**Examining the Direct and Indirect E**ff**ects of Climatic Variables on Plague Dynamics**

**Ricci P.H. Yue \*,**† **and Harry F. Lee \*,**†

Department of Geography and Resource Management, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

**\*** Correspondence<EMAIL_ADDRESS>(R.P.H.Y<EMAIL_ADDRESS>(H.F.L.)

† These authors contributed equally to the work.

Received: 15 March 2020; Accepted: 10 April 2020; Published: 15 April 2020

**Abstract:** Climate change can influence infectious disease dynamics both directly, by affecting the disease ecology, and indirectly, through altering economic systems. However, despite that climate-driven human plague dynamics have been extensively studied in recent years, little is known about the relative importance of the direct and indirect effects of climate change on plague outbreak. By using Structural Equation Modeling, we estimated the direct influence of climate change on human plague dynamics and the impact of climate-driven economic change on human plague outbreak. After studying human plague outbreak in Europe during AD1347–1760, we detected no direct climatic effect on plague dynamics; instead, all of the climatic impacts on plague dynamics were indirect, and were operationalized via economic changes. Through a series of sensitivity checks, we further proved that temperature-induced economic changes drove plague dynamics during cold and wet periods, while precipitation-induced economic changes drove plague dynamics during the cold periods. Our results suggest that we should not dismiss the role of economic systems when examining how climate change altered plague dynamics in human history.

**Keywords:** climate change; plague; direct and indirect effects; Structural Equation Modelling


**Acknowledgements**

The authors would like to express their hearty gratitude to Can Tho University of Medicine and Pharmacy. The authors also thank all of their colleagues for their excellent assistance.


*Article* **Natural Fluorite from Órgiva Deposit (Spain). A Study of Its Pozzolanic and Mechanical Properties**

**Domingo A. Martín 1,2, Jorge Luis Costafreda 1,\*, Esteban Estévez 2, Leticia Presa 1, Alicia Calvo 1, Ricardo Castedo 1, Miguel Ángel Sanjuán 3, José Luis Parra 1 and Rafael Navarro 4,5**

**Abstract:** This work presents the results of the partial substitution of Portland cement (PC) by natural fluorite (NF) and calcined fluorite (CF) in mortars, at 10%, 25% and 40%. To meet these objectives, a sample of fluorite was initially studied by XRD, SEM and Raman Spectroscopy (RS). A chemical quality analysis (CQA) and a chemical pozzolanicity test (CPT) at 8 and 15 days were carried out in a second stage to establish the pozzolanic properties of the investigated sample. Finally, a mechanical compressive strength test (MCST) at 7, 28 and 90 days was carried out on specimens made up with PC/NF and PC/CF mixes, at a ratio of 10%, 25% and 40%. XRD, SEM and RS results indicated fluorite as the major mineralogical phase. The CPT and CQA showed an increase in the pozzolanicity of the samples from 8 to 15 days. The MCST showed an increase in compressive strength from 7 to 90 days for both PC/NF and PC/CF specimens. The results obtained establish that fluorite produces positive effects in the mortar and contributes to the gain of mechanical strength over time, being a suitable material for the manufacture of cements with pozzolanic addition with a reduction of CO2 emissions, and by reducing the energy costs of production.

**Keywords:** fluorite; cement; pozzolan; mortar; mechanical strength; reduction of CO2 emissions

## **1. Introduction**

The use of fluorite in the making of cement has been known for several decades. In studies such as those presented by Gilvonio and Dominguez [1] the use of fluorite as a flux in the clinkerization process is mentioned; this significantly reduces the clinkerization temperature and obtains cements with greater mechanical strength, which contributes to the reduction of high energy costs during the production of Portland cement [2]. Fluorite has been used in the manufacture of cement with special characteristics, such as fast setting and high initial strength, as demonstrated by Najafi Kanil and Allahverdi [3]. Dominguez et al. [4] and Endzhievskaya et al. [5] have used natural fluorite in the characterization of the clinker sintering process, concluding that phase transformation and, above all, the presence of alite, occur at lower temperatures than usual when this mineral is incorporated. Other researchers, such as Fridrichova et al. [6], have managed to raise the reactivity of the common included of Portland clinker by the controlled addition of natural fluorite. Additionally, Zeta-Garcia et al. [7] have recently shown that the addition of fluorite in the standard clinker with alite, belite and ye'elimite (ABY) causes an increase

**Citation:** Martín, D.A.; Costafreda, J.L.; Estévez, E.; Presa, L.; Calvo, A.; Castedo, R.; Sanjuán, M.Á.; Parra, J.L.; Navarro, R. Natural Fluorite from Órgiva Deposit (Spain). A Study of Its Pozzolanic and Mechanical Properties. *Crystals* **2021**, *11*, 1367. https://doi.org/10.3390/ cryst11111367

Academic Editor: Francesco Capitelli

Received: 24 October 2021 Accepted: 9 November 2021 Published: 10 November 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in the mechanical strength of mortars and favourably affects the production of "ecological cements". Recently, the use of natural fluorite residues as substitutes for traditional aggregates in concrete has been used in proportions of 5%, 10% and 20%, achieving an improvement in mechanical properties [8]. The works of Zhu et al. [9] mention the use of waste from the mining of serpentine and kaolin by means of mixtures with fluorite slurry, obtaining mechanical strength of up to 150 MPa. Achmania et al. [10] detail the uses of fluorite in other fields, such as steel, aluminium metallurgy, the manufacture of plastics, solvents, refrigerants, welding and glass manufacturing, as well as pointing out the use of this mineral in the cement industry. It is pertinent to mention that the fluorite has been used as an index mineral for locating possible deposits of rare earth (REE) elements and niobium, related to carbonatites, as established by Makin et al. [11].

This research aims to demonstrate the efficacy of natural fluorite from a site in the province of Granada, south of the Iberian Peninsula (Spain). For this, natural and calcinated fluorite have been used to partially replace the Portland cement in the mortars, and to establish two fundamental parameters: the proportion of PC/NF and PC/CF mixtures, as well as the ideal calcination temperature threshold of the samples, in order to achieve optimal compression strength.

This work has been structured and developed in two phases: first, natural fluorite has been characterized by XRD, SEM and Raman Spectroscopy (RS) to determine the constitution and morphology of the mineralogical phases present in the sample; secondly, a study was carried out to establish technological quality through calcination tests, chemical analysis, pozzolanicity and compression strength at different ages.

This work is considered novel for several reasons, such as obtaining resistant mortars, the possible manufacture of cements with more environmentally friendly pozzolanic additions, the inclusion of fluorite and its residues within the circular economy cycle and the revitalization of the local industry.

#### **2. Materials and Methods**

#### *2.1. Materials*

The materials used in this research consist of natural fluorite, Portland cement and standardized sand. Natural fluorite comes from the Órgiva deposit, located in the Province of Granada, in the south of the Iberian Peninsula. The Órgiva Mining Company exploits the deposit. A volumetric sample of fluorite of 50 kg was taken from the La Candelaria mine, by lithogeochemical fragment sampling.

A Portland cement (PC) type 1, 42.5 N, of normal resistance, with the characteristics and parameters indicated in the standard UNE-EN 197-1:2011 [12], was used in this research.

A standardized sand (NS) of Normsand-CEN EN 196-1 typology, consisting of 98% silica, was used in this work as a fine aggregate.

#### *2.2. Methods*

#### 2.2.1. Preparation of the Sample

The sample was crushed in two phases. In the first, a reduction in the size of the particles was made up to 3 cm in diameter, with the help of a crusher model Alas. In the second phase, the size of the particles was reduced to 1 cm, by using the Controls brand crusher. The sample was then ground in a Siebtechnick mill model Scheibenschwingmühle TS 1000 in order to obtain two different particle sizes or blaine fineness (BPF): 2000 cm2/g and 5000 cm2/g, respectively. To obtain the BPF of 2000 cm2/g the sample was ground for 50 s, whereas for the fraction 5000 cm2/g the time was 30 s.

#### 2.2.2. Calcination Test

Two subsamples, CF-01 and CF-02, representative of both particle sizes 2000 cm2/g and 5000 cm2/g, were calcinated. The calcination process was carried out at a temperature of 900 ◦C, for one hour, with the use of a Binder stove, model 9010-0101 ED240; Serial number: 206493; Measuring range and scale division: 1300 ◦C/1 ◦C; Electrical power and connection voltage: 100–240 V; Measuring accuracy: +/−3 ◦C; Made in Germany. After the heating treatment, the samples were crushed again for 40 s, following the procedure described in Section 2.2.1, to remove the compaction produced during the calcination process. All the materials and instruments used in this research belong to the laboratories of the Escuela Técnica Superior de Ingenieros de Minas y Energía and the Laboratorio Oficial para Ensayos de Materiales de Construcción (LOEMCO), both of the Universidad Politécnica de Madrid, Spain.

#### 2.2.3. X-ray Diffraction Analysis (XRD)

A study to determine the mineralogical phases present in the natural fluorite sample was performed by X-ray diffraction (XRD) (All the materials and instruments used in this research belong to the laboratories of the Escuela Técnica Superior de Ingenieros de Minas y Energía and the Laboratorio Oficial para Ensayos de Materiales de Construcción (LOEMCO), both of the Universidad Politécnica de Madrid, Madrid, Spain). The equipment used consisted of a Rigaku Miniflex-600, with the capacity to carry out qualitative and quantitative tests. It is equipped with an X-ray tube operating at 600 w, plus a graphite monochromator and a standard scintillation counter. The diffractometer contains a sixposition automatic sampler, and has a HyPix-400 MF-2DHPAD, as well as a ShapeFlex Brand sample stand. The software used was the SmartLab Studio II. This equipment has an interface for profile views, data view, 3D views, and view of the crystal structure of the sample. The power used in the analysis process is 1ø, 100–240 V and 50/60 Hz. About 500 mg for each sample were weighed, ground up and screened to a fineness of 74 μm. The standard tablets were then manufactured in their respective moulds and then placed on the sample holder for analysis.

#### 2.2.4. Scanning Electron Microscopy (SEM)

A Hitachi S-570 scanning electron microscope from the Laboratorio Centralizado of the Escuela Técnica Superior de Ingenieros de Minas y Energía of the Universidad Politécnica de Madrid was used in the characterization of the morphological properties of the investigated sample.

The equipment has a Kevex-1728 analyser, a Polaron BIORAD, a power supply for evaporation and a Polaron SEM coating system. The equipment attains a resolution of 200 × 103. Other components are part of the Scanning Electron microscope such as: a lithium-doped silicon, a liquid nitrogen reservoir, an electronic cannon, a filament chamber, control panel for placing the sample in the high vacuum chamber and for varying the angles of the sample position, and an interface module to display the image during electronic scanning of samples.

The equipment operates with two pieces of software, Winshell and Printerface, to handle the information obtained during the study of the analysed sample and to take microphotographs, respectively. To perform the analysis by SEM, the samples in their natural state were previously reduced to a particle diameter of 0.2–0.5 cm; they were then placed on a graphite adhesive tape and later on the sample holder. Next, samples were covered with a layer of vacuum graphite and then placed in the sample holder of the scanning electron microscope.

#### 2.2.5. Raman Spectroscopy (RS)

Raman Spectroscopy is a non-destructive analytical technique that is based on the analysis of scattered light and molecular vibrations produced by the incidence of monochromatic light on the surface of the sample, which yields information on the chemical and structural composition.

The sample was analysed using a Spectrometer brand Gemmo Raman-532, Class 1, Laser product, Reference No. 18-Raman/PL Broad Scan with spectral artifacts-Sinhalite, Brownish Yellow. The equipment also has a laser cannon for the concentration of displaced

photons; a filter to separate elastic scattered light from inelastic light, a monochromator to separate the wavelengths of inelastically scattered light and a detector that collects and digitizes the signal of the different wavelengths.

#### 2.2.6. Chemical Pozzolanicity Test (CPT)

Pozzolanicity is the property of certain materials to react with Ca(OH)2 in solution when these materials have a very fine particle size, usually below 50 microns [13]. Pozzolans can be of natural origin [14] as well as artificial [15]. Two samples of fluorite, one in its natural state (NF) and the other calcinated (CF), were analysed by CPT to determine their pozzolanic properties, following the indications of the standard UNE-EN 196-5:2011 [13] at 8 and 15 days. A portion equivalent to 100 mL of demineralized water was heated to 40 ◦C for 60 s; then 20 g of a mixture of PC/NF and PC/CF, respectively, were added to this solution. Once the 8 days had elapsed, the solutions were filtered. This procedure was also repeated for a period of 15 days. The concentration of hydroxyl ions [OH−] was calculated using the following equation:

$$\left[\mathrm{OH}^{-}\right] = \frac{1000 \times 0.1 \times \mathrm{V\_3} \times \mathrm{f\_2}}{50} = 2 \times \mathrm{V\_3} \times \mathrm{f\_2} \tag{1}$$

where:

The concentration of calcium oxide (CaO) was calculated with the following equation:

$$\text{[CaO]} = \frac{1000 \times 0.025 \times \text{V}\_4 \times \text{f}\_1}{50} = 2 \times \text{V}\_4 \times \text{f}\_1 \tag{2}$$

where:

The pozzolanicity test is positive when the hydroxide concentration of calcium in dissolution is lower than the saturation concentration [13]. The results are seen in the graph, where all samples below the solubility isotherm curve are considered as pozzolan.

#### 2.2.7. Chemical Quality Analysis (CQA)

A chemical quality analysis (CQA) was carried out to determine the main major compounds of the natural fluorite (NF) sample analysed in this research; in addition, to determine its quality as a natural aggregate of pozzolanic cement. The different stages of the test were developed following the parameters of the standard UNE-EN 196-2-2014 [16]. The main compounds determined were the following: Total SiO2 (TS), reactive SiO2 (RS), total CaO (TC), reactive calcium (RC), MgO, Al2O3 and Fe2O3. In addition, the insoluble residue (I.R.) was calculated in a standardized solution of hydrochloric acid [16].

#### 2.2.8. Mechanical Strength Test (MST)

This method was carried out to determine the compression strength of specimens cured at 7, 28 and 90 days, using the procedures indicated in the standard UNE-EN 196- 1 [17]. The specimens were developed with standardized mixtures of PC/NF and PC/CF. The percentage of PC replacement by NF and CF, respectively, was 10, 25 and 40%. In addition in the manufacture of the specimens, two blaine particles fineness (BPF) were taken into account: 2000 and 5000 cm2/g, for both NF and CF. A total of 12 samples of specimens were prepared for the mechanical strength test, with three PC/NF-CF replacement ratios (10, 25 and 40%) and two types of BPF (2000 and 5000 cm2/g). All data referring to the preparation and proportion of the components of the specimens are presented in Table 1.

**Table 1.** Proportions of mixtures of PC, NF, CF and NS used in the preparation of the specimens.

**1** Portlan cement; **2** Natural fluorite; **3** Calcined fluorite; **4** Normalised sand; **5** Distilled water.

#### **3. Results and Discussion**

#### *3.1. X-ray Diffraction (XRD)*

The study of the mineralogical phases determined that the analysed sample is composed of a main phase of fluorite, of up to 98.3%; it also has subordinate secondary phases of quartz and calcite, indicating that the ore is practically monomineral (Figure 1). The study detected the three main peaks of fluorite at the following 2θ angular positions: 2θ = 28.2508/Intensity Cps = 50.766 (peak 5); 2θ = 46.9905/Intensity Cps = 47.552 (peak 17); 2θ = 55.7479/Intensity Cps = 14.129 (peak 19) and 2θ = 26.61/Intensity Cps = 235 (peak 4) (Figure 1).

**Figure 1.** X-ray diffraction patterns of the natural fluorite from the research area.

The peaks of quartz and calcite have low intensity and are confined to the background of the X-ray diffraction patterns. The major presence of fluorite in the investigated

sample has been confirmed by the analyses of SEM and Raman Spectroscopy (RS) in Sections 3.2 and 3.3.

#### *3.2. Scanning Electron Microscopy (SEM)*

The analysis of the sample under study shows a majority of fluorite, as shown in Figures 2a–d and 3a–d. In Figure 2a–d the fluorite crystals are large and they usually have a typical orientation that provides an apparent banding. This orientation was possibly caused by tectonics. The direction of accretion and orientation of the fluorite crystals are indicated by arrows in Figure 2a,b.

**Figure 2.** Microphotographs (**a**–**d**) of the natural fluorite (NF) sample investigated, showing typical banding.

The crystals are enlarged and show an anomalous increase in size, possibly by recrystallization. They form compact crystalline aggregates of zebra appearance that are reflected in the sample as list or bands. Sometimes these aggregates are in the form of impregnations and granular compact masses.

According to the Figures 2a–d and 3a–d, it seems that fluorite comes from two origins: a primary one, prior to tectonic deformation processes (Figure 2a–d), and a postectonic one, with the formation of new generation fluorite, in which the crystals are well preserved (Figure 3a–d).

Calcite forms small aggregates of rhombohedral shapes in empty spaces, as well as druses that develop greater in fractures (Figure 2c). Calcite is very scarce in the samples analysed.

The microphotographs, in Figure 3a–d, show compact crystalline aggregates of fluorite without deformation. Its origin may be related to hydrothermal solutions caused by autometamorphism [18]. All the crystals grow and are arranged in a very tight way, without spaces or pores, it is common to see interpenetrations and minerals twins. Figure 3a–d shows poorly developed calcite crystals.

**Figure 3.** Microphotographs (**a**–**d**) of the investigated natural fluorite sample without banding.

#### *3.3. Raman Spectroscopy (RS)*

Figure 4 indicates the presence of virtually pure natural fluorite (NF) in the Raman fingerprint zone to the left of the diagram. The intensity of the peak of the NF exceeds 5500 counts, while the wavelength is 541 nm. The application of the Raman spectroscopy method in this research confirms the results obtained by XRD and SEM, Section 3.1 and Section 3.2, respectively, where the fluorite phase was determined to be the major presence; thus, it was established that the methods used are adequate and complement each other.

**Figure 4.** Results obtained from the analysis of the NF sample by Raman Spectroscopy.

#### *3.4. Pozzolanicity Test (CPT) at 8 and 15 Days*

Figure 5a,b demonstrates the pozzolanic character of the natural fluorite (NF) and calcinated (CF) samples investigated. The 8-day test (Figure 5a) fixes both samples immediately below the isothermal solubility curve at 40 ◦C, indicating an adequate pozzolanic reaction.

**Figure 5.** Evolution of the pozzolanic behaviour of the NF and CF samples analysed: (**a**) at 8 days; (**b**) at 15 days.

After 15 days of testing, both samples varied their position under the isotherm, which is interpreted as an increase in pozzolanic reactivity over time. It follows that the factors that caused this trend are the percentages of reactive SiO2 and CaO present in the composition of both samples, as shown in Table 2.

**Table 2.** Chemical composition of the NF and CF samples investigated.

\* Insoluble residue.

Another factor that can determine the pozzolanic capacity of the researched fluorite is its electrical conductivity (EC); in this sense, Liu et al. [19] have determined that the EC is dominated by the fluorine ion, reaching ~0.01 S/m at 650 ◦C and is potentially important as a carrier of electric charge. Costafreda et al. [20] have demonstrated a similar situation in their study on the electrical conductivity of altered volcanic tuffs. In the works of Rosell et al. [21] it is established that by means of electrical conductivity the index of pozzolanic activity of the materials can be rigorously evaluated.

#### *3.5. Chemical Quality Analysis (CQA)*

Table 2 shows the result of the chemical analysis of two fluorite samples, one in the natural state (NF), which has already been described in Sections 3.1–3.3, and the other calcinated (CF). Virtually all silica in the NF sample is reactive (51.36%). A similar case occurs with reactive CaO (28.38%). Compounds such as MgO, Fe2O3 and Al2O3 are exceptionally low. The contents of I.R. are moderate with a tendency to reach the limit indicated by the standard UNE EN 196-2:2014 [16] and UNE-EN 197-1:2011 [12].

However, the analysis of the CF sample indicates an increase in reactive SiO2 and reactive CaO of 55.70% and 28.40%, respectively; a similar case has been demonstrated in the work of Pei et al. [22]. In addition, the I.R. is comparatively lower in the CF, which is caused by the influence of the increase in temperature and the elimination of the most unstable residues [16].

The percentages of reactive SiO2 and reactive CaO indicated in Table 2 confirm the pozzolanic character of both samples, both calcinated and in their natural state, which is reinforced by what was discussed in Sections 3.4 and 3.6.

#### *3.6. Mechanical Strength Test (MST)*

The results shown in Figure 6a,b and Figure 7a,b indicate an evident increase in mechanical compressive strength, both for specimens made with PC/NF and those with PC/CF, over the investigation period between 7 and 90 days of curing. Note how the values of compressive strength vary depending on two fundamental factors: the BPF of the NF and CF particles (2000 and 5000 cm2/g) and the degree of substitution of the PC by natural and calcinated fluorite. First, it considers the specific case of the PC/NF-01 to 06 sample series; within this series, the PC/NF-01 to 03 samples replace the PC in the specimens in 10, 25 and 40% with the BPF of the NF particles equal to 2000 cm2/g. The PC/NF-04 to 06 series has the same PC/NF ratios as the above-mentioned series, but the NF BPF is 5000 cm2/g. A substantial increase in mechanical strength is observed in specimens containing 10% NF and BPF 2000 cm2/g, and this increase is even more noticeable in specimens made with BPF of NF equal to 5000 cm2/g, as is the case with the PC/NF-04 to 06 specimen series. According to these results, it can be established that the mechanical strength increases as the portion of NF in the specimen decreases, and when the BPF is reduced from 2000 cm2/g to 5000 cm2/g [23].

**Figure 6.** Behaviour of the mechanical strength of specimens elaborated with substitution of PC by NF in 10, 25 and 40%. In (**a**) the specimens have BPF of fluorite of 2000 cm2/g, while in (**b**) the BPF is 5000 cm2/g.

**Figure 7.** Mechanical strengths of specimens elaborated with replacement of PC by CF in 10, 25 and 40%. The BPF of CF in (**a**) is 2000 cm2/g, while in (**b**) it is 5000 cm2/g.

In the specimens made with CF the trend seems to be similar to that seen in Figure 6a,b. An evolution of the compressive strength is observed in the series of specimens PC/CF-07 to 09 and PC/CF-10 to 12, where the highest values correspond to the specimens with the lowest amount of CF and with the lowest BPF (Figure 7a). The analysis carried out in the groups of samples PC/NF-01 to 06 and PC/CF-07 to 12 confirms an increase in mechanical strength from 7 to 90 days in the specimens made with CF and with B.P.F of 2000 cm2/g, with respect to the NF. Similarly, this fact is seen in specimens where the BPF is 5000 cm2/g (Figure 7b).

The comparison of these results with those of the previous subsection shows the relationship that exists between pozzolanicity and mechanical strength [24]. According to Figure 6a,b and Figure 7a,b, in no case were the compressive strength values of the reference Portland cement (PCS-REF) exceeded.

#### **4. Conclusions**

The researched fluorite sample has a high degree of purity, according to the results of the XRD, SEM and RS analyses.

Both NF and CF have contents of reactive compounds, such as reactive SiO2 and reactive CaO, which cause the pozzolanic reactivity of the studied sample; therefore, this is considered a solid argument to establish the pozzolanic character of the fluorite of the Órgiva deposit.

The mechanical strengths obtained are adequate, and this fact has guaranteed that the tested specimens have reached good compressive strengths at 28 and 90 days. However, an increase in resistance can be forecast beyond 90 days. With a view to large-scale industrial processes, the PC/NF ratio of 10% would be the most effective for the manufacture of pozzolanic cements; even the PC/NF ratio of 25% could also be used, which would lead to a significant decrease in production costs. Depending on the economic solvency of the local company, the PC/NF ratio of 40% could be used as a last resort. In the case of calcinated fluorites, the best PC/CF ratio, 10%, would be the most suitable, followed by a PC/CF of 25%. However, since the difference between the compressive strengths of the PC/NF and PC/CF specimens is not very noticeable, it would be recommended to not initially opt for the large-scale fluorite calcination method, due to the high costs that this represents. In this sense, it should also be considered that excessive grinding in the industrial process, with, for example, a BPF equal to 5000 cm2/g, would increase costs significantly.

The results obtained in this research could serve as a guide for the local Mining Company Órgiva, which would have an impact of added value for its fluorite deposits.

**Author Contributions:** Conceptualization, D.A.M., J.L.C., L.P. and A.C.; methodology, D.A.M., J.L.C. and L.P.; software, D.A.M., J.L.C., L.P., A.C. and R.C.; validation, D.A.M., J.L.C., M.Á.S., J.L.P. and R.N.; formal analysis, D.A.M., J.L.C., E.E., L.P. and A.C.; investigation, D.A.M., J.L.C., E.E., L.P., A.C., R.C., M.Á.S., J.L.P. and R.N.; resources, D.A.M., J.L.C., E.E., L.P., R.C. and R.N.; data curation, D.A.M., J.L.C., L.P., A.C. and R.N.; writing original draft preparation, D.A.M. and J.L.C.; writing review and editing, D.A.M., J.L.C., L.P., R.C., M.Á.S. and J.L.P.; visualization, D.A.M., J.L.C., L.P. and A.C.; supervision, J.L.C. and D.A.M.; project administration, D.A.M. and J.L.C.; funding acquisition, D.A.M., J.L.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work has been funded for the Fundación Gómez Pardo and the Laboratorio Oficial para Ensayos de Materiales de Construcción (LOEMCO), Madrid, Spain.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Acknowledgments:** The authors wish to thank the Laboratorio Oficial para Ensayos de Materiales de Construcción (LOEMCO) for the preparation of the samples, the performance of the tests and the interpretation of the results, as well as the financial support offered to cover the costs of editing and publication of this work. The authors would also like to acknowledge the financial support provided by the Gómez Pardo Foundation. D.A.M., J.L.C., L.P.M. and A.C.P. thank the Centralised Laboratory of the Escuela Técnica Superior de Ingenieros de Minas y Energía (Universidad Politécnica de Madrid) for the SEM analysis. The authors thank the Laboratorio de Estratigrafía Biomolecular of the Escuela Técnica Superior de Ingenieros de Minas y Energía (Universidad Politécnica de Madrid) for support with X-ray diffraction analysis. The authors are very grateful for the help given by the Laboratorio de Gemología of the Instituto Gemológico de España in the study of fluorite by Raman spectroscopy. The authors are especially grateful to the mining company of Órgiva (Granada, Spain) (Minera de Órgiva, S.L.) for their support with geological work and sampling.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


*Edited by Seloame Tatu Nyaku*

*and Agyemang Danquah*

Tomato cultivation is a major economic activity in many countries of the world. Thus, strategic efforts should be directed towards mitigating production constraints that limit overall yields and quality. In addressing some of these constraints, researchers are developing and using varieties of modern and innovative techniques to improve local tomato germplasm, make rapid genetic gains, and breed for varieties with resistance to biotic and abiotic stress. This book focuses on recent advances in genomics and genetic improvement of the tomato crop, and production systems, and center around the following themes: (i) disease and pest management in tomato production, and (ii) breeding tools and improvement of the tomato.

ISBN 978-1-78985-033-8

Recent Advances in Tomato Breeding and Production

Published in London, UK © 2019 IntechOpen © mini444 / iStock


**2. Literature review**

#### **2.1 Research on infectious diseases during disasters**

Infectious diseases are often prevalent after the occurrence of natural disasters [5]. There are many cases of infectious diseases and psychological stress caused by a

## *Disaster Management Process Approach: Case Study by BOSS for Disaster Response under… DOI: http://dx.doi.org/10.5772/intechopen.94954*

massive change in the living environment due to the way of life in evacuation centers [6], food shortages, and unsanitary environments increase the risk of infectious diseases [5]. Also, an unspecified number of people live together in evacuation centers, but the area per person is small, and the rate of infections spread is high in dense evacuation centers [7]. Medical workers get tired during disasters, and they may be affected, and insufficient medical resources provision compared with typical phases contributes to the spread of infection [6].

There were many acute respiratory symptoms and acute gastrointestinal symptoms in the 2011 Great East Japan Earthquake disaster [8]. During the 2016 Kumamoto earthquake disaster, infectious gastroenteritis due to Norovirus was prevalent [9]. After natural disasters, although not limited to Japan, the outbreak of infectious diseases such as aspiration pneumonia, skin, and wound infections occurred during the 2004 tsunami disaster in Indonesia [10], and the hurricane Katrina in 2005. At that time, Norovirus infection spread in evacuation centers [11].

#### **2.2 Research on disaster response work**

**Keywords:** BOSS, process management, disaster management, disaster response,

COVID-19 is prevalent all over the world. Different countries applied a largescale lockdown [1], and although the lockdown is currently relaxed and economic activities have started again, the expected second wave of infection has occurred.

In Japan, a disaster-prone country, there is concern about the COVID-19 as it spreads. Particularly in recent years, disasters such as the 2011 Tohoku-Pacific Ocean Earthquake-tsunami disaster, the 2016 Kumamoto Earthquake disaster, and the 2018 massive rain disaster in western Japan have become more severe and frequent.

It is necessary to manage the entire disaster response processes in such a situation. Also, it is essential to manage the disaster response work process and effectively communicate the disaster response work so that there is no duplication,

Regarding the operation of shelters among various disasters, schools and public halls become shelters for many displaced residents [3, 4], and various operations may coincide with the potential high risk of infection among the crowded residents. In the current COVID-19, outbreaks of infectious diseases at shelters can occur. There is currently no mention of the Basic Act on Disaster Countermeasures in Japan requiring improved living conditions in evacuation centers, such as distributing food, clothing, medicines, and healthcare services (Article 86–6). Therefore, there is an urgent need to establish an evacuation center management system that

As there is a concern that the COVID-19 outbreak will result in a lack of human and physical resources, the following five components are essential for sufficient disaster response work: (1) Standard workflow for disaster response, (2) Information sharing and distribution to understand the situation of dispersed evacuation centers, (3) Effective allocation and management of human resources for various kinds of situations, (4) Information management and distribution of materials and equipment, (5) Continuous follow-up on measures/policies against infectious diseases. Regarding the standard workflow for disaster response, research on standardization and systematization of disaster response has been conducted to realize effective disaster response. The author developed a Business Operation Support System (BOSS) for effective disaster management. BOSS is a workflow system with a database that summarizes disaster response works in a workflow chart.

In this study, the BOSS was used to create a comprehensive workflow focus on shelter management for COVID-19, and the created workflow was verified in the situation of the scenario disaster. In particular, we compared the responses for shelter management with the BOSS or without the BOSS (with a conventional paper manual) and analyzed the effect of using BOSS. This study verified shelter man-

Infectious diseases are often prevalent after the occurrence of natural disasters [5]. There are many cases of infectious diseases and psychological stress caused by a

agement operations in Kawasaki City, Kanagawa Prefecture.

**2.1 Research on infectious diseases during disasters**

**2. Literature review**

**100**

rework, or waiting by concerning human resources in an organization.

COVID-19

**1. Introduction**

Re-expansion occurred [2], and it has continued.

*Natural Hazards - Impacts, Adjustments and Resilience*

incorporates measures against infectious diseases.

Local governments often create manuals as a prevention measure. Since many disaster response works are performed in parallel and complicated activities, omissions occur in manuals [12]. Due to detailed descriptions [13], many studies have been conducted to clarify effective manual creation methods. Also, [14, 15] clarified the effects of the new system replacing manuals in the form of empirical experiments. From the operational perspective, according to [16, 17], the actual disaster response work is quantitatively analyzed.

However, although all of these analyzes actual disaster response, the effects of documents such as manuals created in advance are not mentioned.

Therefore, in this study, using an evacuation shelter case as an example, shelter operation using the BOSS under COVID-19 was conducted to quantitatively clarify the effect of the BOSS by comparing the evacuation shelter operation manual (with or without the BOSS).


**Abstract**

Swift progress in urban areas can pilot to urban sprawl, where new components are urbanized around the external boundaries of urban areas, frequently taking up important farmland. Many tenants want better residence, comfortable space, and calm and safe environment, which they discover in the new suburban growth far separate from the center of the city. This kind of expansion requires the addition of utilities and the transport network, as well as the precautions of services such as education, amusement, medical facilities, and commercial services. A number of Indian cities, for example, have a "Delhi," a "Mumbai," and other cultural-based communities that reflect our multi-cultural people. Historical information said, many Indian immigrants who lived in these neighborhoods were poorly treated and subject to racism and discrimination by the general people. Urbanization is a truth of life in most cities in India and around the world.

**Keywords:** town and city, migration, growth, sprawl, urbanization


*Article* **Utilization of Suberinic Acids Containing Residue as an Adhesive for Particle Boards**

**Raimonds Makars 1,2,\* , Janis Rizikovs 1 , Daniela Godina 1 , Aigars Paze 1 and Remo Merijs-Meri 3**

**Abstract:** The birch (*Betula* spp.) outer bark is a valuable product rich in betulin. After removal of betulin extractives, suberin containing tissues are left. Suberin is a biopolyester built from α,ω-bifunctional fatty acids (suberinic acids), which after depolymerization together with lignocarbohydrate complex is a potential adhesive as a side-stream product (residue) from obtaining suberinic acids for polyol synthesis. In this work, we studied the utilization possibilities in particleboards of the said residue obtained by depolymerization in four different solvents (methanol, ethanol, isopropanol and 1-butanol). The adhesives were characterised by chemical (acid number, solubility in tetrahydrofuran, epoxy and ash content) and instrumental analytical methods (SEC-RID, DSC, TGA and FTIR). Based on the results of mechanical characteristics, ethanol was chosen as the most suitable depolymerization medium. The optimal hot-pressing parameters for particleboards were determined using the design of experiments approach: adhesive content 20 wt%; hot-pressing temperature 248 ◦C, and hot-pressing time 6.55 min.

**Keywords:** birch outer bark; suberinic acids; particle boards

## **1. Introduction**

Birch (*Betula* spp.) trees are very common in the Northern hemisphere. Veneer production is considered the most efficient way of processing birch wood, and birch bark is accumulated as the by-product in this process and mostly used for the production of thermal energy by incineration. Birch bark is composed of two layers: the outer bark and the inner bark [1]. The outer bark proportion of the total birch biomass is roughly 3 to 5 wt% [2]. Although the amount of bark is relatively small, the large production volumes of veneer result in a noticeable accumulation of this material.

The birch outer bark (BOB) contains a high proportion of lupane-type triterpene extractives (betulin, lupeol, betulinic acid) which can be isolated by various methods [3–5]. After the removal of extractives from BOB, mainly suberin-containing external protective tissues are left [6].

Suberin is a biopolymer of plant origin occurring in specific parts of tissue, where protection from the surrounding environment is required. Chemically, suberin comprises two chemically distinct parts: (a) an aliphatic region built from α,ω-bifunctional fatty acids (suberinic acids (SA)) and glycerol through ester linkages; (b) polyaromatic lignin-like structures [7]. These regions are cross-linked and various depolymerization methods are proposed to obtain SA monomers and oligomers. The most common method is alkaline hydrolysis in water [8,9] and other solvents, such as isopropanol [2,6]. Our previous research showed that SA obtained by hydrolytical depolymerization can be further used as a bio-based adhesive in the preparation of environmentally friendly wood composites, such as plywood and particle boards. The particle boards made from SA-based adhesive

**Citation:** Makars, R.; Rizikovs, J.; Godina, D.; Paze, A.; Merijs-Meri, R. Utilization of Suberinic Acids Containing Residue as an Adhesive for Particle Boards. *Polymers* **2022**, *14*, 2304. https://doi.org/10.3390/ polym14112304

Academic Editors: Petar Antov, Pavlo Bekhta, Yonghui Zhou and Viktor Savov

Received: 13 May 2022 Accepted: 3 June 2022 Published: 6 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

in hot-pressing process exhibited satisfactory water-resistant properties as determined by low thickness swelling values. In addition, the high internal bonding values showed that the SA-based adhesive has as good compatibility with wood particles as a filler [9,10]. The satisfactory water resistance is explained by the lipophilic properties of suberin [11].

In addition, in our previous work [12] we have demonstrated that SA can be obtained from extractive-free BOB by alkaline depolymerization in ethanol-water solution for further utilization in polyol synthesis. During this process, residue containing SA and lignocellulosic components is accumulated. One way to utilize this residue after suberin depolymerization is as solid fuel or fuel filler [13]. However, these products have low added-value, therefore the aim of this study was to investigate the utilization of the residue obtained after depolymerization in four different solvents (methanol, ethanol, isopropanol and 1-butanol) in particle board preparation. For the most promising adhesive, the design of experiments (DOE) approach was used to determine the optimal hot-pressing parameters.

#### **2. Materials and Methods**

#### *2.1. Raw Material—Birch Outer Bark*

Isolated and fractionated BOB was kindly supplied by AS Latvijas Finieris (Riga, Latvia). BOB samples were dried at room temperature (moisture content 4–5 wt%) and milled in an SM 100 cutting mill (Retsch GmbH, Haan, Germany) to pass through a sieve with holes measuring 4 mm in diameter. Milled BOB was fractionated by sieving using an AS 200 Basic vibratory sieve shaker (Retsch GmbH, Haan, Germany). Fraction of >4 mm was collected and repeatedly milled in the same cutting mill to pass through a sieve with 2 mm aperture. BOB was further extracted with ethanol twice, as described by Godin, a et al. [14]. Extracted BOB was dried at room temperature and further used as a feedstock for depolymerization.

#### *2.2. Other Materials and Chemicals*

Ethanol (96,3% *v*/*v*) was supplied by SIA Kalsnavas elevators (Jaunkalsnava, Latvia). Methanol (≥99.8%), hydrochloric acid (HCl) (≥37%), and tetrahydrofuran (THF) (anhydrous, ≥99.9%) were obtained from Sigma-Aldrich (Steinheim, Germany). Isopropanol (Reag. Ph Eur, ≥99.8%) was received from Merck (Darmstadt, Germany). 1-Butanol (99%) was purchased from Acros Organics (Geel, Belgium). Potassium hydroxide (KOH) (Reag. Ph Eur, 85.0–100.5%) was provided by VWR International (Leuven, Belgium). Nitric acid (HNO3) (≥65%) and acetone (≥99%) were obtained from Honeywell (Seelze, Germany).

#### *2.3. Obtaining SA-Based Adhesive*

BOB depolymerization was carried out in KOH solution for 60 min at 66 to 80 ◦C (depending on the solvent). Four different solvents were used: methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) and 1-butanol (BuOH). The depolymerization conditions are given in Table 1.

**Table 1.** Depolymerization conditions.

1 ethanol-water solution 1:10 (m/m).

After depolymerization, the solution was cooled down and filtered. The filtrate can be further used for preparing polyols as described by Rizikovs et al. [12]. The precipitate (residue) was further used to prepare the adhesive by suspending it in water to the ratio of 100 g per litre. The suspension was acidified with HNO3 to pH 2.0, then it was filtered

and rinsed with deionized water. As a result, four different SA-based adhesive samples were obtained.

#### *2.4. Characterization of SA-Based Adhesive*

For SA characterization, adhesive samples were dried at room temperature and milled with a CryoMill cryogenic mill (Retsch GmbH, Haan, Germany) under LN2 purge at −196 ◦C.

#### 2.4.1. Acid Number

To about 0.2 g of the sample, 5 mL of DMSO was added and stirred for 1 h. Afterwards, 20 mL of i-PrOH and 5 mL of water were added, and the solution was titrated with 0.1 M KOH solution. Two replicate experiments were performed for each sample.

#### 2.4.2. Epoxy Groups

To about 0.2 g of the sample, 10 mL of 0.2 M HCl in acetone was added. The solution was stirred for 1 h and titrated with a known concentration of 0.1 M KOH solution. Two replicate experiments were performed for each sample.

#### 2.4.3. Soluble Substance in THF

To about 0.1 g sample 5 mL of THF was added. The mixture was heated at 50 ◦C for 0.5 h, cooled and stirred for another hour. The soluble substance in THF was determined by filtration through a filter crucible (porosity 2). Two replicate experiments were performed for each sample. The filtrate was further used for size exclusion chromatography analysis.

#### 2.4.4. Ash Content

The ash content was determined at 715 ± 10 ◦C according to ISO standard 1171:2010 [15].

#### 2.4.5. Size-Exclusion Chromatography

GPC analysis of sample (100 µL) was performed using an Agilent Infinity 1260 HPLC system with degasser, auto sampler, RI detector and MALS (miniDAWN) detector. Two GPC analytical columns connected in line were used for the analysis: PLgel Mixed-E (3 uL, 300 × 7.5 mm). The flow rate was 1 mL/min and the RI detector temperature was 35 ◦C.

The polystyrene calibration graph was obtained by preparing polystyrene standard solutions in THF at a mass concentration of 2 mg/mL and analysing them with GPC instrument. Molecular weights of polystyrene standard substances were 500, 850, 1000, 2500, 5000, 9000, 17,500, 20,000, and 30,000 Da.

#### 2.4.6. Differential Scanning Calorimetry (DSC)

DSC822 differential scanning calorimeter (Mettler-Toledo, Greifensee, Switzerland) was used to analyse the thermal behaviour of adhesive samples. The analysis was performed in pierced aluminium pans under N2 purging at a rate of 10 ◦C min−1 in two stages: (1) heating from 20 ◦C to 260 ◦C; (2) cooling from 260 ◦C to 20 ◦C.

#### 2.4.7. Thermogravimetric Analysis (TGA)

TGA analysis was performed using a TA Instruments Discovery TGA 5500 thermogravimetric analyzer. Mass loss was determined in Pt sample pans under an N2 purge at 50 mL min−1 by isothermally treating the sample at 30 ◦C, followed by heating to 700 ◦C at a rate of 10 ◦C min−1 .

#### 2.4.8. Fourier Transform Infrared Spectroscopy (FTIR) Analysis

FTIR spectrometry data was collected with a Nicolet iS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at a resolution of 4 cm−1 , 32 scans. The FTIR data were collected using the attenuated total reflectance (ATR) technique with a diamond crystal prism.

#### *2.5. Particle Board Preparation*

The SA-based adhesive with moisture content ∼80 wt% was mixed with fractionated (0.4–2.0 mm) birch wood particles generated from veneer shorts. Afterwards, the mixture was oven-dried at 100 ◦C to moisture content <1 wt% and hot-pressed with thickness bars by a LAP 40 single-stage press (Gottfried Joos Maschinenfabrik GmbH & Co. KG, Pfalzgrafenweiler, Germany). The dimensions and density of the boards were designed to be 180 × 150 × 7 mm and 0.8 g cm−3 , respectively.

2.5.1. Comparison of Particle Board Properties Depending on SA-Based Adhesive Sample

To choose the most suitable adhesive, MeOH, EtOH, i-PrOH, and BuOH adhesivebased particle boards were made, as described above. Based on previous experience [9] particle board hot-pressing temperature (T) was set to 225 ◦C and the pressing time (t) was 5 min. Pressure (p) was varied in two cycles: 3.5 MPa in the first cycle (t = 2 min) followed by pressure release to 0.1 Mpa per 30 s; during the second cycle, the pressure was increased to 1.7 Mpa (t = 1 min 50 s) followed by another pressure release to 0.7 Mpa for 40 s. The adhesive content (c) in the particle boards varied from 20 to 40 wt% (dry basis).

#### 2.5.2. Experimental Design to Determine Optimal Hot-Pressing Parameters

After choosing the most suitable adhesive, the design of experiments (DOE) approach was used to determine the optimal hot-pressing parameters. Three variable factors were defined: c, T, and t. At first, full factorial design methodology (23 ) using Design Expert 13 software (Stat-Ease Inc., Minneapolis, MN, USA) was used, which consisted of 8 runs. The variable factor levels are given in Table 2. Depending on t, the hot-pressing was performed in 2 or 3 cycles (at t = 2 and t = 8 min, respectively). The duration of the hot-pressing cycles is given in Table 3.

**Table 3.** Duration of the hot-pressing cycles.

The effects of the hot-pressing parameters on response values (modulus of elasticity (MOE), bending strength (MOR), and thickness swelling after a 24 h immersion test (TS 24 h), density) were evaluated using the software. To improve the resolution, the full factorial experimental plan was augmented by 8 additional runs using D-optimality criteria.

#### *2.6. Evaluation of the Particle Board Properties*

The obtained PBs were conditioned (RH = 65 ± 5%, T = 20 ± 2 ◦C) and characterized according to relevant standards by density [16], MOE and MOR in the 3-point bending test [17], and TS 24 h [18]. The particle boards obtained using the optimal hot-pressing parameters were characterized by internal bonding (IB), determining the tensile strength perpendicular to the plane of the board [19]. Mechanical tests (MOE, MOR, IB) were performed on a Z010 (Zwick Roell AG, Ulm, Germany) universal machine for testing the resistance of materials. The obtained particle boards were compared with the EN 312 standard requirements [20].

#### **3. Results and Discussion**

## *3.1. Characterization of SA-Based Adhesive Samples*

#### 3.1.1. Acid Number, Epoxy Group Content, Soluble Substance in THF, Ash Content

Chemical analysis (acid number, epoxy group content, soluble substance in THF, ash content) was carried out to determine the properties of adhesive samples. The results are summarized in Table 4. Acid number values show that the highest acid functionality was for i-PrOH samples (122 mg KOH g−1 ), which could be a desirable characteristic for an adhesive, because acid groups are involved in adhesive cross-linking reactions. However, the i-PrOH adhesive had the lowest epoxy group content (0.11 mmol g−1 ). A high epoxy group content is a desirable property since epoxy groups play a role in the formation of cross-linked networks [21]. From that point of view, the epoxy group value for the EtOH adhesive was almost 6 times higher—0.61 mmol g−1 . i-PrOH and BuOH adhesives showed higher values for the soluble substance in THF (58.1 and 57.5 wt%, respectively), compared to the MeOH sample (44.0 wt%), which suggests that a higher proportion of suberin-based monomers and oligomers were present in those samples. On the contrary, the MeOH sample had a relatively high ash content (12.9 wt%). This could be because of the higher SA particle aggregation in the acidification process. As a result, unacidified SA salts remained in the agglomerates, so the adhesive potentially could be less effective. The lowest ash content values were for SA obtained in the BuOH and EtOH (6.6 and 6.7 wt%, respectively).

**Table 4.** Chemical properties of SA-based adhesive samples.

Because of the high amount of acid number, epoxy groups, soluble substance in THF, and the low ash content adhesive obtained in ethanol showed the most promising properties for obtaining wood-based panels.

### 3.1.2. Size-Exclusion Chromatography

The adhesive molecular weight is also an important characteristic of the adhesive. During SEC-RID analysis of the soluble substance in THF, seven to eight (depending on the sample) different molecular weight fractions were separated, ranging from 94 to 69,909 Da. Since adhesives have limited solubility in THF, it is possible that larger molecular weight fractions (including lignocellulose) were left out. The chromatograms with the relevant molecular weight fractions are given in Figure 1. For a more convenient result comparison, lower (oligomeric + monomeric) molecular weight fraction (<3000 Da) percentages were estimated.

The adhesive molecular weight is also an important characteristic of the adhesive. During SEC-RID analysis of the soluble substance in THF, seven to eight (depending on the sample) different molecular weight fractions were separated, ranging from 94 to 69,909 Da. Since adhesives have limited solubility in THF, it is possible that larger molecular weight fractions (including lignocellulose) were left out. The chromatograms with the relevant molecular weight fractions are given in Figure 1. For a more convenient result comparison, lower (oligomeric + monomeric) molecular weight fraction (<3000 Da) per-

3.1.2. Size-Exclusion Chromatography

centages were estimated.

**Figure 1.** SA-based adhesive sample SEC-RID chromatograms. **Figure 1.** SA-based adhesive sample SEC-RID chromatograms.

Overall, the SEC-RID analysis shows similarities in the separation patterns. The analysis showed that the highest low molecular weight portion was for the i-PrOH sample, which corresponds to the higher acid number for the sample (Table 4). In addition, the solubility in THF was the highest and the Mn and Mw values were the lowest for this sample (Table 5), both indicating that this adhesive contained a higher proportion of low molecular weight fraction. Consequently, the polydispersity was higher for MeOH, EtOH and BuOH adhesives. The high polydispersity values show that a variety of monomers and oligomers were represented in all the samples. Suberin's aliphatic structure mostly comprises C16 to C24 α,ω-diacid and ω-hydroxyacid monomers [7] with molecular weights ranging from around 270 to 400 Da. Thus, the most pronounced peak at tR ≈ 14.5 min for molecular weights ranging from 662 to 834 Da is most likely attributed to SA dimers in the adhesives. Overall, the SEC-RID analysis shows similarities in the separation patterns. The analysis showed that the highest low molecular weight portion was for the i-PrOH sample, which corresponds to the higher acid number for the sample (Table 4). In addition, the solubility in THF was the highest and the Mn and Mw values were the lowest for this sample (Table 5), both indicating that this adhesive contained a higher proportion of low molecular weight fraction. Consequently, the polydispersity was higher for MeOH, EtOH and BuOH adhesives. The high polydispersity values show that a variety of monomers and oligomers were represented in all the samples. Suberin's aliphatic structure mostly comprises C16 to C24 α,ω-diacid and ω-hydroxyacid monomers [7] with molecular weights ranging from around 270 to 400 Da. Thus, the most pronounced peak at tR ≈ 14.5 min for molecular weights ranging from 662 to 834 Da is most likely attributed to SA dimers in the adhesives.

**Table 5.** Molecular weights of adhesive samples.

## 3.1.3. DSC Analysis

DSC thermograms in Figure 2 show that all four adhesives have two common crystalline phase melting zones (see red curve) at 84–87 ◦C and 133–139 ◦C. Heating curves for MeOH, EtOH and i-PrOH samples also had an additional shoulder at 65–70 ◦C, and for the MeOH adhesive, an additional peak at 78 ◦C was observed. Both EtOH and i-PrOH samples exhibited more pronounced crystalline phase melting areas, showing that more crystalline SA were present on these samples. The acid number values were also

higher for these samples (Table 4) and this may suggest that acidic groups form more crystalline structures, most likely due to intermolecular hydrogen bonding. The cooling curves (in blue) showed crystallisation zones of the corresponding melting peaks. The areas of the corresponding crystallisation peaks at around 110 ◦C and 50 ◦C were smaller than the melting peaks, which could indicate to the thermal destruction (see TGA curves in Figure 3) and to intramolecular reactions in the heating process, and, thus, resulting in cross-linked structures. This suggests that all adhesives have thermosetting properties, which is necessary for obtaining wood composites with desirable properties. crystalline SA were present on these samples. The acid number values were also higher for these samples (Table 4) and this may suggest that acidic groups form more crystalline structures, most likely due to intermolecular hydrogen bonding. The cooling curves (in blue) showed crystallisation zones of the corresponding melting peaks. The areas of the corresponding crystallisation peaks at around 110 °C and 50 °C were smaller than the melting peaks, which could indicate to the thermal destruction (see TGA curves in Figure 3) and to intramolecular reactions in the heating process, and, thus, resulting in crosslinked structures. This suggests that all adhesives have thermosetting properties, which is necessary for obtaining wood composites with desirable properties.

DSC thermograms in Figure 2 show that all four adhesives have two common crystalline phase melting zones (see red curve) at 84–87 °C and 133–139 °C. Heating curves for MeOH, EtOH and i-PrOH samples also had an additional shoulder at 65–70 °C, and for the MeOH adhesive, an additional peak at 78 °C was observed. Both EtOH and i-PrOH samples exhibited more pronounced crystalline phase melting areas, showing that more

**Sample Mn, kDa Mw, kDa Mw/Mn** MeOH 9.886 7526 761 EtOH 9.036 6577 728 i-PrOH 3.926 1600 407 BuOH 10.502 7525 717

*Polymers* **2022**, *14*, x FOR PEER REVIEW 7 of 16

**Table 5.** Molecular weights of adhesive samples.

3.1.3. DSC Analysis

**Figure 2.** DSC thermograms for (**a**) MeOH, (**b**) EtOH, (**c**) i-PrOH, and (**d**) BuOH adhesive samples (the red curve represents heating from 20 °C to 260 °C and the blue curve represents cooling from 260 °C to 20 °C). **Figure 2.** DSC thermograms for (**a**) MeOH, (**b**) EtOH, (**c**) i-PrOH, and (**d**) BuOH adhesive samples (the red curve represents heating from 20 ◦C to 260 ◦C and the blue curve represents cooling from 260 ◦C to 20 ◦C). *Polymers* **2022**, *14*, x FOR PEER REVIEW 8 of 16

As seen in the TGA DTGA curves in Figure 3, all the adhesive samples showed similar weight loss patterns. The first weight loss was observed below 100 °C due to the mois-

EtOH adhesives were the lowest at 207.6 °C and 207.8 °C, respectively, whereas MeOH was more heat resistant with an onset temperature of 232.2 °C. The second decomposition in DTGA starts at around 200 °C, which could be attributed to the decomposition of suberin-like structures. This was confirmed by acquiring an additional TGA curve from pure SA obtained from the filtrate, as mentioned in Section 2.3. (sample EtOH\_SA). In addition, there is a possibility that suberin mass loss over 200 °C temperature overlaps with lignin decomposition [22], resulting in a second peak at 255–277 °C. According to Şen et al. [23] the hemicellulose decomposition of *Betula pendula* bark samples occurs at 309 °C, which coincides with the third peak at 310–325 °C. A sharp peak at 358–373 °C is characteristic because of the degradation of cellulose [23] followed by a decomposition of suberin components at ∼400 °C and 443–450 °C, attributing to ω-hydroxy acids and α, ω-diacids respectively [22,23]. When comparing the DTGA curves of adhesive samples to feedstock (extracted BOB), there is a clear decrease in SA-related decomposition. This is due to the removal of some of the suberinic acids after depolymerization for further use in polyols. Based on experimental findings from *Betula pendula* bark pyrolysis [23], suberin composition can be estimated using a multi-peak fitting technique of the DTGA signal. With the help of Origin Pro 2021b software deconvolution tool, we estimated the cellulose and suberinic acid content using Lorentzian multi-peak fitting. Aromatic suberin and lignin content were estimated from resulted char (at 700 °C) by subtracting ash content from the residue after TGA analysis. The results of chemical composition are summarized in Table 6. Cellulose content, as well as aromatic suberin plus lignin values, were close to all the samples. EtOH sample had the highest ω-hydroxy acid and total suberinic acid (ω-hy-

droxy + α, ω-diacids) content, which is important for the adhesive effectiveness.

**Lignin, wt%**

MeOH 9.2 20.3 12.2 13.3 EtOH 9.0 21.4 17.5 11.9 i-PrOH 11.3 22.3 13.8 13.9 BuOH 8.3 22.1 14.2 11.2

**Sample Cellulose, wt% Aromatic Suberin +**

**Table 6.** Estimations of chemical composition (dry-basis) of adhesive samples from TGA analysis.

**ω-Hydroxy Acids, wt%**

**α, ω-Diacids, wt%**

**Figure 3.** TGA (**a**) and DTGA (**b**) curves for adhesive and feedstock samples. **Figure 3.** TGA (**a**) and DTGA (**b**) curves for adhesive and feedstock samples.

3.1.4. TGA Analysis

#### 3.1.4. TGA Analysis

As seen in the TGA DTGA curves in Figure 3, all the adhesive samples showed similar weight loss patterns. The first weight loss was observed below 100 ◦C due to the moisture content in the sample. Onset temperature values in TGA curves for the i-PrOH and EtOH adhesives were the lowest at 207.6 ◦C and 207.8 ◦C, respectively, whereas MeOH was more heat resistant with an onset temperature of 232.2 ◦C. The second decomposition in DTGA starts at around 200 ◦C, which could be attributed to the decomposition of suberin-like structures. This was confirmed by acquiring an additional TGA curve from pure SA obtained from the filtrate, as mentioned in Section 2.3. (sample EtOH\_SA). In addition, there is a possibility that suberin mass loss over 200 ◦C temperature overlaps with lignin decomposition [22], resulting in a second peak at 255–277 ◦C. According to ¸Sen et al. [23] the hemicellulose decomposition of *Betula pendula* bark samples occurs at 309 ◦C, which coincides with the third peak at 310–325 ◦C. A sharp peak at 358–373 ◦C is characteristic because of the degradation of cellulose [23] followed by a decomposition of suberin components at ∼400 ◦C and 443–450 ◦C, attributing to ω-hydroxy acids and α, ω-diacids respectively [22,23]. When comparing the DTGA curves of adhesive samples to feedstock (extracted BOB), there is a clear decrease in SA-related decomposition. This is due to the removal of some of the suberinic acids after depolymerization for further use in polyols.

Based on experimental findings from *Betula pendula* bark pyrolysis [23], suberin composition can be estimated using a multi-peak fitting technique of the DTGA signal. With the help of Origin Pro 2021b software deconvolution tool, we estimated the cellulose and suberinic acid content using Lorentzian multi-peak fitting. Aromatic suberin and lignin content were estimated from resulted char (at 700 ◦C) by subtracting ash content from the residue after TGA analysis. The results of chemical composition are summarized in Table 6. Cellulose content, as well as aromatic suberin plus lignin values, were close to all the samples. EtOH sample had the highest ω-hydroxy acid and total suberinic acid (ω-hydroxy + α, ω-diacids) content, which is important for the adhesive effectiveness.

**Table 6.** Estimations of chemical composition (dry-basis) of adhesive samples from TGA analysis.

#### 3.1.5. FTIR Analysis

Infrared spectra (Figure 4) for adhesive samples and feedstock (extracted BOB) showed a difference for C=O absorption bands. The band at 1734 cm−1 was attributed to esters, whereas the band at 1702 cm−1 corresponds to C=O vibration in carboxylic acids [24,25]; thus confirming that adhesive samples consisted mostly of depolymerized SA. Another band at 1160 cm−1 was present just for the feedstock sample. The C–O–C vibrations at this wavenumber were most likely attributed to ester linkages [26]. Only for BuOH sample, the peak at 1734 cm−1 was present, as well as a band at 1160 cm−1 was more pronounced than for other adhesives. This may suggest that 1-butanol was the least suitable solvent for BOB depolymerization. Asymmetric (2922 cm−1 ) and symmetric (2852 cm−1 ) vibrations, as well as bands at 1464, 1455, and 1374 cm−1 were another characteristic of suberin, corresponding to C–H bonds in alkyl chains [24–27]. The vibrations at 721 cm−1 are inherent to suberin resulting from aliphatic R1CH=CHR2 structures [24,27]. Absorbance at the 1243 cm−1 region can be attributed to epoxy groups in suberin structures and to cellulose and lignin [26,27]. It appears that this band was slightly less pronounced for the MeOH sample, although MeOH epoxy group content (Table 4) was higher when compared

*Polymers* **2022**, *14*, x FOR PEER REVIEW 9 of 16

3.1.5. FTIR Analysis

to i-PrOH and BuOH samples, confirming that there is an overlap in the epoxy, cellulose, and lignin signals. the MeOH sample, although MeOH epoxy group content (Table 4) was higher when compared to i-PrOH and BuOH samples, confirming that there is an overlap in the epoxy, cellulose, and lignin signals.

Infrared spectra (Figure 4) for adhesive samples and feedstock (extracted BOB) showed a difference for C=O absorption bands. The band at 1734 cm−1 was attributed to esters, whereas the band at 1702 cm−1 corresponds to C=O vibration in carboxylic acids [24,25]; thus confirming that adhesive samples consisted mostly of depolymerized SA. Another band at 1160 cm−1 was present just for the feedstock sample. The C–O–C vibrations at this wavenumber were most likely attributed to ester linkages [26]. Only for BuOH sample, the peak at 1734 cm−1 was present, as well as a band at 1160 cm−1 was more pronounced than for other adhesives. This may suggest that 1-butanol was the least suitable solvent for BOB depolymerization. Asymmetric (2922 cm−1) and symmetric (2852 cm−1) vibrations, as well as bands at 1464, 1455, and 1374 cm−1 were another characteristic of suberin, corresponding to C–H bonds in alkyl chains [24–27]. The vibrations at 721 cm−1 are inherent to suberin resulting from aliphatic R1CH=CHR2 structures [24,27]. Absorbance at the 1243 cm−1 region can be attributed to epoxy groups in suberin structures and to cellulose and lignin [26,27]. It appears that this band was slightly less pronounced for

**Figure 4.** FT-IR spectra for adhesive samples and feedstock (extracted BOB). **Figure 4.** FT-IR spectra for adhesive samples and feedstock (extracted BOB).

Lignin and suberin aromatic components in the adhesive and feedstock samples were identified at 1628 and 1609 cm−1 resulting from C=C vibrations from the conjugated carbonyl groups of aromatic components. Absorbances at 1512 and 824 cm−1 are another characteristic band for lignin, corresponding to aromatic C=C stretching and ring vibrations [27]. The more pronounced band at 1628, 1609, and 824 cm−1 points to the fact that the lignin content has increased in the adhesive again, confirming the removal of some of the suberinic acids after depolymerization for further use in polyols. The peak at 1033 cm−1 is associated with C–O vibrations in polysaccharide components [24,27]. The stretching at around 3370 cm−1 can be attributed to O–H stretching in polysaccharides and due to the moisture in the sample. Lignin and suberin aromatic components in the adhesive and feedstock samples were identified at 1628 and 1609 cm−1 resulting from C=C vibrations from the conjugated carbonyl groups of aromatic components. Absorbances at 1512 and 824 cm−1 are another characteristic band for lignin, corresponding to aromatic C=C stretching and ring vibrations [27]. The more pronounced band at 1628, 1609, and 824 cm−1 points to the fact that the lignin content has increased in the adhesive again, confirming the removal of some of the suberinic acids after depolymerization for further use in polyols. The peak at 1033 cm−1 is associated with C–O vibrations in polysaccharide components [24,27]. The stretching at around 3370 cm−1 can be attributed to O–H stretching in polysaccharides and due to the moisture in the sample.

#### *3.2. Choosing the Most Suitable Adhesive for PB Hot-Pressing*

To assess the properties of MeOH, EtOH, i-PrOH, and BuOH adhesive-based composites and compare their mechanical properties, particle board samples from the corresponding adhesives were made. The adhesive content was the only variable in these experiments (c = 20–40 wt%).

The data collected in Table 7 show that the density values for all the composites obtained were relatively similar and were just slightly higher than the designed density (0.830 g cm−3 ). The differences between the other properties of the particle boards were more noticeable. Higher TS 24 h value was achieved with higher adhesive content. TS 24 h requirements were met by all samples made from i-PrOH, BuOH and by two samples from EtOH adhesive (c = 30 wt% and 40 wt%). Overall, MeOH adhesive-based particle boards had the poorest mechanical properties. Regarding the MOE values, very good mechanical properties were achieved by composites based on EtOH and i-PrOH adhesives. BuOH adhesive-based composites showed satisfactory MOE values at c = 20 and 30 wt% when compared to standard P2 requirements. Overall, with an increase in adhesive content, the MOE value decreases for the particleboards, and, therefore, higher adhesive content

corresponds to less stiff composites. MOR values are amongst the lowest for BuOH adhesive-based boards. EtOH and i-PrOH adhesive-based particle boards showed the highest values of MOR. However, none of them met the requirements of standards P2 and P3.

**Table 7.** Mechanical properties of particle boards obtained from MeOH, EtOH, i-PrOH and BuOH adhesives.

1 Standard requirements for non-load-bearing boards (>6–13 mm thick) for use in humid conditions. 2 Standard requirements for boards (>6–13 mm thick) for interior fitments (including furniture) for use in dry conditions.

It seems that the higher acid number value (Table 4) corresponded to the overall better mechanical properties of the particle boards. The poor mechanical properties of MeOH adhesive-based boards could also be explained both by too little content of suberinic acids and their oligomers in the sample (low solubility in THF, Table 4) and because this adhesive had higher thermal stability, which requires a higher hot-pressing temperature. In addition, unwanted condensation reactions during the drying stage of adhesive and wood particle mixture may have resulted in low particle board mechanical properties.

In summary, the most perspective depolymerization mediums for obtaining particle board adhesive from BOB seem to be EtOH and i-PrOH. From a technological point of view, obtaining the EtOH adhesive was less complicated. In addition, the cooling of i-PrOH depolymerizate resulted in rapid precipitation. It seems that this precipitation was a reason that i-PrOH adhesive exhibited highest acid number (Table 4). However, the yield of SA that can be obtained from filtrate after depolymerization in an i-PrOH was about 18% lower when compared to EtOH-obtained SA, which is important for SA-based polyol synthesis [12]. Therefore, it is important to add that the SA-based adhesives that are studied in this paper are obtained as a side-stream product. Based on this evidence, the EtOH adhesive was considered the most suitable for further investigation of optimal hot-pressing parameters.

#### *3.3. Experimental Design for Obtaining Particle Boards*

As mentioned, the EtOH adhesive was chosen as the most suitable adhesive from the obtained data on the properties of the particle boards, for the full factorial experiment (FFE). The adhesive was further used to make particle board composite materials for investigation of the effects of the hot-pressing parameters on the properties of the particle boards through the FFE approach.

Depending on hot-pressing parameters, the properties (MOE, MOR, TS 24 h, density) of particle boards were studied. The results in Table 8 show that lower c and higher T and t values corresponded to better mechanical properties. It can be seen that temperature plays

a very important role in the curing properties of SA-based adhesive, judging by MOE and MOR values. It also seems that the ability of the adhesive to penetrate the filler (wood particles) improves with an increase of the temperature, indicated by lower TS 24 h values. Overall, the statistical analysis showed that all hot-pressing variables had a significant effect on the particle board properties (*p* < 0.05).

**Table 8.** Properties of particle boards, depending on the hot-pressing parameters.

To improve the resolution, the experimental plan was augmented with eight additional runs using D-optimality criteria. The mechanical properties of the obtained particle boards are summarised in Table 9. The boards with t = 2 min were hot-pressed according to the parameters given in Table 3. The rest of the boards were hot-pressed in two cycles: during the first cycle, the pressure was 3.5 MPa (t = 120 s) followed by a reduction in the pressure to 0.1 MPa for 30 s; in the second cycle, the pressure was 1.7 MPa (t = 50 to 180 s) with a subsequent pressure release to 0.7 MPa for 15 to 28 s.

**Table 9.** Characteristics of particle boards, depending on the pressing parameters (additional points).

Overall, the densities were slightly higher than the designed values. MOE values for most of the boards met the EN 312 P3 and P2 requirements, however, this was not the case with the MOR values. At this stage of the study, it seemed that the standard requirements P2 can only be met due to the comparatively low MOR value. In addition, for P2 requirements TS 24 h value was not relevant. However, most of the particle boards exhibited good water resistance (TS 24 h values below 15%) confirming the beneficial water-proofing properties of SA.

After summarizing the data, 4D mathematical models depending on the obtained results were developed and their visual representations are further discussed in the following subsubsections.

#### 3.3.1. MOE of the Particle Boards

Changes in MOE of the obtained boards depending on the hot-pressing parameters are shown in Figure 5. A quadratic numerical model was chosen, as suggested by the software. To simplify the model, statistically insignificant values were excluded. According to ANOVA results, T, t, s . . . T, s . . . t, T . . . t and t2 variables had a statistically significant effect (*p* < 0.05) on the MOE value. It can be seen that higher MOE values can be achieved at higher hot-pressing temperatures. These results corroborate that the formation of more cross-linked structures, and, thus, higher stiffness can be achieved at higher temperatures. In addition, as the hot-pressing time increases from 2 to 5 min, the value of MOE increases, but it decreases as it approaches t = 8 min, suggesting that at prolonged hot-pressing periods the degradation of both wood particles and adhesive occur. ing to ANOVA results, T, t, s … T, s … t, T … t and t2 variables had a statistically significant effect (*p* < 0.05) on the MOE value. It can be seen that higher MOE values can be achieved at higher hot-pressing temperatures. These results corroborate that the formation of more cross-linked structures, and, thus, higher stiffness can be achieved at higher temperatures. In addition, as the hot-pressing time increases from 2 to 5 min, the value of MOE increases, but it decreases as it approaches t = 8 min, suggesting that at prolonged hot-pressing periods the degradation of both wood particles and adhesive occur.

39 240 5.03 2679 8,74 6.9 0.896 20 233 5.30 2840 9,03 15.6 0.889 30 250 3.59 2951 8,76 7.0 0.870

Overall, the densities were slightly higher than the designed values. MOE values for most of the boards met the EN 312 P3 and P2 requirements, however, this was not the case with the MOR values. At this stage of the study, it seemed that the standard requirements P2 can only be met due to the comparatively low MOR value. In addition, for P2 requirements TS 24 h value was not relevant. However, most of the particle boards exhibited good water resistance (TS 24 h values below 15%) confirming the beneficial water-proof-

After summarizing the data, 4D mathematical models depending on the obtained results were developed and their visual representations are further discussed in the fol-

Changes in MOE of the obtained boards depending on the hot-pressing parameters are shown in Figure 5. A quadratic numerical model was chosen, as suggested by the software. To simplify the model, statistically insignificant values were excluded. Accord-

*Polymers* **2022**, *14*, x FOR PEER REVIEW 12 of 16

**Figure 5.** Changes in the MOE value of the boards depending on the hot-pressing parameters. **Figure 5.** Changes in the MOE value of the boards depending on the hot-pressing parameters.

#### 3.3.2. MOR of the Particle Boards 3.3.2. MOR of the Particle Boards

ing properties of SA.

lowing subsubsections.

3.3.1. MOE of the Particle Boards

Changes in MOR of the obtained boards depending on the hot-pressing parameters are shown in Figure 6. A quadratic numerical model was chosen, as suggested by the software. To simplify the model, statistically insignificant values were excluded. According to ANOVA results, T, t, s … T, s … t and t2 variables had a statistically significant effect (*p* < 0.05) on MOR value. The graphic interpretation of the model shows that at higher hotpressing times and higher temperatures, it is possible to reach higher MOR values at relatively low adhesive contents. Changes in MOR of the obtained boards depending on the hot-pressing parameters are shown in Figure 6. A quadratic numerical model was chosen, as suggested by the software. To simplify the model, statistically insignificant values were excluded. According to ANOVA results, T, t, s . . . T, s . . . t and t2 variables had a statistically significant effect (*p* < 0.05) on MOR value. The graphic interpretation of the model shows that at higher hot-pressing times and higher temperatures, it is possible to reach higher MOR values at relatively low adhesive contents. *Polymers* **2022**, *14*, x FOR PEER REVIEW 13 of 16

**Figure 6.** Changes in the MOR value of the boards depending on the hot-pressing parameters. **Figure 6.** Changes in the MOR value of the boards depending on the hot-pressing parameters.

3.3.3. TS 24 h of the Particle Boards 3.3.3. TS 24 h of the Particle Boards

3.3.4. Particle Board Density

temperatures, resulting in lower density boards.

pronounced.

Changes in TS 24 h of the obtained boards depending on the hot-pressing parameters are shown in Figure 7. A quadratic numerical model was chosen, as suggested by the Changes in TS 24 h of the obtained boards depending on the hot-pressing parameters are shown in Figure 7. A quadratic numerical model was chosen, as suggested by the

software. To simplify the model, one statistically insignificant value (T2) was excluded. All the rest of the values showed statistically significant effects (*p* < 0.05) on the water-

(lower TS 24 h values) was significantly improved at t = 5 min (compared to t = 2 min). After a further increase of duration to t = 8 min, the improvement of the 24 h value is less

**Figure 7.** Changes in the TS 24 h value of the boards depending on the hot-pressing parameters.

Changes in the density of the obtained boards depending on the hot-pressing parameters are shown in Figure 8. The two-factor interaction numerical model was chosen, as suggested by the software. Statistical analysis showed that all hot-pressing parameters significantly (*p* < 0.05) affected the density of the boards. An interesting pattern can be seen for higher t values (8 min): at lower hot-pressing temperatures and higher c, the density is rather high. However, with an increase in T, the density further decreases. It could be explained by the rapid release of condensation and decomposition products at higher software. To simplify the model, one statistically insignificant value (T2 ) was excluded. All the rest of the values showed statistically significant effects (*p* < 0.05) on the water-resistance of the boards, which is also shown in the graphic interpretation of the model. It can be seen that the hot-pressing time had a noticeable effect as the moisture resistance (lower TS 24 h values) was significantly improved at t = 5 min (compared to t = 2 min). After a further increase of duration to t = 8 min, the improvement of the 24 h value is less pronounced. All the rest of the values showed statistically significant effects (*p* < 0.05) on the waterresistance of the boards, which is also shown in the graphic interpretation of the model. It can be seen that the hot-pressing time had a noticeable effect as the moisture resistance (lower TS 24 h values) was significantly improved at t = 5 min (compared to t = 2 min). After a further increase of duration to t = 8 min, the improvement of the 24 h value is less pronounced.

**Figure 6.** Changes in the MOR value of the boards depending on the hot-pressing parameters.

Changes in TS 24 h of the obtained boards depending on the hot-pressing parameters are shown in Figure 7. A quadratic numerical model was chosen, as suggested by the software. To simplify the model, one statistically insignificant value (T2) was excluded.

*Polymers* **2022**, *14*, x FOR PEER REVIEW 13 of 16

**Figure 7.** Changes in the TS 24 h value of the boards depending on the hot-pressing parameters. **Figure 7.** Changes in the TS 24 h value of the boards depending on the hot-pressing parameters.

#### 3.3.4. Particle Board Density 3.3.4. Particle Board Density

3.3.3. TS 24 h of the Particle Boards

Changes in the density of the obtained boards depending on the hot-pressing parameters are shown in Figure 8. The two-factor interaction numerical model was chosen, as suggested by the software. Statistical analysis showed that all hot-pressing parameters significantly (*p* < 0.05) affected the density of the boards. An interesting pattern can be seen for higher t values (8 min): at lower hot-pressing temperatures and higher c, the density is rather high. However, with an increase in T, the density further decreases. It could be explained by the rapid release of condensation and decomposition products at higher temperatures, resulting in lower density boards. Changes in the density of the obtained boards depending on the hot-pressing parameters are shown in Figure 8. The two-factor interaction numerical model was chosen, as suggested by the software. Statistical analysis showed that all hot-pressing parameters significantly (*p* < 0.05) affected the density of the boards. An interesting pattern can be seen for higher t values (8 min): at lower hot-pressing temperatures and higher c, the density is rather high. However, with an increase in T, the density further decreases. It could be explained by the rapid release of condensation and decomposition products at higher temperatures, resulting in lower density boards. *Polymers* **2022**, *14*, x FOR PEER REVIEW 14 of 16

**Figure 8.** Changes in the density of the boards depending on the hot-pressing parameters. **Figure 8.** Changes in the density of the boards depending on the hot-pressing parameters.

3.3.5. Determination of Optimal Parameters for Particle Board Hot-Pressing 3.3.5. Determination of Optimal Parameters for Particle Board Hot-Pressing

The constraints of the variables and response values given in Table 10 were used for the determination of optimal hot-pressing parameters. The constraints of the variables and response values given in Table 10 were used for the determination of optimal hot-pressing parameters.

**Value Goal Lower Limit Upper Limit**

T, °C minimize 210 250 t, min minimize 2 8 MOE N, mm−2 maximize 1800 3917 MOR N, mm−2 maximize 11.00 12.59 TS 24 h, % minimize 1.88 44.94 Density, g cm−3 is target = 0.83 0.671 0.952

After setting the goals, the software suggested the following optimal hot-pressing parameters: c = 20 wt%; T = 248 °C and t = 6.55 min. To confirm the results, particle boards were hot-pressed according to the suggested parameters in two cycles. During the first cycle, the pressure was 3.5 MPa (t = 150 s) followed by a reduction in the pressure to 0.1 MPa for 30 s; while in the second cycle, the pressure was 1.7 MPa (t = 180 s) with a subsequent pressure release to 0.7 MPa for 34 s. All four response values as well as additionally IB were determined for the resulting boards and the results are given in Table 11. It can be seen that all values were within the limits of the model's 95% prediction interval and

**Interval Lower Limit**

**95% Prediction Interval Upper Limit**

**Table 11.** Mechanical properties of particle boards obtained under optimal conditions.

**Response Value Result 95% Prediction**

**Table 10.** Constraints for hot-pressing parameters and response values.

**Table 10.** Constraints for hot-pressing parameters and response values.

After setting the goals, the software suggested the following optimal hot-pressing parameters: c = 20 wt%; T = 248 ◦C and t = 6.55 min. To confirm the results, particle boards were hot-pressed according to the suggested parameters in two cycles. During the first cycle, the pressure was 3.5 MPa (t = 150 s) followed by a reduction in the pressure to 0.1 MPa for 30 s; while in the second cycle, the pressure was 1.7 MPa (t = 180 s) with a subsequent pressure release to 0.7 MPa for 34 s. All four response values as well as additionally IB were determined for the resulting boards and the results are given in Table 11. It can be seen that all values were within the limits of the model's 95% prediction interval and also met the EN 312 P2 requirements (Table 7).

**Table 11.** Mechanical properties of particle boards obtained under optimal conditions.

1 EN 312 P2 requirement ≥ 0.40 N mm−2 .

#### **4. Conclusions**

The present work studied the possibilities of obtaining and utilising suberinic acidcontaining residues after birch outer bark depolymerization in the alkanol environment. Four different adhesives obtained in four different solvents (methanol, ethanol, isopropanol, 1-butanol) were compared by chemical and thermal properties, as well as their performance in particle board bonding. The highest acid number (122.0 mg KOH g−1 ) was reached when the adhesive was obtained in isopropanol, but this adhesive was characterised with the lowest epoxy content (0.11 mmol g−1 ). For ethanol-based adhesive, the epoxy content (0.61 mmol g−1 ) and ω-hydroxy suberinic acid (17.5 wt%) content as determined with DTGA was the highest. TGA analysis also showed that all obtained adhesives showed heat resistance from 207 ◦C for adhesives obtained in ethanol and isopropanol to 232 ◦C (methanol adhesive). Based on particle board mechanical tests, ethanol was chosen as the most appropriate depolymerization medium for obtaining adhesive. The optimal hotpressing parameters were determined using the design of experiments approach: adhesive content 20 wt%; hot-pressing temperature 248 ◦C and hot-pressing time 6.55 min. The obtained optimal particle boards met the EN 312 P2 requirements and had a satisfactory water resistance. One limitation of this process is the relatively high hot-pressing temperature, and, therefore, the focus on the future research can be aimed at reducing it with polymerization catalysts or cross-linkers.

**Author Contributions:** Conceptualization, R.M. and J.R.; methodology, J.R., D.G. and A.P.; formal analysis, R.M.; data curation, R.M., J.R. and D.G.; writing—original draft preparation, R.M. and J.R.; writing—review and editing, R.M., J.R., A.P. and D.G. and R.M.-M.; visualization, R.M.; supervision, J.R. and R.M.-M.; project administration, J.R.; funding acquisition, J.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by ERDF project no. <IP_ADDRESS>/19/A/089 "Birch bark as a valuable renewable raw material for producing formaldehyde-free particle boards and suberinic acids polyols for the development of polyurethanes".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**