Hydrothermal treatment of sisal fiber for composite preparation

Abstract

The influence of hydrothermal carbonization of sisal fibers on the mechanical properties of composites based on recycled polypropylene matrices was investigated in this paper. The fibers were characterized by X-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The composites, with short fibers randomly distributed, processed by extrusion and injection processes, were characterized using quasi-static tensile and impact mechanical tests and scanning electron microscopy. The hydrothermal carbonized sisal fibers, proved by Fourier-transform infrared spectroscopy analysis, besides increasing the material's crystallinity, caused a larger adherence on the interface fiber/polymer matrix as verified in the scanning electron micrographs. The addition of 10% and 20% of 2 h hydrothermal carbonized sisal fibers treatment produced a composite with, respectively, an increase of 17.9% and 32.2%, on the modulus of elasticity, of 9.25% and 52.3% on the resistance to impact and of 19.06% and 29.85% on yield strength, in comparison to the recycled polypropylene. The hydrothermal carbonization technique changed the concept of recycling the polypropylene allowing new applications to the produced materials due to its mechanical properties.

Keywords

Polymer–matrix composites fibers recycling mechanical properties hydrothermal carbonization

Introduction

In 2015, the worldwide production of plastic reached the amount of 322 million tons.¹ The first global association to support the interests of recycling industries, Bureau of International Recycling, estimates that the world recycling plastics commerce counts a total of 12 million tons per year.² In Brazil, plastic is the most used recycled material, representing 13.5% of the total urban solid residues generated per year.¹ Polypropylene (PP), used in industry for making plastic packages for food, plastic bags, lids, and labels, is one of the most commonly recycled materials.^3–5 Polymeric matrix composite materials may be produced with the matrix coming from this recycling process and can be reinforced with natural fibers, due to their resistance, low weight, and low cost properties.^6,7 Natural fibers, like sisal fibers, have numberless advantages in relation to synthetic fibers, mainly for being a renewable source and for being biodegradable.⁸ They are basically composed of cellulose, hemicelluloses, and lignin; therefore, they are of hydrophilic nature and need to be modified to reach an interfacial compatibility with the hydrophobic polymer matrices.^8–11

The techniques to modify surfaces, classified as biological, physical, and chemical, have become more and more popular in different fields in the last 30 years.^12,13 Diverse chemical treatments have already been applied, and include acetylation,^14,15 bleaching,¹⁶ treatment with silane,¹⁷ treatment with enzymes,¹⁸ alkalization,¹⁹ and mercerization.¹⁴ The alkaline treatment is the most commonly used raw fibers, for it partially removes the lignin, waxes, and oils from the external surface of the fiber's cell wall.²⁰ The main disadvantage of those treatments is the addition of a third component for carrying out the chemical conversion.⁹ The hydrothermal carbonization (HTC) has been greatly highlighted due to the absence of chemical reagents and to the low temperature of the synthesis.²¹

Basically, the HTC heats the sample at 180–250℃ into water under pressure, converting the organic content, such as biomass, in a hydrochar which decreases the power consumption significantly. Any kind of biomass can be hydrothermally carbonized¹⁹; tests already performed reinforce even more the advantages of HTC for the production of carbonaceous materials,²¹ once it produces a kind of fuel that is easier to deal with and store, being stable and nontoxic.²² The HTC is considered to be a green technology, due to the use of only water in the process, and still working with less energy and time, presenting a great importance from the economic and environmental points of view.²¹ Thus, the use of the HTC presents better results in improving the interaction between wood composite materials.^23,24 In this study, we developed a polymer composite based on recycled PP reinforced with hydrothermal carbonized sisal fibers (HCF) enhancing the mechanical properties due to better interaction between the matrix and the fibers.

Material and methods

Materials

The composite was prepared using as the matrix phase, recycled polypropylene (rPP); supplied by GAPLAST, Maringa, PR, Brazil and for the disperse phase, sisal fibers (Agave Sisalana Perrine; supplied by cotton unity of EMBRAPA, Campina Grande, PB, Brazil).

Fibers treatments and composites preparation

Sisal fibers with a length of 5 mm were previously washed with deionized water and, subsequently, treated hydrothermally at 180℃, in a stainless steel Teflon-coated autoclave, under the proportion of 3 g to every 80 mL of deionized water, under different treatment times (0 h, 1 h, 2 h, 3 h, 4 h, 5 h, and 6 h). The treated and raw sisal fibers were mixed to the rPP matrix, according to the composition presented in Table 1. The samples were processed in the twin-screw extruder Thermo Scientific Mini Lab II HAAKE Rhemex CTW 5, under a temperature of 190℃, speed of 65 r/min with 5 min of internal recirculation. The specimens for the mechanical tests were processed in an injection machine Thermo Scientific HAAKE Mini Jet II, with 210℃ of cannon temperature, 40℃ of mold temperature, 650 bar of injection pressure, 15 s of injection time, 300 bar of repression pressure, and 30 s of repression pressure time.

Table 1.

Composition and nomenclature of the composites.

Sample	Treatment time (h)	Recycled PP (%)	Sisal fiber (%)
rPP	–	100	–
rPP/FS0	0	90	10
rPP/FS0	0	80	20
rPP/FS1	1	90	10
rPP/FS1	1	80	20
rPP/FS2	2	90	10
rPP/FS2	2	80	20
rPP/FS3	3	90	10
rPP/FS3	3	80	20
rPP/FS4	4	90	10
rPP/FS4	4	80	20
rPP/FS5	5	90	10
rPP/FS5	5	80	20
rPP/FS6	6	90	10
rPP/FS6	6	80	20

rPP: recycled polypropylene; FS: sisal fibers; PP: polypropylene matrices.

X-ray diffraction (XRD)

The fibers were characterized by XRD by a Bruker D8 Advance equipment, with Cu Kα (λ = 0.1542 ηm) as radiation source, 40 kV of tension, 30 mA of current, 2°/min of speed, and 2θ between 10° and 80°. Based upon the XRD results, it was possible to calculate the crystallinity indexes (CIs) through the Segal method, using equation (1)²⁵

I_{c} = \begin{matrix} \frac{I_{002} - I_{am}}{I_{002}} \end{matrix}

(1)

in which I_c is relative CI, I₀₀₂ is the maximum diffraction intensity of the (002), and I_am is the diffraction intensity of the amorphous part.

Thermal analysis of the compounds

The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the fibers were performed in the thermal analyzer STA6000 of PerkinElmer. The analyses were performed in an open platinum sample holder and 10℃/min of heat in the grate in the temperature range from 50℃ to 850℃ for TGA and 50℃ to 500℃ for DSC. The tests were carried out under nitrogen flow atmosphere at 20 mL/min, with 10℃/min of heating rate.

Scanning electron microscopy (SEM)

The morphology of the fibers and of the composites was analyzed by SEM, in the QUANTA 250 FEI equipment. Images of the fractured area of the composites submitted to the impact test were obtained.

Fourier-transform infrared spectroscopy (FTIR)

The fibers were characterized by FTIR, in KBr pellets, with the Vertex series of the Bruker Optics equipment, in the range of 4000 to 400 cm⁻¹, with a resolution of 4 cm⁻¹ and 64 scans.

Analysis of free O–H

The processing of FTIR spectra, baseline corrections, derivatives, and the deconvolution method of the bands were carried out with the software OriginLab® version 8.5. The frequencies and the maximum number of sub-bands, of the spectral region of 3700–3100 cm⁻¹, which corresponds to the stretch band of O–H, were determined by using the FTIR spectra (in absorbance) of the second derivate.^26,27 Afterwards, the combination of the ranges to the best spectral performance was automatically adjusted; the adjustment of the curve for the deconvolution of peaks was carried out in the Gaussian function, and a correlation factor of 0.999 was obtained. The maximum deviation of bands position was ±15 cm⁻¹.

Quasi-static tensile test

The tensile strength test of rPP and composites (n = 8 to each group) was carried out in a universal testing machine EMIC DL10000, using a load cell of 5 kN, according to the standard ASTM D638, under the speed of 20 mm/min up to the failure.

Izod impact test

The impact resistance tests were carried out in the equipment CEAST, model Resil Impactor Junior, according to the Method A of the ASTM D256-02, with a notch and pendulum of 2.75 J. Five tests were performed for each sample by means of Izod method.

Statistical analysis

In order to investigate the statistical influence of the hydrothermal treatment time on the mechanical properties of the composites were analyzed the normality test and the one-way analysis of variance (ANOVA) used to the Design-Expert® v.7.0 software, followed by Tukey's test evaluated under significance level of 5%.

Results and discussion

Fibers treatment and characterization

XRD

The results of the X-rays analysis of raw (FS0) and HTC fibers (FS1, FS2, FS3, FS4, FS5, and FS6) are presented in Figure 1. Despite the interferences of the amorphous regions, one can observe the presence of peaks referring to the crystallographic planes in the following Bragg's angles (2θ) along the fiber: 15.45° (plane 101); 22.36° (plane 002), and 36.42° (plane 040). For Martin et al.²⁸ and Klemm et al.,²⁹ these results can be referred to the crystalline planes characteristic of lignocellulosic materials. The most intense peak in 22.36° corresponds to the type I cellulose and the amorphous halos in 15.45° and 36.42° correspond to the amorphous macromolecules of the fibers hemicellulose and lignin.²⁸

A gradual narrowing of the main peak (corresponding to plane 002) is noticed up to sample FS6 when compared to the peak of sample FS0. The calculated CIs are also presented in Figure 1. The CI for FS0 was 57%, a value similar to the one reported in the literature.^18,30 The CI increase of hydrothermally carbonized samples may have occurred due to the removal of the amorphous material that involves the microfibers of cellulose.^18,30

TGA/DTG

The TGAs of the raw (FS0) and HTC (FS1, FS2, FS3, FS4, FS5, and FS6) fibers are shown in Figure 2. It was noticed that samples FS0, FS1, and FS2 present three mass loss events, whereas samples FS3, FS4, FS5, and FS6 present only two. Water evaporation was observed in the first event in temperatures below 100℃. In this event, a reduction on the mass loss is noted with the increase of the fiber's treatment time. There was a lower mass loss for FS6 (0.5%) and a temperature difference of 12.7℃ in relation to FS0. This fact suggests that the HTC process made the fibers less hydrophilic.³¹

When samples FS0, FS1, and FS2, are compared among themselves at a temperature of around 297℃, it can be noticed that sample FS2 obtained the lowest mass loss (11.5%), indicating depolymerization of hemicellulose, pectin, and cellulose glycosidic bonds.⁹ Besides that, after 3 h of treatment (FS3, FS4, FS5, and FS6), the mass loss related to hemicelluloses was not observed, indicating the removal of this polysaccharide.³¹ In the third event of mass loss, in the temperature range of 310–400℃, all the samples showed a reduction of approximately 4% and an increase in the degradation temperature. Comparing sample FS3 to FS0, a temperature difference of 11.8℃ is obtained.

According to Saheb and Job,³² the fiber's thermal stability is evidenced by the increase of the degradation temperature and a reduction in the mass loss. In this context, despite the fact that the mass loss difference among all samples is not significant, the modification of the fibers caused the material to be more stable toward temperature. Besides that, the mass loss observed in this last event was attributed to the high removal of α-cellulose and the pyrolysis of smaller fractions of lignin.^9,33 Above 400℃, there was not a significant mass loss due to the slow degradation of lignin and the generation of solid residue.³⁴

DSC

The DSC curves of the raw (FS0) and HTC (FS1, FS2, FS3, FS4, FS5, and FS6) fibers are presented in Figure 3. The endothermic peak verified in temperatures below 100℃ was attributed to water evaporation, like was also evidenced in TGA analysis. The curve of sample FS0 showed an endothermic peak at the temperature 355℃ and a change to an exothermic peak after the fiber's modification (FS1, FS2, FS3, FS4, FS5, and FS6). This fact was attributed to the decomposition of cellulose, and it can be inferred that the HTC allowed modifications in the chemical bonds among hemicellulose, cellulose, and lignin.¹⁷ Similar results were also observed on jute fiber, with the use of alkaline treatment.³⁵ The change in the baseline observed in all samples at 230℃ is due to an artifact generated by the calorimeter.

SEM

The scanning electron micrographs of fibers FS0, FS2, FS4, and FS6 are presented in Figure 4. In sample FS0, granules could be observed on the fiber's surface, due to the residues from a set of parenchymal cells of the spongy profile.³¹ Besides that, the elementary fibrous structure is connected by lignin, forming continuous filaments along the fiber length.²⁸ In sample FS2, the morphological transformations reveal the characteristics of cellulose,²¹ and an increase of the superficial quality can be observed, due to the partial removal of hemicellulose, waxes and oils that cover the surface of the fibers' cell wall.⁹ This behavior was also observed for the mercerization treatment¹⁴ and for the treatment with silane.³⁶ These changes help the interaction fiber-matrix for the composite production.¹⁹ When the sisal fiber remains in HTC for longer (FS4), its fibrous net is interrupted, presenting micro-cracks on the surface of the treated fiber. The presence of carbon microspheres distributed in a nonuniform way was also observed.³⁷ For sample FS6, it was noticed that the microspheres quantity increased substantially, indicating the transformation of lignin and hemicelluloses into carbon.³⁸ Because of the time space of HTC, with the fiber structure completely exposed, it was also observed that the defibrillation occurs as the bonding materials (lignin) are removed.¹⁵

FTIR

The FTIR spectra of the raw (FS0) and HTC (FS1, FS2, FS3, FS4, FS5, and FS6) fibers are presented in Figure 5. The band in 3418 cm⁻¹, with baseline corrected, present in all samples refers to the O–H stretching of cellulose and/or hemicelluloses,^39,40 whereas the C–H stretching of hemicellulose is observed in 2913 cm⁻¹.⁴¹ For the spectrum of sample FS0, the band in 1742 cm⁻¹ was attributed to the stretching of C=O group present in lignin and in hemicelluloses.^15,42 The bands in 1612 cm⁻¹ and 1421 cm⁻¹ were evidenced in all the samples and refer to the bonds C=C present in the lignin aromatic ring and acetyl group of hemicelluloses, respectively.^34,39 Nevertheless, still in the spectrum of sample FS0, the bands in 1244 cm⁻¹, 1159 cm⁻¹, and 1057 cm⁻¹ were attributed to the stretches C–O, C–O–C and deformation C–OH, respectively, referent to the acetyl groups present in hemicelluloses and cellulose,^34,39,41 and the band observed in the region from 760 to 899 cm⁻¹ is characteristic of β-glycosidic bonds between sugar units.⁴¹

From the spectrum of sample FS2, it can be observed an increase of band intensity in 1421 cm⁻¹, attributed to the group C=C.³⁹ This growth is due to an addition of aromatic groups in the process of HTC⁴² and to the transformation of lignin into carbon.¹³ Moreover, the HTC is evidenced by the increase of the band referring to the group C=C, what confirms the HTC of the sisal fiber.⁴³

The fibers FS4 presented the disappearance in band at 1742 cm⁻¹, indicating a partial removal of hemicelluloses and lignin⁴⁴ and the disappearance of the band at 1244 cm⁻¹, attributed to the removal of hemicellulose, due to the weakening of the bond C–O up to the disruption.⁴⁵ Besides that, literature reports the growth and precipitation of spherical hydrochar in wood fibers due to the decomposition of cellulosic components during the treatment.³⁸

Analysis of free O–H

The spectral characteristics associated to the different types of stretches vibrations O–H were identified with the second derivate of the FTIR absorption spectrum, as shown in Figure 6(a).^26,27,46,47 Six different bands were identified in all the samples. Samples FS3, FS4, FS5, and FS6 presented displacements in five bands, while the band in 3339 cm⁻¹ (OH-bonding (3)) remained at the same wavenumber. It was possible to notice that HTC does not alter the chemical composition of the natural fiber, but with the increase of the treatment time, it can modify the structure and the surface of the sisal fibers.⁴⁸ The band from 3570 to 3550 cm⁻¹ was attributed to the free-OH groups.^26,27,47 The formation of intramolecular hydrogen bonds was observed in bands from 3530 to 3339 cm⁻¹, which corresponds to the OH-bonding (1), (2), and (3).^26,27,47 To the other signals, OH-bonding (4) and (5), in the bands from 3279 to 3216 cm⁻¹, were combined to the formation of the intermolecular hydrogen bonds in cellulose and cellulose II.^26,27,46 The percentage of relative free O–H, presented in Figure 6(b), decreases over the treatment time. In fact, the natural fibers are in the amorphous state, in which the cellulose chains reach a certain level of movement liberty, and the active sites, such as hydroxyl groups, are more available.^26,27 With the HTC time increase, the chains are regularly aligned, and the intermolecular and intramolecular attraction forces increase,²⁷ i.e., the CI grows, as observed in XRD analysis. In this case, the stretching and the vibrations of OH-bonded predominate and lead to a high absorption band intensity in bands from 3530 to 3216 cm⁻¹.

Composites characterization

SEM

The scanning electron micrographs of the fracture surface of rPP and composites rPP/FS0, rPP/FS2, rPP/FS4, and rPP/FS6 are presented in Figure 7. The rPP image shows a totally homogeneous interface, as there are no fracture points, different when one has reinforcement fibers inside of the matrix polymer. In the composite rPP/FS0, it was observed a gap between the fiber and the matrix; it happened because of a weak interaction between the matrix and the raw fiber.^49,50 The weak interaction between the raw sisal fiber and the polymer matrix of PP was already evidenced by Ibrahim et al.⁵¹ and Krishnaiah et al.⁵² For the sample rPP/FS2, it was possible to verify the best interaction matrix/fiber. This fact is related to the reduction of hydrophilic groups (free OH)⁴⁷ and to the removal of superficial impurities.⁵³ Similar results of effective adherence of the matrix with the fiber are also presented by Subramonian et al.⁵⁴ For the composite rPP/FS4, the fibers are very dispersed along the whole matrix, due to the loss of fibrous structure highlighting fibers randomly oriented in the composite. However, one can also notice the nonexistence of phase adherence in some areas, with the fiber pullout at the fracture.^14,52 Since the increased discontinuities fibrous occurs and the microspheres carbon presence in the sisal fiber with the treatment time, potentially leading to decreased in adhesion between the phases. The sample rPP/FS6 image also shows crevice lines between fiber–matrix, indicating low adherence.⁵¹ It can be inferred that the formation of carbon microspheres, derived of the lignin and hemicellulose carbonization, on the exposed fiber structure, that may have contributed to the adherence reduction with the polymer matrix.

Quasi-static tensile test

The statistical data of the tensile strength of rPP, rPP/FS0, rPP/FS1, rPP/FS2, rPP/FS3, rPP/FS4, rPP/FS5, and rPP/FS6 with addition of 10% (P = 0.447) and 20% (P = 0.132) of sisal fiber are presented in Figure 8. The composites with 10% of fiber do not present a significant statistical difference in the tensile strength for groups rPP, rPP/FS2, rPP/FS3, rPP/FS4, and rPP/FS5, however, these samples presented higher tensile strength compared with rPP/FS0, rPP/FS1, and rPP/FS6. For the composite rPP/FS2 with 20% of fiber, there was an increase of 13.73% of tensile strength when compared to rPP/FS0. This improvement can be attributed to the increase of the interaction between the matrix and the treatment fiber, as one can observe in SEM analysis. When the tensions are applied in composites that have a strong interaction between the fiber and the matrix, as happens on the tensile strength test, the matrix yields and deforms plastically, while the fibers continue to stretch elastically. Once the limit of resistance to tension of the fibers is significantly higher than the yield strength of the matrix, this results in more resistant materials.⁵⁵

The results of the yield strength of rPP, rPP/FS0, rPP/FS1, rPP/FS2, rPP/FS3, rPP/FS4, rPP/FS5, and rPP/FS6 with addition of 10% (P = 0.663) and 20% (P < 0.050) of sisal fiber are presented in Figure 9. The yield strength increased significantly with the increase of the fibers percentage in the composite. When compared to rPP/FS0, the composites rPP/FS2 with addition of 10% and 20% of fibers had an increase of 8.67% and 17.17% in yield strength, respectively, is no statistic significant according to ANOVA. This small increase in the yield strength has its importance, due to the better superficial interaction between the two phases, with sisal fiber treated for 2 h. The growth of this resistance is very important for the engineering projects that are designed to operate within the material's elastic regime.⁵⁶ The composites showed a decline in tenacity, due to fact that the forces above the yield strength are applied and the deformation located can occur on the matrix, making that the linkage between the fiber and matrix burst.⁵⁷ Besides that, micro-cracks and vacancies were not observed in the morphological analyses, what justifies the increase of the yield strength.

The data of the modulus of elasticity of rPP, rPP/FS0, rPP/FS1, rPP/FS2, rPP/FS3, rPP/FS4, rPP/FS5, and rPP/FS6 with addition of 10% (P = 0.887) and 20% (P = 0.158) of sisal fiber are presented in Figure 10. In both cases, the value of the modulus of elasticity increased significantly for all samples when compared with recycled PP. The composite rPP/FS2 with 20% of sisal fiber hydrothermally treated presented the best result, reaching 224 MPa, although it was not much higher than the rPP with raw sisal. This indicates that the modification on the sisal fiber surface increased the interfacial adhesion between the fiber and the matrix,⁵¹ as was demonstrated in SEM analysis. The modulus of elasticity value is kept within a small range between samples rPP/FS3 to rPP/FS6, showing significant differences related to the longest HTC time.

Izod impact test

The results for the resistance to impact of rPP, rPP/FS0, rPP/FS1, rPP/FS2, rPP/FS3, rPP/FS4, rPP/FS5, and rPP/FS6 with addition of 10% and 20% of sisal fiber are presented in Figure 11. There was a statistically significant increase in impact resistance of 27% in the composite rPP/FS0 with 20% of sisal fiber when compared to the rPP. The literature reports that the inclusion of fibers to the polymer matrix increases the resistance to impact.^14,48,55 The sample that presented the greatest resistance to impact was rPP/FS2 with 20% of sisal fiber addition, with an increase of 52.3% when compared to rPP. Nevertheless, upon 2 h of HTC, the material starts a process of rigidity growth, absorbing less impact energy during the failure.⁴

Conclusion

The influence of HTC sisal fibers on the mechanical properties of the composites was successfully investigated. The CI for sisal fiber showed to an increased with the increase of the HTC process time due to the removal of the amorphous material. In addition, based on the TGA and DSC analyses, the fibers presented the removal/depolymerization in the fiber's surface inducing in a carbon-covered surface which improves the interaction with the rPP matrix. The morphological characterization confirmed the efficacy of HTC to control the surface structure of the fibers. The FTIR analysis validated to the removal of hemicellulose as of 4 h (FS4) and presented to a decrease of relative free O–H on fiber sisal surface with the treatment time. The mechanical tests showed that the addition of sisal fibers with 2 h of HTC produced a composite with an elevated modulus of elasticity, high yield strength and high resistance to impact. The composites obtained are very important from the economic and environmental points of view, which make their use very promising.

Footnotes

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) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to acknowledge to State University of Maringa (UEM), Araucaria Research Foundation of Parana State, Coordination for the Improvement of Higher Education Personnel (CAPES), and Brazil's National Council of Scientific and Technological Development (CNPq) for the scholarship concession and financial support.

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