Mark J. Kirwan Paper and Paperboard Packaging Technology

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through 180° and heat sealed, face to face, with the preceding winding in the

overlapped area. This is known as an ‘Anaconda fold and seal’ (see Figure 6.7).

Aluminum foil is used under the contact surface where barrier and product

protection are both required. A kraft (paper) layer completes the liner structure.

It serves as a carrier through the converting process that produces the liner

structure.

The ratings in terms of ‘High’, ‘Medium’ and ‘Low’ for moisture vapor

transmission rate and oxygen permeability are shown in Table 6.5 and examples

of typical liner structures, their barrier ratings and typical uses are shown in

Table 6.6.

Table 6.5 Ratings for various levels of functional barrier

* Moisture vapor transmission rate.

Approximate barrier parameters

MVTR* Grams/24 hours/container

38 °C/90% RH

permeability cc/24 hours/container

25 °C/50% RH

High <0.02 <0.01

Medium <1.00 <0.50

Low <5.00 <1.00

Table 6.6 Barrier ratings for typical liner structures

Liner structures commonly used Barrier properties Typical uses

Kraft/foil/polypropylene High Hermetic food (coffee, nuts, snacks,

shortening, peanut butter, infant formula,

etc.) solvent- and latex-based paints

Kraft/foil/Surlyn

High Hermetically sealed packs for food,

including nuts and snacks

Kraft/foil/High density

polyethylene (HDPE)

High Solid shortening, powder beverage, snacks

Kraft/MPET/Surlyn

High Hermetic snack foods

Kraft/PE/foil/Coex HD High Refrigerated dough

Kraft/foil/vinyl slipcoat Medium Refrigerated dough, adhesives, caulk,

powder beverage, non-hermetic food

Kraft/PE/PET/Surlyn

Medium Pet food, paper-bottom can

Kraft/LDPE/OPP Medium Solid shortening

Kraft/LLDPE/White HDPE Low Hard to hold frozen concentrate

Kraft/Surlyn

/White HDPE Low Hard to hold frozen concentrate

Kraft/White HDPE Low Frozen citrus concentrate

Kraft/HDPE Low Motor oil

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6.7.2 The paperboard body

The paperboard body of the composite can is its source of strength, Figure 6.8.

The body must provide axial strength sufficient to withstand multi-pallet

stacking of finished goods, and sidewall strength necessary to absorb impact

during distribution.

There are generally two types of paperboard used in composite cans:

unbleached (brown) kraft and recycled. Both provide similar axial and sidewall

strength, but kraft may resist more shearing because of its longer fibers. Recycled

board is an affordable alternative, and due to regulations in some geographic locations,

is a required component of packaging.

In order to achieve a neat joining between adjacent spirally wound layers,

the edges are ‘skived’. This means that the edges are cut at an angle so that

the adjacent edges mesh neatly together. This also adds strength to the

composite can.

6.7.3 Labels

The label is the outer layer of the composite can that meets a variety of needs. It

serves as a billboard for product information and attention-grabbing graphics, as

well as providing additional barrier protection.

Membrane to liner seal.

Poly-to-poly seal along top

edge of container allows

opening to occur.

Body ply

1 or 2 ply recycled board

Liner

Customized liner to match

product needs with full

range of barriers

Metal end bottom

Metal end closure

Membrane top

Composite makeup

Figure 6.8 Composite-can construction. Source: Sonoco.

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6.7.4 Nitrogen flushing

The materials and methods of constructing a composite can depend on its end use

and desired performance characteristics. When composite cans were being

developed for the snack industry, the concept involved creating an hermetically

sealed container that could replace the vacuum-packed steel can. This process –

known as nitrogen flushing – was developed to accomplish this, and it is still in

use today to extend the shelf life of certain products, such as nuts, snacks and

chips (potato crisps).

The most common way to remove oxygen is called flushing. It works on the

principle of flooding the container with a non-reactive gas, such as nitrogen, just

before it is filled.

The type of gas flush is generally determined by the nature of the product.

For example, powders have a tendency to pack together, i.e. particles

compact together, making it more desirable to purge in several stages. Firstly,

to purge the can prior to filling; secondly, to purge the product in the filler;

and finally, after filling, prior to seaming, to remove oxygen from the head-

space. For snacks, it may be only necessary to flush the filled can prior to

seaming.

6.8 Printing and labeling options

6.8.1 Introduction

The two most common label materials are aluminum foil and paper. Foil is most

often used for aesthetics and barrier properties when packaging products such as

cleanser and some food stuffs. An example is the Kraft

Parmesan Cheese canister.

On paper, there is more of a matte, less glossy finish, such as the Pringles

Potato

Crisps canister.

A thermoset or thermoplastic coating is applied over the foil or paper for print

protection and added barrier properties, particularly moisture. These coatings also

provide shelf-appeal, delivering a high-gloss appearance.

Ensuring proper registration on graphics is a complex process. Both alumi-

num foil and paper provide good offset litho, flexographic and rotogravure

print surfaces. Paper is less expensive, but aluminum foil adds additional barrier

protection.

It is important to remember that a package is a communication vehicle, which

plays an important role in the purchasing process – both as a billboard for the

brand and as a bearer of product information. Therefore, the impact the label has

on the effectiveness of the composite can is paramount to the end-user, the consumer.

It must be eye-catching and appealing.

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There are several printing options for composite can labels. In the following

sections, we will focus on the most common printing options: flexographic,

rotogravure and off-set printing.

6.8.2 Flexographic

Flexographic printing is an efficient, cost-effective and versatile printing method,

widely used for packaging products and envelopes. It provides high print quality

on a wide variety of absorbent and non-absorbent substrates, including flexible

materials, label stock, corrugated substrate, rigid plastics, envelopes, tissue paper

and newsprint.

Flexography is a relief printing process, in that the image is pressed onto the

label material. This printing process is so named for its flexible rubber plates, which

are also the key to the method’s versatility and increasing popularity. The increasing

use of flexography also increases the demands on the print quality of the end product.

Flexography is the fastest growing printing process, due to:

•

its ability to print on practically any substrate, absorbent or non-absorbent,

including very thin extensible films

•

the use of fast-drying inks, be they solvent-based, water-based or UV curable

•

development of photopolymer plastic plates with good half-tone reproduction

and low dot gain

•

quick make-ready and changeover times make it profitable for short-run

printing and

•

its cost-effectiveness for many applications.

The European Flexographic Technical Association (EFTA) estimates nearly 40%

of all packaging printed globally is through the flexo process with annual growth

of 5%. Additionally, it is in label printing where the greatest growth has occurred;

over 70% of new machine installations in Europe are believed to be flexo. The use of

ultraviolet (UV) inks has contributed to the growth and popularity of this printing

process (EFTA).

6.8.3 Rotogravure

Once an art form that has been totally digitized, rotogravure is a printing technique

characterized by high print quality and large quantity runs – hundreds of

thousands or even many millions. Tiny ink volumes are transferred from the steel-

based gravure printing cylinder to printing dots on the label substrate. Millions of

printing dots show up to the human eye as letters, text and images.

Material knowledge, papermaking competence and production facilities are

the secrets to ensuring superior printing performance. Gravure is a quality printing

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process producing excellent quality and constant reproductions throughout the

entire print run. The secret of the gravure process lies in the cylinder. Increased

robotics and total plant computer control systems will bring many further advantages

for gravure.

Gravure’s environmentally friendly printing process allows the use of:

•

papers with higher recycled-fiber content

•

highly effective solvent-recovery installations

•

industry-leading methods to save paper, ink and energy and

•

less residual ink solvent content in gravure products.

In short, rotogravure is a very simple printing process that can produce millions

of perfect copies at enormous speed. It also produces superb colors and good gloss

on relatively low quality paper. Because the gravure process can guarantee

consistent reproduction of high quality graphics, it suits the packaging market

where buyers are greatly influenced by the quality of the printed image. The ideal

substrates are generally smooth in finishing (clay-coated paper, films and foils)

since effective ink transfer depends on thorough cell contact with the substrate to be

printed. Presses for packaging gravure printing must meet different requirements

due to the variety of substrate colors and finishing processes. Running at lower

speeds enables the processing of difficult materials and drying of special inks.

Packaging presses can combine in-line finishing processes including laminating,

cutting, creasing, embossing, etc. (ERA).

6.8.4 Lithography (litho/offset) printing

Lithography is an ‘offset’ printing technique. Ink is not applied directly from the

printing plate (or cylinder) to the substrate as it is in gravure or flexography. Ink is

applied to the printing plate to form the image to be printed and then transferred or

offset to a rubber blanket. The image on the blanket is then transferred to the

substrate to produce the printed product.

Lithography is based on the principle that oil and water do not mix (hydrophilic

and hydrophobic process). Lithographic plates undergo chemical treatment that

renders the image area of the plate oleophilic (oil-loving) and therefore ink-receptive

and the non-image area hydrophilic (water-loving). Since the ink and water essentially

do not mix, the fountain solution prevents ink from migrating to the non-image areas

of the plate.

The structure of the label substrate has a strong influence on print quality and

ink drying, making this a field of high priority for research and development.

Properties such as the base paper’s formation and topography and the surface’s

porosity and smoothness must be optimized for the end-use and printing

method.

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6.8.5 Labeling options

•

Spiral labels roll fed

– printed using rotogravure or flexographic printing processes

•

Convolute (strip) labels – roll fed

– more cost-effective

– printed using rotogravure or flexographic printing processes

•

Convolute (strip) labels – sheet fed

– can only be printed off-set/lithographic

•

Single wrap

– pre-printed on the body blanks

– liner, body and graphics are all together as one piece – wrapped to

make the body

•

Post-filling labeling techniques

•

No label; no printed surface

– customer applies labels

While many improvements can be readily made, there has to be a balance between

cost and market demand. For the future, efforts will be focused to enhance label

appearance with a more glossy finish. The differences between rotogravure and

flexographic processes are narrowing as printers have made tremendous strides to

improve the quality of the latter. Rotogravure printers are working toward reduc-

tions in pre-press costs.

6.9 Environment and waste management issues

6.9.1 Introduction

During the latter part of the twentieth century, the composite can received extensive

scrutiny in terms of meeting current environmental initiatives underway at that

time. The package’s body plies are made from recovered and recycled fiber and

can have a post-consumer recovered waste content of over 50%. In many

communities, this qualifies the composite can for placement in the material

flow streams of curbside recycling programs. In addition to the body, the metal

ends of the can are also recyclable.

Today, the composite canister continues to fit into the majority of environmental

initiatives demanded by the marketplace, and continues to be recognized for its

environmentally friendly attributes. These attributes include the introduction and

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use of a paper bottom end, which can increase the amount of recycled post-

consumer content up to 70%, reduction of material into the waste stream and other

efforts to enhance the can’s ability to be recycled.

6.9.2 Local recycling considerations

When selecting packages, manufacturers and co-packers take a close look at

local and regional recycling laws. In the US, some states require that certain

types of packaging are recyclable and use a minimum of recycled content while

still meeting Food and Drug Administration (FDA) food contact standards.

The wide availability of paper and plastic recycling resources in the US make

composite cans an excellent package for a variety of products. While there is a

preference for foil composites in Europe, there is also a good deal of aluminum

can packaging due to its easy recyclability.

6.10 Future trends in design and application

6.10.1 Introduction

The ongoing development of new materials, sizes, shapes and technologies

indicates that market applications for the composite canister will continue to grow.

Packaging engineers are focusing on ways to increase performance and

convenience by:

•

enhancing existing features and materials

•

identifying ways to reduce costs associated with materials and processes and

•

redesigning closure systems.

Some of the more interesting opportunities include research into the use of sorbent

materials in liners, the recent commercialization of a valved-membrane end and

the delivery of improved closing features, specifically the plastic overcap.

6.10.2 Sorbents

Sorbents provide high moisture and odor absorbing capability. Generally included

in the product mix as a non-edible packet consisting of silica gel, clay or other

compound, there is promising new research underway focusing on ways to build

sorbents into the liner structure. By continuing to absorb oxygen, the product

inside the can will remain fresher before and after opening, Figure 6.9.

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6.10.3 Valved membrane end

Designed for coffee, this one-way release valve allows for packing and sealing

immediately after roasting. This eliminates the need for extended hold times for

degassing, and at the same time maximizes flavor and aroma. A vacuum is no longer

necessary. Because the end is a peelable foil membrane, a can opener is not needed.

6.10.4 Resealable plastic overcap

In another effort to increase product life span, the resealable plastic overcap offers

great promise for providing increased shelf life after opening. Redesign of the

typical overcap to increase performance levels can be accomplished through the

addition of a gasket that will snap to the opening of the composite can.

6.11 Glossary of composite can related terms

Anaconda seal – In order to maintain a complete barrier where the edge of one

spiral winding overlaps the preceding winding, the edge is turned through 180°

and heat sealed, face to face, with the preceding winding in the overlapped area.

Body stock – A term used to identify the inner plies of a composite can body –

excludes the liner or label.

Caliper – Thickness of a sheet of paper or paperboard, measured under certain

specifically stated conditions, expressed in thousandths of an inch. Units are called

‘mils’, when referring to paper, and ‘points’ when referring to paperboard.

Functional barrier

Scavenger layer

Barrier layer

Figure 6.9 Oxygen scavenging. Source: Sonoco.

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Canboard – A generic name for the board used in composite cans. It is generally a

medium-strength board with good surface smoothness.

Can dimensions – Can diameter and height measurements are expressed in inches

and sixteenths of an inch. The standard 12-ounce juice can measures 211×414,

which translates to 2

in. in diameter by 4

in. in finished can height. The

diameter of the can is given first. In Europe, dimensions are measured in millimetres

(mm), which refer to the internal diameter of the can.

Composite can – A term used to describe a can made from more than one type of

material. The tops and bottoms can be made of metal, plastic, paper or a combination

of materials.

Convolute can – A laminated fiber can made by winding material around a form

with the material fed at right angles to the axis of the form.

Finished can height – The overall height of a finished can with one end seamed

on. Often referred to as ‘one end on height’.

Gas retention – The measured ability of a can to retain gas during a specified

time.

Gassing – The act of removing air from a can and replacing it with a non-reactive

gas, such as nitrogen or carbon dioxide.

Hermetic can – Air or gas-tight canister.

Hermetic seal – The bonding of two parts together so that it is air or gas-tight.

Kraft – A term meaning strength applied to pulp, paper or paperboard having a

minimum of 85% virgin wood fibers and produced by the sulfate process.

Label – The outermost ply of a composite can. This layer may be printed. Even if

it is not, it is still called the label.

Liner – The innermost ply of a composite can, constituting the can body’s principal

barrier to moisture or gas transmission.

Linerboard – Solid bleached or unbleached kraft paperboard, either fourdrinier or

cylinder, combination paperboard produced from a furnish containing less than

80% virgin kraft wood pulp.

Mira strip tape – Plastic strip adhered to the top of a can-opening feature, which is

used with crimp seams (such as frozen concentrate cans).

Nitrogen flushing – During the filling process, the can is filled with a nitrogen gas

before inserting the product. The nitrogen gas displaces oxygen allowing for

longer shelf life.

Overcap – A cap fitting over another closure that will allow resealing once the can is

opened. An overcap must be slightly larger than the end on the can for a proper fit.

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Paperboard – A broad classification of materials made from fibrous matter on

board machines, encompassing linerboard and corrugating medium. Most com-

monly made from wood pulp or paper stock.

Ply – The layers of paper used to form the can body. The paper on a can is usually

described by the number of plies wound into the can: two-ply versus one-ply can.

Reclosure – A term used to identify or classify the type of end, plug or cap which

can be replaced after opening a container.

Recycled stock paper – A paper made from pulp consisting of reclaimed paper

waste materials.

Shelf life – The length of time a composite can, or material in a container, remains

in a saleable or acceptable condition under specified conditions of storage.

Spiral can – Any composite can which has been formed by winding material

around a mandrel at an angle to the axis (less than 90°).

Self-opening can – A method of construction that allows a refrigerated dough can

to open when the label is removed, the body ply joint is exposed, allowing the

liner heat seal to release and dough pressure pops the can to open.

Skive – In order to achieve a neat joining between adjacent spirally wound layers,

the edges are ‘skived’. This means that the edges are cut at an angle so that the

adjacent edges mesh neatly together. This also increases the strength of the com-

posite can.

Wicking – The tendency of liquid to be absorbed by osmosis through a sheet of

paper.

Reference

Packworld, 2002, Rothwell, T. ‘The packaging marketplace: A global view’. Packaging World, vol. 9,

no. 10, 191, visit http://www.packworld.com/articles/Features/15082.html.