Strength of resin-coated-adhesive-bonded double lap-shear pultrusion joints at ambient temperature

The inherited adhesion limitation of polyester and vinyl ester resin-based pultruded GFRP makes pultrusions difficult to bond, especially when a thixotropic adhesive is used. While such an adhesive is necessary for gap filling, it has a limited wettability. Therefore, coating the adherend with low-viscosity epoxy resin, prior to bonding, improves wetting and hence increases joint strength. The paper describes the experimental methodology to achieve this, using double lap-shear (DLS) joints with various materials combinations. A significant strength improvement was reached as a result of coating the inner adherend in conjunction with using a “high adhesion” outer adherend. To further understand the effect of coating, numerical stress analysis was undertaken, including preliminary micro-models representing the composite/adhesive interface as well as overall DLS models.     

Scanning electron microscopy of styrene-divinylbenzene copolymers

Summary Styrene-divinylbenzene copolymers were synthesized by suspension polymerization in presence of di(2-ethylhexyl) phosphoric acid and methyl ketone as diluents. The surface and bulk morphology of the beads were investigated through scanning electron microscopy. The influence of diluent-copolymer interaction, dilution degree and interfacial tension on the porous structure formation are discussed.     

Scanning electron microscopy of coal macerals

Individual macerals separated from some United Kingdom coals of Carboniferous age and bituminous rank were examined by scanning electron microscopy. In each case a specific morphology characteristic of the macerals studied could be recognized. Collinite (a member of the vitrinite maceral group) was recognizable in all samples by its angular shape and characteristic fracture patterns, the particles (30–200 μm) frequently showing striated or laminated surfaces. Sporinite particles had no well defined shape and were associated with more detrital material than were the other macerals studied. This detritus was shown by conventional light microscopy to be the maceral micrinite. Fusinite was remarkable in having a ‘chunky’ needle form, with lengths of up to 200 μm.     

Scanning electron microscopy of soybean protein bodies

Protein bodies prepared from defatted soybean flour contained numerous spherical particles 1–3 μ in diameter, plus amorphous material, when examined in a scanning electron microscope. Full-fat and defatted soybean flours contained particles 1–10 μ in diameter. The larger protein bodies apparently disrupted during isolation. The scanning technique is a simple and rapid method for observing the effects of various treatments on subcellular seed particles of this size.     

Scanning electron micrographic studies of ettringite formation

The morphology of ettringite, formed by various reactions (C₃A + gypsum, C₃A + anhydrite, CA + magnesium sulfate, in the presence and absence of calcium hydroxide) was studied by scanning electron microscopy. Methods included immersion of solid samples into saturated solutions of the other reactant, and paste hydration. It is concluded that slender needles and spherulites are formed only if sufficient space is available; alternately (e.g. in pastes of lower water-to-solid ratios) ettringite occurs as short, prismatic crystals. Ettringite is formed in C₃A-gypsum pastes near the surface of C₃A grains by a through-solution mechanism; a topochemical mechanism can be definitely excluded. Results are significant for a better understanding of mechanism of expansion and set retardation by gypsum.     

Effect of neutral salt spray (NSS) exposure on the lap-shear strength of adhesive-bonded 5052 aluminum alloy (AA5052) joints

This work explores the mechanisms for the non-linear degradation of the lap-shear strength of adhesive-bonded 5052 aluminum alloy (AA5052) joints exposed to neutral salt spray (NSS) environment up to 1200?hours. The lap-shear testing results show that a mixed failure dominated by adhesive failure occurred in both unexposed and exposed adhesive-bonded AA5052 joints. The lap-shear strength of adhesive-bonded AA5052 joints declined sharply in the preliminary stage of NSS exposure (e.g. shorter than 240?hours), then it decreased moderately as the NSS exposure time increased. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electrochemical noise measurement were applied to exploring the degradation mechanisms of the lap-shear strength of adhesive-bonded AA5052 joints. The adhesive properties remained nearly unchanged after exposure in the NSS environment. The decreased hydrogen bond at the AA5052/adhesive interface was responsible for the sharp degradation of the lap-shear strength of adhesive-bonded AA5052 joints in the preliminary stage of NSS exposure. In the later stage of NSS exposure (e.g. more than 240?hours), the surface oxidation of AA5052 substrates dominated the lap-shear strength degradation of adhesive-bonded AA5052 joints.     

Scanning electron microscopy in the detergent industry

In 1965, scanning electron microscopes became available commercially. The use of these instruments and other new equipment (such as X-ray microanalysis and cathodoluminescence) created new possibilities for evaluation of detergent qualities permitting further insights into the morphology and inorganic chemical composition of powder particles, including the surfaces and the inside regions. The new instruments also can be used in testing procedures during product development. Use of scanning electron microscopy in evaluation of detergents was discussed using selected examples related to investigations concerning (a) stability of bleaching activator prills, (b) coating of enzyme marums, (c) distribution of optical brighteners, (d) distribution of the inorganic constituents of powder particles, (e) thermal stability of sodium triphosphate hexahydrate, and (f) development of sodium aluminum silicates (SASIL) as phosphate substitutes. The examples were chosen to show the possibilities of scanning electron microscopy. Together with its additional features and in combination with other methods of physical and chemical analysis, scanning electron microscopy can provide important information concerning solid surfaces, which are relevant in the field of detergent chemistry.     

Scanning electron microscopy and ultrasonic studies of drawn polypropylenes

Ultrasonic measurements and scanning electron microscope studies are reported on a series of drawn samples of polypropylene. The ultrasonic measurements were performed over the frequency range 15 to 35 MHz and over a temperature range 283 to 343K. The scanning electron microscope observations confirmed the predictions from density data that the ‘void’ content initially increases on drawing and then decreases. The acoustic measurements on the samples with the highest draw ratios indicated that severe restriction of the motion of the polymer occurs when the fibrous structure is formed.     

Scanning electron microscopy study of chemically modified coir fibers

Chemical‐surface modification of coir fibers was done by dewaxing, using an alkali treatment (5% and 10% NaOH), vinyl grafting with methyl methacrylate (MMA) and cyanoethylation. The chemically modified fibers were characterized by Fourier transform infrared (FTIR) spectroscopy. In addition, the surface features of untreated, dewaxed, alkali‐treated, grafted, and cyanoethylated coir fibers were studied using scanning electron microscopy (SEM). Progressive changes in surface morphology were observed. SEM observations showed the removal of tyloses from the surface of coir as a result of alkali treatment (5%), resulting in a rough fiber surface with regularly spaced pits. At a lower percentage of grafting (PMMA), the surfaces became more or less uniform, while the surfaces of the coir fibers with a higher percentage of grafting were increasingly covered with grafted materials, resulting in canal‐like cavities between the overgrowths of the grafted materials on the unit cells. Cyanoethylated coir‐fiber surfaces showed an insufficient deposit of cyanoethyl groups. SEM analysis of the samples was corroborated by measurements of a mechanical property (maximum stress at break). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1169–1177, 2001     

Scanning electron micrographs of high density polyethylene fracture surfaces

Three high density polythylenes with widely differing molecular weights have been subjected to a fast fracture process and studied by scanning electron micrography. Two polymers in the low to medium molecular weight range show evidence of substantial melting during facture, in support of some previous work. On the other hand the ultra high molecular weight material seems to deform on a more massive scale without recognisable relics of a melting process.     

Notch sensitivity and fractography of polyolefins

The notch sensitivity of high-density polyethylene (HDPE), polypropylene (PP), and polybutylene (PB) was evaluated using uniaxial tensile deformation and fractographic analysis. Each polyolefin was tested at relatively low and high molecular weights (MW). Only the lower MW HDPE was found to be clearly notch-sensitive. The lower MW PP exhibited some tendency toward notch sensitivity. The lower and higher MW PB, the higher MW HDPE, and the higher MW PP displayed notch strengthening. Whereas PB showed similar notched tensile performance regardless of molecular weight, both HDPE and PP showed higher susceptibility to notch sensitivity at lower molecular weights (and concomitant higher crystallinity). Tendencies toward notch sensitivity or notch stengthening were evidenced in the failure modes of these materials.     

Stress redistributions in adhesively bonded double-lap joints, with elastic–perfectly plastic adhesive behavior, subjected to axial lap-shear cyclic loading

A shear-lag model is developed in order to evaluate stress redistributions in double-lap joints under axial (tensile) lap-shear cyclic loading. The adherend materials exhibit linear elastic behavior, whereas the material of the adhesive layer satisfies the elastic–perfectly plastic shear stress–strain constitutive relation. The reference state (from which the stresses are redistributed) is based on the standard elastic–perfectly plastic shear-lag analysis for double-lap joints. The main conclusion of the current analysis is that, during unloading, shear stresses of opposite sign may develop in the plastic zones of the adhesive layer, at the ends of the overlap, without reversing the direction of the applied load. A simple model for evaluating the variation of the maximum peel stress in the adhesive layer, based on the variation of the peak shear stress, demonstrates that the sign of peel stresses may alternate, as well. Under cyclic (fatigue) loading, the range of the peak stresses in the adhesive layer is the basic parameter for the evaluation of the variation of the energy release rate and the associated crack growth rate in the overlap. In this framework, the current simplified analysis may provide a reference model for comparisons with experimental data or with results which are based on more complex numerical models. The current model can be readily extended to cover the cases of development of plastic zones in the adhesive layer with shear stresses and plastic strains of opposite sign (during unloading or during load direction change).     

Scanning electron microscopic study on the catalytic gasification of coal

The behaviour of nickel catalyst during coal gasification of Leopold coal (West Germany) was examined by means of scanning electron microscopy. Microscopic observations of the same field were made several times as the reaction proceeded. In addition to the uncatalysed gasification, the nickel-catalysed gasification was clearly observed under the microscope. With steam gasification, catalysts moved very actively, and they seemed to accelerate the gasification mainly by pitting holes into the char. The topological changes on char surfaces by hydrogasification were not so pronounced. The function of catalyst may not be restricted to the pit formation, for it seems to accelerate the gasification over all of the char surface. Because of the high hydrogasification temperature, agglomeration of catalyst takes place to a considerable extent. Finely dispersed nickel catalysts were observed when the coal had been pretreated with liquid ammonia. The catalytic activity of these fine particles was so large that the char beneath them was gasified rapidly.     

Scanning electron microscopy studies on tear fracture of natural rubber

Tear fracture surfaces of peroxide- and sulphur-cured natural rubber vulcanizates, both unfilled and filled, have been studied using scanning electron microscope. Peroxide-cured unfilled vulcanizate shows stick-slip tear whereas the HAF black-filled vulcanizate gives rise to a layered fracture surface. Tear branching, observed in the unfilled sulphur-cured vulcanizate changes to a high level of tear deviation resulting in a rough surface, with the addition of HAF black. FT black improves tear resistance only slightly and the fracture surface appears comparatively smooth. The low level of polymer-filler interaction in clay filled vulcanizates causes the filler agglomerates to come out of the matrix during the test thus decreasing tear resistance and giving rise to a pitted surface.     

Low temperature tensile lap-shear testing of adhesively bonded polyethylene pipe

This work studies the lap-shear strength performance of polyethylene pipeline bonded with acrylic adhesive in the temperature range -10 to +20 °C. Single lap shear test samples were firstly prepared at 20 °C under various clamping pressures and curing times to determine suitable conditions under which to prepare and test further samples at temperatures of -10, -5, 0, +5 and +20 °C. It was found that a decrease in curing/testing temperature to zero degrees resulted in a steady reduction in the lap-shear strength performance of the bonded joints from a mean value of 2.72 MPa at +20 °C to 1.15 MPa at 0 °C. Below zero degrees the strength of the bonded substrates was significantly reduced; no samples bonded at -5 °C had sufficient strength to test and only one sample bonded -10 °C was tested, which had very low strength of 0.105 MPa.     

Scanning electron microscopy study of raw and chemically modified sisal fibers

Mercerization and acetylation treatments were applied to sisal fibers to enhance adhesion with polymer matrices in composites. The structures of the untreated and treated fibers were assessed with scanning electron microscopy. The waste from sisal‐fiber decortication consisted of mechanical, ribbon, and xylem fibers, and their ultimate cells varied considerably in size and shape. After mercerization and acetylation, the fibers and conductive‐vessel surfaces were successfully changed. The parenchyma cells were partially removed, and the fibrils started to split, because of the alkali action. This increased the effective surface area available for contact with the matrix. The mercerized and acetylated fibers were coated with cellulose acetate by the grafting of the acetyl group in the fibrils. The treatment used to remove lignin and hemicellulose caused changes in the fiber surface but did not damage the fiber structure because the fibrils remained joined in a bundle. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2333–2340, 2004     

Scanning electron micrograph study of the impact-improved MBS/PVC blends

The improvement of the impact strength of the telecommunication equipment using injection moldable PVC/MBS and other modifiers were studied. The addition of MBS to the PVC was found with rapid increases in the impact stregths in Kg-cm/cm: 5.5 of pure PVC; 80 of 12 phr and the maximum of 131 of 20 phr of MBS. This increasing impact strength phenomena were subjected to close examinations of SEM, DMTA, and strain-stress diagrams. The DMTA analysis of the retaining of the individual T_g's of MBS and PVC in the MBS/PVC blends, indicates that no miscibility of MBS and PVC has occurred. The DMTA diagrams of 5, 12, 20, and 30 phrs of MBS in PVC were analyzed and the same samples were also subjected to the impact strength measurements and the SEM examinations. SEM photos showed a gradual transition of the linear to the three-dimensional arrangements had occurred in the PVC/MBS blends. The formation of three-dimensional arrangement occurred between 10–20 phr of added MBS. The spacial change from linear to the three-dimensional arrangements observed in SEM photos correlates the rapid increases of the impact strengths of the specimens tested. Injection specimens of the telephone sets made of PVC/MBS blends with 20 phr of MBS in PVC indicate that the impact strength is averaged at 115 Kg-cm/cm with tensile strength of 330 Kg/sq cm and elongation of 150%. The similar change of the spacial arrangement is also observed in their SEM photos.     

Scanning transmission electron microscopy of multiphases in graphite-alkali metal intercalation compounds

Structural and micro-analytical evidence is presented for the presence of multiphase regions in graphite-Rb intercalation compounds for stages n ? 2. The intercalate layers are composed of islands of alkali metal, ordered incommensurately with respect to the adjacent graphite layers and embedded in a background of disordered rubidium in the intercalate layer. The results confirm the non-integral stoichiometry of graphite alkali metal intercalation compounds for stages n ? 2.