February 2011 Teacher's Guide Table of Contents

Background Information (teacher information)


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Background Information (teacher information)

More on the history of recycling bottles
First recycle bill (for bottles only)
Recycling of plastics didn’t happen on a large scale until several decades after plastics had become ubiquitous in our daily lives. It was only after people realized that the ever-increasing percentage of plastics in the huge amount of waste we were dumping in landfills would stay there for decades, and maybe centuries, that companies and municipalities saw the need for, and the opportunity to make money from, recycling.
The first recycling program (for any material) was the result of a law passed in 1971 in Oregon, called the “Bottle Bill”. It involved placing a refundable 5-cent deposit on all beer and soft drink beverage containers, which at that time were mainly glass and aluminum. The bill originally was designed to cut down on litter along roadways and park paths. It was successful at this task in that it reduced the number of bottles in roadside litter from 40% of the total litter in 1971 to about 6% by 1979. (It was so successful that nine other states passed similar bills.)
Another goal of the bill was to promote the use of refillable bottles, in order to minimize the amount of raw material needed to produce new bottles and to reduce the amount of energy needed to manufacture the new bottles. Returning bottles meant they could be washed, sterilized and refilled, instead of throwing them “away”. [Note: So, one might say, this wasn’t a “recycle” bill at all, but a “reuse” bill. Reusing is actually preferable to recycling because it uses far less energy to reuse an object for the same purpose as that for which it was designed, as it would be to reprocess the material into a new object.]

From the Oregon Department of Environmental Quality website, Oregon Bottle Bill: Then and Now: “Refillable bottles were common prior to passage of the bottle bill. In 1971, about 36 percent of beer and 53 percent of soft drink bottles were true refillable bottles. Non-refillable bottles made up 31 percent of beer and 7 percent of soft drinks while cans made up 33 percent of beer and 40 percent of soft drinks. The immediate effect of the bottle bill was a sharp reduction in both cans and non-refillable bottles. Shortly after passage of the law, more than 90 percent of beer and soft drinks sold in Oregon were sold in refillable glass bottles.” (http://www.deq.state.or.us/lq/sw/bottlebill/thenandnow.htm)

A gradual shift from local bottling plants to national production plants serving the whole country reduced the need and financial rewards of the refillable bottle. This is because it cost too much to transport refillables long distances to regional or national plants, reducing the profit margin considerably and opening the door for beverage manufacturers to lobby to return to non-refillable bottles.
A most noticeable effect of the bill has been the waste reduction aspect: since its inception, and ongoing, the bill has resulted in return rates in Oregon for beverage containers covered under the bill have exceeded 80%, with some years reaching 94%. Although rates have dropped back to 80% in recent years, the rates for other containers not covered by the bill are considerably lower, in the 36% range. Of course, 80% return means about 20% of bottles go into landfills and are unredeemed at the stores, which allows the store owners to pocket those refunds as profit.
Although only glass and aluminum were included in the original Bottle Bill, this was not because of the design of the bill, but because these were the only two materials used for beverage containers in 1971. The bill, however, did not state the composition of the container; it only said that it applied to beer and soft drink containers. That meant that when PET finally arrived on the scene about 10 years later, the bill was flexible enough to cover that material also. Eventually, PET bottles drastically outnumbered glass bottles in the refundable-deposit bottle arena.

The Oregon Bottle Bill lasted in its original state until 2007. It was not the advent of plastics that caused its eventual change, rather the bill was expanded in 2007 to include water and flavored water containers, drinks that hadn’t even been “invented” way back in 1971. Interestingly enough, the expanded bill wasn’t as all-encompassing as the original bill, as it failed to include teas, juices and other new beverages, containers of which now litter roads, since they have no deposit requirements, and thus no value to consumers. The deposit fee remained 5 cents in the expanded bill, even though a nickel in 1971 would now be worth about 26 cents.

(Oregon Department of Environmental Quality’s Bottle Bill webpage: http://www.deq.state.or.us/lq/sw/bottlebill/index.htm)

(Oregon.gov’s Bottle Bill & Redemption Center Info webpage: http://www.oregon.gov/OLCC/bottle_bill.shtml)
The idea of widening the net of states that have “bottle-bill” legislation apparently is not dead. The “Bottle Bill Resource Guide” is a web site dedicated to increasing the number of states that legislate a deposit-refund system to greatly increase the recycle rate of plastic, glass and aluminum beverage containers. (http://www.bottlebill.org/)
More on recycle codes
As noted in the article, the waste stream of plastics contains many different types of plastics, each of which must be processed separately from the rest, in order to ensure the integrity of the resulting recycled plastic. Recognizing this growing problem, in 1988 the Society for Plastics Industry (SPI) issued the now-familiar plastics recycling code (although they call it the resin recycling code). The goal of the code, now recognized internationally, was to make it easier for the public to recognize the various types of resins, to make it easier to separate the various high-volume polymer types from the comingled waste stream—to make recycling easier for all (and therefore more probable and more profitable).

For more information on the Society for Plastics Industries recycle code, directly from SPI, see http://www.plasticsindustry.org/AboutPlastics/content.cfm?ItemNumber=823&navItemNumber=2144. A short 4-minute video explains the recycle code numbers 1-6 and provides a bit more about the meaning of recycle code 7.

More on recyclable plastics
The first 2-liter bottles
In the interest of reducing the amount of plastic going into landfills in the first place, the plastic bottle industry did much research in the early days of PET bottle manufacture into reducing the weight of the 2-liter soda bottle (and other PET bottles as well). Original 2-liter PET bottle molds formed a round-bottom bottle. That needed a stabilizer cup glued to the bottom of the bottle to help them stand up. This cup was made of a different polymer, most likely polyethylene. This, of course, added cost to the bottle, requiring two separate pieces of plastic, and dabs of glue to hold the two together. They were able to eliminate the need for the bottom stabilizer cup that first appeared on plastic soda bottles by including in the molds for the bottles five small indentations in the bottle itself, making feet” to stabilize each bottle.
This also eliminated the second type of plastic from the waste stream, making it easier to recycle PET bottles. It also reduced the weight of the bottle. From 1980-1995, over 15 years, the PET plastic bottle industry was able to lower the total mass of the PET in two-liter bottles from 68g to 48g, according to the National Association for Plastic Container Recovery (NAPCOR).
Polystyrene recycling

Most municipal recycling systems do not include polystyrene (PS, #6 recycle code). There are several reasons for this. The first is that much polystyrene waste is expanded polystyrene, EPS. This is plastic that has a blowing agent added to it in processing so that the actual plastic contains closed cells that contain gas from the blowing agent that was produced with the heating and processing of the resin and resulted in the bubbles forming in the plastic. This blowing agent used to be CFCs, until they were outlawed by Federal regulation. The blowing agent of choice now is pentane, although for food packaging containers the agent is carbon dioxide. The density of “normal” polystyrene, the plastic is about 1.05 g/cm3. The density of expanded polystyrene, EPS, is very low, from 0.016-0.64 g/cm3. When recycling firms discuss the amount of PS recycled, they will typically say that the mass of PS in the municipal waste stream (MWS) is extremely low, about 1% of the total waste. But that hides the fact that the volume of polystyrene in the waste stream is much larger, since its density is so low. This large volume is difficult to handle in a municipal waste stream and adds much bulk to the waste collected, while adding little actual material (plastic) and thus little financial value.

A second reason for PS not being included in recycling programs is that there is almost no secondary market for recycled PS, resulting in a low resale value for recycled polystyrene. This is a bit of a “catch-22” since, if there were a secondary market, that would raise the scrap

Value of PS, and that in turn would encourage more municipalities to recycle PS to provide more income.

Recycling statistics
The Container Recycling Institute maintains databases to show the production, use, recycling and wasting of PET vs. aluminum cans and steel cans. The graph below shows the production share for each of the categories noted. Note that aluminum cans, both soda and beer, account for almost half the total beverage container market. Note also that production of PET bottled water containers now (2006) outnumbers PET soda bottle production.

More on plastics in a landfill
Trash bags or grocery bags are made of polyethylene. In attempts to make them biodegradable, scientists have added corn starch to the plastic. Corn starch will degrade in the presence of air and water, so adding some to the plastic should make it more degradable over the long term. According to Dr. Ramani Narayan, a researcher at the Michigan Biotechnology Institute in Lansing, these experiments have met with limited success.

For very strong, very stiff trash bags, cellulose or woody materials can be used,” he says, “but if you want a cheaper, more biodegradable trash bag material, and are willing to sacrifice some strength, then cornstarch can be used.” Presently [1991], most biodegradable trash bags contain around 6% cornstarch. Some bags contain 30-60% cornstarch.

In a six-month study, chemist Michael Tempestra at the University of Missouri in Columbia exposed polyethylene films, with and without 6% starch, to conditions simulating a landfill, a compost heap, an anaerobic (without oxygen) waste-treatment plant, and surface litter. His findings showed the starch was removed from the polyethylene in all these environments, and that the polyethylene degraded to smaller molecules of waxy materials. However, the extent of the degradation was unpredictable-sometimes the samples lost a lot of polyethylene; other times very little.

(Downey, C. Biodegradable Bags ChemMatters, October, 1991, 9 (1), pp 4-6.)

It appears more work was necessary at that time.
More plastics today are being made to be degradable. Biodegradableplastics.org’s website contains information on biodegradation and photodegradation, as well as bioplastics and composting, a newer mechanism for degrading plastics that is designed to keep them out of landfills. For another take on the story of biodegradability, see “Biodegradable Plastics: True or False? Good or Bad?” on the sustainableplastics.org website at http://www.sustainableplastics.org/spotlight/biodegradable-plastics-true-or-false-good-or-bad. It shows 6 companies’ claims of their products’ biodegradability, but calls into question the actual properties of the products. (Of course, you should note that the goal of sustainableplastics.org is to manufacture biodegradable plastics, so they have a vested interest in making petroleum-based plastics look bad.)

Yet another website, mindfully.org, has prepared a brief fact sheet describing three generations of starch-based plastics. The first plastic bags made with starch contained only 5-20% starch, and were not very biodegradable, since they still contained 80% synthetic polymer material (probably polyethylene). Second generation starch-based plastics contained larger amounts of starch (50-80%), and the starch was actually incorporated into the plastic, making it more biodegradable, but still not completely so. The third generation is actually 100% starch-based biopolymeric plastic. See the site at http://www.mindfully.org/Plastic/Biodegradable-Plastic.htm.

More on thermoplastics and thermoset plastics
Thermoplastics are typically solids at room temperature, but will melt or become soft when heated. At these higher temperatures, they can be molded into useful objects that retain their shapes when cooled again. Thermoplastics are usually linear or slightly branched polymer chains. As such, when heated they do not chemically bond with each other, but instead the long chains are attracted to each other by van der Waals forces. These weak forces cause the chains to clump together and entangle one another, like a clump of cooked spaghetti strands.
If heated again to the same higher temperatures, they will re-melt and can be re-molded, over and over again. When heated, the long polymer strands slide past one another (called plastic flow) and they tangle around one another (due to increased molecular motion). Weak van der Waals forces arise between adjacent atoms on juxtaposed polymer strands. Once cooled, these forces maintain the entanglements to hold the long strands in place, and the object maintains its shape—so long as the plastic is not heated sufficiently to once again agitate the polymer chains, weakening the effect of the van der Waals forces, enabling the chains to move and realign once again.
It is this ability to be heated and re-molded that makes thermoplastics so valuable in the recycling industry, as they can be collected, processed, heated and reformed into new useful objects. Some degradation of the polymer does occur, however, after each reheating and cooling, so thermoplastics cannot be recycled indefinitely. That is why virgin resin is often added to recycled plastic in processing—to increase its strength and durability. You can read more about the characteristics of thermoplastics at Wikipedia: http://en.wikipedia.org/wiki/Thermoplastic.

Thermosetting plastics (thermosets—[set with heat], for short) are soft solids, or even liquids, at room temperature. They are sometimes considered to be pre-polymers. However, at higher temperatures they will melt and become malleable. At these high temperatures (generally above 200oC) they will form crosslinks between the long polymer strands, forming a three-dimensional network of polymer molecules. These crosslinks are permanent chemical bonds, not just intermolecular (secondary) bonds (as in thermoplastics). When molded and then cooled, the polymer strands are held by the crosslinks; they are said to be “cured”, as in curing rubber (also a thermoset). Usually a curing agent is needed to serve as the permanent crosslink between polymer strands.

These thermoset plastics are locked in that shape permanently. Upon reheating to the same high temperatures as before, they will not melt because the crosslink chemical bonds that formed the first time still remain in place. During the formation of the crosslinks between polymer strands in the first heating, the polymer itself became larger molecules, resulting in higher melting temperatures than the pre-polymer. Thus reheating the plastic to its original melting temperature will have no effect on the plastic. Further (higher temperature) heating will only result in the degradation or charring of the thermoset.
The permanence of thermosets makes them practically useless in recycling efforts because there is no after-market for these materials. Their only value might be as a fuel, since they are still primarily hydrocarbons and will therefore still burn. Gases produced from the burning of thermosets then become a problem for industries using them for fuel.
This table, from a NASA publication, shows some of the differences between thermoplastics and thermosets.


  • Description

  • Melt, flow and cool polymer matrix into final shape

  • Remeltable

  • Advantages

  • Directly recyclable with some loss of properties

  • Shorter cycle time

  • Disadvantages

  • Dimensional stability at high temperatures

  • Paint offline

  • Examples: Nylon, PPS, PEEK, [PE, PET]


  • Description

  • Mix and flow resin, and curing agent

  • React to form polymer matrix with final shape
  • Crosslinked, cannot be melted

  • Advantages

  • Higher use temperatures

  • Paint online with steel

  • Disadvantages

  • Secondary recycling

  • Examples: Polyurethanes, epoxies

More on the methods of recycling of plastics
As mentioned in the article, physical recycling, also called mechanical recycling, involves collection, sorting, chopping, washing, heating, and remolding or re-spinning the plastic (into fiber). These are physical changes in the material, and at the end of the process we have essentially the same material as we had at the start of the process.
Chemical recycling can be done in one of four ways:

  • Pyrolysis, where plastic waste is heated to very high temperatures in a vacuum to produce a mixture of gas and liquid hydrocarbons that are not unlike crude oil before its processing

  • Hydrogenation, where plastic waste is heated (again, to very high temperatures) with hydrogen, which results in “cracking” of the polymers into a liquid hydrocarbon

  • Gasification, whereby plastic waste is heated in air, which results in a mixture of carbon monoxide and hydrogen gases. These are then used to prepare new raw materials, such as methanol, which can also be used as a fuel.

  • Chemolysis, whereby individual plastics are treated chemically or de-polymerized and returned to their original monomers. (This is the process described in the article.

Any of the above four processes involves changing the plastic chemically into new substances; hence the term, chemical recycling.


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