polymers and Fiber reinforced polymers Flashcards
what is a polymer
manmade material generally consisting of a small molecular unit (monomer), combined in a chain or a network
what is a small molecular unit called
monomer
what is polymerization
stitching together monomers into chain
what has an influence on the properties of polymers
specific type of chain and technique of polymerization
what are the two types of polymerization
- addition
- condensation
what is an addition polymerization
monomers combined together in a line or network, with no by-products
- can be of one or more monomer in a wide variety of configurations
- polymers with two monomer species are called ‘copolymers’
what do you call a polymer with two monomer species
copolymers
what is condensation polymerization
a chemical reaction that stitches together the polymer chain and produces by-products
in addition polymerization what are the two types
- homogeneous type A+A….
- copolymer trype A+B+A+B
where does most macroscopic strength come from
- strong covalent bonds
- van der waals forces will attract adjacent chains together but less strength than covalent
- stronger polymers have higher degree of cross-linking between polymer chains
The more possible sites for chains to attach on a monomer, the more….
- more chances for cross linking
- moving to a 2D or 3D structure instead of 1D
- the stronger it will be
consequence of having larger and longer molecule chains
more possibilites for cross linking
- higher utlimate strength (more resisting links)
- higher melting point ( more links to expend energy and break down)
- less flexibility (less chance of plastic deformation)
what other than chain molecules can be added to change polymer properties
- plasticizers: improve plastic behavior
- fillers: inert material used to pad bulk volume and lower casts
- reinforcers: improve mechanical properties
- stabilizers: improve durability resistance of material
what are the three categories of polymers based on differences in material properties and production
- thermoplastic
- thermoset
- elastomer
what consists of a thermoplastic polymer
- polymer which displays plastic behavior at a high temperature (above glass transition temperature Tg and below melting point)
- solidifies into solid material upon cooling below Tg
what are the characteristics of thermoplastic polymer
- recyclable: reversible process
- can be reshaped and reformed if brought above Tg range
- polymer chains plastically deform past each other
- anisotropic properties depending on orientation of long molecules
what type of polymer falls into thermosets
- polymers usually formed by condensation polymerization which produces an irreversible final product
characteristics of thermosets
- more 2D and 3D covalent bonds as compared to the thermoplastic type
- net result is a more isotropic and strong material
- doesn’t become plastic upon reheating
- stiffness lowers upon heating
- may be susceptible to UV radiation
what type of polymer falls into elastomers
- they possess linear polymer structures with some cross-linking between molecules, thus helping to provide the unique elastic properties
what type of elastic moduli do elastomers possess, and how does that benefit them
a very low elastic moduli
- giving them the ability to elastically deform under relatively low loads without permanently changing in shape
what causes a risk to the durability of a polymer
- susceptible to UV radiation damage
- heating
why does UV radiation damage affect polymers
- reaction between photons and polymer may break links or form new by-products
- results in weaker and more brittle material
- sunlight can be detrimental
why does heating damage polymers?
- causing brittleness in thermosets and plastic behaviour in elastomers
- may off-gas toxic fumes
the microstructure composition of an FRP
- matrix
- interface
- fibres
- additives
what is the matrix
holds fibres in place and protects them from the environment
what is the interface
area in matrix that transfers stress between matrix and fibres
what do the fibres do
provide primary strength and stiffness resistances
- only capable of resisting along fibre direction due to high aspect ratio
what do additives do
admixtures to provide durability, improve material characteristics
comment on the variety of macaroscopic configurations of FRP composites
- enormous
- unidirectional sheets, 2 ( or more) ply sheets
what type of fibres are used in FRP
- can use fibres made from glass, aramid, carbon fibre
- need small diameter, long strings for optimal behavior
name some types of fabrication methods fro RCP
- pulltrusion
- injection molding
- hand lay up
- spray up
- filament winding
- etc
what are the three characteristics that high performance fibres must possess
- Small Diameter (increases tensile strength)
- high aspect ratio (increases surface area/volume ratio)
- high flexibility (allows for easy fabrication of FRPs)
why are thermoplastics usually recyclable while thermosets are not recyclable?
- in thermoplastics, usually manufactured via addition polymerization, the polymer can be torn apart into individual monomers
- in thermosets, usually manufactured via condensation poly., recycling is not posisble bc there is no economic or efficient way of getting the by-product(ex: water molecule) back into the individual components
why do fibres gain strength when the fibre diameter decreases?
- due to size effect (larger volume, lower strength, since higher probability of critical flaw)
- fibres work in other direction: making fibres as small as possible and reducing volume = increases strength
- manufacturing process eliminates critical flaws in the longitudinal direction or else there wouldnt be continuous fibres
- perpendicular direction to the length of the fibres, the fibres get down to a size relative to the size of flaws, so no flaw in fibres
- makes overall strength much higher
why do FRPs have good impact resistance?
- in an impact, there is damage
- damage uses energy
- bc many fibres in FRP, if one breaks, energy is used up
- with many fibres breaking, lots of energy is used up
- reduces kinetic enregy of the impacting object, until it is zero
- but, once impact damage is done, impact resistance of an FRP is decreased
- in extreme cases, the FRP has to be replaced bc fibres can only break once
how is it possible to increase the surface bond of pultruded FRP rebar with concrete? why is it difficult to take the same approach as done in steel rebar?
- in steel rebar, there are raised ridges. In FRP, using pultrusion, the specimen must be cast with a constant cross-section, which makes it not possible to create ridges on the bar
- but, during the pultrusion process, the epoxy is still not cured, once can add sand to the surface of the bar to increase surface area and thus bond with concrete, and this also increases mechanical interlock
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: might have strength issues due to denaturing of polymer covalent bonds due to UV radiation
interior: in case of a fire, toxic fumes would be hazardous to occupants health
