Plywood Glue adhesive : ADHESIVES Knowledge
All veneer-based products involve the use of adhesives during manufacture. For example, plywood is manufactured from sheets of cross-laminated veneers or plies, arranged in layers and bonded with an adhesive. Adhesive systems and
gluing protocols are therefore critical components in veneer-based product manufacture.
A variety of adhesives can be used for bonding veneers during product manufacture. Australian and New Zealand standard AS/NZS 2754.1:2016 provides information relating to the suitability of adhesives for the manufacture of plywood and laminated veneer lumber (LVL). Factors that influence adhesive selection include cost, compatibility with the assembly process, glue line strength and durability requirements and intended end use for the product. Bond durability is related to the expected conditions the glue line will be exposed to .
Many adhesive systems used to manufacture veneer-based products are
composed of resin, fillers and extenders. The resin content is the amount of resin
solid in the glue mix and it is the resin that interacts with the substrate to form the
adhesive bond. This should not be confused with the general term ‘resin’ that is
often used to describe the overall adhesive mix. Fillers and extenders are added
to some adhesive systems and can assist adhesion and reduce costs. Common
fillers include nutshell flour (e.g. macadamia nut), wood flour (e.g. alder bark)
and mineral flours, while common extenders include wheat flour, maize flour and
starches. These are inert materials added in small quantities to improve adhesive
working properties such as viscosity, tack, transfer and cohesive strength. Correct
viscosity is important to prevent the adhesive from being over- or under-absorbed
into the timber face. Water is often added to adhesives to assist with viscosity
control. Specific viscosities may be required to suit the type of equipment used to
apply the adhesive and the overall manufacturing protocols.
There are two main categories of wood adhesive: thermosetting and
thermoplastic. Thermosetting adhesives provide a rigid glue line that, once
cured, will not re-plasticise if heated. Examples include the formaldehyde-based
adhesives. It is generally accepted that these will not creep when subjected to
long-term stress. Thermoplastic adhesives will re-plasticise when reheated.
Examples include polyvinyl acetate (PVA) adhesives. It is generally accepted that
thermoplastic adhesives will creep under long-term stress and for that reason they
are not acceptable for any structural plywood