Going “green” with inorganic pigments for coatings

Pigments manufacturers have been challenged to increase the “green” profile of their products to meet the growing Coatings market requirements. Inorganic pigments have been under high pressure for years, mostly as regards their heavy-metals content. In the early 2000s Nubiola Inorganic Pigments identified this environmental concern as an opportunity to offer a differentiated product portfolio of higher added value. To develop “eco-friendly” anticorrosive pigments and to upgrade the pigment performance of an “already-green” product as Ultramarine Blue, hardly used in coatings, were the targets. The successful results of all the work carried out since then are now in the market. By Alex Capuz, Global Marketing Manager at Nubiola Inorganic Pigments

“Greener” coatings demand more environmentally-friendly pigments: a threat or an opportunity for inorganic pigments producers
During the last decades the European Union authorities have become very much aware of the need to make all the possible to ensure a sustainable and healthy development of our world. One of the consequences of this is that Paints&Coatings producers have been facing a tremendous challenge to meet more demanding “environmentally-friendly” requirements for their products. Since the most important components of any paint are binders, pigments, fillers, solvents and additives, the “green” challenge has been directly transmitted to the manufacturers of such raw materials.

Pigments provide two types of main benefits to any coating: aesthetics and functionality. Being very important the decorative aspect pigments offer, they are more than just color. They can increase the overall performance of the protecting color coating by enhancing its durability and increasing the protection of the substrate. Pigments’ characteristics such as chemical resistance, weather fastness, corrosion protection, etc are some of the key ones to deliver such benefits.

A general and basic way to classify pigments divides them into organics and inorganics. Taking into account the raw materials to produce them it can be said that organic pigments are those that derive from oil and coal tar distillation. Inorganic pigments do not have that origin but usually the precipitation of synthetic metal oxides and salts. Due to their chemical nature, organic pigments have been historically associated to bright shades and high tinting strength properties that enable them to provide intense colors to paints. However, organic pigments are mostly transparent and have limited weatherability performance. On the other hand, inorganic pigments have been traditionally linked to high opacity and durability so that they have been mostly used in outdoor applications. In the case of anticorrosive pigments, they become active corrosion-prevention players of metal substrates. Some inorganic pigments like Iron Oxides have been used for centuries, therefore there is a long background of their good performance. As major drawbacks, they tend to be duller and with weaker tinting strength than the organic ones.

From the environmental standpoint both organic and inorganic pigments face a growing pressure to meet the “green” requirements of the 21st century. This pressure is related in each case to the presence of chemical compounds that are intimately linked to the chemical nature and manufacturing process of the two families. The presence of harmful halogenated organic compounds as well as VOCs (Volatile Organic Compounds) are some of the most important issues usually associated to organic pigments. When it comes to inorganic pigments, the most important challenge manufacturers face is the prohibition/limitation of the use of paints containing certain pigments made of very specific heavy-metals found to be harmful for human health and/or the environment e.g. Lead, Chrome (VI), Cadmium, Zinc. These heavy-metals are major components of several inorganic pigments that historically played a major role in the high performance of decorative and industrial coatings. Some examples are Chrome Yellows and Molybdate Oranges, Zinc Chromates, Strontium Chromates and to a lesser extent, Zinc Phosphates.

Nubiola Inorganic Pigments has focused its business on the production and commercialization of some of the most popular inorganic pigments since it was founded in 1914 by the Nubiola family i.e. Ultramarines (we are the world’s largest producer), Iron Oxides, Chrome Yellows and Molybdate Oranges, Green Chrome Oxides and Anticorrosive pigments.

In the early 2000s we identified the “green” challenge of the Coatings market as a major driver of the future of the business of our customers, and hence of our growth. By regularly talking to them we learnt that customers were just starting to develop future products that had to comply with all the environmental regulations that were going to be rapidly in place. High pigment performance/cost ratio was not anymore the only characteristic searched, but also “green” compliance. We therefore decided to focus our R&D efforts on improving our existing product portfolio in order to better fit the new market requirements. The two main product development areas were Anticorrosive pigments and Ultramarine Blues. On one hand, the target was to develop “eco-friendly” corrosion inhibitors beyond Zinc Phosphate and its derivatives. On the other hand, we aimed at upgrading the historical limited performance in coatings of an “already-green” pigment as Ultramarine Blue.

“Green” Anticorrosive pigments, the future beyond Zinc derivatives
The technology to prevent corrosion in metal structures by using anticorrosive pigments has been one of the most rapidly evolving ones in the last decades. The driver of such development has been the fact that some of the most popular pigments used for such application have been declared dangerous substances for human health and the environment so that they require special labelling. This happened, among others, to Zinc Chromate, Zinc Tetraoxychromate and Strontium Chromate.

In 2004 Zinc Phosphate was also classified as harmful for the aquatic media by the European Directive 2004/73/CE and hence it required special labelling. Nubiola then decided to further optimize the Zinc-based product technology the company acquired in the past in order to generate a range of anticorrosive pigments that outperformed Zinc Phosphate as a corrosion inhibition solution without requiring any labelling. The result of this was the development of two new series of the Nubirox anticorrosive pigments range: the Nubirox 100 and 200 series.

Nubirox 100 series is based on the combination of a small spherical particle size Zinc Phosphate with Zinc Molybdate and an organic surface treatment. The resulting products are much more effective than conventional Zinc Phosphate even at lower dosages, improve the pigment-binder interphase, show a high binder compatibility both for water and solvent-born systems and are very suitable for thin film coatings applications e.g. coil coating. Moreover they are very compatible with other potentially synergetic corrosion-inhibition solutions.

Nubirox 200 series are multiphase pigments consisting of hydrated Zinc and Iron Phosphates. They again defeat Zinc Phosphate as regards its anticorrosion ability even at lower dosages and they are close to being universally compatible with all sorts of binders, both in water and solvent-born systems.

However, one step further in the journey towards “eco-friendly” corrosion inhibitors is to produce high performing anticorrosive pigments that do not contain Zinc, either because of the environmental pressure this metal is under, or the price instability of the raw materials to make it. So far “eco-friendly” products available in the market have shown limited performance. They show poor early corrosion resistance, are not cost effective and tend not to be user-friendly.  Years of R&D work at Nubiola has resulted in the development of the Nubirox 300 series that target to excel at “eco-friendly” corrosion inhibitors. The first product of such a new range is Nubirox 301, a Calcium Strontium Phosphosilicate that does not contain any reportable heavy-metal in its composition. This Zinc-free pigment behaves as a direct anodic inhibitor of high performance thanks to the sphere-shape of its fine particles (average particle size = 1.0 µ, with a significant rate of “active” nanoparticles in its composition).  It shows an excellent early corrosion resistance as well as good cost effective performance on multi-substrates. The excellent results of the massive amount of in-house corrosion tests carried out with this new product are being confirmed by customers worldwide, from Europe to North America through India and other emerging coatings markets.

Ultramarine Blue, more than just a “green” product but a high performance pigment for Coatings
Ultramarine Blue (PB 29) has not been historically considered as a high-performance pigment for Coatings. It has mostly been used to tint white paints in order to provide a blue undertone highly appreciated by paint makers. It is however a pigment that shows outstanding technical characteristics that could make it very appealing for coatings producers in the 21st century.  The most important is the uniqueness of its reddish undertone. Such a blue color space cannot be achieved by any other pigment or combination of pigments. Therefore it becomes an exceptional differentiation factor for coatings producers. Moreover Ultramarine Blue is a very easy-to-disperse and to wet pigment, neither migrates nor flocculates, has a high light fastness, is heat resistant, has a high solvent and alkali fastness and does not show any metamerism. On top of all these attributes, it has a well recognized “green” profile because it does not contain any reportable chemical compound e.g. heavy-metals, VOCs. The “mild” chemical nature of the pigment, i.e. a Sodium Aluminum Sulpho-Silicate, is definitively a guarantee of excellent “environmentally-friendly” performance.

On the other side of the coin, Ultramarine Blues do not certainly have high tinting strength in comparison to organic pigments, nevertheless it is better than other inorganic pigments such as Cobalt Blues. And it is true that Ultramarines are very transparent and do not provide opacity, but in many applications this is not a con but a pro e.g. metallic paints.

What then is the reason why this pigment has never been on the list of high-performance pigments for Coatings? The answer to this question is the historical perception industry technicians have had of the pigment’s poor weather fastness derived from its lack of acid resistance.

Due to its polysulfured composition, conventional Ultramarine Blues are not acid resistant. The pigment has a very porous structure. When Ultramarine Blue comes into contact with an acid, this acid penetrates into the pigment and breaks the polysulfur groups damaging the 3D structure of the pigment. The polysulfur groups are the chromophores of the colorant. Once they have been broken there are no more chromophores available and the reddish blue color becomes white.

When could any Ultramarine Blue incorporated into a coating potentially be exposed to acids? Mostly in the case the coating is exposed outdoor. On one hand, exterior coatings are subjected to UV radiation, high temperatures, water (both humidity and rainfall) and oxygen for long periods of time. Even in the case of using the most stable organic binders they will progressively degrade in the presence of such conditions. This degradation might generate acid radicals that could be released from the binder and attack the Ultramarine Blue structure. On the other hand, when exterior coatings are located in highly industrial areas they can be exposed to the “acid-rain” impact, hence:

• traces of acid from the atmosphere can slowly penetrate into the paint (if it is not well protected from them) and/or
• the binder degradation, if the system is not highly resistant to industrial atmospheres, can lead to a deterioration process that gives as a result acid components.

This “acid rain” could then lead to the discoloration of Ultramarine Blue used in paints located in such environments.
In addition to all these, any coating made with binders of acid-nature e.g. PVDF, PVC plastisols or containing acid additives is likely to have in its composition little traces of acid components that could equally cause the color fading of Ultramarine Blue.

Therefore to make Ultramarine Blue become a high-performance pigment for Coatings, Nubiola Inorganic Pigments faced the challenge to develop a new manufacturing technology that protected Ultramarine Blue from acid attack in order to be used in exterior coatings. From all the technologies tested, the silica-encapsulation of the pigment proved to be the most effective. This technology has already been in the market for several years but the performance of this encapsulation was not the best one. It certainly protected Ultramarine Blue from acids to a limited extent but at the cost of other characteristics of the pigment e.g. lower tinting strength, high oil absorption, poor dispersibility, which limited the use of the pigment in Paints&Coatings.

The newly developed silica encapsulation technology overcame all the limitations of the conventional acid resistant Ultramarine Blues from the past. This new technology delivered a new product called Nubicoat HWR (High Weather Resistance). It is based on the fact that the new technology encapsulates elemental particles and not agglomerates. A thinner silica layer is deposited therefore on each Ultramarine Blue particle. This gives the encapsulated pigment a smaller average particle size as well as a narrower particle size distribution. Moreover it provides higher abrasion resistance of the protective silica coating.

In order to demonstrate the upgraded weather fastness of Nubicoat HWR vs conventional Ultramarine Blues, Pthalocyanine Blues and Cobalt Blues, we carried out dozens of accelerated and long-term/natural weatherability studies in different coatings. The results prove that the weather fastness of Nubicoat HWR is similar if not better than high quality Phthalocyanine Blues (PB 15) and just a bit lower than Cobalt Blue (PB 28), the highly durable blue pigment benchmark. More than two years after the launch of such a breakthrough product this pigment has gained approvals and is regularly used in a wide variety of applications e.g. façade paints (water-borne acrylics, styrene acrylics, silicate emulsion), interior decorative paints, powder coatings, coil coatings, coatings for plastics, bicycle and motorbike paints, tinting systems for architectural and industrial coatings.

Summary
Inorganic pigments manufacturers have faced the challenge to improve the “green” profile of their products to meet the fast-growing environmental requirements of coatings producers. Nubiola Inorganic Pigments proved that this potential threat became an opportunity to develop high added value solutions: Nubirox 300 series, “eco-friendly” anticorrosive pigments; and Nubicoat HWR, an Ultramarine Blue that is more than just a “green” pigment but a high performance pigment for Coatings.

About Nubiola Inorganic Pigments
Nubiola Inorganic Pigments is a specialized world-wide producer of Inorganic Pigments, with headquarters in Barcelona, Spain. Nubiola is a family-owned company with more than 90 years experience in the marketplace, committed to sustainable growth based on the technical and production capabilities that permits to market innovative and efficient products. That commitment explains its position of leading world supplier of Ultramarine pigments as well as a valid alternative in a wide range of pigments such as Iron Oxides, Zinc Ferrites, Anticorrosive pigments, Green Chrome Oxides and Chromates and Molybdates. Nubiola employs about 600 people, has five production plants all over the world and sells to more than 85 countries. Customers are mainly in the plastics, coatings, construction, printing ink, artist’s colours, cosmetics, soap, detergents, rubber, ceramic, mosaic and other industries.

About the Author
Alex Capuz holds a Chemical Engineering degree from the Institut Químic de Sarrià (Barcelona, Spain) and an MBA from ESADE (Barcelona, Spain). He started his professional career as a researcher in the R&D department of Uriach, a leading pharmaceutical lab in Spain. 3 years later he moved to the Surface Sciences Division of Huntsman International, where he occupied several marketing posts for 5 years until becoming European Market Manager for Industrial markets. He joined Nubiola Inorganic Pigments in 2003 to become the Product Manager of Ultramarine Pigments. He is the Global Marketing Manager of Nubiola Inorganic Pigments since July 2006.