The Case for Micro-porosity
Does it hold water?
||Lead paint is arguably the most effective protective coating for timber, but its use has been banned on all but the most historic buildings because it is so toxic. Modern alternatives are often marketed as being 'microporous'.
At the start
of the last century, wood-coating technology enjoyed a period of
relative stability when little energy was expended in the development
of new or improved coatings. Then, joinery was manufactured from
large-sectioned, slow-grown wood and protected with flexible lead-based
paints which seldom showed the type of failures that have become
so characteristic of their modern day equivalents.
One could be
forgiven for saying that, based on the frequency of observed failures,
today's coating technology has shown a seemingly retrograde development.
The reason for this apparent incongruity has often been blamed
on shifts in attitudes in silviculture, which saw timber make
the transition from being an abundant building commodity to that
of a scarce and valuable resource. The consequence was that the
definition of acceptable timber quality was redefined to maximise
the use of a shrinking resource. This in turn placed seemingly
unrealistic demands on existing coatings which were expected to
perform well despite the fact that the nature of the substrate
beneath them had changed. This is reflected in our historic buildings,
where the original woodwork is manufactured from large-sectioned,
slow grown temperate hardwoods that do not exert the same demands
on paint coatings as today's more rapidly produced counterparts.
this is an oversimplification, since it does not take into account
legislative pressures exerted against the use of lead paints which
began as early as 1921, when the International Labour Organisation
(ILO), in recognition of its toxicity, implemented a convention
restricting the use of white lead in paints. This eventually culminated
in the UK Paint Regulations of 1927 (now superseded by
the Control of Lead at Work Regulations 1998). Today, the
UK regulations allow the manufacture and use of white lead paint
only in controlled and special circumstances for the decoration
of Grade I and II listed historic buildings (England and Wales)
and categories A, B and C(S) (Scotland). Strict regulations apply
to its use and the general sale of lead paint in the UK is prohibited.
backdrop, the wood coatings industry was forced to adapt and manufacturers
responded by developing new types of coatings, including the development
of systems where the lead component (lead carbonate) was substituted
with titanium dioxide. One of the main advantages of lead carbonate
is that it reacts with linseed oil to produce fatty-acid soaps
which are both tough and elastic, with good wetting properties.
This is arguably the principal reason why white lead was used
so commonly for the protection of wood substrates: it could produce
a resilient, relatively impermeable protective film capable of
responding to movements of the wood caused by changes in air humidity
and substrate moisture content. It was the inherent ability of
the old lead-based paints to accommodate movements within the
substrate which was the key to their success when used to protect
exterior wood. Formulation technology has responded to restrictions
in their use through the development of alternatives, among them
the acrylic and vinyl-based systems and polyester products which
led to the development of the traditional 'alkyd systems'.
|Paint cracking around the joints is a common problem in modern paint systems due to the relative brittleness as they age. The eventual result is flaking and peeling (illustrated below right).
In many ways
history is repeating itself: in just the same way as health and
safety controls signalled a radical change in the direction of
developments during the 20th century, imposition of environmental
legislation has set in motion further changes at the start of
the 21st. These are apparent in the proliferation of low volatile
organic compound (VOC) - or 'compliant' - coatings, including
waterborne dispersions, hybrid products and high-solids coatings.
The upsurge in the availability of alternatives has provided today's
consumer with a wide choice of products, and established a highly
competitive market. Manufacturers have been forced to develop
alternative coating strategies backed up by claims which have,
over the years, had a runaway effect on the way coatings are perceived
in the context of joinery protection. Nowhere has this been more
noticeable than in the case of 'micro-porous' or 'breathable'
paints and paint systems, where justifiable technical claims have,
over the years, been heavily exaggerated to increase marketing
concept of 'micro-porosity' stemmed from the need to control and
reduce the build-up of moisture within wood joinery to below 21
per cent - generally accepted as the point at which timber starts
to become susceptible to attack from wood-destroying organisms.
Before the advent of micro-porous coatings, alkyd paints were
widely used as the main protection strategy for window joinery.
Alkyd resins are polyesters which have been chemically reacted
with a drying oil. This process results in the formation of a
three-dimensional interconnecting network of molecular 'chains'
which resist the passage of water and form a hard film. It has
the benefit of being easy to clean with solvents and is less prone
to sticking or blocking when freshly painted. However, its main
drawback is that it tends to embrittle with age, becoming progressively
less responsive to dimensional movements within the woodwork.
this occurs, cracking of the paint film around joints is the usual
result, with the consequence that rainwater can be drawn into
the wood at the point of fracture by capillary attraction.
inside, the water has no effective means of being vented out since
it cannot re-emerge past the alkyd paint. The result is that moisture
build-up within the fabric of the joinery occurs to a point at
which it can both trigger the development of rot and also interfere
with the surface adhesion of the intact coating, causing its eventual
delamination. The result is peeling and flaking paint, a scenario
which is all too familiar in the context of restoration work.
were quick to realise that one of the most effective ways of controlling
moisture in joinery was through the formulation of paints which
were more permeable and which could allow any entrapped moisture
within the joinery a ready means of escape.
is defined as the amount of water which can pass through a given
area over a given period of time and is usually expressed in terms
of grams per square metre per day (g/m²/day). Coating permeability
is defined as the transmission of moisture across a film, usually
in vapour form. It can be determined either on the basis of the
thickness of the applied coating (film permeability) or in terms
of the permeability when applied at a specific thickness, usually
100µm (specific permeability).
pitfalls in the use of the practical guidance offered by permeability
data insofar as the measurements may differ according to whether
they have been calculated on the basis of water loss or moisture
absorption. In practice, permeability may be influenced by the
density and interaction of the substrate, as well as being significantly
influenced by variances in temperature, humidity, the moisture
gradient present within the joinery and the age of the coating.
As with many things, there is a balance to be struck. If coating
permeability is too high, it may lead to splitting and deformation
of timber components. If it is too low, it may not allow moisture
to escape readily and may lead to decay. The real problem from
the user's standpoint is that no agreement currently exists on
what the optimum figure for any given end use should be. Consequently,
providing data on permeability becomes largely academic, since
there is little of practical value to guide the consumer.
the years there has been a tendency to use permeability as a marketing
tool, referring to it in terms such as 'open', 'ventilating',
'breathing' and 'micro-porous' to promote sales. This has led
to the erroneous perception that micro-porous paints somehow constitute
a specific category or generic class of coating. In reality it
can be argued that most paints are 'micro-porous' to a certain
extent, being capable of allowing at least some moisture to pass
through the cured coating.
It is also a common misconception that
'micro-porous' paints can let water escape from painted wood without
allowing it to enter. In essence, if a coating is permeable it
will allow moisture in vapour form to cross it in both directions,
as dictated by the relative vapour pressure differential which
exists on both sides of the paint film. Herein lies a paradox
for the consumer. Does he select a coating which can vent excess
moisture from within a window frame if climatic conditions are
dry, in the knowledge that the same coating might equally be responsible
for a rise in the moisture content of the window if conditions
are wet? Much has been written and said about the issue of micro-porosity
and the illusion still persists that it is in some way a 'cure'
for many of the problems which are a common feature of painted
woodwork. In reality, it is used by specifiers as a palliative.
Yet there are much more important issues to consider. Many of
the arguments in favour of using micro-porous paints can be dismissed
by careful detailing and good design, which can go a long way
to excluding water uptake in the first place. The irony here is
that conservation work by its nature deals with the restoration
of original building features irrespective of whether they work
to the benefit of long-term function or not. One can make a case
for using a permeable paint system as a measure to offset or delay
the inevitable consequences of bad design. However, one can make
an equally valid argument for using less permeable paint systems
if, because of their greater properties of extensibility, they
can effectively prevent moisture penetration and resist the onset
of premature cracking - white lead paint gives that option.
technology is able to offer the conservationist a range of alternative
systems which are neither very permeable nor toxic to health.
However, it is important to select coatings based on a logical
treatment of the specific issues in a given project and to resist
marketing pressures to choose a coating which may not be right
for the job in hand.
- J Graystone, The Care and Protection of Wood, ICI Paints Division, Slough,
- EA Hilditch
and RJ Woodbridge, 'Progress in Timber Finishing in Great
Britain', Journal of the Oil and Colour Chemists' Association,
Wood Information Sheet 2/3-1, Finishes for Exterior Timber,
article is reproduced from The Building Conservation Directory, 2008
PETER KACZMAR is head of chemical and analytical laboratories
at the Timber Research and Development Association (TRADA) and manager of its research on surface coatings and timber
preservation. He has been responsible for co-ordinating
a number of national and EC-funded research projects on
wood coatings and preservation.
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