Thixotropy is one of the important properties of sealants. While most paste sealants are easy to stir, they are often accompanied by construction slumps, especially when building three-dimensional structures or working on the back. The sealants are easily affected by gravity. The components flow out, which not only pollutes the substrate, but also causes waste of colloid.
A sealant with good thixotropy, similar to toothpaste, is easy to extrude when sizing. After trimming, it can fill the glue seam and stay in the glue seam without sagging or flowing, keeping the appearance neat and beautiful. In general, sealants with good thixotropy can accelerate the construction process, and have obvious good effects on improving labor efficiency and reducing the construction period.
What is thixotropy?
Thixotropy refers to the characteristic that the shear stress of a colloid decreases with time under the action of a certain shear rate.
Specifically:
①When stirring is used, the viscosity of the colloid decreases, which is convenient for stirring and construction;
②After sizing, the viscosity of the colloid increases, which has a good shaping effect and is not easy to flow.
Generally, there are three main ways for the formation of thixotropy:
1. It works by forming hydrogen bonds, which is also the most common way.
This type of thixotropic agent is usually fumed silica, which is divided into a single undisturbed free particle and an aggregated state in which multiple particles are connected together. Since the surface of silica contains a large amount of -Si-OH, most of which exist in an aggregated state through the formation of hydrogen bonds. The bonding of silanols in the oily system is very easy to form a three-dimensional network structure. Under the action of force, the viscosity decreases, and the system restores good fluidity; when the shear force is eliminated, the three-dimensional structure will automatically recover and the viscosity will rise.
2. The interaction between filler and surface modifier.
In addition to providing chemical energy to help the filler disperse, the surface modifier forms a gel structure through the interaction between polar groups. The hydrocarbon chain on the surface of the filler thickens the system and produces a thixotropic effect through its strong solvation ability. Under the action of external force, the viscosity decreases when the structure is destroyed, and the original viscosity and structure are restored when the external force is eliminated.
3. Through dispersion and activation, the swollen long chains are entangled with each other and have thixotropy.
Such common thixotropic agents are polyamide waxes, castor oil derivatives, etc., which have thixotropy through the intertwining of macromolecular chains to form a network structure. When sheared, the entangled molecular chains are pulled apart and the viscosity decreases; when the shearing stops, the macromolecular chains are re-wound and the viscosity increases. Since this new winding process is relatively slow, viscosity recovery is also slower, giving the wet film longer time to flow and level without causing runaway sag. Usually the thixotropy of a system is often formed by the combined action of the above factors.
Similarly, thixotropy also plays a very important role in architectural silicone sealants, which directly affects its construction performance. Good construction performance not only improves work efficiency, but also brings a good working experience to construction workers.
Silicone sealant is a linear polyorganosiloxane based polymer, which is prepared by adding various additives such as reinforcing powder, coupling agent, crosslinking agent, catalyst, etc., and is in contact with moisture in the air. After that, it is cross-linked and cured into an elastomer at room temperature. Among them, the proportion of filler mass is suitable (about 50-60%), which is beneficial to the preliminary adjustment of thixotropy of silicone sealant.
The most commonly used reinforcing filler in the silicone sealant industry is modified calcium carbonate. According to the particle size, it is divided into nano-calcium carbonate and heavy calcium. In this paper, by adding the same mass fraction of bulk nano-calcium carbonate, spherical nano-calcium carbonate and heavy calcium as reinforcing fillers (see Figure 1 for the surface morphology), the effect on the thixotropy of silicone sealants was explored. Thixotropy is usually measured in the industry by sag and extrusion.
Only the sag and extrudability of spherical nano-calcium carbonate meet the requirements of the national standard at the same time, among which sag: massive nano-calcium carbonate < spherical nano-calcium carbonate < heavy calcium; plasticity: massive nano-calcium carbonate > spherical nano Calcium carbonate > heavy calcium. This is mainly because the aspect ratio of massive nano-calcium carbonate is larger than that of quasi-spherical ones, the overlap between massive nano-calcium carbonates is a surface-to-surface bridging, and the overlap between quasi-spherical nano-calcium carbonates is a point-to-point contact. The filler is the action site. Under the interaction of the surface modifier, the intermolecular force rises sharply, which is manifested as plasticity. Similarly, the force to destroy the structure is also greater, which is manifested as the extrusion of massive nano-calcium carbonate. Properties < spherical nano calcium carbonate < heavy calcium. From this, it can be seen that the massive nano-calcium carbonate is accompanied by a thickening phenomenon at the same time when the plasticity effect is excellent.
Therefore, a silicone sealant with excellent performance often needs to balance various properties from all angles, and design products with excellent performance according to the needs of customers.