Storage Stability in Paints and Coatings

Long-term storage and extreme temperature variation can affect the performance of paint and coatings formulations. Factors, such as long storage time, exposure to extreme cold or hot temperatures can have drastic effects on the storage stability of the coatings.

Storage stability, the ability of a paint formulation to retain its original properties under normal storage conditions, becomes an important economic factor as some changes occurring in the paint system can be irreversible. Besides this, the storage stability of the paint system indicates the degree of chemical compatibility between individual ingredients in the paint.

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

Let’s discuss more about this crucial property, the factors affecting it and the methods to evaluate it in more detail.

Factors Affecting Storage Stability

Storage stability of paints after production is important to consider as it can directly impact the performance of your product and in turn your profits.

Several common factors could be the cause behind the instability during storage.

Let’s discuss them in detail.

1. Dispersion stability

Dispersants help maintain a state of dispersion by preventing settling or aggregation even during storage. A wrongly chosen dispersant can cause sedimentation or loss of stabilization of solid particles in your formulation.

The selection of the right dispersing agent is not only crucial during manufacturing but also to maintain the right stabilization of the system during storage and transport. You can go in-depth to understand the basics of dispersion here.

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 of solid particles. See the figure below.



For avoiding flocculation and stabilizing the solid particle in the formulation an efficient stabilization strategy is needed. Understanding which type of dispersing agent will provide you with which type of stabilization is the correct way to go.

Want to strategically approach the flocculation issue? Talk to one of our specialists today!

2. Role of Rheology

Rheology plays a vital role in the storage stability of the paints. The rheological properties like viscosity or thixotropy if incorrect can adversely affect the stability and disrupt the properties and performance of your paint.

For example, a high-viscosity paint will have more stability during storage as compared to a low-viscosity one as the high viscosity will prevent the solid particles from settling down and causing sedimentation.

With the use of a suitable rheology modifier, these types of issues can be easily fixed. Also, the right storage temperature and conditions add to maintaining the stability and performance of the paints.



Colloid Stabilization

Colloid stability is also very important for the long-term storage of a paint formulation. The colloid stability of any dispersed system is determined by the balance between attractive Van der Waals and repulsive electrostatic forces.

There are many solid particles in the paints which are suspended in a liquid medium. If the paint lacks stability, these solid particles tend to settle down throughout storage. This aggregation of particles leads to the poor performance of the paint, such as:

• Opacity issues
• Color instability
• Durability and performance issues

Factors that can cause colloid instability in paints include the viscosity of the medium, the charge of the solid particles, the absence or low amount of stabilizers, or even the size of the solid particles in your formulation.

Typically, the stabilization of colloids can be achieved by the addition of stabilizers. The stabilizers, when selected correctly, prevent a particle from interacting with other particles, so they can prevent aggregation or flocculation.

There are several sorts of stabilizers available in the market for example electrostatic stabilizers, steric stabilizers, or polymeric ones. To achieve the right stabilization choosing the correct type of stabilizer is important.

Understanding the type of stabilization your formulation requires, studying the solid particles and liquid medium, and then deciding the right dosage of additives can help in achieving an effectively stable system that maintains performance over a period.

3. Microorganism-caused Spoilage

Microorganisms can cause havoc when it comes to spoilage in stored products. 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

Although any coating is susceptible to spoilage, water-based coatings are more prone to it.

An efficient in-can biocide should have broad-spectrum antimicrobial efficacy and long-term protection. Paints may be kept in storage for a 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

Apart from biocides, the use of antioxidants or film formers is also common. Maintaining the right pH level and using solvents with apt volatility can also help prevent in-can spoilage.

4. Instability in Storage Temperature

Temperature variation significantly affects the storage stability of paints. Both too high or too low temperatures can impact the properties of paint and lead to low performance.

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

5. Chemical Reactions Occurring in the Can

As the paint is stored for long durations multiple types of chemical reactions can take place causing instability and other performance issues.

Some of the most common chemical reactions include:

• Oxidation: In this oxygen reacts with certain components in the paint formulation which lead to the degradation of organic compounds, such as resins or binders, resulting in changes in color, viscosity, and overall performance of the paint.
• Hydrolysis: Hydrolysis involves the breakdown of chemical bonds in the presence of water. If moisture enters the paint can, it can react with various components, such as pigments, additives, or emulsifiers, leading to hydrolysis. This can cause changes in viscosity, pH, and overall stability of the paint.
• Acid-Base Reactions: Acid-base reactions can occur between different components in the paint formulation, resulting in changes in pH. For example, acidic components can react with alkaline materials, leading to pH shifts that can affect the paint's stability and properties.
• Photochemical Reactions: Light, particularly ultraviolet (UV) radiation, can trigger photochemical reactions in paints. This can result in the degradation or discoloration of pigments, fading of colors, and overall deterioration of the paint's appearance. Light-sensitive components, such as certain dyes or organic compounds, are particularly prone to photochemical reactions.
• Metal Corrosion: If the paint can is made of metal and comes into contact with water or acidic components, corrosion can occur. This can lead to the formation of rust or other metal oxides, which can contaminate the paint and affect its stability and appearance.

These reactions can be avoided if regular monitoring is done when paints are in storage.

6. Quality default of the paint container

During storage the container is the only shield between the environment and your paints. therefore, using a container of great quality is super important.

A good quality container helps in:

• Protecting the paint from external environmental factors
• Preventing leakage and spilling
• Preserving paint properties
• Maintaining the structural integrity of the product
• And most importantly in passing regulatory compliance

Visible Signs Your Coating Lacks Storage Stability

Once your coating is out of manufacturing the next step for it is to undergo transportation and storage. It is not always that the storage conditions would be ideal for our product to maintain its quality throughout the whole storage duration.

As storage stability can be impacted by various factors we need to employ various strategies to ensure our product remains in the top-notch condition until it is ready to use.

Regular quality control testing and stability studies should be conducted to assess and monitor the performance and stability of paints over time. For maintaining the quality and stability of paint products in storage it is crucial to:

• Follow recommended storage guidelines
• Maintain proper temperature ranges in the storage facility
• Ensure proper ventilation
• Keep the product protected from light

But in some cases, even when the storage conditions are maintained the product still shows instabilities and signs of spoilage.

Below are some of the common signs you may witness in case your product lacks storage stability.

Phase Separation

In some cases, paints may experience phase separation, where different components separate into distinct layers. Proper mixing or agitation before use is necessary to restore homogeneity.

Phase separation can arise due to various factors, including:

• Ingredient Incompatibility: Coating formulations consist of multiple components and when these ingredients are not compatible with each other, they can separate into different phases during storage.
• Solvent Evaporation: Coating formulations often contain volatile solvents that can evaporate over time. As the solvent evaporates, it can lead to changes in the concentration and ratios of the remaining components, potentially causing phase separation.
• Temperature and Humidity Variations: Fluctuations in temperature and humidity levels can affect the stability of coating formulations. These variations can promote phase separation by altering the solubility and interactions between the different components.

Maintaining stable storage conditions, such as controlled temperature and humidity, can help prevent fluctuations that may trigger phase separation.

Sedimentation and Settling

Sedimentation occurs when the dense solid particles (fillers and pigments) in the coating are pulled down due to gravitational force. Settling refers to the process in which heavier solid particles in the paint formulation sink and accumulate at the bottom of the container. This results in an uneven distribution of these solid components throughout the paint. Settling is particularly common in paints with high pigment concentrations or those with larger particle sizes.



Sedimentation could be happening because the dispersion is stable (no re-agglomeration), but rheology is unable to prevent the migration of particles toward the bottom of the storage can. Choosing the right rheology additive can help in this case to avoid sedimentation so that there is the presence of a reversible three-dimensional network.

Or, it may be happening that the rheology is perfectly fine for the intended particle size, but poor pigment stabilization is causing the larger particle size than intended and leading to sedimentation.  

In this case, the formulation requires a suitable dispersing agent in the right ratio.  

Dispersing agents can make or break the performance of your final coatings, therefore becoming a crucial part of your formulation. Understanding how much dispersant should be added to your formulation is the key to a high-performing coating.  

Do not know which dispersing agent can be the best fit for your coating? Talk to our experts today and get a custom solution!

Change of pH

There are chances that the color or pH of the paint may change while in storage.

The coating formulation often has reactive or volatile components that can undergo chemical changes with time. These reactions can result in the release or consumption of acids or alkaline, leading to a change in pH. Adding buffering agents or stabilizers could help avoide the change in pH. Also, if the right storage conditions are maintained and the packaging of the product is good it is less likely that a change of pH could occur.

Change of Color

The color of paints may change during storage due to factors such as oxidation or UV degradation if correct conditions are not maintained in the storage facility.

Oxidation impacts the color to become darker or get an effect of yellowing specifically where organic pigments or dyes are used. This majorly happens due to changes in temperature, light exposure or reactive compounds present in the formulation.

UV rays from the sun and other artificial light sources can impact the color of the paints during storage where they fade or shift into another hue completely. Color shifts can also happen because of the incompatibility of the paint components.

To prevent this, we should:

• Use stable pigments and colorants, to reduce color degradation or reactivity
• Prevent air exposure by properly sealing paint containers
• Store paint in a controlled environment, avoiding extreme temperatures, high humidity, and exposure to direct sunlight or strong artificial light sources
• Follow storage guidelines provided by the paint manufacturer
• Conduct regular quality control testing to monitor any changes

Change of viscosity

Factors like solvent evaporation, particle settling and temperature variations can lead to viscosity changes in paints that are in storage.

The viscosity of the paint can both increase or decrease depending upon the conditions the paint is stored in. The formulations often have volatile substances and hence under high temperatures, the solvent evaporates leaving behind solid particles which in turn increases the viscosity of the paint.

Even particle settling could lead to a viscosity increase as the solid particles settle at the bottom of the container.

Another problem, problem of aging can also impact the viscosity of your coating when in storage. Aging occurs when the components in the coating undergo any chemical or physical change.

To avoid viscosity issues with your product when it is in storage you can periodically conduct viscosity measurements and ensure the conditions in the storage facility are up to mark.

Gelling

Gelling in paints refers to the phenomenon where the paint formulation undergoes a transformation from a liquid to a gel-like or solid state over time. This can render the paint unusable as it loses its original fluidity.

It may be happening due to bacterial degradation of the protein binder or other thickening agents. The use of contaminated tools and water/solvents or the mixing of different brands or types of paints are some of the causes leading to this paint defect during storage.

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

Other factors leading to the problem of gelling could be component incompatibility, chemical instability or temperature variation in the storage facility.

To prevent it we can:

• Use stabilizers or inhibitors that can delay gelling effect
• Make sure the storage conditions are optimal as per the product
• Ensure regular stirring and agitation
• Maintain proper sealing to minimize air exposure

Skinning

Skinning is the process of the formation of a skin layer on the container when moisture is lost from paint in high temperatures or due to air exposure.

The paint is exposed to air when the container is partially filled or improperly sealed. This leads to oxidation and in turn the surface layer of the paint dries and forms a skin.

Solvent evaporation could also be the reason behind the skinning issue. The solvent evaporates and the concentration of non-volatile components increases which leads to the formation of skin.

In cases where the paint is stored for too long or under high temperatures the skin is formed faster.

To prevent this we should ensure proper sealing, timely agitation, and surface protection. We can avoid using larger containers so that there is enough product to fill the containers and the product is not exposed to air.

Gassing

During paint storage gas bubbles may release or foaming may occur in paints. This phenomenon is called gassing which can adversely affect the paint quality and performance.

All issues like temperature variations, improper storage conditions, and incompatibility of components could be behind the issue of gassing.

To mitigate this issue we can ensure:

• The foaming agents are added to the paints and they are compatible too with the rest of the ingredients
• The formulation is properly balanced and compatible
• The handling of the containers remains gentle throughout the journey
• The storage conditions are apt for the product

Bad Odor Development

In some cases, the product develops a peculiar odor after it is stored for some time. This could impact the sale of such products and in turn a significant loss for the companies as the product is rendered useless.

This could be happening due to volatile organic compounds (VOCs). Paints that contain VOCs which are responsible for the characteristic smell of fresh paint. If the paint has been stored for a long time, the release of VOCs may diminish, resulting in a stale or unpleasant odor.

Contamination or microbial growth in the paint containers could also be leading to stale odors.

To overcome the issue we can:

• Ensure proper ventilation
• Check the containers for any leakage or damage to avoid future contaminations
• Evaluate the shelf life

Methods to Test Storage Stability in Paints

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, souring, lower layer rigidity, consistency, and the presence of lumps 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 determining 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.