Sustainable composites with self-healing capability: Epoxidized natural rubber and cellulose propionate reinforced with cellulose fibers

Aligned with the UN's Sustainable Development Goals (SDGs), self-healing elas-tomers stand out as a cutting-edge field in Rubber Science and Technology. These materials have the potential to reduce resource consumption, prolong the lifespan of infrastructure and products, and contribute to the Circular Economy. This study presents the development of bio-based self-healing elastomeric composites prepared from blends of epoxidized natural rubber (ENR) and cellulose propionate (CP) reinforced with cellulose fibers (CFs). The ENR/CP ratio was optimized, with a 70/30 ratio enhancing the tensile strength (TS) of the base rubber and slightly reducing the elongation at break. This blend demonstrated a TS healing efficiency of 75% after a temperature-driven healing protocol (200 bar at 150 (cid:1) C during 12 h). Then, the CF content was varied to enhance both mechanical performance and self-healing capabilities. Remarkably, from medium-high (5 phr to 15 phr) CF content, healing efficiencies higher


| INTRODUCTION
The Circular Economy (CE) envisions a world where the continual reuse of products and materials reduces the demand for finite resources and curbs environmental pollution. 13][4][5][6][7][8] These materials align with several of the UN's Sustainable Development Goals (SDGs). 9,10For example, SHM guarantees an infrastructure that is both longer lasting and more resilient.This aids in building robust industries and motivates further innovations that prioritize longevity and sustainability over short-term gains (SDG 9: Industry, Innovation, and Infrastructure).Their integration into urban planning and construction could be transformative.Cities that incorporate SHM would not only be more sustainable but also more cost-effective (SDG 11: Sustainable Cities and Communities).
However the essence of SHM in extending the life of products and minimizing waste directly encourages responsible consumption (SDG 12: Responsible Consumption and Production).Fewer frequent substitutions mean that fewer resources are extracted, processed, and transported, resulting in a smaller overall environmental footprint (SDG 13: Climate Action).Besides, the reduced waste from longer-lasting products means less ocean (SDG 14: Life Below Water) and land (SDG 15: Life on Land) pollution, thus preserving our ecosystems.
Although the potential of SHM is clear, it still faces persistent challenges.Many current developments rely on synthetic or non-sustainable materials. 4,5This can sometimes lead to a paradox in which the extended lifecycle provided by self-healing capabilities is offset by the environmental impact of the origin of the material and eventual waste.To address this challenge, there is a growing trend towards the development of SHM using bio-based and/or biodegradable products. 11poxidized natural rubber (ENR) is an eco-friendly material that can be used in self-healing applications.ENR is obtained after epoxidation of natural rubber (NR), the only commercial rubber that is not of synthetic origin.ENR offers enhanced polarity, improved compatibility with other polymers, and increased reactivity owing to the presence of epoxy groups, but is detrimental to mechanical performance. 124][15] A self-healable nanostructured Ti3C2MXenes/rubber-based supramolecular elastomer (NMSE) for intelligent sensing, inspired by the supramolecular interactions in proteins was developed. 13MXene nanoflakes were modified with serine through an esterification reaction, integrating them into an elastomer matrix to create dynamic supramolecular hydrogen bonding interfaces.This NMSE exhibits excellent toughness (12.34 MJ/m 3 ) and near-perfect self-healing ability ($100%) at room temperature.A similar approach was followed, and Ti3C2 MXene/ENR elastomer composites were also developed. 15These composites exhibit superior mechanical and self-healing properties.This was achieved by controlling the confined structure and the number of surface hydroxyl groups on Ti3C2 MXene.Synchrotron radiation x-ray three-dimensional nanocomputed tomography confirmed the uniform distribution of Ti3C2 MXene within ENR.This strategic design maximizes dynamic non-covalent bond interactions between Ti3C2 MXene and ENR.Consequently, the composite elastomer demonstrated high mechanical strength (4.93 MPa) and excellent self-healing efficiency of approximately 98% at room temperature.
Regarding bio-based fillers, different types of natural fibers have been explored in ENR composites.Citric acidmodified bentonite (CABt) with abundant carboxyl groups, serves as an effective crosslinker and reinforcer for ENR. 16This composition allows transesterification reactions, resulting in self-healing capability.With 20 wt % of CABt, the material displayed significantly enhanced mechanical properties compared to neat ENR, with tensile strength (TS) of 4.5 MPa and elongation at break (EB) of 400%.Notably, the post-healing efficiency rates of TS and EB were approximately 96% and 94%, respectively, after heating at 150 C for 3 h.Another potentially sustainable filler is alginate, 17 which was recently used in the form of sodium alginate (SA) for the development of ENR composites crosslinked via a supramolecular hydrogen-bonding network. 18These composites presented a substantial TS (6.5 MPa with 20 parts per hundred rubber, phr, of SA) and rapid self-healing at room temperature, recovering 60% in 2 min and 80% in 10 min.This film has potential applications in protective coatings, soft robotics, and wearable electronics.ENR composites reinforced with chitin nanocrystals (CNCs) without traditional crosslinking agents were also developed. 19A supramolecular network, which was also formed through hydrogen bonds, was designed.Specifically, with 20 wt.% CNCs, the TS nearly doubled compared to pure ENR (1.19 MPa).Remarkably, the composite restored 83% of its original strength and 95% extensibility at room temperature in the absence of external stimuli.Given that CNCs are abundant renewable resources, the combination of ENR/chitin or chitosan is among the most explored in the field. 20,21ecently, the self-healing capabilities of ENR by integrating another sustainable filler, lignin, with 1,3-bis (citraconimido-methyl) benzene (CIMB) was explored. 22ost previous ENR self-healing methods have focused on supramolecular chemistry, but this research emphasizes reversible covalent bonds through Diels-Alder interactions between ENR and CIMB.Optimal healing was achieved at 180 C for 15 min with 83% healing efficiency using 6 phr of lignin and 15 phr of CIMB.This results from the combined effects of lignin and CIMB, although both additives reduce the initial mechanical strength of the ENR.
On the other hand, cellulose, the most abundant organic polymer on Earth, has a rich history in materials science due to its renewability, biodegradability, and mechanical properties. 23This material can manifest in thermoplastic forms like cellulose acetate propionate (CAP) or cellulose acetate butyrate (CAB), and also as reinforcing additives in forms like cellulose nanocrystals (CN) or cellulose fibers (CFs). 23,24Dual-crosslinked networks in ENR nanocomposites using tunicate cellulose nanocrystals (t-CNs) has been developed. 25The t-CNs surface was modified with carboxyl groups through a grafting reaction, allowing covalent crosslinking with the epoxy groups on ENR.Additionally, hydrogen bonds between the hydroxyl groups on the t-CNs and epoxy groups of ENR formed a physically crosslinked network.This dual system offers strength and flexibility.With only 5 phr t-CNs, the materials displayed an enhancement in TS and toughness (up to 52% and 45%, respectively) compared to the solely physically crosslinked ENR nanocomposites.Many studies have explored the reinforcing character of CNC in rubbers; and its use in SHM, although less widespread, has experienced an increasing trend in recent years. 26,27he primary goal of this research was to work, for the first time, with other forms of cellulose in rubber-based SHM.For this, bio-based self-healing elastomeric composites were designed and developed, based on blends of ENR and cellulose propionate (CP) reinforced with CFs.By selecting a thermoplastic phase for the matrix and a filler that is also cellulose-based, an improved fibermatrix interaction was anticipated.Dicumyl peroxide (DCP) was selected as a crosslinking agent, being organic peroxides the second most used in the rubber industry for the vulcanization of different materials such as ENR, but also natural rubber (NR), styrene-butadiene rubber (SBR), nitrile rubber (NBR) and butadiene rubber (BR). 28he study meticulously optimized the blend ratios, delved into the impact of CFs content on the mechanical and self-healing properties and elucidated the underlying healing mechanisms.All the materials were thoroughly characterized.This research seeks to contribute to the UN's SDGs, advancing the cause of sustainable materials in a world that is increasingly aware of its environmental responsibilities.

| Materials
The elastomeric phase used for preparing the blends was epoxidized natural rubber (ENR) with 25 mol % epoxy units (ENR 25).This rubber is commercially available as Ekoprena 25 and was supplied by the Tun Abdul Razak Research Centre (TARRC) of the Malaysian Rubber Board.The thermoplastic phase of the blend was cellulose propionate (CP, Merck), an organic biodegradable polyester obtained from the reaction between cellulose and propanoic acid.The reinforcing filler was cellulose fibers (CFs, Merck), a natural, renewable, and biodegradable fibers composed of 10,000 D-glucose units linked by β-(1,4)-glycosidic bonds.The CFs have a density of 1.23 g mL À1 , with an average particle size between 50 and 350 μm, an average fiber diameter between 12 and 15 μm, and a degradation temperature of $300 C. Supporting Information S1 lists the technical specifications of the three main ingredients (rubber, thermoplastic, and filler), which were used as received.To complete the rubber recipe, dicumyl peroxide (DCP, Merck) was used as the crosslinking agent.The prepared rubber recipes are shown in Section 3.

| Compounding and vulcanization
Prior to the compounding process, CP and CFs were dried in a vacuum oven at 80 C for different times (from 30 min to 12 h) to remove the absorbed water resulting from the formation of hydrogen bonds between the hydroxyl groups and water molecules in the environment.It was determined that drying for 2 h was sufficient to ensure the complete elimination of water without affecting the thermal stability of both products.
The compounds were mixed in one stage at room temperature using an open two-roll mill (MGN-300S, Comerio Ercole).A friction ratio of 1:1.15 was used for 20 min.Moreover, a water circulation system was maintained throughout the process to prevent excessive heating of the equipment and avoid pre-vulcanization.The following order was used to incorporate the ingredients into the rolls: first, the rubber was masticated for 4 min, then the CP was added; followed by CFs (if required) at 12 min; and finally, DCP, 5 min before the end.During this process, the distance between the rolls was progressively reduced, and cuts were made in alternate directions to guarantee sufficient homogeneity of the obtained blend.The rubber band was also rolled up and reintroduced into the rolling mill thrice before a satisfactory surface was achieved.After mixing, the blends were stored in a freezer for at least 24 h before characterization.
The curing parameters of the rubber compounds were obtained by analyzing the curing curves at least 24 h after mixing using a Moving Die Rheometer (MDR 2000, Monsanto) under isothermal conditions.For this experiment, 4 g of each sample were placed between two polyester films in a die with an arc of 0.5 and 1.7 Hz frequency for 120 min at 180 C. The curing time (t90) was determined using the tangent method owing to the marching character of the curing curves.
All compounds were vulcanized by compression molding using a hydraulic press (P 200 P, Collin) at 180 C and 200 bar.An uncured sample of each compound was placed in a rectangular mold between two sheets of an anti-adhesive material (Teflon).An additional 5 min was added to cool the material to room temperature after t90.Vulcanized square sheets of 2 mm thickness and 110 mm long were obtained.

| Characterization
The crosslink density (ν) of each compound was calculated using Equation (1): where ρ r is the rubber density and M c is the average molecular weight between crosslinks.The affine model approximation (assuming a tetra-functional network, f = 4) of the Flory-Rehner expression was required to calculate ρ r M c À1 Equation (2): where χ is the Flory-Huggins interaction parameter between the rubber and toluene (0.39), V s is the molar volume of the solvent (106.28 cm 3 mol À1 ), and V r is the volume fraction of rubber determined by a regular equilibrium swelling test in toluene for 70 h at room temperature.Five square section samples of each vulcanizate (1 cm Â 1 cm Â 2 mm) were used.The morphology of the compounds was observed using a scanning electron microscope (SEM, XL30, Phillips) with a tungsten filament and an accelerating voltage of 25 kV.The cross-section samples were obtained by cryogenic fracture of vulcanized samples and then sputtercoated with a gold alloy to enhance their conductivity.
The thermal transitions of the CP were determined using a differential scanning calorimeter (DSC 214 Polyma, Netzsch).The measurements were performed under a nitrogen atmosphere.Four temperature sweeps were conducted at a heating rate of 10 C min À1 .The first heating sweep (from room temperature to 100 C) eliminated the thermal history of the material.Then, a second cooling down sweep (from 100 C to À90 C) was performed, followed by a third heating sweep (from À90 C to 250 C) and a fourth cooling sweep (from À90 C to 25 C).The third and fourth sweeps are reported.
The thermal stability of the vulcanizates was determined using a thermal analyzer (TGA 2, Mettler Toledo).The samples were heated from room temperature to 600 C in a nitrogen atmosphere at a heating rate of 10 C min À1 .
The mechanical properties of the compounds were determined by performing uniaxial tensile tests using a universal testing machine (4204, Instron) with a load cell of 500 N at room temperature.A crosshead speed of 200 mm min À1 was set.The initial gap between clamps was fixed at 35 mm.At least five samples from each vulcanizate were tested.Beforehand, the specimens were cut in an Universal Sample Cutter (P-VS 3000, MonTech) using a type 3 dumbbell-shaped die cutter (according to UNE-ISO 37:2013 standard).Each test was performed until the sample broke completely.The equipment recorded the evolution of the stress as a function of strain.Then, the following parameters were extracted from the data: tensile strength (TS) and elongation at break (EB), which correspond to the values of stress and strain obtained at the breaking point, respectively, and the values of the stress at different strain rates (called as "modulus").Typically, the following values are considered: 50% strain (M50), 100% strain (M100), and 300% strain (M300).

| Self-healing protocol
Damage was performed on five rectangular-shaped samples by cutting them into two pieces using a razor blade.Then, the two pieces of the sample were manually placed in contact inside a mold that matched the dimensions of the pristine sample.They were then subjected to thermal treatment in a hydraulic press at a pressure of 200 bar for 12 h at 150 C. The resulting material was a healed sample.The tensile mechanical properties of pristine and healed samples were determined.Figure 1 shows the schematic of the self-healing protocol.
The healing efficiencies (η) of the compounds were calculated using Equation (3): where P is a selected property before healing (P pristine ), after damage (P damaged ), and after the healing protocol (P healed ).As the serious damage of cutting a sample in two pieces leads to a loss of any tensile property (P damage ¼ 0Þ, the equation can be simplified as the ratio between the property before and after the healing protocol.

| Optimization of the rubber matrix: the ENR/CP ratio
The novel character of the system used in this study requires the design of several rubber recipes and their prior optimization to select the most suitable combination.The first stage of this work involved determining the optimized weight ratio of ENR/CP in the composite material, by selecting three ratios 90/10, 80/20, and 70/30.Table 1 lists the recipes used for this stage in phr.The optimization of this parameter is extremely important for any blend between an elastomer and a thermoplastic.Usually, in this type of combination, the best of both worlds is sought: first, the thermally driven response given by the thermoplastic phase, and second, the elasticity of the elastomeric phase.The latter is severely affected by the presence of the thermoplastic, which contributes to the viscous phase of the viscoelastic behavior of the rubber.Consequently, precise optimization is required to ensure the desired performance.Figure 2 shows the curing curves of the prepared compounds, and Supporting Information S2 lists the values of the different rheometric parameters and crosslink densities.
An increase in the maximum torque (MH) and the torque difference (ΔM) was evidenced with the CP content.This can be explained by the higher content of the thermoplastic phase in the composite, which strengthens the material; thus, a higher torque is required to deform the sample by shear during the test.Regarding the minimum torque (ML), the variations are very small and almost negligible; thus, it can be deduced that this parameter is slightly affected by the variation in CP content in the compounds.It is expected that with the incorporation of solid particles in the rubbery matrix, the viscosity of the mixture and thus the ML would increase; however, the unaffected viscosity is a good indicator of the processability of the prepared materials.
Another positive aspect of optimum processability is the small variations in the curing time (t90) (<1 min) between the compounds.It is therefore presumed that the presence of CP does not negatively affect the reactivity of DCP with ENR; in fact, the values of the peak cure rate (PCR, the maximum slope in the vulcanization zone) revealed a slight speeding effect.In addition, the crosslink density seems to increase with the CP content.This could be related to the poor solubility of CP in toluene, acting as an insoluble physical crosslinking point.
The morphological characteristics of the four unfilled compounds were studied.The photomicrographs obtained by SEM are shown in Figure 2B.A clear distinction between the ENR and CP phases was noted.Moreover, the obtained photomicrographs clearly highlight that the more CP added, the more thermoplastic domains are present inside the rubber matrix.This may indicate an incompatibility effect between both phases that is not necessarily negative, since according to the results observed in the increase in MH and ν, it could suggest that these domains act as physical reinforcement points of the ENR, as discussed above.
The mechanical properties of the rubber compounds were determined through tensile tests.The representative stress-strain curves obtained for the unfilled ENR/CP compounds are shown in Figure 3A, and Supporting Information S3 summarizes the values of the mechanical properties.The results show that the modulus at low, medium, and high strains (M100, M300, and M500) and the TS increased globally with the addition of CP to the rubber compounds, as expected, similar to findings reported for other rubber/thermoplastic blends. 29,30These observations can be well correlated to the previously discussed rheometric properties.Indeed, MH and ΔM increase with the addition of CP; thus, the material has a higher stiffness and a lower capacity for deformation.The overall reduction in EB highlights the reinforcing character of the CP on the ENR matrix.Therefore, A2 and A3 appear to be better candidates for optimizing the matrix of the novel composite, as far as the mechanical properties are concerned.
Before considering A2 and A3 for self-healing applications, it is imperative to perform thermal stability analysis to determine the temperature resistance limits of the prepared compounds.For this purpose, thermogravimetric analysis (TGA) was performed on the vulcanized compounds, as shown in Figure 2B.The incorporation of CP slightly affected the degradation temperature of pure ENR (Supporting Information S2), reducing it by approximately 20 C, from 365.1 C to 343.1 C.However, it is also evident that the material is thermally stable up to at least 300 C, a temperature well above the conventional healing protocols found in the literature, 5,31 as well as the real service conditions of general-purpose rubber materials.
Equivalent results were recently found by Tanpichai et al. 32 In their study, ENR nanocomposites were prepared with hyacinth-extracted cellulose nanofibers (CNF).Despite the construction of a robust network based on covalent sulfur bonds (a conventional sulfur-accelerant vulcanization system was employed) the thermal stability of the composites was altered by the incorporation of cellulose which usually has a lower onset degradation temperature than the ENR.
The self-healing capacities of the unfilled compounds were studied using the protocol described in Section 2. The healing temperature chosen (150 C) was higher than the glass transition temperature of the thermoplastic phase (approximately 132 C according to Figure 3C) to guarantee the mobility of the polymeric chains under pressure.The chosen time (12 h) was based on previous works in the research group, which revealed that better healing efficiency is usually achieved with longer healing times. 17The results (Figure 3D) revealed a full recovery of the M100 values, with healing efficiencies close to 100% for all unfilled compounds.Regarding the TS, a clear trend of increasing repair efficiency with increasing CP content was observed, whereas EB recovery did not vary considerably and remains at approximately 40%-50% recovery.
The healing efficiency can be explained by two distinct mechanisms.On one hand, there is the interdiffusion of rubber chains and the mobility of the thermoplastic phase (CP) into the damage zone due to a healing temperature higher than their glass transition, resulting in a reparation of the damage.The action of an external agent (CP) on the rubber matrix is identified as an extrinsic mechanism, as shown in Figure 4A.On the other hand, when the two sides of the damaged specimen are put back together, hydrogen bonds that are initially present before the damage are formed again.These bonds can be formed between an epoxy group of ENR and a hydroxyl group of CP, or between the hydroxyl groups of both phases.One of ENR comes from the ring-opening reaction typical during the peroxide crosslinking process.This is identified as an intrinsic mechanism, as shown in Figure 4B.At this point, it is important to mention that the ring-opening reaction of epoxidized isoprene can occur in the presence of an arbitrary ring-opening reagent.This additive can be acid, base, alkyl halide, or metal salt. 33Some of the agents used have included p-phenylenediamine, 34 acetic acid, 33 poly(lipoic acid), 35 F I G U R E 3 (A) Stress-strain curves of unfilled compounds, (B) thermogravimetric curves, (C) DSC thermogram of CP, and (D) M100, TS, and EB self-healing efficiencies of unfilled compounds.performic acid and a reducing agent (NaNO 2 ), 36 dicarboxylic acids, 37,38 and hexamethylene diamine (HMDA), 39 to mention a few, but it is particularly important in the presence of organic peroxides, 40,41 being the second most used crosslinking agent in the rubber industry.Supporting Information S4 shows a comparison between the FTIR spectra (in ATR mode) of compound A3 before and after vulcanization, where an increase in the intensity of the zone corresponding to 3580 and 3200 cm À1 associated with inter-and intramolecular hydrogen bonds between hydroxyl groups can be clearly seen. 424][45][46][47][48][49][50][51][52][53][54] Therefore, it can be confirmed that as the CP content increased, both healing mechanisms were favored, increasing the healing capacity of the compound.A third effect of lower intensity (because the material was crosslinked using the optimum cure time, t90) may be due to post-crosslinking processes, a common occurrence in peroxide crosslinking. 55,56This could lead to an increase in the crosslinking density after the repair protocol, promoting the recovery of the mechanical performance, and contributing to the self-healing efficiency synergistically with the extrinsic and intrinsic mechanism described.However, further studies are required to properly quantify the effect of these minor side reactions.In the self-healing field, the compromise between mechanical performance and healing efficiency is a common issue that needs to be solved.These two properties are antagonistic, meaning that an improvement in one implies the detriment of the other. 57n this work, we have successfully developed a blend between a rubber and a thermoplastic in which the TS and the healing efficiency both increase with the CP content, solving the main limitation of self-healing elastomers.
In this sense, the compound selected was A3, which corresponds to an ENR/CP ratio of 70/30, owing to its higher healing efficiency, higher moduli at low and medium strains (M100, M300, and M500), and very good mechanical performance at the breaking point.In the next stage, this compound was used as the matrix for the designed reinforced compounds.

| Development of rubber composites reinforced with CFs
In the second stage of this work, five recipes of ENR/CP blends reinforced with CFs were designed and characterized.The aim of this stage was to optimize the CF content in the composite materials and to evaluate a possible improvement in the compatibility between ENR and CP, simultaneously achieving satisfactory mechanical performance and healing capacity.The designed rubber recipes are listed in Table 2.
The curing curves, rheometric parameters, and mechanical properties of the composites are presented in Figure 5A and summarized in Supporting Information S5, respectively.A clear trend was observed according to the rheometric parameters.At low CF contents (1.25 phr and 2.5 phr), the stiffness of the material is detrimental.This is reflected in the significant reduction of the MH and ΔM values with respect to the unfilled compound.These results have also been extrapolated to the crosslink density values, with a reduction from 2.49 Â 10 À5 to 2.14 Â 10 À5 mol cm À3 at 1.25 phr of CFs.However, it is also evident that as CFs increased, these values started to recover, especially after 5 phr, when the values of the unfilled compound (A3) were exceeded.Thus, it could be said that the slightly reinforcing effect of CFs seems to occur from medium content (>5 phr) onwards.
As for the parameters associated with curing kinetics, the values of t90 and PCR show a substantial change with the incorporation of the smallest amount of CFs (CF1.25).A reduction of t90 and a respective increase of PCR is observed.This result contrasts with most of the studies available in the literature.Several authors have reported a decrease of t90 in NR composites reinforced with different natural fibers such as jute fibers, 58 pineapple leaf fibers, 59 oil palm fibers, 60 sisal fibers, 61 and grass fibers. 62However, they all attribute this increase to a longer mixing time, that optimizes dispersion and thus improves vulcanization.However, in our case, a constant mixing time was set for all the compounds prepared (20 min, as indicated in Section 2).Therefore, it is possible that the observed changes are linked to a reduction in the viscosity due to the incorporation of small amounts of CFs.Interestingly, the lower ML values confirmed that CFs generate a decrease in the viscosity of the blend.It is important to note that a slight improvement in the F I G U R E 5 (A) Curing curves, (B) crosslink density vs. torque variation, and (C) SEM photomicrographs of the filled compounds.flowability of the system could be always useful for selfhealing.However, in terms of vulcanization, this effect might be counterbalanced by the presence of functional groups on the surface of CFs.Recently, Kulshrestha et al. 63 prepared NR composites reinforced with CNF.
According to the authors, the functionalities (hydroxyl groups) present on the surface of CNF restrict the movement of free radicals generated during vulcanization, thus reducing the cure rate.This explains why, with a continuous increase in CF content (starting from 2.5 phr, CF2.5), there is an observed upward trend in the t90 value.
The mechanical properties determined from the uniaxial tensile tests and the surface hardness of the composites are summarized in Supporting Information S6.The results correlate well with the trends observed for the curing parameters.A reduction in stiffness (M100, M300, M500, and hardness values) was observed at low CF concentrations, which also recovered from CF5 until it reached a maximum value at CF15.Where up to 60% increase in TS from 1.07 to 1.7 MPa was also achieved.The incorporation of CFs at medium and high contents generates a substantial reduction in deformability, but remains at optimum values of EB, around 500%.
Photomicrographs of selected compounds were obtained by SEM, as shown in Figure 5C, to correlate the previous results with the morphology of the materials.The mechanical performance of the composites can be inferred from the distribution, quantity, and interaction of CFs within the rubber matrix.In the CF1.25 micrograph, the distribution of CFs within the matrix appears to be sparse, as indicated by the small and isolated bright spots.These isolated fibers may not provide effective reinforcement within the polymer matrix because the interfacial adhesion between the fibers and the matrix is a critical factor in transferring stress.Insufficient bonding or poor distribution can lead to decreased mechanical performance as the load transfer between the matrix and the fibers becomes less efficient.Additionally, the presence of voids or gaps, as indicated by the light blue arrows, can act as stress concentrators, thereby compromising the strength and leading to inferior mechanical properties.
In contrast, the CF15 micrograph shows a more uniform matrix with a smoother interface.This could be related to the increased number of fibers and their optimal distribution.This could translate into a higher degree of reinforcement, contributing to an interconnected network that can effectively distribute the applied stress throughout the composite material.The fibers are not readily observed at these high magnifications, which may suggest that they are well embedded within the matrix.This would result in better interfacial adhesion and more efficient transfer of stress from the matrix to the fibers.This improved fiber-matrix interaction enhances the overall mechanical properties of the composite, 64 making the CF15 composite superior in terms of mechanical performance compared to A3 and CF1.25.Supporting Information S7 shows photomicrographs taken at the same magnification for all compounds (A3, CF1.25, CF2.5, CF5, CF10, and CF15), which corroborate this improvement in the interaction between the matrix and the fiber, evidenced in a smooth interface with the increase of CFs content.
Similar results were found by Yang et al. 27 on ENR composites reinforced with sodium carboxymethyl cellulose (SCMC).In their study, a dual network formed by hydrogen bonds and dynamic covalent bonds based on disulfide exchange was constructed.The covalent part of the network ensures excellent mechanical performance but with a less conventional recipe.Through SEM they were able to visualize that at similar contents from 5 phr to 20 phr of SCMC a good compatibility between the ENR and the particles was achieved; however, the filler starts to agglomerate considerably from 30 phr of SCMC onwards.In that research, contents lower than 5 phr of the filler were not explored.
Finally, the self-healing efficiency of the reinforced composites was studied by applying the same pressure, temperature, and time as in the first stage (200 bar, 150 C, and 12 h).Figure 6 shows the obtained results.The ability of the composites to fully recover their mechanical performance at low strains (M100) was not affected by the incorporation of CFs.Regarding the properties at the breaking point, with the addition of 1.25 phr and 2.5 phr of CFs (CF1.25 and CF2.5), an increase in healing efficiency was observed; however, this is entirely motivated by the reduction of the crosslink density and the mechanical performance.However, the most interesting values were observed at 5 phr of CFs onwards.Composites CF5, CF10, and CF15 had higher mechanical performance and crosslink density than the base compound A3 but also achieved a higher healing efficiency for both TS and EB.CF5, CF10, and CF15 exhibited an increase of 21%, 50%, and 60% of TS, respectively, compared to the TS of A3.These compounds also exhibited an increase in TS healing efficiency, from 75% in A3 up to 100%, 94%, and 88%, respectively.As previously mentioned, one of the mechanisms involved in the healing of these materials is the formation of multiple hydrogen bonds between the functional groups of the ENR and the CP.By introducing the CFs, a greater number of hydroxyl ( OH) groups are incorporated, which would increase the formation of hydrogen bonds, resulting in an improvement of the self-healing capacity. 43hile the system explored in this study is unique and has no direct equivalent in the literature, as noted in the Introduction, it can still be evaluated in relation to other systems that incorporate bio-based fillers and are constructed through supramolecular interactions.It is crucial, however, to recognize the inherent challenges of comparing self-healing materials.The lack of international standards, coupled with the sheer number of variables at play, makes it difficult to establish a fair and consistent basis for comparison across different selfhealing systems.
Bearing the previous in mind, Nie et al. 19 developed ENR compounds with CNCs by forming a supramolecular network of just hydrogen bonds.Their approach, which used a substantial 20 wt.% CNC content resulted in a TS of 1.19 MPa.This figure is notably less than what has been achieved in the current study with just 5 phr of CFs.Additionally, their material demonstrated a selfrepair efficiency of 83% in TS and 95% in EB, remarkably achieved without the need for external stimuli.In contrast, the current research utilizes minimal amounts of DCP within the composite recipe to ensure the formation of a non-dynamic covalent bond network.These nondynamic covalent bonds potentially limit the mobility of the rubber chains, necessitating more stringent repair conditions.However, they also impart thermal stability to the compounds, which is essential for practical applications, as the TGA results have shown.

| CONCLUSIONS
This study explored self-healing rubber composites as a sustainable alternative to traditional rubber, aligning with the need for eco-friendly materials.New bio-based ENR/CP composites with CF reinforcement have been developed, showing promising rheometric, mechanical, and selfhealing properties.The investigation detailed herein meticulously delineates the optimization of the ENR/CP ratio as a key factor for enhancing the mechanical performance and self-healing efficiency of the bio-based elastomeric composites prepared.Self-healing was optimized 150 C for 12 h, utilizing an extrinsic mechanism facilitated by chain interdiffusion and intrinsic hydrogen bonding between the rubber and cellulose additives.The incorporation of CFs has further improved the performance, culminating in a composite material that not only upholds the mechanical strength essential for low-strength-demanding applications (less than 2 MPa) but also embodies the healing capability that is highly desired in sustainable material sciences.An increase of 21% in TS and an improvement in the healing efficiency of up to 100% was achieved in a composite with 70/30 ENR/CP and 5 phr CFs.This suggests a positive balance between mechanical strength and selfhealing capacity, which is a significant advancement in self-healing elastomers.These developments hold the potential to contribute to the UN SDGs by minimizing rubber waste and promoting sustainable industries and communities as well as responsible consumption and production.In this context, a possible application for the developed bio-based self-healing elastomeric composites could be in the field of low-stress, environmentally friendly packaging materials, specifically in elastic wrapping or cushioning products.This can include lightweight consumer goods, delicate items, or even agricultural applications, such as wrapping fruits and vegetables to prevent bruising during transportation.The soft, flexible, and elastic nature, combined with the self-healing efficiency higher than 75% makes these composites suitable for applications where high mechanical strength is not a critical factor.The bio-based nature of the materials aligns with the increasing demand for eco-friendly packaging solutions, while the selfhealing property could extend the lifespan, reducing waste and the utilization of single-use thermoplastics and thermosets.In future work, methodological improvements can consider the use of an internal mixer for enhanced dispersion and mixing at the thermoplastic melting point to foster phase compatibility.

F I G U R E 1
Schematic representation of the self-healing protocol.T A B L E 1 Rubber recipes for the optimization of ENR/CP ratio (in phr).

F
I G U R E 2 (A) Curing curves at 180 C, (B) crosslink density versus torque variation, and (C) SEM photomicrographs of the unfilled compounds.

F I G U R E 4
Scheme of self-healing (A) extrinsic and (B) intrinsic mechanisms.

F
I G U R E 6 (A) M100 and (B) TS and EB self-healing efficiencies of the reinforced composites.
T A B L E 2 Rubber recipes of reinforced ENR/CP/CFs composites (in phr).