Magnesium oxide (MgO) board has moved from a fire-door core material to a mainstream skin for SIPs, partition panels and modular construction — and it glues differently from OSB, steel or gypsum. We have supplied adhesive for MgO-faced fire doors and composite panels for years, and the same handful of process questions comes up every time. Here is how we answer them, with the actual numbers.
Sample test: two-component PU glue bonding an MGO+XPS+MGO panel.
Why MgO Bonds Differently
MgO board is mineral, porous and slightly alkaline. Three consequences for bonding:
- It drinks low-viscosity adhesives. A thin glue partly disappears into the surface before the joint closes, leaving a starved bond line.
- Its surface strength is finite. A well-designed joint fails inside the board (substrate failure), not in the adhesive — which is also how you should read test results.
- It carries moisture. For a moisture-curing PU that is actually helpful: the board itself feeds the cure.
The Adhesive We Recommend: 1K Moisture-Cure PU (MPU-20)
For MgO-to-core and MgO-to-frame bonding we recommend our single-component moisture-curing polyurethane, MPU-20 — the same grade used in fireproof door assembly, where MgO and calcium silicate boards are standard face and core materials. Key figures from the TDS:
| Property | Value |
|---|---|
| Type | Single-component, solvent-free, moisture-cure PU |
| Appearance | Dark brown / tan viscous liquid |
| Density | 1.15±0.05 g/cm³ |
| Viscosity | 5–20 Pa·s |
| Non-volatile matter | ≥94% |
| Coating weight | 180–200 g/m² |
| Open time | >45 min |
| Curing time | 4–5 h (accelerated by light water mist) |
| Bond strength | ≥15 MPa metal-to-metal (GB 6329); substrate failure on insulation and fireproof boards |
The viscosity range is the point: MPU-20 is thick enough not to vanish into the MgO surface, and the 45-minute open time covers panel lay-up on a normal line. After curing the adhesive layer is insoluble and non-melting, and it foams slightly as it cures — useful gap-filling on a board that is never perfectly flat.
Process That Works on MgO
- Dust is the enemy. MgO board sheds fine mineral dust from cutting. Brush or vacuum the faces — glue over dust and the bond is to the dust, not the board.
- Spread 180–200 g/m² with a serrated scraper on one face. On very absorbent batches, a first thin pass followed by the main pass keeps the bond line fed.
- Lay up within the open time and press. Even, moderate pressure until cured; a light water mist on the glue line shortens the 4–5 hour cure.
- Test to substrate failure. Accept the joint when the board fails before the glue line — on MgO, chasing adhesive-layer numbers beyond that adds nothing.
Where This Shows Up
The combinations we are most often asked to support: MgO skins on EPS or rock-wool cores for partition and modular panels, MgO faces in fire-door assemblies (with perlite, rock-wool, magnesium and calcium-silicate cores), and MgO as a non-combustible alternative to OSB in SIP-type sandwich panels. If your build pairs MgO with a skin we have not listed, ask — the answer is usually a coating-weight adjustment, not a different product.
FAQ
Do I need a primer on MgO board?
Usually no. With a 5–20 Pa·s moisture-cure PU at 180–200 g/m², clean dust-free board bonds to substrate failure without priming. Extremely absorbent low-density boards may want a doubled first pass.
Can the same adhesive bond MgO to steel?
Yes — MPU-20 is specified at ≥15 MPa metal-to-metal (GB 6329), and MgO-to-steel is a standard fire-door construction.
Is the bond affected by MgO board's chloride content?
Choose sulfate (MOC-free) board for humid service in any case; the adhesive layer itself is insoluble and non-melting after cure and is not degraded by the board chemistry in normal indoor service.
What packaging and shelf life?
25 kg plastic drums or 50/100 kg iron drums; one year sealed indoors. Reseal part-used drums promptly — the product cures with moisture.
Full substrate lists, certifications and a TDS are on the fire-door adhesive page, or contact us with your panel build-up for a grade confirmation and sample.
