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Article

High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers

by
Dmitry S. Konovalov
1,
Natalia N. Saprykina
2 and
Vjacheslav V. Zuev
1,2,*
1
Chemical Engineering Department, ITMO University, Kronverkskiy Pr. 49, 197101 Saint-Petersburg, Russia
2
Institute of Macromolecular Compounds of the Russian Academy of Sciences, Bolshoi Pr. 31, 199004 Saint-Petersburg, Russia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(14), 8258; https://doi.org/10.3390/app13148258
Submission received: 15 June 2023 / Revised: 11 July 2023 / Accepted: 13 July 2023 / Published: 17 July 2023
(This article belongs to the Special Issue Functional Polymers: Synthesis, Properties and Applications)
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Abstract

:
A new method for the preparation of coatings based on renewable biomaterials such as castor oil and birch flour is suggested in this study. The introduction of birch flour in a polyurethane matrix synthesized from castor oil and oligomeric methylene diphenyl diisocyanate (MDI) leads to a more than doubled value of tensile strength and almost doubled strength of adhesion to steel at 20 wt.% loading. The composite with such level loading has tensile strength equal to 7.1 MPa at an elongation at break of 31%, with an adhesion to steel of 3.71 MPa. Hence, the use of such level loading allows for an increase in tensile strength of 887.5% in comparison with that of polyurethane based on neat (as received) castor oil, leading to a decrease in the value of elongation at break. The adhesion to steel of these composites increases by 185.5% in comparison with starting polyurethane. FTIR and SEM studies identified the mechanism of the reinforcement effect of birch fibers. This reinforcement is based on the good wetting of birch fibers by polyurethane with the formation of chemical bonds between them, and the cellulose and lignin components of wood fillers. As a result, we obtained cheap bio-based coatings with acceptable mechanical and adhesion properties.

1. Introduction

Striving for a clean world that is free from polymeric waste, including microplastics, stimulates the development of polymeric materials based on renewable feedstock. The use of products obtained from vegetable sources allows for a solution to this problem because they are renewable and, as a rule, biodegradable. Polyurethanes (PU) are some of the most commonly used industrial polymers from medicine to construction and even in the space industry. The broad range of PU applications leads to a wide range of requirements of their material properties. This problem can potentially be addressed by variation of the components used at PU synthesis. The world flora provides a number of chemicals which can be applied at PU production.
Castor oil, obtained from castor beans as the vegetable source, is one of the most used polyols for PU synthesis [1]. However, the mechanical performance and protective qualities of PU materials based on castor oil are far from perfect [2]. One of the ways to improve the performance of castor oil-based PU is the chemical modification of castor oil itself to increase its reactivity and functionality [3]. Pathways of the chemical modification of castor oil that have been used include epoxy ring construction on the double C=C bonds of castor oil [4,5,6,7], ester exchange using ester fragments of castor oil [8,9], the hydroformylation of double C=C bonds [10] and its ozonolysis [11,12]. Other possible modifications of C=C double bonds include a metathesis reaction [13] or being subjected to a click reaction using a thiol–olefin reaction [14,15,16,17]. However, these methods are very laborious and their transfer to industry is highly problematic.
The synthesis of composite materials based on the PU matrix is one of the many possible ways to overcome these disadvantages [18], and the developments in this field have recently been reviewed [19].
Bio-based fillers are used for the preparation of composites [20]. Recommendations concerning the choice of fillers in bio-based composites are given in review [21]. The forest is an inexhaustible source of fillers based on wood. The human population has used wood since prehistoric times for products with different properties, shapes, and sizes. Wood is used for the production of paper, household products, construction, furniture, and in industry, including aerospace [22]. Wood–polymer composites are also well studied, and their development has a long history [23]. The intensive exploitation of forests, primarily tropical, leads to a decrease in their area, causing irreparable damage to the environment, and resources of valuable wood are constantly being depleted. In Russia, this applies to conifers that substitute “weed” trees such as the birch tree. Wood fibers from birch trees are the shortest in comparison with fibers obtained from conifers, resulting in a negative effect on the properties of polymer composites with thermoplastics [24]. Birch is a fast-growing tree, making this resource quickly renewable. This has led to the idea of introducing wood fibers of birch origin to the production of polymer composites.
One of the ways to improve the mechanical performance of polymer composites filled with birch wood fibers is the formation of chemical bonds between these fibers and the polymer matrix [25]. Cellulose has free hydroxyl groups in its polymer chains that can be used in the process of polymer composite preparation [26]. This approach was used in the preparation of polymer composites filled with different vegetable fibers [27]. However, the number of such studies is limited.
The economy requires lower production costs to ensure sustainable development. This stimulates the search for inexpensive starting chemicals while maintaining high consumer characteristics of the final products. Birch flour matches the necessary environmental and economic conditions. Hence, the development of a new polyurethane coating for steel constructions filled with this additive and showing a high mechanical performance aligns with modern composite science.
The aim of the present study is the development of polyurethane–birch wood fiber composites with high tensile strength (more than 5 MPa) at acceptable elongation at break (more than 20%), which provide high wire resistance [1]. The obtained coatings should have tolerable values of adhesion to steel (more than 2.5 MPa) [2].

2. Materials and Methods

Castor oil was received from OOO TexChim (Moscow, Russia) and birch flour (trademark M400) from OOO Legnus Resurs (Saint-Petersburg, Russia) (Figure 1). Methylene diphenyl diisocyanate (MDI) (Wannate PM-200 from Yantai Wanhua (Shandong, China) (molecular weight Mn = 340, 30.2–32% NCO, viscosity at 25 °C 150–250 cps, specific gravity d = 1.25 g/cc)) was used as received. Zeolites (molecular sieves), trademark 3A, were provided by OOO PromCoat (Saint-Petersburg, Russia). Castor oil was dried with 1 wt.% zeolites for 3 days. Birch flour was dried in a thermostat at 120 °C for 24 h.

2.1. Preparation of Composites (Scheme 1)

Castor oil with the calculated amount of birch flour was kept in a thermostat at 113 °C for 24 h for better penetration of the oil into the birch wood. Following this, the concentrate was skipped three times through three-roller machines (Shanghai Root Mechanical and Electrical Equipment Co., Shanghai, China) to destroy the birch flour aggregates. This concentrate was mixed with the calculated amount of oligomeric MDI and, after adding 0.35 wt.% of catalyst (tin dibutyl dilaurate), was poured into silicone forms for film formation. The films were ready for measurements after staying at 50 °C for 24 h.
Applsci 13 08258 sch001 550
Scheme 1. PU synthesis from castor oil and oligomeric MDI.
Scheme 1. PU synthesis from castor oil and oligomeric MDI.
Applsci 13 08258 sch001

2.2. Optical Microscopy

Micromed Met 400 (Micromed Met Co., Moscow, Russia) with magnification up to 400 was used to analyze the birch flour.

2.3. Fourier-Transform Infrared Spectroscopy (FTIR)

Attenuated total reflectance (ATR)-infrared spectra were acquired using a Vertex Fourier-transform infrared spectrometer (Bruker, Germany). A spectral resolution of 2 cm−1 was maintained, and 32 scans were co-added for acceptable signal-to-noise ratio. A diamond micro-ATR crystal accessory was used. Composite samples were placed between the crystal and a metal tip, and sufficient load was applied to ensure good contact of the sample with the ATR crystal.

2.4. Mechanical Testing

The mechanical properties of the composites, such as tensile strength and elongation at break, were measured according to ISO 527-2 [28] using the Universal Testing Machine Instron 5966 with sample dimensions: 100 × 10 × 0.35 mm. At least 5 samples were used for each composition.

2.5. Adhesion to Steel Testing

The adhesion of the composites to steel was tested according to ISO 4624:2002 by the destructive testing of the bonded coupons using PosiTest AT-M Manual Adhesion Tester (DeFelsko, Ogdensburg, NY, USA). An average of 5 measurements is reported.

2.6. Hardness of Composite Testing

The hardness of the composites was measured according to ISO 868 using the Zwick 3105 combi test instrument (Zwick, Ulm, Germany) with sample dimensions: 100 × 100 × 30 mm. At least 5 samples were used for each composition.

2.7. DSC and TG Measurements

DSC measurements were performed using the DSC 204 F1 Phoenix (Netzsch, Dubai, UAE) calorimeter at a heating rate of 5 °C/min. A TG analysis was performed using the TG 209 F1 Libra (Netzsch, Dubai, UAE) instrument at a heating rate of 10 °C/min in nitrogen.

2.8. SEM Studies

The SEM studies were carried out on a SUPRA 55VP scanning electron microscope (Zeiss, Jena, Germany). To ensure the electrically conductive properties of the cryogenic prepared studied samples (films or powders) and to eliminate interference due to the accumulation of surface charge during scanning, as well as to increase the contrast, the samples were pre-sputtered with gold (Au) or platinum (Pt) by cathode sputtering (15–20 nm) on Quorum 150 (Oxford, UK). Then, they were glued with double-sided electrically conductive tape to the microscope stage. The secondary electron mode (SE2) was used to study the surface morphology.
The IR and SEM studies, DSC and TG measurements, and adhesion and mechanical testing were performed as described previously [29,30].

3. Results

For the synthesis of PU-based coatings, we chose the trifunctional oligomer of methylene diphenyl diisocyanate because it is the cheapest commercial isocyanate, which, combined with the three hydroxyl functionalities of castor oil, offers the simplest combination of the PU polymer matrix. The problem of commercial castor oil is its relatively high content of water that is connected to its origin from the vegetable raw materials processed by pressing castor beans. The presence of water induces the formation of bubbles during PU synthesis [31], which negatively affects the mechanical performance (Figure 2 and Figure 3). Hence, one of the most important tasks of this study is the search for methods and additives to castor oil that can help solve this problem. The tensile strength and elongation at break of PU prepared from the as-received castor oil and PM 200 are given in Table 1. As one can see, the obtained value of tensile strength is low.
The IR spectrum of synthesized PU was recorded (Figure 4). As one can see, the band corresponding to the valence vibrations of carbonyl groups is broad and structured. The deconvolution of this band was carried out using OPUS software (Figure 5). As a result, the number of bands was obtained, which was attributed to the valence vibrations of carbonyl groups in urethane (1730–1700 cm−1) and in urea units (1680–1660 cm−1) [32].
The presence of urea units in the synthesized polymer is due to the presence of water in castor oil. The reaction of water with isocyanate forms amine groups, further reacting with isocyanate with the formation of urea units [33]. Normally, the presence of urea groups in a PU structure leads to an improvement in the mechanical performance of the composition [34]. However, the presence of urea groups in synthesized polymer leads to the formation of two types of urea units (H-bonded (1664 cm−1) and free (1684 cm−1)), and two types of urethane units, which are attributed to free carbonyls (i.e., non-H-bonded), and hydrogen bonded corresponding to the bands at carbonyl groups 1728 and 1702 cm−1, respectively (Table 2). The presence of urea units causes an increase in the fraction of free carbonyl groups in the urethane fragments. Hence, the presence of urea units leads to the formation of “loosened” rigid domains in PU, contributing as an additional factor to the deterioration of the mechanical properties [35]. The band at 1742 cm−1 is connected to the presence of ester bonds in castor oil [35]. The presence of wood fibers in the composites has practically no effect on the IR spectra (Figure 4). The band at 3050 cm−1 is attributed to the valence vibration of aromatic C-H bonds presented in lignin contained in wood [36].
The analysis of the IR spectra (Table 2) shows that the introduction of birch flour leads to an increase in the fraction of H-bonded carbonyl groups in the urethane units. Therefore, the degree of phase separation (DPS) in the PU matrix grows, although not as significantly. Typically, the increase in DPS leads to an improvement in the mechanical performance [37]. The increase in DPS can be connected with the surface of the birch particle, which is covered by hydroxyl functionalities from cellulose and lignin macromolecules. The interactions of these functionalities with urethane units leads to the ordering of hard segments of the PU matrix near the surface of the birch fibers, which contributes to phase separation. This is one factor in the improvement of the mechanical performance of the PU composites after the addition of birch flour.
To solve this problem, we used the most common antifoaming approaches: the drying of castor oil and the addition of polydimethylsiloxanes (PDMS) [38]. As one can see, both methods give good results. However, the IR spectra measured using ATR units shows that PDMS extrudes from the PU matrix when pressured using a diamond needle (Figure 6). Therefore, the use of PDMS in the preparation of wood fiber-filled composites in a three-roller machine can lead to the loss of mechanical properties and the sliminess of materials. Hence, we chose the drying of castor oil by molecular sieves as the working method for composite preparation.
This choice was supported by the results of the IR and SEM analysis. As one can see from the deconvolution of the IR spectrum of the prepared polymer in the region of the valence vibrations of carbonyl groups (Figure 5b), the frequencies characteristic of the urea units are absent. The SEM images of PU prepared from neat castor oil show (Figure 7) the disordered amorphous polymer matrix around the bubbles. The PU prepared from dried castor oil forms a pseudo-crystalline ordered structure. Together, these factors lead to gaining a maximal mechanical performance attainable for such composition (the tensile strength σ = 3.2 MPa), as was shown previously by Ionescu et al. [39].
The choice of birch flour as an additive is based on its fiber-like structure (Figure 1), with fiber lengths from one to dozens of µm. To estimate the mean size of the fiber and its distribution on size, a freely available software was used (https://imagej.net/software/fiji/ (accessed on 25 May 2023)). The data are given in the Supplementary Materials. Such length of fibers is optimal for obtaining a good mechanical performance of polymer composites [40]. The use of a three-roller machine for the preparation of a prepolymer mix of birch fibers and castor oil was found as an optimal method for the preparation of PU composites with the best mechanical performance. The use of ultrasound mixing did not allow us to completely destroy the wood fiber bundles in castor oil. As one can see from Table 1, the introduction of 5–20 wt.% of birch flour leads to an increase in tensile strength up to ten times, with growth of elongation at break as well. At a wood fiber fraction of 30 wt.%, a decline of tensile strength and elongation at break was observed. On the one hand, this is a typical effect of a limit of loading [41]; on the other hand, it can be connected to the intensive foaming at a high fraction of filler (Figure 3).
The method of preparation of the prepolymer mix is very important [42]. The use of manual, mechanical, or ultrasound mixing did not allow us to achieve the mechanical performance of composites with the best properties.
Hardness is one of most important properties for the performance characteristics of the coating. The starting PU obtained from native castor oil is relatively soft. The use of antifoaming methodology allows coatings with satisfactory values of hardness to be obtained (Table 1). The use of wood fibers increases the hardness values proportionally to the filler fraction (Table 1). The composite with 20 wt.% of birch fibers has the optimal values of mechanical parameters.
The thermal characteristic of composites was studied using the DSC and TGA methods. The filling of the PU matrix with birch fibers did not change their thermodegradation behavior (Figure 8). All destruction processes observed in neat PU polymer exist in composites too. The introduction of birch fibers shifts the position of the degradation process by 5–6 °C to higher temperatures. With the increase in the fraction of wood fibers, the amount of forming cinder increases.
This result is unexpected. The introduction of cyclohexane units in the PU matrix leads to a considerable increase in thermostability in comparison with aromatic units (on 30–50 °C [43]). The absence of such an effect for castor oil-based PU with the addition of birch flour, in the presence of chemical bonding between urethane units and cellulose as well as lignin macromolecules on the flour particle surface (according to IR spectra), speaks to the inclusion of these fragments in the common degradation process with castor oil sub-units of the PU matrix. Therefore, bio-based fragments have a similar thermodegradation mechanism that motivates the search for the biodegradability of the obtained composites.
The glass transition temperatures hardly change with the introduction of birch fibers (Table 1). The increase in Tg for PU synthesized from neat castor oil can be attributed to the presence of urea units, leading to the formation of a more rigid polymer matrix as a result of stronger H-bonds. The absence of urea bonds leads to a decrease in the glass transition temperature by more than ten grades (Table 1). This is important for the environmental conditions of Russia because such an effect increases the frost resistance of the coating. In Russia, where more than 20% of gas and oil pipelines are located in the Arctic Circle, such an effect increases the operational and marketing opportunities of bio-based coatings [44].
The reinforcement effect of birch fibers is determined by the presence of the hydroxyl groups from cellulose and lignin on their surface [45]. As shown in the SEM images (Figure 9), the wood fibers are wetted well by the PU matrix and are incorporated into their structure. This leads to a strong reinforcement effect [46]. Such effects are typical for the influence of flexible chains on rigid molecules [47]. As a result, at 20 wt.% loading of birch fibers, the tensile strength of the PU composite more than doubles (Table 1).
One of the most important coating parameters is the adhesion to steel. We measured this parameter for neat PU and composites (Table 1). The introduction of birch flour in the PU matrix leads to an almost two-fold increase in the adhesion strength to steel (at 20 wt.% of birch flour in the composite). This effect conditioned both the improvement of the mechanical performance of the composites and an increase in hydroxyl functionalities [48]. Hence, we developed new bio-based coatings with satisfactory properties using the cheapest renewable components.

4. Conclusions

In this study, bio-based PU coatings were successfully prepared from renewable vegetable sources such as castor oil and birch flour. The developed manufacturing process allows cheap coatings with acceptable mechanical and adhesion properties to be obtained. The fiber-like structure of birch flour allows a more than doubled value of tensile strength to be achieved, and an almost doubled adhesion strength to steel at 20 wt.% loading as a filler in the PU matrix. The introduction of birch flour in the PU matrix leads to the formation of a more phase-separated structure (with a large degree of phase separation), which is due to the presence of hydroxyl functionalities on the surface of the birch fibers. The presence of hydroxyl groups on the surface of the wood fibers leads to their good wetting at composite synthesis. These results are grounds for the improvement of the mechanical and adhesion characteristics of composites.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app13148258/s1, Figure S1: Distribution of birch fibers on size (From Figure 1). On axis Y- number of particles, on X- its diameter.

Author Contributions

Conceptualization, V.V.Z.; methodology, V.V.Z.; formal analysis, V.V.Z.; experiments, D.S.K.; recording of SEM images, N.N.S.; investigation, D.S.K. and V.V.Z.; writing—review and editing, V.V.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The other data are not available.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 08258 g001 550
Figure 1. Microphotographs of birch flour in optical microscope with measuring scale.
Figure 1. Microphotographs of birch flour in optical microscope with measuring scale.
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Figure 2. The films prepared from samples 1–3.
Figure 2. The films prepared from samples 1–3.
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Figure 3. Stress–strain curves of samples 1 (1), 2 (3), and 6 (2).
Figure 3. Stress–strain curves of samples 1 (1), 2 (3), and 6 (2).
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Figure 4. IR spectrum of sample 1.
Figure 4. IR spectrum of sample 1.
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Figure 5. FTIR spectra in the carbonyl stretching region with deconvolution ((a) sample 1; (b) sample 6)).
Figure 5. FTIR spectra in the carbonyl stretching region with deconvolution ((a) sample 1; (b) sample 6)).
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Figure 6. FTIR spectra of sample 3 and press out from its PDMS.
Figure 6. FTIR spectra of sample 3 and press out from its PDMS.
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Figure 7. SEM images: samples 1 (a) and 2 (b).
Figure 7. SEM images: samples 1 (a) and 2 (b).
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Figure 8. TG and DTG curves of sample 1 (green), 4 (blue), and 6 (red).
Figure 8. TG and DTG curves of sample 1 (green), 4 (blue), and 6 (red).
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Figure 9. SEM images of sample 6 with different magnification.
Figure 9. SEM images of sample 6 with different magnification.
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Table
Table 1. Mechanical and thermal properties of PU composites filled with birch flour.
Table 1. Mechanical and thermal properties of PU composites filled with birch flour.
SamplesDescriptionTensile Strength, σ, MPaElongation at Break, ε, %Hardness, Shore DAdhesion to Steel, MPaTg, °C
1With neat castor oil0.8 ± 0.135 ± 114 ± 12.0 ± 0.05−17.6
2Castor oil dried with zeolites3.2 ± 0.135 ± 118 ± 12.0 ± 0.05−25.5
3With PDMS2.8 ± 0.160 ± 128 ± 10.52 ± 0.05−26.5
45 wt.% birch flour3.5 ± 0.151 ± 126 ± 12.5 ± 0.05−29.2
510 wt.% birch flour5.3 ± 0.144 ± 126 ± 13.53 ± 0.05−29.5
620 wt.% birch flour7.1 ± 0.131 ± 132 ± 13.71 ± 0.05−29.6
730 wt.% birch flour3.7 ± 0.128 ± 138 ± 12.75 ± 0.05−29.6
Table
Table 2. Deconvolution of valence band of C=O in FTIR spectra of PU composites filled with birch flour.
Table 2. Deconvolution of valence band of C=O in FTIR spectra of PU composites filled with birch flour.
SamplesDescription1664 cm−1, %1684 cm−1, %1702 cm−1, %1728 cm−1, %
1With neat castor oil10.93.342.443.4
2Castor oil dried with zeolites0050.349.7
3With PDMS8.5047.244.3
510 wt.% birch flour0054.845.2
620 wt.% birch flour0052.347.7
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MDPI and ACS Style

Konovalov, D.S.; Saprykina, N.N.; Zuev, V.V. High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers. Appl. Sci. 2023, 13, 8258. https://doi.org/10.3390/app13148258

AMA Style

Konovalov DS, Saprykina NN, Zuev VV. High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers. Applied Sciences. 2023; 13(14):8258. https://doi.org/10.3390/app13148258

Chicago/Turabian Style

Konovalov, Dmitry S., Natalia N. Saprykina, and Vjacheslav V. Zuev. 2023. "High-Performance Castor Oil-Based Polyurethane Composites Reinforced by Birch Wood Fibers" Applied Sciences 13, no. 14: 8258. https://doi.org/10.3390/app13148258

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