Shape stability of laminated veneer products – a review – defining and achieving shape stability

Abstract

Laminated veneer products (LVPs) consist of veneers bonded together with an adhesive into a predetermined shape. Deviation from the intended shape is a problem for both manufacturers and customers of the final products. The shape stability of LVP depends on a variety of material and process parameters, and this review identifies several important factors that influence shape stability, the purpose being to find ways of decreasing distortion and improving shape stability so that product quality can be improved.The review ends to the following statements:The fibre orientation of the veneer strongly influences the shape stability of LVP. The products can show considerable distortion (particularly twist) if the fibre orientation of the veneer is oriented in an unsuitable way in the assembly before moulding.The effect of fibre orientation on shape stability can be very small directly after moulding, but it can increase considerably when the moulded product is subjected to a change in moisture content (MC). In general, a change in moisture leads to distortion. Moisture changes alone, however, result in a controlled distortion.The orientation of the loose and tight sides of the veneers in LVP affects the shape stability. An asymmetrical construction, coupled with different levels of moisture in the different veneers in an assembly before moulding, will result in poor shape stability of the product.The choice of wood species affects shape stability. Beech shows more distortion and poorer shape stability than birch.Based on existing knowledge to produce shape-stable LVP, the veneers should be conditioned to uniform MC and sorted with regard to fibre orientation and loose or tight side of the veneer before bonding. End-user climates should govern the veneers' MC and added moisture from the adhesive during the process. Straight grain veneers and symmetry should always be sought.This review reveals several areas that need to be further clarified in order to achieve shapestable LVP, for example the influence of adhesive, the distribution of pressure, temperature, stresses and strains during moulding and the development of numerical methods to better predict the final shape.

Lignin Biorefinery: New Horizons in Catalytic Hydrodeoxygenation for the Production of Chemicals

“…The cell wall consists of different layers, and the concentration of the lignin gradually decreases from the outer to the inner layer. The innermost lignin provides rigidity with cellulose and hemicellulose microfibrils through chemical bonding, whereas the outermost lignin acts as a binding agent to hold the adjoining cells together . Lignin can be extracted from different biomasses, for example, grass, hardwood, and softwood, and the physicochemical properties of lignin depend upon the extraction process .…”

“…The innermost lignin provides rigidity with cellulose and hemicellulose microfibrils through chemical bonding, whereas the outermost lignin acts as a binding agent to hold the adjoining cells together. 33 Lignin can be extracted from different biomasses, for example, grass, hardwood, and softwood, and the physicochemical properties of lignin depend upon the extraction process. 34 It is quite complex to accurately measure the molecular weight of isolated lignin due to high crosslinking and the presence of different functional groups, whereas other properties can be determined accurately.…”