Direct Correlation Between Adhesion Promotion and Coupling Reaction at Immiscible Polymer-Polymer Interfaces

Hugh Brown has shown that interfacial entanglements govern adhesion between two polymers. We demonstrate this for three systems by adding interfacial chains via chemical coupling. The adhesion between polypropylene (PP)/amorphous polyamide (aPA) was reinforced by the coupling reaction of maleic anhydride grafted PP (PP-g-MA) and the primary amine groups on aPA; huge increases in adhesion were observed. A good correlation between critical fracture toughness, G _c , and PP-g-MA concentration squared follows Brown's crazing mechanism. For a polystyrene (PS)/aPA interface reinforced by the coupling reaction of poly(styrene-r-maleic anhydride) (PS-r-MA)/aPA only modest adhesion increases in G _c were observed through the whole PS-r-MA concentration range. This different behavior of G _c vs. functional polymer concentration is believed to be caused by segregation of the formed graft copolymers at the interface. The relationship between G _c and the extent of coupling was studied quantitatively with a model PS/PMMA system. The interface was reinforced by the coupling reaction of 0–10% PS-NH₂/PMMA-anh. G _c was measured with the asymmetric dual cantilever beam test (ADCB) and the amount of copolymer formed at the interface was determined by a fluorescence labeling technique. G _c is low and is linear in block copolymer interfacial coverage (Σ), indicating a chain scission mechanism. Reasonable agreement was achieved between experiment and theoretical prediction based on the energy to break C–C bonds.     

Determination of fracture toughness of epoxy using fractography

Fracture toughness of epoxy was determined by quantitative fractography, one of the techniques for brittle materials based on fracture mechanics. Two different epoxy systems, an anhydride‐cured and an amine‐cured epoxy based upon diglycidyl ether of bisphenol A (DGEBA) were studied. Epoxies with different average molar mass between crosslinks (M_c) or crosslink density were prepared by varying the cure profiles. The materials were characterized using differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMS), and density measurements. Optical microscopy was used to measure the dimensions of the different regions on the fracture surfaces of unnotched samples that were tested to failure under tension. The fracture toughness values were calculated from the relationship between the measured sizes and fracture stress. Epoxies with lower M_c values or higher crosslink densities have lower fracture toughness values. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 257–268, 1999     

Mode I and Mode II delamination resistance and mechanical properties of woven glass/epoxy–PU IPN composites

The effect of polyurethane on the mechanical properties and Mode I and Mode II interlaminar fracture toughness of glass/epoxy composites were studied. Polyurethanes (PU) synthesized using polyols and toluene diisocyanate were employed as modifier for epoxy resin by forming interpenetrating polymer network. The PU/Epoxy IPN was used as matrix material for GFRP. PU modified epoxy composite laminates having varying PU contents were prepared. The effect of PU content on the mechanical properties like interlaminar fracture toughness (Mode I, G_1c and Mode II, G_IIc), tensile strength, flexural strength, and Izod impact strength were studied. The morphological studies were conducted on the fractured surface of the composite specimen by scanning electron microscopy (SEM). Tensile strength, flexural strength, and impact strength of PU‐modified epoxy composite laminates were found to increase inline with interlaminar fracture toughness (G_1c and G_IIc) with increasing PU content to a certain limit and then it was found to decrease with increase in PU content. It was observed that toughening of epoxy with PU increases the Mode I and Mode II delamination toughness up to 17 and 120% higher than that of untoughened composite specimen, respectively. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

The Adhesive Fracture Energy of Bonded Thermoplastic Fibre-Composites

The present paper first discusses the problems that occur when thermoplastic-based fibre-composite materials are bonded using structural engineering adhesives, such as epoxy and acrylic adhesives. A double-cantilever beam joint has been employed and it is shown that the value of the adhesive fracture energy, G_c , is very low when a simple abrasion/solvent wipe pretreatment is used for the thermoplastic fibre-composites. This arises from crack growth occurring along the adhesive/composite interface, which is relatively weak when such a pretreatment is employed. Secondly, it is demonstrated how very effective a corona surface pretreatment may be for these materials. Indeed, when such a pretreatment is used, interfacial crack growth is no longer observed but the crack now propagates either cohesively in the adhesive or through the composite substrate; both failure modes lead to relatively high values of G_c , with the former resulting in the highest values of G_c being recorded. Finally, from measuring the fracture properties of the composites and combining these data with a detailed analysis of the stresses in the DCB joint, calculated using a finite element analysis, the reasons for these different loci of failure may be readily understood and predicted.     

A relationship between the fracture mechanics and surface energetics failure criteria

The reversible part of the fracture mechanics (F-M) fracture energy γ_c is redefined in terms of current theory for surface energetics (S-E) interactions at regular interfaces. These new failure criteria are applied to the definition of surface energy criteria for spontaneous interfacial failure, where γ_c = 0, produced by selected conditions of liquid-phase immersion. For cases where γ_c > 0, the total fracture energy W = γ_c + W_p, where the irreversible plastic work of surface formation W_p ≈ W ? γ_c. A qualitative relation between γ_c^1/2 ∝ W_p is observed for the case of steady-state crack propagation in peeling. For adsorption bonds, the theory provides a new method of mapping the surface energy effects of the immersion phase upon the Griffith fracture energy γ_c. Essential factors which determine water sensitivity of interfacial bonds are incorporated into the analysis and experimentally verified.     

Toughness response of vinylester/epoxy‐based thermosets of interpenetrating network structure as a function of the epoxy resin formulation: Effects of the cyclohexylene linkage

Vinylester/epoxy (VE/EP)‐based thermosets of interpenetrating network (IPN) structures were produced by using a VE resin (bismethacryloxy derivative of a bisphenol A–type EP resin) with aliphatic (Al‐EP) and cycloaliphatic (Cal‐EP) EP resins. Curing of the EP resins occurred either with an aliphatic (Al‐Am) or cycloaliphatic diamine compound (Cal‐Am). Dynamic mechanical thermal analysis (DMTA) and atomic force microscopy (AFM) suggested the presence of an interpenetrating network (IPN) in the resulting thermosets. Fracture toughness (K_c) and fracture energy (G_c) were used as the toughness characterization parameters of the linear elastic fracture mechanics. Unexpectedly high K_c and G_c data were found for the systems containing cyclohexylene units in the EP network, such as VE/Al‐EP+Cal‐Am and VE/Cal‐EP+Al‐Am. This was attributed to the beneficial effects of the conformational changes of the cyclohexylene linkages (chair/boat), which were closely analogous to those in some thermoplastic copolyesters. The failure mode of the VE/EP thermoset combinations was studied in scanning electron microscopy (SEM) and discussed. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2124–2131, 2003     

The effect of moisture and loading rate on the interfacial fracture properties of sandwich structures

The skin‐core interfacial fracture properties of a number of dry and moistureconditioned sandwich structures have been investigated over a range of crosshead displacement rate using the three point bend sandwich (TPBS) structure. It has been shown that the interfacial fracture toughness, G_c, of a crosslinked PVC system decreases rapidly with loading rate, whereas the toughness of a linear PVC remains roughly constant. In contrast, the interfacial fracture toughness of the balsa core material increased steadily with increasing crosshead displacement rate, an effect that was attributed to the rate dependent properties of the glass fibers in the wake of the primary crack. Prolonged seawater exposure in an aluminum honeycomb structure was found to attack the bond between the epoxy matrix and the aluminum core, facilitating crack advance along the skin‐core interface. Finally, it is concluded that great care should be exercised before selecting sandwich structures for hostile marine environments.     

Temperature and Time Dependence of Copolymer Mixtures on Interfacial Adhesion at PS/PMMA Interfaces

The effect of copolymer mixtures on the interfacial adhesion between slabs of PS and PMMA was investigated as a function of composition, time and temperature using the asymmetric double cantilever beam (ADCB) method. The nature of the interface was further probed using atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (D-SIMS). The results show that mixtures of graft and block copolymers are much more effective than pure block copolymers in enhancing the interfacial adhesion. The most effective mixture consisted of a block copolymer of molecular weight 70K and a copolymer with two PS grafts of molecular weight 30K. This mixture yielded an interfacial fracture toughness of G_c = 127.5 J/m² as compared with G_c = 38.2 J/m² and G_c = 3.5 J/m² for the pure block and graft copolymer, respectively.

G_c at the PS/PMMA interface reinforced only with block copolymer was maximal after an annealing temperature of 150°C for 1 hr. It decreased by an order of magnitude when the temperature was increased to 180°C or the joining time was increased from 1 to 10 hours. G_c at the interface reinforced with a graft/diblock copolymer mixture was also maximum at an annealing temperature of 150°C but it decreased only by a factor of 2 with increasing joining time or temperature. Dynamic Secondary Ion Mass Spectroscopy (DSIMS) data show that this effect may be due to decrease in the diffusion of the copolymer from the interface when the mixture is present, i.e, the diblock copolymer is trapped within the graft copolymer.     

Toughening of vinylester–urethane hybrid resins by functional liquid nitrile rubbers and hyperbranched polymers

Liquid nitrile rubbers with vinyl (VTBN), carboxyl (CTBN) and epoxy (ETBN) and hyperbranched polyesters with vinyl (VHBP) and epoxy (EHBP) functionalities were added in 10 wt% to a bisphenol-A based vinylester–urethane hybrid resin (VEUH) for its toughening. The fracture energy (G_c) was determined on compact tensile specimens at ambient temperature. High toughness improvement was achieved by adding ETBN and CTBN to VEUH. It was established that a change in the initial stoichiometry of OH/NCO may affect G_c.The combination of CTBN with other additives in 1:1 ratio yielded a synergistic effect with respect to G_c. Changes in G_c were explained by differences in the fracture mode based on fractographic inspection of the fracture surface of the specimens. It was shown the same G_c may be derived from completely different failure scenarios.     

Mode II interlaminar delamination of composite laminates incorporating with polymer ultrathin fibers

This study makes use of electrospinning to produce epoxy polymer ultrathin fibers. Those fibers were collected as nonwoven sheets with various thicknesses. The collected ultrathin fibrous sheets (UFS) were incorporated into ply interfaces of a glass/epoxy composite laminate. End‐notch flexure (ENF) specimens were employed to measure the mode II interlaminar fracture toughness (G_IIC) of the resulting laminate. Scanning electron microscopy (SEM) was carried out to investigate the fracture surface and fracture mechanism of tested samples. Results indicate that there were some reinforcing effects of the UFS on the G_IIC when the thickness of the sheets was not more than 0.13 mm for a single sheet and 0.06 mm for multi‐sheets incorporated. The mechanical properties were quickly decreased and became poorer when a higher thickness of the sheets was used. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Impact properties of short carbon fiber reinforced nylon 6.6

The impact properties of injection-molded nylon 6.6 composites containing different loadings of short carbon fibers have been studied using an instrumented falling weight impact tester (IFWIT). Analysis of the impact data using linear elastic fracture mechanics (LEFM) has enabled the evaluation of the critical strain energy release rate, G_c. Instrumentation of the impact machine has facilitated the determination of another fracture mechanic parameter, the fracture toughness, K_c. Both parameters are observed to increase with increasing volume fraction of fibers. Examination of fracture surfaces using scanning electron microscopy (SEM) has revealed that the main energy dissipative processes responsible for toughening the composites is the fiber pull-out mechanism.     

On the significance of the standardized notched impact strength for materials selection and design

Experimental data on the standard Charpy notched impact strength (CNIS) for a large sampling of particulate-filled and rubber-modified polypropylenes were analyzed. To determine the significance of the CNIS for material selection and design, CNIS data were compared with fracture toughness measurements expressed as the critical strain energy release rate G′_c, measured under impact loading. A scale factor representing the state of stress at the crack tip was calculated, assuming small scale yielding, Class I linear elastic fracture mechanics (LEFM). Based on the data, three principal groups of materials were identified. In the smallest of the three, CNIS and G′_c measurements showed the same functional dependence on the material variables studied and were characterized by a scale factor independent of the composition. In these cases, one can justify comparisons of toughness of different compositions using single values of the CNIS, since the materials are probably being compared in equivalent states of stress. In a second group, CNIS and G′_c also show the same functional dependence on material variables, but exhibit large variations in the scale factor with composition. One should not compare the toughness of different materials from this group based on single values of CNIS, since it is likely that the comparisons would not be made relative to equivalent states of stress. In these cases, only measurements of G′_c can separate the effect of specimen geometry from those of the intrinsic properties of the material. The majority of materials studied fell into a third group, in which the CNIS and G′_c exhibited substantially different functional dependencies on the material properties, and the scale factors depended also on composition. In these latter cases, a comparison of CNIS values for different compositions is not a reliable indication of the relative toughness and is of little value as a parameter for material selection and design.     

Effect of molecular weight between crosslinks on the fracture behavior of rubber‐toughened epoxy adhesives

The effect of molecular weight between crosslinks, M_c, on the fracture behavior of rubber‐toughened epoxy adhesives was investigated and compared with the behavior of the bulk resins. In the liquid rubber‐toughened bulk system, fracture energy increased with increasing M_c. However, in the liquid rubber‐toughened adhesive system, with increasing M_c, the locus of joint fracture had a transition from cohesive failure, break in the bond layer, to interfacial failure, rupture of the bond layer from the surface of the substrate. Specimens fractured by cohesive failure exhibited larger fracture energies than those by interfacial failure. The occurrence of transition from cohesive to interfacial failure seemed to be caused by the increase in the ductility of matrix, the mismatch of elastic constant, and the agglomeration of rubber particles at the metal/epoxy interface. When core‐shell rubber, which did not agglomerate at the interface, was used as a toughening agent, fracture energy increased with M_c. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 38–48, 2001     

Strength of Epoxy/Glass Interfaces after Hygrothermal Aging

The stability of epoxy/glass interfaces subjected to hygrothermal aging was assessed using a fracture-mechanics approach. An epoxy system consisting of diglycidyl ether of bisphenol F cured with 2-ethyl-4-methyl-imidazole was bonded to borosilicate glass adherends that were treated with various types of adhesion promoters to provide a variety of interfaces. Adhesive strength was measured under dry, as-processed conditions and as a function of exposure time to an 85°C/85% relative humidity (RH) environment. As expected, the strain-energy-release rate, G _c , dropped significantly with aging time for the bare epoxy/glass interface. The drop in G _c is assumed to be due to a loss of interfacial forces. The use of two silane-based adhesion promoters, 3-aminopropyltriethoxysilane (APS) and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECH) resulted in improved adhesive strength both before and after hygrothermal aging. The improvement in adhesive strength can be explained by the introduction of chemical bonds at the interface. The drop in G _c is assumed to be due to a loss of interfacial forces and hydrolysis of siloxane bonds. In addition to the use of organosilane-based adhesion promoters, a series of polyhydroxyaminoethers (PHAE) thermoplastic adhesive resins was also investigated as potential adhesion promoters. It was found that 2% PHAE in Dowanol® PM, a hydroxyl-group-containing solvent, was the best system for the PHAE-based adhesion promoters. Interestingly, both the acetic acid concentration in the solvent and maleic anhydride content in the PHAE resin were shown to affect the adhesive strength.     

Comparison of the fracture and failure behavior of injection-molded α- and β-polypropylene in high-speed three-point bending tests

The fracture and failure mode of α- and β-isotactic polypropylene (α-iPP and β-iPP, respectively) were studied in high speed (1 m/s) three-point bending tests on notched bars cut from injection-molded dumbbell specimens and compared. The fracture response of the notched Charpy-type specimens at room temperature (RT) and −40°C, respectively, was described by terms of the linear elastic fracture mechanics (LEFM), namely fracture toughness (K_c) and fracture energy (G_c). K_c values of both iPP modifications were similar, while G_c values of the β-iPP were approximately twofold of the reference α-iPP irrespective of the test temperature. It was demonstrated that β-iPP failed in a ductile and brittle-microductile manner at RT and −40°C, respectively. By contrast, brittle fracture dominated in α-iPP at both testing temperatures. Based on the fracture surface appearance, it was supposed that β-to-α (βα) transformation occurred in β-iPP. The superior fracture energy of β-iPP to α-iPP was attributed to a combined effect of the following terms: morphology, mechanical damping, and phase transformation. Results indicate that their relative contribution is a function of the test temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 2057–2066, 1997     

Impact fracture behavior of continuous glass fiber-reinforced nylon 6

The impact fracture toughness of nylon 6/continuous glass fiber composites at four levels of fiber content has been studied. The composites were produced by anionically polymerizing caprolactam within a glass mat using a vacuum injection technique. Application of linear elastic fracture mechanics to characterize the impact fracture toughness of the composites, using an energy approach (G_IC), has been found to be applicable provided that a correction is made for the size of the damage zone. The concept of J_c, fracture energy per unit ligament area, has also been applied to the composites and agreement between G_IC and J_c has been found to be reasonably satisfactory. The ratio of crack propagation energy to the total energy absorbed (ductility index) has also been calculated. The ductility index was found to be close to one for the composites, indicating that additional energy is involved in propagating the fracturing cracks probably due to fiber debonding and/or crack blunting and fiber pullout. Fractographic examination of the impact fracture surface confirmed the presence of these features.     

Impact Fracture and Failure Behavior of in‐situ Polymerized,Ethylene/Butyl Acrylate Rubber Toughened Polyamide‐12

The fracture and failure behavior of in‐situ polymerized polyamide‐12 (PA‐12) blends prepared by reactive extrusion were studied in instrumented high‐speed (v = 1.2 m/s) impact bending tests using the linear elastic fracture mechanics approach. PA‐12 was polymerized in presence (up to 9 wt.‐%) of ethylene/butyl acrylate copolymers (E/BA) of varying BA content and melt viscosity. From the tests performed on injection molded specimens at ambient temperature and –40°C, respectively, the fracture toughness (K_d) and initiation fracture energy (G_d,i) were derived. K_d was less sensitive to either testing temperature or E/BA type and content. G_d,i, on the other hand, went through a maximum at room temperature and monotonously increased at T = –40°C as a function of modifier content. E/BA with higher melt viscosity and lower polarity (lower BA content) performed better than the lower melt viscosity, higher polarity E/BA counterpart. The dominant failure modes and their change both with temperature and modifier content were studied by fractography and discussed.     

Impact and dynamic fracture resistance of crystalline thermoplastics: Prediction from bulk properties

The behavior of tough, crystalline thermoplastics in notched impact tests leads to the definition of crack initiation resistance and propagation resistance as two distinct properties, G_c and G_D. It is shown here that a single criterion—adiabatic thermal failure of a crack-tip cohesive zone—can be applied to predict both. Dynamic fracture resistance G_D emerges as a geometry independent, though crack speed and temperature dependent, material property, whose minimum value G_D,min depends only on temperature and bulk physical properties. G_D,_min can be measured using a simple pressurized-tube test. Crack initiation resistance G_c, however, is inherently influenced by geometry and impact speed, although its lower bound is also G_D,min. Craze extension and failure of a notched impact specimen, and hence G_c, can be predicted for a specific temperature, given bulk thermal property data and a dynamic stress/strain curve measured by impact bending of an unnotched beam. For materials that comply with the model, sharp-notched Charpy type impact tests will not arrive at a unique G_c value, while Izod type tests, for which a revised compliance calibration is presented, may fail to establish any G_c value at all.     

Studies on the mechanical and mechanical interfacial properties of carbon–carbon composites impregnated with an oxidation inhibitor

In this work, the relationships between work of adhesion and fracture toughness parameters, such as work of fracture (W_f), the critical stress intensity factor (K_IC), and the specific fracture energy (G_IC), of carbon–carbon composites (C/C composites) were investigated. The impact properties of the composites were also studied in the context of differentiating between the initiation and propagation energies for failure behavior. Composites consisting of different contents of the oxidation inhibitor MoSi₂ displayed an increase of the work of adhesion between the fibers and the matrix, which improved both the fracture toughness and impact properties of the composites. The 12 wt% MoSi₂ composites exhibited the highest mechanical and mechanical interfacial properties. This was probably due to the improvement of the London dispersive component, W_A^L, of the work of adhesion, resulting in an increase in the interfacial adhesion force among the fibers, filler, and matrix in this system.     

Studies on fracture behavior of tough PA6/PP blends

Fracture toughness of injection-molded PA6/PP blends compatibilized with SEBS-g-MA was studied using deeply double-edge notched tension (DDENT) specimens according to the essential work of fracture procedure. The fracture mechanical studies also included tensile impact tests on the DDENT specimens and characterization of the fracture surfaces by electron microscopy. The results were compared with those of traditional tensile tests and Izod impact tests on single-edge notched samples, and the sensibility of the methods was evaluated. Effects of sample position, ligament length, testing direction, and test speed were studied as well. It was found that the essential work of fracture concept, earlier applied to thin sheets, can also be applied to injection-molded tough blends. High deformation of the skin may, however, interfere with the measurements and cause a “tail” in the load-deformation curves. The plastic work of fracture (w_p) was found to correlate with the impact strength, and thus, it described the toughness. The highest values for work of fracture were recorded for the compatibilized blend with a PA6/PP ratio of 80/20. The essential work of fracture (w_e) in turn increased with increasing PA6 content and behaved like tensile strength. The test speed was found to affect the fracture behavior substantially: differences between the materials were more pronounced in high-speed tensile impact tests, which revealed signs of cavitation in addition to large-scale plastic deformation for the tough PA6-rich blend compositions. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2209–2220, 1997