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726 / Grades

Glassman A (1985) Fundamentals. Atlanta, GA: fiber is important, the greatest factor affecting Technical Association of the Print and Industry. bending stiffness is sheet thickness or caliper. Rela- Lebtinen E (ed.) (2000) Papermaking Science and Technol- tively small changes in thickness will result in large ogy: Pigment and Surface of Paper: changes in bending stiffness since the effect of sheet Helsinki: Fapet Oy. thickness on stiffness is cubic rather than linear. Pulp Oittinen P and Saarelma H (eds) (1998) Papermaking selection for greater fiber stiffness leads papermakers Science and Technology: Printing: Helsinki: Fapet Oy. to choose naturally thicker fibers such as softwood or Parker JD (1972) Sheet-Forming Process. Atlanta, GA: Technical Association of the . hardwoods with noncollapsible, round cross-sections. Paulapuro H (ed.) (1998) Papermaking Science and High yield pulping processes such as thermomecha- Technology: Paper and Board Grades: Helsinki: Fapet Oy. nical pulp (TMP) or semichemical that produce pulp Scott WE and Abbott JC (1995) Properties of Paper: An with high percentages of are also desirable for Introduction, 2nd edn. Atlanta, GA: Technical Associa- paperboard because of their inherently stiffer fibers. tion of the Pulp and Paper Industry. High yield pulps are very desirable for the manufac- Smook GA (1992) Handbook for Pulp and Paper ture of paperboard because the fiber is not only stiff Technologists, 2nd edn. Vancouver, Canada: Angus but produces bulky sheets that enhance bending Wilde. stiffness through both fiber stiffness and sheet thick- ness. For these two reasons, recycled fiber is also used to a large degree in paperboard manufacture. Paperboard Grades Manufacturing Multilayer Paperboard G J F Ring, University of Wisconsin–Stevens Point, Stevens Point, WI, USA The same basic paper machines used to produce writing and printing paper are also used to form & 2004, Elsevier Ltd. All Rights Reserved. paperboard. However, modern paper machines are limited in their ability to produce a single-layer paper À sheet with a above 150 g m 2. There are a Introduction number of reasons for this limitation. Primarily, thicker single-layer sheets are more difficult to Paperboard is a heavyweight paper that has a dewater requiring excessive reductions in machine grammage or basis weight generally greater than speed. Furthermore, the increased drainage forces À 150 g m 2 and is thicker than 300 mm. The addi- applied to thicker sheets in the forming section tional thickness results in a stiffer sheet that is ideal would cause greater fines removal from the bottom for and providing protection for of the sheet resulting in a rougher surface. The saleable products during transportation, handling, topside of a very thick sheet would also be adversely and storage. Paperboard grades include - affected since paper is formed on fourdrinier boards, containerboards, and specialty boards, and machines layer by layer from the wire side up, which are used in either food or non-food applications. The would allow extra time for the fibers in the top layer English units for paperboard are given in pounds per to flock and produce a ‘hill and valley’ appearance. 1000 square feet (lb/1000 ft2) and the thickness is The combination of these two effects would produce measured in points or mils (thousands of an inch). an unacceptably two-sided product. A is a rectangular with or without a Manufacturing multilayered paperboard from top designed to transport and store a variety of goods separately formed sheets provides a solution to the and products in either liquid or solid form. A box above-mentioned problems. The forming section of must be strong enough to hold its contents not only paperboard machines are composed of two, three, or during transportation and storage but also during the even four forming sections that bring individual packaging operation. Because boxes are frequently sheets together at the wet press. Paperboard stored in stacks, compression resistance in their machines are for this reason large and complex vertical sides is required to avoid bulging and having heights that are two to three times greater cracking that would result in loss or damage to the than single-former machines. Any one of the former product contained within. sections in a multilayer machine can be either a Boxes manufactured from paperboard are made to traditional fourdrinier or a modified fourdrinier resist compressive forces by using stiff pulp fibers equipped with a top-wire unit for additional dewa- formed into thick sheets. Paperboard meeting these tering capacity. The use of different furnishes in each two requirements will have high levels of bending former produces a final sheet that is engineered for stiffness. Although the stiffness requirement for the specific stiffness and smoothness requirements. PAPERMAKING / Paperboard Grades 727

Although initially forming two to three separately (FBB), white lined chipboard (WLC), solid bleached formed sheets of paper, a multilayer machine forms a sulfate board (SBS), solid unbleached sulfate board single sheet of paperboard when the individual sheets (SUS), and (LPB) are major of paper are combined together in the wet press. The types of cartonboard grades. Many of these grades individual single-layered sheets prior to the wet press will be pigment coated to produce a highly printable are ‘vacuum dewatered’ with a typical consistency of top surface. These need to be strong and 20% (80% moisture) and are simply assemblages of stiff to form the walls of a carton. Because fibers held together by capillary forces exerted by the are stacked during storage, compression resistance is continuous matrix of water surrounding the fibers. also important. In particular, cross-machine direction When the sheet continues it progress through the wet (CD) stiffness is required to prevent carton bulging press and the dryers, this continuous matrix of water since the carton is manufactured with the CD parallel is decreased and the fibers are progressively drawn to its height. High-speed converting and mechanized together through surface tension. Eventually, at the packaging lines mandate excellent runnability; there- end of the drying process with a final moisture fore, resistance to curl is also a necessity. Finally, content of 4–8%, the surface tension forces between folding these paperboards into containers must occur individual fibers will produce pressures sufficiently without cracking, which requires not only good high enough to form fiber-to-fiber hydrogen bonds strength, but also good stretch. The ream area for resulting in a mechanically strong sheet. During paperboard grades is 1000 ft2. multilayer forming, a single sheet of paperboard is formed from the individual sheets of paper by merging the water matrices of each sheet into a Folded Boxboard single, hydraulically connected matrix in the wet grades are used to make packages press. The net result is that the multilayer sheet for numerous products including: dry foods, liquid continues through the wet press and dryer section foods, frozen foods, confectionaries, cosmetics, forming fiber-to-fiber bonds inside layers and be- detergents, and numerous other items. The gram- tween layers as if they were initially formed together. À mage range for folding boxboard is 160–450 g m 2. Theoretically, the fiber-to-fiber bonding between Folding boxboard is a multilayer sheet having three separately formed layers will be identical to fiber- to four layers. The top and bottom layers are to-fiber bonding within a single layer. Differences in frequently made of bleached chemical pulp. The interlayer bonding strength (measured by z-direction middle layers consist of either mechanical pulp or strength tests) will be found when the individual recycled fiber for stiffness enhancing bulk. The top sheets are wet pressed at moisture contents lower layer is usually surface sized for good picking than what is necessary to form a hydraulically resistance in printing. Because the board must have connected matrix. (z-direction strength is the max- a smooth top surface for good printability, yet have imum tensile force per unit area which a paper or adequate thickness for good bending stiffness, paperboard can withstand when applied perpendi- smoothness is obtained through a combination of cularly to the plane of the test sample.) limited calendaring and coating. It is important for The advantage of manufacturing a multilayer sheet folding boxboard grades to also have good folding is that key paper properties can be engineered into and scoring properties. the paperboard that would not be obtainable by single-layer forming. Special top layers can be incorporated that are white and smooth, therefore, White Lined Chipboard having excellent printing properties. Middle layers can be used that are bulky and thus inherently White lined chipboard (grammage range of 200– À thicker producing the stiffest possible board. These 450 g m 2) uses recycled fiber in its middle layers, middle layers can also contain recycle fibers or pulp and has many similar applications as folding box- fibers of lower quality that can be covered or masked board except for certain applications by higher quality top and/or bottom layers. where contaminants in the recycled pulp might spoil or taint the food. WLC grades may sometimes have Cartonboard Grades four sheet layers. The fourth layer is actually an additional top layer used to maximize use of less Cartonboard produces consumer product packaging bright recycled pulps, therefore minimizing the use of for food and retail items. The market for these grades expensive highly bleached top layers. The top layer is is large and product development is great due to frequently coated to increase printability and an dynamic marketplace forces. Folding boxboard optional bottom coating may be found. 728 PAPERMAKING / Paperboard Grades

Solid Bleached Sulfate Board embossing. Because cartonboard boxes are often an important part of the merchandizing of the contained Solid bleached sulfate board is very similar to folding product, rotogravure printing is necessary to provide boxboard and white lined chipboard except that it is the highest possible printed quality. Where self- a single layer sheet (hence the term ‘solid’) made of advertising on store shelves is not critical, offset or virgin bleached kraft pulp. Hardwood is the pre- flexography printing is used instead, or the box may dominant furnish but softwood is also used. Con- tamination and tainting of food and cigarette products be unprinted. Printed boxes, unlike magazines and books, is the primary reason for using virgin kraft pulp. SBS experience considerable abrasion during their use as board may be coated on one or both sides with the a product package, which requires surface protection topside receiving as many as three layers of coating. through either varnishing or film . Solid Unbleached Sulfate Board Varnishing is accomplished through coating during the printing process. The varnishes of choice are Solid unbleached sulfate board is a heavier multilayer usually aqueous dispersions that are cured on-line o À 2 board (grammage 500 g m ) that uses unbleach- with ultraviolet light. Varnishing provides a gloss to ed pulps where extra board strength is required. For the surface of the box further enhancing its attrac- this reason, SUS boards are used in beverage and tiveness. When a more durable or glossier surface is other packages intended specifically for carrying required, off-line film lamination is also used. consumer items. Because SUS is not intended to Quality printing requires cartonboard grades to come directly in contact with food, recycled pulps have a relatively high brightness from either the pulp are frequently used in the middle layers. However, or an applied surface coating. Printing also requires the high strength requirements (which includes tear high smoothness, which is obtained from coating the strength) necessitate using kraft pulps in both the top sheet and using hardwood furnishes. and bottom layers. Sheet strength and surface Cartonboard boxes are specifically designed for smoothness are augmented through the application each product and have no standard sizes. Products of surface size. Packaging applications for SUS such as cigarettes, camera film and software are sold in require printing on the topside, which is accom- boxes that have unique sizes and designs, which may plished by coating the topside two or three times to include intricate product-inspection windows or cut- prevent ‘show through’ of the darker unbleached outs. These potentially complex designs are produced furnish. Where it is important to have high quality by die-cutting a cartonboard sheet in either a flat-bed printing, high-whiteness pigments such as titanium or a rotary press that has cutting rules in the shape of dioxide (TiO2) are used. the unfolded box mounted to a cutting die, which is then applied through the sheet against a counter die. Liquid Packaging Board The cutting rules or knives that are attached to the Liquid packaging board is a multilayer board having cutting die have compressible rubber gaskets on either a -coated liquid barrier in conjunction side. These rubber gaskets hold the sheet motionless to with single or multiple layers of virgin chemical pulp. produce an accurate cut and then expand to cleanly , juice containers, microwave, and ovenable eject the sheet after the cutting compression stroke has trays are typical uses for this grade. Polyethylene taken place. Any cutouts are prevented from accumu- provides not only a liquid barrier, but also the ability lating in the cutting die area by keeping them to heat seal during container manufacture. The connected to the sheet through small ‘nicks’ or polyethylene is coated on only one side when a perforations that are produced with notched cutting moisture vapor barrier is required for long shelf-life, rules. Cutouts are then removed in a stripping step that but on both sides for liquid packaging such as milk. breaks the nicks and collects the waste material. Aseptic packaging uses liquid packaging boards that Die-cutting requires cartonboard sheets to have have high cleanliness and purity standards. good strength properties to produce a clean cut. Poor fiber-to-fiber bonding, a function of low sheet strength, will result in fiber linting or dusting. Low- Manufacturing Cartonboard Boxes strength sheets are also more subject to tearing after Cartonboard sheets or reels are converted into boxes cutting and exhibit poor runnability on converting through a process that involves cutting and creasing machines. Moisture content is also important for and then gluing the box into the final carton. converting machine runnability. Excessively dry Additional steps are frequently taken to produce a sheets are harder and will wear out the die-cutting visually attractive box that may include printing, rules sooner. Drier sheets will also produce more varnishing, film laminating, foil blocking, and cutter lint or dust. PAPERMAKING / Paperboard Grades 729

Cartonboard sheets must also be creased to form Typically, compression testing such as the edge crush, folding boxes. Creasing is a step that occurs simulta- flat crush, ring crush, and short-span compression is neously with die-cutting on flat-bed machines, but useful in predicting linerboard performance in the after cutting on rotary machines. A folding crease is final product. Burst testing, which formed with a creasing rule that has a rounded edge measures tensile strength in an empirical manner, has rather than a cutting edge. Creasing rules are attached also been used to assess linerboard strength, but is to the cutting die together with the cutting rules; now considered inferior to compression testing. Dry however, the creasing rules work in conjunction with a strength agents such as may be applied at a creasing channel located directly opposite them on the sizing press to increase linerboard’s surface strength, counter die. A properly working creasing rule will and sizing may be applied in the headbox to form a sufficiently deep and narrow crease that will enhance water resistance and . Printable allow the cartonboard sheet to precisely fold where top plies are made from bleached chemical pulps intended and produce a sharp attractive box corner. (either hardwood or softwood) that are sometimes The creasing rule will be pressed against the side of the pigment coated for more demanding printing re- sheet that is intended to form the inside of the box. quirements. The grammage range for linerboard is À Cartonboard grades that exhibit good creasing 125–350 g m 2. Linerboard made from at least 85% properties have good tensile strength and elasticity in virgin kraft pulp is called ‘kraft linerboard.’ Liner- order to survive the out-of-plane deformation during board meeting the requirements of Rule 41 of the creasing. When a box is folded along its crease, the Consolidated Freight classification in the US is called inside portion of the crease will undergo compression ‘test linerboard,’ and meets either a minimum while the outside of the crease will undergo tension. bursting strength or a minimum edge crush strength. Cracking will occur when the elastic limit is reached Because kraft linerboard easily exceeds either mini- and the sheet fails in either tension or compression. mum strength criteria, the term ‘test linerboard’ is In general, the compression limit is reached before usually applied to linerboard containing predomi- the tension limit. Cartonboard sheets that resist nantly recycled pulp. Modern linerboard machines cracking tend to have multilayered construction that are multi-ply machines using twin-wire/gap-forming delaminate around the crease with the inside layers technology. Many new machines are being outfitted buckling rather than undergoing compression failure. with the latest in shoe or extended nip wet presses. Corrugating medium, used as the fluting between two linerboard layers, is a single-layer uncalendered Corrugated Containerboard Grades À sheet with a grammage range of 112–180 g m 2. Corrugated containerboard boxes are lightweight Compression resistance rather than tensile strength is structures that can transport significant internal weight particularly important for corrugated medium, be- and withstand buckling and collapsing when multiple cause, as a composite structural material, corrugated container units are stacked on top of each other. containerboard boxes will fail at the box corners in an Corrugated containerboard boxes are manufactured identical compression mode when either excessive by gluing a fluted corrugated medium between two external loads or excessive internal loads are present. strong outer linerboard faces. In this manner, max- In general, failure occurs in either condition because imum thickness is obtained with minimum weight to the containerboard is bending outwards and the ensure high bending stiffness. Corrugated container- linerboard on the inside of the bend is compressed board may also have two or even three corrugated to failure. The primary role of the corrugating layers sandwiched between alternating layers of liner- medium in preventing containerboard failure is to board when extra strength and rigidity is required. provide adequate separation of the linerboard faces Linerboard’s primary characteristic is high bending such that the degree of bending is minimized during stiffness and is usually produced with two or more loading. For this reason, the portion of the flute that plies from high-quality kraft or recycled pulp. To spans from one linerboard face to the other must be maximize bending stiffness, pulp fibers that will stiff enough to resist compressive collapse. This produce the highest tensile strength are placed in the particular property can be obtained in relatively outer or top layer that comprises about 30% of the low-cost, minimally refined, semichemical pulps total sheet. For example, the top layer will have the where the fibers are naturally stiff due to the presence highest percentage of virgin fiber while the lower of lignin. Recycled pulps also having characteristically base layer will have a higher percentage of recycled stiff fibers can be successfully used in this grade. These pulp. As another example, the top layer may be a pulps produce sheets with bad formation and low kraft pulp cooked to a lower degree of lignin content tensile strengths. Handsheets made from these pulps À and refined to a higher degree than the bottom layer. are specified to be formed at 150 g m 2 rather than 730 PAPERMAKING / Tissue Grades

À 60 g m 2 to compensate for their lack of wet handling scorers before being cut into the final board sheet. strength. Corrugating medium is also produced with a Converting machines capable of producing double high machine-direction fiber alignment to enhance and triple wall container board operate in a similar compression resistance in the direction of fluting. manner to that just described with the exception that flutes on the corrugation side of a single face board Manufacturing Corrugated can be glued to the linerboard side of a second single face board to form the final multilayer construction. Containerboard Final boxes are manufactured using similar die- Producing corrugated containerboard requires con- cutting, creasing, and printing techniques as de- verting machinery that combines separate rolls of scribed above for boxboard. linerboard and medium into a glued structure. The simplest structure is one sheet of linerboard glued to See also: Packaging, and Printing: Packa- single sheet of corrugated medium and is known as ging Grades; Science and Technology; single faced corrugated. Gluing an additional sheet of Printing. Papermaking: Paper Raw Materials and Tech- linerboard to the corrugated side of single face nology; World Paper Industry Overview. Pulping: Bleach- ing of Pulp; Physical Properties. produces a single wall corrugated board. Gluing either two or three single faced corrugations together with a final linerboard face produces a double wall or Further Reading triple wall corrugated board respectively. During the production of single face corrugated, Attwood B and Morre G (1995) An Introduction to the linerboard is unrolled and preheated to a tempera- Theory and Practice of Multiply Forming. Leatherhead, ture of 160–1901C in a unit known as a ‘single facer.’ UK: Pira International. In this unit, medium is also unrolled and preheated, Biermann CJ (1993) Essentials of Pulping and Papermak- ing. New York: Academic Press. but is passed over a corrugating roll to form the flutes Cakebread D (ed.) (1993) Paper-Based Packaging. Leather- or corrugations. During this operation, the medium head, UK: Pira International. must be held securely by either pressure or vacuum in Hunter D (1947) Papermaking: The History and Techni- the fluting roll to maintain the shape of the flutes. que of an Ancient Craft. New York: Knopf. Glue, generally starch, is applied to one side of the ISO (1998) ISO Standards Handbook: Paper, Board and flute tips, and then pressed firmly to the underside of Pulps. Geneva: International Organization for Standar- the linerboard instantaneously bonding them to- dization. gether. The bond line formed by the glued tips is Kouris M (ed.) (1990) Pulp and Paper Manufacture: observable through the linerboard, and is preferably Coating, Converting, and Specialty Processes. Atlanta, used as the inside surface of corrugated boxes. Single GA: Joint Textbook Committee of the Paper Industry. face is flexible, bending easily at the glue lines. As a Paulapuro H (ed.) (1998) Papermaking Science and Technology: Paper and Board Grades. Helsinki: Fapet Oy. material by itself, single face is used as a packing Paulapuro H (ed.) (2000) Papermaking Science and material due to its ability to wrap around items such Technology: Papermaking, Part 1, Stock Preparation as providing them with excellent crush and Wet End. Helsinki: Fapet Oy. protection during shipping. Savolainen A (ed.) (1998) Papermaking Science and Single wall corrugated boards are produced in a Technology: Paper and Paperboard Converting. Helsin- converting machine where the single facer unit is ki: Fapet Oy. coupled with a ‘double facer’ or ‘double backer’ unit. Smook GA (1992) Handbook for Pulp and Paper In this unit, the single face material is again Technologists. Vancouver, BC: Angus Wilde. preheated and glued to the remaining unbonded flute tips. The prepared single face is then pressed to another preheated linerboard sheet, which produces Tissue Grades the rigid containerboard. Less pressure is applied in P Parker, Western Michigan University, Kalamazoo, the double facer than in the single facer to avoid MI, USA crushing the corrugations. However, because of the reduced pressure the linerboard glued in this manner & 2004, Elsevier Ltd. All Rights Reserved. does not have the same observable glue line as the single face material, and is more suitable for use as the printable outside surface of a containerboard Introduction box. Once the single wall board is produced, further heating to set the glue must take place over hot plates ‘Tissue’ is a generic name for a variety of light-weight in order to convey the board flat through slitters and paper products. In normal use, the word refers to the