Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part III. The Effects of Postcure

A novel method for tailoring the interphase of carbon fiber-polymer composites by resistive electric heating is presented. The single-fiber fragmentation test is used to investigate the adhesion and fracture properties of the interphase. Electric resistive heating is shown to increase adhesion and toughness at the interphase region. In analyzing the results, the strength and fracture energy of the interphase are related to the thermal postcure conditions created by resistive electric heating. For this purpose, a difference analysis method is used to obtain a numerical solution for the heat conduction problem in the single-fiber test specimen and the temperature distributions are determined. Improvements obtained by using resistive electric heating of the carbon fiber are compared with those obtained by postcuring of the whole sample via convective thermal postcuring. The results obtained using these two different postcure methods seem to be similar.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part II. The Effects of Cure Temperature Time and Curing Agent Content

The effects of curing agent content, cure temperature and time on the adhesion strength between carbon fibers (treated with epoxy sizing) and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Theoretical equations describing a majority of the phenomena affecting the adhesion process are also reviewed. The possibility of superposition between percent curing agent content, cure temperature and time is illustrated based on the analytical models presented. Experimental data are presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. Such presentation of the data allows for interpretation of the skewness in the data population. Optimum curing agent, cure temperature and time values resulting in highest interfacial strength are also determined.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part II. The Effects of Cure Temperature Time and Curing Agent Content

The effects of curing agent content, cure temperature and time on the adhesion strength between carbon fibers (treated with epoxy sizing) and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Theoretical equations describing a majority of the phenomena affecting the adhesion process are also reviewed. The possibility of superposition between percent curing agent content, cure temperature and time is illustrated based on the analytical models presented. Experimental data are presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. Such presentation of the data allows for interpretation of the skewness in the data population. Optimum curing agent, cure temperature and time values resulting in highest interfacial strength are also determined.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate,Test Temperature,Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate, Test Temperature, Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

The Effects of Interphase Properties on Interfacial Shear Strength in Polymer Matrix Composites

The effects of interphase elastic modulus, glass transition temperature, and thickness on interfacial shear strength were investigated both experimentally and theoretically. Single fiber fragmentation tests were performed over a range of temperatures on samples with tailored interphases. Three different types of interphases were investigated: high modulus/high glass transition temperature, low modulus/low glass transition temperature and uncoated (no tailored interphase). A reduction in interfacial shear strength with temperature was observed for all three types of samples tested. The magnitude of this decrease was found to correlate with the glass transition temperature of the interphase. The low T_g interphase samples showed large reductions in IFSS, while samples with a higher T_g coating showed only a small decrease. A three-phase, axisymmetric elasticity solution was developed to predict the sensitivity of the stress state to the interphase material properties and temperatures used in the experimental studies. Predictions which incorporated the change in modulus of both the matrix and interphase with temperature were in good agreement with the experimental trends. Both the theoretical and experimental results supported the existence of an interphase with reduced glass transition in the uncoated samples.     

Adhesion Mechanisms of Polyurethanes to Glass Surfaces. III. Investigation of Possible Physico-Chemical Interactions at the Interphase

Surface free energies of polyurethanes made from toluene diisocyanate and 1, 4 butanediol-based hard segments and caprolactone polyol-based soft segments were calculated using additive functions. Good agreement was found between the calculated values based on additive functions and the calculated values based on contact angle measurements. The phase-separated polyurethanes were found to have a higher polar surface free energy component (γ^P). This was linked to the preferential segregation of butanediol/butanediol-derived moieties to the polyurethane surfaces due to phase separation. The adhesion values of these polyurethanes to soda-lime glass were correlated with their respective γ^P values and a linear relationship was found. It was also shown that the adhesion values of the low γ^P polyurethanes improved substantially when the glass surfaces were coated with a thin layer of butanediol prior to the bonding. The modulus of the interphase region rich in butanediol was evaluated. Although a modulus increase was found at the interface, this increase was found to play a secondary role in the adhesion. The chemical interactions at the polyurethane/glass interphase were investigated by pre-treating the glass surfaces with methyl-trimethoxysilane and trimethylchlorosilane prior to adhesion testing. The adhesion data showed no significant difference between the uncoated and the silane-treated glass substrates. Based on this experimental evidence, the possibility of any covalent or ionic bonding at the polyurethane/glass interphase was assumed negligible. It was determined that the mechanism of adhesion between the polyurethanes and the glass surface could be through the formation of an interphase region in which hydrogen bonding between the butanediol-rich interphase region and the hydroxylated glass surface plays a key role.     

Adhesion Mechanisms of Polyurethanes to Glass Surfaces. III. Investigation of Possible Physico-Chemical Interactions at the Interphase

Surface free energies of polyurethanes made from toluene diisocyanate and 1, 4 butanediol-based hard segments and caprolactone polyol-based soft segments were calculated using additive functions. Good agreement was found between the calculated values based on additive functions and the calculated values based on contact angle measurements. The phase-separated polyurethanes were found to have a higher polar surface free energy component (γ^P). This was linked to the preferential segregation of butanediol/butanediol-derived moieties to the polyurethane surfaces due to phase separation. The adhesion values of these polyurethanes to soda-lime glass were correlated with their respective γ^P values and a linear relationship was found. It was also shown that the adhesion values of the low γ^P polyurethanes improved substantially when the glass surfaces were coated with a thin layer of butanediol prior to the bonding. The modulus of the interphase region rich in butanediol was evaluated. Although a modulus increase was found at the interface, this increase was found to play a secondary role in the adhesion. The chemical interactions at the polyurethane/glass interphase were investigated by pre-treating the glass surfaces with methyl-trimethoxysilane and trimethylchlorosilane prior to adhesion testing. The adhesion data showed no significant difference between the uncoated and the silane-treated glass substrates. Based on this experimental evidence, the possibility of any covalent or ionic bonding at the polyurethane/glass interphase was assumed negligible. It was determined that the mechanism of adhesion between the polyurethanes and the glass surface could be through the formation of an interphase region in which hydrogen bonding between the butanediol-rich interphase region and the hydroxylated glass surface plays a key role.     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

Effects of Microwave Processing on Fiber-Matrix Adhesion. II. Enhanced Chemical Bonding of Epoxy to Carbon Fibers

The effect of microwave processing on the chemical interactions occurring between the carbon fiber surface and the epoxy matrix constituents was investigated using X-ray Photoelectron Spectroscopy (XPS). Monofunctional model compounds selected to duplicate the matrix constituents were exposed to the carbon fibers at temperatures similar to those encountered during composite processing. After solvent extraction, chemisorbed species were quantified by XPS. Differences were apparent in the C 1s and O 1s core electron regions of the microwave treated samples when referenced to the same elemental regions of thermally (convection) treated samples. Specifically, the atomic percentage of oxygen (in the form of carbon oxides) was increased to a greater degree when using the microwave treatment as opposed to the thermal treatments. The microwave treatment resulted in a substantial increase in the amount of chemical interaction between the fiber surface and the epoxy resin and amine components of the matrix. An epoxy resin/amine hardener adduct compound was also used to investigate the possible interaction of the adduct hydroxyl group with the carbon fiber surface. XPS results indicate a low to insignificant interaction of the hydroxyl with the carbon fiber surface under the conditions used in this study.     

XPS of the Interphase Between PMMA and Metals. A Study of Degradation and Interaction

XPS was applied to PMMA layers of varying thickness in the nanometer range on gold and natural aluminium surfaces, respectively. The numerical decomposition of the core levels into the contributions from the various atomic bonding states reveals some peculiar features of the interphase region to the metals. First, for both metals special mechanisms of thermal destruction are deduced. On Au the splitting of complete methacrylate side groups and, even more intense, the stripping of (-O-CH₃)-units is found while on Al the side group scission is verified only. Secondly, the metals reveal different effects on the electron density in the methacrylate groups. Au appears to be inert but the natural Al surface exerts a withdrawing effect on the electrons of both the carbon and the oxygen atoms in the side groups.     

Effects of Simple Abrasive Combing and Pretreatments on the Properties of Pineapple Leaf Fibers (Palf) and Palf-Vinyl Ester Composite Adhesion

Despite being mechanically and environmentally sound, pineapple leaf fibers (PALF) are of little use in Malaysia and the least studied for composite applications. In this study effects of abrasive combing and simple pretreatments on PALF and their adhesion to vinyl ester were investigated. In pineapple leaves, PALF are present in top lamina as large vascular bundles and bottom lamina as fine strands. Tensile strength and modulus of fine PALF strands are 155% and 134% higher than those of vascular bundles respectively. Abrasive combing reduced PALF diameters by 50.3% resulting in finer bundles with 48.6% higher modulus and 51.5% greater strength without much negative effects on fiber integrity. Water-soak did not change PALF tensile properties significantly, while dilute sodium hypochlorite (NaOCl) solution improved PALF modulus and strength by as much as 123% and 35% respectively while reducing elongation at break by 47%. Dilute solution of NaOCl changed PALF structurally through higher crystallinity and closer packing resulting in increased tensile strength and modulus. PALF thermal stability was also enhanced. PALF-vinyl ester adhesion improved due to bleach pretreatment indicated by significantly reduced fiber pull-out length of broken PALF-vinyl ester composites. Morphological study using scanning electron microscope was used to confirm the findings. This study indicated that abrasive combing and simple pretreatment of dilute sodium hypochlorite are potential techniques to produce cost-effective PALF for reinforcing plastics.     

The Role of Adhesion Promoters on the Molecular and Mesoscopic Structure of the Interphase

New experimental results are described from Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM) on the molecular orientation, chemical composition, and topography of PMDA/ODA Polyimide (PI) deposited by the Langmuir-Blodgett (LB) technique onto Si (100) surfaces covered with a native oxide (SiO_x) and onto SiO_x substrates pre-treated with a variety of alkylsilanes. The alkylsilane monolayers used as adhesion promoters were found to be chemically and structurally stable during temperature treatments used to imidize the polymer precursor films. On both the oxidized silicon surface and on the silane-treated surfaces, we find a thickness-dependent, preferential orientation of the pyromellitimide unit in PI, but randomly-oriented oxydianiline fragments. We can not identify any specific chemical bonds between the polymer and the aminosilane film or, in the case of the untreated surface, the SiO_x substrate. However, a quantitative analysis of the NEXAFS data reveals a pronounced deficit in oxydianiline in the monolayer (ML) films, which decreases with film thickness, and a higher degree of imidization on the 3-aminopropyltrimethoxysilane (APS) surface indicating that imide linkages are formed between the polymer and the aminosilane. The AFM images show a heterogeneous surface topography with nucleation of PI polycondensates on both the SiO_x and aminosilane-treated surfaces. On the aminosilane surface additional nuclei are detected, believed to be polycondensates of aminosilane formed during imidization of the PI precursor. Nucleation of aminosilane polycondensates is not observed in the absence of polyamic acid. As suggested by a comparison of Lateral Force Microscopy (LFM) and Atomic Force Microscopy measurements, these aminosilane nuclei are covered by PI with increasing film thickness and, hence, provide mechanical connections between the polymer and substrate on a mesoscopic scale. Delamination experiments of 2 μm thick PI films spun onto ω-amino and methyl terminated alkylsilanes indicate that bond fracture always occurs in the aminosilane/PI and alkylsilane/PI and interphase. We conclude that the adhesion promotion effect of LB-deposited polyimide films by aminosilanes can be explained by chemical stabilization of the interface against decomposition and mechanical interlinking surface via polycondensates of aminosilane on a mesoscopic length scale. Our study is not conclusive as to whether covalent bonding of the polymer to the substrate via the silane is important for macroscopic adhesion.     

The role of the vapour phase in fluid coker cyclone fouling: Part 1. Coke yields

The cyclone sections of fluid coking reactors at oil sand upgrading facilities have been observed to undergo severe coke fouling during each operational cycle. This work was conducted to determine if vapour phase deposition mechanisms could be responsible for the formations observed in the cyclones. The effects of vapour phase temperature (from 490°C to 560°C) and residence time (from 0.4 to 4.4 s) were studied. Raising the vapour phase temperature above the temperature at which the vapours were derived was found to significantly increase both the yield and rate of production of vapour phase coke.     

On characterizing microscopically the adhesion interphase for the adhesion between metal and rubber compound Part III. Effect of brass-plating amount for brass-plated steel cord

Three kinds of brass-plated steel cords with different brass-plating amounts from 2.62 to 5.01 g/kg were prepared and their adhesion properties to rubber compounds were studied. Adhesion properties decreased with the increase in the brass-plating amount from 2.62 to 3.88 g/kg. Furthermore as the increase of brass-plating amount over 3.88%, adhesion property increased. The strong adhesion of brass-plated steel cord with low brass-plating amount to the rubber was shown not only after curing but also after hostile aging treatments. The good adhesion stability against humidity aging of adhesion property for brass-plated steel cord with low brass-plating amount to rubber compound was discussed in terms of the formation and growth of adhesion interphase determined by AES depth profile. The good adhesion stability of brass-plated cord with low brass-plating amount can be resulted from the suppression of the excessive growth of copper sulfide and the inhibition of dezincification due to the low brass-plating amount.     

The role of the vapour phase in fluid coker cyclone fouling: Part 2. Liquid and vapour yields and compositions

The vapours derived from thermally cracked Athabasca bitumen were exposed to temperatures phase 490°C and 560°C in order to investigate the effects of vapour phase thermal cracking on product yields and campositiom. Raising the vapour phase temperature above the temperature at which the vapours were derived increased both the yield of non‐condensable vapours and the amount of olefins in the produced gas. In addition, the liquid yield decreased with increasing vapour phase temperature and changes in liquid product composition were also observed. The H/C ratio of the liquid product decreased with increasing vapour phase temperature.     

On the Effects of Processing Conditions and Interphase of Modification on the Fiber/Matrix Load Transfer in Single Fiber Polypropylene Composites

The scope of this study was the investigation of the effects of both processing conditions (in terms of thermo-mechanical history) and interphase modification (fiber sizing and/or matrix coupling) on the interfacial shear strength (τ_i) of fiber reinforced isotactic polypropylene (iPP). Fiber/matrix load transfer efficiency was investigated by modified single fiber pullout and microdroplet pulloff test methods, respectively. It was established that τ_i of the neat microcomposite (unsized fiber/uncoupled matrix) is improved by quenching of the samples rather than by various spherulitic or transcrystalline supermolecular structures set under isothermal crystallization conditions. Enhanced interfacial shear strength for the quenched samples was attributed to a better wetting behaviour and a fine dispersion of the amorphous PP (aPP) fraction formed. An adhesion model was proposed based on which optimum τ_i is linked to both matrix strength and its wetting behaviour. It was demonstrated that the results from pullout and pulloff tests correlate very well with each other for the particular glass fiber/iPP model composite systems studied. It was shown further that matrix modification (coupling) or fiber sizing enhances τ_i practically to the same level, whereas a combination of matrix coupling and fiber sizing yields an even higher interfacial shear strength (synergistic effect).     

The Effects of Laser Radiation at 248 nm on the Surface Characteristics and Joint Properties of Aluminum Adherends

Procedures presently used to prepare surfaces of aluminum adherends for bonding with structural adhesives entail the use of corrosive solutions that are environmentally hazardous.

As an alternative, we investigated whether eximer laser radiation can effectively be used to prepare the surfaces of aluminum adherends. The results indicate such a methodology to be very promising. Using a KrF laser, at a radiant intensity of 1.67 × 10¹³ W/m²/pulse, successive mm² regions of aluminum coupons were irradiated for fixed numbers of pulses/region. This resulted in changes to the topographies and oxidation states of the surfaces. Symmetric lap shear joints from coupons so treated had considerably increased strains at fracture and 24% greater joint strengths as compared with controls. The results further indicate that fracture toughness can be increased above that of presently used procedures. The topographies of the treated and control surfaces were characterized using a new topographic characterizing system. An elastic model is presented that relates failure characteristics to experimentally-determined topographic parameters. The results suggest that different mechanisms for joint enhancement are operative.     

On characterizing microscopically the adhesion interphase for the adhesion between metal and rubber compound Part II. Effect of copper content for brass-plated steel cord

Three brass-plated steel cords with different copper contents from 51.4 to 75.5% were prepared and their adhesion properties to rubber compound were examined. Adhesion properties improved with the decrease in the copper content. The high adhesion of brass-plated steel cord with low copper content to the rubber was obtained after cure and after hostile aging treatments. The stability against humidity aging and the cause for the high adhesion of the brass-plated steel cord with low copper content were discussed in conjunction with the formation and growth of adhesion interphase determined by Auger electron spectroscopy depth profile. The excellent adhesion stability of brass-plated cord with low copper content can be explained by the suppression of the excessive growth of copper sulfide and the inhibition of dezincification due to the low copper content.     

The Influence of Curing Agent Content on the Mechanical and Adhesive Properties of Dicyandiamide Cured Epoxy Systems

Mechanical and adhesive properties were evaluated for an elastomer-modified model epoxy system cured with varying amounts of dicyandiamide (Dicy). The modulus and bulk fracture toughness values were found to be independent of Dicy content, whereas the adhesive performance was shown to be greatly influenced. For increases in Dicy content, single lap shear (SLS) failure values increased, while quasi-static double cantilever beam (DCB) performance was decreased. These adhesion test results are discussed in a comparative fashion. Preliminary x-ray photoelectron spectroscopy (XPS) surface evaluations are presented to discuss the drastic changes in DCB performance. It is postulated that in these model adhesive systems the Dicy variations produce significant chemical changes only in the epoxy/steel interphase region.