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Material Science > Chapter 15 > Flashcards

Chapter 15 Flashcards

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1
Q

elastomers

A

a polymeric material that may experience large and

reversible elastic deformations

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2
Q

viscoelasticity

A

a type of deformation exhibiting the mechanical

characteristics of viscous flow and elastic deformation.

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3
Q

relaxation modulus

A

time-dependent elastic modulus for viscoelastic polymers

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4
Q

leathery or glass transition region

A

deformation will be time dependent and not totally recoverable on release of an applied load

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5
Q

viscoelastic creep

A

time-dependent deformation when the stress level is maintained constant

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6
Q

craze

A
  • regions of very localized plastic deformation, which lead to the formation of small and interconnected microvoids
  • can support a load across its face
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7
Q

fatigue limit

A

a stress level at which the stress at failure becomes independent of the number of cycles

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8
Q

tear strength

A

energy required to tear apart a cut specimen that has a standard geometry

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9
Q

drawing

A

a deformation technique in which polymer fibers are strengthened by elongation

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10
Q

entropy

A
  • measure of the degree of disorder within a system

- entropy increases with increasing disorder

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11
Q

criteria for a polymer to be an elastomer

A

-it must not easily crystallize; elastomeric materials are amorphous, having molecular chains that are
naturally coiled and kinked in the unstressed state
-chain bond rotations must be relatively free for the coiled chains to readily respond to an applied force
-for elastomers to experience relatively large elastic deformations, the onset of plastic deformation must be delayed

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12
Q

vulcanization

A

a nonreversible chemical reaction involving sulfur or another suitable agent in which crosslinks are formed between molecular chains in rubber materials. The rubber’s modulus of elasticity and strength are enhanced

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13
Q

three phenomena that are important with respect to the design and processing of polymeric
materials

A

crystallization, melting, and the glass transition

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14
Q

crystallization

A

process by which, upon cooling, an ordered solid phase is produced from a liquid melt having a highly random molecular structure

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15
Q

melting

A

reverse process that occurs when a polymer is heated

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16
Q

glass transition

A

occurs with amorphous or noncrystallizable polymers that, when cooled from a liquid melt, become rigid solids yet retain the disordered molecular structure that is characteristic of the liquid state

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17
Q

melting temperature

A

the temperature at which, upon heating, a solid (and crystalline) phase transforms into a liquid

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18
Q

glass transition temperature

A

temperature at which the polymer experiences the transition from rubbery into rigid states

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19
Q

plastics

A

materials that have some structural rigidity under load and are used in general-purpose applications

20
Q

fiber

A
  • any polymer, metal, or ceramic that has been drawn into a long and thin filament
  • have a high tensile strength (over a relatively wide temperature range) and a high modulus of elasticity, as well as abrasion resistance
21
Q

coatings

A

are applied to:

  • protect the item from the environment, which may produce corrosive or deteriorative reactions
  • improve the item’s appearance
  • provide electrical insulation
22
Q

latex

A
  • common coating

- stable suspension of small, insoluble polymer particles dispersed in water

23
Q

adhesive

A
  • a substance used to bond together the surfaces of two solid materials
  • bonds: mechanical and chemical
24
Q

mechanical bonding

A

there is actual penetration of the adhesive into surface

pores and crevices

25
Q

chemical bonding

A 
involves intermolecular forces between the adhesive
and adherends (the two surfaces)
26
Q

films

A

have thicknesses between 0.025 and 0.125 mm are fabricated and used extensively as bags for packaging food products and other merchandise, as textile products

27
Q

foams

A

plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles

28
Q

polymerization

A

process by which monomers are linked together to generate long chains composed of repeat units

29
Q

addition polymerization

A
  • monomer units are attached one at a time in chainlike fashion to form a linear macromolecule
  • initiation, propagation, & termination
30
Q

condensation polymerization

A
  • formation of polymers by stepwise intermolecular chemical reactions that may involve more than one monomer species
  • usually a low-molecular-weight by-product such as water that is eliminated
31
Q

additives

A

intentionally introduced to enhance or modify many of these properties and thus render a polymer more serviceable

32
Q

fillers

A

added to polymers to improve tensile and compressive

strengths, abrasion resistance, toughness, & dimensional and thermal stability

33
Q

plasticizers

A
  • improve flexibility, ductility, and toughness of polymers

- produce reductions in hardness and stiffness

34
Q

stabilizers

A

additives that counteract deteriorative

processes

35
Q

colorants

A

impart a specific color to a polymer

36
Q

flame retardants

A

enhance the flammability resistance of the remaining combustible polymers

37
Q

fabrication of thermosetting polymers

A
  • first comes the preparation of a linear polymer (sometimes called a prepolymer) as a liquid having a low molecular weight
  • the second stage, termed curing, may occur during heating and/or by the addition of catalysts and often under pressure
38
Q

molding

A

shaping a plastic material by forcing it, under pressure and at an elevated temperature, into a mold cavity

39
Q

compression molding

A

the mold is closed, and heat and pressure are applied, causing the plastic to become viscous and flow to conform to the mold shape

40
Q

transfer molding

A

molten material is injected into the mold chamber, the pressure is distributed more uniformly over all surfaces

41
Q

injection molding

A
  • material is fed into cylinder by a ram
  • pushed forward into heating chamber, which melts it
  • impelled through tube into mold
  • very fast
42
Q

extrusion

A

-a mechanical screw or auger propels the pelletized material through a chamber, where it is successively
compacted, melted, and formed into a continuous charge of viscous fluid
-molten mass is forced through a die orifice

43
Q

blow molding

A

-a parison, or length of polymer
tubing, is extruded
-while still in a semimolten state, the parison is placed in a two-piece mold
-the hollow piece is formed by blowing air
or steam under pressure into the parison, forcing the tube walls to conform to the contours
of the mold

44
Q

spinning

A

the process by which fibers are formed. A multitude

of fibers are spun as molten or dissolved material is forced through many small orifices

45
Q

melt spinning

A
  • material is first heated until it forms a relatively viscous liquid
  • pumped through a plate called a spinneret, which contains numerous small, typically round holes
  • as the molten material passes through each of these orifices, a single fiber is formed, which is rapidly solidified by cooling with air blowers or a water bath
46
Q

metal vs polymers

A

Metal: higher ultimate tensile strength and modulus of elasticity
Polymers: higher level of ductility

Material Science (6 decks)

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