Flexibility of Paints and Coatings: Everything That You’ll Want to Know

1. Mechanical Properties of Coatings
2. Basics of Coating's Flexibility
3. What Factors Govern Flexibility of Paints and Coatings?
4. Popular Methods to Test Flexibility of Coatings

Mechanical Properties of Coatings

The mechanical properties of coatings strongly influence their performance. The coating on substrates is regularly subjected to several types of mechanical or physical stresses during a lifetime resulting from impact, harsh environment, or dimensional variations of the substrate. These stresses can result in film damage.

So, it is important for coatings to show the desired balance of mechanical properties, such as impact resistance, flexibility, hardness, toughness, etc. to meet service requirements for a specific application.

As we have already discussed the importance of impact resistance, flexibility and toughness are the other two important mechanical properties of coating and paint formulations that influence the final performance and contribute to coating longevity. For example, for achieving good outdoor durability of coated wooden substrates, the coating must be flexible enough to compensate for the changing wood dimensions due to temperature or humidity variations.

Flexibility and toughness describe the ability of a coating to withstand different stresses.

• Flexibility is the ability of a coating to bent or flexed without getting cracked or undergoing other failures. It is the coating’s resistance against damaged when the substrate and coating are deformed.
• On the other hand, toughness is the coating’s ability to withstand great strain imposed in a short time period without tearing, breaking, or rupturing.

Let's understand the basics of flexibility in paints and coatings along with key factors and test methods governing it.

Basics of Coating's Flexibility

Flexibility is a material property that is described by flexural modulus or bending modulus of elasticity. Flexural Modulus denotes the ability of coating material to bend. It is a measure of a coatings stiffness/flexibility when a force is applied perpendicular to the long edge of a sample.

Flexural modulus is called “modulus of elasticity in bending,” but also known by the modulus of elasticity, elastic modulus, or simply modulus.

• The International Standard unit of Flexural Modulus is the pascal (Pa or N/m² or m^-1.kg.s^-2).
• The practical units used are megapascals (MPa or N/mm²) or gigapascals (GPa or kN/mm²).
• In the US customary units, it is expressed as pounds (force) per square inch (psi).

The standard flexural strength test for materials, including coatings, paints and films, is ASTM D790. The different varieties of this test include three-point and four-point methods. For ASTM D790, the pressure is applied to the central region of a bendable sample. The test is stopped when the specimen reaches 5% deflection or the specimen breaks before 5%.

This test holds prime importance to evaluate the coatings that are applied to flexible substrates, like unsecured strips of thin metal or plastic, to determine the material’s quality. (Other flexibility tests are described below)

What Factors Govern Flexibility of Paints and Coatings?

It is usually found that flexibility is the property majorly influenced by binder. The binder cements the pigment particles into a uniform paint film and makes the paint adhere to the surface. The nature and amount of binder decide most of the paint's performance properties. However, there are several other factors as well that govern the flexibility of a coating.

Let us discuss them separately.

Binder System

The basic property that affects the flexibility of coating is the viscoelastic behavior of the (coating) formulation. Viscoelasticity is the property of materials that show both viscous and elastic characteristics when undergoing deformation.

Coatings are made up of polymer systems and polymers that are viscoelastic in nature. Therefore, coatings show elastic recovery and yet will flow with time when placed under stress.

The main factors how binder system governs flexibility also include:

• Molecular weight and flexibility of resin (and crosslinker, if present)
• Crosslink density of the binder system
• Compatibility of binder system with all ingredients (no phase separation)

Most synthetic binders are softer and more flexible than hard resins. Consequently, they impart good elasticity, impact resistance, and improved adhesion. For example:

• Acrylic polymers are the binder of choice in producing quality high-performance latex paints. Typically, the highest quality latex paints are used for a wide variety of architectural coatings where 100% Acrylic Latexes offer superior adhesion, long-term flexibility, breathability, toughness, etc.
• Radiation-curable coatings based on polyether, polyester, or urethane acrylates offer a wide range of properties. Polyester acrylates combine good abrasion resistance with high toughness, while polyether or urethane acrylates provide flexibility and elasticity. The flexibility of a given urethane acrylate can be enhanced by increasing the linear molecular weight between crosslinks. UV-curable coatings with high flexibility and abrasion-resistance are needed for flooring applications.
  
  
  
  Flexibility of UV-curable EPD coatings
  Source: Research Gate

The physical properties that you wish in your final formulation are to be achieved by proper choice of flexible/rigid monomers, or proper binder.

→ View All Commercially Available Polymers Suitable for Coatings

Solid Particles and Pigments

The solid particles or pigments influence several properties, such as flattening, hue, gloss as well as flexibility, amongst others in a given coating system.

Flexibility is governed by the degree of loading of the system with solid particles. The pigment loading of a coating is quantified by Pigment Volume Concentration. Pigment volume concentration or PVC, is the term used to describe the volume (not weight) of pigment in a dry paint film.

When formulating paint, the pigment/binder ratio used plays a vital role. This ratio is accounted through the PVC and, and it is key to paint’s aesthetics and physical properties (durability, flexibility...). It tells us how much of the volume of the paint film is made up of pigment versus the amount made up of binder.

It is known that there is a critical point, named as critical pigment volume concentration (CPVC), that represents a point when the amount of binder added is the minimum to fill the voids between the pigment particles. Paints made at pigment levels above the critical PVC result in a paint film that has an increasingly large number of voids, which in turn leads to a layer that is increasingly fragile and less resistant to physical damages.


The type of solid particles also plays a key role in determining the flexibility of coating. For example, platelet-shaped solid particles of high flexibility will improve the flexibility of the coating.

Select the right pigment type for paints and coatings in order to achieve the desired color, physical and chemical property in your formulations.

Curing Mechanism

During curing or film formation, the binder matrix contracts because of reaction shrinkage, for example in solvent-free and UV-curing systems. This can result in stress build up in the binder matrix during film formation.

Also, curing any coatings below or beyond the proper range can generate difficulties for flexibility of coating. An indication of under cure would be open cracks or even shattering when test panels are bent.

Addition of Additives (E.g. Plasticizers)

Plasticizers are added to paints and coatings to impart toughness and flexibility. Plasticizers lower the Tg of the resin and elasticize the coating. When added to a coating formulation, plasticizers show the following features:

• They improve film formation and gloss.
• They increase the resistance of a coating to heat and light.
• They enhance mechanical properties like adhesion, elongation, etc.

Plasticizers interact physically with the polymer binder molecule, without a chemical reaction and form a homogeneous system. Most binders used today are inherently flexible and can be regarded as "internally plasticized" resins.

Plasticizers are used to allow initial film forming and to achieve the right amount of film flexibility to avoid cracking and peel-off.

Learn how to select the perfect plasticizer for your coating application by understanding the role of plasticizers along with their benefits and sub-categories.

Other Crucial Factors

Interface Strength – The strength of the interfaces within the coating, especially the interfaces between the solid particles and binder matrix govern the flexibility of a coating film

Coating Thickness – When the coating film is applied below the target thickness range, the physical properties of the film itself may be jeopardized. The decreased flexibility of the coating corresponds to the reduced cohesive strength of the film and can result in cracking or delamination.

Temperature – Flexibility decreases if the coating temperature decreases. The glass transition temperature (Tg) is one of the most vital characteristics of the coating to determine its flexibility. For a coating to have reasonable flexibility, the Tg must be above the use temperature. This is particularly true of thermoplastics coatings.

Strain Rate – Flexibility is also considerably influenced by the strain rate. This is the rate at which the coating is elongated and is expressed in % per minute. In general, flexibility decreases if the strain rate increases.

Adhesion – Flexibility is closely related to adhesion between coating and substrate, being another key property of coatings. Good adhesion tends to give better flexibility than does poor adhesion.

Popular Methods to Test Flexibility of Coatings

When coatings are applied to substrates that can undergo deformation (e.g. plastic or wood), then it is imperative for a coating to be able to withstand the material's deformation without failure. Testing the flexibility of the coating can be helpful to prevent this type of coating failure from occurring. A common way to test coating stiffness is to apply the coating to a base material, let it cure, and then bend the base material and the coating around a mandrel.


Bend and impact tests are employed to evaluate the flexibility of coatings. These tests measure the deformation due to bending and impact. Deformation is distinguished with respect to (testing) the flexibility of a coating (on a substrate) in two types:

• Slow deformation (determined using Erichsen Slow Penetration (ESP) and bending of substrate and coating by using a mandrel)
• Fast deformation (e.g. determined by impact by letting a weight fall on the coated substrate, either on the coating, called direct impact or on the backside of the panel, called reverse impact).

Overall, the different test equipment used to test flexibility (and toughness) of coatings can be categorized into the following groups:

• Mandrel bend test
• Impact resistance test
• Cupping test

Mandrel Bend Test

To determine the flexibility of a coating, there are two types of mandrel test – conical and cylindrical that are often employed. ASTM D522 is a standard test method for mandrel bend test of attached organic coatings. It is used to determine the coating resistance to cracking, upon bending once they were applied and cured on sheet metal or other flexible materials.

In this test method, fully cured coated panels (substrates of sheet metal or rubber-type materials) are bent over a mandrel and the resistance to cracking of the coating is determined.

• In Test Method A the coated substrates are bent over a conical mandrel.
  
  
  Source: TQC


• In Test Method B the coated substrates are bent over cylindrical mandrels of various diameters.
  
  
  Source: TQC

Coated panels are elongated when the substrates are dimensionally unstable or are bent during the manufacture of articles or when the articles are abused in service. This elongation, if the coating/curing process is not adequate can lead to cracking causing problems ranging from poor appearance to premature corrosion of the panel substrate. ASTM D522 is used in rating the coatings for their ability to resist cracking when this happens. They have been useful in evaluating the flexibility of coatings on flexible substrates.

Impact Resistance Test

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.

The test standards used for impact resistance testing include:

• 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

Learn more about commonly used test methods to measure the impact resistance of paints and coatings.

Cupping Test

Cupping tests are designed to test the elongation and deformability of lacquers and protective coatings applied to metal substrates. In this test, a coated sheet metal is subjected to gradual deformation by a polished die. This deformation is caused by the die being pushed from beneath the coating – i.e. from the reverse side of the sheet metal. The endpoint is when the coating begins to crack. 

The related test standards are:

1. ASTM E643 - 15: Standard Test Method for Ball Punch Deformation of Metallic Sheet Material
   
   ASTM E643 testing covers the procedure for conducting the ball punch deformation test for metallic sheet materials intended for forming applications. The test applies to specimens with thickness between 0.008 and 0.080 in. (0.20 and 2.00 mm). (This includes the Erickson Test and Olsen Cup Test).


2. ISO 1520:2006: Paints and Varnishes — Cupping Test (BS 3900-E4: Methods of Tests for Paints)
   
   ISO 1520:2006 specifies an empirical test procedure for assessing the resistance of a coating of paint, varnish, or related product to cracking and/or detachment from a metal substrate when subjected to gradual deformation by indentation under standard conditions.

Note: The tests are performed on relatively fresh coating films or freshly coating substrates, so it is imperative to note that over time due to the volatilization of plasticizers and chemical changes like crosslinking, coating films tend to lose flexibility.