1. Source, Types, and Current Situation of MS Sealant
MS sealant was first launched in Japan in 1979 and has over 40 years of development history. It was not until 2010 that MS sealant began to gain attention in China. Currently, China accounts for less than 5% of the market, while Japan holds about 30%. This indicates that with proper application, MS sealant has strong potential for rapid growth in the Chinese market.
Strictly speaking, most products currently available in China can only be classified as silane-modified resins rather than true MS resin. Furthermore, their molecular structures differ significantly from Japanese products.
The full name of MS sealant is Modified Silane Polyether Sealant, a new generation of construction sealant developed after polysulfide, silicone, and polyurethane sealants. In China, “MS resins” currently include: MS, STPE, SMP, STPU, and SPU, among others. Their relationships are summarized below:
1) MS: Modified Silane (Silyl) Polymer
MS Polymer® is a registered trademark of Kaneka Japan.
2) STPE: Silane-Terminated Polyether
STPE, also called “hybrid sealant,” is a high-performance elastic sealant (e.g., from WACKER) with silane-terminated polyether as the base polymer. Domestically, it is sometimes referred to as “silicone-modified polyether” or “modified silicone sealant.”
3) SMP: Silane-Modified Polymer
SMP, launched by Dow as VORASIL™ SMP, combines polyether with silane end groups. It features low viscosity and rapid room-temperature curing.
Note: MS, STPE, and SMP are all silane-terminated polyether resins. The different names arise from intellectual property considerations and brand distinctions: SMP respects IP rights, STPE can refer to both the generic “Silane-Terminated Polyether” and the WACKER brand.
Technically, “modified silicone” is misleading because it is not silicone-based; its main component is silane-terminated polypropylene ether. The more accurate term is “silane-modified polyether sealant.”
4) STPU: Silane-Terminated PU
STPU refers to polyurethane chains capped with siloxane groups at both ends. The polyurethane chain forms the backbone, while the siloxane groups are terminal functionalities.
5) SPU: Silane (Modified) PU
SPU is a polyurethane-based material end-capped with silane. It retains the advantages of traditional PU sealants while offering improved heat resistance, aging resistance, and weatherability. It bonds well to common substrates such as metal, glass, and PVC, with non-polluting cured products that meet environmental requirements.
Summary: STPU/SPU differ significantly from MS, STPE, and SMP; they are closer to polyurethane-based systems.
2. Performance Differences Between Silicone, MS, and SPU Sealants
Taking common RTV silicone sealant (main component: hydroxyl-terminated polydimethylsiloxane, “107 sealant”) as a reference:
2.1 Chemical Structure and Bond Energy
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Silicone’s main chain consists of Si–O bonds (bond energy ≈ 444 kJ/mol), higher than the energy of ultraviolet photons (≈399 kJ/mol at 300 nm). Therefore, silicone sealants have excellent weather resistance and UV stability.
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Japanese MS resin is synthesized via hydrosilation, contains no C–N bonds, and mainly consists of C–C and C–O bonds, which are slightly weaker than Si–O bonds. Its weather resistance is slightly lower than silicone but still good. Low-activity alkoxy end groups require chelated tin or large amounts of organotin catalysts, and cured surfaces may feel slightly tacky.
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Domestic SMP resin uses polyether coupled with isocyanate silane for end-capping. Although no CNO bonds remain, C–N bonds form at both ends. End groups are highly reactive, allowing rapid curing. However, the presence of C–N bonds slightly reduces heat and weather resistance, usually compensated with antioxidants and UV absorbers.
2.2 STPU/SPU Resin
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Polymers are formed by low-viscosity polyether reacting with polyisocyanate, containing many C–N bonds in both the main chain and end groups. The C–N bond energy is only 284 kJ/mol, much lower than UV energy.
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As a result, STPU/SPU-based sealants are prone to cracking and powdering under heat or UV exposure. They have poor outdoor durability and are generally restricted from exterior applications.
2.3 Market Considerations
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Genuine MS/SMP sealants use MS/SMP resin as the base, with complex synthesis and high costs. Some manufacturers replace the polyether backbone with polyurethane and end-cap with ordinary aminosilane or NCO, selling the product as “MS sealant.” These are actually STPU/SPU-based, prone to outdoor failure.
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Polyurethane often contains residual NCO, which is harmful to health and the environment.
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The inclusion of UV absorbers, light stabilizers, and heat stabilizers also significantly affects long-term performance. Tensile strength tests alone are insufficient to assess quality.
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