What is a One-Part Cold Applied System?

A one-part cold applied system is a joint sealant that arrives ready to use. It cures by drawing moisture out of the air. There is no mixing, no pot life, and no plant beyond a gun or a pump. Most are polyurethanes. They are the straightforward option for floor joints, car park decks and trafficked areas, where the surface does not have to reopen the same shift.

At Shepherd and Sons we install both one-part and two-part cold applied systems. From working with clients on warehouse floors, ports and commercial decks, we've found the choice between them is almost always decided by programme rather than by chemistry. What matters is how fast the joint has to take traffic.

Two-part sealants cure because you mix a base with a hardener and a chemical reaction starts. A one-part cures by reacting with atmospheric moisture. That means it cures from the surface inwards.

That single fact explains almost everything about how the material behaves on site. It skins over quickly and goes hard slowly. The moisture has to work its way down through material that is already curing above it.

Typical published figures for a one-part polyurethane are a skin time of around 50 minutes and a tooling time of about 40 minutes at 23 degrees. Both are short. The manufacturer's own data then gives a cure rate of roughly 3.5mm per 24 hours, and that is the number people miss.

Take a joint 16mm deep. At 3.5mm a day, curing through takes about four and a half days, not the 24 to 48 hours people often assume from the skin time.

Then apply a British working temperature. Published figures for time to 80 per cent of final hardness run at two days at 23 degrees. At 10 degrees it is five days, and at five degrees it is six. A Kent site in February is looking at most of a week.

None of that is a fault in the material. It is simply what moisture cure means, and it is the thing to programme around rather than discover.

In our experience, a two-part system works better than a one-part where the surface has to carry traffic the same shift. A two-part cures throughout at once, instead of a few millimetres a day. Thioflex 555 machine grade returns to service in about 30 minutes at 20 degrees. Nothing one-part comes close to that.

Simplicity, and the absence of plant. There is no mixing, no ratio to get wrong, no pot life ticking, and no equipment to clean out at the end of a shift.

That matters on scattered work more than on a single long run. A day of small joints across a large building is a bad fit for a two-part pump. It is a good fit for a cartridge or a bulk gun.

It also matters where a two-part cannot be justified commercially. A modest joint length in a warehouse does not warrant mobilising machine-grade equipment. The material cost per litre is usually a little lower as well.

A good one-part polyurethane is an elastic sealant with real movement capability. Published figures include plus or minus 25 per cent to one test method. To the cold applied sealant standard it is plus or minus 35 per cent, with elastic recovery around 90 per cent.

The standard that matters is BS EN 14188-2, which covers cold applied joint sealants. Older documents cite BS 5212, and that has been withdrawn, so a specification still quoting it is quoting a superseded standard.

This is where specifications go wrong most often. One-part and two-part are not shorthand for non-fuel-resistant and fuel-resistant.

What decides whether a sealant survives fuel exposure is whether it carries a declared fuel-resistant classification. Under the cold applied standard that is a Type F designation. It is stated on the data sheet, or it does not exist.

So a one-part is the wrong choice for an aircraft fuelling apron, a fuel farm or a forecourt. The common one-part polyurethanes carry no such declaration. That is why our aviation work runs on two-part polysulphide instead. It is not because two-part is inherently better, it is because that is where the fuel-resistant classifications live.

The material is only half of it. A sealant fails at the geometry more often than at the chemistry.

A movement joint wants a bead roughly twice as wide as it is deep. It also wants to be bonded on two faces only, so it can stretch between them as the slabs move.

If the sealant also bonds to the bottom of the slot it is held on three sides. It then cannot stretch at all. It tears instead, usually at the weakest face.

A backer rod prevents that. It sets the depth of the bead and stops the third bond forming, and it is not the place to save money.

Depth matters twice over with a one-part. Too shallow and the bead has no movement capacity. Too deep and you have added days to the cure for no benefit at all.

Priming is the other thing worth confirming. Some substrates need a primer and some do not, and the manufacturer's data sheet is the authority rather than habit.

Width is worth checking against movement as well. A joint that has to accommodate real slab movement needs a bead wide enough to stretch, and widening the slot is cheaper than replacing a failed seal a year later.

One-part sealant cures with moisture, which means the tube is fighting the air from the day it is made.

Shelf life is therefore real and it is shorter than people expect. Unopened cartridges have a dated life, and part-used ones have effectively none once the nozzle has been opened.

Cold storage extends it. Heat shortens it, and a box left in a van through a hot week can go off before anyone has used it.

The practical consequence is ordering. Buying a large quantity on price, then using it across a long programme, tends to end with the last of it unusable.

It is worth checking the batch date on delivery rather than at the point of use, because the second is too late to do anything about it.

• How soon must the joint take traffic? Same shift means two-part.
• Is there fuel, de-icer or aviation chemical exposure? That needs a declared fuel-resistant grade.
• How much joint is there? A long continuous run justifies plant; scattered joints do not.
• What is the site temperature going to be? Cold slows a one-part far more than a two-part.
• How deep is the joint? Depth multiplies a one-part's cure time and does not affect a two-part's.
• Is the joint horizontal or does it run up a face? A self-levelling grade will not stay put on an upstand.

Both are legitimate systems and we install both. There is more on how cold applied sealing works generally, and on the two-part system where speed or fuel resistance decides it.

For one-part cold applied joint sealing across Kent, London and the South East, get in touch with our team.