BENEFITS OF
ACRYLIC PAINT
TECHNOLOGY


THE BREATHABILITY OF ACRYLIC GLOSS PAINTS


Number
in a series of technical bulletins issued by the Paint Quality Institute, the leading source of quality paint information worldwide.


© 2000 Paint Quality Institute,
All rights reserved.

 

The main cause of failure of paint applied to exterior joinery and weatherboards is the presence of excessive moisture in the wood. Wood is a porous material which can absorb or lose water depending on the moisture content and temperature of the environment.

When water is present in wood in excessive amounts, it reduces the adhesion of the paint to the wood and can place severe stress on the coating via swelling and shrinkage of the grain as water is absorbed and desorbed. Ultimately, this cycling process leads to raised grain, checking and splitting of both the wood and the paint film.

Simultaneously, vapour pressure will be exerted on the coating from the excessive moisture seeking to escape, leading to blistering of the point, particularly as its adhesion to the wood has already been reduced by the waterpresent.

In addition, the presence of water in wood at high levels over a period of time leads to decay by micro-organisms if the timber has not been adequately pre-treated with preservative. The principle cause of decay is the wet rot fungus, which will propagate at wood moisture contents above approx. 22%.

In the case of window frames, moisture may come from both rain and internal condensation, and enter the wood at several weak points:

  • where the putty has cracked
  • at unsealed end-joints
  • where the paint has cracked or flaked off
  • at contact points with masonry

    The end-grain of wood is particularly prone to rapid water uptake, as the cells are aligned in the same direction, permitting absorption by capillary action.

    Both solvent based (alkyd) gloss paints and acrylic emulsion gloss paints provide a barrier to water in liquid form. However, solvent based gloss paints are also largely impervious to watervapour, and once the water has entered the window frame via one of the weak points, it is trapped. Paint failure occurs at the points of highest water concentration, notably along the bottom rail of windows, and in particular at joints.

    By contrast, acrylic gloss paints have sufficiently high water vapour permeability to allow the excess moisture to escape in the form of water vapour. These paints can therefore be described as 'microporous' or 'breathable'(fig 1). With wood being kept at its natural moisture content, the paint is much less exposed to stress, and resists the eventual cracking and flaking of ordinary solvent based gloss paints.

    • fig. 1
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    It should be pointed out that although acrylic gloss paints have higher permeability than alkyd gloss paints, their permeability is not as high as, for example, low build solvent based woodstains. Highly permeable, coatings will let in water quickly as well as release water quickly, and with rapid cycling, there is a risk of wood splitting. By having moisture vapour permeability at a healthy, intermediate level, acrylic gloss paints can be said to have 'controlled' permeability.

    The acrylic gloss paint is also better suited to

    		accommodate any stress to which it may be exposed, as it is inherently moreflexible, through not changing in nature during its lifetime. In comparison, the solvent based gloss paint continues to cross-link by reacting with oxygen in the atmosphere and so becomes progressively more brittle.

    The superior moisture vapour permeability of acrylic paints can be demonstrated in a number of ways. At paint testing laboratories, three experiments have been carried out with the following results:

    VAPOUR PERMEABILITY
    MEASUREMENTS
    .
    fig. 2
    In this experiment, permeability tests were made on various acrylic and solvent based (alkyd) paint systems, with the following typical results (fig. 2):-

    The higher numbers obtained for the acrylic systems demonstrate the higher vapour permeability of these paints.

    The test method used (fig. 3) involved sealing the open end of a cup containing calcium chloride with a paint film. It was then placed in a humidity cabinet for 24 hours at 40°C and 100% Relative Humidity. The moisture uptake (in grams) of the calcium chloride was then recorded, and the permeability calculated from the following equation:-



    fig 3.
    Paint Film

    Calcium Chloride

    BLISTER BOX TEST
    .
    fig. 4
    The excellent adhesion of paint systems based on quality acrylic polymers, which incorporate adhesion promoting technology, is well illustrated (fig. 4) by the severe blister box test.

    In this test, wooden panels, with the topsides painted with an acrylic primer and gloss topcoat, are placed over a steam bath for 50 hours. Steam penetrates the uncoated underside of the wooden panels, and passes through the coated upper surface. Properly formulated acrylic paints can pass this test with the paint showing no evidence of blistering or loss of adhesion.

    Panels

    Electrically Heated
    Water-Bath
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    WATER RELEASE
    .
    fig. 5
    In this experiment, blocks of wood were soaked in water to give a high water content(40-50%), and after drying the surfaces, were painted with acrylic and alkyd gloss paints. The moisture content was measured as the blocks were allowed to dry out at room temperature.

    The results (fig. 5) show that the acrylic gloss paints allow the water to escape much more easily than alkyd gloss paints. Wood saturated with water will dry out to 'safe levels' of water content (below 22%) much more quickly if painted with acrylic rather than alkyd gloss paint.

    CONCLUSIONS
    .
    fig. 6
    The conclusion from this work is that a paint system based on a high quality acrylic emulsion polymer provides a much superior protection to wood which may be exposed to the ingress of water. Unlike a solvent based alkyd paint, the acrylic paint permits the excess water to evaporate through the coating, leading to a much lower average water content. The acrylic coatings also maintain a much higher degree of flexibility throughout their lifetime.

    The adjacent photographs (fig. 6) are of two window frames (with close-ups of the bottom rail) which are incorporated into the Building Research Establishment test house at the Princes Risborough Laboratory U.K. The windows have been exposed, facing south, for 7 years. The frame on the left hand side, showing typical cracking and flaking, was painted with a 3 coat commercial alkyd system: the frame on the right hand side with a 3 coat acrylic system.


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    		 For further details about the contents of this bulletin, and other bulletins in this series,
    please contact the Paint Quality Institute, care of the Rohm and Haas Company in Australia or New Zealand.
    Paint Quality Institute, PO Box 115, Camberwell, Vic. 3124. Australia. Phone 1800 338 015.
    Paint Quality Institute, PO Box 22-220, Otahuhu, Auckland 6, New Zealand. Phone (09)276 4866.
    By email at Ask PQI