JAHRGANG 61, AUSGABE 3/2020

In a joint project with producers of fl ame retardants and insulation material scientists of Technische Universität Dresden developed and tested a flame retardant (FR) based on oak bark. Oak bark as residue of wood industry is a suitable raw material for the production of biopolymer based FR. A chemical modification of the bark powder and its application on celcellulose-based insulating materials leads to comparable burning properties as references with commercial flame retardants. At comparable dosages, the results show that the bark based FR achieve a standard-compliant fl ame retardant effect. First tests indicate that the synthesis of this bark based fl ame retardant should be realized using extruder technology.

In Sweden and other Scandinavian countries, wood waste materials are normally used for biofuel production or waste incineration. From environmental perspective, it is favourable to recycle wood material and use the concept of multiple sequential use, rather than just burning it for shortterm energy production. One of the objectives of this study was to investigate the feasibility of manufacturing different variants of low formaldehyde emission particleboard from wood waste bonded by an adhesive system based on leftover cakes of rape oil in natural state. In this study, untreated wood waste material (type AI) and slightly treated wood waste (type AII) were used to produce three-layered particleboards bonded by rape resin based on leftover cakes of rape oil in natural state. The particleboards were produced with a target density of 640 kg/m³. For each panel variant the extractable formaldehyde content and formaldehyde release (3 h and 24 h) were analyzed. The results point out that it is possible to manufacture ultra-low formaldehyde emission three-layered particleboard variants based on recycled wood waste material (AI, AII) bonded by rape resin based on leftover cakes of rape oil in natural state complying with the European standard EN 312 (2010). Moreover, the extractable formaldehyde values of the three-layered rape resin particleboard variants comply with the European standard EN 312 (2010) and fulfil the requirement for particleboards of type P2.

It is well known that wooden building components show significant changes in shape as a function of changes in wood moisture content. As a result of the increase in relative moisture content, different volume expansions along the three anatomical directions can be observed below the fibre saturation range. These effects are known as free swelling behaviour of wood. If the expansions that occur are restricted, so-called swelling pressure forces are induced. In the context of this work, these absolute swelling forces as well as their evolution over time during water storage with complete swelling restriction were investigated. In addition to the pure direction-dependent rheological material behaviour, the structural bonding of individual components with different fibre orientation leads to locking effects in wood-based materials. For this reason, the investigations were carried out on spruce (picea abis) specimens with different internal structures with one to 21 layers.

A novel wood composite based on specially produced wooden particles, the so-called macro-fibres, is presented. Forest thinnings were used as raw material for the production of macro-fibres. The disintegration process was a combined-squeezing-cutting process, which takes into account the naturally grown fibre orientation of the wood. The resulting macro-fibres were subsequently processed to wood-based materials using three different adhesive systems. Varying amounts of applied adhesive and material density provided information about the achievable mechanical properties of the macro-fibre wood composites. A density range of 200-1,300 kg/m³ could be investigated, whereby density and the different amounts of applied adhesive showed strong influences on the mechanical material properties. A maximum bending strength of 225 N/mm² and a bending modulus of elasticity of up to 34,000 N/mm² could be achieved with the macro-fibre wood composite.

Additive manufacturing (AM) processes enable new designs and part functions through a layered structure and the use of specifi c materials. In the wake of climate change, we have decided to investigate how AM technologies can be used to produce parts from bio-based residual/waste material, provide an up-cycling strategy for these materials and at the same time exploit their individual properties. The Binder Jetting (BJ) and Fused Filament Fabrication (FFF) technologies are particularly interesting from this perspective, as different materials can be used as fi llers in combination with biodegradable thermoplastics as binders. In this study we present the concept of manufacturing parts from peach kernels, which are ground to powder in combination with polyvinyl alcohol (PVA). Parts made of such materials show a high decomposition rate when exposed to water. This makes them ideal for applications such as disposable packaging or products with a short service life. The aim of the research is to determine the feasibility of using these materials in BJ and FFF.

The cold tack of urea-formaldehyde (UF) resins is an important property used in the production of plywood and veneer lumber to ensure that the veneers adhere to each other during the manufacturing process, thus ensuring transport and feeding into a multi-daylight press. Various factors that can affect cold tack, such as lay-up time, resin amount, resin age, veneer moisture content, veneer temperature and pre-press time, were systematically analyzed based on the tensile shear strength of uncured glue joints. Veneer temperature and moisture content showed the highest influence, but also lay-up time and pre-press time had an effect on cold tack. Models were created to calculate the best conditions for cold tack of UF resins. Based on these findings, the influence of cold tack on the behaviour of the cured bond was then investigated. Factors that significantly influence the cold tack did not show any effect on the tensile shear strength in the cured state.

Due to their good mechanical properties, compressed laminated woods are suitable as potential construction materials for technical applications with high load requirements. In addition to the absolute values, the designer also needs informations about parameters influencing the mechanical properties for calculation within the conteption phase. One of these influencing parameters is the fiber-load angle as a deviation of the load direction with respect to the central plane of the board (90 ° = classic load with transverse tensile). As the material data sheets generally only give values for fiber-load angles parallel and perpendicular to the board center plane, this article investigates the influence of the fiber-load angle on the mechanical properties of selected synthetic resin presswoods under transverse tensile load in a critical test area (45 ° - 90 °). Experimental methodology and results are presented.