Abstract
The effect of surface treated sisal fiber on the mechanical, thermal, flammability, and morphological properties of sisal fiber (SF) reinforced recycled polypropylene (RPP) composites was investigated. The surface of sisal fiber was modified with different chemical reagent such as silane, glycidyl methacrylate (GMA), and O-hydroxybenzene diazonium chloride (OBDC) to improve the compatibility with the matrix polymer. The experimental results revealed an improvement in the tensile strength to 11%, 20%, and 31.36% and impact strength to 78.72%, 77%, and 81% for silane, GMA, and OBDC treated sisal fiber reinforced recycled Polypropylene (RPP/SF) composites, respectively, as compared to RPP. The thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), and heat deflection temperature (HDT) results revealed improved thermal stability as compared with RPP. The flammability behaviour of silane, GMA, and OBDC treated SF/RPP composites was studied by the horizontal burning rate by UL-94. The morphological analysis through scanning electron micrograph (SEM) supports improves surface interaction between fiber surface and polymer matrix.
1. Introduction
With the international governmental policies, ecological risks and increasing global energy crises have been the driving force behind the more and more research interest owing to their potential of serving as alternative for natural fiber reinforced thermoplastics composites [1–7]. Accordingly, extensive studies on the attractiveness of a plant based fiber reinforcement materials come from its low abrasion, multifunctionality, low density, unlimited availability, ecofriendliness, high specific mechanical performance, and renewability [8–16]. These properties have made fibers very attractive candidate material for many applications such as automotive, building, construction, and aerospace. In spite of the advantages, natural fibers are hydrophilic in nature as they are derived from lignocellulose, which contain strongly polarized hydroxyl groups. These fibers, therefore, are inherently incompatible with hydrophobic thermoplastics, such as recycled polypropylene; the main difficulty with using natural fiber arises due to improper adhesion with hydrophobic polymer matrix and formation of agglomerates during processing [17–23]. This has resulted in an increasing demand towards improving the adhesion between the fiber and matrix by the modification of the fiber and/or polymer matrix using chemical methods. Fiber modifications include mercerization, acetylation, silanation and treatment with strontium titanate (SrTiO3), glycidyl methacrylate (GMA) & O-hydroxybenzene diazonium chloride (OBDC) by different authors for enhancing better surface interaction towards polymer matrix [15, 19, 24–31].
In the age of “going green” it is becoming easier and easier to recycle. A sustainable polymer product cannot be land filled but has to be recycled. Polymer recycling is classified into four distinct categories depending on the level of conservation of the polymer chemical structure. Recycling of polymeric material has been practiced for many years by industries without any great accuracy. The new environmental, economic, and petroleum crises have induced the scientific community to deal with polymer reprocessing and sustainability. Recycled polypropylene (PP) is of great interest, particularly for the automotive industry. One problem with recycled resins is that tensile toughness and impact strength usually decrease during recycling and restoration requires additives. Polypropylene is an economical material that offers a unique combination of excellent physical, chemical, mechanical, thermal, and electrical properties which is not found in any other thermoplastic [32].
Again, PP is one of the most versatile thermoplastics polymers as a consequence of its recyclability and its capability low processing cost for massive production. The use of recycled polypropylene (RPP) obtained from the above application is not as easy as virgin one due to the inconsistent material properties in presence of impurities. To overcome this limitation several methods have already been developed but reinforcement with natural fiber are yet to be studied [33].
The sisal fibers used in this investigation is one of the most widely used natural fibers extracted from the leaves of the plant Agave sisalana. This was originated from Mexico and is now mainly cultivated in East Africa, Brazil, Haiti, India, and Indonesia. The name “sisal” comes from a harbor town in Yucatan, Maya, Mexico. It means cold water. Agave plants were grown by the Maya Indians before the arrival of the Europeans. They prepared the fibers by hand and used it for ropes, carpets, and clothing. Some clothes were called “nequen,” and this is where the present name of Mexican agave, henequen, probably originates from. The outstanding mechanical properties of SF associated with high cellulose content and low microfibrillar angle have been taken into consideration for enhancing strength and performance characteristics in the current study. However, its hydrophilic character has been the main cause of poor compatibility between natural fibers and polymers matrix, which leads to unsatisfactory mechanical and thermal properties of reinforced composites [34, 35].
The interaction between the natural fiber and the matrix can be further improved by surface modification of the fibers using chemical treatment techniques, such as acetylation, benzylation, plasma treatment, and mercerization. Mechanical properties of natural fiber-reinforced composites can be further influenced by adhesion characteristics of the fiber-matrix interface. Similar investigation was carried out by Singleton et al. in his work on “On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite” [8].
In this study, an attempt has been made to study the effect of chemically treated sisal fiber on mechanical, thermal, morphological, and flammability properties of fiber reinforced recycled polypropylene composites.
2. Experimental
2.1. Materials
Recycled polypropylene (RPP), obtained from M/s. Cargil Dow (Bair, US-NE), was used as a matrix material. The MFI for RPP is between 1 and 12 g/10 min (230°C, 2.16 kg), and density is 0.9 g/cc. Sisal fibers (Agave sisalana) obtained from M/s Sheeba fibers and handicrafts, Poovancode, Tamilnadu, India, with a density of 1.5 g/cc were used as reinforcement. Surface modifiers: 3-aminopropyltriethoxysilane (APS), a product of M/s Evonik Degussa (China) Co., Ltd. supplied by Aroma Chemical Agencies (India) Pvt. Ltd. and bis-(3-triethoxy silyl propyl) tetrasulfane (Si69) from M/s Sigma Aldrich. Co., Germany, were used as modifying agents. Common chemicals such as sodium hydroxide, acetic acid, acetic anhydride, and sulphuric acid were collected from Merck Specialities Pvt. Ltd., Mumbai, India.
2.2. Methods
2.2.1. Fiber Surface Modification
Prior to chemical treatment bundles of sisal fiber were cut in to a length of 13–15 cm and scoured in mild detergent solution at 60°C for 2 hrs to remove dust and other impurities. Finally the fibers were washed in distilled water and dried in air for 2 days.
NaOH Treatment (Mercerization)
Mercerization of the fibers was carried out by immersing the fibers in 1 N sodium hydroxide (NaOH) solution for 1 hr at room temperature. Then the fibers were washed with distilled water containing few drops acetic acid, followed by washing under continuous stream of water until the complete removal of NaOH residue. Subsequently, the mercerized fibers were dried at room temperature for 24 hrs and then in a vacuum oven at 80°C for 12 hrs.
Acetylation
The fiber bundles were immersed in the mixture of acetic acid/acetic anhydride of ratio 1 : 1 along with 1 mL of conc. sulphuric acid as a catalyst. Then the fiber was drained from the mixture, washed with distilled water until the complete removal of unreacted reagents, and then finally dried in a oven for 24 hrs at 80°C.
Silane Treatment
The detergent washed fibers were dipped in 5 wt% silane solution in alcohol water mixture (60 : 40) for 1 hr at room temperature. Bis-(3-triethoxysilylpropyl) and 3-amino propyltriethoxysilane (APS) were used as coupling agents. The pH of the solution was maintained to 4 with acetic acid. Then the fibers were washed in distilled water and dried.
Strontium Titanate (SrTiO3) Treatment
The fibers were dipped in 5 wt% alcoholic solution (ethanol) for 30 min in presence of 2% strontium titanate (SrTiO3) as a coupling agent. The pH of the solution was maintained between 4.5 to 5.5 with the addition of ethanoic acid; further the fiber was washed with ethanol and dried in oven for 2 hrs.
Glycidyl Methacrylate (GMA) Treatment
Detergent washed sisal fiber was reacted at 90°C with 60 mL of glycidyl methacrylate solution. Hydroquinone (2% by weight of GMA) was added to minimize the free radical reaction at the unsaturated end of GMA molecules. After 2 hrs, the modified fibers were separated through filtration and rinsed with acetone. The residual solvent was removed with excess of acetone and finaly dried under vacuum at 90°C for 24 hrs.
O-Hydroxybenzene Diazonium Chloride (OBDC) Treatment
O-hydroxybenzene diazonium chloride solution was prepared by the addition of O-hydroxyaniline (dissolved in HCl) with NaNO2 in an ice bath temperature ≤5°C. The prepared solution was mixed with water in the ratio of 10 : 90. Then the fibers were dipped into the solution. After treatment the fibers were washed with distilled water followed by drying in oven for 4 hrs.
All the treated fibers were chopped using an electronic fiber cutting machine in to short fiber short fiber of length 2–3 mm. The surface treatments of sisal fiber have been confirmed using FTIR spectroscopy.
2.3. Fourier Transformation Infrared Spectroscopy (FTIR)
FTIR spectra of untreated and treated sisal fiber were recorded using Nicolet 6700, USA spectrometer. Each spectrum was obtained by adding 64 consecutive scans with a resolution of 4 cm−1 within the range of 500–4000 cm−1.
2.4. Fabrication of Composites
RPP/SF composites were prepared by melt mixing in a batch mixer (Haake Rheomex 600, Germany) at various percentages of fiber loading from 10, 20, and 30 to 40 wt%, respectively. The mixing was carried out at 180°C with a rotor speed of 40 rpm for 10 mins. The melt mixer obtained was cooled to room temperature, granulated, and conditioned at 80°C for 2 hrs prior to specimen preparation. Finally molded sheets of 3±0.1 mm thickness were prepared using a 100 T compression press (M/s Delta Malikson, Mumbai) at 190°C, 80 kg/cm2 pressure over a total cycle time of 15 min. Further, specimens were prepared from these sheets as per various ASTM standards using a count ceast copy milling machine (M/s ceast, Italy).
2.5. Mechanical Testing
Tensile properties were measured as per ASTM-D 638 with gauge length of 50 mm, at a cross head speed of 50 mm/min by using the Universal Testing Machine (3382 Instron, UK). Notched izod impact strength of the specimens was evaluated using an impactometer (Tinius olsen, USA) as per ASTM-D 256 with a notch depth of 2.54 mm and notch angle of 45°.
2.6. Thermal Characterization
The melting, crystallization, and thermal stability of RPP and RPP/SF composites have been studied using differential scanning calorimetry (DSC Q 20, TA Instruments, USA) and TGA (TGA Q 50, TA Instruments, USA), respectively. DSC analysis was carried out using 5–10 mg of samples at a scanning rate of 10°C/min with a temperature range of 30–200°C under nitrogen atmosphere. Similarly TGA of 5–10 mg samples were carried out from 30 to 600°C at a heating rate of 10°C under nitrogen atmosphere. An HV-2000A-C3, GOTech, (Taiwan) analyzer was used to measure the heat-deflection temperature (HDT) of the composites.
2.7. Flammability Study
Flammability of the samples was studied by a horizontal burning test as per UL-94. In the horizontal burning test, the sample was held horizontally and a flame fuelled by natural gas was applied to light one end of the sample. The time for the flame to reach from the first reference mark (25 mm from the end) to the second reference mark, which is at 100 mm from the end, was measured.
2.8. Morphological Analysis: Scanning Electron Microscopy (SEM)
The SEM of impact fractured composite specimens was carried out using EVO MA 15, Carl zeiss SMT (Germany). The samples were sputtered with platinum and were dried for half an hour at 70°C in vaccum, before study.
3. Result and Discussion
3.1. FTIR Analysis
Untreated and treated Sisal fiber were investigated by FTIR analysis as shown in Figure 1 from wave number 4000 to 450 cm−1 and the possible reaction mechanism represented in Figure 2. Peaks in the region of 1030–1150 cm−1 are mainly due to the C–O–C and C–O symmetric stretching of primary and secondary hydroxyl group in the cellulose, lignin, and their glycoside linkages. Peak near to the 1654 cm−1 corresponds to –C=O stretching due to the presence of aliphatic carboxylic acid in cellulose chain, whereas ketonic groups indicate –OH stretching peak at around 3400 cm−1. The peak area around 1600 cm−1 reveals absorbed moisture content in SF. In addition, –C–H stretching absorption around 2900 cm−1 also can be seen in the SF spectra.
FTIR spectra of (a) untreated, (b) NaOH, (c) Acty, (d) Si, (e) SrTiO3, (f) GMA, and (g) OBDC treated sisal fiber.
Mechanism of chemical treatments (a) NaOH, (b) acetylation, (c) Silanation, (d) SrTiO3 treatment, (e) GMA treatment, and (f) O-hydroxybenzene diazonium chloride (OBDC) treatment.
Surface modification of SF exhibits the absence of characteristic peaks at 1643 and 1156 cm−1, accompanied with reduction in peak intensity at 1425 cm−1. This is because of the decomposition of hemicellulose and partial leaching out of lignin by sodium hydroxide [7]. The disappearance of the peak at 1643 cm−1 is due to the dissolution of a portion of uranic acid, a constituent of hemicelluloses xylan. The shift of the peak from 2900 cm−1 to 2515 cm−1 indicates participation of some free hydroxyl groups in the chemical reaction [24].
Similarly, acetylation of SF also leads to changes in the appearance or increment in absorbance peak in the region around 1735–1737 cm−1. The appearance of peak at around 1500 cm−1 is associated with the bending of aromatic C–H bond, present in lignin. The peak corresponding to 1735–1737 cm−1 is due to the esterification of the hydroxyl groups, which results in an increased stretching vibration of the carbonyl (C=O) group present in the ester bonds. The spectrum of fibers near to 1732 cm−1 is primarily assigned to the C=O stretching vibration of the carboxyl and acetyl groups in “xylan” of hemicelluloses. The appearance of the peak at around 2372 cm−1 for the acetylated SF confirms formation of ester linkage between the acetyl and hydroxyl groups in the fibres [25].
Silane treated SF introduces organosiloxy group to the fiber surface. The expected change in FT-IR spectra with silane treatment of SF is symmetric –(Si–O) and (–C–Si) stretching frequencies and asymmetric O–Si–O–and –Si–O–C– stretching frequency in a region around 510–1160 cm−1. Thus the peaks around 825 cm−1 and 1020 cm−1 are attributed to the Si–C symmetric stretching also contributes a peak around 891 cm−1 of the spectra. The asymmetric stretching frequencies of (–Si–O–C–) are due to the condensation reaction between the Silane and SF. These condensed Siloxanes also contribute to the Si–O–Si symmetric stretching frequency. In addition the above general characteristics peaks of Siloxane like C–N stretching frequencies around 2187 cm−1 can also have been observed [24, 25].
Treatment of SF by SrTiO3 shows the disappearance of peak in the region of around 1630–1750 cm−1 conforming the partial dissolution of hemicelluloses group to carbonyl functionalities. The –OH stretching band becomes broad in a region around 2800–2900 cm−1 indicating the presence of increased number of hydroxyl group occurs due to the reaction of cellulosic group with sensitive SrTiO3 [20].
Whereas treatment of SF by GMA shows a remarkable absorption peak in the carbonyl region at around 1728 cm−1 and peaks at 1418 and 1595 cm−1, which are associated with the methacrylic ester bond and C=C group of GMA. The peak intensity in the modified samples increased with the reaction time, thus supporting a high reaction extent of the epoxide groups of GMA with the OH groups on the fiber surface [26].
However treatment with OBDC indicates absorption band at around 1700–1600 cm−1 and 1400–1000 cm−1 for N–O and C–O stretching, respectively; thus the result gives a clear indication of chemical modification of SF with O-hydroxybenzene diazonium chloride [15].
3.2. Mechanical Properties
3.2.1. Effect of Untreated Sisal Fiber Loading on Mechanical Properties of RPP/SF Composites
Figure 3 represents the mechanical properties of RPP/SF composites at a variable percentage of fiber loading from 10 to 40 wt%. Test results indicate that RPP has tensile strength, tensile modulus, and impact strength of 18.90 MPa, 1204.61 MPa, and 16.31 j/m respectively. Incorporation of SF into the RPP matrix reduces the tensile strength to about 41.57% at 30 wt% of fiber loading as compared with RPP. Likewise the percentage in tensile strain at break is found to be decreased confirming the discontinuity of dispersion in polymer matrix [28]. However tensile modulus and impact strength are found to be increased to small extent at 30 wt% of SF loading due to the reinforcing effect of fiber where stress is transfer from the matrix to the fiber. The results imply that untreated fiber does not have better interaction with the polymer matrix due to the presence of surface impurities. At higher fiber content the agglomeration of fiber is found to be more, which further results in deterioration in mechanical properties at 40 wt% of fiber loading [24]. The quality of a fiber reinforced composites depends considerably on the fiber matrix interface because only a well form interface allows better stress transfer from matrix to the fiber. Therefore good interfacial adhesion between the matrix and fiber is essential to improve the mechanical strength of composites.
Mechanical properties of RPP/SF composites at a different wt% fiber loading whereas A: RPP, B: RPP/SF (90/10), C: RPP/SF (80/20), D: RPP/SF (70/30) and E: RPP/SF (60/40).
3.2.2. Effect of Compatibilizer (MA-g-PP) on Mechanical Properties of RPP/SF Composites
The hydroxyl acid and the polar groups located in the branched heteropolysaccharide present in the Sisal fiber are active sites of water absorption, which results in incompatibility with hydrophobic RPP matrix. This incompatibility leads to weak interface and poor mechanical properties. With the addition of compatibilizer (MA-g-PP) in RPP/SF composites at 3–7 wt% of MA-g-PP loading result in significant improvement in mechanical properties, represented in Figure 4. Here addition of MA-g-PP acts as a dispersing agent between the polar fibers and the nonpolar matrix resulting in improved interfacial adhesion, which contributes towards enhanced stress transfer from the matrix to the fiber [6, 14].
Mechanical properties of RPP/SF/MA-g-PP composites at different wt% of MA-g-PP loading whereas F: RPP/SF/MA-g-PP (67/30/3), G: RPP/SF/MA-g-PP (65/30/5) and H: RPP/SF/MA-g-PP (63/30/7).
As the RPP/SF composite at 30% fiber loading shows increased tensile modulus and impact strength, so this composition has been taken for preparation of RPP/SF/MA-g-PP composites for further characterization studies. Incorporation of MA-g-PP at 5 wt% shows increased tensile strength of 28.22% and tensile modulus of 2.11% as compared to RPP/SF composites without compatibilizer, which is as per the strength observed in case of RPP. This behaviour primarily attributed to the fact that the anhydride group of MA-g-PP reacts with the hydroxyl group of sisal fibers forming an ester linkage at the interface. Furthermore, the high molecular weight MA-g-PP has more flexible PP chains, which are able to diffuse into the matrix leading to interchain entanglements, thereby contributing to the mechanical continuity of the systems. Gassan and Bledzki [36] have reported similar behaviour for jute and flax reinforced PP composites. However, increase in the MA-g-PP content to 7 wt% resulted in the decrease of mechanical strength of composites. This is due to self-entanglement among the compatibilizer chains rather than with polymer matrix resulting slippage. Similar results are also indicated by Biswal et al. in their work on “Influence of Organically Modified Nanoclay on the Performance of Pineapple Leaf Fiber-Reinforced Polypropylene Nanocomposites” [7].
3.2.3. Effect of Surface Treated Sisal Fiber on Mechanical Properties of RPP/SF/MA-g-PP Composites
The different chemical modifications of natural fibers aimed at improving the adhesion with a polymer matrix were performed by a number of researchers [7, 15, 19, 24, 27, 33]. Some examples of chemical modification of fiber surface such as Mercerization, Acetylation, Silane, Strontium titanate, GMA and O-hydroxybenzene diazonium chloride treatments are represented in our current study. Different chemical treatments of SF intrinsically increase the interacting ability of the fiber with the polymer matrix which results improved mechanical properties as represented in Figure 5.
Mechanical properties of RPP/SF/MA-g-PP composites at ratio (65/30/5) whereas A: RPP, B: RPP/Acty-SF/MA-g-PP, C: RPP/NaOH-SF/MA-g-PP, D: RPP/Si-SF/MA-g-PP, E: RPP/SrTiO3-SF/MA-g-PP, F: RPP/GMA-SF/MA-g-PP and G: RPP/OBDC-SF/MA-g-PP.
NaOH Treated SF
Mercerization of SF results in an improvement in interfacial debonding of fibers with RPP, which is probably due to the additional sites created for mechanical interlocking [25]. This treatment increases the tensile strength 5.5%, tensile modulus 32.02%, and impact strength 72.75% as compared to RPP. Mercerization also provides better fiber surface adhesion characteristics by removing natural and artificial impurities, thus producing better surface topography.
Acetylated SF
Incorporation of acetylated SF (Acty-SF) into RPP indicates improvement of 0.21% in tensile strength, 28.93% in tensile modulus, and 65.64% in impact strength as compared to RPP. This improvement in tensile strength is due to the loss of hemicelluloses from the fiber surface [25] and tensile modulus due to the cross-linking reaction during melt mixing [24], whereas impact strength increases due to the minimum interfacial debonding according to Mishra et al. [37].
Silane Treated SF
Silane treated SF significantly increased the tensile strength, tensile modulus, and impact strength to 10.97%, 31.61%, and 78.72%, respectively as compared to RPP. This is due to that the ethoxy group of the silane reacts with the carbonyl group of the cellulosic fiber in ethanol media which further interact with RPP through hydrogen and covalent bonds. The sulphur atom present in silane can also impart polarity in the system to enhance the interaction with the RPP matrix. Thus it forms a bridge between fiber and matrix, enhancing the interfacial interaction between them [24–26]. Furthermore it can also be explained by increased nucleation ability as a result of silane surface treatment yielding smaller and better crystals in a transcrystalline interphase region improving the bonding capability between the fiber and matrix.
SrTiO3 Treated SF
The improvement in tensile strength, tensile modulus, and impact strength is found to be 19.43%, 26.12%, and 74.03% with the addition of SrTiO3 treated SF. These results indicate improved bond durability and increased interaction between fiber and polymer matrix. This is also responsible for the hardening of fiber morphology and decreased elasticity [20].
GMA Treated SF
GMA treated SF indicates the improvement of tensile strength to 19.81%, tensile modulus to 30.48%, and impact strength to 77.55% respectively. This improvement is probably due to the effect of interfacial interactions in the presence of compatibilizer. The interactions between the epoxy groups grafted on fiber and the hydroxyl group on cellulose fibers are enhanced as the compatibilizer ratio increased [26].
OBDC Treated SF
Mechanical results due to OBDC treated fiber indicate improvement in tensile strength, tensile modulus and impact strength to 31.37%, 31.88%, and 81.13%, respectively. This indicates that interfacial bonding between the fiber and matrix has significantly improved upon chemical treatment leading to increased stress transfer efficiency from the matrix to the fiber. This further confirms the capability of the fiber to absorb energy that can stop crack propagation [15, 24, 25].
3.3. Thermal Analysis
3.3.1. Differential Scanning Calorimetry (DSC)
The DSC heating and cooling thermograms of RPP, RPP/SF, and RPP/untreated and treated SF/MA-g-PP composites are depicted in Table 1 and Figure 6. The glass transition temperature (T g ), crystalline temperature (T c ), and melting temperature (T m ) of all samples are listed in Table 1. RPP shows a T g value of −12.18°C, which is decreased to about −14.23°C with the incorporation of untreated SF. This indicates the plasticization effect occurs with the incorporation of natural fiber. Plasticization due to untreated fiber increased the chain mobility and free volume of the matrix chains by loose packing of fiber within the matrix confirms the improper surface interaction. This interaction can be further enhanced by addition of compatibilizer (MA-g-PP) and surface treatment of SF. In case of RPP/Si-SF/MA-g-PP composites, T g of RPP increased from −12.18°C to −3.91°C. This confirms efficient adhesion between fiber and matrix upon treatment with silane. RPP/GMA-SF and RPP/OBDC-SF composites with MA-g-PP also an increase of −3.36°C and −4.48°C due to the poor adhesion and moisture absorption effect by GMA and OBDC surface treated fiber, respectively [15, 27].
Glass transition, melting, and crystallization properties of RPP/SF composites.
DSC graph temperature versus exothermic heat flow for (a) melting temperature (T m ) and (b) crystalline temperature (T c ).
The crystalline temperature (T c ) of RPP, RPP/SF, and RPP/untreated and treated SF/MA-g-PP are represented in Table 1. The results indicate that the T c of RPP/SF with and without MA-g-PP and treated SF (Silane, GMA, OBDC) was slightly increased as compared with RPP. This may be due to the reorganization of amorphous domains in to crystalline regions with increased macromolecular flexibility and mobility at higher temperature [7–9].
Melting temperature (T m ) of RPP was observed around 167.8°C whereas its RPP/SF decreased to 165.83°C due to loss of chain alignment and conformational purity of RPP matrix during melt process with SF. This indicates the formation of crystallites with different sizes and/or perfection of ordering [38]. The melting transition has been enhanced by addition of MA-g-PP and surface treated SF as compared with RPP/SF composites. The improved interfacial interaction of fiber with polymer matrix leads to the enhancement in melting temperatures due to the addition of MA-g-PP and surface treated fiber [7].
3.3.2. Thermogravimetric Analysis (TGA)
The thermal stability of RPP, RPP/SF, RPP/SF/MA-g-PP (65/30/5), and RPP/SF (Silane and GMA and OBDC treated SF)/MA-g-PP (65/30/5) composites were investigated using TGA and DTG curve as represented in Table 2 and Figure 7. It is evident that the thermal degradation of RPP started at 356°C with the final degradation temperature of 460°C. It also can be seen that the weight loss of RPP occurred in a one-step degradation process from 356°C to 450°C. The mass loss of RPP 234.2°C continues very slowly at temperature below 401.5°C. Above 400.6°C this process takes place very rapidly and the residue amount is almost zero at 460°C. On the other hand the thermal degradation of RPP/SF composites occurred in two-step degradation process. This result is also confirmed from DTG curve as represented in Figure 7. Addition of SF up to 30 wt% decreases its thermal stability as compared to RPP. This may be due to the lower decomposition temperature of SF present in the composites [32].
Thermogravimetric analysis (TGA).
TGA and DTG analysis curve for thermal analysis.
However RPP/SF composites with MA-g-PP exhibited increased thermal stability with an initial degradation temperature of 246.8°C and final degradation temperature of 412.5°C. This is probably due to dehydration from cellulose unit and thermal cleavage of glycosidic linkage by transglycosylation and scission of C–O and C–C bonds. At 436.5°C, RPP got completely decomposed, whereas in the composites a charred residue of carbonaceous product of 2.13% was left [7]. The RPP/SF composites with treated SF such as silane, GMA, and OBDC indicate higher thermal stability. Treated Silane is found to be increased to about 362.6°C and 472.5°C in initial and final degradation temperature. This behaviour is probably due to the increase in molecular weight by cross-linking reaction between matrix and fiber or molecular chain extension of the matrix itself, whereas GMA and OBDC are found to be increased in 363.4°C and 494.3°C in initial temperature and 379.2°C and 512.8°C in final degradation temperature. This is due to better surface adhesion towards the polymer matrix [15].
3.3.3. Heat Deflection Temperature (HDT)
Heat deflection temperature (HDT) of RPP and RPP/SF composites was investigated using HDT analysis, which is depicted in Figure 8. The HDT values of RPP are found to be lesser with the addition of 30 wt% of SF and 5 wt% of MA-g-PP, whereas with the incorporation of treated fiber in RPP/SF composites show a significant increase in thermal stability to about 25°C–42°C. RPP/Si-SF/MA-g-PP, RPP/GMA-SF/MA-g-PP, and RPP/OBDC-SF/MA-g-PP composites indicate improved HDT to about 3.35%, 6.7%, and 14.77% as compared to RPP, respectively. The HDT value depends on the modulus and glass-transition temperature of a material. The modulus-temperature relationship plays a critical role in determining the HDT. Thus the improvement in the HDT values for the RPP/SF composites approches towards higher modulus values at elevated temperature as discussed earlier in case of mechanical properties [27].
HDT graph where A: RPP, B: RPP/SF (70/30), C: RPP/SF/MA-g-PP (65/30/5), D: RPP/Si-SF/MA-g-PP (65/30/5), E: RPP/GMA-SF/MA-g-PP (65/30/5), and F: RPP/OBDC-SF/MA-g-PP.
3.4. Flammability Study
The burning rate of all the systems was investigated through horizontal burning test using UL-94 standards. All the systems burned up to 100 mm marks. Burning rates of RPP and RPP/SF composites measured by horizontal burning test are represented in Table 3 and Figure 9. The composites with the loading of 30 wt% of SF showed lower burning rate than that of the RPP. This indicated the lower sensitivity of sisal fiber filled RPP composites, to the flame. On the other hand the burning rate of the RPP/SF composites compatibilized with MA-g-PP was slightly decreased as compared to that of the uncompatibilized composites and was even less than that of RPP. This revealed the addition of compatibilizer help to reduce the burning rate of the RPP. Among the composites filled with treated fibers, the RPP/Si-SF/MA-g-PP showed the lowest rate of burning as it decreased by 16% and 7.42% as compared with RPP and RPP/SF composites, respectively. Therefore among all the systems RPP/Si-SF/MA-g-PP composite showed the highest stability towards flame [39].
Rate of burning properties of RPP/SF composites.
Rate of horizontal burning.
3.5. Morphological Study (SEM)
The surface morphology of fractured RPP/SF composites reflects the reason about higher mechanical properties. The Morphological impact fractured surface of RPP/SF (70/30), RPP/SF/MA-g-PP (65/30/5), RPP/Si-SF/MA-g-PP (65/30/5), RPP/GMA-SF/MA-g-PP (65/30/5), and RPP/OBDC-SF/MA-g-PP (65/30/5) composites and treated sisal fiber shown in Figure 10, SEM images of RPP/SF, RPP/SF/MA-g-PP implies poor interfacial bonding between the matrix and the untreated fiber. On the other hand chemically treated SF composites show almost no sign of fiber agglomeration and microvoids in the fractured surface of the composites. These results suggest that interfacial adhesion between the chemically treated SF and RPP matrix has become much more favorable upon treatment of the fiber with Silane, GMA and OBDC [7]. In general, fiber treatments can significantly improve adhesion at the former interface and also lead to ingress of the matrix into the fibers, obstructing pullout of the cells. This further confirms the removal of wax, oils and other low molar mass impurities which make the fiber surface rougher towards better adhesion with the matrix. The micrographs show that the untreated fibres are covered with a layer, whose composition is probably mainly waxy substances, as has been reported also previously [15–32]. It can be seen that the layer is not evenly distributed along the fibre surface, but its thickness varies from point to point. As seen in SEM images the surface of the treated fiber became smoother as compared to that of untreated fiber. Removal of the waxy and oil substances on the surface of lignocellulosic materials and replacement of surface hydroxyl groups by acetyl and propionyl groups could explain smoothening of the fibre surface after treatments [40].
SEM image of impact fractured RPP/SF composites, untreated and treated sisal fiber, whereas (a) RPP/SF, (b) RPP/SF/MA-g-PP, (c) RPP/Si-SF/MA-g-PP, (d) RPP/GMA-SF/MA-g-PP, and (e) RPP/OBDC-SF/MA-g-PP, (f) untreated SF, (g) NaOH-SF, (h) Acetylated SF, (i) Si-SF, (j) SrTiO3-SF, (k) GMA-SF, and (l) OBDC-SF.
4. Conclusions
In this present study the mechanical, thermal, flammability, and morphological properties of the RPP/SF (untreated and treated) with and without compatibilizer composites have been investigated. RPP/SF composites were prepared using melt compounding techniques. Composites prepared at 30 wt% of fiber loading with 5% MA-g-PP showed optimum mechanical performance. Incorporation of treated sisal fiber such as silane and GMA and OBDC additionally increases the mechanical and thermal properties of PP matrix. Thermal degradation temperature of RPP/SF composites shows higher values as indicated by TGA and HDT analysis. DSC study shows higher T g and T c as compared to RPP indicating increased macromolecular flexibility and mobility. The flammability of RPP/SF composites studied by horizontal burning (UL-94) method shows the decrease level of burning rate of RPP/SF composites due to the presence of flammable sisal fiber. The morphological observations by SEM indicate efficient removal of cementing agents from raw sisal fibers enhances the fiber adhesion properties with matrix due to surface treatment. Thus RPP/SF composites with balanced mechanical and thermal properties can be achieved at an optimal concentration of treated SF and compatibilizer with RPP matrix.