Two-way benefits

Amphiphilic amine both neutralises and aids pigment dispersion

Jean-Pierre Lallier*

The latest neutralising and co-dispersing amine for waterborne coatings from Arkema also has the useful property of reducing the surface tension of water. This ensures good paint performance, even in challenging formulations (new emulsions, high pigment and filler loadings, and even high binder content) with low odour, reduced volatility, and no formaldehyde emissions.

"Alpamine N41" secondary-butyl ethanolamine (sBEA) is an alkyl alkanolamine with both alcohol and amine functionalities. Its chemical formula is:
CH3CH2CH(CH3)NHCH2CH2OH
Pigment concentrates are formulations that are difficult to prepare because of a number of critical factors, including their very high pigment content and the poor chemical compatibility between pigments, mostly hydrophobic due to their organic nature, and water, highly hydrophilic (by definition). sBEA is a highly effective additive which can help to resolve this type of difficulty.
The various steps in the preparation of pigment concentrates are first considered. The second part of this paper discusses when and how sBEA imparts its beneficial functions. An example based on a blue pigment concentrate then shows the benefits from both technical and economic points of view, as it helps to reduce the levels of both the dispersant and the pigment concentrate required.

When preparing a pigment concentrate, the objective is to obtain sufficient stabilisation of the pigments in order to retain them in the dispersed state. Dispersion and stabilisation can be broken down into three steps (Figure 1):

››› Wetting of dry pigment agglomerates by the liquid;
››› Separation of pigment particles in the dry pigment agglomerates (separation phase) with a dispersant;
››› Stabilisation of the pigment particles.

Pigment particles are not initially free but are bonded together to form dry pigment agglomerates. In contact with a liquid (such as water), these dry pigment agglomerates have to be wetted as much as possible. This will be the case if the surface tension of the liquid is lower than the surface energy of the dry pigment agglomerates.This condition can be satisfied if the surface energy of pigment particles is high or if the surface tension of the liquid is low. But this is in fact the opposite of what normally happens. Organic pigments in particular are hydrophobic with a low surface energy, while liquids (and water in particular) have a high surface tension. Thus the objective is to reduce the surface tension of the liquid and/or increase the surface energy of the pigments.
These are the two technical aspects in which sBEA has been found to be very effective. This alkanolamine molecule is able to decrease the surface tension of the water efficiently while at the same time making the surface of the agglomerates more hydrophilic.

Whereas most amines or other alkaline bases (such as ammonia and sodium hydroperoxide) do not reduce the surface tension of the water very efficiently, sBEA does reduce it very efficiently down to 27 mN/m as a result of its amphiphilic structure, shown in Figure 2.
Adsorption on the hydrophobic surface of dry pigment agglomerates occurs because of this amphiphilic structure. The hydrophobic alkyl group (sec-Bu) is adsorbed onto the pigment, while the other part of the molecule (hydrophilic aminoethanol group) remains outside the agglomerate, producing a hydrophilic layer on the surface of the dry pigment agglomerates. This effect increases the compatibility between the pigment agglomerates and water.Following this wetting phase, the wetted pigment agglomerates are split into individual pigment particles. This grinding step requires a great deal of energy as well as perfect wetting of the dry pigment agglomerates. The smaller the pigment particles, the more energy is required to separate them. In the work discussed below, a mill with glass balls was used to carry out this step.
Once the particles are separated, they must be stabilised with a dispersant to prevent flocculation. Most dispersants are large molecules (oligomers) with an amphiphilic structure. Thus one part of the dispersant exhibits good chemical affinity with some chemical sites on the pigment surface, while another part of the dispersant shows good chemical affinity with the liquid phase (Figure 3).
Two stabilisation principles are used to prevent the pigment particles from flocculating: electrostatic stabilisation and steric stabilisation. In the former case, all particles have the same electrostatic charge, which produces electrostatic repulsion between the particles. In the latter case, the particles are covered by a layer of polymer that dissolves in the liquid and so produces steric stabilisation.This is the classical principle used in stabilisation. Molecules such as sBEA with the structure of a surfactant can help dispersant molecules to achieve good adsorption on pigments using less dispersant. Effective stabilisation prevents the particles from flocculating.

A formulation with a very low dispersant content (see Table 1) was prepared containing a copper phthalocyanine pigment (Ciba "Irgalite blue GLO"), which is hydrophobic. For the dispersant, "Coadis 123K" was selected. This is a potassium polycarboxylate in aqueous phase, which has a hydrophobic nature and a very high water resistance.
Conventionally, the dispersant content is approximately 15 % of the pigment weight, therefore the test formulation should have contained 4.2 % dispersant. For this preparation, only half this amount was used, the other half being the amine.
Under these very demanding conditions, ammonia, sodium hydroxide, monoethanolamine or other classical bases are not able to produce effective wetting or dispersion. Wetting of the dry pigment is poor, with significant flocculation, and the final result is heavy sedimentation (Figure 4).
Only with sBEA was it possible to obtain a homogeneous pigment concentrate without flocculation or sedimentation. Thus, with sBEA, it is possible to halve the dispersant content and achieve cost benefits (Figure 5).

In the second part of this study, an answer was sought to the question: how does this improved wetting power of sBEA lead to cost savings? Several blue pigment pastes were prepared with a low dispersing agent loading (only the neutralising agent was changed in each case).
These pigment pastes were compared in term of their stability. The stable ones were then introduced into a classical white paint base in different amounts. The shade of blue obtained in the final paints with different amounts was then assessed and cost calculations were carried out to establish the advantages of using sBEA as neutralising agent in the paste.
The formulation given in Table 1 was used as the basis for the pigment pastes. Four amines were evaluated, always with the same amount in the formula: 2.1 %.

››› Ammonia
››› Monoethanolamine (MEoA)
››› Dimethylethanolamine (DMEoA)
››› sBEA.

Dimethylethanolamine is the amine most commonly used in pigment pastes. At this level of dispersing agent, the use of ammonia does not lead to acceptable stability: this paste was not further evaluated.
The other three amines led to acceptable stability and were tested further. Then, these three pigment pastes with each different amine were introduced into a white base paint (semi-gloss), based on styrene acrylic latex. The colour impact on the final paint was determined in relation to the base used in the paste (determination of the value b*) and the results are given in Figure 6.
The conclusion is that 2 grams of paste with sBEA additive are required to obtain the correct b* whereas 4 grams of paste containing MEoA or DMEoA are required to obtain the same b*. Thus, using "Alpamine N41" in the pigment paste leads to 50 % savings on paste loading in the final paint.

A cost analysis has been produced using sBEA. The price of sBEA is higher than that of MeoA and DMEoA, but the difference is at most 2 €/kg. As the amine loading in the pigment paste is 2.1 %, this means a cost increase of about 50 €/metric ton of pigment paste.
Then considering the final paint: assuming a medium price of 10 €/kg for a blue pigment paste, the reduction from 4 % to 2 % addition leads to cost savings of 200 €/metric ton of final coating. As the volume of paint or ink produced is very much higher than that of the pigment pastes used, the economic benefits for a coating formulator when the paste is based on sBEA can easily be deduced.
These results show that secondary-butyl ethanolamine is a very effective alkyl alkanolamine for the preparation of pigment concentrates. Because of its amphiphilic structure, sBEA acts as a surfactant to produce good wetting of the pigments (classical function).
Additionally, it has been shown that sBEA can help dispersant molecules to achieve good adsorption on pigments and so make use of a smaller amount of dispersant. These various enhancements help to solve dispersing problems in pigment pastes, slurries and other dispersions, and even to reduce pigment and dispersing agent loading.
Using sBEA is not only a way to lower odour and avoid stability problems in waterborne coatings, it is also a way to save money. ?

Jochum Beetsma, SpecialChem webseminar, 9 December 2008: Pigment Dispersion & Stabilisation in Practice.

Dr. J.P. Lallier
R&D Engineer
Solvents and
Amines Applications
Arkema
jean-pierre.lallier@arkema.com

»› A newly introduced neutralising amine for waterborne coatings, secondary-butyl ethanolamine or sBEA, has the useful property of having an amphiphilic structure, which enables it to reduce the surface tension of water and assist dispersants in separating and binding to pigment particles.»› Tests on a highly loaded blue tinting base show that when the dispersant addition is reduced from its normal level, a stronger tinting power can be obtained by using sBEA rather than classical amines such as DMEA.»› This product, marketed as "Alpamine N41", is more expensive than classical amines, but the reduction in use of dispersant and/or stronger tinting power obtained lead to significant savings in the final tinted paint.