6 Applications of fillers

This chapter is intended for people who use mineral fillers in coating system for-

mulations. The examples provide an idea of formulation parameters where fillers

exert a direct influence, along with their significance and hints on interpretation.

Users will also learn how to select the optimum fillers for imparting desired

prop-erties in a given application. Numerous worked examples and practical

illustrations round out the discussion on filler applications.

6.1 Importance of fillers in paints and coatings

Fillers were once commonly perceived as nothing more than cheap material for

bulking up profits, their sole purpose being to reduce the manufacturing cost of

coatings. How times have changed. Users of fillers have since discovered addi-

tional, technical functionality in these unprepossessing materials, not least be-

cause filler development activity has made progress despite the nay-sayers.

There is indeed a great deal of ongoing research and development in the filler

field, directed at imparting extra functionality or enhancing established features –

see also Chapter 7. Yet existing, tried and true fillers have a major part to play in paint and coating

formulations. Fillers can be pivotal to a variety of key technical properties: •_ Increased solids content and filling capacity •_ Reduced level of volatile organic compounds •_ Enhanced optical properties •_ Regulation of coating reflectivity •_ Enhanced mechanical properties, e.g. strength •_ Reinforcement of coating materials •_ Rheology control Depending on the application and the property requirements profile, selecting the

right filler is not always easy. Frequently, that means resorting to tried and true

formulas. It is unfortunate that fillers are not always tested and evaluated with the

same diligence applied to other classes of raw materials. An oft-heard argument here

is that fillers, after water, are the cheapest formula ingredients and therefore do not

merit thoroughgoing tests. Accordingly, binders, pigments and additives tend to

attract higher priority – this despite the fact that fillers comprise the dominant class

of raw material in formulations for interior emulsion paint, façade paints, renders,

plasters and other thick film systems, marking paints, primers, and many

more besides, see also Chapter 1.2. Such products frequently contain 30 to 40%

filler, rising as high as 80% in certain applications. It follows that fillers can also

play a governing role in the overall property requirements profile of a coating

ma-terial, and this should not be underestimated. Meanwhile, natural fillers have attained levels of quality where users may easily

forget that these are, in fact, products of nature. It is well known that natural prod-

ucts are subject to variations in their composition, and these subsequently affect the

properties of the coating materials into which they are made. Producers of fill-ers are

aware of this, and use selective mining techniques to minimise such varia-tions. But

to eliminate them altogether would be unrealistic, so at this juncture it needs to be

pointed out once and for all: natural fillers are variable products. 6.2 Important formulation parameters Special formulation parameters exist, which are important for characterising

coat-ing materials that use filler ingredients. Formulation parameters make it

possible to classify coating materials, as well as predict the property profile of a

given formula. 6.2.1 Non-volatile matter Solids, solids content (SC) or stoving residue are frequent terms for describing “non-

volatile matter” (NVM). ISO 4618 defines what is considered non-volatile matter in

coating materials, binders and other raw material ingredients, while ISO 3252 covers

the quantitative determination aspects. Non-volatile matter is de-termined

analytically as a means to deducing the composition of an unknown mate-rial.

Manufactured materials are subjected to similar analysis for quality control purposes.

The temperature and drying time used are major factors in the stated result: one

popular determination method for non-volatile matter involves exposure to a

temperature of 105°C for 2 hours. The initial weight of the material under test is

divided by the output weight after drying and multiplied by 100, see Equation 6.1. Equation 6.1: Calculation of non-volatile matter from empirically determined values For material of known composition like a laboratory test formulation, it is also

possible to determine non-volatile matter by arithmetic means alone, without prior

measurement. The arithmetic method requires knowledge of the NVM in each raw

material ingredient of the formulation. Figure 6.1 shows a worked ex-ample based on

a simplified formulation for demonstration purposes. Starting with the NVM in each

individual ingredient used and the ingredient’s percentage by weight of the overall

formulation, it is then possible to calculate the NVM of each

Function Substance NVM in Weight [kg] NVM in

substance [%] formula [%]

Binder Styrene-acrylic emulsion 50 160.0 8.0 Pigment Titanium dioxide 100 80.0 8.0 Fillers Calcium carbonate 100 300.0 30.0 Talcum 100 80.0 8.0 Precipitated aluminium 100 30.0 3.0 silicate

Additives Thickener, defoamer, 33 18.0 0.6 wetting and dispersing

agents, preservatives, etc.

Solvent Water 0 332.0 0.0

Total 1,000.0 57.6 Figure 6.1: Worked example of calculating non-volatile matter in a simplified emulsion paint formulation formulation ingredient, and from that, the cumulative NVM. Our sample

formula-tion contains 57.6% NVM of which over two thirds, 41% to be exact, is

attribut-able to the filler content. It is fillers that give coating materials their “body”, so it is logical that they are

major contributors of non-volatile matter. It is possible to reduce the amount of

volatile matter present by substituting fillers that have higher oil absorption by

others which are less absorbent, with low binder consumption. The result is

higher filler and solids content, yet viscosity holds steady. Although very high

levels of non-volatile matter are desirable from an environmental viewpoint,

there are prac-tical upper limits, due by the requisites of the production process

and the way the product is designed to behave when applied to a substrate. 6.2.2 Spreading rate Spreading rate according to ISO 4618 part 1 is understood to mean the average

substrate area that can be covered by a given volume or mass of coating material.

Practically speaking, the method of application needs to be capable of producing

a single layer in one pass. The resultant spreading rate is stated either in m²/l, or

m²/kg. Spreading rate is often considered in tandem with coating material

opacity. EN 13300, for example, describes classification of coating materials by

their opti-cal and mechanical properties. The practical method for determining

opacity meas-ures the contrast ratio between films of differing thickness; they

are dried and subsequently weighed to determine their mass per unit area. The

result is convert-ible to spreading rate by factoring-in non-volatile matter and the

coating material density. A plot of spreading rate vs. contrast ratio allows

interpolation and com-parison of contrast ratios for various paints that have a

similar nominal spreading rate: 7.5m²/l in the example shown in Figure 6.2 [1].

100 Class 1

99

[%]

98 Class 2

rati

o

97 Coating material 1

Con

tras

t

Coating material 2

96 Coating material 3

Class 3

95

Class 4

94

0 2 4 6 8 10 12

Spreading rate [m²/l]

Figure 6.2: Graphic example of correlation between spreading rate and contrast ratio

Filler content can exert a marked influence on spreading rate in pigmented

coating materials. As the filler content increases, so does the solids content and,

as a rule, spreading rate as well. These observations only apply where the

pigment content is comparable in each case: if the pigment level changes, the

spreading rate will change too. Fillers of differing density produce a similar

effect, likewise changing the spreading rate expressed in terms of area per unit

volume. High-density fillers reduce the volume spreading rate more than

lightweight fillers, so coating consump-tion increases as a result. 6.2.3 Pigment volume concentration Pigment volume concentration (PVC) is a key parameter of paints and coatings.

Along with a few other parameters, PVC makes it possible to predict numerous

interrelationships between a coating system’s composition, and the resulting prop-

erties. PVC makes an equally effective tool for characterising and classifying paints

and coatings. ISO 4618 part 1 defines PVC as the ratio of pigment and filler volume

in a coating film to the aggregate volume of non-volatile matter. This further implies

that the binder is considered in its final, solid form, rather than as supplied. Equation

6.2 shows the mathematical definition of PVC.

Equation 6.2: Definition of pigment volume concentration (PVC)

To illustrate the theory, the following is a PVC calculation example based on the

numbers in Figure 6.1. Calculating PVC requires details of the individual raw

material ingredients and the masses used, their non-volatile matter and density.

Mass is divided by the material’s density to convert the mass of raw material to

its corresponding volume, see Equation 6.3.

ρ kgl = mvV [ l ] =

mρ

Equation 6.3: Conversion of raw material mass to volume For each of the raw material ingredients involved in the worked example, Figure 6.3

lists NVM, mass as supplied and the NVM component of that mass, plus density. The

last column shows the calculated volume of each raw material ingredient.

Substance NVM in Weight NVM in Density Volume

substance [kg] weight [kg/l] [l]

[%] [kg]

Styrene-acrylic emulsion 50 160.0 80.0 1.05 76.2

Titanium dioxide 100 80.0 80.0 4.00 20.0

Calcium carbonate 100 300.0 300.0 2.70 111.1

Talcum 100 80.0 80.0 2.75 29.1

Precipitated aluminium 100 30.0 30.0 2.10 14.3

silicate

Thickener, defoamer, 33 18.0 6.0

neglected

wetting and dispersing

agents, preservative, etc.

Water 0 332.0 0.0

0.0

1.00

Total 1,000.0 57.6 250.7

Figure 6.3: Worked example of calculating the volume of individual raw material ingredients in a simplified demonstration emulsion paint formulation Once the respective raw material ingredient volumes have been calculated, the

PVC can be derived as shown in Equation 6.4. The volumes used in the example

result in a PVC of 69.6%. Equation 6.4: PVC calculation for a simplified demonstration emulsion paint formulation based on the data provided in Figure 6.3 As mentioned previously, PVC by itself is already a highly significant parameter.

Yet something more is required for a really meaningful interpretation: the critical

pigment volume concentration (CPVC) [2]. The relation of PVC to CPVC allows

qualitative statements about coating materials and their properties, see also

Chap-ters 6.2.4 through 6.2.6. Equation 6.2 and the worked example reveal just how strongly PVC is affected

by the type and amount of filler used. With coating materials, switching from

one mineral filler to another without accounting for their density difference can

have drastic results, see Chapter 6.4.1. 6.2.4 Critical pigment volume concentration ISO 4618 part 1 covers not just pigment volume concentration, but critical

pigment volume concentration (CPVC) as well. CPVC is defined as the PVC at

which binder just fills the voids between adjacent solid particles of pigment,

filler, etc. in a coating film. Above this point, a number of properties experience

significant change, see Figure 6.4. Various properties are predictable based on a

coating ma-terial’s CPVC and PVC, see also Chapter 6.2.5. When the PVC lies below the CPVC. The excess binder present (plus wetting agents

and dispersants) completely envelopes the pigment surfaces and fills the voids

between particles. In this case, the excess binder can produce a glossy

Dev

elop

men

t of p

rope

rty

high Porosity

Corrosion

Density

med. Gloss

Wet scrub

low resistance

20 40

80

0 60 100

PVC [%]

Figure 6.4: Changes in coating properties vs. PVC around the CPVC region