Pigment concentrates

positive influences of additives on the stability and compatibility of pigment concentrates in water based paint formulations

Sandra Gurt
Dr. Nicholas Büthe

MÜNZING CHEMIE GMBH
Salzstrasse 174
D-74076 Heilbronn
Germany

1 Introduction

Pigment concentrates are important products in the fabrication of paints and coatings[1]. Various types of pigment concentrates exist as the requirements for their use differ strongly depending on the application. For the use in decorative paints and coatings as well as for industrial paints a broad compatibility is essential to cover a large range of binder types and paint systems. Universal concentrates for solvent and water based systems are mainly used in decorative paints where they have to cover the whole range of different paint formulations. Specific concentrates, formulated with grinding resins[2] and/or highly efficient dispersing agents, are used in general for industrial paints and inks.

However, numerous interferences may occur from a pigment concentrate on the base paint as well as from the base paint on the pigment concentrate due to ingredients or formulation strategies used either in the pigment concentrates or in the base paint. This article wants to explain the reason of some observed interferences and wants to give some help how it is possible to solve these problems with the right selection of additives.

The pigment concentrate is able to cause several problems in the base paint. The type and quantity of wetting and dispersing agent used in production of the pigment concentrate can destabilize

the dispersion of the white pigments and fillers in the base paint (especially if their performance and amount in the concentrate is not sufficient),
the binder (especially if unsuitable wetting agents or incompatible grinding resins are used) or even
the viscosity of the base paint (especially if a lot of glycols or other surfactants are used).
The other way round the pigment concentrate may be sensitive
on wetting and dispersing agents used in the base paint,
on thickeners used in the base paint,
on defoamers used in the base paint,
on the pH level of the base paint or
on other specific formulation ingredients of the base paint.

Beside the negative influences also positive interference can be observed. Polyurethane (PUR) associative thickeners used in the base paint can exhibit a positive influence on the stability of pigment concentrates in tinting systems due to their hydrophobic-hydrophilic character and can improve colour strength. Also fillers like kaolins in the base paint can improve the pigment stability in certain paint formulations.

In this lecture influences of specific additives like defoamers, wetting agents and PUR associative thickeners on the stability and compatibility of pigment concentrates in paint and tinting systems are discussed.

1.1 Types of pigment concentrates and their formulation

Pigment concentrates, if they are used as a prefabricated product in the paint and coating production, are requested to be as universal and economical as possible. The requirements however, are very specific, depending on the paint system and application.

A differentiation can be made in pigment concentrates which are based on grinding resins and others, in which dispersing and wetting agents are used as principle grinding agents. In the first case the grinding resin provides strong wetting properties and dispersing agents have only to be added in smaller quantities to support the dispersion. Due to various binder types used in industrial paints incompatibilities can limit the use of grinding resins
Resin free pigment concentrates generally show a better compatibility with respect to the binder which is used in the paint formulation. As universal concentrates they are mainly used in solvent and water based decorative paints. But also here the development is going to specific concentrates for use in either solvent or water based paint systems.

The applications in which pigment concentrates are used also determine the formulation of a concentrate. Pigment concentrates for architectural coatings, with less requirements of compatibility towards the binder, may be formulated with a minimum quantity of standard wetting agents just in order to obtain a complete coverage of the pigment’s surface. Anti-settling or thickening agents can be used to stabilize the concentrate if they do not influence gloss and colour strength significantly.
Industrial coatings[3] however, require a much better dispersion and quality of the concentrate. Here, the colour development, gloss, haze gloss and water sensitivity are the most important factors in order to obtain a perfect quality of the applied film[4]. The pigment concentrates for industrial use therefore require high quality dispersing agents and mainly higher dosages than concentrates which are used in architectural coatings only.

1.2 Dispersing agent technologies for pigment concentrates -
advantages and disadvantages

In the preparation of pigment concentrates with a grinding resin a good stability of the concentrate is obtained by the high molecular resin which has sufficient wetting properties. Further wetting and dispersing agents only have to be added in lower quantities in order to obtain a better dispersion and stabilization. The use of thickening or anti-sedimentation agents is dispensable. However, the compatibility limits the „universal“ use of resin containing concentrates in industrial paints.
A resin free concentrate requires a higher content of dispersant. This dispersant demand[5] is given by the quality and strength of the dispersant and the type of pigment, as the pigments surfaces vary strongly in their chemical and physical properties. The minimum dosage of dispersant to cover the whole pigment surface, however, is not enough to obtain a stable concentrate. The needed quantity of dispersant to obtain a stable concentrate can exceed in this case the dispersant demand strongly.
In addition the rising requirements for better performing pigment concentrates and environmental concernings about alkylphenolethoxylates (APE) leads to the substitution of APE[6] in pigment concentrates. Polymeric dispersing agents can substitute these compounds in an environmental friendly way. This parallels the increasing demand for solvent free (especially glycol free) pigment concentrates requiring thus also solvent free dispersing agents.

2 Results and Discussion

2.1 Influences of defoamers on the stability of pigment concentrates

Defoamers in water based paints can exhibit strong influences on pigment concentrates due to their surfactant absorbing character. This is especially pronounced in cases, where the quantity of wetting agent in a concentrate is rather low and/or where a strong defoamer has already absorbed wetting agents from the base paint.
The influence of defoamers on the stability of pigment concentrates in paint formulations is studied with the tinting of a white interior wall paint of high pigment volume concentration (PVC) with a conventional, commercially available pigment concentrate of a black iron oxide pigment. Different types of defoamers with variations in their hydrophobic properties and chemical composition are added to the paint. The base paint is prepared in this experiment without defoamers.
The defoamer and the concentrate are added at the same time to the paint and are incorporated during 5 minutes by stirring at 1000 rounds per minute (RPM). To 100 g of paint 3.0 g of pigment concentrate is added together with 0.2 g of defoamer.
Defoamer 1 and defoamer 2 are strongly unpolar, hydrophobic defoamers, based on pure mineral oils of pharmaceutical grade („medical white oils“). Defoamer 3 is also strongly hydrophobic, based on medical white oils, but contains an emulsifier. The hydrophobicity of the polysiloxane defoamer 4 is controlled by the organic modifications of the silicone chains.
After application of 200 µm paint on degreased glass plates a rub out test is done and after drying the colour strength for the rubbed and unrubbed surfaces is determined. Values are reported in Table 1.

	Defoamer composition	colour strength [%] (integral)
		unrubbed	rubbed

without defoamer	---	100.0	108.3
Defoamer 1	medical white oil, nonionic fatty compounds, hydrophobic silica strongly unpolar, hydrophobic	59.2	49.0
Defoamer 2	medical white oil, nonionic fatty compounds, hydrophobic silica strongly unpolar, hydrophobic	46.7	55.6
Defoamer 3	medical white oil, special waxes, nonionic emulsifier strongly unpolar, hydrophobic	85.9	99.9
Defoamer 4	modified organo polysiloxane, nonionic alkoxylated compounds hydrophobic properties controlled by modifications	94.6	108.3

Table 1: Comparison of colour strength values using different defoamers

In table 1 a comparison of the colour strength values of rubbed and unrubbed parts are displayed in function of the defoamer. The colour strength of the unrubbed part of the sample without defoamer serves as reference for the comparison with different defoamer types.

The influence of the defoamer on the colour strength values of the applied films is significant. Based on the 100% colour strength which are obtained without defoamer, all defoamers containing samples show a reduction of colour strength. The values of colour strength with defoamer range from 46.7 to 94.6 for the unrubbed parts and from 49 to 108 for the rubbed parts.

Figure 1: Colour strength of rubbed and unrubbed parts in relation of the defoamer

Although defoamer 1, 2 and 3 consist of a medical white oil as principle active substance there is a big difference in the influences they exhibit. Defoamer 3 in comparison to the two other defoamers of similar chemical composition, has a much lower influence on the colour strength. However, the initial value of the sample without defoamer cannot be reached. The lowest influence is observed with defoamer 4, which is a organic-modified polysiloxane defoamer. As already observed before in other occasions, this defoamer type in general shows the lowest influence on the stability of pigment concentrates in paint formulations.

In contrast to the other applications, defoamer 1 shows an inverse behaviour regarding the rub out properties. In general, after rubbing of the applied film, the rubbed part turns darker than the unrubbed part. That means that the pigments of the concentrate have flocculated and agglomerations are destroyed by this application of mechanical forces. Only in the case of defoamer 1 the rubbed surface is brighter than the unrubbed surface. This means, that mainly the titanium dioxide was flocculated by defoamer 1.

2.2 Influences of defoamers, addition of wetting agents

Defoamers act as „absorber“ of wetting agents and therefore can influence the stability of pigment concentrates. As a result flocculation occurs upon tinting a base paint with pigment concentrates. In order to reduce the influence of the defoamer the above displayed experiment was modified in such a way that a wetting agent was added together with the defoamer. The type of wetting agent and the way of addition to the paint was modified.

For all trials defoamer 1 was used due to the strong reduction of colour strength in the previous experiment. Paint and concentrates are the same as mentioned under II.1. The wetting agents which are used in this trials to modify the defoamer behaviour, are nonionic glycol esters (wetting agents 1 to 3) and a modified sulfosuccinate (wetting agent 4).

Every wetting agent was introduced in three different ways:
a) mixing wetting agent and defoamer before adding to the paint,
b) addition of the defoamer to the paint first, then stirring for one minute and subsequent addition of the wetting agent, then stirring for four minutes,
c) addition of the wetting agent to the paint first, then stirring for one minute and subsequent addition of the defoamer, then stirring for four minutes.

No		incorporation	colour strength [%] (integral)
		method	unrubbed	rubbed
1	without defoamer		100.0	108.3
2	defoamer 1		59.2	49.0
3	defoamer 1 + wetting agent 1	a	100.7	112.3
4		b	68.6	51.8
5		c	76.0	65.0
6	defoamer 1 + wetting agent 2	a	53.9	61.3
7		b	57.7	49.5
8		c	59.3	66.4
9	defoamer 1 + wetting agent 3	a	100.2	113.4
10		b	65.1	52.8
11		c	75.4	60.5
12	defoamer 1 + wetting agent 4	a	49.1	57.3
13		b	64.1	56.0
14		c	69.9	31.8

Table 2: Comparison of colour strength values obtained by different incorporation methods of the wetting agent

In table 2 the colour strength values of the applied samples are displayed in function of the way of incorporation and wetting agent. As comparison the values of the colour strength of the samples without defoamer and with defoamer 1 from the previous experiment are also given. The unrubbed part of the sample without defoamer was again taken as reference at 100%.

In figure 2 the colour strength values of the rubbed and unrubbed parts of the samples containing different wetting agents, are displayed. Trial No 1 is the reference and No2 the defoamer application without wetting agent from the previous experiment.

As can be seen immediately from figure 2 the wetting agents 1 and 3 have a positive influence in order to avoid stability problems of the concentrate (trials No 3 and 9). In these cases the values for the unrubbed parts are like the reference value and the rubbed parts are even more intense compared to the reference. In these two examples, the negative influence of the defoamer (No 2) could be neutralised by the use of a wetting agent. However, the nature of the wetting agents is of subordinate importance: the third wetting agent of the same chemical nature (wetting agent 2; glycol ester) as well as the sulfosuccinate (wetting agent 4) do not increase colour strength values.

The way of incorporation of the wetting agent is more important. The high colour strength values with wetting agents 1 and 3 can only be obtained, if the wetting agent and the defoamer are mixed together before addition to the paint (incorporation method a). In the case of the incorporation methods b and c, in which one additive is added to the paint before the other one, no influence is observed and significant lower colour strength values are obtained.

Figure 2: Colour strength of rubbed and unrubbed parts in dependence on the defoamer/wetting agent combination and the way of incorporation

Independent from the chemical nature of the wetting agent is also the behaviour, whether the black or the white pigments flocculate and therefore a higher or lower colour strength of the unrubbed surfaces compared to the rubbed surfaces are obtained. Here again the way of incorporation of the wetting agent is more influent. As can be seen from figure 2, in all incorporation methods „a“ the colour strength of the unrubbed parts is inferior to the rubbed parts, independent from the wetting agent. The wetting agent only has an influence on the relation of colour strength between rubbed and unrubbed parts. The higher colour strength values of the rubbed parts indicate a flocculation of the pigments of the pigment concentrate, which is added after defoamer and wetting agent. The addition of premixed wetting agent and defoamer does not influence the titanium dioxide; however, an influence on the pigment concentrate cannot be excluded.

In contrast, all trials following incorporation method „b“ lead to higher colour strength values of the unrubbed surfaces. In this case the defoamer, which is added to the paint before the wetting agent, leads to a flocculation of the titanium pigments. This flocculation is subsequently eliminated by rubbing, resulting thus in a lower colour strength of the rubbed part.

Incorporation method „c“ mainly also leads to lower colour strength values of the rubbed parts. Only in trial No 8 the results are contrary. In the case of No. 14 an extreme difference in the colour strength of the rubbed and unrubbed surfaces is observed. In this case the wetting was unsuitable and caused an additional destabilization of the system.

2.3 Influences of associative PUR thickeners on the stability of pigment concentrates

Associative PUR thickeners consist of a three-segmented structure: hydrophobic-hydrophilic-hydrophobic. Due to hydrophobic-hydrophobic interactions associative PUR thickeners can interact with hydrophobic surfaces of pigments, obtaining thus an immobilization (stabilization) of the pigment in the paint system[7]. Due to their hydrophobic-hydrophilic structure PUR thickeners additionally can act surfactant-like.
In order to examine the influences of PUR thickeners on the stabilization of pigment concentrates a pigment concentrate of an organic pigment was added to an interior wall paint which contained only cellulosic ethers as thickeners. Two pigment concentrates were prepared, one with a minimum stabilization of the pigments and one with an optimum stabilization.
Pigment concentrate A contains an organic brilliant phthalocyanine green pigments (PG 7) with 15% of a polymeric dispersing agent calculated on the pigment. The concentrate is stable to sedimentation but contains only the minimum dispersant amount. Pigment concentrate B contains 35% polymeric dispersant on pigment and is stable and compatible with all paint formulations. The further ingredients are listed in table 3.
The concentrates were added in 1.72 g quantity to 100 g of paint to the paint formulation. This corresponds to 0.5% pigment solid in the paint. The paint contains only a MHEC cellulosic thickener of a 6000 grade and was prepared without defoamer.

Ingredients	Pigment Concentrate A	Pigment Concentrate B
water	58.80	51.90
Polymeric dispersing agent	5.25	12.25
Defoamer	0.50	0.50
Biocide	0.10	0.10
PG 7 pigment	35.00	35.00
Organically modified smectite clay	0.35	0.25
*Total*	100.00	100.00
Dispersant on pigment	15%	35%

Table 3: Two pigment concentrate formulations with different dispersant amounts

Four different trials are performed in order to determine the influence of the thickener. In a first trail the concentrate was added to a paint sample which did not contain neither defoamer nor thickener. In a second trial the concentrate was added to a sample which contained only defoamer (defoamer 1 from the previous trials), the third only PUR thickener, without defoamer and in the fourth sample both, defoamer and PUR thickener were present. The paint samples were applied on glass plates and one part of the surface was rubbed. After drying colour strength and rub-out values were determined.

When concentrate A with 15% dispersant on pigment is added to the paint which does not contain neither defoamer nor PUR thickener, low colour strength and high rub-out values are obtained. The defoamer contained in the second sample decreases slightly the colour strength compared to the previous application and the rub out value is increased. This indicates the flocculation of the green pigment.
This changes significantly when the PUR thickener is added to this paint (without defoamer). A strong increase in colour strength and a strong reduction of the rub out can be observed. If defoamer and PUR thickener are added to the paint, colour strength and rub out are not as good as in the trial with the PUR thickener only but still a much higher colour strength value and lower rub-out value are obtained compared to the sample where only the defoamer is added.

Pigment Concentrate A	colour strength	rub out [DE]
without defoamer, without PUR thickener [I]	0.45	36.7
0.35% Defoamer 1 [II]	0.41	41.8
0.30% PUR thickener [III]	1.75	19.9
0.35% Defoamer 1 + 0.3% PUR thickener	1.67	20.5

Table 4: Colour strength and rub out values for pigment concentrate A

In table 5 the colour strength and rub out values for the samples with concentrate B, containing the optimum stabilization of 35% dispersant on pigment, are displayed.

Pigment Concentrate B	colour strength	rub out [DE]
0.35% Defoamer 1	3.88	1.1
0.35% Defoamer 1+ 0.3% PUR thickener [IV]	3.88	0.9

Table 5: Colour strength and rub out values for for pigment concentrate B

The results change in the case of the pigment concentrate B, which is sufficiently stabilized by dispersing agents from the beginning. Pigment concentrate B contains 35% dispersant on pigment which ensures a perfect dispersion and stabilization of the pigments in any paint formulation. If the PUR thickener is added together with the defoamer the influence of the stabilization, which was observed in the concentrate with a lower dispersant concentration, is not as strong as before. The colour strength (see table 5) is not affected by addition of the thickener. The rub out value, however, can be lowered again.

Figure 3: Colour strength and rub out for I, II, III and IV in an interior wall paint formulation

3 Summary

The compatibility and stability of pigment concentrates in paint formulations can be influenced by additives. Due to their chemical nature defoamers are surfactant absorbers, due to which they can lead to destabilization effects on a pigment concentrate. However, it is possible to find suitable defoamers which prevent destabilization effects due to their chemical nature. The right choice of a defoamer therefore is one possibility to obtain a stable and compatible concentrate.
It could be demonstrated that wetting agents can improve the compatibility of the defoamer. Negative influences of the defoamer can be prevented if defoamer and wetting agent are premixed and then added to the paint formulation. Not all wetting agents however are suitable and lead to an improvement.
Associative PUR thickeners can show a stabilizing effect if the used pigment concentrate contains a low amount of dispersant. Especially in basic architectural paints where still often only cellulosic ethers as thickeners are used the addition of PUR thickeners can give several additional benefits. Apart from better rheological properties as better levelling and lower spattering pigment concentrates are stabilized and rub out problems can be reduced.
The choice of the right additives is essential for the preparation of tinted paint systems.
Apart from the use as unique wetting and dispersing additive the polymeric dispersing agents are also used in order to improve the compatibility of a given pigment concentrate with the base paint. It could be demonstrated that low dosage levels of a polymeric dispersant in the base paint are able to improve significantly the acceptance and the homogenous distribution of the pigment concentrate[8]. Polymeric dispersing agents therefore increase the compatibility between pigment pastes which are produced with different kinds of dispersing agents and the basic paints with also varying dispersing agents and binder systems.

4 Literature

[1] Brock, Groteklaes, Mischke, Lehrbuch der Lacktechnologie, Vincentz Verlag,
Hannover (1998)

[2] M. Dalton, R. Krause, P. Braun, Ink Wold, Vol. 3, No. 3 (1997) p. 64

[3] W. Herbst, H. Hunger, Industrial Organic Pigments, Verlag Chemie, Weinheim (1993)

[4] L-J. Calbo, Handbook of Coatings Additives, Marcel Dekker, New York (1992)

[5] W. Heilen, G. Feldmann-Krane, S. Silber; European Coatings Journal, 10 (1995) p. 712

[6] J.H. Bielmann, Farbe & Lacke, 4 (1996), p. 91

[7] J. Schrickel, Pitture e Vernici, 5 (1999)

[8] Münzing Chemie, Dispersing Technology and Recommendations, Technical Report (2006)