Building Construction Handbook, Eighth Edition

Adhesives ~ although timber laminates are carefully machined, the

minimum of cupping permitted and efficient cramping methods

employed it is not always possible to obtain really tight joints

between the laminates. One of the important properties of the

adhesive is therefore that it should be gap filling. The maximum

permissible gap being 1.25 mm.

There are four adhesives suitable for laminated timber work which

have the necessary gap filling property and they are namely:-

1. Casein † the protein in milk, extracted by coagulation and
   precipitation. It is a cold setting adhesive in the form of a
   powder which is mixed with water, it has a tendency to stain
   timber and is only suitable for members used in dry conditions
   of service.


2. Urea Formaldehyde † this is a cold setting resin glue
   formulated to MR/GF (moisture resistant/gap filling). Although
   moisture resistant it is not suitable for prolonged exposure in
   wet conditions and there is a tendency for the glue to lose its
   strength in temperatures above 40°C such as when exposed to
   direct sunlight. The use of this adhesive is usually confined to
   members used in dry, unexposed conditions of service. This
   adhesive will set under temperatures down to 10°C.


3. Resorcinol Formaldehyde † this is a cold setting glue
   formulated to WBP/GF (weather and boilproof/gap filling). It is
   suitable for members used in external situations but is
   relatively expensive. This adhesive will set under temperatures
   down to 15°C and does not lose its strength at high
   temperatures.


4. Phenol Formaldehyde † this is a similar glue to resorcinol
   formaldehyde but is a warm setting adhesive requiring a
   temperature of above 86°C in order to set. Phenol/resorcinol
   formaldehyde is an alternative, having similar properties to,
   but less expensive than resorcinol formaldehyde. PRF needs a
   setting temperature of at least 23°C.

Preservative Treatment † this can be employed if required,

provided that the pressure impregnated preservative used is

selected with regard to the adhesive being employed. See also

page 464.

Ref. BS EN 301: Adhesives, phenolic and aminoplastic, for load-

bearing timber structures. Classification and performance

requirements.

Laminated Timber