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Abstract

Blends of isotactic polypropylene (iPP) and ethylene-octene-copolymer (EOC), having different proportions, were prepared in a co-rotating twin screw extruder. The tensile and flexural properties of iPP-EOC blends containing up to 20% EOC content were determined. Impact strength of blend containing 5% EOC increased drastically to the tune of >200% whereas the modulus and strength decreased marginally. The effect of EOC content on the mechanical properties was found to be linear suggesting uniform dispersion of EOC domains in polymer matrix. Various proportions of treated sisal and banana fibers were blended with iPP-EOC blend containing 5% EOC polymer matrix by melt blending technique. Both tensile and flexural properties increased whereas impact strength decreased with increase in fiber content. The extent of reinforcing effect of both sisal and banana fiber on iPP-EOC matrix is at comparable level. The melting point of composites was lower than virgin iPP, and the degree of crystallinity of composites was found to be lower than virgin iPP but higher than iPP-EOC blend. The extent of decrease in melting point and degree of crystallization remained same in both banana and sisal fiber composites. Thermal stability of composites was found to be lower than virgin iPP and iPP-EOC blend. Poor adhesion between polymer matrix and fiber was observed.

Keywords

polypropylene natural fibers composites toughening

Introduction

Blending two or more polymers can give rise to a better balance of properties than that of individual components. Polypropylene (PP) based blends are no exception to this, and have attracted much attention due to easy processability and broad spectrum of properties which can also be obtained at a competitive price. PP has lower impact strength particularly at about 0°C but its working temperature and tensile strength are superior to low density polyethylene (LDPE) or high density polyethylene (HDPE).¹ Semi crystalline polymers such as PP have a high craze stress and deform easily by shear yielding; however the notch, these polymers often fracture in a brittle manner. An efficient way to toughen these materials is to incorporate a dispersed rubbery phase, which increases the fracture energy by many folds, at the expense of modulus and yield strength to a smaller extent. Such blends and alloys are used in engineering and packaging applications where toughness is required. Impact-modified PP is a class of thermoplastic olefin (TPO), which has been the fastest growing segment for the last few decades. The automotive industry is a major growth market for TPOs with new applications such as interior trim and exterior fascia. To improve the impact strength, PP is often toughened with ethylene-propylene copolymer,^2,3 ethylene-propylene-diene monomer (EPDM),^4–6 styrene-ethylene-butylene-styrene (SEBS) block copolymer⁷ and ethylene-octene copolymer (EOC).^8–11 Blends of PP-EOC have attained significant commercial importance due to better processability when compared with PP-EPDM blends. Substantial work has been carried out where both homopolymer and copolymer of PP has been blended with EOC for improving the impact strength.^12–16 Studies using nucleating agents,¹⁷ conducting fillers,¹⁸ nanoclay,¹⁹ and inorganic fillers such as talc²⁰ were also carried out. However, natural fiber-reinforced PP-EOC composites have received less attention. Recently, natural fiber-reinforced polymer composites have experienced a tremendous growth in the composite industry because of their eco-friendliness and cost effectiveness.²¹ Low-cost lignocellulosic fibers such as bamboo, jute, sisal, hemp, flax, ramie, coir, etc., provide unique opportunities for their utilization in fabricating inexpensive composite materials due to their high performance in terms of mechanical properties, significant processing advantages, excellent chemical resistance and low density. These advantages place natural fiber composites among the high performance composites having economic and environmental advantages. The combination of unique mechanical and physical properties together with their environmentally friendly characteristics have encouraged several industrial sectors for applications in aerospace, automobiles, construction etc.^22,23 Therefore, it is of immense interest to study natural fiber-reinforced PP-EOC composites.

In this study, we have used banana and sisal fibers for reinforcement because of their high cellulose content, low microfibrillar angle, easy availability, low cost and superior mechanical properties. For practical and economic reasons, iPP is taken in this study. The primary objective of the present investigation is to study the effect of banana and sisal fiber content on the mechanical and thermal properties of composites containing a fixed blend-ratio of iPP-EOC blend. Initially, iPP-EOC blends were prepared and mechanical properties were studied to optimize the blend ratio. The optimized blend ratio was reinforced with varying fiber content.

Experimental

Materials

Injection molding-grade of Isotactic Polypropylene (iPP) pellets, (grade name H110MA) was obtained from M/s Reliance Industries Ltd., Mumbai, India. The melt flow index (MFI) at 230°C/2.16 Kg and density of iPP were 11 g/10 min and 0.91 g/cc, respectively. EOC was procured from DuPont, under the trade name Fusabond MN493D. MFI at 2.16 Kg load and density of Fusabond were 1.2 g/10 min and 0.87 g/cc, respectively. Banana and sisal fibers obtained from M/s Sheeba Fibers and Handicrafts, Poovancode, Tamil nadu, India were used as a reinforcing agent. These fibers were extracted by retting process, a water-intensive process employing the action of bacteria and moisture on plants to dissolve or rot away much of the cellular tissues and gummy substances surrounding bast fiber bundle facilitating the separation of fiber from the stem. Diameter of banana and sisal fibers was 80–250 and 50–200 µm, respectively. The density of sisal and banana fibers is 1.45 and 1.35 g/cc, respectively. Sisal fiber consists of 67–78 wt.% of cellulose, 10–14.2 wt.% hemicelluloses, and 8–11 wt.% of lignin, 10 wt.% of pectin, 2 wt.% of wax and 11 wt.% of moisture content. Banana fiber consists of63–67.6 wt.% of cellulose, 19 wt.% hemicelluloses, and 5 wt.% of lignin and 8.7 wt.% of moisture content. Both types of fibers were cut into small pieces of 5–6 mm length and to remove the lignin content these fibers were treated by sodium hydroxide method.

Preparation of iPP-EOC Blends

iPP-EOC blends were prepared by melt blending technique, wherein shear force was used to disperse EOC in iPP matrix. Berstoff ZE25 (Germany) twin screw extruder, a self wiping co-rotating extruder with a rotating speed of 100 rpm, was used. L/D ratio of extruder is 48:1. Temperature profile in the range of 160–210°C from feed-to-die zone was used. Five different compositions were prepared by varying the EOC content and the details are given in Table 1. Blends were named by two letters, namely E followed by number. E represents EOC and the number represents the percentage of EOC content in the blend. For example, material code E5 means the blend contains 5% of EOC and 95% of iPP.

Table 1.

Blend composition.

Blend	iPP%	EOC%
E0	100	0
E5	95	5
E10	90	10
E15	85	15
E20	80	20

Preparation of iPP-EOC Composites

Processing of natural fiber is relatively very difficult due to its high bulk density, therefore compounding natural fiber with the polymeric material was done in Haake Rheocord 9000 with a counter-rotating screw. Composite material was extruded at a screw speed of 40 rpm, and a temperature of 160–185°C from feed-to-die zone was used. Processing was carried out at lower temperature to prevent the degradation of natural fiber because fiber is susceptible to degradation at higher processing temperature. Three different grades having different compositions were prepared for both sisal and banana fibers. Composites are named by two letters, the first letter represents fiber type and second letter for fiber content. For example, S5 means 5% sisal fiber. For banana fibers, letter “B” was used instead of “S”. Ratio of iPP and EOC was maintained throughout, namely 19:1. Composition details of the composites are tabulated in Table 2.

Table 2.

Composite composition.

Composite	iPP%	EOC%	Fiber%
S5	90.25	4.75	5
S10	85.5	4.5	10
S15	80.75	4.25	15
B5	90.25	4.75	5
B10	85.5	4.5	10
B15	80.75	4.25	15

Testing and Characterization

Test specimens for the measurement of mechanical properties were prepared by injection molding on a Windsor SP-130 injection molding machine with a reciprocating screw. Test specimens were injection-molded with a mold temperature of 40°C, injection time of 5 s, injection pressure of 70 MPa, holding time of 12 s, holding pressure of 50 MPa and a cooling time of 25 s at temperature profile of 160–210°C from feed-to-die zone. Specimens were conditioned at 23 ± 2°C and 50 ± 2% RH for 48 hours before testing. A minimum of five samples were tested, and the average was taken.

Tensile specimens of dumbbell shape and dimension of 165 × 12.7 × 3 mm were tested using Universal Testing Machine (UTM, LR 100K Lloyds Instruments, UK) as per ASTM D638. A cross-head speed of 50 mm/min and gauge length of 50 mm was used for tensile test. Rectangular bar of 127 × 12.7 × 3 mm dimension was used for flexural test using UTM (LR 100K Lloyds Instruments, UK) in accordance with ASTM D790. Three-point bending mode was used. A cross-head speed of 1.3 mm/min and span length of 50 mm was used for carrying out the test. According to ASTM D256, impact test was carried out using impactometer 6545 (CEAST, Italy). Rectangular bar specimen of 63.5 × 12.7 × 3 mm dimension with a V-notch depth of 2.54 mm and notch angle of 45° was used for impact strength determination.

Water absorption test was carried out according to ASTM D75-95. Three samples of dimension 25 × 25 × 3 mm were dried in an oven at 50°C for 24 h, cooled in a desiccator and immediately weighed to the nearest three decimals. The weighed samples were immersed in distilled water at 30°C for 24 h. Samples were removed from water and the excess water was wiped off with a dry cloth and weighed again to the nearest three decimals. Percentage of water absorbed was determined using the following formula:

W = (W_{f} - W_{i}) \times 100 / W_{i}

where W_f and W_i are the weight of the sample after immersion in water and initial dried sample weight respectively.

Rockwell Hardness was measured in R scale using FIE Hardness Tester (model RASN-E) ½” stell ball indenter and 60 Kg load. Thermal behavior was measured by differential scanning calorimetry (DSC) (Perkin Elmer: Pyris Diamond DSC). A heating rate of 10°C/min was employed. Thermal stability in the temperature range of 100–600°C was evaluated using a Perkin Elmer: Pyris 7 TGA at a heating rate of 10°C/min in nitrogen atmosphere. Surface morphology was investigated using a Jeol JSM-5600LV Scanning Electron Microscope (SEM) instrument with high tension voltage of 20 KV.

Results and Discussion

Specific Gravity

Specific gravity values are given in Table 3. The specific gravity of blend containing 5% EOC (E5) was found to be lower than that of virgin iPP. This was attributed to the lower density of EOC when compared to that of virgin iPP (0.87 vs. 0.91 g/cc). Density of sisal and banana fiber (1.45 and 1.35 g/cc respectively) was higher than that of both virgin iPP and EOC, which resulted in higher specific gravity of the composites. As the fiber content increases the specific gravity was also found to increase in both sisal and banana fiber-based composites. At equal fiber content, the specific gravity value of sisal fiber composites was higher than that of banana fiber composites. This was due to the denser sisal fibers.

Table 3.

Physical and flow properties of composites.

Material code	Specific gravity	MFI (g/10 min)	Water absorption %
Virgin iPP	0.904	10.78	0.032
E5	0.876	9.92	0.028
S5	0.906	9.22	2.13
S10	0.925	8.86	3.38
S15	0.954	8.21	4.62
B5	0.900	9.13	1.97
B10	0.913	8.77	3.10
B15	0.931	8.3	4.24

Melt Flow Index

Table 3 shows the MFI values of virgin iPP, blend and composites. MFI of E5 blend was lower than that of virgin iPP. This was attributed to the addition of 5% EOC which has very low MFI, i.e. 1.2 g/10 min when compared to that of virgin iPP. The addition of fiber to the polymer blend further reduced the MFI value of the composite. When the fiber content of the composite was increased, MFI value was also found to decrease. At a particular fiber content, MFI value almost remained the same, irrespective of the nature of fiber used, i.e. sisal or banana.

Water Absorption

Water absorption values are reported in Table 3. Both virgin iPP and E5 polymer blend has low water absorption values when compared to composite samples because of their hydrophobic nature. Both banana and sisal fibers have hydrophilic groups, namely hydroxyl groups, which have a tendency to absorb more water than hydrophobic polymer matrix. This accounts for higher water absorption value for composite samples. Water absorption value increased with an increase in fiber content. Sisal fiber-reinforced composites absorbed more water than banana fiber-reinforced composites.

Tensile Properties of Blends

Tensile strength and modulus values of iPP-EOC blends are given in Table 4. Upon increasing the EOC content, tensile strength and modulus decreased. When compared to virgin iPP, decrease in tensile strength and modulus with the addition of 5% EOC were found to be 3.9 and 4.5%, respectively, whereas with 20% EOC addition, the decrease was found to be 27 and 23.9%, respectively. Decrease was attributed due to the presence of soft elastomeric phase.²⁴ Figure 1 shows the effect of EOC content on the tensile strength and modulus of iPP-EOC blends. Both tensile strength and modulus decreased linearly with respect to EOC content, as evident from Figure 1, and regression coefficient (r²) values of the best fit straight line equations of tensile strength and modulus. This indicated that the rule of mixture (additivity principle) was observed, and the dispersion of EOC in iPP matrix was uniform. Using Figure 1, tensile properties could be easily determined for the compositions not exceeding 20% EOC content without doing the experiment.

Table 4.

Impact strength and Rockwell Hardness of iPP-EOC blends.

Blend	Impact strength (J/m)	Rockwell Hardness
E0	53.6	98.4
E5	115.8	89.9
E10	122.4	84.4
E15	135.9	76.2
E20	157.9	66.0

Figure 1.

Plot of tensile strength and modulus of iPP-EOC blends as a function of EOC content.

Flexural Properties of Blends

The effect of EOC content on the flexural strength and modulus is given in Figure 2. As the EOC content is increased, both flexural strength and modulus decrease linearly. Best fit straight line indicated that the rule of mixture was observed and the dispersion of EOC was uniform. Similar to Figure 1, flexural properties could be calculated easily for other compositions without the need for performing the experiment. This decreasing trend was probably due to the formation of pores by the elastomer phase at the interface region. The presence of elastomer in the blends reduced the stiffness of virgin polymer due to an associated reduction in the effective cross-sectional area of sample as investigated by Lotti and Canevarolo.²⁵

Figure 2.

Plot of flexural strength and modulus of iPP-EOC blends as a function of EOC content.

Impact Strength of Blends

The variation of impact strength of iPP-EOC blends as a function of EOC content is enumerated in Table 4. Incorporation of EOC resulted in substantial increase in the impact strength of iPP-EOC blends from 53.62 J/m to 157.92 J/m, respectively. Unless the dispersion of EOC in the polymer matrix is uniform, the impact strength would not have increased substantially. In other words, the effective dispersion of smaller EOC domains in the iPP matrix helped the dissipation of more impact energy.²⁶ Impact strength increased significantly when EOC content was increased from 0 to 5%, and afterwards there was a marginal improvement. Our main objective was to improve the impact strength of the composite with probably a small loss in the strength and modulus of tensile and flexural properties. Therefore, the optimized composition is 5% EOC containing iPP-EOC blend (E5), wherein the impact strength increased drastically and the tensile and flexural strength decreased marginally. This composition (E5) was used to study the effect of fiber type and content on the properties of composites.

Mechanical Properties of Composites

Table 5 shows the values of tensile strength, tensile modulus, flexural strength and flexural modulus of both sisal and banana fiber based composites. In both types of fibers, tensile strength and modulus increased with increase in the fiber content. At a given fiber content, tensile strength and modulus of sisal and banana fiber-based composites were comparable. The increase in tensile strength and flexural strength was 10.7 and 21.1%, respectively, with incorporation of sisal fiber. In the case of banana fiber, tensile strength increased by 12.6%, and flexural strength increased by 19.6%. Flexural strength and modulus increased with an increase in fiber content. The values of flexural strength and modulus of both types of composites were comparable at particular fiber content. The extent of the reinforcing effect of both sisal and banana fibers on the iPP-EOC matrix was found to be at comparable levels. The introduction of sisal and banana fibers decreased the impact strength of the composites. The extent of decrease increased with an increase in fiber content. For the given fiber content, the impact strength of banana and sisal fiber-reinforced composites were comparable.

Table 5.

Mechanical properties of sisal and banana fiber reinforced iPP-EOC composites.

Composite	Tensile strength (MPa)	Tensile modulus (GPa)	Flexural strength (MPa)	Flexural modulus (GPa)	Impact strength (J/m)	Rockwell Hardness
E5 (S0/ B0)	29.8	0.64	41.8	1.49	115.8	89.9
S5/ B5	30.7/31.0	0.67/0.65	43.4/43.2	1.5/1.59	107.3/106.8	83.2/84.7
S10/ B10	31.4/32.0	0.70/0.71	45.9/46.3	1.70/1.84	99.8/98.8	82.3/83.4
S15/ B15	33.0/33.5	0.72/0.72	50.6/50.0	2.22/2.12	96.0/94.3	82.1/83.1

Rockwell Hardness of Blends and Composites

Table 4 shows the Rockwell Hardness values of blend samples. Hardness is the measure of a polymer’s resistance to surface indentation. Rockwell Hardness decreased with increase in EOC content in iPP-EOC blend. This means that the resistance to surface indentation decreased with increase in the elastomeric component, EOC, in iPP-EOC blends. In other words, the material had lost rigidity and became softer with respect to increasing EOC content. Rockwell Hardness values of composites are given in Table 5. Rockwell Hardness marginally decreased with introduction of sisal/banana fiber. Both fiber content and type had no effect on the hardness values.

Melting Point and Percent Crystallinity

Melting point (T_m) and percent crystallinity of iPP-EOC natural fiber composites were investigated using DSC, and the values are shown in Table 6. The melting point (T_m) was taken from the maxima of endothermic peak of second heating cycle whereas the crystallization temperature (T_c) was taken as the maximum of the exothermic peak from the cooling cycle. The crystallinity percentage was calculated using the following equation:

Table 6.

DSC data of composites.

Sample	T_m (°C)	T_c (°C)	X_c (%)
iPP	176.2	115.8	47.1
E5 (S0/ B0)	164.1	113.6	37.9
S15	166.9	115.7	40.7
B15	166.8	112.0	40.4

X_{c} (%) = \frac{Δ H_{f}}{Δ H_{100 %}} \times 100

Where X_c is % crystallinity; ΔH_f is heat of fusion from DSC; ΔH_100% is heat of fusion of 100% crystalline PP.

It was observed that the T_c of iPP was about 115.8°C, and the X_c was 47.1%, as shown in Table 6. When 5% EOC was introduced into iPP, T_m was reduced from 176.2 to 164.1°C, and T_c and X_c were reduced to 113.6°C, and 37.8%, respectively, suggesting that the EOC reduced the perfection of crystals in polymer matrix. When 15% sisal fiber was added to iPP-EOC blend, there was a slight increase in the degree of crystallization suggesting that the fiber had induced marginally the growth of crystals. The degree of crystallization remained the same irrespective of whether the fiber used was uniform or not. The melting point of banana and sisal fiber-reinforced composites was found to be higher than iPP-EOC blend but lower than virgin iPP.

Thermal Stability

TGA data of composites are given in Table 7. The addition of EOC elastomer enhanced thermal degradation in the constituent polymer matrix. It might be due to the soft nature of the elastomer than polymer chain. The ash content at 600°C for the iPP-EOC blends was increasing compared to that of the virgin PP.

Table 7.

TGA data of composites.

Material code	T_10% (°C)	T_50% (°C)	T_Max (°C)	Ash content (%)
iPP	374.9	430.5	450.2	1.0
E5	356.5	406.9	413.3	2.1
E20	294.7	344.6	358.7	2.3
B5	295.4	352.3	350.6	2.3
B15	287.6	341.3	345.6	3.0
S5	338.0	402.7	416.0	2.8
S15	328.3	401.4	415.2	3.0

The reduction in thermal stability of both sisal and banana fiber composites was attributed to the dehydration of cellulose unit and thermal cleavage of glycosidic linkage by transglycolisation and scission of C–O and C–C bonds. But the thermal degradation of the material had increased for both fibers, indicating that the material might degrade easily compared to that of virgin PP. The ash content of the blend (E5 and E20) was higher than iPP, indicating that EOC helped in high char formation. The ash content of the composites increased because the fiber gets charred and remained as ash at 600°C, though most of the material degrades even before that temperature. This behavior was observed in both types of fiber-reinforced composites.

Surface Morphology

The morphology of the tensile fractured surfaces of iPP-EOC blend containing sisal and banana fibers is depicted in Figure 3. SEM Micrographs of S5 and B5 revealed that at 5% fiber loading, the fiber quantity was very less in proportion as compared to the polymer matrix, the dispersion of fiber was not clearly visible. When the fiber content increased to 15%, as in the case of S15 and B15, fibers were extensively visible and dispersed throughout the polymer matrix. The interfacial adhesion and wetting of the treated fiber was poor with iPP matrix and there was no bonding taking place between them.

Figure 3.

SEM micrographs of iPP-EOC composites.

Conclusions

iPP-EOC blend composites reinforced by banana and sisal fibers were prepared by melt blending technique. Both tensile and flexural properties and degree of crystallization increased whereas Rockwell Hardness, impact strength and thermal stability decreased upon introduction of natural fiber to polymer matrix. SEM micrograph indicated poor adhesion between fiber and polymer matrix.

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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Studies on natural fiber reinforced isotactic polypropylene-ethylene-octene copolymer blend composites

Abstract

Keywords

Introduction

Experimental

Materials

Preparation of iPP-EOC Blends

Preparation of iPP-EOC Composites

Testing and Characterization

Results and Discussion

Specific Gravity

Melt Flow Index

Water Absorption

Tensile Properties of Blends

Flexural Properties of Blends

Impact Strength of Blends

Mechanical Properties of Composites

Rockwell Hardness of Blends and Composites

Melting Point and Percent Crystallinity

Thermal Stability

Surface Morphology

Conclusions

References