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Mixed Mineral Thixotropic Agents – A Strong Leap in Organoclay Performance

Turkchem 09 Jun 2017 33 9 dk okuma

TURKCHEM

The use of additives has always been an important part of success in the paints and coatings industry. Small amounts can significantly change performance. Among the broad range of additive classifications, thickeners play a significant role.

Mixed Mineral Thixotropic Agents – Organic Clay

A Strong Leap in Performance

The use of additives in the paints and coatings industry has always been an important part of success. Small quantities can change performance considerably. Among the broad classification of additives, viscosity modifiers play an important role.

Although all rheology agents are known as "viscosity modifiers," they are essentially thixotropic agents that provide application and storage properties to coatings. Commonly used thixotropic agents are organic clays, fumed silicas, and castor oil derivatives.

This article discusses a new type of thixotropic agent – mixed mineral thixotropic agents – primarily based on organic clay chemistry but with unique application properties – or in short: MMT – a new type of thixotropic agent.

MMTs – How Do They Work?

The chemical bases of organic clays and MMTs are quite similar; both are clay-based and have an ionic non-clay surface coated with cationic quaternary ammonium compounds to make the material hydrophobic and compatible with organic coatings. During dispersion, hydrophobic clay platelets are separated from each other by applied force. Only the surfaces of the platelets are coated with hydrophobic quaternary ammonium compounds; the edges remain uncoated with hydrophobic clay. In an organic medium, the hydrophilic portions of different platelets interact with each other through hydrogen bonds. This creates a three-dimensional network and simultaneously forms a thickened gel structure known as the "house of cards" effect. The gel has a certain strength that can be eliminated by applied force and reforms when the force is removed – this is always reversible. MMT clays give paint thixotropic behavior. Under low or no-force conditions such as storage or sagging application, viscosity is high because the gel's network structure is not disrupted. During brush or spray application, force is applied to the paint, disrupting the network structure and lowering viscosity. This explains why organic clays are used to prevent sagging, settling, and phase separation.

Figure 1. Clay surface acquiring hydrophobic properties through reaction with cationic surfactants

Figure 2. Gelation through hydrogen bonding, creating a "house of cards" structure

Figure 3. Thixotropy governed by reversible destruction of the "house of cards" structure by shear/breaking force

Up to this point, organic clays and MMTs are quite similar, so what is the difference between them? Typical organic clays, such as bentonite or hectorite, consist of uniformly distributed clay particles. MMTs are different. They are made up of several types of clay by design. MMTs consist of different clays with particles of different shapes. This is not simply from impure clay sources but is deliberately designed this way. Clays of different shapes are extracted separately and purified individually. Only at the step of making surfaces hydrophobic are they packed together in a reaction vessel. These mixed minerals give MMTs their unique performance. Because of a mixture of plate-shaped, ribbon-shaped, and tube-shaped clays, they do not pack together as densely as the uniform shapes of conventional organic clays would allow. The particles within MMTs are not as closely packed to each other. They cannot bind to each other as strongly. When translated into application properties, this means that particles within MMTs can be more easily separated from each other than in conventional organic clays. This provides faster and easier dispersion, even easier than conventionally used self-dispersing organoclay.

Figure 4. Easier dispersion of MMT

The density of MMTs is lower compared to conventional organic clays but still somewhat higher than fumed silica. Even though MMTs show performance similar to fumed silica, their higher density makes them easier to handle and process. MMTs leave less dust in the environment and take up less space compared to fumed silica.

Figure 5. MMTs produce less dust than fumed silica because their density is higher.

Application

Although MMTs perform similarly to conventional organic clays and resemble the performance of fumed silica, MMTs have certain advantages. First is the ability to create pregels at higher concentrations. Pregels can be made at 10-15% MMT concentrations in fluid consistency and are easy to use, and moreover, MMTs do not require much force to be dispersed. In some cases, circulation pumps alone can do this job.

With organic clays, pregels can only be made at around 6%, and these are at the edge of pumpability.

For fumed silicas, this value is even lower. MMT pregels can be made not only in solvents but also in monomers such as styrene, without requiring any activator. This is an advantage in high-solids systems.

Application properties are also unique. MMTs show pseudoplastic behavior. This means that during paint application, low force is sufficient, and when the force is removed, the rapid viscosity increase plays an important role in improving resistance to storage conditions and paint sagging.

Figure 6. MMT provides high concentration but low-viscosity and flowable pregel properties.

Examples

Organic clays fail in high-solids systems. They either give too much application viscosity or are very difficult to disperse. A system where conventional organic clays fail is unsaturated polyester systems. Here, the classic alternative is fumed silica. This formulation based on Gremopal unsaturated polyester resin is shown in a graph. The organic clay did not show the desired sagging viscosity. Fumed silica performs better. But MMT shows the most suitable performance: For example, during spraying, due to the pseudoplastic nature of MMTs, viscosity drops rapidly, and when force is removed, viscosity increases very quickly, providing the best resistance to sagging.

Formulation

GREMOPAL 181.10 91.6 (unsaturated polyester resin)

Styrene 5.3

MMT 1.0

Byk R 605 0.05 − (rheology assistant)

Byk A 500 0.05 (air release agent)

Sicoflush P blau 6880 0.10 Styrene 2.1

Figure 7. MMT significantly increases low sagging viscosity with virtually no negative impact on application viscosity.

The second example is a system made with Palatal P4-01 unsaturated polyester resin. In this case, organic clay was not preferred, and under appropriate conditions, superior performance was shown by MMT compared to fumed silica. This means that when MMT is added as a powder, its performance is very similar to fumed silica, and the sagging and flow curves are comparable. But MMTs have a unique advantage in this case: with MMTs, it is possible to make a fluid pregel at 12% concentration in styrene monomer using low-speed mixing. With fumed silica, it is not possible to make a pregel in this formulation with the low amount of styrene present. When MMT is prepared as a pregel, performance doubles. MMT at 0.5% addition level, compared to 1% fumed silica, provides the same sagging resistance and a similar flow curve. This is shown in detail in the graph. MMT, due to its good storage stability and absence of any phase separation and settling compared to all competing products, has delivered the best results achieved.

Unsaturated Polyester Formulation:

Palatal P4-0 91.6 (ortho-UPR, DSM)

Styrene 7.4

Byk A 555 0.1 (foam suppressant)

Byk A 515 0.2 (foam suppressant) (or Byk 057 / A500)

Thixotrope 1.0 (MMT, powder or 12% pregel

in styrene)

Byk R 605 0.1 − (rheological modifier)

Sicoflush P blau 6880 0.1

Figure 8. Adding MMT as a pregel provides nearly 50% savings compared to fumed silica.

The reasons for MMTs' excellent performance can be demonstrated by conducting a sagging relaxation test. A paint containing a thixotropic agent is mixed at high speed to disrupt the three-dimensional structure of the thixotropic agent responsible for the paint's viscosity. Then, the mixing speed is reduced to 1/s low mixing speed. The time taken for the paint to return to its original viscosity is measured; this is the time required for the three-dimensional structure to form again. As can be seen from this graph, the return to original viscosity in this system is faster with MMT than with fumed silica. MMT's low viscosity tendency at high forces provides easier spray application.

Figure 9. MMT shows faster recovery to original viscosity after application.

MMTs generally do not require any activator, but in some cases, the use of small amounts of polar activator accelerates gel formation. Particularly in unsaturated polyesters, polar activators are used to gel fumed silicas; approximately 10% activator ratios are sufficient to make MMTs form an improved gel.

In tests conducted among activators, propylene carbonate, the typical activator for organic clays, is not the best. Preferred are Byk R605 or DGA (di-glycol amine). In some cases, even a small addition of water is sufficient to provide the best hydrogen bonding and best gelation.

Another system that presents difficulty for organic clays is epoxy resin systems. Particularly these are solvent-free epoxy systems. They do not provide adequate opportunity for low-concentration pregels of conventional organic clays to be well dispersed; however, for MMT dispersion, only the epoxy resins themselves can work – compared to castor oil and amide paraffins, MMTs do not require increased system temperature for good dispersion and show no particle settling problems.

The image shows a two-component epoxy floor coating containing quartz sand as filler. Due to the high density of quartz sand, it settles rapidly and the mixture cannot be stored stably without a thixotropic agent. MMT was compared with conventional organic clay and fumed silica. The image shows storage test results. As expected, organic clay is completely unsuitable for this system.

Fumed silica and MMT work in this system, but MMT is more advantageous.

Epoxy Floor Coating Containing Quartz Sand:

Polypox E 403 36.00

Byk 341 0.16

Byk A 530 0.50

Quartz Sand GS 13 36.80

Plastorit O 12.00

Polypox E 403 14.00

Byk A 530 0.50

Thixotrope 0.40

Byk R 605 0.04

Figure 10. MMT shows improved settling and storage stability in solvent-free epoxy systems.

To reduce the required amount of MMT working in solvent-free systems, use of 10-15% pregel is recommended when solvents are used in the formulation. Additionally, monomers or low-viscosity components in the system can also be used to prepare a pregel.

In filled systems, there is no need to make a pregel because there is sufficient filler material available to fully disperse the MMT.

The third system shown is a conventional organic clay that does not show the desired performance and can be replaced with MMT. This is an unfilled 2K PU system. All conventional organic clays failed in creating thixotropic behavior in the polyol portion. They show Newtonian viscosity behavior. This indicates that conventional organic clays were inadequately dispersed. The only clay product that works as desired is MMT. MMT shows pseudoplastic viscosity behavior. When tested immediately after adding the sagging hardener, MMT remains the product that provides the best sagging resistance compared to all conventional organic clays tested.

Figure 11. In critical systems, MMT disperses faster

Summary

Mixed mineral thixotropic agents, or "MMTs," have more advantages compared to conventional organic clays and fumed silicas. MMTs can form low-viscosity fluid pregels and are easily dispersed. Their high density during use produces less dust compared to fumed silica. They are easy to use in paint production and easy to disperse. MMTs are also suitable in solvent-free, high-filled systems. Due to their pseudoplastic viscosity behavior, in unsaturated polyesters and 2K epoxy systems, viscosity drops very quickly at high force application and returns to original viscosity very quickly at low force application. This brings improved application properties such as better sagging resistance and better storage stability. MMTs are a stronger and easier-to-use alternative to fumed silicas. MMTs are patent-protected.

MMTs are available at BYK Additives & Instruments under the trade name GRAMITE®.

Author: Dr. Klaus Dziwok – Research and Development Manager – Additives and Equipment Division – BYK – Chemie GmbH

Translator: Özlem Tığlı Özcan – Regional Sales Manager – BYK Additives and Equipment Division – BYK – Chemie GmbH