The effect of temperature on the behavior of the interphase in polymeric composites

The single-filament fragmentation method for measuring the fiber/matrix stress transfer was used for the identification of interphase perturbations. This technique is based on the measurement of the fiber length resulting from the multiple fracture of a single fiber embedded in a resin specimen during tensile loading. A series of single-fiber fragmentation experiments was conducted over a wide range of temperatures on the AS4-carbon-fiber/Epon-828/PACM20-epoxy-resin system. Critical aspect ratios, the magnitude of which is considered to be inversely proportional to the square root of the matrix modulus, showed a significant increase from ambient to elevated temperatures, at temperature levels much lower than the glass transition point of the bulk matrix. This increase was consistent with the existence of an interphase of lower glass transition temperature than the bulk matrix. A three-concentric-cylinder elastic model was employed to correlated the effect of material properties.     

Properties of modified anhydride hardener and its cured resin

Methyl-nadic-tetrahydric-methylanhydride (MNA), nadic-tetrahydric-methylanhydride (NA), anhydride hardener was modified by solid diol molecule to improve the impregnation resin fracture toughness in cryogenic temperature. The lap-shear strength, transverse tension as well as the thermal shock test showed that the resin cured by the modified anhydride hardener had higher bond strength and more toughness at 77 K. After the experiment of vacuum pressure impregnation (VPI) processing, it was found that this resin had a longer usable life, better impregnating properties, but higher initial viscosity than the resin hybrid HY925 as hardener.     

Matrix-dominated tensile behaviour of unidirectional T300/914 and structural modelling of the material

Unidirectional T300/914 carbon epoxy composite has been tested under transverse tensile loading at various temperatures and test rates. The test programme revealed a different viscoelastic response for the bulk resin modulus from that of the composite modulus and associated fractographic examination showed the structures of the resin and composite to be complex. The resin consisted of an epoxy-based particulate phase embedded within a thermoplastic-based connecting phase. The addition of carbon fibres resulted in the formation of an irregular epoxy-based interphase region. A parameter, , has been defined that easily characterizes the viscoelastic behaviour of the composite. Structural models have been successfully developed for both the resin and the composite and a cure mechanism suggested. Any plastic deformation occurring on failure was within the connecting phase and the weakest point of the composite was identified as being the particle/interphase boundary. Property modelling has successfully accounted for the interphase influence on the transverse tensile modulus and the composite transverse tensile strength was found to be higher than the bulk resin tensile strength.     

Interphase investigation in glass fibre composites by micro-thermal analysis

The micro-thermal analysis combines the advantages of both microscopy (visualisation) and thermal analysis (characterisation), with high spatial resolution. The experiment was performed on glass fibre composites based on cyanate resin matrix in order to highlight the interphase in these composites. Two types of glass fibres, which differed only by the sizing, were inserted in the resin. The subsurface distribution of the glass monofilaments can be observed into the conductivity images. Localised thermal analysis were carried out by positioning the probe over selected regions: glass monofilaments, bulk resin and interphase. The glass transition decreases from the bulk resin value to a value about 40 °C lower, for composites with sizing glass monofilaments. A less dense cross-linkage can be observed in the interfacial area.     

Cooling rate influences in carbon fibre/PEEK composites. Part 1. Crystallinity and interface adhesion

The effect of cooling rate on the fibre–matrix interface adhesion for a carbon fibre/semicrystalline polyetheretherketone (PEEK) composite was characterised based on the fibre fragmentation, fibre pullout and short beam shear tests. The interface adhesion was correlated to the degree of crystallinity and the crystalline morphology, as well as the bulk mechanical properties of neat PEEK resin, all of which were in turn controlled by cooling rate. It was shown that the interface bond strength decreased with increasing cooling rate; the tensile strength and elastic modulus of PEEK resin decreased, while the ductility increased with increasing cooling rate through its dominant effect on crystallinity and spherullite size. The improvement of crystalline perfection and flattened lamella chains with high crystallinity at the interphase region were mainly responsible for the strong interface bond in composites processed at a low cooling rate. The interphase failure was characterised by brittle debonding in slow-cooled composites, whereas the amorphous PEEK-rich interphase introduced in fast cooled specimens failed in a ductile manner with extensive plastic yielding.     

Investigation of properties of fiber/matrix interphase formed due to the glass fiber sizings

Sizings on glass fibers consist of a silane-based network that is chemically bound to the fiber and other compounds that are adsorbed onto the glass surface. Formation of interphase involves dissolution of adsorbed species and inter-diffusion of these compounds and resin monomers into the interphase region and chemical reaction of available functional groups. All these phenomena occur at the presence of the silane-based network. In this study, the effects of the silane-based network on the properties of the interphase region are investigated for an epoxy/amine resin system and compatible sized glass fibers. The composition of the sizing material bound to glass was determined using nuclear magnetic resonance (NMR) spectroscopy. Based on this information, model interphase materials were synthesized that were a blend of an epoxy/amine matrix and inclusions. The inclusions consist of an interpenetrating network of silane-based polymer and epoxy/amine thermoset that represents the interphase material formed during processing. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) techniques were used to characterize the glass transition temperature and flexural modulus of the model materials. The properties of the model interphase material were obtained using the DMA results and established micromechanics models. The results show that the glass transition temperature of the model interphase is about –5°C, and its flexural storage modulus at room temperature is about 50% of that of the bulk matrix. This work has also shown that a reduction in the cross-link density of the bound network might significantly reduce the modulus within the interphase region by a factor of 5 to 8.     

The influence of fibre surface treatment on the formation of an interphase in CFRP

In the scheme of a EURAM programme the influence of a wet oxidative surface treatment on the formation of an interphase, on the fibre-matrix bond strength and on the mechanical properties is investigated. The fibre CG 43–750 was supplied by Courtaulds treated to four different surface treatment levels (designated STL=0% or untreated, 10, 50 and 100%) sized (1% by weight) and impregnated with the resin HG 9106. This resin consists of di-, tri- and tetrafunctional epoxies with the hardener 3.3 DDS and also contains polyethersulphone. During the cure this resin separates in a continuous (thermoset-rich) phase which completely covers the fibres and a discontinuous (thermoplastic-rich) phase with a roughly globular structure. From water uptake experiments and matrix (interphase) sensitive composite properties (shear modulus G ₁₂, transverse modulus E ₂₂) it was concluded that the activated carbon fibre surface gives rise to a more fully crosslinked interphase, resulting in a reduction of the modulus of this interphase.     

Fracture in epoxy matrix resins

The occurrence of fracture-energy-enhancing steps and welts on fracture surfaces of crosslinked matrix resins has been studied in an epoxy obtained from a trifunctional epoxy resin cured with an anhydride. It is suggested that the steps and welts arise from an underlying basic longitudinal texture, which was revealed by strongly tilting fracture specimens toward the collector in a scanning electron microscope. A model for the development of the basic longitudinal texture is proposed involving a meniscus instability of the propagating crack front, which gives rise to a series of fingers protruding into the bulk resin ahead of the nominal crack front. The periodicity of the basic longitudinal texture seen in the epoxy specimens studied was roughly 350 nm, which was independent of the epoxy resin: hardener ratio within at least 10% of stoichiometry. Because the periodicity of the basic longitudinal texture is roughly equal to the separation of the fracture surfaces immediately behind the crack, a considerable blunting of the crack by plastic deformation or yielding is suggested, a property that should depend on the matrix resin.     

基于峰值力纳米力学模量成像技术的碳纤维/树脂复合材料界面尺寸与性能的原位表征

为准确测定复合材料界面结构的尺寸与性能,提出一种原位峰值力纳米力学模量成像(PF-QNM)技术,对其测试原理、校准方法和适用性进行分析,并采用PF-QNM技术对碳纤维/聚醚醚酮(T300/PEEK)、碳纤维/聚醚砜(T300/PES)和碳纤维/环氧树脂(T700/TR1219B)三种复合材料的界面尺寸和各组分原位模量进行测试。结果标明:该方法的横向分辨率可以达到纳米尺度,测得树脂、界面、纤维区域的弹性模量数值呈梯度上升趋势,区分度明显,T300/PEEK、T300/PES和T700/TR1219B复合材料界面厚度分别为(69.3±7.9)nm、(101.3±10.2)nm和(48.4±5.4)nm。实验范围内,热固性复合材料界面厚度小于热塑性复合材料。对模量成像图统计分析可得,T300/PEEK、T300/PES和T700/TR1219B复合材料的树脂区平均弹性模量分别为4.36 GPa、4.96 GPa和3.59GPa,与其宏观弹性模量数值较为接近。     

Ultrasonic characterization of the quality of an epoxy resin polymerization

This work concerns monitoring the polymerization of an epoxy resin and its hardener. An ultrasonic pulse echo technique was used to monitor the attenuation, the phase velocity, and the acoustic impedance of the resin as a function of time. The first two parameters give information about the average state of the hardening of the resin itself. The third parameter, acoustic impedance, indicates the state of the hardening of the resin at the interface with the vessel. These parameters are determined from spectral properties of echoes extracted from the experimental echo waveforms. Experiments were made for different proportions of hardener and allowed a determination of the best mixture (10% of hardener) that corresponds to the manufacturer's recommended value. Analysis of the results shows a progressive hardening from the center of the resin toward the walls of the vessel.     

Hardener type as critical parameter for the electrical properties of epoxy resin/polyaniline blends

Epoxy resin/polyaniline-salt (EP/PANI-DBSA) blends were prepared and cured with different types of hardeners. The effect of hardener type on the volume resistivity of the blends was investigated. It was found out that acidic curing agents support the conductive character of polyaniline, while alkaline hardeners conflict this property. Moreover, colour changes of the mixtures after the addition of the hardener revealed possible transformation of the conductive PANI-DBSA salt to the nonconductive PANI base form. An UV-vis analysis confirmed these assumptions. Despite of the acidic character of the anhydride type hardener, low conductivity was obtained. This was related to its insufficient curing action in the presence of PANI-DBSA. Furthermore, IR spectra of EP/PANI-DBSA blends varying the hardener type and the wt% of PANI-salt (< 10 wt%) were recorded. These indicate a possible connection of the shoulder creation at the 1585 cm^{– 1} band, spectral region 1575 to 1560 cm^{– 1}, with the degree of volume resistivity.     

Flexural and interlaminar shear strength properties of carbon fibre/epoxy composites cured thermally and with microwave radiation

The ease of heating an epoxy resin with microwaves depends, among other factors, on the dielectric properties of its components at the frequency of the radiation used. The majority of the papers published on the microwave curing of reinforced epoxy resin composites have used widely available DGEBA type resins and amine hardeners such as 4,4′-diaminodiphenylsulphone (DDS). This paper investigates the use of two epoxy systems where the choice of resin and hardener was based on their measured dielectric loss factors. System 1 contained a resin and hardener with higher loss factors than those used in System 2. The two systems are formulated with polyetherimide (PEI) as a toughening agent. Unidirectional carbon fibre prepregs were prepared from both systems. Composites were laid up from these prepregs, which were then cured in three different ways: autoclave curing, partial autoclave curing followed by microwave post-curing, and microwave curing. System 1 composites had greater flexural properties and interlaminar shear strengths than System 2 composites when autoclave cured. Flexural properties and interlaminar shear strengths were greater for System 2 in the microwave post-cured composites. When fully microwave-cured the properties were similar. In the microwave-cured composites the flexural and interlaminar shear properties were influenced by the structure of the phase separated PEI and the void content.     

The influence of interphase regions on local thermal displacements in composites

Although the exact physical and chemical mechanisms are not clearly understood, it is widely believed that an interphase region with properties that differ from those of the plain matrix is developed near fiber surfaces in polymer matrix composites. The current study involves experimental investigation and theoretical modeling of the influence of the interphase on local thermal displacements. Experimental studies have centered on the development of a scanning microinterferometer for in-situ measurements of thermal displacements in the interphase. Thermal displacement measurements have been successfully made for specimens containing a single carbon fiber embedded in an epoxy matrix. A three-phase composite cylinder model is adopted to predict the thermal displacements of the single fiber specimen. Comparison of the theoretical displacement predictions with the experimental profiles measured by the interferometer indicate that the value of the matrix properties near the fiber surface differs from the value in the bulk resin. The data provide evidence of the existence of a lower glass transition temperature in the interphase.     

Characterization of epoxy resin-based anion-responsive polymers: applicability to chloride sensing in physiological samples

The potentiometric properties of an epoxy resin-based anion-responsive polymer and its feasibility as a chloride sensing membrane are described. The response mechanism of the epoxy resin-based membrane is studied by varying the types and contents of epoxy components (e.g., resin, hardener, diluent, and plasticizer). It is found that epoxy resins cured with polyamine-type hardeners have excellent characteristics as chloride-sensing membranes, in terms of their selectivity, stability, sensor lifetime, and adhesion for constructing an all-solid-state sensing device. In addition, effects of the diluent or plasticizer added in an epoxy resin/hardener system on the potentiometric behaviors are investigated. To demonstrate the practical analytical utility of the epoxy resin-based electrodes, chloride levels in various controls and biological specimens (e.g., human sera and whole blood) are determined with a flow-cell system.     

Characterisation of interphase nanoscale property variations in glass fibre reinforced polypropylene and epoxy resin composites

The local microstructure can be altered significantly by various fibre surface modifications, causing property differences between the interphase region and the bulk matrix. By using tapping mode phase imaging and nanoindentation tests based on the atomic force microscope (AFM), a comparative study of the sized fibre surface topography and modulus as well as the local mechanical property variation in the interphase of E-glass fibre reinforced epoxy resin and E-glass fibre reinforced modified polypropylene (PPm) matrix composites was conducted. The phase imaging AFM was found a highly useful tool for probing the interphase with much detailed information. Nanoindentation experiments indicated the chemical interaction during processing caused by a gradient profile in the modulus across the interphase region of γ-aminopropyltriethoxy silane (γ-APS) and polyurethane (PU)-sized glass fibre reinforced epoxy composite. The interphase with γ-APS/PU sizing is much softer than the PPm matrix, while the interphase with the γ-APS/PP sizing is apparently harder than the matrix, in which the modulus was constant and independent of distance away from the fibre surface. The interphase thickness varied between less than 100 and ≈300 nm depending on the type of sizing and matrix materials. Based on a careful analysis of ‘boundary effect’, nanoindentation with sufficient small indentation force was found to enable measuring of actual interphase properties within 100 nm region close to the fibre surface. Special emphasis is placed on the effects of interphase modulus on mechanical properties and fracture behaviour. The interphase with higher modulus and transcrystalline microstructure provided simultaneous increase in the tensile strength and the impact toughness of the composites.     

Polymerization of epoxy resins studied by positron annihilation

Positron annihilation lifetime spectroscopy (PALS) has been applied to study polymerization of epoxy resins of cresole novolac with a hardener of phenol novolac. PALS uses positrons to probe the microstructure of a nanometer (nm) size. Using PALS polymerization can be followed through three states: powder (monomer), liquid and solid. PALS is a unique method for the detection of intermolecular spaces, hence polymerization was followed from the point of view of free spaces (inter-molecular spaces) between polymer networks. The glass transition temperature (T_g) was determined from the temperature dependence of the positronium (Ps) lifetime. Although Tg determined by PALS is usually lower than that determined by a mechanical analysis (TMA), it was observed that T_g approached the value determined by TMA after long curing. Ps can form bubbles in a liquid, and the surface tension of a mixture of the resin and the hardener was calculated from a simple empirical formula using the Ps lifetime; the resulting value is similar to that of the bisphenol-A epoxy resin. Gelation was observed as an increase in the intensity of Ps and a sharp decrease in the lifetime. Received: 9 October 2000 / Reviewed and accepted: 10 October 2000     

Crack propagation in an amine-cured epoxide resin

Crack propagation has been studied in a DGEBA epoxide resin cured with two different series of amine hardeners. Both series produced a cured resin in which cracks propagated in a stick-slip manner in a series of jumps. With a primary diamine hardener, the initiation energy depended on the molecular weight of the hardener, but with a mixed amine hardener the initiation energy was independent of molecular weight. There were also significant differences in fracture surface morphologies between the two series. The effect of temperature and testing rate on the fracture energy and fracture surface morphology of ethylene diamine cured resin has also been studied. The fracture initiation energy increased with increasing temperature and decreasing testing rate and there is some indication of a peak occurring in the 22° C data. Long term static loading experiments have failed to show the occurrence of a static fatigue effect. Different batches of nominally identical resin produced different results, leading to problems of irreproducibility of behaviour.     

碳纤维复合材料界面结构的形貌与尺寸的表征

为了准确测定碳纤维增强树脂基复合材料界面结构的形貌和尺寸, 本文中介绍了一种原位纳米力学动态模量成像技术, 并采用该方法对碳纤维增强热固性树脂基复合材料进行了测试, 对该技术在界面结构测试上的参数设置、 数据处理方法以及适用性等方面进行了分析。研究表明, 该方法的横向分辨率可以达到纳米尺度, 适于测量界面尺寸在纳米级别的碳纤维复合材料界面形貌与尺寸。对于碳纤维/环氧树脂和碳纤维/双马树脂体系, 界面区的储能模量呈梯度变化, 根据储能模量成像图的统计分析可得到界面的形貌和厚度。所得界面平均厚度在100nm左右, 横截面上界面形貌呈不均匀的“河流状”, 并与碳纤维表面形貌相似。      

Shear strength of polymers and fibre composites: 2. carbon/epoxy pultrusions

Pultrusions were made with carbon fibres and an epoxy resin. Three different curing agents were used, so that the matrices were resins with different glass transition temperatures. The composites were tested for shear strength at different temperatures, so that the effect of the resin shear strength on composite shear strength could be observed, with a fixed fibre architecture. It was found that the composite was always much stronger than the resin both for the 0 and 90° fracture modes. The 90° fracture surfaces contained many broken fibres, and shear hackles were observed in the resin-rich regions. These suggested that shear failure (rather than tensile failure) took place in the Iosipescu test for the 90° specimens. It was concluded that the fibre architecture played a dominant role in the composite shear strength, with interphase effects being involved also.     

一种中温定型剂及其RTM预制体性能

研究了ES-T321定型剂及其配对的RTM专用环氧树脂3266的混合使用性能(如相容性、流变性和热性能等).ES-T321可以制备成丙酮溶液或粉末定型材料,使用方便.ES-T321的熔点比3266树脂的RTM注射温度窗口高,具有较好的抗流动冲刷性能,能够保证定型织物在树脂注射时不被冲散.ES-T321在3266树脂的固化温度下能溶解在树脂中,并与3266的固化剂反应.加入定型剂对树脂混合体系的热性能几乎没有影响.定型处理的预成型体的回弹效应小,溶液法定型的预成型体的回弹比粉末法更小.