02 Composite Resins (2)

Composite ResinsA. 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 ElementsMetalsCeramics (Ionic Bonds)

Non-MetalsPolymers (Covalent Bonds)

Metals (Metallic 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

M M

MonomersM M M M M

PolymersMM

M

M

M M

M

Covalent Bonds

M M M

Polymer

PolymersHH

H

HH

H

C

H

CH

C

H

C

CHH

CHH

H

CH

CH

Monomer

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

Crystal Regions

Amorphous (random entanglement)

Semi-Crystalline or Crystalline (regular packing)

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)Tm Liquid Tg Semi-rigid Td Degraded Td

Semi-crystalline thermoplastic HDT Tg Hard, stiff Thermoset Hard, stiff Leathery HDT

(Tm)

Degraded, Char

Temperature

The Great Dilemma in Polymers Polymers must have good properties Good properties are favored by high molecular weightMechanical Properties

Polymers must have good processing Good processing is favored by low molecular weightEase of Processing

Molecular Weight

Molecular Weight

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

...C C C C...

...C ( C

C )nC...

C C C C C C

Polyethylene (no aromatic)C C O C C O C C C C C C C C C C O C...

Polystyrene (pendant aromatic)

C C Epoxy (aromatic backbone)

HOC C ...C C C C C N H O C C C C

HO 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

OH C C C C C OH C C C C

....CC C C C...

C C

C C C C C

C...

Kevlar (aromatic backbone)

HO C C ....C

C...

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

PolarityN S S N S S N N

d+

C O

d-

d+

C O

d-

O d+ ...C C O C...d-

d-

d+

O H

H

O H

H

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

BondingOH OH d+ ...O Si O Si O... OH d-OHCH3 H3C Si O C C C C C C C... CH3dNonpolar regions (weak attraction) d+ Fiberglass A highly polar molecule Sizing (alkylsilane) Mixed polar/non-polar

O C C...

....C C O C C Cd+ Polyester Largely non-polar

Thank youA. Brent Strong