Bonding timbers treated with preservative is notoriously difficult because the preservative, by its very nature, hinders the adhesive from curing. These preservative deposits and water repellents (which are retained near the surface), together with the problem of the moisture content of the timber being very high, resist the adhesive from wetting out on the substrate thereby interfering with the curing process. High moisture contents will also interfere with the adhesive curing, and in addition, may set up high stress concentrations due to subsequent shrinkage of the glued structures. On the plus side, kiln-drying of timber treated with a water-borne preservative should present no significant bonding problems.
Which adhesive should I use for bonding preserved or treated timber?
The best adhesives to use for bonding timber with preservative treatment on are PRF (phenol resorcinol) and MUF (melamine urea formaldehye) adhesive systems. These adhesives are fully resistant to moisture and to long term weathering, but are able to cope with the difficulty of bonding to the preserved, or treated, wood. However, it may necessitate special care when gluing if bond strength is to be fully developed. Fully cured PRF adhesives are not affected by even the severest treatment, such as vacuum-pressure impregnation with heated solutions.
Preservative treatment may be carried out either before or after gluing. Ideally the preservative treatment should be carried out after the timber has been glued and the glue-line is fully cured, but practical considerations often preclude this.
There aren't many manufacturers making PRF and MUF adhesive systems. The main ones are Akzo Nobel, Momentive and Dynea. Ureka distribute Dynea's range of PRF and MUF systems. Here are two options;
PRF - Woodstikk PRF. A two component, 1:1 mix ratio system which gives the ultimate bond in terms of strength and weather resistance.
MUF - Woodstikk MUF. A two component, 5:1 mix ratio system which gives an opaque glueline for applications where a dark glueline, like Prefere 4050, cannot be used.
HINTS AND TIPS FOR BONDING PRESERVED OR TREATED TIMBER SUCCESFULLY
- Kiln-dry to a suitable moisture content for the chosen adhesive.
- Sand the surfaces to be bonded. Heavy sanding may be necessary where salt deposits are high. Note: After sanding, the surfaces must be flat enough to ensure a close- fitting joint.
- Glue the joint not more than 24 hours after sanding.
- If the treatment is to be carried out after gluing, the adhesive must be allowed time to gain sufficient strength to withstand the effect of the tretment.
For more detailed information as to the different types of preservatives and what is needed to successfully bond them, download the guide "Gluing techniques for bonding preserved timber" in the downloads section on this page.
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