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Density is one of the fundamental properties of fibres which reinforce polymer matrix composites, and is used both to estimate composite weight and to evaluate fibre content for property predictions. For traditional composites, reinforced by glass or carbon fibres, unique density values are well known for a particular fibre grade and provide reliable fibre content estimations and composite property predictions. However, this is not the case for natural fibres. This paper first reviews published density values for flax (Linum usitatissimum L.) fibres and describes the limitations of techniques used to measure fibre density. Significant variations in published densities are found, which can be related to the measurement method. New data quantify the influence of measurement technique, fibre extraction route, moisture content, and reinforcement geometry. Values obtained by buoyancy are around 10% lower than those obtained by pycnometry. Finally the consequences of measured density variations on property estimations for long fibre composites are discussed; volume fraction values calculated from buoyancy provide more accurate tensile modulus values compared to experimental values than those from gas pycnometry; the former are recommended for predictive use.

Density is one of the fundamental properties of fibres which reinforce polymer matrix composites, and is used both to estimate composite weight and to evaluate fibre content for property predictions. For traditional composites, reinforced by glass or carbon fibres, unique density values are well known for a particular fibre grade and provide reliable fibre content estimations and composite property predictions. However, this is not the case for natural fibres. This paper first reviews published density values for flax (Linum usitatissimum L.) fibres and describes the limitations of techniques used to measure fibre density. Significant variations in published densities are found, which can be related to the measurement method. New data quantify the influence of measurement technique, fibre extraction route, moisture content, and reinforcement geometry. Values obtained by buoyancy are around 10% lower than those obtained by pycnometry. Finally the consequences of measured density variations on property estimations for long fibre composites are discussed; volume fraction values calculated from buoyancy provide more accurate tensile modulus values compared to experimental values than those from gas pycnometry; the former are recommended for predictive use.

Density is one of the fundamental properties of fibres which reinforce polymer matrix composites, and is used both to estimate composite weight and to evaluate fibre content for property predictions. For traditional composites, reinforced by glass or carbon fibres, unique density values are well known for a particular fibre grade and provide reliable fibre content estimations and composite property predictions. However, this is not the case for natural fibres. This paper first reviews published density values for flax (Linum usitatissimum L.) fibres and describes the limitations of techniques used to measure fibre density. Significant variations in published densities are found, which can be related to the measurement method. New data quantify the influence of measurement technique, fibre extraction route, moisture content, and reinforcement geometry. Values obtained by buoyancy are around 10% lower than those obtained by pycnometry. Finally the consequences of measured density variations on property estimations for long fibre composites are discussed; volume fraction values calculated from buoyancy provide more accurate tensile modulus values compared to experimental values than those from gas pycnometry; the former are recommended for predictive use.

This paper presents results from an experimental study of three types of non-woven preforms (needlepunched, spunlaced and mat manufactured using a paper-making process) intended as composite reinforcement. These are potentially very attractive for transport applications. First, the influence of processing on elementary fiber tensile properties is shown to be limited. Then the preforms are evaluated in polypropylene matrix composites and mechanical properties are determined. The structure of non-woven reinforcements is strongly dependent on the manufacturing route. By varying the fiber content it is shown that the most efficient reinforcement for flax fibers is the mat produced by paper processing. The new spunlaced reinforced composites are shown to have slightly lower tensile properties (15% lower strength, and 25% lower stiffness) compared to mat composites at equivalent volume fraction, but further optimization is possible for these materials. Based on the measured constituent properties micromechanics models have been used to estimate composite stiffness. A good correlation is obtained between test results and model predictions.

This paper presents results from an experimental study of three types of non-woven preforms (needlepunched, spunlaced and mat manufactured using a paper-making process) intended as composite reinforcement. These are potentially very attractive for transport applications. First, the influence of processing on elementary fiber tensile properties is shown to be limited. Then the preforms are evaluated in polypropylene matrix composites and mechanical properties are determined. The structure of non-woven reinforcements is strongly dependent on the manufacturing route. By varying the fiber content it is shown that the most efficient reinforcement for flax fibers is the mat produced by paper processing. The new spunlaced reinforced composites are shown to have slightly lower tensile properties (15% lower strength, and 25% lower stiffness) compared to mat composites at equivalent volume fraction, but further optimization is possible for these materials. Based on the measured constituent properties micromechanics models have been used to estimate composite stiffness. A good correlation is obtained between test results and model predictions.

This paper presents results from an experimental study of three types of non-woven preforms (needlepunched, spunlaced and mat manufactured using a paper-making process) intended as composite reinforcement. These are potentially very attractive for transport applications. First, the influence of processing on elementary fiber tensile properties is shown to be limited. Then the preforms are evaluated in polypropylene matrix composites and mechanical properties are determined. The structure of non-woven reinforcements is strongly dependent on the manufacturing route. By varying the fiber content it is shown that the most efficient reinforcement for flax fibers is the mat produced by paper processing. The new spunlaced reinforced composites are shown to have slightly lower tensile properties (15% lower strength, and 25% lower stiffness) compared to mat composites at equivalent volume fraction, but further optimization is possible for these materials. Based on the measured constituent properties micromechanics models have been used to estimate composite stiffness. A good correlation is obtained between test results and model predictions.

The industrial production of flax fibers yields two fibrous products: scutched fibers are valuable for their length, fineness and cleanness, while tows are entangled, impure and are low-priced. There is a need for technical data to compare these two kinds of fibers beyond the difference of appearance, and to assess their potential as composite material reinforcements. In this work, we have compared the properties of scutched flax and flax tows from the same batch. The morphology was first assessed by measuring the length, cleanness and fineness of fiber bundles. The tensile properties of single fibers and of unidirectionally reinforced epoxy composites were then measured. The results show that the morphology of flax tows is different from but close to the morphology of scutched flax. Tow bundles were 15% shorter than scutched flax bundles and tow bundles were 25% thicker than scutched flax bundles. The tensile properties of single fibers were in the same range. Tensile properties of the unidirectional composites show a similar evolution of properties versus fiber volume fraction. The use of scutched flax allowed volume fractions of 0.7 to be achieved while tow volume fraction was limited to 0.6. Despite the visual and morphological difference between tows and scutched flax both can be good candidates for composite reinforcement in high performance applications.

The industrial production of flax fibers yields two fibrous products: scutched fibers are valuable for their length, fineness and cleanness, while tows are entangled, impure and are low-priced. There is a need for technical data to compare these two kinds of fibers beyond the difference of appearance, and to assess their potential as composite material reinforcements. In this work, we have compared the properties of scutched flax and flax tows from the same batch. The morphology was first assessed by measuring the length, cleanness and fineness of fiber bundles. The tensile properties of single fibers and of unidirectionally reinforced epoxy composites were then measured. The results show that the morphology of flax tows is different from but close to the morphology of scutched flax. Tow bundles were 15% shorter than scutched flax bundles and tow bundles were 25% thicker than scutched flax bundles. The tensile properties of single fibers were in the same range. Tensile properties of the unidirectional composites show a similar evolution of properties versus fiber volume fraction. The use of scutched flax allowed volume fractions of 0.7 to be achieved while tow volume fraction was limited to 0.6. Despite the visual and morphological difference between tows and scutched flax both can be good candidates for composite reinforcement in high performance applications.

The industrial production of flax fibers yields two fibrous products: scutched fibers are valuable for their length, fineness and cleanness, while tows are entangled, impure and are low-priced. There is a need for technical data to compare these two kinds of fibers beyond the difference of appearance, and to assess their potential as composite material reinforcements. In this work, we have compared the properties of scutched flax and flax tows from the same batch. The morphology was first assessed by measuring the length, cleanness and fineness of fiber bundles. The tensile properties of single fibers and of unidirectionally reinforced epoxy composites were then measured. The results show that the morphology of flax tows is different from but close to the morphology of scutched flax. Tow bundles were 15% shorter than scutched flax bundles and tow bundles were 25% thicker than scutched flax bundles. The tensile properties of single fibers were in the same range. Tensile properties of the unidirectional composites show a similar evolution of properties versus fiber volume fraction. The use of scutched flax allowed volume fractions of 0.7 to be achieved while tow volume fraction was limited to 0.6. Despite the visual and morphological difference between tows and scutched flax both can be good candidates for composite reinforcement in high performance applications.

The flax quality required for composite applications is not yet well established. Retting is one of the steps that are not well defined for these applications, and is a critical parameter during flax production. In this study, the influence of the degree of retting of flax on the properties of short flax fiber/polypropylene composites has been assessed. First, the degree of retting of gradually retted flax was measured by both qualitative and quantitative experimental techniques. In addition, water sorption studies were performed. Furthermore, tensile tests were carried out on both single fiber and injected composite materials. The microstructure of the composites was analyzed by scanning electron microscopy and X-ray microtomography. Single fiber and composite tensile properties, as well as water sorption behavior of flax fibers, were found to depend upon the degree of retting.

The flax quality required for composite applications is not yet well established. Retting is one of the steps that are not well defined for these applications, and is a critical parameter during flax production. In this study, the influence of the degree of retting of flax on the properties of short flax fiber/polypropylene composites has been assessed. First, the degree of retting of gradually retted flax was measured by both qualitative and quantitative experimental techniques. In addition, water sorption studies were performed. Furthermore, tensile tests were carried out on both single fiber and injected composite materials. The microstructure of the composites was analyzed by scanning electron microscopy and X-ray microtomography. Single fiber and composite tensile properties, as well as water sorption behavior of flax fibers, were found to depend upon the degree of retting.

The flax quality required for composite applications is not yet well established. Retting is one of the steps that are not well defined for these applications, and is a critical parameter during flax production. In this study, the influence of the degree of retting of flax on the properties of short flax fiber/polypropylene composites has been assessed. First, the degree of retting of gradually retted flax was measured by both qualitative and quantitative experimental techniques. In addition, water sorption studies were performed. Furthermore, tensile tests were carried out on both single fiber and injected composite materials. The microstructure of the composites was analyzed by scanning electron microscopy and X-ray microtomography. Single fiber and composite tensile properties, as well as water sorption behavior of flax fibers, were found to depend upon the degree of retting.