Can Coating Application
Can Coatings
The nature of the can or packaging coatings industry is dictated by the packaging manufacturers need to cut costs and produce better quality cans more quickly. As a result, can coating formulations are generally seen to be in a state of continuous change. In most white pigmented can coatings titanium dioxide often represents almost half of the total solid content and up to one third of the total formulation. It must therefore be seen as making a considerable contribution to the properties and to the cost of the coating formulation. Improvements in coating properties and cost due to titanium dioxide choice are thus very valuable to the paint formulator.
The Important Properties
Important Formulation Notes
Regulatory Compliance
As can coatings are frequently used for food contact purposes, it is essential that all raw materials in the coating comply with relevant national clean food regulations. These vary from country to country, but in all cases the Titanium Dioxide pigment must meet individual country's legislation. These regulations normally cover materials from within the coating as its ingredients may leach into the contents of the can under service conditions, including in-can cooking. Such leaching would obviously contaminate the can contents. Although Titanium Dioxide is completely non-toxic, it must still be tested against these regulations to ensure that it does not interact with other coatings raw materials or that its own surface treatment chemicals are not leachable. USA FDA compliance is frequently taken by other countries as a mark of the compliance of the coating and its raw materials.
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Pigment Dispersion
Pigment dispersion is particularly important part in optimizing the use of Titanium Dioxide pigments in can coatings. If the dispersion of the pigment is not optimized, significant problems will be encountered in achieving the correct level of dispersion (correct particle size distribution) resulting in loss of gloss, opacity and tint strength. Can coatings probably represent the most demanding application of Titanium Dioxide pigments in terms of dispersion due to the very thin films applied. Very few Titanium Dioxide pigments meet the requirements of the can coatings industry and the millbase, even for acceptable pigments needs to be perfectly optimized to ensure maximum performance from the pigment.
Important Properties Top
Dispersant Demand
Dispersant demand plays a very important part in the dispersion of Titanium Dioxide pigments into water borne media. If the correct amount of dispersant is not used, significant problems in achieving the correct level of dispersion (correct particle size distribution) resulting in loss of gloss, opacity and tint strength.
The dispersant demand results for a variety of common dispersants using three pigments are shown in figure 2. The potential benefits to you of low dispersant level are:
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Lower raw material costs by a reduction in the quantity of dispersant needed.
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Possible improvements in the gloss and the ink acceptance of the cured film due to a reduction in precipitation or "blooming" of excess dispersant onto the film surface. This blooming often results in reduced gloss, or more importantly a change in the surface tension of the cured film. This in turn may have major effects on ink lay.
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Possible improvements in stability, with some low dispersant demand pigments showing excellent flocculation resistance.
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A reduction in the water sensitivity of the cured film. In can coating systems this may result in benefits in condensation and process resistance.
It is apparent therefore that Dispersant Demand must be considered when formulating water Borne Can Coatings systems.
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Opacity
You can use the potential benefits of increased opacity from the pigment in any one of several ways:
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To reduce TiO2 content and hence lower raw material cost.
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To run at reduced film weights, giving better mileage.
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To obtain an overall improvement in product quality at no extra cost.
The performance of a modern, high quality, multipurpose pigment is tailor made for the can coatings industry, offering improved opacity in the final coatings with the possibility of reduced cost.
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Viscosity
In solvent borne systems, using a pigment having a low oil absorption results in a lower solvent and binder demand and hence lower viscosity in the millbase and final coating. Figure 1 shows a graph of viscosity versus pigment concentration for the millbase of a typical solvent borne paint formulation for dispersion on a High Speed Disperser. The same can be true for the viscosity difference produced in water borne systems. In both cases, the final viscosity of the paint produced will be lower than that produced by pigments with higher water and binder demand.
The benefits of these effects to you, the coatings formulator, can again be realized in several ways:
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You can convert lower viscosity into higher paint solids and thus obtain better mileage.
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Lower viscosity will allow you more freedom of choice of coalescing solvents, which will improve flow and levelling of the coating.
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Lower viscosity allows you to use higher molecular weight or higher glass transition temperature resins. The higher viscosity of these resins can be counterbalanced by the lower viscosity produced by the use of a low oil absorption pigment. The use of such resins can result in improvements in cure, flexibility or the flow of the finished coating.
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Gloss
The gloss potential of Titanium Dioxide pigment has a significant influence on the final film gloss of the coating. The Millennium Inorganic Chemicals' range of pigments has excellent gloss potential.
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Important Formulation Notes
Solvent Borne Systems
Low oil adsorption pigments compared to other grades of titanium dioxide will give a lower millbase viscosity for a given pigment concentration (see figure 1.) You must take great care therefore to avoid thin millbases. These lead to poor dispersion with reduced gloss and opacity. It is therefore extremely important that you optimize the millbase viscosity in order to generate the appropriate shear forces during dispersion. Maximum shear (at optimum viscosity) can usually be obtained by the addition of more pigment, rather than less solvent or more resin to the millbase. Since the optimized millbase is now richer in TiO2, the output volume of the finished coating will increase, leading to greater productivity. If the volume of the finishing tank does not allow for the above method of optimisation, then you can reduce the millbase volume to compensate. If you add 10% more pigment (producing 10% more coating) then reducing the overall millbase quantity by 10% will revert to your original output, whilst still giving you the required improvements in opacity and gloss.
Formulation Notes Top
Water Borne Systems
There are numerous advantages in using low water and dispersant demand pigments. It is important to note however that these advantages cannot be realised unless you take the appropriate formulation measures.
As with solvent borne systems, using low water demand pigments can lead to low viscosity millbases. Unless you compensate for this it can lead to poor gloss and dispersion. The same advantages of higher millbase loading also apply. It is therefore important that you optimize the millbase to suit the particular pigment selected.
You will not realise the advantages of low dispersant demand unless you optimize the dispersant level specifically for each pigment. In fact it is probable that a coating formulated for a pigment with a high dispersant demand, when made with a low demand pigment will have an excess of dispersant present. This excess dispersant (the level above that required to stabilise the pigment dispersion) may not only cause film sensitivity and possible ink lay problems but may also cause flocculation of the dispersed pigment leading to reduced opacity.
Figure 3 shows how the film opacity may vary with dispersant concentration for two pigments with different dispersant demands. It shows that the excess dispersant can "bridge" the dispersed pigment particles and lead to flocculation.
It is clear that the dispersant level must, therefore, be optimized in order to take advantage of the numerous benefits that low demand pigments have to offer.
Formulation Notes Top
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