Paper unit i

PAPER

INTRODUCTION

• Paper is a thin material mainly used for writing upon, printing upon or for packaging.

• It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets.

Raw materials

Five different fiber classifications include:– bast or stem fibers, which are the fibrous

bundles in the inner bark of the plant stem running the length of the stem;

– leaf fibers, which run the length of leaves; – seed-hair fibers; – core, pith or stick fibers,– Grass and all other plant fibers not included

above

Bast fibers

Examples of bast or stem fibers include;– jute, – flax, – hemp, – kenaf,– ramie, – roselle, – andurena.

BAST FIBER

• Obtained from the flax plant.• Located inside the stem.• Only 5% usable for

papermaking.• Hemp and jute are other forms

of bast fibers.• Hemp has been used for

cigarette and Bible papers.• It is used for heavy duty

shipping tags and heavy pattern boards.

Leaf fibers• Leaf fibers include;

– banana, – sisal, – henequen, – abaca, – pineapple,– cantala, – caroa, – mauritius, and phormium

Cont…Seed-hair fibers include;

– coir, – cotton,– kapok, – and milk weed floss.

Center or pith fibers include;– kenaf – and jute.

Seed Hair Fibers—Cotton Plant

• Cotton fibers are very expensive.

• Used primarily for bank notes, high grade writing papers, maps—anything that requires exceptional strength and durability.

Grass Fibers

• Wheat Straw– Absorbs water readily.– Used to make low grade paper.

• Bagasse fiber– Made from crushed stalks of sugar cane.– Used as a source of fuel for Sugar Mills.

• Esparto Plant– Has very little strength and does not split into fibrils easily.

Grass Fibers

• Kenaf plant– Shows great potential

for papermaking.– May produce 5 to 7

times more pulp/acre than pine.

– Grows to maturity in 120 days compared to 20 years for most trees.

The remaining fibers include;• roots, • leaf segments, • flower heads, • seed hulls and• short stem fiber

Wood Fibers

• 95% of all paper is made from wood fibers.• Deciduous hardwood trees, e.g., Oak,

Gum, Maple, Aspen, etc.• Coniferous softwood trees, e.g., Pine,

Spruce, Fir, etc. (the softwoods produce a stronger pulp).

WOOD

STRUCTURE OF CELLULOSE

Cellulose properties

• It is abundant and replenishable

• It can be easily harvested and transported to its usage site

• Good tensile strength

• Great affinity for water

• They resist change or degradation by any chemicals bcoz of this unique characteristics.

Pulping

• The objective of pulping is to separate the wood into individual fibers.

• A pulp mill is a manufacturing facility that converts wood chips or other plant fiber source into a thick fiber board which can be shipped to a paper mill for further processing.

Pulping methods

Three broad classifications of pulping methods:– Mechanical (groundwood)– Mechanical (refiner)– Chemical (kraft process)– Chemical (sulphite process)– Combination (chemi-mechanical)– Combination (chemi-thermo mechanical)

Mechanical pulping

• Grinding-Stone Groundwood (SGW)– Logs (very occasionally

chips) are pressed into a turning stone thus releasing fibers.

Ground wood

• The bark is removed from the logs.

• Then, the cut logs are forced by hydraulic or steam pressure against a revolving grinding stone in the presence of water.

• This treatment converts the wood into a pulp consisting of minute particles of both fibrous and non fibrous portions of wood.

• The nonfibrous materials deteriorate when left for some time in contact with air.

• This type of pulp is not as strong as chemical pulp.

• Groundwood pulp has to be mixed with other pulps, e.g., Newsprint—contains 80% groundwood pulp and 20% chemical pulp.

Advantages

• Low cost and high yield

• This pulp has high bulk and opacity

• Excellent printing, cushioning and ink absorbency

• Disadvantages;– Low strength and brightness

Refining pulp

• Refining-Refiner Pulp – Chips are fed between 2 disks. One disk is always

turning while the other can be fixed or turning.

Cont…

Cont…

• Small cylinder or wood chip is placed between two opposite rotating plastic disc

• Chip disintegrates into coarse fibers

• Compression and decompression of rotating disc

• Heat generated splits the individual fibers.

Advantages

• Refiner mechanical pulp also utilizes waste chips and low grade wood than ground wood

• It has the high opacity and yeild

Chemical pulping

• It is used to isolate fibers from wood compartment is to remove the lignin

• Delignification is done by degrading the lignin molecules, bringing them into solution and removing them by washing

• Also a certain amount of carbohydrates (cellulose and hemicelluloses) is lost in this process

Sulphate process• The kraft process (or sulphate process) is

the dominant chemical pulping method worldwide

• Which involves cooking the chips in a solution comprising sodium hydroxide ( NaOH) and sodium sulphide (Na2S), with OH and HS- as the active anions in the cooking process

• The hydrogen sulphide is the main delignifying agent and the hydroxide keeps the lignin fragments in solution

Sulphite process

• The sulphite process involves dissolving lignin with sulphurous acid (H2SO3) and hydrogen sulphite ions (HSO3-) as active anions in the cooking process

• More recently devolped pulping methods include the use of organic solvents as ethanol, methanol and peracetic acid (CH3CO3H) for delignification .

TMP• To make TMP, wood chips or sawdust are

first softened by steam and then subjected, under pressure, to the defibering action of a disk-type refiner.

• This causes the fibers to completely separate from each other and suffer less damage than those produced by the conventional groundwood pulp process.

• TMP is cleaner and stronger when compared with groundwood pulp.

Advantages

• TMP is stronger than RMP and stone groundwood and it eliminate more costly chemical pulps that are blended with mechanical pulp

• It has high yield and high opacity

• It has better runnability on the presses.

CTMP

• In the chemi-thermo mechanical pulping, where the wood chips are treated with mild chemicals prior to the refining

• This mild chemical pretreatment improves pulp brightness to some extent

Bleaching technique

• All pulp requires bleaching to remove residual lignin before papermaking process

• The purpose of bleaching is to remove stains caused by lignin

• There are many different types of bleaching process, involving different chemicals and conditions

Bleaching agents

• Major bleaching agents– chlorine, – sodium hypochlorite,– hydrogen peroxide,– chlorine-dioxide, – and oxygen

Cont…

• Chlorine gas is one of the different bleaching systems that are used to remove lignin that remains in the pulp.– Chlorine gas is passed into the pulp-water

mixture.– The gas reacts with whatever lignin that is left

and becomes chlorinated.– The chlorinated lignin is then removed when

treated with the caustic soda NaOH (Sodium Hypochlorite)

Mechanical pulp bleeching

• After the refining process, the resulting fibers do not have the brightness required for most paper applications. Bleaching is therefore required.

• For the mechanical pulp, hydrogen peroxide, sodium hydrosulphite or formamide sulphuric acid are used for bleeching

• Chlorine was widely used in the past but few mills use it today.

Chemical pulp bleeching

• In chemical pulping, the majority of the lignin has been removed in the pulping stage

• In general kraft or sulphate process requires more bleeching than sulphite

• Sodium hypochlorite or hydrogen peroxide may be used for brightening chemically pulped fibers

Cont…

• Once the pulp has been screened, cleaned and bleached it is ready for the stock preparation

Stock preparation

• It involves many operations, including stock blending, refining, broke management, operation of the several system, and mixing and blending of wet-end additives.

• All these are important operations in determining the quality of paper and paper machine efficiency.

Cont…

• The pH is controlled and various fillers, such as whitening agents, size and wet strength or dry strength are added

• Additional fillers such as clay, calcium carbonate  and titanium dioxide to increase opacity

Fibers & non fibers

• Fiber - Selected for properties & cost

• Non-fibrous Additives - Selected to obtain properties not inherent in fiber

– Strengthening Agents (Starch)

– Fillers (Clay, CaCO3, TiO2, etc)

– Internal Sizing Agents (Rosin,etc.)

– Coloring agents

– Other

Process • Pulp is pumped through a sequence of

tanks that are commonly called chests.

• Chests may be either round or rectangular made with special ceramic tile faced reinforced concrete, mild and stainless steels are also used

• Pulp slurries are kept agitated in these chests by propeller like agitators near the pump suction at the chest bottom.

Different levels

• Production level– To break down the raw material into individual

fibers, – to separate fibers and contaminants, – to separate fibers/solids and water, – to treat the fibers, and – to treat the residual contaminants.

• Recovery level – where fibers and other solids, and water are

recovered from the rejects of the separation processes applied in the production level. Again fibers and contaminants as well as solids and water have to be separated.

• Discharge level– for ecological and economic reasons the

final rejects are separated into wastewater and residues with high dry content.

Effect of adding long fiber

Effect of refining on pulp/paper properties

Pulp Composition• High Hemicellulose content - Easy to

refine, strong & dense sheet.

• High Lignin - Difficult to refine, and

inhibits swelling.

• High Carboxylic Acid Content – Causes fiber swelling, and ease in refining

• pH - Easier at high pH, but difficult to

generalize.

FILLERSFillers are applied to the paper mainly;

• To improve the optical properties, such as brightness and opacity

• To improve the smoothness of the sheet surface (i. e. decreased roughness, especially after calendering)

• To improve the sheet formation by filling the voids between the fiber matrix

Cont…

• To enhance printability in the various printing processes due to a more uniform paper surface, higher opacity and better ink receptivity.

• The latter resulting in reduced printing ink penetration, wicking and ink strike-through to the opposite side of the sheet

Cont…

• To improve the dimensional stability of the paper as most fillers remain inert when wetted, unlike the natural fibers usually used in papermaking.

• To improve the permanence of the paper (alkaline papermaking, calciumcarbonate (CaCO3) filler)

Sizing

• Sizing makes the fiber hydrophobic and thus prevents or reduces the penetration of water or other aqueous liquids into the paper.

• Sizing prevents the spreading and strike through of ink or printing colors.

Cont…

• Papermaking fibers have a strong tendency to interact with water.

• A high absorbency is important for a few paper grades such as toweling and tissue.

• Also corrugated medium paper must be “absorbent” to a certain degree to convert properly in the corrugating process.

Cont…

• On the other hand such properties are disadvantageous for many paper grades e. g. liquid packaging, top layer of corrugated board, writing and printing papers and most of the specialty papers.

• The water and liquid absorbency can be reduced by the addition of sizing agents to the paper stock and/or by their application to the paper surface.

Process

• Sizing is usually performed in a size press or a film press.

• In a size press, the web is passed through a pond of the sizing agent, which is located above a roll nip.

• As a result of both capillary action in the pond and the hydraulic pressure in the roll nip, the paper web absorbs the sizing liquor.

Cont…

• The amount of size pick-up and the degree of penetration depend upon the– the concentration and viscosity of the size, – the absorption behavior (porosity, moisture

content, temperature, etc.) of the paper web,– and the nip pressure and nip length. – Control of the size pick-up is mainly by

variation of the size concentration, but also by variation of the nip pressure.

Dying

• Stock dying

• Surface dying

• Dip dying

Stock dying

• This process is called internal dyeing and is the most widely used paper dyeing process.

• Because of clean working conditions and the most efficient usage, the dyes are now mostly added continuously and fully automatically into the stock flow.

• choice of dye and the fixing and dyeing conditions largely depend on the raw materials used in papermaking (recycled fibers, stone groundwood, TMP, CTMP, unbleached or bleached chemical pulp, type and portion of filler) and on its preparation process

• Eg. higher degree of beating of the pulp results in a deeper coloring

• Fillers increase the required amount of dyes because they absorb dyes and, at the same time, reduce the coloration

Cont…• Addition of dye is determined by a few

factorse. g. high consistency dyeing at a stock consistency of 3–4 % (before mixing with white water ahead of the headbox)

• Alternatively low consistency dyeing at a stock consistency of 0.5–1.5 % (in front of the mixing pump or pressure screens).

• The pH conditions are very important.

• The addition of aluminum sulfate usually promotes the absorption of dyes and yields less colored waste water and effluent.

• In general, there is a trend towards paper production in the neutral or alkaline pH range. These conditions need dyes with a very good affinity to the paper stock in a neutral medium and/or very effective fixatives and retention aids.

Advantages and disadvantages

• Batch addition has the advantage of thorough mixing of the additives with the paper stock and optimal fixation due to longer contact time between the fiber and dye.

• The disadvantages are that the time required for color correction and color change is relatively long (loss of productivity).