**3.1. Characterization of the fillers**

Filler particle size and shape are important factors for reinforcement materials. **Figure 1** shows selected SEM images of RH and SD fillers. The morphologies differ strongly: SD is made up of lengthwise agglomerated fibers, whereas RH is made up of platelets with protrusions aligned in a regular pattern (**Figures 1b**, **d** and **2**). Detailed analysis shows a hollow cellular structure in both fillers (**Figure 1c**, **d**). The milling leads to different modes of fracture: In saw dust, the fibers separate, partially branch off or break, sometimes the cells collapse or the cell walls open, resulting in a highly nonuniform morphology with an irregular surface, which will allow to interact with the molten polyolefine during compounding and increase the chance of mechanical interlocking. In contrast, in RH the platelets have a highly regular pattern of 30 μm protrusions in a 90° array on the outside and a smooth inside. In the detailed image (**Figure 2a**), additional hair-like structures become visible between the protrusions. Platelet fracture occurs mainly along the lines between the protrusions, leading to platelets of angular shape and of rather uniform thickness (50 μm), and the cellular structure is contained within the platelet.

**Figure 1.** SEM micrographs of the fillers: surface morphology of (a, c) saw dust and (b, d) rice husk.

**Figure 2.** SEM micrographs of the morphology of rice husk (a) outer and (b) inner surface.

The filler surface layer plays an important role in surface tension and thus in wettability. It has been postulated that the inner RH surface contains lipid and proteinaceous compounds bound to the protein molecule by ester or thioester bonds [9]. The amount of lipid on the filler surface has an influence on hydrophobicity and surface tension.

**Figure 3** shows particle size distribution and aspect ratio of the rice husk and saw dust fillers. About 80% of RH and SD fillers were in the range of 180–500 μm. The aspect ratio of both saw dust and rice husk fillers is quite small. The fibrous saw dust has a higher aspect ratio in the range of 4–5 compared to rice husk filler formed of rectangular platelets with an aspect ratio of 2–3. Note that for better comparison, length-to-width ratio is used for both filler types. The commonly used aspect ratio for platelets—diameter to thickness—scales with the particle size as the thickness is around 50 μm for all platelets.

**Figure 3.** Size distribution (a) and aspect ratio (b) of filler particles (saw dust SD and rice husk RH). inset: Model of filler shape: platelets (a × b × c) for rice husk, circular rods (length l, diameter d) for saw dust. Note: Here calculation of aspect ratio is l/d for a rod and a/b for platelets.

A simple model is used to estimate the surface area of the particles, cf. **Figure 3b** inset: RH platelets may be modeled as rectangular blocks of 50 μm thickness and SD as spherical rods. The surface-to-volume ratio of a thin platelet is dominated by thickness (or, 2/side length a + 2/ side length b + 2/thickness), whereas that of a long circular rod is roughly proportional to the inverse of the diameter (4/diameter + d/rod length). A rough estimate of the surface area per volume (with a size distribution and aspect ratio as shown in **Figure 3** and considering the densities of 1.3 g/cm3 for SD and 1.5 g/cm3 for RH) shows that in a given weight, RH contains a factor of two higher number of particles, but the filler surface area vs. particle size distribution is only slightly shifted to smaller particle sizes in SD. The total filler surface area of the two filler types is equal according to this simple model.

#### **3.2. Mechanical properties of the composites**

**Figure 1.** SEM micrographs of the fillers: surface morphology of (a, c) saw dust and (b, d) rice husk.

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**Figure 2.** SEM micrographs of the morphology of rice husk (a) outer and (b) inner surface.

has an influence on hydrophobicity and surface tension.

The filler surface layer plays an important role in surface tension and thus in wettability. It has been postulated that the inner RH surface contains lipid and proteinaceous compounds bound to the protein molecule by ester or thioester bonds [9]. The amount of lipid on the filler surface

**Figure 3** shows particle size distribution and aspect ratio of the rice husk and saw dust fillers. About 80% of RH and SD fillers were in the range of 180–500 μm. The aspect ratio of both saw The good specific mechanical properties are the prime reason for the application of filled polymer composites. The mechanical properties of the filler/polymer composites depend strongly on filler loading, interfacial adhesion, the degree of dispersion, and the filler particle size [10, 11]. **Figures 4**, **5**, and **6a** show the effects of filler content on the mechanical properties of bio-filler/polyolefine composites. The stated filler content is the wt.%, with a density of 1.3 g/cm3 for SD and 1.5 g/cm3 for RH, and the volume contents are approximately 4% lower for SD and 7% lower for RH.

**Figure 4.** Influence of filler content on bending modulus of the three composites, no compatibilizer added.

**Figure 5.** Effect of filler content on (a) bending strength (b) tensile strength of the composites, no compatibilizer added.

The addition of stiff fillers to the polyolefine matrices improves the stiffness of the composite, and the bending modulus increases linearly with filler content (**Figure 4**). The modulus reflects the capability of fiber and polymer matrix to transfer the elastic deformation in the case of small strains without interface fracture. The modulus of filled PP is 25% higher than that of both filled PEs. It is noted that at high content of saw dust (above 50 wt.%), the viscosity of melt compound is very high, leading to voids and reducing the modulus.

**Figure 6.** Left: Effect of filler content on impact strength of the composites, no compatibilizer added. Right: Influence of compatibilizer content on impact strength of the composites at 50% filler content.

The bending strengths (**Figure 5a**) are mainly influenced by the matrix type: PP/RH has 40% increased bending strength compared to the PE composites. Considering the volume filler content, the data for the bending strength of PE/RH and PE/SD coincide.

The saw dust/polyethylene composites show 20% higher tensile strengths compared to the rice husk/polyethylene composites (**Figure 5b**). This is probably caused by the higher cellulose content, better adhesion, and the higher aspect ratio of saw dust compared to the rice husk filler [12]. The tensile strength of the PP/RH composite is 20% higher than that of the PE/RH.

**Figure 4.** Influence of filler content on bending modulus of the three composites, no compatibilizer added.

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**Figure 5.** Effect of filler content on (a) bending strength (b) tensile strength of the composites, no compatibilizer added.

The addition of stiff fillers to the polyolefine matrices improves the stiffness of the composite, and the bending modulus increases linearly with filler content (**Figure 4**). The modulus reflects the capability of fiber and polymer matrix to transfer the elastic deformation in the case of small strains without interface fracture. The modulus of filled PP is 25% higher than that of both filled PEs. It is noted that at high content of saw dust (above 50 wt.%), the viscosity of

melt compound is very high, leading to voids and reducing the modulus.

There is a clear decrease in tensile and bending strength in dependence on filler content throughout the investigated range. The fillers introduce a large interface area; in unmodified polymers, this is dominant due to weak interfacial interaction between the polar bio-filler and the apolar polyolefine matrix. As filler is added, this surface area increases, resulting in a slight decrease in the tensile, bending, and impact strengths of three composite systems PE/RH, PE/ SD, and PP/RH in the absence of compatibilizer. The decrease is linear up to 50 wt.% (i.e., 46 vol.% of SD, 43 vol.% of RH) of filler. Specimens of 60 wt.% SD in PE could not be injection molded with the current set up, and samples of 60 wt.% RH in PE showed a stronger-thanlinear decrease. At concentrations approaching the theoretical packing limit (e.g., 74 vol.% for oriented spherical rod fillers), defects and voids occur, the viscosity increases, and the weak, unbound interphase plays an increasing role.

**Figure 6** presents the notched Izod impact strengths of rice husk and saw dust composites. The impact strength of a composite is influenced by many factors, especially the toughness of the filler and matrix components, and the dynamic stress transfer of the interphase, which in turn is determined by particle size, shape, and filler surface properties [13–15]. The notched impact strength of RH/PE composites, **Figure 6a**, is better than that of SD/PE composites when no compatibilizer is added. The PP/RH composites show lower impact toughness than PE composites. This trend is similar to a polypropylene composite system published by Bledzki et al. [15], where it is attributed to the brittleness and local internal deformation found more commonly in wood composites. Particle size, shape, and filler surface properties have influence on the impact strength [14]. The nature of the interphase plays an important role, and in the case of fibrous material, the frictional work involved in pulling the fibers out of the matrix.

A matrix modification was performed by using compatibilizers (MAPE for PE matrix composites and MAPP for PP matrix composites) in order to improve the bonding strength between the bio-fillers and the matrix polyolefine. **Figures 6b** and **7** show the effect of compatibilizer content on the impact strength of the composites containing 50 wt.% filler. Tensile, bending, and impact strengths increased while adding compatibilizer and leveled off at above 2 wt.% MAPP for PP matrix composites and at above 4 wt.% MAPE for PE matrix composites. The increase in strengths of the composites is due to the improved interfacial adhesion between the fillers and polyolefine matrices. The maleic anhydride groups of compatibilizer interact with the polar filler surface (through chemical coupling or hydrogen bonding), while their polyolefine chains diffuse into the polyolefine matrices. Therefore, the interfacial strength is improved, as seen before, for example, by Marti-Ferrer et al. and Correa et al. [4, 5]. The increase in tensile, bending, and impact strengths due to the compatibilizers is higher than the losses due to the inclusion of fillers.

**Figure 7.** Influence of compatibilizer content on bending (left) and tensile strength (right) of the composites at 50% filler content.

#### **3.3. Composite fracture surfaces and cross- sectional analysis**

**Figure 8** shows SEM images of the tensile fracture surfaces of the composites at 50 wt.% filler without and with compatibilizers (2 wt.% MAPP for PP matrix composites and 4 wt.% MAPE for PE matrix composites). Adding the compatibilizers changes the fracture behavior of the composites. The unmodified composites fail at the interphases of the filler particles. Voids at the interphase (especially the outside surface of RH particles) and pull-out cavities (especially in the higher aspect ratio SD particles) are visible. In the modified composite, the interfacial interaction is improved, shifting the failure into the matrix with far less pull-out cavities and polymer-coated or partially coated filler particles.

on the impact strength [14]. The nature of the interphase plays an important role, and in the case of fibrous material, the frictional work involved in pulling the fibers out of the matrix.

A matrix modification was performed by using compatibilizers (MAPE for PE matrix composites and MAPP for PP matrix composites) in order to improve the bonding strength between the bio-fillers and the matrix polyolefine. **Figures 6b** and **7** show the effect of compatibilizer content on the impact strength of the composites containing 50 wt.% filler. Tensile, bending, and impact strengths increased while adding compatibilizer and leveled off at above 2 wt.% MAPP for PP matrix composites and at above 4 wt.% MAPE for PE matrix composites. The increase in strengths of the composites is due to the improved interfacial adhesion between the fillers and polyolefine matrices. The maleic anhydride groups of compatibilizer interact with the polar filler surface (through chemical coupling or hydrogen bonding), while their polyolefine chains diffuse into the polyolefine matrices. Therefore, the interfacial strength is improved, as seen before, for example, by Marti-Ferrer et al. and Correa et al. [4, 5]. The increase in tensile, bending, and impact strengths due to the compatibilizers is higher than the losses

**Figure 7.** Influence of compatibilizer content on bending (left) and tensile strength (right) of the composites at 50%

**Figure 8** shows SEM images of the tensile fracture surfaces of the composites at 50 wt.% filler without and with compatibilizers (2 wt.% MAPP for PP matrix composites and 4 wt.% MAPE for PE matrix composites). Adding the compatibilizers changes the fracture behavior of the composites. The unmodified composites fail at the interphases of the filler particles. Voids at the interphase (especially the outside surface of RH particles) and pull-out cavities (especially in the higher aspect ratio SD particles) are visible. In the modified composite, the interfacial interaction is improved, shifting the failure into the matrix with far less pull-out cavities and

**3.3. Composite fracture surfaces and cross- sectional analysis**

polymer-coated or partially coated filler particles.

due to the inclusion of fillers.

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filler content.

**Figure 8.** SEM micrographs of tensile fracture surfaces of PE/RH composites (top), PE/SD composites (middle), and PP/RH composites (bottom), (a) without and (b) with compatibilizer. Scale bar: 100 μm.

**Figure 9** shows SEM micrographs of polished cross-sections for the composites in the longitudinal and transverse specimen direction. The saw dust is oriented along the specimen long axis due to the injection molding process. Here, an anisotropic mechanical behavior is expected. In rice husk, no preferred orientation of the platelets is seen for either PP/RH (as **Figure 9** bottom) or PE/RH (not shown here).

**Figure 9.** SEM photomicrographs of polished samples of PE/SD composites (top) and PP/RH composites (bottom), in xdirection (a) and in z-direction (b).

#### **3.4. Hardness**

**Figure 10** shows the effect of the fillers on the hardness of three modified matrix composites [PE/RH (MA), PE/SD (MA), and PP/RH (MA)]. The fillers increased the hardness of polyolefines significantly. Neat and filled polypropylene had a higher hardness than neat resp. filled polyethylene. The addition of 50 wt.% fillers improved significantly the hardness of three composite systems. The hardness of PE and PP matrix composites increased about 78 and 65%, respectively. This was expected, since the chosen bio-fillers display considerably higher hardness than the soft polyolefine matrices, the hardness of RH being higher than that of SD. Thus, it was also expected that for the PE composites, the hardness of the PE/RH composites would be higher than that of the PE/SD composite. In fact, they had the same value at 50 wt.% filler content. This result could be caused by the higher volume content of SD filler (43 vol.%) compared to that of RH (39 vol.%). The hardness values can be considered as a measure of the wear resistance, since hard materials resist friction and wear better [15].

**Figure 10.** Hardness of polyolefine and filler/polyolefine composites at 50 wt.% filler content.

#### **3.5. Thermomechanical analysis (TMA)**

**Figure 9.** SEM photomicrographs of polished samples of PE/SD composites (top) and PP/RH composites (bottom), in x-

**Figure 10** shows the effect of the fillers on the hardness of three modified matrix composites [PE/RH (MA), PE/SD (MA), and PP/RH (MA)]. The fillers increased the hardness of polyolefines significantly. Neat and filled polypropylene had a higher hardness than neat resp. filled polyethylene. The addition of 50 wt.% fillers improved significantly the hardness of three composite systems. The hardness of PE and PP matrix composites increased about 78 and 65%, respectively. This was expected, since the chosen bio-fillers display considerably higher hardness than the soft polyolefine matrices, the hardness of RH being higher than that of SD. Thus, it was also expected that for the PE composites, the hardness of the PE/RH composites would be higher than that of the PE/SD composite. In fact, they had the same value at 50 wt.% filler content. This result could be caused by the higher volume content of SD filler (43 vol.%) compared to that of RH (39 vol.%). The hardness values can be considered as a measure of the

wear resistance, since hard materials resist friction and wear better [15].

direction (a) and in z-direction (b).

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**3.4. Hardness**

**Figure 11** shows the thermal expansion as determined by TMA in x-direction and the coefficient of thermal expansion (CTE) values of the modified matrix composites [PE/RH (MA), PE/SD (MA), and PP/RH (MA)], respectively. The CTE values of pure PE and PP were determined to 134 × 10−6/°C and 123 × 10−6/°C for the temperature range of −10 to 50°C, respectively. Thermal expansion is higher for the range 50–100°C (210 × 10−6/°C for PE and 163 × 10−6/°C for PP). CTE values of pure PE and PP decreased to 30–62% by adding 50 wt.% filler. While the CTE values of rice husk composites are isotropic, the difference in CTE values in longitudinal and transverse directions of the saw dust composite is quite high, especially in the high-temperature range 50–100°C (94 × 10−6/°C in x-direction and 187 × 10−6/°C in zdirections). This reflects on the one hand the anisotropic behavior of the saw dust [16] and on the other hand the aspect ratio of the SD particles which in injection molding lead to orientation of the filler particles, confer **Figure 9**.

**Figure 11.** Coefficient of thermal expansion of polyolefine and filler/polyolefine composites. inset: Dimension change of polyolefine and filler/polyolefine composites in x-direction.

#### **3.6. Dynamic mechanical thermal analysis (DMTA)**

Dynamic mechanical thermal analysis (DMTA) is a sensitive technique that characterizes the mechanical responses of materials by monitoring property changes with respect to the temperature and/or frequency of oscillation [17]. It has been commonly used as a technique for investigating the viscoelastic behavior of the composites for determining their dynamic modulus such as storage modulus (E′), viscous behavior (loss modulus E"), and energy damping (tan δ) as a function of temperature [18–19]. The elastic component describes the energy stored in the system, while the viscous part describes the energy dissipated.

**Figure 12.** DMTA: storage modulus (left), loss modulus, and tan δ (right) of PP, PE, wood, and composites with 50% filler loading, without and with compatibilizer.

**Figure 12** shows the storage modulus E′, the loss modulus E″, and tan δ values of the composites without and with compatibilizers as well as the stock materials PP, PE, and wood. The storage modulus (E′) of polypropylene and polyethylene matrix composites was higher than those of pure polypropylene and polyethylene matrices, respectively. This is the expected effect caused by the addition of high surface rigid fillers into semi-rigid polyolefine matrices. In all the systems, the storage modulus drops with increasing temperature due to the increased segmental mobility of the polymer chains. The E′ value of polypropylene systems decreased rapidly at the glass transition above 0°C, whereas the E′ value of wood and polyethylene systems decreased in a wider range around 30°C.

**3.6. Dynamic mechanical thermal analysis (DMTA)**

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Dynamic mechanical thermal analysis (DMTA) is a sensitive technique that characterizes the mechanical responses of materials by monitoring property changes with respect to the temperature and/or frequency of oscillation [17]. It has been commonly used as a technique for investigating the viscoelastic behavior of the composites for determining their dynamic modulus such as storage modulus (E′), viscous behavior (loss modulus E"), and energy damping (tan δ) as a function of temperature [18–19]. The elastic component describes the

**Figure 12.** DMTA: storage modulus (left), loss modulus, and tan δ (right) of PP, PE, wood, and composites with 50%

filler loading, without and with compatibilizer.

energy stored in the system, while the viscous part describes the energy dissipated.

The temperature dependence of loss modulus and tan *δ* for the polyolefine, wood, and three composite systems without and with compatibilizers is presented in **Figure 12**. The loss modulus (E″) is a measure of the absorbed energy due to the relaxation and is associated with viscous response of the viscoelastic materials. E″ of polyolefine and composites increased with temperature and had a peak in the transition region about 0 and 30°C for PP and PE systems, respectively.

The damping factor tan δ, defined as the ratio of the loss modulus to the storage modulus (E″/E′), is commonly used to characterize viscoelastic behavior of the materials, for example, *T*<sup>g</sup> and energy dissipation of composite materials. With increasing temperature, the tan δ values of PP and PP matrix composites increased due to the increased polymer chain mobility of the matrix and exhibited two relaxation peaks in the vicinity of 5 and 70°C. The low-temperature peak is related to the glass transition of the amorphous polymer fractions [20–21]. The high-temperature peak corresponds to the α transition related to the PP crystalline fractions. The α transition peak of the modified PP composite (81°C) was higher than that of the unmodified one (72°C) that can be a result of the existence of enhanced transcrystallinity around the fibers in the modified composites [20, 22].

For the polyethylene system, tan δ curves of neat PE and the composites had less distinctive α transition processes compared to the loss modulus curves and there was no peak corresponding to the *T*g of polyethylene (approximately −130°C) because it was not sufficiently cooled down to this temperature while carrying out the test. The α relaxation is generally attributed to segmental motions in the noncrystalline phase [23]. The α transition of the composites shifted to higher temperature compared to neat polyolefine. An addition of compatibilizer also led to shift slightly the α transition curve to the higher temperature, that is an indication of the presence of some processes, which have restricted the mobility of the chains in the crystalline phase so that more energy is required for the transition to happen. Therefore, the natural fibers somehow restricted the matrix polymer chains and increased the α transition temperature [17].
