s Recognizing Plastics at Home S-1 · Recognizing Plastics at Home Objective: ... corrosion deformation sagging shattering stretching warping thermal degradation Discuss these terms

Many things around you are made ofplastic materials.

In a small group, brainstorm and listthings made of plastic. Group these thingsby how they are used. Present your re-sults in a web.

Thinking in Terms of Properties

Recognizing Plastics at Home

Objective: To identify and list products made of plastics

You may want to repeat the brainstorm forother materials. Wood, metal, glass, brickand stone are some possibilities.

Toys____________________________________

S-1

} Examples


Recognizing Plastics in Cars

Objective: To identify some of the plastic components of carsStudents can review the diagram and textbelow and add to it other plastic compo-nents not identified (such as infant or childsafety seats).

BodyFront & rear end bumpersBumper over-ridersAir intake grill AccessoriesAir outlet grill Heating & ventilationRadiator grill grillsWindow fitting Heating & airHard top conditioning ductingTrunk lid Ash traysHeadlight fitting Interior fittingsHood Door handlesSunroof fitting Grab handlesGasoline cap Mirror housingsDoor seal Door lock housingFoam cavity Washer reservoir insulation EmblemsPaint Hub capFenders Decorative trim

Interior light

Engine InteriorBelt drive Fascia & fittingsFan Glove compart-Timing chain drive ment & cover ShelfFuel pump diaphragm Upholstery &Fuel tank seatingRadiator hose Door arm restClutch lining Horn buttonAir filter Knee protectionAir filter housing Head restraintsGaskets Speaker grillSwitch gear Steering wheel &Cable ducting coverCylinder head gasket Side paneling &

roof liningSun visor

Electrical System Door pocketFuse box & dust cover FootmatsIgnition cable, insulation Floor covering & clips Seat beltsInsulation on wiringBattery case

Objective: To identify some of the plastic components of cars

S-2

Adapted from The World of Plastics


Extending Your Vocabulary

Objective: To increase your vocabulary of properties

Through your observations and experi-ments in science class, you may havebecome familiar with some terms used todescribe materials. The best way tobecome really familiar with properties ofmaterials is through direct, hands-onexperience. But materials have manyproperties. Most of these can only beinvestigated using complex equipmentnot usually found in science classes.

Here are some words to describe proper-ties of materials.

flexibilityflame resistanceimpact strengthresistance to:

abrasionbendingbreakingcrackingcorrosiondeformationsaggingshatteringstretchingwarpingthermal degradation

Discuss these terms with others until youhave a general understanding of what theymean. Don’t worry about formal defini-tions.

Add at least three more properties to thelist. Describe situations where you’veencountered some of these properties ofmaterials. Keep in mind these properties asyou explore the issues and questions inother activity cards.

S-3


Physical Properties

Objective: To identify physical properties

Certain materials are chosen for certainproducts because their physical or chemi-cal properties make them suitable for agiven use. We don’t often think about theproperties of materials or describe thoseproperties carefully, but, with the scienceknowledge you have, you can begin torecognize some properties of materials,and find the best words to describe them.

1. Review what you already know aboutphysical properties. Define the term.

2. List as many physical properties ofmaterials as you can.

3. Look through a room at home, in thefamily car, or in your classroom. Fromeach location, choose one item madefrom each of these materials: metal,wood, paper, glass, plastic. In a chart,describe the physical properties of eachitem. Use words you have alreadylearned

PRODUCT MADE FROM METAL WOOD PAPER GLASS PLASTIC

4. We make use of an amazing variety ofproducts made from a wide range ofmaterials. In your history class, youmay have talked about the Stone Age,the Iron Age or the Bronze Age.

These names refer to the materials mostwidely used during those time periods.What term or terms would you use to de-scribe our “age”? Why?

S-4


Why Plastics?

Objective: To identify some properties of plastics that make them suitable materials forspecific applications

Here is a list of properties that describemost plastics to some degree:

light weight (low density)corrosion resistancechemical resistanceelectrical non-conductivityload-bearing strengthwater repellancehigh and/or low temperature tolerancenon-permeabilityimpact resistanceoptical propertiesshatter resistanceflexibilityability to be mouldedability to be coloured

1. From your lists of plastic products (CardS-1), identify at least three products foreach of the properties listed here.

2. Think of a product that has most of theproperties on the list.

3. Find advertisements that show plasticproducts. Try looking in magazines orcatalogues. As a class, create a collage ofpictures or sketches of the products.Include only pictures of products youfind interesting. Be prepared to saywhy.

S-5


Matching Properties To Products

Objective: To apply the vocabulary of properties of materials to properties of products

Any one product has many properties —it would be difficult to list them all.Some of its properties will be more im-portant than others. Think about theproperties a product needs to have. Thenthink about the materials that providethese properties.

1. For each of the products in the chart,list what you think are its three mostimportant properties. Make yourown copy of the chart

2. In the third column are materials thatcould be used to make each product. Foreach important property, put a checkmark below the material that you thinkhas that property.

3. Add at least three more products to the list. Repeat steps 1 and 2 for each product.

ITEM DESIRABLE PROPERTIES MATERIALS

water pipes

skylights

home insulation

baseball bat

insulation forelectrical wires

householdfurniture

automobile seats

tennis racquets

picnic tables

Plastics Woods Metals Glass

S-6

Objective: To describe the failure of a product in terms of a property of the material

Sometimes a product doesn’t work prop-erly, or doesn’t work the way you want itto. This may cost you time, money, ormay simply annoy you.

Have you had a bad experience with aproduct? What happened? Choose asituation where you can easily identify theproblem — an ice cube tray that broke, ahandle that snapped, a cup that melted,etc.

Now try to describe what happened inmore scientific terms. What property didthe product fail to exhibit?

Describe the failure using terms from yourextended vocabulary list. (See Card S-3)Do you think the failure could have beenprevented? How?

What kinds of technical words would youuse to tell the product designer what kindof improvement you want?

Write to manufacturer of the product de-scribing the defect. Suggest how it might becorrected.

These situations worry a manufacturer,because customer dissatisfaction is bad forbusiness.

Start the classroom collection of productsthat just haven’t “made the grade”. Thesemight be products that have broken, orproducts that just aren’t as useful as theycould be.

Note: Keep this card active through theyear. If you haven’t had a bad experienceyet, you may have one later!


Not All Products Are Perfect S-7

Getting to Know Plastics

What Are Plastics Made From?Objective: To identify the raw materials used for plastics and how much is used for different

purposes

Plastic products have so many differentproperties and uses that it is hard toimagine that they are all made from thesame starting materials. The very firstplastics were made from a substancecalled coal tar. Most modern plastics aremade from petroleum resources, crudeoil and natural gas.

There are several steps in making a plas-tic product. First, the crude oil or naturalgas is transported from the well to arefinery. Refineries use various processesto produce fuels and petrochemicals.Petrochemicals are used to make

many products, including fertilizers, lubri-cants, and plastic resins. Plastic resins areused to make the plastic products you arefamiliar with.

The flow chart on this card shows howpetroleum is used. Remember, petroleumincludes both crude oil and natural gas.Study the diagram and note what the vari-ous percentages refer to.

1. Use the information in the flow chart tomake a pie chart. Your pie chart willshow where crude oil and natural gasend up.

S-8

Use ofPetroleumResources

Getting to Know Plastic

Major Categories of Plasticss

Objective: To understand the atomic structure of plastics and understand how they differ

You now have a long list of products inyour home and at school made of plastics.You might classify different plastics bytheir properties. Here are some examples:those that are soft and pliable, such asgarbage bags; those that are hard and rigid,such as telephone casings. Scientists di-vide plastics into three groups based onsome of their most important properties.

To understand why different plastics havedifferent properties, it helps to know some-thing about their structure.

Here is an example of a chemical unit calledethylene. It is made of carbon and hydro-gen, held together by chemical bonds. Thischemical unit is called a monomer.

Ethylene monomers can hook up witheach other lengthwise. They are also heldtogether by chemical bonds. This forms along chain of repeating ethylene units. A

long chain of repeating monomers is calleda polymer. The polymer made from repeat-ing ethylene units is called polyethylene.

Here is another example of a monomer. It is called vinyl chloride.

The polymer formed from vinyl chloride is called polyvinylchloride.

S-9A


A polymer is like a string of beads, witheach identical bead joined to the next.However, the string would have to holdmany thousands of beads to have thenumber of units in most polymerchains.

The three major categories of plastics havevery different properties because theirpolymer chains link together in differentways.

There are two basic types of binding forcesin polymers:

a) bonds between adjacent monomer unitsin a polymer chain. These are calledchemical bonds. They are strong andrelatively resistant to rising temperature.

b) bonds joining polymer chains. These arecalled secondary bonds. They are usu-ally much weaker, and are weakenedfurther with rising temperature. Sec-ondary bonds produce crosslinks be-tween main polymer chains.

S-9A


Major Categories of Plastics

Objective: To identify three major categories of plastics and understand how they differ

ThermoplasticsPlastics with few crosslinks betweenpolymer chains become soft when heated.They are called thermoplastics. Thermo-plastics can be heated until soft and then

moulded into new shapes many times.Polyethylene is a common thermoplastic.It’s made into household products such asbleach and liquid detergent bottles.

ThermosetsPlastics with many crosslinks betweenpolymer chains do not become soft whenheated. They are called thermosets.

These plastics are often hard and brittle,and decompose at high temperatures.Counter tops are made of thermosets.

Thermoplastics have few cross-links between polymer chains.

Thermosets have many cross-links between polymer chains.

S-9B


elastomers and the crosslinks help themreturn to their original shape. Rubber is anelastomer.

ElastomersPlastics with some crosslinks betweenpolymer chains are usually flexible. Theyare called elastomers. You can squeeze

Elastomers have an intermediate number of cross-links

1. Elastomers are easily identified bytheir elastic properties. Make a list ofproducts you think are made ofelastomers.

2. Thermoplastics andthermosets can be identi-fied by the effect of heaton the plastic.

Test different plastic mate-rials to find out whetherthey are thermoplastics orthermosets.

a) Hold a metal needle withtongs. Heat the needle ina flame until it is red hot.

b) Touch each piece of plastic with the hotneedle. If it is a thermoplastic it willmelt. If there is no effect it is a thermo-set.

c) Complete thefollowing table. De-scribe each productyou tested, the resultof the test, its proper-ties and the uses forvarious thermoplas-tics and thermosets.

SAFETY NOTE:

DO NOT heat plastics in anopen flame. Caution shouldbe taken to avoid accidents orinjury.This test must be supervisedby your teacher.

Descriptionof product

Effect ofhot needle

Type of PlasticThermosetThermoplastic Elastomer

3. Which type ofplastic do you thinkcan be recycledmost easily? Why?

S-9B

Objective: To identify major types of plastic resins used in packaging

In Canada, most plastics are used for pack-aging. Examine plastic containers in your

home to identify the types of plastic poly-mers used in packaging.

The table below shows the logostamped on the packaging and namesthe polymer used. Look on the bottomof plastic containers in your home tofind the logos. Make a copy of thetable and add your examples to the listof packaging.Describe the properties of the plasticused for each product.

Major Markets forPlastic Products

Packaging 34%

Construction 26%

Transportation 18%

Furniture and Furnishings 5%

Electrical and Electronics 5%

Other 12%

Polymer Logo Packaging PropertiesTable 1


Plastics in Packaging

Polyethyleneterephthalate

High-densitypolyethylene

Polyvinylchloride

Low-densitypolyethylene

Other

Polypropylene

Polystyrene

2-L soft drink bottlewater bottle

crinkly department store bagbleach bottlelarge vegetable oil bottlewindow cleaner bottlefurniture polish bottle

grocery baggarbage bag

margarine tubfast-food microwave tray

meat traydisposable cup

peanut butter jar

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Table 2

PETE

HDPE

V

OTHER

PS

LDPE

PP

3

4

5

6

2

7

1

1. A packager must choose a material thatbest fits the function of the product.Examine your list of plastic containersand the properties of the material used.Summarize some impor-tant properties for thekind of container.

2. Repeat the hot needle test(card S- 9b) on a productfrom each category.Determine if the product is athermoplastic or a thermoset.

3. Compare the masses of a plastic bottleand a glass bottle having the same ca-pacity. Which has less mass? What arethe advantages of having as little mass

as possible?

4. Why are plastic foodcontainers collected forrecycling rather thanrefilling.

5. What are things a manufacturer shouldthink about in producing a good pack-age?

SAFETY NOTE:Remember a hot needlecan cause small burns.

Getting to Know Plastics S-10


High-density andLow-density Polyethylene

Objective: To model the key molecular differences between HDPE and LDPE

After a shopping trip, you might bringhome bags from different stores. Somestore bags have a crinkly feel, whileothers feel waxy. Yet these bags are madefrom the same plastic, polyethylene. Achemist makes plastics with differentproperties by using polymer chains withdifferent connections. An importantproperty of a polymer chain is how muchbranching it has.

An unbranched polymer chain resemblesa string of beads, each bead representing arepeating unit of the polymer (Figure 1).

Each bead is joined to only two other beads,one in front and one behind.

A branched polymer has strands coming offthe main polymer chain (Figure 2). Bycontrolling how the polymers are made,chemists can make polymers with varyingamounts of branching. For example, twotypes of polyethylene can be produced —high-density polyethylene (HDPE) and low-density polyethylene (LDPE). The differ-ence in density between HDPE and LDPE isdue mainly to the different amount ofbranching in these polymers. Models helpyou understand how branching affects thedensity of a plastic.

Figure 1: High-density polyethylene has relatively few branches off the mainpolymer chains and has a crinkly feel.

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Figure 2: Low-densitypolyethelyne: has arelatively large numberof branches and has awaxy feel.

PROBLEMConstruct a model for branched andunbranched polyethylene and predictwhich has the greater density.

MATERIALSsmall box,connectible cubes (e.g. Lego® cubes)

METHODa. Connect cubes to make straight chains.

When a chain is long enough to just fitinto the box, place it there. Makeenough chains to fill the box.

b. When the box is full, find the mass ofthe unbranched “polymer” chains.

c. Next connect cubes to make chains withbranches. Place the branched chains inthe box until it is full.

d. When the box is full, find the mass ofthe branched “polymer” chains.

QUESTIONS1. Density is the ratio of mass/volume. If

two objects have the same volume, theone that has a higher mass has the higherdensity. Which of your models repre-sents high-density polyethylene? DoesHDPE have many branches or few?How does this fact explain the differencein density between HDPE and LDPE?

2. HDPE is used to make crates, bleachbottles and other household containers.It is harder and more rigid than LDPE.Use your model for HDPE to account forthese properties.

3. The connectible cubes allow you tomodel different polymer chains. How-ever, models do not always representeverything properly. In what ways doyou think your models for HDPE andLDPE are not like the actual polymerchains?

EXTENSIONS4. Measure the dimensions of the box you

used in this activity. Calculate its vol-ume. (Remember, the formula for vol-ume is V=LxWxH).

5. Calculate the density for your models ofbranched and unbranched polymerchains.

Objective: To develop further understanding of crosslinked polymer chains by using spaghetti asa model for elastomers

Elastomers are fun to play with, but theyalso have important uses. Elastomers,such as rubber, bounce when droppedand return to their original shape afterbeing stretched or squeezed. Rubber canbe made from liquid found in certaintrees, but much of the world’s supply isnow made of synthetic chemicals, whichduplicate the properties of natural rubber.

Rubber is a polymer. That means it’smade of long chains of many identicalchemical units. The polymer chains canbe joined at various points along thechain. These joins, called crosslinks, havean important effect on the properties

of a polymer. A polymer with somecrosslinks is called an elastomer. When anelastomer is squeezed or stretched, thepolymer chains deform but the crosslinkshelp them return to their original shape.

If there are only a few, easily broken cross-links, the elastomer may flow very slowlylike Silly Putty® . More solid elastomershave crosslinks that are not easily broken.The more crosslinks in the material, themore rigid it is. By varying the conditionsof the chemical reaction, chemists can varythe numbers of crosslinks in the final prod-uct.

b. Have an adult drain the water and pour the spaghetti onto a plate or a tray.

c. Notice how the spaghetti strands movepast each other. Record your observa-tions.

d. Leave the spaghetti for about 30 minutesNow try pouring it intoanother container. Thespaghetti will be coolenough to touch, so take ahandful and squeeze it.Record your observations.

e. Leave the spaghetti for several hours.Again try to pour it and touch it. Recordyou observations.

PROBLEMUse the changing properties of cookedspaghetti as a model to explain the prop-erties of cross-linked polymer chains.

MATERIALS pot of boiling water spaghetti

METHODAsk your teacher to pourboiling water for you. Spa-ghetti will stay very hot forseveral minutes after it’sremoved from the boilingwater.

a. Boil about 2L of water in a large pot. Addabout 150g of spaghetti (a quarter of apacket) and cook uncovered for about 10minutes. Stir the spaghetti occasionally.

Do not boil water without an adult present

SAFETY NOTE:


A Model for Elastomers S-12

Getting to Know PlasticsS-12

QUESTIONS

1. How does freshly cooked spaghetti be-have like a liquid?

2. How is spaghetti that has been left forabout 30 minutes like an elastomer?What changes in the spaghetti accountfor this?

3. What is spaghetti like after it has beenleft for several hours? Explain this interms of changes in the spaghetti’s prop-erties.

EXTENSION

4. Experiment with Silly Putty®. Describeits properties and try to explain them.

5. Divide the Silly Putty® in two. Put onehalf in hot water, and the other half in afreezer. Repeat step 4. Note how the hotSilly Putty® is different from the coldsample.

Objective: To compare the mass and volume of cups used in fast food outlets

It is becoming increasingly difficult tofind suitable landfill sites for garbage.We need to reduce the amount we throwaway. The quantity of garbage can bemeasured by either mass or volume.

Which type of disposable cup has theleast mass? Which type occupies the leastvolume?

Collect a variety of disposable cups madeof different materials. Choose cups thathold the same volume of liquid. (If you’recollecting from restaurants, explain whatyou’re doing and ask permission first.)

1. How many different types of cups didyou collect? Describe the properties ofeach type.

2. Determine the mass of the cups thathold the same volume of liquid.Record your observations. Which typeof cup has the least mass? What is theadvantage of a cup with low mass?

Many fast-food restaurants use disposablecups. Most of these end up in a landfill,unless a community has a recycling pro-gram.

3. Determine the volume of each type ofmaterial used in the cups. Record yourobservations. Which type of material hasthe least volume? What is the advantageof a cup with a low volume of material?

4. Which property, mass or volume, is ofgreater concern in objects going into alandfill. Why? What other properties areimportant?

5. Why are disposable cups used by somebusinesses? Discuss alternatives to dis-posable cups. What are some advantagesand disadvantages of cups that are meantto be reused?

EXTENSIONMake a list of questions you would like toask the manager of a fast-food restaurantregarding its use of disposable cups. If yourteacher agrees, contact a manager and ar-range an interview.

Testing Properties of Plastics

Mass and Volume S-13

Objective: To separate different plastics using a physical property

We use many different types of plastic. Inmany recycling operations, different typesof plastics are separated. One method

recyclers use to separate a mix of differentplastics makes use of their different densi-ties.

PROBLEMUse a salt solution to separate a mix ofplastics.

MATERIALSlarge beakersaltstirring rodsmall strips of different plastics

METHODa. Fill a beaker about three-quarters full

of water. Add the strips of plastic andstir. Stirring will remove air bubblesand break surface tension. After stir-ring, any plastic that floats must havea density less than that of water.Record the identity of any strip thatfloats.

b. Add salt to the water a bit at a time.Stir after each addition. Adding saltraises the density of the salt solution.When the density of the salt solutionmatches that of a particular plastic,the plastic strip will begin to float.Record the order in which the differentplastics float.

QUESTIONS1. The plastic strips used in your experi-

ment may not have had the same mass.Would this affect your results?

2. Suppose you want to start a plasticsrecycling operation. How could youuse the principles encountered in thisactivity to design your plant? Explain.


Relative Density S-14

Objective: To compare heat conductivity through plastic and other materials

More plastics are poor conductors of heatand electricity. That’s the same as sayingthey are good thermal and electrical insula-tors. Most wires are covered with plasticbecause it is an excellent electical insulator.For the same reason, plastics are used tomake electrical plugs and other connections.Some plastics, such as foamed polystyrene,hold trapped air.

PROBLEMCompare how rapidly hot water cools whenit’s placed in cups made of different materi-als.

MATERIALSheat source timerthermometer graph papercardboard scissorsmeasuring cup

Cups of similar size made fromexpanded polystyrenepaperceramicmetal

METHODa. Make a table to record temperature and

time. You will need a line for everyminute from 0 to 10 minutes.

b. Make a cardboard cover for your cup.Use the cup with the largest diameter.Turn it upside down on the cardboard.Trace around the cup with a pen orpencil, and cut around the tracing. Makea hole in the centre of the cardboard discjust large enough for thethermometer.

c. Fill a kettle or beakerwith water and heatuntil it reaches about70°C.

This characteristic makes them good materi-als for insulating buildings.

Heat ConductivityIn this activity you will compare the thermalinsulating ability of cups made of differentmaterials.

d. Use a measuring cup to find the volumethat will fill each of your experimentalcups about three-quarters full. You mustuse the same volume of water, at thesame temperature, with each cup youtest. Pour this volume of hot water intoone of the experimental cups.

e. Place a cardboard disk over the cup andput the thermometer through the hole inthe center. Record the initial temperatureat time t = 0 minutes.

f. Measure and record the temperatureevery minute for 10 minutes.

g. Repeat steps c. to f. for each cup

h. Make a graph with temperature on the y-axis and time on the x-axis. Plot each setof data on the same graph, using differ-ent symbols or colors for each cup. Drawa smooth curve through each set of data.

QUESTIONS1. In which cup was there the smallest

change in temperature?

2. Which set of data produced a line withthe smallest slope? What is the signifi-cance of the slope of this line?

3. Rank the cups from best insulator toworst.

4. Look at a piece of foamedpolystyrene under amicroscope. Describe whatyou see.


Conductivity S-15

Use hot water under the supervi-sion of your teacher or anotheradult.

SAFTEY NOTE:

Objective: To list the properties of various food wraps and identify appropriate applications

In your kitchen, you may have threedifferent types of wraps: plastic film, waxpaper, and aluminum foil. These filmshave different properties, and these prop-erties determine how the wraps are used.

Make a table to compare the properties anduses of the different types of wraps.

1. Which properties are most important for each of the uses you’ve identified?

2. Describe some uses for which either plastic wrap or wax paper are suitable. Whichproperties of these two materials are similar?

3. Describe one use for which aluminum foil is the only suitable choice. Which propertyor properties of aluminum foil are important in this case?

4. Describe a use for which aluminum foil is unsuitable. Explain why, using properties ofaluminum foil.

5. List all the properties that you think a food package should have.

Plastic Film

Wax Paper

AluminumFoil

Wrap Properties Uses


Properties of Food Wraps S-16

Compare how different plastic food wrapswithstand a weight being dropped on them.

PROBLEM

a. Place some packing material in thebottom of the jar to protect the bottom ofthe jar.

b. Tear off a piece of wrap equal to thediameter of the jar plus about 6 cm.Place the wrap tightly across the mouthof the jar and over the rim. Use an elas-tic band to hold it in place.

Hold the long cardboard tube just abovethe centre of the film without resting onit.

c. Place a few washers over the bolt so theycan rest on the nut. This is the weightyou will drop onto the wrap.

d. Drop the weight through the tube from aheight of 10 cm above the film. Examinethe wrap for any effects due to the fall-ing weight. Record your observations.

e. Replace the wrap after each drop, even ifits not broken. Add a few more washersand repeat step d. Take care to drop theweight from the same height each time.

METHOD

Objective: To compare the puncture resistance af various food wraps

Plastic film often needs to be reasonablystrong. There are several ways to test thestrength of plastic films and other foodwraps. This method is called a punctureresistance test.

MATERIALSjar nut, bolt, and identical washers newsprint paperpaper wax paper writing paperpacking material balance wrapping paperlong cardboard tube aluminum foil various paper bagselastic band plastic film


Testing some Materials for PunctureResistance

S-17

jar

bolt

washers

nut


S-17

f. Find the mass that just breaks thewrap.

g. Compare the “breaking strength” onthe same wrap found by others in theclass. What was the average “break-ing weight” for each wrap.

QUESTIONS1. Why should you replace the film after

each drop even if it does not break?

2. Why is it important to drop the weightfrom the same height each time?

3. Suppose you have a nut and bolt, butno washers. How should you changeyour experiment to test the film?

EXTENSIONS• Compare different brands of plastic film

wrap. which was strongest in this test?What other factors are important whenchoosing among brands of film?

• Repeat your test using writing paper,wax paper, newsprint and thenaluminum foil. Which of the three mate-rials best withstands a mass falling on it?

• Puncture resistance is only one physicalproperty of a food wrap. What otherproperties might be important to con-sumers?

Pure polyvinyl chloride (PVC) is hard, brit-tle, and decomposes at the temperatureneeded to process it — in its pure formPVC is almost useless. However, PVC isone of the most useful andversatile plastics. That’s because whensubstances are added to resin, theychange the basic properties of PVC.

The process of mixing additives into aplastic resin is called compounding.Many plastic resins are compounded, thatis, they contain additives that give themcertain properties. For example, pigmentsare added to resins to give the final prod-uct an attractive colour. Other additivesmake the plastic easier to process, ormake it stiffer, tougher, or more resistantto attack by chemicals.

Here is a brief description of somecommon types of additives and their uses.

Antiblock agents prevent two plasticsurfaces from sticking together. Whenyou have difficulty opening up a plasticbag, it may be because the manufacturerdid not add enough antiblock agent to theresin.

Antistatic agents prevent electric chargesfrom building up on the surface of plas-tics. Plastics are usually good insulators.An antistatic agent allows the surface ofthe plastic to conduct charge onto some-thing else.

Biocides prevent attack by bacteria orfungi.

Colourants give the final product anattractive colour.

Fillers such as mica make plastics stronger.Other fillers, such as talc, allow products tobe made with less plastic. They end upbeing less costly.

Fire retardants hinder the burning ofplastics. Because plastics are petroleum-based materials, they are basicallyflammable substances. Fire retardants donot prevent plastics from burning, but theycan slow the spread of fire.

Plasticisers make plastics more flexible.

Processing aids make plastics flow betterwhen they are soft. This makes mouldingeasier.

Slip agents decrease the friction betweensheets of plastic film during and immedi-ately after processing.

UV stabilizers prevent or reduce the dam-age sunlight can do to plastics.

1. What additives would you add to theresin for a plastic raincoat?

2. List some plastic articles that might con-tain fire retardants.

3. How might the use of additives affect therecycling of plastics?

Objective: To describe how resin properties can be changed with different additives

Working with Plastics

Changing Properties with Additives S-18

S-19Working with Plastics

Engineering Plastics

Objective: To identify properties of engineering and composite plastics that distinguish them fromother plastics, and to identify reasons why there might be growth in sales for these plastics

Engineering PlasticsSome plastics are particularly well suitedfor construction and automotive applica-tions. These resins are called engineeringplastics. Their properties include very highstrength, atmospheric corrosion resistance,thermal and electrical insulation, and lowthermal expansion. They can be mouldedinto load-bearing shapes, and used in theplace of metals. By understanding thedetails of their chemical structure, chemicalengineers and plastics designers can customdesign their properties for many applica-tions.

Engineering plastics are high-performancematerials that are replacing traditionalmaterials such as metals and ceramics.They are more expensive than plastics suchas polyethylene. Often they’re more expen-sive even than the traditional materials theyreplace. However, engineering plasticsoffer several advantages, such as lightweight, and their use is increasing.

Composite PlasticsComposites are one type of engineeringplastic. A composite is a plastic that con-tains another material embedded in it,usually in the form of fibres or flakes. Com-posites are very strong for their mass.

Composites are usually developed forapplications that require special strength orstiffness. Fibre composites made of glass,Kevlar®, or carbon provide strength alongthe direction of the fibres. The fabricatorcan choose a reinforcing fibre and plasticresin to form the best composite for a givenuse.

Glass fibre composites are the least expen-sive and the least stiff. They are findingincreasing use as automobile parts.Kevlar® composites are stiffer than glasscomposites. Composites with carbon fibresare the stiffest of all. However, high per-formance composites made with Kevlar® orcarbon are expensive. They are used inaerospace and aircraft manufacturing andfor sporting goods, such as lightweightcanoes, when performance is more impor-tant than cost.

Composites made using flakes providestrength in the plane of the flake. The mostcommon and cheapest flake reinforcementis mica. Mica-reinforced polypropylene isused in the automobile industry.

1. Explain why some plastics are called“engineering plastics”.

2. Think of a specific piece of sports equip-ment. Describe the properties it shouldhave. What is it commonly made of? Isthis a product that could be made of anengineering plastic or composite?

3. Identify a product made of a glass fibrecomposite. What other material mighthave been used to make the same object?List factors that would lead a manufac-turer to choose one material over others.

4. Why would you expect that there wouldbe a growth in sales for these products?


Designing with Plastics S-20

Objective: To consider a career possibility

The first uses for polymers required littledesigning. Items such as plastic spoonsand electrical outlet covers could be madesimply and cheaply in large quantities.

In the past twenty years the major growthin the plastics industry had been in pack-aging. Plastic film, tubs, bottles and cratesare in every store and home.

The next major growth area is expected tobe in what are called durable products —those that are now made of metal or ce-ramics. To be successful, durable plasticsproducts will have to be carefully de-signed to take full advantage of the prop-erties of plastics.

A plastic product can be more successfulthan one made of traditional materials if itis cheaper, stronger, more appealing, lastslonger or offers some other advantage.The price of PVC is about the same as thatof steel but some engineering resins maycost several times as much as steel on aper pound basis. So, considering cost , itseems odd that plastics would sometimesbe chosen to replace steel. This is happen-ing because of the design flexibility ofplastics and of course its much lowerdensity.

The cost of making a product is muchmore than the cost of material used. Thisis especially true when many pieces mustbe put together to make the finishedproduct. A common lawn rake madeusing steel takes 21 pieces. These piecesmust be packaged, transported, stored ina warehouse, painted and assembled. Thesame sort of rake made of plastic has only

three pieces, which do not need paintingand are much easier to assemble. Thesavings result in a plastic rake that may beless expensive and more attractive than asteel one in that its lighter in weight andwill not rust.

The design engineer cannot simply replaceall metal parts with plastics ones becausemetal may be both cheaper and stronger.However, the design of the product cantake advantage of the unique ability ofplastics to be formed into any desired shapeand to take on almost any desirable prop-erty. If you were designing a durable prod-uct, you might begin with questions such asthese:

What is the purpose of the product?

What job will it do?

How long must it last?

Will it be exposed to a harmful environ-ment?

How will different parts of the product bejoined together?

Which parts must be strong and what partscan be weaker?

Answering the last question can be verydifficult. The design engineer may use acomputer to help find some solutions. Theengineer will use a computer to calculatethe forces acting on different parts of theitem, and how thick each part must bemade to perform well.

The next step is to choose the best mate-rial for the product. The plastics fabrica-tor can choose from a wide range of mate-rials which have different properties.Usually the fabricator will work closelywith the resin supplier to find answers tothese kinds of questions:

Which resins should be used?Can a better product be made using acomposite?

1. Find a large product that uses morethan one type of material (e.g. anappliance, a wheelbarrow, an automo-bile). Examine it closely. If possible,compare one made mostly of metalwith another made mostly of plastic.Consider these questions.

How many separate parts are there tothe product?Which are made of plastic?Which are made of metal?What advantages do the plastic partshave over the metal ones?What advantages do the metal partshave over the plastics ones?Which parts, if any, can be recycled?

What additives should be compoundedwith the resin?

The final decisions are affected by the proc-ess and machines to be used for the prod-uct. Sometimes the fabricator can use exist-ing machinery with some modifications, forexample, a new mould.

2. Find out what is meant by CAD/CAM(computer aided design/computer aidedmanufacturing ) technology. Try to locatea company that uses CAD/CAM. Writea one-page report on CAD/CAM.Describe the impact of this technologyon design and engineering practices.

3 What sort of skills do you think design-ers of plastic products should have?How can these people help make acontribution to society?

4. Write a paragraph on whether youwould be interested in a career as aplastics designer. Explain your reasons.

Working with PlasticsS-20


Some Environmental Aspects of Plastics:The Automobile Bumper — A Case Study

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In the mid 1970s the average NorthAmerican car had a mass of 1760 kg. Atabout that time, legislation was passedrequiring cars to become more fuel effi-cient. By the end of the 1980s the mass ofan average car was about 1350 kg. Part ofthis reduction is due to the popularity ofsmaller cars; part of it is due to the use oflighter materials in the place of heaviermaterials such as steel. Plastics are themost important of these lighter materials.

A typical car in the mid 1970s containedabout 60 kg of plastic. (That figure wasjust under 12 kg in 1960.) In 1998, theaverage car contained 125 kg of plasticmaterials, reducing the mass of an aver-age car by about 150 kg. This substitutionof plastics for steel reduced the energy

needed to make a car by 55%. That energywould have been required to manufacturethe steel replaced by plastics.

The energy not used also reduced the asso-ciated pollution. As a result of this reducedweight, North American drivers avoidburning 3.8 billion litres of gasoline a year.That figure represents the energy used todrive 3 million cars more than 16 000 kmeach year. The impact of this substitutionon air pollution and hence public health issignificant.

In the past, manufacturers generally aimedonly to make a better, cheaper product.Increasingly today, the whole life cycle of aproduct is considered. For an automobilebumper this means considering not only thecost of making the parts, but also the costsof repairs and replacement due to collision.The life cycle concept includes the possibil-ity of recycling, and the cost of disposal.Let’s consider some aspects of a life cycleanalysis for an automobile bumper.

A bumper has three main compo-nents, the fascia, bumper beamand energy absorbing system.The fascia, or bumper cover, isnow usually made of plastic,instead of chrome-coated steel.The plastic fascia is corrosionresistant, and resists damage

from small stones. It can be made usingeither injection moulding or compressionmoulding, producing rounded edges thatare more difficult to achieve by stampingsteel. When the Ford motor companyswitched to a polyurethane bumper coverfor its Mustang, it saved $9 per car anddecreased its mass by 6 kgs.

Objective: To consider environmental complications and applications

Frame

(Bumper cover)

The bumper beam provides rigidity, andmost are still made of steel. Some cars dohave bumper beams made using engi-neering plastics such as glass reinforcedpolypropylene and a blend ofpolycarbonate with polybutyleneterephthalate. Beams made of these plas-tics have less mass, but cost more thansteel beams.

Automobile bumpers are designed tominimize damage during collisions at lowspeeds. Bumpers made using plasticstend to be damaged less than all steelbumpers by such collisions. This is be-cause the plastic bumpers tend to bounceback, whereas those made using steel tendto crease and bend.

Other factors to consider are manufactur-ing cost and mass. The least expensivebumper to make would be one using allsteel, but it would also have the greatestmass and would contribute to poor fuelefficiency. A decrease of 70 kg in the massof a car increases its fuel efficiency byabout five per cent.

The final stages of life cycle involve recy-cling and disposal. Recycling of automobileparts and scrap metal is well established,but traditionally plastic material has beensent to landfills. With rapidly increasinglandfill costs and growing quantity of plas-tic used in cars, there is a need to be able torecycle the plastics from automobiles.

Recycling automobile plastics is difficultbecause they contain many different poly-mers, together with contaminants. Onepossibility is to recycle shredded plastic intoa construction material similar to particleboard. Another option is to recover energyfrom the plastic by burning it in a well-designed and maintained energy fromwaste plant.

1. List the advantages and disadvantagesof the change in bumper design.

2. Do you think the advantages outweighthe disadvantages? Give reasons foryour answer.

3. What are the advantages of a plasticgasoline tank for automobiles?


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s Recognizing Plastics at Home S-1 · Recognizing Plastics at Home Objective: ... corrosion deformation sagging shattering stretching warping thermal degradation Discuss these terms