der Waals Bond
The van der Waal bonds occur to some extent in all materials
but are particularly important in plastics and polymers. These
materials are made up of a long string molecules consisting of
carbon atoms covalently bonded with other atoms, such as hydrogen,
nitrogen, oxygen, fluorine. The covalent bonds within the molecules
are very strong and rupture only under extreme conditions. The
bonds between the molecules that allow sliding and rupture to
occur are called van der Waal forces.
ionic and covalent bonds are present, there is some imbalance
in the electrical charge of the molecule. Take water as an example.
Research has determined the hydrogen atoms are bonded to the oxygen
atoms at an angle of 104.5°. This angle produces a positive
polarity at the hydrogen-rich end of the molecule and a negative
polarity at the other end. A result of this charge imbalance is
that water molecules are attracted to each other. This is the
force that holds the molecules together in a drop of water.
This same concept can be carried on to plastics, except that
as molecules become larger, the van der Waal forces between molecules
also increases. For example, in polyethylene the molecules are
composed of hydrogen and carbon atoms in the same ratio as ethylene
gas. But there are more of each type of atom in the polyethylene
molecules and as the number of atoms in a molecule increases,
the matter passes from a gas to a liquid and finally to a solid.
Polymers are often classified as being either a thermoplastic
or a thermosetting material. Thermoplastic materials can be easily
remelted for forming or recycling and thermosetting material cannot
be easily remelted. In thermoplastic materials consist of long
chainlike molecules. Heat can be used to break the van der Waal
forces between the molecules and change the form of the material
from a solid to a liquid. By contrast, thermosetting materials
have a three-dimensional network of covalent bonds. These bonds
cannot be easily broken by heating and, therefore, can not be
remelted and formed as easily as thermoplastics.