A Brief Chemistry of Paper
Published on 5 March 2024
Introduction
Isn’t is somewhat puzzling that the printing industry is still growing! And if that holds some water, it won’t be unfair to revisit my old college days and go through a brief chemistry of paper once again. It was a piece that I wrote those days and found just a few days back while searching some documents.
An already amused me found it astonishing to come across an Authority Hacker post that quotes one report saying that the global print-on-demand industry worth $6.2 billion in 2022 is expected to surpass $39 billion by 2030. The print industry, over the last two decades, has diversified in many ways.
It’s now offering different types of solutions catering to a range of demands from large and bulk quantity offset printing at one end to short format digital printing in few, as-much-as-you-need to the other ends. Therefore, the process of documentation for many different organizations has opened up new horizon with opportunities aplenty.
So the need of paper and its business are no way going to die soon and that sets the stage for studying a brief chemistry of paper.
Someone living with videos, post, effects and simulation, it’s somewhat weird to go down the memory lanes and write on ‘A Brief Chemistry of Papers’. Right! But that’s something I’ll do today. It’s good to take a break from the monotony of topics at times too.
Let’s get straight into the business. Here is the list of topics.
Table of Contents
What is Pulp? – Used in Paper Manufacturing [Compositions]
Paper is one of the main substrates used in the printing industry for publishing works. While studying the chemistry of paper, it’s important that we start right from the stage of pulping during paper manufacturing with its compositions.
Paper is made from pulp through a series of processes. Pulp is at the core of paper manufacturing, which is made up of the cellulose fibers.
Now, depending upon the nature of the ingredients and their sources, pulp can be broadly classified into two different categories.
- Softwood pulp with the fiber length from 3μn (nanometer) to 4.5μn.
- Hardwood pulp with the fiber length from 1μn (nanometer) to 1.5μn.
The Hardwood pulp adds smoothness to the paper while the softwood pulp increases its tensile strength.
Good quality paper can be obtained by mixing hardwood pulp with softwood pulp in the ratio of 2:1. The pulp contains 40% fibrous material i.e. Cellulose, and the rest is the non-fibrous material like Hemicellulose – 25%, Lignin – 25% and Resin – 4%.
A cellulose molecule is a carbohydrate. The degree of polymerization of cellulose molecule ranges from 4000 to 5000, while the length of fibers varies from 3 nanometer to 4.5 nanometer.
When cellulose is soaked into water, the fibers get separated due to the formation of hydrogen bonds between the cellulose and water molecules. This results in swelling of the fibers.
Now, just in case you’re interested to know what is the degree of polymerization, here is a short note.
What is the ‘degree of polymerization’?
Polymerization is a chemical process in which the monomer molecules are joined together to form polymer chains of three-dimensional structures. There are many types of polymerization and we won’t get into the details of polymer chemistry here.
All we’ll know here, while studying the chemistry of paper pertinent to its manufacturing is, usually it takes several thousand of monomer molecules to form a single polymer chain.
Understanding the Constituents for the Chemistry of Paper
The Structure of Cellulose [(C6H10O5)n; 4000 to 5000 nm or μm]
Cellulose is a linear polysaccharide and its molecule is made up of about 5000 glucose units joined to each other by β-1, 4 – glycoside bonds as shown in a Cellobiose unit (in the right).
Since a Cellulose molecule on partial hydrolysis yields Cellobiose, it would be more appropriate to consider that the chains of Cellobiose units constitute the linear molecule of cellulose.
These long molecules give ‘fibrils’ that are held together by hydrogen bond between OH groups on adjacent chains (monomer). The Cellulose fibers are built with these fibrils that spirally wound in opposite direction. This lends the molecular structure considerable physical strength.
Hemicellulose
It is a branched carbohydrate (C6H10O5) n, but of far less a degree of polymerization with only about 200. Naturally, the fiber length is much shorter.
Hemicellulose is soluble in alkali. Cellulose and Hemicellulose molecules are linked together by means of Lignin. Unless this lignin is dissolved in a certain medium, it’s not possible to separate Cellulose fibers, which is the prime ingredient of paper during pulp manufacturing.
Lignin – Why Old Paper Stock often looks yellow or brownish!
Lignin is a constituent of wood chips which is to be removed completely to get the Cellulose fiber separated for pulp production.
It is not soluble in alkali at the room temperature. So, it must be subjected to a drastic condition i.e. a high temperature and pressure. Any lignin content present in a paper stock is susceptible to aerial oxidation and does gradually turn the stock look brownish or yellowish.
This is why old books or stocks of paper often look pale yellow or brownish due to the presence of Lignin in the paper. It’s obsious that a complete eradication of Lignin was possible during
So, it’s necessary to remove lignin from the cellulose fibers that are used in pulp manufacturing. Here is the structure of a simple Lignin given below.
Resin
It’s soluble in alkali. So, it can be easily removed from the wood chips.
Manufacturing of Paper
The wood stock used in the paper making process, is converted into chips or grounds. In this process, the collected wood is cut into pieces or logs. These logs are then debarked and ground until they’re reduced to tiny particles.
The Preparation of Chemical Wood Pulp
There are several types of chemical wood pulp used in paper making and each type has certain characteristics of its own. These are as follows.
- Sulfite pulp
- Sulfate pulp
- Alpha Cellulose pulp
- Neutral Sulphite Chemical pulp
Among all the above varieties of chemical wood pulp, we’ll be interested in the Sulfate pulping process. The Sulfite process of pulping is applicable when there is no resin present in the pulp.
Sulfate Pulping Process of Paper Manufacturing
The advantage of this process is that any kind of wood can be subjected to this process. Also, the efficiency of the process in recovering and recycling its effluents results in less pollution of air and water.
The Sulfate pulp adds high tensile and tearing strength to the paper. The production of Sulfate pulp occurs in an alkaline method as opposed to an acidic medium used in the Sulfite process of pulping.
Constituents of the Sulfate Process of Pulping
- Sodium sulfate, Na2SO4
- Sodium Sulfide, Na2S
- Sodium Hydroxide, NaHO
The required temperature for the Reaction: 1600C to 1800C
Mechanism
Steam is is passed over the mixture and wood chips.
The Reactions involved:
- Na2S + H2O = NaOH + NaHS
- Na2SO4 + Na2S = Na2S + 2O2
The mixture of Sodium Hydroxide and Sodium bi-Sulfide plays a vital role in Sulfate process. But the rate of reaction slows down as Sodium bi-Sulfide gets consumed. Therefore, to maintain a constant supply of products, more reactant i.e. Na2S must be added.
The purpose of reaction in the above equation no. 2:
When Na2S gets exhausted, the Na2SO4 decomposes at the temperature of the digester to produce Na2S.
In this way, the amount of Na2S is kept at a constant level. With the amount of Na2S is getting continually replenished, the point of equilibrium of the equation no. 2 shifts toward left diminishing the formation of the Na2S and finally stopping it.
A direct removal of Lignin from the wood fibers is not possible. It is converted into a complex compound through the reaction of Sodium Hydrosulfide (NaHS), which makes it dissolve in the alkali.
The chemical reactions involved in the Sulfate process of pulping are as below. It also has the process of bleaching with Sodium hypochlorite. (NaOCl).
What are Beating & Hydration in the Chemistry of Paper Making?
The final form of pulp which attributes to the characteristics of paper is developed during the process of beating. This makes beating a highly important stage of in the paper making process.
Three things happen to the pulp fibers while they are in the beater.
- They are fibrillated.
- They are frayed at the ends.
- They are shortened and thereby do undergo better formation when furnishing takes place inside the paper making machine.
Fibrillation and Hydration in effect serve the ultimate purpose for bonding of the fibers during paper manufacturing.
Initial beating ruptures the outer wall of the fiber. Simultaneously, the internal fiber walls absorb water and swell.
The force because of swelling together with the rubbing of fibers and the force of the beater bars causes rupture of the internal Hydrogen bonds i.e. freeing of Hydroxyl or OH groups and fibrillation or the actual exposure of the thread like structure (fibrils).
With the opening of crystals or the freeing of OH groups (the Hydroxyl radicals, it is possible for long chain molecules in the adjacent fibrils to bend during the drying operation.
Effect of Beating & Hydration
The primary effect of beating, during paper manufacturing, is to increase the tensile strength of the sheet. At the same time, it decreases the tearing strength properties of the paper. The beating of the pulp roughens and frays the minute fibers so that they tend to cling to each other. The gelatin like substance that stimulated by hydration serves as an adhesive agent (producing chemical bonds) that helps the fibers attach to each other.
Additive Added in the Beating stage
Additives are generally introduced at the beater to improve optical and physical properties. They also add color to the paper.
The additives are as follows:
Starch
Starch (C6H10O5) n is added to the pulp at the time of beating to increase the strength and stiffness of the paper. Besides the fiber-starch-fiber bonding is also enhanced.
Filler loading material
The most used material filler is clay (Al2O3.2SiO2.2H2O) which is a natural earth product that can be reduced to a fine powdery form much like talcum powder.
Clay imparts smoothness and opacity to paper. Another filler material is Calcium Carbonate (CaCO3). This adds an affinity for printing ink to the paper. But for the papers used in offset printing, Calcium Carbonate (CaCO3) must not be used as it can react with the fountain solution.
Among other filler materials are Titanium di-Oxide (TiO2), Zinc Sulfide (ZnS), Barium Sulphate (BaSO4). These substances contribute to the properties of brightness, smoothness, and a high degree of opacity to the paper.
Internal Sizing during Paper Manufacturing – Rosin Size
Internal rosin sizing is used to make paper repellent to water or fluids.
Aluminum Sulfate or Alum is added to fix the rosin. This Alum or Aluminum sulfate helps rosin adhere to the paper fibers. It is the mixture of Sodium salt of rosin and free rosin. The mixture contains about 70% Sodium salt and 30% rosin.
The use of rosin and alum for internal sizing adds acidity to paper to some extent. They are responsible for the rapid yellowing and crumbling with age typical of acid paper.
Rosin size + Al2(SO4)3 —— > Rosin – Al (OH)3 complex
Al2(SO4)3 is added to decrease the pH of the solution roughly to 4.5.
Al2(SO4)3 + 6H2O = 2Al (OH)3 + 3H2SO4
The free rosin colloids contain -ve charge due to the presence of carboxylic acid while the Al (OH)3 is positively charged.
R COOH – Al – Cellulose bond is formed due to the interaction between rosin size and Aluminum Sulphate.
As a result, the contact angle of the complex is increased from 900 to 1050 and thus it prevents the penetration of water into the fiber of cellulose.
It is to be noted that the R COOH – Al – Cellulose bond formed is a physical one while the bond between the Cellulose and synthetic resin e.g. Stearic Anhydride is chemical. The chemical bond once formed is very stable in nature.
Synthetic sizing agent [Stearic anhydride]
Another sizing material is Urea Formaldehyde resin that increases the strength of paper even when it is wet.
The pulps used in making paper for the off-set or lithographic process of printing receive generous amount of sizing material.
This protects the fiber structure from softening due to moisture and does prevent the fibers from being pulled apart when the paper undergoes tension i.e. the pull of the blanket in an off-set press.
So, the ultimate purpose of internal sizing of paper is to prevent penetration of liquid into its fibers.
External Surface Sizing
The size press applies a solution of starch (most often) or animal glue which tend to affect the paper characteristics in many respects.
The surface strength or folding strength is increased, while the resistance to water is also increased to some degree. Besides, the opacity is somewhat reduced too. The external surface sizing results in some benefits while printing.
- Increases pick resistance.
- Reduces tendency of fluffing.
- Increases dimensional stability.
- Increases surface smoothness and finish.
- Improves resistance to ink, oil, and water penetration.
The moisture content of paper prior to sizing remains at about 5% to 10%. This reduces to 4.5 % to 8% after external or surface sizing depending upon the type of paper being made.
Paper Coating
The purpose of coating paper is to create a very smooth surface that may be glossy or matte, depending on the very grade of paper meeting certain requirements of the printing industry. In addition to a favorable surface for a good ink reception, there are a few other advantages that result in coating of paper.
These include resistance to
- Fluids and chemicals like water, grease, solvents, air, gases, and other compounds
- Physical damage due to abrasion, folding or scuffing.
- Environmental damage due to extreme heat and light.
There are three different types of basic coating processes that are in place e.g. water-base, organic solvent, and non-solvent (hot melt).
The water-based coatings are applied in varieties of paper that are used in the printing industry. Colorants, Pigments, and Dyes are blended with adhesives in the water, and these constitute the coating material for the water-based coating.
Among the adhesives of water-based coating are Starch, Casein, Soybean protein, Animal glue, Vinyl acetate, Butadiene-acrylonitrile.
The predominant constituent of water-based coatings are the white mineral pigments including Clay, Barium Sulfate, Calcium Carbonate, Titanium DI-Oxide etc.
Clay is the Hydrated Aluminum Silicate. It has an affinity to the printing ink and on using it in Coating, the size of the interstitial space on paper surface becomes so small that the pigment cannot pass through the holes (0.1 micron) while the oil can pass through the holes.
Calcium Carbonate adds brightness and opacity to the paper. The Carbonate particles used in coating are much finer than those used as the filler material.
This is because a superior grade of Clay is used as the constituent of water-based coating. Another inorganic white pigment used in coating is the Titanium-DI-Oxide or TiO2. It has an affinity to ink and does make a low-gloss finish of the paper.
Calendaring
This is a process by which the paper is given the characteristics of smoothness and gloss.
Papers are passed through the nips of two steel rollers to form a smooth-surface finish. The heavier the coating of paper, the smoother and glossier it will be after calendaring.
Besides, the process of calendaring reduces brightness using excess pressure and moisture. It also decreases porosity and ink absorption.
Super Calendaring
Super- calendaring of uncoated paper greatly improves their smoothness. A super-calendar is a stack of rolls supported in a frame. The rolls iron and polish the paper giving it a very smooth surface.
Paper Grain & Side
There are two characteristic directions on a sheet of paper. The machine direction or grain direction is the direction of the paper parallel to its flow-through on the paper machine.
The cross-grain direction is the direction on the paper at right angle to the machine direction. This distinction between two grain directions on a sheet of paper is necessary because of marked difference in the paper properties in two directions.
There is always a tendency for the fibers to orient parallel to the machine direction. It’s therefore easy to tear a paper along the machine direction or ‘along the grain’ and not ‘across the grain’.
For any change in the moisture content, paper expands or contracts far more across the grain than along the grain. This is because on absorbing moisture, the fibers swell along its width and not along the length of the paper sheet.
pH of Paper
The degree of acidity or alkalinity of paper is found by measuring its pH. The pH of a paper stock is very important because it affects the rate of drying of ink and influences the paper properties.
For fast drying of ink, the pH of the paper stock must be above 5 and the relative humidity should be over 65%.
Conclusion
With the ‘paperless office’ being the norm of today, it’s kind of going back writing a brief chemistry of paper. But the point is paper is not going away anytime soon.
The documentation and education still revolve around the use of paper. The oldest know evidence of paper manufacturing dates back to the 2nd century China to what is now a global pulp and paper industry with China leading its production and the United States following.
Much more used in Offset and lithographic process, than the Gravure or intaglio process of printing, it’s still a long way to go before our lives could completely get rid of paper. The writer of this article no way admires any process of industry that continually adds pollutant to our environment.
So, ‘A Brief Chemistry of Paper’ could be regarded as a quick run down of the chemistry behind the modern pulp and paper making process.
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