Adhesives

Types

There are two groups of adhesives: thermosetting and thermoplastic.

Thermosetting adhesives are cured by heat and will not break down when heated. They are the most common structural adhesive.

Thermoplastic adhesives will soften when heated and creep under long-term loads. They can break down when subjected to moisture.

Types of adhesives - Printable PDF

Resorcinol formaldehyde adhesive visible along the veneer lines and at veneer laps in LVL

Source:  Courtesy David V Reid

Application

Manufacturers’ instructions must be strictly followed when adhesives are used because insufficient adhesive will cause bonding failure, while excess will cause low bond strength.

It is critical that the timber surfaces are prepared properly, and the type of adhesive used is appropriate for the moisture content of the timber.

Care must be taken with the assembly time (to avoid curing of the glue before the components are clamped), the ambient temperature, the adhesive mix and the pressure used to clamp components.

Glue laminated (glulam) manufacture

Strict procedures are followed for the mixing, application and spreading of glues during glue laminated timber component manufacture.

After application of a controlled quantity of adhesive by the glue spreader, the laminations are assembled in a clamping jig and pressures of between 400 kPa and 700 kPa are applied.

The clamping jigs are set out to the required camber or shape of the timber member and pressures are carefully checked by torque wrench or load cell.

Once timber members are secured in the press jigs temperatures are maintained during the curing process.

Depending on the type of adhesive, it may be necessary to provide heating to the pressing area to keep temperatures above the minimum curing temperatures specified by the glue manufacturer.

Site gluing

When site gluing components consideration must be made of the environment and the skill of the on site personal.

The accurate mixing of the hardener, a controlled temperature environment, a narrow band of moisture content of the timber and clamping of the components is not likely to be successfully achieved on site.

For this reason it may be appropriate to restrict site gluing to elastomeric construction adhesives which come in an easy to apply tube and don’t require mixing.

This is likely to mean that mechanical fixings will be required as well.

Alternatively the design could incorporate factory glued connections for transportable components then mechanical fixing on site.

Glulam beams being clamped during manufacture

Uses

Resorcinol formaldehydes

Resorcinol formaldehydes are used for the manufacture of glulam beams, laminated veneer lumber (LVL), plywood and durable finger jointing, particularly when components will be used in a moist environment. However, they are relatively expensive.

Melamine urea formaldehyde (MUF)

Melamine urea formaldehyde (MUF) is the most widely used adhesive for glulam that will be used internally (where the timber moisture content will be less than 18 percent), although an isocyanate adhesive may be used where a greater moisture resistance than MUF is required.

Epoxy resins

Epoxy resins have not yet proved to be effective or cost efficient for laminating purposes, but are used extensively for repairs or in fixing metal work dowels and plates into beams for jointing purposes.

Fabrication of an epoxy steel dowel joint

Source: Courtesy Timber Design Guide, 2007

Specification

There are variations in the performance of different brands of adhesives, so the manufacturer and adhesive product should be specified, particularly for structural applications.

For example, epoxy dowel joints have been tested using several different epoxy resins, so only those tested, with a known performance, should be used. Refer to Epoxy grouted steel rods.

Performance/Design

The PDF below includes the following information on performance/design data:

• Elastomeric adhesive design

• Cross-lapped glue joint design

Adhesives Performance and design data - Printable PDF