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Flashcards

BLOCK 2 POLYMERS

Martin Sejas Engineering English 27 cards

Cards in this deck (27)

  1. 1What was the material of choice for low pressure distribution mains until the 1950's? And what were the pipes subjected to?Cast Iron.
    They were sealed by jute packing plus cement or molten lead. Leaks developed in the packing over time due to the overhead traffic, freeze-thaw cycles, shifting soil, and the switch to dryer natural gas
  2. 2What negative properties cast iron has making it unsuitable for modern distribution mains?Cast iron has the problems of:
    Corrosion
    Brittleness
    Leakage
    Expertise needed to replace pipes
  3. 3What are desirable properties for a material used in a distribution main?Chemically Resistant
    Toughness
    High Yield Stress
    High Stiffness
    Ease of fusion
    Resists Pressure requirements
    Creep Resistance
    Cost
  4. 4It is essential that pipes "yield before break". To determine this fracture toughness equation is used. Explain the terms on it.K = stress intensity factor
    K(1)<-- Represents the type of loading
    K1c = Critical stress intensity factor (after this point cracked structure will fail)
    Y is dimensionless and depends on shape
    a is related to the length of the crack
  5. 5What are the terms in the Materials Index called "Ductility factor"?Its to prevent plain strain fracture, the crack exceeding a cricial length. Hence its Critical Stress intensity factor/ yield stress
  6. 6What is the general objective of a gas pipe?To deliver low pressure natural gas, safely with low gas leakage for 50 years at a low cost.
  7. 7What is the main disadvantage of PE Pipes used in gas distribution?Methane diffuses through PE pipes, however the leak is very small and controlled (in comparison to cast iron)
  8. 8What impacts does the external environment have on PE gas pipes?Exposure to UV radiation and oxygen degradation influence heavily on the material properties of PE pipes.
  9. 9What does PE stand for?Polyethylene
  10. 10How are polymer crystals arranged?They are always separated from each other by amorphous layers.
  11. 11What is an amorphous layer?A layer in which atoms are not arranged uniformly in a 3d lattice (not crystalline). Therefore even crystalline polymers are actually only semi-crystalline.
  12. 12Name 2 types non-crystallizing polymers, and some examples of them.Glassy Polymers: Polystyrene, PMMA, polycarbonate, (transparent)
    Elastomers (rubbers): Polyisoprene, or butyl rubber, filed with particles to ncerase stiffness and resistance to wear.
  13. 13What does LDPE stand for?Low density Polyethylene
    (Used for films, bags, packaging).
  14. 14What does HDPE stand for?High Density Polyethylene
    (Used in pipes and tanks)
  15. 15What does LLDPE stand for?Linear low density polyethylene, processed at lower temperatures than LDPE, with higher tensile strength, impact and puncture resistance than LDPE.
  16. 16What does MDPE stand for?Medium density polyethylene, processed by mixing LDPE and HDPE.
  17. 17What does UHMWPE stand for?Ultra High Molecular Weight Polyethylene. Has high toughness and difficult to form crystal structure. Only processed by compression or machining.
  18. 18What does MFI stand for and what is it used for?Melt Flow Index, empirical method to determine melt viscosity of polymers.
  19. 19What affects the MFI of a polymer?Molecular weight distribution, presence of co-monomers, degree of chain branching, crystallinity and heat transfer.
  20. 20What is the difference in creep in metals and creeps in a polymer?When metals suffer creep they experience permanent strain, while polymer its temporary, and they might recover from the strain if no further strains near yield point occur.
  21. 21What is a severe stress cracking agent?A potentially hazardous liquid absorbed by a polymer in a short period.
  22. 22What is Runaway Crack Growth, and when does it occur?Its fracture energy, and occurs when its bigger than the critical value G1c
  23. 23What does PP stand for?Polypropylene, its more rigid than PE and used in lids, tapes, caps, plastic bottles and car parts.
  24. 24Is PP a thermoplastic or a thermoset?PP is a thermoplastic.
  25. 25What is PU and what is it used for?Polyurethane, used for elastic seals, its an elastomer and best used especially when cost is a factor.
  26. 26How is PU made?Usually made during the manufacture of the project, its also a thermoset.
  27. 27What does PDMS stand for?Poly dimethyl siloxane, its a silicone with methyl groups.

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Flashcard for revising block 2 of materials in design.
Created by
Martin Sejas
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