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2005 Schweighofer Prize

LIKE METAL, WOOD CAN "MELT" AND BE WELDED, A DISCOVERY AWARDED THE
2005 SCHWEIGHOFER PRIZE

The Schweighofer prize for innovation in wood science was awarded in June 2005 in Vienna to a joint Swiss-French team led by Tony Pizzi (France) and by Balz Gfeller (Switzerland) for their work on “wood welding”.

The research team has developed a wood bonding process that eliminates the need for adhesives. Almost 100,000 tonnes of furniture adhesives of petrochemical origin are used in France alone each year. They sometimes contain small quantities of toxic, or polluting chemicals. Their use is expensive and requires several hours of hardening to obtain a finished bonded joint.

There are two processes for glueless wood welding, both of which are very quick. The first consists of applying rapid, alternating mechanical friction to the two wood surfaces to be welded, under some pressure (see Figure 1).

Figure 1 Vibrational movements of two solid wood surfaces during wood fuston welding

The equipment for this type of work already exists for welding thermoplastics, for example in the car industry. At the temperatures attained by mechanical friction (>200°C) the characteristics of the lignin and hemicellulose between wood cell walls change, and start to flow, hence the initial phase of “melting”. The wood fibres released by this flow then become entangled, and form a high density composite and are “drowned” in the molten material and bonded in it, once it has cooled. This composite constitutes the bond, or weld line of the joint (Figure 2). The mechanical resistance of the joints formed by welding in 2-4 seconds is comparable to that obtained 24 hours after gluing.

Figure 2 Welding beech 150 x 20 x 30 mm. The black line indicates the weld line

The process can be applied for welding any two flat pieces of timber, of the same or different species, and can be used in the manufacture of furniture and joinery. The only limitation is that the joints are not of exteriorgrade, but only suitable for use indoors. One can now speak of “weldlam” where one now talks of “glulam”.

The second process developed is high-speed rotational welding of dowels. A dowel in rapid rotation, inserted by means of a simple electric drill into a substrate, welds to the substrate with a mechanical resistance 20 times greater than the traditional hammer inserted dowels, and with a strength comparable to dowels glued-in for 24 hours with PVA adhesive. This approach is particularly suitable for any small to medium sized company, workshop and DIY enthusiast as a standard inexpensive wood drill, is the only equipment needed.

Figure 3X-ray micrograph of a fluted groove beech dowel inserted and welded to two separate blocks of beech wood. The white areas are those of higher density. At the microcrack indicated by arrow (1), welding has not occurred or has broken. Arrow (2) indicates molten intercellular material which has seeped into the fissure between the two blocks.

It is perhaps paradoxical that the process was discovered by accident. Originally the Swiss partners used linear friction equipment to glue wood by inserting and melting a plastic between the two surfaces. On one occasion, a technician forgot to insert the plastic but the wood still welded well. The French team then analysed the welded joints and determined what had happened at the molecular and wood anatomical levels. This knowledge was used to further improve the results.

Wood welding is not protected by any patents. This was deliberate, to ensure that anybody could use the processes (and to ensure that the technology could not be stopped).

We are grateful to Dr Antonio Pizzi for contributing to this article.

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