Flexible packaging: Paper and board, metallic foil, films and multi-layer constructions (2025)

When looking at food packaging materials for example, the particular type of food product, its chemical make-up, size, expected storage conditions and required shelf life, moisture content, aroma/flavor and product appearance are all just a few of the many characteristics that must be taken into account when selecting the right material for a specific food or other product.

An ongoing trend in food packaging for example, is the way the pack is designed so as to extend the shelf life of foods while at the same time maintaining a fresh-like quality. Such requirements places a high demand on the selection of materials that not only need to provide the necessary properties to maintain the quality of the food but must also be done at an economic and cost-effective price.

Good design is of course critical in persuading consumers to purchase, but the repeat purchase will only be made when the promise of the design is matched by the quality of the product inside the packaging. The quality of the product inside the pack can only be guaranteed over the period of the shelf life by selecting a good-quality packaging paper, film, foil or barrier material and matching the capability of the material used to the protection requirements of the product. Get that step right, and the product will reach the consumer in the condition intended by the manufacturer.

Another aspect to be considered in food packaging is the permeability of the packaging material. This is one of the most critical features of the package that can impact on the quality of the food product. Materials can definitely be selected to provide a very long shelf life, but do they provide the best barrier. Added to that is the question of whether the extension of the shelf life justifies the cost of the material and the quality of the food? Knowing the key factors for material selection just based on permeability can be an essential part of the package design process.

Outside of foodstuffs, papers used for wrapping soaps and detergents for example, will need to protect the consumer from coming into contact with the chemical ingredients of these products. Hot melt coatings, anti-mold treatment and anti-fungus papers are all likely requirements for soaps and detergents. The papers will also have to perform well on high-speed packaging lines.

Enhanced storage and distribution requirements, retail and consumer shelf life demands, and the running of printed materials on filling, forming and sealing machines may also be important considerations when making the selection of which flexible packaging materials are to be used.

With so many varied and different requirements placed upon flexible packaging materials today, much innovation and development has been placed on the evolution of laminated films and high barrier constructions that prevent the permeation of water, water vapor, oil, oxygen, aroma, flavor, gas or light.

Lamination of a wide range of specific substrate types means that it is today possible to provide the best available packaging formats. Laminates now available provide the necessary strength and barrier properties, as well as print clarity, with encapsulated and laminated print eliminating scuff and offering peelable webs for consumer convenience. Each specific barrier material and formulation has its own unique barrier properties.

With printed flexible packaging, the mono-web, multi-web or specific barrier construction materials selected must also be able to provide printed shelf appeal, provide optimum printability, meet the necessary food or other labeling requirements, and perform as required in materials, print and usage tests.

In general, brand owners should know the performance and barrier properties required to protect their products. They have food scientists working for them, so any converter intending to make a speculative approach, or propose a new structure, will need to do their research carefully, and have an idea of what they are proposing and why it will work.

Look at the structures already being used in the market place; generally, they are used for good reason. If the product is successful it means that the packaging is probably fulfilling its purpose and preserving the product, allowing it to be enjoyed with the texture, flavor and aroma with which it left the factory.

Knowledge around barrier protection is undoubtedly crucial for servicing the diverse flexible packaging market. Suppliers must be able to provide the right sealant for a given application and be able to support clients with Ôfitness for use' testing.

To summarize, what general information does the printer/converter need to understand before ordering flexible packaging materials? Shelf life, as already explained, is important and defined as the period in which consumer acceptability is maintained. The quality of most products changes over a period of time through changes to color, texture and flavor.

Shelf life is shortened by a number of factors, including: moisture, where a gain or loss can affect the texture and make a product go stale or go soft, or it can act as a catalyst to degradation in products containing fat; oxygen (which causes oxidation of products that contain fat or oil), and can assist in color changes and the onset of mold.

Light is another catalyst for oxidation that causes rancidity, and oils and fats to break down, causing odors and unwanted contaminating smells. Highly flavored foods are likely to lose aroma compared to bland foods that are likely to absorb odors. Additionally, the environment must also be considered. Secondary packaging, warehousing, transportation, distribution, temperature and in-store situations can all affect the shelf life of a product.

The table shown in Figure 2.1 is intended as a general guide to the list of food types that require protection, and shows why selection of the optimum flexible packaging material is important. It should be noted that products within these categories can vary in terms of need.

So, having discussed the basic requirements of flexible packaging and having a general knowledge of products that require protection, it now becomes possible to go back to the flow chart shown in Figure 2.2. This provides a basic guide to the main types of materials available - paper and paper-based materials, metallic foils, cellulose and plastics/polymer films.

FLEXIBLE PACKAGING PAPERS
While polymer films now dominate the usage of flexible packaging materials it has to be noted that papers continue to be important. They have a relatively low cost when compared with other materials. They are widely used in laminated webs (as light barrier materials), have a tactile feel and touch and offer good environmental performance, including low production energy levels, stiffness, breathability and cleanliness. They are additionally one of the easiest of the flexible packaging materials to recycle (over 60 percent re-use in Europe).

Various grades of paper are used for flexible packaging, representing different chemical (e.g., sizing) and physical (e.g. calendaring) treatments during paper making. Coatings on paper are used for visual and functional effects that enhance the material's utility in packages. Paper's dead fold and tear property combinations are unique in the array of substrates used in flexible packaging.

Paper properties in lamination with other materials - providing better stiffness and puncture resistance - can often provide cost effective and unique package performance attributes for customer applications. Along with Cellophane and some bioplastics, paper represents one of the few flexible packaging materials produced from renewable natural materials.

Flexible packaging paper grades can range from simple wrapping papers (frequently made from mixed recovered paper) to kraft papers, both of which are predominately supplied in rolls. They are made from various virgin pulps, from recycled fibers or from a mixture of chemical pulp and recycled fibers.

Whether coated or uncoated, matte or gloss, flexible packaging papers offer versatility alongside consistently high standards. Most grades are recyclable and available as FSC and PEFC certification on request, and there are grades suitable for a wide range of end-use applications - from shopping bags, food wrappers, dry and fatty food wrapping, small bags, soup packs, soaps, and tea envelopes to cigarette soft packs, inner liners and tobacco pouches.

Clay coated papers provide the ultimate printing surface and are used where multi-color printing is required for high-quality color printed results. They can also be laminated to a range of different films or foils to produce the finished packaging material.

In extrusion-coated formats, papers are used for the flexible packaging of dried food, prepared meals and savory snacks, while wrapping papers and paper sachets/pouches can be widely found in fast-food restaurants, cafés, coffee shops and for savoury snacks and baked products. Laminations with aluminum foil are also used within the food sector.

Food products that have perishable properties, such as butter, cheese, curd etc. are known as perishable food. Butter, soft spreads and cheeses are the best example of food items that all contain fats. When these products come into contact with moisture and atmosphere, they will lose their taste, color and aroma, requiring a quality food paper packaging material to keep all these intact. Uncoated grease and oil resistant wet-strength flexible packaging papers are also available for food applications.

On the other hand, food items like wheat flour, rice, tea, coffee, sugar, salt, seasoning, soup mix, snack foods, chips, noodles, etc. can be termed as non-perishable foods. These have a longer shelf life over other items such as curd or ghee but still they need quality packaging materials to further extend their shelf and consumer life term. Food paper grades must also fulfil the requirements of the relevant food packaging/labeling legislation.

Special paper packing materials can be produced and are available for most packaging purposes, e.g. greaseproof papers used for packing butter, margarine, meat, sausages, etc. Such papers include vegetable parchment and glassine and are provided with barrier polymers.

One-sided double coated flexible packaging papers, applicable for flexo, offset and gravure printing are available for use in both food and non-food applications, from dairy products and confectionery to tobacco. Such papers may have a rough back side, useful for lamination applications.

Vapor proof papers are papers that have been chemically treated or laminated with a vapor barrier that will resist the passage of gases or vapor through it, again, typically used for food packaging. Greaseproof wrapping papers are made from chemical wood pulps which are highly hydrated in order that the resulting paper may be resistant to oil and grease.

Outside of foodstuffs, papers used for wrapping soaps and detergents for example, will need to protect the consumer from coming into contact with the chemical ingredients of these products. Hot melt coatings, anti-mold treatment and anti-fungus papers are all likely requirements for soaps and detergents. The papers will also have to perform well on high-speed packaging lines.

When studying paper-based flexible packaging materials the key initial choice is usually between kraft papers - primarily used for paper bags and sacks - and wrapping papers, which are used on their own or in laminations with other materials, but particularly for food packaging applications.

Kraft paper sack and bag kraft are high strength papers made from sulfate pulp which are used to manufacture multi- and single-wall sacks and a range of bags, either in form-fill-seal applications or other automated packaging processes, or also in loose bags used to pack products at the point of sale, such as bakeries, street vendors, etc. Kraft bag and sack papers normally have a greater bulk, a high tensile strength, and a rougher surface than the more usual kraft wrapping papers which are used in a wide range of multi-substrate applications as well as in plain wrapping functions.

Pulp used to make kraft papers is stronger and darker than that made by other pulping processes, but it can be bleached to make a white pulp. Fully bleached kraft pulp is used to make high quality paper where strength, whiteness and resistance to yellowing are important. Sack kraft paper, is a porous kraft paper with high elasticity and high tear resistance, designed for packaging products with high demands for strength and durability. Figure 2.3 provides a summary of kraft paper properties, performance and applications.

Candy wrapping paper and twisting paper are primarily thin 30-40 g/m2 kraft papers and are mostly flexo or offset printed. These papers require a good strength, with highly oriented fibers. Twisting paper is mostly opaque and often supercalendered.

CELLULOSE AND BIOPLASTICS FILMS
Prior to the 1950s there was no availability of large quantities of thermoplastic film. The major transparent packaging film used at that time was a non-thermoplastic regenerated cellulose film. Made by chemically regenerating a dissolved cellulosic compound into a thin film or sheet, cellulosic film was commercialized under the trade name of Cellophane.

While the development of petroleum-based plastic films began to erode cellulosic films from the 1960s onwards, they nevertheless still remain an important and viable flexible packaging material today. Indeed, in the light of the Paris Climate Agreement, which was aimed at strengthening the ability of countries to deal with the impacts of climate change, the ongoing development of environmentally sustainable packaging materials has led to the market for cellulosic, bio-based and PLA (Polylactic acid) materials now increasing slowly but steadily - largely due to ever-increasing environmental and legislative concerns, and due to the largely unpredictable price of petroleum oil.

Recent years have certainly seen extensive research and development undertaken into bio-based packaging materials, especially in Europe, although at the present time, despite the ready availability of several bio-based solutions, most packaging products still heavily rely on fossil-based (polymer) materials which continue to contribute to enhanced emissions of greenhouse gases and generate a lot of waste.

However, cellulosic flexible packaging materials are forecast to have a bright future in bio-based packaging applications. Cellulose is a highly hydrophilic material due to a great number of hydrogen groups on the surface. Made using a dissolving-grade sulphite wood pulp, steeped in caustic soda, shredded and then converted to film-ready viscose which, in turn, is pumped to the wet-end of a film forming machine, the resulting cellulosic film is wound on a core ready for coating, slitting, or both.

Softeners may be added for dimensional stability for when the film is used in unsupported form or to provide best durability for bag and pouch applications.

Cellulosic coatings and films efficiently prevent the permeation of oxygen, grease, and oils - so giving the film its broad packaging functionality - but, on the other hand, provide very poor moisture barrier properties as water easily breaks the hydrogen bonds that hold the chains together. At high humidity conditions, the cellulosic materials tend to swell as a result of moisture absorption.

Without adding moisture-proof coatings, the cellulosic film will tend to lose moisture and can become brittle and shrink. If there is excessive moisture pick-up the film will lose its, otherwise excellent, gas and aroma barrier properties.

If the cellulosic film is to be used on high-speed packaging machines then heat sealability in the coatings used will become an important requirement. Uncoated cellulosic film generally has limited packaging uses, such as decorative wrappings. To supply the best technical solution and functionality, high barrier films such as NatureFlex are laminated to an internal sealing bio-polymer so that the final structure, certified compostable, can be heat sealed to compostable base trays made from bio-polymers or wood pulp.

NatureFlex films are produced from sustainable and responsibly sourced wood pulp harvested from managed plantations and are certified to both EU (EN13432) and US (ASTM D6400) composting standards. They are considered as the next generation of Cellophane film. In addition to industrial composting, the product has reached the standard required for home composting. NatureFlex films are engineered to provide a high barrier to moisture, aroma and gasses, have excellent transparency and high gloss; making them an ideal solution for a number of flexible packaging applications, compostable lidding structures, and as barrier layers for biolaminates.

More recently, a new generation of biodegradable films made from renewable resources (PLA bio-based resin) has been introduced as a key step in driving the flexible packaging industry towards even more sustainable solutions. The films offer the possibility to choose natural products and contribute to the reduction of greenhouse gas emissions and post-consumer waste.

In many applications today, biaxially-oriented PLA films can replace oil-based plastics like polypropylene, polyester or polyethylene, so providing two key environmental advantages: bio-based origin, certified by Vin.otte, and compostability according to the EN13432 norm.

The range of PLA films includes heat sealable transparent and solid white films and are designed to cover a wide range of food and non-food packaging applications, using existing converting and packaging technologies. Applications for PLA films are expected to continue to increase steadily.

Bioplastics are rapidly becoming one of the key raw materials to be used by flexible packaging manufacturers. Growing demand for bio-based PLA films in food, bakery, confectionery and snack packaging applications, owing to their easy recyclability and biodegradable nature, is expected to be a key market driver over the next seven years.

Of the newer bioplastic film materials, Cellulose nanofibrils (CNF), also referred to as nanocellulose, are seen as one of the most promising candidates for use as a sustainable material in the packaging industry. As well as being completely renewable, biodegradable, and safe to use CNF also possess exceptional physical and chemical properties. Unfortunately, the strength and barrier properties are highly dependent on the humidity conditions, hence further surface modifications have been developed to strengthen the position of cellulosic films in high-performance film applications.
A summary of the properties and applications for cellulosic, bio-based and PLA films are shown in Figure 2.4.

ALUMINUM FOIL AND METALIZED SUBSTRATES
Aluminum foils are used where their better barrier properties and aesthetic appeal give them the edge over flexible films or papers and are widely used in multi-layer flexible packaging laminations for products such as confectionery, ready-meals, pharmaceuticals, soups and sauces, preserved foods and liquid foods.

Foil has the best deadfold properties of the flexible substrates and provides an excellent 100 percent barrier to all gases and to moisture. It is also a good reflector of radiant heat.

For guidance on the thicknesses of foils it should be noted that household foil is typically 17.5 µm (0.0005 inches), but it can be made available in gauges as low as 7 µm (0.00028 inches). Below a gauge of 12 µm (0.0005 inches) pin holing starts to become likely. Aluminum foil is also susceptible to flex cracking when folded. Consequently, most foils are supported with plastic and/or paper.

Consequently, foil may be readily combined into multi-layer structures with any of the other flexible packaging materials to form composites that are then able to meet a range of specific end-use requirements. For example, the oxygen barrier is significantly improved - up to fifty times for BOPP films, and up to ten times for BOPET films, which are two of the most commonly metalized flexible packaging films, along with BOPA (nylon) films. These three films, and other flexible packaging films, will be described later.

Aluminum foil is also used to seal the opening on plastic jars as well as acting as a very effective tamper-proof lid. Aluminum foil die-cut lids prevent the product from deterioration and pilferage during distribution and storage. They can be crimped over the top of the jar or, more frequently, heat sealed to provide a more secure closure and sealing system.

For instance, in Figure 2.5, aluminum foil scores the highest in all the parameters that determine the shelf life of products, which clearly suggests that any laminate structure involving aluminum foil as one of the layers will be best in terms of barrier properties and shelf life of the product.

The use of aluminum foil lids offers secure seals for sealing plastic container for valuable products packed in a variety of plastic jars and containers, thereby providing products with super protection against pilferage and making them tamper proof. Applications for aluminum foil lids are as diverse as food stuffs, cosmetics, beverages, flavored drinks, powders and yogurt.

The properties and applications for aluminum foils and metalized papers and films are summarized in Figure 2.6.

PLASTICS/POLYMERS FILMS
The primary reason for the increasing popularity and growth of plastics in flexible packaging is the highly versatile nature of a wide variety of polymer films now available, which enables them to be converted into a large number of shapes, sizes, and designs, with a whole portfolio of performance characteristics. Also, plastics are more flexible, durable, and cost-effective than many other materials used for flexible packaging which has led to their increased adoption.

Film flexible packaging options available to the converter include both single and multi-layer films: film/foil/paper laminates; metalized films, film sealing materials; form-fill seal processes and pouch styles.

A great many of the flexible packaging films used today are biaxially-oriented. Indeed, most plastic packaging films tend to be oriented. The orientation process simply consists of stretching the film in one (monoaxial, eg. OPP) or two (biaxial, e.g. BOPP) directions. Orientation dramatically improves film properties, such as stiffness and tensile strength, while at the same time reducing elongation. Orientation also increases film yield.

Biaxially-oriented films undoubtedly play a major role in the flexible packaging industry, due to the unique combination of mechanical, optical and barrier properties they are able to offer. Between them, these films provide the state of the art, as well as the ongoing future trends, for the most important of the flexible packaging film types, such as BOPP, BOPET, BOPA, BOPS, BOPE and BOPLA. Each of these film types will be expanded upon.

Of the biaxially-oriented films mentioned, BOPET and BOPP in particular, can be regarded as the workhorses of flexible packaging films, principally in laminations, where they can be used in the as-oriented form, but are often vacuum-metalized for applications where the films themselves do not offer sufficient barrier protection. BOPP is widely used in salty, dry snack packaging - as it offers better moisture vapor barrier than BOPET. BOPP also has the lowest density of the commonly oriented packaging films.

In addition, there are various additives that can be introduced into flexible packaging films during the film manufacturing process. These include:

• Process stabilizers (heat stabilizers)
• Environmental stabilizers (anti-oxidants, UV stabilizers)
• Surface modifiers (slip agents, anti-static, anti-blocking)
• Optical modifiers (pigments, nucleating agents)
• Functional additives (mechanical property enhancers).

BOPET (Biaxially-oriented polyethylene terephthalate) is a popular surface film in laminations where its superior stiffness, chemical inertness, heat-resistance, and oxygen-barrier properties - and a wide operating window (compared to BOPP) - make it a good choice for a wide variety of products. Produced by biaxial orientation of PET resin, BOPET has proved to be particularly suitable for converting and various other industrial applications.

Available as a clear, transparent or translucent material, BOPET film is manufactured commercially in a range of widths, thicknesses and properties depending upon the needs of end users. It can be made as a single layer or can be co-extruded with other co-polymers into a multilayer film encompassing the desired characteristics of each material. They are also said to offer the highest tensile strength of all the packaging polymers. They have good moisture and gas barrier properties and low elongation.

Biaxial orientation of PET film makes it suitable for such applications as food packaging by increasing the product's crystallinity and thereby improving its tensile strength, heat resistance, and gas-barrier properties. The distinct physical properties of various types of PET film can be imparted into the product either during the polymerization of the PET resin, by the addition of chemicals such as slip modifiers (surface modifiers) or color additives, or subsequently during the PET film production process where various surface finishes may also be imparted by externally treating the film's surface(s).

More stable through printing and laminating processes than BOPP, it is often preferred where high-quality graphics are required. It is the surface film of choice for retort pouches because of its dimensional stability through retort

The improvements in mechanical, optical, and barrier properties of oriented films makes them compelling choices for flexible packaging structures. Of the biaxially-oriented films, BOPET offers performance at low thickness, high stiffness, good heat-resistance, and a reasonable balance of oxygen and moisture vapor barrier.

The largest application of thin BOPET films is in flexible packaging. White BOPET is used as a lidding material for dairy goods such as yogurts; clear BOPET finds applications in lidding for fresh or frozen ready meals; laminates containing metalized BOPET are used to protect food against oxidation and aroma loss in products such as coffee packaging and pouches for convenience foods. A summary of the properties and applications for BOPET films is shown in Figure 2.7.

BOPP (Biaxially-oriented polypropylene) is another of the commonly-used plastic film materials, this time made from stretched polypropylene (PP) resin. It offers superior moisture vapor barrier properties and has the lowest density of the BOPET, BOPP, and BON triumvirate. Against this, it has a poor oxygen barrier.

BOPP itself is a low density high performance film and has excellent mechanical, optical and barrier properties and can be regarded as one of the most efficient and competitive biaxially-oriented film solutions for many flexible packaging applications.

With a relatively low cost, easy processability and good chemical compatibility, BOPP films are an attractive product for food pouches and bags, clear wraps and most snack foods. With a higher softening point than, say, polyethylene (PE) it becomes suitable for hot filling. The film is often metalized and printed.

Recent studies predict that BOPP films are forecast to grow at 5.8 percent CAGR up to 2024, with the food packaging sector anticipated to be the fastest growing application segment, driven by the high demand for BOPP films for the production of a wide variety of package types and labels required in the food industry and tapes for industrial packaging purposes.

Variations of BOPP films include transparent, metalized, solid white, cavitated and matte films and these are available with certificates of compliance with EU and USA legislation for material intended to come into contact with food.

A summary of the properties and applications for BOPP films is shown in Figure 2.8.

BOPE (Biaxially-oriented polyethylene). Before looking at BOPE films it is perhaps useful to have a better understanding of polyethylene itself. Polyethylene is a film material manufactured from ethylene - predominately from natural gas or petroleum - and, although it can be produced from renewable resources, it is not readily biodegradable.

Polyethylene packaging doesn't allow water vapor to pass through, meaning it can seal easily contaminated products away from dangerous elements. Many kinds of polyethylene packaging can be heat sealed, meaning the material can be wrapped tightly around the product and secured with an airtight seal.

Overall, polyethylene is the most commonly used plastic in the world and there are many variations and grades. Different grades of polyethylene can each have a different molecular arrangement that enables the production of different families of PE films - low-density polyethylene (LDPE), high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE).

Higher density polymers have their molecules more closely packed and are stiffer in nature, while low density polymers have loosely packed molecules are more flexible. Such are the properties and applications of LDPE, LLDPE and HDPE, that they are probably best considered as different films with different characteristics. Each of the films can be biaxially-oriented.

LDPE films are relatively low cost and easily processed. They are soft and clear, have the lowest softening and melting point (making them good for heat-sealing), are compatible with most foods and household chemicals, provide a fair moisture barrier (but poor oxygen barrier) and have a very high elongation. Being a very flexible material LDPE is particularly suitable for applications like shopping bags. Indeed, everything from food products to nails are packed in polyethylene bags - perhaps more commonly known as Poly Bags.

LDPE-based flexible packaging films are regarded as ideal for perishable goods, like packaging of food items, pharmaceuticals, liquids, pesticides, spices, edibles, industry goods, cosmetics etc. LDPE products are durable and moisture proof and are frequently combined with other films (such as PP) to give them heat sealability.

LLDPE film is very similar to LDPE, but with the added advantage that the properties of the film can be easily changed by adjusting the formula constituents. The overall production process is less energy intensive than LDPE. The film provides more stretchability and is tougher than LDPE. It also has a better heat-seal strength.

HDPE is a strong, high density, moderately stiff plastic with a higher temperature resistance and much improved water vapor barrier than LDPE, but is hazier. It is frequently used for applications such as laundry detergents.

Overall, polyethylene resins can provide a range of benefits, including: helping to keep food fresh, enhancing barrier properties and providing product protection. It's also inexpensive and highly chemically resistant. It can either be used by itself or combined with other substrates to provide barrier performance. High barrier structures can be produced by co-extruding PE, and PE with high barrier materials, such as nylon.

BOPA (Biaxially-oriented polyamide) is a widely used resin for the production of flexible packaging films, providing the best oxygen barrier performance, a good barrier to chemicals and aroma substances, and exceptional toughness and puncture-resistance. Standard versions of BOPA films, which tend to offer a good balance in mechanical properties between MD and CD, are widely sourced and specified for their unique contributions to packages.

BOPA films are increasingly in demand and widely used or the packaging of perishable food (especially fatty and oily food) due to its unique combination of properties. It is a highly transparent packaging film, which is also used for distilled goods packaging, agricultural products packaging or medical products.

A commonly used generic name for the polyamide family of polymers is ÔNylon', DuPont's original brand name.

A summary of the properties and application for BOPA is shown in Figure 2.9.

BOPE (Biaxially-oriented polyester) films are highly appreciated for their features like moisture proof and durability. These are manufactured using high grade materials and advanced technology.

PVC films provide a range of twist wrapper, shrink labels, cosmetic shrink labels, flexible packaging materials and multi-purpose pvc bags. PVC twist wrappers come in a bright and quality construction finish and can be provided in widths from 10 to 1500mm and thickness of 20 to 100mic. Further, they can be provided in transparent or color finishes.

BOPS (Biaxially-oriented polystyrene) is hard, stiff, brittle, and crystal clear material which is readily expanded with gases to make expanded PS. It provides poor solvent resistance (can be solvent bonded) and poor overall barrier properties and is most commonly found in the form of expanded PS label stock or used for envelope windows.

LAMINATES, BARRIER MATERIALS AND SEALABILITY
There is no perfect, universal flexible packaging material, which means that many materials used for flexible packaging involve the use of multi-layers which are created by bonding together two or more materials, whether plastic, paper or foil. An ideal laminate for a particular application assembles materials with individually desirable properties to create an optimum performance combination.

There are generally three components in a flexible packaging bonded or laminated structure - the exterior, the barrier, and the sealant. The exterior layer is the print surface. The second component is the barrier layer (or layers) 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 multi-layer structure on the side that comes in contact with the product. There are a number of heat-sealing materials that the converter can use. These are shown in Figure 2.10.

In flexible packaging the term barrier is most commonly used to describe the ability of a material to stop or retard the passage of atmospheric gases, water vapor, and volatile flavor and aroma ingredients. A barrier material, as earlier described, is one that has been designed to prevent, to a specified degree, the penetrations of water, oils, water vapor, or certain gases, as desired. Barrier materials may serve to exclude or retain such elements without or within a package.

There are many variations in the properties of substrates from different manufacturers. Substrates with an improved barrier are generally more expensive and more difficult to obtain. So, it is important not to over-specify the barrier required, just to play safe.

However, unless there is good seal integrity there is no point in spending money on a barrier film.
Whether the sealant is a coextruded heat seal layer, a coating, cold seal, CPP or PE, it must hermetically seal the inside of the pack from the outside.

As narrower web printers move into flexible packaging they must understand barrier and sealant layers and make sure they understand how the film will be sealed, and which side seals to which side, so that they can purchase the appropriate films. Most flexible packaging films are heat sealed, but not all sealant layers are compatible. Generally, sealant layers will always be compatible with themselves.

But do not assume that side A will seal to side B. Ensure to understand how the film will form into the package, and where there is an A to B seal make sure the two sides will actually seal, and there is no ink or lacquer preventing this.

When combining two or more films to make a laminate (Figure 2.11), the actual barrier performance created will be a combination of the barrier of the top and bottom (or more) layers. This is a complex issue as the converter actually needs to have the equipment to measure the above properties. Ideally, the aim would be to work with one film that provides the necessary barrier, with the other being used just to protect the inks.

With regard to laminates, it should perhaps be noted that ever greater importance is being placed on the development of sustainable packaging. Some of the key flexible packaging producers are working extensively to protect the environment through producing laminates which do not contain aluminum foil, but are made from the mono polymer family. Such laminates can be easily recycled and better fit in to today's more circular economy.

Lamination can be undertaken using either extrusion coating or adhesive laminating.

Extrusion laminating is a process in which layers of multi-layer 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 multi-layer packaging materials are laminated to each other with an adhesive.

As previously discussed, a substantial barrier can also be achieved through coatings or metalization. Film manufacturers have been applying these for years. However, converters do have the opportunity to apply their own barrier to film. Originally, these coatings were the same as those applied by the main suppliers of coated film. Now however, there is a newer type of coating available which allows gravure and standard flexo converters to achieve the highest levels of barrier in a flexible coating that is not subject to flex cracking. These are based on mica or silica particles that provide a difficult path for the water vapor or oxygen particles.

However, a word of caution. If the coating is undertaken in-house, the converter then becomes responsible for the barrier and the protection of the food in the package, and there is a risk of very high claim levels if the film underperforms.

GETTING STARTED
One of the first challenges that the flexible packaging converter will be faced with is deciding what paper, film or films, metalized film, etc., to use or include in a flexible packaging structure with barrier performance. Plenty of polymers, barrier materials, coatings and sealants are available and in common use in the industry. A basic understanding of the material choices available (set out in the preceding pages) and the requirements of different product, commercial and technical considerations can undoubtedly simplify the selection process. These can perhaps best be summarized in the table in Figure 2.12.

If it helps to simplify decision making then Figure 2.12 can be used in the form of a tick box to ensure that all the essential elements in materials selection have been identified for a particular job, then specified and quantified. But do not over-specify. That will quickly lead to increased costs and being uncompetitive.

On the other hand it is essential to be sure to include all the elements that are required for successful substrate, pack, print, packing line, quality and end-use performance. A failure to include important items can lead to failure at one or more stages in the whole integrated flow line process. The aim should be to ask the right questions and agree the key specification requirements (Figure 2.13) with the customer . not only what matters, but also what is known to work. To a large degree this comes down to experience.

The aim should be to agree all the key materials specification requirements with the customer on what has to be delivered at an appropriate and acceptable cost. It is important to understand that the value of a substrate has to be set in the context of the end use. High-value goods with possible high failure costs can perhaps justify a more costly, higher-performance packaging material so as to guarantee a secure and fully functional delivery.

Lower-value commodity type goods may enable the selection of a lower pack performance and, where consumer or industrial end customer switching costs are small to non-existent, a critical need for cost efficiency may drive the selection to a lower package performance and overall cost-in-use.

The correct substrate specifications and selection will undoubtedly ensure a happy customer. Poor or wrong substrate selection can adversely affect the supply relationship, and may result in high wastage, rejected goods, higher costs and even non-payment.

No flexible packaging supplier can guarantee that its package is suitable for every type of product under all possible usage conditions. However, if the product manufacturer and the packaging materials supplier, ink and coating manufacturer and the converter all effectively collaborate, they usually can discover a successful solution. It is therefore crucial for all parties to maintain open communication lines and have a clear understanding of accountability for different aspects of product performance, quality, safety, standards and the environment.
The impact of packaging waste on the environment can be minimized by prudently selecting materials, following EPA guidelines.