MEMS 0040 Exam 3 Spring 2022 key
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Apr 3, 2024
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MEMS 0040 Materials and Manufacturing Exam 3 Spring 2022 Name:
____________________________________________________ •
50 minutes •
Closed book •
You may use a calculator I have neither given nor received unauthorized assistance on this test: (Signature):
________________________________________________
Section 1: True/False (2 points each, 20 points total) __
T
__ 1. Branching in a thermoplastic polymer decreases the density of the polymer. __
F
__ 2. Polymers which exhibit tacticity can crystallize more easily than polymers which do not exhibit tacticity. Tacticity is related to asymmetry of side groups, and this always reduces crystallinity compared to polymers with identical side groups. __
F
__ 3. Polymer melts usually exhibit pseudoelastic behavior, so that the viscosity increases with increasing shear rate. Polymer melts exhibit pseudoplastic behavior, not pseudoelastic behavior. __
F
__ 4. Thermoset polymers are hard to recycle because different thermoset polymers usually do not mix well after they melt. Thermoset polymers do not melt. __
T
__ 5. Plastic bags are commonly made by the blow film extrusion process. __
F
__ 6. During injection molding, the auger moves forward while spinning to deliver a shot of molten thermoplastic polymer into the mold. The augur does not spin during its forward motion in injection molding. __
T
__ 7. Polymer fibers have high elastic modulus (compared to bulk polymer) because of the high degree of molecule alignment along the axis of the fiber. __
T
__ 8. The fused deposition modeling (FDM) process for additive manufacturing of polymers is essentially an extrusion process. __
F
__ 9. The fracture strength of carbon fibers increases as the fiber diameter increases. Larger fibers have a higher likelihood of critical flaws, so fracture strength is lower. Strength is a measure of stress to failure, and is measured as force per unit area. __
T
__ 10. The pultrusion process can be used to make a composite tube with continuous fibers oriented parallel to the axis of the tube.
Section 2: Problems and Short Answer Questions (80 points total) Polymer Synthesis (20 points total)
11. (a) Polyethylene is synthesized by addition polymerization, whereas polyethylene terephthalate is synthesized by condensation polymerization. Explain the basic difference between addition polymerization and condensation polymerization processes. (4 points) In addition polymerization, monomer molecules are simply added stepwise to the growing polymer. In condensation polymerization, each step of monomer addition also produces a small byproduct molecule, such as water. (b) Polystyrene is a thermoplastic polymer. Draw the repeat unit for this polymer, and list the number of each type of atom in the repeat unit. (4 points) The repeat unit of polystyrene is
There are eight carbon atoms and eight hydrogen atoms in the repeat unit. (c) Polypropylene is a thermoplastic polymer which can show tacticity. List two types of tacticity which can be exhibited by polypropylene. Which type would be expected to have a higher degree of crystallinity? (4 points) Two types of tacticity are the isotactic
configuration and the atactic
configuration. The isotactic
configuration is expected to have a higher degree of crystallinity. (d) If the average molecular weight of polytetrafluoroethylene is 60,000 amu, calculate the average degree of polymerization for the polymer. Assume that the atomic weight of carbon is 12 amu and the atomic weight of fluorine is 19 amu. (4 points)
Repeat unit has one carbon atom and two fluorine atoms. Repeat unit weight = 12 + 2(19) = 50 amu Degree of polymerization = 60,000/50 = 1200
(e) If the average degree of polymerization of polyvinyl chloride is 3000, calculate the average molecular weight of the polymer. Assume that the atomic weight of carbon is 12 amu, the atomic weight of hydrogen is 1 amu, and the atomic weight of chlorine is 35 amu. (4 points)
Repeat unit has two carbon atoms, three hydrogen atoms, and one chlorine atom. Repeat unit weight = 2(12) + 3(1) + 1(35) = 62 amu Average molecular weight = (3000)(62 amu) = 186,000 amu
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Polymer Properties (20 points total)
12. Materials can behave in several different ways when a stress is applied. When the stress is below the yield stress, crystalline solids behave in an elastic manner. Simple liquids respond to applied stress in a viscous manner. The behavior of solid polymers is described using the term viscoelastic. The figure below is a plot of strain versus time for a viscoelastic polymer. Explain (briefly) what the three linear segments in the plot represent in terms of the behavior of the polymer under stress. (6 points) The vertical segment at time t
o
represents instantaneous elastic deformation when a stress is applied. The vertical segment at time t
f
represents elastic recovery when the stress is removed. The sloped segment connecting these vertical segments represents viscous (creep) deformation under constant load, which increases with time and is permanent. 13. The percentage of polymers which is recycled is much less than for many metals. List four obstacles that prevent the recycling of a larger percentage of polymers. (8 points) Four of the following would be suitable: Collection of polymer waste products is inefficient. Polymers are usually mixed with one another and are often contaminated. It is difficult to separate, sort, and clean the polymers to get high quality recycled material. Polymers do not mix well to produce homogeneous product. Additives cannot easily be removed from the polymer melt. The economics of recycling (often by downcycling) are not favorable. 14. (a) A filler is added to polypropylene to make injection molded thermoplastic polymer parts. Does the filler decrease or increase the viscosity of liquid polypropylene? Does the filler decrease or increase the strength of the final parts? Circle the correct answers. (2 points)
Viscosity
: decrease / increase Strength
: decrease / increase (b) An extender is added to polypropylene to make injection molded thermoplastic polymer parts. Does the extender decrease or increase the cost of making the polypropylene? Does the extender decrease or increase the recycling value of the polypropylene? Circle the correct answers. (2 points)
Cost
: decrease / increase Recycling value
: decrease / increase (c) A plasticizer is added to polypropylene to make injection molded thermoplastic polymer parts. Does the plasticizer decrease or increase the strength of intermolecular bonds in the polypropylene? Does the plasticizer decrease or increase the glass transition temperature of the polypropylene? Circle the correct answers. (2 points)
Bond strength
: decrease / increase Glass transition temp
: decrease / increase
Polymer Shaping (20 points total)
15. For extrusion, the equation for net (overall) forward flow rate is: Q
x
= 0.5 p
2
D
2
N d
c
sin A cos A - p
D d
c
3
sin
2
A p/(12 h
L) An extruder barrel has a length of 2 m, a diameter of 100 mm, a channel depth of 5 mm, and a flight angle of 30°. The extruder is operated with a polymer melt viscosity of 100 Pa·s, a screw rotation rate of 1 revolution per second, and a head pressure of 10
7
Pa. (a) Calculate the drag flow rate of the polymer for the extruder. (5 points)
Q
d
= 0.5 p
2
D
2
N d
c
sin A cos A = 0.5 p
2
(0.1 m)
2
(1/s)(0.005 m) sin(30°) cos(30°) = 1.07 x 10
-4
m
3
/s
(b) Calculate the backflow rate of the polymer for the extruder. (5 points)
Q
b
= p
D d
c
3
sin
2
A p/(12 h
L) = p
(0.1 m) (0.005 m)
3
sin
2
(30°) (10
7
Pa)/[12(100 Pa·s)(2 m)] = 4.09 x 10
-5
m
3
/s
(c) If the viscosity of the melt were decreased, what would happen to the values of drag flow rate and backflow rate? You do not need to calculate the new values for this answer. (4 points)
The drag flow rate (Q
d
) is unchanged
, since viscosity is not a variable in that equation. The backflow rate (Q
b
) will increase
when the polymer viscosity decreases. (d) How could the viscosity of a thermoplastic polymer for extrusion be decreased without introducing any additive to the polymer? (2 points)
The viscosity of a thermoplastic polymer be decreased by increasing the temperature
. Lowering the average molecular weight would also reduce viscosity. (e) What would be the expected effect of an addition of chopped glass fibers on the shrinkage of a thermoplastic polymer during cooling after extrusion? Indicate the direction of the change and explain (briefly) why the addition causes that change. (4 points)
The addition of chopped glass fibers will decrease the shrinkage
of the polymer during cooling after extrusion. This is due to the fact that polymers have higher thermal expansion coefficients than glass does, so the shrinkage of the combination of polymer and glass will shrink less than the polymer itself would shrink.
Composite Processing (20 points total)
16. (a) Processes for manufacture of composites can be classified into two categories: Open Mold processes and Closed Mold processes. Briefly describe the differences between these categories in terms of the surface finish on the final composite parts and in terms of the possibility for automation of the processes. (5 points)
Open mold processes have only one mold surface, which has a smooth finish, while the other surface is rough. Closed mold processes have two mold surfaces, which gives a smooth finish on both surfaces. Open mold processes are not easily automated unless discontinuous (chopped) fibers are used in a spray-up process. Closed mold processes are generally easier to automate to some extent. (b) In elastic reservoir molding, the final composite product is a sandwich structure with a low density polymer core and a higher density shell containing polymer and continuous fibers. Briefly describe the type of loading for which this type of composite is ideally suited, and why the low strength of the polymer core is not a problem. (5 points)
A sandwich structure with fibers near the surface is ideally suited for an application in which there are bending stresses, because the highest stresses are at the surface and the strong fibers are located there. The low strength of the polymer core is not a problem because the stresses are very low near the neutral axis, which is near the centerline of the cross-section. (c) It is possible to add chopped fibers to polymer resin in order to produce composites. Explain briefly why this type of discontinuous reinforcement is not as effective as continuous reinforcement using long fibers in improving the stiffness and strength of composites. (5 points)
Any tensile loads being carried by the fibers must be transferred to the matrix at the ends of the fibers. Chopped fibers are short, and so the loads must be supported in part by the matrix, which is not stiff or strong. Continuous fibers can carry most of the load without the need for the matrix to carry high stresses, and this provides most of the stiffness and strength in the composite. (d) Composites are often produced from thermosetting resins and strong fibers made of glass or carbon. Explain briefly why this type of composite is not easily recycled. (5 points) By design, it is difficult or impossible to separate the fibers from the polymer matrix, so it is difficult to recover the materials. In addition, thermoset polymers do not soften or melt, so they cannot be reshaped into new polymer products.
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