Experimental and Numerical Analysis of T-peel Joints for the Automotive Industry

The T-peel joint is commonly used in the automotive industry, especially in the panels of the load compartment in vans. In order to determine the effect of using a structural adhesive instead of spot-welding, a detailed series of tests and finite element analyses were conducted. The adhesive was a toughened epoxy and the adherend was mild steel used in the manufacture of the car bodyshell. Various parameters were investigated such as the bondline thickness and adherend radius. The spew fillet was maintained flush in all cases. Contrary to the case of lap joints, there are no stress concentrations around the fillet area and, therefore, it is possible to use the maximum uniaxial tensile stress as a failure criterion for these joints. The bending moment at failure was found to be constant across the different geometries modelled, and it was also similar to that found in lap shear joints in previous studies.     

Investigation of optimal surface treatments for carbon/epoxy composite adhesive joints

Although an adhesive joint can distribute the load over a larger area than a mechanical joint, requires no holes, adds very little weight to the structure and has superior fatigue resistance, but it not only requires a careful surface preparation of the adherends but also is affected by service environments. In this paper, suitable conditions for surface treatments such as plasma surface treatment, mechanical abrasion, and sandblast treatment were investigated to enhance the mechanical load capabilities of carbon/epoxy composite adhesive joints. A capacitively coupled radiofrequency plasma system was used for the plasma surface treatment of carbon/epoxy composites and suitable surface treatment conditions were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time by measuring the surface free energies of treated specimens. The optimal mechanical abrasion conditions with sandpapers were investigated with respect to the mesh number of sandpaper, and optimal sandblast conditions were investigated with respect to sandblast pressure and particle size by observing geometric shape changes of adherends during sandblast process. Also the failure modes of composite adhesive joints were investigated with respect to surface treatment. From the peel tests on plasma treated composite adhesive joints, it was found that all composite adhesive joints failed cohesively in the adhesive layer when the surface free energy was higher than about 40 mJ/m², because of high adhesion strength between the plasma treated surface and the adhesive. From the peel tests on mechanically abraded composite adhesive joints, it was also found that the optimal surface roughness and adhesive thickness increased as the failure load increased.     

Surface modification of glass by oligomeric fluoroalkylating agents having oxime-blocked isocyanate groups

A new type of oligomeric fluoroalkylating agent with oxime-blocked isocyanate groups was synthesized and the modification of a glass surface using this oligomer was studied. Based on contact angle measurements, the surface free energy of the modified glass was calculated. The results show that the glass surface was effectively fluoroalkylated, the surface free energy was lowered enough and the glass surface became both water- and oil-repellent. The longer the chain length of the fluoroalkyl group (R _f) is, the lower is the surface free energy of the modified glass. There existed a good relationship between the surface free energy and the F 1s /Si 2 p peak area ratio measured by XPS. The surface free energies decreased with an increase of this peak area ratio and above a ratio of 2, they became constant. The effects of modification by fluoroalkyl end-capped oligomers having oxime-blocked isocyanate groups and fluoroalkyl end-capped silanes were interpreted in terms of the structure of the modified layer, i.e. in terms of the monolayer interphase and network interphase models, respectively.     

Effects of boron compounds on the bonding strength of phenol-formaldehyde and melamine-formaldehyde adhesives to impregnated wood materials

Wood materials are increasingly being used in the construction of structural beams, sports equipment, etc. This study was carried out to determine the bonding strength of phenol-formaldehyde (PF) and melamine-formaldehyde (MF) adhesives to impregnated wood materials. For this purpose, brutia pine (Pinus brutia Ten) and elm (Ulmus compestris L.) woods were impregnated with borax (Bx), boric acid (Ba), Bx + Ba (wt:wt 50:50%), di-ammonium phosphate (D), [D + (Bx + Ba)]/(50 + (25 + 25%), w/w) and Tanalith-C 3310 (T-C 3310) using the vacuum method according to ASTM-D 1413-76. The effects of wood species, impregnating material and type of adhesive on the bonding strength were determined. The highest shear strength (11.09 N/mm²) was obtained from elm wood control (i.e., without any impregnating materials) samples with MF; thus, the impregnation process negatively affected the adhesive bonding strength. Impregnating materials, especially those containing oily or similar solutions such as T-C 3310, are not advised for wood elements which are subjected to shear.     

Characterization of epoxy resin surface energetics

This study has characterized the energetics of both the liquid state and the solid state of two commercially available epoxy resins: a DGEBA- and a TGMDA-based epoxy system. The surface properties of the liquid epoxies were evaluated by wetting measurements using a dynamic contact angle analysis (DCA). The Lifshitz-van der Waals components of the surface tension were found to be similar for both epoxy systems, while the acid-base components were found to be slightly different. Two different techniques were used to characterize the cured epoxy surface properties: wetting measurements and vapor adsorption measurements by means of inverse gas chromatography (IGC). The Lifshitz-van der Waals components of the surface energy were observed to be nearly the same for both epoxies, confirming that both resins have the same potential for non-specific interactions, in both liquid and solid states. Evaluations of the acid-base components of the work of adhesion by DCA and the Gibbs free energy change by IGC suggest that both cured epoxies show non-negligible specific interactions with both acidic and basic probes. However, computations of the accepticity and donicity parameters showed that both cured epoxies are predominantly basic, but also possess non-negligible acidity. It is likely that the presence of water on the solid surface contributes to the acidic character of the cured epoxies. The temperature dependence of the liquid surface tension for both epoxy systems was investigated. The same temperature dependence was observed: the surface tension decreased with temperature, following a linear regression. Corrections for viscous-drag effects on the liquid surface tension measurements were also made.     

Surface analysis of some polymer-based materials by means of a soft X-ray emission technique

This paper reviews the performances and limits of low-energy electron-induced X-ray spectroscopy (LEEIXS) in the surface and thin film analysis of polymer-based materials. The major interest of this soft X-ray emission technique results from the use of a windowless gas discharge tube operating in the primary vacuum of the X-ray spectrometer as an electron excitation source (1-5 keV). The capabilities of LEEIXS are illustrated through the analysis of light elements (C, O, F,Si, etc.) (i) in thin polymeric films deposited on metallic substrates from various wet (dipping, electrodeposition, electropolymerization) and dry (plasma polymerization, thermal evaporation) processes and (ii) on polymer substrates subjected to chemical or physical surface treatments. These examples also show that LEEIXS analysis, when it concerns organic or polymeric materials, is limited to those materials which present a sufficient chemical and thermal stability, as the electron beam bombardment, in spite of the low current density (0.1 mA cm^-2) used, can cause degradation of many materials.     

Surface characterization of polymers modified by keV and MeV ion beams

The present work deals with two different surface modification techniques for altering the surface properties of polymers: plasma treatment and ion implantation. Polymer foils were exposed in an inductively-coupled r.f. (13.56 MHz) plasma system with and without applying a negative high voltage pulse to the sample stage. The influence of low pressure plasmas of oxygen, nitrogen, or argon on the chemical composition, topography, and wettability of polymer surfaces was studied in detail. Etch rates of poly(ethylene terephthalate) for different plasma parameters were monitored. The polymer surface was also modified by a high energy ion beam process. Polyimide films were implanted with different ion species such as Ar⁺, N⁺, C⁺, He⁺, and O⁺ at doses from 1 × 10¹⁵ to 1 × 10¹⁷ ion/cm². Ion energy was varied from 10 to 60 keV for the plasma source ion implantation (PSII) experiment. Polyimide samples were also implanted with 1 MeV hydrogen, carbon, and oxygen ions at a dose of 1 × 10¹⁴ ion/cm². Depending on the ion energy, dose, and ion species, the surface resistivity of the film was reduced by several orders of magnitude. A study on the plasma-treated and ion beam-treated polymer surfaces was performed using TOF-SIMS, XPS, SEM, AFM, and water contact angle measurements.     

Adhesion characteristics of plasma surface treated carbon/epoxy composite

The load transmission capability of adhesive joints can be improved by increasing the surface free energy of the adherends with surface treatments. In this paper, suitable plasma surface treatment conditions for carbon/epoxy composite adherend were investigated to enhance the strength of carbon/epoxy composite adhesive joints using a capacitively coupled radio-frequency plasma system. Effects of plasma surface treatment parameters on the surface free energy and adhesion strength of carbon/epoxy composite were experimentally investigated with respect to gas flow rate, chamber pressure, power intensity, and surface treatment time. Quantitative chemical bonding analysis determined with XPS (X-ray photoelectron spectroscopy) was also performed to understand the load transmission capabilities of composite adhesive joints with respect to surface treatment time.     

Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.     

Effect of structural modification of polyester fibres on the strength of nonwoven materials

The positive effect was established and the optimum technological parameters of treating the surface of polyester fibres with a solution of NAOH, a quaternary ammonium compound — Catamine AB, and organosilazanes on the strength characteristics of nonwoven materials in autoadhesive bonding of fibres were determined. The advantage of organosilicon modifiers was demonstrated. A 13-fold increase in the strength of nonwoven materials was obtained with the optimum concentration of the preparation on the fibre of 0.27 wt. %. Moscow State Textile Academy. Translated fromKhimicheskie Volokna, No. 4, pp. 54–55, July–August, 1999.     

Surface modification of bioceramics by grafting of tailored allyl phosphonic acid

Abstract

A new route to interfacial bonding between ceramic and matrix in biocomposites is identified. A tailored allyl phosphonic acid is used as a coupling agent bound to the surface of a bioceramic to form a 'grafted' calcium phosphate (CAP). The allyl phosphonic acid coupling agent is synthesised by reaction of allyl halide and trialkyl phosphite. Successful synthesis was confirmed by nuclear magnetic resonance and Fourier transform infrared spectroscopy (FTIR). The allyl phosphonic acid was incorporated onto calcium phosphate using a wet chemical coprecipitation synthesis route. The resulting 'grafted' CAP was characterised using FTIR coupled with photoacoustic sampling, and Fourier transform Raman spectroscopy (FTR). The spectroscopic data suggest an interaction between the allyl phosphonic acid and calcium phosphate resulting from observed reductions in intensity of the hydroxyl (3570 cm^?1) and phosphate ν ₃ (1030 cm^?1) peaks. The continued presence of C=C functionality on the surface of the grafted CAP was indicated by FTIR and FTR spectra (peaks at 1650 and 1635 cm^?1 respectively) and confirmed by X-ray photoelectron spectroscopy (XPS). On the basis of these results, it is concluded that grafted CAP may be used to produce a chemically bonded composite with superior mechanical properties.     

Surface modification of hybrid fibres for fire protection

The formation of polyaluminosiloxane networks through surface modification of cellulose-polysilicic acid hybrid fibres with inorganic aluminium compounds enhances flame retardancy and laundry performance of these fibres. Fibres of cellulose-polysilicic acid (VISIL) have been reported as a flame retardant. In contrast to their thermal property, these fibres undergo a significant change, in terms of flame retardancy, when subjected to alkaline conditions (pH > 10). Surface modification of these fibres with inorganic aluminium compounds not only reduces the solubility behaviour but also increases the flame retardancy.     

Surface quality prediction of thermoset composite structures using geometric simulation tools

Abstract

The surface quality of polymer composite laminates was examined via geometric modelling techniques and compared to experimental data. TexGen software provided the platform for the development of a surface roughness simulation tool which accounted for textile architecture and specific cure kinetics of the matrix. The study focused on the influence of thermal and chemical shrinkage during cure and the change in localised volume fraction across the surface of a unit cell. A one-dimensional analysis was used to determine proportional dimensional changes to the matrix region, with the results stitched together to form a three-dimensional topological plot. Three demonstrator geometric models were developed to represent a carbon 2 × 2 twill weave fabric with 3000, 6000 or 12 000 tows. These models were analysed with low and high shrink resin properties. Optical microscopy was used to determine accurate tow forms for compacted tows which aided the development of the geometric model. Simulated profiles, topography and surface roughness measures were compared to experimental data which demonstrated the significance of matrix contraction and fabric architecture on the final surface quality. The simulations were shown to represent experimental data typically within 6%.     

Ultraviolet surface treatment for adhesion strength improvement of carbon/epoxy composite

—As the applications of composite structures have increased, various techniques to join composite parts to the structures have been developed in order to meet the required adhesion strength. In this work, surface modification of carbon/epoxy composites was investigated using ultraviolet (UV) surface treatment to increase the adhesion strength between the carbon/epoxy composite and the epoxy adhesive. After UV surface treatment, X-ray photoelectron spectroscopy (XPS) analysis and contact angle measurements were performed to analyze the surface characteristics of the carbon/epoxy composites. From the results of XPS analyses and adhesion strength tests, it was found that the increase of C O bond density on the surface of carbon/epoxy composite caused the enhancement of adhesion strength. Also it was found that the UV-B (wavelength 280–315 nm) surface treatment resulted in a superior adhesion strength compared to the UV-A (wavelength 315–400 nm) surface treatment.     

Surface modification of polytetrafluoroethylene by Ar+ irradiation for improved adhesion to other materials

A surface of thin square polytetrafluoroethylene (PTFE) samples (1 × 1 × 0.2 cm³) was irradiated with Ar⁺ at 1 keV with varying ion dose from 5 × 10¹⁴ to 1 × 10¹⁷ ions/cm² with and without an oxygen environment. The irradiated surface of the samples was examined by scanning electron microscopy (SEM) for surface textural changes and x-ray photoelectron spectrometry (XPS) for changes in chemical structure. A wettability test was conducted on the irradiated surface of PTFE samples by water droplets. A Scotch ™ tape adhesion test, after a thin film of Cu or Al was evaporated on the irradiated surface, and a tensile test after irradiated samples were glued to sample holders by an adhesive glue (Crystal Bond) was also run. The SEM micrographs showed increasing roughness with fiber forest-like texture with increasing ion dose. The Ar⁺ with an O₂ environment produced finer and denser fiber forest-like texture than that without O₂. The high-resolution XPS spectra showed decreased intensity of the F1s peak and formation of the O1s peak when irradiated with the O₂ environment. The increase of the O1s peak may be attributed to the reaction of oxygen atoms and the free radicals created by Ar⁺ bombardment. The wettability of water droplets on the irradiated surfaces was found to be inversely proportional to the surface roughness. Adhesion tests were conducted on 2000 Å thick Al or Cu film. Full detachment of the metal films was observed when PTFE samples were not modified. Partial detachment of the Al film occurred when PTFE was irradiated without the O₂ environment, regardless of ion dose. No detachment of the film occurred when PTFE was irradiated with the O₂ environment with the ion dose exceeding 1 × 10¹⁶ ions/cm². Partial detachment of Cu film was observed with or without the O₂ environment when the ion dose was 5 × 10¹⁴ ions/cm². No detachment occurred with or without the O₂ environment when the ion dose was 1 × 10¹⁵ ions/cm² or greater. The tensile test showed that adhesion of an adhesive cement (Crystal Bond) to the irradiated PTFE samples increased significantly with increasing ion dose up to 1 × 10¹⁶ ions/cm². Possible mechanisms for the improved adhesion are given. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1913–1921, 1997     

The effect of chemical modification on the fracture toughness of montmorillonite clay/epoxy nanocomposites

The change in fracture toughness and its dependence on the content of clay nanoplatelets and adhesion at the interface between clay nanoplatelets and anhydride-cured epoxy matrix are discussed. Three clay nanoplatelets with different chemical modifications were used in this investigation. To fabricate nanocomposites, the clay nanoplatelets were sonicated in acetone for 2 h. The role of the clay nanoplatelets in the mechanical/fracture properties was investigated by transmission electron microscopy (TEM). Bright-field TEM micrographs showed excellent dispersion of clay nanoplatelets in epoxy matrix. Both intercalation and exfoliation of clay nanoplatelets were observed depending on clay modification. Compact tension specimens were used for fracture testing. The fracture toughness increased with increasing clay content. The fracture toughness of clay/epoxy nanocomposites varied with the clay morphology in the epoxy matrix. Different morphologies of the fracture surfaces, highly dependent on the morphology of dispersed clay nanoplatelets, were observed using environmental scanning electron microscopy (ESEM). The fracture toughness was found to be correlated with the fracture surface roughness measured by confocal laser scanning microscopy (CLSM).     

Surface modification of polypropylene film using N2O-containing flames

Contact-angle measurements and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize polypropylene (PP) films that were exposed to laminar premixed air: natural gas flames containing small quantities of nitrous oxide. During combustion, the nitrous oxide generates gas-phase nitrogen oxides that lead to the affixation of nitrogen-containing functional groups to the PP surfaces. Treatment of PP in nitrous oxide-containing flames also leads to an increase in surface oxidation and markedly improves wettability when compared with standard flame treatments. The chemical form of the nitrogen affixed to the PP surface is strongly dependent on the flame equivalence ratio. Fuel-lean flames tend to affix highly oxidized forms of nitrogen such as nitrate and nitro groups, while fuel-rich flames tend to affix less-oxidized nitrogen groups such as nitroso, oxime, amide, and amine. A computational model, SPIN, was used to elucidate the chemistry of the flame as it impinges upon the cooled PP surface. The SPIN modeling indicates that the principal reactive gas-phase species at or near the PP surface are O₂, OH, H, NO, NO₂, HNO, and N₂O. A number of possible reactions between these species and the PP can account for the formation of the various nitrogen functional groups observed.     

Plasma modification of cellulose fibers for composite materials

Composites of natural fibers and thermoplastics can be combined to form new enhanced materials. One of the problems involved in this type of composites is the formation of chemical bonds between the fibers and the polymers at the interface. This work presents a study where low energy glow discharge plasmas are used to functionalize cellulose fibers implanting polystyrene between the fibers and the matrix that improve the adhesion of both components. The interface of polystyrene was synthesized by continuous and periodic glow discharges on the surface of the cellulose fibers. The results show that the adhesion in the fiber–matrix interface increases with time in the first 4 min of treatment. However, at longer plasma exposures, the fiber may be degraded reducing the adhesion with the matrix. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3821–3828, 2006     

Characterizing the mechanism of improved adhesion of modified wood plastic composite (WPC) surfaces

To have a better knowledge of the phenomena that affect the adhesion characteristics of wood plastic composites (WPCs) a series of surface treatments was performed. The treatments consisted of chemical, mechanical, energetic, physical, and a combination of energetic and physical WPC surface modifications. After each treatment, the composite boards were bonded using a commercial epoxy adhesive, and bond shear strength was determined according to ASTM D 905. All the surface treatments, except the mechanical one, were performed and presented in a previous paper (W. Gramlich et al., J. Adhesion Sci. Technol. 20, 1873–1887 (2006)). Mechanical treatment and surface characterization for all the treatments were performed in the present study. The surface characterization included application of thermodynamic and spectroscopic techniques. Most of the surface treatments improved the adhesive bondability of wood plastic composites and, particularly, the smoothest WPC surfaces increased the shear strength by 100% with respect to the control. Thermodynamic measurements indicate that the WPCs low surface energy of about 25 mJ/m², is likely due principally to the surface migration of a lubricant component used in the extrusion formulation. The surface energy increased over 45% with respect to the control samples after the chemical treatments. X-ray photoelectron spectroscopy analysis indicated that high oxidation levels of the WPC surfaces resulted in high surface energy and high bond shear strength.     

Selective use of rubber toughening to optimize lap-joint strength

This paper introduces a novel approach to increasing the lap joint strength, different from the traditional methods of either increasing the lap joint area or altering the joint geometry. This is accomplished by the selective use of rubber toughening in epoxy to optimize lap joint strength. This was accomplished in three stages. In the first stage an adduct was prepared, this was used to make bulk tensile specimens to calculate the bulk properties for various concentrations of rubber, i.e. 0, 10, and 20 parts per hundred parts of resin (epoxy). In the second stage finite element models were developed using the bulk properties previously obtained. Interfacial stresses were used to access the trends obtained by the selective use of rubber toughening at different locations of the overlap in different configurations. The modeling of adhesive joints was done using ALGOR 2-D, linear and nonlinear finite element analyses (FEA). In the third stage, tensile shear tests conducted on the lap joints validated the trends from the finite element models. Finite element modeling and meshing of the lap joints having 25.4 and 50.8 mm adhesive overlap lengths were completed. Different configurations of rubber toughened and untoughened adhesive were tried in these two overlaps. The validation was done by lap joint tests conducted on an Instron mechanical tester coupled with an extensometer. Comparable strengths were obtained for completely toughened overlap and the configuration where only the edges of the adhesive overlap were toughened and the region in-between was untoughened. Also, the nonlinear FEA was shown to represent the experimental results more closely than the linear approach.