The chemistry of quality

Bradite’s chief chemist Sean Tormay explains why only the best makes sense when it comes to paint formulation.

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? Photooxidation by the UV component of sunlight is an important process during the deterioration 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 properties also depend upon the ratio in which these are combined, the degree of dispersion 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!

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 cracking and abrasion. They are often used in maintenance paints for steel, concrete and other high quality exterior features, making them ideal for housebuilding.

Two key properties of silicone alkyds can be directly linked to the excellent outdoor durability of these polymers. First, the bonds in the polymer are strong, implying that a high amount of energy is needed to break down the backbone. Secondly, silicone is nearly transparent towards UV radiation.

This is a premium technology for top quality protective coatings and can be used as a pure polymer i.e. 100 per cent silicone, or incorporated into a more traditional alkyd resin.

Pros:

  • Superior exterior durability
  • Superior flexibility
  • Superior heat resistance
  • Superior chalk resistance
  • Superior yellowing resistance
  • Superior water repellence
  • Improved solvent/pigment compatibility
  • Improved adhesion

Cons:

  • Cost – at least three times more expensive than conventional alkyd technology

A good manufacturer will to look to optimise its products based on technical performance first, cost second and actively invest in the highest performing technology to meet the expectations of its customers. This ensures a commitment to quality.

Sean Tormay is chief chemist at Bradite