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Abstract

This work is devoted to investigate the available agricultural Tunisian waste: the date pits as reinforcing filler for thermoplastic matrix. The chemical composition of this reinforcing filler is found to be comparable to nonwood plants: its content comprises of 13% of extractibles, 22% of lignin, and 61% of holocellulose. Then, the lignocellulosic filler was used to prepare different composites films using Brabender mixing device. A series of composite film was established by different loadings of the date pits waste with 10–50% of the filler in 10% as an interval. The ensuing composites materials were then characterized by several techniques such as the morphology of the composites, which was investigated using scanning electron microscopy. The thermal properties of prepared materials were studied using differential scanning calorimetry and thermogravimetric analysis. Finally, the mechanical and water absorption properties were involved. The obtained results indicated that date pits–based particles enhanced the thermomechanical properties of the thermoplastic matrix and demonstrated that this available lignocellulosic biomass can be considered to be a promising filler material.

Keywords

Date pits LDPE composite Brabender machine mechanical properties thermal properties

Introduction

The innovation of eco-materials and the term sustainable development are increasingly mentioned. The concept of sustainable development requires consideration of environmental issues. It is in this context that the study seems to be more attractive and interesting. It is suggested to use materials from biomass materials for the future so that after their uses, they rapidly degrade naturally. Over the last decade, there has been a growing interest in the use of lignocellulosic fibers or particles as reinforcing elements in polymeric matrices.^1,2 A large number of researchers have focused in investigating the examination of lignocellulosic fibers as load-bearing constituents in composite materials. Compared to inorganic fillers, the main advantage of lignocellulosic are their low cost, low density, renewability, biodegradability, abundant availability in variety of forms through art the wood, and so on.^3

–7

In many industries, structural and household supplies polymer-based materials are playing an important role.⁸ However, these materials lack sufficient stiffness and are highly susceptible to creep, especially at elevated temperatures. One way to improve these properties is to combine the polymer matrix with filler or a reinforcing fiber.^1,9 The resulting materials are called composites, which possess enhanced physical and mechanical properties. Traditionally synthetic fibers have been used to reinforce the polymer-based products.¹⁰ Problems frequently encountered in preparing composites from wood fiber or lignocellulosic particle are inadequate dispersion and bounding element between fiber and polymer matrix, moisture absorption by natural fibers, and nonuniform mechanical and physical properties of natural fibers.¹¹ Nowadays, the date pits (Figure 1) have been used as natural fillers to develop thermoplastic-based composites. Important quantities of date stone accumulated every year in the south of Tunisia. The date pits represent about 10% of the weight of the fruit produced each year in Tunisia.¹² Date pits are by-products of some date-processing industries. The pits generally have a specific use; sometimes they are utilized as a soil organic additive or as feed for various livestock.^13,14 They could be suitable biosorbents in wastewater under specific conditions.¹⁵ The pits date contain about 8.6–17.7% moisture,¹⁶ 73–84% carbohydrates,^17,18 5–13.5% oil,^19,20 and 1.5% ash.²⁰

Figure 1.

Agricultural waste from date palm: Date pits.

Preliminary study on using date pits as filling additive for production material composite indicated that, in comparison with some lignocellulosic residues such as pistachio shell flour and flax fiber, the date pit fillers had significantly higher melt flow index.²¹ This behavior was attributed to the effect of coupling agent during the production of composite.^6,7 To the best of our knowledge, no data regarding the use of date pits (without any fractionation and any additional purification treatment will induce extra costs and will make meaningless the full approach of valorization) as a filler material for thermoplastic-based composite using low-density polyethylene (LDPE) as matrix have been reported in the literature. This idea is not only limit to Tunisia but it can be benefiting all the countries, which present large fields oases. Furthermore, LDPE is one of the most commonly used thermoplastic to develop composites reinforced by different natural materials (cellulose,²² lignin,²³ starch,²⁴ etc.). Therefore, LDPE was chosen as the model base polymer matrix to prepare date stone-filled composites. In various stages of composites production, especially during molding, the blends are heated to increase the flow of the material. Softening or melting of a blend is necessary for ease of processibility and product forming; meanwhile, overheating causes permanent damage to the processed materials and reduces the quality of the final products.²⁵ Thus, knowledge of thermal properties of the material is crucial for an optimum design and process monitoring for manufacturing composite products. Moreover, strength and toughness are very important for many composite materials. The mechanical properties of composites depend on the factors such as raw material selection, formulation, manufacturing technique, and processing parameters. The interfacial adhesion between the fillers and matrix is an important factor that determines the strength and toughness of the resulting composites.²⁶ Particle size distribution of the fiber or particles and the blend proportion also significantly influence the ultimate tensile properties. Understanding the mechanical properties of composites is crucial for their proper fabrication and applications. In this study, a step forward in the valorization of date pits was presented. The main objective of this work is to prepare several composites with different proportion with LDPE and date pits using Brabender mixing device (Germany). Moreover, the effect of date pits’ weight on the process ability, mechanical, absorption water capacity and thermal stability was systematically studied.

Materials and methods

Preparation of the date pits

The date was supplied by Horchani date in Tozeur Tunisia during the month of March 2014. The pits were firstly cleaned and washed in order to eliminate external impurities and then were oven dried at 75°C during 24 h. The ensuing materials were milled and sieved. The particle size distribution of date pits fragments was investigated by laser diffraction using a Mastersizer Malvern (UK) and the granulometry was found to be around 50 μm.

Chemical composition of date pits

The analysis of chemical composition of the date pits was determined by measuring the contents of holocellulose α-cellulose, Kalson lignin, solvent extractives as well as the ash using different standard methods. Before the analysis, the fragments of studied material were milled and sieved to 40–60 mesh. As there are several standard methods used to determine the chemical composition of the studied lignocellulosic biomass, since we think that they are not always familiar, we decided to describe them very briefly.

Solvent extractives of wood and pulp, test method T 204 cm-07: this test determines the amount of solvent-soluble, nonvolatile compounds in the present raw material. The method consists of an extraction step that was conducted for 6 h in a toluene/ethanol mixture (62/38, v/v) using a Soxhlet extractor. The proposed method was found to give the highest amount of extractives owing to the additional dissolution of low molecular weight carbohydrates and polyphenols.

Ash in wood and annual plants (test method T 211 om-07): The sample was placed in a ceramic crucible mineralized at 525 ± 25°C for 4 h. The ash content is calculated by weighting the residue remaining after the mineralized process.

α-, β-, and γ-Cellulose in pulp (test method T 203 cm-99): α-Cellulose is the pulp fraction which resistant to 17.5% and 9.45% sodium hydroxide solutions under conditions of the test. β-Cellulose is the soluble fraction, which is precipitated on acidification of the solution. Concretely, at 25°C, the pulp is extracted consecutively with 17.5% and 9.45% sodium hydroxide solutions. The soluble fraction, consisting of β- and γ-celluloses, is determined volumetrically by oxidation with potassium dichromate, and the α-cellulose, as an insoluble fraction, is derived by difference.

Holocellulose²⁷: The date pits fibers were introduced into a flask containing distilled water (200 mL). Subsequently, 1.5 g of sodium chlorite and 0.5 mL of glacial acetic acid were added. The reaction mixture was heated at 80°C during 1 h. The bleaching process was repeated more time (at least three times) until obtained white fibers.

Kalson lignin (test method T 222 om-06): The lignin content was determined by the Klason method. In fact, a dry fiber (1 g) was suspended in a 72% sulfuric acid solution (15 mL) for 2 h at room temperature under stirring. The hydrolysis is continued even after dilution to 3% for 4 h. Afterward, the precipitate was filtered and washed thoroughly with hot water in order to remove residual acid. Each experimental test was carried out, at least in duplicate and the difference between the various values obtained was within an experimental error of 5%.

Matrix

LDPE was used as a matrix of the prepared composite film with lignocellulosic particle. The LDPE (PB-608) supplied by Braskem has a density of 0.915 g cm⁻³ and a melt flow index (160°C/2.16 kg) of 3 g min⁻¹. All chemicals were purchased from Sigma-Aldrich (St Louis, Missouri, USA) and were used without further purification.

Preparation of the composite films based on date pits and LDPE

Composite films at different loadings, namely, 0, 10, 20, 30, 40, and 50% (w/w with respect to the matrix) were prepared by mixing the thermoplastic matrix (LDPE) with date pits. The condition of mixture was established using a Brabender machine (with the characteristics of L/D ratio and diameter used of 30:1 and ¾″, respectively), at 50 r/min and 160°C for 10 min. After mixing, the prepared samples were molded by hot pressing in a hydraulic press (at 150°C, under 10 tons of pressure for 5 min) using suitable steel, yielding rectangular films (10 × 5 cm²) of thickness of 0.5 mm. After that, the prepared samples were left to cool down at room temperature (23 ± 2°C), and the composite films were recovered when the temperature was between 40°C and 30°C.

Characterization

Morphology of the prepared films

A Quanta 200 environmental scanning electron microscope (FEI, Hillsboro, Oregon, USA) was used in order to study the analysis morphology of the different composites samples. Before the analysis, each sample was frozen under liquid nitrogen for 5–7 min and later fractured. The fractured surface was subsequently coated with a gold/palladium layer to prepare the sample for observation using the scanning electron microscope.

Thermal analysis differential scanning calorimetry

The thermal properties of the samples were studied by differential scanning calorimetry (DSC) and a thermogravimetric analyzer.

DSC measurements were performed on a DSC Q100 (TA Instruments New Castle, DE, USA) (DSC¹ STAR^e System). For each prepared composite, about 5–10 mg of the sample was placed in hermetically closed DSC capsules. Each sample was heated from −50°C to 250°C under a heating rate of 10°C min⁻¹. The melting temperature (T _m) and the enthalpy of fusion (ΔH _m) of the sample were determined. The crystallinity degree (χ _c) of prepared composite films was then calculated using the equation (1):

% x_{c} = 100 \frac{Δ H_{mi}}{Wf \times Δ H_{100 % cristallirity}^{°}}

where, ΔH _mi is the fusion enthalpy of prepared composite (ΔH _m, (J g⁻¹)) determined by DSC, ΔH_100% crystallinity is the fusion enthalpy of LDPE : 285 J g⁻¹ ²⁸, and W _f is the fraction of lignocellulosic material weight present in the composite.

Thermal behaviors of composites were also examined using a thermo gravimetric analyzer (TA Instruments Q50, New Castle, Delaware, USA). Each composite was heated from 0 to 800°C, at a rate of 10°C min⁻¹. Thermal decomposition temperatures of the composites were examined under 20 mL min⁻¹ of nitrogen using platinum pans. Each experimental test was performed at least in duplicate.

Mechanical properties

The mechanical properties of the prepared samples were established by the dynamic mechanical analysis (DMA) in the tensile mode. The measurements of the prepared composite films were carried out using an RSA 3 DMA apparatus from Rheometrics (Piscataway, New Jersey, USA). During this experiment, the films were cut into thin rectangular strips with dimensions of 30 × 5 × 0.4 mm³. The measurements were performed under isochronal conditions at 1 Hz, and the temperature-scanning interval varied from −90°C to 100°C at a heating rate of 2°C min⁻¹. These measurements were repeated at least in triplicate.

Water absorption

The water absorption of the prepared composites was established using the procedure described by Mansouri et al. and Abdelmouleh et al.^29,30 The prepared films were cut into small samples with dimensions of 1 × 1 cm² × 0.5 mm. The ensuing samples were weighed and then soaked in distilled water at 25°C for different periods. Next, the samples were removed from the blotted to remove the excess water on the surface and then immediately weighed. The difference between the mass after a given time of immersion and the initial mass is used to determine the water absorption. The water uptake and the diffusion coefficient behavior were evaluated for the different samples using equations (2) ^31,32 and (3), respectively:

WU (%) = 100 \times (\frac{M_{t} - M_{0}}{M_{0}})

D = \frac{π}{16} \times {(\frac{\frac{Mt}{M \infty}}{\frac{\sqrt{t}}{h}})}^{2}

where, WU is the amount of water absorbed, M _t is the mass of the sample soaked for time t, and M ₀ is the dry mass. D is the diffusion coefficient, M _∞ is the maximum water uptake, M _t is the water uptake at a time t, which is the equilibrium time, and h is sample thickness.

Results and discussions

Chemical composition of date pits

The natural fiber-reinforced polymer composites performance depends on several factors, including fibers chemical composition, cell dimensions, structure, physical properties, and also the interaction of a fiber with the polymer. In order to evaluate the application of natural fibers for composites and improve their performance, it is essential to know the fiber characteristics.^33,34 The chemical characterization of obtained particles from date pits was established and the results were reported in the Table 1.

Table 1.

Chemical composition of date pits.

References	Cold water	Hot water	Solvent extractives solubility	Ash	Kalson lignin	Holocellulose	α-cellulose
This work	22.2	27	13.1	2	22.6	61	41
¹¹	–	–	10.7	1.2	–	–	–
¹²	–	–	10.2–12.7	1.1–1.2	–	–	–
¹³	–	–	5.2–7.2	1.3–3.2	–	–	15.2–19.3

From this table, we could conclude that the amounts of extractives in water (cold, hot) are slightly higher (or/lower) when compared with wood which around 6%.

The ethanol-toluene extractives, ashes, holocellulose, and α-cellulose contents are comparable to those of other annual plants or agricultural crops.³⁵ The obtained value demonstrated clearly that the polysaccharide content is close to that associated with nonwood materials, which justified their valorization in such biodegradable composites applications. In addition, the Klason lignin was found to be relatively lower (around 22%). On the opposite, ash content in date pits (2%) is lower but remains at the same order of magnitude as most of the nonwood plants.^36,37

Characterization of the composite films based on date pits and LDPE

Different composites films based on LDPE filled with date pits fragments were successfully prepared with different homogenous films of up to 50 wt%. Thus were confirmed by morphological analyses, which explain clearly the filler dispersion within the matrix and the interfacial adhesion between the two components (filler and matrix). The results of the scanning electron microscopy (SEM) analysis of fractured surface of the composites films were performed in Figure 2. From this figure, the homogeneity of the composite material was demonstrated even with a high loading percentage of date pits particles (50 wt%). Subsequently a good dispersion and strong interfacial adhesion were clearly observed. These features of homogeneity can also be justified by the absence of holes or cracks inside the prepared films. The idea here is the use of the entire material without any fractionation, because we are dealing with the valorization of a waste. Any additional purification step will induce extra costs and will make meaningless the full approach. Nevertheless, it is worth to mention that the grinding treatments cannot break the noncellulosic material from cellulose species such as hemicelluloses and lignin induce though the surface specific of obtained particles it can be increased and consequently increases a physical adhesion can be observed between matrix and the filler. During the second part of this work, we focused on examining the characterizations of the thermal and mechanical properties of ensuing materials.

Figure 2.

SEM images of cross sections and surfaces of composites with different amounts of date pits fillers (a) 0%, (b) 10%, (c) 20%, (d) 30%, (e) 40%, and (f) 50%. SEM: scanning electron microscopy.

Thermal properties

DSC and thermogravimetric analysis (TGA) investigated the thermal properties of prepared composites. The analyses of DSC were established and the results are described in Figure 3 and Table 2.

Figure 3.

DSC tracings of all the composite films. DSC: differential scanning calorimetry.

Table 2.

Thermal properties of LDPE-based composites reinforced with date pits.

R/M	T _m (°C)	ΔH _m (J g⁻¹)	ΔH _mc (J g⁻¹ PELD)	χ _c (%)
Neat matrix	107	97.9	97.9	34.35
10 wt%	107	90.7	100.77	35.36
20 wt%	109	80.2	100.25	35.18
30 wt%	109	70.3	100.42	35.24
40 wt%	108	60.3	100.5	35.26
50 wt%	108	50.4	100.8	35.37

R/M: %raw material/% LDPE; T _m: melting temperature; ΔH _m: the fusion enthalpy; ΔH _mc: the fusion enthalpy of prepared composite determined by DSC; χc: cristallinity; DSC: differential scanning calorimetry.

Figure 3 shows obviously that the date pits particle with increasing slightly content into the melting point as well as the glass transition temperatures remained constant. This behavior is quite interesting and useful in practice because the produced films retain their flexibility even for the highest filler loading investigated here.³⁷ In addition, the effect of added particles into crystallinity degree of the thermoplastic matrix-based composites was established and the result was summarized in Table 2.

It can be indicated that the crystallinity of the prepared composites increases slightly from virgin film of PET and the film prepared with 10 wt% filler (date pits particles, w/w with respect to the matrix). From this value, the crystallinity of prepared film remains constant and equal to 35%. The phenomena can be observed as reported by Mansouri et al.²⁹ and Khiari et al.²⁸, which is attributed to the presence of crystalline regions in the matrix of LDPE which is influenced by the added fraction of the lingoparticles. Thus, it can be also attributed to the chemical composition of filler that this proved the lack of chemical bond between the matrix and the filler. The T _m of composite showed no significant difference. No effect of filler on LDPE melting was also observed.²

TGA of prepared film was also investigated by the TGA and the result is presented in the Figure 4. It can be shown that LDPE the thermal degradation occurred in a single stage at around 400°C. Compared with the pure sample, the samples reinforced with date pits showed the kinetic of degradation was slightly lower which subsequently a higher thermal stability can be observed. This is due to the higher and longer thermal resistance of the lignocellulosic particles.³⁵ From the figure, the virgin film with LDPE and 10 wt% presents the same curves. Thus, means clearly that the addition of particles date pits until 10% (w/w) present no effect. This result also in accordance with those find in the DSC, prove that the crystallinity degree remains constant. However, the prepared films up 10 wt% of loading particles present different phenomena and phases. Thus, it can be explained by the effect of adding the lignocellulosic particles. Thus, several phases were showed when increasing the temperature: (i) T <200°C: This step consists of drying phase, the residual moisture is removed; (ii) when the temperatures are between 350°C and 400°C, the hemicelluloses (the most thermally unstable compounds) degrade. After this temperature, it can be remarked that changing the slope of the curve indicates a change in chemical kinetics, (iii) when the temperatures are between 400°C and 450°C, the degradation of cellulose was taken up; (iv) when the temperature exceed the 450°C, lignin degradation was established and the kinetics of degradation is slower than the other compounds; however, this occurs over a wide temperature range, which is not easily detectable by ATG: TGA-STA 6000 (Perkin Elmer Instruments, Buckinghamshire, England); and (i–v) T≈600°C the end of the degradation phenomena.

Figure 4.

TGA curves for LDPE-based composites reinforced with date pits particles. TGA: thermogravimetric analysis; LDPE: low-density polyethylene.

Mechanical properties of the prepared materials

The DMA of the composite films filled with the different lignocellulosic particles contents (0, 10, 20, 30, 40, and 50% w/w) was also accomplished. The results on the evolution of the storage modulus (log E′) and of the loss factor (tan δ) against the temperature, in isochronal conditions at a frequency of 1 Hz, is presented in Figure 5(a) and (b), respectively.

Figure 5.

Evolution (a) of the logarithm of the E′ and (b) tan (δ) vs. temperature at 1 Hz for LDPE-based composites reinforced with date pits particles. LDPE: low-density polyethylene; E′: storage tensile modulus.

The curve of log E′ obtained for the neat matrix LDPE (Figure 5) is typical to a fully amorphous thermoplastic polymer. Below the glass transition temperature (T _g), that is, −20°C, the LDPE matrix is in its glassy state, with a storage modulus of approximately 1 GPa; it starts decreasing slightly with temperature, but remains roughly constant. A rapid and significant decrease in the storage modulus was observed at approximately −20°C (T _g), corresponding to the relaxation behavior associated with the transition of the matrix from the glassy state to the rubbery state. However, above the T _g, the presence of date pits fragments induced a significant mechanical reinforcement.

The log E′ curves of the composite films (Figure 5(a)), in turn, seemed to be normalized with that of the matrix at low temperatures, revealing that below the T _g the difference of modulus between the cellulose and the matrix was not important enough to induce a significant reinforcing effect. However, above the T _g, the presence of date pits fragments conducted to a significant mechanical reinforcement. Indeed, the storage modulus of the composite materials containing 10, 20, 30, 40, and 50 wt% of particles (Table 3) at room temperature (25°C), were 7.94, 8.03, 8.09, 8.13, and 8.19 MPa, respectively. Such an increase is relatively significant when compared to that of the pure matrix (7.72 MPa). This behavior can be attributed to the incorporation of the lignocellulosic fillers, which reinforced the matrix and yielded a homogeneous and continuous structure of the final composite films.³⁷ In fact, as previously discuss by Faruk et al. (2012)³⁴ that increasing fiber content in the composites increases the composite’s stiffness significantly. Additionally its strength is increased through the addition of natural fibers. Higher fiber content improves the impact strength and unfortunately increases the water uptake. Moreover, the composite’s ductility can be affected. The fiber length and its geometry also play a decisive role in composites. Usually, most mechanical properties of a fiber can be enhanced by increasing the aspect ratio. Besides, this high reinforcing effect could be attributed to a phenomenon of mechanical percolation cellulose particles forming a rigid continuous network of particles bound by hydrogen bonds.²⁸

Table 3.

DMA properties of LDPE-based composites reinforced with date pits.

	Neat matrix	10 wt%	20 wt%	30 wt%	40 wt%	50 wt%
T _f (°C)	36.82	54.43	57.99	63.6	67.34	72.34
E′_25°C (MPa)	7.72	7.94	8.03	8.09	8.13	8.19

T _f: flow Temperature; E′_25°C (MPa): storage modulus at 25°C; DMA: dynamic mechanical analysis; LDPE: low-density polyethylene.

Moreover, Figure 5(a) makes (d) conclude that above the T _g, the fillers extended significantly the rubbery plateau of the composites, as confirmed by the increase of the flow temperature from 37°C to 72°C the melting temperature (T _f).

Figure 5(b) shows the evolution of the tan (δ) versus temperature at 1 Hz for the different LDPE-based composites. From this figure, we could observe the presence of two relaxations located at approximately between −18 and −20°C (denoted α) and 60−65°C (denoted αc). These phenomena are related to the mobility of the polymer chains and the molecular motion in the crystalline phase. As expected, this result was observed by many research studies^37
–39 and justified the interpretation that the transition αc (60–65°C) can be attributed to the relaxation process corresponding to the rearrangements associated with the crystalline phase. Especially for the composite film with 10 wt% of lignocellulosic particles, it can be observed that the curve shifted toward higher temperatures, which was attributed to a good interaction between lignocellulosic particles and matrix, corroborated by the SEM results presented above. The overall conclusion to be deduced from this part; that despite the quite difference of mechanical performance of date pits fragments when compared to some annual plants as well as the wood, this biomass remains an interesting source and comparable with those of annual plant such as Posidonia oceanica and vine stem fragment’s, which proves also their advantages for use as new filler based the thermoplastic materials.^28,29

Absorption properties of the composite materials

The water absorption test can provide information about the adhesion between the lignocellulosic particles and the matrix in the interface region, because the higher the adhesion at the interface region the fewer the number of sites that could store water molecules and, consequently, the lower the water absorption. It is very well known that the water sensitivity of the composite based on lignocellulosic fillers constitutes the main issue that hampers the development of these class of composites, namely, in a damp atmosphere where the contact with water is likely to adversely affect their mechanical performance. The higher water sensitivity is the consequence not only of the inherent hydrophilic character of the filler but also of the presence of voids and holes at the interfacial area between the filler and the matrix. The relative mass gain for the samples increased with increasing the immersion time in the distilled water till reach it to saturation moisture mass (m _∞) was established and the results are shown in Figure 6 and Table 4.

Figure 6.

Water uptake versus immersion time at 25°C for LDPE-based composites reinforced with date pits particles. LDPE: low-density polyethylene.

Table 4.

Water absorption and diffusion coefficient of LDPE-based composites reinforced with date pits.

	Neat matrix	10 wt%	20 wt%	30 wt%	40 wt%	50 wt%
WU (%)	0.81	6.34	13.89	16.05	23.06	24.95
D (mm² s⁻¹) × 10⁻¹³	0.34	2900	3228	3436	3461	3489

LDPE: low-density polyethylene; WU: amount of water absorbed; D: diffusion coefficient.

Moisture diffusion in polymeric composites has shown to be governed by three different mechanisms. The first involves diffusion of water molecules inside the micro gaps between polymer chains. The second involves capillary transport into the gaps and flaws at the interfaces between lignocellulosic particles and the matrix. The third involves transport of micro-cracks in the matrix arising from the swelling of fibers.⁴⁰ The water may occupy preexisting space within the host material. This would be characterized by an increase in total mass with no overall volume change. Assuming that the preexisting space was occupied by gas (e.g. atmospheric gas) this would result in an increase in density and no swelling.

Water uptake under the immersion condition of composites versus time at different date pits loading is varied following the amount of added lignocellulosic particles. However, all the prepared composites film excepted the virgin (LDPE film’s), the water content M_t increased rapidly at the first stage of absorption and then gradually slowed down until saturation plateau was attained after about 2 months immersion in water. The equilibrium water absorption M _∞ increased with date pits particles content, which is in accordance with the results noted for other lignocellulosic fillers.^29,30

The diffusion coefficient (D) of water in the different composites was determined using the equation described by Alamri and Low.³⁹ The diffusion coefficient of water (D) and water uptake at equilibrium (WU) increased when increasing the lignocellulosic particles content. From the obtained result of the diffusion coefficient of water, as well as the water uptake, the film prepared up to 20 wt% of cellulosic fragments should be modified to decrease the amount of water absorption.^29,31 This is not surprising since the lignocellulosic particles are highly sensible to the water which per consequence is a limiting factor in the use of such prepared films especially if low water absorption is desired.

Conclusion

During this study, the chemical composition of Tunisian date pits was established. The ensuing data confirmed that the date pits contain high amount of cellulose, which justifies their valorization in cellulose derivatives or as a source of fibers for cellulose fiber-reinforced composites or in papermaking applications. The obtained particles fragments of date pits, after milling and sieving, was investigated to be used as reinforcing elements in cellulose-based composite materials. Different composites were successfully prepared with various amounts of raw materials from 0 to 50 wt% with interval of 10 wt%. The homogeneity of the composite material was demonstrated even with a high loading percentage of date pits particles (50 wt%). These features of homogeneity can also be justified by the absence of holes or cracks inside the prepared films. The analysis by DSC and TGA demonstrated that the films composites present good thermal properties, which justified the obtained degree of crystallinity, melting, and glass transition temperatures. Moreover, this paper is an evidence of the mechanical properties of ensuing samples and seems to be interesting when observing the obtained data got from Young Modulus.

Footnotes

Acknowledgments

The authors thank Bertine Khelifi (LGP2, Saint-Martin-d’Hères, France) and Pierre Sailler (CERMAV, Grenoble) for their help and availability. The authors also gratefully express their sincere gratitude to Prof. Mohamed Naceur BELGACEM, Director of PAGORA-INP Grenoble, for his help and availability.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Composite materials based on low-density polyethylene loaded with date pits

Abstract

Keywords

Introduction

Materials and methods

Preparation of the date pits

Chemical composition of date pits

Matrix

Preparation of the composite films based on date pits and LDPE

Characterization

Morphology of the prepared films

Thermal analysis differential scanning calorimetry

Mechanical properties

Water absorption

Results and discussions

Chemical composition of date pits

Characterization of the composite films based on date pits and LDPE

Thermal properties

Mechanical properties of the prepared materials

Absorption properties of the composite materials

Conclusion

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Funding

References