02 Composite Resins (2)

Composite Resins

A. Brent Strong, PhD

August 2003

What we will discuss

• Basics of composites• Matrix properties in general• Basics of polymers• Polyesters• Epoxies• Vinyl esters• Phenolics• Specialty thermosets• Thermoplastics• Properties and Testing

Composites

• Very important in our 21st Century world

• Listed as one of the top 10 greatest engineering developments of last quarter of the 20th Century– Others: Apollo moon landing, unmanned

satellites, microprocessor, CAD, CT scan, jumbo jet, lasers, fiber-optic communication, genetic engineering

What are composites?

• Solid materials composed of a binder or matrix that surrounds and holds in place reinforcements.– The material consists of two (or more) phases– One of the phases is continuous (the matrix)– The other phase is discontinuous (the

reinforcement)– The phases can be thought of as a group of

islands (discontinuous) in a sea (continuous)

Matrix purposes

• Hold the reinforcements together • Give shape to the object• Transfer loads to the reinforcements

Protect the reinforcements– Heat– Weather– Flammability– Impacts– Solvent/water

Reinforcement purposes

• Carry the load (most mechanical properties)

• Give directionality of some properties (optional)

Types of composites

• Engineering– Fiberglass reinforced– Matrix of unsaturated polyesters and vinyl esters or

common engineering thermoplastics– Uses: tub/shower, boats, automotive, pipes,

architectural, etc.

• Advanced– Carbon fiber, aramid fiber, or other high performance

reinforcements– Matrix of epoxies and specialty resins– Uses: aerospace, sporting goods, specialty

Basic Materials – a primer

• Three types of solid materials– Ceramics– Metals– Polymers

• These differ, at the most fundamental level, in the types of bonds between the atoms

Periodic Table of the ElementsMetals Non-Metals

Ceramics(Ionic Bonds)

Metals(Metallic Bonds)

Polymers(Covalent Bonds)

Polymers

• Polymers can be natural (like wood, cotton, wool, leather)

• Polymers can be man-made (plastics)

• Polymers can be easily shaped (molded)

• Polymers have other advantages over ceramics and metals

Polymers

• Made from small molecules (monomers) which are linked together– “mono” means one– “mer” means unit

• The linked monomers form a chain-like structure called a polymer– “poly” means many

• The links are the covalent bonds between the atoms

Polymers

M

M M

M

M

M

M

M

M

M

M

M

M

M

M

M

M

Monomers

Polymer

CovalentBonds

Polymers

H

H

C

C

H

H

C

H

H

C

H

H

C

H

H C

H

H

H

H

C

CH

H

Monomer

Polymer

H

H

C

C

H

H

C

H

H

C

H

H

C

H

H C

H

H

H

H

C

CH

H

Polymer

Polymers

• Many millions of chains exist in the typical polymeric part

• The chains are intertwined – Like a mass of spaghetti

What determines physical, chemical and mechanical properties of

materials?

• Molecular shape and movement– Crystallinity– Thermal transitions and crosslinks – Aromaticity– Pendant groups– Chemical nature of the backbone

• Bonding between matrix and reinforcements • Polarity (like attracts like)

Polymers − Physical structure

• Amorphous − Polymers that have no regular internal structure (just like the spaghetti)

• Semi-crystalline − Polymers that have some internal structure (regular packing)– Semi-crystalline polymers vary in the amount of

packing (crystallinity)– Semi-crystalline polymers with high percentage of

packing are sometimes called crystalline – No polymers are 100% crystalline

Amorphous and Crystalline

Amorphous(random entanglement)

Semi-Crystalline or Crystalline(regular packing)

CrystalRegions

Polymers − melting, molecular weight, crosslinking

• Polymers are classified into two groups depending on whether they are crosslinked– Thermoplastics (not crosslinked)– Thermosets (crosslinked)

• Crosslinks are covalent bonds that link between the polymer chains

• When crosslinking occurs, the polymers will no longer melt– When heated to a high temperature, they burn or char

Thermoplastics• Thermoplastics are not crosslinked and so they

will melt• Thermoplastics are processed (molded) as

molten liquids • Thermoplastics are cooled to solidify• Thermoplastics can be re-melted repeatedly• Kitchen example:

candy

• Examples of thermoplastics: polyethylene, polystyrene, nylon, polycarbonate, acrylic, Teflon®, PET (thermoplastic polyester)

Thermosets

• Thermosets are crosslinked and do not melt • Crosslinking is sometimes called curing• Thermosets are processed as room

temperature liquids • Thermosets are heated to solidify• Kitchen example:

cake

• Examples of thermosets: polyesters, vinyl esters, epoxies, phenolics, polyimides, silicones

Thermal Transitions• Heat Distortion Temperature (HDT)

• Glass transition temperature (Tg)

• Melting point (Tm)

• Decomposition temperature (Td)

Semi-crystalline thermoplastic

Thermoset

Temperature

HDT TgTm Td

HDT TgTd

(Tm)

Hard, stiff Leathery Liquid

Degraded, Char

Degraded

Hard, stiff Semi-rigid

The Great Dilemma in Polymers

• Polymers must have good properties– Good properties are

favored by high molecular weight

• Polymers must have good processing– Good processing is

favored by low molecular weight

Molecular Weight

Me

ch

an

ica

l Pro

pe

rtie

s

Molecular Weight

Ea

se

of

Pro

ce

ss

ing

The Great Dilemma In Polymers

• Thermoplastics meet the dilemma by compromise– High enough molecular weight to get adequate

properties– Low enough molecular weight to process OK

• Thermosets meet the dilemma by crosslinking– Low molecular weight initially (for wetout and

processing) followed by curing to increase molecular weight

– No compromise is required

Polymers − Molecular shape• Aromatic − Contains the benzene group

(sometimes called phenyl group)– Named aromatic because it tends to have a strong

smell (like styrene)– Increases stiffness– Increases strength– Increases non-flammability

• Aliphatic −Does not contain the benzene group– Increases flexibility– Increases toughness– Increases weatherability

( )n...C C C C...CC...C C...

CC

CC

C

C

C...OC

C

C

C

C

CC

C

C

C

C

C

C

OCC C

O

C......C

H

NC

C

C

C

C

C C C

C

C

C

C

C

O

OH OH

OHOHOH

C

C

CC

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

....C C...

C.......C

Polyethylene (no aromatic)

Polystyrene (pendant aromatic)

Epoxy (aromatic backbone)

Kevlararomatic backbone)

Phenolic (aromatic network)

Bonding

• Bonding is strongest when electrons are:– Transferred (ceramics)– Shared by many atoms (metals)– Shared by two atoms (covalent)

• Weak bonding occurs without electrons being transferred or shared

• These weak bonds depend upon polarity

Polarity

N S S N

S N S N

...C C O

O

C...

-

-

O

HH

O

HH

C O --

C O

Polyester is attacked by water molecules

Bonding in polymers by polarity

• Polar areas on the polymers attract other polar areas on other molecules.– Opposite charges attract– The most electronegative atoms are those

that cause polarity– The electronegative atoms are: F, O, N, Cl– These are all in the upper right corner of the

periodic table

• Non-polar areas attract other non-polar areas

Bonding

Fiberglass

Sizing (alkylsilane)

...O Si O Si O...

OH

OH

OH

OH

....C C O C C C

O

C C...

CH3 Si O C C C

CH3

C C C C...

CH3 Nonpolar regions (weak attraction)

-

-

− A highly polar molecule

− Largely non-polarPolyester

− Mixed polar/non-polar

Thank you

A. Brent Strong