Showing papers on "Epoxy published in 1978"

Constrained cracking in glass fibre-reinforced epoxy cross-ply laminates

Abstract: Specimens of 90° cross-ply glass-reinforced epoxy resins were tested in tension parallel to the direction of reinforcement in the outer plies. The thickness of the inner ply was varied and cracking constraint was observed at small thicknesses. At large inner-ply thicknesses the specimens showed uniform transverse cracking, and at very low inner-ply thicknesses this transverse cracking could be suppressed completely prior to total specimen failure. Fracture toughness values were determined on transverse unidirectional laminates of the same volume fraction. It was found that the cracking constraint observed can be accounted for, using the theory of Aveston and Kelly.

Two rapid and simple methods used for the removal of resins from 1.0 μm thick epoxy sections

Abstract: SUMMARY Ultrathin epoxy sections are commonly used in electron microscopy. More useful information can often be obtained by examining thicker (0.5–3.0 μm) resin sections under the light microscope. However, there are some histological stains that are unable to penetrate these resins. This short note describes the easy and rapid preparation of two resin-removal solutions which allow subsequent staining of 1.0 μm thick sections without obvious tissue damage and with excellent results using standard histological dyes.

The mechanisms of elevated temperature property losses in high performance structural epoxy resin matrix materials after exposures to high humidity environments

Abstract: : The objective of this research program was the determination of the mechanisms by which epoxy resins utilized in high performance composites and adhesives exhibit losses in their elevated temperature properties as a result of exposure to a humid environment. The study specifically considered an epoxy resin formulation that is characteristic of a variety of composite and adhesive systems. The moisture absorption and diffusion characteristics of this resin system were first investigated as a function of simple humidity environments and simulated real-life environments which included therm spikes. In addition, the effect of absorbed moisture on the resin system's glass transition temperature, T(g), was determined as a function of both simple humidity and real-life exposures. This was followed by a study into the tensile and creep behavior of the epoxy resin as a function of absorbed moisture. A variety of experimental techniques were utilized to accomplish these studies: constant strain rate and dynamic tensile measurements, infrared spectroscopy, heat distortion tests, bomb creep tests, scanning electron microscopy, polarized-light photomicroscopy, and ESCA. The results established that moisture plasticized this particular epoxy resin, causing a lowering of the Tg which in turn affected mechanical response, such as by shifting the relaxation moduli to shorter times. This phenomenon is described and quantified in terms of an existing free volume theory relationship.

On multiple transverse cracking in glass fibre epoxy cross-ply laminates

Abstract: An investigation has been made of multiple transverse cracking in glass fibre epoxy cross-ply laminates. Four laminates of differing transverse ply thicknesses were investigated. Transverse crack spacing was found to decrease with increasing applied stress and decreasing transverse ply thickness. Very close agreement has been found between the experimental results and a multiple cracking theory based on shear lag analysis in which the plies remain essentially elastically bonded. In these composites a small modulus change is observed at a strain lower than that at which cracking initiated. This phenomenon is associated with a visual, under some circumstances reversible, whitening effect.

Relationship between structure and mechanical properties in rubber‐toughened epoxy resins

Abstract: Toughened polymers were prepared by adding CTBN rubbers to DGEBA-type epoxy resins. Structure was varied by altering the type and concentration of hardener, the initial molecular weight of the resin, the amount of Bisphenol A added, and the conditions of cure. Electron microscopy showed that these factors affected both particle size and degree of phase separation: rapid curing inhibited phase separation, and produced small particles. Increasing the molecular weight of the resin, either directly or by reaction with Bisphenol A, improved phase separation. Dynamic mechanical measurements of rubber phase volume proved possible, although Tg of the CTBN rubber coincided with a β process in the epoxy resin. Fracture resistance, measured by GIC, increased linearly with rubber phase volume. Creep and yield behaviour were also affected by the degree of phase separation.

Fatigue and Residual Strength Degradation for Graphite/Epoxy Composites Under Tension-Compression Cyclic Loadings:

Abstract: A fatigue residual strength degradation model has been generalized to account for the effect of tension-compression fatigue loading. The model is verified experimentally by use of graphite/epoxy unnotched composite laminates. It is shown that the correlation between the experimental results and the theoretical predictions on both the residual strength degradation and fatigue life is very reasonable.

Liquid crystal display and photopolymerizable sealant therefor

Abstract: A method of making a liquid crystal display cell wherein the planar glass plates of the cell are bonded together with a photopolymerizable epoxy-containing material to form a hermetically sealed cell and wherein the photocured seal is chemically resistant to the contents of the liquid crystals cell. Adhesion of the photopolymerizable epoxy material to the glass plates of the liquid crystal cell is improved by addition of glycidoxypropyl triethoxy silane or its precondensate.

Photocurable epoxy-acrylate compositions

Abstract: This invention relates to radiation curable compositions comprising mixtures of epoxy resin and di- or polyacrylate or methacrylate terminated monomers in combination with a catalyst system comprising a carbonyl type photoinitiator with an aromatic onium salt capable of effecting the cure by release of a Lewis Acid.

The Durability of Epoxies.

Abstract: : This report reviews the state of knowledge of the basic factors that control the durability of epoxies in service environments. The structure-property relations of amine (diethylene triamine, DETA)-cured bisphenol-A-diglycidyl ether (DGEBA) and amine (diaminodiphenyl sulfone, DDS)-cured tetraglycidyl 4,4'diaminodiphenyl methane (TGDDM) epoxies, the effects of sorbed moisture, and the fabrication and environmental factors that control the durability of epoxies are reviewed. The modes of deformation and failure of these materials, which occur primarily by crazing, are controlled by their network structure and microvoid characteristics. The pertinent, basic, physical phenonema induced and/or modified by sorbed moisture that affect the durability of epoxies are reviewed. These include the plasticization, swelling stresses, and modification of the crazing process in epoxies caused by sorbed moisture together with the diffusion characteristics of moisture in these materials. The durability of epoxies in service environments are discussed in terms of the structure-property relations of epoxies, the effect of fabrication and environmental factors and their complex interactions on the formation of permanent damage regions. (Author)

Ultrasonic wave and moduli changes in a curing epoxy resin: Ultrasonic techniques are examined as a means for monitoring reaction extent and the development of solid-phase moduli in a curing epoxy

Abstract: During the cure cycle, thermosetting polymeric resins change in physical character from a viscous liquid to a gel and finally to a hardened solid. Ultrasonic-wave-propagation techniques can be used to monitor both the extent of cure and the development of solid-phase moduli. This paper outlines such a program and presents results for a room-temperature, amine-cured epoxy system. Signal attenuation and wave-speed data are given as a function of cure time for 1.0 MHz longitudinal and shear waves. The behavior observed is first interpreted in terms of linear-viscoelastic wave-propagation theory. Then using the wave speeds, apparent elastic moduli are calculated as a function of cure time. Finally, relations are developed which suggest a mapping between the moduli and a normalized reaction-extent variable. Implicit in these relations is that the moduli themselves can serve or be viewed as reaction-extent variables.

Effect of Stacking Sequence and Lay-Up Angle on Free Edge Stresses Around a Hole in a Laminated Plate Under Tension

Abstract: One mode of failure in fiber reinforced laminated plates is delamination initiating at a free edge. Experimental results and theoretical stress investi gations found in the literature indicate that the interlaminar normal stress, σz, is a factor in this mode of failure. In this paper, the stress behavior around a circular hole in a laminated plate is investigated using a three- dimensional finite element stress analysis. First, computed results for the tangential strain distribution around a circular hole for a (0/±45/0) s lamin ate under uniaxial loading are compared with laboratory data to provide a level of confidence in the analysis. Next, attention is focused on the inter laminar normal stress distribution around the free edge of the circular hole and the effects of changes in stacking sequence and lay-up angle. Initially, laminates with stacking sequences of (0/90)s are analyzed. Graphite/epoxy laminates of the type (02/±θ/∓θ)S, (±θ/∓θ/0 2)S, (902/±θ/∓θ) 2 , and (±θ/∓θ/902)S where θ is 30° , 45°, an...

The effect of glass‐resin interface strength on the impact strength of fiber reinforced plastics

Abstract: The effect of glass-resin interface strength on the impact energy of glass fabric (style 181) reinforced epoxy and polyester laminates has been determined. The interface strength was altered by surface treatment of the fabrics with silane coupling agents and with a silicone fluid mold release and the interlaminar shear strength was determined as a means to evaluate the interface strength. An instrumented Charpy impact test was used on unnotehed specimens and thus both initiation and propagation energies could be determined as well as dynamic strength. It was found that the initiation energy for both polyester and epoxy laminates increased with increasing interlaminar shear strength, The propagation energy and thus the total energy for polyester laminates displays a minimum at a critical value of interlaminar shear strength (ILSS). Below this critical value, the total impact energy increases with decreasing shear strength and the dominant energy absorption mode appears to be delamination. Above the critical value, the impact energy increases with increasing values of ILSS and the fracture mode is predominantly one of fiber failure. In all cases, even with mold release applied, the shear strength of epoxy laminates was above this critical value and-thus the total impact energy increases with Increasing values of ILSS. The maximum energy absorbed for the epoxy laminate and the polyester laminate is nearly identical. However, the maximum for the epoxy laminate occurs when the shear strength is maximized while for the polyester laminate the shear strength must be minimized. For the polyester laminate when delamination is predominant, it was found that the glass surface treatment affects the amount of delamination as opposed to the specific value of delamination fracture work.

Effect of Moisture on Epoxy Resins and Composites

Abstract: The effect of absorbed moisture on the properties of six neat epoxy resins (3501-5, 3501-6, 3502, 5208, 934, and NMD 2373) and three epoxy composites (AS/3501-5, B/5505, and hybrid B/5505-AS/3501-5) is explored. Excellentagree. ment among the diffusion coefficients, activation energy, and activation entropy is observed for all resins with the exception of NMD 2373. Good correlation exists between neat resins and corresponding composites. The effect of a weekly thermal cycle on the diffusivity and equilibrium moisture content of two resins and three composites is presented. Correlations are developed between preconditioning temperature and relative humidity on the magnitude of the anomalous moisture absorption. In addition, the effect of absorbed water on the glass transition temperature of six neat epoxy resins and two composites using thermomechanical analysis is considered. The water generally behaves as a classical plasticizer of the epoxy resin, lowering the glass transition temperature as predicted from theory. Comparison between neat 3501-5 resin and AS/3501-5 reveals generally good correlation in T g at comparable resin moisture levels, but plasticized hybrid boron-graphite/epoxy laminate exhibits a somewhat lower T g than predicted from theory.

Tensile, impact and fatigue behavior of an amine‐cured epoxy resin

Abstract: Although crosslinked networks are commonly used as adhesives and composite matrixes, structure-property relationships are not as well established as with thermoplastics. For this reason, an extensive study was begun to systematically examine effects of stoichiometry, morphology, and distribution of crosslink density on viscoelastic behavior and ultimate properties. The system selected was based on a bisphenol-A-type epoxy cured with methylene dianiline. This paper describes and discusses results obtained for resins in which the amine/epoxy ratio ranged between 0.7 and 2.2. In agreement with reports by others, the tensile strength, modulus, and ultimate elongation were relatively insensitive to stoichiometry but did not show slight maxima or minima when the amine was somewhat in excess. Impact strengths, tensile energies-to-break and fracture toughness were, in contrast, quite sensitive, though the patterns of each differed significantly. Both fracture toughness and the stress intensity factor required to drive the crack at a given rate varied directly with the amine/epoxy ratio, as did estimates of the characteristic flaw size. Fatigue striations were observed on the fracture surfaces and corresponded to the incremental advance of the crack in one loading cycle.

Swelling of Composite Laminates

Abstract: : The swelling behavior of a graphite/epoxy (AS/3501-5) is shown to be described by a model which is based on the assumption that the swelling is negligible until the moisture concentration reaches a threshhold value and then increases linearly thereafter. The model also provides a means of measuring the volume fraction of voids. The analytical results, which take into account the curing stresses, are compared with the experimental data for the transverse and thickness strains of symmetric laminates as well as for the warpage of unsymmetric laminates. The initial absorption on virgin specimens facilitates the subsequent diffusion and the residual stresses appear to be responsible for the absorption being slower than the desorption during the early stages of moisture conditioning. Warpage of unsymmetric laminates is discussed in light of the cracks found in constituent plies. Finally derived is a temperature-relative humidity relationship which renders the graphite/epoxy composite laminates free of residual stresses. (Author)

Evaluation of the tensor polynomial failure criterion for composite materials

Abstract: A comprehensive experimental and analytical evaluation of the tensor polynomial failure criterion was undertaken to determine its capability for predicting the ultimate strength of laminated composite structures subject to a plane stress state. Results are presented demonstrating that a quadratic formulation is too conservative and a cubic representation is required. Strength comparisons with test data derived from glass/epoxy and graphite/epoxy tubular specimens are also provided to validate the cubic strength criterion.

Sub‐Tg annealing studies of rubber‐modified and unmodified epoxy systems

Abstract: Epoxy network systems based on DGEBA (bisphenol‐A‐diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl‐terminated butudiene‐acrylonitrile copolymer) rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time‐dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near‐linear behavior with the logarithm of time at sub‐Tg annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time‐dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.

Mutagenic action of aromatic epoxy resins.

Abstract: EPOXY RESINS are used extensively for protective coatings and in paints and adhesives; the consumption of epoxy resins during 1973 was 190,000 metric tons, which is an almost twofold increase over a period of 10 years1. About 90% of the epoxy resins used are manufactured by condensation of two molecules of epichlorhydrin with one or more molecules of bisphenol A (2,2-bis (4-hydroxyphenyl)-propane). The mutagenic action of simple epoxides such as ethylenoxide and epichlorhydrin was first shown in fruit flies2 and has subsequently been demonstrated in a wide range of organisms. Epoxy resins are diepoxides and bifunctional alkylating agents, and bifunctional alkylating epoxides are known to be mutagenic3,4. Mutagens are suspected of having a carcinogenic effect because of the relationship between carcinogenicity and mutagenicity5 and most diepoxides are carcinogenic in mice and rats6. We report here that aromatic epoxy resins are mutagenic in Salmonella typhimurium, and may thus represent a cancer risk in man.

Weldable coating compositions

Abstract: An improvement in a weldable coating for metallic substrates containing a high molecular weight epoxy or phenoxy resin, an electroconductive pigment such as zinc and a diluent such as a glycol ether wherein the improvement resides in replacing at least about 70 weight percent of said resin with a composition containing a low molecular weight epoxy resin such as the diglycidyl ether of bisphenol A, a phenolic hydroxyl-containing compound such as a phenolic hydroxyl terminated adduct of a diglycidyl ether of a diphenolic compound such as bisphenol A and a catalyst for effecting the reaction between the epoxy resin and the adduct.

Polyester resin composition

Abstract: PURPOSE: A polyester resin composition moldable into articles having improved thermal and mechanical characteristics free from warping, comprising an aromatic polyester, specific amounts of platy glass flakes, a rubber-like copolymer, and an epoxy compound CONSTITUTION: A composition comprising (A) 100 parts by wt of an aromatic polyester, eg polytetramethylene terephthalate, (B) 5W120 parts by wt of platy glass flakes having a major axis ≤ 1,000μ and an aspect ratio ≥ 5, (C) 0W120 parts by wt of an inorganic solid, eg calcium carbonate, (D) 1W100 parts by wt of a rubber-like copolymer comprising a vinyl monomer, and (E) 001W10 parts by wt per 100 parts by wt of the total amount of (A), (B), (C), and (D) of an epoxy compound, eg diglycidyl polyether The ingredient (D) is preferably a copolymer prepared by grafting a vinyl monomer, eg methyl methacrylate or styrene, onto a rubber-like polymer, eg a diene or acrylate polymer COPYRIGHT: (C)1979,JPO&Japio

Some anomalies associated with moisture diffusion in epoxy matrix composite materials

Abstract: Diffusion of moisture into graphite/epoxy composites was investigated and some anomalies observed. In particular, data are presented which indicate a departure from classical Fickian behavior. An apparent two-stage diffusion process is observed in both the neat epoxy resin and derived graphite/epoxy composites. Evidence is presented which suggests time-dependent matrix cracking as the mechanism associated with the two-stage diffusion process. Comparison of laminate diffusion data with unidirectional data suggests that the diffusion coefficient is stress dependent.

Diffusion of Water Vapor in Graphite/Epoxy Composites

Abstract: T300/5208 graphite-fiber-reinforced epoxy laminates were exposed to 33 different combinations of temperature and relative humidity. The diffusivities and equilibrium solubility for both nonpostcured and postcured laminates were determined at 21, 43, and 63°C (70, 110, and 145°F). Non-Fickian absorption anomalies were observed at five different relative humidities and water immersion for specimens at 82°C (180°F). Moisture diffusion was examined with respect to possible ply thickness, stress, and moisture concentration effects.

Surface-coating binders for cathodic electrocoating

Abstract: Surface-coating binders for cathodic electrocoating which comprise a protonized heat-curable reaction product (A), substantially free from epoxide groups, of (a) one or more Mannich bases, prepared from (a 1 ) one or more polyhydric condensed phenols which may or may not contain ether groups. (a 2 ) one or more secondary amines which contain one or more hydroxyalkyl groups, with or without one or more secondary amines which do not contain such groups, and (a 3 ) formaldehyde or a formaldehyde donor, with (b) one or more epoxy resins, Which binder also contains from 1 to 40% by weight, based on the total of (A) + (B), of a water-insoluble non-ionic resin (B), free from epoxide groups and containing urethane groups, which resin has been obtained by reacting a partially blocked diisocyanate or higher polyisocyanate with a reaction product of a polyepoxide and one or more low molecular weight compounds containing SH and/or OH groups. These surface-coating binders are especially used for the cathodic electrocoating of metal articles.

Simple methods for demonstration of epoxy resins of bisphenol A type.

Abstract: Low molecular weight oligomers of epoxy resins of bisphenol A type are common sensitizers. For demonstrating the presence of sensitizing oligomers of these resins, two simple methods are described. The first one, a colour reaction, demonstrates the presence of the bisphenol A skeleton. If this test is positive, thin-layer chromatography is carried out to demonstrate the presence of low molecular weight oligomers of epoxy resins. Some practical applications are reported.

The fracture energy of hybrid carbon and glass fibre composites

Abstract: The fracture energy of a model carbon fibre/glass fibre/epoxy resin hybrid composite system has been evaluated as a function of the carbon fibre/glass fibre ratio. Work of fracture measurements were less than a rule of mixtures prediction and a pronounced negative synergistic effect was observed at high carbon fibre and high glass fibre contents. Fibre debonded lengths and fibre pull-out lengths for the carbon and glass fibres were accurately measured using a projection microscope technique. Models of microscopic fracture behaviour, together with these measurements, were successful in quantitatively describing the observed fracture behaviour of the hybrid fibrous composites. It was found that post-debond friction energy provided a major contribution to the fracture energy of the glass fibres. The post debond sliding mechanism was also shown to be primarily responsible for the non-linear behaviour of the work of fracture of the hybrid composite.