Papermaking Raw Materia & Their Characteristics II

8/12/2019 Papermaking Raw Materia & Their Characteristics II

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PAPERMAKING RAW MATERIALS AND

THEIR CHARACTERISTICS

ByDr. Chhaya Sharma

INDIAN INSTITUTE OF TECHNOLOGY, ROORKEE

SAHARANPUR CAMPUS,DEPARTMENT OF PAPER TECHNOLOGY

SAHARANPUR247001 (INDIA)


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CONFORMABLE

DEVELOP STRONG BONDS


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Paper is a felted sheet of Fiber.

The basic requirement of fibers to be useful

for papermaking, Conformable & developstrong bonds

The degree of fiber conformability can be

measured as sheet formation, while the

degree of bonding is measured by the burstor tensile strength of the sheet.

Some pulps are useless for papermaking in

their raw state because the fibers are

relatively non-conformable and non-bonding.

For example, cotton and linen rags (which

are still used to produce high quality, durable

papers) must be mechanically treated to

develop the desired properties.


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Table. Properties of Cellulosic Fibers

high tensile strength

flexibility, conformabilityresistance to plastic deformation

water insoluble

hydrophilicwide range of dimensions

inherent bonding ability

ability to absorb modifying additives

chemically stable

relatively colorless (white)


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The hydrophilic nature of cellulosic fibers plays an

important role, since the papermaking process

occurs in an aqueous medium.

The fibers readily absorb water and are easily

dispersed in a water suspension.

When wet fibers are brought together during the

sheet-forming operation, bonding is promoted bythe polar attraction of the water molecules for each

other and for the hydroxyl groups covering the

cellulose surface.

As the water is evaporated the hydroxyl groups ofcellulose surfaces ultimately link together by

means of hydrogen bonds.


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Illustrating different levels of hydrogen bonding

(A) Loosely through water molecules

(B) More tightly through monolayer of water molecules

(C) Directly


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While individual cellulosic fibers generally have

high tensile strength, the strength parameters of

paper are also dependent on the bonds

between fibers.

Beating or refining tends to optimize bonding at

the expense of individual fibers strength.

Of course, the original fiber strength depends

on the raw material and the method of pulping.

Since most paper products utilize non-fibrous

additives in their manufacture, the ability of the

pulp fibers to absorb or retain a wide variety of

modifying materials is important.


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FIBER MORPHOLOGY

INTRODUCTION

Fibers constitute the basic raw material for paper. Thiscategory of fiber includes nonplant fibers and plant fibers.

Non Plant FibersNon plant fibers can be of following type:

Animal Fibers (wood, hair, silk)

Mineral Fibers (asbestos, glass)

Synthetic (Rayon, Nylon etc.)

All these nonplant fibers are incorporated only in some speciality

products.


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Plant Fibers

Almost all growing plants have been tried

for pulp & papermaking, and technically it

has been possible to produce satisfactory

products also.

However economics plays an important

role here and the consideration of

collection, storage, and processingsometimes put limit to the use of some

materials.


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All these fibrous raw materials range

from tiny grass plants to all trees.

Since they all are the metabolic products

of living organism, it is certain that they

will have infinite variations due to geneticand environmental factors.

There is a need to understand

scientifically and technically these rawmaterials w.r.t its anatomical and

morphological characteristics in order to

obtain a correct and optimized utilization.


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THE PLANT KINGDOM

Plants kingdom can be divided into 4 main

divisions:

Thallophytes

Bryophytes

Pteridophytes

Spermatophytes

These seed bearing plants are only of use to

pulp and paper maker.


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Spermatophytes (Seed bearing plants)

Gymnosperm (Naked seeds)

Conifers, Evergreen, SoftwoodAll woody

Angiosperms (Covered seeds)

Monocotyledonous

Non woody

Dicotyledonous

Woody or

Non-woody


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DEFINITION OF WOOD

Plants living for many years. (Perennials)

The stem must not die back every year. Even if

the roots may survive and produce a stem next

year, these plants are not classified as woody.

Plants must possess a vascular system and have a

specialized conducting system. (Xylem and

phloem).A cambium between xylem and phloem

producing secondary growth.

Classifying a plant as woody means that the

following criteria must be fulfilled.


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PRIMARY GROWTH

The woody stem has an axial core of vascular tissues,

protected by the bark on the outside.

When a tree grows it increases with age both in

length and diameter.

The growth is due to the activity of specialized cellscalled MERISTEMS (greek means divisible).

The meristems are situated at the tips of the shoots

and the roots.

These cells divide, grow, differentiate, and cause the

extension of the plant.


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SECONDARY GROWTHXYLEM-PHLOEM

A tree grows in thickness by the activity of the

CAMBIUM, a single layer of living meristematiccells.

These cells produce sapwood or XYLEM on the inside

and bark or PHLOEM on the outside.

As the tree grows larger, the inner core of theSECONDARY WOOD (xylem) ceases to function

actively. This part called the HEARTWOOD functions

only as mechanical support.

In many species of wood the heartwood is darker than

the outside active part called the SAPWOOD.

That part is a complex tissue both strengthening and

conducting water and solutes.

The main function of the phloem (the INNER BARK)

is transport of photosynthetic products.


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Outer bark

Inner bark


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Plant organization

Three major tissue system: - 1. Dermal 2. Vascular 3. Fundamental.

Dermal: -

In its early life plant is covered by one to several

layers of cells called epidermis which function in

photosynthesis, gives protection, regulate gasexchange.

Epidermal cells are impregnated with hydrophobic

wax like substance cutin.


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Vascular System: -

It is the conducting or circulatory system of higher plants and

is composed of two major complex system xylem andphloem.

The xylem functions principally in movement of water, soil

nutrients and stored food from root system upwards to

developing leaves and buds.

Hormones and photosynthates manufactured in trees crown are

then transported downward direction by phloem to developing

stem and roots.

Another function of xylem in woody plants is to provide

mechanical support.

This role is fulfilled by Fiber cells which are dead and

heavily lignified at maturity.


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Fundamental System: -

These cells are relatively unspecialized and can have various

physiological functions.

These cells are commonly referred as ground tissue also often

acts as a sort of filler tissue. These categories can be

distinguished.

Parenchyma.

Collenchyma.

Sclerenchyma.


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Parenchyma:-

In softwoods and hardwoods parenchyma are small, often

living cells functioning for photosynthesis, storage, secretion

and wound healing.

They are commonly thin walled, but in central regions of

stems and roots may be thick walled and heavily lignified.

They may also accumulate crystals (e.g. cal. oxalates,

Tannins (polyphenols) or food materials like starch fats, oil

etc.

In monocots (non wood) parenchyma are much more varied

in size, and can be as large as fibers in some plants.


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Collehchyma:-

They have a large similarity to that of parenchyma, however,

these cells are a bit longer and have special wall thickening.

The cell walls are rich in pectin.

These are often located near the surface of young stems andin leaf veins.

They are not of any relevance to wood pulping but are

present in some grasses in large volumes along with

parenchyma and it is desirable to separate them from fibercells e.g. bagasse.

Sclerenchyma:


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Sclerenchyma: -

These are usually dead cells at maturity; they are thick walled and

heavily lignified functioning primarily for mechanical support.

There are two types of cells which can be distinguished (a) fiber(b) sclereids.

Fibers are similar to xylem, sclerenchyma fibers are extra xylary

in location.

Such as phloem of dicots or bast fibers (jute, help etc.) present in

vicinity of vascular bundles.

Sclereids are variably shaped cells often branched .

They give hardness and rigidity to tissues like phloem, leaves,

seed coats, shells etc.

In wood pulping they may enter by way of bark, this may give

rise to stone cell problem or dirt or speck in high quality papers.


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Sl. No. Pulp FIBERS Parenchyma Vessels Epidermal

Cells

Length Width SpecialFeatures

(1) (2) (3) (4) (5) (6) (7) (8)

i) Rag 2900

(780-8000)

mostly

incomplete

24 (10-34 ) Flat, ribbon-

like and

twisted,

occasionally

with torn,

base

- - -

STRUCTURE OF SOFTWOOD


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More than 90% of the cells in softwood are tracheids. A

TRACHEID is a hollow tubular cell with closed pointed ends

with a major function as a liquid conducting element.

The cavity inside the cell is called Lumen.

The length of the tracheids ranges from 2.5 to 7 mm. (Average is

3.5 mm).

The width is more constant and ranges from 30 to 45 um. This

gives average length versus diameter 100:1 for softwood fibers.The tracheids in the summerwood develop thick walls and a

narrow lumen and may be termed FIBER-TRACHEID.

Softwood fibbers are highly appreciated because of their fiber

length and conformability, and the source of so called long

fibered woodpulps.Neighbouring tracheids are joined by so called Bordered pits for

inter fiber conduction.

Usually the bordered pits are present only on the radial walls of

the tracheids.


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Major types of cell composing softwood are listed below.

Orientation of Cells Major Axis in the Tree.

Vertical (Longitudinal) Horizontal (Radial)

1. Trachied (Fiber) 1. Ray Trachied

2. Storage parenchyma 2. Storage parenchyma

3. Epithetial cells 3. Epithelial cells

Wall Anatomy


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Wall Anatomy

Softwood fiber wall are interrupted by pits which makes the

major path-way for upward or interfiber condution of liquid sap.

Which are called borderedpits. Some times they have another

wall making called spiralthickening.


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(A & . B)Softpine., (C)Hardpine, (D)Doughlasfire

Sl.

No.

Pulp FIBERS Parenchyma Vessels Epidermal

Cells

Length Width Special Features

(1) (2) (3) (4) (5) (6) (7) (8)

viii) Spruce

(mechanical)

Incomplete 40

(30-60

)

Tracheids with large

uniseriate bordered

pita and piceoid

cross-field pitting

Torn ray cells

attached to

tracheids

- -

HARDWOODS (DICOTYLEDONS)


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HARDWOODS (DICOTYLEDONS)

The botanical group of plants larger than that of

softwoods.

Hardwoods grow in almost all regions of the world,

from temperated to tropical zones.

They have to be adaptable to variable growing

conditions and have therefore a more complicated

anatomy than the softwoods.

Hardwoods are rapidly growing trees and consequentlythey need an effective conducting system.

The water conducting function is undertaken by so

called vessel elements.

Thus hardwood is called porous wood. In hardwoods the

annual rings are visible mainly due to the regularity of

the vessels.

Large vessels are usually present in the springwood

zone and later they are gradually decreasing.

This pattern is very characteristics for each species


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STRUCTURE OF HARDWOOD

Major cell types comparing most hardwoods used for pulp and paper

are listed below.

Orientation of Cells Major Axis in the Tree.

Vertical (Longitudinal) Horizontal (Radial)

1. Fiber Cellsa) Libriform fibers

b) Fiber Trachieds

1. Ray parenchymaa) Horizontal Parenchyma

b) Vertical Orientation

2. Storage parenchyma -

3. Vessel Elements -

The vessels are non fibrous tube like elements running vertically in


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g y

the tree. They have large diameter compared to the fibers. They are

joined together end-to-end and appear as pores in the cross section.

Cross-sectional section of Betula (Birch)

A. Vessels, B. Fibers, C. Medullary rays

Two-dimensional view of Betula (Birch)


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The hardwood fibers are truefibersand are termed

libriform fibers (liber means hard tissue)

Libriform fibers only have a reinforcing function, and

are not water leading.

Usually they have thick walls, a narrow lumen and

reduced pitting.

The average fiber length is 1 to 1.6 mm (in woodpulps ax. Only 1 mm).

Hardwood fibers are referred to as shortfibers.

Many species (like oak and eucalyptus) also have so-

called vasicentric tracheids, short irregular fibrous

cells with small bordered pits.

They are usually surrounding the large vessels.


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(A,B & C - Fiber) (DVessel) (E,F,G,&HParenchyma)

Sl.

No.


Cells


(1) (2) (3) (4) (5) (6) (7) (8)

vi) Hard wood

(chemical)

850

(350-

1300 )

25

(15-45

)

Fibers markedly

variable in width

often with abruptly

pointed ends. Septa

sometimes present

Comparatively few.

Length 120(40-320

). Width 35 (20-

70 )

Present.

Width

170 (70-

280 )

-

STRUCTURE OF NONWOOD


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STRUCTURE OF NONWOOD

The sugar cane stalk, as it enters the sugar mill after the leaves,

dirt and extraneous matter are removed, contains 3 main parts

pith, fiber bundles and epidermis.

They behave differently during pulping processes because they

each have different physical and to some extent chemical

characteristics.

Pith or center portion

Approximately 50% of the oven dry weight of stalk consists of

pith or parenchyma cells which do not have fibrous value and

located throughout the cross section with individual fiber bundles

embedded in this pith.

Within the center portion of stalk, also there are scattered fiber

bundles.

The fiber of the Ring


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The fiber of the Ring

Approximately 50% of the oven dry weight of stalk consists of

high quality fiber bundles concentrated in the hard, dense rind of

stalk.

The fiber bundles located in this rind layer are all oriented parallelto the axis of stalk except for these at the nodes, and they give

rigidity to the stalk.

The fiber in the rind layer are longer than the scattered fibrous

elements in the interior of stalk and are more resistant to chemical

action than either the pith or interior fiber or node fibers.

The Epidermis

Outside the rind layer of sugar cane stalk at the surfaces is found a

thin, but very dense, epidermis.

It contains waxes and other materials and is very resistant topulping. The epidermis layer usually represents about 5% of dry

weight of cane stalk and is perhaps the most undesirable element

of stalk so far pulping is concerned.

If it is not removed during depithing it leaves troublesome residue

in the finished pulp, usually appearing as dark specks.


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Morphological characteristics of Bagasse Pulp

Sl.

No.


Cells


(1) (2) (3) (4) (5) (6) (7) (8)

iii) Bagasse 1750

(250-4 000)

23

(10-60

)

Compressed areas

with transverse

marking common.

Fibre pits fairlynumerous

Abundant Length

375(100-900).

Width 100

(30-180)

Present

Width

1001

(30-220 )

Few with

undulating

margins


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MAIN FUNCTION OF THE VARIOUS CELL TYPES IS WOOD.


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Softwood Hardwood

Mechanical function Trachieds Libriform fibers

Fiber trachied

Conductivity

Function

Early wood

TrachiedRay trachied

Vessel

Vessel trachied

Storing function Ray parenchyma

Longitudinal

Parenchyma

Ray parenchyma

Longitudinal

Parenchyma

Secretioning Function Epithetial Epithetial cells


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COMPARISON BETWEEN SOFTWOOD HARDWOOD AND


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COMPARISON BETWEEN SOFTWOOD, HARDWOOD AND

NON-WOOD

Soft

wood

Pine

Nonwood

Bamboo

Hardwood (Tropical

zone) Tectona grandis

Cell dimensions

Trachieds/Fiber Length

mm

Diameter m

3.1

30

2.7-4.0

15

0.7-1.4

---

Vessels

Length mm

Diameter m

---

---

650

120

---

50-370

Cell percentage

Trachieds/Fibers%

Vessel %

Longitudinal

parenchyma %

Ray parenchyma %

93.1

---

1.4-5.8

5.5

58

16

---

---

66.3

11.6

11.6

15.5


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PAPERMAKING FIBER

A plant fiber, to pulp and paper maker is an elongated or

tubular, cylindrical very small cell obtained from certain plantsor parts of plants.

Its diameter is quite thin and considered to be microscopic, i. e.

less than 0.1 mm (100 m).

However its length can be significant varying from about 0.5

mm to over 120 mm. for common paper making fibers, the

length/diameter (L/D) ratio lies in the range of 50-200:1.

Fibers from different sources have different physical properties,length, width, wall thickness, cavity diameter in addition to

their varying amounts of three main constituents i. e.

Cellulose, Hemicellulose and Lignin.


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ULTRA STRUCTURE AND CHEMISTRY

The fibers a built up of several different layers and has a cavity

inside.

The internal organization of fiber wall, the percentage chemical

present in it and its dimensions are very important feature in

deciding the pulp and papermaking characteristics of fibers.

The internal organization of the fiber wall is referred to as its

"ultra structure".

The fiber wall is a composite of organic materials.

The organic materials are the polymers, which with special

arrangement in producing a complex, highly ordered material

with distinct structural and mechanical properties.


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Framework SubstanceCellulose

The skeletal or framework substance of all wood cell walls is"cellulose"-a straight - chain, unbranched, hydrophilic

polysaccharide-composed of repeating sugar units or

monomers.

The latter are all the same - a six carbon ring sugar, "glucose"'.

The number of these monomers in a single cellulose molecule

or polymer (degree of polymerization or DP) averages about

10,000 in wood.

Structurally, due to the particular linkage of these monomers,the smallest identically repeating segment of the chain is

actually a pair of adjacent glucose units known as the

cellobioseunit.


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A measure of the order in cellulose is referred to as its "index of


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order" and is related to the amount of so-called "crystalline"

cellulose.

Highly ordered regions are thus referred to as "crystallites", and

the zones where cellulose is not so ordered are termed"amorphous".

Crystalline Regions

Schematic of Molecular organization within a Cellulose Microfibril

Amorphous

Regions


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The order in wood cellulose is highly variable and depends on

tree age, the particular wood tissue, and the tissue's overall

chemical composition.

It is an important parameter, however, affecting the physical

and chemical properties of both wood and pulp fibers.

More specifically, higher degrees of order (crystallinity)

normally imply higher (fiber) density and hardness, stiffness,and rigidity, tensile strength, and dimensional stability.

At the same time, other factors are lower, namely, fiber

flexibility, toughness, elongation, swelling on water

absorption, and chemical reactivity.

Paper properties will be affected by these fiber properties.


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Organization of Cell Wall Layers


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Organization of Cell Wall Layers

Microfibril occurs in small bundles or

macrofibril.

These, in turn, can be organized into thin sheets or

lamellae which give the wall a layered architecture.

At the fiber surface, the microfibril forms a thin,net like covering surface, the primary wall.

However, in the bulk of fiber wall or secondary

wall, the microfibrils occur in parallel arrays, or

sheets of preferred orientation which is spiral about

the fiber, producing layered construction.

The orientation of different wall lamellae from the

fiber axis is termed as microfibril angle.


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Primary Wall


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Primary Wall

In the thin primary wall, microfibrils form a more or less

irregular, interwoven pattern.

This arrangement facilitates the walls expansion activity,during fiber elongation.

Microfibrils in the outer part of the primary wall (the first

formed part) tend to be oriented somewhat along the fiber axis.

In the innermost part, next to the outer part of the secondarywall, they are oriented more transversely, i.e., at a high

microfibril angle.

The cellulose content of the primary wall is difficult to

determine.

However, it has been estimated to be about 10% in the living

tree and is thought to be embedded in a matrix of pectic

materials (largely carbohydrate derivatives of polygalacturonic

acid), other hemicelluloses, and lignin.

The DP of cellulose here is thought to be about 5000.


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Secondary WallIn the inner most part of primary wall the cellulose

microfibril begin to exhibit an ordered arrangement.

The layers have three different layers S1, S2and S3.

In S1 (0.10.2 mthick) the microfibril have large

angel to the fiber i.e. 55 75o

C, S2makes a thicklayer about 2 10 m thick and has microfibril

orientation close to fiber axis.

In S2the fibrillar angel is typically 5oand 20 oC.

S3 layer has similar construction to that of S1 it is

thinner than S1 (0.07-0.08 m) and has microfibril

angle between 60o90o.


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In high-purity pulps (dissolving grades or "high-alpha" pulps) it

is common practice to remove most of the hemicelluloses as

well as lignin.

However, in paper-grade pulps, hemicelluloses are retained asmuch as possible to maintain pulp yield and to promote

desirable fiber properties.

More specifically, hemicelluloses are very hydrophilic (water-

loving) and play a major role in the fiber's ability to adsorb

water during beating and refining.Consequently, they promote internal lubrication of the fiber,

leading to improved flexibility, ease of mechanical refining, and

increase sheet density.

Hemicelluloses also act as an interfiber bonding agent or

adhesive to strengthen paper.

However, during pulp drying them also tend to help harden or

stiffen the fibers (hornification) which can impede-subsequent

pulp rehydration.


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The types and simplified structures of the major hemicelluloses in wood


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EXTRANEOUS SUBSTANCES

In addition to the major chemical components of the

wood fiber, the wall complex usually contains small

amounts of various extraneous, largely organic

materials which are known simply as 'extractives".

As this name implies, these substances can beextracted from the wood tissue or fiber wall with

either water or with various organic solvents, the

choice of solvents varying with the nature of the

extractive.

Typical solvents include alcohols, ethers, acetone,

and others.

. Major Classes of Wood Extractives

CLASSES PRIMARY LOCATION


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CLASSES PRIMARY LOCATION

1. ALIPHATIC

Fats

Waxes

Sterols

-Mostly fatty acid esters

(mainly triglycerides)

-Esters of long-chain aliphatic

alcohols or terpenoid alcohols-Esters of steroid compounds

RESIN CANAL

COMPLEX

PARENCHYMA

Softwood

Hardwood

2. TERPENES/TERPENOIDS

Volatile monoterpenes

Terpenoid

Resin acids

Other (high melting point)

RESIN CANAL

COMPLEX

PARENCHYMA

Hardwood

3. PHENOLIC COMPOUNDS

Simple phenols

Lignans

Stilbenes

Tropolones

Polyphenols

Hydrolyzable tannins

Flavonoids

Condensed tannins

HEARTWOOD

Substituted C6or C3C6units

Two C.3C6 units (e.g., conidendrin -

spruce, hemlock)

Extremely reactive (e.g., pinosylvins

- pines)

Fungicidal properties (e.g.,

thujaplicin - w. redcedar)

Restricted essentially to hardwoods

e.g., taxifolin - Douglas-fir, larch

Flavonoid polymers; essentially in

softwoods

Softwoods

Hardwoods

"Compose "nonsaponitiables", which contribute to pitch problems in pulping.

Of h d h l i i l l h


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Of concern to the wood technologist, extractives are largely the

source of wood color and odor and can have a strong influence

on its liquid/gas permeability, dimensional stability natural

durability, density, and strength properties.

The pulp maker is not generally concerned so directly with

extractives as they are found in the wood.

However, the variable effects that extractives can have on wood

chipping refining, penetrability and diffusion of pulping liquors,by-product yield type from pulping, cost/difficulty of extractives

control in pulping/bleaching/washing, and the effects of residual

extractives or pitchin the final pulp are direct concerns of the

pulp mill and ultimately of the pulp converter.

The relative importance of wood species and extractives type

will vary with the particular pulping process (e.g., mechanical or

chemical, kraft or sulfite).

Incrustant - Lignin


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g

The major constituent of the fiber wall is

"lignin", an amorphous, highlybranched, and three dimensional,phenolic polymers.

Lignin is manufactured by maturing

fibers (or other wood cells) andpermeates the fiber walls andintercellular regions (middle lamellae).

In mature xylem, lignin lends rigidity and

cohesiveness to wood tissue as a whole. On a weight basis lignin comprises

about 25-35% of normal softwood xylem,and about 15-25% of hardwood xylem.

Lignin constitutes 80% or more by


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g yweight of the middle lamella inmature xylem. However, most of the

wood lignin (two thirds or more) islocated in the wood cell walls.

Except in some species or certaintree parts or tissues, lignin is

distributed essentially in a uniformpattern across the fiber wall.

It is also chemically liked to woodhemicellulose, which further

complicates its removal duringchemical pulping.

I it t l t t i th d ll ll


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In its natural state in the wood cell wall,lignin is referred to as "protolignin" ornative lignin and is markedly

thermoplastic. It is also very it much less hydrophilic

than either cellulose or hemicellulose,almost to the point of being

hydrophobic.As such, lignin in the pulp fiber has thegeneral effect of inhibiting waterabsorption and fiber swelling, and canrender the fiber less responsive to

mechanical refining. However, since lignin is it does possess

a characteristic that can be used toadvantage in mechanical pulping, where

high temperatures soften the lignin.

Th b i t t f li i diff t


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The basic structure -of-lignin-differs tosome extent between softwoods andhardwoods.

In commercial softwoods, the prominentrepeating structure is known as a"guaiacyl" unit, which contains

a single methoxyl group, on the

phenylpropane ring; hardwood-lignin, onthe other hand, is a copolymer ormixture of guaiacyl and "syringyl"'lignin, the latter containing two methoxylgroups per phenylpropane nucleus.

The ratio of guaiacyl to syringyl unitsvaries from 4:1 to 1:2 among differenthardwoods.


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PROPERTIES OF FIBER IMPORTANT TO PAPERMAKER

Fiber Length


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Fiber Length

Usually it is a belief that a paper made from a long fiber will

give more paper strength than that made form a shorter fiber.

However with usage of different kinds of raw materials it ispossible to produce good quality papers from even short fibers.

Clark established a few empirical relationships between pulp

strength and fiber length.

Burst factor = K1LBreaking length = K2L

0.5

Tear factor = K4L1.5

Fiber length also influences the general structure and surface

properties of a paper sheet.Although fiber length is an important property of fiber yet it is

not considered to be very important as its required according to

the specific paper product demand.

Cell Wall Thickness


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The importance of the fiber wall on properties of

paper hand been recognized for a number of years.

The early wood portions of the growth ring may

have fibers with comparatively thin wall and late

wood fibers have thick walls.

The pulps obtained form wood having thin walled

fibers give dense and well bonded sheets, and

those from thick wall give bulky sheets with high

tearing resistance.

It is apparent that thin walled cells collapse and

confirm to other fibers easily to give a dense

bonded sheet of paper, due to their high flexibility.


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Coarseness or FlexibilityIt is very important property of fiber It depends largely on fiber wall


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It is very important property of fiber. It depends largely on fiber wall

thickness. It can be reported as

Flexibility ratio 1/D.

Coarseness: defined as weight in mg. of the fibers needed to

give a total length of 100 metres.

STRENGTH TABLE FOR MORPHOLOGICAL FACTORS

Trend Tensile and

burstingstrength

Tearing

strength

Folding

strength

Sheet

density

Fiber length rising 0 to + . ++ 0 to + 0 to -

Cell wall thickness late wood

fraction rising (tube structure)

rising - 0 to + -- --

Cell wall thickness early wood

fraction falling (ribbon structure)

falling + 0 to + ++ ++

Ratio fiber length to fiber width rising +

Curling of fibers rising -- + -

* Porosity, absorbency, air permeability, bulk have a contrary trend.

STRENGTH TABLE FOR CHEMICAL AND PHYSICO-CHEMICAL FACTORS


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STRENGTH TABLE FOR CHEMICAL AND PHYSICO-CHEMICAL FACTORS

Trend Tensile and

bursting

strength

Tearing

strength

Folding

strength

Sheet

density

Average degree of

polymerization (D.P.)

rising 0 to + 0 to + 0 to + 0

D.P. very low - - - -

Hemicellulose content rising Optimum Optimum Optimum +

Lignin content

Stiffness

rising* - - - - -

Water retention value rising** + Optimum

to +

++ +

* Porosity, absorbency air permeability, bulk have a contrary trend.

** Plasticity, bendability has a contrary trend.

CELLS CREATIVE PROBLEMS DURING PAPERMAKING

Vessel elements


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Vessel elements

These cells are present in all angiospermic plant

whether woody or non-woody hardwoods containvessel volume tends to lower wood density.

They range in size from 0.2-1.3 mm. in length and

20-300 m in diameter. In general shorter vessel

elements are also wider (hardwoods).

The larger vessels are generally more narrower (non-

woods).

These vessel elements tend to pull away or pick

from the sheet surface during printing.

Vessels play an important advantageous role intransport of pulping liquor in wood chips.

Parenchyma


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Parenchyma can be axial or ray parenchyma.

It is present about less than 10% by volume in softwood

whereas can be as high as 50% in non-woods.These cells are normally the site of most inorganics which

can be K, Mg, Mn, Ca, Si. Crystallized deposits of these

material and amorphous of silica are present in parenchyma

cells.

These inorganic materials contribute to ash in wood or non-wood.

They are problem for some very high purity pulp and

promote scale formation in equipments used for recovery of

pulping chemicals.

Parenchyma cells are also the originators of organic

extractives which are undesirable in dissolving grade pulp

and cause pitch problem in pulp and paper mills.

In absorbent grades of pulps they render the fibers less wet

table reducing liquid absorption rate.


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Parenchyma cells are some times as small

as 10-100 m thus are responsible for

giving fines content to pulp causing higherdrainage rate of stock.

These cells also contain gums, oils, resins,

latex chemicals during pulping.

They are very thin walled cells of poor

strength so not desirable in sheet and are

generally preferred to be screening out e.g.

during depithing process.

Epidermal Cells


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These cells present in abundance in

monocotyledonous pulps.

Since epidermis constitute the outer most coveringof non-woody plants.

It is hydrophobic and contains wax like substance

called cut in. In week epidermal cells give strength

and protection to internal tissue.

These cells are also containing silica, as in case of

rice straw.

These cells are not easily separated by cooking

chemicals and appear in pulp as masses with sharp

toothed margins.Epidermal cells also consume more amounts of

chemicals; contain siliceous material thus

undesirable for pulping.

Hardwoods yield pulp fibers that on average are about

to the length and about width of softness fibers


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to the length and about width of softness fibers.

Hardwoods are generally preferred for printing papers

where surface smoothness is increased by short fibers.

Softwoods fibers are long and coarse thusadvantageously used in packaging or high strength

papers. Softwoods can yield any type of mechanical

pulp as well.

Non-woods are having large variety of cell types.Their cell dimensions vary to very large extent from

outer part to inner part.

They are having very thin walled and low diameter

fibers and contain a large percentage of smaller

parenchyma cells.

Non-woods are not very suitable for mechanical pulp

production and are to be blended with other raw

materials pulp to give variety of quality papers.

TABLE : Chemical Composition, and Fibre Dimensions of Some Typical Cellulosic

R M t i l A il bl i I di


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Raw Materials Available in India.

Lignin

%

Ash

%

Hemi-

cellulose

%

Gross &

Bevan

cellulose %

Ave.

Length

mm.

SEED FIBER:Cotton (Gossypium hirsutum)

- - 2.0 92-97 18

BAST FIBER:

Hemp (Cannabis Sativa)

5.2 - 5.5 79.3 22

SOFTWOOD:

Chir (Pinus longifolia)

Sikkim Spruce (Picea Spimulosa)

Fir (Abies spectablia)

26.6

28.6

29.2


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RAW MATERIALMOISTURE

Water in the raw material cell wall is held on and betweenmicrofibrils at hydrogen-bonding sites.

As water is absorbed/adsorbed into the cell walls, the latter

expand, giving rise to a gross change in the raw material

tissue as a whole.

As this bound water is removed from the walls (bydrying), the cell walls and raw material tissue shrink.

If there is just enough water to completely saturate the cell

wall substance and no liquid water is present in the cell

lumens, the raw material is said to be at its fibersaturation

point(FSP).

Here the water is essentially that confined or boundto the

wall substance and any microvoids therein.

Raw material exhibits its maximum swollen volume at FSP;

any gain in moisture content above FSP induces no further

changes in raw material dimensions.


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When there is sufficient raw material water to cause

in accumulation of liquid in the "cell lumens, is said

to be "free water" (or capillary-held water") inaddition to the bound water.

Since this type of water is held in the raw material's

void system, gain or loss of free water, causes no

raw material swelling or shrinkage, respectively.

If all possible cell lumen and intercellular spaces are

filled with water, the raw material is at its

maximummoisture content".

Raw materials with thin-walled cells have a higher

void volume (greater lumen volume) than thosewith mostly thick-walled cells, and hence have a

greater maximum MC.

Raw material (and paper) at any MC will eventually achieve


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( p p ) y y

an "equilibrium MC" or EMC if exposed to an atmosphere

containing water vapor, and the EMC will always be less

than FSP.

The EMC attained will vary with relative humidity (RH) of

the environment, temperature, and drying history of the raw

material (or paper), i.e., whether the wood is approaching

equilibrium by gaining moisture or by losing moisture.

Shows that once raw material tissue has been dried

(desorption), the next time it is exposed to water (absorption)it will not gain as much water at the same temperature and

RH.

In general, raw materials of higher density will have a

greater overall volumetric shrinkage or swell, property.

For most commercial raw materials, total volumetricchanges from green to oven-dry (or vice versa) are normally

less than 15% and are approximately equal to the sum of the

R and T dimensional changes.


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Calculation of moisture content (MC) on a:


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( )

Wet Basis

% MCw= (W-D)/W x 100

Dry Basis

% MCd= (W-D)/D x 100

Where:

W = wet (as-is) wt. of sample

D = oven-dry wt. of same sample

% oven-dry = % solids = 100-MCw

If either of the above formulae is applied to wood with high

levels of organic extractives, the MCs so obtained are not

accurate relative to wood substance since it is not possible to

use the true weight of dry wood substance only.

Relevance to Pulping

Whether raw material is purchased in chip or log form the pulp


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Whether raw material is purchased in chip or-log form, the pulp

mill is interested in paying for only the weight of the cell wall

substance and not the raw material water .

Thus, knowledge of raw material moisture has a direct financialimpact on the total cost of pulpwood.

Another situation in which it is important to know how much

water is in the raw material is in the determination of the proper

ratio of liquor to raw material in the pulping digester.

Fresh raw material chips are rarely more than about 40% oven-

dry (60% MC on green basis), and chips as wet as this,

containing water-swollen - cell walls, are more easily pulped by

both kraft and sulfite processes than much drier raw material .

As softwood chips dry, the interfiber pits become aspirated,sealing off intercellular pathways for free movement of pulping

liquor.

In hardwood chips the effects of drying may or may not be as

noticeable, varying with the extent of "tylosis" formation (which

plugs the vessels).

In both raw material types, however, dry cell wall

b i diffi l d


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substance is more difficult to penetrate and saturate

with pulping liquor than are sufficiently hydrated cell

walls.As one might suspect, overly dry raw material

necessitates special procedures for liquor penetration.

This is of particular importance in sulfite pulping, but

a high level of raw material moisture, either natural or

introduced at the pulp mill, is also a prerequisite forvarious mechanical and thermo mechanical processes.

While a certain amount of water in wood is an

obvious advantage in pulping operations, in some

cases it can also be very troublesome.Specifically, in climates where winters are severe,

snow and ice in outside chip piles cause chip handling

problems.


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Frozen bark is also more difficult to remove.

A related problem is frozen pulpwood, which causes

chipping problems (more energy, knife dulling) and

overall inferior chip quality.

Chips from frozen raw materials are thinner than

normal and are often undersized (pin chips).

The latter pack more tightly and have a tendency tointerfere with liquor circulation in the pulping

digester.


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RAW MATERIAL SPECIFIC GRAVITY

The wood physical property most commonly examined by thepulp technologist in an effort to evaluate overall wood quality

is "specific gravity".

This parameter is used to give a idea of how much wood fiber

or wood substance can be obtained per unit volume of a given

type of pulpwood.

In addition, the potential behavior of pulp fibers in

papermaking or other manufacturing processes and in the final

product can often be correlated to wood specific gravity.


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Relationship to Raw Material Anatomy


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Relationship to Raw Material Anatomy

Wood specific gravity, even for extractive-free wood,varies with wood type (hardwood, softwood), species,

within species (due to site and geography), and within

the same tree.

This variability can be attributed directly to the

combined effects of (a) cell wall thickness, (b) cell

size, and (c) the number of cells of a given type, as

defined by (a) and (b).

Softwoods

The proportion of earlywood and latewood (fibers) within annual


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The proportion of earlywood and latewood (fibers) within annual

growth increments, together with fiber wall thickness, are

important variables governing the SG of coniferous pulp woods.

Within a given growth increment, SG is greater in the thicker-

walled latewood.

Earlywood/latewood ratio, as well as the fiber wall thickness of

each zone, varies with species, tree age, growing conditions, and

imposed forest management practices.The difference in earlywood/latewood morphology is

particularly noticeable in woods, such as hard pines, Douglas-fir

and larch.

Species with gradual earlywood/ latewood transition show less

SG difference between earlywood and latewood and, on theaverage, are lower in overall wood SG.

Pulp fiber coarseness (wt./unit length) in softwoods varies

directly with wood SG and has a strong influence on the

behavior of resulting paper products.

Hardwoods


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In angiosperms, fiber wall thickness is still major determinant of

gross wood SG, but the volume of wood occupied by the vessel

system (essentially void space) is also important.

Both of these factors, together with fiber cell volume, combine to

produce a broader range of SGs between species than exists

among softwoods.

Vessel arrangement and fiber/vessel ratios can also vary with tree

growth conditions, making general statements and predictions on

hardwood quality difficult.

This situation is further illustrated in the fact that hardwoods

how less consistency than softwoods in trends or changing SG in

both the radial and axial directions (e., with tree age and tree

height).


One of the first locations at the pulp mill where wood SG can


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exert an effect is at the chipper.

High, density woods are actually harder, more difficult to chip,

and necessitate adjustment in chipper operation maintenance.

Excessively hard" Woods also require more energy for

chipping a produce more variable chips.

These species include the higher SG hardwoods such as maple,

oak, hickory, ash, and birch.

Lower SG hardwoods, such as aspen and cottonwood, and

essentially all coniferous pulp-woods are considered "soft"with respect to their demands for chipping energy.

The pulp mill is also concerned with maintaining a prescribed

level of production, and the mill manager would like to get the

most pulp possible from the chips going into the pulping

digesters.Since these units will hold only a certain volume, higher SG

wood will produce (at a given pulp yield) a greater weight of

pulp per cook.

Lower SG wood results in a lower chip bulkdensity(kg/m3)

and reduced digesterloading.

These reductions can also be caused by improper chip geometry,

but in general, the most influential factor here is wood SG.


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g

A change in wood SG can also affect the millscooking schedule,

since a change in wood density (usually meaning a change in

fiber wall thickness) must be accommodated for by changes incook chemistry, time, and / or temperature.

For mechanical-type pulps, energy, mill production, and possibly

pulp quality can all be affected by significant changes in wood

specific gravity.

Since low-coarseness and high-coarseness fibers will usually

collapse to a greater or lesser extent, respectively, in the wet state

such fibers will process differently during pulp washing,

bleaching, and screening operations.

High specific-gravity woods yield stiff, rod-like fibers whichdrain water more easily (freepulp) than thin-walled and easily-

collapsed (less free) fibers from low specific-gravity wood.

These two fiber types also respond differently to mechanical

refining.

Relevance to Papermaking

Changes in raw material specific gravity also have an influence


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Changes in raw material specific gravity also have an influence

on the papermaking process and the characteristics of the final

product.

Such changes are generally less noticeable within and among

hardwood species - in comparison to softwoods - although pulp

freeness, sheet wet strength, and sheet density can vary

noticeably between certain species.

On the case of softwoods, however, a major change in chip SG

will noticeably alter paper machine drainage, sheet wet strength,

and final sheet density.

Thinner-walled and lower-coarseness fibers drain slower (are

less free) but collapse more readily to yield higher wet strength

and higher sheet density, changes in wood SG and pulp fiber

coarseness can often be traced to changes in wood species or

species blends, but even within the same species, chip SG can

vary with forest geography, tree age, and/or tree growth rate.

Raw material SG or its counterpart in pulp fiber coarseness is one

of the most influential factors controlling the strength and several


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g g

other physical characteristics of the paper sheet.

The statistical technique of multiple analysis used to predict the

performance of different raw material sources always reveals thevery strong influence of raw material specific gravity and

resulting fiber flexibility or collapse.

This is true for both softwoods and hardwoods. Paper properties

like tensile and burst decrease with increasing fiber coarseness.

Nonpaper uses for raw material pulps, such as in absorbent

disposables (diapers and related products), tissues and towels or in

nonwoven fabrics, are also strongly dependent on fiber

characteristics related to raw material specific gravity.

Raw material s of higher fiber coarseness will produce bulkier,usually more absorbent, disposables.


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RAW MATERIAL STRENGTH

The ability of wood to resist tensile,

compressive, and shear forces under

various circumstances is probably of

little direct technical interest to the pulp

converter or papermaker.

General Nature

When subjected to an imposed mechanical stress (force per unit


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When subjected to an imposed mechanical stress (force per unit

area), wood tissue will deform or undergo a certain level of

strain (deformation per unit area or dimension).

For small strains of short duration, wood is "elastic"; that is,strain is proportional to stress, and the strain is fully recovered if

the time of application is short.

On the other hand, large strains and/or extended times of stress

application can lead to unrecoverable or "plastic" deformation,

or to eventual wood failure.This situation is depicted graphically, which illustrates a

stress/strain (/) curve for wood strained beyond its

"proportional limit" (where and are no longer linearly

related).

Over the linear range of deformation, the /ratio is called the

modulusof elasticity", MOE, or Young's modulus for tensile or

compressive stresses.

"When measured at constant thickness or at constant

weight/area, the term "extensional stiffness" is also used.

Stiffness is a term important to both the wood products and the

paper industry.


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Moisture Effect

Since water swells wood and has the general of separating the

cell wall substance on a molecular level, it is logical that an

increase in moisture (up to its fiber saturation point), causes a

corresponding reduction in measured strength.

As wood gains moisture, less is needed to induce a given strain -

other factors held constant.

On the other hand, as wood dries over the moisture content range

below FSP, more energy is required to strain the wood, including

tensile, compressive, and shear forces.


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Temperature Effect

This effect is not simply described due to an interaction with

wood moisture.

Generally speaking, raw material strength is reduced by

increasing temperature at a given level of raw material moisture,

and below FSP, higher moisture content at a given temperature

usually means weaker raw material .

Interactions between -temp time, and pH of the system will

determine the overall effect on raw material strength.

For raw material treated to high temperature (150oC and up)while in water, hydrolysis of the carbohydrate fraction takes

place, and the raw material can be weakened substantially.

Hardwoods are more affected by this type of treatment than


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Hardwoods are more affected by this type of treatment than

softwoods.

At elevated temperature and in the wet state, major woodcomponents become thermoplastic or soften over different

ranges of temperature - hemicelluloses: 50-60oC; lignin: 90-

100oC; and cellulose: 230-250oC.

Importance to the producer of thermo mechanical pulps, ligninin dry wood must be heated too much higher temperatures

(130-190oC) before it becomes thermoplastic.

At ambient temperature and moisture content, raw material

becomes weaker with age (time) due to very gradual hydrolysis

of the cellulose.

Since this is an extremely slow process, such degradation is

normally of no consequence to the pulpwood user.


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Specific Gravity Effect

If one considers the factors that control wood SG, it is easily

seen why it has a strong influence on wood mechanics at any

level of wood moisture and temperature.

For the various tree species used for pulp, a given wood strength

property, S, is related to SG by the equation, S = k (SG)n, where

n and k are constants.

Thus, wood chemistry, anatomy, and ultra structure at a given

temperature and moisture content all combine to produce

differences in the strength behavior of different wood sources.



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Raw material strength, as reflected in the factors of hardness and

rigidity, determines the mechanical behavior of the compositeraw material matrix and its resistance to imposed stresses,

thereby determining the ease with which raw material is

processed in the aforementioned operations.

The raw material strengths probably of most concern to the pulpmill are compressive and shear strengths (mostly parallel to

grain or tree axis) that must be overcome for chipping and chip

refining.

The energy needed here to cut and fiberize can be a significant

portion of the total cost of pulp production, particularly those

operations that rely to a large extent mechanical energies.


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Papermaking Raw Materia & Their Characteristics II

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Transcript of Papermaking Raw Materia & Their Characteristics II