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MATSE 151: Composites > Finals: All In > Flashcards

Finals: All In Flashcards

1
Q

Why is it necessary for resins to have high shear strength?

A

If shear stress is applied to the composite, the matrix will be displaced at a different rate than the fibers because of the properties of these components. High shear strength will lead to a more uniform displacement

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2
Q

Why does a matrix need low creep?

A

to prevent the deformation of the material under long term stress and to be able to keep the fibers in place as well

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3
Q

Why is it important for matrixes to have the same CTE as the fiber?

A

If these two have different CTE, they will expand at different rates and apply load or forces onto one another. When this happens, stresses will develop between their interface, which is detrimental to the mechanical properties of the composite, and this may also cause delamination and debonding of the fiber.

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4
Q

Why do you want the matrices to have low moisture absorption?

A

So that water molecules don’t adhere to the surface and interior during processing, which may reduce interaction and contact between the fiber and matrix, so their bond strength may not as be as strong as it could be.

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5
Q

Why do you want the matrix to have a low viscosity during processing?

A

A low viscosity will lead to more contact time between the fiber and matrix, which increases the chances of good matrix and fiber wetting, which therefore leads to better interaction between the components. High viscosity will have shorter contact time and less wet-out

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6
Q

What is fiber wet out?

A

It is the full impregnation and surface coating of the fibers with the resin, which is what allows the fiber and the matrix to bond and interact

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7
Q

Why do you need good flow characteristics during processing for the matrix?

A

Good flow characteristics will lead to the mold filling fully, corners are reached, and it won’t take to long or too short of a time

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8
Q

Why is rapid curing and solidification of matrix important?

A

Because curing is the step which takes the most amount of time

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9
Q

Why would one want the curing temperature of the matrix to not greatly be above the product usage temperature?

A

So that more energy is not required/wasted or expended for curing

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10
Q

How come it’s important for shrinkage to be low during and after molding?

A

This leads to better dimensional stability. Shrinkage entails poor dimensional stability, which may make the composite unsuitable for the application. Also, if only the matrix shrinks and not the fiber, they will push against each other, which leads to stresses, poor bonding, and delamination/debonding.

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11
Q

Why is the shelf life of thermosets more limited compared to thermoplastics?

A

There is potential for thermoset matrices to crosslink during storage

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12
Q

Why to thermosets have a tendency to shrink more than thermoplastics?

A

For thermosets, due to the cross-linking and structure, there is going to be the formation of bonds due to the presence of leading groups. When these are formed, there is going to be bond alignment, and this alignment/arrangement allows the composite to be more compressed (i.e. shrinkage).

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13
Q

What are the two thermoplastic resins we learned about and the four thermoset resins?

A

TP: PEEK and Polyimide
TS: PF Resin, Polyester, Vinylester, Epoxy

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14
Q

For composites, what is the component that dictates composite processing? Why?

A

The matrix, because fibers are the ones that tend to be more stable and more easily processible.

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15
Q

In processing, why is it important to account for the viscosity of the matrix, what does it dictate?

A
  1. fiber-wet out (due to contact time)
  2. flow time and flow pattern
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16
Q

True or false, curing time is often what dictates the overall process cycle time. Why?

A

True, because it is the longest step in composite processing. So it naturally takes the most time, and is like the “rate-limiting” step.

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17
Q

In the processing of thermosets, why are fabricators or manufacturers given only incompletely polymerized pre-polymers (stage-B polymers) in the semi-liquid/solid form, and the ones who convert the polymer to its C-stage?

A

Stage B polymers are more fusible and can be handled to be brought to the desired shape, however, they are not too rigid to the point that they can’t flow (Stage C).

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18
Q

What are the two types of PF resins, and what are their main differences?

A
  1. Novolac - manufactured with less formaldehyde (less OH groups), and they have an acid catalyzed reaction
  2. Resole - manufactured with excess formaldehyde (more OH groups), and these have a base-catalyzed reaction
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19
Q

For PF resins, formaldehyde exists in what form typically?

A

Formalin

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20
Q

Resole is known to have an excess of formaldehyde and therefore more OH groups. What does this entail and why is this relevant for the final product?

A

OH groups are crosslinking sites, and more crosslinking entails more bond formation (and leaving groups) and therefore more bond alignment. This bond alignment will lead to a greater amount of shrinkage for the product, making it dimensionally unstable for its application.

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21
Q

Novolac has less OH groups compared to Resole because it was manufactured without an excess of formaldehyde. What is the advantage of this?

A

To drive the crosslinking reaction, usually more formalin is added. This means that you can control WHEN crosslinking occurs and the DEGREE to which the resin will crosslink, based on when you add formalin or how much you add.

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22
Q

What are the main advantages of PF resin?

A

cheap, thermally/dimensionally stable, and there’s generally good chemical, mechanical, and electrical properties

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23
Q

What are the main disadvantages of PF resin?

A

Think of OH groups:
1. leaving groups -> many byproducts -> water -> evaporation -> voids
2. high shrinkage
3. high water absorption
4. easily attacked by aqueous bases

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24
Q

Where are PF resins most used?

A

“Bakelites” or Laminates for high voltage insulators (i.e. plugs, switches, etc.)

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25
Q

What are polyester resins made from, and how does this influence the polyester?

A

Dibasic acids and diols. Combination of these due to the difference in their structures impart variation in mechanical properties like flexural strength, tensile strength, and compressive strength.

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26
Q

What is the main drawback of the polyster resin due to its structure containing ester bonds?

A

It is very prone to hydrolysis and isn’t environmentally stable because of the ester bonds.

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27
Q

Why do polyester resins have the ability to gel together, and what can be done to slow this down?

A

The free ends of chains are reactive, and therefore can interact to form longer chain networks, which leads to gel formation. To slow this down, inhibitors can be added.

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28
Q

What are some of the main applications of polyester matrices?

A
  1. dinghies/workboats
  2. chairs
  3. sportscar bondies
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29
Q

What are the pros of polyester as a matrix?

A
  1. cheap
  2. rigid
  3. easy to handle/shap
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30
Q

What are the cons of polyester as a matrix?

A
  1. easy gelling and crosslinking
  2. prone to hydrolysis
  3. short shelf-life
  4. only moderate mechanical properties
  5. high cure shrinkage
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31
Q

For vinyl esters, what is this usually comprised of?

A

Styrene monomer (in PE solution)

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32
Q

Styrene as a monomer minimizes gelation and viscosity for vinyl esters. How?

A

Since styrene is now surrounding the polyester chains, the free ends can no longer interact, and so there’s no more attachment between the chains.

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33
Q

How come styrene in vinylester helps in pore reduction?

A

Due to its structure styrene can crosslink PE chains without any byproducts because there’s no longer any leaving group. Since there’s no leaving group, there’s no more formation of byproducts (i.e. water) that will evaporate and cause pores on the surface.

34
Q

Styrene allows the vinylester resin to be formed with low pressure. Why?

A

Curing can happen without the formation of byproducts which inhibit contact/interaction between chains. So chain formation is more efficient and extra pressure is no longer needed.

35
Q

Since vinyl esters have less ester groups than polyester, what does it mean for its chemical/water resistance?

A

It has better chemical and water resistance.

36
Q

Vinyl esters shrink less than polyesters. Why?

A

Vinyl ester has less ester groups and therefore has less reactive sites for cross-linking. Cross-linking leads to bond formation and alignment, which can lead to these chains being compressed. Also, because of styrene’s structure, there’s no realignment of bonds when the leading group leaves.

37
Q

What is the main application of vinylester?

A

Chemical storage tanks, pipelines, etc.

38
Q

Describe the rate crosslinking of epoxy resins. Why is it like this?

A

It is very fast because epoxy has very highly reactive groups. Therefore it’s good as an adhesive

39
Q

Epoxy resins have a lot of OH groups that are very reactive, causing them to bond or crosslink quickly. In spite of these OH groups, how come epoxy has the least shrinkage among the discussed thermosets? What else does this entail for the material’s properties?

A

It doesn’t have a leaving group. So there’s no bond realignment leading to shrinkage, but there are several strong bonds that lead to the material having very high-performing properties. Stresses are not generated during curing.

40
Q

What are some of the most common epoxy applications?

A
  1. Adhesives
  2. electrical components (doesn’t disrupt frequencies)
  3. aerospace/transport applications
  4. sports
41
Q

What are the disadvantages of epoxy?

A
  1. expensive
  2. high deg. of smoke liberation in fire
  3. UV degradation sensitive
  4. corrosive handling
42
Q

Why is it important to mitigate shrinkage, give the 2 main reasons?

A
  1. matrix will shrink, fiber will resist. when the matrix and fiber press against each other, this causes built-in stresses which are bad for mechanical properties.
  2. poor dimensional stability, making it unusable for the application
43
Q

What leads to a matrix having a “chicken-pox-like” surface?

A

Osmosis can take place, and the matrix acts like a semi-permeable membrane because of hydrolyzable components. Water is drawn into cell-like pockets, and pressure increases as water increases until the pocket bursts and forms a hole.

44
Q

What are the advantages of polyimide?

A
  1. high temp applications
  2. high intermolecular forces due to ring structure -> high strength and temp applications
45
Q

What are the drawbacks of polyimide?

A
  1. difficult to process because it emits water during curing
  2. expensive
  3. toxic raw materials
46
Q

What are the four factors that account for how a fiber contributes to a composite?

A
  1. orientation
  2. volume fraction
  3. mechanical properties
  4. interfacial interaction
47
Q

What are the main advantages of organic fibers and inorganic fibers?

A

Organic - low density and high elasticity

Inorganic - higher stiffness, more thermally stable, but harder to combine with matrix

48
Q

What are the main advantages of FRPs?

A
  1. lightweight
  2. shock absorption and dampening
  3. less prone to corrosion and less maintenance
  4. less thermal expansion
  5. better fatigue strength
49
Q

What are the advantages of glass fibers?

A
  1. cheap
  2. easy to manufacture
  3. high strength
  4. color changing
  5. isotropy in thermal expansion
  6. low density
50
Q

What are the disadvantages of glass fibers?

A
  1. brittle
  2. prone to surface damage
  3. oxide dissolution via oxygen
51
Q

What are the two treatments of glass fibers?

A
  1. drawing and cooling (to prevent crystallization)
  2. Fiber finishing
52
Q

What are the best fibers for telecomm applications?

A

Glass fibers

53
Q

What are the advantages of aramid fibers?

A
  1. high impact strength
  2. more flexible than glass
  3. high strength due to packing efficiency
  4. less abrasion
  5. heat resistance
54
Q

What are the disadvantages of aramid fibers?

A
  1. hard to cut
  2. discoloration (quinoid reorientation of benzene)
  3. moisture absorption
55
Q

Why is carbon fiber sourced from PAN?

A

higher MW due to less impurities nad more process control for in-lab synthesis

56
Q

How are carbon fibers made from pan?

A
  1. heating (cyano sites become cyclic and eliminate some H)
  2. oxidation (more H removal and rings become aromatic)
  3. carbonization (adjacent chains join to form networks, barely any H, many double bonds)
57
Q

What are the drawbacks of carbon fibers?

A

Lowest impact strength, quite brittle, always black, expensive

58
Q

What are the 4 types of of constructions?

A
  1. unidirectional, 2. woven (continuous), 3. multiaxial (discontinuous), 4. braids
59
Q

Why is the matrix ratio very important, what happens if there’s too much or too little?

A

Too much = wasteful and strength is not maximized

Not enough = strength is compromised due to poor wetting and fibers not being held well

60
Q

Why is polyester both a good and not a good fiber?

A

It cannot compete in terms of strenght, but it has good impact resistance and low modulus.

61
Q

What is the drawback of quartz fibers?

A

High price and hard to manufacture (but better properties than glass)

62
Q

What are boron fibers used for and why?

A

Aerospace applications kasi mahal sila

63
Q

Boron is usually paired with what other fiber to make a hybrid?

A

Carbon

64
Q

Ceramic fibers are usually found in the form of what and used for what type of composites?

A
  1. Whiskers
  2. MMCs
65
Q

Why do fibers need finishing, and what is usually used? Why?

A

For cushioning after manufacturing so that fibers are less abraded. Starch is used because it’s viscous, cheap for a one-time use item, and can be removed via H2O washing.

66
Q

For particulate reinforcement, what is the mechanism behind this?

A

The matrix and chains within it are immobilized through introducing mechanical restraint which acts as a barrier.

67
Q

Degree of restrains is dependent on particulate spacing. So if more particles are added, what happens?

A

Chains are prevented from wiggling, flexing, bending, etc. Mobility and deformation are restricted, stiffness is increased

68
Q

What is the difference between a compatibilizer and a coupling agent?

A

Compatibilizer - makes 2 polymers blending-compatible

Coupling agent - improves fiber-matrix adhesion and optimises interaction between them for better load transfer

69
Q

CMCs and MMCs don’t need coupling agents. Why?

A

They don’t need because they have a similar bond-type between the fiber and matrix (assuming the fibers aren’t made of polymers)

70
Q

Whta are the four mechanisms for fiber-matrix adhesion?

A
  1. Adsorption by wetting (PMCs)
  2. Electrostatic Charges (+/- on fiber and matrix)
  3. Chemical Bonding
  4. Mechanical Adhesion (for MMCs and CMCs)
71
Q

Coupling agents may be added in the form of solids and liquids. Can they be added to both the resin and fiber or only one?

A

They can be added to both.

72
Q

What is the structure of silane?

A

Y-Si(OR)3

73
Q

What is the strength of silanes?

A

It helps bonding of organic resins to inorganic materials.

74
Q

What happens when Y-Si(OR)3 is hydrolized, and what part bind to what?

A

It becomes Y-Si(OH)3, and these OH groups bond to the glass, whereas the Y groups bonds to the polymer. The glass then become hydrophobic because of the OH groups.

75
Q

What are the benefits of silane?

A
  1. improved wet-out and mech. properties
  2. durable in wet environment
  3. electrical property retention
  4. can be used in hot and cold cycling
76
Q

ow does silane help in PMC applications involving hot/cold cycling?

A

Typically MMCs and CMCs have fibers and matrices that expand and contract unifromly. For PMCs they don’t, the matrix resists expansion as an insulator, and cracks can form. If coupling agent is added, the fiber is adhered better and the matrix is ALSO PULLED during expansion and contractions. Minimizes fracture.

77
Q

For are the three ways silanes can be incorporated?

A
  1. Grafting Reaction
  2. Chemical Reactions
  3. Copolymerization
78
Q

The tensile strength is usually attributed to that of the fiber. Why?

A

If you have fibers well-boned through coupling agent, in bending, the fiber will transfer energy to matrix so it won’t breaking, but without the coupling agent, the matrix will flex but the fibers will break.

79
Q

Silane impart improve electrical properties. Why does adding silane prevent increase in the dielectric constant when the material is exposed to moisture?

A

OH groups = more polar = more conducting; adding silane makes the material more hydrophobic thus removing the OH groups, and so the material remains less conducting

80
Q

Adding clay will increase the modulus, stiffness, and tensile strength of the composite. Why?

A

Since it’s hydrophilic, it’s inserted between chains to ensure incorporation. Clay is a particulate reinforcement that can immobilize chains like a barrier to prevent deformation.

MATSE 151: Composites (5 decks)

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