Friction between viscosity and thixotropy

Anyone who has tried to putty or paint a car has probably noticed that one of the basic parameters is given in the technical information of the products - viscosity. When you talk to a salesperson, you are likely to hear the sentence: This product has very good thixotropic properties. Does the fact that the product is thixotropic affect its viscosity?

An important property of paint

Let's start with the answer to the question of what viscosity is. Simply put, viscosity is internal friction resulting from the relative motion of molecules. The ratio of dynamic viscosity to the density of a fluid determines its kinematic viscosity. It is the kinematic viscosity that determines the properties of a paint product. For example, we can try to mix an epoxy primer and then a base coat for comparison. Which substance will be more resistant? Definitely the primer, as it is thick, which means that the molecules are more tightly packed, resulting in a higher product weight. The viscosity will change depending on the temperature – it will be lower in summer (the product is thinner) and higher in winter. This is why it is so important to maintain a constant temperature, especially when it is below zero outside.

Why is it important to measure the viscosity of paint?

The viscosity parameter is extremely important for the correct painting process. The products used in the repair of our cars (including paint primers, car putties, car paints) are representatives of rheological liquids, i.e. liquids that change over time and under its influence. The longer the shear stress (in this case, our stirring) acts, the greater the changes in the structure. The viscosity of the paint decreases under stirring, giving the impression of a thinning material, and increases when stationary, making the paint appear thicker. The viscosity of the paint is measured with a device called the Ford flow cup. 100 cm3 of paint is poured into a special DIN 4 cup. The time it takes for the material to flow through a four-millimetre hole is then observed. The kinematic viscosity is measured in degrees (st) and centistokes (cst).

Characterisation of thixotropy

The properties of thixotropy are determined by the qualitative and quantitative composition of the dispersed phase of a chemical substance. The values used to determine thixotropy are the highest dynamic viscosity, the lowest dynamic viscosity and the final shear stress. These are important values to consider when working on vertical or horizontal surfaces. Thixotropy literally means change during touch (thixis – touch and trop – change). In practice, thixotropy is the property of dispersed substances, i.e. substances composed of many components, usually a mixture of liquid and a dispersed dense phase, to restore the yield point at rest, i.e. in the absence of mechanical influence on the substance.

Thixotropy and rheopexy

It is important not to confuse the concept of thixotropy with the phenomenon of pseudoplasticity. Substances characterised by the latter property are characterised by a decrease in viscosity when subjected to shear stress. In the case of thixotropic substances, however, the opposite is true - their viscosity decreases when subjected to constant shear stress. When thixotropic substances are subjected to a constant deformation rate, the shear stress decreases over time. The viscosity and shear rate of some substances under constant ambient conditions change over time. A substance is thixotropic if its viscosity decreases over time, whereas if the viscosity increases, we are dealing with rheopexy, also known as antithixotropy. Each of these types of substances can reach a constant value in a different amount of time, ranging from a few seconds to even a few days.

Thixotropy and the phenomenon of pseudo-plasticity

Thixotropy is a characteristic of colloidal and polymeric systems that are under mechanical influence at a constant temperature. Thixotropy characterises the property of a substance that allows it to overcome the limit of its plasticity after the mechanical influence has ceased, in the case of paint materials, mixing. This determines the possibility of reversing changes in the structure of a given paint substance through coagulation of colloidal particles in a dispersion system. The concept of thixotropy is often mistakenly used as a substitute for the pseudo-plasticity of a substance. This is incorrect because the two properties are completely opposite. A pseudoplastic substance differs from a thixotropic substance in that it reduces its viscosity under the influence of a temporary shear stress, while a thixotropic substance reduces its viscosity over time under constant mechanical influence.

Thixotropy and changes in material properties

When we start to mix a rheological liquid, we initially face high resistance (high viscosity), but after a while the resistance decreases (we obtain the right viscosity). Thanks to this phenomenon, we can paint or fill large surfaces without worrying about drips. One might now wonder if the bonds break down through mixing, the product is already destroyed after the first mixing. Nothing could be further from the truth, it turns out that a given liquid has a memory, thanks to which, after mixing ceases, it returns (after some time) to its initial state, and it is this memory that is the property called thixotropy. It depends on the time and speed of shearing, as well as on the mechanical phenomena occurring in the material. The rheological profile of a thixotropic product can be obtained by adding various cellulose ethers, polyacrylic acids, etc.

How are thixotropic properties tested in the laboratory?

When a thixotropic liquid is sheared, e.g. in a rotational viscometer, the structure of the liquid is destroyed. The destruction of the structure ends when the residual structure of the liquid is generated at a certain shear rate. By comparing the flow curve plotted at increasing shear rate with the curve at decreasing shear rate, the extent of the destruction of the thixotropic structure can be determined. If, before starting to determine the curve (when the shear rate is still low), the substance is subjected to shear for a certain period of time at a specific maximum rate (γ), the shear stress (τ) corresponding to this rate will decrease in proportion to the breakdown of the thixotropic structure.

How can this be used in practice?

From the point of view of our industry, the painter wants to receive a product that is easy to apply, easy to apply with spray guns - therefore spray viscosity is very important. On the other hand, we want the paint not to run off the surface after spraying (no streaks) and to be able to apply a thick layer in one go - to increase the covering power. Using the phenomenon we have learned about, we can produce paints with these properties. Work should start as early as the product design stage. This is a very difficult task because we want the final product to combine many advantages that are often difficult to reconcile due to the properties of the individual components. As you can see, knowledge of the rheology of the system is extremely important in the production process of paint materials, as this is the only way to ensure a fully informed process and production planning. The combination of optimal spray viscosity and high hiding power is a real challenge for the manufacturer.

Thixotropy in the context of the paint material production process

The so-called functional additives used in car paint to give the paint the desired properties are playing an increasingly important role in the production process of materials. By using these additives, the production technology of paint materials and their properties during use can be better controlled. The most important additives used in paint are thickeners, i.e. rheology modifiers, dispersants and various types of photo-stabilisers. By using knowledge of the rheology of the paint system, it is possible to improve its flowability, ensure even coverage of the substrate and prevent viscosity reduction, which can occur during colouring, for example.

The specifics of working with a spray gun and the viscosity of the paint

During the painting process, the paint is distributed with the help of a spray gun by means of compressed air. The air flows inside the device at a high pressure, adjusted to the power of a particular paint material. During the painting process, the paint is broken down into microscopic particles, creating an aerosol at the nozzle outlet. The particles settle on the object to be painted, creating a painted surface. The spray should leave an even mark on the surface, resembling an elongated oval or rectangle with very rounded edges. This ensures that the paint is evenly applied to the surface.

Paint viscosity – adjustable

The thixotropy of the paint means that it can be easily applied using a spray mechanism and at the same time quickly returns to the appropriate level of viscosity, which guarantees even coverage of a horizontal surface. If the viscosity of the paint is too high, we observe a reduction in the amount of paint in the centre of the spray pattern. This problem can be solved by adding a little thinner to the paint, for example. This can only be done by experienced painters and only when spraying the surface. However, it is important to remember that thinning the paint with a solvent significantly reduces the thixotropy of the coating. Dilution is possible up to a maximum of 20%.

Thixotropy is a property of various substances. It is important to note that thixotropy is characterised by the ability of a plastic substance to assume a liquid state, but does not determine the property of liquid substances to assume a thick consistency. This is why the viscosity of paint is related to the concept of thixotropy.