CH 7

7.23 Describe how a rechargeable lithium battery works.

o They carry current from the negative electrode to the positive electrode through an electrolyte. When charging a voltage higher than the regular battery voltage is applied forcing lithium ions. It is here they become embedded in the porous negative electrode material through a process know as intercalation.

7.29. Describe your opinion regarding the recycling of plastic versus the development of plastic that are biodegradable.

o 29. Biodegradable plastic are not really a viable at this stage of development, and may never to that point. These plastics must be made with intentional weak spots to allow the degradation process to take place, and as a result may actually create failures of the product in service. Biodegradable plastics are also very costly to produce, and take away too long to degrade anyway. o Recyclable plastics continue to lose their properties the more they are recycled, making them weaker and less reliable than their virgin counterparts. Also, the fuel to run a recyclable collection program and to run the melting / processing furnace may expend more in natural resources and pollution output than it saves environmentally or financially.

7.30. Explain how you would go about determining the hardness of plastics.

o 30. The only viable of assessing a polymer's hardness is by using a Durometer. Other tests, like brinell or knop hardness tests are indeterminate in polymers because of the high impact absorption capabilities that polymers exhibit. Rockwell hardness does not work because it is time-dependent on the stress relaxations characteristics of the polymer.

7.31. Why is there so much variation in the stiffness of products made of polymers? Explains.

o 31. Stiffness will vary with the polymerization process and the characteristics of the polymer itself, and most can be intentionally changed through one or more variables to affect different stiffnesses. The degree to which the polymerization process is allowed to proceed and the type and ability of cross-linking to take place can dramatically affect the end product's stiffness. o Reinforcing materials can also be added to or formed with the polymer, changing its stiffness characteristics even further.

7.32. Explain why thermoplastics are easier to recycle than thermosets.

o 32. Stiffness will vary with the polymerization process and the characteristics if the polymer itself, and most can be intentionally changed through one or more variable to affect different stiffnesses. The degree to which the polymerization process is allowed to proceed and the type and ability of cross linking to take place can dramatically affect the end product's stiffness. o Reinforcing materials can also be added to or formed with the polymer, changing its stiffness characteristics even further.

7.33. Explain why thermoplastics are easier to recycle than thermosets.

o 33. Thermoplastics are made by melting plastics pellets and then injecting them or forming them in a mold. When it comes for them to be recycled, the material can be cut up and sized similarly to the original plastic product and melted and reformed again, with a slight lose in strength and density, but not much more. o Thermoset plastics can't be broken down into their former components and separated. So thermoset plastic recycling has to be done by a completely different method than the original process used to form the part. Also, thermoset plastics do not melt like thermoplastic plastics, making them even harder to recycle and reuse.

7.34. Give an example of process where crazing is desirable.

o 34. The old label makers used machine to squeeze the imprint of number or letter into a specially designed plastic tape with glue on one side. Dymo was one such manufacturer of these items. When the letter was squeezed into the plastic tape, it embedded the tape, causing local crazing at the deformation sides. This deformation turned the sticky take a different color so would stand out from the original background color.

7.35. Describe the principal behind shrink wrapping.

o 35. Specially- designed plastic sheet made with branched thermoplastics is heated to above its glasstransition temperature. When it reaches that temperature, the branches in the plastic align in one direction. The sheet is then cooled quickly to keep the strands oriented in such linear fashion. When the shrink-wrap plastic is then wrapped around a pallet of boxes or material and reheated, the strands in the plastic relieve and return to their former non-oriented state, causing the plastic to shrink.

7.36. List and explain some environmental pros and cons of using plastic shopping bags vs. paper bags.

o 36. The only real advantage plastic bags have over paper is that they cost less to manufacture and are subsequently less costly to the consumer. o Plastic and paper shear the properties of recyclables, so there is no advantage to either product there, except that plastic is cheaper to recycle. o Paper bags are made from a renewable resource that, when alive, helps remove greenhouse gases from the atmosphere. Paper is biodegradable naturally, while plastic is not.

7.37. List the characteristics required of a polymer for (a) a bucket, (b) a golf ball, (c) an automobile, (d) clothing, (e) flooring, (f) fishing nets.

o 37. Polymers used in a car's dashboard must be easily formable, must maintain their properties in the heat of the sun or the cold of winter, must be pliable for cushioning during a collision, and able to be made in a variety of colors. o Polymers used for clothing manufacture must be able to be produced in long, thin, continuous strands. It must be pliable, flexible, and soft so it can be woven into fabric for clothing. It must have good heat transfer characteristics, and must be able to withstand normal wear and the chemicals and abuse of the washing machine. o Polymers for laminated flooring need to be hard, coatable or moldable in a wide variety of colors and patterns, be abrasion resistant, tough, and resistant to water based chemical for cleaning. It should also have good traction characteristics. o Polymers used for fishing nets must be capable of being produced in long, thin continuous strands, invisible in water, strong with low elasticity, and be chemically resistant to salt water and fresh water.

7.38. How can you tell whether a part is made of a thermoplastic or a thermoset?

o 38. One obvious way is by looking at the overall part and shape. If it is a thin film, it must be a thermo because it is made by expanding tubing inside a mold. o A tensile test will quickly determine whether the material is a thermoplastic or thermoset plastic. Thermoplastics will yield a deform without recovery. Thermosets are very hard and brittle, and will accept a high tension before fracturing. o Holding the material in front of a butane burner is another easy test. If the plastic begin to melt, it is a thermoplastic. It has a glass transition temperature.

7.14 inspect various plastic components in an automobile and state whether they are made of thermoplastic materials or of thermosetting plastics.

o In a car thermo plastics parts found as a cup holders, door handles, dash boards, door trim, fasteners and clips, seat belt trim, and arm rests. o Auto parts made from thermosetting plastics include battery casings, emission system components, steering wheel parts, and exterior body parts.

7.39. As you know, there are plastic paper clips available in various colors. Why are there no plastic staples.

o 39. Paperclips operate like spring with their limits, and as long a paperclip stays below its yield point, it will maintain its spring force. Plastic are easily capable of this. o Staples require that the plastic penetrate sheets of paper and then fold plastically back over itself to hold the papers together. If a thermoplastic was used in this application, it would spring back (recover) dramatically and the paper would not stay clamped together. If a thermoset were used, the staple would shatter as soon is it started to bend.

7.40. By incorporating small amounts of a blowing agent, it is possible to manufacture hollow polymers fibers with gas cores. List possible applications for such fibers.

o 40. Gas core would be excellent insulators, and could be former into thin sheet with high thermal resistance properties. This could be used as window film, building material, or clothing material. o Gas filled fibers would be a lighter weight than their solid relatives. The fibers could be spun into a thread, and the thread woven into clothing that would be lightweight without having to be thin.

7.41. In injection molding operation it is common practice to remove the part from its runner, to place the runner into a shedder, and tor recycle the resultant pellets. List the concerns you would have in using such recycled pellets as opposed to so called virgin pellets.

o 41. Rechopping the runner may result in degraded plastic that will not produce parts of equal quality. Also the lubricants used in the plastic in the plastics forming process will contaminate the recycle mix. Dirt and other contaminants can get into the recycle stream and result in contaminants in future products. o For high purity or zero-contamination acceptable parts, any process involving recycle introduces potential contaminants into the system, including metal from the runner cutter and shredded drums.

7.26. How can polymers be made to conduct electricity?

o 7.26. Some polymers are conductive naturally. Polymers can also be made to be conductive by impregnating them with metal flakes or whiskers. o Wire can be cast into polymer during processing, either in a single direction, or in a single plane if wire mesh is used. o Polymers can also be coated with nickel plating to make them conductive.

7.27. Explain the reasons for which elastomer were developed?

o 7.27. Elastomer were the result of attempts to produce materials that could undergo large strains and deformations without losing their permanent shape or failing. In shear they are used for traction added friction, in compression they are used as vibration isolators. They also work to absorb shock.

7.28. Give several examples of plastic products or components in which creep and stress relaxation would be important considerations.

o 7.28. Creep is present in plastics subject to low strain, high temperature environments or applications. Microwavable cookware must be able to withstand the temperature of hot food without deformation. Handles on cookware must be able to similarly able to not creep. Plastic parts in the engine compartment of car must be creep resistant. o Stress relaxation occurs when high strain and normal temperature are present. A plastic container for a hanging plant would be one example where stress relaxation could be an issue, particularly if the hanging eye and attachments are plastic as well.

7.12 what effects does a plasticizing agent have on a polymer?

o A plasticizer may be added to a polymer in order to induce higher flexibility and softness. It does this by reducing the glass transition temperature of the polymer my reducing the strength of the secondary bonds between long chain molecules.

7.2 What are the major differences between the mechanical and the physical properties of plastics and metals?

o A primary advantage to plastics over metals is the relative ease, which they can be machined, cast, formed and joined into complex and varied shapes. Metals are not nearly as easily shaped and bent. o In addition to the ease of shaping, plastics are also available in a wide array of colors that metals simply cannot be offered in. plastics also cost less.

7.6 What is the glass transition temperature?

o An amorphous or glass like polymer will not have a specific melting point. Rather they have a temperature at which a distinct mechanical behavioral change occurs. This temperature is known as the glass transition temperature. o Typically an amorphous polymer will be hard rigid brittle and glassy at low temperature. At high temperatures these polymers will be rubbery.

7.11 what is an elastomer?

o An elastomer is a highly elastic material, which is capable of retuning to its original shape, and size after a significant load, which caused its deformation, is removed.

7.7 list and explain the additives commonly used in plastics.

o Carbon black is sometimes is sometimes added to plastics in order to provide adequate protection from ultraviolet radiation. In general carbon black will absorb a large percentage of the ultraviolet radiation passing through the plastic. o Colorants are added in order to produce the wide array of colors available in plastics. These colorants are dispersed particles, which have greater resistance to temperature and light than a normal dye would. o Lubricants such as linseed oil, mineral oil, waxes and metallic soaps such as calcium stearate and zinc stearate added to polymers in order to reduce friction during processing casting.

7.8 What is crazing?

o Crazing is an effect present in some thermoplastics such as polystyrene and polymethacrylate. When these materials undergo high tensile stress or bending they can develop narrow, wedge regions of highly deformed material.

7.24 Explain how cross linking improves the strength of polymers.

o Crossed linked polymers contain additional bonds between the molecules that provide resistance to flow. All of the extra links or bonds must be broken before the molecules can move relative to each other.

7.22 Discuss the significance of the glass transition temperature Tg in engineering applications.

o From engineering perspective any thermoplastic subject to an external force in operation fluid pressure compressive or tension loads would be required to have a glass transition temperature well above the maximum expected operating temperature of the service system to prevent the plastic from softening and failing.

7.4 what is the difference between condensation polymerization and addition polymerization?

o In condensation polymerization, polymers are formed by the release of small particles as by product of a condensation reaction. The molecules released are typically water or methanol. o In addition polymers are formed by bonding monomers together. Unlike in condensation of polymerization no molecules in the material are released to form the polymers.

7.18 Do you think that the substitution of plastics for metals in products traditionally made of metal may be viewed negatively by the public at large? If so why?

o Initially this has been the case with every product converted from metal to plastic in the past, and there's no reason to think that'll change the future. Once the product is put into wide use, however plastic becomes accepted as the standard. o One example of this is power tools, which were traditionally made of metal and started getting cased in plastic 30 years ago. The initial perception was negative but once consumers understood the advantageous of shock protection and durability became a non-issue.

7.5 explain the differences between linear, branched, and cross linked polymers.

o Linear polymers are chain like. They are so called because their structure is linear and sequent.o As their name suggest branched polymer has a structure which branches. This process can change the properties of a polymer when compared to linear polymers. A branched polymer typically has lower density since the branching structure is harder to pack with in polymer chains. o A cross linked polymer is one in which the polymers are linked together by covalent bonds. This linking typically happens in all three dimensions.

7.13 Define the following abbreviations PMMA, PVC, ARS, HDPE, and LDPE.

o PMMA stands for polymethlymethacrylate a transparent thermoplastic, which can be used in place of glass for most applications. o PVC stands for polyvinylchloride and inexpensive, water resistance material which can be either kept rigid or allowed flexibility. o ABS stands for Acrylonitrile butadiene styrene, a materiel that maintains high dimensional stability and rigidity. o HDPE stands for high-density polyethylene, a thermoplastic normally made from petroleum. o LDPE stands for low-density polyethylene, also a thermoplastic made from petroleum.

7.1 Summarize the important mechanical and physical properties of plastics

o Plastics are corrosion and chemical resistant o Plastics have low electrical and thermal conductivity rating o Plastics have low density rating o Plastics have high strength to weight ratio o Plastics are available in a wide array of colors and transparencies o Plastics are useful in noise reduction applications o Plastics are easy to manufacture in a wide array of complexities and design possibilities

7.16 what characteristics make polymers advantageous for applications such as gears? What characteristic are drawbacks in such applications?

o Plastics have lower coefficient of friction meaning they wear less even if lubricant is poor of nonexistent o Plastics are great vibration dampers and noise deadeners. They would help reduce gear train noise in service. o Plastic gears are easier to fabricate and require less production processing. Plastics are not advantageous for gears because they have lower stiffener than steels. Especially at high temperatures. o Plastics have lower strength than steels, which would mean an equally sized gear would be able to transfer nearly the same amount of load as steel gear.

7.9 what are polyblends?

o Polyblends are utilized when different characteristics found in various polymers are required. Various polymers will be selected for their high ratings in the necessary characteristics and blended together. A process similar to the alloying of metals can be used in order to make these polyblends more ductile.

7.3 list properties that are influenced by the degree of polymerization.

o Polymerization is the process by which polymers are formed by linking and cross linking various monomers. o The degree of polymerization is the ratio of polymer molecular weight to the molecular weight of the repeating unit. This means that the degree of polymerization indicates how the ratio of the molecular weights increases. o Polymerization and in particular the degree of polymerization has an impact on many of its properties. The affected properties are tensile strength, impact strength, resistance to cracking, and viscosity. Each would increase as the degree increase.

7.17 what properties do polymers have that thermoplastics in general do not have?

o Polymers are elastic meaning they can remain their original shape after deformation. Whereas plastics cannot. Polymers are much better at dampening vibration that plastic because they cab absorb a large amount of energy.

7.25 Describe the methods by which the optical properties of polymers can be altered.

o Polymers can be changed to different colors and different levels of translucency through the addition of addition of additives in the forms of dyes and pigments. o Polymer sheet can be brightened and have its opacity increased through a process called whitening which is essentially stretching the sheet. o Some other optical properties are affected by the crystalline structure of the polymer itself.

7.15 Give applications for which flammability of plastics would be of major importance.

o The flammability of a plastic would be of major importance on the fabricant of portable gasoline containers. o Plastic flammability is a major concern to airplane manufactures and airline companies because they do not want seats, inner cabins, and floors catching on fire. o Housing construction take flammability of plastics very important. Plastics and polymers are being more used in plumbing, insulation and vapor barriers and interior florin. Multiple unit housing or apartments add yet another level of concern because of toxic vapors. o Beauty products such as blow dryers, curlers, curling irons, and hair strengtheners must all select plastics for use wisely to prevent them from catching of fire when the device is hot.

7.10 list the major differences between thermoplastics and thermosets.

o Thermo set plastics will have a stress and strength characteristics that is not affected by temperature or rate of deformation. Thermoset polymers will also typically have better mechanical, thermal and chemical characteristics than thermoplastics. o A thermo set plastic is formed in two stages. The first involves polymerization of molecules into linear chains. The second takes place during parts production. Because of their nature, thermoset polymers don't have a well-defined glass transition temperature.