Fabrication and Design of Wood-Based High-Performance Composites

In this video, we show the fabrication of densified, delignified wood which represents a new light-weight, high-performance, and bio-based material. The presented, closed mold densification and the particular vacuum processing combines shaping, densification, and drying in a simple and scalable approach. Densified, cellulose material could be an attractive alternative to other plant fiber or glass fiber composites, and may potentially find application in automotive industry.

To begin, mount a stainless steel sample holder in a crystallizing dish and place a magnetic, stir bar below the sample holder. Stack one point five millimeter thick, radial cut spruce veneers on top of the holder, and separate them by metal meshes or metal mesh stripes. Prepare a one to one volume mixture of 30 weight percent hydrogen peroxide and glacial acetic acid, and pour the mixture into the crystallizing dish until the veneers are fully covered.

Soak the samples in the solution at room temperature overnight while stirring at 150 rpm. In the morning, heat the solution to 80 degrees celsius on the stir plate, and run the reaction for six hours for full delignification. Adjust the delignification time depending on the sample thickness.

After delignification, pour the delignification solution into an empty beaker and let it cool down before disposal. Gently rinse the delignified veneers multiple times with deionized water. Then, fill the crystallizing dish with deionized water to continue washing the veneers without stirring.

Replace the water twice a day until the washing water reaches a pH above five. Process the wet, delignified veneers within two to three weeks, or alternatively, dry the sheets between metal meshes for storage. Use molds made out of a open, porous material.

For example, ceramic molds or porous, 3D printed polymer molds to enable water removal and sufficient drying. For curvature radii in the centimeter range, or plane structures, use samples that are conditioned at 95 percent relative humidity, at 20 degrees celsius. For smaller curvature radii, drape the veneer in the water-saturated state.

Pre-dry the draped material in an open mold at 95 percent relative humidity, or pre-dry the material in an oven at 65 degrees celsius for five to 30 minutes to remove the free water. Densify the veneer in the closed mold using screw clamps. Speed up the drying process by placing the mold into an oven at 65 degrees celsius.

After full drying, de-mold the composite part and reuse the mold for a new run. Use a porous, open mold or a non-porous mold with a porous layer on top of the mold or on top of the delignified wood, to enable drying. Apply a textile layer to protect the mold from contamination.

Then, drape a water-saturated delignified veneer on top of the textile, and cover it with a second textile layer and a flow mesh. Place the mold on top of a stainless steel plate, apply sealing tape and vacuum tubing, and wrap the mold with a vacuum bag. The porous layer enables the water flow to the vacuum tubing.

Apply a vacuum for drying and simultaneous densification of the composite. For accelerated drying, place the set up into an oven at elevated temperature at 65 degrees celsius. Use cold traps to avoid water entering the vacuum pump.

Turn on the vacuum and keep the oil pump in a pressure range of ten to the minus two millibar. After drying, de-mold the dry composite and reuse the mold and vacuum set up for a new composite part. Choose the fiber orientation angle of the layers as in traditional composite manufacturing, and manufacture thick, multi-layer composite parts by lay up techniques.

Increase bonding between delignified wood layers by applying a water-based adhesive, for example, 16 point 5 weight percent starch solution between the layers during the draping process. After densification and drying, de-mold the composite part and machine finish by hand or with standard wood tooling. To recycle, place the delignified wood composites in water and let the material disintegrate.

Then, re-shape the pulp material to obtain a new product. Complete delignification resulted in a fragile, shape-able cellulose material when in the wet state. Closed mold densification of water-saturated delignified wood caused fiber deviations and cracks in the final material due to free water in the scaffold.

With moist, pre-conditioned delignified wood, reasonable shape was maintained, and its densification did not lead to fiber alignment distortions and defects. The open mold process was used to manufacture a helmet by placing water-saturated delignified wood veneers in a gypsum mold. Draping of two outer layers with hexagon flakes was followed by draping the inner four layers in a zero over 90 lay up.

After densification and drying of the part by vacuum, the dry part was de-molded and finished with a cutter. Possible problems during vacuum processing include the occurrence of small cracks, which are caused by shrinkage or incomplete drying. The cellulose starch composite is all bio-based which allows for disintegration in water.

After disintegration, the fibrous slurry was used for the production of new fiber-based products. Degradation of the delignified wood fibers was achieved by placing them in a Petri dish filled with soil and water. Different biodegradation degrees were shown after one day, eight days, and 26 days.

The most important factor for successful delignified wood densification is to ensure the appropriate humidity condition of the cellulose material that is tailored for the applied densification technique. For complex shapes, large-scale production, we recommend to use our newly developed, vacuum shaping technique, allowing for simultaneous densification and drying. Delignified wood is a versatile material.

In addition to its potential application as a high-performance material, it further provides attractive and diverse design options.