Bonding treated and dense timber with PUR

Treated timber is where a wood adhesive earns its keep and where assumptions fail. The treatment that protects the timber can also interfere with the bond, and the effect varies by preservative, retention and drying history.

What changes with treatment

• Surface pH and chemistry. Some treatments leave alkaline or acidic residues that affect cure.
• Wetting. Treatment can raise or lower surface energy; the adhesive may sit on the surface rather than penetrate.
• Moisture. Re-wetting after treatment raises MC; bonding too soon gives a foamed line.
• Density. Dense species absorb less adhesive, so spread rate and pressure need adjusting.

What to do before a structural run

1. Confirm the treatment type and retention with the timber supplier.
2. Check MC after treatment and drying — treated stock is often wetter than assumed.
3. Bond a sample joint and break it; look for wood failure, not glue-line failure.
4. If the sample fails, abrade the surface and retest before changing the adhesive.

Species notes

Dense hardwoods and high-extractive species (some oak, teak, merbau) are the usual suspects. The extractives can migrate to the surface and form a weak layer; planing shortly before bonding helps.

Treatment types and what they change

Treatment	Typical effect on bonding	What to do
Water-borne copper (ACQ, MCQ)	Surface residues; higher MC after treatment	Dry to 8–12% MC, abrade, test
Oil-borne (creosote, PCP)	Oily surface; poor wetting	Not normally bonded; test if required
Borate	Minimal effect at normal retention	Test as usual
Thermal modification	Lower surface energy, brittle surface layer	Abrade and test; expect lower strength

The common thread: the treatment is designed to resist biological attack, and some of that chemistry also resists adhesive wetting. The test joint is the only reliable filter.

Retention and penetration are different questions

A treatment specification usually states retention (kg/m³) and sometimes penetration depth. For bonding, penetration matters more: a treatment concentrated at the surface leaves a chemically hostile layer exactly where the adhesive needs to bond, while a deep, uniform treatment may have little effect on the surface chemistry.

Ask the treater two questions:

1. What is the surface retention, not just the average?
2. Was the timber re-dried after treatment, and to what MC?

A “dry” treated board can be 18% at the core for weeks. Bonding it produces the foamed line that gets diagnosed as an adhesive fault.

Thermal modification is the hardest case

Heat-treated timber is chemically different at the surface: the hemicelluloses have been broken down, the surface energy is lower, and the outer layer is brittle. PUR bonds it, but the strength is typically lower than the same species untreated, and the failure mode is often a thin layer of modified wood pulling away rather than wood failure.

If a project specifies thermally modified timber for a structural application, the bonding specification needs its own test data — a standard glulam approval does not transfer. Run a sample joint, break it, and record the result with the batch.

What this means for buyers

1. Treat “treated timber” as a different substrate, not a variant of the same one.
2. Test a sample joint for every treatment batch you buy.
3. Check MC after treatment; do not assume kiln-dried means dry.
4. Record treatment type and retention with the batch — it is the first question after a failure.