Comparative analysis of the qualitative characteristics of formaldehyde and acetaldehyde resins based on styrene-modified oil shale alkylresorcinols

Reducing the amount of volatile compounds in alkylresorcinol-aldehyde resins, reducing the impact of their components on the environment, and improving their performance can be achieved by replacing formaldehyde with acetaldehyde and by preliminary aralkylation of the resorcinol components of the raw material. To prove this, a comparative analysis of the properties of resins synthesized based on oil shale alkylresorcinols pre-treated with styrene, formaldehyde, or acetaldehyde was carried out. The effects of the molar ratio of feedstock/aldehyde and the amount of catalyst on the yield and characteristics of the resins were considered. Both individual alkylresorcinols (R, 5-MR, 2.5-DMR) and industrial fractions (REZOL, HONEYOL) were used as the raw materials. The following indicators were used to compare the obtained resins: softening point, ash content, coke number, moisture content, volatile substances, and solubility in organic solvents. The thermal properties of the resins were studied by TG/DTG/DTA in inert and oxidative atmosphere. Qualitative and quantitative analyses of resins and distillates for the content of unreacted resorcinol, individual alkylresorcinols, and styrene were carried out using iodometry, thin-layer chromatography (TLC), and gas chromatography (GC). It has been shown that the use of acetaldehyde makes it possible to obtain resins with the stated characteristics. The formaldehyde (SF) and acetaldehyde (SAc) resins obtained consist of oligomers with different chain lengths. Synthesized SAc resins are solids with a softening point of 51 °C–103 °C. Resins are soluble in acetone, EtOH, acetonitrile, and THF, and insoluble in benzene. SAc resins contain 3–4 times less unreacted original resorcinol components compared to SF resins. The conversion of styrene in the synthesis of Rez100SF resins was 42 wt%, Rez100SAc and Hon100SAc are 98 and 97 wt%, respectively. A comparative analysis of resins synthesized using acetaldehyde instead of formaldehyde helped identify both the advantages and disadvantages of the proposed synthesis variation.

Hon 100 SAc resorcinol-alkylresorcinol-acetaldehyde resin modified with styrene based on the HONEYOL fraction.

Introduction
Currently, in the construction, paint, varnish, woodworking, and tire industries, both natural and synthetic polymers are used to produce aerogels, rubber, and electrical products. The main advantages of using synthetic polycondensation resins are their high adhesion to a large spectrum of materials, water resistance, mechanical strength, and chemical and thermal stability. Among synthetic polymers, the most common are polycondensation resins based on phenol (Ph) and formaldehyde (F) [1,2].
However, the production and use of phenol-formaldehyde resins do not meet modern environmental requirements [3,4]. According to the World Health Organization, the resin components Ph and F are highly toxic, carcinogenic, and mutagenic [5][6][7]. When released into water bodies, Ph negatively impacts natural biocenoses. This causes several environmental problems. While F negatively affects the genetic apparatus of all the living organisms.
One way to reduce the impact of F and Ph on the environment is to replace them with more environmentally friendly compounds, including the lower members of the homologous series of aliphatic aldehydes, such as acetaldehyde, propionaldehyde, butyraldehyde, and resorcinol (R) or alkylresorcinols (ARs) [8,9]. The use of aliphatic aldehydes in the reaction of R and its derivatives enables the synthesis of resinous novolac products [10]. Novolac resorcinol-formaldehyde (RF) resins are produced without F relative to R [11].
By preliminary aralkylation of the resorcinol components of the resin with an aromatic alkene (S), it is possible to reduce the amount of emitted volatile substances in the RF resin at the time of processing as well as during its storage and operation. As shown in previous research [12], RF resins without S modification are characterized by a low softening point, high moisture content, volatile components, and unreacted R. It is the most hygroscopic among SF resins modified with S. Preliminary aralkylation of the HONEYOL alkylresorcinol fraction with S makes it possible to obtain resins with a low content of ARs without compromising their technological characteristics. It should be noted that the content of the main component (5-MR) in the fraction is 84 wt% [13].
Earlier studies [10,[13][14][15][16][17][18][19] have shown the possibility of obtaining Ph-and R-acetaldehyde resins. However, there are no data in the literature on the synthesis of polycondensed SAc resins based on the alkyl(aralkyl) derivatives of R. The properties of resins are described in [10]. Resorcinol-acetaldehyde resins without preliminary modification of resorcinol with styrene have a softening point of 100°C-130°C, content of unreacted R is 9-15 wt%. An increase in the softening temperature of the resin above the melting point of resorcinol (>110°C) leads to the release of resorcinol on the surface of the resin.
Both R, 5-MR, and others preliminarily aralkylated with aromatic or aliphatic alkenes, as well as industrial ARs fractions obtained by processing oil shale using the Kiviter technology, can be used as raw materials for the synthesis of resins [10,13,20,21]. The above sources provide data on the synthesis of resins and some physicochemical characteristics. The results of the analysis of these resins by methods: TG/DTG/DTA, TLC, GC are not available.
The main difficulty in the synthesis of resins based on industrial mixtures of oil shale ARs lies in the different reactivity of the components [22,23]. Consequently, the final product (resin) may contain unreacted ARs.
The purpose of this work was to study the synthesis of SAc resins based on R and ARs preliminarily modified with S, to determine the technical and physicochemical characteristics of the resulting resins, and to compare them in all respects with SF resins [12] synthesized under different conditions. For the first time, acetaldehyde resins were synthesized from raw materials (R, 5-MR, and 2.5-DMR) preliminarily modified with styrene and oil shale fractions of alkylresorcinols REZOL and HONEYOL, containing 5-MR ∼ 43 and ∼ 55 wt%, respectively.
The HONEYOL and REZOL fractions, in addition to the main components, also contain nitrogen (N) and moisture (W). Their amounts were N HONEYOL = 0.3 wt% and N REZOL = 0.3 wt%, W REZOL = 0.5 wt% and W HONEYOL = 0.2 wt%. These components influence the course of the synthesis.
Along with the listed ARs, HONEYOL and REZOL may contain other substituted ARs with an alkyl chain of various lengths (9 or more carbon atoms) in the 5-position of the ring, as well as with cyclic side chains [24]. The REZOL fraction contains more such compounds than HONEYOL. This can be explained by differences in their boiling points. Styrene (99.9 wt%) was used as the aralkylating agent. The polycondensation reaction was carried out in a slightly acidic medium using an inorganic catalyst (2N H 2 SO 4 solution). An equivalent amount of NaOH was used to neutralize it.
The resins were synthesized in two stages. In the first step, a mixture of R and oil shale ARs was aralkylated with freshly distilled S in the presence of H 2 SO 4 as an acid catalyst.
In the process of aralkylation of R with styrene, mono-, di-, and trimethylbenzyl-substituted derivatives of R can be obtained [10]. Figure 1 shows possible variants of the aralkylation reaction of alkylresorcinols (5-MR, 5-ER, and 2.5-DMR) with styrene. Subsequently, aldehyde, -F, or acetaldehyde (Ac) was added to the aralkylated mixture. The structures of the novolak-type resins obtained using F and Ac with R and its derivatives are shown in figure 2.
Preliminary experiments showed that the most complete interaction between S and R occurred when the latter was fractionally introduced into the reaction flask. Therefore, all further experiments on aralkylation of R derivatives, individual ARs, and fractions of oil shale ARs were carried out as follows. The initial amount of the resorcinol component was divided into two parts. The first part was placed in a reaction flask and allowed to melt. Then, a calculated amount of acid catalyst was added, and S was added dropwise with constant stirring of the reaction mixture. This avoids the occurrence of side and secondary processes and prevents the polymerization of S. After introducing the entire volume of S, the second part of the initial resorcinol component was added to the reaction flask. The aralkylation reaction was performed for 30-90 min at 150 ± 10°C. The completeness of the aralkylation reaction was controlled by both the decrease in the concentration of S in the reaction mixture by GC and the content of S in the distillate. The end of the reaction was considered as the time corresponding to the cessation of the change in S concentration. The number of moles of the HONEYOL and REZOL fractions was calculated from their number-average molar mass M n , which was obtained using equation (1): where N i -the number of moles of the i-th component of the HONEYOL or REZOL fraction, M i -the molecular weight of the i-th component of the HONEYOL or REZOL fraction. The molar ratios of the raw materials, aldehyde and catalyst are presented in table 2. The modifying agent (S) was added in an amount of 0.2 mol per mol of the resorcinol component.
The conditions for the synthesis of SF resins (RSF, 5-MRSF, R 90 Rez 10 SF, R 70 Rez 30 SF, and Rez 100 SF) were described in our previous paper [12].
When carrying out the synthesis with Ac, its low boiling point of 20.2°C was taken into consideration. Therefore, the temperature in the vessel from which Ac entered the reaction flask was maintained at approximately 10°C. Before the introduction of Ac, the reaction flask containing the mixture was preliminarily cooled to 80°C-90°C, after that the calculated amount of Ac was added dropwise into it. After adding the entire amount of Ac, the reaction was conducted for 2 h at a temperature of 100 ± 5°C. After the completion of the reaction, the catalyst was neutralized with NaOH. The pH of the medium was determined using a universal indicator paper. Next, distillation was performed to eliminate the reaction mixture of water and unreacted components. Distillation conditions: atmospheric -temperature 140°C-145°C, vacuum -until the temperature reached 165°C in the flask and pressure 0.03-0.04 MPa. The mixing speed was 150 rpm. The hot resin was then poured into a metal container. The temperature and stirring rate were adjusted and controlled during synthesis using an IKA RET control-visc device (IKA ® -Werke GmbH & Co. KG, Germany) with a temperature setting range of 0°C-340°C at a heating rate of 7 K min −1 .

Extraction of unreacted resorcinol and alkylresorcinols in the resins
The amounts of unreacted R and ARs in the resins were determined as follows: approximately 1 g of pre-crushed resin was dissolved in 2 ml of acetone and added to a solution of 100 ml of distilled water. The mixture was boiled for 10-15 min, filtered through a white ribbon filter and then through a blue ribbon filter using a vacuum pump. Filtration was performed until the solution became clear. 10 ml of the filtrate was transferred to a separating funnel, and an equivalent amount of diethyl ether was added, shaken, and allowed to settle until the two phases Mater. Res. Express 10 (2023) 035304 A Jurkeviciute et al were completely separated. The resulting ether layer was evaporated to a constant weight in an oven at 105°C and then weighed again. The dry residue was subsequently used for qualitative and quantitative analyses of the resin composition by TLC and GC.

Gas chromatography
The GC method was used to determine the composition of the initial fractions of the oil shale ARs and the residual S content during aralkylation. Before entering the gas chromatographic system, the total ARs residue obtained after extraction, starting material, and aralkylated mixture was silylated with 1.1.1.3.3.3hexamethyldisilazane (HMDS) (Sigma-Aldrich, USA). 0.004-0.005 g of the mixture to be analyzed was placed in a test tube for derivatization, 0.2 cm 3 of acetonitrile and 0.5 cm 3 of HMDS were added, and the contents were thoroughly mixed. The tube was stoppered with an air condenser, calcium chloride tube, and placed in a thermostat or Labnet dry bath (USA), and maintained for 1 h at 130°C. The tube was then cooled to room temperature. The contents of the tubes were transferred to vials for chromatography. GC analysis was performed using a Thermo Finnigan TRACE GC chromatograph (USA) equipped with a flame ionization detector with electronic pressure and flow controls. GC conditions: T initial of the thermostat was 150°C, with a heating rate of 7.5°C min −1 , and T final of the thermostat was 260°C., T evaporator and T detector were 260°C and split 70:1. Separation was carried out on a capillary column Petrocol TM DH (100m × 0.25 mm × 0.5 μm) with a non-polar dimethylpolysiloxane phase at a He carrier gas feed rate of 2.5 ml min −1 . The sample (volume 0.2 μl) was injected using a Hamilton ® syringe for chromatography (HAM80075, Germany).

Gas chromatography mass spectrometry
Gas-liquid chromatography with mass spectrometric detection (GC-MS) was used to identify the compounds formed during the aralkylation of individual ARs (nt. 5-MR, etc.). The analysis was performed using an Agilent 7820 gas chromatograph (Agilent Technologies, USA) with an Agilent 5975 mass selective detector (Agilent Technologies, USA). The conditions for obtaining the chromatograms and column were identical to those used for the GC. The pumping action was achieved through the electron impact ionization of gas molecules (ionization energy: 70 eV). Mass spectrum measurements were performed in scan mode over the entire m/z range from 10 to 500.

Thin layer chromatography
The qualitative determination of R, 5-MR in the resins, as well as the composition of the resins, was performed by TLC. TLC was run on analytical Silica gel 60 GF 254 (Sigma-Aldrich) plates using a mixture of toluene, i-PrOH and EtOH (85:4:11 V/V/V ) as a solvent system. The plate was initially treated with hydrogen peroxide, followed by drying in hot air. 2 μl of a solution of the original resin in acetone (1 wt%) was applied to the plate using a microsyringe. It was then placed in a chamber with an eluent. The plates were then air dried. The dried plate was dipped into 1 wt% ammonium peroxodisulfate ((NH 4 ) 2 S 2 O 8 ) solution to develop the spots. Subsequently, it was dried again using hot air. The resulting spots were scanned and processed using JustTLC program to determine the Rf value.

Softening point
The softening point of the resins was determined by the standard method of Ring and Ball [25] with the replacement of water with glycerin, because the softening points of some resins were above 100°C.

Resin solubility
The solubility of the obtained polycondensation resins was investigated by dissolving 10 mg of the polymer in 1 ml of solvent containing different compounds (benzene, acetone, distilled water, lower alcohol, etc). The solution was kept at room temperature for two hours, and the contents of the test tubes were occasionally shaken. In the case of partial dissolution or swelling of the resin, its solubility during heating was evaluated. The contents of the tubes were heated in a water bath for 15-30 min at the boiling point of the solution.

Thermogravimetric analysis
Thermal analysis (TG/DTG/DTA) was performed using equipment of Mettler Toledo (Switzerland) and Setaram LabSys (France) in inert gas (N 2 or Ar) and air flow 60 ml min −1 at a heating rate of 5 deg min −1 with ∼ 10 mg of the sample.

Determination of styrene by the iodometric method
The iodine value in the distillate after polycondensation was determined according to the standard method ASTM D1510 [26].

Results and discussion
The study results show that the aralkylation of oil shale ARs fractions takes twice as long as that of individual ARs [27]. It was determined that S reacts with R within 30 min The reaction of S with 5-MR occurs almost instantly, which explains the higher reactivity of the latter than that of R and other ARs.
The direction of the ARs aralkylation process is determined by the magnitude of the negative charges on the carbon atoms of the benzene ring. According to the digital values of the charges on carbon atoms [28], the predominant direction of attack of the carbocation is the ortho-position 5-MR, 5-ER, and 2.5-DMR, where the carbon atoms have the highest negative charge (table 3).
This does not exclude the possibility of simultaneous formation of para-substituted ortho-and paradisubstituted ARs with para-substituted ones.
The possible acid polycondensation products of ARs with F and Ac, leading to the formation of novolac resins, are shown in figure 2. Addition of an aldehyde to the benzene ring occurred at the ortho-position. Oligomers with different chain lengths were formed. Presumably, fragments of both the pure R and aralkylated components can be included in the oligomeric chain structure.
The polycondensation reaction of 5-MR with F and Ac proceeded quite rapidly compared with the R and ARs fractions. This is since the rate of interaction of pure 5-MR with F is approximately 80 times greater than R [29]. According to the relative increase in reactivity with F, the alkyl derivatives of R can be placed in the following row:4.5-DMR < 5-ER < 2.5-DMR < 2-MR < 5-МR [30]. It should also be noted that the reactivity of Ac is lower than that of F [31].
The HONEYOL and REZOL fractions contain nitrogen-containing compounds, predominantly pyridine bases [32]. In turn, they interact with sulfuric acid to neutralize it. Therefore, to obtain resins in the formulation in which REZOL and HONEYOL were used, the amount of catalyst was increased in proportion to the volume of the added fraction. An increase in the amount of catalyst by approximately 1.5 times during the   4 show that more than 90 wt% conversion of the main components of the fraction (R, 5-MR, and 5-ER) can be achieved when the resin is synthesized using the REZOL fraction and Ac. In 2.5-DMRSAc resin, the conversion of the main component is approximately 44 wt%. This can be explained by the steric factor of the 2.5-DMR molecule. The degree of conversion of 2.5-DMR in the HONEYOL and REZOL fractions ranged from 73 to 97 wt%. The 2.5-DMR content in the REZOL fraction was 2 times less than that in HONEYOL. It can be assumed that, during aralkylation, it does not react with S and remains in its original state. Such a 2.5-DMR molecule has 2 free positions for the reaction with an aldehyde.
Qualitatively, TLC showed that the SF and SAc resins consisted of a set of oligomeric structures ( figure 4). It was found that the distillates obtained after polycondensation with Ac contained significantly less unreacted ARs than those obtained after polycondensation with F.
The appearance of unreacted S in the resin and distillate was observed (table 2) when the HONEYOL and REZOL fractions were used in the synthesis. In the case of the Rez 100 SF resin, the conversion of styrene reached 42 wt%, while in the synthesis of the Rez 100 SAc and Hon 100 SAc resins, it was 98 and 97.3 wt%, respectively.
The synthesized SF and SAc resins are solid-colored brittle or plastic substances with softening point of 51.5°C -103°C. SF resins have a rich crimson color, SAc -from yellow to crimson. An increase in the proportion of the alkyl resorcinol fraction in the formulation led to a change in the color of the resin to brown. The main characteristics of solid resins used in the rubber industry are their softening point (not lower than 80°С) and solubility in acetone.
The lowest softening temperature of 51.5°C is 2.5-DMRSAc resin. The unreacted 2.5-DMR content exceeded 50 wt%. This may be since the addition of the Ac molecule to the aralkylated 2.5-DMR molecule is very difficult owing to the spatial structure of the latter ( figure 5).
The softening temperatures of the acetaldehyde resins R 70 Rez 30 SAc and 5-MRSAc compared to formaldehyde resins R 70 Rez 30 SF and 5-MRSF changed from 78 to 102.5°C and 81.5°C to 103.0°C, respectively.
Replacing 30 mol.% R with the REZOL fraction in the formulation results in an increase in the softening point from 73 to 102.5°C. This was due to the increase in the molecular weight of the obtained oligomers. The softening temperature of SF resins begins to decrease with an increase in the composition of their fraction of oil shale ARs. The low softening temperature of the Rez 100 SF and Rez 100 SAc resins (about 69°C) is since the REZOL fraction contains ARs, in the structure of which there are long alkyl chains and benzene rings [24], some of which were subsequently aralkylated with S. The polycondensation reactions of such compounds are difficult. According to the literature, the softening point of resins depends not only on the ratio of the initial resorcinol component and aldehyde, but also on the amount of unreacted R present in the resin [10]. For resins based on individual R (RSAc) and its derivatives 5-MR and 2.5-DMR (5-MRSAc, 2.5-DMRSAc), this pattern was preserved (tables 2-5, 10, 11). The results of the thermal analysis presented in figure 6 highlight that the resins based on (containing) SF behave very similarly. Decomposition takes place in one step at 150°C-400°C in an inert atmosphere. The decomposition maxima varied at 27°C and the total mass loss at 600°C varied 4 %. The decomposition of SF samples in air occurs in two steps: between 170°C-350°C the mass loss is 40%-45% and between 400°C-600°C additionally 50%-55%. In the heat flow curves, small endothermic effects were followed at 80°C-120°C related to the release of water and volatile components and at 200°C-270°C related to melting and decomposition of unreacted components (5-MR, 5-ER et al) and low-molecular-weight oligomers. The intensive exothermic effect above 400°C indicates oxidation reactions of the rest of the resins.
The TA analysis results of RSAc and 5-MRSAc ( figure 7) indicate a greater impact of 5-MR on the thermal properties of the Ac resins, despite the lower content of the unreacted part in the resin (table 5). The main decomposition in the inert atmosphere also occurred in one step: for RSAc in the temperature interval of  150°C-300°C but for 5-MRSAc up to 400°C without remarkable thermal effects. The TA curves in air were more complicated and indicated numerous decomposition reactions up to 400°C, particularly for 5-MRSAc resin. The oxidation reaction, which is more intensive for 5-MRSAc above 400°C, led to a mass loss of 96 %. The higher mass loss of RSAc at 50°C-200°C can be explained by the higher water content (table 2).
The thermal stability of the resins was also evaluated based on the temperatures at certain mass-loss levels, as presented in table 5. In the initial stage of heating in an inert atmosphere, a mass loss of 10 % was achieved at     Table 6 shows data on the solubility of resins in various organic solvents. From the presented results, it follows that almost all resins exhibit good solubility in polar (acetone, EtOH), aprotic (acetonitrile) and moderately polar (THF) organic solvents at room temperature. The Rez 100 SF and Rez 100 SAc resins, which have low softening points, are insoluble in water. Resins based on R and individual ARs are partially soluble in water. This is caused by the high content of unreacted components with OH-groups in the resin and the presence of oligomers whose molecules do not contain aralkylated components. Partial solubility in water makes possible to introduce them into emulsion systems while maintaining stability. They remain in the aqueous phase and do not migrate into the polymer phase.
To reduce the moisture content and volatile components of the RSAc, 5-MRSF, 5-MRSAc, and 2.5-DMR resins, it is necessary to carefully distill and dry the resin. After aralkylation with S, the resins exhibited reduced hygroscopicity compared to non-aralkylated resins.

Conclusions
In the present work, the possibility of aralkylated polycondensation resins synthesis based on R, 5-MR, 2.5-DMR, with partial replacement of R by the oil shale ARs fraction (REZOL), as well as the REZOL and HONEYOL fractions, and aldehyde -Ac, is shown.
The interaction patterns of individual resorcinols (R, 5-MR, and 2.5-DMR) and their mixtures with S were studied. The SF and SAc resins were analyzed using GC, GC-MS, TLC, and TG/DTG/DTA methods. Their various technical characteristics (softening point, moisture content, etc) were determined.
In the aralkylation process, the addition of S to the 5-MR, 5-ER, and 2.5-DMR molecules occurs at the 2-, 4, or 6 positions of the aromatic ring.
It has been found that the use of Ac makes it possible to obtain resins with characteristics that are not inferior to SF resins, and in some cases even better. Resins synthesized with Ac (R 70 Rez 30 SAc, 5-MRSAc) had a higher softening point than similarly formulated resins with F (R 70 Rez 30 SF, 5-MRSF).
SAc resins contain 3-4 times less unreacted original resorcinol components compared to SF resins. It has been shown that SF and SAc resins are formed by oligomers with different chain lengths.
All the prepared SF and SAc resins were soluble in organic solvents, such as acetone, acetonitrile, THF, and EtOH. In addition, RSF resins have good solubility in water, whereas SAc resins are only partially soluble in it.
Upon the receipt of SAc resins, the volume of distillation decreases, and accordingly, less waste requires further purification and disposal.
The use of environmentally friendly raw materials, natural shale ARs, and Ac can be considered as alternatives to Ph, R, and F.
The synthesis of resins based on such raw materials will expand their range and improve their properties compared to those of traditional resins. This opens prospects for obtaining not only those listed in the article but also many other multifunctional polymer products. Thus, the presence of reactive functional groups and fragments in the SAc resins makes possible to modify them with other organic compounds. The next steps will be studies introduction of such resins into the industry, considering all the modern requirements. The improvement of properties also leads to the expansion of the use of resins, for example, in the production of adhesive components and additives, electrical, paint, varnish, rubber products, and composite materials.
Thus, obtaining SAc resins based on industrial HONEYOL and REZOL fractions can be considered a very promising direction, which makes it possible to reduce environmental damage during their synthesis and further use in composites.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Funding statement
This research was conducted under the doctoral study of the Tallinn University of Technology.

Ethical compliance
This article does not contain any studies with human participants or animals performed by any of the authors. This article is original. The article has been written by the stated authors, who are ALL aware of its content and approve its submission.