FORMULATING THE RAINBOW

Transcription

1 4 TINTING SYSTEMS Source: Marco Fotolia.com FORMULATING THE RAINBOW Label-free universal colourants created with matched set of dispersants. By Steffen Onclin, Heinz-Günther Schulte and Paula Gomez-Perea, BASF. Because the usage of solvent-borne architectural paints is decreasing, the established use of separate water-borne and solvent-borne colourant lines is becoming less attractive. A new toolbox consisting of four different types of eco-label compliant dispersants allows a full range of universal colourants to be produced. Continuous advances in water-borne architectural paint technology have led to a steady decrease in the use of solvent-borne alkyd paints. Nevertheless, the majority of paint producers continue to offer a solventborne line. This is primarily for high gloss finishes in exterior metal and anticorrosion application, where solvent-borne alkyds are an established class of coatings. As solvent-borne paints represent a small market share today, they do not really justify a separate tinting line. Therefore, universal colourants are becoming increasingly important. The general composition of a water-borne colourant is shown in Figure 1. Water and pigment are the main components of a colourant. Pigment loading depends on the pigment class. Opaque inorganic pigments are usually formulated at high (> 5%) pigment loading, while some organic pigments or certain blacks are formulated at much lower pigment concentrations. BASIC REQUIREMENTS FOR COLOURANT FORMULATIONS Modern colourants have to comply with a vast range of requirements. The technical requirements of the colourant can be divided into the properties of the colourant itself and its application properties in a paint film. The colourant itself needs to have a rheology suitable for accurate dispensing. Pigment loading has to be adjusted so that the tinting strength fits with the desired colour space at suitable dispensing quantities. The formulation has to be stable, which means no settling or changes in colour strength or rheology under storage or low-shear dispenser conditions. In addition, it needs to have the right drying properties in the can and the dispenser nozzle. After mixing the colourant with the paint, the application of that tinted paint and many of its paint film properties are tested. These include colouration properties, appearance and gloss, but also mechanical properties such as hardness, blocking, and scrub and water resistance. The key to achieving good paint film properties is optimal compatibility with the binder system that is used. Compatibility with a broad range of binder technologies is required; these include acrylic, styrene/acrylic and vinyl-acetate dispersions, but also silicates, silicones and PU dispersions. One key component in the colourant formulation is the dispersant package. It determines to a significant extent the colouration properties of the colourant and also governs its compatibility with different paint bases. DISPERSANT STABILITY IS CRITICAL IN COLOURANTS The pigment grinding step takes place in an aqueous environment, so excellent stabilisation in water is needed. Upon mixing with an aqueous let-down paint, the colourant components are exposed to a range of surfaces that are stabilised with surface-active species, such as titanium dioxide, the extender package and the binder. The pigment in the colourant needs to be stabilised well enough to prevent migration of the dispersants to other surfaces and vice versa. Such instabilities can lead to well-known undesirable phenomena in the applied paint film, such as flooding/floating and rub-out. For universal colourants in particular, the

2 TINTING SYSTEMS 41 RESULTS AT A GLANCE Because the usage of solvent-borne architectural paints is decreasing, the established use of separate water-borne and solvent-borne colourant lines is becoming less attractive. Universal colourants can tint both types of paint. However, the dispersant package significantly affects the properties of a colourant, and the dispersant requirements for universal colourants are challenging. Regulatory and sustainability considerations are increasingly important in the coatings industry and are a major driver for the development of new coatings raw materials. A new toolbox consisting of four different types of ecolabel compliant dispersant has been developed, which enables a full range of water-borne colourants to be formulated which can also be used in solvent-borne paints. Colourants using these dispersants comply with the major eco-labels, without compromising performance. requirements for the dispersant package are challenging. In this case, the aqueous colourant formulation is also used to tint solvent-borne alkyd paints. Therefore, besides providing excellent stabilisation in an aqueous environment, the dispersants should also provide compatibility with a solventborne paint system. With paint producers moving away from aromatic solvents to aliphatic solvents, this task has become even more difficult to achieve due to reduced system polarity. REGULATIONS EFFECTIVELY REQUIRE LABEL-FREE COLOURANTS In addition to technical properties, regulatory and sustainability considerations have become increasingly important in the coatings industry and are an important driver for the development of new coatings raw materials. Although this trend is global, the growing importance of sustainability is especially noticeable in Europe. Turning visions into reality Bühler engineering services. Considering extending or building a new plant? Then you are in the right hands at Bühler. Benefit from our long-term experience and vast capabilities in handling complex projects all over the world! We provide single-sourced, complete solutions from start to finish. No matter whether for small or large-scale production our experts will support you in turning your vision into reality. Figure 1: General composition of a water-borne colourant. biocide <1% rehology <1% defoamer <1% dispersant 1-1% humectant -1% pigment, extender 1-65% water Bühler AG, Grinding & Dispersion, CH-924 Uzwil, T , F , grinding.dispersion@buhlergroup.com, Innovations for a better world.

3 42 TINTING SYSTEMS Figure 2: Influence of a phosphoric acid ester-based dispersant on compatibility. Pigment loading 6%, 7% dispersant (solids) on pigment. Figure 3: Poor colour development in an aromatic-free alkyd paint with universal polymeric dispersant. 12 Colour development rubout PY Relative colour development (%) ,6 1,2,8,4 Rubout (DE) Pr. A/ Pr. A/ WB paint Fsrel Typ 2 SB alkyd Table 1: New dispersant toolbox for universal colourants. Technology Solids % VOC ISO VOC ISO Product labelling Eco-label compliant * Product A Phosphoric acid ester sodium salt 26 <.1% <.5% H315 Yes Blend of amine-and acid-functional polymers 91 Ca..3% < 1% H32, H315, H319 Yes Product C Non-ionic polyalkoxylate 1 <.1% <.1% None Yes Product D Non-ionic alkyl polyalkoxylate 1 <.1% <.1% None Yes * To 214/312/EU Figure 4: Application example using Pigment Black 7; pigment loading 3%, 2% dispersant (solids) on pigment. 2 12% Colour development PBk 7 Viscosity (mpa.s) at 1-1s Rel. colour strenght (%) 1% 8% 6% 4% 2% Benchmarkt 2 Viscosity fresh Viscosity after 2 weeks 5 C % fresh 2w 5 C fresh 2w 5 C fresh 2w 5 C fresh 2w 5 C WB paint 1 WB paint 2 SB paint 1 SB paint 2 Benchmarkt 2

4 TINTING SYSTEMS 43 Because of this growing awareness, the importance of paint directives and eco-labels has increased in recent years. Paint directives and eco-labels provide guidelines for paint formulations, and this has put pressure on coatings raw materials suppliers to offer raw materials that comply with these guidelines. Maximum VOC concentrations, for example, continue to decrease and even semi-volatile organic compounds (SVOC) guidelines are now listed. Also, risk labels are explicitly mentioned and are no longer permitted on the paint can. This means that colourants, which can be added to paints in large quantities, also need to be label-free. This is especially important because of the CLP regulation EC 1272/28 that requires proper labelling of mixtures that includes paints. In paint stores where customised colours are prepared, complex labelling calculations have become obsolete with label-free colourants. as those of organic pigments. Historically and for economic reasons, alkyl phenol ethoxylates are still being used in many parts of the world but for environmental concerns this chemistry clearly needs to be replaced by alternative ethoxylates [1]. ANIONIC AND PHOSPHORIC ACID ESTER DISPERSANTS Anionic surfactant-like dispersants are usually similar in structure to their non-ionic counterparts. The main difference is that one part of the molecule is functionalised with one or more anionic groups. These can be carboxylic, sulfonic or phosphoric groups. The products can be supplied in a free acid form or as (partly) neutralised structures. As the free acids often carry a severe eye/skin irritation or damage label, or can even be corrosive, the neutralised forms are preferred from a regulatory perspective. Such functional groups have a strong affinity for metal oxide surfaces, making them ideal products to disperse inorganic pigments and extenders. They can be used on their own, but are often combined with non-ionic surfactant types or polymers. CHOOSING DISPERSANTS FOR MAXIMUM COMPATIBILITY A colourant line usually consists of 12 to 2 individual colourants in order to cover the desired colour space. Many different pigment surfaces need to be stabilised, ranging from metal oxides to organic pigments and carbon blacks in addition to a variety of extenders. To manage the complexity of the raw materials, a dispersant or dispersant package with a broad window of applications is desirable. Dispersants for universal colourants can be roughly divided into three categories: non-ionic and anionic surfactant-like dispersants, and polymers. Non-ionic surfactant-like wetting agents and dispersants are a well-known class of materials. Their general surfactant-like structure consists of two distinct parts. One part is hydrophobic, providing affinity for hydrophobic pigment surfaces, and one part is hydrophilic, usually through use of an ethoxylated component, which provides a certain degree of stabilisation by the formation of a hydrated poly(ethylene oxide) shell around the pigment surface. Because of the poly(ethylene oxide) part, the products are considered polymers. Non-ionic surfactant-like products are available without hazard labels and can be (S)VOC-free, depending mainly on composition and molecular weight. This class of dispersants is available in a broad range of chemical structures in all HLB (hydrophilic-lipophilic balance) values and in molecular weights ranging from around 2 g/mol to several thousand g/mol. However, what all these structures have in common is that pigment affinity is based on hydrophobic interactions, which are naturally favoured by non-polar surfaces such The better choice for sustainable coating systems Environmentally friendly alkyd resins for wood and metal applications. high renewable raw material content water-based and water-dilutable cobalt and meko free free of coalescing agents flexible and fast drying Worlée-Chemie GmbH. Söllerstraße D Lauenburg service@worlee.de.

5 44 TINTING SYSTEMS Phosphoric acid esters are known to improve the universal character of a colourant. Figure 2 shows an application example of a Colour Index PY 184 colourant (i.e. a bismuth vanadate) dispersed with a benchmark dispersant for universal colourants. Colour development and rubout results in a waterborne paint are good. However, in a solventborne alkyd, poor colour development and strong rubout issues are observed. The addition of a newly developed phosphoric acid ester (product A ) as co-dispersant drastically improves colouristic properties in the alkyd system, while the results in the waterborne paint remain virtually unchanged. THE VALUE OF POLYMERIC DISPERSANTS Polymeric dispersants often show superior dispersing properties when compared to surfactant-like products. Because of their higher molecular weight, they provide more robust stabilisation, which can be steric, electrostatic or electrosteric in nature. Polymers that stabilise exclusively through electrostatic stabilisation, such as polyacrylic acid (co)polymers, are only used to disperse TiO 2 and extenders. Polymeric additives contain multiple pigment affinity groups. This multivalency results in strong attachment to the pigment surface, which makes displacement by other surface-active species in the let-down paint unlikely. Amine-based anchoring groups in particular are known to have a strong affinity to a broad range of organic pigment surfaces. Additional reasons for the use of polymeric additives include their often mild labelling and low (S)VOC content. Although polymeric dispersants can provide excellent stabilisation against flocculation, they tend to be less universal in compatibility when compared to surfactant-like products. A possible explanation for the poor performance of aqueous colourants with polymeric dispersants in alkyds is polymer/polymer-incompatibility [2]. Figure 3 shows an application example of a blue colourant, which was prepared with a polymeric dispersant that is suitable for aqueous and solvent-borne applications. In spite of the universal character of the dispersant, poor colour development is observed in aromatic-free alkyd paints. SET OF DISPERSANTS CAN PROVIDE BROAD APPLICATION Formulating a range of label-free universal colourants is a challenging task. The multitude of different pigment classes and paint formulations makes it virtually impossible to work with a single dispersant. The development of a new dispersant toolbox that was developed to formulate a complete set of label-free universal colourants is discussed below. The toolbox products comply with the major eco-label standards. Table 1 shows some of the characteristic parameters of the products. A starting point was to develop a new base dispersant with affinity for a broad range of pigment surfaces and good compatibility with both water-borne and solventborne paints. Based on innovative new dispersant chemistry, a combination of amine- and acidfunctional polymers was found that provides universal affinity for pigment surfaces. With this dispersant (product B, a mixture of amine- and acid-functional polymers) it is possible to formulate inorganic pigments at high pigment loading while also achieving good results with organic pigments and carbon blacks. Table 2: PBk 7 ( Special Black 1 ) universal colourant guide formulation. Component Level 6.7 Loxanol PL Water, demineralised 53.7 NaOH 25% 1.1 Pigment (PBk 7) 3. FoamStar SI Preservative.2 Total 1. The product carries a mild labelling, which allows its use in label-free colourant formulations at a maximum concentration of 1%, provided no other components with similar labelling are used. An example of a label-free universal colourant formulation is shown in Table 2. In this case, the polymer blend product is used as the sole dispersant. Figure 4 shows an application example of the same formulation. On the left, colourant viscosity and storage stability is indicated. Product B provides improved storage stability when compared to an industry benchmark dispersant for universal colourants, which carries a severe hazard label. The diagram on the right shows very constant colour strength data in two waterborne- and two solvent-borne paints, both freshly made and after storage of the colourant. Stability tests on tinted paints have not been carried out yet. Figure 5: Effect of dispersant combinations in a PB 15:3 colourant on viscosity and relative colour strength in an SB alkyd paint; pigment loading 42%, 2% dispersant (solids) on pigment Viscosity (mpa.s) at 1s Relative colour strength (%) 9:1 8:2 D 9:1 8:2 D Colourant (PB15:3) viscosity Relative colour strength in SB alkyd paint

6 TINTING SYSTEMS 45 With a boiling point of 188 C propylene glycol is clearly VOC. 3 questions to Heinz-Günther Schulte Water contamination in alkyds can cause a range of physical and appearance problems. What tests have you carried out to ensure that your water-borne colourants will not affect the performance of solvent-borne alkyds? The toolbox dispersants are very compatible with water and alkyds and enable the incorporation of small amounts of water easily. No negative optical effects were observed in the normal dosing rates up to a few per cent. Deep alkyd colours and anticorrosion formulations are normally not prepared via the tinting route. BASF has developed FAME (Fatty Acid Modified Emulsifiers) dispersants specifically to allow water to be incorporated into solvent-based paints without problems. Why did you decide not to use these products? One of the new dispersants in the toolbox belongs to the FAMEgroup. Indeed there is a product which allows water incorporation into an alkyd paint to fulfil e.g. VOC-regulations. However, this chemistry was not suitable to prepare high quality tinters with such broad performance as the toolbox dispersants allow us to. Dr Heinz-Günther Schulte Technical Marketing Manager Formulation Additives BASF heinz-guenther.schulte@basf.com What advantage do you see in preparing universal colourants in water rather than existing long-established universal systems such as those based on propylene glycol? Technically propylene glycol is an ideal liquid for universal colorants as it is nicely compatible with water and many other solvents. It also prevents from drying of the colorant in tinting machine nozzles. However, with a boiling point of 188 C it is clearly VOC and therefore restricted by almost all eco-labels. ADDITIONAL DISPERSANTS TO MATCH PIGMENT TYPE The toolbox additionally consists of a set of co-dispersants. Besides the phosphoric acid ester mentioned earlier it contains two newly developed non-ionic surfactant-like dispersants. Products C and D (alkyl polyalkoxylate) are label-free, VOC-free and have less than.5% SVOC. They perform well on a selected range of pigments. The main advantage of a toolbox lies in the combination of the different dispersant types to develop an optimal result for each pigment and base paint. The alkyl polyalkoxylate, for example, provides very good colouration properties on phthalocyanine pigments, but needs the addition of the polymer blend dispersant for rheology control of the colourant. The effect of dispersant combinations on viscosity and relative colour strength of a PB 15-3 (phthalocyanine blue) colourant is shown in Figure 5. Guide formulations with most of the commonly used pigments are available. Colourants and paints are complex multicomponent systems where interactions between raw materials are difficult to predict. Therefore, it is advantageous to work with a dedicated toolbox of dispersants. Because of the different nature of the dispersants, a broad application profile can be covered. Moreover, detailed fine-tuning is possible to match the properties desired through combinations of the toolbox components. REFERENCES [1] ECHA, Recommendation List for Annex XIV, July 1st 215. [2] Elias H.-G., Makromoleküle, 4. edition, p.211.