Polymer Matrix Composites (PMC)

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Polymer Matrix Composites (PMC)
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Reasons for the use of polymeric materials as
matrices in composites

• i. The mechanical properties of polymers are
  inadequate for structural purposes, hence
  benefits are gained by reinforcing the polymers
• Processing of PMCs need not involve high pressure
  and high temperature
• The equipment required for PMCs are much simpler

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Disadvantages of PMC

• Low maximum working temperature
• High coefficient of thermal expansion-
  dimensional instability
• Sensitivity to radiation and moisture

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Classification of Polymer Matrices

• 1. Thermoset
• 2. Thermoplastic- crystalline amorphous
• 3. Rubber

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Thermoset

• Thermoset materials are usually liquid or
  malleable prior to curing, and designed to be
  molded into their final form
• has the property of undergoing a chemical
  reaction by the action of heat, catalyst,
  ultraviolet light, etc., to become a relatively
  insoluble and infusible substance.
• They develop a well-bonded three-dimensional
  structure upon curing. Once hardened or
  cross-linked, they will decompose rather than
  melt.
• A thermoset material cannot be melted and
  re-shaped after it is cured.
• Thermoset materials are generally stronger than
  thermoplastic materials due to this 3-D network
  of bonds, and are also better suited to
  high-temperature applications up to the
  decomposition temperature of the material.

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• Some examples of Thermosets are
• Polyester resin (used in glass-reinforced
  plastics/fibreglass (GRP))
• Epoxy resin (used as an adhesive and in fibre
  reinforced plastics such as glass reinforced
  plastic and graphite-reinforced plastic)
• Polyimides used in printed circuit boards and in
  body parts of modern airplanes
• Vulcanized rubber
• Phenolic

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• Polyester
• polyester resins are generally copolymers of
  unsaturated polyesters with styrene
• Styrene is the crosslinking monomer and curing is
  effected by the use of an organic peroxide
  initiator which generates free radicals leading
  to the formation of 3-D network
• Are relatively inexpensive and have low
  viscosities, which is beneficial in many
  fabrication processes

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• Epoxy
• is a thermosetting epoxide polymer that cures
  (polymerizes and crosslinks) when mixed with a
  catalyzing agent or "hardener".
• More expensive and more viscous than polyester
• Epoxies have a major advantage in that they are
  usually cured in two or more stages. This allows
  preforms to be pre-impregnated with the epoxy in
  a partially cured state
• The pre-preg may be stores, before moulded into
  the final shape and then cured

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• Generally start as linear low molecular weight
  polymer, curing agents such as polyamides
  polyamines were used as curing agents
• The mechanical properties depend on the
  particular resin system and the curing generally
  epoxies are stiffer and stronger, but brittle
  than polyester
• Epoxies maintain their properties to higher
  temperature than polyester

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Phenolic

• Produced by reacting phenol and formaldehyde,
  characteristics of the resin product depending on
  the proportions of the reactant and catalyst
• Good fire resistance
• An undesirable feature of phenolic resin-
  volatile by-product are evolved during curing
  hence high pressures are often necessary in
  composite production

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Polyimides

• More expensive, less widely used than polyester
  and epoxies, but can withstand relatively high
  service temperature
• The presence of ring structure, results in high
  stiffness, low CTE, and service temperature as
  high as 425C for several hours
• Like other thermoset, polyimides are brittle

where R' and R? are two carbon atoms of an
aromatic ring.
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Thermoplastic

• is a plastic that melts to a liquid when heated
  and freezes to a brittle, very glassy state when
  cooled sufficiently.
• Most thermoplastics are high molecular weight
  polymers whose chains associate through weak van
  der Waals forces (polyethylene) stronger
  dipole-dipole interactions and hydrogen bonding
  (nylon) or even stacking of aromatic rings
  (polystyrene).
• The bondings are easily broken by the cobined
  action of thermal activation and applied stress,
  thats why thermoplastics flow at elevated
  temperature
• unlike thermosetting polymers, thermoplastic can
  be remelted and remolded.

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• Thermoplastics can go through melting/freezing
  cycles repeatedly and the fact that they can be
  reshaped upon reheating gives them their name
• Some thermoplastics normally do not crystallize
  they are termed "amorphous" plastics and are
  useful at temperatures below the Tg. They are
  frequently used in applications where clarity is
  important. Some typical examples of amorphous
  thermoplastics are PMMA, PS and PC.
• Generally, amorphous thermoplastics are less
  chemically resistant

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• Depends on the structure of the thermoplastics,
  some of the polymeric structure can be folded to
  form crystalline regions, will crystallize to a
  certain extent and are called "semi-crystalline"
  for this reason.
• Typical semi-crystalline thermoplastics are PE,
  PP, PBT and PET.
• Semi-crystalline thermoplastics are more
  resistant to solvents and other chemicals. If the
  crystallites are larger than the wavelength of
  light, the thermoplastic is hazy or opaque.
• Why HDPE exhibits higher cystallinity than LDPE?

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• Polyetheretherketone (PEEK) is a semicrystalline
  polymer having 20-40 crystallinity.
• It has a rigid backbones, which gives high Tg and
  Tm (Tg 143C and Tm 343C). Can be employed at
  temperature as high as 230C)
• It is possible to blend two or more polymers to
  obtain a multi-phase product with enhances
  properties

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Comparison of typical ranges of property values
for thermoset and thermoplastics

• Properties t/set t/plastic
• Youngs Modulus (GPa)1.3-6.0 1.0-4.8
• Tensile strength(MPa) 20-180 40-190
• Max service temp.(ºC) 50-450 25-230
• Fracture toughness,KIc 0.5-1.0 1.5-6.0
• (MPa1/2)

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Thermoplastics are expected to receive attention
compared to thermoset due to

• Ease of processing
• Can be recycled
• No specific storage
• Good fracture modulus

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Rubber

• Common characteristics
• Large elastic elongation (i.e. 200)
• Can be stretched and then immediately return to
  their original length when the load was released
• Elastomers are sometimes called rubber or rubbery
  materials
• The term elastomer is often used interchangeably
  with the term rubber
• Natural rubber is obtained from latex from Hevea
  Brasiliensis tree which consists of 98
  poliisoprena
• Synthetic rubber is commonly produced from
  butadiene, spt styrene-butadiene (SBR) dan
  nitrile-butadiene (NBR)

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• To achieve properties suitable for structural
  purposed, most rubbers have to be vulcanized the
  long chain rubber have to be crosslinked
• The crosslinking agent in vulcanization is
  commonly sulphur, and the stiffness and strength
  increases with the number of crosslinks

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PREPREG

• It is short form for pre-impregnation material
• It is a semifinished product
• It will be used in next processing technique to
  obtain a finish product
• It can be produced from thermoset or
  thermoplastic matrix

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Thermoplastic prepregs are getting attention due
to

• Easy storage
• High toughness
• Fast easy processing
• Can be recycled