physical-mechanical properties of polypropylene … from renewable resources, vol. 1, no. 2, 2010...

105Polymers from Renewable Resources, Vol. 1, No. 2, 2010

Physical-Mechanical Properties of Polypropylene Filled with Wood Fibre, Rice-Husk, Bagasse

©Smithers Rapra Technology, 2010


Abdulrasoul Oromiehie* and Jalal Faghihi

Iran Polymer and Petrochemical Institute, P.O. Box: 14965-115, Tehran, Iran

Received: 20 December 2009, Accepted: 23 March 2010

SUMMARY

The effects of various reinforcing fi llers in polypropylene have been studied.Composites containing different amounts (40, 50, and 60) of rice-husk (RH), wood fi bre (WF) and bagasse (BG) fi bres as the reinforcing fi llers with polypropylene were prepared using a co-rotating twin screw extruder. The physical and mechanical properties of the samples were characterized. In order to increase the interphase adhesion between the components, polypropylene grafted maleic anhydride was added as a coupling agent to all compositions. It was found that the PP/WF composites had higher tensile and fl exural strength compared with other composites. Flexural strength and fl exural modulus of composites for all reinforcing fi llers were improved, and impact strength and tensile strain decreased with increased fi bre content. Water absorption of the composites increased with increased fi ber content. In this respect, PP/BG composites, among other composites, showed the highest level of water absorption. SEM observations showed that, up to 50 wt% of fi bres, more voids were formed in the composites which led to weak adhesion between the polymer matrix and the fi bres and as a result the products showed lower tensile strength. Thermal analysis of the composites fi lled with 50 wt% fi ber showed that better thermal stability was attained for PP/WF and PP/RH composites compared to their PP/BG counterpart.

Keywords: Composites, Wood fi bre, Rice husk, Bagasse, Mechanical properties

A.Oromiehie @ippi.ac.ir. Tel: 0098-21-44580060; Fax: 0098-21-44580060

106 Polymers from Renewable Resources, Vol. 1, No. 2, 2010

Abdulrasoul Oromiehie and Jalal Faghihi

INTRODUCTION

The plastic materials reinforced by natural ligno-cellulose fi bers are inexpensive, readily available and have low density and low specifi c properties compared with composites reinforced by inorganic fi llers and synthetic fi bers. Moreover, being biodegradable they are helpful in the efforts against environmental pollution [1].

Among the commodity thermoplastics, polypropylene (PP) has been one of the most popular candidates as a matrix material due to its versatility in accepting numerous types of fi llers and reinforcement.

A special interest has grown in composites based on thermoplastic matrices reinforced with raw ligno-cellulose materials such as rice husk (RH), wood fi ber (WF) and bagasse (BG) [2-5]. The use of these natural fi llers in polymer matrix has resulted in a signifi cant enhancement of tensile modulus or fl exural modulus of the composites produced.

However, due to the geometrical nature of the fi llers, that is their irregular shapes with low aspect ratio (length-to-diameter), the improvement in the tensile strength and impact strength are rather limited compared to synthetic fi bers such as glass, carbon, or Kevlar [6].

The combination of ligno-cellulosic material with thermoplastic matrix can present considerable problems: (a) incompatibility between the polar and hygroscopic fi ber and the non-polar and hydrophobic matrix adversely lowers adhesion between the two components and as a result it may lead to weak interfacial bonding. (b) The high level of moisture absorption of the wood fi bers and the poor adhesion with hydrophobic polymeric materials also lead to debonding with ageing which would certainly reduce the mechanical properties of the composite system.

A possible solution to counter this problem is through chemical modifi cation of the fi bers by inclusion of hydroxyl groups, which are highly susceptible to chemical reaction [7].

Figure 1 shows esterifi cation reaction and H-bond interactions of the cellulose fi ller and PP-MAH interface [8]. A non-polar group can be inserted into the fi bers, resulting in hydrophobic characteristic compatible with a polyolefi n matrix [9].

It is known that the interfacial compatibility between fi bers and matrix plays a key role in the reinforcing properties of the composites [10-16]. There are several reports on the chemical modifi cation of natural fi bers aiming to achieve better fi ber-matrix interfacial compatibility in the production of composites [17-20].



The aim of this research work is to explore the effect of three types of ligno-cellulose fi bers: rice husk, bagasse and wood fi ber on mechanical and physical properties of polypropylene composites as well as the fracture surface of the composite.

These methods include silane treatment [21, 22], alkali treatment [23-25], and treatments of fi ber with maleated polyolefi n [26-28] are also found to be very effective in improving fi ber-matrix adhesion. The role played by each type of fi ber and its effect on the mechanical and physical properties of polypropylene by fi ber content up to the 60% by weight are investigated. Further examination of the chemical composition of three fi bers showed that bagasse fi bers are mainly composed of cellulose. Wood fi bers are composed of cellulose, hemi-cellulose and lignin [29]. Rice husk contains 35% cellulose, 25% hemi-cellulose, 20% lignin and 17% ash (94% silica) by weight [30]. Silica is inorganic; therefore it would be possible to obtain composites with better thermal properties than with conventional wood fi ber; possibly with lower impact properties because of lower fl exibility of this fi ber [2].

Figure 1. Esterifi cation reaction and H-bond interactions at the ligno-cellulose fi ber and PP-MAH interface [8]



EXPERIMENTAL

Materials

Wood-fi ber (WF) from beech trees was obtained from Aria Cellulose Co., Iran. The average particle size of the fi bers was 0.17-0.26 mm. Chopped rice husk (RH) and bagasse (BG) fi bers were obtained from a local market. The WF fi bers originated from natural wood and RH and BG fi bers were from cellulose pulp. Bulk density of WF fi bers was 1.5 g/cm3 and for BG and RH fi bers were 1.88 and 1.3 g/cm3, respectively. They were air-dried and then ground in a Wieser grinding machine (WG-LS 200/200). To minimize the moister content, ligno-cellulose fi bers were dried in an oven at 120°C for 24 h before being blended with polypropylene.

Polypropylene (PP), homopolymer, with a melt fl ow index of 8 g/10 min, and a density of 0.9 g/cm3, was supplied by Bandar Imam Petrochemical Co., Iran. Maleated polypropylene (MAPP) was obtained from Dupont, trade name of MD353D, with melt fl ow index of 450 g/10 min, and MA content of 1-1.4 wt% used as a compatibilizer.

Compounding

Composites including different amounts of polypropylene with three content levels of rice husk, wood fi ber, and bagasse were formulated according to Table 1. The MAPP was added as a compatibilizer at 3 wt% of PP content. This was used as a reference matrix.

Table 1. Composition of PP/ ligno-cellulose composites

Formulation Code of samples

PP (wt%) Rice husk (wt%)

Wood fl our (wt%)

Bagasse (wt%)

PP - 100 0 0 0

PP + RH A1 60 40 0 0

PP + RH A2 50 50 0 0

PP + RH A3 40 60 0 0

PP + WF B1 60 0 40 0

PP + WF B2 50 0 50 0

PP + WF B3 40 0 60 0

PP + BG C1 60 0 0 40

PP + BG C2 50 0 0 50

PP + BG C3 40 0 0 60



The compounded materials were blended in an intermeshing co-rotating twin-screw extruder ZSK-25 with fi ve temperature zones and equipped with the screws of 25 mm in diameter and L/D (length to diameter of screw) ratio of 40. The temperature profi le for different barrel zones and the die were set at 165°C, 185°C, 190°C and 185°C from hopper to die zones. The composites were then pelletized and dried at 120°C for 24 hr. Samples for different experiments were prepared by injection of the material into an appropriate mould, using injection moulding machine (Imen Machine Co., Iran). Processing temperature for injection moulding was set at 195°C with pressure of 11 MPa, respectively.

CHARACTERIZATION

Mechanical Properties

The mechanical properties of all PP/ligno-cellulose fi bers composites were assessed via tensile, fl exural and impact strength determinations. All the results on mechanical properties reported in this work were averages of fi ve tests, except impact strength which was at least an average of ten specimens of each composite.

The tensile properties of dumbbell-shaped specimens were measured on an Instron testing machine (Model 6025) at a crosshead speed of 5 mm/min. The thickness, length and width of the samples were 4, 120 and 10 mm, respectively. Flexural properties were examined in a three point bending mode on the same Instron testing machine and at a constant defl ection rate of 2 mm/min. The samples were rectangular bars of 110 mm ×10 mm ×10 mm and the span length was taken to be 50 mm.

The impact strength of the composites was studied by using a Zwick impact pendulum tester (Model 5102) in the Izod impact mode according to ASTM D-256. The experiment was carried out on the notched samples and at least ten specimens of each composite were tested to obtain a reliable average and standard deviation.

Scanning Electron Microscopy

The surface fracture of various composites was studied by using a Cambridge scanning electron microscope (Model S360). The fracture surfaces were prepared



by snapping the composites to half at liquid nitrogen temperature. The samples were mounted on a sample stub and the surface was sputtered with gold.

Density Measurement

This test method was based on observing the level to which a test specimen sinks in a liquid column exhibiting a density gradient in comparison with standards of known density according to ASTM 1505.

Water Absorption Measurement

The fl exural samples were dried in an oven for 24 hr at 110°C, before being immersed in water at room temperature. The samples were removed from water at different time intervals, dried by piece of clean cloth and weighed. Each value obtained was the average of fi ve samples.

Percentage of water absorption was obtained according to ASTM D-570.

The oven-dried weight (Wd) was determined and used to calculate the degree of moisture absorption as follows:

Water absorption (%) =

W Wd

Wd

100

Where W is the weight of the sample after water uptake in de-ionized water at 25°C and atmospheric pressure.

Thermal Properties

TGA analyses were performed using a Perkin Elmer PYRIS 1500 analyzer model at 10°C min-1 heating rate from 25 to 600°C under air fl ow. The weighed samples, each weighing ~20 mg, were placed in a platinum pan. The precision on temperature measurement was ±1.5°C.

RESULTS AND DISCUSSION

Mechanical Properties

Mechanical properties of the composites are presented in Table 2. Tensile strength of the composites shows that it is improved up to the 50 wt% of



fi ber content and beyond this level it is reduced. In other words, for the fi ber contents above 50 wt%, the fi ber adhesion to the matrix is not strong to transfer applied load through the interface. Furthermore, the interfacial area is also increased which leads to a poor interfacial bonding between fi ber and the matrix. Comparing the PP/WF, PP/BG and PP/RH composites together, it is evident that the tensile strength of PP/WF composites at various fi ber loadings are higher than PP/BG and PP/RH composites. This is attributed to the compatibility between the matrix and fi bers which have resulted in increased interfacial strength. Addition of MAPP to polypropylene enhances the interfacial adhesion at fi ber/matrix interface. Usually in composites, lower elongation-at-break values are observed with increased modulus [31].

Tensile-strain-at-yield and Izod impact resistance of the composites have decreased with increased fi ber content. Reduction in tensile strain and impact resistance can be attributed to the plasticity decrement, strong cohesive interaction between the fi bers and the matrix and increased brittleness of the systems by using MAPP. Tensile strain of PP/WF composites is lower than PP/BG and PP/RH composites due to strong interfacial adhesion at system’s interface compared with other composites. Compared to the PP matrix, the PP/WF composites having 50 wt% of fi ber, showed an almost 60% reduction in strain-at-break. The higher tensile strength of PP/WF composites compared with PP/BG and PP/RH can be attributed to the difference in chemical compositions of the fi bers.

Table 2. Mechanical properties of various composites

Sample code

T.S(MPa)

SD T.St (%)

SD I.S (J/m)

SD F.S (MPa)

SD F.M (GPa)

SD

PP 18.00 0.4 12.90 0.45 18 0.44 39 0.1 0.60 0.3

A1 34.10 0.7 11.40 0.92 25.2 0.65 36.01 0.43 2.03 0.7

A2 42.30 0.6 8.20 0.74 21 0.77 46.45 0.33 3.42 0.2

A3 33.00 0.8 3.50 0.62 17.1 0.34 52.16 0.38 4.80 0.1

B1 39.20 0.8 8.30 0.66 34.1 0.73 54.2 0.57 3.78 0.5

B2 46.30 0.4 5.10 0.75 28.1 0.88 58.07 0.47 5.14 0.3

B3 38.30 0.3 1.90 0.79 22.4 0.76 61.3 0.27 6.38 0.2

C1 31.20 0.5 14.30 0.68 29 0.55 43.34 0.39 2.42 0.6

C2 38.30 0.5 9.10 0.65 26.3 0.63 53.84 0.42 3.78 0.7

C3 29.40 0.7 5.70 0.76 21.4 0.48 58.51 0.44 5.10 0.4

T.S: Tensile Strength I.S: Impact Strength (Izod) F.M: Flexural ModulusT.St: Tensile Strain F.S: Flexural Strength



In Table 3, chemical composition of the fi bers is listed. Wood fi bers have higher cellulose content than other fi bers. Furthermore, there are more OH groups available to make H-bonding with MA groups on the matrix.

Table 3. Chemical compositions of the lignocellolosic fi bers [1, 40]

Hemicellulose % Lignin (%) Ash (%) Others (%)

Wood 62.5 26.2 0.4 10.9

Rice husk 60 20 17 3

Bagasse 48 14.3 2.4 28.7

Therefore, PP/WF composites have presented higher tensile strength compared to other composites. As the fi bre loading increased beyond 50 wt%, thereby increasing the interfacial area, the weaker interfacial bonding between the two phases lowered the tensile strength of the composites.

The impact strength of composites is even more complex than those of the unfi lled polymers because of the part played by the fi bers and the interface in addition to the polymer. Generally, fracture process may be divided into crack initiation and crack propagation stages. The energy required for crack propagation was measured with a notched Izod impact test. Izod impact strength of the composites decreased at higher fi ber loading due to the greater surface area of weakly bonded regions between the two phases. This poor interfacial bonding induces microspacing between the fi bers and the matrix, and these cause numerous microcracks under impact, which readily induces crack propagation and lowers the impact strength of the composites. Thus lower plasticity is obtained which in turn reduces the impact strength [32].

RH composites show lower impact resistance compared with other composites. RH fi bers contain 15 wt% silica. As silica is an inorganic and infl exible fi ller, composites with lower impact resistance are obtained at 15 wt% silica content [2]. Rice husk fi ber is easily agglomerated, which is the characteristic of this fi ber, and the presence of these agglomerates results in the generation of voids between the fi bers and matrix. This leads to lower tensile strength of RH composites as compared with the WF composites [33]. WF composites show higher impact resistance relative to other composites.

PP/WF composites show improved interfacial adhesion and better stress transfer between the fi bers and the matrix at their interfaces which have led to greater resistance to crack propagation in the matrix. Lower impact strength of BG/PP composites compared to the WF/PP composites can be attributed to lower cellulose content in bagasse fi bers with formation of gaps and fl aws and lower interfacial adhesion at fi ber/matrix interface.



Flexural strength and modulus of composites increased with higher fi ber contents as compared to PP matrix composite. It is observed from Table 2 that RH composites at 40 wt% of fi bers showed slightly lower fl exural strength. This is probably attributed to the non-uniformity of fi ber particles dispersion and irregular-shaped fi llers. Therefore, tensile strength of this composite is decreased due to the inability of fi ller to support stress transferred from the matrix.

The fl exural strength of WF composites was higher than other composites. WF composites showed the highest fl exural strength of 57% and 963% modulus at 60 wt% fi ber loading, in comparison with PP matrix.

Addition of fi bers into the PP matrix produced composites of higher modulus, because of the high modulus of the fi bers.

Flexural modulus is largely dependent on fi ber content in the composites [34].

If the fi bers are distributed and aggregated inside the matrix, a higher fl exural modulus for the material can be obtained. Composites of better performance with random distribution into the matrix are attained due to homogeneous distribution of the fi bers [3].

Previous studies showed that increased brittleness of the composites can change the mode of failure from fi ber pull-out to fi ber breakage while the crack propagates due to enhanced interaction between the fi bers and polymer.

SEM Investigation

Optical studies show bagasse fi bers that has originally a honeycomb structure. The percentage of the area of cells in the honeycomb structure including cavities was~ 25%. RH fi bers shows a large number of knob exist on the outer surface of fi bers. Hair-like structures were also observed in the gaps between the bridges in some regions whereas in wood fi bers short, thin-walled fi bers contribute to the smooth surface and cell wall consists of several layers of cellulose embedded in lignin and hemicellulose. These layers, made up of fi brils differently oriented [4, 5, 35].

SEM studies of tensile fracture surfaces of composites confi rm the results from mechanical properties investigation. Figures 2(A1-C3) show the SEM micrographs of tensile fracture surfaces of various composites. In composites containing 40 wt% of fi bers, low wettability and insuffi cient interaction between OH groups on the fi ber surface and matrix leads to void formation and weak adhesion at interface resulting in lower tensile properties compared with composites containing 50 wt% fi bers. Probably, the fi ber particles which are



coated with a thin and irregular polymer layer result in deformation. It should be noted that, polymer causes plastic deformation in composites as evident in Figure 2(A1-C1). Previous studies showed MAPP did not infl uence the tensile properties at low fi ber content because of the poor interaction between OH groups on the fi ber surface and PPMA [36]

Addition of fi bers up to 50 wt% have also led to different morphologies and produced a more homogeneous and less voids of better wetting of the fi llers with matrix. Therefore, better stress transfer from matrix to fi bers has occurred and tensile strength of these composites is higher than composites with 40 wt% and 60 wt% of fi bers as in Figure 2(A2-C2). Lower tensile and impact strength of the composites with more than 50 wt% fi bers compared to those at 40 and 50 wt% fi ber loading may be attributed to the formation of more voids and

Figure 2. SEM micrograph of surface fractures of various composites: (C1-C3): 40-60 wt% bagasse (B1-B3):40-60 wt% wood fi ber (A1-A3): 40-60 wt% rice husk



less fi ber wetting by the matrix. In other words, above 50 wt% fi ber loading there was brittle fracture under applied load. In Figure 3(A1-C3) the fractured surfaces of the composites are shown at higher magnifi cation.

Figure 3(A3-C3) shows that by increased fi ber loading above 50 wt% there were agglomerates and voids observed in these composites. Furthermore, weak interfacial adhesion between fi bers and matrix at the interface occurred and there was less stress transfer from the matrix to the fi bers. Therefore, tensile and impact strength of these composites were lower than those at 40 and 50 wt% fi ber loading.

Figure 3(C1-C3) shows that, wetting of the BG fi bers by matrix is not suffi cient as they can be clearly observed through the matrix. This may be attributed to lower cellulose content and (OH) groups on the BG fi ber surface which

Figure 3. SEM micrograph of surface fractures of various composites: (C1-C3): 40-60 wt% bagasse (B1-B3): 40-60 wt% wood fi ber (A1-A3): 40-60 wt% rice husk. (Circles show agglomeration regions)



result in insuffi cient adhesion between the two phases of the interface. Thus this leads to lower tensile strength of these composites compared to those of WF and RH composites.

Figure 3(B2) shows the wetted WF fi bers at 50 wt% loading by where fi llers can hardly be observed through the matrix. This can be attributed to higher cellulose content and (OH) groups on the fi llers’ surface which results in an improved bonding with the matrix that leads to higher tensile strength of the WF/PP compared to rice husk and bagasse composites at 50 wt% fi ber content.

Water Absorption and Density

Water absorption and moisture penetration into composites are conducted in three ways: (1) microgaps between polymer chains; (2) gaps and fl aws at interfaces between the fi ber and polymer because of incomplete wettability and impregnation (3) microcracks in the matrix formed during the compounding process. The results of water absorption and density properties of composites are shown in Table 4. The densities of the composites were increased by increased fi ber content. Compared with other composites PP/WF shows higher density at all fi ber contents which may be due to better compatibility between the WF fi bers and matrix [37]. As it was expected, by increased fi ber content, the amount of water absorption increased due to increase in the fi bers’ hygroscopic hydroxyl groups [6]. The lower water absorption of WF/PP composites was assumed to be the result of better adhesion between the fi bers and polymer matrix. Water absorption of BG composites is higher than other systems. The difference in the amount of water absorption of composites refl ects the difference between polarity, chemical structures, water diffusion coeffi cient and distribution of fi ber reinforcement in polymer matrix [38, 39]. Among all the composites PP/BG type shows the highest water absorption. According to the works of Graziela et al. the bond formation between (-OH) groups on the fi ber surface and the matrix prevents any bond formation between the cellulose fi bers and water, thus limiting water absorption takes place [10]. Therefore, PP/WF composites due to higher cellulose content and -OH groups showed lower water absorption and in contrast, the PP/BG composites showed higher water absorption. Cellulose fi ber is the main component in bagasse fi bers and each cellulose fi ber is hollow and contains a lumen at its center. Therefore, there are three main regions where the absorbed water in the composites can reside: The lumen, the cell wall and the gap and fl aw at the interface between the fi ber and matrix. Physical structure of RH fi bers is like BG fi bers. Lower water absorption of RH composites compared to BG composites can be attributed to higher -OH groups on RH fi ber surface for bonding to the matrix and less void formation in RH composites.



Table 4. Density and water absorption of various polypropylene/ligno-cellulose composites

Code of samples Density (g/cm3) Water Absorption (wt%)

PP 0.805 0.02

A1 0.912 0.415

A2 0.932 0.52

A3 0.998 0.625

B1 0.914 0.39

B2 0.935 0.42

B3 0.981 0.575

C1 0.832 0.61

C2 0.845 0.69

C3 0.947 0.73

Thermal Properties of Composites

The thermal stability of lignocellulosic-fi lled polymer matrix composites is a very important parameter for the processing and fi nal application of these materials. TGA and DTG curves of PP matrix and composites at 50 wt% fi bers are shown in Figures 4 and 5, respectively. The results from TGA curves show that PP has thermal degradation temperature of 258-381°C. It is evident that, composites show lower initial degradation temperature than PP matrix. The initial degradation temperature of PP/WF composite appears at the temperature range of 240-342°C. This for PP/RH and PP/BG composites is in the range of 238-359°C and 257-345°C, respectively. The initial degradation temperature of composites may correspond to thermal degradation of cellulose component of the composites. Besides, the second degradation temperature of PP/WF composite appears at the temperature range of 345-426°C and for PP/RH and PP/BG composites it is within 360 to 425°C and 355 to 393°C, respectively. The second degradation temperature of composites may be related to the thermal degradation of lignin and matrix component. Beyond 500°C the composites showed further defragmentation of decomposed products formed during the thermal analysis. The weight loss for PP is 7.6 wt% and for PP/WF composite is 13.7 wt%, whereas this is 11.5 wt% and 10.3 wt% for PP/RH and PP/BG composites at 600°C, respectively. The temperatures at 50 wt% weight loss of PP, BG, RH and WF composites correspond to 367°C, 380°C, 412°C, 422°C, respectively. The shift of the second transition temperature of composites towards higher temperature may be related to the thermal stability of composites. This indicates that compatibility and interfacial adhesion between fi bers and matrix improved in the presence of MAPP coupling agent. PP/WF



and PP/RH composites showed higher degradation temperature compared to PP/BG composite. The higher degradation temperature increased by 45°C compared with PP. Furthermore, MAPP as a coupling agent helped to

Figure 5. DTG curves of the composites (50 wt% fi bers)

Figure 4. TGA curves of the composites (50 wt% fi bers)



improve interfacial adhesion between the fi bers and matrix in PP/WF and PP/RH composites compared to PP/BG composite.

CONCLUSIONS

The results of this study indicate that composites containing wood fi ber, rice husk and bagasse fi bers as reinforcing components and polypropylene showed different mechanical and physical properties. These differences may be related to the composition of fi bers. Tensile strength of the composites up to 50 wt% fi ber content increased and then decreased, although the impact resistance and tensile strain of composites decreased with increased fi ber loading.

It should be noted that, up to 50 wt% of fi bers, strong chemical interactions between MA groups of MAPP and cellulose hydroxyl groups is probably formed and as a result the tensile strength of the composites increased. SEM observations showed that at higher fi ber loading, formation of voids in composites leads to weak adhesion bond between matrix and fi bers and as a result tensile and impact strength of the composites decreased. The reduction of tensile strength indicates poor interaction and insuffi cient fi ber length to receive proper tensile load transfer from the matrix.

PP/WF composites showed higher mechanical properties compared with other composites. In composites containing up to 50 wt% of fi ber, MAPP improved interfacial adhesion between fi bers and matrix. PP/RH composites showed lower impact resistance compared to other composites and PP/WF composites showed higher impact resistance compared to other systems. Flexural strength and fl exural modulus of the composites increased with increased fi ber content. Water absorption increased at higher fi ber content and for PP/BG composites it was higher than other composites. Generation of agglomerates in BG composites led to poor adhesion between fi bers and matrix but PP/WF composites showed lower water absorption compared with other composites. In the other words, better interfacial adhesion between PP and WF was formed.

Thermal analyses of composites at 50 wt% fi ber loading showed higher thermal stability for PP/WF and PP/RH composites compared to PP/BG composite. This means that, PPMA as a coupling agent improved interfacial adhesion between fi bers and matrix in PP/WF and PP/RH compared to PP/BG composite.



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physical-mechanical properties of polypropylene … from renewable resources, vol. 1, no. 2, 2010...

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