COMPOSITE WOOD PRODUCTS
When woods are broken down mechanically or chemically then reformed into a solid product with or without adhesive, these reconstructed woods are called composite woods. OR;
Composite wood is a general term for built-up bounded products consisting either wholly of natural wood products. OR;
Wood in combination with metal or plastic, etc
Composite Wood products:
Any wood product consisting of two or more than two types or raw materials is called a composite wood product.
Laminated Board/ Laminated Wood/ Laminates
- A cross bounded assembly generally made up of veneers (or) of veneer in combination with lumber core/ pile, joined with adhesives are called plywood.
- Generally, the grains of one veneer is the right angle to the other.
- Usually odd numbers of piles are used.
- All piles and core panels made of veneer are oriented parallel to panel plane.
- Core board made from up to 1-inch wide wood blocks together with or without glue b/w each block placed to form a slab sandwiched b/w outer veneers
- Grain direction is at a right angle to core grain
- Batten board:
- Variation of blockboard core built up of battens not extending a 3inch width
- Lamin board:
- Core built up from strips of veneer (3.7 mm wide)
- Heavier than blockboard
- Higher cost
- Composite board:
- Plywood with insulting material core such as cork, fiber, foam, rubber, etc. ie cold storage rooms and also for sound damping in studios, telephone boots, etc.
- A sheet or board generally made up of fibers of wood or other lingo-cellulosic material is called fiberboard.
- It is produced by interfiling of fibers in such a way as to produce mat/ sheet.
- Wastes from plywood, veneer, furniture, and logs of smaller diameter, branches, thinning material, grasses, waste paper.
- According to the density:
- Medium/ Intermediate density F.B 4 – 0.8 g/cm2
- Hard board 8 – 1.2 g/cm2
- Special denisified Hard board 2 – 1.45 g/cm2
- Semi-rigid insulation board 02 – 0.15 g/cm2
- Rigid insulation board 15 – 0.40 g/cm2
- As panels for insulation and covering material in buildings and construction where flat sheets are required.
- As a part of the door, cabinets, cupboard, furniture, etc.
- Debarking machine make use of friction forces b/w logs or b/w tools and wood machine like cambio, drum, hydrauling, etc.
- Log chipper are used
- Upto 14inch diameter
- Slab chipper as also used for 44 inch dia material.
- Oversized chips are chipped again
- Chips are led over a magnetic separator which removes iron particles.
- Chips are soaked in water for mechanical defrate and chips must have 40-60% moisture content to have unbroken fibers.
- By using a thermo-mechanical pulping process
- It involves preliminary treatment with heat under pressure called digestion
- This is to improve pulping quality
- We use cooking (digestion) in Autocleve (just like a pressure cooker) in witch the wood chips are cooked under pressure
- The are refined, two processes are included:
- Mechanical process of pulping
- The chips are breaked mechanically into fibers by feeding them with an Archemediean screw b/w two segmented grinding discs.
- Prior the chips are treated by steam under pressure to soften the lignin
- Chips are converted into a pulp (powder)
Mesognite/ Explosion Process:
- It involves sudden ejection of chips from steam heated pressure to vessels causing them to explode into fibrous mass
- Temp 150 – 200 oC
- Pressure 1000 Pascal
- To improve the water resistance properties and exposure resistance properties of the board, we use paraffin wax, rosin, cumarene resin are added during pulping before sheet formation.
Mat or Sheet Formation:
- Wet felting is chiefly used for the formation of mat/ sheet for fiber board by two methods:
Deckle Box Method:
- Consists of bottom less frame with screen
- For 1 sheet stock amount pumped into the box and vacuum is applied from the bottom.
- After drainage of water bulk, the pressure is applied from top to dewater and to reduce sheet thickness.
- After the reckle frame is raised and sheets are remove.
Four dinner Method:
- Stock (1-2%) fiber is first sent through the screen to head box from which it flows to the pulp fiber remains on screen while a great portion of water drains through suction boxes/ rubber section belt
- Pressing of the wet mat is accomplished by a series of rollers.
- Moist sheets out by circular blades (without teeth)
- Heated presses for all types of hard board production.
- Dry of the wet mat by live methods:
- Tuner kiln
- Continuous roller type dryers
- Dry of the wet mat by live methods:
- Transport the wet sheets to the hot press at temp 160-130 C and pressure 50 kg/cm.
Heat treatment and oil tempering:
- Heat treatment is at a higher temp to increase the mechanical strength properties
- MOR, lower the stock resistance or impact strength
- Temp 160 C hot air for 8 hours
- Hard board not previously subjected to the heat treatment is treated with oil to improve strength, weathering properties, abrasion resistance, water resistance
- Humidify the board to the moisture of about 5-8 % to avoid warping
- Carried out the chamber/ tunnel
- Time 5-8 hours
- After conditioning, the boards are trimmed to standard specified dimensions wrapped in convenient packages.
ADVANTAGES OF PLYWOOD (OVER SOLID WOOD):
- Distributed wood strength
- Non-splitting qualities
- Dimensional stability
- Strength/ wt ratio
|Plywood||3/ 16 inches||1|
|Aluminum alloy||3/ 16 inches||0.88|
|High tensile steel||3/ 16 inches||0.62|
|Mold steel||3/ 16 inches||0.39|
- So plywood is strongest where strength/ wt ratio is the deciding factor.
S/w Ratio = ultimate tensile strength of material / weight
- Rigidity of stiffness
Rigidity = E × I
(Where; E = MOE; and I = Moment of inertia)
|Material||Wt/ sq ft||Rel stiffness|
|Mild steel plate 1/16in||2.56||1.00|
- The strip of plywood will be much stiffer than the strip of steel or aluminum of equal weight.
- Impact Resistance:
- Resistance against a sudden shock is called impact resistant
- In solid wood à a line of cleavage is present but in plywood, it is absent so impact resistant is more in plywood than solid wood.
- Fire Resistance:
- It is very difficult to make solid wood as fire retardant but in plywood, it is much easier.
- Asbestos is used in the core of plywood to make it fire retardant.
USES OF PLYWOOD:
- Agricultural work
- Aircraft industry
- Building industry
- Marine uses
- Musical instruments
- Packing instruments
- Transport and motor vehicles
ADHESIVES FOR PLYWOOD:
WBP (Weather and boil proof):
- Highly resistant to weather, microorganism, cold and boiling water, steam and dry heat
- More durable
- Eg phenol formaldehyde resin
BR (Boil Resistant):
- Eg urea melamine formaldehyde
- Good resistant to weather and boiling water but not for long exposure to weather
- Can withstand to cold water for many years.
- Highly resistant to micro-organisms attack
MR (Moisture Resistant and moderately weather resistant):
- Eg urea formaldehyde resin
- Good resistant to cold water
- Fair resistant to hot water
- Negligible resistant to boiling water
- Excellent resistant to organic solvent
- Complete resistant to fungi, woodworm, termites
- In case of full exposure to weather, resistant for only a few years.
- Urea formaldehyde resin
- Resistant to high temp
- Resistant to cold water
- Can be attacked by microorganisms
- Less resistant to hot water
- Composed of animal glue, blood albumins, casein, soya glue
MANUFACTURE OF PLYWOOD:
- Straight, long, clean boles
- 5 ft in length at least
- 3ft in diameter
- Free from knots, cracks, and other defects
- Than 8 ft veneer is obtained
- By total immersion in water
- In some tropical logs, boiling/ stream treatment is done
- The fibers are softened for the peeling process
- Soft, smooth, tight veneer is produced
- The bark is removed by hand/ machine
- Low-density logs are conditioned in cold water while high-density logs in hot water.
- For very hardwoods are sawn by a circular saw, to prevent from a change in color ie Cedrella toona
- 90% product depends on peeling
- It is carried out in large rotating peeling lathe
- Logs are rotated against the lathe blades which lies against full log length
- Gear, feed the blade towards log so that a continuous ribbon of veneer is produced of constant and accurate thickness
- For decorative purposes vertical and horizontal slicer
- Wood is converted into a block called Flitch
- Flitch is fixed and moved towards the knife
- A slice of wood of required thickness is obtained
- It is adjusted in such a way that after one slicing it moves one step up than another
- Veneer is cut and the process is continued
- Veneer thus produced is used for face piles
- Shisham, Teak, Walnut are used for such purposes.
- After peeling we veneer is then fed through a continuous drier (a long heated chamber through which the veneer is carried on continuous belts/ rollers)
- Controllable feeding rate and temperature and humidity is fixed according to spp
- Ensure that veneer are at a moisture content appropriate for bonding
- Passes through clipper to cut it into predetermined press size
- Veneer is sorted by visual/ computer inspection into face and core veneers.
- Subdivided into 4-5 ft appearance classes
- Defective or blemish veneer may be repaired to improve the grade
Jointing and Edge gluing:
- Edge and joined by gluing, taping or stitching to form piles of standard panel sizes
- Passed through a glue spreader
- Glue spreader roles in both sides of the core veneer
- Presses for plywood consists of no of heated plates b/w which loosely assembled panels of plywood placed (pr = 12-20 kg/cm2)
- Outer piles are over dried but not core
- Moisture content gradient is removed with the help of
- Close stacking: For 15 days it is kept to bring a uniform temp and M.C
- Press through air conditions channel but expensive
Sanding and trimming:
- Panels are taken to a suitable sizing saw, where they are trimmed to the derived length and width and then sanded.
- A board/ sheet constituted of fragments of wood (discrete pieces) and lingo-cellulosic material, bounded with adhesives with the help of one or more agents like heat, pressure, humidity, are called Particle Boards.
- Eg chipboard, shoving board, flake board,
Flat – platen Pressed Particle Board:
- Most common in Pakistan
- Mass of particles coated with a binding agent (synthetic resin) b/w two plates
- Pressed and perpendicular pressure is applied
- Obtained from extrusion through a dye
- Molded products
Classification based on Density:
- Low-density PB à less than 0.59 sp gravity
- Medium density PB à b/w 0.59 – 0.8 sp gr
- High-density PB à greater than 0.8 sp gr
RAW MATERIALS USED FOR P.B.
- Wood, thinning materials, offcuts from saw miller, chips, etc
- Nowadays lingo-cellulosic material, bagasse flax shives, cotton, hemp, jute stock, etc.
- Binders: Urea, melamine, phenol, formaldehyde, resin
- 90% PBs are produced with them
- Additives: water repellent, fungicide/ insecticides, fire retardants.
MANUFACTURE OF P.B:
Debarking of logs by:
- Cambio de-barker:
Logs are passed through rings to remove the bark
Logs are soaked before debarking
- Dum debarker:
In rotating drum the logs are rolled OR
At low-speed water is sprayed to wash the bark
Through nozzles, the water is sprayed to the rotating log surface at 85 – 90 kg / cm2
Reduction of Wood:
Wood size is reduced (disintegrated) into a smaller size
If the log dia < 24 cm then it should only cross-cut
If the log dia > 24 cm then it should be splitted
The log is fed in flock of the size < 24 cm dia easily
Hammering for disintegration:
Hammering for the disintegration of various shapes of wood/ material to get a uniform small piece
For particle preparation machine used are:
- Hammer mills: Used to the disintegration of irregular parts into particles
- Refiner: Separate cellulose from lignin. Cut the fiber in length
- Flackers: Ring, drum, disc flacker for manufacturing of geometrically shaped particles
- Attrition mills: Here the moisture content should be very high while preparing the particles (wood may be soaked before working with)
Chip/ Glue blending:
(Glue in powder form) are applied a solution b/w 40-65% solid content, minimum percentage of glue (7-10%) as a binder.
Glue is mixed with hardener within 10 minutes
Transported from glue mixer to pressure vessel
By pressing air into the upper part of the vessel, the glue is fed to the spray nozzles; the glue is fed to the spray nozzles
Liquid glue is sprayed in a relatively small amount on the great mass of chips.
The purpose is to prepare a consistently uniform mat/ carpet of resonated chips
To spread a uniform mat wind shifter machine is used.
It helps to maintain three layers in the particles
Coarser particles at the bottom and lighter particles are in the for/ rare side of heavy particles
- Compression of mat b/w platens of a single opening cold hydraulic press
- Roller through which caul with its mat is pressed.
The mat of chips formed on a stainless steel caul
Caul: A sheet/ steel plate placed below the mat to feed into a single opening cold press on the roller to reduce the thickness of mat. Then met is fed into a hydraulic press, provided with loaded and unloaded equipment.
Temp 130-160 oC
Pressure 14-35 kg/ cm2
- Conditioning: After pressing the board are stacked for few days in order to equalize the moisture contents
- Sizing: To produce chipboard with predetermined width and length
- Sanding: For this veneer to be glued directly to surface. Sanding machines with a 2-4 cylinder covered with abrasive paper. Panels are finely ground on both sides by passing under cylinders. Sanded are free from dust by rotation brushed and the power cut section inside the machine.
Trimming: Sizing means to trim all four edges. (8ft × 4ft) (6ft × 4ft) standard size, water resistance, insect resistance may be needed with the addition of different chemicals.
- Hard gelatin obtained from hides, tendons, cartilage, bones, etc of animals.
- Adhesives are prepared from these substances by heating with water.
- Glue and adhesive (any substance capable of holding the material together by surface catchment) are synonyms in general use
- On the basis of chemical composition:
- Starch Glue:
- From vegetable sources as cereals, roots, tuber, pith or plants
- The principal raw material used in the manufacture of starch glue is from Cassava root
- Used in American plywood industry.
- Manufactured from hides, bones, fish wastes
- Give excellent bond in dry conditions
- Not water resistance and distracted by microorganisms under humidity
- Commonly used in the world because of low cost and strong bonds for the furniture industry and veneering.
- Blood albumin glues:
- Manufactured from blood obtained from slaughtered animals/ from dried soluble blood albumin
- Gives moderately strong bond in boiling water
- In damp conditions break down by microorganisms
- Casein glues:
- Made by dissolving casein obtained from a mile in the aqueous alkaline solvent
- OR; Casein produced from skimmed mike by precipitation with acid
- Provide a strong bond in hot dry conditions
- Destroyed by chemicals and microorganisms.
- Used for high stresses like aircraft construction
- Vegetable protein glues (soybean glue):
- Oil extracted from beans and glues are made
- Also made from crushed residues by the addition of chemical similar properties used as in Casein glue.
- Used for the manufacture of plywood and laminated board
- Soften on heating
- Polymers formed by condensation
- Reaction products of formaldehyde with phenol, urea, melamine, resorcinol, etc
- Vinyl type addition polymers and their derivatives
- Phenol formaldehyde glues:
- Manufactured from phenolic resins, phenol, and formaldehyde
- Give good joints even under bad conditions
- Highly water resistant
- Used for plywood products
- For the impregnation of laminated products
- For furniture
- For ship boats construction etc.
- Resorcinol-formaldehyde resin glues:
- Set quickly at low temp
- Consists of resorcinol and phenol formaldehyde resins
- Urea and melamine formaldehyde resin glues (Amino resins):
- Cured rapidly in presence of a catalyst with heat application
- Good resistance to cold water
- Fair resistance to hot water
- Negligible resistance to boiling water
- Polyvinyl Acetate (PVA):
- Eg Polyvinyl ester glue
- Used for gluing of solid wood as a cold binder
- Used in room temperature
- It replaces the animal and casein glue because
- Unlimited storage life
- Sap filling properties
- Low pressure for setting
- Resistance to micro-organisms
- High sensitivity to water
- Rapid reduction of joint strength under the influence of heat and moisture
- Nor for smooth, non-porous surfaces.
- The term used for better quality wood (or) A general term used for wood that has been specially treated in various ways to reduce the retard, wrapping or movement to increase its strength or other properties
- Types of improved wood:
A form of improved wood in which the wood is treated or is impregnated with synthetic resins
+ Anti-shrink efficiency of impregnated wood increases from 35 – 100% by treating it with different resins chemicals
+ Face checking of plywood is eliminated when the faces are treated with 25-30 of resin
+ Phenolic resin treatment imparts considerable decay resistance, electrical resistance and acid resistance to wood.
+ Generally used for switch panels, pulleys, tools of pipe bending, mold, and jig, etc.
+ Also used for various shell molding dies due to its good resistance to heat.
A form of improved wood in which the wood is compressed
+ This wood is heavier in weight due to an increase of lignin, then normal wood.
+ This wood acts as angle bracket supporting
A form of improved wood in which the wood is treated or impregnated with synthetic resin and compressed
+ Much more dimension stable than untreated compressed wood
+ Can be easily cut or turned using metal work tools operated at slower than normal speed.
+ Can be glued to the normal wood with hot press phenolic or room temp setting resorcinol glue
+ Electric resistance is far better than normal wood and acid resistance is better that impreg
+ It is much more flame resistant than impreg
+ Most of the strength properties of compreg are increased than normal wood.
Uses of Compreg:
+ Compreg can be used for roots of wooden properties and for ship screw bearing
+ Compreg can also be used for forming dies, jigs, wearing, shutters, glass doors, pulls, knife handle, etc.
Image: Build Abroad
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