Effect of polyvinylpyrrolidone on dispersion fluid denaturation of carbon black

Abstract: Viscosity and thixotropic index are simple and quantitative characterizations of rheological properties of carbon black dispersions. Carbon black dispersions with different rheological properties were prepared by adding different relative molecular weights and different amounts of PVP into the matrix of carbon black dispersions. The viscosity, shear rate curve and thixotropic index of each carbon black dispersions were analyzed to prove the influence of PVP on their rheological properties. The results show that the parent carbon black dispersion has shear thinning behavior. After adding PVP, the apparent viscosity and thixotropic index of the system can be significantly improved. The apparent viscosity of the system decreases linearly with increasing temperature and is temperature-dependent.
Carbon black pigment dispersion is a colloidal dispersion system formed by dispersing carbon black into water with dispersant, which is widely used in paint, ink, plastic, rubber and other fields. Generally speaking, fluids can be divided into Newtonian fluids and non-Newtonian fluids, and carbon black pigment dispersions belong to non-Newtonian fluids and have some basic properties of non-Newtonian fluids. The rheological properties of the colloidal system are significantly affected by the composition characteristics of the system, such as the change of the amount of material added and the change of relative molecular mass, which will directly reflect the rheological behavior. Therefore, the rheological behavior of carbon black dispersion can be used to control the consistency and quality of the product. It is of great significance to study the rheological behavior of carbon black dispersion in the processing of raw materials.
Viscosity and thixotropy are two important parameters of fluid rheological properties. When the fluid flows and deforms when subjected to external shear forces, its internal resistance to deformation is generated accordingly, and it is manifested in the form of internal friction, which is an inherent physical property of the fluid, called the viscosity or viscosity of the fluid. Thixotropy and viscosity are two different physical concepts, thixotropy refers to the viscosity of the system under the action of external shear force decreases with time, and then recovers after rest, which has the shear thinning phenomenon of time factor. It can also be understood that thixotropy is a representation of changes in fluid viscosity.
Polyvinylpyrrolidone, referred to as PVP, is a class of non-ionic water-soluble polymer fine chemicals with excellent properties and wide applications. As a synthetic water-soluble polymer compound, PVP has the general properties of water-soluble polymer compounds, such as colloidal protection, film formation, adhesion, solubilization, etc. Its molecular structure makes it with surface activity, and it can be applied to colloidal systems to obtain excellent dispersion stability. At the same time, it has the characteristics of polymer compounds. The ability to significantly regulate the rheological properties of dispersions or solutions.
The viscosity – temperature curve, viscosity – shear rate curve and thixotropic index of carbon black dispersion prepared by adding different relative molecular weight and different content of PVP to the bulk matrix show the influence of PVP on the rheological properties of carbon black dispersion system, and preliminarily explain the cause of its rheological regulation ability. It provides a theoretical basis for technology development, quality control and performance expansion of carbon black dispersion-related products.
Effect of PVP on viscosity – temperature curve of carbon black dispersion
Quantitative PVP with different molecular weights (K15, K30, K60, K90) was added into the carbon black dispersion matrix, and the viscosity values of the system at different temperatures were measured to obtain the viscution-temperature curve. The experimental results are shown in Figure 1.

FIG. 1 Viscosity-temperature curve after adding PVP of different relative molecular weights (the mass fraction of PVP is 6%)

The carbon black dispersion of the carbon black system was prepared by adding K30 with different mass fractions (6%, 12%, 18%, 24%) into the carbon black matrix, and its viscous-temperature curve was measured. The experimental results are shown in Figure 2.

Comparing FIG. 1 and FIG. 2, it is found that the two have some similar characteristics: the addition of PVP significantly improves the apparent viscosity of the dispersion: the apparent viscosity of the dispersion basically decreases linearly with the increase of temperature, and the apparent viscosity of the dispersion basically remains unchanged with the rate of temperature decrease, which is temperature dependent.
The addition of PVP leads to a significant increase in the viscosity of the system, which can be attributed to two aspects. On the one hand, PVP is dissolved in the dispersion of polyethylene glycol, diethylene glycol and other solvent molecules, and after mixing with these solvents, a gel reaction occurs, forming part of the gel and dispersed in the solution, which greatly increases the viscosity of the solution. On the other hand, as an active filler, the surface of carbon black ion can adsorb several large molecular chains at the same time, forming a tangle-like point, resulting in an increase in viscosity.
It can be seen from FIG. 2 that with the increase of K30 addition at the same temperature, the change curve of the apparent viscosity of the system with the increase of temperature basically keeps parallel, which indicates that the increase of K30 addition does not change the trend of the apparent viscosity of the system decreasing with the increase of temperature, but only increases its viscosity accordingly. When the addition of K30 is increased, the number of molecules in the system increases, the number of internal friction between each other increases, and the mutual entanglement leads to the increase of the viscosity of the system. After the temperature rises, the irregular movement of molecules is intensified, the molecular distance increases, the molecular chain desorption is increased, and more energy causes more “holes” to form in the system, so the chain segment is easier to move, resulting in a decrease in the viscosity of the system.
Effect of PVP on viscosity shear rate curve of carbon black dispersion
FIG. 3 and FIG. 4 show the viscosity curves of carbon black dispersions prepared by adding different relative molecular weight PVP and different mass fraction K30 to the matrix of carbon black dispersions with shear rate, respectively. It can be seen from the two figures that the shear viscosity of all samples decreases with the increase of the shear rate, and the larger the shear rate, the smaller the shear viscosity of the sample, showing a typical shear thinning behavior. The addition of PVP did not change the shear thinning property of the original dispersion. This indicates that the carbon black dispersion is a pseudoplastic fluid, and shear thinning is a reaction of its own structure.

FIG. 2 Viscosity-temperature curves of carbon black dispersions prepared with different mass fractions of K30

FIG. 3 Viscosity shear rate curve after adding PVP with different relative molecular weight (PVP accounted for 6% of the system mass fraction)

FIG. 4 viscosity-shear rate curves of carbon black dispersions prepared with different mass fractions of K30

As can be seen from FIG. 3, at the same rotational speed, the viscosity of the system decreases after the addition of K15 and K30, and the viscosity of the dispersion after the addition of K15 is greater than that of the addition of K30, while the shear viscosity of the system after the addition of K60 and K90 increases. This showed that the shear viscosity of the sample was not directly proportional to the relative molecular mass of PVP, but showed an irregular fluctuation. This is because the dispersion effect of K15 and K30 with relatively small molecular weight is stronger than the thickening effect, while the thickening effect of K60 and K90 with relatively large molecular weight is more obvious, which is reflected in the irregular change in the influence on shear viscosity.

It can be seen from FIG. 4 that the shear viscosity of the system decreases slightly compared with the blank sample when the mass fraction of K30 is 6% at the same rotational speed. When the mass fraction of K30 exceeds 12%, the shear viscosity of the system is high. This is because when the mass fraction of K30 is 6%, there are fewer K30 molecules in the system, which is easy to adsorb on the interface to form a stable interface adsorption film, and it is easier to flow when receiving shear, resulting in a slight decrease in viscosity. When the addition of K30 increased, the number of K30 molecules in the system increased, and the thickening effect of K30 on the dispersion also appeared, resulting in an increase in shear viscosity.

The shear thinning behavior of carbon black dispersion can be explained by the change of polymer conformation: with the increase of shear rate, the corresponding shear stress of macromolecular chains in PVP also increases, and the macromolecular chains are oriented along the flow direction, resulting in the deviation of long-chain macromolecules from the equilibrium conformation. The flow resistance between the microscopically oriented macromolecules decreases, showing that the macroscopic viscosity of the system decreases and shear thinning occurs.

It can be seen from the above that PVP with different relative molecular weight and different amount of PVP are added to the carbon black dispersion matrix to prepare carbon black dispersion. The influence of PVP on rheological properties of carbon black dispersion is analyzed through the viscosive-temperature curve and viscosive-shear rate curve of carbon black dispersion:

1. The addition of PVP can significantly increase the apparent viscosity of dispersions. The greater the amount of PVP, the greater the relative molecular mass, and the greater the apparent viscosity and thixotropic index of dispersions.

2. The apparent viscosity of the dispersion with added PVP basically maintains the same rate of temperature increase and decrease, which is temperature-dependent.

3. Carbon black dispersion has a shear thinning phenomenon, and its shear thinning behavior does not change after adding PVP. The shear viscosity of the system decreases with the increase of shear rate.