Relationship between Phenolic Adhesive Chemistry, Cure and Joint Performance. Part I. Effects of Base Resin Constitution and Hardener on Fracture Energy and Thermal Effects During Cure

In this study the relationships between the composition of phenol resorcinol-formaldehyde resins and paraformaldehyde concentration in the adhesive were explored, using DSC, IR, GPC, and solubility measurements. Differences of chemical composition between base resins and adhesives were compared to the fracture toughness of adhesive bonds.

The cure temperature and cure time effects upon fracture toughness were also investigated. Fracture toughness tests were performed with bonded hard maple tapered double-cantilever beam cleavage specimens.     

The Effects of Cure Temperature and Time on the Bulk Fracture Properties of a Structural Adhesive

The purpose of this investigation is to determine experimentally the possibility of optimizing the room temperature bulk fracture properties of structural adhesives with respect to cure temperature, time and cool-down conditions. The model adhesive, Metlbond, is a solid film modified nitrile epoxy resin supplied in two forms: Metlbond 1113 (supported with synthetic fabric carrier cloth) and Metlbond 1113-2 (without carrier cloth). The effects of carrier cloth on the bulk fracture properties are investigated as well. The uniaxial tensile strength and rigidity values were determined over a wide range of cure temperatures and times with fast and slow cool-down conditions during a previous investigation by the authors. For the present investigation, the fracture toughness of the model adhesives, subjected to opening mode failure, are experimentally determined, with the use of single-edge-cracked specimens, for different cure and cool-down conditions. It is found that the optimum fracture toughness values are obtained at low temperature-long time cure conditions in the absence of carrier cloth when slow cool-down condition is employed. Using the elastic-plastic material behavior assumption, it is shown that an average crack tip plastic zone radius can be determined using the fracture toughness and tensile strength values obtained corresponding to a given cool-down condition. These average plastic zone radii values are used along with the available tensile rigidity values to evaluate the optimum fracture energies of the model adhesives for a number of cure schedules. It is found that the optimum fracture energy levels are obtained at high temperature-short time cure conditions, using slow cool-down in the absence of carrier cloth.     

Fracture and structure of highly crosslinked polymer composites

The fracture toughness of silica filled dimethacrylate resins has been found to increase with the degree of cure of the network, and was previously attributed to increased crosslinking or the concomitant reduction in unreacted monomer (sol species). Studies of the dependence of the polymerization on the photoinitiator concentration and cure time of dimethacrylate resins illustrate the interrelation of the wt % sol and the degree of cure and reveal that a considerable amount of residual monomer remains in undercured networks, as predicted by simple gelation theory. In an attempt at separating the effects of residual monomer and crosslinking on the fracture behavior, several series of filled dimethacrylate and epoxy resins were studied in which the crosslink density and sol levels were independently varied without introducing significant changes in the chemical composition of the network. Although the fracture energy and toughness were raised by increasing the crosslink density in the epoxy resins, no significant variation was found for the dimethacrylates. Saturated analogues of dimethacrylates (used to represent residual monomer) significantly impaired the fracture resistance, suggesting that the reduction in residual monomer is responsible for the improved fracture toughness observed with postcured dimethacrylate networks.     

Microstructural effects and the toughening of thermoplastic modified epoxy resins

We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol-A (DGEBA), modified with phenolic hydroxyl-terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near-infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two-step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔG_IC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.     

Comparison of laboratory techniques for evaluating the fracture toughness of glassy polymers

Several test methods were employed to determine polymer fracture toughness (??_Ic, the opening-mode strain energy release rate) at room temperature. The materials used included DGEBA epoxies and those modified by the addition of CTBN elastomers. Double-cantilever beam specimens were used to determine the fracture toughness both of bulk resins and of an adhesive layer bonded between two aluminum half-beams. The adhesive fracture toughness of a 0.025-cm bond was slightly less than the bulk ??_Ic value, attributed to the bond thickness effect. Fracture toughness of bulk resins was also evaluated by using both rectangular and round compact tension specimens. The results, when compared with those obtained with the bulk double-cantilever beams, are quite acceptable. The thickness of compact tension specimens, ranging from 0.64 to 1.0 cm, might not give pure plane-strain conditions, and thus some plane-stress contribution to ??_Ic should be expected for the tougher materials. Izod impact tests were also carried out to determine sample fracture toughness at high loading rate.     

Deoxybenzoin-based epoxy resins

The diepoxide of bishydroxydeoxybenzoin, termed BEDB, was prepared and used as a diepoxide in adhesive formulations with various aromatic diamine cross-linkers. These novel epoxy resins were characterized and compared to the properties of bisphenol A (BPA)- and 3,3′,5,5′-tetrabromobisphenol A (TBBA)-based epoxies in terms of their thermal and mechanical properties. Cured formulations were characterized to determine glass transition temperatures by differential scanning calorimetry (DSC). The char residue, heat release capacity, dynamic mechanical properties, fracture toughness, and adhesion strength of the cured resins were investigated by thermogravimetric analysis (TGA), microscale combustion calorimetry, dynamic mechanical analysis (DMA), plain-strain fracture toughness tests, and lap shear tests. The BEDB-based resins exhibited significantly higher fracture toughness and adhesion strength compared to the BPA-epoxy resins, as well as low heat release properties (i.e., lower flammability) despite the absence of halogen.     

Relationship between phenolic adhesive chemistry and adhesive joint performance: Effect of filler type on fraction energy

The performance of adhesive bonded joints depends on many factors, one of which is the adhesive formulation. The effects of organic and inorganic fillers upon the fracture toughness of phenol–resorchinol–formaldehyde adhesive in hard maple joints were explored in this study. Analytical techniques (DSC, IR, SEM, and GPC), and solubility studies were employed to relate physical effects to chemical effects of teh fillers. The resin showed two distinct stages of cure: (1) a low temperature exotherm associated with resorcinol and paraformaldehyde and (2) a high temperature endotherm associated with the base resin. Filler was found not to affect the cure. Fillers did have a profound effect on the morphology of the wood-adhesive interphase and upon the bulk adhesive properties. These effects, revealed in both SEM and fracture toughness studies, are discussed at lenghth.     

Off-Optimum Cure of Aerospace Epoxy Adhesives

Four epoxy film adhesives used in aircraft manufacture and repair have been examined to establish the effect of deviation from the cure cycle specified by the manufacturer. In addition to the variation of the cure cycle, two surface preparations of the aluminium adherends (chromic acid etch or grit blast followed by silane treatment) were evaluated. Thermal analysis was used to examine the cure envelope of the adhesive, and its extent of cure and glass transition temperature. The adhesive properties were assessed by shear strength (in both single lap joints and in Iosipescu configuration), durability (Boeing wedge test) and chemical resistance to selected aggressive fluids. The sensitivity of the performance of a particular adhesive to offoptimum cure conditions depends on its composition and needs to be determined, not predicted.     

Off-Optimum Cure of Aerospace Epoxy Adhesives

Four epoxy film adhesives used in aircraft manufacture and repair have been examined to establish the effect of deviation from the cure cycle specified by the manufacturer. In addition to the variation of the cure cycle, two surface preparations of the aluminium adherends (chromic acid etch or grit blast followed by silane treatment) were evaluated. Thermal analysis was used to examine the cure envelope of the adhesive, and its extent of cure and glass transition temperature. The adhesive properties were assessed by shear strength (in both single lap joints and in Iosipescu configuration), durability (Boeing wedge test) and chemical resistance to selected aggressive fluids. The sensitivity of the performance of a particular adhesive to offoptimum cure conditions depends on its composition and needs to be determined, not predicted.     

Model Adherend Surface Effects on Epoxy Cure Reactions

Adherend surface effects on the amine cure of epoxy resins were investivated using finely divided aluminum oxide as high surface area models for aluminum. Calorimetric analysis of simplified crosslinking systems revealed significantly faster reactions which led to lower glass transition temperature materials for activated aluminum oxide filled samples. A monofunctional amine and epoxy were then utilized to obtain soluble reaction products amenable to molecular characterization. These studies similarly showed an increase in the rate of epoxy consumption in the presence of activated aluminum oxide which was attributed to both an increase in the rate of amine addition to epoxy as well as to epoxy homopolymerization. The latter was not observed in the unfilled mixtures. Such changes in reaction mechanism at the adherend surface have implications for the strength and durability of actual adhesive bonds.     

Model Adherend Surface Effects on Epoxy Cure Reactions

Adherend surface effects on the amine cure of epoxy resins were investivated using finely divided aluminum oxide as high surface area models for aluminum. Calorimetric analysis of simplified crosslinking systems revealed significantly faster reactions which led to lower glass transition temperature materials for activated aluminum oxide filled samples. A monofunctional amine and epoxy were then utilized to obtain soluble reaction products amenable to molecular characterization. These studies similarly showed an increase in the rate of epoxy consumption in the presence of activated aluminum oxide which was attributed to both an increase in the rate of amine addition to epoxy as well as to epoxy homopolymerization. The latter was not observed in the unfilled mixtures. Such changes in reaction mechanism at the adherend surface have implications for the strength and durability of actual adhesive bonds.     

Solvent Effects on Cure 1-Benzyl Alcohol on Epoxy Cure

The effect of benzyl alcohol and phenol on the cure of epoxy resins is reported. The epoxy resin, a blend of the diglycidyl ethers of bisphenol A and bisphenol F, was cured with 4,4′-methylenebis(cyclohexylamine) diamine. Measurements of the cure and vitrification times were obtained using a vibrating plate curometer. Mechanical properties were assessed using dynamic mechanical thermal analysis (DMTA). Time temperature transformation (TTT) diagrams were constructed. Benzyl alcohol lowers the viscosity, aids cure and plasticizes the final product. Phenol and triethylenetetramine [TETA] significantly enhance the rate of cure and have a catalytic effect.     

Modulation of Adhesion at Silicone Elastomer-Acrylic Adhesive Interface

To characterize the role of small silica like nanoparticles (MQ resins) in the modulation of adhesion at polydimethyl siloxane (PDMS) elastomers-acrylic adhesive contacts, we have designed systems in which the roles of MQ resins in enhancing interactions at the interface and in increasing viscoelastic dissipations in the elastomer layer could be separated. First, the contact between elastomers with various MQ resin contents and PDMS layers made of densely grafted short chains has been investigated through Johnson-kendall-Roberts (JKR) tests, in order to characterize how the dissipations in the elastomer depend on the resin content. The same elastomers in contact with thin-surface-anchored acrylic layers were then tested through JKR tests to determine the role of enhanced interactions in the modulation of adhesion at the interface due to the resins. In these experiments, the thickness of the acrylic layer was kept small enough so that dissipations in the acrylic adhesive could be neglected. Both G₀, the adhesive strength at zero fracture velocity, and G( V), the velocity-dependent fracture toughness, strongly depend on the MQ resin content and on the contact time, suggesting the progressive building of strong interactions between acrylic and elastomer chains.     

The Effects of Cure Temperature and Time on the Stress-Whitening Behavior of Structural Adhesives. Part II. Analysis of Fractographic Data

In this second part of the paper on the effects of cure conditions on the stress-whitening behavior of structural adhesives, fractographic data are presented and discussed. For this purpose, the size and nature of crack-tip-whitening zones obtained using single edge notched tension specimens of the model adhesive Metlbond with and without carrier cloth are studied. Scanning electron photomicrographs are utilized to investigate the effects of cure temperature and time on the crack-tip stress-whitening behavior. A brief discussion on the formation of inherent voids during the cure process is also presented since they are observed to enhance the crack-tip-whitening zones. Experimental results reveal that both the extent of voids produced during the cure process and the size of the crack-tip-whitening zones on the fracture surface increase with increasing cure temperatures. The presence of carrier cloth produces a similar effect with increases in both inherent voiding and stress-whitening sizes. The creation of stress-whitening zones are linked to stable crack propagation in those areas.     

The influence of the core–shell structured meta-aramid/epoxy nanofiber mats on interfacial bonding strength and the mechanical properties of epoxy adhesives at cryogenic environment

The adhesives for adhesively bonded joints at cryogenic environment should be enhanced by reinforcement with low coefficient of thermal expansion (CTE) and high fracture toughness because the materials become quite brittle at cryogenic temperature. Aramid fibers are noted for their low CTE and have been used to control the CTE of thermosetting resins. However, aramid composites exhibit poor adhesion between the fibers and the resin because the aramid fibers are chemically inert and contain insufficient functional groups. In this work, core–shell structured meta-aramid/epoxy nanofiber mats were fabricated by electrospinning with polymer blending method to improve the interfacial bonding between the adhesive and the fibers under cryogenic temperature. The CTE of the epoxy adhesives reinforced with modified nanofiber mats was measured, and the effect on the adhesion strength was investigated at single lap joints at cryogenic temperature. The fracture toughness of the adhesive joints was measured using a double cantilever beam (DCB) test.     

Modulation of Adhesion at Silicone Elastomer–Acrylic Adhesive Interface

To characterize the role of small silica like nanoparticles (MQ resins) in the modulation of adhesion at polydimethyl siloxane (PDMS) elastomers–acrylic adhesive contacts, we have designed systems in which the roles of MQ resins in enhancing interactions at the interface and in increasing viscoelastic dissipations in the elastomer layer could be separated. First, the contact between elastomers with various MQ resin contents and PDMS layers made of densely grafted short chains has been investigated through Johnson–kendall–Roberts (JKR) tests, in order to characterize how the dissipations in the elastomer depend on the resin content. The same elastomers in contact with thin-surface-anchored acrylic layers were then tested through JKR tests to determine the role of enhanced interactions in the modulation of adhesion at the interface due to the resins. In these experiments, the thickness of the acrylic layer was kept small enough so that dissipations in the acrylic adhesive could be neglected. Both G ₀, the adhesive strength at zero fracture velocity, and G ( V), the velocity-dependent fracture toughness, strongly depend on the MQ resin content and on the contact time, suggesting the progressive building of strong interactions between acrylic and elastomer chains.     

Structural heterogeneity in epoxies

The effects of different cure procedures on the structure and properties of epoxy samples made from diglycidyl ether of bisphenol A (DGEBA) and mixtures of two linear aliphatic diamines were studied. The elastic modulus, fracture toughness, impact resistance, and glass transition temperature were determined for various cure schemes. The morphologies of the cured resins were characterized with small angle X-ray scattering. The results show that samples with the same average morphology (molecular network structure) have similar elastic moduli and glass transition temperatures. If some heterogeneity is introduced in the molecular network structure without changing the average structure, however, the experiments indicate that the toughness can be increased without significantly sacrificing other properties.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

Cure characterization of soybean oil—Styrene—Divinylbenzene thermosetting copolymers

Bio‐based resins are an alternative to petroleum‐based resins in the production of fiber‐reinforced polymers (FRPs) by processes such as pultrusion. A detailed understanding of the cure behavior of the resin is essential to determine the process variables for production of FRPs. In this work, the cure kinetics of soybean oil‐styrene‐divinylbenzene thermosetting polymers is characterized by differential scanning calorimetry (DSC) measurements. By varying the concentration of the cationic initiator from 1 to 3 weight percent (wt %), the most viable resin composition for pultrusion is identified. The ability of phenomenological reaction models to describe the DSC measurements for the optimum resin composition is tested and kinetic equations, which can be used to determine the degree of cure at any temperature and time, are determined. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009