NCSU Pulp & Paper Education Pak 1
Papermaking: Fibers Into Paper
Goals
In this activity, students will carry out the steps necessary to convert pulp into a sheet of well-formed, strong paper. First, the pulp fibers will subjected to refining, a mechanical treatment which converts the hollow-tube-like fibers into collapsed, flat ribbons with hairy surfaces. Second, the refined pulps will be placed into a dilute suspension in water and drained on a fine-mesh wire to form a wet sheet of paper. The sheet will then be dried to cause the fibers to bond with each other.
Key Learning Concepts
In conducting this activity, students should learn the theory and application of the following concepts:
The physical structure and nature of cellulosic fibers from plants
The use of refining to make pulps suitable for papermaking
How papermaking is done
Hydrogen bonding between fibers
Consistency of a fiber/water slurry
Measuring out the amount of fiber for proper sheet weight
Basic Equipment Required
* kitchen blender with lid * handsheet forming apparatus
* paper towels * iron
* scale or balance
Safety Equipment Required
* safety glasses or goggles
Background and Key Concepts
The Structure of Cellulosic Fibers
Fibers for papermaking are made up of cellulose, a polymer of glucose, the sugar manufactured by the plant during photosynthesis. A single glucose molecule has a total of five carbons linked together. One end of the molecule has an alcohol group, while the other has an aldehyde group. The rest of the carbons contain hydrogen and hydroxy (OH) groups as side constituents.
COH
H OH
HO H
H OH
H OH
CH2OH
For the production of cellulose, the plant converts each glucose molecule into a six-membered ring and then links the rings together to form a linear polymer. Even with these changes, the outside of the cellulose chain is still covered with hydrogen and hydroxy groups.
Chemically, a hydroxy group has a partial negative charge. By bonding with a positively-charged group like hydrogen, the group can be satisfied and achieve a more neutral, stable state. In fact, it can be said that an OH group wants to be water, and it will freely associate with the hydrogen on another molecule to do so. The resulting bonds – hydrogen bonds – are very strong and are the bonds that hold fibers together in paper.
In the wet fibers of pulp, the hydroxy groups along the cellulose chain are associated with hydrogen atoms on the water molecules. For this reason, cellulosic fibers are very hydrophilic (water-loving). There is a thin layer of water surrounding the fibers. In order to bond fibers together and form paper, it is necessary to a) bring the fibers close enough together so that their hydrogen and hydroxy groups can access each other, and b) remove almost all the water in the fibers, so that the hydrogen bonds between fibers and water will be converted to hydrogen bonds between fibers.
Remember – nothing (glues, resins, etc.) has to be added to paper to get it to bond together. When dried, the fibers bond themselves together.
Hydrogen bonds may seem small, but they are strong. A typical strip of printing and writing paper one inch in width, if suspended from one end, would have to be over 6 kilometers in length before it would break of its own weight. On a strength per unit weight basis, paper qualifies as one of the strongest engineered materials.
The Definition and Structure of Paper
Paper is defined as a thin, flexible web consisting of individual cellulosic fibers deposited randomly on each other in a water suspension and then dried to form interfiber hydrogen bonds.
For a typical printing and writing grade, a sheet of paper is about 6-7 fibers thick. The following is a photomicrograph the edge of a paper sheet:
Refining
Even when liberated during pulping, plant fibers are not in a form most suitable for papermaking. The fibers still are hollow conducting tubes, which limits the surface area available for bonding to other fibers.
Try this test: take a common plastic drinking straw place it next to another. The point of contact is limited to a thin line between the two. Even when a number of straws are placed around the original straw, there is a significant amount of surface area on the straw which does not contact another straw.
If the straw could be collapsed into a flat ribbon, with no open area in the middle, the entire periphery of the fiber would be available for bonding.
Refining is an intense mechanical process carried out on pulp to cause the fibers to be ready for papermaking. Strong paper cannot be made without this treatment. Refining actually performs two key tasks to increase fiber-to-fiber bonding:
Collapses the fibers into flat ribbons
Shreds the outer layer of the fibers (called fibrillation), making it hairy-looking and producing even more surface area for bonding
The illustration below shows the effect of refining on papermaking fibers:
Strong paper cannot be made without the use of refining. Without refining, sheets of paper feel like felt -- fluffy and weak.
In the pulp and paper industry, refining is carried out by passing the pulp between grooved plates rotating at high speed less than 1 mm apart. In the laboratory, a very similar action can be achieved using a common household blender.
Although refining causes some bad effects in the pulp — water drains more slowly from it, and some of the fiber is damaged severely in the process — papermakers need the strength of refining so much that they are willing to put up with these other effects.
Considerations in Making Paper
Paper is judged on four main characteristics:
1. Strength – paper must be able to resist pulling apart, tearing, and puncturing when being used.
2. Optical properties
Ø Brightness
Ø Opacity – the ability of a sheet to resist print show-through from the other side
3. Surface properties – printing and writing grades require a smooth and printable surface
4. Appearance – the sheet must appear uniform and free from flaws
These properties are directly affected by the papermaking process.
The goal of the papermaking process is the following:
· dilute the fibers in a large quantity of water, so that they have less chance to collide and form blobs
· deposit the fibers in a uniform, random mat by draining the bulk of the water through a fine mesh wire
· press the sheet so that the fibers come into intimate contact
· dry the sheet
Papermaking is done continuously in the industry on the fourdrinier paper machine. In the laboratory, sample sheets of paper are made using a handsheet mould.
Papermaking Experiment:
Forming Handsheets in a Mould
In this activity, students will first carry out refining on different types of pulps in a kitchen blender. The pulps will then be made into handsheets, using a handsheet mould. Students will learn how to transfer (“couch”) the wet sheet off the mould wire and do simultaneous pressing and drying using an iron.
Concept to Remember:
Consistency and Fiber Quantity Measurement
“Consistency” refers to the % solids (fiber) by weight in a fiber/water suspension or slurry. Using the consistency, it is possible to determine the actual amount of dry fiber in a given sample of wet pulp or a pulp slurry, using the following formula:
OD fiber, grams = (Wet Weight of Fiber, grams) x (% consistency/100)
Conversely, the formula can be rearranged so that it can be determined how much wet/air-dry pulp or slurry to weigh out to get a specified quantity of dry fiber:
Wet/AD fiber or slurry to weigh out, grams = OD Fiber desired, g (% consistency/100)
TASK 1:
REFINING
Refining can be done at the laboratory scale in a kitchen blender at 1 % consistency. Depending on how long, thick, and strong the fibers are, a blending time of between 10-45 minutes is normally required to get the fibers into a state suitable for making strong paper. Pulps from recycled paper, since they have been refined before, normally require less refining than for virgin pulps.
For simplification, the basic laboratory will be conducted using the following assumptions:
Ø the consistency of wet pulp drained in the Buchner filter flask until no mater water is observed leaving will be assumed to be 7 %
Ø the consistency of paper to be repulped and recycled, at room conditions, is assumed to be 90 %
It is most accurate to measure the moisture content of a pulp slurry using an oven method before refining. A method for this technique is described in the Advanced Applications section.
1. The basic recipe for each batch of pulp to be refined is 8 dry grams of fiber and 530 ml (530 g) of cold tap water. Never use hot tap water, since the refining action will add a significant amount of heat to the slurry – if the temperature gets too high, it can change the refining action.
2. If using paper to be recycled (dry paper at room conditions), then, according to the formula described earlier:
Dry paper to weigh out, g = 8 grams OD fiber desired
(90 % consistency / 100)
= 8.9 grams
Note: cut paper into small pieces before placing into blender.
3. Similarly, if using wet pulp drained in the Buchner filter funnel:
Wet pulp to weigh out, g = 8 grams OD fiber desired
(7 % consistency / 100)
= 114 grams
4. Place the weighed-out paper or pulp into the blender, along with 791 ml (791 g) of cold tap water if using recycled paper or 686 ml (760 g) of cold tap water if using filtered wet pulp.
Processing ANY material in a kitchen blender can be very hazardous. Before turning on the blender, do the following:
a) make sure that the spout of the blender container is turned away from you or anyone else; b) make sure the container lid is on securely; c) place a gloved hand on the lid to insure that it doesn’t pop off during the initial surge of material. YOU MUST BE WEARING SAFETY GLASSES!
5. Energize the blender for 10 minutes. Use the “Blend” setting or similar mid-range speed. Use the same speed for all experiments!
6. The slurry is now ready for measuring out for handsheets.
TASK 2:
PAPERMAKING
Sample sheets of paper made in the laboratory are known as handsheets.
They are made using a standard device known as a handsheet mould. The process of making paper involves the following steps:
a. dilution of the fibers with water
b. drainage and formation of a wet sheet on a wire mesh
c. “couching” (pronounced COO-CHING) – transfer of the wet, embryonic sheet of paper on the wire to a dryer surface, such as a blotter
d. pressing to remove more water and bring the fibers into intimate contact
e. drying
Concept to Remember:
Basis Weight of Paper
One of the most critical attributes of a manufactured sheet of paper is its “heaviness” – how much fiber there is in the sheet per unit area. The terms for this heaviness is “basis weight.” It is expressed as
Basis Weight = grams of dry fiber
square meter paper surface (one side of sheet)
Since the sheet mould is about 5 by 8 inches, the corresponding surface area of sheets made in it is 0.0257 square meters. To determine the amount of dry fiber required to achieve a specified sheet basis weight,
Dry fiber required, grams = (Basis weight desired, g/square meter)
x 0.0257 square meters
1. The target basis weight for this activity is 100 g/square meter. The amount of dry fiber required, according to the formula above, is
Dry fiber, grams
= (100 g/square meter) x (0.0257 square meters) = 2.6 grams
2. Since the slurry consistency after refining is supposed to be 1 %, then the amount required to measure out for each handsheet is
Slurry to weigh out, g = 2.6 grams dry fiber desired
(1 % consistency / 100)
= 260 grams
3. Using a stirring device of some kind, make sure that the slurry in the blender container is properly mixed prior to measuring out a sample for handsheet making.
4. See attached “Handsheet Preparation” manual for the use of the handsheet mold to make your sheet of paper. Use the amount described above instead of the amount specified in the manual as the quantity to make paper.
Basic Application
For Student Projects
1. To demonstrate that refining is necessary to produce strong paper, students can make handsheets with identical basis weight from a variety of pulps in unrefined and refined (5-10 minutes) states.
Suggestions:
a. Wood market pulp (pulp that has never been refined) – available by request from NCSU
b. Wood recycled pulp – newsprint, old brown boxes, xerographic or writing paper
c. Nonwood pulp made in the laboratory
d. Toilet Tissue
The effect of refining is most noticeable on the wood market pulp.
After making handsheets, students can test each for basic tensile strength, using the the Paper Testing Lab manual. In this manner, students can compare the strength of the various pulps and demonstrate how refining greatly increases sheet strength.