polymers and Fiber reinforced polymers

Question 1

what is a polymer

Answer 1

manmade material generally consisting of a small molecular unit (monomer), combined in a chain or a network

Question 2

what is a small molecular unit called

Answer 2

monomer

Question 3

what is polymerization

Answer 3

stitching together monomers into chain

Question 4

what has an influence on the properties of polymers

Answer 4

specific type of chain and technique of polymerization

Question 5

what are the two types of polymerization

Answer 5

• addition
• condensation

Question 6

what is an addition polymerization

Answer 6

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’

Question 7

what do you call a polymer with two monomer species

Answer 7

copolymers

Question 8

what is condensation polymerization

Answer 8

a chemical reaction that stitches together the polymer chain and produces by-products

Question 9

in addition polymerization what are the two types

Answer 9

• homogeneous type A+A….
• copolymer trype A+B+A+B

Question 10

where does most macroscopic strength come from

Answer 10

• 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

Question 11

The more possible sites for chains to attach on a monomer, the more….

Answer 11

• more chances for cross linking
• moving to a 2D or 3D structure instead of 1D
• the stronger it will be

Question 12

consequence of having larger and longer molecule chains

Answer 12

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)

Question 13

what other than chain molecules can be added to change polymer properties

Answer 13

• 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

Question 14

what are the three categories of polymers based on differences in material properties and production

Answer 14

• thermoplastic
• thermoset
• elastomer

Question 15

what consists of a thermoplastic polymer

Answer 15

• 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

Question 16

what are the characteristics of thermoplastic polymer

Answer 16

• 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

Question 17

what type of polymer falls into thermosets

Answer 17

• polymers usually formed by condensation polymerization which produces an irreversible final product

Question 18

characteristics of thermosets

Answer 18

• 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

Question 19

what type of polymer falls into elastomers

Answer 19

• they possess linear polymer structures with some cross-linking between molecules, thus helping to provide the unique elastic properties

Question 20

what type of elastic moduli do elastomers possess, and how does that benefit them

Answer 20

a very low elastic moduli
- giving them the ability to elastically deform under relatively low loads without permanently changing in shape

Question 21

what causes a risk to the durability of a polymer

Answer 21

• susceptible to UV radiation damage
• heating

Question 22

why does UV radiation damage affect polymers

Answer 22

• reaction between photons and polymer may break links or form new by-products
• results in weaker and more brittle material
• sunlight can be detrimental

Question 23

why does heating damage polymers?

Answer 23

• causing brittleness in thermosets and plastic behaviour in elastomers
• may off-gas toxic fumes

Question 24

the microstructure composition of an FRP

Answer 24

• matrix
• interface
• fibres
• additives

Question 25

what is the matrix

Answer 25

holds fibres in place and protects them from the environment

Question 26

what is the interface

Answer 26

area in matrix that transfers stress between matrix and fibres

Question 27

what do the fibres do

Answer 27

provide primary strength and stiffness resistances - only capable of resisting along fibre direction due to high aspect ratio

Question 28

what do additives do

Answer 28

admixtures to provide durability, improve material characteristics

Question 29

comment on the variety of macaroscopic configurations of FRP composites

Answer 29

- enormous - unidirectional sheets, 2 ( or more) ply sheets

Question 30

what type of fibres are used in FRP

Answer 30

- can use fibres made from glass, aramid, carbon fibre - need small diameter, long strings for optimal behavior

Question 31

name some types of fabrication methods fro RCP

Answer 31

- pulltrusion - injection molding - hand lay up - spray up - filament winding - etc

Question 32

what are the three characteristics that high performance fibres must possess

Answer 32

1. Small Diameter (increases tensile strength) 2. high aspect ratio (increases surface area/volume ratio) 3. high flexibility (allows for easy fabrication of FRPs)

Question 33

why are thermoplastics usually recyclable while thermosets are not recyclable?

Answer 33

- 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

Question 34

why do fibres gain strength when the fibre diameter decreases?

Answer 34

- 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

Question 35

why do FRPs have good impact resistance?

Answer 35

- 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

Question 36

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?

Answer 36

- 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

Question 37

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?

Answer 37

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