Laura Tomppo

Utilization of agricultural and forest industry waste and residues in natural fiber-polymer composites: A review

Natural fiber-polymer composites (NFPCs) are becoming increasingly utilized in a wide variety of applications because they represent an ecological and inexpensive alternative to conventional petroleum-derived materials. On the other hand, considerable amounts of organic waste and residues from the industrial and agricultural processes are still underutilized as low-value energy sources. Organic materials are commonly disposed of or subjected to the traditional waste management methods, such as landfilling, composting or anaerobic digestion. The use of organic waste and residue materials in NFPCs represents an ecologically friendly and a substantially higher value alternative. This is a comprehensive review examining how organic waste and residues could be utilized in the future as reinforcements or additives for NFPCs from the perspective of the recently reported work in this field.

Combined acoustic and electric method for monitoring wood drying process: A review

Abstract The combined method is based on using an electrical method for moisture gradient monitoring and an acoustic emission method for detection of microcracking. In the method, electrodes are used to create an electric field in drying wood and to measure the electric complex spectrum using the impedance spectroscopy method, while at the same time measuring acoustic emissions from drying wood. The result can be used for calculating the parameters required for determining the stress state prevailing in the wood. The spectrum is affected by the properties of the wood, such as moisture content, moisture gradient, temperature, density and structure. When the electric complex spectrum and acoustic emission response are determined at the same time, it is possible to estimate both the main reason for the stresses (moisture gradient) and the outcome (microcracking and macrocracking). The results may be used to control drying to achieve wood products of superior quality. The method was studied and tested both in the laboratory and in industrial kilns.

Air-coupled ultrasound and electrical impedance analyses of normally dried and thermally modified Scots pine (Pinus sylvestris)

Abstract Wood is graded according to strength in various applications. The ultimate strength can only be determined by breaking the specimen, and thus other characteristics like density and modulus of elasticity (MOE) are used for estimation of the strength. In this study, the properties of normally dried and thermally modified Scots pine were studied using electrical impedance and air-coupled ultrasound (ACU) methods. Density, hardness, MOE and strength were analysed and compared with the electrical and ultrasonic responses. The measurements were conducted in stable laboratory conditions with well equilibrated samples and the moisture content was not determined. Both the ultrasonic and electrical parameters correlated with the density and mechanical properties. Using multivariate analysis, density could be estimated with accuracy of 21 kg/m3 (SD) for normally dried timber and 13 kg/m3 (SD) for thermally modified timber (TMT; N = 15). MOE could be estimated with accuracy of 0.7 kN/mm2 (SD) for normally dried timber and 1 kN/mm2 (SD) for TMT (N = 14). According to the study, electrical impedance spectroscopy combined with ACU measured across the grain is a potential non-destructive technique for the strength estimation of wood.

Softwood Distillate as a Bio-Based Additive in Wood-Plastic Composites

The possibility of using softwood distillate as a bio-based additive or filler in wood-plastic composites (WPCs) was studied by adding various amounts (1–20 wt%) of distillate to a commercial WPC consisting of thermally treated sawdust in a polypropylene (PP) matrix. Softwood distillate was obtained as a secondary product from industrial ThermoWood® processing and it was further processed in the laboratory. The addition of softwood distillate significantly enhanced the mechanical properties of WPC when the distillate content was 2 wt%; tensile strength increased by 5%, tensile modulus by 3%, flexural strength by 3%, and modulus of elasticity by almost 2% compared with the unmodified WPC. In addition, a considerable decrease (over 16%) in water absorption was observed on distillate addition. Proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) analyses revealed that the addition of softwood distillate increased release rates of volatile organic compounds (VOCs) in general, and that the odor of acetaldehyde and guaiacol is detectable in several WPCs. Overall, softwood distillate had positive effects on this particular WPC.

Improving the properties of wood–plastic composite through addition of hardwood pyrolysis liquid

Wood–plastic composites (WPCs) present a class of materials originally developed to decrease the dependency on the mineral oil-based plastics. However, additives and coupling agents incorporated in these composites are usually derived via synthetic routes. The effect of hardwood distillate (HWD) on the properties of WPC was determined by adding various amounts (1–8 wt%) of distillate to a commercial WPC consisting of thermally treated saw dust (Scots pine) in a polypropylene matrix. The distillate was derived by converting hardwood (birch) into several liquid fractions in a two-part slow pyrolysis retort. The certain addition of particular HWD enhanced the tensile and flexural properties of the WPC studied. Furthermore, the water absorption of the samples decreased substantially, up to over 25%. Proton-transfer-reaction time-of-flight mass spectrometric analyses indicated that the addition of the distillate increased the emission rates of studied volatile organic compounds (VOCs) but had no considerable effects on the emission rates of VOCs that are harmful for humans. The conversion of the emission rates into real room concentration revealed that guaiacol and monoterpenes exceed their odor thresholds and therefore can be smelled from the WPCs studied. The findings of this study show that this particulate HWD displayed good potential as an ecological additive in WPCs.

Study of stilbene and resin acid content of Scots pine heartwood by electrical impedance spectroscopy (EIS)

Abstract Scots pine (Pinus sylvestris L.) heartwood samples were measured with electrical impedance spectroscopy (EIS) at frequency range 1 Hz–10 MHz in green and relative humidity (RH) 65% conditioned moisture content (MC) after oven-drying. Complex impedance parameters were studied in relation to the density, moisture, resin acid and stilbene contents. The measurements were conducted in tangential (T) and longitudinal (L) directions with 36 samples in each analysis. For green MC, there were significant correlations between impedance phase angle and contents of stilbenes and resin acids at frequencies below 400 Hz. For the resin acid content, the strongest correlation with phase-angleT was -0.45 (P<0.01) at 100 Hz. Impedance magnitude correlated significantly with MC throughout the frequency range, e.g., at 10 kHz, r was -0.71 (P<0.001) for L-direction. In moisture conditioned state, the correlation between stilbenes and T-measured phase angle was strongest at 250 Hz, (r=-0.56, P<0.001). Equivalent circuit model of two ZARC-Cole elements in series was fitted for the measurements in green MC. Several model parameters correlated with MC (e.g., R1, L r=-0.64, P<0.001), but only one parameter correlated weakly with stilbene content (R2, T r=0.35, P<0.05). The study shows that EIS has a potential for independent determination of resin acid or stilbene contents and MC for green pine heartwood.

The effect of moisture content on electrical impedance spectroscopy response of natural fibre-polymer composite granules

Two types of natural fibre-polymer composite (NFPC) granules were measured with electrical impedance spectroscopy (EIS). The granules were immersed in water for 70 h, after which the excess water was removed and EIS measurements were conducted. Then, the granules were let dry in open containers at normal room temperature, and EIS measurements were repeated at increasing time intervals. The results show that the EIS response as a function of moisture content (MC) depends on the fibre content of the NFPC. In addition, the results indicate that the EIS could be used for the estimation of MC of certain type of granulate, especially at low MCs, which is relevant for the manufacturing of NFPCs. For single material type, a model with impedance modulus at a single frequency was able to predict 87–95% of the MC variation. Therefore, EIS as a non-destructive on-line technique would allow the evaluation of moisture in NFPC granules.

Predicting the bending properties of air dried and modified Populus tremula L. wood using combined air-coupled ultrasound and electrical impedance spectroscopy

Air-coupled ultrasound and electrical impedance spectroscopy are non-destructive measurement methods, which can be used, for example for quality assessment of sawn timber. Both methods may be used in through-transmission and one-sided reflection mode to measure internal properties and detect defects in wood materials. The ultrasound method is based on mechanical waves and is mainly affected by the mechanical properties of wood. Density affects both methods, and the electrical impedance method is especially affected by moisture content and the chemical properties of wood. In this study, the relations between the methods and the bending properties of air dried and modified aspen (Populus tremula L.) specimens were examined. The modification method was a combination of compression and thermal modification. According to the study, electrical impedance spectroscopy combined with air-coupled ultrasound measured across the grain is a potential non-destructive technique for the strength estimation of aspen wood.

A rapid technique for monitoring volatile organic compound emissions from wood–plastic composites

Wood–plastic composites (WPCs) have numerous indoor applications, including framing, decoration and flooring. However, the impact of WPCs on indoor air quality has not been widely studied. Proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) was utilized to monitor the release of volatile organic compounds (VOCs) from a commercial WPC for 41 days since its day of manufacture. Additionally, the emission rates of VOCs from seven different WPC samples were compared and converted into air concentrations to evaluate whether the odour thresholds would be exceeded. The VOCs studied were formaldehyde, acetaldehyde, acetic acid, cyclohexene, furan, furfural, guaiacol and monoterpenes. The results from the 41-day test revealed that the emission rates of monoterpenes, guaiacol, furfural and acetaldehyde declined by 75%–93%, whereas an opposite phenomenon was observed for cyclohexene (nearly a threefold increase). The comparison of VOC emission rates from seven WPC samples indicated that none of the samples had the lowest or highest emission rate for every VOC studied. The present results are significant in at least two aspects; this study shows that the VOC emission rates from WPCs can be determined by using PTR-TOF-MS. Furthermore, it seems that guaiacol and acetaldehyde exceed their odour thresholds and therefore humans will be able to detect these compounds from the WPCs studied.