44 COATINGS, SEALANTS & PAINTS
WHY DO EXTERIOR COATINGS DEGRADE? The major requirement of interior/exterior protective coatings is durability for long term protection. This is especially critical when products are specified for long-lasting exterior protection.
Outdoor durability of coatings, often called weatherability or exterior durability, refers to the resistance to degradation during outdoor exposure.
So how does degradation occur? Photo- oxidation by the UV component of sunlight is an important process during the deterio- ration of coatings during outdoor exposure. Also, a coating can yellow, crack, lose gloss or lose adhesion because of degradation.
SPEED OF DEGRADATION The speed of degradation of a coating during outdoor exposure depends upon two key factors. Firstly, the strength of the bonds in the resin system. The higher the bond energy, the more difficult it is to break the system down.
Secondly, the amount of UV absorption by the resin governs how many radicals can be formed. Radicals act as the ‘scissors’ for chemical bonds during photo-oxidative degradation of coatings.
DEFINITION OF A PAINT To go back to basics, the definition of paint is ‘A liquid material, that when applied to a substrate in a thin film, dries to form a cohesive, solid film that changes the properties of the substrate.’ There are five main components in paint: • White pigment — to give the paint whiteness, opacity and durability
• Extenders — to add bulk, reduce gloss and boost selective properties
• Binders — to stick the paint together, form a film and give adhesion
• Additives — to provide a degree of control over the vast amount of molecular interactions providing stability to complex systems
•Colourant tinters — to colour the paint, control gloss, prevent corrosion, add bulk and other properties
The chemistry and physics of each of these components determines the role they play in the formulation. Paint proper- ties also depend upon the ratio in which these are combined, the degree of disper- sion and the order of their addition to the formulation.
There are hundreds, if not thousands of each component commercially available at a range of prices, from ‘value’ paints to paints of the highest quality. It’s always good to remember — if you use higher quality baking ingredients, you get a nicer tasting cake!
WWW.HBDONLINE.CO.UK SILICONE ALKYDS
A silicone alkyd coating is a special type of coating in which alkyd resins are modified by adding silicone. The backbone of silicone alkyds consists of silicon and oxygen atoms. When silica is added to
alkyd resins, the end result is a specialised series of products that can be further used to formulate coatings. These coatings are tough, durable and offer resistance to crack- ing and abrasion. They are often used in maintenance paints for steel, concrete and
Enq. 151
THE CHEMISTRY OF QUALITY
Bradite’s chief chemist Sean Tormay explains why only the best makes sense when it comes to paint formulation.
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