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Storage Stability in Paints and Coatings

Storage Stability in Paints and CoatingsLong-term storage and extreme temperature variation can possess performance concerns in the paint and coatings formulations. Factors, such as long storage time, exposure to extreme cold or hot temperatures, sedimentation, spoilage and so on can have drastic effects on the chemical and physical properties of coatings.

Any changes can cause paints to have different properties than when they were originally manufactured. Therefore, storage stability is an important property of paints and coatings formulations which provides an indication of the degree of chemical compatibility between individual components in the formulation.

It helps to understand the complex dynamics of particles in the formulation and their impact on the shelf life of paints and coatings. It is one of the important properties to evaluate the quality and performance of coating products.


Let's discuss the factors affecting storage stability & methods to evaluate it...


Factors Affecting Storage Stability


In powder systems, the storage of ready-made paints in many cases is not a problem because the raw materials are in solid-state. While, in liquid systems (solvent-based and especially in water-based systems), the storage stability can be a problem both in a chemical and physicochemical sense.

Properties of prime importance to consider while storing paints and coatings products include:


Some of the important factors that can have a direct impact on these properties are discussed below.


Sedimentation and Settling


Solid particles like pigments and fillers play a key role in the coatings manufacturing and their final performance. If the dispersed pigments and fillers do not remain in suspension, they will build up a layer of sediment on the bottom of the container. This will ultimately lead to inhomogeneity in the paint or coating.

The important factors contributing to the sedimentation speed include:

  • Density
  • Size of particles
  • Viscosity of liquid


Role of Rheology


Rheology plays a vital role in avoiding sedimentation and settling. Rheology and yield stress are important properties that determine storage stability in the paint. Paint with high yield stress has a strong internal network, thus assures storage stability.

With the use of a suitable rheology modifier, a strong three-dimensional physical network can be created which can resist the gravitational force during storage.

The higher coating viscosity prevents pigment from settling during storage and ensures proper application viscosity. There is a wide range of organic and inorganic thickeners to choose from as listed below:

Associative Thickeners   Non-associative Thickeners   Inorganic Thickeners




Flocculation


Flocculation is an undesired process that can take place during production, storage, application and film formation. It is the spontaneous gluing together of separated solid particles in liquid systems.

Flocculation can be prevented by introducing repulsive forces onto the solid particles. It can be achieved through adsorbed dispersing agent molecules onto the surface solid particles. See the figure below.

Solid Particles Stabilized Using Dispersant
Solid Particle Stabilized in a Liquid System by Using Dispersant

Colloid Stabilization


Colloid stability is also very important for the long-term storage of a paint formulation. Aggregation of particles leads to the poor performance of the paint, such as its opacity, color, and durability. The colloid stability of any dispersed system is determined by the balance between attractive Van der Waals and repulsive electrostatic forces. The factors that affect colloid stability, such as the addition of electrolytes, are described in terms of theories flocculation.

Typically, stabilization of colloids can be achieved by the addition of stabilizers. The stabilizer prevents a particle from interacting with other particles, so they can prevent aggregation.


Rheology & Viscosity Made Easy


Spoilage – In-can Preservation


Water-based coatings are prone to microbiological spoilage. Conditions, such as microorganisms presence, different packaging and storing conditions can pose negative effects in the formulation, such as:

  • Foul odor
  • Loss of viscosity
  • Gassing
  • Phase separation
  • Discoloration
  • pH drop

Hence, in-can preservatives are widely used to protect the coating from the above-mentioned issues and extend the storage life.

An efficient in-can biocide should have a broad-spectrum antimicrobial efficacy and long-term protection. Paints may be kept in storage for long and exposed to wide temperature variations. Optimum preservation can only be achieved if the in-can preservative is compatible with coating ingredients and stable over time.

 »  Also Read: Tips to Select the Right Biocide for Paints and Coatings


Frosting – Storage Temperature


All water-based paints must be protected from frost in storage. If damaged by frost, then emulsions can become unstable and the solids separated. The damaged products are unsuitable for use even after warming. Ideally, products should be stored between 5°C and 20°C.


Skinning and Gelling


Skinning is the process of formation of a skin layer on the container caused due to:

  • Use of non-airtight container
  • Poor formulation, such as lack of anti-skinning agent
  • Storage under excessively warm condition

Gelling (decreased viscosity) is caused by bacterial degradation of the protein binder or other thickening agents. The use of contaminated tools and water/solvents or mixing of different brands or types of paints are some of the causes leading to this paint defect during storage.


Methods to Test Storage Stability in Paints


Methods to Test Storage Stability

ASTM D1849 – Standard Test Method for Package Stability of Paint


This test method covers the change in consistency and certain other properties that may take place when liquid paint of either the solvent-reducible or water-reducible type is stored at a temperature above 0 °C (32 °F). The samples are observed for any skinning, corrosion, the odor of putrefaction, rancidity, or souring, lower layer rigidity, consistency, and the presence of lump or streaks in the brushed films.


ASTM D3925 – Standard Practice for Sampling Liquid Paints and Related Pigmented Coatings


This practice describes the test methods of taking representative samples of fluid paint or pigmented coating products from containers of any type and determine their uniformity and compliance with specification requirements. Related ISO standards include ISO 1513 and ISO 15528.


ASTM D869 – Standard Test Method for Evaluating Degree of Settling of Paint


Paints, if not formulated or processed properly may settle excessively. Paint that settles excessively is difficult to reincorporate into the paint system causing time delays or valuable pigment being left in the drum. This test method covers the determination of the degree of pigment suspension and ease of remixing a shelf-aged sample of paint to a homogeneous condition suitable for the intended use.


ASTM D2243 – Standard Test Method for Freeze-Thaw Resistance of Water-Borne Coatings


With the rising use of water-borne coatings due to environmental concerns, the major concern associated with these coatings is their “freeze-thaw stability” when they are shipped during cold weather.

Further, these coatings may experience cycles of freezing and thawing. Cycles of freezing and thawing cause more damage to waterborne coatings than when the coatings are subjected to steady freezing.

ASTM method “ASTM D2243 – Standard Test Method for Freeze-Thaw Resistance of Water-Borne Coatings” covers a procedure to evaluate the effect of freeze/thaw cycling on the viscosity and the visual film properties of water-borne coatings at −18°C (0°F).

Also, there are test methods devised to speed the evaluation of a coating’s behavior under storage conditions. In such test methods, coatings performance is evaluated by subjecting them to extreme conditions.


Applied Rheology for Improved Performance in Paints and Coatings

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