https://orcid.org/0000-0002-6532-2110
ABSTRACT
Natural fibers can be an effective filling of composites and improve their properties. The aim and novelty of the research was to investigate the effect of modifying natural fibers with silanes and, especially synthesized, polysiloxanes with various functional groups in combination with alkali treatment on the properties of composites based on polylactic acid. For this purpose, composites with a 20% content of flax fibers were produced. Scanning electron microscopic (SEM) tests of the composites showed that the modification of the fibers had a positive effect on the adhesion to the polymer. Their thermal stability (TGA) was slightly improved by the addition of modified fibers, especially by polysiloxane with long alkyl chain. Flammability (PCFC) of composites was also improved up to 36% HRR reduction for mercerized fibers. The mechanical properties of the composites were tested on the basis of a tensile test. It has been shown that properly modified flax fibers can be a good filling for biocomposites, and especially synthesized polysiloxanes used to modify can be more effective than silanes.
摘要
天然纤维可以作为复合材料的有效填充物，改善其性能. 本研究的目的和新颖性是研究硅烷改性天然纤维，特别是合成的具有各种官能团的聚硅氧烷与碱处理相结合对聚乳酸复合材料性能的影响. 为此，生产了亚麻纤维含量为20%的复合材料. 扫描电子显微镜（SEM）测试表明，纤维的改性对聚合物的粘附性有积极影响. 改性纤维的加入，特别是长烷基聚硅氧烷的加入，使其热稳定性略有提高. 复合材料的燃烧性（PCFC）也得到了改善，丝光纤维的HRR降低了36%. 在拉伸试验的基础上测试了复合材料的力学性能. 研究表明，适当改性的亚麻纤维可以很好地填充生物复合材料，尤其是用于改性的合成聚硅氧烷比硅烷更有效.
Introduction
The awareness of society in the subject of broadly understood environmental protection, and what is associated with it: pollution, waste, energy, has been growing exponentially in recent decades. In addition, new regulations are being developed by the government and other groups to reduce pollution and increase recycling. That is why limiting the use of plastic and searching for biodegradable products has become so important today.
Composites are a product of combining the properties of several elements. The conventional ones are a combination of polymers and additives, such as glass, alumina, carbon, etc., which can be dangerous (Bos Citation2004). A great substitute can be natural fibers, e.g. flax, hemp, jute, sisal, etc. These fibers have indisputable advantages over synthetic fibers. They are renewable, cheap, completely or partially recyclable, biodegradable and absorb CO2 during their growth (Elfaleh et al. Citation2023; Tserki et al. Citation2005).
Natural fibers can be used in composites with polymers. However, it should be borne in mind that the use of materials, such as PE, PP, PET, PS and PVC for composites, which take several hundred years to decompose, may also have a negative impact on the environment. There is an increasing emphasis on the need to significantly reduce the amount of plastic waste (Single-Use Plastics Directive) (European Council. Directive (EU) Citation2019). Therefore, material innovations in the form of biodegradable materials are sought. An example is polylactic acid (PLA), which is one of the leading bioplastics on the market (Getme and Patel Citation2020; Manral, Alam, and Chaudhary Citation2020). A blockade in the wider use of biodegradable plastics are high production costs, higher than in the case of traditionally used plastics. The addition of natural fibers to this biopolymer can significantly reduce these costs.
When using natural fibers in composites, however, their greatest limitation should be taken into account. Namely, natural fibers are hydrophilic in nature and the polymer matrix is hydrophobic. As a result of these differences, ineffective stress transfer at the composite interface was observed due to reduced adhesion (Doan, Gao, and Mäder Citation2006; Kabir et al. Citation2012). Meanwhile, for a composite to be a good substitute for plastic and to be successfully used in various industries, it should have appropriate mechanical properties. In order for the natural fiber to exhibit good adhesion to the polymer matrix, it must be appropriately modified. There are a whole range of fiber modification methods. The most important include mercerization, peroxide treatment, silanization, acetylation and many others (Ray et al. Citation2001; Tserki et al. Citation2005; Li, Tabil, and Panigrahi Citation2007; Nair, Thomas, and Groeninckx Citation2001; Pradeep et al. Citation2022; Fuqua, Huo, and Ulven Citation2012, 52, 259–320; Datta and Kopczyńska Citation2015; Freddi et al. Citation2003; De Prez et al. Citation2019).
PLA composites with flax fiber can be obtained by various methods, such as melt compounding and solution dipping processes. Kodal et al. (Citation2015) used polymer coating on the surface of the flax fibers combined with silane treatments and showed an improvement in tensile and impact strength in PLA/flax composites. Polymer coating also showed a decrease in the hydrophilicity nature of the flax fibers. Similar composites obtained by using a twin-screw extruder, reporting an improvement in mechanical properties for PLA/Flax composites when compared to the neat PLA (Xia et al. Citation2016).
The fiber used in our work has been modified in one or two stages using mercerization followed by silanization, or only silanization. Silanization offers the possibility of using more complex reagents and introducing appropriate functional groups to achieve a “tailor-made” modification effect. It is possible to obtain additional benefits depending on the structure of the silicon compound used for modification, which leads to better adhesion, thermal stability, mechanical parameters of the composite structure, and even lower flammability (Gieparda, Rojewski, and Różańska Citation2021).
The whole procedure of the fibers’ modification curried out is described in our previous publication (Gieparda et al. Citation2023). It turned out that the results indicate that the fiber was cleaned during the mercerization process and covered with a layer of silanes or polysiloxanes as a result of silanization. Additionally, FT-IR studies showed that stable bonds were formed between the fiber and silanes and polysiloxanes. Mercerization also improved the thermal stability of the fibers. Flammability tests clearly showed the influence of individual functional groups present in the silanes and polysiloxanes used. The synthesis of polysiloxane with a built-in alkyl chain enabled a significant improvement in the flammability properties of natural fibers, and also allowed to expect an improvement in adhesion when used in composites, due to the uniform coverage of the fiber with a layer of polysiloxane visible in the SEM images.
In the current article, composites based on polylactic acid with modified flax fibers (prepared according to the procedure described in the previously cited publication) were obtained. The novelty is the use of flax fibers modified by, especially synthesized for this purpose, polysiloxanes with various functional groups (alkoxy groups and long alkyl chain). The aim of these modifications is to improve thermal stability, reduce flammability, and improve the mechanical properties of composites.
Modern biocomposites can be used in various sectors of the economy – especially in the automotive industry (Dua et al. Citation2023). The global market for the production and sale of biocomposites is constantly growing. The reason for the increased interest in biocomposites is primarily the high demand from industries that are the main recipients of these materials. The area of their application is also constantly expanding, e.g. packaging, furniture (Weng and Zhang Citation2023), construction (Kim and Chalivendra Citation2020) industries, as well as the production of consumer goods, such as office supplies, cups, toys and many other everyday items (Partanen and Carus Citation2016).
Materials and methods
Materials
Flax fibers modified with silanes and polysiloxanes were prepared by IWNiRZ – PIB and PPNT (Poznań, Poland) according to the procedure in the article by Gieparda et al. (Citation2023). Polylactic acid Ingeo 3251D, density 1.24 g·mL−1 and MFR 80 g·10 min−1 (210°C, 2.16 kg) was purchased from NatureWorks (Minnesota, USA).
Flax fibers modifiers: N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane, vinyltrimethoxysilane, polysiloxane with alkoxy groups, difunctional polysiloxane with alkoxy groups and alkyl chains. The synthesis of polysiloxanes and the fibers modification procedure were described in our publication Gieparda et al. (Citation2023).
Composites preparatioln
Before the process of combining with the natural filler, PLA was dried in a dryer Digicolor Drywell DW12/10 (Digicolor, Herford, Germany) – drying conditions: temperature 85°C, dew point −40°C. Drying was carried out until the moisture content of the material was below 0.025%. The humidity was determined on the MA.X2 moisture analyzer (Radwag, Poland).
The composites were prepared by mixing previously dried PLA polymer and prepared, appropriately modified flax fibers. The composites were prepared using a Dynisco LME laboratory extruder (Dynisco, MA, USA), the hopper temperature was 190°C, the die temperature was 180°C, and the cylinder speed was 25%. The composites were pelletized using a DyniscoTUS (Take-Up System) and an LEC pelletizer (Dynisco, MA, USA). The filler content was 20% w/w. composite. The granulate was then dried under the same conditions as PLA before injection molding of the test pieces for mechanical properties testing. All samples prepared in the studies are listed in the .
Table 1. All composites prepared in the research.
Multipurpose test specimens in accordance to ASTM D-1708-18 (Citationn.d.) were molded by the versatile Laboratory Mixing Molder LMM (Dynisco, MA, USA). Barrel temperature profile: 200°C, mold temperature was set at 60°C. Example specimens as well as test specimen dimensions were presented in the below:
Figure 1. Pictures of test specimens: (a) dimensions of test specimen according to ASTM D-1708-18 (ASTM D-1708-18 Citationn.d.), (b) composite – polylactic with raw flax fibers, (c) polylactic acid.
Test methods
Scanning electron microscopy
Microscopic test – photos of longitudinal views of PLA composites were made with a Hitachi S-3400N scanning electron microscope (SEM) in a high vacuum mode (a secondary electron detector SE). The samples were sprayed with a gold layer under a pressure of 0.1 mbar prior to the tests. The magnifications of 500 were selected, the working distance was 20 mm and the value of the accelerating voltage was 20 kV.
Thermal stability tests
Thermogravimetric study (TGA) – was performed with TA Instruments, Analyser Q50. A 15 ± 1 mg composite sample was heated to 700°C at a heating rate of 10°C·min−1 under a nitrogen atmosphere with a constant gas flow rate of 90 ml·min−1. The mass loss curve and the first derivative of TG (DTG) were determined.
Flammability tests
Flammability tests were performed using a pyrolytic combustion flow calorimeter (PCFC) by FTT for composite samples weighing 5 ± 1 mg. Testing was carried out in accordance with ASTM D7309–Citation2007. The heating rate was 1°C·s−1. The pyrolysis temperature range was 75–500°C, and the combustion temperature was 900°C. The flow was a mixture of oxygen and nitrogen gases at a ratio of 20:80 cm3·min−1. The maximum heat release rate (HRRmax) was determined.
Mechanical properties
Composites’ tensile tests were carried out at room temperature with a Universal Testing Machine Inspekt Table 50 kN (Hegewald & Peschke MPT, Nossen, Germany) as recommended by ASTM D-1708-18 (ASTM D-1708-18 Citationn.d.). A crosshead speed was set to 1 mm·min−1. At least five specimens were tested for every material.
Results
Scanning electron microscopy
In order to confirm that the modification of the flax fibers has positively affected the possibility of using them in composites, SEM tests were performed for the composites of the tested fibers in the amount of 20% and PLA. The fractured surfaces of tensile test specimens of composites were analyzed using scanning electron microscopy to evaluate the degree of adhesion between fiber and matrix. shows SEM images of composites with unmodified and modified flax fibers. Electron micrograph images of the fractured composites were taken in two magnifications of 500 and 1000 times. The photos are summarized in a figure divided into two parts. Left side – composites with one-stage modified fibers – only with silicon compounds; right side – composites modified in two stages – Na OH treatment followed by modification with silicon compounds.
Figure 2. Scanning electron microscopy images of composites with flax fibers without mercerization process and flax fibers with mercerization process in two resolution: 500 and 1.0 k.
The analysis of the obtained SEM images suggests that in most cases the two-stage modification of the fibers brought positive results in terms of the effect on the adhesion of the fibers to the polymer. In all composites, the fibers are uniformly distributed in the polylactide. In the PLA/F composite with unmodified fibers, a lot of empty spaces between the fibers and the polymer can be seen (marked in red in the ). In practice, this means that the compatibility of the fiber with the matrix is poor, and consequently when stress has been applied to the composite, the fibers will be easily pulled out from the matrix. After modifying the fibers with vinyl- and aminosilanes (PLA/F15, PLA/F611), these spaces are still visible, but they have been reduced, especially in the two-stage modification (PLA/FM15, PLA/FM611) (marked in red in the ). In composites with fibers modified with polysiloxane 1 (PLA/FPS1), the situation is similar, but with two-stage modification (PLA/FMPS1), a significant improvement in interfacial adhesion can be observed (marked in red in ). The best result was obtained for the fibers modified with polysiloxane 2, i.e. with a difunctional polysiloxane with an alkyl chain, using isopropanol as a solvent, both in two-stage (with NaOH in the first stage) and in one-stage (only modification with polysiloxane) modification. In this case, a very good adhesion of the fibers to the polymer was obtained (marked in red in the ). There are no visible voids between the fiber and the polymer as well as fibers pulled out of the polymer matrix are also not observed (PLA/FPS1ip, PLA/FMPS1ip, PLA/FPS2, PLA/FMPS2ip) (marked in red in ).
There are not many reports in the literature on the modification of natural fibers with polysiloxanes; however, information can be found that this process increases the adhesion of fibers to the polymer matrix in composites. Seki (Citation2009) reported in his study that oligomeric siloxane treatment of alkalized jute fabrics improved interfacial adhesion between jute fibers and polymer matrix in the polyester composites. The microscopic investigation of the fractured surfaces of composites revealed enhanced bonding between the matrix and the fiber.
Thermal stability tests of flax/PLA composites
In PLA-based natural fiber composites, thermal stability may be reduced due to the degradation of natural fibers that occurs at PLA processing temperatures (i.e. 200–210°C). PLA degradation, on the other hand, starts at about 300°C (Ngaowthong et al. Citation2019). That is why it is so important to study the thermal degradation of PLA composites with natural fiber.
The analysis of the TGA/DTG curves in the range of 200–500°C were showed in . The composites decomposition process can be divided into three main stages. In addition, the second stage in some cases can be divided into two or three substages. However, in most cases, only three main stages can be clearly seen, as additional stages may overlap.
Figure 3. Thermogravimetric analysis of flax/PLA composites in the range of 200–500°C: (a) composites with flax fibers without mercerization process, (b) composites with flax fibers with mercerization process. The solid lines represent the TGA curves, while the dotted lines represent the DTG curves.
To better present all stages of decomposition of individual composites, the results are summarized in a . The analysis of TGA thermograms was started with the onset temperature, i.e. the temperature at which the decomposition of the material begins (Tonset). According to the standard ISO 11,358–1 this is the point of intersection of the starting-mass baseline and the tangent to the TGA curve at the point of maximum gradient. For pure PLA this temperature is 338°C, while for PLA/F composite is only 2°C lower. A slight increase in this temperature is visible for composites with mercerized fiber, and the highest temperature (345°C) was obtained for composites with a two-stage modified fiber – by mercerization and then silanization with polysiloxane with an alkyl chain (PS2). Surprisingly, the use of isopropanol as a solvent to modify the fibers slightly, but still, lowers this temperature. As expected, the aminosilane used to modify the fibers negatively affected the thermal stability of the composites as well (onset temperature 307°C), but mercerization reduced this effect (332°C).
Table 2. Thermal stability results of flax fibers.
Then, taking into account decomposition regions, in the first stage, a weight loss from 0.64% for pure PLA can be seen to approximately 1.5% in the composites. This is due to water loss from the natural fibers. Normally, natural fiber contains about 5–6% of water (Gieparda et al. Citation2023) while with 20% of fiber in composite, this water will be much less. Therefore, this stage is almost unnoticeable in thermograms and does not differ between individual samples (). The second stage of degradation (for composite with fibers modified by aminosilane, mercerized and mercerized-aminosilanized this stage is divided, see , Stage II a, II b, II c) may correspond to the degradation of the PLA polymer matrix as well as to the decomposition of hemicelluloses and cellulose present in the fibers (Beall and Herbert Citation1970). The degradation of PLA includes the following four processes: trace water hydrolysis, intramolecular transesterification to form cyclic oligomers, cis-elimination to acrylic acid, and formation of acetaldehyde and CO2 due to fragmentation (Haafiz et al. Citation2013; Valapa, Pugazhenthi, and Katiyar Citation2014). Stage II b is the main stage of decomposition with the greatest mass loss. At this stage, in the range 341–368°C, depending on the type of fiber modification, the first derivative peak temperature (DTG peak) occurred. The peak of the first derivative indicates the point where the rate of weight loss is the greatest. Additionally, for the composite with fibers modified by aminosilane PLA/F15 second DTG peak occurred in the II a stage at 267°C, while for the composite with mercerized fibers PLA/FM – second DTG peak occurred in the stage II c at 480°C and for the composite with fibers mercerized and silanized PLA/FM15 DTG peak occurred at 463°C. The additional peak appearing in the II a region of PLA/F15 results from the reduction of the thermal stability of the fiber itself, therefore a group of compounds such as: hemicelluloses, amorphous cellulose or low-molecular compounds begin to decompose faster. In contrast, the additional peak in the II c region for PLA/FM coincides with the additional peak visible in the PCFC microcalorimeter graph. A proposed explanation of this fact is described in the section “3.3. Microcalorimeter tests of flax/PLA composites.”
It is also worth noting that the intensity of DTG peak for composites with aminosilane-modified fiber was significantly reduced compared to the other samples (from 2.5%/°C to approx. 1.5–1.7%/°C), which means that these composites began to decompose earlier (onset temperature), while the decomposition is slower and calmer.
The main DTG point is similar for all composite samples (361–368°C), except for three mentioned samples for which there are additional DTG peaks – in these cases the temperature at this point is lowered. The highest temperature at this point applies to composites with fiber modified in two stages – mercerization and silanization in isopropanol with polysiloxane with an alkyl chain (PLA/FMPS2ip). The weight loss at stage II b was over 90% for all samples except the three already described. Finally, the third stage of degradation can be attributed to the thermal degradation byproducts. The final residue that is obtained at 700°C is known as char. The residue in the case of composites comes primarily from the fiber. With pure PLA, there is virtually nothing left. The largest residue remains in the case of composites with aminosilane-modified fiber (both in combination with and without mercerization) – approx. 4%. There are no major differences between the residue in composites with mercerized fiber and only silanized fiber.
Goriparthi, Suman and Rao (Citation2012) in their publication on various ways of modifying jute fibers, draws attention to the difficulty in unambiguous interpretation of thermal stability results for PLA composites. Depending on the type of modification used, the thermal stability is lowered or slightly increased. Alkali, permanganate and peroxide treated composites exhibited lower thermal stability, whereas modification with silane coupling agents, had slightly improved the thermal stability of the composites. Improved thermal stability may be due to remove some easily hydrolyzed substances, which decompose at temperatures lower than the main constituents (cellulose and lignin) (De Freitas Rosa et al. Citation2009).
Microcalorimeter tests of flax/PLA composites
shows the HRR curves from the pyrolysis and combustion flow calorimeter (PCFC) test for composites with fibers modified only with silicon compounds (a), and first mercerized and then modified with silicon compounds (b).
Figure 4. Microcalorimetric results of: (a) composites with flax fibers without mercerization process, (b) composites with flax fibers with mercerization process.
The addition of natural fibers to the PLA polymer matrix reduced the HRR peak. Different chemical compounds can be used for further HRR reduction (Bocz et al. Citation2014; Dorez et al. Citation2014; Sonnier et al. Citation2015). In this work, NaOH fiber mercerization, silicon compounds – silanes and polysiloxanes of various functional groups – or a combination of both methods of modification was carried out. The mere addition of 20% of unmodified natural fibers to the polymer matrix reduced flammability by 6%. Interestingly, the same amount of mercerized fiber reduced flammability by as much as 36% and there is an additional peak around 500°C. This peak may be the result of additional binding of PLA to hydroxyl groups activated as a result of the mercerization process, as well as the result of the reaction of these groups with decay products formed during the pyrolysis process in the PCFC furnace. The basis for such reasoning is the fact that after the silanization process, the presence of this peak in composites with mercerized fiber disappears as a result of the reaction of silicon compounds with -OH groups on the surface of the fiber.
All used silica compounds lowered the HRR of obtained composites in case of unmercerized fibers. The best results were obtained for aminosilane due to the presence of amino groups in its structure. Modification with vinylsilane caused an approx. 10% increase in HRR, which is a normal phenomenon for this type of modification (Gieparda, Rojewski, and Różańska Citation2021).
The results for fibers modified with silica compounds are similar in both cases (a) and (b) despite the mercerization process – reduction of HRR by an average of 25%.
Because polysiloxanes are sensitive to water it was decided to use different solution to prevent those compounds from premature condensation which could hinder proper silanization. The solution of 60% ethanol and 40% of water was replaced with the solution of 95% isopropanol and only 5% of water. Unfortunately, as a result of such a change, an increase in HRR can be observed for both of polysiloxanes. At the same time, it does not mean that the reaction did not proceed properly. Properly applied polysiloxanes modification unfortunately slightly increases the HRR, while improving the mechanical properties.
Mechanical tests
show the effect of the different modifications of flax fibers on the tensile properties of the polylactide composites.
Figure 5. Tensile properties of PLA/flax composites.
Figure 6. Stress–strain curves for PLA/flax fibers composites: (a) flax fibers without mercerization process, (b) flax fibers with mercerization process.
The use of flax fibers in the amount of 20% in composites based on PLA – Ingeo 3251D resulted in a reduction in tensile strength by about 20–30% in majority of samples. However, for the composite with alkali treated and vinylsilane-modified fibers, tensile strength was at the same level as pure PLA polymer. This is confirmed by our previous research (Gieparda et al. Citation2021). The greatest effectiveness of the modification in the field of improving mechanical properties in epoxy laminate was obtained with the use of vinylsilane. In turn, the greatest deterioration in tensile strength, in our current research, was obtained for the composite with amino silane-modified fibers, which also confirms previous studies. Aminosilane is an excellent modifier in the context of reducing the flammability of natural fibers, but it does not work well when it comes to influencing the mechanical properties of the composite.
In the tested composite samples, mercerization had a positive effect on the improvement of tensile properties in the case of modification with silanes. However, in the case of modification with polysioxanes, mercerization did not have such a significant impact on this parameter. However, changing the solvent from ethanol to isopropanol caused a positive effect and small increase in the strength. The smallest decrease in tensile strength among composites with polysiloxane-modified fibers was obtained for modification with polysiloxane 1 in isopropanol without mercerization. The reduction here was only 15% compared to pure PLA polymer. Other composites with polysiloxane-modified fibers in various combinations performed slightly worse, but it can be assumed that they were at a similar level.
The surface quality of the fibers is a very important element in the interfacial bonding between the fibers and the polymer, which can lead to the better mechanical properties in composites. Meon et al. (Citation2012) improved tensile properties of alkaline treated kenaf fibers. Asumani et al. (Citation2012) showed that the interfacial bonding between the kenaf fibers and the polymer matrix is strengthened after using a silane as a fibers treatment. Another very important parameter that may affect the mechanical properties of composites may be processing parameters such as fiber volume fraction or molding temperature. Jai Inder Preet Singh (Citation2021) in his research work use flax fibers as reinforcement and Poly-lactic acid (PLA). The effects of processing parameters on mechanical properties of flax/PLA composites were investigated. It was concluded from the results that maximum tensile and flexural strength are recorded at 35% fiber volume fraction of flax/PLA composites developed at 170°C molding temperature.
Conclusions
In the presented work, it was shown that the obtained composites based on PLA with 20% addition of modified flax fibers show very good adhesion in the case of fibers modified by polysiloxanes with prior mercerization. In particular, this relates to long chain alkyl polysiloxane (PS2) using isopropanol as solvent. After replacing polysiloxane 2 with polysiloxane 1, the adhesion is slightly worse but still satisfying.
The thermal stability of composites was slightly increased or one the similar level in most cases (except for the composite with aminosilane-modified fiber). The onset temperature was the highest for composites with fiber modified by polysiloxanes (340–345°C). For the composite with fiber modified by polysiloxane 2 using ethanol as a solvent, the main stage (II b) of decomposition began at the latest (291°C). The highest DTG temperature, one the other hand, was achieved for the FMPS2ip composite.
As far as the flammability of composites is concerned, the greatest reduction in HRR was obtained for composites with mercerized fiber and with aminosilane-modified fiber.
Aminosilane is an excellent modifier in the context of reducing the flammability of natural fibers, but it does not work well when it comes to influencing the mechanical properties of the composite. Tensile properties for the composite with aminosilane-modified fibers are the lowest. They are slightly better for the composite with vinyl silane, but it is still not a satisfactory level. It looks much better in the case of composites with polysiloxane-modified fiber. In the mechanical properties, we can definitely see a correlation with the quality of adhesion visible in the SEM images.
Studies of the produced PLA composites have clearly shown that newly synthesized polysiloxanes can be effective modifiers for flax fibers. Therefore, it is planned to continue research in the field of synthesis of various polysiloxanes, which will be able to further improve the properties of natural fibers, and thus biocomposites.
Highlights
obtained composites based on PLA with 20% addition of modified flax fibers show very good adhesion in the case of fibers modified by polysiloxanes with prior mercerization
The thermal stability of composites was slightly increased or one the similar level in most cases (except for the composite with aminosilane-modified fiber)
the greatest reduction in HRR was obtained for composites with mercerized fiber and with aminosilane-modified fiber.
polysiloxanes are the best modifiers in terms of the mechanical properties of composites
the correlation of mechanical properties with the quality of adhesion is visible in the SEM images
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Consent
All participants in the manuscript consent to publish.
Ethical approval
I confirm that all the research meets ethical guidelines and adheres to the legal requirements of the study country.
Disclosure statement
No potential conflict of interest was reported by the author(s).
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