FIBREBOARD
Definition:
It has been defined as: sheet material generally exceeding 1.5 mm in thickness, manufactured from lignocelluloses fibre with the primary bond from the felting of the fibre and their inherent adhesive properties. Bonding materials or additives may be added. The primary bond is made by an adhesive.
Fibreboard is reconstituted from single fibres or bundles of them as opposed to particle boards in which pieces of wood, i.e., particles/chips are re-assembled.
Types of fibreboard:
It is generally classified by its density and is manufactured in rather a wide range of densities, from 20 to 1440 kg/m.
3 types of fibre board: recommended by ISO-
1. Soft fibreboard –density up to 250kg/m3.
2. Medium fibreboard – density up to 350 to 800 kg/m3.
3. Hard fibreboard – density above 800 kg/m3.
Manufacture of fibreboard:
It is manufactured from refined or partially refined wood fibre or other vegetable fibre, interfelted in such a manner as to produce a mat which is hot pressed under high or low pressure. Adhesives or other chemicals may be added, singly or in any combination thereof, during the manufacture of fibreboard to improve various properties, e.g., increased strength and resistance to moisture, decay, insect or fire, etc.
Raw materials:
· Pulpwood and fuel wood.
· Inferior quality and damaged wood.
· Thinning up to a 50mm or 2 inch in diameter.
· Logging wastes lops and tops branches and stumps of forest felling.
· Wastes from sawmills and veneer plants, e.g., slabs, edging, off-cuts, etc., and waste veneer, peeler, cores, etc., respectively.
· Sawdust.
· Planer shaving and other wastes from wood working operations.
· Bark.
· Agricultural residues, such as baggage, wheat and rice straw, corn, cotton and mustard stalks, coconut and area –nut husks, palm leaf ribs, flax shays, jute sticks, etc.
· Bamboo and other tropical grasses.
· Waste paper and board.
Chemicals additives used singly or in any combination:
· Sizes such as resin, paraffin wax about 1% by weight, to improve resistance to water.
· Oils such as linseed, tong , soybean, peril, tall, etc. about 1% of board stalk, to improve strength and water repellency.
· Asphalt and asphalt emulsion for boards subject to severe exposure.
· Resin, generally phenol formaldehyde, to improve strength properties of ‘dry process’ boards, and increased resistance to moisture.
· Alum, to adjust ph to about 4.5
· Preservatives for protection against bio-deterioration.
· Fire-retardants chemicals, e.g., ammonium phosphates, boric acid, etc.
Methods of fibre production:
A. Mechanical Pulping
B. Explosion pulping
A. Mechanical pulping:
There are broadly two methods for the production of mechanical pulp, One is the grinding of debarked logs and the other is the refining of wood chips and agricultural residues.
1. Bolts of soft wood and hard wood are pressed against a revolving wet grindstone for defibration. The pressure and the rotatory motion increase the temperature in the grinding zone. The higher temperature softens the lignin and polyoses in the intercellular substance and thus helps in the separation of fibre.
2. The other method involves either treatment of the wood chips or agricultural residues with chemical; or thermal pretreatment prior to pulping in disk refiners or attrition mills. Solid wood is debarked and then chipped in a chipper and screened. The coarse chips are rechipped and the fines sent to the boiler.
ü In the chemo–mechanical pulping, neutral sulphite cook, sodium hydroxide cook or a lime cook is use.
ü Thermo – mechanical pulping is based on the softening of the middle – lamella or inter –cellular layers between the fibres due to heating at 150*C to 180*C. this facilitaties separation of fibre. The chemical composition of the pulp remains almost identical to that of the original wood or other lingo- cellulosic materials. The original fibre structure is also preserved and very high yield, 90 to 93 percent is obtained.
B. Explosion process:
This process is better known as the masonite process. Here 100 kg of wood chips on agricultural residus are charged into a high pressure cylinder called a gun and steam is admitted. The pressure is raised to 40 kp/cm in about 30 seconds. After a steaming period of another 30 seconds under this pressure, the steam pressure is quickly raised to 70 to 80kp/cm. raising the temperature to 285*C to 295*C and held for only about 5 seconds, before suddenly releasing the pressure by a hydraulically operated quick opening valve “The gun” is blown. First under the influence of high steam pressure, moisture, and high temperature, the wood under goes a hydrolytic reaction which breaks down the ligno-cellulosic bond. Secondly, the sudden release of the hydrolyzed chips to atmosphere pressure tears them apart to produce a characteristically brown fluffy fibre’. The fibres are washed with clear hot water to remove hydrolytic products and buffered before mat formation.
Method of Mat formation:
1. Wet Process
2. Dry Process
3. Semi- Process
Wet process:
In the wet process, water is added to fibre to produce a slurry of around 1 percent consistency with constant agitation to prevent settling of the fibre. Additives such as sizes for water repellency, e.g, resin, wax, bitumen emulsion, etc. Phenol formaldehyde resin, pesticides and fungicides may be incorporated at 4 stages-
a. Fourdriner machine
b. Cylinder machine
c. Deckle box
d. Hot pressing
Fourdrinier machine:
The slurry is passed to a continuous forming machine, furdrinier machine, with an enrless wirel-mash belt for the felting of the fibre. Water is gravity drained through the wire mesh screen aided by suction pumps and light squzeeing between rollers.
Cylinder machines:
A cylinder with a wire mesh surface partially submerged in a vat of pulp rotates and the pulp is carried on the surface of the wire. The wet web is lifted off the cylinder surface before it disappears below the surface of the pulp solution once more. It is passed through a roll press to further dewater and control the thickness of the sheet.
Deckle box:
For small scale production, deckle box is used for mat formation. It consists of a bottomless frame which can be raised or lowered on a screen. A measured amount of slurry is poured into the deckle box and vacuum is applied from the bottom. Water is drained and a pressure is applied from the top to further dewater and compress the mat.
Hot processing:
· Insulation Board:
For producing soft or insulation board the partially dry wet-lap, as it is now called, is trimmed in width and cut into the desired sheet length and cut into the desired sheet length and dried in tunnel-kilns, continuous multi-deck roller type dryers or hot presses with stops, at a temperature of 120c to 190c.
· Hardboard:
For the production of hardboard single or multiple daylight presses heated by steam or hot water are used. Temperature of 200*C with a high initial pressure of 50 kp/cm, followed by a lower pressure of about one –fifth of the initial pressure, then bringing the pressure upto the original level with a time schedule of 2-1-3 minutes is typical for a 3.2mm thick wet process hardboard.
2. Dry and Semi-Dry process:
For duo-faced boards having two sides smooth, S-2-S, dry or semi dry process is used. Here the fibres are pre-dried in a high temperature tuned drier on counter flow principle to 20-40 percent moisture content in the case of semi-dry process and 5-12 percent moisture content for dry process and are carried in air-suspension as opposed to water suspension and air-felted into a mat. The mat is pressed between two smooth platens, as the danger of blow-up is absent due to low moisture content of the fibre, in the hot press using a higher pressure , 60 to70 kp/cm and temperature up to 260*C but shorter time than in the case of “wet process” boards. As dry process boards are inherently weaker than wet process boards synthetic resin adhesives, namely, phenol formaldehyde resin, must be added as supplementary bonding materials during defibration or during refining. Also larger quantities of waxes, up to 2% are added.
On leaving the press, the dry boards are conditioned to a moisture content in equilibrium with the surrounding service atmosphere. This is accomplished by spraying the board with water and keeping closely stacked for a few days or putting these in a humidifying chamber. The boards are usually trimmed using tungsten carbide tipped saws.
Post Manufacturing Treatment:
Boards may be sold as it is, i.e., as plane boards,or may be given further treatments.
1. Heat treatment:
Wet process boards made without resin are usually given a heat treatment at 160 to 170*C for 4 hours for improved strength and water resistance.
2. Tempering:
Tempered hardboard are made by immersing in hot natural oils, e.g., linseed, tung, etc., for several hours so that boards take up 5 to 6% of their weight in oil. These boards have higher density, strength, and resistance to moisture and abrasion.
3. Surface treatment:
Surface may be sealed, primed, enameled and lacquered, pulp-faced, molded and embossed, laminated, provided with wood-grained effects, perforated, or flame-retardant treated.
Uses:
Uses of fibre wood are many and diverse. Use of it will satisfy a need better or more economically than any other material. Some of the uses are-
· Fibre board is used for construction. In construction, hardboard is used as floor underlayment, facings of flash doors, interior wall linings, concrete forms, etc.
· It is used in furniture and furnishing includes drawer bottoms, dividers, mirror backs, insert panel, television, radio, stereo cabinet, etc.
· In automobiles, trucks, buses and railroad cars, hardboard is used in interior linings, doors and interior sidewall panels.
· Hard board is also used in store fixture, appliances and automotive and rolling stock, etc.
· Special densified hardboard is used for making templets and jigs for manufacturing, as electrical panel materials, laboratory work surfaces, gears, cams, etc.
· Fibre board of low density is used mainly for insulation purposes.
