Polymer Tensile Strength

1. Polymer Tensile Strength Michelle Fong and Daniel Matthews Engineering 45: Properties of Materials Santa Rosa Junior College December 5, 2005

2. Polymers: A polymer is a generic term used to describe a substantially long molecule with structural and repeating units called monomers. In chemistry, a monomer is a small molecule that may become chemically bonded to other monomers to form a polymer. The process of converting these units to a polymer is called polymerization. Polymerization is a process of reacting monomer molecules together in a chemical reaction to form linear chains or a three-dimensional network of polymer chains. There are many forms of polymerization and different systems exist to categorize them. The first ‘synthetic’ polymers of the 19th century were actually formed by modifying natural polymers. The first man-made polymer, the phenol-formaldehyde resin bakelite, was made in 1872. However, research into polymers and polymerization only really started to grow in the 1930s after the discovery of polyethylene by the chemical company ICI (Imperial Chemical Industries). The mechanical properties of polymers can be described with much of the vocabulary introduced with metals. Tensile strength and modulus of elasticity are important design parameters for polymers as well as inorganic structural materials. There are few natural polymers generally considered to be “plastics”.

3. History of the Plastic Bag 1957 The first baggies and sandwich bags on a roll are introduced. 1958 Poly dry cleaning bags compete with traditional brown paper. 1966 Plastic bag use in bread packaging takes over 25 to 30 percent of the market. Plastic produce bags on a roll are introduced in grocery stores. 1969 The New York City Sanitation Department's "New York City Experiment" demonstrates that plastic refuse bag curbside pickup is cleaner, safer and quieter than metal trash can pick-up, beginning a shift to plastic can liners among consumers. 1974/75 Retailing giants such as Sears, J.C. Penney, Montgomery Ward, Jordan Marsh, Allied, Federated and Hills make the switch to plastic merchandise bags. 1973 The first commercial system for manufacturing plastic grocery bags becomes operational. 1977 The plastic grocery bag is introduced to the supermarket industry as an alternative to paper sacks. 1982 Kroger and Safeway start to replace traditional craft sacks with polyethylene "t-shirt" bags. 1990 The first blue bag recycling program begins with curbside collection. Consumer plastic bag recycling begins through a supermarket collection-site network. 1992 Nearly half of U.S. supermarkets have recycling available for plastic bags. 1996 Four of five grocery bags used are plastic.

4. Has this ever happened to you?

5. Project Overview Performed Tensile Tests on Kitchen/Trash/Grocery plastic bags to find: • Stress, σ • Strain, ε • Modulus of Elasticity, E • Yield Strength, σY.S. • Tensile Strength, σT.S. • Toughness • Ductility • Price Comparisions

6. Procedure: • Cut a 2.54 cm x 30.48 cm strip vertically starting from the bottom crease and cutting through both sides of the plastic bag. • Record the initial thickness with a micrometer. • Record the initial width with a metric ruler. • Clamp one end of the plastic strip to the top of the apparatus. • Record the mass of the clamp device that the hanger will be attached onto. • Record the mass of the hanger that masses will be added onto. • Clamp the other end of the plastic strip with the clamp device and record the initial length of the plastic strip before letting it hang. • Add a 20 gram mass and immediately record the new length.; Repeat until plastic strip breaks. • Calculate it’s stress (s= P/Ao) and strain (e = DL/Lo) for each point recorded and graph Stress vs. Strain.

7. Experiment Samples: 13 Gallon Kitchen Bags: • Safeway • Costco Kirkland • Glad Force Flex • Hefty Ultra Flex 30 Gallon Trash Bags: • Glad Force Flex • Hefty Ultra Flex Shopping Bags: • Safeway Grocery Bag • Longs Drugs Bag

8. Modulus of Elasticity: 128.35 MPa Yield Strength: 7.93 MPa Tensile Strength: 10.47 MPa Toughness: 8.69 MPa Ductility: 89%

9. Modulus of Elasticity: 132.83 MPa Yield Strength: 6.64 MPa Tensile Strength: 9.01 MPa Toughness: 11.25 MPa Ductility: 150%

10. Modulus of Elasticity: 10.75 MPa Yield Strength: 7.11 MPa Tensile Strength: 8.97 MPa Toughness: 40.53 MPa Ductility: 108%

11. Glad 13-Gal Kitchen Bag

12. Modulus of Elasticity: 149.22 MPa Yield Strength: 8.3 MPa Tensile Strength: ~8.99 MPa Toughness: 40.53 MPa Ductility: ~490%

13. Hefty 13-Gallon Kitchen Bag

15. 13-Gallon Kitchen Bags

16. Modulus of Elasticity: 14.57 MPa Yield Strength: 8.56 MPa Tensile Strength: 10.59 MPa Toughness: 13.21 MPa Ductility: 168%

17. Modulus of Elasticity: 169.46 MPa Yield Strength: 8.91 MPa Tensile Strength: ~11.96 MPa Toughness: 53.95 MPa Ductility: ~543%

18. Hefty 30-Gal Trash Bag

20. 30-Gallon Trash Bags

21. For Trial 2: Modulus of Elasticity: 476.28 MPa Yield Strength: 21.5 MPa Tensile Strength: 25.99 MPa Toughness: 18.87 MPa Ductility: 78%

22. Why Safeway Trial 1 was Inconclusive:

23. Modulus of Elasticity: 304.95 MPa Yield Strength: 24.4 MPa Tensile Strength: 28.95 MPa Toughness: 39.16 MPa Ductility: 150%

24. Longs Drugs Bag

26. Grocery Bags

28. All Bags

29. Recycled Plastic

30. What you get for your money!

31. References: • http://en.wikipedia.org • http://www.plasticbag.com/environmental/history.html • http://www.budgetpackaging.com/inofgaba.html • http://www.santarosa.edu/~yataiiya/E45/LAB/TENSILE%20TEST.pdf • http://www.americanplasticscouncil.org/s_apc/sec.asp?CID=313&DID=931