Lecture 2 - Reinforcement and matrices for polymeric matrix composites (PMCs) Flashcards
What does a high and low Deborah number indicate?
Low = Liquid High = Elastic solid
where on the pipkin diagram are composites located?
And why?
On the boarder between Linear viscoelasticity and elastic solid. This is because the matrix and the reinforcement has different properties.
What does anisotropy mean?
The material have different properties in different directions.
What is the coupling phenomena?
When the material is bend-twisted due to the reinforced fibers.
Name why the matrix is important for the composite
- important for the performance and sometimes even control the performance.
- Matrix protects the fiber surface from abrasion
- Flammability, temperature properties, processing, surface performance, appearance
Which is the main thing that determines which polymer matrix to use?
The service temperature
What is kinking?
Buckling
In what direction does only the matrix and not the fiber affect the properties?
Transverse
What defines a polymeric material?
- Chemistry
- Mass
- Configuration/conformation/arrangement/topology
- 1 Termoplastic
- 1.1 Amorphous or Semi crystalline
- 2 Thermosets
Is thermosets or thermoplastics most common as a matrix in composites?
Thermosets
Name a polymeric matrix from thermosets
Epoxy
What is the main problem with thermosets?
They can not be recycled
Name a polymeric matrix from thermoplastics
PP, PS, PUR
How do you use a thermoset as a polymeric matrix?
Uncured liquid (H20) + curing agent –> Solid thermosetting polymer.
Cross-linking gives us a solid material
What can curing be?
Heat, pressure, radiation, curing agents or combination of these.
When do you add the reinforcement?
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.
What is vitrification?
When the molecules are too long to move. So it becomes “solid”
What is the difference between addition polymerization and condensation polymerization when it comes to cross-linking?
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.
Do we want the degree of curing to be high or low?
As high as possible.
What can be a problem with fillers when we talk about degree of curing, c?
Fillers can change Tg so that the material won’t cure properly
What are the 3 stages of curing?
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.
How does temperature effect degree of curing?
Temperature help the curing process
Why is it not possible to reverse curing?
The energy to make the molecule chains to move again is too high and would only make the material catch fire.
Give some pros for thermosets
- 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.
Give the main cons with thermosets
- (some thermoplastic polymers can offer similar to thermosets service temperatures)
- reuse/recycling - curing is not reversible
What is pre-preg?
Pre-impregnated: The thermoset matrix is only partially cured to allow easy handling
What does the thermoset cocktail include?
- 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)
How can you reduce the shrinkage?
- 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)
What is voids?
Air
Where does voids originate from?
• 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
How can you remove voids?
- 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).
Which are the families of thermosets?
And why are they divided like that?
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)
What applications are common for UP?
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)
What are the 3 different kinds of UP?
- 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).
What does UP stand for?
Unsaturated polyester
Which is the most common thermosetting polymer?
UP
What are common applications for epoxy?
adhesive, glass fiber reinforced automotive leaf springs, carbon fiber reinforced bicycle frames, aircraft wings and fuselage (often with CF)
Compare epoxy vs UP
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.
What does epoxy look like in RT?
Crystalls
How do you cure epoxy?
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).
When is cold-curing suitable?
when contact moulding
Does hot or cold curing epoxies have the best mechanical properties?
Hot-curing
What control the cross-linking density in epoxy?
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.
What does VE stand for?
Venyl ester
Where do you usually use VE?
composites for windmills blades, automotive and marine applications.
How does VE stand agains Epoxy and UP when it comes to mechanical properties and cost?
VE lay in between
What pros does VE have compared to UP?
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
Name a disadvantages with VE
High shrinkage on cure (5-10%)
How is the viscosity for VE?
It is low which makes it a good material får composite matrices
What does MF stand for?
Melamine formaldehyde
What do you use MF for?
Coatings and cross-linker in the lamina, paper and textile industries.
What special properties does MF have?
The hardest surface of any commercial plastic material
Which group of thermosets does MF belong to?
Amino resins or aminoplastics
What does PF stand for?
Phenol formaldehyde
What is the trade name for PF?
Bakelite
What applications do you use PF for?
Heat-resistant handles for pans, irons and toasters, electric hylene tetramine (HMTA).
Why have the interest for PF become higher?
Due to its superior high T properties, good fire resistance and low smoke emission as well as cold-cure varieties
What are the 2 main types of PF and what are they depending on?
Resoles and novolacs. They depend on the intermediates.
What does PUR stand for?
Polyurethane
What kinds of PUR is available?
thermosetting, thermoplastic, rubbers and thermoplastic elastomers.
What does PI stand for?
Thermoset Polyimide
What kind of applications do you use PI for?
High T applications
Name some advantages and disadvantages of epoxy
- 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
Name some advantages and disadvantages of UP
- 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
What does fire resistance depend on?
- Surface spreading of flame
- fuel penetration
- oxygen index
What effects the flammability of a composites?
- Matrix type
- fire-retardant additives
- reinforcements
What is good with Phenolics when they burn?
They have very low smoke emission and give out no toxic by-products
Which thermosets are nonflammable
PI
Order these by decreasing fire resistance
Epoxies, Phenolics, Polyesters, Vinyl esters
- Phenolics 2. Epoxies 3. Vinyl esters 4. Polyesters.
Why is thermoplastic matrices so good?
• 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
What are the disadvantages with thermoplastics
- 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.
Name some properties of polypropylene
- 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.
Name some high temperature polymeric matrices
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).
