Jussi Ruponen

Factors influencing properties of parallel laminated binderless bonded plywood manufactured from rotary cut birch (Betula pendula L.)

Abstract In this study, the moisture resistance properties of self-bonded plywood were enhanced by thermal modification. The plywood was prepared without any adhesive, using only heat, moisture and mechanical compression. It is known that self-bonded plywood or the wood welding joints suffer from delamination under moist conditions. The results show that the moisture resistance of the joints can be enhanced, i.e. the tendency of delamination can be reduced or eliminated (almost totally) by post-manufacture thermal modification. This was most probably caused by lowered water absorption, relaxation of stored strain energy and formation of crosslinks within the lignin–hemicellulose matrix. The changes in bond integrity in moist conditions as affected by different lay-up types and initial veneer moisture contents were also evaluated. Plywood from initially wet veneers was found to have greater bond stability when soaked. This might be caused by increased crosslinks, because the lignin–hemicellulose matrix is more mobile in wet conditions.

Reducing the moisture sensitivity of linear friction welded birch (Betula pendula L.) wood through thermal modification

Linear friction welding of wood is a bonding process applied to wood and during which a stiff bond line is formed by the softening and rehardening of wood components to form a composite material composed mainly of wood fibres embedded in a modified lignin matrix. Unfortunately, the bonds tend to spontaneously delaminate or lose their strength when exposed to moist conditions. Some approaches were previously applied to overcome this problem, but so far a suitable solution has not been found. This paper presents results of applying post-welding thermal modification to reduce the moisture sensitivity of welded wood. The experiments included welding of birch wood, thermal modification under superheated steam at atmospheric pressure, internal bond (IB) and tensile-shear strength testing and soaking tests. As supposed, the non-modified reference specimens performed poorly after the seven days soaking test (on average 0.33 MPa IB strength), whereas the thermally modified specimens yielded almost the same IB strength in dry and wet condition (on average e.g. 1.15 and 0.93 MPa, respectively). Such a similar load bearing capacity in very different moisture conditions was previously reported only in the case of paduk wood. Similar to the reduction of IB strength occurred during the soaking test, also delamination was observed more clearly in non-modified reference specimens (e.g. 4 vs. 0 total delaminations after seven days soaking). Therefore, the authors suggest that post-welding thermal modification could provide a suitable bond-stabilisation method against moisture, although the process parameters must be optimised in further research, for instance, to ensure scalability.