Aging mechanisms in cellulose fiber reinforced cement composites

This paper examines the effects of laboratory scale accelerated aging exposures on the changes in physical and mechanical properties of commercially produced cellulose fiber reinforced cement composites. Two different accelerated aging methods were used to simulate the possible aging mechanisms for which the material may experience under service conditions, both methods being compared to material naturally weathered for 5 yr in roofing. The first aging method consisted of different cycles of water immersion, carbonation, and heating exposures whereas in the second method, cycles of water immersion, heating and freeze-thaw exposures were used. The porosity, water absorption, permeability of nitrogen and compressive shear strength of the composites were examined before and after aging exposures. The surface morphologies of the composites fractured in compression shear tests were examined using scanning electron microscope. Experimental results showed that the compressive shear strength of the accelerated aged composites were related to the microstructures within the composites. Both natural weathering and accelerated aging in CO₂ environment reduced the porosity, water absorption, and nitrogen permeability in the cement matrix, and enhanced the durability of the cellulose fiber-cement composites. The aging test based on artificial carbonation was more effective in simulating natural aging performance of the composites, while the freeze-thaw cycling method failed to induce significant aging effects on the composites even after 21 cycles.     

天然纤维/环氧树脂-混凝土的力学性能及老化规律天然纤维/环氧树脂-混凝土的力学性能及老化规律

环氧树脂-混凝土是由混合树脂、固化剂和砂石骨料等原料固化成型的一种复合材料,因其优异的性能已成为土木与建筑应用中富有潜力的新型工程材料。本文将高性能的天然纤维（剑麻和苎麻）引入环氧树脂-混凝土中,以进一步增强其力学性能。实验结果表明,极少含量的天然纤维便能够提升环氧树脂-混凝土的抗弯拉强度,体积分数为0.36vol%的剑麻纤维和苎麻纤维可以将混凝土的抗弯拉强度分别提高10.5%和8.4%。天然纤维对环氧树脂-混凝土抗弯拉强度的增强效应可以用基于混合率的并联模型描述,模型预测结果与实测结果相比,相对误差低于5.1%。利用紫外光耐气候试验箱模拟华南地区自然环境的日照辐射和湿热条件,研究了天然纤维/环氧树脂-混凝土抗弯拉性能随老化时间的衰减。当等效老化时间为6年时,天然纤维/环氧树脂-混凝土的抗弯拉强度分别下降了14.3%（剑麻纤维）和15.9%（苎麻纤维）。实验观察到的衰减规律可采用复合材料湿热老化的剩余强度模型描述。      

植物纤维改性方法及其增强复合材料研究进展

植物纤维增强的复合材料具有低成本、降解性能良好的特性,但植物纤维与被增强材料之间存在着界面相容性较差的问题。因此,需通过合适的处理对植物纤维进行表面改性,以改善纤维与被增强材料之间的界面相容性,提高复合材料的力学性能。简要介绍了植物纤维的结构和性质,综述了近年来植物纤维改性方法对纤维和纤维增强的复合材料性能的影响。最后分析了各种方法的优缺点并对植物纤维的改性技术和复合材料的未来发展趋势进行了展望。     

Effect of Structure on the Mechanical Behaviors of Three-Dimensional Spacer Fabric Composites

Three-dimensional (3-D) spacer fabric composite is a newly developed sandwich structure, the reinforcement of which is integrally woven by advanced textile technique. Two facesheets of 3-D spacer fabric are connected by continuous fibers, named pile in the core, providing excellent properties like outstanding integrity, debonding resistance, light weight, good designability and so on. Usually the 3-D spacer fabric composite without extra reinforcement is a kind of core material. In comparison with the facesheet reinforced spacer fabric composite, here the composite without additional weaves is called mono-spacer fabric composite. In this paper, two kinds of mono-spacer fabric composites with integrated hollow cores have been developed, one with 8-shaped piles and the other with corrugated piles. The mechanical characteristics and the damage modes of these mono-spacer fabric composites under different load conditions have been investigated. Besides, effects of pile height, pile distribution density and pile structure on the composites mechanical performances were analyzed. It is shown that the mechanical performances of mono-spacer fabric composites can be widely adapted to the respective requirements through the choice of the structural factors.     

Use of styrene butadiene rubber industrial waste devulcanized by microwave in rubber composites for automotive application

The aim of this work was to develop and characterize styrene butadiene rubber (SBR) composites filled with SBR industrial scraps (SBR-r) devulcanized by microwave. The SBR-r was ground under ambient conditions and subjected to microwave exposure for 1, 2 and 3 min. The composites were developed by the incorporation of 80 phr (parts per hundred of rubber) of the devulcanized SBR-r into the rubber matrix and subsequent revulcanization. The rheometric and physical–mechanical properties (shore A hardness, compression set, tensile and tear strength) were determined. The samples were submitted to two types of accelerated aging processes, in an air oven (thermo-oxidation) and an artificial weathering test in a UV chamber (photo-oxidation). The results were compared with a control sample submitted to the same conditions. For further application in automotive profiles, the set of results indicated that the best performance was achieved by the composite containing SBR-r devulcanized for 2 min in a microwave. For this composite, the changes in the chemical characteristics after aging were evaluated with cross-link density and Attenuated Total Reflectance–Fourier Transform Infrared Spectroscopy (ATR–FTIR) analyses. The results revealed that the aging mechanism was dependent on the presence of SBR-r and on the time of aging. The cross-link density analysis confirmed the occurrence of cross-link scissioning at shorter aging times and the formation of additional cross-linking in the postcure process. The ATR–FTIR results indicated an oxidative process occurring on the surface of the composites for both aging processes. Moreover, artificial weathering presented a more pronounced change in the chemical structure of the samples.     

Development of vegetable fibre–mortar composites of improved durability

The primary concern for vegetable fibre reinforced mortar composites (VFRMC) is the durability of the fibres in the alkaline environment of cement. The composites may undergo a reduction in strength and toughness as a result of weakening of the fibres by a combination of alkali attack and mineralisation through the migration of hydration products to lumens and spaces. This paper presents several approaches used to improve the durability performance of VFRMCs incorporating sisal and coconut fibres. These include carbonation of the matrix in a CO₂-rich environment; the immersion of fibres in slurried silica fume prior to incorporation in the ordinary Portland cement (OPC) matrix; partial replacement of OPC matrix by undensified silica fume or blast-furnace slag and a combination of fibre immersion in slurried silica fume and cement replacement. The durability of the modified VFRMC was studied by determining the effects of ageing in water, exposure to cycles of wetting and drying and open air weathering on the microstructures and flexural behaviour of the composites. Immersion of natural fibres in a silica fume slurry before their addition to cement-based composites was found to be an effective means of reducing embrittlement of the composite in the environments studied. Early cure of composites in a CO2-rich environment and the partial replacement of OPC by undensified silica fume were also efficient approaches in obtaining a composite of improved durability. The use of slag as a partial cement replacement had no effect on reducing the embrittlement of the composite.     

Porosity and water permeability of rice husk ash-blended cement composites reinforced with bamboo pulp

Cellulose fibres have already been applied commercially as an alternative to asbestos in fibre-cements composites. In spite of their industrial scale production for more than 20 years, these composites still require much research efforts, which focus mainly on durability aspects. The influence of the most relevant deterioration mechanisms can be minimized if mineral admixtures with high pozzolanic activity replace ordinary Portland cement (OPC). The improvements then achieved are due to the decrease in Ca(OH)₂ content and the more compact matrix and interfaces in the composite. In this respect, rice husk ash (RHA) is one of the most promising materials to be applied as a partial cement replacement in the cellulose-reinforced cement-based composites. This is due to the high active silica content of the ash and the widespread availability of the husks. To assess the influences of different chemical compositions of RHA, and the effects of autoclave curing on the pore characteristics of bamboo-pulp-reinforced cement composites, a comparative study was carried out in which pore characteristics were assessed by mercury intrusion porosimetry (MIP). Complementarily, the effects exerted by changes in the pore structure of the composites on their water permeability are evaluated by analytical and experimental approaches. It was observed that the incorporation of RHA in the composites could cause an extensive pore refinement in the matrix and in the interface layer, thereby decreasing water permeability. The results indicate that partial replacement of cement by RHA can improve the durability characteristics of cellulose–cement composites.     

聚乙烯醇基相变复合材料研究进展

亲水性聚乙烯醇(PVA)具有相对较好的可加工性、安全性及生物可降解性,被广泛用作支撑材料,可将相变材料封装在其三维网状结构中以解决相变组分泄露的问题并使复合材料获得良好的导热性。基于此,综述了分别利用分子间作用力、共价键将PVA基体与相变材料复合的物理共混法及化学接枝法,对比了两种方法用于封装相变材料的优缺点;总结了目前PVA基相变复合材料的类型、成型方法及性能,包括经湿法纺丝、干法纺丝及静电纺丝等方法制备的相变复合纤维,经物理共混法制备的相变复合膜以及经一步法原位复合制备的相变复合多孔材料;分析了复合材料中相变组分及成型工艺等对材料结构及相变蓄热、力学及热稳定性等性能的影响;同时展望了功能化PVA基相变复合材料的研究方向及发展前景。     

Fibre reinforcement and fracture response in geopolymeric mortars

The inorganic polymeric cement called geopolymer or PSS, has been studied in recent years as a binder for mortar and concrete. The present work reports the fracture toughness studies in mortars made of PSS cement matrix reinforced by wollastonite microfibers (Ca(SiO₃)). K_I‐curves for PSS cement composites were determined according to the superposition asymptotic assumption and compared with reference Portland cement (PC) composites. The maximum toughness gain occurs in both composite systems with V_f = 2%. For higher fibre volumes (3 and 5%), K_I values decrease, due to an increase in porosity. Microstructural analyses showed that toughening mechanisms, as debonding and pullout of the fibers, are more common in PSS cement composites than in the reference PC composites. The difference of toughness between PSS and PC cement (0% of fibers) is about 80%. This demonstrates the high performance of these geopolymeric materials.     

废旧塑料在复合材料领域中回用技术的研究进展

介绍了国内外废旧塑料现状以及废旧塑料在复合材料领域利用新进展,综述了废旧塑料在植物纤维/废旧塑料复合材料、废纸/废塑料复合材料、木塑复合发泡材料、可生物降解塑料复合材料及其他复合材料领域的再利用技术新进展,并分析了废旧塑料在复合材料领域回收再利用技术的发展趋势,提出应进一步探讨不同种类废旧塑料对复合材料力学性能及植物纤维/废旧塑料界面相容性的影响。     

Mechanical characterization of epoxy composite with multiscale reinforcements: Carbon nanotubes and short carbon fibers

Carbon nanotubes (CNT) and short carbon fibers were incorporated into an epoxy matrix to fabricate a high performance multiscale composite. To improve the stress transfer between epoxy and carbon fibers, CNT were also grown on fibers through chemical vapor deposition (CVD) method to produce CNT grown short carbon fibers (CSCF). Mechanical characterization of composites was performed to investigate the synergy effects of CNT and CSCF in the epoxy matrix. The multiscale composites revealed significant improvement in elastic and storage modulus, strength as well as impact resistance in comparison to CNT–epoxy or CSCF–epoxy composites. An optimum content of CNT was found which provided the maximum stiffness and strength. The synergic reinforcing effects of combined fillers were analyzed on the fracture surface of composites through optical and scanning electron microscopy (SEM).     

Flexural behavior of geopolymer composites reinforced with steel and polypropylene macro fibers

Like ordinary Portland cement concrete, the matrix brittleness in geopolymer composites can be reduced by introducing appropriate fiber reinforcement. Several studies on fiber reinforced geopolymer composites are available, however there is still a gap to understand and optimize their performance. This paper presents the flexural behavior of fly ash-based geopolymer composites reinforced with different types of macro steel and polypropylene fibers with higher aspect ratio. Three types (length-deformed, end-deformed and straight) of steel fibers and another type of length-deformed polypropylene fiber with optimum fiber volume fraction of 0.5% are studied. The effects of different geometries of the fibers, curing regimes (ambient cured and heat cured at 60 °C for 24 h) and concentration of NaOH activator (10 M and 12 M) on the first peak strength, modulus of rupture and toughness of the geopolymer composites are investigated. The quantitative effect of fiber geometry on geopolymer composite performance was also analyzed through a fiber deformation ratio. The compressive strength, splitting tensile strength and flexural toughness are significantly improved with macro fibers reinforcement and heat curing. The results also show that heat curing increases the first peak load of all fiber-reinforced geopolymers composites. End-deformed steel fibers exhibit the most ductile flexural response compared to other steel fibers in both heat and ambient-cured fiber reinforced geopolymer composites.     

非均质秸秆纤维复合材料保险杠蒙皮刚度分析

目的 研究非均质秸秆纤维复合材料保险杠蒙皮的刚度性能.方法 采用试验与模拟分析的方法,通过共混挤出与化学发泡注塑工艺制备微发泡秸秆纤维/聚丙烯(SF/PP)复合材料试样,通过试验测试非均质结构试样的力学性能与微观结构,通过有限元分析手段建立非均质微发泡秸秆纤维/PP复合材料结构分析模型,并分析非均质材料保险杠蒙皮的刚度...     

碳纤维表面结构对复合材料吸湿性能的影响

通过改变阳极氧化处理程度得到了具有不同表面结构的碳纤维, 然后将其与环氧树脂加工成碳纤维/树脂基复合材料, 研究了碳纤维的化学结构与湿热环境下复合材料吸湿之间的关系。结果显示: 阳极氧化处理后碳纤维表面的活性显著提高, 碳纤维表面含氧官能团的含量大幅增加, 尤其是-OH由处理前18.62%提高到处理后的34.84%。随着湿热处理条件的改变, 复合材料的吸湿机理也有所差异, 且温度是影响复合材料吸湿的重要因素。碳纤维表面活性越高, 复合材料达到吸湿平衡时的平衡吸湿量越大, 而平衡吸湿量的增加又会导致复合材料层间剪切强度(ILSS)下降幅度增大。     

Effect of operating parameters and chemical treatment on the tribological performance of natural fiber composites: A review

The demand for high-performance engineering products made from natural resources is increasing because of the low-cost, low-density, biodegradability, renewable nature and lighter than synthetic fibers. With these characteristics, the tribological performance of natural fiber composite has become an important element to be considered in most industrial and manufacturing functions. This paper presents an overview of the factors that influence the tribological performance of natural fiber composites, which include applied load, sliding distance, sliding velocity and fiber orientation. Influences of chemical treatment is also reviewed and illustrated through scanning electron microscope (SEM) observations. This review will focus on kenaf fibers (KFs) and oil palm fibers (OPFs) which have been widely exploited over the past few years among the available natural resources. The results show that the operating parameter, fiber orientation and chemical treatment has significant effects on the tribological performance of natural composite. A clear understanding of the factors that affect the tribological performance is very essential in performance improvement on natural fibers reinforced polymer composite for potential applications.     

PET基定形相变复合纤维的制备

采用静电纺丝技术成功制备了以5种脂肪酸二元低共熔混合物(LA-MA、LA-SA、MA-PA、MA-SA、PA-SA)为固液相变材料,聚对苯二甲酸乙二酯(PET)为支撑材料的定形相变复合纤维。研究了不同种类的脂肪酸二元低共熔物对复合相变纤维的形貌结构、储热性能以及力学性能的影响。研究结果表明这5种定形相变复合纤维的表面均呈现褶皱的形貌特征,同时纤维直径也明显增大。热分析结果表明当改变纤维中脂肪酸二元低共熔物的种类时,复合相变纤维的熔化温度和熔化焓值均随之而变化,其中熔化温度最低为33.23℃,最高为52.82℃,熔化焓值最低为62.75kJ/kg,最高为94.76kJ/kg。力学性能测试结果表明,由于脂肪酸二元低熔物的加入复合相变纤维的拉伸强度减小,断裂伸长率增大。     

不同种类废纸纤维形态及结构特性研究

目的研究4种最常见废纸纤维的性能,探索废纸纤维更广泛的用途,从而实现废纸的高值化和资源化利用。方法通过纤维分析、FT-IR、XRD、SEM、TG等手段研究旧报纸、箱纸板、打印纸、书刊纸纤维的形态、结构特性、结晶情况和热解性能。结果旧报纸纤维表面较粗糙,纤维受损严重,且扭结程度较大,细小纤维含量多,热稳定性最差;旧箱纸板纤维表面粗糙度相对较高,扭结程度最低;打印纸纤维长径比最大,形态和表面相对完整;书刊纸纤维表面较粗糙,结晶度最大。经FT-IR分析发现,箱纸板纤维表面羟基相对较少,打印纸和书刊纸存在较多的填料。结论旧箱纸板和书刊纸纤维的综合性能相对较好,更利于与其他材料复合成型。     

Degradation kinetics and aging mechanisms on sisal fiber cement composite systems

The kinetics of vegetable (sisal) fiber degradation and the mechanisms responsible for deterioration of continuous sisal fiber cement composites are presented in this paper. Two matrices were used: one with 50% partial cement replacement by metakaolin (PC–MK) and a reference matrix having as binder only Portland cement (PC). The durability performance of the composite systems is examined and the mechanisms for the significant delay in the fiber degradation when the total amount of calcium hydroxide is reduced from the matrix discussed. The composites were subjected to 5, 10, 15, 20 and 25 cycles of wetting and drying and then tested under a four point bending load configuration in order to determine the flexural behavior and cracking mechanisms with progressive aging. Furthermore, composites stored under controlled lab conditions were tested under bending load at ages ranging from 28 days to 5 years. Fibers extracted from the aged composites were subjected to thermal analysis, Fourier transform infrared spectroscopy and microscopical observations in order to evaluate the changes in chemical composition and microstructure. Two fiber degradation mechanisms were observed in the PC composites: fiber mineralization due to the precipitation of calcium hydroxide in the fiber cell and surface and degradation of cellulose, hemicellulose and lignin due to the adsorption of calcium and hydroxyl ions. The degradation process occurs rapidly and after 10 cycles of wetting/drying a quite expressive modification in the flexural behavior is observed. The residual mechanical parameters after 25 cycles were the same as those observed in the unreinforced matrix. For the PC–MK composite fiber mineralization was not observed due to the low content of CH in the matrix.     

Bamboo and wood fibre cement composites for sustainable infrastructure regeneration

This paper describes the development of eco-friendly bamboo and wood fibre cement composites from agriculture wastes for applications in the housing and building industries, and for sustainable infrastructure regeneration. Bamboo flakes and fibres from oil palm tree fronds were produced and tested for their sugar content and effect on the setting and strength development of the portland cement matrix. To counteract the adverse effects on cement hydration, chemical accelerators, cement replacement materials or a combination of both were used in the manufacture of the composite boards. With the bamboo, the composition of the particleboard was optimized in terms of bamboo–cement ratio and the type and amount of chemical admixture to produce a composite with satisfactory strength and dimensional stability. For the production of wood fibre cement composites, cement replacement materials such as fly ash, rice husk ash and latex were used in conjunction with chemical admixtures to counteract the adverse effect on the hydration characteristics of the cement matrix. Tests were then carried out to optimize the amount and type of cement replacement material and chemical admixtures to produce boards with adequate strength and dimensional stability. All the strength and dimensional stability tests reported in the paper were carried out according to Malaysian Standard MS 934. The paper emphasizes the need for holistic design combining chemical admixtures, cement replacement materials and modern production technology to produce a wide range of cement-bonded composite boards, which will satisfy international standards and can be widely used for infrastructure regeneration.     

Durability of kraft pulp fiber–cement composites to wet/dry cycling

If pulp fiber–cement composites are to be used for exterior applications, the effect of cyclical wet/dry exposure must be known. In this research program the effects of three fiber treatments—beating, bleaching, and drying—were investigated to identify those that may minimize effects of environmental aging and degradation during wet/dry cycling. After 25 wet/dry cycles, all composites showed significant losses in first crack strength, peak strength, and post-cracking toughness. The majority of losses in mechanical properties occurred within the first 5 wet/dry cycles, though ductile fiber failure was still observed by scanning electron microscopy (SEM). A three-part progressive degradation mechanism during wet/dry cycling is proposed: (1) initial fiber–cement debonding, (2) reprecipitation of hydration products within the void space at the former fiber–cement interface, and (3) fiber embrittlement due to fiber cell wall mineralization. Unbeaten fiber–cement composites exhibited greater peak strength and post-cracking toughness, prior to cycling, while no significant differences were seen after 25 cycles. The effects of fiber beating varied prior to and after cycling. Unbleached fiber–cement composites exhibited the slowest progression of degradation during cycling. The initial drying state appeared to have no effect on composite performance after 25 wet/dry cycles.