Wood technology researchers at BRE have been studying ‘wet gluing’ since the early 1990s, having initially liaised with the New Zealand Forest Research Institute which patented the brown coloured Greenweld system. BRE-CTTC has also evaluated another wet gluing system based on a clear polyurethane adhesive.
Conventional adhesives can be used only on dry timber at room temperature and, before jointed components can be robustly handled, the adhesive needs to be cured for some time under pressure or by using high energy curing techniques. By comparison, says BRE-CTTC, these new wet systems ‘are effective on wet, unseasoned timber as well as on dry timber’.
The Greenweld system has been found to be effective at any temperature and self cures in a matter of seconds, while initial curing takes a little longer with the polyurethane adhesive system. The Greenweld system produces a brown glue line which is almost completely obscured once jointed components are stained or painted; however, the polyurethane system creates an almost invisible, colourless joint – especially when wood components are end-matched to give the appearance of continuous natural grain.
Tests at the centre indicate that both these wet gluing systems have ‘adequate strength and durability for external joinery when used on British-grown pine’, although the polyurethane route has been found to be preferable for gluing green (wet) oak.
According to BRE-CTTC, potential products from green glued British-grown timbers include: high class joinery (windows and doors) from pine, oak and spruce; character flooring from oak and ash; internal mouldings from pine, oak and spruce; agricultural joinery (high class gates) from oak; and cladding from pine, oak and spruce.
Dr Bravery said the two adhesive systems were provoking a high level of commercial scrutiny since they offered a means of adding value to a variety of timbers. Interest in the Greenweld system, he added, included upgrading softwood timber for structural purposes through defect cutting and end jointing.
‘Market acceptance is slow,’ he acknowledged, ‘because there is scepticism that a piece of wood cut and joined with glue will be stronger than other parts of the same piece of wood. Therefore, there is need for more research and more input into standards, as well as further marketing and promotion work.’
One UK company already to have benefited from the development of Greenweld is Stadium Packaging Services, which has used the adhesive to finger-joint short lengths of wood to make longer lengths and has thereby created a viable outlet for previously unusable waste (TTJ December 16/23, 2000).
The process allows the removal of weaknesses such as knots, while the strength of the joints exceeds BS 5291. Even more important from the company’s perspective is the ability of the adhesive to work on wet, unseasoned timber. According to Stadium, ‘this means that when it is approved for constructional use on timber with more than 18% moisture, the timber can be used for high quality studs, trusses, windows and door frames in the building industry’.
