039 Teflon Tape

Expt 039 -- Teflon Tape

Plumbers Teflon tape (*PTFE = polytetrafluoroethylene) is a silky material with interesting properties. Use the tape as an open ended exploration of polymer properties. Messages written on the tape are easily distorted and reformed with the polymers.

Introduction

When one of my pipes sprung a leak last month, I did what anyone would do: I called a plumber. He arrived and told me that fixing the leak would cost a minimum of $75.00 for labor (translation: "Be prepared to fork over next month's paycheck!"). Wait a second -- I thought to myself -- iron, corrosion, copper, water, hydrostatic pressure...I'm a chemistry teacher; I teach this stuff (and besides, doesn't the word "plumber" come from the same Latin prefix responsible for lead's chemical symbol "Pb"?). This should be easy! "Well, thanks anyway," I told Mr. Rotowrench, "I'll try fixing it myself..."
So off to the hardware store to learn about valves and fittings and faucets and a wonderful product called "Teflon tape" (AKA: "PTFE THREAD SEAL TAPE"*). Plumbers wrap it around the threads on the end of a pipe before screwing on the next length of pipe. It acts to form a water tight seal and prevent leaking. So, about half-way through the job, my two daughters (AKA: "helpers") ask to see some of this tape. They discovered very quickly that it has some interesting and fun properties: its very thin and soft, almost like silk -- and its not at all sticky as they had expected. Also, it also has some curious stretching properties. In short, the tape makes a great inexpensive open-ended exploratory activity.

*PTFE = polytetrafluoroethylene = ÒTeflon.Ó It is made from the polymerization of tetrafluoroethylene into long chains. See below.

Safety

Wear goggles if flame tests are being conducted in the laboratory.
If the optional flammability test is performed, use only small (1 cm x 1 cm) pieces, and light them over a cup of water to catch any molten drips and to extinguish the flame when desired.

Procedure

Cut a 10 cm piece of Teflon tape with scissors. Make your observations as quantitative as possible.
Investigate the properties of the Teflon tape. Describe its texture, color, rigidity. Test how well it stretches lengthwise and widthwise. How easily does it break lengthwise and widthwise? Design your own tests. Carefully record your observations of all of the properties tested.
Investigate the rubber band in a similar manner. Compare the properties of these two polymers.
Obtain a length of low-density polyethylene from a six-pack soda can holder, and investigate this polymer in a similar manner. Compare it to the previous two.
For fun, obtain a new piece of Teflon tape 15-20 cm in length. Hold it down on top of 2-3 layers of paper towel and write a message on the tape with a permanent pen. Allow the message to dry for a few seconds, then stretch the tape widthwise in several places to fully distort the message, but leave 2-3 cm on each end intact. Then try pulling the ends of the tape, and see what happens to the message.
Try this same procedure on the rubber band and the low-density polyethylene.

Explorations

One set of possible tests is described here. Students may think of many more.

Write a message on the Teflon tape. Pull the tape both crosswise and lengthwise to observe any configuration shift that occurs. Repeat several times.

!!!Click here to See Movie.
!!!Click here to See Movie.
Try pulling a second small piece of tape crosswise as far as possible. Note some slipping is lengthwise.
Write on the tape. Pull from diagonal corners of the tape. Note the direction of movement of the polymers.
Stretch the rubber band both crosswise and lengthwise to observe any configuration shift that occurs. Repeat several times. Note the temperature of the rubber band.

!!!Click here to See Movie. Click |> or <| to step the slides forward or back.
Stretch the plastic (low-density polyethylene) six-pack soda can holders to observe any configuration shift that occurs.

Stretch lightly and release.
!!!Click here to See Movie. Click |> or <| to step the slides forward or back.
Stretch strongly until the ring distorts. Release.
!!!Click here to See Movie. Click |> or <| to step the slides forward or back

Optional Variation (note safety): Compare the flammability of the three samples: Use a forceps to hold a 1 cm x 1 cm piece by the corner; use a Bunsen burner or match to try to light the opposite corner. Note: Perform this test on small pieces only and over a cup of water to catch any molten drippings. Once they are observed, extinguish any flames by submerging them in the water.

Questions

From your observations with the Teflon tape:
1. Try to explain on a molecular level: how the molecules in the tape must be shaped and arranged.
2. Which is stronger -- the intramolecular forces or the intermolecular forces, and what evidence is there to support your answer?
3. What evidence is there that tape must be several molecules thick?
4. What is happening to the molecule chains when the tape is being stretched widthwise?
5. What is happening when it shears widthwise?
Try to explain on a molecular level how the molecules in the rubber band are shaped differently than the Teflon tape.
When the polymers are stretched, what happens to the energy used for stretching? Describe the molecular interaction changes in each.

Teflon Tape
Rubber Band
Can holder
The density of Teflon tape is very close to 1.0 g/cm³. Note its nearly neutral buoyancy in water. Determine the thickness of a piece when it is first cut. A 1 meter length of the tape (1.27 cm in width) has a mass of only 0.33 g.
1. Calculate its thickness.
2. Stretch a small length of polymer widthwise and measure as its widthwise dimension until it begins to shear. Use your calculation in part a to estimate an upper limit for its thickness at the time it shears.
Compare the flammability of the three samples.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 039 Teflon Tape

Watch the movies and carefully record your observations.

Answer the questions.

Curriculum-

This makes a great open-ended exploratory activity for students, maybe as a way of introducing the concept of polymers.

Activity-

Laboratory or Home Experiment
The activity can be as simple as a passing around a message on the tape for stretching and pulling or a full fledge student laboratory, as described above.
The activity is best as a hands-on, open-ended student exploration. After the initial exploration, conduct a group discussion where student results are pooled and recorded on the board. Do not distribute the Exploration section for an open-ended laboratory.
Give students some questions that could be answered in the laboratory. Data for some of the questions may already be posted. Ask students to continue investigations and answer the questions posed. See "Questions" above.

Safety-

No special precautions recommended.

Time-

Teacher Preparation: 2 minutes

Class Time: 15 minutes (The open-ended exploration requires a bit of planning time for the students.)

Materials-

(per student group)

25 cm Teflon tape
1 rubber band (wide ones shaped like tape)
plastic holder from a six-pack of soda
permanent marking pen
paper towels
rulers

Optional:

candle or Bunsen burner for flame test.

Disposal-

Dispose with ordinary solid trash.

Lab Hints-

If you are passing a message around, tape the ends of the Teflon tape with scotch tape to hold them in place.

!!!Click here to See Movie.
!!!Click here to See Movie.
Writing on the Teflon tape takes practice; the tape tends to catch on the tip of the pen. The paper towels help, because they act as a cushion. It also helps to angle the pen towards you and write using only down strokes. Use a light touch.

Observations-

Most remarkably, once it is stretched widthwise into a very distorted shape, it may be pulled back lengthwise into its precise original shape. This is especially impressive if some message is written on the piece of tape at the beginning. When the tape has been stretched out widthwise, the message becomes completely distorted and unintelligible. When the tape is then stretched lengthwise (and the polymer chains have been pulled back to their original parallel configuration), the message become visible again. What's more, the same piece may be used over and over with little or no signs of wear and with virtually no distortion of the original message!! Note: If the tape is stretched too far width-wise, it becomes almost transparent and eventually tears. When it does tear, it shears with almost no resistance and always in a clean line parallel with the length of the piece.

Answers-

Q1. From your observations with the Teflon tape: Q1a. Try to explain on a molecular level how the molecules in the tape must be shaped and arranged. A1a. The molecules are very long and aligned parallel, length-wise in the tape. Q1b. Which is stronger -- the intramolecular forces or the intermolecular forces, and what evidence is there to support your answer? A1b. The intramolecular (covalent bonds) are stronger, for it stretches and shears widthwise much more easily than it does lengthwise. Q1c. What evidence is there that tape must be several molecules thick? A1c. It stretches widthwise quite a bit -- to as much as 10-20 times its original width -- before shearing. Q1d. What is happening to the molecule chains when the tape is being stretched widthwise? A1d. They are slipping sideways over one another but still sticking together. Q1e. What is happening when it shears widthwise? A1e. The chains have spread out to a point where they are perhaps only one molecule thick in places; they then have no choice but to tear apart from one another.
Q2. Try to explain on a molecular level how the molecules in the rubber band are shaped and arranged differently than the molecules in the Teflon tape. A2. The molecules contain several bends and loops and are cross-linked. The cross-linking keeps the band from stretching out permanently in any direction.
Q3. When the polymers are stretched, what happens to the energy used for stretching? Describe the molecular interaction changes in each. A3. Teflon Tape Teflon tape is difficult to stretch lengthwise. When it does break energy is changed to potential energy in the form of broken covalent bonds. Crosswise the energy is used to separate long molecules. Little energy is required because only weak intermolecular interactions are interrupted. Rubber Band The energy used to stretch the band is released as heat when the band is released. No net change in bonding occurs. The energy is released as heat when the band contracts. Can holder The holder is permanently distorted in any direction when pulled. The energy is used to break intramolecular and intermolecular bonds.
Q4. The density of Teflon tape is very close to 1.0 g/cmÜ. Note its nearly neutral buoyancy in water. Determine the thickness of a piece when it is first cut. A 1 meter length of the tape (1.27 cm in width) has a mass of only 0.33 g. a. Calculate its thickness. A4a. (0.33g/1 meter ) x (1 meter/100cm) = 0.0033 g/cm of length (0.0033 g/cm)/(1.27 cm (the width)) = 0.0026 g/cm³ of tape (0.0026 g/cm³ ) x (1 cm³/1 g) = 0.0026 cm thickness (0.0026 cm thickness) x (10⁶µm/10²cm) = 26 µm b. Stretch a small length of polymer crosswise and measure as its crosswise dimension until it begins to shear. Use your calculation in part a to estimate an upper limit for its thickness at the time it shears. A4b. Exact measurement is not possible because some lengthwise shear takes place at the same time as the crosswise shear. When it gets stretched out width-wise, it probably reaches a thickness of less than a single µm before it shears. Pulling carefully results in strips 30 to 40 cm in the crosswise direction. 26µm x(1.27/30) = 1µM
Q5. Compare the flammability of the three samples. A5. Both rubber and polyethylene burn, but Teflon does not. Although it can be charred, it does not maintain a flame the way most other polymers do. This is due to the relatively strong bond between C and F.

Key Words 1-

polymers, Teflon