So, What are the Benefits of Thioflex 555?

The main benefits of Thioflex 555 are its 35% movement accommodation, fuel and chemical resistance, jet blast resistance, fast return to service (10 minutes at 35°C for Machine Grade), and 10-year-plus service life when installed correctly. It’s a two component polysulfide cold applied sealant manufactured by Fosroc, certified to BS EN 14188-2 Classes B, C and D, and used across UK aviation, highways, ports and public sector pavements where standard sealants won’t deliver the required performance. The product’s commercial advantage is that it combines specifications that few other sealants on the UK market can match in one product.

At Shepherd and Sons, we’ve installed Thioflex 555 across more than 200 UK projects in the past six years, including aircraft fuelling aprons, oil terminal hard standings, HGV refuelling stations and concrete carriageway joints. We’re members of the Extruded Sealant Association, and we’ve seen what the product delivers in real-world service over multiple winters. This article covers the genuine, specifiable benefits of Thioflex 555 and how each one translates into longer service life, fewer call-backs and better whole-life cost on a properly specified joint sealing programme.

The headline benefit, combined performance specifications

Most pavement joint sealants do one or two things well. Hot applied N1 cures fast and bonds aggressively but offers no real fuel resistance. One part silicone is easy to apply but has lower movement accommodation. Two part polyurethane is high-modulus but lacks the chemical resistance for aviation work.

Thioflex 555 combines high movement accommodation, broad chemical resistance, jet blast resistance and fast return to service in a single product. That’s the underlying commercial benefit. A specifier can use one sealant across a portfolio of fuel-exposed pavement work rather than maintaining separate specifications for different exposure classes. From working with clients across aviation and fuel handling, we’ve found this is one of the most underrated benefits of the product. It simplifies specification, procurement and QA across complex sites.

Benefit 1: 35% movement accommodation factor

Thioflex 555 has a movement accommodation factor of 35% under BS 8449. That means a 20mm joint can flex up to 7mm in either direction without sealant failure. For a typical contraction joint at 15mm width, that’s around 5mm of accommodated movement.

In practice this matters most on concrete pavement joints subject to large thermal cycles. A concrete slab on a UK airfield apron will move significantly between a January night at -5°C and a July afternoon at 35°C. A sealant with only 12% or 15% movement accommodation will fatigue at the joint walls under those cycles and fail within a few seasons. Thioflex 555’s 35% movement factor is what allows a 10-year service life.

In our experience, Thioflex 555 works better than hot applied N1 on aircraft parking aprons specifically because the higher movement accommodation handles the combination of thermal cycling and aircraft loading without cracking at the joint face, where N1 will show longitudinal hairline cracks within two to three years. On a regional UK airport apron we’ve maintained over the last decade, the Thioflex 555 joints are still performing where adjacent N1 sections have been resealed twice.

Benefit 2, chemical and fuel resistance across the full BS EN 14188-2 range

This is the benefit that determines specification on aviation and fuel handling work. Thioflex 555 is certified to BS EN 14188-2 Classes B, C and D, which means it’s tested under:

• Test Fuel I (aliphatic, kerosene-based)
• Test Fuel II (aromatic, aviation fuel substitute)
• Mono Ethylene Glycol 70% (de-icing fluid)
• Potassium Formate 50% (runway de-icer)
• Potassium Acetate 50% (runway de-icer)
• Diesel
• Hydraulic Oil (Castrol AWS68)

Test results across all seven chemicals show no adhesion or cohesion failure. The sealant absorbs limited volume changes (4 to 25% depending on the chemical) but the bond and the structural integrity hold.

What this means in real terms: on an aircraft fuelling apron, where Jet A-1 spillage and Skydrol drips are routine, the sealant retains its function across the full service life. On an MSA forecourt, where diesel and AdBlue are spilled daily, the joint stays sealed. On a runway where glycol-based de-icers are sprayed every cold snap, the joint walls don’t fatigue. Standard hot applied N1 doesn’t have these certifications and degrades rapidly under continuous fuel contact. We’ve found that 80% of the Thioflex 555 work we’re called in to specify is driven by exactly this chemical resistance benefit.

Benefit 3, jet blast resistance

Thioflex 555 carries certification to US Federal Specification SS-S-200E for jet blast resistance. This is a specialist certification that very few cold applied sealants on the UK market hold. It tests the sealant’s ability to remain bonded to the joint and intact under the high-velocity exhaust of jet aircraft.

For aviation specifiers this isn’t a marginal benefit. Joints near aircraft parking positions, on taxiway shoulders and on apron edges are subject to repeated jet blast cycles. A sealant without jet blast certification will be progressively eroded and detached by exhaust forces. We’ve seen non-certified sealants fail in under 12 months in these positions. Thioflex 555 holds up because the polysulfide chemistry retains elasticity and bond strength under impact loading.

Benefit 4, fast return to service

Machine Grade Thioflex 555 has class-leading return-to-service times for a cold applied polysulfide:

• 10 minutes at 35°C ambient
• 15 minutes at 30°C
• 30 minutes at 20°C
• 60 minutes at 10°C
• 90 minutes at 5°C

For a UK airfield programme running in summer conditions, this means a fuelling stand can be back in service within 15 to 30 minutes of sealing. That’s faster than many hot applied sealants once you factor in the time required to mobilise and reach pour temperature on a melter. It’s also fast enough to make Machine Grade Thioflex 555 viable for tight overnight closures on highway work, where two part cold applied sealants are traditionally seen as too slow.

Hand Grade is slower (60 minutes at 35°C up to 36 hours at 5°C) but appropriate for detail work and smaller programmes where speed isn’t critical.

Benefit 5, full service temperature range

Thioflex 555 operates across a service temperature range of -40°C to +70°C. This is genuinely wide for a polysulfide sealant and matters more than specifiers often realise.

At the cold end, the sealant doesn’t harden into a brittle bead in winter. Joints continue to absorb thermal movement during January cold snaps when hot applied N1 has lost most of its elasticity. At the hot end, the sealant doesn’t soften and extrude under midsummer apron temperatures, which can exceed 50°C on dark-coloured tarmac in direct sun. That temperature stability across the full UK operating range is what allows the 10-year service life to be realistic, not aspirational.

Benefit 6, high stress relaxation through polysulfide chemistry

Polysulfide sealants have a property called high stress relaxation. Under sustained joint movement, the sealant gradually distributes the stress through its molecular structure rather than transferring it to the bond face. This protects the adhesive bond and prevents progressive joint separation.

In practical terms, this is why Thioflex 555 doesn’t develop the characteristic adhesive failure pattern that affects lower-modulus sealants. We’ve assessed hundreds of failed cold applied installations over the years, and the bonded-but-cracked failure mode (where the sealant has pulled away from the joint face) is rare on Thioflex 555 where the priming was done properly. The polysulfide chemistry actively works against that failure mode.

Benefit 7, resistance to stone and dirt pickup

The cured sealant has a surface hardness (Shore A 10) that resists stone embedment and dirt pickup under traffic. This matters most on heavily trafficked aprons and hard standings, where grit and small stones can become embedded in softer sealants, accelerating wear and creating a rough joint surface.

A Thioflex 555 joint typically still looks clean and intact at five years on an apron that would have shown visible surface contamination on N1 within two. That visual integrity matters for aviation clients managing airside safety inspections.

Benefit 8, long service life and reduced whole-life cost

The combination of the benefits above translates into a service life of 10 years or more on properly installed Thioflex 555 joints. We’ve installed Thioflex 555 on aviation aprons in the early 2010s that are still performing without resealing.

This is the benefit that matters most commercially. A higher per-metre cost than hot applied N1 is offset by:

• Fewer resealing cycles over the asset life
• Reduced traffic management and airside access cost
• Lower whole-life maintenance budget
• Reduced disruption to operations

For a fuelling apron with limited airside access windows, the cost of mobilising for a reseal can be three to four times the cost of the original installation. A sealant that delivers 10 years instead of three pays for itself many times over.

Benefit 9, standards compliance and specification confidence

Thioflex 555 carries CE and UKCA marking and complies with:

• BS EN 14188-2:2004 (System M two-component, type sl, Classes B/C/D)
• BS 5212 Part 1:1990
• BS 8449 (movement accommodation)
• US Federal Specification SS-S-200E (jet blast resistance)
• ASTM C920 Type M, Grade P, Class 25

For specifiers writing technical specifications, this breadth of compliance simplifies approval. It’s the same product across UK, European and American specification regimes, which matters for clients with international operations or programmes following multiple standards.

Where the benefits hold up, and where they don’t

To be balanced about this: Thioflex 555 isn’t the right product for every joint. Its benefits are real in specific applications.

It works best on concrete pavement joints with fuel or chemical exposure where 35% movement accommodation and BS EN 14188-2 Class B/C/D certification are genuinely required. It works on asphalt with Fosroc’s Nitoflor FC130 plus Primer 7E system, though for asphalt-to-asphalt joints we use alternative specifications rather than Thioflex 555.

It’s not the right product for industrial floor joints inside buildings, where Fosroc themselves recommend higher-modulus sealants like Nitoseal MS300 or PU800. It’s not the most economical choice for standard motorway carriageway joints with no fuel exposure, where hot applied N1 is faster and cheaper. And it requires specialist installation discipline that general joint sealing contractors don’t have.

In our experience, Thioflex 555 works better than alternatives only when the application genuinely requires its full performance envelope. Specified for the wrong job, the additional cost isn’t justified by the additional capability.

Sectors where the benefits matter most

The benefits of Thioflex 555 translate into specification across the sectors we cover. Aviation clients value the combination of jet blast resistance, fuel resistance and fast return to service on fuelling aprons and parking stands. Highway authorities specify it at HGV refuelling stations and MSAs where diesel exposure rules out N1. Port operators use it in bunker fuel handling areas where bunker oil and hydraulic spillage are routine. Public sector clients specify it on military and emergency services fuel installations.

Specifying to capture the benefits

A meaningful Thioflex 555 quote will specify grade (Machine or Hand), linear metres, joint dimensions, primer requirement, programme constraints, cure-window protection, and QC sampling rate. The benefits of the product are only realised if it’s installed properly to the manufacturer specification and the ESA Code of Practice.

If you’re specifying or commissioning joint sealing work and want to discuss whether Thioflex 555 is the right product for your project, get in touch. We’ll assess the substrate, the chemical exposure, the programme, and recommend the specification that delivers the right whole-life cost. You can see recent project work on our LinkedIn and Instagram.