Demystifying polyurethane sealant

Fri Apr 29 2022 JITAI

foreword

Polyurethane sealant is a room temperature curing material for in-situ filling and bonding of joints. Before curing, it is in the form of paste, which can be extruded to fill the gap. After curing, it is an elastomer, which plays the role of interface bonding, anti-leakage, waterproof, dustproof, and withstands vibration or displacement caused by thermal expansion and contraction. .

Polyurethane sealants are available in one-component and two-component packaging styles. Two-component polyurethane sealant is packaged with two components, one is a component containing NCO, and the other is a curing agent containing active hydrogen. When using, the two components are mixed in a certain proportion, and the two components are mixed with active hydrogen through NCO. The reaction is cured into an elastomer. Its advantages are that the room temperature curing speed is faster than that of one-component, and the cost is low, but it is inconvenient to operate. One-component polyurethane sealant is composed of NCO-terminated prepolymers, fillers and additives. It is sealed and stored. When used, it reacts with moisture in the air to cure into an elastomer with a network structure.

Because polyurethane sealant has good adhesion to various materials (such as concrete, metal, glass), and has the characteristics of UV resistance, low temperature resistance, chemical resistance, and high cost performance, it has been widely used in construction engineering, Transportation, aerospace and automotive fields.

Raw material composition


Polyurethane sealants are mainly composed of macromolecular polyols, isocyanates, solvents, additives and fillers.

macromolecule polyol


Macromolecular polyol is the soft segment in the PU structure, which will directly affect the flexibility and low temperature resistance of the PU sealant. Polyether polyols are mainly based on high-activity polyether polyols in polyurethane sealants. High-activity polyether polyols contain high content of primary hydroxyl groups and have high reactivity; at the same time, polyether polyols contain a large number of ether bonds (-C-O-C- ), not easy to be hydrolyzed, and the prepared sealant has excellent hydrolysis resistance, good low temperature resistance and high molecular chain flexibility. Sometimes a small amount of low molecular weight polyether polyol is added for compound use.

Isocyanate


Common classes of isocyanates are aromatic isocyanates and aliphatic isocyanates. MDI is the most commonly used in polyurethane sealants. MDI has high reactivity and relatively minimal toxicity, and can be used in fast-reacting polyurethane adhesives. In addition, the molecular structure of MDI is symmetrical, and it can be designed into a micro-domain structure with a relatively regular and orderly molecular chain, so the elasticity, bonding strength and wear resistance of the adhesive layer are excellent. Sometimes a small amount of TDI is added for compounding.

Fig.1 Molecular structure of MDI

solvent

In order to improve fluidity, a small amount of solvent, such as mixed xylene, is sometimes added to the polyurethane sealant.

Auxiliary

·catalyst


Polyurethane sealant catalysts are mainly divided into tertiary amine catalysts and organometallic catalysts according to their chemical structures. Commonly used PU catalysts include organotin and tertiary amines. Organometallic catalysts have obvious effect on -NCO/-OH reaction (commonly used T-12), while tertiary amine catalysts have obvious effect on -NCO/H2O reaction (commonly used bismorpholinyl diethyl ether).

As a catalyst used in the polyurethane sealant industry, the principle of its use should not only be based on the catalytic activity of the catalyst, but also consider the use state, chemical toxicity, its mutual solubility with other raw material components, use temperature and cost price, in the reaction The chemical stability of the raw material system, the influence of catalyst residues in the product on the performance of the sealant and other factors.

·Plasticizer

In order to reduce the viscosity of the material (easy to mix) and increase the flexibility and elongation of the product, a certain amount of plasticizer is often added to the polyurethane sealant. The commonly used plasticizers are mainly diisooctyl phthalate (DOP), diisononyl phthalate (DINP), diisodecyl phthalate (DIDP), and diisooctyl adipate. (DOA), triethyl phosphate, etc.

·A silane coupling agent

In order to improve the compatibility between inorganic fillers and polymer resins and improve the bonding force between organic resins and inorganic fillers, a small amount of silane coupling agent, such as KH-560, is often added to polyurethane sealants.

Figure 2 Molecular structure of KH-560

·Water remover

In order to maintain the storage performance of the product, a small amount of water scavenger is often added to the polyurethane sealant, such as molecular sieve, p-toluenesulfonyl isocyanate (PTSI), calcium oxide, etc.

Figure 3 Molecular structure of PTSI

filler

Inorganic fillers are closely related to the rheological properties and mechanical strength of polyurethane sealants. Many experiments have proved that different types of fillers, such as pigments (carbon black, etc.), nano-calcium carbonate, can improve the adhesive strength of polyurethane sealants. After organic surface modification, fumed silica can also improve the rheological and mechanical properties of polyurethane sealants, but because it is difficult and expensive to handle, inorganic fillers to replace fumed silica are still being closely watched. Calcium carbonate is a commonly used filler in the polyurethane sealant industry. Because calcium carbonate particles are large and have not undergone surface treatment, it has little effect on the rheological properties and mechanical properties of polyurethane sealants. Studies have found that calcium carbonate treated with stearate or cationic surfactant can improve the rheological and mechanical properties of polyurethane sealants. Some scientists have also found that the use of calcite calcium carbonate fillers in polyurethane sealants will have good thixotropy and high tensile strength. Compared with ordinary calcium carbonate, nano-calcium carbonate has a smaller particle size and a larger specific surface area. Because its particle surface is organically modified, when it is used as a polyurethane sealant filler, its particles are easier to seal in polyurethane. It is dispersed in the glue and has good compatibility with the polyurethane prepolymer, which is beneficial to improve the reinforcing effect and sagging resistance of the polyurethane sealant system.

application

achitechive

In the field of construction, polyurethane sealants are widely used for the sealing of construction joints such as concrete slab curtain walls, reinforced concrete and slate, thin plates, and glass fiber reinforced concrete. At the same time, polyurethane sealant is also widely used in the caulking of roads and runways and the bonding and sealing of various pipes and bridges, especially the joints subject to dynamic stress and thermal contraction and expansion.

Figure 4 Construction industry

Aerospace

Due to advances in sealing technology, polyurethane sealants have also opened up new markets in the aerospace field. In the mid-20th century, sealants have been used in the aerospace field, and there have been reports of the development of an Low-volatility high-grade sealants, and with the tremendous progress in aerospace technology, the demand for this new type of sealant will continue to grow.

Figure 5 Aerospace field

Automobile industry

As far as the automotive industry is concerned, in recent decades, due to the development of the automotive industry requiring lightweight automotive components, a large number of plastic parts have been used, especially high-strength ERP (glass fiber reinforced plastic) and SMC (sheet molding material). , is used in automobile manufacturing. The polyurethane sealant not only has a good bonding and sealing effect on traditional metal materials, but also has a very good bonding effect on thermoplastics and new composite materials. Compared with traditional mechanical welding and riveting technology, polyurethane sealant has very obvious advantages, such as: able to bond substrates with different thermal expansion coefficients, firm bonding, high mechanical strength, good weather resistance, better insulation and Less corrosion, etc., and reduce vibration with screw and rivet assemblies, and improve efficiency in bonding of automotive assemblies.

At present, moisture-curing polyurethane sealant is the best bonding and sealing material for installing automotive windshields, and 95% of the world's automotive windshields use one-component moisture-curing polyurethane sealants. More importantly, the outstanding feature of lightweight, green and environmentally friendly polyurethane sealant is to meet the requirements of policies and regulations such as environmental protection, industrial hygiene, and resources.

Figure 6 Automotive industry

Analysis of Polyurethane Sealants

At present, the development of polyurethane sealants has been relatively mature, and with the development of science and technology, many fields have put forward higher requirements for the performance of polyurethane sealants, which requires us to continuously improve the formulation system to achieve product performance upgrades.

Demystifying polyurethane sealant