One-component polyurethanes (1-c-PUR) are commonly used adhesives for the manufacture of cross-laminated timber (CLT). Typically, these adhesives do not govern the mechanical response of CLT under normal service temperatures. However, when subjected to heating from a fire, failures may transition from within the timber to the bond line interphase zones. CLT bonded with 1-c-PUR has been shown to be prone to heat induced delamination (HID), which may compromise its structural performance at elevated temperatures. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic thermo-mechanical analysis (DTMA) were used to study the thermal and thermo-mechanical responses of engineered wood products and components (i.e. adhesives and timber) and their interphase (i.e. bond line). Two commercially available 1-c-PUR adhesive films from the same manufacturer, two timber species (Norway Spruce and Radiata Pine, as sawdust or as veneers), and four different veneer shear lap combinations, were studied. Shear lap specimens bonded with a conventional 1-c-PUR adhesive consistently experienced bond line mechanical failure in DTMA at about 220-240 °C. The same adhesive films tested via DSC and TGA softened at 240-260 °C. Conversely, a different 1-c-PUR adhesive, formulated specifically for enhanced performance at elevated temperature, displayed no detectable softening in DSC and TGA, and shear lap specimens in DTMA consistently failed within the timber; even at elevated temperatures. The presented thermo-mechanical methods allow identification of failure modes and temperatures, whilst the thermal microscale methods (i.e. TGA, DSC, DTMA) assist in understanding the various factors contributing to failures.

In this work, a dual resin application system using commercial phenol formaldehyde (PF) resins with different molecular weight (MW) was investigated to improve bonding performance of bamboo and wood composite laminates. Water droplet contact angle was deemed to be unreliable for assessing resin wettability on bamboo due to its unique tissue structure compared with wood. Microscopic observation of the resin penetration showed high MW PF largely remained in the glueline and only entered the lumens of cut or damaged bamboo cells near the bondline. Low MW PF appeared in cell corners of bamboo parenchyma but not lumens. Applying low MW PF to the bamboo and high MW PF to the wood surface separately significantly improved bond shear strength with reduced difference between dry and wet conditions. The dry and wet bond strengths using the new method were enhanced by 36.5% and 97.4%, respectively, compared to high MW PF alone. The results suggest that low MW PF can permeate bamboo cell walls and fortify them against swelling and stress on the bamboo-resin interface in wet conditions. Further modifications are required to produce a stronger adhesive than the bamboo tissue to improve wet shear fiber failure rates and develop a viable structural bond qualification test for bamboo and bamboo-wood composites.

Carbon fiber reinforced polymer (CFRP) is an engineering composite material with excellent performance. After metallization, the CFRP composite exhibits unique properties such as electromagnetic shielding and high electrical conductivity. In this study, we utilized laser treatment process to enhance the adhesion strength of the copper layer electrolessed on CFRP composite. The surface microstructure of CFRP and the copper layer was determined using an optical microscope, and the adhesion force of the copper layer on CFRP composite was measured through 3M tape and pull-out tests. The results indicate that with an increase in the number of laser treatment cycles, the trenches depth on the surface of CFRP composite also increases, leading to high surface roughness and thus enhancing the adhesion strength between the copper layer and the composite. The adhesion state of the copper layer on laser-treated CFRP composite can be qualitatively classified as grade 5B. Additionally, both mechanical cutting and laser treatment can improve the adhesion strength of the samples. The samples treated by mechanical cutting and the laser scan with ±45° exhibit the highest adhesion strength of 5.48 MPa. This is 415 % higher than that of the untreated sample, with a minimum damage area after pull-out testing, approximately 10 %. Compared to the sandblasting pretreatment process, the adhesion strength of the sample by laser treatment increased by 119 %.

Soybean-based adhesive has been commercialized in the wood industry owing to its several advantages, such as no formaldehyde, good bonding properties, and renewability. However, the large-scale application of soybean-based adhesive is greatly limited by its significantly higher cost than urea-formaldehyde resin. Thus, in this study, a cost-effective polyamidoamine-epichlorohydrin (PAE) resin with improved crosslinking efficiency to soybean meal was prepared from branched polyamidoamine (PAA) with reduced dehydration time at high temperatures. Test results from GPC, FTIR and NMR analyses showed that traditional PAA synthesis involving 3-h dehydration time at 180 °C resulted in over-branching of PAA resin and poor crosslinking efficiency of PAE resin and decreased water resistance of obtained soybean-based adhesive. PAA resin synthesized at appropriate dehydration time (1 h) remained sufficient secondary amine groups for being grafted by epichlorohydrin to form effective PAE resin with more azetidinium groups. As a result, the water resistance of soybean-based adhesive crosslinked by the optimal PAE-1 resin prepared from 1 h-dehydrated PAA-1 significantly improved by 42.9 % compared with that of soybean-based adhesive crosslinked by traditional PAE resin, attributing to forming denser and stronger crosslinking networks after thermally cured. Consequently, this optimal PAE-1 could reduce the cost of soybean-based adhesive due to decreasing 16.7 wt% of PAE dosage without compromising bonding property and the energy consumption of PAA/PAE synthesis. Therefore, an appropriate decrease in the dehydration time of PAA resin at a high temperature provides an effective, economical, and energy-saving strategy to improve the bonding property of soybean-based adhesives.

As the medium between the reinforcement and the matrix, the interface plays a critical role in the mechanical properties of silicon carbide particle reinforced aluminum matrix composites. This study used first-principles calculation methods to systematically analyze 14 different 6H-SiC/Al low-index interfaces, including atomic configuration, interface bonding strength, and electronic structure and bonding principles between interfaces. The adhesion work calculations reveal that the C-top-SiC(0001)/Al (111) and SiC(0001)/Al (100) interfaces have larger adhesion work values, specifically 5.09 J/m2 and 5.021 J/m2, respectively. Additionally, the rigid tensile testing confirms that the tensile stress values at the interfaces of C-top-SiC(0001)/Al (111) and SiC(0001)/Al (100) are higher, measuring 36.4 GPa and 32.5 GPa, respectively. The above results show that the interface bonding strength of these two configurations is the highest, the most stable, and most likely to appear in the 6H-SiC/Al interface configuration. The results of the analysis on charge density difference and partial density of states indicate that the interfaces of C-top-SiC (0001)/Al (111) and SiC (0001)/Al (100) are primarily composed of strong ionic and covalent bonds.

ObjectiveTo evaluate the shelf-life effect of a silane-containing universal adhesive (UA) on its shear bond strength (SBS) to lithium disilicate glass-ceramic (LiSi2). MethodsSBS to mirror-polished (‘MP’) and hydrofluoric acid-etched ground (‘HF’) lithium disilicate glass-ceramic (Initial LiSi, GC) with/without prior separate ceramic primer application (G-Multi Primer, GC: ‘G-MP’) was measured. Scotchbond Universal Plus (‘SBUp’) (3M Oral Care) was used 33 (‘fresh’) and 6 (‘expiring’) months before the expiration date. ‘MP’ specimen preparation involved (1) ‘G-MP’ priming (or not), (2) ‘SBUp’ application, (3) composite (Clearfil AP-X, Kuraray Noritake) placement, and (4) 20 s light-curing (SmartLite Pro, Dentsply Sirona). ‘HF’ specimen preparation involved (1) HF (Porcelain Etch, Ultradent) etching, (2) phosphoric acid (K-Etchant, Kuraray Noritake) post-etching, with (3) and (4) being the same as for ‘MP’. Upon light-curing, all specimens were stored in 37 °C water for either 1 week (‘immediate’) or 3 months (‘aged’) prior to SBS measurement. Statistics involved Linear Mixed-Effects modelling (α = 0.05). ResultsSBS was not significantly affected by SBUp's shelf life, except for a significantly higher SBS recorded for ‘fresh’ SBUp applied on HF-etched and separately silane-primed LiSi2. Aging did not significantly decrease SBS. In fact, a significant increase in SBS upon aging was recorded for HF-etched LiSi2 that was not separately silane-primed. The significantly highest bond strength was measured when LiSi2 was HF-etched followed by separate silane-priming. SignificanceDespite the UA investigated has silane in its formulation, the most effective and durable bonding to LiSi2 was achieved by HF-etching followed by separate silane-priming, irrespective of the UA's shelf life.

ObjectiveTo evaluate microtensile bond strengths (μTBS), nanoleakage (NL), and degree of conversion (DC) of two universal adhesives, using etch-and-rinse (ER) or self-etch (SE) strategies on eroded dentin submitted to in vitro and in situ erosive challenges. MethodsDentin blocks were prepared from 120 human molars and categorized based on dentin condition (sound, in vitro eroded, and in situ eroded), adhesive system (Scotchbond Universal [SBU] and Zip bond Universal [ZIP]), and adhesive strategy (ER and SE). In the in situ erosive challenge, 20 volunteers wore acrylic resin palatal devices with dentin blocks, immersing them in cola soft drink for 90 s, six times daily for 15 days. The same erosive protocol was used in vitro, followed by rinsing and remineralization. Sound dentin blocks served as controls. Afterward, all dentin blocks were restored with composite resin and sectioned into resin-dentin bonded sticks for μTBS, NL, and DC assessments. Data were analyzed using three-way ANOVA and Tukey's test (α = 0.05). ResultsSound dentin exhibited the highest μTBS and DC values and the lowest NL values, while in situ eroded dentin showed the lowest μTBS and DC values and the highest NL values (p = 0.000001). While some differences in the μTBS values were observed between universal adhesives when evaluated on sound dentin (p = 0.0001), no significant differences between adhesives were observed when tested on in vitro and in situ eroded dentin. Regarding NL and DC, no significant differences were found between SBU and ZIP, as well as among adhesive strategies (p > 0.05). ConclusionErosion in dentin, especially under in situ conditions, presents significant challenges to the adhesion of restorative materials. The choice of an effective adhesive system is crucial, as dentin eroded in situ showed lower adhesion strength and greater nanoleakage. These results highlight the need for specific clinical strategies to improve the durability and effectiveness of restorations.

Nowadays, bonded joints have increasingly become the most used joining process in many industrial fields and even in civil engineering due to their simple geometry and structural efficiency and especially with the development of structural adhesives. Several solutions have been proposed in order to improve the mechanical strength of bonded joints by taking into consideration modifications to the adhesive edges and adherends to attenuate as much as possible the high stress concentration at the level of the adhesive. In this study, a 3D numerical model was developed in Abaqus to evaluate the influence of geometric changes in the adherends’ and adhesive edges on the mechanical strength of a single-lap joint under uniaxial tensile stress. Two modified geometric configurations of the bonded joint were proposed, taking into account on the one hand the presence of an adhesive fillet as the first modification and on the other hand, a removal of material at the level of the free edges of the adherends (adherends notching) as the second modification. The objective is to analyze the impact of these geometric modifications on the reduction of stress concentration in the adhesive and to explore how this new joint design can contribute to improve the strength of bonded joints. The results clearly show that a geometric modification at the level of the two free edges of the two substrates improves the strength of the joint and reduces the high stress concentrations in the adhesive. The joint strength is greatly improved if these modifications are optimized in relation to the overlap length and especially in relation to the thickness of the adherends and the adhesive. Adherend notching or applying an adhesive fillet resulted in a considerable reduction in peel stresses.

Adhesive bonding plays a pivotal role in timber engineering, enhancing structural integrity, sustainability, and aesthetic appeal, while also addressing environmental concerns. The assessment of strength in adhesively bonded timber joints involves cohesive strength, adhesive strength, and substrate failure, all of which are crucial considerations for designing dependable timber structures. A comprehensive investigation was carried out with the aim of improving adhesive bonding for construction by revealing the relationship between curing progress and mechanical adhesive properties. For that, dynamic DSC measurements, kinetic modelling, tensile tests to determine the cohesive and adhesive strength, as well as tests for the evaluation of stiffness and hardness were performed using a two-component polyurethane adhesive. The investigation yielded valuable insights, particularly regarding the time- and temperature-dependent development of the curing degree and the aforementioned material properties. Additionally, the correlation between the curing degree and the respective material properties could be determined, showing that cohesion and stiffness built up occurs quite similar while the build-up of adhesive strength correlates well with hardness. It was thus concluded that Shore D hardness might represent a practical indicator for monitoring the progress of curing at low temperatures, which is a suitable means of improving adhesive applications in the construction industry.