How strong are plastic wraps?

Introduction: (Initial Observation)

Mechanical properties of polymers are an important factor on selecting them for special applications. These properties include tensile strength, elongation, flexural strength, and impact resistance.

Information Gathering:

Find out about plastic wraps and their properties. Read books, magazines or ask professionals who might know in order to learn about how you may measure physical properties of plastic wraps including their tensile strength. Keep track of where you got your information from.

Following are samples of information that you may find.

What is tensile strength?

The resistance of a material to a force tending to tear it apart, measured as the maximum tension the material can withstand without tearing.

Properties of plastic wraps:

Among the more important mechanical properties of polymers are tensile strength, elongation, flexural strength, and impact resistance. A large number of standard tests have been developed. Standards are set by the American Society for Testing Materials (ASTM).

To measure tensile strength, a test specimen of uniform cross-section is clamped at each end and stretched until it breaks. Tensile strength is defined as the stress force necessary to break the sample at a constant rate of stretching. It usually varies from about 1,000 to 12,000 pounds per square inch (psi) for most common commercial polymers. These values would be equal to 6.9 to 82.8 megapascals (MPa) or newtons/square millimeter (N/mm2).

1 Mega Pascal = 1 Newton/square millimeter

To convert a tensile strength from psi to MPa, multiply the value by 0.006895
(or divide it by 145)

Elongation is the increase in length of a sample at the breaking point. Elongation is associated with the uncoiling of polymer molecules and their movement relative to other molecules. Highly crosslinked polymers have a low elongation relative to linear polymers. Elongation can vary widely among polymers and is usually expressed as a percent of the original length of the sample.

Flexural strength is measured by supporting a sample test bar of uniform cross-section at each end, in a horizontal position. The sample is then subjected to a vertical stress until it yields or breaks. Most common polymers have flexural strengths ranging from 3,000 to 20,000 psi (20.7 to 138.9 MPa or N/m2). Crosslinked polymers are more rigid and have a higher flexural strength than linear polymers.

Impact resistance is a measure of the toughness of a polymer. It can be determined by striking a vertical sample with a weighted pendulum and measuring the distance the pendulum travels after the sample breaks. Values for impact resistance for common polymers range from 0.5 to 10 foot-pounds per inch (0.1 to 0.2 J/cm2).

Question/ Purpose:

The purpose of this project is to identify a method and measure the strength of different plastic wraps. We focus on those used for food packaging and compare them with a few other wrapping material.

Most foods are very hot when they are first prepared. They get very cold when they are in a refrigerator. Food warping material must maintain certain physical properties and remain useable in such a wide range of temperatures. The tensile strength of plastic wraps are among the subjects that can be studied in this project. A specific question for this study is:

• How does temperature affect the tensile strength of a specific type of plastic wrap?

Another specific question/purpose for this project can be like this:

• Compare different brands of plastic wraps for their strength.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

For the first question: Temperature is the independent (manipulated) variable. Tensile strength is the dependent variable. Controlled variables are the brand and the thickness of plastic wrap. Constants are procedures and test method.

For the second question: Brand name of plastic wrap is the independent variable. Tensile strength is the dependent variable. Controls are temperature and thickness of plastic wraps. Constants are procedures and test method.

Hypothesis:

Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Following are two sample hypothesis:

Sample Hypothesis for the first question: The tensile strength of plastic wraps decrease by increasing the temperature. My hypothesis is based on my common sense and my experience with plastics.

Sample Hypothesis for the second question: Among the three brands of plastic wraps, X will have the highest strength, followed by Y and Z. My hypothesis is based of the difference in prices and I think the one that is more expensive is supposed to have a higher strength.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

How to measure tensile strength?

Tensile Strength

1. Assemble two ring stands with a crossbar attached horizontally using clamps. Weigh down or clamp the base of the ring stands for stability. See figure below.If you don’t have ring stands, the cross bar may be placed (like a bridge) between two tables or chairs about 1 foot apart.

2. Cut 2-inch x 8-inch strips of each type of food wrap and use duct tape to suspend each strip from the horizontal crossbar.

3. Attach a second piece of duct tape at the base of each sample and pierce a small hole in the tape. A hooked spring scale will be suspended to measure force.

4. To measure tensile strength, attach a spring scale to the sample. Pull down at a constant rate until the sample wrap breaks. Record the force (N) at the moment of break. This will give a relative value for tensile strength. The final tensile strength value is usually obtained by dividing the force by the cross-sectional area, but in this case the cross-sectional areas of the samples should be fairly uniform.

How do I calculate the cross section?
To calculate the cross section in square millimeters, multiply the width of the sample (in millimeters) by the thickness of the sample (in millimeters). Thickness of the product may be written on the packaging or measured using a micrometer.

5. Collect all the tensile strength data from your multiple experiments and calculate the average value +/- the deviation.

How do I do that?
If you get 3 values of 10, 12 and 13 the average is 11.66. The deviation is +/- 1.66

6. Repeat the procedure for each type of food wrap.

Elongation

1. Use the assembled ring stands with a crossbar from the tensile strength procedure above.

2. Cut 2-inch x 8-inch strips of each type of food wrap and use duct tape to suspend each strip from the horizontal crossbar.

3. Attach a second piece of duct tape at the base of each sample and pierce a small hole in the tape. A hooked spring scale is suspended to measure force.

4. To measure elongation, attach a spring scale to a sample and vertically pull down at a constant rate. Record the force (N) at various points of the stretch (cm) until the sample wrap tears. See figure below.

5. Graph force (N) versus stretch (cm). Determine the slope of the graph to get a relative measure for the elongation of each type of food wrap. Record.

6. Repeat the procedure for each type of food wrap. Record.

Flexural Strength

(Just for your information. Not needed for this project)

1. Cut a sample of each wrap large enough to be secured over the mouth of an open-ended coffee can.

2. Mount the sample wrap over the mouth of the can and adhere it to the can with duct tape. Make sure the sample is pulled taut. See figure below.

3. Add weight (vertical stress) to the center of the sample until the wrap breaks. Record this force (weight) in Newtons. It will be used to calculate the flexural strength.

4. Use pr2 to compute the area of a circle in cm2 (area of can). Divide the force (N) by the area (cm2) to get the flexural strength.

5. Repeat the procedure for all sample wraps.

Impact Resistance

(Just for your information. Not needed for this project)

1. Use steps 1 and 2 from the flexural strength procedure to prepare samples for impact resistance. You may need several samples of each wrap.

2. Construct an impact resistance assembly using a hammer, an eyelet, and the crossbar assembly from the tensile strength procedure. Screw the eyelet into the handle of the hammer such that the crossbar will allow the hammer to swing freely, like a pendulum. See figure below.

3. Determine the mass of the hammer (in kilograms).

4. Adjust the height of the crossbar so that the hammer’s head strikes the center of the mounted sample on the face of the can, at the base of the hammer’s swing.

5. Pull back the hammer to a set height above the base of its swing. Record height (in meters).

6. While another lab partner holds the sample still, release the hammer, using caution to avoid injury. Change the starting height until the sample wrap breaks. (A fresh sample should be used for each change in starting height. Otherwise, the sample may be stressed enough by the first hit that a second hit of equal value will break it.) The height will be used to calculate the energy (potential) exerted per sample area required to break the sample. This is a relative measure of the impact resistance of the food wrap.

7. Calculate the impact resistance (joules per square centimeter) using the potential energy formula (mgDh) divided by area of the hammer head ( pr2).(D=delta; p=pi)

8. Repeat the procedure for all sample wraps.

Experiment 1:

How does temperature affect the tensile strength of a specific type of plastic wrap?

Procedure:

1. Cut 15 identical pieces of plastic wraps.

2. Prepare all pieces for test by attaching duct tapes.

3. Place 5 of the pieces in refrigerator for about 10 minutes. Take them out of refrigerator one by one and test their tensile strength immediately. Record the results and calculate the average tensile strength for cold samples.

4. Test five other samples at room temperature. Record the results and calculate the average tensile strength for room temperature samples.

5. Place 5 of last pieces in hot water water (about 60ºC) for about 30 seconds. You can insert them in hot water in a way that duct tapes do not get wet. Take them out of hot water one by one and test their tensile strength immediately. Record the results and calculate the average tensile strength for hot samples.

6. Record your final results in a table like this:

7. Use your above results table to draw a bar graph.

Experiment 2:

Compare different brands of plastic wraps for their strength.

Procedure:

1. Cut 5 identical pieces of plastic wraps from each of the three brands that you are testing.

2. Prepare all pieces for test by attaching duct tapes and label them with the brand.

3. Measure the tensile strength of the five samples of each brand. Record the results and calculate the average tensile strength for that specific brand.

4. Record your final results in a table like this:

7. Use your above results table to draw a bar graph.

Make a graph:

Use the above results table to make a bar graph that can visually present your results. Make one vertical bar for each of the brands you are testing. The height of the bar will be the tensile strength.

Materials and Equipment:

• 5 (2-inch x 8-inch) strips of the following food wraps:
  1. polyvinylidene chloride film (e.g., Saran Wrap* Brand Plastic Film)
  2. polyethylene film (e.g., Handi-Wrap** Plastic Film)
  3. waxed paper (is not plastic, but is a food wrap and is good for comparison purpose)
  4. aluminum foil (is not plastic, but is a food wrap and is good for comparison purpose)
• 2 ring stands and a crossbar with 2 clamps (or something similar)
• 1 (10 N) spring scale (includes in  MiniScience part# PSS_US5)
• 1 wooden handle hammer
• 1 screw eyelet with a diameter fitting a crossbar
• 1 coffee can (one end open)
• 1 (30 cm) metric ruler
• duct tape
• 1 set of weights (0.2 N to 10 N)
• 1 pair of scissors

*Trademark of the Dow Chemical Company
**Trademark of Dow Brands

Results of Experiment (Observation):

Record the results of your experiments in form of tables and graphs:

Tensile Strength

Data Table

Elongation

Sample Data Table (required for each wrap)

Sample Graph (required for each wrap)

Data Table

Flexural Strength

Data Table

Impact Resistance

Data Table

D=delta

Calculations:

Description

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Conclusion:

Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.

References:

Almost all books related to material science, material engineering and material strength can be used as a reference for this project. You may find such books in your local library or the library of local colleges. Following are some web references:

http://matse1.mse.uiuc.edu/~tw/metals/e.html

http://palimpsest.stanford.edu/don/dt/dt3469.html

http://www.lib.umich.edu/dentlib/Dental_tables/Ulttensstr.html

Tensile Performance

http://www.ce.memphis.edu/1101/notes/concrete/strength%20of%20materials.html

Beer, F.P., and Johnston, E.R. (1992). “Mechanics of Materials,” Second Edition, McGraw Hill Publishing Co., New York, NY