Nondestructive characterization of prepreg ageing using nuclear magnetic resonance techniques

The ageing of prepregs of and fiber reinforced duroplastic polymers (glassfiber and carbonfiber reinforced polymers; GFRP and CFRP) during storage before manufacturing and during the manufacturing process at elevated temperatures is an essential technical problem. The resulting mechanical properties of cured components are strongly influenced by the ageing state of the prepregs. Nondestructive techniques up to now are unsufficient or time consuming. Nuclear magnetic resonance (NMR) techniques have been well known for 40 years and applied in analytical chemistry in order to characterize the microstructure and binding state of organic chemical compounds in fluid states. In medical diagnostics NMR-imaging is used for tissue characterization. Publications on solid state NMR-imaging are rare in technical literature because of the fact that the strong spin-spin-interaction in solids results in a broadening of the spectral lines, i.e., very short transversal relaxation times, T₂ (some µs), compared to fluids (up to some 100 ms). Therefore special electronics are needed in order to produce short pulses and to switch the gradient fields in short time periods.The emphasis of the following contribution will be on the presentation of first results on the application of NMR techniques at prepegs in order to characterize the crosslink state after exposition at room and elevated (50°C) temperature. The results correlate with results of destructive tests and document the potential of NMR as a NDT tool. The experiments were performed with a MSL-400 Bruker NMR-spectrometer and micro-imaging system which works at 400 MHz (¹H resonance frequency at a magnetic field of 9.4 T).This article is dedicated to Professor Dr. Paul Höller on the occasion of his 65th birthday.     

Phase contrast micro-tomography and morphological analysis of a short carbon fibre reinforced polyamide

The mechanical properties of components made of short fibre reinforced composites, generally obtained by injection moulding, are strongly linked to fibre orientation. Therefore, it is of great importance to be able to verify the results of manufacturing process simulations obtained by commercial software. From the experimental point of view, the definition of carbon short fibre structure within a polymeric matrix in a micro-tomography is a non-trivial task, as the X-rays absorption properties of the two phases are very similar. This paper presents how this problem was overcome by using phase contrast imaging techniques. High resolution fibre structure reconstructions could therefore be obtained. The reconstruction of a large sample volume by overlapping of successive tomographies was also discussed. Moreover, this work shows that the anisotropy identification techniques based on morphological parameters, previously introduced by some of the Authors for short glass fibre reinforced polymers, can also be adopted for fibre arrangement identification in this type of materials.     

Luminescence of heat-treated silicon-based polymers: promising materials for LED applications

A new strategy to obtain transparent, thermally stable, and formable photoluminescent materials for LED applications is presented. Starting from commercially available silicon-based polymers, luminescence properties are developed by means of simple heat treatment. Solid polymethylsilsesquioxane MK (Wacker-Besil^®PMS MK) and liquid poly(ureamethylvinyl)silazane Ceraset (Kion Ceraset^® PUVMS) were thermally treated between 200 and 700 °C for 2 h under Ar atmosphere. Photoluminescence properties were observed in all the samples. The structural rearrangements during thermal annealing were effective in order to red-shift the emission spectra of the untreated polymers to the visible range. The formation of dangling bonds and carbon sp², associated with the annealing procedure and confirmed by means of Electron Paramagnetic Resonance (EPR) spectroscopy and solid state Magic Angle Spinning NMR (MAS-NMR) contribute to the red-shift of the photoluminescence emissions of the polymers. After heat treatment at low temperatures (200, 300, and 400 °C), both the polymers show fluorescence in the UV range. While the polysiloxane reveals white luminescence after annealing at 500 and 600 °C, the polysilazane heat-treated at 500 °C exhibits emission in the blue-green range and is transparent. At higher temperatures the presence of free carbon counteracts the luminescence properties.     

Possible Nature of Field-Induced Antiferromagnetic Ordering in SC Cuprates

In the present work, the results of recent neutron scattering, STM, and NMR experiments concerning the structure of the vortex core in resistive state of the cuprates are discussed. It is demonstrated that “field-induced” antiferromagnetic (SDW) ordering observed in the SC state is the same as “temperature-induced” one arising above T _c in the pseudogap (SDW) state. It is pointed out that in resistivity measurements, due to short mobile charge carrier relaxation time, a magnetic structure is sampled on much shorter time scale as compared with other techniques such as neutron scattering, Mossbauer effect, etc. It is noted that existing theoretical models mainly consider the SDW order only as competing to the SC one rather than stimulating the SC to appear at higher temperature.     

Application of X-ray refraction topography to fibre reinforced plastics

The effect of X-ray refraction employs an unconventional small angle X-ray scattering (SAXS) technique which has been developed and applied to meet actual problems for improved non-destructive characterisation of advanced materials. The X-ray refraction technique makes use of X-ray optical effects at micro interfaces of composite materials. This method reveals the inner surface and interface concentrations in nanometer dimensions due to the short X-ray wavelength near 10⁻⁴ μm. Sub-micron crack and pore sizes are easily determined by “X-ray refractometry” without destroying the structure by cutting or polishing for microscopic techniques. The non-destructive characterisation of microfailure e.g. voids, fibre debonding, fibre cracks and microcracks of a short glass fibre reinforced polyoximethylene (POM-GF) after mechanical loading and accelerated ageing is investigated. X-ray refraction topographs are illustrated, showing the damage accumulation of POM-GF specimens after the fatigue test.     

A method of measuring energy dissipation during crack propagation in polymers with an instrumented ultramicrotome

In order to characterize very local energy dissipation during crack propagation in polymers, an ultramicrotome was instrumented to measure the energy dissipated during sectioning. The work to section per unit area, W _s, was measured for five different amorphous polymers [polymethyl methacrylate (PMMA), polystyerene (PS), polycarbonate (PC) and two epoxy resins] in the glassy state. When the section thickness was varied between 60 and 250 nm, W _s varied between 15 and 100 Jm^–2, depending on the material and section thickness. The method and the results are compared with other methods used for determining the energy dissipation at a local level as well as at a macroscopic level in polymers. The differences between different polymers were found to be contradictory to macroscopic fracture toughness, G _lc, measurements. The material that showed the highest W _s had the lowest G _lc values reported. Possible mechanisms for energy dissipation during sectioning are also discussed.     

Computational intelligence aspects for defect classification in aeronautic composites by using ultrasonic pulses

Production of carbon fiber reinforced polymers (i.e., one of the basic material of the modern airplanes) is an elaborate process unfree from faults and problems. Errors during the manufacturing or the plies' overlapping, in fact, can cause particular flaws in the resulting material, so compromising its same integrity. Within this framework, ultrasonic tests could be useful to characterize the presence of defect, depending on its dimensions. On the contrary, the requirement of a perfect state for used polymers is unavoidable in order to assure both transport reliability and passenger safety. Therefore, a real-time approach able to recognize and classify the defect starting from the measured ultrasonic echoes could be very useful in industrial applications. The ill-posedness of the so defined process induce a regularization method. In this paper, an heuristic approach is proposed for this aim. Particularly, the proposed method is based on the use of support vector machines. Obtained results assure good performances of the implemented classifier, with very interesting applications.     

Characterization of aluminosilicate on rutile surface by high resolution solid state NMR

Pigments of titanium dioxide rutile, coated with SiO₂>-Al2O₃, have been characterized by high-resolution solid state NMR. The model samples were prepared following four synthesis routes differing by the aluminate and silicate salts addition sequence. From ²⁷Al single pulse MAS and 3Q-MAS NMR experiments, we characterize short range order around Al and distinguish several types of environment: Al_IV, Al_V, Al_VI and Al_VI linked to Ti, depending upon synthesis route and thermal treatment. The major difference between the different samples is observed after heat treatment at 750 °C. Rotational echo double resonance ²⁷Al-¹H and cross-polarization H-Si experiments provide longer range distance information through dipolar coupling to proton. Two types of surface treatment can be distinguished from ²⁷Al-¹H Redor by the presence of proton free alumina domain in the surface treatment.     

Study of the degradation of non-conventional MgO-SiO2 cement reinforced with lignocellulosic fibers

This paper assesses the use of low alkaline composites based on magnesium oxide and silica (MgO-SiO₂) cement and reinforced with cellulose fibers for the production of thin elements to resist bending loads. The strategy adopted was to study the durability of lignocellulosic fibers in a lower pH environment than the ordinary Portland cement (OPC), by comparing the flexural performance of samples at 28 days and after 200 accelerated ageing cycles. Two types of vegetable fibers were used: eucalyptus and pine pulps. For both types of fibers, composites made out of MgO-SiO₂ cement after ageing treatment show a better mechanical performance than OPC samples (modulus of rupture of ∼10.5 and 9 MPa respectively). When used in MgO-SiO₂ cement matrices, eucalyptus fibers offer excellent specific energy (SE) values (∼5 kJ/m²) compared to OPC samples in which SE drastically decreases after ageing from 4.97 kJ/m² to 0.14 kJ/m². The preservation of the reinforcing capacity of the composite materials after ageing was also proved by SEM techniques. In the light of the results, the use of MgO-SiO₂ cements is an effective way to apply cellulosic fibers as reinforcement in fiber-cement products since no signs of degradation were found, even improving flexural properties over time.     

Effects of 400 keV electrons flux on two space grade silicone rubbers

Two different space grade silicone rubbers were irradiated by an electron flux of 400 keV. The irradiation impact strongly depends on the chemical structure of rubbers (one reinforced with MQ resins, and the other one functionalized with phenyl groups at the silicon atoms and reinforced with silica). The irradiated rubbers were studied by means of solvent swelling, solid-state ²⁹Si NMR, and ATR–FTIR spectroscopy. Physical properties were evaluated by thermal (differential scanning calorimetry), mechanical (dynamic mechanical analysis), and thermo-optical (ultraviolet–visible–near infrared spectroscopy) analyses. The formation of silicium T units and Si–CH₂–Si networks were evidenced by ²⁹Si NMR, and the increase of the glass transition temperature and of modulus reflect the substantial increase in the macromolecular chain rigidity of the irradiated material. Dramatic damages of mechanical properties were observed, depending on the reinforced materials used. Slight changes of thermo-optical properties were highlighted independently to the initial chemical structure.     

Characterisation of Ductile Prepregs

This study is focused on the analysis of micro-perforated prepregs created from standard, off the shelf prepregs modified by a particular laser process to enhance ductility of prepregs for better formability and drapability. Fibres are shortened through the use of laser cutting in a predetermined pattern intended to maintain alignment, and therefore mechanical properties, yet increase ductility at the working temperature. The increase in ductility allows the product to be more effectively optimised for specific forming techniques. Tensile tests were conducted on several specimens in order to understand the ductility enhancement offered by this process with different micro-perforation patterns over standard prepregs. Furthermore, the effects of forming temperature was also analysed to assess the applicability of this material to hot draping techniques and other heated processes.     

NMR imaging of materials

Nuclear magnetic resonance (NMR) and NMR imaging (MRI) are being used more and more to characterize the heterogeneity and the distribution of relevant properties in materials like gels, polymers, elastomers, and catalysts. Moreover, spatially resolved NMR can be used for the investigations of particle dynamics in granular media and parameters of mass transport, a topic of interest in chemical engineering. The possibility to enhance the nuclear polarization by laser-polarized ¹²⁹Xe and ³He can be combined with MRI of materials. This method opens up the possibility to obtain NMR parameter images of material surfaces.     

Strengthening of thin-webbed castellated beam using CFRP

Abstract

The utilisation of fibre reinforced polymers (FRP) in the rehabilitation of steel structures increased in recent years. This article presents the advantages of using carbon fibre reinforced polymers (CFRP) in strengthening of the thin-webbed castellated beam (TCB). The performances of CFRP strengthened TCBs were analysed using finite element (FE) tool ABAQUS^®. Nonlinear FE analysis was carried out to find the optimum geometrical. Nine different designations of TCBs were arrived based on weld lengths, perforation sizes and strengthening technique (three each). Though all three techniques increased the strength considerably, the third one had much greater efficiency and was more suitable.     

Design and preparation of high-performance amine-based poly(ether ketone)s with strong photonic luminescence

Amine-based poly(ether ketone)s (A-PEKs), as novel high-performance functional polymers, have been obtained by the polycondensation of dibromo ketones with aromatic ether diamines via palladium-catalyzed aryl amination reaction. The structures of the polymers are characterized by means of FT-IR, ¹H NMR spectroscopy and elemental analysis, the results show a good agreement with the proposed structures. DSC and TGA measurements exhibit that polymers possess high glass transition temperature (T _g ≥ 175 °C) and good thermal stability with high decomposition temperatures (T ₅ ≥ 400 °C). Based on the hydrogen bonds between the polymer chains, thin films of A-PEKs show great mechanical behaviors with high tensile strength up to 89.5 Mpa. In addition, due to the photoinduced intramolecular charge-transfer (ICT) of A-PEKs, these synthesized polymers are endowed with significantly strong photonic luminescence in N,N′-dimethylformamide.     

Fiber Reinforcement in Injection Molded Nylon 6/6 Spur Gears

Injection molded polymer composite gears are being used in many power and or motion transmission applications. In order to widen the utilization of reinforced polymers for precision motion transmission and noise less applications, the accuracy of molded gears should be increased. Since the injection molded gear accuracy is significantly influenced by the material shrinkage behaviour, there is a need to understand the influence of fiber orientation and gate location on part shrinkage behaviour and hence the gear accuracy. Unreinforced and 20% short glass fiber reinforced Nylon 6/6 spur gears were injection molded in the laboratory and computer aided simulations of gear manufacturing was also carried out. Results of the mold flow simulation of gear manufacturing were correlated with the actual fiber orientation and measured major geometrical parameters of the molded gears. Actual orientation of the fibers near the tooth profile, weld line region and injection points of molded gears were observed using optical microscope and correlated with predicted fiber orientation.     

自动铺放过程双马树脂预浸料温度与黏度

通过对自动铺放成型工艺的分析,探索了自动铺带成型过程中预浸料温度对复合材料黏性的影响规律。对影响预浸料温度的热风温度和铺放速度两个因素进行了研究,从理论上建立了热风加热温度和铺放速度与预浸料温度之间的关系,提出了控制预浸料温度的具体实施方案,指导自动铺带成型过程。根据双马树脂预浸料黏度和温度曲线,分别在不同的热风温度和铺放速度条件下进行实验,得到在热风温度为90℃、铺放速度为10~20m/min时,预浸料黏度满足铺覆性要求,验证了预浸料热风温度和铺放速度对复合材料制品质量的影响,为自动铺带工艺提供参考。     

Bis(ethynyl)-polymers

Poly(2,5-thiophenediyl)ethynylene and poly[ethynylene(2,5-thiophenediyl)ethynylene], synthesized in our laboratory, have been investigated in order to characterize their physicochemical properties. The optical and infrared spectra of the two polymers were studied as well as their thermal properties by means of differential scanning calorimetry and thermogravimetric analysis. The electrical conductivity of the two polymers in the pristine state and in the I₂-doped one has been measured and the effects due to the doping on the spectroscopic and thermal properties have been studied.     

Glass transitions of hygrothermal aged pultruded glass fibre reinforced polymer rebar by dynamic mechanical thermal analysis

The changes that can occur in glass fibre reinforced polymer (GFRP) composites with ageing can affect its application, performance and lifetime. Hygrothermal ageing (i.e. accelerated ageing by moisture absorption and temperature change) is a very useful technique to evaluate durability as well as development of GFRP composites in a reasonable timeframe. Dynamic mechanical thermal analysis (DMTA) is essentially able to detect all changes in the state of molecular motion in polymer composites as temperature is scanned. In this work, pultruded GFRP rebars were accelerated aged in an alkaline aqueous environment at 60 °C for 1, 2, 3, 4 and 6 months to evaluate the changes in glass transition of viscoelastic GFRP rebars by DMTA. Five different glass transitions in an average temperate range from 110 to 165 °C were observed at storage modulus, loss modulus and damping factor traces of DMTA. It was also found that glass transition temperature (T_g) of the aged samples changed up to maximum 6 °C compared with that of controlled sample. This change in T_g with ageing time was believed to be due to moisture absorption by rebars.     

Time Domain 1H NMR as a New Method to Monitor Softening of Gelatin and HPMC Capsule Shells

ABSTRACT

Defined mechanical properties are an essential requirement for any pharmaceutical dosage form and this is particularly important in the case of liquid-filled capsules. Changes in the mechanical properties may be induced by exposure of the capsules to humidity or by a shift of the water equilibrium that typically occurs when hydrophilic or amphiphilic fill masses are used, for example, in self-emulsifying drug delivery systems. This study aims to characterize the softening of empty hard gelatin and hydroxypropyl methylcellulose (HPMC) capsules by means of mechanical tests, a Bareiss hardness test, and a stiffness test using a texture analysis method. A benchtop time domain NMR method is applied in addition to characterize the physico-chemical state of water in the capsule shells and to correlate this with the results of the mechanical tests. Hardness and stiffness measurements resulted in corresponding values, showing a softening for both capsule materials in a humid environment, which was most pronounced beyond 60% relative humidity. The capsules made of gelatin exhibited in general higher stiffness and hardness values compared to the HPMC capsules. The physico-chemical state of water in the capsule shells, as probed by a time domain NMR method, was interpreted in terms of a population balance model. Three different water populations were identified that differ in their molecular mobility, as indicated by their characteristic spin-lattice relaxation times, T1. The most loosely bound water fraction dominated in the capsule shells in the range beyond 60% relative humidity. Numerical correlation of the data led to a heuristic equation between the NMR-derived fraction of loosely bound water in the capsule shells and their mechanical stiffness and hardness. Adequate models were obtained for both capsule types, gelatin, and HPMC. Mechanical measurements of pharmaceutical capsules are generally destructive and time consuming. Testing is usually performed in an analytical laboratory, off-line from the manufacturing process, and involves only a small number of samples. Based on the here presented correlation between mechanical stiffness measurements and benchtop time domain NMR data, the latter method may be used as a nondestructive alternative for mechanical testing. This study also opens the possibility to investigate liquid-filled capsules and to establish a process analytical technology (PAT) during manufacturing.