Structural Adhesives Research Articles

Convolutional Neural Network for Interface Defect Detection in Adhesively Bonded Dissimilar Structures

This study presents an ultrasonic non-destructive method with convolutional neural networks (CNN) used for the detection of interface defects in adhesively bonded dissimilar structures. Adhesive bonding, as the weakest part of such structures, is prone to defects, making their detection challenging due to various factors, including surface curvature, which causes amplitude variations. Conventional non-destructive methods and processing algorithms may be insufficient to enhance detectability, as some influential factors cannot be fully eliminated. Even after aligning signals reflected from the sample surface and interface, in some cases, due to non-parallel interfaces, persistent amplitude variations remain, significantly affecting defect detectability. To address this problem, a proposed method that integrates ultrasonic NDT and CNN, and which is able to recognize complex patterns and non-linear relationships, is developed in this work. Traditional ultrasonic pulse-echo testing was performed on adhesive structures to collect experimental data and generate C-scan images, covering the time gate from the first interface reflection to the time point where the reflections were attenuated. Two classes of datasets, representing defective and defect-free areas, were fed into the neural network. One subset of the dataset was used for model training, while another subset was used for model validation. Additionally, data collected from a different sample during an independent experiment were used to evaluate the generalization and performance of the neural network. The results demonstrated that the integration of a CNN enabled high prediction accuracy and automation of the analysis process, enhancing efficiency and reliability in detecting interface defects.

Debondable Epoxy-Acrylate Adhesives using β-Amino Ester Chemistry.

The reuse of multilayered materials, which are held together by structural epoxy adhesives, is a major challenge since the bonded substrates cannot be easily separated for recycling. In this research, we explore a one-pot strategy based on β-amino ester chemistry for the development of modified epoxy adhesives with on-demand debonding potential. For this, a formulation of commercially available acrylate, epoxy and amine compounds is used. The research starts with a systematic study, demonstrating the influence of the different compounds on the thermal and adhesive properties of the materials. Subsequently, the potential for debonding is demonstrated using rheological measurements and tensile tests. The fast, catalyst-free Aza-Michael reaction enables the straightforward preparation of such epoxy-based adhesives, while the reverse reaction allows for debonding at 120 °C. In general, a chemical design is demonstrated for producing an industrially attractive generation of debondable epoxy-based adhesives.

Molecular pathology of skin fragility

The skin's barrier function is primarily maintained by the cohesion of its layers and the specialized stratum corneum. Genetic and autoimmune disorders that result in skin fragility have significantly contributed to understanding the role of various molecular components in the skin. These conditions, characterized by blisters, erosions, wounds, and impaired wound healing, are rare but must be considered in clinical differential diagnoses. Key cutaneous adhesion structures include the epidermal basement membrane, anchoring fibrils, cell-matrix adhesions (e.g., hemidesmosomes and focal adhesions), and cell-cell adhesions (e.g., desmosomes and corneodesmosomes). These multiprotein suprastructures not only provide structural support but also participate in signaling processes and physiological or pathological conditions such as skin aging and wound healing. Additionally, dermal connective tissue plays astructural role and serves as areservoir for proteases, growth factors, and cytokines. Modern techniques like single-cell and spatial transcriptomics, combined with artificial intelligence algorithms, are advancing the molecular mapping of human skin. Moreover, this review highlights the molecular composition of adhesion structures and the specific diseases associated with deficiencies in these components.

Identification of hub genes and pathways in Uterine corpus endometrial carcinoma (UCEC): A comprehensive in silico study

BackgroundUterine corpus endometrial carcinoma (UCEC), derived from the endometrium, is the most common type of endometrial malignasis. This gynecological malignancy is very common all over the world, especially in developed countries and shows a potentially rising trend correlated with the increase in obese women. MethodsDifferentially Expressed Genes (DEGs) analysis was conducted on GSE7305 and GSE25628 datasets from the Gene Expression Omnibus (GEO). DEGs were identified using GEO2R (adjusted p-value <0.05, |logFC| > 1). Pathway analysis employed KEGG and Gene Ontology databases, while protein-protein interactions were analyzed using Cytoscape and Gephi. GEPIA was used for target gene validation. ResultsWe have identified 304 common DEGs and 78 hub genes using GEO and PPI analysis, respectively. The GO and KEGG pathways analysis revealed enrichment of DEGs in extracellular matrix structural constituent, extracellular space, cell adhesion, and ECM-receptor interaction. GEPIA analysis identified three genes, ENG, GNG4, and ECT2, whose expression significantly differed between normal and tumor samples. ConclusionThis analysis study identified the hub genes and associated pathways involved in the pathogenesis of UCEC. The identified hub genes exhibit remarkable potential as diagnostic biomarkers, providing a significant opportunity for early diagnosis and more effective therapeutic approaches for UCEC.

Mechanical Strength of Additive Manufactured and Standard Polymeric Components Joined Through Structural Adhesives.

The main aim of this work is to evaluate the mechanical properties of additive manufactured polymeric parts joined with standard plastic parts through structural adhesives. The primary advantage of this technique is its ability to significantly increase the size of the final assembly by using additive manufacturing (AM) for complex joints and inexpensive, reliable extruded plastic parts for load-bearing components. This hybrid assembly combines the flexibility and shape adaptability of AM with the structural strength and cost-effectiveness of extruded polymer parts, resulting in a final design that performs comparably to the base material. The materials used in the paper are rigid acrylic adhesive and toughened acrylic, both applicable with almost no surface preparation and fast curing. The 3D-printed parts are produced in ABS, while the standard parts are in PVC. First, the work is devoted to estimating the performance of the adhesives using pin-collar joints and a combined numerical and experimental methodology. The second section presents and discusses the results of two more realistic applications of adhesive bonding to hybrid complex joints. For the pin-collar joints, the results show failure mostly in the adhesive, with an average shear stress of 11.5 MPa and 5.22 MPa and a stiffness of 4449 N/mm and 3649 N/mm for the rigid and toughened adhesives, respectively. The results of the adhesive bonding of structural joints show that the adhesive is always capable of providing the load-carrying capacity required to achieve the strength of traditionally manufactured polymeric parts. The paper shows that adhesives are a feasible way to expand the potential of 3D-printed equipment to obtain larger hybrid parts partially realized with traditional technology, especially with inexpensive off-the-shelf bars and sections.

Debonding Detection of Thin-Walled Adhesive Structure by Electromagnetic Acoustic Resonance Technology.

The detection of debonding defects in thin-walled adhesive structures, such as clad-iron/rubber layers on the leading edges of helicopter blades, presents significant challenges. This paper proposes the application of electromagnetic acoustic resonance technology (EMAR) to identify these defects in thin-walled adhesive structures. Through theoretical and simulation studies, the frequency spectrum of ultrasonic vibrations in thin-walled adhesive structures with various defects was analyzed. These studies verified the feasibility of applying EMAR to identify debonding defects. The identification of debonding defects was further examined, revealing that cling-type debonding defects could be effectively detected using EMAR by exciting shear waves with the minimum defect diameter at 5 mm. Additionally, the method allows for the quantitative analysis of these defects in the test sample. Due to the limited size of the energy exchange region in the transducer, the quantitative error becomes significant when identifying debonding defects smaller than this region. The EMAR identified debonding defects in clad-iron structures of rotor blades with a maximum error of approximately 15%, confirming its effectiveness for inspecting thin-walled adhesive structures.

Enhancement and Compatibilization of Waste-Sourced Biocomposites Through Elastomer Blending and Matrix Grafting Modification

A novel elastomer-modified multicomponent, multiphase waste-sourced biocomposites, was prepared for converting waste biomass and plastic into value-added products. The effects of blending elastomer–olefin block copolymer (OBC) and maleic anhydride (MAH), and divinylbenzene (DVB) co-grafting of recycled polypropylene (rPP) matrix on the adhesion interface, structure, and properties of high wood flour-filled (60 wt.%) composites were thoroughly investigated. The results indicated that DVB introduced branched structures into the polymer matrix molecular chain and increased the MAH grafting rate. Co-grafting rPP/OBC blends enhanced the interfacial adhesion among rPP, OBC, and wood flour. Additionally, MAH-grafted OBC was prone to encapsulating rigid wood flour, thereby forming an embedded structure. Notably, the tensile modulus and impact strength of the final three-component composites increased by 60% and 125%, respectively, compared with the unmodified composites. Additionally, dynamic mechanical analysis revealed that DVB-induced branching promoted the formation of microvoids in the OBC shell layer surrounding the wood, which in turn induced significant plastic deformation in the polymer matrix. This work offers a facile and efficient method for preparing high-toughness, high-stiffness, and low-cost waste PP-based composites for automotive interiors, and indoor and outdoor decoration.

Robust Self-Healing Adhesives Based on Dynamic Urethane Exchange Reactions.

Thermoset polyurethanes (PUs) have been successfully reprocessed as covalent adaptable networks (CANs) by catalyzing carbamate exchange. Here we extend bond exchange beyond the internal network cross-links to create a dynamic urethane adhesive. Interfacing PU CANs to substrates with nucleophilic functional groups creates adhesives capable of reversible transcarbamoylation with the substrate, which has not been demonstrated previously by CAN adhesives. Two types of thermoset PU films were synthesized, one containing the green carbamate exchange catalyst Zr(tmhd)4 and the other containing no catalyst. Although otherwise identical in chemical and network properties, as indicated by FT-IR spectroscopy and dynamic mechanical thermal analysis (DMTA), the film containing catalyst showed dynamic bond exchange behavior through stress relaxation analysis. When evaluated as an adhesive, the CAN film exhibited self-healing properties and retained its adhesive strength for five cycles, which is attributed to reversible covalent bonding to the glass substrate. This work expands industrially relevant CANs to structural adhesives and demonstrates their potential value in an application that presently employs PUs as single-use materials.

Augmenting bonding properties of OSB and Chinese fir hybrid cross-laminated timber by sandblasting pretreatment

The investigation was conducted into the adhesive performance within hybrid cross-laminated timber (HCLT) composed of sandblasted Chinese fir and oriented strand board (OSB) which was originally covered by cured adhesive during the manufacture leading to very fragile bonding. HCLT specimens with three layers were bonded utilizing three structural adhesives which were polyresorcinol formaldehyde (PRF), polyurethane (PUR), and polyisocyanate (EPI). Surface roughness and wettability testing ascertained that sandblasting could effectively change the original resin layer and microtopography on the surface of OSB laminates, significantly modified the level of roughness on the surface, thereby providing a basis for vertical spreading and permeating of the adhesives. Sandblasting pretreatment consistently augmented the dry bonding shear strength about all the three adhesives, achieving the most substantial increases of 53 %, 39 %, and 37 % for PRF, PUR, and EPI, respectively. The pretreatment further reducing delamination rates to about a half in cyclic delamination tests of all three adhesives. Fluorescence microscopy detection of the bonding interfaces revealed a significant increase in adhesive penetration depth which had been further confirmed by the bonding interface thickness measurement. Surface roughness generally had a negative effect on dry shear bonding strength, especially with EPI adhesive, while surface wettability was positively correlated with bonding strength and adhesive layer thickness. This study introduced an environmental and efficient method to enhance the bonding performance of HCLT.

Ultraviolet Irradiation Surface Treatment to Enhance the Bonding Strength of Polyamide-Based Carbon Fiber-Reinforced Thermoplastic Polymers.

Adhesive bonding is a suitable joining method to satisfy the increasing industrial demand for carbon fiber-reinforced polymers without the need for a machining process. However, thermoplastic-based carbon fiber-reinforced polymers have small adhesive strength with structural thermoset adhesives. In this study, an ultraviolet irradiation surface treatment was developed to improve the adhesive bonding strength for polyamide-based carbon fiber-reinforced polymer. The type of ultraviolet wavelength, irradiation distance and irradiation time were optimized. Surface treatment with simultaneous UV irradiation of 185 nm and 254 nm wavelength generated unbound N-H stretching that was capable of chemical bonding with epoxy adhesives through a photo-scission reaction of the amide bond of polyamide matrix. Therefore, ultraviolet irradiation treatment improved the wettability and functional groups of the polyamide-based carbon fiber-reinforced polymers for adhesive bonding. As a result, the adhesive strength of the polyamide-based carbon fiber-reinforced polymers was increased by more than 230%.

Finite element modeling for cohesive/adhesive failure of adhesive structures with a thermosetting resin

In this study, we established a novel method based on the finite element method (FEM) for predicting the strength of adhesive structures. Viscoplasticity, void growth, and cohesive zone model were introduced into the FEM to create a nonlinear damage growth (NDG) model. This model was used to comprehensively analyze the process zones within the adhesive layer. Furthermore, the embedded process zone approach was used to develop an interface constitutive law that averages the mechanical response of the adhesive layer. This modified Ma–Kishimoto (MMK) model can accurately represent the adhesive layer as an interface element and is computationally efficient. Furthermore, the study obtained the necessary interface properties for the MMK model from the NDG model, creating a numerical material test that can approximate the effect of the process zone. To validate the proposed method, single-lap shear tests were performed, and the accuracy of the predicted strength and deformation field was evaluated. The damage evolution in the NDG model and the MMK model were compared, and the scope of application of the MMK model was discussed. The results of this study can be used as a reference for the failure mechanism of thermosetting adhesives and establishment of design indices for adhesive structural strength.

Influence of hygrothermal aging on the lifetime and mechanical properties of GFRP/Al adhesive joints

The study aimed to investigate the impact of hygrothermal aging on the degradation of mechanical properties and predict the lifetime of adhesive joints between aluminum alloy (AA6061) and glass fiber-reinforced epoxy (GFRP) composites. The adhesive joints were prepared in a single lap joint (SLJ) configuration and subjected to hygrothermal conditions to induce aging. Immersion tests were conducted at a water temperature of 70°C with varying aging periods (10, 18, 40, 54, 60, and 75 days) to assess the long-term performance and predict the lifetime of the joints. The tensile and shear strength of the adhesive joints were evaluated using a tensile test. The test was performed at room temperature (RT) with a speed rate of 1.3 mm/min. Among the different methods available for lifetime prediction, a semi-empirical approach was selected to analyze the changes in the mechanical properties of the adhesive joints. A comprehensive comparison was made between aged and unaged specimens. The experimental results revealed a reduction in the ultimate tensile and shear strength of the adhesive bonds over the 75-day aging period. The semi-empirical relationship used for long-term durability prediction of adhesive joint structures indicated a 78% decrease in ultimate tensile strength for the adhesive joints over a one-year aging period. This highlights the significant influence of hygrothermal aging on the mechanical properties of the investigated adhesive joints. During the aged periods, various failure modes were observed, including adhesive failure, cohesive failure, and structural failure.

Microstructure and wear behavior of WC-reinforced AlCoCrFeNiSi high entropy alloy coatings prepared by high-velocity arc spraying

In this study, AlCoCrFeNiSi/WC (WC from 0 to 40 wt%) high entropy alloys (HEAs) coatings were deposited on Q235 steel substrates using high-velocity arc spraying technique. The coatings' phases, microstructure, microhardness, nano-hardness, wear properties, and the mechanisms of wear were examined. The results have shown that the WC reinforced HEA had faced-centered cubic, body-centered cubic solid solution and carbide phases, while faced-centered cubic solution and Cr3Si, Al2O3 phase were observed in AlCoCrFeNiSi coating. The coatings possess a dense, lamellar structure and strong adhesion to the substrates. With the addition of reinforced particles, the porosity of the coatings initially decreases and then increases. The average microhardness of the coatings is significantly improved as WC content increased, reaching a maximum of 530 HV0.1. The ratio of nano-hardness to reduced elastic modulus rises as more WC is introduced. The coatings with the WC particles exhibit good wear resistance with the optimum wear rate at WC content of 40 wt%. Under the same conditions, the coating with 40 wt% of WC displayed the lowest wear rate, which decreased by 60.5 % compared to the AlCoCrFeNiSi coating. The AlCoCrFeNiSi coating experienced a mix of adhesive, abrasive, oxidative, and fatigue wear mechanisms. The wear mechanism changed to more abrasive, adhesive, and oxidative in the reinforced coatings. The load-bearing capacity and secondary phase strengthening effects of WC reduce the wear rate of the coatings.

Thermally-responsive dismantlable adhesion system for steel plates using methacrylate copolymers containing a tert-butoxycarbonyl group

A drastic change in the bulk properties of adhesive materials as well as a reduction of interfacial interaction is an important key for the development of dismantlable adhesion systems, i.e., reliable bonding systems with a function of on-demand debonding. The deprotection of tert-butoxycarbonyloxy (BOC) groups included in adhesive polymers can be applied to the construction of a dismantlable adhesion system for steel plates combined with acrylic and epoxy adhesives. In this study, we evaluated the adhesive properties of the copolymers of 2-(tert-butoxycarbonyloxy)ethyl methacrylate (BHEMA), glycidyl methacrylate (GMA), and 3-trimethoxysilylpropyl methacrylate (SMA) for the dismantlable bonding of steel plate cold commercial (SPCC). Spontaneous dismantling of the SPCC adhesive structures was achieved after heat treatment under the conditions at 210 °C for 15 min when the BHEMA/GMA/SMA copolymers with an appropriate copolymer composition were used as the acrylic adhesives or the primers for epoxy adhesion. We discuss the interfacial interactions between the adherend and the adhesives for reliable adhesion and the effects of the deprotection of the BOC groups for dismantling to exhibit both an adhesion strength required for metal bonding and a rapid decrease in the strength by thermal stimulus.

Nb-doped hydrogenated diamond-like carbon coated biodiesel injectors material: Synthesis, structure and properties

Despite the economic and environmental benefits of biodiesel, it poses significant challenges, including coking on metal surface and increased wear on diesel engine injector nozzles. Therefore, this study investigates the synthesis of niobium-doped diamond-like carbon (Nb-DLC) coatings on two types of injector nozzle materials (viz. H13 steel, AISI 420 stainless steel) using the Closed-Field Unbalanced Magnetron Sputtering (CFUBMS) technique. Comprehensive characterizations, including microstructural analysis, crystal structure evaluation, hardness assessment, and tribological property assessment, were conducted. The Nb-DLC coatings demonstrated a stable friction coefficient, with values four times lower than that of the substrate materials. The present findings demonstrate significant enhancements in terms of structural composition, crystalline phase, hardness, friction coefficients, and adhesion properties due to the Nb-DLC coatings. Therefore, Nb-DLC coated materials could be the promising candidate material for biodiesel engine injector nozzle applications. This emphasizes their potential to contribute significantly to the sustainable consolidation of biodiesel within automotive engine.

Structurally-colored adhesives for sensitive, high-resolution, and non-invasive adhesion self-monitoring

Polymeric adhesives are critical in many applications, from daily life to implantable devices and soft robotics. Monitoring adhesion in a real-time and convenient manner before premature failure is essential yet challenging. Herein, we present structurally-colored adhesives for sensitive, high-resolution, and non-invasive adhesion self-monitoring via distinct color change for detecting subtle deformation and debonding. The structurally-colored adhesives are designed by integrating one-dimensional photonic nanochains into exemplified acrylate-copolymer-based adhesives and demonstrate distinct color-changing capability through a unique tilting mechanism of nanochains under shear. Our structurally-colored adhesives can be customized as pressure-sensitive and structural adhesives, exhibiting ultrafast response (<60 ms), high sensitivity, and high resolution (~120 μm). Moreover, predicting adhesion states and premature failure can be achieved assisted by imaging systems and machine learning algorithms with an average accuracy of up to 97.2%. Our structurally-colored adhesives are expected to offer a practical paradigm for structural health monitoring in the Internet of Things era.

Effects of transition elements additions on interfacial properties of Al (111)/B4C (0001) interface based on first-principles study

In this study, the structural stability and adhesion performance of the Al (111)/B4C (0001) interface are explored by performing first-principles calculations, as well as the effects of the additions of the transition elements Sc, Zr, and V on the interfacial adhesion work and electron structures are further investigated. The results of interfacial structure calculations demonstrate that the B-terminated interface with FCC site stacking modality is the optimal configuration of the Al (111)/B4C (0001) interface. The results of adhesion work and electronic properties indicate that the d orbitals of Sc, Zr, and V elements are hybridized with the p orbital of B to form Sc-B, Zr-B, and V-B bonds, respectively, at the interface, which are more covalent than the Al-B bond. In particular, the hybridization between the d orbital of the V element and the p-orbital of B is more intense. Therefore, the best improvement of the interfacial wettability is obtained by the V element, in which the interfacial adhesion work is enhanced by 41.18% compared to the undoped interface. A theoretical guide to enhance the wettability and interfacial properties of the Al/B4C system by adding transition elements can be provided by our calculations.

Next-Generation Structural Adhesives Composed of Epoxy Resins and Hydrogen-Bonded Styrenic Block Polymer-Based Thermoplastic Elastomers

Next-Generation Structural Adhesives Composed of Epoxy Resins and Hydrogen-Bonded Styrenic Block Polymer-Based Thermoplastic Elastomers

Non-destructive testing for thickness measurement of the adhesive layer and defect detection in ceramic-metal bonded structures using THz waves

Ceramics, owing to their superior thermal barrier properties, are employed by bonding them to primary structures exposed to elevated temperatures, such as the external components of aircraft and spacecraft. However, when these bonded structures are subjected to thermal impact resulting from high-temperature environments or rapid temperature variations, issues such as debonding, crack formation, and degradation may ensue. These issues can significantly undermine the reliability of the structures, necessitating nondestructive testing (NDT) to monitor the condition of the adhesive layer before and after thermal impact. In this article, we evaluated the integrity of adhesive structures before and after thermal impact using Terahertz (THz) waves, which offer exceptional penetration and resolution for NDT of nonmetallic materials without causing damage to the structure. To achieve this, ceramic-metal bonded structures were mounted on a robotic arm, and scanning was performed over a 70 mm area at 1 mm intervals and 0.5° increments. The peak-to-peak time of the THz signal through the adhesive layer was calculated to determine the thickness distribution. Based on experimental results, the average thickness of the ceramic layer before thermal shock was measured at 9.48 mm (standard deviation: 0.03) and the average thickness of the adhesive layer was 0.234 mm (standard deviation: 0.03). Additionally, four suspected defect areas within the adhesive layer were identified. After thermal shock, the ceramic layer’s thickness decreased by an average of 0.02 mm (0.17%), resulting in an average thickness of 9.46 mm, and the adhesive layer’s thickness decreased by an average of 0.002 mm (0.99%), resulting in an average thickness of 0.232 mm. The four suspected defect areas were confirmed to be in the same locations before thermal impact. To validate the NDT method using THz wave, the ceramic-metal bonded structure was cut to identify debonding defects in the suspected areas. The difference between the thickness distribution obtained using the THz wave-based NDT and that measured using optical microscopy after cutting was 4.4%, confirming the accuracy of the THz wave-based method for defect detection and thickness measurement.

Fungal decay-provoked degradation of cross-linking networks in phenol-formaldehyde adhesive: From fragmentation process towards damaged mechanical robustness

The mechanical robustness of structural adhesives as represented by phenol formaldehyde (PF) adhesive is vital for the safe service of engineered wood/bamboo products facing environmental aggressors. Here, fungal decay-provoked hazard towards the mechanical robustness of PF adhesive was demonstrated, which was usually overlooked in previous works. Obtained results proved that the white rot (Trametes versicolor, T. versicolor) and brown rot (Gloeophyllum trabeum, G. trabeum) fungus can both grow on the surface of cured commercial PF adhesive. An evident oxidization can only be observed on the T. versicolor-decayed PF adhesive, as along with the appearance of 2,6-bis(1,1-dimethylethyl)-1,4-benzenediol after decay. It further demonstrated the ruptured and oxidized cross-linking structures. The evident degradation of PF adhesive can be assigned to the favored activity of laccase (40.9 U mL−1) and lignin peroxidase (60.5 U mL−1) during the decay of T. versicolor. The elastic modulus and hardness of PF adhesive reduced by over 31.6 % and 50.2 %, respectively, which is also accompanied by the elevation in the creep deformation (45.4 %), after a 40-day T. versicolor decay. This work revealed the decay-sensitivity of PF adhesive, and the cleavage of cross-linking structures is the major trigger for the degraded mechanical robustness of PF adhesive after decay.