A Comparison of Folding Carton Substrates & Appropriate Applications

In today’s packaging landscape, the choice of raw materials can be daunting. Substitutions are sometimes made with little or no regard to suitability; nonetheless, the differences in paperboard properties and appropriate application are creating some interesting nuances in today’s recovering economy. The choice of substrate is influenced by a number of key parameters, including the requirements of print performance, product protection, design features and requirements, sustainability, and overall value.

Print performance is a key object for paperboard packaging. Traditional print technologies such as sheet and web offset lithography, flexography, and rotogravure are being supplemented by digital printing, laser imaging, inkjet, and rotary screen. It’s a modern miracle that packaging substrates can and do perform suitably with each of the processes. Inks and overprints may be aqueous, solvent-based, oil-based, and so on. They may cure by a variety of mechanisms, such as oxidation/polymerization, UV, IR, EB, etc. Finally, it’s interesting to note that packaging solutions often employ more than one printing process in a single package.

Product protection is important, since the contents—as well as the package itself—must survive the rigors of distribution through filling, packing, palletizing, transportation, retail, and consumer handling. Damaged packaging can result in unsalable products, which take an enormous toll on profitability. Dry and frozen foods, personal care products, beverages, pharmaceuticals, automotive, hardware, toys, and electronics all require differing sorts of protection—temperature, shock resistance, barriers for water, humidity, bleach, alcohol, grease, blood, oxygen, and so on.

Design features and requirements, such as the size and style of the primary package, can greatly influence the choice of substrate. Panels, windows, opening, closure, filling line requirements, end-use and end-of-life scenarios may favor one material over another. The grain orientation of a package is also influential, since properties such as stiffness and tensile strength are favorable in the one direction, whereas tear strength excels in the opposing direction. The proportional differences depend on the type of fiber, density, and forming method of each material.

Sustainability is increasingly important in packaging substrate decisions. While paperboard packaging may represent less than 5 percent of a product’s overall carbon footprint, it is perceived to be much more than that. Indeed, 60 percent of all packaging in the United States is ultimately landfilled, and initiatives such as packaging scorecards and extended producer responsibilities are raising the bar. Factors such as the use of certified fiber, renewable materials, recycled content, recyclability, biodegradability, compostability, package-​to-content weight ratios, and cube utilization are all tied to these decisions.^1,2

Overall value can be defined as the expectation of benefits gained for the price paid.3 Sustained value can be described as the customer’s willingness to specify, utilize, pay for, and reorder a given level of quality or service at a given price. Low-cost materials and packaging solutions are acceptable for some lower-margin product categories, but would be unacceptable for others. So, with these considerations in mind, an introduction to the types of paperboard follows, including solid bleached sulfate, coated recycled board, coated unbleached kraft, and uncoated kraft boxboard.

Solid bleached sulfate (SBS) is a prominent packaging material in North America, made from mostly hardwood, first-generation, chemically pulped fibers that are subsequently bleached, refined, and formed in one ply on Fourdrinier-type paperboard machines. SBS is white on both sides and throughout the sheet. The relatively short hardwood fibers are coated and polished, creating an impressively smooth print surface that accommodates the highest-quality imaging by offset, flexography, and gravure. This ultrasmooth surface also forms an ideal foundation for film and metallized foil lamination. Most SBS products are coated on one side only (C1S), but some may be partially or fully coated on the back side as well (C2S). SBS generally performs well for embossing, gluing, and high-speed filling. It cuts cleanly and has sufficient internal and surface strength properties to accommodate high-speed printing with tacky oil-based lithographic inks. SBS is often higher-priced compared with other paperboard substrates.

SBS is not known for tear strength, but it generally has good stiffness and compression strength. It may be susceptible to yellowing over time with exposure to white light. It is traditionally the first choice, however, for high-quality graphics in personal care packaging, medical, pharmaceutical, confectionery, media, and tobacco packaging. The pristine white back side and internal characteristics are preferred by purists for bakeries and other products involving direct food contact.

Primary North American suppliers of SBS include WestRock, International Paper Co., Clearwater, and Georgia Pacific.

Decision Matrix

Selection criteria: Cost, quantity, appearance, strength, environmental perception, finish

1 Life Cycle Assessment results: www.guardian.co.uk/environment/2009/feb/17/cadbury-dairy-milk-cows, beeractivist.files.wordpress.com/2008/07/carbon-impact.jpg

2 Greenliving: United States Recycling Statistics:

www.greenliving.lovetoknow.com/united_states_recycling_statistics

3 Wikipedia: en.wikipedia.org/wiki/value%28marketing%29

Coated unbleached kraft (CUK) is also made from mostly first-generation, chemically pulped fibers on Fourdrinier-type paperboard machines; however this substrate is formed in two or three layers and is composed mostly of longer softwood (pine) fibers, which contribute to strength. A thin concentration of hardwood fibers is added to the top surface for somewhat enhanced smoothness. CUK is known mostly for durability and resistance to tear. It is white on the top side due to its extremely opaque, titanium-​based coating and otherwise brown throughout the sheet. It’s somewhat less expensive than SBS.

CUK is safe for direct food contact and performs well in frozen food applications. It is commonly used for packaging large club-store multipack cereals. A wet-strength version of this grade is used for the packaging of brewery and soft drink bottles and cans, where the paperboard retains 90 percent of its dry tear strength when exposed to saturated, wet conditions. A general folding carton version of this grade is well-suited for frozen foods, snacks and cookies, multipack pudding cups, automotive, electronics, and sports equipment applications requiring superior tear, tensile, and compression strength in a relatively dry environment.

Primary North American suppliers of CUK include Graphic Packaging and WestRock.

Coated recycled board (CRB), also known as clay-coated news, is a dominant material in North America and elsewhere in the world. CRB is generally made from a blend of recycled newsprint, old corrugated containers, and mixed secondary paper. Its fibers are often 100 percent recycled with 35 percent or more of the content considered “post-consumer.” It is composed of as many as eight or more layers of material with the top ply made of bleached or semibleached fibers. A variety of machines, including Fourdrinier, cylinder, and hybrid forming systems are used in its manufacture.

CRB is the most common substrate for low-cost folding cartons when recycled content is preferred or required. It is often used for single-pouch cereal boxes and other multipurpose packaging where tear strength and premium print quality aren’t paramount. It is not always used in direct food applications, although food-grade versions of CRB are not uncommon. It often has a one-direction stiffness advantage that can be beneficial for bulge resistance in packaging of granular products such as rice, pet foods, pancake mix, and powered laundry detergent.

The specification tolerances of CRB are somewhat less stringent than with SBS and other virgin grades. It is often not as strong and tends to carry slightly higher moisture content than other grades. CRB is more absorbent than SBS and CUK, and therefore it requires a slightly thicker ink film and/or stronger inks to achieve a given strength of color. As with all other materials, die cutting, creasing, and gluing conditions should be adjusted to optimize its performance.

Primary North American suppliers of CRB include Graphic Packaging, WestRock, Cascades, PaperWorks, Caraustar, and White Pigeon.

Uncoated kraft boxboard (UKB) is the final grade to be considered. Similar to coated unbleached kraft, UKB is composed mostly of first-generation, chemically pulped pine fibers. It is similarly formed on a Fourdrinier-type paperboard machine in two layers, where the top layer houses a concentration of hardwood fiber for somewhat enhanced smoothness. Since it’s uncoated, however, UKB can be a cost-effective alternative to coated paperboard, with applications that appreciate an environmentally friendly, natural brown surface. It is lower in density than other grades; therefore, cartons of equal caliper weigh less.

A sturdy material, UKB is safe for direct food contact. It can be reasonably printed by offset, flexo, or gravure and performs well in embossing and foil stamping. The quality of graphic reproduction doesn’t match that of coated materials; however, designers

are sometimes creative with the earth-tone appearance. An interesting technique is to press-apply a white pigmented spot color in certain areas, then build process color on top of that. UKB is a popular choice for institutional food packaging, lawn and garden, hardware, sporting goods, and other durable-packaging needs.

A primary North American supplier of UKB is KapStone Charleston Kraft. WestRock produces an alternative product that is made from 100 percent recycled old corrugated containers.

Hybrid materials also exist. Some CRB products may have a brown kraft back side, hence the term “coated kraft back” (CKB), or a white layer of fiber on the back side. Mechanically pulped (ground wood) fibers may team up with chemically pulped fibers, and varying percentages of recycled fiber content are now seen with some of the grades. Nonetheless, it is incumbent on package designers to understand the effect that these treatments may lend to the performance characteristics of the various grades. In every situation, adjustments must be made in the printing, cutting, creasing, gluing, and filling line conditions to optimize performance given a substrate substitution.

Primary North American suppliers of CKB include Graphic Packaging and WestRock. White back CRB is produced by WestRock. Ground wood fibers are employed in some products produced by Tembec.

The choice of substrates can be summarized in a decision matrix as on the facing page, understanding that these are guidelines only. Creative license allows for some products to play the field; for example, bar soaps have been successfully packaged in all four materials, and sometimes utilizing the back side of a material as the outward-facing side opens up a new world of possibilities.

Dan Malenke is president of PKGPRO LLC. He may be reached at 334-703-2114 or pkgpro@hotmail.com.

This article was reprinted with permission. © Zumbiel Packaging.