Introduction

A film is a semi-permeable material that is self-supporting, yet is thin enough to be flexible without breaking. Thickness is generally less than 0.003 inch (< 75 micrometers), and in the US, the thickness is often designated by gauge. Most often, flexible films are manufactured from synthetic polymer resins through the processes of extruding and casting, or they are air-cooled or water-quenched blown.  

History

When, in 1908, Swiss Chemist and textile manufacturer Jacques E. Brandenberger invented his machine and process for continuous production of a strong, transparent, cellulose film, he set a world of more effective, flexible barrier solutions in motion. 

Overtaking metal foil as the most commonly used packaging barrier material of the time, the cellulose film patented by Charles H. Stern in 1898, gained the name cellophane, and, in 1920, landed in the hands of Dupont. (Bellis) Here, its full potential was realized, and its water-resistance improved. Cellophane became the standard for barrier film in numerous packaging applications worldwide.

But a new revolution in barrier technology was on the horizon. After dominating the flexible film market for close to 10 years, Cellophane production began to give way to the synthetic polymer industry’s scientists, who were rapidly advancing flexible film innovations of their own.

Polyvinyl Chloride (PVC) was at the forefront of this polymerization revolution, though there was a lapse of 40 years between its invention by German chemist Eugen Baumann and its patent as an invented polymer by Friedrich Klatte in 1913. Moreover, another 13 years passed before PVC was made useful through advanced polymerization techniques developed by Waldo Lonsbury Semon in 1926. (Bellis) 

Polyvinyl Chloride resin has since been used in the production of puncture-resistant, semi-permeable flexible film for industrial stretch film and food, non-food, and pharmaceutical packaging applications.

In the 1940s, the demand for industrial and commercial supplies outpaced the availability of natural raw materials. (SPE) New synthetic resins and advances in their manufacturing introduced innovative solutions to meet the needs of all industries. By the 1950s, a variety of synthetic polymer films were becoming more widely available on the market, and new applications for them were conceived of every day.

Polyethylene (PE) invented in 1933 by E.W. Fawcett and RO. Gibson became one of the first synthetic resin derived films to see wide distribution, beginning in the 1950s. The offspring of PE, HDPE, LDPE, and LLDPE, with their ease of processing, high tensile strength, flexibility, and excellent water barrier performance, are among the most widely used plastic films in the food and packaging industry today.

Polyethylene Terephthalate (PET), also known as Polyester, was the invention of Rex Whinfield and James Dickson, based on the previous work of Wallace Carothers. In 1941, PET stepped into the barrier film landscape, bringing with it, strength, high gas barrier performance, and an ability to withstand high temperatures. PET flexible film has gained popularity as a barrier solution within both the medical and food industries. (Hoppe)

The 1950s also introduced the moisture, fat, and chemical resistant qualities of Polypropylene film to the market, thanks to Paul Hogan and Robert Banks. Though a poor gas barrier, Polypropylene found solid footing within the food and shrink wrap industries, and, often, the gas barrier quality of PP is improved through the application of a PVDC coating.

By 1985, the flexible film industry had certainly been revolutionized. Cellophane had been replaced as the barrier of choice in many industries. Synthetic polymers decreased the manufacturing cost of flexible films and increased their effectiveness with enhanced mechanical performance, as well as improved water, gas, and oxygen exchange qualities. 

Film Technology

A film is a semi-permeable material that is self-supporting, yet is thin enough to be flexible without breaking. Thickness is generally less than 0.003 inch (< 75 micrometers), and in the US, the thickness is often designated by gauge. Most often, flexible films are manufactured from synthetic polymer resins through the processes of extruding and casting, or they are air-cooled or water-quenched blown.

The function of a film is to act as a barrier to inhibit, prevent, or moderate the exchange of oxygen, water vapor, gas, odor, chemicals, fats, or flavors between two substances. 

Films are also used to protect, mask, or lend specific qualities to a material or to the surface of a substrate. While a flexible film’s post-production structure is that of a thin, flat continuous layer, it can be comprised of a single polymer, a copolymer blend, or multiple coextruded or laminated layers.

Films are formulated and manufactured to achieve or enhance mechanical or barrier qualities according to their end-use application. Strength, puncture and tear resistance, heat resistance, permeability, visual characteristics, printability, and slip, are only a few of the qualities that flexible film manufacturers consider.

Film Types

Since the purpose of a flexible film is to function as a barrier, with particular emphasis on the food and pharmaceutical industries’ specific tolerance requirements, film types can be categorized in terms of inherent barrier qualities.

Low-barrier Films

Low-barrier films are films with a high exchange rate in one or more aspects, such as Polystyrene (PS), Polyethylene (PE), Polypropylene (PP), and Cellophane. Though they may exhibit excellent mechanical properties in some manner, their vulnerability to water vapor, gas, or oxygen exchange may render them undesirable for specific applications in their base form. However, high permeability might be a requirement in some circumstances, in which case, they would make suitable single polymer films or, more likely, good candidates for copolymer or multilayer solutions. For instance, a film designed for dressing wounds would need to be resistant to tearing, but maintain high gas and oxygen permeability to promote healing.

Average-barrier Films

Average-barrier films have neither low nor high exchange rates of oxygen, gases, or water vapor, including Polyvinyl Chloride (PVC), Polyamide/Nylon (PA), Polyethylene Terephthalate (PET). 

PVC film is semi-permeable and allows enough oxygen exchange to maintain the freshness of red meat, where PA allows for more carbon dioxide exchange than oxygen, making it ideal for cheese. PET, has excellent mechanical qualities, but an average exchange rate is often metalized for food and pharmaceutical packaging, where its higher heat resistance makes it more suitable over other resins.

High-barrier Films

High-barrier films have very low exchange rates and are usually manufactured as coated films, coextrusions, or laminates. Polyvinylidene Chloride (PVDC), Ethylene Vinyl Alcohol (EVOH), Polyacrylonitrile (PAN), PolyAmide MXD6 (PAMXD6), and Coated Cellophane all have one or more excellent barrier qualities. 

They are, however, more expensive polymer resins, so are most often applied as coatings to improve another polymer’s performance. Or, they are a constituent of a specialized barrier created for a specific application, as in modified atmosphere packaging for food preservation.

Film Compositions

The lion’s share of resins used in the manufacture of flexible film belongs to the synthetic polymer families, which are derived from petroleum oil. Though advances are being made in the development of new cellulose and starch-based bioplastics, they have yet to see a wide-spread application in the production of flexible films. The diverse barrier qualities of synthetic polymer films remain unmatched, but recyclability is increasingly manufactured into plastics, as well as the intent to continually down-gauge.

Cellophane Films

Cellophane is made from regenerated cellulose derived from wood pulp, which is processed then plasticized with glycerin. Its high water vapor permeability and low gas permeability make it ideal for applications where a breathable film is required, such as packaging for bread. It’s a clear, glossy film that is printable with conventional methods, and also resistant to oils, grease, and water. It is a stiff material, yet it’s tearability is high, which makes it suitable for some industrial applications. 

Cellophane film is used primarily for retail food packaging and is valued for its permeability and compostability. To improve its permeability as well as make it heat-sealable for broader applications, cellophane is sometimes coated with PVDC.

Polymer Resins

Given the diversity of monomer combinations possible and the array of manufacturing techniques to manipulate their linkages, thousands of synthetic polymers and copolymers with a wide variety of mechanical and barrier properties are available. Through the addition of plasticizers, coupling agents, processing aids, and stabilizers, many of the most commonly produced resins are further transformed in composition and function.

Polyvinyl Chloride

Polyvinyl Chloride (PVC) film is achieved through the addition of small plasticizing molecules that reduce intermolecular forces to improve the elongation of this otherwise rigid material. 

PVC is a versatile and cost-effective thermoplastic with good heat, puncture, and impact resistance that is printable by conventional means. It’s a polymer film that’s less resistant to chemicals, but with excellent resistance to oils and grease. It’s high oxygen permeability, clarity, and cling make it desirable for use in the food industry. 

PVC is one of the most widely produced plastic films along with PE and PP and is used for shrink-wraps, liners, adhesive backings, and for a variety of applications within the medical industry, including blood and IV bags.

Polyethylene

Polyethylene (PE), the most common thermoplastic produced, is generally formulated with additives to improve functionality. It can be manufactured by polymerizing ethylene to produce a homopolymer or ethylene combined with another monomer to create a copolymer blend. 

As a homopolymer film, PE is highly ductile and impact-resistant, but with low tensile strength and tear resistance. It is a good water barrier, but with high oxygen permeability and low melting temperature. These attributes make PE a general use plastic film, highly suitable for disposable shrink-wrap, refuse bags, or similar applications.

When formulated with the addition of terephthalic acid, PE becomes the polyester Polyethylene terephthalate (PET). PET is also considered general-purpose, but with improved strength, heat resistance, and barrier qualities, this film finds a broader range of applications in the retail and medical industries.

HDPE & LDPE

High-Density Polyethylene (HDPE) and Low-Density Polyethylene (LDPE) are achieved through reducing or increasing the degree of PE molecular branching. 

During HDPE polymerization, free radical formation is influenced by a catalyst and controlled reaction condition that results in less branching and, therefore, more densely compacted PE molecules. This process improves moisture barrier, temperature resistance, as well as increased stiffness of the film to make down-gauging possible, though clarity may be somewhat compromised. 

Free-radical polymerization of LDPE, on the other hand, initiates a high degree of branching with less compact crystallization and weaker intermolecular forces. This improves ease of processing, but most importantly, heat-seal ability.

Polypropylene

Polypropylene (PP) film is used for many of the same purposes as PE films, and shares close to equal popularity across industries. But with applications requiring high-temperature sterilization, Polypropylene is more suitable than Polyethylene. 

Polypropylene is a low-cost thermoplastic of high clarity with good moisture barrier and tensile strength properties. When metalized or PVDC coated, its average gas barrier quality can be improved for use in extended shelf-life packaging. 

Its tensile strength, breathability, and resistance to moisture make it a desirable solution for sterile wound wrap and other biomedical applications, as well as for FDA approved pharmaceutical packaging. 

Ethylene Vinyl Acetate 

Ethylene Vinyl Acetate (EVA) and Polyethylene Vinyl Acetate (PEVA) are referencing the same copolymerized resin of ethylene and vinyl acetate. 

This inert, tough, and highly elastic thermoplastic has gained wide use as a copolymer constituent, primarily due to its adhesion quality being of benefit in multilayer films. It is also valued as an addition to olefin copolymers, such as PE, as a melt temperature reducer to enhance sealability. 

With poor water and gas barrier qualities, EVA is rarely used on its own. In copolymer blends, the concentration of EVA can reduce the barrier qualities of companion monomers, but it can also improve their clarity and impact resistance.

Ethyl Vinyl Alcohol

Ethyl Vinyl Alcohol (EVOH) is a thermoplastic copolymer with exceptional oxygen, carbon dioxide, and nitrogen barrier properties; however, the presence of water will negate that advantage due to its hydrogen bonding ability. As a result, it is most often used in copolymer PE blends rather than alone. 

Resistant to chemicals, oils, and solvents, EVOH multilayer film is utilized for medical and pharmaceutical packaging, and its unique barrier properties are valued in the production of specialized packaging requiring a modified inner atmosphere.

Polyamide

Polyamide (PA) or Nylon films are good aroma and oxygen barriers, but with high water permeability. Similar to Polypropylene, PA has a high melting temperature, which makes it suitable for applications that involve heat. It is an easily manufactured, high tensile, tear-resistant film. 

Though it can be processed as a homopolymer, it most often serves as an element for lamination onto low oxygen barrier substrates, like paper, or is coextruded to impart its temperature and barrier properties to copolymer blends.

Film Structures

Films are structured during manufacture to enhance or inhibit the inherent characteristics of the polymers according to the film’s intended use. Homopolymer films consist of identical monomer units that may or may not include coloring agents or other secondary, non-polymer additives. These single monomer resin films can subsequently be laminated to become multilayer films, and though some qualities may be enhanced, they remain a homopolymer product. 

If an adhesion layer is required, for example, PE laminated with a Polyamide, the film becomes a copolymer composite. Copolymer films can contain two or more unique monomer units that are combined at the molecular level during melt. Copolymer blends can also be combined at the micro-level through coextrusion, such as HDPE coextruded with EVA, a film used in pharmaceutical packaging applications.

Though some polymer characteristics call for one structuring process over another, as with polymers prone to heat degradation, coextrusion for multilayer films is often preferred. Coextrusion allows for single-step processing that lowers manufacturing costs and eliminates the need for adhesives or coatings in some polymer blends. 

However, lamination is the only way to achieve superior mechanical and barrier performance from some copolymer composites, as with Polyethylene-Cellophane or Polycarbonate-Polyethylene films.

Manufacturing Methods

The three most common manufacturing methods for flexible films are extrusion, casting, and blown. 

Extrusion

Extrusion is the process of melting the plastic pellets, granules, or powder in a heated extruder chamber by means of an internal, continuously revolving screw. The molten polymer is then forced through a die opening and fed onto a conveyor belt, rollers, or immersed in water for cooling. The means by which the polymer is cooled directly impact crystal formation, which, in turn, affects the film’s functional properties. 

Calendering is a rapid cooling method of extrusion, whereby the warm plastic is immediately cooled on chilled polished rollers. This technique is used most often for high volume PVC production, though the long-term durability of the end product is greatly reduced.

Cast-extrusion

In the cast-extrusion process, the molten polymer is filtered then fed through a die, directly onto water-filled quenching rolls. The edges are then trimmed, and if they are required, surface treatments are applied in-line before the film is drawn onto the winders. If the polymer is recyclable, the waste from the trim can be reused. If it is not, this becomes a disadvantage of the cast-extrusion process. 

However, due to the efficiency of the rapid cooling system, the overall production of film is increased. Cast films are well suited to plastic intended for thermoforming, due to their high clarity and elongation properties. But as a result of amorphous crystallinity, they may exhibit lower tensile strength and barrier performance.

Blown

A blown polymer film is created by extruding a ring of semi-molten plastic up through a vertical chamber where it is stretched by a bubble of air into a tube. The cooled, flattened tube is then nipped, wound, and slit to create the rolls of film. 
Blown film can be water-quenched or air-cooled depending on the desired crystalline structure, and like extruded and cast processes, incorporate coextrusion of polymers as well. 

One of the biggest advantages of blown polymer film is the ability to easily produce biaxially oriented films. Though this bi-directional stretching is also achieved with rollers in both extrusion and casting processes, blown manufacturing achieves it more efficiently. 

This technique of modifying the molecules’ orientation as the film cools produces polymers such as BOPP (Biaxially Oriented Polypropylene), BOPET (Biaxially Oriented Polyethylene Terephthalate), and BOPA(Biaxially Oriented Polyamide), with highly improved transparency, strength, and barrier properties.

Modification Techniques

There are many reasons a flexible film would need to be modified and an abundance of methods for doing so. Altering molecular orientation through bi-directional stretching, layering of polymers for improved barrier performance, and changing color, texture, or transparency, are all ways in which polymers are customized to meets specific application requirements. 

Surface treatments are the most commonly applied modifications and can broaden the scope of a film’s usefulness by degrees. Adding a coating to a film, as with CoPET, or increasing dyne level through corona treatment at the time of production enables ink to adhere to the surfaces of otherwise unprintable polymers. 

Metalized films are another form of treatment, which enables low barrier films to perform well in perishable content applications. 

And a more recent and innovative surface modification that has gained momentum is anti-microbial coatings that inhibit bacteria, yeast, and mold for use in hygienic environments such as the medical and food industries.

Pricing

The cost of flexible film can vary widely depending on the constituent polymers, film structure, treatments required, and production method used. Additionally, polymer prices will fluctuate according to the availability of raw materials and rise or fall in market demand.

The Future of Films

The landscape of flexible film manufacturing is changing. Consumer demand for sustainable plastics is driving the development of biodegradable, compostable, and recyclable polymers. Technological advances within the electronic, biomedical, and pharmaceutical industries are challenging producers to create ultra-thin deposition films and functional films with “smart” capabilities. With the US barrier film market forecast to grow to $5.7 billion in 2021, the future of flexible film looks pretty clear.