Plastics in Packaging

April 6, 2001

CE 435

Scott H. Boyle

Brian D’Amico

Janine Horn

Mark Przybylski

Introduction

In the early twentieth century, the advancement of plastic technology began to fuel a revolution in the packaging industry. Plastic brought benefits that other materials simply couldn’t deliver. But the advent of plastics was also accompanied by many adverse side effects. This purpose of this paper is to provide a detailed analysis of plastics as they pertain to the packaging industry. This will include a discussion on the positive and negative impacts that plastic packaging has had in our lives.

A plastic is defined as an organic material that has the ability to flow into a desired shape when heat and pressure are applied and to retain that shape when they are withdrawn. Plastics are composed primarily of a binder with plasticizers, fillers, pigments, and a host of other additives. The binder, which is the actual polymer, gives the plastic its main characteristics as well as its name. The plasticizers are added to the binder to increase flexibility and toughness. Fillers are added to improve particular features such as hardness or resistance to shock, while pigments are used to impart various colors.

The popularity of plastics in packaging can be attributed to its wide variety of different qualities. Plastics can be rigid when protection is needed, or flexible for convenience sake. They can be clear or opaque. They can also be molded into an almost infinite variety of shapes and sizes. On top of these benefits is perhaps the most important one of all, cost. The effect of plastic packaging on a company’s bottom line will be examined in a later section.

According to Market Tracking International estimates, world sales of primary packaging materials, such as boards, plastics, metal cans, and glass, will reach $434 billion by the end of 2001. This marks a growth of 12.5 percent from $385 billion in 1997. [1] World sales of finished packaging materials, which include the value added from conversion, such as printing, are estimated at roughly twice the value of raw packaging materials, or $800 billion.[2] The industry’s revenues are generated by four main categories of raw materials: Paper and board, plastic, glass, and metal. Figure one on the next page shows the percentage of industry sales attributed to each category in 1997 as well as the expected sales distribution for 2001.

Figure One – Percent of Industry Sales According to Raw Material[3]

As illustrated by the charts above, the use of plastics in packaging is increasing. In 2001, it is estimated that plastics, at the expense of glass and metal, will become the most popular packaging material in the industry.

Surprisingly, six resins account for almost 97 percent of all plastic used in packaging.[4] A detailed percentage breakdown is shown in figure three below. A brief description of the uses of each also follows.

Figure Three – Most Common Packaging Plastics[5]

Polyethylene Terephthalate (PET) – Used mainly in soft drink bottles, as well as other containers such as cough syrup.

High-density polyethylene (HDPE) – Used in milk, water, juice, soap and laundry product bottles, margarine tubs, yogurt containers, etc.

Poly-vinyl chloride (PVC) – Used for some cooking oils, water bottles, film for meat packaging, car care products, etc.

Low-density polyethylene (LDPE) – Used in film products such as bags, flexible lids, and “squeeze” bottles.

Poly-propylene (PP) – Used in ketchup bottles, plastic screw tops, opaque maple syrup bottles, etc.

Polystyrene (PS) – Used as a rigid plastic that can be clear, as in bakery containers, or foamed as in coffee cups, meat trays, etc.

The Manufacturing Process

Since plastics are petroleum products, the first phase of the plastic manufacturing process is the extraction of crude oil or natural gas from the ground. Through a series of chemical processes the oil is transformed into monomers, which react with catalysts to form the polymers that characterize specific plastics. Plastics are then shaped through compression, transfer, injection, or extrusion molding. In compression molding, materials are placed in mold cavities where the application of heat and pressure makes them first plastic, then hard. In the transfer method, the compound is plasticized by outside heating and then poured into a mold to harden. This method is used for designs with intricate shapes and variations in wall thickness.[6] In the injection blow molding process, the plastic powder is placed in a heating chamber with a rotating screw that forces it into cold molds. In extrusion blow molding, the mold is placed around the molten plastic and then air is used to expand the molten plastic to fit the mold.

Types of Plastics Used in Packaging

One of the most predominant plastics used in packaging is polystyrene, a vinyl polymer made up of long styrene chains. A figure depicting the structure of polystyrene is shown below as figure four. Polystyrene offers strength and durability without adding significant weight. It is also a constituent of poly(styrene-butadiene-styrene), which is a hard rubber that gives it such durable properties.[7] The properties of polystyrene that make it an effective packaging material are shown below in figure five.

Figure Four – Polystyrene Structure[8] Figure Five – Properties of Polystyrene[9]

Properties	Values
Density	0.035 g/cc
Water Absorption	9%
Compressive Yield Strength	0.5 MPa
Maximum Service Temperature	167 ⁰F
Minimum Service Temperature	-58 ⁰F
Thermal Conductivity	0.027 W/m-K

Since the water absorption is so low and the compressive yield strength so high, it provides good protection against moisture and is able to maintain its shape for long periods of time. Due to the large range in service temperatures, polystyrene can be used in almost any climate. Also, since the thermal conductivity is so low, polystyrene provides for a great insulator. Polystyrene packaging is available in two forms: solid and foam. Foam polystyrene is used in packaging meat trays and egg cartons and in protective packaging for electronics. Solid polystyrene is used in yogurt and cottage cheese packages.[10] Two different types of blowing agents primarily produce polystyrene foam: carbon dioxide and pentane. This produces polystyrene that is 95 percent air and 5 percent polystyrene, which provide excellent insulation. Polystyrene also only compromises only a small fraction of the waste in landfills since it is primarily made of air.[11]

Primarily most meat is packaged in heat-shrinked bags and tray-ready containers. Some of the advantages of shrink bags are that it provides great oxygen protection and there is less chance of punctures in the packaging. The other type of packaging, tray-ready containers, is how meat is typically packaged for sale in supermarkets. These consist of a polystyrene tray bottom and a vacuum packed plastic seal.[12] Polystyrene packaging helps meat maintain enough oxygen content in order to protect against physical and chemical change. It is also sturdy enough to withstand the weight of the meat as well as provide a good barrier against leaks.[13] Egg cartons, like meat cartons, also provide for study protection. Additionally, foam egg cartons are processed extremely well in automated equipment.

Polystyrene is also widely used in protective packaging for electronics. The polystyrene packaging can come in two different forms: loose fill “peanuts” and “shape molded packaging”. Loose “peanuts” allow for more than one item to be shipped in the same package. Where “shape molded” allows for a tight fit around equipment, such as stereo sets and televisions. Both are lighter than other materials, which in turn save money and energy. Electronics polystyrene packaging also resists moisture and does not attract insects. It can also be used over and over again as opposed to other packaging methods.[14]

Polyethylene terephthalate is another polymer used in plastics packaging. PET is a polyester with a phenylene group within the chain. A molecular image of polyethylene is shown below in figure six and the structure in figure seven. Polyethylene terephthalate can exist in both transparent and opaque, or white materials. PET is often found in soft drinks and non-carbonated mineral water bottles.

Figure Six - Polyethylene Terephthalate[15]

Figure Seven - Structure of Polyethylene Terephthalate[16]

For large bottle manufacturers, polyethylene terephthalate has many desirable properties, which are shown in figure eight on the next page.

Properties	Values
Density	1.64 g/cc
Water Absorption	0.05%
Tensile Strength	120.5 MPa
Elongation Break	1.81%
Compressive Yield Strength	172.7 MPa
Glass Transition Temperature	156.2 ⁰F
Maximum Service Temperature	435.2 ⁰F

Figure Eight - Properties for Polyethylene Terephthalate[17]

PET’s low density and water absorption make it a good material for beverage packaging. Polyethylene terephthalate is found to be extremely strong, stiff and hard. Typical molecular weights for polyethylene terephthalate range from 10,000 to 15,000. These high molecular weights provide for high strength under industrial applications.[18] PET also provides good barriers against oxygen and carbon dioxide, which help keep drinks carbonated. It also generally has good resistance to mineral oils, solvents and acids, but not bases. Polyethylene terephthalate is also used in beer bottles, mouthwash bottles, and non-food containers.[19] PET is also one of the most environmentally safe plastics because it does not produce any acid-rain elements.[20] There are other derivates of polyethylene that are used for other commercial items as well.

Other polymers that are used quite extensively in packaging are high and low density polyethylene. A representation of each is shown in the following figures.

Figure Nine - LDPE –A Molecule of Branched Polyethylene[21]

Figure Ten - HDPE – A Molecule of Linear Polyethylene[22]

HDPE is primarily used in milk, water and juice containers. High-density polyethylene can be made either unpigmented or pigmented. The unpigmented bottles have good barrier properties and are well suited for short shelf life products. HDPE also has excellent chemical resistance, which makes it a good packaging material for detergents and bleach. Pigmented HDPE bottles have better stress crack and chemical resistance.[23] The properties of high-density polyethylene are shown in the following table.

Properties	Values
Density	0.95 g/cc
Hardness	65 Rockwell R
Melt Flow	8 g/10 min
Tensile Strength	24.1 MPa
Elongation Break	650%
Maximum Service Temperature	156.2 ⁰F

Figure Eleven - Properties of High Density Polyethylene[24]

Low-density polyethylene plastic is primarily used to manufacture flexible lids and bottles. It is also used to produce bread bags and frozen food bags. The properties of LDPE are found in figure twelve. It is often used where heat seals are necessary in packaging. Also, due to the low water absorption rate, LDPE is good for packages that need a good barrier to moisture.[25]

Properties	Values
Density	0.928 g/cc
Elongation Break	50%
Water Absorption	0.01 %
Tensile Strength	13 MPa
Maximum Service Temperature	113 ⁰F

Figure Twelve – Properties of Low-Density Polyethylene[26]

Polypropylene is used in many types of packaging ranging from automobile parts to food packaging, most notable peanut butter containers and ketchup bottles. PP is very chemically resistant and has the lowest density of all the plastics used in packaging. In addition to those features it is also a very durable polymer with a high melting point, which makes it ideal for holding hot liquids. Below are figures illustrating the chemical structure and the physical properties of polypropylene.

Properties	Values
Density	0.902 g/ml
Low Temperature Brittle Point	10°F
Tensile Strength	4500 psi
Yield Strength	4000 psi
Elongation	300%
Modulus of Stiffness	200000psi
Resistance to Heat	230°F

Figure Thirteen - Structure of Polypropylene[27] Figure Fourteen - Properties of Polypropylene[28]

Polypropylene is similar to polyethylene, but each segment of the chain has a methyl group attached to it. It is translucent and has no known solvent at room temperature. Products made of polypropylene are brittle at 10°F and may crack or break if dropped from waist height. However at room temperature objects made out of PP can sustain falls from much greater heights without damage. Another advantage of PP is the fact that it can be produced as a rigid plastic or a stretchy film.

The use of polypropylene is increasing due to the fact that many more uses are being discovered and an increasing number of firms are manufacturing this polymer. The increased use of this polymer is due to a couple of factors. Its physical properties make it good for durable, lightweight, and inexpensive packaging. Also the physical properties can be modified over a wide range of conditions, giving polypropylene the ability to have many different uses.

PVC is another inexpensive polymer used in packaging. It can be used as a flexible or rigid material. Some of its uses include piping and packaging electronic materials. When used for packaging electronic equipment it is produced in sheet form and coated with an electrostatic discharge (ESD) agent. Another factor that makes PVC so widely used is the fact that it can be machined with standard metal working tools. PVC is a chemically non-reactive polymer, so it can be coated with some form of antistatic agent; some examples of these are ammonium salts, amido-amines, or salts of octanoic acid.[29] These agents fight static charge build up in the packaging of electronic devices. Some quantitative properties of PVC are listed in figure sixteen below. Also PVC is self-extinguishing which is why it is used to house bundles of electrical wires. PVC comes in two basic forms, plasticized and unplasticized. The general difference is that unplasticized PVC, which includes PVC I and PVC II, is more rigid. Unplasticized PVC is normally available in rod sheet slab pipe tubular bar fittings and valves. Plasticized PVC is available in sheet film, fittings, flexible tubing, and pipe.

Properties	Values
Density	1.4 g/cc
Tensile Modulus	426000 psi
Yield Strength	7500 psi

Figure Fifteen -Structure of Polyvinyl Chloride Figure Sixteen - Properties of Polyvinyl Chloride[30]

Polyvinylidene Dichloride, or Saran, is used for protection for a number of products including, food, consumer, and industrial. It is ideal for this purpose due to the co-polymerization process used to create this polymer, because this results in a film with molecules so closely spaced together that very little gas or water can get through. This provides an excellent barrier against oxygen, moisture, chemicals and heat-qualities. PVDC is resistant to oxygen, water, acids, bases, and solvents. This will have a positive effect on the length of time that food can be stored. Saran was discovered in 1933, as an accident by a scientist at DOW.[31] Before it was used as a food packaging aid it was sprayed on airplanes to stop corrosion from salt water. Originally it was green and had a pungent smell to it, eventually these characteristics were removed and then it was used for meat storage. The most common form of PVDC is Saran Wrap®, a food storage wrap. Below is a table illustrating some of the physical properties of PVDC.

Properties	Values
Density	1.7 g/cc
Elongation	250%
Compression Strength	8000 psi
Yield Strength	4300 psi
Flexural Strength	5800 psi
Flexural Modulus	7250 psi

Figure Seventeen – Structure of Polyvinylidene Dichloride Figure Eighteen – Properties of Polyvinylidene Dichloride[32]

Plastic vs. The Alternatives

Over the past twenty years and continuing today, the packaging industry has been witnessing a trend towards plastics and away from alternatives such as glass, aluminum, wood/paper, steel, etc. Is plastic truly a better material? A simple analysis of the mechanical stress strengths of several packaging materials would seem to indicate that it isn’t. As shown in figure nineteen, we see that plastic (PE) has the weakest stress strength among the group. However, this is not a great method of comparison, since the stress strength is primarily

Figure Nineteen - Stress Strengths of Various Packaging Materials[33]

a measure of when the material yields. At a high stress level PE will give and bend due to its flexible nature while glass and aluminum will crack or crush. Another major benefit of plastic use is that it is incredibly lightweight when compared to glass, wood, and even aluminum. This can mean a lot to a manufacturers bottom line since transporting product in plastic can save thousands of dollars a year in transportation costs. It’s staggering to consider that just 2 pounds of plastic can deliver 1000 ounces of liquid, while it would take 3 pounds of aluminum, 8 pounds of steel, and 27 pounds of glass to carry the same amount.[34] Compared specifically to glass, plastic drink bottles allow a distribution truck to carry up to 63% more drink and 83% less packaging. A 39% fuel saving can be realized simply by switching from glass bottles to plastic bottles.[35] When compared to paper, it was found that seven trucks were needed to deliver the same quantity of paper bags contained in just one truckload of plastic bags. Not only is plastic lightweight and virtually unbreakable, but it is also relatively cheap when compared to alternative materials. As of mid-2000, PET was the most expensive packaging plastic, selling for approximately $.62/lb.[36] At around the same time, the price of aluminum alloy was estimated at $1.28/lb[37] while good quality wood was considerably more.

Plastic is not without its downfalls. Glass, and aluminum to a lesser extent, is less porous than plastic bottles. Glass is also inert, meaning that there is no migration of components of the plastics to the food or beverage. This results in a safer and purer taste. This theory is supported by the actual practice of the packaging industry. PET bottles are being used increasingly for soft drinks, because the migration of plastic chemicals is not as noticeable to the consumer because of the strong flavoring of the drink. On the other hand, mineral water with carbon dioxide shows off this chemical taste immediately. This is the reason glass bottles are still used for this kind of beverage.[38]

Plastics are coming under fire recently because of their perceived negative impact on the environment. It is a known fact that consumers don’t recycle plastics as frequently as they do aluminum and glass. This phenomenon is shown in figure twenty below. Plastics that do not get recycled are sent to landfills where they remain indefinitely since plastics do not decompose like their paper/wood counterparts. Although plastic undoubtedly does have some negative impacts on the environment, activists tend to overlook the environmental benefits that plastic provides. These benefits can be illustrated by comparing the

Sources: Aluminum Association; American Plastics Council;

Glass Packaging Institute: "The State of Garbage in America", Biocycle

Figure Twenty -Recycling Rates of Common Packaging Materials

manufacturing processes of polystyrene and paper cups. The paper cup uses: 15 times more chemicals, more than six times as much steam, 13 times more electricity, 30% more cooling water, and 170 times more process water.[39] Also remember the fact that the lightweight nature of plastics results in fewer truckloads and less fuel usage, which ultimately equates to lower levels of air pollution.

Reusing Plastic Packaging

Plastic packaging is many times reused before it is even recycled. Wirthlin Worldwide conducted a survey in 1997 that showed that 80 percent of Americans reuse plastic products and packaging.[40] People reuse plastic grocery bags for everything from trash to holding wet swimsuits. The use of milk jugs and detergent bottles filled with either sand or water to hold down above ground winter pool covers is a common practice in suburban life. Plastic beverage containers are extensively reused as extra water containers, while plastic butter tubs and whip cream containers are commonly reused as “leftovers” containers. These types of plastic packaging are usually long lasting, durable, and chemically inert which makes them perfect candidates for use and reuse. Reusing plastic packaging products delays the filling of landfill space and lowers the cost of garbage disposal. For example many products now implement a reusable container, for which smaller packages of concentrated product are to be placed in and diluted.[41]

Lately, American businesses have been shipping and receiving products in reusable plastic shipping containers (RPSC’s). Businesses buy these returnable containers and ask their customers to send them back for reuse. These types of containers are replacing single use corrugated cardboard containers across the United States. Using these containers has significantly reduced wood and cardboard waste in landfills.[42]

Recycling Plastic Packaging

In recent years there has been an enormous increase in community collection and recycling programs for plastic packaging. More than 80 percent of all U.S. households have access to plastic collection and recycling programs.[43] Plastic bottle packaging is the most familiar form of packaging being reused and recycled. The amount of plastic bottle packaging recycled in 1998 was 1.45 billion pounds by volume.[44] Post-consumer recycled plastics are more and more widely used in products and products packaging.

The recycling process involves collection, sorting, and reclamation. The following is a pictorial example of this process.

Figure Twenty-One - Pictorial Example of Recycling Process.[45]

Collection programs include curbside, drop off, and state deposit. After the plastics are collected, they are taken to materials recovery facilities (MRF) or intermediate processing centers (IPC) to be manually or automatically sorted. They are then shipped to reclamation centers where the plastic resins are separated away for reuse. Below are statistics for the Average Reclaimer Yield Values for a variety of plastic containers.

Average Reclaimer Yield Values
Bottle Type	Base Resin Yield (%)
Two-piece PET soda bottles (w/base cup) One-piece	65-75 (PET)
PET soda bottles and custom PET bottles	75-85 (PET)
Natural HDPE bottles (e.g., milk, water)	85-95 (HDPE)
Pigmented HDPE bottles (e.g., soap, detergent)	75-85 (HDPE)
PVC bottles	85-92 (PVC)
PP bottles	85-95 (PP)

Figure Twenty-Two - Average Reclaimer Yield Values for a Variety of Plastic Bottles.[46]

This shows that most kinds of plastic products can be recycled back to their main resins in very high yield if there are not a lot of additives or attachments in the original product. Companies will use these recycled plastics in their products to save money as well as the environment.

Conclusion

In summary, this report has analyzed the reasons for the increasing use of plastics in packaging applications. Plastic is lighter, cheaper, and more durable than its competitors, but unfortunately, the need for more environment-friendly plastics still persists. As we move through the twenty first century, plastic technology will improve further, making it the material of choice for an even wider variety of packaging applications.