BYU MFG 355 Plastics Midterm Exam (Chemistry)

Name three methods of reducing the amount of thermal degradation that might occur in a heat-sensitive plastic. How does each reduce thermal degradation?

(1)Reduce the amount of regrind (as a percentage) that is used during processing. This assures that the heat history of the total material will never become large, thus reducing the likelihood that degradation from accumulated thermal processing will occur. (2) Add thermal stabilizers—materials that preferentially accept the heat and, therefore, reduce the amount of heat encountered by the polymer. (3) Add processing aids—materials that facilitate processing, generally by allowing the processing to occur at lower temperatures. Hence, the total amount of thermal energy added to the system is reduced. (4) Use some mechanical energy to melt the plastic material so that all of the energy is not from thermal sources.

Which of the following means basic repeating unit in organic chemistry nomenclature? a. Mono- b. Poly- c. -mer d. -ene

-mer

Approximately what percent of U.S. petroleum consumption is used directly for the production of plastics?

5%

Describe the bonding in a carbon-carbon double bond. Include in this description an explanation of the mechanism by which the various bonds are formed. What does the existence of a double bond tell about the other atoms bonded to the carbon atoms?

A carbon-carbon double bond assumes that each carbon is bonded to three atoms (the other carbon with the double bond and two additional atoms). All carbons must have four bonds. The approach of the three atoms creates a rearrangement of the carbon atomic orbitals to accommodate the three bonds. This means that the s atomic orbital and two of the p atomic orbitals will be oriented in space at 120o angles from each other in a flat trigonal arrangement. The atomic p orbitals on each of the carbon atoms are dumbbell-shaped and will not be forced to rearrange, but will overlap and create a second bond between the carbons. Each carbon will, therefore, have four bonds. Three of these bonds are sigma bonds. And one of these bonds is a pi bond. One of the sigma bonds and the pi bond are between the carbons.

What is a copolymer and how are copolymers formed?

A copolymer is formed when more monomers are mixed together than are required for normal homopolymer formation. For addition polymerization this means that two different monomer types are required. For condensation polymerization three different monomer types are required. When these monomers are mixed, the resultant polymer has some of each of the monomers within it. These monomers can be arranged in four different patterns. The properties of the copolymer are normally intermediate between the properties of the homopolymers that would have been formed from pure, single-type monomers.

Define a free radical and explain why it is so reactive.

A free radical is an atom or molecule which has at least one electron which is not paired with another electron. It is so reactive because the natural state of matter is for that unpaired electron to form a bond.

Define monomer and polymer. Write typical polymeric repeating unit structures for both addition and condensation polymerization, and explain the various symbols contained therein.

A monomer is a molecule that can be chemically reacted to form a long chain. A polymer is that long chain of monomer units. In a typical repeating unit structure for a polymer made by addition polymerization, the polymer unit is represented without the carbon-carbon double bond and with two bonds extending beyond the basic monomer unit. These bonds indicate that the basic unit bonds to additional units on either side of the unit shown. The entire basic unit is enclosed in parentheses (which normally go through the lateral bonds) to indicate that the basic unit exists as a group that is repeated many times down the chain. A subscripted n follows the second parenthesis and represents the number of times the basic unit is repeated down the chain. For polymers made by the condensation polymerization method, the basic unit is the combination of the two monomers with the monomers joined at the active sites but with the small, condensate molecule removed. All the other symbols are the same as for the addition polymerization case.

Which polymerization method, addition or condensation, is expected to result in branched molecules? Why?

Addition polymerization will give branched polymers. This branching results from the loss of an atom, usually hydrogen, during the polymerization process which causes a free radical to form at the site where the hydrogen atom was previously located. This free radical is then able to bond with other free radicals, as on the end of a growing polymer chain, or with a monomer and begin a chain which grows as a branch off the main backbone. Condensation is less likely to branch because there is no mechanism that will create a reactive site in the middle of a chain.

Explain why the properties of a polymer below the glass transition temperature are different from the same properties in the same polymer above the glass transition temperature.

Below the glass transition temperature polymers are hard. This occurs because the polymer structure is relatively fixed in space. Energy input at temperatures below the glass transition temperature can generally only result in small vibrations and rotations which do not lend movement to the polymer structure. Above the glass transition temperature much more movement (vibration, rotation and translation) is present. This increased movement, especially the long-range concerted movements of segments of chains, imparts flexibility to the polymer structure. Hence, above the glass transition temperature, the polymer is pliable and flexible.

Explain bimodal distributions of polymers and their usefulness.

Bimodal distributions have two peak areas in the MWD plot. These two peaks indicate that some lower molecular weight material has been mixed with some higher molecular weight material. This mixing is often done to improve the processability of a material with a high, narrow MWD. The lower molecular weight material will melt at lower temperatures and provide lubrication for the higher molecular weight polymers, thus increasing the ease with which they can be processed.

When high temperatures or direct flame is applied to a plastic part that is made from a thermoset material the typical response is what?

Blacking and charring

Most polymers in use today are molecules with a backbone consisting primarily of which basic element?

Carbon

Describe the type of bonding between carbon and chlorine.

Carbon and chlorine are both nonmetals and will therefore share electrons to form covalent bonds. Carbon will always form four bonds by sharing its outer four electrons. Since each chlorine needs only one electron to complete its octet, four chlorines are required to give carbon its four bonds. The resulting material is CCl4.

Describe the differences between the carbon and oxygen atoms.

Carbon has six protons in the nucleus and six electrons. (the number of neutrons determines the isotope of carbon but is unimportant for the differences in behavior between carbon and oxygen.) Oxygen has eight protons and eight electrons.

What is a commodity resin?

Commodity resins are those that are sold in high volumes and are generally similar in properties from one supplier to another.

Step-growth polymerization is also known as?

Condensation Polymerizaiton

Describe crosslinking and the resultant properties that it will create.

Crosslinking is the process of connecting polymer chains with covalent bonds. When this occurs, the total length of the polymer chains (or molecular weight) increases tremendously which raises the melting point of the polymer, usually beyond the decomposition temperature. Therefore, polymers that have been crosslinked will decompose before they melt.

Explain the effects of crosslinking on the glass transition temperature.

Crosslinks tie the molecular chains together and make long-range movements more difficult. Therefore, as the amount of crosslinking increases, the glass transition temperature should also increase.

Which type of material does not generally have a detectable Glass Transition Temperature? a. Amorphous Thermoplastics b. Crystalline Thermoplastics c. Thermosets

Crystalline Thermoplastics

A molder is hoping to increase the stiffness of a high-volume polyethylene plastic part. He asks you for your opinion on using a copolymer of polyethylene and polystyrene, instead of the plain (homopolymer) polyethylene. Discuss how you would expect the crystallinity, glass-transition temperature, solvent resistance, and gas permeability to be affected by changing to this copolymer.

Crystallinity: TG: Solvent resistance: Gas permeability:

For a typical amorphous thermoplastic material, which temperature designation corresponds to the highest temperature in degrees? a. Glass Transition Temperature b. Melting Temperature c. Heat Distortion Temperature d. Decomposition Temperature

Decomposition Temperature

Give several reasons for the development and use of plastics.

Demand for highly formable materials with properties that are not easily obtained with natural materials. Plastics permit greater flexibility in design and allow many of the innovative modern products to be made economically.

Describe the key features of a polymer that would make it electrically conductive.

Electrical conductivity results from the ability of electrons to move freely within the material's structure. This can occur in polymers when the electrons are delocalized. One method to achieve delocalization is when the bonds in the polymer are conjugated double bonds. Another method to achieve conductivity is to add a conductive filler to the plastic material.

Explain the flammability of aromatic versus aliphatic molecules from an energy perspective assuming the bond strength of a C==C bond is approximately twice as much as the bond strength of a C—C bond. Also, that the bond strength of a hydrogen attached to a benzene is about 20% higher than the bond strength of hydrogen attached to propylene.

Flammability is a measure of a material's ability to perpetuate a flame. This is done as heat is released when bonds are formed. Typically the bonds that reform are those between hydrogens and oxygens. As the energy in bonds increases, it becomes more difficult to break the bonds (requires higher temperatures) and change it to heat energy. Therefore, the carbon double bonds in the benzene ring of the aromatics will increase the energy required for burn as well as the bond between the hydrogens and the benzene. In order to burn the aromatics, the applied heat to break bonds and begin (and perpetuate) the burn is significantly higher.

When naming organic compounds, the prefix but- (or buta-) is used to indicate which quantity?

Four

Identify gun cotton and indicate why it was important in the development of modern plastics.

Gun cotton or nitrocellulose is made from cellulose fibers that have been treated with nitric acid. This material was, perhaps, the first plastic since it was a substantial modification of a natural polymer and it could be shaped or formed. It served as the model for several other acid-derived cellulose products such as rayon, celluloid and cellophane.

What is a heat history, as applied to polymers, and why is it important in polymer technology?

Heat history is the accumulated total amount of thermal energy input into the polymer system. Hence, if the polymer has been processed twice, the heat history will be roughly twice as much as if the polymer were only processed once. This is important, especially for heat-sensitive polymers, because excessive processing, or processing at very high temperatures, can lead to degradation.

Explain the relationship between plastic crystallinity and sensitivity to solvent attack.

Highly crystalline polymers have strong secondary bonds that resist the formation of solvent-polymer bonds or, perhaps, require the formation of more or stronger solvent-polymer bonds to achieve solvent penetration into the polymer network. Hence, crystallinity reduces solvent sensitivity.

Discuss the implications in MWD when the number average molecular weight and the weight average molecular weight are widely different.

If all the polymers were the same length, the number and weight average molecular weights would be the same. Therefore, when the number and weight averages are vastly different, it implies that the molecular weights are also vastly different and that the MWD is wide.

How does changing the molecular weight distribution (MWD) alter the processability of the plastic?

If the MWD is broad, the short molecules will melt before the long molecules and serve as lubricants for the entire plastic mass, even before complete melting occurs. This results in some residual entanglement among the long molecules which gives tensile strength to the melt. Processes that require melt strength, such as extrusion and blow molding, are thus improved in processability by broad MWD. On the other hand, narrow MWD will result in a sharper melting point and a lower viscosity melt conditions that are favored in injection molding. The cooling of the polymer is also faster with a narrow MWD to further improve injection molding processability.

Contrast and discuss the difference between amorphous and crystalline regions in a polymer.

In crystalline regions the polymer chains are tightly packed which results in a high density. The polymers in these crystalline regions have relatively strong intermolecular bonds which increase the stability of the polymer and increase most mechanical and physical properties. Amorphous regions are zones within the polymer structure where no crystallinity exists. Even highly crystalline polymers usually have some amorphous regions. The physical and mechanical properties for largely amorphous polymers are lower than analogous crystalline polymers.

Discuss the relationship between melt index and molecular weight.

In general, as molecular weight increases, melt index will decrease. This is not always true from one polymer type to another, but is generally true within a particular polymer type. This relationship exists because the melt index measures the ease with which a polymer is melted. Melting is generally easier (that is more polymer is melted at any temperature) as the molecular weight is reduced.

Explain why isotactic polypropylene can crystallize and the other polypropylene stereoisomers cannot crystallize.

In isotactic polypropylene all of the methyl groups are on the same side of the tertiary carbon. This reduces the intermolecular interference between these groups and allows the molecules to pack tightly together, thus forming crystals.

Contrast the interatomic or intermolecular forces present in solids, liquids and gases. Explain the consequences of these forces and how these forces are normally overcome.

In solids the inter-atomic and intermolecular forces are very strong. The particles (atoms or molecules) are relatively fixed in relation to all the surrounding particles. These materials resist movements of the particles. Hence, solids are rigid. In liquids the inter-particle forces are less strong than in solids, but there is still some association between the particles. The particles can move freely to take the shape of the container but will not fill the entire volume of the container. In gases the inter-particle forces are nearly non-existent. Gases can take both the shape and fill the volume of the container. The inter-particle forces are normally overcome by adding thermal energy to the material. This thermal energy causes the particles to vibrate, rotate, and translate, thus eventually breaking the inter-particle forces.

Why is it harder to make very long polymer chains using the condensation polymerization method than by using the addition polymerization method?

In the condensation reaction every monomer molecule can react with equal probability upon encountering any monomer of the opposite type. This situation favors the formation of many small chains which grow simultaneously. As the chains get larger, their ability to continue reacting is slightly less than small chains because the large chains have less mobility and, therefore, less chance to encounter a monomer with which it can react. Also, in condensation polymerization the byproduct of water is formed as the monomers react. The presence of water molecules in the reaction will hinder chain formation by diluting the reactants. In addition polymerization, encounters between monomers do not form bonds unless one of the monomers has been initiated, that is, reacted with an initiator so that the monomer contains a free radical. Therefore, in addition polymerization, since only a few of the monomers are initiated, the growth of the chains will be largely at the sites that are on the already formed polymers, thus making those polymer chains even longer.

Explain in thermodynamic and molecular structure terms why small molecules are more readily solvated than large molecules.

In thermodynamic terms, solvation will occur when the free energy for the solvation is negative, that is, the energy is more favorable for solvation than for staying in the solid state. Either the enthalpy term (H), the entropy term (TS), or both, must be negative and dominate whichever of the terms might be positive. The enthalpy term will only be negative when the solvation bonds are stronger than the polymer bonds. This rarely happens. Hence, the entropy term must be more negative than the enthalpy term is positive. This can happen at high temperatures because T becomes very large. It can also happen when the entropy, S, is large. The entropy will be large when great randomness is associated with the system. Because small polymer molecules can become more random when solvated than can large molecules, small molecules are more favorable to a dominating entropy term and will therefore solvate more easily. (The increased randomness with small molecules arises from the inherent structure that is always associated with large molecules, just because they are large and structurally more defined.) The molecular structure view of solvation is simply that small polymer molecules require fewer solvent secondary bonds to equal the secondary bonds between the polymer molecules. Hence, solvation is easier than for large polymer molecules.

Discuss the three major methods of increasing the strength of a polymer. Explain why each method works.

Increase chain length (molecular weight), add pendant groups and crosslink. Increased chain length allows for greater secondary bonding, pendant groups increase entanglement and crosslinking increases force to failure because forces are now applied to primary bonds instead of secondary bonds.

What is an initiator and why is it important in a polymerization reaction?

Initiators are materials that start a chemical reaction. In the case of polymers, the reaction is usually either a polymerization or a crosslinking reaction. The most common initiator is a peroxide which breaks apart, usually when heated or through certain chemical reactions, to form free radicals (chemical groups containing an unshared, and therefore very reactive, electron).

Discuss how creep is likely to be affected by aromatic pendant groups.

Large pendant groups such as the aromatics will decrease creep because they cause interference between the molecular chains. This interference raises the energy required to move the chains relative to each other and, as a result, creep is decreased.

Given the following data, calculate the number average molecular weight and the weight average molecular weight, and discuss whether the MWD distribution is wide or narrow compared with a normal commercial ratio of 20. Ni (x 10^24) 5 7 8 9 10 8 6 3 2 1 Mi (x 10^4) 1 2 3 4 5 6 7 8 9 10

MWn = 4.10 x 104, MWw = 5.77 x 104, MWw/ MWn = 1.38 (very narrow MWD)

Why is the flexural modulus so important for many plastic applications?

Many plastics are used in applications where the plastic is a cover, a seat, or a board-like structure. The most common type of force that these materials would encounter would be flexural. The modulus is often more important than the strength because these materials rarely fail (crack) but will become unsuitable for the application because they sag too much when the force is applied. A plastic seat is a good example. If the seat deflects too much, it is uncomfortable. The force on a seat is a flexural force.

Explain why laboratory identification of a plastic so often involves burning the plastic.

Many plastics can be readily distinguished by their burning characteristics and the burning tests are so easy to perform.

For any sample of plastic, it is generally too costly to count the number of repeating units on each of the polymer chains. Instead, for processing reasons, which value is given to designate polymer chain length? a. Sample Density b. Melt Index c. Molecular Weight Distribution d. Parts Per Pound

Melt index

What is the relationship between melt index and viscosity? Explain the relationship using microstructure concepts.

Melt index is a measure of a polymer's ability to flow through an orifice within a given time. Time through the orifice gives insight as to the chain length and structure. Viscosity also measures chain length and structure as an interaction between the measuring device and the viscous fluid.

How is modulus likely to be affected when toughness modifiers are added to a plastic? Why?

Modulus will likely be reduced because the toughness modifiers will have longer elongations than will the base polymer and will therefore decrease stiffness (modulus).

What is the likely effect on modulus of adding a filler? Why?

Modulus will likely increase by adding most common fillers. The reasons are that the filler is itself a stiff material. Secondly, the filler will likely fill into the spaces between the molecules and thus inhibit the molecules from moving over each other. This resistance to intermolecular motion is seen as an increase in modulus.

Discuss how you would expect the glass transition to be affected if a large, pendent group were added to the monomer unit.

Most large pendent groups will require more thermal energy to achieve long-range chain movements. Therefore, the glass transition temperature would be expected to increase.

Can all materials exist as solids, liquids, and gases at various temperatures? Explain.

Most simple, small atoms and molecules can exist in all three states of matter - solids, liquids, and gases. In some cases, especially when the molecules are large, the decomposition temperature of the material is lower than the melting point and therefore, increasing the temperature will cause decomposition before melting. Very large molecules will rarely exist as gases.

Which currently manufactured consumer product does not typically contain plastic components? a. Sewing machines b. Automobiles c. Home furnishings d. Athletic equipment e. None of the above

None of the above

Wallace Carothers was commissioned by DuPont in the early 20th century to develop a new synthetic giant molecule that could be used as a replacement for natural materials such as silk. Carothers' research yielded which synthetic material?

Nylon

Achieving a high molecular weight was a major problem in the development of the nylon polymerization process. Why was this so? Suggest a method to overcome the problem.

Nylon is made through the condensation polymerization process. Water is a byproduct of the polymerization. After a certain point in the process, the reaction begins to slow because of the abundance of water. Continued reaction becomes increasingly more difficult and chain growth is halted. This gives shorter chain length and lower molecular weight. One method to overcome the overabundance of water in the reaction vessel is to apply a vacuum to the polymerization chamber. By removing the water from the reaction, the molecular weight can continue to grow.

Why does processing of a crystalline material like nylon require more critical thermal control as opposed to an amorphous material like ABS?

Nylon, like all crystalline materials, has a sharp melting point. Processing below this temperature is virtually impossible. Processing much above this temperature can make the melt excessively runny (low viscosity). Amorphous materials (like ABS) have a broad melting range and processing 50 to 75F below Tmp is possible. Most grades of nylon, for example, do not process below 508F, the point where it loses crystallinity and is capable of flow. ABS can be processed from 420 to 550F.

What is the first modern plastic that was synthesized with a specific set of properties in mind, who sponsored the work, and when was it done?

One correct answer would be Celluloid, invented by James Wesley Hyatt in 1868. However, the first truly synthetic plastic that was made to a specification was nylon. The work was sponsored by the DuPont Company (Wallace Carothers was the chief scientist) and the work was done in the 1930s.

What is the purpose of notching Izod or Charpy samples?

One of the important aspects of impact toughness is the tendency of the material to fail through cracking. Cracks often begin at sharp locations as is created when a notch is installed in the part. Therefore, a key reason to notch the sample is to determine the tendency to fail by cracking. Another, perhaps equally important reason, is to provide a point at which failure is likely to occur so that the failure values from one test to another will be less dependent on the initiation of the failure and more dependent on the crack propagation through the material.

Which plastic is relatively the most expensive? a. Polytetrafluoroethylene (PTFE) b. Polystyrene (PS) c. Polyethylene (PE) d. Polycarbonate (PC)

PTFE

Consider the behavior of polyethylene in a tensile test. Why doesn't it fail at the thin spot when necking begins? Why does a "necked" sample not return to its original shape like an elastomer after removal from the test grips? Polyethylene as used in containers is normally translucent. Why does it transmit less light (becomes chalky white) after tensile pulling?

Part 1: Part 2: Part 3:

A company is thinking of making a new sail boat. They have decided to make the boat out of fiberglass reinforced plastic and the polymer they have chosen is unsaturated polyester. a) Illustrate/describe how this polymer crosslinks (cures). b) The company has several choices of grades of unsaturated polyester to choose from. One grade has substituted a long chain aliphatic di-acid (adipic acid) for some of the aromatic di-acid (orthophthalic acid). Comment on two changes in properties you would expect from the use of this adipic acid grade compared to an all orthopthalic acid grade. Explain these changes from a molecular standpoint.

Part a: Part b:

Explain plastic deformation in terms of molecular chains.

Plastic deformation occurs when the molecular chains move relative to each other in a nonrecoverable way. In other words, the changes in shape are permanent because the sliding of the molecules has used up some energy and not enough energy remains to force the molecules into their original positions.

Explain why plasticizers increase flexibility and elongation in plastics. What is the relationship between plasticizers and the glass transition temperature?

Plasticizers are solvents that are added to swell and partially solvate the polymer. By swelling the polymer the plasticizer expands the polymer network and, therefore, allows the polymer atoms to move more freely. This greater mobility is seen as increased polymer flexibility and elongation. Raising the temperature above the glass transition temperature has the same general effect. Therefore, adding a plasticizer lowers the glass transition temperature.

Define the term polymer and relate it to the term plastics.

Polymer is a very long molecule made by joining many small molecules together. All plastics are polymers, but some polymers, for instance, natural polymers such as fur and leather, are not plastics.

You are molding a toy bridge (6 inches high with a 24 inch span) that is failing (bending) when kids step on the span. The part is currently made of a plastic with a melt index of 10 and a density of 0.94. The market is highly competitive, so ideal solutions should take cost into account. Indicate 4 possible methods of solving the problem, and then choose one of the methods and defend your choice.

Possible answers include thickening the bridge in key areas, finding a material with higher melt index or higher density, or even using mold-in supports. Answer should also include a valid discussion of the chosen solution.

Why should the concentration of regrind PVC be kept low in any rigid PVC extrusion operation?

Regrind material is rejected product that has been chopped into small particles so that it can be reprocessed to make new parts. This is an advantage of thermoplastics. With PVC, however, the tendency to degrade is so pronounced that each time the PVC is heated, the degrading gets worse. To further complicate the matter, the degradation of PVC is autocatalytic, meaning that the decomposition happens faster as more is formed (like a chain reaction). Therefore, if the concentration of regrind PVC is too high, the accumulation of degrading material will be too high in one place and the material will degrade rapidly.

What is a resin?

Resins are plastic materials that are unformed or require additional forming to be useful. Resins are often liquids, but can be solids in a raw material or intermediate form.

Identify and describe the forms of plastic resins

Resins are usually liquids (like honey), granules (like laundry soap powder), flakes (like uncooked oatmeal), or pellets (like very short spaghetti).

Why must average molecular weights be used for polymers rather than exact molecular weights based upon the molecular formula as is done for small molecules?

Small molecules have exact formulas which always apply. In polymers, the chains can be of varying length and so a general formula must be used. This general formula represents the number of monomer units by an unknown (usually an n). Hence, if the exact number of units in each of the chains is not known, some statistical measure of the polymer chain length must be used. The average molecular weight is a convenient statistical measure to use.

Suggest 4 possible solutions that you might use to increase the molecular weight of polymer that is made using an addition polymerization process. Explain each of the solutions and why you think it will work.

Solution 1 Add fewer initiators to decrease the number of chains and allow them to grow longer. Solution 2 Crosslink the plastic to effectively join the chains and increase the molecular weight. Solution 3 Add a solvent to improve the mobility of the chains to help them find more monomers Solution 4

Describe the processes of solvent/solute interaction that lead to swelling and, in some polymers, dissolving of a plastic by a solvent.

Solvent molecules form secondary bonds with the polymer molecules, usually at sites on the polymer that are chemically similar to the nature of the solvent. When many of these secondary bonds form, there is sufficient energy in the molecules to allow the penetration of the solvent molecules into the polymeric structures. This causes swelling. If even more of the secondary bonds between solvent molecules and the polymer are formed, or if the secondary bonds are extremely strong, then the total polymer molecules can be separated from the other polymer molecules and total solvation occurs.

A company has approached you with an idea for a basketball hoop support pole that they would like to make from plastic. What are the most important mechanical aspects of this new part that need to be considered? Why?

Some of the aspects that should be included would be the creep nature of plastics, the impact resistance of both the part and the polymer chosen, the flexural strength of the material and also the torsional strength of material. There may be other considerations that can be validated through appropriate reasoning.

Why are strength and hardness of a plastic usually linearly related?

Strength is a measure of the polymer chains' tendency or ability to limit inter-chain movement. Similarly hardness is defined by a polymer chain's ability to move among other chains. Both properties measure similar inter-chain interactions.

What is ASTM and why is such an organization important?

The American Society for Testing and Materials (ASTM) is an organization that publishes uniform testing procedures for common tests. It gives one reference for standards that all materials and tests can be compared.

What is the limiting oxygen index (LOI)? Which burns more easily, a material with an LOI of 21 or of 17?

The LOI is a test for flammability of a plastic sample. In this test the sample is supported in a candle-like arrangement and ignited. The composition of the gases surrounding the burning sample is controlled and gradually the amount of oxygen is reduced or increased until the sample will just barely burn. That level of oxygen is called the limiting oxygen index. The sample with an LOI of 17 is more easily burned.

Low-density polyethylene is normally translucent. If a piece of this material is bent into a tight radius, it translates less light (becomes chalky white). What is this phenomena called? Explain why this happens. Explain why the material often breaks after repeated bends.

The change in light transmission due to stress like bending is called blushing. Blushing forms where areas of crystallinity are created caused by these stresses. As a rule, crystalline areas transmit less light than an amorphous region. The break will come as the induced crystallinity increases and those areas become brittle. Failure will occur when that crystallinity becomes significant.

Why is crystal polystyrene clear?

The clarity comes from the ability of light to pass through the material unimpeded by any internal barriers. Crystal polystyrene has no crystalline structure to interfere with the passage of light (it is amorphous) and generally has few additives. It is therefore transparent.

Identify the type of bond and the product formula expected between magnesium (Mg) and chlorine (Cl), and explain the basic nature of this bond. Show the resulting outer electron configurations of Mg and Cl after the bond has formed.

The compounds will form an ionic bond since one is a metal (loses electrons) and the other is a non-metal (gains electrons). The product formed will be MgCl2. After the bond is formed, Mg will have transferred its outer shell electrons to two Cl atoms. Two Cl atoms are required because Mg wants to lose two electrons to have the stable octet outer electron configuration. Each Cl will accept one electron and so two chlorines are needed for both electrons. The resulting Mg ion will have a charge of +2 and each of the Cl ions will have a charge of -1.

What are three major determining factors in the price of plastics?

The current price of raw materials, the cost efficiency of the processing method and the amount of plastic produced (price-volume relationship) all affect the price of plastics.

Identify three tests for thermal properties of plastic materials.

The heat distortion test (HDT), the coefficient of thermal expansion (CTE) and the thermal conductivity test.

What are the three necessary and sufficient criteria that must be satisfied by all plastics materials?

The material must be (a) composed of very large (polymeric) molecules, (b) must be synthetic, and (c) must be formable.

Identify the type of bond and the product formula expected between potassium (K) and bromine (Br), and explain the basic nature of this bond. Show the resulting outer electron configurations of K and Br after the bond is formed.

The materials will form an ionic bond since one is a metal (loses electrons) and the other is a nonmetal (gains electrons). The product formed will be KBr. After the bond is formed, K will have transferred its outer electron to Br. Potassium will then have a stable octet and a net charge of +1. Bromine will have a stable octet and have a net charge of -1.

A stress-strain experiment was done on a plastic and the following data was noted on the plot of the experiment: ultimate tensile strength = 9000 psi, yield strength = 5000 psi, proportional limit = 4000 psi, strain-to-failure = 0.025, strain at yield = 0.020, stain at proportional limit = 0.015. The original length of the sample was 4 inches. What was the modulus?

The modulus is calculated by examining only the slope of the stress-strain curve before yield, that is before and non-linear behavior is seen. Hence, the slope at the beginning of the experiment is stress at yield (5000 psi) divided by strain at proportional limit (.015).

Discuss how copolymerization would affect crystallinity.

The nature of copolymerization would cause more randomness to occur along the molecular chain than would the corresponding homopolymers. Therefore, crystallinity would be lower.

What is the difference between a number average and a weight average molecular weight?

The number average is dominated by the actual number of molecules and gives equal value to both large and small molecules. The weight average molecular weight gives greater value to the large, heavy molecules.

Describe the Izod toughness test.

The sample (cut to the correct dimensions) is held upright in a rigid holder. A pendulum is swung so that it impacts the sample. The energy which was absorbed by the sample is identified by the extent of rotation of the pendulum after the impact.

Explain the cause of the differences in structure of LDPE, HDPE, and LLDPE.

The structures are dependent on the amount of branching in the molecules. When the branches are long and frequent, the molecules cannot pack tightly together thus preventing crystallization. This is LDPE. When the branches are short and infrequent, the molecules can pack together and crystals can form. This is HDPE. When the structure is similar to HDPE but with slightly longer and more frequent branches, the polymers cannot pack together and the density is low like LDPE, but other properties are like HDPE, this arrangement gives LLDPE.

What is the molecular difference between thermoset and thermoplastic materials?

The thermoset materials are crosslinked and thermoplastics are not. This crosslinking raises the total length (molecular weight) of the thermoset and thereby prevents the material from being remeltable.

Discuss the physical factors in a plastic that affect permeability.

The tightness or denseness of the polymer structure is one of the most important physical factors affecting permeability. Therefore, crystallinity reduces permeability. Likewise, crosslinking reduces permeability. Other factors affecting permeability would be the similarity of chemical natures between the polymer and the gas passing through. If these are similar, permeability is increased. Other factors affecting permeability include temperature (increases permeability) and size of the permeating molecules (decreases permeability).

What are thermal stresses and how are they caused in plastic materials? How can they be relieved?

Thermal stresses are energy concentrations that are retained in a material resulting from the inability of the material to change shape as would normally occur upon cooling or heating. These thermal stresses can be relieved by slowly heating the material to a moderate temperature (annealing), thus allowing the molecules to move and the stresses to relax.

True or False. All plastics, thermoplastic or thermoset, will degrade at some temperature?

True

Every covalent bond is made up of how many electrons?

Two

Describe how ultraviolet light degrades plastics and why it does not generally degrade metals.

Ultraviolet (UV) light interacts with the electrons in covalent bonds, often raising the energy of the electrons so they are no longer in a favorable bonding energy level. When this happens, the bond is broken. This rarely occurs in metals because the energy of the sea of electrons can be excited with little effect on any individual bond between the atoms. Hence, the bonds remain intact even when some of the electrons are excited to higher energy levels.

Explain what is meant by a viscoelastic material, and relate its response to applied stresses by comparing the material with an elastic solid and a viscous fluid.

Viscoelastic materials have some properties that are similar to elastic solids and some properties that are similar to viscous fluids. When a stress is applied to a viscoelastic material that is a solid, the solid will deform much like an elastic solid but will not immediately recover its original shape when the stress is removed. The viscoelastic material has used up some of the applied stress through internal heating, hence, not all of the energy is available to assist in the shape recovery.

Which material EVA or PS, would be expected to be a better barrier material against water? Why?

Water is a polar molecule so it would be excluded best by a non-polar polymer. EVA has many polar atoms in the pendent group and would allow water to permeate easily. Polystyrene is nonpolar and would exclude polar molecules like water. The size of the side chain for EVA is large, as is the pendent group for polystyrene. Both plastics would have about the same openness so size would not be a factor in determining the permeation differences between these two plastics. Therefore, polystyrene is a better barrier material for mater than is EVA.

Using a molecular view, explain why compression strength is generally less than tensile strength in polymers.

When a compressive load is applied to a polymer, the molecules are forced closer together. If the polymer is highly crystalline, this force will simply press on the crystals with little damage, much as would happen in a crystalline solid. However, in an amorphous material, such as most plastics, there is considerable space between the polymer chains, which will allow deformation of the polymers at lower stress levels. Furthermore, the spaces allow the polymers to collapse into the holes, thus reducing the ultimate compressive strength. Hence, the compressive forces are lower for polymers than for highly crystalline materials.

Why is an octet of electrons a stable configuration?

When both the s and p orbitals of an atom are filled, a certain stability is achieved. There are two electrons which fill the s orbitals and six electrons that fill the p orbitals. Therefore, eight electrons will fill both, giving a stable configuration.

Two samples of the same resin are pulled in tension at two different rates. One is pulled at 1 inch per minute and the other is pulled at 10 inches per minute. Discuss the differences in elongation you would expect, and explain why the differences occur.

You would expect to see a higher elongation in the pieces that were pulled at a slower rate. The slower rate allows the polymer chains time necessary for disentanglement. The forces seen in this case are those of the secondary bonding. At a faster rate, the forces that you see are applied to the backbone of the molecule itself and are generally higher.

A polymer that is said to have toughness is able to a. Withstand slow moving forces for extended periods of time at elevated temperatures b. Absorb relatively large amounts of energy without rupture c. Leap tall buildings with a single bound

b. Absorb relatively large amounts of energy without rupture

When surface cracks develop due to exposure to environmental chemical embrittlement this is generally called a. Blushing b. Crazing c. Oxidization d. Charring

b. Crazing

When a fluid is said to have a high viscosity that means that it a. Flows quickly, like water b. Flows slowly, like honey c. Is slightly yellow or tan, like motor oil d. Absorbs moisture from the environment

b. Flows slowly, like honey

Polyvinylchloride (PVC) is used in industrial applications as a rigid material but is also used in many consumer applications as a soft pliable material called vinyl. The main difference between these two applications is based around the use of a. Fillers b. Plasticizers c. Colorants d. Modified polymerization methods

b. Plasticizers

If a highly polar plastic like Polyvinylalcohol is placed into a polar solvent like water, the plastic will a. Swell or soften b. Do nothing c. React d. Dissolve

d. Dissolve

Values derived from a melt index test can be used as a valid measurement when a. compared between plastics of differing chemical structures b. compared between plastics of similar chemical structures c. for personal use only d. compared between different grades of the same plastic

d. the most accurate and reliable use for melt index results is in comparing between two different grades of the same resin. The nature of the polymer chains and their interactions make this test less reliable as a comparison between different plastics.