Pulp and paper chemistry

Definition: Pulp chemistry and paper chemistry represent discrete yet dependent fields in the same industry. The pulping procedure utilizes mechanical and chemical processes that convert wood and other natural material into individual fibers. Pulp produced in this manner is then treated with additional chemicals to create paper products that vary based on both the type of pulp utilized and the style of paper desired. Throughout the process, pulp and paper chemists seek to make stronger, more durable, and increasingly environmentally sound products that meet the needs of the modern world.

Basic Principles

Since the emergence of writing, humankind has sought better methods of recording the fruits of literacy, starting with stone, clay, and papyrus. It was not until the early second century CE that the Chinese created the first paper as it is understood in modern times. The Arab world, in turn, borrowed the concept before transporting it to the European continent. Linen, rather than wood, served as the basis of Western papermaking, a craft that grew dramatically following the fifteenth-century German invention of the printing press. At the time, papermaking was a slow process that produced individual sheets rather than large rolls.

The nineteenth century witnessed the gradual introduction of wood into the pulp and paper process. Utilizing wood made the papermaking process more complex. The introduction of a variety of chemical solutions pioneered by scientists such as Hugh Burgess (who created the soda process of pulping), Benjamin Chew Tilghman (who is credited with the sulfite process), and Carl F. Dahl (credited with the kraft process) all contributed to making wood an effective pulping agent. These chemical advances, in conjunction with Carl Scheele’s discovery of the bleaching qualities of chlorine and the invention of the Fourdrinier machine that continuously turns pulp into paper, established the foundation of the modern paper industry.

Broadly, the papermaking process begins with chemical, semichemical, or mechanical efforts to separate individual cellulose fibers from the lignin that binds them in wood to create pulp. Once this separation is completed, the pulp is washed and, if a lighter color is desired, bleached before being strained and pressed into paper.

Core Concepts

Raw Materials. With any paper product, it is important to consider the parts that constitute the whole. Since the advent of literacy, many items have been used to form paper. Wood, the resource most utilized today, was not employed in the Western world until chemical advances in the nineteenth century made it a suitable pulping agent. Before that, cotton and linen rags typically served as the principal material from which paper pulp was produced. Hundreds of other items were also explored with varying degrees of success. Until the widespread use of wood, however, the growth of the paper industry was constricted by a scarcity of resources and the technological limitations that made paper production a challenge. The four-stage Fourdrinier papermaking machine, coupled with advances in technology utilizing wood for pulp, transformed the paper industry. The prevalence of assorted hard- and softwood trees in North America made the United States one of the world’s leading producers of paper products.

Pulp Processing. How wood is processed ultimately determines its later usage. Chemical processing, as the name implies, necessitates adding chemicals to a heated wood mixture that permits the separation of cellulose fibers from the lignin that binds them. In the end, pulp made in this fashion yields a strong paper. When weaker paper is needed, mechanical pulping is ideal. In this method, wood is crushed against a grinder that separates its constituent parts. Semichemical pulping commences with a standard chemical procedure but concludes with mechanical separation of fibers from lignin. This process produces the strength typical of the chemical process, yet affords greater control over the fibers during the paper-production stage.

Kraft Process. In the United States, approximately 80 percent of the chemical pulp produced comes from the kraft process, owing in part to its use of an alkaline procedure that is considered by some to be less environmentally destructive than earlier sulfite-based techniques. This method places wood chips in a water-based chemical mixture of sodium sulfide and sodium hydroxide that is exposed to high heat and pressure. This “white liquor” separates the lignin from the fibers before a washing process extracts the chemicals from the pulp. The pulp is further washed and bleached before it is converted to paper. The white liquor that remains is diluted with the wash water, forming “black liquor”—a combination of the waste water, pulping chemicals, and lignin—which then proceeds through an evaporator, with the remaining concentrate fired in a recovery furnace. Steam resulting from the process is used to fuel the papermaking portion of the procedure. Smelt left in the furnace is mixed with water, creating a “green liquor” that is treated in a caustic bath and converted back to white liquor for reuse.

Pulp Washing and Bleaching. Once pulp is created, it is washed in very hot water to remove the cellulose from the lignin. Should a lighter color be desired, the pulp must then be bleached, since even the most technically advanced pulping processes leave behind some lignin, which darkens the pulp and, by extension, the paper produced from it. Bleaching procedures typically require several steps, from injecting the pulp with the lightening agent to washing with water, before restarting the process until the desired color is reached. Historically, bleaching agents have ranged from chlorine to hydrogen peroxide to sodium hypochlorite. With the pulping process complete, the finished product is, in an integrated mill, transferred to the paper-producing portion of the plant or, in a nonintegrated site, transported to a paper mill, which will then transform the pulp into paper.

Paper Production. At the paper mill, raw pulp is deposited into a head tank, where it is mixed with water to form a slurry often referred to as “white water.” By this stage in the operation, the pulp has been treated to maintain proper pH and cut with assorted fillers to suit the standards of the paper being made. This solution is then spread over a screen and pressed in a process that captures the fibers while removing much of the water; industry participants call this the “wet end” of the papermaking process. Heat is then applied to the mixture to facilitate drying and fiber binding. Next, a calender, or series of rollers, presses what remains, joining the fibers together and smoothing the surface area. The continuous operation of modern Fourdrinier machines necessitates a continuous supply of pulp in addition to careful management of the entire process to guarantee paper consistency and quality. The finished product is rolled onto storage wheels from which it will be packaged and marketed.

Applications Past and Present

Early Papermaking. The papermaking craft began in China by a court official named Cai Lun in 105 ce. Chinese craftsmen used cloth scraps, mulberry bark, and other items such as hemp and fishing nets. Raw materials were placed in mud for several weeks, then removed and pounded on a mortar with a pestle. What remained was placed in a tub of water, where a variety of fillers were added until the substance contained inside reached the necessary consistency. It was then framed, drained of water, and heated. After multiple repetitions of the process, paper was formed. From China, the rest of the world would derive their own papermaking processes. By 800 ce, Arabs had introduced papermaking to their land and replaced Chinese plants with linen as the basis of their pulp. Spanish Moors brought papermaking to Europe, where the Italians and the French fast became the principal producers of the commodity. Germany received its first paper mill at the start of the fourteenth century. In the mid-fifteenth century, German goldsmith Johannes Gutenberg invented the printing press. The advent of movable type forever changed the world, bringing with it the rapid dissemination of information and the need for new and improved methods of producing paper.

Despite the ravenous need of the presses, paper production remained a time-consuming, labor-intensive affair that yielded superb-quality paper of limited size. A machine capable of producing larger rolls of paper was clearly needed. In 1798, Louis-Nicolas Robert, a French printer’s assistant, fashioned a machine capable of creating sheets of paper over forty feet in length. A cash-strapped Robert sold his patent to Saint-Léger Didot, who in turn enlisted the assistance of the English brothers Sealy and Henry Fourdrinier. By 1804, the Fourdrinier brothers had a fully functioning papermaking machine that, when taken in conjunction with the printing press, helped to create the modern paper industry. The Fourdrinier machine remains the basis of papermaking technology and the papermaking industry today.

Introduction of Wood. As the need for paper increased and the technology behind papermaking improved, so too did efforts to find new materials from which to form pulp and, ultimately, new types of paper. The nineteenth century would see a revolution in the chemistry of papermaking that would usher in the modern era. Many experimented with materials to replace rags as the basis of paper production. Despite many efforts, most experiments yielded only limited results. A new pulp source was needed that would prove both plentiful and capable of making a wide array of products.

In 1844, Friedrich Gottlob Keller invented a mechanical wood-pulping machine that paved the way for additional experimentation and discovery. Hugh Burgess in 1851 created a chemical pulping process in which wood chips were placed in a caustic chemical soda bath and boiled. Burgess’s invention represented a dramatic advance in quality from the paper made by Keller’s mechanical process. Several years later, American chemist Benjamin Tilghman broadened the science of papermaking by pioneering the use of sulfites in the pulping process. But German paper-mill worker Carl F. Dahl would introduce perhaps the most influential chemical process of all: sulfate or kraft pulping. What made Dahl’s process so revolutionary was that the chemicals used in it could be recycled, and the energy released while pulping helped to fuel papermaking machines. In North America, the kraft process was revolutionary because it worked on all types of wood, including that of the pine tree, which was prolific there. The kraft process made the United States one of the world’s leading producers of paper.

Following the remarkable success and widespread applicability of the kraft process, the need emerged for a semichemical process that would more effectively handle the largely unused hardwoods prevalent in North America. This procedure began with an initial chemical bath, briefer and less substantial than in the kraft process, before being followed by mechanical pulping. The new procedure, which emerged in the 1920s, served several purposes, including controlling pulpwood cost inflation and permitting the expansion of the paperboard industry. The latter concern was especially significant, as it fit the packaging needs of a nation that had entered an era of mass production and mass consumption of consumer products. As in the past, the pulp and paper industry answered the needs of a changing nation and world. In this instance, hardwood corrugated board produced by the semichemical pulping process enabled the continued expansion of the American economy.

Coinciding with improvements in the pulping process were more efficient bleaching techniques. As with the pulping process, most of the bleaching performed in the early nineteenth century was done on rag stock. As pulpwood replaced cloth, extant bleaching practices became ineffective. Early twentieth-century changes in bleaching techniques that focused on multistage exposure to whitening agents dramatically improved the process, and shifts in the bleaching agent from chlorine to hydrogen peroxide have made the process less environmentally damaging.

Modern Concerns. As the twentieth century progressed, pulp and paper chemists focused on improving quality and yield while also experimenting with consistency. All the while, their efforts remained guided by the drive to increase company profits. Scientists working in conjunction with mill managers at both the pulp and the paper level helped foster the explosive growth of the business, which became a top-ten manufacturing industry in the United States.

In the second half of the twentieth century, many became more aware of the fragile nature of the earth’s ecosystem and noted that human activities were adversely affecting the planet. It was not long before the pulp and paper industry came under scrutiny for its use of a wide assortment of hazardous chemicals and its exploitative relationship with woodlands and forests. Change would once again be forced on the industry.

Today, one of the greatest challenges confronted by pulp and paper chemists is balancing the needs of consumers with the demands of environmentalists. Making the entire industry an efficient custodian of resources has assumed paramount importance, as has improving the quality and productivity of the entire processing procedure. Reducing the environmental impact of the industry must take place in a holistic manner that spans the entire process. Reforestation efforts in cutover timberlands have yielded positive results. Many environmentally conscious paper companies currently replant felled trees in an effort to create a sustainable resource. Gone are the days of clear-cutting followed by abandonment of the region. Scientists are busy designing fast-growing seeds that can replace lost timber at a much faster rate, while many in the industry have committed themselves to less exploitative practices. It is a slow process, but one that clearly represents the next wave in the industry’s development.

Once trees are cut, they are sent to the pulping plant, where less environmentally suspect chemicals are used instead of the known carcinogenic agents of the past. Despite growing national and state-level regulation, environmental pollution remains a major concern. Pulp plants dump treated water into local waterways and burn a variety of compounds that emit toxic chemicals into the air. Strange smells, random fish kills, and high cancer rates are often the unfortunate byproducts of the paper industry. Minimizing these impacts represents perhaps the greatest challenge confronting the pulp and paper industry. It is a problem that has drawn the attention of environmental groups and even the federal government. The United States Environmental Protection Agency (EPA) routinely issues critical reports on the processes that produce the nation’s paper products.

Aside from environmental concerns, the biggest issue shaping the future of the pulp and paper industry is the computer revolution that began in the second half of the twentieth century. The volume of paper a country produced was once deemed an effective measure of its relative civilization and literacy. In modern times, an increasing number of individuals from industrialized countries own or have access to computers that make viewing the news and reading books a simple affair accomplished by clicking a few buttons. In the process, how people consume information has changed. Where once the local newspaper afforded a window on the world’s happenings, today a person can scan headlines online or receive messages sent directly to their cell phones alerting them of news items relevant to them. As a result, many newspapers are struggling to compete and increasingly losing their market share to online vendors; other newspapers have cut costs by switching to online-only editions and only publishing articles digitally. The future of the newspaper industry, and to a greater extent the entire paper publishing world, is in peril. Adding to the threats posed to the once-limitless future many once envisioned for the pulp and paper industry is the increased use of plastic in the packaging process. Although not without environmental risks of its own, plastic is often seen as a cheaper alternative to paper.

Some forecasters fear that the pulp and paper business will seek refuge in underdeveloped nations where environmental laws are lax and virgin timber stands abound. Rather than creating a more environmentally sound processing procedure so that they may continue producing in nations such as the United States, many corporations might relocate their operations elsewhere, making environmental exploitation and degradation rather than careful stewardship the standard in the industry, and indeed the only means of staying competitive in the global marketplace. The industry offers many opportunities for enterprising pulp and paper chemists whose investigative talents might not only save the environment but also maintain the industry’s viability in industrialized nations.

Social Context and Future Prospects

Some have posited that American paper production is entering the same period of long-term decline and eventual demise that once befell the steel and textile industries. Wages remain stagnant, and factories that lack the resources to embrace the latest technologies have shut down. In the future, even currently fiscally solvent enterprises could lose their competitive edge as pressure from third-world producers, who face neither the wage nor the environmental concerns of industrialized nations, challenge the industry once dominated by North American interests. Some hope that improvements in technology, plant upgrades, and greater investment will stop the decline.

Interest in greener technology and industrial processes might also increase the solvency of the enterprise in North America at the same time it makes it a better neighbor and employer for those who reside in the shadow of pulp and paper plants. The future remains uncertain, as the ongoing clash between profits and environmental regulations in North America and northern Europe has led many firms to relocate to the more permissive business climates found in places such as Russia and China. Pulp- and paper-industry researchers continue their efforts to make the chemical processes involved in paper production less detrimental to the environment and to the people who live near their plants. These improvements must not only be effective, they must also be reasonably priced. Without the discovery of environmentally safer, more cost-efficient chemical processes, the pulp and paper industry will inevitably find other, less regulated countries in which to do business. Green technology needs to be affordable, or efforts to safeguard the environment will collapse in a wave of outsourcing.

Bibliography

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"Chemists and Materials Scientists." Occupational Outlook Handbook, Bureau of Labor Statistics, 17 Apr. 2024, www.bls.gov/ooh/life-physical-and-social-science/chemists-and-materials-scientists.htm. Accessed 28 Aug. 2024.

"The History of Paper." American Forest and Paper Association, 9 July 2021, www.afandpa.org/news/2021/history-paper. Accessed 6 Sept. 2023.

"Identifying Greener Paper." Environmental Protection Agency, 28 May 2024, www.epa.gov/greenerproducts/identifying-greener-paper. Accessed 28 Aug. 2024.