Low-modulus Adhesives Research Articles

Stress mitigation for adhesively bonded photovoltaics with fibre reinforced polymer composites in load carrying applications

Structural loads, especially in-plane compression, may cause local buckling and debonding of the photovoltaic (PV) cells that are mechanically integrated with structural members and this may lead to degradation in their electrical performance. This paper proposes an approach to mitigate the strains transferred from structural members to PV cells through the partial composite action provided by low-modulus adhesives. Specimens were fabricated by bonding amorphous silicon (a-Si) PV cells to glass fibre reinforced polymer (GFRP) structural components by an adhesive layer of 0.5- or 2.0-mm thickness. Two types of adhesives were used including a two-part rigid epoxy adhesive and a low-modulus silicone adhesive. These integrations were then submitted to in-plane compressive loadings. PV cells bonded by the silicone adhesive showed no damages during loading. While for PV cells bonded by epoxy adhesives, obvious electrical performance degradations were observed, when the strain reached 0.62% or 0.23% for specimens bonded by epoxy with a layer thickness of 0.5 mm or 2.0 mm respectively. Debonding and local-buckling of the PV cells were also witnessed. Theoretical analysis was conducted to understand the strain mitigation of the adhesive as a result of the induced partial composite action. Results demonstrate that such strain differences between the GFRP and the bonded PV cell are dominated by the shear modulus and thickness of the adhesive layer as well as elastic modulus, thickness and length of the PV cell. The theoretical analysis was validated by finite element (FE) modelling and design suggestions are provided accordingly.

Silicone‐Based Adhesives with Highly Tunable Adhesion Force for Skin‐Contact Applications

A fundamental approach to fabricating silicone-based adhesives with highly tunable adhesion force for the skin-contact applications is presented. Liquid blends consisting of vinyl-multifunctional polydimethylsiloxane (V-PDMS), hydride-terminated PDMS (H-PDMS), and a tackifier composed of a silanol-terminated PDMS/MQ resin mixture and the MQ resin are used as the adhesive materials. The peel adhesion force of addition-cured adhesives on the skin is increased by increasing the H-PDMS molecular weights and the tackifier content, and decreasing the H-PDMS/V-PDMS ratio. There is an inverse relationship between the adhesion force and the Young's modulus. The low-modulus adhesives with a low H-PDMS/V-PDMS ratio exhibit enhanced adhesion properties. The low-modulus adhesives with the high MQ resin content show significantly enhanced adhesion properties. These adhesives exhibit a wide range of modulus (2-499 kPa), and their adhesion force (0.04-5.38 N) is superior to commercially available soft silicone adhesives (0.82-2.79 N). The strong adhesives (>≈2 N) provide sufficient adhesion for fixing the flexible electrocardiogram (ECG) device to the skin in most daily activity. The human ECG signals are successfully recorded in real time. These results suggest that the silicone-based adhesives should be useful as an atraumatic adhesive for the skin-contact applications.

Controlled multiple neutral planes by low elastic modulus adhesive for flexible organic photovoltaics

To protect brittle layers in organic photovoltaic devices, the mechanical neutral plane strategy can be adopted through placing the brittle functional materials close to the neutral plane where stress and strain are zero during bending. However, previous research has been significantly limited in the location and number of materials to protect through using a single neutral plane. In this study, multiple neutral planes are generated using low elastic modulus adhesives and are controlled through quantitative analyses in order to protect the multiple brittle materials at various locations. Moreover, the protection of multiple brittle layers at various locations under both concave and convex bending directions is demonstrated. Multilayer structures that have soft adhesives are further analyzed using the finite element method analysis in order to propose guidelines for structural design when employing multiple neutral planes.

Повышение весовой эффективности гибридных каркасно-панельных кузовов за счет соединений с управляемой сдвиговой жесткостью

The paper presents a new approach for design of long-length car bodies with predetermined fatigue strength, which utilizing bonded joints with low-modulus adhesives and transversal reinforcement. Ready-to-use composite sandwich panels with GFRP skins are bonded into a metal frame during the production process of a frame-panel car body. Joints used for this process can be classified as single-lap ones. High requirements for strength/weight ratios of modern car bodies demand smoothing the loads on panels and a frame, especially in the case of steel frames with significant deviations from ideal design geometry (alternating gaps between frame and panels). Smoothing can be satisfied by means of stiffness control of bond lines with alternating thickness. The new conception of stiffness control by using of small diameter self-tapping screws is proposed. The conception implemented by usage of developed nomograms, which relates bond line thickness, reinforcement pitch (distance between reinforcement elements) and shear stiffness per unit length of lap joints. The nomograms allow determining reinforcement pitch in respect with given both a bond line thickness and a shear stiffness of joints. The mentioned design approach is demonstrated on the example of a low-floor tram car body. After the determination of a reinforcement pitch the appropriate estimation of loads on joints and frame is carried out. Loads obtained from simplified FE car body model and refined by usage special adjustment factors, which calculated on the base of detailed FE modeling of typical frame-panel structures. The static and fatigue strength analysis performed by verified FE models of bonded/screwed joints and analytical experimental dependencies respectively.

Dragonfly wing venous cross-joints inspire the design of higher-performance bolted timber truss joints

This communication concerns the design and development of high performance biomimetic timber joints as inspired by the venous cross-joints of dragonfly wings. A special cross-joint configuration in Crocothemis servilia from the Libellulid family is identified. Between the veins of this cross-joint is a resilin envelope. Finite element modelling reveals that the presence of this resilin envelope reduces the levels of localised stress in the jointing region. By gaining an understanding of the structure and function of this dragonfly wing joint, dragonfly-joint inspired timber trusses are developed by integrating low-modulus adhesives around bolted connectors. We find that not only are the properties of strength, stiffness, toughness and deformability of bolted truss joints vastly improved on applying dragonfly-mimicking technology, but that the fatigue resistance is also enhanced. This preliminary work is an important step forward in the design and development of high-performance biomimetic joints for timber construction.

Dynamic analysis of adhesively bonded joint under solid projectile impact

A three-dimensional (3-D) dynamic finite element analysis (FEA) was performed to study the transient stress distribution of adhesively bonded joints under solid projectile impact. The influence of the processing parameters is numerically studied including the thickness of the adhesive layer, the solid projectile size and its velocity, the material properties of the adhesive, and the strain-rate effect on the dynamic stress of the joint. The numerical results show that there exists a complex 3-D stress state in the adhesively-bonded joints under the solid projectile impact. When the solid projectile touches the joint surface, a stress concentration is imparted at the location of contact. The maximum stress of the adhesive layer varies with time as a function of the impinging solid projectile such that the location moves farther from the center of the overlap zone after the solid projectile rebounds from the joint surface. A lower stress is expected at the adhesive layer for low-modulus adhesives with respect to the case for high-modulus adhesives under the same transverse impact load. The numerically-modeled transverse deflection of the single-lap joint for the three impactor sizes with different kinetic energies and three adhesive types was in good agreement with the experimentally observed values. Experiments were performed by using developed equipment for impact testing and a high frame rate videography. The proposed dynamic model is useful in providing a reference for the design of adhesively-bonded structures.

A Low Modulus Adhesive Characterization by Means of DMTA Testing

The main objective of this paper is to investigate a way to characterize experimentally the mechanical behavior of low modulus adhesives. In this context, the relaxation and complex moluli master curves of a SIKAFLEX® 505 adhesive are obtained by means of dynamic mechanical thermal analysis. Firstly, the procedure carried out to prepare and to validate the test specimens is explained. Next, the influence of the test specimen thickness and strain level is studied. Finally, the relaxation and dynamic master curve sunder tension strain are constructed via a procedure based on the time-temperature superposition principle.

Perfluoroether elastomer for wide fluid and temperature capabilities

Perfluoroether elastomer is based on a perfluoroether polymer backbone combined with silicone crosslinking. This combination produces products that have excellent low temperature flexibility and heat resistance, together with improved performance in fuels, solvents, acids, and alkalis compared to standard fluoroelastomers. The material family is available in a number of different forms including low viscosity flowing liquid, gum solid and also intermediate types of viscous fluid and paste. Variation of chemical compositions, filler loadings and rubber formulations make it possible to manufacture a variety of elastomeric products such as molded rubber parts, low modulus adhesives, soft coatinglayers, and very soft and sticky gels for a wide range of engineering applications.

Moisture durability of four moisture cure urethane adhesives

Moisture durability of four low modulus adhesives was examined. The four low modulus adhesives all had the same basic moisture cure polymer but contained different fillers and additives. Hot-dipped galvanized steel was bonded to random fiber-reinforced unsaturated polyester and aged in two moisture environments for various durations. Adherend surface wipes included acetone, isopropyl alcohol and a typical surface contamination for galvanized steel (an organic lubricant). Diffusion coefficients, moisture uptake and modulus changes due to moisture environment were determined for the adhesives and the fiber-reinforced plastic (FRP). The moisture cure adhesive with clay and poly(vinyl chloride) (PVC) as fillers (adhesive 252) had the highest retained lap joint strengths. Initially, adhesive 252 had single lap joint strengths of 1.47 ± 0.08 MPa for acetone-wiped joints and 1.39 ± 0.33 MPa for organic lubricant-wiped joints. After accelerated aging in a cataplasma environment for 9 weeks, lap joint strengths fell to 0.61 ± 0.08 MPa for acetone-wiped joints and to 0.65 ± 0.11 MPa for organic lubricant-wiped joints. Environmental scanning electron microscopy (ESEM) and energy-dispersive spectrometry (EDS) showed that the actual failure locus was through a corroded zinc layer and between the adhesive and the zinc surface after aging. Dynamic mechanical analysis (DMA) showed that the modulus for adhesive 252 dropped from 21.7 to 13.9 MPa after cataplasma aging. From finite element analysis (FEA), this modulus drop corresponded to a drop in normal stress concentration from 0.75 to 0.57, and a drop in shear stress concentration from 1.41 to 1.36 at a point 0.5 mm from the end of the single lap joint overlap.

The Shear Stress–Strain Behaviour of Low-modulus Structural Adhesives

The shear stress–strain behaviour of two low-modulus structural adhesives has been measured using the butt-torsion test. The Nadai correction for non-linear shear behaviour is explained as it is necessary to understand how this correction can be applied to butt joints. The results for one adhesive were accurately used to predict the strength of a lap joint, and it was shown that the strength of such a joint can approach that of a conventional, modern, structural epoxy. Structural adhesives are usually reckoned to be those with a high strength (50 MPa and upwards) and (these days), a strain to failure of at least 10% in tension, and which usually have a tensile modulus of 2 GPa or so. However, adhesives which are significantly less stiff, less strong, but much more ductile are entering the ‘structural’ arena. In order to evaluate their effectiveness and use in design, it is necessary to be able to measure accurately their stress–strain behaviour. Two such materials are 3M 9245 Structural Bonding Tape (SBT) and 3M 7838 B/A.

New Reworkable High Temperature Low Modulus (in Excess of 400–500 °C) Adhesives for MCM-D Assembly

Large area substrate processing is a key solution for improving the productivity of multi-chip module deposition (MCM-D) technology. This project is focused on high temperature low modulus polymeric adhesives for attachment of silicon tiles to suitable pallets to facilitate large area film processing of MCM structures. Current high temperature polymeric materials are predominately polyimide-amide, siloxane modified polyimide-amide, Bisbenzocyclcobutene (BCB), and polynorbornene materials, all of them are neither reworkable nor flexible, which creates an obstacle to remove the coated substrates and to reuse the high cost pallets. However, one of the approaches to address this demand is introducing a low modulus elastic material combined with a unique thermal stabilizer, which will be thermally stable at 400°C for 1 h and thermally degradable at 450°C after 1 h. A series of novel reworkable high temperature (in excess of 400–450°C) adhesives have been developed in this study, that can meet the requirements of adhesion, viscosity, low modulus, thermal stability, and reworkability for MCM-D mass production. The mechanism of thermal stabilizer functionality and adhesive reworkability is presented in this study. In addition, finite element modeling is used to predict the out-of-plane warpage, axial, and interfacial stresses of the tiles-on-pallet palletized assemblies. [S1043-7398(00)00501-6]