Composite Resins

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Composite Resins

• A. Brent Strong, PhD
• August 2003

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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

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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

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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)

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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

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Reinforcement purposes

• Carry the load (most mechanical properties)
• Give directionality of some properties (optional)

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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

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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

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Periodic Table of the Elements
Metals
Non-Metals
Ceramics (Ionic Bonds)
Polymers (Covalent Bonds)
Metals (Metallic Bonds)
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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

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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

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Polymers
Monomers
M
M
M
M
M
M
M
M
M
M
M
Covalent Bonds
M
M
M
Polymer
M
M
M
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Polymers
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Polymers

• Many millions of chains exist in the typical
  polymeric part
• The chains are intertwined
• Like a mass of spaghetti

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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)

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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

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Amorphous and Crystalline
Crystal Regions
Amorphous (random entanglement)
Semi-Crystalline or Crystalline (regular packing)
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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

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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)

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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

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Thermal Transitions

• Heat Distortion Temperature (HDT)
• Glass transition temperature (Tg)
• Melting point (Tm)
• Decomposition temperature (Td)

Semi-crystalline thermoplastic
Tm
Td
HDT
Tg
Hard, stiff
Leathery
Degraded
Liquid
(Tm)
Td
HDT
Tg
Thermoset
Degraded, Char
Hard, stiff
Semi-rigid
Temperature
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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

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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

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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

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Polyethylene (no aromatic)
Polystyrene (pendant aromatic)
Epoxy (aromatic backbone)
Kevlar (aromatic backbone)
Phenolic (aromatic network)
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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

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Polarity
N S
S N
S N
S N
d
d-
d
d
d-
d-
d
d-
Polyester is attacked by water molecules
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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

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Bonding
Fiberglass
- A highly polar molecule
d
Sizing (alkylsilane)
- Mixed polar/non-polar
d-
d
Nonpolar regions (weak attraction)
d-
d
Polyester
- Largely non-polar
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Thank you

• A. Brent Strong