Quiet as a virgin, quick as a hare—this ancient proverb aptly describes the magical property of a key performance "thixotropy" the field of structural adhesives.
In building reinforcement projects, construction workers often face a dilemma: if the adhesive is too thick, it is difficult to stir and apply; it is too thin, it is easy to flow on vertical surfaces. The solution to this problem benefits from the thixotropic properties of structural adhesives. In recent years with the application of new material technologies such as nanotechnology, structural adhesive thixotropy technology has achieved new breakthroughs.
01 What is thixotropy?
Thixotropy, derived from the Greek words "thix" (touch pressure) andtropos" (change), refers to the reversible phenomenon where a material exhibits a decrease in viscosity when subjected to shear force, and gradually recovers its viscosity after the shear is stopped.
From a rheological perspective, thixotropy is a rheological property of a colloid in which the system‘s viscosity changes over time when subjected to shear. Inman‘s terms, thixotropy is the property of "changing at the touch."
The microscopic essence of thixotropy is the formation of structures between theed particles within the colloid. These structures break down during flow and recover when the flow stops. This process can be viewed as a reversible transformation between "gel-sol" at temperature.
In the field of civil engineering, GB 50728-2011 "Technical Code for Identification of Safety of Reinforcement Materials for Structures" clearly stipulates the thixotropy of structural adhesives, where the "thixotropic index" is a key indicator for measuring this property.
02 Typical Characteristics and Mechanism of Thixotropy
Thixotropic fluids have several typical characteristics: reversible structural changes, on shear history, and time dependence. When shearing starts from a static state or the shear rate is increased, the viscosity of a thixotropic fluid decreases with;
when shearing stops or the shear rate is decreased, the viscosity gradually recovers with time. This property is different from simple shear thinning, which only refers to the change in-state shear viscosity with shear rate, without considering the time factor.
Thixotropy originates from the destruction and reconstruction of the internal network structure of the fluid. In a state, the internal structure of the binder forms a fragile three-dimensional network structure through interactions such as hydrogen bonds and van der Waals forces, making it resistant to flow like gel.
When a shear force is applied, this network structure is destroyed, and molecular chains and filler particles orient along the flow direction, reducing internal resistance and causing a decrease in. After the shear stops, Brownian motion gradually restores the random distribution of molecules and particles, and the network structure is reconstructed, leading to a recovery of viscosity.
03 Characterization Methods and Standards for Thixotropy
There are three main methods for characterizing thixotropy: the thixotropic method, the thixotropic loop method, and the structure destruction-reconstruction method.
In the field of adhesive technology, the thixotropic index It is the commonly used characterization quantity. The determination method is to measure the apparent viscosities η1 and η2 of the adhesive at two widely different shear rates at a specified (usually 23℃), with η1>η2, then It=η1/η2.
The thixotropic loop method is make a closed curve of shear stress-shear rate (thixotropic loop) by changing the shear rate, and the greater the area of the loop, the stronger theixotropy. The structure destruction-reconstruction method (three-stage thixotropic method) can quantitatively compare the degree of shear thinning and the speed of destruction, well as the degree and speed of recovery.
Structural adhesives with excellent thixotropy have become the standard requirement for modern building reinforcement projects. With the continuous innovation of new material technologies such nanotechnology, the thixotropic performance of structural adhesives will be further optimized, providing solutions for more complex and demanding construction environments.
In the future, Txiang Technology‘s requirements for the thixotropic performance of structural adhesives will continue to improve, and this "touch-and-change" property will undoubtedly play more critical role in ensuring project safety.
