Paper and Pulp Notes | NEB Class 12 Chemistry | NEPALI EDUCATE

Unit: 20

PAPER AND PULP

Introduction:

It is believed that paper originated in China in the 2nd century as a writing material. In the early days, papers were developed from papyrus grass, mulberry barks, hemp rags, banana fibers, bamboo, etc. Production was introduced to Europe in the 12th century.

In Nepal, it is presumed that paper making was adopted in the Lichhavi dynasty (400-750AD). Paper was developed from pulp extracted from plants available in hilly regions like Allo plant, Lokta plant, and Argeli. The paper obtained from these plant products is known as Nepali Kagaj. In the industrial level, Bhirkuti Kgaj Karkhana was established in 2039 in Gaidakot, Nawalparasi. This industry used to produce nearly eight to twelve metric tons of paper per day and could supply nearly 605% of the national demand. These days, this industry is not working.

Pulp:

Pulp is commercial fibrous material derived from cellulose of plants or plant products like bamboo, wood, bagasses, rice straw, wheat straw, reeds, etc. by chemical or mechanical methods. Pulp is the major raw material for the industrial production of paper. The process of production of pulp is called pulping in which long and bulky fibrous materials are disintegrated into individual, small agglomerate fibers. Pulp can be manufactured by chemical or semi-chemical or fully chemical method (Kraft and sulphite process).

Paper:

Paper is a thin sheet made of finer fibers derived from cellulose of wood and other lignocelluloses materials such as cotton, rice and wheat straw, reeds, grasses, bamboo, etc. In the paper, fibers are well sized, bonded, and intertwined to each other to form a dense and smooth sheet. Different degrees of quality of paper can be produced by comprising variation of raw material during production of papers.

Types of paper:

1. Bond paper: Stronger and more durable paper than average sheet paper. Perfect for letterheads, typed reports, and envelopes.
2. Glossed coated paper: It has a high shining surface. Typically used for brochures and flyers (advertising circular).
3. Matte coated paper: It is coated with matte materials. Its surface is dull. Perfect for reports, flyers, and leaflets.
4. Silk coated paper: It is smooth to touch without shine. Usually used for magazines, books, and catalogues.
5. Uncoated paper: It has advantages of being used by both printer and pen. It is used for textbook paper, forms, bills, letterheads, memo paper (brief written report).
6. Recycled paper: It is made of reused paper products. It can be used for reports, forms, documents, memo papers, etc.
7. Watermarked paper: It is high-quality paper and gives a feeling of luxury. It is used for important documents like exam certificates, grade sheets, transcripts, etc.
8. Tissue paper: It is used as napkin paper, toilet paper, in cigarettes.
9. Groundwood printing paper: For catalogues, posters, wallpaper, paper tablets.

Raw material of paper:

The main raw material for paper production is fibrous cellulose, which is obtained from lignocellulosic biomass. For high-grade paper, the fiber should be ideally long, have high cellulose content, and low lignin content. Bamboo is the ideal raw material for the production of paper, which can be used alone or can be blended with other sources like bagasse and hardwood.

Types of raw materials:

The raw materials used in the paper industries are of following types:

1. Conventional material types: e.g. bamboo, wood, etc
2. Non-conventional material types: e.g. rice straw, wheat straw, grass, reeds, etc.
3. Cotton type: e.g. cotton, cotton rags, jute
4. Waste product type: e.g. sugar bagasse, sawdust

On the basis of material used in the production of paper, raw materials are broadly two types:

Fibrous raw materials: All kinds of cellulose pulps obtained from wood, bamboo, reed, reuse pulp

Non-fibrous materials: Inorganic raw materials such as Na₂S, NaOH, Na₂CO₃, CaCO₃ etc. and organic raw materials such as wax, glycerol, resin, etc.

Followings are the requisites of cellulose raw materials for the pulp and paper industry.

1. They should be rich in cellulose content.
2. The raw material should be present in large quantity.
3. Raw material should be available throughout the year.
4. They should not undergo instant deterioration for storage.
5. They should yield good quality paper.
6. They should have low cost of collection, storage, and transportation.

Sources of raw materials:

Raw materials for paper production are obtained from natural resources or from a recycle base. The major parts of raw materials are of fibrous type while some non-fibrous raw materials like inorganic compounds (clay, NaOH, Na₂CO₃, CaCO₃, Na₂S etc.) and organic compounds (wax, glycerol, resin etc.) are also used during processing for pulp and paper production. Wood is the major source of pulp for paper production which covers nearly 90% of global pulp production. Other non-wood fiber sources are straw, bagasse, hemp, waste paper that share about 5-10% of global pulp production.

The sources of raw materials are:

1. Wood type: Woods are of two types; hard wood and soft wood. Hard wood contains vessels and are more porous such as acacia, eucalyptus, gum, birch, rubber plant wood, paper mulberry etc. Soft wood may be coniferous like pine, spruce and non coniferous wood like fir, larch, hemlock etc.
2. Grasses and reeds: Non conventional source of raw material.All tall plants with hollow stem e.g. Bamboo, jute, lemon panni, siru, babiyo, kush etc.
3. Straws: rice, barley, wheat, reeds, bagasses
4. Waste products: It includes sugar bagasses, saw dust, etc
5. Recycling materials:

Stages in production of paper:

The commercial method of paper production basically includes two steps in which fibrous raw material is first converted into pulp and then pulp is converted into paper. Several stages such as raw material preparation and handling, pulp manufacturing, pulp washing and screening, chemical recovery, bleaching, stock preparation and paper making involve in the production of paper. The main stages of paper production are

1. Raw material preparation and handling:

Pulp manufacture begins with the raw material preparation. Proper raw materials are collected and selected for the preparation of desired quality of paper. It includes various steps timbering, debarking (if wood is raw material), chipping, screening, chip washing, handling, and storage.

1. Timbering: Timbers are obtained from matured trees. The quality of paper depends upon the quality of timber. Especially coniferous plants like pine, spruce, and some other plants like eucalyptus, paper mulberry, rubber plants, hemlock etc. are used for paper manufacturing.
2. Debarking: Debarking is the process of removing the bark of trees. It is performed hydraulically with high-pressure jets or mechanically by rubbing logs with cutting tools. (Bark contains grits which are necessary to remove for increasing quality and on the other hand, it has low fibers content)
3. Chipping process: The debarked and stripped are chipped into small pieces (2-5 cm) using rotary disc and heavy knives. Oversized chips make handling problem for chemical pulping process. So, they are rejected by screening.
4. Screening and cleaning: The chips are further sent for the screening process where oversized chips are rejected. Then, the chips are cleaned by washing to ensure that they are free from bark and dirt. The chips are stored temporarily for further subsequent pulping operation.
2. Pulp manufacturing (Pulping):

The wood chips are carried for pulping operation where fibers are separated and lignin is removed or dissolved. One or more of the following processes are adopted for pulping.

1. Chemical pulping process:

The wood chips are fed into a digester (pressure cooker) and digested with steam at a specific temperature to separate the fibers and partially dissolve lignin as well as other extractives. Chemical pulping process is made by using the Kraft (sulphate) and sulphite process. The Kraft process is a modern pulping process in which wood chips are digested with water, sodium sulphide, and sodium hydroxide (white liquor) at elevated temperature 170°C -180°C and 10 atm pressures for approximately 2-5 hours. Here sodium hydroxide is used to dissolve lignin from the fiber surface. Then the cooked pulp is returned to the chemical recovery cycle to recover the chemical. The pulp is screened and filtered to remove uncooked wood and washed with water to remove spent cooking mixture. Then it is subjected to bleaching.

2. Mechanical process:

Rotating steel discs with teeth are used to tear wood part in the mechanical process. Mechanical process is further two types namely ground pulping and thermo-mechanical pulping (TMP).

3. Hydro pulping:

The wood fibers are brought into a circular tank provided with water arranged having a powerful agitator at the bottom which breaks the wooden fibers into small pieces.

3. Bleaching process:

After digestion (cooking), fibers pulp still contains some amount of lignin and other impurities that impart fibers brownish color. So, bleaching is carried out to refine and brighten raw pulp by removing the remaining lignin with bleaching agents like chlorine, hypochlorite, chlorine oxide, and caustic soda in several phases.

4. Screening and cleaning:

The pulp is pumped through a series of screens and cleaners to remove undesirable fibrous and non-fibrous materials.

5. Paper making process:

The bleached or unbleached pulp is again refined to enhance the formation and bonding of fibers before entering the paper machine. The pulp slurry is diluted with water to less than 1% fiber content and then cleaned in cyclone cleaners and screened in centrifugal screens. Then the dilute stock passes through a head box that distributes the fiber slurry uniformly over the width of the paper sheet to be formed.

The conversion of fiber into paper involves the following steps

1. Forming a wet web (paper web): The dilute fiber slurry is fed into the paper machine with the help of a moving belt of clothes wire at a speed of 50m/minute- 500m/minutes where water is drained under gravity to form a wet web.
2. Pressing the wet sheet: Paper web still contains 80% water which is passed through a pressure section containing steam heated cylinders to squeeze out the excess water. Here water amount is reduced to 60-65% with the help of mild pressure.
3. Drying process (Drying of sheet): The sheet thus obtained from the pressure section is passed to the drying section through smoothing roller followed by steam heated metal cylinders to obtain smooth and well-finished paper. Finally, paper is cut, coated, and packaged.

FLOW SHEET DIAGRAM OF MANUFACTURE OF PAPER:

Quality of paper:

The quality of paper depends upon the composition of raw materials of paper. Typically, a ‘paper weight’ directly relates to the thickness and stiffness of the paper. Different physical and mechanical properties are very important in determining the overall quality of the paper. Tensile strength, compression strength, bending, stiffness, tear and burst resistance, opacity, etc are some of the mechanical properties of paper.

Followings are the characteristics of paper to be considered for good quality of paper:

1. Brightness: The percentage of light that the paper reflects determines the brightness. Typically, coated papers have a better appearance than uncoated paper. Coated papers are heavier and glossier which influences the overall quality of paper.
2. Opacity: It is the measurement of light passing through a sheet of paper. It is directly related to the thickness and presence of high filling agent.
3. Strength: The mechanical properties paper sheet like tensile strength, breaking strength, elongation, tearing and burst resistance, folding resistance, and stiffness are considered and determined by using different testing methods for quality control of paper.
4. Texture: Texture of paper including smoothness and finish quality is the basic requirement for most printing papers for proper ink transfer. Surface smoothness of paper is well known to correlate with printability.
5. Finish: It characterizes the finish quality of paper surface such as glossy, stain, matte coated surface. It depends upon the purpose of paper applications.
6. Sizing: The role of sizing is to bind fibers and filling agents. It must be uniform and closed so that no bleeding takes place during the use of ink or drawing ink.
7. Clarity: It indicates the paper is either coarsely (large pieces) ground or finely ground.
8. Color: Color is used to impart colorful paper as required by its application.
9. Weight: The weight of paper is measured in grams per square meter. This quality is commonly called Grammages. The grammages of up to 200gsm are considered as paper and above 200 gsm are referred to as paperboard. Typical office paper has weight 80gm/m2.

These above are the important properties of paper; however, the requirements may vary based on the printing process.

Challenge in establishing paper industries:

Major challenge in establishing paper industries in the countries like Nepal is as follows:

• High cost of raw materials
• Expensive effluent treatment process.
• Lack of power supply
• Non-availability of skill manpower. etc.

Technical difference between sulfate (Kraft) process and sulfite process:

	Sulfate (Kraft) process	Sulfite process
1.	It is an alkaline process	It is an acidic process
2.	Pulp must be bleached more efficiently.	Pulp is bleached less efficiently.
3.	It is carried at 170-180°C and 10 atm.	It is carried at 125-160°C and 4-6 atm.
4.	Fibers have better strength properties.	Fibers have low strength properties.

Difference between pulp and paper:

	Pulp	Paper
1.	Pulp is the primary material that is used to make paper.	Paper is used as a sheet used for writing or printing.
2.	It is moist, soft and shapeless matter.	It is dry, smooth or matte and shaped.
3.	It is fibrous material obtained from wood chips.	It is a sheet obtained from cellulose fibers.
4.	It is an unrefined suspension of fibers.	It is refined, pressed, and finished sheet.