Rheology of Waterborne Paints – Part II

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Waterborne Paints
Udaya Bhaaskar Bulusu, Director – Projects & Operations

Sanitech Engineers & Consultants Private Limited. &

Kinshuk Infra Private Limited

Sagging

Sagging, the flow behavior in a vertical position, is determined with a sagging doctor blade. Ten stripes of the paint are applied using the doctor blade in thicknesses of 75 to 300 µm on a Leneta sheet. Immediately after application, the card is lifted into a vertical position so that the stripes lie parallel to the horizontal. The stripes with the lowest film thickness are at the top. After drying, the stripe at which the paint starts to sag is given on a scale of 0 to 10: 0 meaning that all stripes sagged and 10 meaning that no stripes sagged (fig).

Alternatively, it is possible to determine at which film thickness sagging starts to occur by applying the paint, in various thicknesses or as a wedge, to a vertical surface.

Brushing resistance

Brushing resistance is determined by applying a specified amount of paint to a test surface and assessing the resistance felt on distributing the paint with a brush.

Spattering characteristics

The spattering characteristics can be determined by applying a specified amount of paint uniformly on a previously conditioned roller and rolled several times over a grid. The paint spray thrown from the roller is collected on a black card under the grid. The test card is compared with a standard tested at the same time.

These test methods are shown in a video on our Homepage entitled “Various methods for measuring viscosity”.

Selection methods

At this point, to select what type of thickener need to use. Start to see which type of thickener and the different shear level required. These methods of incorporation are extremely important as the higher the shear, the more potential foaming issues may have to be addressed. Figure below illustrates four specific attributes that the viscosity of the product addresses.

In the above example, the direct relationship that paint rheology has on specific attributes. The thickener selection and chemistry type determine the shear rate needed to incorporate into the paint.

After choosing the ingredient for viscosity build, levelling and sagging directly affect the final paint’s performance.

With the newest paint and coating formulations reducing volatile organic compounds (VOC), waterborne paint formulations have become extremely complex. Older versions of many coatings contained a mere five to seven ingredients and were loaded with glycol ethers. That is no longer the case, with zero-VOC paints flooding the market. Chemists must ensure that EVERY ingredient is VOC-free. So today, the chemical manufacturers have modernized additives to meet these requirements.

The choice of additives, such as wetting aids, dispersing aids, and the products that address sagging and viscosity consistency, will affect that overall formulation.

Now, let’s discuss what additives are available and how they affect these flows. Most people refer to these as associative thickeners.

Rheology modifiers

Rheology modifiers that thicken by volume exclusion include cellulosic ethers and alkali soluble (or swellable) emulsions (ASE). Cellulosic ethers are nonionic, water soluble polymers derived from natural fibers. They thicken by absorbing water and creating chain entanglement and flocculation.

There are basically three types that are commonly used in waterborne paints to control overall viscosity and flow. They are:

  • ASE = Alkali swellable emulsions
  • HASE = Hydrophobically modified ASE
  • HEUR = Hydrophobically modified ethoxylated urethane resins

Thickener selection affects many things with the paint, such as adequate covering power, good film build, brush-ability and viscosity. Then flow, sag and leveling are also affected. Even paint splatter resistance can be controlled with the correct thickener!

As an example, in medium and high PVC paints, the HEUR thickeners also improve water resistance and even wet scrub resistance in the final paint.

Associative and non-associative thickeners

The viscosity stability during colorant addition, there are some cases where the use of a combination of both associative and nonassociative thickeners help tailor the needs of the rheological profile for the best performance.

The “associative thickening involves non-specific interactions of hydrophobic end groups of a thickener molecule both with themselves and with components of the coating. The thickener produces a reversible, dynamic network of thickener molecules and other components of the coating. The thickening effect is caused by interactions of the hydrophobic end groups of the thickener with other components of the formulation.”3

The non-associative thickening as “thickening by an entanglement of water-soluble, high molecular weight polymer chains. The effectiveness of a thickener is mainly determined by the molecular weight of the polymer. Formulations thickened non-associatively have pseudo-plastic rheology with highly elastic properties. This produces good stabilization against settling out and low sagging even with high build coatings.”3

Surfactants are normally used in waterborne paints for different effects to the coating. These can affect the efficiency of thickeners as well, look at this possibility.

Adding universal colorants to paint can also disrupt and contribute to a lower viscosity due to the normal glycol ethers that are used in these products.

Recent Advances in Waterborne Coatings Technology

An overview of technological advances enabling superior performance in the architectural segment versus historical technologies. In subsequent articles, we’ll expand on these topics, as well as provide an overview and details of emergent waterborne coatings technologies that are closing the gap in the high performance industrial coatings market versus solvent-borne and solvent-less systems.

Many of the technological advances that exist today are the result of increasingly stringent regulations. Volatile Organic Compound (VOC) regulations automatically come to mind first, but there are several others. The elimination of the use of alkylphenol ethoxylates (APE’s) led to new classes of surfactants developed by many.

With the emergence of nanomaterials for applications spanning abrasion resistance, UV-protection, antimicrobial protection and thermal and electrical conductivity, there is a cautious concern that we may not have enough information to deem all of these materials as “safe.” Bioaccumulation and non-degradability add to this concern.

Technological advances have been made in the development of coalescing solvents for waterborne resins that meet the new regulations. Some of these have boiling points that exceed the test protocol and, therefore, are not volatile. Some approaches include the use of reactive diluents or materials made from plant products (“green chemistry”). Plasticizers, such as most phthalates and n-methyl pyrollidone (NMP), have been replaced with much less toxic materials.

In the preceding years, there has been a preponderance of advancements in waterborne resin technologies. In the past few years, they have been very large. Examples of large advancements include core-shell technology, latent crosslinking, 1K technologies that perform similar to 2K, resins with easy-clean properties, resins based on green chemistries, as well as many others. Fluoropolymers, which were once used only in OEM coil coatings, have made their way into other segments of the industrial market, and have some uses in architectural exterior coatings that can tolerate cost weatherability. Fluoroadditives for slip and block resistance, as well as mar reduction, have been in production for years. The resins will be discussed in more detail in future articles.

Although not as many advances have been made in pigment technology for the architectural coatings market, there still have been some significant ones. Infrared (IR) reflective pigments mitigate heat, providing as much as 25-30°F reduction in heat vs. conventional pigments in similarly painted structures.

There are new colored pigments that are more colorfast not only in masstones but in pastels. Surface chemistry has improved the weatherability characteristics of TiO2 and inorganic nanomaterials are highly synergistic to organic UV-absorbers and hindered amine light stabilizers (HALS) in exterior semi-transparent stains and varnishes.

In this general category are colorants. Many paint companies have eliminated ready-mix colors in favor of all colors prepared via custom mixing in the store. Therefore colorants had to be reformulated to be VOC-free, in addition to being compatible in both waterborne and solventborne paints and in all types of finishes. This was a particularly difficult challenge due to the high levels of glycols, among other things.

There are a multitude of new additives, including the UVA/HALS mentioned previously. Due to the changing waterborne resin chemistries as well as VOC regulations, additive suppliers have been busy creating chemistries that work as universally as possible.

Defoamers are a good example of a real challenge. Traditional defoamers might include mineral oil or a hydrocarbon carrier with hydrophobic silica and perhaps a fatty amide. They worked well for flat paints, but negatively affected gloss levels in semito high-gloss paints and could have a significant impact on the overall VOC level of a finished paint. All things have to be considered in the formulation of a zero-VOC paint.

At about the same time new defoamers were introduced to the waterborne flexographic ink market, there was a need for similar chemistry for different reasons in coatings. Analogs of the flexo defoamers are found in the waterborne architectural market, along with others.

Formulating Waterborne Coatings: a Checklist

What are formulating?

What traits?

Once these determined, some simple but necessary questions to start to round out the first series of items.

This technology has soared for virtually every surface imaginable. Most people think of house types of latex paint when one thinks of water-based products. And although that is still a very large market, other resin technology has allowed for water products to be applied to things like baseball bats, automobiles, trains, planes and most places in between.

The initial concern when products started going to water formulations: is this as durable as the solvent counterpart? The adage was that when adhesion was questionable, use solvent. We threw that one out the window! With the new polymer developments, newer additives development, and regulations that favor the water product over solvent, welcome to the world of waterborne/water-based products.

This list is always dependent on several factors, so let’s walk through the necessary steps to properly formulate these products. To use a checklist and find out:

  • what exactly the product will be
  • how it will be used
  • what inherent traits it needs to have

This may sound basic, but to those who are just starting out, or even seasoned professional formulators, it is a very necessary step.

What properties?

Next comes the starting qualities of the products are formulating. Again simple, but necessary. if the product should incorporate:

  • Adhesion qualities
  • Chemical resistance
  • UV resistance
  • Elongation factors

What application methods?

If the method of end user application will have any bearing on product development. They may use:

  • Brush
  • Rollers
  • Airless Sprayers
  • Conventional Spray
  • Curtain Coating
  • Roll Coating
  • Automatic Spray
  • Dipping
  • Robotics

There may be many additional questions, other than this intial list. Now that it was laid the groundwork for what to formulate, start to sift through the polymer selections and those technologies to start to what will fit with initial list.

Original Source

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