Interfacial Adhesion and Mechanical Properties of Wood-Polymer Hybrid Composites Prepared by Injection Molding

Abstract

Birch (Betula pendula Roth.) and beech (Fagus sylvatica L.) solid wood and plywood were overmolded with polyamide 6 (PA 6) and polypropylene (PP) to investigate their mechanical properties and interfacial adhesion. In the case of PA 6, maximum tensile shear strengths values of more than 8 to 9 MPa were obtained for birch and beech, respectively. The values are comparable to bond strengths of commercial joints bonded with formaldehyde-containing amino-plastics. Perpendicular to the wood elements, bond strength values of 3 MPa was achieved for PA 6. The penetration of the polymers into the wood structure results in a non-densified interphase and subsequent plastic deformation of the wood structure beyond the interphase. These compressed areas influenced the interfacial adhesion and mechanical interlocking. SEM and XPS analysis revealed different interpenetration behavior of the polymers into the wood structure, with chemical interaction confirmed only for wood and PA 6 but not PP.

Keywords: XPS; interfacial bond strength; wood-polymer composites; wood-polymer interface.

Figure A1

Representative deconvoluted XPS spectrum with relative areas of the C 1, C 2, C 3 and C 4 binding energy regions of the birch solid wood specimens, overmolded with PA 6, within the cross section of a sample.

Figure 1

Schematic of the overmolded plywood boards (a) and schematic representation of the specimens for the tensile strength perpendicular to the plane of the board σ_P (b), tensile strength perpendicular to the edge σ_E (c) and tensile shear strength σ_S (d).

Figure 2

Schematic of the overmolded solid wood (a) and representation of the specimens for the tensile shear strength σ_S (b) according to ÖNORM EN 302-1 [34].

Figure 3

Schematic of the test setup for overmolded plywood specimens for strength measurements according to DIN 52 188 [37] and DIN EN 302-1 [34]: (a) test setup for tensile strength perpendicular to the plane of the board σ_P, (b) test setup for tensile strength perpendicular to the edge σ_E, and (c) test setup for tensile shear strength σ_S.

Figure 4

Micrograph of the overmolded solid wood specimens for X-ray photoelectron spectroscopy analyses (XPS) used to determine the elementary distribution of C, O and N and for high-resolution deconvoluted XPS spectra within the samples cross section, indicating the position of the X-ray beam.

Figure 5

Strength properties of 121 birch plywood specimens and 118 beech plywood specimens overmolded with PP at three different injection temperatures. σ_P is the average tensile strength perpendicular to the plane of the board, σ_E is the average tensile strength perpendicular to the edge, σ_S is the average tensile shear strength and n is the number of the samples tested. The whiskers show minimum and maximum values. X is the mean value. ° indicate values of statistical outliers and—is the median.

Figure 6

Average tensile shear strength σ_S of birch solid wood specimens and beech solid wood overmolded with PA 6 and PP, respectively; n is the number of the samples. The whiskers show minimum and maximum values. X is the mean value. ° indicate values of statistical outliers and—is the median.

Figure 7

Representative SEM micrographs recorded on the cross-section of the overmolded specimens for birch solid wood (1a–2b) and beech solid wood (3a–4b); b represents always the detailed inset of a. (1a), (1b), (3a) and (3b) show samples overmolded in radial direction and (2a), (2b), (4a) and (4b) represents samples overmolded in tangential direction.

Figure 8

Mean values and standard deviation of the XPS results: Carbon peak components at C 1 and C 3 for birch solid wood (a) and beech solid wood (b) overmolded with PA 6 and the C 1 and C 2 for birch solid wood (c) and beech solid wood (d) overmolded with PP.