Timo Kärki

Physical and Mechanical Properties of Wood-Polypropylene Composites Made with Virgin and/or Recycled Polypropylene

The physical and mechanical properties of wood-polymer composites made with virgin or recycled polypropylene, or a mixture of these were studied. The composites made with recycled polypropylene had higher density, lower porosity, and higher dimensional stability compared to the composites made with virgin polypropylene. Although the composites made with recycled polypropylene exhibited lower tensile strength than those made with virgin polypropylene, they had higher Charpy impact strength. Scanning electron microscopy analysis of the fractured surfaces of the composites showed no significant differences in the fracture mechanisms of the studied composites. The degree of crystallinity was estimated to be higher for the virgin polypropylene than for the recycled one.

Mineral fillers for wood–plastic composites

Abstract Five mineral fillers were tested for wood–plastic composites (WPCs): calcium carbonate, two different types of wollastonite, soapstone and talc. The impact of the fillers on the mechanical properties of the composites was studied. The experiments included bending tests, tensile tests, Brinell hardness and scanning electron microscopy experiments. The amount of wood, mineral and plastic (polypropylene) was kept steady. Only the mineral type was changed during the tests. A control sample without any mineral added was also manufactured. The mineral addition improved the tensile strength of the WPCs. The hardness of the composite was also improved when the minerals were added, and along with the increasing mineral hardness, the hardness of the composite increased. The wollastonite acicular shape was crushed during the manufacturing process, so the phase of the process in which the minerals are added requires careful consideration.

Weathering of wood-polypropylene composites containing pigments

Outdoor applications of composites have raised questions about their durability. In this study, the effects of outdoor weathering on the properties of wood-polypropylene composites with and without pigments were examined. The composites were placed outdoors for one year, and their colour changes were evaluated after 1, 3, 6, 9 and 12 months of weathering. The durability of the composites was assessed by testing flexural strength and density. Scanning electron microscopy was applied to evaluate the surface degradation of the composites. The weathering resulted in considerable colour fading of the composites. The composites containing darker colour pigments had better colour stability. After weathering, the general trend was a decrease of the flexural strength and density of the composites. The decrease in flexural strength was found to be lower for the composites having higher density.ZusammenfassungDer Einsatz von Verbundwerkstoffen im Außenbereich hat Fragen bezüglich ihrer Dauerhaftigkeit aufgeworfen. In dieser Studie wurde der Einfluss einer Freibewitterung auf die Eigenschaften von Holz-Polypropylen-Verbundwerkstoffen mit und ohne Pigmente untersucht. Die Verbundwerkstoffe wurden über einen Zeitraum von einem Jahr im Freien gelagert und die Farbänderungen wurden nach 1-, 3-, 6-, 9- und 12-monatiger Bewitterung gemessen. Anhand von Biegefestigkeits- und Dichteprüfungen wurde die Dauerhaftigkeit der Verbundwerkstoffe bestimmt. Die Veränderung der Oberflächen wurde mittels Raster-Elektronenmikroskopie beurteilt. Die Bewitterung führte zu starkem Ausbleichen der Farbe. Verbundwerkstoffe mit dunkleren Farbpigmenten zeigten eine bessere Farbbeständigkeit. Nach der Bewitterung wurde generell eine Abnahme der Biegefestigkeit und der Dichte der Verbundwerkstoffe festgestellt. Bei Verbundwerkstoffen mit höherer Dichte nahm die Biegefestigkeit weniger stark ab.

Color Changes of Birch Wood During High-Temperature Drying

In low-temperature drying, a high drying rate has been found to be adequate for silver birch (Betula pendula) and European white birch (B. pubescens) timber as far as wood colour is concerned. During high-temperature drying, however, birch timber darkens significantly if steam is used as the drying medium. The objective of this research was to study the effects of drying force (wet-bulb depression), timber thickness, and initial moisture content on the color of high-temperature-dried birch wood. The reflectance spectrum of wood was measured and transformed to the CIEL*a*b* color scale. The increase in drying force increased the lightness and decreased the redness and the yellowness of wood. At the same time, the difference in color between the surface layer and the interior of boards increased. Increase in thickness and in initial moisture content increased the difference in color between the surface and the interior of boards. Pretreatment of timber with water soaking decreased the difference in color between the surface layer and the interior of boards when low drying force was used, but this difference was increased when high drying force was used.

Research progress in wood-plastic nanocomposites A review

The interest towards wood-plastic composites (WPCs) is growing due to growing interest in materials with novel properties, which can replace more traditional materials, such as wood and plastic. The use of recycled materials in manufacture is also a bonus. However, the application of WPCs has been limited because of their often poor mechanical and barrier properties, which can be improved by incorporation of the reinforcing fillers. Nano-sized fillers, having a large surface area, can significantly increase interfacial interactions in the composite on molecular level, leading to materials with new properties. The review summarizes the development trends in the use on nanofillers for WPC design, which were reported in accessible literature during the last decade. The effect of the nanofillers on the mechanical properties, thermal stability, flammability and wettability of WPC is discussed.

Comparison of water absorption and mechanical properties of wood-plastic composites made from polypropylene and polylactic acid.

Abstract This study focuses on the water absorption and mechanical properties of composites made from softwood sawdust and plastics, such as virgin and recycled polypropylene and polylactic acid (PLA). The composites were processed by extrusion, and their properties were investigated by a water immersion test, mechanical tests and a cyclic test for moisture resistance. Scanning electron microscopy was used to study the morphology of the fracture surfaces of the composites. The composites made with recycled polypropylene had the lowest water absorption and thickness swelling of the studied composites. The PLA composites made with heat-treated sawdust showed the highest flexural strength. Of the polypropylene based composites, virgin polypropylene resulted in composites with higher flexural strength. The Charpy impact strength of the composites was found to have an inverse trend compared to flexural strength. Cyclic treatment of the studied composites resulted in 20–60% loss of flexural strength, depending on type of composite.

Characterization of plastic blends made from mixed plastics waste of different sources

This paper studies the recyclability of construction and household plastic waste collected from local landfills. Samples were processed from mixed plastic waste by injection moulding. In addition, blends of pure plastics, polypropylene and polyethylene were processed as a reference set. Reference samples with known plastic ratio were used as the calibration set for quantitative analysis of plastic fractions in recycled blends. The samples were tested for the tensile properties; scanning electron microscope–energy-dispersive X-ray spectroscopy was used for elemental analysis of the blend surfaces and Fourier transform infrared (FTIR) analysis was used for the quantification of plastics contents.

Effects of wood flour modification on the fire retardancy of wood–plastic composites

Wood–plastic composites (WPC) are materials that combine the properties of wood and plastics. Both are flammable organic materials. Fire safety is considered to be an important issue especially in construction applications. In this research, the fire safety of WPC material is improved by using modification chemicals for wood flour treatment. The effectiveness of the modification is studied with a cone calorimeter device. Mechanical properties and water absorption are also examined. Scanning electron microscopy is used for analyzing the interfacial adhesion of the fiber and matrix. The results show that the fire performance of WPC material can be enhanced through wood modification, which can also improve the moisture uptake performance and the interfacial adhesion of the material. However the mechanical properties decrease as a result of modification with the used fire retardants.

Effects of Atmospheric Plasma Treatment on the Surface Properties of Wood-Plastic Composites

Wood-plastic composites are a group of materials with potential to penetrate markets currently dominated by plastic or wood products. The surface properties of wood-plastic composite materials have been found to be similar to those of polyolefin materials, thereby presenting a challenge to the use of adhesive joining methods. Plasma chemistry can be performed to improve the adhesive properties of polyolefin materials. In this research, the effect of atmospheric plasma treatment on polypropylene and spruce (Picea abies) wood-plastic composite surfaces is investigated by contact angle measurement with sessile drop method and tensile strength tests of glued samples. The plasma treatment is performed on extruded WPC profiles. Confocal Raman microscopy and scanning electron microscopy are used for analysis of the material surface. The results show an increase in the contact angle of plasma treated WPC materials and an improvement in the tensile strength of glued samples following plasma treatment. Observation of the Raman spectrum indicates an increase in polar groups after plasma treatment. Scanning electron microscopy shows changes in the surface of the treated samples, which can be seen as increased porosity, possibly due to etching as a result of the plasma treatment. It is concluded that atmospheric plasma treatment for adhesion improvement of WPC materials can therefore be applied successfully, although it has a mechanical effect on the surface of the material.

The effect of mineral fillers on the thermal properties of wood-plastic composites

Abstract The thermal properties of wood-plastic composites with five different mineral fillers were studied. The tested mineral fillers were calcite (CaCO3), two different qualities of wollastonite, soapstone, and talc. The amount of wood, mineral, and plastic (polypropylene) was kept constant. Only the mineral type has been changed during the tests. The thermal behavior of the samples was studied by using a differential scanning calorimeter, a thermogravimetric analyzer and by determining the heat build-up. The analyzed properties were compared with a reference sample made without adding any minerals. The results show that the addition of mineral fillers does not remarkably change the thermal stability of composites. All the studied mineral fillers except soapstone had a small effect on the heat build-up.