Page 205 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 7 Polymers: Structure, General Properties, and Applications

7.4 Thermosetting Plastics

When the long-chain molecules in a polymer are cross-linked in a three-dimensional
arrangement, the structure in effect becomes one giant molecule with strong cova-
lent bonds. These polymers are called thermosetting polymers or thermosets, be-
cause (during polymerization) the network is completed and the shape of the part is
permanently set. This curing (cross-linking) reaction, unlike that of thermoplastics,
is irreversible. The response of a thermosetting plastic to a sufficiently elevated tem-
perature can be likened to what happens in the baking of a cake or in the boiling of
an egg: Cnce the cake is baked and cooled, or the egg boiled and cooled, reheating
it will not change its shape. Some thermosets (such as epoxy, polyester, and ure-
thane) cure at room temperature, because the heat produced by the exothermic reac-
tion is sufficient to cure the plastic.
The polymerization process for thermosets generally takes place in two stages.
The first occurs at the chemical plant, where the molecules are partially polymerized
into linear chains. The second stage occurs during the final step of part production,
where cross-linking is completed under heat and pressure during the molding and
shaping of the part (Chapter 19).
Thermosetting polymers do not have a sharply defined glass-transition tem-
perature. Because of the nature of the bonds, the strength and hardness of a ther-
moset (unlike those of thermoplastics) are not affected by temperature or by rate of
deformation. If the temperature is increased sufficiently, the thermosetting polymer
instead begins to burn up, degrade, and char. Thermosets generally possess better
mechanical, thermal, and chemical properties; electrical resistance; and dimensional
stability than do thermoplastics. A typical and common thermoset is phenolic,
which is a product of the reaction between phenol and formaldehyde. Common
products made from this polymer are the handles and knobs on cooking pots and
pans and components of light switches and outlets.

7.5 Additives in Plastics
In order to impart certain specific properties, polymers usually are compounded
with additives. These additives modify and improve certain characteristics of the
polymer, such as stiffness, strength, color, weatherability, flamrnability, arc resist-
ance (for electrical applications), and ease of subsequent processing.

° Plasticizers are added to polymers to impart flexibility and softness by lower-
ing their glass-transition temperature. Plasticizers are low-molecular-weight
solvents with high boiling points (nonvolatile); they reduce the strength of the
secondary bonds between the long-chain molecules and, thus, make the poly-
mer flexible and soft. The most common use of plasticizers is in polyvinyl chlo-
ride (PVC), which remains flexible during its many uses; other applications are
in thin sheets, films, tubing, shower curtains, and clothing materials.
° Most polymers are affected adversely by ultraviolet radiation (such as from sun-
light) and by oxygen; they weaken and break the primary bonds and cause the
scission (splitting) of the long-chain molecules; the polymer then degrades and
becomes stiff and brittle. On the other hand, degradation may be beneficial, as
in the disposal of plastic objects by subjecting them to environmental attack (see
also Section 7.8). A typical example of protection against ultraviolet radiation is
the compounding of certain plastics and rubber with carbon black (soot). The
carbon black absorbs a high percentage of the ultraviolet radiation. Protection