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Packaging: Corrugated Paperboard
Packaging: Corrugated Paperboard$ SEM Selke, Michigan State University, East Lansing, MI, USA r 2016 Elsevier Inc. All rights reserved.
1 2 3 4 5 5.1 5.2 5.3 5.4 6 7 8 9 Reference Further Reading
1
Introduction Production of Paperboard Paperboard Grades Main Properties of Paperboard Corrugated Fiberboard Flute Sizes Stacking and Compression Environmental Factors and Stacking Strength Carrier Requirements for Corrugated Boxes Fiberboard Containers Folding Cartons Environmental Considerations Summary
1 1 2 2 3 4 4 4 5 5 5 5 6 6 6
Introduction
Paperboard, a very popular packaging material based on wood fibers, is available in many grades, based on the type of wood fiber and the fabrication process. Virgin fibers are obtained directly from trees, either softwood or hardwood trees; recycled fibers are obtained from recycled paper and paperboard. Manufacture of paperboard requires similar processes to paper manufacture: pulping, optional bleaching, refining, sheet forming, drying, calendering, and winding. Paperboard can be fabricated on different machines, with the fourdrinier and cylinder machines being the most common. Paperboard can be classified in different ways: virgin or recyclable, cylinder or fourdrinier-produced, and single-ply (known as ‘solid’ paperboard) or multi-ply. Often a board grade involves more than one criterion and depends on its intended application or use. Important properties of paperboard include: basis weight, thickness (or caliper), density, bulk, stiffness, bending, ply bonding, and glueability. These properties are interrelated to various degrees and are a function of the ratio of virgin to recycled fibers, as well as of production conditions. As with paper, paperboard properties are highly affected by humidity, and should generally be evaluated at the standard conditions of 5072% RH and 23.071.0 1C. Paperboard packaging applications cover a wide variety of physical forms and shapes, including folding cartons, trays, blister backings, set-up boxes, solid-fiber boxes, convoluted and spiral-wound cans, and fiber drums. By far the largest application of paperboard is the fabrication of corrugated fiberboard, which is a structured composite material combining linerboard, corrugating medium, and adhesive (usually starch-based). Corrugated fiberboard is mainly used for fabricating shipping containers, but some is used for cushioning, mechanical protection, and pallet construction. Paperboard containers are employed in almost every industry around the world, including pharmaceuticals, cosmetics, chemicals, food, bakery, tobacco, soap, textiles, hardware, toys, office products, sporting goods, electronics, and beverages. The importance of paperboard in packaging is clearly indicated by its uses as primary, secondary, and shipping (tertiary) containers. Figure 1 shows the approximate proportions of corrugated fiberboard boxes and other paper-based packaging materials produced in the USA.
2
Production of Paperboard
Solid paperboard uses predominantly virgin kraft fibers and is mainly produced by fourdrinier machines similarly to single-ply paper. In contrast, there are several methods for producing multi-ply board using recycled and/or virgin fibers. Multifourdrinier machines have secondary head boxes that apply a second ply after the first is formed, and are used for producing linerboard. Rotary formers consist of a felt roll passing by a series of vats, with pulp suspension added to the roll at each ☆
Change History: March 2015. S. Selke added abstract and updated Figure 1, Section 5.2, References, and Further Reading.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02196-2
1
2
Packaging: Corrugated Paperboard
Other paperboard packaging 0%
Bags & sacks 3%
Other paper packaging 4%
Folding cartons 14%
Gable top & asepc cartons 1%
Corrugated 78%
Figure 1 Relative proportions of paper and paperboard packaging materials in the USA (U.S. EPA 2014).
vat. Rotary formers can be either cylinder-mold machines (with two basic types, uniflow and contraflow), or roll formers (cylindermolds with a flow box to deliver the fibers). Cylinder machines are very common throughout the world. Inverform twin-wire machines form the web paper between two forming rotating screens. The more recently developed multiformer machines combine features of both fourdrinier and cylinder machines. Historically, cylinder machines have used mostly recycled (or secondary) fibers to produce paperboard, while fourdrinier machines employ mostly virgin fibers. However, modern cylinder and fourdrinier machines can process both virgin and secondary fibers. Fourdrinier and cylinder machines use different approaches to produce a thick web. In the fourdrinier machine, fibers come solely from the head-box on the top wire; in the cylinder machine, paperboard is produced by adding several layers of stock from different vats. After forming the wet web, it is pressed, compacted, dried, and wound. Other operations include sizing and calendering. In general, fourdrinier machines are good for producing a web of the same type of fiber throughout, while cylinder machines are good for producing a multilayer web. An important quality issue is the higher fiber orientation produced by the cylinder machine compared to the fourdrinier. For example, at equal thickness boards made on cylinder machines have similar machine direction (MD) stiffness compared to fourdrinier boards, but they have less stiffness than fourdrinier in the cross-machine direction (CD). Fourdrinier boards are more balanced (or squared) than cylinder boards. The higher fiber orientation in the cylinder machine, however, cannot be entirely attributed to the machine itself. During drying, high tension and stretching due to the relatively thicker multi-ply web also tend to increase fiber orientation.
3
Paperboard Grades
As for paper, the primary source of virgin fibers for paperboard is softwood trees. The strong softwood fibers are separated by chemical pulping and are available in unbleached or bleached grades. While bleached paperboard is often associated with pure and clean fibers for food and cosmetic applications, unbleached sulfate boards are characterized by excellent stiffness, high tearing strength, water resistance, and a more ‘natural’ appearance. The various types of paperboard can be grouped into containerboard, which includes both linerboard and corrugating medium used in the fabrication of corrugated fiberboard; boxboard, used for folding cartons and set-up boxes; and paperboard for other special applications. Table 1 lists the major types of paperboard used in packaging.
4
Main Properties of Paperboard
In addition to basis weight, caliper, bulk, and density, other important characteristic properties of paperboard materials are burst, stiffness, folding, ply bonding, and glueability. Relevant optical properties of paperboard include opacity, whiteness, brightness, gloss, and color. Basis weight, or grammage, an indicator of the paperboard mass per unit area, has a direct relationship to the cost per unit area of a package when the board cost is based on weight. Caliper affects the machineability during carton-making, and influences carton bulging, density, stiffness, and scoring. Bulk, which is thickness divided by basis weight, indicates the volume per unit of package mass (inverse of density), and is an important determinant of stiffness. Stiffness can be measured by the force required to bend a board sample through a 151 angle (Tauber stiffness). Alternatively, stiffness can be reported as the force needed to crush a formed carton. Stiffness is proportional to paperboard caliper. When paperboard is bent, the concave ply is extended and the convex ply is compressed. Therefore, increasing caliper and fiber orientation in both external plies tends to increase stiffness.
Packaging: Corrugated Paperboard
Table 1
3
Paperboard grades and typical uses
Board grade
Common furnish
Typical applications
Liner board Test liner Corrugating medium
Corrugated fiberboard Lower quality corrugated fiberboard Corrugated fiberboard
Solid bleached sulfate (SBS)
2-ply virgin kraft softwood Virgin fiber layer on a recycled fiber base Semi-chemical hardwood pulp, virgin, and recycled fiber Virgin softwood fiber, bleached
Solid unbleached sulfate (SUS)
Virgin softwood fiber, unbleached
Kraft-lined chipboard Chipboard White-lined
Unbleached kraft on a recycled fiber base Recycled fiber Outside liner: bleached virgin pulp. Underliner: mechanical pulp. Middle: recycled. Back: mixture of fibers Various
Can board Lined board Double-lined board Multifoudrinier board (MFB) Food-board Special-food board Frozen-food board Tube board Machine-glazed board
Chipboard lined with virgin fiber Two bleached virgin pulp liners sandwiching a layer of mechanical pulp Bleached kraft and thermomechanical pulp Bleached or unbleached pure virgin softwood pulp, single or multi-ply Plastic-coated food grade board Bleached virgin chemical pulp with moistureresistant additives, claycoated Various, with unsized and smooth finished Various
Strong stiff material. Used for frozen food, milk, and ice cream cartons. Can be plastic-coated Strong and stiff. Heavy-duty boxes, beverage carriers Hardware packaging Generic low-grade packaging Rigid and folding cartons
Composite cans and drums for powders. If coated with plastics can be used for liquids Folding cartons and set-up boxes High-quality board for food and cosmetic products Stiff, strong board for folding cartons, trays, etc. High-quality food packaging Cartons for liquid products such as milk and fruit juices High-quality frozen food applications Spiral or convolute composite tubes One-side glossed board
Good scoring and folding behavior are important in boards for folding cartons. Folds, or bends, are made after the board is scored or creased. The plies will separate locally at the fold, but the board must be able to sustain up to a 1801 bend without splitting or cracking. For this reason, folding boards have liners with a high percentage of long wood fibers (softwood). Ply bonding refers to the force required to separate the paperboard plies, which is determined by the adhesive bonding strength between the fibers of adjacent plies. During paperboard formation, ply bonding is controlled by the ply water content and the chemical similarity between fibers in contacting plies. Low ply bonding yields a poor paper with a tendency to split between plies, at locations other than the scoring lines. The glueability of paperboard is related to the adhesion characteristics of folding cartons. Adhesives must be chemically compatible with the board outer ply, penetrate it, and quickly produce adequate tack. Evaluating glueability requires conducting tests under controlled conditions of temperature and setting time, to determine whether fiber tear occurs rather than adhesive failure. Glueability depends on several factors such as adhesive/surface board compatibility, surface smoothness, board porosity, and surface roughness. Good board printability depends on the surface smoothness, ink receptivity, sheet flatness, dimensional stability, and absence of curling in the presence of water or high humidity.
5
Corrugated Fiberboard
The extraordinary strength/weight ratio of corrugated board is based on the column-arch construction provided by the undulated corrugated medium anchored on both sides to linerboard by starch-based adhesive. When properly dry, corrugated fiberboard is a rigid material that resists bending, flat crushing, and edgewise compression, as well as providing cushioning protection and good thermal insulation. The undulated corrugated medium is known as ‘flute’ and is typically made from short fiber hardwood, using a semi-chemical pulping process, or from recycled paper or a combination. Medium thickness is about 9 point (225 mm), with a standard basis weight of 130 gm2. Some amount of recycled fiber is commonly incorporated into the medium. Corrugated fiberboard is generally formed in 90 m long corrugators, which form the flutes in the corrugating medium board, glue it to the linerboard, dry the structure, and cut the combined board into sheets. The linerboards (or facings) are typically made from unbleached, kraft longfiber softwood pulp, with basis weights between 150 and 250 gm2.
4
Packaging: Corrugated Paperboard
Table 2
Common flute types in corrugated fiberboard
Type of flute
Flutes per meter
Height of flute (mm)
Take up factor (medium:liner length ratio)
A B C F E N
110710 155710 130710 295710 420710 560710
4.7 2.5 3.6 1.6 1.1 1.1
1.54 1.32 1.43 1.27 1.23 1.20
The simplest corrugated construction, beyond the medium alone, consists of one medium and one liner, and is called singlefaced board. This material is fairly flexible, and provides reasonable cushioning but not much compressive strength. It is often used as the primary package for light bulbs, and as a cushioning material for a variety of other fragile products. Single-wall (also called double-faced) fiberboard contains two liners sandwiching the corrugating medium and is the ‘typical’ corrugated board. The addition of a second medium and another liner produces double-wall corrugated fiberboard. Similarly, triple-wall corrugated contains three mediums and four liners. While paperboard (unless it is extremely thick) and single-faced corrugated can be wound in a roll, single-, double-, and triple-wall corrugated must be cut into sections and stored flat.
5.1
Flute Sizes
A variety of flute sizes are available, designated by capital letters. The major ones are described in Table 2. A-flute was produced first, historically. It has the largest wave of the major types. Its primary attribute is excellent compression strength, but it has the lowest flat-crush resistance. The smaller B-flute was introduced to provide better flat-crush resistance and hence printability. C-flute, intermediate in size between A and B, delivers better compression strength than B-flute, and better flat-crush resistance than A-flute. E-flute (for elite), the smallest of these four, was introduced later, primarily to provide a good printing surface, so that corrugated could be used as an attractive retail package for products that required more protection than could be obtained from packages such as folding cartons. Soon after the introduction of E-flute, with its break in the tradition of orderly alphabetical designation of flutes, a large number of flute variations were introduced by a number of corrugators, each with their own performance claims. One can now find J (jumbo), K, S, N, and F flutes, and probably more. The most used flutes at present are C and B, with A and E the next most common, and the others used only in small quantities. In double- or triple-wall boards, flutes of differing sizes are normally used. The combination of B- and C-flutes, in a BC doublewall board, is a popular choice. As a general rule, increasing the height of the flutes increases the stacking strength in line with the flutes. Increasing the number of flutes per linear foot (flute density) increases the strength across the flutes. Small flutes produce superior printing surfaces, while large flutes provide better cushioning.
5.2
Stacking and Compression
By far, the major application of corrugated board is transport packaging. During distribution and storage, packages are commonly subjected to rough handling and stacking compression, the two main distribution hazards besides humidity. Puncture resistance and burst (Mullen) tests are common evaluation measures, and are related to the total combined weight of the corrugated liners. The initial box compression strength (ICS) can be estimated from the McKee equation, originally developed by the Institute of Paper Chemistry in 1963 and later simplified to the formula shown in eqn [1]. It applies only to regular slotted containers (RSC) with a perimeter-to-depth ratio no greater than 7 and length less than 3 times the width. The McKee equation is: ICS ¼ 5:87 ECT
pffiffiffiffiffi tP
½1
where ECT is the edge crush test strength of the corrugated board, t is the board thickness, and P is the load-bearing perimeter of the box. Variability in ICS from calculated values is estimated at about 15%.
5.3
Environmental Factors and Stacking Strength
The box compression strength at the end of the distribution/storage cycle, or final compression strength (FCS), is diminished by several factors including length of storage time, t; relative humidity, RH; stacking patterns, S; palletization conditions; and the intensity of rough handling. FCS can be estimated by: FCS ¼ ðICSÞ Ft FRH FS FO
½2
Packaging: Corrugated Paperboard
5
where Ft is the storage time factor, FRH is the humidity factor, and FS is the stacking pattern factor. FO includes several multiplicative subfactors: pallet deck gap, pallet overhang, and excessive handling. Ft varies from 0.63 for 10 days down to 0.5 for 180 days. FRH varies from 0.9 for 60% RH down to 0.15 for 100% RH. FS for columnar misalignment ranges from 0.9 to 0.4. FO includes subfactors of 0.9 to 0.75 for pallet deck gap, 0.8 to 0.6 for overhang, and 0.9 to 0.6 for excessive handling. Another way to express the negative effects of relative humidity, storage time, and stacking patterns on the compression strength of corrugated boxes is to simply divide the ICS by a safety factor, as described in ASTM D4169 (ASTM International, 2014): 1. Use a safety factor of 8 for extremely severe conditions: frequent high humidity, long-term storage, overhung, and interlocked pallets. 2. Use a safety factor of 4.5 for average conditions: occasional high humidity, medium-term storage, and well-loaded pallets. 3. Use a safety factor of 3 for best conditions: controlled humidity, short-term storage, no pallets, and good stack alignment.
5.4
Carrier Requirements for Corrugated Boxes
In general, packaging itself is not regulated by any federal agency in the U.S. However, articles in the package or the printing outside the package may be regulated. For example, hazardous materials are regulated by the Department of Transportation, and paper and paperboard in direct contact with foods are regulated as indirect food additives by the Food and Drug Administration. In order for carriers to insure goods shipped in corrugated boxes, packages should comply with transportation rules. Rules for shipping by truck or rail are the National Motor Freight Traffic Association's National Motor Freight Classification (NMFC) and the National Railroad Freight Committee's Uniform Freight Classification (UFC), respectively. Item 222 of the NMFC and Rule 41 of the UFC set the standards for corrugated and solid fiberboard boxes. Both Item 222 and Rule 41 require minimum combined weight of facings and minimum burst test values, as a function of maximum values of box plus product weight and maximum outside dimensions. Minimum ECT values can substitute for combined facing weight and burst test values. Complying boxes must be printed with a manufacturer's box certificate indicating the applicable rule values.
6
Fiberboard Containers
Two other types of paperboard shipping container are solid fiberboard boxes and fiber drums. Solid-fiber boxes have greatly decreased in use, with the growth in use of corrugated. In general, solid-fiber shipping containers are subject to the same freight classification rules as corrugated. Fiber drums are available in a wide range of sizes, from about 8 in (200 mm) diameter to 23 in (584 mm), and capacities from 0.75 gal (2.8 L) to 75 gal (285 L). They are produced from convolutely wound plies of high-strength kraft fiberboard, glued together, usually with sodium silicate. No single paperboard ply may be less than 0.012 in. (0.30 mm) thick. The plies may include metal foil, plastic, or other materials, either as individual layers or in laminated form. In addition, the outer ply must be sized or waterproofed to resist loss of strength on exposure to moisture. Lightweight drums may have as few as four plies, while large drums intended for heavy loads may have as many as 11. The ends of the drums may be steel, fiberboard, or plastic, and are attached by crimping and toggle action of metal chimes, adhesive, stitching, taping, or other means.
7
Folding Cartons
Folding cartons are made from paperboard that has been cut and scored for folding into the desired shape. Ends may be closed using adhesive, or by tucking or interlocking. Cartons are usually preprinted in web form, cut and scored, the manufacturer's joint sealed, and scores broken, and then shipped in flat, knocked down form to the filler. However, modern cartoning machines increasingly are designed to accept flat, preprinted cut and scored blanks. A wide variety of carton styles are available, which can be grouped into tube style, tray style, and special construction. Tube style is the most common, with the machine direction of the board oriented to run horizontally around the carton, in order to provide the best resistance against bulging. A variety of grades of paperboard can be used, depending on the intended application. Cartons may also be laminated or coated to improve water resistance.
8
Environmental Considerations
Corrugated boxes are among the most recycled items in municipal solid waste. Folding cartons are often constructed from 100% recycled paperboard.
6
9
Packaging: Corrugated Paperboard
Summary
Paperboard is the largest single material used for packaging. The versatility of paperboard permits the construction of packages in a wide variety of shapes and sizes. By far the single most prevalent type of package in the world today is the corrugated box.
Reference ASTM International, 2014. D4169−14 Standard Practice for Performance Testing of Shipping Containers and Systems. West Conshohocken, PA: ASTM International.
Further Reading Attwood, B.W., 2009. Paperboard. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 913–921. Fibre Box Association, 2005. The Fibre Box Handbook. Rolling Meadows, IL: Fibre Box Association. Gordon, G.A., 2009. Drums, fiber. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 368–373. Obelewicz, P., 2009. Cartons, folding. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 235–341. Twede, D., Selke, S., Kamdem, D., Shires, D., 2015. Cartons, Crates and Corrugated Board, Handbook of Paper and Wood Packaging Technology, second ed. Lancaster, PA: DesTech. U.S. Environmental Protection Agency, 2014. Municipal Solid Waste Generation, Recycling, and Disposal in the United States, Tables and Figures for 2012. Washington, DC: EPA.
1 2 3 4 5 5.1 5.2 5.3 5.4 6 7 8 9 Reference Further Reading
1
Introduction Production of Paperboard Paperboard Grades Main Properties of Paperboard Corrugated Fiberboard Flute Sizes Stacking and Compression Environmental Factors and Stacking Strength Carrier Requirements for Corrugated Boxes Fiberboard Containers Folding Cartons Environmental Considerations Summary
1 1 2 2 3 4 4 4 5 5 5 5 6 6 6
Introduction
Paperboard, a very popular packaging material based on wood fibers, is available in many grades, based on the type of wood fiber and the fabrication process. Virgin fibers are obtained directly from trees, either softwood or hardwood trees; recycled fibers are obtained from recycled paper and paperboard. Manufacture of paperboard requires similar processes to paper manufacture: pulping, optional bleaching, refining, sheet forming, drying, calendering, and winding. Paperboard can be fabricated on different machines, with the fourdrinier and cylinder machines being the most common. Paperboard can be classified in different ways: virgin or recyclable, cylinder or fourdrinier-produced, and single-ply (known as ‘solid’ paperboard) or multi-ply. Often a board grade involves more than one criterion and depends on its intended application or use. Important properties of paperboard include: basis weight, thickness (or caliper), density, bulk, stiffness, bending, ply bonding, and glueability. These properties are interrelated to various degrees and are a function of the ratio of virgin to recycled fibers, as well as of production conditions. As with paper, paperboard properties are highly affected by humidity, and should generally be evaluated at the standard conditions of 5072% RH and 23.071.0 1C. Paperboard packaging applications cover a wide variety of physical forms and shapes, including folding cartons, trays, blister backings, set-up boxes, solid-fiber boxes, convoluted and spiral-wound cans, and fiber drums. By far the largest application of paperboard is the fabrication of corrugated fiberboard, which is a structured composite material combining linerboard, corrugating medium, and adhesive (usually starch-based). Corrugated fiberboard is mainly used for fabricating shipping containers, but some is used for cushioning, mechanical protection, and pallet construction. Paperboard containers are employed in almost every industry around the world, including pharmaceuticals, cosmetics, chemicals, food, bakery, tobacco, soap, textiles, hardware, toys, office products, sporting goods, electronics, and beverages. The importance of paperboard in packaging is clearly indicated by its uses as primary, secondary, and shipping (tertiary) containers. Figure 1 shows the approximate proportions of corrugated fiberboard boxes and other paper-based packaging materials produced in the USA.
2
Production of Paperboard
Solid paperboard uses predominantly virgin kraft fibers and is mainly produced by fourdrinier machines similarly to single-ply paper. In contrast, there are several methods for producing multi-ply board using recycled and/or virgin fibers. Multifourdrinier machines have secondary head boxes that apply a second ply after the first is formed, and are used for producing linerboard. Rotary formers consist of a felt roll passing by a series of vats, with pulp suspension added to the roll at each ☆
Change History: March 2015. S. Selke added abstract and updated Figure 1, Section 5.2, References, and Further Reading.
Reference Module in Materials Science and Materials Engineering
doi:10.1016/B978-0-12-803581-8.02196-2
1
2
Packaging: Corrugated Paperboard
Other paperboard packaging 0%
Bags & sacks 3%
Other paper packaging 4%
Folding cartons 14%
Gable top & asepc cartons 1%
Corrugated 78%
Figure 1 Relative proportions of paper and paperboard packaging materials in the USA (U.S. EPA 2014).
vat. Rotary formers can be either cylinder-mold machines (with two basic types, uniflow and contraflow), or roll formers (cylindermolds with a flow box to deliver the fibers). Cylinder machines are very common throughout the world. Inverform twin-wire machines form the web paper between two forming rotating screens. The more recently developed multiformer machines combine features of both fourdrinier and cylinder machines. Historically, cylinder machines have used mostly recycled (or secondary) fibers to produce paperboard, while fourdrinier machines employ mostly virgin fibers. However, modern cylinder and fourdrinier machines can process both virgin and secondary fibers. Fourdrinier and cylinder machines use different approaches to produce a thick web. In the fourdrinier machine, fibers come solely from the head-box on the top wire; in the cylinder machine, paperboard is produced by adding several layers of stock from different vats. After forming the wet web, it is pressed, compacted, dried, and wound. Other operations include sizing and calendering. In general, fourdrinier machines are good for producing a web of the same type of fiber throughout, while cylinder machines are good for producing a multilayer web. An important quality issue is the higher fiber orientation produced by the cylinder machine compared to the fourdrinier. For example, at equal thickness boards made on cylinder machines have similar machine direction (MD) stiffness compared to fourdrinier boards, but they have less stiffness than fourdrinier in the cross-machine direction (CD). Fourdrinier boards are more balanced (or squared) than cylinder boards. The higher fiber orientation in the cylinder machine, however, cannot be entirely attributed to the machine itself. During drying, high tension and stretching due to the relatively thicker multi-ply web also tend to increase fiber orientation.
3
Paperboard Grades
As for paper, the primary source of virgin fibers for paperboard is softwood trees. The strong softwood fibers are separated by chemical pulping and are available in unbleached or bleached grades. While bleached paperboard is often associated with pure and clean fibers for food and cosmetic applications, unbleached sulfate boards are characterized by excellent stiffness, high tearing strength, water resistance, and a more ‘natural’ appearance. The various types of paperboard can be grouped into containerboard, which includes both linerboard and corrugating medium used in the fabrication of corrugated fiberboard; boxboard, used for folding cartons and set-up boxes; and paperboard for other special applications. Table 1 lists the major types of paperboard used in packaging.
4
Main Properties of Paperboard
In addition to basis weight, caliper, bulk, and density, other important characteristic properties of paperboard materials are burst, stiffness, folding, ply bonding, and glueability. Relevant optical properties of paperboard include opacity, whiteness, brightness, gloss, and color. Basis weight, or grammage, an indicator of the paperboard mass per unit area, has a direct relationship to the cost per unit area of a package when the board cost is based on weight. Caliper affects the machineability during carton-making, and influences carton bulging, density, stiffness, and scoring. Bulk, which is thickness divided by basis weight, indicates the volume per unit of package mass (inverse of density), and is an important determinant of stiffness. Stiffness can be measured by the force required to bend a board sample through a 151 angle (Tauber stiffness). Alternatively, stiffness can be reported as the force needed to crush a formed carton. Stiffness is proportional to paperboard caliper. When paperboard is bent, the concave ply is extended and the convex ply is compressed. Therefore, increasing caliper and fiber orientation in both external plies tends to increase stiffness.
Packaging: Corrugated Paperboard
Table 1
3
Paperboard grades and typical uses
Board grade
Common furnish
Typical applications
Liner board Test liner Corrugating medium
Corrugated fiberboard Lower quality corrugated fiberboard Corrugated fiberboard
Solid bleached sulfate (SBS)
2-ply virgin kraft softwood Virgin fiber layer on a recycled fiber base Semi-chemical hardwood pulp, virgin, and recycled fiber Virgin softwood fiber, bleached
Solid unbleached sulfate (SUS)
Virgin softwood fiber, unbleached
Kraft-lined chipboard Chipboard White-lined
Unbleached kraft on a recycled fiber base Recycled fiber Outside liner: bleached virgin pulp. Underliner: mechanical pulp. Middle: recycled. Back: mixture of fibers Various
Can board Lined board Double-lined board Multifoudrinier board (MFB) Food-board Special-food board Frozen-food board Tube board Machine-glazed board
Chipboard lined with virgin fiber Two bleached virgin pulp liners sandwiching a layer of mechanical pulp Bleached kraft and thermomechanical pulp Bleached or unbleached pure virgin softwood pulp, single or multi-ply Plastic-coated food grade board Bleached virgin chemical pulp with moistureresistant additives, claycoated Various, with unsized and smooth finished Various
Strong stiff material. Used for frozen food, milk, and ice cream cartons. Can be plastic-coated Strong and stiff. Heavy-duty boxes, beverage carriers Hardware packaging Generic low-grade packaging Rigid and folding cartons
Composite cans and drums for powders. If coated with plastics can be used for liquids Folding cartons and set-up boxes High-quality board for food and cosmetic products Stiff, strong board for folding cartons, trays, etc. High-quality food packaging Cartons for liquid products such as milk and fruit juices High-quality frozen food applications Spiral or convolute composite tubes One-side glossed board
Good scoring and folding behavior are important in boards for folding cartons. Folds, or bends, are made after the board is scored or creased. The plies will separate locally at the fold, but the board must be able to sustain up to a 1801 bend without splitting or cracking. For this reason, folding boards have liners with a high percentage of long wood fibers (softwood). Ply bonding refers to the force required to separate the paperboard plies, which is determined by the adhesive bonding strength between the fibers of adjacent plies. During paperboard formation, ply bonding is controlled by the ply water content and the chemical similarity between fibers in contacting plies. Low ply bonding yields a poor paper with a tendency to split between plies, at locations other than the scoring lines. The glueability of paperboard is related to the adhesion characteristics of folding cartons. Adhesives must be chemically compatible with the board outer ply, penetrate it, and quickly produce adequate tack. Evaluating glueability requires conducting tests under controlled conditions of temperature and setting time, to determine whether fiber tear occurs rather than adhesive failure. Glueability depends on several factors such as adhesive/surface board compatibility, surface smoothness, board porosity, and surface roughness. Good board printability depends on the surface smoothness, ink receptivity, sheet flatness, dimensional stability, and absence of curling in the presence of water or high humidity.
5
Corrugated Fiberboard
The extraordinary strength/weight ratio of corrugated board is based on the column-arch construction provided by the undulated corrugated medium anchored on both sides to linerboard by starch-based adhesive. When properly dry, corrugated fiberboard is a rigid material that resists bending, flat crushing, and edgewise compression, as well as providing cushioning protection and good thermal insulation. The undulated corrugated medium is known as ‘flute’ and is typically made from short fiber hardwood, using a semi-chemical pulping process, or from recycled paper or a combination. Medium thickness is about 9 point (225 mm), with a standard basis weight of 130 gm2. Some amount of recycled fiber is commonly incorporated into the medium. Corrugated fiberboard is generally formed in 90 m long corrugators, which form the flutes in the corrugating medium board, glue it to the linerboard, dry the structure, and cut the combined board into sheets. The linerboards (or facings) are typically made from unbleached, kraft longfiber softwood pulp, with basis weights between 150 and 250 gm2.
4
Packaging: Corrugated Paperboard
Table 2
Common flute types in corrugated fiberboard
Type of flute
Flutes per meter
Height of flute (mm)
Take up factor (medium:liner length ratio)
A B C F E N
110710 155710 130710 295710 420710 560710
4.7 2.5 3.6 1.6 1.1 1.1
1.54 1.32 1.43 1.27 1.23 1.20
The simplest corrugated construction, beyond the medium alone, consists of one medium and one liner, and is called singlefaced board. This material is fairly flexible, and provides reasonable cushioning but not much compressive strength. It is often used as the primary package for light bulbs, and as a cushioning material for a variety of other fragile products. Single-wall (also called double-faced) fiberboard contains two liners sandwiching the corrugating medium and is the ‘typical’ corrugated board. The addition of a second medium and another liner produces double-wall corrugated fiberboard. Similarly, triple-wall corrugated contains three mediums and four liners. While paperboard (unless it is extremely thick) and single-faced corrugated can be wound in a roll, single-, double-, and triple-wall corrugated must be cut into sections and stored flat.
5.1
Flute Sizes
A variety of flute sizes are available, designated by capital letters. The major ones are described in Table 2. A-flute was produced first, historically. It has the largest wave of the major types. Its primary attribute is excellent compression strength, but it has the lowest flat-crush resistance. The smaller B-flute was introduced to provide better flat-crush resistance and hence printability. C-flute, intermediate in size between A and B, delivers better compression strength than B-flute, and better flat-crush resistance than A-flute. E-flute (for elite), the smallest of these four, was introduced later, primarily to provide a good printing surface, so that corrugated could be used as an attractive retail package for products that required more protection than could be obtained from packages such as folding cartons. Soon after the introduction of E-flute, with its break in the tradition of orderly alphabetical designation of flutes, a large number of flute variations were introduced by a number of corrugators, each with their own performance claims. One can now find J (jumbo), K, S, N, and F flutes, and probably more. The most used flutes at present are C and B, with A and E the next most common, and the others used only in small quantities. In double- or triple-wall boards, flutes of differing sizes are normally used. The combination of B- and C-flutes, in a BC doublewall board, is a popular choice. As a general rule, increasing the height of the flutes increases the stacking strength in line with the flutes. Increasing the number of flutes per linear foot (flute density) increases the strength across the flutes. Small flutes produce superior printing surfaces, while large flutes provide better cushioning.
5.2
Stacking and Compression
By far, the major application of corrugated board is transport packaging. During distribution and storage, packages are commonly subjected to rough handling and stacking compression, the two main distribution hazards besides humidity. Puncture resistance and burst (Mullen) tests are common evaluation measures, and are related to the total combined weight of the corrugated liners. The initial box compression strength (ICS) can be estimated from the McKee equation, originally developed by the Institute of Paper Chemistry in 1963 and later simplified to the formula shown in eqn [1]. It applies only to regular slotted containers (RSC) with a perimeter-to-depth ratio no greater than 7 and length less than 3 times the width. The McKee equation is: ICS ¼ 5:87 ECT
pffiffiffiffiffi tP
½1
where ECT is the edge crush test strength of the corrugated board, t is the board thickness, and P is the load-bearing perimeter of the box. Variability in ICS from calculated values is estimated at about 15%.
5.3
Environmental Factors and Stacking Strength
The box compression strength at the end of the distribution/storage cycle, or final compression strength (FCS), is diminished by several factors including length of storage time, t; relative humidity, RH; stacking patterns, S; palletization conditions; and the intensity of rough handling. FCS can be estimated by: FCS ¼ ðICSÞ Ft FRH FS FO
½2
Packaging: Corrugated Paperboard
5
where Ft is the storage time factor, FRH is the humidity factor, and FS is the stacking pattern factor. FO includes several multiplicative subfactors: pallet deck gap, pallet overhang, and excessive handling. Ft varies from 0.63 for 10 days down to 0.5 for 180 days. FRH varies from 0.9 for 60% RH down to 0.15 for 100% RH. FS for columnar misalignment ranges from 0.9 to 0.4. FO includes subfactors of 0.9 to 0.75 for pallet deck gap, 0.8 to 0.6 for overhang, and 0.9 to 0.6 for excessive handling. Another way to express the negative effects of relative humidity, storage time, and stacking patterns on the compression strength of corrugated boxes is to simply divide the ICS by a safety factor, as described in ASTM D4169 (ASTM International, 2014): 1. Use a safety factor of 8 for extremely severe conditions: frequent high humidity, long-term storage, overhung, and interlocked pallets. 2. Use a safety factor of 4.5 for average conditions: occasional high humidity, medium-term storage, and well-loaded pallets. 3. Use a safety factor of 3 for best conditions: controlled humidity, short-term storage, no pallets, and good stack alignment.
5.4
Carrier Requirements for Corrugated Boxes
In general, packaging itself is not regulated by any federal agency in the U.S. However, articles in the package or the printing outside the package may be regulated. For example, hazardous materials are regulated by the Department of Transportation, and paper and paperboard in direct contact with foods are regulated as indirect food additives by the Food and Drug Administration. In order for carriers to insure goods shipped in corrugated boxes, packages should comply with transportation rules. Rules for shipping by truck or rail are the National Motor Freight Traffic Association's National Motor Freight Classification (NMFC) and the National Railroad Freight Committee's Uniform Freight Classification (UFC), respectively. Item 222 of the NMFC and Rule 41 of the UFC set the standards for corrugated and solid fiberboard boxes. Both Item 222 and Rule 41 require minimum combined weight of facings and minimum burst test values, as a function of maximum values of box plus product weight and maximum outside dimensions. Minimum ECT values can substitute for combined facing weight and burst test values. Complying boxes must be printed with a manufacturer's box certificate indicating the applicable rule values.
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Fiberboard Containers
Two other types of paperboard shipping container are solid fiberboard boxes and fiber drums. Solid-fiber boxes have greatly decreased in use, with the growth in use of corrugated. In general, solid-fiber shipping containers are subject to the same freight classification rules as corrugated. Fiber drums are available in a wide range of sizes, from about 8 in (200 mm) diameter to 23 in (584 mm), and capacities from 0.75 gal (2.8 L) to 75 gal (285 L). They are produced from convolutely wound plies of high-strength kraft fiberboard, glued together, usually with sodium silicate. No single paperboard ply may be less than 0.012 in. (0.30 mm) thick. The plies may include metal foil, plastic, or other materials, either as individual layers or in laminated form. In addition, the outer ply must be sized or waterproofed to resist loss of strength on exposure to moisture. Lightweight drums may have as few as four plies, while large drums intended for heavy loads may have as many as 11. The ends of the drums may be steel, fiberboard, or plastic, and are attached by crimping and toggle action of metal chimes, adhesive, stitching, taping, or other means.
7
Folding Cartons
Folding cartons are made from paperboard that has been cut and scored for folding into the desired shape. Ends may be closed using adhesive, or by tucking or interlocking. Cartons are usually preprinted in web form, cut and scored, the manufacturer's joint sealed, and scores broken, and then shipped in flat, knocked down form to the filler. However, modern cartoning machines increasingly are designed to accept flat, preprinted cut and scored blanks. A wide variety of carton styles are available, which can be grouped into tube style, tray style, and special construction. Tube style is the most common, with the machine direction of the board oriented to run horizontally around the carton, in order to provide the best resistance against bulging. A variety of grades of paperboard can be used, depending on the intended application. Cartons may also be laminated or coated to improve water resistance.
8
Environmental Considerations
Corrugated boxes are among the most recycled items in municipal solid waste. Folding cartons are often constructed from 100% recycled paperboard.
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Packaging: Corrugated Paperboard
Summary
Paperboard is the largest single material used for packaging. The versatility of paperboard permits the construction of packages in a wide variety of shapes and sizes. By far the single most prevalent type of package in the world today is the corrugated box.
Reference ASTM International, 2014. D4169−14 Standard Practice for Performance Testing of Shipping Containers and Systems. West Conshohocken, PA: ASTM International.
Further Reading Attwood, B.W., 2009. Paperboard. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 913–921. Fibre Box Association, 2005. The Fibre Box Handbook. Rolling Meadows, IL: Fibre Box Association. Gordon, G.A., 2009. Drums, fiber. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 368–373. Obelewicz, P., 2009. Cartons, folding. In: Yam, K. (Ed.), The Wiley Encyclopedia of Packaging Technology, third ed. New York: Wiley, pp. 235–341. Twede, D., Selke, S., Kamdem, D., Shires, D., 2015. Cartons, Crates and Corrugated Board, Handbook of Paper and Wood Packaging Technology, second ed. Lancaster, PA: DesTech. U.S. Environmental Protection Agency, 2014. Municipal Solid Waste Generation, Recycling, and Disposal in the United States, Tables and Figures for 2012. Washington, DC: EPA.