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What are polymers? How are polymers made? What is a plastic? What are polymers? All plastics are polymers, but not all polymers are plastics. Plastic more commonly refers to the way a material behaves under applied forces, or behaves when it melts and flows. Wool, cotton, silk, wood and leather are examples of natural polymers that have been known and used since ancient times. This group includes biopolymers such as proteins and carbohydrates that are constituents of all living organisms. Synthetic polymers, which includes the large group known as plastics, came into prominence in the early twentieth century. Chemists' ability to engineer them to yield a desired set of properties (strength, stiffness, density, heat resistance, electrical conductivity) has greatly expanded the many roles they play in the modern industrial economy. Polymers are just very big molecules made of smaller molecules linked together into long, repeating chains. You may not know it, but you're in touch with polymers every day more than any other kind of material. Rubber bands are made of polymers, so are paints and every kind of plastic. And by the way, most of the food you eat is made of natural polymers! Polymers are formed through chemical reactions in large vessels under heat and pressure. Other ingredients are added to control how the polymer is formed and to produce the proper molecular length and desired properties. This chemical process is called "polymerization". How are polymers made? Synthetic polymers are produced by chemical reactions, termed "polymerizations." Polymerizations occur in varied forms--far too many to examine here--but such reactions consist of the repetitive chemical bonding of individual molecules, or monomers. Assorted combinations of heat, pressure and catalysis alter the chemical bonds that hold monomers together, causing them to bond with one another. Most often, they do so in a linear fashion, creating chains of monomers called polymers. Some polymerizations join entire monomers together, whereas others join only portions of monomers and create "leftover" materials, or by-products. Co-polymers can be formed using two or more different monomers. And two or more polymers can be combined to produce an alloy, or blend, that displays characteristics of each component. For an example, let's consider the common plastic polyethylene, which is found in such items as grocery bags, toys and bottles. The monomer ethylene is composed of two carbon atoms, each bonded to two hydrogen atoms and sharing a double bond with one another. Polyethylene consists of a chain of single-bonded carbon atoms, each still carrying its two hydrogen atoms. Linear LINEAR POLYETHYLENE One way to produce polyethylene is called "free radical polymerization." As in other polymerizations, the process has three stages, known as initiation, propagation and termination. To begin, we need to add a catalyst to our supply of ethylene. A common catalyst is benzoyl peroxide, which when heated has the habit of splitting into two fragments, each with one unpaired electron, or free radical. These fragments are known as initiator fragments. The unpaired electron naturally seeks another and finds a convenient target in the double bond between the carbon atoms in the ethylene molecule. Taking an electron from the carbon bond, the initiator fragment bonds itself to one of the monomer's carbon atoms. The radical is now happy, but this initiating reaction creates another free radical associated with the ethylene molecule's other carbon atom. The new radical also seeks a partner. And so ethylene monomers begin attaching themselves in a chain, creating new radicals each time and lengthening the chain. This stage is called propagation. Growing chains may also attach themselves to one another. Most commonly, chains join end to end, but sometimes they join end to backbone, making branched polyethylene molecules. Eventually, free radical polymerization stops due to what are called termination reactions. For example, instead of stealing an electron from double-bonded carbons or a nearby propagating chain, the carbon atom with the free radical sometimes steals an entire hydrogen atom from another chain end. The polymer end--robbed of its hydrogen--easily forms a double bond with its adjacent carbon atom, and polymerization stops. Because every part of the ethylene monomer is included in the finished polymer, the free radical polymerization of polyethylene is referred to as an addition polymerization; the ethylene molecules are simply added together. Polymerizations that use only portions of a monomer, however, are known as condensation polymerizations. The monomers that condense with each other must contain at least two reactive groups in order to form a chain. Condensation reactions result in "condensed" polymers that have less total mass than the monomers used to create them and the by-products, or "condensates," combined. For example, poly(ethyleneterepthalate), a polyester known as PET that is commonly found in soda bottles, forms from a reaction of two monomers: ethylene glycol and terephthoyl chloride. At the reaction's end, an atom of hydrogen and an atom of chlorine are left out of each PET molecular junction, resulting in a by-product of hydrogen chloride (HCl) gas. A polymer is composed of many repeating units called monomers. Starch, cellulose, and proteins are natural polymers. Nylon and polyethylene are synthetic polymers. Polymerization is the process of joining monomers. Polymers may be formed by condensation or by addition polymerization. 1. Condensation polymerization? a. What is it a result of? What is the usual by-product? We call a polymerization a condensation polymerization if part of the monomer molecule is kicked out when the monomer becomes part of the polymer. The part that gets kicked out is usually a small molecule like water, or HCl gas. http://www.psrc.usm.edu/macrog/synth.htm Sketch out an example of how two monomers can join to form a dimer. http://www.psrc.usm.edu/macrog/synth.htm c. How can this lead to the formation of long-chain polymers? What is the prerequisite for this to occur? It can of course react with one of the monomers to form a trimer. It may react with another dimer to form a tetramer. Or it may even get really crazy react with a trimer to form a pentamer. these tetramers and pentamers can react to form even bigger oligomers. And so they grow and grow until eventually the oligomers are big enough to become polymers. http://www.psrc.usm.edu/macrog/synth.htm d. What are five kinds of important, everyday condensation polymers? Nylon (a polyamide), Dacron (a polyester), polyethyleneglycol, or PEG (a polyether), urea-formaldehyde foam (another polyamide), and polycarbonate resins. http://www.noahsays.com/article.asp?aid=2873 2. Addition polymerization. a. What is meant by the label �addition polymerization�? In one process, the small molecules, or monomers, simply add to each other in a linear fashion, and in a stepwise manner. A molecule of polyethylene, for example, is formed as ethylene molecules bond together end-to-end. Polymers that are formed in this way are called 'addition polymers', and the process is called 'addition polymerization'. http://www.psrc.usm.edu/macrog/synth.htm b. Show how addition polymerization can occur. http://www.psrc.usm.edu/macrog/synth.htm c. What are the three basic steps in addition polymerization? What has to happen during each step for polymerization to occur? The first step in producing polymers by free radical polymerization is initiation. This step begins when an initiator decomposes into free radicals in the presence of monomers. The instability of carbon-carbon double bonds in the monomer makes them susceptible to reaction with the unpaired electrons in the radical. In this reaction, the active center of the radical "grabs" one of the electrons from the double bond of the monomer, leaving an unpaired electron to appear as a new active center at the end of the chain. After a synthesis reaction has been initiated, the propagation reaction takes over. In the propagation stage, the process of electron transfer and consequent motion of the active center down the chain proceeds. In theory, the propagation reaction could continue until the supply of monomers is exhausted. However, this outcome is very unlikely. Most often the growth of a polymer chain is halted by the termination reaction. Termination typically occurs in two ways: combination and disproportionation. Combination occurs when the polymer's growth is stopped by free electrons from two growing chains that join and form a single chain. Disproportionation halts the propagation reaction when a free radical strips a hydrogen atom from an active chain. Disproportionation can also occur when the radical reacts with an impurity. This is why it is so important that polymerization be carried out under very clean conditions. http://plc.cwru.edu/tutorial/enhanced/files/polymers/synth/Synth.htm http://matse1.mse.uiuc.edu/~tw/polymers/prin.html d. What are three important, common polymers formed by addition polymerization? Polyethylene (what sheet plastic and plastic bags are made of), polymethylmethacrylate (Plexiglas), polytetrafluoroethylene (Teflon), polyacrylonitrile (CrazyGlue), polypropylene (what plastic ropes and plastic tarps are commonly made of), and natural rubber. http://www.noahsays.com/article.asp?aid=2873 3. Polymers can be described as being thermosetting or thermoplastic. a. Why are the thermosetting polymers so different from thermoplastic polymers? As plastics become easier to mold and shape when they're hot, and melt when they get hot enough, we call them thermoplastics. This name can help you tell them apart from crosslinked materials that don't melt, called thermosets. Thermosets are hard and stiff crosslinked materials. Thermosets are different from thermoplastics, which become moldable when heated. Thermosets are crosslinked, so they don't. They don't melt because the crosslinks tie all the polymer chains together, making it impossible for the material to flow. http://www.psrc.usm.edu/macrog/plastic.htm http://matse1.mse.uiuc.edu/~tw/polymers/prin.html b. What decides if a polymer will be thermosetting or thermoplastic? Thermosetting polymers are crossed-linked where as thermoplastic polymers are in long chains. c. What are three important, common thermosetting polymers? Polyurethane, polyester, and nylon �two part� epoxy, Phenolic (Bakelite) d. What are three important, common thermoplastic polymers? Polyethylene, polypropylene, and polystyrene, SBS rubber, TPU (thermoplastic polyurethane) http://www.polyurethane.org/polyurethane_material/thermoplastic_poly/index.html 4. During polymerization, the �-mers� can form long straight chains, long side chains that give a branched configuration, or long chains chemically linked together and are said to be �cross-linked�. a. Describe each of these three types of molecular arrangements separately comparing their physical and chemical properties. Linear polymers are made up of one long continuous chain, without any excess appendages or attachments. Branched polymers have a chain structure that consists of one main chain of molecules with smaller molecular chains branching from it. A branched chain-structure tends to lower the degree of crystallinity and density of a polymer. Cross-linking polymers occur when primary valence bonds are formed between separate polymer chain molecules. b. What are three important long straight-chained polymers? Polypropylene, polybutylene, and polystyrene, polyvinylchloride c. What are three important long branched polymers? HDPE d. What are three important long cross-linked polymers? Polyurethane, polyester, and nylon. Carbon fiber? 5. Polymers can be described as being amorphous or crystalline polymers. a. Compare the physical and chemical properties of amorphous and crystalline polymers. Crystalline polymers have higher chemical resistance, shrinkage, warpage, tensile strength, tensile modulus, creep resistance, flow, maximum exposure temperature, and density but lower elongation. Crystallinity makes a material strong, but it also makes it brittle. A completely crystalline polymer would be too brittle to be used as plastic. The amorphous regions give a polymer toughness, that is, the ability to bend without breaking. http://www.dow.com/polypro/techctr/primer/fig_tab/cryst.htm http://www.psrc.usm.edu/macrog/crystal.htm b. What are three important amorphous polymers? Lexan-Polycarbonate, atatic polystyrene, Lucite/Plexiglas-poly(methyl methacrylate), Natural rubber-polyisoprene, polybutadiene (ABS rubber), PVC. c. What are three important crystalline polymers? Polypropylene, polyethylene, syndiotactic polystyrene (SBS rubber), Nylon, Kevlar, polyketones, and PET. http://www.dow.com/polypro/techctr/primer/fig_tab/cryst.htm d. How does light penetration qualities depend on the degree of crystallization? The higher the degree of crystallinity, the less light can pass through the polymer. Therefore, the degree of translucence or opaqueness of the polymer is directly affected by its crystallinity. http://www.plasticsresource.com/s_plasticsresource/sec.asp?TRACKID=&CID=124&DID=226 6. General attributes: a. Polymers can be very resistant to chemicals. What makes them this way? Some polymers have excellent thermal insulating properties. Why or how do they do this? What are three examples of such polymers? Some polymers are excellent insulators because they are combined with so much air which conducts heat poorly. This is why the temperature outside of a polystyrene foam cup full of hot coffee is a lot lower than the temperature inside. Ex. PVC, PS, PTFE, PP, acrylics. c. Many polymers are excellent electrical insulators. Why? What are three examples of such polymers? d. Research is being done on producing polymers that will conduct an electrical current? Why would this be desired? What kind of polymer has been produced so far that will conduct a current? To replace other conductive materials with conductive polymers that would be better for the job. PPP can conduct a current. 7. Polystyrene a. How is polystyrene turned into Styrofoam? During manufacture, the distinctive Blue* color rigid foam insulation is extruded under enormous pressure, creating a dense mass of tiny, closed cells so tightly packed together that water cannot penetrate the insulation, nor can the insulating air escape from the cells. Because of this closed-cell structure, STYROFOAM Brand Insulation has a superior insulation value, excellent water resistance, and high compressive strength. Polystyrene foam products are 95 percent air and only five percent polystyrene. When polystyrene foam packaging is produced, a blowing agent is used in the process. Polystyrene foam products are now manufactured primarily using two types of blowing agents: Pentane and Carbon Dioxide. Pentane gas has no effect on the upper ozone layer, although, if not recovered, it can contribute to low-level smog formation. Therefore, manufacturers use state-of-the-art technology to capture pentane emissions. With ever-evolving technology, some manufacturers use carbon dioxide (CO2 or other hydrocarbons in some cases) as an expansion agent for polystyrene foam. CO2 is non-toxic, non-flammable, does not contribute to low-level smog, and has no stratospheric ozone depletion potential. In addition, the carbon dioxide used for this technology is recovered from existing commercial and natural sources. As a result, the use of this blowing agent technology does not increase the levels of CO2 in the atmosphere. http://www.polystyrene.org/polystyrene_facts/facts.html b. What are some other polymers that are also �foamed�? Polyethylene and polypropylene. http://www.eob-consult.ch/data/SignalAutumn00.pdf Polyurethanes are formed by reacting a polyol (an alcohol with more than two reactive hydroxyl groups per molecule) with a diisocyanate or a polymeric isocyanate in the presence of suitable catalysts and additives. Because a variety of diisocyanates and a wide range of polyols can be used to produce polyurethane, a broad spectrum of materials can be produced to meet the needs of specific applications. Most polyurethanes are thermoset materials; they cannot be melted and reshaped as thermoplastic materials can be. Polyurethanes exist in a variety of forms including flexible foams, rigid foams, chemical-resistant coatings, specialty adhesives and sealants, and elastomers. c. Why �foam� a polymer? Polystyrene protective packaging comes in two primary forms � loose fill "peanuts" and shape-molded packaging. Loose fill "peanuts" allow variously shaped items, such as office supplies or cosmetics, to be shipped in the same box. Shape molded packaging fits snugly around delicate products like computers, television sets, stereo equipment and appliances to protect them during shipping. Both shape molding and loose fill are lighter in weight than other protective packaging, saving energy and money during shipment. They also resist moisture and do not attract rodents or insects. An added benefit, polystyrene protective packaging can be used over and over again through reuse and recycling. http://www.polystyrene.org/polystyrene_facts/facts.html http://www.tuscarora.com/prod01.htm d. What are three important uses of these foamed polymers? The most recognizable forms of polystyrene packaging are expanded and extruded foams (sometimes incorrectly called Styrofoam�, a Dow Chemical Co. trademarked form of polystyrene foam insulation). Foamed polystyrene is used to make cups, bowls, plates, trays, clamshell containers, meat trays and egg cartons as well as protective packaging for shipping electronics and other fragile items. http://www.polystyrene.org/polystyrene_facts/facts.html Rigid polyurethane foams are used as insulation for buildings, water heaters, refrigerated transport, and commercial and residential refrigeration. These foams are also used for flotation and for energy management. Flexible polyurethane foams are used as cushioning for carpet and in upholstered furniture, mattresses, and automobiles. They are also used for packaging. Polyurethane adhesives and sealants are used in construction, transportation, marine, and other applications where their high strength, moisture resistance and durability are required. The term "polyurethane elastomers" includes such diverse products as thermoplastic polyurethane, cast elastomers and Reaction Injection Molded (RIM) products. These materials go into a wide variety of applications from footwear and skate wheels to machinery housings, to athletic tracks to electronic media. http://www.polyurethane.org/about/applications/ 8. Consider each of the six common resins studied in Experiment 8. Give the name for each of the six; example- polystyrene, PS, #6, and draw out the structural formula for each. Polyethylene Terephthalate (PET or PETE) : Qualities: Clarity, strength/toughness, barrier to gas, resistance to grease/oil, stiffness, resistance to heat. Uses: Plastic soft drink bottles, mouthwash bottles, peanut butter and salad dressing containers. Recycled Products: Tote bags, dishwashing liquid containers, clamshells, laser toner cartridges, picnic tables, hiking boots, lumber, mailbox posts, fencing, furniture, sweatshirts. High Density Polyethylene (HDPE) : Qualities: Stiffness, strength/toughness, low cost, ease of forming, resistance to chemicals, permeability to gas, ease of processing. Uses: Milk, water and juice containers, grocery bags, toys, liquid detergent bottles. Recycled Products: Recycling bins, benches, bird feeders, retractable pens, clipboards, fly swatters, dog houses, vitamin bottles, floor tile, liquid laundry detergent containers. Vinyl (Polyvinyl Chloride or PVC) : Qualities: Versatility, ease of blending, strength/toughness, resistance to grease/oil, resistance to chemicals, clarity, low cost. Uses: Clear food packaging, shampoo bottles. Recycled Products: Air bubble cushioning, flying discs, decking, film, paneling, recycling containers, roadway gutters, snowplow deflectors, playground equipment. Low Density Polyethylene (LDPE) : Qualities: Ease of processing, barrier to moisture, strength/toughness, flexibility, ease of sealing, low cost. Uses: Bread bags, frozen food bags, grocery bags. Recycled Products: Shipping envelopes, garbage can liners, floor tile, furniture, film, compost bins, paneling, trash cans, landscape timber, mud flaps. Polypropylene (PP) : Qualities: Strength/toughness, resistance to chemicals, resistance to heat, barrier to moisture, low cost, versatility, ease of processing, resistance to grease/oil. Uses: Ketchup bottles, yogurt containers and margarine tubs, medicine bottles. Recycled Products: Signal lights, battery cables, brooms and brushes, ice scrapers, oil funnels, landscape borders, bicycle racks. Polystyrene (PS) : Qualities: Versatility, insulation, ease of processing, low cost, clarity. Uses: Videocassette cases, compact disc jackets, coffee cups, knives, spoons and forks, cafeteria trays, grocery store meat trays and fast-food sandwich containers. Recycled Products: Thermometers, light switch plates, insulation, egg cartons, vents, desk trays, rulers, license plate frames, concrete. http://www.teachingplastics.org/hands_on_plastics/intro_to_plastics/teachers.html b. Describe separately the following properties: compare the density of each with that of water, indicating specifically which, if any, will float. 2. compare their �burning� properties, such as how they burn, how rapidly, if there is an odor, etc. Polyethylene (PE) burns rapidly, drips flames, smells like candle wax and, when extinguished, will produce a white smoke. Polypropylene (PP), on the other hand, burns more slowly, smells like burning fuel, and does not drip flames while burning. Polyvinyl chloride (PVC) can be ignited but will self-extinguish as soon as the fire source is removed. PVC has a very acidic odor when burning because hydrogen chloride is a burning by-product. Polystyrene (PS), on the other hand, burns rapidly, has a strong gas odor, and produces tremendous amounts of soot. http://www.handsonplastics.com/intro_to_plastics/teachers.html 3. compare their reactions in acetone, alcohol, and other organic solvents. PE is impervious to chemical solvents while PP will dissolve in hot toluene. PS will swell readily in acetone. Rigid PVC will become rubbery in the presence of benzene or will dissolve in methyl ethyl ketone. http://www.handsonplastics.com/intro_to_plastics/teachers.html 9. Once a polymer has been produced it has to be processed into some useful form, such as a bottle. a. What is the extrusion process? What common objects are produced by this process? Extrusion is a process makes parts of constant cross section like pipes and rods. Molten polymer goes through a die to produce a final shape. It involves four steps: Pellets of the polymer are mixed with coloring and additives. The material is heated to its proper plasticity. The material is forced through a die. The material is cooled. Some common objects produced include drinking straws, molding strips, hose and tubing, seamless gutters, window frames, and vinyl siding. http://matse1.mse.uiuc.edu/~tw/polymers/prin.html http://www.handsonplastics.com/intro_to_plastics/teachers.html b. What is the sheeting process? What common items are produced by this process? A process similar to extrusion is the production of sheeting. Plastic is extruded and fed through two hot rollers producing sheets, which continue to be fed through hot rollers to produce the desired thickness. The process of material being fed through a series of hot and/or cooled rolls is called calendering. Sheeting has a final thickness of 0.25 mm or more; whereas film, which can also be produced by this method, is less than 0.25 mm in thickness. http://matse1.mse.uiuc.edu/~tw/polymers/prin.html http://www.handsonplastics.com/intro_to_plastics/teachers.html c. How are thin polymer fibers or filaments produced? What common materials are produced by this process? Fibers and filaments are also extruded through multiple openings of a spinneret die. The specially shaped strands of molten plastics are cooled, then drawn and stretched by passing through a series of rollers. The drawing process will orient the molecule chains to produce the desired strength, while the shape of the fiber will affect other characteristics. All natural fibers have distinguishing cross-sections that fiber producers mimic to create a synthetic that resembles a natural fiber. http://www.handsonplastics.com/intro_to_plastics/teachers.html d. What are the injection molding and blow molding processes? How are they similar and how are they different? What common items are produced by this process? Injection Molding is a very common process for forming plastics involves four steps: Powder or pelletized polymer is heated to the liquid state. Under pressure, the liquid polymer is forced into a mold through an opening, called a sprue. Gates control the flow of material. The pressurized material is held in the mold until it solidifies. The mold is opened and the part removed by ejector pins. It has the most precise control of shape and dimensions, is a highly automatic process, has fast cycle time, and the widest choice of materials. It has high capital cost, is only good for large numbers of parts, and has large pressures in mold (20,000 psi). Blow molding produces bottles, globe light fixtures, tubs, automobile gasoline tanks, and drums. It involves: A softened plastic tube is extruded The tube is clamped at one end and inflated to fill a mold. Solid shell plastics are removed from the mold. This process is rapid and relatively inexpensive. It can make hollow parts (especially bottles), stretching action improves mechanical properties, has a fast cycle, and is low labor. It has no direct control over wall thickness, cannot mold small details with high precision, and requires a polymer with high melt strength. http://matse1.mse.uiuc.edu/~tw/polymers/prin.html e. What is the rotational molding process? What common items are produced by this process? Rotational molding is a simple, four-step process that uses a thin-walled mold with good heat transfer characteristics. This closed mold requires an entrance for insertion of plastic, and the capability to be opened, so that the cured parts can be removed. Usually, dry, powdered plastic is put into the mold, which rotates simultaneously about two axes located perpendicular to one another. With slow rotation about the axes, the material inside the mold tumbles to the bottom creating a continuous path that covers all mold surfaces equally. This process is capable of molding small to large hollow items with relatively uniform wall thickness. http://matse1.mse.uiuc.edu/~tw/polymers/prin.html Table 1: Comparison of polymer processing techniques for thermoplastics and thermosets. Process Thermoplastic (TP) or Thermoset (TS) Advantages Disadvantages Injection Molding TP, TS It has the most precise control of shape and dimensions, is a highly automatic process, has fast cycle time, and the widest choice of materials. It has high capital cost, is only good for large numbers of parts, and has large pressures in mold (20,000 psi). Compression Molding TS It has lower mold pressures (1000 psi), does minimum damage to reinforcing fibers (in composites), and large parts are possible. It requires more labor, longer cycle than injection molding, has less shape flexibility than injection molding, and each charge is loaded by hand. Transfer Molding TS It is good for encapsulating metal parts and electronic circuits. There is some scrap with every part and each charge is loaded by hand. Blow Molding TP It can make hollow parts (especially bottles), stretching action improves mechanical properties, has a fast cycle, and is low labor. It has no direct control over wall thickness, cannot mold small details with high precision, and requires a polymer with high melt strength. Extrusion TP It is used for films, wraps, or long continuous parts (ie. pipes). It must be cooled below its glass transition temperature to maintain stability. What is your area of expertise? What's your area and level of expertise? What are the different types of Polymers? http://www.qureshiuniversity.com/polymers1.html ; etQkT1MNpIdGbRoAIJNpapOehCYTExOZ |