**3.1. Preparation of bagasse microfibers for biocomposite fabrication**

In order to produce and tune a lignocellulosic material to improve the mechanical performance of natural fiber reinforced polymer composites (NFPC), it is very important to conceptualize adhesion as one of the most important factors to achieve such challenge [20, 21]. Adhesion on the polymer-fiber interface is said to follow one of the four common mechanisms: mechanical interlocking, electrostatic interactions, molecular entanglement, or chemical bonding [22]. Many commercial polymer-coupling additives like maleic and acrylate grafted thermoplastics work as adhesion enhancers in polyolefins by generating chemical bonds with the free

**Figure 2.** Granulometry of milled sugarcane fibers after lignin removal and silanization with hexadecyl triethoxysilane.

alcohol functionality on the fiber's cellulose [23]. In those aforementioned cases, the adhesion increases by conditioning the polymer to the fiber's surface. Instead, when silanes are used to increase adhesion, in the fiber's surface which is conditioned to interact with the olefin polymer matrix by promoting electrostatic interactions or chain molecular entanglement [24, 25].

When nonpolar silanes, like dodecyl, hexadecyl, or octadecyl triethoxysilanes, are used for fiber modification, there is a lowering effect of the fiber surface energy which increases compatibility with the matrix by matching the polarities [26]. A practical and quick way of estimating the surface energy of surfaces is by measuring the contact angle of the surface. However, in many NFPC applications, the size of the fibers used is in the range of micrometers, as shown in the granulometry in **Figure 2**, for the case of sugarcane bagasse fibers.

Measuring contact angle on rough surfaces, such as those formed for a bed of microsized fibers, can be challenging since the observed angle is the manifestation not just of the molecular interactions at the solid/liquid interphase but also of the microstructure of the surface (**Figure 3**).

This effect is counted by the model of Wenzel which predicts that if a molecularly hydrophobic surface is rough, the appearance is that of an even more hydrophobic surface [27, 28]. An interesting evidence of this phenomenon in particulate fibers is shown in **Figure 4**, where the contact angle of several groups of sugarcane bagasse fibers treated with solutions of variable silane concentrations appears almost invariant (between 122° and 129°), even though the absorbed amount of silane changes nearly in one order of magnitude (5.5 × 10−<sup>5</sup> to 1.04 × 10−<sup>4</sup> mol of silane per gram of fiber).

Another critical factor to achieve a good reinforcing material made of small natural fibers is the generation of anchoring points on the rough surface in order to produce enough silanization. Natural fibers in contrast with fiberglass, for example, do not have a well-defined geometry and instead lack of the advantage of having a highly energetic surface prone to react during silanization. Fiberglass has a surface populated with free hydroxyl groups from Si-OH functionality, but

**Figure 4.** Effect of silane absorbed on microsized sugarcane bagasse fibers and their contact angle.

**Figure 3.** SEM photograph of a silanized sugarcane fiber. Inset shows the rough surface of a bed of fibers used to measure

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contact angle of the fibers.

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**Figure 3.** SEM photograph of a silanized sugarcane fiber. Inset shows the rough surface of a bed of fibers used to measure contact angle of the fibers.

**Figure 2.** Granulometry of milled sugarcane fibers after lignin removal and silanization with hexadecyl triethoxysilane.

alcohol functionality on the fiber's cellulose [23]. In those aforementioned cases, the adhesion increases by conditioning the polymer to the fiber's surface. Instead, when silanes are used to increase adhesion, in the fiber's surface which is conditioned to interact with the olefin polymer matrix by promoting electrostatic interactions or chain molecular entanglement [24, 25]. When nonpolar silanes, like dodecyl, hexadecyl, or octadecyl triethoxysilanes, are used for fiber modification, there is a lowering effect of the fiber surface energy which increases compatibility with the matrix by matching the polarities [26]. A practical and quick way of estimating the surface energy of surfaces is by measuring the contact angle of the surface. However, in many NFPC applications, the size of the fibers used is in the range of micrometers, as shown in the

Measuring contact angle on rough surfaces, such as those formed for a bed of microsized fibers, can be challenging since the observed angle is the manifestation not just of the molecular interactions at the solid/liquid interphase but also of the microstructure of the surface (**Figure 3**).

This effect is counted by the model of Wenzel which predicts that if a molecularly hydrophobic surface is rough, the appearance is that of an even more hydrophobic surface [27, 28]. An interesting evidence of this phenomenon in particulate fibers is shown in **Figure 4**, where the contact angle of several groups of sugarcane bagasse fibers treated with solutions of variable silane concentrations appears almost invariant (between 122° and 129°), even though the absorbed

to 1.04 × 10−<sup>4</sup> mol of

granulometry in **Figure 2**, for the case of sugarcane bagasse fibers.

amount of silane changes nearly in one order of magnitude (5.5 × 10−<sup>5</sup>

silane per gram of fiber).

136 Characterizations of Some Composite Materials

**Figure 4.** Effect of silane absorbed on microsized sugarcane bagasse fibers and their contact angle.

Another critical factor to achieve a good reinforcing material made of small natural fibers is the generation of anchoring points on the rough surface in order to produce enough silanization. Natural fibers in contrast with fiberglass, for example, do not have a well-defined geometry and instead lack of the advantage of having a highly energetic surface prone to react during silanization. Fiberglass has a surface populated with free hydroxyl groups from Si-OH functionality, but

**Figure 5.** (a) TG and (b) DTG curves of bagasse fibers at heating rates of 10°C/min.

natural fibers are usually covered by a nonreactive lignin layer. That is why for natural fibers, the surface area that will react with silanes is determined by a good process of lignin removal, using alkaline or oxidative solutions, which will expose cellulose at the natural fiber's surface [29, 30].

The performance of the delignification treatment can be estimated from thermal gravimetric analysis (TGA) of fibers. **Figure 5** shows the TGA of sugarcane bagasse fibers before and after delignification with alkaline treatment and after silanization.

When a good delignification is carried out, fibers gain some thermal stability. As shown in **Figure 5**, the T<sup>0</sup> for sugarcane bagasse goes from 262 to 282°C when lignin has been removed. Also, as noted in the DTG, the typical signal of hemicellulose around 290°C disappears [31]. Furthermore, after most of lignin goes away, it is possible to observe a cleaner DTG signal with no shoulders that make evident the presence of residual compounds in the fibers. With only cellulose, the maximum degradation (Tmax) in DTG occurs around 325°C. Silane presence also increases Tmax to 335°C, mostly due to the formation of refractory siloxane network after silanization. Additionally, as observed in the TGA results, when surface modification by silanization with hydrophobic moieties has occurred, there is a clear decrease of water evaporation after 60°C, since fibers absorb less water when silanized. Changes in water uptake can go from 5 to 1%. This result indicates that silanization process reduces water absorption of the fibers and may give resistance against fungal decay [32].

Another factor that plays a role in the reinforcing ability of fibers is the distribution of silane on their surface. Few works have detailed how silane gets distributed in the rough surface of natural fibers. Using chemical maps from scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), it is possible to survey the surface and locate silicon at specific locations on the fiber [33]. **Figure 6** shows the chemical maps for oxygen and silicon, as an example of chemical mapping of silanized fibers.

From SEM–EDX spectra, the concentration of surface atoms can be estimated using the intensity of signals at the specific energies of each atom. In this case, sugarcane bagasse fibers modified with hexadecyltrimethoxysilane were interrogated for the content of oxygen, silicon, and carbon before and after silanization. **Figure 7** reviews the results. Spectra revealed

that atomic oxygen content changed from 28.57 to 17.44%, carbon from 70.86 to 81.67%, and silicon from 0.29 to 0.57% before and after silanization, respectively. The variations in the atomic content are in agreement with the process performed. For example, the increment in

**Figure 7.** Percentage of total atomic content of carbon, oxygen, and silicon at fiber surface (black) and after (gray)

**Figure 6.** SEM–EDX chemical maps of sugarcane bagasse fiber treated with hexadecyltrimethoxysilane after deligni-

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fication with NaOH 8%.

silanization, measured by SEM–EDX.

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**Figure 5.** (a) TG and (b) DTG curves of bagasse fibers at heating rates of 10°C/min.

delignification with alkaline treatment and after silanization.

fibers and may give resistance against fungal decay [32].

silicon, as an example of chemical mapping of silanized fibers.

**Figure 5**, the T<sup>0</sup>

138 Characterizations of Some Composite Materials

natural fibers are usually covered by a nonreactive lignin layer. That is why for natural fibers, the surface area that will react with silanes is determined by a good process of lignin removal, using alkaline or oxidative solutions, which will expose cellulose at the natural fiber's surface [29, 30]. The performance of the delignification treatment can be estimated from thermal gravimetric analysis (TGA) of fibers. **Figure 5** shows the TGA of sugarcane bagasse fibers before and after

When a good delignification is carried out, fibers gain some thermal stability. As shown in

Also, as noted in the DTG, the typical signal of hemicellulose around 290°C disappears [31]. Furthermore, after most of lignin goes away, it is possible to observe a cleaner DTG signal with no shoulders that make evident the presence of residual compounds in the fibers. With only cellulose, the maximum degradation (Tmax) in DTG occurs around 325°C. Silane presence also increases Tmax to 335°C, mostly due to the formation of refractory siloxane network after silanization. Additionally, as observed in the TGA results, when surface modification by silanization with hydrophobic moieties has occurred, there is a clear decrease of water evaporation after 60°C, since fibers absorb less water when silanized. Changes in water uptake can go from 5 to 1%. This result indicates that silanization process reduces water absorption of the

Another factor that plays a role in the reinforcing ability of fibers is the distribution of silane on their surface. Few works have detailed how silane gets distributed in the rough surface of natural fibers. Using chemical maps from scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), it is possible to survey the surface and locate silicon at specific locations on the fiber [33]. **Figure 6** shows the chemical maps for oxygen and

From SEM–EDX spectra, the concentration of surface atoms can be estimated using the intensity of signals at the specific energies of each atom. In this case, sugarcane bagasse fibers modified with hexadecyltrimethoxysilane were interrogated for the content of oxygen, silicon, and carbon before and after silanization. **Figure 7** reviews the results. Spectra revealed

for sugarcane bagasse goes from 262 to 282°C when lignin has been removed.

**Figure 6.** SEM–EDX chemical maps of sugarcane bagasse fiber treated with hexadecyltrimethoxysilane after delignification with NaOH 8%.

**Figure 7.** Percentage of total atomic content of carbon, oxygen, and silicon at fiber surface (black) and after (gray) silanization, measured by SEM–EDX.

that atomic oxygen content changed from 28.57 to 17.44%, carbon from 70.86 to 81.67%, and silicon from 0.29 to 0.57% before and after silanization, respectively. The variations in the atomic content are in agreement with the process performed. For example, the increment in carbon content after silanization is due to the additional carbons brought to the surface by the long chains (C16) of hexadecyltrimethoxysilane. For oxygen instead, the surface atomic concentration becomes lowered since a not-significant amount of oxygen is added by silanes. Silicon, as expected, almost doubles his surface concentration.

increase of 40% was observed. This result shows that silanization increases the capacity of PP to absorb energy. This phenomenon can be explained by a possible energy absorption promoted by fracture mechanisms, which involve detachment, slippage, and fragmentation of the fiber.

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**Figure 8** shows the results of the strain sweep tests of the PP matrix. Images of the PP specimen are included at a strain of 0.01% (linear region) and 0.6% which corresponds to the nonlinear zone. In this zone it is observed that the specimen has been highly deformed. From

**Figure 9** shows the thermograms obtained in the DMA for the PP matrix and its biocomposites. In these graphs the values of the storage modulus (E'), loss modulus (E"), and tan delta are shown. Neat PP tan delta plot shows two relaxations located near 6°C (β relaxation or Tg) and 60°C (α relaxation) [35]. It is also observed that the values of E´ are temperature dependent. At 25°C the value of E´ is 2708 MPa, while at 75°C, this value is 1199 MPa, which

In the tan delta plot of the biocomposite PP-Bag (**Figure 10**), a Tg of 5.3°C is observed, while the α relaxation increased 17.5°C compared to the neat PP. Also, E'values at 25°C is 2454 MPa,

These mechanisms do not occur in neat PP and PP biocomposites without silanization.

these results a strain of 0.01% was used for subsequent temperature ramp tests.

*3.2.2. Dynamic mechanical analysis*

*3.2.2.1. Strain sweep tests*

*3.2.2.2. Temperature ramp tests*

represents a decrease of 55%.

**Figure 8.** Strain sweep test curves for neat PP at 0, 30, and 60°C.

In general, thermal, morphological, and chemical characterization of fibers is necessary when lignocellulosic materials are prepared as reinforcing fillers. The knowledge of important factors like the degree of lignin removal, distribution of silane, and hydrophobic character of fibers are very important to ensure that the material will behave successfully during compounding with polymeric matrices and then to obtain suitable mechanical properties of biocomposites.
