PIGMENTS CONCENTRATE PASTES Marco Zuccadelli, Elisa Tarasco, Marco Canini AB Chimica S.r.l., Italy Introduction WHAT ARE THEY? Preparations containing PIGMENTS, BINDER, SOLVENT and ADDITIVES. Pigments are supplied by the industries in the form of dry powders. These can be named primary particles, which are sized to give the best results in paint films. For example, white pigments (Titanium Dioxide – TiO2) scatters light most efficiently (whiteness and opacity) when the particle size is 0.22 microns. Some pigments are much smaller, some larger. The small primary particles of pigment clump together when the powder is dried and form relatively large clumps (a clump of Titanium Dioxide may contain 5,000,000 primary particles). These clumps are known as aggregates and in this aggregated form the pigments are supplied. To produce good pigments dispersions, it is necessary that the pigment should be at its primary particle size when mixed with the vehicle. Otherwise most of the expensive coloured pigments could not be used efficiently, and the dry film will not be smooth, glossy, opaque or have the correct colour. It is therefore necessary to break down the aggregates of pigment particles in the presence of the vehicle and to prevent them from re-aggregations. For this reason, it is not possible simply to stir the pigment into the vehicle. Rather, work has to be done on the mixture in order to break down the aggregates and ensure that the resin coats the small particle surfaces. This process is known as pigment dispersion and the following processes must occur: - Wetting of the pigment surface by the vehicle - De-aggregation of the pigment aggregates to small primary particles - Stabilisation of these small particles by the resin to prevent re-aggregation PIGMENTS They are substances that impart coloration to the support in which they are incorporated or to which have overlapped. They distinguishes from the colorants which act on the sub-layer being in it dissolved. They are classified through the Colour Index number (international method of classification). The Pigments can have both natural origin and synthesis and both organic and inorganic nature. Some pigments nature: Colour Inorganic Pigments Organic Pigments Black Iron Oxide Carbon Black Yellow Blue Violet Green Red White Iron Oxide Lead Chromates Ultramarine Blue Cobalt Blue Chrome Oxide Iron Oxide Molibdate Red Titanium Dioxide Azoic Pigments Isoindolinone Phthalocyanine Quinacridone Phthalocyanine Manganese Toner Quinacridone BINDER The binder can be a resin or a mixture of resins. For chemical reaction or for simple evaporation of the volatile substances a resinous mixture is formed, polymeric, resistant, endowed with filming properties, or rather able to give origin to a continuous film. The binder is loosened or dispersed in volatile substances and this solution or dispersion is called Vehicle. It is defined water or solvent vehicle depending if the volatile substance is made of water or organic volatile substances. The type of resin used can be varied to enable the paint to have whatever final properties are required. Commonly used resins: Solvent-base Water-base Long-oil Alkyd resin Medium-oil Alkyd resin Short-oil Alkyd resin Ketonic resin Epoxy resin Water-soluble Acrylic resin Aqueous mix of Polymeric Surfactants SOLVENT It is a substance that in the liquid state it is able to solve other substances. Dissolve or suspend the binder and give the paint the correct consistency (viscosity) for easy application. It must also evaporate after application to allow the paint to dry. Functions of the solvent in a coating: - It solves the binder resulting in the vehicle in which the pigment is dispersed - It wets the support on which the coating product will be applied (i.e. in the case of the wood it also has to penetrate) - It influences the formation of the film that, according to the demands, has to be resistant, compact, homogeneous, continuous and smooth - It evaporates completely in the time and under the desired conditions Commonly used solvents: White spirit Xylol Water PMA Butyl acetate ADDITIVES Commonly used additives: Defoaming agents, used in water-based paints for the foam formation prevention; Dispersing and wetting agents, active in the pigment particles demolition, helping the bonding with the resin; Coalescent, help the formation of an homogeneous film during the application; Antiskin, avoid the formation of superficial skin in solvent-based systems; Anti-sedimentation, avoid the formation of sedimentation; Hardeners, added to the finished paint, help the exsiccation; Preservatives, used into the water-based paints, avoid the proliferation of bacteria and mildews. Meaning WHAT ARE THEY FOR? Semi-processed preparations for the production of varnishes, industrial, construction and do-by-yourself enamels, wood stains and printing inks. They simplify and facilitate the productive trial, guaranteeing to the user constant and continuous respect of the specifics and high quality level. PRODUCTIVE TRIALS: THE PIGMENTS DISPERSION PHASE Whoever produces varnishes, for whatever type and use, is today facing a big dilemma: Producing from the powdered pigments or from the concentrate pastes? The dispersion phase implicates the use of machineries, called mills, with employment of manpower both for the production and maintenance and the stock of numerous raw materials, with the relative problems of logistic, control and management. THE CONCENTRATE PASTES ADVANTAGES • • • • • • Reduction of the production time, avoiding the grinding phase, including the relative costs, difficult to calculate, i.e. the expenses of cleaning the machineries and general manpower Reduction of money investments dedicated to the machines used for the dispersion Reduction of the pollution inside and outside the business premises Increasing of the productive rhythm without relative costs Reduction of the warehouse costs, avoiding the production of an entire batch, purchasing only the necessary quantity for the immediate use Reduction of the raw materials quality control costs QUALITY, CONTROL AND RELIABILITY The numerous and accurate tests done on all the entering raw materials, on the preparations in phase of production and on the final product, guarantee the constant acquiring of the followings principal characteristics: • • • • • • Maximum grinding fineness High covering Tone purity by the high saturation value Low flocculation tendency Respect of the quality standards required Assured conformity to the specifications Problems of the pigments preparations Being high pigments concentrate preparations, the pastes have few problems related mainly with the way the users handle them. For instance, the shock of the pigments once suddenly diluted into a low concentrated solution is a common factor influencing the reaction between the paste and the converter. A pre-dilution of the paste with a 1:1 ratio is suggested, all the time. The characteristics of the different pigments have to be considered using the preparations, any known problem will affect the rheology and the stability of the paste, as well as the finished paint. For example, the Phthalocyanine Blue and the Carbon Black pigments have the tendency to flocculate, so a 1:1 pre-dilution with the converter is warmly suggested; the heavy inorganic pigments (i.e. Iron Oxides) could form a pigment/solvent stratification and so a constant agitation of the preparation is suggested. In few words, we can summarize that any pigment problem is a paste problem, but being the latter an high pigments concentration preparation, that problem become critical. FLOCCULATION Flocculation or reagglomeration refers to the congregation of already dispersed pigment particles. It is a consequent of the high energy state of the pigment particles with their large surface areas, combined with inadequate stabilization. Destabilizing forces of attraction, depending on the chemical character of the pigment surface, include: Van der Waals forces (dispersion forces, London forces, dipolar interactions, induced dipolar interactions) Hydrogen bridge bonds Ionic interactions (Coulomb forces) Flocculation is usually an undesirable state since the colour of flocculated pigments is less intense than that of their finely divided (flocculated Red Iron Oxide has a bluer tint than its finely divided equivalent), or cause flooding. There are some perspective, however, from which a certain, or selective, flocculation is actually desirable, to prevent hard sediment, for example. SEDIMENTATION (Settling-out) The sedimentation problem is due to the following main characteristics: Pigment density: the higher it is, the more quickly the pigment settles-out System viscosity: the greater it is, the more slowly a sedimentation is formed Particle radius: large, un-grinded or flocculated particles sink much more quickly than small ones Agglomerates or flocculates are very large particles and hence they settle-out more quickly but form only a soft sediment. Fine particles sink more slowly to the bottom but form a tightly compacted, solid sediment that in extreme cases can be virtually free from binder. PH VALUE The pH value is closely linked to electrical conductivity but is not the only factor that influences it. As a measure of the concentration of H+ ions, i.e. for the acid/base behaviour of aqueous solutions in the ranges: pH < 7: acid behaviour pH = 7: neutral pH > 7: alkaline, basic behaviour the pH value is particularly important for the stability of the dispersion or emulsion binders, but also for the dissolved film formers, for which the following principles must be taken into consideration: If binder or coatings/paints are acid-neutralized, i.e. acid stabilized, their resistance is usually increased (to a certain limit) due to an additional acid excess, i.e. a lower pH value; a rise in the pH value due to the addition of alkali, on the other hand, leads to rapid coagulation Conversely, alkali-neutralized binders and coatings (neutralized with amines or alkali solutions) can readily tolerate a somewhat higher pH value but definitely not an addition of acid The colour intensity and stability of pigments as well as the subsequent weather-proofing and corrosion protection action on the coating system also depend on the correct pH value. Measurement errors in pH determination are unfortunately a common occurrence and one of the most frequent sources of error in the development, production and application of water-borne coatings. Materials and Methods The following are the values and tests every manufacturer of pigments pastes should verify batch after batch, in order to guarantee the constant compliance to the given specifications: Reduction with white for the tinting strength (spectrophotometer) Full shade for the brightness and covering characteristics (spectrophotometer) Viscosity with a (Brookfield) viscosimeter Specific weight by pycnometer pH value, for water-based products, by a pH-meter Grinding fineness with a grind-meter When the grinding fineness value is the lower obtainable, depending on the pigment type, further tests are done to verify the colour strength, shade and brightness. If they are under the specifications, it is necessary to verify the viscosity, the specific weight and, in case of water-based products, the pH value. COLOUR STRENGTH Colour intensity is not only important for supply agreements, it is also a good means to determine an optimum dispersion time and other dispersion parameters, since it increases sharply during the dispersion process as a result of the growing number of particles, until it reaches a limiting value which thereafter remains unchanged. As a consequence of over-dispersion and possible subsequent flocculation, the colour intensity may even decrease again over a continued dispersion period. Colour Strength (%) Dispersion Curve 100 90 80 70 60 50 40 30 20 10 0 1 2 3 4 5 6 7 8 9 10 Dispersion Time (h) The slope and course of the dispersion are very much dependant on the coherence and wetting ability of the pigments particle size and shape. Finely divided, organic pigments are usually significantly more difficult to disperse than inorganic pigments containing coarser particles. Accordingly, micronised, Transparent Iron Oxides pigments are likewise more difficult to disperse than conventional, opaque Oxide pigments. PARTICLE SIZES The modern methods of determining particle size distributions are now indispensable in the development, production and quality control of pigments, binder dispersions and even for coating materials. The most important methods are: Coulter Counter Photon Correlation Spectroscopy Laser Light Diffraction These three methods can only be used with extremely diluted dispersions, in which the particle sizes can quite easily deviate somewhat from their original state. RUB-OUT TEST To reveal flocculation or flooding, a RUB-OUT test is performed by rubbing the film with a finger while it is still damp but already starting to dry. In doing so the flocculated or flooded, i.e. separated, pigments are rubbed together and the colour of the homogeneous blend is reinstated by means of a kind of re-dispersion. The difference in colour intensity from the unrubbered film indicates the extent of the problem. If a RUB-OUT test on white/coloured pigmented systems shows the colour intensity of the rubbed coated surface to be greater than that of the rest of the surface, we refer to a positive RUB-OUT effect, and in the opposite case to a negative effect. Tinting Systems The demand of quality products is increasing year after year and not only for industrial coatings. At today even the final user for do-by-yourself applications is demanding more and more quality, in terms of constancy of the colorimetric characteristics and environmental safety matters. The market demand of water-based varnishes for wood, industrial coatings and road signs is sensibly increasing, the new European regulations for the VOC’s emissions and the high costs for the research and development departments are forcing more and more paint producers to take the decision of quitting the dispersion phase into the productive trial, quitting the costs of the lab, avoiding to do colorimetric compliance tests as a routine job and quitting to stock numerous raw materials like the pigments, reducing the warehouse stock cost and manpower. Also, the difficulties to find well prepared young technical persons and the reluctancy that most of the senior technical managers demonstrate in training youngsters, make the demand of complete tinting systems enormously increasing in the last few years. At today it is possible to find in the market suppliers of tinting systems for all the different coating applications. It is necessary to highlight the differences between the two main tinting systems way to operate: volumetric and gravimetric. Years ago the totality of the tinting machines dedicated to the small shops were calibrated to work exclusively with the volumetric system, being easier for the workers dose and sell the finished products without making extra activities like involving balances, taking into consideration the specific weight data and making calculations. The numerous problems created by the sudden viscosity data variations (i.e. in-stock temperature and humidity data variations) affect the colorimetric characteristics of the pigments concentrate pastes and the formulations have to be settled time by time. On the opposite way, the gravimetric controlled pigments dispersions guarantee a reliable constancy in maintaining their colorimetric characteristics unchanged in the time. The modern way to produce tinting machine has included the possibility to switch the system from volumetric to gravimetric even in the smaller machines, just adding a good balance and asking to the supplier a dedicated gravimetric operating software. Conclusions The traditional production methods are more and more expensive and therefore anti-economic and inadequate. It is necessary to specify that the pigments pastes are able, at today, to satisfy any particular demand; these preparations are formulated to be compatible with the most used resins having the purpose to produce industrial and resale paints, not altering but increasing the quality specifics required by the final users. Their complete adaptability to the modern industrial tinting systems fully satisfies all the production requisites. The introduction of these preparations is an occasion to revolutionize the traditional productive method, furnishing the means with which the paint manufacturers can economize the procedures satisfying the always greater demand of ready products, either in different packaging and in different colours. Besides, the use of the pastes allows to give a valid technological answer to the legislation in subject of environmental and ecological respect. Bibliography [1] T. Brock, M. Groteklaes, P. Mischke: European Coatings Handbook (2000)