Delignified densified wood represents a new promising lightweight, high-performance and bio-based material with great potential to partially substitute natural fiber reinforced- or glass fiber reinforced composites in the future. We here present two versatile fabrication routes and demonstrate the possibility to create complex composite parts.
Delignified densified wood is a new promising and sustainable material that possesses the potential to replace synthetic materials, such as glass fiber reinforced composites, due to its excellent mechanical properties. Delignified wood, however, is rather fragile in a wet state, which makes handling and shaping challenging. Here we present two fabrication processes, closed-mold densification and vacuum densification, to produce high-performance cellulose composites based on delignified wood, including an assessment of their advantages and limitations. Further, we suggest strategies for how the composites can be re-used or decomposed at the end-of-life cycle. Closed-mold densification has the advantage that no elaborate lab equipment is needed. Simple screw clamps or a press can be used for densification. We recommend this method for small parts with simple geometries and large radii of curvature. Vacuum densification in an open-mold process is suitable for larger objects and complex geometries, including small radii of curvature. Compared to the closed-mold process, the open-mold vacuum approach only needs the manufacture of a single mold cavity.
The development of novel natural fiber (NF) based composites equipped with superior mechanical properties represents one of the main tasks in materials science, as they can be sustainable alternatives for current synthetic systems such as glass fiber composites1,2,3. Besides traditional NF composites (flax, hemp, kenaf, etc)4,5, the densification of wood after partial or complete removal of matrix components has received increasing attention in recent years6,7,8,9,10,11. The top-down fabrication route, based on delignification of bulk wood followed by densification, is conceptually contrary to rather complex bottom-up processes for pulp and slurry based products12. In pulp and slurry based products, the beneficial wood fiber alignment is not retained as fibers are separated in the process. In contrast, structure-retaining delignified wood, which is obtained in a top-down process, transfers the sophisticated architecture with aligned cellulose fibers into the new material. To achieve densification of delignified wood without fiber alignment distortions, new processing routes must be developed.
Direct densification of water-saturated delignified wood samples leads to a limited densification degree, cracks, and fiber alignment distortions due to the wet-sample-inherent free water that creates a counter pressure during densification. Current solutions to avoid structural integrity loss upon densification includes utilization of partially delignified wood followed by high-temperature densification9 or pre-drying of delignified wood prior densification6. Both methods enhance connectivity between neighboring cells, either due to the remaining lignin that acts as glue or free water removal between cells.
In both cases, reduced formability occurs, which limits design applications; the required sample pre-conditioning also leads to longer processing times. Therefore, a fast and scalable process that combines shaping and densification in a single step is necessary.
In this regard, we present here open/closed-mold densification and vacuum processing of delignified wood as methods to combine shaping, densification, and drying in a simple and scalable approach. Figure 1 shows delignified densified wood-composite parts, which were obtained by using the techniques described in this work.
Figure 1: Examples of delignified densified wood composite parts. (A) Door panel, (B) side mirror, (C) door handle of a car, (D) orthosis, (E) cut open helmet, and (F) tachometer cover of a car. Please click here to view a larger version of this figure.
1. Delignification of wood veneers
NOTE: This delignification protocol is based on our previous works, published by Frey et al. 20186 and Segmehl et al. 201813.
Figure 2: Delignification setup. (A) Crystallizing dish with metal mesh sample holder and wood veneers stacked on top of the sample holder. Metal mesh stripes separate the individual veneers from each other. (B) Delignfied veneers covered by water during the washing process. Please click here to view a larger version of this figure.
2. Storage and "cellulose prepreg" production
3. Densification and forming of delignified wood in closed molds
4. Vacuum shaping and densification of delignified wood in open molds
Figure 3: Schematic illustration of the open-mold process. (A) Porous mold with smaller pores towards the surface. (B) Delignified wood draped on top of the porous mold (grey) and optional textile layer for mold protection (green). (C) Textile, flow mesh and vacuum bag placed on top of delignified wood. Pressure is applied through the vacuum bag and leads to densification and drying of the material. (D) Final composite after demolding. Please click here to view a larger version of this figure.
5. Manufacturing of laminated composite parts
6. Re-use and recycling of composite parts
Delignification and handling of wood veneers.
Complete delignification leads to a mass reduction of around 40% and a volume reduction of around 20% after drying at 65% RH6. Besides lignin, a fraction of hemicelluloses gets removed too. Removal of these components results in a fragile cellulose material (see Figure 4). Using metal meshes as supports eases handling and draping.
Figure 4: Handling of delignified wood in wet state. (A) Fragile delignified wood in its wet state. (B) Handling of the material is eased by using a metal mesh for transportation or (C) for draping the material to a mold. (D) Delignified wood draped on top of a porous 3D-printed mold. Please click here to view a larger version of this figure.
Densification and forming of delignified wood in closed molds.
Densification of water-saturated delignified wood (Figure 5A-C) is demanding, as free water in the scaffold creates a counter pressure upon densification and allows the material to flow during processing. This causes fiber deviations and cracks in the final material (Figure 5B,C). One possibility to bypass these limitations is to use moist pre-conditioned (95% RH and 20 ˚C), delignified wood. In this condition, delignified wood is still reasonably shapeable and its densification does not lead to fiber alignment distortions and defects.
Pre-conditioned material, however, is more rigid compared to the water-saturated state, which makes it difficult to obtain small curvature radii without material damage. For small curvature radii, wet draping followed by conditioning in an already shaped state prior densification can be used. However, conditioning is rather time consuming and therefore not recommended for large-scale applications.
Figure 5: Closed-mold densification of delignified wood in a wet and moist state. (A) Densification of the water-saturated cellulose material leads to (B,C) cracks and fiber misalignment. (D-F) Densification of moist material, conditioned at 95% RH results in a better preservation of fiber alignment and less defects. Please click here to view a larger version of this figure.
Vacuum shaping and densification of a laminated part in an open mold.
Exemplarily for vacuum shaping, we manufactured a helmet in a self-made gypsum mold using an open-mold process (Figure 6A,B). As lay-up, we draped 2 layers of hexagon-flakes for surface texturing followed by 4 layers of delignified wood veneer in a [0˚/90˚] lay-up (Figure 6C). The flakes provide an attractive surface design, whereas the unidimensional (UD) layers add strength and stiffness to the composite. We applied 16.5 wt% starch as adhesive between layers to prevent delamination14.
Vacuum densification (Figure 6D) leads to full drying of the part within 48 h and densification down to a thickness of 3 mm (1/3rd of the initial thickness). After the vacuum processing, the composite part is demolded (Figure 6E) and the edges are trimmed with a cutter (Figure 6F).
The maximum layup thickness that could be densified and fully dried with the open-molding approach was an 8-layer layup (8 x 1.5 mm veneer) with an end thickness of this part of 2.5 mm, which corresponds to a densification down to approximately one quarter of the initial thickness of dry delignified wood, taking into account the layer shrinkage upon delignification and drying. To obtain such high densification degrees, a low vacuum in the range of 10-2 bar is needed.
Delignified wood composites that are densified to around one quarter of their initial thickness typically achieve elastic moduli values around 25 GPa and strength values in the range of 150-180 MPa, as shown in our previous work (Table 1)7.
<!–FVC | 20% | 33% | 50% | 66% | 85% |
Density (g/cm3) | 0.3 | 0.5 | 0.72 | 1 | 1.3 |
Tensile elastic modulus (GPa)7 | 5 | 10 | 15 | 25 | 35 |
Tensile strength (MPa)7 | 60 | 90 | 120 | 180 | 250 |
Table 1: Literature values for tensile elastic modulus and tensile strength of densified delignified wood. The vacuum processing results in a densification down to 1/4th of the initial thickness, which corresponds to an FVC of 66%.
Figure 6: Manufacturing of a helmet by open-mold processing. (A,B) Molding of the original helmet using a gypsum mold. (C) Draping of two outer layers with hexagon flakes followed by draping the inner 4-layers in a [0/90] layup. (D) Densification and drying of the part by vacuum. (E) Demolding of the dry part and (F) finish using a cutter. Please click here to view a larger version of this figure.
Utilizing flow meshes typically results in a mesh-imprint in the sample. This can either be considered as a process-inherent design strategy or can be prevented by placing an additional thicker textile layer between delignified wood and flow mesh.
Alternatively, closed molds combined with vacuum processing as described in protocol step 4.2 can be used. Regular patterning can be obtained by placing small pieces of delignified veneers in a defined order, as shown before for our example with the hexagon patterning on the helmet.
Problems that can arise during vacuum processing include warpages in the composite part, which are caused by incomplete drying and the occurrence of cracks (Figure 7). Cracks mainly result in delignified wood that was stored in EtOH prior composite fabrication. Therefore, after EtOH storage, we recommend to carefully soak delignified wood in water before further processing. Additionally, careful draping followed by slight densification by hand to remove some free water reduces the risk of cracking.
Figure 7: Possible problems arising in fabrication of complex geometries. (A) Back view and (B) side view of the manufactured helmet. (C,D) Small cracks due to shrinkage of the material during processing. Please click here to view a larger version of this figure.
Re-use or decomposition of composite parts.
Our cellulose-starch composite is all bio-based and can disintegrate in water. On the one hand, the hydrophilicity of the material is a disadvantage, as it leads to reduced mechanical performance when in contact with water. A simple method to protect the composite from liquid water comprises hydrophobic coatings, as we have shown in Frey et al. 20197. On the other hand, a hydrophilic behavior of the material can also be beneficial when it comes to end-of life use and recycling aspects. The sample can simply be disintegrated in water to smaller pieces and the fibrous slurry can further be used for the production of new fiber-based products as shown in Figure 8. Furthermore, the fibrous material is fully biodegradable, as shown in Figure 9.
Figure 8: Re-use of delignified wood fibers. (A-C) Reduction of delignified wood veneers into small pieces by dispersing the material in water. (D-F) Re-use of the fiber slurry for producing the lining of a helmet. (D) Reveting of a silicon mold with fiber slurry. (E) Final lining of the helmet. (F) Lining made out of disintegrated delignified wood inside of the hard shell of the helmet. Please click here to view a larger version of this figure.
Figure 9: Degradation of delignified wood fibers. (A) Petri dish filled with soil. (B) Placing the fiber slurry on top of the soil and (C) filling it with water. (D) Bio-degradation after one day, (E) after eight days, and (F) after 26 days. Please click here to view a larger version of this figure.
We present versatile fabrication techniques to obtain high-performance delignified wood-based composites and suggest possible re-use and recycling strategies. Closed-mold processing prerequisites pre-conditioning of the material, as it cannot be processed in water-saturated state. Utilizing a closed-mold process, however, could be the method of choice especially if e.g. there is no vacuum setup available or if a nice (smooth) surface finish on both sides is desired.
Open-mold vacuum processing of delignified wood allows for combining shaping, densification, and drying of water-saturated samples in a simple and scalable approach. The technique is applicable for the production of complex geometries and offers a scalable alternative for closed-mold processes. We have manufactured composites by stacking delignified wood veneers using starch as adhesive between layers. Densification down to one quarter of the initial thickness resulted in a final thickness of 2.5 mm of the 8-layer thick composite part. For obtaining a smoother surface finish in the vacuum process, the use of a closed porous mold could be an appropriate alternative.
For both processing methods, we recommend the use of an adhesive system in between delignified wood layers in order to decrease the risk of delamination. For the given example, we choose starch, as it is a well-known bio-based glue for pulp and paper products, such as paper bags, and is water based. Future works will focus on the fabrication of thicker laminates to resolve current limitations in terms of drying and fiber flow deviations.
In general, vacuum processing of delignified wood has the potential for an easy and fast production of large-scale densified cellulose fiber composites. After addressing the material's durability issue by applying proper coatings, water-stable adhesive systems or chemical modification, possible industrial applications may include automotive components such as door panels, floors, and dashboards. Our material could replace metals or fiber reinforced composites in order to reduce weight for better fuel efficiency and to improve recyclability.
The authors have nothing to disclose.
The authors thank Silvan Gantenbein for the 3D printing of porous molds.
Acetic acid | VWR Chemicals | 20104.312 | |
Breather | Suter Kunststoffe AG | 923.015 | |
Flow mesh/bleeder | Suter Kunststoffe AG | 180.007 | |
Gypsum | Suter Kunststoffe AG | 115.3002 | |
Hydrogen peroxide, 30% | VWR Chemicals | 23622.298 | |
Oven | Binder GmbH | ||
Press | Imex Technik AG | ||
Seal tape | Suter Kunststoffe AG | 31344 | |
Stainless steel mesh | Drawag AG | ||
Starch | Agrana Beteilungs AG | ||
Textile, peel ply | Suter Kunststoffe AG | 222.001 | |
Vacuum bag | Suter Kunststoffe AG | 215.15 | |
Vacuum bag, elastic | Suter Kunststoffe AG | 390.1761 | elastic vacuum bag for complex shapes |
Vacuum pump | Vacuumbrand | ||
Vacuum tubing | Suter Kunststoffe AG | 77008.001 | |
Wood veneers | Bollinger AG |