Material of the month – Bamboo

Materials World magazine
,
3 Nov 2015

It is the fastest growing plant on the planet and is great for the environment. What is not to love about bamboo? Anna Ploszajski investigates

This month, we return to the ‘unruly’ class of materials – the natural ones. Like cork (Material of the Month, May 2015), bamboo has properties unmatched by man-made alternatives, not to mention its carbon-positive environmental credentials and proud place at the top of the menu for a panda.

The familiar bamboo structure is a cylindrical stem called a culm, periodically intersected with nodes along its length. The culms are predominantly hollow, with a thin woody layer stretching over the cross section at each node. Certain species of bamboo can grow more than 30m tall and 20cm in diameter, but although the plants can be tree-like in scale, bamboo is actually a member of the grass family. Many bamboo species are woody, but the lack of secondary growth means that the stems are recognisably columnar in shape, rather than tapering like trees.

Wild bamboo outcrops may be found in diverse terrains across the globe, and occur mainly in the tropics and subtropics including India, China, northern Australia, sub-Saharan Africa, and Central and South America. Bamboo has an unusually long flowering cycle of 65–120 years, depending on the species. Even more remarkably, all of the plants of the same species will flower simultaneously, regardless of their location around the world. The reason behind this and the mechanism itself are still something of a mystery.

Growing at rapid rates

Bamboo holds the world record for the fastest growing plant at 91cm per day (3.8cm per hour) under the right soil and climate conditions. These rates are achievable because, as a bud at the base of the plant, all of the cells, which will become the culm, are ready in place. These cells grow by elongation, which is a much faster process than mitotic cell division. On emerging out of the ground, the culms are already at their full diameter, and will grow to their full height within the first year. During the second year, branches extend from the nodes along the culm’s length and produce long, elegant leaves. The pulpy walls of the culms gradually harden around this time too, and the plant reaches full maturity after three years. After this time, it can be harvested. 

After harvesting, the sap must be removed by a process called leaching. This can be done by suspending the cut bamboo vertically in air or water, weighted down in a stream, or pumping water through it to remove the sap more forcefully. After leaching, the bamboo is carefully dried in the shade. Bamboo destined for construction is harvested at the time of year when the sugar levels in the sap are at their lowest, and the culms are treated with borax and boric acid to minimise the risk of pest infestation later down the line.

Bamboo is a natural fibre-reinforced composite material. The fibrous vascular bundles that run along the length of the stem are made from stiff, fibrous sclerenchyma cells that surround channels, and are responsible for transporting water and nutrients up and down the stalk. The bundles are embedded in a soft matrix of parenchyma cells and this composite structure makes the tensile strength of bamboo greater than steel, weight-for-weight. This, combined with its low price and high availability in some areas of the world, is what makes bamboo such an attractive building material. There is evidence that bamboo cables made from split bamboo have been used in simple suspension bridges in China and India for more than 2,200 years. Even today, bamboo is used as scaffolding, and in housing, fencing, flooring, furniture, and as a reinforcer in a concrete composite. Bamboo releases about a third more oxygen when it is alive and photosynthesising than the equivalent stand of trees, and this makes it an even more sustainable choice than alternative construction materials.

Construction, fabrics and music 

The usefulness of bamboo stretches far beyond the building of houses and bridges. Many of us are familiar with bamboo socks made from fabrics boasting their eco-friendly and anti-bacterial credentials. Strictly speaking, these fibres should really be described as semi-synthetic, although bamboo-derived cellulose is the raw material, it is exposed to an array of chemicals, such as caustic soda, carbon disulphide and sulphuric acid, and treated to an extensive list of processes including rolling, ageing and extrusion through a spinneret before the fibres are woven. By this time the cloth is called rayon, of which viscose is a well-known variety. Thus, bamboo socks are a somewhat controversial choice for the sustainability-inclined hosiery consumer.

Given the geographical location of many native bamboo species, it is unsurprising that bamboo features heavily in the culture of the countries where it grows. All sorts of traditional musical instruments from flutes to rattles, didgeridoos to marimbas, panpipes to organs have been made from bamboo for centuries. Even novel iterations of modern western instruments, such as saxophones and clarinets, have been made from bamboo. 

In Chinese art, bamboo is one of the plants called the Four Gentlemen or the Four Noble Ones, alongside plum, orchid and chrysanthemum. These plants are celebrated for their beauty as well as their moral character. Traditionally, bamboo represents uprightness and elegance due to its shape, while its hollow insides signify tolerance and open mindedness. Its strength and flexibility also represent integrity, in which one yields but does not ever break.

The earliest writing surface of the ancient Chinese was made up of strips of bamboo, tied next to each other with string to produce a flat, rollable writing surface. In April 2014’s Material of the Month, I described how this medium was improved upon in the discovery of the method of manufacturing paper from wood pulp, 2,200 years ago in that region. It turns out that bamboo was another important material for this pulp, and so-called xuan paper made from bamboo pulp is still a popular material for Chinese calligraphy and paintings. 

Weapon of choice

Its shape, high strength-to-weight ratio, flexibility, low cost and high availability has rendered bamboo an attractive choice for basic weaponry in martial arts. Staffs, swords and bows may all be made from bamboo, as well as a weapon called the fire lance, which consists of a hollow bamboo tube filled with gunpowder and projectiles or poison darts. Upon ignition of the gunpowder, the contents are ejected at speed and, although the early designs only had a range of a few feet, the fire lance is one of the earliest examples of explosive weaponry and paved the way to modern firearms.

Sadly, the violence doesnt stop there. Japanese soldiers during the Second World War also came up with a disturbing use for the extraordinarily fast growth rates of young bamboo shoots. The gruesome torture method involved securely tying the enemy above a young bamboo shoot and, over the following days, the sharp shoot would pierce the body, and eventually completely penetrate it, to emerge on the other side.

On a lighter note, the humble lightbulb has become a recurring theme in my Material of the Month series – first wire (December 2014), then tungsten (April 2015) and now, unpredictably, bamboo. Thomas Edison played a big part in the development of the first commercially viable lightbulb and his success is thanks to the grass to which we dedicate this article. During the 19th Century, Edison, and many others, had experimented with different filament materials, such as platinum and various modifications of carbon. These filaments had limited success but, in 1880, Edison and his team demonstrated that carbonised bamboo filaments could produce an electric light, which lasted approximately 1,200 hours – a lifetime that finally made the electric lightbulb a commercially viable product. And the rest,
as they say, is history.