Non-aldehyde resins based on resorcinol and natural alkylresorcinols modified with styrene

The use of natural alkylresorcinols produced in Estonia (5-methylresorcinol, HONEYOL80 fraction) instead of the expensive resorcinol reduces the cost of the synthesized resins while maintaining their high technological characteristics. Resins based on resorcinol (R), 5-methylresorcinol (5-MR), HONEYOL80, dicyclopentadiene (DCPD), and styrene (S) were synthesized in the presence of an acid catalyst. Their physical and chemical characteristics have also been studied. It was shown that depending on the mole ratio of the components of the formulation, resins with different softening temperatures (66 to 97 °C), ash content of 0.04 to 0.39 wt%, and a low content of volatile components at 105 °C, including moisture, were obtained. The best conditions for the synthesis of resins with a softening point of 94 °C–97 °C are molar ratio of R (5-MR): DCPD: S = 1: 0.5: 0.3 at temperature 135 °C–170 °C, synthesis time 5 h 30 min. The synthesized resins did not require additional processing under a vacuum. The presence of unreacted resorcinol and 5-methylresorcinol (quantitatively and qualitatively) in the final product, the homogeneity, and molecular weight characteristics of the resin composition (qualitatively) were determined by TLC and GPC.


Introduction
Polymers based on phenol (Ph) and resorcinol (R) condensations are widely used in the chemical industry.They are used in the tire and rubber industry to increase adhesion, preparation of hot-and cold-curing adhesives, sealants, and tire cord impregnation, in medicine, to obtain aerogels, in the manufacture of aerospace and aviation equipment, etc [1][2][3][4].Modification of the polymerization stage or the addition of modifiers to the product can expand their field of application.Composite materials are the most promising materials from the point of view of further development of production technologies and subsequent applications [5][6][7][8].The matrix of polymer composites can be either phenolic [9,10] or resorcinol-formaldehyde (RF) resins [11,12].
The resins produced by condensation with formaldehyde (F) have several disadvantages.They are unstable due to increased volatile components content, are hygroscopic, and, accordingly, have a limited storage period.The actual problem is to decrease toxic emissions to the environment [13,14].The possibility of replacing F for environmentally and human-friendly acetaldehyde (Ac) was shown in previous studies [15,16].However, Ac is difficult to use because of its low boiling point (T b ∼ 20 °C).The production of such resins requires specialized equipment with additional cooling.
Polycondensation reactions release low molecular weight substances.In our case, this is water, which can remain in the synthesized resin even after drying.It is bound by hydrogen bonds to the OH-groups of the resin.In the molding process, after breaking these bonds, residual moisture leads to the formation of water vapor in the molten polymer mass.Voids are formed in the composite; therefore, the mechanical properties of the resin are reduced [17,18].
Currently, environmentally safe technologies for polymer production and manufacturing are becoming a priority [19,20].Requirements for the performance characteristics of these materials based on them are becoming stricter every year.To achieve this goal, the introduction of the vinyl compound styrene (S) into the aromatic ring of (alkyl)resorcinols has been investigated [21,22].These studies demonstrated the possibility of obtaining modified (alkyl)resorcinol resins with improved technical and environmental characteristics.The addition of S to the polymer matrix of the composite reduced its viscosity.This facilitates the processing of the resin and promotes better dissolution in the butadiene-styrene-rubber base of the composite.
Table 1 shows the comparative properties of vulcanized rubbers derived from R and resorcinoldicyclopentadiene-styrene resin (RDS) synthesized with different component molar ratios [23].The results illustrate the curing behavior and physical and mechanical properties of the composite with the addition of individual R and RDS resins.
As turns out from the above data, RDS resins improve the sub-vulcanization time of rubber compounds, with a slight increase in the vulcanization time.Mechanical and dynamic test results show that rubber compounds containing RDS resins have improved properties, such as strength, hardness, and dynamic modulus of elasticity (G').The tangent of the dielectric loss angle (tg Δ) decreased.
One of the drawbacks of resins with a high free resorcinol content is its release during the preparation of some compositions and the appearance of fumes [23].The use of modifiers consisting of DCPD and S eliminates this disadvantage [23,29] and allows the production of resins with low R (< 1wt%) use.In addition, mechanical properties such as the elastic modulus and relative elongation were improved.The use of (alkyl)resorcinol increases the level of adhesion between the surfaces of the fibers, cords, and the polymer matrix [2,22,29].The method described in [23] does not produce a large amount of waste and is associated with the use of low temperature and pressure.
Ph and R are typically used for the production of formaldehyde-free resins.However, Ph is a carcinogenic substance with mutagenic properties.R can be used instead.It is mainly synthetically produced.This leads to a significant increase in the cost of the product, despite its advantages over Ph.Natural alkylresorcinols (ARs) can be a substitute for expensive R. [15,21,22].They are obtained during oil shale processing using the Kiviter technology [30].Replacing R with 5-methylresorcinol (5-MR), a fraction of the total alkylresorcinol HONEYOL80 (Viru Keemia Grupp AS, Estonia), and using DCPD and S opens opportunities to produce new resins in oil shale chemistry plants.
The aim of this study was to investigate the possibility of obtaining an (alkyl)resorcinol-dicyclopentadienestyrene formaldehyde-free resin with improved characteristics based on industrial oil shale raw materials.
Natural ARs, such as 5-MR and HONEYOL80 oil shale alkylresorcinol fractions, have been used to produce dicyclopentadiene-styrene resins for the first time.
In the analytical part, reagents from Sigma-Aldrich (USA) were used with an ACS reagent purity ( 99.5%).Commercial fraction HONEYOL80, R and 5-MR, and p-TSA were provided by Viru Keemia Grupp AS.

Synthesis of resins
(Alkyl)resorcinol-dicyclopentadiene-styrene resins were synthesized according to recommendations given in the literature [29].A block diagram is shown in figure 1.
The raw material for the first three syntheses, R, was used, which was later partially or completely replaced by 5-MR and the HONEYOL80 fraction.The formulations of the synthesized resins are shown in table 2.
R was placed in a four-neck flask equipped with a mechanical stirrer, a thermometer, a reflux condenser, and a dropping funnel and heated to 125 °C-135 °C.When R was completely melted, the first portion of the p-TSA catalyst was added, and the mixture was stirred for 5 min.Subsequently, a calculated amount of DCPD was added to the reaction mixture using a drop funnel for 60-90 min at 135°C-145 °C.Then the contents of the flask were incubated at 140 °C-145 °C for an additional 60 min.A second portion of p-TSA catalyst was introduced, and styrene was slowly added using a dropping funnel for 60 to 90 min at 140 to 145 °C.The synthesis was continued at 155 °C-160 °C for another 60 min.The reaction temperature was then slightly decreased and an equimolar amount of NaOH (40 wt% solution) was added to neutralize the p-TSA catalyst.The removal of unreacted DCPD and S was carried out under vacuum (7-17 mm Hg) at 150 °C-160 °C.
Based on the literature data [2,29], the synthesis of a formaldehyde-free resin can proceed according to the following scheme (Scheme 1).

Extraction of unreacted resorcinol and alkylresorcinols in the resins
Unreacted R and ARs were extracted from the resins as follow: 1 g of resin was boiled in 100 ml of distilled water.The suspension was filtered through a blue-ribbon filter.The mass and volume of the filtrate were measured.The pH of the solutions was then checked because in a basic environment, there may be phenolates that are insoluble in diethyl ether.These were destroyed by adding a weak solution of hydrochloric acid to pH 7.Then, a 20 ml aliquot of the filtrate was transferred to a separating funnel, 10 ml of diethyl ether was added, and the mixture was shaken.The ether layer was poured into a bottle and evaporated in air until a constant weight was reached.The resulting residue was used to determine the unreacted R and ARs using GC and TLC.

Gas chromatography
The component composition of the initial (alkyl)resorcinol (R and ARs fractions) was determined using an Agilent Technologies 7890A chromatograph with a PID detector, a DB-1 capillary chromatographic column with an inner diameter of 0.

2.3.3.Thin layer chromatography
TLC was used to determine unreacted R and 5-MR quantitatively and qualitatively in the resins, as well as to establish the homogeneity of the resin composition.For this, Silica gel 60 GF 254 plates from Sigma-Aldrich were used.Then, in two test tubes, 0.1 g of R and 5-MR were dissolved in 10 ml of diethyl ether.The solutions were applied using a micro-syringe at a volume of 3 μl per plate and eluted using the ascending method.A mixture of toluene, propanol, and ethanol taken in volume ratios of 46:3:9.5 was used as the eluent.The plates were then air dried.The dried plate was lowered into a developer solution, Na 3 [Co(NO 2 ) 6 ].Subsequently, it was dried again with hot air until the appearance of brown spots.The resulting spots were scanned and processed using JustTLC software to determine the R f value of the standard and the R and 5-MR extracted from the resin.The obtained R f retention coefficients were used to calculate the amount of unreacted R and 5-Mr Scheme 1. Synthesis reaction of (alkyl)resorcinol-dicyclopentadiene resins modified with styrene.

Softening point
The softening point of the resins was determined using the standard method of Ring and Ball [31] with the replacement of water with glycerol, as the softening points of some resins were above 100 °C.

Resin solubility
Samples of the formaldehyde-free resin were ground to a particle size of 1-3 mm.One gram of crushed sample was placed in test tubes, and 100 g of the solvent (distilled water, acetone, ethanol, toluene, and tetrahydrofuran) was added to a measuring cylinder.The mixture was shaken and allowed to stand for 2 h at room temperature.Periodically shaken, the degree of dissolution indicated fully dissolved, partially dissolved, not dissolved, suspended, or settled particles.In the case of partial dissolution or swelling of the resin, its solubility upon heating was evaluated.To achieve this, the contents of the tubes were heated in a water bath for 15 min at the boiling point of the solution.

Determination of styrene and DCPD by the iodometric method
The unsaturated hydrocarbon content in the resin was determined using the standard method of iodometric titration [33].

Gel permeation chromatography
Samples for GPC measurements were dissolved in THF and passed through a 0.22 μm PVDF filter to remove all insoluble material.GPC measurements were performed on an Agilent Technology 1260 Infinity system with a three-column configuration.The column set was provided by Polymer Standards Services GmbH, Mainz, Germany and consisted of 8×300 mm SDV-type columns with 5 μm beads and porosities of 100000 Å, 1000 Å, and 100 Å, respectively.The system was operated at flow rate 1 ml•min −1 at 35 °C.An RI detector, poly(methyl methacrylate) (PMMA), and polystyrene (PS) standards were used.Calibration sets were provided by Polymer Standards Services GmbH (Mainz, Germany).

Volatile and ash content
Volatile and ash content were determined using the standard methods [34] and [35], respectively.

3.Results and discussion
The molar ratio of the initial components (table 2) affects both the characteristics of the resins: softening point, ash content, the content of volatile and unreacted components, etc (table 3), and the molecular composition of the final product (Scheme 2).An increase in the mole fraction of the resorcinol fraction in the resin formulation to 30 mol% (R 70 Hon80 30 DS) led to an improvement in the technical characteristics of the resin.The content of unreacted R (5-MR) decreases from 1.9 wt% (RDS 5 ) to 0.5 wt% (R 70 Hon80 30 DS).The degree of conversion of the initial resorcinol components reaches 98.9 wt%.A further increase in the content of the HONEYOL80 alkylresorcinol fraction lowered the softening point and increased the quantity of unreacted components; therefore, the conversion of the initial (alkyl)resorcinols decreased.This is because the alkylresorcinol fraction contains, in addition to the main component 5-MR, also other alkylresorcinols with different side chain lengths, which are less reactive compared to 5-MR [15].
It was established that due to the ratios of the components of the formulations (table 2) during the processing of the resin under vacuum (distillation), the distillate was not formed, and no unreacted unsaturated compounds were present in the resin.Their concentrations were determined by iodometric titration [33].From this, we can conclude that DCPD and S fully reacted with (alkyl)resorcinol and that the resins obtained by this synthesis method, described in this work, do not require the distillation stage.A comparison of the properties of the resins (free R content and softening point) based on the molar ratio of the components are shown in table 4.
From the data shown in table 4, it follows that the results of the laboratory experiments differ from those in the literature.This can be explained by differences in the quality of the starting substances used in the synthesis.However, the softening temperature depends on the molar ratio of the resin components.Changing the molar ratio made it possible to obtain resins with the desired softening point.Notably, R reacts easily with DCPD in the presence of an acid catalyst at elevated temperatures.In the first stage of the synthesis, resorcinol molecules interacted with DCPD, and a linear oligomer was formed (Scheme 1).In the second stage, S was introduced.S and DCPD bound unreacted resorcinol molecules.The oligomer molecules probably have different structures depending on the molar ratio of the components used for the synthesis (Scheme 2).
The studied resins exhibited moderate molecular weights (table 5), which are typical for this class of materials.By GPC analysis content of molecules with low-MW (180 g•mol −1 and 250 g•mol −1 ) were detected, which can be attributed to the starting compounds R, 5-MR, S, DCPD, or Hon80.Their content can be roughly estimated from GPC.However, any detailed analysis is behind the scope of this study aimed at study of replacement of formaldehyde-based materials with new resins with good/excellent properties.
Figures 2 and 3 shows a comparison of the MW of the samples based on the HONEYOL fraction.The molecular weight of R 70 Hon 30 DS was remarkably higher in comparison with samples R 70 Rez 30 SF and R 70 Rez 30 SAc.Highest MW achieved in the sample with HONEYOL80 was several times higher than for Rez/Ac samples.Dispersity was also higher, which can be explained by the increased ratio of low-M w compounds (starting materials), which strongly affects the M n value.The higher values of DCPD and S-containing samples can be explained with increased reactivity of the double bonds and the increased number of possible connections.The second plausible explanation is the different characteristics of the macromolecular architecture.Macromolecules with more linear character show higher molecular weight in GPC than its branched counterparts with identical molecular weight.As shown in Schemes 1 and 2, the introduction of DCPD and S led to the formation of linear segments.The addition of F or Ac leads to branching through methylene bridges.
The first three resins (table 3) were synthesized with the same molar ratio of R: DCPD: S = 1:0.4:1,but at different temperature conditions and process times.The obtained resins exhibit low softening temperatures.The molar ratio of the was adjusted to increase softening temperature.Table 3 shows that simultaneous increase in DCPD amount from 0.4 to 0.5 mol and decrease in styrene amount from 1.0 to 0.7 mol increase resin softening temperature by 10 °C.The RDS 5 resin synthesized with 0.3 mol of S has a softening point at 97 °C.Varying the molar ratio of the components made it possible to obtain a product with the required softening point.In the subsequent synthesis, 5-MR was used instead of R, which is characterized by increased reactivity owing to the presence of an alkyl group at the meta-position of the benzene ring [36].The introduction of 5-MR instead of pure R hardly reduced the softening point.The complete replacement of R by the HONEYOL80 fraction obtained by Viru Keemia Grupp AS resulted in a reduction of the softening point to 75 °C.This can be explained by the presence of alkylresorcinol compounds with long-chain and cyclic groups, as well as the higher content of pyridine bases in HONEYOL80 [21].
The TA results (figures 4 and 5) shows that the amount of water evolved at temperatures of 70 °C-150 °C is inappreciable quantity, since in this temperature range, there is practically no loss of mass of the resin sample and the release of H 2 O (m/z = 18).The synthesized resins were stable up to a temperature of 200 °C (table 6).The RDS 5 resin exhibited the highest stability and lowest weight loss among the studied samples.The Hon 100 DS resin is unstable.Weight loss for all the samples occurred in almost three stages.In figure 4 is seen that because of decomposition of the resin at a temperature of 230 °C-300 °C, light hydrocarbons, as well as styrene, begin to release.The intensity of evolvement depended on the composition of the resin.From stable resins, the release of components with a cyclic structure (m/z = 78) (RDS 2 , RDS 5 and R 50 Hon80 50 DS) occurred in the temperature interval of 300-350 to 450 °C.For the unstable resins, this temperature was reduced by approximately 100 °C.Oxidation (burning) was revealed by an exothermic effect in the DTA curve, as well as by the release of CO 2 and H 2 O.It becomes dominant for resorcinol resins at approximately 350 °C, but for resins obtained using HONEYOL fraction at 400 °C-430 °C.
The successful application of a polymer material under various conditions depends on its ability to maintain its performance during long-term storage.Using the TGA method, it is possible to make preliminary conclusions regarding the durability of the polymer [32].The mass loss data convey this information most accurately.As a rule of thumb, the higher the decomposition temperature, the longer is the shelf life of the polymer.The DCPDS resins can be operated at high temperatures.This was indicated by the mass loss temperatures (τ % ) obtained by TGA (table 6).
It should be noted that the obtained resins had low ash content.The ash content index is an important technical characteristic of the resins.This affects the mechanical resistance of both the resin and the composite when the matrix (resin) is added.The lowest values of ash content were noted for RDS, R 50 Hon80 50 DS ─ 0.04 and 0.06 wt%, respectively.Thermogravimetric analysis (TGA) revealed that the volatile compounds in these resins were insignificant.
The resins obtained were solid and brittle substances with a dark red or brown color.Red resins were synthesized based on R and 5-MR and brown based on the fraction HONEYOL80.The primary parameters are listed in table 3. The composition of such resins is more homogeneous than that of resorcinol-aldehyde resins [15,21].It should be noted that they practically do not contain volatile components, including moisture, at 105 °C, as shown by the results of TGA results (figure 4).
Resins have different solubilities in various organic solvents.They were soluble in acetone, ethanol, and tetrahydrofuran; slightly soluble in toluene; and insoluble in water.Very good solubility of the resins of all formulations was observed in acetone.It was found that the 1% solutions of the synthesized resins differed sharply in color.This makes it possible to expand their application areas from the tire industry to the paper industry [37,38].In the latter, in addition to a decrease of softening point, a low chromaticity of the adhesive additive is required, the role of which is played by the resorcinol resins.

Conclusions
Natural alkylresorcinols, such as 5-MR and HONEYOL80 oil shale alkylresorcinol industrial fraction, have been used to produce new non-aldehyde resins (dicyclopentadiene-styrene resins), and their physical and chemical characteristics were determined.It was shown that depending on the mole ratio of the components of the formulation resins with different softening temperatures (66 to 97 °C), ash content of 0.04 to 0.39 wt%, and a low content of volatile components at 105 °C, including moisture, were obtained.The best conditions for the synthesis of resins with a softening point at 94 °C-97 °C are molar ratio of R (5-MR): DCPD: S = 1: 0.5: 0.3 at temperature 135 °C-170°C, synthesis time 5 h 30 min.The best technical parameters have the resin (R70Hon8030DS) obtained using HONEYOL80: softening point 95 °C, ash content − 0.2 wt %, and total content of unreacted R and 5-MR in resins 0.5 wt %.
It has been shown that the synthesized resins do not require additional processing under a vacuum.The negligible water and volatile content in the resins would allow to implement molding without additional ventilation for vapor escape.
Variation in the initial composition is reflected in the MW characteristic of the final material.This is observed in the resins with high double-bond content.Such resins are suitable for replacing synthetic materials with natural ones.It has been proven that the combination of moderate-molecular-weight macromolecules and oligomeric/monomeric fractions corresponds to the proposed chemical mechanisms.
Therefore, it is possible to replace (partial or fully) synthetic R with natural ARs (5-MR) or oil shale ARs industrial fraction (HONEYOL80) in the formulations of non-aldehyde resins.Depending on the molar ratio of the initial components of the formulations, it is possible to synthesize resins with different properties without using a solvent.This makes possible to achieve considerable energy savings.
25 mm, length of 50 m, film thickness 0.25 μm.Temperature program: T injector and T detector − 260 °C, T initial − 150 °C (isotherm time − 3 min), T end − 260 °C (isotherm time − 15 min), temperature rise rate − 7.5 °С•min-1, split ratio − 1:70.The mass content of individual components in the sample was calculated automatically using ChemStation software.The initial resorcinol components of HMDS were preliminarily derivatized according to the method described in the work [15].

Figure 1 .
Figure 1.Block-diagram of the synthesis of DCPD resin.

Figure
Figure GPC traces of samples RDS 4 , RDS 5 and 5-MRDS converted in MWD distribution by calculation using PS calibration.

Figure 3 .
Figure 3. GPC traces of samples R 70 Rez 30 SAc, R 70 Hon80 30 DS and R 70 Rez 30 SF converted into MWD distribution by calculation using PS calibration.

Table 2 .
Resins formulations and synthesis conditions.
12with ∼ 10 mg of the sample.Thermal analyser was coupled with the Pfeiffer OmniStar Mass Spectrometer (Pfeiffer Vacuum Technology AG, Asslar, Germany) by a heated transfer line kept at T = 180 °C.The ion currents of the selected mass/charge (m/z) numbers were monitored in multiple ion detection (MID) mode (Quadera version 4.20 software) with the collection time of 1 s for each channel.The m/z ratios selected for analysis were12for C or C 3 H 7 ; 18 for H 2 O, 26 and 27 for C 2 H 2 and C 2 H 3 ; 44 for CO 2 ; 45 and 43 for C 3 H 7 ; 52, 78 and 106 for C 6 H 6 (styrene).

Table 3 .
Characteristics of samples.

Table 4 .
Resin properties depending on the molar ratio of the components.

Table 5 .
MW-characteristics of the samples.

Table 6 .
Comparative characterization of the mass loss temperatures of different resins.