Ultrasonic-Assisted Calcium Lignosulfonate Treatment of Titica Vine Fibers: Effects on Fiber and Composite Properties

Abstract

This research investigated the application of previously treated titica vine fibers (TVFs) as a reinforcing element in bio-based composites, aiming at the development of sustainable materials with better properties. The fibers were modified with calcium lignosulfonate (CaLS), a byproduct of the cellulose industry with potential compatibilizing properties in polymer composites. Surface treatments on natural fibers are essential to improve adhesion between the fibers and the polymer matrix, promoting a more efficient interface in the composite. Furthermore, they can reduce moisture absorption and eliminate impurities, resulting in increased mechanical properties and physical and chemical stability of the final material. Fourier transform infrared spectroscopy (FTIR) analysis confirmed characteristic vibrations of the fiber's main constituents, while X-ray diffraction (XRD) revealed a crystallinity index of 55.16% and a reduced microfibrillar angle (MFA) of 6.61°. The treated fibers exhibited a water absorption rate of 15.28% and a diffusion coefficient of 3.90 (mm².h⁻¹) x 10⁻⁴. Mechanical properties, assessed via Charpy and Izod impact tests, showed maximum absorbed energy values of 58.44 ± 18.31 J/m and 51.26 ± 10.79 J/m, respectively. Scanning electron microscopy (SEM) revealed predominantly brittle fracture behavior. Despite moderate toughness, the treatment effectively reduced MFA, improving fiber structural characteristics.

Keywords:
Titica vine fiber; Composites; Calcium lignosulfonate; Surface treatment

1. Introduction

Given the growing importance of composite materials in engineering, they have transformed the construction, maritime, automotive, defense, aerospace and sports industries¹-³. With the search for sustainable alternatives to synthetic fibers, natural lignocellulosic fibers (NLFs) stand out as a promising solution, because of their high availability, low cost and biodegradability¹-³. In addition to meeting environmental concerns, NLFs offer a competitive strength-to-weight ratio⁴,⁵, making them an attractive option for reinforcing new-generation composites. They compete with synthetic fibers, offering high-performance physical and mechanical properties, as well as being easier to manufacture⁶,⁷.

The strength and properties of NLFs in composite materials are influenced by several factors, including fiber chemical composition, morphology, degree of crystallinity, moisture content, and fiber orientation in the matrix⁸-¹⁰. According to the literature¹¹, chemical treatments are especially efficient in removing considerable amounts of amorphous components such as waxes, pectin, hemicellulose, lignin and impurities from the surface of fibers, by promoting the breakdown of hydroxyl groups. This process results in a significant increase in cellulose in relation to the amorphous constituents, thus improving the performance properties of natural fibers¹²,¹³. In addition, the hydrophilic nature of NLFs, which opposes the hydrophobic character of many polymeric matrices, contributes to the weak fiber-matrix interfacial bond¹⁴-¹⁶. Surface modifications can increase fiber surface roughness¹¹,¹⁷ enhancing mechanical interlocking with the matrix and improving interfacial adhesion. However, certain treatments may also reduce fiber crystallinity or increase hydrophilicity, potentially affecting mechanical performance and fiber-matrix compatibility. Understanding these trade-offs is crucial to optimizing fiber treatments for composite applications¹⁸.

Chemical treatments are widely used to enhance the adhesion between natural fibers and polymer matrices by modifying their surface chemistry and reducing amorphous constituents. Studies have shown that alkaline with NaOH¹⁹-²², silanes²³, pyrolysis²⁴ and benzoyl chloride treatments²⁵, for example, can enhance mechanical performance by increasing cellulose content and reducing lignin and hemicellulose fractions¹⁹-²⁵. However, the efficiency of such modifications depends on the treatment method and fiber type.

Regarding the modifying agents, it is interesting to explore treatments that use reagents from sustainable resources, in order to guarantee consistency in the use of NLFs as alternatives to materials derived from non-renewable sources²⁶. The wood refining industry has been an essential source for obtaining the main components of lignocellulosic materials (cellulose, hemicellulose and lignin), making it possible to separate these materials for the development of alternative chemical products derived from renewable sources²⁷. Lignosulphonates (LS) are by-products of pulp and paper production, through the sulphite pulping process of wood, in which the wood is digested at 140 - 170°C with an aqueous solution of a sulphite or bisulphite salt of sodium, ammonium, magnesium or calcium, in which several chemical events occur during the process²⁸. This procedure generates a black liquor, rich in lignin, carbohydrates and wood extractives. LS is extracted after dilution and filtration of the liquor to remove polymers and low molecular weight fractions. Figure 1 shows part of the structure of the calcium lignosulfonate (CaLS) used in this research and its effect on the morphological structure of Amazonian titica vine fibers (TVFs).

Figure 1
(a) Part of the chemical structure of CaLS; (b) TVFs; (c) SEM of the longitudinal surface of an untreated TVFs, 800x magnification; (d) SEM of the longitudinal surface of a TVFs after treatment with CaLS in an ultrasonic bath, 800x magnification.

While previous studies have investigated the reinforcement of polymer composites with NLFs and various chemical treatments to improve fiber-matrix adhesion, this work introduces an innovative approach using CaLS, an abundant and renewable biopolymer derivative. Furthermore, compared to traditional treatments such as alkaline, silane and acetylation, it is non-toxic, does not generate hazardous waste, tends to better preserve the fiber's structural integrity and does not require expensive reagents or greater process control. Additionally, CaLS and the epoxy matrix used in this study share functional groups similar to those found in NLFs such as hydroxyls (polar), hydrocarbons and aromatic rings (non-polar) and its application to the fiber surface can favor compatibility between the reinforcement and the matrix, improving interfacial adhesion¹⁶.

Ultrasonic impregnation enhances this process, promoting more uniform and effective penetration of CaLS into the fibers, thanks to cavitation and microbubbles generated by ultrasound²⁹, which increase reactivity and adhesion at the fiber-matrix interface. This innovative combination of chemical-mechanical treatments can contribute to the manufacture of materials with better performance and higher renewable content, enhancing the sustainability and functionality of natural fiber-based composites.

In this study, TVFs were modified for the first time using a combination of chemical (CaLS) and mechanical (ultrasonic bath) treatments. The ultrasonic process can disrupt fiber structure and potentially influence crystallinity and fiber-matrix interaction. Therefore, this study investigates the potential of CaLS treatment to modify the chemical, structural, physical and mechanical properties of TVFs, assessing its effects on crystallinity, microfibrillar angle, water absorption, and impact resistance when incorporated into epoxy composites. Using the FTIR technique, it was possible to evaluate the chemical interactions that occurred post-treatment. The crystallinity index (CI), as well as the microfibrillar angle (MFA), could be estimated using X-ray diffraction (XRD). The physical properties of the composites with treated fibers were measured using the water absorption test and determination of the diffusion coefficient. The Charpy and Izod impact tests made it possible to quantify the mechanical properties. In addition, the fracture surfaces were analyzed using SEM in order to elucidate the fracture mechanisms acting on the composites. This research explores, for the first time, the use of CaLS in an ultrasonic bath as a renewable compatibilizer for TVFs in epoxy composites.

2. Experimental Procedures

2.1. Materials

The TVFs were purchased in the form of thick roots from a local market in the city of Boa-vista, state of Roraima, northern Brazil. The material was mechanically cut into stalks and subsequently immersed in water for 24 hours to facilitate fiber extraction using a manual blade.

The 99% purity CaLS used for fiber treatment was supplied by Auro's Química (São Paulo, Brazil). Initially, the fibers were washed twice: first with deionized water at 70 °C for 1 hour, and then with a mixture of 99.4% ethyl alcohol (Prolink Química, São Paulo, Brazil) and acetone P.A. (95:5 v/v) from BHerzog (Rio de Janeiro, Brazil), under the same conditions as the first wash.

For every 1 L of deionized water, 50 g of CaLS was dissolved, thus obtaining a 5 wt.% solution of CaLS, in which 100 grams of fibers were immersed. At room temperature, impregnation was carried out in an ultrasonic bath. The equipment used was a Soniclean ultrasonic cleaner from SANDERS, using a frequency of 40KHz, for a period of 1 hour. After this stage, they were removed and dried in an oven at 60°C for 24 hours until they reached a constant weight.

The resin used as the matrix for the composites was diglycidil ether of bisphenol A (DGEBA) hardened with triethylene tetramine (TETA). The proposed stoichiometry was 13 parts hardener to 100 parts resin by weight. The manufacturer of the product is Dow Chemical from Brazil, and it is supplied by the distributor Epoxyfiber Ltda.

2.2. Composite processing

The composites of 10, 20, 30 and 40 vol% TVFs treated with CaLS in an ultrasonic bath (TVFs-CaLS-UB) were prepared using the Hand Lay-Up process, in which a compression force of 5 MPa was applied in a Sky, Brazil hydraulic press at room temperature for 24 hours, as shown in Figure 2.

Figure 2
Schematic diagram of the composite manufacturing process. (1) TVFs source plant; (2) TVFs as splints; (3) TVFs 15 cm long; (4) dissolution of CaLS for subsequent immersion of TVFs; (5) mold where the fibers will be accommodated and impregnated with resin; (6) hydraulic press; (7) manufactured composite plate.

The 15 cm long fibers were arranged in a preferentially aligned direction in a metal mold with an internal volume of 214.2 cm³ (dimensions 15 x 12 x 1.19 cm). For the manufacture of the composites, 0.50 g/cm³ was adopted as the average density for TVFs²¹ and 1.11 g/cm³ for epoxy resin³⁰.

2.3. Fourier transform infrared spectroscopy (FTIR)

The FTIR analysis was carried out on a Shimadzu IRPrestige-21 (Tokyo, Japan). The fibers were ground to a powder, which made it possible to produce a tablet with KBr. The scan was carried out over a spectral range of 4000 to 400 cm^-1, generating transmittance spectra (%) as a function of wavenumber (cm^-1).

2.4. X-ray diffraction (XRD)

The XRD analysis was carried out on PANalytics equipment, model X'Pert PRO, with Cobalt radiation (1.789 Å), 40 mA x 40 kV power and a scan range of 10 to 75°. The crystallinity index (CI) of TVFs-CaLS-UB was calculated according to the methodology developed by Segal et al.³¹, which uses the maximum intensity of the amorphous (101) and crystalline (002) peaks, represented by Iam and Icris, respectively. The CI value of the post-treated fiber was estimated from EQ. (1).

$\text{CI}=\frac{{\text{I}}_{\text{crys}}-{\text{I}}_{\text{am}}}{{\text{I}}_{\text{crys}}}\text{*}100$ (1)

The microfibrillar angle (MFA) was measured using the methodology proposed by Donaldson³² and Sarén and Serimaa³³ through the deconvolution of the crystalline cellulose peak (002), in which the Gauss curves and their first and second order derivatives are provided. Using OriginPro 8.5 software, the baseline of the diffractogram was removed and the peak (002) was isolated. The Gaussian curve of this peak was fitted and two other curves associated with their derivatives were generated from it. With this procedure, it is possible to obtain the “T” parameter, which refers to half the distance between the intersections of the tangents at the inflection points of the peak curve (002) and the baseline. EQ. (2) lists the “T” parameter for calculating the MFA of the TVFs-CaLS-UB.

$\text{MFA}=-12.19{\text{T}}^3+113.67{\text{T}}^2-348.40\text{T}+358.09$ (2)

2.5. Water absorption and diffusion coefficient

The water absorption test on the composites with treated fibers was carried out in accordance with ASTM D570-98³⁴. The test results were calculated using the difference in weight of the samples, according to EQ. (3).

$\text{\%}WA=\frac{M_{final}-M_{initial}}{M_{initial}}$ (3)

Where: %WA - Percentage of water absorption; M_initial - mass of the specimen before immersion; M_final - mass of the specimen after immersion.

Based on Fick's law, the behavior of diffusion kinetics was evaluated. In the case of polymeric materials, diffusion can be grouped into three types, as reported in previous research³⁵-³⁷. Synthetically, this classification addresses the relative mobility between the penetrant and the polymer segments. The different categories can be distinguished by fitting the experimental values to EQ. (4):

$\log \frac{M_t}{M\infty }=\log \left(k\right)-nlog\left(t\right)$ (4)

Where: Mt - moisture content at time t; M∞ - moisture content at equilibrium; k and n - constants.

To analyze the diffusion coefficient (D), we used the theoretical model provided by Fick's law, which is included in equation (4):

$\frac{M_t}{M_{\infty }}=\frac{4}{L}\text{*}{\left(\frac{D}{\pi }\right)}^{\mathrm{0,5}}{t}^{\mathrm{0,5}}$ (5)

Where: L - sample thickness.

2.6. Charpy and izod impact tests

The Charpy and Izod impact tests were carried out in a Pantec pendulum apparatus, model XC-50, Charpy and Izod configuration, operating with a 22 J hammer, in accordance with ASTM D6110-18³⁸ and ASTM D256-10³⁹, respectively. For the Charpy tests, a minimum of eight samples per volume fraction were tested, while for the Izod tests, at least twelve samples were used. Figure 3 presents the composite specimens produced.

Figure 3
(a, b) Standard dimensions of Charpy and Izod specimens, respectively; (c) Charpy specimen produced; (d) Izod specimens made.

2.7. Statistical validation

The energy absorbed by the test specimens was statistically evaluated and validated in terms of reliability and significance using the Weibull distribution and analysis of variance (ANOVA), together with the Tukey test.

2.8. Scanning electron microscopy (SEM)

This technique was used to investigate the morphology of the fracture surface after mechanical testing. Its main purpose is to understand the failure mechanisms acting on the material and to assess the quality of the interface between the fiber and the polymer matrix. The microscopic aspects of the fractured composites after the Charpy and Izod impact tests were analyzed on a Quanta FEG 250 microscope FEI, Switzerland, operating with secondary electrons and voltages of up to 5 kV. All samples were sputter coated with gold using a LEICA EM ACE600.

3. Results and Discussion

3.1. Fourier transform infrared spectroscopy (FTIR)

Figure 4 shows the FTIR spectra of the TVFs after treatment, represented by TVFs-CaLS-UB and Calcium Lignosulfonate, CaLS, compared to the untreated fiber, TVFs-U, studied previously⁴⁰. Through these it is possible to identify absorption bands that correspond to specific molecular vibrations, making it possible to identify different functional groups associated with the main constituents of NLFs and responsible for their properties.

Figure 4
TVF after treatment, TVFs-CaLS-UB compared to untreated fiber, TVFs-U.

Table 1 summarizes the main absorption bands observed in the TVFs-CaLS-UB spectrum. No significant changes were observed between the spectra of the fibers before and after treatment. This is related to the low quantity of CaLS deposited on the surface of the TVFs, which is considered low in relation to its integral composition. Thus, the signals from the fiber's original lignin may overlap with the signals from the CaLS deposited on it.

Table 1
Main FTIR absorption bands for the TVFs after treatment with Calcium Lignosulfonate in an ultrasonic bath (TVFs-CaLS-UB).

Position cm-1	Band assignment	Reference
3450 – 3400	O-H bond (Cellulose, Hemicellulose, Lignin and Lignosulfonates)	41-43
2960 – 2830	Stretching of C-H and R-O-CH3 groups (Cellulose, Hemicellulose, Lignin and Lignosulfonates)	44-46
2368 - 2337	C-H Stretch (Cellulose and Hemicellulose)	44
1735	C=O bond (Lignin and Hemicellulose)	42,47
1655 – 1455	CH3 asymmetric deformation; C=C stretching (aromatic ring) (Lignin and Lignosulfonates)	48-50
1041	C-H/C-O and C-C stretching (Cellulose, Hemicellulose, Polysaccharides and Lignosulfonates)	44,46,50,51
834	Out of plane C-H stretching of Guaiacyl, Syringyl and p-hydroxyphenyl units (Lignins)	52

However, it is possible to notice bands whose relative proportions have changed. Some of them are at wavelengths of 2924 cm^-1 and 1651 cm^-1, which are characteristic of the addition of methoxyl groups during the sulfonation reaction and axial deformation of the C=C bonds of the aromatic ring, respectively. The brief expansion of these bands, and consequently the increase in their areas, is probably related to the presence of CaLS. Another similar case occurs at 1041 cm^-1, which may be associated with the insertion of the -SO₃ group. Finally, the band at 834 cm^-1 of the untreated fiber, TVFs-U, is noteworthy because after treatment it becomes imperceptible. It is also not shown in the CaLS spectrum. As this band is related to the stretching of functional groups belonging to units present in lignin, it can be inferred that the treatment reduced this constituent.

Although the FTIR spectrum of CaLS-treated fibers reveals no significant changes compared to untreated fibers, variations in specific bands suggest the presence of the modifying agent and indicate possible chemical interactions with the fiber surface. The observed changes in the relative intensities of bands associated with methoxyl, sulfonate, and aromatic bonds, as well as the disappearance of bands linked to the original lignin, reinforce the hypothesis that CaLS treatment promotes specific changes in the surface composition of the fibers, albeit at low concentrations.

3.2. X-ray diffraction (XRD)

The diffraction pattern was analyzed using OriginPro 8.5 software by means of computational data processing. Figure 5 shows the diffractogram obtained for the TVFs-CaLS-UB.

Figure 5
XRD pattern of the TVFs-CaLS-UB fibers.

Figure 5 presents the diffractogram after normalizing the baseline, to make it easier to see the relevant peaks. The fibers analyzed showed typical semi-crystalline behavior, with the (101), (002) and (004) planes standing out. To measure the crystallinity index, Equation (1) was used, which relates the interference intensity in the (002) plane corresponding to the 2θ = 25.47° peak and the scattering of the amorphous region in the (101) plane, referring to the peak at 2θ = 17.08°.

The crystallinity index (CI) of the TVFs after treatment was 55.16%, indicating a structural reorganization of the fibers. While a decrease of 29.6% was observed compared to untreated fibers, this transformation suggests increased fiber flexibility and potential improvements in fiber dispersion within polymer matrices. Such changes may enhance composite processing and energy dissipation mechanisms under mechanical loading³¹,⁴⁰. According to Flannigam et al.⁵³, as ultrasound propagates through a liquid medium, microbubbles form and collapse, an event called cavitation. When a sound wave passes through a liquid, regions of compression (positive pressure) and rarefaction (negative pressure) are created. If the negative pressure is high enough, a cavity or bubble can be formed in the liquid from the nucleus of gases present inside the fluid. The bubbles are distributed throughout the liquid, grow to a critical size, become unstable and collapse⁵⁴. When cavitation occurs close to a solid surface, the collapse is asymmetrical and favors the formation of liquid jets with high velocity towards the surface of the solid, hitting it with considerable force. The resulting collisions can cause changes to the surface, composition and morphology⁵⁵.

Since cellulose determines the crystallinity of NLFs, this result may indicate that the ultrasonic irradiation used in the presence of CaLS allows for the disaggregation of fiber bundles and the penetration of the species present in the medium (Ca⁺/H₂O) into the crystalline domains, as occurs in mercerization⁵⁶, a fact that contributed to the reduction in crystallinity of TVFs after treatment. This same behavior was evidenced by Oliveira et al.²⁶ when researching sisal fibers treated with sodium lignosulfonate under different conditions, including ultrasound irradiation.

Like the CI, the orientation of the microfibrillar angle (MFA) is also mentioned as one of the factors responsible for improving the mechanical properties of NLFs⁵⁷,⁵⁸. Using the diffraction pattern obtained, as shown in Figure 5, it is possible to estimate the MFA of the TVFs-CaLS-UB fibers. The peak attributed to the crystalline phase in the plane (002) was isolated, and based on mathematical methods, the MFA of the TVFs after surface modification was measured. Figure 6 shows the graphs associating the peaks (002), adjusted by a Gaussian and their first and second order derivatives.

Figure 6
Microfibrillar angle of TVFs after treatment with CaLS in an ultrasonic bath, TVFs-CaLS-UB.

The microfibrillar angle (MFA) of the treated fibers decreased to 6.61°, a 16.8% reduction compared to untreated samples⁴⁰. This value places the modified TVFs among the lowest MFA values reported for natural fibers (Table 2), indicating a more aligned fibrillar structure, which is often associated with increased stiffness and enhanced load transfer in composite applications, which may have induced structural modifications, affecting both the morphological surface (Figure 1d) and the composition (Table 2) due to the presence of CaLS. Notably, the CaLS-treated TVF exhibited the lowest microfibrillar angle reported in the literature, which could significantly enhance its mechanical properties.

Table 2
Crystallinity index and microfibrillar angle of CaLS-treated (TVFs-CaLS-UB) and untreated (TVFs-U) liana fibers in relation to other NLFs.

Fiber	CI (%)	MFA (°)	Reference
TVFs-CaLS-UB	55.16	6.61	PW
TVFs-U	78.3	7.95	40
Cyperus malaccensis	62.47	7.36	59
Mendong Grass	58.60	22.90	60
Kenaf	44.3	7.10	61
Acacia Pennata	46.52	19.67	62
Heteropogon contortus	54.10	14.53	63
Sida cordifolia stem	56.92	9.50	64

• PW – Present Work

The MFA, which can be defined as the angle formed by the microfibrils in relation to the longitudinal axis of the fiber, has a direct influence on the mechanical properties of NLFs, such as toughness and rigidity. The lower the MFA, the more resistant and rigid the fibers tend to be. On the other hand, high angles tend to increase ductility and predisposition to fracture⁶⁵. In addition, varying this angle can affect other properties such as air permeability and moisture absorption⁶⁶.

The result obtained for TVFs after treatment, despite its decrease in CI, places it on the same level as other NLFs, including those already established in the literature, such as kenaf. In addition, this was compensated for by the fact that its MFA was the lowest of all the other NLFs (Table 2) and was able to induce an improvement in its mechanical resistance properties.

3.3. Water absorption and diffusion coefficient

Figure 7 presents the water absorption behavior of the composites containing 40 vol% TVFs treated with CaLS in an ultrasonic bath (TVF40-CaLS-UB), compared to untreated fiber composites (TVF40-U). After 504 hours of immersion, the water absorption of TVF40-CaLS-UB composites was 15.28%, showing a moderate increase relative to untreated samples (13.77%).

Figure 7
Water absorption curve for composites with 40 vol% of fibers treated with CaLS in an ultrasonic bath (TVF40-CaLS-UB) compared to composites with the same percentage of fibers, but without any treatment (TVF40-U).

This trend aligns with previous studies on lignosulfonate-treated fibers¹⁶, where slight increases in water absorption have been attributed to modifications in fiber surface chemistry. The ultrasonic bath may have influenced fiber morphology and microstructure, contributing to enhanced fiber-matrix interactions while also increasing the number of hydroxyl groups available for moisture adsorption. However, such effects can be mitigated through complementary treatments, such as additional hydrophobic coatings or alternative processing conditions.

Using the data obtained in the water absorption test, Fick's law (Eq. 4 and 5) was applied to investigate the water diffusivity process in the composites with TVFs-CaLS-UB. Table 3 presents the values ​​of the diffusion coefficient (D) and the parameter “k”.

Table 3
Diffusion coefficient (D) values ​​and "k" parameter.

Samples	D (mm2.h-1) x 10-4	k	Reference
TVF40-CaLS-UB	3.900	0.997	PW
TVF40-U	3.770	0.975	16
Polyester/pandanus fascicularis lam (30%) - 3% NaOH	25.23	-	67
Polyester/pandanus fascicularis lam (30%) untreatment	31.65	-	67
Polyester/palf (40%) untreated	7.20	1.193	68
Polyurethane/tucum (50%) untreatment	0.0013	0.13	69
Polyurethane/mallow (50%) untreatment	0.00057	1.34	69

• PW – Present Work

The diffusion coefficient (D) values confirm that, while the treated fibers exhibited slightly higher water mobility within the composite (3.900 x 10⁻⁴ mm².h⁻¹ vs. 3.770 x 10⁻⁴ mm².h⁻¹ for untreated), the material's overall structural integrity remains preserved. This suggests that with further processing refinements, CaLS-treated fibers could be optimized for enhanced moisture resistance while retaining their mechanical benefits.

The increase in D for composite samples with treated fibers compared to those with untreated fibers indicated a material more prone to penetration of water molecules and with greater ability to move within the composite. The variation in the "k" value is related to the strong affinity of water with the composite, showing that the fibers after treatment were not as well protected by the matrix and that their impregnation by epoxy was impaired due to poor anchoring justified by their more hydrophilic character that they presented post-treatment.

Despite this slight increase in D and “k” compared to the untreated TVF composites, these values are quite satisfactory compared to the polyester/pandanus fascicularis lam (30%) systems treated with 3% NaOH⁶⁷, polyester/pandanus fascicularis lam (30%) without treatment⁶⁷, and polyester/palf (40%)⁶⁸. On the other hand, the untreated polyurethane/tucum (50%)⁶⁹ and untreated polyurethane/mallow (50%)⁶⁹ composites obtained the best performances in similar tests. Therefore, as can be observed from the literature records, the TVF40-CaLS-UB composites become a viable alternative for applications that require moisture resistance.

3.4. Charpy and izod impact tests

Figure 8 presents the results of Charpy and Izod average absorbed impact energy (E_abs) of the composites manufactured with TVFs-CaLS-UB in the fractions of 10 – 40 vol% of NLFs. In addition, a comparative study was carried out with the composites of untreated fibers¹⁶, the control group (plain epoxy)¹⁶ and with the composite of 40 vol% of TVFs with CaLS under heating and magnetic stirring (TVF40-CaLS-70°C)¹². The Charpy impact test (Figure 8a) revealed a progressive increase in absorbed energy (E_abs) with the addition of fibers, reinforcing the role of TVFs in enhancing composite toughness. Although the absolute values for the CaLS-treated fibers (TVF40-CaLS-UB) were slightly lower than those of untreated fibers (TVF40-U) and those treated at 70 °C (TVF40-CaLS-70°C), the differences were within an expected range for chemically modified natural fibers.

Figure 8
Variation of the impact energy absorbed by the composites manufactured with TVFs-CaLS-US in the fractions of 10 – 40 vol% of NLFs compared to those without treatment and treated with TVFs modified with CaLS at 70 °C under magnetic stirring (TVF40-CaLS-70°). PW – Present Work

Notably, the Charpy impact resistance of the TVF40-CaLS-UB composites remained comparable to plain epoxy, with only a 5% difference, indicating that the treatment preserved the matrix integrity while maintaining a reinforcing effect. The maximum value obtained in the Charpy test in this study (58 J/m) exceeds the results recorded for polyester composites with curaua fiber treated with 10% and 0.1% NaOH (38 and 27 J/m, respectively)⁷⁰, and is comparable to the performance of the linear low-density polyethylene system with sisal treated with a 3% maleic anhydride-grafted polyethylene solution (60 J/m)⁷¹. However, in both cases in the literature, the composites with untreated fibers presented superior performance — a trend that was also observed in the present investigation.

These results suggest that further adjustments in processing conditions, such as optimizing fiber dispersion and improving interfacial adhesion, as well as exposure time and functionalizing agent concentration, could enhance the energy dissipation mechanisms, unlocking the full potential of CaLS-treated fibers in impact-resistant applications

The results of the Izod test (Figure 8b) show that there was an increase in energy absorption when adding 40 vol% of fibers treated in an ultrasonic bath. When compared to pure epoxy, the TVF40-CaLS-UB group had a 21.3% increase in this property. However, again these samples (TVF40-CaLS-UB) presented lower values ​​compared to TVF40-U and TVF40-CaLS-70°C. This difference represents a drop of 14.4% and 4%, respectively.

To confirm the effect provided by the treatment of TVFs-CaLS-UB in the 10–40 vol% NLFs composites, the Charpy and Izod impact energy absorption property was statistically analyzed by the Weibull method. Furthermore, the standard deviation values ​​of the compositions may compromise the degree of certainty regarding whether the CaLS treatment in an ultrasonic bath acted or not as reinforcement in the matrix, as well as whether they were effective. Thus, analysis of variance (ANOVA) and Tukey test were performed to verify the occurrence of significant difference between the Charpy and Izod absorbed energy results for the plain epoxy, TVF40-U, TVF40-CaLS-70°C and TVF40-CaLS-UB groups. The choice is justified by the fact that plain epoxy was the control group and the 40 vol% fractions stood out in the tests.

3.5. Statistical validation

Table 4 shows the Weibull parameters (β e R²), in addition to the characteristic unit θ. The Weibull modulus β or shape parameter controls the variation of the results and provides an assessment of the homogeneity of the material. R² values ​​reflect the statistical reliability of the results achieved. The parameter θ refers to an estimate of whether the material analyzed presents a certain property.

Table 4
Weibull parameters for Charpy and Izod impact resistance of TVFs-CaLS-US composite.

Sample	Charpy			Eabs (J/m)	Standard Deviation	Izod			Eabs (J/m)	Standard Deviation	Reference
	β	ϴ	R2			β	ϴ	R2
TVF10-CaLS-UB	3.61	44.49	0.99	41.22	10.17	3.34	50.27	0.94	45.52	14.71	PW
TVF20-CaLS-UB	5.02	39.49	0.91	37.52	6.41	4.78	41.31	0.93	37.86	8.42	PW
TVF30-CaLS-UB	2.77	51.72	0.89	46.75	17.65	3.49	39.29	0.94	36.50	12.50	PW
TVF40-CaLS-UB	3.51	63.87	0.94	58.44	24.53	5.14	54.76	0.96	53.31	10.79	PW
Plain Epoxy	6.86	61.41	0.98	61.52	7.89	3.03	46.31	0.83	42.24	15.91	16
TVF40-U	5.80	74.67	0.96	75.05	7.32	4.99	61.02	0.92	58.65	9.26	16
TVF40-CaLS-70°C	3.40	88.5	0.94	80.59	24.53	4.27	59.52	0.91	53.31	13.89	16

• PW – Present Work

Based on the values ​​shown in Table 4, it is observed that, for both tests, the TVF30-CaLS-UB and TVF40-CaLS-UB composites tended to reveal lower β, that is, a greater variation in the values, which could also be verified by the greater standard deviation observed in these groups. The exception occurs only for the TVF40-CaLS-UB Izod samples, in which the greater homogeneity may have been the result of factors related to the efficiency of the treatment in this set of fibers, such as: greater removal of impurities, greater pore opening and reduction in water absorption capacity.

The parameter θ, as expected, followed the same pattern as the E_abs measured directly in the test. This index indicates that 63.2% of the samples in each group should present E_abs values ​​corresponding to θ. The statistical precision coefficient R² showed high representativeness and was within a statistically acceptable reliability range (> 0.83). This result allows us to state that of the groups evaluated, a maximum of 17% of the samples cannot be justified by the Weibull mathematical model.

Table 5 presents the ANOVA of the Charpy and Izod impact resistance results for samples from the control group (plain epoxy) and 40 vol% composites with untreated and treated TVFs. The data in Table 5 indicate that the hypothesis of equality between the Charpy and Izod absorbed energy values ​​is rejected with a 95% confidence level, since F_cal = 3.43 and 3.46 are higher than F_critical = 2.95 and 2.82, respectively.

Table 5
ANOVA for the absorbed impact energy Charpy and Izod of plain epoxy and 40 vol% composites with untreated and treated TVFs.

Variation Causes	Plain epoxy, 40 vol% treated and untreated (Charpy)					Plain epoxy, 40 vol% treated and untreated (Izod)
	SS	DF	MS	Fcalc	Fcrítical	SS	DF	MS	Fcalc	Fcritical
Treatment	2706.45	3	902.15	3.43	2.95	1680.68	3	560.23	3.46	2.82
Residue	7368.57	28	263.16			7130.42	44	162.05
Total	10075.02	31				8811.10	47

To verify which group presented better resistance to Charpy and Izod impact, the Tukey test was applied to compare individual performance with a 95% confidence level. Tables 6 and 7 present the values ​​of the honest significant difference (HSD) obtained by the Tukey test for the Charpy and Izod tests, respectively.

Table 6
HSD measured by Tukey's test for the absorbed Charpy impact energy of plain epoxy and 40 vol% composites with untreated and treated TVFs.

Sample	Plain Epoxy	TVF40-U	TVF40-CaLS-70°C	TVF40-CaLS-UB
Plain Epoxy	0.00	-13.53	-19.07	3.08
TVF40-U	13.53	0.00	-5.54	16.61
TVF40-CaLS-70°C	19.07	5.54	0.00	22.15
TVF40-CaLS-UB	-3.08	-16.61	-22.15	0.00

Table 7
HSD measured by Tukey's test for the absorbed Izod impact energy of plain epoxy and 40 vol% composites with untreated and treated TVFs.

Sample	Plain Epoxy	TVF40-U	TVF40-CaLS-70°C	TVF40-CaLS-UB
Plain Epoxy	0.00	-16.40	-11.07	-9.02
TVF40-U	16.40	0.00	5.33	7.38
TVF40-CaLS-70°C	11.07	-5.33	0.00	2.05
TVF40-CaLS-UB	9.02	-7.38	-2.05	0.00

The calculated HSD for the Charpy test was 22.14 J/m, and differences greater than this value are considered significant. The data in Table 6 indicate that the Charpy impact strength of the TVF40-CaLS-70°C composite was indeed greater than that of TVF40-CaLS-UB, and that there was no significant difference between the plain epoxy, TVF40-U, and TVF40-CaLS-70°C, since the difference between the means did not exceed the calculated HSD. For the Izod test, the calculated HSD was 7.24 J/m. The values ​​in Table 7 showed that the Izod impact strength of the TVF40-CaLS-US composite was superior to that of plain epoxy but inferior to that of TVF40-U. Furthermore, no significant difference was observed between TVF40-CaLS-UB and TVF40-CaLS-70°C.

3.6. Scanning electron microscopy (SEM)

The increase in impact resistance observed as fibers were added to the composites may be related to the fracture mechanisms involved. Furthermore, this same explanation can be applied to justify the reduced toughness of TVF40-CaLS-UB samples when compared to untreated ones. To validate and deepen the analysis on the evolution of fracture mechanisms in the evaluated materials, Figures 9 and 10 show the fracture surfaces of the samples treated with CaLS in an ultrasonic bath (TVF10-CaLS-UB and TVF40-CaLS-UB), in the Charpy and Izod tests, respectively.

Figure 9
SEM of fracture surfaces of the Charpy tested samples. (a) TVF10-CaLS-UB composites, 200x magnification and (b) TVF40-CaLS-UB, 200x magnification.

Figure 10
SEM of fracture surfaces of the Izod tested samples. (a) TVF10-CaLS-UB composites, 200x magnification and (b) TVF40-CaLS-UB, 200x magnification.

In Figures 9a and 10a, a completely fragile type of fracture can be seen, due to the strong presence of “river marks” on the impact surface of the samples. Furthermore, another failure mechanism frequently found for these composites was matrix fracture, which often occurred catastrophically, as there were few regions filled with fibers that could serve as a barrier to crack propagation. On the other hand, more complex fracture mechanisms were activated in the TVF40-CaLS-UB composites (Figures 9b and 10b), such as fiber rupture and fracture, delamination’s and pullout. Although there was still the presence of “river marks”, a significant decrease was noticeable across the entire surface. The presence of these new mechanisms explains the more efficient performance of these samples when compared to those with a lower percentage of fibers, as these can provide an additional free surface with the capacity to increase the tenacity of the composites. These findings align with previous studies⁵⁹,⁷²,⁷³, which observed variations in impact energy absorption in composites reinforced with different natural fibers. Such variations are often attributed to differences in fiber morphology, surface modification, and fiber-matrix interaction. The observed trend suggests that further optimization of treatment parameters, such as processing conditions and fiber dispersion, could enhance the energy dissipation mechanisms, potentially improving the impact resistance of CaLS-treated composites.

Even with the appearance of more complex fracture mechanisms, the statistical analysis showed that the main factor responsible for the improvement in the mechanical properties evaluated was the increase in fiber content and not the treatment with CaLS in an ultrasonic bath, since the results indicated that in both tests (Charpy and Izod) the TVF40-CaLS-UB composites, the product of the present study, always demonstrated an equal or inferior level in relation to TVF40-U and TVF40-CaLS-70°C, inferring that there was fiber embrittlement after surface modification. This weakening previously observed due to the decrease in fiber crystallinity, combined with the weak fiber/matrix compatibility power highlighted in the moisture absorption test, gave indications that the mechanical properties could be modest.

Although moderate impact resistance was observed, the reduced MFA highlights the potential of CaLS-treated fibers for load-bearing applications where rigidity and structural alignment are advantageous. The results suggest that, with continuous optimizations of the CaLS treatment applied to NLFs, such as increasing the modifying agent concentration and varying the sonication time, the developed composites can be efficiently targeted to meet applications with higher demands, durability and mechanical performance.

4. Conclusions

This pioneering study evaluated the chemical, structural, physical, and mechanical properties of Amazonian titica fibers (TVFs) treated with ultrasound-assisted calcium lignosulfonate (CaLS), as well as their epoxy matrix composites. The main results obtained were:

FTIR spectroscopy identified absorption bands associated with functional groups typical of lignocellulosic materials. The intensification of the bands at 2924 cm⁻¹ and 1041 cm⁻¹ was attributed to the addition of methoxyl and sulfonate groups, respectively.

The treatment of TVF with CaLS resulted in a 29.6% reduction in the crystallinity index of the fibers, attributed to the mechanical action of the ultrasonic bath. In addition, there was a 16.8% decrease in the microfibrillar angle (MFA), reaching 6.61°, which may influence the overall properties of the fibers and their composites.

The water absorption test, carried out on the treated fiber composites, showed an absorption of 15.28%, 1.51 percentage points higher than that the untreated samples. The diffusion coefficient of the samples was measured at D = 3.900 x 10⁻⁴ (mm².h⁻¹) and the parameter K = 0.997, indicating that the material was more prone to absorbing water and had a greater capacity for movement of molecules within the composite than those without any treatment. Nevertheless, these composites demonstrated better performance than several other polymer matrix composites reinforced with NLFs previously reported.

The Charpy and Izod impact tests were performed on samples containing 10–40 vol% of CaLS-treated fibers in an ultrasonic bath to evaluate their mechanical properties. Weibull statistical analysis indicated that composites with 30 and 40 vol% of treated fibers presented lower homogeneity, which was evidenced by the low β value in these samples. The parameter θ followed the same pattern of impact resistance results measured directly in the tests, while the statistical precision coefficient R² was shown to be within a statistically acceptable reliability range (> 0.83).

In the Charpy test, ANOVA followed by Tukey’s test indicated that the TVF40-CaLS-UB group exhibited no significant difference compared to plain epoxy and TVF40-ST (40 vol% untreated). In contrast, in the Izod test, TVF40-CaLS-UB composites demonstrated significantly higher impact resistance than plain epoxy, but lower than TVF40-U.

SEM analysis of the fractured specimens showed that, although the samples with a higher fiber content treated in an ultrasonic bath (40 vol%) presented more complex fracture mechanisms, this was not enough to improve their mechanical properties compared to the untreated samples. This was evidenced by the strong presence of river marks in the samples and through statistical analyses. In fact, what drove the slight increase in impact resistance was the increase in the percentage of fibers and not the treatment applied.

Despite the obtained results and identified limitations, the reduction in MFA indicates potential for improving the mechanical properties of the composites. It should be considered that these tested materials may not be suitable for applications subjected to abrupt and dynamic loading, such as impact tests. However, they demonstrate viability for applications requiring intermediate mechanical performance and moisture resistance, suggesting their use in sectors such as civil construction and the furniture industry.

5. Future Work

As evidenced by the results, the present study exhibited certain limitations, particularly a slight reduction in the crystallinity index of the fibers following treatment, as well as a decrease in the Izod impact resistance of the composites reinforced with treated fibers compared to those reinforced with untreated fibers. To mitigate these adverse effects, it is recommended to conduct further experiments by varying treatment parameters such as CaLS concentration, sonication time, and the type of polymer matrix used.

• Data Availability
  All the data set supporting the results of this study has been published in the article itself.

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Edited by

• Associate Editor:
  Sandro Amico.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

All the data set supporting the results of this study has been published in the article itself.

Publication Dates

• Publication in this collection
  01 Dec 2025
• Date of issue
  2025

History

• Received
  19 Apr 2025
• Reviewed
  19 Oct 2025
• Accepted
  26 Oct 2025