DEV Community: Krypton Chemical, S.L.

Aliphatic vs Aromatic Polyurethane Topcoats

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:58:32 +0000

Aliphatic vs aromatic polyurethane is the choice that decides how a roof coating looks after a few summers. It has little to do with whether the roof stays watertight. Aromatic systems are tough, economical and fast, but they change colour in sunlight. Aliphatic systems keep their colour and gloss, which is why they are used as the top layer.

Most exposed liquid roofing systems use both: an aromatic membrane for waterproofing, and an aliphatic topcoat for UV stability. This article explains why, with values from Krypton Chemical technical data sheets (TDS).

The chemistry in one paragraph

Polyurethanes and polyureas are built from isocyanates. Aromatic isocyanates (such as MDI) contain a benzene ring. Under UV light, that ring structure forms coloured by-products, so the surface yellows or darkens. Aliphatic isocyanates have no aromatic ring. They are far more resistant to UV-driven colour change. The Wikipedia article on polyurethane gives more background on isocyanate types.

What the data sheets say about UV

The manufacturer's own wording is the clearest evidence:

Product (TDS)	Chemistry	UV statement in TDS
Polyurea Rayston	Aromatic	"A colour change is to be expected under sunlight. This change does not affect its mechanical properties."
Impermax	Aromatic PU	"A colour change is expected… This discolouration does not affect its properties."
Impertrans	Aliphatic PU	"High retention of brightness, color and mechanical resistance when exposed to weather and UV radiation."
Colodur	Aliphatic PU	"Does not yellow on exposure to sunlight."
Impermax A	Aliphatic PU membrane	"Does not yellow if exposed to sunlight."

So the aromatic layer keeps working when it discolours. The aliphatic layer is there for appearance, gloss retention, wear resistance and reflectance.

Aliphatic vs aromatic polyurethane topcoats: TDS values

Property	Impertrans (colourless / pigmented)	Colodur (colourless / pigmented)	Impermax A
Hardness (ISO 868)	65–70A / 85–90A	53D / 60D	80A
Elongation at break	276% / 380%	173% / 70%	above 250%
Tensile strength	10.4 / 11.7 MPa	27.4 / 15 MPa	above 6 MPa
Solar reflectance, white	83.4% (ASTM E903)	84.7% (ASTM E903)	81.1% (ASTM E903)
Thermal emittance, white	—	0.82	0.83
SRI, white	103	105	100
Service temperature	-40 to 80 °C	-40 to 80 °C	up to 140 °C
Abrasion (Taber)	—	11 mg (CS-10, 1 kg)	43.5 mg (CS-17, 1000 cycles)

A dash means the value is not stated in that TDS.

How to read this table

Impertrans is the elastic topcoat. With 276–380% elongation, it follows a highly elastic membrane without cracking. The TDS lists it for waterproofing membranes "of high elasticity".
Colodur is the hard topcoat. At 53–60D, with low Taber abrasion, it suits foot traffic and wear. With quartz sand broadcast, the TDS reports slip-resistance Class 3.
Impermax A is an aliphatic membrane, not just a topcoat. Its TDS says it is normally applied as a second, exposed layer over an aromatic 1K polyurethane, which "would save a step" because no separate thin topcoat is needed.

Matching topcoat elongation to the membrane

A topcoat that is much stiffer than the membrane below can craze when the membrane stretches. That is a common cause of "cracked roof coating" complaints where the waterproofing underneath is still intact.

Practical pairing from the TDS:

Very elastic polyurea such as Impermax Polyurea H Supreme (650% elongation): its TDS recommends "aliphatic finishes with good elasticity", such as Impertrans Pigmented or Impertrans Eco.
Trafficked balconies and terraces: Colodur over Impermax, as shown on the roofs, balconies and terraces page.
Two-layer liquid roof without a separate topcoat: aromatic Impermax, then Impermax A.

A waterborne, almost solvent-free aliphatic option also exists: Colodur Eco. Its TDS positions it mainly for heavily used floor coatings, including outdoor areas.

Cool roof finishes

White aliphatic finishes double as cool roof coatings. They reflect solar radiation and lower surface temperature. The relevant metrics are solar reflectance (ASTM E903), thermal emittance (ASTM C1371) and the Solar Reflectance Index (ASTM E1980). All three appear in the data sheets above.

An SRI of 100 corresponds to a standard white reference surface. Values above 100, such as Colodur's 105 and Impertrans' 103, indicate a cooler surface than that reference under the test conditions.

Independent product ratings are published by the Cool Roof Rating Council. Remember that dirt pick-up reduces reflectance over time, so the TDS cleaning and maintenance guidance matters.

See Krypton's cool roofs application page and the case note on the Impermax Aqua cool roof.

Application notes from the TDS

Impertrans: 1–2 layers at 200–250 g/m², depending on the opacity required. Apply by brush, roller or airless.
Pot life: colourless Impertrans and Colodur about 6 h; pigmented versions about 2 h (1 kg, 20 °C, 50% RH).
Shelf life: colourless 12 months, pigmented 6 months.
VOC: Impertrans 450 g/l (colourless) and 380 g/l (pigmented), below the 500 g/l limit of 2004/42/EC. Ventilate accordingly.
Maintenance: reapply decorative layers when worn by traffic or weather, as the membrane TDSs recommend.

Key takeaways

In aliphatic vs aromatic polyurethane, the difference is UV colour stability, not waterproofing.

Aromatic membranes discolour without losing mechanical properties; an aliphatic topcoat protects appearance.

Match topcoat elongation to the membrane: Impertrans (276–380%) for elastic membranes, Colodur (53–60D) for wear.

White aliphatic finishes reach SRI 100–105 in the TDS, which supports cool roof designs.

FAQ

What is the difference between aliphatic and aromatic polyurethane?

Aromatic polyurethanes use isocyanates with a benzene ring and yellow under UV. Aliphatic polyurethanes do not have that ring and keep their colour and gloss outdoors.

Do aromatic polyurethane membranes need a topcoat?

For appearance, yes, if exposed to sunlight. Krypton's data sheets state the colour change does not affect mechanical properties, but they recommend an aliphatic finish such as Impertrans or Colodur.

Why is my polyurethane roof coating turning yellow?

It is almost certainly an aromatic layer exposed to UV. Overcoat it with an aliphatic topcoat after cleaning, following the TDS recoating guidance.

What is a good SRI for a cool roof coating?

Higher is cooler. The white aliphatic finishes here declare SRI 100–105 (ASTM E1980). Check local building-code thresholds and rated values.

Polyurethane Waterproofing Membrane for Roofs

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:58:12 +0000

A polyurethane waterproofing membrane is a liquid that cures on the roof into a continuous elastic sheet. There are no laps, no torch-on seams and no mechanical fixings. That makes it well suited to roofs with many penetrations, upstands and changes of level, which are hard to detail with sheet membranes.

Not all polyurethane membranes behave the same way. One-component (1K) moisture-cured products are rolled or brushed. Two-component (2K) systems can be hot-sprayed and cure in seconds.

This article compares them using Krypton Chemical's Impermax technical data sheets (TDS).

How a liquid polyurethane membrane works

A 1K membrane such as Impermax contains an isocyanate-terminated prepolymer. It cures by reacting with moisture in the air into "an elastomeric, cold-applied polyurethane membrane", fully bonded to the substrate.

A 2K membrane such as Impermax 2K mixes a polyol/polyamine component with an aromatic isocyanate prepolymer in a hot-spray machine. It contains no solvents or mineral fillers. Cure is much faster.

For the polymer background, see the Wikipedia article on polyurethane.

Polyurethane waterproofing membrane: TDS comparison

Property (TDS)	Impermax (1K)	Impermax ST (1K, thixotropic)	Impermax 2K (hot-spray)
Application	Roller / brush, cold	Roller / brush, cold, verticals	Hot projection machine
Hardness (ISO 868)	65–70A	65–70A	83A
Max. elongation (EN ISO 527-3)	450–600%	421%	350%
Tensile strength	2.5–4.3 MPa	3.4 MPa	13 MPa
Tear strength (ISO 34-1 B)	14 N/mm	—	43 N/mm
Adhesion: concrete / ceramic / PU foam	2.0 / 2.6 / 1.4 MPa	2.0 / 2.6 / 1.4 MPa	—
ETA	06/0263 (10 and 25 years)	06/0263	10/0296 (W3, 25 years)
Working time / reactivity	Pot life 4–6 h	—	Gel 16 s at 25 °C
Typical consumption	2 coats at 1 kg/m² each	—	1.8–2.0 kg/m², min. 1.9 mm

Sources: Impermax TDS, Impermax ST TDS, Impermax 2K TDS. A dash means the value is not stated in that TDS.

Reading the table

1K Impermax has the highest elongation and the easiest application. It is the practical choice for balconies, terraces, wet rooms and roofs with complex details.
Impermax ST is slightly thixotropic, so it can be applied to vertical surfaces "with minimal sagging". Its TDS also lists canals and water tanks.
Impermax 2K is much stronger (13 MPa, 43 N/mm tear) and cures in seconds. It suits large roofs, green roofs and fast-track projects.

What else the Impermax TDS confirms

Beyond the mechanical values, the 1K Impermax data sheet states:

External fire: Broof(t1) for any roof slope and Broof(t4) (EN 13501-5).
Thermal stability up to 140 °C. With Geomax fabric, it resists the 160 °C heat of poured asphalt.
Water vapour resistance µ above 1000 (EN 1931).
Root resistance certificates with and without reinforcement.
BBA certificate 11/4836.
VOC 184 g/l, below the 500 g/l limit of 2004/42/EC.

The European Technical Assessment matters most. It is the document that ties working life, climate zone and imposed load categories to a defined build-up. EOTA publishes the assessment documents used for liquid-applied roof waterproofing kits.

Green roofs and buried membranes

A green roof places the membrane under permanent moisture, root pressure and soil load. That is a harder service condition than an exposed roof.

Impermax 2K is "specially designed for the waterproofing of vegetable roofs (extensive and intensive) and planters". Its ETA includes root-penetration resistance to EN 13948, and the TDS says it "resists continuous contact with stagnant water (neutral) on decks". It is also partly bio-based, with renewable raw materials of vegetable origin.

See Krypton's notes green roofs written as a system and the Oviedo green roof with Impermax 2K, plus the gardened roofs application page.

Ponding water

Flat roofs hold water at low points. A membrane specification should treat ponding as a load case, not an accident.

The 1K Impermax TDS rates resistance to water at 24 h / 90 °C as 5 (best). Impermax 2K explicitly claims stagnant-water resistance. Read Krypton's note ponding is a design condition for the detailing side.

UV exposure: aromatic membranes need a topcoat

All three products above are aromatic. The Impermax TDS states that a colour change is expected and "does not affect its properties". The Impermax 2K TDS recommends an aliphatic finish (Colodur, Impertrans) to preserve appearance.

The pedestrian-roof system on the roofs, balconies and terraces page shows this: Porosity Sealer, then Impermax, then a Colodur topcoat.

Specification checklist

[ ] ETA number and working life (10 or 25 years) stated for the full kit
[ ] Primer matched to the substrate (concrete, ceramic, metal, old bitumen)
[ ] 1K: two differently coloured coats at 1 kg/m² each, so coverage is visible
[ ] 2K: 1.8–2.0 kg/m², minimum 1.9 mm, sprayed with wind below 25 km/h
[ ] Reinforcement at edges, corners and drains
[ ] Root resistance confirmed for green roofs
[ ] Aliphatic topcoat where exposed to UV

Key takeaways

A polyurethane waterproofing membrane gives a seamless, fully bonded roof layer with no laps.

1K Impermax: 450–600% elongation, cold-applied, ETA 06/0263.

2K Impermax 2K: 13 MPa, cures in seconds, root-resistant, ETA 10/0296 with 25-year W3.

Aromatic membranes need an aliphatic topcoat where appearance matters.

FAQ

How long does a polyurethane roof membrane last?

Look at the ETA, not the brochure. Impermax is assessed for 10 and 25 years; Impermax 2K for 25 years (W3), for the build-ups defined in those assessments.

Can a polyurethane waterproofing membrane be used on a green roof?

Yes, if it has root-penetration resistance. Impermax 2K is designed for extensive and intensive green roofs and its ETA covers EN 13948 root resistance.

Is a polyurethane membrane better than bitumen?

They suit different situations. A liquid polyurethane membrane has no seams and handles complex details well. The Impermax TDS also lists use over existing bituminous products with a suitable primer.

How many coats of liquid polyurethane membrane are needed?

For 1K Impermax, the TDS describes two coats at 1 kg/m² each, in different colours so missed areas are easy to see.

Open Cell vs Closed Cell Spray Foam Insulation

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:46:34 +0000

Open cell vs closed cell spray foam is not a quality ranking. The two are different materials made from the same polyol and isocyanate chemistry. They solve different problems in the building envelope.

Open-cell foam is light, soft and vapour-open. It is mainly an acoustic and thermal fill for interiors. Closed-cell foam is denser and rigid. It resists water and vapour and carries some load.

This article compares them using real values from Krypton Chemical's Rayston Spray Foam technical data sheets (TDS). It does not use generic "typical" numbers.

How spray polyurethane foam works

Spray polyurethane foam (SPF) is made on site by mixing a polyol blend (component A) with an isocyanate (component B) in a plural-component spray gun. A blowing agent expands the mixture within seconds.

Two things decide whether the cells end up open or closed:

The blowing agent. Water reacts with isocyanate to make CO₂, which tends to escape and leave open cells. Physical blowing agents such as HFO stay trapped in closed cells.
The formulation and density. Low-density foams are generally open-cell. Higher-density foams are closed-cell.

The trapped gas is why closed-cell foam insulates better per millimetre. For a general overview, see the Wikipedia article on spray foam.

Open cell vs closed cell spray foam: TDS data

Property (TDS)	Spray Foam 8W (open cell)	Spray Foam HFO 35	Spray Foam HFO 40	Spray Foam HFO 50
Cell structure	~95% open cells (ASTM D2856)	above 90% closed	above 90% closed	above 90% closed
Blowing agent	CO₂ from water reaction	HFO	HFO	HCFO-1233zd(E)
Applied density	8–10 kg/m³	30–40 kg/m³	42–47 kg/m³	48–55 kg/m³
Declared aged λ at 100 mm	0.038 W/m·K	0.027 W/m·K	0.027 W/m·K	0.026 W/m·K
Thermal resistance at 100 mm	2.63 m²K/W	3.70 m²K/W	3.75 m²K/W	3.80 m²K/W
Vapour resistance factor μ	4.6	≥ 80	≥ 80	80 (CE code MU80)
Short-term water absorption	—	≤ 0.2 kg/m²	≤ 0.2 kg/m²	0.2 kg/m² (CE code W0,2)
Compressive strength	—	—	—	above 200 kPa (stated application)

Sources: Rayston Spray Foam 8W TDS, HFO 35 TDS, HFO 40 TDS, HFO 50 TDS. A dash means the value is not stated in that TDS.

The closed-cell grades carry CE marking to EN 14315-1 and are classed Reaction to Fire Class E for all thicknesses.

What the numbers mean in practice

To reach about 3.7 m²K/W:

Closed-cell HFO 35 needs about 100 mm (3.70 m²K/W per its table).
Open-cell 8W needs about 140 mm (3.68 m²K/W per its table).

Where depth is limited, such as a rafter zone or a retrofit wall, that difference often decides the choice.

When to choose open-cell foam

The 8W TDS lists internal thermal and acoustic insulation of ceilings, attics, timber walls, masonry and steel structures, and lightweight buildings.

Its strengths are:

Acoustics. The TDS reports sound insulation of 40 dB at 195 mm plus one 13 mm plasterboard, and 44 dB with two boards (EN 10140-2).
Vapour openness. μ = 4.6 lets the construction dry, which can suit timber.
Low density. 8–10 kg/m³ adds very little weight.
Blowing agent. CO₂ from the water reaction only (ODP = 0, per the TDS).
Indoor air. VOC emissions Class A (ISO 16000).

Its limits: it is not a water barrier, and it should not be left exposed on a roof.

When to choose closed-cell foam

The HFO grades are intended for direct spraying on surfaces that do not carry permanent loads: interior walls and ceilings, and façades. The TDS notes that on façades the foam "provides waterproofing to water, maintaining some transpiration".

Choose closed-cell foam when you need:

More R per mm. About 0.026–0.028 W/m·K aged.
Moisture control. μ ≥ 80 and water absorption ≤ 0.2 kg/m².
Rigidity. HFO 50 is designed for applications needing compressive strength above 200 kPa.
Adhesion and air sealing on irregular substrates, including metal roofs.

A water-blown rigid option also exists. The Rayston Spray Foam 50W TDS declares a thermal conductivity of 0.030 W/m·K and no HFC or HCFC content.

Protecting foam on roofs

Spray foam on an exposed roof needs a waterproof, UV-stable skin. Krypton's membranes list adhesion to polyurethane foam in their data sheets. For example, the Polyurea Rayston TDS gives more than 1.5 MPa on 150 kg/m³ high-density PU foam (foam failure). Impermax 2K lists "protective finish of polyurethane insulating foam" among its applications.

See the systems for thermal and acoustic insulation of roofs, metal roofs and industrial roofs.

Application conditions checklist (HFO grades, TDS)

[ ] Component temperature 30–60 °C (HFO 35/40), 25–45 °C (HFO 50)
[ ] Working pressure 60–120 bar
[ ] Ambient 0–40 °C, RH below 90%, wind below 30 km/h
[ ] Substrate 5–40 °C with no surface condensation
[ ] Mix ratio 100:100 by volume
[ ] Respiratory protection and ventilation, since component B contains isocyanates

The Spray Polyurethane Foam Alliance publishes industry guidance on safe SPF installation and re-occupancy.

Key takeaways

In open cell vs closed cell spray foam, closed cell gives more R per mm (0.026–0.028 vs 0.038 W/m·K aged).

Open cell (8–10 kg/m³, μ 4.6) is a light, vapour-open thermal and acoustic fill for interiors.

Closed cell (30–55 kg/m³, μ ≥ 80) controls moisture and adds rigidity.

Exposed roof foam needs a waterproof, UV-protected membrane on top.

FAQ

Which is better, open cell or closed cell spray foam?

Neither in general. Closed cell is better where space is tight or moisture control matters. Open cell is better for acoustic fill and vapour-open timber construction.

What is the R-value of closed cell spray foam per inch?

The TDS values are given in metric. At 100 mm, HFO 35 declares 3.70 m²K/W, which is roughly 0.94 m²K/W per 25 mm. Always use the declared aged value from the product sheet.

Can open cell spray foam be used on a roof?

Only on the inside, as insulation under the deck. It is vapour-open and not water-resistant, so it cannot act as an exposed roof layer.

Does spray foam need a vapour barrier?

It depends on the climate and the build-up. Closed-cell foam has high vapour resistance itself (μ ≥ 80 here). Open-cell foam (μ 4.6) often needs a separate vapour control layer, so run a hygrothermal check.

Polyurea Water Tank Lining for Potable Water

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:46:15 +0000

A polyurea water tank lining is one of the few coatings that can do three jobs at once in a drinking-water reservoir. It forms a seamless barrier, it bridges cracks in the concrete, and it can be certified for contact with water intended for human consumption.

The third point is the one that matters most. In potable water, a lining is judged first on hygiene approvals, then on mechanical performance. This article covers both, using data from the Polyurea Rayston and Impermax Aqua 2K technical data sheets (TDS).

What a potable water tank lining has to do

An enclosed reinforced-concrete reservoir has a floor slab, walls, expansion joints, plinths and pipe penetrations. The lining has to stay continuous across all of them.

The engineering requirements are:

Hygiene: certified for contact with drinking water.
Watertightness: under constant hydrostatic head.
Crack bridging: concrete tanks crack with shrinkage and load cycles.
Chemical resistance: to water, chlorinated water and cleaning agents.
Continuity: no seams at corners, wall-to-floor junctions or penetrations.

Krypton's engineering note on lining drinking water tanks with polyurea sets out the same list as a design brief.

Certifications for polyurea water tank lining

The Polyurea Rayston TDS lists these approvals relevant to water:

Approval / test (TDS)	Reference
WRAS Material Approval (UK, water for human consumption)	Approval no. 2208579
Drinking water migration test (Applus)	no. 928/09/8505
CE marking, EN 1504-2	0370-CPR-2247
European Technical Assessment (ETAG 005)	ETA 16/0148
BBA certificate (UK, roofing)	18/5582
Environmental Product Declaration	EPD-IES-0020897

Always check that the approval covers the exact product and system you are specifying. Krypton's own note makes the point directly: do not conflate the Polyurea Rayston certifications with those of a different product. The UK scheme is run by WRAS / Water Regs UK, where approvals can be checked.

Performance data from the TDS

Property	Polyurea Rayston
Hardness (ISO 868)	87A / 35D
Elongation at break	324%
Tensile strength	16.2 MPa
Tear strength	69 N/mm
Watertightness (EN 12390-8)	Watertight at 5 bar (50 m water column)
Liquid water permeability (EN 1062-3)	0.002 kg/m²·h^0.5
Static crack bridging (EN 1062-7)	Class A5 at -10 °C
Adhesion to concrete with epoxy primer	4.0 MPa
Heavy metals (Sb, As, Pb, Cr, Ni, Se, Co)	each below 1 mg/kg

Immersion resistance

The TDS reports immersion tests on a 0–5 scale (5 = best). Relevant results for water tanks:

Water, 15 days at 80 °C: 5
Saturated salt water, 15 days at 80 °C: 5
Sodium hypochlorite 1%, 7 days at 80 °C: 3
Bleach, 7 days at 80 °C: 4

These are accelerated tests at 80 °C, which is far more severe than a cold-water reservoir. Use them to compare products, not to predict service life.

Typical system build-up

For enclosed concrete reservoirs, the build-up described in Krypton's tank note is:

Surface preparation: abrasive blasting or grinding, with defects repaired before spraying.
Concrete primer: 0.2–0.4 mm, for example Rayston Epoxy 100, or a humidity-tolerant primer where needed.
Hot-sprayed Polyurea Rayston: 1.8–2 kg/m², around 2 mm nominal thickness.
No internal aliphatic topcoat. An enclosed tank has no UV exposure, so the polyurea itself is the wet-contact surface.

The fourth point matters. A topcoat inside the tank adds another material to the water-contact surface, and that material would need its own approval.

Cold-applied alternative: Impermax Aqua 2K

Hot-spray rigs are not always practical, for example in small tanks with tight access. The Impermax Aqua 2K TDS describes a solvent-free, two-component polyurethane membrane "certified according to provisions of European regulation 98/83/EC" for contact with drinking water.

Property	Impermax Aqua 2K
Elongation at break	above 130%
Tensile strength	4 MPa
Hardness (Shore A)	66–69
Coat thickness	about 1 mm per 1.5 kg/m² coat
Chlorinated water (20 ppm)	5 (best)
Use temperature	-15 °C to 80 °C

Because it is solvent-free, it suits indoor and poorly ventilated spaces. It is a different product with different approvals, so specify it by name rather than as "equivalent to" the hot-spray polyurea.

Fire water and steel tanks

Not every tank holds drinking water. Fire-water reserves and process tanks need watertightness and corrosion protection, but not necessarily potable approval. See Krypton's pages on fire water tanks and steel tanks and containers. For potable use, start from the drinking-water reservoirs page.

Site QA checklist

[ ] Approval reference checked against the exact product and system
[ ] Substrate pull-off at least 1.5 N/mm²; moisture below 4% for standard epoxy primers
[ ] Primer thickness matched to porosity and moisture
[ ] Polyurea consumption recorded by weight, plus thickness readings
[ ] Corners, joints and penetrations detailed and inspected
[ ] Holiday or visual inspection for pinholes before filling
[ ] Cure, flushing and disinfection as required by the water authority

Key takeaways

In a polyurea water tank lining, the hygiene approval comes first: Polyurea Rayston lists WRAS approval 2208579.

Typical build-up: primer 0.2–0.4 mm, then 1.8–2 kg/m² hot-sprayed polyurea, with no internal topcoat.

Watertight at 5 bar and Class A5 crack bridging at -10 °C per the TDS.

Where hot-spray is impractical, Impermax Aqua 2K is the cold-applied, solvent-free option.

FAQ

Is polyurea safe for drinking water tanks?

Only if the specific product is certified for potable contact. Polyurea Rayston lists WRAS material approval and a drinking-water migration test in its TDS. Uncertified polyureas should not be assumed safe.

How thick should a polyurea tank lining be?

Krypton's tank build-up specifies about 2 mm, applied at 1.8–2 kg/m² over a primer.

Does a polyurea tank lining need a topcoat?

Not inside an enclosed tank. There is no UV exposure, and a topcoat would add another material to the water-contact surface.

Can you line a concrete water tank without spray equipment?

Yes, with a cold-applied membrane such as Impermax Aqua 2K, which is certified under 98/83/EC.

Polyurea Spray Application: Surface Prep Guide

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:39:57 +0000

Most failures in polyurea spray application are not chemistry failures. They are preparation, primer or equipment failures. Pinholes, blisters, soft spots and poor adhesion nearly always trace back to the substrate or to the spray rig.

This guide walks through the sequence that an applicator or site engineer should check. All the numbers come from Krypton Chemical technical data sheets (TDS), mainly Polyurea Rayston, a pure polyurea that "can only be applied by hot mechanical spraying equipment".

Step 1: confirm the substrate before polyurea spray application

The Polyurea Rayston TDS sets out clear support requirements. The substrate must be:

Flat, levelled, even and regular.
Compact and cohesive, with a pull-off value of at least 1.4 N/mm².
Free of cracks and fissures, or repaired beforehand.
Clean and dry, free of dust, oil, organic residue and laitance.

Moisture and temperature limits

Parameter (Polyurea Rayston TDS)	Requirement
Substrate moisture	below 4%
Substrate temperature	10–40 °C
Air temperature	10–40 °C
Relative humidity	below 85%

The TDS notes that higher humidity does not stop polymerisation, but it makes adhesion "increasingly difficult" because of condensation. A practical rule on site: measure the dew point and keep the substrate comfortably above it.

Step 2: mechanical preparation

Concrete must be prepared mechanically, by high-pressure sand or abrasion, to remove the surface layer and obtain an open pore. Sharp irregularities are removed with an abrasive disc. Dust is then removed by brushing or vacuum.

For steel, the substrate must be free of rust, oil and grease, and primed with a PU primer.

Surface preparation standards for steel are published by AMPP (formerly NACE and SSPC). Use them to write the blast grade into the specification instead of "clean steel".

Step 3: choose the primer

Primer choice is the most common place to cut corners. It is also where pinholes start. The Polyurea Rayston TDS troubleshooting table lists "bubbles or open holes in the membrane" and gives the cause: porous substrate, no primer.

Primer (TDS)	Type	Key data	Use
Rayston Epoxy 100	2K epoxy, ~100% solids	Mix 100:47 by weight; pot life 40 min at 25 °C; 80D; for substrates below 4% moisture	General concrete priming and sealing
Polyurea Primer	2K fast-curing polyurea	300–400 g/m²; pot life 60 min at 25 °C; adhesion to concrete >5 N/mm²	Rigid, porous, dry substrates
PU primer	Polyurethane	Steel adhesion 5.3 MPa with Polyurea Rayston	Metal substrates

The Polyurea Primer TDS adds a few substrate conditions: cohesion at least 1.5 MPa, compressive strength at least 25 MPa, and a 21-day wait on fresh concrete or mortar.

Where underlying moisture is suspected, the Polyurea Rayston TDS recommends two coats of epoxy primer: the first neat, the second with quartz sand broadcast. Spray polyurea only after the primer has fully cured.

Step 4: set up the hot-spray equipment

Pure polyurea gels in seconds (4 s at 25 °C for Polyurea Rayston). The machine does the mixing, so the machine must be right.

Recommended settings from the Polyurea Rayston TDS:

Component A: 65 °C
Component B: 75 °C
Hose: 65 °C
Spray pressure: at least 140 bar
Mix ratio: A=1, B=1 by volume (A=1, B=1.17 by weight)

Before loading, precondition both sides to 25–30 °C. Mix component A, including the pigment paste, then recirculate both components while heating. A compressed-air dryer is recommended.

Ratio errors show up as soft or sticky membrane. Krypton's note on 2K mix errors covers the warning signs.

Step 5: thickness and consumption

Polyurea Rayston is applied at 1.5–2.0 kg/m² to obtain 1.5–2 mm. The TDS recommends reaching the full thickness in a single application, and a minimum of 1 kg/m² to avoid open holes.

Check thickness during spraying. On site, track consumption by weight alongside a wet or ultrasonic thickness gauge.

Step 6: cure and return to service

The TDS gives a reference hardness curve (1 mm film, 25 °C, 50% RH):

Time after spraying	Hardness (Shore A)
5 min	28
20 min	55
1 h	70
24 h	80
4 days	88

The membrane is rain-resistant after about 10 minutes under most conditions. That speed is the main reason to choose hot-spray polyurea on roofs and bridge decks with short possession windows.

For bridge work, see the bridge decks application page and the note on ETAG 033 for bridge decks.

Pre-spray checklist

Run through this list before every polyurea spray application, not just on the first day.

[ ] Pull-off test at least 1.4 N/mm² (membrane) / 1.5 MPa (Polyurea Primer)
[ ] Moisture below 4%; substrate above dew point
[ ] Open-pore surface; laitance and dust removed
[ ] Primer selected, applied and fully cured
[ ] A 65 °C / B 75 °C / hose 65 °C, at least 140 bar
[ ] Test patch sprayed and checked for ratio and adhesion
[ ] Target 1.5–2.0 kg/m², single pass

Key takeaways

Good polyurea spray application starts with the substrate: at least 1.4 N/mm² pull-off and below 4% moisture.

Prime porous concrete. Pinholes are usually a primer problem, not a polyurea problem.

Follow the TDS temperatures and pressure; ratio errors show as soft or sticky film.

Spray full thickness in one pass and verify it during application.

FAQ

What surface preparation is needed for polyurea?

Mechanical preparation (abrasive blasting or grinding) to an open pore, a sound substrate with adequate pull-off strength, low moisture, and a suitable primer.

Why does sprayed polyurea get pinholes?

Usually because air escapes from a porous, unprimed substrate while the fast-gelling polyurea is curing. An epoxy primer seals the pores.

What temperature should polyurea be sprayed at?

For Polyurea Rayston, the TDS gives 65 °C for component A, 75 °C for component B and 65 °C for the hose, at a minimum of 140 bar.

Can polyurea be applied without a spray machine?

Pure hot-spray polyurea cannot. Where a rig is impractical, a cold-applied product such as Impermax Cold Polyurea Supreme is the intended alternative. See hot-spray vs cold-applied.

Polyurea vs Epoxy for Roof and Deck Waterproofing

Krypton Chemical, S.L. — Sun, 18 Oct 2026 13:38:10 +0000

The polyurea vs epoxy question comes up on almost every flat roof, podium and parking deck specification. Both are two-component reactive resins, both are sold as "seamless", and both bond well to prepared concrete. But they do very different jobs. One is an elastomeric membrane that follows the movement of the structure. The other is a rigid, hard-wearing coating that does not.

This article compares the two using real technical data sheet (TDS) values, not generic marketing ranges. It also shows where they work together in the same build-up.

What each material is, chemically

Polyurea forms from an isocyanate reacting with an amine-terminated resin. The reaction is extremely fast, so pure polyurea is normally hot-sprayed with plural-component equipment. The result is a tough elastomer with high elongation.

Epoxy forms from an epoxide resin reacting with an amine hardener. It cures slowly (tens of minutes of pot life) into a hard, highly cross-linked thermoset. That gives excellent adhesion, chemical resistance and compressive hardness, but very little stretch.

For background on the chemistry, the Wikipedia entry on polyurea is a reasonable neutral starting point.

Polyurea vs epoxy: TDS comparison

The table below uses three products from the same manufacturer, so the test methods are consistent. Polyurea Rayston is a pure polyurea membrane. Impermax Polyurea H Supreme is a hot-sprayed polyurea system designed for substrates with large movement. Rayston Epoxy 100 is a universal epoxy primer.

Property (TDS)	Polyurea Rayston	Impermax Polyurea H Supreme	Rayston Epoxy 100
Hardness (ISO 868)	87A / 35D	90A / 40D	80D
Elongation at break (ISO 527)	324%	650%	7.5%
Tensile strength	16.2 MPa	17 MPa	23 MPa
Tear strength (ISO 34-1 B)	69 N/mm	46 N/mm	n/a
Reactivity	Gel 4 s at 25 °C, tack-free 30 s at 70 °C	Gel 12 s, tack-free 20 s	Pot life 40 min at 25 °C
Static crack bridging (EN 1062-7)	Class A5 at -10 °C	"Excellent at extremely low temperatures"	not a crack-bridging product

Sources: Polyurea Rayston TDS, Impermax Polyurea H Supreme TDS, Rayston Epoxy 100 TDS.

The key number is elongation. A 7.5% epoxy film cannot bridge a live crack in a roof slab. A 324–650% polyurea membrane can.

Why crack bridging decides roof and deck waterproofing

Roofs and decks move. Thermal cycling, deflection and shrinkage open and close cracks every day. A waterproofing layer has to stay continuous across those cracks, especially in winter when the polymer is stiffest.

That is why a crack-bridging class should always be read together with its test temperature. Polyurea Rayston declares Class A5 static at -10 °C and Class B4.2 dynamic at 23 °C and -20 °C. Krypton's note on why the crack-bridging class must be stated in the cold explains how to read those classes.

Other roof-relevant values from the Polyurea Rayston TDS:

Low-temperature foldability: no cracking at -45 °C.
Glass transition temperature: -47 °C.
External fire: Broof(t1) (EN 13501-5).
Watertight at 5 bar (EN 12390-8).
European Technical Assessment ETA 16/0148 and BBA certificate 18/5582.

An epoxy floor coating has none of these roof-membrane approvals, because it is not designed to be one.

Where epoxy belongs in a waterproofing system

Epoxy is not the loser in the polyurea vs epoxy comparison. It does a different job, and most polyurea systems depend on it.

Epoxy as a primer under polyurea

Fast-curing polyurea has very little time to wet out a porous substrate. Air leaving the concrete pores can cause pinholes. The Polyurea Rayston TDS lists "bubbles or open holes" as a typical fault and gives the fix: apply an epoxy-type primer first.

With an epoxy primer, the TDS reports adhesion of 4.0 MPa on concrete and 1.6 MPa on plywood (cohesive wood failure). Where underlying moisture is suspected, it recommends two coats of epoxy primer, the second broadcast with quartz sand.

Epoxy as a wearing surface

Indoors, under forklifts and trolleys, a hard epoxy wins. EP Coat 100 is a 100% solids epoxy for industrial floors and parking decks. Its TDS lists 80D hardness, impact resistance of at least 14.7 Nm (EN ISO 6272-1) and Bfl-s1 reaction to fire.

That makes it a good choice for an internal car park floor, but not for an exposed roof that moves.

Decision checklist: polyurea or epoxy?

Use this as a quick screen before you open the data sheets:

Is the substrate expected to crack or move? Choose an elastomeric polyurea membrane.
Is the surface exposed to rain, ponding and frost? Choose a membrane with an ETA and a crack-bridging class at low temperature.
Is the main load abrasion and wheel traffic indoors? An epoxy coating is usually the better wearing layer.
Is the concrete porous or slightly damp? Prime with epoxy before spraying polyurea.
Is the membrane exposed to sunlight? Add an aliphatic topcoat. Polyurea Rayston is aromatic and will darken; the TDS recommends an Impertrans or Colodur topcoat.
Is a spray rig impractical? Look at a cold-applied polyurea instead of switching to epoxy.

Typical build-up for an exposed parking deck

A common deck build-up combines both chemistries:

Mechanical preparation to an open-pore surface.
Epoxy primer (with sand broadcast where needed).
Hot-sprayed polyurea membrane, typically 1.5–2 mm.
Aliphatic polyurethane topcoat for UV and wear.

Krypton's car park roof application page shows this kind of layered system. The pure polyurea vs hybrid on roofs note covers the next decision: which polyurea family to use.

Key takeaways

In the polyurea vs epoxy choice, elongation is the deciding property: 324–650% for the polyureas above vs 7.5% for the epoxy primer.

Polyurea is the waterproofing membrane; epoxy is the primer or the wearing course.

Always read crack-bridging classes with their test temperature.

Aromatic polyurea needs an aliphatic topcoat when exposed to UV.

FAQ

Is polyurea better than epoxy for roof waterproofing?

For an exposed roof, yes. A roof needs elongation and crack bridging, and polyurea membranes provide both. Epoxy is too rigid to act as the waterproofing layer, though it is often used as the primer.

Can you put polyurea over epoxy?

Yes. It is the standard approach on concrete. The Polyurea Rayston TDS says to spray only after the epoxy primer has fully cured.

Does polyurea turn yellow in the sun?

Aromatic polyureas change colour under UV. The Polyurea Rayston TDS says this does not affect mechanical properties. An aliphatic topcoat prevents the visual change.

What is the difference between polyurea and epoxy deck coatings?

Polyurea is an elastic membrane that bridges cracks. Epoxy is a hard, rigid coating that resists abrasion and chemicals but cannot follow structural movement.

DEV Community: Krypton Chemical, S.L.

Aliphatic vs Aromatic Polyurethane Topcoats

The chemistry in one paragraph

What the data sheets say about UV

Aliphatic vs aromatic polyurethane topcoats: TDS values

How to read this table

Matching topcoat elongation to the membrane

Cool roof finishes

Application notes from the TDS

FAQ

What is the difference between aliphatic and aromatic polyurethane?

Do aromatic polyurethane membranes need a topcoat?

Why is my polyurethane roof coating turning yellow?

What is a good SRI for a cool roof coating?

Further reading

Polyurethane Waterproofing Membrane for Roofs

How a liquid polyurethane membrane works

Polyurethane waterproofing membrane: TDS comparison

Reading the table

What else the Impermax TDS confirms

Green roofs and buried membranes

Ponding water

UV exposure: aromatic membranes need a topcoat

Specification checklist

FAQ

How long does a polyurethane roof membrane last?

Can a polyurethane waterproofing membrane be used on a green roof?

Is a polyurethane membrane better than bitumen?

How many coats of liquid polyurethane membrane are needed?

Further reading

Open Cell vs Closed Cell Spray Foam Insulation

How spray polyurethane foam works

Open cell vs closed cell spray foam: TDS data

What the numbers mean in practice

When to choose open-cell foam

When to choose closed-cell foam

Protecting foam on roofs

Application conditions checklist (HFO grades, TDS)

FAQ

Which is better, open cell or closed cell spray foam?

What is the R-value of closed cell spray foam per inch?

Can open cell spray foam be used on a roof?

Does spray foam need a vapour barrier?

Further reading

Polyurea Water Tank Lining for Potable Water

What a potable water tank lining has to do

Certifications for polyurea water tank lining

Performance data from the TDS

Immersion resistance

Typical system build-up

Cold-applied alternative: Impermax Aqua 2K

Fire water and steel tanks

Site QA checklist

FAQ

Is polyurea safe for drinking water tanks?

How thick should a polyurea tank lining be?

Does a polyurea tank lining need a topcoat?

Can you line a concrete water tank without spray equipment?

Further reading

Polyurea Spray Application: Surface Prep Guide

Step 1: confirm the substrate before polyurea spray application

Moisture and temperature limits

Step 2: mechanical preparation

Step 3: choose the primer

Step 4: set up the hot-spray equipment

Step 5: thickness and consumption

Step 6: cure and return to service

Pre-spray checklist

FAQ

What surface preparation is needed for polyurea?

Why does sprayed polyurea get pinholes?

What temperature should polyurea be sprayed at?

Can polyurea be applied without a spray machine?

Further reading

Polyurea vs Epoxy for Roof and Deck Waterproofing

What each material is, chemically

Polyurea vs epoxy: TDS comparison

Why crack bridging decides roof and deck waterproofing

Where epoxy belongs in a waterproofing system

Epoxy as a primer under polyurea