Lecture 2 - Reinforcement and matrices for polymeric matrix composites (PMCs)

Question 1

What does a high and low Deborah number indicate?

Answer 1

Low = Liquid
High = Elastic solid

Question 2

where on the pipkin diagram are composites located?

And why?

Answer 2

On the boarder between Linear viscoelasticity and elastic solid. This is because the matrix and the reinforcement has different properties.

Question 3

What does anisotropy mean?

Answer 3

The material have different properties in different directions.

Question 4

What is the coupling phenomena?

Answer 4

When the material is bend-twisted due to the reinforced fibers.

Question 5

Name why the matrix is important for the composite

Answer 5

• important for the performance and sometimes even control the performance.
• Matrix protects the fiber surface from abrasion
• Flammability, temperature properties, processing, surface performance, appearance

Question 6

Which is the main thing that determines which polymer matrix to use?

Answer 6

The service temperature

Question 7

What is kinking?

Answer 7

Buckling

Question 8

In what direction does only the matrix and not the fiber affect the properties?

Answer 8

Transverse

Question 9

What defines a polymeric material?

Answer 9

1. Chemistry
2. Mass
3. Configuration/conformation/arrangement/topology
4. 1 Termoplastic
5. 1.1 Amorphous or Semi crystalline
6. 2 Thermosets

Question 10

Is thermosets or thermoplastics most common as a matrix in composites?

Answer 10

Thermosets

Question 11

Name a polymeric matrix from thermosets

Answer 11

Epoxy

Question 12

What is the main problem with thermosets?

Answer 12

They can not be recycled

Question 13

Name a polymeric matrix from thermoplastics

Answer 13

PP, PS, PUR

Question 14

How do you use a thermoset as a polymeric matrix?

Answer 14

Uncured liquid (H20) + curing agent –> Solid thermosetting polymer.

Cross-linking gives us a solid material

Question 15

What can curing be?

Answer 15

Heat, pressure, radiation, curing agents or combination of these.

Question 16

When do you add the reinforcement?

Answer 16

Hard question and very important to master. When cross-linking the material gets “sticky”. So you need to add the reinforcement before it gets too sticky to get a even spread of the reinforcement.

Question 17

What is vitrification?

Answer 17

When the molecules are too long to move. So it becomes “solid”

Question 18

What is the difference between addition polymerization and condensation polymerization when it comes to cross-linking?

Answer 18

addition polymerisation: such reactive site is carbon-carbon double bond. If two or more carbon-carbon double bonds are present in the monomer, crosslinking can occur

condensation polymerisation: if the reacting molecules have three or more reactive sites, crosslinking can occur.

Question 19

Do we want the degree of curing to be high or low?

Answer 19

As high as possible.

Question 20

What can be a problem with fillers when we talk about degree of curing, c?

Answer 20

Fillers can change Tg so that the material won’t cure properly

Question 21

What are the 3 stages of curing?

Answer 21

Stage A – a thermoplastic stage, used for casting
Stage B – polymerised but not crosslinked; nearly insoluble in organic solvents but still fusible under heat and pressure, used for injection moulding
Stage C – final, infusible, cross-linked polymer.

Question 22

How does temperature effect degree of curing?

Answer 22

Temperature help the curing process

Question 23

Why is it not possible to reverse curing?

Answer 23

The energy to make the molecule chains to move again is too high and would only make the material catch fire.

Question 24

Give some pros for thermosets

Answer 24

• relatively high service temperature
• stiffness
• fatigue resistance
• low cost for low volume manufacturing
• relative ease of processing due to lower viscosity/lower processing temperatures
• good to excellent pre-pregging
• resistance to moisture etc.

Question 25

Give the main cons with thermosets

Answer 25

* (some thermoplastic polymers can offer similar to thermosets service temperatures) * reuse/recycling - curing is not reversible

Question 26

What is pre-preg?

Answer 26

Pre-impregnated: The thermoset matrix is only partially cured to allow easy handling

Question 27

What does the thermoset cocktail include?

Answer 27

* antidegradants, accelerators, flame retardants etc * fibrous reinforcements (composites; for increased strength and rigidity) * Particulate fillers (reduce costs and/or improve properties) * flow promoters (plasticizer type compounds)

Question 28

How can you reduce the shrinkage?

Answer 28

* The addition of fibres or fillers reduces the volumetric shrinkage of a resin * The shrinkage in UP or VE resins can be reduced significantly by the addition of thermoplastic polymers, such as PE, PMMA, PVAc, and polycaprolactone (PCL)

Question 29

What is voids?

Answer 29

Air

Question 30

Where does voids originate from?

Answer 30

• incomplete impregnation, wetting-out of fibres Voids are formed during processing of the non-solidified matrix, and originate from: • by resin cavitation during deformation • incomplete impregnation, wetting-out of fibres • from air or vapours entrapment • by resin cavitation during deformation • incomplete impregnation, wetting-out of fibres • too high viscosity • from air or vapours entrapment • by resin cavitation during deformation

Question 31

How can you remove voids?

Answer 31

* degassing the liquid resin (for example, important for epoxies) * applying vacuum during the moulding process * allowing the resin mix to flow freely in the mould, which helps in carrying the air and volatiles out through the vents in the mould (drainage).

Question 32

Which are the families of thermosets? | And why are they divided like that?

Answer 32

These groupings are mainly based on the nature of the crosslinking reaction that is characteristic of the family • unsaturated polyesters, UP (carbon-carbon double bond plus initiator (usually peroxide)) • epoxies (epoxy ring plus curing agent like amine) • vinyl esters (VE) (closely related to UPs) • amino plastics: urea-formaldehyde, UF, and melamine-formaldehyde, MF (amine Reinforcements & matrices plus formaldehyde) • phenol-formaldehyde or phenolics, PF (phenol plus formaldehyde) • polyurethanes, PUR (polyol plus diisocyanate) • thermoset polyimides, PI (imide condensation or imide and carbon-carbon double bond)

Question 33

What applications are common for UP?

Answer 33

Examples: toaster sides, iron handles, satellite dishes, glass fiber reinforced breaker switch housing and automotive panels, reinforced plastic laminates for marine construction, bathware and shower stalls (v. often combined with GF)

Question 34

What are the 3 different kinds of UP?

Answer 34

* orthophthalic polyester, made by combining maleic and phthalic anhydrides with glycol (commonly propylene glycol);is the least expensive and widely used for small boats * isophthalic polyester, where phthalic anhydride is replaced by isophthalic acid; is more expensive, has superior mechanical properties and water resistance, used for high performance boats and marine gel coats * bisphenol polyester, where anhydride or isophthalic acid is replaced by bisphenol A, much enhanced water and chemical resistance is achieved (at higher price).

Question 35

What does UP stand for?

Answer 35

Unsaturated polyester

Question 36

Which is the most common thermosetting polymer?

Answer 36

UP

Question 37

What are common applications for epoxy?

Answer 37

adhesive, glass fiber reinforced automotive leaf springs, carbon fiber reinforced bicycle frames, aircraft wings and fuselage (often with CF)

Question 38

Compare epoxy vs UP

Answer 38

Mechanical properties and water resistance offered by epoxies can be superior to that of UPs. Epoxies show less shrinkage (about 3%) during cure compared to UPs (5-8%). They can be several times more expensive than UPs.

Question 39

What does epoxy look like in RT?

Answer 39

Crystalls

Question 40

How do you cure epoxy?

Answer 40

The cure process for epoxy requires addition of a curing agent (hardener), together possibly with an accelerator and most cases application of heat (60-150°C). Its also possible to cold-cure some epoxies (20-25°C).

Question 41

When is cold-curing suitable?

Answer 41

when contact moulding

Question 42

Does hot or cold curing epoxies have the best mechanical properties?

Answer 42

Hot-curing

Question 43

What control the cross-linking density in epoxy?

Answer 43

Factors that control the cross-link density in epoxies are the chemical structure of the starting liquid resin (e.g., number of epoxide groups per molecule and spacing between epoxide groups), functionality of the curing agent (e.g., number of active hydrogen atoms in DETA), and the reaction conditions like temperature and time.

Question 44

What does VE stand for?

Answer 44

Venyl ester

Question 45

Where do you usually use VE?

Answer 45

composites for windmills blades, automotive and marine applications.

Question 46

How does VE stand agains Epoxy and UP when it comes to mechanical properties and cost?

Answer 46

VE lay in between

Question 47

What pros does VE have compared to UP?

Answer 47

Superior toughness, less micro-cracking and great chemical and fatigue resistance. This is due to less carbon-carbon double bonds, that has less cross-links

Question 48

Name a disadvantages with VE

Answer 48

High shrinkage on cure (5-10%)

Question 49

How is the viscosity for VE?

Answer 49

It is low which makes it a good material får composite matrices

Question 50

What does MF stand for?

Answer 50

Melamine formaldehyde

Question 51

What do you use MF for?

Answer 51

Coatings and cross-linker in the lamina, paper and textile industries.

Question 52

What special properties does MF have?

Answer 52

The hardest surface of any commercial plastic material

Question 53

Which group of thermosets does MF belong to?

Answer 53

Amino resins or aminoplastics

Question 54

What does PF stand for?

Answer 54

Phenol formaldehyde

Question 55

What is the trade name for PF?

Answer 55

Bakelite

Question 56

What applications do you use PF for?

Answer 56

``` Heat-resistant handles for pans, irons and toasters, electric hylene tetramine (HMTA). ```

Question 57

Why have the interest for PF become higher?

Answer 57

Due to its superior high T properties, good fire resistance and low smoke emission as well as cold-cure varieties

Question 58

What are the 2 main types of PF and what are they depending on?

Answer 58

Resoles and novolacs. They depend on the intermediates.

Question 59

What does PUR stand for?

Answer 59

Polyurethane

Question 60

What kinds of PUR is available?

Answer 60

thermosetting, thermoplastic, rubbers and thermoplastic elastomers.

Question 61

What does PI stand for?

Answer 61

Thermoset Polyimide

Question 62

What kind of applications do you use PI for?

Answer 62

High T applications

Question 63

Name some advantages and disadvantages of epoxy

Answer 63

* very good adhesion to most fibres * higher service temperatures * lower shrinkage on cure and lower thermal expansion * can reside in stage A, this being of importance for preparation of prepregs (see Moulding compounds ahead) * practically no volatiles are emitted when cured at room temperature * have higher viscosity * water in-take causing onsets after long service * difficult to de-gas * longer curing times * need accurate metering * more expensive * can cause nickel allergy

Question 64

Name some advantages and disadvantages of UP

Answer 64

* very good adhesion to most fibres * higher service temperatures * lower shrinkage on cure and lower thermal expansion * can reside in stage A, this being of importance for preparation of prepregs (see Moulding compounds ahead) * practically no volatiles are emitted when cured at room temperature * have higher viscosity * water in-take causing onsets after long service * difficult to de-gas * longer curing times * need accurate metering * more expensive * can cause nickel allergy * hazards: styrene emission; wrongly added peroxide. * unattractive surface appearance can be caused by shrinkage around glass fiber * undesirable sharp edges on fracture needs attention

Question 65

What does fire resistance depend on?

Answer 65

* Surface spreading of flame * fuel penetration * oxygen index

Question 66

What effects the flammability of a composites?

Answer 66

* Matrix type * fire-retardant additives * reinforcements

Question 67

What is good with Phenolics when they burn?

Answer 67

They have very low smoke emission and give out no toxic by-products

Question 68

Which thermosets are nonflammable

Answer 68

PI

Question 69

Order these by decreasing fire resistance | Epoxies, Phenolics, Polyesters, Vinyl esters

Answer 69

1. Phenolics 2. Epoxies 3. Vinyl esters 4. Polyesters.

Question 70

Why is thermoplastic matrices so good?

Answer 70

• possibility to re-process by melting + reshaping (generally, thermoplastics can be re-processed by melting + reshaping with the exception where the degradation temperature is lower than the processing temperature, like for poly(acrylonitrile), PAN, or when the thermoplastic is very difficult to process by melting like poly(tetrafluoroethylene), PTFE) • Short cycle production • Long shelf-life • Increasing service T being enabled by several thermoplastics • Dimensional stability • Resistance to moisture • Good to excellent dielectric properties • Very high compressive strength after impact and open-hole compressive strength of some best thermoplastics • Abrasion resistance

Question 71

What are the disadvantages with thermoplastics

Answer 71

* tools (moulds) are expensive and processing equipment can be very expensive due to high pressures and temperatures involved, and mainly cannot be afforded in small and medium size companies * Viscosity during processing is higher which can pose manufacturing difficulties compared to thermosets * Less stiff and the stiffness easily decrease with increasing temperature.

Question 72

Name some properties of polypropylene

Answer 72

* low cost * lightweight (0.9 g/cm3) * glass transition temperature -18 to -10 ̊C * tough but not as high as polyethylene and this particularly applies to low temperatures; thus rubber toughening is used * stiffness gradually decreases up to about 60 ̊C, and next rapidly drops * does not absorb water * highly processable.

Question 73

Name some high temperature polymeric matrices

Answer 73

``` high temperature thermosets: • special epoxies • polyimides (PI) • bismaleimides (BMI) • polybenzimidazole (PBI) • polyphenyl-quinoxaline (PBQ) • cyanate ester (CE) ``` high temperature thermoplastics: • polyimide (PI) • poly(etheretherketone)(PEEK) and poly(etherketoneketone)(PEKK) • polyimide-amide(PI-A) • polysulfone (PSF or PSO or PSUL) Reinforcements & matrices • polyphenylene sulfide(PPS) • polyether imide (PEI).