Paper and Board Making

DEFINITION

The Board and Paper Industry, for convenience, consider that board is composed of substances whose combined weight exceeds 200 g/m2. Anything with a lower weight is termed paper.

Board and paper can be described as: "An interlaced web of fibers formed by the action of felting from a water suspension which is dried and finished, The fibers are normally of a vegetable origin, but they may also be from other sources, as in the case of animal or synthetic fibers".

FIBER TYPES

The most common fibers from which board is manufactured are from wood sources and are composed of cellulose which forms the structural body of the fiber, and lignum which binds the fibers together giving wood its rigidity. It is the cellulose in fibers that is the main raw material required for board making. Fiber size varies depending on the parent plant, and from which part of the plant they are derived. On the basis of their location in the plant, board making fibers can be divided into five categories:

Category Fiber Dimensions Cellulose
Content
Length mm Width mm
Seed Hairs
Cotton 		  10 - 60   0.020   92%
Bast Fibers
Flax
Hemp
Jute 		5 - 60
5 - 55
1.5 - 5.0 		0.012 - 0.027
0.025 - 0.050
0.020 		80%
77%
60%
Leaf Fibers
Esparto
Manila 		0.05 - 2.0
2.50 - 12 		0.013
0.025 - 0.040 		55%
53%
Wood Fibers
Deciduous (Hardwood)
Coniferous (Softwood) 		 
1.0 - 1.18
3.5 - 5.0 		 
0.030
0.030 		 
62%
58%
Grasses
Cereal Straws
Bamboo
1.0 - 3.4
1.5 - 4.0
0.023
0.030
Varies
60%

Some of the uses of these fibers are:

MANUFACTURING PROCESSES

The processes of converting wood into board can be considered in three distinct phases:

Fiber Processes

1) Wood preparation - Reducing logs to chips used in chemical and mechanical pulping.

2) Pulping and digestion - Removing lignum from the chips, thereby releasing individual fibers.

3) Stock preparation (beating and refining) Mechanically treating fibers in water to increase surface area flexibility and promote bonding when dried.

Web forming

4) Web Forming - Laying the fibers onto a moving belt.

Drying and Finishing

5) Pressing, drying and calendering - Removing the water from the fibrous web, drying the board, consolidating and producing an even surface.

1) Fiber Processes - Wood Preparation

a) For Chemical Processing

Debarked logs are fed into machines called "chippers" to produce chips about 3/8" x 1" long, which are screened to remove sawdust and contraries. Small chips can more easily be reduced to fibers in the following chemical pulping and digesting processes.

b) For Mechanical Processing

Logs are debarked prior to feeding into mechanical grinding machines.

2) Fiber Processes - Pulping and Digesting

Pulping consists of breaking down the wood chips to individual or small groups of fibers, which are then further processed by digestion. To separate the fibers it is necessary to remove the lignum and this is done commercially by three methods:

a) Chemically.
b) Mechanically.
c) Semi-chemically.

a) Chemical Pulping

Cellulose fibers have good resistance to dilute acids and alkalis in which lignum is soluble. Therefore, the lignum can be chemically leached away leaving the individual cellulose fibers. The yield of chemical fibers from chips is 40 - 60%, the remainder

being lost in stocks etc. One very important aspect of chemical pulping is that the natural fiber length is not reduced by the processing. The low yield of fibers makes this process expensive.

Three chemical processes are used for the different fiber types:

Soda Process (Alkaline):

Several alkaline methods are in use, each based directly or indirectly on the use of Sodium Hydroxide. The chips are fed into pressurized vessels under controlled temperature and pressure conditions, and are cooked, thereby dissolving the fiber bonding materials. This process is mainly used on hardwood.

Sulphate Process (Alkaline):

This is also referred to as the Sodium Sulphide process and can be used for both hard and soft woods particularly in the production of Kraft.

Chips are fed into a solution of Sodium Sulphide and caustic soda in a large sealed vessel. The liquid is cycled through a steam heater and the fibers in suspension pass into a receiving tank beneath. The pulp is then washed and screened to remove the chemicals. Turpentine and methanol are by-products of this process.

Sulphide Process (Acid):

This chemical process uses Sodium Sulphite and is most applicable for hardwoods. The treatment is conducted in vessels similar to those previously described, and is harder on fibers than the Sulphate process.

Sodium dioxide and Calcium Bisulphite are added to the wood chips in a digester and heated to 140 deg. C, dissolving the lignum. The fibers are then washed and screened.

b) Mechanical Pulping

A physical method of fiber separation consisting of grinding debarked logs under pressure against a grindstone in the presence of water. Water serves to lubricate and cool the grinding surface of the stone and to carry away the pulp. The logs are sometimes boiled or steamed prior to grinding, thereby loosening the fibers and making the grinding easier. The effect of this treatment is to darken the fibers.

Mechanical pulping is cheaper than the chemical process, but is only used on softwoods. This is because fiber length is shortened by mechanical breakdown and decreases in the length of hardwood fibers could not be accepted. The fiber yield from wood is high (90 - 95%) but the resulting fibers have little permanence when formed into board, as they fade and turn yellow on exposure to light. Newspaper is made from softwood mechanical pulp.

Less severe mechanical treatment of fibers has been achieved with the disc wood or refined ground wood pulping method. Wood chips are fed into a disc refiner containing stationary and rotating abrasive discs cooled by water.

c) Semi-Chemical Pulping

Chemical pulping whilst producing long fibers, has a low yield (fibers to wood). The object of the semi-chemical process is to produce as high a yield as possible commensurate with the best possible fiber lengths. By this method which combines chemical and mechanical processes up to 75% yield can be achieved, and the pulp is very strong, suitable for wrapping papers, boards, newsprint.

The Neutral Sodium & Sulphite Process:

A pulp yield of 60 - 70% is obtained by cooking wood chips in a 15% solution of Sodium Sulphate, in the presence of Sodium Bicarbonate.

Further fiber separation can subsequently be obtained by mechanical means using disc refiners. This process is particularly suitable for hardwoods e.g. aspen, birch.

Acid Sulphite Process:

By modifying this normal chemical process, a pulp yield of 65% can be obtained. The process consists of digesting wood chips with hot sulphite liquor at high pressure, and then using disc refiners. The pulp produced is used for newsprint and some mechanical printing grades.

After pulping, fibers are bleached and chlorinated. Chlorination is a continuation of the digesting process and the object is to remove the lignum remaining in the fibers without unduly attacking the cellulose. Because Chlorination has little effect on the color of the fibers, bleaching is also necessary when white fibers are required.

Comparison of Pulping Methods

  Chemical Mechanical Semi-Chemical
Cost Expensive process Cheapest Between Chemical and Mechanical
Yield 40 - 60% 90 - 95% 60 - 75%
Fiber Treatment No Damage Reduces Length of Fibers Not severe reduction in Length
Fiber Strength Strong Fibers Weakened Fibers Strong Fibers
Color Properties Good Little Permanence Good

Pulp Types, Characteristics and Uses:

Chemical Pulps:

Mechanical Pulps:

Semi-Chemical Pulps:

Re-Used Fiber Sources:

3) Fiber Sources- Stock Preparation

This is a two stage process consisting of beating and refining. The object of beating fibers from which to form a sheet of board is to develop the fiber condition described as "hydration". Fibers absorb water and become frayed or fibrillated. This condition, on drying is the source of the compacting bonding force, which holds the fibers and by contraction, pulls the fibers more closely together. This reduces the air space between the fibers and increases the density of the finished sheet of paper.

Beating the virgin fiber for boardmaking is traditionally a batch process but continuous methods are being introduced.
Pulp is fed into beaters (e.g. Hollander type shown) which consist of a hard cylinder situated in a drum cavity. The gap between the roller and the base plate is gradually reduced as the stock is circulated. The pulp at this stage is 99% water. Beaten stock is fed to storage chests prior to refining.

Reused materials (e.g. wastepaper for container middles) are disintegrated into individual fibers by Shartle pulpers. Bales of waste are fed into a continuous channel of hot water and pulp mixture. They pass into a disintegrating chamber fitted with a large toothed roll where they are broken down. The pulp is then pumped up to a higher level for recirculating. When the fibers are sufficiently freed, the pulp is emptied into a tank or fed into refiners.

The effect of this mechanical beating on fibers is to:

a) Shorten the fiber length by transverse cutting.

b) Remove the outer wall of the fiber.

c) Absorb water causing the fiber to swell and rupture, thereby releasing small "tentacle like" projections called fibrils. (This is known as fibrillation).

d) Split longitudinally forming ribbons of fibrils.

e) Decrease the stiffness of fibers causing their collapse.

Fibrillation occurs over the whole surface of chemical fibers which account for the very versatile fiber produced. On mechanical fibers, however, fibrillation only takes place where the fibers are actually split or broken owing to the percentage of lignum still present.

The greater the beating, and, therefore, formation of fibrils, the greater the bonding of the paper web, and the smaller the gaps between fibers. Chemical fibers for Kraft board would be beaten heavily compared with mechanical fibers used for tissue where large gaps between fibers in the mat give the paper its absorbency properties.

Refining is the second stage in stock preparation and is a continuation of beating, whereby the fibers reach full hydration without being reduced in length. Cone refiners such as Jordans are used in series, the stock being cycled.

Cone refiners consist of a central rotating cone with wooden longitudinal strips which fit closely into an outer shell. Stock at this stage is approximately 99% water, and is pumped with the direction of the central cone.

The passage of stock in refiners is much faster and smoother than in beaters. "Rosin" size is added to the pulp at the refining stage; this is used to impart water resistance properties in the finished board.

Stock of "finished stuff" as it is known at this stage, is fed directly to the board machines for web forming or to stuff chests for storage.

A continuous treatment of fibers, particularly from reused sources, is with hydrapulpers. These are large steel vessels into which waste paper etc. is conveyed. A series of rotating blades in the base reduce the waste to fibers and contraries are channeled out on a vertical bucket elevator connected to the base of the pulper, or attach themselves to a dangling derager rope. From hydrapulpers the stock would pass to Jordans for refining.

Web Forming

The action of web forming is common to all types of board and involves the fibers in suspension being fed in one or more layers onto the moving wire or felt of A board machine. They are shaken, some of the water is removed, and the fibers are consolidated. Three types of machine are used in the production of the board;

  1. Fourdrinier machine

  2. Cylinder machine

  3. Inverform machine

a) Fourdrinier Machine

This type of machine was first used early last century and produces a homogeneous board, In essence, it consists of a box filled with stock, feeding a continuous moving wire on which the fibers form the board web.

Stock at 1/2% consistency is held in stuff chests prior to feeding into the machine headbox. This regulates the amount and consistency of the stuff which flows into the machine breastbox. Transversing the machine, the breastbox feeds the fiber suspension onto the wire, ensuring the even discharge of fibers at constant velocity with the minimum of turbulence. Situated at the mouth of the breastbox is the "slice" which is an adjustable metal blade or bar designed to maintain an even flow of randomly oriented fibers. The fibers on the flat wirebed form together, interbinding at the fibrils. Specific mesh sizes are required for different boards. On either side of the wire are adjustable deckle straps which ensure no fibers spread beyond the required deckle.

As the speed of the wire is greater than that of the stock when fed on, the water which flows out carries with it the smaller fibers, dies etc., leaving behind the longer fibers which are oriented in the machine direction. To overcome this, the wire is shaken from side to side as it moves forward to move some of the fibers into the cross direction of the sheet as formation begins to take place.

Backwater, containing fibers, drains downwards from the web, and is collected in trays pumped back to the stock preparation department for reuse. At this stage, the web has about 10% fiber consistency. More water is removed by atmospheric pressure as the wire passes over a series of vacuum boxes. The amount of water removed depends on the resistance of the fibers as the matt becomes progressively thicker, and the number of vacuum boxes in use. The maximum calmer of board depends on the effectiveness of water removal; this is approximately 305 microns.

Finally, the wire carries the web through a "suction couch roll" which removes as much water from the paper as can be drawn out by suction. The couch roll consists of an outer shell of bronze or stainless steel with a large number of perforations, connected to a suction box. Sufficient water is removed to make the fibrous mat self-supporting; it has a consistency of 20% fiber at this stage. The matt is then transferred to an endless "wet felt" of woven fabric, by which it is supported through the press section. The board machine wire is spray cleaned as it passes underneath for the next cycle.

2) The Cylinder Machine

Cylinder machines were introduced to make board much thicker than is possible on the single wire mesh of the Fourdrinier machine. It consists of a series of web forming cylinders (up to 8) which build up a thick matt of fibers onto a continuous felt.

Stock at about 1% consistency is fed into a series of vats in which a large mesh cylinder rotates. Two adjustable sills inside the vat are used to control the flow of stock and reduce turbulence. As the cylinder rotates it picks up randomly oriented fibers and permits the water to drain through the wire, leaving the fibers on the outside of the mesh. After it has been formed on the wire, the fiber mat is picked off, by an endless band of felt which passes over all the cylinder moulds of the machine and is pressed against each mould by a separate couch roll covered with felt or rubber. The action of the couch roll is to press as much water from the web as is possible, leaving the mat in a condition to receive further layers of fiber from successive cylinder moulds.

The consistency of the fiber on the underside of the felt at this stage is about 15%. Just below the felt, a shower sprays the mould surface with water and so washes it ready for further immersion in the stock. Most of the water draining through the wire, passes to the circulating pump, which returns it to the stuff chests. Successive layers of fiber are picked up from further cylinders as the felt moves. By using different fiber stocks in the vats, a great variation in board types can be made, for example, the board might have a sulphite lining, a Kraft exterior, with the middle layers made from pulped waste papers.

The lamination of the succeeding sheets, one to another is effected by pressure and water alone, while still in the formation or wet area of the machine.

From the last couch, the felt passes round a suction drum and into a pair of light press rolls. A top felt, which is used in conjunction with the pick up felt, joins the top of the sheet and thus sandwiches the sheet before it passes into the press section. Further water is removed from the underside of the felt by vacuum boxes situated under the felt. Before returning for fiber pick up, the felt is washed. The fiber mat is transferred to a second felt which transports it through the press section.

The Inverform Machine

This machine has been recently developed with the object of providing a new method for the manufacture of single and multiple sheets at higher speeds than could be obtained with the Fourdrinier or Cylinder machines.

The fiber mat on a Fourdrinier machine is drained downwards, and only one layer of fibers is formed as efficient drainage beyond this cannot be achieved at high speeds. The Inverform machine overcomes this problem by upward removal the water from the formed web. Each Inverform unit (up to 5) can lay and remove water from the web of fibers, thus heavy boards can be made with different fiber "layer" make-up.

Each Inverform unit has a flow box which is kept full from stuff boxes. This regulates the flow of fibers onto the bottom wire. A shaking action on the bottom wire ensures that all fibers will be randomly oriented.

A top wire sandwiches the fibers shortly after their formation and they pass into the "Autoslice". This is a stiff, beveled scraper blade pressed lightly against the top wire. Water is forced upwards through the top wire, swept up the sloping blade and returned to the machine pit. With a single ply board, or the bottom layer of a multiple, a quantity of water passes downwards, but very little from other plies can drain downwards and, therefore, must be removed upwards. Further water is removed after the autoslice by inverted suction boxes, conventional boxes are used on the first layer. More water is removed by a rotary vacuum box and the bottom wire then carries the formed web to the next Inverform unit. The top wire passes overhead, is washed and recycled. Up to 5 layers of fibers can be built up with the Inverform system. The formed web is transferred to a felt before passing into the drying section.

The main advantages of the Inverform machine over the Fourdrinier are the speed of formation and the greater weights of board that can be achieved.

Drying and Finishing

Press Section

This normally consists of a series of light presses gradually increasing in weight, whose function is to remove as much water as possible from the sheet after it leaves the wire and before it enters the drying cylinders. The fiber web is transferred from the board machine wire or felt to a separate transporting felt. At this stage, the water content is about 80% and the wet board is protected from the crushing action of the rolls by a top felt. The water content is reduced to 60% by the presses some of which may incorporate suction pressing.

Before leaving the press section, the board passes through a series of finishing presses, carried on close weave felt to remove any felt marks made by the main presses on the wet sheet. Finishing presses are highly polished rolls of stone or metal. When the board leaves the last of the main presses, it is dry and firm enough to be self supporting, and the felts are cleaned and recycled.

Drying Section

The drying section consists of a series of hollow cylinders heated by steam, the purpose of which is to drive moisture out of the board by evaporation. The cylinders are 3 - 5 ft. in diameter, are accurately bored inside and the outer surfaces are polished. Doctor blades are pressed against those portions of the cylinders not in contact with the board, to ensure the cylinders are kept clean and polished. Board is threaded between the cylinders and water vapor blown from the spaces between the cylinders by air jets. The board is dried down to about 4% moisture and during this time, the fibers lose their water, shrink and bond, producing an increase in strength of the web as it travels along the drying section.

Callendering

The water content of the board on leaving the drying section is lower than the surrounding atmosphere. It is, therefore, raised to normal by being passed over a "sweat roll" through which cold water is circulated. Water vapor in the atmosphere condenses on the roll and the moisture is taken up by the board. The board finally passes through a callender stack which consists of a series of highly polished rolls, designed to impart a smooth surface to the board, which is effected by consolidation. These rolls are arranged in a vertical stack and are friction driven by the bottom roller.

Following this surface treatment, the board is reeled up, thus completing its manufacture.