Between a glue and a seal

4 min read

Silyl-modified polymers (also known as silane-modified polymers, or SMPs), offer a wide applicability and share properties of both sealants and adhesives


Christian Tan Eng Teng, global business development manager, Bostik transportation sealants, answers questions about SMPs.

FAST: What does ‘SMP’ mean?

Christian Tan Eng Teng (CT): SMP stands for silyl modified polymer. It is a one-component technology that reacts with humidity from the air or the substrates. The reactive function is an alkoxysilane instead of an isocyanate in the polyurethane technology. Different SMP families exist and they differ by the polymer chain between the alkoxysilane functions. MS was the historical structure of SMPs, but there are now others, such as SPUR and STPE. They have different reactivity and final properties, and are formulated together with other raw materials to meet the needs of the market.

The technology was invented 40 years ago in Japan. About 30 years ago, the inventors approached us, with the result that Bostik in Europe was one of the first to extend SMPs to general industrial markets.

FAST: What’s the distinction between sealing and bonding?

CT: SMPs can be formulated for sealing and bonding applications. Sealing products are used to fill a gap between two parts that have been assembled. Compared to bonding products, they tend to have lower shore A, lower viscosity, higher elongation at the breaking point and higher elastic recovery. Some of them are self-levelling. Bonding products are used to bond two elements together; they have higher initial strength, final lap shear and higher cohesive strength.

FAST: What are the benefits of SMPs?

CT: Regulations regarding the use of polyurethane are more and more stringent due to the presence of di-isocyanate monomers. With repeated contact, even in small quantities, people can become allergic to the molecule, resulting in severe respiratory issues. SMPs are 100% free of di-isocyanante monomers.

New EU regulations on di-isocyanates coming in 2023 will require users of polyurethane products with more than 0.1% monomers to train all their permanent and temporary employees on dangers and the safe use of those products (see also Training has to be documented and repeated every five years.

SMPs are also known for their superior adhesion properties. After cleaning the substrate, typically with a PU you would need to dispense a primer and an activator, creating two extra steps. With SMP technology, these additional steps can be avoided. That’s beneficial for productivity and quality, as with every step that you add, there’s a risk of a mistake, therefore impacting overall lifecycle cost.

On the technical side, compared to most PUs used nowadays, SMPs are more stable when fresh or fully-cured, and are easier to apply at low temperature. PU technology has a tendency to become hard and less flexible when the temperature goes down. With SMP technology, this happens much less and they have overall better ageing properties.

FAST: What are the drawbacks?

CT: Most current SMPs tend to have lower elongation-at-break percentage and force. However, we have been selling this technology around the world in industrial and transportation markets without any issues for more than 30 years. Most specifications have been written based on the PU technology that they have been using for many years, rather than the applications’ technical needs. In most cases those requirements are overkill.

Once this is accepted, people can embark into a transition that is more in line with future trends and needs such as sustainable development. The SMP technology is isocyanate-free, can work without primers and reduces packaging waste – creating a more efficient process overall. We have been seeing much higher interest for the technology over the last couple of years and there seems to be no sign of this abating.

Typically, the €/kg price of SMPs is higher than PU, however most recently costs of raw materials have been very dynamic with many shortages and price increases. Today, the price disparity is lower. Through all its benefits, the SMP technology offers excellent lifecycle value cost.

FAST: How are SMPs applied?

CT: The most common packages are 200L/20L drums, 600ml aluminium foil sausages and 290ml plastic cartridges. Sausages and cartridges are applied using manual or electric/pneumatic guns. The product is pushed through a nozzle, creating a bead of sealant on the substrates. Automated drum unloaders are used with the drums when consumption is high.

Once applied, the product starts reacting with humidity from the air to create a skin at the surface. This will take place from between 10 to 90 minutes, depending on the product and the application. It will take a couple of days to fully cure, but the initial strength of the product is high enough so that you do not have to wait for full cure.

We also offer a two-part boosted system. With a specialised gun, you can inject and mix in humidity into the sealant. The static mixer ensures efficient mixing, the product will react much faster and users will not depend on the surrounding humidity in the room.

FAST: What are new developments in SMPs?

CT: Today, we are working on bridging the gap with polyurethane in terms of strength and elongation while maintaining the other advantages of the SMP technology.


Second opinion

Bob Orme (BO), senior applications engineer at Henkel, makers of Loctite, answers more SMP questions


FAST: What are the advantages and disadvantages of SMPs compared to silicones and butyls, which I see that you offer as well?

BO: SMPs’ advantages over silicones are typically more universal adhesion (more substrates); can be painted; lower cost than high-performance silicones. Their drawback is temperature resistance (80-100°C versus 250°C). Butyls are generally non-curing, so provide sealing but effectively near-zero strength.

FAST: Please would you explain the similarities and differences between elastic sealing and elastic bonding applications?

BO: All sealants must bond to the surface to create a robust seal. There is a continuum between elastic sealing, elastic bonding and structural bonding. We would normally think of elastic bonding when strength is around 2N/mm² or above.

FAST: How much flexibility do SMPs offer in terms of reducing viscosity for thinner bond lines? How thin in diameter can dispensing nozzles go for use with SMPs?

BO: In very thin bond lines, the flexibility provided by the adhesive/sealant is reduced, for example an elongation of 250% but in a gap of a few microns still only allows a small deflection to failure (although greater than a rigid sealer). We typically dispense beads down to around 1mm diameter.

FAST: What health labels do your SMPs attract?

BO: Generally there are no H&S warnings on these products.

FAST: What are the recent trends and new developments in SMPs?

BO: Two-part for faster cure; fully-transparent grades; improved temperature resistance.