Improvement of the mechanical strength of laminated veneer lumber
The lamination of wood is a well-known technology by means of which round timber is reduced to boards, veneers and sticks and subsequently glued together again in layers to form wooden slats. Wood-based materials such as glued laminated timber, plywood, laminated veneer lumber (LVL), multipex boards and blockboards are hereby created. With decreasing lamellar thickness, the influence of strength-reducing growth features is reduced and the standard deviations of the lamella-based materials are greatly reduced. Materials on the basis of veneers are particularly effective as the ratio of the usable raw timber mass (raw material efficiency) to the adhesive requirement is very favorable. Through the possibility of continuous production of the materials, the manufacture is very economical. In order to further increase the mechanical strength of materials on the basis of veneers, a sub-goal of the project is the optimization of the manufacturing procedure for veneers.
The properties of wood are very strongly direction-dependent. The directional dependency depends on the orientation of the fibers in the direction of the trunk axis as well as the distribution and orientation of the main components of the wood (cellulose, polyoses, lignin). As a result of this anisotropy, cracks develop in the peeled material during machining which can lead to a collapse of the shavings. Whilst this phenomenon is absolutely desirable in the processing of wood, cracks are undesirable in veneer production. The effects, which are also called peeling cracks, lead to rough surfaces, facilitate capillary water penetration and reduce the strength of the veneer and the materials made therefrom. Within the framework of the collaborative project, we are developing process conditions at the Fraunhofer WKI which lead to fewer peeling cracks, a lower peeling crack frequency and a lesser peeling crack depth. Furthermore, conditions are to be developed which lead to the slightest possible surface roughness.
Improvement of the durability of laminated veneer lumber from indigenous deciduous species
If less-durable wood species such as beech, birch and poplar - which are predominant among the indigenous hardwood species - are used for the production of veneer laminated wood, the material utilization in the construction sector is limited, despite the high mechanical properties. This limitation can be solved through a modification. With regard to a possible wood modification, veneers are advantageous due to their lesser thickness. For the purpose of wood modification, procedures are anticipated by means of which the chemical and/or physical structure of the wood can be selectively altered. If it is possible to avoid the exchange of moistening and drying, the undesirable crack formation can be effectively prevented. Since a certain water content in the wood is necessary for the biological degradation, dry wood is furthermore protected from biological destruction. The known procedures for achieving this goal are either extremely expensive or the desired effects cannot be achieved. Effective but expensive is, for example, the impregnation of the wood with phenol-based substances. As regards the costs, the high consumption of phenol and solvents must be taken particularly into account. In order to be able to offer a more economical procedure for impregnation, the Fraunhofer UMSICHT is investigating, within the framework of this collaborative project, the possibilities of an impregnation of beech veneers with PF resins in a compressed CO2 atmosphere. The successful impregnations in a compressed CO2 atmosphere all have in common the characteristic that the CO2 diffuses very quickly into the target matrix compared to other solvents. By means of CO2, impregnates can be introduced into the matrix (wood), where they are deposited or where they can react.
As the Fraunhofer UMSICHT was able to prove, impregnates must not necessarily be soluble in CO2, but can also be available
- in particulate form, as a dispersion in an additional solvent (water, alcohols) or
- as a solvent in an additional solvent.
A further advantage of the impregnation in a compressed CO2 atmosphere is the residue-free separation of the CO2 from the impregnated matrix (impregnated veneer). The CO2 separation takes place through the pressure reduction in the high-pressure system. The CO2 escapes from the already-impregnated matrix, whilst the impregnates remain in the matrix.