cubic boron nitride hybrid materials as functional components for abrasive tools

International Journal of Biological Macromolecules 122 (2019) 88–94

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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Kraft lignin/cubic boron nitride hybrid materials as functional components for abrasive tools Łukasz Klapiszewski a,⁎, Artur Jamrozik a,b, Beata Strzemiecka a, Paulina Jakubowska a,b, Tadeusz J. Szalaty a,b, Małgorzata Szewczyńska c, Adam Voelkel a, Teofil Jesionowski a,⁎ a b c

Poznan University of Technology, Faculty of Chemical Technology, Institute of Chemical Technology and Engineering, Berdychowo 4, PL-60965 Poznan, Poland Wielkopolska Centre of Advanced Technologies, Umultowska 89C, PL-61614 Poznan, Poland Central Institute for Labour Protection – National Research Institute, Czerniakowska 16, PL-00701 Warsaw, Poland

a r t i c l e

i n f o

Article history: Received 2 September 2018 Received in revised form 8 October 2018 Accepted 24 October 2018 Available online 25 October 2018 Keywords: Lignin Cubic boron nitride Organic-inorganic materials Abrasive tools

a b s t r a c t In this study, the kraft lignin/cubic boron nitride hybrid materials have been obtained and characterized for the first time. The effectiveness of the combination of lignin and boron nitride was evaluated on the basis of Fourier transform infrared spectroscopy. Furthermore, it was confirmed that the addition of cubic boron nitride (cBN) improved the thermal stability of the inorganic-organic material. Upswing in thermal properties allowed to apply the prepared materials in preparation of model abrasive composites. Beneficial influence of the lignin/ cBN filler was also proven by a noticeable decrease in the amount of harmful phenol released from the compositions during headspace gas chromatography analysis. Mechanical properties of the lignin/boron nitride hybrids and resin systems were investigated by the three-point flexural test. The obtained results show that the used additives can be promising materials for abrasive tools combining the good properties of lignin as a plasticizer and of cubic boron nitride as a filler which improves the thermal and mechanical properties of finished products and, at the same time, limits the negative impact on human health and environment. © 2018 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, grinding is one of the most common methods used to shape and finish metals, in addition to the cutting and annealing treatment. Finishing is used to provide appropriate final properties, such as size, hardness or surface smoothness [1,2]. Due to the increasingly higher requirements set for the modern abrasive industry in terms of quality, durability and performance in operation, the grinding process is constantly improved. The type and structure of the abrasive tool have the greatest impact on the effect of the process. Abrasive tools are complex materials composed of abrasive particles (alumina, silicon carbide) acting as the dispersed phase, which are surrounded and bonded together with a polymer matrix (phenol-formaldehyde resin, imide resin). One of the most effective ways to increase the efficiency of the tool is to introduce innovative, advanced fillers to the composition of abrasive tools. Fillers are an important component of bonded abrasive tools. Aside from self-lubrication of the tool, fillers increase the strength of the bonding, enhance heat and burst resistance, but they can also act as a secondary abrasive which supports the grinding process [3–5]. They are mostly inorganic compounds which are added to the composition ⁎ Corresponding authors. E-mail addresses: [email protected] (Ł Klapiszewski), teofi[email protected] (T. Jesionowski).

https://doi.org/10.1016/j.ijbiomac.2018.10.163 0141-8130/© 2018 Elsevier B.V. All rights reserved.

for many purposes: they induce porosity, improve binding properties, change aesthetic features and more [6,7]. Pyrite, zinc sulfide, and potassium sulfate are often used as a lubricants. They reduce the friction as a result of melting but unfortunately they may emit sulphur during decomposition [8]. They also improve the overall versatility and quality of the finished surface [2,9,10]. Cryolite and potassium fluoroborate decompose at elevated temperature (700 °C and 950 °C respectively) greatly absorbing heat from the process [1,11], but at the same time releasing harmful fluorine and its compounds to the atmosphere [12]. Due to all these drawbacks, the actual development of abrasive technology is focused to a large extent on the search for new, eco-friendly, functional fillers, such as increasingly common hybrid materials. The main goal for the actual development is to create systems less environmentally aggravating with improved properties which have a decisive impact on the efficiency of the final product. The combination of organic polymers or biopolymers with inorganic compounds makes it possible to obtain modern additives with previously unknown properties. The unique properties of functional materials result from the combination of resistance, stability and strength of inorganic products and various physicochemical and structural properties of organic compounds [13]. Biopolymers such as lignins and lignosulfonates can be applied as the organic part of the hybrid due to highly developed aromatic structure and the presence of free hydroxyl groups in their molecules [14,15]. They were mostly employed as a source of natural phenol for

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phenol-formaldehyde resins synthesis [16–20], but there are known the examples of utilization of lignin in more advanced applications such as green hydrogels preparation [21,22]. Unfortunately, these polymers as are not efficient enough and thermally stable to use them as a single, standalone filler in such demanding applications as abrasive articles and friction materials, due to the relatively low decomposition temperature [23–28]. Combining biopolymers with exceptionally thermally stable and mechanically durable inorganic compounds can create outstandingly performing advanced materials for high performance applications [29–34]. Extensive literature studies report the incorporation of alternative fillers in abrasive tools. Aluminosilicates, such as synthetic zeolite Micro20 and other zeolites, exhibit enhanced surface activity which increases adhesion between grain and resin bond [35]. Inclusion of βcyclodextrin and dialkyl pentasulfide complex wheel greatly reduces the grinding force and surface roughness, as well as enhances the grinding ratio in comparison with a standard grinding wheel [36]. However, organic-inorganic hybrid materials seem to be the most promising materials for the role of modern fillers in grinding tools. It was confirmed that such hybrids based on lignin and silica [37] or alumina [38] can be successfully incorporated into the abrasive composition. Tools with the addition of hybrid materials based on lignin and inorganic oxides exhibit better thermo-mechanical properties and emit smaller quantities of harmful compounds during manufacturing and exploitation of the tools. In the present work an attempt has been made to prepare and comprehensively characterize innovative a hybrid material based on the alkali kraft lignin and cubic boron nitride (cBN). The addition of a lignin/ boron nitride type filler into the composition should allow good distribution in the resin matrix due to the affinity of the chemical structure of the biopolymer for phenol-formaldehyde resins, which has a positive effect on the homogeneity of the final product [39]. Combination of highly thermally stable cubic boron nitride and spatially developed lignin may modify the rigidity and brittleness of abrasive composites and, at the same time, reduce the emission of harmful compounds. Extremely important is the fact that the proposed lignin/boron nitride systems are definitely a scientific novelty and have not been developed and applied in any way yet. It is also worth to mention that they have not been used until now in the formation of abrasives. 2. Experimental 2.1. Preparation of kraft lignin/cubic boron nitride hybrid materials Kraft lignin/cubic boron nitride materials have been obtained using a mechanical method. Appropriate amounts of kraft lignin (KL, Sigma Aldrich, Germany) and cubic boron nitride (cBN, material provided by Andre Abrasive Articles Sp. z o.o. Sp. k., Poland) were placed in a planetary ball mill and subjected to grinding for 6 h. To prevent the material from overheating, the ball mill was turned off every 2 h for a period of 5 min. A detailed procedure has been presented in our previous publications [37,38]. Within the framework of this publication, hybrid materials were obtained with the following weight ratios of lignin to cubic boron nitride: 8:1, 8:2, 8:4 and 8:6. 2.2. Physicochemical and dispersive-morphological characteristics of materials The surface morphology and microstructure of materials were examined on the basis of the SEM images recorded from an EVO40 scanning electron microscope (Zeiss, Germany). Before testing, the samples were coated with Au for a time of 5 s using a Balzers PV205P coater (Switzerland). Particle size distributions were determined using a Zetasizer Nano ZS and Mastersizer 2000 (both produced by Malvern Instruments Ltd., UK) enabling measurement of particle diameters in the range 0.6–6000 nm

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(non-invasive backscattering technique, NIBS) and 0.2–2000 μm (laser diffraction method), respectively. Identification of functional groups characteristic for lignin, cubic boron nitride and the final materials were conducted using Fourier transform infrared spectroscopy (FTIR) analysis. It allows to register vibration bands of functional groups in the spectral range of 4000–450 cm−1. FTIR spectra were registered using a VERTEX 70 spectrophotometer (Bruker Optics GmbH, Germany), equipped with a sensitive MCT (mercury cadmium telluride) detector works at temperature of liquid nitrogen. Furthermore, this apparatus ensures very high resolution of sample scanning, which exceeds 0.5 cm−1. Thermal analysis was performed using a Jupiter STA449 F3 (Netzsch, Germany). Samples weighing approximately 10 mg were placed in an Al2O3 crucible and heated at a rate of 10 °C/min from 30 to 1000 °C in a nitrogen atmosphere, at a flow rate of 40 cm3/min. 2.3. Preparation of abrasive composites with the addition of organicinorganic systems The model abrasive composites were prepared in the three step process using resole, organic-inorganic filler, novolac and abrasive grain, at a ratio of 3:5:12:80 by weight. The quantity of the components were selected as the standard proportion used in the abrasive tools manufacturing. In the first step, abrasive grain was layered with liquid resole resin. At the same time the filler was mixed with powdered novolac resin. In the second step both mixtures were combined to form an abrasive composition. All the components were mixed using a mechanical mixer at a slow rate of 200 rpm for approx. 3 min — the process was carried out at room temperature. The last stage included placing the abrasive mixture in the PTFE molds and hardening according to the following temperature program: heating from 50 °C up to 180 °C, heating rate 0.2 °C/ min, then heating at 180 °C for 10 h. White fused alumina with a 120 mesh granulation was used as an abrasive (Imerys Fused Minerals, Austria). Novolac contains 9% hexamethylenetetramine (HMTA). The composites prepared this way were formed into cuboids. 2.4. Quantitative analysis of phenol and formaldehyde emitted from the compositions using headspace – gas chromatograph (HS-GC) system The amount of phenol emitted from the compositions with phenolic resins in the presence of KL/cBN fillers was determined by the HS-GC technique. The measurements were carried out using a Clarus 580 gas chromatograph equipped with FID and coupled with the PerkinElmer® TurboMatrix HS 40 automated headspace sampler. The novolac-resoleorganic-inorganic filler compositions were placed in a 20 mL vial and sealed. The vials were then placed in an automatic sampler from where they were automatically placed in the thermostat in which they were heated under static conditions (without gas flow in the vial) for 5 min at 180 °C. Subsequently, the specific volume of the gas sample from the headspace was collected and transferred to the chromatographic column via a heated transfer line. Chromatographic analysis was carried out at 210 °C. For the quantitative analysis of phenol released from the tested compositions, the procedure of full evaporation from the headspace was used, i.e. analysed sample was collected several times (6 times in the described analysis) until the phenol peak was no longer present, at the noise level (total phenol extraction from the sample). In order to evaluate the reproducibility of the method, the measurements for each composition were repeated three times each time preparing a fresh composition, placing and sealing it tightly in the headspace vial. The analytical procedure of absolute calibration was used to determine the amount of separated phenol from the investigated compositions. The standard samples consisted of a mixture of novolac and resole resin with a known content of free phenol and a known amount of added phenol. Due to analysis of the emission of formaldehyde the tested compositions were placed in an oven and heated at 180 °C. A sorption tube filled

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with 2,4‑dinitrophenylhydrazine modified silica gel (100/50 mg) was placed at the outlet of the furnace. Samples were collected for 10 min at a flow rate of 20 L/h. Three litres of air were collected. The highperformance liquid chromatography (HPLC) with UV–vis detector was used to determine carbonyl compounds (aldehydes and ketones). In the HPLC experiment, the Ultra C18 column (25 cm × 4.6 mm) with a grain size of 5 μm was applied at temperature of 15 °C. Isocratic phase was a mixture of acetonitrile and water in the ratio 70:30 (% vol.) with a flow rate 1 mL/min. Volume of injected samples was equal to 10 μL. The analytical wavelength of the UV–Vis detector was 365 nm. The analysis was performed using the standard addition method, which was added in an amount of 10 μg/mL. Both analyses were conducted for the following compositions: (i) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g cBN; (ii) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g KL/cBN at a weight ratio of 8:6; (iii) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g KL/cBN at a weight ratio of 8:4; (iv) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g KL/cBN at a weight ratio of 8:2; (v) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g KL/cBN at a weight ratio of 8:1; (vi) 0.1 g resole + 0.47 g novolac (containing 9% HMTA) + 0.18 g KL. 2.5. Mechanical properties of abrasive composites The three-point flexural test of investigated compositions was carried out with the use of a Zwick/Roell Z020 universal testing machine (Zwick, Germany). The results of the flexural modulus, flexural strength and flexural strain at break were determined according to the ISO 178:2011 standard. The crosshead speed was at 1 mm/min. Barshaped samples of all composites were tested. The obtained results were reported as an average value calculated from five samples.

3. Results and discussion 3.1. Physicochemical and dispersive-morphological characteristics of KL/ cBN fillers 3.1.1. Dispersive-morphological analysis Particle size distribution of the prepared materials and their precursors were presented in Table S1 (see Supplementary Information). The obtained results show that depending on the ratio of components there are no major differences in the size and distribution of filler particles. The size of the particles for the KL/cBN 8:2 sample changes in the range from 255 nm to 955 nm at the maximum contribution of particles of size 531 nm. The data obtained with the Mastersizer 2000 analyzer and the SEM microphotograph (see Supplementary Information) also confirm the presence of agglomerate forms in the material structure. It can be observed that 50% by volume of the KL/cBN 8:2 sample is occupied by particles with diameters smaller than 5.2 μm, while 90% of the sample volume is taken up by particles with diameters smaller than 9.6 μm. The average particle size in the hybrid system is 5.0 μm (see Table S1). Pristine cBN particle size complies with the specification provided by the supplier (3–8 μm). Micrometric sizes do not exclude the use of prepared fillers in applications such as abrasive tools, there is no need for further fragmentation to nanometer size. To determine morphological properties, a scanning electron microscope (SEM) was used. SEM images of prepared fillers were presented in Fig. S1 (see Supplementary Information). The presented images (Fig. S1) confirm the similar size of the filler particles for all products. Addition of lignin and its co-milling with cBN particles caused a slight tendency to aggregate and agglomerate. This phenomenon was already observed in the authors' previous work [38].

Fig. 1. FTIR spectra of cubic boron nitride (cBN), kraft lignin (KL), and prepared organic-inorganic materials with a weight ratio of 8:1, 8:2, 8:4, 8:6, named KL/cBN (8:1), KL/cBN (8:2), KL/ cBN (8:4) and KL/cBN (8:6), respectively.

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Fig. 2. TGA curves of cBN, KL and organic-inorganic materials.

3.1.2. FTIR spectroscopy The aim of the conducted research was to develop a functional material with potential application as an abrasive tool. For this reason, cubic boron nitride was used as a commonly used component of abrasive mixtures. However, the high price of receiving this polymorphic variety encourages research to develop new functional materials with appropriate mechanical parameters. For this reason, it seems reasonable to select lignin as a component of such a functional system. It is a waste product from the production of cellulose fibres [40]. Its extensive structure is rich in numerous functional groups which can determine the physical and chemical affinity for inorganic precursors. In addition, it has been documented in the literature that it may be used as a substitute for phenol in the production of phenol-formaldehyde resins [41–43]. The description of the chemical structure and confirmation of the effectiveness of the combination of synthesized products was made on the basis of FTIR analysis. FTIR spectra

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of kraft lignin, cubic boron nitride and prepared systems were presented in Fig. 1. However, the values of wavenumbers at which characteristic bands for precursors were recorded were presented in Table S2 (see Supplementary Information). For cubic boron nitride, the bands assigned to the oscillations of the N\\H (3310 cm−1), B\\H (2099 cm−1) and cBN (1130 cm−1) were recorded in the analysed infrared range, which were marked by a dashed line. In turn, the spectrum of lignin is much more varied, because the biopolymer consists of three basic phenylpropanoid units [40]. As a consequence, bands confirming the aromatic-aliphatic character of lignin can be distinguished. Hydroxyl groups are significant, which were recorded in the form of stretching vibrations at the wavenumber value 3416 cm−1. The bands attributed to the aromatic ring (marked with blue backlight) are noteworthy. These were signals at wavenumbers 1595 cm−1, 1506 cm−1 and 1421 cm−1. These bands, together with the signal at the wavenumber of 1371 cm−1, confirm the phenolic nature of lignin. The mentioned bands have also been registered for hybrid materials prepared by mechanical method. The bands assigned to the aromatic ring and the signal characteristic for cBN (1130 cm−1) are the most important confirmation of the appropriate amount of organic or inorganic precursor. In addition, the band assigned to the B\\H bond was recorded at an appropriate intensity to the weight ratio of the boron nitride used. The used inorganic precursor has a small number of hydroxyl groups attached to the boron atom on its surface. Their presence was observed as a broad band at the 3420 cm−1. Its intensity is low and obscured by the vibrations of N\\H and B\\H bonds. However, with their participation interaction with the hydroxyl groups of lignin may occur. As a result of the mechanical connection, hydrogen bonds and physical interactions (mainly van der Waals forces) are formed. For this reason, changes in the intensity of the hydroxyl group in the prepared materials have not been registered. Other types of systems with lignin can also be observed in the literature, e.g. SiO 2-lignin [37], Al 2O 3 -lignin [38], which can successfully form the basis for setting functional abrasive tools.

Fig. 3. Mold filled with abrasive composition after curing for KL/cBN 8:4 sample (a) and SEM images of model abrasive composites: reference sample (b), cBN (c), KL (d), KL/cBN 8:1 (e), KL/cBN 8:2 (f), KL/cBN 8:4 (g) and KL/cBN 8:6 (h).

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Fig. 4. The mass of emitted phenol and HCHO from studied compositions.

3.1.3. Thermal analysis Thermal stability analysis plays a very important role in the potential use of organic-inorganic materials as functional fillers for abrasive tools. These materials heat up to very high temperatures due to friction during operation, therefore an adequately high thermal stability of potential fillers is very important (see Fig. 2). Pristine kraft lignin does not have high thermal stability. Depending on the type and batch provided by the manufacturer, the mass loss of lignin at temperatures up to 1000 °C ranges from 50 to 65% [44,45]. In the case of our study, the weight loss was equal to 52%. From a thermogravimetric curve it can be seen that the lignin is characterized by a three-step weight loss. A detailed description together with the specified degradation steps was presented in our previous works [44,45]. In order to improve the thermal properties, a regular boron nitride proved to be an excellent material, the loss of mass at temperatures up to 1000 °C was only 2%. Thus, the obtained lignin/boron nitride hybrid systems were characterized by increased thermal stability compared to a pure biopolymer. The thermal stability increased with increasing content of boron nitride in relation to lignin (for the KL/cBN 8:6 system, the mass loss was 31% of the original mass sample). However, each of the mentioned systems qualified the material for its potential use as a filler in abrasive tools.

3.2. Morphology of compositions Surface morphology of the composite samples was determined by the SEM technique. Fig. 3a presents mold filled with composites after curing, while Fig. 3b–h show SEM images of all composites. All composites exhibited uniform structure, no cracks or defects were observed. In all samples, the filler was located on the interface between grain and binder. On the images d-h lesser agglomerates can be observed. It is mostly caused by the lignin which has been explained in the authors' previous paper [38,40]. It is worth to mention that the

Fig. 6. The flexural modulus of studied compositions.

prepared composites have a porous structure which helps coolants and lubricants to penetrate the tool and, at the same time, improves the efficiency of the grinding process and extends the tool life. 3.3. Quantitative analysis of phenol and formaldehyde emitted from the compositions The mass of emitted phenol from each tested composite was estimated by GC coupled with HS. Such a system was most appropriate as it has been widely used in the quantification of volatile compounds for the samples with very complicated matrices [46]. However, HCHO emission cannot be easily measured by HS-GC system as HCHO gives no or very small signal when using FID and due to its unstable nature in air or other gas phase matrices at elevated temperatures [47]. Thus, the sorption method and then HPLC analysis were used for determination of HCHO (Section 2.4). The results of quantitative analysis of emitted phenol and formaldehyde from the studied compositions were presented in Fig. 4. The emission of phenol decreased approx. 20% in the presence of all studied fillers in comparison to reference composition (composition without filler). The highest decrease of phenol emission was observed for the composition with lignin. In case of HCHO, the emission decrease is within the error limit. However, the tendency to lower the HCHO emission is visible. Similarly to phenol, the lowest emission was observed for composition with lignin. The phenomenon of low HCHO emission might be the effect of high cross-linking of lignin and phenol molecules, which leaves less free places for HCHO substitution in aromatic rings [48]. On the other hand, the\\CH2OH groups present in lignin can react with free phenol

Fig. 5. Possible reaction of free phenol with\ \CH2OH groups present in lignin.

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Fig. 7. The flexural strain at break of studied compositions.

by polycondensation reaction [49]. These side reactions could bind phenol and incorporate it into the lignin structure. The possible effect of free phenol reacting with lignin from the filler was presented in Fig. 5. 3.4. Mechanical properties of compositions The stress-strain relation of abrasive tools has effects on the bond abrasion and may affect the possibility and feasibility of released grains getting embedded into the bond [50], therefore, the study of the flexural test is significant for investigated composites. The mechanical properties of the phenolics-based composites depend mainly on interactions between the fillers and the matrix, the cross-linking density of the resins achieved in the course of the curing process as well as on the curing conditions and the structures of the hardeners and fillers [7,51]. The effect of lignin, cBN and organicinorganic fillers content on the flexural properties (i.e., flexural modulus (Ef), flexural strength (σfM) and flexural strain at break (εfB)) of studied composites was shown in Figs. 6–8. The obtained results show that, despite no major differences in the size and distribution of filler particles (see Table S1 and Fig. S1 – Supplementary Information), the mechanical properties of the tested abrasive materials depended on the ratio of the components. Addition of lignin and its co-milling with cBN particles caused a tendency to decrease the mechanical properties of investigated composites.

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The results of the flexural modulus exhibited decreases in all composites. The highest Ef was obtained for the reference sample without lignin and cBN content. It is well-known that the flexural modulus is correlated with the sample elasticity. Moreover, the filler hardness affected the flexural modulus values [52]. The addition of lignin caused an increase of the elasticity of investigated materials probably due to the low hardness of the filler, which facilitated the material deformation. Moreover, the lignin chains introduced into the matrix can increase the flexibility of the composites, interact with resin ingredients, resulting in the decrease of resin–resin network and may contribute to energy dissipation through internal friction [53,54]. It should be noted, however, that the obtained lignin/cubic boron nitride systems were characterized by decreased flexural modulus compared to a pure biopolymer but simultaneously the Ef was higher, the higher the content of boron nitride in relation to lignin (for the KL/cBN 8:1 system the Ef was about 30% lower than reference sample but for the KL/cBN 8:6 system this value was only about 19% lower than reference sample). The flexural modulus for the sample containing separately KL and separately cBN was equal to 3.11 ± 0.08 and 3.26 ± 0.08 GPa, respectively. Therefore, the increase in the content of cBN in the filler influenced the higher Ef value of the abrasive material. The cubic boron nitride is a factor that improved the mechanical properties of the finished products in the filler. The plasticizing effect of the lignin was also confirmed by the results obtained for flexural strain at break. The εfB increased by approx. 17% relative to the reference sample for a lignin-containing composite which implies a non-reinforcing nature of this filler [55]. Then the flexural strain at break decreased with the increase in cubic boron nitride content in hybrid materials. For the KL/cBN 8:6 system εfB was equal 0.43 ± 0.03% and it was only 4.8% higher than for the reference sample. The results of the flexural strength also exhibited decreases in all composites. This may be the result of the presence of bulky lignin particles in the phenol matrix. Similar results were indicated by [56]. It is worth emphasizing, however, that the higher the cBN content in the hybrid filler, the higher the strength of the final product. For KL/cBN 8:4 and KL/cBN 8:6 systems, the flexural strength was comparable with the reference sample. Obtained results show that lignin fillers can be promising materials for a phenolic binder combining the good properties of lignin as a plasticizer and of cubic boron nitride as a filler which improves the mechanical properties. According to [4], the plasticizing effect of fillers that decrease the fragility of composites materials used as abrasive tool (in this case lignin/cubic boron nitride material) may have a positive impact on the efficiency of the final products. Confirmation of this assumption requires additional research which will be published soon.

4. Conclusion

Fig. 8. The flexural strength of studied compositions.

Due to the relatively low thermal stability of lignin proved by the TG analysis, combination of both lignin and cBN increases the stability of the filler system and, at the same time, optimally reduces the amount of phenol and formaldehyde released to the environment. All compositions with addition of lignin/cBN filler system emitted lower quantities of harmful phenol and formaldehyde. The plasticizing effect of hybrid fillers decreased the fragility of composites and lowered flexural modulus and strength for investigated composites in comparison to reference sample. Despite this fact, the application of lignin/cubic boron nitride fillers to phenolic resin may be environmentally and economically beneficial, and at the same reducing the cost of products and improving the efficiency of the final abrasive tools. Additionally, the presence of cubic boron nitride, which can also act as a secondary abrasive in the composition, may be advantageous in terms of performance and quality of the finished surface which will be studied in further experiments.

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Acknowledgements This work was supported by the National Science Centre Poland under research project no. DEC-2014/15/B/ST8/02321. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2018.10.163. References [1] S. Malkin, C. Guo, Grinding Technology: Theory and Application of Machining With Abrasives, 2nd ed. Industrial Press Inc., New York, 2008. [2] I.D. Marinescu, M. Hitchiner, E. Uhlmann, W.B. Rowe, I. Inasaki, Handbook of Machining With Grinding Wheels, 2nd ed. CRC Press, Boca Raton, 2016. [3] K. Colleselli, K.H. Schwieger, Schleifscheiben und Schleifkörper, in: Becker/Braun (Ed.), Kunststoff-Handbuch 10-Duroplaste, Hanser Verlag, München 1988, pp. 894–908 , (in German). [4] A. Gardziella, L. Pilato, A. Knop, Phenolic Resins: Chemistry, Applications, Standardization, Safety, and Ecology, 2nd Completely Revised, Springer, Berlin, 2000. [5] L.A. 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