Paper Engineering
Paper engineering refers to the design, production, and innovative application of paper, a versatile material that has evolved significantly since its invention in China around 105 CE. Originating from the processing of cellulose fibers, true paper is created through a series of intricate steps that involve pulping various materials, forming a slurry of fibers, and compressing it into sheets. This process not only allows for a wide range of paper types but also enables tailored properties based on the source material, such as wood or cotton.
Historically, the emergence of paper has had profound impacts on human civilization, facilitating communication and record-keeping. Different categories of paper serve specific functions—from absorbent papers used in sanitary products to barrier papers that provide moisture resistance. Advances in technology and environmental awareness have also influenced the paper industry, prompting efforts to reduce energy consumption and emissions during production.
As research continues, there is a growing intersection between traditional paper engineering and modern materials science, especially with innovations involving cellulose-like structures such as graphene. This synergy indicates promising future applications, including advancements in electronics and sustainable materials, positioning paper engineering as a critical aspect of ongoing technological development.
Paper Engineering
Summary
Paper is an important technological development. Although paper-like materials had already been in use for thousands of years, true paper from separated cellulose fibers was invented in China in the year 105 CE. The manner of its production was a closely guarded secret. Since then, paper has become one of the most versatile and useful materials known, and it is used for many purposes.
Definition and Basic Principles
At its most basic, paper is a random collection of cellulose fibers manipulated and compressed to form a flat sheet. The variety of fiber-refining and fiber-processing methods used makes an infinite variety of paper types possible.
True paper is a semisynthetic material made by physically and chemically treating a cellulose source material to isolate and separate the individual cellulose fibers. Through the pulping process, different source materials provide cellulose fibers of different molecular lengths and properties. The cellulose fibers are made into a slurry, which can then be drawn into a sheet of a desired thickness, typically in a Fourdrinier machine. The sheet of loose cellulose fibers then goes through a calendaring process, in which heated rollers express the residual water from the slurry and compress the loose fibers into a thin, hardened sheet. This dry finished sheet is then made into the final form of the paper by trimming and rolling processes.
Background and History
The word paper is taken from the ancient material called papyrus, a paper-like writing material developed and used approximately five thousand years ago in ancient Egypt. Papyrus was a constructed material made by weaving together the fine fibers of the papyrus bush and compressing the resulting weave into a hardened, paper-like sheet.
True paper was invented in China by a court official, Cai Lun, in 105. Cai Lun found that by treating cotton rags in a certain way, the material would break down into a mushy mass of fibers. When more water was added, the mixture could be pressed and dried into a thin, rigid sheet that could be used for writing. This true paper became popular because it was much lighter than bamboo mats and less expensive than silk, both used as writing materials at the time. Cai's paper-making method eventually made its way to Europe, becoming the material of choice for the printing press in the fifteenth century. Industrialization and mechanization since that time have led to the modern paper industry and the thousands of types and forms of paper that are a basic commodity of modern society.
How It Works
Paper is produced by processing different materials to isolate cellulose fibers, which are formed into a sheet structure. Essentially, any cellulosic material can be used as the source of cellulose fibers. Still, the exact nature of the fibers and the type of processing required to obtain them depend on the source.
Most paper is made by chemically pulping wood from softwood trees, yet the finest papers are made from cotton. The difference is in the type of fibers each material contains and the required processing. Cellulose molecules in wood are combined with shorter molecules called hemicelluloses and with a hard substance called lignin, which gives trees and other plants their structural strength. The cellulose fibers must be separated from these components before they can be used to make paper. This requires a detailed chemical and physical treatment to obtain usable wood cellulose fibers. Compared with wood cellulose, cotton fiber provides long, pure cellulose fibers that require little or no additional treatment. The length of the cotton fibers also ensures that the intertwining of the fibers in the slurry stage is more extensive, which in turn produces a much tougher final paper structure. The difference is readily seen by comparing a sheet of cotton-based paper with a sheet of ordinary bond paper.
Paper formation begins when the cellulose fibers, from whatever source, are blended with water to form a slurry. The density of the slurry can be adjusted to any consistency according to the type of paper being produced. Colorants and other additives are introduced to the slurry at this point. The Fourdrinier machine, named for its English inventor, is commonly used to produce continuous sheet paper. In a Fourdrinier machine, a continuous mesh belt passes through the slurry, drawing a uniform coating of the mixture. Most of the liquid in the slurry passes through the mesh structure of the belt, leaving behind the randomly mixed cellulose fiber.
The fiber sheet then goes into calendaring machinery, in which heated rollers express the remaining liquid from the sheet and dry the cellulose fiber mass while compressing the fibers together into the harder and more rigid structure of the paper. After calendaring, the paper sheet is trimmed and taken up on rolls to be used as feedstock for final production. Rolled stock is subjected to further trimming, shaping, and finishing treatments based on the ultimate use of the particular type of paper.
Applications and Products
Cellulose fiber is one of the most versatile materials known. The range of properties imparted to paper produced from cellulose fiber is essentially infinite. The intrinsic properties of the paper depend on the average length and purity of the cellulose fibers themselves. Other properties are imparted during the physical processing of the paper, and still others are provided by combining the paper with other materials during formation and finishing procedures.
Papers are typically named according to their intended use, their method of production, or the source of the cellulose fibers used. A conservative listing of paper types contains approximately two hundred individual references in several general categories. A list of the various pulps and pulping procedures used is similarly long.
Abrasive Papers. Paper is used as the backing or support material for several products designed for abrasive purposes, the most familiar being common sandpaper. Sandpaper is produced in several standard grit designations, ranging from the extremely coarse 30 grit to the fine polishing paper called crocus cloth. Sandpapers are normally used dry, but they are also formulated to be wet with water or other solvents.
Sandpaper is produced by coating the backing paper with an appropriate adhesive, which is then applied to an even and consistent distribution of the desired abrasive grit material. Once the adhesive has cured, the sandpaper is cut to a standard size.
Absorbent Papers. Various papers are produced without extensive calendaring or sizing, leaving a relatively thick, fiber-like paper that readily absorbs liquids. Most commonly used in producing various sanitary paper products, such as toilet paper and paper towels, this class also includes blotting paper, essential for absorbing excess ink from written documents.
The bulky nature of absorbent papers also makes them permeable to liquids. Based on this property, absorbent papers are used as filter media in coffee makers, in laboratory procedures for the recovery of crystalline solids, in fuel and oil filters for all types of machinery, in air filters for computer hard drives, and in many other similar applications.
Barrier Papers. Barrier papers are produced with treatments to make them impermeable and nonabsorbent. The treatment may be incorporated into the paper during its formation, added after as a finishing procedure, or through lamination of the paper to an impermeable material. Barrier papers resist or prevent the passage of fluids such as air, water, and oils. This class includes papers for several different applications, including aluminum-foil-laminated paper (for chocolate bar wrappers), bacon paper and blood-proof or butcher's paper (for wrapping fat- and blood-containing meats), Bakelite paper, waxed paper (such as candy-twist tissue papers, butter wrapping-paper, common household waxed paper), and all manner of cellophane papers. Polyethylene plastic sheeting has been substituted for some barrier papers, especially in food wrapping applications.
Cellophane. Cellophane looks and acts like a hydrocarbon-based plastic material but is not one. The viscose process extracts cellulose as cellulose xanthate, which is then treated and processed to regenerate the cellulose as Rayon, a material used extensively as fibers. The same material, however, can be made in sheet form, which is essentially a modified cellulose paper. Though cellophane is biodegradable, compostable, and made from natural materials, its manufacturing requires significant energy, and carbon disulfide, a toxic chemical, is often used.
Industrial Papers. The industrial papers comprise one of the largest uses of paper products worldwide. Industrial papers are not subject to the refining, bleaching, and other finishing treatments required for finer papers. Instead, some industrial-grade papers may actually contain small wood fibers among the cellulose. This paper class is normally used for packaging material and utility applications that do not call for a more refined paper.
One application is making low-grade newsprint-type paper used as separators for sheets of glass, plastics, and other materials. Another application is heavy brown manila paper, which is used for wrapping bulk matter such as raw, cured tobacco and manufacturing heavy-duty envelopes and corrugated cardboard boxes. The industrial papers include numerous construction-grade cardstocks and fiberboard materials such as tar-coated roofing paper and exterior wall sheathing, as well as low-, medium-, and high-density construction fiberboard. Seemingly more like wood products than paper in their applications, they are manufactured from cellulosic pulps using papermaking methods.
Laminated Papers. This designation of paper types crosses into many other classes, including barrier and industrial papers. Laminated papers are produced by laminating sheets of a particular type of paper to either other sheets or other sheet materials. Cardstocks and fiberboards are typically made by laminating together several sheets of a thinner construction or industrial paper. Barrier papers are often made by laminating a base paper to aluminum or other metal foil or some kind of plastic sheet.
Supercalendered Papers. This class includes all paper types processed with a high degree of calendaring to provide an extremely smooth, high-gloss finish. Papers used for printing magazines are typically supercalendered. The paper sheet known as glassine is a special case of supercalendered paper because of the extra processing required to prepare the pulp from which it is made and the higher calendaring pressures used in its formation. The result is an almost impermeable, translucent paper with a texture reminiscent of very thin glass, hence the name glassine.
Social Context and Future Prospects
Paper engineering is such an intimate part of the historical development of human civilization that society without it is unimaginable. For that reason alone, it can be expected that paper will continue in that role as new forms and applications of the material are developed. Paper engineering serves another purpose related to the most advanced materials known. The long history of paper made from cellulosic fibers has provided a wealth of information and understanding regarding the behavior of fibers in general. This historical fact also applies to knowledge of fiber-like molecules of other materials.
Graphene, a natural material, has been recognized and characterized as perhaps the most significant material. It has a rather paper-like molecular structure and unique chemical and physical properties that could revolutionize electronics and electronic technology. Related compounds called fullerenes are essentially small graphene molecules that have folded over and “zipped together” through chemical bonds. They range in size from the relatively small buckminsterfullerene (C60) molecule to the macromolecules known as carbon nanotubes.
The physical behavior of large carbon nanotubes (CNT) closely mimics the behavior of cellulose fibers, enabling the material to be formed into a carbon nanotube paper. This represents a relatively new area of research and development that likely would only have been discovered with the scientific understanding of paper and paper engineering. Scientists use carbon nanotube technology to create buckypaper, which is 250 times more powerful than steel yet 10 times lighter. In this role, paper engineering will have the greatest effect on future applications and technologies, including body armor, electrical conduction, fire prevention, energy storage, batteries, and engineering biological tissues like nerve cells.
In the twenty-first century, managing energy use and limiting environmental impact is increasingly important in paper engineering. The third largest operating cost for paper manufacturers is energy, including electricity, steam, and fuels. The American Forest and Paper Association reported its members reached their goal of producing over 50 percent of their required electricity between 2005 and 2020 by updating technology and adopting renewable energy sources like biomass. The industry continues aiming to reduce emissions.
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