Composites- Fibre Phase 1 Flashcards
Why is a small diameter fibrous form stronger than the equivalent bulk material form in brittle materials?
Lower probability of critical flaws
3 types of fibrous reinforcement
Whiskers: 10-100nm, e.g graphite, SiC
Fibres: 5-15μm, e.g glass, carbon, polymeric
Wires: 0.1-1mm, e.g steel wire
Whiskers information
Very small diameter fibres. Highly crystalline, very low number of flaws, very high strength. Very high aspect ratio. Highly efficient reinforcement (if in right place/orientation). Difficult to combine into a matrix as high crystallinity means few surface flaws or functional groups to adhere to. Very expensive. HARN (high aspect ratio nanoparticles) health concerns (need breathing apparatus). Very high specific tensile strength and specific tensile modulus
Wires information
Basically larger diameter fibres. Limited true composite applications. Much lower specific tensile strength and modulus than fibres or whiskers
How do fibres lie between extremes of whiskers and wires?
Good properties and efficient reinforcement.
Processing is economically viable
Cost, modulus and strength of glass, carbon and polymeric fibres
Glass: low cost, medium modulus, medium-high strength.
Carbon: high cost, high-very high modulus, high-very high strength.
Polymeric: medium-high cost, medium-high modulus, medium-high strength
Structure of glass in glass fibres
Based on SiO2 with other oxides added. Series of interconnected tetrahedral SiO2 groups repeated in 3D with Si atoms at centre, O atoms at vertices and strong covalent bonds between atoms. Usually amorphous. Some crystallisation possible at high T leading to reduction in strength
Physical properties of glass in bulk or fibre
Bulk: transparent, hard, corrosion resistant, chemically inert.
Fibre form: stiff, strong, relatively flexible
Isotropy of glass fibres
Same properties parallel to fibre direction (longitudinal) as perpendicular to fibre direction (transverse). Contrast to carbon/polymeric fibres
What do low valency elements do in glass?
Low valency elements in the oxide mixture preferentially bond to the O. E.g Ca, Na, K. Reduces stiffness and strength but improves processing
Types of glass fibres used in composites
E-glass (electrical): good strength, stiffness, electrical and weathering properties, cheapest so most common
C-glass (corrosion): better corrosion resistance, lower strength.
S-glass (strength): more expensive, higher strength, tensile modulus and temperature resistance.
ECR and AR glasses: for acid and alkali resistance
Glass fibre applications
Marine: boat/ship/yacht hulls
Automotive: car/truck/bus shells
Military: minesweepers, aircraft radomes and interiors
Construction: panels, beams, gratings, pipes, ladders, handrails, trench covers, platforms.
Storage tanks and silos. Elec equipment, printed circuit boards. Bath and shower enclosures
How are glass fibres made ?
Weigh and dry mix oxide powders in correct proportions. Pass to refractory furnace and melt and homogenise at 1370C. Pass to refiner at 1340C which completes homogenisation and allows for removal of bubbles. Pass to forehearth at 1250C which allows extrusion of molten glass out through Pt bushings with thousands of holes. Draw fibres down rapidly at 60m/s. Quench fibres using air or sprayed water to solidify them. Size fibres (coat). Process (wind or chop)
What is diameter of fibres controlled by?
Viscosity of melt (dependent on composite and temperature).
Size of bushing holes.
Drawing speed.
E-glass usually 8-15μm diameter
Processing of glass fibres
Continuous strand (over 100mm long) is long fibres.
Chopped strand (3-100mm long) is short fibres.
Hammer milled (30μm-3mm long) is particulate.
Functions of a sizing
Protects fibre from damage. Binds fibres together to aid processing. Lubricate fibres to reduce abrasion. Impart anti-static properties to fibres. Provide chemical link (adhesion) between fibres and matrix (most important). Prevent water affecting the fibre surface
What does a typical sizing consist of?
A protective polymeric film (like polyvinylacetate PVA).
A small molecule to act as lubricant.
A coupling agent to promote adhesion, such as organosilane
Problem of glass fibres absorbing water
Glass fibres readily absorb water from atmosphere onto their surface. In particular the SiO2, Fe2O3 and Al2O3 form bonds to incoming water. These complexes then hydrogen bond to more water exacerbating the problem. Over time important elements like Na and Ca are leached out of the fibre leaving it porous and weak. Water also significantly reduces surface energy of fibres. If this drops below that of liquid matrix, wettability of fibres reduced and fibre-matrix interface weakened.
Ideal, less ideal and non-ideal situations for sizing
Ideal: strong covalent chemical bonds (approx. 350kJ/mol)
Less-ideal: weaker H bonds (approx. 10kJ/mol)
Non-ideal: weak physical interactions (just friction, van der Waals forces, approx. 3kJ/mol)
The process of glass fibre surface treatment
Use organosilanes (silanes) which are R-SiX3. In aqueous solution the X groups hydrolyse with water creating the corresponding silanol (R-Si(OH)3). Silanol molecules compete with incoming water from atmosphere to H bond with hydroxyl groups on fibre surface. When fibres dried, free water is driven off and polysilane layer created by condensation reactions (loss of H2O) between fibre and silanol and neighbouring silanols. R groups on fibre appear polymeric to incoming matrix promoting reaction and hence adhesion.
Thermoset matrices with silane
Silane reacts when polymer cures. For epoxy and phenolics use γ-Aminopropyl Triethoxy Silane (GAPS). For unsaturated polyesters use Methacryloxy Propyl Trimethoxy Silane
Thermoplastic matrices with silane
Polymer is already formed so peroxide radical initiator can be used. For polyethylene or polypropylene use Cinyl Trimethoxy Silane and peroxide. For polycarbonate or nylons use GAPS
Fibre appearance with and without coupling agent
Without: clean fibres and long pull-out lengths, empty holes and unaffected matrix.
With: fibres retain matrix on surface, short pull-out lengths, no empty holes and matrix suffers damage
