Bond quality at the FRP–wood interface using wood-laminating adhesives

Fibre-reinforced polymer (FRP)-strengthened glulam would be a more economically viable product if a single adhesive type could be utilised at all the bonded interfaces. This paper describes a test programme that examines the hygrothermal compliance of five commercial wood-laminating adhesives when bonding commercially viable FRP materials to wood. It was seen that the integrity of the bond depended not only on adhesive type but also on the FRP type under consideration. For one of the FRP types, moisture-cycled FRP–wood bonded specimens obtained high wood failure percentages and good shear strength results that compared well with non-moisture-cycled FRP–wood specimens, non-moisture-cycled wood–wood bonded specimens and solid control specimens taken from the same board. This encouraging result suggests an alternative to the expensive structural epoxy adhesives, which are generally accepted as the appropriate adhesive in FRP-strengthened glulam.     

Bonding of FRP materials to wood using thin epoxy gluelines

The use of fibre-reinforced polymers (FRPs) in the construction industry is becoming increasingly common. One application of these materials is in the stiffening and strengthening of glue-laminated timber. The research programme discussed in this paper examined the bonding of commercially available FRPs to wood using three commercial epoxy adhesives. The programme involved comparative testing of non-moisture cycled FRP–wood specimens, non-moisture cycled wood–wood bonded, and solid control specimens with moisture cycled FRP–wood specimens all manufactured using wood from the same boards. Findings showed that with specific adhesives, cost-effective thin bondlines have the capacity to resist severe hygrothermal stresses imposed at the FRP–wood interface. It was further noted that the integrity of the bond depended not only on the epoxy adhesive in question but also on the FRP type.     

Effect of adhesive type on Strengthening-By-Stiffening for shear-deficient thin-walled steel structures

Strengthening-By-Stiffening (SBS) is a novel technique whose purpose is to improve structural strength by stiffening buckling-prone regions in thin-walled steel structures using pultruded composite sections. A proof of concept study showed that SBS can achieve gains in shear strength of up to 56% using glass fiber reinforced polymers (GFRP) sections. This paper presents experimental results showing the effect of adhesive type on the efficiency of SBS for shear-deficient thin-walled steel beams. Specimens strengthened with two adhesive types were tested; a generic type (Type I) that is typically used for FRP-strengthening of concrete structures and a relatively new type (Type II) that is particularly promoted for steel structures. Like most FRP-strengthened structures, a debonding failure mode was observed for SBS specimens strengthened using adhesive Type I. Conversely, specimens strengthened using adhesive Type II did not fail by debonding, but rather by buckling of the smaller (less slender) shear panels. The resulting ductile failure mode is uncommon for FRP strengthening techniques and can lead to new applications of FRP strengthening for steel structures that were not possible using more brittle adhesives with lower capacity to absorb inelastic energy.     

Bonding adhesive joints with radio-frequency dielectric heating

The feasibility of using radio-frequency (RF) dielectric heating to cure thermoset adhesives has been evaluated. Thermoset and thermoplastic polymer panels have been bonded to steel using conventional one-and two-part epoxy and two-part urethane adhesives. Process cycle times for adhesive cure using RF heating were about 20 to 60 seconds, compared with about 20 to 30 minutes for the same materials using conventional oven-cure methods. Thermoset substrates bonded included glass fibre-reinforced composite panels based on sheet moulding compound (SMC) and resin transfer moulding styrene-vinylester (RTM). Thermoplastics such as polycarbonate, polyarylate, Noryl, ABS and polymethylmethacrylate were also successfully bonded. RF bonding experiments were performed by preparing and testing lap-shear joints as well as by joining a large test structure with a 25 mm x 1.25 m bondline. Bonding of painted steel to SMC composite, SMC to SMC and steel to RTM composite were also accomplished using the RF dielectric heating process.     

Effects of adhesive type and polystyrene concentration on the shear strength of bonded polystyrene/high-density polyethylene blends

The lap-shear strengths of adhesively bonded polystyrene (PS), high-density polyethylene (HDPE), and their blends, were studied as a function of adhesive type and blend composition. The performance of virgin and recycled polymer systems was examined. The lap-shear strength depended strongly on the amount of PS in the blend and the type of adhesive, and the acrylic adhesives demonstrated the best performance for all compositions. Bonded strengths of HDPE increased by approximately 50% when HDPE was blended with 34% PS, the co-continuous composition. The results indicate that structural elements made from PS/HDPE immiscible blends may be effectively bonded with adhesives without expensive surface treatments.     

碳纤维布浸胶及与钢粘接用胶的性能研究

研究了用以浸碳纤维布的环氧胶及粘接浸胶碳布与钢的胶粘剂组成及粘接性能。发现高温性能主要取决于浸碳布胶,室温性能较好的胶,高温性能则明显偏低。制成了室温～250℃剪切强度都在10MPa以上,300℃为8～9MPa的浸碳布胶和粘接用胶。该胶具有不燃性、无烟,以及优异的耐烧蚀性能。     

Development of metal-bonding spray adhesives for use in aerospace repair applications

In many aerospace applications, the use of film and paste adhesive materials is not conducive to either good manufacturability or good bonding properties. To address this concern, the development of high-performance, epoxy-based, spray-adhesive resins has been undertaken from a research and development standpoint. These materials were investigated in the light of their potential repair applications, with focus on adhesive joint designs based on lap-shear performance and processibility of the adhesive. It was found that the resins investigated displayed adequate lap-shear strengths for the bond types of interest. However, some resins proved to be a considerable challenge during processing and solvent removal presented a problem for some materials. As expected, the combined use of mechanical testing and scanning electron microscopy revealed that the lap-shear strengths of the resins could be tailored with changes in their chemistry, and that these changes also affected the solvent removal process.     

Fracture and fatigue response of a self-healing epoxy adhesive

A self-healing epoxy adhesive for bonding steel substrates is demonstrated using encapsulated dicyclopentadiene (DCPD) monomer and bis(tricyclohexylphosphine)benzylidine ruthenium (IV) dichloride (Grubbs’ first generation) catalyst particles dispersed in a thin epoxy matrix. Both quasi-static fracture and fatigue performance are evaluated using the width-tapered-double-cantilever-beam specimen geometry. Recovery of 56% of the original fracture toughness under quasi-static fracture conditions occurs after 24 h healing at room temperature conditions. Complete crack arrest is demonstrated for fatigue test conditions that render neat resin and control samples failed. Inspection of fracture surfaces by electron microscopy reveals evidence of polymerized DCPD after healing. These results are the first mechanical assessment of self-healing for thin (ca. 360 μm) films typical of adhesives applications.     

The chemical,kinetic and mechanical characterization of tannin-based adhesives with different crosslinking systems

Network formation, cure characteristics and bonding performance of tannin-based resins were investigated in order to establish structure–property relationships between the stage B and stage C. Tannin–aldehyde and base-catalyzed autocondensed tannin resins were synthesized and characterized for molecular weight distribution, cure kinetics and cure chemistry by means of GPC, DMA and ¹³C CP/MAS NMR spectroscopy and solvent stability tests. The resins performance as wood adhesives was further established from lap-shear tests and microscopic observation of the bondline. Resins prepared with highly reactive aldehydes, such as formaldehyde or glyoxal, exhibited a significant extent of hetero-condensation reactions, fast cure kinetics, a high storage modulus and good solvent stability of the stage C-resin. In contrast, resins prepared with bulky aldehydes of low reactivity, such as citral, were dominated by autocondensation reactions, and exhibited slower cure kinetics, a lower storage modulus and solvent-stability of the stage C-resin, alike those neat autocondensed tannin resins. However, all resin systems fulfilled the standard requirements for wood adhesive bonding for interior applications. Additionally, storage modulus increase during cure was found to be a good predictor of the stiffness of the wood-bonded assembly, useful to discriminate between the autocondensation and heterocondensation cure chemistries.     

Adhesion Studies of Mixtures of Ethyl Cyanoacrylate with a Difunctional Cyanoacrylate Monomer and with other Electron-deficient Olefins

Alkyl cyanoacrylate instant adhesives are widely used because of their fast cure speed and versatility on a large number of substrates. Recent performance improvements, such as increased thermal resistance, resulted from the addition of latent acids and polymers, which do not copolymerize with the adhesive monomer, to the adhesive formulations. However, use of these additives can increase fixture time or reduce the final adhesive strength.

Two methods for possibly improving alkyl cyanoacrylate instant adhesives, without loss of cure speed or adhesive properties, could be either crosslinking the alkyl cyanoacrylate monomer with a dicyanoacrylate or copolymerizing it with a second 1, 1 disubstituted electron-deficient olefin. A crosslinker. 1,4 butanediol dicyanoacrylate (BDDCA) and two monofunctional monomers, diethyl methylenemalonate (DEMM) and N,N diethyl-2-cyanoacrylamide (DECA), were prepared, in good purity, for adhesion studies with ethyl cyanoacrylate (ECA). Crosslinking ECA with BDDCA does improve solvent resistance, as determined by solvent swelling experiments. Glass fixture times are approximately the same for ECA, crosslinked ECA, the pure monomers, and monomer mixtures with ECA, while steel fixture times are generally slower. Crosslinking ECA with BDDCA does not improve lap-shear adhesion, either at room temperature or after thermal exposure at 121°C. Lap-shear strength data, before and after heat exposure, revealed that the ECA/DEMM and the ECA/DECA monomer mixtures exhibit weaker lap-shear adhesive strength than ECA alone.     

Peel resistance of adhesive joints with elastomer–carbon black composite as surface sensing membranes

The peel resistance of four adhesives (“J-B Weld” by J-B Weld (adhesive A), 3 M Scotch-Weld DP 125 Gy (adhesive B), Loctite PL Premium (3x) Construction Adhesive (adhesive C), and Henkel Hysol EA9394 (adhesive D)) is investigated for their bonding performance of a styrene‐ethylene/butylene‐styrene– carbon black (SEBS–CB) composite membrane used in structural health monitoring (SHM) applications. Tests are performed on membrane samples bonded on four common structural materials, namely aluminium, steel, concrete, and fiberglass, to obtain the peel resistance of adhesives. Results show that adhesive B has the highest strength for aluminium, steel, and fiberglass substrates, and that adhesive C has the highest strength for the concrete substrate. The performance is also evaluated versus adhesive cost, a critical variable in SHM applications. Here, adhesive C performed best for all substrates. Lastly, membrane residuals resulting from the peel tests are compared. Tests show that Adhesive B resulted in the highest residual percentage for aluminium, while adhesive C performed better for all other substrates. However, membrane residuals for adhesive C do not show a positive correlation with the peel resistance.     

Debonding failure characteristics of multi-laminated bonding system under cryogenic temperature

In the present paper, the debonding failure characteristics of epoxy and polyurethane adhesives embedded using a multi-laminated bonding system (MLBS) under cryogenic temperatures are evaluated. A series of pull-off and lap-shear tests under various temperatures (293, 163, and 110 K) are performed to identify the debonding failure characteristics of the MLBS. A modification of the Ramberg-Osgood constitutive model is carried out to predict the temperature-dependant elastic-inelastic stress-strain behaviour of the MLBS. The temperature-dependant material parameters are predicted using an empirical formulation. The debonding failure phenomenon is also studied.     

Stress Analysis of Adhesively Bonded Electroprimed Steel Lap Shear Joints

Structural applications for adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for precise numerical modeling of adhesive joint behavior, particularly along bondline interfaces where low surface energy adhesives contact high surface energy metallic oxides. The purpose of the present study is to determine the effect of electrodeposited organic paint primer (ELPO) on the stress and strain distributions within an adhesively bonded single-lap-shear joint. Initial experimental studies have shown that bonding to ELPO-primed steel adherends has enhanced strength and durability characteristics compared to conventional bonds to unprimed steel surfaces. Recent studies based on finite element analysis of varied single-lap-shear joint moduli and thicknesses, and subsequent testing of joints with two different adhesive moduli, have indicated the mechanisms involved in this phenomenon. The presence of the ELPO-primer reduced peak peel and shear stresses and allowed for more uniform stress distribution throughout the joint.     

Stress Analysis of Adhesively Bonded Electroprimed Steel Lap Shear Joints

Structural applications for adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for precise numerical modeling of adhesive joint behavior, particularly along bondline interfaces where low surface energy adhesives contact high surface energy metallic oxides. The purpose of the present study is to determine the effect of electrodeposited organic paint primer (ELPO) on the stress and strain distributions within an adhesively bonded single-lap-shear joint. Initial experimental studies have shown that bonding to ELPO-primed steel adherends has enhanced strength and durability characteristics compared to conventional bonds to unprimed steel surfaces. Recent studies based on finite element analysis of varied single-lap-shear joint moduli and thicknesses, and subsequent testing of joints with two different adhesive moduli, have indicated the mechanisms involved in this phenomenon. The presence of the ELPO-primer reduced peak peel and shear stresses and allowed for more uniform stress distribution throughout the joint.     

Water-based adhesive formulations for rubber to metal bonding developed by statistical design of experiments

Waterborne adhesives for rubber to metal bonding have been available since 1990. However, published information about their formulation has been limited, as proprietary restrictions are exercised by companies. As a consequence, the way these adhesives interact with substrates has not been studied extensively. With the aim of investigating the effect the components of a waterborne adhesive have on rubber to metal bonding, fractional factorial and surface response methodologies of design of experiments were employed in this study. Twenty six formulations were prepared with a polychloroprene latex as the adhesive polymer. Viscosity, wettability and non-volatile solids content were measured with each liquid adhesive, while the mechanical strength was evaluated by applying a tensile mechanical stress over cured solid adhesive films. Adhesion properties were evaluated by using a single lap-shear test on metal to metal joints and a pull-out test on rubber to metal joints. The results showed that the components with the largest relative influence on cohesive and adhesives forces were tackifier resin, silicon dioxide and polychloroprene latex type. In order to better understand the contributions of these variables, mathematical models correlating them with the response variables were obtained. This study is valuable in explaining how, through statistical methods, a waterborne adhesive for rubber to metal bonding can be formulated with a reasonably low number of experiments.     

Bonding performance after aging of fusion bonded hybrid joints

The investigations of this article are showing the bonding performance after aging of hybrid fusion bonds in combination with a laser pre-treatment. The investigated materials are a galvanized steel (HX340 LAD Z100MB) and two glass fiber reinforced Polyamide 6 materials. In order to achieve a structural strength of the fusion bond a laser pre-treatment is used. The investigation is focusing on the changes from the laser pre-treatment to the galvanized surface by analyzing the steel surface with a scanning electron microscope, energy disperse X-ray spectroscopy, micro sections and surface roughness measurements. The composition of the applied thermoplastic materials is not in the focus of the article which is why different manufacturers for the fiber reinforced thermoplastic material have been chosen. The aging of the samples is done by a climate change (PV1200) and a salt spray (PV1210) test in order to evaluate different aging mechanisms. Furthermore the investigation is providing a crucial finding by showing the impact of a batch change on the achievable lap-shear strength by comparing two batches from the same manufacturer. The results of the laser surface pre-treatment show that the zinc coating of the steel adherend is reduced significantly and does not provide a closed coating. The climate change test after 100 cycles showed no decrease of lap-shear strength compared to the reference when the highest investigated pre-treatment intensity is applied to the surface. The salt spray test showed a corrosion of the pre-treated area for laser pre-treatment settings which generate a low amount of oxygen on the surface. The pre-treatment settings generating a more oxidized surface (medium and high intensity) showed only a minor influence on the achievable lap-shear strength after 90 cycles.     

Synthesis and properties of polyurethane wood adhesives derived from crude glycerol-based polyols

Crude glycerol, a waste stream of the biodiesel production process, is low-cost renewable feedstock for the production of chemicals and polymers. In this study, polyurethane (PU) adhesives were synthesized from crude glycerol-based polyols (CG-based polyols) for wood bonding applications. Effects of different variables, including hydroxyl values of CG-based polyols, chain extenders, and the molar ratio of NCO/OH on the properties of PU adhesives were investigated. The chemical structures of PU adhesives were characterized, and their thermal, mechanical, and chemical resistance properties were evaluated. The experimental results indicated that an increase of the NCO/OH molar ratio (1.3) substantially improved bonding strength by up to 38 MPa. Higher thermal stability and stronger chemical resistance to hot and cold water and to alkali and acid solutions were observed comparing to vegetable oil-based adhesives. However, the effect of the hydroxyl value of polyols on bonding strength was not significant. Additionally, bond strength of crude glycerol-based PU adhesives was comparable to that of some commercial PU wood adhesives. All these properties demonstrated the potential of CG for PU wood adhesive applications, particularly for fast-curing uses.     

The Effect of Adhesive Thickness on the Mechanical Behavior of a Structural Polyurethane Adhesive

One parameter that influences the adhesively bonded joints performance is the adhesive layer thickness. Hence, its effect has to be investigated experimentally and should be taken into consideration in the design of adhesive joints. Most of the results from literature are for typical structural epoxy adhesives which are generally formulated to perform in thin sections. However, polyurethane adhesives are designed to perform in thicker sections and might have a different behavior as a function of adhesive thickness. In this study, the effect of adhesive thickness on the mechanical behavior of a structural polyurethane adhesive was investigated. The mode I fracture toughness of the adhesive was measured using double-cantilever beam (DCB) tests with various thicknesses of the adhesive layer ranging from 0.2 to 2 mm. In addition, single lap joints (SLJs) were fabricated and tested to assess the influence of adhesive thickness on the lap-shear strength of the adhesive. An increasing fracture toughness with increasing adhesive thickness was found. The lap-shear strength decreases as the adhesive layer gets thicker, but in contrast to joints with brittle adhesives the decrease trend was less pronounced.     

氯盐干湿循环作用下嵌入式CFRP筋与混凝土界面粘结性能研究

海岛工程建设中,潮汐作用下的海水干湿循环是影响纤维增强复合材料(FRP)加固混凝土结构耐久性能的主要因素之一。针对碳纤维增强复合材料(CFRP)筋嵌入法加固混凝土结构,通过CFRP筋从混凝土块中的拔出试验研究高性能纤维增强水泥基复合材料(HPFRC)和环氧树脂胶作为加固粘结材料时,海水干湿交替循环作用下嵌入式CFRP筋与混凝土界面的粘结性能,并对其破坏模式、极限粘结承载力进行分析。结果表明:环氧树脂胶作为粘结材料时,随干湿交替循环次数的增加,极限粘结承载力略有下降;而HPFRC作为粘结材料时,干湿循环90 d内的极限粘结承载力得到持续增长,可以达到作为粘结材料时极限承载力的70.3%,因此HPFRC可以作为粘结材料应用于潮汐环境下的FRP筋嵌入法加固海工混凝土结构中。     

Process-induced effects in thin-film bonding of PEKEKK in metal-polymer joints

In this work, poly ether ketone ether ketone ketone (PEKEKK) was used as a thermoplastic adhesive for joining metals. Titanium and cobalt chromium alloys were joined to form tensile butt joints. These tensile specimens were used to evaluate bond performance. A controlled thermal processing cycle was used to modify and enhance the polymer crystallinity during bonding. The resulting effects on bond performance were examined. The process window for a thermal bonding process was identified. Factorial experiments were conducted to determine the effects of modifications to adhesive and adherend material, and bonding pressure on bond performance. A titanium alloy and a cobalt chromium alloy were used as the adherends. Understanding changes in the thermoplastic adhesive joint with the variation of process parameters will allow for proper application and processing of thermoplastic structural adhesives.