Development of friction welding of wood

For several decades friction welding technology has found broad application in many fields of the metal and plastic connection industry. The fabricated joints are laminar or punctiform depending upon the shape of the parts to be welded. The work presented here deals with the application of this technology to wood connection. Its application to the material wood is still very young and the process is to a large extent unexplored. The type of connections investigated in this study is of laminar shape. The wood in the contact zone is heated by means of frictional energy. This causes a thermal alteration of the wood cell structure, which leads to the formation of a viscous layer. After cooling, the material in this layer forms the connection. The welding machine used for these investigations applies a circular movement to the parts to be welded. Within the scope of the work presented here the behaviour of the material during the welding process is examined. Measurement of the frictional force and interfacial temperature during the welding process shows that this process bears distinct resemblance to friction welding of metals and thermoplastic synthetics. As with metals and plastics, this process can be divided into different phases by means of measuring the frictional force. These phases reflect the particular states of the interface (dry friction, transition phase, viscous state). The influences of the machine settings, welding frequency and welding pressure, on the process cycle are examined by means of the friction force and the interfacial temperature as a function of welding time and welding displacement. To investigate the influence of the annual rings and their orientation these investigations are carried out for samples with different cut direction. Welding frequency and welding pressure affect the welding duration as well as the friction force in a distinct manner. The results show that circular friction welding is influenced by the orientation of annual rings in a significant way. The moisture content of the material is part of the investigations as well. Its influence on the behaviour of the material and the resistance of the connection is of substantial importance. This result arises from investigations on test pieces welded with different moisture contents. In accordance with the experiments carried out, the atmosphere of the welding environment (ambient air, argon atmosphere) seems to be not very important with regard to the process flow. Investigations concerning the evolution of the interfacial resistance showed that the welded connections do not meet the resistance achievable by conventional glues. However, the relatively high initial resistance permits a continuous welding of multilayered laminates. The initial strength of the newly formed connection clearly exceeds the load applied by the newly introduced vibration. Therefore the existing joint is not damaged by welding of an additional layer. Microscopic examinations reveal that the cell structure at the contact layer is completely destroyed. It becomes apparent that the decomposed wood forms a viscous layer during the welding process. This layer encloses the adjacent cell structure, embeds the cell walls, and contributes in this manner to the adhesion. Thermal degradation of the adjacent cell structure occurs only within a thin layer close to the contact zone. This is due to the good insulation properties of wood. The thermal influence becomes evident by a visible dark discoloration of the cell structure. The joint consists of a consolidated mass of thermally-altered wood decomposition products. This is also reflected in the results of chemical analyses carried out within the scope of this research. This study shows that the main components of wood (cellulose, polyoses and lignin) experience a thermal degradation. Within the material, which forms the interfacial layer the cellulose was detected to remain in a relatively high proportion. Due to its predominantly crystalline structure the cellulose behaves in a thermally relatively stable manner. Polyoses are thermally significantly less stable. Compared with thermally unchanged spruce wood, polyoses remain only in a small percentage in the joint substance. The molecular structure of lignin, the third chemical compound, experiences distinct changes. However, its total mass stays relatively stable. The investigations indicate reactions between the decomposition products of polyoses and cellulose and the thermally altered units of lignin, which could lead to a cross-linking of the joint material and thus contribute to cohesion. Advantages of this technology are seen particularly in the rapidity of the joint formation. In addition, wooden compounds, which are fabricated by this method consist of natural wood and thermally altered wood only. Compared to glued compounds where the adhesives usually contain solvents, this leads to advantages with regard to environmental compatibility. With respect to machining, welded connections offer advantages when compared to traditional methods used for wood connection. Disadvantages result from the resistance of the joint, which is significantly less stable than the resistance of most conventional glues. One application of this new method is seen in the fabrication of solid wood elements with rather small stress loads at the interface.

Joining Wood by Friction Welding - Fabrication of Multi- layered Components

Bernhard Stamm, Yves Weinand

Since decades traditional friction welding methods are applied industrially to metal and thermoplastic synthetics. Research showed that for different applications this method is promising for wood too. The process does not necessitate any additional material. One of the process’ advantages, compared to glued connections, is the short time, which is necessary to realise a connection. The consolidation or curing of the decomposed wood at the heat-affected zone, and thus the development of the interfacial shear strength, is of major importance for the efficiency of friction welding of wood laminates. Seven test series of ten samples each from Norway spruce (Picea abies) were welded and tested with regard to the evolution of the connection’s shear strength. Compression shear tests were carried out at different times shortly after the termination of the welding process. One minute after the termination of the frictional movement, the joints show average shear strength of about 2 MPa. During the following fifteen minutes, the shear strength increases to about 2.6 MPa. This demonstrates that the shear strength obtained immediately after the interruption of the welding process, reaches about 70% of the maximum value, reached after fifteen minutes with nearly ambient temperature at the heat-affected zone. According to measurement of the interfacial shear forces during welding, the maximum shear stress applied by the vibrational motion is approximately 0.5 MPa. Thus, the resistance of the connection, reached immediately after termination of the process, exceeds this value noticeably. As the initial shear resistance is higher than the shear stress applied by the vibration, continuous welding of wood laminates composed of various layers is possible. In spite of the rather low shear strength obtained for welded spruce wood samples, the method could potentially be applied to the lamination of wooden slabs by timber planks with low shear load at the connection, forming solid wall and ceiling elements. The experimentation carried out dealt with small surfaces of mainly 25 cm². However, the results gained, led to the development of a new friction welding machine, conceived to be able to weld significantly larger surface areas and to be more appropriate for the material than traditional machines.

2006

Chemical investigations on the thermal behaviour of wood during friction welding

Julius Natterer, Bernhard Stamm

Friction welding is a new technology in the course of which wood pieces are subjected to circular or linear movements in a welding machine. The wood in the contact zone is heated and melted. Aiming at a better understanding of the thermal alteration of wood and the formation of a viscous layer acting as adhesive, the thermally changed material was taken to be analysed by chemical and spectroscopic methods. As a manifestation of thermally splitting of wood, the total amount of extractable compounds within the joint material (JM) is higher than that of unaltered spruce. Cellulose was found less altered than the other essential wood components. Polyoses, on the other hand, are less stable under the conditions of friction welding; they are detectable only in small amounts in the altered zone. Lignin also suffers distinct changes as demonstrated by the increase of free phenolic groups and the decrease of the typical bonds between the phenolpropane units. Furan derivatives were detected within the volatiles of the smoke gas, arising mainly from the polyoses. It is obvious that reactions between furfural and other furan derivatives with lignin belong to the main reactions in the friction zone leading to cross-linking of the JM.

2006

Processing of Sustainable Cellular Biocomposites

Carole Boissard

Petroleum-based polymers and composites are commonly used in a wide variety of application fields. However ecological concerns, increasing oil prices and dwindling natural resources explain why nowadays industrial and public interest is more than ever focused on new bio-based – “greener”- products. In order to find viable substitutes for petroleum-based materials, major efforts are devoted to the development of sustainable solutions combining fully compostable matrices and fibres from renewable resources. Polylactide (or polylactic acid) (PLA) is currently one of the most versatile biodegradable polymers derived from renewable resources such as non-food crops of corn or sugar cane. Its transformation into foamed cellular structures would decrease its density and at the same time may improve its specific mechanical properties. This could provide cost-effective and sustainable products for packaging and construction. In the context of developing “green” foams, the focus in the present work has been on a solvent-free process based on physical foaming with supercritical carbon dioxide (ScCO2) as a foaming agent. This foaming process includes thermally driven but kinetically controlled phenomenon. Solubility and diffusion of the gas in the molten polymer, cell nucleation and growth, as well as stabilization of the final foam structures were studied for neat and composite materials. At low processing temperatures and low depressurization rates, homogeneous structures were obtained thanks to a balance between cell growth mechanisms and cell stabilization, thus limiting uncontrolled cell coalescence. The density of the resulting polylactide foams ranged from 0.12 to 0.3 g/cm3 with corresponding compression moduli of 6 to 73 MPa respectively. The processing windows were strongly dependent on the rheological melt properties of the PLAs. PLA is generally considered to be a high strength, high modulus thermoplastic but its brittleness has limited its industrial use. PLA was therefore combined with wood fibres (WF) and microfibrillated cellulose (MFC) in order to improve its mechanical performance without compromising its environmental impact. Two compounding methods, a wet mixing papermaking-like process and a solvent casting process were investigated for the production of WF/PLA and MFC/PLA biocomposites. Degradation of PLA occured during wet-mixing, due to the water used as a dispersant, and to several heating steps applied during compounding. Improvement of the drying and processing conditions and/or the replacement of water by isopropanol limited this degradation. The presence of cellulose fibres considerably modified foaming mechanisms; gas solubility was decreased and the coefficient of diffusion increased, leading to lower quantities of CO2 available for foaming. In addition, stiffness of the fibre network impeded full expansion of the polymer matrix. Thus smaller cells were created. Densities from 0.15 (neat PLA) to 0.37 and 0.55 g/cm3 and moduli ranging from 35 (neat PLA) to 125.5 and 373 MPa were obtained by the addition of 5wt% WF and MFC respectively. Addition of a chain extender enabled viscosity of the matrix to be increased causing increased orientation of the fibre network during flow and hence fibre alignment in the cell walls. Density was thus decreased by 95 % to 0.017 g/cm3 and compression modulus decreased to 0.32 MPa for 5wt% WF composite. Cell wall shrinkage was observed in the neat PLA foams obtained with the chain extender. This was reduced by adding wood fibres while maintaining similar density and modulus. Cellulose fibres act as an extra scaffold in the cellular structures. Industries are currently considering the use of these materials for applications in packaging e.g. as cushioning materials, or in architecture as display panels, or as sandwich core materials for the replacement of expanded polystyrene, polyvinyl chloride or polyurethane foams.

EPFL2012

Joining Wood by Friction Welding - Fabrication of Multi- layered Components

Bernhard Stamm, Yves Weinand

2006

Processing of Sustainable Cellular Biocomposites

Carole Boissard

EPFL2012