Process for producing redispersible pigment granules, and redispersible pigment granules thereof

Abstract

A discontinuous high-shear wet granulation process for producing redispersible pigment granules comprising the following steps: a. Providing at least one powder pigment in its natural aggregated and agglomerated form; b. Blending the at least one pigment with at least one insoluble grinding aid and / or disintegrant in powder form obtaining a blend; c. Dispergating the blend with a first liquid mixture comprising at least two non-polymeric surfactants by means of high-shear mixing, obtaining pigment particles with an average particle size lower than the average particle size of the pigment agglomerates and aggregates; d. Wetting the pigment particles with a second liquid mixture comprising at least one polymeric surfactant under high-shear mixing and letting the wetted pigment particles to nucleate; e. Adding at least one granulating polymeric binder in solid form to the wetted pigment particles under high-shear mixing, resulting in the formation of a cohesive solid bed of particles; f. Feeding the second liquid mixture onto the solid bed of particles under high-shear mixing, obtaining wetted pigment granules; g. Massing the wetted pigment granules to grow by coalescence; and h. Drying the wetted pigment granules of step g. obtaining redispersible pigment granules.

Description

[0001] "Process for producing redispersible pigment granules, and redispersible pigment granules thereof'

[0002] TEXT OF THE DESCRIPTION

[0003] Field of the invention

[0004] This disclosure concerns a process for producing redispersible pigment granules, and redispersible pigment granules thereof.

[0005] Background of the invention

[0006] Pigments are a key component in formulating and tinting coatings and plastics and play a critical role when it comes to performance. Pigments only develop their beneficial action when they are evenly distributed within matrices (vehicles), where they are in liquid solutions or melted forms. Most pigments are produced and marketed as dry powders that "agglomerate" due to mutual attraction and therefore present themselves as larger, fairly spherical entities. The destruction of these structures by mechanical forces in polymer solutions or melts, yielding a homogenous distribution of the single pigment particles, is called "dispersing". In that sense, dispersing is the elemental step in the production of any composite materials, especially in the case of coatings.

[0007] Despite the huge importance of dispersion processes in the production of composite materials, dispersing itself is still often looked upon as being more of an art rather than a fundamental, scientifically underlain technical process. The reason for this may lie in the fact that several separate steps take place simultaneously during dispersions. These steps are the wetting of pigment surfaces, the mechanical disruption of agglomerates, and the stabilization of the single "primary" pigment particles obtained against renewed agglomeration.

[0008] Pigment dispersions often are marketed in liquid form at relatively high pigment concentrations and are used in additive quantities to impart color to a wide variety of products. The term "pigment dispersions" is often used synonymously with colorants, color concentrates, and pigment preparations. They are used to color many types of materials, such as paint and coatings or plastics. Applications include decorative and industrial applications, thermoset composites (powder coatings), inks, plastics, and textiles.

[0009] In liquid form, pigment dispersions are typically added to another material by either dispensing (volumetrically) or adding by weight (gravimetrically), which can be an automated or manual process. Liquid pigment dispersions present several disadvantages, including a low environmental profile due to the relatively high content of VOCs (Volatile Organic Compounds) and biocides. Additionally, they pose challenges in maintaining clean workspaces and preventing mold infections. There is also the issue of pigment separation within liquid systems, leading to inaccurate dosing and fouling of dosing systems. The evaporation of solvents alters the concentration and viscosity of the liquid, further compromising dosing accuracy. Moreover, these dispersions have a short shelf-life.

[0010] Such drawbacks have increased interest in the use of formulated pigments as dry granules which have several advantages over liquid formulations: reduced transport and handling costs through not having to transport large amounts of solvent, typically water, over large distances; improved physical and chemical stability resulting in longer shelf life and improved safety; allowing to design-in complex product performance; multicomponent products.

[0011] Formulated or structured pigments in solid form also eliminate the issues presented when pigments are used as fine powders, such as: high dustiness; safety and health issues; product losses; low flow properties and handling (which impede controlled metering); low bulk density for packaging; caking over storage; etc.

[0012] As an effective alternative to liquid pigment dispersions, structured solid pigments have to readily achieve a comparable degree of fineness when applied to the medium for which they are intended.

[0013] Most of the available pre-dispersed dry pigment dispersions are produced by spray-drying a conventional liquid dispersion of pigments containing various types of additives. Whilst the resulting products attain good performance, the process is highly energy intensive due to the removal of large amounts of liquid (solvent mostly water) by either conventional thermal evaporation or other methods like freeze-drying. The burden of such high costs makes the value proposition of such products less appealing.

[0014] Although many attempts have been made to produce pigments in solid form that are easily dispersible, this goal has not been achieved yet.

[0015] Summary of the invention

[0016] The object of this disclosure is to provide a process by which pigments can be produced in granular form that are readily dispersible in the final medium.

[0017] According to the invention, the above object is achieved thanks to the subject matter recalled specifically in the ensuing claims, which are understood as forming an integral part of this disclosure.

[0018] The present invention provides a discontinuous high-shear wet granulation process for producing redispersible pigment granules comprising the following steps: a. Providing at least one powder pigment in its natural aggregated and agglomerated form; b. Blending the at least one pigment with at least one insoluble grinding aid and / or disintegrant in powder form obtaining a blend; c. Dispergating the blend by applying a first liquid mixture comprising at least two non-polymeric surfactants by means of high-shear mixing, obtaining pigment particles with an average particle size lower than the average particle size of the pigment agglomerates and aggregates; d. Wetting the pigment particles with a second liquid mixture comprising at least one polymeric surfactant and letting the wetted pigment particles to nucleate under high-shear mixing; e. Adding at least one granulating polymeric binder in solid form to the wetted pigment particles under high-shear mixing, resulting in the formation of a cohesive solid bed of particles; f. Feeding the second liquid mixture onto the solid bed of particles under high-shear mixing, obtaining wetted pigment granules; g. Massing the wetted pigment granules to grow by coalescence; and h. Drying the wetted pigment granules to obtain redispersible pigment granules.

[0019] The present inventors found out that wet granulation of pigments performed in batch operated high shear mixers can attain semi-permanent aggregates in which dispergated pigment particles are still distinguished and are easily redispersible in the final medium.

[0020] Brief description of the drawings

[0021] The invention will now be described, by way of example only, with reference to the only drawing, wherein a schematic representation of the process subject of the present invention is provided.

[0022] Detailed description of the invention

[0023] In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the embodiments.

[0024] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0025] The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0026] With the term "dispergation" is meant a process of breaking down agglomerates or clusters of solid particles into their primary particles and smaller aggregates. Dispergation consists of two steps: 1. wetting the solid particles to reduce inter-particle forces and facilitate separation, and 2. shearing to physically break down agglomerates and distribute the particles. The outcome of dispergation is a wetted mixture of primary particles and smaller aggregates ready for further processing or incorporation into a medium. Dispergation is different from "dispersion". "Dispersion" refers to the distribution and stabilization of solid particles within a medium to form a stable system where the particles remain suspended and do not re-agglomerate. The key steps of dispersion are: 1. wetting; 2. breaking down and distributing particles throughout a medium, and 3. introducing stabilization mechanisms, like adding surfactants or modifying the zeta potential, to prevent particles from settling or re-agglomerating. The outcome of the dispersion is a stable colloidal dispersion where solid particles are evenly distributed and maintained in suspension over time. While dispergation and dispersion relate to the distribution of particles, dispergation in the context of the present invention emphasizes the initial breakdown and the conditions under which particles may spontaneously disperse. On the other hand, dispersion generally refers to the broader process of distributing and stabilizing particles within a medium to ensure they remain suspended. In essence, while dispergation focuses on the initial breaking down of particle clusters, dispersion encompasses the broader process, ensuring not only distribution but also stabilization of particles within a medium. For further references on dispergation, please see Makarewicz, Edwin, and Agnieszka Michalik. "Research on the Influence of the Type of Surfactant and Concentrator in Aqueous Dispersion of Pigments" Journal of Surfactants and Detergents 17, no. 4 (July 2014): 773-84, https: / / doi.org / 10.1007 / sl l743-013- 1518-5; and Rusanov, A.I. "Thermodynamics of Dispergation: Development of Rehbinder’s Ideas" Colloids and Surfaces A: Physicochemical and Engineering Aspects 160, no. 2 (December 1999): 79-87, https: / / doi.org / 10.1016 / S0927- 7757(99)00353-2.

[0027] With the term "redispersibility" is meant the ability of a dispersion or suspension (in the colloidal physical -chemi cal context) to be redispersed. It refers to how easily or difficult it is to redisperse a sample after it has been at rest for a certain period. Redispersible pigments are pigments that, once dispersed in a medium and then dried or settled, can be redispersed back into the medium upon the addition of a suitable solvent without losing their properties. In the context of redispersibility, the phrase "without losing their properties" signifies that redispersible pigments have the capability to return to their original state with all their inherent characteristics and qualities intact. When redispersing these pigments after they have dried or settled, they should maintain their essential properties, such as color, particle size, stability, and any other relevant attributes that make them suitable for their intended application. Essentially, it means that the pigment can be effectively restored to its initial state without any significant degradation or alteration of its desirable features. The term "dispersible" primarily refers to the pigment's ability to be distributed throughout a medium without necessarily implying that it can be redispersed once settled or dried. Please see the following references on redispersibility: EP0859036; Chi Liu and Yanmin Jia "Effect of Redispersible Latex Powder and Fly Ash on Properties of Mortar" Coatings 2022, 12(12), 1930, https: / / doi.org / 10.3390 / coatingsl2121930; Sara R. Gomes de Sousa and Antonio J. Faria Bombard "Redispersibility in magnetorheological fluids and its relevance for MRF formulations" https: / / arxiv.org / pdf / 2302.07617.pdf.

[0028] As the aim of the present invention is to provide redispersible pigment granules, a key objective is to retain or improve the available surface area of the input pigment powders. This is because, for a given pigment, quality is not only a function of the purity and morphology of the pigment crystal structure, but it is also strongly dependent on the available surface area of the pigment particles. It is the interaction of the pigments with incident light that results in selective shares being removed (absorbed) from the white light and the remaining being scattered. The first property results in chroma, while the second is the prerequisite for good hiding power. The ability to produce chroma is then proportional to the pigment surface area which can interact with the light. This means that for a given pigment quantity the degree of selective absorption increases with decreasing particle size.

[0029] The High Shear Wet Granulation (HSWG) process is a well-established process largely used in the pharmaceutical sector for the production of oral solid dosage forms. HSWG process is conventionally performed by spraying a liquid (normally referred to as binder) onto the particles as they are agitated in a high- shear mixer. The liquid forms inter-particle bonds by a combination of capillary and viscous forces until more permanent bonds are formed by subsequent drying.

[0030] Traditional HSWG has its limitations, especially in producing granules with controlled dissolution properties due to inherent low porosity of the granules.

[0031] The present inventors modified the standard HSWG process to target the production of redispersible granular pigments (like for example titanium dioxide and iron oxides) optimized for paint and coatings applications. Under the process object of the present disclosure the pigment is converted into a mechanically stable granular form that is easy to handle, store, and transport. Yet, this transformation is reversible as the granules are designed to effortlessly disperse again when introduced into a medium such as water for paints or resin for powder coatings. The redispersibility unlocks the limitations traditionally associated with granular or powdered pigments. Such a product attains the convenience of granules and the performance of a finely dispersed pigment.

[0032] In one embodiment, the present invention provides a discontinuous high- shear wet granulation process for producing redispersible pigment granules comprising the following steps: a. Providing at least one powder pigment in its natural aggregated and agglomerated form; b. Blending the at least one pigment with at least one insoluble grinding aid and / or disintegrant in powder form obtaining a blend; c. Dispergating the blend with a first liquid mixture comprising at least two non-polymeric surfactants by means of high-shear mixing, obtaining pigment particles with an average particle size lower than the average particle size of the pigment agglomerates and aggregates of step a.; d. Wetting the pigment particles with a second liquid mixture comprising at least one polymeric surfactant and letting the wetted pigment particles to nucleate under high-shear mixing; e. Adding at least one granulating polymeric binder in solid form to the wetted particles under high-shear mixing, resulting in the formation of a cohesive solid bed of particles; f. Spraying the second liquid mixture onto the solid bed of particles under high-shear mixing, obtaining wetted granules; g. Massing the wetted granules to grow by coalescence, preferably in a cascade flow regime; and h. Drying the wetted granules to obtain redispersible pigment granules, wherein steps a. to f. are carried out in a high-shear wet granulator, step g. either in a high-shear mixer or, preferably, in a tumbler, and step h. in a fluid bed dryer, wherein the granulator is equipped with at least one impeller, at least one side-chopper, some nozzles for liquid spraying (into the granulator) and a vacuum pump.

[0033] The high-shear wet granulator, usually equipped with at least one agitator / impeller that imparts high shear to particles for effective agglomeration, comprises one or more nozzles that spray liquid over the material bed. The agitator / impeller typically features a three-blade impeller, and the motor is fitted with a variable frequency drive (VFD) for speed control throughout the granulation process, reaching blade tip speeds up to 15 m / s. The motor may also include a power or torque meter to assess the rheology of the wet mass. The granulator may also comprise a secondary impeller, known as a chopper. Smaller and faster than the main agitator, the chopper breaks down lumps during granule growth, and can be mounted either on the top or side of the vessel. The granulator usually comprises a vacuum pump and a bag filter that are used for maintaining vacuum and filtering solids, respectively. The granulator's bowl may also have a jacket for heating or cooling. Granulators that can be used to implement the process of the present inventions are (i) Vertical Axis Granulators (having top or bottom-mounted agitators, with the vessel itself being cylindrical or tulip-shaped to enhance the impeller's roping action. Key manufacturers include Ima, Diosna, Gea, Glatt, Freund Vector, and Syntegon); (ii) Horizontal Axis Granulators (manufactured i.a. by Lbdige); (iii) Conical Shape Granulators (where the vessel's shape creates a higher shear field along the wall, like e.g. Cyclomix by Hosokawa); (iv) Multifunctional One-Pot Granulators (that uniquely integrate multiple processes within a single machine, encompassing mixing, deagglomerating, dispersing, granulating, coating, kneading, and drying functions, combining the dynamic tumbling effect, achieved through a rotating and tilted vessel, with the traditional efficiency of a conventional mixer; e.g. Eirich’s One-Pot Granulators).

[0034] A HSWG apparatus may also comprise a set of sensors to detect physical quantities indicative of properties of the HSWG processing as well as a control unit coupled to the sensors and to the actuators in the HSWG apparatus to control operation thereof based on one or more of the sensed signals.

[0035] The pigment powders of step a. consist of aggregates / agglomerates of similar particles.

[0036] In the dispergation phase (step c.) of the process, it is the magnitude of the cohesive forces between the individual particles that determine the extent of deagglomeration before the mechanical action (high shear mixing) is applied. But the most difficult part of the process to define is that concerned with the breakdown of the pigment aggregates and agglomerates into finer particles after all the available surface has been wetted with the first liquid mixture comprising at least two non-polymeric surfactants. This step comprises two distinct phases, a first one of wetting the pigment powder with a mixture of surfactants, and a second one of high-shear mixing the wetted particles to obtain pigment particles with an average particle size lower than the average particle size of the pigment agglomerates and aggregates of step a.

[0037] Dispergation can be achieved through mechanical methods, such as milling and grinding, and chemical methods, such as the use of surfactants and polymers.

[0038] In the dispergation phase, it is more important to disrupt pigment agglomerates and aggregates than to secure a stable dispersion of primary particles. This means maximizing the speed of wetting and the speed of deagglomeration. Mechanical energy is required to destroy aggregates or break down single crystals into smaller units. The fine breaking down of solids to create new interfaces is facilitated by the adsorption of the non-polymeric surfactants (contained in the first liquid mixture) at structural defects in the surface, which are normally present in the natural state and might also appear as microcracks during the high-shear mixing of step c. The efficiency of deagglomeration is related to the degree of lowering of the free surface energy by surfactant(s) adsorption, which is the dominating effect.

[0039] The weakening of solids by controlled adsorption of surface-active agents (in the present case the at least two non-polymeric surfactants) is known as the Rehbinder effect, also known as Adsorption-Induced Reduction of Strength (AIRS). It is a phenomenon in physics that causes a decrease in strength and specific work of fracture in solids due to the interaction between the solid and the surfaceactive agents of an environment. This effect can be applied to the dispergation phase occurring in the high-shear mixing of powders, where it can facilitate the breakdown of agglomerates and aggregates by weakening the interatomic bonds in the solid. The Rehbinder effect is sensitive to the superatomic structure of the solid, strain rate, and temperature. In this context, the Rehbinder effect can be considered a mechanism of the dispergation process, as it aids in the dispersion of solids by reducing the strength of agglomerates and aggregates and facilitating their breakdown into smaller particles, leading to a more efficient process.

[0040] By leveraging the Rehbinder effect in the context of the claimed process, the inventors found out that surface-active agents with affinity for pigments surface, e.g., the non-polymeric surfactands, can be effectively adsorbed in sufficient amounts to lower free surface energy of the solid-liquid system, hence improving the breaking down efficiency as exerted by the mechanical work of the high-speed mixer impeller and the lateral chopper. It was surprisingly noted that enough shear and impact energy can be transferred to the solid particles for an effective dispergation phase. The key to this phase is to avoid the formation of liquid bridges among the solid particles which would have a detrimental effect on the break-up of the agglomerates and aggregates.

[0041] Furthermore, the present inventors discovered that the dispergation phase is greatly improved, in terms of robustness, by adding to the input pigment a small quantity of insoluble grinding aid(s) and / or disintegrant(s) in powder form (step b.).

[0042] In one embodiment, the at least one insoluble grinding aid is selected from fumed silica, magnesium stearate, colloidal silicon dioxide, corn starch, calcium silicate, talc, sodium stearyl fumarate, stearic acid, and MCC microcrystalline cellulose.

[0043] In one embodiment, the at least one disintegrant is selected from croscarmellose-sodium, sodium starch glycolate, crosslinked polyvinylpyrrolidone, carboxymethylcellulose, microcrystalline cellulose, naturally derived fibrous cellulose, magnesium silicate coprecipitated on chitin, magnesium aluminum silicate, potassium polacrilin (Amberlite), alginic acid, sodium alginate, gellan gum, xanthan gum, guar gum, chitosan and chitin derivatives.

[0044] In one embodiment, the at least one insoluble grinding aid(s) and / or disintegrant(s) is(are) added in the range of 0.1 to 2%wt relative to the dry pigment weight, preferably in the range of 0.5 to l%wt relative to the dry pigment weight, respectively.

[0045] Fumed silica and croscarmellose serve different purposes in the dispergation phase. Fumed silica acts as a grinding aid, helping to reduce particle agglomeration and improve the milling process efficiency. On the other hand, croscarmellose is a (super)disintegrant that facilitates the breakdown of agglomerates and aggregates, promoting a more homogeneous liquid distribution and reducing the formation of liquid bridges (pendular) that could negatively affect the milling process. In the dispergation process, where the amount of liquid is limited to achieve full wetting of the particles, it was found out that croscarmellose is very effective due to its rapid swelling and wicking properties. When exposed to fluids, croscarmellose helps clusters become wetted by the liquid, allowing the liquid to penetrate through cavities and promoting the breakdown of agglomerates and aggregates. This leads to a more efficient dispergation process and better dispersion of the solid particles in the liquid medium.

[0046] In one embodiment, the dispergation of step c. comprises (preferably consists of) two phases, wherein:

[0047] - the first phase involves spraying the first liquid mixture on the pigment agglomerates and aggregates under vacuum and under high-shear mixing, and

[0048] - the second phase involves stopping the spraying of the first liquid mixture and high-shear mixing under a mixing condition of a Froude number equal or greater than 20, under vacuum. The Froude number is a dimensionless number used in fluid dynamics to compare inertial forces to gravitational forces. It is defined as: ) 2 V Fr = -

[0049] 9 wherein, m is the angular velocity in (rad / s), r is the radius of the rotation in meters (m), and g is the acceleration due to gravity in (m / s2).

[0050] In one embodiment, the spraying of the first liquid mixture is stopped when the amount of liquid sprayed onto the particles is sufficient to cover their surface with a surfactant adsorbed layer lower than 10 monolayers.

[0051] Monolayer adsorption refers to the process where a single layer of atoms or molecules adheres to a surface. This layer is exactly one atom or molecule thick. Calculating monolayer adsorption typically involves determining the surface area of the material and the size of the adsorbing molecules. One common approach is to use the BET (Brunauer, Emmett, and Teller) method, which measures the amount of gas adsorbed onto a surface at different pressures to calculate the surface area and thus infer the surfactant amount needed for a monolayer. For liquids, similar principles apply, but factors like molecular size and surface tension are considered. The Rehbinder effect, named after Russian scientist Pavel Rehbinder, is an observed phenomenon in the spontaneous dispersion of solids in liquids, particularly relevant to the reduction of strength and hardness of solids in contact with surface-active media, like certain liquids. This effect contributes to increased mechanical wear or erosion. When solids interact with liquids containing surfaceactive agents (surfactants), these agents adsorb onto the solid surface, modifying its surface energy and mechanical properties. This change makes the solid more susceptible to fracturing or disintegration under stress, leading to smaller particle fragments and enhanced dispersion in the liquid. The Rehbinder effect is significant in industries requiring fine dispersions of solids in liquids, such as in paint, ink, pharmaceutical, and cosmetic production, where understanding and optimizing this phenomenon is key to efficient solid dispersion. A monolayer, within surface chemistry and materials science, is a single, continuous layer of atoms or molecules adsorbed onto a surface, each in direct contact with the surface and just one atom or molecule thick. In a monolayer, liquid molecules adsorb in one layer on the particle surface. As additional layers form, the adsorbed liquid volume significantly increases relative to the solid weight, illustrating how adsorbed liquid amounts can rapidly grow with more layers beyond the initial monolayer. To calculate the amount of liquid needed to form a monolayer on a solid surface, consider factors like the solid surface area, the liquid or surfactant molecular size, and the molecules packing density in the monolayer. The step-by-step method includes:

[0052] 1. Determine the Pigment Surface Area (A): Measure or estimate the pigment particles total surface area, using techniques like BET (Brunauer-Emmett- Teller) analysis for accuracy.

[0053] 2. Estimate the Surfactant Molecules Molecular Size (c): Determine the molecular dimensions, using molecular models or literature values. For surfactants, use the surfactant molecule's size, approximating from the Van der Waals radii of the atoms.

[0054] 3. Calculate the Number (n) of Surfactant Molecules per Unit Area: Determine the molecules packing density in the monolayer, estimating using the molecular size and a close-packed arrangement assumption. Use the area of a circle formula (7tr2) to estimate one molecule area, calculating n as I / (nr2).

[0055] 4. Calculate the Total Number (N) of Surfactant Molecules Needed: Multiply the molecules per unit area (n) by the solid's total surface area (A). N = n X A.

[0056] 5. Determine a Single Surfactant Molecule Volume (V mol): Estimate this from the molecular weight and liquid density (e.g., water: 18 g / mol, 1 g / cm3).

[0057] 6. Calculate the Surfactant Monolayer Total Volume (V): Multiply the total number of molecules (N) by a single molecule volume (V_mol). V = N x V mol.

[0058] This theoretical estimate may vary in practice due to factors like solid surface roughness, molecular orientation, packing density, and impurities presence, requiring experimental validation for precise calculation adjustments.

[0059] In the first phase of dispergation the solid-air interface is replaced by one between solid and liquid. This means that when the first liquid mixture comprising the mixture of at least two non-polymeric surfactants is sprayed over the mass of powder, air has to escape from the capillaries through the liquid and the voids. This is the reason why proper wetting requires low surface tension of the 1-v interface, contrary to what is normally stipulated in the HSWG process or by the Washburn equation.

[0060] In one embodiment, the first liquid mixture comprises:

[0061] - at least one non-polymeric non-ionic gemini-type surfactant; and

[0062] - at least one non-polymeric anionic surfactant, if the redispersible pigment granules are to be dispersed by the final user in an aqueous medium; or

[0063] - at least one non-polymeric cationic or amphoteric surfactant, if the redispersible pigment granules are to be dispersed by the final user in a non-aqueous medium.

[0064] Moreover, the inventors found out that operating under mild to strong vacuum surprisingly boosts the quality of step c., as better breakdown of the agglomerates and aggregates is achieved. As a result of such discovery, the claimed process starts with blending the powder pigments with the at least one insoluble grinding aid(s) and / or disintegrant(s) above described (step b.) while applying vacuum to the system.

[0065] In one embodiment, step b. is carried out under vacuum, preferably at a pressure P equal to or lower than 0.85 bar, more preferably at a pressure P comprised between 0.4 and 0.05 bar.

[0066] The inventors found that vacuum starts to promote step c. at P=0.85 bar, and the effect increases almost linearly up to a P=0.05 bar. A good level of performance is attained at a level of P comprised between 0.4 and 0.2 bar.

[0067] The vacuum is then held for the entire step c. (dispergation phase) up to the completion of step e. (granulating polymeric binder addition).

[0068] From a process operating conditions standpoint, dispergation is carried out at the maximum transferable mechanical momentum, which corresponds to a fully developed roping flow of the powder as defined by Iveson and Litster in their seminal works. In the context of powder dispersion in process engineering, a "roping flow" regime refers to a specific flow pattern observed in the movement of powders, particularly in pneumatic conveying systems or similar environments. This regime is characterized by the formation of a concentrated, rope-like stream of powder particles that moves along the boundaries of the conveying medium, typically air. This state is achieved at a specific flow rate and pressure that results in the formation of this roping flow, where the particles behave almost like a fluid, ensuring effective and uniform dispersion as described by Iveson and Litster.

[0069] In one embodiment, step c. is carried out at a powder flow regime corresponding to a Froude number of at least 20.

[0070] The inventors also found out that the side-chopper, traditionally used to break large lumps of over-grown granules, is surprisingly effective during the dispergation phase when high shear is applied to break up agglomerates and aggregates. For instance, when sizing a chopper for a 700-liter granulator, the critical parameter to consider is the tip velocity, which can be calculated based on the chopper's diameter and rotational speed. To ensure effective performance, the chopper should be configured to achieve a tip velocity exceeding 1500 rpm, preferably up to 3000 rpm, while featuring a min three multi disk knife-type design with an outer diameter of at least 120 mm or greater.

[0071] As far as the spraying of the first liquid mixture, such a step is carried out to operate in the drop-penetration regime (see K.P. Hapgood and J.D. Litster): a high- pressure hydraulic multiple fan-shaped type nozzles arrangement is used, to minimize the volumetric flow rate for each nozzle and simultaneously reduce the spray area. Measured in water-like viscosity, nozzles operate at an average drop size of < 100 microns. Such conditions are achieved with hydraulic pressure ranging from 6 to 45 barg. Under a water-like viscosity level, the hydraulic pressure of the nozzle is about 25 to 30 barg.

[0072] With the combination of powder flow (incipient roping flow regime) and multiple nozzles arrangement, a spray-flux number target for step c. is lower than 0.15.

[0073] The dimensionless spray-flux number (see K.P. Hapgood, D.J. Lister) is a critical parameter in the context of granulation processes. It quantifies the coverage of the powder surface by liquid droplets in relation to the movement of powder through the spray zone. The spray-flux number can be expressed analytically as follows:

[0074] To expand this into a more detailed expression, consider the following parameters:

[0075] - Let QLbe the liquid spray rate (volume per unit time, e.g., — ).

[0076] - Let d be the average diameter of the droplets (e.g., in meters).

[0077] - Let w be the width of the flat spray nozzle (e.g., in m).

[0078] - Let vpbe the velocity of the powder through the spray zone (e.g., in m / sf It can be estimated by the Froude number as vp« 0.15^ / r - r ■ g

[0079] The rate of wetted area coverage by droplets can be approximated as the product of the number of droplets per unit time and the average area covered by each droplet.

[0080] The area flux of powder through the spray zone can be defined as w x vprepresenting a linear measure of the powder flow through the spray zone.

[0081] Putting these together, the dimensionless spray-flux number can be represented as:

[0082] Substituting the expressions, we have:

[0083] The given expression quantifies the density of droplets covering the powder surface in the granulation process, enabling the understanding and optimization of wetting and granulation efficiency.

[0084] In one embodiment, in step c. the first liquid mixture (liq) is sprayed at a rate ranging between 0.1 and 3 kgiiq / kgsoi / h, preferably between 0.5 and 1 kgnq / kgsoi / h with respect to unit mass of the dry pigment (sol).

[0085] In one embodiment, in step c. the first liquid mixture is sprayed in an amount ranging between 0.1 and 5%w, preferably between 0.5 and 2%w, with respect to dry pigment weight.

[0086] In one embodiment, in step c. the first liquid mixture is sprayed in an amount ranging between 1 and 10 monolayers, preferably between 1 and 3 monolayers, with respect to the total surface area of the particles.

[0087] In the dispergation phase, following the completion of the targeted liquid mixture addition, which consists of non-polymeric surfactants, the spray is discontinued and the chopper mechanism is activated. The termination of Step c. is then contingent upon one of two criteria: it may be considered complete after a predefined period of time, such as 4 minutes, subsequent to the initiation of the chopper; or, as an alternative condition, the completion can be determined when the power consumption (or torque) profile, as a function of time, consistently displays a zero first derivative (f) for a minimum duration of two minutes. This latter criterion is indicative of a stable and optimized state in the breakdown process of the pigment agglomerates and aggregates.

[0088] Once the step c. is complete, step d. is initiated by deactivating the chopper and starting the addition of the second liquid mixture under the drop-penetration flow regime described earlier.

[0089] In step d., granules nuclei (seeds) are formed as pendular liquid bridges are initially established. The wetting phase commences with the spraying of a liquid mixture (liq) onto the particles at a flow rate (V ) ranging between 0.5 and 5 kgiiq / kgsoi / h, preferably between 1.0 and 3 kgiiq / kgsoi / h with respect to unit mass of the dry pigment (sol).

[0090] In one embodiment, in step d. the second liquid mixture is sprayed in an amount ranging between 5% and 40%wt, preferably between 10% and 25%wt with respect to the dry pigment weight.

[0091] In one embodiment, step d. is carried out at a powder flow regime corresponding to a Froude number of at least 20.

[0092] Adding of at least one granulating polymeric binder in solid form (powder or granules) into the system (step e.) is carried out while keeping the roping flow regime of the now-formed wetted mass unchanged. By keeping this flow regime unchanged, the binder is effectively mixed with the wetted particles, ensuring uniform granulation and efficient binding. This step allows achieving desired properties in the final product, such as consistent particle size, strength, and stability. Such granulating polymeric binders are needed to form a more coherent mass at the intragranular level with a high degree of plasticity needed to ensure homogeneous, dense, dry granules with low friability, whilst ensuring adhesion and cohesion in the wet state, as well as, excellent plasticity and re-dispersibility after the granules are dried. A stable structure of sufficient strength is thus formed in order to withstand the hydrophilic forces arising due to the evaporation of water. A structure-forming 'cement' allows the pigment granules to be soluble in the medium into which it is desired to disperse the pigment in order to regenerate the fine particles.

[0093] A binder in the context of wet granulation is a substance used to impart cohesiveness to the granules, facilitating the formation and maintenance of their structure. Binders can be natural or synthetic, and they function by providing adhesive forces that bind the primary powder particles together. This results in the formation of larger, multi-particle entities known as granules. The binder's structural characteristics, such as molecular weight, viscosity, and solubility, influence its effectiveness. A well-chosen binder will enhance the flow properties of the granules, improve compressibility, and ensure the uniformity of the final product, all while maintaining the desired release profile of the active ingredients.

[0094] A specific advantage offered by the granulating polymeric binder(s) is to contribute to minimizing the total amount of surfactants. This is particularly important given the adverse effect of excess surfactants on wet-scrubbing resistance and water permeability of the paint systems. Excess surfactant does not improve dispersion stability and may alter the water sensitivity of the resulting paint.

[0095] In one embodiment, the granulating polymeric binder is selected from copovidone PVP-PVA copolymer; polyethylene glycol (PEG) in the range 400 to 10000 Da, preferably 4000; polyvinyl alcohol (PVOH); PEG-PVA; polyvinyl pyrrolidone (PVP); partially gelatinized starch (PGS); sodium carboxymethyl cellulose; cellulose ethers; starch; microcrystalline cellulose; hydroxypropyl methylcellulose; gelatin; acacia gum; sodium alginate; and mixtures thereof. These binders are selected based on their compatibility with other ingredients, desired properties of the final product, and specific requirements of the granulation process. For example, a very effective polymeric binder is copovidone PVP-PVA copolymer, a nonionic polymeric binder with high molecular weight (500-200,000 Da). In water-borne paint applications, the obtained granules displayed a low water sensitivity and excellent gloss.

[0096] In one embodiment, the granulating polymeric binder(s) is(are) either soluble or miscible in the liquid mixtures sprayed in steps c. and d.

[0097] In one embodiment, the granulating polymeric binder(s) has(have) a Tg lower than 60° Celsius.

[0098] In one embodiment, the granulating polymeric binder(s) is(are) added in dry form and in amounts ranging between 0.5% to 2%wt to dry pigment.

[0099] The addition of the second liquid mixture (step f.) is then stopped as soon as pore saturation of min 80% is reached, indicating that the majority of the pores within the granules are filled with the liquid. This level of saturation allows optimal granule formation, as it ensures sufficient wetness for granule growth while preventing excess liquid that could lead to over-wetting and undesirable agglomeration. In terms of granulation dynamics, this point corresponds to the formation of full funicular liquid bridges and the onset of capillary state formation in the granules.

[0100] In one embodiment, in step f. the second liquid mixture is sprayed in an amount ranging between 1% and 20%wt, preferably between 5% and 15%wt with respect to the dry pigment weight.

[0101] In one embodiment, in step f. the second liquid mixture (liq) is sprayed at a rate ranging between 0.1 and 3 kgiiq / kgsoi / h, preferably between 0.5 and 1.5 kgnq / kgsoi / h with respect to unit mass of the dry pigment (sol).

[0102] Step g., known as the 'massing' step, is preferably carried out in a tumbler, a device widely recognized in the art of granulation. In this phase, the granule growth behavior is intricately linked to the liquid content, especially as the saturation level approaches 80%. During massing, the onset of capillary liquid bridges leads to a rapid coalescence of nuclei and granules into larger agglomerates. The tumbler's mixing mechanism, involving the free fall and tumbling of solid particles down its walls, plays a pivotal role in this process. This tumbling motion is particularly beneficial for granulation, promoting the coalescence and enlargement of granule size in a controlled and uniform manner. In the cascade flow regime, which is achieved at a Froude number (Fr) of 0.5 or lower, the material is gently lifted and cascaded down inside the tumbler. This regime is identified as the optimal condition for granulation, as it ensures a gentle yet effective massing of the wet mass. The tumbling effect under these conditions is conducive to a uniform and gentle blending of the granules, ensuring that their integrity is maintained while achieving a very narrow particle size distribution.

[0103] Tumbler blenders, used in this step, vary in shape, including cylindrical, rectangular, and with conical or bi-conical bottoms. Notably, these tumblers lack internal baffles or rotating parts, simplifying the construction and maintenance. The rotation speed and direction of these tumblers are adjustable, typically ranging from 4 to 20 rpm, allowing for precise control over the granulation process. A variant, the bin blender, features a removable container that facilitates easy transfer to subsequent processing stages.

[0104] The present inventors have found that this stage of massing, under a cascading flow regime with a Froude number of 0.5 or lower, promotes the coalescence of wet granules while maintaining a narrow particle size distribution. This approach to massing ensures that the quality of the granules is consistent and meets the desired specifications.

[0105] In one embodiment, the massing step is carried out in a cascading flow regime corresponding to a Froude number < 0.5.

[0106] If the wet granules are tumbled in a drum or vessel at a flow regime identified as cascading, corresponding to a Froude number < 0.5, the wet granules steadily coalesce without deteriorating the consolidation degree, conversely the porosity, achieved in the process, or even improving the particle size distribution. This provides a means to improve the particle size and its distribution in an economical way. Under these conditions, as detailed previously, the wet granules from step f. appear still surface-dry, but as minimum force is applied, liquid squeezes out to the surface promoting the coalescence.

[0107] At the end of step g. wet granules of pigments are obtained with a very narrow particle size distribution (mono-modal) and a d50 that can range from 50 to 1000 microns with a variance of the distribution less than 0.25. With an input pigment powder having a size normally < 10 microns, the d50 ranges from 100 to 200 microns. In the case of very fine organic pigments or nano-size inorganic pigments (such as transparent iron oxides) the d50 of the obtained wet granules is normally lower and in the range of 50 to 100 microns.

[0108] Wet granules post tumbling-growth are ready for the last phase of drying (step h.). Given the high bulk density and size of the wet pigment granules, the best option is to dry them in a continuous fluid bed dryer where fines can be separated and collected, further improving the particle size distribution. The fines can either be recycled back to the tumbler or the granulator. The fluid bed dryer is fast, operates at a relatively low temperature, and by the effect of a bubbling gas only limited shear to the granules is applied. Very limited particle growth takes place in the fluid bed and the extraction of the liquid from the granules improves the final porosity of the product. The fluid bed dryer operates on the fluidization principle: material in powder or granular form is suspended in a flow of hot air, passing through a perforated plate. The air flow causes the particles to behave like a fluid, ensuring uniform drying. The orientation of the perforated plate in the fluid bed ensures even distribution of hot air, promoting uniform drying throughout the batch. Other types of dryers, continuous or discontinuous, are suitable for the task provided they do not apply high strain on the granules.

[0109] A schematic representation of the process object of the present invention is provided in Figure 1.

[0110] As exemplified in Figure 1, a high shear wet granulation, HSWG apparatus comprises, in a manner per se known: a vessel comprising an internal volume (or space) 12 to perform HSWG granulation processing of (powder) materials inserted therein; at least one input duct (or filling spout or hatch 13) configured to couple the internal volume 12 of the vessel to a set of (buffer) tanks or volumes of powder materials Po, Pi, such as a first tank of at least one pigment powder (and / or filler) Pi and at least one second tank of solid powder Po (such as grinding aids, disintegrants, granulating polymeric binders); for instance, in case the first powder material comprises pigment powder, it is possible to synchronize, with respect to transition points of the HSWG processing, inserting a granulating polymeric binder in the volume 12 via a control unit CU, as discussed in the following; a mixer device 14, such as an agitator 14, comprising a rotating body (e.g., comprising at least two blades with elongated side wings) coupled to an actuator configured to impart rotation thereto, the first mixer device configured to impart (high) shear to particles for effective processing; a liquid distribution sub-system L configured to spray at least one liquid substance on the solid bed of the internal volume 12 in a controllable manner (e.g., via a set of a metering system) during the operation of the apparatus 10.

[0111] As exemplified in Figure 1, the liquid distribution sub-system L comprises at least one source of liquid substances LI, L2, and one or more nozzles 15, 17 coupled to the sources LI, L2 and configured to spray the at least one liquid substance over the bottom of the internal volume 12 after it has been filled with powder materials and after blending thereof.

[0112] For instance, the central body of the mixing device 14 is coupled to a set of blades (not visible in Figure 1), e.g. three helical blades forming a three-blade impeller. It is noted that the exemplified number and shape of blades of the agitator 14 is purely exemplary and in no way limiting as one or more embodiments may employ virtually any number of blades greater than two. As appreciable to those of skill in the art, in order to perform HSWG, the agitator 14 has a diameter substantially equal to the diameter of the internal volume 12.

[0113] As exemplified in Figure 1, the apparatus 10 comprises a conventional vacuum pump 19 coupled to the apparatus 10 via the outlet of a bag filter installed thereon or on a sealing lid 18 of the internal space 12 of the apparatus 10. For instance, the vacuum pump 19 is configured to vary (e.g., reduce) the pressure within the internal space 12 of the apparatus 10 based on process parameters / phases.

[0114] Still referring to the exemplary apparatus of Figure 1, a set of sensors is coupled to the (parts at least partly within) the internal volume 12 of the vessel of the apparatus 10, e.g., to detect properties of the HSWG processing.

[0115] As appreciable to those of skill in the art, the granulator 10 may also comprise a jacket for heating or cooling the vessel, in a manner per se known.

[0116] Advantageously, the control unit of the HSWG apparatus is configured to control the granulation process dynamically, preferably varying the process parameters, such as liquid source parameters (e.g., via high-pressure positive displacement pumps with variable speed control) and / or frequency of the motors driving the agitator, based on at least one sensing signal received from the set of sensors.

[0117] In the exemplary scenario in which the HSWG apparatus 10 is used to produce pigments, powder pigments Pi are blended with additives Po like grinding aids and / or disintegrants while absolute pressure is dropped (via the vacuum 19) to favor air removal and improve milling efficiency.

[0118] Inventors have observed that using torque (also referred to as the moment of force, used to describe the rate of change of angular momentum) exerted via the agitator 14 as a control variable for operating the apparatus 10 it is possible to gain insights into the wet mass's rheological behavior. Therefore, it is possible to provide an estimate of amounts of liquid to provide for effective granulation and understanding the granules plastic deformation strength.

[0119] There are several types of online torque meters suitable for coupling to a motor shaft driving the agitator 14 for a high shear wet granulator. In general, torque meters may be configured to measure the torque applied to the shaft in real-time, sampling the values at constant time intervals. Conventional torque meters suitable for use in one or more embodiments may comprise strain gauge torque sensors, rotary torque transducers, in-line torque sensors, magnetic torque sensors, optical torque sensors, digital torque sensors and so on.

[0120] A method for estimating the liquid-to-solid ratio (L / S) during the granulation process performed by the apparatus 10 comprises: performing discrete increments in the amounts of liquid distributed L, (e.g., via a set of control knobs of liquid feeding pump(s)) and monitoring (e.g., via the set of sensors) corresponding changes in torque exerted by the agitator 14 (e.g., via monitoring the shaft thereof).

[0121] A method of determining an equilibrium point for the liquid-to-solid ratio (L / S) for the HSWG processing, thereby controlling the HSWG apparatus accordingly, comprises: after introducing the first powder material Pi (such as a pigment, for instance) in the internal volume 12, spraying L at least one liquid substance thereon and computing a second derivative of the torque of the impeller 14 as a function the normalized cumulative liquid volume inside the processing space 12; and detecting transition points in the evolution of the torque profile by detecting zero-crossing points and local minima for the second derivative of the torque as a function of the normalized cumulative liquid volume.

[0122] For instance, a second zero-crossing is indicative of transitions in the granulation process, for instance the shift from pendular to funicular liquid bridges.

[0123] As exemplified herein, the HSWG processing method further comprises: in response to detecting a first transition point (e.g., at a first zero crossing in the second derivative of the torque as a function of the normalized cumulative liquid volume), reducing (via the liquid distribution sub-system L) a flow rate of the liquid substance being sprayed L; preferably, also varying a pressure inside the internal volume 12 via the vacuum pump 19; and / or inserting the second powder material Po, such as a granulating polymeric binder, in response to detecting a second transition point (e.g., at a zero-crossing of the third derivative of the torque as a function of the normalized cumulative liquid volume); and / or stopping liquid spraying L in response to detecting a third process transition point (e.g. in response to the second derivative reaching a second zero-crossing after detecting a minimum value thereof).

[0124] One or more embodiments exploit a process control approach that hinges on a detailed analysis of the mechanical forces at play during the granulation process.

[0125] One or more embodiments exploit the second derivative of torque with respect to the normalized cumulative volume of liquid sprayed. For instance, this parameter serves as a pivotal indicator for rheological transitions during the formation of wet granules.

[0126] In one or more embodiments, HSWG processing may encompass various stages, such as:

[0127] - a first processing phase (corresponding to step d.) comprising modulating operational parameters in response to detecting a change of sign in the second derivative (that is, a zero crossing of the function); for instance, such a first phase involves reducing the mass flow rate of the liquid being sprayed (the second liquid mixture) while adjusting the internal pressure to a threshold level (e.g., about 0.85 bar); this first processing phase facilitates maintaining consistency in the early stages of granulation;

[0128] - a second processing phase (corresponding to step e.) involving adding the granulation polymeric binder in response to detecting that the second derivative reaches a minimum, corresponding to a zero crossing of the third derivative; introducing a pre-determined quantity of solid granulating polymeric binder into the HSWG apparatus at this point facilitates ensuring uniformity in the granulation process; - a third processing phase (corresponding to step f.) of transition to massing which is triggered when the second derivative of the torque as a function of the normalized cumulative liquid volume exhibits a second zero crossing; during the third phase, the completion of the second liquid mixture spraying phase is reached; stopping the liquid at this point correlates with the onset of capillary liquid bridges within the wet granules.

[0129] In one or more embodiments, the strategic use of the second derivative of torque of the agitator 14 facilitates producing wet granules with high consistency in particle size distribution (PSD) and yield.

[0130] One or more embodiments provide an approach that facilitates improving quality in the formed wet granules for their intended application in pigment preparation, thereby enhancing their performance in paints and coatings.

[0131] The primary focus of the process object of the present disclosure is to obtain redispersible pigment granules based on colored and non-colored (i.e. white and black) pigments in both aqueous and non-aqueous media. Redispersibility of granules produced by the claimed process is strongly dependent on their size and more specifically their porosity. During the growth phase (step g.), granules collide with each other and the process equipment to consolidate (densify). In a batch process, the consolidation period corresponds to the total batch time, typically spanning several minutes. The inventors discovered that the final pigment quality is influenced not only by the total amount of liquid content but also by the rate at which the liquid is added. For a specific formulation, in order to achieve optimal granule porosity, the duration of liquid addition is minimized, thereby ensuring the rate of addition is as rapid as feasible within process constraints.

[0132] The pigment content of the resulting granules is determined by the wettable surface area of the feed pigment particles in their finely divided form that can range from few sqm / g (< 15 sqm / g) to hundreds of sqm / g (> 150 sqm / g). As a result, it is possible to obtain redispersible pigment granules with a pigment solid content from 40% to up to 97%wt. The targeted average particle size of these granules is between 50 and 1000 microns, preferably between 100 and 200 microns.

[0133] Further ingredients of the redispersible pigment granules are polymeric binder(s), disintegrant(s), and grinding aids(s). No additional compounds as solvents, defoamers, antioxidants, biocides, etc. are required.

[0134] As already stated above, the term redispersibility is used herein to refer to the ability of pigment granules to re-disperse into a medium, either liquid or melt, to regenerate highly dispersed, stable colloidal dispersions.

[0135] Dispersibility has been defined as the ease with which the particles in the powder are distributed in a continuous medium so that each particle is completely surrounded by the medium, and no longer makes permanent contact with other particles. It is extremely difficult to calculate the minimum amount of energy required to separate the particles as yet no satisfactory theoretical treatment has been developed.

[0136] There are three stages in the dispersion process that although distinct in their nature, overlap in practice, and allow to reach the object of having redispersible pigment granules. These three steps are wetting, breaking up clusters and flocculation (i.e., the problem of maintaining in the dispersed state the particles).

[0137] In wetting, the addition of surfactant(s) ensures that the contact angle between the solid and liquid phases (Dupre - Young equation) is close to zero (spreading liquid), the interfacial tension 1 / v is greatly reduced, and upon adsorption, the s / 1 interfacial tension also decreased. Both effects lead to better wetting. However, another important factor in the wetting process is the rate of penetration of the liquid into the capillaries, and it is desirable that this be as large as possible (Washburn equation). The optimal condition for a fast penetration requires a high interfacial tension 1 / v and a low contact angle. A low contact angle is an important requisite, but when close to zero further lowering the surface tension 1 / v reduces the penetration rate.

[0138] In breaking up the clusters, the fine grinding of solids to create new interfaces is greatly facilitated by the adsorption of non-polymeric and polymeric surfactants at structural defects in the pigment surface. These defects are normally present in the natural state and also appear as microcracks during the milling process. The decrease in the surface energy associated with the surfactant adsorption can be remarkable even rendering the colloidal state more stable than the condensed state. The weakening of solids by adsorbed material is known as the Rehbinder effect.

[0139] In flocculation, the validity of the DLVO theory is questionable. The electrostatic charge mechanism cannot assure stabilization of concentrated dispersions, either in aqueous or non-aqueous media, and to prevent flocculation some sort of protective action by polymeric species is required. The protective action could arise from the effects associated with adsorption or through the polymer in the bulk forming a gel structure through which diffusion of the particle is restricted. The inventors found out that for concentrated disperse systems, the effective stabilizing action by the adsorbed polymeric layer is only possible if the layer is structured, i.e. it has a much higher viscosity than the medium and thus serves as a mechanical barrier, assuming the cohesive energy between the external surfaces of the layers is low.

[0140] The inventors thus came to the realization that effective redispersibility could be attained in the process disclosed herein only if surfactants are sequentially deployed in the distinct steps c. to f.

[0141] There are several ways in which surfactants can be adsorbed but the most practical and effective manner and the way which can be most generally predicted lies in the amphiphilic behavior of certain molecules (especially in aqueous medium).

[0142] The amphiphilic character gives rise to adsorption in which the hydrophobic groups are oriented away from the water and the polar group towards it. Based on the nature of the solubilizing group, these agents are classified into anionic, cationic, and non-ionic types. When strong attractions are set up between the polar groups of a surfactant and specific groups on the surface of a solid phase, specific adsorption occurs. In such cases, adsorption with reversed orientation can occur, in which the hydrophobic groups are forced to orientate towards the aqueous phase. This is a highly specific phenomenon exploited in the process described herein.

[0143] Polymeric surfactants display a different adsorption mechanism compared to either amphipathic or specific adsorption.

[0144] Specific interactions between certain sites on the solid phase and groups in the polymeric surfactant occur but, because of the cumulative effect of several attachment points between the surface and one macromolecule, the specific bonds do not require to be as strong as with the non-polymeric surfactant molecules. Because of the liquid mixture properties of polymeric surfactants, a contributory factor in their surface activity is related to a sort of configurational entropy, with the most probable configuration depending on both liquid mixture properties and behavior at the surface.

[0145] In the case of pigments, wetting is a prerequisite for all dispersion processes both in aqueous and non-aqueous media, as well as in the final paint with reference to gloss, haze, texture, and adhesion properties. The use of surfactants for the dispersion of pigments in a variety of media is undeniably ubiquitous. The prime requirement for dispersion is adsorption, reversible or not, and unless this occurs dispersion cannot be improved.

[0146] Although a necessary condition, dispersion is not sufficient as other factors involving the forces between the particles have also to be satisfied. Surfactants modify the electric charge on surfaces, they can introduce steric barriers which increase the pigment stability and they can alter the adsorption characteristics of the pigment surface to increase the adsorption of polymeric materials in pigment systems (paints), thus increasing the barriers to flocculation.

[0147] The stability of a sol (liquid colloidal system) is due to an energy barrier that is set up by one or more of three complementary mechanisms, viz. electric double layer, a steric protective layer, and a solvation energy effect. The inventors found out that the steric and solvation sheath plays a role in the preparation of redispersible pigment granules, which are aimed to redisperse into a liquid medium to give highly dispersed, stable colloidal dispersions. The enhancement of stability shown by nonionic and ionic surfactants cannot be explained without involving the steric and solvation barrier effects.

[0148] In aqueous media, amphiphilic adsorption is most commonly encountered as this is by far the most predictable behavior. This aids in the selection of molecules for particular uses.

[0149] In non-aqueous media, the majority of surfactants used as dispersing agents are very specific to the systems involved. Such complexity is given by the lack of any predictive theory of dispersion stability in non-aqueous systems. The lack of understanding of all the factors involved in the dispersion of pigments in such media means that we must rely to a very large extent on empirical sorting tests when seeking a dispersing agent for a particular system.

[0150] The surface activity of a surfactant depends on the size and polarity of the lipophilic and hydrophilic groups in the molecule. The inventors adopted the HLB (Hydrophile-Lipophile-Balance) system classification as a convenient yardstick to measure the surface activity of polymeric surfactants in pigment systems. The main reason is that the required HLB values for a number of organic and inorganic pigments are constant in both aqueous and non-aqueous paint systems. For organic pigments the polymeric surfactant(s) has(have) an HLB comprised in the range 8- 16, and for inorganic pigments the polymeric surfactant(s) has(have) an HLB comprised in the range from 13 to well over 20.

[0151] The results from experimentation confirmed that a combination of non- polymeric non-ionic and anionic surfactants enabled great control over particle size and rheological behavior, when the redispersible pigment granules are to be dispersed by the final user in an aqueous medium. In case the redispersible pigment granules are to be dispersed by the final user in a non-aqueous medium, a combination of non-polymeric non-ionic and cationic or amphoteric surfactants enabled as well great control over particle size and rheological behavior.

[0152] In one embodiment, the first liquid mixture comprises (i) a non-polymeric non-ionic gemini-type surfactant and (ii) at least one of

[0153] - a non-polymeric anionic surfactant if the redispersible pigment granules are to be dispersed by the final user in an aqueous medium, or

[0154] - a non-polymeric cationic or amphoteric surfactant if the redispersible pigment granules are to be dispersed by the final user in a non-aqueous medium.

[0155] In case the redispersible pigment granules are to be dispersed by the final user in an aqueous medium, good conditions for triggering the dispergation are achieved by the adsorption by the pigment of a liquid mixture of low viscosity non- polymeric non-ionic gemini-type and non-polymeric anionic surfactants of relatively low MW (between 200 and 2,000 g / mol, preferably < 800 g / mol, more preferably between 200 and 500 g / mol). Because of the non-polymeric surfactant small size their rate of adsorption is high relative to polymeric surfactants, and although in time the non-polymeric molecules could be replaced on the pigment surface by polymeric surfactants employed in steps d. and f. , the improved wetting obtained initially by these non-polymeric surfactants increases the rate of attainment of fine particles size and thus lowers the grinding energy. The two non- polymeric surfactants used in the process are found to be synergistically and very effectively adsorbed on the surface of the powder pigments forming a partial monolayer molecular film of selectively adsorbed surfactant molecules which drop the contact angle and the surface tension of the water-vapour interface.

[0156] In case the redispersible pigment granules are to be dispersed by the final user in a non-aqueous medium, the non-polymeric anionic surfactant is replaced with a non-polymeric cationic or amphoteric one with comparable molecular weight. Cationic or amphoteric surfactants offer considerable advantages in improving pigment dispersion, soft-texturing, and dispersibility in non-aqueous media.

[0157] Gemini-type surfactants, also known as dimeric or bis-surfactants, are a unique class of surfactants characterized by having two hydrophilic head groups and two hydrophobic tail groups, linked by a spacer. Non-polymeric gemini surfactants are those where the spacer does not contain a polymer chain. These surfactants are known for their high efficiency and effectiveness at lower concentrations compared to conventional monomeric surfactants. Below is a list of possible types of non-polymeric gemini-type surfactants that can be used in the claimed process, along with their nature, features, and properties:

[0158] 1. Alkyl Chain Gemini Surfactants (Nature: Typically consist of two long alkyl chains connected by a short spacer. Features: High surface activity, good wetting, and foaming properties. Properties: Effective in reducing surface and interfacial tension, often used in formulations requiring strong wetting agents).

[0159] 2. Ester-Linked Gemini Surfactants (Nature: Contain ester linkages in the spacer or tail group. Features: Biodegradable due to the ester bond, lower irritation potential. Properties: Useful in personal care and pharmaceutical applications where biodegradability and skin compatibility are important).

[0160] 3. Amide-Linked Gemini Surfactants (Nature: Incorporate amide linkages in the spacer or tails. Features: Good stability, lower toxicity, and skin irritation. Properties: Suitable for applications requiring mild surfactants, such as in cosmetics and topical pharmaceutical formulations).

[0161] 4. Quaternary Ammonium Gemini Surfactants (Nature: Contain quaternary ammonium groups as the hydrophilic head. Features: Excellent antimicrobial properties, good stability. Properties: Commonly used in disinfectants, antiseptics, and as conditioning agents in personal care products).

[0162] 5. Sulfonate and Sulfate Gemini Surfactants (Nature: Sulfonate or sulfate groups as the hydrophilic head. Features: High solubility in water, strong cleansing ability. Properties: Often used in high-performance detergents and industrial cleaning agents).

[0163] 6. Ethoxylated Gemini Surfactants (Nature: Contain ethylene oxide units in the spacer or head groups. Features: Adjustable hydrophilic-lipophilic balance (HLB), reduced skin irritation. Properties: Versatile in applications ranging from emulsification to solubilization, used in cosmetics and pharmaceuticals).

[0164] 7. Peptide-Based Gemini Surfactants (Nature: Incorporate peptide sequences in the spacer. Features: Biocompatibility, potential for specific interactions. Properties: Interesting for biomedical applications, including drug delivery systems).

[0165] 8. Sugar-Based Gemini Surfactants (Glycolipids) (Nature: Sugar moieties as part of the head groups. Features: Biocompatibility, low toxicity, and often biodegradable. Properties: Suitable for food, cosmetics, and pharmaceutical formulations, particularly where eco-friendly and mild surfactants are desired).

[0166] 9. Acetylenic diols (also known as molecular defoamers or nonionic gemini surfactants are a significant and unique category of surfactants that can be used for dispersion of solids like pigments and in defoaming applications. Acetylenic diols have a unique structure characterized by an acetylenic (triple-bonded carbon) group and typically two hydroxyl groups. They are classified as nonionic surfactants and are known for their high surface activity and ability to reduce surface tension effectively. Here's where acetylenic diols fit in the context of surfactants for pigment dispersion: Nature: Nonionic, with a unique molecular structure featuring acetylenic bonds. Features: Excellent surface tension reduction, low foam characteristics, and good wetting properties. Properties: Acetylenic diols are particularly effective in non-aqueous systems and are used in coatings, inks, and paints to improve wetting on low-energy surfaces and to control foam. Applications: They are commonly used in formulations where foam control is critical, and their ability to improve wetting makes them suitable for pigment dispersion in various industrial applications).

[0167] In one embodiment, the non-polymeric non-ionic gemini-type surfactant is selected from acetylenic diols, like 2,4,7,9-tetramethyl-5-decyne-4,7-diol or its derivatives (e.g., alkoxylates).

[0168] In one embodiment, the non-polymeric non-ionic gemini-type surfactant is 2,4,7,9-tetramethyl-5-decyne-4,7-diol (TMDD).

[0169] In one embodiment, the first liquid mixture contains two non-polymeric non-ionic gemini-type surfactants, preferably Surfynol 104 (2, 4,7,9- tetramethyldec-5-yne-4,7-diol) and Surfynol 465 (2,4,7,9-tetramethyldec-5-yne- 4,7-diol, ethoxylated).

[0170] In one embodiment, the non-polymeric anionic surfactant is selected from polymers based on: ethylenically unsaturated carboxylic acids (e.g., Poly(acrylic acid), poly(methacrylic acid), and copolymers of acrylic acid with other monomers such as acrylamide); polyurethane-based surfactants (e.g., Anionic polyurethane dispersions and anionic polyurethane-urea copolymers); acidic phosphoric, phosphonic, sulfuric, or sulfonic esters of poly ethers (e.g., Phosphate esters of polyethylene glycol, sulfonate esters of polypropylene glycol); polycondensation products of aromatic sulfonic acids with formaldehyde (e.g., Naphthalene sulfonate-formaldehyde condensates, benzene sulfonate-formaldehyde condensates). In one embodiment, the non-polymeric anionic surfactant is sodium bis-(2-ethylhexyl) sulfosuccinate. In one embodiment, the non-polymeric anionic surfactant is Disodium Alkyl Polyglucoside Citrate (A sugar-based green anionic surfactant derived from the esterification of citric acid with alkyl polyglucoside).

[0171] In one embodiment, the non-polymeric amphoteric surfactant is selected from betaine-based surfactants and amphoacetates. Examples include: Cocamidopropyl Betaine (CAPB), Lauryl Betaine, Oleamidopropyl Betaine, Cocoamidopropyl Hydroxysultaine. Examples of Amphoacetates include Disodium Cocoamphoacetate and Sodium Lauroamphoacetate.

[0172] In one embodiment, the non-polymeric cationic surfactant is selected from stearyl n-propylenediamine dioleate, N-alkylaminobutyric acid, alkyl trimethyl ammonium chloride, and alkyl trimethyl ammonium bromides (such as Cetyl Trimethyl Ammonium Bromide). In one embodiment, the non-polymeric cationic surfactant is Cetyl Trimethyl Ammonium Bromide (CTAB).

[0173] The inventors found out that when a relatively small quantity of liquid mixture of the non-polymeric non-ionic gemini-type surfactant 2,4,7,9-tetramethyl- 5-decyne-4,7-diol (TMDD) in ethylene glycol (or its water-soluble derivative with ca. 10 oxy ethylene segments) and the anionic surfactant sodium bis-(2-ethyl-l- hexyl)sulfosuccinate (Aerosol OT or AOT) is spray-atomized on a porous bed of pigment aggregates and agglomerates, surprisingly good break up of aggregates and agglomerates is achieved in the dispergation phase under high-shear mixing conditions. The inventors have reason to believe that thanks to the highly branched structure and the low surface concentration relative to the surface area covered, the non-ionic gemini-type surfactant provides lower surface viscosity and cohesive strength, which, in turn, reduces the stability of the solid agglomerates. The most relevant synergistic effect of mixing the two surfactants is the significant lowering of the critical micelle concentration, further boosted by the nanoparticle size grinding aid (e.g., fumed silica) blended with the pigments. Also surprising was the positive interaction of the non-ionic gemini-type surfactant with the disintegrant (e.g., sodium croscarmellose) dispersed with the pigments easing the air stripping from the particle interstices as a result of reduced surface tension and specific molecular defoaming properties. The effects described above were achieved with the addition of an amount of each the insoluble grinding aid and disintegrant in the range of 0.1% to l%wt relative to pigment weight.

[0174] In one embodiment, the first liquid mixture comprises the non-polymeric non-ionic gemini-type surfactant in an amount ranging between 0.5%w and 20%w with respect to the dry pigment weight.

[0175] In one embodiment, the first liquid mixture comprises the non-polymeric anionic surfactant in an amount ranging between 0.5%w and 20%w with respect to the dry pigment weight.

[0176] In one embodiment, the first liquid mixture comprises the non-polymeric cationic or amphoteric surfactant in an amount ranging between 0.5%w and 20%w with respect to the dry pigment weight.

[0177] As indicated in the process description above, in step c. the solid particles are kept under constant high-shear mixing for the entire stage, whereas the liquid addition is halted upon reaching the minimum wetting requirement to enhance the breakdown of the agglomerates and aggregates. Once an energy equilibrium is reached - optimal dispergation - the liquid spraying addition is resumed (step d.) but with the second liquid mixture comprising of at least one polymeric surfactant.

[0178] Following the completion of the dispergation phase, which is performed using non-ionic gemini and anionic (or cationic) surfactants, the process advances to the wetting-nucleation phase (step d.). In this phase, the surfactant mixture is composed of one or more polymeric surfactants. The purpose of using these polymeric surfactants is to form a stabilizing steric or electrosteric barrier around the pigment particles and nuclei.

[0179] Non-ionic and anionic polymeric water-soluble surfactants are highly valuable in various applications. Nonionic surfactants, often based on polyethers, are preferred. These nonionic surfactants can be in the form of unmixed polyalkylene oxides, block copolymers (including those with polypropylene oxide and polyethylene oxide blocks), or random copolymers. These polyalkylene oxides are created by adding alkylene oxides to starter molecules such as aliphatic or aromatic alcohols, amines, carboxylic acids, carboxamides, and sulfonamides. The choice of the starter molecule and the extent of alkylene oxide addition determine the characteristics of the resulting surfactant, whose properties and utility in the field are widely known to the skilled man.

[0180] The choice of surfactant depends on the specific requirements of the paint formulation and its intended properties. Different surfactants may offer unique benefits, such as improved stability and dispersion, leading to enhanced paint quality.

[0181] In one embodiment, the polymeric surfactant present in the second liquid mixture has a molecular weight comprised between 1500 and 20000 Daltons.

[0182] In one embodiment, the polymeric surfactant is present in the second liquid mixture in an amount less than 2 %wt with respect to the dry pigment weight.

[0183] A simple but effective polymeric surfactant useful in step d. is offered by ammonium polyacrylate, a low-viscosity polymeric anionic surfactant intended for paint products. With a low molecular weight (~ 3500 Da) ammonium polyacrylate (NHPA) has been extensively characterized as a dispersant for the colloidal powder processing of high solid content pigment dispersions. The mechanism involves the adsorption of either uncharged polymers or the adsorption of charged polymers referred to as polyelectrolytes onto an electrostatically charged pigment surface. As NHPA is classed as a 'weak' polyelectrolyte, electro-steric dispersion is the proposed mechanism. Polyelectrolytes are polymers that possess a charge that is present along the length of the polymer chain, as opposed to other polymers that have a charged species at one end of the molecule only. A stable adsorption, corresponding to a loose loop and tail configuration, is reached at a relatively high concentration of NHPA (> 4 mg / g), yielding a favorable stabilizing barrier resisting drying.

[0184] The high concentration of NHPA required to achieve stable stabilization is difficult to achieve within the constraint of the total amount of liquid available to reach the intended level of pore saturation in the granulation process. To overcome such limitations a more powerful amphiphilic steric surfactant was tried. A commercial polymeric non-ionic triblock BAB copolymer of polyethylene oxidepolypropylene oxide-polyethylene oxide (PEO-PPO-PEO) was used. The non-ionic triblock (BAB) PE0100-PP065-PE0100 copolymer (e.g., Pluronic®-F127), having an approximately MW of 12600 Daltons and a fully extended PEO segment length of - 40nm, was used. The PEO block is hydrophilic and water soluble while the PPO block is hydrophobic and water insoluble. PPO can form adducts with alcohols, glycols or amines to give an almost infinitely adjustable hydrophobic block which can then be further reacted with ethylene oxide (EO) to incorporate one or more hydrophilic entities into the molecule. In an aqueous environment, these triblock copolymers self-assemble into micelles with a hydrophobic PPO center core and a hydrophilic PEO outer shell that interfaces with water. These nonionic copolymers are extensively used as steric stabilizers. The stabilizing performance of a copolymer is controlled by the conformation of the adsorbed layer at the solid-liquid interface, influenced by the structure and MW of the macromolecule.

[0185] The efficacy of a steric stabilizer in a given situation depends principally on three factors. First, it must be well anchored to the particle surface such that it does not desorb or migrate laterally upon particle-particle interaction. Thus, block copolymers, whose solvent-incompatible blocks provide "anchors", are commonly used for this purpose. Second, the un-adsorbed portion of the block copolymer (the "buoy" moiety) must have good solvent compatibility. Since the solvent is selective for only the buoy moiety of the polymer, it is expected that at bulk concentrations yielding a plateau in the adsorbed amount, the copolymer remaining in solution will be in the form of micelles. Last, for effective steric stabilization, the buoy moiety must provide sufficient effective thickness to prevent the approach of the colloid pigment particles within range of the attractive van der Waals forces.

[0186] It is known that polyelectrolytes are more effective at producing depletion forces than nonionic polymers due to counterion contributions to the osmotic pressure difference between the interparticle gap and the bulk solution. The inventors thus used a combination of at least two polymeric surfactants, i.e., a nonionic polymeric surfactant and a polymeric anionic surfactant.

[0187] In one embodiment, the nonionic polymeric surfactant present in the second liquid mixture is a non-ionic triblock BAB copolymer, wherein block B is hydrophilic, and block A is hydrophobic.

[0188] In one embodiment, the second liquid mixture further comprises at least one anionic polymeric surfactant.

[0189] In one embodiment, the at least one anionic polymeric surfactant is selected from sodium polyacrylate (A widely used anionic polymer, known for its thickening, dispersing, and emulsifying properties. It can form complexes with Pluronic F127, enhancing the stability and viscosity of formulations); Ammonium Polyacrylate (Similar to sodium polyacrylate but with ammonium counterions, offering different solubility and interaction characteristics with Pluronic F127); Polystyrene Sulfonate (PSS - A strong anionic polymer with sulfonate groups. It can interact with the hydrophilic PEO segments of Pluronic Fl 27, potentially enhancing the stability of colloidal systems); Carboxymethylcellulose (CMC - A cellulose derivative with carboxymethyl groups, CMC can provide viscosity control and stabilization in mixtures with Pluronic F127, particularly in aqueous systems); Alginates (e.g., Sodium Alginate - Natural polysaccharides extracted from brown seaweeds, alginates can form gels and stabilize emulsions when mixed with Pluronic F127); Poly(methacrylic acid) (PMAA - An anionic polymer with carboxylic acid groups, PMAA can interact with Pluronic Fl 27 to form hydrogels and improve the release profile of active agents in pharmaceutical formulations); Dextran Sulfate (A sulfated polysaccharide that can provide high charge density and interact with the hydrophilic segments of Pluronic Fl 27, enhancing the solubilization of poorly soluble drugs); Sulfonated Polymers (e.g., Sulfonated Poly(ether ether ketone) [SPEEK] - These polymers offer high thermal and chemical stability and can be used to improve the mechanical strength and ionic conductivity in mixtures with Pluronic Fl 27); Hyaluronic Acid (Sodium Hyaluronate - A naturally occurring anionic polysaccharide, known for its biocompatibility and moisturizing properties, it can enhance the biocompatibility and hydration properties of formulations with Pluronic F127); Poly(vinyl sulfate) (An anionic polymer with sulfate groups, it can be used to modify the rheological properties and stability of Pluronic F127-based systems).

[0190] In one embodiment, the anionic polymeric surfactant present in the second liquid mixture has a molecular weight comprised between 2,000 and 15,000 Daltons.

[0191] In one embodiment, the anionic polymeric surfactant is present in the second liquid mixture in an amount comprised between 0.5% and 2% with respect to dry pigment(s) weight.

[0192] The combination of polymeric, non-ionic and ionic surfactants mixture is widely used in many industrial processes to take advantage of their different properties. The main characteristic of these liquid mixtures is that the nonionic and ionic surfactants often associate and form complexes via van der Waals forces, hydrogen bonding, electrostatic forces, and / or hydrophobic forces. In addition, when the interacting surfaces have a net charge of the same sign as the polyelectrolyte, electrostatic effects contribute to excluding the polymers from the gap region, in addition to steric and / or entropic effects.

[0193] By forming F127-NHPA complexes, the bound ionic surfactant (NHPA) imparts a polyelectrolyte character to the otherwise neutral nonionic copolymer (F127). When both NHPA and F127 are present in solution, the onset of the depletion force occurs at a very low concentration of NHPA in the presence of a mild concentration of F127 (10,000 ppm). These results surprisingly indicate the synergistic effect of a nonionic and an ionic surfactant on depletion forces. The inventors have reason to believe that this effect results from the complexation of the species that effectively transforms the neutral nonionic Fl 27 triblock copolymer into a polyelectrolyte, leading to a strong depletion force plus a long-range structural force at increasing concentrations. The overall result is the surprising synergistic effect of the mixture such that a much lower concentration of these polymeric surfactants (anionic and nonionic) is required to yield a very stable stabilizing effect of the adsorbed layer which can effectively withstand the drying effect.

[0194] Such a greater efficiency results in the economizing effect of the overall liquid amount needed in the wetting-nucleation stage of the agglomeration process, permitting greater latitude to control the granulation process by monitoring the pore saturation level of the agglomerates.

[0195] As reported above, on the onset of the funicular liquid bridges solid (powder or granular) polymeric binders are added to the wetted pigment mass. Such polymeric binders are needed to form a more coherent mass at the intragranular level with a high degree of plasticity needed to ensure homogeneous, dense, dry granules with low friability, whilst ensuring adhesion and cohesion between the pigment particles in the wet state, as well as, excellent plasticity and redispersibility after the granules are dried.

[0196] A stable structure of sufficient strength needs to be formed in order to withstand the hydrophilic forces arising due to the evaporation of water. A structure-forming 'cement' allows the pigment granules to be soluble in the medium into which it is desired to disperse the pigment in order to regenerate the fine particles.

[0197] A specific advantage offered by the granulation polymeric binders is to contribute to minimizing the total amount of surfactants. This is particularly important given the adverse effect of excess surfactants on wet-scrubbing resistance and water permeability of the paint systems. Excess surfactant does not improve dispersion stability and may alter the water sensitivity of the resulting paint.

[0198] In one embodiment, the at least one granulation polymeric binder is selected from PVP-PVA copolymer in the range 500-200,000 Da; polyethylene glycol (PEG) in the range 400-10,000 Da, preferably 4,000; polyvinyl alcohol (PVOH); PEG-PVA; polyvinyl pyrrolidone (PVP); partially gelatinized starch (PGS); sodium carboxymethyl cellulose; cellulose ethers.

[0199] In one embodiment, the at least one granulation polymeric binder is added in an amount ranging between 0.5% to 2%wt to dry pigment. For example, a very effective granulation polymeric binder is PVP-PVA copolymer, a nonionic binder with high molecular weight (500-200,000 Da). In water-borne paint applications, the obtained granules displayed a low water sensitivity and excellent gloss.

[0200] In one embodiment, the process herein disclosed employs a weight ratio of the sum of the first and second liquid mixtures versus the weight of the dry powder pigment(s):

[0201] - less than 25%wt, preferably less than 20%wt, if the powder pigment(s) has(have) a surface area lower than 15 sqm / g (BET); or

[0202] - less than 60%wt, preferably less than 40%wt, if the powder pigment(s) has(have) a surface area lower than 150 sqm / g (BET).

[0203] The redispersible pigment granules resulting from the process object of the present disclosure show these features: solvent-free, residual moisture lower than 1% at the time of production (dryer outlet), typical mono-modal particle size distribution with a variance of the distribution lower than 0.25.

[0204] The redispersible pigment granules produced according to the process described herein comprise:

[0205] 1. one or more disintegrant, preferably insoluble,

[0206] 2. one or more grinding aid in solid form, preferably insoluble,

[0207] 3. two or more non-polymeric surfactants,

[0208] 4. one or more polymeric surfactant, and

[0209] 5. one or more polymeric binder in solid form.

[0210] The pigments are preferably chosen from the oxides / hydroxides pigments, mixed metal oxides pigments, carbon black pigments, monoazos, naphtol AS, benzimidazolone, isoindolin(one)e, and polycyclic pigments.

[0211] A typical composition of the single redispersible pigment granule based on powder pigments with a surface area lower than 15 sqm / g (BET) is: pigment(s) 80 - 97 %wt; grinding aids(s) 0.1 - 2 %wt; disintegrant(s) 0.3 - 2 %wt; non-polymeric surfactant(s) 0.5 - 5 %wt; polymeric surfactant(s) 0.5 - 8 %wt; polymeric binder(s) 0 - 2 %wt; and residual moisture of less than 1% at the drying outlet.

[0212] A typical composition of the single redispersible pigment granule based on powder pigments with a surface area greater than 150 sqm / g is: - pigment(s) 40 - 80 %wt; grinding aids(s) 0.1 - 5 %wt; disintegrant(s) 0.3 - 5 %wt; non-polymeric surfactant(s) 0.5 - 20 %wt; polymeric surfactant(s) 0.5 - 25 %wt; - polymeric binder(s) 0 - 4 %wt; and residual moisture less than 1% at the drying outlet.

[0213] References

[0214] Iveson, Simon M., James D. Litster, Karen Hapgood, and Bryan J. Ennis. "Nucleation, Growth and Breakage Phenomena in Agitated Wet Granulation Processes: A Review" Powder Technology 117, no. 1-2 (June 2001): 3-39. https: / / doi.org / 10.1016 / S0032-5910(01)00313-8.

[0215] Hapgood, Karen P., James D. Litster, Edward T. White, Paul R. Mort, and Damian G. Jones. "Dimensionless Spray Flux in Wet Granulation: Monte-Carlo Simulations and Experimental Validation" Powder Technology 141, no. 1-2 (March 2004): 20-30. https: / / doi.Org / 10.1016 / j.powtec.2004.02.005.

[0216] Knight, P.C, J.P.K Seville, A.B Wellm, and T Instone. "Prediction of Impeller Torque in High Shear Powder Mixers" Chemical Engineering Science 56, no. 15 (August 2001): 4457-71. https: / / doi.org / 10.1016 / 80009-2509(01)00114-2.

[0217] Malkin, A. I. "Regularities and Mechanisms of the Rehbinder’s Effect" Colloid Journal 74, no. 2 (April 2012): 223-38. https: / / doi.org / 10.1134 / S1061933X12020068.

[0218] Rehbinder, P. A. "Formation of Structures in Disperse Systems" Pure and Applied Chemistry 10, no. 4 (January 1, 1965): 337-58. https: / / doi.org / 10.1351 / pacl96510040337.

[0219] G.R. Feat and S. Levine "The Double-layer Interaction of Two Charged Colloidal Spherical Particles of a Concentrated Dispersion eina Medium of Low Dielectric Constant", Journal of Colloid and Interface Science, Vol. 54, No. 1 January 1976.

Claims

Claims1. A discontinuous high-shear wet granulation process for producing redispersible pigment granules comprising the following steps: a. Providing at least one powder pigment in its natural aggregated and agglomerated form; b. Blending the at least one pigment with at least one insoluble grinding aid and / or disintegrant in powder form obtaining a blend; c. Dispergating the blend with a first liquid mixture comprising at least two non-polymeric surfactants by means of high-shear mixing, obtaining pigment particles with an average particle size lower than the average particle size of the pigment agglomerates and aggregates; d. Wetting the pigment particles with a second liquid mixture comprising at least one polymeric surfactant under high-shear mixing and letting the wetted pigment particles to nucleate; e. Adding at least one granulating polymeric binder in solid form to the wetted pigment particles under high-shear mixing, resulting in the formation of a cohesive solid bed of particles; f. Feeding the second liquid mixture onto the solid bed of particles under high-shear mixing, obtaining wetted pigment granules; g. Massing the wetted pigment granules to grow by coalescence; and h. Drying the wetted pigment granules of step g. obtaining redispersible pigment granules.

2. The process of claim 1, wherein the at least one insoluble grinding aid is selected from fumed silica, magnesium stearate, colloidal silicon dioxide, corn starch, calcium silicate, talc, sodium stearyl fumarate, stearic acid, and microcrystalline cellulose.

3. The process of claim 1 or claim 2, wherein the at least one insoluble disintegrant is selected from croscarmellose-sodium, sodium starch glycolate, crosslinked polyvinylpyrrolidone, carboxymethylcellulose, microcrystalline cellulose, naturally derived fibrous cellulose, magnesium silicate coprecipitated on chitin, magnesium aluminum silicate, potassium polacrilin (Amberlite), alginic acid, sodium alginate, gellan gum, xanthan gum, guar gum, chitosan and chitinderivatives.

4. The process of any one of the preceding claims, wherein the first liquid mixture comprises (i) a non-polymeric non-ionic gemini-type surfactant and (ii) at least one of:- a non-polymeric anionic surfactant, if the redispersible pigment granules are to be dispersed by the final user in an aqueous medium, or- a non-polymeric cationic or amphoteric surfactant, if the redispersible pigment granules are to be dispersed by the final user in a non-aqueous medium.

5. The process of any one of the preceding claims, wherein the dispergation of step c. comprises two phases, wherein:- the first phase involves spraying of the first liquid mixture on the pigment agglomerates and aggregates under vacuum and under high-shear mixing, and- the second phase involves stopping the spraying of the first liquid mixture, and high-shear mixing under a mixing condition of a Froude number equal or greater than 20, under vacuum.

6. The process of any one of the preceding claims, wherein the second liquid mixture comprises at least one polymeric non-ionic surfactant and at least one polymeric ionic surfactant.

7. The process of any one of the preceding claims, wherein the pigment granules at the end of step g. have an average particle size (d50) in the range of 50 - 1000 micron with a variance of the distribution less than 0.25.

8. The process of any one of the preceding claims, wherein in steps c. to f. the first liquid mixture and the second liquid mixture are fed onto the pigment particles by atomization.

9. The process of any one of the preceding claims, wherein steps b. to e. are carried out under vacuum.

10. The process of any one of the preceding claims, wherein the weight ratio of the sum of the first and second liquid mixtures versus the powder pigment is lessthan 60%wt.

11. Redispersible pigment granule based on powder pigments with a surface area lower than 15 sqm / g (BET), wherein the single granule comprises the following ingredients:- pigment(s) 80 - 97 %wt;- grinding aids(s) 0.1 - 2 %wt;- disintegrant(s) 0.3 - 2 %wt;- non-polymeric surfactant(s) 0.5 - 5 %wt;- polymeric surfactant(s) 0.5 - 8 %wt;- polymeric binder(s) 0 - 2 %wt; and having a residual moisture less than 1%.

12. Redispersible pigment granule based on powder pigments with a surface area lower than 150 sqm / g (BET), wherein the single granule comprises the following ingredients:- pigment(s) 40 - 80 %wt;- grinding aids(s) 0.1 - 5 %wt;- disintegrant(s) 0.3 - 5 %wt;- non-polymeric surfactant(s) 0.5 - 20 %wt;- polymeric surfactant(s) 0.5 - 25 %wt;- polymeric binder(s) 0 - 4 %wt; and having a residual moisture less than 1%.

13. Redispersible pigment granule of claim 11 or claim 12, wherein the at least one powder pigment is selected from oxides / hydroxides pigments, mixed metal oxides pigments, carbon black pigments, monoazos, naphtol AS, benzimidazolone, isoindolin(one)e and polycyclic pigments.

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