PAPER MANUFACTURE



A/ Stock Preparation

-  the interface between the pulp mill or pulp warehouse and the paper machine
-  in integrated mill begins with dilution of the heavy stock at the discharge of the high-density pulp storage chests and ends with the blended papermaking furnish in the machine chest
-  in the independent paper mill begins by feeding pulp bales into the repulping system
 

Objectives:

- to take the required fibrous raw materials (pulps) and non-fibrous components  (additives)
- treat and modify each furnish constituent as required
- combine all the ingredients continuously and uniformly into the papermaking  stock
 

Operations:

Repulping – baled pulp (or other fibrous material) is dispersed into water to form a slush or
                   slurry. The operation can be either batch or continuous.

Refining (or beating) -  the fibers are subjected to mechanical action to develop their optimal papermaking properties with the respect to the product being made. The
operation is usually continuous, but some non-wood and specialty pulps are still treated batch-wise.

Utilization of Wet-End Additives – a wide variety of mineral and chemical agents are added to the stock, either to impart specific properties to the paper product or
to facilitate the papermaking process. Preparation is usually carried out batch-wise.

Metering and Blending – the various fibrous and non-fibrous furnish components are continuously combined and blended to form the papermaking stock.

Additional Operations – pulp screening and cleaning, secondary stock preparation system, white water and broke handling systems, etc. Classification of wet-end chemical and mineral additives
 

Functional additives

- Fillers                                (e.g., kaolin clays, talc, TiO2) – improve optical and surface and optical and surface properties
- Sizing agents                      (e.g., rosin) – control penetration of liquids, help to repel

water
- Dry-strength adhesives       (e.g., starches, gums) – improve burst and tensile; add stiffness and pick resistance
- Wet-strength resins            add wet strength to such grades as towelling and wrapping
- Dyes                                 (dyes and pigments) – impart desired color
- Optical brightners               improve apparent brightness
 

- Control additives

- Retention aids                   improve retention of fines and fillers, polymers bind to fines, overcome surface charge
- Drainage aids                   increase water removal (rate) on wire

- Acids and bases               control pH
- Alum                                [  Al2(SO4)3 ]  control pH, fix additives onto fibers, improve retention
- Fiber flocculants                improve sheet formation
- Defoamers                        improve drainage and sheet formation
- Pitch control chemicals      prevent deposit/accumulation of pitch
- Slimicides/biocides             control slime growth and other microorganisms
- Specialty chemicals           corrosion inhibitors, flame proofing and antitarnish, chemicals etc.
 

B/ Paper Machine (PM)

1803 – 1807    patents issued to Fourdrinier brothers for improved continuous paper machine
1809 patent issued to John Dickinson for the cylinder paper machine (England)
1817 first cylinder machine in America
1827 first Fourdrinier machine in America


Sections of PM:

1/  Forming                                application of slurry to screen
2/  Draining                                water removal by gravity or vacuum
3/  Pressing                                squeezing water out
4/  Drying                                   removal of water by heated surface
5/  Post drying operations            surface processing

1/ Forming section

a) Flow spreader – takes the incoming pipeline stock flow and distributes it evenly across the machine from back to front.
b) Head box – is a pressurized device that delivers a uniform pulp slurry on the wire, through the slice, at approximately the same velocity as that of the wire.  Original headboxes were open, unpressurized, and used hydrostatic head for the necessary pressure. A secondary headbox may be used part of the way down the table to give a
top coat of high quality fiber relative to the rest of the sheet. This is done, for example, to produce a white printing surface on linerboard or to put secondary fiber in the middle layer of linerboard, where contaminants such as polymers and wax are hidden. Paper machines operating above 2500 ft/min (800m/min) require a special, high pressure headbox known as a hydraulic headbox.
 

2/ Draining section

c) Wire, forming fabric -  endless, moving fourdrinier fabric forms the fibers into a continuous matted web while the fourdrinier table drains the water by suction forces. Continuous loop or belt of finely woven screen made from wire or plastic; the mesh size varies from 40 to 100 mesh (opening per inch). A coarse wire allows faster drainage but gives a coarser paper; as in most aspects of pulp and paper, there is always a tradeoff in goals. The forming media has three functions: 1. to
transport the fibers, 2. to permit draining the sheet, 3. to transmit power.
d) Deckle board  -  are used to prevent the stock from flowing off the two sides of the forming  fabric when a thick layer of stock is delivered to the fabric by a large slice opening.
e) Foils  -   stationary blades 5-10 cm wide. With stock on the fabric, and the fabric in motion, the suction that develops causes the fabric to drawn down towards the foil surface. Foils have two functions apart from supporting the fabric: 1. to provide hydraulic shear, 2. to give uniform, controlled water removal.
f) Lovacs, Hivacs  -  suction boxes to drain the sheet using low or high vacuum.
g) Dandy roll  -  hollow, light, wire covered roll that rides on the top of the fourdrinier wire just ahead of the suction boxes.
This roll has four purposes:

1. to impart a water mark to the sheet,
2. to improve the top surface for printing,
3. to improve formation by mechanical shear,
4. to increase the drainage capacity of the flat wire.

h) Couch roll  -  guide or turning roll for the fourdrinier wire, where the paper web leaves the wire and the wire returns to the breast roll.
It has two purposes:
1. to transmit power to the fabric,
2. to increase the dryness of the sheet applying the suction by the vacuum.

i) Twin wire formers – machines that use a jet of stock imparted on two converging wires to accelerate water removal and maintain better web uniformity. Theses are particularly useful for the high speed machines, where the fourdrinier wet end would tend to give two-side sheet, since both sides are wire sides and the sheet is formed symetrically on the two sides.
j) Cylinder machine – with a cylidrical forming wet end was invented in 1809.the web is formed an a rotating cylindrical screen 36 to 60 in. in diameter and immediately picked off. Typically , 5 to 10 of these operate in series to make a multi-layer sheet. Used to make heavy weight board from secondary fiber, which is not de-inked, for folding boxboard, chipboard and gypsum board. A high quality fiber surface may be added for printing upon.

3/ Press section

the sheet is conveyed through a series of roll presses where additional water is removed and the web structure is consolidated (i.e., the fibers are forced into  intimate contact). A press is a pair of squeeze or wringer rolls designed to remove water mechanically and smooth and compress the sheet.

4/ Drying section

most of  the remaining  water is  evaporated and fiber-to-fiber bonds are developed as the paper contacts a series of steam–heated cylinders. Removing water   from the web is accomplished by adding heat to the web and circulating the air. Heat is applied by the pressurized, steam filled circular steel or cast iron dryer drums. The rate of water removal depends on many factors:
1. the temperature and amount of steam entering the dryer,
2. adequate removal of the steam condensate and air from the interior of the dryer,
3.  the amount of sheet-dryer surface contact, contact time, and contact pressure,
4. cleanliness of the drum’s exterior and interior surface,
5. type of felt and condition of felt,
6. circulation of hot, dry air.
The initial drums cannot be too hot, otherwise large amounts of liberated steam will cause the paper to cockle. The paper often stretches a few percent in the machine direction while contracting in the cross direction through the course of drying, causing the machine direction to be stiffer and to have a higher tensile strength than the cross machine direction. Sophisticated control of individual dryer motors controls the tension throughout the dryer section and reduces web breaks.

Dryer hood  -  an enclosure around the dryer section and is used to improve drying efficiency by removing the moist air near the surface of the web. If this air is not removed, it quickly becomes saturated with water, preventing further water removal from the web.

Yankee dryer  -  is a large dryer drum (3.5 – 4.5 m in diameter) for drying tissue papers that are not strong enough to endure numerous felt transfers. The Yankee dryer is normally the only dryer used to dry tissue. The creping blade (doctor blade) is the thin metal blade that scrapes the dry tissue off the Yankee dryer.

Size press  -  is located between dryer sections and consists of pair of squeeze rolls mounted horizontally, vertically, or at a 45 o angle. It is used to apply surface size (usually a starch solution) to paper.
 

5/  Post drying operations

k/   Calander  -  calander stack is a series of solid rolls, usually steel or cast iron, mounted horizontally and stacked vertically. Dry paper passes between the rolls under pressure, thereby improving the surface smoothness (for example, from imperfections caused by felt marks, cockles, lumps, fibrils, etc.) and gloss and making  the caliper more uniform, if not decreasing the caliper and porosity. The bottom roll is called the king roll. The roll above the king roll is the queen roll.

l/    Supercalander  -  is similar to the calander but uses alternate hard and soft, heated rolls. Supercalandering may actually increase fiber bonding with an increase in strength, but the compact sheet has decreased opacity. It is usually done off-machine because of the relatively low speed required.

m/   Reel  -  is the last unit on the paper machine that collect the paper. The paper is wound on a spool that rotates against a drum. A new spool is started on a secondary arm before the filled reel is discharged, for changing reels “on the fly”.
 

Moisture profile on PM:

0.3 – 0.6 %  - dilute suspension of fibers is applied to an endless wire screen or plastic fabric
18 – 23 %    - consistency of the web leaving wet end. Water is removed by gravity, or the vacuum developed by table rolls, foils or suction equipment, and the drilled couch.
35 – 55 %    - consistency of the web leaving the press section
94 – 98 %    - dryness of final paper leaving drying section

4. Paper Machine:

The paper machine operation starts with stock preparation where wood fiber and recovered wood fiber is mixed with water and mineral (commonly clay or calcium carbonate and titanium dioxide). A mixture of 99+% water and slightly less than 1% papermaking material is blended prior to use on the paper machine. Water is drained
from the fiber slurry on modern machines in both upward and downward directions to form a web of paper. The web of paper is approximately 20% fiber and 80% water at the point it enters the press section. In the press section, the paper is squeezed between rollers and a felt, reducing its moisture content to 40-50%, at which point it enters the papermaking dryer section. Here steam-heated dryers further reduce the moisture content of the paper to a level of 2-6%. In our high quality machines, a size press coater applies a light coating of starch and mineral (calcium carbonate and clay), which enhances the surface strength and smoothness of the paper. Since this coating is applied in a water suspension, the paper must be dried in the after-dryer section. Finally, the paper is spooled into a giant roll of
paper.

5. Blade Coater:

In our coated paper processes, a blade coater is used to apply coatings that provide high quality print characteristics for sheet offset, web offset, or rotogravure printing applications.

6. Supercalender:

The supercalendering machine is a series of rollers. Paper passes through rollers alternatively made of steel and cotton or a synthetic material. This calendering takes place under high pressure and temperature, and creates a glossy, smooth and attractive surface. After supercalendering, the paper is cut into roll sizes that will eventually be shipped to our web offset or rotogravure customers, or to our sheet converting operation for converting to sheets.

7. Sheet Converting:

In the sheet converting operation, rolls of paper are cut into sheet dimensions that are used by our sheet offset customers or by our office paper customers. The converting operation also packages our products and prepares it for shipment to our customers.
*Two other pulping methods are used at Consolidated: Mechanical pulping and TMP. The most common method of mechanical pulping is the groundwood process where wood is pressed lengthwise against a grinding stone. This causes the surface of the wood to abrade away. The fiber is then mixed with water into a slurry. TMP, or thermomechanical pulping, is another important process at Consolidated. In the TMP process, chips are subjected to high temperatures and pressures between rotating disks, which shred wood into fiber bundles or individual fibers. Both of these processes convert 95% or more of the dry wood into pulp.
 



 

FIBER LENGTH, PRINTING AND STRENGTH PROPERTIES

Effect of fiber length on paper properties:
    Property  Effect on paper as Fiber Length is
                                                   Increased             Decreased

Bursting Strength                          increases            decreases
Folding Endurance                        increases            decreases
Formation                                    becomes wilder   becomes less wild
Print Quality                                 becomes poorer  becomes better
Surface levelness                         decreases           increases
Surface Smoothness                     decreases           increases
Tearing resistance                         increases           decreases
Tensile strength                             increases           decreases
 

Effect of refining on paper properties:
    Property  Effect on Property as the Degree of refining is
                                               Increased        Decreased

Apparent Density                     Increases        Decreases
Caliper                                    Decreases       Increases
Compressibility                        Decreases       Increases
Dimensional stability                Decreases       Increases
Formation                               More uniform   Less uniform, wilder
Ink holdout                              Greater           Becomes less
Internal bond strength               Increases       Decreases
Porosity                                   Decreases      Increases
Smoothness                             Increases        Decreases
 

Filler’s Influence

Fillers- beneficial influence on optical properties related to printability Tend make paper less sensitive to moisture change, improve dimensional stability
Reduce bulk and stiffness, make paper softer
Reduce paper strength-burst, fold, tear, tensile, internal bond strength

Effect of fillers on paper properties
                                               Increased        Decreased

Property                                  Increased        Decreased
Brightness                               increased         decreased
Bursting strength                     decreased        decreased
Caliper                                   decreases         increases
Dimensional stability                increases         decreases
Folding endurance                   decreases         increases

Ink absorbency for uncoated      more uniform  becomes less uniform

Opacity                                   increases          decreases
Picking resistance                   decreases          increases
Rattle                                     decreases          increases
Smoothness                            tends to increase tends to decrease
Tensile, tear                           decrease            increase