8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
1/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
2/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
3/115
CONFORMABLE
DEVELOP STRONG BONDS
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
4/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
5/115
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)
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
6/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
7/115
Illustrating different levels of hydrogen bonding
(A) Loosely through water molecules
(B) More tightly through monolayer of water molecules
(C) Directly
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
8/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
9/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
10/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
11/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
12/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
13/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
14/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
15/115
Spermatophytes (Seed bearing plants)
Gymnosperm (Naked seeds)
Conifers, Evergreen, SoftwoodAll woody
Angiosperms (Covered seeds)
Monocotyledonous
Non woody
Dicotyledonous
Woody or
Non-woody
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
16/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
17/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
18/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
19/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
20/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
21/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
22/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
23/115
Outer bark
Inner bark
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
24/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
25/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
26/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
27/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
28/115
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:
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
29/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
30/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
31/115
STRUCTURE OF SOFTWOOD
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
32/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
33/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
34/115
STRUCTURE OF SOFTWOOD
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
35/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
36/115
(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)
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
37/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
38/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
39/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
40/115
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)
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
41/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
42/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
43/115
(A,B & C - Fiber) (DVessel) (E,F,G,&HParenchyma)
Sl.
No.
Pulp FIBERS Parenchyma Vessels Epidermal
Cells
Length Width Special Features
(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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
44/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
45/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
46/115
Morphological characteristics of Bagasse Pulp
Sl.
No.
Pulp FIBERS Parenchyma Vessels Epidermal
Cells
Length Width Special Features
(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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
47/115
MAIN FUNCTION OF THE VARIOUS CELL TYPES IS WOOD.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
48/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
49/115
Sl.
No
Pulp FIBERS Parenchyma Vessels Epidermal
Cells
Length Width Special
Features
(1) (2) (3) (4) (5) (6) (7) (8)
ii) Bamboo 2100
(400-
5 000 )
15
(5-40)
Compres
sed areas
with
transvers
e
markings
common,
Pitting
extremely
sparse
Abundant.
Length
110
(30-500 ).
Width
35.(10-60 )
Present.Wi
dth
120
(40-250)
Rare with
wavy
margins
COMPARISON BETWEEN SOFTWOOD HARDWOOD AND
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
50/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
51/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
52/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
53/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
54/115
A measure of the order in cellulose is referred to as its "index of
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
55/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
56/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
57/115
Fibers of higher stiffness and rigidity will produce
bulkier papers.
Fibers with greater dimensional stability will produce
papers of greater dimensional stability (lower curl
potential).
Cellulose molecules are aggregated laterally to form
threadlike structures called microfibrils, which are
visible only with an electron microscope.
The length of microfibrils is not well defined, but
their diameters are reported to measure about 10-20
nm.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
58/115
Organization of Cell Wall Layers
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
59/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
60/115
Primary Wall
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
61/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
62/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
63/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
64/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
65/115
Matrix MaterialHemicellulose
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
66/115
The cellulose framework is penetrated and encased
by, but not bonded to, carbohydrates known as
"hemicelluloses".
These materials differ substantially from cellulose inthat they are composed of combinations of five-
carbon and six-carbon ring sugars, possess side
groups on the chain molecule, and are amorphous in
the fiber wall.Also, the DP of these sugars is in the range of only
50-300.
Softwood hemicelluloses are largely galactogluco-
mannans while those in hardwoods are mostly xylan.
In both cases, they serve as a supportive matrix for the
cellulose microfibrils and appear to have a relatively
uniform distribution across the fiber wall.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
67/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
68/115
The types and simplified structures of the major hemicelluloses in wood
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
69/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
70/115
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 tanninsFlavonoids
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
71/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
72/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
73/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
74/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
75/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
76/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
77/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
78/115
PROPERTIES OF FIBER IMPORTANT TO PAPERMAKER
Fiber Length
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
79/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
80/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
81/115
Coarseness or FlexibilityIt is very important property of fiber It depends largely on fiber wall
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
82/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
83/115
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** + Optimumto +
++ +
* Porosity, absorbency air permeability, bulk have a contrary trend.
** Plasticity, bendability has a contrary trend.
CELLS CREATIVE PROBLEMS DURING PAPERMAKING
Vessel elements
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
84/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
85/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
86/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
87/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
88/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
89/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
90/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
91/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
92/115
( 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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
93/115
Calculation of moisture content (MC) on a:
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
94/115
( )
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
95/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
96/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
97/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
98/115
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.
TerminologySpecific gravity is defined as the ratio of the weight of a wood
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
99/115
p g y g
sample to the weight of water displaced by that sample.
Consequently, specific gravity (SG) is a pure number and has no
units.
This ratio is often but incorrectly given the units of g/cm3 and is
therefore often mistakenly equated to wood "density," which is
defined as:
Density = weight of wood substance (g)/volume of wood substance (cm3)
Both SG and density can be used to provide the same practicalinformation.
Since both the volume and the weight of raw material usually
change with change in moisture content.
Common pulp raw materials have a basic specific gravity range of
about 0.35-0.65.Raw materials outside this range are considered either light or
heavy, respectively, including both temperate and tropical species.
Factors such as tree age, tree part, and growth rate will give rise to
variability between species, within species, and within the same
tree..
Relationship to Raw Material Anatomy
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
100/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
101/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
102/115
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).
Relevance to Pulping
One of the first locations at the pulp mill where wood SG can
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
103/115
One of the first locations at the pulp mill where wood SG can
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
104/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
105/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
106/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
107/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
108/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
109/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
110/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
111/115
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
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
112/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
113/115
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.
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
114/115
8/10/2019 Papermaking Raw Materia & Their Characteristics Presentatiofn
115/115
Top Related