Polymers in Food Packaging
With a Special Look at Potato Chips
April 10, 2000
Imagine waking up one morning with a definite feeling that something is terribly wrong. You look for your alarm clock, but it is nowhere in sight. You overslept and missed your 8:00am CE212 class! Mumbling to yourself, you stumble into the kitchen. You open the refrigerator door, and realize that all of your cartons have been replaced with glass bottles. You prepare to pour yourself a bowl of cereal, but the slippery milk bottle slips out of your hand and shatters on the floor. Shrugging off what had just happened, you decide to sit down with your bowl of Fiber Log Cereal you bought last night, but realize it’s stale. Confused, you look in the cereal box and notice that the liner is missing. At that point you had had it, "Is there a better way to package food?" Although most people never think about it, they would probably be lost without polymer products. The food packaging industry utilizes polymers to make containers easy to use and to keep foods as fresh as possible.
The Purpose of Packaging
The basic reasons for packaging include sanitation, safety, and marketing. 1 Traditionally, food packaging has been involved with the protection of the product. But today, marketers have recognized that packaging is a very important part of most products, because an attractive package can contribute to a substantial increase in sales. Packaging is the message of a product, which determines how customers will perceive the product and how that item will attract the consumer’s attention and interest.
A food package should inform the consumer about the usability of the product, give instructions on how to use the product correctly, and describe the dangers of the product. It should also give an expiration date for perishable goods, specify the ingredients of the product, mention the brand name, and state the name of the manufacturer. Seals, emblems, and other symbols are also used to show the quality standard, the safety standard, and other standards of the product.
Polymers are becoming an increasingly popular packaging material. Figure 1 shows the relative amounts of various polymers used in packaging.
Figure 1: Polymers used for Packaging
An Exciting Example with Potato Chips
Potato chips are expected to account for one third of the global snack food market share by the year 2000, according to Euromonitor, the international market analysis firm. Chips racked up $13.6 billion in global sales in 1995. Potato chips now account for $15.7 billion in worldwide sales. 3 Since the products differ very little in the potato chip market, packaging can be a crucial component due to its promotional aspects as effective as advertising.
Research was conducted on customer response to potato chips in wax-coated paper bags and polyvinyl bags at two large super markets. A significant proportion of consumers perceived the chips in polymer packages to be crisper and tastier than chips in wax packages. Consumers were also willing to sacrifice ease of opening for a higher quality of crispiness and taste. Since taste is the most important purchase motivation, paper bags are out of the competition. Also, polymer bags offer a much longer shelf life than paper bags. 2
A Multi-Layered Bag
To examine the complications involved in food packaging, let us examine a potato chip bag, a multi-layered structure. Most commonly, the layers of a potato chip bag are metallized BOPP (inside layer), LDPE (middle layer), BOPP (middle layer), and SurlynÒ (outermost layer and sealant).
Figure 2: Bag Structure
Properties of the Layers
In recent years, BOPP has become one of the most popular high growth films in the world market. It is packaging material with varied features which include high strength, minimum thickness, good appearance and machinability, and excellent fat, oil, and water protection. The film is also chemically neutral and nontoxic. It does not pollute the environment and may be reused or incinerated causing no danger to the environment. 6
LDPE is known for its flexibility, moisture protection, toughness, chemical resistance, lightweight, sealing properties, and low cost. However, it cannot be used alone because of its poor gas resistance, inability to retain ink, and its strong tendency to develop a static charge that may attract dust, which can be unsightly on a retail shelf. 7
BOPP has a high oxygen transmission rate, and if it were used alone, it would permit oxygen into the package. This would oxidize the fat in the potato chips causing spoilage. This is the reason that an extremely thin layer of aluminum is sprayed onto one of the layers, a process called metallizing. This metallized layer is about 400 to 500 Angstroms thick, which is three times thinner than the thinnest commercial foils. Besides providing an effective barrier to atmospheric gases and aroma constituents, metallizing also prevents light from entering. Light is undesirable since it may also be a catalyst for the oxidation of the fat.
SurlynÒ delivers outstanding impact toughness, abrasion resistance, and chemical resistance in a variety of consumer and industrial products. Injected molded, extruded, foamed, thermoformed, or used as a powder-coating or resin modifier—SurlynÒ offers a broad range of processing options. Unlike other clear plastics, SurlynÒ is highly resistant to chemicals and oils, but holds ink well enabling unique packaging options. 15
Maintaining the Freshness of "Fat-Free" Snacks
When products get thrown away due to expiration of "sell by" dates, the profit potential of that product gets discarded too. However, food manufacturers recognize that they can extend the product’s shelf life by carefully choosing optimum packaging systems. The growing reduced-fat and fat-free baked goods and snacks category presents some special shelf life challenges. The lack of fat in these products makes their texture less stable and more dependent on moisture content, so the packaging used must exhibit enhanced moisture barrier properties.12
A new research study by Mobil Films and Aspen Labs has demonstrated that low-fat and fat-free baked products are more susceptible to off-odor contamination and loss of desirable flavor or aroma. Fat solubilizes organic compounds, such as desirable food flavors or undesirable contaminating odors, more effectively than water. This means that bad odors will be less absorbed by and more volatile around low-fat foods when compared to regular fat foods.
The study suggested that a combination of nitrogen flushing and high-barrier metallized packaging could significantly deter development of rancid odors and flavors by hindering oxidation. Metallization, as well as oxygen barrier, is key because nitrogen flushing does not eliminate the need for a light barrier to help guard against oil rancidification.
Nitrogen and carbon dioxide are the gases of choice for most food applications of modified atmosphere packaging. Carbon dioxide may be said to be an active ingredient – its bacteriostatic properties inhibiting growth of spoilage microorganisms. This can lead to significantly extended shelf life of products. Nitrogen is used to maintain anaerobic conditions and to pad the package. By altering the ratio of gases in the pack pressure, it is possible to design a package that not only protects the product from spoilage, but from mechanical damage or pack collapse. 16
When it comes to fat free chips—a favorite for those who try to watch what they are eating—packaging issues multiply. For example, Dana Alexander Inc. experienced difficulties shipping low-fat chips over the mountain regions west of the Mississippi River. Depending on the altitude, air pressure changes caused some of the bags to burst at the seams, especially during the heat of the summer months.
Surlynâ by Dupont was the only material that could meet this challenge. To solve the issue, the company selected a multi-layer packaging structure consisting of a biaxially oriented polypropylene (BOPP), low-density polypropylene (LDPE), high barrier metallized polypropylene and Surlynâ 9. This is an example of how polymers have efficiently increased sales and protected a product, thus increasing profit.
Production of the Packaging Polymers
Polymers can be divided into two major groups based on their thermal processing behavior. Polymers that can be heat softened in order to process into a desired form are called thermoplastics. Thermosets are polymers whose individual chains have been chemically linked by covalent bonds during polymerization.10 Polyethylene and polypropylene are one of the earliest and most important groups of thermoplastics.
LDPE (Low Density Polyethylene)
LDPE, a crystalline structured polymer, was the first commercialized polyolefin produced in 1939 by ICI in England. Low density polyethylene is produced by free-radical bulk polymerization using traces of oxygen or peroxide as the initiator. This is the simplest technique that gives the highest purity of polymer. Polymerization is conducted either in high-pressure autoclaves or in continuous tubular reactors operating at a temperature near 250° C and pressures as high as 3000 atm. Since the heat of ethylene polymerization is high (105 kJ mol-1), the exotherm needs to be carefully controlled. For this reason, the polymerization may be conducted in stages of low conversion. 11 The mechanism of free radical polymerization of LDPE is illustrated in Figure 3.
Figure 3: Mechanism of free-radical polymerization of LDPE
The U.S. production of LDPE for packaging in 1998 was 2.9 billion pounds.7 The price of low-density polypropylene in the United States is approximately $0.22 per pound. 14 Eastman Chemical Co. is one of the major suppliers of LDPE.
BOPP (Biaxially Oriented Polypropylene)
Polypropylene is a versatile polymer with a crystalline structure. PP can be made from the monomer propylene by Ziegler-Natta polymerization and by metallocene catalyst polymerization. A Ziegler-Natta catalyst is a metal-organic complex of a metal cation from I-III in the periodic table and a transition metal compound from groups IV-VIII10.
Figure 4: Polymerization of polypropylene
Research is being conducted on using metallocene catalyst polymerization to synthesize polypropylene. By using metallocene catalyst, polypropylene can be made with different tacticities. Most polypropylene used in the food packaging industry is isotactic. The polypropylene that you buy off the shelf at the store has about 50-60% crystallinity.13
Orienting, or stretching the film under carefully controlled temperatures can improve certain properties of polypropylene. This causes a realignment of molecules and a much tougher film, increasing strength and barrier properties. The film itself is stretched in one or two directions with a tentering frame, a device that grips the edges of the web in clamps that move outward to stretch the sheet in the cross-machine direction. Stretching in the machine direction is accomplished by altering the film-winding tension. For biaxial orientation, polypropylene film is gently stretched in a transverse direction. Although strength and barrier properties of PP are improved upon orientation, they still are not fully impermeable to oxygen, light, and moisture.
BOPP film is now being supplied by overseas firms (Japan, Korea, Turkey) since the current selling price for BOPP is very low (less than $1/pound).4 The U.S. demand for polypropylene in flexible packaging is 900 million pounds per year.7
SurylnÒ , produced by Dupont, is an ionomer resin available for use in conventionally blown and cast film extrusion and co-extrusion equipment. SurlynÒ utilized in snack structures is normally pressed at melt temperatures ranging from 190-227oC in blown film equipment. Materials of construction used in processing this resin are corrosion resistant. For example, stainless steel of the types 316, 15-5 PH and 17-4 PH are excellent.
Once each layer of the bag is produced, the layers are laminated together.
Figure 5: Laminating the Bag Layers
As shown in Figure 5, a film of metallized BOPP comes up from the bottom and is carried over the adhesive roller on the left. Films of LDPE and Surlyn coming in at the top left meet the adhesive coated bottom film (BOPP) in the nip of the two rolls, which are one above the other on the left. The combined layers pass around the snub rolls to the right and are carried to the next operation.
The next steps include printing the Surlyn side of the film, cutting the film into the proper dimensions, and heat-sealing the sides of the bags. The bags can then be loaded with chips and sealed, sometimes with packing gases, to be distributed and sold in the stores.
The next time you go into the kitchen to grab a snack or to prepare a meal, remember that chefs do not have the monopoly on creativity in the kitchen. Engineers and scientists are all part of the creative team. If it weren’t for the polymers in packaging, we would miss out on the tasty quality we enjoy daily.