Chemistry Reference
In-Depth Information
sufficiently fluid to be used conveniently in surface coatings, adhesives, matrices
for glass cloth reinforcement, and in casting or encapsulation formulations. Their
molecular sizes must, however, eventually be increased by curing reactions such
as
Eq. (1-9), (1-10)
, or others to make durable final products. Such reactions
move the molecular weights of these polymers very far to the right in
Fig. 1.1
.
The general reaction shown in
Eq. (1-8)
can be modified and carried out in stages
to produce fairly high-molecular-weight polymers without terminal epoxy groups:
CH
3
x HO
C
OH
CH
3
CH
3
H
+
- HCI
C
CH
C
CH
2
O
O
CH
3
H
x
O
x Cl CH
2
C
CH
2
H
1-22
(1-11)
When x in
formula 1-22
is about 100, the polymers are known as phenoxy
resins. Although further molecular weight increase can be accomplished by reac-
tion on the pendant hydroxyls in the molecule, commercial phenoxy polymers
already have sufficient strength to be formed directly into articles. They would be
in the finite-strength region of curve B in
Fig. 1.1
. (The major current use for
these polymers is in zinc-rich coatings for steel automobile body panels.)
The curves in
Fig. 1.1
indicate that all polymer types reach about the same
strength at sufficiently high molecular weights. The sum of intermolecular forces
on an individual molecule will equal the strength of its covalent bonds if the mole-
cule is large enough. Most synthetic macromolecules have carbon
carbon,
carbon
nitrogen links in their backbones and the strengths of
these bonds do not differ very much. The ultimate strengths of polymers with
extremely high molecular weights would therefore be expected to be almost equal.
This ideal limiting strength is of more theoretical than practical interest
because a suitable balance of characteristics is more important than a single out-
standing property. Samples composed of extremely large molecules will be very
strong, but they cannot usually be dissolved or caused to flow into desired shapes.
The viscosity of a polymer depends strongly on the macromolecular size, and the
temperatures needed for molding or extrusion, for example, can exceed those at
which the materials degrade chemically. Thus, while the size of natural rubber
molecules varies with the source, samples delivered to the factory usually have
molecular weights between 500,000 and 1,000,000. The average molecular weight
and the viscosity of the rubber are reduced to tractable levels by masticating the
polymer with chemicals that promote scission of carbon
oxygen, or carbon
carbon bonds and
