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.

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

PHENOL FORMALDEHYDE RESIN The OH of the phenol activates the O & P position of the ring. OH OH + H2C-H-O CH2OH   CH2OH CH2OH CH2OH Further reaction leads to the formation of methylene bridge and dimethylene ether link

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

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. OH OH CH2 Curing achieved by the addition of hardeners (curing agent).

UREA AND MELAMINE FORMALDEHYDE RESIN The reaction of urea and melamine formaldehyde resin involves the formation and condensation reaction of N-methylol groups. 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 Same reaction is used to prepare the melamine-formaldehyde.

EPOXY RESINS They are formed from prepolymer containing the epoxide end group 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

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

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

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. O = C = N - - CH2 - - N = C = O     CH3 CH2 [ O-CH2 - CH]n OH CH2 CH2 [ O - CH2 - CH]n OH CH3  Cross linking density and molar mass control the flexibility of the network formed.

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