Polymers And Composites Flashcards
Why are thermoplastics usually recyclable while thermoses are not recyclable?
Thermoplastics: usually manufactured through addition polymerization - polymer can be torn apart into individual monomers
Thermosets: usually manufactured through condensation polymerization - recycling isn’t possible since no economic or efficient way of getting byproduct back into individual components
Why do fibers gain strength when the fiber diameter decreases?
Size effect
Usually larger volume - lower strength since higher probability of critical flaw
For fibers, it’s the opposite: reduce the volume as much as possible by reducing the cross sectional area to increase the strength
Through the manufacturing process, if there were any critical flaws in the longitudinal direction, there wouldn’t be continuous fibers
The fibers get down to a size relative to the size of the flaws, leaving no flaws in the fibers, making the overall strength much higher
Why do FRPs have good impact resistance?
The impact results in damage which uses up energy
With FRPs, there are many fibers so if one fiber breaks, energy is used up
A lot of fibers breaking uses up a lot of energy
The energy that breaks the fibers takes away from the kinetic energy of the impacting object until it is at 0 KE
HOWEVER once impact damage occurs, the resistance against future impacts is decreased since the fibers can only break once (in most extreme cases the FRP needs to be replaced - motorcycle helmet impact)
How is it possible to increase the surface bond of a pultruded FRP rebar with concrete? Why is it difficult to take the same approach as done in steel rebar?
Steel rebar has raised ridges
FRPs made using pultrusion must have a constant cross-section so you can’t just have ridges on the bar
BUT, immediately after the pultrusion process, the epoxy isn’t cured so add sand to the surface of the bar to increase surface area and therefore bond with concrete (and also better mechanical interlock)
Which type of polymer is recyclable?
Thermoplastic
Why durability issues might be an issue if FRP is used on the exterior of a structure? What other issues might be a concern if FRP is used on the interior?
Exterior: strength issues from denaturing of polymer covalent bonds due to UV radiation
Interior: if there is a fire, toxic fumes would be a hazard
Do elastomers have a high or low elastic modulus?
Low
What are the characteristics that a high quality fiber must have?
Small diameter (increased tensile strength)
High aspect ratio (increased SA/volume ratio)
High flexibility (easy fabrication of FRPs)
