The chemistry of painting cars and the recent development of water based paints

It is necessary to paint cars because the paint protects the steel of the car from corrosion, paint fills and smooths out any imperfections in the steel surface and it provides a durable, decorative, glossy appearance.

If rusting continually takes place the car becomes unsafe. When water and oxygen come in contact with the metal of the car surface, several redox reactions happen. The oxygen is reduced to hydroxide ions and iron is oxidized to Fe2+ ions which pass into solution. More reactions happen which ultimately produce rust – Fe2O3H2O.

The components of car paint

There are three components of car paint, each one has different roles:

* The pigment provides a barrier to protect car from atmosphere slowing the rate of corrosion. It disguises appearance of the steel car body, gives colour and other effects which enhance the appearance of the car.

* The polymers in paint protect the metal from damage that may be caused by the paint sticking to the surface. It also allows the paint to have a continuous, unbroken film by flowing under the effect of surface tension. This gives the car a smooth, glossy finish.

* The solvent in paint aids the transfer of paint between the container and the surface (when being sprayed) because the paint polymer is dissolved in it. The solvent helps the paint flow while it dries and then evaporates. This causes health and environmental problems.

When painting cars three different layers of paint are applied. Each one has a different function.

Layer

Function

Properties

Primer

Protects the metal from the atmosphere and so from corrosion.

Flexible – does not bend/crack when car bends.

Adheres to the metal

Surfacer

Fills and smoothes discrepancies in the steel surface.

Flows smoothly (due to an appropriate viscosity).

Flexible so no cracks

Adheres to the primer.

Top Coat

Gives durable and pleasant appearance.

Resistance to U-V radiation.

Flexible so no cracks.

Adheres to the surfacer.

The Structures of Polymers in Paint

Polimers give paint many of it’s properties because of their structures. Some properties are needed in all layers, some are more specific.

Properties Of All Layers

* A short chain length means the paint can be applied better.

* A low relative molecular mass is also required to allow the paint to be easily applied.

* Co-polymers with randomly distributed side groups are versatile and therefore help give the desired physical and chemical properties.

Properties Of The Primer Layer

Amine groups in chains mean that ionic groups are produced when in contact with acid. This makes the polymer soluble in water. When a car is immersed in the primer (containing the ionic polymer and rest of paint) an electrolysis cell is made (car body is cathode). The polymer then becomes electrically neutral and therefore insoluble in water. This means the car has better coverage of paint and is consequently protected, due to amine groups in the polymer.

Aromatic ether units in the polymers mean the paint adheres to the steel surface better and therefore gives better protection against rusting.

A greater mechanical flexibility is achieved by Three-Carbon aliphatic links.

Hydroxyl groups allow cross-linking when the paint is heated, therefore improve the strength of the paint.

The diagram below (Fig. 1) shows the structure of part of a polymer of an epoxy resin present in primers.

Properties Of The Surfacer Layer

Branched mixed polyesters have unreacted hydroxyl groups which help the surfacer stick to the primer and promote cross-linking during heating. This increases the strength of paint.

The structure of one polymer often present in surfacers is shown in the diagram below – Fig. 2.

CH2 __ OH

l

HO __ CH2 __ C __ CH2__ OH

l

CH2 __ OH

Fig. 2

Properties Of The Top-Coat Layer

Acrylic co-polymers are often used in the top-coat because they are resistant to UV-radiation, retain glossiness and are not too hard and brittle.

When polar side groups are added, the ability to stick to the surfacer is increased as well as the strength – due to cross linking between unreacted side groups.

Health and Environmental Problems with Painting Cars

12-15 litres of organic solvent is released into the atmosphere for every car that is painted. This is both damaging and wasteful.

This is a problem because there is a strong link between the presence of low level smog and the concentrations of sulphuric acid, nitric acid, ozone and dust with the release of organic molecules.

Low level smog causes respiratory problems, cancer of the lungs and poor visibility in humans, as well as damaging crops and car tyres.

Sulphuric acid and nitric acid are the main components of acid rain which can cause fish and trees to die. Also damages buildings and contaminates water which can lead to ill health.

Overcoming Problems

There has been lots of research into cutting pollution from car paints.

Two main areas are being looked at:-

* Adapting pre-existing equipment.

* Designing new paints.

Adapting Pre-Existing Equipment

This involves creating solvent removal systems such as incinerators, carbon absorption units and scrubbers. The disadvantages of these are that they are very expensive – both to set up and maintain running.

Designing New Paints

There are three main developments which eliminate the use of solvents:-

* Powder Coatings – These require specific equipment and need a lot of energy to ‘cure’. In addition, they’re difficult to repair if damaged, to match the colour of car accurately and it’s difficult to apply them to complex structures.

* Higher Solid Products – These use existing equipment but unfortunately have poor flow properties, are sticky and not good in metallic colours.

* Water Based Systems – Can use existing equipment but difficult to control flow in variable humidity conditions. Have high boiling points and enthalpy changes of evaporation, which prevents the paint thickening in the air between the gun and metal surface.

Aquabase

Aquabase began when it was realised that polymer particles could be dispersed in water in a similar way to oil and grease being dispersed by soap. This led to Alan Blackhouse realising that polymer particles surrounded by a special layer could make a water based paint.

Further research and development created a paint which has many advantages over the original method of painting cars, as it can be applied in a range of humidities and requires less baking (consequently saving energy).