Coating rheology and its application solutions for measurement

Paint concept

“Coating, because most of the early coatings used vegetable oil as the main raw material, it is also called paint. Now synthetic resin has replaced vegetable oil, so it is called coating. Coating is not only liquid, powder coating is a large category of coatings.”

 

Rheology of the coating

The coating is generally a viscous liquid or a powdery substance. Since the coating must pass through the fluid phase during the coating process, rheological properties are an important property of the coating. Fluids can be divided into Newtonian and non-Newtonian types according to a large class, and non-Newtonian fluids are divided into shear rate dependent and time dependent types. The shear rate dependent type means that the flow behavior of the fluid varies with the shear rate, including pseudoplastic, dilatant, and plastic. Time-dependent type refers to the flow characteristics of fluids changing with time at a certain shear rate, including thixotropic and fluidic types. The actual coatings are mostly thixotropic fluids.

 

In the production, storage, construction and film formation process of the coating, the forces received can be divided into pure shearing, tensile shearing and simple shearing, etc., mainly simple shearing. When the coating is subjected to simple shearing and unidirectional laminar flow, there is a speed difference between the layers. If the shear stress is τ and the shear rate is Y, the viscosity η=τ/Y is called the dynamic viscosity, and the unit is Pa.s. Common units are mPa.s or cP (centipoise). Viscosity is an important indicator of coating rheology and is closely related to shear rate and shear stress.

 

The coating is mainly composed of three parts: a film-forming substance, a solvent and a filler. The effects of these substances on the rheology of the coating are mainly in the low shear rate, such as flocculation of the pigment, the presence of various auxiliaries, the resulting structure changes the viscosity greatly; at high shear rates, the structure is destroyed The viscosity exhibited is close to the effect of the resin solution itself and the dispersed particles on the viscosity. The viscosity at high shear rates allows the coating to have a rheological property for storage and construction.

 

For example, in the storage of paints, it is desirable to have a higher viscosity of the system to prevent the precipitation of pigments and fillers; at the beginning of construction, the viscosity of the system is required to be low, which is favorable for the leveling of the coating film, but the viscosity of the coating film is required to reach a higher viscosity at a certain time. In order to avoid sagging and drooling of the coating film; the powder coating has sufficient leveling only when its melt has a sufficiently low viscosity or good fluidity, and the viscosity also has a great influence on the dispersion of the pigment in the coating. .

 

From the above analysis, it can be seen that the parameters of the rheological properties of the coating: yield value, thixotropy, viscosity recovery rate and viscosity at the construction shear rate have a great influence on the quality of the coating, so the measurement of these parameters is The production, development and use of coatings are highly valued.

 

Coating Rheology Measurement Solutions

Brookfield, USA, is one of the world's leading manufacturers of rotational viscometers/rheometers. Since the invention of the world's first dial-type rotational viscometer in 1932, a series of viscometers/rheometers for coating production and development have been introduced for rheology and fluidity research and measurement during coating production and development. Provides a complete application solution.

 

01 standard rotary viscometer

Represented by DV2T

 

The rotors of this type are usually common disc rotors, and other rotors can be configured according to user requirements. The new DV2T model is equipped with a 5.7-inch full-color touch-screen display that can be connected to a computer for measurement with user-friendly software. It can be used to measure the viscosity of coating materials, the viscosity of semi-finished products, the viscosity of finished products, etc., and is widely used in QC and R&D.

 

For different customer needs, Brookfield also introduced small sample adapter (SSA), screw adapter (SA), ultra low viscosity adapter (ULA), lifting bracket, paddle rotor and other accessories. The use of these accessories in some production or development processes allows the range of measured viscosities to be greatly expanded and the measurement accuracy to be greatly improved. Such viscometers are also DV1, DV3T, dial type, DVS+ and the like.

DV2T rotary viscometer

 

 

02 Stormer Viscometer

Represented by KU-3

 

Commonly used in coatings is the Stormer viscometer. The development of this instrument is to imitate the painter's paddle-stirred paint, and the consistency of the paint is judged by the resistance, expressed as Krebs Unit (KU). Brookfield's next-generation Stormer Viscometer (KU-3) is equipped with a 200RPM fixed-speed drive motor to completely replace the weight required for conventional viscometers. Such viscometers can also directly read the viscosity of the coating in grams, cP. It is quick and easy to use, and it makes the measurement more accurate and repeatable to meet the needs of different test conditions.

 

KU-3 Stormer Viscometer

 

Old KU-2 viscometer

 

 

03CAP series cone and plate viscometer

Represented by CAP2000+

 

Mainly used to determine the viscosity of the more viscous material and its rheology. Because of its high shear rate (high shear rate of 13,300 1 / S), it is very suitable to simulate the shear rate during paint coating, which is an essential tool for quality control and development in paint production. The instrument requires less sample, easy cleaning, fast test speed, and comes with a temperature control system. In addition to the CAP2000+ viscometer, there is a single-speed pure QC model CAP1000+ viscometer.

CAP2000+ cone and plate viscometer

 

 

 

04RST series rheometer

Represented by RST-CPS

The RST Series rheometers control shear stress and shear rate and are more convenient for measuring the thixotropy and yield stress of coatings. The appearance of the coating after painting depends on the rheological behavior of the coating at the beginning of the baking. The fluidity after coating can only be studied at very low shear rates, so the RST series rheology is used. The instrument uses a high to low shear rate change that responds well to the thixotropic behavior of the coating. By controlling the shear stress, the RST series of rheometers can characterize the curve change of the coating just beginning to yield, resulting in a more comprehensive coating rheology.

 

 

 

 

05 online viscometer

Represented by AST-100

The online viscometer provides real-time monitoring of the viscosity of the paint production process. The viscosity is used to reflect the quality of the product, the quality control is carried out, the consistency of the quality of different batches of products is ensured, the problems in production can be found in time, various wastes are reduced, and production costs are saved. There are currently two types of online viscometers used in the coatings industry: the TT-100 rotary online viscometer and the AST-100 vibrating online viscometer.

AST-100INK online viscometer

 

 

 

06 powder flow tester

Represented by PFT

The PFT powder flow tester can analyze the flow properties of powder coatings. The flow properties of the powder reflect the coating effect of the powder coating and the effectiveness of the product formulation, and thereby predict the clogging potential of the powder coating during transportation, storage and use due to fluidity behavior or equipment limitations.

 

The rheology of the coating is complex. With the expansion of coating applications and the deepening of research, some practical rules have been summarized for practical guidance, especially the use of some high-performance instruments including rotary viscometers/rheometers, so that the measured data can be used to a large extent. Reflecting the rheology of the coating, promoting the technological advancement of the coating in R&D, QC and production, will also make the research of coating rheology deeper and deeper!

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