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

3. 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

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

5. 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).

6. 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-NH H2N-C-NH-CH2-OH
O O
CH2OH CH2OH
NH HOCH N-CH2OH +H2O
C =O C =O
NH NH2
Urea formaldehyde
Same reaction is used to prepare the melamine-formaldehyde.

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

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

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. 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 CH 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.

11. Thank You
See You Next Lecture