Beverage containers pose an interesting engineering and policy challenge to society. They must satisfy a number of physical and structural cirteria, must be inexpensive, and sould have a minimal impact on the environment. In this paper we will explore the engineering and policy choices made in the product and life cycles of glass, aluminum, and plastic beverage containers. We will look at the technologies involved in the manufacturing and recycling of the three different types of containers and conclude with a recommendation of a beverage container policy.
The beverage container industry has been driven by many technological forces and constant innovation to take advantage of the enormous economy of scale which exists in a market of its magnitude. Several technological limitations exist in container production, whose understanding is important when recyclability and container design are taken into account.
Glass containers have the simplest manufacturing process of modern containers. Higher purity silica from sand is mixed with glass cullet (crushed glass from any of a variety of sources) and melted in furnaces. The glass liquefaction is enhanced by adding salts like soda ash (sodium carbonate) to the melt. Using glass cullet as an input product is actually preferred to pure silica, because its lower viscosity upon heating allows the furnaces which co-melt the cullet and sand to operate at lower temperatures. Several other salts are added to improve the hardness, finish, and handling qualities of the finished product, and the glass is ready for the final forming steps. The liquid is cooled to a intermediate viscosity, machines cut pieces of the viscous glass liquid off, and the bottle shape is formed by blowing or press forming the glass into molds. The glass is cooled, annealed, inspected and ready for the rest of the filling and canning process.
Several manufacturing attributes lend glass well to recycling, The raw materials are quite abundant. The recycled product is more economical to produce than the virgin product, and has indistinguishable quality. Glass is strong, stackable, heat resistant, and relatively unreactive with container contents. Various glass formulations can be made to protect reactive or sensitive container contents of virtually any type. The heat treatability of glass also allows the packaging of contents which require aggressive aseptic canning to maintain product freshness.
Aluminum beverage containers are presently made by the Draw and Iron method, which is an efficient way to produce two piece metal containers which have an internal pressure, such as carbonated beverages (whose CO2 partial pressure keeps the can supported), and beverages which have a nitrogen pressurized headspace. The pressure is needed because the side walls of the can are too weak to allow tall stacking of the containers without additional support. Sheets of aluminum are fed into a press which forms cups from the sheet by stamping from the top. The shape of the bottom is formed, and the container is cut from the roll and trimmed. At this stage the cans are washed and dried. Decoration is applied, and an organic polymer is applied to the interior of the can to protect the contents from the relatively reactive aluminum. The diameter at the top of the can is reduced by a rolling step to reduce the size of the top used, which enhances container economy. The cans are filed and the perforated aluminum top is attached to the can.
Aluminum can be recycled well, and the manufacturing technology responds well to the use of recycled aluminum. Once the aluminum is reclaimed, purified, and remixed to provide the correct alloy it produces a can indistinguishable from the virgin product. Aluminum cans stack relatively well and are resistant to breaking and cracking. Products which have a high internal pressure, such a carbonated beverages, help keep the can rigid and are particularly suited to Draw and Iron type cans. Aluminum cans, however, cannot be made in arbitrarily large sizes. The largest cans in commercial use today are less than a liter in volume. The cans also cannot be resealed in any manner which allows internal pressure to be maintained.
Plastic beverage container production is substantially more complicated than both glass and plastic container technology. PET and HDPE are the main polymers used for beverages. HDPE's main market is in large volume waters, juices, and milk (four liters and larger) and is the largest resin consumer by volume, but PET is dominant in most small and medium volume beverage markets (up to three liters). PET competes most closely with glass and aluminum and thus will be considered here. Virgin PET resin is a petrochemical commodity, usually supplied to bottlers in the form of a resin by chemical processing firms. PET bottles are produced by first injection molding miniature preforms with an appropriate threaded top, known as the finish. In the two-step bottle forming process (by which pressurized soda bottles are made), these preforms are usually cooled and then fed to a blow molding machine. Several variations on the technique exist, but all involve mounting the preforms by the finish, heating them, and then inflating them with compressed air inside a molding cavity. Several additional processing steps have been developed to increase strength, clarity, and enhance control over shape, but the basic process remains the same. The bottles are then filled and capped. Because of the low glass transition point of PET bottles, these containers can only be used in 85 degree Celsius hot fill applications, which limits their application in certain market sectors which require more aggressive canning to insure product sterility.
Plastic bottling technology does not lend itself well to using recycled material. Virgin resin is less expensive than post consumer product, and post consumer resin often has impurities which can migrate out of the container and contaminate the contents. Furthermore, because of the delicate chemistry of the polymers involved residual contamination by other plastics, labels, metal flakes, and other materials which are also present in the same waste streams can degrade the properties of the recycled material or altogether ruin entire batches of resin. Furthermore, all known methods of automatically cleaning the bottles with caustic washes and heat treatments ruin the bottles. The economy and technology of producing the bottles does not allow a closed-loop manufacturing process, and it is for this reason that post consumer PET resin finds use in secondary applications such as plastic logs and synthetic fibers, and is not currently formed into new bottles.
Like most solid waste, packaging is mainly disposed of through incineration or landfilling. According to the Environmental Protection Agency, 33.4 million tons, about 26% of all packaging, was recycled in 1990. Of the 33.4 million tons, half was packaging; the recovery rate for packaging was higher than any other municipal solid waste category tracked by the EPA. The usage of materials recovery facilities (MRF's) is responsible for these recovery rates, and is currently the option preferred over incineration or landfilling whenever possible. The following is a description of how aluminum, glass and plastics are separately recovered.
Aluminum can be recovered by several methods in the aluminum module of a basic MRF (see figure "Basic MRF"). Most often, aluminum is separated from other municipal solid waste with eddy currents. The principle operation of this method is that when an electrically conductive material such as aluminum is placed in a region of changing magnetic field, circulating electrical currents called eddy currents are produced in the material. The interaction of the eddy currents with the changing magnetic field results in a force on the metallic particle. It is the variation in this force that causes particles of different materials to trace out different paths and thus become separated. Eddy currents are primarily used to separate aluminum from any number of non-metals and are especially useful in an MRF since the primary feeder line is always one of commingled municipal solid waste. Other, less desirable methods of separating aluminum from non-metals include air classification and several wet processes such as jigging, water elutriation, and heavy media asportation. Depending on specific alloy specifications, postconsumer aluminum, usually in the form of UBCs, can be reused in amounts up to 100% of finished product.Glass. The two critical specifications of glass recovery are that the glasses be sorted by color (to control the cosmetic appearance of end products) and that the glass be free of all contaminants. Both specifications are difficult to meet, as glass often breaks in transport the from the curbside or point of consumer discard to the MRF; different colored glasses become mixed with each other and with other materials. The most successful methods of separating glass from commingled waste without subsequent color separation are trommeling, screening, air classification or combinations thereof. These techniques simultaneously break and densify the mixed glass cullet; a discrete densification step, usually pulverizing or glass crushing, may be avoided. Manual picking on a conveyor belt is the most common method of color separation, but other means are currently being tested as well. When specifications are suitably met, recovered glass beverage and food containers can be resold to glass container manufacturers for up to 100% of virgin materials. Plastics. The variety of resins and colors makes it difficult for the generator, curbside collection crew, MRF workers, or MRF mechanical devices to distinguish on type of plastic from another. Also, because plastics represent such a small fraction of MSW by weight, recycling plastic is usually not profitable enough. Therefore, plastics recycling technology is slow to develop. Polyethylene terephthalate and high density polyethylene are easiest to identify among other plastics and are the most commonly recycled plastics. They are usually separated from other resins by manual picking on the conveyor belt prior to such processes as shredding and baling or granulating followed by packing and shipment. In addition to manual sorting, plastics can be separated from other material with the help of air classification and vibration screening. The usefulness of these mechanically assisted methods heavily depends upon the design approach to glass recycling, glass breakage, and cross contamination. Recovered plastics only substitute about 20% of virgin resin.
Glass, plastic, and aluminum UBC's are all currently recycled in communities around the country. However the recyclability of the three containers differs; aluminum is the easiest to recycle, then glass, and lastly plastic. In addition, each type of beverage container has advantages and disadvantages depending on the recycling program used by the town.
Plastic beverage containers are the least recycled container. The current recycling rate of plastic bottles is 8.6%. Bottle manufacturers pay $40-$160 per ton of recycled plastic, however; it takes 15,000 bottles to make a ton. Currently the average plastic bottle contains 0-25% recycled plastic. The bottles are primarily turned into carpet fiber, and industrial scrapping, and fiberfill for such items as sleeping bags and ski jackets. Recycling plastic beverage containers has some severe limitations. The containers have a very high weight to volume ratio which makes them unprofitable to collect in curb side recycling programs. Collecting plastic bottles is like picking up air; the containers fill up the truck and are not worth very much. Pittsburgh and other cities have dropped plastic containers from their recycling program because of this problem. One solution to this problem is to have trucks equipped with shredders which decrease the volume of the plastic bottles. However the equipment is costly and it adds extra time to a recycling route. Contamination is another severe limitation in plastic beverage container recycling. Bottle caps are made of a different material then the bottles. The bottles are made from a resin called PET or polyethylene terephthalate and the tops are made from PVC's or polyvinyl chlorides. Just one top among thousand of bottles can ruin a batch of recycled plastic; the PVC's cause the batch to be to runny. These limitations but a damper on the success of plastic beverage container recycling.
Glass containers are recycled extremely efficiently with virtually no loss of materials. Glass containers are either recycled through curbside recycling programs or collected from bars and restaurants and refilled. The recycling rate for glass bottles is between 20% and 25%. The highest cost involved in glass recycling is in the separation of materials. There is no efficient way to separate the colored glass. Approximately 2/3 of the recycled glass beverage containers are clear, 1/4 brown, and the rest green. Clear glass can be negatively sorted down the line in a recycling facility, but brown and green must be separated. There is virtually no market for mixed colored. Clear glass brings in the most profit, recycling companies net $40-$50 per ton. Brown glass is usually sold for between $20-$30 per ton and green glass is sold for $0-$20 per ton. The price differences reflect the market sizes of the different colored glasses. Clear glass is primary used in soda bottles, brown in beer bottles and green glass is used in very few bottles. There is a gross of recycled green glass in the US market because foreign beer manufactures use green glass bottles but US manufactures use brown bottles. Most glass bottles have between a 30% -60% recycled content depending on how pure the recycled glass used to make the bottles was. Other uses for recycled glass include fiberglass production, glass foam beads in reflective paint, and "glassphalt" in road construction.
Recycling glass beverage containers has two major limitations which severely effect the quality of the glass. The first problem is that the glass bottles tend to break in transport to the recycling facility. The shards of different colored glasses then mix and there is currently no method for economically separating the shards. Since there is no market for mixed glass, the shards end up in garbage dumps. Studies have found that recycle trucks with separate bins help limit glass breakage en route to the recycling plant. However, the cost of converting from a fleet of trucks without separate compartments to one with separate compartments is expensive. Ceramic contamination in a glass furnace is the other major problem. Ceramic containers look and feel like glass containers but ceramic melts at a much higher temperature then glass. In the furnace, tiny bits of ceramic become lodged in the glass and make it brittle. The ceramic bits can also cause the furnace to explode. Consumer Glass, a Canadian company, has found that by grinding the glass and ceramic into a fine sand like powder the ceramic contamination is made harmless. However expensive mineral crushing equipment is required to crush the glass and ceramic mixture and most MRFs can not afford the cost. The profit the MRFs make on glass is limited by the quality of the glass they can produce.
Aluminum cans are the most profitable recyclable; MRFs receive $700-$880 per ton of recycled aluminum. The sale of aluminum accounts for an average of 50% of the revenue of an MRF. Currently approximately 63% of all the aluminum cans are recycled. There is only a 5% to 15% loss of materials when the cans are recycled back into cans. The average aluminum can consists of 53% recycled aluminum. The turnover time of the aluminum from one can to the next is around 6 weeks. The primary market for recycled aluminum is the can manufactures. Some aluminum is also sent to smelters which manufacture aluminum ingot from scrap.
The technology involved in recycling aluminum cans is extremely efficient and the effect of most contaminates has been removed. The primary limitation to aluminum recycling is the water content of the cans. Aluminum cans were designed to hold in water but aluminum buyers who pay by weight don't want the cans to have a high water content. MRFs are able to reduce the water content of the can to around 6%, however; most companies want the water content to be 4% or less. The companies then deducted the percentage over 4% from the price they pay for the aluminum. For example if a shipment of aluminum had a 6% water content, the company would deduct 2% from the overall price they would normally pay for the aluminum. Shredded cans have less moisture then whole cans but many MRFs can not afford to buy a shredder. A wire tumbler reduces the moisture content of the cans but not to the 4% desired by the can manufactures. The acceptable water content of aluminum cans is currently a big debate in the industry.
It is hard to assign cost values to the processing price of the different materials. However, it is clear from the high price recycling companies relieve for aluminum that it is the most profitable recyclable. Glass is a very sound second choice for ease of recyclability. Glass recycling has been going on longer then any other type of recycling and recycled glass has lots of uses in the market place. Plastic is the clear loser in recyclability. It is not economical to collect plastic in curbside recycling programs and contamination is hard to avoid.
It is pretty fair to say at this point that in the environmetnal race for a practical beverage container, plastic is the clear loser. However, what is not so clear is the differentiation between aluminum and glass in terms of an overall winning material; fortunately, a choice does not have to be made. Both glass and aluminum each have a different market. Glass is prinarily made for larger quantities and aluminum for single servings. A policy can be made promoting both materials.
The first thing to realize in defining a policy in a capitalistic economy is that nothing should be mandatory. The government should not have any absolute authority over a company (with exception taken to public safety). The key to enforcing a successful policy is making the people want to follow it. In most cases the best incentive is an economic one.
There are many relatively simple ways to convince a company to stop using plastic in their beverage container production. One is to provide tax breaks. For example, money spent on conversion from plastic can be made tax deductible. Another way is to provide research and development funding asistance for glass and aluminum processing and recycling technologies. This will encourage an increase in the efficiency of these processes.
In order to convine the consumer to buy glass and aluminum over plastic a different approach must be taken, economics isn't enough. There should be a premium placed on plastic containers in an attempt to dissuade the consumer from purchasing plastic. Additionally a larger deposit should be placed on all the containers to promote their recyclability. Finally a national marketing scheme, funded by the plastics premium, should be employed to convince the public that they should not want to buy plastic.
Any change is policy is a difficult and daunting task. It will never be met without resistance. But, that is okay. There comes a time where even the extreme effort of change is more feasable that the danger which lay ahead if that change is not incurred. We have reached that time. Let us make a change.
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