Effect of pigmentation on organic coating characteristics

aiyuanzh

DanY.Perera 发表在Progress in Organic Coatings上的一片综述性质的文章,摘要如下：
The paper reviews the effects of pigmentation,i.e.type of pigment and pigment volume concentration,on organic coating characteristics,
such as curing and film formation,mechanical properties,thermal expansion coeffcient,glass transition temperature,stress development,
physical ageing and water transport.The main families of pigments used in the fabrication of organic coatings and the effect of nanopigments
incorporation in binders are also discussed.

全文共16页，参考文献169篇。

下载地址：http://www.91files.com/?6F4FDSWIQSF9CWPZO10P

hujian

谢谢提供，颜料作为涂料中很重要的组分，对涂料性能影响巨大，这个专题很有价值。

lyuanqing

很实际很具体很好

lyuanqing

我给贴出来吧，以后这个链接失效了，照样能看到

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1. Introduction
The pigments, indispensable components of any paint,
have been known and used as long as the humans have been
painting. Organic or inorganic, inert or active, the pigments
are fine particle-sized substances that are incorporated in a
binder for a variety of reasons. A simple way to classify
pigments is to consider them as true, extender, anti-corrosive
and effect pigments, e.g. [1–9].
A true pigment provides colour—white, black and
colour—and covering. Titanium dioxide (TiO2), zinc oxide
(ZnO), zinc sulphite (ZnS), lithopone (a mixture of ZnSO4
and BaSO4) and white lead are the main commercial white
pigments. TiO2 is the dominant white pigment since it is
the whitest and brightest due to its high refractive index and
its relatively low and uniform absorption of visible light,
e.g. [3,10]. Carbon blacks are the main black pigments, e.g.
[11]. Among the coloured inorganic pigments are the red,
yellow and black iron oxides, green chromium oxides and
cadmium pigments, e.g. [7–9]. The colour of the cadmium
pigments varies from greenish, through orange, to red and
maroon. The organic coloured pigments are often classified
as azo or monoazo pigments. The polymeric materials or
plastic pigments, e.g. styrene/acrylic (Rohm & Haas) and
vinylidene chloride/acrylonitrile (AKZO) copolymers, are
relatively new products used by the paint industry, and they
can probably also be included among the true pigment, e.g.
[7,12,13].
E-mail addresses: danperera@belgacom.net, cori@cori-coatings.be,
piens.m@cori-coatings.be (D.Y. Perera).
The “extender pigments”, mainly inorganic pigments that
absorb little of any light, are, generally, white or light grey
in colour. Due to their low refractive indices of less than
1.65, close to those of binders, these extender pigments
have negligible hiding power when incorporated in a binder,
e.g. [3,6,14]. They include, among others, calcium carbonates,
kaolins (aluminium silicates), talcs (magnesium silicates),
mica (hydrous aluminium potassium silicates), silica
(silicon dioxide), wollastonites (calcium metasilicates) and
barite (barium sulphate). Their tinting strength is generally
low and their density is less than 3, except the barite, which
has a density of around 4.5. In most cases because the extender
pigments are inexpensive, they are used to reduce the
cost of paints by filling the coating volume with minimal
impact on performance, e.g. [14]. Some of the extender pigments
are affecting the paint characteristics. For example,
those changing the rhelogical properties, usually by increasing
the apparent viscosity, are known as thickening agents,
while those able to reduce the tendency of other pigments
to settle or reduce the gloss of paints are called unsettling
and flatting agents, respectively. Acicular and lamellar extender
pigments can act as mechanical reinforcement and
thus improve the mechanical properties of paints. Other extender
pigments increase chemical resistance and reduce the
coating flammability, e.g. alumina trihydrate and silica, and
enhance the electrical characteristics [15].
The “anti-corrosive pigments”, incorporated mainly in
primer paints, protect metallic substrates against corrosion,
e.g. [16].
Among the effect pigments, such as the metallic, pearlescent,
interference and heat reflection, the first two are the

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most common. The “metallic pigments” provide the coatings
with aesthetic, polychromatic effects, and functional
qualities, such as anti-corrosive properties, e.g. [17]. The
“pearlescent” pigments, known also as nacreous pigments,
provide the coating with optical effects imitating natural
pearl lustre, e.g. [18,19].
Aiming at improving the quality of paints, a great number
of studies have been carried out to understand the role
played by pigments in paint formulation, fabrication, application
and durability. All the studies indicated that pigment
characteristics, such as chemical nature, particle size, state
of dispersion, morphology and level of pigmentation, determine
the coating properties in its liquid and solid state.
Despite their great importance, topics such as the processes
related to the incorporation of pigments in binders
(dispersion process), e.g. [10,11], and their effect on coating
rheology, e.g. [3,5,6,20] and paint application, e.g. [2,6] will
not be discussed in this paper. The objective of this paper is
to review the effect of pigmentation on a number of coating
characteristics, such as film formation, mechanical properties,
certain thermal properties, water permeability, stress development,
physical ageing and glass transition temperature
(Tg). The characteristics that have already been thoroughly
presented in the literature or for which no new insights have
been revealed, recently, will only briefly be discussed here.
2. Generalities
The simplest manner to consider the effect of pigmentation
on coating characteristics is to assume that the effect
is proportional to the pigment content incorporated in the
binder, regardless of the type of pigment and binder. While
this way of proceeding might be a good approximation for
coatings containing a low proportion of inert pigments, for
the majority of coatings met in practice such an approach
can lead to significant errors. With a few exceptions, all the
coating properties discussed in this article will be described
as a function of pigment volume concentration (PVC) or pigment
volume fraction (φ) (Eq. (1)), since they are mainly volume
dependent, as first mentioned in 1925 [21] and clearly
demonstrated in 1949 [22].
PVC =
Vp
Vp + Vb × 100 =
Wp/dp
(Wp/dp) + (Wb/db) × 100 (1)
where V, W and d represent the pigment’s volume, weight
and density, respectively. The subscripts p and b represent
the pigment(s) and binder(s), respectively.
An important practical and theoretical coating concept,
often referred to in this paper, is the critical pigment volume
concentration (CPVC) introduced by Asbeck and Van
Loo [22]. The CPVC represents the PVC corresponding
to the “random tightest possible packing pigment particles
and the minimum amount of binder necessary to fill the
interstices between particles” [1]. The great importance of
this concept consists in the fact that at this level of pigmentation
practically all-coating properties change drastically
[22–53]. Below the CPVC the coating film is continuous
and made only of binder and pigments. Above the CPVC
the film is discontinuous due to the presence of air pockets
around pigment particles that replace the binder.
It is known that the CPVC is affected by many factors
such as the chemical nature of the filler and binder, filler
particle size and morphology, drying conditions and the type
of substrate. For this reason, whenever the CPVC has to be
known more accurately, we suggest to determine it directly
rather than evaluate it from mathematical models that do not
take into consideration all the parameters mentioned before.
While the determination of the CPVC is useful for all type of
coatings, e.g. industrial and architectural, for paper coatings
it is imperative since they are formulated above the CPVC
for economical reasons, e.g. [50,51].
In certain cases the coating properties will be represented
as a function of Λ (reduced PVC, PVC/CPVC),
e.g. [32,36,37,42–44]. Such a representation enables one to
compare properties of coatings at identical pigment volume
packing effects [32].
3. Curing and film formation
Certain pigments are inert while others interact with the
binders. A consistent deviation from Newtonian flow, and
changes in induction time, reaction rate, extent of reaction,
reaction enthalpy, gel time and Tg are among the indicators
that pigments and binders are interacting. Since the surface
of pigments has a basic or an acidic character, the pigments
can react with the binder functional groups to form hydrogen
and even covalent bonds. As a result, in some cases the
incorporation of pigments in a binder can affect the curing
process of the respective paints. Despite its practical importance,
this aspect has been relatively little investigated.
In earlier works on ZnO (still used as a non-toxic fungistat)
[3,53] and red lead (currently little used due to its toxicity)
[54–56], it was shown that these pigments can react
with ester groups of linseed oil, stand oil, alkyds and similar
products to form zinc and lead soaps, respectively.
More recent studies indicated that certain pigments could
have a great influence on the curing process, and even to
eliminate it completely [57]. For example, differential scanning
calorimetry (DSC) curves obtained with diallyl phtalate
prepolymer, catalysed by dicumyl peroxide, show that
the type of pigment incorporated affects the reaction process
(Fig. 1). Three pigments (silica, wollastonite and asbestos)
present exothermal peaks, practically identical to that of
the non-pigmented sample, indicating that they affect only
slightly the reaction process. On the contrary, the kaolinite
clay virtually eliminates the curing, as indicated by the presence
of a small reaction enthalpy. The decomposition of the
initiator by the acidic groups present at the clay surface was
considered to cause this effect. A report indicated that the incorporation
of pigments in blocked isocyanates polyol could

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 249
Fig. 1. DSC-curves of diallyl phtalate prepolymer catalysed by dicumyl
peroxide containing four fillers [57].
affect the unblocking temperature, by raising, or lowering or
not affecting it [58]. The powder coating chemists [59] are
well aware of the fact that pigment particles, e.g. TiO2, and
resins, e.g. with carboxyl functionality, can interact leading
to higher yield values and higher viscosities. Another paper
reported that, depending on the type of yellow iron oxides
the curing process of powder coatings is affected differently
[60].
Today, to decrease or increase their reactivity, many inorganic
pigments are treated with inorganic or organic products
[3,61,62]. The modifications produced at the pigment
surface particles are often bringing also other benefits such
as an improved coating durability, better pigment dispersion,
change in rheology of paints and improved coating mechanical
properties.
A number of studies were dedicated to the effect of pigmentation
on the film formation of latex coatings [63–67].
For example, despite the difficulty of such investigation,
some studies were designed to look at the ways the latex
particles are spreading and ordering in inorganic pigments.
It was found that, in general, the ordering and spreading
of latex particles are affected by the physical and chemical
characteristics of latex and inorganic pigment particles as
well as by the drying conditions.
A study indicated that an extender pigment, CaCO3, incorporated
in a styrene/butyl acrylate copolymer latex does
not modify the polymer structure [68]. It was concluded that
at high PVC, the extender pigment particles are fixed by isolated
latex particles and that the polymer is not completely
spread over pigment. Another investigation used a methyl
methacrylate/(2-ethylhexyl) acrylate copolymer latex filled
with two extender pigments (250–300 nm CaCO3 and 25 nm
silica) to determine how a high PVC affects the coalescence
and the subsequent polymer diffusion processes [69].
The results obtained indicated the importance of the type of
filler, its particle size and chemical nature, and the spatial
arrangements of the pigment and latex particles. While both
pigment extenders do not interfere in the coalescence of the
latex particles, they affect the rate of polymer interdiffusion
differently. Up to 80% (by weight) of the 250–300 nm
CaCO3 has an insignificant effect on the rate of polymer interdiffusion,
whereas the 25 nm silica particles slows it, even
in a small amount. This retardation effect was attributed to
the relatively strong interaction between the polymer and
the nanosilica surface. A similar retardation effect was also
reported when plastic pigments, e.g. poly(methyl methacrylate)
and Ropaque (Rohm & Haas), are incorporated in organic
coatings [70].
4. Mechanical properties
The effect of pigmentation on elastic modulus, tensile
strength and strain of organic coatings were among the mechanical
properties the most investigated in the last four
decades, as indicated by a large number of publications, including
review papers [1,6,28,71–78]. Among the numerous
types of coatings investigated were alkyds, epoxies,
polyvinyl acetates, polyesters/melamines and acrylics filled
with one or a mixture of pigments such as TiO2, ZnO, china
clay, CaCO3, iron oxides, BaSO4, silica and wollastonite.
4.1. Elastic modulus
The elastic modulus, an important coating property directly
reflecting the coating rigidity, can be determined by
static, e.g. stress–strain, and/or dynamic mechanical measurements
under a variety of deformation modes, such as
tensile, compression, bending, shear and torsion. The choice
of the technique and the mode of deformation is mainly dependent
on the type of material, i.e. hard or elastomer, and
material’s geometry.
The elastic modulus is affected by the introduction of rigid
particles in a binder. As shown in Fig. 2, the elastic modulus
increases with PVC up to CPVC. Above the CPVC, due to
the presence of air pockets, the film becomes discontinuous
and the elastic modulus decreases.
A comprehensive summary of the main mathematical
equations describing largely the relationship between the
elastic modulus of pigmented coating (E), pigment volume
fraction (φ) and the elastic modulus of the non-pigmented
coating (Eo) is given in [28]. Some of these equations are:
• Guth’s equation for spherical particles and 0 < φ < 0.1
[78,79]:
E = Eo(1 + 2.5φ + 14.1φ2) (2)
is an expansion of Einstein’s known equation [80]:
E = Eo(1 + 2.5φ)
• Guth’s equation for particles having a shape factor f (ratio
length/diameter) much higher than one [78]:
E = Eo(1 + 0.67φ + 1.62f 2φ2) (3)

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262
Fig. 2. Elastic modulus (Ec, GPa) as a function of PVC (%) for a
thermoplastic acrylate filled with: (×) a TiO2; (
) a read iron oxide; ()
a yellow iron oxide; () a talc [37].
• Nielsen’s equation applicable when a good adhesion exists
between the pigment particles and the polymer [81]:
E =
Eo
1 − φ1/3 (4)
• The equation of Landel et al. considers the maximum
packing fraction (φm) [82]:
E = Eo
1 −
φ
φm −2.5
(5)
• Pliskin and Tokita equation takes into consideration the
“truly pigment volume fraction φe” that includes the
“strongly” adsorbed polymer layer on pigment particles
[83]:
E = Eo(1 − φe)−n (6)
• Heertjes and De Jong model [84] assumes that the pigment
particles are spherical, distributed in a cubic arrangement
on which the binder adheres perfectly well, and do not
change the viscoelastic properties of the binder:
E = Eo  πn2d2
4(1 − nd) + 1 −
πn2d2
4  (7)
where nd = (6φ/π)1/3, d is the particle diameter, and n
the number of particles/cm3 in the film.
• Narkis’ semiempirical equation takes into consideration a
stress factor, K, with usual values in the range of 1.4–1.7
[85].
E = Eo[K(1 − φ1/3)] − 1 (8)
Fig. 3. Storage elastic modulus (E) as a function of temperature (T) for a
polyacrylate coating containing different volume fractions (φ) of a TiO2
(adapted from [76]).
Other used mathematical models to predict the elastic
modulus of filled polymers are those of Mooney [86], Kerner
[87], Thomas [88], Frankel and Acrivos [89], Quemada [90]
and Nielsen [91]. In general, the above mathematical models
describe relatively well the dependence of elastic modulus
on PVC for spherical and inert particles, but deviations are
observed for systems with other morphologies, or containing
reactive pigments or agglomerates.
As it is well known, the elastic modulus is temperature
dependent, especially in the glass transition region where an
important change occurs. The replacement of binder with
rigid pigment particles provokes an increase in the elastic
modulus, especially noticeable in the glass transition and
rubbery regions (Fig. 3). The results presented in Fig. 3 also
show that the CPVC is situated somewhere between PVC
45 and 55% as indicated by the lower E values in the glassy
region of the coatings pigmented at PVC 55 and 60%.
It is important to mention that in most cases the pigment
incorporation in a binder affects the viscoelastic character
of the respective coating as reflected by the changes taking
place in the loss tangent (tan δ) defined as the ratio storage/
loss moduli. With PVC, the loss tangent peak that defines
the glass transition region shifts to higher temperatures
and broadens indicating also an increase in coating heterogeneity.
The incorporation of pigments in a binder can transform
an isotropic coating into an anisotropic one, an effect dependent
on the type of filler, e.g. its morphology and alignment,
and the way the coating is applied. The example presented
in Fig. 4 shows that the storage elastic modulus values are
different when determined perpendicularly or in parallel to
the alignment of pigment particles, usually being lower in
the latter case.

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 251
Fig. 4. Storage elastic modulus (E, MPa) as a function of temperature (T, ◦C) for a polypropylene/wollastonite (50% w/w) system determined perpendicularly
(×) and in parallel () to alignment of pigment particles (bending mode) (unpublished results).
4.2. Ultimate properties
Stress–strain measurements are often used to determine
the ultimate properties—tensile strength (σ) and strain
(ε)—of organic coatings. These properties are important
because they are related to coating strength and extensibility.
The area under stress–strain curves, called toughness
or work-to-break, represents the energy per unit volume
necessary to break the material. The toughness is related
to material cohesion, and combines the resistance and
ductility properties of an organic coating. Tensile testing
machines, usually capable to carry out the tests at different
temperatures and elongation rates, are used to determine
these characteristics. The effect of pigmentation on ultimate
properties has been subject of numerous investigations
[52,92–104] and a number of review papers have been
written [30,72–74,76].
It is useful to mention that contrary to the elastic modulus,
the ultimate properties are difficult to be correctly evaluated.
This is mainly due to the fact that the ultimate properties are
depending directly on the local stress concentrations, a consequence
of coating heterogeneity created by the presence
of pigment agglomerates, impurities and/or air pockets in
the coating. In most cases, to obtain statistically significant
data, 10–15 measurements are needed.
A literature search indicates that two behaviours characterise
the tensile strength dependence on PVC (or φ) (cases
A and B in Fig. 5). In case A, σ is increasing with PVC
up to the CPVC, and decreases above it. This behaviour
is considered to occur when a good adhesion between the
binder and the pigment exists, the prevailing situation with
most organic coatings. Above the CPVC, σ decreases with
PVC due to increase in stress concentrations caused by the
binder replacement with air pockets. In case B, σ continuously
decreases with PVC. This is considered to occur when
the adhesion between the binder and pigment is inexistent or
weak. For both cases, the actual tensile strength dependence
on PVC (or φ) is determined by the pigment characteristics,
Fig. 5. Schematic representation of the relative tensile strength (σ/σo)
dependence on PVC. Case A: good adhesion between the binder and
pigment. Case B: weak or no adhesion between the binder and pigment.

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262
Fig. 6. Relative tensile strength (σ/σo) dependence on PVC for an acrylic
binder system containing: (
) a CaCO3; (        ) a micro-talc; () a TiO2;
() a baryte [30].
such as morphology, particle size and reactivity. A few examples
of the relative tensile strength dependence on PVC
are shown in Fig. 6.
Despite difficulties in predicting the tensile strength dependence
on PVC (or φ), a number of tentatives have been
made. For case A, it has proposed that σ be calculated from
Eq. (9) [105] or Eq. (10) [77].
σ = (σ1 + Cτm)φ + σo(1 − φ) (9)
σ = σo(1 − aφb + cφd) (10)
where σ and σo are the tensile strengths of the pigmented and
non-pigmented coatings, respectively; C a constant considered
to be 0.83; τm the shear strength of the non-pigmented
coating; a, b, c and d are the “adjustable” coefficients.
Eq. (9) shows a linear dependence between σ and φ that
does not account for the maximum observed at the CPVC.
The empirical Eq. (10) does account for the σ-maximum observed
at the CPVC due to its four adjustable coefficients. It
has been suggested that the values of “c” and “d” are indicators
of pigment–binder adhesion [77]. Despite difficulties of
determining them quantitatively, these coefficients are useful
for those interested in increasing the reinforcement of
organic coatings via chemical modifications or through the
use of coupling agents.
Relationship (11) is considered to represent the case B,
i.e. when a weak or no adhesion between the pigment and
binder exists [106].
σ = σo(1 − aφb) (11)
The value of “a” is 1.21 for spherical pigment particles
having no adhesion at all with the binder, and <1.21 when
some adhesion exists. The value of “b” is related to the type
of sample failure: two-third for random fractures and one
for planar fractures.
In most cases, the “strain at break” decreases with PVC,
a decrease that becomes more important above the CPVC.
A few examples are illustrated in Fig. 7. The only case
when the “strain at break” of a filled binder is higher than
that of the unfilled one occurs when the binder gives rise to
micro-cracks [30].
A number of mathematical models have been proposed
to predict the dependence of “strain at break” on pigment
volume fraction [81,94,107,108]. As in the case of “stress at
break”, the equations are valid for relatively simple cases,
therefore useful only to evaluate a general tendency. To determine
the actual (real) behaviour, sample testing is needed.
5. Thermal expansion coefficient
The thermal expansion coefficient (αTF
) is an important
coating characteristic since, as it will be discussed later, it affects
directly the thermal stress, one among the main factors
acting against the coating integrity. αTF
can also provide interesting
information about the coating characteristics such
as the degree of cure [107]. The measurement of αTF
as a
function of temperature allows also the determination of Tg,
the “glass transition temperature”. In general, the increase
of αTF
with temperature is small and gradual in the glassy
region, but greater in the rubbery region.
αTF
can be measured by means of thermal mechanical analysis
(TMA) in different deformation modes such as tensile,
penetration, expansion and bending. The choice of the deformation
mode and the experimental conditions, e.g. relative
humidity, heating rate, and load applied, are dependent
on the material’s characteristics, such as hardness and thickness,
and on the conditions to which the coating is exposed
in practice.
The incorporation of pigments in the binder can greatly
affect the αTF
. Particle shape and size, and the pigment

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 253
Fig. 7. Relative strain at break (ε/εo) dependence on PVC for an acrylic
binder system containing: (
) a CaCO3; (        ) a micro-talc; () a TiO2;
() a baryte [30].
content are the main factors determining the values of αTF
.
In general, αTF
decreases with the decrease of particle size
and the increase of pigment content (Fig. 8).
As already mentioned, the coatings containing pigments
can be not only heterogeneous, but also anisotropic. For
such materials, the αTF
, as the elastic modulus, is affected
by the direction of the expansion measurement (length,
thickness or width). Fig. 9 illustrates the dependence of αTF
on temperature for a polymeric material containing wollastonite.
For this material, the highest αTF
values are obtained
when the measurements are made in the direction of the
thickness.
6. Glass transition temperature (Tg)
Tg delimits the glassy and the rubbery regions. In principle,
Tg can be determined by using any material property
that exhibits a measurable discontinuity at this temperature.
Dynamic mechanical analysis, differential scanning
Fig. 8. Thermal expansion coefficient (αTF
) dependence on PVC at 21 ◦C
and 0% RH for an epoxy coating containing a TiO2 (unpublished results).
calorimetry and thermal mechanical analysis, techniques
that measure mechanical properties, heat flux and deformation,
respectively, as a function of temperature under a
variety of experimental conditions are among the most used
Fig. 9. Thermal expansion coefficient (α) dependence on temperature
(T, ◦C) for a polypropylene containing wolastonite (50% w/w) measured
in three directions: (×) length; (
) width; () thickness (unpublished
results).

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262
Fig. 10. Schematic description of the loss tangent (tan δ) as a function of
temperature (T) for a pigmented (P) and non-pigmented (NP) coating.
methods to determine the Tg. It is important to recall that
while being a fundamental property of a material, the value
of the Tg is dependent on the method and the experimental
conditions, e.g. frequency and heating rate, used. Also, while
in the literature the Tg usually refers to one temperature,
due to the coating heterogeneity, we consider that it is more
appropriate to consider the whole glass transition region.
The model often used to describe the pigmented coatings
assumes a three-phase system, similar to one describing the
semi-crystalline polymers, consisting of a mobile amorphous
(bulk) binder, pigment and an adsorbed binder “inter-phase”
[108,109]. The strength and the thickness of the inter-phase
are dependent on the type of bonding, e.g. covalent, ionic
and van derWaals, between the pigment and the binder. Such
a model considers that when the Tg of the inter-phase is less
than that of the bulk binder, the Tg of the pigmented coating
will show a decrease with addition of pigment; when the
Tg of the inter-phase is larger than that of the bulk binder,
the Tg of the pigmented coating will show an increase in Tg
with addition of pigment [110].
A literature search has shown that in some cases the presence
of pigments in a binder can leave the Tg unchanged
[30,113] or cause a decrease in its value [111,112]. In the
majority of cases, however, as expected, the presence of
pigments in a binder induces an increase of Tg of the pigmented
coating [114–117]. For this case, the model depicts
the inter-phase fraction as a constrained, rigid amorphous
phase, restraining the mobility of adjacent binder segments,
and having a broad distribution of relaxation times. While
the thickness of the adsorbed layer is 20–60 Å, its effect
is considered to extend to a much larger distance. Certain
materials show even two Tgs, one being attributed to the
inter-phase fraction [118,119].
With respect to the damping peak (tan δ), the pigment
incorporation in a binder, in general lowers, broadens and
shifts the maximum peak value of (tan δ)max to higher temperatures
(see Fig. 10). The broadening and the shift effects
increase with decreasing particle size, i.e. increased particle
surface area, and are more pronounced when lamellar
particles are incorporated when compared to spherical particles
being incorporated [120].
Notice that for coatings consisting of binary blends of
binders possessing different Tgs, it is proposed to use the
lowering of (tan δ)max to determine the level of pigments
present in each binder [121]. The method is based on the
assumption that the lowering is proportional to the amount
of pigment incorporated.
7. Stress
The main causes of stress in organic coatings are the film
formation, and variations of temperature and relative humidity.
The respective stresses are known as internal, thermal
and hygroscopic. Because under usual environment conditions
the organic coatings are in the glassy state, it follows
that they are practically always under stress. Above the Tg,
i.e. in the rubbery region, although stresses appear, they can
relax.
One of the factors contributing to the interest in understanding
and evaluating the stresses arising in organic coatings
is the fact that they act against the coating adhesion and
cohesion, possibly leading to delamination and cracking of
coatings.
In practice, the internal (SF), thermal (ST) and hygroscopic
(SH) stresses can interact in such a way that the total stress
(Stot) can be small or high [48,122]:
Stot = SF ± ST ± SH (12)
The positive and negative sign designates the tensile and
the compressive stresses, respectively, reflecting the coating
tendency to contract or expand.
Eq. (12) also indicates that high tensile stresses develop
in the coating in a dry and a cold environment, and high
compressive stresses develop in a humid and a warm environment.
Among the methods proposed to measure the different
stresses, such as optical, strain gauges, X-ray diffraction, and
cantilever-beam, it is this last method that appears to be the
most used and adequate for organic coatings [123–127]. The
stressmeter instrument [48,122] based on cantilever-beam
principles enables one to determine all the stresses mentioned
above in a relatively simple manner.
A number of studies were dedicated to the effect of stress
development in organic coatings [37,43,128–133]. Examples
of the development of internal stress as a function of
time are shown in Figs. 11 and 12. These figures illustrate
the results obtained with a solvent and a water borne coating
containing TiO2 at different levels of pigmentation. The
stress arising during the different stages of film formation
can be recognized [43,48,133,134]. After the paint application
on a substrate, there is a period during which the stress
does not develop because the paint is still liquid and mobile
enough to permit volume contraction. Once the Tg of the
film becomes at least equal to the ambient temperature, the

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 255
Fig. 11. Stress (S) dependence on time for a polyisobutyl methacrylate containing a TiO2. The numbers in the figure indicate the PVC (%), CPVC = 51%
[37,48].
stress starts to develop because the rate of its development
is higher than that of its relaxation. In general, for PVC <
CPVC, the decrease is mostly due to relaxation processes,
while for PVC > CPVC, the decrease is due to relief processes
such as filler/binder dislocation and/or formation of
micro-fissures (see the curves corresponding to PVC = 55%
in Figs. 11 and 12).
The maximum internal stress as a function of PVC for
two coating systems is shown in Figs. 13 and 14. They
indicate that the maximum internal stress increases with
PVC up to a value that corresponds to the CPVC, and then
decreases. Such dependence shows the possibility of determining
the CPVC of coatings more accurately from stress
measurements than by other methods, e.g. permeability,
hiding power, gloss and blistering.
The effect of pigmentation on the stress development can
be understood by considering the following general stress
Fig. 12. Stress (S) dependence on time latex (vinyl versatate copolymer) containing a TiO2 at different PVCs: () 45%; () 50%; (×) 55%; () 60%;
CPVC = 52% [48,122].
equation:
S =
Eε
1 − ν
(13)
where E is the elastic modulus, ε the internal, thermal or
hygroscopic strain and ν the Poisson’s ratio.
Expression (13) indicates that the stress magnitude is a
resultant of the way E, ε and ν are affected by the pigment’s
characteristics and the PVC. As discussed in Section 4.1,
for PVC < CPVC, an increase in PVC causes an increase
of E, and a decrease of ε and ν.
The results in Figs. 13 and 14 as well as other literature
data attest that the elastic modulus is the main coating characteristic
affecting the stress dependence on PVC. Figs. 13
and 14 also show that the stress magnitude is greatly determined
by the type of pigment, e.g. TiO2 and red iron oxide
induce higher stresses than CaCO3 and talc, a consequence

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262
Fig. 13. Maximum internal stress (S) dependence on PVC and reduced
PVC (Λ) for a solvent borne thermoplastic binder containing: (×) a TiO2;
(
) a red iron oxide; () a yellow iron oxide; () a talc [37,48].
of pigment/binder interaction controlled by the characteristics
of both the pigment and binder.
Since in practice most organic coatings contain a mixture
of fillers, the stress was also measured in such systems (see,
e.g. Fig. 15). An examination of results obtained with a large
number of systems indicates that the maximum stress (Smi )
Fig. 14. Maximum internal stress (Sm) dependence on PVC (%) for a
latex (styrene acrylic copolymer) containing: (
) a TiO2; (×) a CaCO3;
() a talc [43,48].
Fig. 15. Maximum internal stress (Sm, MPa) dependence on PVC (%) for
a solvent borne thermoplastic binder containing: ()a yellow iron oxide;
() a talc; (
) their mixture (n = 0.5) [48,135].
can be calculated for PVC < CPVC with good accuracy
from [43,133]
Smt = n1Sm1 + n2Sm2 +· · ·+niSmi =
i
x
=1
nxSmx (14)
This simple equation based on additivity requires only the
knowledge of the volume fraction of the different pigments
present in the mixture (n1, n2,. . . , ni), and the stress of different
single pigment coating systems (Sm1, Sm2, . . . , Smi )
at a given Λ below 1, i.e. below the CPVC.
It is probably useful to mention that the CPVC of many
coatings containing a mixture of pigments [(CPVC)T ] can be
approximated if the pigment volume fraction and the CPVC
of different single pigment coating systems are known [133]:
1
(CPVC)T =
n1
(CPVC)1 +
n2
(CPVC)2 +· · ·+
ni
(CPVC)i
=
i
x
=1
nx
(CPVC)x
(15)
This discussion on the influence of PVC on the development
of internal stress is also valid for thermal and hygroscopic
stresses. Examples of this influence for thermal stresses arising
in a thermosetting coating are presented in Figs. 16
and 17. One figure shows the stress dependence on temperature
and the other on PVC. Notice that the CPVC of the
coating, as expected, is the same regardless of the temperature
considered, above or below the Tg, or of the way the
stress is obtained, i.e. by heating or cooling.
In summary, the stress dependence on PVC can be
schematically described as follows (Fig. 18) [136].
(a) When the environmental temperature (T) is higher than
the Tg of the binder (T > Tg), the stress increases with

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 257
Fig. 16. Stress (S) dependence on temperature (T) for a polyester/melamine
coating (cured at 150 ◦C for 15 min) containing a TiO2 at different PVC
determined during cooling (2 ◦C/min) from 65 to 2 ◦C (unpublished results).
PVC in a significant way only when the PVC approaches
the CPVC. This means that at PVCs close to the respective
CPVC, the majority of polymer chain segments are
adsorbed (immobilized) at the pigment’s surface. Such
a situation causes an increase of the coating’s mean
Tg, such that it becomes higher than the environmental
temperature, a condition that allows the stress to develop.
Above the CPVC, as already discussed, the stress
decreases due to relief processes.
(b) When the Tg > T, the stress increases almost continuously
with PVC until this reaches the CPVC, and then
it decreases.
Fig. 17. Dependence of thermal stresses (S) arising during cooling
(2 ◦C/min) at 10, 20 and 30 ◦C on PVC for a polyester/melamine coating
containing a TiO2 at different PVCs (cured at 150 ◦C for 15 min)
(unpublished results).
Fig. 18. Schematic representation of the stress dependence (Sm) on PVC
for coatings with (a) Tg < T and (b) Tg > T [136].
8. Physical ageing
Physical ageing is the spontaneous structural relaxation
of a material in the glassy state, a consequence of
its non-equilibrium state [137–142]. The importance of
the physical ageing is in the changes accompanying the
material’s properties that, in most cases, adversely affect
the material’s durability. Among the effects observed are an
increase in rigidity and relaxation time, and a decrease in
durability and fracture resistance. Physical ageing, alone or
in combination with other factors, e.g. photochemical degradation
and stresses arising in the film, contributes to coating
failure. An overview of the effects of this phenomenon on
the coating properties, such as enthalpy relaxation, film
contraction, elastic modulus, thermal expansion coefficient,
stress relaxation, ultimate properties and thermal stress,
and the techniques used to investigate are presented in
[143].
The term “physical” in “physical ageing” indicates that
the changes in coating properties mentioned above occur in
absence of chemical changes, e.g. photochemical reactions.
The changes induced by physical ageing are affected by the
ageing temperature: closer this temperature is to Tg, faster
the changes occur, and smaller are their magnitudes. Physical
ageing is also a reversible process meaning that the reheating
of the material to a temperature above the Tg can
eliminate it.
Although, a number of phenomenological models describe
well the kinetics of physical ageing, on molecular
scale this phenomenon is not completely understood. The
phenomenon is considered to be associated with conformational
polymer segments rearrangements and increased
molecular packing and densification.

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Fig. 19. Dependence of enthalpy relaxation (!Hrelax) on ageing time (t)
for a polyester/melamine coating: (
) non-pigmented; () pigmented
with a TiO2 (PVC = 35%), physically aged at 25 ◦C; (×) !Hrelax (values
calculated per amount of binder) [142].
Despite the importance of pigments as major components
of paints, the number of studies dedicated to their effect on
physical ageing is very limited. The results found in the literature
are shown in Figs. 19 and 20. They illustrate the
dependence of the enthalpy relaxation on the ageing time
for two non-pigmented and pigmented with TiO2 coatings.
Figs. 19 and 20 show that the enthalpy relaxation rate of pigmented
coatings calculated per amount of binder is practically
identical with that of the non-pigmented coating. This
indicates that for these coatings, the pigments have only a
diluent effect and, therefore, do not interfere in the physical
ageing process. The situation can, however, be different for
other type of pigments and binders and/or for higher pig-
Fig. 20. Dependence of enthalpy relaxation (!H) on ageing time (ta) for
a polyester/TGIC powder coating: (×) non-pigmented; () pigmented
with a TiO2 (PVC = 20%), physically aged at 65 ◦C; (
) !H (values
calculated per amount of binder) [142].
ment volume concentrations. In addition, phenomenological
models predict the linear increase of the enthalpy relaxation
with logarithm of ageing time, in agreement with the results
shown in Fig. 20.
9. Water transport
The organic coatings are required to protect a great variety
of substrates against numerous aggressive agents, including
moisture. The destructive action of water can be important as
shown by the deterioration of building materials manifested,
for example, by the development of micro-organisms, frost
damage, efflorescence, decrease of thermal insulation, and
corrosion of metallic substrates.
To protect a substrate against moisture, the organic coatings
must possess well-defined “hygro” properties [144]. In
other words, this means that the organic coatings must be
formulated with specific moisture characteristics, such as coefficients
of permeability, diffusion and solubility, for each
type of substrate. To conceive such kind of organic coatings,
it is necessary to know how the pigmentation affects the
water transport. The transport process, or permeation, is a
result of solubility and diffusion phenomena [145,146]. The
solubility and diffusion coefficients describe, respectively,
the number and the mobility of penetrant molecules present
in the film. The permeability coefficient (P) is related to the
diffusion (D) and solubility (S) coefficients by
P = DS (16)
A literature search shows that inclusion of pigments in a
binder can greatly affect the transport process in organic
coatings [147–150]. Depending on the type of pigmented
binder, P, D and S can increase, decrease or be practically
independent of PVC. For relatively simple cases, described
by models which regard the pigmented films as composed
of two phases, a binder and an inert pigment perfectly dispersed,
the dependence of D, S and P on pigment volume
fraction (φ) is given by:
D = KDo (17)
S = (1 − φ)So + φSp (18)
P = K(1 − φ)Po + KφPp (19)
where K is the tortuosity factor, representing the “real” distance
covered by a penetrant in crossing the film, and described
by the ratio between the real distance and the film
thickness; So and Sp are, respectively, the solubility coefficient
of the binder and pigment; Do is the diffusion coefficient
of the binder. It follows that for impermeable pigments
(Pp = 0) that do not take up water (Sp = 0), the Eqs. (18)
and (19) become
P = K(1 − φ)Po (20)
S = (1 − φ)So (21)

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262 259
Fig. 21. Dependence of permeability coefficient (P, g/cm h mmHg) on
pigment volume fraction (φ) for an alkyd resin filled with: (×) a TiO2;
() a red lead; an epoxy resin filled with () a TiO2 at 20 ◦C and
RH1–RH2: 100–0% (adapted from [56]).
Examples of P, S and D dependence on pigment volume
fraction determined for one experimental humidity condition
(100% RH on one side of the film, and 0% RH on the
other side) are shown in Figs. 21–23. At other humidity
conditions, the magnitudes of the “hygro” characteristics
might be different if they are water concentration dependent,
which is the case of the majority of organic coatings, e.g.
[54].
For an alkyd resin filled with a TiO2, P (Fig. 21) and S
(Fig. 22) decrease with increase of φ, in accordance with the
Eqs. (20) and (21), while D (Fig. 23) decreases only slightly.
This indicates that in this case the pigment is impermeable
Fig. 22. Dependence of solubility coefficient (S, g/cm3 mmHg) on pigment
volume fraction (φ) for an alkyd resin filled with () a TiO2; () a red
lead; an epoxy resin filled with (
) a TiO2 and calculated from Eq. (20)
(continuous lines), at 20 ◦C and 100% RH (adapted from [56]).
Fig. 23. Dependence of mean diffusion coefficient (D) on pigment volume
fraction (φ) for: (×) an alkyd resin; () an epoxy resin, both filled with
TiO2, at 20◦C and RH1–RH2: 100–0% (adapted from [56]).
to water and well dispersed in the binder, K is close to 1,
and the water taken up by the pigment is negligible.
For an epoxy resin filled with a TiO2, P is practically
independent of φ (Fig. 21), while S decreases (Fig. 22) and D
increases (Fig. 23) with increase of φ. It was shown that this
behaviour is due to the presence of porous aggregates in the
film containing air, a consequence of an incomplete wetting
of pigment particles by the binder. Separate density and
permeability measurements had shown that the volume of air
entrapped in the film was sufficient to modify the expected
dependence of P and D on φ. Since the permeability of
water in air is about 105 higher than that of water in organic
coatings films, air volume fractions as low as 2 × 10−4 are
sufficient to change the dependence of P and D on φ.
The alkyd resin–red lead paints, which for toxicity reasons
are now only rarely used, are typical of systems where
pigments and binders are chemically interacting. The strong
decrease of P (Fig. 21) and S (Fig. 22) with increase of φ,
much higher than predicted by Eqs. (20) and (21), indicates
that the pigment and the binder interact chemically by forming
a new component (lead soaps) much less permeable to
water. Viscosity and electron microscopy tests confirm these
conclusions.
10. Nanofillers
An examination of coating properties discussed, as well
those not discussed in this paper, such as rheology, show that
they can be greatly affected by the pigment particle size: the
smaller the particle size, the larger the effect. This indicates
that the incorporation of nanofillers instead of micro-fillers
should favour phenomena associated with atomic and molecular
interactions that would lead to new macroscopic properties
for organic coatings [151–155].

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D.Y. Perera / Progress in Organic Coatings 50 (2004) 247–262
Nanocomposites are already used in the preparation
of a variety of materials, such as catalysts, electronics,
photonics, magnetic, biomedical and high performance
coatings [155]. A large number of studies dedicated
to coating systems containing nanofillers have already
been published, such as on epoxy/TiO2 [156,157], latex/
silica [158], poly(vinyl acetate), poly(styrene copolymer)
and poly(methyl methacrylate)/silica [159], poly(vinyl
acetate)/CaCO3 [160], and acrylic-polyurethane/silica [161].
These studies show that compared to conventional organic
coatings containing micro-fillers, the use of nanofillers
has advantages, such as improvement in scratch, abrasion,
heat, radiation and swelling resistance; decrease in
water permeability; and increase in hardness, weatherability,
modulus and strain-to-failure while maintaining
toughness [151–162]. When the size of nanoparticles is
smaller than the wavelength of visible light, no scattering
and reflection occur in the visible light range, and the
nanocomposite is transparent. This provides the possibility
of formulating transparent organic coatings with good
weathering resistance since the fillers can still absorb UV
light.
The above special qualities of nanocompounds are commonly
attributed to formation of an inter-phase region, a
consequence of strong interactions taking place between
the polymer and the nanofiller [153,154,159,163–165].
Due to the high surface area of nanofillers, lots of interfaces
are produced. They are considered to consist
of immobilized polymer chains that form “special constrained
matrix structures”. These inter-phases extend
further away from the filler particles affecting the properties
of the bulk sample up to 60–70%. Among the
properties affected by the restriction of polymer chains
mobility are viscosity, diffusion coefficient, NMR, relaxation
time, specific heat and Tg. In certain cases, a second
tan δ peak, attributed to the region of the restricted
mobility, is present in tan δ as a function of temperature
curve [119,159,160,166]. For thermosets, it is suggested
that the special mechanical properties are due not only to
the presence of the inter-phase, but also to other factors,
such as the absorption of hardener onto the nanoparticles
[167].
To make a successful “nanoorganic coating”, it is imperative
to disperse uniformly the nanopigments throughout
the binder. If this is not achieved, agglomerates will form
and the expected properties will not be obtained. An uniform
dispersion process is not easy to achieve due to the
nanoparticles’ tendency to agglomerate, a consequence of
their high surface energy. The methods mentioned in the literature
to incorporate nanoparticles in polymers consist from
simple mixing [160,167,168] to ultrasonic radiation [157],
emulsion polymerisation [163] and in situ polymerisation in
presence of nanofillers [155]. When the uniform incorporation
is difficult, such as with lamellar silicates, a chemical
modification of the nanofiller surface might be necessary
[169].
11. General remarks
All studies indicate that the incorporation of pigments in
binders affect the properties of organic coatings. The magnitude
of the effect is dependent on the pigmentation (nature
and volume of pigment), nature of the binder and the level
and nature (physical and/or chemical) of the pigment/binder
interaction. Most properties behave differently below and
above the CPVC. For micrometer pigments, representing the
great majority of pigments used by the paint industry, for
PVC < CPVC, in general, the increase in PVC leads to an
increase in the elastic modulus, stress at break and stress
arising in coatings, while the strain at break and thermal expansion
coefficient decrease.
The dependence of Tg and physical ageing rate on PVC
seems to be related to the strength and the level of binder
adsorbed at the pigment surface. The water transport appears
to be dependent not only on PVC, but also—among
other factors—on the quality of pigment dispersion. The inclusion
of nanopigments in binders results in new and interesting
macroscopic properties for organic pigments due to
the extended “inter-phase” created around the nanopigment
particles. Understanding the different effects on coating
properties resulting from the pigment incorporation should
favour balancing the various imperatives, such as high quality,
environmental protection and low cost, to which the
paint industry must respond.
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