Why PU sealant costs more than silicone per cartridge

A PU cartridge usually costs more than a silicone cartridge of the same volume. Buyers notice the invoice line and miss the three lines below it.

The raw material is genuinely more expensive

Polyurethane prepolymers, isocyanates and the plasticisers and fillers that go into a structural grade cost more than the siloxane chemistry in a general-purpose silicone. That is the visible part of the difference, and it is the smallest part.

Coverage per cartridge is not the same

Coverage depends on density and on how much of the bead you can actually use. A 310 ml cartridge of PU at density 1.45 g/cm³ delivers roughly 450 g of sealant. The same volume of a filled silicone may be denser or lighter depending on filler load.

The practical number is metres per cartridge at your bead size — and that number moves with the joint, not with the price tag.

The rework line

This is where PU usually wins the calculation:

• A silicone bead in a body shop contaminates the paint line. One fish-eye repair can cost more than a pallet of cartridges.
• A sealer that is not sandable adds labour at every seam.
• A bead that cracks at 18 months means a warranty visit, and warranty visits are priced in hours, not in grams.

Where silicone is the cheaper answer

Exposed glazing joints, sanitary work, and any joint that will never be painted and moves a lot. Paying for PU strength in a bathroom is paying for performance nobody uses.

A way to compare quotes

Ask for cost per linear metre at your bead geometry, not price per cartridge:

cost per metre = cartridge price / metres per cartridge
metres = usable volume / bead cross-section area

A triangular bead 10 mm wide and 10 mm high has a cross-section of 50 mm². A 310 ml cartridge, allowing 5% waste, gives about 5.9 m. At that point the comparison is between two numbers that mean the same thing.

The comparison, with numbers

Take a 20 mm × 10 mm joint, 1,000 m of it, and two products at realistic cartridge prices.

bead section            20 × 10 ÷ 2 = 100 mm²  → 100 ml/m
PU,   310 ml at USD 6   usable 295 ml → 2.95 m per cartridge → USD 2.03/m
silicone, 310 ml at USD 4  usable 295 ml → 2.95 m → USD 1.36/m
difference              USD 0.67/m → USD 670 over 1,000 m

Now the rework side. One paint defect caused by silicone contamination costs a panel preparation and a repaint — hundreds of dollars, and a day of booth time. Three such incidents over the life of the project exceed the entire PU premium.

That is the break-even: the PU premium is recovered if the silicone route causes roughly three paint reworks per 1,000 m. In a body shop or a painted façade, that threshold is met quickly; on an unpainted sanitary joint, it is never met, and silicone is simply cheaper.

Where the comparison is usually made dishonestly

Suppliers compare cartridge prices, or cost per litre, and stop. Both ignore:

• Coverage, which depends on bead section and waste, not on cartridge volume.
• Process steps, which differ by chemistry — primer, topcoat, cure window.
• Rework risk, which is a probability multiplied by a cost that is much larger than the material.

The only fair number is cost per finished, accepted metre. Everything else is a price, not a cost.

What this means for buyers

1. Compare cost per metre at your bead size, never price per cartridge.
2. Count the rework risk: paint contamination and sanding labour belong in the comparison.
3. Do not over-specify: silicone is correct for joints that will never be painted.
4. If a quote is far below the market, ask which grade of raw material changed.