BENEFITS OF
ACRYLIC PAINT
TECHNOLOGY


THE WEATHERABILITY 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.
Acrylic gloss paints are formulated with resins with a different chemical composition than those used in solvent based (alkyd) gloss paints. The acrylic chemistry is very similar to that used in acrylic plastic sheet and moulding powder, well-known in applications such as outdoor illuminated signs, street-light covers, and rear light covers on cars. Acrylic paints are also widely used for protecting cars and aircraft. Acrylic resins for decorative gloss paints are designed to maximize resistance to weathering without being too hard or inflexible for use as house paints.

As well as acrylic resins differing in chemical structure from alkyd resins, they also dry by different mechanisms. The acrylic emulsion contains tiny spheres of polymer held in suspension in water. When a thin film is applied to a surface, the water evaporates and the spheres are forced together by capillary pressure. With coalescents present to soften the surface, the spheres fuse together to form a continuous film of polymer, which is then insoluble in water after all the water and coalescents have evaporated from the film (fig. 1).

By comparison, alkyd resins are oxidising resins, which means that they must react with oxygen from the atmosphere to dry. This reaction with oxygen continues during the lifetime of the paint, causing it to progressively embrittle. This leads eventually to failure by cracking and peeling, particularly over substrates which have any tendency to move.

The loss of flexibility and extensibility on ageing of alkyd systems can be measured in the laboratory. Indeed, it has recently been pointed out that even the initial extensibility of alkyd undercoats is inadequate to cope with normal movement in the wood substrate.

The Paint Quality Institute's experience of weatherability has been built up over more than thirty years of scientific testing of paint formulations at exposure stations around the world. In the Asia - Pacific region, the PQI maintains exposure stations at Geelong in Australia, and also in Singapore and Shanghai. Around the globe, major exposure stations are located at Spring House near Philadelphia in the USA and in southern France near Valbonne. Other sites include Chicago and Florida in the USA, and Puerto Rico.

Commercial exposure facilities, eg. Allunga Test Services at Townsville, in north Queensland, Australia, are also used to reinforce our own results.









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fig. 1

Aqueous dispersion deposited on surface.

Water evaporation and polymer fusion.

Continuous polymer coating.

fig. 2


Paint Quality Institute test site at Spring House, Pennsylvania, U.S.A.

fig. 3


Paint Quality Institute test site at Geelong, Victoria, Australia.
Exposure data from the major sites is fed into a computer giving speedy access and interpretation of results to assist both our own research and development and customers needs.

In addition, to obtain data in special environments, smaller exposure facilities are utilized in New Jersey, Florida, Puerto Rico, and the North East of England.

The experience gained from this massive accumulation of know-how enables polymer chemists to design acrylic polymers with durability a key feature in the overall balance of properties.

The adjacent photographs of panels from a number of representative exposure series illustrate the difference in performance in exterior durability between various solvent based (alkyd) gloss paints and typical acrylic gloss paint formulations.

fig. 5

RESISTANCE TO CRACKING
AND FLAKING
.
fig. 4
The panel shown in fig. 4 was exposed facing South at 45?for 6 years at Spring House U.S.A. The acrylic paint on the right hand side is still intact, whilst the solvent based alkyd paint on the left is showing severe cracking and flaking along the wood grain. This is one of many panels which demonstrates the superior long term flexibility and extensibility of acrylic gloss paints.


RESISTANCE TO CHALKING
AND COLOUR LOSS
.
fig. 6A
The paints shown in fig. 5 were exposed for 10 years. Due to the acrylic polymer composition, the acrylic gloss paints on the right have retained their original colour, whilst the alkyd gloss paints on the left have 'chalked' badly. Again, this exposure series is typical of many which demonstrate the better resistance of acrylics to the effects of ultra-violet light.

Figure 6a shows a comparison of two alkyd enamels (upper right and lower left) versus two low sheen exterior acrylics on fibrous cement sheet after 34 years north vertical at the exposure test farms near our Research Centre, Spring House, Pennsylvania, U.S.A.

Figure 6b shows a solvent based semi gloss soya alkyd trim paint (left) versus acrylic semigloss finishes over primed cedar after 19 years south vertical at Pennsylvania. These results speak for themselves.


fig. 6b
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GLOSS RETENTION
.
Measurements of the degree of gloss were made of paints exposed vertically at Spring House, U.S.A., facing south over a 3 year period. Although the acrylic gloss paint started with a lower initial gloss level, the gloss retention was significantly better than the solvent based alkyd paint (fig. 7).
The photograph (fig. 8) was taken after two years exposure of acrylic gloss paints in various colours. There is substantially no change in colour or gloss from the original appearance.


fig. 8

RESISTANCE TO DIRT PICK-UP
AND MILDEW
.
fig. 9
Alkyd gloss paints have a tendency to be affected by 'rain streaking', caused by a combination of rain and atmospheric pollution, which often appears in the early life of the paint film and cannot be easily removed. These paints also are more likely to support mould growth on the surface, as the oil acts as a nutrient (fig. 9).

By comparison, the acrylic gloss paints stay cleaner, brighter and whiter throughout their life.


CONCLUSION
.
It can be predicted from the chemical composition and non-oxidizing nature of acrylic gloss paints that they will be highly resistant to the effects of weather. This theory is borne out in practice, and has been demonstrated in very many long term exposure series, where the superior gloss retention and resistance to cracking, flaking, chalking, colour loss, dirt pick-up and mildew has been repeatedly proven.
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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 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