Heiko Thoemen

Bio: Heiko Thoemen is an academic researcher from Bern University of Applied Sciences. The author has contributed to research in topics: Urea-formaldehyde & Ultrasonic testing. The author has an hindex of 6, co-authored 15 publications receiving 118 citations.

Effects of raw materials and process parameters on the physical and mechanical properties of flat pressed WPC panels

Abstract: For the production of WPC, the flat pressing technology can be considered as an alternative to common techniques, particularly when manufacturing large-dimensioned panels. The aim of the present study was to identify and analyze the main influencing parameters on sheets made of wood flour (WF) imbedded in a polypropylene matrix. Test panels were made to measure the effects of panel density, polymer melt flow rate (MFR), WF content, coupling agent, WF size and press temperature on water absorption (WA), thickness swelling (TS), internal bond strength, MOE and MOR. Density, WF content and MFR were identified as main influencing parameters. The increase of density typically leads to an improvement of properties. Lower WF contents cause reduced WA and TS, while the MOE seems to have a maximum at a WF level of 50–70%, depending on raw material used. As an explanation for improving properties when using a high-MFR polymer, a better distribution on the wood surface is hypothesized.

Preferences of furniture manufacturers for using lightweight wood-based panels as eco-friendly products

Abstract: Lightweight panels, as an eco-friendly element for furniture manufacturing, offer several potential market and production opportunities for many secondary woodworking companies. In this study, the perceptions of Iranian furniture manufacturers were evaluated by means of ANP-based multi-type questionnaires, regarding the conditions and requisite features of lightweight panels in furniture manufacturing. The ANP results revealed that product design, quality and price were effective sub-criteria for furniture manufacturers. The most important attributes for choosing panels during the manufacturing process were: lack of panel warping, product abundance in the market, and withdrawal resistance of connectors. Manufacturers indicated that they were aware of the advantages lightweight panels would bring them, but they were still in need of further persuasion concerning the quality and workability of lightweight panels. Combining lightweight and conventional panels for furniture manufacturing could lead to an increased interest in assuring furniture quality. In addition, further development of lightweight panels is necessary to increase the variation in the market.

Effects of panel density and particle type on the ultrasonic transmission through wood-based panels

Abstract: Air-coupled ultrasonic inspection for the detection of delaminations inside panels is a widely applied technique in the wood-based panel industry. Additionally, the ultrasonic signal is used to monitor the constancy of the production process as unsteady material properties and process parameters cause variations in the transmitted ultrasonic signal. This article examines the physical effects of air-coupled ultrasonic testing and analyses the influences of panel density and particle type on the transmitted ultrasonic signal. The experiments were carried out for a broad range of wood-based panels. The experimental results indicate that the tested properties influence the transmission in two ways. Firstly, density affects the reflection and transmission factors at the material boundaries, and secondly, particles and pores act as scattering centres inside the material, leading to an attenuation of the ultrasonic signal. The results of this study may be used to efficiently include the ultrasonic signal into a trending system for the wood-based panel production process.

Utilization of inorganic mineral filler material as partial replacement for wood fiber in medium density fiberboard (MDF) and its effect on material properties

Abstract: The influence of inorganic mineral filler addition on the material properties of medium density fiberboard (MDF) was investigated at varying densities. Fibreboards with 10, 20 and 30% fibers replaced by calcium carbonate weight basis (dry/dry) and with target densities of 550, 700 and 850 kgm−3, respectively, were manufactured in a laboratory scale environment. The fiberboards were prepared by adding the mineral filler in dry powder form to the wood fibers to which urea formaldehyde UF resin in liquid state had previously been added. The obtained fiber-filler furnish, with the filler distributed uniformly by sticking to the UF resin on the fibers, was then pressed into panel forms. The effect of inorganic filler on elastic and strength properties, thickness swelling, permeability, as well as formaldehyde and volatile organic compound (VOC) emissions was quantified. The results demonstrate that the addition of mineral filler to MDF up to levels of 10 wt.% only have a small effect, if at all, on the material properties. The addition of calcium carbonate, and the associated increase in pH, does not have any effect on the strength development in the cured MDF, even though the results clearly demonstrate the negative effect on the curing kinetics of urea formaldehyde (UF) resin. Likewise, thickness swelling and formaldehyde and volatile organic compounds (VOC’s) emissions are not affected by filler addition levels of up to 10 wt.%. However, exceeding this level leads to a significant decline in strength, which is believed to be caused mainly by a reduction in the number of fiber-to-fiber stress transfer points. Further, increased emissions in VOC’s are expected due to a combination of the catalytic effect of the calcium carbonate that causes a breakdown of fatty acids to VOC’s and also increased emission rates due to a higher permeability of the MDF with filler addition levels above 10 wt.%.

Observation of interference effects in air-coupled ultrasonic inspection of wood-based panels

Abstract: The air-coupled ultrasonic inspection method is a widely applied non-destructive measuring technique in the wood-based panel industry. The technology is mainly applied to detect panel delaminations by analyzing the transmitted signal. Recent research deals with the use of ultrasonic techniques not only for the qualitative but also for the quantitative characterization of wood-based panels. To achieve a fundamental understanding of the behavior of ultrasonic waves in wooden panels, it is necessary to study the mechanisms that affect ultrasonic transmission and velocity during testing. Impedance and attenuation effects have been examined in previous studies. This article focuses on the interferences of ultrasonic waves. The interferences can be detected in experiments where the ultrasonic transmission is tested against the panel thickness. The results are verified with a mathematical model that explains the interferences due to multiple reflections inside the tested panels. By fitting the experimental data to the model predictions, the ultrasonic velocity and attenuation can be determined. So far, interference effects have not been considered for the non-destructive testing of wood-based panels. This research is a contribution to a better understanding of the mechanisms influencing the air-coupled ultrasonic methods.

3D Printing of Polymers with Hierarchical Continuous Porosity

Abstract: Cellular thermoplastic structures with multiscale porosity (1–300 µm microporosity to 0.3–10 mm macroporosity) are produced from an integrated approach of 3D printing by Fused Deposition Modeling printing and Supercritical CO2 Foaming. Porous strands and related hierarchical structures are processed in one continuous step. Influence of printing parameters (e.g., deposition temperature and speed) on foam porosity and pores distribution is described for a semicrystalline food grade Poly-Lactide Acid. The process is then applied to other thermoplastics filament with different mechanical properties. In particular, a composite blend of Poly-l-Lactide and βTCP (90–10) ceramic particles and a soft copolymer Poly-Lactide-co-Caprolactone are processed to print foamed strands with various internal cellular microstructures. The dynamic material transformation phenomena controlling the microstructure upon deposition and foaming are determined. This new additive manufacturing method introduces for the first time the possibility to tune cellular morphology in situ and on line, offering an unmatched freedom to design anisotropy in 3D.

A Critical Review on Wood-Based Polymer Composites: Processing, Properties, and Prospects

Abstract: Waste recycling is one of the key aspects in current day studies to boost the country’s circular economy. Recycling wood from construction and demolished structures and combining it with plastics forms wood-polymer composites (WPC) which have a very wide scope of usage. Such recycled composites have very low environmental impact in terms of abiotic potential, global warming potential, and greenhouse potential. Processing of WPCs can be easily done with predetermined strength values that correspond to its end application. Yet, the usage of conventional polymer composite manufacturing techniques such as injection molding and extrusion has very limited scope. Many rheological characterization techniques are being followed to evaluate the influence of formulation and process parameters over the quality of final WPCs. It will be very much interesting to carry out a review on the material formulation of WPCs and additives used. Manufacturing of wood composites can also be made by using bio-based adhesives such as lignin, tannin, and so on. Nuances in complete replacement of synthetic adhesives as bio-based adhesives are also discussed by various researchers which can be done only by complete understanding of formulating factors of bio-based adhesives. Wood composites play a significant role in many non-structural and structural applications such as construction, floorings, windows, and door panels. The current review focuses on the processing of WPCs along with additives such as wood flour and various properties of WPCs such as mechanical, structural, and morphological properties. Applications of wood-based composites in various sectors such as automotive, marine, defense, and structural applications are also highlighted in this review.