Brittleness: How to Determine the Performance of Coatings?

1. What is Brittleness?
2. Role of Binders and Additives in Defining Coating’s Brittleness
3. Coating Deterioration – What are the Factors Influencing Brittleness?
4. Popular Test Methods to Measure Coatings Brittleness

What is Brittleness?

The strength and durability of a coating and its end-use performance go hand-in-hand. Coatings are exposed to different stresses, such as dimensional variations of substrates like wood, environmental & service conditions, etc., during their lifetime and require superior durability to withstand different stresses thus to keep performing.

Brittleness is one such important indicator that dictates the quality and durability of a coating. It is a common characteristic of many coatings that determines the performance of the coating for a given application under given conditions. For example, when a substrate coated with a more brittle coating is stressed, it is very much prone to failure by developing cracks and eventually lost adhesion.

So, how brittleness governs the long-term performance of any coating? Let us find out.

Role of Binders and Additives in Defining Coating’s Brittleness

Brittleness describes the property of a coating or paint film that fractures when subjected to stress. It is the lack of resistance of a dry coating film to cracking or breaking when bent or flexed. Brittleness can result from varied factors, such as the drying/curing method, plasticizer migration, film thickness, moisture, and temperature, etc.

We will discuss these factors in detail later but first, understand the role of binders and additives in defining the coating’s brittleness.

Binders and Brittleness

Binders used in paints and coating can be either linear or cross-linked.

• Linear binder molecules are like randomly intertwined strands of spaghetti. They are softer, more flexible, less brittle, more soluble, more water permeable, and less heat resistant than cross-linked binders.
• Cross-linking means that the linear, threadlike binder molecules have become laterally bonded together at various positions along their length. The resulting 3-D networks are usually stiffer, harder, more brittle, less soluble, less water permeable, and more heat resistant than linear binders.

List of Some of the Binders in Coatings with Brittle Behavior

Alkyd resins are very versatile but short and medium oil alkyds-based solvent-borne paints can embrittle upon long-term exposure.

Coatings based wholly or primarily on phenolic resins possess some desirable properties, but they suffer from being extremely brittle. The brittleness in phenolics is a consequence of both their rigid aromatic structure and their high crosslink density.

The more densely crosslinked epoxies (or any densely crosslinked coating) possess inherent brittleness.

PVC (Polyvinyl Chloride) by itself is not a suitable resin for coatings. It is inherently brittle, difficult to dissolve and has poor adhesion. Therefore, they are seldom used for coatings without some kind of modification. The modification was a large amount of plasticizer, which accounts for the generic name of ‘plastisol’ or further dispersions in organic media referred to as 'organosol.’

Pure silicone coatings, such as those used for very high - temperature applications such as heat stacks, have limited use because of their brittleness. Polysiloxane coatings have been developed to circumvent these limitations.

LMW resins have relatively high glass transition temperatures and are generally brittle.

Paint Additives and Brittleness

The mechanical properties, such as hardness, flexibility, brittleness, etc., are often controlled by additives used in paints and coatings.

For example, pigmentation gives strength and stiffness to a paint film. This is because the pigment particles can:

• Act as a load-bearing part of the film, and
• Resist movement within the film.

However, a higher degree of pigmentation concentrations (i.e., higher than the CPVC (Critical Pigment Volume Concentration)) can sometimes make coating films brittle and chalky.

Another interesting class of additives that govern paint film brittleness is “plasticizers.” Plasticizers are selected to meet the requirements of the coating that may include low-temperature flexibility. They soften and decrease the brittleness of a paint film. They are small molecules that are thoroughly mixed to separate the binder molecules and pigment particles from themselves and each other. This separation reduces the attractive forces between the paint components and provides a more flexible, softer, less brittle film.

But plasticizers can slowly evaporate or migrate from the film over a period of months or years. The result is that many paints tend to become harder and more brittle with age.

Coating Deterioration - What are the Factors Influencing Brittleness?

Now, not only restricted to binders and additives, but different conditions e.g., environment, temperature, and strain rate can affect the embrittlement of coatings. Let us discuss these factors in detail. 

Curing and Internal Stress

Upon drying of liquid coatings, shrinkage occurs in the film due to the loss of volatile materials. The shrinkage provides some initial stress to the coating and to the adhesion of the coating. As the paint dries further, and particularly if chemical crosslinking occurs, further stresses are applied leading to further hardening, increasing brittleness, and tensile stress to the coating. For example, oil-based house paints become harder and embrittle on exposure as they continue to cross-link.

• Under-curing generally results in a film that has a poor solvent and chemical resistance.
• Over-curing can cause a coating to be too brittle for it to withstand environment or service conditions without cracking.

Another type of internal stress is known as ‘aging’ that refers to change in a coating’s physical or chemical properties as it is exposed over a prolonged period to the degrading effects of chemicals, light, moisture, or heat. These environmental factors can alter the chemical structure of the coating, resulting in changes in gloss, brittleness, adhesion, etc.

Temperature

The flexibility and toughness of coatings are dependent on temperature. For example, as we know, materials are more brittle at lower temperatures. A coating may appear flexible at 70°F, yet will crack badly when tested at 0°F. This difference in behavior is a consequence of the material’s glass transition temperature, Tg.

Brittleness dependency on temperature is particularly true of thermoplastic coatings, but it also is a factor for thermoset coatings. Coatings at a temperature below Tg are hard and brittle. Above Tg, the film will be more flexible and less susceptible to the formation of fracture.

Impact

Mechanical damage on the coating surface due to sudden impact (dropped tools or stones) can cause the coating to crack and/or spall. Impact damage is greatest when a coating has high internal stress and is very brittle or is at or below its glass transition temperature (Tg).

Energy

Energy acting on a coating (or material) can degrade a material by breaking or interfering with the chemical bonds holding the resin (or a molecule) together and to a substrate. For example, prolonged exposure to UV light can degrade and fade coating over time, thus affecting the mechanical properties. Also, long exposure to heat energy can result in molecular vibration and thus can form free radicals. The presence of free radicals can lead to additional crosslinking between independent macromolecules, which, in excess, may reduce impact strength and create brittleness in the coating film.

Abrasion

Abrasion occurs as a result of scraping, scuffing, or erosion due to contact with small moving particulate matter such as sand or slurries. More brittle coats are more susceptible to abrasion damage than rubbery softer coatings.

Paint Mixing

Ambient-cure, two-component, or catalyzed binders such as epoxies and urethanes require a proper mixing ratio to allow the finished film to have its desired properties. Improper ratio mixing can lead to a host of final film problems, including inadequate curing, brittleness, loss of extensibility, etc.

Chemical Permeation

Exposure to strong alkalis such as sodium, potassium, and calcium hydroxides attack susceptible chemical groups in coatings. All coating resins containing ester groups are susceptible to such attack.

For example, in oil-base coatings and alkyds, alkali attacks the ester linkage of drying oils resulting in bond breaking which reduces molecular flexibility and embrittles the film; this ultimately leads to resin deterioration and the formation of a sticky soft coating under damp conditions, or a brittle powdery coating when dried.

Permeation of Oxygen

Oxygen species (Ozone) is a strong oxidizer that reacts with most organic materials, including coatings, to form free radicals and ultimately photochemical embrittlement degradation.

It is important to note that there are several other factors that govern brittle behavior of the coatings which are not discussed here for now.

Popular Test Methods to Measure Coatings Brittleness

The brittle-coating method of experimental stress analysis technique for solving/determining a coating cracking problem. The method is based on the adhesion of a coating with brittle characteristics to the specimen being studied.

When the specimen is subjected to external loads, the thin brittle coating cracks under tensile stress. However, the behavior of the coating is quite complicated as it depends on the number of parameters, such as:

• Coating Thickness
• Coating Temperature
• Creep in coating during testing
• Moisture
• Curing time of the coating

Impact Resistance & Hardness Testing of Coatings

Impact tests signify toughness, or impact strength, of a coating to absorb energy under mechanical load. The drop impact test (or falling-weight impact test) is a commonly used test method to determine the impact resistance.

Impact resistance tests provide methods for obtaining information about the degree of cross-linking and cure.

• An under-cured coating may exhibit a low impact value, but as cross-link density increases, impact values improve.
• An over-cured coating may be brittle and have a low impact value.

There are several standard test methods to study the rapid deformation of paints or coatings under the influence of impact forces and thus evaluate the effect of deformation. Related standards are mentioned in the table below.

Standard	Title
ASTM D2794	Test Method for Resistance of Organic Coatings to the Effects of Rapid Deformation (Impact)
ASTM G14	Standard Test Method for Impact Resistance of Pipeline Coatings (Falling Weight Test)
ISO 6272	Paints and Varnishes - Rapid-deformation (Impact Resistance) Tests
ASTM D2137	Test Methods for Brittleness Point of Flexible Polymers and Coated Fabrics
ASTM D3363	Test Method for Film Hardness by Pencil Hardness
ASTM D2134	Test Method for Determining the Hardness of Organic Coatings with a Sward-Type Rocker
ASTM D4366	Test Methods for Hardness of Organic Coatings by Pendulum Damping Tests