Influence of pre-treatments on the
mechanical properties of palmyra palm leaf stalk fiber–polyester composites
M Thiruchitrambalam
1and D Shanmugam
2Abstract
Palmyra palm leaf stalk fibers were subjected to various chemical pre-treatments like Mercerization, benzoylation, and permanganate treatment with the aim of improving the adhesion with the matrix. Composites were prepared by reinforcing untreated and pre-treated fibers in unsaturated polyester matrix and mechanical performances are studied.
The Mercerized and benzoyl-treated fiber composites had an improvement of around 60% in tensile strength while the tensile modulus increased by 37% and 60%, respectively. In the case of permanganate-treated fiber composites, the flexural strength increased by 70% and flexural modulus increased by 110% in comparison to the untreated composites.
The impact strength for the Mercerized and permanganate-treated fiber composite improved by 55% and 42% in comparison to the untreated fiber composites, respectively. Chemical pre-treatment of fibers reduced water absorption of the composites. The benzoyl chloride treated fibers absorbed less water in comparison to the untreated fiber composites. Thermogravimetric analysis showed that the reinforcing-treated fiber has enhanced the thermal stability of the composites. Scanning electron microscope fractographs of the composites revealed the presence of good adhe- sion between the fibers and matrix compared to the untreated fiber composites. The composites exhibited comparable properties with other composites based on natural fibers.
Keywords
Natural fibers, chemical treatments, mechanical properties
Introduction
Natural fibers are used as an alternative for synthetic fibers such as glass fibers due to comparable properties and their composites are finding increasing usage in automotive industries.1 The properties of natural fibers depend on age, nature of soil where it was grown, and climatic conditions that affect the proper- ties of the fiber.2,3The use of natural fibers as reinforce- ment in polymer matrix has increased dramatically in the recent year. The biodegradability of cellulosic fibers related to biological, chemical, mechanical, thermal, photochemical, and aqueous conditions has increased its scope for use in many applications.4Glass fibers are used as reinforcement for plastics widely due to their excellent mechanical properties; however, the disposal remains as a problem due to shortage of land fill.5The major limitation of natural fibers is its hydrophilic nature which results in swelling due to moisture
absorption and this in turn leads to poor interfacial bonding between the fibers and the matrix.6–8 Aathijeyamani et al.9 conducted experiments on the effect of moisture absorption on hybrid composites with sisal and roselle fibers and their results showed that moisture absorption reduced the mechanical prop- erties of the composites. Chemical treatments of the fiber can be used to reduce the hydrophilicity and also to improve the mechanical properties of the
1Department of Mechanical Engineering, Tamilnadu College of Engineering, Coimbatore, India
2Department of Mechanical Engineering, Dr Mahalingam College of Engineering and Technology, Pollachi, India
Corresponding author:
D Shanmugam, Department of Mechanical Engineering, Dr Mahalingam College of Engineering and Technology, Pollachi, Tamil Nadu, India.
Email: [email protected]
Journal of Reinforced Plastics and Composites
31(20) 1400–1414
!The Author(s) 2012 Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684412459248 jrp.sagepub.com
natural fiber reinforced composites. Alfa fibers were subjected to various surface treatments involving acet- ylating and it was found that the treatments improved the resistance of fiber to moisture.10Sree Kumar et al.11 studied the effect of pre-treatments like Mercerization, thermal treatment, permanganate treatment, and ben- zoyl chloride treatment on the properties of sisal/poly- ester composites and their results showed increased mechanical properties and reduced water intake due to treatment. Studies on the behavior of pineapple leaf fibers treated with NaOH and modified with two different functionalities were attempted by Threepopnatkul et al.12 and their results showed that the treated fiber composites had better mechanical properties. Their studies on the thermal behavior of the composites showed that the thermal degradation of the fibers are less compared to that of neat resin and also the thermal degradation decreased with increased fiber content. Sisal fibers were treated by admicellar polymerization with a poly (methyl meth- acrylate) film to enhance the adhesion of the fiber and matrix. The results showed improved adhesion was achieved as evident by the improved tensile, flexural, and impact strength of the composites compared to those of untreated fibers.13 Chemical treatments like Mercerization and acrylic acid on bagasse fiber have resulted in superior tensile and flexural properties and also resulted in composites with reduced water intake compared to that of the untreated fiber reinforced composites.14
Presently, the fibers extracted from the leaf stalks of the palmyra palm tree (Borassus flabellifer) are used to manufacture brooms and brushes in large quantities.
This tree is a native of Africa and Asia. It can be found in abundance in India. During the current inves- tigation, the fiber extracted from the leaf stalk of the palmyra palm tree was used as reinforcement in a poly- mer matrix. Some studies on the use of this fiber as reinforcement was reported by Velmurugan and Manikandan.15 Various length of the fibers with vary- ing weight percentage were reinforced in rooflite resin and they have found that reinforcing fibers with 50 mm length and 55 wt% of palmyra palm fibers have yielded good mechanical properties. According to the authors, works related to chemical treatments on the palmyra palm leaf stalk fibers (PPLSF) and their influence on mechanical properties have not yet been published and the property of PPLSF reinforced with unsaturated polyester matrix has not yet been studied. So, an effort was made to study the influence of chemical pre-treatments on the mechanical and thermal proper- ties of PPLSF/unsaturated polyester composites. The fibers were treated with NaOH—alkali treatment (mer- cerization), benzoyl chloride (benzoylation), and potas- sium permanganate (permanganate treatment) and the
effects of these treatments on the changes in mechanical and thermal properties while being reinforced with unsaturated polyester matrix were investigated and comparison of the properties of PPLSF/polyester com- posites with other natural and synthetic fiber compos- ites was also discussed.
Materials and methods Materials
The fibers were extracted from the leaf stalk of the pal- myra palm tree. The thorns on the sides of the leaf stalk and the skin of the leaf stalk were removed manually and the leaf stalks were retted in water for 20 days followed by gently hammering and separating the fibers manually from the stalk. The removed fibers are then cleaned, washed, and dried to remove the moisture and other impurities sticking to the fibers.
Unsaturated polyester resin, cobalt naphthalene (accel- erator), and methyl ethyl ketone peroxide (catalyst) was procured form Covai Seenu and Company, Coimbatore, India. The chemicals for pre-treatments such as sodium hydroxide pellets, benzoyl chloride, potassium permanganate, ethanol, and acetone all of analytical reagent grade was procured form The Precision Scientific Company, Coimbatore, India.
Figure 1 shows the untreated and treated fibers after separation form the leaf stalk, which were then sub- jected to treatment. The properties of fibers before chemical treatments are given in Table 1.
Pre-treatments on fibers
Mercerization (alkali treatment). PPLSF were cut manu- ally in to 50 mm long fibers and were treated with 5%
NaOH for 30 min. The fibers were washed with tap water, subsequently washed with very dilute hydro- chloric acid. Washing was continued for several times till the fibers were alkali free followed by drying the fibers in an oven maintained at 70C.
Benzoylation (benzoyl chloride treatment). PPLSF were alkali treated followed by agitating the fibers in benzoyl chloride for 15 min. The mixture was filtered and washed thoroughly with distilled water, and then, the fibers were soaked in ethanol for 1 h to remove the unreacted benzoyl chloride. Finally, the fibers were washed with distilled water several times and dried in an oven maintained at 70C.
Permanganate treatment. Alkali-treated PPLSF were soaked in 0.02% KMnO4/acetone for 3 min followed by washing several times with distilled water and dried in an oven maintained at 70C.
Fabrication of composites
A steel die of dimension 1601603 mm3was fabri- cated for preparation of composites. The mold has two parts, the top plate and the bottom mold cavity. The PPLSF were randomly distributed inside the mold cavity and the top plate was moved till the complete closure of the mold, applying a force of 2 tons by hydraulic compression to produce a non-woven fiber mat of thickness 3 mm in such a way that the weight percentage of the fiber was 30 after the composite fab- rication. The matrix was prepared by mixing acceler- ator and catalyst by 1.5% by weight in unsaturated polyester resin and stirred well to insure homogeneity of the system. The mold was cleaned using air bellows after the non-woven mat is removed and later mold release agent (silicon) was sprayed inside the mold cavity for easy removal of the composite plate after curing. The pre-pressed fiber mat was then placed again inside the cavity and the resin was poured into the die cavity and a force of 2 tons was applied by hydraulic compression to compress the composite.
The mold was kept under pressure for 12 h and post- cured at room temperature for 12 h and specimens were cut to the required size. The composite specimens are named as UTC for untreated fiber composite, ATC for alkali-treated fiber composite, BTC for benzoyl chlor- ide treated fiber composite, and PTC for potassium permanganate treated fiber composite.
Characterization of the composites
Tensile test. The tensile test was conducted according to ASTM D638-03. Dog bone shaped specimens were cut from the composite plates fabricated. Tensile testing was carried out using Instron universal testing machine model 3369 with a crosshead speed of 5 mm/min and gage length of 50 mm. Five specimens of each formulation were tested and average values were reported.
Flexural test. The flexural test were conducted as per ASTM D790-03 using Kalpak universal testing machine of capacity 20 kN, with a crosshead speed of 2 mm/min. Specimens of size 12712.73 mm3 were cut from the composite plates fabricated. Five speci- mens of each formulation were tested and average values were reported.
Impact test. IZOD impact tests were conducted as per ASTM D256-05. Notched test specimens of size 64133 mm3 were cut from the composite plates fabricated. Five specimens of each formulation were tested and average values were reported.
Moisture absorption tests. Moisture absorption tests were conducted as per ASTM D 570-98. The specimens of size 76.225.43 mm3 were cut from the composite plates and were pre-conditioned by drying the Figure 1. Optical photograph of: (a) untreated PPLSF and (b) treated fibers.
PPLSF: palmyra palm leaf stalk fiber.
Table 1. Chemical and mechanical properties of PPLSF
Fiber
Average diameter (mm)
Density (g/cm 3)
Cellulose (%)
Hemicellulose (%)
Lignin
(%) Wax (%)
Tensile strength (MPa)
Strain at break (%)
Young’s modulus (MPa)
PPLSF 300–320 1.2 58.58 22.8 13.48 0.35 2765 3.08 899012
PPLSF: palmyra palm leaf stalk fiber.
specimens in an oven and weighed before being dipped in distilled water. Five specimens of each for- mulation were tested and the average values were reported. The samples were removed from the water at regular intervals and wiped with tissue paper to remove the excess fluid on the surface.
An electronic balance of accuracy 0.00001 mg, SHIMADZU make and model AY220 was used to measure the weight of samples in the moisture absorp- tion test.
Thermogravimetric analysis
The thermogravimetric analysis (TGA) analysis was performed according to ASTM E 1131 on the raw and treated PPLSF reinforced composites using Perkin Emler Pyris by passing nitrogen gas at 20 ml/min and heating from 50C to 800C at a heating rate of 10C/min.
Scanning electron microscopy
The surface morphology of fractured surface of untreated and treated PPLSF reinforced composites were examined using scanning electron microscopy (SEM) JEOL JSM 6390 model at an accelerating volt- age of 10 kV.
Results and discussions Tensile properties
The effect of treatment on the tensile properties of the PPLSF-reinforced composites was investigated and the tensile stress, tensile strain, and tensile modulus were determined and shown in Figures 2 to 4, respectively.
The tensile strength and tensile modulus of the compos- ites improved for treated fibers compared to those of the UTCs. In comparison to the neat resin, the untreated fiber reinforced composites showed 7%
Figure 3. Tensile strength of untreated and treated fiber composites.
Figure 2. Tensile stress vs tensile strain for untreated and various treated composites.
increase in tensile strength while all the other treated fiber composites resulted in much better properties. The tensile stress vs tensile strain curves (Figure 2) showed up to certain limit Hooke’s law behavior on increase in load. In the case of UTCs, the failure is due to the poor adhesion between the fiber and matrix that has lowered the tensile strength. The alkali and benzoyl chloride treated fibers reinforced composites showed around 60% improvement in tensile strength while the tensile modulus increased by 37% and 60% for alkali and benzoyl chloride treated composites, respectively, com- pared to the UTCs. It was found that the value of tensile strength was not influenced by the method of pre-treatment. The tensile modulus of BTC was found to be the highest (1869 MPa) in comparison to that of ATC (1608 MPa) and PTC (1566 MPa).
It has been reported that alkali treatment led to fiber fibrillation and also changed the crystallinity of the fibers due to the exclusion of cementing materials that led to better packing of cellulose chains.16 Increase in effective surface area of the fiber due to the removal of lignin, hemicelluloses, and other substances have resulted in larger area available for contact with the matrix and the formation rephrase of rough surface with many pits on the surface giving rise to better mech- anical interlocking by providing better interfacial adhe- sion (enhanced mechanical interlocking) between the fiber and the matrix.16–19 This has attributed to 60%
improvement in the tensile strength and 37% improve- ment in tensile modulus of the ATC compared to UTCs.
Benzoylation caused reduction in the thickness of the fibers which may be due to the removal of alkali soluble fractions like waxy layer, ligin, etc.
Improvement in adhesion between the fibers and
matrix may be due to the availability of benzene rings in benzoyl group attached to the fibers and the styrene of the polyester resin,20 and also due to the formation of small voids that led to rough surface which has pro- vided place for the matrix to lock inside the fibers.
Permanganate treatment improves the adhesion between the fiber and matrix.3,21During permanganate treatment, the reactive permanganate ions which were responsible for initiating graft copolymerization have resulted in the improved adhesion between the fiber and matrix.8 The permanganate treated PPLSF com- posites had 47% and 33% improvement in tensile strength and tensile modulus, respectively, in compari- son to the UTCs.
Table 2 summarizes the tensile properties of the untreated and treated PPLSF composite with other nat- ural fibers and glass fiber composites. From the previ- ous works on the performance of composites with treated fiber reinforcements, the Sansevieria cylin- drica/polyester composites with 40 wt% exhibited 88% improvement in tensile properties which is the highest among all the other natural fibers. In this study, it can be noted that around 65% improvement in properties was achieved with only 30 wt% of the fibers in polyester matrix. In the case of palmyra fiber/rooflite resin matrix composites,15 the tensile strength of 42.65 MPa was obtained with 55 wt% of fibers, i.e. 30% improvement compared to neat rooflite resin; whereas, in this study, 65% improvement in ten- sile strength was obtained with only 30 wt% of treated fibers and this shows that a better properties of com- posites can be obtained by the use of treated fibers. The lower values of strength in the case of untreated pal- myra palm/rooflite resin composite may be due to the poor interfacial adhesion between the fibers and
Figure 4. Tensile modulus of untreated and treated fiber composites.
Table2.Comparisonoftensilestrengthofvariousfiber-reinforcedcomposites Fiber/matrixCompositefabricationmethodFiberorientation/weight%Typesof chemicaltreatments Tensile strength (MPa) (untreated)
Tensile strength (MPa) (treated)References PPLSF/polyesterHandlayupandcompression moldingRandomlydistributed50mm/ 30wt%Alkali,benzoylchloride,per- manganatetreatment180.7282.8Presentwork Palmyra/roofliteresinCompressionmoldingRandomlydistributed50mm/ 55wt%–42.65–Velmuruganand Manikandan15 Sisal/polyesterResintransfermoldingRandomlydistributed30mm/ 40vol%Alkali,benzoylchloride,per- manganate,andthermal treatment
&67&79SreeKumaretal.11 Sisal/polyesterHandlayupRandomlydistributed 10–40mm/30vol%Admicellarpolymerization&5563–73Supraneeetal.13 Bagasse/polyesterVacuumbaggingRandomlydistributed/20wt%Alkaliandacrylicacidtreatment17–2322–35Vilayetal.14 Jute/polyesterHandlayupandcompression moldingFabric/36vol%Alkaliandsilanetreatment&4750–74Severetal.23 S.cylindrica/polyesterCompressionmoldingRandomlydistributed30mm/ 40wt%Alkali,benzoylperoxide,potas- siumpermanganate,andste- aricacidtreatment
&7582–141Sreenivasanetal.24 Coir/polyesterHandlayupNon-wovenmat/17wt%Alkaliandbleaching&2122–26Routetal.19 L.cylindrica/polyesterHandlayupRandomlydistributed50mm/ 30vol%Alkali,formicacid,andacetic acidtreatments&35&56DilaraKocak25 Banana/polyesterHandlayupandcompression moldingRandomlydistributed30mm/Silaneandalkali5870Pothanetal.16 Pineapple/polycarbonateInjectionmoldingRandomlydistributed/20vol%Alkaliandsilanetreatment60–6565–70Threepopnatkuletal.12 Sugarpalm/epoxyHandlayupRandomlydistributed/10vol%Alkalitreatment4232–50Bachtiaretal.26 Bamboo/HDPEInjectionmoldingRandomlydistributed6mm/ 40wt%MAPE&25&28MohanthyandNayak27 Glass/polyesterHandlayupRandomlydistributedchopped/ 50wt%Treatedwithpolyalkenyl-poly- maleic-anhydride-ester,poly- alkenylpoly-maleic-anhydride- amide,andpolyalkenyl-poly- maleic-anhydride-ester-amide 99.4110.4Vargaetal.28 Glass/epoxyHandlayupWovenmat–&83–Harishetal.5 PPLSF:palmyrapalmleafstalkfiber;HDPE:high-densitypolyethylene;andMAPE:maleicanhydridegraftedpolyethylene.
matrix.22 The tensile strength of the PPLSF/polyester composites are comparable with that of bagasse/poly- ester and coir/polyester composites. However, the trea- ted glass/polyester composites exhibited better tensile properties compared to all other natural fibers except Sansevieriafiber composites. However, the overall per- formance of treated PPLSF composites indicates a better alternative for all other existing natural fibers.
Flexural properties
The extent to which the material resist the bending forces applied perpendicular to the longitudinal axis is called the flexural strength of the material. The flexural properties of the composites are controlled by the inter- laminar forces; therefore, if a material has high value of
flexural strength, it can be concluded that there exists a good adhesion between the fiber and matrix.29The flex- ural tests showed the permanganate treated fiber com- posites withstood more load in bending than the UTCs.
The flexural strength of the composites improved by 70% for PTC, 40% for BTC, and 10% for ATC and the flexural modulus of the composites increased by 110% for PTC, 80% for ATC, and 45% for BTC in comparison to the untreated fiber reinforced compos- ites (Figures 5 and 6). The changes in the surface rough- ness of the fiber due to chemical treatments has resulted in better wettability and good bonding between the fiber and matrix. This has enhanced the strength and rigidity of the composite.30
Table 3 presents the comparison of flexural strengths of several natural and glass fiber composites that resulted from the reinforcement of treated fibers.
It is evident that the flexural strength of the treated PPLSF composites is comparable to other natural fiber composites. This study shows that the treated fibers have around 80% improvement in flexural properties.
In the case of palmyra fiber/rooflite resin composites, a maximum value of 59.19 MPa was obtained with 55 wt% of fibers of 50 mm length,15but during the cur- rent investigation, the flexural strength around 80 MPa was obtained by reinforcing with only 30 wt% of trea- ted PPLSF in unsaturated polyester matrix. It can be concluded that the use of treated PPLSF can influence the strength of the composites. The bagasse/polyester14 and Alfa/polyester composites31 showed more than 100% improvement in properties. Of course, it is a well-known fact that glass fiber reinforced composites have superior properties due to their inherent strength and properties as discussed earlier. However, further investigation need to be carried out with respect to
Figure 6. Effect of treatments on the flexural modulus of composites.
Figure 5. Effect of treatments on the flexural strength of composites.
Table3.ComparisonofFlexuralstrengthofvariousfiber-reinforcedcomposites Fiber/matrixCompositefabrication methodFiberorientation/weight%Typesofchemical treatments Flexural strength (MPa) (untreated)
Flexural strength (MPa) (treated)References PPLSF/polyesterHandlayupandcompres- sionmoldingRandomlydistributed50mm/ 30wt%Alkali,benzoylchloride, permanganatetreatment483.1802.3Presentwork Palmyra/roofliteresinCompressionmoldingRandomlydistributed50mm/ 55wt%–59.19–VelmuruganandManikandan15 Sisal/polyesterResintransfermoldingRandomlydistributed30mm/ 40vol%Alkali,benzoylchloride, permanganateandthermal treatment
84&105SreeKumaretal.11 Sisal/polyesterHandlayupRandomlydistributed 10–40mm/30vol%Admicellarpolymerization75–8495–100Supraneeetal.13 Bagasse/polyesterVacuumbaggingRandomlydistributed/20wt%Alkaliandacrylicacidtreatment31–4835–76Vilayetal.14 Jute/polyesterHandlayupandcompres- sionmoldingFabric/36vol%Alkaliandsilanetreatment&6362–81Severetal.23 S.cylindrica/polyesterCompressionmoldingRandomlydistributed30mm/ 40wt%Alkali,benzoylperoxide,potas- siumpermanganate,stearic acidtreatment
83.85102–150Sreenivasanetal.24 Coir/polyesterHandlayupNon-wovenmat/17wt%Alkaliandbleaching&5248–61Routetal.19 Banana/polyesterHandlayupRandomlydistributed30mmSilaneandalkali&6045–72Pothanetal.16 Alfa/polyesterHandlayupRandomlydistributed60mm/ 40wt%Alkalitreatment&2217–56Rokbietal.31 Bamboo/HDPEInjectionmoldingRandomlydistributed6mm/ 40wt%MAPE18.7323–28MohanthyandNayak27 Glass/epoxyHandlayupWovenmat–&132–Harishetal.5 PPLSF:palmyrapalmleafstalkfiber;HDPE:high-densitypolyethylene;andMAPE:maleicanhydridegraftedpolyethylene.
performance of treated PPLSF/glass and other biofiber hybrid composites.
Impact properties
Impact strength of the composites determines the abil- ity of the material to withstand sudden impact load and also the total energy dissipated in the material before final failure occurs.32The treated fiber reinforced com- posites showed better improvement in impact proper- ties compared to the UTCs. The impact behavior of the
composite is directly related to the bond strength between the fiber and matrix, the properties of fiber and matrix. The treated fiber composites had 75%, 50%, and 25% increase in impact energy absorbed for failure which correspond to 55%, 42%, and 12%
increase in impact strength for permanganate, alkali, and benzoyl chloride treated fiber composites, respect- ively, compared to the UTCs (Figures 7 and 8). In the case of treated fiber composites, better interlocking between the fiber and matrix has allowed for more energy absorption and to stop the propagation of crack resulting in a notable improvement in impact properties (impact strength and impact energy) com- pared to UTCs.
Table 4 summarizes the impact strength of some natural and synthetic (glass) fibers.
The impact strength of the composites using treated fiber as reinforcement has increased the impact proper- ties13,19,24,25
by considerable percentage, due to better interfacial adhesion between the fibers and matrix which has enhanced the capability of the composite to absorb more energy and also to stop crack propaga- tion. In this investigation, the maximum impact strength of the treated PPLSF composites with 30 wt% fiber loading was 21.6 kJ/m2which is compar- able with impact strength of Luffa cylindrica/polyester (20 kJ/m2), pineapple/polycarbonate composites (7 kJ/
m2), sisal/phenol formaldehyde resin (10 kJ/m2), and coconut/phenol formaldehyde resin (19.1 kJ/m2).
Velmurugan and Manikandan15have investigated and found the impact strength of the untreated palmyra composites with 55 wt% fiber loading was 60.5 kJ/m2.
Figure 8. Impact strength of untreated and treated fiber composites.
Figure 7. Impact energy of untreated and treated fiber composites.
Table4.Comparisonbetweenimpactstrengthofvariousfiber-reinforcedcomposites Fiber/matrixCompositefabrication methodFiberorientation/weight%Typesof chemicaltreatments
Impact strength (kJ/m2 ) UTCs
Impact strength (kJ/m2 ) Treated fiber compositesReferences PPLSF/polyesterCompressionmoldingRandomlydistributed 50mm/30wt%Alkali,benzoylchloride, permanganate treatment
142222Presentwork Palmyra/roofliteresinCompressionmoldingRandomlydistributed 50mm/55wt%–60.5–VelmuruganandManikandan15 Sisal/polyesterHandlayupRandomlydistributed 10–40mm/30vol%Admicellarpolymerization&11&12.5Supraneeetal.13 S.cylindrica/polyesterCompressionmoldingRandomlydistributed 30mm/40wt%Alkali,benzoylperoxide, potassiumpermangan- ate,stearicacid treatment
9.459.55–23.41Sreenivasanetal.24 L.cylindrica/polyesterHandlayupRandomlydistributed 50mm/30vol%Alkali,formicacidand aceticacidtreatments720DilaraKocak25 Pineapple/polycarbonateInjectionmoldingRandomlydistributed/ 20vol%Alkaliandsilanetreatment–4.5–7Threepopnatkuletal.12 Sisal/phenolformaldehydeHandlayupandcompres- sionmoldingRandomlydistributed 50mm–10–Sreekalaetal.33 Coconut/phenolformaldehydeHandlayupandcompres- sionmoldingRandomlydistributed 50mm–19.1–Sreekalaetal.33 Glass/epoxyHandlayupWovenmat–&52.66–Harishetal.5 PPLSF:palmyrapalmleafstalkfiber;UTC:untreatedfibercomposite.
This high value was due to the higher amount of fiber content in the composite. The fiber content will also influence the impact properties. The glass/epoxy com- posites had impact strength around 52 kJ/m2. However, hybrid composites can be produced with treated PPLSF/glass to achieve the desirable properties of com- posites compared to glass fiber composites.
Moisture absorption properties
The major drawback in the use of natural fibers as reinforcement is that the fibers are more sensitive to water which will increase the dimension of the com- posites and also will reduce the mechanical proper- ties.34 Moisture absorption of the composites would lead to swelling and degradation at the fiber matrix Figure 9. Moisture absorbance of untreated and treated PPLSF/polyester composites for every 30 min.
PPLSF: palmyra palm leaf stalk fiber.
Figure 10. Moisture absorbance of matrix, untreated and treated PPLSF/polyester composites for every 2 h interval for 24 h.
PPLSF: palmyra palm leaf stalk fiber.
interface which may result in poor stress transfer resulting in poor mechanical properties and dimen- sional stability.35Figure 9 shows the moisture absorp- tion as function of time for untreated and treated fiber composites after every 30 min while Figure 10 records the measurements made after every 2 h interval. The moisture absorption tests showed that the treated fiber absorb less water compared to the untreated fiber reinforced composites. The water uptake is more for the UTCs due to the presence of high amount of hemicelluloses and large amount of porous tubular structure.11 Treatment removes the hemicellouse and waxes, reducing the hydrophilicity of the fibers. Due to chemical treatments, the nature of surface changes, i.e. surface becomes rougher, voids being created on the surface leading to better bonding between the fiber and matrix and fiber becomes hydrophobic. Improved adhesion reduces the rate of diffusion of water mol- ecules. The moisture absorption in the composites was found to be around 9%, 6%, 5%, and 4% for UTC, ATC, PTC, and BTC, respectively, after 24 h of immersion in water. Benzoyl-treated composites absorbed less water compared to all other treated composites and this may be due to benzoyl chloride which includes benzoyl which has contributed for the decrease in hydrophilic nature of the fibers.20
Thermogravimetric analysis
The TGA of matrix, untreated fibers composites, and chemically treated fiber composites are shown in Figure 11. Matrix processes a single stage thermal
degradation at around 30–420C. All the composites and the matrix exhibit similar decomposition behav- ior. The onset temperature of degradation was found to be 344C, 312C, 325C, and 316C for UTC, ATC, BTC, and PTC, respectively. Similar type of behavior was reported12,36 for pineapple leaf fiber/
polycarbonate composites and chicken feather/poly- lactic acid composites, respectively. A second transi- tion can be seen around 420C which is due to the decomposition of the composite. The weight loss of the treated fiber composites above 400C is seen to be less compared to the matrix and UTCs. So, the treated fiber composite has better thermal resistance compared to UTCs.
Scanning electron microscopy
The SEM fractographs of tensile tested specimens are shown in Figure 12. In case of untreated fiber reinforced composite, it can be seen from Figure 12(a) that there is a poor adhesion between the fiber and matrix leading to low interfacial strength and also large cracks in the matrix. Due to low interfacial strength, the stress transfer efficiency is much less compared to that of composites reinforced with treated fibers, which in turn has resulted in very low mechanical properties for the untreated fiber reinforced composites. It can be seen from Figure 12(b) that the fibers remain in the matrix and some amount of fiber pull out is also seen, but the extent to which this had occurred for ATCs are considerably less compared to the UTCs.
It is also clear from SEM images (Figure 12(c) and (d)) that only little evidence of fiber pull out is vis- ible, which indicates that chemical treatments led to good interfacial adhesion between the fiber and matrix leading to better stress transfer efficiency with increased mechanical properties compared to those of the UTCs. The fiber pull out length is con- siderably shorter which can be seen from Figure 12(b) and (d) where failure of the composites had occurred due to fiber shearing and not due to inter- facial failure. Similar types of results were obtained by Herrera Franco and Valadez Gonzalez17 and Vilay et al.14 All of these factors contributed for improved mechanical properties of the composites.
With respect to the view of producing a cost-effective composites, it is clear from the above discussions that properties of composites can be increased to a consid- erable amount by reinforcing them with treated fibers.
A lightweight, biodegradable, and cost-effective prod- ucts can be produced by the use of treated PPLSF, thereby increasing the level of utilization of these fibers for varied applications.
Figure 11. Thermogravimetric curves for matrix, UTC, ATC, BTC, and PTC.
UTC: untreated fiber composite; ATC: alkali-treated fiber com- posite; BTC: benzoyl chloride treated fiber composite; and PTC:
potassium permanganate treated fiber composite.
Conclusions
The influence of chemical pre-treatments of the fibers on the mechanical and thermal properties of the non- woven PPLSF/polyester composites have been experi- mentally evaluated and the following conclusions are drawn.
1. The performance of the PPLSF/polyester compos- ites is very much influenced by the surface properties of the fibers. Alkali, benzoyl chloride, and perman- ganate pre-treatments have improved the mechanical properties, i.e. tensile, flexural, and impact proper- ties of the composites in comparison to composites
reinforced with untreated fibers due to improved interfacial bonding.
2. The alkali and benzoyl chloride treated fiber com- posites had an increase of 60% in tensile strength while the tensile modulus increased by 37% and 60%, respectively, compared to those of the UTCs due to better load transfer between the fibers and matrix fibers during the tests.
3. The permanganate-treated composites had better flexural and impact properties compared to the other treated and UTCs which was due to better interfacial adhesion between the fibers and matrix.
4. All chemical pre-treatments have decreased the amount of water absorption by the composites.
Part fiber pullout and fiber failure
Good adhesion Fractured fiber Fiber pull out
Part fiber pullout and fiber fracture
Good adhesion
Fiber pullout (c)
(b)
Matrix crack Fiber pullout
Poor adhesion
Matrix crack
Fiber pullout
Poor adhesion
(a) (a)
Good adhesion Part fiber pullout and
fiber fracture
Fiber fracture
Good adhesion
Fiber failure (d)
(d)
Figure 12. SEM of the tensile fractured specimen: (a) UTCs; (b) ATC; (c) BTC; and (d) PTC.
SEM: scanning electron microscopy; UTC: untreated fiber composite; ATC: alkali-treated fiber composite; BTC: benzoyl chloride treated fiber composite; and PTC: potassium permanganate treated fiber composite.
BTC had least water absorption of 4.3% followed by PTC 5.1% and ATC 6.1% compared to UTCs which had 9.1% water absorption.
5. The SEM fractographs showed the evidence of better fiber matrix adhesion due to chemical treatments.
6. The treated PPLSF exhibited comparable properties with other natural fibers, so the fibers can be of potential use as reinforcement for composites man- ufacturing with desirable properties. Hybridization of treated fibers with natural and synthetic fibers can be another alternative that can be used to enhance the properties further.
Funding
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Acknowledgments
The authors thank the necessary support provided by the Sakthi Auto Components Limited, Perundurai, Erode District, Tamil Nadu, India and The management of Dr Mahalingam College of Engineering and Technology, Pollachi, Tamil Nadu, India for carrying out the study.
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