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Higher solids – more than just a numbers game

Over the past twenty years, enormous strides have been made in the improvement of solids content and coverage of paints in many areas of the industrial coatings market. It still remains the focus of many technical papers and much research and development effort. In this article, James Maxted of Beckers' LTD Laboratory, and leader of the Becker “Polyester Technology Group”, cites economics and efficiency as the prime motivator for higher solids in coil coatings.

In common with the coatings industry as a whole, the coil coating industry is keen to improve solids content, if for partially different reasons. The release of VOC (volatile organic components) is dramatically less in the coil coating process than in other areas of industrial paint application.

A recent survey gave VOC emissions of 0.02 – 0.4 g/m2 for a typical modern coil coating line with abatement systems compared with 32 – 163g/m2 for a post-finish, wet paint line without post-incineration.

Modern coil coating lines utilize after-burners or incinerators wherein solvents released in the stoving process are oxidized and the energy released is reused in heating the curing ovens. The drivers for higher solids systems in coil coatings are therefore not only for environmental reasons; rather they are related to the yield and cost and ultimately the economics of the painting process. The incentive for the paint producer and the end user is then improved economics and efficiency.

A soupēon of arithmetic

Before a discussion can be undertaken on how we are improving the "yield" and "coverage" of Becker coil coating paints, it is important to understand just what the terms mean and how formulating chemists with Becker are looking to improve matters.

The most usual expression for yield is:

This expression tells us that not only do the volume solids of the paint influence the yield but also the density of the paint and of course the thickness applied onto the coil. Whilst the latter is very much in the hands of the coater, the formulator has the possibility for both increasing the volume solids and reducing the paint density to improve yield.

Volume solids?

There is a major impact on the yield from the method used to determine the volume solids of the paint. This can be measured directly using a method based on the Archimedes principle or calculated indirectly from measured weight solids of the paint.

It became clear in the early stages of this project, that measured weight solids depends significantly on the technique used (especially the oven temperature and dwell time). A survey of methods used by the coil coating industry worldwide has shown that at least a dozen different methods have been adopted. In an extended study in the Long Term Development laboratory a series of “standard” and “higher solids” coil coating topcoats were made in an un-tinted white polyester with identical parameters other than their polyester resins.

Their weight and volume solids were assessed by nine different methods including BS, SIS, AFNOR and ASTM standards and the summary of the resulting impact on the final yield calculation is shown in Figure 1.

Figure 1: Effect of solids method and polyester resin on paint yield,

Those yield values marked by * (blue box) indicate that a direct measured volume solids, rather than a calculated value was used as a basis for the yield calculation. In all cases the paint density was measured directly.

Several things are clear from this data:

The temperature and time of the test method used has a significant impact on the final yield result.(/LI>
The typical coil coating bake schedule (red box) cannot be recommended because of the difficulties of control and reproducibility in testing over such short dwell times (232°C peak metal temperature for 35 secs. and 20 micron film).
Tests made at 165°C – 180°C show the closest agreement with solids determined from a coil coating bake schedule.

When making any comparison between methods and formulations it is vital to understand any differences between systems. Small differences in viscosity, pigment loading, the density of the solvent blend and, of course, test sample size, oven temperature and time all have a significant effect on the final yield value.

When is “non volatile”, volatile?

There is a direct correlation between the polyester resin structure and the ”yield”. The data in Figure 1 demonstrates that optimisation of the structure of the polyester resin is necessary to improve yield – an experimental “ultra high solids” resin actually gives the same yield value under the highest temperature solids tests as the conventional polyester. The simplified diagram in Figure 2 shows the sources of volatile loss in a typical polyester paint and provides some explanation for this surprising result.

Figure 2: Sources of volatile loss in polyester melamine paint systems under curing.

It should be remembered that the extent and nature of these losses will vary depending on the design of ovens, airflow and so on.

The diagram highlights that it is not only the solvent content that contributes to volatile loss, but also the residual monomers and low molecular weight material there may be in the polyester resin. This point is best understood by looking at the molecular weight (MW) distribution curves measured by gel permeation chromatography (GPC). Figure 3 shows the curves for both a conventional and a high solids resin. It is clear from this that the “higher solids” resin has a lower weight- and number- average MW and is richer in lower MW polymer chains and thus has more potentially volatile material.

Figure 3: Gel Permeation Chromatography – molecular weight distribution.

If the molecular weight of the high solids resin is too low, then unacceptable losses in these lighter fractions (oligomers) occur. This degradation of the resin can have a negative impact on the final dry film performance, although loss of very low molecular weight material might even improve formability.

The diagrams also show that if the system is crosslinked with methylated melamine formaldehyde resin than there will also be volatile losses associated with this material and the by-products of the crosslinking reaction.

The design and formulation of high solids paints must therefore involve a degree of compromise and optimisation to ensure that such unwanted losses do not occur.

Optimisation not maximization

The Becker Polyester Technology Group have been evaluating a wide range of resins and other formulation variables, using PC based DOE (design of experiment) analysis. The key objective was to produce a range of topcoat systems with high solids and yield and improved economics without compromising the final dry film performance.

This has necessitated considering the contribution of every part of the formulation to the final yield value – and has involved considerable lateral thinking too – looking at other coating businesses and their approach to high solids formulation and application technologies. It has also involved us working closely with our suppliers.

Another major issue to be addressed in this project was that of application and set-up of the roller coater. The use of higher solids formulations demands the application of a thinner liquid coating to achieve the same dry film thickness. Tighter gaps between the application and pickup rolls can lead to application difficulties and higher roller wear. In addition it also raises the questions of whether a reduction of solvent passing into the abatement system and therefore available to help heat the ovens air would have a negative effect on oven energy consumption. Trials on the Becker Pilot line, have also demonstrated that optimisation rather than minimizing volatile content is key. As part of this exercise the Polyester Technology Group have considered a variety of different application methods to improve the ease and economics of paint application.

As with many areas of paint formulation today, the search for improved economics and reduction of environmental impact is necessitating a thorough review and questioning of traditional approaches to formulating and more than a modicum of lateral thinking. Suffice to say that this approach is being used in other areas of Becker’s coil coating technology too.

Conclusions

Whilst work continues in this important area, the Polyester Technology Group have come to a number of important conclusions regarding the formulation and use of high yield paints.

Be aware of the impact of any method chosen to calculate yield and non-volatile content and check its relevance for a particular line.
Be aware that non-volatile content is not just a function of the solvent – many other aspects of the coating system contribute to this.
Be aware that producing a high yield coating requires more than a minimization of solvent – it is more about optimising solid content for all aspects of performance.
Be aware of the impact of higher solids materials on processing and the hidden costs that such a change may bring.

What is also clear from this exercise is that improving the yield and economics of Becker Coil coating systems is far more than just a numbers game!

Reference

1. Bade,T., van Ham,J., Hoeflaak,M., ’Solvent Emissions and Energy Consumption Involved in Coil Coating’ TNO Report for the ECCA-Nederland Group. Sept. 1993