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MATSE 151: Composites > Lecture 2: Resins > Flashcards

Lecture 2: Resins Flashcards

1
Q

Are resins are expected to have low creep at the temperature that it’s being used? Why?

A

Yes, so that the material does not plastically deform due to the different stresses that it undergoes.

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2
Q

Why should the CTE of the fiber match that of the matrix?

A

So that these expand at a uniform rate under particular temperatures. If these expand at different rates, there will be force applied from one material to the other, thus causing stress to be present on the interface between the resin and dispersed phase.

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3
Q

Why do resins need high shear strength?

A

When a normal force is applied, shear stresses may be acting as well on the material. Due to the force, the matrix may be displaced, however there is no guarantee that the fiber is being displaced proportionally, which may affect the loading at the interface. A high shear strength will help prevent this.

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4
Q

Why is a high heat distortion temperature needed for resins?

A

When you increase the temperature, the polymer should not undergo deformation.

Example: if you have a low heat distortion, the material might unnecessarily expand. Not good for engineering applications.

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5
Q

What are the three important chemical properties of polymer matrices?

A
  1. good bond with fiber
  2. resistant to solvents or chemicals
  3. low moisture absorption
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6
Q

Why does the resin need good flow characteristics?

A
  1. it might not fill the mold adequately during processing
  2. might take too long to fill
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7
Q

What factors lead to good fiber wetting?

A

Low viscosity and appropriate surface tension

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8
Q

Why is fiber wetting necessary?

A

The material will not act as a composite w/o wetting. Wetting helps the resin and fiber interact. Fiber and resin have to bond, if the viscosity is too high or surface tension is not ideal, the wetting cannot happen properly because it’s too thick, and the contact time is shorter.

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9
Q

Why is it ideal for matrices to have rapid curing or solidification time?

A

Curing and solidification take the longest time in processing, so lessening this minimizes the time it takes for the overall process.

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10
Q

Why is it preferred that the cure temperature for matrices is not greatly above the usage temp?

A

If curing temperature is too high, you’re wasting energy on a higher temperature.

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11
Q

Why is it ideal for resins to have low shrinkage during and after molding?

A

It loses its dimensional stability. So it might not be suitable for the application anymore. Also, the fiber isn’t shrinking, just the matrix, so the contact is going to become less/poorer. So you’re reducing the fiber and polymer bonding and interface interaction. Poor load transfer from matrix to fiber. Poorer properties.

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11
Q

Why is it ideal for resins to have low shrinkage during and after molding?

A

It loses its dimensional stability. So it might not be suitable for the application anymore. Also, the fiber isn’t shrinking, just the matrix, so the contact is going to become less/poorer. So you’re reducing the fiber and polymer bonding and interface interaction. Poor load transfer from matrix to fiber. Poorer properties.

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12
Q

Which has a longer shelf life, thermoplastics or thermosets? Why?

A

Thermoplastics. Thermosets have a shorter shelf life because they are more prone to undesirable crosslinking due to their structure.

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13
Q

What is prepeg?

A

Partially processed polymer with the reinforcing agent.

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14
Q

Thermoplastics are more expensive than thermosets. True or false?

A

False thermosets are more expensive.

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15
Q

Which undergoes more shrinkage, thermoplastics or thermosets? Why?

A

Thermosets are more prone to shrinkage because there is a greater tendency for bond alignment, and there is the possible formation of new bonds due to the leading groups present. When bonds are aligned and form, this causes the composite to compress.

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16
Q

Which is tougher and not brittle, thermoplastics or thermosets?

A

Thermoplastics.

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17
Q

Why are thermoplastics easier to process?

A

You are no longer considering the step of crosslinking, crosslinkers involved, additional time for crosslinking and other related processes.

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18
Q

What are the three common thermoplastic resins used?

A

Polyimides, PEEK, polysulfones

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19
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible

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19
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible

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19
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible

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19
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible

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20
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible

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20
Q

Why would thermoplastics be better for a garden hose?

A

Because of its application, the resin needs to be resistant to wear and tear, and must also be tougher. Thermosets are less rigid and more flexible, so if there is an increase in water pressure, the material is able to expand instead of rupturing.

21
Q

What is the main drawbacks of thermoplastics in terms of their structure?

A

They don’t have as much structural rigidity especially at high temperatures.

22
Q

Which component of the composite mainly influences how it will be processed? The matrix or the dispersed phase? Why?

A

The matrix. Fibers tend to be more stable than the resin involved.

23
Q

The viscosity of the resins determines what during processing?

A
  1. Flow time and pattern of resin inside mold (more difficult to fill mold)
  2. Wettability
24
Q

Does the fiber impact the time of the process cycle?

A

No. Processing is mainly influenced by the matrix.

25
Q

Process cycle length for a composite and time that the mold will be kept closed is dependent on what?

A

Curing time of the thermoset matrix

26
Q

For thermoplastics, the polymerization is completed by the manufacturer of the product. Fabricators receive incompletely polymerized polymer-prepolymers (in the B stage where they are fusible). Why?

A

So that this polymer can be given the desired shape by applying heat and pressure to form the final product. If its viscosity is too low and it hasn’t been polymerized, it may be too difficult to control, but having it fully polymerized makes it too viscous and unable to flow adequately.

27
Q

Phenol-Formaldehyde (PF) resins were the first commercially available thermoset. What are the two kinds?

A

Novolac and Resole

28
Q

What are the differences between resole and novolac?

A

Resole has excess formaldehyde over phenol, and can be cured by just heating. Novolac has excess phenols over formaldehyde, and requires formaldehyde to be added.

29
Q

What are the differences between resole and novolac?

A

Resole has excess formaldehyde over phenol, and can be cured by just heating. Novolac has excess phenols over formaldehyde and requires formaldehyde to be added for curing to take place.

30
Q

How come resole can be cured by just heating?

A

Resole has an excess of formaldehyde over phenol in composition. More formaldehyde implies the presence of more OH- groups in the structure, and therefore there are more available sites for crosslinking to take place.

31
Q

What shrinks more, novolac or resole? Why?

A

Resole shrinks more. Resole has an excess of formaldehyde over phenol in composition. More formaldehyde implies the presence of more OH- groups in the structure, and hence there are more sites for bonding, and bond alignment, which will lead to shrinkage.

32
Q

In what form is formaldehyde supplied?

A

Formalin (36-50% aq solution)

33
Q

When resoles are polymerized to a high molecular weight network, they are now known as?

A

Resite

34
Q

Why does additional formaldehyde need to be added to crosslink novolac?

A

Since it has more phenol compared to formaldehyde, it has only a limited number of OH groups. For it to crosslink, a greater number of OH groups are needed, so formaldehyde is added.

35
Q

What is the main advantage for novolac in terms of processing even if formaldehyde or hexamethylenetetramine needs to be added?

A

You can use novolac when you want to CONTROL the time or when you’ll crosslink, which is the advantage of this matrix.

36
Q

What are the main disadvantages of PF resins as matrices?

A
  1. by-products produced during curing.
  2. shrinkage during curing
  3. phenolic laminates are porous and thus weaker
37
Q

What are the main disadvantages of PF resins as matrices?

A
  1. by-products produced during curing.
  2. shrinkage during curing
  3. phenolic laminates are porous and thus weaker
38
Q

Why is it a disadvantage for by-products to be produced during curing?

A

Because byproducts like water may evaporate from the system, causing pores to be left behind, which make the material weaker.

39
Q

Why do PF resins have a water absorption of 1-10%?

A

Because of the presence of OH groups.

40
Q

What is the main drawback of PET as a resin?

A

It is prone to hydrolysis and not environmentally stable -> attributed to the ester bonds.

41
Q

Why does PET gel together?

A

Look at the structure: These ends are free and can interact with one another and form a longer chain. Which is why they can form gels. These bonds can begin breaking as well and crosslink.

42
Q

In resin manufacture, what is added to prevent the gelling of polyester resins?

A

Inhibitors.

43
Q

For use in moulding, a polyester resin requires the addition of several ancillary products. What are these?

A

Catalysts, accelerators, additives

44
Q

Why are epoxy resins used in the fabrication of dinghies and workboats?

A

Epoxy is cheap. Boats have to be cheap because it needs to be accessible to the people who need it. Polyester is cheap while also having good strength.

45
Q

Cured polyester resin has styrene in its structure. What does styrene enable in resin and why is it advantageous?

A

Enables resin to cure from liquid to solid through cross-linking without the formation of byproducts. These can be formed through low-pressure methods. The advantage if you’re crosslinking with styrene: there will not be any voids or pores. Water is usually a byproduct, and without it, there’s going to be no voids.

46
Q

Cured polyester resin has styrene in its structure. What does styrene enable in resin and why is it advantageous?

A

Enables resin to cure from liquid to solid through cross-linking without the formation of byproducts. These can be formed through low-pressure methods. The advantage if you’re crosslinking with styrene: there will not be any voids or pores. Water is usually a byproduct, and without it, there are going to be no voids.

46
Q

Cured polyester resin has styrene in its structure. What does styrene enable in resin and why is it advantageous?

A

Enables resin to cure from liquid to solid through cross-linking without the formation of byproducts. These can be formed through low-pressure methods. The advantage if you’re crosslinking with styrene: there will not be any voids or pores. Water is usually a byproduct, and without it, there are going to be no voids.

47
Q

Why does the use of styrene make cured polyester resin less prone to gelling?

A

Groups are surrounded by styrene. Because it’s surrounded by styrene, and thus they won’t attach to each other.

47
Q

Why does the use of styrene make cured polyester resin less prone to gelling?

A

Groups are surrounded by styrene. Because it’s surrounded by styrene, and thus they won’t attach to each other.

48
Q

Why does vinyl resin undergo less shrinkage?

A

Less shrinkage because there is no bond realignment when the leading groups leaves.

49
Q

Why does epoxy resin act as an excellent adhesive?

A

It crosslinks very fast. It has reactive groups -> therefore it’s a good adhesive .

50
Q

How come epoxy resins don’t shrink very much when they’re cured?

A

This is because there is no significant leaving group, and thus there is not rearrangement or elimination of present atoms.

51
Q

Shrinkage is associated with built-in stresses that can weaken the material. What happens in terms of stress?

A

The composite will be under stress during shrinkage -> fiber is strying to resist the difromation -> this builds tremendous stresses in the composite.

52
Q

If water is present in the sample due to moisture absorption, why are there “dotted bulges” on the surface of the composite?

A

If there’s moisture absorption -> the surface may have “dotted bulges” -> this is because the surface which is the resin acts as a semi-permeable membrane. Water is going inside, and making the surface bulge. If the water is too much it may even rupture.

53
Q

What resins are prone to water degradation and why?

A

Both polyester and vinyl ester resins are prone to water degradation due to the presence of hydrolysable ester groups in their molecular structures.

MATSE 151: Composites (5 decks)

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