STRUCTURAL, MECHANICAL AND THERMAL PROPERTIES OF LOW DENSITY POLYETHYLINE/BIOMASS COMPOSITE: EFFECTS OF PARTICLE SIZE

ABSTRACT This study focuses on the use of Alibizia Lebbeck Benth pod particles (ALBp) as reinforcement on low density polyethylene (LDPE). Composites were processed via casting where 408 and150 μm ALBp where used in reinforcing LDPE. Samples were subjected to Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC). Thermogravimetry Analysis (TGA) impact strength and tensile characterizations. Composites showed improved thermal stability and crystallinity compared to unreinforced LDPE. Tensile strength (UTS) of composite increased by 331% as it improved from 0.36MPa for unreinforced LDPE to 1.55MPa using ALBp of 150μm. Additional C=C and C-O-C groups observed on the composite’s spectrum could be responsible for the improvement in mechanical properties. Reinforcing LDPE with larger ALBp (408μm) culminated in the formation of gaps and voids in the composite.


INTRODUCTION
Development necessitates the use of different kinds of materials in areas such as housing, clothing, transportation, medical, defence, food, etc. Success in materials development has recently propitiated the creation of advanced materials which includes polymer matrix composites (PMCs). Composites consist of more than one materials merged to create a lone piece with improved features compared to that of its individual components if used alone.
Polymers are light in weight, adhere readily to other materials, flexible and easily processed to desired shapes due to their ease of flow at temperatures beyond its melting point (Sabu et al., 2012;Bilyeu et al., 2001). Often times, investigations affirm that mechanical properties of the matrix are being enhanced as a result of reinforcement inclusion which possesses better resistance to external loads (Adeosun et al., 2015). The reinforcement could either be fiber or particle. The particles could be spherical, platelets, or of any other regular or irregular geometry.
Particle -reinforced composites are less expensive than fiber reinforced composites and in addition, they usually require less reinforcement (up to 40 to 50 wt. %) due to processing difficulties and ease of fracture (Uygunoglu et al., 2012). Particlereinforced polymers have been found useful in manufacturing, electrical, commercial and aviation industries (Kim et al., 2004). Polymeric materials that have been used as matrix include synthetic polymers such as epoxy resins, polyethylene, polypropylene, unsaturated polyester 5 and biopolymers such as polylactide (Wan et al., 2004;Gupta et al., 2001;Shehu et al., 2014;Huang et al., 2013;Flandez et al., 2012;Adeosun et al., 2016). Fillers sourced from nature have been proven to yield nontoxic products and impart mechanical strength on plastics owing to their high stiffness (Ishidi, 2014) compared to the synthetic ones including carbon and glass fiber.
During the last few years, biomass from crops has been the main target in the search for new materials applicable in several industrial areas. These materials however, are ubiquitous and hence cheap to source, easily interact with matrix and offer good thermal properties (Raju et al., 2012). The filler used for this study is sourced from Albizzia lebbeck Benth (ALB). This tree is being regarded as "all purpose" tree and belongs to leguminosae family and widely distributed in Asia, South Africa, Australia and West Africa (Nazneen et al., 2012). The plant is often used in medicine because it possesses antimicrobial and antioxidant features (Shahid and Firdou, 2012). The use of this plant as a reinforcement in polymer has not been explored.
Pods of ALB often litter the environment when detached from the stalk (result of ripening) and are mostly being discarded. This study examines the potential of converting these perceived Kufa Journal of Engineering, Vol. 11, No. 2, April 2020 69 wastes to useful materials for engineering applications by investigating the influence of ALB pods particles on the mechanical properties of LDPE.

Materials
TASNEE LD 1925AS pellets with melt flow rate 1.9g/min and density of 0.925g/cm 3 was used for this experiment.

Preparation method
Dry pods of ALB were gathered and milled to particle sizes of 408 and150μm. Particle of 30 wt. % was measured as reinforcement contents introduced to the LDPE matrix. Materials were charged into the heating chamber of a compounding machine designed for this work. Thorough ALBp distribution in molten LDPE was achieved via stirring powered by electric motor. The mixture was poured into moulds after LDPE had reached its molten state.

Fourier Transform Infrared spectroscopy (FTIR)
Functional groups in matrix, reinforcement and composite were detected with the use of Nicolet 6700M spectrometer. Each sample of 10mg was compressed to pellets after being dispersed in KBr Spectra measurement in absorbance mode were processed at a resolution of 4 cm −1 between 500-4000 cm− 1 .

Water absorption
Dry samples were initially weighed and immersed in distilled water at 32˚C for 8 weeks. At the end of each week, soaked samples were separated from the medium, cleaned to remove surface moisture and weighed. The quantity (%) of the water absorbed by LDPE and composites in terms of weights measured were calculated using Equation 1.
Quantity of water absorbed, weights before and after immersion are represented by Abs, Wd and Ww respectively.

Thermal test
Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) were used in determining thermal characteristics of samples. With the use of a Mettler Toledo DSC equipment, samples were appropriately weighed and heated from 0 to 150 ºC at the rate of 10 ºC/min where flow of heat was plotted against temperature. Crystallinity (Xc) was calculated using Equation 2 (Chun et al., 2015): Samples' heat of fusion is given by Hf while that of fully crystalline LDPE (taken as 288 J /g) is represented by ∆H f o .
Samples assigned for TGA analysis were heated from 25ºC to 600ºC at a similar rate with that of DSC with the use of Shimadzu -DTG-60 equipment in a nitrogen atmosphere purged at 50 ml/min. Thermogravimetric curves were plotted from the results.

X-Ray Diffraction (XRD)
A PANanalytical Empyrean was used for this study and samples were exposed to a

Dhkl = kλ/ βcos θ 4
Indication of crystallite perfection is denoted by K, usually taken to be 1 while possesses a default magnitude of 1.5406. The equation above calculates the crystallite sizes of samples.

Tensile test
Tensile specimens were prepared and tested in accordance with ASTM D412 with the use of an Instron Tensometer. Each sample was fixed and held firmly at both ends by the gauge and load was applied at 10 mm/min at room temperature until the sample finally failed. The tensile strength (UTS) and extent of deformation response were measured from the stress-strain results recorded.

Impact test
An impact testing equipment (Izod) was used in determining the shock absorbing strength of samples in conformity with ASTM D256.

Scanning Electron Microscopy (SEM)
Samples to be investigated for morphological studies were coated win Au to enhance proper electrical conductivity which were scanned using a Phenom Prox. 800-7334 model SEM.

FTIR
The FTIR spectrum of ALBp in Fig. 1a . 1b) with that of LDPE, the OH group occupies a wider region at 3422cm -1 which could be as a result of cellulose and hemicellulose present in ALBp. Additional peaks of 1644 (C=C) and 1057cm -1 (C-O-C) will impart improved mechanical strength on the composite.

Water absorption
Amount of water absorbed by all samples during 8 weeks of immersion illustrated in Fig. 2 as it produces a wider contact area for water molecules. Poor wettability promoted by these large particles may have also encouraged penetration of water molecules through the composites' structures.

TGA
A single step decomposition of LDPE is observed within 453 -477 o C in the TGA and DTG curves shown in Fig. 3a (408µm) composites respectively (Fig. 3b). This further justifies a slight improvement in the thermal stability of LDPE/ALBp composites which could be attributed to the residue formed during the heating process of ALBp which serves as barrier that prevents the thermal degradation of composites. Table 1 shows the Tonset, Tfinish Tmax and residue content obtained from the thermal investigation.

XRD
The diffractogram of ALBp exhibits a typical crystalline structure of native cellulose (Fig. 5a), where peak at 2θ = 22 o corresponding to (002)  Where A020, A040 and Aamr represent areas under the crystalline (for (002) and (040)) and amorphous peaks respectively. The Xc is calculated to be 47.1% which is comparable to that obtained using the peak intensity method. This gives the amount of crystallinity of cellulose that can be obtained in ALB pods in an untreated state.
The XRD patterns of unreinforced LDPE and 70LDPE/30ALBp composites are shown in Fig.   5b with two distinct peaks. Each sample exhibits typical diffraction peak of LDPE where the stronger peak (representing (110)  with ALBp engenders narrower peaks with greater intensity compared to unreinforced LDPE diffraction peaks. This is an indication of improved crystallinity which is calculated from DSC results in this study (see Fig. 4). Comparing the two composites, reinforcement with a finer ALBp of 150µm shows much narrow diffraction patterns than that reinforced with 408µm ALBp. It can thus be said that the presence of cellulose in ALB enhances the molecular chain arrangement of LDPE which is better improved with a finer one. The improved crystalline sizes (increase in magnitude) of 70LDPE/30ALBp composite compared to unreinforced LDPE suggests that Xc of composites should be higher with 70LDPE/30 ALBp (150µm) possessing the highest of the three samples. This is illustrated in Table 3.

Tensile test
The experimental results in Fig. 6 reveal that the reinforcement used have significantly created strengthening effect on LDPE, which is responsible for the steady increase in UTS compared Unreinforced LDPE 70LDPE/30ALBp (150µm) 70LDPE/30ALBp (408µm) to the unreinforced sample (100 wt. % LDPE). This property improves from an initial strength of 0.36MPa in unreinforced LDPE to 1.07 and 1.55MPa when reinforced with 408 and 150µm ALBp respectively. There is a sudden drop in stress of the two composites after UTS have been exceeded. The volume fraction of the reinforcement (which is less elastic than the matrix) used in this study has acted as a rigid constituent which obstructs the mobility of crazes. Gradual load increase is thus required during the deformation process until the maximum strength is reached. Beyond this point, there will be matrix/filler de-bonding caused by reduction in inter particle spacing which must have led to such drop. Effect of the ALBp on strain at break, whose percentage is a function of samples' ductility, is also illustrated in Fig 6. Ductility of 70LDPE/ 30ALBp composites is lower than unreinforced LDPE, measured to be 9.5%. Addition of particulates may have caused the matrix to lose its elastic properties. Incorporation of 30 wt. % of 150 and 408µm ALBp into LDPE matrix both reduce the elongation at break to 8.9%. It can be concluded that the reinforcement acts a rigid constituent which obstructs the mobility of craze during the deformation process. Ductility of LDPE is reduced to the same magnitude on addition of ALBp irrespective of the different particle sizes.

Impact test
The energy absorbed on sudden load application on samples is shown in Fig. 7 respectively. This implies that the finer the ALBp the better they act as terminator of craze, which will contribute to the improvement of composites' impact strengths. Fig. 7. Impact strengths of LDPE and 70LDPE/ 30ALBp composites at 150 and 408 µm particle sizes.

SEM
A defect free micrograph of 100 wt. % LDPE fractured surface is shown in Fig. 8a . 8b) with even distribution of reinforcement in its structure devoid of particles detachment. The good 70LDPE/30ALBp (150µm) interfacial bonding is accountable for its superlative UTS. Existence of gaps and cavities as a result of particle detachment after fracture is shown in Fig. 8c. This shows that interfacial adhesion between the LDPE and 408µm ALBp reinforcement is poor.
These defects are initiators of cracks in LDPE which have culminated in their lowest magnitudes of impact energies when compared to LDPE reinforced with 150µm ALBp. Fig.   9a, b and c shows the fractured morphologies of samples after the immersion period. Bump-

CONCLUSION
This study has revealed other areas where ALB can be used asides its medicinal potentials.
Additional peaks of 1644 (C=C) and 1057cm -1 (C-O-C) have imparted improved mechanical properties on LDPE with 150µm ALBp particles offering the best reinforcing effect. Poor matrix/reinforcement bonding is witnessed when 408µm ALBp are used as reinforcement. This leads to existence of cavities and voids thus imparting a less mechanical strengthening effect compared to LDPE reinforced with 150µm ALBp. Thermal stability and Xc are improved with ALBp addition, 70LDPE/30ALBp (150µm) possessing the maximum values. In summary, the finer the ALBp, the better they serve as good reinforcements on LDPE.