People make extensive use of raw materials and chemicals derived from plants. Products such as rubber, oils, textile fibers, dyes, and pigments are present in many aspects of our daily lives. Apart from food crops, by far the most important plant resource we use is wood. It provides us with lumber and is manufactured into other products, the most significant of which is paper.
Of the 10 billion acres of land covered by forest, only about 40 per cent is exploited. The most heavily forested areas in the world include central Africa, the Amazon region, northern and southeastern Asia, and northern North America. However, Brazil, Canada, China, India, Indonesia, Nigeria, the United States, and the countries of the former Soviet Union collectively produce nearly two-thirds of the world’s wood-based products. These countries have the capacity to process lumber into boards, panels, or pulp.
In many countries, the timber industry relies on conifers. Conifers are softwood and make the best pulp for paper because of the long fibers of the wood. Nearly all the trees in the forest can be utilized, and conifer forests are, thus, extremely valuable, producing more timber per acre than any other kind, including tropical forests.
Despite the increasing use made of steel and concrete, lumber is still often employed for heavy constructions, such as bridges, harbors, and mine shafts. Some treated softwoods and some temperate and tropical hardwoods have the necessary strength, durability, and rot resistance for such building work, for example greenheart (Nectandra rodiei) and some members of the genus Shorea. Softwood plywood, which is manufactured mainly from Douglas fir and southern pine, is used extensively in the housing industry, which accounts for more than half the world s consumption of this material.
The combined physical properties of solid wood often give it an advantage over other materials. It is, for example, an excellent heat insulator. The specific heat capacity of wood is similar to metals but it is a poorer conductor of heat because it contains air in its cells. For this reason, wood is, for example, used for saucepan handles. The lightest woods, such as balsa (Ochroma lagopus), are good insulators because of their large, air-filled cells and are used for containers for chemicals and liquefied gases because they keep them cool and stable.
Different types of timber may have specific qualities that can be used. One variety of willow, Salixalba, for example, is the only lightweight wood with the right degree of resilience to be suitable for cricket bats. Mallets and the sheaves and blocks of pulleys are made from Lignum Vitae (Guaiacum officinale), a wood so dense that it sinks in water. Many musical instruments are also made from wood, but only a few high-density timbers are suitable for xylophones. Rosewood (Dalbergia sp.), for example, is one of the few woods to produce a musical note when struck, the length and thickness of each strip determining its pitch. Conversely, low-density materials, such as cork bark, absorb sound and are used in concert halls to reduce echoes.
Worldwide demand for pulpwood-based products, such as chipboard, fiberboard, and paper, is growing faster than the market for solid wood products. For example, in 1994, about 189 million short tons (171.5 metrictons) of pulp were produced. All of this pulp was used to satisfy the world’s huge demand for paper and paperboard products.
Recent developments have created a process whereby the advantages of wood can be combined with plastic. Low-quality wood is impregnated with such chemicals as vinyl acetate and exposed to gamma rays. The radiation polymerizes the vinyl compound to a hard plastic, producing a rock-hard material known as Novawood. Wood alone lacks such strength because, although the tensile strength of each cellulose fiber is stronger than steel, there are no linkages between them. It is for this reason that natural vegetable fibers, such as cotton, need to be spun and woven in order to make strong textiles.
Other plant products
The wide use of wood and other vegetable materials is dependent as much on their chemical composition as on their physical properties. Cellulose is the world’s most abundant natural organic compound; it constitutes 42 per cent of wood and 95 per cent of cotton. It is a complex carbohydrate made up of about 3,000 glucose units. When chemically processed, it can be manufactured into materials, such as cellulose acetate, triacetate, and nitrate, that have a wide range of applications from plastic films to propellant explosives. Cellulose is also a beginning ingredient for making methanol and other alcohols in the chemical industry, some used as fuels.
The textile industry makes use of both raw materials and chemicals derived from plants. For example, cotton is woven in a relatively unaltered state and is used extensively for clothing and furnishing fabrics, whereas synthetic textiles, such as viscose, are chemically derived from cellulose.
The pharmaceutical industry also draws on the natural chemicals found in many plants. Aspirin was originally made from chemicals extracted from the bark of several kinds of willow (Salix spp.) although now the drug is artificially made.
Other plant extracts, such as oils, dyes, resins, and gums are valuable raw materials for the cosmetic, food, and printing industries. Yet another important plant extract is rubber, which is obtained by tapping the latex produced by some species of tropical and subtropical plants. More than 85 per cent of the world’s natural rubber is grown in Malaysia and the Far East, the greater part of it destined for the production of tires for motor vehicles. Other uses for natural rubber are footwear, carpet backing, and conveyor belts.