Changing climate for coatings testing

The worst-case scenario for an exterior paint coating is early failure when at the mercy of demanding environmental conditions. Whilst companies can test samples in challenging climates over the long term, it is possible to approach the problem within a shorter time scale by studying product behaviour under simulated conditions in the laboratory.

Materials characterisation specialists at Exeter Advanced Technologies (X-AT), based at the University of Exeter, in the UK, are doing exactly this by examining the effects of environmental conditions on the mechanical properties of standard coil coatings from Becker Industrial Coatings (BIC) in a three year research program.

What are coil coatings?

Coil coatings are liquid coatings applied to continuous strips of metal, usually zinc coated steel or aluminium, under carefully controlled conditions. The paint is cured using energy efficient ovens, in periods as short as 20 seconds, and any evaporated solvents are incinerated in an environmentally friendly manner.

The coated strips are then recoiled and sent to a roll former or fabricator, where post-forming of the painted metal takes place. The superior mechanical performance of coil coatings allows the end-user to cut and form the metal into the end product without cracking or causing other damage to the paint surface. It is vital to test such products to ensure the surface coating will not break when the coated metal strips are rolled, bent or profiled.

Demanding applications

Coil coatings are used to protect internal and external building cladding, domestic appliance carcasses, automotive body panels and many other industrial end uses. Buildings can be exposed to any number of environmental conditions such as: extremes of heat and cold, high and low humidities and those promoting corrosion. The last scenario occurs especially close to coastlines.

Therefore, the exterior paints must be resistant to corrosion, possess colour stability when exposed to strong UV light and have good durability, which includes resistance to paint delamination. Dirt pick up resistance is another major requirement for exterior coating used in some parts of the world. For use on domestic appliances, the paint needs to be resistant to knocks and stains, and have the ability to bend smoothly into shape without cracking. The range of coatings BIC manufactures are aimed predominantly at industrial and construction markets across the world. Their coil coatings products have become so successful that BIC has been the number one supplier in Europe for this type of paint for many years.

The project

To fully understand the behaviour of external coatings wherever in the world they are exposed, whatever the climate, ideally the paints need to be tested and examined in location. However, this is not always possible, so obtaining good quality information on the behaviour under a given set of circumstances is critical.

Beckers is particularly concerned with understanding the effects that extreme environmental conditions have on its products, particularly coil coatings. The company's Long Term Development (LTD) Laboratory decided that this could be achieved by supporting a university to develop an environmental control chamber that could be placed around samples situated in a tensile testing machine. For such specialists expertise and equipment, the company looked to Exeter Advanced Technologies (X-AT), based at the University of Exeter to come up with a solution.

The Materials Characterisation Laboratory at X-AT is equipped with various materials characterisation equipment such as the EZ20 universal testing machine from Lloyd Instruments. As well as boasting a powerful 20 kN force capacity, this machine also features a 1000 Hz data sampling rate for unrivaled sensitivity to test thin paint film samples that BIC require.

Furthermore, the robust twin column frame means accuracy is not compromised when used in conjunction with the company's own thermal cabinets and environment chambers. It is therefore possible to carry out tensile to break tests on thin film coatings at precisely controlled humidity and temperatures ranging from -30 to +60°C.

Aims

BIC hopes the laboratory tests will:

• Reveal the nature and magnitude of the effects of humidity on the mechanical properties of the paint systems in use;
• Relate the changes in properties to difficulties experienced during the processing of painted coils, ie, cracking during forming, paint layers sticking to each other in the recoiled strip (blocking) and poor abrasion resistance;
• Relate any changes in properties to performance in the field, ie, weathering in humid environments and dirt pick up resistance;
• Establish a relationship between the tensile test data, glass transition temperature and brittle tough transitions.

Solution

By adapting the environment chambers and thermal cabinet attachments, specialists at the Exeter laboratory are able to replicate accurately the environment conditions that the final end-users are faced with. The effects of humidity and liquid immersion on the physical properties of the system can be predicted so that adjustments can be made prior to exposure in extreme conditions of humidity, temperature, UV dose and atmospheric contamination.

To simulate different humidity conditions as well as changing temperature conditions, samples are also placed within specially designed humidity chamber, which in turn is fitted inside the thermal cabinet. In addition, an oven has been designed to be filled with water to simulate wet conditions. Using a special immersion jug, samples are tested in tension under water.

Exeter are also using Lloyd Instruments NEXYGEN MT control data analysis software package to control the EZ20. This software package possesses a huge library of international standard test set-ups for manufacturing compliance. The company's Ondio Applications Builder Software is also being used to design specific tests according to the application needs.

To test coil coatings, two types of tests are being performed: a tensile test and a peel test. Tensile samples only 30 µm in length are prepared by coating a non-stick surface, curing in the normal way, removing the sample and using specially designed tensile grips to clamp the sample. Tests performed include tensile to break and creep tests under controlled load.

The tests are repeated under different environmental conditions and data is stored and analysed using the standard tensile test set-ups within NEXYGEN MT software. Peel tests are also being performed on paint films.

Meeting the standard

BIC is operating to a proprietary internal standard. The project will ultimately define proven internal test standards to which future coil coating formulations produced by the company will need to conform.

Other international standards for the physical testing of paints, varnishes and coatings include:

• ISO 4624:1978 - Paints and Varnishes: Pull-off test;
• BS EN 24624:1993 - Paints and Varnishes: Pull-off test for adhesion;
• ASTM D 4541 - Coatings: Pull-off test for adhesion.

Summary

This flexible approach to laboratory testing is ideal for any paints, coatings and printed inks manufacturer or any other industry where access to powerful and sensitive materials testing solutions would benefit their product development. BIC continues to send panels to exposure sites around the globe, but with this new capability to reproduce environmental conditions much closer to home the company is able to drive its R&D program quicker and more cost-effectively than ever before.

So far the project has demonstrated that humidity has a considerable influence on the glass transition temperature of the paint systems, this in turn will affect the mechanical properties. Ultimately it is hoped the data can be used to define testing protocols against which all further products could be tested.