The term “paper” denotes sheets composed of densely interwoven fibers that have been compacted by either matting or pressing processes. The composition of these sheets frequently consists of cellulosic materials and is typically manufactured through the process of suspending fibers in water and shaping them on a fine wire screen.
Pulp is a term used to describe the cellulose material that is commercially utilized in the creation of paper. This material is derived from several sources, including bamboo, bagasse, and wood, using either mechanical or chemical processes. The process of producing pulp is often known as pulping.
History and Development
- Papyrus, an ancient writing material derived from plants, holds the distinction of being the earliest known medium for written records. Its origins can be traced back to ancient Egypt.
- The initial recorded method for producing authentic paper can be traced back to China’s Eastern Han period (25–220 AD). The credit for this achievement is commonly attributed to Cai Lun, an esteemed court official.
- Farmers in China had domesticated the mulberry tree by the 6th century. They specifically cultivated it for the purpose of producing pulp used in the papermaking process. Apart from mulberry, pulp was also derived from various other sources, such as bamboo, hibiscus bark, blue sandalwood, straw, and cotton.
- The utilization of hemp and linen fibers derived from worn-out clothing, fishing nets, and fabric bags in the production of paper gained significant traction in Europe during the 13th century.
- The extensive utilization of rags played a pivotal role in enhancing the accessibility and affordability of rag paper, thereby facilitating the progress of printing technology.
- In the 19th century, the papermaking and printing sectors experienced notable transformations in order to meet the growing demands for production. Significantly, a discernible transition occurred in the selection of primary resources, wherein pulpwood and various tree-based commodities emerged as the prevailing sources. Currently, these materials constitute more than 95% of the total global pulp production.
- The incorporation of wood pulp and the introduction of automated paper technology throughout the late 18th and early 19th centuries played a pivotal role in establishing paper as an economically viable item in modern society.
Raw Materials of Pulp
The ideal fiber for the manufacturing of high-quality paper should have elongation properties, a significant proportion of cellulose, and a low concentration of lignin. Bamboo exhibits inherent qualities that render it very appropriate as a raw material. The paper industry uses the following raw materials:
- Softwood consists of both coniferous and non-coniferous types of wood. Conifers, a subgroup of gymnosperms, are distinguished by their ability to produce seeds in cones.
- Non-coniferous woods, however, are sourced from trees commonly found in regions characterized by moderate climatic conditions.
Grasses and Reeds
- There are several species of grasses and reeds, including lemongrass, cogon grass (known for its extensive rhizome network), bamboo, and giant reed (a clumping, perennial grass with hollow stems measuring 1/4 to 2 inches in thickness), among others.
- Hardwoods come in a variety of forms, including oak, rubberwood, and eucalyptus.
- A well-known hardwood is oak, whereas rubberwood, a medium-density tropical hardwood from the Pará rubber tree, is a lighter-colored alternative.
- On the other hand, eucalyptus is a fast-growing evergreen tree that is native to Australia.
- Rice straw, which is obtained as a byproduct during rice harvest, is one of the materials used to make straws.
- Straws can also be manufactured from a variety of materials, including wheat, bagasse (the dry fibrous waste left over after extracting juice from sugarcane or sorghum stalks), and barley.
- Cotton is a commonly employed seed fiber within the papermaking industry. During the ginning process, the extraction of the elongated fibers from the cotton seed occurs.
- These fibers are employed in the production of cotton rag, which is widely utilized in the manufacturing of textiles and paper products.
Stages of Paper Making
There are three different stages of paper making which include:
- The quality of the paper is primarily determined by the quality of the timber utilized.
- Timber is sourced from a variety of forested areas, including the rainforest.
- Debarking refers to the procedure of extracting the outer layer of bark from trees.
- The bark is removed from the logs utilizing various tools such as a machete, drum, abrasion, or hydraulic barker.
- The bark that has been removed is utilized as a source of fuel or as a means of enriching soil through fertilization.
- The logs that have undergone debarking are subsequently fed into the chipping machines, where they undergo fragmentation into smaller pieces.
- The chips are then stored in sizable tanks or bins for further processing.
Following the aforementioned procedures, the process of pulping is carried out utilizing the chipped wood. There exist various techniques for pulping, and this discussion will focus on highlighting some of the most significant ones.
- Mechanical pulping is a pulping technique that employs mechanical force to extract fibers from lignin.
- The wood chips or logs undergo an abrasion process, either by being pressed against a rotating grinding stone or by being passed through a mill.
- The mechanical process effectively disintegrates and breaks down the wood fibers, although a significant portion of the lignin remains attached to them.
There are three different types of mechanical pulping that include:
- Groundwood pulping is considered to be the most basic and traditional method of mechanical pulping.
- The wooden logs are subjected to pressure against a rotating stone, resulting in their transformation into pulp.
- The pulp exhibits a significant yield of approximately 95%, however, it demonstrates relatively low levels of strength and brightness.
- Thermomechanical pulping (TMP) is an advanced variant of groundwood pulping in which the wood undergoes a treatment involving the application of heat and steam to promote its softening before the grinding procedure.
- The strength and brightness of pulp are relatively greater as compared to groundwood pulp, although they are inferior to those of chemical pulp.
- Chemi-Thermomechanical pulping (CTMP) is a comprehensive process that combines mechanical and chemical methods.
- The wood chips are pretreated with chemical substances, such as sodium sulfite, to partially break down the lignin before the grinding process.
- The strength and brightness of the pulp are higher compared to the thermomechanical pulp (TMP) but lower compared to the chemical pulp.
- Chemical pulping is a pulping technique that employs chemical solutions to dissolve lignin and facilitate the separation of fibers from each other.
- The wood chips undergo a cooking process in digesters, where they are subjected to chemical treatment at elevated temperatures and pressures.
- The solubility of lignin is achieved by employing chemical agents, followed by its removal from the pulp through washing or bleaching procedures.
There are three different types of chemical pulping that include:
- The Kraft (sulfate) pulping method is widely acknowledged as the primary and extensively used chemical pulping process.
- The wood chips are subjected to a cooking process using a solution composed of sodium hydroxide and sodium sulfide.
- The process efficiently breaks down the lignin found in the wood chips, leading to the creation of a strong and versatile pulp.
- The pulp has a relatively low yield, approximately 45–50%. However, it has notable strength and brightness characteristics.
- The sulfite pulping process is a chemical pulping method that, although older and less widely used, still remains relevant.
- The wood chips are cooked in a solution that includes sulfuric acid along with either calcium, magnesium, sodium, or ammonium bisulfite.
- The solution efficiently dissolves the lignin, resulting in a pulp that is well-suited for the production of high-quality paper.
- Kraft pulp has a lower yield, approximately 50–55%, compared to regular pulp. However, it is known for its superior strength and brightness.
- Soda pulping is an affordable and straightforward chemical pulping process that utilizes sodium hydroxide as the sole cooking agent.
- The pulp has a low yield, typically around 35-40%, and possesses low strength and brightness.
- However, it is easily bleached and finds application in specialty papers.
- The hydro-pulping process involves separating thin layers of plastic and aluminum from cellulosic fibers using a hydro pulper.
- The high-quality fibers obtained from this process are then utilized in the production of various paper products, including tissue, paper towels, and fine writing paper.
- The stock is transferred to a blending chest, where various chemicals can be added to achieve the desired characteristics for the final paper product.
- Dyes are added to the paper as needed in order to provide color. Dyes adhere to cellulose fibers and exhibit excellent resistance to both light and water.
- To produce each grade of paper and board, it is crucial to achieve a precise combination of pulps and additives.
- Throughout the manufacturing process, computers constantly monitor the properties of the paper to ensure accuracy.
- Waste paper is obtained through the collection process from Waste Paper Banks and Commercial collections.
- When individuals deposit their old papers into a waste paper bank, they engage in the process of sorting the paper into different classes prior to its collection by the merchant.
- The waste paper merchant engages in the collection of used paper, after which it is subjected to manual sorting into various categories. The paper that is not suitable for recycling is eliminated.
- Subsequently, the waste paper merchant will proceed to compress the waste paper into bales in preparation for its transportation to the paper mill.
In order to streamline the recycling of printed paper, such as office waste and newspapers, it is crucial to remove the ink. If the ink is not removed, it will mix with the pulp and result in the paper having a dull gray appearance. There are two main processes of de-inking that are:
- The process of waste paper pulverization involves the utilization of a pulper, which is equipped with a substantial amount of water.
- Centrifugal screens are capable of effectively eliminating staples.
- Slots or screens remove the majority of water and ink that has dispersed while keeping the pulp.
- The process of fine screening effectively eliminates stickies.
- The waste is transformed into a slurry, and then the contaminants are eliminated.
- Surfactant chemicals are added to create a sticky froth on the surface of the pulp.
- The pulp is infused with air bubbles, which effectively transport the ink to the surface.
- When the bubbles rise to the surface, they create a layer of foam that effectively captures and contains the ink.
- It is necessary to remove the foam before the bubbles burst, otherwise, the ink will return to the pulp.
- The flotation system does not require a large water treatment plant because the ink is removed from the flotation machine in a concentrated form.
- This is the location where the cellulose fibers undergo a crucial refining process that plays a vital role in the craft of papermaking.
- Prior to the refining process, the fibers exhibited the characteristics of being rigid, lacking flexibility, and forming a limited number of bonds.
- The stock is conveyed through a conical machine comprising a sequence of rotating discs. The forceful and abrasive action causes the fibers to be cut, opened, and decluttered, resulting in the division of their ends.
- In this particular state, the fibers exhibit pliability and possess an increased surface area, thereby leading to a notable enhancement in fiber bonding.
- The characteristics of the paper are intricately linked to the refining process. The process of refining was previously referred to as beating.
Screening and Cleaning
- Pulps typically contain both fibrous and non-fibrous materials that are considered undesirable and need to be removed before the pulp can be converted into paper or board.
- The process of cleaning involves using rotating screens and centrifugal cleaners to remove tiny particles of dirt and grit.
Formation of Paper
The process of converting fiber suspension into paper sheets involves three fundamental steps:
Formation of Wet Web
- To prepare a wet sheet, a water-fiber slurry with a water content of 99.5% is introduced onto a moving belt made of wire cloth.
- The slurry is fed onto the belt at a speed of 60 miles per hour for fine paper production and 500 meters per minute for newsprint production.
- The force of gravity is primarily responsible for draining water. In addition to that, it is also removed through the use of a pressure roll and a suction roll.
- The collected water, referred to as white water, is reused to achieve optimal fiber recovery, water, and additive recovery, and to prevent water pollution.
Pressing the Wet Sheet
- The paper sheet, comprising approximately 80% water content, is subsequently introduced into the press section via the felt roll.
- In this particular section, water is efficiently extracted from the paper using a delicate application of pressure.
- The water content has been reduced to a range of 60-70%.
Drying the Sheet
- The sheet from the press section is subjected to a series of steps involving a smoothing roller and a sequence of steam-heated metal cylinders.
- During this stage, the efficient transfer of heat and moisture occurs to a felting or canvas belt that is positioned on top of the paper. The paperboard is dried without the utilization of felt material.
- Once the sheet has reached a moisture content of 5–6% on the final drying roll, it proceeds to undergo a concluding procedure that encompasses a sequence of pressure or calendaring rolls.
- Ensuring the completion of this step is essential for attaining a sophisticated and flawless paper surface.
- The large paper rolls are transported to the finishing department, where they undergo various processes, including cutting, coating, and packaging.
Video on Manufacturing of Paper