PAPER AND CORRUGATED PAPERBOARD: THE WHO, WHAT, WHEN, WHERE, WHY, AND HOW OF THE MOST COMMONLY USED PACKAGING MATERIAL
PREPARED BY HERBERT H. SCHUENEMAN, CP-P, MH
Presented by WESTPAK, INC. 83 Great Oaks Blvd., Blvd., San Jose, CA 95119 (408) 224-1300, FAX (408) 224-5113 www.westpak.com __________________________________________
I. OVERVIEW PAPER This very common term is the name for all kinds of material, typically matted sheets of fiber, usually vegetable, but sometimes mineral, animal or synthetic, formed on a fine screen from a water suspension. Paper derives its name from papyrus, papyrus , a sheet made by pasting together thin sections of an Egyptian reed and used in ancient times as a writing material. Paper is also widely used in the packaging business as one of the two broad subdivisions of paper; namely, paper or paperboard. The distinction between paper and paperboard is not a sharp one, but generally speaking, paper is lighter in basis weight, thinner, and more flexible than paperboard. The largest use for paper is in the printing, writing, wrapping, and sanitary purposes, although it is widely used for other purposes as well. Paperboard normally refers to sheets of paper .012” (12 points) or more in thickness. There are some exceptions such as in the case of blotting paper or drawing paper. Paperboard is made from a wide variety of materials (called furnishes) on a number of different types of machines, principally cylinder and Fourdrinier. Broad classes of paperboard include: A. Container board used for corrugated boxes; B. Box board principally used to make cartons; C. All other paperboard. The paper and paperboard industry touch mankind in many different ways. Practically every moment of every day we find ourselves relying on products made from pulp, paper, and paperboard. Most of these products are easy to recognize such as computer paper, newsprint, paper towels, corrugated boxes, and grocery bags. Other commonly used papers include coffee filters, postage stamps, money, milk cartons, baseball cards, and pizza boxes. Products made from pulp chemicals include cologne, shoe polish, cosmetics, toothpaste, roofing, food flavorings, and detergents just to name a few. In all, over 12,000 different kinds of products are made from paper and its byproducts. Today, large papermaking machines in mills operate around the clock to prepare vast quantities of paper for writing, printing, wrapping, and packaging purposes among others. The machines use cellulose fiber primarily from wood, cotton, fibrous plants such as bagasse (sugar cane waste) or other common raw materials. The remainder of this paper will deal with the history of the papermaking process, identification of the important properties of paper for packaging purposes, how these properties are tested, how paper is combined into a useful structure such as corrugated paperboard, and how that structure is tested and used in the packaging of goods for distribution.
II. HISTORY OF THE PAPERMAKING PROCESS Paper is a sheet of material made up of many small discrete fibers bonded together. It is generally conceded that paper was invented and first produced in China. The date of the first papermaking operation is unknown, but some reports put it at approximately 105 A.D. The art of papermaking was a Chinese secret for many years. The knowledge spread slowly throughout the country and eventually reached the Korean Peninsula (then an eastern part of China) at approximately 600 A.D. It was then brought to the Japanese island where the process was further refined. The westward expansion of papermaking occurred when the Muslims captured some Chinese papermakers and brought them to Baghdad. This occurred in about the 8 th century and the process of papermaking slowly expanded through Africa and Europe to reach Damascus and Cairo in approximately the 10 th century. The papermaking art expanded slowly through the European continent, reaching Germany in the late 1300’s and England sometime thereafter. In 1690, William Rittenhouse built the first paper mill in the United States at Germantown, Pennsylvania. Prior to this, all papermaking was done by hand. The process involved first reducing bark or grass to a fibrous pulp by beating it in a stone mortar. Then a slurry in water was formed, poured into a box-like mold with a porous bottom, and the water was allowed to drain forming a sheet of fibers. The sheet still in the mold was allowed to dry in the sun. Improvements in the technique came along over the centuries including dipping the mold into a slurry in a vat, removing the sheet from the screen by a felt, and pressing stacks of sheets and felts together on a hot surface. The process remained tedious and time consuming and production was small. However, demand was small also, so there was no great problem. The invention of the printing press by Guttenberg in the 1500’s changed all that. Demand for paper skyrocketed. Soon the industry was faced with two major problems. There was a shortage of fibers, particularly cotton, and the tedious hand forming methods of the papermaking were no longer adequate. Neither problem was solved for several centuries. The solutions, when them came, led directly to the paper industry technology that we know today. Throughout the entire history of papermaking, the principal fibrous raw material has been cellulose which which is the major product of photosynthesis. It is the most abundant organic material on earth with an estimated growth rate of about 100 billion tons per year. Unknowingly, the early papermakers had found the inner bark of a certain shrub and trees with a relatively pure form of cellulose fibers. In addition, cotton was also used which is the purest form that occurs naturally.
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In the early 1800’s, a French chemist identified cellulose as a separate entity in wood. Prior to this, wood had been considered to be a homogeneous chemical compound. Within a few years, wood fibers were being used in paper and wood has served as the major papermaking raw material ever since.
Corrugated Collar
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III. CELLULOSE AND THE PAPERMAKING PROCESS CELLULOSE Chemically speaking, wood is a complex mixture of substances, exact nature and amount being dependent on many factors. Tree species often are classified roughly as conifers (softwoods) and deciduous (hardwoods). Both contain about 50% cellulose by weight. The remaining 50% contains hemicellulose compounds and lignin which is a substance that essentially glues the cellulose fibers together.
Cellulose Springwood
Cellulose 160X
Cellulose 2875X
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Considerable variation in the amount of cellulose in wood is quite common. Nevertheless, on a dry basis, most species contain one half cellulose and approximately 25% lignin. Lignin is the phenolic material often said to encase the fibers and cement them together. The fibers from hardwoods and softwoods vary considerably in their characteristics, especially their length. Typical softwood fibers will average approximately 3.5 to 4 mm in length while hardwood fibers are generally in the range of 1 to 1.5 mm in length.
For use in papermaking, these fibers must be separated from each other into discrete entities. The separation process may involve rather drastic action, the net result of which the fibers will compress from a tubular structure to a ribbon structure.
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PULPING The first process in papermaking involves changing wood into an aqueous pulp solution. This is generally done by using a mechanical, chemical, or “semichemical” processes. Semi-chemical is a combination of both mechanical and chemical. The first approach to pulping was mechanical. That is, simply grinding up the wood. However, the method of grinding is critical. Wood can be ground in ways which completely destroy fibrous nature or in other ways retain a significant fibrous portion. Obviously the latter is desired. An important point is that no purification or selection is achieved in mechanical pulping. The pulp has essentially the same composition as the wood from which it was made, except for a small percentage of water soluble material. This means that all the lignin is retained in the pulp with the resulting paper showing poor strength and bonding, poor permanence, but good printability. Examples of mechanically pulped paper are short utility papers such as newsprint.
Unbeaten Softwood 130X
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Chemical pulping is used on most papers produced commercially in the world today. Traditionally, this has involved a full chemical treatment in which the object is to remove non-cellulose wood components leaving in tact the cellulose fibers. In practice, separation of the components is never completely realized. Yet satisfactory compromises are reached in the processes which yields somewhere between 45% and 55% of the wood mass. It has been recognized recently that some of the other products in wood (i.e. lignin) may impart desirable properties for pulp in the papermaking process. Such processes are generally designated at semi-chemical and may operate at yield levels above 60%. They generally require some mechanical agitation to disintegrate the wood chip after chemical treatment. Each process has its advantages and disadvantages. The major advantage of mechanical pulping is its high yield of fibers up to 90%. Chemical pulping yields approximately 50% but offers higher strength properties and the fibers are more easily breached because the mechanical pulping process does not remove lignin. Even with subsequent bleaching, these fibers are susceptible to yellowing. This is the reason that paper grades containing high quantities of mechanical pulp fiber such as newsprint discolor quickly, especially when exposed to sunlight. Recycled fibers greatly affect the characteristics of the paper. For example, old corrugated containers used in new box making require less virgin fiber and maintain a high degree of the original strength properties from the Kraft paper process. Mixed post-consumer waste contains many contaminants and often requires more processing. This results in a lower yield of fibers and produces a higher level of sludge in chemical pulping processes. This topic will be treated in depth later. KRAFT PULPING By far, the majority of papermaking in the United States involves a process of pulping referred to as Kraft pulping . In this process, trees converted to wood chips undergrowth screening to a uniform size to ensure that all chips will cook evenly. A very small chip would obviously cook faster than a large one. Wood chips and the chemical pulping solution referred to as white liquor are mixed in a vessel called a digester and cooked at high temperatures under pressure. Digesters use either a batch or continuous process. After cooking, a washing process separates the wood pulp and cooking chemicals. Because of the cooking, approximately 50% of the original weight of the wood, primarily lignin, dissolves and is washed out with the cooking chemicals. The resulting solution, referred to as black liquor , is a mixture of dissolved lignin and other organic constituents as well as inorganic cooking chemicals. After washing, the pulp is suitable for further processing of the paper. It may undergo bleaching for white or colored grades of paper.
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One of the important side products of the Kraft pulp processing features the excellent recovery capability of the chemicals used in the system. A recovery process consists of separation of the spent liquor from the pulp by washing, concentration of the liquor by evaporation, combustion of the organic portion of the liquor, and conversion into sodium compounds and the dissolution of the sodium in water. Emergence of the Kraft process required only the addition of a smelting furnace in which the sodium sulfur compounds were reduced to sodium sulfate.
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PAPERMAKING Paper mills are integrated or non-integrated by type. An integrated mill is one that conducts pulp manufacturing on-site. Non-integrated mills have no capacity for pulping but must bring pulp to the mill from an outside source. Integrated mills have the advantage of using common auxiliary systems for both pulping and papermaking such as steam and electric generation and waste water treatment. Transportation cost is also reduced. Non-integrated mills require less land, energy, and water than integrated mills. Their location can, therefore, be in a more open setting where they are closer to large work force populations and perhaps to their customers. A paper mill can house a single paper machine or several machines. Each machine can make a single grade of paper or a variety of papers. A dedicated machine usually manufactures a commodity grade paper such as liner board or tissue. Machines designed to make specialty grades typically have more operating flexibility and will manufacture many types of paper. A paper machine can be longer than a football field and widths vary from 100” to more than 400”. The basic process of papermaking remains the same despite the type of paper manufactured or the size of the machine.
Paper Machine
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STOCK PREPARATION Before pulp can be made into paper, it must undergo several steps called stock preparation. This amounts to a mechanical beading process that will make the fibers more readily suited to laying in a uniform web and for interconnecting to other fibers in order to make a strong sheet. For example, the fibers may become kinked or curled, fiber flexibility may increase, fibers may be cut or shortened, and external fibrillation may occur making the fibers look “fuzzy”. All of these changes affect the way the fibers bond together in the sheet. This strongly influences the strength of the sheet and its light scattering properties.
Unbeaten Softwood 130X
Beaten Softwood 130X
Fibrillation 1920X
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The furnish (as it is now referred to) can also be treated with may chemical additives. These include resins to improve the wet strength of the paper, dies and pigments to affect the color of the sheet, fillers such as talc and clay to improve optical qualities, and sizing agents to control penetration of liquids and to improve printing properties. After stock preparation, the next step is to form the slurry into the desired type of paper at the wet end of the paper machine. FORMATION The pulp is pumped into the head box of the paper machine at this point. The slurry consists of approximately 99.5% water and approximately 0.5% pulp fiber. The exit point for the slurry is the “slice” or head box opening. The fibrous mixture pours onto a traveling wire mesh in the Fourdrinier process, or onto a rotating cylinder in the cylinder machine. The Fourdrinier machine is named after its French inventors, the Fourdrinier brothers, and is essentially a table over which the wire moves. Greater quantities of slurry released from the head box result in thicker paper. As the wire moves along the machine path, water drains through the mesh. Fibers align in the direction of the wire travel and interlace to improve the sheet formation. After the web forms on the wire, the task of the remaining portion of the paper machine is to remove additional water. Vacuum boxes located under the wire aid in this drainage. One of the characteristics inherent in the performing of the sheet on a Fordrinier paper machine is that all the water is removed through one side of the sheet. This can lead to differences in the sheet properties on one side as opposed to the other. This two-sided property increases as machine speed increases. In response to this, manufacturers developed twin wire and multiple Fordrinier machines. Manufacturers of such equipment use different engineering designs that can be vertical or horizontal. After the paper web has completed its short forming distance, it continues along the second wire loosing water as it travels.
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Double Screen Wet End
Double Screen Paper Machine 13
PRESSING AND DRYING The next stop for the paper is the pressing and drying section where additional dewatering occurs. The newly created web enters the press section and then the dryers. As the paper enters the press section, it undergoes compression between two rotating rolls to squeeze out more water. The extent of water removal from the forming and press sections depends greatly on the design of the machine and the running speed. When the paper leaves the press section, the sheet usually has about 65% moisture content. The paper web continues to thread its way through the steam heated dryers loosing moisture each step of the way. The process evaporates many tons of water. Paper will sometimes undergo a sizing or coating process. The web in these cases continues into a second drying operation before entering the calendaring stacks that are part of the finishing operation. Moisture content should be about 4% to 6% as predetermined by the mill. If the paper is too dry, it may become too brittle. About 90% of the cost of removing water from the sheet occurs during the pressing and drying operations. Most of the cost is for the energy required for drying. At the end of the paper machine paper continues onto a reel for winding to the desired roll diameter. The machine tender cuts the paper at this diameter and immediately starts a new reel with the additional paper falling as an endless web.
Paper Mill Overview 14
FINISHING AND CONVERTING For grades of paper used in the manufacture of corrugated paperboard, the process is now complete. For those papers used for other purposes, finishing and converting operations will now occur, typically off line from the paper machine. These operations can include coating, calendaring, or super calendaring and winding. COATING Coating is the treatment of the paper surface with clay or other pigments and/or adhesives to enhance printing quality, color, smoothness, opacity, or other surface characteristics. There is a great demand for paper with a very smooth printing surface. Coated paper gives better results than uncoated paper and all major printing processes including better press, lithography, rotogravure, or Xeroxgraphy. Solid color areas can be reproduced more easily in photographs (half tones) or have more sharpness on coated paper. Four-color process printing especially offers sharper images with coated paper because of the smooth surface and reflective whiteness. In addition, the ink will not absorb into the paper as much and, therefore, stays on its surface in a dot format resulting in better color reproducibility. Various grades of paper, including paperboard, printing, writing and industrial or packaging grades sometimes have coatings. Most coated paper is ground with paper made from mechanical pulp. The term “coated free sheet” describes paper made from ground wood free fibers being produced from chemical pulp. Three major coated paper categories exist – glossy, dull, and mat. Many people equate coated paper with the gloss stock of a magazine. Books and other products may use dull coated paper to retain the advantages of coated paper while reducing light glare. Two popular coating methods are air knife and blade coating. In the air knife process, a jet of air acts like a blade to remove excess coating applied to the paperboard. The blade coating process using a flexible blade set in an adjustable angle to remove excess coating across the web. Following the coating operation, the sheet must again be dried and rewound. CALENDERING Calendering is an on-machine process where the paper passes through a series of polished steel rolls to smooth the paper surface before rewinding on a reel. Besides imparting smoothness, calendering can reduce variations in the sheet and create a higher density sheet. It can also affect the water absorption properties of the paper.
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WINDING Winding may appear to be a simple process, but anyone who has ever tried to rewind a roll of bathroom tissue after a small child has played with it will think differently. Maintaining proper tension on the reel so that the sheet lies flat and attains proper alignment for both edges is a difficult task. Further complications occur with the higher speeds (up to 6,000 ft/min) of the paper machine. At this rate, the paper web is moving faster than a car at highway speed and paper the length of 20 football fields would wrap on a roll every minute. Other operations can also take place including cutting, sorting, counting, and packaging. For some products such as tissue and copy paper, the typical paper mill will conduct all of these operations. In most cases, however, the rolls are wrapped and readied for shipment to their final destination. The rolls going to a corrugated paperboard plant are of specific interest to us here. SUMMARY The nature of paper and papermaking has changed very little over the past 150 years since the introduction of the Kraft Fordrinier process. However, the techniques and equipment necessary to make paper have changed dramatically. Because of this, we can rely on a consistent supply of high quality graded papers for almost any need we can imagine. It’s hard to envision life in the United States without this consistent supply of high quality paper. Equally true is the ability of most packages to survive without high quality paper. The United States used an average of 750 lb. of paper per capita in 1995 compared with approximately 550 lb. 20 years earlier (so much for the paperless office).
Paper Production
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It’s hard to envision how the packaging industry could exist at all without a steady supply of high quality container systems such as we have in corrugated fiberboard boxes. This is all made possible by the abundant supply of high quality paper from which the containers are made.
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IV. IMPORTANT PROPERTIES OF PAPER FOR PACKAGING APPLICATIONS PRECONDITIONING AND CONDITIONING When judging the quality and performance characteristics of paper, it is necessary to understand that sampling and conditioning greatly affect the accuracy and repeatability of test results. Based on this, the reader is encouraged to review TAPPI T400 techniques for sampling of paperboard and T402 for conditioning. As a hydroscopic material, paper and paperboard will have very different strength properties when judged at various humidity levels. Therefore, it is important that the conditioning requirements be followed explicitly and that testing be conducted in a conscientious and accurate fashion. For almost all standard testing, conditions of 23 °C and 50% RH are required. In addition, it is known that paper will approach standard moisture content asymptotically and, therefore, it is important that these conditions be applied to the paper after it is first brought to a lower humidity state (preconditioning). Preconditioning requires that the paper be held at 39 °C and 30% RH for a period of sufficient time to allow the paper to stabilize, normally 24 hours. Following this drying sequence, the paper is then brought to standard conditions from a dryer state. This helps assure that the required humidity conditions of the paperboard can be reached in a minimum period of time. INTERNAL TEAR RESISTANCE This test is conducted on a Elmendorf type device. It measures the ability of paper to resist a tear once it has been initiated. This test is conducted according to TAPPI T414 and is basically an indication of the strength of the individual fibers within the paper, the length of those fibers, and the formation of those fibers into a coherent and uniform sheet. Tear resistance is not a property that is normally specified when ordering corrugated boxes. However, this property does have a great affect on the ability of the boxes to properly form and carry their contents through the normal distribution process. For example, the slotting of flaps on a corrugated box produces what amounts to a cut in the paper surface. The paperboard must have the ability to resist propagation of this cut through the normal impacts and handling operations that will occur from the time the box is produced until it is used and recycled. Refer to TAPPI T414 for more details.
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STIFFNESS The stiffness properties of paper and paperboard contribute directly to one of the primary characteristics of corrugated paperboard, namely the ability to stack loads on top of one another without falling over. The beam formed by the corrugated paperboard process works in compression only if the outside members, the liner boards, are very stiff in response to an applied load. The stiffness properties of paperboard are determined on a bending resistance apparatus such as the Taber stiffness tester. To conduct this test on paper, a properly sized and conditioned specimen is placed in the apparatus which is then forced to bend in each direction. The resistance to that bending force is measured by the machine.
Stiffness Tester
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Stiffness is a property that is directly affected by the fiber mix used in the head box of the paper machine, the caliper or thickness of the paperboard sheet, and the finishing or calendering operations conducted on the paperboard. Refer to the test method TAPPI T489 for more details on the test procedure. TENSILE The tensile characteristics of paper and paperboard are important in their ability to resist tearing or ruptures that are typical of the distribution environment of corrugated boxes. The test is conducted according to TAPPI T494 or similar test procedures. To conduct this test, a properly conditioned specimen is subjected to a constant separation until failure occurs somewhere in the paper sample. The rate of separation is specified in the standard. The maximum load at failure is a measure of the tensile properties of the paperboard. The test can be conducted on any suitable tensile test machine provided the jaws are properly aligned and can separate the sample at the required rate (1 in/min). In addition to the maximum tensile strength, the energy absorbed by the sample during the test is also important. This quantity is referred to as TEA or tensile energy absorption. It is numerically equivalent to the area under the graph produced by the stress-strain curve developed during the test. It is a measure of the toughness of the paper sample or its ability to resist impacts. Refer to the test procedure for more details. INTERNAL BOND STRENGTH (Z AXIS DIRECTION TENSILE) This test describes the procedure for measuring the internal bond strength of paperboard referred to as the z axis direction tensile. The procedure consists of applying double coated pressure sensitive tape to both sides of the test specimen. The specimen is then placed between two platens and compressed uniformly. Uniform tensile is then applied to the test specimen. This tensile load is applied perpendicular to the plane of the sample and is designed to separate the test specimen. This property has become more important in recent years with the advent of multiple head box paper machines where different layers of paper are laid down at different points in the forming process at the wet end of the paper machine. In addition, the continued use of recycled fibers in the furnish will yield different results in the internal bond strength of paper and, therefore, this property is important to both the paper maker as well as the users of paper in general.
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Of particular interest is the practice whereby a paperboard sheet is comprised of three separate layers, each formed by a separate head box on the paper machine. The outside layers are typically virgin Kraft while the center head box is 100% recycled fiber. This practice generally results in a high quality exterior on the sheet but potentially poor internal bonding strength. To conduct the test, specimens are prepared and conditioned according to normal TAPPI procedures. A layer of double sided tape is then applied to both sides of the paperboard sheet. This specimen is then placed in the test apparatus where positive pressure is applied for a period of 6 seconds in order to properly bond the adhesive to the paper. At this point, a tensile load is applied to the test apparatus designed to pull the specimen apart. The load at which the specimen fails is referred to as the “z-axis bonding co-efficient” of the paperboard. Refer to TAPPI T541 for more details on the test procedure. BURSTING STRENGTH This test procedure describes the strength of the paperboard in resisting a disk shaped diaphragm attempting to burst it. The test was originally designed and utilized for the clothing industry in an attempt to determine the resistance of clothing to protrusions of the human body such as elbows. To conduct this test, a specimen of board is clamped between two platens with circular openings in the center. An expansible diaphragm is distended through the lower platen by means of hydraulic pressure until the specimen bursts. The maximum hydraulic pressure when the specimen ruptures is recorded. The test is a measure of some basic properties of the paperboard sheet, primarily tensile and elongation. The test is primarily a measure of sheet formation, although other properties including fiber length, fiber quality, internal sizing, and surface treatment also have an affect on the burst strength of paperboard. To conduct the test, samples of paperboard are properly selected and conditioned according to TAPPI Standards. Specimens are then clamped in the test apparatus and a burst test to failure is conducted. The test requires a minimum of five acceptable bursts from both sides of the paper sheet. Refer to TAPPI T807 for more details on the test procedure.
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RING CRUSH This test measures the edgewise compression strength of paperboard. test procedure is intended for paperboard between .011” and .020” thick.
The
The edgewise compression strength of corrugated paperboard is the principle element in determining the dynamic compression strength of the container made from that board. Since fiberboard shipping containers are frequently subjected to top loads and are expected to resist those loads, the ring crush capability of paperboard is an important characteristic. To conduct this test, a compressive force is exerted on a specimen held in a ring form in a special fixture placed between two platens of a compression test machine. By causing the upper platen to approach the lower platen at a uniform speed, the specimen collapses resulting in a force level referred to as the ring crush level. The test is performed according to TAPPI T818 (flexible beam method) or TAPPI T822 (rigid support method). The differences between these two are a slight difference in the fixturing used to support the specimen during the ring crush test. As with other test procedures, the sampling and conditioning of the paperboard is an important characteristic in the overall test results. Refer to the test procedure for more information. CALIPER (THICKNESS) This test measures the thickness of paper and paperboard and is a useful procedure in determining the overall thickness of the paperboard. This is an important property and variations in thickness are also important for papers used for mechanical purposes. The test is useful for research work, routine control and design of end use products, and for acceptance testing for conformance to specifications. The thickness of combined board is one of the characteristics that importantly affects the flexural stiffness of the corrugated paperboard made from these products. The test is conducted according to TAPPI T411 or similar procedures. It requires a caliper with a known area and a given force applied between the two pressure feet. Normally, a dial indicator is used with an accuracy of .00002”. Ten readings are taken from each side of the paper surface.
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GRAMMAGE OF PAPER (WEIGHT PER UNIT AREA) This procedure is conducted according to TAPPI T810 and measures the bulk or weight of paper per unit area. Most paper is bought and sold in accordance with its mass per unit area and, therefore, the grammage has significance both to the consumer and the producer in defining price. The values effect many physical properties such as bursting strength, thickness and bulk. These are interpreted and specified with regard to grammage. The grammage test must be conducted under standard conditions with samples properly sized and properly conditioned. This is particularly important for this particular procedure. Refer to TAPPI T410 for more information on this test.
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V. THE EFFECT PROPERTIES
OF
RECYCLING
ON
IMPORTANT
PAPER
To understand the effect of recycling, it is important to understand that paper and paperboard is made from either virgin fibers or recycled fibers, or in most cases, a combination of both. The term virgin fiber is referred to fibers from trees or plants. Broad classifications of virgin fibers are hardwood or softwood, depending on the type of tree used. Normal virgin pulp can contain up to 20% of pre-consumer waste, typically from the paper machine itself. The recycled paper implies post-consumer content in the feed stock used to make a new product. Post-consumer content in paper is defined by the U. S. Congress in Section 6002 of the Resource Conservation and Recovery Act to be "Paper, paperboard, and fibrous waste from retail stores, office buildings, homes, and so forth, after they have passed through their end use as a consumer item, including used corrugated boxes, newsprint, tabulating cards, used cordage, and all paper, paperboard, and fibrous waste that enter and are collected from the municipal solid waste stream." Defining exactly what constitutes recycled paper has been an issue of bureaucratic entanglement for some time. ASTM is currently working on definitions for recycled paper as are a number of state and local governments. The EPA has its own definition. So do a number of foreign countries. This includes Canada's Environmental Choice Program. This program defines pre-consumer waste as broke, which many American mills claim as recycled content under EPA guidelines. The Conservatree Paper Company has issued a ranking standard which lists four separate grades of recycled paper. C4 is paper which meets the EPA 50% waste paper standard. • Papers containing secondary post-mill waste are rated C3. • Papers containing de-inked fibers such as printer waste rate C2. • Papers containing 15% post-consumer fiber and 60% secondary post-mill fiber receive a rating of C1. •
Symbology of Recycling The familiar chasing arrow recycling symbol is everywhere these days. Its design conveys three integral phases of recycling: the collection, processing, and manufacturing of recovered materials into new products. A stylized outline of a tree can be seen in the center of the symbol.
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The recycling symbol is created by Container Corporation of America, a major producer of recycled paperboard. In the late 1960's the company identified a marketing opportunity to promote the benefits of its products in response to the emerging environmental consciousness of the United States.
In 1970 the original Earth Day celebration also included a design contest for graphic art students to design a recycling symbol for paper recovery. Over 1000 entries were submitted. The winning entry was submitted by Gary Anderson, an art student from the University of California, Berkeley. This symbol was later modified into the now familiar chasing arrows design by William Lloyd. Originally two versions of the symbols were created to convey two different messages. Solid arrows within a black circle designated a box or container that was made from recycled paperboard. Another version where the arrows appeared in outline meant that the box or container was recyclable. This symbol quickly became the centerpiece of an ongoing campaign to promote the use of recycled paperboard. A now defunct New York environmental group challenged the service mark registration application on the basis that the symbol would create consumer confusion. Today the original symbol, and many versions of it, have inspired generic recycling labels. Sometimes the three chasing arrows appear against a green background, stand alone within a circle, or show up reversed out or upside down. Some versions of the symbol include words, codes, logos, or other symbols.
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It is ironic that the very problem feared from registering the symbol; namely, consumer confusion, is now widespread because of the diversity of the symbol's use, appearance, and meaning. The American Paper Institute uses the symbol as a marketing tool for the members of its Recycled Paperboard Division, and it provides guidelines for the symbol's use on recycled paperboard. API does not explain the symbol's implications when it appears on other paper grades, such as printing paper or letterhead. This situation invites abuse and confusion. Until environmental claims are regulated in a more uniform fashion, consumers must rely on ethics of those promoting their products as recycled or recyclable.
Table 1 New York-Rhode Island Emblem Program
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The following table describes recent attempts at defining what constitutes a recycled container system or recycled content in various paper streams. The standard from the New York/Rhode Island emblem program is an attempt at a political definition, while Table 2 shows an attempt to define paper recycled content in terms of both percentage of weight and percentage of fiber.
Explanation of Recycled Content
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STRENGTH PROPERTIES In general, the use of recycled fiber will reduce the values of the following tests conducted on paperboard: 1. 2. 3. 4.
Tensile Burst Tear Ring Crush
The use of recycled fiber is generally considered to be neutral on properties of stiffness, caliper, and grammage. It should be emphasized that in all cases, the perceived negative effect of using recycled fiber can be overcome by proper paper chemistry and wet end formulation of the furnish. In addition, additives that improve the paper qualities can all but negate the effect of high levels of recycled content. There are some paper properties that are actually enhanced by using recycled fibers; smoothness, perosity and printability are examples. In addition, depending on the circumstances, paper recycling may end up using more resources than it saves. This depends largely on the type of recycled paper being used and the type of paper being produced. In fact, about 35% of the raw materials used in U. S. paper mills is recovered paper. Twenty-two of the 400 paper-making mills in the United States rely exclusively on recycled paper furnish. Recovery of office paper has more than doubled in the United States since 1990. In 1996, 3.3 million tons of office paper were recovered. In addition, more paper and paperboard packaging is recovered for recycling than all glass, plastic, metal, and other materials combined on a weight basis. In 1996 nearly 20 million tons of paper and paperboard were recovered for recycling in the U. S. As much as 60% of all corrugated boxes and containers used in the United States were recycled in 1996. As much as 63% of all newsprint used in the United States is recovered for recycling. Approximately 1/3 of that goes back into making more newsprint. Other products made from this recycled stock include cereal boxes, corrugated containers, books, insulating materials, printing and writing paper, tissue, egg carton and other molded pulp products. It is the announced goal of the U. S. Paper Recovery Industry to recover 50% of all paper made and to re-use that in the paper-making and related industry processes.
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VI. THE MANUFACTURE OF CORRUGATED PAPERBOARD The strength and durability of corrugated and the secret of its versatility lie in the very concept of the material itself. It can contain and protect its contents effectively because it utilizes simple engineering forms. First a sheet of corrugating medium is shaped into a continuous rolling wave. These are called flutes. Observed vertically, they form a row of columns, basically a structure form capable of supporting great weight. Viewed horizontally, the flutes are arches, another basic form, and the strongest way to span a given space.
Singlewall Corrugated The combination of columns and arches produces a material far stronger than the paperboard from which it is made, yet the sheet of corrugated board can easily be cut into shapes and creased or scored for folding in any direction.
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The process of making corrugated paperboard starts with a corrugator, which is a large machine, often as big as a football field. Three large rolls of paper are necessary. One forms the corrugated medium, and the other two form the liner boards on either side of the medium. The top board is normally called the single-face liner board , and the bottom layer often called a double-face liner board .
Singlewall Corrugator To make the board, the corrugated medium is softened with steam in a preconditioner and then formed into flutes by pressing it in between meshed rolls which fit together like huge gears. As the web emerges from the forming operation, it is held close to the corrugating roll while glue is placed on the tips of the flute. The glue is normally starch-based and is applied to the tips of the flutes which are then pressed against the liner board, also been pre-heated. The adhesive sets up quickly following which the single-faced material is moved up onto a bridge where the glue finishes the setting operation. In the next section of the machine, known as the double-backer or double-facer, a second layer of liner board is added. This layer is also pre-heated and adhesive is applied to the outside of the flutes just as before. Again, the glue set up is rapid. However, in order to cure the adhesive, more heat is used. This is followed by an unheated section which allows the board to cool. The completed corrugated structure is then slit or cut into sheets as required for the individual application. For double-wall or triple-wall board, the double backing process is repeated, normally in line with the original single back operation. Most corrugators have the ability to add double-wall capability to their operation by extending the length of the corrugator.
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Approximately 70% of all corrugated paperboard made in the U.S. is C-flute. C-flute board measures approximately 9/64” and has 39+ 3 flutes per linear foot. Normal combination of board is 42 lb. liners with a 26 lb. medium.
200 lb. Singlewall B-flute is the next most common board utilization which is 3/32” thick with 47+ 3 flutes per linear foot. The most common double-wall board is a combination B-flute and C-flute with 69 lb. liners on each extreme with a 42 lb. liner in between and 26 lb. mediums. This is generally referred to as a 275 lb. BC double-wall corrugated paperboard.
275 lb. Doublewall The manufacturer of corrugated boxes from paperboard is heavily influenced by regulations such as Rule 222 of the National Motor Freight Classification and Rule 41 of the Uniform Freight Classification. Both of these regulations combine to call out minimum construction and/or edge crush performance testing of single-wall, double-wall, triple-wall, and solid fiber boxes used in interstate commerce. Refer to Appendix X for more details.
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VII. PROPERTIES OF CORRUGATED PAPERBOARD AND TEST PROCEDURES The following is a partial list of properties important to corrugated containers and the test procedures used to check those properties. Sampling and Conditioning – As with paperboard, it is important that the sampling and conditioning of the test specimens be done according to recognized standards and procedures. Reference is made to TAPPI T400 and T402 for sampling and conditioning. Box Compression Test – One of the primary functions of a shipping container is to resist stacking loads during warehousing and/or shipping. In essence, this test describes how to determine the resistance of the fiberboard shipping container to these compressive forces. This is accomplished by placing the container between two flat platens, one of which is mechanically or hydraulically driven to compress the container. A recording device is incorporated to indicate the force and deformation required to compress the container. It is important that the container be first conditioned according to TAPPI T402. The hydroscopic nature of corrugated paperboard lends significant variation to test results based on testing and other than standard conditions. To conduct the test, the container is placed between the platens of the test machine and compressed at a rate of .5 in/min. The maximum load and deflection at failure is recorded. The container must be properly set up in the test machine in order to obtain meaningful results. This is generally accomplished by prebending of the flaps 90 ° outward and then 270 ° inward to the required positions. Some test procedures require that the flaps be glued in place. However, this is normally neglected for most sampling runs. the ASTM D642 procedure also allows for a swivel or pivoting platen on top of the box in order that a uniform load be applied. The TAPPI test procedure does not allow for this option. The test procedure also requires that a preload depending on the box structure be applied prior to actually starting the test. This preload consists of a 50 lb. load for single-wall boxes, 200 lb. load for double-wall, and 500 lb. for triplewall. A minimum of five containers is required and the testing can be conducted in any axis of the box depending on where the compressive force is applied. For example, testing a box horizontally may be very useful for determining its ability to resist forces associated with clamp truck handling. Refer to TAPPI T804 or ASTM D642 for more details. 32
Burst Strength – Corrugated paperboard is subjected to burst testing just as paperboard is. The significance of the burst test of combined board is primarily an indication of the character of the materials used in the manufacture of the box. On the other hand, this test gives no direct information regarding the ultimate performance of a design or construction of a finished container. In other words, this test correlates very poorly with actual performance of the container in shipment. Bursting strength of combined board is a requirement of various carrier regulations and federal specifications for shipping containers. The bursting strength of the paperboard is of critical importance as a control test in the paperboard mill since the conformity of the finished container is generally controlled by burst strength. Triple-wall corrugated board cannot be tested suitably by the burst test method. Testing of double-wall is of questionable accuracy since it is rarely possible to get sufficiently simultaneous burst of the multiple facings. The test is simple and easy to execute but must be recognized that it is subject to serious errors if instrument, diaphragm, and gauges are not properly maintained or if improper procedures are used.
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The test uses the same Mullen burst apparatus as described in TAPPI T403 burst strength of paper. Typically a minimum of 20 bursts are required, 10 from each side of a corrugated sheet. The carrier regulations such as Rule 222 for National Motor Freight and Rule 41 for Rail Carriers normally requires that no value of the burst test be below the minimum. This information is stamped in the Box Maker Certificate which is a requirement of the carrier regulations.
Box Maker Certificates It is important that the testing be conducted at standard temperature and humidity conditions and that the box be preconditioned according to the required TAPPI T402 procedures. Refer to the test method for more details.
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Edgewise Compression Strength (Short Column Test) – This test describes the procedure for determining the edgewise compression strength parallel to the flutes of a short column of single, double, or triple-wall corrugated fiberboard. Research has shown that the edgewise compression strength of specimens with flutes and vertical columns relate directly to the top to bottom compressive strength of a corrugated shipping container. The method may be used for comparing the edgewise compression strength of different lots of similar board or for comparing different material combinations. More recently, the procedure has been utilized as a defining characteristic of paperboard material that complies with carrier regulations. To conduct this test, a sample board is cut the size of which is dependant on the nature of the corrugated board. For C-flute board, a specimen 1” tall by 2” wide is utilized. The edges of the specimen are dipped in wax in order to increase their strength and allow for failure in the center of the specimen. Alternately, a neck down sample may be used assuming that the proper cutting instrument has been employed in cutting the sample. In either case, the short column is crushed vertically at a rate of .5 in/min. The maximum load sustained by the board per unit length of specimen is referred to as the edge crush value or ECT number for the corrugated board.
Refer to TAPPI T811 for more information on the test procedures.
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Pin Adhesion of Corrugated Board – This method is used to determine the force required to separate corrugated board between the flute tips of the corrugated medium and its liner board facings. Good adhesion of the corrugated medium to its facings is important in obtaining high quality corrugated boxes. The pin adhesion test provides a means of determining the nature and strength of the bond formed during the combining operation and may be used to detect some manufacturing defects such as poor adhesive penetration, spotty adhesion, and container board with low internal bond strength. The test will measure the force required to separate selectively either the inside or outside facings of corrugated board. The test procedure utilizes a pin adhesion device that when inserted into a given size corrugated board will force the upper or lower facing to be removed from the corrugated medium. The force necessary for this removal is referred to as the pin adhesion value. As with all other paper tests, it is important that sampling and conditioning be done according to the required test procedures. Reference is made to TAPPI T400 and T402 for more information on sampling and conditioning.
Pin Adhesion Fixture
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Flat Crush Test – The flat crush test is a measure of the resistance of the flutes in corrugated board to crushing applied perpendicular to the surface of the board under prescribed conditions. The test is satisfactory for single faced or single-wall corrugated board but should not be used for double-wall or triplewall board. In this method, the specimen rests on the rigid support and is tested at a constant strain rate. The flat crush test is a measure of the flute rigidity of the corrugated board. A high flat crush value indicates a good combination of flute formation and at least adequate medium strength. A low flat crush indicates a number of faulty conditions. Among these are low strength medium, leaning flutes, and/or crushed flutes. Care must be taken when measuring flat crush resistance of finished containers to ensure that the measure of resistance adequately simulates the reaction to crushing forces observed during the container’s actual use. This method generally does not simulate these conditions. Therefore, the correlation may be low between flat crush and the ability of the box to perform in the field. This test is conducted by selecting samples and properly condition them according to the required procedures (TAPPI T400 and T402). Generally speaking, a 5” circular sample is used. The specimen is placed between the rigid platens of a compression test machine and compressed at a constant rate of .5 in/min. A minimum of 10 specimens should be used to conduct the test. The end result of the test is a flat crush value in lb./square inch. Refer to TAPPI T825 for more information on the test procedure.
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VIII. COMPRESSION STRENGTH OF CORRUGATED BOXES The ability to withstand and hold a load in a static or dynamic environment is perhaps the single most highly sought after property in corrugated paperboard boxes. It is this property that most readily distinguishes a corrugated box from other structures such as a paper bag or sack. As we have already observed, the structure of paperboard itself and corrugated paperboard in particular lends itself well to the job of stacking. When combined in the proper portions and with proper care, corrugated paperboard will hold many times its own weight in stacking for long periods of time. There are, however, many factors involved in the distribution of products in boxes that will adversely affect the ability of a box to withstand top load. These will be investigated along with the procedure necessary to judge top load compression strength, either as a function of the corrugated board itself or the box structure. Testing A corrugated box is normally tested for top load compression strength using standards such as TAPPI T811, ASTM D642, or ASTM D4577. The TAPPI procedure and ASTM D642 are essentially the same document. The D4577 test procedure is intended for longer term storage applications where a dead load is applied to the container for a specified period of time. The value obtained from the compression test is referred to as the ultimate yield strength of the box. This is the maximum value that one can hope to obtain during any type of top loading. In actuality, the maximum top load applied to the container can be only a fraction of this amount for good practical results. These issues will be discussed shortly.
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Another reasonably accurate method of determining top load compression strength of the containers is by use of the McKee formula. The formula is as follows:
The formula indicates that with knowing the edge crush test value of the corrugated paperboard, its caliper, and the perimeter inches of the box, one can accurately determine (within 5%) the top load compression strength of a container made from that material. The formula is surprisingly accurate.
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IX. FACTORS THAT NEGATIVELY COMPRESSION STRENGTH
AFFECT
TOP
LOAD
Humidity - As a hydroscopic material, it should come as no surprise that higher humidity levels in the air and therefore in the corrugated paperboard will tend to affect the paperboard properties adversely. What may not be known, however, is that dryer conditions will positively affect corrugated paperboard and top load compression performance. The relationship between these two is not linear. Refer to the following figure for some indication of the relationship between top load compression strength and moisture content of the paperboard.
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Age - If nothing else changes but the number of days a box is under compression, that same box will loose compression strength at an alarming rate. This is primarily due to creep within the cellulose fiber structure of the paper and the corrugated material itself. The rate of decay of the strength properties is somewhat dependent on the temperature of the aging process, although the relationship is not direct. What is known, however, is that a corrugated box subjected to nothing other than a top load compression will loose about 50% of its stacking strength over a six month period.
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Misalignment - Corrugated containers stacked one on top of the other will have a certain compression strength as a stacked column. Misalignment of that column can result in as much as 29% decrease in the overall compression strength of the stack.
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Column Stack versus Interlock - If nothing else changes other than the pallet pattern and, therefore, the stacking pattern of the boxes, as much as 45% decrease in compression strength can be anticipated.
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Pallet Overhang - If boxes are stacked on a pallet with as little as 1” overhang, up to 32% of the compression strength of the containers can be lost.
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Rough Handling - Mishandling including dented vertical edges on the containers can result in as much as 50% decrease in the box compression strength when compared to the virgin container. Recycling - Testing has shown that boxes made from 100% recycled fiberboard may be as strong in compression as virgin Kraft boxes. Much depends on the source of the recycled fiber and since raw material sources and pulping processes vary widely, so does the quality of the finished board. Recycling tends to shorten the length of fibers affecting properties of fiberboard such as tear strength and burst strength. This, in turn, will affect performance of the finished box by reducing its rough handling capabilities. Recycling processes which control fiber length properly can mitigate strength losses to some degree. Note that Kraft liner board which averages about 12% recycled fiber, can still be defined as virgin Kraft with up to 25% recycled content. It is also worth noting that most corrugated mediums contain 50% or more recycled fiber. One study recently concluded that about 25% of all boxes contain 100% recycled content medium. This trend is likely to continue. The corrugated industry currently recycles approximately 50% of the corrugated boxes it produces. The goal in the next 15 years is to increase that number to 80% total recycled boxes. High Performance Boards – With the advent of more recycled content in corrugated, many paperboard manufacturers have attempted to develop stiffer paperboards, that is boards with higher short column strength. The direction was enhanced when the fiber box industry adopted the ECT standard for corrugated as a substitute for the Mullen burst test. Since that time, paper manufacturers have developed a variety of new materials which produce higher ring crush values in liner boards and mediums. Note that there is a direct relation between ring cut crush results of paperboard and the ECT values in the combined board product. The new container boards generally fall into one of three categories. 1.
High Performance – These are densified sheets made by high pressure forming typically in the calendaring and finishing stack operations on the paper machine.
2.
X Linked – These are processes whereby fibers are cross linked in the papermaking operation in order to align the fibers better for a stronger finished box structure. This is sometimes done with an oscillating spray head box that alternates the direction of flow of the furnish onto the Fordrinier belt at the wet end of the paper machine. Other procedures are also used.
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3.
Additives – These are generally chemical compounds that enhance fiber bonding. This area is often approached very cautiously due to the chemical nature of the additives and the restrictions on most chemicals from the Environmental Protection Agency. It is also true that these chemicals can have a great effect, often negative, on the recycleabiltiy or corrugated containers.
At the box plant, chemical additives are sometimes applied to the corrugator and stiffer liner boards can be achieved by laminating two or more layers together. This requires running the corrugator at slower speeds which significantly increases cost but does result in a stronger box board structure. The bottom line is that high performance boards are generally a manipulation of some property of the paper or paperboard at some point in the manufacturing process, normally increasing one value at the detriment of another. Thus, short column crush values may be increased at the expense of burst or tear resistance.
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