Chapter 15 Flashcards
elastomers
a polymeric material that may experience large and
reversible elastic deformations
viscoelasticity
a type of deformation exhibiting the mechanical
characteristics of viscous flow and elastic deformation.
relaxation modulus
time-dependent elastic modulus for viscoelastic polymers
leathery or glass transition region
deformation will be time dependent and not totally recoverable on release of an applied load
viscoelastic creep
time-dependent deformation when the stress level is maintained constant
craze
- regions of very localized plastic deformation, which lead to the formation of small and interconnected microvoids
- can support a load across its face
fatigue limit
a stress level at which the stress at failure becomes independent of the number of cycles
tear strength
energy required to tear apart a cut specimen that has a standard geometry
drawing
a deformation technique in which polymer fibers are strengthened by elongation
entropy
- measure of the degree of disorder within a system
- entropy increases with increasing disorder
criteria for a polymer to be an elastomer
-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
vulcanization
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
three phenomena that are important with respect to the design and processing of polymeric
materials
crystallization, melting, and the glass transition
crystallization
process by which, upon cooling, an ordered solid phase is produced from a liquid melt having a highly random molecular structure
melting
reverse process that occurs when a polymer is heated
glass transition
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
melting temperature
the temperature at which, upon heating, a solid (and crystalline) phase transforms into a liquid
glass transition temperature
temperature at which the polymer experiences the transition from rubbery into rigid states
plastics
materials that have some structural rigidity under load and are used in general-purpose applications
fiber
- 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
coatings
are applied to:
- protect the item from the environment, which may produce corrosive or deteriorative reactions
- improve the item’s appearance
- provide electrical insulation
latex
- common coating
- stable suspension of small, insoluble polymer particles dispersed in water
adhesive
- a substance used to bond together the surfaces of two solid materials
- bonds: mechanical and chemical
mechanical bonding
there is actual penetration of the adhesive into surface
pores and crevices
chemical bonding
involves intermolecular forces between the adhesive and adherends (the two surfaces)
films
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
foams
plastic materials that contain a relatively high volume percentage of small pores and trapped gas bubbles
polymerization
process by which monomers are linked together to generate long chains composed of repeat units
addition polymerization
- monomer units are attached one at a time in chainlike fashion to form a linear macromolecule
- initiation, propagation, & termination
condensation polymerization
- 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
additives
intentionally introduced to enhance or modify many of these properties and thus render a polymer more serviceable
fillers
added to polymers to improve tensile and compressive
strengths, abrasion resistance, toughness, & dimensional and thermal stability
plasticizers
- improve flexibility, ductility, and toughness of polymers
- produce reductions in hardness and stiffness
stabilizers
additives that counteract deteriorative
processes
colorants
impart a specific color to a polymer
flame retardants
enhance the flammability resistance of the remaining combustible polymers
fabrication of thermosetting polymers
- 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
molding
shaping a plastic material by forcing it, under pressure and at an elevated temperature, into a mold cavity
compression molding
the mold is closed, and heat and pressure are applied, causing the plastic to become viscous and flow to conform to the mold shape
transfer molding
molten material is injected into the mold chamber, the pressure is distributed more uniformly over all surfaces
injection molding
- material is fed into cylinder by a ram
- pushed forward into heating chamber, which melts it
- impelled through tube into mold
- very fast
extrusion
-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
blow molding
-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
spinning
the process by which fibers are formed. A multitude
of fibers are spun as molten or dissolved material is forced through many small orifices
melt spinning
- 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
metal vs polymers
Metal: higher ultimate tensile strength and modulus of elasticity
Polymers: higher level of ductility
