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)
Melamine (f
NH2-C-NH2
O
= 6)
H2 N
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

CH2


OH
CH2
acid


OH
Novolak
CH2
OH
+ H2C-HO



Phenol +
formaldehyde
base
OH
OH
CH2OH HOH2C



CH2OH
+
CH2OH
Resoles
CH2OH

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
OH
OH
CH2
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 Nmethylol groups.
 H2C
= O + H2N-C-NH2

O
CH2OH
NH
+
HOCH2
C =O
NH2

Same
H2N-C-NH-CH2-OH
O
CH2OH
N-CH2OH +H2O
C =O
NH2
Urea formaldehyde
reaction is used to prepare the melamineformaldehyde.
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
aqueous O
CH2 - CH-CH2 [ O
NaOH
CH3
-CCH3
-O CH2 CH-CH2 ]n

These resin are either viscous liquid or solid depending
on n.
 Curing usually achieve by using of a multifunctional
amines. CH CH
CH
CH
2
2
H
O
H
O
N–R–N
O
H
CH2
CH
CH
OH
CH2
O
H
OH
CH – CH2
CH2 – CH
N–R–N
CH – CH2
OH
CH2 – CH
OH
Epoxy
resin are
shrinkage on curing,
electrical insulators,
matrix materials
composites.
characterized as low
and used as adhesives,
surface coatings and
for fiber reinforced
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 [ O-CH2 - CH]n OH

CH3
CH2
CH2
CH2
CH2 [ O - CH2 - CH]n OH

CH3
Cross
linking density and molar mass control the flexibility of the network
formed.
Thank You
See You Next Lecture