We produced Wood-Polymer Composites (WPCs) with phenolic resin (PR) filled with saw dust (SD) and rice husks (RH) in a PR:fillerratio of 60:40 wt.%. RH and SD were grinded and sieved into particles <160 μm. The aim of this research work was to evaluate sawdust and rice husks as fillers for sustainable phenolic resin based WPCs. Therefore, we investigated the thermal stability of PR/RH and PR/SD WPCs then we studied and compared the tensile, flexural properties of PR/SD and PR/RH WPCs samples, as well as their dimensional stability after water absorption test. Furthermore, through ultraviolet light exposure, we evaluated the effects of photo-oxidation on the water stability and mechanical properties of PR/RH and PR/SD WPCs samples compared to unexposed ones. PR filled with SD presented better mechanical properties compared to PR/RH WPCs samples. However, PR/RH WPCs showed good mechanical properties, and better thermal resistance and better water repulsion capabilities compared to PR/SD WPCs samples. Although, long time UV exposure ended up lowering considerably the mechanical properties and water resistance of PR/SD and PR/RH WPCs, both RH and SD offer great added value as fillers for PR based WPCs; SD having better interactions with PR matrix compared to RH.
The world’s global population is steadily increasing. Along with it, waste generation, decreasing of natural resources and global warming effects are major concerns for the next generation. Throughout the world, tremendous research works are being conducted in order to bring sustainable solutions to those issues [
Phenolic resins are synthetic polymers obtained from reaction between phenol and formaldehyde [
Rice is one of the most cultivated crops across the world. In fact, around 600 million tons of rice paddies are produced globally each year [
Furniture and wood processing factories are producing huge amount of sawdust waste. Sawdust is mainly used to make wood pellets as firewood, and offer good value added when used for particle board, or used as reinforcement in composite materials [
In regard to these waste generation and the disposal issues, it appeared relevant to us to combine the desirable properties of phenolic resin, rice husks and sawdust. Therefore, in this research work, we manufactured wood-polymer composites made with phenolic resin (PR) filled with sawdust (SD) and rice husks (RH) and evaluated their sustainability. Samples were manufactured through hot-press method then subjected to thermal, tensile, flexural, and water absorption tests. Furthermore, as WPCs are mainly used for outdoor application such as fencing, railing, flooring [
Cedar wood sawdust SD obtained from Aomori Japan, and rice husks RH from a rice processing factory in Saint-Louis, Senegal, were grinded into particles between 160 μm and 250 μm in size, with a WARING Commercial Laboratory Blender. SD and RH were then oven dried at 150˚C for 30min to remove moisture content. Phenolic resin with a molecular density of 134.134 g/mol was used. We prepared then formulations of 60 wt.% of Phenolic resin and 40 wt.% of SD and RH respectively; matrix and filler were mixed together in a ITO SEISAKUSHO PSL-1M ball mill machine for 10 hours. In
With an AS ONE hot-press machine, samples were manufactured through Hot-press method at 180˚C for 20 min, applying 20 MPa of pressure after 10 min of heating. We manufactured PR/RH and PR/SD WPCs samples (PRH and PSD) for tensile test, water absorption test and three-point bending test. For each test, five samples were tested. The samples dimensions were as below:
Tensile test: Length: 119 mm, Grip section Width: 14 mm, Cross-section area: 18 mm2, Gage length: 29 mm);
Flexural test: (Length: 94 mm, Width: 25 mm, Thickness: 4 mm);
Water absorption test: (Diameter: 10 mm, Height: 10 mm).
Some samples were exposed to ultraviolet light using an AS ONE Handy UV lamp SLUV-6:
UV intensity light source to 50 mm:
Long-Wavelength (365 nm): 1274 μW/cm2
Short-Wavelength (254 nm): 1112 μW/cm2
For our experiment, sample were exposed to UV light at 60 H of frequency and 365 nm wavelength, range of UVA which represent 95% of UV light that reach the Earth surface. The distance from samples to light source was set at 13.5 cm and
Tests performed on UV exposed and unexposed samples are described below.
Thermo-gravimetric analysis (TGA) was performed on PR, RH and SD to study their thermal degradation process. For this analysis, we used a BRUKER AXS―TG-DTA2020SA. Using 10 mg as samples’ weight under air atmosphere, we set the target temperature at 1000˚C starting from 25˚C at a heating rate of 20˚C/min up to 600˚C then 10˚C to 1000˚C; the reference sample was 10 mg of
Filler | Cellulose % | Hemi-cellulose % | Lignin % | Others % |
---|---|---|---|---|
Cedar SD | 45 | 13.2 | 29.3 | Extractive: 10.2 Ash: 0.2 |
RH | 25 - 35 | 18 - 21 | 26 - 31 | Silica (SiO2): 15 - 17 Others: 2 - 5 |
Matrix | Density g/mol | Molecular Formula | Glass Temp. ˚C | Max Curing Temp. ˚C |
PR | 134.134 | C8H6O2 | 53 | ~160 |
Samples | Exposure time (days) | Total UV dose (J) | Total UV dose/area (J/cm2) |
---|---|---|---|
PSD | 0 | 0 | 0 |
PRH | |||
PSD_1 | 7 | 19262.88 | 105.695 |
PRH_1 | |||
PSD_2 | 12 | 33022.08 | 181.191 |
PRH_2 |
Alumina (α-Al2O3) powder. Mass loss as a function of heating temperature was plotted to analyze the thermal degradation process of samples.
The tensile test was performed at room temperature, with an SHIMADZU Tension and Compression Testing Machine (AG-50KNX). The testing speed was set at 0.5 mm/min. Tensile strength ( σ T ) and Young’s modulus (ET) were calculated using the following equations.
σ T = F max A (1)
E T = σ ε (2)
Fmax: max load supported
A: Cross-sectional area
σ and ε: Stress and Strain at any point of tangent of linear part of stress-strain curve portion between 5% and 15% of maximum strain, where no crack has not yet occurred.
Three-point bending test were performed at room temperature with a Techno Graph NMB TG-50 kN, and TU3D-10kN as load cell model. The Crosshead speed was set at 0.5 mm/min and the length of the support span was L = 60 mm. The flexural strength ( σ f ), and the flexural modulus Ef were calculated using the following equations.
σ f = 3 F L 2 w t 2 (3)
E f = L 3 m 4 w t 3 (4)
F = Load at fracture
L = support span length
W = width
t = thickness
m = gradient/Slope of the initial straight line portion of load/deflection curve.
E f = L 3 m 4 w t 3 E f = L 3 m 4 w t 3 Water absorption test
The water absorption was performed by soaking samples in room temperature water. To investigate water absorption properties of PR/SD and PR/RH WPCs samples, we measured the %water uptake of samples after 4 hours, 8 hours and 24 hours. Prior to soaking into water, samples were oven dried at 120 for 30 min then weighed. After 4 h, 8 h, 24 h and 72 h, samples were weighed again. The percentage of water uptake was calculated using the following equation.
% water uptake = w t − w i w i × 100 (5)
Wi: initial weight; wt: weight after a time t in water.
Scanning Electron Microscopy (SEM)
The fractured surface morphology and fiber/matrix interactions were observed and analyzed through scanning electron microscope. The SEM analysis was performed with a SHIMADZU Field Emission SEM JEOL JSM-7610F at 20 KV.
The TGA analysis was performed on the PR/SD, PR/RH composites and on PR, to evaluate their thermal degradation process. As shown in
composed of hemi-cellulose which thermally degrade faster, from 220˚C up to 315˚C, cellulose degrading from 315˚C to 400˚C, and lignin which degrade earlier, at 140˚C, but slower than hemi-cellulose and cellulose. It can be present beyond 400˚C [
However, we can clearly see in
We evaluated the tensile properties of PR/SD and PR/RH WPCs.
UV are strong radiation that can initiate bond breaking, especially C-H bonds which are weaker, thus leading to creation of unstable functional groups then polymer inter-chain linkage that lead to increasing polymer molecular weight and further to polymer degradation for long UV exposure time [
Samples | Thermal Stability region | Decomposition temperature (˚C) | mass loss 25˚C to 600˚C | Residuals 1000˚C |
---|---|---|---|---|
PR | Up to 350˚C | 600 | 32.26% | 63.22% Char |
PR/RH WPC | Up to 220˚C | 580 | 40.918% | 42.49% Char |
PR/SD WPC | Up to 220˚C | 556 | 42.93% | 36.01% Char |
sensitive to photo-oxidation compared to cellulose and hemi-cellulose [
In
After seven days of UV exposure, we noticed an increase in flexural strength in both composites, PR/SD more than PR/RH. UV radiation initiated bond breaking and inter-chain linkage that increased the fiber/matrix interface and the composites stiffness just enough to confer them higher flexural strength. Nevertheless, for the reasons stated earlier for the tensile test results, the increase in flexural strength was lower for PR/RH (+8.5%) than PR/SD (+26.96%). However, when we exposed the composites for longer time, further reactions occurred and led to polymers degradation and stiffening that explain the loss in elasticity thus loss in flexural strength and modulus. In fact, after 12 days of UV exposure, PR/SDWPCs showed a flexural strength of 55.395 MPa and a modulus of 2.382 GPa while PR/RH had 41.825 MPa of flexural strength and 1.915 GPa of flexural modulus; these values being lower than that of unexposed PR/SD and PR/RH WPCs. Again, with regards to these results, PR/RH WPCs have better weathering properties than PR/SD. After long UV exposure, PR/RH WPCs lost 9.99% in flexural strength and 5% in modulus; while PR/SD WPCs lost 10.4% in flexural strength and 8.19% in modulus.
In
The SEM analysis was performed to analyze fractured surface morphology of composites and the interface between fillers and phenolic resin. Fiber and matrix interactions are mainly through mechanical interlocking, hydrogen or covalent bonding and through wetting process [
As well, after analysis of the fiber/matrix interface and composites fractured surface morphology after UV exposure, we could better understand and sustain the results of tensile and flexural test. In
These observations helped us have an overview of the mechanism through which UV affects the mechanical performances of PR/RH and PR/SD WPCs.
We evaluated the thermal, mechanical and water absorption properties of wood-polymer composites made with phenolic resin filled with rice husks and sawdust. PR/SD showed better mechanical properties, but lower thermal stability and water resistance compared to PR/RH PWCs. However, considering the main applications of these types of composites, both composites showed relatively good properties. Nevertheless, the weathering effects considerably affects and lowers the mechanical and water resistance properties of both composites. Thus, we should consider using sodium hydroxide/silane treatment, plasma or ozone treatment on the fillers to increase the filler/matrix interfacial bond strength; as well as UV stabilizers, to limit photo-degradation effects, and Nanoclay to increase fire resistance. These treatments and additives could considerably optimize the composites performances and get the most of the desirable properties of SD and RH. In fact, rice husk and sawdust are among the most abundant biomass products produced worldwide every year. Therefore, using them as fillers for phenolic resin based wood-polymer composites could provide good added value to construction sector, especially in developing countries. Furthermore, regarding alarming Global warming effects, Green technologies are nowadays more than welcome in order to build a sustainable world for the future generation.
The authors declare no conflicts of interest regarding the publication of this paper.
Lette, M.J., Ly, E.B., Ndiaye, D., Takasaki, A. and Okabe, T. (2018) Evaluation of Sawdust and Rice Husks as Fillers for Phenolic Resin Based Wood-Polymer Composites. Open Journal of Composite Materials, 8, 124-137. https://doi.org/10.4236/ojcm.2018.83010