A strong increase in environmental concerns has arisen in the last years. This fact, together with the continuous increase in petroleum prices and the overall depletion of fossil fuels, has encouraged researchers to develop new environmentally friendly materials  . One of the engineering fields that have experienced a more valuable growth is that related to composite materials with natural fillers/reinforcements such as natural fiber reinforced plastics  . The use of natural fillers/reinforcements into polymeric matrices could lead to multiple advantages, such as clear cost reduction, lightness and good balance on mechanical properties, and a marked low environmental impact as well, due to their abundance, availability, low density and high specific strength compared to synthetic fibers . For these reasons, some technological sectors such as aerospace, automotive, building among others and other sectors, such as packaging, have shown a clear interest on these materials    . This study focuses on the characterization of fibers from coffee shell (FCS) which was extracted from waste coffee hulls using alkaline treatment method. Waste coffee hulls are abundantly available at the end of each coffee season. It’s either burned as waste or used as fertilizer due to its excellent biodegradable properties    . This material, considered as waste, can be used as reinforcement of thermoplastic like linear low density polyethylene. The objective of this work is to develop new coffee hull fibers-based polyethylene composites with mechanical properties similar to the virgin LLDPE. The composites were characterized by mechanical properties.
Linear Low Density Polyethylene (LLDPE) is a type of plastic widely used in industrial products and household goods and especially as a matrix material in composites, due to low production cost, design flexibility, and recyclability, compared with other polymers. Other potential properties of LLDPE include heat distortion temperature, flame retardant, transparency, and dimensional stability. Besides, LLDPE also suits filling, reinforcing and blending . This research is aimed at valorizing these waste coffee hulls, by using it to reinforce LLDPE, thereby giving it an added value and increasing farmer’s incomes. Farmers will no more grow coffee only for their seed but also for their hulls.
2. Materials and Methods
In this study, we used the shells pulp from coffee Canephora and as matrix, granulate of Linear Low Density Polyethylene LLDPE granules obtained from the Department of chemical engineering at the Faculty of Science in Laval University of Quebec
2.1.1. Raw Coffee Hulls
Robusta coffee tree is a shrub of the Rubiaceae family. After its seed has been removed, coffee shell pulp is obtained. There are two varieties of coffee, the hulls used in this study is that of coffee Canephora (Robusta coffee).
Coffee hulls were collected from a coffee factory in a rural area of Cameroon (Nkongsamba) (Figure 1).
2.1.2. Linear Low Density Polyethylene
Linear-low-density PE LL1201 XV used is a blown film type polyethylene from Exxon Mobile Chemicals; It was used because it does not contain slip enhancing additives, with the following properties: melting temperature of 122˚C, density of 0.916 g·cm−3 (25˚C) and melt-flow index (MFI) of 40 g/(10 min) at 230˚C (Figure 2).
The preparation of the raw coffee hulls material for different analysis has been done, according to the following stages: (Figure 3).
2.1.4 Physicochemical Characterization of Coffee Hulls
The following tests have been carried out:
1) Water and dry matter content
Figure 1. Coffee hulls at the moment of collection (May 2021).
Figure 2. Linear low density polyethylene.
Figure 3. Synoptic diagram of the process of transformation of raw coffee hulls into powder.
The determination of the water content was carried out by AFNOR method (1982) reported by . A mass of the fresh sample is dried at 105˚C to constant weight in an oven for 24 hours.
2) Crude fiber content
The crude fiber content of the samples was determined by Weende method . This method consists of treating the sample at the boiling point with sulfuric acid and then with soda. The residue obtained is dried and then calcined and weighed.
3) Lignin content
The lignin content was determined by the Klason method reported by . This so-called Klason lignin method uses the property of the insolubility of lignin in a concentrated acidic medium which hydrolyzes and dissolves all other constituents. Its objective is to obtain an essentially woody residue.
4) Cellulose content
Cellulose is insoluble in ethanol as long as nitric acid converts lignin to alcohol soluble nitro products and hydrolyzes hemicelluloses.
5) Apparent density
The method described by Ernesto de la Torre Chauvin in 2015  was used to evaluate the apparent density (Equation (1)). The measure of apparent density consists to place empty volumeter of capacity 1 m3 on the SEDITECH balance with a precision of 1/1000 and tare. Fill the volumeter with coffee hulls until 1 m3. For a volume Vi of powder, record the mass mi. Determine the value of mean and the standard deviation which represents the density ρ of the micronized coffee hulls powder.
where mi = mass of the powder and vi = volume of the powder.
6) Wetted density
The method described by Ernesto de la Torre Chauvin in 2015  was used to evaluate the wetted density. It consists to take an empty flask of volume Vo and weight the mass of the flask (mo). Add a mass m1 of water, and a mass m2 of coffee hulls powder in the flask and let stand. Let all the mixture stand avoiding the formation of air bubbles in the flask and note the mass (m3). The wet density is given by Equation (2). Calculate the average and the standard deviation of the records.
7) Gravimetric Thermal Analysis (ATG)
The thermal gravimetric analysis of coffee hulls was conducted by thermo gravimetric (TG-DTG) and differential scanning calorimetry (DSC). 20 to 22 mg of very fine particles of coffee hulls sample was placed in an alumina crucible and taken for analysis. The samples were heated up, steadily at a rate of 10˚C/min from 24˚C to 1000˚C in the ambient air of the laboratory, and an isotherm at 900˚C for 30 minutes under argon medium. A device of the brand LINSEIS STA-PT 1000 (from physic-chemistry of Mineral Materials Laboratory, Faculty of Science, University of Yaoundé 1, Cameroon) was used for the thermal analysis.
8) Fourier transform infrared spectrum (FTIR)
Coffee hulls were analyzed using the Perkin Elmer Frontier attenuated total reflectance (ATR) apparatus equipped with a Diamond/ZnSe crystal. 2 mg of grounded powder was deposited on the crystal device. Each spectrum was obtained by 32 scans with a resolution of 4 cm−1 from 4000 to 600 cm−1.
9) Scanning Electron Microscopy (SEM) Observation
The morphology evaluation of coffee hulls surface was performed using a Hitachi S-4800 scanning electron microscope (Hitachi, Japan). The samples were analyzed under nitrogen atmosphere, according to the following cycles: in the first cycle the sample was heated from 20˚C to 200˚C, at a heating rate of 10˚C·min−1.
3. Composites Production
In order to evaluate the importance of coffee hulls fiber composites, different coffee hulls fiber fractions were used in the composite besides the use of the compatibilization agent (MAPE). The impact of fiber proportion on absorption properties was assessed, it can caused changes in the mechanical properties of the composite. Coffee hulls fiber was blended with LLDPE and maleic anhydride polyethylene as coupling agent. The processing temperature in the extruder was 160˚C in order to prevent coffee hulls fiber degradation and providing the fusion between the materials. The material was processed in twin screw extruder Werner & Pfleiderer Krupp model ZSK-25, and chopped a SAGEC Model GS mill 70. The next step was to inject LLDPE and coffee hulls fiber as the specimen in injection Semerano Model 650/247 .
3.1. Mechanical Test
Tensile and flexural properties of the composites were measured by Emic (Model DL10000), testing machine, according to standards ASTM D 638 and ASTM D 790 respectively. The notched Izod impact strength was measured with an Olsen Tilsen Impact tester (model No. 43–02–01) according to ASTM D 256 using a pendulum of 5 ft-lb .
3.2. Water Absorption Test
The water absorption tests of pure LLDPE and for elaborated composites of polyethylene (LLDPE) and fiber from the coffee shell (FCS) at the proportions of 60% - 40% and 80% - 20% polymer - filler ratio were carried out following ASTM D 570-99 standard. Rectangular samples were cut with the dimension of 39 × 10 × 3 mm, dried at 105˚C until the weight remained unchanged, cooled to room temperature in a desiccator using silica gel, and immediately weighed with an accuracy of 0.001 g. To investigate the water absorption of LLDPE/FCS composite, the sample was immersed in distilled water for 24 h at room temperature. Then, the sample were taken, with the excess water on their surface removed using a soft cloth, and weighed. The percentage of water absorption (W) of the samples was calculated using the following:
w1 weight of the specimen before immersion,
w2 weight of the specimen after immersion
4. Results and Discussion
4.1. Chemical Composition of Coffee Hulls
The results are given in Table 1 below.
The coffee hulls studied has a dry matter content of 88.1% and an organic matter content of 92.1%. This high organic matter content shows that coffee hulls can be an important source of nutrients for microorganisms in anaerobic digestion, this can explain why it’s used as fertilizer. These results are close to those of  On the other hand, these values are slightly higher than those of  which had 85.9% for dry matter and 87.7% for organic matter. In addition,  had high values, i.e. a dry matter content of 91.5% and 93.3% for organic matter. As regards the lignocellulosic composition, there are 40.1% crude fibers, 35.3% lignin and 41.4% cellulose. These results are in line with those of  who for these same compounds had 42.22%, 38% and 43.13% respectively. In addition, these contents are higher than that of  whose work revealed a crude fiber content of 36.6% against 21.4% of lignin. Such lignin (between 10% and 35%) and cellulose (between 20% and 50%) contents immediately make coffee shells a lignocellulosic biomass according to . All these results demonstrate the variability in the composition of coffee shells. This can be explained by differences in the soil composition of the place of origin, the difference in the degree of maturity as well as the harvest period. But the fact remains that coffee hulls constitute a promising source of fibers that can be used as reinforcement or filler.
Table 1. Chemical composition of coffee hulls.
+ Dry base, * Wet base.
Density is an important parameter for plastic composites in view of weight reduction. The average value obtained in the case of apparent density of coffee hulls powder is
This density can be compared to the density of plant fibers such as oil palm fiber.
The average value obtained in the case of wet density is ρm = 0.789 ± 0.003 g/cm3. This value compare to the density values of some plant fibers used to reinforced polymer such as kenaf fibers (0.6 - 1.5 g/cm3), Oil palm fiber (0.7 - 1.6 g/cm3) or sisal (1.2 g/cm3 ) shows that composites obtain after processing will be light    .
4.3. Thermal Analysis
The ATG-DTG curves of the coffee hulls are shown in Figure 4. Thermogravimetric analysis (ATG) (in red) provides the loss of mass of the material during thermal degradation. The heat treatment phenomenon is described by . The second derivative of thermogravimetric analysis (DTG) (in blue) gives the degradation temperatures of the hulls as defined by .
Observation of the curve shows that around 440˚C, the biomass has been completely consumed.
Figure 4 shows four main phases of mass loss. The first mass loss of 8.00% by weight is located between 40˚C to 100˚C and has a peak around 86˚C; it corresponds to the evaporation of the water present in the hulls. The second loss of mass correspond to mass loss of 64.50% is observed between 300˚C and 400˚C, with a DTG maximum at 321˚C, It is assigned to the degradation of polysaccharides (hemicelluloses and cellulose) in the fiber of coffee hulls . The third loss of 22, 60% observed between 321˚C and 401˚C would be due to the degradation of cellulose; in fact cellulose would degrade around 401˚C. Between 401˚C and 423˚C, a loss of 5.12% is observed having a peak around 423˚C, which would be due to the degradation of the lignin at 423˚C .
We note that these results are in agreement with the chemical composition of coffee hulls: the major constituents of which are cellulose, hemicellulose and lignin; they degrade with a strong loss of mass .
In addition, hemicelluloses decompose at low temperatures because their molecular chains are short. In addition to their low molecular mass, they have less regular structures in their chains .
The curve of Figure 4 shows four thermal phenomena for coffee hulls. One endothermic peak observed at lower temperature around 86˚C and three exothermic peaks range between 321˚C, 401˚C and 423˚C. The first thermal incident was attributed to evaporation of water molecules from fibers; the second third and fourth respectively represent the degradation of holocelluloses, celluloses and lignin from the coffee hulls  (Table 2).
Figure 4. Thermogravimetric and differential scanning calorimetry of raw coffee hulls.
Table 2. Types of thermal phenomena.
4.4. Infrared Fourier Transform Analysis
Figure 5, FT-IR analysis of coffee hulls shows different peak intensities
The large peak intensity at 3299 cm−1 can be attributed to O-H stretching group of alcohol (cellulose content in coffee pulp). The intensity 2926 cm−1 can be attributed to C-H stretching group of alkanes or the vibration of methoxy group of lignin. The intensity peak between 1300 - 1150 cm−1 and those of 1238 - 1016 cm−1 could be attributed to C-O stretch of alcohols, esters or ethers.
These results are in agreement with data found in the literature concerning coffee pulp. The study indicates that coffee skin can be an interesting filler for polymeric composites; its properties can be ameliorated by pretreatments techniques.
4.5. Morphological Investigation (SEM)
Figure 6(a) shows that the neat coffee hulls exhibits a rough surface due to the presence of non cellulosic materials, while figure (b) shows mercerized fibers that were made to react with caustic soda; In this image (b) gaps between the fibers can be clearly observed in, which indicate the partial removal of lignin and hemicellulose. Also, this can be related with mechanical properties enhancement that can be observed in the composites after addition of treated coffee pulp.
4.6. Water Absorption and Mechanical Test
4.6.1. Mechanical Test
This analysis allows us to measure the maximum stress at which the composite can resist the extreme conditions of solicitation. The values of the tensile module and its tensile strength are recorded in Table 3. The observation of the table reflects that the resistance to the fraction decreases with the addition of the fiber of the coffee shell. This result can be explained by the poor adhesion between the fiber which is hygrophilous and the LLPDE which is hydrophobic. Similarly, according to Sahoo et al.  the incorporation of coffee shell fiber into the LLPDE gradually reduces the tensile strength of composites .
Figure 5. Infrared of raw Coffee shell pulp.
Figure 6. Scanning electron micrographs of coffee hulls (a) (untreated or, neat coffee skin); (b) (mercerized coffee skin).
Table 3. The constituents of the manufactured composite materials with their tensile strength, elongation at break and water absorption.
4.6.2. Water Absorption Test
Figures in Table 3 shows that an increase in the percentage of filler leads to an increase of water absorption. Study has shown that water absorption by bio composites occur at the level of fibers, matrix being generally thermoplastics. The higher the proportion of fibers, the more water is absorbed.
The properties of coffee hulls fiber-based LLDPE composites, in general, depend on coffee hulls fiber percentage, as well as the addition of coupling agent to ensure better interaction between coffee hulls fiber and matrix. The addition of coffee hulls fiber produces an environmentally friendly material without loss of the desired characteristics of the virgin LLDPE. It is clear that coffee hulls fiber has great potential as a filler and reinforcement for composites requiring similar properties to LLDPE.
The authors acknowledge to the Research Unit for Macromolecular Chemistry, Applied Inorganic Chemistry Laboratory, Faculty of Science, University of Yaounde I, Yaounde, Cameroon for the technical support.
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