How To Make A Paper? (Chemists' Recipe)
In this section, we have a discussion about the chemistry of paper manufacturer. We will see the chemistry of paper especially the processing to make a paper and it will be concluded the future trend of paper. To begin with, we will have a time travel through the history of paper.
The first paper was invented for the first time by ancient Egyptian where they used papyrus paper (left). Papyrus paper was made from papyrus tree which was common in the bank of Nile river but this paper is fragile, rough and had limited supply. After that, in 200 BC ancient Greek used a parchment which was made from stretched, dried calf, sheep, or goat skin; but this paper was expensive, sensitive to humidity, and also limited supply. Then, a Chinese court official, Cai Lun, inspired by bee and wasp nests to make paper from wood pulp (especially mulberry tree) in 105 AD and this paper was used for writing, toilet paper, banknotes, etc. The first paper mill was founded in Baghdad, Iraq at 739 AD but this paper-making by hand is a slow and batch process. The modern paper-making uses the fourdrinier machine (1801) allows high speed and continuous paper-making.
Then, as we travelled through time we will have a look the market of the paper nowadays. Total annual world paper production is currently around 350 million tonnes (UK: 6 million tonnes) and mainly used for packaging, printing, and writing.
For example the scale of this paper usage, each issue of the Sunday Times uses 15 000 - 20 000 trees and one ream (500 sheets) office paper uses 6% of tree, but 1.2 trees planted in Sweden per tree cut down. Besides that, global wood usage for paper pulp are 35% softwood evergreens (e.g. fir, pine, spruce) and 65% hardwood deciduous trees (e.g. birch, maple, aspen).
The main ingredients of paper is obviously the woods, so what is wood exactly? In general, a wood is mostly composed of cellulose fibres which gives tensile (stretching) strength and lignin matrix provides compressive strength. To make it easier to imagine, wood is similar with reinforced concrete where the steel bars in similar way with cellulose and concrete as the lignin matrix.
Cellulose is a linear poly(β-glucose) where the fibres are insoluble due to hydrogen bonding interaction between chains. Moreover, cellulose is formed by condensation reaction of β-glucose and pure cellulose is white coloured.
Meanwhile, lignin is aromatic, highly crosslinked, and coloured; and the monomer of lignin is monolignol. Lignin is formed by radical oxidation reaction and it causes paper to yellow on ageing due to oxidation.
In paper manufacturing, the first step is pulping where the wood is broke down into fibres by either mechanical or chemical pulping (Kraft pulping). In mechanical pulping, wood is simply ground down so the pulp still contains lignin which makes paper yellows with age. Besides that, in grinding process it shortens the fibres which makes the paper weaker but it is high yield and low cost so it is suitable for newsprint industry. Meanwhile, chemical pulping dissolves and removes lignin matrix from cellulose fibre. From this process, longer fibres can be produced so stronger paper can be made, but this process has lower yield and more expensive. Besides that, the pulp remains brown and usually bleaching is needed; non-bleaching pulp is used to made brown paper bags or bleaching pulp is used to made high quality white paper.
The chemical pulping or Kraft process is used OH− and SH− to degrade cross-linked lignin to form water-soluble phenoxide fragments. However, this process is quite inefficient as cellulose is partially degraded by the aggressive conditions with overall yield of cellulose from wood is approximately 50%.
After pulping, the pulp is bleached as lignin fragments contain conjugated chromophores that remain after chemical (Kraft) pulping. The conjugation is destroyed by oxidation using bleach, so bleaching destroys residual lignin to reduce yellowing. Traditionally, Cl2 is used but now replaced by ClO2 for Elemental Chlorine Free process. Based on its reduction to Cl−, ClO2 is 2.5 time more oxidising than Cl2 per unit mass. After the addition of bleaching agent, NaOH is added to solubilise oxidised lignin fragments and partially removes 'organic' chlorine and this process is repeated for certain sequences. This process is 5 - 10 times less chlorinated by-products than Cl2, but this process is expensive, but highly selective oxidant for lignin. Furthermore, there is another bleaching agent which is called 'Totally Chlorine Free' process which uses O2/O3/H2O2.
The bleaching process that showed above could determine the paper longevity. To begin with, in pre-19th century paper was made from cotton-fibres and cotton-based paper is alkaline, typically lasts for centuries. Besides that, the original wood pulp-based paper was acidic, so light exposure causes yellowing and embrittlement. Therefore, many books from 1870-1950 now very badly degraded especially if acidic inks, alum rosin used in manufacture as paper degradation problem first identified in 1950's. Nowadays, modern paper manufactured by either alkaline or acidic process. Furthermore, high-quality paper is alkaline, contains CaCO3 filler/buffer and this 'acid-free' paper is more expensive but much longer-lasting.
In paper manufactured, additives are also added, and of the example is for internal sizing. As we acknowledged cellulose surface is hydrophilic so it needs to increase cellulose contact angle to prevent water penetration. Therefore, to overcome this problem, introducing surface with hydrophobic group is known as sizing (not related to paper size). In unsized paper, an extensive spreading and water penetration, but with introducing the hydrophobic group (increase sizing) it reduces wetting to make a sharper image on the paper.
One of the common compounds that is used in sizing is rosin (pine rosin) where the carboxylic acid group is bound to hydrophilic cellulose surface by ionic interaction with help of Al3+. Furthermore, modern sizing uses chemically reactive anhydrides.
The others additives that are used in paper manufacturing are fillers and pigments. Commonly, inorganics added to improve optical and physical properties (denser, brighter, softer, and smoother) and these fill up spaces between fibres as inorganics comprise 5 to 15% by mass of paper. Furthermore, inorganics are cheaper than pulp so it reduces cost but weakens paper.
In general, paper are more stable when slightly basic at pH 7.5 - 9.0, so it needs to add less TiO2 to give stronger paper. Furthermore, CaCO3 is added as buffer to counteract the acidity, e.g. sweat.
The next process in paper-making is what happens inside the fourdrinier machine. So to begin the process, the pulp is diluted to 1% solids aqueous suspension and sprayed onto a moving screen. Then, it dewatered by gravity and suction to 20% solids (2). After that, paper then pressed and dried over a steam-filled cylinders to a few percent water (3). This is an expensive process due to energy-intensive process and a modern paper-making machine is essentially an extremely fast and continuous filtration system. At 10 m wide rolls of paper is produced at a speed of 1700 m per minute (100 km per hour) which means 12 kg s−1 (1000 tonne per day) or this machine has to be fed with 10 000 - 20 000 trees per day.
As pulp comprises of large fibres (1 - 5 mm x 25 μm) and much smaller fillers (1 - 5 μm) in water, there are 2 problems often occur during dewatering. The problems are fillers are too small to be retained on the mesh screen and fillers also block the pores between fibres, causing slower water drainage. Therefore, the technical solution is to stick small filler particles onto larger fibres, but most fibres and fillers have negative surface charge, so they are mutually repulsive. To overcome this problem, high molecular weight cationic polymer is used as 'glue' to stick fillers onto fibres.
The final step in manufacturing of paper is surface coatings where final layer is added to improve gloss, printing resolution and strength. The coating typically consists of an aqueous suspension of pigment (e.g. clay or TiO2). Finally, paper is super-calendered or compressed between hot rollers to allign clay platelets, improving gloss. Furthermore, latex binder can also be used as a surface coating as latex comprises microscopic film-forming polymer particles in water.
Some say, paper manufacturing in non-environmental friendly, but it is not completely true as there are some environmental considerations. Firstly is 'black liquor' (degraded lignin solution) is concentrated by evaporation and burned to provide energy for paper mill processes (e.g. produces steam for paper drying). Secondly, molten inorganics (Na2CO3, Na2S) can be recovered from furnace for recycling for example Na2CO3 is converted to NaOH using Ca(OH)2 as shown below.
Lastly is chlorine-free bleaching reduces waste-water contamination.
Besides the ordinary printing paper, there are some speciality paper and the examples are followed
Most made from cotton and linen fibres which is stronger than wood pulp and Australian dollars are made entirely of polypropylene. Besides that, many security features such as UV dyes and holograms, and also watermarks are pressed into the pulp as it dries.
The future trend of paper such as E-ink is a US-based electrophoretic display company where electric field rearranges white (+) and black (-) encapsulated particles. Besides that, E-ink requires no power to hold the image which means a possibility for low power consumption for electronic paper application. The displays can be made with conducting polymers where circuitry is inkjet-printed onto flexible substrates and allows production of very light rollable displays. Furthermore, it is compatible with either E-ink or OLED technology. Hence, the question is will rollable and/or flexible displays replace books and newspapers?
As the conclusion of this part, here a summary of paper making process.
 |
| The fourdrinier machine |
Then, as we travelled through time we will have a look the market of the paper nowadays. Total annual world paper production is currently around 350 million tonnes (UK: 6 million tonnes) and mainly used for packaging, printing, and writing.
 |
| Paper usage and global paper production |
 |
| The common trees for paper manufacturing |
The main ingredients of paper is obviously the woods, so what is wood exactly? In general, a wood is mostly composed of cellulose fibres which gives tensile (stretching) strength and lignin matrix provides compressive strength. To make it easier to imagine, wood is similar with reinforced concrete where the steel bars in similar way with cellulose and concrete as the lignin matrix.
 |
| Wood structure and the reinforce concrete analogy |
 |
| Cellulose formation |
 |
| Lignin formation |
In paper manufacturing, the first step is pulping where the wood is broke down into fibres by either mechanical or chemical pulping (Kraft pulping). In mechanical pulping, wood is simply ground down so the pulp still contains lignin which makes paper yellows with age. Besides that, in grinding process it shortens the fibres which makes the paper weaker but it is high yield and low cost so it is suitable for newsprint industry. Meanwhile, chemical pulping dissolves and removes lignin matrix from cellulose fibre. From this process, longer fibres can be produced so stronger paper can be made, but this process has lower yield and more expensive. Besides that, the pulp remains brown and usually bleaching is needed; non-bleaching pulp is used to made brown paper bags or bleaching pulp is used to made high quality white paper.
 |
| Pulping processes. The result of mechanical pulping (left) and chemical pulping (right) |
 |
| Kraft process |
After pulping, the pulp is bleached as lignin fragments contain conjugated chromophores that remain after chemical (Kraft) pulping. The conjugation is destroyed by oxidation using bleach, so bleaching destroys residual lignin to reduce yellowing. Traditionally, Cl2 is used but now replaced by ClO2 for Elemental Chlorine Free process. Based on its reduction to Cl−, ClO2 is 2.5 time more oxidising than Cl2 per unit mass. After the addition of bleaching agent, NaOH is added to solubilise oxidised lignin fragments and partially removes 'organic' chlorine and this process is repeated for certain sequences. This process is 5 - 10 times less chlorinated by-products than Cl2, but this process is expensive, but highly selective oxidant for lignin. Furthermore, there is another bleaching agent which is called 'Totally Chlorine Free' process which uses O2/O3/H2O2.
 |
| The bleaching mechanism (left) and its product (right |
In paper manufactured, additives are also added, and of the example is for internal sizing. As we acknowledged cellulose surface is hydrophilic so it needs to increase cellulose contact angle to prevent water penetration. Therefore, to overcome this problem, introducing surface with hydrophobic group is known as sizing (not related to paper size). In unsized paper, an extensive spreading and water penetration, but with introducing the hydrophobic group (increase sizing) it reduces wetting to make a sharper image on the paper.
 |
| Sizing |
 |
| Sizing mechanism |
The others additives that are used in paper manufacturing are fillers and pigments. Commonly, inorganics added to improve optical and physical properties (denser, brighter, softer, and smoother) and these fill up spaces between fibres as inorganics comprise 5 to 15% by mass of paper. Furthermore, inorganics are cheaper than pulp so it reduces cost but weakens paper.
 |
| Common fillers and pigments |
 |
| Paper making and the fourdrinier machine |
As pulp comprises of large fibres (1 - 5 mm x 25 μm) and much smaller fillers (1 - 5 μm) in water, there are 2 problems often occur during dewatering. The problems are fillers are too small to be retained on the mesh screen and fillers also block the pores between fibres, causing slower water drainage. Therefore, the technical solution is to stick small filler particles onto larger fibres, but most fibres and fillers have negative surface charge, so they are mutually repulsive. To overcome this problem, high molecular weight cationic polymer is used as 'glue' to stick fillers onto fibres.
 |
| Retention and drainage aids |
The final step in manufacturing of paper is surface coatings where final layer is added to improve gloss, printing resolution and strength. The coating typically consists of an aqueous suspension of pigment (e.g. clay or TiO2). Finally, paper is super-calendered or compressed between hot rollers to allign clay platelets, improving gloss. Furthermore, latex binder can also be used as a surface coating as latex comprises microscopic film-forming polymer particles in water.
 |
| Surface coating |
Some say, paper manufacturing in non-environmental friendly, but it is not completely true as there are some environmental considerations. Firstly is 'black liquor' (degraded lignin solution) is concentrated by evaporation and burned to provide energy for paper mill processes (e.g. produces steam for paper drying). Secondly, molten inorganics (Na2CO3, Na2S) can be recovered from furnace for recycling for example Na2CO3 is converted to NaOH using Ca(OH)2 as shown below.
 |
| Sodium carbonate cycle |
Besides the ordinary printing paper, there are some speciality paper and the examples are followed
Banknotes
 |
| Pound sterling notes (left), Australian dollar (centre), and watermark |
Thermal ('receipt') paper
This paper uses leuco dye and a solid acid which melt to react and give strongly purple coloured dye. |
| Thermal paper receipt |
Carbonless copy paper
 |
| Carbonless copy paper |
The future trend of paper such as E-ink is a US-based electrophoretic display company where electric field rearranges white (+) and black (-) encapsulated particles. Besides that, E-ink requires no power to hold the image which means a possibility for low power consumption for electronic paper application. The displays can be made with conducting polymers where circuitry is inkjet-printed onto flexible substrates and allows production of very light rollable displays. Furthermore, it is compatible with either E-ink or OLED technology. Hence, the question is will rollable and/or flexible displays replace books and newspapers?
 |
| E-ink |
As the conclusion of this part, here a summary of paper making process.
 |
| Paper making process |
Comments