5
Additives and Particulates

As indicated in Chapter 1, coatings contain essentially four types of components, namely, binders, particulate components, additives, and solvents (usually transient). In this chapter we present an overview of the types of additives in order to provide a flavor of their use. They are used for different purposes and, as we will see, are often considered as the problem solvers, although they may introduce additional problems in a coating formulation as well. The chapter ends with a brief discussion on particulates, that is, pigments and fillers.

5.1 Types of Additives

Additives are typically seen as the problem solvers for coating technology: they are added to solve a practical production or application problem [1]. Therefore a possible definition for additives is substances that are added in small quantities to a paint or coating material to improve or modify certain properties of the coating during its manufacture, storage, transportation, or application. The amount of additives added is typically <5 wt% and on average ≅1.5% of the total formulation. Their use obviously requires a proper balance of interactions with the other components, that is, the pigments (via adsorption), binders (via association), and solvents (via solubilization).

Additives can be labeled according to their function. We distinguish between:

  • Thickeners. There are two categories: inorganic and organic thickeners.
  • Surface active agents. The most important ones are wetting and dispersing agents, antifoaming agents, and adhesion promoters.
  • Surface modifiers. Here we have matting, leveling, and coalescing agents.
  • Catalytically active additives. Here we distinguish between catalysts for the crosslinking process and those related to dryers.
  • Special effect additives. Here there is a longer list comprising antiskinning agents, light stabilizers, corrosion inhibitors, biocides, flame retardants, and photoinitiators.

In the next paragraphs we discuss briefly each type of additive, starting with what type of problem is intended to be solved.

Illustration of a clay structure. (a) Crystallographic structure of montmorillonite; (b) SEM image of partially exfoliated clay platelets.

Figure 5.1 Clay structure. (a) Crystallographic structure of montmorillonite. (b) SEM image of partially exfoliated clay platelets.

5.2 Thickeners

Thickeners are added to solve problems with the rheological behavior, that is, to optimize the flow behavior of the paint for a particular system by adjusting the viscosity for storage, processing, and application. Thickeners are usually added before dispersing.

5.2.1 Inorganic Thickeners

Organoclays, that is to say, organically modified laminar natural silicates or clays, form the most important category of inorganic thickeners, mostly used for waterborne coatings. Typical clays used are hectorite and montmorillonite. The crystallographic structure of the latter (Figure 5.1a) contains strongly bound sheets of SiO2 tetrahedra and Al2O3 octahedra, held together by relatively weak interatomic forces between the layers, resulting in platelet‐like particles (Figure 5.1b).

The mechanism of thickening occurs via a three‐dimensional hydrogen‐bonded network of the platelets (i.e. individual sheets or a few layer stacks of sheets) with water. To disperse the platelets a strong shear force is applied, which sees to it that the agglomerated platelet stacks come apart. The addition of polar solvent leads to an increase of distance between the sheets within the platelets, while the attractive van der Waals forces are reduced and the stacks break up into elementary sheets with increasing shear force. Solvation of the organic cations on the planar surfaces takes place at the same time, and edge‐to‐edge H‐bonding between the hydroxyl groups produces a three‐dimensional network (Figure 5.2b). This network is reversible, breaks down under low shear forces, and can be regenerated in the rest state. The dispersion mechanism is illustrated in Figure 5.2 [1].

Schematic illustration of the thickening mechanism by a layered type of thickener. (a) Formation of a hydrogen-bonded network of sheets via water molecules; (b) The configuration of water molecules around a sheet.

Figure 5.2 Thickening mechanism by a layered type of thickener. (a) Formation of a hydrogen‐bonded network of sheets via water molecules. (b) The configuration of water molecules around a sheet.

The preferred rheological behavior is pseudoplastic flow, in which the viscosity decreases with increasing shear rate at a given pressure and temperature. Most coatings show this behavior, and it is normally explained by changes in the position of the particles in the coating under the influence of applied forces. For example, the orientation of polymer molecules or dispersed particles may be induced to move parallel to each other, flocculated particles may deflocculate, or loops between macromolecules may disrupt. Another type of behavior is thixotropy, in which under constant shear rate and predetermined pressure and temperature, the viscosity decreases with ongoing time, and after the application of stress is ended, the viscosity increases again. The mechanism is time dependent and involves the breakdown of the structure of the dispersion by stirring, followed by recovery. Ideally the behavior should recover completely, but often the behavior shows hysteresis, that is, there remains a difference between the increasing shear rate and decreasing shear curves. Both types of flow behavior are illustrated in Figure 5.3.

Graphical illustration of rheological behavior. (a) Pseudoplastic flow behavior; (b) Thixotropic flow behavior.

Figure 5.3 Rheological behavior. (a) Pseudoplastic flow behavior. (b) Thixotropic flow behavior.

In a number of cases, there are extra benefits by introducing exfoliated (nano)clays in polymer coatings, which lead to polymer nanocomposites. These benefits include the low density of these fillers, their good flexibility and moldability, heat stability, and chemical resistance. Through the addition of clay particles, one obtains also an improvement in mechanical properties (strength, impact/scratch resistance) and better barrier properties so that the permeability for vapors and gases is reduced and flame retardancy is increased.

Nanoclays can also be used in the encapsulated form, as obtained by nonaqueous dispersion polymerization. This leads to anisotropic nanocomposite particles, which, if some layering during application occurs, might lead to improved barrier properties. The process entails the exchange of the ions associated with the (charged) clay platelets by polymers. In this process the individual platelets are first exfoliated and, if sufficient polymer present, thereafter dispersed, as illustrated in Figure 5.4. The exfoliated and dispersed platelets can also be obtained during the preparation of the polymer binders, using inverse emulsion polymerization. In emulsion polymerization the emulsion droplets have to be stabilized so that polymerization inside the (stabilized) droplet can occur [2] (Figure 5.5). This is normally done with (a relatively large amount of) surfactants but can also be achieved by particles, in which case it is often called Pickering stabilization (Figure 5.6a). Apart from isometric particles, also clay platelets have been used [1], an example of which is shown in Figure 5.6b. Hence, the platelet clays can be used both as stabilizers and thickeners, providing many advantages as explained above.

Schematic illustration of the dispersion of clay particles, depending on the amount of polymers present. (a) With a limited amount of polymer, the ions originally present are exchanged by polymers; (b) the original clay morphology with ions present between the various platelets; (c) with a substantial amount of polymers leading to individual dispersed platelets.

Figure 5.4 Dispersion of clay particles, depending on the amount of polymers present. (a) With a limited amount of polymer, the ions originally present are exchanged by polymers. (b) The original clay morphology with ions present between the various platelets. (c) With a substantial amount of polymers leading to individual dispersed platelets.

Images of clay platelet encapsulation. (a) A schematic showing an organically modified platelet further encapsulated by emulsion polymerization; (b) A SEM and TEM image of a dumbbell particle containing exfoliated platelets.

Figure 5.5 Clay platelet encapsulation. (a) A schematic showing an organically modified platelet further encapsulated by emulsion polymerization. (b) A SEM and TEM image of a dumbbell particle containing exfoliated platelets.

Images of the stabilization of emulsion droplets. (a) A schematic picture showing, respectively, stabilization by surfactants (1), by particles (2), and by platelets (3); (b) an SEM image of clay platelet stabilized emulsion droplets.

Figure 5.6 Stabilization of emulsion droplets. (a) A schematic picture showing, respectively, stabilization by surfactants (1), by particles (2) and by platelets (3). (b) A SEM image of clay platelet stabilized emulsion droplets.

5.2.2 Organic Thickeners

These types of thickeners are used for waterborne systems as well as for solventborne systems. For the former case one can distinguish between water phase thickeners (typically cellulose and starch derivatives and acrylic thickeners) and associative thickeners (typically hydrophobically modified polyoxyethylenes, acrylic thickeners, and cellulose ethers). The cellulose derivatives influence the viscosity and stability but suffer from water retention. Starch derivatives are mainly used in highly pigmented, cheap paints with a pigment volume concentration (PVC) > 85%. Acrylic thickeners have a limited use for interior walls and are available in an emulsion form. The drawback is that they are affected by pH.

While water phase organic thickeners interact predominantly with the water phase, associative thickeners interact with several components of the coating. Typically, associative thickeners have a molecular weight ranging from 10 000 to 50 000, to be compared with that of surfactants (with a comparable role) with molecular weight of 200–500. The process is schematically shown in Figure 5.7.

Schematic of associative thickening. (a) Network between binder particles and thickeners; (b) Schematic of a thickener and a surfactant with as only essential difference that a thickener contains a hydrophobic functional group at both ends (indicated by the black block) while a surfactant contains one such a block.

Figure 5.7 Schematic of associative thickening. (a) Network between binder particles and thickeners. (b) Schematic of a thickener and a surfactant with as only essential difference that a thickener contains a hydrophobic functional group at both ends (indicated by the black block), while a surfactant contains one such a block.

Organic thickeners are partially soluble in water and thicken the coating formulation by interaction with other components. One distinguishes between:

  • Hydrophobically modified polyoxyethylenes, in particular hydrophobically modified ethoxylated urethanes (HEUR).
  • Acrylic thickeners, for example, hydrophobically modified alkali‐swellable emulsions (HASE).
  • Cellulose ethers, in particular hydrophobically modified hydroxyethyl cellulose (HMHEC) ethers.

The first category, the HEUR additives, comprises several hydrophilic segments consisting of urethane groups, provided with at least two hydrophobic terminals that can adsorb on or associate with the binders, while some micelles can also be formed in the dispersion. They show a combined thickening mechanism as the PU increases the viscosity of the solvent (water), the additive promotes emulsion formation and PU connections and leads to association between binder and pigment particles.

For (organic) solventborne coatings, aspects such as stability, solvent retention, rheology, and film formation are important. The balance of the various factors involved will be different for different applications. The rheology determines the flow not only during application but also after application, and an improper rheological behavior can lead to defects, such as sagging, while the structure buildup due to crosslinking after application controls whether a proper distribution of, for example, metallic flakes is realized. A few examples of thickeners for solventborne coatings are shown in Figure 5.8, while Table 5.1 indicates some advantages and disadvantages of the various types.

Schematic structures of some examples of (organic) solvent-borne thickeners. (a) Hydrogenated castor oil; (b) polyamines; (c) sulfonated salts.

Figure 5.8 Examples of (organic) solventborne thickeners. (a) Hydrogenated castor oil. (b) Polyamines. (c) Sulfonated salts.

Table 5.1 Types of thickeners for (organic) solventborne coatings.

Thickener Advantages Disadvantages
Hydrogenated castor oil Thixotropic rheology, flow, and leveling Heat sensitive, seeding, solvent dependency
Polyamides Stability, universal Difficult incorporation, affects intercoat adhesion
Sulfonated compounds Easy incorporation, high gloss, temperature stability Water sensitive, high alkalinity
Cellulose acetobutyrate Good weathering characteristics, low moisture absorption Swelling by many solvents

5.3 Surface Active Agents

As coatings are covering surfaces, coatings themselves have a surface and contain interfaces with pigment particles. Interfaces form a crucial aspect in coatings, and to be able to control their behavior is of the utmost importance for the final properties of the coatings. Surface active agents are just meant to do so. Possible problems to be solved are improving the liquid spreading or wetting properties of a system by reducing the surface tension and enriching the solution with interfaces. In coatings several types of interfaces are important. Liquid–air interfaces play a role in pigment wetting, leveling, film formation, and defoaming. Solid–air interfaces play a role not only in pigment wetting and stabilization but also in substrate wetting and the final film quality. Liquid–solid interfaces also influence pigment wetting, while solid–solid interfaces are relevant in adhesion and the mechanical properties of coatings. Within the surface active agent category, we distinguish here between wetting and dispersing agents, antifoaming agents, and adhesion promoters.

5.3.1 Wetting and Dispersing Agents

The purpose of wetting and dispersing agents is to facilitate the dispersion of solids in the liquid phase. The degree of dispersion influences many aspects of a coating, to name a few, color strength, opacity, gloss, outdoor properties, and stability. For example, it may be difficult to obtain a clear color with agglomerated particles. Stabilization can occur by electrostatic and steric mechanisms and, for polar media, also by polyelectrolytes. The moiety responsible is called a surfactant. Surfactants, often also addressed as dispersants, generally contain strongly hydrophobic tails and strongly hydrophilic heads. One can distinguish between anionic surfactants consisting of negative molecules (and a positive counterion) and cationic surfactants consisting of positive molecules (and a negative counterion). Typical small molecular mass examples are SDS (sodium dodecyl sulfate; C12H25OSO3Na) and CTAB (hexadecyl trimethylammonium bromide; C12H25N(CH3)3Br). Also high molecular mass surfactants are frequently used, for example, for the dispersion of TiO2. Moreover, one has nonionic surfactants containing highly polar groups, such as polyethylene oxide, and amphoteric surfactants carrying a positive and negative charge while being on the whole neutral. The cationic and anionic compounds have a strong affinity for the interface because their tails prefer to be out of the water, while their heads prefer to be in the water. In a similar way the polar groups of the nonionic surfactants prefer the water phase. Most surfactants practically used are anionic, followed by nonionic surfactants. Anionic surfactants often interfere less with the other components of the coating, while their dispersibility is usually better as compared with the other type of surfactants. Nonionic surfactants are usually less sensitive to pH but contain generally larger molecules, and hence they may be more difficult to dissolve or to disperse. Cationic surfactants often pose environmental problems, while amphoteric surfactants are generally expensive and therefore only used for special applications.

The characteristics of the particulates to be dispersed have an influence on their final wetting and dispersing state. The shape of the particles in the formulation codetermines the type of aggregation and agglomeration (Figure 5.9). However, also size plays a role as small particles have a large specific surface area that influences often the reactivity but can lead to easier agglomeration through van der Waals interactions.

Schematic illustration of the influence of shape (upper row) on aggregation (middle row), and agglomeration (lower row).

Figure 5.9 Schematic of the influence of shape (upper row) on aggregation (middle row) and agglomeration (lower row).

For nonsolvent coatings, such as powder coatings, special surfactants may be used. Here one uses compounds that are miscible and/or react with the binder so that their presence later on in the process may be useful. An example [3] is provided by block copolymers of poly(vinyl pyrrolidone) (PVP), which adsorbs on the pigments, and polycaprolactam (PCL), which adsorbs on the binder.

5.3.2 Antifoaming Agents

In several process steps in a coating preparation, intensive stirring is applied, and this may lead to foaming. Moreover, since one often uses surfactants that stabilize air interfaces, this can lead to the entrapment of air (foaming) and in a later stage to crater and pinhole defects in the coating. Hence several types of defoamers are used, typically mineral or vegetable oils and poly(dimethyl siloxanes) (-Si-O-(CH2)nCH3). They provide a low surface energy while being insoluble in the phase to be defoamed. In order to be effective, they should have a high evaporation enthalpy. Figure 5.10 shows defects due to foaming and pinholing and a schematic picture of the effect of a surfactant and defoamer.

Images of some defects in coatings. (a) Foaming; (b) Pinholing or cratering; (c) (non)-bubble formation in the absence and bubble formation in the presence of a surfactant; (d) using a defoamer that breaks up the surfactant stabilized interface.

Figure 5.10 Defects in coatings. (a) Foaming. (b) Pinholing or cratering. (c) (Non)‐bubble formation in the absence and bubble formation in the presence of a surfactant. (d) Using a defoamer that breaks up the surfactant stabilized interface.

Structures of organosilanes showing the backbone and typical end groups Y and X.

Figure 5.11 Organosilanes showing the backbone and typical end groups Y and X.

5.3.3 Adhesion Promoters

The problem tried to be solved with these additives is (obviously) poor adhesion, and hence their function is to improve the physical interactions or chemical bonding with substrate. While physical interactions typically have a value less than 50 kJ mol−1, chemical bonding can result in much higher values, say, for covalent bonds 60–700 kJ mol−1 and for ionic bonds 600–1000 kJ mol−1. A typical example is provided by organofunctional silanes (Figure 5.11). Such a compound, say, an amine methoxy silane, will be hydrolyzed by the presence of a small amount of water, and the methoxy groups will be exchanged by hydroxyl groups. On a metallic substrate, on which always hydroxyl groups are present, the silane–hydroxyl groups can react covalently with the hydroxyl groups present on the substrate by splitting off water. This leads to a substrate with the surface modified by a covalently bound silane that still has the amine group to react with the binder of the coating.

Images of matting. (a) Typical mat surface; (b) Cross-section of mat film showing the particles embedded.

Figure 5.12 Matting. (a) Typical mat surface. (b) Cross section of mat film showing the particles embedded.

5.4 Surface Modifiers

Since often a coating has also an aesthetic function, the surface is important from this perspective. Small and well‐dispersed pigments are used to obtain a homogeneous color, and usually this also leads to high gloss. In a number of cases though, a mat surface is required for aesthetic or other reasons, and so one requires a uniform rough surface, obviously still having a good wear resistance, stability against shear stress, and scratch resistance (Figure 5.12). To this purpose a matting agent is used to realize such a diffuse light scattering surface so that a proper gloss results. As additives for this purpose, one often uses silica gels, natural or synthetic silica particles (amorphous SiO2), waxes, or polymers, such as polyolefins or urea–formaldehyde condensates.

5.5 Leveling and Coalescing Agents

Referring again to the surface of a coating, in other cases a high gloss is required. Moreover, one often requires leveling, that is, the ability of a paint to spontaneously level out the uneven surface that originates from its application. The determinant factors are the viscosity η (the lower the viscosity, the easier the leveling) and the interfacial tension γ (the higher the surface tension, the higher the driving force). However, local differences in surface tension may occur from solvent evaporation, and in that case the liquid paint moves toward the high surface tension areas. This might lead to the orange peel and Bénard cell effects (Figure 5.13). In the former the surface tension only changes the level locally, but in the latter a flow is induced resulting in a (more or less hexagonal) pattern that after solidification is still visible.

Images of some defects of coatings. (a) Orange peel effect; (b) Benard cell effect; (c) Schematic of the Benard cell effect.

Figure 5.13 Defects of coatings. (a) Orange peel effect. (b) Bénard cell effect. (c) Schematic of the Bénard cell effect.

Practical factors that might lead to the orange peel effect related to the coating are a too high viscosity during application, poor pulverization, and a solvent that too rapidly evaporates. Application also influences the appearance. If the substrate to be coated is too far removed from a (spray) nozzle, the equipment is unsuitable for the paint to be applied. Another example is if the temperature is too high and the substrate to be coated is too hot, which may also lead to the orange peel effect. The remedies are then clearly diluting the paint or use a slower evaporating thinner, while for the practical factors one considers moving the substrate coated closer to the nozzle, optimizing the nozzles and working pressure, using more application systems for producing smooth surfaces (e.g. mixed air), or applying at a lower ambient temperature and on a cooler substrate. Table 5.2 provides an overview of flow leveling agents. The dosage normally used is 2–5% for solvents, 0.1–1.0% for polymers, and 0.01–0.2% for silicones.

Table 5.2 Flow leveling agents.

Products/examples Reduction in γ Application areas Remarks
Solvents/cyclohexanol None or very little Solventborne and waterborne systems Leveling, but slower drying speed, and lower viscosity may bring reduced sag resistance
Polymers/copolymer based on BA and 2‐EHA Little or none Solventborne, waterborne, solvent‐free and powder systems Leveling and anti‐popping effects, good intercoat adhesion, but may be tacky at high content and have turbid appearance when too incompatible
Silicones/dimethyl polysiloxane Little to high Solventborne, waterborne and solvent‐free systems Leveling and flow, slip, but may stabilize foam, intercoat adhesion possibly poor
Fluorinated surfactants/ammonium perfluorodecyl sulfonate High to very high Solventborne, waterborne and solvent‐free systems Leveling and flow, but may stabilize foam, intercoat adhesion usually poor

5.6 Catalytically Active Additives

Frequently also catalytically active additives are used. They are applied not only to accelerate the chemical reactions involved, such as thermosetting, that is, crosslinking, but also to enhance other chemical reactions taking place during film formation. Among the catalytic active additives, we distinguish between dryers and other catalysts, which are usually present in formulations. For the latter, we discuss only melamine‐based and polyurethane (PU)‐based systems.

5.6.1 Dryers

In many cases oxidative crosslinking is used, and that process is promoted by soaps coordinated to a metal cation (Mx+):

  1. Neutral soaps (R‐COO)2M2+, (R‐COO)3M3+
  2. Acidic soaps    (R‐COO)2M2+ •R‐COOH, (R‐COO)3M3+ •R‐COOH
  3. Basic soaps     (R‐COO)3M22+ •−OH

In the past Co‐based dryers were extensively used, but due to their toxicity they are now replaced by Fe‐ and Mn‐based dryers [4], often in combination with additional dryers based on Ba, Ca, or Zr.

The basic mechanism for oxidative drying (crosslinking), that is, drying without dryers (Figure 5.14), is a slow process. Normally dryers are used, and to that purpose one makes use of hydroperoxide formation, illustrated in Figure 5.15. The first two steps are accelerated whereby the metal ions act as O2 carriers, followed by hydroperoxide decomposition, recombination of the radicals R, chain addition, and propagation.

Schematic structures illustrating drying without drier making use of radicals and peroxide formation.

Figure 5.14 Drying without dryers making use of radicals and peroxide formation.

Schematic illustration of oxidative drying. (a) Hydrogen abstraction leading radical formation; (b) Peroxide formation in which the metal acts as catalyst; (c) Possible crosslink reactions; (d) Regenerating new radicals via the metal; (e) Possible side-products via β-scission.

Figure 5.15 Oxidative drying. (a) Hydrogen abstraction leading radical formation. (b) Peroxide formation in which the metal acts as catalyst. (c) Possible crosslink reactions. (d) Regenerating new radicals via the metal. (e) Possible side products via β‐scission.

Dryers can be deactivated via various processes. Chemisorption of the dryer onto the pigment surface leads to permanent immobilization of the dryer. Moreover, insoluble complexes can be formed with short‐chain aliphatic solvents. These newly formed complexes usually have a low solubility and crystallize out. Another possibility is hydrolysis of the dryer, and this is often the main reason for drying ability loss in waterborne systems. It occurs via rapid hydration with water, leading to an insoluble basic metal soap.

In order to increase shelf life, sacrificial or feeder dryers are used (Figure 5.16a). During storage the catalyst is slowly consumed, and in order to have sufficiently catalyst available, a sacrificial dryer is introduced that releases metal ions later on. Recently also environmentally friendlier alternatives for Co‐based systems are introduced. Figure 5.16 shows two examples, one for solventborne [5, 6] and one for waterborne [79] systems.

Schematic illustration of alternatives. (a) A feeder drier used to release metal ions after metal ion consumption due to degradation; (b) Mn-based driers for solvent-borne systems; (c) The system Fe2+/H2O2/ascorbic acid for waterborne systems.

Figure 5.16 Alternatives. (a) A feeder dryer used to release metal ions after metal ion consumption due to degradation. (b) Mn‐based dryers for solventborne systems. (c) The system Fe2+/H2O2/ascorbic acid for waterborne systems.

5.6.2 Other Catalysts

Crosslinking applies to many systems, and here we mention only two of the most important systems. For melamine systems, crosslinking occurs via hydroxyl and carboxyl resins (Figure 5.17). As melamine systems have a high functionality, usually a relatively dense network is obtained. To improve shelf life also, here blocking is applied for which typically para‐toluene sulfonic acid (p‐TSA) is used. This compound can be used as a salt or as covalently bonded to the melamine. By applying elevated temperature, deblocking occurs. Using blocking enlarges the storage time of the resin considerably, while, dependent on curing temperature and type of blocking, the reactivity is not or only limitedly decreased (Figure 5.18).

Schematic illustration of melamine crosslinking. (a) Melamine where R = H or CnH2n+1 with n = 1-4; (b) The reaction with functionalized polymers; (c) Self-condensation of melamine.

Figure 5.17 Melamine crosslinking. (a) Melamine where R = H or CnH2n + 1 with n = 1–4. (b) The reaction with functionalized polymers. (c) Self‐condensation of melamine.

Illustrations of blocked p-TSA. (a) Mechanism and structures of blocked p-TSA, via ionic and covalent bonding; (b) Storage behavior of polyester polyol + HMMM showing the effect of adding p-TSA on the viscosity increase versus time; (c) Hardness change as a function of curing temperature with curing time for 20 min for the same system showing the increase in hardness.

Figure 5.18 Blocked p‐TSA. (a) Mechanism and structures of blocked p‐TSA, via ionic and covalent bonding. (b) Storage behavior of polyester polyol + HMMM showing the effect of adding p‐TSA on the viscosity increase versus time. (c) Hardness change as a function of curing temperature with curing time for 20 min for the same system showing the increase in hardness.

For PU systems one uses often polyisocyanurates of hexamethylene diisocyanate(HDI) or isophorone diisocyanate (IPDI) as crosslinker (Figure 5.19). The catalysts involved are typically Zr(octoate)4 (0.2–0.5%), Ti(OBu)4, SnCl4, Bu3SnCl, or dibutyl tin laurate (DBTDL, 0.001–0.01%) (Figure 5.19c).

Schematic illustration of alternatives. (a) Urethane formation; (b) urea formation; (c) crosslinkers often used, respectively, HDI, IPDI, and polyisocyanurate. Also shown is the catalyst DBTDL (dibutyl tin laurate).

Figure 5.19 Alternatives. (a) Urethane formation, (b) urea formation, and (c) crosslinkers are often used, respectively, HDI, IPDI, and polyisocyanurate. Also shown is the catalyst DBTDL.

PU coatings are often employed as two‐pot (2 K) systems, for which the gel time depends, apart from the temperature, also on the crosslinker used. For example, at room temperature over the concentration range 0.001–0.2% (of solid resin), HDI appears to lead much faster to gelling than IPDI. For 2 K systems a critical factor is the balance between pot life and curing speed. To prolong pot life of Sn‐catalyzed PU systems via blocking, one adds typically acetylacetone, or another reactive compound, in particular a thiol. Deblocking occurs either via evaporation or reaction.

PU coatings are also employed as one‐pot (1 K) systems, in which blocked isocyanates are used. In this case the use of a catalyst is essential. Figure 5.20 shows the mechanism involved. Deblocking occurs via elimination–addition or transesterification. The blocking agents as used for blocking isocyanate are shown in Figure 5.21, while the catalyst often used is DBTDL (Figure 5.19c), normally added in extremely small amounts (0.001–0.01%) as PU systems are generally rather reactive already. This catalyst itself is normally blocked, for example, by acetyl acetonate or a thiol.

Schematic illustration of blocked PU. (a) Blocking mechanism with BH as blocking agent; (b) Deblocking by elimination-addition; (c) deblocking by transesterification.

Figure 5.20 Blocked PU. (a) Blocking mechanism with BH as blocking agent. (b) Deblocking by elimination–addition. (c) Deblocking by transesterification.

Schematic illustration of blocking agents as used for blocking polyurethanes.

Figure 5.21 Blocking agents as used for blocking polyurethanes.

5.7 Special Effect Additives

Finally, there is a wide range of special effect additives that are very much related to the environment and overall use of the coating. As their effect is generally very specific, we do not extensively describe them here but just mention a few important categories for which there is plenty of specialized literature available [1]. The first category to mention is antiskinning agents, meant to retard film formation on the surface of a liquid (skinning). An often used compound is methyl ethyl ketoxime (MEKO), which blocks the catalyst activity (Table 5.3). The mechanism proceeds via the formation of stable complexes with the metal ions, for example, [Co(MEKO)6]3+ (applicable for Mn catalysts as well) that renders the catalyst metal inactive for autoxidation. Due to its high vapor pressure, MEKO evaporates immediately after the coating material is applied, and following the evaporation of MEKO, the complex with Co is broken up, and the catalytic capability of Co is recovered.

Table 5.3 Effect of MEKO on drying.

Without MEKO With MEKO
Totally dry after 4.75 h 4 h
Skinning after 3 d >250 h

System: alkyd with dryer (0.05% Co, 0.03% Zr, and 0.1% Ca, drying temperature 23 °C.

Another type of important additive is light stabilizers. These additives should prevent photodegradation, a kind of reverse of polymerization in which the polymer breaks down in smaller fragments under the influence of UV radiation. Photodegradation of coatings can lead to cracking, chalking, delamination, and color change, while a nondegraded coating protects the substrate meanwhile providing good adhesion, proper gloss, and color retention. Introducing UV absorbers is done to prevent the formation of free radicals by UV absorption. Typical UV absorbers are shown in Figure 5.22. The absorption mechanism is often based on a structural change in the absorber. Another mechanism is based on the destruction of the free radicals formed by radical scavengers (Figure 5.23). A combination of UV absorbers and radical scavengers offers the best protection.

Schematic illustration of UV absorbers. (a) Structural change in two absorbers and the (b) corresponding absorption spectrum.

Figure 5.22 UV absorbers. (a) Structural change in two absorbers. (b) Corresponding absorption spectrum.

Schematic illustration of radical scavengers. (a) Mechanism where R = -H, -CH3, -OCnH2n+1,...; (b) HALS-4; Effect of HALS-4 on the gloss retention.

Figure 5.23 Radical scavengers. (a) Mechanism where R = -H, -CH3, -OCnH2n+1, … (b) HALS‐4; Effect of HALS‐4 on the gloss retention.

For outdoor applications under severe conditions, one adds corrosion inhibitors to coatings. They are often based on a sacrificial compound, like Zn [10], but conductive polymers and hybrid conductive coatings are also considered nowadays [11].

As in many cases organic coatings are in contact with the environment, frequently biocides are added to prevent the growth of bacteria on the coating.

Finally, we mention flame retardants, introduced to lower the flammability of organic coating. Often they are based on inorganic materials like Al(OH)3 and Mg(OH)2, but also organic halogen‐containing compounds such as H(CHCl)nCl and phosphor‐based compounds like O-P(OCH2CH3) are used.

5.8 Particulates

To many coatings particles are added. For coloring purposes these particles are usually referred to as pigments, while for other purposes, such as increasing wear resistance or increasing toughness, one normally addresses them as fillers. We will address them here collectively as particulates. For coloring, not only colored pigments are added but also often white pigments, mainly TiO2. Apart from the color, other important aspects are:

  • Crystallographic structure. Many inorganic compounds do have more than one crystallographic modification, of which the properties may differ significantly.
  • Particle size (distribution) and the related specific surface area. Inorganic powders usually consist out of primary particles, each of which contains many crystallites. While crystallites do have a more or less perfect long‐range order that can be probed by X‐ray diffraction, the orientation between the various crystallites in the primary particles differs from the average orientation by a variable small angle. Crystallites usually have a size below 10 nm and the size of primary particles ranges typically from 10 nm to 50 µm. The size and dispersity of the size of the primary particles are normally important, for example, for uniformity of color and hiding power (see Chapter 11), while the specific surface area codetermines their adsorption behavior.
  • Particle shape. More or less isometric particles are used in many cases, but a platelet‐like shape may be required for effect coatings or decreasing the permeability by offering a more tortuous path for diffusing species when dispersed in the final film.
  • Degree of agglomeration and/or aggregation (Figure 5.9). Here the term agglomerates refers to loosely bonded clusters of primary particles for which the complete surface area of the primary particles is virtually available. Typically they can be easily broken down by stirring but do reform upon standing of the stirred dispersion. The term aggregates refers to clusters of primary particles that are relatively strongly bonded to each other. Only part of the surface of the primary particles constituting the aggregate is available, and separation usually requires strong forces, such as occur, for example, in milling. For uniformity, obviously, both agglomerates and aggregates should be avoided.

The (many types of) existing pigments and fillers can be conveniently divided in inorganic and organic pigments. Here we only deal briefly with just a few of the most important of them.

Table 5.4 Rutile and anatase physical–chemical properties.

Rutile Anatase
Density (g cm−3) 4.23 3.78
Refractive index (−) 2.61 2.49
Oil absorption value (g oil per 100 g filler) 16–48 18–30

Although the crystallographic density is fairly accurately determined, pigments often show a somewhat variable density, dependent on the purity and porosity.

The most important inorganic white pigment is titania (TiO2), although this may change in future in view of ongoing legislation (see Chapter 1). It exists (at room temperature) in three stable crystallographic modifications, of which rutile and anatase are important for coatings. Table 5.4 provides some of their characteristics. Apart from its use in paints, it is also used in plastics, paper, sunscreen, and food coloring. For 2015 the world production of titania was estimated to be about 7.2 million metric tons, of which about 60% for paints, 20% for plastics, 12% for paper, and 8% for other applications [12]. An extensive review of titania technology is given in [13]. Its isoelectric point (see Chapter 7) ranges from 4.5 to 6.0 pH units. Rutile absorbs some violet light, while anatase absorbs almost no light. Hence, a rutile‐pigmented coating has yellowish hue. Titania can have a substantial deviation from strict stoichiometry. Charge neutrality is compensated by changing the valency of (some of) the titanium ions. With band gap of 3.05 eV, it is photoactive absorbing UV radiation and generating electrons. To reduce the effect of these electrons (on degradation), titania particles are often coated with (primarily) silica and/or alumina, but affecting also their isoelectric point (see Chapter 7). To improve their compatibility with the binder, also organic modifiers are used.

Other white pigments include ZnO, ZnS, and mixtures of typically 30% ZnS and 70% BaSO4. For colored pigments, either iron hydroxide FeO(OH) for brownish and yellowish colors or regular and mixed iron oxides for reddish colors are frequently employed, the latter typically in the spinel form, containing both divalent and trivalent ions. The relations between optical properties and crystallographic structures of these pigments are extensively discussed in the dedicated literature [14].

Silica (SiO2) is mainly used as a filler (for example, to improve wear resistance) and is used both in crystalline and amorphous form. Before use, these powders are mostly organically modified, for example, by silanes.

Carbon black (CB) is nominally pure paracrystalline carbon material that has a high surface‐area‐to‐volume ratio and is made by the incomplete combustion of heavy petroleum products, such as coal or ethylene cracking tar and a small amount from vegetable oil. The primary size ranges from 5 to 500 nm, but CB is usually heavily aggregated, and the aggregates are on their turn agglomerated. Deagglomeration is therefore essential for almost all applications. Although CB is mainly used as a reinforcing filler in pneumatic tires and other rubber products, it is also used in paints, plastics, and inks as a pigment. They are used as black color as such but also to darken other colors. All CBs have chemisorbed oxygen complexes (i.e. carboxylic, quinonic, lactonic, phenolic, and other groups) on their surfaces to varying degrees depending on the conditions of manufacture. In this form CB is typically a poor electrical conductor due to these adsorbed components, but high purity CB can also be made and used to produce electrically conductive coatings. For a review on CB, see [15, 16]. Nowadays, carbon nanotubes (CNTs) and (probably more important) graphene are researched for reinforcing purposes and increasing conductivity.

Finally, we discuss pigments for effect coatings. To obtain a metallic effect, one uses metallic flakes of which Al flakes are by far the most important. They have a diameter typically ranging from 1 to 200 µm and a thickness of 0.1 to 2 µm. As Al oxidizes rapidly (and finely divide Al is pyrophoric), the materials are usually supplied as a paste containing about 35% volatile hydrocarbons. Metallic flakes can be divided in leafing and nonleafing pigments. Leafing pigments are surface treated, resulting in a low surface energy, for ease of dispersion. They orient at the surface upon application. Nonleafing pigments are also surface treated but mainly to reduce chemical reactions with water and/or the acids used in processing and have a higher surface energy. Consequently, they remain mainly in the bulk of the coating, orienting themselves largely parallel to the surface of the substrate. Their main application is in automotive coatings, to obtain a so‐called metallic effect. For pearlescent and iridescent effects, one uses mica, typically 5–100 µm in lateral size and 0.3–0.6 µm thickness. They are normally treated with a thin layer giving an interference effect. These pigments are also widely used in automotive coatings for the metallic effect, but with additional color effects.

Organic pigments typically have relatively sharp absorption bands, and the color results as a complement of the absorption. The absorption is due to unsaturated groups, denoted as chromophores (see Figure 1.5), often enhanced by substituents groups, called auxochromes. The chromophores are frequently based on compounds containing (one or two) diazo groups, thereby providing a wide range of low‐cost, bright, yellow, orange, and red colors. However, their outdoor stability is limited. Phthalocyanines, providing blue and green colors, are more stable. Copper phthalocyanine is the main representative, of which the color can be tuned by substitution. Phthalocyanines do show an α‐ and a β‐form, and a transition between these forms affects the particle size. Moreover, they are prone to flocculation.

Dispersing particles in a paint is a major issue and involves several steps:

  • The first step comprises breaking up of the agglomerated and/or aggregated particles (deagglomeration). This is done using dispersers, ball mills, or roller mills. A disperser consists of a shaft connected to a disk provided with circumferential ribs perpendicular to the disk. This disk rotates at high speed, typically 5000 rpm, in a vertical cylindrical tank containing the dispersion. A ball mill is a horizontal cylindrical container containing milling balls together with the dispersion. It is loaded approximately half with milling balls and is rotating at such a rate that the content is cataracting instead of centrifuging or rolling at the bottom to provide maximum efficiency. A roller mill contains two (sometimes three) rolls with a prescribed spacing through which the dispersion is fed. To realize a paste nowadays also extruders are frequently employed.
  • Second, stabilizing the individual primary particles either by steric or by electrostatic means via using the appropriate amounts of the appropriate surfactant(s).
  • Third, the selection of the solvent, as related to the nature of the binder as well as the surfactants and other additives.

The degree of dispersion and the resulting stability of the dispersion determine the quality of the paints. Its assessment comprises rheological measurements and particle size (distribution) measurement as well as the flocculation rates.

References

  1. 1 Bieleman (2000).
  2. 2 Voorn, D.‐J., Ming, W. and van Herk, A. (2006). Macromolecules 39: 4654.
  3. 3 (a) See, e.g., Hiemenz, P.C. and Rajagopalan, R. (1997). Principles of Colloid and Interface Chemistry, 3e. New York: Marcel Dekker.(b) Butt, H.‐J., Graf, K. and Kappl, M. (2006). Physics and Chemistry of Interfaces, 2e. Weinheim: Wiley‐VCH.
  4. 4 Gezici‐Koc, Ö., Thomasa, C.A.A.M., Michela, M.‐E.B. et al. (2016). Mater. Today Commun. 7: 22.
  5. 5 Oyman, Z.O., Ming, W., van der Linde, R. et al. (2005). Polymer 46: 1731.
  6. 6 Oyman, Z.O., Ming, W., and van der Linde, R. (2006). Eur. Polym. J. 42: 1342.
  7. 7 Miccichè, F., van Haveren, J., Oostveen, E. et al. (2005). Arch. Biochem. Biophys. 443: 45.
  8. 8 Miccichè, F., van Straten, M.A., Ming, W. et al. (2005). Int. J. Mass Spectrom. 246: 80.
  9. 9 Miccichè, F., Oostveen, E., van Haveren, J. et al. (2006). Appl. Catal., A 297: 174.
  10. 10 Mathazhagan, A. and Joseph, R. (2011). Int. J. Chem. Eng. Appl. 2: 225.
  11. 11 (a) Sitaram, S.P., Stoffer, J.O. and O'Keefe, T.J. (1997). J. Coat. Technol.69: 65.(b) Zarras, P., Anderson, N., Webber, C. et al. (2003). Rad. Phys. Chem. 68: 387.
  12. 12 U.S. Geological Survey. (2016). Mineral Commodity Summaries, U.S. Geological Survey, January 2016.
  13. 13 Braun, J.H., Baidins, A. and Marganski, R.E. (1992). Progr. Org. Coat. 20: 105.
  14. 14 Chromy, L. and Kaminska, E. (1978). Progr. Org. Coat. 6: 31.
  15. 15 Medalia, A.I. and Rivin, D. (1976). Carbon black. In: Characterization of Powder Surfaces (ed. G.D. Parfitt and K.S.W. Singth), 279. London: Academic Press (Chapter 7).
  16. 16 Wang, M.‐J., Gray, C.A., Reznek, S.R. et al. (2014). Carbon black. In: Encyclopedia of Polymer Science and Technology, 4e, vol. 2 (ed. H.F. Mark), 426. New York: Wiley.

Further Reading

  1. Bieleman, J. ed. (2000). Additives for Coatings. Weinheim: Wiley‐VCH.
  2. Calbo, L.J. ed. (1987). Handbook of Coatings Additives. New York: Marcel Dekker.
  3. Orr, E.W. (1998). Performance Enhancement in Coatings. Munich: Hanser Publishers.
  4. Parfitt, G.D. (1981). Dispersing Powders in Liquids, 3e. London: Appl. Sci. Publ.
  5. Wicks, Z.W. Jr. Jones, F.N., Pappas, S.P. and Wicks, D.A. (2007). Organic Coatings: Science and Technology, 3e. Hoboken, NJ: Wiley.
  6. Brezinski, D., Koleske, J.V. and Springate, R. (2014). 2014 and 2013 Additives Reference Guide, 2012 Additives Handbook. Troy, MI: Paint and Coatings Industries (PCI).
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