Decorative/Architectural Finishes
The Decorative or Architectural paints segment is the largest in the coatings market. Whilst estimates of its size vary, the general view is that it accounts for between 60 and 65% of the overall volume of Titanium Dioxide used in the coatings market. Change, innovation and continued growth characterize the industry. Millennium Inorganic Chemicals has been a major supplier of titanium dioxide pigments to this industry for over eighty years. Of this segment, trade paints (those products sold to professional painters) are thought to account for just over two thirds, although this varies considerably by country.
One of the major changes that has taken place in the Decorative or Architectural market, from a TiO2 supplier's viewpoint, has been in the choice of pigment used. In the past it was not uncommon to find that a paint manufacturer would use three types of TiO2; one type for solvent based gloss paints, another for interior water based gloss paints and another for Matt emulsion paints. Nowadays paint manufacturers may use a multipurpose TiO2 product across their formulations. This leads to cost savings through reduced inventory for the manufacturer. Multipurpose and general purpose TiO2 pigments account for nearly 75% of the TiO2 pigments used in Decorative paints.
With the growing influence of environmental legislation it is inevitable that the trend to replace solvent borne paints with water borne equivalents will continue. Millennium Inorganic Chemicals supplies a number of their products in slurry form in North America. The use of slurry in the manufacture of water borne coatings can result in significant savings.
Important Considerations
Four considerations stand out as very important in the formulation of Decorative/Architectural Paints. These are
Dispersing TiO2
It is common practice to disperse modern pigments using only a high speed disperser (HSDM) to make Decorative Paints. While there are numerous different designs of HSDM equipment ranging from small laboratory sized mills to units capable of manufacturing in excess of 20,000 liters of paint, the basic principles of design and operation are essentially the same. Best results are obtained using the mill geometry shown in figure 1.
Correct blade speed (20 to 30 m/s) is essential. When the mill is operated correctly, a rolling doughnut type of circulation is induced by the rotation of the blade and a small inner portion of the blade should be just visible. Correct dispersion is critical to obtaining the best performance from the pigment. The process can generate considerable heat leading to solvent loss and viscosity reduction. For solvent borne millbases, it is generally accepted that the best dispersion will be obtained with resin solids in the millbase between 30% and 40% non-volatile content, the exact level depending on the type and quality of resin used. The ideal pigment loading is usually 75-80% pigment by weight or even higher. Water borne millbases use similar parameters. The millbase liquids would normally be a combination of water and glycol. A good starting ratio is 3:1. Normal levels of wetting, dispersing and anti foam agents should be used. A pigment loading of 80% or higher is again recommended.
Compared to conventional products, it is usually possible to incorporate up to 10% more of Millennium Inorganic Chemicals' leading multipurpose products, into a millbase. This can be achieved without incurring excessive viscosity or increasing the amount of energy necessary to reach the required dispersion level. Thus power demand can be kept to a minimum and a potential overload of the power unit can be avoided.
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Cost and Opacity
Fundamental studies in relation to optimum cost effective formulation have been carried out at Millennium Inorganic Chemicals and a short summary follows. Figure 2 shows the classical relationship between opacity and pigment volume concentration (PVC). At PVC's greater than 10% opacity no longer has a linear relationship to TiO2 content. At these higher levels, the rate of increase in opacity diminishes with increasing PVC until it reaches zero at approximately 25% PVC. This relationship will vary slightly with individual resins and pigments. This is due to pigment crowding effects and as PVC increases further, hiding power decreases. However, at lower, more efficient PVC's the raw material costs are higher. Therefore it is necessary to strike a balance between maximizing opacity and realistic costs. This is possibly one of the most difficult compromises to make when formulating coatings.
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Flat/Matt Latex Paints
Flat/Matt latex paints, designed for interior use, are usually formulated above the critical pigment volume concentration CPVC Considerations which is defined as the pigment volume concentration (PVC) at which there is just sufficient binder to fill the interstices between all the pigment and extender particles. It should be noted that the degree of "wet hiding" (apparent opacity during application of the paint) is proportional to the amount of TiO2 in the formulation. This means that for a given level of pigment, the higher the surface treatment on the pigment the lower will be the wet opacity.
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CPVC Considerations
At PVC levels above CPVC the extenders and the TiO2 will both contribute to the overall opacity of the dry paint film due to the presence of air/extender, air/pigment and polymer/pigment interfaces within the film. Below CPVC only pigment/polymer interfaces will contribute to opacity since the refractive indices of the extender and binder are very similar. The extenders in such paints will only contribute to opacity when the paint has dried and air has replaced the evaporating water, giving rise to what is known as the dry hiding effect.
Traditionally these emulsion paints have been formulated using titanium dioxide pigments with a high surface treatment level, and high oil absorption. This type of pigment effectively lowers the position at which CPVC is obtained and gives a greater contribution to dry hiding. Hence for a given total PVC, greater opacity or hiding power is obtained. High treatment levels also improve the spacing of TiO2 particles throughout the film. This allows each particle to give its maximum contribution to the opacity of the film and hence improves the cost effectiveness of the coating.
Figure 2 demonstrates the effect of PVC on emulsion paint opacity and also shows the opacity of a highly surface treated product in comparison with a multipurpose product. It should be noted that the degree of "wet hiding" (apparent opacity during application of the paint) is directly proportional to the amount of TiO2 in the formulation. This means that for a given level of pigment, the higher the surface treatment on the pigment the lower will be the wet opacity.
In recent years, the use of highly treated pigments in flat finishes has come under scrutiny and some paint manufacturers are now using multipurpose pigments with no loss of performance or increase in cost.
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