Oil absorption of pigments and fillers

The oil absorption value is an integral record in technical data sheets of fillers and pigments used in paints. But what is the significance and what are the methods of determining the oil absorption? There are many myths about this parameter and there are several methods of laboratory OAV (oil absorption value) determination. In this article, we will discuss the importance of this parameter, especially when using fillers and pigments in latex paints, as well as discuss methods for its determination and dispel myths about it.

A set of equipment for determining oil absorption using the ASTM D281 method – the most popular of the test methods

Introduction

Let’s start with a few definitions. Oil absorption value according to the terminology developed by Paolo Nanetti and described in Coatings from A to Z it is expressed as the mass of linseed oil in gram per hundred gram pigment or filler (extender) and it is a measure for the oil requirement of the pigment. In the case of pigments and fillers with relatively coarse particles, the determination of the oil absorption value gives important information for the formulation of the approximate composition of a mill base.

According to the same book by Paolo Nanetti, oil requirement in practice it is determined by the evaluation of the oil absorption value and it expresses how much binder has to be absorbed from a pigment or filler (extender), in order to wet completely the particles and to fill all interstices between particles. The oil requirement depends directly on the specific surface area of the pigment but also on its state of agglomeration or of dispersion.

The third definition in this book is binder requirement – amount of binder, which is needed to produce a paste with defined flow properties, starting from a specified weight of pigment or filler (extender). The binder requirement can be numerically defined, according to the used test method. Most frequently it is determined as oil absorption value, although the determining of the yield point or the Daniel titration with different binders are other methods used in practice. The binder requirement has major importance for mill-base formulations.

These are the most common definitions in industry terminology. Let’s move on to the analysis of the definitions themselves and how they relate to the paint production process on the market, with particular emphasis on the production of water-based latex paints.

From the definition it follows that in the production of paints it is necessary to know the value of the amount of linseed oil needed to prepare the paste with fillers and pigments which is necessary for the preparation of the paste. The problem is that such a process in which linseed oil could be used to prepare pigment paste or fillers is currently only used in art paints. Indeed, using linseed oil, a very high viscosity paste is prepared which is passed through a roller machine, usually a three-roller mill, to break up aggregates and agglomerates and wetting each pigment and filler particle with oil. Oil absorption is therefore a parameter that can be directly used in the production of artistic paints in order to find the minimum amount necessary to grind dry ingredients on a three-quartering machine. However, architectural and industrial paints, both water-based and solvent-based, are produced in a completely different production process and with the use of binders other than linseed oil.

Industrial three-roller mill – today it is used only in artistic paints and other industries, such as rubber

In the case of the production of artistic paints in a three-roller mill, it is really important to prepare an appropriate paste that will allow for a perfect oil coating of particles that will be grinded as a result of squeezing between the rollers. However, the production process of architectural and industrial paints is not based on such a production process. In the case of solvent borne paints, the grinding is usually carried out in basket mills, ball mills or bead mills after the pigment and filler slurry has been initially dispersed in the solvents with and without a binder (resin). In solvent-based paints, the key is to prepare paints with appropriate PVC, which is smaller than CPVC. In turn, in the case of water-based paints based on polymer dispersions (e.g. acrylic wall paints), the dispersion process is carried out in a suspension without a binder (latex). The dispersion is carried out in a cowles dissolver as an aqueous suspension of pigments and fillers with additives. The binder (polymer dispersion) is added at the let-down stage. In such a production process, the particles are wetted and dispersed as a result of reglomeration and reaggregation due to the use of appropriate additives (surfactants as wetting additives and dispersants) in devices ensuring appropriate shear forces.

Covering the pigment particles and fillers with the binder is irrelevant here, because it does not participate in the grinding process – for water-based paints (without polymer dispersion), and in the case of solvent-based paints it can take place without the use of resin. In water-soluble paints, the coating of pigment particles and fillers does not matter at all, because as I mentioned before, the binder (latex) is added at the let-down stage after grinding (after dispersing), and moreover, architectural water-borne paints are often prepared depending on PVC > CPVC which is completely excludes the importance of oil absorption in any binder requirement (binder damand) calculations. The demand for binder is determined at the stage of laboratory tests of coatings, and so in the case of latex paints, a number of parameters are determined that are key to the amount of binder that will be used in the formulation by determining PVC and performing tests such as scrubbability, weatherability, gloss, etc.

So what is the oil absorption parameter for and does it matter?

The importance of the oil absorption parameter in reality

In art paints prepared in three-roll mills with the use of linseed oil, it can really be important and translate into the amount of oil needed to prepare the grinding paste. But what is the importance of oil absorption in water based paint formulations?

Oil absorption is an important parameter that serves something other than determining the amount of oil to be paste on the three-quartering machine. Oil absorption is used to calculate CPVC (critical pigment volume concentration). CPVC is such a value of PVC, where the amount of film former, which is contained in the coating film, just suffices to fill all hollow spaces between the pigment and extender particles. In the formula for calculating CPVC, it is necessary to insert OAV, which enables the calculation of this value. The CPVC value alone is needed to calculate the Q factor, i.e. the PVC/CPVC ratio, in order to calculate how porous (Q > 1) or tight the coating is (Q < 1). A special case is Q = 1 or Q close to 1, which means PVC = CPVC. In the case of water-based latex paints, balancing on the border of Q = 1 and exceeding Q > 1 is possible and when formulating recipes, calculating this ratio is necessary to design an appropriate formulation.

It does not matter that the binder used to determine OAV is linseed oil, because in this way we determine the value of CPVC in which “some substance” covers the particles of pigments and fillers to a minimum extent, which can be translated into a coating with another substance, e.g. copolymer from polymer dispersion. Formulating recipes without calculating PVC, Q or determining CPVC for fillers in the formulation is very difficult and correct understanding of handling these values is particularly helpful for designing paints with specific coating properties directly resulting from these dependencies, e.g. for tightness for water vapor transmission, porosity which can be used in the opacity or formulation of coatings with a specific output retention of dirt (dirt pick-up).

Methods for determining oil absorption

The principle of the method for determining oil absorption is to rub the oil into the test pigment or filler. The method of rubbing, rubbing time and the parameters of the linseed oil used are very important for the repeatability of the test and the determination result. There are basically two methods of determining oil absorption – the spatula rub-out method and the Gardner-Coleman method. The Spatula rub-out method is dedicated to typical pigments and fillers, while the Gardner-Coleman method is most suitable for fillers with a very high specific surface area (very porous and light), such as for example diatomaceous earth.

Determining the oil absorption requires the skill of the test technician and the development of a relatively quick rub-out of the oil. The rubbing time affects the repeatability of the result, because the prepared paste may dry out depending on the type of fillers. The most important thing in repeatability is a well-noticed endpoint, which is often the source of the greatest error, as is the wrong linseed oil. In the Spektrochem laboratory, a number of tests were carried out on the same types of fillers and pigments in order to demonstrate the influence of linseed oil on the obtained result of oil absorption in the rub-out method.

A set of linseed oils during case studies in the Spektrochem laboratory regarding the influence of the acid number on the results of oil absorption

The ASTM methods clearly define the parameters of the linseed oil that should be used to determine the OAV of pigments and fillers, both according to the ASTM D281 – spatula rub-out method and the ASTM D1483 – Gardner-Coleman method. The importance of a suitable linseed oil, especially in terms of acid number, saponification number and iodine number, is crucial for obtaining reproducible results. Rubbing linseed oil into a pigment or filler does not only consist in coating the particles of the tested powder and breaking up agglomerates and aggregates, but also in reactions, e.g. saponification, occurring between the oil and the filler or pigment. Hence, the quality and repeatability of linseed oil with certain standardized parameters is crucial. The characteristics of raw linseed oil according to ASTM standards are given in the table below.

Specific gravity
Boiling point
Acid number
Saponification value
Iodine value
Gardner color
0.926 – 0.931
>149 °C
3 ± 1
185-196
minimum 170
maximum 13
Specification of raw linseed oil (ASTM D281 and ASTM D1483)

Spatula rub-out method

In the ASTM D281 a stiff, putty-like paste is formed by the dropwise addition of linseed oil to pigment that is being thoroughly rubbed with a spatula. The amount of oil required to produce the end point is used to calculate an oil absorption value. The method is divided into two sub-methods: A – adding linseed oil from a weighing bottle and B – adding linseed oil from a burette. The method uses linseed oil indicated in this article (with the parameters from the table) and rubs it in with a specific spatula. It is very important to keep the time of performing the determination and determine the end point correctly.

Near the oil absorption end point as determined by ASTM D281 spatula rub-out method

A pictorial representation of the ASTM D281 method A test was presented in the video from our laboratory. The link to the video is below.

How is oil absorption determined by spatula rub-out method? See the video from our laboratory

Gardner-Coleman method

The Gardner-Coleman test method described in ASTM D1483 differs from the previous one. A soft paste is formed by the dropwise addition of linseed oil to the gently stirred pigment. The amount of oil required to form the paste is used to calculate an oil absorption value. This method is much more suitable for very highly absorbent fillers and pigments, which would be difficult to process with spatula rub-out. The Gardner-Coleman method determines the oil absorption of porous and high surface area fillers, e.g. diatomaceous earth, some kaolins, etc.

Equipment for the Gardner-Coleman method of determining oil absorption ASTM D1483

In the method described in ASTM D1483, a different spatula is used, and the whole operation is performed not on the glass rub-up plate or marble slab, but in the beaker, forming a ball that does not drip oil and does not stick to the wall of the beaker.

The photo below shows an intermediate step in performing an oil absorption using the Gardner-Coleman method.

High surface area filler for the Gardner-Coleman oil absorption determination

Below is a link to another video from our laboratory showing the principle of performing an oil absorption determination using the Gardner-Coleman method ASTM D1483.

How is oil absorption determined by the Gardner-Coleman method? See the video from our laboratory

Summary

Determining the oil absorption for pigments and fillers is necessary, however this parameter must be correctly interpreted. However, for that to happen, education about its meaning needs to be widely disseminated, hence this article on our blog. It should be remembered that in the 1960s, when solvent-based paints were first prepared by grinding, for example, iron oxide red in a three-roller mill, and then adding the rest of the binder (linseed oil) and other additives, this parameter was indeed important for the proper preparation of the load for grinding.

Today, it is relevant in art paints that are still produced in this way. The current production processes and the use of other raw materials in both water-based and solvent-based paints as well as grinding processes in cowles dissolver or pearl mills mean that oil absorption is not used to calculate the amount of binder needed to grind pigments and fillers. Oil absorption is an excellent support for paint formulators in terms of support in determining the relationship between PVC and CPVC, which are the basic parameters when determining paint recipes, especially water-based latex paints, in which paint formulations are prepared practically in the entire range of PVC (from a few percent to almost 99% in in the case of so-called ceiling paints and an extremely modest budget)

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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