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Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
Polymer Processing Demonstrations Using PET Bottles Alfredo Luis M. L. Mateus*
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Colégio Técnico, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 31270-901, Brazil ABSTRACT: Polyethylene terephthalate (PET) bottles are readily available materials that can be used in a series of demonstrations highlighting the technological aspects of their production. The Make Your Own Bottle demonstration shows a simplified version of the blow stretch mold process used in industry. The Plastics Don’t Forget and Making Spirals demonstrations allow for discussions about polymeric glass transition and what happens to the plastic after stretching and cooling.
KEYWORDS: Polymer Chemistry, High School/Introductory Chemistry, Materials Science, Demonstrations, Hands-On Learning/Manipulatives
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to manufacture almost everything that we use in our daily lives and the impacts of these processes on the environment are discussed. Concepts like product life-cycle analysis, green chemistry, and sustainable development are discussed in several activities. The chemical processes involved in the production of polymers, paper, cement, and metals like steel and aluminum are researched by the students. Afterward, a series of experiments and demonstrations like the ones described here are conducted with each of the materials to give the students a better grasp of the materials’ properties. To assess student understanding of the chemistry involved in the demonstrations and to start discussions about it, the students are separated into small groups and asked to answer some questions in writing. For all three demonstrations, they should describe in their own words what happened in the experiment. For the Plastics Don’t Forget demonstration, the students are asked to compare the PET piece being stretched to a common office rubber band and explain what changes when we go from one material to the other. In the two other demonstrations, the students are invited to propose explanations for the behavior of the plastic before and after the experiment.
olymers are materials that are present everywhere in modern society. One of the main uses of plastics is for packaging, which accounts for more than 30% of the total production in the United States. Global production has doubled every 15 years since the 1950s, up to 230 million tons per year in 2012.1 Polyethylene terephthalate (PET) is the most commonly used polyester type of thermoplastic polymer. Laboratory experiments and demonstrations about polymer science have been the subject of several articles in this Journal.2−9 Still, some authors have pointed out that the number of experiments available for discussing polymer chemistry is smaller than the importance of the field would require.10 There have been descriptions of outreach activities using polymer science as a theme.11,12 Some of the polymer related experiments and kits that are available to teachers use specialty polymers, like poly(vinyl alcohol), 2,3 sodium polyacrylate,4 and sodium alginate,4,9 which are not similar to the bulk of the plastics used daily. Many articles specifically involving PET polymer lab activities are intended for undergraduate courses and use techniques that are beyond the reach of most high school teachers, such as thermal analysis,5 mechanical tests,6 or organic chemistry procedures like depolymerization or synthesis.7,8 The demonstrations presented here use readily available materials and show some very interesting properties of polymers. They can be used in an undergraduate or high school chemistry course, providing a starting point for relevant discussions about polymer properties and their use in our society. These experiments have been used in an environmental chemistry class that is part of a high school level Chemical Technician course in Brazil. In the first part of the class, the role of chemistry in the production of materials that are used © XXXX American Chemical Society and Division of Chemical Education, Inc.
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BACKGROUND
Plastic soda bottles are made with polyethylene terephthalate (PET). PET belongs to the polyester family of polymers and most of it (61%) is used for fiber production.13 Received: October 29, 2018 Revised: June 21, 2019
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DOI: 10.1021/acs.jchemed.8b00890 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 1. Steps for the Make Your Own Bottle demonstration. The preform is heated in boiling water (a), and the cap with the tire valve is screwed on (b). The bicycle pump is connected to the tire valve (c), and air is pumped in the preform until the expanded bottle is ready (d).
place where the bottles are filled is much more energy efficient than transport of full-sized empty bottles, which would require many more delivery trucks and fuel. A simple procedure for blowing a preform without using a mold is described in the Make Your Own Bottle experiment below. PET bottles are transparent, a desirable property for a packaging material. The transparency is associated with the degree of crystallinity of the polymer. Highly transparent polymers are amorphous or semicrystalline. Amorphous solids undergo a glass transition between a solid state and a more flexible and rubber-like state. This change occurs over a range of temperatures that is characteristic for each polymer and that depends on the level of crystallinity. PET has a glass transition temperature (Tg) that is close to the boiling temperature of water, around 75 °C.14 Above this temperature, the plastic is soft and can be stretched. Cooling the plastic below its Tg in this elongated state will “lock” the molecules in this new position. When the plastic is reheated above the Tg, we obtain a surprising result: the molecules “relax”, and the plastic returns to its original shape. This is demonstrated in the second experiment, called Plastics Don’t Forget. The sides of a soda bottle are examples of a stretched polymer that has been locked in this elongated position. In this case, heating the plastic above the Tg will not return it to the preform shape. The thick preform walls have been stretched beyond return. However, the tension remains, and we can observe the polymer relaxing by heating it. This is explored in
PET can be obtained by a polycondensation reaction of ethylene glycol and terephthalic acid, with water as a byproduct:
or by a transesterification reaction of dimethyl terephthalate with ethylene glycol. In this case, methanol is the byproduct:
The polymer obtained in the reaction is processed into pellets, which can be injection molded into preforms for bottle production. PET bottle production through the stretch blow molding process has been detailed elsewhere.14 The preforms are then transported to the beverage industry where the bottles are made. The preform is heated in an oven, placed in a mold, and blown with pressurized air. The mold is chilled with cold water so the plastic can quickly harden, and the bottle is released, ready for use. A single blowing machine can produce 10,000 bottles per hour. Transport of the small preforms to the B
DOI: 10.1021/acs.jchemed.8b00890 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 2. Plastics Don’t Forget demonstration. The threaded part of the bottle is heated in boiling water (a) and stretched with pliers (b,c). When the plastic piece is cooled, it remains stretched (d), but when it is placed back in hot water, it returns to its original shape (e,f).
easier to perform this part of the procedure with help of another person. While one person holds the preform and shows it to the class, the other quickly pumps the air. Place the bottle under tap water for a few seconds to cool it before opening it. If the bottle is opened before it is cooled, it may shrink and become deformed. The one-of-a-kind bottle can be passed around the class for inspection. The demonstration may be followed by showing a video of the industrial plastic bottle blowing process. Many of such videos can be found on YouTube.18,19
the reuse of the plastic in the bottles to make spirals, as described in the last experiment (Making Spirals).
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PROCEDURE
First Demonstration: Make Your Own Bottle
The procedure is shown in Figure 1. Before the presentation, drill a hole in a plastic bottle cap in such a way that a tire valve will fit tightly. Pull the tire valve through the hole, locking it in place. The construction of such a connector has been described before.15 During class, show the students a soda bottle preform (you can pass one around the class). Bring to their attention the fact that the preform has walls that are much thicker than the final bottle. We used 2 L preforms obtained from a local beverage company for free. Many companies that offer educational materials for science teaching sell preforms (also called “baby bottles”) in their catalogues, mainly for use as unbreakable test tubes.16,17 Discuss the reasoning behind the soda bottle preforms and the process used to make PET bottles. Tell the students that you will make a plastic bottle in class. Put water in the preform up to 1−2 cm below the threaded top. Place the preform in the beaker and fill it with water, making sure the water level is below the screw ring. Bring the water to a boil by using either an immersion heater or a microwave oven. The microwave oven is our preferred method for heating, because it heats the water inside the preform as well. The hot water on the inside of the preform helps to keep it hot enough for blowing. Carefully remove the preform from the beaker using pliers or thermal gloves and discard half of the water. Quickly place the cap with the tire valve on the preform and tighten it. This is important to avoid heating the top part of the preform where the screw is located. If this part becomes too soft, it may become difficult to screw the cap on. Connect the pump to the valve and pump air inside the preform. It is very important to work quickly so that the preform does not cool down. Keep pumping until it has grown to a size smaller than the regular bottle size of the preform (in our case, smaller than a 2 L bottle). It may be
Second Demonstration: Plastics Don’t Forget
The steps for the demonstration are shown in Figure 2. Cut the threaded part of a PET bottle with a hacksaw just below the ring. Fill a beaker with 150 mL of water. Heat it until the water is boiling. Turn off the heat and place the screw top in the hot water. After a few seconds, remove it from the beaker using pliers. Place the head of the pliers inside of the ring and force it open. The plastic easily deforms and stretches. Hold the pliers in this position for a few seconds while the plastic cools down. Remove the stretched plastic and show to the class that it is stuck in the new shape now. You may ask the students to make a prediction about what would happen if the deformed plastic piece is placed in the hot water again. After a few seconds in the hot water, the piece returns to its previous round shape. The process can be repeated many times. Different noses in the pliers will give different shapes to the stretched plastic. The stretched plastic pieces can be given away to the students. To show that the plastic really “remembers” that it once had a round shape, place the ring in recently boiled water, remove it with pliers and cut it open with scissors. Open the ring and hold it in this position until it cools down. Placing the now straight piece of plastic in hot water will make it slowly return to the circular shape. A projection system can be used to show the changes to a large room. Alternatively, this experiment can easily and safely be performed by small groups of students. C
DOI: 10.1021/acs.jchemed.8b00890 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
Figure 3. Making Spirals demonstration. The plastic bottle is cut in a long strip (a) and wound around a pipe or dowel (b). Boiling water is placed on top of the plastic (c); after it cools down, the plastic can be removed (d). The spiral is ready for use (e) and can be made in several different colors, widths, and shapes (f).
Third Demonstration: Making Spirals
Students were surprised and engaged throughout the demonstrations and actively participated in the discussions that followed. The phenomena demonstrated in the experiments can be presented to different audiences to generate curiosity about science in general and polymers in particular.
Figure 3 shows the procedure step by step. Remove the top of a soda bottle by making a small cut with a craft knife and then cut around the top with scissors. Cut a strip of plastic parallel to the bottom of the cylinder and keep going around the bottle to make a continuous band at least 1 m long. Wrap the plastic strip around a wood dowel or metal pipe. Hold the edges in place with clamps or tape. Place the whole set in a Pyrex bowl. Boil water and carefully add the hot water on top of the plastic until it is submerged. After 10 to 15 s, the plastic may be removed from the hot water and left to cool. The plastic spiral is removed from the dowel and shown to the class. Many shapes (circular, rectangular, hexagonal, etc.) can be created in this way by using different profiles for the wood dowel. The width of the plastic strip and the distance between turns as you wrap the plastic will also lead to different final results.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alfredo Luis M. L. Mateus: 0000-0001-7640-2389 Notes
The author declares no competing financial interest.
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ACKNOWLEDGMENTS The author would like to thank Iberê Thenório for assistance in the production of the photographs in this article.
HAZARDS Care must be taken while handling the beaker with hot water. Heat resistant gloves may be used. Check the preform for defects such as bubbles. We recommend that the Make Your Own Bottle experiment be performed only as a demonstration by the teacher. The plastic material used in these demonstrations can be safely sent for recycling. Be aware that there are plastic tubes that resemble bottle preforms in that they have threaded tops and caps but are used for packaging. The walls of these tubes are much thinner, because they are not intended for blowing into a bottle. Only use real, thick walled preform tubes, as the other tubes may burst upon pumping.
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REFERENCES
(1) Allwood, J. M.; Cullen, J. M.; Carruth, M. A.; Cooper, D. R.; McBrien, M.; Milford, R. L.; Patel, A. C. Sustainable materials: with both eyes open; UIT Cambridge: Cambridge, 2012. (2) Casassa, E. Z.; Sarquis, A. M.; Van Dyke, C. H. The Gelation of Polyvinyl Alcohol with Borax. J. Chem. Educ. 1986, 63 (1), 57−60. (3) McLaughlin, K. W.; Wyffels, N. K.; Jentz, A. B.; Keenan, M. V. The gelation of poly (vinyl alcohol) with Na2B4O7.10H2O: killing slime. J. Chem. Educ. 1997, 74 (1), 97−99. (4) Criswell, B. A diaper a day and what’s going on with gaviscon?: two lab activities focusing on chemical bonding concepts. J. Chem. Educ. 2006, 83 (4), 574−576. (5) Iler, H. D.; Rutt, E.; Althoff, S. An introduction to polymer processing, morphology, and property relationships through thermal analysis of plastic PET bottles. Exercises designed to introduce students to polymer physical properties. J. Chem. Educ. 2006, 83 (3), 439−442.
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CONCLUSION The demonstrations use an easy to obtain material and show surprising results. The goal of the demonstrations was to show students the behavior of polymers after heating and how that relates to their use in the production of everyday objects. D
DOI: 10.1021/acs.jchemed.8b00890 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Demonstration
(6) Erk, K. A.; Rhein, M.; Krafcik, M. J.; Ydstie, S. Demonstrating the Effects of Processing on the Structure and Physical Properties of Plastic Using Disposable PETE Cups. J. Chem. Educ. 2015, 92 (11), 1876−1881. (7) Donahue, C. J.; Exline, J. A.; Warner, C. Chemical recycling of pop bottles: The synthesis of dibenzyl terephthalate from the plastic polyethylene terephthalate. J. Chem. Educ. 2003, 80 (1), 79−82. (8) Kaufman, D.; Wright, G.; Kroemer, R.; Engel, J. ″New″ Compounds from Old Plastics: Recycling PET Plastics via Depolymerization. An Activity for the Undergraduate Organic Lab. J. Chem. Educ. 1999, 76 (11), 1525−1526. (9) Friedli, A. C.; Schlager, I. R. Demonstrating Encapsulation and Release: A New Take on Alginate Complexation and the Nylon Rope Trick. J. Chem. Educ. 2005, 82 (7), 1017. (10) Hodgson, S. C.; Bigger, S. W.; Billingham, N. C. Studying Synthetic Polymers in the Undergraduate Chemistry Curriculum. A Review of the Educational Literature. J. Chem. Educ. 2001, 78 (4), 555−556. (11) Ting, J. M.; Ricarte, R. G.; Schneiderman, D. K.; Saba, S. A.; Jiang, Y.; Hillmyer, M. A.; Bates, F. S.; Reineke, T. M.; Macosko, C. W.; Lodge, T. P. Polymer day: Outreach experiments for high school students. J. Chem. Educ. 2017, 94 (11), 1629−1638. (12) Cersonsky, R. K.; Foster, L. L.; Ahn, T.; Hall, R. J.; van der Laan, H. L.; Scott, T. F. Augmenting Primary and Secondary Education with Polymer Science and Engineering. J. Chem. Educ. 2017, 94 (11), 1639−1646. (13) Weissmann, D. PET Use in Blow Molded Rigid Packaging. In Applied Plastics Engineering Handbook, 2nd ed.; Kutz, M., Ed.; William Andrew Publishing: New York, 2011; pp 717−741. (14) Belcher, S. L. Blow Molding. In Applied Plastics Engineering Handbook, 2nd ed.; Kutz, M., Ed.; William Andrew Publishing: New York, 2011; pp 265−289. (15) Zipp, A. P.; Kavanah, P. Gas experiments with plastic soda bottles. J. Chem. Educ. 1998, 75 (11), 1405−1406. (16) Soda Bottle Preforms and Caps. Educational Innovations, Inc. https://www.teachersource.com/product/preforms-and-caps/labequipment-general (accessed June 21, 2019). (17) World’s Best Plastic Test Tubes − Baby Soda Bottles. Steve Spangler Science. https://www.stevespanglerscience.com/store/babysoda-bottles-giant-test-tubes.html (accessed June 21, 2019). (18) How Its Made. How Its Made - 157 Plastic Bottles and Jars, 2014. YouTube. https://www.youtube.com/watch?v=Gt9DRifRwn0 (accessed June 21, 2019). (19) Sidel. Blowing step by step, 2011. YouTube. https://www. youtube.com/watch?v=eyiu18DsItk (accessed June 21, 2019).
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DOI: 10.1021/acs.jchemed.8b00890 J. Chem. Educ. XXXX, XXX, XXX−XXX
