Improved Synthetic Route of Incorporation of Nanosilicon Species into Phenol-Formaldehyde Resin and Preparation of Novel ZnAl-Layered Double-Hydroxide Hybrid Phenol-Formaldehyde Resin

Hybrid phenol-formaldehyde (PF) resins represent one of the most important niche groups of binding systems for composites. New industrial needs, environmental requirements, and price fluctuations have led to further research on materials with enhanced mechanical and thermal properties. The preparation of novel hybrid materials can be achieved by inclusion of various elements or functional groups in the organic polymer phenolic framework. Herein, we report the synthesis and characterization of a PF-based hybrid material with different nanoscale silicone species and ZnAl-layered double hydroxide (LDH). The main goals of this study were to improve the synthetic pathways of hybrid resin, as well as to prepare granulated composite materials and test samples and determine their characterization. Added inorganic species increased the glass-transition temperature by a minimum of 8 °C, which was determined using differential scanning calorimetry (DSC). Rheological properties (melting viscosity and flow distance) of the hybrid resin were measured. The homogeneity of distribution of added species across the organic matrix was evaluated with scanning electron microscopy (SEM). With synthesized new hybrid-binding systems, we prepared different granulated composite materials and evaluated them with the measurements of rheological properties (flow curing characteristics). Tensile strength of samples, prepared from granulated composite material, improved by more than 5%.


Introduction
Phenol-formaldehyde (PF) resins have had one key role in the manufacturing of polymer composite materials for decades. They are an essential component in the production of paints, adhesives, and coatings, as well as in the automotive and electronic industries [1]. The research field of PF resins is nowadays divided into two streams. The first one is interested in improving specific properties through the incorporation of different organic or inorganic species [2][3][4]. The second one is focused on moving from phenol and formaldehyde to biodegradable or environmentally friendly raw materials [5][6][7].
The main goal of the synthesis of organic-inorganic hybrid materials is the synergy of both materials. While the organic part contributes to toughness, elasticity, and also enables functionalization, the inorganic part provides hardness and thermal and mechanical stability [8]. Silicon compounds and precursors are one of the most represented species for the preparation of various hybrid PF resins [9][10][11][12][13]. It has already been confirmed that the addition of silicon species improves both thermal and mechanical properties [14][15][16]. Most of the hybrid resins were prepared by the sol-gel process, in which the biggest problem represents the phase separation and agglomeration that occur during the synthesis [17][18][19]. The preparation of the layered double hydroxide (LDH)-containing hybrids from PF novolac resin is a less-researched area. LDH can be described by a formula [M 2+ 1−x M 3+ x (OH) 2 ] x+ A n− x/n ·mH 2 O, where M 2+ and M 3+ represents a divalent or a trivalent metal cation (e.g., Mg 2+ , Zn 2+ , and Al 3+ ). A n− is a charge-compensating inorganic or organic anion (e.g., CO 3 2− , SO 4 2− , NO 3 − , and RCO − ) [20]. Several positive effects have already been noticed when combining LDH with different materials. In biomedicine applications, they can successfully serve as a controlled drug delivery and release system [21]. In the field of polymer materials, it has already been proved that the addition of LDH can improve the mechanical properties and thermal stability of polymers [22].
The aim of this work is to examine the in situ synthesis of nano-SiO 2 phenol-formaldehyde hybrid resin and the preparation of ZnAl-hybrid phenol-formaldehyde resin. For the nano-SiO 2 phenol-formaldehyde hybrid resin, we focused on the homogeneity of the system, which is crucial for uniform mechanical properties. We tried to achieve it with different synthetic paths. We also tested the concentration dependence of added species on the selected properties. Individual types of zinc and aluminum species together are very common partners in composite materials, as they facilitate processability and help to improve specific properties [23]. We prepared the hybrid resin of ZnAl-LDH with phenol-formaldehyde resin and characterized it with different analytical methods. We investigated the effects of both added species on glass-transition temperature, melt viscosity, and reactivity of hybrid resins. Based on synthesized hybrid binding systems, we prepared different granulated composite materials and explored their rheological characteristics, the content of volatile organic components, and reactivity. For the evaluation of mechanical properties and dimensional stability, test samples from granulated materials via hot-press machine were prepared.

Synthesis of Novolac Resin with 3-Aminopropyltriethoxysilane (AMEO)
Phenol, formaldehyde, and oxalic acid were put into the four-necked round-bottom glass flask, equipped with a thermometer, mechanic stirrer, and condenser. The reaction took place at about 100 • C until the concentration of formaldehyde fell below 0.5%, determined according to ISO standard 11402. Then, we started the atmospheric distillation until the temperature of the mixture rose to about 180 • C. At 180 • C, we started with vacuum distillation until the concentration of the free phenol in the reaction mixture fell below 0.5%. The mixture was then cooled to about 130 • C and the 3-aminopropyltriethoxysilane was added. The reaction mixture was then poured into a tray and allowed to cool to room temperature.

Synthesis of Hybrid Novolac Resin with Colloidal Dispersion of Nano-SiO 2 (COSI)
Phenol, formaldehyde, colloidal dispersion of nano-SiO 2 , and oxalic acid were put into the four-necked round-bottom glass flask, equipped with a thermometer, mechanic stirrer, and condenser. The reaction took place at about 100 • C until the concentration of formaldehyde fell below 0.5%. Then, we started the atmospheric distillation until the temperature of the mixture rose to about 180 • C. At 180 • C, we started with vacuum distillation until the concentration of the free phenol in the reaction mixture fell below 0.5%. The reaction mixture was then poured into a tray and allowed to cool to room temperature.

Preparation of Hybrid Novolac Resin with ZnAl-Layered Double Hydroxide
Phenol, formaldehyde, and oxalic acid were put into the four-necked round-bottom glass flask, equipped with a thermometer, mechanic stirrer, and condenser. The reaction took place at about 100 • C until the concentration of formaldehyde fell below 0.5%. Then, we started the atmospheric distillation until the temperature of the mixture rose to about 180 • C. At 180 • C, we started with vacuum distillation until the concentration of the free phenol in the reaction mixture fell below 0.5%. The reaction mixture was then poured into a tray and allowed to cool to room temperature. ZnAl-layered double hydroxide (LDH) was then added in three different ways. Firstly, the novolac resin was heated to 70 • C and the LDH was added. Secondly and thirdly, the novolac resin and LDH were crushed together and heated to 70 • C.

Preparation of Granulated Composite Materials
The novolac resin or hybrid novolac resin was mixed together with hexamethylenetetramine (HMTA), nontreated and silane-treated kaolin, lubricant, release agent, dye, and milled glass fibers ( Figure 1). The mixture was heated to about 75 • C and the water was added. The mixture was then kneaded, cut to small granules, and dried at 86 • C.

Synthesis of Hybrid Novolac
Resin with Colloidal Dispersion of Nano-SiO2 (COSI) Phenol, formaldehyde, colloidal dispersion of nano-SiO2, and oxalic acid were put into the four-necked round-bottom glass flask, equipped with a thermometer, mechanic stirrer, and condenser. The reaction took place at about 100 °C until the concentration of formaldehyde fell below 0.5%. Then, we started the atmospheric distillation until the temperature of the mixture rose to about 180 °C. At 180 °C, we started with vacuum distillation until the concentration of the free phenol in the reaction mixture fell below 0.5%. The reaction mixture was then poured into a tray and allowed to cool to room temperature.

Preparation of Hybrid Novolac Resin with ZnAl-Layered Double Hydroxide
Phenol, formaldehyde, and oxalic acid were put into the four-necked round-bottom glass flask, equipped with a thermometer, mechanic stirrer, and condenser. The reaction took place at about 100 °C until the concentration of formaldehyde fell below 0.5 %. Then, we started the atmospheric distillation until the temperature of the mixture rose to about 180 °C. At 180 °C, we started with vacuum distillation until the concentration of the free phenol in the reaction mixture fell below 0.5%. The reaction mixture was then poured into a tray and allowed to cool to room temperature. ZnAl-layered double hydroxide (LDH) was then added in three different ways. Firstly, the novolac resin was heated to 70 °C and the LDH was added. Secondly and thirdly, the novolac resin and LDH were crushed together and heated to 70 °C.

Preparation of Granulated Composite Materials
The novolac resin or hybrid novolac resin was mixed together with hexamethylenetetramine (HMTA), nontreated and silane-treated kaolin, lubricant, release agent, dye, and milled glass fibers ( Figure 1). The mixture was heated to about 75 °C and the water was added. The mixture was then kneaded, cut to small granules, and dried at 86 °C.

Methods of Characterization
Methods of characterization were used in two different stages of the process. The first stage of characterization took place after the synthesis/preparation of resins and the second stage was completed for the characterization of the granulated composite material.

• Karl Fischer titration
Water content was determined with a volumetric titrator (DL38, Mettler Toledo, Greifensee, Switzerland) based on ISO standard 15512:2016 [26]. The reagent for volumetric titration was pyridine free one-component HYDRA POINT composite 5 (J. T. Baker, NJ, USA). The sample mass was 800 ± 20 mg. The analysis of free phenol content was performed using gas chromatography with a flame ionization detector (GC-FID, Trace 1300, Thermo Fischer Scientific, Waltham, MA, USA). The method was based on the ISO 8974:2002 [27] standard for determination of residual phenol content by gas chromatography. The separation column used was the Rxi-5Sil MS column (30 m × 0.32 mm × 0.25 µm). For evaluation, Chromeleon TM Chromatography Data System software was used.
Flow-distance test of phenolic resin powder is specified with ISO standard 8619:2003 [28], according to which some internal adjustments were made: the height of the pellet was 5 ± 0.2 mm and the diameter was 14.5 ± 0.3 mm. Flow distance is dependent on the reactivity and on the melt viscosity of the resin.

•
Melt-viscosity analysis Melt viscosities (80 ± 2 mg of sample) were measured using a cone and plate rotational viscometer (RM100 CP2000, LAMY Rheology, Champagne au Mont d'Or, France) at shear rate 600 s −1 . The temperature was 150 • C and time of measurement was 60 s.
• Differential scanning calorimetry (DSC) Glass-transition temperature was determined using a DSC823 e Mettler Toledo Instrument (Mettler Toledo, Greifensee, Switzerland). We used 40 µL aluminum pans with pierced lids. Samples (10-15 mg) were analyzed at the temperature range from 25 to 160 • C at a heating rate 10 K/min under air atmosphere. The data collection and analysis were performed using the program package STARe Software 16.2 (STAR e , Software 16.2, Mettler-Toledo GmbH, (2019)).

•
Scanning electron microscopy (SEM) Samples were investigated using a scanning electron microscope (JEOL JSM-IT500, Japan) with SEM operational software. The resin samples of approximately 10 × 5 × 3 mm 3 were prepared using epoxy resin (EpoFix Resin, Struers, Denmark) and hardener (EpoFix Hardener, Struers, Denmark) and were polished. Samples were placed on double-sided carbon tape and, additionally, were Au/Pd (n(Au):n(Pd) = 80:20)-coated using (Quroum Q150T ES). The thickness of the Au/Pd coating was 20 nm. The images were carried out in high-vacuum mode; a working distance of 10 mm, an accelerated voltage of 15 kV, and a secondary electron detector (SED) were used.

•
Volatile organic compounds The test samples were heated and held at 168 • C using a halogen moisture analyzer apparatus (HG 63, Moisture Analyzer, Mettler Toledo, Greifensee, Switzerland). Aluminum sample pans were filled with 13 ± 0.1 g and the analysis time was 12 min. • Rheology analysis The flow-curing behaviors of PMCs were determined using a torque rheometer (Brabender Plastograph EC plus, MB30 mixer, Brabender GmbH & Co. KG, Duisburg, Germany). The 25 g of test samples were loaded into the mixer bowl at 140 • C. The speed of the triangle-shaped mixing blades was 30 rpm. The measurements shown on the plastogram represent the time dependence of the torque at a given temperature with respect to the changes in material structure. The evaluation was made with Brabender data correlation software according to standard DIN 53 764 [29].

Characterization of Hot-Pressed Cylindrical Samples
• Density The density of cured cylindrical samples was measured in water at room temperature. We used a precision analytical balance (XP205, Delta Range, Mettler Toledo, Greifensee, Switzerland) with the associated density kit.

•
Mechanical test The mechanical characterization of cured final cylindrical test samples was carried out using a static tensile machine (Z100, Zwick/Roell, Ulm, Germany) at a speed of 86 mm/min. The tensile strength was measured on hollow cylindrical samples. The height of the sample was 15.00 ± 0.05 mm (H), outer diameter 20.00 ± 0.05 mm (D), and inner diameter 12.00 ± 0.05 mm (d).

Results
The samples we prepared followed each other in the following order. Sample R1 represents the basic resin without additives. Samples R2-R4 were prepared with the addition of AMEO, COSI was added to samples R5-R7, and samples R8-R10 contained ZnAl-LDH. In all of them, the lowest concentration of additive was 1.5 wt% and the highest was 3 wt%. From all hybrid resins (R1-R10), we prepared granulated composite material (GC1-GC10), in which 25 wt% represents resin with hardener, 30 wt% fillers with additives, and 25 wt% fibres.

Water Content and Free Phenol Content
The water content and free phenol content of none of the measured samples exceeded 0.5%, which was set for two of the basic goals in all syntheses (Table 1).

Flow Distance and Melt Viscosity
Flow distance and melt viscosity of the resins varied with the content of the water, free phenol, and concentration of additions of different species in the polymer matrix. Since the concentrations of water and free phenol content in all synthesized hybrid systems were in the same range, they did not play a decisive role in the changing melt viscosities and flow lengths. The hardener concentration was also the same in all samples. It is observed that melt viscosity and flow length are reciprocally correlated. The results show that the melting viscosity of the hybrid resins increases with the addition of inorganic species, while the Polymers 2022, 14, 4684 6 of 11 flow distance decreases. Both are also dependent on the concentration of added species (Table 1).
Flow distance and melt viscosity of the resins varied with the content of the water, free phenol, and concentration of additions of different species in the polymer matrix. Since the concentrations of water and free phenol content in all synthesized hybrid systems were in the same range, they did not play a decisive role in the changing melt viscosities and flow lengths. The hardener concentration was also the same in all samples. It is observed that melt viscosity and flow length are reciprocally correlated. The results show that the melting viscosity of the hybrid resins increases with the addition of inorganic species, while the flow distance decreases. Both are also dependent on the concentration of added species (Table 1).

Differential Scanning Calorimetry (DSC)
The DSC measurements were performed in order to determine the glass-transition temperatures (Tg) of the synthesized resins. The glass transition was determined using the STAR e software function. The sp-line was determined as the baseline. The glass-transition temperature is the temperature at which increased molecular mobility

Differential Scanning Calorimetry (DSC)
The DSC measurements were performed in order to determine the glass-transition temperatures (Tg) of the synthesized resins. The glass transition was determined using the STAR e software function. The sp-line was determined as the baseline. The glasstransition temperature is the temperature at which increased molecular mobility results in changing the glassy state of the resin into a rubbery or melted one. The results show that with all additives, the glass-transition temperatures increased compared to the basic resin ( Figure 3). A trend can be observed in how, at a concentration of 1.5 and 2% of any chosen additive (R2-R6 and R8-R10), the glass-transition temperature moves in the same range by approximately 8 • C higher than the basic resin. A larger deviation occurs with the addition of 3% COSI (R7), when the temperature of the glass transition rises by an additional 6 • C (Table 1).
the STAR e software function. The sp-line was determined as the baseline. The glass-transition temperature is the temperature at which increased molecular mobility results in changing the glassy state of the resin into a rubbery or melted one. The results show that with all additives, the glass-transition temperatures increased compared to the basic resin (Figure 3). A trend can be observed in how, at a concentration of 1.5 and 2% of any chosen additive (R2-R6 and R8-R10), the glass-transition temperature moves in the same range by approximately 8 °C higher than the basic resin. A larger deviation occurs with the addition of 3% COSI (R7), when the temperature of the glass transition rises by an additional 6 °C (Table 1).

Scanning Electron Microscopy (SEM)
Using a scanning electron microscope, we determined how homogeneously nano-SiO2 and ZnAl-LDH were distributed along the organic matrix during one-pot synthesis. Nanoparticles are known to have a high tendency to aggregate and agglomerate, which was also confirmed in this case. A bright area, nearly in the middle of Figure  4b,c represents aggregates of colloidal SiO2 and ZnAl-LDH particles, around which PF resin is organized. Nevertheless, it was further shown that partial agglomeration does not affect the deterioration of mechanical properties of the final hot-pressed cylindrical samples prepared from granulated composite material.

Scanning Electron Microscopy (SEM)
Using a scanning electron microscope, we determined how homogeneously nano-SiO 2 and ZnAl-LDH were distributed along the organic matrix during one-pot synthesis. Nanoparticles are known to have a high tendency to aggregate and agglomerate, which was also confirmed in this case. A bright area, nearly in the middle of Figure 4b,c represents aggregates of colloidal SiO 2 and ZnAl-LDH particles, around which PF resin is organized. Nevertheless, it was further shown that partial agglomeration does not affect the deterioration of mechanical properties of the final hot-pressed cylindrical samples prepared from granulated composite material. results in changing the glassy state of the resin into a rubbery or melted one. The results show that with all additives, the glass-transition temperatures increased compared to the basic resin (Figure 3). A trend can be observed in how, at a concentration of 1.5 and 2% of any chosen additive (R2-R6 and R8-R10), the glass-transition temperature moves in the same range by approximately 8 °C higher than the basic resin. A larger deviation occurs with the addition of 3% COSI (R7), when the temperature of the glass transition rises by an additional 6 °C (Table 1).

Scanning Electron Microscopy (SEM)
Using a scanning electron microscope, we determined how homogeneously nano-SiO2 and ZnAl-LDH were distributed along the organic matrix during one-pot synthesis. Nanoparticles are known to have a high tendency to aggregate and agglomerate, which was also confirmed in this case. A bright area, nearly in the middle of Figure  4b,c represents aggregates of colloidal SiO2 and ZnAl-LDH particles, around which PF resin is organized. Nevertheless, it was further shown that partial agglomeration does not affect the deterioration of mechanical properties of the final hot-pressed cylindrical samples prepared from granulated composite material.

Volatile Organic Compounds
The content of the volatile organic components in the granulated composite material, which was estimated on the basis of preliminary tests not to affect the dimensional changes of the test samples after the curing process, was set between 2.7 and 3.3 wt%. The

Volatile Organic Compounds
The content of the volatile organic components in the granulated composite material, which was estimated on the basis of preliminary tests not to affect the dimensional changes of the test samples after the curing process, was set between 2.7 and 3.3 wt%. The value of the volatile organic components of all composites prepared from hybrid resins ranged between 2.8 and 3.3 wt%.

Rheology Analysis
The flow-curing behavior plastograms illustrate the torque measurement over the period of the test duration and are a reflection of structural changes in the granulated material. The characteristic values of the measurements are the torque minimum (B) and the residence (tV) and reaction (tR) times. The torque minimum determines the melt viscosity of a granulated sample before the curing reaction. The residence time determines the time duration of material in a molten state before starting the curing reaction. After reaching the maximum torque peak and the torque stabilizes, the greater part of the curing reaction is completed, which defines the reaction time. The results show a slight increase in minimum viscosity for all granulated composite materials prepared from resins with additives, but no significant deviation occurs for any. The residence and reaction time of composite material prepared from hybrid resins with aminosilane are slightly shorter, which means that the curing reaction takes place a little faster than with other composites. The addition of inorganic additives in resin has no noteworthy effect on the crosslinking rate (Table 2, Figure 5). value of the volatile organic components of all composites prepared from hybrid resins ranged between 2.8 and 3.3 wt%.

Rheology Analysis
The flow-curing behavior plastograms illustrate the torque measurement over the period of the test duration and are a reflection of structural changes in the granulated material. The characteristic values of the measurements are the torque minimum (B) and the residence (tV) and reaction (tR) times. The torque minimum determines the melt viscosity of a granulated sample before the curing reaction. The residence time determines the time duration of material in a molten state before starting the curing reaction. After reaching the maximum torque peak and the torque stabilizes, the greater part of the curing reaction is completed, which defines the reaction time. The results show a slight increase in minimum viscosity for all granulated composite materials prepared from resins with additives, but no significant deviation occurs for any. The residence and reaction time of composite material prepared from hybrid resins with aminosilane are slightly shorter, which means that the curing reaction takes place a little faster than with other composites. The addition of inorganic additives in resin has no noteworthy effect on the crosslinking rate (Table 2, Figure 5).  Figure 5. Examples of flow-curing behaviors of some prepared GC composites. Figure 5. Examples of flow-curing behaviors of some prepared GC composites.

Density
The density of a hot-pressed cylindrical samples was measured before and after the postcuring process. In all test samples the precuring density was around 1900 ± 20 g/cm 3 , indicating that the hybrid resin does not affect the density of the final cylindrical samples. However, a trend is observed, in which the density of the postcuring samples was about 1.0% lower than the density before postcuring.

Mechanical Test
The dimensional measurements of the cylindrical samples before and after the postcuring processes showed very small dimensional changes (about 0.3%) in all three measured parameters. It was observed that the outer diameter and height slightly decreased, while the inner diameter slightly increased. With the selected composition of the composite material, the addition of aminosilane had no additional effect on the tensile strength, which can be partly attributed to the fact that one of the fillers was already coated with aminosilane [31]. The tensile strengths of all composites, prepared with hybrid resins, which contained COSI, were 7-14% greater than in basic resin composites. The addition of ZnAl-LDH to the resin increased the tensile strength of the composite by just under 6% (Table 2).

Discussion
The addition of nano-SiO 2 and ZnAl-LDH compared to aminosilane and basic resin without additives, the influence of selected additives on the properties of hybrid resins, and the granulated composite material prepared from them were experimentally investigated.
The water content has a significant role in plasticity of novolac phenolic resins and even more so affects the melt viscosity. Its content, from 0.1 to 3%, can reduce melt viscosity by up to 90%. When the water content increases, the flow distance decreases due to the higher reactivity of the resin with the hardener, despite the fact that the melt viscosity drops. Free phenol content plays a similar role to water content, except that its effect is less rigorous [1,32].
In the FTIR spectrum of the hybrid resins with nano-SiO 2 , we expected a peak at approximately 1100 and 810 cm −1 [33]. However, the concentration of the additive was too low, and at the same time, the peaks of the basic resin were prominent in this region, so that they overlapped. The peak, representative of ZnAl-LDH (1357 cm −1 ) is characteristic of a free carbonate anion and represents the carbonate stretching mode [34,35].
The glass-transition temperature varies with the addition of inorganic additives and their concentration. Inorganic additives, dispersed in a matrix, can suppress the local mobility of the polymer material and, therefore, increase its glass-transition temperature.
The large specific surface area of nanoparticles and the attractive forces between them lead to the formation of agglomerates. This is greatly helped by the high viscosity of the polymer matrix, which prevents their optimal dispersion. Nevertheless, the agglomerate regions were not so pronounced as to affect the mechanical properties of the test samples [31].

Conclusions
The key findings of the research on the one-pot synthesis of nano-SiO 2 hybrid resins and solid-solid mechanochemical milling addition of ZnAl-LDH are as follows: The formation of the hybrid resins via in situ synthesis with nano-SiO 2 was successful, but the aggregation of nanoparticles into larger agglomerates was partially confirmed. The formation of the agglomerate did not affect the final properties of the resin, which were nevertheless homogenous, as well as the properties of the prepared composite material from this hybrid resin.
Melt viscosities, flow distances, and glass-transition temperatures change markedly with the addition of an inorganic additive. With the addition of nano-SiO 2 , individual properties depend on the concentration of the additive itself, while with aminosilane and ZnAl-LDH the concentration does not play a significant role. Crosslinking rate of granulated composite material does not depend on the addition of nano-SiO 2 and ZnAl-LDH to the hybrid resin, nor on their concentration, while the addition of the aminosilane slightly accelerates the reaction.