Physical Properties of Food Wraps

What We'll Study...
The mechanical properties of polymers in the form of food wraps. Properties include tensile strength, elongation, flexural strength, and impact resistance.

Did You Know...
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. This lesson contains simulations of analytical work to give you an appreciation for the technical work needed by industry. You should become familiar with the terms "accuracy" and "precision" as the work is performed. For all mechanical properties testing of polymers, a wide spread of values is obtained. Therefore, it is necessary to test several samples and calculate an average for each property.

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 meter (N/m2).

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).

OBJECTIVES

To test the physical properties of various food wraps and compare polymer-based wraps to nonpolymer-based wraps.

MATERIALS (PER LAB STATION)

*Trademark of the Dow Chemical Company
**Trademark of DowBrands

SAFETY AND ENVIRONMENTAL CONCERNS

 

PROCEDURES

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

  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 p r2 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
  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 (mgD h) divided by area of the hammer head ( p r2).(D=delta; p=pi)
  8. Repeat the procedure for all sample wraps.

 

DATA AND OBSERVATIONS

Tensile Strength

Data Table


Type of Food Wrap

Relative Tensile Strength
Force (N)
   
   
   
   

 

 

 

 

 

 

 

Elongation

Sample Data Table (required for each wrap)

Force Applied as Sample Is Stretched
(N)

Amount of Stretch
(cm)
   
   
   
   
   

Sample Graph (required for each wrap)


Data Table


Type of Food Wrap

Relative Elongation Taken From Slope
(N/cm)
   
   
   
   

 

 

 

 

Flexural Strength

Data Table


Type of Food Wrap

Force at Break
(N)

Area of Open Can Top
(cm2)

Flexural Strength, Force/Area
(N/cm2)
       
       
       
       

 

Impact Resistance

Data Table

Type of Wrap

Mass of Hammer
(kg)

g
(m/s2)

Change in Height,
Dh
(m)

Potential Energy, mgh
(joules)

Area of Hammer
(cm2)

Impact Resistance
(J/cm2)
   

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D=delta

QUESTIONS

  1. Research and describe the differences between linear polymers and branched polymers.
  2. Classify the materials used in this lab as branched polymers, linear polymers, or nonpolymers.
  3. Describe each physical property and relate it to the branched polymers, linear polymers, and nonpolymers. Which wraps had the highest and lowest value for each tested property? Relate these data to the polymer type.
  4. Compare your results to the class results. Explain why many results give a better picture of the "true" result as opposed to only one reading. Relate this to accuracy and precision.
  5. Design a procedure to evaluate a physical property of a new ceramic you have developed for tennis rackets.










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