Tensile Strength and Water Vapor Transmission of Plastic Food Bags
Researched by Carmen L.  2003-04	PURPOSE  HYPOTHESIS  EXPERIMENT DESIGN  MATERIALS  PROCEDURES  RESULTS  CONCLUSION  RESEARCH REPORT  BIBLIOGRAPHY  ACKNOWLEDGEMENTS  ABOUT THE AUTHOR

The first purpose of this experiment was to find out how much water vapor could pass through different food grade plastic bags.

The second purpose of this experiment was to find out the tensile strength of various food grade plastic bags

I became interested in this idea because people use plastic bags in their everyday lives, to keep food fresh and sanitary. Last year I tested tensile strength of garbage bags, and I am interested in doing more with plastics. 

The information gained from this experiment will help product consumers to make wise purchasing decisions.

My first hypothesis was that the greater the wall thickness of the bag, the less water vapor would pass through.

My first hypothesis was that the thicker the bag the stronger its tensile strength will be.

I based my 1st hypothesis on a statement made by Scott McGregor, a chemist specializing in plastic: “The thicker the bag, the less water vapor will pass through it“. I based my 2nd hypothesis on another statement made by Scott McGregor: “the thicker the bag, usually means the stronger the tensile strength.

The constants in study 1 were:
* Temperature in the room 
* Humidity the film is exposed to 
* Size of plastic tested
* Machine used (Moisture Vapor Transmission Rate Analyzer)
* The age of the bag (unused)
* Way of cutting plastic 

The constants in study 2 were:
* Temperature in the room 
* Humidity the film is exposed to 
* Machine used (Tensile Tester
* Size of plastic tested (2.54x2.54 cm.)
* The age of the bag (unused)
* Way of cutting plastic into 2.54x2.54 cm. Squares
* Stress on plastic when put into jaws (.15 -.25 psi.)

The manipulated variable was type of food bag that was tested.

The responding variable in study 1 was the amount of water vapor that passed through the bag.

To measure the responding variable in study 1, I used a Mocon„ moisture vapor transmission rate analyzer. (MVTR.)

The responding variable in study 2 was the tensile strength.

To measure the responding variable in study 2, I conducted an elongation test, with a Tensile Tester machine.

Test 1
1. Prepare bag A samples by cutting plastic bags into the die shape, by placing the die over the bag, and running a razor blade around the edges.
2. Gauge the sample pieces thickness, in at least 10 different places, and find the average thickness of the sample piece.
3. Moisten the sponge, and place it in the machine. 
4. Grease the outer edges of the sample piece, and place onto the machine
5. Run Test.
6. Run procedures 1-5 on each bag.

Test 2
1. Look at bag “A” very closely, and try to find out which is transverse direction and machine direction
2. Label the machine direction with the red pen
3. Label the transverse direction with the black pen
4. Carefully fold bag “A” in half, while leaving the red and black marks you have made showing
5. Carefully fold bag “A” in half again, while leaving the red and black marks you have made showing
6. Clip two clips on the top of the folded bag
7. Label bag “A” by taking a small piece of paper, and writing the brand of bag on it, and clip the paper with one of the two clips
8. Cut the folded bag “A” into 2.54x2.54cm. Test strips in transverse direction with die cut. Do this twice
9. Label the top test strip of bag “A” transverse direction with the black marker
10. Put the transverse direction pieces clipped to label
11. Cut the folded bag “A” into 2.54x2.54cm. Test strips in machine direction with die cut. Do this twice
12. Then label the top test strip of bag “A” machine direction with red marker
13. Put the machine direction pieces clipped to label, but do not get them confused with the transverse direction ones
14. With thickness tester machine, measure thickness of bag “A” 
15. Input thickness of bag “A” into computer
16. Place one test piece of bag “A” into the jaws of the tensile tester with around .30 to .40 psi
17. Don’t forget to check if the piece is lined up straight when you lock the jaws
18. Press the lock jaws button
19. Press button on computer to start test
20. When test is complete, unlock the jaws and allow tested piece to fall
21. Repeat steps 14-20 until all pieces of “A” have been tested
22. Repeat steps 1-21 only instead of using bag a use bag “B”
23. Repeat steps 1-21 only instead of using bag a use bag “C”

The original purpose of my 1st experiment was to find out how much water vapor could pass through different food grade plastic bags.

The results of experiment 1 were that the thickness did have much to do with how much moisture vapor that could pass through food grade plastic bags. Fred Meyer brand bags were the thickest and also the most resistant to water vapor movement.  Western Family brand was the thinnest and had the most moisture passing though. All the other bags allowed moisture to pass through in the same order as their rank in thickness.

The original purpose of my 2nd experiment was to find out the tensile strength of various food grade plastic bags

The results of experiment 2 were that the thickness didn’t have much to do with the tensile strength of food grade plastic bags. In fact, the Western Family bags were thinnest but had the greatest tensile strength in both the transverse and machine directions.
 

See the tables and graphs below.

My first hypothesis was that the greater the wall thickness of the bag, the less water vapor would pass through.

The results indicate that this hypothesis should be accepted, for the thicker the bag, the less water vapor passed through.

My second hypothesis was that the thicker the bag the stronger its tensile strength will be.

The results of experiment 2 indicate that this hypothesis should be rejected, for the bag with the highest tensile strength was also the thinnest bag.

Because of the results of experiment 1, I wonder what characteristics having to do with plastics that affect their moisture vapor transmission. 

Because of the results of experiment 2, I wonder if there are any other characteristics that having to do with plastics that allow their tensile strength to be stronger.

If I were to conduct this project again I would test more samples, and I would conduct more tests on each sample and take the averages.

RESEARCH REPORT INTRODUCTION Plastic has changed the way we live, and many people don’t realize how hard our lives would be without it. It is used in a variety of ways from health matters such as tooth fillings and medication bottles, to commercial industries such as garbage cans and juice machines. USES Plastic is used in many different ways. It is used to replace metal on cars, because it does not rust. It is used in building structures, and in surgeries. It’s used to replace leather, fur, clothing, stuffing, wood, stone, glass, metal, and fiberglass.  HISTORY Plastic has been around for a very long time. Back in the late 1700’s and early 1800’s natural resins were made of tortoise shells, and animal horns. It was also found in the Gutta Percha Malaya tree sap. These natural resins were placed over fires, made by Native Americans, and were brushed or poured over various things to preserve and water proof these items. It has also been said that the ancient Egyptians created natural plastic, and covered their dead with it to preserve them during the mummification process. Alexander Parkes created the first man-made plastic in 1862. He was at the Great International Exhibition in London, when he claimed that his great invention would do anything that rubber could do but at a much lower price. “It can be transparent, and can mold into 1000+ shapes” he claimed. Thanks to him, now we have what we know as plastic. In 1866, John Wesley Hyatt spilled celluloid all over the floor in his workshop and discovered when it dried it became very flexible. This was considered the first thermoplastic and was used in the first flexible photo film.  Rayon was first developed in Paris in 1891, by Louis Marie Hibire Bernigaunt. At the time, he was looking for a way to make manmade silk, after a recent study of the silk worm. He made the rayon, but then found out it was highly flammable. In 1892 Charles Topham used Louis Bernigaunt’s idea but made some changes to his original idea, and fixed the factor that it was highly flammable. The 1920’s were considered the plastic craze. Cellophane was invented and everyone wanted it. Then in 1933 polyethylene was invented.  In 1957 James Wright discovered that when silicone oil and boric acid were mixed it created an incredibly bouncy and stretchy mixture. It could bounce 25% higher than a bouncy ball, and was then named “Silly Putty”. George de Maestral also created Velcro this year.  CHEMISTRY Plastic consists of long, chain like molecules called polymers. The polymers are made of small molecules called monomers, which contain hundreds of thousands of alcanes, which are found in crude oil. The plastic can have a higher tensile strength due to the way the polymers are aligned. They can be aligned to make structures that are sturdier than others. TYPES Plastic is sorted into two major groups, thermosetting and thermo plastic.  The difference is that thermosetting plastic can only be heated once, and then it can never again be reshaped. When this type of plastic is heated for the first time it undergoes chemical reaction that causes the polymers to link and bind together. When this type of plastic is reheated, it doses not become runny and watery like thermoplastic, but instead it burns. This type of plastic is used in places where there may be high amounts of heat, such as light fixtures, and for some medical devices. When products that are made out of this type of plastic are unable to be used, they are ground down into a fine dust and are then disposed of.  The other type of plastic is called thermoplastic. This is the type of plastic that will become soft when it comes in contact with heat. It can be melted and reshaped over and over again. Polyethylene is used in bottles, packaging, and carpet. It will break with scissors, has some bend, floats, and will soften when heated. When put over a flame it will turn yellow/blue, and will smell like wax.  Polystyrene can be broken with a hammer, does not bend, floats in water, and will soften in heat. An example of a product made of this is toys.  Polyvinyl chloride can be broken with scissors, does not bend, does not float in water, and will soften when heated.  Allcylic is a strong plastic material that is used outdoors, and in electrical plug-ins. Epoxy is a type of plastic resistant to water and weather. It hardens quickly, and is a protective coating. Phenolic is resistant to heat and cold, it is used often on appliance handles. PROCESS There are 7 main processes of shaping plastic. They are molding, casting, extrusion, calendaring, laminating, foaming, and thermoforming. The molding process is the 1st procedure. During this procedure, the resin is put into a mold, heated, and then is released. Some of the products made this way are boats, and car bodies and so much more.  Another process is casting. During this process, the melted resin is poured into a mold, and there is no external pressure. Some products made this way are paperweights, and other solid objects. Extrusion is another process used on plastic. During this procedure, the melted resin is pushed through a hole, and creates long, continuous, sameshaped products. Some of these products are pipes, tubing, and wire covers.  Calendaring is the 4th procedure. During this procedure, resin is pushed between rollers to create long sheets of plastic. Table clothes are usually made this way.  Laminating is a process where two separate pieces of plastic are bound together; this creates a protective layer over the product. The sheets are coated and covered with resin.  Foaming is a process where gas is used to blow plastic bubbles. Some products made this way are Styrofoam cups, and coolers.  Thermoforming is an inexpensive way to mold sheets. In this process plastic is secured over the object, then it is heated, and then the air is sucked out of the mold by a pump. Then it hardens to the mold’s shape.  Blow molding is the process in which water bottles are made. The plastic is placed in a mold, and then blown into so hard that the plastic sticks to the sides of the mold to recreate a copy of the mold. Injection molding is a process in which plastic is injected into a mold, and then once the mold is full and cooled, the mold is opened, and the product is done. Products made this way are rulers, and steering wheels.  RECYCLE Plastic is not biodegradable, and is a very big problem with recycling. Now there is a new type of plastic called TDPA, or Totally Degradable Plastic Additive. When it is in contact with heat, or humidity, it starts to disintegrate. It does this because they add chemicals to the plastic that create these effects.  It has helped the environment tremendously and now junkyards, and dumps, are less full of plastic products. WATER VAPOR Water vapor is an invisible gas that is a result from boiling water. It can change back into liquid through condensation, and is measured through humidity. Its boiling point is 100*C, or 212*F. CONCLUSION Plastic may not be something you think about every day, but it is something you use every day. Plastic affects our lives, in many positive ways.

BIBLIOGRAPHY Ardley, Neil. Dictionary of Science. New York, New York: Dorling Kindersley Publishing Inc, 1994. Pp. 162-163. Beisenberger, Joseph A. “Plastic.” The World Book Encyclopedia. 1991. Cash, Terry. Plastics. Limited, London: A&C Black, 1989. Pp. 8,9,16,24. Chisholm, Jane and Johnson, Mary. Chemistry. London England: Usborn Publishing LTD, 1983. Dempsley, Donna. “The Society of Plastic Industry Inc.” October 30, 2002 <http://www.plasticindustry.org/>. “History of plastic.” October 30, 2002 <http://www.americanplasticscouncil.org/benefit/about_plastic/history.html?src=over>. “Major Plastic Materials.” Microsoft Encarta Encyclopedia Deluxe. 2001 Ed. Richardson, Terry L. “Plastics and Environments.” Microsoft Encarta Encyclopedia Deluxe.2001 Ed. “Tensile Test.” November 6, 2002 <http://www.enged.com/students/matpro/matprosl.html>. Warriers, Gopikrishna S. “Scottish Co. Offers Tech.” <http://proquest.umi.com/pqdweb?did=000000207932351&fmt=3deli=1&mtd=1&1dx=1&sid=2&rqt=309>.

I would like to thank the following people for helping make my project possible:
* My mom for taking me back and forth so many times to Shields, and for bringing me everything that I needed.
* Mr. Newkirk for dedicating his free time to allow us to work on our science experiments after and before school.
* Everybody at Shields, especially Janna, Deb, and Scott, for all the hours they allowed me to work in their lab, and for all their advice, and shared knowledge.
* Mrs. Helms for helping me when Mr. Newkirk was unavailable, and for helping me with my pictures.

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