Reinforcements

Reinforcements

A reinforcement is the strong, stiff

integral component of a composite

which is incorporated into the matrix

to achieve desired properties.

The term ‘reinforcement’ implies

some property enhancement

Forms of Reinforcements Fibers

Discontinuous fibres: L/D ratio<100 Continuous fiber cross-section can be circular, square or

hexagonal Textile Structure

Unidirectional Woven Braid

Particulate Particles Flakes

Materials for Fibres

Glass – most widely used filament Carbon – high elastic modulus Boron – very high elastic modulus Polymers - Kevlar Ceramics – SiC and Al2O3 Metals - steel

Composite Parameters

For a given matrix/dispersed phase system:

Concentration Size Shape Distribution Orientation

Concentration

SizeShape

Distribution Orientation

ParametersParameters

Particles and Flakes

A second common shape of imbedded phase is particulate, ranging in size from nanoscopic to macroscopic

In most of the cases, the distribution of particles in the composite matrix is random, and therefore strength and other properties of the composite material are usually isotropic.

Parameters Size

Shape （ aspect ratio） Concentratio

n

Dispersion

OrientationIsotropic or anisotropic？

Particle-carbon black

Flake-graphite nanoplatelet

Flakes are basically two‑dimensional particles

Nano-size, 2D oriented-graphene

Types of Fibers

The fibers are divided into two main groups: Glass fibers: There are many different kinds of

glass, ranging from ordinary bottle glass to high purity quartz glass. All of these glasses can be made into fibers. Each offers its own set of properties.

Advanced fibers: These materials offer high strength and high stiffness at low weight. Boron, silicon, carbide and graphite fibers are in this category. So are the aramids, a group of plastic fibers of the polyamide (nylon) family.

Fibers - Glass Most widely used fiber Applications: piping, tanks, boats, sporting goods Fiberglass properties vary somewhat according to the type of glass

used. However, glass in general has several well–known properties that contribute to its great usefulness as a reinforcing agent:

Tensile strength Chemical resistance Moisture resistance Thermal properties Electrical properties Low cost

Disadvantages Relatively low strength High elongation Moderate strength and weight

Fibers - Glass There are three main types of glass used in fiberglass:

E-glass: Most common glass used for reinforcement, especially for PCB laminates of electronic applications. High strength and good resistance to chemical attach

C-glass: Formulated for greater chemical corrosion resistance

S-glass: Stiffer (20% higher modulus) and stronger formulation for structural composite application. Higher creep rupture resistance and higher temperature application. Contain a higher alumina content and are most difficult to process-hence most costly

Fibers - Glass Glass structure

Three-dimensional polyhedral network containing oxygen atoms around a silicon atom with strong covalent bonds.

Molecular structure of glass

Fibers - Glass Format

Fibre diameter: this may be varied between 10-20m. Finer filaments give better properties, but uneconomical to produce fibres of less than 10m.

Fibres are produced in bundles or strands which are treated with size as they are drawn. Several strands are put together to form roving.

Fibers - Glass Fibre Production

Glasses are produced from minerals (sand, lime, borax, etc.) based on silica (SiO2) with addition of oxides of Ca,

B, Na, Fe, Al. Continuous drawing from molten glass at high speed

(~100m/s) through holes in a platinum alloy bushing

Surface treatment To protect glass fibres from abrasion and mechanical

damage, fibres are treated with water-based size immediately after forming.

For glass fibres intended for subsequently textile operation such as weaving, spooling, the size contain a mixture of starch and lubricant. The size also contain a coupling agent to improve adhesion with polymer resin.

Fibers – Aramid (Kevlar)

Applications: High performance replacement for glass fiber Aerospace and military applications Examples ： Armor, bullet vest, protective

clothing, sporting goods Advantages:

Higher strength and lighter than glass More ductile than carbon High use temperature compared to other

organic fibres High chemical resistance

Fibers – Aramid

Fibre Production Poly-phenylene-terephtahlamide (PPTA) is

prepared by a condensation polymerisation of exact ratio of phenylenediamine with terephthaloyl chloride

The PPTA fibres are extruded at about 80°C in the form of liquid-crystalline PPTA-H2SO4 dopes from a spinneret

The resulting yarn is washed with water, neutralised with NaOH, subsequently dried under tension on rollers at 65-130°C (Kevlar 29)

Further drawing at 550 ° C for 1-6 sec produces yarns of higher modulus (Kevlar 49)

Fibers – Aramid

Fibers – Aramid Fibre

Structure The molecules form

a planar array with weak interchain hydrogen bonding: easily fibrillated upon fracture

Radially arranged axially pleated crystalline supramolecular sheets