Composites- Fibre Phase 2 (Polymeric Fibres)

Question 1

What do high performance polymeric fibres rely on?

Answer 1

Alignment of the polymer chains with the fibre axis

Question 2

Two approaches to obtain high modulus

Answer 2

Extend chains when polymer is flexible: UHMWPE, dyneema, spectra are examples.
Align chains when polymer is rigid: para-aramids and Kevlar are examples

Question 3

Properties of UHMWPE fibres

Answer 3

Ultra high molecular weight polyethylene. Remarkably high specific modulus and specific strength. Very low density, tough and chemically inert. Low maximum usage temperature.
Zig-zag structure aligns very well leading to high crystallinity

Question 4

Manufacturing UHMWPE fibres

Answer 4

Gel-spinning hot-drawing method. Dissolve PE in 0.5-5% decalin solution at 150C to remove most of chain entanglements. Spin fibres (extrude) to form the fibre and remove some entanglements and provide some orientation. Draw fibres under tension at 155C to provide most of orientation

Question 5

Properties of aramid fibres

Answer 5

Class of aromatic polyamides. Have high specific modulus and specific strength. Very low density and very tough. Poor in compression, degrade easily in UV light and absorb moisture readily.
Backbone para-phenyl rings add strength and stiffness. H bonds between adjacent chains increase crystallinity

Question 6

Manufacturing aramid fibres

Answer 6

Dry-jet wet-spinning method. Dissolve aramid in 15-20% concentration H2SO4 solution to form liquid crystal solution which allows rigid polymer chains to align. Spin fibres (extrude) at 80C to form the fibre. Drawing needed only to reorient on exiting the spinneret in the air gap.

Question 7

Composite applications for UHMWPE

Answer 7

Same for dyneema, spectra. Low weight, very high specific energy absorption. Ballistic protection for personnel, vehicles, buildings, etc

Question 8

Composite applications for aramids

Answer 8

Kevlar and twaron. Fabrics in aircraft, pressure vessels, sporting goods. Ballistics too but often non-composite applications

Question 9

Why is surface treatment of polymeric fibres more challenging than with glass or carbon?

Answer 9

Fibres contain highly aligned, stable polymer chains which depend on strong inter-chain interactions (e.g H bonds). Therefore it’s difficult to bond to the surfaces without compromising the fibres.

Question 10

Oxidative plasma surface treatment for PE fibres

Answer 10

Can improve adhesion but reduces strength. A short treatment oxidises the surface and improves wetability. An intermediate treatment develops crosslinking at the surface and eliminates any weaker material. A long treatment causes a pitted surface that can be penetrated by the resin. Development of sizes to chemically treat polymeric surfaces is a current research area