Art Classes with Marc Surrency

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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.

 

O

O

O

CH2OH

H

CH2OH

H

CH2OH

H

O

O

OH

H

OH

OH

H

H

H

H

H

H

OH

H

CELLULOSE

N-2

OCH3

OH

C

C

C

CH3O

OCH3

OH

PHENYL PROPANE UNITS

C

C

C

OH

H

OH

H

OH

H

OH

H

 

 

 

 

 

 

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.