Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 137
on brittleness is not precisely defined but a figure
of 1–3% elongation at break has been suggested
for picture varnish coatings (Feller and
Curran, 1985). Knowledge of viscosity grade is
useful in comparing polymers in a series based
on the same monomer. However, it does not
necessarily follow that polymers of different
types having the same viscosity grade will have
precisely the same strength and brittleness.
This applies equally to the hardness of different
materials. In general, however, low viscosity
grade polymers are more brittle than those
of higher viscosity grade. For example,
dammar and mastic (viscosity grade 1.5 cp) are
much more brittle than Elvacite 2044 (poly (nbutyl
methacrylate)) of viscosity grade 48 or
AYAF (PVAC) of 80 cp. Brittleness can be
reduced by using plasticizers. However, a better
result may be achieved by suitable co-polymerization.
The inclusion of material which is
not part of the polymer structure can result in
problems caused by its migration into paint
layers. Plasticizers are generally avoided in
conservation materials but have been widely
used in the manufacture of plastic objects.
Plasticizers reduce the modulus of elasticity,
effectively reduce Tg, decrease hardness,
increase creep and permeability and may result
in more rapid deterioration. It is therefore better
to use polymers with intrinsically desirable
properties.
4.3.3 Barrier properties
Permeability is a general term used to describe
the property of a material to allow the passage
of some substance through it. Permeation takes
place by diffusion. Measurements of permeability
refer to the rate of transmission through
a film.
Diffusion can be defined as the movement of
atoms, molecules or ions through a gas, liquid
or solid. In diffusion, gases, vapours (i.e. water
vapour) and liquids pass through permanent or
transient voids between polymer molecules.
The diffusion rate therefore depends considerably
on the size of the small molecules and the
size of the gaps. It also depends on the partial
pressure gradient across the film and intermolecular
forces (hydrogen bonding, Van der
Waals forces). For polymers, the size of the
gaps depend on the physical state of the polymer,
that is, whether it is glassy, rubbery or
crystalline. In the case of amorphous polymers
above Tg, molecular segments have considerable
mobility and there is an appreciable ‘free
volume’ in the polymer mass. Also, because of
segment mobility, there is a good chance that
a molecular segment will at some stage move
out of the way of a diffusing small molecule.
Below Tg, the segments have little mobility
and there is a reduction in free volume. There
will be fewer voids and a diffusing molecule
will have to take a much more tortuous path
to get through the polymer. Around the Tg
there are often complicating effects as the diffusing
molecule may plasticize the polymer
thus effectively reducing the Tg (e.g. PVAL and
water). Crystalline structures have a much
greater degree of packing and the individual
lamellae can be considered to be almost impermeable.
Diffusion can therefore only take place
through amorphous zones or through zones of
imperfection. Hence, crystalline polymers tend
to resist diffusion more than either rubbery or
glassy polymers. Unfortunately penetration of
solvent into the polymer mass will be similarly
restricted and these materials are difficult to
formulate and use as coatings. Crosslinked
polymers tend to resist diffusion but are generally
unsuitable for conservation coatings.
Permeation can be defined as a three-part
process. It involves dissolution of small molecules
in the polymer, migration or diffusion
through the polymer according to the concentration
gradient and emergence of the small
particle at the other side. Hence, permeability
is a product of chemical compatibility and diffusion.
Henry’s law states that the solubility of
a gas in a liquid is directly proportional to the
partial pressure of the gas at a given temperature.
Most atmospheric gases have relatively
low solubilities and obey Henry’s law but diffuse
freely through amorphous regions of solid
polymer. Vapours of organic substances with
similar solubility parameters to the polymer
have relatively high solubilities, with deviations
from Henry’s law, but diffuse more slowly as a
result of comparatively larger molecular size
and often strong interactions with polymer
chains.
Polymers with low permeability to both
gases and vapours include poly(vinylidene
chloride) and co-polymers, acrylonitrile-styrene
co-polymers, epoxides, poly(vinylidene fluoride),
poly(ethylene terephthalate) (polyester),