Tikalon Blog by Dev Gualtieri

Strong Wood

January 14, 2019

Once again I'm revealing my age when I recall a popular song from my teenage years, Surf City by Jan and Dean, released in 1963.[1] This song was popular in my inland Eastern United States hometown despite it's being a 5-6 hour trip from the nearest desirable surfing location, Long Beach Island, New Jersey. It might have been because of its "two girls for every boy" catch phrase.

Physics of a stone skipping on water.

There aren't any arXiv papers about surfboarding, but there are a few on the mechanism by which flat stones will skip across water when thrown at a proper angle.

In this diagram, the stone hits the surface at a velocity V, an incidence angle β, and a tilt angle θ. The stone's trailing edge is immersed to a depth z, causing a contact surface area S_im.

(Modified arXiv figure from ref. 2.[2] Click for larger image.)

Surf City also mentions a type of automobile body type called a woodie. As its name implies, some of the automobile body was wood. As manufacturing techniques changed, the wood was simulated in vinyl plastic. Today, wood is not used in automobiles for both safety and fuel economy reasons, but wood has been used as a structural material since prehistoric times, and it's used today for making things from baseball bats to buildings.

Wood is a combination of several chemical compounds, but its major constituent is cellulose, a crystalline polymer that comprises about 42% of its weight. Hydrogen bonding between straight, parallel chains of cellulose polymer gives wood its strength, as shown in the figure.

Polymer chains of cellulose.

The polymer chains are straight, and the strength of wood derives from hydrogen bonding, not the coiling or branching found in other polymers.

(Illustration by Luca Laghi, via Wikimedia Commons.)

The strength of wood has something in common with the highly optimized creation of the gas turbine blades used in jet engines. These blades are directionally-solidified so that the polycrystalline grains of their alloy align to give them additional strength. Wood has a texture, its wood grain, along its growth direction. This gives trees the strength they need to resist high winds and other insults. This also means that the wood that we harvest has different mechanical properties in different directions. My father, who was a carpenter, had a crosscut saw designed to cut lumber across its grain, which is a difficult cut.

Wood is weaker longitudinal to the grain, and it's much stronger in the transverse direction across the grain. This isn't a problem when constructing a typical wood-frame building, since the wall studs are cut along the length of the tree, but it is a problem when an isotropic strength is needed. Immanuel Nobel (1801-1872), who was the father of Alfred Nobel of the eponymous Nobel Prize devised a method in the mid-nineteenth century to create wooden sheets with strength in all directions.

A sheet of plywood used as a shelf material. This sheet rests on a "2x4" pine stud frame.

The five layers of this 3/4-inch thick laminate, known as C/D grade plywood, can be seen.

(Photo by the author)

Nobel manufactured laminated boards, now called plywood, made from several thin layers of wood that were bonded together in a cross-grain arrangement in which alternate layers have their grain aligned perpendicular to each other. This same technique can be applied at the nanoscopic level, since cellulose nanocrystals are straight and needle-shaped with highly anisotropic mechanical properties. They have an estimated Young's modulus along the [001] (c-axis) of 206 GPa, which is comparable to that of steel, while their Young's modulus along [100] (a-axis) is estimated to be only 19 GPa.

Just as metallurgists have chemically-enhanced the elemental metals mined from ores by creation of alloys, scientists have been devising methods to enhance the strength of wood. One early example of such research appears in a 1944 paper in Industrial & Engineering Chemistry.[3] Earlier this year, a team of materials scientists and engineers from the University of Maryland (College Park, Maryland), the USDA Forest Service (Madison, Wisconsin), the University of California Merced (Merced, California), and Northeastern University (Boston, Massachusetts) published a chemical bath method that renders wood into a material that's stronger than steel.[4-5]

While wood has been extensively used as a structural material for construction of furniture and small buildings, its strength and toughness are not adequate for advanced structures and applications.[4] Wood has been enhanced in the past through treatment with ammonia, steam, or heat, such treatments result in just a small densification, and wood treated by such means tends to lose its dimensional stability in humid environments.[4] Such treatments have resulted in an increase in strength by just a factor of three to four.[5]

The recent advance in wood treatment that results in a more than tenfold increase in strength, toughness and ballistic resistance comes from the simultaneous application of chemicals, high temperature, and pressing.[4-5] The treated wood is also dimensionally stable.[4] The treatment process involves a first step of boiling in an aqueous mixture of sodium hydroxide (NaOH) and sodium sulfite (Na₂SO₃) that results in a partial removal of lignin and hemicellulose from the natural wood.[4] This is followed by hot-pressing at 100 °C for a day that leads to the total collapse of cell walls and a densification of the natural wood by a factor of three by alignment of cellulose nanofibers.[4-5] The chemicals are used also in conversion of wood pulp to paper.[5]

This technique was proven for a variety of wood species, and it resulted in a specific strength higher than that of most structural metals and alloys.[4] The density increased from 0.43 g/cc to 1.3 g/cc.[5] This makes it too dense to float on water.[5] The partial removal of lignin by the chemical process might make the processed wood susceptible to bacterial or fungal attacks.[5] scanning electron microscopy showed that the process crushes the cellulose tubes, forcing them to crumple and interlock, so all the nanofibers are aligned in the growth direction.[6]

Ballistic test of a strengthened wood laminate.

Natural wood has limited impact resistance, but the impact resistance is improved in a single sheet of strengthened wood.

A 46 gram steel projectile fired at 30 meters per second was stopped by a 3 mm thickness five-layer strengthened wood laminate.

(Still image from a YouTube video by Liangbing Hu/NPG Press[8])

Projectiles were fired at wood specimens from a ballistic air gun, and 3 mm thickness five-layer laminates were able to stop a 46 gram steel projectile fired at 30 meters per second (see figure).[6] This is about the impact that an automobile would have in a collision; so, there might be a revival of the woodie body type.[6] Removing just the right amount of lignin is important for maximizing performance. The wood was strongest when about 55% of the lignin remains.[6] This material would be an environmentally-friendly alternative to plastics, and the processing chemicals are relatively benign.[6-7] Such materials might be an alternative to steels or alloys in buildings and bridge construction, and as a material to make lightweight vehicles for better fuel efficiency.[7]

Although the typical limit for the size of a wooden structure is three stories (about 45 feet), higher structures are now being built with engineered wood, such as cross-laminated timber.[9] In 2012, a 90 foot wood building was constructed in Prince George, British Columbia, Canada, there's a 34 story wood tower planned for Stockholm, Sweden, and an architect's study shows that 42 stories are possible.[9] The energy savings between wood and other materials is considerable. Per weight, brick needs four times the energy than wood to be made into a building material, concrete five times, steel 24 times, and aluminum 126 times. In the building itself, wood is a five times better thermal insulator than concrete and 350 times better than steel.[9] A proposed Tokyo skyscraper, built mostly of wood with just 10% steel, would be 70 stories (1,148 feet) tall.[10]

Strong Wood

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