Importance of Yield Stress in Paints and Coatings

1. What is Yield Stress?
2. Types of Yield Stress and How to Measure Them
3. Common Approaches to Calculate Yield Stress
4. Yield Stress in Paints and Coatings Industry – Practical Aspects

What is Yield Stress?

Paints and coatings display complex flow behavior owing to their highly structured particle dispersions. The dispersions are formulated using several components, such as:

• Binders,
• Pigments,
• Solvents, and
• Several other additives, including defoamers, dispersing agents & surface modifiers.

Yield Stress is an important property to study the flow behavior of such complex structured fluids that include paints, inks, emulsions, dispersions, pastes, grease and foams.

Complex structured fluids have an internal structure that must be broken down to allow them to flow. Minimum stress required to break down the structure of fluids to initiate or maintain the flow is called “Yield Stress”.

Over their lifetime, paints are exposed to a wide range of shear rates and shear environments.

• During storage and transport, paints are expected to behave as a relatively low viscosity, free-flowing liquid. They experience low levels of shear (<1 s^-1) and to prevent phase separation, the paint is expected to have a high, solid-like viscosity.
• During processing, the pump-ability and energy required for mixing directly correlates with a viscosity at medium-to-high shear rates (10 to 1000 s^-1)
• Finally, during application, such as brushing, rolling and spraying, the paints are subjected to high shear rates (>100 s^-1) where it is expected to behave as a relatively low viscosity, free-flowing liquid.

Thus, paints need to be evaluated for their rheological properties to examine the overall performance, and yield stress is one of the important properties to consider while analyzing formulations’ rheological properties.

Theoretically, Yield Stress, τy, is defined as the minimum shear stress required to initiate flow. It is sometimes also referred as yield value or yield point. Its formula is:


where:

Yield stress is an important parameter during formulation to better understand product performance in several industries, including paints and coatings. The yield stress is a point before which the complex fluid does not flow unless the applied stress exceeds a certain value. The molecular network of paint stays intact when the applied shear force is smaller than the yield stress of the paint.

Below the yield stress, the paint behaves as a stable elastic solid and inhibits flow under relatively low stresses induced by gravity. Yield stress is an important property playing a significant role in the storage, transfer, packaging, and end-use performance of paints and coatings.

Types of Yield Stress and How to Measure Them

Yield stress measurements can be classified and measured as either static or dynamic.

1. Static yield stress is measured in an undisturbed fluid. It is more relevant for long-term suspension of solids in dispersion or to initiate flow in pumps.
2. Dynamic yield stress is a shear stress required by a fluid to go from a flow to a rest state. It governs leveling and sag resistance after an application of paint or coating.

At times, both dynamic yield stress and static yield stress may appear in the product due to the presence of two different structures of fluids.

Common Approaches to Calculate Yield Stress

Rheograms from rotational viscometer measurements are used to calculate yield stress. Applying rheological mathematical models also allow to obtain the required yield stress.

The two common approaches used in rotational rheometers are:

• Controlled rate approach – In this approach, the material being studied is placed between two plates. One of the plates is rotated at a fixed speed and the torsional force produced at the other plate is measured. Hence, speed (strain rate) is the independent variable and torque (stress) is the dependent variable.
• Controlled stress approach – In this approach, the situation is reversed. A torque (stress) is applied to one plate and the displacement or rotational speed (strain rate) of that same plate is measured.

The controlled stress approach is a preferable approach to determine apparent yield stress. In this method, it is possible to gradually increase the stress applied to the material and detect the point at which movement (yield) first occurs.

The methods to determine yield stress of paints and coatings formulations are:

• Vane Technique – The technique involves an apparatus with vane which turns slowly and stretches the internal structure of a material. As the structure is progressively stretched, the force required by the viscometer to maintain its rotational speed increases. The yield stress is determined as the minimum stress required for continuous rotation of the vane.


• Stress Ramp – In this method, the applied stress is slowly increased, and the movement of sensor is monitored. Initially there is a slight movement and as the structure stresses, and once the yield stress is reached, the rate of deformation increases drastically.
  
  Plotting the deformation vs. stress on log scales shows two distinct linear regimes – before and after yielding. The intersection of the two liner segments is the yield stress.


• Flow Curve Extrapolation Technique – In this technique, a flow curve is generated displaying a relationship between shear stress and applied shear rate. The curve is extrapolated back to the shear stress axis and the intercept is the yield stress.


• Oscillatory Amplitude Sweep Test – In this test, the measurements are made in oscillatory mode on a rotational rheometer, where the oscillatory strain amplitude is increased incrementally and the elastic stress (σ’) or component of the stress associated with G’ (elastic modulus) recorded. By recording σ’, only that stress required to stretch and yield any connected structure is measured associated with solid-like behavior.
  
  
  
  Oscillatory Amplitude Sweep Test by WEE-Solve

The stress ramp is the most frequently used technique to measure the yield stress today; however, the most appropriate method is one that best represents the process to estimate yield stress.

Yield Stress in Paints and Coatings Industry – Practical Aspects

In the paints and coatings industry, the yield stress is an important property used to measure performance-critical rheological changes occurring during the life cycle of paints and coatings. It:

• Controls formulation stability,
• Influences ways to remove paint from containers / dispensing,
• Governs application method (spraying, brushing, coating, or dipping), and
• Imparts sag and slump resistance.

The yield stress is critical for leveling and thickness uniformity in the coating application process. Its stress may also affect drying behavior and cause flow instability.

How Yield Stress Governs Leveling, Sag Resistance and Stability of Paint or Coating?

Leveling is an important property affecting smoothness, gloss, color, and mechanical behavior. A coating can flow laterally and diminish differences in the thickness of adjacent areas of the coating. Thixotropy measurements are important to measure the ‘flow’ attributes of a paint or coating and its leveling behavior. Thixotropy is the time-dependent viscosity change under the application of shear. For example, thixotropy is usually important for paint leveling or coating finish since lower viscosity will facilitate flow out before high viscosity is regained.

Sagging is an undesirable flow of a coating down a vertical surface. Whether a coating will sag or not depends on its thickness and its viscosity at low shear rates. The product of its density, gravitational constant, and its thickness must not exceed its yield stress for a coating to resist sagging.

Product stability at the time of manufacturing, transportation, storage and application is critical for paints and coatings. Yield stress measures the dispersion’s physical strength or ability to hold its shape.

The yield stress for paints and coatings must be optimized such that:

• The solid particles in the paint will not sink during storage (yield stress is high enough) and guarantee good storage stability of paint.
• The product can be worked easily (i.e., is mobile when some stress is applied) but then holds its shape rather than slumping after application.

Coatings Professionals – Stay Alert!

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