However, where the packaging itself comes into contact with food products, either directly or indirectly, there are numerous rules and regulations in place to protect the consumer and to ensure that inks, adhesives and other materials do not migrate into the product, or contaminate them in any way. These issues will be highlighted in this article.
We will also explore a number of the main flexible packaging systems in use and their role as a carrier of product decoration and branding, in a variety of market sectors.
MARKETS FOR FLEXIBLE PACKAGING
The largest sector of the flexible packaging market are for applications in food and retail, but other non-food sectors are growing fast (Figure 8.1).
Figure 8.1 Examples of printed filmic packaging used in the food sector
Major applications for flexible packaging are diverse and include the following;
Multi-colored frozen food packs
Confectionary packs – e.g. Mars wrappers etc.
Garden centre and horticultural packs – fertilizer, bark, grass seed etc.
DIY packs – wallpaper paste, plaster etc
Stand-up pouches – washing liquids, detergents, soups etc.
Lidding for yogurt, cream, dessert, pots etc.
Agricultural packs – feeds, seeds etc.
Some product categories have notably shifted almost entirely to filmic packaging, for example baby food and some tobacco products.
The key end-use markets for flexible packaging are featured in the chart (see Figure 8.2).
Figure 8.2 Global flexible packaging market - the key end-use markets for flexible packaging
THE CONVERTING PROCESS
A typical converting process for flexible packaging from material through to the end-product is illustrated in Figure 8.3 below. Each of these steps will be covered in the paragraphs that follow.
Figure 8.3 Typical converting process for flexible packaging
Flexible packaging uses a combination of materials to provide properties that protect the package contents.
Flexible packaging materials are typically produced from three main materials types:
Aluminium foils and foil laminates
Although paper is widely used, it is the growth in the use of filmics that has been most dramatic.
Of these materials, some 75 percent are plastics, of which the main film materials used are polyethylene (PE), low density polyethylene (LDPE) and bi-axially orientated polypropylene (BOPP).
Aluminum foils are used where their better barrier properties give them the edge over flexible films or papers. They are used for confectionery, ready-meals, pharmaceuticals, soups and sauces, preserved foods, liquid foods.
Paper and paper based materials are used for a wide range of printed bags and sacks. Paper face constructions are typically used for dry goods like sugar, seasoning, soup mix and cocoa. Constructions between three to four mils can be surface printed with a varnish. By adding over-lamination, the material provides better stiffness and puncture resistance.
Many materials used for flexible packaging involve the use of multi-layers which are created using two different laminating processes.
Extrusion laminating is a process in which layers of multilayer packaging materials are laminated to each other by extruding a thin layer of molten synthetic resin, like polyethylene (PE), between the layers.
Adhesive laminating is a process in which individual layers of multilayer packaging materials are laminated to each other with an adhesive (described later in the section on in-line laminations).
There are generally three components in a flexible packaging structure – the exterior, the barrier, and the sealant. The exterior layer is the print surface.
The second component is the barrier layer which provides protection based on the product being packaged, the desired shelf life and the storage and distribution conditions required.
The sealant layer is a material that will adhere to itself, or to another film when heat and pressure are applied to produce hermetic seals that prevent gases from penetrating through the seals into the package.
It is typically applied to the inside layer of a multilayer structure on the side that comes in contact with the product.
PRINTING OF FLEXIBLE PACKAGING
All flexible packaging formats are printed in multiple colors on web-fed (reel printing) machines, including wide-web, mid-web and some narrow-web presses. The printing of films, some often quite thin, means that presses may require sophisticated tension control, perhaps heat management systems, static control, corona treating and registration, etc.
In terms of the printing of flexible packaging, gravure is regarded as being superior in print quality, although the quality of flexo has improved dramatically and is now considered cost-effective compared to gravure.
With more and more installations around the world, many converters are now seeing the value in the production of flexible packaging using digital printing.
Digital printing’s ability to react swiftly to market demands and to produce small print runs without the corresponding loss of time that used to accompany the production of the new tooling required for flexo, offset or rotogravure printing, offers significant and positive benefits.
Traditional flexible packaging run lengths continue to trend downwards as wide web converters struggle to print the smaller run lengths economically. There are however many narrow web in-line style printing presses (servo-driven machines in narrow web widths) that are now able to handle the shorter runs. Servo drives and controls have helped overcome the main challenges that have historically plagued the flexo market, such as gear marking.
Some 60 percent of flexible packaging printing is direct onto the film surface, whilst 40 percent is printed on the reverse and laminated.
Apart from print performance, flexible packaging films may have to withstand lamination, sterilization, pasteurization and thermo sealing and therefore may be surface coated or treated.
Inks and varnishes play a key role in not only providing the decorative appeal, but also adding barrier properties, coatings and seals.
As most flexible packaging is used for food or for sensitive applications (body-care, cigarettes, etc.), it is important that neither the materials nor the inks should be able to contaminate the product by odor, migration or through trace particles. A small, but increasing use of UV inks is tending to push flexibles printing towards flexo (covered in the following section on migration).
Standard tests for flexible packaging may include sniff tests, color assessment and flavor tests.
As there are a number of different foil laminations available, substrate surface preparation is key to achieving high quality print. Most plastics – polyethylene, polypropylene and polyester, have chemically inert and non-porous surfaces. Corona treatment is a popular option used to increase the surface tension of a given material to promote adhesion.
Laminating is the process through which two or more flexible packaging webs are joined together using a bonding agent to improve the appearance and barrier properties of the substrate.
The choice of the most suitable web laminating process is mainly dictated by the end-use of the product. A number of different technologies are available that cover the wide variety of applications in the food and non-food packaging industries.
Laminating machinery can be classified according to the type of bonding agent used to produce the laminates. These types are:
Solventless lamination: The adhesives used do not contain solvents. They dry by chemical reaction and therefore do not requiring a drying system. This method is used widely in flexible packaging since the chemistry is relatively simple and the applications broad.
Wax lamination: The adhesive is a wax or hot melt which is applied in a liquid state to one of the substrates prior to the substrates being brought together. This process allows the production of paper-paper or paper-aluminum foil laminates that are widely used for the packaging of biscuits and bakery products.
Dry lamination: Is the process where the bonding agent, dissolved into a liquid (water or a solvent), is applied to one of the webs, before being evaporated in the drying oven. The adhesive coated web is laminated to the other under strong pressure and using heated rollers, which improves the bond strength of the laminate.
Wet lamination: With wet lamination the adhesive is in a liquid state when the laminate and base substrates are brought together. It is commonly used to produce a paper-aluminum foil laminate that is widely used in flexible packagingand in the self-adhesive labeling industry. In the process of wet lamination the adhesive is applied to either the reverse side of the film laminate or alternatively to the face of the printed substrate to be laminated. The liquid adhesive is applied by a roller coating system. The two substrates, one of which has been coated with the adhesive, are then nipped to form the bond (see Figure 8.4).
Figure 8.4 - Wet Lamination Process
In all of the laminating processes described the resulting laminated web is then rewound into a finished roll.
A barrier coating or sealing coat can be applied to prevent migration of ink, adhesive, or other substances through the face material.
Barrier and other functional coatings encompass materials that are coated onto substrates to provide a barrier to protect selected packaged goods.
Barrier coatings, providing barriers for food packaging requirements, may include protection against oxygen and aromas, liquid water and water vapor, oils, and grease.
An effective barrier can prevent both losses from the packaged product, and penetration into the package, both of which can affect quality, and shorten product shelf life.
Packaged food products are being maintained fresh longer as a result of new materials, and food processing developments. For example, O. scavengers are now being used that work within a sealed package to limit O. reaction with a food product. Combined with effective O. barrier packaging, food packagers have the ability to improve shelf life, preserve product appearance, and flavor, while minimizing preservative use.
Additionally, antimicrobials, while under siege, have been proven effective as additives to coatings, and packaging films, in combating food sourced illnesses.
Nanotechnology is being applied to improve the gas barrier properties of coatings. In doing so, nanoclay is dispersed in barrier coatings, resulting in a platelet orientation that creates a 'torturous path' for gas molecules to traverse, yielding a very thin film, effective gas barrier.
FORMING THE PACK
The final process in creating a filled and decorated pack requires that the printed film is converted into a bag, pouch, sachet, tube, sack or other shapes using equipment that typically forms, fills and seals the pack. This process will vary depending on the type of pack required. The main types of end-use formats for flexible packaging will be dealt with in the next section.
FLEXIBLE PACKAGING END-USES
Printed flexible pouches can be a pre-formed, three-side sealed pouch, or formed as an in-line operation with the filling and sealing combined on a forming and filling line.
A typical pouch forming machine is illustrated in Figure 8.5. This equipment unwinds a pre-printed roll of film, folds and seals the film on the edges, leaving the top open. The pouch is then filled from above with product before being sealed to form the sealed pouch.
There are numerous pouch formats, styles and shapes that are available.
Figure 8.5 A typical pouch forming and filling machine
Stand up pouches are a laminated film bag, typically made of plastics or a blend of plastic film and aluminum foil. They can be printed with any color, logo, or design, so the potential to really make an impact on retail shelves is very high.
Stand up pouches are excellent options for both dry food packaging and liquid products.
Made from a continuous web of material, the first step in the stand-up pouch manufacturing process happens when the material is passed through a set of plows that fold a W-shape gusset into the bottom, so it can stand up.
The popular retort pouch is a flexible laminated food package that can withstand thermal processing. The choice of materials for the manufacture of retort pouches is very important. The material must have sound structural integrity and be able to withstand retort temperatures as well as normal handling conditions. A hermetic seal is achieved in retortable pouches by the fusion of two heat-sealable layers (such as polypropylene) to each other.
Single or re-fill pouches in particular, are being used for an increasing range of products, from liquid detergents to beverages and lubricants – and this is expected to continue to grow rapidly. Examples of stand up pouches can be seen in Figures 8.6 and 8.7.
Figure 8.6 Digitally printed stand-up pouches with personalization
Figure 8.7 Stand up pouches for the food sector
Pre-printed wrappers and decorative bands are an effective way for pack branding and packaging. Wrappings are typically used on confectionary, sweets, gift wrapping, butter packs, etc.
In many cases products are completely over wrapped in paper or film to form an airtight seal. Figure 8.8 is a good example of this style of decorative packaging.
Figure 8.8 Typical wrapper style packaging (gravure printed)
A Form Fill Seal (FFS) process called ‘flow wrapping’ is used to form a close fitting and air tight bag around the product. A flow wrapping machine is a horizontally operated machine with the packaging material mounted above the operating level. Typically, the product is loaded horizontally with a longitudinal seal formed below the pack.
Over-wrapping is another method of packing a product that does not create an airtight seal, but creates a pack like a neatly wrapped gift.
Both flow wrap and over wrap methods typically use polypropylene film (BOPP) to create the wrap, although other film and paper substrates can also be used.
Cold sealing methods are used, particularly in confectionary applications. The adhesive is coated onto the material. During the wrapping process the material is folded onto itself and sealed via a sealing wheel.
Single use packs
Consumers are finding significant benefits in single serve flexible packs that minimize waste, supports eating on-the-go lifestyles or allow users to sample small quantities of product (see Figure 8.9).
Figure 8.9 Typical single serve packs for a home improvement range
Single use packs are typically smaller versions of stand-up pouches or sachets.
Stick packs are another method of accommodating single use products. Named after its long and slender shape, stick packs are a type of vertical form, fill and seal (FFS) pouching (Figure 8.10)
Figure 8.10 Typical stick pack construction (Constantia Flexibles)
Some containers such as tubs, trays or jars, now have a clear printed plastic film heat sealed onto the container: This is often called a lidding film.
Typical lidding materials include paper, metalized PET or PP. Quite widespread is the use of aluminium foil for lidding, particularly for water cups, dry foods, yogurt pots, ice-cream, etc. A typical lidding film construction is illustrated in Figure 8.11.
Lidding films not only seal and protect the product but can perform an important decorative function. Most lidding films are designed to be peelable to allow easy access to pack contents (Figure 8.12).
Figure 8.11 Structure of a typical aluminium foil lid
Figure 8.12 A heatsealable polyester film which peels cleanly from trays in ambient or chilled conditions. (KPeel 3G - KM Packaging Services)
Peelable seal lids require a polymer layer on the inside to facilitate the heat sealing.
Some lidding applications require a heat seal coating applied to the film construction. The coated film passes over a pre-heat station where it warmed before it is sealed to the tray or pack, via a sealing bar or platen set at a desired heat, pressure and dwell time. The sealed tray or pack is then die-cut to shape.
Pot applications such as yogurts use heat seal pre-cut lids or diaphrams matched to the shape of the container.
Form Fill Seal (FFS)
Form-fill-seal (FFS) machines are used to form the package, fill it with a wet or dry product and seal it closed. Most FFS systems use a roll of flexible packaging film which is shaped and sealed to form the primary package, such as a wrapper or pouch (Figure 8.13).
Figure 8.13 Digitally printed vertical form filled seal, 3-ply laminated stand-up bag for cocoa and chocolate drink powders (Uni Packaging - Bensdorp’s Barista range)
FFS machines can be positioned either vertically or horizontally. Vertical machines form and cut packages. The product is then dropped into the package before final sealing (Figure 8.14).
Horizontal machines are used in cases where dropping a fragile product (such as a cake or biscuit) vertically may cause damage and instead the product is placed into the package horizontally (Figure 8.15).
Figure 8.14 Typical vertical form fill seal system
Figure 8.15 Typical horizontal form fill seal system
FFS machines are able to fill the flexible pack from either the top or the side. In most cases heat sealers apply heat to the sides of the package and melt the substrate material together to form a seal.
Ultrasonic sealing is a new development which tends to be used for heat-sensitive products and permits sealing through liquids.
A variety of materials can be used for sachet packing including aluminium foil, paper backed foil and PET foil.
Individual packs can be designed and printed in a range of colors to reinforce branding, as well as displaying all the required regulatory and product information.
Many different sizes of sachets can be filled with a range of products including powders, tablets and capsules or liquid.
Packaging machines are used which take flexible packaging material, to form a package which is then filled and sealed in a sequence of operations to form a three or four side sealed sachet. The process is similar to that illustrated in Figure 8.6.
There are two basic types of sachets, a fin seal type which is a face-to-face seal around the pack and a pillow style which is a crimp seal on the top and bottom edges and a flat seam running down one side.
The majority of machines used for these type of operations, are of the vertical form fill seal type, although horizontal form fill seal equipment is sometimes employed.
ALUMINIUM FOIL BOTTLE CAPS AND NECK FOILS
There are many types of bottle cap liners in use. Whilst they can be printed, their main function is to seal and protect the contents of the pack.
A popular seal liner is the induction seal which contains a foil laminate (known as an inner-seal) which is welded to the top lip of jars and bottles and creates a hermetic, tamper evident seal. Aluminium foil’s ability to be a total barrier to light, atmosphere and liquids is the principle reason for its use in caps, capsules and lids.
The sealing process takes place after the filling and capping operation. The capped containers pass underneath an induction sealing system which produces an electromagnetic current and the foil laminate generates electrical resistance, heating the foil. The hot foil in turn melts a polymer coating on the inner-seal. The heat, coupled with the pressure of the cap, causes the inner-seal to bond to the lip of the container.
Heat seal closures are compatible with the wide variety of plastic containers – PS, PP, PE, PVC and PET.
Aluminium neck foiling is used on a large scale particularly for premium brand beers, wines and champagnes to convey the impression of high quality.
Unsupported neck foils are often printed and are totally malleable, and can cover curved and shaped areas of the bottle neck and closure, to create a decorative finish and provide evidence of tampering. Neck foils are frequently embossed giving a distinctive appearance (Figure 8.16).
Figure 8.16 Bottle neck foiling conveys the impression of high quality
FOOD LABELING APPLICATIONS
In the food sector consumers are increasingly concerned about labels and/or packaging contaminating products. This concern is heightened in instances where packaging materials are in direct contact with food contents, but many of the issues relating to migration apply to all types of labeling and packaging.
Most countries have standards which determine which label and packaging materials may come into direct contact with food-stuffs and human skin. These products are prohibited along with those where there may be some transfer or migration of substances to food. The label user and/or label manufacturer are held liable for any failure on their part to comply with standards.
The labeling requirements for foods are set out in the relevant EU, FDA and national regulations around the world. They generally relate to most prepared foods, such as canned and frozen foods, breads, cereals, snacks, desserts, etc. Nutritional labeling may be voluntary, while other foods may have more detailed separate labeling requirements.
There are a number of directives and regulations governing food labeling, but these are complex and not always easy to understand. The EU started to harmonize legislation on food contact materials several years ago, but fully harmonized legislation does not yet exist for all materials.
Much of the basic regulatory background is contained in Regulation (EC) N° 1935/2004 and in particular article 15 of this regulation.
The article lays down common rules for packaging materials which come, or may come, into contact with food, either directly or indirectly. It also seeks to protect human health and consumers’ interests throughout the European Economic Area. It covers a wide range of different materials, including all papers and boards, plastics, inks, adhesives and coatings Any substances which can reasonably be expected to come into contact, or which can transfer their constituents to food are covered by the regulation. It seeks to ensure that the labeling of foods ‘shall not mislead the consumer’.
In America the FDA (US Food and Drugs Administration) is the U.S. federal agency responsible for ensuring that foods are safe, wholesome and sanitary; that human and veterinary drugs, biological products, and mechanical devices are safe and effective; that cosmetics are safe; that electronic products that emit radiation are safe.
The FDA also ensures that these products are honestly, accurately and informatively represented to the public, including issuing regulations governing the use of self-adhesive labels for contact with foodstuffs (FDA 175.125 for direct contact, and FDA 175.105 for indirect contact).
The U.S. Federal Food, Drug and Cosmetic Act (FFDCA) defines food ‘labeling’ very broadly. It covers all labels and other written, printed, or graphic matter upon any article or any of its containers or wrappers, or accompanying such article. The term 'accompanying' extends to tags, leaflets, circulars, booklets, brochures, instructions, and even websites.
The Nutrition Labeling and Education Act (NLEA), which amended the FFDCA, requires most foods to show specific nutrition and ingredients on the label. Food, beverage and dietary supplement labels that show nutrient content claims (for example ‘low fat’ or ‘contains vitamin XYZ’) and certain health messages have to comply with specific legal requirements.
Furthermore, the Dietary Supplement Health and Education Act (DSHEA) has amended the FFDCA, in part, by defining ‘dietary supplements’. It also adds specific labeling requirements for dietary supplements, and provides for optional labeling statements).
A US government advisory body exists to check compliance of any label with current rules. This service (which must be paid for) is recommended for manufacturers and label/leaflet vendors. A recent regulation (May 2016) changes US requirements regarding health criteria to be mentioned on labels.
Figure 8.17 highlights some of the most important aspects of food labeling regulations and directives.
Figure 8.17 A guide to some of the most important aspects of food labeling regulations and directives
There are rules and regulations in place to ensure that inks, adhesives and other materials do not migrate into the product.
For Europe, regulation (EC) No 1935/20041 requires that materials and articles which, in their finished state, are intended to be brought into contact with foodstuffs, must not transfer any components to the packed foodstuff in quantities which could endanger human health, or bring about an unacceptable change in the composition or deterioration in organoleptic properties.
Figure 8.18 highlights some of the key migration issues to be considered when labeling or packaging food.
Figure 8.18 Key areas of migration to be considered in food labeling
Although the risk that chemicals may leach through labels and packaging is real, the fear arises through most people’s ignorance of what chemicals there are in ink. As a result, label converters are turning pre-emptively to more expensive low-migration inks.
Some ink manufacturers are promoting UV LED inks not only for the energy economy, but also because this drying technology can be better controlled than traditional mercury lamp drying.
There has been some concern about the risk of contamination through mineral oils contained in substrates. Paper materials however are mostly made from virgin fibers, and synthetic substrates have (so far) been less often accused, and are available in ‘high barrier’ options from several producers.
Adhesives used for self-adhesive labels can also be a potential source of migration and contamination, particularly when these labels are applied directly to foodstuffs. The two most commonly used adhesives for pressure-sensitive labels are hotmelt and acrylic emulsion.
Nearly all adhesives stick because they contain resins, and the lower the resin content, the lower the initial tack. This problem is even more acute with filmic labels.
However recent developments using a multi-layer adhesive technology have resulted in a virtually resin-free adhesive, which can be used for filmic food labels as well as for moist or fatty surfaces.
Knowledge around barrier protection is crucial for servicing the market. Suppliers must be able to provide the right sealant for a given application and to support clients with 'fitness for use' testing.
One of the main factors driving the growth in flexibles is cost savings, particularly when flexibles are compared with labeled or decorated rigid packaging. Users of flexible can claim an improved carbon footprint, because of the weight and volume savings achieved. As mentioned previously migration and food labeling regulations need to be carefully considered in those sectors.
Recycling, environmental friendliness and sustainability have become issues for all packaging, providing the opportunity for new packaging materials development. As a result, continual improvements in polymer films, surface treatments, and coatings are yielding new packaging alternatives while respecting environmental impact.
Film structures are clearly moving to thinner laminations with stronger barriers and improved seal properties, which may require investments in servo-driven presses with special tensioning capabilities. At the same time, packaging machinery is being built to run faster.
In recent years the market for flexibles has been driven by innovative new origination, print and press technologies.
There is also a growing realization that narrow-web digital presses also have a significant role to play in meeting manufacturer’s demands for shorter runs, lead times, variable data and more product personalization.