Series editors' preface - Wood Tools

CH 2—OOCR l
R ll COO—CH O
CH 2—O—P—OH
O –
(a)
Plastics and polymers, coatings and binding media, adhesives and consolidants 165
H +
O
H2—C—O—C—R1
R2 —C—O—C—H
O O
H2—C—O—P—O—CH2CH2N + (CH3) 3
O –
(b)
Figure 4.12 (a) Phosphatidic acid; (b) phosphatidyl
choline (lecithin)
so named because they contain a phosphorous
atom. Phosphatides are tribasic (i.e. there are
three atoms of acidic hydrogen in the molecule
thus giving rise to three possible series of
salts), phosphatidic acid and may combine further
with various compounds (Figure 4.12a).
In combination with the strong nitrogenous
base choline, for example, phosphatidyl
choline or lecithin is formed (Figure 4.12b).
Lecithin is an emulsifier, that is it confers on
egg yolk the property of forming and stabilizing
emulsions.
An emulsion is a colloidal system. A colloidal
solution is one in which the solute (disperse
phase) is present as a system of particles of
10 –4 –10 –6 mm in the dispersion medium. Such a
system has properties distinct from a true solution
because of the larger size of the particles
(approximately 10 –4 –10 –6 mm across). In an
emulsion, one liquid is dispersed in the form
of fine droplets throughout another liquid with
which it cannot evenly mix. The most common
examples are made up of oil and water. For
example, mayonnaise and cream are emulsions
of fat dispersed in water, and butter is a water
in oil emulsion. Many paints rely on the formation
of emulsions to achieve the combination
of properties desired. In creating
emulsions, energy is required to overcome the
surface tension between two immiscible
liquids. Once this has been achieved a method
of stabilizing the result is required so that the
components do not simply re-coalesce into
two separate phases. Certain proteins, plant
resins and gums and other large carbohydrate
molecules (e.g. starch) can do this by surrounding
the droplets of the dispersed phase
and interfering with their coalescence. An
important class of emulsifier relies on the properties
of fatty acids which have a fat soluble tail
attached to a water soluble head. With one end
immersed in the droplet phase and the other in
the continuous phase, such molecules, known
as surfactants, are extremely effective at preventing
one droplet from recognizing another.
Soaps belong to this class as do lecithin, and
other related substances, present in egg yolk.
4.7.2 Waxes
True waxes are esters of long chain fatty acids
ranging from C24 to C36 and monohydroxy alcohols
ranging from C16 to C36. When obtained
from natural sources, waxes also contain varying
amounts of hydrocarbons, free fatty acids
and alcohols and may also contain plant sterols
and triterpenoids and their esters (Mills and
White, 1987). Waxes are non-greasy solids at
room temperature, soften or melt at fairly low
temperatures, are highly insoluble in water and
have very low chemical reactivity. The term
wax is also applied to fully saturated high molecular
weight hydrocarbons having similar
properties (e.g. paraffin wax).
Waxes and wax resin mixtures have been
used as adhesives, consolidants and fill materials.
Waxes, especially beeswax, were finishes
in early and continuous use. Beeswax is
known to have been in use on wooden artefacts
as early as the time of the ancient
Egyptian craftsmen and its use in the fine arts
can be traced back over 2000 years (Kirk-
Othmer, 1985). Wax finishes have very low
permeability to moisture (Gettens and
Bogelow, 1933) though a buffed wax layer on
top of a continuous film is exceptionally thin,
much thinner than a typical resin coating, perhaps
only a few molecules thick. Waxes show
low optical saturation when applied cold but a
high degree of optical saturation when applied
hot. Their natural colour varies from white to
brown depending on type but they are easily
coloured using pigments or fat-soluble dyes.