Silanized cereal straw as a novel, functional filler of natural rubber biocomposites

The thermal stability of the straw was studied using a TGA/DSC1 (Mettler Toledo, Greifensee, Swiss) analyzer. The sample weight was kept at approx. 10 mg. Straw was heated from 25 to 600 °C in a argon atmosphere with a heating rate of 10 °C/min.

The chemical structure of straw used in this study was characterized by Fourier Transform Infrared Spectroscopy (Thermo Electron, Nicolet 6700 spectrophotometer, Waltham, USA), 128 scans were taken between 400 and 4000 cm⁻¹ with the resolution 8 cm⁻¹. Single reflection diamond ATR crystal on ZnSe plate and DTGS/KBr detector was applied.

Straw was compressed into the form of pressurized discs using a hydraulic press. The static advancing contact angle between a water droplet (10 µl) and the pressed fiber disc was measured using the sessile drop method on a goniometer (DataPhysics, OCA 15EC, San Jose, USA). The contact angle was measured from the captured images.

Rubber compounds with the formulations given in Table 1 were prepared using measuring mixer and laboratory two–roll mill. The mixing in a Brabender measuring mixer N50 at 50–60 °C lasted 8 min which included 4 min of plastification of rubber and 4 min of mixing rubber with the straw. Next, two–roll mill was used to mix the blend with the sulfur curing system and obtain rubber sheets.

Cure time and rheological parameters of the rubber mixtures were determined by an oscillating disk rheometer (Mon‐Tech, MDR 300, Buchen, Germany) according to ISO 3417 standard. Measurements of rheological properties were performed twice for each sample. The obtained results were repeatable, usually the measurement error did not exceed 0.05. Next, the composition were cured in molds by using hydraulic press with electrical heating at 160 °C according to their respective t₉₀ values, obtained from the test of cure characteristics.

The crosslink densities (υ_e) of the vulcanizates were determined by their equilibrium swelling in toluene, based on the Flory–Rehner equation (Eq. 1) (Flory and Rehner 1943).where: \( \upupsilon_{\text{e}} \)—the crosslink density (mol/cm³); V₀—the molecular volume of solvent (106.7 cm³/mol); µ—the Huggins parameter of the NR-solvent interaction was calculated from the equation µ = 0.478 + 0.228 V_r, where V_r is the volume fraction of elastomer in the swollen gel (Eq. 2).where: Qw-equilibrium swelling reduced by the filler content (x [phr]) − Qw = (100 + x/100);ρ_r-density of rubber [g/cm³] (0.99 g/cm³); ρ_s—density of solvent [g/cm³] (0.86 g/cm³).

Mechanical properties of composites were measured by using an universal machine (Zwick, Ulm, Germany). Tensile strength was determined in accordance with standard ISO 37 by using standard dumbbell‐shaped samples. The samples were subjected to a stress–strain test at a cross‐head speed 500 mm/min. The measurements were performed at room temperature. From the obtained stress–strain curve, tensile strength (TS), elongation at break (Eb), and modulus (SE) were determined. Obtained results from the 5 specimens for each composite were averaged.

Hardness of vulcanizates was determined according to ISO 868 standard using a Shore type A Durometer (Zwick/Roell, Ulm, Germany) and showed results are average from random ten points for each sample.

The relative damping of composites was measured according to the PN-C-04289 standard by using disc shape of the samples. Dimensions of specimens: diameter—35 mm and height—17.8 mm. The analysis was carried out at room temperature by using an universal machine (Zwick, Ulm, Germany). Each sample was stressed from 0 to 0.7 MPa and then stress was reduced. Hysteresis loops were recorded and the relative damping values were determined according to the Eq. 3:where: T_τw—relative damping, ΔW_i—the difference between the compression work and the work during reducing the compressive stresses, W_ibel—compression work.

Dynamic mechanical analysis (DMA) were performed with the application of ARES Rheometer (plate–plate system, plate diameter: 20 mm; gap 2 mm). DMA test parameters: temperature: 25 °C, sample deformation rate: 10 rad/s, stress: from 0.1 to 100%, test force: 5 N. The Payne effect (ΔG′) values of the composites have been calculated based on the Eq. 4.where: G′_min (lim 10⁻¹)—a composite storage modulus determined under the deformation of 10⁻¹%; G′_min (∞)—a composite storage modulus determined under the maximum deformation.

The thermo-oxidative degradation of the vulcanizates was performed at a temperature of 70 °C for 14 days. To estimate the resistance of the samples to aging, their mechanical properties after aging were determined and compared with the values obtained for vulcanizates before the aging process. The aging factor (K) was calculated as the numerical change in the mechanical properties of the samples upon aging (Eq. 5) (Masek et al. 2012):where: TS is the tensile strength of the vulcanizate, and EB is the elongation at break.

The flammability of vulcanizates was studied by the method of oxygen index (OI) using 50 × 10 × 4 mm samples, at a constant nitrogen flow rate in a measurement column (D = 75 mm) amounting to 40 ± 2 mm × s⁻¹. The concentration of oxygen was selected so that the sample could be completely burned within 180 s. The sample tip was ignited for 5 s by means of a gas burner supplied with propane–butane mixture. The value of OI was calculated from the following Eq. (6):where: [O₂] is the final flow rate of oxygen, at which a sample is burned within 180 s [l/h]; [N₂] is a constant flow rate of nitrogen [l/h].

Barrier properties were evaluated based on the through-plane air permeability of composites using manometric method in accordance with the ASTM D1434 standard. The tests were conducted by using atmospheric air at room temperature. Measurement of the barrier properties was based on a pressure difference in the chambers on both sides of the sample.

The barrier properties was determined by gas transmission rate (GTR) and coefficient of gas permeability (P), that were calculated from the following Eqs. (7), (8):where: d—the thickness of the sample [m]; V_c—volume of low-pressure chamber [l]; T—temperature [K]; P_u—the gas pressure in the high-pressure chamber [Pa]; A—area permeation of gas through the sample [m²]; dp/dt—pressure changes per unit time [Pa/s]; R—gas constant 8.31 · 10³ [(l Pa)/(K mol)].

The morphology of the composites surface and the dispersion of the filler were evaluated by means of Scanning Electron Microscopy with field emission (Hitachi, TM-1000, Tokio Japan). Before the SEM measurements, the samples were placed on carbon plasters and coated with the use of carbon target by the Cressington 208 HR system. The accelerating voltage was 25 kV.

In order to assess the influence of the shape and size of solid phase particles on the macroscopic properties of composite samples, scanning electron microscopy images (SEM) of used fillers and composites were prepared and studied (Figs. 2, 3).

Both modified and unmodified fillers exhibit inhomogeneity in shape and particle size. Their size ranges from a dozen to several dozen microns. Varied morphology of used fillers could be related to the efficiency of the milling processes. The type of filler (particle shape, particle size, specific surface area, concentration of disperse phase) effects on the multifunction properties of composites. Non-homogeneous structure of the filler particles can result differentiation of properties and incomplete exploitation of the reinforcing potential of the dispersed phase.

Analyzing the presented SEM photographs of selected composites (10 phr), no significant effect of straw silanization on the improvement of the degree of dispersion in natural rubber was observed. Both materials were characterized by good dispersed filler in the elastomer. There are straw particles of various sizes present in the matrix, which is related to the heterogeneous characteristics of the filler used. The continuous morphology of composite indicate that the cereal straw is well connected to the elastomeric matrix. The proper distribution of the filler particles has a significant impact on its reinforcing effect, and thus the mechanical properties of the vulcanizates.

The thermal properties of straw (unmodified and silanized) were determined by means of thermogravimetric analysis. Based on the obtained data, the influence of straw modification on thermal decomposition of lignocellulosic material (Fig. 4) was described. The curves represent the mass loss (TG curve) and mass loss rate (DTG curve) of both types of tested fibers.

Thermal decomposition of straw occurred in two stages, which is reflected by two peaks on the DTG curve. The differences visible in the first step of the decomposition were related to the loss of water content in the samples, which resulted in a slight loss of mass. The loss of mass due to the evaporation of moisture contained in the unmodified fiber was greater than in the case of a silanized straw. The silane treatment may have contributed to reducing the hygroscopicity of the material.

The next stage was related to the degradation of the lignocellulosic material. The process can be divided into main steps: degradation of hemicellulose (220–315 °C); lignin and cellulose decomposition (315–400 °C) and lignin degradation (> 450 °C) (Asadieraghi and Wan Daud 2014). DTG curve shows the speed of mass loss during thermogravimetric measurements. Silanization of straw has improved its thermal properties compared to unmodified fibers. Silanized straw also showed a lower rate of weight loss than unmodified fibers. The temperatures of the 5, 10, 50% weight loss for the unmodified fibers were 130, 253 and 329 °C, respectively, whereas for silanized straw fibers the temperatures were higher and amounted to 187, 261 and 345 °C.

Modification by silanization is effective in removal of large amounts of lignin and hemicelluloses, which is indicated by reduction in intensity of the peak at 1250 cm⁻¹, and around 1600–1650 cm⁻¹ (Fig. 5). Silane treatment reduces pectin and wax, at the peak around 1750 cm⁻¹. The presence of the peak around 1100 cm⁻¹ in silane treated fibres is due to the formation of asymmetric stretching vibrations of Si–O–Si and Si–O–C bonds, which indicate the reaction between silane and the fibres and the presence of a polysiloxane network. The vibration peak present at 805 cm⁻¹ due to Si–C stretching bond revealed silane presence on the straw surface. This was an indication of very good interfacial interaction between the silane coupling agent and the surface of the lignocellulose materials (Asim et al. 2016).

The contact angles between water and the pressed surfaces of unmodified and silanized straw are shown in Fig. 6. When the droplet was placed on the straw surface, the more hydrophilic nature showed pure straw, since the contact angle between the surfaces was about 74°. In the case of silanized straw, this angle was almost twice as large and amounted to 133°. On the surface of the samples, the drop of water spread over time, the drop was soaked. The difference during the disappearance of the drop was significant. The time of absorption of the drop on the surface of sialinated straw was much longer and occurred after 15 s, while on the surface of pure straw the drop disappeared after 0.5 s. The results indicate that the silanized straw fibers were characterized by reduced hydrophilicity. This could contribute to greater compatibility of silanized straw and natural rubber, which in turn will affect the stronger connection and adhesion between components of the composite.

The vulcanization time (t₉₀), scorch time (t_s2), also minimum (M_min) and maximum (M_max) torque gain values obtained in rheometry measurements of rubber mixtures filled with pure and silanized straw are shown in Table 2.

The results indicated that the time of crosslinking and the scorch time of natural rubber slightly decreased after introducing the filler into the polymer medium. Silanization had no considerable effect on the vulcanization prosses.

The increase in the maximum torque with the higher content of straw indicates that the presence of straw in the polymer medium has reduced the mobility of the macromolecule chains. Both untreated and silanized straw showed an upward trend. However, the modified filler exhibited a higher level of torque gain compared to untreated straw. This was most likely related to the creation of a more extensive structure of the spatial network. The results confirmed that the addition of the filler and the increase in its content influenced the increase in the vulcanizates crosslinking density (Fig. 7), and consequently the increase in torque during vulcanization. The use of silanized straw as a filler caused a further increase in the torque gain during the vulcanization process due to the increase in the crosslinking density of the vulcanizate.

Changes in the values of ΔM affected by the presence of fillers’ structure in elastomer matrix are an indirect measure of the hydrodynamic effect. This effect is related to addition of a non-deformable phase to the polymer matrix, which can manifest itself by improving the mechanical properties of the composites. Also the increase of the rheometric torque gain probably resulted from the increment of biocomposites cross-link density. The increasing fillers loading resulted in the increase of crosslink density. Filler–polymer interactions behave like physical network nodes and act as additional elements in the system’s network. As a result of the increase in the overall crosslink density due to the presence of the filler in the polymer, the strength of the network, as a consequence, also the stiffness of the composites increases with the increase of the filler–polymer interactions.

Achieving greater compatibility between the elastomeric matrix and filler particles through the modification process resulted in the expected improvement in the tensile strength of the vulcanizates. Such changes were probably possible due to the increase in interactions at the interface between rubber and silanized straw, which was reflected in the increase in cross-linking density. The largest reinforcing effect was observed for samples containing 10 and 20 phr of silanized straw. A further increase in content up to 30 phr caused a slight decrease in the TS value, however not below the reference system (Fig. 8). This may have resulted from a significant increase in the cross-link density, and consequently also in larger modules at 100, 200 and 300% elongation and a reduction in elongation at break (Table 3). The research also confirmed that the introduction of straw into the rubber (regardless of its modification) contributes to the increase in the hardness of the composites relative to the reference sample. The hardness values obtained were the higher, the higher the proportion of filler in the composites.

The full mechanical characteristics of rubber is obtained as a result of the evaluation of its dynamic properties, at a given temperature and constant deformation frequency, uniquely determined by the dynamic module (Fig. 9) and angle of the phase shift-tanδ (Fig. 10). The rubber properties are strongly non-linear, hence the values of the modules strongly depend on both the temperature and the size of the deformation as well as its frequency.

The addition of the filler to the elastomeric medium causes an increase in the elastic modulus G′, which is caused by the creation of its own structure by the filler, the so-called “network” in the elastomer. The dynamic module of vulcanizates filled with straw decreases as the deformation amplitude increases, which is the result of breaking the interactions—mainly van der Waals and hydrogen bonds between aggregates. This phenomenon has been thoroughly examined in the works of A. R. Payne and M.-J. Wanga (Payne 1965; Wang 1998).

The increase in modulus, especially in the small deformation range (Fig. 9) of vulcanizates containing a silanized filler compared to composites filled with pure straw, could indicate the formation of a more extensive and developed filler–filler network. The fact that the composites showed a larger G ‘modulus at small strain amplitudes proved that the straw particles showed a stronger tendency to mutual interactions in the elastomeric medium, as well as a greater tendency for agglomeration, rather than filler–polymer interactions.

The decrease in the storage module (Payne effect) in the whole strain range numerically expressed as ΔG′ (Table 4) also confirmed greater activity of the modified straw. In all cases, an increase in ΔG′ values was observed for composites containing a silanized filler. This effect is the more pronounced the higher the content of the filler in the composition. It means that the structure created by its particles has become more developed, and thus the scale of filler–filler interactions that are damaged during sample deformation has increased.

The maximum tanδ loss angle (Fig. 10) is in the area of strain amplitude, in which the fastest dynamic module decrease is observed, so additional mechanical energy dissipation is associated with disruption of physical bonds both filler–filler and filler–elastomer. It is known that the higher the angle of the tested material, the greater is the damping of mechanical vibrations and greater dissipation of mechanical energy. Increasing the filler content in vulcanizates caused a significant increase in the loss angle value almost in the entire range of strain amplitudes, which led to a decrease in elasticity and thus an increase in damping properties of the composites. The improvement of the quality of the natural-straw rubber connection by silanization of the filler positively influenced the attenuation characteristics.

Good damping properties of composites were also confirmed by tests of damping coefficient under compressive stress conditions. The vibration damping coefficient is a measure of the material’s ability to absorb the vibration energy of the element subjected to a cyclic load. The addition of straw resulted in the improvement of the damping properties of the composites produced, which indicates an increase in their ability to dissipate energy during deformation (Fig. 11). With the increase in its content, the relative attenuation values Tτw have increased, reaching at 30 phr twice the value of the unfilled system. The damping occurring at the interface between the matrix and the dispersed phase (interface damping) is related to, inter alia, the quality of their connection. Both weak and very good adhesion can potentially improve the composite’s ability to dampen vibrations. The use of the straw silanization process to increase compatibility and improve interfacial adhesion has increased the ability to dissipate the energy of composites filled with it.

The obtained composites showed an improvement in barrier properties (Table 5). The reduction in gas permeability may have resulted from the method of distribution of the straw particles in the polymer matrix. Due to their complex, fiber-like structure, particles can form layers, making it difficult to diffuse air between successive batches of filler. Vulcanizates containing silanized straw were characterized by much more limited gas permeability compared to samples with pure cereal straw. A significant decrease in the P value compared to the unfilled system was already observed at 10 phr. In the case of pure straw vulcanizates, a similar P value was obtained for a composite filled with 30 parts by weight. Silane straw compatibilization for better interfacial connection with the elastomer probably reduced the mobility of the polymer chain, which hindered the penetration of air through the modified vulcanizate surface.

Straw as a flammable material supports the burning of vulcanizates filled with it, as evidenced by the decrease in the oxygen index for composites compared to the reference sample (Fig. 12). The flammability of the materials increased as the straw content increased. A slight increase in IO was observed for all composites prepared using modified cereal straw. Silanes acting as coupling agents can bind the fillers and polymer matrix very easily with little energy by simple synthetic rout, with moreover a great contribution to forming a compact and dense char layer during combustion (Shah et al. 2017).

The resistance to thermo-oxidative aging was evaluated on the basis of changes in mechanical properties of vulcanizates before and after the simulation of aging processes at elevated temperature with air access (Fig. 13). The thermo-oxidation aging factor obtained for vulcanizates of pure natural rubber was 0.64. On this basis, it was found that the mechanical properties of these samples deteriorated as a result of the thermo-oxidative aging process. The poor resistance of the elastomer to aging is related to its structure, because the poly-isoprene molecule has unsaturated bonds that are active in the degradation process. The obtained results showed that even the smallest straw addition improved the resistance of composites to thermo-oxidative aging. Regardless of the type and amount of introduced filler, the aging coefficients obtained for the composites filled with it were higher than the reference sample, however the values varied. It proves that their mechanical properties have not changed as much as unfilled natural rubber. This effect is most likely caused by the presence of lignin in the straw. Lignin contains phenolic hydroxyl group that also has a strong antioxidant function.