Fibreboard

The term fibreboard(1) includes hardboard, medium-density fibreboard (MDF)(2) and insulation board. Because wood is fibrous by nature, fibreboard exploits the inherent strength of wood(3) to a greater extent than does particleboard.

To make fibres for composites, bond between the woodfibres must be broken.

In its simplest way, this is done by attrition milling(4) whereby material is fed between two disks, one rotating and the other stationary. When the material is forced through between the disks it is separated into fibres.

Attrition milling, or refining(5) as it commonly called, can be done more effective by water soaking(6), steam cooking or chemical treatment of the wood material. Steaming (7) weakens the lignin bonds(8) between the cellulosic fibres. As a result, the fibres are more readily separated and usually are less damaged than processed by dry processing methods.

Fibreboard is normally classified by density and can be made by dry or wet processes. Dry processes are used for boards with high-density (hardboard) and medium-density (MDF). Wet processes are used for both high-density hardboards and low-density insulation boards.

Dry-process fibreboard. Dry-process fibreboard is made in a similar way to particleboard. Adhesives (synthetic resins) may be applied to the fibres by spraying in blenders or introduced as the wet fibres are fed from the refiner(9) into a drier. The adhesive-coated fibres are then uniformly distributed to form a mat. After the fibre mat is formed it is typically pre-pressed in a hand press. The densified(10) mat is then trimmed by a disk cutter and transferred to the press. Dry-process boards(11) are pressed at 140 degrees to 190 degrees depending on the type of the adhesive used. Board density is a basic property and an indication of board quality. The density is greatly influenced by moisture content and that is why it is constantly monitored by moisture sensors.

The board with density 600 to 800 kg/ m3 is called medium density fibreboard (MDF). MDF is widely used. It is a valuable material because while having characteristics which are very similar to those of natural solid wood, MDF does not have disadvantages of solid wood, such as non-uniformity, low density, presence of knots, and others. It is easier to work with solid wood because its physical and mechanical properties are much the same in all directions. The main application of MDF is furniture making.

Wet-process hardboards. Wet-process hardboards differ from dry-process fibreboards in several ways. First, water is used for forming the fibres into a mat. Secondly, some wet-process boards(12) are made without additional binders(13) if the lignocellulosic contains enough lignin and if lignin is still present, lignin can serve as a binder. Under heat and pressure, lignin will act as thermosetting adhesive.

The wet mass which is called pulp, flows onto a mesh screen. Water flows through the screen and then the mass goes through a series of press rolls to remove excess water.

Wet-process boards are pressed in presses heated by steam. Heat is a very important factor during pressing to make fibre-to-fibre bond(14).

Insulation board. Insulation boards are low-density, wet-process product used for heat insulation and sound absorbing. Insulation boards typically do not use an additional binder, they make use of lignin (which is present) as a binder to hold the board components together. Binding agents are usually added during manufacture to improve certain properties. They include resin, starch, paraffin and others. Insulation board is usually cold-pressed to remove most of free water after the mat is formed. The wet mats are then dried to the final moisture content. Insulation board has a density of less than 496 kg/ m3.