An Experimental Investigation on Short and Long Fiber Reinforcement in Tampico and Palymira Reinforced LLDPE Produced by Rotational Moulding Process.

Rotational moulding is a powder processing technology widely used to manufacture hollow plastic products. It is a well-known process used for the manufacturing of large containers, water tanks, fuel tanks, refrigerated containers etc. The present work aimed at the incorporation of Tampico and Palmyra bers as reinforcement in Linear Low-Density Polyethylene (LLDPE) matrix. The effect of ber length on various properties like mechanical, morphological and vibration damping characteristics of the composites were investigated. The bers were initially treated with 5% NaOH solution for better reinforcing eciency and to improve the degradation temperature. Thermo Gravimetry Analysis (TGA) reported that a sucient increment in degradation temperature is achieved by NaOH treatment on bers. The short ber composites considered for the study has obtained better mechanical strength than long ber reinforced LLDPE composites. The results proved that ber length has less effect on the vibration damping characteristics of the composites, but the percentage of ber content is a signicant factor for improving the natural frequency of the composites.


Introduction
Rotational moulding is a well-known technique used to produce hollow non-metallic stress-free products. The process is also known as roto moulding or roto casting. The process involves heating the polymer powder in a closed mould which is rotating bi axially inside a heating chamber or furnace [1].
Theoretically, it is possible to rotomould any plastic which melts while heating but practically most of them degrade during rotational moulding. The thickness of the nished product depends upon the weight of the polymer powder fed into the mould. The mould is rotated biaxially at a very slow speed, and then it is cooled to room temperature either by natural cooling or induced cooling [2]. The strength of the product, surface roughness and sintering e ciency depends upon the quality, size and shape of powder used for processing. Since there is no external force applied, the product obtained is relatively stress-free as compared to other polymer processing techniques.
Recent advancements in the rotational moulding are the development of composite hollow products.
Natural ber nds its application as a good reinforcement in polymer composites due to its better strength and also the biodegradable nature [3]. Studies reported that the mercerization treatment done on natural bers leads to better tensile properties [4]. The use of natural bers as reinforcement has some vital advantages like cost reduction, weight reduction, bio degradable nature etc., supports the usage of natural bers than synthetic bers now a days [3], [5]. Earlier studies reported on banana and abaca brereinforced polyethylene (PE) composites prepared with 5% ber addition results in improved elastic modulus and reduced impact strength as compared with unreinforced PE [6]. Cabuya and Sisal bers were used as the potential reinforcement to High-Density Poly Ethylene (HDPE) with a ber length of 5 mm and reported that Cabuya ber composites show better impact strength than sisal ber composites [7]. The optimum ber content for better mechanical properties was estimated to be 5 wt% for long bers of 5mm length. Fiber addition beyond that optimum composition drastically reduced the mechanical properties. Natural ber nds its application as potential reinforcement in polymer composites due to its complicated structures, better mechanical strength, and biodegradable nature.
Many research works extended the investigation towards the biodegradability, moisture absorption, vibration damping and thickness swelling of natural ber reinforced polymer composites (NFRPC)[8], [9].
Luffa ber reinforced epoxy composites were investigated and found that the addition of ber increases the moisture absorption and hence due to this increased moisture absorption, the mechanical properties were decreased [10]. Many researchers recommended the use of bamboo bers since it possesses better tensile strength and exural strength compared to other bers [10]- [14]. The use of ller particles such as Rice Husk in LLDPE matrix has been recently reported with the addition of nano silica and nano clay [15].
The Rotational moulding process with natural ber as reinforcement has limited number of literatures reported. The primary focus of the present work is to investigate the mouldability of natural ber reinforced polymer composites produced by rotational moulding process. Two novel bers were identi ed as a potential reinforcement in LLDPE matrix. Reinforcement with Palymira bers and Tampico bers were not yet reported in LLDPE matrix and hence these bers are considered for the present work.
Palmyra bers are medium coarse bers available in southernmost regions of India whereas Tampico bers are imported from Mexico. The present work investigates the mouldability as well as the mechanical, morphological, and vibration damping properties of palymira and Tampico ber reinforced LLDPE by rotomoulding process.

Fiber treatment
Tampico and palymira bers were collected and chopped to the speci ed length with the help of a chaff cutter. The chopped bers were manually cleaned in distilled water and dried exposed to sunlight for 8 hours after rinsing with the help of a centrifuge. The dried bers were collected and immersed in 5% NaOH solution for 24 hours to remove the lignin and extractives on the surface of bers. Then it is dried in sunlight for two days to remove the moisture content. The stearic acid is applied to ensure uniform ber dispersion and to eliminate agglomeration. The palymira and Tampico bers chemical compositions are shown in Table 1.

Preperation of composites
The composites were prepared using a lab model rotational moulding machine with a mould dimension of 32cm X 32cm X 34cm. The charge weight is calculated for a mould thickness of 5mm. The metered quantity of LLDPE powder with different ber content is fed to the stainless-steel mould and rotated biaxially. Silicon spray is applied gently over the inner mould surface in order to remove the nal product easily. Both Tampico and palymia beres were added at different wt% of 5, 10 & 15. The LLDPE powder along with the bers was dry blended in a blending machine for a few minutes. The speed ratio of the rotomoulding machine was set as 1:4. An electric motor attached to the major arm ensures the driving torque. The heating phase of 30 minutes is followed by fan cooling until room temperature is achieved

Mechanical Characterization
Estimation of mechanical properties such as tensile strength, exural strength, impact strength and hardness is essential for the novel composite developed. The specimens are prepared as per ASTM standards with the help of a water jet cutting machine in order to avoid excessive pressure and delamination of bers. Tensile strength was determined as per ASTM D638. Samples are cut from all the six sides of the composite cube. The values obtained are averaged to obtain the tensile strength of the composite product. ASTM D 790 standards were followed for investigating the exural strength of the composites. Instron Universal testing machine was used for both the analysis. Shore D Hardness values were measured by taking the average of 8 different indentation points. The toughness of the composites was evaluated from the impact strength determined as per ASTM D256.
A Scanning Electron Microscope (SEM) is used to determine the bre-matrix adhesion and interaction.
TESCAN-VEGA3 machine was used for the characterization. JEOL-JFC 1600 auto ne coater is used for gold sputtering over the exposed surface. Sputtering enhances the electron interaction with the sample surface thereby initiating a conductive layer over a non-conducting polymer. Fractured surfaces were analysed to determine the bre-matrix adhesion and to investigate micro-macro cracks and void formation inside the matrix.
Experimental Modal Analysis (EMA) is conducted to determine the natural frequency of the composites. bers. The peaks indications in UTF and UPF between 500 and 750cm − 1 con rms the presence of cellulose which is absent in the spectra of treated bres. The reaction of natural bre (N.F*) with NaOH is expressed as [1] N.F* -OH + NaOH→N.F* -O − Na + + H2O + impurity Thermo Gravimetric Analysis (TGA) TGA is conducted using Perkin Elmer STA 6000 Thermo -Gravimetric Analyzer with a resolution of 0.1µg. The experiment is conducted in a controlled atmosphere containing nitrogen. Heating rate is selected as 10-degree Celcius per minute starting at room temperature to 950 o C. The nitrogen gas is supplied at the rate of 20ml/minute. An alumina crucible supported by a precision balance is used to hold the powdered specimen weights 5mg. TGA is done in order to characterise the materials by measuring the change in mass as a function of temperature. Treated and untreated ber specimens were investigated and TGA -DTG curve has been plotted as shown in the Fig. 3. The rst stage of degradation for Tampico bers ( Fig. 3a) was improved from 232 o C to 251 o C for NaoH treated bers. The second stage of degradation was found to be shifted to 362 o C from 343 o C for treated Tampico bers compared with untreated one. A similar hike in degradation temperature is observed for palymira ber also (Fig. 3c). The derivative thermogravimetric (DTG) curve plotted for Tampico and palymira bers are shown in Fig. 3 respectively. From the DTG curve it can be inferred that the degradation peak is obtained at a higher temperature for treated bers than untreated bers. This proves that the ber treatment done with NaOH was effective.

Mechanical Characterization
Effect of ber content on tensile strength Tensile properties of all the 12 composites were conducted as per ASTM D638 standards and compared with unreinforced LLDPE. From Fig. 4, it can be observed that Tampico ber composites showed better tensile properties than palymira bre-reinforced composites. It was found that by the addition of bers in the form of long bers, the tensile properties of TL3 composite was decreased to 14% as compared to TL1. At the same time the introduction of short bers enhanced the tensile strength of TS3 composite to 18.25MPa compared with 16.5 MPa of TS1. A similar trend was observed for palymira ber reinforced composites also. Tensile strength of PL3 is decreased by 19% compared to PL1, but the inverse effect was observed in the case of short palymira ber composites. During tensile deformation, the short bers were able to impart better adhesion with the LLDPE matrix, and the long bers shows debonding and poor interfacial properties with LLDPE matrix. Since the process in a low shear process with no external pressure is applied while manufacturing the composites, the void formations will be more as compared with other polymer processing techniques. The void formation inversely affects the tensile properties sine the crack formation is initiated through the voids.

Effect of ber content on Flexural strength
Flexural strength is the stress experienced in the material at the yield moment. It is measured as per ASTM D 790 and plotted in Fig. 5. Flexural strength shows a similar trend as that of tensile strength results. The reinforcement with short Tampico ber at 15wt% shows a percentage increase in exural strength of 22.5% whereas short Palymira ber at 15wt% gives an increment of 20%. Both TL3 and PL3 composites marked poor exural strength compared to other composites. While processing the composites, the long bers are randomly oriented and found to be agglomerated at the corners of the cubical mould. But the short bers were evenly distributed to all the sides of the mould. This is due to the process characteristics. Rotational moulding is a powder processing technology, and long bers incorporation reduces the slushing effect required for perfectly cooked mould. This is the reason for the reduction in exural strength when processing with long bers and an increase in strength when processing with short bers.
Effect of ber content on Impact Strength ASTM D 256 standards were followed for the investigation of impact strength of the composites. A signi cant hike in impact strength was inferred when comparing long and short ber composites, as shown in Fig. 6. On 5% ber addition, TS1 marked a percentage increase of 5.5% in impact strength than TL1. But on 15% ber addition, an increase of 65% in strength was observed. The same trend was observed in the case of Palymira bre-reinforced composites also. PS1 marked a percentage increase of 6% than PL1. But on 15% ber addition, the percentage hike improved to 41.3% for PS3 compared to PS1. The powdered LLDPE has better bonding with short bers than long bers thereby resulting in homogeneous reinforcement than long ber composites. The number of ber particles per square centimetre in extrados of mould for short ber composites is more compared with long ber composites. This is the reason for achieving high impact strength for short ber composites than long ber composites.

Effect of ber content on Hardness.
Hardness values of all the 12 composites were estimated and compared with unreinforced LLDPE are shown in Fig. 7. It can be inferred that by the addition of natural bers, the hardness values of the composites increased for both long and short ber reinforcement. Even though the percentage increase is very marginal, the hike in hardness values promises the surface properties are improved by the addition of natural bers. There is an improvement of 3.5 and 2.5 shore D points for TS3 and PS3 respectively compared with a hardness value of 58 obtained for Untreated LLDPE. The surface treatment done on bers were effective in improving the mechanical properties of the composites. But the hardness values depend on the surface quality of the product. In rotational moulding, the bubble formation is found on the surface of the composite product also. The void formation is more in rotational moulding process compared to other polymer processing techniques. No external pressure is applied to remove the air bubbles entrapped in this process. But it can be concluded from the hardness results that the surface void formation is less by the addition of natural bers.
The fractured surface of tensile test specimens was analysed morphologically in order to determine the bre-matrix adhesion and interaction. The SEM images of PL1, PS1, PL3 and PS3 samples are included as a,b,c and d respectively. In Fig. 8 (a) it is clearly visible that the ber and matrix bonding is improper, and the presence of voids and microvoids are also visible. But when considering the PS1 samples, the bre-matrix bonding is perfect. The bers surface seems to be peeled off during the application of tensile force and still the matrix ber adhesion seems to be perfect. This is the reason for improved mechanical strength observed for PS1 when compared with PL1. Figure 8 (c) & (d) depicts the images of fractured surfaces of PL3 and PS3 composites, respectively. The bre-matrix adhesion and bonding are better in PS3 composites where matrix crack, void formations etc. like defects are not visible at all. But the defects like ber agglomeration and debonding of bers can be observed in Fig. 8 (c) which represents PL3 composite. This is the reason for the decrement in mechanical properties obtained when long bers are further considered upto 15 wt %. Since rotomoulding is a powder processing technique, the bers can perform better when it is added in powdered form. This can be clearly inferred from the SEM images of PS1 and PS3 composites.
In the Fig. 9 (a), it can be observed that the matrix resin fails to penetrate into the bundle of bers which leads to the ber matrix debonding and nally results in poor mechanical properties. Even though the random ber dispersion is achieved for TL3 composites, the matix crack and void formations lead to a drastic reduction in mechanical strength when compared to TL1 and TL2 composites. The Fig. 9 (b) indicate the strong interfacial bonding of Tampico bers with LLDPE matrix, which results in improved mechanical properties of TS3 composites.

Vibration Damping characteristics
The frequency at which the composite material resonates is termed as its natural frequency. The aim of incorporating the bers in the present study is to enhance the natural frequency of the composites, thereby achieving better stiffness. The most commonly used experimental modal analysis technique is hammer testing method. Here the composite bar is excited with an impact hammer attached with force transducer at a number of points, and the response is measured at a single point. A fast fourier transform analyser is used to generate a frequency response function (FRF) as output. The peak indications in the FRF curve represents the natural frequency of the composites. Figure 10 represents the Frequency response curve of Tampico ber reinforced LLDPE compared with that of unreinforced LLDPE. As the percentage of bers increases, the natural frequency of the composites also found to be increased for both short and long ber composites. It is clear from the results that the length of bers is not signi cant, but the percentage of bers is the signi cant parameter. The natural frequency of TL3 composites has obtained an increase of 131% compared to unreinforced LLDPE, whereas TS3 composite shows an increment of 120% when compared to LLDPE. The results conclude that ber length plays an insigni cant role in EMA results for Tampico ber composites.
When considering the palymira ber composites, the results obtained was much better compared to that of Tampico ber composites results, as shown in Fig. 11. PL3 composites obtained an increase in natural frequency of 166% when compared to unreinforced LLDPE whereas PS3 composites could mark an increase of 135%. The results conclude that the ber, when included as long bers, shows better vibration damping properties than short ber composites. Also, Palymira bers are suggested as a potential vibration damping material for natural ber reinforced polymer composite structures produced by rotational moulding process.

Conclusions
The present work investigates the mouldability of LLDPE with palymira ber and Tampico ber as reinforcement and successfully incorporated these bers under three different weight percentages. The mechanical properties and vibration damping properties of unreinforced LLDPE is compared with short ber and long ber reinforced LLDPE, and the results are summarized as follows.
The FTIR results showed that the NaOH treatment done on bers were effective in removing the excess dirt and cellulose, thereby reducing the hydrophobic nature of bers. This results in improving the mechanical properties of the composites. The thermal degradation of both treated and untreated bers were examined using thermogravimetric analysis and the results shows that the degradation temperature can be effectively improved by proper NaOH treatment on bers. The tensile properties of the composites were improved by the addition of short bers. By increasing the ber content, the long bers nd di cult to adhere with the matrix, thereby decreasing the tensile strength. There is an increment of 30% in tensile strength reported for PS3 compared to PL3. In the case of Tampico ber reinforced composites, the TS3 composite shows an increase of 50% in tensile strength when compared with TL3 composite. Flexural properties also depict a similar trend as that of tensile properties. The PS3 composites obtained a exural strength of 17.96MPa which is 38% more than that of PL3 composites. The better adhesion of short bers with LLDPE matrix, as shown in morphological studies, helps to achieve this increment in exural properties. The TS3 composites have better exural strength than TL3 composites as an increment of 44% is observed in the results. The ber content and ber length have a signi cant role on the impact strength of the composites prepared by rotational moulding process. Short ber composites have better impact properties compared to that of long ber composites. By the addition of long bers from 5-15% by weight, the impact strength is found to be decreased in the case of long ber reinforcement and found to be improved in the case of short ber composites. The reinforcement as both short and long bers has no signi cant impact on the hardness values. Since the ber addition only up to 15wt% is considered for the study, the hardness values are found to be almost same as that of unreinforced LLDPE. The vibration damping experimentation results are most signi cant in the present study. There is a signi cant hike in the natural frequency of the composites considered for the study when compared to the unreinforced one. Thus, it can be suggested that the palymira and Tampico ber reinforced rotomoulded products are a better alternative for automotive-related parts since the natural frequency of the product is improved, which leads to minimal vibrations.

Declarations
Funding: No funding was received to assist with the preparation of this manuscript.

Con icts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

Availability of data and material
Any data related to the present work may be disclosed under request.
Code availability Not applicable Figure 9 The SEM images of the fractured surfaces of TL3 and TS3 composites after the tensile test. Effect of ber length and weight percentage on Natural frequency of the composites (a) Short Palymira ber and Unreinforced LLDPE (b) Long Palymira ber and Unreinforced LLDPE