Determining the optimal dispersant loading for pigments and fillers

Blog Archive | 7 minutes  | Author: Adam Morgan , Ph.D.

There are many different dispersant chemistries available to the formulator. Some will be much more effective than others at dispersing particular pigments in aqueous or non-aqueous systems. This article offers tips on how to optimise the loading of a suitable dispersing agent, as the amount of dispersant required to fully cover the surface of a pigment or filler will vary. This will depend on the particular dispersing agent chosen as well as variations in surface area, surface energy, absorption kinetics and other formulation specifics. It is essential that the primary particle surface is covered completely, as this will optimise pigment stability and performance.

Lawrence Industries represents MÜNZING CHEMIE in the UK and IE - offering a comprehensive range of additives for coatings and inks. Amongst these additives are the high-performance dispersing agents for organic pigments, inorganic pigments and carbon black - marketed under the well-known EDAPLAN® and METOLAT® brand names. Get in touch with our technical sales team on 01827 314151, to see how we can help you with your dispersion goals.

In aqueous pigment slurries, the optimal amount of dispersant is relatively easy to determine. It relies on the fact that viscosity reaches a minimum when the pigment surface is completely covered. A common test method is as follows:

 

METHOD A - An approximate measure of required dispersant

1). Prepare the pigment slurry – Pigment content is high enough, such that a slurry is obtained after mixing in the absence of a dispersing agent. Usually, a loading of 30 wt.% for organic pigments, or 55 wt.% for inorganic pigments, is necessary.

Dispersing time is about 30 minutes with a dissolver (or equivalent).

 

2). Addition of dispersant and determination of viscosity – Dispersant is added in portions to the stirred pigment slurry. After addition and mixing, the viscosity is measured at low shear rates with a Brookfield viscometer or a cone and plate system.

A dispersing agent is added until a minima in viscosity is obtained. Two typical viscosity curves are observed as a result (Figure 1). Curve 1 is typical of conventional wetting agents and dispersants for inorganic pigments, such as polyphosphates and polycarboxylic acids. Curve 2 is usually seen for other types of polymeric dispersing agents.

Curve showing viscosity change with addition of dispersant in pigment slurry

Figure 1. Viscosity decreases to a minimum as a polymeric dispersant is added to the pigment slurry. Different dispersant types give different curves. Optimal dispersion loading is the point at which the viscosity minimum is achieved.

In general, the amount of dispersing agent which is determined in this experimental method is too low. This is because:

  • The optimum degree of grinding is not achieved because the amount of dispersant added during dispersion is too low.
  • Reagglomeration starts immediately after dispersion is stopped and there is not sufficient dispersant to stabilise the pigment particles i.e. there are changes in between stopping dispersion and taking the viscosity measurement.

This can be demonstrated if the procedure is changed to the following method:

 

METHOD B - Refining the optimal amount of dispersant required to stabilise a pigment slurry

1). Dispersion of a pigment slurry with glass balls in a shaking mixer e.g. Red-Devil.

 

2). Addition of dispersant until a minimum of viscosity is obtained. Dispersion is continued for approximately 10 minutes. In many cases, the viscosity rises again - so that more dispersing agent has to be added to the slurry. The procedure is continued until a clear minimum in viscosity is obtained.

For a polymeric dispersant the curve of viscosity reduction is as follows in Figure 2.

Figure 2. Reagglomeration starts immediately after the removal of shear when there is insufficient dispersant. Using method B (rather than method A) will help you to determine the correct loading of dispersant required.

Values of viscosity decrease after each addition of dispersant to a minimum, but rise again after a longer dispersing time. Viscosities can be lowered stepwise until no final increase in viscosity is observed. Quantities of dispersing agent found via this method are not indicative of the obtained particle size, stability against reagglomeration and the stability of adsorbed dispersants on the pigment surface. For determining these factors refer to the next section.

 

Testing the quality of a dispersion

There are a number of test methods that can be carried out to determine different aspects of the quality of a dispersion. In the following method we look at how to prepare a drawdown, ready for testing:

 

1). Determination of dispersant demand – Follow Method B above to calculate the exact amount of dispersing agent required.

 

2). Preparation of pigment pastes – Pigment pastes are prepared with the correct amount of dispersant. Additionally, pastes are prepared with 10% less and 10% more dispersant than the optimal amount. This helps to corroborate that the optimum amount is what was determined from Method B.

Dispersing is performed on a shaking mixer or sand mill (Red-Devil or Scandex). Always use the same device and procedure when comparing samples. The ratio of pigment paste to glass balls = 1:1.5. Dispersing time = 40-60 minutes. Addition of a relatively high level of persistent defoamer (0.5-1.0%), such as AGITAN® 731, will ensure that air bubbles do not interfere with the subsequent measurements. To summarise, the paste compositions are given in Table 1. Note that a pH modification may be required for some types of dispersant.

 

  Organic Pigments Inorganic Pigments Carbon Black
Pigment 30-40% 55-60% 10-20%
Dispersant Value determined in Method B
Defoamer 1.0% 0.5% 1.0%
Water Rest up to 100%

Table 1. Composition of the pigment paste made up for subsequent testing of dispersion quality.

 

After grinding, clear lacquers are pigmented with the prepared tinters and applied onto clean glass plates. The ratio of the pigment to binder solids should be as follows:

  • Inorganic pigments = 20-30%.
  • Organic pigments = 10-15%.
  • Carbon black = 5-10%.

 

3). Evaluation of the dispersion – The following test methods in Table 2 can be performed to determine and compare the quality of a dispersing agent and the dispersion.

Test Method What it Reflects
Gloss and Haze The degree of grinding
Colour point, colouring power Degree of grinding, eventual colour changes caused by chemical reaction of dispersant and pigment. Changes of primary particle size or structure
Rub-out Reagglomeration, flocculation, floating/flooding
Different application methods e.g. pouring, spraying etc. Flocculation, formation of agglomerates
Transparency or hiding power Fineness of grinding

Table 2. Test methods for determining the quality of an obtained pigment or filler dispersion.

 

An example of how dispersion quality can affect rub-out and transparency are shown in Figure 3.

rubout and transparency of inks with and without proper dispersion

Figure 3. Dispersant type, loading and dispersion protocol have a huge impact on the rub-out testing and transparency measurements.

 

What to do now?

Lawrence Industries have a team of highly trained technical sales staff on hand to help with your formulation challenges. Give us a call to see how we can help you with your particular dispersion requirements. If you already know what product you are looking for then request a sample through our EDAPLAN® and METOLAT® homepage.

Author: Adam Morgan , Ph.D.

Adam studied chemistry at the University of Warwick for 8 years, where he obtained a Ph.D. in the field of polymer and inorganic colloid science. He has been with Lawrence Industries since 2014 as a technical sales manager covering all industry areas. He is now responsible for marketing within the company as well.