Introduction

Continuous progress in materials engineering related to the production of functional and advanced polymeric materials, the growing ecological awareness of society and increasingly stringent legal regulations regarding environmental protection and the strategy of implementing the principles of sustainable development require the search for and improvement of pro-ecological solutions (Abdulkhani et al. 2020; Chauhan et al. 2022). One of the directions that meet these expectations is the use of raw materials from renewable sources in the production of polymer composites. The biocomposites obtained in this way usually consist of natural fibers and biopolymers, they are an environmentally friendly alternative to conventional materials. A significant part of research in the field of polymer reinforcement bio-additives focuses on composites based on thermoplastics and resins, the topic of using natural fillers in elastomeric matrices is less developed (Ku et al. 2011; Mohammed et al. 2015). Natural rubber (NR) is the only bio-based elastomer. It is considered a biomaterial with high-performance parameters, such as high tensile strength, high tear strength, high flexibility, good resistance to cracking and low heating (Liu et al. 2015). The importance and position of NR are inextricably linked to technical and economic development, especially in means of transport and the automotive industry (Nair and Joseph 2014).

In turn, nature has provided us with many examples of materials that can act as bio-additives in polymer composites, these are raw materials obtained from plants (plant fibers, biomass) or animals (bird feathers, wool, silk) (Ramamoorthy et al. 2015). Among natural vegetable fibers used in the technology of polymer composites, research attention is focused on materials obtained from kenaf, sugar palm, flax, jute, hemp, pineapple, rice and cotton, etc. (Jagadeesh et al. 2021). The use of these fibers in polymer matrices has already been extensively described and investigated. In addition, these plants are mostly exotic raw materials from the point of view of European agriculture and climate. Therefore, alternative sources of bio-additives for the production of biocomposites are still being sought. An interesting proposition of a natural filler may be the use of horsetail biomass. Although field horsetail, due to its health-promoting properties, has been used in natural medicine and cosmetology (Sandhu et al. 2010), it is still a common weed (James and Rahman 2010). It is a plant widespread on our continent, therefore it is a valuable source of material that is easily available and requires management. Thus, horsetail aroused interest and became a potential source of biomass for modifying the properties of natural rubber. Horsetail, due to the high content of active antioxidant compounds, in previous works was characterized as a source of extracts with anti-aging properties for use in polymer composites (Masłowski et al. 2020a, b).

Field horsetail as a lignocellulosic material due to the presence of hydroxide groups on its surface is more hydrophilic compared to the polymer matrix. As a consequence, the bond between the plant fiber and the polymer matrix is weak. In order to ensure better compatibility and full use of the potential of the fiber, it is necessary to improve the compatibility between the components of the biocomposite. In elastomer technology, a commonly used method of increasing adhesion at the polymer-filler interface is the addition of a silane coupling agent (Shokoohi et al. 2008). These agents are mainly used to modify carbon fillers (Choi and Son 2016; Szadkowski et al. 2020; Wang et al. 2022), and silica (Ko et al. 2012; Zheng et al. 2018), moreover, numerous works of literature have indicated the surface modification of metal oxide particles (Ahangaran and Navarchian 2020) such as TiO2 (Zhang et al. 2019), Al2O3 (Abdelsalam et al. 2023), and Fe3O4 (Raa Khimi and Pickering 2016). However, more and more often attempts are made to use silanes as compounds in the compatibilization of natural fillers with polymer matrices (Xie et al. 2010). The literature on the subject indicates that they are mainly used for thermoplastic and epoxy matrices and mainly concern the modification of natural fibers, and to a lesser extent, finely divided natural lignocellulosic fillers (Abdelmouleh et al. 2007).

The natural fiber is treated with a silane coupling agent, changing the hydrophilic surface to an organophilic surface, which only helps to avoid particle agglomeration in the system, but also to improve the mutual wettability of the polymer and fillers. Silane coupling agents belong to the class of organosilane compounds containing at least two different types of reactive groups bound to the silicon atom in the molecule (Goyal 2006). One of the reactive groups of different types (e.g. methoxy, ethoxy and silanol hydroxyl groups) can react with foreign groups on the surface of the filler (e.g. cellulose hydroxyl groups), while other reactive groups which are usually non-polar (e.g. vinyl groups, epoxy, methacrylic, amine and mercapto) are more compatible with rubbers and can also participate in the cross-linking process to form chemical bonds (Sae-oui et al. 2006). The functional groups of the silane molecules play an important role in the filler/rubber system, affecting filler-to-elastomer interfacial adhesion (Kaewsakul et al. 2015). The silanization reaction enhances filler-elastomer interactions, reduces filler-filler interactions, and lowers the compound viscosity, which is beneficial for both processing and vulcanizate properties (Hayichelaeh et al. 2018).

Due to the diversity of the chemical structure of silanes, they can affect the interactions between the composite components in different ways. The paper presents a review of the use of selected silanes to modify the natural filler in the form of field horsetail. The effect of silanized horsetail fibers on the processing and functional properties of composites and the effects of a modified filler in natural rubber were described. The coupling compounds were selected in such a way that some of them could participate in the rubber cross-linking processes due to the presence of double bonds (Vinyltriethoxysilane) or the presence of sulfur atoms (3,3′-Tetrathiobis(propyl-triethoxysilane)) in their structure. In addition, a silane with a polar amino group (3-Aminopropyl)triethoxysilane) was used, as well as a compound containing a non-polar octyl group (Octyltriethoxysilane) for comparison.

Experimental

Materials

Polymer

Natural rubber (NR) in the form of Ribbed Smoked Sheets (RSS 1), cis-1,4-polyisoprene, with a density of 0.93–0.98 g/cm3 (country of origin—Malaysia, supplier—Torimex-Chemicals Ltd. Sp. z o. o., Poland).

Curing system

Sulfur (S) (Siarkopol, Poland), zinc oxide (ZnO) (Sigma–Aldrich, USA), stearic acid (SA) (Avantor Performance Materials, Poland) 2-mercaptobenzothiazole (MBT) (Sigma–Aldrich, USA).

Coupling agents

Vinyltriethoxysilane (VTES) putity 97%, density 0.903 g/mL at 25 °C; v 3,3′-Tetrathiobis(propyl-triethoxysilane) (TESPTS) purity technical, ≥ 90%, density 1.08 g/mL at 20 °C; 3-(aminopropyl)triethoxysilane (APTES), purity ≥ 98.0%, density 0.929 g/mL at 25 °C; 3-(chloropropyl)triethoxysilane (CPTES).

purity 95.0%, density 1.000 g/mL at 25 °C and octyltriethoxysilane (OTES) putity 97%, density 0.88 g/mL at 25 °C. All components were provided by Sigma–Aldrich, USA. The structure of the silane compounds used is illustrated in Table 1.

Table 1 Characteristics of applied silane compounds
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Filler

Horsetail (HT) (ManuTea, Poland). Preparation of the filler included mechanical processing using Pulverisette 5 Classic Line planetary ball mill (Fritsch, Germany) for 30 min, with a speed of 300 rpm. The ground product was dried at 70 °C. Next, the filler was chemically modified. The treatment of the horsetail material was carried out in a mixture of water and ethanol (volume ratio 1:4). Silane and natural filler were added to the solution in a weight ratio of 1:10. The following concentration of silane compounds was used to modify the filler (Table 2):

Table 2 Concentrations of silanes applied in studied systems
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Ultrasonic treatment was used to ensure sufficient mixing. After 2 h, the solvent was evaporated from the mixture using an evaporator Laborata 4001 (Heidolph, Germany). The modified filler was dried at 70 °C. The plan of modifications to the surface of the fillers is shown in Fig. 1.

Fig. 1
figure 1

Proposal of horsetail modification with the use of silanes

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Biocomposites (sample)

Composition of typical elastomer mixture: NR rubber (100 phr—Parts Per Hundred Rubber), sulfur (2 phr), mercaptobenzothiazole (2 phr), zinc oxide (5 phr), stearin (1 phr) and fillers—(0 for reference sample and 10, 20 and 30 phr for biocomposites).

Methods

Fourier Transform Infra-Red Spectroscopic analyses (FTIR) were carried out in Nicolet 6700 FTIR spectrometer and OMNIC 3.2 software (Thermo Scientific, USA) between 400 and 4.000 cm−1, using 64 scans and 4 cm−1 resolution.

Thermal analysis was carried out using the Thermogravimetric Analyzer TGA (Mettler Toledo, Switzerland) for filler samples weighing approx. 10 mg. The measurement consisted of two stages. In the first segment, the sample was heated from 25 to 600 °C in an argon atmosphere, after exceeding this temperature, the gas was changed to air and further heated to 800 °C. The heating rate was 10 °C/min.

Scanning electron microscopy (SEM) was used to assess the morphology of both powdered fillers and vulcanizate fractures. Before the measurement, the samples were covered with carbon by the Cressington 208 HR system. SEM images were taken using the LEO 1530 Gemini scanning electron microscope (Zeiss/LEO, Germany).

In order to determine the contact angle, the filler powders were compressed into a pellet with a smooth surface. Then, drops of water (10 µl) were placed on the surface of the obtained samples, and the image was immediately captured. A goniometer (DataPhysics, OCA 15EC, San Jose, USA) was used for the measurements.

Elastomeric mixtures according to the recipes described in the material section were prepared in two stages. First, rubber and filler were mixed at 50 °C using the Measuring Mixer N50 (Brabender, Germany). Subsequently, the cross-linking agents were added to the previously prepared masterbatches and sheets of the rubber mixture were formed. This step was carried out using a two–roll mill.

Rheometer tests were carried out at 160 °C using a rotorless rheometer Model–MDR (Alpha Technologies, USA). Based on the obtained rheometric curves, specific parameters were determined, such as the increase in the rheometric moment (ΔM) and the optimal curing time (t90).

The range of NR curing temperatures and the enthalpy of this process were explored by employing a differential scanning calorimeter DSC1 (Mettler Toledo, Switzerland). A sample of the rubber mixture (approx. 10 mg) was cooled to −100 °C and stabilized isothermally for 5 min. In the next measurement segment, the sample was heated to 250 °C. The analysis was performed under an argon atmosphere (gas flow 20 ml/min) at a heating rate of 10 °C/min. Characteristic parameters for rubber mixtures were determined using the specialized STARe software.

In order to obtain samples of vulcanizates, rubber mixtures were crosslinked in hydraulic presses using appropriate molds shaping the material. The vulcanization process was carried out at a pressure of 15 MPa, at 160 °C for 10 min.

The hardness was measured for three disc-shaped specimens of the NR vulcanizates according to the ISO 48 standard procedure. A hardness tester to Shore Zwick Roell 3105 (Zwick Roell, Germany) was applied to determine the hardness value.

Mechanical properties were tested with dumbbell specimens (75 × 4 × 1 mm) on a universal test machine Zwick (Roell Group, Germany) at an extension rate of 500 mm/min according to ISO 37.

The crosslinking density of the NR sample was determined by equilibrium swelling in toluene, based on the Flory-Rehner (Flory and Rehner 1943) (Eq. 1):

$${\upgamma }_{\mathrm{e}}=\frac{\mathrm{ln}\left(1-{\mathrm{V}}_{\mathrm{r}}\right)+{\mathrm{V}}_{\mathrm{r}}+\upmu {\mathrm{V}}_{\mathrm{r}}^{2}}{{\mathrm{V}}_{0}\left({\mathrm{V}}_{\mathrm{r}}^\frac{1}{3}-\frac{{\mathrm{V}}_{\mathrm{r}}}{2}\right)}$$
(1)

\({\upgamma }_{\mathrm{e}}\)—the crosslinking density (mol/cm3), V0—the molar volume of solvent (toluene—106.7 cm3/mol) µ -the Huggins parameter of the polymer–solvent interaction, was calculated from the Eq. (2) (Roovers and Toporowski 1990):

$$\upmu ={\upmu }_{0}+\upbeta \cdot {\mathrm{V}}_{\mathrm{r}}$$
(2)

µ0—the parameter determine of non-crosslinked polymer /solvent relations, β—the parameter determine of crosslinked polymer/solvent relations (µ0 = 0.478, β = 0.228),

Vr—the volume fraction of elastomer in the swollen gel (Eq. 3) (Ahmed et al. 2013).

$${\mathrm{V}}_{\mathrm{r}}=\frac{1}{1+{\mathrm{Q}}_{\mathrm{w}}\frac{{\uprho }_{\mathrm{r}}}{{\uprho }_{\mathrm{s}}}}$$
(3)

ρr—density of rubber (0.99 g/cm3), ρs—density of solvent (0.86 g/cm3), Qw—the weight of equilibrium swelling (Eq. 4):

$${\mathrm{Q}}_{\mathrm{w}}=\left(\frac{{\mathrm{m}}_{\mathrm{sw }}-{\mathrm{m}}_{\mathrm{s}}}{{\mathrm{m}}_{\mathrm{s}}}\right)\cdot \left(\frac{100+\mathrm{x}}{100}\right)$$
(4)

msw—the weight of the swollen sample; ms—the weight of the dry sample; 100—the elastomer content in the sample; x—the filler content in the sample.

The Payne effect of biocomposites was characterized by the difference \((\Delta {\mathrm{G}}^{\mathrm{^{\prime}}}={{\mathrm{G}}^{\mathrm{^{\prime}}}}_{\mathrm{min}}(\mathrm{lim}{10}^{-1})\)-\({{\mathrm{G}}^{\mathrm{^{\prime}}}}_{\mathrm{max}}(\infty ))\) between the storage modulus determined under the deformation of 0.1% and storage modulus determined under the max deformation in the dynamic strain amplitude range. Measurements were performed using the rotational rheometer ARES-G2 (TA Instruments, USA) at a frequency of 10 rad/s by varying strain amplitude from 0.1 to 100%.

The flammability of biocomposites was performed by the method of burning vulcanizates in air using 50 × 10 × 4 mm samples. The sample tip was ignited for 5 s by means of a gas burner supplied with propane–butane mixture. Then, the burning time of a specific section of the composite was measured.

Statistical Analysis: Data in repeated analyzes were presented as the arithmetic mean of the results obtained, and the uncertainty of measurement was determined using the standard deviation.

Results and discussion

Analysis with FTIR spectroscopy

Spectroscopic analysis was carried out to evaluate the effect of horsetail particle modifications on their chemical structure. The results of the analysis are shown in Fig. 2. Horsetail, like all natural fibers, consists mainly of lignin, cellulose, hemicellulose, pectin and waxes. Chemical treatment of natural fibers usually cleans the surface of impurities and waxes and prevents moisture absorption (Seisa et al. 2022; Mohammed et al. 2022). Silanization is a process that enables the formation of chemical bonds between the fiber and the modifier, which allows for the creation of structures more compatible with the elastomer matrix (Barczewski et al. 2020).

Fig. 2
figure 2

FTIR spectra of horsetail materials

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Modified field horsetail samples were characterized by lower absorbance in the range of 3600–3000 cm−1 in comparison with untreated fiber. This was due to the decrease in the intensity of the peak corresponding to the vibrations of the –OH group. As a result of silanization, the hydroxyl groups of cellulose participated in the formation of new structures with modifiers, and hydrogen bonds could also be broken. In addition, the intensity of the bands at the wavenumber.

1650–1600 cm−1, which allows monitoring of the moisture content in the fibers, was lower for all the treated samples (Kabir et al. 2013). The silanization process may have contributed to increasing the hydrophobicity of the natural filler samples, which reduced water absorption by the fibers. The greatest decrease in absorbance in this range was observed for the fillers treated with VTES and TESPTS, which corresponds to the results of thermal analysis, where the weight losses of the sample at temperatures up to 120 °C were also the smallest (Fig. 3).

Fig. 3
figure 3

The thermogravimetric (TG) curves (up) and the derivative of the TG versus time (DTG) curves (down) of modified fillers

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In the case of spectroscopic spectra of silanized fibers and untreated horsetail, changes are visible directly indicating a change in the chemical structure caused by the reaction of the silane with the filler. An increase in the intensity of the peaks was observed in the bands 1420–1400 cm−1 corresponding to vibrations within SiCH2 (Barczewski et al. 2020) and at 1110 cm−1, which can be attributed to Si–O–C stretching vibration (Lu et al. 2014), indicating that the pre-hydrolyzed silane reacted with hydroxyl groups on fiber surface (Valadez-Gonzalez et al. 1999). The intensity of the bands recorded in this region of radiation was the highest for the APTES and OCTES samples, so probably the degree of reaction of these silanes with groups present on the surface of the fibers was the highest. Moreover, natural fibers modified with silanes showed three intense increases in absorbance bands compared to pure horsetail around wavenumbers 1050, 820, and 460 cm−1, which correspond to the Si–O bond stretching, bending, and rocking, respectively (Wang et al. 2020), that is indicative of the existence of polysiloxanes deposited on the filler. These results show that several reactions occur between the silane coupling agents and the surface of lignocellulosic materials during processing (Zhou et al. 2014). The appearance of a band associated with the presence of –Si–O–Si– bonds indicates the formation of polysiloxanes deposited on the fiber, while the peak coming from the –Si–O–C– bond may confirm the condensation reaction between the silane coupling agent (Valadez-Gonzalez et al. 1999).

Thermal analysis of fillers

In order to determine the thermal stability of natural fillers, a thermogravimetric analysis (TGA) was carried out, the results of which are presented in Fig. 3. In addition, Table 3 shows a summary of the characteristic parameters recorded during the test.

Table 3 Parameters determined based on thermogravimetric analysis for bioadditives modified with silanes (T10-temperature of 10% weight loss, T50-temperature of 50% weight loss, R800-residues in 800 °C)
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Determining the thermal stability of natural fillers is a very important parameter in the processing and use of composite materials. The production of this type of composition often requires the use of pre-treatment, e.g. mixing, and vulcanization at high temperatures, consequently leading to thermal degradation of lignocellulosic materials, which may lead to undesirable effects on the properties (Norul Izani et al. 2013). The assessment of thermal stability presented in research reports usually applies to polymer composites with natural fibers, much fewer researchers focus on the thermal analysis of the biofiller itself. Thermal stability studies of pure and treated field horsetail fibers were an important tool for visualizing the physicochemical nature of biofillers, which determines their application significance.

Thermal decomposition of natural fibers takes place in several stages, i.e. moisture evaporation, hemicellulose decomposition, cellulose decomposition and lignin decomposition, similarly in the case of horsetail. The course of the thermogravimetric curves of all samples is similar but shows slight differences in the individual stages of degradation resulting from the modifications carried out. The first mass loss of samples, about 5–8%, occurred in the temperature range from 70 to 120 °C, which is related to the removal of physisorbed water contained in the filler. The largest weight loss in this area was observed for unmodified horsetail and amounted to approx. 8%. Significantly lower mass loss (approx. 3–4%) in this range was recorded for silanized samples, but no significant differences were noted between the modifiers used. The coupling agents treatment could cause silane deposition on the surface of the fibers, as well as its penetration into the cell walls through the pores and the accumulation of its particles in the interfibrillar spaces. In this way, the silane agents can provide a barrier to the penetration and absorption of water by the filler. As a consequence, silanes can change the hydrophilic surface of lignocellulosic fillers, making them more organophilic and compatible with the polymer medium (Pan et al. 2020). A similar effect was observed by Balan et al. (2017) if modified with silanes, and coconut shell powder. Analyzing the further course of thermogravimetric curves, the stages of degradation occurring from 200 to 500 °C include the thermal decomposition of lignocellulosic material. Decomposition of hemicellulose corresponded with weight loss occurs mainly at 220–315 °C. In the case of horsetail fibers, the maximum mass loss for hemicellulose degradation appeared at a temperature of approx. 270°C. Cellulose decomposes in the temperature range of 315–400 °C; for horsetail, its maximum weight loss was observed at about 320 °C. Of the three components, lignin is the most difficult to decompose (Yang et al. 2007). Lignin, due to its structure as an aromatic polymer with three-dimensional linkages in an alkyl-benzene structure, is characterized by a low decomposition rate and a wide range of temperatures (Akubo et al. 2019). Comparing the course of TG curves recorded for horsetail samples, silanized fibers showed a higher temperature value at which a 50% weight loss was noted, which indicates their greater thermal stability compared to the reference sample. On the DTG curves (Fig. 3), it can be seen that the modified horsetail was characterized by a higher decomposition rate in the temperature range of 270–350 °C, which could be related to the degradation of the modifier used for processing, which consequently accelerated the decomposition process of the filler. After reaching the temperature of 700 °C, the test atmosphere was changed to air in order to burn the residues after thermal decomposition. The sample residue at 800°C for the untreated horsetail was slightly lower compared to the modified fibers. The increase in the value of R800 (Table 3) for the silanized material could be due to the presence of an additional residue after the degradation of the modifier. To sum up, field horsetail fibers can be successfully used as fillers in elastomer composites, as their initial decomposition temperature does not exceed 160 °C, i.e. the rubber processing temperature during vulcanization. In addition, the modification of the lignocellulosic material with silanes slightly increased its thermal stability.

Morphology of the fillers

SEM images of field horsetail filler samples are shown in Fig. 4. Particles of plant material, regardless of the type, were characterized by different shapes and sizes. Among them, there were both elongated fibrous structures and small crushed fragments of lignocellulosic material. The HT_untreated sample was characterized by a compact and smoother structure compared to the modified fillers. Silanization insignificant contributed to a change in the morphology of the field horsetail material. However, it can be seen that after treatment, the fibrous structures underwent a slight delamination. The SEM images of the modified samples showed some cracks within the fiber and their surface became a bit rough. Slight smoothing surface of untreated horsetail could be due to the presence of pectin, wax, etc. The pure organization could be lost after chemical treatment due to structural changes during modification. Perhaps, as a consequence of the resulting changes, i.e. an increase in roughness, the specific surface of the filler, responsible for contact with the polymer, increases. Treatment with silane agents, in addition to attaching groups more compatible with the matrix to the lignocellulosic material, also ensured a change in the morphology of the filler surface, which should favor the mechanical connection of the fiber and the matrix.

Fig. 4
figure 4

Influence of applied modifications with silanes on the morphology of the horsetail filler

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Contact angle measurement

Wetting characteristics of natural, surface-modified fibers by measuring the contact angle is useful in predicting fiber-matrix compatibility and interactions for polymers with different surface energies. Knowing the contact angles can also help predict the ability of the polymer to penetrate the spaces between the fibers when formulating composites (Schellbach et al. 2016). Most reflective pictures of water drops deposited on the surface of compressed field horsetail fillers are shown in Fig. 5, in turn determined on the basis of testing the contact angle values listed in the Table 4.

Fig. 5
figure 5

Images of water drops on a smooth surface of horsetail bioadditives with the determined contact angles

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Table 4 The contact angle values of filler
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The CA values obtained for the reference sample were significantly lower compared to the results obtained with the silanized fibers. The determined value for the HT_untreated sample was approx. 77.5°, which indicates the hydrophilic nature of the horsetail surface. All modifications carried out contributed to an increase in the CA value, the recorded contact angles for these fillers were higher than 90°, which allows them to be classified as hydrophobic materials. The analysis shows that the highest CA values were recorded for the HT_TESPTS and HT_OTES samples. The functional groups on the surface of the silanes were characterized by different polarities. In the case of the APTES-modified sample, the amino group increased the polarity of the system, which resulted in a low CA value compared to samples such as OTES, VTES or TESPTS, where the polarity of the functional groups of these compounds is much lower. In addition, during the measurements, rapid spreading of the drop on the surface of the unmodified filler was also observed, while on the surface of the samples treated with silane agents, the stability of the drop was much higher.

Cure characteristics of NR compounds

The influence of the modification and the amount of filler used on the rheometric properties of rubber mixtures were examined. This study provides a lot of useful information on the processing and properties of rubber products. The cure curves of selected composites are presented in Fig. 6. Figure 7 shows the values of rheometric torque increase obtained for composites with NR. This parameter is considered an indirect indicator of the cross-linking density of elastomeric materials, as its increase is a consequence of the increase in composite stiffness. Additionally, the ΔM value of the filled rubber materials is also affected by the hydrodynamic effect of the additive used.

Fig. 6
figure 6

Cure curves of composites filled with 10 phr of field horsetail

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Fig. 7
figure 7

The effect of applied modifiers on torque gain (ΔM) during vulcanization of NR mixtures

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The analysis of the provided data on the rheometric increase of rubber mixtures shows that the addition of the filler contributed to the increase in the value of ΔM. All mixtures containing silanized field horsetail were characterized by a greater rheometric increase compared to the unfilled sample and the NR_HT_untreated composites. In addition, an increase in the value of ΔM was observed with the increasing degree of filling. The most pronounced increase in this parameter was noted for elastomeric materials containing field horsetail treated with OTES and TESPTS. It was observed that the composites filled with silanized fibers have about 1.23 dNm (for TESPTS) higher ∆M compared to the reference system. Torque difference represents shear dynamic modulus which indirectly related to the crosslink density of a rubber compound (Shi et al. 2019). In the case of TESPTS silanization, the modifier could increase the effectiveness of the vulcanization reaction due to the presence of sulfur atoms in the structure of this compound. In turn, VTES silane has an unsaturated bond, which can also participate in cross-linking reactions. The remaining silanes can improve the compatibility of the components in the rubber compound and contribute to increased intermolecular interactions.

The influence of the applied filler systems on the optimal vulcanizate time of natural rubber composites is shown in Fig. 8. The introduction of a natural additive to the elastomer matrix resulted in a reduction of the t90 value, regardless of the type of filler. A similar effect was also observed in our previous studies, when we used cereal straw biomass as a filler (Masłowski et al. 2020c). The introduction of a natural filler increases the maximum torque and the curing time (t90) decreases, because the presence of fibers in the matrix limits the mobility of the polymer chain. This may be the likely reason for the reduction of the t90 value with increasing fiber content. The decrease in cure time with increasing filler load indicates that increasing the fiber load increases the mixing time with the matrix, which generates more heat due to friction during processing (Samant et al. 2023). The effect of the applied silane agents on shortening the time was observed because the samples containing silanized fibers were characterized by a shorter optimal vulcanization time than the reference composite. The most significant differences concerning the unfilled system were obtained for the mixture containing horsetail modified with aminopropyltriethoxysilane. The use of this modification allowed reducing the t90 value by approx. 1 min compared to the 0 NR sample. In addition, a shortening of the scorching time was also observed, which adversely affects the safety of processing this type of composites. Silan APTES contain an amine compound, which can absorb the polar sites of filler. This has reduced the possibility of the accelerator absorbing onto the fiber surface. As a result, silane decreases both the scorch and curing time. Another possible reason may be due to the alkaline nature of amine-based silane, which increases the pH of the rubber compound. In most cases, the increased pH in rubber compounds will increase the vulcanization rate (Hayeemasae et al. 2022).The obtained results confirm their accelerating effect on the vulcanization process. The similar effect of the use of coupling agents on the cross-linking kinetics of elastomeric materials was also confirmed by other researchers (Yan et al. 2004; Bach et al. 2020; Sowińska-Baranowska et al. 2022).

Fig. 8
figure 8

The optimal curing time of natural rubber mixtures filled with horsetail

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Kinetics of vulcanization of rubber mixtures

Table 5 presents the parameters determined on the basis of DSC analysis measurements of natural rubber mixtures containing 20 phr of the bioadditive. The results indicate that both the addition of the filler and its modification did not significantly affect the glass transition temperature of the rubber mixtures, which was about −63.5 °C–(−61.5°C), which is typical for natural rubber compositions. Changes related to the segmental mobility of polymer chains as a result of interactions with the filler are reflected in the change of the glass transition temperature. There is a slight difference in the determined Tg values between the filled systems and the reference sample. In general, the glass transition temperature of the filled samples was about 1–2 °C lower than the reference mixture. This was probably due to the increased stiffness of the material caused by the limited mobility of the elastomer chains by the filler network. Insignificant changes in the shift of glass transition temperatures for samples containing silanized filler may indicate a small degree of modification of the horsetail surface.

Table 5 Vulcanization kinetics parameters of NR mixtures obtained on the basis of DSC analysis
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The vulcanization process of the reference mixture was carried out in the temperature range of 166–224 °C with the process enthalpy of 4.56 J/g. The addition of pure field horsetail did not change the beginning of the vulcanization temperature, but it reduced the final temperature of the process by several degrees and increased the energy effect of the cross-linking reaction to 7.35 J/g. This is probably an effect related to the presence of the filler network, which hinders heat diffusion through the elastomer matrix during crosslinking (Maciejewska and Sowińska-Baranowska 2022).

However, the use of a modified biofiller significantly affected the cross-linking temperature. The initial crosslinking temperature was lowered by about 10–15 °C, which increased the energy effect of crosslinking compared to rubber compounds without these additives. The positive effect of the use of silane compounds on the cross-linking reactions could result from the improvement of the compatibility of horsetail fibers with the rubber matrix by creating additional polymer-filler connections, thus influencing the heat transfer inside the elastomeric matrix. The energy effect of the vulcanization process for mixtures containing modified lignocellulosic material ranged from 7.96 J/g for NR_HT_APTES to 9.95 J/g for NR_HT_CPTES.

The morphology of natural rubber composites

In order to determine the morphology of the produced biocomposites, a microscopic analysis of the vulcanizates was performed. The SEM images of fractures of the NR samples are shown in Fig. 9.

Fig. 9
figure 9

Scanning electron microscopy (SEM) images of natural rubber vulcanizates containing untreated and modified filler

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Unfortunately, the analysis of the obtained structure of the composite containing the unmodified filler did not provide a clear answer about the changes that occurred after the modifications. However, it can be presumed that in the case of unmodified vulcanizate, field horsetail fibers could be inaccurately bonded to the elastomeric matrix. Free spaces and voids at the interface were then formed, probably due to the different surface properties of the hydrophilic lignocellulosic material and the hydrophobic polymer. Such places could negatively affect the mechanical properties of composites when transferring stresses occurring during the use of elastomeric materials.

In contrast, the chemically treated fillers bonded better to the natural rubber, creating a slightly more continuous phase of the composite. Nevertheless, in the structure of silanized samples, we can observe interfacial imperfections. The presented SEM pictures are not sufficient proof of the positive effect of silane coupling agents on the compatibility of horsetail fibers and natural rubber. However roughness of the horsetail material after modifications could result in better adhesion of the filler surface to the NR and affect the quality of the polymer-filler connection. As a consequence, these composites should be characterized by improved performance properties.

Cross-linking densities of vulcanizates

Studies of rheometric properties, as well as differential scanning calorimetry (DSC) analysis, confirmed the positive effect of silanization of horsetail fibers on the natural rubber cross-linking reaction. As a consequence, vulcanizates containing silanized fillers were also characterized by increased cross-link density compared to the unfilled system and the sample containing untreated horsetail (Fig. 10). This is due to stronger rubber-filler interactions related to chemical interactions between NR, HT and silane molecules. The formation of additional network nodes in the composite could result from several factors of silane interactions, including improved compatibility, better dispersion of composite components, as well as chemical interactions between the modified fiber and the elastomeric matrix. The highest values of the ve parameter were recorded for biocomposites containing the TESPTS-modified material. It is a bifunctional silane coupling agent that can function in two different ways in a filler–polymer system. Firstly, this compound is characterized by high efficiency of coupling, which can increase the hydrophobicity of the filler surface by reacting with the hydroxyl groups of the lignocellulosic material. In addition, this effect is attributed to TESPTS and its ability to donate sulfur atoms during vulcanization, leading to rubber-filler interactions and an increase in the concentration of effective network nodes (Siriwong et al. 2017). Another active modifier in cross-linking reactions is also VTES, the presence of a double bond in the structure of the compound may also contribute to increasing the cross-linking efficiency. In the case of composites filled with HT_VTES, a significant increase in ve value was observed compared to the reference samples. Also noteworthy is the significant increase in ve value for vulcanizates containing amine-functionalized horsetail fibers. In general, compounds containing amino groups have a positive effect on cross-linking reactions, contributing to the increase in the efficiency of this process. The positive effect of the use of this type of compound in vulcanization reactions has also been confirmed by other researchers (Masłowski et al. 2020a). For vulcanizates containing modified horsetail fibers using CPTES and OTES, an increase in the value of ve was also noted, it was slightly smaller than for the previously discussed systems. The increase in the parameter was probably due to the increase in the hydrophobicity of the filler, which contributed to better compatibility and improved dispersion of the composite components.

Fig. 10
figure 10

Influence of applied modifications of horsetail on the cross-linking density of vulcanizates

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Mechanical properties

It is known that the performance properties of cross-linked rubbers strongly depend on the cross-linking density (Zhao et al. 2011). For this reason, the hardness results (Fig. 11) obtained for composites with the addition of modified horsetails correspond to the determined ve values. For biocomposites filled with functionalized fibers, an increase in the hardness value was obtained concerning the reference systems. In addition, the hardness values increased with an increasing degree of filling. The highest values of this parameter were again obtained for vulcanizates NR_HT_TESPTS and NR_HT_APTES. The obtained results are correlated with the determined values of cross-linking density (Fig. 10). The content of the filler also had an impact on the obtained hardness results. The introduction of rigid particles into the matrix reduces the mobility of polymer chains, causing an increase in the stiffness and hardness of composites.

Fig. 11
figure 11

Hardness of biocomposites filled with lignocellulosic materials

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Compatibilization of lignocellulosic fillers with silane agents contributed to the improvement of the mechanical strength of rubber composites. The course of stress–strain curves for biocomposites containing horsetail fillers is shown in Fig. 12. The increase in stress both at low strain (Se100) and at break (TS) (Table 6) recorded both for vulcanizates containing modified additives proves the improvement of the reinforcing effect for silanized fillers.

Fig. 12
figure 12

Stress–strain curves of NR biocomposites filled with horsetail

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Table 6 Mechanical properties of NR composites (Se100-stress at 100% relative elongation; TS-tensile strength; Eb-elongation at break)
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Silane functional groups (organofunctional) attached to the horsetail particles acted as compatibilizers and effectively improved the interaction between HT-NRs. In addition, the basic amino group present in APTES could have a positive effect on cross-linking reactions, contributing to the effectiveness of the vulcanization process, and thus increasing the mechanical strength of the composites. In turn, the unsaturation present in the surface-modified HT, as well as the sulfur atoms contained in the TESPTS groups attached to the horsetail, favored cross-linking with NR molecules. The silane-treated horsetail promotes enhanced bonding at the filler-elastomer interface and uniform dispersion, resulting in improved tensile strength. The most pronounced increase in the TS value for the systems containing the modified filler was observed at the content of 10 and 20 phr. As the filler concentration increases, agglomeration occurs and thus increased filler-filler interaction, which in turn results in poor interaction and bonding between filler and matrix. This effect has also been confirmed by other researchers (Balan et al. 2017).

In the case of biocomposites containing horsetail modified with APTES, VTES and TESPTS, a decrease in elongation at break was observed. This could be related to the increase in the cross-linking density of these composites, due to the content of functional groups present on the surface of the fillers, which could actively participate in the cross-linking reactions.

Payne effect

Determination of the Payne effect (ΔG′) allows for the study of interactions between filler particles in vulcanizates. The numerical value was determined based on the decrease of the storage modulus (G′) with increasing strain (strain) in the dynamic (oscillation) tests. In measurements, the increase in the strain amplitude causes changes in the microstructure of the material, which in turn leads to partial destruction of the weak physical interactions in the network created by the filler. An increase in the value of ΔG′ means a greater number of filler-filler interactions and may indicate a high tendency for the occurrence of filler clusters in the structure of composites. The use of silane modifying agents contributed to the reduction of the value of the Payne effect recorded in dynamic tests, the results are presented in Fig. 13. The activity of silane compounds leading to a reduction of filler-filler interaction or filler network for a typical elastomer-silica filler has been confirmed in studies conducted by Siriwong et al. (2014).

Fig. 13
figure 13

Influence of used fillers on the Payne effect of natural rubber composites

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Untreated horsetail showed the greatest decrease in storage modulus, which resulted from a large number of interactions between filler molecules. The hydroxide groups present in the cellulose structure can form internal networks through hydrogen bonds. In turn, the modification contributed to the reduction of the number of OH groups on the surface of the lignocellulosic material as a result of the reaction between silane and cellulose hydroxyl groups. This effect was confirmed by FTIR analysis. In turn, in the case of modified fillers, self-condensation of silanes could occur. In addition, for composites containing 10 phr of the bioadditive treated with APTES, a high dG value was observed, perhaps hydrogen bonding between the amino groups present in the structure of this compound could also occur (Fig. 14). On the other hand, the smallest differences between the registered modules were found in the samples treated with TESPTS and OTES, whose functional groups show a lower activity of intramolecular interactions. As a consequence, the amount of filler-filler interactions leading to a reduction in the value of the Payne effect for composites containing silanized natural additives was limited.

Fig. 14
figure 14

Relation of storage modulus to oscillation strain for composites containing 10 phr of filler

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Flammability

The influence of the type of fillers used on the flammability of natural rubber composites was noted (Table 7). The addition of unmodified horsetail extended the burning time of the vulcanizates by about 5 s at the content of 10 phr, while in the case of using 30 phr HT_untreated by about 1 min. The probably observed phenomenon during the combustion of the vulcanizate filled with lignocellulosic material resulted from the rapid decomposition of the natural material with low thermal stability, causing increased oxygen absorption and smoke thickening, which contributed to the formation of char, which hindered further combustion of the elastomer.

Table 7 The combustion time (in air) measurements for rubber vulcanizates filled with natural horsetail fibers
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A further increase in the burning time of NR composite samples was observed when filled with silanized horsetail. As confirmed in previous studies, the use of silane agents for biofiller modification led to an improvement in the degree of dispersion and distribution of the additive in the matrix compared to the powder not subjected to surface treatment. Homogenously distributed filler particles could lead to the creation of a barrier limiting the diffusion of gases and gaseous products of thermal decomposition, and thus increase the stability of the composites, leading to an extended burning time of biocomposites.

Conclusions

The characteristics of the fillers showed changes in the spatial structure confirming the attachment of the silane groups of the modifying agents to the surface of the natural filler. The attached functional groups could actively couple the filler particles and polymers with each other, however, selected parameters such as changes in the glass transition temperature of composites, crosslinking density indicated that the degree of silane modification was probably limited. During the treatment, apart from the reaction of cellulose hydroxyl groups with silanol groups (after hydrolysis) of modifiers, self-condensation reactions of silanol groups could occur, limiting surface modifications of bioadditives. Nevertheless, the use of the modified material processing contributed to changes in the properties of the filler, which are important from the point of view of their application to the elastomer matrix, such as improving thermal stability or increasing the hydrophobicity of the surface. In the case of the analysis of the filler, the effect of the applied modifications was confirmed, but no significant differences were noted regarding the type of compatibilizers used. However, when referring to the properties of composites containing treated horsetails, differences in their activity were noted. The results showed that the use of modified fillers contributed to the increase of the reinforcing effect obtained for biocomposites. In systems filled with lignocellulosic material treated with VTES APTES and TESPTS, improvements in gum-filler interaction, filler dispersion and cross-linking density were noted, which were largely responsible for the improved mechanical properties. This effect was probably related to the presence of groups that could participate in or actively influence the natural rubber cross-linking processes. Cross-linking active centers present in VTES (double bond) and TESPTS (sulphur donors) modifiers may probably actively participate in cross-linking processes and strengthen the structure of the composite with treated fillers, creating a chemically bonded network. On the other hand, the positive effect of the HT_OTES and HT_CPTES fillers on the functional properties of biocomposites was related to the increase in the hydrophobicity of the bio additive and better compatibility with the non-polar polymer matrix related to physical interactions. The use of compatibilising agents in the modification of natural fibers also allowed reducing the interactions between the filler particles (filler-filler interaction), which contributes to the reduced tendency of the additive to form their clusters.. On the other hand, polar groups of silanes, such as amine, can interact with each other to form hydrogen bonds. Obtaining a more homogeneous structure of biocomposite components also resulted in reducing the flammability of NR, due to the formation of a charred layer limiting mass and heat flow.