Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 125
Figure 4.1 The arrangement of atoms that make up a
polymer fundamentally affects the way in which
adjacent molecules can fit together, which in turn
radically affects the mechanical and other properties of
the molecule. Some examples of polymer terminology:
(a) linear; (b) branched; (c) crosslinked; (d)
homopolymer; (e) homopolymer; (f ) alternating copolymer;
(g) random co-polymer; (h) block co-polymer;
(i) graft co-polymer; (j) atactic form of polypropylene –
has a random arrangement of units; (k) isotactic form of
polypropylene – all units have a spatially identical
arrangement of atoms
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
(k)
dom and block. Polymer terminology is illustrated
in Figure 4.1.
Poly(vinyl acetate) (PVAC), one of the simplest
polymers, consists of up to 20 000 repeating
units of vinyl acetate joined together. This
can be expressed by writing the chemical formula
as [CH 2CHOCOCH 3] n where n specifies
the number of monomer units in the chain and
is called the degree of polymerization (DP).
Synthetic polymers are formed by two major
classes of chemical reactions, addition reactions
and condensation reactions (Hall, 1981).
Because of the nature of the reactions by
which polymers are formed, not all molecules
of a given compound are identical in size. Even
under controlled conditions of manufacture
there will be a range of molecular weights and
the DP quoted for any product will be an average.
The plastic may also contain some unreacted
monomer and small quantities of other
impurities. All these factors are important
because the mechanical, chemical and ageing
properties depend to a considerable extent on
them (Brydson, 1991).
The simplest macromolecules are composed
of fundamental units linked end to end in
chains with relatively few branches or side
chains. The links or primary (covalent) bonds
along each chain are strong compared to the
secondary forces (e.g. Van der Waals forces
and hydrogen bonding) holding adjoining
chains together and it is therefore possible to
separate the molecules from each other. Such
materials are usually both soluble and fusible,
that is they are thermoplastic, becoming soft
and fluid when heated and returning to the
solid state on cooling. If secondary forces are
sufficiently strong, a linear polymer may be
insoluble or restricted in solubility and it may
be infusible or fusible only at temperatures at
which the polymer begins to decompose. If the
composition is such that the chain molecules
are flexible, but the inter-chain forces are very
weak, random coiling occurs and the material
shows long range elasticity. Such materials are
known as elastomers (e.g. rubber). If more
than two primary linkages can be formed by
each fundamental unit, the structure of the
polymer becomes one of chains with cross
chains and may approach a fully crosslinked
network which is effectively a single molecule.
Polymers of this type are much less tractable
than linear polymers being both insoluble and