Topic-11-Papermaking-Introduction-text.pdf

Papermaking
OVERVIEW AND INTRODUCTION
1. Introduction and Overview
Papermachine quick facts:
- 10m wide
- 100m long
- 100 km/hr
- $250,000,000++
2. Unit Operations
The unit operations of papermaking are shown in Figure 2. Each is
described below in some detail.

Figure 2
2.1Stock Preparation
In this step, pulps are repulped (if delivered to the mill in dry form),
refined, and blended to give the desired furnish for the particular
grade of paper. This blended stock is then pumped to the machine
chest. From here it is pumped as thick stock through a tickle refiner,
stuff box, and lastly the basis weight valve which controls the fibre
delivery to the paper machine.
2.2 Approach Flow
At the fan pump, the thick stock is diluted with whitewater filtrate from
the forming section. The mixed stock is pumped by the fan pump to
the paper machine. It passes through centrifugal cleaners (to remove

heavy material), a pressure screen (to remove large material), and
then to the paper machine. There is sometimes a secondary fan
pump between the cleaners and screen to assist in pumping.
After the pressure screen, the stock enters a manifold where it is
drawn off over the width of the paper machine into the headbox.
Stock leaving the headbox is made into a sheet by filtration. The
fibrous mat is called a wet web, which is pressed, dried, and wound
into a reel of paper on the paper machine.
Paper machines may vary in width from about 5 to over 30 feet, and
operate at speeds up to 1800 m/min. They may produce from a few
tonnes to 1000 tonnes per day. The paper basis weight may vary
from light tissue (about 10 grams per square metre) to paper board
(up to 500 grams per square metre).
Paper machines may be divided into three general types:
- Fourdrinier machines
- twin-wire formers
- multi-ply formers
By far the most common type of paper machine in use today is
Fourdrinier. In this former, stock is drained on a moving horizontal
screen called a fabric. The various parts of a typical Fourdrinier
machine are illustration in Figure 3. The full machine is shown in
Figure 4. It consists of a stock distribution system; headbox; forming
section; press section; dryer section; calender stack; and reel.
In twin-wire formers, the stock is filtered between two fabrics.
Multi-ply formers are used in the production of paperboard. The most
common type is cylinder formers. These consist of a series of screen-
covered cylinders, each rotating in a vat of dilute paper stock. Web
formation occurs on the screen as a result of suction inside the
cylinder which removes the filtrate.

Figure 3
Figure 4
2.3 The Headbox

Stock leaving the flow distributor passes through a headbox before
the filtration process begins. The major functions of the headbox are:
to assure uniform distribution of flow across the paper machine; to
provide velocity control of the jet leaving the headbox by the pressure
in the headbox; and break up of pulp flocs by turbulence within the
headbox. These functions are achieved by causing the stock to flow
through several rotating perforated rolls within the headbox or, in
modern headboxes, past stationary flow elements. After passing
through these turbulence-generating elements, the stock is
accelerated in a sharply converging orifice called a slice. On leaving
the slice, the stock becomes a free jet, which then impinges upon the
forming screen to begin the filtration process of forming.
2.4 Forming Section
The Fourdrinier table of a paper machine consists of foils, table rolls,
breast roll, couch roll, suction boxes, wire rolls, and other parts. The
fabric on which the sheet of fibre is formed is a finely woven synthetic
fabric of strands made of specially fine drawn and filaments, woven
into a screen. Various types of weave are used to obtain maximum
fabric life and to reduce wire marking on the wet sheet.
The foils are located under the forming wire. In addition to supporting
the wire, the negative pressure created in the expanding nip on the
underside of the fabric, causing drainage of the water through the
wire. Older paper machines sometimes use table rolls for this
purpose. However, most modern paper machines now use foils.
After the foil section on the forming table, the moving fabric passes
over a series of vacuum boxes and then over a couch roll. Often, a
dandy roll is located on top of the forming fabric over the suction
boxes. This is a light open structured roll covered with wire cloth,
resting lightly upon the surface of the sheet. Its function is to flatten
the top surface of the sheet and improve the finish. A pattern on the
dandy roll may leave translucent patterns on the wet paper, in the
form of names, insignia or designs. These are called watermarks.
The last roll over in the forming section is called the couch roll. It is a
suction roll to remove further water.

2.5 The Press Section
The press section mechanically squeezes water from the wet web,
increasing the solids content of the sheet of paper. It also reduces the
bulk or thickness of the paper. This compaction assists in subsequent
bonding of fibres.
2.6 The Dryer
Paper leaves the press section and enters the dryer at a consistency
of 40-45%. The dryer causes further water removal by evaporation. A
typical dryer section consists of from 40 to 70 steam-heated dryer
cylinders. The sheet is held in intimate contact with the heated
surfaces by means of dryer felts.
2.7 Calendering
After the dryer, the paper web is passed through a calender stack.
This is a vertical stack of solid iron rolls which impart successively
higher compression cycles to the paper as the paper passes
downward through the stack. The function of the calender stack is to
reduce the thickness and to impart a smooth surface to the paper
web for good printability.
2.8 Reel
After the calender stack, the paper web is wound into a large roll at
the end of the paper machine, called a reel. The calendering and
reeling operations are the last part of the continuous paper machine.
2.9 Winder
When reels are wound to full size, the web is transferred to a new
spool, without machine shut down. The full reel is transferred by a
crane to a winder where it is unwound and slit into rolls based on

customer specifications. In newsprint mills, the rolls then goes to the
wrapping station, and then into storage.
2.10 Coating
High gloss papers are produced by coating the paper surface with a
pigment-binder compound, either on-machine or off-machine. The
pigment consists of mineral particles having platelet shapes, e.g.
clays.
2.11 Supercalendering
Coated papers are passed through an off-line supercalender made
up of hard and soft rolls. These impart gloss to the coated surface.
2.12 Converting
This term describes further operations after papermaking to make
products from paper. Usually these are carried out in separate
converting plants, but some converting may be done in a paper mill,
e.g. production and packaging of sheets from continuous webs.
Water Removal
As is evident, much of the paper machine is devoted to water
removal, taking consistency from 0.5-1% to greater than 90%. The
relative amount of water removed in each section is shown in Figure
5. The cost of water removal at each section is shown in Figure 6. As
is evident, the cost of water removal increases down the machine,
with a particularly large increase at the dryer. Accordingly, as much
water as possible must be removed in the forming and pressing
sections.

3. Pulp Quality Measurements
A number of analytical tests can be performed on pulp samples.
Some measure chemical characteristics while others measure
physical attributes. Most provide information that is useful in
predicting the properties and performance of the end product made
from that pulp. The most common tests are described below. Many
of the following descriptions will relate to TAPPI (Technical
Association of the Pulp and Paper Industry) standard test procedures.
It is important to recognize, however, that there are many European
standards which are equally applicable and, in some cases, more
widely practiced.
Moisture Content. For pulp that has been formed into sheets,
moisture content is determined by weighing a representative sample
collected from several locations within a bale and then oven drying
to remove all water. The difference between the original sample
weight and the oven dried sample weight represents the amount of
moisture present in the original sample. A percentage is determined
by dividing the amount of moisture by the original sample weight.
For dry pulp sheets, moisture content is usually between 5-15%.
For wet lap pulp, moisture content is typically 50%-60%.
Consistency. This test measures the dry solids concentration of a
suspension of fiber in water. A representative sample of pulp is
collected, it is filtered to remove free liquid, and then oven dried to
remove all remaining water. In stock preparation refining systems,
consistency is normally between 2% and 6%. This permits relatively
easy pumping of the slurry using conventional centrifugal pumps.

Kappa Number. The result of a quantitative chemical analysis, the
Kappa number of a pulp indicates the extent to which lignin has been
removed in the chemical pulping process. A high Kappa number,
indicating a greater amount of retained lignin, is common for
unbleached pulps used in the production of linerboard or bag paper. If
the pulp is to be bleached, the Kappa number of the pulp before
bleaching predicts the bleachability of the pulp. Low Kappa pulps are
easier to bleach. High Kappa pulps usually require more energy in
refining, but often produce stronger paper or board (particularly with
regard to tear strength).
Viscosity. There are a number of standard tests in which a sample of
pulp fibers is dissolved in a suitable solvent, and the viscosity of the
resulting solution is measured. This viscosity indicates the degree of
polymerization of the cellulose and is a measure of the degradation of
the cellulose in the pulping and bleaching process. The lower the
molecular weight (or degree of polymerization), the more degraded is
the cellulose and, in general, the lower the physical strengths of the
paper or board produced.
Fiber Length. The average fiber length of a pulp has a direct bearing
on many of the important physical characteristics of the paper
produced. In particular, tearing resistance is very closely related to
average fiber length: the longer the fiber, the greater the tear strength
of the sheet produced. Screening classifiers (e.g. Bauer McNett) have
been used in the past to determine the weight percentage of fibers
collected on various screen sizes, but these have largely been
replaced by computer automated optical devices. The most
commonly used of these optical instruments are the Kajaani FS-200
and the Optest FQA. The data output of these devices can provide
information such as the average length and length distribution curve

for the fibers in that pulp.
Fines Content. An additional measure of pulp particle size is the
percentage of fines. This consists of particles measuring less than
0.2 mm in length as measured by an optical analyzer, or the weight
percentage of the P200 fraction obtained from a Bauer McNett
classifier. Fines can have a significant impact on processing,
particularly with regard to filtering or drainage operations. Fines
content of a kraft pulp may be in the range of 5– 15%. For a
groundwood mechanical pulp, the fines content may exceed 40%.
Coarseness. This is a measure of the average weight of fiber per
unit length, often reported in units of mg/m. It is most conveniently
measured using an optical analyzer. For fibers of a given average
length, it is a measure of the cross sectional area of the fiber. For a
given average diameter, it is measure of wall thickness. Coarse
fibers are considered to be less conformable than fine fibers and do
not bond as readily. Coarser fibers also result in fewer fibers per
mass of pulp, which has a significant impact on sheet formation and
light scattering potential.
Zero-Span Tensile. This test is a measure of the intrinsic strength of
individual fibers. For normal tensile strength tests (described in the
next section), the jaws of the tensile machine gripping a paper
sample are spaced about 7” apart. As the sample is pulled apart,
many fibers pull out of the sheet by breaking fiber-to-fiber bonds. The
resultant breaking load is a measure of both fiber strength and bond
strength. For the Z-Span test, the gripping jaws are moved together
until they are touching so that, in principle, all fibers in the tensile
zone are gripped by the jaws. The fibers are then stressed to the
breaking point and a measure of fiber strength is obtained.

Freeness. Freeness is a measure of the drainage resistance of a
pulp slurry. In the Canadian Standard Freeness (CSF) test, one liter
of dilute slurry (0.3% consistency) is drained through a standard
screen which captures the fibers to form a mat. The amount of water
overflowing a weir and collected from a side orifice is then a measure
of how fast the water drains through the mat. CSF values may be as
high as 750 ml for an unrefined, unbleached softwood kraft, and as
low as 30-40 ml for a fine groundwood mechanical pulp. Stock prep
refining of kraft pulp will typically reduce freeness to between 600 and
250 ml depending on the starting freeness and the paper grade being
produced. Freeness is a good general predictor of sheet density and,
as such, is routinely used to predict strength, opacity and other
physical properties of paper. Freeness is the most widely used
control test in the stock preparation area of the paper mill, at least in
North America. Other standard measures of pulp drainage include
Schopper-Riegler (SR
o
), Williams slowness, and TAPPI drainage
time. All of these measures can be roughly converted to equivalent
CSF values.
Beating Response. There are a number of standardized bench top
beating devices that can be used to refine small pulp samples. The
Valley Beater and the PFI Mill are two such commonly used devices.
The nature of the fiber treatment in these devices is very different
compared to each other and to industrial refining, and it is not
possible to directly compare the results of one type of test to results
of another. As beating takes place, samples are withdrawn from the
test device to generate a refiner curve. In the case of the Valley
Beater, samples are typically collected at 5 or 10 minute intervals
over a period of 20 to 40 minutes depending on the ease of refining
for a given pulp. Pulp samples taken during the beating cycle are
typically tested for freeness, fiber length, fines content, and

handsheet properties.
Handsheet Testing. To test for sheet properties, pulp is diluted and
formed into handsheets using a sheet mold and a very specific
forming procedure. The wet sheets from the handsheet mold are
carefully removed and pressed under standard conditions. Pressing
is performed on several sheets at a time, with thin highly polished
steel plates between them. The pressed sheets adhere to the steel
plates which are placed on separator rings and stored in a
temperature and humidity controlled room. After a specified period of
drying and conditioning, the handsheets are peeled from the polished
plates and set aside for physical testing. One very important factor to
remember when using handsheet test results to predict machine
made paper is that handsheets are fully restrained by the polished
plates during the drying process. In other words, they are able to
shrink in the thickness direction but cannot shrink in the X-Y plane.
Machine-made paper, on the other hand, shrinks considerably in both
the machine and cross directions. Consequently, sheet properties
and the trends observed from beating can be very different for
differing conditions of restraint during drying.
Physical and optical tests – such as tensile strength (or breaking
length), tear strength, burst strength, caliper and light scattering - are
performed on pieces of the handsheets cut to specification. The
results of these tests are typically plotted against beating time,
freeness, and/or density in order to demonstrate the beating
response of the pulp being tested. Tests performed on handsheets
are similar to those performed on samples of machine made paper,
as described in more detail in the following section.
3. Paper Quality Measurements

There are a variety of tests that can be performed on paper
samples. Some measure strength or toughness, others measure
surface properties such as smoothness or coefficient of friction,
and still others measure optical properties such as opacity or
brightness.
Nearly all paper properties are a function of the density of the paper
which is, in turn, largely a function of refining. In evaluating
processing alternatives, it is therefore generally appropriate to
examine how refining affects the sheet properties of interest at a
given density. More importantly, density can have a direct economic
impact since many paper and board grades are sold not by weight
but by area, subject to a set of test criteria. If a producer can meet
sheet specifications at a lower density (i.e. at higher bulk), there is a
significant profit incentive to do so.
It is important to recognize that machine made paper will exhibit
significantly different properties in the direction of travel of the web
(machine direction or MD) compared with the perpendicular direction
(cross-machine direction or CD). As mentioned earlier, tests
performed on standard Tappi handsheets are not dependent on
direction.
Density. Density of paper is determined by measuring the weight
and thickness (or caliper) of a sheet of paper of known area. Basis
weight and caliper are usually measured using several sheets of
paper in order to reduce local variations. The inverse of density is a
measure of specific volume, most commonly referred to as sheet
bulk.
Tensile Strength. Tensile strength is measured by clamping a strip

of paper between two jaws of a tensile testing machine and
recording the resulting load as the moving jaw stretches the paper
to its breaking point. The maximum total force applied is called the
breaking load. The elongation (or strain) of the strip is also
measured, and the machine usually provides an output plot of load
versus elongation as well. The area under the load-elongation
curve represents the tensile energy absorption (TEA), and the slope
of the curve is called the elastic modulus.
From this we can define a Tensile Index, as
R
F
T
wR
=
Where,
Fr is the force to rupture the paper
w is the width of the paper (standard is 1.5cm)
Rupture
Elongation (mm)
Load (N)
ER
FR
A

R is the basis weight of the specimen.
One of the most common means of expressing tensile strength of
paper which is given as the length of paper that the paper can hold
up without breaking. This is called ‘Breaking Length’
That is,
3
10
101.97 101.97
R R
F F
L T
wRg wR
= = =
Tear Strength. Measurement of the out-of-plane tearing strength (or
tearing resistance) is done using a pendulum type device that
measures the energy absorbed in tearing a paper sample. A starting
slit is cut into the sample to initiate the tear, and the load is applied to
simulate a piece of paper being torn by grabbing it with two hands
and ripping it down the middle. Tear strength is a function of fiber
length, fiber strength, and the degree of bonding in the sheet.
The tear Index is given by
t
F
TI
R
=
Where Ft is the force required to tear the paper and R is basis wt.
Bursting Strength. This test is performed by clamping a paper
sample between two steel rings over a rubber diaphragm. The
diaphragm is then inflated, and the inflating pressure is measured at
the moment that the diaphragm bursts through the sample. Burst
strength is an indicator of sheet bonding and often trends with tensile
strength.

We can define a Burst Index as,
P
B
R
=
Where P is the burst pressure (kPa) and R is the basis weight.
It has been suggested that Burst strength is related to Tensile
strength as
1/2
R R
P KF E
=
Compressive Strength. There are several methods available to
measure the edgewise compressive strength of a paper or
paperboard sample. The Ring Crush Test is a traditional method in
which a ½” x 6” strip of paper is rolled end-to-end to form a very short
cylinder about 2” in diameter. The cylinder is then compressed axially
between two plates and the maximum load is measured. In recent
years, a short span compression test (STFI) has become more widely
used. Other similar tests have been developed for corrugated board
samples. Compressive strength increases with refining in a nearly
linear relationship to density.
Internal Bond or Z-Direction Strength. There are several test
methods, the most common of which is the Scott Bond test which
measures the energy or force required to separate a paper sample in
its thickness direction. This is a very important property for paper or
board grades that are printed on high-speed offset printing presses,
where high tack inks act to separate the paper into two layers at the
exit of the printing nip.

Stiffness. As implied, the stiffness test measures the resistance to
bending of a sample of paper. The Gurley type tester is the most
common. It can be used for a variety of paper and paperboard grades
by adjusting the length and width of the sample to keep the
measurement within a specified range. Stiffness can be a very
important characteristic of paper and is affected by refining in a
complex way. It is increased as the amount of bonding increases, but
decreased by the reduction in sheet caliper. Therefore, increasing the
amount of bonding at a given sheet density always improves
stiffness.
Porosity. Sheet porosity or air permeability is determined by
measuring the air flow through a known area of paper. It can be
reported in two ways: 1) as the time required for a known volume of
air to pass through a sample of paper, in units of sec/100 cc (Gurley
porosity); or 2) as the volume of air flow per minute, reported in
ml/min (Sheffield or Bendtsen). Refining always acts to close up a
sheet and make it less porous. Porosity is often used as an indicator
of the potential absorbency of the paper, particularly for coated
grades.
Smoothness. Smoothness (or, conversely, roughness) is often
measured indirectly using a Sheffield, Bendtsen or Parker Print Surf
(PPS) test. These tests measure the extent to which air flows
between the land area of a smooth ring and the surface of a paper
sample on which the ring rests. Both the air pressure and the
contact pressure of the ring are carefully controlled. More
sophisticated methods are available for measuring the microscopic
topography of a paper sample, but none are in common use yet.
Folding Endurance. This test measures the number of double folds
that a paper sample will endure while subjected to a fixed load in

tension. The most common test device is the MIT tester. The effect
of refining on MIT fold is not clearly predictable. Folding endurance is
very dependant on fiber length and coarseness, where longer and
less coarse is better.
Brightness. The brightness test is a measure of the reflectance of
light by paper. It is indicative of the apparent whiteness of a paper
sample. The brightness of pulp can also be determined by preparing
pulp pads or handsheets according to standard methods.
Opacity. Opacity is a measure of the relative reflectance of light by
paper on a pure black background compared with the reflectance
when backed by several sheets of the sample paper (or a calibrated
“white” backing). It is indicative of the usually undesirable tendency
for printing on one side of the paper to show through to the other
side. Opacity always decreases with refining due to the fact that the
bonded area between fibers is nearly transparent. Any increase in
the bonded area will increase the transparency and reduce the
opacity.
Scattering Coefficient. Light scattering coefficient is an alternative
form of presentation of the opacity test above, and it is often used
as an indirect measure of the relative bonded area (RBA) in a
paper sample.
Ash Content. This test measures the non-combustible portion of a
pulp or paper sample which is primarily made up of filler or coating
materials. Virgin pulps also have measurable ash content as a
result of minerals absorbed into the wood of the source trees. In
this test, a sample of known weight is placed in an oven at a
temperature of 525
o
C (or 900
o
C depending on the nature of the
fillers present) and fully combusted. The weight of the residual ash

after combustion is reported as the ash content.

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