**3. Results and discussions**

#### **3.1 Mechanical properties**

**Table 1** presents the tensile and bending properties for the studied polymers and composites. Polymer variability significantly affected all WPC mechanical properties (**Table 2**). PP showed better mechanical properties compared to the HDPE. Pure HDPE samples showed the lowest tensile and flexural properties (**Table 1**) compared to PP and WPC. Adding wood fibers to HDPE, PP or polymer mixes improved the tensile and flexural moduli of elasticity and strengths and decreased the elongation at maximum strength (**Table 1**). Several factors explain this decrease, including the stiff nature of the wood fiber, the poor adhesion between the fiber and the polymers, and the incompatibility of the non-miscible polymeric chains in the case of WPC made with a mix of polymers. The heterogeneity of composition, poor adhesion and lack of polymers' miscibility lead to increased microstructure cavities and voids, which negatively affect the WPC strength and ductility.

#### **3.2 Physical properties**

Water absorption increases with the duration of immersion and remains constant upon saturation [37]. **Figure 1** illustrates the water absorption for the investigated formulations after 45 days of immersion in distilled water. HDPE and PP samples maintained a constant weight. They did not absorb water after 45 days of immersion (**Figure 1**) due to the polymers' hydrophobicity, weak surface energy, and free hydroxyl groups' absence.

For WPCs, water uptake increased with time of immersion according to the same pattern of evolution. PP-based composites showed lower water uptake compared to HDPE-based composites. The water uptake of polymers mix-based composites is in between.

Wood fibers are responsible for water absorption in WPCs because of their hydrophilic character. Adding wood fiber to PP, HDPE, or polymer mixes increased the water uptake in agreement with previous findings [37–41]. The WPC water absorption phenomenon is due to the capillary transport into the gaps and flaws at the interfaces between the fibers and polymers because of poor fiber-polymer adhesion, incomplete wettability, and impregnation, which lead to water transport by micro-cracks formed during the processing [39–41].


## *Properties of High-Density Polyethylene-Polypropylene Wood Composites DOI: http://dx.doi.org/10.5772/intechopen.101282*

**Table 1.**

*Average and standard deviation of HDPE, PP, and WPCs' tensile and flexural properties.*


#### **Table 2.**

*Analysis of variance (F values) on the effect of polymer variability on WPC properties.*

#### **Figure 1.**

*Evolution of water uptake of WPC made from black spruce fibers and HDPE, PP polymers and their mixes after 45 days of water immersion.*

#### **3.3 Thermal stability**

**Figure 2** illustrates the TGA curves of all studied formulations. The maximum degradation of PP occurred at 490°C, while the maximum degradation of the WPC made with 100% PP occurred at 520°C. The same tendency occurs for the WPC made with HDPE. These results indicate that the presence of the wood fibers improves the thermal stability of PP and HDPE. For all studied formulations, total degradation occurs at around 600°C. The lowest degradation is obtained for the composites made with 20% PP and 80% HDPE polymer mix. This composite was the most thermally stable among the different composites. The HDPE and PP curves show a one-stage degradation (**Figure 2**) and WPC's two stages of degradation (**Figure 2**). The first stage corresponds to the wood fiber component, which begins to degrade at 220°C, and the second stage corresponds to the polymer degradation. The obtained patterns of variation of the TGA curves are typical of those reported for WPC [42].

**Table 3** shows the DSC results for the tested polymers and WPCs, including the melting temperature (Tf), the enthalpy of fusion (ΔHf), and the crystallinity index (Xc), the crystallization temperature (Tc), and the enthalpy of crystallization (ΔHc). WPCs made with polymers mixed showed two different fusion peaks. The melting temperatures of 80% HDPE + 20% PP and 20% HDPE + 80% PP WPCs were 132.3°C and 165.7°C, respectively. The HDPE WPC showed the highest *Properties of High-Density Polyethylene-Polypropylene Wood Composites DOI: http://dx.doi.org/10.5772/intechopen.101282*

#### **Figure 2.**

*TGA curves of HDPE and their WPCs made with black spruce fiber and HDPE, PP, (80% HDPE + 20% PP) and (20% HDPE + 80% PP) matrices.*


*Tf: the melting temperature,* Δ*Hf: the enthalpy of fusion, Xc: the crystallinity index, Tc: the crystallization temperature,* Δ*Hc: the enthalpy of crystallization.*

#### **Table 3.**

*Thermal properties of pure HDPE, pure PP, and WPC made with black spruce fiber and HDPE, PP, (80% HDPE + 20% PP) and (20% HDPE + 80% PP) matrices.*

crystallinity index (Xc = 74.5%) due to its better thermal stability than PP WPC (Xc = 64.1%). The 20% HDPE-80% PP showed lower crystallinity (Xc = 69.7%) than the 80% HDPE-20% PP WPC PP (Xc = 72.7%). Thus, increasing PP proportion in the polymer mix decreases the crystallinity.

Pure PP showed the lowest crystallization temperature at 110.1°C (**Table 3**). The crystallization temperature of all WPCs is higher than the pure polymers crystallization temperature because of the degradation of the wood fibers during the heating process. Adding wood fibers decreased the fusion and the crystallization enthalpies due to the dilution effect of the wood fiber within the polymers. The decrease in the polymer content reduces the heat of fusion, and the increase in wood fiber content limits the thermal movement of the polymer molecular chain and results in a reduction in released heat fusion is reduced.

#### **3.4 Surface chemistry**

**Figure 3** shows the FTIR absorbance spectra range (4000–400 cm−1) of wood, HDPE, PP, and the studied WPC formulations. Spectra of the wood fibers are similar to those previously reported [43, 44]. These spectra show the presence of a

#### **Figure 3.**

*FTIR spectra of wood; HDPE, PP, and WPCs made with HDPE, PP, and PP-HDPE mixtures.*

broad stretching band for intermolecular bonded hydroxyl groups (OH) at around 3400 cm−1. The OH groups may include absorbed water, aliphatic primary and secondary alcohols found in carbohydrates and lignin, aromatic primary and secondary alcohols in lignin and extractives, and carboxylic acids in extractives [43]. This OH stretching band is flanked by prominent methylene/methyl bands appearing at around 2900 cm−1. These bands are shifted and divided into two peaks at 2922 cm−1 and 2853 cm−1, respectively. An ester carbonyl vibration occurs at about 1728 cm−1, emanating from carbonyl (C〓O) stretching of acetyl groups in hemicelluloses and carbonyl aldehyde in lignin and extractives. This vibration emanates from the carbonyl (C〓O) stretching of carboxyl groups in hemicelluloses, lignin, and extractives, as well as esters in lignin and extractives [43]. Between 1500 and 400 cm−1, we observe several absorption bands due to various functional groups of wood constituents. The bands around 1457 cm−1, 1424 cm−1, and 1373 cm−1 are associated with methylene deformation and methyl asymmetric and methyl symmetrical vibrations [43]. The strong bands appearing at 1270 cm−1 are due to either a carbon single-bonded oxygen stretching vibration or an interaction vibration between carbon single-bonded oxygen stretching and in-plane carbon single-bonded hydroxyl bend in carboxylic acids [43].

Papp et al. [44] attributed bands containing no other nearby absorption maxima to one chemical component (1510 cm−1: aromatic rings, 1270 cm−1: guaiacyl units, 1158 cm−1: C▬O▬C bonds of cellulose). The absorption band at 1158 cm−1 is due to the asymmetric stretching of C▬O▬C in the cellulose and hemicelluloses [43] or the saturated fatty acid ester carbon single-bonded oxygen stretching associated with the ester carbonyl at lower wavenumber [43]. The strong intensity bands at 1059 cm−1 and 1036 cm−1 correspond to cellulose [43]. The vibrations between 896 cm−1 and 810 cm−1 are due to ring stretching and out-of-plane carbon singlebonded hydrogen [43].

The infrared spectrum of HDPE shows peaks at 2916 cm−1 and 2849 cm−1 associated with methylene asymmetric and symmetric C▬H stretching, respectively. The peak at 1472 cm−1 is due to the methylene asymmetrical C▬H bending, while the peak at 1463 cm−1 is associated with methylene scissoring. The peaks at 730 and 720 cm−1 are associated with crystalline and amorphous methylene. The absorption peaks on the infrared spectrum of PP are related to the methyl group (▬CH3) and

#### *Properties of High-Density Polyethylene-Polypropylene Wood Composites DOI: http://dx.doi.org/10.5772/intechopen.101282*

the methylene group. The typical peak at 1375 cm−1 is associated with the symmetric bending vibration mode of methyl group CH3. The Peak located at 840 cm−1 is assigned to C▬CH3 stretching vibration, while the absorption peaks displayed at 972, 997, and 1165 cm−1 are associated with ▬CH3 rocking vibration. The peak observed at 2952 cm−1 is related to ▬CH3 asymmetric stretching vibration. The absorption peaks at 1455, 2838, and 2917 cm−1 are attributed to ▬CH2▬ symmetric bending, ▬CH2▬ symmetric stretching, and ▬CH2▬ asymmetric stretching, respectively [45–48].

The HDPE WPC is similar to that of HDPE, with only minor changes in the spectrum. This similarity is because the polymer coats the fiber. The PP WPC is also identical to that of PP for the same reason. For the WPC made with polymer mixes, the 80% HDPE-20% PP WPC FTIR spectrum is similar to the HDPE WPC, while

#### **Figure 4.**

*Scanning Electron microscopic observation of fractured surfaces of the different WPCs made with black spruce fiber (35 wt%) and HDPE ((a) ×100; (b) ×500); PP ((c) ×100; (d) ×450); 80% PP-20% HDPE ((e) ×100; (f) ×600); 20% PP-80% HDPE ((g) ×100; (h) ×250).*

20% HDPE-80% FTIR spectrum is similar to that of PP. The disappearance of peaks associated with wood in the two spectra is also due to the fibers coating with the polymer mixes.

Among the slight differences between the composites and the polymer, the spectrum is the absorbance peak at 1031 cm−1. This peak is associated with the carbon-oxygen (C▬O) bonding between cellulose and hemicellulose.

#### **3.5 WPC microstructure by scanning electron microscopy**

SEM observations (**Figure 4**) show distinct phases without evidence of interfacial adhesion. Voids and traces of pullout appear in all figures indicating the weak interfacial adhesion between the different phases. **Figure 4d** shows a wood fiber (the element with punctuations) and a crack between this element and the polymer, demonstrating poor contact and adhesion in the interface. **Figure 4e, f** and **h**  shows a complete fiber pulled out from the polymers, confirming the weak interfacial adhesion. In addition, the bad dispersion seen in **Figure 4e**–**g** confirms the non-compatibility of the two polymers and the absence of interfacial adhesion between the polymers and the wood fibers.
