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Roland Baumstrak

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Herausgeber: Vincentz Network
Kategorie: Wissenschaft und neue Technologien
Sprache: Englisch
Veröffentlichungsjahr: 2022

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The Mission: This book provides comprehensive knowledge on water-based acrylic dispersions for architectural coatings. Covering a wide range of topics, from the basics to suitable binders for dispersions to formulations for various substrates and applications through to the relevant test methods, it makes essential reading for any formulator of competitive modern architectural coating systems. The Audience: Novices to the technology or those switching specialisms, along with students and experts who wish to expand and deepen their knowledge and who are looking for essential background information that will be useful to them in the formulation and testing of water-based acrylic dispersions. The Value: This second, updated edition of this established standard work offers a clear overview of everything one needs to know about the various types of binders, systems and test methods associated with the application of water-based acrylic dispersions in architectural coatings.

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Bibliographische Information der Deutschen BibliothekDie Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliographie; detaillierte bibliographische Daten sind im Internet über http://dnb.ddb.de abrufbar.

Roland Baumstark and Roelof Balk

Water-based Acrylic Dispersions: Applications in Architectural Coatings, 2nd Revised Edition

Based on 1st Edition Waterbased Acylates for Decorative Coatings

Hanover: Vincentz Network, 2022

European Coatings Library

E-Book ISBN 978-3-7486-0490-7

Vincentz Network GmbH & Co. KG, Plathnerstr. 4c, 30175 Hanover, Germany

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European Coatings Library

Roland BaumstarkRoelof Balk

Water-basedAcrylic Dispersions

Applications in Architectural Coatings

2nd Revised Edition

Foreword

Water-based polyacrylates, as emulsion binders, dispersing resins or thickening polymers, are nowadays impossible to do without as raw materials in the paint and coatings industry. Since their introduction in the fifties and sixties of the last century pure acrylic and styrene-acrylic dispersions have established themselves as environmentally friendly high-performance alternatives to solvent-borne, air-drying alkyd resin binders mainly used before. They show a particularly wide performance spectrum, excellent levels of durability and pigment binding ability combined with a superior chemical robustness. In addition, their structural variability allows their still ongoing development as binder class for coating materials.

This book gives an overview of the preparation and properties of water-based acrylic dispersions and the special features pertaining to their use in the architectural coatings sector. This completely revised 2nd edition also includes innovations in these fields of the last 20 years. Following a general introduction to acrylics and polymer dispersions, a separate chapter on the basics of emulsion paint formulation is included. In the following chapters, deeper insight is given into the diverse types of paints, such as primers, masonry paints, including the special topics dispersion-silicate paints, silicone-resin paints and elastomeric crack-bridging paints, interior paints, wood coatings, trim paints and textured finishes or plasters.

The aim of the book is to present an easy to read, up-to-date picture of the preparation of acrylic binders and their formulation to diverse types of architectural paints. Owing to the breadth of the applications of acrylic dispersions and the diversity of present-day knowledge concerning colloid chemistry, it is impossible for this book to deal with every theory and aspect of dispersions and paints manufacture. The reader is therefore referred to the quoted literature, which offer more in-depth views on the various topics.

In addition, the architectural coatings market differs strongly between the regions. This cannot fully be covered by this small book. The focus in the formulation chapters is therefore on the European paint market.

The book is aimed both at students and newcomers in the field of paints and coatings, but it also gives the experienced dispersion user and paint formulator in the industry relevant information and specific hints. The provided information gives assistance with the selection and daily use of modern polymer dispersions.

Acknowledgments

The authors wish to thank the co-author of the first edition Manfred Schwartz for his initial contribution. Our thanks also goes to Konrad Roschmann and Bastiaan Lohmeijer and their lab teams for their support, among others with graphics, and to diverse other colleagues and experts such as Ivan Cabrera, Heribert Kossmann, Alan Smith, Oliver Wagner and Chen-Le Zhao, all of them present or former employees of BASF, without whose previous work, their knowledge and publications this book would not have been possible to write.

We would also like to thank Jürgen Kaczun for his contribution to the wood coatings chapter.

Finally, thanks are due to BASF SE, Ludwigshafen for approving the manuscript for publication.

Ludwigshafen, September 2022

Roland Baumstark and Roelof Balk

1Introduction and basic principles

1.1Architectural coatings and binders

Architectural coatings have a dual function: on the one hand, they make a significant contribution to the aesthetics of the building or the structural components, for instance by colouring or accentuating their structure. On the other one they protect the building material against external influences, such as moisture, sunlight, or mechanical or chemical damage.

The majority of water-based architectural coatings are complex mixtures of a wide variety of chemical components, as shown by the following compilation:

Main components

–Water

–Binders

–Pigments

–Fillers

Additives/auxiliaries

–Dispersants and wetting agents

–Thickeners/rheology modifiers

–Defoamers

–Preservatives/biocides

–Solvents/film-forming auxiliaries

It is not uncommon for aqueous architectural coatings to contain between 10 and 20 different ingredients.

The function of the binder is to give the coating the necessary cohesion, durability, weathering stability, good mechanical properties such as flexibility or hardness, and to give the paint its advantageous processing properties. The binder embeds the colouring pigments and fillers in a stable matrix and joins them to the substrate. This distinguishes the finished coating from, say, classroom chalk, which is based on pressed chalk, acts without a binder and is therefore easily washed off.

Besides the pure inorganic material water-glass, which has already been in use for a long time in silicate systems, today the binders used for water-based architectural coatings are predominantly polymer dispersions.

According to Chem Research GmbH, in 2019 ca. 42 million tonnes of coatings material (coatings and lacquers) were produced worldwide, the largest part of which were architectural coatings (21.4 million tonnes). In Europe alone, that year the demand for architectural coatings was approx. 5.2 million tonnes [1]. IRL reported for 2017 even a market size of 7.6 million tonnes for Europe. According to that study [2], today already more than 80 % of architectural coatings are water-based and formulated with water-based polymer dispersions. This would mean that 800,000 to more than 1 million tonnes of polymer dispersions per year are used for this application in Europe today.

The solvent-borne alkyd resin systems which formerly dominated the sector are increasingly being replaced by more environment-friendly, aqueous coating systems bound with polymer dispersions; they are still available and have a certain place, especially in applications such as wood coatings and trim paints. Moreover, also the alkyd technologies developed towards either water-based or high solid systems to meet the present-day requirements [3]. In Germany, the production statistics of the paint and printing ink industry association (VDL) [4] for 2018 record a total of 683,296 metric tons of aqueous emulsion paints for interior and masonry application alone, whereas of high solids and aqueous alkyd systems only 63,930 metric tons are produced.

1.1.1Polymer dispersions [5–8]

Generally, a dispersion is a multi-phase system in which at least one phase is present in a state of microscopically fine distribution (the disperse phase: liquid or solid, for example) within a continuous phase (e.g. liquid or gas). In polymer dispersions, the disperse phase consists of (spherical) polymer particles with a diameter which is usually less than 1 µm; the continuous phase is water.

Figure 1.1: A polymer dispersion at various magnification levels

Water-based polymer dispersions are usually milky white liquids whose viscosity varies from low, like water, to high, like whipped cream. In analogy to the milky sap of the plants which provide natural rubber, they are also often referred to as latex, and the polymer particles as latex particles (Figure 1.1). A single millilitre of polymer dispersion contains on average approximately 1015 particles. In turn, from 1 to 10,000 macromolecules are present per particle, each of these macromolecules being composed of about 50 to 106 building blocks (monomers).

Polymer dispersions are not thermodynamically stable per se. The polymer system has the tendency to minimize its large internal surface area by agglomeration of the particles, coming together in lumps (coagulation), followed by settling or creaming (dependent on relative density of polymer and medium). By adding charge carriers (charge or Coulomb stabilization) or uncharged spacers of medium to high molecular weight (steric or entropic stabilization) to the surface of the polymer particles, however, the disperse state can be stabilized [9,10]. Nevertheless, external influences, such as shearing (as a result of shaking, pumping or stirring, for example), freezing, pressure or salt exposure, may in adverse circumstances cause the stabilization to fail; the dispersion then coagulates.

Among polymer dispersions a distinction is made between primary dispersions, prepared by polymerizing the basic building blocks (monomers) directly in the liquid phase (via emulsion polymerization in water, for example), and secondary dispersions, for which a preformed polymer, such as a solution polymer or film-forming resin, is distributed or dispersed in the medium in a second step, usually involving input of mechanical energy [11]. The primary dispersions possess the greatest importance; they are industrially readily obtainable by emulsion polymerization and show good cost performance properties. Among the secondary dispersions, the last 15 years alkyd emulsions have gained an important position besides the class of the polyurethane dispersions [3, 4, 12]. These last ones are primarily used in the industrial coatings sector, for wood coatings and for trim paints.

1.1.2Binder classes, polymerization and polyacrylates

The chemistry of the polymers in the field of aqueous architectural coatings is very diverse. The most important classes of binders in the coating sector are:

–Acrylate copolymers (pure or straight acrylics),

–Styrene-acrylate copolymers (styrene-acrylics),

–Alkyd and alkyd-acrylate polymers,

–Vinyl acetate homopolymers and copolymers (vinyl acetate-ethylene, vinyl acetate-ethylene-vinyl chloride terpolymers, vinyl acetate-versatic acid vinyl ester, vinyl acetate-maleate, vinyl acetate-acrylate).

Table 1.1: List of the most frequently used monomers in architectural dispersions

Acrylates/acrylic esters

Methacrylates/methacrylic esters

Other monomers

n-Butyl acrylate (n-BA)

Methyl methacrylate (MMA)

Styrene (S)

2-Ethylhexyl acrylate (2-EHA)

n-Butyl methacrylate (n-BMA)

Vinyl acetate (VAc)

Ethyl acrylate (EA)

Methacrylic acid (MAA)

Acrylonitrile (AN)

Acrylic acid (AA)

Methacrylamide (MAM)

Vinyl chloride (VCl)

Acrylamide (AM)

Vinyl versatate

(“VeoVa” a)

Ethylene (E)

aregistered trademark of Hexion, Columbus, Ohio, USA

Other dispersions, such as styrene-butadiene copolymers or polyurethane dispersions play only a minor role in the coatings field. The reason for this is the poor weathering stability and/or strong yellowing tendency of the styrene-butadiene dispersions and the high price of the (secondary) polyurethane dispersions (PUD’s), respectively. Consequently, the use of the very hydrophobic styrene-butadiene dispersions is restricted to non-topcoat applications such as primers, especially anti-corrosion primers, and the use of polyurethane dispersions to high-performance wood coatings for especially parquet and furniture because of their excellent mechanical properties and chemical resistances.

1.1.3Free-radical polymerization[13–20] and emulsion polymerization [10, 21–34]

Apart from vinyl acetate homopolymers, the most important binder types are exclusively copolymers, where the fundamental properties are brought about by free-radical polymerization of a specific combination of different α,β-unsaturated organic building blocks (monomers).

Reaction scheme

Free-radical polymerization is a chain reaction initiated by the decomposition of an initiator molecule (I2) to form fragments having a reactive, unpaired electron. These initiator radicals (I∙) then attack the double bond of a monomer molecule (M), forming chain radicals (I-M∙). These radicals can react with a further monomer molecule to produce extended chain radicals (I-M-M∙). The chain reaction continues to propagate until the growth of the chains (I-MnM∙) is terminated by recombination (e.g. dimerization) or disproportionation (hydrogen transfer). The resulting polymer chain length can be controlled by introducing so-called chain transfer agents (R-X). These compounds have a labile carbon-hydrogen, carbon-halogen or sulphur-hydrogen bond (e.g. mercaptans), which stop the growing chain by hydrogen or halogen atom transfer. The residual chain transfer agent radical R∙ starts a new chain. The primary effect of the chain transfer agent is a controlled reduction of the degree of polymerization.

A free-radical polymerization is characterized by its rapid, exothermic course. The high-molecular polymers formed are referred to as addition polymers.

The overall mechanism of the reaction is represented by the following scheme:

The monomer building blocks most frequently used for architectural dispersions are given in Table 1.1; the basic structures of the principal classes of monomer are shown in Figure 1.2.

Mechanism of emulsion polymerization

As already described the industrial preparation of the (primary) water-based dispersions takes place via a specific form of free-radical polymerization known as emulsion polymerization.

In this process, the monomers react in water in the presence of surface-active compounds of low molecular mass (emulsifiers) or oligomers/polymers (protective colloids) by adding a water-soluble free-radical initiator and heating. The emulsified monomers undergo polymerization to form the dispersed macromolecules. The micellar mechanism of the emulsion polymerization of styrene was first described by Harkins[35] and by Smith and Ewart[36] (see Figure 1.3).

According to them, before the addition of initiator the monomers in the polymerization reactor are distributed between emulsifier-stabilized monomer droplets (having a diameter of 1 to 10 µm) and so-called micelles, i.e. aggregates of 20 to 100 emulsifier molecules (with a diameter of 5 to 15 nm). The fraction of monomer present in molecular solution in the water is very small. On heating, the initiator breaks down in the water phase to form radicals which initially grow by attachment to the water-dissolved monomers to form oligomeric radicals. At a certain chain length, depending on type of attached monomers, these radicals are no longer soluble in water and they start to precipitate at available surfaces. Since the number of micelles in the reactor per unit volume (approximately 1018 per cm3) is significantly higher than that of the monomer droplets (approximately 1010 per cm3), and since the total surface area of the micelles is substantially greater than that of the monomer droplets, the oligomeric radicals enter almost exclusively the micelles.

Figure 1.2: Structures of four important monomer classes

There, the chains continue to grow, as a result of which the micelles should in fact become increasingly depleted of monomer. This does not occur as long as the transport of monomer molecules from the monomer droplets via the water phase to the micelles is sufficient. Consequently, the monomer concentration in the water phase remains a constant as long as there are monomer droplets present in the reactor. Meanwhile, in the micelles, the polymer chains grow to build up latex particles until all monomer droplets have disappeared. In the course of the polymerization, therefore, the growing, polymer-filled micelles turn into emulsifier-stabilized latex particles.

Figure 1.3: Mechanism of emulsion polymerization

A: monomer droplet with monomer (E) and surfactant molecules (F)

B: micelle with monomer molecules

C: polymer particle, stabilized with surfactant molecules, containing several macromolecules, one of these with reactive radical chain end (x), and monomer molecules (A)

D: water soluble initiator radical (x)

E: monomer in the water phase (dissolved)

F: molecularly dissolved surfactant molecule

G: water molecules

To supplement the theory of micellar particle formation as described above, Fitch and Tsai[37] postulated the principle of “homogeneous nucleation”, later further developed by Ugelstad and Hansen[38]. According to this principle, initiation by a water-soluble charged peroxide radical which adds to monomer units in the water phase is the trigger for the formation of oligomeric macroradicals. Above a chain length defined for each monomer (2 to 100 units), the limit of solubility is exceeded and primary particles are formed. These primary particles are usually unstable and undergo agglomeration until they reach a state of colloidal stability: secondary particles. The diameter of the secondary particles is limited by the amount of emulsifier and the polarity of the polymer.

The mechanisms discussed above for the formation of the particles are limiting cases [39]. Depending on the choice of monomers and how the polymerization is performed, both mechanisms come into play to a certain extent. For instance, for monomers with higher solubilities in water (such as vinyl acetate, ethyl acrylate and methyl methacrylate) initiated with peroxides, homogeneous nucleation is important and can even become dominant.

Irrespective of the mechanism discussed, a prerequisite for the emulsion polymerization is that the monomers used are at least slightly soluble in water. Consequently, while monomers such as styrene or 2-ethylhexylacrylate will still readily undergo emulsion polymerization, polymer dispersions of very hydrophobic, long-chain and hence virtually water-insoluble (meth)acrylates, such as lauryl (meth)acrylate or stearyl acrylate, are not obtainable by conventional emulsion polymerization. To polymerize this kind of monomers in an aqueous environment, a so-called mini-emulsion polymerization process can be used, in which the polymerization takes place in the droplets of a preformed mini-emulsion of the monomer(s). These droplets must be sufficiently small, i.e. from 50 to 300 nm, and stable in order to get an appropriate reaction rate and good control over the process. To get the small droplet size, a macro-emulsion of monomer(s) plus emulsifier(s)/stabilizer(s) has to be subjected to high shear (e.g. by using an ultrasonic device or (high-pressure) mechanical homogenizer), and to gain the necessary stability often a very hydrophobic costabilizer is used that is soluble in the monomer, but very insoluble in water (e.g. hexadecane) [40].

For the emulsion polymerization process on the industrial scale, the monomers are usually pre-emulsified in water (i.e. to give a macro-emulsion). The emulsion thus prepared, and the initiator solution are then metered separately into the polymerization reactor over a defined period of time. With this semi-continuous or feed process, the instantaneous conversion of the monomers is very high (usually >90 %), so that a randomly assembled copolymer is formed, largely irrespective of differences in reactivity and copolymerization parameters. Moreover, the semi-continuous process offers the advantage over the formerly used batch process, that the heat of polymerization produced can be regulated by way of the metering time and dissipated in a controlled fashion by external cooling.

Before starting the emulsion polymerization process, often a small amount of a small-sized seed latex is added to the reactor. This results in a better control of the overall polymerization process, and of the final particle size of the dispersion.

In comparison to solution polymerization, emulsion polymerization, in which the resulting polymer particles are finely distributed in water, has the additional advantage that even high molecular weights (up to more than 1 million Dalton) can be obtained with a low system viscosity. The industrial polymer dispersions therefore usually have high polymer contents of from 40 to 60 % by weight.

Table 1.2: Solubilities of the principal monomers for acrylic dispersions in the architectural coatings sector and glass transition temperatures of their homopolymers [41]

Monomer building blocks

Water solubility at 25 °C in g/100 cm3

Glass transition temperature (Tg) of the homopolymer [°C]

Acrylates

Methyl acrylate (MA)

5.2

+22

Ethyl acrylate (EA)

1.6

-8 (or -17 [16])

n-Butyl acrylate (n-BA)

0.15

-43

iso-Butyl acrylate (i-BA)

0.18

-17

t-Butyl acrylate (t-BA)

0.15

+55

2-Ethylhexyl acrylate (2-EHA)

0.04

-58

Lauryl acrylate (LA)

<0.001

-17

Methacrylates

Methyl methacrylate (MMA)

1.5

+105

n-Butyl methacrylate (n-BMA)

0.08

+32

iso-Butyl methacrylate (i-BMA)

0.13

+64

Styrene (S)

0.02

+107

Acrylonitrile (AN)

8.3

+105

Vinyl acetate (VAc)

2.4 to 2.5

+42 (or +28 [16])

1.2Polyacrylates, pure acrylics and styrene-acrylic copolymers [5, 41–50]

Within the group of the acrylate copolymer dispersions to which this book is devoted, two classes of copolymers can be differentiated: pure acrylic and styrene-acrylic dispersions. Pure acrylics are polymer dispersions composed exclusively of acrylate and/or methacrylate monomers. Styrene-acrylic copolymers contain styrene as well. For both types of copolymers there are a lot of monomers available (Table 1.2) with a wide variety regarding polarity and glass transition temperature (Tg) of the homopolymers prepared from them.

The glass transition temperature is the temperature at which a polymer undergoes a transition from a brittle, glassy to a rubbery, flexible and more or less pronounced tough state.

Acrylic acid and the acrylates are nowadays industrially prepared starting from propene, whereas methacrylic acid and the methacrylates are prepared starting from 2-hydroxy-2-methyl propionitrile (adduct of acetone and prussic acid) or from isobutene or isobutyraldehyde [44, 51]. The multistage preparation processes make these classes of monomers more expensive than styrene or vinyl acetate, in turn leading to a higher price for the pure acrylic copolymers as compared with styrene-acrylic copolymers and polyvinyl acetate.

The special features of the poly(meth)acrylates, which justify their relatively high price, are the generally very good weathering and UV stability, high transparency (to UV radiation as well), good water and yellowing resistance, and great ease of variation in toughness, flexibility and hardness. The high film gloss level with good gloss retention under weathering, in combination with the chemical resistance to alkali, acid and water (resistance to hydrolysis), likewise contribute to the excellent suitability of this class of polymers for the architectural coating sector.

Structure and properties

The main polymer properties, such as glass transition temperature, film mechanics and polarity, are determined by the composition and structure of main and side chains.

The water solubility of the monomers (see Table 1.2) may be regarded as a measure of the polarity of the homopolymers. As this solubility in water increases, there is an increase in the polarity of the resultant polymers. The water solubility of the free acids, acrylic and methacrylic acid is infinitely, especially in the neutralized state. For the esters, a decrease of water solubility is seen with increasing side-chain length. The C-C linked main chain is chemically largely inert and provides the poly(meth)acrylates with chemical and weathering stability. Owing to the low bond strength of the α-CH group adjacent to the carbonyl centre (C=O), however, the polyacrylates are somewhat less stable than the methyl-substituted polymethacrylates. Thus, polyacrylates exhibit a somewhat lower stability than the corresponding polymethacrylates, both with respect to UV radiation and under strongly oxidative conditions. The hydrolysis tendency of the polyacrylates is higher than that of the polymethacrylates as well, due to the missing steric shielding of the carbonyl centre by the adjacent methyl group of the last group of polymers.

The strong influence of the side chain on the properties is evident, for example, from the falling saponification tendency of the polymers as this length increases and its degree of branching goes up.

Polymethacrylates possess increased chain rigidity relative to the homologous polyacrylates, because of the additional methyl groups and the resulting steric hindrance of the main chain to rotate. This increased rigidity leads to increased glass temperatures, a rise in hardness, and reduced flexibility in comparison with the homologous polyacrylates. As the side chain becomes longer, there is a decrease in hardness and glass transition temperature and an increase in stretchability (up to 8 carbon atoms in the case of acrylates and about 12 carbon atoms in the case of methacrylates; with longer chains, the hardness increases as a result of increasing side chain crystallinity) (see Figure 1.4 and Table 1.3). A rise in chain length is also accompanied by an increase of the tackiness of the polymers, as shown by measurements of the tack on different homo-polyacrylates (Figure 1.5).

Table 1.3: Film mechanics of different homopoly(meth)acrylates [46b]

Tensile strength at break [N/m2]

Elongation at break [%]

PMMA

68970

PEMA

37240

P-n-BMA

3450

300

PMA

6930

750

PEA

230

1800

P-n-BA

2000

Figure 1.4: Glass transition temperature (Tg) as a function of the side chain length for poly(meth) acrylates

Table 1.4: Influence of increasing side chain length on the properties of poly(meth)acrylates

Properties of poly(meth)acrylates

Hardness falls

Flexibility and stretchability rise

UV stability rises

Gloss retention improves

Tackiness rises

Alkali resistance rises

Water sensitivity falls

Alcohol resistance rises

Hydrocarbon solubility rises

The trends of the influence of the side chain length are illustrated in Table 1.4. Also the branching of the side chains has a profound influence on properties. Poly(meth)acrylates with unbranched side chains formed from unbranched alcohols, are softer and more stretchable than the species which start from the branched isomers. This is clearly reflected by their different glass transition temperatures (see Table 1.5).

Figure 1.5: Tack and glass transition temperature of polyacrylates

Table 1.5: Comparison of the glass transition temperatures of polybutyl (meth)acrylates with branched and unbranched side chains [41]

Substituent

Acrylate

Methacrylate

n-Butyl

-43 °C

+32 °C

iso-Butyl

-17 °C

+64 °C

tert.-Butyl

+55 °C

+102 °C

Glass transition temperature of copolymers

Copolymerization of different monomers makes it possible to produce polyacrylate dispersions with wide variations in properties. The glass transition temperature of copolymers can approximately be calculated using the (empirical) Fox equation:

Fox equation:

For architectural coating binders it is usual to combine monomers having low glass transition temperature homopolymers (“soft” monomers), such as n-BA and 2-EHA, with monomers having high glass transition temperature ones (“hard” monomers), such as n-BMA and MMA, in order to get a copolymer with a glass transition temperature between 0 and 40 °C. Only in case a high level of low-temperature elasticity is required, by example for crack-bridging exterior coatings, lower glass transition temperatures (down to -45 °C) are established in the copolymers by using appropriately high fractions of soft monomer. On the other hand, owing to the high level of coating hardness required for interior wood coatings (e.g. for parquet) and special industrial coatings, the glass transition temperatures of such binders are often 40 °C or more.

Glass transition temperatures are usually measured by Differential Scanning Calorimetry (DSC), for instance according to DIN 53 765 (testing of plastics and elastomers, thermal analysis, DSC method), ISO 11 357 or ASTM D 3418. Normally, the glass transition extends over a certain temperature range, depending on homogeneity of the (co) polymer. As Tg, often the midpoint of this range is taken. A glass transition is, in contrast to a melt or crystallization transition, not a first but a second order phase transition; its position depends on the used measurement method and a parameter like heating rate. Most (co)polymers produced by emulsion polymerization are amorphous and lack a crystalline state; they only show a glass transition.

Table 1.6: Binder properties using styrene or methyl methacrylate as hard monomers

Property

MMA

Hardness

Light stability

+/- to -

Water resistance

+/-

Water vapour permeability

Chalking/gloss reduction

+/- to -

Dirt pick-up resistance

Saponification resistance

+/- to +

Pigment binding power

+/-

Film gloss

Price

++ very good; + good; +/- not so good;- unsatisfactory

Copolymer variants

In the case of the styrene-acrylic copolymers, costs can be reduced, and properties optimized by replacing at least part of the MMA usually used for pure acrylic copolymers (owing to the necessary hardness), by the less expensive monomer styrene. This is possible because of the good copolymerization tendency (comparable with that of MMA) of styrene with acrylates and the approximately equal glass transition temperature of the two homopolymers poly-MMA and -S. In the resultant copolymers, incorporation of the non-polar styrene monomer as a substitute for MMA leads to an improvement in water and alkali resistance, and to an increase in the pigment binding power, expressed e.g. in a better wet scrub resistance of the formulated coatings. Moreover, because of the higher refractive index of PS relative to PMMA, higher coating gloss is achievable when styrene is incorporated in the binder. With high styrene content in the polymer and low levels of pigmentation of the coatings, however, the inherent UV absorption of the phenyl groups may result in increased photo-induced binder degradation, chalking, loss of gloss and yellowing in the course of long-term weathering (for comparison of properties see Table 1.6).

Comparing the binder class of the poly(meth)acrylates with the polyvinyl esters based on vinyl acetate, the coatings which result when using poly(meth)acrylates are more hydrophobic, resistant to water and stable to saponification and hence more stable to weathering. Moreover, due to the higher refractive index and the often smaller-sized nature of the acrylic dispersions, higher degrees of gloss can be achieved in the high gloss coating segment using acrylates.

Accordingly, simple polyvinyl acetate copolymers and high-pressure polymers of vinyl acetate and ethylene are preferably used in coatings for interior application with relatively high levels of pigmentation, where the nature of the polymer is not so dominant a factor in the coating properties and where the lack in water resistance and its negative influence on substrate protection ability is minimal. The properties required for exterior applications are only achieved with this class of coating when vinyl acetate is copolymerized with large amounts of expensive, sterically bulky co-monomers, such as the versatic acid vinyl esters. In that case, however, the cost advantage over the poly(meth)acrylates disappears.

In Europe, pure acrylics are principally used for exterior applications, for clear varnishes, for stains, gloss emulsion coatings, exterior masonry and for house paints, i.e. in clear coats or at low to moderate levels of pigmentation. Because of their favourable price/performance ratio, styrene-acrylic dispersions are almost universal in their applicability. Only with clear varnishes and wood stains, or generally in coatings having a very low UV-screening pigment and filler fraction, there are limits on their use.

1.3Film formation of polymer dispersions [52–62]

1.3.1Mechanism and minimum film forming temperature

The process of film formation by a dispersion is significantly more complex than for a solution polymer, where following the evaporation of the solvent formation of a continuous film readily takes place by interlooping of the polymer chains. In contrast, for polymer dispersions film formation takes place via a multistage process (Figure 1.6).

Drying phases

During the first stage of the drying process, the dispersion particles move closer and closer together (see Figure 1.6a) as the water evaporates, until they contact each other (first phase of film formation, see b). As the evaporation of water continues, the capillary and surface tension forces press the particles against one another. If the temperature during the drying process is above the minimum film formation temperature (MFFT), which is a characteristic temperature for each polymer dispersion, these forces start to deform them. Up to this point, however, the original particle boundaries are still present (second phase of film formation, see c). In a last step, then, interdiffusion of polymer chains across these particle boundaries takes place. As a result, the polymer particles coalesce with each other at the points of contact to form a continuous film (third phase of film formation, see d). The boundaries may disappear during this stage, at least in part.

As a consequence of the fact that the water-soluble components, such as emulsifiers and salts, in the course of the film formation process are enriched at the particle boundaries forming a so-called interstitial phase, the dispersion film never completely “forgets” its particulate past, even at temperatures far above the MFFT. Consequently, in electron micrographs, it is often possible to observe a hexagonal network, resembling a honeycomb, which originates from the starting particles (examples in [60], see Figure 1.7). The hydrophilic components enriched in the interstitial phase are an important reason why dispersion films and emulsion paints are more sensitive to water than films or coatings based on solution polymers.

Another reason is the fact that failures can easily occur during the process of particle packing, resulting in incomplete coalescence and reduced film quality [63].

1.3.2Parameters determining the minimum film forming temperature

In addition to the Tg there are a number of other parameters having some effect on the MFFT, such as particle size, particle size distribution, molecular weight, crosslinking type and density, surfactants in use.

Figure 1.6: Scheme of film formation of a polymer dispersion

Figure 1.7: Electron microscopic picture of the film structure of a pure acrylic dispersion (monolayer preparation, magnification 40,000 : 1)

Small-sized dispersions better form films than coarse dispersions of the same composition [64], which is usually also reflected in a somewhat lower MFFT [65], improved film quality and water resistance, and a higher level of film gloss. For this reason, acrylate-based architectural binders are usually fine-sized dispersions, with mean particle diameters smaller than 300 nm, for instance from 100 to 200 nm, and today often smaller than 100 nm, down to 50 nm or even less.

The MFFT is usually situated just below the glass transition temperature of the polymer. The difference between MFFT and Tg is more pronounced with polar dispersions. In this case the phenomenon of hydroplasticization occurs, i.e. the polymer is plasticized by water resulting in swelling and softening of a surface layer of the particles or even of the complete particles. A consequence of this is that polar dispersions can have an MFFT which is up to, or even more than 15 °C lower than that of non-polar dispersions with the same glass transition temperature [66].

The MFFT of a dispersion is usually determined on a so-called Kofler bench (e.g. in accordance with DIN EN ISO 2115). To this purpose, the dispersion is applied to a metal bar having a temperature gradient and dried under controlled atmospheric conditions. The MFFT is then the lowest temperature at which a homogeneous, transparent and crack-free film is formed.

1.3.3Coalescents/solvents and plasticizers

In order to be able to reliably form films at room temperature even with “hard” polymer dispersions having glass transition temperatures higher than 20 °C it is common to use temporary plasticizers – solvents which evaporate after the film formation. In contrast to true or permanent plasticizers (for typical representatives see Table 1.7) which are likewise used to reduce the MFFT, these solvents do not remain long-term in the film. They are emitted to the environment at different rates depending on drying conditions like temperature and humidity, on boiling point, and on the resulting vapour pressure.

The solvents in emulsion coatings are therefore frequently referred to as film-forming aids or coalescents [67]. Besides the classical white spirit, today preference is given to water-miscible glycol ethers (butyl glycol, butyl diglycol, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, etc.) and their acetates. Though, as part of the VOC discussion (volatile organic compound) and eco label provisions (e.g. the EU sunflower, the German Blue Angel) there is an increasing trend to use more hydrophobic coalescents with boiling points >250 up to 400 °C, such as the mono-ester type “Texanol” d, di-esters of dicarboxylic acids, such as “Loxanol” CA 5308c and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (TXIBd) or tripropylene glycol mono-n-butylether (“Solvenon” TPnB c), for various solvents see Table 1.8.

The reason for this is that according to provisions of the European Union and of the German paint industry association, the term solvent is used only when the boiling point is 250 °C at most (at 1 atm) 68, [69]; in this case it contributes to the VOC content of the paint (according to the EU-directive 2004/42/EG “Decopaint”) [70]. All film-forming auxiliaries having boiling points above 250 °C are by definition plasticizers. They may be used without restriction in emulsion paints labelled as “solvent-free”, despite the fact that they do not remain permanently in the coating: they belong to the so-called SVOCs (semi volatile organic compounds) [68].

Table 1.7: List of solvents/plasticizers of high boiling point

Solvents/plasticizers

Example of trademarks

Dibutyl phthalate

Dioctyl phthalate

Diethylenglycol hexyl ether, hexylene glycol

Polypropylene glycol alkyl phenyl ether

“Loxanol” PL 5060c

Tributoxyethyl phosphate

2,2,4-Trimethyl-1,3-pentanediol di-isobutyrate (TXIBd)

“Optifilm Enhancer” 300 d

Triethylene glycol--bis-(2-ethyl hexanoate)

“Optifilm Enhancer” 400 d

Tripropylene glycol mono-butylether

In general, modern coalescents should enable excellent film formation of the paints by softening the polymer particles and helping them to coalesce to a continuous film. They should preferably be easy in use and compatible with a broad range of polymer dispersions. In addition, they must be non-toxic, mild in odour, non-VOC, improving the overall coating performance and, today, should be even bio-based or biodegradable.

The film-forming auxiliary particularly affects the position of the MFFT. There are three major aspects influencing the effectiveness of a coalescent: its distribution between polymer particle and water phase, its softening ability of the polymer and the rate it evaporates during drying of the film.

An important part is played by the compatibility and solvation capacity of the coalescent with respect to the latex particles [71]. Its efficiency can be estimated by determining its Tg and using this value in theoretical Tg calculations of binder plus coalescent. This was done by Taylor and Klots[72] – they found values from -67 down to -129 °C for the Tg’s of the coalescents they studied.

Hydrophobic solvents, such as the ester types “Loxanol” CA 5308c and “Texanol” d or the classical paraffin based white spirits, are highly compatible with hydrophobic polymers. Accordingly, depending on their affinity to the latex particles, they swell and plasticize them already in the wet state to a greater extent than hydrophilic solvents such as the glycol ethers like butyl glycol or butyl diglycol. These last ones are then predominantly present in the aqueous phase, however, during the last stages of drying they partition towards hydrophilic regions of the latex particles, also resulting in plasticization and swelling and contributing to an improved film formation [73].

Table 1.8: List of commonly used solvents/film-forming auxiliaries in water-borne architectural paints

Hydrophilic solvents, such as ethylene glycol or propylene glycol, have virtually no plasticizing effect. However, by hydrogen-bonding with water they slow down its evaporation and so retard the formation of the film. Polymer dispersions and their formulations generally dry fast and the open time is short – often too short to brush out surface defects, to smooth the surface or to apply the paint without brush marks or a stripy appearance. These two solvents are thus particularly suitable as auxiliaries for prolonging the open- and wet edge time and thus also the phase during which a freshly painted surface can be over-coated and corrected with a fresh paint load without optical defects. For this reason, they are used to improve the workability, e.g. by brush; in addition, they may make the coatings more frost resistant by lowering the freezing point of the medium water. The latter is the major reason to use mono ethylene glycol specifically.

The effect of different solvent additions on the course of the MFFT is evident from the example of the styrene-acrylic dispersion “Acronal” S 790c, a standard binder for paints and plasters, having a glass transition temperature of ca. 22 °C and an MFFT of 18 to 20 °C (see Figure 1.8).

Service properties/performance aspects

High boilers, such as “Texanol”d or “Loxanol” CA 5308c, remain in the film long-term, sometimes for weeks or months, and therefore have the disadvantage that the final properties of the coating, such as freedom from tackiness, good hardness and high blocking resistance, are achieved not until fairly long after the application of the paint (see Figure 1.9 on the pendulum hardness of “Acronal” S 790c with different solvents).

In contrast to film-forming auxiliaries, true plasticizers remain permanently in the film and permanently alter its properties. Owing to the relatively strong film tackiness which they generally entail, there is a strong increase in the dirt pick-up tendency of the coatings. Because of this, the use of permanent plasticizers remains restricted to specialities, e.g. elastic, crack-bridging coatings. Often, plasticizers have a certain tendency to migrate to the surface, which not only worsens the soiling tendency of coatings throughout their lifetime, but also enables the transport of these low-molecular weight compounds to the environment, which is of course undesired.

In general, when forming a film of a polymer dispersion or a dispersion-based architectural coating, it should be ensured that the processing temperature is high enough and the drying time is sufficiently long. Only at temperatures above the MFFT the system forms a continuous, clear film, which can mechanically be stressed. If the water evaporates at temperatures below the MFFT, fissured, non-continuous and turbid films are formed; in an extreme case even powdery residues are found.

Figure 1.8: Influence of solvents on the MFFT of “Acronal” S 790 c

Other than by the choice of film-forming auxiliaries, the drying rate and film quality are considerably affected by temperature, relative atmospheric humidity and rate of air exchange [74]. But as well, the substrate plays an important role. On soaking substrates such as wood or mineral porous surfaces especially hydrophilic coalescents may soak quickly together with the water into the substrate – later on they are missing for the film formation of the coating.

Coalescents also influence the wetting behaviour and the rheology of the coating. Upon addition of coalescent, the polymer particles generally swell, resulting in an increase in viscosity. Especially in case associative thickeners are used in the formulation they may have a quite strong influence on the thickener activity (see Chapter 5).

A negative aspect is that addition of especially water-friendly coalescents to the dispersion (e.g. butyl glycol, PnB, DPnB), but sometimes also more hydrophobic types, may cause a solvent-shock, leading to formation of grit or even to complete coagulation. Such effects can be avoided by slow dosage of the coalescent at stirring or, still better, by predilution of the coalescent with water.

Figure 1.9: Pendulum hardness of “Acronal” S 790c as a function of solvent and drying time; 100 µm wet film thickness on glass, without pigment, 20 °C and 65 % relative humidity, 3 % solvent on dispersion

Generally, the colloidal stability of the system decreases after addition of the coalescent. This is consequence of the swelling and softening of the polymer particles.

In summary, the paint formulator should be aware of the fact that type and amount of coalescent not only influences the MFFT – for obtaining crack-free film formation at application temperatures – but also stability, viscosity, surface tension, foam tendency and a lot of other parameters, including important final coating film properties such as water resistance, whitening tendency, hardness, toughness, tackiness, stack ability, dirt pick-up resistance and wet scrub resistance.

An overview of coalescents is given in Table 1.8; for selected types it also contains information about boiling points (BP), relative evaporation rates (with respect to n-butyl acetate) and water solubilities as measure for their polarity.

1.3.4Environmental aspects

Increased environmental awareness among end users in Northern Europe in particular, but also in Germany, the Netherlands, Austria and Switzerland, has resulted in paint and coating manufacturers increasingly offering low-solvent or solvent-free products. Accordingly, raw materials manufacturers offer product ranges including “internally plasticized” binders, with an increased proportion of copolymerized soft monomers (old product reviews can be found in [75]; for current ones see the homepages of the suppliers). Good film formation without addition of solvent, even under adverse weather conditions, requires a binder MFFT of not more than 5 °C, better below 3 °C [76]. According to the present day definition of the German paint industry association described in the VDL guideline 01 (January 2017), “eco paints” produced using such binders are allowed to be labelled as “solvent and plasticizer free” if the residual VOC-, SVOC- and plasticizer content is each less than 700 ppm (mg/kg) measured according to test methods DIN EN ISO 17 895 or DIN EN ISO 11 890-2 [68].

1.4Parameters and properties of architectural coatings binders [77]

The key parameters of polymer dispersions for architectural coatings are as follows:

–Total solids content/polymer content,

–pH,

–viscosity/rheology,

–coagulum and micro-coagulum (grit) content,

–particle size (particle diameter)/particle size distribution,

–surface tension,

–colloidal stability (to shearing, electrolyte addition, freeze/thaw cycling),

–residual monomer content/total VOC content/odour,

–minimum film formation temperature/glass transition temperature,

–molecular weight/molecular weight distribution and crosslinking density,

–mechanical properties of polymer and coating films.

The first three characteristics, total solids content, pH and viscosity (at a certain measurement geometry and applied shear rate) are often used to specify the polymer dispersion.

The significance of glass transition Tg and minimum film formation temperature MFFT has already been discussed in Chapter 1.3. The remaining items are addressed in more detail below.

1.4.1Total solids content

The total solids content (TSC) is a measure of the amount of active substance in the dispersion. It is the ratio of the dry mass of the dispersion (following evaporation of all volatile fractions) to the overall mass. The solids content is composed of the polymer, the stabilizers and the inorganic salts (initiator decomposition products or buffer substances, for example). The total solids content is normally determined in a drying oven at elevated temperature, e.g. at 140 °C for 30 min (DIN EN ISO 3251 and DIN EN ISO 4618). Alternatively, IR drying or a microwave oven can be used.

1.4.2pH value

The pH value of architectural coating binders is usually positioned within the neutral to weakly basic range (pH 6 to 9, measured in accordance with DIN ISO 976). This is a consequence of the fact that the stability of the binder, which is usually obtained by copolymerization of AA or MAA and accordingly functionalized with carboxyl groups, increases strongly as the pH goes up (especially above a pH of 5 to 7). The reason for this is the increasing deprotonation of the acid groups and, as a result, improved charge or Coulomb stabilization. Moreover, standard dispersant agents based on polycarboxylic acids exert their dispersing and agglomeration-preventing activity on fillers and pigments likewise only above a pH of 6.5 in the finished paint, which must therefore similarly be neutral to slightly alkaline.

The pH also has an influence on the viscosity of dispersion and paint. As the pH goes up, the viscosity rises as a function of the amount and type of copolymerized carboxylic acid.

Normally, the polymerization is performed under acidic conditions. At too high pH-values, this may result in retardation of the conversion of especially protic and water-soluble monomers. The pH is mostly adjusted at the end of the process, by adding a base, like ammonia, a higher boiling amine (e.g. 2-amino-2-methyl-1-propanol, “AMP-90” g) or a solution of sodium or potassium hydroxide. Through this neutralisation step, the dispersion gains its necessary stability for processing and formulating. Of course, the neutralisation step has to be carried out carefully in order to avoid any electrolyte shock which would result in the formation of grit and coagulum.

Because of legislation, ways are explored to completely omit biocides in the water-based binders. One of the possibilities is to increase the pH to a range of 10 to 11.5. The challenge is then of course to cope with the viscosity control and the tendency of the acrylate moieties to hydrolyse at these harsh alkaline conditions

1.4.3Viscosity and rheology

The flow properties, i.e. the viscosity of the polymer dispersion in the liquid phase, are of considerable significance for its preparation, handling and processing. Besides the particle diameter, the polymer content of the dispersion and its surface functionalization (by using water-soluble comonomers, such as MAA, AA or AM) control the viscosity and flow behaviour. The overall viscosity is a function of the viscosity of the aqueous phase, the volume fraction and the packing of the particles. The viscosity of a dispersion can be described mathematically by the Mooney equation:

Figure 1.10: Relationship of solid fraction, particle size distribution and viscosity [78]

Accordingly, with a low volume fraction, the viscosity is essentially determined by the water-soluble polymer fractions and increases slowly as the volume fraction of the polymer particles goes up. However, if the volume fraction is close to the maximum packing factor, the viscosity rises very rapidly (see Figure 1.10 from [78]). As the particle diameter decreases, the increase in viscosity takes place at lower and lower volume fractions of the polymer, due to increasing packing density and particle interaction.

This increase occurs more readily for monomodal dispersions (i.e. dispersions with particles of uniform size) than for dispersions with a broad or even multimodal particle size distribution [78–80] (see also Figure 1.10).

Influence of solids content on viscosity

Very small-sized, monomodal dispersions (particle size 30 to 80 nm) are fluid only up to solids content of 35 to 45 % by weight, whereas standard binders with particle sizes of 100 to 200 nm retain their fluidity for solids content of 48 up to a maximum of 52 % by weight. For dispersions, unlike polymer solutions, the effect of molecular weight on the viscosity is relatively small. Given acceptable viscosity, solids content of more than 60 % by weight can be obtained only for dispersions having a bimodal or multimodal particle size distribution, i.e. for mixtures of particles differing in size [78–80]. In such cases the small particles fill the gaps between the coarse particles. This is evident from the surface micrograph of a film of an essentially bimodal dispersion as obtained by atomic force microscopy (AFM) (see Figure 1.11). Multimodal, highly concentrated dispersions of this kind can be produced in a controlled fashion by means of special emulsion polymerization techniques. This is normally done by generating one or more new particle generations in the course of the polymerization by means of tailored additions of emulsifier or seed latex, for instance by so-called “shot” additions.

Figure 1.11: Atomic force microscopic picture of the surface of a film obtained from a bimodal dispersion with high solids content (particle size of the big particles is about 300, that of the small ones <100 nm)

Viscosity control

Water-based polymer dispersions and architectural coatings without added thickener generally exhibit more or less pronounced pseudoplastic flow behaviour, i.e. their viscosity falls as the shear rate increases. This contrasts with solution polymers and conventional solvent-borne coating systems (e.g. conventional alkyd coatings), which tend to display Newtonian-like behaviour, i.e. virtually no shear dilution (see also Chapter 2.4.4).

A simple method of determining the flow behaviour is to work with flow cups (Ford-cup, DIN-cup, or in accordance with ISO 3219, for example). These are funnel-shaped cups which are filled with a defined volume of the dispersion. The efflux time as a function of the outflow geometry (length and diameter) is then a measure of the viscosity of the dispersion.

Alternatively, the viscosity can be determined using a rotational viscometer (e.g. in accordance with DIN EN ISO 3219 or Brookfield measurements in accordance with ISO 2555, ISO 1652). In the first case a metal cylinder turns concentrically in a dispersion-filled cup. By measuring the torque on this cylinder, it is possible to measure the shear stress when the shear rate is varied. Using the relationship whereby the viscosity is equal to the ratio of shear stress and shear rate, it is then possible to determine the viscosity at different shear rates (flow curve). Normally, manufacturers specify the viscosity of the dispersion at a certain shear rate after a defined pre-shearing time at controlled temperature (e.g. at 100 or 250 s–1 after 1 min at 23 °C). Architectural coating binders are normally situated within the range from 50 to 1,500 mPa∙s (measured at 100 s-1 in accordance with DIN EN ISO 3219).

1.4.4Coagulum

Coagulum or micro-coagulum is the term used to refer to coarse or fine grit or precipitate of polymer material in the dispersion. It can be quantified by retaining it in a filter of defined mesh size (e.g. 100 or 125 µm) and weighing it (e.g. in accordance with DIN ISO 4576). It comprises fragments of skin, dried-on foam or coarse aggregates of polymer particles, all of which may have a disruptive effect on the quality of the dispersion and coating film. Fine coagulum fractions (so-called specks or grit; in German “Stippen”) become visible, when the dispersion is applied with a doctor blade or alternative film applicator onto a smooth glass plate and viewed in transmitted light. These may lead to defects in the coating film, for instance speckles and dots, or pinholes after spray application of the paint.

For this reason, the polymer dispersions are filtered through fine-mesh filters (usually from 20 to 200 µm in mesh size, e.g. 45 or 100 µm) by the manufacturer. Since it is usually impossible to preclude foaming and skinning completely during transport and handling of the dispersions, it is advisable – especially for high-quality coating systems or lacquers with low or no pigmentation – to filter the dispersion with a coarse filter (e.g. 500 µm mesh size) during deloading and storage tank filling and if necessary once again with a finer mesh size before using them in paint production.

Abacus method

A special technique to quantify and classify the fractions of over-sized particles in a polymer dispersion is the Abacus or single particle counter. During the measurement, the diluted dispersion is passed through a narrow capillary. The passing particles are “counted” by a laser. Particles >2 µm can be measured. These particles are normally due to agglomerates of the primary polymer particles, they are counted as fine coagulum or grit. Correlation of these values with those obtained by filtration techniques is not always given. One of the reasons is, that during filtration sometimes grit bigger in size than the filter mesh is pressed through the filter, another that smaller-sized coagulum can plug the pores of the filter.

1.4.5Particle size/particle size distribution

The particle size of a polymer dispersion is not a distinct value, it is normally expressed as a mean value of a distribution of particle sizes. Depending on measurement method a number-, weight- or z-averaged value of the particle size distribution (PSD) is obtained (a mono-modal PSD can be represented by a log-normal distribution, the various moments of this distribution correspond with the mentioned averages). The particle size has an influence on many important binder properties, such as viscosity, film forming tendency (see Chapter 1.3), film gloss (see Chapter 6), binding power (see Chapter 2 and 7) and penetration capacity into porous substrates (see Chapter 3). Moreover, the particle size strongly determines the internal surface area of the system and thus the amount of stabilizer required as well as the interaction strength with associative thickeners (see Chapter 2.4.4). For spherical particles, following equations can be used (where r is the particle radius):

Typical values are listed in Table 1.9.

Table 1.9: Particle number and internal surface area for 1 g of dispersion of density of 1 g/cm3