Anticipating Student Questions (answers to questions students might ask in class)
“Why don’t we recycle polystyrene (Recycle code #6)?”See “Polystyrene recycling” under “More on recyclable plastics” above.
“What do the recycle code numbers mean?” The article shows you what each type of plastic is, according to the recycle code numbers. The numbers themselves were established based on the volume of each type of plastic in the waste stream. PET (or PETE) was the most used—and discarded—plastic at the time the recycle code was established. The thinking was that the most used and discarded plastics would be the most logical ones to recycle since they would likely provide profit due to their higher volume for recycling and to their after-processing resale value.
“Why don’t thermoset plastics soften when heated?”Cross-links between polymer strands are not easily broken. Heat causes increased molecular motion in polymers (as in all material), but the heat is as likely to break bonds along the polymer strands as it is to break the bonds in the cross-links. This results in shorter strands in the polymer, which destroys the integrity of the polymer. The long strands can’t be easily reformed to restore the polymer to its original condition. In a thermoplastic, heating breaks the weaker intermolecular bonds between the strands, but most of the strands themselves remain intact, and when the heated plastic is remolded and cooled, the attractions between the unchanged strands can re-establish themselves and restore the plastic to close-to-its-original condition.
“If we can produce PET indistinguishable from the original virgin PET by chemical recycling, why don’t we do it for all the plastics? That way we’d never run out of plastic.”While this is logical from a materials point of view, what is missing from this equation is the chemical needed (methanol) to make the reverse reaction happen, the heat and pressure (both energy-intensive) needed to make it happen, and the machinery needed to make it happen. It is a very expensive process. Perhaps someday we will come to the point where we will have to do this reverse polymerization process because we have no more raw materials from which to make new plastic, but by then materials will be very expensive, indeed.
“Why don’t we have small-scale recycling here in the US, like in India?”First, it is very labor-intensive, as well as energy-intensive. It takes a lot of energy (fuel) to collect the plastic for recycling (think how much energy it would take if we each drove our waste to a small-scale recycling facility, instead of having one truck come and pick it all up for all of us), and it takes a lot of energy to run the machinery. It is more energy- and labor-efficient to centralize the process in a large-scale plant. Second, as you can see from the article, there are serious health hazards to humans associated with the small-scale recycling operations in India and elsewhere. Our government regulations would never allow people to be exposed to the machinery and dust, etc. Our large-scale operations are automated, in part, to avoid human exposure to these hazards, and they contain the dust so that it is not released into the atmosphere.
In-class Activities (lesson ideas, including labs & demonstrations)
Recycle PET into fibers by heating a small piece of a 2-Liter PET soda bottle (or smaller PET bottle) on a piece of aluminum foil on a hot plate and use a wooden splint to draw the molten PET info long fibers. You can find the write-up for the activity at the Macrogalleria website, http://pslc.ws/macrog/demos/recycle.htm, or at the Polymer Ambassadors website, http://polymerambassadors.org/fibers.pdf. The Polymer Ambassadors page also includes a simulation of ram extrusion of plastic involving polyvinyl alcohol in a syringe, injected into acetone.
You can view this YouTube video to show the industrial process of recycling PET from bottles into fibers and cloth: http://www.youtube.com/watch?v=zyF9MxlcItw&feature=related.
The American Chemistry Council has an updated version of their “Hands on Plastics” kit for middle/high school students. It contains samples of the major plastic resins from the recycle codes, a PET pre-form and a sample of plastic lumber recycled from milk jugs, as well as lesson plans to cover several topics in plastics, including a lab to separate and identify the six major recyclable plastics. The site below gives a good description of the kit, as well as online access to the teacher materials and 8 short videos for students. The kit costs $12.99 and can be ordered online at http://www.americanchemistry.com/store/detail.aspx?ID=259&CategoryID=14&ProductType=-1&ParentID.
To show how many PET bottles we recycle in the US daily—and how many bottles are discarded—see this animated video, “Animated Water Bottle Recycling Rates”: http://www.youtube.com/watch?v=OZbTXDkrD1o. It shows a “trickle” of bottles being recycled, and then a “waterfall” of bottles being discarded, a bit of an eye-opener.
To show that recycling of plastics isn’t the “only game in town”, you might want to have students try their hand at recycling paper. See “Making and Sizing Paper” on the Chemical Heritage Foundation’s “Science Alive!” website at http://www.chemheritage.org/percy-julian/activities/6a.html. Click on the “For Teachers” button at the top to access background material and alignment with National Standards.
The Fun Science Gallery website has a section called “Making and Recycling Paper at Home” that describes the paper-making process in detail, and has nice photos to support the steps in the process. It also discusses a bit of recycling of paper at the end. The site also discusses a bit of the history of paper before the paper-making activity, and it gives a brief web bibliography of paper-related sites. View it all at http://www.funsci.com/fun3_en/paper/paper.htm#3.
The Cornell Center for Materials Research provides a series of polymer activities for high school students and teachers, called “Polymer Investigations”. It consists of a series of four days of class activities that could be used as part of a unit on organic chemistry, or on a short unit just on polymers. The activities include drawing fibers from plastics, as noted in 1) above, as well as an introduction to polymers, and conducting trash audits to discover more about recycling. Teacher information and student investigation forms are all available at http://www.ccmr.cornell.edu/education/modules/documents/PolymerInvestigations.pdf. Student work includes extensions involving research and group presentations on the uses and disposal of petroleum-based and/or bio-polymer materials.
You can have students study the effects on various plastics of being buried in a landfill for a class experiment. Here is a site that uses this topic as a science fair project for students. It suggests allowing at least three months to complete the project. It deals with paper bags, plastic bags, and newspaper. (http://www.education.com/science-fair/article/biodegrade-plastic-paper-newspaper/)
Although the following web site is from a PBS video that focused on recycling of the Trabant, a Russian car, it contains two activities that simulate 1) sorting materials from a mixture of waste for recycling and 2) a landfill. Each gives students a bit better idea of these two processes that municipal waste managers face in their daily routine. The level is probably middle school, but the ideas are relevant to high school. The sorting activity uses static electricity, magnetism, density, coefficient of friction and air pressure to separate materials. (http://www.pbs.org/safarchive/4_class/45_pguides/pguide_402/4542_trabant.html)
You can show the difference between thermoplastics and thermosetting plastics using craft store materials. Friendly Plastic® and Sculpey® III are two materials available in most craft/art stores. Friendly Plastic® is a thermoplastic, which is fairly rigid at room temperature, but it can be heated and then molded into a specific shape, and when it cools it will retain that shape. Sculpey® III is a thermoset; it is a polymer clay that is soft and can be shaped at room temperature, but when baked (“fired”) in an oven, it becomes rigid, just like fired natural clay. Upon reheating, the Friendly Plastic® can be re-shaped (thermoplastic). The Sculpey® III is permanently held in its baked shape; further heating will not change the Sculpey® III (thermoset).
A lesson plan for a student activity using these materials can be found at http://www.midmichiganspe.org/pdfs/documents/thermoplasticandthermoset.pdf. This activity also uses two-part epoxy putty as another example of a thermoset plastic. The instructions give teachers a few pieces of background information, including mention of polycaprolactone and its unavailability at the time of writing the activity. That is not true now (2010); it is available as Friendly Plastic®.
The MSDS for Friendly Plastic® can be found here: http://cdn.dickblick.com/msds/DBH_60602XXXX.pdf. It does NOT give its chemical name or formula, but Wikipedia states that Friendly Plastic® is one of several trade names for polycaprolactone.
Glencoe publisher has a WebQuest project on recycling plastics for its middle school book Physical Science. You could take this idea and expand on it as a way to cover the topic of recycling plastics (or for the topic of polymers) in a controlled assignment, without spending much class time teaching the topic. (http://glencoe.mcgraw-hill.com/sites/0078779626/student_view0/unit5/webquest_projects.html)