Bo Madsen

Physical and mechanical properties of unidirectional plant fibre composites: an evaluation of the influence of porosity

Abstract Unidirectional composites were made from filament wound non-treated flax yarns and polypropylene foils. With increasing composite fibre weight fractions from 0.56 to 0.72, porosity fractions increased from 0.04 to 0.08; a theoretical model was fitted to the data in order to describe the composite volumetric interaction between contents of fibre, matrix and porosity. In the model two porosity components were proposed, a process governed component and a structurally governed component. The composite axial stiffness and strength were in the range 27–29 GPa and 251–321 MPa, respectively. A modified version of the “rule-of-mixtures”, supplemented with parameters of composite porosity content and anisotropy of fibre properties, were developed to improve the prediction of composite tensile properties.

Cellulosic Fibers: Effect of Processing on Fiber Bundle Strength

A range of differently processed cellulosic fibers from flax and hemp plants were investigated to study the relation between processing of cellulosic fibers and fiber bundle strength. The studied processing methods are applied for yarn production and include retting, scutching, carding, and cottonization. There was a monotonically decreasing relationship between the strength and the number of processing steps, which was well fitted by an exponential regression line. The reduction factor was determined to be 0.27, indicating that the fiber bundle strength was on average reduced by 27% per processing step at the applied conditions. No large changes in cellulose content and crystallinity were observed, so the reduction in strength must be explained by other changes in the fiber ultrastructure. Altogether, the study presents a quantitative basis for reduction in strength of cellulosic fibers due to processing.

Experimental Study of Fiber Length and Orientation in Injection Molded Natural Fiber/Starch Acetate Composites

Composite compounds based on triethyl citrate plasticized starch acetate and hemp and flax fibers were prepared by melt processing. Plasticizer contents from 20 to 35 wt% and fiber contents of 10 and 40 wt% were used. The compounded composites were injection molded to tensile test specimens. The effect of processing, melt viscosity and fiber type on the fiber length was investigated. The lengths of fully processed fibers were determined by dissolving the matrix and measuring the length of the remaining fibers by microscope analysis. A clear reductive effect of the processing on the fiber length was noticed. A reduction of fiber length along the increasing fiber content and the decreasing plasticizer content was also detected. This reduction originated from the increasing shear forces during compounding, which again depended on the increased viscosity of the material. Hemp fibers were shown to remain longer and fibrillate more than flax fibers, leading to higher aspect ratio. Thus, the reinforcement efficiency of hemp fibers by the processing was improved, in contrast with flax fibers. In addition, the analysis of fiber dispersion and orientation showed a good dispersion of fibers in the matrix, and a predominant orientation of the fibers in the melt flow direction.

Short cellulosic fiber/starch acetate composites — micromechanical modeling of Young’s modulus

This study is presented to predict the Young’s modulus of injection-molded short cellulosic fiber/plasticized starch acetate composites with variable fiber and plasticizer content. A modified rule of mixtures model is applied where the effect of porosity is included, and where the fiber weight fraction is used as the basic independent variable. The values of the input model parameters are derived from experimental studies of the configuration of the composites (volumetric composition, dimensions, and orientation of fibers), as well as the properties of the constituent fiber and matrix phases (density and Young’s modulus). The measured Young’s modulus of the composites varies in the range 1.1—8.3 GPa, and this is well predicted by the model calculations. A property diagram is presented to be used for the tailor-making of composites with Young’s modulus in the range 0.2—10 GPa.

Properties and performance of flax yarn/thermoplastic polyester composites

Aiming at demonstrating the potential of unidirectional natural fiber-reinforced thermoplastic composites in structural applications, textile flax yarn/thermoplastic polyester composites with variable fiber volume fractions have been manufactured by a filament-winding process followed by a vacuum-assisted compression molding process. The microstructure of the composites shows that the flax fiber yarns are well impregnated by the polyester matrix, and this supports the measured low porosity content of the composites. The experimental tensile modulus and ultimate tensile stress of the composites in the axial and transverse directions are well simulated by rule of mixtures models. In the axial direction, at a fiber volume fraction of 0.50, the experimental tensile modulus and ultimate tensile stress are 32 GPa and 350 MPa, respectively. In comparison, for glass fiber composites at a fiber volume fraction of 0.50, the tensile modulus and ultimate tensile stress are calculated to be 38 GPa and 1800 MPa, respectively. The flax yarn composites show better specific tensile modulus than the glass fiber composites with values of 23 GPa/g/cm3 and 20 GPa/g/cm3, respectively. An analysis of data from previous studies of unidirectional natural fibre composites demonstrates comparatively good reinforcement efficiency of the flax yarn fibers with an effective tensile modulus and ultimate tensile stress of the fibers in the area of 70 GPa and 800 MPa, respectively. Altogether, it is demonstrated that composites with high-quality textile flax yarn are well suited for structural applications when stiffness and weight saving are the central selection criteria.

Strength of cellulosic fiber/starch acetate composites with variable fiber and plasticizer content

In this experimental study, the performance of injection-molded short flax and hemp fibers in plasticized starch acetate were analyzed in terms of strength. Parameters involved in the analysis are a variable fiber and plasticizer content. The measured strength of the composites varies in the range of 12–51 MPa for flax fibers and 11–42 MPa for hemp fibers, which is significantly higher than the properties of the unreinforced starch acetate matrix. The micro-structural parameters used in modeling of composite strength were obtained from optical observations and indirect measurements. Some of these parameters were qualitatively verified by X-ray microtomography.

Volumetric composition and shear strength evaluation of pultruded hybrid kenaf/glass fiber composites

In the present study, six different combinations of pultruded hybrid kenaf/glass composites were fabricated. The number of kenaf and glass rovings was specifically selected to ensure constant local fiber volume fractions in the composites. The volumetric composition of the composites was determined by using a gravimetrically based method. Optical microscopy was used to determine the location of voids. The short-beam test method was used to determine the interlaminar shear strength of the composites, and the failure mode was observed. It was found that the void volume fraction of the composites was increased as a function of the kenaf fiber volume fraction. A linear relationship with high correlation (R2 = 0.95) was established between the two volume fractions. Three types of voids were observed in the core region of the composites (lumen voids, interface voids and impregnation voids). The failure of the samples started with horizontal shear cracks that propagated into the core region, and ultimately the samples failed by a vertical crack. The interlaminar shear strength was found to decrease as a function of the hybrid fiber mixing ratio.

Development of new biomass-based furan/glass composites manufactured by the double-vacuum-bag technique

The present study addresses the development of new biomass-based furan resin/glass fibre composites manufactured by the double-vacuum-bag technique using a two-stage cure cycle to allow removal of water from the resin. The volumetric composition and mechanical properties of the composites are measured and analysed with focus on the porosity content. The so-called matrix correlated porosity factor is determined to be 0.096, meaning that the furan matrix itself contains 8.8% porosity. In the optimal case of no matrix porosity, stiffness of the composites compares well with the stiffness of conventional thermosetting/glass composites, but with lower strength. The findings of the present study show that a more efficient water removal during manufacturing, a lower porosity content and a less brittle stress–strain behaviour of the furan matrix are to be addressed to further improve the properties of the composites.

Protocol for Quantification of Defects in Natural Fibres for Composites

Natural bast-type plant fibres are attracting increasing interest for being used for structural composite applications where high quality fibres with good mechanical properties are required. A protocol for the quantification of defects in natural fibres is presented. The protocol is based on the experimental method of optical microscopy and the image analysis algorithms of the seeded region growing method and Otsu’s method. The use of the protocol is demonstrated by examining two types of differently processed flax fibres to give mean defect contents of 6.9 and 3.9%, a difference which is tested to be statistically significant. The protocol is evaluated with respect to the selection of image analysis algorithms, and Otsu’s method is found to be a more appropriate method than the alternative coefficient of variation method. The traditional way of defining defect size by area is compared to the definition of defect size by width, and it is shown that both definitions can be used to give unbiased findings for the comparison between fibre types. Finally, considerations are given with respect to true measures of defect content, number of determinations, and number of significant figures used for the descriptive statistics.

Biobased composites: materials, properties and potential applications as wind turbine blade materials

Abstract: This chapter about biobased composites starts by presenting the most promising types of cellulose fibres; their properties, processing and preforms for composites, together with an introduction to biobased matrix materials. The chapter then presents the typical mechanical properties of biobased composites, based on examples of composites with different fibre/matrix combinations, followed by a case study of the stiffness and specific stiffness of cellulose fibre composites vs glass fibre composites using micromechanical model calculations. Finally, the chapter presents some of the special considerations to be addressed in the development and application of biobased composites.