Composites- Fibre Phase 1

Question 1

Why is a small diameter fibrous form stronger than the equivalent bulk material form in brittle materials?

Answer 1

Lower probability of critical flaws

Question 2

3 types of fibrous reinforcement

Answer 2

Whiskers: 10-100nm, e.g graphite, SiC
Fibres: 5-15μm, e.g glass, carbon, polymeric
Wires: 0.1-1mm, e.g steel wire

Question 3

Whiskers information

Answer 3

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

Question 4

Wires information

Answer 4

Basically larger diameter fibres. Limited true composite applications. Much lower specific tensile strength and modulus than fibres or whiskers

Question 5

How do fibres lie between extremes of whiskers and wires?

Answer 5

Good properties and efficient reinforcement.
Processing is economically viable

Question 6

Cost, modulus and strength of glass, carbon and polymeric fibres

Answer 6

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

Question 7

Structure of glass in glass fibres

Answer 7

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

Question 8

Physical properties of glass in bulk or fibre

Answer 8

Bulk: transparent, hard, corrosion resistant, chemically inert.
Fibre form: stiff, strong, relatively flexible

Question 9

Isotropy of glass fibres

Answer 9

Same properties parallel to fibre direction (longitudinal) as perpendicular to fibre direction (transverse). Contrast to carbon/polymeric fibres

Question 10

What do low valency elements do in glass?

Answer 10

Low valency elements in the oxide mixture preferentially bond to the O. E.g Ca, Na, K. Reduces stiffness and strength but improves processing

Question 11

Types of glass fibres used in composites

Answer 11

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

Question 12

Glass fibre applications

Answer 12

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

Question 13

How are glass fibres made ?

Answer 13

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)

Question 14

What is diameter of fibres controlled by?

Answer 14

Viscosity of melt (dependent on composite and temperature).
Size of bushing holes.
Drawing speed.
E-glass usually 8-15μm diameter

Question 15

Processing of glass fibres

Answer 15

Continuous strand (over 100mm long) is long fibres.
Chopped strand (3-100mm long) is short fibres.
Hammer milled (30μm-3mm long) is particulate.

Question 16

Functions of a sizing

Answer 16

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

Question 17

What does a typical sizing consist of?

Answer 17

A protective polymeric film (like polyvinylacetate PVA).
A small molecule to act as lubricant.
A coupling agent to promote adhesion, such as organosilane

Question 18

Problem of glass fibres absorbing water

Answer 18

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.

Question 19

Ideal, less ideal and non-ideal situations for sizing

Answer 19

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)

Question 20

The process of glass fibre surface treatment

Answer 20

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.

Question 21

Thermoset matrices with silane

Answer 21

Silane reacts when polymer cures. For epoxy and phenolics use γ-Aminopropyl Triethoxy Silane (GAPS). For unsaturated polyesters use Methacryloxy Propyl Trimethoxy Silane

Question 22

Thermoplastic matrices with silane

Answer 22

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

Question 23

Fibre appearance with and without coupling agent

Answer 23

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