*Polymers* **<sup>2022</sup>**, *<sup>14</sup>*, 4060 *Polymers* **<sup>2022</sup>**, *14*, 4060 12 of 16

(**a**) (**b**) (**c**)

*Polymers* **2022**, *14*, 4060 12 of 16

*Polymers* **2022**, *14*, 4060 12 of 16

takes place (Figures 11b and 12b).

**Figure 8.** (**a**) Tensile modulus of FL-LI composites; (**b**) tensile strength of FL-LI composites; (**c**) elongation of FL-LI composites. **Figure 8.** (**a**) Tensile modulus of FL-LI composites; (**b**) tensile strength of FL-LI composites; (**c**) elongation of FL-LI composites. **Figure 8.** (**a**) Tensile modulus of FL-LI composites; (**b**) tensile strength of FL-LI composites; (**c**) elongation of FL-LI composites. (**a**) (**b**) (**c**) **Figure 8.** (**a**) Tensile modulus of FL-LI composites; (**b**) tensile strength of FL-LI composites; (**c**) elongation of FL-LI composites.

yarn in the composite is longer and has thereby a larger contact area with the matrix system as well as a larger clamping length than the warp yarns. When the consolidated sample is subjected to tension during the tensile test, the weft yarns can therefore absorb more load (Figure 10). The meandering yarn course in the weft direction is also helpful against the weft yarns being pulled out of the matrix. The higher elongation can also be explained by the increased load transfer from the weft yarns to the matrix system. The undulation of the weft yarns is reduced during the tensile test in 0° direction, which results in a plastic deformation of the sample along the diagonal twill weave lines (Figures 11a and 12a), compared to the tensile test in 90° with straight warp yarns where no plastic deformation

yarn in the composite is longer and has thereby a larger contact area with the matrix system as well as a larger clamping length than the warp yarns. When the consolidated sample is subjected to tension during the tensile test, the weft yarns can therefore absorb more load (Figure 10). The meandering yarn course in the weft direction is also helpful against the weft yarns being pulled out of the matrix. The higher elongation can also be explained by the increased load transfer from the weft yarns to the matrix system. The undulation of the weft yarns is reduced during the tensile test in 0° direction, which results in a plastic deformation of the sample along the diagonal twill weave lines (Figures 11a and 12a), compared to the tensile test in 90° with straight warp yarns where no plastic deformation

yarn in the composite is longer and has thereby a larger contact area with the matrix system as well as a larger clamping length than the warp yarns. When the consolidated sample is subjected to tension during the tensile test, the weft yarns can therefore absorb more load (Figure 10). The meandering yarn course in the weft direction is also helpful against the weft yarns being pulled out of the matrix. The higher elongation can also be explained by the increased load transfer from the weft yarns to the matrix system. The undulation of the weft yarns is reduced during the tensile test in 0° direction, which results in a plastic deformation of the sample along the diagonal twill weave lines (Figures 11a and 12a), compared to the tensile test in 90° with straight warp yarns where no plastic deformation

(**a**) (**b**)

(**a**) (**b**)

**Figure 9.** (**a**) Cross section showing the course of warp yarn of the FL-LI composite; (**b**) cross section showing the course of weft yarn of the FL-LI composite. **Figure 9.** (**a**) Cross section showing the course of warp yarn of the FL-LI composite; (**b**) cross section showing the course of weft yarn of the FL-LI composite. **Figure 9.** (**a**) Cross section showing the course of warp yarn of the FL-LI composite; (**b**) cross section showing the course of weft yarn of the FL-LI composite. **Figure 9.** (**a**) Cross section showing the course of warp yarn of the FL-LI composite; (**b**) cross section showing the course of weft yarn of the FL-LI composite.

**Figure 10.** Load transfer directions of the weft yarn during the tensile test with idealised yarn course. The red arrows indicate the force (F). The blue line shows the weft yarn in the matrix background. **Figure 10.** Load transfer directions of the weft yarn during the tensile test with idealised yarn course. The red arrows indicate the force (F). The blue line shows the weft yarn in the matrix background. **Figure 10.** Load transfer directions of the weft yarn during the tensile test with idealised yarn course. The red arrows indicate the force (F). The blue line shows the weft yarn in the matrix background. **Figure 10.** Load transfer directions of the weft yarn during the tensile test with idealised yarn course. The red arrows indicate the force (F). The blue line shows the weft yarn in the matrix background. *Polymers* **2022**, *14*, 4060 13 of 16

**Figure 11.** Fracture zones of selected tensile test specimens (**a**) in 0° (weft direction, above front view and below back view); (**b**) in 90° (warp direction, above front view and below back view). **Figure 11.** Fracture zones of selected tensile test specimens (**a**) in 0◦ (weft direction, above front view and below back view); (**b**) in 90◦ (warp direction, above front view and below back view).

**Figure 12.** Blue coloured detailed view on the structure of the fracture zones of selected tensile test specimens:(**a**) tensile test in weft direction with plastic deformation along the diagonal twill weave

The bending properties of the FL-LI composites were determined in a 4-point bending test according to DIN EN ISO 14125 and are shown in Figure 13a to c. The flexural modulus of elasticity of FL-LI 90° is only slightly lower at 3.52 GPa compared to that of FL-LI 0° at 4.43 GPa. Due to higher yarn density in warp direction (6 yarns per cm) compared to that in the weft direction (5 yarns per cm) of the FL-LI fabric, a higher flexural modulus in the warp direction (90°) was expected than in the weft direction (0°). However, the 4-point bending tests proved a higher flexural modulus in the weft direction. The higher flexural modulus in the weft direction (0°) is due to the fact that the weft yarns are located further outside the central neutral bending axis, in the FL-LI composite, than the warp yarns (90°). Below the neutral bending line, the yarns are subjected to tensile stress, and above it, to compressive stress. Thus, the weft yarns, which are further away from the neutral bending line are significantly more stressed during the 4-point bending test. The higher flexural modulus in the weft direction is therefore mainly generated by the weft

The same effects as for the flexural modulus are evident in the flexural strength tests. The FL-LI composite has a higher flexural strength in the weft direction (69.05 MPa) than that in the warp direction (39.63 MPa). The weft yarns, which are located more off-centre than the warp yarns in the composite, generate a higher section modulus than the warp

 (**a**) (**b**)

lines; (**b**) tensile test in warp direction without plastic deformation.

3.4.2. Flexural Properties

yarns below the neutral bending line.

 (**a**) (**b**)

**Figure 12.** Blue coloured detailed view on the structure of the fracture zones of selected tensile test specimens:(**a**) tensile test in weft direction with plastic deformation along the diagonal twill weave lines; (**b**) tensile test in warp direction without plastic deformation. **Figure 12.** Blue coloured detailed view on the structure of the fracture zones of selected tensile test specimens:(**a**) tensile test in weft direction with plastic deformation along the diagonal twill weave lines; (**b**) tensile test in warp direction without plastic deformation.

**Figure 11.** Fracture zones of selected tensile test specimens (**a**) in 0° (weft direction, above front view

and below back view); (**b**) in 90° (warp direction, above front view and below back view).

#### 3.4.2. Flexural Properties 3.4.2. Flexural Properties

The bending properties of the FL-LI composites were determined in a 4-point bending test according to DIN EN ISO 14125 and are shown in Figure 13a to c. The flexural modulus of elasticity of FL-LI 90° is only slightly lower at 3.52 GPa compared to that of FL-LI 0° at 4.43 GPa. Due to higher yarn density in warp direction (6 yarns per cm) compared to that in the weft direction (5 yarns per cm) of the FL-LI fabric, a higher flexural modulus in the warp direction (90°) was expected than in the weft direction (0°). However, the 4-point bending tests proved a higher flexural modulus in the weft direction. The higher flexural modulus in the weft direction (0°) is due to the fact that the weft yarns are located further outside the central neutral bending axis, in the FL-LI composite, than the warp yarns (90°). Below the neutral bending line, the yarns are subjected to tensile stress, and above it, to compressive stress. Thus, the weft yarns, which are further away from the neutral bending line are significantly more stressed during the 4-point bending test. The higher flexural modulus in the weft direction is therefore mainly generated by the weft yarns below the neutral bending line. The bending properties of the FL-LI composites were determined in a 4-point bending test according to DIN EN ISO 14125 and are shown in Figure 13a–c. The flexural modulus of elasticity of FL-LI 90◦ is only slightly lower at 3.52 GPa compared to that of FL-LI 0◦ at 4.43 GPa. Due to higher yarn density in warp direction (6 yarns per cm) compared to that in the weft direction (5 yarns per cm) of the FL-LI fabric, a higher flexural modulus in the warp direction (90◦ ) was expected than in the weft direction (0◦ ). However, the 4-point bending tests proved a higher flexural modulus in the weft direction. The higher flexuralmodulus in the weft direction (0◦ ) is due to the fact that the weft yarns are located further outside the central neutral bending axis, in the FL-LI composite, than the warp yarns (90◦ ). Below the neutral bending line, the yarns are subjected to tensile stress, and above it, to compressive stress. Thus, the weft yarns, which are further away from the neutral bending line are significantly more stressed during the 4-point bending test. The higher flexural modulus in the weft direction is therefore mainly generated by the weft yarns below the neutral bending line. *Polymers* **2022**, *14*, 4060 14 of 16 yarns. The further the weft yarns are displaced under the neutral bending line of the FL-LI composite, the greater their influence on a higher flexural strength. Elongations of 2.54 % and 3.05 % are achieved in the weft and warp directions, respectively. The more even distribution of the warp yarns in the FL-LI composite increases the elongation in the warp direction, because the warp yarns are closer to the neutral bending line than the weft yarns. In contrast to the weft yarns, the yarns in the warp direction are not stressed as much in the 4-point bending test. Therefore, the warp yarns only break at a larger deflection.

**Figure 13.** (**a**) Flexural modulus of FL-LI composites; (**b**) flexural strength of FL-LI composites; (**c**) elongation of FL-LI composites. **Figure 13.** (**a**) Flexural modulus of FL-LI composites; (**b**) flexural strength of FL-LI composites; (**c**) elongation of FL-LI composites.

**4. Conclusions**  This paper presents an experimental study on the use of lignin as a matrix material The same effects as for the flexural modulus are evident in the flexural strength tests. The FL-LI composite has a higher flexural strength in the weft direction (69.05 MPa) than

and as a coating of yarns as an alternative for the semi-finished production of composite components. A commercial flax yarn is coated with a lignin matrix using an extrusion

posite panels. The semi-finished products thus produced, yarn and composite material,

The coating by thermal extrusion of flax yarn with lignin was successful. The lignin product passes quickly from the liquid to the solid aggregate state and make the yarn very stiff. Temperature adjustments in the individual extruder zones, as well as varying the production speed result in an acceptable but not satisfactory coating process. To increase uniformity of the coated yarn in future work, a yarn without splices will be used to prevent yarn breakage during the coating process. Furthermore, the lignin formulation has to be improved regarding a longer processing time, a wider temperature processing range and for a smooth surface of coated yarn in addition with reduced add-on. A coating degree of 336 % was attained. However, this could not be brought down to the desired 225

The coated flax yarns exhibited moderate elongation behaviour and good strength with a fibre volume content of nearly 35 vol.%. These properties allow for further processing by weaving, but the low flexibility transverse to the yarn axis and the saw wire-

The fabrics were produced with the lignin coated flax yarn (1099 ± 15 tex) in the warp direction and the commercially flax yarn (205 ± 5 tex) in the weft direction. The warp and weft densities are 6 and 5 yarns per cm, respectively. The lignin coated yarn is very brittle and therefore, gentle and slow processing are required. Nevertheless, the processing of the coated yarn remains intricate. To minimise damage to the yarn, the weaving shed should be reduced in size and the warp tension should be as low as possible. The width of the woven fabric is 500 mm. Another problem is the sharp-edged nature of the lignin coated flax yarn. This causes increased wear on the healds and the weaving reed. Large

were examined and analysed with regard to their characteristic properties.

like surface structure posed a considerable challenge.

healds and a coarser weaving reed could bring improvement.

%.

that in the warp direction (39.63 MPa). The weft yarns, which are located more off-centre than the warp yarns in the composite, generate a higher section modulus than the warp yarns. The further the weft yarns are displaced under the neutral bending line of the FL-LI composite, the greater their influence on a higher flexural strength.

Elongations of 2.54% and 3.05% are achieved in the weft and warp directions, respectively. The more even distribution of the warp yarns in the FL-LI composite increases the elongation in the warp direction, because the warp yarns are closer to the neutral bending line than the weft yarns. In contrast to the weft yarns, the yarns in the warp direction are not stressed as much in the 4-point bending test. Therefore, the warp yarns only break at a larger deflection.

#### **4. Conclusions**

This paper presents an experimental study on the use of lignin as a matrix material and as a coating of yarns as an alternative for the semi-finished production of composite components. A commercial flax yarn is coated with a lignin matrix using an extrusion process. This coated yarn was processed into a woven fabric and consolidated into composite panels. The semi-finished products thus produced, yarn and composite material, were examined and analysed with regard to their characteristic properties.

The coating by thermal extrusion of flax yarn with lignin was successful. The lignin product passes quickly from the liquid to the solid aggregate state and make the yarn very stiff. Temperature adjustments in the individual extruder zones, as well as varying the production speed result in an acceptable but not satisfactory coating process. To increase uniformity of the coated yarn in future work, a yarn without splices will be used to prevent yarn breakage during the coating process. Furthermore, the lignin formulation has to be improved regarding a longer processing time, a wider temperature processing range and for a smooth surface of coated yarn in addition with reduced add-on. A coating degree of 336% was attained. However, this could not be brought down to the desired 225%.

The coated flax yarns exhibited moderate elongation behaviour and good strength with a fibre volume content of nearly 35 vol.%. These properties allow for further processing by weaving, but the low flexibility transverse to the yarn axis and the saw wire-like surface structure posed a considerable challenge.

The fabrics were produced with the lignin coated flax yarn (1099 ± 15 tex) in the warp direction and the commercially flax yarn (205 ± 5 tex) in the weft direction. The warp and weft densities are 6 and 5 yarns per cm, respectively. The lignin coated yarn is very brittle and therefore, gentle and slow processing are required. Nevertheless, the processing of the coated yarn remains intricate. To minimise damage to the yarn, the weaving shed should be reduced in size and the warp tension should be as low as possible. The width of the woven fabric is 500 mm. Another problem is the sharp-edged nature of the lignin coated flax yarn. This causes increased wear on the healds and the weaving reed. Large healds and a coarser weaving reed could bring improvement.

The analysis of the composite material with regard to the tensile and bending properties showed clear differences between the weft (FL-LI 0◦ ) and the warp (FL-LI 90◦ ) direction, which can be attributed to the position and orientation of the flax yarns in the warp and weft directions in the composite. In addition, investigations are needed on the general fibrematrix adhesion between flax yarn and lignin as well as on the influence of the extrusion process on the fibre-matrix adhesion. Furthermore, the penetration depth of the lignin matrix into a compact structure, such as the flax yarn, has to be analysed.

To summarise, the use of lignin as a matrix offers a possibility for the cascading use of biomass, in which the use of by-products or waste products contributes to closing material cycles instead of their purely thermal utilisation, and leads to new advanced bio-based materials. Coating of (natural fibre) yarns with lignin presents an alternative way to manufacture hybrid yarns, which can be used to produce semi-finished products for composite components.

**Author Contributions:** Conceptualization, C.M.; methodology, C.M., M.C., T.W., T.H., H.-J.B., T.S.; investigation, C.M., M.C., T.H.; validation, C.M., M.C., T.W., T.H.; writing—original draft preparation, C.M., T.W., T.H.; visualization, C.M., T.W., T.H.; project administration, C.M.; writing–review and editing, S.B., H.-J.B., T.S., W.W., G.T.G.; supervision: S.B., H.-J.B., T.S., G.T.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the German Institute of Textile and Fibre Research (DITF).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within this article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

