- June 30, 2006, Steve Sargent, PhD., Toray Plastics (America)
When selecting a packaging film, converters and consumer product goods manufacturers often choose among oriented polyethylene terephthalate (OPET), oriented polypropylene (OPP), and oriented polyamide [(OPA) or nylon], the three films commonly used for food and other substances that are sensitive to oxygen and moisture. In most cases, the choice is not simply one film versus another, since most packaging structures today are multilayer laminates, often made up of two or even all three films.
Developing the best structure for an application often entails combining the films’ respective strengths after having compared those materials’ mechanical, thermal, and barrier properties. But cost is also a consideration.
This article compares PP, polyester, and nylon in terms of downgauging, barrier, and density, and includes a look at retort applications, as well as recent technological developments and the future of those materials.
Because packaging film is sold by the pound, the thicker the film, the more expensive it is per unit of area. Thickness often is measured in terms of gauge (ga). A lower gauge number indicates a thinner film. Downgauging, or reducing the thickness, can help reduce overall costs, but some films can be downgauged more than others without significant loss of mechanical, thermal, and/or barrier properties. Because of this and differences in basic resin costs, a thinner but higher-cost-per-pound film with acceptable performance properties still may be less expensive than a slightly thicker but lower-cost-per-pound film of a different material with similar properties.
In order to maximize the value of the three films under consideration, there has been a considerable amount of process development work done in the area of downgauging. Of the three, OPET is the easiest for material suppliers to downgauge. In fact, outside the packaging area, OPET capacitor films as thin as 1.5μm (6 ga) are being produced.
The development of gas/electric hybrid automobiles also has spawned a number of significant developments in this area. In fact, it is feasible that an all-film capacitor could replace a traditional chemical battery within the next five years, thanks to the availability of thin metallized films. Within the packaging area, the gauge-limiting factor is heat distortion problems encountered during the converting process.
OPP is downgauged to 45 ga by some manufacturers on a regular basis. Below this thickness, because of the extensional properties of OPP, problems with web breaks during processing commonly are encountered. As a result, it is difficult to imagine downgauging OPP much below 40 ga in the existing commercial environment.
OPA is probably the most difficult to downgauge because of the intrinsic water uptake properties of the film. It absorbs moisture readily from the atmosphere, and the amount and rate of absorption vary with the relative humidity. This has a pronounced effect on the properties of the film during manufacture. As a result, nylon is available in thicknesses lower than 40 ga, but such thicknesses are very uncommon. As with OPET, downgauging is limited by heat distortion problems. This is summarized in Table I.
|Material||Current||Future||Converting Process Issues|
|OPET||48 ga||32-36 ga||Heat Distortion|
|OPP||60 ga||45-50 ga||Web Breaks|
|OPA||48 ga||44 ga||Heat Distortion|
OPET, OPP, and OPA films all have intrinsic oxygen and moisture vapor barrier properties, although they vary by material. They also are affected by the stretch ratios, or degree of orientation, applied to the film during the manufacturing process. In general, higher stretch ratios increase both crystallization and the density of the amorphous phase, both of which improve barrier performance. This is true of all three films, which normally are tentered and biaxially oriented.
Table II shows a comparison of the oxygen transmission rates (OTR), measured in cubic centimeters per 100 sq in./day, and the moisture vapor transmission rates (MVTR), measured in grams per 100 sq in./day, of the three films.
|48 g OPET||-6 cc||1.9 g|
|48 ga OPP||-200 cc||0.64 g|
|48 g OPA||-2.6cc||13.3 g|
When even lower OTR and MVTR are required, common practice is to use a metallized substrate instead of simply the clear film itself, or to coat the film off-line with a clear, high-barrier material such as polyvinylidene chloride (PVdC), commonly known as Saran.
In the case of metallizing, the adhesion of the metal to the substrate is critical in most applications. Typically, adhesion is due to chemical bonding of the aluminum metal to the surface on which it is deposited through a covalent bond. The strength of the bond increases with the polarity, or difference between the most electromagnetically positive and negative charges, on the surface of the substrate.
As a result, anything that can be done to increase the likelihood of a covalent bond will increase metal adhesion.
Of the three films under consideration, OPP, a pure hydrocarbon polymer chain containing no oxygen or nitrogen molecules to polarize it, requires the highest degree of functionalization because of its poor polarity. This functionalization can be imparted either during the film-making process or later in the metallizing chamber.
Table III shows a comparison of the OTR, measured in cubic centimeters per 100 sq in./day, and the MVTR, measured in grams per 100 sq in./day, of metallized versions of the three films under consideration. Note that for this data an average of 2.2 OD (optical density) metal deposition is used, since metal deposition also affects barrier greatly.
|0.1 cc||0.1 ga|
|48 g MOPP||1.5 cc||0.01 g|
|48 g MOPA||0.2 cc||0.5 g|
Density a Cost Consideration
Density, or weight per unit of volume, can be an important consideration in choosing a packaging film. In general, higher density films have lower yields, or area per unit of weight, than lower density films. Because packaging films are sold by weight, a thinner but higher density film actually can cost more than a thicker but lower density alternative.
In terms of thermal and mechanical properties, for example, OPET films generally are better than either OPP or OPA. However, OPET is not always the film of choice for packaging applications because of its higher density, which often results in a higher cost per unit of area. Although OPA is intermediate in density between OPP and OPET, its density advantage over OPET film is more than overcome by its higher resin price disadvantage, making it the most expensive per unit area of the three films under consideration. The densities of metallized versions of all three films are approximately the same.
In addition to its yield advantage over OPET at the same thickness, OPA offers good mechanical and thermal properties. It also has a lower modulus of elasticity than OPET, giving it a “stretchiness” that can result in better puncture and flex-cracking resistance in certain applications such as retort pouches and balloons. Recent advances in film development, however, are beginning to yield OPET and OPP films that compete with OPA in some of these applications, especially since OPET can be downgauged so easily.
Although the mechanical and thermal properties of OPP are not as good as those of OPET, OPP very often is the film of choice. Its mechanical and thermal properties are sufficient for many applications, and its much lower density compared with OPET (OPP = 1.0, OPET = 1.4) and similar resin cost make it much more economical on a cost-per-unit area ($/MSI) basis.
One of the fastest growing applications for flexible packaging is retort, named for the cooking process in which the pouch is filled with the food product and then heated to 125 deg C. The consumer then reheats the food in the single or multiple serving pouches in which it is packaged. The Japanese, Europeans, and the US military long have known this method of food preparation not only is more convenient than heating in separate containers but also results in tastier food because of less overcooking near the outside of the container and superior moisture retention. Now American consumers are discovering the benefits of retort. Rice foods and tuna are just two examples of products that have adopted this form of packaging successfully.
Common retort structures involve combinations of OPET and OPA substrates along with an FDA-compliant functional barrier layer. Retorting, however, is a very aggressive thermal process. As a result, OPP normally is unsuitable for such applications, although cast PP often is used for the inner sealant web.
Recent developments in retort films include clear barrier vacuum deposition for OPET substrates, OPET materials with excellent adhesion on both sides, high-elongation OPET to provide superior puncture resistance, and metallized PET (MPET) with retortable adhesion-promoting layers to improve the bond between the metal and the base sheet.
What the Future Holds
Further improvements in packaging films will be through increased functionalization. In some cases, suppliers will modify the resin itself to provide improved performance. Using a highly crystalline core, for example, can produce an improved clear barrier film. In other cases, desirable properties will be imparted by coextruding functional layers on the surface or surfaces of the film. This technique can be used to produce a film that is primed for ink adhesion on one side and is heat sealable on the other.
In addition, in-line coating of films during the film-making process will make possible increased functionalization, such as adding a high-barrier coating during film manufacture with a minimal increase in cost.
Unlike PVdC, which cannot be metallized, many of these new technologies lend themselves well to the metallization process, actually improving metal adhesion in some cases, and permitting the production of ultra-high-barrier substrates.
Efforts also are underway to improve the films themselves. Recent advances in multilayer die/feedblock designs have allowed researchers to improve vastly the tear propagation and puncture resistance of lower-cost OPET and OPP materials. As noted earlier, OPA often is used as one of the layers in retortable films. Although its use as a single film is limited by its higher price, new coextruded films with OPA as one of the layers currently are being developed. The moisture absorption properties of OPA also are proving useful in applications such as laminating it to paper to avoid curl issues. Nylon’s popularity with coffee packaging applications probably will remain strong also.
Significant efforts are underway to improve the heat seal properties of both OPP and OPET substrates. Film developers are working to improve sealing performance to provide a more hermetic seal for better package stability and shelf life, at the same time improving throughput on packaging lines and resulting in a more robust design.
Finally, developments in the area of retortable sealant OPET and coextruded OPA are underway as the retort market continues to expand and enter new segments.
Toray Plastics (America)—PFFC-ASAP 310. torayfilms.com
Steve Sargeant, PhD., is director, new business development, Toray Plastics (America), North Kingstown, RI, a manufacturer of PET and PP films for flexible and rigid packaging, graphic, magnetic, and industrial applications. Contact him at firstname.lastname@example.org or 401/294-4511, ext. 4442.