Polyurethane sealants have dominated automotive and construction for decades. MS polymer sealants have grown in market share for clear reasons. Understanding the difference helps you buy the right product for the job rather than defaulting to what you've always used.
How PU Sealants Work
Polyurethane sealants cure through a moisture reaction. As they absorb moisture from the air, the polymer chains cross-link and harden. Most automotive PU sealants contain free isocyanates as part of this process.
Isocyanates are sensitisers. With repeated exposure they can cause occupational asthma. The UK's Health and Safety Executive identifies isocyanates as one of the leading causes of work-related asthma in the UK. Workshops using PU sealants regularly are required to assess and manage this risk under COSHH regulations.
PU sealants typically cure to a rigid or semi-rigid finish. Bond strength is high. Resistance to water, fuel, and oils is good. For high-throughput accident repair where cure speed is the priority, they perform well.
How MS Polymer Sealants Work
MS polymer (modified silicone polymer, also called silyl-modified polymer or SMP) was developed to take the best properties from silicone and polyurethane without the main drawbacks of either.
Like PU, MS polymer cures through moisture. Unlike PU, it does not use isocyanates. The result is a sealant that is safer to handle in enclosed environments, bonds well to most substrates, stays flexible after cure, and can be painted over.
PU vs MS Polymer: Side by Side
Isocyanate content: PU contains isocyanates. MS polymer does not. Direct impact on COSHH compliance.
Flexibility after cure: MS polymer stays more flexible. PU cures harder. On panels that flex, vibrate, or expand with temperature, MS polymer holds better long-term.
Paintability: MS polymer is paintable once skinned. Most PU formulations, including Tiger Seal, are not reliably paintable.
Working time: MS polymer gives slightly more open time before skinning. Useful when positioning panels or working longer joints.
Cure speed: PU cures faster to handling strength. If turnaround time is critical, PU has an advantage here.
UV resistance: Both perform well outdoors. MS polymer has a slight edge for exterior applications in direct sunlight.
Removal and repairability: MS polymer's flexible cure means cleaner removal when a panel needs to come off. PU's rigid bond makes clean separation difficult without damaging substrates.
Which to Choose
For maximum cure speed and rigid bond strength in high-volume accident repair, PU is still the practical choice. The trade-off is isocyanate exposure and limited repairability.
For finish quality, COSHH compliance, and long-term performance on panels that move and flex, MS polymer is better suited. Restoration workshops, van converters, coachbuilders, and prestige bodyshops doing work where finish matters tend to prefer it.
Mansory Automotive Adhesive & Sealant
Mansory is an isocyanate-free MS polymer sealant made for UK automotive workshops. Paintable, UV resistant, strong bond on metal and plastic. 310ml cartridge. Tested with 200+ UK workshops.