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F O A M C O N T R O L

FOOD CONTACT COMPLIANT DEFOAMINGA new organo-modified silicone defoamer comound is effective in many application areas. By Peter Bene, Sascha Oestreich and Andrea Schamp, BASF.

A new defoamer based on organo-modified silicones provides high defoaming efficiency even after storage at elevated tem-peratures. Test results show excellent performance across a wide range of binder and formulation types, in both grind stage (pigment concentrate) and final applications. This product is reg-istered globally and has broad food contact compliance.

F oam is a very undesirable phenomenon, especially in industrial situations that can emerge during dispersing processes, pump-

ing, stirring or while applying paints. Foam can, for example, increase the production time, create difficulties in filling vessels, reduce the ef-ficiency of many high speed operations and promote surface defects such as craters, fish eyes, pinholes and weak points in the dried film.Antifoam agents are therefore essential to eliminate undesired foam and (micro-) air bubbles from paints, coatings and inks. Various ac-tives, e.g. mineral/natural oil-based products, modified polyacrylates or modified polydimethylsiloxanes are available to provide the best de-foaming/deaerating effect in a broad range of coating or ink systems.Besides top technical performance, today’s customers also demand antifoam agents that are compliant with all relevant food contact reg-ulations and have a global registration.

WHY FOAM FORMS AND HOW IT IS STABILISED

It is a general observation that pure liquids such as water do not foam, whereas liquids which contain surfactants or surfactant-like products tend to have a more or less strong tendency to foam. Foam can be de-fined as a meta-stable dispersion of gas (usually air) in a liquid where surfactants stabilise foam lamellae.A pure liquid has a constant surface tension all over its surface, and it will remain constant in any situation as long as no other substance is added. The situation changes only when surfactants or other surface active ingredients are added. Surfactants stabilise the foam lamellae by two effects: ą They can create an osmotic pressure, which works against drainage of the foam lamellae.

ą Additionally, they strengthen the foam lamellae via what is called the Gibbs-Marangoni effect.

ą If the foam lamella is being stretched, the surface concentration of surfactants decreases, which in turn causes the surface tension to increase locally. In this way surfactants selectively strengthen the weakest parts of the foam lamella and tend to prevent them from stretching further.

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RESULTS AT A GLANCE

ű Effective defoaming of water-borne coatings requires ma-terials which show ‘controlled incompatibility’ with the formu-lation. By carefully adjusting this incompatibility it is possible to avoid surface defects while mainting long term persistency.

ű A new commercial product based on organo-modified sili-cones, “FoamStar SI 2240”, shows high defoaming efficiency, re-tained after storage at elevated temperatures and has minimal effect on gloss.

ű Test results are presented showing its efficiency across a wide range of binders and formulations. High shear stability al-lows its use in (binder containing) pigment concentrates and the let-down phase. It is registered globally and has broad food contact compliance.

Figure 1: Entering, spreading and bridging of a defoamer droplet (green) in a foam lamella.

Entering Spreading Bridging

Figure 2: Structure of the ‘molecular defoamer’ molecule.

Hydrophobic

Hydrophilic

A foam lamella that fails to change its surface tension during a con-traction or expansion process will burst due to its rigidity [1].

BASIC PRINCIPLES OF DEFOAMING

There is no single overarching defoamer theory in scientific literature. However, for aqueous systems all theories have some explanations and prerequisites in common. It is generally accepted that a defoamer active should [2]: ą Possess a ‘controlled incompatibility’ with the formulation to be defoamed. This ‘controlled incompatibility’ must remain even after long-term storage;

ą Have a low surface tension; ą Exhibit a de-wetting effect; ą Have the ability to enter into foam lamellae and to spread effec-tively on the liquid/gas interface (Figure 1).

Due to this “controlled incompatibility”, the defoamer active forms droplets that are able to enter the foam lamellae. Its low surface tension allows the droplet to spread and push aside the stabilising surfactants at the air-liquid interface. Finally, it can bridge the foam lamella. The bridge will be unstable and the foam lamella will break.This can also be explained by the following equations, where g aw = interfacial tension air/water, g ow = interfacial tension oil/water, g oa = interfacial tension oil/air.Entry coefficient: E = g aw + g ow + g oa (1)If E is negative, the oil droplet is wetted by the aqueous phase. So E must be positive for an antifoam.Spreading coefficient: S = g aw - g ow - g oa (2)If S is positive, then the defoamer can spread and push aside the sur-factants.Bridging coefficient: B = g2 aw + g2 ow + g2 oa (3)If B is positive, then if E allows the drop to enter the wall of a foam lamella, and if the drop radius is large enough, it will bridge the wall.

ACHIEVING DEFOAMING WITHOUT SURFACE DEFECTS

Understanding the mechanism of defoaming is thus all about under-standing interfacial processes and the physico-chemical behaviour of interfacially active substances. Table 1 gives a good overview of the entry and spreading coefficients of various liquids.The table also explains why silicone is one of the preferred defoaming actives. It exhibits positive entry and spreading coefficients, which al-lows it to effectively enter the foam lamella and spread and push aside the stabilising surfactants.Unfortunately, defoamer formulations based on pure silicone oil might create surface defects in coatings (turbidity, craters, fisheyes), because they are rather too incompatible in aqueous systems. No

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surface defects such as craters should be generated in the final paint film due to defoamer incompatibility. The defoamer must therefore be sufficiently compatible with the binder.In order to combine the excellent defoaming characteristics of sili-cone with good compatibility, organo-modified silicones have been invented. Their organic modifications can contain side chains such as polyethers or polyesters, which allow efficient control over the de-gree of incompatibility. Organo-modified silicones achieve excellent defoaming characteristics with little or no side effects such as loss of gloss. This makes them highly suitable for use in modern water-borne coatings and inks.Another very special defoaming active is the award-winning “Foam-Star” polymer (Figure 2). This is a hyper-branched molecule with a 3-D star-shaped structure, containing hydrophilic as well as hydrophobic elements [3].Unlike conventional mineral oil and silicone defoamers, this molecular structure defoams at a molecular level. It acts as a unique surfactant, interacting with the foam-stabilising surfactants and destabilising the foam bubbles. When combined with silicone-based defoamer actives,

Figure 3: Defoamer performance (density measurement) in a resin-based pigment concentrate (“Joncryl HPD 396” resin) in the premix, after the bead mill and after 24h storage.

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Figure 4: Defoamer performance (density measurement) in a let-down varnish based on “Joncryl 669” for corrugated board. Measurements were done initially, after 15 min. and after 30 min after stirring.

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Figure 5: Defoamer performance (density measurement) in a formulation based on “Acronal S 790” immediately and after t wo weeks of storage.

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Figure 6: Defoamer performance (density measurement) in a for-mulation based on “Mowilith LDM 1871” immediately and after t wo weeks of storage.

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it achieves synergistic effects such as faster foam knock-down and improved overall efficiency. Due to its surfactant-like structure, it also achieves improved wetting characteristics [4].Besides top technical performance, today’s customers also ask for antifoam agents that are compliant with relevant food contact regula-tions and have a global registration. This can only be achieved by a careful selection of all raw materials involved. It is not sufficient to only evaluate the main ingredients of an additive formulation.This is the reason why a completely new defoamer formulation has been built up with carefully selected unique raw materials.

DEFOAMERS COMPARED IN APPLICATION AND DENSITY TESTS

Defoamer performance was evaluated according to two distinct test procedures.The stirring test (density test) allows the efficiency and the separation tendency of a defoamer in liquid systems to be evaluated. The com-plete formulation (including the defoamer) is stirred with a toothed dissolver at high speed (3 minutes @ 5,000 rpm). Then the density of

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the formulation is determined by weight with a pycnometer. The higher the resulting density (g/cm³), the higher the de-aeration ef-ficiency of the defoamer.In order to simulate conditions of paint applications, the sponge roller test is carried out. Therefor 15 g of each test system (includ-ing the defoamer) is applied on 0.04 m² polyester film and dis-tributed with a high-porosity sponge roller. The knock-down effect during and after application and the surface appearance of the dried film are rated by optical inspection.Rating is from one to five where five reflects an excellent appear-ance with no foam, and one indicates bad de-aeration. A com-mercially available defoamer based on a strong organo-modified silicone compound was used in all cases as a ‘standard defoamer’.

VERSATILE PERFORMANCE IN PRINTING INKS

As mentioned above, compliance with all major food contact regulations is essential in printing and packaging applications. The defoamer evaluated below, “FoamStar SI 2240” (referret to as “Product 2240” in graphics and tables) is compliant with most relevant food contact regulations (e.g. several FDA paragraphs, EU 10/2011 and Swiss Ordinance List A) and it has a global country registration status.For a defoamer in printing and packaging applications, it is not only necessary to have good defoaming characteristics in pigment concentrates, but it should also perform well in the let-down. Tests were carried out in two typical test formulations: ą In a resin-based pigment concentrate based on a grinding resin;

ą In a let-down varnish for corrugated board based on an acrylic colloidal emulsion.

Usually, two separate defoamers are needed to defoam the two stages in the formulation. In this case, the molecular defoamer shows very good efficiency in both applications (Figures 3 and 4).

GOOD RESULTS IN WOODSTAIN FORMULATIONS

Coatings formulations such as trim paints are usually not easy to defoam and sometimes rather sensitive to surface defects. It is essential to use very efficient, but also compatible, defoamers for such paints.Tab. 2 reveals performance characteristics of the new defoamer in a wood stain for brush applications. As can be seen from these figures, the molecular defoamer achieves excellent results in the sponge roller test compared to the standard.A similar picture can be seen in a low-build stain based on a dif-ferent resin where both defoamers evaluated show excellent defoaming performance. In the sponge roller test, however, the

Table 1: Over view of interfacial tensions goa and gow and the corresponding entry and spreading coefficients of different fluids.

Substance goa (mN/m) gow (mN/m) E (mN/m) S (mN/m)

n-pentane 16.1 50.2 107.1 6.7

n-octane 21.7 51.7 103.1 -0.4

Silicone oil 21.2 36.9 87.2 13.4

Water 73.0 -- -- --

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molecular defoamer achieves better de-aeration and improved com-patibility compared to the standard defoamer (see Tab. 3).

STABLE PERFORMANCE IN ARCHITECTURAL PAINTS

Additionally, the new type of defoamer was tested in the broadly used acrylic binder “Acronal S 790” (Figure 5). In such systems, (interior and exterior) the most frequently used defoamers are mineral oil based. However, in formulations with high binder content, the use of high-efficiency organo-modified silicone-based defoamers is preferred.Compared to the standard defoamer, the new defoamer compound shows excellent defoaming persistency after two weeks storage at 40°C.Another class of binder systems, especially for interior paints, are vinyl acetates and vinyl acetate copolymer-based dispersions. This type of binder is particularly difficult to defoam. However, the new

defoamer shows an excellent defoaming effect in one such resin, which remains at a high level even after storage at elevated tempera-ture (Figure 6).

REFERENCES

[1] Garrett P.R., The Science of Defoaming – Theory, Experiment and Applica-tions, Taylor & Francis Group LLC, 2014.[2] A Practical Guide to Defoamers; BASF, ED2 0414e (downloadable from www.basf.com).[3] Mangano J., The role of molecular defoaming actives in low VOC paints, Asia-Pacific Coat. Jnl., Dec.. 2014.[4] Nejad M. et al, Caught on camera, ECJ, 2015, No. 10.

“Performance synergy with silicones.“

Peter BeneBA SFpeter.bene@basf.com

3 questions to Peter Bene

Could the equations possible be used to predict the optimum dosage of a defoamer in different coating systems? No, the equations only describe the fundamental working mechanism of defoamers. Real coating formulations contain varying amounts of ingredients, e.g. wetting agents, emulsifiers, dispersing agents, etc., which might influence the necessary dosage of the defoamer. Could your hyper branched polymer be used in in combination with silicon based defoam-ers to control the negative surface defects of those? Indeed the combination of our hyper-branched polymer (FoamStar) with silicones shows in many cases a performance synergy – combin-ing the excellent spontaneous defoaming effect of the hyperbranched polymer with the long term persistency of the silicone compounds. In fact, several products of our FoamStar range are based on such a synergistic combination. How does the new defoamer affect the wetting and dispersing of pigments when used in the grinding process? First, the new defoamer compound supports removal of entrapped air from the dry pigments during the pigment wetting phase. Secondly, the new product effectively destroys foam during the grinding process allowing faster and more effective grinding of the pigments.

Table 2: Defoamer performance (density test) - before and after storage, as well as deair ing properties (sponge roller test) of a wood stain based on Acronal LR 9014.

Table 3: Defoamer performance (density test) - before and after storage, as well as deair ing properties (sponge roller test) of a wood stain based on Joncryl 8386.

Density test – Sponge Roller test (rating 1 (bad) - 5 (excellent))

immediately in g/cm3

after 2 weeks in g/cm3

Wood stain based on “Acronal LR 9014”

Wo defoamer 0.68 0.57 1.5

Standard defoamer 1.00 0.90 2.5

Product 2240 0.99 0.96 3.5

Density test – Sponge Roller test (rating 1 (bad) - 5 (excellent))

immediately in g/cm3

after 2 weeks in g/cm3

Low-build wood stain based on “Joncryl 8386”

Wo defoamer 0.61 0.42 3.5

Standard defoamer 1.02 0.96 3

Product 2240 0.97 0.91 4.5