Lecture 7: Raw Materials for part fab 3: Resin 1 (matrix materials)

Function of matrix

-Transfers the load(s) to reinforcement

-Protects reinforcement from environmental effects

Types of matrix materials

-Polymers aka resins, most commonly used (thermosets and thermoplastics

-Ceramics and metals both used for high temp applications

What composite properties are dominated by the matrix?

-Thermal properties

-Resistance to solvents, water, gases, fire, and UV

-electrical properties

-optical properties

-Aka, matrix mostly deals with environmental properties, fiber deals with strength/stiffness

What happens to thermoplastics matrix if temp increases?

it could melt

What happens to thermoset matrix if temperature increases?

It could char

Glass transition temperature

Tg, temperature at which a polymeric material changes from a rigid glassy solid into a softer, semi-flexible material

+Cross-links are no longer locked in position

Polymerization

The process of joining monomers together to form polymers

+Split up into Addition polymerization and Condensation polymerization

Addition polymerization

-Chain growth

-A molecule, usually a peroxide, is activated- often by heat- and creates a highly reactive free radical

-Free radical have an unpaired electron

-Unpaired electron reacts with a monomer (attacking to it), making it reactive itself, so another monomer attaches

-Process repeats until terminating agent added

Condensation polymerization

-Two monomers present (M1 and M2)

-Two active end groups present

-Active end group of one monomer must react with active end group of other monomer (M1 and M2 not reacting with one another, but with a different pair of M1 and M2).

-This forms linkage element L and by-product condensate C

-This large polymer with now 4 polymers must now find another polymer with M1 and M2 in it to link again and grow

Crosslinking

-Chemical ties between 2 or more polymer chains

-More of it gives high strength and rigidity

Thermosets vs Thermoplastics: Thermosets

-Both composed of molecular chains

-Molecular chains are interconnected with chemical bonds (crosslinking)

-More rigid

-Higher temp performance

-Require a cure to achieve crosslinking

Thermosets vs. Thermoplastics: Thermoplastics

-Both composed of molecular chains

-Higher toughness

-Can be re-melted (no crosslinking)

-Are heated, molded to desired shape then cooled

Toughness

Measure of a material’s energy absorption capacity, before fracturing at the presence of crack and discontinuities

Do thermosetting resins have a high or low toughness and why?

Low, because of their rigid, cross-linked and glass polymer structure

Toughening approaches for thermosetting resins

-Network alteration

-Rubber elastomer second-phase toughening

-Thermoplastic elastomer toughening

-Interlayering

Network alteration toughening approach

-Lower cross-link density, turning thermoset more like thermoplastic

-Decreases glass transition temp Tg and resin modulus (bad effects)

Rubber elastomer second-phase toughening approach

Rubber particles help to bluny crack growth, promotes greater plastic flow at crack tip

thermoplastic elastomeric toughening toughening approach

Add thermoplastics to resin, this increases toughening while cross-linked thermoset retains properties

Interlayering toughening approach

-Add thin layer of a tough ductile resin between each prepreg ply

-Helps reduce interlaminar shear and normal forces

What materials can you add to resin and reinforcement?

-Tougheners

-Flame retardants

-Fillers (ground solids added to resin reduce cost)

-Surface agents (can help with foaming and wetting)

Polyester resins

-Most widely used thermoset resin

-Low cost

-Shrinks during cure

-Low temperature, mechanical, and weather properties means limited in use for high performance composites

Polyester resins uses

-Commercial applications (boats, corrugated sheets, golf carts, etc)

-Not many high performance composites

Polyester resin chemistry

-Cured through addition reaction involving crosslinking at unsaturated C=C double bonds

-Consists of three ingredients: polyester, crosslinking agent (properties depend on this), and initiatior

-Can be formulated to cure at (T)room and (T)elevated

Epoxy Resins

-Second most widely used thermoset resin

-Good thermal stability, strength, and adhesion

-Low shrinkage

-Versatility in processing

-Tcure and Pcure around 350 F and 100 psi

Epoxy chemistry

-Molecular chains of thermosets interconnected with crosslinks

-Crosslinking occurs at three-member epoxy rings, which are “active sites”

-Usually more viscous then polyesters

Epoxy cure first step

Resin flow:

-Temperature is ramped up and held

-This allows resin to flow and volatiles to escape

Epoxy cure second step

Polymerization:

-Temperature is increased again and held

-Resin undergoes polymerization → gelation → crosslinking

Why is pressure applied to epoxy cure process?

100 psi of pressure is applied to provide ply compaction and to reduce voids. Vacuum pressure is applied during temperature increase between resin flow and polymerization stages, and autoclave pressure is held throughout polymerization stage

Epoxy pot life

Working time of resin during fiber we out and other pre-cure activities such as bagging

Shelf life

Time resin can be stored w/o reacting

Post curing or annealing

When part is heated for a time, advancing cross linking and improving mechanical properties

How are epoxy resins different then polyester resins?

-Processing is more difficult

-Fiber wet out more difficult (higher viscosity)

-Cure rates longer

-Less shrinkage

-Properties generally better