International Journal of Adhesion and Adhesives (v.25, #5)

Synthesis and characterization of a new adhesion-activator for polymer surface by Dong Ri Zhang; Sun Wha Oh; Yong Pyo Hong; Young Soo Kang (371-378).
We developed a new type of adhesion-activator, Lichlor-6, consisting of lithium chlorohexylisocyanurate (LCI) and lithium dichloroisocyanurate (LDI), which can be soluble in water very well (more than 30 g/water 100 g), to replace trichloroisocyanuric acid (TCIA) which is only soluble in organic solvent. LDI acts as an adhesion-activator and LCI is used as additive to improve wetting ability of LDI on the polymer surface. The surfaces of styrene-butadiene rubber (SBR) and ethylene vinyl acetate (EVA) were treated with Lichlor-6 solution and bonded together using water-soluble polyurethane adhesive. Contact angle measurements, attenuated total reflectance-FTIR spectroscopy and scanning electron microscopy were carried out to characterize the treated surfaces. Tear resistance was also tested to compare adhesion strengths. After treatment with 16 wt% solution of Lichlor-6, the water contact angles decreased from 89° and 88° to 34° and 36° for SBR and EVA, respectively. The treatment with the higher concentration of Lichlor-6 produced the more marked chemical and morphology modifications on the SBR and EVA surfaces. The increased surface roughness of SBR and EVA caused by the treatment with Lichlor-6 have larger effects on the decrease of the contact angle than chemical modification. Tear resistance values obtained are 4.9, 6.5, 8.4 and 10.2 kgf/2 cm for the treatment with 0, 2, 5 and 8 wt% solutions of Lichlor-6, respectively, and the tested tear resistance is almost inversely proportional to the larger one between the two contact angles of SBR and EVA surfaces.
Keywords: Adhesion-activator; Trichloroisocyanuric acid; Styrene-butadiene rubber; Ethylene vinyl acetate;

The objective of the present work is to assess factors that influence thermo-mechanical stresses developed in adhesively bonded tools. Plain and filled adhesives, dry cutting and cutting with coolant, and the effectiveness of bond line thickness were investigated and their effect has been assessed. The finite element technique was adopted in the present work to study the influence of these factors. The present work showed that the thermal stresses developed in bonded tools decrease tremendously when cutting fluids are applied, however, the thermo-mechanical stresses seems to be unaffected with cutting fluid application.
Keywords: Thermo-mechanical stresses; Bonded tools; Finite element analysis; Cutting fluids;

Surface characterization of the specially prepared Toray high-strength sample carbon fiber was undertaken with a view to evaluate and understand adhesion to polymer matrices. Surface chemical analysis by XPS, surface free energy determination from the dynamic contact angle, and microstructural characterization by STM was conducted and compared with before and after surface treatment of this fiber and with other PAN-based high-strength carbon fibers as well. It was found that the surface of Toray sample carbon fibers has higher O/C and N/C ratios than those of other PAN-based carbon fibers, such as the IM-7 and AS-4. The fiber surface free energy appears slightly lower than those of the IM7 and AS4. Adhesion of Toray Lab-carbon fibers measured by the single fiber fragmentation was determined to be in a similar level to AS-4 and IM-7 in various epoxy-amine thermoset systems and polycarbonate thermoplastic matrix.
Keywords: Carbon fibers; Carbon fiber surface characterization; Single fiber fragmentation test; Adhesion;

Composite materials manufactured from woods with different adhesives are being used increasingly in the construction of furniture frames and buildings. Yet there is little information available concerning the dimensional stabilization and shear strength of laminated veneer lumbers (LVL) after being exposed to steam. In this study, LVLS were manufactured from pine (Pinus sylvestris L) and Black sea fir (Abies nordmanniana) veneers with Phenol-formaldehyde (PF), poly (vinyl acetate) (PVAc), Desmodur-VTKA (D-VTKA) and urea-formaldehyde (UF) adhesives. Dimensional stability of LVL were measured after being exposed to steam for 2, 6, 12, 24, 48 and 96-h-period according to the procedure of the Turkish Standards (TS) 3639, then shear strength tests were applied (based on BS EN 205). As a result, the highest density was determined for LVL with UF adhesive in both samples. The highest weight increment as 57.53% for black sea fir LVL with PF, the highest radial swelling as 9.74% for pine LVL with PF, the highest tangential swelling as 4.1% for pine LVL with VTKA and the highest longitudinal swelling as 0.44% for pine LVL with PF adhesive were determined after being exposed to 96 h steam. The highest shear strength was obtained in pine control samples with VTKA adhesive (5.36 N/mm2) and the lowest shear strength was obtained in Black sea fir LVL with PVAc adhesive (4.48 N/mm2).
Keywords: Adhesives for wood; Durability; Mechanical properties of adhesives; Composites; Steam;

The mechanisms governing adhesion are often complicated as adhesion properties depend on several physical and chemical parameters. Owing to the growing importance of adhesion in several fields of research, there are an increasing number of studies in the hope to bring a better understanding of these phenomena. The aim of the present paper is to show the correlation between the surface characteristics of composite materials and adhesion performances of corresponding surfaces assemblies. To this purpose, surface free energy assessment, roughness measurement, scanning electron microscopy (SEM) analysis, and infra-red Fourier transform (IRFT) coupled with microscopic study have been performed. Relationships between carbon/epoxy and glass/epoxy composite surface characteristics and results from single lap shear tests are established. Conclusions clearly show the importance of considering a polymer composite surface not only as a polymer entity, but also as a fibre/polymer unit.
Keywords: Epoxide; Composites; Surface roughness; Lap shear tests; Contact angle;

Finite element (FE) analyses were conducted for double lap, metal-to-metal bonded repair joints with different linear edge taper angles, or optimal taper profiles using the shape optimisation approach. Various adhesive failure criteria were applied and compared. A fracture mechanics approach was also applied in the FE analyses, in which an initial crack was pre-set at the edge of the adhesive bond line. The energy release rates were calculated and the results were used to evaluate the fatigue resistance. FE results predicted that the taper angle should strongly affect the fatigue performance of the repair patch. Compared with the 90° taper case, the peak stresses in the 6° taper case reduced by around 60%, and the stresses in the 3° taper case reduced by around 80%. The optimum design was able to reduce the peak stresses by about 50% compared with the widely used 6° linear taper (i.e. 1:10) with the same taper length. Thus, it appeared to be the best in terms of the fatigue resistance vs. taper region length. The calculated energy release rates also indicated a similar trend for the fatigue resistance as a function of the edge taper configuration. Experimental results agreed well with the numerical predictions. In particular, the predicted crack initiation loads, based on estimated threshold value of mode 1 energy release rate, correlate well with experimental results and the high fatigue resistance of the optimal design was confirmed.
Keywords: Finite element stress analysis; Fracture mechanics; Lap-shear; Fatigue; Repair;

Modeling of long-term creep behavior of structural epoxy adhesives by C.-W. Feng; C.-W. Keong; Y.-P. Hsueh; Y.-Y. Wang; H.-J. Sue (427-436).
The mechanical properties of polymeric materials change over time, especially when they are subjected to long-term loading regimes. It is imperative that reliable accelerated tests be developed to determine the long-term time-dependent performance of polymers under different environmental conditions. The long-term creep behaviors of a neat epoxy resin and a commercial structural adhesive for bonding aluminum substrates were investigated. The time–temperature superposition method produced a master curve, allowing for the long-term creep compliance to be estimated. The physics-based coupling model was utilized and found to fit well with the long-term creep master curve. The equivalence of the temperature and moisture effects on the creep compliance of the epoxy adhesives was also addressed. Finally, a methodology for predicting the long-term creep behavior of epoxy adhesives was proposed.
Keywords: A. Epoxy/epoxides; D. Creep; Coupling model; D. Viscoelastic;

Effect of nano-sized silver particles on the resistivity of polymeric conductive adhesives by Hsien-Hsuen Lee; Kan-Sen Chou; Zong-Whie Shih (437-441).
In this work, conductive adhesives were made by adding micro-sized silver flakes, mixed-sized silver particles or nano-sized silver colloids to the polyvinyl acetate (PVAc) emulsion. Film resistivity was then measured as a function of silver volume fraction. Our results indicated that the addition of nano-sized silver colloids to micro-sized Ag flakes usually increased its resistivity, probably due to increased contact resistance. Only near the percolation threshold, would the addition of nano-sized silver particles decrease the resistivity by helping to form the conductive path. Films made with only nano-sized silver colloids at the volume fraction of 0.848 showed a rather low resistivity of 1.93×10−4  Ω cm. Increase in heating temperature helped to form necks between particles and thus improve the conductivity to some extent.
Keywords: Nano-sized silver colloid; Micro-sized silver flake; Conductive adhesive;

The work described investigates the development of photoreagents as surface primers for the enhancement of adhesion with heavy-duty leather surfaces (Salz). Several reagents were investigated which included 4-azidophenol (1), 3-azidophenol (2), 4-(2,2′-iminodiethanol)-3-nitrophenylazide (4) and 4-azido-(2,2′-iminodiethanol)benzamide (6) and used as an intimate mixture with commercial polyurethane adhesive (Solibond PU39) to enhance the adhesion of Salz leather surfaces. The photoreagents upon UV irradiation covalently attach to the leather surface enriching it with new functionalities to provide sites for bonding with the adhesive layer. The adhesion strength of photo primer treated leather samples were tested using the T-peel test against abraded leather surfaces under dry and wet conditions. Compounds 4 and 6 on T-peel testing were found to enhance adhesion strength by 99% under dry conditions and 163% and 157%, respectively, under wet conditions. The observed failure type was cohesive within the substrate when photoreagents were used, suggesting covalent bonds formation at the interface in addition to the usual van der Waals forces and hydrogen bonds affecting adhesion.
Keywords: Coupling agents; Surface treatment; Destructive testing; Chromium tanned leather;

The curing process of polymeric diphenylmethane diisocyanate (pMDI) with wood is affected significantly by moisture in the wood since the reaction of isocyanate is highly sensitive to water. This study determined the curing kinetics of pMDI and wood mixtures containing various moisture contents by means of differential scanning calorimetry (DSC). Model-free analysis of the kinetics suggested that the curing reactions for the oven-dried wood/pMDI mixture are different from those of the mixtures with moisture. Peak temperatures indicated that the activation energy, reaction enthalpy, and reaction rate were lower for the oven-dried wood and resin mixture because the curing process was controlled by diffusion in the absence of moisture. The activation energy increased when moisture was present, but decreased when the moisture content further increased. Both the reaction enthalpy and reaction rate increased with the increase in moisture content and remained almost unchanged or increased slightly after the moisture content reached 12%.
Keywords: Adhesives for wood; Thermal analysis; Cure; pMDI;

The objective of this research was to investigate the effect of adding polyvinyl acetate (PVAc), for reducing the formaldehyde emission level, on the adhesion properties of melamine-formaldehyde (MF) resin for fancy veneer and plywood in engineered flooring. We controlled the hot-press temperature, time and pressure to determine the bonding strength and formaldehyde emission. Blends of various MF resin/PVAc compositions were prepared. To determine and compare the effect of PVAc content, 0, 30, 50, 70 and 100% PVAc floorings, by weight of MF resin, were used. Wheat flour, 25% by weight of adhesive, was added as material to increase the quantity. To determine the level of formaldehyde emission, we used the desiccator method. The formaldehyde emission level decreased with increased additions of PVAc. At a PVAc replacement ratio of only 30%, the formaldehyde emission level of the coated sample by UV-curable coat was under E1 grade. Curing of the high MF resin content in this adhesive system (MF resin with PVAc) was well processed indicating that the bonding strength was increased. In the case of PVAc only, the bonding strength was much lower due to the already high temperature of 120 °C. The adhesion layer was broken by high temperature and pressure. The sample with 30% PVAc added to MF resin (MF resin: PVAc=70:30) showed good bonding strength compared with MF resin only in all cases, hot-press temperature, time, pressure and boiling test.
Keywords: PVAc; Bonding strength; Formaldehyde emission; Flooring; Fancy veneer;