Epoxy biocomposites-based chemically treated coffee dystrophy and castor oil

Abstract

An agro-waste such as coffee beans has been used to generate cellulose particles. Coffee roast which is brown in color, was treated with 6% sodium chlorite solution, followed by alkali treatment. This chemically treated mass was subjected to acid treatment with 20% sulfuric acid. Cellulose microwhiskers were released. These micro particles were examined by an X-ray diffractometer (XRD). X-Ray diffraction study of these white cellulose particles and the residue showed a highly crystalline nature of the cellulose particles. The fourier transform infrared (FTIR) spectra were carried out to further investigate any structural changes after chemical treatment. The spectra of the treated powder showed lesser peak intensity at 1630 cm⁻¹. This peak is related to the aromatic ring of lignin. Thus, indicating that the removal of aromatic rings of lignin and polysaccharides after hydrolysis process, simultaneously increases degree of crystallinity. Composites of epoxy resin with a conventional amide-type hardener reinforced with renewable materials were investigated in the presence of castor oil (CO). The renewable material was extracted from coffee beans using various chemical agents. The extracted renewable material has been incorporated into epoxy resin. The composites were evaluated by FTIR to check any interactions. The remarkable hint is the increased intensity of the peak located at 3941 cm⁻¹ being assigned to the –NH₂ of the amine cured epoxy. The increment in the intensity is being attributed to the enhanced degree of interaction between the multifunctional CO and the amine cured epoxy resin as mentioned earlier. Tests of tensile and impact strength properties were carried out and Izod impact was determined at room temperature. It has been found that the incorporation of CO has significantly increased the elongation at break. The impact resistance of the composites with CO has significantly increased as compared to the control and the samples without CO. Scanning electron microscopy (SEM) images were taken to assess the effects of reinforcement and homogenization of the composites. It was noticed that the incorporation of the CO has turned the topography of the samples to a smooth surface with respect to rugged phase of the samples without CO.

Keywords

Epoxy natural fillers mechanical properties composites

Introduction

Preparation of green cellulose particles from renewable resources is becoming an important area of research. In this regard, renewable materials from natural resources are gaining momentum as alternative promising reinforcement to inorganic and synthetic fibers.^1–5 It is accepted that polymer composites containing bio materials such as cellulose and lignocellulose to the polymer matrix are designated as bio-composites.^6-8

Examples of cellulose materials used to produce biocomposites are jute fibers, sesame olive husk etc.^5,6,9,11 Another important resource is the cellulose extracted from coffee beans which is the most consumed beverage worldwide. Such particles have been incorporated in non-polar polymer matrix such as polypropylene in the presence of silane coupling agent resulting in improved stronger interactions with the matrix. In this regard, biocomposites reinforced natural fillers can be utilized in various end applications which helps to protect the environment from pollution as they can be safely returned to the natural carbon cycle by simple decaying. Referring to the impact of the renewable fillers into polymeric materials, it increases the stiffness of the biocomposites made on this basis via the increment of young’s modulus of elasticity.

Due to their hydrophilic nature, one obstacle using natural fillers is their modest compatibility with hydrophobic polymer matrices. To overcome such shortcoming, the naturally occurring fillers are subjected to surface modification or incorporated into polar polymer matrix such as epoxy resin.^12–14 Recall that epoxy resins are of high performance such as excellent thermal and mechanical properties. Nevertheless, the crosslinking of the epoxy resin turns the material into a hard and brittle material which limits their end-use applications to a high extent. Therefore, it is necessary to enhance the toughness of the epoxy biocomposites. This is achievable by the addition of various types of synthetics rubbers and thermoplastics.¹⁵ Note that biodegradability is crucial aspect when dealing with polymer biocomposites due to environmental concerns.^1,3,15 This has triggered the addition of biodegradable additives. Examples of biodegradable materials such as soybean oil and castor oil (CO) have been mixed into crosslinked polymer matrices.^15,16

The rationale behind the selection of such biodegradable materials is the presence of multifunctionalities within the structure such as OH, COOH, and unsaturation. Such reactive sites offer the possibility for strong interactions with reactive groups of the epoxy and the long alkyl chains which lead to soften the biocomposites and diminish their brittleness. The aim of this study is to extract microcrystalline cellulose (MC) from agricultural resources and to utilize such MC in the fabrication of epoxy biocomposite material. These MC was subjected to scanning electron microscopy (SEM), X-ray diffraction (XRD), and FTIR spectroscopy. The biocomposites will be evaluated with respect to their strength properties at room temperature as well as their resistance to influence of water absorption. The microstructure of the samples will be examined for any interaction between the matrix and filler. This will open new avenues for using such abundant, renewable, and inexpensive agro-waste materials for developing value-added products.

Experimental methods

Materials

Coffee beans (Tchibo Arabica) were collected from the local market. Sodium chlorite and sodium bisulfite were used for the delignification step. Sodium hydroxide and acetic acid, from Merck, were used for removal of the hemicellulose fraction. Sulfuric acid (laboratory-grade Merck product) was used for acid hydrolysis.

Pretreatment of coffee beans

Residual coffee beans were roasted at 250°C using a coffee roasting machine followed by grinding using a Retsch ZM 200 mill.¹⁷ The powder was sieved with Retsch AS 200 shaker to classify the powder into different particle size. The shaking time was set up at 10 min and the lowest particle size was selected at ≤45. The selected size (≤45 μm) was subjected to steam current for 5 min before further treatment. To isolate the cellulose particles, it is important to remove the lignin and hemicellulose fractions from the coffee beans to be fully exploited, as leaving them embedded within the amorphous bridging will not allow complete usage of the hard cellulose. Therefore, the raw coffee powder was subjected to multistep removal processes. These cementing constituents were removed out from the roasted coffee beans by delignification at pH 4 and heated for 2 h while stirring, followed by filtration and washing. The filtrate was soaked in 2wt.% sodium bisulfite solution.¹⁰ The obtained residue was filtered, washed, and dried in an air circulated oven until constant mass was achieved. The obtained mass displayed an off-white color where nearly 28 weight% loss was observed due to the delignification process mentioned earlier. This dried mass was then subjected to mercerization process using 16% NaOH solution in order to breakdown the hemicellulose constituent. The total weight loss observed after alkali treatment was nearly 22%. The solid mass after hemicellulose elimination was subjected to acid hydrolysis. This process was carried out with 20% sulfuric acid while stirring for 5 h at 40°C.¹⁰ The solid residue precipitated at the bottom of the flask as well as the floating particles at the surface of the solution were filtered with suction filtration apparatus and washed continuously by adding distilled water to remove any impurities followed by sonication for 2 h. The obtained residue was dried under sun for few days before incorporated into the epoxy resin.

Composite preparation

Biocomposites-based epoxy resin was fabricated according to the following formula: 70:30 wt% epoxy to MC. The curing agent was a polyamide type curing agent as recommended by the distributor. The ratio of hardener to resin was 1:2 wt%. The castor oil was 10 wt% based on the epoxy resin. The ingredients were intensively mixed using an electrical mixer for 5 min at 2500 round/min. The paste solution was casted into a Teflon mold and left to cure at room temperature for 72 h. The following codes were used throughout this study; the control sample was coded as “E.” The epoxy with MC was designated as “EL,” the epoxy with the castor oil was coded as “ECO,” and the formula with the MC and CO was coded as “ELCO.”

Mechanical testing

Stress–strain curves were carried out according to ASTM D-638 on a Zwick 1456 universal tensile testing machine. Dumb-bell shaped specimens (3 mm thick) were casted into a Teflon mold and cured at room temperature for 72 h. A crosshead speed of 2 mm/min was used and the tests were performed at 25 ± 3°C. Tensile toughness was calculated by integrating the area under load-extension curves. Rectangular specimens with 64 × 10 × 3 mm dimensions for un-notched Izod impact test specimens were prepared. The test was carried out according to ASTM, D-356–88 using CEAST model 6545 impact testing machine. The hammer energy was 7.5 J and velocity was 3.0 m/s. The density of the materials was measured by a hydrostatic method.

FTIR analysis

The FTI-IR spectra of the untreated, chemically treated roast of coffee beans, resin, hardener, castor oil, and the various compositions of the epoxy/MC composites were directly obtained in the 400–4000 cm⁻¹ region using transmittance mode on a Bruker ATR-IR spectrometer with 32-scan in each case at a resolution of 4 cm⁻¹ in the transmission mode.

Morphological studies

The izod impacted surfaces of the prepared samples were viewed under scanning electron microscope model ZEISS GeminiSEM. The aim was to overwhelm the filler dispersion and bonding quality between the filler and matrix as a result of the coffee beans chemical modifications; and to detect any micro-defects, if any. The fractured surfaces of the specimen were sputtered with a thin layer of gold to avoid electrostatic charging.

X-Ray diffraction analysis

The wide-angle X-ray scattering spectra of the untreated and treated powders were recorded on an XPert-Philips diffractometer. The generator operated at 45 kV and 40 mA. All scans were performed in transmission and step-scan mode at a scan speed of 4°/min in steps of 0.05°. All samples were scanned in a 2θ range from 0 to 60°.

Hardness test

Rectangular samples of 4 mm thickness were tested on a Zwick 3140 shore D hardness tester according to DIN EN ISO 868.

Results and discussion

The XRD spectra of the raw sample, and the bleached as well as the hydrolyzed sample, that is, the MCC powder are displayed in Figure 1. It can be seen that the control sample derived from roast coffee beans was highly amorphous as reflected by less and broad peaks in the diffractogram shown in Figure 1, which indicates an amorphous structure due to the presence of hemicellulose and lignin within the structure. Recall that the presence of amorphous aromatic compounds such as lignin, polysaccharide should act as a plausible reason to explain the amorphous nature of the control displayed in Figure1. The spectra of the bleached sample are displayed in Figure 1 as well.

Figure 1.

XRD curves of raw coffee residue, bleached and hydrolyzed samples.

Note that the XRD pattern shows two peaks with different areas and heights for both the bleached and the hydrolyzed counterpart, which is attributed to the removal of the amorphous lignin as a consequence of bleaching treatment process. Similar findings were reported on solid state characteristics of micro crystalline cellulose from oil palm empty fruit bunches [15]. The XRD spectra of MCC resulted from the sulfuric acid treatment of the control sample displayed in Figure 1 too. It shows that the highest crystalline structure is reflected by the MCC. Again, this should be related to the hydrolysis of the amorphous regions of alpha cellulose, consequently, the release of the crystallites domains. The improved degree of crystallinity mentioned earlier is evidenced by the crystallinity index calculated by the integration of the peaks shown in Figure 1. This is in line with earlier findings reported by other workers on Kenaf, cotton linter, groundnut shells, baggas, rice straw, cotton stalks, and sisal [12, 16, 14,10,5].

Infrared spectroscopy

The molecular structure of the control, the bleached and the hydrolyzed samples are shown in Figure 2. The FT-IR spectra of the three samples were scanned in the range of 4000–500 cm⁻¹. Basically, the spectra of the reference sample can be analyzed as follows. In the first place, it can be seen that a broad peak is located at 3200–3400 cm⁻¹ region which is related to the hydroxyl group of carboxylic acid. This is confirmed by the peak located at 1747 cm⁻¹ being assigned to the carbonyl group of carboxylic acid. The bands located at 2918 and 2848 cm⁻¹ were assigned to the C-H vibration of the caffeine. The band at 1635 cm⁻¹ is attributed the C = C of the carbohydrates (aromatic ring of lignin). The band at 1149 cm⁻¹ is associated with the polysaccharides and, namely, with C-O-C of the cellulose molecules. The influence of chemical treatment on the intensity of the aforementioned functional groups can be summarized by Figure 2 as well. It is observed that the peak intensity for the OH group of MCC displayed the highest value as compared with other two samples of cellulose, respectively. Furthermore, the location of the peak has been shifted to 3353 cm⁻¹ for the MCC which is a hint on an increased degree of crystallinity. This is in line with a previous study on cellulose structure entitled “Preparation and Characterization of Cellulose Microcrystalline from Fiber of Empty Fruit Bunch Palm Oil.”¹⁵ This could indicate that hydrolyzed sample obtained an improved degree of crystallinity which is attributable to the degradation of the hydrogen bond between the cellulosic chains as a result of sulfuric acid treatment (hydrolysis process). The peak at 1149 cm⁻¹ being related to the hemicellulose is used as a precursor for the effectiveness of the treatment method. Note that the broadness of this peak is being reduced with treatment. This implies that the removal of hemicellulose, as a result cellulose content is expected to increase. The medium peak at 1630 cm⁻¹ is related to the aromatic ring of lignin. Note that the intensity of this peak is less for the MCC structure. Thus, indicating that the removal of aromatic rings of lignin and polysaccharides after hydrolysis process, simultaneously increases degree of crystallinity. Interestingly such observation is confirmed by the appearance of a functional group located at 2324 and 2333 cm⁻¹ for the bleached sample and the hydrolyzed counterpart consequently. This peak can be assigned to the CN group from the caffeine which appeared as a result of bleaching and hydrolysis due to the removal of aromatic groups and polysaccharides as mentioned earlier.

Figure 2.

FTIR curves of coffee residue, bleached and hydrolyzed samples.

Tensile Properties

The stress–strain curves and the related tensile properties of the epoxy composites are displayed in Figures 3, 5–7 consequently. Figure 3 depicts the load-elongation curves of the epoxy composites with various compositions. The corresponding tensile properties are shown in the subsequent Figures. It can be seen that the control sample (plain epoxy) displayed a stiff behavior with limited elongation. On the other hand, the addition of the MCC increased the rigidity of the epoxy resin. This was evidenced by the reduction of the elongation as shown in Figure 3 as well. Referring to the epoxy modified with castor oil, it can be seen that the addition of castor oil has tremendously increased the elongation of the epoxy as shown in Figure 3. It is obvious that the castor oil has converted the rigid epoxy to a tough material. This could be due to the presence of various functional groups such as OH, COOH, and aliphatic un-saturations in the molecule. Such reactive sites are responsible for many chemical interactions with materials having similar functionality, including epoxy resins, provided that the long alkyl chains can act as internal plasticizer providing some flexibility resulting in soft resin with enhanced elongation at break. Hence, it can be concluded that CO is a good example of a natural plant-based resin precursor with the potential to plasticize other functional resin systems. The role of castor oil as a plasticizer is evidenced by the FTIR results shown in Figure 4. The remarkable hint is the increased intensity of the peak located at 3941 cm⁻¹ being assigned to the –NH₂ of the amine cured epoxy. The increment in the intensity is being attributed to the enhanced degree of interaction between the multifunctional CO and the amine cured epoxy resin as mentioned earlier.

Figure 3.

Stress–strain behavior of epoxy composites with and without fillers.

Figure 4.

FTIR curves of plain CO cured epoxy and epoxy/CO.

Figure 5 compares the effect of MCC and castor oil on the tensile strength of epoxy composites. It can be seen that the addition of the MCC powder has reduced the tensile strength of the composite as compared to the control. However, it is observed that the sample with castor oil displayed higher tensile strength than the sample with the hybrid counterpart containing MCC and castor oil and the sample with MCC.

Figure 5.

The effect of CO, LC, and their combination on tensile strength at break of epoxy composites.

Figure 6 shows the elastic modulus of the epoxy composites with various additives. The trend displayed here is at odds to the trend detected for tensile strength. That means the samples with the MCC showed the highest modulus. The increased modulus suggests improved stiffness of the samples with MCC. The less modulus for the sample with CO confirms the plasticization role of the plant-based oil as mentioned earlier.

Figure 6.

The effect of CO, EL, and their combination on the E-modulus of epoxy composites.

The influence of MCC and castor oil on the EB of all composites is presented in Figure 7. It can be seen that the incorporation of the CO has increased the EB tremendously as compared to the epoxy with and without MCC. The increased EB is related to the interaction between the CO and the epoxy as mentioned in the previous section and to the plasticization role played by the CO. On the other hand, the decrement of EB with MCC addition should be related to the rigid nature of the MCC particles which confers rigidity to the sample as well, hence, reduced EB is recorded.

Figure 7.

The effect of CO, LC, and their combination on elongation at break of epoxy composites.

Figure 8 presents the un-notched Izod impact energy of the epoxy composites with various additives, as shown in Figure 8. It can be seen that the sample with MCC was very brittle, exhibiting the lowest impact energy. The impact energy increased significantly with the incorporation of castor oil. This is in line with the EB mentioned earlier. When the composites are under load, their tough interface can efficiently absorb the fracture energy, resulting in increases in the impact resistance. Thus, the plasticizing role of the castor oil is confirmed. The hardness of the epoxy with various additives is shown in Figure 9. It can be noted that the control sample displayed the highest hardness value, whereas the sample with castor oil displayed the lowest value. This is attributable to the plasticizing role of the castor oil. Recall that hardness is a toughness-related property. The implication is that the sample with the CO is more ductile which is harmony with the EB and the impact behavior elaborated earlier.

Figure 8.

The effect of CO, LC, and their combination on the absorbed impact energy of epoxy composites.

Figure 9.

The effect of CO, LC and their combination on shore D of epoxy composites.

SEM of epoxy/MCC-based biomass composites

Figure 10(a) and (b) show the tensile fractured surface of the plain cured epoxy resin and the MCC containing sample. It can be seen that the fracture surface of the control displayed brittle fracture mode. Referring to the composite with the MCC shown in Figure 10(b), it is noted that a good bonding between the resin and the filler was achieved and the MCC was well wetted by the matrix. This is related to the reactive nature of both components, that is, the matrix and the MCC, recall that both components are polar. Further, it can be seen that the fracture mode was via tearing of the matrix. Notice that some holes on the surface are displayed, thus it can be concluded that the fracture behavior of the matrix is a particle pullout failure mechanism. Considering the fracture surface of the formula displayed in Figure 10(b), it is clear that the surface was rugged compared to the smooth surface of the plain matrix. Figure 10(c) displayed the fracture surface of the formula with castor oil. It is obvious that the incorporation of castor oil turned the surface to a smooth one with tough fracture mode. This is evidenced by the matrix extensibility upon failure. This observation is in line with mechanical performance of this formula such as impact energy and EB elaborated earlier. Figure 10(d) shows the fracture surface of the hybrid sample with MCC and CO. It is clear that the degree of adhesion between the MCC and the matrix has been improved with incorporation of the CO. This is due to the presence of the various reactive groups within the three components. The improved degree of adhesion is evidenced by the less number of holes of the fractured surface. The implication is that the pull-out of the particles was curbed due to the improved degree of adhesion between the plasticized particle matrix.

Figure 10.

(a) SEM micrograph of tensile fractured surface of cured plain epoxy resin. (b) SEM micrograph of tensile fractured surface of cured plain epoxy resin with MCC, (c) SEM micrograph of fractured surface of cured plain epoxy resin with CO, (d) SEM micrograph of tensile fractured surface of cured plain epoxy resin with both CO and LC.

Conclusions

Based on the results obtained in this work, it has been found that it is possible to extract a microcrystalline cellulose from the coffee dystrophy by chemical means. This was evidenced by the increased degree of crystallinity as shown by XRD curves. Further proof on the successful extraction of MCC was obtained by the FTIR studies as evidenced by the less intensity of the lignin peak of the MCC structure compared to the control and bleached sample as elaborated earlier. The quality of the produced epoxy composites has been found to change with extracted MCC-based agricultural wastes. The addition of modified ground coffee increased the modulus of elasticity. Further, the incorporation of castor oil has significantly increased the toughness as well as the elongation at break of the epoxy composites. The plausible explanation for this was the improved degree of interaction between the filled and matrix due to the various functional groups in both the MCC and the epoxy resin. The improved impact resistance of the composites was attributed to the plasticizing role of the CO for the epoxy. Again this was related to the polar–polar interactions between the two due to the various functional groups in both components. The improved interaction between the CO and the epoxy composite was reflected by the fracture mode–based SEM microstructure. The SEM pictures revealed a ductile fractured mode for the samples with castor oil. Thus, indicating enhanced fracture mode of the brittle epoxy resin.

Footnotes

Acknowledgements

One of the authors, namely, Prof. Dr Ahmad Mousa is thankful to the Alexander von Humboldt-Stiftung for the scholarship to carry out this research and to the Leibniz-Institut für Polymerforschung Dresden e. V. (IPF) for online cooperation.

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.

ORCID iD

Ahmad Mousa

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