CANADIAN PLASTICS INDUSTRY ASSOCIATION

Transcription

1 CANADIAN PLASTICS INDUSTRY ASSOCIATION 1

2 2 TABLE OF CONTENTS SCIENCE: MATERIALS & TECHNOLOGY Pages 3-12 TEACHER S NOTES ACTIVITY CARDS Card Number THINKING IN TERMS OF PROPERTIES S-1 Recognizing Plastics at Home S-2 Recognizing Plastics in Cars S-3 Extending Your Vocabulary S-4 Physical Properties S-5 Why Plastics? S-6 Matching Properties to Products S-7 Not All Products Are Perfect GETTING TO KNOW PLASTICS S-8 What Are Plastics Made From? S-9 Major Categories of Plastics S-10 Plastics In Packaging S-11 High-density and Low-density Polyethylene S-12 A Model for Elastomers TESTING PROPERTIES OF PLASTICS S-13 Mass And Volume S-14 Relative Density S-15 Conductivity S-16 Properties of Food Wraps S-17 Testing Some Materials for Puncture Resistance WORKING WITH PLASTICS S-18 Changing Properties with Additives S-19 Engineering Plastics S-20 Designing with Plastics S-21 Some Environmental Aspects of Plastics: The Automobile bumper A Case Study

3 SCIENCE: MATERIALS & TECHNOLOGY THINKING IN TERMS OF PROPERTIES 3 Card S-1 Recognizing Plastics At Home An amazing variety of products are made from plastics. You may want to refer to Card MB-21 for some types of products to consider. You may want to organize a scavenger hunt. Label several cards, Things in the gym made of plastics, Things in a car made of plastics and Things in a classroom made of plastics. Send students out to search in groups. Set a time limit. Ask students to categorize their findings and share results with the class in the form of a chart. According to the Kirk-Othmer Encyclopedia of Chemical Technology (Wiley, New York, 1982), a standard chemical reference, a plastic is defined as a material that: a) contains one or more organic polymeric substances of high molecular weight as an essential ingredient, b) is solid in its finished state, and c) can be shaped by flow at some stage in its manufacture or processing. However, in a court case in the United States, an expert witness for the State of New York stated, Plastics is a vague amorphous term that does not inform the Court about the issue at hand. The very first plastics were made from a group of substances called phenols, which were derived from coal tar. Today most plastics are made from oil and gas, but some plastics are now being made entirely from substances of vegetable origin. Card S-2 Recognizing Plastics in Cars Card S-3 Extending Your Vocabulary The physical properties most often introduced at this level do not include many properties students encounter in their lives. Some of these properties have very specific technical definitions that are usually defined in engineering schools. The intent of this activity is to encourage a general understanding of terms like these, and to encourage students to use the words without worrying about technical definitions. Encourage students to start thinking about products in terms of their properties, and to use a wide vocabulary. As an alternative, you may wish to write the words on cards and distribute them among small groups of students. Have them check a dictionary for definitions, or have the groups develop working definitions. Each group should share the following information with the whole class: a) a working definition of the property, b) a material having this property, and c) a circumstance where the property is important. Some students may wish to create a chart for a wall display. Card S-4 Physical Properties Focus on properties identified in you curriculum. You may wish to add brief discussions of chemical or biological properties. Such a discussion may help students identify the difference between a physical property and other kinds of properties. Card S-5 Why Plastics? You may want to design a few simple experiments to demonstrate some of these properties. The experiments found in Testing Properties of Plastics (see Cards S-13 to S-17) may fit well here.

4 4 Card-S-6 Matching Properties to Products Use this card to extend students understanding of properties to other materials besides plastics. You may want to ask the class to prepare a collage of pictures of products and use this to make the link between products and properties. Card S-7 Not All Products Are Perfect Cards S-6 and S-7 are intended Do not heat plastics in an to make explicit the connection between products and open flame. Caution should be taken to avoid accidents their properties. Use this card & injury. to emphasize that there is always room for improvement and there are always business opportunities to make a better product. GETTING TO KNOW PLASTICS Card S-8 What Are Plastics Made From? Plastic resins are usually derived from oil and natural gas using complex chemical processes. Although they are very important materials and are manufactured in large quantities their production uses a small fraction of Canada s annual consumption of oil and natural gas. Card S-9 Major Categories of Plastics Thermoplastics Thermoplastics start as resins. They melt when heated and harden when cooled. They are easily recycled because they can be melted and remoulded repeatedly. Many household containers are thermoplastics. They can be identified by a symbol stamped into the plastic. The table on the following page lists most of the thermoplastics students will encounter, along with their identification symbols. Other thermoplastics of commercial importance are high cost, low volume engineering thermoplastics, which have higher physical and mechanical properties for very specific structural applications. Here are some examples: nylons polycarbonates thermoplastic polyurethanes polyphenylene oxides acetals thermoplastic polyimides Thermosets Thermosets usually start as liquids. When they are heated they form chemical bonds. This leads to very strong crosslinks. The process is similar to the process of cooking an egg. The cooled product is solid. It does not soften or melt when reheated. Scrap material from thermoset products cannot be recycled using technologies designed for the recycling of thermoplastics. There are several common types of thermosets: polyesters epoxies polyurethane foams phenolics

5 5 Common Thermoplastics 1 PETE Polyethylene terephthalate Properties and Uses Density g/cm 3. Sinks in water. Transparent and shiny. Uses: carbonated drinks bottles. 2 HDPE 3 PVC High-density Polyethylene Polyvinyl chloride Density g/cm 3. Floats on water. Relatively hard, difficult to scratch with finger nail. Film has a crinkly feel. Uses: department store bags, bottles for milk, bleach and other household chemicals, drums, crates, pails and toys, pipes and coatings for wire and cables. Density g/cm 3. Sinks in water. Rigid PVC is hard and stiff. Fairly easy to cut. Uses: bottles for cooking oil, window cleaner, liquid detergent, house siding, sewer pipes. 4 LDPE Low-density Polyethylene Density g/cm 3. Floats on water. Flexible, softer than HDPE, easily scratched with fingernail. Waxy feel. Uses: grocery and garbage bags. 5 PP Polypropylene Density 0.90 g/cm 3. Floats on water. High resistance to heat and chemicals. Uses: margarine tubs, yogurt containers, fast food microwave trays. 6 PS Polystyrene Density 1.05g/cm 3. Sinks in water. Rigid polystyrene is hard and stiff. It has a characteristic ring when tapped. Uses: food containers, pen barrels. Expanded polystyrene is usually white. Crumbles when cut. Uses: meat trays, disposable cups, insulation, packing material.

6 6 Many polyesters are made into fibres for clothing, carpets and other fibre products. They can be reinforced to form fibreglass reinforced plastics (FRPs). Epoxies are used in paints, adhesives, and composite or reinforced plastics. Polyurethanes are most often made into foams. Phenolics have many uses. They are the easiest thermosets for students to test. Small squares of countertop material or electrical wall switch plates make convenient testing samples. Elastomers Natural rubber, synthetic rubber, synthetic sponges, chewing gum all are examples of elastomers. Elastomers processed into foam are used for cushions, padding and insulation. The amount of crosslinking in an elastomer has an important effect on its properties. For example, rubber used for tires must have many crosslinks to make it more durable. These types of plastics are both elastomers and thermoplastics: styrene-elastomer polyurethane- and polyester-polyether polyolefine blends; thermoplastic polyolefines Thermoplastic elastomers combine the elastic properties of rubber with the strength and melting properties of thermoplastics. They have the Do not allow students to heat plastices in an open flame. Use a hot needle to test whether a plastic melts. Remind students that a hot needle looks like a cold needle. Making a mistake can cause a small burn. Students should not test epoxies. crosslinks of elastomers, but the links can be undone by heat. They can be melted and remoulded repeatedly. Thermoplastic elastomers are used for footwear, adhesives, sealants, moulded tires and bumpers, and extruded gaskets, hoses and covers for wires and cables. Card S-10 Plastics in Packaging This topic is examined from an environmental point of view in Cards E-17 - E-23 found in the Plastics and the Environment module. 3. Plastic bottles weigh much less than glass bottles with the same capacity. Plastic bottles and other plastic packaging use up less energy for manufacture and distribution. The mass of a one-litre plastic bottle is about 6 per cent of the mass of a one-litre glass bottle. 4. In Canada the refilling of plastic food containers is discouraged. One never knows to what use a container may have been put prior to reuse. For example someone might have stored gasoline or pesticide in a soft drink bottle. Plastics may absorb the substance and simple washing can not remove it. Over the least decade Canada has developed a substantial infrastructure to collect and recycle plastic containers. These containers may be remoulded into new containers for non-food applications, for example oil bottles.

7 5. Students might identify factors such as: environmental impact of the package manufacturing process cost of manufacturing the package weight and bulk of the package. The higher these are, the more transportation will cost, and the greater will be the amount of fuel used in transportation and the environmental impact of that use of fuel. ability to protect the product attractiveness of the package ability of the package to present information to the consumer end fate of the package Card S-11 High-density and Lowdensity Polyethylene Density is a difficult concept for many students. This model may help students better understand density. Collect glass, metal, wooden and plastic beads of the same size. The beads can represent molecules. Put them in identical glass jars. This produces different masses in the same volume. The jar with the highest mass in the same volume has the highest density. You might introduce this activity by displaying a liquid bleach detergent bottle made from LDPE and a bleach bottle made from HDPE. Look for these signs on the bottles: 2 4 HDPE LDPE While the difference in density may not be apparent, students should notice the difference in hardness and rigidity. Challenge students to account for these differences in properties. 1. High-density polyethylene contains few branches. As a result, more polymer strands can pack together in a given volume. 2. The tighter packing of unbranched polymer chains also accounts for HDPE being harder and more rigid, although other factors such as crosslinks and additives also have an effect. 3. Polymer chains typically contain from several hundred to a few thousand repeating units. It would be impractical and unnecessary to use this many connectible cubes. A more important problem with the model is that unbranched chains can only be constructed straight, and branches can only be made at certain angles to the main chain. 7 Low-density polyethylene has a density in the range of g/cm 3, while the density of high-density polyethylene is g/cm 3. The densities of the models will, of course, be different from these, but the model can be helpful in introducing the idea of density. Card S-12 A Model for Elastomers Method The quantity of spaghetti and boiling water can be varied according to the class size. The cooking instructions are standard directions taken from a packet of spaghetti. 1. Freshly cooked spaghetti can be poured from one container to another, much like a liquid. Also, the spaghetti takes on the shape of the container, an important property of liquids. The few crosslinks between strands can easily be broken. 2. After about 30 minutes, spaghetti will no longer pour as it did when freshly cooked.

8 8 It will not take on the shape of its container but it can be easily deformed. These are properties of a solid rubber material, an elastomer. Students are likely to describe the spaghetti as rubbery. This change is the result of spaghetti strands sticking together, forming crosslinks. 3 After several hours the spaghetti will be harder and less rubbery. There will be more crosslinks between strands and the crosslinks will be stronger. The spaghetti will resemble a thermoset rather than an elastomer. Students must be supervised if they boil the water for this exercise. An adult should drain the water from the spaghetti. Caution students that cooked spaghetti will stay hot for several minutes Extensions Silly Putty behaves like a viscous liquid. When pulled gently it deforms but is not elastic. When pulled quickly it breaks, and, when dropped it bounces. The crosslinks TESTING PROPERTIES OF PLASTICS Card S-13 Mass and Volume Students should have samples of both paper and expanded polystyrene cups. They do not need the same number of each type of cup if they find the average mass for each type. There are different ways to compare the volume of material used to make the cups. Students might simply crush the cups and make a pile of those that are the same, and then compare heights. A similar, but slightly more accurate method, is to tear the cups into tiny pieces. The pieces could be packed into a graduated cylinder, or measuring cup, and the volumes measured. 4. A landfill can handle a certain volume of garbage, so volume is more important than mass in landfills. Other important landfill considerations include the production of methane gas and leaching of toxic chemicals into the groundwater. Because polystyrene is chemically inert, it does not break down and release gases or other problem chemicals. between polymer chains in Silly Putty are very weak. A gentle force allows the Silly Putty to flow, but a quick pull will break the crosslinks, snapping the material. Reference Gail Marsella, ChemMatters, 1986, pages Students may mention that it is important for materials to biodegrade in a landfill. This is a common misconception. Landfills consist of layers of crushed garbage and soil. Biodegradation requires moisture, bacteria, warm temperatures and especially air. Even biodegradable materials do not biodegrade in landfills, because they are not exposed to air. For example, if last night s leftover vegetables were dumped in a landfill instead of being composted, they would still be recognizable many years later. 5. The alternatives to landfilling disposable cups are incineration, recycling, or reusable cups. Students should be made to understand that each option has drawbacks: incineration must take place in special plants at high temperatures; paper cups cannot be recycled because of the glue used on the seams; and reusable cups must be washed, creating a different waste and energy problem.

9 Extension Students might like to know the relative costs of different types of cups, how many the restaurant uses, and justification for using this type of cup, especially if the student s investigations suggest a different type might be better. There are several cards found in Plastics and the Environment that could be integrated at this point. If you wish to discuss landfills now, use cards E-10 to E-16. Card S-14 Relative Density A variety of different plastics should be used. Some types of plastics have a range of densities because of factors such as branching. The following values may be useful: Substance Density (g/cm 3 ) polypropylene 0.90 low-density polyethylene high-density polyethylene polystyrene 1.05 acrylonitrile butadiene styrene 1.1 (ABS) polymethylmethacrylate 1.2 rubber 1.2 polyvinyl chloride 1.4 polyethylene terephthalate The density of a saturated salt solution is 1.2 g/cm 3. Polypropylene and polyethylene will float on water. As salt is added, polystyrene will begin to float, followed by ABS and perhaps polymethylmethacrylate and rubber. Polyvinyl chloride and polyethylene terephthalate will not float even when the solution becomes saturated. Students using hot water must be supervised by an adult The plastic strips do not have to have either the same mass or volume. If the volume increases, so does the mass. The key property is density, the ratio of mass to volume. This is a fixed property of the material. Both forms of polyethylene, LDPE and HDPE, and polypropylene (PP) will float on water. Corn oil has a density of 0.92 g/cm 3 and can be used to separate polypropylene (floats) and polyethylene (sinks). Linseed oil has a density of 0.94 g/ cm 3, so it could be used to separate HDPE from LDPE. However, it would be easier to rely on other characteristics such as the waxy feel of LDPE and harder, more rigid properties of HDPE. Card S-15 Conductivity This experiment provides experience in graphing, and in interpreting the meaning of the slope of the graph. This exercise does not require mathematical calculations of specific heat capacities of the different cups. Card S-16 Properties of Food Wraps This topic is also in cards E-18 and E-23. It helps establish connections between pure science topics (properties of materials) and applications of science. A food package must a) ensure the safety and nutritional value of food yearround; b) protect the quality of the product, from its manufacture to its consumption; and c) meet consumer needs and preferences. To meet food safety requirements, the package must: meet all regulations that apply to the package materials, provide acceptable package integrity through final consumer usage, provide a barrier to microorganisms and other potential contaminants,

10 10 provide protection against infestation by insects, rodents and other vermin, and withstand distribution, storage, and any other functions. To maintain product quality, the food package must provide: a barrier to prevent transfer of flavours and odours an oxygen barrier to preserve colour, flavour, and nutrients, a barrier to prevent moisture transfer a carbon dioxide barrier to retain internal pressure (e.g. carbonated beverages) a barrier to contain gases such as nitrogen or carbon dioxide (used as preservatives in controlled or modified atmosphere packaging), a light barrier for vitamins or colours that break down in light, a thermal barrier for products above or below room temperature. To meet consumer needs and preferences, the package should: provide critical information on labels (e.g. product brand identification, weight, ingredients list, manufacturer s name and address, codes, date, preparation instructions, nutritional profile), discourage and give evidence of tampering, display contents (in some cases), and open and reclose, be microwavable, dispense or serve, resist shattering, and ensure safe handling. Specific package requirements will vary from food to food, but these lists highlight the many concerns of the packager. Reference Horizons, Volume 1, Issue 2, December Card S-17 Testing Film for Puncture Resistance Students should realize that this test provides information about how different films behave in a very specific way. Films have other properties that should be considered after all, we do not buy films to drop weights on them. Encourage students to think about the properties of films, and how these properties relate to how we use them. For example, we would use only aluminium foil in an oven, but never in a microwave. WORKING WITH PLASTICS Card S-18 Changing Properties with Additives 1. Plastics for raincoats might contain plasticizers, UV stabilizers, fire retardants, biocides, antistatic agents, and antiblock agents. 2. Home installation (polystyrene), electrical wire insulation, electronic components all contain fire retardants. 3. Additives often remain in plastic after it s been recycled. These additives, for example colourants, limit the possible uses for the recycled plastic. The recycled plastic is often used as a middle layer in multilayered containers. Card S-19 Engineering Plastics The ability to tailor a raw resin for a wide range of specific applications by suitable additives is one of the great advantages that plastics offer.

11 Card S-20 Designing with Plastics Use this card as a means of getting students to think about characteristics of careers which might suit them. Have them identify reasons for their choice. Some science teachers may want to consider integrating a selection of cards from The Plastics Manufacturing Business section. Card S-21 Case Study: The Automobile Bumper You may want to choose another product and have students investigate its use following this model. Supplemental Card 1 - Teacher Demonstration Recycling a Plastic This activity card can be given to students if you wish. Students can prepare the mould, tear up the pieces of polystyrene, and set up the heating apparatus. You should actually heat the polystyrene. Method The key to a successful demonstration is to use very small pieces of polystyrene and to heat them slowly at a low temperature. 1. Thermoplastics soften when heated and can go through many cycles of heating and moulding. Thermosets do not soften when heated. Wear safety glasses during this demonstration, both for your own safety and to set a good example for the students. Perform this demonstration in a fume hood. If one is not available, use a well vented area. If the polystyrene catches fire, avoid breathing the gases and extinguish it with water immediately Different thermoplastics soften at different temperatures. As a result unsorted mixtures of different plastics have limited value and applications. A major problem with recycling household plastics has been that so little plastic has been collected in the past. That situation is changing. Many major collection initiatives have been started. Recyclers can now collect sufficient raw material to ensure an economically viable operation. Students should be made aware that plastic waste from industry is recycled. One alternative to recycling household plastic waste is to incinerate it. The energy released through incineration is about the same as that released by an equal mass of oil. Incinerators that burn plastics operate at high temperatures. That keeps potentially harmful gases and ash well below regulated limits.

12 12 TEACHER DEMONSTRATION: RECYCLING A PLASTIC Thermoplastics become soft when heated and can be moulded into a new shape. This process can be repeated, which makes thermoplastics good materials for recycling. Problem To melt polystyrene (a thermoplastic) and mould it into a new shape. Materials polystyrene coffee cup or tart cup aluminium foil wire gauze bunsen burner tripod Do this demonstration in a well vented area. A fume hood is ideal. Avoid overheating polystyrene. If it catches fire, cover it with water and remove from heat. Supplemental Card 1 a. Make an aluminium mould in any shape, using either a piece of aluminum foil, or a small tart cup. b. Tear the coffee cup into the smallest pieces possible. Each piece should be no more than a few square millimetres. Place the pieces into the mould. c. Use a low flame to gently heat the polystyrene in the mould. The polystyrene will melt and solidify in the shape of the mould when cool. Questions 1. Why are thermosets not recyclable in the same way as thermoplastics. 2. A mix of different thermoplastics is difficult to heat and remould. Why do you think this is so?