COATING AND LAMINATING

Coating and laminating are two of the most widely used processes for transforming flexible films and sheets into products having properties useful in food packaging. Coating is the process of apply-ing one or more layers of a fluid or melt to the surface of a material, while laminatapply-ing is the bondapply-ing

of two or more webs (Brown, 1992). A laminate is defined as any combination of distinctly different plastic film materials or plastic plus nonplastic materials (typically paper and aluminum foil), where each major web is generally thicker than 6 μm, regardless of the method of manufacture. There is no upper limit to the possible number of webs, but two is the obvious minimum and one of these must be thermoplastic.

Two major techniques are employed in the fabrication of laminates: adhesive lamination and extrusion coating. However, before coating and laminating films, it is necessary to treat the surface to ensure good adhesion; four treatments are used commercially.

5.3.1 S^URFACE T^REATMENT 5.3.1.1 Surface Energy

The surface energy of a packaging film is a critical film property when printing, coating or laminat-ing packaglaminat-ing films. Surface tension γ has the dimension of force per unit length (mN m⁻¹ which is equivalent to dyn cm⁻¹), or of energy per unit area (mJ m⁻²). The two are equivalent but when refer-ring to energy per unit area, the more general term surface energy is used as it applies to both solids and liquids. Surface tension is responsible for the shape of liquid droplets. For a liquid, the surface tension and the surface energy are identical. For example, water has a surface energy of 72 mJ m⁻² and a surface tension of 72 mN m⁻¹.

The main objective of any surface treatment method is to increase the surface energy of the film surface to improve wet-out and adhesion of coatings, inks and adhesives used in converting the film into packaging. Wet-out refers to how completely a coating or adhesive flows and covers a surface to maximize the contact area and the attractive forces between the adhesive or coating and the film surface. Typical polymers used for packaging films have a surface energy without surface treatment of from 29 to 45 mN m⁻¹. A typical rule of thumb to insure good wet-out and adhesion to a substrate is that the surface energy of the substrate should be 7–10 mN m⁻¹ higher than the surface tension of the coating being applied. For example, water has a surface tension of 72 mN m⁻¹ compared to the surface tension of ethanol which is 22 mN m⁻¹. The surface energy of a PET film is ∼42 mN m⁻¹, thus allowing the ethanol to easily wet out the surface of the film. However, the water, which is 50 mN m⁻¹ higher than the ethanol, will bead up (form a sphere, which is the smallest surface area for a given volume) and not wet out the film. There are several different ways of surface treating a film to increase the surface energy and, thus, improve wet-out and adhesion of coatings, inks and adhesives, including corona discharge, flame treatment, priming and chemical etching.

5.3.1.2 Corona Treatment

Corona treatment involves the application of a high voltage (10–40 kV) low frequency (10–20 kHz) discharge across a fixed air gap between an electrode (treater bar) and an earthed conductive roller that carries the film. The air between the two surfaces ionizes and consists of positively charged ions, electrons and excited or metastable species of O₂ and N₂, as well as other forms of radiation.

In addition, metastable O₂ species react with O₂ molecules to generate ozone, a powerful oxidizing agent. Particle energies are about 10–20 eV, which is high enough to break C–C and C–H bonds (2.54 and 3.79 eV, respectively) and generate free radicals in the polymer-surface region. A continu-ous arc discharge (corona) is generated at the surface of the film.

A corona generator is usually mounted in-line and prior to the extrusion coating head; the degree of treatment is regulated by rheostat adjustment of the power fed to the discharge electrode. Treatment will decay over time and is adversely effected by high humidity conditions. In continuous operation, the corona discharge appears to be a random series of faint sparks in a blue–purple glow (UV radiation).

The corona treatment cleans, oxidizes and activates the surface by converting it from a nonpolar to a polar state by bombarding it with ozone, O₂ and free radicals of O₂. This increases the surface energy and results in the substrate surface being more compatible with a freshly oxidized polyeth-ylene surface, thus promoting adhesion between the two.

Pretreating substrates with corona discharge is a widely used method for polyolefin films. Corona treatment has been found to increase the O₂ content and carbon–oxygen functionalities on LDPE (Hirvikorpi et al., 2010a).

Corona-treated films should be used immediately for further applications such as extrusion coat-ing or printcoat-ing because of the diminishcoat-ing effect of the improved properties with time (Ozdemir et al., 1999). Ozone is a by-product of the corona discharge method, and provision must be made for its removal for health and safety reasons. Corona treatment is often done during film manufacturing and can be done again in-line with a secondary converting process such as printing to “bump” or increase the film surface energy. For printing and laminating, it is common to increase the surface energy of an LDPE film from 34 mN m⁻¹ to close to 50 mN m⁻¹.

5.3.1.3 Flame Treatment

In flame treatment, the polymer surface is passed through a flame (1000°C–2800°C < 1 s) generated by the combustion of a hydrocarbon (HC) (typically natural gas). The film passes directly through the flame tips that have formed an O₂-rich plasma. This produces an oxidized layer on the surface by a mechanism similar to that of corona discharge but it is more difficult to control; if the heat pen-etrates too far into the film, it degrades and becomes weak. This treatment method is said to produce high surface energy levels and longer lasting treatment levels than corona discharge.

5.3.1.4 Priming

In this method, primer treatments are applied in very thin coatings to the substrate. Optimum primer coating weight is usually in the order of a few milligrams per square meter. The primers seem to function by being of such a chemical nature that the oxidized, extruded polyethylene surface adheres strongly to them, and the primer, in turn, adheres strongly to the substrate. If the primer coating is too thick, loss of adhesion results since the primers have low cohesive strength. Primers consisting of polymers such as styrene–butadiene latexes are often applied to paper to prevent coat-ings from penetrating too deeply into the substrate (Brown, 1992).

5.3.1.5 Chemical Treatment

Treating polymer surfaces with chemicals can alter the surface chemistry to improve the surface energy by providing active chemical bonds or groups on the polymer surface.

Chemical treatment of a film typically involves cleaning, etching and rinsing steps. The cleaning removes any surface contaminants. The etching involves the use of acid, base or oxidizing agents such as nitric acid or potassium chromate to chemically change the polymer surface. Finally, the film is rinsed clean of the etching chemicals and dried. This process is usually done following film manufacturing, which significantly adds to the final cost of the film. This treatment method is often slow and creates waste disposal issues as the chemicals used are problematic in terms of handling and environment.

5.3.2 C^OATING P^ROCESSES

There are two types of coating processes: in one, an excess coating is applied to the web and the surplus removed, while in the other, a predetermined amount is applied to the web using rollers or other equipment. Nitrocellulose and PVdC copolymer are the most common surface coatings used, but synthetic resins, acrylics and many other formulations are used for varnishing, barrier formation and/or heat sealing.

Extrusion coating was first practiced on a commercial scale in the production of LDPE-coated paperboard for milk cartons in the mid-1950s as a replacement for wax-coated board (Robertson, 2002). Compared to wax, LDPE is superior with greater strength, seal integrity and resistance to cracking and flaking off. It also provides greater resistance to moisture, thus protecting the paperboard substrate from the damaging effects of water for much longer periods of time.

Today,  almost all applications for wax-coated paperboard have been replaced by polyolefin-coated paper and board.

In theory, there is no reason why any thermoplastic that is normally processed by extrusion tech-niques cannot be coated onto paper or other substrates. In practice, most extrusion coating technol-ogy that has been developed utilizes the lower-density polyethylenes, although PP, PAs and PET are also used. For example, PET-coated paperboard is used in dual ovenable trays, where a 38 μm PET coating is put on 500–625 μm paperboard.

Extrusion coating with polyethylene has several advantages over adhesive lamination of a pre-fabricated polyethylene film to paper. First, thin films of polyethylene are difficult to handle and maintain flat, and handling them requires very low tensions that are difficult to control at high speeds. Second, extrusion coating temperatures are sufficiently high so that good mechanical bonds are obtained by resin penetration into the porous paper substrate. The same adhesion level can be obtained only by the use of adhesives when free films are laminated to paper, thus making extrusion coating less expensive.

The development of adhesion between polyethylene and various substrates is an aspect of extru-sion coating that has received a great deal of attention. Substrates, whether polymer films, paper-board, aluminum foil or RCF, require some type of surface pretreatment to obtain an adequate level of adhesion of the extruded polyethylene.

5.3.3 LAMINATING PROCESSES

Methods that combine two or more webs by bonding them together are called laminating processes.

Bonding is usually accomplished by thermal or chemical means with adhesives and curing systems.

After the adhesive is adequately dried or cured, the coated web is combined with an uncoated web through the application of heat and/or pressure in a nip. A wide range of materials may be laminated to each other, and the process continued if required until the laminate has the desired protective properties.

Thermal laminating is the joining of two webs with an adhesive that is first applied to and cooled and dried on one of the webs. The webs are heated before pressing them together in the nip of two rollers that provide the force needed to establish the intimate contact required for the bond (Brown, 1992). The adhesives most commonly used are polyolefins such as EVA, and the webs that are most commonly laminated this way include plastic films, and aluminum foil joined with heat-seal-coated film or paper.

Wet bond laminating uses solvent or aqueous-based adhesives and can only be used when one or more of the webs are permeable to the water or other solvent used, thus allowing it to escape.

Wet bonding is not generally successful with plastic films, even when laminating them to paper.

Usually aqueous adhesives such as casein, sodium silicate, starch, PVA latex, rubber latex or dex-trin are used.

Dry bonding is considerably more versatile in that any two materials can be laminated once an adhesive system has been developed. Either aqueous or solvent-based adhesives are used, and they are dried or cured, if necessary by the application of heat, prior to laminating. Adequate drying of the solvent is particularly important where the solvents cannot be absorbed into the film as excess solvent is a major cause of delamination and may permeate into the package and affect the food.

However, the use of organic solvent-based adhesives has been largely phased out because of legisla-tion limiting the release of VOCs (volatile organic compounds) into the atmosphere.

Solventless laminating consists of bonding together two webs by curing in the absence of sol-vents. It has now become the dominant laminating method in commercial use because of legislation limiting the release of VOCs. A reactive chemical system (either a single- or two-component system) is used to cure the adhesive. Because the adhesive layer is formed by curing (polymerization), it releases neither solvents nor water, although small amounts of CO₂ may be emitted. Single-component urethanes are the most widely used; polyester isocyanates are also used.

Extrusion laminating is a specialized use of extrusion coating, where a hot extruded film is trapped between two other webs and cooled. This process is used mainly for producing a triple lam-inate of such materials as paper, aluminum foil, RCF and PET with LDPE, where the latter material is extruded and acts as the bonding agent between the two substrates. As in the case of extrusion coating, this process is applicable to any thermoplastic material, but the technology has been highly developed mainly for polyethylene and associated copolymers, including ionomers.

5.4 BLENDING

Generally, multilayer structures (either coextrusions or laminates) have been used to improve the barrier properties of plastic packaging. They utilize complex and expensive technology and the final product is not easily recyclable. These structures, which have at least three layers, comprise a barrier material as the middle layer and PET or PP as the outer and inner layers. Polymer blending is a low-cost alternative route to multilayer extrusion and has attracted increasing attention over the last two decades in packaging applications to enhance properties, improve processing or lower cost.

Some of the attributes that can be achieved by blending in addition to improving the barrier proper-ties include tailoring surface properproper-ties such as COF, adding color, promoting adhesion, improving stability and obtaining easy-opening features (Morris, 2009). Achieving a consistent quality blend with the desired properties requires proper attention to both process and product design.

The properties of a polymer blend are influenced by specific interactions between the molecules (thermodynamics, which determines whether the blend forms a miscible single phase or immiscible multiple phases) and their response to deformation (rheology) (Morris, 2009). The term compatibil-ity is used to describe the degree to which polymers interact.

A masterbatch is a highly concentrated blend of an additive with a carrier resin that should be compatible and miscible in the resin into which it is being blended. Film manufacturers commonly blend a masterbatch into the resin at the extruder feed hopper when making film or sheet. In more sophisticated (and expensive) compounding devices such as twin-screw extruders, ingredients can be added at various stages along the extruder.

Blends of PET and aromatic polyamides such as nylon-MXD6 are of interest in food packaging because of their potential for combining the low O₂ and CO₂ permeability of PAs with the good toughness, clarity and economics of polyester, although these blends suffer from low optical clarity and the generation of an undesirable yellow color during processing. Incompatibility of the constitu-ent polymers makes it difficult to achieve the optimum barrier structure of PET/PA blends, and to overcome this problem, small amounts of an ionomer are added.

A new, alternative technology is smart blending (Zumbrunnen, 2009). Smart denotes an ability to control in situ fine-scale structure development in polymer blends and composites. Thus, even at a fixed composition, the structure and composition of extrusions can be optimized to obtain a bal-anced combination of physical properties or impart functionality. Extrusions have been produced with more than 10,000 layers and layer thicknesses less than 200 nm. This is an advantage because it is often preferable to have large numbers of thin layers in lieu of just a few thick layers. Figure 5.4 shows the essential elements of a smart blender system.

Smart blending machines operate on the principle of chaotic advection, a recent subfield of fluid mechanics. The term advection denotes movement, and chaotic advection refers to the cha-otic (i.e., nonperiodic) motion of a particle in a fluid that can occur even where the flow field is simple and periodic. Chaotic advection provides a template for forming structured plastic materi-als in situ because minor and major polymer components in a melt are stretched and folded recur-sively about one another so that a layered polymer blend morphology first arises. Initially, large polymer melt streams are organized into multilayers in lieu of being broken down into droplets as in conventional mixing.

A desired structure type can be obtained from a smart blender (shown schematically in Figure 5.4) by extruding the structured melt through a conventional die. By this processing method, a wide

variety of structural types are attainable even where overall composition is held constant. Because the structure of a composite material greatly influences additive diffusion, this processing method provides a means for producing packaging films with tailored release rates of active compounds (Jin et al., 2009).