The efficiency of isocyanate coating on the basalt fibre surface for the mechanical performance of elastomeric polyurethane composites

Introduction

Recently, the research on fibre-reinforced polymeric composites has gained importance due to the enhanced demand for multifunctional materials with superior mechanical performance and low density due to their reduced weight characteristics, mainly for the transportation industry. In this scope, chemistry-driven innovations help address current and future logistics needs with the help of research studies.^1–5 In recent decades, the polymer industry has seen a surge in research interest in integrating basalt fibres (BF) due to their superior mechanical strengths. These fibres are being employed to create lightweight, high-performance hybrid composite materials for construction, logistics, and other engineering applications. BF is an artificially manufactured fibre made from melted naturally occurring basalt rock. Accordingly, composite materials involving BF are categorized as environmentally friendly composites. The major producers of BF are located in Ukraine, Österreich, Türkiye, Russia, Germany, Korea, Japan, Ireland, and China.^6–9 Silane sizing agents are applied commercially to the BF surface to improve the compatibility of fibre-matrix adhesion.^10–13

Elastomeric polyurethane (EPU) is comprised of rigid block parts known as hard segments and alternating elastic networks or soft segments that are principally created by the interaction of the diisocyanate with the chain extender. Because of the different chemical components and thermal dissimilarities of the hard and soft portions, phase separation is perceptible in the structure of EPU. The molecular structure of EPU varies between hard and soft segments, providing it with desirable mechanical behavior such as increased fracture toughness and resilience. Nonetheless, EPU’s weak UV resistance, high demand, and poor water tolerance limit some of its uses. EPU has increasingly been employed in a wide spectrum of technical applications, including vehicles, electronics, packaging films, medicinal products, sports equipment, aerospace, appliances, cables, and wires.^14–17

According to the literature survey, a few number of studies dealing with BF-loaded EPU composites were published. Anwer et al. investigated the wear properties of various composite systems, including BF-reinforced EPU-based composites in order to evaluate their antislip applications. ¹⁸ In another EPU/BF-related study published as a conference proceeding, Bek and Emri postulated the damping performance of BF-filled EPU in noise and vibration reduction purposes for railway technology. ¹⁹ Polyurethane covering on BF surface yielded enhancement in the mechanical performance of epoxy-based composites involving BF fabrics fabricated via hand lay-up technique by Yu et al. ²⁰

The primary objective of this study was to point the finger at the effect of BF surface coatings on the mechanical, thermo-mechanical, melt-flow, and morphological parameters of EPU-based composites. The impact of the isocyanate-functional silane layer on the BF surface was compared with desized BF due to the enriched interface adhesion between the BF and EPU phases in terms of property improvements on investigated characteristics. Since amino-functional silane is used commercially as a sizing layer in BF production steps, isocyanate-functional silane has the potential to be a new coating option for BF surfaces where a high level of compatibility is required in various polymers reinforced with BF. Achievement of strong adhesion between fibre and polymer phases can result in the developing of multifunctional and mechanically durable BF-reinforced EPU composite structures in the logistics sector, where weight reduction and mechanical resistance are specific requirements.

Experimental

Composite production

EPU pellets and BF samples were dried at 100°C for 2 h to remove moisture before processing steps. Composite samples were produced using a laboratory scale twin-screw micro-compounder (MC 15 HT, Xplore Instruments BV, Netherlands). The loading ratio D-BF and S-BF was kept at 20 wt% since the optimum performance of BF-reinforced composites was achieved at that composition.^23,24 EPU and BF-filled composite samples obtained from extrusion were shaped by a laboratory-scale injection molding device (Micro-injector, Daca, USA). The dimensions of dogbone-shaped test specimens were 7.6 × 2.0 × 80 mm³. Processing parameters applied during the extrusion and molding stages are listed in Table 1.

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Table 1.

Extrusion and injection molding parameters of composites.

Parameter	Value	Unit
Extrusion temperature	205	oC
Mixing time	300	sec
Screw speed	100	rpm
Injection pressure barrel	1	MPa oC oC
Temperature	210
Mold temperature	25
Holding time	60	sec

Measurement techniques

The characterization steps of test samples were carried out by recording experimental data as an average of three specimens. All the results represent an average value of three samples with the standard deviations. Density measurements were conducted on composite samples using a digital density meter (Easy D30, Mettler Toledo) for thermal tests (TGA and DSC) and density measurements. Infrared spectroscopy (IR) analysis was performed for BF samples in attenuated total reflectance (ATR) mode by Bruker Vertex 70 spectrometer. The wavenumber range of 650-4000 cm⁻¹ with 32 scans was applied during IR analysis. Energy dissipative X-ray spectroscopy (EDX) was used on the BF surface for elemental analysis with the help of scanning electron microscope (SEM) images, which were obtained from JSM-6400, JEOL Ltd. Morphological characterization of composites was visualized by this SEM device as well. The surfaces of cyro-fractured composite samples were coated with a thin layer to form a conductive surface. A tensile test of EPU and composites was carried out using Lloyd LR 30 K with a 5 kN load cell and cross-head speed of 50 mm/min. TQC Sheen BV digital Shore hardness device was utilized to measure the Shore hardness parameters of EPU and composites. DMA test was conducted in dual bending mode at −80°C to 100°C with 1 Hz frequency and 10°C/min heating rate using Perkin Elmer DMA 8000. Coesfield Meltfixer LT was utilized to investigate the melt-flow behavior of EPU and composites. At least 10 MFI values were recorded during measurements.

Results and discussion

Surface properties of BF

The surface functionality of BF samples was examined with the help of the IR spectrum as displayed in Figure 1.

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Figure 1.

IR spectra of basalt fibre samples.

The characteristic BF peak can be seen around 900 nm owing to the existence of basaltic silica-related compounds in the BF structure. This peak was found to be more intense for S-BF due to the silane layer on its surface.^25–27 The main difference between D-BF and S-BF samples was observed at the wavelength range of 2800-3000 nm. The presence of oxygen-containing groups, such as C-O vibrations, yielded weak bands at this range.^28–32 In addition, the characteristic isocyanate peak can be seen around 1620 cm⁻¹ stem from the C = O stretching vibrations.^33,34

According to SEM/EDX data of D-BF and S-BF represented in Figure 2, the Si content of D-BF was raised to a higher level in the case of S-BF thanks to isocyanate-silane coating on the BF surface. SEM image of S-BF revealed that silane sizing caused a smoother surface to be observed compared to the surface of desized BF.

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Figure 2.

SEM/EDX data of basalt fibre samples.

Tensile behaviors of composites

Characteristic stress versus strain curves of EPU and relevant composites are visualized in Figure 3.

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Figure 3.

Tensile test curves of EPU and its composites.

The yield stress of EPU shifted to higher tensile strength values as well as greater percentage strain was obtained, according to Figure 3. Ultimate stress of unfilled EPU at breaking point gave nearly identical tensile strength as it was compounded with D-BF. However, S-BF inclusion led to improvement in tensile strength at break. The increasing amount was found to be nearly 3.0 MPa as the stress values of EPU and EPU/S-BF were compared. BF additions resulted in a remarkable decline in the percentage strain of EPU. Similarly, according to the literature, short fibre-reinforced composites exhibited a lowering in the strain value of polymer matrices since the high aspect ratio characteristic of fibres contributes to the brittleness of polymer structure.^35–39

Hardness measurements of composites

The a and D types of shore hardness results are displayed as bar graphs in Figure 4. Recorded hardness values were located at the top of each bar of samples.

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Figure 4.

Hardness values of EPU and its composites.

Both types of Shore hardness parameters of EPU exhibited an increasing trend with the BF loadings. Enhancement in Shore hardness of composite involving S-BF was found to be more distinctive relative to desized BF. These findings indicated that optimizing fibre-matrix adhesion with the help of compatible isocyanate-modified surface of BF donated a stiffer network in the composite structure, resulting in elevated hardness values.^40–44

Thermo-mechanical studies of composites

DMA thermographs of EPU and relevant composites which are composed of storage modulus, loss modulus, and Tan δ loss factor curves, are represented in Figure 5.

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Figure 5.

DMA curves of EPU and its composites.

The storage modulus of composites displayed higher results compared to unfilled EPU. The increasing amount in storage modulus was obtained, which is more evident in the case of S-BF incorporated composite than D-BF. Similar to storage modulus findings, the loss modulus of the EPU/S-BF composite became apparent at a higher level relative to the D-BF-loaded EPU. The peak value of the loss modulus curve corresponds to the glass transition temperature (T_g) of polymeric material in which glassy to rubbery transition occurs in the polymer structure. Isocyanate-modified BF addition led to an increase in T_g stem from the restriction effect of BF filaments for chain motions.^45–48

In the case of Tan δ curves, unfilled EPU and D-BF containing EPU displayed nearly identical trends, whereas S-BF inclusion resulted in an expansion in the Tan δ curve in addition to enhancement in the peak value. This observation was related to the increasing damping property of elastomeric polymer with the integration of S-BF thanks to the establishment of strong interactions between fibre-matrix phases.^49–54

The findings regarding improvement in storage modulus and Tan δ curves revealed that BF-reinforced EPU composites fit the application requirements in the case of high mechanical resistance, and damping property is required.

Melt-flow and density of composites

MFI results using two different loads were demonstrated in Table 2.

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Table 2.

MFI and density data of EPU and its composites.

Sample name	MFI @ 2.16 kg (g/10 min)	MFI @ 5.0 kg (g/10 min)	Density (g/cm3)
EPU	6.4 ± 0.2	14.0 ± 0.3	1.18 ± 0.02
EPU/D-BF	11.7 ± 0.1	21.9 ± 0.2	1.09 ± 0.01
EPU/S-BF	13.8 ± 0.2	27.3 ± 0.2	1.12 ± 0.02

The melt-flow rate of EPU was positively affected by BF inclusions regardless of surface functionality. MFI values of BF-included composites recorded under a 2.16 kg load were found to be nearly twice the MFI of neat EPU. EPU/S-BF sample gave slightly higher MFI values than D-BF, which might be due to the contribution of a strongly bonded BF filaments-matrix system to level up the flow rate of molten polymer. Additionally, fibre’s high aspect ratio (L/D) promotes the flowing polymer phase along one direction with higher rates.^55–58 In terms of the processing view, MFI results were found to be moderated values since no dramatic changes were obtained.

MFI measurements are indicative parameters for molds and tooling, which are essential in manufacturing 3D-printed polymeric composite parts. Since MFI values of composites were found to be in the acceptable range, their processing stage can be applicable to additively manufactured composite parts.

According to the density measurements listed in the last column of Table 2, the inclusion of BF led to a decline in the density of unfilled EPU. The D-BF-containing sample gave the lowest density value, which was related to the formation of gaps between the fibre and the EPU matrix, which was caused by the lack of BF adhesion to EPU. The reduction of the density after introducing BF enables the production of lower-weight composite parts, which is crucial to application requirements for the logistics sector.

Morphological investigations of composites

SEM micro-images of composites with ×2.000 magnification are represented in Figure 6.

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Figure 6.

SEM micro-images of composites.

Composite reinforced with S-BF displayed a more homogeneous morphology than EPU/D-BF, according to Figure 6. The volumes of gaps caused by the pull-out of BF filaments during cyro-fraction were found to be more prominent in the SEM image of the D-BF-containing sample. Weak adhesion between EPU-BF phases was observed for composite filled with D-BF. In contrast to desized BF, S-BF filaments were found to be trapped in the EPU matrix thanks to the strong adhesion of BF to the matrix, promoting resistance to deformation during fracture. These visual shreds of evidence support the findings regarding performance improvements in terms of mechanical strength, as discussed in earlier sections.

Conclusion

The effect of the isocyanate-containing sizing layer upon the fibre surface on the mechanical, thermomechanical, processing, and morphological properties of basalt fibre (BF) reinforced elastomeric polyurethane (EPU) composites was investigated using tensile test data and Shore hardness parameters, dynamic mechanical evaluation (DMA), melt flow index (MFI) and scanning electron microscopy (SEM). In addition to functional group variance via infrared spectroscopy, SEM analysis was employed to observe the sized and desized BF surfaces. Traditional processing methods like extrusion and injection molding were implemented to manufacture composite samples. Isocyanate coating on fibre seemed to favorably influence the tensile strength and tensile modulus of composites. With the addition of both sized and desized BF, the percent elongation of EPU dropped. BF inclusions enhanced the shore hardness of EPU. DMA results indicate that surface-modified BF-filled composites possess greater storage modulus values than desized BF ones. BF additions had no discernible effect on EPU’s MFI parameter. Density of EPU declined with BF inclusion. Developing lightweight and strong BF-reinforced EPU-based composites opens ways to provide large-scale, cost-effective production of reduced-weight composite components for the automotive sector. SEM images provided visible evidence for improved mechanical performance by comparing the adherence of sized BF layers to the EPU matrix versus desized BF. The significance of surface sizing on BF surfaces in BF-reinforced EPU composite applications was assessed via the compatibility of composite phases. The reinforcing effect of isocyanate-functional silane coating on the BF surface was confirmed by examined findings linked to the achievement of strong adhesion through the fibre-matrix interface.