SCIENCE EXPERIMENTS ON FILE Revised Edition 5.25-1 Density and Stress in Plastics Mary V. McCrary Topic Density and birefringence of plastics Time Part A: 30 to 45 minutes; Part B: 30 to 45 minutes! Safety Please click on the safety icon to view the safety precautions. Adult supervision is necessary when cutting and stressing the plastics. Be careful when using glass; it can break if dropped or handled roughly. Brittle plastic may shatter when cut; always wear safety goggles, and cover the sample with a towel when cutting. Materials FOR PART A various plastic containers (should be of different colors, thicknesses, textures, and uses) FOR PART B various types of clear plastic, e.g., plastic wrap, six-pack ring for soda cans, cups, bags, etc. beam balance scale tall glass or jar towel 600-mL beaker (to place over 25-W light source) ring stand with ring water scissors two pieces polarized film (an old pair of sunglasses you can break in half will work) light source: 25-W bulb and socket or a flashlight Procedure PART A: DENSITY OF PLASTICS Density is the ratio of mass measured in grams (g) to the volume measured in milliliters (ml). The density of water is 1 g/ml; in other words, 1 ml water will weigh 1 g. 1. Collect various samples of plastic (the more varieties the better), and cut each into 5-cm 2 5-cm squares. Give each sample a number for identification purposes. 2. List each sample on data table 1. Record its use (for example, milk bottle) and physical properties: its texture, thickness, whether or not it s flexible, etc.
5.25-2 SCIENCE EXPERIMENTS ON FILE Revised Edition DATA TABLE 1 Sample Use Physical properties Mass Hypothesis 1 2 3 4 5 3. Using the scale, determine and record the mass for each sample. 4. Fill a tall jar or glass three-quarters full of water. Make sure that your samples will fit into the glass if placed flat. 5. Based on the information recorded on data table 1, draw hypotheses as to which samples will float, sink, or hang in the middle of the glass. Record your hypotheses on the data table 6. Place one sample at a time in the glass, and press it under water completely for 1 or 2 sec. When you let go, observe what happens to the sample. After it comes to rest, enter your observation on data table 2. DATA TABLE 2 Sample Floats Hangs Sinks 1 2 3 4 5 PART B: BIREFRINGENCE OF PLASTICS UNDER STRESS Stress can occur in five basic ways: a. Shearing, which is the stress applied when two joined members are pulled away from each other b. Bending, when something is forced to move oppositely to the way it is shaped c. Pushing (compression) d. Pulling (tension) e. Torsion, which is twisting 1. Collect various samples of clear plastic, e.g., plastic wrap and bags, bottles, a protractor or other, hard, brittle plastic. 2. Give each sample a number for identification, and list each sample on a separate sheet of paper. Next to each sample, record your observations about it, as
SCIENCE EXPERIMENTS ON FILE Revised Edition 5.25-3 well as its physical characteristics and use. Leave enough room between each listing to record further observations. 3. Set up the ring stand. Place the light source on the base (with the light shining upward if you are using a flashlight). Place one polarized filter on top of the light source (see figure). (If you are using a lightbulb, place a beaker over the lightbulb to prevent the polarized film from burning.) ringstand with ring polarized film polarized film light source 4. Adjust the ring on the stand to allow enough room for your hands and a sample of plastic, which you will be manipulating, between the polarized light source and the ring. Place the second piece of polarized film on top of the ring. (If you are using polarized lenses from sunglasses, you will probably need to make a holder for the lens out of cardboard to support it in the ring.) 5. Turn on the light source (avoid looking directly into it unless it is covered by polarized film). While looking through both pieces of polarized film, turn the upper polarizer clockwise until the light from below is no longer visible, or is as dim as it gets. This is called the point of extinction, because the light is said to be extinguished. 6. Hold your first plastic sample between the polarizers. Observe and record anything of interest. While holding the sample between the polarizers, apply different types of stress to your sample. This can include tearing, cutting, and breaking (refer to the descriptions of stress at the beginning of Part B, above). Record all observations, including location and direction of any changes. ABOUT PART A 1. Of the samples you tested, which are denser than water and which are less dense? 2. How can you explain the samples (if any) that floated near the middle of the glass? How does their density compare with that of water? ABOUT PART B 1. Where did most of the colors appear in the stressed plastics? 2. What caused the color fringes in the plastic samples you tested? 3. Why do you think a sample would show color before being stressed, and not after or during the application of stress?
5.25-4 SCIENCE EXPERIMENTS ON FILE Revised Edition What s Going On Part A: The samples that sink are denser than water, and those that float on top of the water are less dense than water. This means that if you measure the volume of a plastic that is denser than water and an equal volume of water, the plastic will have greater mass. The reverse will hold true for a plastic that has a density lower than water. The samples that float in the middle of the glass have the same density as water or are very close. Part B: Colors appear most often in stressed regions. The polymers, which make up different types of plastics, refract light. As a sample of plastic is stressed, the direction in which the polymers line up is changed, causing the direction in which the refracted light travels to change. Light waves traveling in the same direction combine (this is called interference), and the new light wave becomes visible as a color. Samples that show birefringence without being stressed by the experimenter are exhibiting polymers that are out of line with each other as a result of the manufacturing process. If a sample shows colors that lessen or disappear when stress is applied, this means that the stress is actually reorienting the polymers so that they are in line and exhibit less optical activity. Connections Plastics are organic (carbon-containing) compounds, the smallest unit of which is called a monomer. A monomer is simply a molecule, meaning that it is the smallest amount available of the substance. When a group of monomers of the same or different compound(s) are linked together to form long chains or a network, they are called polymers. The way in which polymers are linked, the amounts used, and the compounds joined together have an effect on the density of the plastic. Density is the mass (weight) of a measured volume (amount) of a substance. The direction in which the polymer chains are lined up can have an effect on light passing through the substance. Stress will affect the polymer chains so that different bands of color appear when the polymers are out of line and polarized light passes through them. This is called birefringence. Engineers use birefringence to check for stress lines in products, to find ways to strengthen their construction. In this experiment you tested the different densities of plastics and observed birefringence in areas of stress.
Safety Precautions READ AND COPY BEFORE STARTING ANY EXPERIMENT Experimental science can be dangerous. Events can happen very quickly while you are performing an experiment. Things can spill, break, even catch fire. Basic safety procedures help prevent serious accidents. Be sure to follow additional safety precautions and adult supervision requirements for each experiment. If you are working in a lab or in the field, do not work alone. This book assumes that you will read the safety precautions that follow, as well as those at the start of each experiment you perform, and that you will remember them. These precautions will not always be repeated in the instructions for the procedures. It is up to you to use good judgment and pay attention when performing potentially dangerous procedures. Just because the book does not always say be careful with hot liquids or don t cut yourself with the knife does not mean that you should be careless when simmering water or stripping an electrical wire. It does mean that when you see a special note to be careful, it is extremely important that you pay attention to it. If you ever have a question about whether a procedure or material is dangerous, stop to find out for sure that it is safe before continuing the experiment. To avoid accidents, always pay close attention to your work, take your time, and practice the general safety procedures listed below. PREPARE Clear all surfaces before beginning work. Read through the whole experiment before you start. Identify hazardous procedures and anticipate dangers. PROTECT YOURSELF Follow all directions step by step; do only one procedure at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure that there is adequate ventilation. Do not horseplay. Wear an apron and goggles. Do not wear contact lenses, open shoes, and loose clothing; do not wear your hair loose. Keep floor and work space neat, clean, and dry. Clean up spills immediately. Never eat, drink, or smoke in the laboratory or near the work space. Do not taste any substances tested unless expressly permitted to do so by a science teacher in charge. USE EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives and other sharp or pointed instruments with caution; always cut away from yourself and others. Pull plugs, not cords, when inserting and removing electrical plugs. Don t use your mouth to pipette; use a suction bulb. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges. Clean up broken glassware immediately. v
vi Safety SCIENCE EXPERIMENTS ON FILE REVISED EDITION Do not use reflected sunlight to illuminate your microscope. Do not touch metal conductors. Use only low-voltage and low-current materials. Be careful when using stepstools, chairs, and ladders. USING CHEMICALS Never taste or inhale chemicals. Label all bottles and apparatus containing chemicals. Read all labels carefully. Avoid chemical contact with skin and eyes (wear goggles, apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING INSTRUCTIONS Use goggles, apron, and gloves when boiling liquids. Keep your face away from test tubes and beakers. Never leave heating apparatus unattended. Use safety tongs and heat-resistant mittens. Turn off hot plates, bunsen burners, and gas when you are done. Keep flammable substances away from heat. Have a fire extinguisher on hand. WORKING WITH MICROORGANISMS Assume that all microorganisms are infectious; handle them with care. Sterilize all equipment being used to handle microorganisms. GOING ON FIELD TRIPS Do not go on a field trip by yourself. Tell a responsible adult where you are going, and maintain that route. Know the area and its potential hazards, such as poisonous plants, deep water, and rapids. Dress for terrain and weather conditions (prepare for exposure to sun as well as to cold). Bring along a first-aid kit. Do not drink water or eat plants found in the wild. Use the buddy system; do not experiment outdoors alone. FINISHING UP Thoroughly clean your work area and glassware. Be careful not to return chemicals or contaminated reagents to the wrong containers. Don t dispose of materials in the sink unless instructed to do so. Wash your hands thoroughly. Clean up all residue, and containerize it for proper disposal. Dispose of all chemicals according to local, state, and federal laws. BE SAFETY-CONSCIOUS AT ALL TIMES