The roles of the degree of substitution and the degree of polymerization on the behaviour of cellulose ethers applied as adhesives for artwork conservation

Quite recently cellulose ethers have been introduced into the conservation field as an adhesive agent. Cellulose ethers are available in a wide range of products. Their properties are related not only to the nature of the substituent but also to the degree of polymerization (DP) and the degree of substitution (DS). Thus, the number of OH groups that are bound to C1 and C4 or to C2, C3 and C6 carbon atoms of the cellulose ether molecule affect their properties. Paper traction resistance is increased by using cellulose ethers as an adhesive to paste Tenguyo paper, a paper with a low density of fibres usually used by restorers. This increase is directly related to the molecular structure of the cellulose ethers. Therefore, the nature of substituents, the DP and the DS of cellulose ethers have to be taken into account in conservation practice. DP equals the number of anhydroglucose units in the chain and DS equals the number of hydroxyl groups replaced by the substituent in every anhydroglucose unit in the chain. As this information is not usually provided by suppliers, viscosity measurements are proposed as a means of predicting the behaviour of different products in relation to the conservation of paper artefacts, or to any other field where cellulose ethers solutions are used.     

New two-part epoxy paste structural adhesives for a thick bondline

The properties of three different adhesive pastes J-101, J-114, and EA9309.3N/A are evaluated. These adhesives are two-part thermosetting pastes which can be cured at 80-90°C and are designed for inlay or dowel bonding application, and thus are especially suitable for a bondline thickness from 0.8 to 1.6 mm. The compositions are based on CTBN-modified DGEBA and multicomponent polyamine curing agent systems. Durability and fatigue life tests are also discussed. The microstructure and fracture behavior of these adhesives is studied by scanning electron microscopy (SEM). Different toughening mechanisms are proposed to account for the toughness of these adhesives. The experimental results show that both J-101 and J-114 adhesives develop good mechanical properties when cured at 80-90°C. Furthermore, their durability performance compares favorably with, or even exceeds, that of commercially-available EA9309.3N/A adhesive.     

The effect of residual strains on the progressive damage modelling of environmentally degraded adhesive joints

This work is concerned with developing numerical modelling techniques for predicting the environmental degradation of adhesively-bonded joints. Associated experimental data are also reported. The moisture-dependent mechanical properties of the adhesive were obtained by testing bulk specimens also exposed to various moisture contents. The diffusion parameters for moisture in the adhesive were determined by carrying out gravimetric experiments on bulk adhesive samples. The moisture-dependent interfacial bond strength of the adhesive system investigated has been determined by testing a mixed mode flexure (MMF) specimen, exposed to obtain various levels of moisture content at the interface. Progressive damage in the joints was modelled with a two-parameter cohesive zone model (CZM). The CZM parameters were determined by correlating the experimental data obtained from the MMF test with results from the numerical simulation. The parameters were then used to predict the response of another configuration, the notched coating adhesion (NCA) specimen. When the residual stresses were neglected in the modelling, the predicted NCA response was seen to be in good agreement with the experimental data. However, initial simulations that included the residual stresses resulted in poor predictions of the NCA response. Creep tests on the saturated adhesive, at the ageing temperature, showed large viscoplastic deformations at low loads. Coupled diffusion-stress modelling, including viscoplastic material properties for the adhesive continuum, showed that the residual stresses for the aged specimens decreased significantly and thus did not contribute strongly to the environmental weakening. Good predictions were then obtained for the NCA tests.     

An investigation on the initiation and propagation of damaged zones in adhesively bonded lap joints

In this study, the initiation and propagation of damaged zones in the adhesive layer and adherends of adhesively bonded single and double lap joints were investigated considering the geometrical non-linearity and the non-linear material behaviour of the adhesive and adherends. The modified von Mises criteria for adherends and Raghava and Cadell's failure criteria (J. Mater. Sci. 8, 225 (1973) [1]) including the effects of the hydrostatic stress states for the epoxy adhesive were used to determine the damaged adhesive and adherend zones which exceeded the specified ultimate strains. The stiffness of all finite elements corresponding to these zones was reduced so that they could not contribute to the overall stiffness of the adhesive joint. This approach simplifies to observe the initiation and propagation of the damaged zones in both the adhesive layer and adherends. A tensile load caused first the damaged adhesive zones to appear at the right free end of the adhesive-lower adherend interface and at the left free end of the adhesive-upper adherend interface, and then to propagate through the adhesive regions near the adhesive-adherend interfaces (interfacial failure). In the bending test, the damaged zone initiated at the left free end of the adhesive-upper adherend interface in tension, and similarly propagated through the adhesive regions close to the adhesive-adherend interface (interfacial failure). In the double-lap joint subjected to a tensile load, the damaged adhesive zones initiated first at the right free end of the adhesive-middle adherend interface and then propagated through the adhesive region near the adhesive-adherend interface. After the damaged zone reached a specific length it also grew through the adhesive thickness, and the adhesive joint failed. The SEM micrographs of fracture surfaces around the free edges of the overlap region indicated that the failure was interfacial. An additional damaged zone growth was observed in the side adhesive regions due to lateral straining, called the Poisson effect.     

Determination of Structural Damping and Optimal Vibration Control of an Adhesively-Bonded Double Containment Cantilever Joint

In this study, the loss factors of an adhesively-bonded double containment cantilever joint were determined for different plate and support lengths. The response of the adhesive joint subjected to a transverse excitation force was measured with a contactless eddy-current sensor and the first bending natural frequency was determined using the Fast Fourier Transform method. The loss factor was calculated using the half-power bandwidth method based on the power spectrum of the joint vibration. After an excitation force was applied to the joint, the damped free vibration analysis was carried out using the finite element method and its measured loss factor. The transverse vibration attenuation was actively controlled with different numbers of actuators located on the top surface of the plate. The optimal control of the vibration attenuation was achieved based on a performance index by considering the strain energy, the kinetic energy, the work done on the adhesive joint by the actuators as well as the vibration attenuation time. Genetic Algorithm was implemented to this optimization problem in which the optimal control force histories, the optimal locations and the optimal numbers of the actuators were searched. Eight actuators exhibited the best control force history minimizing the performance index to 3.34 × 10^–2. Thus, the attenuation time was reduced from 16 s to 0.15 s and the absolute displacement was decreased from 13.1 mm to 17.15 × 10^–3 mm for 0.15 s. In addition, the modal strain energy and kinetic energy were found to be at lowest levels. As the actuator number was increased only a minor decrease in the performance index was observed after four actuators.     

Three-dimensional finite element analysis of single-lap adhesive joints subjected to impact bending moments

This paper deals with the stress wave propagations and stress distributions in single-lap adhesive joints subjected to impact bending moments with small strain rate. The elastic stress wave propagation and the stress distribution in single-lap adhesive joints of similar adherends subjected to impact bending moments are analyzed using three-dimensional finite-element method (FEM). A three-point impact bending moment is applied to the joint by dropping a weight. FEM code employed is DYNA3D. The effects of Young's modulus of the adherends, the lap length, the adherend thickness and the adhesive thickness on the stress wave propagation at the interfaces are examined. It is found that the maximum value of the maximum principal stress, σ₁, appears at the interface between the adhesive and the upper surface of upper adherend which is impacted. The maximum stress, σ₁, increases as Young's modulus of adherends, the lap length and the adhered thickness increase. It is also found that the maximum stress, σ₁ increases with decreasing adhesive thickness. In addition, experiments were carried out to measure the strain response of single-lap joints subjected to impact bending moments using strain gauges. A fairy good agreement was observed between the numerical and experimental results.     

Characterization of thermoplastic polyurethane adhesives with different hard/soft segment ratios containing rosin as an internal tackifier

Three thermoplastic polyurethanes (TPUs) containing different hard/soft (h/s) segment ratios (1.05-1.4) were prepared using the prepolymer method. MDI (diphenylmethane-4,4′diisocyanate) and polyadipate of 1,4-butanediol (M _w = 2440) were allowed to react to produce the prepolymer. To provide the polyurethanes with high immediate adhesion to different substrates, a rosin + 1,4-butanediol mixture (1 : 1 equivalent%) was used as chain extender (TPU-Rs). These TPU-Rs had two types of hard segments: (i) Urethane hard segments, produced by reaction of the isocyanate and the 1,4-butanediol, and (ii) Urethan-amide hard segments, produced by reaction of the isocyanate and the carboxylic acid functionality of the rosin. The TPUs and TPU-Rs were characterized using FTIR spectroscopy, gel permeation chromatography, differential scanning calorimetry, stress-controlled plate-plate rheology, stress-strain measurements, and Brookfield viscosity. The TPUs and TPU-Rs were used as raw materials to prepare solvent-based polyurethane adhesives, the adhesion properties of which were obtained from T-peel tests on PVC/polyurethane adhesive/PVC joints. The addition of rosin as an internal tackifier increased the average molecular weight, more markedly in the TPU-Rs containing higher hard/soft segment ratios, but the elastic and viscous moduli decreased. An increase in the hard/soft segment ratio of the TPU-Rs retarded the kinetics of crystallization (which was determined by the soft segment content in the polyurethane), and increased the immediate T-peel strength in PVC/polyurethane adhesive/PVC joints (which was determined by the urethan-amide hard segments). Furthermore, addition of rosin to the polyurethanes decreased the final adhesion, although always reasonably high peel strength values were obtained.     

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB)-modified epoxy: mechanical and thermal properties

Amine-terminated poly(ethylene glycol) benzoate (ATPEGB), synthesized from the esterification reaction of poly(ethylene glycol) (PEG) with 4-amino benzoic acid, was used to modify the toughness of bisphenol-A diglycidyl ether epoxy resin (DGEBA) cured with room temperature curing agent, triethylene tetramine. ATPEGB was characterized by FT-IR and H-NMR spectroscopies, viscosity measurements, solubility parameter calculation and molecular weight determination with gel-permeation chromatography (GPC). The modified epoxy network was evaluated for its impact, adhesive, tensile, flexural and thermal properties. Improvement in mechanical properties depends upon the concentration of the ATPEGB modifier. The optimum properties were obtained at 12.5 phr (parts per hundred parts of resin) concentration of the modifier. The ATPEGB modified cured epoxy was thermally stable up to 315°C. The morphology of cured epoxy was also analyzed by scanning electron microscopy (SEM) investigation of fracture surfaces.     

On the non-linear elastic stresses in an adhesively bonded T-joint with double support

Structures consisting of single or more materials, such as adhesive joints, may undergo large displacements and rotations under reasonably high loads, although all materials are still elastic. The linear elasticity theory cannot predict correctly the deformation and stress states of these structures, since it ignores the squares and products of partial derivatives of the displacement components with respect to the material coordinates. When these derivatives are not small, these terms result in a non-linear effect called geometrical non-linearity. In this study, the geometrically non-linear stress analysis of an adhesively bonded T-joint with double support was carried out using the incremental finite element method. Different T-joint configurations bonded to a rigid base and to a flexible base were considered. For each configuration, linear and geometrically non-linear stress analyses of the T-joint were carried out and their results were compared for different horizontal and vertical plate end conditions. The geometrically non-linear analysis showed that the large displacements had a considerable effect on the deformation and stress states of both adherends and the adhesive layer. High stress concentrations were observed around the adhesive free ends and the peak adhesive stresses occurred inside the adhesive fillets. The adherend regions corresponding to the free ends of the adhesive–plate interfaces also experienced stress concentrations. In addition, the effects of the support length on the peak adhesive and adherend stresses were investigated; increasing the support length had a considerable effect in reducing the peak adhesive and adherend stresses.     

Assessment and prediction of long-term mechanical properties of adhesives with high plasticity

Long-term life prediction for adhesives based on laboratory tests requiring only days, weeks, or months, is still a demanding challenge. Testing is carried out with the intention of simulating and often accelerating time-dependent aging effects that may occur in a joint during its lifetime. Initial strength values of bonded joints, such as shear or peel properties, can often be obtained from the adhesive manufacturers or retrieved from the literature. They are useful to compare different adhesives and to demonstrate the effect of parameters such as bondline thickness, overlap length or curing conditions and, in some cases, also the surface state. On the other hand, only few data are available relative to the long-term mechanical properties of adhesives with high plasticity. Due to the specific network structure of elastomer adhesives their viscoelastic-plastic deformation behavior is strongly time- and temperature-dependent. Therefore, the main objective of this paper is to illustrate methods for investigating the viscous properties of elastomer-based adhesives and creating basic data for the design and engineering of adhesive joints with enhanced plasticity.