Until recently the most important product from the rubber tree was its latex and efforts to improve the tree concentrated upon increasing the latex yield. Typically following an exploitation period of about thirty years the trees are felled for replanting with higher yielding clones. Until recently, most of the timber was used as fuel. With the depletion of tropical forests, leading to a shortage of timber for many industrial and engineering uses, attention has turned towards rubberwood as an alternative source of timber.

Rubber trees grow to a height of 25 m and generally have straight trunks. At the time of felling, the girth varies between 100 to 110 cm at a height of 125 cm from the ground and gives 0.62 m³ of stump wood and 0.4 m³ of branch wood: normally 180 to 185 trees will be available per hectare.

Industrial articles made from rubberwood include: benches; bread boards; building components; blackboards; block boards; cabinets; carving boards; chairs; chests; chopping boards; cement boards; charcoal; dining sets; doors; drawer faces; fibreboards; folding chairs; folding tables; furniture and furniture components; garden equipment; gift boxes; hardboards; ice buckets; jewellery boxes; kitchen cabinets; knife blocks; magazine racks; moulded hardboards; mouldings; match splints; match boxes; packing cases; pallets; panelling; paper; particle boards; picture frames; pepper & salt sets; parquet flooring; plant stands; plywood; pulp; restaurant furniture; railings; rocking chairs; rayon; salad bowls; screen partitions; serving trays; shelves; spice racks; steak plates; stools; suit cases; tables; tea trolleys; television cabinets; toilet seats; toys including dolls houses; vegetable boxes; wine racks; etc

Properties of rubber tree bark wood elements

Methods of extraction, conversion and transport of rubberwood have been generally standardised on plantations. The usual implements for felling trees are used: for crosscutting the stems and branches, power chain saws or bow saws are employed. The sites for conversion and storage are chosen to avoid interference with other plantation operations.

Anatomy of rubberwood

The texture of the wood is fairly even with a moderately straight and slightly interlocking grain. It is whitish yellow when freshly cut, but the wood turns to light brown during drying. At this stage latex vessels can be found with a characteristic smell in some parts of the wood. The wood is soft to moderately hard with an average weight of 515 kg m-3 at 12% moisture content. Pores on the cross section are diffused and of medium to large size, mostly solitary, but sometimes in short multiples of two to three, filled with tyloses. Vessel tissues are conspicuous in radial and tangential faces and are of the order of about 200 u in diameter. Wood parenchyma are abundantly visible to the naked eye appearing as narrow. irregular and somewhat closely spaced bands forming a net like pattern with rays. The rays of the wood are moderately broad, rather few and fairly widespread. The pits found between the vessels and rays are half-bordered with narrow width. The length of the fibres is more than 1.0 mm on the average and the width is about 22 : when dry. The cell wall thickness when dry is about 2.8 :. The other characteristics of rubber wood are summarised in Table 2 (Bhatt et al. 1984). These authors further studied the variation in the properties of the wood and bark at different heights and came to the conclusion that there is no significant variation of bole quality between height levels of a tree or between trees in a plantation, in contrast with naturally grown trees in a forest.

There is insignificant heart wood formation and no transition appears between sapwood and heart wood, which is confined near the pith only. Growth rings or annual rings are not visible in rubber wood, unlike many other woods (ring porous woods). However, concentric false rings sometimes appear on the wood, depending on the presence of tension wood (g~latinous cells) which are fairly common in most of the clones. Maximum number of such rings are found in the basal portions with decreasing number towards the top. The tension wood may vary from 15 to 65%. and such erratic distribution tends to give a woolly appearance on the surface of wood. Such distribution and variation are supposed to be responsible for some of the commonly observed defects that may occur during drying and processing.

There are very few natural defects in rubberwood capable of making it unsuitable for general purpose applications. Unlike typical forest based trees, rubberwood is grown on plantations where the trees are carefully nurtured. However, due to the presence of growth stresses and induced drying stresses, a few defects such as splits, cracks and checks are usually observed. These can be avoided or minimised by careful control measures during storage and drying. Decay or rot often occurs in rubber wood due to attack by fungi, which can be avoided by suitable chemical treatments. Similar defects due to other biological agencies like insects and birds or due to weather can also be suitably minimised by chemical treatments. Other defects, like grain orientation, knots, woolly surfaces etc., can be lessened by suitable machining and sorting. Logging defects like ruptured or crushed fibres can be eliminated by employing proper tools and observing necessary precautions while logging and transporting. Thus the defects that are commonly observed in rubberwood are not so serious as to render it useless.

Physical and mechanical properties

Rubberwood, like most woods, exhibits orthotropicity in its properties: that is its properties are different and independent in the three principal directions of growth: longitudinal, radial and tangential. Being non-homogeneous in its structure. its density also varies from site to site inside the material. The variations in properties are attributable not only to the variations in density but also to the presence of latex particles in some locations and to the predominance of tension wood.

Edaphic, agrometeorological and plant factors such as elevation, air temperature. solar radiation, humidity, rainfall, soil characteristics, spacing, clonal difference and age of the tree can influence to a certain degree the properties of any species of wood. However, these changes may be significant with reference to the expected end use, and are generally taken care of in the system of evaluation itself by drawing samples representative of different growth conditions. However, it should be noted that strength in the green condition does not vary with moisture content.

Like most wood species, the dynamic properties of rubberwood are higher than the static properties: under impact loads, rubberwood is capable of taking loads nearly twice that under slowly applied loads. However, it may be noted that the static properties of rubberwood when in dry condition are higher than those when green, but in the case of dynamic properties.

The reverse is the case for fibre stress at elastic limit and modulus of elasticity, and the increase is not significant in the case of maximum height of drop. This shows that in such cases where shocks come into play, the presence of moisture in wood is helpful in taking up higher loads.in some countries it is customary to explain the mechanical behaviour of any species for a specific function or end use, in terms of the mechanical behaviour of a popular species, widely used for a variety of purposes or for the same function and end use. In India teak is one such species and so the mechanical behaviour of all species is compared to that of teak as 100. The comparative figures are known as 'suitability figures' or 'suitability indices' and the same are indicated for rubber wood in the following table.

Comparative suitability figures of rubber wood

Parameters rating taking teak as 100

From the table it may be seen that rubber wood is very near to the weight and shear properties of teak, and fairly comparable in other properties except in suitability as posts. However, rubberwood has to be used cautiously where compressive forces along the grain come into play. The suitability figures are derived by combining suitably the various properties in green and dry condition that become important for the particular function or end use. These figures serve only for comparison and not for any design or calculation of natural forces that come into action.

Machining and finishing properties

The first stage in the utilisation of any wood, involves machining. In these processes, the geometry of cutting tools, the speed of their cutting, the rate of feed and the manner of feed of the material play a prominent role in deciding the quality of the machined material and consumption of energy for the required operations. A suitable combination of these properties, determined quantitatively under standard conditions, is known as the working quality.

The economy, efficiency, and safety factors are governed by the machining properties and the working quality of the wood in question. Rubberwood has been subject to most types of machine operation and qualitative experience is available on its working. Nevertheless, few quantitative data are available on the machining properties.

Wood finishing is the effect of various surface finishes, painting, polishing etc, on wood. In the case of rubberwood, qualitative experience indicates that it can be worked to a good finish suitable for high class furniture. The finish adaptability is rated at 94% of that of teak under standard conditions. Rubber wood can be easily worked on a lathe, but is not that good for boring and mortising. Ammonia treated rubberwood, free from blue stains, has exhibited better finish adaptability and water gloss than untreated or blue stained rubberwood.

Seasoning behaviour

Seasoning reduces and adjusts the inherent moisture content of wood to a predetermined level, usually to a level of equilibrium in the region of that to which the material will be in use. Thus the absorption and desorption of moisture by wood will be very much minimised, and consequent swelling, shrinking, and warping in planks, and cut sizes of rubberwood are avoided. Other advantages of seasoning are reduced surface cracking and splitting, improved physical and mechanical properties, better working quality with different tools and easy finish uptake. Seasoned material, being comparatively lighter than unseasoned material (called green material ie. above the fibre saturation point), is easier and cheaper to transport.

The most popular and economical methods are (1) kiln seasoning and (2) air seasoning or air drying. The former is done in an enclosed chamber in which temperature, humidity and circulation of air can be controlled to ensure a gradual removal of moisture. Humidity is controlled by water sprays or steam jets. Circulation of air is achieved via reversible fans inside the chamber. Starting with low temperature and high humidity, the conditions inside the chamber are gradually changed to low humidity and high temperature and the process of seasoning is continued until the required moisture content of the material is attained.

Air drying of green timber is done in the open: the timber is stacked under cover in a way that enables free air to pass through the stack. Precautions need to be taken to avoid defects: material above 10 cm in thickness must be air dried first to bring down the moisture content to about the fibre saturation point. For a material of similar thickness and initial moisture content, air seasoning takes longer than kiln seasoning. Kiln seasoning requires skilled operations and careful supervision, whilst air drying and higher cost, air seasoning does not require much skill and supervision and is cheaper in spite of locked up capital in the wood stacked for long periods.

Rubberwood is found to be amenable to extremely rapid movement in many other species under the same conditions. Planks about 25 mm thickness take about 55 to 60 days in air drying while only about 6 to 7 days in kiln drying. In solar kilns drying time is about 15 days for the same material.

Durability and preservation

Timbers are usually classified as non-durable or durable depending on whether their heartwood is liable to be attacked by fungi and insects or not. In some countries the durable woods are further divided into three groups as (1) highly durable (2) moderately durable and (3) less durable depending upon the degree of damage of a standard sized piece of wood on exposure directly to wood destroying agencies. Similarly the various species of wood are classified as easily treatable or not, depending upon the capacity of the species to take chemicals for preventing attack by fungi and insects. At present there are no adequate laboratory or field data on rubberwood to classify its durability or treatability, but there are various industrial and field experiences, from which it is known that rubberwood is definitely non-durable, but fairly easily treatable by water soluble chemicals or even oil type preservatives.

Several agents damage rubberwood including fungi and insects which require to be treated chemically. There are several types of fungi which are known to attack rubber wood. Some of the well known among them are Lenzites palisotti, Ganoderma applanatum, Tramates corrugata, Pollyporous zonzlis, Lentinus blapharods and Schizophyllum commune. Similarly some of the insects found to attack rubber wood are those belonging to families Cerambycidae, Bostrychidae, Lyctidae and Platypodidae.

To protect wood from the attack of these agents, it has to be treated with chemical preservatives. The common types of preservatives are oil-based and water-based. In the oil based preservative, coal tar creosote, with or without admixture of fuel oil, is used mostly for exterior use of rubber wood. The discolouration and unpleasant odour restricts its use at many places. However this is toxic to many types of organisms and also prevents any further splitting in wood. Water based preservatives contain toxic inorganic salts dissolved/dispersed in water. The leachable types amongst these are zinc chloride, boric acid or borax and sodium pentachlorophenate. Non -leachable water based preservatives include copper-chrome-arsenic (CCA), acid-cupric-chromate (ACC)