Adapted from 1996 DOW/NSTA Summer Workshop Lesson Plans, "It's In the Wrap", by Jean Olson, Rutherford HS, Panama City, FL and Wayne St. Peter, Somers HS, Somers, CT.

Analytical chemistry is important to industry because the accurate measurement of quantities, composition, and properties is critical to the market success of products. These measurements are usually performed on highly sophisticated and complex instruments that are not available in the high school laboratory, however many of the mechanical properties of plastics can be tested using simple apparatuses.

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

Tensile strength is measured by clamping a test sample of uniform cross-section and stretching it 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 meter (N/m³).

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 21,000 psi (20.7 to 138.9 MPa). 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 joules/cm²).

The purpose of this experiment is to test the physical properties of various food wraps and compare polymer-based wraps to non polymer-based wraps.

Procedure:

Part A. Tensile Strength:

Assemble two ring stands with a crossbar attached horizontally by way of clamps. Weigh down the bases of the rings stands for stability. See Fig. 1 on page 2 for picture of setup.

Carefully, cut 2 inch x 8 inch strips of each type of food wrap and use duct tape to suspend each strip from the horizontal corssbar. Attach a second piece of duct tape at the base of each sample and pierce a small hole in the tape. Now attach a spring scale to the sample. Carefully pull down at a constant rate until the sample wrap breaks. Record the force at the moment of break. This will give a relative value for tensile strength. The final 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.

Repeat for each type of food wrap. Put the value you obtain for each sample into the table on the overhead transparency. Make a similar table in your lab book and copy the data from all five groups.

Part B. Elongation:
Cut another 2 x 8 inch strip of each type of food wrap and hang from the ring stand with crossbar setup as done in Part A. Again hang a hooked spring scale from the free end. This time stand a meter stick on end alongside the sample strip. Now pull down at a constant rate. Record the force (N) at various points of the stretch (in cm) until the food wrap tears. Repeat with each of the different kinds of food wrap.

Part C. Flexural Strength: Cut a sample of each wrap large enough to be secured over the mouth of an open-ended coffee can. Lay the wrap over the can and tape it to the can using duct tape. Be sure to put the wrap taut. Add weight (vertical stress) to the center of the sample until the wrap breaks. Record this force (weight) in newtons. Repeat the procedure for all sample wraps.

Part D. Impact Resistance: Set up the coffee can as in Part C using another sample of the food wrap. Next, 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. Determine the mass of the hammer in kilograms.

Adjust the height of the crossbar so that the hammer's head will strike the center of the food wrap when it's head is at the lowest point of it's swing. Note: the can will be lying on it's side for this test. Pull the hammer back until it is 5 cm above it's lowest point. Be sure to record this height. Release the hammer and let it strike the middle of the food wrap. One person should be holding the can securely in place during this test. Unless the wrap breaks, repeat once more using this change in height. Repeat the procedure, raising the height in 5 cm increments until the food wrap breaks. You must replace the wrap with a fresh piece for each change in height, otherwise, the sample may be stressed enough by the previous hits that a hit from a greater change in height might break it whereas a fresh, unstressed sample would not break. Repeat the procedure for all the food wraps.

Analysis:

Part A. Tensile Strength: Put the class data (should be five values for each wrap) into a spreadsheet and calculate the tensile strength for each sample, then the average and average deviation for each kind of food wrap. Tape a copy of the spreadsheet (showing the formulas you used to do the calculations) on a left-hand page in your lab book and report just the average and average deviation for each kind of food wrap in the Results and Discussion section.

Part B. Elongation: Graph the force (N) vs. stretch (cm), then determine the slope of the graph. This will give a relative measure for the elongation of each type of food wrap.

Part C. Flexural Strength: Calculate the area of the open end of the coffee can, and thus the surface area of the food wrap that was subject to the force (weight) added to it. Divide the force (N) by the area (cm³) to get the flexural strength. Calculate for all sample wraps.

Part D. Impact Resistance: Calculate the impact resistance (J/cm²) by using the formula for potential energy (mg times the change in height) divided by the area of the hammer head.

Discussion:

Describe the differences between linear and branched polymers. Classify the food wraps used in this lab as branched polymers, linear polymers, or non polymers.

Relate each physical property (described in your introduction) to the branched and linear polymers and non polymers. Which wraps had the highest and lowest value for each tested property? Relate these data to the polymer type.

Compare the results of your group for each test to the class results. How close are you to the averages for the class?

Design a procedure to evaluate a physical property of a new polymer you have developed for tennis rackets.