Search

To gain a deeper understanding of the bond durability in adhesively bonded wood products, a multiscale investigation of the mechanical properties of wood joints bonded with a one-component polyurethane (PU) adhesive in successive cycles of accelerated aging treatment was conducted by combining nanoindentation (NI) and conventional tensile-shear techniques. The delamination ratio, shear strength, and wood failure ratio of the wood joints, as well as the reduced elasticity modulus (E r) and hardness (H) of the PU resin, cell walls in the interphase region (CW–I), and control cell walls (CW) were evaluated. The correlations between the macro shear strength and micromechanical properties were further established. The results showed that the average delamination ratios of the wood joints were 10%–26 % after eight cycles of hydrothermal treatment. Macromechanical and most micromechanical properties substantially decreased with an increase in the number of treatment cycles, with maximum reductions of 37 %, 24 %, 60 %, 56 %, 48 %, and 57 % for the shear strength, wood failure ratio, E r of PU, H of PU, E r of CW-I, and E r of CW, respectively. The linear regression for the mechanical properties and the number of treatment cycles indicated that the E r of PU, CW-I, and CW, as well as the H of PU, showed linear downward trends (with R 2 values of 0.765–0.921) during successive aging treatments. Significant and positive linear correlations were found between the shear strength and micromechanical properties of PU, with correlation coefficients of 0.970 (P < 0.01) and 0.941 (P < 0.05) for E r and H of PU, respectively. The wood failure ratio showed significant positive correlations with the E r values of CW-I (P < 0.01) and CW (P < 0.05), whereas the proportion of wood failure in the interphase was significantly (P < 0.05) negatively correlated with the E r value of CW. The shear strength was positively correlated with the E r of the cell walls, with correlation coefficients of 0.861 and 0.868 for CW-I and CW, respectively. In summary, both the micromechanical properties of the PU resin and the elasticity of the wood cell walls in the wood-adhesive interphase were demonstrated to be key factors affecting the bond durability of wood bond joints. [ABSTRACT FROM AUTHOR]

The effect of synergetic surface treatments on the adhesion performance and strength durability of aluminum-lithium alloy bonded joints was investigated. The synergistic modes combine the advantages of different surface treatments, rough morphologies were firstly generated and then porous oxide layer structures were produced on the rough surfaces, so more porous structures and suitable rough characteristics were generated on the substrate to enhance the bonding performance of adhesive joint. The surface characteristics, surface wettability, fracture morphology, shear strength and strength durability of samples subjected to different surface treatments were compared and analyzed. The experimental results indicated that the synergistic treatments had a positive effect on the strength performance and environmental durability of bonded joints, with the highest surface free energy of 80.6 mJ/m2 and shear strength of 40.2 MPa, and also had the best environmental durability with a strength reduction of 8.2 % during the hot-humid exposure. [ABSTRACT FROM AUTHOR]

In this work, a series of fluorinated core-shell polyacrylate latex pressure sensitive adhesives (PSAs) were successfully synthesized through monomer-starved seeded semi-continuous emulsion polymerization. Specifically, (meth)acrylate monomers were employed as the main materials, with dodecafluoroheptyl methacrylate (DFMA) were used as modified monomer. The impact of fluorine monomer DFMA on the resulting core-shell latex and PSA properties was thoroughly examined. FTIR analysis confirmed the successful incorporation of DFMA into the latex copolymer through emulsion polymerization. TEM images illustrated that the morphology of latex particles exhibit a distinct core-shell structure. The results also showed that with the addition of DFMA, the water absorption and water whitening of the latex films decreased, while the water contact angle and thermal stability are improved. Interestingly, the latex films with DFMA added to the shell phase exhibit a larger WCA value when compared to those with DFMA in the core phase, further confirmed by XPS results. Finally, the adhesive properties of the fluorinated core-shell acrylate PSA were evaluated. The results of shear strength and gel content demonstrate that the prepared PSA exhibits excellent cohesive properties. Thus, this work could present a feasible strategy for optimizing the comprehensive properties of acrylate latex PSAs. [ABSTRACT FROM AUTHOR]

Pressure-sensitive adhesives (PSAs) are a critical class of adhesives used in numerous applications. Acrylic copolymers and polyurethanes are two main categories of polymers that PSAs are made from. However, to take advantage of unique properties of both of them, polyurethane-acrylic hybrid PSAs have been proposed. To date, for synthesis of these hybrid PSAs, there should have been an acrylic comonomer containing a specific functional group to be able to react with the isocyanate group. This research was conducted to eliminate the need for such an acrylic comonomer with a specific functional group through silanization. For this purpose, firstly, a silanized polyurethane (SPU) was synthesized through silanization of a polyol. Subsequently, copolymerization of two acrylic comonomers was performed in the absence or presence of SPU. In the next step, prior to adding the acrylic comonomers to the SPU, silane was added to SPU to enable higher grafting. The PSA containing 0.04 wt % of silane showed a peel force, static shear strength, and tack of 1.6, 1.4, and 2.9 times those of the acrylic PSA on stainless steel, respectively. With this proposed silanization strategy, it is possible to design a wide variety of polyurethane-acrylic hybrid polymers. [Display omitted] • A general-purpose hybrid pressure-sensitive adhesive was prepared. • The polyurethane-acrylic hybrid pressure-sensitive adhesive was prepared through silanization strategy. • The hybrid adhesive showed a peel force and tack of 1.6 and 2.9 times of those of pure acrylic adhesive. • The proposed silanization strategy can be exploited for designing a wide variety of polyurethane-acrylic hybrid polymers. [ABSTRACT FROM AUTHOR]

Hybrid joints combining pre-tensioned bolts and adhesive bonding have been shown to have the potential to increase joint loading capacity for steel structures. In this paper, experimental and numerical research on hybrid joints has been reviewed with a focus on load transfer mechanisms. However, there is still no clear consensus on whether hybrid joints are an extension of a pre-tensioned bolted connection with an increased friction coefficient or adhesively bonded ones. This is due to the complexity of hybrid joints, which require knowledge from two different fields of engineering and can result in incomplete characterization of adhesives and documentation of manufacturing and testing large-scale samples. The first part of the review paper discusses the experimental evidence gathered on small-scale samples (Napkin and off-axis tests), although not strictly resulting from observations directly made on hybrid joints. The authors that studied the σ - τ –interaction on both tensile and compressive normal stresses provided evidence of a clear distinction to be made between tensile and compressive normal stresses. For tensile normal stresses, there is a clear lower limit below which no shear strength exists, while compressive stresses significantly increase shear strength. However, there is no clear consensus regarding the exact nature of the interaction between compressive normal stresses and shear strength. Some authors assume a linear relationship involving some cohesive strength, while others identified bi-linear or more complex relationships. Adhesives do not exhibit any noteworthy differences in the friction coefficient, and differences related to the failure mode were identified: cohesive failure is likely driven by the intrinsic strength of the adhesive, while adhesive failure can be modelled as a classic friction problem. Relating the aforesaid to the adhesive, or specific properties thereof, proved difficult, as most of the studies do not provide sufficient information. Additional complexity arises if tests are carried out on galvanized or coated surfaces, in which case the performance of the adhesive may be subordinated to that of the specific steel surface. Overall, the review paper provides valuable insights into the load transfer mechanisms of hybrid joints, but further research is necessary to fully understand their behaviour and to optimize their design for use in steel structures. [ABSTRACT FROM AUTHOR]

In this study, crosslinkable acrylic adhesives with functionalized cellulose nanocrystal (fCNCs) were prepared via in situ emulsion polymerization at 80 °C for 3h. The neat acrylic pressure sensitive adhesives (PSAs) without any nanoparticles were prepared using butyl acrylate (BA), 65 wt%, and methyl methacrylate (MMA), 35 wt%, and various amounts of 2-hydroxyethyl methacrylate (HEMA), 1, 3, 5, 7 wt% based on total BA and MMA monomers. For preparing fCNCs reinforced adhesives, the cellulose nanocrystals (CNCs) were functionalized using various concentrations of 3-methacryloxypropyl trimethoxy silane (MPS), 5, 20, 40 mM. The cellulose nanocrystals (CNCs), functionalized cellulose nanocrystals (fCNCs), and the prepared adhesives were characterized by Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The Adhesion performance of the adhesives was evaluated by lap shear and 180° peel tests. The results showed that increasing the HEMA content as crosslinking agent in the adhesive structure, enhanced its gel content, glass transition temperature (Tg), thermal stability, and adhesive performance, i.e. lap shear and peel strength. Finally, the adhesive containing 5 wt% HEMA was selected as the appropriate sample. Thereafter, the adhesives containing 5 wt% HEMA were synthesized in the presence of various amounts of fCNCs, i.e. 0.5, 1, 1.5 wt%. The Adhesion performance of the neat and reinforced adhesives indicated that increasing the fCNCs to the adhesive enhanced thermal stability, gel content, lap shear and peel strength. Additionally, the fCNCs prepared using higher concentration of MPS (40 mM) was more effective on the adhesion performance than the fCNCs prepared with the lower MPS contents, 5, 20 mM. The results showed that the adhesives prepared using 1 wt% and 1.5 wt% of fCNC40 (containing 40 mM MPS) exhibited better adhesion performance. [ABSTRACT FROM AUTHOR]

Urea formaldehyde (UF) is the most widely used resin in the wood composite board industry. UF has formaldehyde emission, which is harmful to the environment and human health. In this paper, hydroxyapatite (HAP) was surface modified by silane coupling agent (HAP-KH). And hydroxyapatite/urea formaldehyde resin (HAP-UF) was prepared by adding different amounts of HAP-KH. The optimal amount of HAP-KH was determined by wet shear strength and formaldehyde release. The degradation performance and degradation kinetics of the resin were also studied. The results show that the addition of HAP-KH can improve the wet shear strength of plywood and reduce the release of formaldehyde. The optimal content of HAP-KH was 3%, and the formaldehyde emission of plywood reached the lowest level of 0.35 mg L−1, 52.32% lower than that of the control resin. The wet shear strength was 1.14 MPa, 14.4% higher than that of the control resin. HAP-KH had no significant effect on the thermal stability of UF resins. However, HAP-KH has a significant effect on the thermal degradation mechanism of UF resins. The most likely kinetics mechanism models for UF and HAP-UF are G(a) = [−ln(1−α)]2 and G(a) = [−ln(1−α)]3, respectively. [ABSTRACT FROM AUTHOR]

The adhesive bonding of wood is preferably carried out at levels of moisture content close to those expected during service. In this study, the influence of non-ideal wood moisture content (MC) during adhesive bonding is investigated, using lap shear testing, nanoindentation and microscopy. Spruce lamellas were bonded at 5, 12, 25% MC and largely above fiber saturation point (∼55% MC) with melamine-urea-formaldehyde (MUF) and one-component polyurethane adhesive (1C PUR). Tensile shear strength decreased with increasing MC for both adhesive systems, even though the specimens were given ample time to post-cure, as testing was at standard MC 12%. Particularly above FSP, with the presence of free water, tensile shear strength was significantly reduced. For MUF, this is consistent with reduced mechanical properties measured with nanoindentation presuming that excessive moisture hindered adhesive cure. For 1C PUR, micromechanical properties of the bulk adhesive were not significantly affected by MC, the influencing factor was rather insufficient adhesion to the wood surface due to free water. The results propose the applicability of the adhesive beyond the recommended moisture ranges, but advise caution when bonding wood above FSP without process adjustments. [ABSTRACT FROM AUTHOR]

In the present work, urea-formaldehyde (UF) resin was modified by maleic anhydride-modified nanolignin. For this purpose, nanolignin was prepared by an acidic method and then different ratios of virgin lignin, nanolignin and maleated nanolignin (10, 20 and 30 wt%) were added to the UF resin during its synthesis. The synthesized resins were then used in plywood panels manufacturing. The physicochemical and thermal properties of the prepared resins were determined. Formaldehyde emission, water absorption and shear strength of the plywood panels bonded with the modified resins were also measured according to the relevant standards methods. FTIR analysis indicated that the peak corresponding to O –H was decreased by the treatment of nanolignin with maleic anhydride. The intensity of the 2800 cm−1, 1200 cm−1 and 1700 cm−1 bands related to respectively C –O, C C and COOH bonds also increased in maleic anhydride-treated nanolignin. Based on the physicochemical results obtained, the modification of nanolignin by maleic anhydride accelerates the gelation time and increases the viscosity of the lignin-urea-formaldehyde (LUF) resin compared to when virgin nanolignin was used. DSC analysis shows that the addition of maleated nanolignin significantly decreases the curing temperature of the LUF resin. Based on panel analysis results, lower formaldehyde emission and higher mechanical strength and dimensional stability were obtained by treating nanolignin with maleic anhydride. [ABSTRACT FROM AUTHOR]

Gelatin can act as wood adhesive for fabrication of numerous structures. Polyvinyl acetate (PVAc) is also used as wood adhesive because it provides excellent adhesion. Unfortunately, its bonds are sensitive to humid atmospheres especially at elevated temperatures. The current work explores hybridization of PVAc emulsion with finite amounts of gelatin and the resulting latex was fully characterized using different techniques, where it was found that gelatin insertion caused densification of the latex structure without affecting its stability while the viscosity increased without visual phase separation, indicating strong interactions emerged between both. Dynamic light scattering (DLS) and transmission electron microscope (TEM) confirmed a mean size of the latex particles increasing from 265 nm to 290 nm in presence of elevated doses of gelatin, with the surface charge decreasing from −42 mV to −33 mV. Turbidity measurements revealed the latex particles became more likely to coagulate in presence of increasing amounts of gelatin and formation of a more compact film that provides higher resistance to water permeation and stronger adhesion as supported by shear strength evaluations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) proved strong interactions between PVAc with gelatin, causing higher thermal stability and possibility of use at relatively higher temperatures. X-ray diffraction (XRD) showed broadening of the peak at 2θ = 21° indicating partial destruction of the triple helical structure of gelatin, which can be correlated with the spectra of Fourier transform infrared (FTIR) that showed obvious shifts of the main peaks of PVAc due to effective insertion of gelatin. [Display omitted] [ABSTRACT FROM AUTHOR]

Additive Manufacturing (AM) is one of the important technological pillars of Industry 4.0. Thanks to its tool-free manufacturing and extreme flexibility, AM has the potential to respond to individual customer preferences with a customized final product. Nevertheless, the size of 3D printed products is limited by the size of the build chamber of commercially available printers. In addition, deformations, stability, tolerances, and especially cost are highly dependent on the maximum size of the parts. Therefore, a larger final product can only be realized as an assembly group. Therefore, adhesive bonding appears to be an attractive solution for assembling parts made by AM. In this paper, a comparison of the influence of surface texture on the shear strength of 3D-printed metallic and polymeric adhesive joints was performed. A fish scale (FS) bio-inspired surface texture was chosen to examine how surface patterning influences the strength of joints. Fused deposition modeling (FDM) and selective laser melting (SLM) techniques were used to produce polycarbonate (PC) and stainless steel (17-4 PH) specimens, respectively. The effect of bio-surface and material type on the shear strength of adhesive joints was assessed using the ASTM D4501 standardized block shear testing method. The morphological analysis revealed that FDM exhibits reduced resolution and accuracy, making it less suitable for the additive manufacturing of complex surface features in 3D-printed parts when compared to SLM technology. In addition, bio-surface texturing significantly improved the wettability of stainless steel, requiring the synergy of higher surface free energy and increased surface roughness (FS bio-surface) for super-hydrophilicity, while a combination with lower surface free energy (PC material) and the same surface roughness resulted in hydrophobic behavior. On the other hand, the bio-surface texture greatly enhanced the shear strength (+31%) of stainless steel specimens, compared to PC specimens, due to the increase of physical adsorption forces and the enhancement of the mechanical interlocking effect. The analysis of variance (ANOVA) results highlight the predominant influence of material type on surface wettability (84%), while surface texture has a more pronounced impact on the shear strength of bonded joints (71%). [ABSTRACT FROM AUTHOR]

This study reveals the micro-structure of the naturally oxidized shot-blasting steel substrate surface and uses a simple resin pre-coating (RPC) approach to fully wet the substrate surface for a stronger adhesive bonding effect. RPC solutions are made of around 90% acetone and 10% resin without hardener, which can be sprayed or brushed onto the steel substrate surface. To demonstrate the wetting effect of RPC treatment on the oxide layer, a single lap shear (SLS) test is conducted to measure the shear strength of grit-blasting and naturally oxidized shot-blasting steel substrates under six different concentrations of RPC solutions. Results showed that after being treated with a 5 wt% RPC solution, the average shear strength of grit-blasting and naturally oxidized shot-blasting specimens increased by 27% and 20% respectively. Meanwhile, it has been observed that the average shear strength of naturally oxidized shot-blasting specimens treated with a 5 wt% RPC solution (15.53 MPa) is slightly higher than that of grit-blasting specimens without RPC treatment (15.45 MPa). This finding indicated that RPC treatment could effectively wet the oxide layer on the surface of shot-blasting steel substrates, removing the need of grit-blasting for steel substrates. Consequently, RPC treatment can simplify on-site processes and reduce environmental pollution. • The 5 wt% RPC solution greatly improves the wettability of oxidized shot-blasting steel substrates. • The RPC solution can penetrate the micro-cracks present in the oxide layer on the shot-blasting steel substrates. • RPC method can be used as an alternative to grit-blasting during the engineering bonding process. [ABSTRACT FROM AUTHOR]

Industrial flour served as a classical filler in wood industry for many years. However, its high-cost and competition for use as food stuff paved the way to a novel substitution tactic by using agriculture and forestry waste residues. Camellia oleifera shell and palm kernel meal are both by-products of oil-refining with advantages such as wide availability, great-reserve, renewability, and low-cost. Herein, a compound filler strategy was proposed by combining Camellia oleifera shell powder (Cosp) and palm kernel meal (PKM) to solve some drawbacks while Cosp as a single filler. The results showed that the bonding performance of plywood incorporating 20% single Cosp modified MUF was inferior with respect to MUF modified by same amount of industrial flour, due to the significantly increased viscosity, leading to a decrease in wet shear strength (1.63 MPa and 1.47 MPa for cold and hot-water soaking pretreatment, respectively). Encouraged, the overall status (including the viscosity, gel time, and solid content) of MUF modified by the compound filler displayed is regarded the best formulation for Coaps to PKM weight ratio of 5:5. Therefore, the wet shear strength for derived plywood samples with the adhesive incorporating 20% filler content, can reach 1.90 MPa and 1.88 MPa for cold and hot-water soaking pre-treatment, respectively. Interestingly, this bonding performance is similar to MUF resin modified by industrial flour. Thermogravimetric analysis (TGA), fourier transform infra-red spectroscopy (FT-IR), and scanning electron microscopy (SEM) were utilized to further confirm the potency of the compound filler modification strategy. Furthermore, the formaldehyde emission of MUF resin modified by the compound filler was effectively reduced by 8.24–14.12%, compared to industrial flour as filler. Cosp and PKM, as agricultural and forestry waste residue, exhibited the cost advantage for this approach based on calculation, which can save the cost of adhesive by over 40% for filler. The strategy for employing this compound filler provides a sustainable and environmentally friendly approach for green application of waste residues obtained from agriculture and forestry. • A fully bio-based compound filler consisted by Cosp and PKM was prepared. • The Cosp and PKM modified MUF maintained the outstanding bonding performance. • The formaldehyde release of MUF with Cosp and PKM filler was reduced by 8.24–14.12%. • The cost of filler utilized in MUF resin would go down by 40%. [ABSTRACT FROM AUTHOR]

The formaldehyde-based adhesive is commonly used in wood industry; however, the emission of formaldehyde is a fatal defect, which endangers people's health as well as pollutes the environment. Herein, the non-toxic and low-volatility glyoxal (G) is chosen to react with dimethylolurea (DMU) to prepare a novel and excellent water-resistant wood adhesive of glyoxal-dimethylolurea (G-DMU) resin. Quantum chemical calculations, structure characterizations, and performance measurements are carried out to study the reaction mechanism, and structure-performance relations of G-DMU resin with different DMU/G molar ratios. The results show that during the synthesis of G-DMU resin, the hydroxyl compounds N-p-G-DMU and N-p-G1-DMU, as well as the carbocation intermediates C-p-G-DMU and C-p-G1-DMU are formed by the addition reactions between DMU and protonated glyoxal (p-G and p-G1). The plywood bonded with G-DMU resin with the optimum DMU/G molar ratio of 1.2:1.0 has the best water resistance. Its wet shear strength is increased by 145% (cold water) and 193% (63 °C) over the DMU/G molar ratio of 0.8: 1.0. The analyses of structure characterizations reveal that the generation of C=N, C–O–C, N–C–N, and heterocyclic rings, simultaneously, the main cross-linked components concentratedly are characterized in 400–500 Da, 500–550 Da, 698 Da, and 712 Da regions. In addition, the high molecular weight oligomers at 658–712 Da, 874 Da, 1037 Da and 1213 Da promote the formation of a highly cross-linked network during the curing process. Meanwhile, these results are in good agreement with the results of quantum chemical calculations. Thus, this work establishes the theoretical foundation for the novel glyoxal-based resin of G-DMU resin. [Display omitted] • A novel eco-friendly glyoxal-dimethylolurea (G-DMU) resin has been successfully prepared. • The molar ratio DMU/G is a key parameter to influence the properties of G-DMU resin. • N-p-G-DMU, N-p-G1-DMU, C-p-G-DMU and C-p-G1-DMU are key intermediates. • The addition reactions of G or G1 with DMU are thermodynamically unfavorable. • The high molecular weight oligomers promotes a highly cross-linked network. [ABSTRACT FROM AUTHOR]

Polyethylene terephthalate (PET) recycling has been carried out by various methods, e.g., mechanical recycling, chemical recycling, and energy recovery method. In this study, chemical recycling of PET was carried out by aminolysis using ethanolamine and converted into bis(2-hydroxyethyl) terephthalamide (BHETA). We have studied the effective polymeric catalysis of the aminolytic depolymerization of waste polyethylene terephthalate (PET) using sulfated polyborate producing a broad range of crystalline terephthalamides. The reaction was performed by varying the PET: ethanolamine ratio, reaction time and catalyst. Recycled PET bottles, film, and fibres were used for conversion. A yield of 95 % was obtained for PET: ethanolamine ratio of 1:4 (w/w), with 4 h reaction time, at 160 o C with sulfated polyborate as a catalyst. BHETA was characterized with FTIR, 1H,13C NMR, and DSC analysis. BHETA further gets reacted with dimer acid along with Ethylenediamine (EDA) at different molar ratios which produces polyesteramide (PEA) based hot melt adhesive (HMA). Thermal, mechanical, and rheological tests are performed and compared with traditional HMA. The effect on the properties of the hot melt adhesive, such as mechanical properties: tensile strength, shore D hardness, elongation at break; thermal properties: glass transition temperature (Tg), melting temperature (Tm), the heat of crystallization (Hc), crystallization temperature (Tc); adhesion properties: lap shear strength (LSS) and T-peel strength (TPS); rheological properties and degree of crystallinity were investigated. It was observed that the mechanical and thermal properties are increased by optimizing the concentration of BHETA. These HMA materials may have the capacity of being eco-friendly and have high adhesion properties. [ABSTRACT FROM AUTHOR]

EN 302-1 and EN 205 are among the most commonly used standards for the approval as well as research of wood adhesives to evaluate bond strength. The present study investigates the sensitivity of the lap-joint samples to a possible error in groove cutting as well as the influence of using steam-treated and untreated beech wood on tensile shear strength. The lap-joint specimens cut with 0.5 mm too shallow and 0.5 mm too deep grooves was compared with samples cut precisely within the tolerances mentioned in the standards. A comparison of tensile shear strength between the joints manufactured out of steam-treated and untreated beechwood has been made. The results show a significant influence of groove depth on the resulting tensile shear strength. While deep groves (+0.5 mm) underestimate, shallow grooves (−0.5 mm) overestimate it dramatically. Additionally, the joints with steam-treated beech wood delivered lower tensile shear strength than those of untreated beech wood bonds when there is a considerable wood failure percentage. When the ultimate tensile shear strength of the adhesive bond approaches the shear strength parallel to the grain of wood, the impact of steam-treated beech wood on the estimation of adhesive bond strength is higher. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental studies aimed at determining the effect of holes and notches at the front edge of adherends on the strength of adhesive joints. Single-lap joints made of S235JR steel sheets joined with Araldite 2014-2 epoxy adhesive were tested. Comparative tests of static strength in the shear test as well as high-cycle fatigue strength tests were carried out. Joints with three holes with a diameter of 3 mm filled with adhesive and notches 3 mm wide and 4 mm long also filled with adhesive were tested and compared with reference joints, i.e. without modification. The assumption was that the structural changes applied were to contribute to the reduction of the peak of peel and shear stresses that typically occur in this type of joint. Based on the static strength tests, no significant effect of the applied modifications on the strength of the connections was demonstrated. However, in the case of fatigue strength, a significant improvement in fatigue lifetime was demonstrated, which in the area of low cycle fatigue increased by 328.6 % for the joint with notches and by 640.8 % for the variant with holes. For the fatigue fracture surfaces, a topography analysis was carried out by determining the roughness parameters. Significant differences were exposed since at the leading edge, the selected parameters were significantly lower for the reference variant than for variants with notches and holes. [ABSTRACT FROM AUTHOR]

The principal methods of predicting the strength of lap shear bonded joints have been introduced for metallic and composite adherends. The early theories of Volkersen and of Goland and Reissner provided indications of the shear and transverse [peel] stresses. These equations were not easy to use and were increasingly displaced by finite element analysis [FEA]. The importance or otherwise of the high stresses around the embedded corner of the unloaded adherend were examined and shown to be the most likely cause of initial cracking, but not necessarily of joint failure. By combining an extensive experimental programme of testing both adhesives and joints with FEA, knowledge was gained of the reasons why lap shear joints fail. Some simple equations were developed with failure criteria based on yield of the adherend and/or the adhesive which produced predictions in good agreement with experimental results. Finally, the bonding of composites was considered, and it was shown that the key factor is the transverse tensile strength of the composite. [ABSTRACT FROM AUTHOR]

The paper deals with the behaviour of glued joints of wooden sections exposed to long and short term elevated temperatures. If the adhesive is not sufficiently temperature resistant, the glued joint may lose adhesion, which could result in a change in behaviour of the glued element cross-section when exposed to fire. Moreover, detailed knowledge of the behaviour of bonded joints under such long and short term thermal exposures will enable us to make informed decisions about the need for major structural intervention in structures post exposure. Three types of adhesives: phenol-resorcinol-formaldehyde (PRF), polyurethane (PUR) and emulsion polyisocyanate (EPI), commonly used in load bearing wooden structures, were chosen for the experimental work. Thermal analysis showed that the PRF and PUR were not sufficiently cured, especially the PRF. Calorimetric analysis showed the point at which thermal degradation of all three adhesives began, at 135 °C for EPI and at about 160 °C for PRF and PUR. Spruce lap joints, with a cross section of 40 × 48 mm, bonded using these adhesives were exposed to temperatures of 140 and 170 °C. To properly distinguish the effect of thermal exposure duration on the lap joints, different thermal durations were used: 20, 40, 60 and 80 min, plus 3 h and 1 day. The thermal resistance at each of the elevated temperatures was investigated through measurement of the reduction in tensile shear strength. At an elevated temperature of 140 °C there was no significant strength reduction in all of the tested adhesives, it was only a longer duration (1 day) that produced a reduction in the tensile shear strength properties (31.0% for PRF, 16.2% for PUR and 34.1% for EPI). A temperature of 170 °C produced a greater decrease in strength, a significant drop (more than 15%) was found for PRF and EPI after an exposure of 40 min; for PUR after 60 min. Moreover, the longer duration exposures to a temperature of 170 °C resulted in a total loss in strength for PUR and a half loss of strength for EPI; for both these isocyanates adhesives this was confirmed by the total loss of wood failure criterium. On the contrary, a strength reduction of 50.0% and a sufficient wood failure criterium for a 1day temperature exposure at 170 °C was found for PRF. [ABSTRACT FROM AUTHOR]

Semibatch emulsion polymerization of high oleic soybean oil-based monomer (HO-SBM), 2–ethylhexyl acrylate (2-EHA), and styrene (St) was applied to synthesize pressure-sensitive latex adhesives with up to 40 wt% of plant oil-based content. Ternary latex copolymers with similar chemical composition and broadly different molecular weight distribution (MWD) and modality were synthesized at varying temperatures and the mechanism of free-radical initiation. Conventional thermal initiation with ammonium persulphate at 80 °C and ascorbic acid/ tert -butyl hydroperoxide redox system of lower reaction temperature up to 20 °C were employed. Linear, short-chain, and long-chain branched macromolecules are formed due to chain-breaking reactions of HO-SBM (chain transfer on allyl groups) and 2-EHA (inter- and intramolecular chain transfer on the polymer). The dominant effect of chain transfer on allyl group protons and subsequent decrease in molecular weight was observed at higher reaction temperatures, while extensively branched polymer with high molecular weight was obtained at reaction temperature close to 20 °C. The effect of MWD and branching on the PSAs performance of synthesized biobased latexes was studied using loop tack and peel strength and lap shear testing and compared to two commercial PSA tapes as a benchmark. A significant increase in shear strength (up to 0.23 MPa) was observed for latex adhesives due to high molecular weight fractions, maintaining satisfactory level of tack (3–7 N) and peel (3–6 N/in). Broad MWD of the copolymers can be beneficial for providing materials with high shear strength (0.25–0.33 MPA) while preserving the significant level of tack (2–4 N) and peel strength (9–12 N/in) on both glass and steel substrates. [ABSTRACT FROM AUTHOR]

To evaluate the effect of distinct surface treatments on Peek (polyetheretherketone) adhesion to the resin cement and the mechanical fatigue behavior of lithium disilicate adhesively bonded to it. Peek slices embedded in plastic rings with self-curing acrylic resin were randomized into 4 groups for adhesion testing, considering the following surface treatment factors: Ctrl (just cleaned with 78 % isopropyl alcohol); Grinding (4219F diamond bur - 46 μm grain size); Air abrasion (45 μm aluminum oxide for 10 s); Sulfuric acid (acid etching for 30 s). All slices received a layer of adhesive agent; resin composite cylinder specimens (Ø = 3 mm; h = 3 mm) were produced and bonded onto the Peek slices using a dual resin cement for the shear bond strength tests (n = 20) through the wire-loop technique. Next, disc-shaped specimens of Peek (Ø = 10 mm; thickness: 2 mm) and lithium disilicate (LD) (Ø = 10 mm; thickness: 1 mm) were produced for the fatigue tests (n = 20) and paired. The LD was treated (5 % hydrofluoric acid for 20 s + silane agent) and adhesively bonded (with dual resin cement) onto Peek (treated with the previously mentioned surface treatments). The fatigue tests were performed with a step-stress approach (20 Hz; 10,000 cycles/step; initial load: 100 N; step-size: 50 N). Roughness, contact angle, topography and fractographic analysis were performed. The adhesive data was analyzed by one-way ANOVA and Tukey post-hoc tests, while the fatigue data were subjected to survival analysis (Kaplan Meier and Mantel-cox post hoc tests). Only the air abrasion surface treatment was able to improve the bond strength between Peek and resin cement (p < 0.05); the Grinding and Sulfuric acid groups were similar to the Ctrl. All surface treatments promoted similar fatigue performance. Air abrasion promoted increased bond strength between Peek and resin cement, having no detrimental effect on fatigue behavior. Overall, the mechanical behavior under fatigue of a lithium disilicate glass-ceramic was not influenced by the Peek substrate surface treatments. • The air abrasion with alumina particles increased the bond strength of Peek. • Peek treatment was not detrimental for the fatigue behavior of lithium disilicate. • The failure pattern of the set was not affected by the Peek surface treatment. [ABSTRACT FROM AUTHOR]

Urea-formaldehyde (UF) resins are the most used resins in manufacturing particleboards. However, UF resins are susceptible to hydrolytic degradation, which causes formaldehyde emission and limits their use as wood panel adhesives for interior applications. In this study, we investigated the effects of the addition of nano MnO 2 into urea-formaldehyde resin (UF) on the physicomechanical properties and formaldehyde emission of the bonded plywood. Two UF resins (a conventional as control and an innovative one with MnO 2 nanoparticles (UFL)) were analyzed for their specifications with FTIR measurements both in their cured state. Differential scanning calorimetry (DSC) analysis was used to investigate the curing process. The formaldehyde emission decreased by 72.8 %, and the 3 h hot water soaking wet tensile shear strength improved by 74.5 % with the addition of 1.2 % (w/w)MnO 2 nanoparticles compared to the UF resin control. [ABSTRACT FROM AUTHOR]

This review paper is focused on the load transfer mechanisms and mechanical behaviour of hybrid joints that combine adhesive bonding and pre-tensioned bolts. The first part of the review presented discussions related to small-scale tests that highlighted the significant effect of compressive stresses on the corresponding shear strength. Based thereupon, two models were presented for the hybrid joint: either a classical pre-tensioned bolted connection with the adhesive layer merely increasing friction or as an adhesively bonded joint with pre-tension reducing the peeling stresses. In the first case, we would expect no significant influence of either the overlap length or the disposition of the bolts, but a significant influence of the level of pre-tension. However, as shown in this second part the importance of bolt disposition about joint strength is not clear, and the role pre-tension plays is yet not fully elucidated. While the significance of the overlap length for joint capacity tends to support the idea that hybrid joints are a form of bonded joints, the relatively little influence of the adhesive itself behaviour mitigates this. Similarly, assuming hybrid joints are adhesively bonded with pre-tensioned bolts only reducing tensile stresses at the end of the overlap, the effect of bolt positioning remains inconclusive. The inconclusiveness is largely because the studies presented in this review paper never covered all aspects of the full complexity of hybrid joints. Many studies that investigated the influence of adhesives failed to fully characterize them, and some experimental campaigns varied more than one parameter but not independently. The complexity of hybrid joints, which require knowledge from different fields of engineering, is further compounded by the difficulty in manufacturing and testing large-scale samples. Many publications report a low number of replicates, and cross-comparison between different experimental campaigns is challenging due to the involvement of many variables. Further research is necessary to fully understand the load transfer mechanisms and behaviour of hybrid joints and improve their design and practical applications. [ABSTRACT FROM AUTHOR]

Cross laminated timber (CLT) was fabricated from Acacia mangium wood by using phenol resorcinol formaldehyde (PRF) and one component polyurethane (PUR) as binders. The purpose of the study was to evaluate the bond integrity of A. mangium CLT produced using different working parameters. The assemblies were pressed at 30 °C for 30 min using three pressing pressures (0.9 N/mm2, 1.2 N/mm2, and 1.5 N/mm2). Delamination and block shear tests were conducted on the CLT according to European Standards, EN 391 and EN 392, respectively. The results revealed that PRF-bonded CLT experienced lower percent delamination compared to that bonded with one component PUR. It appears that a higher clamping pressure i.e. 1.5 N/mm2, is needed to sufficiently bond A. mangium lumbers as indicated by a marked increase in bond shear strength with an increase of pressing pressure. PRF was found to be a more superior adhesive than PUR irrespective of cramping pressure and loading direction. A. mangium wood is relatively dense thus requires quite high pressure, 1.5 N/mm2, irrespective of adhesive used. PRF appears to bond A. mangium wood better compared to PUR with shear bond strength of 21% and wood failure percentage of 220% higher. [ABSTRACT FROM AUTHOR]

Ultraviolet(UV) laser treatment on the surface of the carbon fiber reinforced polymer (CFRP) laminate becomes an effective method to benefit the bonding strength of adhesively bonded joint in aerospace industries. In the present research, homomorphic CFRP laminates with different resin distribution on the surface are bonded into single-lap joints. Their shear strengths are tested to evaluate the effect of surface resin distribution on bonding mechanical performance. The different resin distributions on the surface of CFRP laminate are obtained by UV pulse laser with different laser scanning speeds. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) are conducted to analyze the laser treated surfaces and fracture surfaces of tested joints. The experimental results indicate that the residual resin of CFRP surface increases with the increase of scanning speed. Compared with both the reference surface without laser pre-treatment and that with no-residual resin for bonding, the surface with partial residual resin results in an enhancement of the shear strength of bonded joint. Moreover, the shear strength of the reference sample is higher than that bonded by the surface with no-residual resin. The research lays foundation for understanding the relationship between surface resin distribution and bonding strength. [ABSTRACT FROM AUTHOR]

Glass is increasingly used as a structural material in buildings, not only for enclosing spaces but also for bearing loads. To meet architect requirements for a uniform structure without appearance disturbance, adhesively bonded joints are preferred over mechanical fasteners. However, in the construction industry, most technical guidelines cover only soft structural silicone sealants, while stiffer adhesives like epoxy resins are not considered. Alternatively, the specific transparent epoxy adhesives offer a high potential for bonded glass assemblies, enabling thinner joints and a fully transparent junction. Indeed, under real service conditions, elevated temperatures are one of the worst factors affecting the mechanical properties of structural bonded joints. Therefore, in this paper, an experimental program was conducted to evaluate the performance of transparent bonded glass joints with epoxy resin over a wide range of temperatures. Tensile and shear tests were performed on bulk-cured adhesive samples and bonded glass-to-glass block shear specimens respectively, at room temperature (RT), 40, 60, and 80 °C. The differences in surface characteristics such as roughness, wettability, and surface free energy (SFE) between both glass sides were examined. The results showed that tensile and shear strengths decreased with increasing temperatures. Furthermore, the shear strength was influenced by the surface properties of the glass sides. At all tested temperatures, the rougher bonded glass side specimens revealed higher shear strength coupled with a mixed failure while a loss of adhesion characterised the smoother glass surfaces. [ABSTRACT FROM AUTHOR]

Date palm fibers (DPFs) can be potentially used in novel applications including composites. In this study, the Microbond technique was used to measure the interfacial shear strength (IFSS) between DPFs extracted from the mesh (ME), the midrib (MI), and the Spadix stem (SS) and three types of thermoset resins, i.e., unsaturated polyester (P), epoxy (E), and phenolic Resol (R). The fibers' tensile strength was measured and the aspect ratio of the fibers (critical length/diameter) for different fiber-resin systems was determined. It was shown that there is a clear relationship between resin droplet contact angle and IFSS. SS fibers provided the highest and ME fibers the lowest IFSS values among the fibers. Among the resins, E provided the highest and R the lowest adhesion to all fibers. The IFSS values of the E with MI, SS, and ME fibers were 7.62, 8.89, and 5.88 MPa, for the R matrix were 2.94, 3.02, and 2.33 MPa, and for the P matrix were 5.59, 5.97 and 5.17 MPa, respectively. It was shown that there is a clear relationship between IFSS values and contact angles of different fiber/matrix systems. [Display omitted] • Adhesion of date palm fibers with different resins was measured. • Critical length was measured for fiber/resin systems. [ABSTRACT FROM AUTHOR]

Safe design of adhesive joining in multi-materials in engineered structures requires the accumulation of numerous experimental data on the failure behavior of various adhesively-bonded material combinations under different loading conditions. The deep understanding of mechanical performance, fracturing morphologies, and main damage mechanisms is also quintessential for accelerating the development of proper physics-based and multi-scale models for assisting the design. Towards this goal, this work presents a comprehensive characterization of the failure behavior of adhesively-bonded metal–metal, metal–CFRP, and CFRP–CFRP material combinations under global shear deformation via single lap shear testing. Thanks to a synergistic combination of measurement methods by using Digital Imaging Correlation (DIC) and 3D optical profilometry, adhesive features on the adherend after failure were quantified and the main progressive damage mechanisms were identified. The characterization performed in this work provides quantitative data that contributes to a better understanding of shear failure in adhesive bonding across different bi-material combinations. The obtained results have practical implications, including the potential to enhance adhesive bonding design, identify failure causes in adhesive joints, and develop or validate computational models capable of capturing the observed behavior in various adhesively-bonded materials under global shear deformation. [ABSTRACT FROM AUTHOR]

In this study, the prepolymers (TYF) were prepared by formaldehyde and l -tyrosine, different amounts of TYF were added to prepare TYF modified urea-formaldehyde adhesives (LUF), and the gel time of resin, formaldehyde emission and wet shear strength of the plywood were tested. This result indicated that the optimal addition amount was 3 wt%, and the formaldehyde emission of plywood decreased by 23% to 0.57 mg/L, and the wet shear strength increased by 47% to 1.47 MPa. The thermal degradation and curing characteristics of urea-formaldehyde (UF) and LUF resins were characterized by thermogravimetric (TG) and differential scanning calorimetry (DSC), and the kinetic parameters were analyzed. The results showed that TYF could reduce the formaldehyde emission and improve the wet shear strength significantly. Moreover, the thermal degradation activation energy (E α) of UF resin was increased, and the thermal stability was enhanced. Although the initial and peak curing temperatures of LUF were higher, but the LUF cured more easily than the UF once the curing temperature was reached. [Display omitted] [ABSTRACT FROM AUTHOR]

The research investigates the effects of adding Reduced Graphene Oxide (RGO) at various weight fractions (0.25, 0.5, 0.75 and 1 wt%) on mechanical properties of high strength structural adhesive Araldite©️ 2011 and lap shear strength of adhesive joints. For uniform dispersion of RGO across adhesive, an extensive solution mixing technique was carefully carried out for the preparation of RGO/Epoxy combinations. Two types of lap joints: single lap joint (SLJ) and double lap strap joint (DLSJ) were fabricated. The joints were fabricated using Al 5083 as adherends. The adherends were treated with phosphoric acid anodization (PAA) before joining. A comparison of the strength properties was made after tensile testing of the adhesive specimens and lap joints. In the case of adhesive samples, an increase in mechanical properties was observed at lower concentrations. A maximum increase of 9.5% in the tensile strength and 128% in toughness characteristics was observed with the addition of 0.25 wt% RGO. A maximum improvement of 24% for SLJ and 25% for DLSJ was observed at the same filler percentage. Chemical analysis was conducted using Fourier-Transform Infrared Spectroscopy (FTIR), while dispersion analysis was carried out using Ultraviolet–Visible Spectroscopy (UV-VIS-Nir). Digital and Scanning Electron Microscopy (SEM) techniques were utilised in close examination of the fracture surface at high magnification. A finite element analysis based on cohesive zone modelling (CZM) was performed to predict the failure load characteristics of lap joints. Finally, a comparison of strength and fracture behaviour of RGO with GNPs and MWCNTs was critically discused. [Display omitted] • Uniform dispersion of Reduced Graphene Oxide (RGO) via solution mixing technique in the epoxy adhesive. • A significant improvement of 25% in lap shear strength of adhesive joints with the addition of RGO. • A significant improvement of 128% in toughness behaviour of adhesive reinforced with RGO. • Strength prediction of lap joints by FEA analysis using Cohesive Zone Modelling (CZM). • Inferior lap joints strength improvement compared to GNPs and MWCNTs. [ABSTRACT FROM AUTHOR]

The bonding potential of as-printed additively manufactured stainless steel is largely unknown, and the use of additional hierarchical bio-inspired surface texture has not yet been explored or characterized. In this study, it is demonstrated with the use of Selective Laser Melting (SLM) that a hierarchy of three surface features at different length scales can greatly improve the shear strength of metal-metal adhesive joints. The intrinsic stochastic texture of the SLM as-printed surface showed a high contact angle and a mixed failure with a dominance of adhesive failure. However, enhancing the as-printed surface with the Tree Frog (TF) and Fish Scale (FS) bio-inspired textures produced super-wetting surfaces, significantly improving the shear strength due to the synergistic effect of mechanical interlocking and physical adsorption adhesion mechanisms, resulting in mixed failure. The results indicated that the interfacial bonding performance was in line with the super-wetting behavior and was higher for the samples with a hierarchy of three surface features. The highest increase in bond strength was found for the tree frog hierarchical hexagonal texture (+70% compared to the as-printed texture). In contrast, abrasion of the as-printed surface with P1000-grit paper significantly reduced the surface roughness and increased the water contact angle, resulting in the lowest shear strength. [Display omitted] [ABSTRACT FROM AUTHOR]

Despite several experimental, analytical, and numerical attempts to evaluate the hygrothermal impacts of the metal and composite adherents, the effects of hygrothermal effects on the adhesively bonded 3D-printed component have not been sufficiently investigated in the literature. With this motivation, this work aims to experimentally investigate the mechanical strength of adhesively bonded 3D-printed joints under hygrothermal conditions. To accomplish this goal, the joints having different surfaces and adhesive types were subjected to different durations. Taguchi L 16 (42 × 21) orthogonal array was chosen as the experimental plan, and the single lap shear tests were conducted to evaluate the effects of the parameters on the joint strength. Polylactic acid (PLA) was used as adherends, and the adherends were manufactured by fused deposition modeling technique. Analysis of variance (ANOVA) was conducted to identify the correlations between variables, and also regression analysis is performed to show the suitability of experimental measurements. The results revealed that the adhesive type is the most influential parameter (60.76%) on single lap strength, followed by aging duration (23.62%), whereas the surface pattern (1.85%) shows no significant influence. The optimal set of parameters and levels is found as Loctite EA 9466 adhesive, 45° surface pattern, and no aging duration. The current study also revealed that the Taguchi method demonstrated its reliability as a predictive tool for assessing joint strength in hygrothermal environments. [ABSTRACT FROM AUTHOR]

Rational design of environmentally friendly and sustainable bio-based wood adhesives is of great significance to people's health, environmental protection, high value-added utilization of crop resources and national economic development. In this work, we develop new cellulose-based wood adhesives and explore the differences in the spatial structure of branched polyamines and linear binary amines after cross-linking with dialdehyde cellulose (DAC). The bio-based wood adhesives DAC-2N (adhesives obtained from the reaction of DAC with diethylenetriamine) and DAC-3N (adhesives obtained from the reaction of DAC with tris(2-aminoethyl)amine) are prepared by the cross-linking reaction between DAC and linear amines (2 N)/branched amines (3 N). The effects of spatial cross-linking degree, mass ratio of amine to DAC and solid content of adhesives on the bonding performance are systematically studied, and the mechanical properties of wood specimens are also tested by universal mechanical tests. Results show that the best formulation of DAC adhesive is a 1:1 mass ratio of DAC to 3 N, and the optimal solid content is 40%. Compared with the pristine DAC adhesive, the dry bonding strength of the DAC-3N adhesive increased by 103% from 1.35 MPa to 2.74 MPa, and the wet bonding strength increased from 0 MPa to 1.81 MPa. DAC-3N adhesive with dense network structure has stronger glue and water-proof properties than DAC-2N adhesive, and the shear strength test of the three plywood shows the failure mode of the substrate rather than the failure mode of bonding or cohesion. Therefore, this study might offer a prospective strategy for future research, especially the rational design and potential application of bio-based adhesives with branched structure. Sustainable and high-performance bio-based wood adhesive via integration of branched amine and dialdehyde cellulose. [Display omitted] • Cellulose-based environmentally friendly wood adhesive was prepared via Schiff-base reaction. • The cross-linking network formed by covalent bonds and non-covalent bonds is helpful to improve the bonding properties. • The comparison of DAC-2N and DAC-3N adhesives showed that the adhesive with branched structure had higher bond strength. • Compared with pure DAC, the DAC-3N adhesive has splendid bonding strength and excellent water resistance. [ABSTRACT FROM AUTHOR]

This research attempts to create a pressure-sensitive adhesive (PSA) from cardanol as an outset material. The two kinds of formulations were established; the first scheme followed the epoxidation of cardanol, dihydroxylation, and PSA development. In the latter system, cardanol was pretreated with dichlorodimethysilane (DCDMS) in the second scheme, continuing the same procedure to obtain PSA resin. The chemical nature and structure confirmation were done using FTIR and NMR spectroscopy, hydroxyl value, epoxy equivalent weight (EEW), etc. To evaluate PSA resin's crystallinity, thermal stability and hydrophilicity the film was separately obtained for both the schemes by curing whereas to obtain PSA tapes the obtained PSA and tackifying resins were applied onto PET sheets. XRD analysis showed a decrease in the degree of crystallinity after incorporating DCDMS. The gel content values decreased, whereas the water absorption values increased, which could be related to the increase in the hydrophilic nature of PSA with an increasing number of tacky sites. TGA and DSC showed the formation of a high amount of char yield (∼31%) and a higher Tg (−37.72 °C) in Si containing film. The probe tack, peel strength, and shear strength values were high for the formulation with equal amounts of PSA and tackifying resin. Finally, the contact angle measurements showed that the hydrophobicity of film was also improved after incorporating DCDMS. The materials resembling PSA tape and including only small amounts of tackifying resins and crosslinkers displayed the ideal characteristics with satisfactory results. [ABSTRACT FROM AUTHOR]

The efficiency of different surface modifications on alumina nanoparticles on both filler dispersion and the final properties of the resulting adhesive nanocomposites have been investigated. A tetraglycidyldiaminodiphenylmethane (TGDDM) epoxy resin and three sample series of nanocomposites were prepared via in-situ incorporation of alumina nanoparticles into the reactor. The alumina/TGDDM nanocomposites were prepared individually using neat or non-treated alumina nanoparticles and two kinds of silane-grafted alumina nanoparticles, i.e., APS-treated alumina and GPS-treated alumina. The presence of different alumina nanoparticles in the epoxy matrices resulted in different states of nanofiller dispersion as revealed in SEM and AFM micrographs. It was elucidated that the silane treatment on alumina nanoparticles is crucial for the desired dispersion in the epoxy matrix. Besides, the appropriate filler dispersion resulted in improved thermal resistance and high degree of cure, especially for the adhesive nanocomposite containing APS-treated alumina nanoparticles. In adhesion tests, the shear strength was improved in both nanocomposites containing silane-grafted alumina with more pronounced values for the nanocomposite containing APS-treated alumina nanoparticles. The shear strength reached from 6.6 MPa for the neat epoxy adhesive to 10.2 MPa for the adhesive nanocomposite containing 5 wt % APS-treated alumina nanoparticles mainly due to high levels of dispersion of the high modulus alumina nanoparticles and effective interfacial interactions with the epoxy matrix. The adhesive peel strength of alumina/TGDDM nanocomposites showed a similar trend as in shear strength with more pronounced variations. A noticeable increase in the peel strength of the nanocomposites containing silane-grafted alumina nanoparticles appeared to correlate with greater levels of crack deflection and hence dissipation of fracture energy as observed in SEM pictures. [ABSTRACT FROM AUTHOR]

Conventional treatment of complex fractures includes the use of plates and nails, which may compromise the affected limb's functionality. Previous studies have demonstrated promising results through chemical, mechanical, and cytotoxicity tests of a chitosan-based adhesive—proposed as a new method to bond high energy fractures—in dry environments with adequate adhesion, malleability, and biocompatibility. In this study, we focused on performing an evaluation of bio-adhesives' mechanical properties and bone-adhesive joint using two chitosan-based formulations (with and without a cross-linking agent). The texture profile analysis determined adhesive properties, such as cohesiveness, adhesiveness, hardness, and resilience at different cure times. Bone-adhesive joint was evaluated according to the tensile bond strength test and shear bond strength test. Fracture toughness and cohesive strength were calculated through a rigid double cantilever beam test at mode I failure. Bone-adhesive joints were tested in two environments: dry and submersed in water at 37 °C for 1, 6, and 24 h (curing time), an approximation of surgery conditions. The experimental results showed an incremental of adhesiveness and hardness of the cross-linked adhesive during the first 15 min, which was determined as the usage time to spread on the bone fracture. The joint interaction between the adhesive and bone surfaces was studied; chitosan-based formulations showed an adhesive joint failure under dry conditions in most of the cases. However, this behavior changed under aqueous conditions, presenting cohesive failures. Under aqueous conditions, cross-linked bone-adhesive presented an augmented tensile bond strength up to 0.024 ± 0.0036 MPa, a shear bond strength up to 0.031 ± 0.0069 MPa, and fracture toughness of 2.38 ± 0.54 J/m2 was observed with a cure time of 24 h. Finally, the presence of the cross-linking agent in the cross-linked bio-adhesive reduced the sensitivity of the adhesive to water; a promising finding that should be explored in future studies. [ABSTRACT FROM AUTHOR]

Abstract This paper considers the effects of an atmospheric plasma treatment (APT) on the surface properties of an amine cured carbon fibre/epoxy resin composite, and how those effects manifest over time. In particular, the ability of the APT to remove a thin layer of silicone-containing, proprietary, release agent (Chemlease® 41 EZ), typically used in the production of composite components, has been investigated. It was concluded that the reduction in water contact angle (WCA) after APT for both the solvent wiped and contaminated surfaces was as a result of an increase in oxygen containing species at the surface, as determined through X-ray photoelectron spectroscopy (XPS). Further, it was found that the APT slightly reduced the failure strength of lap shear specimens for solvent wiped surfaces, whereas an increase in failure strength was observed for silicone contaminated samples. WCA and XPS results suggest that the contaminant layer was not removed, but instead transformed to a more stable inorganic form. [ABSTRACT FROM AUTHOR]

Abstract The aim of this study was to enhance the water resistance of soy protein (SP) adhesives using laccase/TEMPO-modified lignin. Kraft lignin was depolymerized by laccase enzyme in the presence of TEMPO to expand the oxidation reaction of both phenolic and non-phenolic compounds. This simplified process has the distinct advantage of enhancing lignin-protein interaction. Compared with SP adhesives, lignin-protein adhesives showed a stronger elastic modulus, higher thermal stability, and increased wet adhesion performance. Wet shear strength increased by 106% from 0.693 to 1.429 MPa, and partial wood failure was observed after the test. Better performance was also observed in the three-cycle soaking test. At the same time, the stronger interactions between -COO- and -NH 2 groups of protein and lignin led to a decrease in flowability and spreadability. [ABSTRACT FROM AUTHOR]

Abstract The aim of this research is to study the influence of moisture absorption at low moisture contents on the creep behaviour of an epoxy adhesive in steel bonded joints. Single lap joints were manufactured using high strength steel adherends and a two-component epoxy adhesive. The single lap joints were tested at load levels corresponding to average lap shear stresses of ± 5%, 15%, 30% and 45% of the dry lap shear strength in both 40 °C air and 40 °C distilled water. Specimens were not pre-aged to be able to analyse the coupled effect of moisture and loading. The test results show that an increase in the load level resulted in an increase in the instantaneous strain and in the creep strain rate. The creep strain of single lap joints loaded in water was generally larger than for the ones loaded in air. For joints loaded in water the creep behaviour was found to be dependent on the moisture concentration in the adhesive. At low moisture percentages creep was suppressed, resulting in a lower instantaneous strain. At higher moisture percentages creep was promoted, resulting in a larger strain rate. The suppression of creep at low moisture percentages is attributed to water molecules bonding to the epoxy macromolecules, resulting in a reduction in molecular mobility and a smaller creep strain. At higher moisture percentages the plasticizing effect of the water dominates, resulting in a larger creep strain. The Maxwell three-element solid model and Kelvin-Voigt three-element solid model were used to simulate the creep behaviour of the single lap joints loaded in air and water. The models gave good representations of the creep response across the different load levels in both water and air, they were however unable to give a correct representation of the instantaneous strain of the single lap joints loaded in water. This is attributed to the models being unable to account for the present short-term relaxation process that is dependent on the moisture concentration. [ABSTRACT FROM AUTHOR]

Abstract An inexpensive adhesive bearing polyphenol groups was synthesized in this work. In order to obtain the desired adhesive, gallic acid (3,4,5-trihydroxybenzoic acid) was chosen as the functional compound to modify hydroxyl acrylate copolymer. The rheology and creep properties of the adhesive expressed that its cohesive strength and fatigue resistance were both increased. The resultant adhesive presented a better bonding strength to different substrates, such as steel, polypropylene (PP) and polytetrafluoroethylene (PTFE). In addition, reversible adhesion could be stimulated by modulating the temperature. Shear strength for PP was not less than 0.5 MPa, furthermore that for steel was no less than 4 MPa and the maximum achieved 6 MPa. Graphical abstract fx1 [ABSTRACT FROM AUTHOR]

Abstract The influences of various Al surface treatments, adhesive thicknesses as well as the incorporation of synthesized microcapsules into epoxy adhesive on the shear strength of adhesive/ Al joints have been investigated using lap-shear tensile tests. First, the influence of adhesive thickness on the shear strength of joints has been presented. Then, the effects of various Al surface treatments on the surface roughness of Al and shear strength of joint have been researched. Atomic force microscopy was used to study the Al surface morphologies and textures. Finally the few micron-sized polymeric microcapsules were synthesized and the shear performances of microcapsule filled epoxy adhesives were inspected. It was observed that the HCl acid based etching increased both micro-roughness and nano-texture of the Al surface and led to the peak shear strength. Moreover, HCl-nitric acid treatment offered the maximum value for the cohesive failure. Capsule inclusions into the adhesive displayed different influences on the joint shear performances depending on the capsule morphology and the surface treatment of Al. Graphical abstract fx1 Highlights • The HCl acid based etching increased both micro-roughness and nano-texture of the Al surface and led to the peak shear strength. • By following an in-situ polymerization technique very fine urea-formaldehyde microcapsules have been achieved. • The integration of 10 wt% hollow microcapsule enhanced shear performances of adhesive/Al joint. • The integration of filled microcapsules into the adhesive mostly increased the cohesive failure. [ABSTRACT FROM AUTHOR]

Abstract In the present study, both the effects of accumulated laser fluence as surface treatment and bondline thickness on adhesive bonding of carbon fiber reinforced polymer (CFRP) composite materials were investigated. Proper CFRP composite surfaces for adhesive bonding were obtained by a laser treatment process using pulsed CO 2 laser. Laser treatments were obtained with different accumulated laser fluences and then surfaces were analyzed with roughness and contact angle measurements. Adhesive bonding was performed with various bondline thicknesses ranged between 30–500 µm using two component structural epoxy based paste adhesive (Loctite Hysol ® EA 9396TM). Adhesive bonding strength of bonded samples was determined with single lap shear tests. It is worthy to note that if the accumulated laser fluence which has significant effect on shear strength does not optimize, it causes ineffective adhesion. [ABSTRACT FROM AUTHOR]

Abstract A new testing methodology is presented for determining the adhesion strength and failure at composite interfaces. An automated bonding evaluation system (ABES) has been adapted to process and test thermoplastic sandwich composites formed with various natural solid phase materials such as wood. Composites can be rapidly formed and then tested in situ over a range of temperatures to provide information on interfacial adhesion and shear strength between the thermoplastic and the substrate. This methodology is demonstrated for HDPE, PP, PLA and TPU composites comprising wood as solid phases. The interfacial properties depend on processing and testing temperatures, individual thermoplastic phase properties, and wood species. It is expected that this new test method will have applicability in thermoplastic composite design, manufacture and service life predictions for reinforced or filled thermoplastics. [ABSTRACT FROM AUTHOR]

Abstract Graphene oxide (GO) is prepared and introduced into epoxy resins through a wet-transfer migration technique using a three-roll mill. The results of TEM, XRD and digital microscope observation show that good dispersion of GO is achieved without using any additives. The mechanical and thermal properties of GO/epoxy (GO/EP) adhesives are enhanced with GO incorporated. A 10.2% increase in Young's modulus and a 56.3% increase in elevated-temperature (120 °C) lap shear strength (LSS) was observed on addition of 1.0 wt% GO, compared to the neat epoxy adhesive. Increased glass transition temperature and improved thermal stability of the GO/EP adhesives are also observed in the DMA and TG analysis. Moreover, the toughness of the GO/EP adhesives is improved and much rougher fracture surface can be observed compared with the neat epoxy adhesive. No GO agglomeration can be observed in the SEM images of GO/EP adhesive with 1.0 wt% loading. [ABSTRACT FROM AUTHOR]

Abstract This work presents the development and characterization of a new and environmentally friendly wood bio-adhesive using cassava starch and bio-oil that was from the fast pyrolysis of larch sawdust. Bio-oil contains large amounts of chemicals and is cheap and renewable compared to the traditional modification methods for starch-based adhesives. It will bring great benefits if bio-oil can be used as the chemical additive to improve the quality of the regular starch-based wood adhesive. This study prepared the bio-oil-starch (BOS) adhesive and examined its properties. The structure and thermal decomposition stability were characterized with Fourier transform infrared spectroscopy, scanning electron microscopy, atomic force microscopy and thermogravimetric analysis. Results showed that, compared with a regular starch-based wood adhesive, the BOS adhesive displayed better liquidity, longer storage period (extended from 13 days to 55 days) and higher shear strength (the bonding strength of starch-based wood adhesive with 25% of bio-oil increased 17.4% in the dry state and 50.9% in the wet state, and its water resistance increased by 10.0%). Characterization showed that the improved performance of the BOS adhesive was supported by its strengthened molecular structure and enhanced thermal stability. The introduction of bio-oil significantly improved the properties and modified the structure of the regular starch-based wood adhesive. [ABSTRACT FROM AUTHOR]

Abstract Herein, a green, renewable, value-added environmentally friendly, and cost-effective soybean meal-based resin was prepared in this study inspired by the quality of reinforced concrete structure combined with the concept of increasing the added value of biomass materials. A series of hybrid composite materials were developed consisting of two reinforcements in the soybean meal resins to compare their varying mechanical properties. The soybean meal resin material acted as a matrix constituent with the primary reinforcement of kenaf fibers (KF) and secondary reinforcement of halloysite nanotubes (HNTs). The wet shear strength of hybrid composite adhesive was increased from 0.38 to 1.15 MPa after the addition of 1,2,3-propanetriol-diglycidyl-ether (PTGE), KF, and KH560-HNTs; the solid content improved by 20.50% and the toughness and thermal stability of the adhesive considerably improved. The proposed approach of including PTGE/KF/KH560-HNTs in soybean meal blended to enhance the physical properties, cost-effectiveness, and value-added environmental friendliness of the material would be advantageous in expanding the industrialized applications of soybean meal-based resins. [ABSTRACT FROM AUTHOR]

Abstract In line with the developments in advanced engineering applications such as aerospace and automotive industries, the techniques of joining similar and dissimilar materials have become a crucial issue, i.e. the need for a stronger joint has significantly grown. Compared to conventionally used bolted, riveted and pinned joints, adhesively bonded joints have been increasingly used due to its improved fatigue life and damage tolerance and lower structural weight, especially the case when relatively thin adherends are used. Alternatively, hybrid joints, combination of two or more joining techniques, are presently investigated to create a joint with higher strength than those gained from one technique. In this study, we compared mechanical performance of bolted, bonded and hybrid single lap joints subjected to the tensile loading using three different adherend thicknesses and two different adherend materials with different mechanical behaviors, such as yield and tensile strength and ductility. To this end, a combined experimental and numerical study was performed. In finite element simulations, cohesive zone, ductile and shear damage models were used to model the damage initiation and evolution for the adhesive film layer (AF163-2K), aluminium adherend (AL6061 and AL7075) and the steel bolt materials, respectively. Force displacement curves, the amount of energy absorbed and failure history for each configuration tested, were analysed extensively to elucidate the strength of various joints. [ABSTRACT FROM AUTHOR]

Abstract The process and mechanism of phenol-formaldehyde (PF) resin synthesis and nano copper oxide (CuO) preparation simultaneously in the same reaction vessel were studied in this paper. Curing kinetics, molecular structure changes as well as the bonding performance of the modified PF resin was investigated. The results indicated that loading levels of the derived nano CuO at 4% and 8% reduced the apparent activation energy of the PF resin and accelerated both the addition and condensation reactions, while changes of reaction enthalpy were consistent of these results. With the introduction of the derived nano CuO, the chemical shifts of all samples were generally offset to the low field for the solvent effect. A complexation reaction between copper ions and oxygen on the phenolic hydroxyl groups had an influence on phenoxy carbons as shown by NMR. Quantitative analysis indicated that nano CuO had a positive effect on para substituted reaction and thus enhanced both the addition and the condensation reactions. Though the shear strength under several test conditions decreased slightly, values were within the requirements set by the appropriate Chinese Standard. [ABSTRACT FROM AUTHOR]

Abstract The challenge of developing epoxy-foam adhesives for automotive applications lies in the difficulties associated with finding the optimal composition ensuring both high expansion ratio and mechanical strength. Herein, epoxy-foam adhesives based on epoxy composites were prepared and evaluated for automotive applications. The curing behavior was studied by differential scanning calorimetry, and the morphology was characterized non-destructively by micro X-ray computed tomography. Increasing the content of rubber-modified epoxy (RME) retarded the curing reaction by increasing the expansion ratio and affected the lap shear strength. A high expansion ratio and suitable mechanical strength was observed for epoxy-foam adhesives containing 10 or 15 wt% RME. [ABSTRACT FROM AUTHOR]

Abstract The multicomponent injection molding process enables the combination of different materials in a single step. However, one of the challenges of this new technology is related to the adhesive strength between dissimilar materials. In this work, specimens were overmolded with dissimilar composite materials made of polypropylene and coconut fibers added with coupling agents. By means of a new patented device specially produced to evaluate the adhesive strength of such different materials under pure shear stress, it was possible to measure the force necessary to promote the detachment of dissimilar surfaces. Electron microscopy and contact angle analyses were used to better understand the adhesion phenomenon between such dissimilar materials. Although a maleic anhydride additive promoted better anchoring between fibers and the polymer matrix, it was observed that the composite without coupling agents exhibited the greatest adhesive strength between dissimilar surfaces. [ABSTRACT FROM AUTHOR]