Polymer Composites Flashcards
What are polymer composites 2 phases?
same as copolymer:
1. matrix
2. dispersed
what is the difference between composite and blend?
composites dispersed phase are non-polymer compounds
- composites create designs that can compete with metal and ceramics for mechanical properties
Why do we add composites?
To improve properties or to add a desired functionality (ex. electric conductivity)
fibre-reinforced polymer composites are popular choices for rigid, lightweight structural materials (ex. hockey sticks, canoes)
How are thermoplastic composites made?
- typically prepared with melt blending using extrusion
- polymer and dispersed particles both added into extruder where the polymer is melted and they are mixed
- has a low degree of particle alignment
- maximum dispersed particle fraction is lower (~40%) because need to be able to flow
How are thermoset composites made?
- can use wet layup process where a fibre mat is rolled out and resin is poured around it in a mold
- time intensive
- difficult to make small parts (but good for large materials)
- good alignment of dispersed particles
- up to 80% polymers
- lots of batch processes (can use filament winding for continuous but is a very slow process)
- very difficult to recycle
What are some processes to create thermoset composities?
- wet lay-up process
- filament winding
- pultrusion
- resin transfer moulding
What factors affect composite reinforcement?
- concentration
- (more dispersed = better properties to a certain point) - size
- interfacial area decreases with increasing size - shape, aspect ratio
- aspect ratio = largest dimension/smallest dimension
- high aspect ratio aligns more with flow - distribution
- want homogenous mixed
- want to avoid agglomeration - orientation
- if fibres perpendicular to load, do not get reinforcing effect
What is the fiber reinforced composite effect?
- size of polymer aligned with the force
- the interfacial tension is low enough, will get properties of fibre when stress is applied (fibre often 10-100X stronger than polymer)
- there is a critical length the fibre must be to to have a noticeable stiffening effect (balance of the tensile strength of the fibre and the shear yield strength of the interface)
What happens if below or above the critical length?
equal or above, stress reaches maximum applied load and remains there
below, it never reaches maximum applied load as polymer breaks from fibre
How does continuous aligned fibres impact the strength and stiffness of mixtures?
aligned, follows the typical lower and upper values based on their volume fraction (when L > Lc)
How does discontinuous aligned fibres impact strength and stiffness of mixtures?
If L>Lc, then get a matrix and a fibre phase, same as when L<Lc (but here we have the shear strength at the interface directly)
(just 2 different equations)
How does discontinuous random fibres impact strengh and stiffness of composites?
random orientation are relatively isotropic
- follows the rule of mixture, modified by K on the fibre property
- K less than 1 (usually between 0.1 and 0.6)
How to add conductivity to composite materials?
- need to add conductive particles but also need them to form a pathway (or need percolation)
- There is a threshold where below this volume fraction, conductivity is low but above grows exponentially to a fully formed network of dispersed particles
What are ways to improve conductivity of composite materials with the same or less conductive dispersed particle?
- since conductive materials are expensive, use strategies for controlling percolation
- selection of filler volume fraction and aspect ratio
- cocontinuous blends where conductive particles localize at 1 (or at the interphase)
- Use foaming - this creates bubbles that push dispersed particles together where the bubbles touch
-pathway formation through annealing and crystallite exclusion (same mechanism as foaming but with crystals instead)
What is the desired aspect ratio of dispersed particles for packaging?
- want high aspect ratios
- dispersed is often considered impermeable (or less than polymer), so can create longer flow backs with high aspect ratio particles aligned against the flow path
