Coating carbon fibers with a thermoplastic polyimide using aqueous foam

Many techniques have been developed to coat carbon fibers with thermoplastic resins to form a prepreg. These techniques include melt impregnation and solution/slurry coating. The applicability of these techniques, however, may be restricted by high melt viscosities or significant drying times. Recent emphasis has been towards a dry powder coating technique using a fluidized bed. This technique may be limited by difficulties in fluidization of the polymer powders. To overcome these difficulties, a technique has been developed to continuously coat carbon fibers with thermoplastic resins using aqueous foam as a carrier medium. The polymer is slurried in a surfactant solution, into which air is dispersed using a foam-generating device. A predetermined amount of this foam is then applied to a moving carbon fiber tow using a foam application unit. The tow is then pulled through ovens where the foam is collapsed and the power fused to the fibers. Other aspects that are addressed include the stability of foams in the presence of powders, the mechanical properties of the composites formed using foam prepregging, and the effect of surfactant on composite properties.     

Dual curing of carbon fiber reinforced photoresins for rapid prototyping

Photo-thermal dual-curing of a carbon fiber reinforced photoresin is reported in this study. The primary limitation of carbon fibers is that they are opaque to ultra-violet (UV) light, and the resin present inside the carbon fiber tow remains uncured if only a photolytic mechanism is used. Therefore, a dual-curing resin formulation was developed that consisted of acrylate ester of bisphenol A-epoxy (primary resin), 1-hydroxycyclohexyl phenyl ketone (a photoinitiator), and lauroyl peroxide (thermal initiator). The fibers were dispersed in the resin and photocured by UV radiation. Subsequently, the composites were thermally cured to solidify the resin present inside the fiber tow. Differential scanning calorimetry indicates that the thermal initiator activates at approximately 70°C. The dual-cured composites, processed by a dip-coating technique and also in a desktop photolithography unit, were tack-free and fully solidified in the interior. Thus, it was established that by using a dualcuring resin formulation, the resin present outside and inside the carbon fiber tow could be cured to produce a carbon fiber reinforced photoresin. Such a reaction scheme has application in composite photolithography where incorporation of carbon fibers into photopolymers can enhance electrical and thermal conductivities of rapid tooling materials.     

Fiber Classification and the Influence of Average Air Humidity

The use of man-made vitreous fibers (MMVFs) as a substitute for asbestos in industrial and residential applications has raised the concerns of the potential hazards associated with inhalable aerosolized fibers. The complex movement of fiber makes it difficult to predict the pattern of fiber deposition in human airways from the behavior of spherical particles. Difficulties in producing monodisperse length fibers has been an obstacle to study fibrous particle deposition in the human respiratory system. To address this problem, a narrow length distribution of fibers was generated using dielectrophoretic classification. Dielectrophoresis is the motion of neutral matter in a nonuniform electric field due to an induced dipole moment. It is sensitive to the conductivity of the matter in the field. A fiber classifier has been used to study the influence of atmospheric humidity on the behavior of glass fibers. Glass fibers, as insulators, can not be classified by the dielectrophoretic classifier. However, our study shows that a humidity higher than 15% RH can change the conductivity of the glass fibers so as to permit their effective classification.     

PMR-15/carbon fiber composites produced from powder-coated towpreg

An addition-type thermosetting polyimide, PMR-15, has a relatively high glasstransition temperature, T_g, of approximately 340°C and attractive high-temperature composite mechanical properties after postcuring. PMR-15 prepreg is typically produced by pulling carbon fibers through a methanol solution that contains the PMR-15 monomers. One of the monomers, methylene dianiline, is highly toxic, thereby making the solution-coating technique undesirable. To alleviate the toxicity problem in the prepregging step, “preimidized” PMR-15 is now available in powder form, and this material can be used to produce towpreg using a powder-coating method. The powder-coating method that has been used in this work involves the use of an aqueous foam as a carrier medium to continuously deposit the PMR-15 powder onto carbon fibers. The powder is slurried in a surfactant solution into which nitrogen is dispersed using a foam-generating device. A predetermined amount of this foam is then applied to a moving carbon fiber tow. The foam is collapsed and the powder fused to the fiber in a set of tubular ovens. The powder appeared to be particularly sensitive to the heat treatment, but towpreg was produced and composites were consolidated successfully. The mechanical properties of the composites formed from powder-coated towpreg compare favorably with the properties of composites fabricated from prepreg that was drum wound from a methanol solution.     

Fibrous Particle Deposition in a Turbulent Channel Flow—An Experimental Study

The deposition rate of glass cylinders and dust paper fibers in a turbulent duct flow was studied experimentally. The glass fibers with a minimum diameter of 5 μm and the paper fibers with a minimum diameter of 1–20 μm and aspect ratios from 4 to 20 were deposited on a flat gold plate. The particle concentration at the test section was measured with the aid of an isokinetic probe in conjunction with a digital image processing technique. An oil lubricant was used on the plate to reduce the effect of particle bounce from the surface. The experimental data show that the deposition rate increases with an increase in fiber length and size. For a fixed minimum diameter or a fixed equivalent relaxation time, the deposition rate increases rapidly with fiber aspect ratio. When the equivalent spherical particle relaxation time is used, the deposition rate of the fibers was found to increase only slightly with aspect ratio and resemble those of spherical particles. The measured deposition velocities were in good agreement with the empirical model predictions and previous data.     

Recent Aspects of Glass Fiber-Resin Interfaces

The recent developments in Auger spectroscopy have been used to define the composition of two glass fiber surfaces. The effect of fiber surface area on the interlaminar shear strength was also investigated. The chemistry of several silane “coupling agents” has been studied from the standpoint of its chemical form when it is applied to the glass fibers, and has in part been determined using a gas chromatographic technique. The relative thermal stability of some silanes in high temperature resin matrices was determined. A comparison of a treatment of glass fibers with aqueous and non-aqueous systems is made.     

Deposition of Man-Made Fibers in a Human Nasal Airway

Many occupational lung diseases are associated with exposure to aerosolized fibers in the workplace. The nasal airway is a critical route for fiber aerosol to enter the human respiratory tract. The fiber deposition efficiency in the nasal airway could be used as an index to indicate the fraction of the inhaled fibers potentially transported to the lower airways. In this research, experiments of fiber deposition in the human nasal airway were conducted. Man-made carbon, glass, and titanium dioxide fibers in the inertia regime were used as the test fiber materials. The deposition studies were carried out by delivering aerosolized fibers into a human nasal airway replica at constant human inspiratory flow rates ranging from 15 l/min to 43.5 l/min. The deposition results were compared in detail between these fiber materials to study how the fiber characteristics affected the nasal airway deposition. The results showed that the deposition efficiency of the carbon fiber increases as the fiber impaction parameter increases. Many carbon fibers deposited in the anterior region of the nasal airway. In contrast, very few glass or titanium dioxide fibers deposited in the nasal airway, but relatively more of these two fibers deposited in the turbinate region. This result implies that, if a fiber in the inertia regime is inhaled during normal human breathing, the smaller the fiber, the more easily it could enter the human lower respiratory tract, possibly causing harm to the human respiratory tract.     

Fabrication of Glass Matrix Composites by Tape Casting

A method has been developed to fabricate borosilicate glass matrix composites reinforced with monofilament SiC fibers by tape casting. Green matrix tapes are laminated with fiber mats of a uniform fiber spacing. The resulting laminate is sintered at 710°C to >98% relative density and HIP-consolidated to full density. The final specimens contain a high volume fraction of fibers (>35 vol%) in a uniform array. A variation of this technique can be used to mount "microcomposites" (i.e., coated fibers) in a glass matrix to facilitate fabrication of push-out test specimens.     

Impregnation of thermoplastic resin in jute fiber mat

Impregnation rate of thermoplastic resin (polypropylene) in jute fiber mat and influence of relative factors on impregnation were studied, aiming to develop the continuous melt impregnation technique and to investigate the effect of impregnation rate and temperature on processing conditions and mechanical properties of natural fiber mat-reinforced thermoplastics. Influence of pressure on porosity of fiber mat and effect of melt viscosity on impregnation rate were also investigated. The modified capillary rheometer was used as apparatus and experimental data were analyzed based on the one-dimension Darcy’s law. Results showed that at a given pressure, the impregnation rate is inversely proportional to melt viscosity and jute fiber mat has higher porosity than glass fiber mat. The architecture, compressibility, permeability and fiber diameter of jute fiber mat were compared with those of glass fiber mat and their effects on impregnation were discussed further. It could be seen that the average diameter of jute fiber is much bigger; the porosity of jute fiber mat is significantly higher and inner bundle impregnation does not exist in jute fiber mat. Therefore, it is not difficult to understand why the impregnation rate in jute fiber mat is 3.5 times higher and permeability is 14 times greater. Kozeny constants of jute and glass fiber mats calculated based on the capillary model are 2950 and 442, respectively.     

Fundamental Understanding of Processes Involved in Optical Fiber Manufacturing Using Outside Vapor Deposition Method

Optical fibers are often made using a flame hydrolysis deposition process, in which silica and doped silica particles from a soot-laden flame are deposited on a target rod. The porous preforms thus formed are subsequently sintered into glass blanks and then drawn into optical fiber. We review here various tools that have been developed for better understanding of different processes involved in the making of optical fibers using the outside vapor deposition technique. Fundamental understanding of the different unit operations involved in the optical fiber manufacturing is critical for process optimization and improved product attributes.     

Effects of preferential encapsulation of glass fiber on the properties of ternary GF/PA/PP blends

Long fiber molding materials are expected to play an important role in the near future. This paper describes a series of experiments performed to examine properties of ternary blends containing glass fiber (GF), polyamide (PA), and polypropylene (PP). The continuous glass fiber was impregnated with one of the blend constituent polymers by our specially designed impregnation apparatus and cut into chips of 6 mm length. These chips and the other polymer were used to produce various testing specimens in a twin screw extruder or in injection molding machine. The results indicated that the effect of fiber addition on the mechanical and rheological properties is clearly dependent on the order of impregnation process. In the blends containing the GF/PA + PP, the GFs are preferentially encapsulated with PA, and therefore the mechanical properties are superior to the blends with the GF/PP + PA in which the PP phase is located surrounding the GFs. This improved wetting of fibers by sequential impregnation not only resulted in better properties but also protected the fibers from shear action of the screw, thereby allowing significant increase in average fiber length to be achieved in the injection molding process.     

Polyamide 6—long glass fiber injection moldings

The injection molding ability of long glass fiber reinforced polyamide pellets was studied. The injection moldable materials were produced by a melt impregnation process of continuous fiber rovings. The rovings were chopped to pellets of 9 mm length. Chopped pellets with a variation in the degree of impregnation and fiber concentration were studied. The injection molded samples were analyzed for fiber concentration, fiber length, and fiber orientation. Dumbbell-shaped tensile bars were made to evaluate the mechanical properties. The fibers in the tensile bars had a high orientation in the flow direction and minor fiber concentration gradients were observed. The fiber lengths decreased with fiber concentration from 1.6 mm for a 2 vol% to 0.6 mm for a 25 vol% system. The tensile and impact properties increased considerably with fiber concentration. A low degree of impregnation in the pellets of the fibers resulted in somewhat lower tensile and impact properties.     

基于粒径分布的静电粉末喷涂上粉率研究

为研究粉末粒径分布对静电粉末喷涂上粉率的影响,采用不同目数标准检验筛将商用粉末涂料进行人工筛选.通过测量铝合金试样单位面积增质量来表征静电粉末喷涂一次上粉率,研究粉末涂料粒径分布对静电粉末喷涂一次上粉率的影响.此外,通过设计矩形凹槽模拟喷涂死角区,采用槽内试样单位面积增质量与槽外单位面积增质量的比值表征静电粉末喷涂死角...     

铝粉粉末涂料邦定工艺的研究

本工艺通过设计聚酯/环氧型粉末涂料配方,采用邦定包膜技术将涂料粉末粒子与铝粉牢固黏结,静电喷涂试验表明该粉末涂料高度接近电镀铬效果,可作为电镀铬的替代产品.其中铝粉与粉末粒子良好粘结,铝粉使用量可降低30％;采用静电喷涂时,对不同金属元器件粘附良好,200℃固化10分钟,其涂膜拥有类似铬电镀的高光泽度金属效果,且色泽均一.相比溶剂型涂料60 ％～65％的成膜率,该粉末涂料成膜率接近100％,且未附着于被喷涂物件的粉末可以全部回收再利用.现有的邦定包膜技术具有高效环保、低成本等特点.     

热塑性树脂浸渍黄麻纤维毡的研究

研究了热塑性树脂PP(聚丙烯)浸渍黄麻纤维毡的速率及相关因素,以解决天然纤维毡增强热塑性复合材料中连续熔融浸渍,考察了浸渍速率、温度等参数对其加工成型及力学性能的影响.用改装的毛细管流变仪作为实验装置,以一维Darcy定律处理实验数据,研究了压力对毡体空隙率以及熔体粘度对浸渍速率的影响.结果显示,相同的压力下,浸渍速率和熔体粘度成反比,麻纤维毡的压缩空隙率都要高于玻璃纤维毡.通过对纤维毡体结构、可压缩性、纤维直径以及毡体渗透率进行对比,进一步讨论了纤维毡浸渍速率的影响因素.表明麻纤维平均直径远大于玻璃纤维,纤维堆叠产生的空隙明显大于玻璃纤维且在麻纤维毡中不存在玻璃纤维毡中的束内浸渍,麻毡的浸渍速率约为玻纤毡的3.5倍,平均渗透率K约为玻纤毡的14倍.运用毛细管模型计算了两种毡体的Kozeny常数,其值分别为2950和442.     

Laser densification of alumina powder beds generated using aerosol assisted spray deposition

Layered manufacturing involves a range of techniques in which objects can be constructed in a laminated form. Therefore, the deposition technique is a critical part of direct-layered fabrication technologies. In this paper, aerosol assisted spray deposition has been applied to generate spraying of a suspension to prepare powder beds for subsequent selective laser sintering. First, an investigation on preparation of the alumina suspension by adding polyacrylic acid dispersant is presented. An emphasis has been given to identify the most effective dispersant to enhance the dispersibility of alumina suspension for aerosol spraying deposition. Subsequently, a laser has been used to selectively densify the alumina powder beds to produce ceramics. The effects of the laser processing parameters such as scanning speed, power and beam size on the microstructural evolution of the powder beds are investigated and discussed. Also, a laser densification mechanism is also proposed and discussed.     

COMPARISON OF GLASS TRANSITION TEMPERATURE AND STICKY POINT TEMPERATURE FOR SKIM MILK POWDER

Wall deposition of particles in spray dryers is a key processing problem, and information about the glass transition temperature of the amorphous material that arises from spray drying can be used to guide the selection of operating conditions that may minimise wall deposition. The glass transition temperatures for skim milk powder with various moisture contents were determined using Differential Scanning Calorimetry (DSC), and a repeatable glass transition temperature diagram was established from these results. The glass transition temperature decreased as the moisture content increased, as expected (low moisture content 1.65 g/100 g of dry powder, glass transition temperature 87.7°C; high moisture content 4.52 g/100 g of dry powder, glass transition temperature 46.7°C). The glass transition temperature was found to be virtually the same as the sticky-point temperature measured using a thermo-mechanical test. The difference is essentially due to the difference between doing a mechanical test for viscosity (sticky-point) and a phase transition measurement (DSC).     

Conductive, Polypyrrole Coating on Mullite/Alumina Fibers for Electrophoretic Deposition of Oxide Matrices

Mullite/alumina fibers (Nextel™ 720) have been rendered conductive for electrophoretic deposition (EPD) by coating with polypyrrole (Ppy) from an aqueous solution of pyrrole (Py). The polymer coating was characterized by thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and conductivity measurements. The Ppy coating thickness is ∼0.1 μm and the fiber resistance 1–2 (kΩ/cm)/fiber tow. A maximum conductivity of ∼48 S/cm was achieved from a 0.005 M pyrrole solution. A Nextel 720/Al₂O₃ composite was synthesized using Ppy-coated fibers as the cathode in an EPD cell. The green and sintered microstructures of the resultant composites are reported.     

The influence of ultra-fine glass fibers on the mechanical and anticorrosion properties of epoxy coatings

The effects of different contents of ultra-fine glass fiber on mechanical and anticorrosion properties of epoxy coatings have been investigated. The FTIR and SEM have been used to analyze the surface nature and microstructure of the coatings. Electrochemical impedance spectroscopy (EIS) and a salt spray test have also been used to characterize the contents of ultra-fine glass fibers on the impedance of the coatings. When 10%, 20%, 30% of ultra-fine glass fibers are added to the coatings, their hardness and adhesion increases by 67%, 67%, 200% and 21.6%, 39%, 40%, respectively, compared with the properties of the pure coating. But the anticorrosion properties of the coatings containing high ultra-fine glass fiber content decreased with respect to the pure coatings properties.     

Stamp forming of fabric-reinforced thermoplastic composites

A stamp forming technique has been used to process a fabric woven composite made of glass fibers (GF) and polyetherimide (PEI). A hemispherical mold with a built-in hold-down arrangement was designed and used at room temperature to stamp parts from preheated flat preconsolidated laminates. Tensile properties of the material were measured under similar heating conditions as in the relevant stamp-forming process. Stretch in the fiber direction was found to be smaller than the maximum elastic extension of the glass fibers. Reduction of the angle between the crossing fibers was quite large when the satin woven fabric composite was pulled in the 45° direction. The effect of die geometries and original laminate dimensions on the “shear-buckling” were studied. The results described the correlations between processing parameters and fiber buckling. Finally, the local strain of fiber bundles was investigated in relation to different directions of fiber orientation.