Resin transfer moulding of anionically polymerised polyamide 12

Liquid composite moulding of Lactam 12 monomer and activating system (APLC12) into satin weave carbon fabrics is investigated, with emphasis on minimising the void content in the final part. The main sources for void formation are identified. The solidification shrinkage is quantified to account for at most 9% in the matrix. Optimal flow conditions are determined to minimize void content during liquid moulding. Finally, as the monomer is kept under Nitrogen prior to processing, diffusion and solubility of Nitrogen in the monomer are characterized, to indicate that Nitrogen coalescence during injection is a major cause of voids in the final part. The average void content is reduced from initially 15% to below 1% in polyamide 12 based composite plates with optimised process conditions.     

Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating

Microwave processing holds great potential for improving current composite manufacturing techniques, substantially reducing cure cycle times, energy requirements and operational costs. In this paper, microwave heating was incorporated into the resin transfer moulding technique. Through the use of microwave heating, a 50% cure cycle time reduction was achieved. The mechanical and physical properties of the produced carbon fibre/epoxy composites were compared to those manufactured by conventional resin transfer moulding. Mechanical testing showed similar values of flexural moduli and flexural strength for the two types of composites after normalisation of the corresponding data to a common fibre volume fraction. A 9% increase of the interlaminar shear strength (ILSS) was observed for the microwave cured composites. This enhancement in ILSS is attributed to a lowering of resin viscosity in the initial stage of the curing process, which was also confirmed via scanning electron microscopy by means of improved fibre wetting and less fibre pull-out. Furthermore, both types of composites yielded minimal void content (<2%). Dynamic mechanical thermal analysis revealed comparable glass transition temperatures for composites produced by both methods. A 15 °C shift in the position of the β-transition peak was observed between thermally and microwave cured composites, suggesting an alteration in the cross-linking path followed.     

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.     

Numerical prediction and experimental characterisation of meso-scale-voids in liquid composite moulding

Pressure gradients that drive the resin flow during liquid composite moulding (LCM) processes can be very low while manufacturing large composite parts. Capillary pressure becomes the predominant force for tow impregnation and thus meso-scale-voids can be generated, reducing the part quality. In contrast, micro-voids are created at high resin pressure gradients. In this work, a numerical method is presented to predict the creation of meso-scale-voids and their evolution. Experimental validation is conducted by measuring void content of produced composite parts with micro-computed tomography (μ-CT). Additionally, the void content as a function of the modified capillary number Ca^* is determined and the influence of the fibre volume content in the bundles on the meso-scale- and micro-void content is studied.     

Influence of prepreg conditions on the void occurrence and tensile properties of woven glass fiber-reinforced polyimide composites

The influence of prepreg solvent content on void occurrence in woven glass fiber-reinforced polyimide composites and their tensile properties was studied. A precursor solution of SKYBOND 703 was diluted in an additional solvent (n-methyl pyrrolidone) and the glass woven fabric was immersed in about 40 wt.% polyamic acid, in solvent. Prepregs were dried at 373 K for different time intervals, ranging from 2 to 24 h. Prepregs with varying residual solvent content under each condition were laid up, and their [(0/90)]₄ composite laminates were formed by autoclaving at a hydrostatic pressure of 0.7 MPa. The relationship of drying time with the amount of residual prepreg solvent, as well as with the volume fractions of fiber and voids was investigated. The void geography and content for each composite laminate, and the tensile strength and modulus at room temperature were also evaluated. The results clearly indicated that, depending on the altering residual solvent content in the prepreg, the void geometry and location influenced reduction of the tensile properties of woven fabric composite laminate. An appropriate prepreg resin viscosity during curing, which avoids reduction of the tensile properties, was revealed.     

Processing of co-bonded scarf repairs: Void reduction strategies and influence on strength recovery

Partially impregnated prepreg are attractive materials for scarf bonded repairs as they can be consolidated with vacuum assisted pressure only. Several air evacuation strategies were evaluated for cobonded scarf repairs of monolithic composites skins. Various void levels were measured in the bondline by digital radiography, and tensile tests were performed to assess the repairs strength recovery. Most of the entrapped air could be evacuated prior to cure for the method using a textured adhesive film. This repair strategy consistently led to a significant reduction in bondline porosity and exhibited the highest strength recovery. Overall, it was found an average linear reduction in tensile strength recovery of 4.5% per 1% areal void content in the adhesive. Additionally, a change in final failure mode was observed from cohesive failure for porous bondlines, to net-section tensile failure in case of void-free bondlines.     

Interstrand void content evolution in compression moulding of randomly oriented strands (ROS) of thermoplastic composites

Compression moulding of randomly oriented strands (ROS) of thermoplastic composite is a process that enables the forming of complex shapes with keeping final properties close to that of continuous fibre composites. During forming several deformation mechanisms occur. In this paper we focus on the interstrand void content (ISVC) reduction: the squeezing of each single strand during compression enables filling of the gaps between strands. A modelling of this deformation mechanism was developed. The compaction is ruled by an ordinary differential equation that was solved numerically. The model was validated experimentally using an instrumented hot press with Carbon-PEEK prepreg strands. The model accurately predicted ISVC in four characteristic cases. Using the proposed model, the influence of several process and material parameters were investigated. Finally, a design chart giving the final ISVC for a wide range of pressure, strand geometry and part thickness, was constructed.     

Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: Influence of fibre loading,fibre length,fibre orientation and voids

The influence of fibre loading (20, 30, 40 mass%), fibre fineness, and the processing procedure (compression moulding – CM and injection moulding – IM) on the tensile and impact strength of lyocell/PLA composites was examined. The results revealed a significantly higher tensile and impact strength for CM composites compared to IM composites. An increase in strength up to a fibre loading of 40% was determined for CM composites, while for IM composites the highest values were measured at a fibre loading of 30%. Composites were investigated for their void content, fibre orientation, fibre length and process-induced fibre damage. A better fibre/matrix adhesion and compaction of IM composites was found while fibre orientation as well as mechanical properties of extracted fibres show no significant differences between CM and IM composites. The different mechanical characteristics of CM and IM samples are attributed predominantly to the fibre aspect ratio and the distribution of voids.     

High performance hybrid PP and PLA biocomposites reinforced with short man-made cellulose fibres and softwood flour

This paper presents the research on hybrid thermoplastic biocomposites reinforced with a combination of short man-made cellulose fibres and softwood flour. The introduced short fibre composites are meant to be processed with injection moulding and may be an alternative to glass-fibre reinforced thermoplastics on account of their comparable specific strengths. The occurring positive hybrid effect enables to substitute up to half the weight of short fibre cellulose reinforcement with softwood flour without a significant reduction of material flexural strength. The flexural modulus of investigated hybrid biocomposites remained approximately at the same level, while impact strength was reduced with increasing softwood flour content. The proposed hybridisation leads to establishing biocomposites of suitable performance with competitive density, price and recycling possibilities in comparison to standard glass fibre reinforced counterparts. Moreover, the application of biobased polymers like polylactide as biocomposite matrix, contributes to the development of so called “green” high performance materials.     

孔隙对碳纤维/环氧复合材料层合板层间剪切疲劳性能的影响

研究了孔隙对碳纤维增强环氧树脂基复合材料层合板［(±45)/0₄/(0, 90)/0₂］_S的静态层间剪切强度和层间剪切疲劳性能的影响。采用不同的热压罐压力制备了孔隙率为0.4%~6.6%的试样。采用显微照相法和图像分析技术对孔隙率和孔隙的微观形貌进行了分析。研究结果表明, 随着热压罐压力的降低, 大孔隙(S>7.85×10-3mm2)所占的比例逐渐增加, 平均孔隙率增加。在孔隙率为0.4%~6.6%时, 每增加1%, 复合材料层压板的层间剪切强度下降2.4%。随着孔隙率的增加, 层压板的疲劳寿命降低。与静态试验相比, 孔隙率对层压板疲劳性能的影响比对静态性能的影响大。大孔隙的存在促进了疲劳裂纹的产生和扩展。      

Compression moulding of glass and polypropylene composites for optimised macro- and micro- mechanical properties-1 commingled glass and polypropylene

The continuing desire in the automotive industry to reduce cost and weight while increasing safety requires innovative materials and processing routes. Glass-mat-reinforced thermoplastics have been used to produce semi-structural components but a higher and aligned glass fibre content is required in moulding materials for structural applications. Experimental design was used to investigate the non-isothermal processing of commingled fabrics which were woven from yarns of intimately mingled glass and polypropylene fibres. Processing models were generated by regression techniques to predict laminate properties over a range of processing conditions. Void contents were measured by image analysis techniques. Preheat temperature had the greatest effect on laminate flexural properties and porosity. A compaction time of 54 s was required to consolidate, cool and reduce the void content in laminates. A two-fold increase in stiffness was found compared with equivalent glass-mat-reinforced thermoplastic laminates. The intimate distribution of matrix and reinforcement reduced moulding pressures by a factor of 10.     

Improvement on the properties of polylactic acid (PLA) using bamboo charcoal particles

Bamboo charcoal (BC) derived from bamboo plants is one kind of well recognized multi-functional materials which has been used in various applications such as medical, cosmetic, food processing and health-related products. In this paper, BC particle is used as reinforcement for polylactic acid (PLA) to enhance its mechanical, thermal and optical properties. The comparison on tensile, flexural and impact properties of BC particle reinforced PLA composites (BC/PLA composites) with the content ranging from 2.5 to 10 wt.% is conducted. Experimental results indicated that the maximum tensile strength, flexural strength and ductility index (DI) of BC/PLA composites increased by 43%, 99% and 52%, respectively as compared with those of neat PLA. This phenomenon was attributed to the uniform distribution of high aspect ratio and surface area of BC particles. Further increasing the BC content to 7.5 wt.% would decrease the glass transition temperature of BC/PLA composites. The mechanical properties of BC/PLA composites were reduced as compared with a neat PLA sample when they were exposed to compost degradation. However, less reduction in these properties was found when they were subject to UV irradiation. UV–Vis spectrometer analysis supported the results of UV irradiation. Fracture surfaces of tensile test samples with and without compost degradation or UV irradiation were analysed by using scanning electron microscopy (SEM). SEM images revealed that there was a good BC particle dispersion in the composites through extrusion and injection moulding processes if the particle content was below 7.5 wt.%.     

The influence of voids on the hydrothermal response of carbon fibre reinforced plastics

Measurements are reported of the shear modulus, strength and strain at failure of carbon fibre reinforced plastics exposed to the effects of temperature and temperature plus water. Two types of specimen were employed, one with a low void content and one with a high void content. Properties were determined either at room temperature after specimens had been fully dried, or at high temperature and in some cases at high temperature and under water. For good, low void content specimens, any effects of temperature and water were completely removed by drying. For specimens containing more than 1 vol% voids this was not so. Measurements at high temperature indicated a fall-off in stiffness and strength for either type of specimen, and for void containing specimens an increase in the strain at failure indicative of bond disruption. The added effects of water were complex.     

Flow characteristics of carbon fibre moulding compounds

This paper presents the development of a low-cost carbon fibre moulding compound using an automated spray deposition process. Directed Fibre Compounding (DFC) is used to produce charge packs directly from low cost carbon fibre tows and liquid epoxy resin. A range of material and process related parameters have been studied to understand their influence on the level of macroscopic charge flow, in an attempt to produce a carbon fibre moulding compound with similar flow characteristics to conventional glass fibre SMCs.Charge packs covering just 40% of the mould can be effectively used to process DFC, without detrimentally affecting void content, fibre distribution and mechanical properties. Tensile stiffness and strength values of 36 GPa and 320 MPa are reported for isotropic materials (100% charge coverage), which increase to 46 GPa and 408 MPa with flow induced alignment (50% charge coverage) at 50% fibre volume fraction.     

Optimization of the preparation conditions of polygranular carbons from mesophase

Polygranular carbons were prepared from a coal-tar pitch based mesophase by sintering, using different experimental conditions. The temperature and time of mesophase stabilization, the pressure applied during moulding, and the sintering heating rate were investigated in order to obtain materials with optimum properties. Oxidative stabilization with air between 225 and 250°C causes a significant reduction in the plasticity of the coal-tar pitch based mesophase, allowing moulding and sintering to be performed. An increase in the moulding pressure results in an increase in the bulk density of the green materials. However, sintering must be carried out at low heating rates in order to control the release of gases and thus avoid damage to the sintered material. Higher sintering heating rates are compatible with low moulding pressures and a high degree of stabilization. Whenever the materials do not distort during sintering, a common feature observed is that mechanical and electrical properties improve with increasing moulding pressure, while an increase in sintering heating rate only serves to improve the strength of the materials.     

Strain field inhomogeneities and stiffness changes in GMT containing voids

During compression moulding of glass mat thermoplastics (GMT), voids may form. However, it is not clear whether voids are as critical to mechanical performance in GMT as in thermoset composites. The present investigation also considers the general problem of damage mechanisms in GMT. Conventional tensile tests, acoustic emission, a stiffness degradation test and a speckle technique for strain field measurements are used as well as optical microscopy of polished cross-sections. The void content (up to 5%) does not significantly influence the strength or stiffness degradation process. The reason is the large inhomogeneity of the strain fields in GMT. Failure occurs in locally soft regions and void effects are of secondary importance. Details of the failure process are discussed, emphasising the large local strains in matrix-rich regions.     

The effects of the injection moulding temperature on the mechanical properties and morphology of polypropylene man-made cellulose fibre composites

Composites made of polypropylene and man-made cellulose fibres that are intended for injection moulding applications show potential for use in sustainable and light weight engineering with high energy absorption capacity. Due to the thermal sensitivity of the cellulose fibres, process parameters play an important role during the injection moulding process. A polypropylene and a man-made cellulose fibre were chosen for this investigation. Effective melt temperatures between 200 °C and 269 °C were used to process the compounds into test specimens. Tensile, impact and colorimetric tests, as well as an SEM analysis, and a measurement of the fibre length distribution were carried out in order to characterise the mechanical and optical properties of the composites. It was observed that the fibre length becomes shorter above 256 °C and elongation at break and Charpy strength (notched) of the composites already decrease at lower temperatures than tensile strength. A direct correlation between mechanical properties and discoloration was not observed. Therefore, melt temperatures up to 250 °C are suitable for these composites.     

Tensile and fatigue characterisation of textile cotton waste/polypropylene laminates

Short fibre based cotton flocks from end-of-life jeans fabric (denim twill weave) were introduced in an amount of 16 wt.% in a polypropylene (PP) matrix using a specifically designed manufacturing process to preserve as much as possible the properties of the cotton waste during injection moulding. This involved a first phase of binding the cotton flocks on polyvinyl acetate (PVAc) support, then pelletizing them with PP and finally extruding the final composite. The resulting composites were subjected to morphological, tensile and fatigue characterisation with stress levels from 50 to 90% of ultimate tensile strength. Results indicated that injection moulding offered a sufficient uniformity of properties to the composite, albeit with some occurrence of pull-out during loading. In particular, the tensile performance exceeded that of the pure matrix in a measure compatible with the amount of fibres introduced. In addition, tensile fatigue loading up to 5000 cycles evidenced a limited amount of degradation for maximum applied stresses up to 70% of composite tensile strength.     

A comparative study on mechanical properties of sisal-leaf fibre-reinforced polyester composites prepared by resin transfer and compression moulding techniques

Resin transfer moulding (RTM) is a novel technology, which bridges the gap between labour intensive hand lay-up process and capital-intensive compression moulding. The present study investigates the tensile and flexural behaviour of sisal fibre reinforced polyester composites as a function of fibre length and fibre content. The composites were prepared by RTM and compression moulding techniques. The properties obtained for composites fabricated by both RTM and compression moulding were compared. From the studies it was found that mechanical properties increase with increase in fibre loading in both cases. The void content and water absorption properties at varying fibre loading were evaluated and found maximum for the compression moulded composites. To analyse the fracture surface morphology of the composites scanning electron microscopy was also performed. A good correlation between morphological and mechanical properties has been observed. Finally, the Young’s modulus and water absorption properties of the composites fabricated by RTM were compared with theoretical predictions.     

Effects of exposure to elevated temperatures and subsequent immersion in water or alkaline solution on the mechanical properties of pultruded BFRP plates

In this article, a pultruded unidirectional basalt fiber-reinforced polymer (BFRP) plate was thermally aged at 135 °C and 300 °C for 4 h, and subsequently immersed in distilled water or strong alkaline solution (simulating concrete pore water, pH = 12.6–13) for 3 months. The variation of the tensile and interlaminar shear (ILSS) properties of the BFRP plates was studied. Thermal aging exhibited a slight effect on both the longitudinal tensile properties and the interlaminar shear strength, although thermal decomposition of the resin matrix started at 300 °C and brought in a high void content (4.8%). FTIR and DMTA results indicate that thermal aging lead to postcuring and oxidation of the resin matrix, leading to an increase of the glass transition temperatures. Thermal aging accelerated the degradation of the BFRP plates in distilled water or alkaline solution at 20, 40 and 60 °C. In the studied hash immersion conditions of 60 °C alkaline solution for 3 months, the unaged, 135 °C aged and 300 °C aged BFRP samples showed reduction in the tensile strength by 43.2%, 62.3% and 74.1%, respectively. The higher the thermal aging and immersion temperatures, the more deterioration of the mechanical properties occurred. Alkaline solution immersion showed more adverse effects compared to the distilled water. The detrimental effects of the thermal aging were attributed to the formation of voids and cracks through which water or alkaline solution tended to easily penetrate into the BFRPs. The degradation of the resin due to thermal aging and immersion was analyzed with dynamic mechanical thermal analysis and scanning electron microscopy analysis. The long term variation of the tensile strength of BFRPs was evaluated based on the Arrhenius equation.