NETWORK POLYMERS

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NETWORK POLYMERS

• Three steps are needed to prepare a network
  polymer. (Prepolymer-Shaping and Curing).
• The first commercial network polymer is
  formaldehyde-based resins.
• Formaldehyde prepared by step polymerization in
  two stage.
• 1. Formation of a prepolymer of low molar mass
• 2. Prepolymer is forced to flow under pressure to
  fill a heated mould in which cross linking takes
  place. gt highly crosslinked rigid polymer in
  the shape at the mould.
• Since formaldehyde is a difunctional the
  co-reactants must have a functionality, f gt 2.

2

• The most commonly employed are

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PHENOL FORMALDEHYDE RESIN

• The OH of the phenol activates the O P position
  of the ring.

Further reaction leads to the formation of
methylene bridge and dimethylene ether link
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• OH
  OH OH
• 
  CH2 CH2
• 
  
• 
  Novolak CH2
• OH
• H2C-HO
•  
• OH
  OH
• 
  CH2OH HOH2C CH2OH
• 
  
• CH2OH
  CH2OH
• Resoles

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• There are two types of phenol-formaldehyde resin.
• 1. Resoles prepared with excess formaldehyde with
  base catalysis. The product contain many
  unreacted methylol groups which upon heating
  react to produce the net work structure.
• 2. Novolaks prepared with excess phenol and acid
  catalysis which promotes condensation reaction of
  the methylol groups. The prepolymers contain no
  methylol groups and are unable to crosslink.

• Curing achieved by the addition of hardeners
  (curing agent).

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UREA AND MELAMINE FORMALDEHYDE RESIN

• The reaction of urea and melamine formaldehyde
  resin involves the formation and condensation
  reaction of N-methylol groups.

• Same reaction is used to prepare the
  melamine-formaldehyde.

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EPOXY RESINS

• They are formed from prepolymer containing the
  epoxide end group

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• These resin are either viscous liquid or solid
  depending on n.
• Curing usually achieve by using of a
  multifunctional amines.

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• Epoxy resin are characterized as low shrinkage on
  curing, and used as adhesives, electrical
  insulators, surface coatings and matrix materials
  for fiber reinforced composites.

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POLYURETHANE NETWORKS

• Uses
• Elastomers, flexible foams and rigid foams)
• Preparation
• By the reaction of diisocyanates with branched
  polyester or Polyethers which have hydroxyl end
  groups.

• Cross linking density and molar mass control the
  flexibility of the network formed.

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Thank You

• See You Next Lecture