Evaluation of the properties of natural rubber bio composite and guava residue ( Psidium guajava L. ) as sustainable application

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Introduction
The guava fruit (Psidium guajava L.) is notable in the food and trade industries due to its high orchard productivity -in Brazil, an average of 578,000 tons of guava are produced annually -and its rich nutritional value as a source of vitamin C, vitamin B6, lycopene, potassium, copper, fiber, and antioxidants.India is the world's leading producer of white guava 1 .Although the fruit is popular for consumption in its natural state, it ripens and spoils quickly, leading to the majority of production being processed industrially to manufacture juices, pulps, jams, nectars, yogurts, and sweet and sour sauce 2 .In the pulp processing procedure, approximately 8% to 10% of residues are generated, comprising seeds, peels, pulp, and fibrous materials, which are used as animal feed or frequently disposed of in landfills 3 .
The development of composites reinforced with natural rubber fibers presents a potential solution to both technological and environmental challenges 4 .Lignocellulosic biomass fibers are comprised of three structural componentshemicellulose, cellulose, and lignin -and possess numerous inherent advantages, such as being non-abrasive, requiring low energy consumption, offering high specific properties, having low density and cost, and being biodegradable when compared to synthetic fibers 5 .The most extensively studied plant fibers include jute 6 , flax 7 , hemp 8 , sisal 9 , pineapple 10 , coconut 11 , cotton 12 , rice straw 13 , wood, and roots 14 .
The application of guava residue can be found in the literature in the form of biochar as an adsorbent for polycyclic aromatic hydrocarbons 15 , as an adsorbent for As 3+ in wastewater 16 , and in gasification for the generation of hydrocarbons 17 .
In recent years, the development of lignocellulosic biocomposite materials has been observed, using natural rubber as the polymer matrix with alkalized wheat straw to resist fire 18 , silanized cereal straws to increase mechanical and fire resistance 19 , and black spruce (conifer) cellulose pulps to partially replace carbon black without affecting the quality of its mechanical properties 20 .The advanced application of matrix/filler composites is based on the study of their physical properties in conjunction with their mechanical, thermal, and electrical properties.Additionally, environmental concerns surrounding their production aim to reduce environmental impacts 21 .
In this work, we obtained a new composite formed by natural rubber and guava residue and evaluated its mechanical, thermal, morphological, and structural properties.

Material
Natural rubber (NR) of brazilian light-colored crepe (CCB) type was acquired from DLP Indústria e Comércio de Borracha e Artefatos LTDA (São Paulo, Brazil).Guava *e-mail: renivaldo.santos@unesp.brfruit biomass was produced in the laboratory; firstly, the biomass underwent the grinding and filtration process to separate the biomass from the juice, and then it was dried in an oven at a temperature of 104 °C.After drying, the residue was micronized until obtaining a particle size smaller than 30 mesh.Other reagents were commercially acquired, with a high degree of purity, and used without any pre-treatment, such as activation agents (zinc oxide and stearic acid), curing agent (sulfur), antioxidant (vulcanox), plasticizers (processing oil and polyethylene glycol/PEG 4000), and vulcanization accelerators (benzothiazole disulfide/MBTS and tetramethylthiuram disulfide/TMTD).

Preparation of the composites
The composites were prepared in two stages.In the first stage, the proportions of guava biomass (0, 10, 20, 30 and 40 phr) and the amount of reagents to be incorporated into the natural rubber were defined based on established parameters.The formulation used in the preparation of the composites is shown in Table 1.Next, the natural rubber was masticated in the open roll mill of the Makintec brand, model 379 m, at 65 °C and a friction ratio of 1:1.25.After rubber mastication for approximately 20 minutes, the activation and processing agents (zinc oxide, stearic acid, polyethylene glycol, vulcanox and processing oil) and the filler (guava residue) were incorporated.In the second stage, the crosslinking agent (sulfur) and the vulcanization accelerators (benzothiazole disulfide/MBTS and tetramethylthiuram disulfide/TMTD) were incorporated, according to ASTM D3182 standard 22 .
In this work, the semi-efficient vulcanization (SEV) system was chosen because it provides intermediate mechanical properties.Thus, it would be easier to evaluate if guava waste offers any improvement in the mechanical properties of the composites.

Methodology 2.3.1. Rheometric properties
The rheometric properties were obtained using the MDR 2000 oscillating disk rheometer from Team Equipamentos with an oscillating angle of 1° and at a temperature of 150 °C, in accordance with ASTM D2084 23 .The composites were vulcanized in a hydraulic press under a pressure of 3.0 MPa at 150 °C, using the optimal cure time (t 90 ) obtained from rheometry, so that all composites had the same degree of cure.

Degree of dispersion of fillers in the polymer matrix
The degree of dispersion of guava residue in natural rubber compounds can be quantitatively determined by Equation 1 24 : Where L is the degree of dispersion of the filler in the polymer matrix; η r is the ratio [M Lc /M Lg ]; m r is the ratio [M Hc /M Hg ]; M L is the minimum torque; M H is the maximum torque; c and g represent the filler and pure gum, respectively.

Density
The determination of the density of the composites was obtained according to ASTM D297 25 using ethyl alcohol with a density of 0.79 g•cm -3 and calculated by Equation 2: Where ρ is the density of the sample (g•cm -3 ); ρ L represents the density of ethanol at the analysis temperature (g•cm -3 ); m A is the mass of the sample without wire in air (g) and the m B is the mass of the sample without wire in liquid (g).

Cross-linking density by swelling method and Flory-Rehner equation
The cross-linking densities were estimated by the equilibrium swelling technique in organic solvent, using Equation 3 developed by Flory and Rehner 26 .In this technique, the samples were weighed to a mass of approximately 0.25 ± 0.05 g and immersed in toluene in the dark for 5 days until equilibrium was reached.The samples were then removed, dried to remove excess solvent, and weighed.Next, the samples were placed in an oven at a temperature of 80 ºC for 24 hours and weighed.The values used for the molar volume of toluene (V 0 ) and for the Flory-Huggins interaction parameter (χ Toluene,NR ) for natural rubber and toluene were 106.3 cm 3 •mol -1 and 0.393 respectively.
Where ν is the cross-link density (mol•cm -3 ); ρ B is the density rubber (g•cm -3 ) e V B is the volume fraction of rubber in the swollen form, determined from the weight increase by swelling.

Scanning electron microscopy (SEM)
The Carl Zeiss EVO LS15 scanning electron microscopy equipment at 20 kV was used to investigate the surface morphology of fractured composites.The samples were coated with a thin layer of gold using a Quorum Q 150R ES sputter coater.The chemical elements present at a particular point on the material were identified by energy dispersive X-ray spectroscopy (EDX).

X-ray fluorescence (XRF)
The presence of inorganic compounds was determined using an energy dispersive X-ray fluorescence spectrometer Shimadzu EDX-7000/8000/8100.

Elemental organic analysis (CHNSO)
The elemental compositions of the samples were determined from the masses of 2 to 3 mg of dry sample, in triplicate, using the Thermo Scientific Flash AE 1112 elemental analyzer.The basic operating principle of this equipment is to separate the gases generated in the form of CO 2 , N 2 , H 2 O e H 2 SO 3 , which are carried by helium gas in a column and detected by thermal conductivity.This equipment identifies the contents of (C), hydrogen (H), nitrogen (N), sulphur (S) and oxygen (O).

Fourier transform infrared spectroscopy (FTIR)
FTIR spectroscopy was performed using a Bruker Vector 22 spectrometer in the attenuated total reflectance (ATR) mode, in the range of 4000-400 cm -1 with a spectral resolution of 4 cm -1 and 32 scans.

Test of resistance to tearing
Resistance to tear tests were performed on the Instron/ Emic DL2000 universal testing machine at 500 mm•min -1 with a 5 kN load cell and an internal strain transducer.For these tests, quintuplicate specimens of type C test pieces (test pieces with right angles) were used, in accordance with ASTM D624 standard 27 .

Stress-Strain test
Tensile strength tests were performed on the Instron/ Emic DL2000 universal testing machine at 500 mm•min -1 with a 5 kN load cell and an internal strain transducer.For these tests, type A specimens (straight section and dumbbell specimens) were used, according to ASTM D412 standard 28 .

Hardness test
The determination of surface hardness of the composites was obtained according to ASTM D2240 standard 29 on the Shore A scale, using an analog Digimess durometer with a capacity of 0 to 100 and a 1 Shore A graduation.

Test of resistance to abrasion
The abrasion loss was calculated using Equation 4, according to ASTM D5963 standard 30 , using the MaqTest equipment with an abrasion course equivalent to 40 m and a pressure on the test specimen against the cylinder of 5 N.
Where PA represents the abrasion loss (mm 3 / 40 m); Δm is the mass loss of the composite (mg); S 0 is the theoretical attack index of sandpaper on standard rubber (200 ± 20 mg); S is the actual attack index of sandpaper on standard rubber (mg) and ρ is the density of the composite (mg•mm -3 ).

Thermogravimetric analysis (TGA)
The Thermogravimetric Analysis (TGA) tests were performed on a NETZSCH equipment model 209, with a temperature range from around 25 °C to 900 °C, using a heating rate of 10 °C•min -1 under a nitrogen atmosphere with a flow rate of 15 mL•min -1 .The amount of sample used for the measurements was approximately 10 mg, according to ASTM D6370 standard 31 .2 presents the values of the minimum (M L ) and maximum (M H ) torques, torque variation (ΔM), pre-curing or safety time (t s ), optimal curing time (t 90 ) and cure rate index (CRI).

Rheological properties of composites
The values of the minimum and maximum torques slightly increased with the increase in the amount of guava waste.The increase in the maximum torque values for higher waste contents indicates that the presence of fillers in the polymeric matrix reduces the mobility of the polymer chains.The torque variation is related to the stiffness of the vulcanized material, which in turn is related to the degree of crosslinking and the presence of fillers.Therefore, the increase in torque variation suggests an increase in crosslink density with increasing filler content.By means of the swelling technique, good interaction between the filler and the matrix was observed, since the filler prevents solvent penetration, behaving as if they were cross-linking points.As the swelling technique performed is only quantitative and not qualitative, there is an increase in the cross-link density when considering these interaction points as cross-linking.
One can observe from Table 2 that the (t s ) and (t 90 ) decreased significantly with the addition of fillers, as shown in Table 3 regarding the chemical composition of guava waste, the presence of metal oxides can increase the cure rate.By increasing the amount of filler, the content of metal oxides in rubber blends also increases, contributing to this increase in the vulcanization rate.The addition of metal oxide increases the reactive sulfur complexes, which consequently raises the degree of crosslinking.In the vulcanization of natural rubber (NR), elemental sulfur is activated by the presence of metal oxide and fatty acid, forming the metallic stearate.Several studies report activation and increased cure rate with the presence of metal oxides 32,33,34 .Additionally, the addition of chemical agents such as PEG 4000, antioxidant, and processing oil, incorporated in the filled composites, may have also influenced the processing times of the vulcanized rubber.
The reduction in vulcanization time of the filled composites also led to a decrease in the cure rate index, resulting in a faster curing rate.

Degree of dispersion of fillers in the polymer matrix
Based on the data of minimum and maximum torque properties shown in Table 2, the values of L for the dispersion of guava residue in the natural rubber matrix are shown in Figure 2. A lower value of L, for a given filler, compared to the unfiller composite, indicates a better degree of residue dispersion.The degree of filler dispersion was adequate up to the incorporation of 30 phr of residue, while at 40 phr, the occurrence of inter-agglomeration between fillers weakens the mechanical properties of the composites.A lower viscosity tends to reduce the minimum torque and facilitate the filler dispersion, strengthening the BN/Guava residue interaction.The filler-matrix interaction can be defined as additional physical crosslinks that contribute to the overall crosslink density 35 .

Scanning electron micrography (SEM) and energy dispersive X-ray spectroscopy (EDX)
The SEM images were taken from the test specimens after tensile deformation tests and are shown in Figure 3. Figure 3a shows a photographic image of dehydrated guava waste, and Figure 3b shows the scanning electron micrograph of guava waste at 1000 x magnification.
It is possible to observe that the guava waste powder has an amorphous surface area with considerable roughness, favoring the interaction between the filler and the polymer matrix.In Figure 3c, we can observe that the surface area of the unfilled composite is very rough, while in Figure 3d of its fracture, the presence of grooves and vulcanization reagents can be seen.With the incorporation of fillers, in Figure 3e, 3g, 3i, and 3k, it can be noted that the surface areas became smoother, with punctual presence of fillers encapsulated by the polymer matrix.In the images of the fracture surfaces of the composites, Figure 3f, 3h, 3j, and 3l, it can be observed that the fillers adhered well to the elastomeric matrix, proving that the roughness of the guava waste favored the matrix/filler interaction.
The Figure 4a1 and 4b1 present the test areas of energy dispersive X-ray spectroscopy (EDX) spectra of the chemical elements constituting the natural rubber composites without filler, Figure 4a2, and with 10 phr of guava waste, Figure 4b2.In Figure 4a2, which refers to the composite without filler, we can observe that the analyzed area consists of 64.41%  carbon and 5.08% oxygen, coming from the constitution of the natural rubber; 17.25% zinc oxide and 12.11% sulfur, coming from the vulcanizing additives as activation and crosslinking agents, respectively; and still 1.15% sodium, probably from the processing oil added in the formulation of natural rubber.In Figure 4b2, we observe the presence of all the aforementioned constituents and an additional 0.31% potassium and 0.30% calcium, coming from the chemical composition of the guava waste.

X-ray fluorescence (XRF) and elemental
organic analysis of guava residue.
Table 3 shows the chemical constitution of the guava residues obtained from X-ray fluorescence.The XRF indicates that, in addition to the chemical elements potassium and calcium determined in the EDX assays, guava waste is composed of other inorganic compounds, such as phosphorus, magnesium, copper, iron, rubidium, and manganese.
The organic chemical composition by elemental analysis was carried out to complement the XRF results, and its values are shown in Table 3.The results obtained by the elemental analyzer show that the chemical elements present in greater quantities in the guava biomass are carbon and oxygen, a result similar to that found by XRF.Hydrogen and sulfur also appear in significant amounts in the composition of guava.

Density, hardness (shore A), and abrasion resistance
Table 4 presents the results of density, hardness on the Shore A scale, and abrasion loss tests.All composites had a density above 1 g•cm -3 and gradually increased with the incorporation of fillers up to a value of 1.09 g•cm -3 with 40 phr of residue.The hardness values presented in Table 4 are on the hardness grading scale for vulcanized rubbers, considered soft (40 to 60 Shore A) according to ASTM D2240 29 , and were measured in triplicate samples from different regions of the composite.The incorporation of fillers significantly increased abrasion loss due to the length of the biomass particles and their dispersion in the matrix, which can cause stress points and detachment of the filler during frictional wear.

Resistance to tearing
Figure 5 shows the bar chart of tear strength for NR/G composites with and without filler (reference), and their values are presented in Table 5.It can be observed that the tear strength of the vulcanized composites remained unchanged with the presence of the fillers, considering the calculated statistical error.

Stress-Strain test
The Figure 6 shows the tensile strength curves and Table 6 presents the values of tensile strength at rupture.In Table 6, it can be observed that the addition of 20 phr of filler enhances the tensile strength at rupture.However, beyond this threshold, the tensile strength of the composites tends to decrease.While higher filler loadings result in reduced deformation and tensile strength values at rupture, the elasticity modules at 100% and 300% exhibit improvement.This behavior suggests that 20 phr represents the optimal value for reinforcing the filler due to the favorable interaction   between the filler and the matrix.For higher filler loadings, the filler-filler interaction becomes more predominant, thereby increasing rigidity and reducing the stress at rupture.Evaluation of the Properties of Natural Rubber Bio Composite and Guava Residue (Psidium guajava L.) as Sustainable Application

Cross-linking density -Flory-Rehner
The determination of cross-link density by the swelling method in organic solvent using the Flory-Rehner equation is the simplest, practical, and economical methodology that provides good results 36 .Table 7 shows the obtained values of the cross-link densities using toluene as the organic solvent.We can observe that the cross-link densities increase gradually with the incorporation of fillers, corroborating with the results of torque variations presented in Table 2.

Fourier transform infrared spectra analysis
The chemical composition of guava waste and its vulcanized composites, evidenced by functional groups, are presented in the infrared spectra of Figure 7 and identified in Table 8.The composites with fillers presented all similar functional groups, confirming that the interaction between polymer matrix and filler occurs physically without generating new spectral bands.

Thermogravimetric analysis (TGA)
The results of the TGA and DTG curves obtained from the guava waste powder and its composites are shown in Figure 8a and 8b.Four mass loss events were observed.The first mass loss occurs at a temperature close to 86 °C and corresponds to 1% of the total weight of the composite, being attributed to the loss of water and low thermal stability volatile materials.The second mass loss of approximately 4%, evidenced at a temperature close to 187 °C, corresponds to the decomposition of hemicellulose.The third event occurs at a temperature close to 260 °C and is attributed to the decomposition of cellulose, corresponding to 13% of the mass loss.The last event presents a mass loss of around 77% and corresponds to the overlap of lignin degradation with BN, occurring at temperatures of 318 °C and 347 °C, respectively 43,44 .The residual mass, about 3 to 5%, is attributed to the inorganic compounds present in the sample (potassium, calcium, phosphorus, magnesium, silica, copper, iron, and zinc) and the chemical additives added in the vulcanization process such as zinc from zinc oxide.

Conclusion
In this article, the feasibility of producing a vulcanized biocomposite using micronized guava waste as a filler in a natural rubber matrix was demonstrated.The presence of these fillers in rheometric tests significantly reduced curing time and improved material processability, suggesting energy savings for the crosslinking of rubber composites with fillers.Furthermore, the tensile strength was enhanced with the addition of up to 20 phr of the filler, resulting in an increase in the density of cross-links obtained through the Flory-Rehner method.This allows for the application of the material in rubberized floors and shoe soles.Thermal analysis performed by TGA and FTIR spectra were not affected by the incorporation of the filler, conferring thermal stability to the composite.Scanning electron microscopy demonstrated good interaction between the polymer matrix and the filler.Therefore, this article proves that guava waste can be used as a raw material in rubber artifacts, improving rubber properties performance, generating added value, and reducing environmental impacts.

Figure 1
Figure1illustrates the rheometry curves of the composites, and Table2presents the values of the minimum (M L ) and maximum (M H ) torques, torque variation (ΔM), pre-curing or safety time (t s ), optimal curing time (t 90 ) and cure rate index (CRI).The values of the minimum and maximum torques slightly increased with the increase in the amount of guava waste.The increase in the maximum torque values for higher waste contents indicates that the presence of fillers in the polymeric matrix reduces the mobility of the polymer chains.The torque variation is related to the stiffness of the vulcanized material, which in turn is related to the degree of crosslinking and the presence of fillers.Therefore, the increase in torque variation suggests an increase in crosslink density with increasing filler content.By means of the swelling technique, good interaction between the filler and the matrix was observed,

Figure 1 .
Figure 1.Rheometry curves of natural rubber and guava waste composites.

Figure 5 .
Figure 5. Bar graph of tear strength of NR/G composites.

Figure 8 .
Figure 8.(a) TG curves of guava residue powder and its composites and (b) DTG curves of guava residue powder and its composites.

Table 1 .
Formulation of natural rubber and guava residue.

Table 3 .
Chemical composition of the constituents of guava residue obtained from X-ray fluorescence (XRF) and by elemental organic analysis.Degree of dispersion of guava residue particles in the natural rubber matrix.Evaluation of the Properties of Natural Rubber Bio Composite and Guava Residue (Psidium guajava L.) as Sustainable Application

Table 4 .
Density, Hardness (Shore A) and Abrasion loss of NR/G composites.

Table 5 .
Table of tear resistance values.

Table 6 .
Table of values of tensile strength at break of NR/G composites.
Figure 7. Infrared spectra of NR/G composites.

Table 8 .
Identification of the infrared spectra of guava residue and natural rubber.