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WOOD GRAIN
1. Wood consists of a multitude of cells, which differ from each other in shape, size, thickness of walls, contents, and arrangement. They are more or less firmly grown together. These cells may be as long as 1/3 inch in spruce or only 1/25 inch in some of the hardwoods. Cells of this sort are called fibres, in the softwoods they conduct water longitudinally in the tree. Cells of another kind which have greater diameter are called vessels; we can see them in the hardwoods. In the cells that make up wood rays water or sap is conducted in a radial direction.
2. No two woods have identical structure. The firs, pines, cedars, spruces and other needle-leaved softwoods have relatively few kinds of cells, while the hardwoods, such as oaks, beeches and maples have many kinds. These cells have various functions, such as conducting water, storing materials and reinforcing the structure.
3. The differences in cell structure produce a distinct pattern within the wood. This pattern becomes characteristic for each kind of wood, but it is never identical in any two pieces of the same wood. The grain figure is affected by the way the log is sawed. The woods that have considerable difference between springwood and summerwood are called coarse-grained; when there is little contrast, they are called fine-grained.
4. Here are the main types of wood grain. Straight-grained wood has the fibres running in the same direction as the main axis of the tree. Interlocked grain, occurs when layers of the wood alternate in direction. Such wood is difficult to split or to plane; for example, mahogany and elm. Wavy grain is almost a regular pattern of waves in the grain. Curly grain is a grain pattern with a definite curly appearance.
5. The grain pattern about the knots is circular and is usually more varied in direction than wood above or below the knot.
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What are the main types of wood grain?
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PULPING
1. Any process of pulp preparation begins with barking. Bark is removed in big steel cylinders called barking drums. Logs tumble against each other and as a result bark is stripped. Another common method is using powerful jets of water.
2. If bark is not removed we get darker and dirtier pulp. The bleaching of such a pulp is very difficult, but this disadvantage is overcome by the greater yield of the pulp. The technology of utilizing the whole tree is already known and is used at many mills. But at the majority of pulp and paper mills bark is removed.
3. Mechanical pulp (groundwood) consists of wood obtained by grinding wood into a fibrous condition. The process by which mechanical pulp is produced is very simple: it involves the wet grinding of logs. The composition of the mechanical pulp is practically identical with that of the wood. Groundwood made from fresh wood is of light yellow colour.
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4. The pulp produced by the mechanical process is inferior to that produced by the chemical processes: it is used in making those kinds of paper where the highest quality is not demanded and the price is principal consideration, for example, in newsprint. Such paper turns yellow owing to the action of sunlight and the atmosphere on the lignin contained in the wood pulp.
5. Mechanical pulp is much cheaper to make than any other form of paper pulp. In the first place only about 2 per cent of raw material is lost, as compared to 50 per cent in the chemical processes. Secondly, there are no chemicals required. Finally, the equipment necessary for making mechanical pulp is much cheaper.
6. Another way of making wood pulp which combines mechanical and chemical methods is called the semi-chemical process. It was developed particularly for the pulping of hardwoods and has many variations. The pulpwood chips are cooked in a mild chemical solution, and then the fibers are separated mechanically. Semi-chemical pulps are used in making corrugated paper boards, . g. cartons and many other items.
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Where is the mechanical pulp used?
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