|
This Section is Currently Under Construction
2. PAPER MAKING
MATERIALS
The materials used in
the manufacturing of paper must supply fibers capable of bonding to one
another as to create sheets resistant to tearing, wetting, and bursting.
The paper must also be able to be finished so that it is opaque and
surface smooth enough to print and write on. Fibers are generally
classified into three categories: animal, artificial, and vegetable.
Although some animal fibers, such as silk, are quite suitable for paper
making, most are not capable of bonding easily and are too hydrophobic to
be used in the paper process. Artificial fibers can be designed to bond
and cross-link forming strong bonds, but are not generally used to create
writing or printing substrates. It is much easier and more efficient to
create polymer sheets by heating polymer pellets to their glass transition
points or extruding the polymer as a sheet instead of a fiber. Therefore
artificial fibers are most often used as decorative accents or security
features mixed in with the vegetable pulp. Vegetable fibers, which
consist mainly of cellulose, are composed of fibers which form strong
bonds and create paper with excellent qualities. The vegetable fibers
used to create paper can be classified as woody and non-woody materials.
2.1 NON-WOOD
MATERIALS
It’s not known why the
practice of making paper from wood fibers was lost, but by the 10th
century paper-makers were using materials, such as cotton and linen rags,
to produce paper. (Paper Handbook) Although 95% of the paper produced in
the world is made from wood fibers, non-wood fibers are still the
paper-maker’s choice when producing fine or specialty papers.(mead) Seed
hair, like cotton and Bast fibers (such as linen) form a substrate that
stronger and more stable than their woody counterparts.
2.1.1 SEED HAIR,
BAST FIBERS, ANIMAL HAIR
A. SOURCES
Sources for these raw
materials range from bales delivered directly from the fields where the
cotton, linen, hemp, etc., are grown, to discarded clothing and recycled
materials used in the production of paper (broke).
Cotton (a seed hair) is
supplied to the paper mill in various ways and conditions, mostly in the
form of woven apparel scraps from textile mills and cotton linters from
thread manufacturing. Although not used much anymore, rags composed of
apparel cuttings, old worn discarded garments and dirty rags supplied by
merchants were a primary source of cotton fibers at one time. The use of
artificial fibers in textile blends made rags unsuitable for paper-making
.
Linen is a bast fiber
that is extracted from the stem of the flax plant (Linum usitatissimum).
Because of its strength, linen is used either alone or in combination with
cotton fibers in the manufacture of fine bond, security, and banknote
stock. The linen fiber is very similar to the cotton fiber, allowing
strong chemical bonding, resulting in an excellent blend for pulp.(mead)
Hemp and Jute are the
two other blast fibers used in paper making. Hemp (Cannabis sativa) has
been used to manufacture cordage for centuries, and until the late
nineteenth century, supplied tall ships with their miles rigging. It also
has been a source of raw materials to make specialty papers for cigarettes
and Bibles. A source for hemp is generally waste from rope and canvas
manufacturing. The fibers from the Indian plate, Jute (Corchorus
capsularis), are used for the manufacturing of coarse sacks, bags, and
heavy paperboards.(Sindall)
B. PREPARATION
The raw materials
arrive to the mill as large bales and must go through several procedures
before they are suitable to be used in manufacturing paper. Regardless
whether the material is cotton, linen, jute, or hemp, it must be sorted,
cleaned, cut, boiled, washed and beaten, before it can be shipped or
transferred.
When rags or textile
waste is used it must be sorted for quality and color and cut to aid in
digestion. The sorting of materials is performed by hand, while the
cutting and dusting (a process for removing any extraneous solid material)
are performed by machines. The duster consists of a long hollow rotating
cylinder of screen, inclined at such an angle that textile scraps tumble
toward the lower end discharging solid material attached to them. The
textile scraps then go through a cutter composed of revolving cylinders
containing blades which can be adjusted for length. The cut rags are then
placed through a final duster and shredder, known as a “Willow”. The
willow has one rotating drum with protruding spikes inside of another drum
with similar spikes, shreds the rags or textiles.
The shredded textile
material is finally placed in the boiler where chemicals remove fats,
starches, and oils present, by making them soluble in the boiling solution
(liquor). The shredded textiles, ropes, etc. are boiled in solution
consisting of Sodium Hydroxide (NaOH, caustic soda), Calcium Carbonate
(CaCO3, caustic lime), or a mixture of Calcium Carbonate and
Sodium Carbonate. The boiler can be either cylindrical of spherical with
a diameter of approximately 9 feet. The boiling time varies depending on
the raw materials and other conditions, but generally takes between 7 and
9 hours at pressures around 15 to 30 psi and 5-8% solutions. During the
boiling action, the unwanted substances are converted into soluble soaps,
which are removed with the spent liquor. (Sindall)
After boiling, the
shredded textiles are placed into a breaking and washing engine to
neutralize the liquor residues and any solid substances that might be
present. The breaking engine consists of an oval tub with a center
divider known as a “midfeather”. In one side of the tub, the floor rises
up to a metal plate with fixed blades, known as the bed plate. Above the
bed plate a rotating drum with blades across its surface breaks up the
rags further into fiber masses and removes any debris that might be
present. The floor of the engine falls away steeply behind the drum
causing a great reduction in pressure which greatly enhances the flow of
the pulp and water. On the other side of the engine there is large
hollow drum, covered with fine wire mesh, which continuously removes the
dirty water. As mush as 70,000 gallons of water are used during the
washing process. The bleaching process is often executed using the
breaking engine as an agitator. During the bleaching process the washer
drum is raised out of the tub and bleach (Calcium hyperchlorite) is added
to the water. After the pulp has been bleached to the required
brightness, the washer drum is lowered, the bleaching solution removed.
The pulp is then washed free of residual bleach.(Sindall)
Rag Breaking and Washing Machine
Empty
machine showing the beating blades and bed plate
After the washing and
bleaching cycles are over, the fibrous masses making up the pulp at this
stage are discharged into large cement holding tanks, and the water
allowed to drain through the perforated floors. The pulp at this stage is
termed “half-stuff.” The half-stuff needs to be further refined so that
the pulp exists as individual fibers. This is accomplished by a process
known as beating and is performed by the “Hollander” engine. The beating
process is critical to the physical characteristics of the final product.
Beating not only separates the pulp into single fibers, but also shortens
the fiber lengths and frays the edges into individual fibrils, creating
intra fiber bond breaking and increasing the chemical bonding
surfaces.(IPC)
The Hollander is
similar to the washing and breaking engine, with the exception of the
washing drum and larger beating cylinder. The beater is partially filled
with water and started with the beating drum well off of the bed plate.
The pulp is then slowly introduced, until the right consistency is
acquired. If the finished paper is a blend, the various pulps are added
to the beater in the ratios desired. The beating drum is then gradually
lowered toward the bed plate. The amount of beating time and the distance
between the plate and drum are dependent upon what type of pulp is being
processed. For blotting paper the time is short (about one hour), for
banknote paper, beating times can be as much as ten hours.(CFC, Swindell,
Mead). During the beating process additives can be placed into the pulp.
One class of additives is used to enhance the fiber to fiber bonding of
the pulp. These additives consist of natural gums and modified
cellulose’s (carboxymethyl and hyrdroethyl). Other additives, such as
urea-formaldehyde and melamine-formaldehyde polymers, provide wet
strength. Internal sizing, dye and pigments are also added at this
stage. Internal sizing provides resistance to penetration by water.
“Rosin, various hydrocarbon and natural waxes, starches, glues, casein,
asphalt emulsions, synthetic resins, and cellulose derivatives are some of
the materials which are used as sizing agents.”(ipc,)
After beating the
bleached pulp is fed to a stuff box and is ready for transfer to the paper
machine vat or head box.
2.1.2 ESPARTO AND
STRAW
A. SOURCES
Esparto grass (Stipa
tenacissima) and straw (the stem of oat, barley, wheat, etc.) reach the
processing mill as large bales.
B. PREPARATION
Much like unprocessed
cotton and linen, esparto and straw must go through several processes,
starting with the duster and willow. The process for dusting and cutting
esparto and straw is the same as that used for cotton and linen (described
in the previous section 2.1.1). After the Willow, the cut esparto and
straw are placed in boilers, also known as digesters, and boiled with
sodium hydroxide. The boiler for esparto is different than that used for
straw. The esparto boiler, a Sinclair Vomiting Esparto Boiler, is a
stationary vertical cylinder. The liquor collects at the floor and is
forced up through pipes to the top of the cylinder, where it sprays back
down onto the saturated grass. The boilers used for straw are the same
configurations, as those used for cotton and linen, either a stationary
vertical boiler or a revolving cylindrical boiler. The spent liquors,
“black liquor,” are collected, recovered if possible, or disposed of. The
resulting half-stuff is washed and beaten in the same manner as cotton.
2.2 WOOD MATERIALS
A. SOURCES
The raw materials for
wood pulp are supplied either by trees in the forest or recycled paper.
Wood can be divided into two main categories hard or deciduous and soft or
coniferous woods. Softwoods supply the finished paper with strength,
while hard wood provides better surface characteristics. The main supply
for both is the forests. The stem of the tree is about 50% cellulose, 25%
lignin and 25% hemicellulose.
Cellulose is a straight
chain crystalline polymer containing approximately four to five thousand
glucose units. Cellulose fibers exist in a complicated structure where
the molecular chains are wrapped around the core in three layers.
Hemicellusloses are also polymeric sugars and are often polymers
consisting of a mixture of two or more monomers, such as xylose, arabinose,
galactose, and glucose. Hemicelluloses are shorter chains than cellulose
and are more susceptible to attack by alkalies. Both Cellulose and
Hemicellulose are hydrophilic. Lignin is a hydrophobic,
three-dimensional polymer, containing phenyl propane units as the main
monomers. Lignin is considered to be the glue that holds cellulose fibers
together and is an excellent binder in paper, but is usually lost in the
pulping processes.
The first step in
getting the wood to the mill involves the harvesting of the trees. The
harvesting is accomplished by machinery which essentially cuts the trees
and prepares them for shipping. Shortwood, Longwood, and Full Tree are
the three systems used to harvest trees. Shortwood systems produce bolts
of wood ranging from 96 to 100 inches depending on the shipping
regulations. These bolts of wood are able to be transported on trucks
with their length perpendicular to the long axis of the truck. Long wood
systems produce logs that must be transported parallel to the long axis of
the truck. In full tree systems the trees are transported without being
limbed or topped.(IPC)
If shortwood or
longwood systems are used the resulting bolts must be debarked before they
are chipped. Bark is unsuitable for paper making for several reasons
including low cellulose yield, non-fibrous materials, and soil. The
debarking process is usually accomplished using a cylinder or drum
debarker. This type of debarker has a rotating hollow drum that has
raised narrow plates that run parallel to the length of the drum. As the
logs travel down the drum they scrape against the blades and each other,
effectively removing their bark.(mead) At this point the logs are either
pulped as stone grounded or are chipped for other processes.
If the wood is needed
as chips the logs are moved to either a drum chipper or disk chipper. The
drum shipper consists of teethed feeder rollers that move the logs down to
a rotating drum with cutting knives. As the drum rotates, it shears the
logs into chips. The disk chipper, often used with whole trees, contains
blades, mounted on a rotating disk. As the logs or trees are fed in,
perpendicular to the disk, the blades cut them into chips. The resulting
chips are then placed through a chip screen. Most modern chip screens are
disk screening systems. The screening process removes any bark, large
chips, or soil present. The disk screening system consists of several
rotating cylinders composed of toothed edges. The resulting smaller clean
chips are stored in piles until they are shipped to the paper mill.(Mead,
ICP)
B. PULPING PROCESS
1. MECHANICAL
MECHANICAL PULPING
(MP)
Mechanical pulping is
the oldest method of acquiring wood fibers from trees. It started being
used commercially in the 1840’s and has seen many improvements is the last
century. Using chips instead of logs, grinding under pressure and the
addition of chemicals are only some of the improvements that have
increased yields and efficiency. Historically low strength, poor print
performance, and a lack of permanency, have been the main disadvantages,
but with newer methods and chemical treatments, mechanical pulp is one of
the most popular choices for inexpensive printing papers, such as
newsprint, magazines and paper towels. The lignin present in ground wood
creates opacity in paper and as a result ground wood is often added to
lightweight papers. Unfortunately lignin also yellows and discolors the
paper with time. (gatf)
The main advantages of mechanical process are high yield, lower capitol
cost and no hazardous wastes. The disadvantage are large power
requirements and low pulp quality.(ICP)
STONE GROUNDED (GW)
Stone grounded pulping
involves physically tearing the fibers apart. One of the main advantages
is the high yields, approaching 100%. The disadvantages rest in the
amount of power consumed in this mechanical process. In stone grounded
the debarked logs are forced under pressure against a rotating
grindstone. There are several grinder designs differing in how the logs
are forced against the stone, but the basic theory and process are the
same. The grinding wheel is constantly sprayed with cooling water which
also rinses the fiber masses into a vat creating a pulp slurry. (IPC)
Stone Grinders
Adapted from Mead
REFINER MECHANICAL
PULPING (RMP)
Refiner Mechanical Pulp
is a variation of the previous method, but instead of six foot logs, it
starts of with wood chips that are steamed and placed in one or more disk
refiners. Disk refiners are a newer development in refiners and consists
of two rotary disk pressed together. The stock is fed through the center
and exits throughout the edges of the disks.
THERMOMECHANICAL
PULPING (TMP)
Thermomechanical
pulping is one of the fastest growing processes in North America. Like
RMP, TMP starts with steamed wood chips, which are then subjected to high
temperatures (120-130C) during refining. The process destroys the cell
walls exposing the cellulose bonding surfaces.(IPC) TMP has the largest
yield of the mechanical processes with less damage to the fibers and no
chemical waste.
CHEMITHERMO-MECHANICAL
PULPING (CTMP)
Chemithermo-mechanical
pulping is similar to TMP. Sodium Sulfite is usually added to the
steaming solution to soften the wood even further than the steam could
alone. The softer chips require less energy to be refined into fibers.
The pulp resulting from this process is generally stronger than TMP and do
not require the addition of chemical pulp for strength.
PRESSURIZED GROUND
WOOD (PGW)
In this process the
logs are ground in a pressurized atmosphere. The power consumption is
about the same as atmospheric grinding and the quality of the pulp is
similar to TMP.
2. CHEMICAL
Although physical
pulping produces high yields, the presence of lignin darkens the paper
over time. Chemical pulping processes, on the other hand, removes
undesirable compounds, like lignin, producing a brighter, whiter paper
that will not darken over time. Most of the chemical processes also
remove the hemicellulose as well. Unfortunately chemical processes also
result in yields as low as 45% and chemical waste that must be disposed of
in a safe and environmentally friendly manner. The wood chips are placed
into steamers and then into either batch or continuous digesters. In
these digesters the lignin and hemicellulose are dissolved and the
cellulose is reduced to fibers. The resulting pulp is then screened,
bleached and refined, and placed in the stuff box.
FULL CHEMICAL
SODA PULPING
The first type of
chemical pulping used sodium hydroxide (NaOH) as the active reagent to
dissolve the lignin. First used in 1854, this process involved placing
wood chips into either a horizontal digester approximately seven feet in
diameter by 20 feet in length or a vertical digester approximately 14-15
feet in diameter by 30-50 feet in height. Green (fresh) liquor consisting
of 10-15% NaOH, as the active reagent, and sodium carbonate, as the
make-up, is pumped into the vessel. The vessel is pressurized (90-135
PSI) and the contents boiled (150-170C) for four to thirty hours,
depending on the pulp and digestion vessel. The resulting paper is bulky,
soft and opaque.(Swindell) Although the original soda pulping process is
no longer commonly used, many manufactures are turning back to sodium
hydroxide because of waste disposal concerns. (IPS)
KRAFT PULPING
Kraft pulping is a
variation on soda pulping, basically consisting of a change in cooking
liquor. The Kraft cooking liquor has the addition of sodium sulfide to
create a buffering solution with the sodium carbonate protecting the pulp
from high pH levels. Like soda pulping, wood chips are fed into a
digester, where they are cooked under slightly higher pressures and
temperatures, for shorter periods of time (4-6 hours). The boiling can be
carried out in batch processes or continuously, depending on the abilities
and needs of the mill. The cooking liquor dissolves about 88-92% of the
lignin from the pulp and supplies about 45-55% yield. (IPS) The chemical
reaction for the attack of sodium sulfate on lignin is as follows:
Na2SO4 + 2C + 4NaOH→
Na2S + 4Na2CO3 + 2H2O
The “black” liquor
which contains the dissolved lignin, hemicellulose and other soluble
chemical compounds is incinerated. The waste discharge, which contains
methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3),
dimethyl dissulfide (CH3SSCH3) and other sulfides,
is known for it’s foul smell.(GATF)
SULFITE PULPING
Sulfite pulping
operates at the other end of the pH spectrum from the soda and Kraft
processes. The resulting paper is hard and has good writing and printing
characteristics, but can lack permanence is the acid content is not
properly neutralized. The cooking liquor is created by burning sulfur,
creating sulfur dioxide (SO2) which is pumped into the bottom
of towers containing wet limestone. As the gas ascends through the tower,
four reactions take place converting the sulfur dioxide and lime slurry
into calcium bisulfate (Ca(HSO3)2) and sulfurous
acid (H2SO3). (gatf)
Rxn 1. S + O2 › SO2
Rxn 2. SO2
+ H2O › H2SO3
Rxn 3. CaCO3
+ SO2 › CaSO3 + CO2
Rxn 4. CaSO3
+ SO2 + H2O › Ca(HSO3)2
The resulting solution
is approximately 6-8% SO2 with a pH of 1.5-2.5. (IPC).
Because of problems
with waste disposal, the substitution of magnesium, ammonium, or sodium
acid sulfites or neutral sulfites have replaced calcium in the mills that
still use this process.
BISULFITE PULPING
The bisulfit and
Magnefite processes are variations of the sulfite process. In bisulfite
pulping the cooking liquor is solely bisulfite. The resulting paper is
used mainly as newsprint. If magnesium is substituted in place of calcium
the process is refereed to as Magnefite pulping. In both processes the
pH (3-6) and yield are higher than if the acid was left in the liquor.
SEMI-CHEMICAL-
NEUTRAL SULFITE
PULPING
Neutral sulfite pulping
is a process where the chips are digested using sodium sulfite which
softens and partially breaks down the wood. Pulping is then added by
mechanical refiners which grind them into pulp. The pH of the cooking
liquor approaches seven, thus giving the designation neutral. The
products of this process are generally used as starting materials for
corrugated boards.
C. BLEACHING
PROCESS
Some lignin still
remains attached to the wood fibers in most pulping processes and must be
eliminated by one more process known as bleaching. For esparto and straw
bleaching powder is dissolved in water and mixed with the pulp. For wood
pulps more aggressive bleaching is used in single stages or multiple stage
system with the use of chlorine’s, and oxidizing and reducing agents. The
main objective in the bleaching process is to create white pulp. White
paper increases contrast between the paper and printed matter and provides
a neutral substrate for color.
1. CHEMICAL PULP
SINGLE STAGE
In the early years of
production , chemical pulp was bleached in a single stage, using strong
solutions of chlorine, hyperchlorites, and peroxides. Such aggressive
bleaching often resulted in reduced yields and weaker pulps. An
alternative is to use several weaker processes, which results in whiter,
stronger paper. This alternative method is known as multi-stage
bleaching.
2. CHEMICAL PULP
MULTI-STAGE
Multi-stage bleaching
can involve as few as three steps and as many as six or seven. Each stage
is designed to remove or alter colored substances in the pulp. The main
source of color is lignin and resin.
CHLORINATION
The first step in
multiple stage bleaching is the use of chlorine gas pumped through an
aqueous solution. The chlorine attacks the lignin forming two insoluble
chlorolignin and oxidized lignin compounds and an acid. The acid must be
removed or neutralized to keep pH around 6-8 to preventing the chlorine
from damaging the cellulose. (mead)
ALKALINE EXTRACTION
After chlorination the
insoluble lignin compounds must be separated from the pulp, which now is
orange in color. This is accomplished by the introduction of sodium
hydroxide, which reacts with the lignin compounds, displacing the chlorine
with hydroxyl ions, making them water soluble. The water soluble lignin
compounds are then washed out of the pulp. The pulp is now almost lignin
free and slightly yellow in color.
HYPOCHLORITE STAGE
The hypochlorite stage
further bleaches the pulp, removing any stains and increasing the
whiteness of the paper. Hypochlorites (OCl-) are usually prepared by
absorbing chlorine in sodium hydroxide or calcium hydroxide. The
resulting hypochlorite is a strong oxidizing agent and will oxidize any
remaining lignin. To keep the cellulose from being oxidized, the pH must
be kept high.
CHLORINE DIOXIDE and
OXYGEN STAGES
Chlorine dioxide and
oxygen are used to further oxidize any remaining coloring agents left in
the pulp. Because of their unstable states they are often generated at
the mill and only right before use. Depending on the degree of whiteness
desired, these two processes can be repeated multiple times.
3. MECHANICAL PULP
Mechanical pulping
produces pulp that contains a high percentage of lignin that must be
neutralized from yellowing the paper. If mechanical pulp was bleached
with the same methods used for chemical pulps the yield would drop
drastically countering the main advantage of mechanical pulping.
Therefore the bleaching system for mechanical pulp must eliminate colored
material without degrading the pulp.
PEROXIDES
Peroxides are the most
effective bleaching agent for pulps with large amounts of lignin. Hydrogen
Peroxide and dissociation of the Perhydroxyl ion (O2H-) are the
active ingredients in one bleaching solution. As a result, the bleaching
occurs at high pH levels (9-11 with a 1-3% sol’n at 35-55C for 1-5hrs),
which must be neutralized and stabilized by the addition of SO2.
REDUCING BLEACHES
Sodium and Zinc
Hydrosulfites (Na2S2O4, ZnS2O4)
are also used to increase the brightness of the pulp and are sometimes a
second stage to the use of peroxide bleaching. A four-percent solution
with a pH 5-7 at 60C for 1 -2 hours is common practice (IPC)
CONTINUOUS BLEACHING (RXN
DRIVING)
Another method of
bleaching involves cycling between two bleaching solutions (NaOH and ClO2)
and driving the reaction by continuously adding new solution and removing
the depleted reagent and products.
|