Dissertations in Forestry and Natural Sciences

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