**6. Results and discussion**

#### **6.1 FTIR**

FTIR Spectrum in Fig. 16 shows a spectrum of modified and unmodified wood flour samples. As can be seen, the intensity of the peak around 3400 cm-1, which is evidence of OH band, is decreased after treatment of fibers. The intensity of the band around 1730 cm-1

Fig. 16. FTIR spectra of the chemical treated wood flour.

increased due to formation of ester band from the reaction between OH group and bezoyl chloride. The intensity peak for aromatic ring at 1508.2 cm-1 is decreased after surface modification due to removal of lignin. A strong peak at 1730 cm-1 in the FTIR spectrum indicates the presence of acetyl group in the fiber. The intensity peak at 1037.6 cm-1 is increased after silane absorbance, which is an overlap of si-o-si band and c-o stretching of fiber (Lu & Drazel, 2010). In the presence of moisture, hydrolysable alkoxy group leads to the formation of silanols. The silanol then reacts with the hydroxyl group of the fiber, forming stable covalent bonds to the cell wall that are chemisorbed onto the fiber surface (Agrawal et al., 2000).

#### **6.2 Adhesion factor**

240 Some Critical Issues for Injection Molding

was 5×1×2 cm. The range of testing temperature was from -50 to 150°C and the experiments were performed at 1 Hz frequency and heating rate of 2C/min. During testing DMTA parameters of storage modulus and loss factor were recorded as function of temperature.

**Code\* Polypropylene content (Wt. %) Wood flour content (Wt. %) PP-g-MA** 

\*PP: Polypropylene, W:Wood Flour, U:Unmodified, A:Alkline, B:Benzoylation, C:Acrylication, S:Silane,

Tensile strength tests of the specimens were carried out according to ASTM D-638 by Instron 6025 model from UK at crosshead speed of 5 mm/min. For each test and type of the

The infrared spectra of raw and treated wood flour were recorded on a Bomem, 150-MB series model Spectrophotometer to characterize the chemical change upon treatment of the

The morphology of the wood modified-PP composites and interfacial bonding between the filler and the PP matrix was examined using a scanning electron microscope (JXA-840) supplied by JEOL Company Limited, Japan. The samples were viewed perpendicular to the

FTIR Spectrum in Fig. 16 shows a spectrum of modified and unmodified wood flour samples. As can be seen, the intensity of the peak around 3400 cm-1, which is evidence of OH band, is decreased after treatment of fibers. The intensity of the band around 1730 cm-1

composite, five specimens were tested and the average values are reported.

**5.4.3 Fourier Transform Infrared Spectroscopy (FTIR)** 

PP 100 0 0 UW-P 60 40 0 UW-P-M 60 40 3 AW-P 60 40 0 AW-P-M 60 40 3 BW-P 60 40 0 BW-P-M 60 40 3 CW-P 60 40 0 CW-P-M 60 40 3 SW-P 60 40 0 SW-P-M 60 40 3

**(Wt. %)** 

Then based on equation (4) data analyzed for determining A factor.

M:PP-g-Ma

**5.4.2 Mechanical test** 

fractured surfaces.

**6.1 FTIR** 

**6. Results and discussion** 

Table 2. Composition of the Studied WPCs.

wood flour with chemical components.

**5.4.4 Scanning Electron Microscopy (SEM)** 

Results for the adhesion factor as an evaluation parameter for fillers-polymer interactions versus temperature is presented in Fig. 17 for different chemical modification.

Fig. 17. Adhesion factor versus temperature for the treated WPCs.

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 243

imparted by the fiber, which allowed a greater degree of stress transfer at the interface (Jain et al., 1992). As the temperature is increased, relaxation process of the molecular matrix is initiated. Also, thermal expansion occurs which decreases the intermolecular forces (George et al., 1999). An appreciable improvement in the storage modulus was observed for the treated composite, due to the increase in the interfacial stiffness brought about by the more intense filler-matrix interaction. The composite modified by silane improves the interfacial adhesion more than other composites and this more lessens the molecular mobility in the interfacial region. In Storage modulus plots, around temperature 22C, slop of most curves change that can be considerable as β transition. At the higher end of the temperature range, the curves of

Fig. 19 shows the loss factor (tan δ) versus temperature for wood-PP composites and their corresponding different chemical treatment. The tan δ peak was shifted to higher temperature for filled samples in comparison to neat polypropylene. Relaxation peaks for treated and untreated samples are present for the tan δ curves in the vicinity of -40C (γ), 22C (β) and 100C (α) which are caused by the onset of the various motions of the chain molecules. The dominant β-peak represents the glass-to-rubber transition of the amorphous

Table 3 depicts the shift in glass transition temperature of the sample which is taken from tan δ curves. Depending on the nature of lignocelluloses filler and filler/matrix interaction, glass transition of the composites shifts to higher temperature. In other words, during stress transfer at the interface the strong bonding causes the fiber constraint and the poor bonding leads to dissipation energy. Among composites, those containing unmodified samples have the lowest values, whereas those containing acrylic acid treated samples have the highest tan δ values compared with the other samples. The difference between various chemical

portion in PP and is assigned to the glass transition temperature.

Fig. 19. Tan δ versus temperature for the treated WPC.

modifications becomes more pronounced at higher temperatures.

PP and unmodified composites converge.

**6.4 Tan δ**

Adhesion factor presents a macroscopic quantitative measure of interfacial adhesion during dynamic loading and at high levels of interface adhesion, the molecular mobility surrounding the filler is reduced, and consequently low values of the adhesion factor suggest improved interactions at the matrix–filler interface (Kubat et al., 1990). As can be seen, below of glass transition temperature, the maximum amount of adhesion factors are related to untreated samples which means the weakest interface has been formed for samples containing untreated wood flour. It seems the chemical modification facilitates the interaction between fillers polymer and decreases the adhesion factor. It is important to note that adhesion factor seems to be very sensitive to glass transition temperature of samples. Around this temperature (around 22C), slop of curves changes and the adhesion factor passes through a maximum due to more polymer chains mobility. According to Kubat et al., (1990) by increasing the temperature there was a release of the thermal stresses at the filler surface and reduced filler–matrix friction and should be related to a more cohesive matrix– filler interface (lower A). In other words, a strong interfacial adhesion i.e. samples which are treated with silane, restricts the chain mobility at the filler matrix interface, therefore the adhesion factor decreases and its maximum shifts to higher temperature. At the higher end of the temperature range, the most curves converge.

#### **6.3 Storage modulus**

The variation of the storage modulus (E') value of the composites as a function of temperature is shown in Fig. 18 for different chemical treatment. E' Value determines relevant stiffness of WPCs (Kim et al., 2005).

Fig. 18. Storage modulus versus temperature for the treated WPCs.

The stiffness of the composites is greater than that of the neat PP in the whole temperature range, and this trend is more significant in the higher temperature range. Fillers play an important role in increasing the storage modulus of polymeric materials. As can be seen, a general decrease trend was also observed over the entire range of temperature and with incorporation of wood flour to PP, significant increase in the E*'* values of composites is clearly seen. This is probably due to increase in the stiffness of the matrix with the reinforcing effect imparted by the fiber, which allowed a greater degree of stress transfer at the interface (Jain et al., 1992). As the temperature is increased, relaxation process of the molecular matrix is initiated. Also, thermal expansion occurs which decreases the intermolecular forces (George et al., 1999). An appreciable improvement in the storage modulus was observed for the treated composite, due to the increase in the interfacial stiffness brought about by the more intense filler-matrix interaction. The composite modified by silane improves the interfacial adhesion more than other composites and this more lessens the molecular mobility in the interfacial region. In Storage modulus plots, around temperature 22C, slop of most curves change that can be considerable as β transition. At the higher end of the temperature range, the curves of PP and unmodified composites converge.

#### **6.4 Tan δ**

242 Some Critical Issues for Injection Molding

Adhesion factor presents a macroscopic quantitative measure of interfacial adhesion during dynamic loading and at high levels of interface adhesion, the molecular mobility surrounding the filler is reduced, and consequently low values of the adhesion factor suggest improved interactions at the matrix–filler interface (Kubat et al., 1990). As can be seen, below of glass transition temperature, the maximum amount of adhesion factors are related to untreated samples which means the weakest interface has been formed for samples containing untreated wood flour. It seems the chemical modification facilitates the interaction between fillers polymer and decreases the adhesion factor. It is important to note that adhesion factor seems to be very sensitive to glass transition temperature of samples. Around this temperature (around 22C), slop of curves changes and the adhesion factor passes through a maximum due to more polymer chains mobility. According to Kubat et al., (1990) by increasing the temperature there was a release of the thermal stresses at the filler surface and reduced filler–matrix friction and should be related to a more cohesive matrix– filler interface (lower A). In other words, a strong interfacial adhesion i.e. samples which are treated with silane, restricts the chain mobility at the filler matrix interface, therefore the adhesion factor decreases and its maximum shifts to higher temperature. At the higher end

The variation of the storage modulus (E') value of the composites as a function of temperature is shown in Fig. 18 for different chemical treatment. E' Value determines

The stiffness of the composites is greater than that of the neat PP in the whole temperature range, and this trend is more significant in the higher temperature range. Fillers play an important role in increasing the storage modulus of polymeric materials. As can be seen, a general decrease trend was also observed over the entire range of temperature and with incorporation of wood flour to PP, significant increase in the E*'* values of composites is clearly seen. This is probably due to increase in the stiffness of the matrix with the reinforcing effect

of the temperature range, the most curves converge.

Fig. 18. Storage modulus versus temperature for the treated WPCs.

relevant stiffness of WPCs (Kim et al., 2005).

**6.3 Storage modulus** 

Fig. 19 shows the loss factor (tan δ) versus temperature for wood-PP composites and their corresponding different chemical treatment. The tan δ peak was shifted to higher temperature for filled samples in comparison to neat polypropylene. Relaxation peaks for treated and untreated samples are present for the tan δ curves in the vicinity of -40C (γ), 22C (β) and 100C (α) which are caused by the onset of the various motions of the chain molecules. The dominant β-peak represents the glass-to-rubber transition of the amorphous portion in PP and is assigned to the glass transition temperature.

Fig. 19. Tan δ versus temperature for the treated WPC.

Table 3 depicts the shift in glass transition temperature of the sample which is taken from tan δ curves. Depending on the nature of lignocelluloses filler and filler/matrix interaction, glass transition of the composites shifts to higher temperature. In other words, during stress transfer at the interface the strong bonding causes the fiber constraint and the poor bonding leads to dissipation energy. Among composites, those containing unmodified samples have the lowest values, whereas those containing acrylic acid treated samples have the highest tan δ values compared with the other samples. The difference between various chemical modifications becomes more pronounced at higher temperatures.

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 245

somewhat leached, leading to dissolution of hemicellulose, lignin and pectin. The removal of surface impurities can make the fiber cleaner and rougher than before ( Liu et al., 2009). Wood flour reinforced plastic composites often showed enhancement in tensile strength upon different modification owing to the increased fiber-matrix adhesion. Optimum

Fig. 21. Tensile strength and modulus of neat PP and the ones reinforced with modified and

Fig. 22 illustrated the result of tensile strength conducted on the modified specimens with and without PP-g-MA. The tensile strength of the composites increased with using coupling agent; because of improve the bonding strength between wood flour and PP matrix. So, the tensile strength of composites increased with conjunction of use both modified fiber and

Fig. 22. Tensile strength of modified and unmodified fiber composites with and without

strength is observed for alkali treated composite.

unmodified fiber composites.

coupling agent.

coupling agent.


Table 3. Shift in glass transition temperature of the treated WPC.

The effects of PP-g-Ma coupling agent on the storage modulus and tan δ of the silane treated wood flour /PP composites are shown in Fig. 20. As can be seen, the addition of PP-g-MA improved E*'* of the composites, that was because PP-g-MA could lead to the creation of a thin and irregular polymer layer, which could assist formation of plastic deformation zone around the fiber (Hristov et al., 2004). Further, incorporation of compatibilizer to composites containing silane treated wood flour decreased tan δ due to the more intense filler-matrix interaction. This results indicated that, simultaneous use of silane and coupling agent on storage modulus and tan delta had synergic effect.

Fig. 20. Simultaneous effect of compatibilizer and silane on storage modulus and loss factor of the wood flour/PP composite.

#### **6.5 Tensile properties**

Results for the tensile strength and modulus of composites as function of chemical modification are presented in Fig. 21. Incorporation of wood flour in PP matrix significantly increased strength and modulus of composites. The increase in tensile strength or modulus is primarily attributed to the presence of fiber, which allowed a uniform stress distribution from continuous PP matrix to dispersed fiber phase (Coutinho et al., 1997).

Benzoylation treatment of fiber enhancement the tensile modulus of composites, but all other modulus has no significant variation together. Wood flour after modification are

The effects of PP-g-Ma coupling agent on the storage modulus and tan δ of the silane treated wood flour /PP composites are shown in Fig. 20. As can be seen, the addition of PP-g-MA improved E*'* of the composites, that was because PP-g-MA could lead to the creation of a thin and irregular polymer layer, which could assist formation of plastic deformation zone around the fiber (Hristov et al., 2004). Further, incorporation of compatibilizer to composites containing silane treated wood flour decreased tan δ due to the more intense filler-matrix interaction. This results indicated that, simultaneous use of silane and coupling agent on

Fig. 20. Simultaneous effect of compatibilizer and silane on storage modulus and loss factor

Results for the tensile strength and modulus of composites as function of chemical modification are presented in Fig. 21. Incorporation of wood flour in PP matrix significantly increased strength and modulus of composites. The increase in tensile strength or modulus is primarily attributed to the presence of fiber, which allowed a uniform stress distribution

Benzoylation treatment of fiber enhancement the tensile modulus of composites, but all other modulus has no significant variation together. Wood flour after modification are

from continuous PP matrix to dispersed fiber phase (Coutinho et al., 1997).

Table 3. Shift in glass transition temperature of the treated WPC.

storage modulus and tan delta had synergic effect.

of the wood flour/PP composite.

**6.5 Tensile properties** 

**Sample Shift in Tg (°C)**  PP - Unmodified 2 Acrylation 7 Benzoylation 8 Alkaline 6 Silane 13

somewhat leached, leading to dissolution of hemicellulose, lignin and pectin. The removal of surface impurities can make the fiber cleaner and rougher than before ( Liu et al., 2009). Wood flour reinforced plastic composites often showed enhancement in tensile strength upon different modification owing to the increased fiber-matrix adhesion. Optimum strength is observed for alkali treated composite.

Fig. 21. Tensile strength and modulus of neat PP and the ones reinforced with modified and unmodified fiber composites.

Fig. 22 illustrated the result of tensile strength conducted on the modified specimens with and without PP-g-MA. The tensile strength of the composites increased with using coupling agent; because of improve the bonding strength between wood flour and PP matrix. So, the tensile strength of composites increased with conjunction of use both modified fiber and coupling agent.

Fig. 22. Tensile strength of modified and unmodified fiber composites with and without coupling agent.

Thermoplastic Matrix Reinforced with Natural Fibers: A Study on Interfacial Behavior 247

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**8. References** 

#### **6.6 Surface morphology**

It is also clear from the SEM images in Fig. 23a that the wood fibers in unmodified sample are pulled out easily and some holes are noticed around the fibers which imply that there are weak interactions between the filler and polymer. As it can be seen in Fig. 23e, there is a better polymer-filler adhesion with the silane treatment than in the composite prepared with untreated wood flour, which implies an increase in the thickness of the interface between the particles and polymers. In samples undergone alkali treatment (Fig. 23b), fibers removed from pp matrix and broken, but not the isolated fibrils were observed, which means that the interactions between the phases are not strong enough. Similar trend is also observed for samples containing acrylic acid (Fig. 23c) and benzoyl (Fig. 23d) treated fibers. As in the case of adhesion factor, the best encapsulation of wood fibers with polymer matrix can be seen in samples with silane treatment. This explanation is similar to that of adhesion factor results.

Fig. 23. SEM Micrographs of modified wood polymer composites with: (b) alkali, (c) acrylic acid, (d) benzoyl chloride, (e) silane, and (a) unmodified samples.
