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NETWORK POLYMERS

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

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NETWORK POLYMERS

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  1. 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. ===> highly crosslinked rigid polymer in the shape at the mould. • Since formaldehyde is a difunctional the co-reactants must have a functionality, f > 2.

  2. OH • Phenol (f = 3) • Urea (f = 4) NH2-C-NH2 O • Melamine (f = 6) H2N N NH2 • C C • N N • C • NH2 • The most commonly employed are

  3. OH OH • + H2C-H-O CH2OH • CH2OH CH2OH • CH2OH 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

  4. acid Phenol + formaldehyde base • OH OH OH • CH2 CH2 • Novolak CH2 • OH • + H2C-HO • OH OH • CH2OH HOH2C CH2OH • + • CH2OH CH2OH • Resoles

  5. OH OH • CH2 • 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).

  6. H2C = O + H2N-C-NH2 H2N-C-NH-CH2-OH • O O • CH2OH CH2OH • NH + HOCH2 N-CH2OH +H2O • C =O C =O • NH2 NH2 • Urea formaldehyde 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.

  7. CH2 CH O • The most important are the diglycidyl ether prepolymer. • O CH3 • (n+2)Cl-CH2 - CH-CH2 + (n+1) HO - C - -O • CH3 • Epichlorohydrin bisphenol -A O CH3 CH2 - CH-CH2 [ O - C - -O CH2 CH-CH2 ]n CH3 aqueous NaOH EPOXY RESINS • They are formed from prepolymer containing the epoxide end group

  8. CH2 CH O CH2 CH O CH2 CH O CH2 CH O OH OH CH – CH2 CH2 – CH N – R – N CH – CH2 CH2 – CH OH OH H H N – R – N H H • These resin are either viscous liquid or solid depending on n. • Curing usually achieve by using of a multifunctional amines.

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

  10. O = C = N - - CH2 - - N = C = O   • CH3 • CH2 [ O-CH2 - CH]n OH • CH2 [ O-CH2 - CH]n OH • CH3 • CH2 • CH2 • CH2 • CH2 [ O - CH2 - CH]n OH • CH3 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.

  11. Thank You See You Next Lecture

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