A study of the effect of chemical treatments on areca fiber reinforced polypropylene composite properties

2.1 Materials

Areca fibers possess superior properties like being light weight, strong and having a high strength-to-weight ratio. Further, areca fibers are biodegradable, non-toxic and eco-friendly and have low maintenance cost. The areca husk is a hard fibrous material covering the endosperm and constitutes about 60–80% of the total weight and volume of the areca fruit. The husk fiber is composed of 55.82% cellulose, 34.28% hemicelluloses, 6.82% lignin 1.80% moisture content and 1.28% ash content [23].

Most commercial polypropylene is isotactic and crystallinity lies in between that of low-density polyethylene and high-density polyethylene. Polypropylene is rough and unusually resistant to acids, bases, solvents and many chemicals. In general, polypropylene is opaque and pigments can be used to add color to polypropylene. Polypropylene has good fatigue resistance.

Areca empty fruits were obtained from Madhu Farm House, Nilogal, Davangere, Karnataka, India. The analytical grade reagents were purchased from Qualigens Company (Mangalore, Karnataka, India) and used as received. The thermoplastic polypropylene matrix material in the form of homopolymer pellets and polyvinyl alcohol mold releasing agent were obtained from Akolite Synthetic Resins, Mangalore, Karnataka, India.

2.2 Fiber extraction

The dried areca empty fruits were soaked in deionized water for about 5 days. This process is called retting and it allows the fiber to be removed from the fruit easily. The fibers were removed from the fruit and separated with a comb. After drying in the room temperature, the fibers were combed in a carding frame to further separate the fibers into an individual state. Then, a sieve was used to remove broken fibers and impurities. The resulting fibers were treated in the conditions of temperature 30°C, relative humidity 70%) for 72 h before the chemical treatment [23–25].

2.3 Alkali treatment

Areca fibers were soaked in a stainless steel vessel containing 6% NaOH solution at room temperature (30–32°C) for 1 h. The alkali treated fibers were immersed in distilled water for 24 h to remove the residual NaOH. Final washing was done with distilled water containing a little amount of 2% acetic acid. Then, the fibers were washed again in running water to remove the last traces of acid sticking on to it, so that the pH of the fibers was approximately 7. Alkali treated areca fibers were dehydrated in an oven at 70°C for 3 h [23, 24, 26].

2.4 Potassium permanganate treatment

Areca fibers, pretreated with 6% alkali, were immersed in 0.5% KMnO₄ in acetone solution for 30 min. The permanganate treated areca fibers were then decanted and dried in air [4, 23, 24].

2.5 Benzoyl chloride treatment

During benzoylation treatment, 6% alkali pretreatment was used to activate the hydroxyl groups of the areca fibers. These alkali pretreated areca fibers were soaked in 6% NaOH and agitated with benzoyl chloride for 15 min. The treated areca fibers were soaked in ethanol solution for 1 h to remove benzoyl chloride that adhered to the fiber surface. Then the treated areca fibers were washed thoroughly using distilled water and dried in air [6, 23, 24].

2.6 Acrylic acid treatment

The 6% alkali pretreated areca fibers were immersed in 5% acrylic acid solution at 50°C for a period of 1 h. Then, acrylic acid treated fibers were washed thoroughly using distilled water and dried in air [23, 24, 27].

2.7 Fabrication of areca fiber reinforced polypropylene composites

The fabrication of untreated, alkali treated, potassium permanganate treated, benzoyl chloride treated and acrylic acid treated areca fiber reinforced polypropylene composite process was done by the compression molding technique. First, the areca fibers were weighed according to the required weight fraction and then they were dried in an oven at 70°C for a period of 1 h to evaporate moisture. Sufficient amount of commercial polypropylene in the form of homopolymer pellets was taken in a beaker and weighed. To prevent voids, water bubbles and poor fiber-matrix adhesion, the polypropylene was dried in an oven at about 100°C for a period of 3 h [28].

Uniform mixing of the dried areca fibers and polypropylene pellets was done carefully in a mixing chamber. That is, one half of the polypropylene pellets was placed inside the mixing chamber for about 1 min at 20 rpm, then areca fibers were added over a period of 2 min. Then, the other half of the polypropylene pellets was placed inside the mixing chamber and the mixing speed was increased to 30 rpm for 5 min. During the mixing of the two ingredients, the weight fractions such as 30%, 40%, 50%, 60% and 70% of areca fibers were carefully controlled. Eq. (1) was used in the composite fabrication where W_f is the weight of the areca fiber (g), W_m is the weight of polypropylene matrix (g), V_f is the areca fiber volume fraction (%), ρ_m is the density of polypropylene matrix (g/cm³) and ρ_f is the density of areca fibers (g/cm³):

Mold surface was cleaned thoroughly and the mold releasing agent polyvinyl alcohol was sprayed over the mold surface properly for the easy removal of the fabricated areca fiber reinforced polypropylene polymer composites. The uniformly mixed areca fibers and polypropylene pellets were taken into the mold box with dimensions of 255 mm×255 mm×4 mm. Then, the mold box was placed in a hot pressing machine at 170°C temperature and 40 kN pressure for about 30 min. Then it was cooled slowly using a water cooling system. Finally, the areca fiber reinforced polypropylene composite specimens were carefully discharged from the mold box after complete cooling.

2.8 Tensile, flexural and impact properties testing of areca-polypropylene composites

The tensile, flexural and impact properties testing of untreated and all chemically treated areca fiber reinforced polypropylene composites were measured by following ASTM standard procedures at a standard laboratory atmosphere of 30±2°C and 65% relative humidity (65%). At least five replicate specimens were tested and the results were presented as an average of tested specimens.

2.9 Thermal study

The thermal stabilities of untreated and all chemically treated areca fibers were measured by using an STA 409 PL Luxx instrument operating at a temperature range of room temperature to 500°C at a heating rate of 10°C/min under a nitrogen atmosphere using a TGA-DTG sample carrier and TGA-DTG alumina crucible.

2.10 Morphological study

The SEM images of untreated and all chemically treated areca fibers were taken by using a JEOL JSM-T330A scanning electron microscope at the accelerating voltage of 20 kV to characterize the morphological investigations of areca fibers. Surfaces of the fiber samples were sputter-coated with gold prior to their observation.

3.1 Chemical treatment of areca fibers

Natural fibers are amenable to chemical modification due to the presence of hydroxyl groups. Chemical treatments facilitate efficient coupling with polymeric matrix by exposing more reactive groups on the fiber surface. That is, the interfacial properties can be improved by giving appropriate modifications to the components, which give rise to changes in physical and chemical interactions at the interface [29–37]. Here, areca fiber surface is modified with suitable chemical treatments in order to optimize effective interfacial bonding between the areca fiber and polypropylene matrix so that areca-polypropylene composites with improved properties can be prepared.

3.1.1 Alkali treatment of areca fibers

Alkali treatment of areca fibers is shown in Scheme 1 and it resulted in the formation of Fiber-cell-O-Na groups between the cellulose molecular chains and there is increase in the amount of amorphous cellulose at the expense of crystalline cellulose. The effect of alkali on cellulose fiber is a swelling reaction. Hydrogen bonding present in the network structure is removed and new reactive hydrogen bonds are formed between the cellulose molecular chains, and the natural crystalline structure of the cellulose relaxes and this provides more access to penetration of chemicals. As a result, hydrophilic hydroxyl groups are reduced, the moisture resistance of the fibers property is increased and effective fiber surface area available for good adhesion with the matrix is increased [6, 23, 24, 26, 38, 39].

Scheme 1:

Reaction between areca fibers and sodium hydroxide [6, 26, 38, 39].

3.1.2 Potassium permanganate treatment of areca fibers

Cellulose manganate is formed when potassium permanganate react with alkali pretreated cellulose of areca fibers and the reaction is shown in Scheme 2. Areca fiber surface is carved by the reaction of permanganate ions with lignin constituents. As a result, areca fiber surface becomes physically rough. This reduces the hydrophilic nature of the areca fibers, improves chemical interlocking at the interface and provides better adhesion with the polymeric matrix [4, 23, 24, 38, 40].

Scheme 2:

Reaction between areca fibers and potassium permanganate [4, 38, 40].

3.1.3 Benzoyl chloride treatment of areca fibers

An ester linkage is formed when benzoyl chloride reacts with alkali pretreated areca fibers and the reaction is shown in Scheme 3. This benzoylation treatment results in the reduction of the hydrophilicity of fibers and hence makes the fibers more compatible with the polymer matrix. Benzoylation treatment also enhances thermal stability of the fibers, improves chemical interlocking at the interface and provides effective fiber surface area for good adhesion with the matrix [5, 6, 23, 24, 38].

Scheme 3:

Reaction between areca fibers and benzoyl chloride [5, 6, 38].

3.1.4 Acrylic acid treatment of areca fibers

Acrylic acid provides more access of reactive cellulose macro radicals to the polymerization medium by reacting with alkali pretreated cellulose of areca fibers and an ester linkage is formed with the areca fibers by the carboxylic acids present in acrylic acid and the reaction is shown in Scheme 4. Moisture absorption of areca fibers is reduced due to the replacement of hydrophilic hydroxyl groups by hydrophobic ester groups and effective fiber surface area for good adhesion with the matrix is increased [23, 24, 27, 41].

Scheme 4:

Reaction between areca fibers and acrylic acid [27, 41].

3.2 Possible hypothetical models of interface of areca fiber-polypropylene composites

An interface is a region where the reinforcing natural lignocellulosic fibers and the polymeric matrix phase are chemically and/or mechanically combined or otherwise indistinct. Chemical treatments such as alkali treatment, potassium permanganate treatment, benzoyl chloride treatment and acrylic acid treatment expose more reactive groups on the areca fiber surface, and due to this, the chemically treated areca fiber surface becomes more hydrophobic and there is improvement in surface characteristics such as wetting, adhesion and porosity of areca fibers. Further, there is improvement in interfacial adhesion between the treated areca fiber surface and the polypropylene matrix. In addition, it provides effective areca fiber surface area available for good adhesion with the thermoplastic polypropylene matrix [4–6, 23, 24, 26, 27, 38–41]. For areca fiber reinforced polypropylene composites, the polymeric matrix thermoplastic polypropylene may be anchored to the areca fiber surface by chemical reaction or by adsorption.

On the basis of hypothetical models proposed by various researchers [42–44], the possible hypothetical models of interfaces of untreated, alkali treated, permanganate treated, benzoyl chloride treated and acrylic acid treated areca fiber-polypropylene composites may be represented as follows.

3.2.1 Untreated areca fiber-polypropylene interface

Based on hypothetical models proposed by various researchers [42–44], a possible hypothetical model of the interface of untreated areca fiber surface having hydroxyl groups with the polypropylene thermoplastic polymeric matrix is shown in Scheme 5.

Scheme 5:

Interface of untreated areca fiber-PP composite.

3.2.2 Alkali treated areca fiber-polypropylene interface

Based on hypothetical models proposed by various researchers [42–44], a possible hypothetical model of the interface of alkali treated areca fiber surface having reactive -ONa groups with the polypropylene thermoplastic polymeric matrix is shown in Scheme 6.

Scheme 6:

Interface of alkali treated areca fiber-PP composite.

3.2.3 Potassium permanganate treated areca fiber-polypropylene interface

Based on hypothetical models proposed by various researchers [42–44], a possible hypothetical model of the interface of potassium permanganate treated areca fiber surface with the polypropylene thermoplastic polymeric matrix is shown in Scheme 7.

Scheme 7:

Interface of potassium permanganate treated areca fiber-PP composite.

3.2.4 Benzoyl chloride treated areca fiber-polypropylene interface

Based on hypothetical models proposed by various researchers [42–44], a possible hypothetical model of the interface of benzoylated areca fiber surface having reactive C₆H₅COO- groups with the polypropylene thermoplastic polymeric matrix is shown in Scheme 8.

Scheme 8:

Interface of benzoyl chloride treated areca fiber-PP composite.

3.2.5 Acrylic acid treated areca fiber-polypropylene interface

Based on hypothetical models proposed by various researchers [42–44], a possible hypothetical model of the interface of acrylic acid treated areca fiber surface having reactive CH₂=CHCOO- groups with the polypropylene thermoplastic polymeric matrix is shown in Scheme 9.

Scheme 9:

Interface of acrylic acid treated areca fiber-PP composite.

3.3 Tensile strength and tensile modulus of areca fiber reinforced polypropylene composites

The force per unit area (N/mm² or MPa) required for breaking the composite material is referred to as tensile strength. This tensile testing is done until complete failure or break. The objective of the tensile strength test is to determine the strength of interfacial bonding between the untreated and all chemically treated areca fiber and the thermoplastic polypropylene matrix.

The tensile modulus or Young’s modulus is defined as the ratio of the stress along an axis over the strain along that axis. A solid body deforms when a load is applied to it. If the material is elastic, the body returns to its original shape after the load is removed. The material is linear if the ratio of load to deformation remains constant during the loading process. A perfectly rigid material has an infinite Young’s modulus because an infinite force is needed to deform such a material.

The effect of chemical treatments on tensile strength of untreated and all chemically treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are depicted in Figure 1.

Figure 1:

Tensile strength of areca-polypropylene composites with different fiber loadings.

With respect to fiber content and chemical modifications of areca fibers, a great influence on tensile strength of areca fiber reinforced polypropylene composites was observed. It was observed that areca fiber reinforced polypropylene composites with 60% fiber loading showed maximum tensile strength values when compared to areca fiber reinforced polypropylene composites with other fiber loadings. Irrespective of chemical treatment, all chemically treated areca fiber reinforced polypropylene composites showed higher tensile strength values than the untreated areca fiber reinforced polypropylene composites.

The tensile strength values of untreated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 20.26 N/mm², 21.84 N/mm², 25.04 N/mm² and 18.86 N/mm², respectively. The tensile strength of untreated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 23.59% compared to 40% fiber loading, 14.65% compared to 50% fiber loading and for 70% fiber loading, tensile strength decreased by 24.68% when compared to 60% fiber loading.

The tensile strength values of alkali treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 22.46 N/mm², 22.84 N/mm², 28.04 N/mm² and 20.32 N/mm², respectively. The tensile strength of NaOH treated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 24.84% compared to 40% fiber loading, 22.77% compared to 50% fiber loading and for 70% fiber loading, tensile strength decreased by 27.53% when compared to 60% fiber loading.

The tensile strength values of potassium permanganate treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 24.22 N/mm², 24.96 N/mm², 28.86 N/mm² and 22.86 N/mm², respectively. The tensile strength of KMnO₄ treated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 19.16% compared to 40% fiber loading, 15.63% compared to 50% fiber loading and for 70% fiber loading, tensile strength decreased by 20.79% when compared to 60% fiber loading. The tensile strength values of benzoyl chloride treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 26.28 N/mm², 28.62 N/mm², 30.52 N/mm² and 24.52 N/mm², respectively. The tensile strength of benzoylated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 16.13% compared to 40% fiber loading, 6.64% compared to 50% fiber loading and for 70% fiber loading, tensile strength decreased by 19.66% when compared to 60% fiber loading.

The tensile strength values of acrylic acid treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 28.04 N/mm², 30.24 N/mm², 36.86 N/mm² and 26.52 N/mm², respectively. The tensile strength of acrylated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 31.46% compared to 40% fiber loading, 21.89% compared to 50% fiber loading and for 70% fiber loading, tensile strength decreased by 28.05% when compared to 60% fiber loading.

NaOH, KMnO₄, C₆H₅COCl and H₂C=CHCOOH treated areca fiber reinforced polypropylene composites with 60% fiber loading showed 11.98%, 15.26%, 21.88% and 47.20% increase in tensile strength values, respectively, when compared to untreated areca fiber reinforced polypropylene composites with the same 60% fiber loading.

All of the chemical treatments which were carried out on areca fibers resulted in significant increase in tensile strength values for chemically treated areca fiber reinforced polypropylene composites. This increase in tensile properties may be due to the fact that chemical treatments reduced the hydrophilic nature of areca fibers and thereby increased the interfacial adhesion between the chemically treated areca fibers and polypropylene matrix.

Joseph et al. [45] reported that the tensile strength was improved for chemically modified natural fiber based polymer composites. Mohanty et al. [3] explained that sodium hydroxide treated jute fiber reinforced composites showed improvements >40% for tensile properties.

Since the areca fiber reinforced polypropylene composites exhibited higher tensile strength values at 60% fiber loading, the tensile modulus or Young’s modulus of untreated and all chemically treated areca fiber reinforced polypropylene composites were determined with respect to 60% fiber loadings and are depicted in Figure 2.

Figure 2:

Tensile modulus of areca-polypropylene composites with 60% fiber loading.

The Young’s modulus values of areca fiber reinforced polypropylene composites were found to be increased after surface modification of natural areca fibers by each chemical treatment. The possible reason for the increase in tensile modulus after each chemical treatment may be due to the proper adhesion between the chemically treated areca fibers and the polypropylene matrix.

NaOH, KMnO₄, C₆H₅COCl and H₂C=CHCOOH treated areca fiber reinforced polypropylene composites with 60% fiber loading showed 2.92%, 1.81%, 4.75% and 20.65% increase in Young’s modulus values, respectively, when compared to untreated areca fiber reinforced polypropylene composites with the same 60% fiber loading. The maximum Young’s modulus value was obtained for acrylic acid treated areca fiber reinforced polypropylene composites with 60 wt% of fiber loading.

Sreekala et al. reported that polymer composites strengthened with oil palm fibres withstood tensile properties to a greater strain level when the fibres were chemically modified by isocyanante, silane, acrylation and peroxide treatments and by latex coating. This was due to the changes in chemical structure and the bond ability of treated fibres [46].

And also, the observed increase in tensile properties with chemical modification and as well as with increase in fiber loading up to 60% is in good agreement with the results reported in literature [3, 4, 6, 10, 24, 26, 27, 38, 45, 46].

3.4 Flexural strength and flexural modulus of areca fiber reinforced polypropylene composites

Flexural strength is the ability of the material to resist deformation under load. It is important to know the weight bearing capacity of many materials and hence based on these calculations it is possible to choose appropriate materials for the industrial sector.

The effects of chemical treatments on flexural strength of untreated and all chemically treated areca fiber reinforced polypropylene composites with different fiber loadings are depicted in Figure 3.

Figure 3:

Flexural strength of areca-polypropylene composites with different fiber loadings.

Flexural strengths of areca fiber reinforced polypropylene composites are greatly influenced by the fiber content and chemical modifications of areca fibers. It is observed that areca fiber reinforced polypropylene composites with 60% fiber loading showed maximum flexural strength values when compared to areca fiber reinforced polypropylene composites with other fiber loadings. Irrespective of the chemical treatment, all chemically treated areca fiber reinforced polypropylene composites showed higher flexural strength values than the untreated areca fiber reinforced polypropylene composites.

The flexural strength values of untreated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 36.28 N/mm², 38.26 N/mm², 44.06 N/mm² and 34.52 N/mm², respectively. The flexural strength of untreated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 21.44% compared to 40% fiber loading, 15.16% compared to 50% fiber loading and for 70% fiber loading, flexural strength decreased by 21.65% when compared to 60% fiber loading.

The flexural strength values of alkali treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 38.04 N/mm², 40.28 N/mm², 46.86 N/mm² and 38.02 N/mm², respectively. The flexural strength of alkali treated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 23.18% compared to 40% fiber loading, 16.34% compared to 50% fiber loading and for 70% fiber loading, flexural strength decreased by 18.86% when compared to 60% fiber loading.

The flexural strength values of potassium permanganate treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 38.86 N/mm², 42.58 N/mm², 48.52 N/mm² and 37.56 N/mm², respectively. The flexural strength of KMnO₄ treated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 24.86% compared to 40% fiber loading, 13.95% compared to 50% fiber loading and for 70% fiber loading, flexural strength decreased by 22.59% when compared to 60% fiber loading.

The flexural strength values of benzoyl chloride treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 39.02 N/mm², 44.56 N/mm², 50.56 N/mm² and 38.52 N/mm², respectively. The flexural strength of benzoylated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 29.57% compared to 40% fiber loading, 13.46% compared to 50% fiber loading and for 70% fiber loading, flexural strength decreased by 23.81% when compared to 60% fiber loading.

The flexural strength values of acrylic acid treated areca fiber reinforced polypropylene composites with 40%, 50%, 60% and 70% fiber loadings are 42.52 N/mm², 46.52 N/mm², 52.26 N/mm² and 40.86 N/mm², respectively. The flexural strength of acrylated areca fiber reinforced polypropylene composites with 60% fiber loading increased by 22.91% compared to 40% fiber loading, 12.34% compared to 50% fiber loading and for 70% fiber loading, flexural strength decreased by 21.81% when compared to 60% fiber loading.

Sodium hydroxide, potassium permanganate, benzoyl chloride and acrylic acid treated areca fiber reinforced polypropylene composites with 60% fiber loading showed 6.35%, 10.12%, 14.75% and 18.61% increase in flexural strength values, respectively, when compared to untreated areca fiber reinforced polypropylene composites with the same 60% fiber loading.

A significant increase in flexural strength values was observed for all chemically treated areca fiber reinforced polypropylene composites. This increase in flexural properties may be due to the fact that chemical treatments resulted in increased surface roughness which in turn increased the interfacial adhesion between the chemically treated areca fibers and thermoplastic polypropylene matrix.

Epoxy-phenolic resin composites fortified with acrylated jute fiber showed increments in flexural and tensile strength values by 13.9% and 42.2% in that order [8].

Since the areca fiber reinforced polypropylene composites exhibited higher flexural strength values at 60% fiber loading, the flexural modulus of untreated and all chemically treated areca fiber reinforced polypropylene composites were determined with respect to 60% fiber loadings and are shown in Figure 4.

Figure 4:

Flexural modulus of areca-polypropylene composites with 60% fiber loading.

The flexural modulus values of areca fiber reinforced polypropylene composites were found to be increased after surface modification of natural areca fibers by each chemical treatment. The possible reason for increase in flexural modulus after each chemical treatment may be due to the proper adhesion between the chemically treated areca fibers and polypropylene matrix.

NaOH, KMnO₄, C₆H₅COCl and H₂C=CHCOOH treated areca fiber reinforced polypropylene composites with 60% fiber loading showed 10.76%, 18.99%, 41.53% and 62.09% increase in flexural modulus values, respectively, when compared to untreated areca fiber reinforced polypropylene composites with the same 60% fiber loading. The maximum flexural modulus value was obtained for acrylic acid treated areca fiber reinforced polypropylene composites with 60 wt% of fiber loading.

Flexural properties of epoxy composites reinforced with bamboo fibers enhanced up to 30% fiber weight and thereafter they showed decreased values. Chemical treatments increased flexural properties because of removal of external fiber surface, increment in cellulose content and interfacial attachment [47].

In this study, the observed increase in flexural properties with chemical modification as well as with increase in fiber loading up to 60% is in good agreement with the results reported in literature [4, 6, 8, 10, 24, 26, 27, 38, 47].

3.5 Impact strength of areca fiber reinforced polypropylene composites

The capability of the material to withstand a suddenly applied load is known as impact strength and it is expressed in terms of energy per square cross-section. Impact tests are used in studying the toughness of a material and are determined by the Charpy impact test. Impact strength is calculated as the ratio of impact absorption to test specimen cross-section (J/mm²). It is very important to know optimum fiber loading to get good impact properties.

The impact strength for all untreated and chemically treated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are shown in Figure 5.

Figure 5:

Impact strength of areca-polypropylene composites with different loadings.

It is clearly indicated that the areca fiber content and chemical modifications of areca fibers greatly influenced the impact strength values of areca fiber reinforced polypropylene composites. It is also observed that areca fiber reinforced polypropylene composites with 50% fiber loading showed maximum impact strength values when compared to areca fiber reinforced polypropylene composites with other fiber loadings. Irrespective of chemical treatment, all chemically treated areca fiber reinforced polypropylene composites showed higher impact strength values than the untreated areca fiber reinforced polypropylene composites.

The impact strength values of untreated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are 32.56 J/mm², 34.68 J/mm², 36.82 J/mm², 28.86 J/mm² and 22.86 J/mm², respectively. In the case of untreated areca fiber reinforced polypropylene composites, the impact strength of 50% fiber loading increased by 13.08% compared to 30% fiber loading, 6.17% compared to 40% fiber loading and for 60% and 70% fiber loadings, it decreased by 21.62% and 37.91%, respectively, when compared to 50% fiber loading.

The impact strength values of alkali treated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are 34.25 J/mm², 35.28 J/mm², 37.28 J/mm², 30.28 J/mm² and 24.24 J/mm², respectively. In the case of alkali treated areca fiber reinforced polypropylene composites, the impact strength of 50% fiber loading increased by 8.84% compared to 30% fiber loading, 5.67% compared to 40% fiber loading and for 60% and 70% fiber loadings, it decreased by 18.78% and 34.98%, respectively, when compared to 50% fiber loading.

The impact strength values of potassium permanganate treated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are 34.86 J/mm², 36.82 J/mm², 38.28 J/mm², 32.52 J/mm² and 25.64 J/mm², respectively. For potassium permanganate treated areca fiber reinforced polypropylene composites, the impact strength of 50% fiber loading increased by 9.81% compared to 30% fiber loading, 3.97% compared to 50% fiber loading and for 60% and 70% fiber loadings, it decreased by 15.05% and 33.02%, respectively, when compared to 50% fiber loading.

The impact strength values of benzoyl chloride treated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are 35.28 J/mm², 37.02 J/mm², 39.82 J/mm², 34.24 J/mm² and 27.24 J/mm², respectively. In the case of benzoyl chloride treated areca fiber reinforced polypropylene composites, the impact strength of 50% fiber loading increased by 12.87% compared to 30% fiber loading, 7.56% compared to 40% fiber loading and for 60% and 70% fiber loadings, it decreased by 14.01% and 31.59%, respectively, when compared to 50% fiber loading.

The impact strength values of acrylic acid treated areca fiber reinforced polypropylene composites with 30%, 40%, 50%, 60% and 70% fiber loadings are 36.86 J/mm², 38.28 J/mm², 40.24 J/mm², 36.82 J/mm² and 28.82 J/mm², respectively. In acrylic acid treated areca fiber reinforced polypropylene composites, the impact strength of 50% fiber loading increased by 9.17% compared to 30% fiber loading, 5.12% compared to 40% fiber loading and for 60% and 70% fiber loadings, it decreased by 8.50% and 28.38%, respectively, when compared to 50% fiber loading.

Sodium hydroxide, potassium permanganate, benzoyl chloride and acrylic acid treated areca fiber reinforced polypropylene composites with 50% fiber loading showed 1.25%, 3.97%, 8.15% and 9.29% increase in impact strength values, respectively, when compared to untreated areca fiber reinforced polypropylene composites with the same 50% fiber loading.

Considerable increase in impact strength values were observed for all chemically treated areca fiber reinforced polypropylene composites. These increased impact strength values may be owing to the fact that chemical treatments resulted in strong interfacial bonding between the chemically treated areca fibers and the thermoplastic polypropylene matrix and this could be due to the changes in surface topography observed for all of the chemically treated areca fibers.

Abaca-epoxy polymer composites showed increases in impact strength values up to 40% fiber loading and then they showed a decline. Benzenediazonium chloride treated abaca-epoxy composites with 40% fiber loading showed high impact strength value compared to untreated and other chemically treated abaca-epoxy composites with the same 40% fiber loading. Several researchers showed that impact strength increased with increase in fiber loading [10, 48, 49].

In this study, the observed increase in impact strength values with chemical modification, as well as with increase in fiber loading up to 50%, is in good agreement with the results reported in literature [4, 6, 10, 24, 26, 27, 38, 48, 49].

3.6 Effect of chemical treatments and effect of fiber loadings on tensile strength, flexural strength and impact strength of areca fiber reinforced polypropylene composites

The tensile strength, flexural strength and impact strength of natural fiber reinforced polymer composites depends on the nature of fiber, polymer and fiber-matrix interfacial bonding [50]. Untreated areca fibers are hydrophilic whereas the polypropylene matrix is hydrophobic and hence, there is incompatibility between the areca fibers and the polypropylene polymeric matrix. So, the untreated areca fiber reinforced polypropylene composites showed lower tensile strength, flexural strength and impact strength values when compared to all chemically treated areca fiber reinforced polypropylene composites.

Alkali treatment of areca fibers removed a certain portion of hemicelluloses, lignin, adhesive pectin, waxy epidermal tissue and oil covering materials. It also reduced fiber diameter and thereby increased aspect ratio. As a result, effective fiber surface area available for good adhesion with the polymeric matrix polypropylene is increased [2, 23, 24, 26, 38, 39].

Permanganate ions carve the areca fiber surface by reacting with the lignin constituents. As a result, areca fiber surface became rough and this improved chemical interlocking at the interface and provided better adhesion with the polymeric resin polypropylene [4, 23, 24, 40].

Benzoylation treatment resulted in an introduction of ester linkage with areca fibers. Further, it improved chemical interlocking at the interface and provided effective fiber surface area for good adhesion with the thermoplastic polypropylene polymeric matrix [5, 6, 23, 24, 38].

During acrylic acid treatment, there is an introduction of hydrophobic ester groups and most of the hemicelluloses and lignin are removed. Hence, effective fiber surface area for good adhesion with the polymeric matrix is increased and stress transfer capacity at the interface is enhanced [23, 24, 27].

Due to the above stated reasons, alkali treated, potassium permanganate treated, benzoyl chloride treated and acrylic acid treated areca fiber reinforced polypropylene composites showed higher tensile strength, flexural strength and impact strength values when compared with that of untreated areca fiber reinforced polypropylene composites with respective fiber loadings.

With increase in fiber loading from 40% to 60%, the tensile strength and flexural strength values of untreated and all chemically treated areca fiber reinforced polypropylene composites increased and beyond 60% fiber loading, the tensile strength and flexural strength values showed a decline. That is, areca fiber reinforced polypropylene composites exhibited maximum tensile strength and flexural strength values at 60% fiber loading. This is because of better areca fiber distribution in polypropylene matrix, less fiber fractures and effective transfer of load from the polypropylene matrix to areca fibers at 60% fiber loading. The observed increase in tensile strength and flexural strength values with chemical modification as well as with increase in fiber loading up to 60% is in good agreement with the results reported in literature [3, 6–9, 23].

The decrease in tensile strength and flexural strength values for untreated and all chemically treated areca fiber reinforced polypropylene composites beyond 60% fiber loading is due to the fact that the melted polypropylene could not reach each of the areca fiber surfaces because of smaller amount of thermoplastic polypropylene matrix material and also, there is poor interfacial adhesion and inefficient stress transfer from matrix to fibers at 70% fiber loading [51]. The factors contributing to the lower tensile strength and flexural strength values beyond 60% may be due to the random alignment of short areca fibers and the presence of voids in the areca fiber reinforced polypropylene composites [52].

With increase in fiber loading from 30% to 50%, the impact strength of all untreated and chemically treated areca fiber reinforced polypropylene composites increased, but beyond 50% fiber loading, the impact strength values showed a decline. That is, areca fiber reinforced polypropylene composites exhibited maximum impact strength values at 50% fiber loading. This is due to the fact that with increase in fiber loading, more force is required to pull out the fibers and hence increase the impact strength. The observed increase in impact strength values with chemical modification as well as with increase in fiber loading up to 50% is in good agreement with the results reported in literature [9–11]. These results suggest that the areca fiber reinforced polypropylene composites are capable of showing high impact strength values at 50% fiber loading because of strong interfacial bonding between the areca fibers and polypropylene matrix.

The decrease in impact strength beyond 50% fiber loading may be due to the induced micro spaces between the areca fibers and the thermoplastic polypropylene matrix which initiates micro cracks on impact and results in crack propagation leading to failure [11]. It has also been reported that high fiber content increases the probability of fiber agglomeration and further, it results in regions of stress concentration and requires less energy for crack propagation [53].

The above results clearly evidenced that chemical treatments are very effective in surface modification of the areca fibers and in improving the polymer composite properties such as tensile strength, flexural strength and impact strength of areca fiber reinforced polypropylene composites. Amongst all of the chemical treatments carried out, acrylic acid treated areca fiber reinforced polypropylene composites with 60% fiber loading showed maximum tensile strength and flexural strength values.

Similarly, amongst all the chemical treatments carried out, acrylic acid treated areca fiber reinforced polypropylene composites with 50% fiber loading showed maximum impact strength values. Hence, these chemically treated areca fiber reinforced polypropylene composites with appropriate fiber loadings are best suitable for lightweight materials industries.

3.7 Thermal studies of areca fibers

TGA measures the amount and rate of change in the mass of a sample as a function of temperature/time in a controlled atmosphere. Thermal stabilities of materials are measured by using TGA. Maximum decomposition rates for weight losses of components present in natural fibers are shown by DTG curves [54]. TGA-DTG is specifically useful for the study of polymeric materials including thermoplastics, thermosets, elastomers, composites, films, fibers, coatings and paints. TGA-DTG measurements provide valuable information that can be used to select materials for certain end-use applications.

The thermal degradation behavior (TGA) curves of untreated (AR1), alkali treated (AR2), potassium permanganate treated (AR3), benzoyl chloride treated (AR4) and acrylic acid treated (AR5) areca fibers are shown in Figure 6.

Figure 6:

Thermogravimetric analysis (TGA) of untreated and all chemically treated areca fibers.

The derivative thermogravimetric (DTG) curve of untreated (AR1), alkali treated (AR2), potassium permanganate treated (AR3), benzoyl chloride treated (AR4) and acrylic acid treated (AR5) areca fibers are shown in Figure 7.

Figure 7:

Derivative thermogravimetric (DTG) analysis of untreated and all chemically treated areca fibers.

The thermal stability of untreated (AR1), alkali treated (AR2), potassium permanganate treated (AR3), benzoyl chloride treated (AR4) and acrylic acid treated (AR5) areca fibers are analyzed by using TGA curves and DTG curves and the results are presented in Tables 1 and 2, respectively.

In accordance with other studies on natural lignocellulosic fibers [55–58], the TGA curve of untreated and all chemically treated areca fibers also showed three weight loss steps and their decomposition occurred in two main stages. The initial weight loss, observed around room temperature to first stage degradation temperature is attributed to the vaporization of moisture and decomposition of volatile extractives from the fibers. Above this temperature, it can be observed that the thermal stability gradually decreases and the degradation of untreated and all chemically treated areca fiber occurs.

The first stage degradation temperature range is associated to the thermal depolymerization of hemicelluloses, pectin and cleavage of glycosidic linkages of cellulose and lignin. The second stage degradation temperature range corresponds to the degradation of α-cellulose and lignin present in the fiber. The weight loss observed after second stage degradation temperature up to 500°C is due to the degradation of lignin. Generally, the decomposition of lignin, owing to its complex structure containing aromatic rings with various branches, occurs slowly within the whole temperature range up to 500°C. The decomposition temperatures corresponding to 25% and 50% weight loss of untreated and all chemically treated areca fibers are shown in Table 1. Thermally degraded mass change percentage of untreated and all chemically treated areca fibers at 150°C, 300°C and 500°C are also shown in Table 1. In an inert atmosphere, the final products of degradation of areca fibers consist of carbonaceous residues and possible undegraded fillers [58–60].

These results are confirmed by the DTG curve of untreated and all chemically treated areca fibers wherein the maximum decomposition rates for weight losses are shown. The small first peak corresponds to evaporation of moisture from areca fibers and the second peak observed confirms the maximum decomposition rates for weight losses of cellulose present in the untreated and all chemically treated areca fibers [54, 61].

The thermal stability of areca fibers is increased after all the chemical treatments. This may be due to the fact that alkali treatment of areca fibers leads to the formation of lignin-cellulose complex which is more stable than the native one. Cellulose-manganate is formed during permanganate treatment of areca fibers, which imparts greater strength to the treated areca fibers. In the case of benzoylation and acrylation treatments, ester groups are introduced into the polymer backbone of the chemically treated areca fibers. The introduction of these new highly reactive chemical moieties into the areca fibers is responsible for the improved thermal stability for all chemically treated areca fibers [4–6, 24, 26, 38–41].

The TGA results of untreated and all chemically treated areca fibers from Table 1 revealed that the untreated, sodium hydroxide treated, potassium permanganate treated, benzoyl chloride treated and acrylic acid treated areca fibers are stable until around 241.5°C, 250.8°C, 259.0°C, 257.0°C and 252.0°C respectively. The DTG results of untreated and all chemically treated areca fibers from Table 2 revealed that the temperature of maximum decomposition for untreated, alkali treated, potassium permanganate treated, benzoyl chloride treated and acrylic acid treated areca fibers are 322.2°C, 346.0°C 350.4°C, 350.7°C and 351.1°C respectively.

In conclusion, it is important to note that, this is in good agreement with the decomposition temperature values of many vegetable fibers, as shown in Table 3, which is reported in literature for selected natural fibers [62–65]. Finally, this result revealed that thermal stability of areca fibers is improved after each chemical treatment.

3.8 SEM image analysis of areca fibers

In the SEM image of untreated areca fibers, Figure 8(A), a clear network structure is observed in which the fibrils are bound together by hemicelluloses and lignin. The presence of longitudinally oriented unit cells with almost parallel orientations is also observed. The intracellular gap is filled up by the adhesives, lignin and fatty substances and these hold the unit cells firmly in the untreated areca fibers [24].

Figure 8:

Scanning Electron Microscope (SEM) images of areca fibers: (A) untreated; (B) alkali treated; (C) permanganate treated; (D) benzoyl chloride treated and (E) acrylic acid treated.

In the SEM image of alkali treated areca fibers, Figure 8(B), a large number of pinholes or pits and a rough fiber surface are observed. The observed topographical changes are due to the removal of waxy epidermal tissue, adhesive pectin and hemicelluloses. In the SEM image of potassium permanganate treated areca fibers, Figure 8(C), a large number of pinholes are observed. This may be due to the fact that permanganate ions react with the lignin constituents and carve the areca fiber surface. As a result, permanganate treated areca fiber surface became physically rough. The SEM image of benzoyl chloride treated areca fibers, Figure 8(D), clearly shows a large number of pinholes and a physically rough fiber surface. This is due to the removal of waxy epidermal tissue, adhesive pectin and hemicelluloses by alkali pretreatment and further removal of fatty deposits from the areca fibers by the reaction of benzoyl chloride. The SEM image of acrylic acid treated areca fibers, Figure 8(E), shows a large numbers of pits and a rough fiber surface. This is because acrylic acid treatment removed most of the hemicelluloses, lignin and destroyed the cellulose structure. The topographical changes observed after alkali treatment, potassium permanganate treatment, benzoyl chloride treatment and acrylic acid treatment when compared to untreated areca fibers confirmed the chemical modification of areca fibers by NaOH, KMnO₄, C₆H₅COCl and H₂C=CHCOOH [24, 26, 40, 41].