A comprehensive overview from basic chemistry to application scenarios

Adhesive sealant, as a functional material used to fill gaps and provide sealing effect, has a complex and diverse classification system. Based on the chemical basis, curing mechanism, performance characteristics and application fields, adhesive sealants can be classified into main categories such as silicone type, polyurethane type, polysulfide type, modified silane type and acrylic type. 

The global sealant market continues to grow, with the market size reaching approximately 20.7 billion US dollars in 2023. Among them, silicone sealants account for the largest market share, approximately 35%, while polyurethane sealants come in second, accounting for about 25%. 

01 Chemical Basis Classification: Molecular Structure Determines Performance Characteristics
The chemical basis of sealants determines their core performance features and application boundaries. Silicone sealants have a main chain structure based on silicon-oxygen bonds (Si-O), with side chains connecting organic groups. This unique inorganic-organic hybrid structure endows it with excellent high and low temperature resistance (-60°C to 250°C) and UV resistance. 

Polyurethane sealant is characterized by amine carbamate bonds and is produced by the reaction of isocyanate with polyol. The flexible segments and rigid segments in its molecular chain form a microscopic phase-separated structure, enabling the material to possess both high elasticity and high strength. 

Modified silane sealant (MS sealant) is made from silane-end-capped polyether or polyurethane as the base polymer. It combines the weather resistance of silicone with the mechanical properties of polyurethane, and does not contain free isocyanate, making it more environmentally friendly. 

Polysulfide sealant is composed of sulfur atoms as the main chain. The disulfide bonds in the molecule can undergo reversible cleavage and recombination, enabling the material to have excellent oil resistance and solvent resistance, and also demonstrating outstanding bonding performance on various substrates. 

02 Classification of curing mechanisms: From moisture curing to ultraviolet light curing
According to the curing mechanism, sealants can be classified into various types to meet the process requirements of different application scenarios. The moisture curing type is the most common curing method for single-component sealants, including silicone and polyurethane. 

This type of sealant undergoes cross-linking and hardening through reactions with moisture in the air. The curing rate is significantly affected by the environmental temperature and humidity. Its advantages are simple construction, while its disadvantage is that the curing depth is limited by the rate of moisture penetration, and it is not suitable for applications with overly thick layers. 

Chemical reaction type mainly refers to two-component sealants, such as epoxy resin, two-component polyurethane, etc. These products separate the base material and the curing agent, and are mixed in the correct proportion when used. The curing is achieved through the chemical reaction between the two components. 

Its advantages are that it is not affected by environmental humidity, the curing process is uniform and rapid. However, its disadvantages are that precise measurement and mixing are required, and the construction process is relatively complex. 

Ultraviolet-curable type is mainly found in acrylic sealants. Under ultraviolet light, the photoinitiator decomposes to produce free radicals, which then initiate the polymerization and curing of the monomers. 

This type of product dries extremely quickly (within seconds to several tens of seconds), making it suitable for automated production lines. However, it is not applicable to opaque seams. 

Anaerobic sealant cures under an air-sealed condition and is mainly used for thread locking and surface sealing. In the presence of metal ions, it forms a cross-linked network through free radical polymerization. 

03 Performance Grade Classification: From Ordinary Sealing to Structural Bearing
Based on mechanical properties and application, sealants can be classified into multiple performance grades. Structural sealants have high strength and modulus, and can withstand large loads for a long time. For example, the tensile strength of silicone structural sealant is usually not lower than 0.6 MPa, and the modulus is not lower than 0.4 MPa. 

This type of sealant must pass strict durability tests, such as the 300-hour water-ultraviolet light aging test. After this test, the strength and elongation retention rate must meet the specified standards. 

Weather-resistant sealant mainly provides waterproof and airtight sealing, and can also withstand the repeated expansion and contraction movements of the joint. Its key indicator is the displacement capacity, which refers to the rate of width change of the joint that it can withstand. 

According to the ISO 11600 standard, the displacement capacity of sealants is classified into several grades, such as ±7.5%, ±12.5%, ±20%, ±25%, up to ±50%. 

General-purpose sealant provides basic sealing effect and is suitable for static joints with low displacement requirements. These products have a lower price, good construction performance, but relatively limited durability, and their service life is usually between 5 and 10 years. 

Special functional sealants include flame-retardant, conductive, heat-conductive, and mold-resistant types of sealants. These special sealants achieve their unique properties by adding specific functional fillers, thus meeting the requirements of specific application scenarios. 

04 Application Domain Classification: From Building Curtain Walls to Electronic Chips
The performance requirements for sealants vary significantly across different application domains, leading to specialized product lines. The application domain of building sealants is the largest, including assembly of curtain wall structures, joint sealing, and installation of doors and windows. 

These products need to have excellent weather resistance, aging resistance and a certain degree of displacement capability. The designed service life is usually required to be over 20 years. 

Industrial sealants cover various fields such as automotive manufacturing, rail transportation, and aerospace. Automotive welding sealants need to be resistant to oil and fatigue; battery power sealants need to be flame-retardant and conductive; aircraft cabin sealants need to be able to withstand pressure differences and resist aging. 

The requirements for electronic sealants are extremely high, including for chip packaging, circuit board protection, and display screen sealing, etc. Such products need to have excellent insulation properties, low corrosiveness, low gas release, and reliable durability. Some applications also require optical transparency or specific refractive indices. 

Medical and food sealing adhesives come into direct or indirect contact with the human body or food. They must undergo biocompatibility tests or food contact material certifications to ensure they are non-toxic, non-allergenic, and capable of withstanding disinfection and sterilization processes. 

05 Product Form and Construction Characteristics Classification
The product form of sealant directly affects its construction method and user experience. Monomer-based sealant is the most common form in the market. When it leaves the factory, all components are pre-mixed. During construction, it can be used directly without the need for mixing, making the operation very simple. 

These products achieve curing through environmental conditions (temperature and humidity). The curing rate is greatly influenced by the environmental conditions, and the curing depth is limited. 

The two-component sealant separates the base material and the curing agent for storage. When using it, they are mixed in the prescribed ratio. Its advantages include the ability to control the curing rate, being not limited by environmental humidity, and achieving deep-level curing. 

The drawback is that it requires a dedicated mixing equipment, the construction process is relatively complex, and the mixture needs to be used up within the specified time. 

Preformed sealing materials include sealing tapes and sealing strips, which are usually pre-made in specific shapes using partially cured or un-cured sealing glue. These products are extremely easy to install and can facilitate rapid assembly, making them particularly suitable for large-scale production lines. 

Spray-type sealant is mainly used for large-scale construction, such as sealing of car weld seams and waterproof coatings for buildings. This type of product has a low viscosity and can be evenly applied through spraying equipment, with high efficiency. However, it requires special equipment and has a significant impact on the environment. 

06 Environmental and Safety Classification: From Solvent-Based to Bio-Based
As environmental regulations become increasingly strict, the environmental performance of sealants has become an important classification dimension. Solvent-based sealants use organic solvents as the carrier, and the solvents evaporate to complete the curing process after application. These products have good construction performance and fast curing speed, but they have high VOC emissions, which are not friendly to the environment and their usage is gradually restricted. 

Water-based sealants use water as the dispersion medium, with low VOC content and good environmental friendliness. However, the curing process is greatly affected by the environment, and their water resistance and durability are usually not as good as those of reactive sealants. They are mainly used in indoor environments and scenarios with low requirements. 

Solvent-free reactive sealants are currently the mainstream development direction, including silicone, polyurethane, MS sealants, etc. These products are cured through chemical reactions, without solvent evaporation, and have excellent environmental protection performance. Moreover, their properties are outstanding after curing.