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Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on the Characteristics of Wood-Plastic Composites 2.2.1 Mechanical properties Wood fibers are added to polymers to increase their stiffness and strength (Wolcott and Englund 1999). The presence of wood fibers in the polymer matrix typically increases the strength and modulus of the composite (Bhaskar et al. 2012, Li et al. 2014). However, both the polymer matrix and the fiber reinforcement are responsible for the mechanical performance of the composite. Tensile strength is more sensitive to the properties of the polymer matrix whereas the modulus of elasticity of the composite is primarily dependent on the properties of the fiber. In order to increase tensile strength, a strong fiber-matrix interface, oriented fibers, and low stress concentration are required whereas the maximization of the tensile modulus requires fiber wetting in the matrix phase, a high fiber concentration and fibers with a high aspect ratio. (Saheb and Jog 1999) The fiber must have a certain minimum length, i.e., the critical fiber length, in order to achieve the fully stressed properties to the fiber in the polymer matrix (Stark and Rowlands 2003, Sain and Pervaiz 2008). The critical length depends on the fiber characteristics and shear strength of the fiber-matrix bond. The fiber-matrix interface is likely to fail due to the debonding at lower stresses if the length of the fiber is less than its critical strength (Stark and Rowlands 2003, Bourmaud and Baley 2007). By contrast, exceeding the critical fiber length may reduce the strength of the composite because the effective stress transfer may be impaired due to fiber curling and fiber bending (Sreekumar et al. 2007). Interphase and interface are two important concepts in fiberreinforced polymer composites. The interface is a twodimensional surface between the fiber and the matrix whereas the interphase is the three-dimensional intermediate between the matrix phase and the fiber phase (Pilato and Michno 1994, Oksman Niska and Sanadi 2008, Jesson and Watts 2012). The interface in any fiber-polymer composite system is responsible for transmitting stresses from the matrix to the fibers, and the contribution of surfaces to stress transfer depends on both the 34 Dissertations in Forestry and Natural Sciences No 222
Wood-plastic composites roughness and the surface chemistry of the constituents. The stress in WPCs is transferred not only by shear along the length of the fiber, but also by tension at the fiber-matrix interface. The stress transfer is limited by the fiber strength, the shear yield strength, and the tensile yield strength of the plastic matrix polymer. (Sretenovic et al. 2006, Sain and Pervaiz 2008) A composite failure can occur through several scenarios, and the uneven nature of the surfaces makes the process even more complex. However, in the simplest case, an adhesive failure can occur in the fiber-interphase interface or in the interphasematrix interface. A cohesive failure of the interphase is also possible. The typical techniques to evaluate interfacial interactions and adhesion between the main constituents include surface analysis methods, such as X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR), microscopy, single fiber-pullout and microbond tests, and dynamic mechanical thermal analysis (DMTA). (Sretenovic et al. 2006, Oksman Niska and Sanadi 2008) The mechanical properties of WPCs have been extensively investigated. Changes in the amount of the wood component exert multiple effects on the characteristics of WPCs. When the wood fiber content is increased, the tensile and flexural moduli tend to increase because wood, especially cellulose, is a highly crystalline material compared to PE, PP, and PVC (Bhaskar et al. 2012). However, the moduli of WPCs are highly dependent on the fiber type and source (Bouafif et al. 2009, Butylina et al. 2011, Ashori et al. 2011, Adhikari et al. 2012, Migneault et al. 2015). Although an increase in the wood fiber content may also lead to the higher hardness, it tends to reduce impact and tensile strength (Bledzki et al. 2002, La Mantia et al. 2005, Ndiaye et al. 2013). In addition, the tensile strain at break decreases considerably. Huang and Zhang (2009) and Ashori et al. (2011) concluded that higher loadings of wood flour in WPCs induced the agglomeration of wood particles, which may impair the mechanical durability of WPCs. Interestingly, Bledzki et al. (2002) showed that WPCs consisting of hardwood fibers had a higher elongation at break Dissertations in Forestry and Natural Sciences No 222 35
Page 1: PUBLICATIONS OF THE UNIVERSITY OF E
Page 4 and 5: Grano Oy Jyväskylä, 2016 Editors:
Page 7 and 8: ABSTRACT The use of raw materials d
Page 9: Universal Decimal Classification: 6
Page 12 and 13: Finally, I am grateful to my wife A
Page 14 and 15: TD-GC-FID/MS Thermal desorption/gas
Page 17 and 18: LIST OF ORIGINAL PUBLICATIONS This
Page 19 and 20: Contents 1 Introduction ...........
Page 21 and 22: 1 Introduction The finiteness of cr
Page 23 and 24: Introduction A new and environmenta
Page 25 and 26: 2 Wood-plastic composites By defini
Page 27 and 28: Wood-plastic composites additives u
Page 29 and 30: Wood-plastic composites Cellulose i
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Materials and methods where Evoc is
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Taneli Väisänen: Effects of Therm
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Bibliography Abdolvahaba E, Lashgar
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Bibliography Ayrilmis N, Jarusombut
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Bibliography Cernansky R. State-of-
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Bibliography polypropylene composit
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Bibliography with odor assessments
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Bibliography La Mantia FP, Morreale
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Bibliography Mohan D, Pittman CU, a
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Bibliography Rinne J, Taipale R, Ma
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Bibliography Stark NM and Rowlands
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Bibliography Yeh S and Gupta RK. Im
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