Polyurethane sealant is a sealant mainly composed of polyurethane rubber and polyurethane prepolymer. This type of sealant has high tensile strength, excellent elasticity, wear resistance, oil resistance, and cold resistance, but its water resistance, especially alkali water resistance, is poor. It can be divided into three types: heating vulcanization type, room temperature vulcanization type, and hot melt type. The room temperature sulfurization type can be divided into single component and two-component types. It is widely used as a joint sealing material in buildings, squares, highways, as well as in automobile manufacturing, glass installation, electronic filling, submarine and rocket sealing.

What should I do when the glue has a sagging phenomenon? What is the phenomenon of streaming?
The phenomenon of adhesive flow refers to the specific flow pattern of liquid adhesive when subjected to external force or gravity. This phenomenon is usually observed when the glue flows or drips. As shown in the picture.

The manifestation of sagging phenomenon in glue depends on the viscosity, density, surface tension and other characteristics of the glue. When the interaction between glue molecules is strong and the viscosity is high, the glue will exhibit a denser flow when flowing, manifested as a slower dripping speed and often forming longer, slender fluid lines.
So, controlling the phenomenon of adhesive flow can help engineers design more effective coating and bonding processes. When the glue shows sagging, it can be considered to add a thixotropic agent, also known as an anti flowing agent or anti sagging agent, such as white carbon black (fumed silica).
Thixotropic agent: white carbon black
What is a thixotropic agent?
（1） Also known as anti flow agent.
A substance with a high specific surface area that can form hydrogen bonds or some other structure with polymers. Under stress, it becomes thinner, and when left to stand, it becomes thicker. After adding a thixotropic agent to the adhesive, the adhesive becomes thinner due to shear during stirring and coating; After the operation is completed, it thickens again and does not flow. Mostly used for construction on vertical surfaces. The main varieties include LBCB-1 thixotropic lubricant, fumed silica, precipitated silica, organic bentonite, and other thixotropic agents include asbestos, kaolin, attapulgite, lotion based vinyl chloride compounds, etc.
（2） Also known as anti sagging agent.
Additives that can cause thixotropy in coatings. When the coating is subjected to shear force, its viscosity decreases. The greater the shear force, the greater the decrease. When the shear force is removed, the viscosity slowly returns to its original state. This phenomenon is called "thixotropy". After using a thixotropic agent in the coating, it has a lower viscosity at high shear rates during construction, which helps the coating flow and is easy to apply; High viscosity at low shear rates before and after construction can prevent pigment settling and wet film sagging. Organic bentonite, hydrogenated castor oil, fumed silica, metal soap, etc. are generally used as thixotropes for solvent based coatings. Hydroxyethyl cellulose and other cellulose derivatives, polyvinyl alcohol, polyacrylate and other water-soluble resins are used for water-based system coatings.
What is the mechanism of thixotropy?
Thixotropy is an important property of liquid phase systems, which directly affects the appearance, construction performance, and storage stability of liquid phase systems. Different liquid phase systems also have different requirements for thixotropic agents. For oily systems, most thixotropes work by forming hydrogen bonds. Untreated fumed silica aggregates on the surface contain multiple components, including isolated and undisturbed free, as well as interconnected hydrogen bonds that form hydrogen bonds with each other. These bonds form a three-dimensional network structure in oily systems and are easily disrupted by mechanical forces, leading to a decrease in viscosity and a restoration of good fluidity in the liquid phase system; After the shear force is eliminated, the three-dimensional structure will automatically restore its viscosity and increase.

In completely non-polar liquids, viscosity recovery time only takes a fraction of a second; The recovery time in polar liquids is relatively long, which depends on the concentration and dispersion of fumed silica. This characteristic endows oily liquid phase systems with excellent storage and construction performance, especially thick slurry liquid phase systems. It can ensure good fluidity of the liquid phase system under certain construction shear forces, and also ensure the thickness of the coating film during one construction. Usually, during the construction process, due to the rapid evaporation of solvents at the edge of the coating, the surface tension is uneven, which makes it easy for the liquid to move towards the edge. The silica network can effectively prevent the movement of the liquid and form thick edges, while also preventing the liquid from flowing and hanging during the curing process, making the coating uniform.
At the same time, fumed silica can form hydrogen bonds to increase the medium to low shear viscosity in the system, thereby playing a thickening role. Therefore, the application of gas-phase silica in oily systems is very extensive.