Internal Stress in Coating and Paint Films

1. Why does Internal Stress Occur?
2. Sources of Internal Stress and Their Solutions
3. How to Avoid Coating Film Failure?
4. Test Methods to Determine Internal Stress

Internal stress is considered to be one of the primary causes of premature coating failure as it may reduce coating adhesion and lead to fatigue resistance. If the internal stress exceeds the tensile strength of the film, it can lead to serious defects or film failure, such as cracking, bending and peeling.

Thus, internal stress is an important property to understand the performance of the coating and paint films upon curing and analyze degradation issues.

Learn why internal stress occurs and its sources in detail...

Why does Internal Stress Occur?

Internal stresses occur when the applied coating cures or sets from a liquid to a solid. Over a period of lifetime, the coating tends to shrink due to:

• Drying process and film formation (evaporation of solvent/water)
• Cross-linking during the curing process
• Aging or service conditions (ultraviolet radiation, temperature, and humidity)
• Differences in thermal expansion coefficients between coating and substrate

Sources of Internal Stress and Their Solutions

There are many sources of internal stress, but the four most common reasons are:

1. Stress concentration due to imperfections in the applied material or at the interface

Localized stress concentrations may occur due to irregularities, such as voids and defects within the applied material or at the interface.

Solutions to Reduce Stress Concentration


• Such stress can be reduced by using an adhesive or coating that wets the substrate surface and has the ability to fill all of the surface micro-roughness. To achieve the proper degree of flow, it is required to control the viscosity of the polymer or by using thickening additives or rheology modifiers.
• Another method of reducing stress concentration from voids and other defects is to introduce a toughening agent into the coating formulation. The toughening agent will provide a stress relief mechanism to interrupt the growth of cracks.


2. Stress due to the difference in thermal expansion coefficients between the coating and the substrate

Stresses due to differences in thermal expansion must be especially considered when the adhesive or coating solidifies at a temperature that is different from the normal operating temperature. The thermal expansion coefficients for some common polymeric materials and metal substrates can be more than an order of magnitude apart.

Solution to Reduce Stress Concentration

An effective solution is to adjust the expansion coefficient of the applied material to a value that is near to the substrate. Coatings can be formulated with various fillers to modify thermal-expansion and limit internal stresses.






3. Stress due to shrinkage of a coating as it cures

The shrinkage stress is the most important one of the internal stresses of the coating. Nearly all polymeric materials shrink during solidification. At times, they shrink due to escaping solvent, leaving less mass in the bond line. When a paint film solidifies, volume changes in the film occur as a result of:


• Evaporation of solvent,
• Rearranging of molecules, and
• Polymerization

The shrinkage that occurs due to the loss of volatile materials 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 to the dried film leading to the possible formation of cracks and voids within the bond line itself.

If the substrate is rigid, the molecule movements near the interface are hindered and internal stress may develop.

Solution to Reduce Stress Concentration

A possible solution to the shrinkage problem is to enhance the coating flexibility.

Other common factors that affect internal stress include:

• Glass transition temperature
• Young’s modulus of the coating
• Crosslinking
• Plasticizing effect
• Pigmentation effect
• Effect of coating thickness

Also, it is equally important to note that at temperatures below the glass transition temperature of the coating, rearrangements of the binder molecules are hindered and volume changes in the film are directly converted to internal stress.

How to Avoid Coating Film Failure?

As mentioned above, surface defects, such as cracking, bending and peeling are common during coating film failure and internal stress can be one of the major reasons behind the failure. To avoid such defects, it is required to use the correct coating systems, application techniques, and dry-film thicknesses. Alternatively, using a more flexible coating system can help in minimizing the failure risk.

The Optimum Use of Additives

A number of formulation variables influences the internal stress in a coating film. These variables include the extent of pigment loading and the presence and nature of coalescing agents.

For the pigmented films above critical pigment volume concentration (PVC), there can be a rapid-build-up to maximum internal stress followed by a sharp decrease. The sharp decrease results from the formation of microfissures and dislocations at the boundary between pigment and binder.

It is imperative to select the right pigment and quantity in the formulation imperative to avoid high internal stress and film failure.

Another effective method to enhance the coating film formation process and lower minimum film forming temperature* (MFFT) is the addition of a coalescing agent or plasticizer to the coating formulation. A coalescing agent helps to lower the glass transition temperature (Tg) as well as increase the diffusivity of the polymer chains by increasing the free volume.

The effectiveness of a coalescing agent depends on factors, such as:

• The volatility or evaporation rate of coalescent. It signifies the time period of the presence of plasticizing molecules in the film.
• The compatibility of the coalescent and polymer i.e. how efficiently the coalescing aids lowers the Tg of the polymer.
• The solubility of the coalescing aid which determines the partition of coalescing molecules from each other while in the suspension.

*The minimum film-forming temperature is the lowest temperature required to coalesce a polymer and applied to a substrate into a thin film. The minimum film-forming temperature of a polymer is extremely important to know and understand when applying a coating to avoid a coating failure.

Test Methods to Determine Internal Stress

1. The internal stress of paint film is measured by ASTM D6991 - 17e1 test based on the beam deflection method. ASTM D6991 - 17e1 is a standard test method to measure internal stresses in organic coatings. This method is suitable for the coatings where:
   • The modulus of elasticity of substrate (Es) is significantly greater than the modulus of elasticity of a coating (Ec).
   • The thickness of the substrate is significantly greater than the thickness of a coating.

*This method has been found useful for air-dry industrial organic coatings, but the applicability is yet to be assessed for thin coatings (thickness <0.0254 mm (.001 in.), for powder and thermally-cured coatings.


2. Another test method to determine internal stress in the coating film is ASTM B975 - 18e1 based on deposit stress analyzer method. It is a standard test method to measure internal stress of metallic coatings by split strip evaluation.
   
   This test method is quantitative to determine the internal tensile or compressive stress in applied coatings. It is applicable to metallic layers applied by the processes of electroplating or chemical deposition that exhibit internal tensile or compressive stress values from 200 to 145 000 psi (1.38 to 1000 MPa).
   
   
   
   Deposit Stress Analyzer
   Source: Researchgate.net


3. Next method, ASTM B636 / B636M - 15 covers the use of the spiral contractometer to measure the internal stress of metallic coatings as produced from plating solutions on a helical cathode. The test method can be used with electrolytic and autocatalytic deposits.
   
   Internal stress values obtained by the spiral contractometer do not necessarily reflect the internal stress values found on a part plated in the same solution. Internal stress varies in many factors, such as coating thickness, preparation of substrate, current density, and temperature, as well as the solution composition. Closer correlation is achieved when the test conditions match those used to coat the part.
   
   
   
   Spiral Contractometer by Specialty Testing