Open access peer-reviewed chapter

Properties of High-Density Polyethylene-Polypropylene Wood Composites

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Mourad Saddem, Ahmed Koubaa and Bernard Riedl

Submitted: June 25th, 2021 Reviewed: October 18th, 2021 Published: March 30th, 2022

DOI: 10.5772/intechopen.101282

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We investigated the effects of polymer blend variation on the physical, mechanical, and thermal properties of wood-polymer composites (WPC). We used high-density polyethylene (HDPE) and polypropylene (PP) and a combination of 80% PP, 20% HDPE, and 80% HDPE, 20% PP as polymer blends for WPC formulations to simulate recycled plastics. We used black spruce (Picea mariana Mill.) hammer milled fibers (75–250 μm) at 35 wt% as a filler for all the formulations. A two-step process was used for WPC manufacturing; pellet extrusion followed by test samples injection. Tensile and three bending tests characterized the WPC mechanical properties. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) characterized the WPCs’ thermal properties. Water absorption and contact angle measurements assessed the composite dimensional stability. Infrared spectroscopy (FTIR) and electron scanning microscopy (SEM) investigated the WPCs’ surface chemistry and microstructure. Mechanical properties and dimensional stability varied according to polymer composition, with better performance for WPC containing higher PP proportions. Thermal properties varied with the polymer composition in the WPC, with better thermal stability for the formulation containing higher HDPE proportions. Surface chemistry analysis did not reveal any chemical changes on the WPCs surface. Scanning electron microscopy analysis revealed distinct phases in all WPCs without evidence of interfacial adhesion.


  • wood fiber
  • polypropylene
  • high-density polyethylene
  • wood-polymer composites
  • mechanical properties
  • water absorption
  • surface chemistry
  • microstructure

1. Introduction

Significant progress was achieved during the last years in wood-polymer composites (WPC) research and product development. In addition to the environmental advantages, the use of wood fiber in WPC manufacturing has several benefits, including low cost, renewability, biodegradability, low specific gravity, and high specific strength and stiffness [1, 2]. WPC use in construction and transport is in constant evolution due to these advantages [1, 2, 3]. The increase in environmental awareness increased the demand for environmentally friendly products [4]. WPC presents an excellent alternative to plastic products with lower environmental impact and cost [5]. Lately, many studies have tried to improve WPC properties and competitiveness by introducing new additives and processes [6, 7, 8, 9]. However, the polymers’ prices are very volatile and depend on petroleum prices, affecting the competitiveness of such products. Thus, the economic aspect is also crucial to ensure the sustainability of WPC production. Using recycled materials appears to be an excellent way to lower the raw material cost and consequently the total production cost. The recycled thermoplastic polymer is commonly used as a matrix to produce WPC [10]. Moreover, the environmental advantages of using recycled polymers could decrease the WPC production costs.

In landfills, plastics are not biodegradable. The 2019 global production of plastics was 368 million tons. This important production leads to a large plastic waste stream, making recycling a great challenge [11, 12]. Recycling plastics in relatively long lifecycle products such as WPC could decrease the carbon footprint of the non-renewable polyolefin and avoid their burial in landfills [13]. Waste prevention comes to be an excellent way to avoid recycling energy uses and facilities expenses. In some cases, waste prevention could be less favorable by diminishing the downstream material recycling and preventing the low-impact of secondary production [14]. In recycling centers, sorting wastes is the most expensive part of the recycling process. Recycled plastics are always a mix of different kinds of polymers, such as polyethylene terephthalate (PET), low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP). These polyolefins have different chemical compositions making them incompatible during processing which could significantly affect the properties of the resulting products.

Several reports [4, 15, 16, 17, 18, 19, 20, 21] investigated the effects of recycling on the properties of WPC, both components of these composites could be from recycled materials. Ashori and Sheshmani [15] showed that recycled newspapers in recycled PP composites showed maximum water absorption compared to non-recycled components formulation. Low et al. [4] used epoxy to produce recycled cellulose fiber epoxy composites. Results showed an improvement in mechanical properties. Tajvidi and Takemura [17] showed that recycled fibers increase the composites hydrophobicity due to better fiber-polymer adhesion. Nerenz et al. [16] showed that the addition of a sunflower hull diminished the tensile strength of composites compared to neat PP. Xiaolin et al. [18] studied the feasibility of composites made with recycled newspapers and magazines and recycled PP. The study showed that recycled fibers and PP could be a viable source for producing WPC.

Petroleum-based polymers such as PE, HDPE, PET, and PP serve for WPC production. These petroleum-based polymers are also large-scale products used for many applications such as packaging and constitute the main component of waste landfills.

Many studies investigated the potential of the polyolefin recycled matrix components for WPC [10, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. Chtourou et al. [21] showed that municipal waste plastics composed of 95% PE and 5% PP and pulp fibers produced WPC with average tensile properties compared to virgin polyolefin. Li et al. [22] used PET and PE to produce microfiber-reinforced composites. They reported that the mechanical properties of the PET composites greatly improved compared to the typical PET/PE blend at the same composition. Cui et al. [23] showed good compatibility of the treated wood post-consumer fiber with recycled HDPE and additives, giving WPC good mechanical properties. Beg and Pickering [24] studied the effect of eight times reprocessing on wood PP composites. The study showed a decrease in mechanical properties with the increase in reprocessing, and the thermal stability increased with the repeated process. In another study, Beg and Pickering [25] found that the equilibrium of moisture content of WPC decreases after eight times reprocessing. Ares et al. [26] showed that with more than 10% wood flour content, the reprocessed PP composites showed mechanical and rheological properties similar to those of virgin polymer composites. González-Sánchez et al. [27] showed that the fiber dispersion is not dependent on the polyolefin type, and reprocessed PP showed more pseudo-plasticity loss than the reprocessed PP HDPE. Bhaskar et al. [28] used recycled PP to produce WPC and compared the mechanical properties with virgin PP WPC. Recycled PP with less than 50% fiber load had WPC with good properties [29]. Combining shell core with recycled polyolefin could lead to cost-effective advantageous WPC [30]. Catto et al. [31] showed that recycled polyolefin is a viable alternative due to its comparable physical and mechanical properties to virgin polyolefin. Adding virgin PP to recycled HDPE improves WPC properties [32]. De Oliveira Santos et al. [33] showed that using recycled PET below the melting temperature could enhance the composites processing and mechanical properties. Another study [34] reported that virgin and recycled polymers give similar mechanical properties and water uptake.

The reuse of polyolefins also has environmental advantages. It contributes to decreasing global warming and using non-renewable fossil hydrocarbons [10]. However, the emissions of reprocessed products weaken these advantages. Rangavar et al. [35] reported that recycled PP leads to WPC with similar or even better properties than the virgin polymer in addition to economic and environmental benefits. Thus, recycled polymers exhibit a high potential to replace virgin polymers for producing fiber-filled thermoplastic composites [36].

This study assessed the effect of the variation of PP and PEHD proportions on WPC properties. We used two different combinations of these two polymers (20% HDPE + 80% PP and 80% HDPE + 20% PP) to simulate recycled plastics. We did not use any grafting agent in all formulations to better assess their interaction and the effects of their simultaneous presence on the WPC properties.


2. Experimental

Black spruce fibers were hammer milled with a Wiley Laboratory Mill mounted with a 2 mm opening sieve. Fibers were classified with Ro-tap Laboratory Sieve Shaker to obtain fibers with size in the class of 200–60 mesh (75–250 μm) and oven-dried to reach 3% moisture content. Polymers used in this study are high-density polyethylene (HDPE) (DOW DMDA-8907 NT7, Dow Chemical) and polypropylene (PP 4150H, Pinnacle polymers, USA). The HDPE is a semi-crystalline material (typically 70–80%) has a 0.95 density, 9.0 g/10 min melt index, and a 135°C melting point. The PP has a density of 0.90 g/cm3 and a melt flow index of 55 g/10 min at 230°C.

Composite pellets were processed by a counter-rotating, intermeshing, conical twin-screw extruder (Thermo Scientific HAAKE PolyLab OS Rheodrive 7 with Rheomex OS extruding module). The screw speed was 50 rpm, and the barrel and die temperature was 165°C. The wood fibers proportion in the composites was maintained constant at 35 wt %. Composites were cooled in a water bath and cut up in pellets of 3 mm size. Injection molding with Minijet Haake injection machine produced test samples for bending and tensile tests using a mold temperature of 80°C, and barrel and nozzle temperature of 175°C.

Three-point bending properties were measured on 1 mm thick, 10 mm wide, and 60 mm long test samples and at a test speed of 1.4 mm/min according to the ASTM D 790 standard. Tensile properties were measured on dog-borne shaped samples of 2 mm thickness, 4 mm width, and 75 mm length at a 5 mm/min speed according to the ASTM D 638.

WPC water uptake was assessed according to ASTM 1037 on bending type specimens in triplicates. Samples were soaked in distilled water, and their weight was measured periodically up to 45 days immersion using a laboratory balance with an accuracy of ±0.001 g.

A Shimadzu IR Tracer-100 (Kyoto, Japan) served for FTIR spectroscopy. The analyzed spectrum was 400–4000 cm−1 with a resolution of 1 cm−1. We used the software lab solutions IR de Shimadzu with 50 scans for each measurement. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the WPC samples were conducted in NETZCH leading thermal analyzer using a heating rate of 10°C/min under helium with 25 mL/min flow rate from room temperature to 900°C.

Scanning electron microscopy served to analyze fractured surfaces of WPC tensile specimens. Micrographs were generated at 20 keV at a 5 mm working distance with Hitachi S3500 (Tokyo-Japan) electron microscope. Samples were prepared and gold-coated before observations.

The mechanical properties were subjected to an analysis of variance using the ANOVA procedure of the IBM SPSS statistics software. Polymer variability was the studied factor. Effects and differences between means were considered statistically significant at p < 0.05. ANOVA assumptions were verified using graphical diagnostics and Levene test for equal variances.


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.

Tensile properties
Et, GPa1.11 ± 0.051.58 ± 0.091.77 ± 0.121.85 ± 0.211.02 ± 0.121.27 ± 0.16
σt, MPa24.7 ± 0.423.6 ± 0.228.28 ± 0.9830.21 ± 0.8320.41 ± 0.8831.38 ± 1.65
ε, %3.18 ± 0.074.19 ± 0.373.60 ± 0.463.86 ± 0.146.39 ± 0.427.79 ± 0.94
Flexural properties
Ef, GPa0.72 ± 0.060.86 ± 0.051.40 ± 0.031.52 ± 0.050.40 ± 0.021.12 ± 0.04
MOR, MPa26.1 ± 1.128.8 ± 0.538. 7 ± 1.042.3 ± 0.417.2 ± 0.640.1 ± 1.5

Table 1.

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

Et: tensile modulus of elasticity, σt: tensile strength, ε: elongation at maximum strength, Ef: flexural modulus of elasticity, MOR: modulus of rupture in flexion.

PropertiesF value
Tensile modulus94.4**
Tensile strength40.3**
Elongation at maximum yield36.3**
Flexural modulus431.0**
Flexural strength543.3**

Table 2.

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

Significant at 1% probability level.

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.

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.

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, 38, 39, 40, 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, 40, 41].

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].

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.

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 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.

SamplesTf (°C)ΔHf (J/g)Xc (%)Tc (°C)ΔHc (J/g)
WPC 35% black spruce fibers
80% HDPE-20% PP132.373.872.7115.4381.1
20% HDPE + 80%165.751.369.7115.6062.8

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.

Tf: the melting temperature, ΔHf: the enthalpy of fusion, Xc: the crystallinity index, Tc: the crystallization temperature, ΔHc: the enthalpy of crystallization.

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 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].

Figure 3.

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

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 single-bonded 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 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, 46, 47, 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 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 4eg confirms the non-compatibility of the two polymers and the absence of interfacial adhesion between the polymers and the wood fibers.

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).


4. Conclusions

Polymer variability affected the WPC mechanical, physical, and thermal properties. The WPC formulations with higher PP proportions exhibited higher mechanical properties and dimensional stability than those with higher HDPE proportions. Surface chemical and microstructure analysis showed the lack of adhesion between the different phases. The polymers coated the wood fibers without chemical reaction, and PP and HDPE were not miscible.

The polymer mix simulated recycled plastic. Although WPC made with this mix showed lower mechanical properties and dimensional stability than the composites made with one polymer, these composites from these mixes are suitable for several end-uses, sustainable, and have the economic advantage of being made from recycled polymers.



Canada Research Chairs Program (Grant 557752), Natural Sciences and Engineering Research Council of Canada (Grant 567663), ForvalueNet NSERC Strategic Network (Grant number 504736), and MITACS (Grant IT03894) funded this research. The authors would like to thank Williams Belhadef and Gilles Villeneuve for their technical assistance and Sebastien Migneault for scientific support.


Conflict of interest

The authors declare no conflict of interest.


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Written By

Mourad Saddem, Ahmed Koubaa and Bernard Riedl

Submitted: June 25th, 2021 Reviewed: October 18th, 2021 Published: March 30th, 2022