and gliadin and to investigate the influence of application method and penetration of the dispersions into the wood material. A sodium hydroxide solution (0.1M) was used as dispersing and denaturing agent. Dispersions with different protein concentrations and viscosities were used, employing wheat gluten dispersions as references. Two different application methods, a press temperature of 110°C and a press time of 15 min, were employed. The tensile shear strength and water resistance of the wood substrates were compared, using a slightly modified version of the European Standard EN 204. The bond lines of the substrates were examined by optical microscopy to study the penetration and bond‐line thickness. The results reveal that the adhesive properties of gliadin are inferior to that of both glutenin and wheat gluten, especially in terms of water resistance. However, the tensile shear strength and the water resistance of gliadin are significantly improved when over‐penetration of the protein into the wood material is avoided, rendering the adhesive performance of gliadin equal to that of glutenin and wheat gluten. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Shear Strength of SMR—Based Pressure-Sensitive Adhesives

The shear strength of Standard Malaysian Rubber (SMR)-based pressure-sensitive adhesives was studied using coumarone-indene resin as the tackifier resin. Three grades of SMR, i.e., SMR L, SMR 10, and SMR 20 were used as the elastomers. The concentration of tackifier resin was varied from 0–80 parts per hundred parts of rubber (phr). Toluene was used as the solvent throughout the experiment to prepare the pressure-sensitive adhesives. A SHEEN hand coater was used to coat the adhesive on the polyethylene terephthalate substrate to give a coating thickness of 30, 60, 90, and 120 µm. Shear strength of the adhesive was determined by using a Texture Analyzer. Results indicate that for a fixed coating thickness, shear strength decreases gradually with increasing resin content for all the rubbers studied. This observation is attributed to the decreasing cohesive strength of adhesive as resin loading is increased. However, for fixed resin content, shear strength increases with increasing coating thickness suggesting that shear strength is thickness-dependent. SMR L consistently shows higher shear strength than SMR 10 and SMR 20 for all coating thickness, an observation, which is attributed to higher purity of SMR L, compared with the latter two rubbers. The shear strength passes through a maximum at 5 min of mastication time, after which it decreases gradually with further mastication.     

The bonding effect of adhesive systems and bulk-fill composites to sound and caries-affected dentine

Purpose: To investigate the influence of adhesive type and increment thickness on shear bond strength of flowable bulk-fill resin composites compared with a flowable conventional resin composite, in both sound and caries-affected dentine.

Methods: Shear bond strength was tested in 100 extracted human molars with coronal dentine caries. Half of the teeth were restored with Adper? Easy Bond and the other half with Adper? Single Bond Plus. Bonded surfaces were restored with Filtek? Ultimate Flow and two bulk-fill composites (SDR and X-tra base) (n = 10 teeth for each subgroup). Restorations of 4 mm were performed with incremental or bulk-fill technique. The shear bond was determined and statistically analysed using three-way ANOVA and Bonferroni multiple comparison post hoc tests (p ≤ 0.05). Dentine–resin interfaces were evaluated by scanning electron microscopy.

Results: For both adhesives, the highest shear bond strength values were obtained for sound dentine. In sound dentine, the highest bond strength observed with Easy Bond when used in 2-mm increments of SDR. In caries-affected dentine, Single Bond in combination with SDR displayed significantly lower bond strength.

Conclusion: There was no significant difference in shear bond strength between the incremental and bulk-fill groups for molars with sound dentine when the etch-and-rinse adhesive system was used; however, for the self-etch adhesive system, incremental application caused higher bond strength than bulk application.