Towards Next-Generation Sustainable Composites Made of Recycled Rubber, Cenospheres, and Biobinder

polymers

Article Towards Next-Generation Sustainable Composites Made of Recycled Rubber, Cenospheres, and Biobinder

Kristine Irtiseva 1, Vjaceslavs Lapkovskis 2,* , Viktors Mironovs 2, Jurijs Ozolins 1, Vijay Kumar Thakur 3 , Gaurav Goel 4 , Janis Baronins 5 and Andrei Shishkin 1,5

1 Rudolfs Cimdins Riga Biomaterials Innovations and Development Centre of RTU, Institute of General Chemical Engineering, Faculty of Materials Science and Applied Chemistry, Riga Technical University, Pulka 3, Riga LV-1007, Latvia; [email protected] (K.I.); [email protected] (J.O.); [email protected] (A.S.) 2 Scientiﬁc Laboratory of Powder Materials & Institute of Aeronautics, 6B Kipsalas Str., Faculty of Mechanical Engineering, Riga Technical University, Riga LV-1048, Latvia; [email protected] 3 Bioreﬁning and Advanced Materials Research Center, Scotland’s Rural College (SRUC), Kings Buildings, West Mains Road, Edinburgh EH9 3JG, UK; [email protected] 4 School of Engineering, London South Bank University, 103 Borough Road, London SE 10AA, UK; [email protected] 5 Maritime Transport Department, Latvian Maritime Academy, 12, Flotes Str., k-1, Riga LV-1016, Latvia; [email protected] * Correspondence: [email protected]; Tel.: +371-29536301

Abstract: The utilisation of industrial residual products to develop new value-added materials and reduce their environmental footprint is one of the critical challenges of science and industry. Development of new multifunctional and bio-based composite materials is an excellent opportunity for the effective utilisation of residual industrial products and a right step in the Green Deal’s direction Citation: Irtiseva, K.; Lapkovskis, V.; as approved by the European Commission. Keeping the various issues in mind, we describe the Mironovs, V.; Ozolins, J.; Thakur, V.K.; manufacturing and characterisation of the three-component bio-based composites in this work. The Goel, G.; Baronins, J.; Shishkin, A. key components are a bio-based binder made of peat, devulcanised crumb rubber (DCR) from used Towards Next-Generation Sustainable tyres, and part of the ﬂy ash, i.e., the cenosphere (CS). The three-phase composites were prepared in Composites Made of Recycled Rubber, Cenospheres, and Biobinder. the form of a block to investigate their mechanical properties and density, and in the form of granules Polymers 2021, 13, 574. https:// for the determination of the sorption of water and oil products. We also investigated the properties’ doi.org/10.3390/polym13040574 dependence on the DCR and CS fraction. It was found that the maximum compression strength (in block form) observed for the composition without CS and DCR addition was 79.3 MPa, while Academic Editor: Krzysztof Formela the second-highest value of compression strength was 11.2 MPa for the composition with 27.3 wt.% of CS. For compositions with a bio-binder content from 17.4 to 55.8 wt.%, and with DCR contents Received: 18 December 2020 ranging from 11.0 to 62.0 wt.%, the compressive strength was in the range from 1.1 to 2.0 MPa. Accepted: 11 February 2021 Liquid-sorption analysis (water and diesel) showed that the maximum saturation of liquids, in both Published: 14 February 2021 cases, was set after 35 min and ranged from 1.05 to 1.4 g·g −1 for water, and 0.77 to 1.25 g·g−1 for diesel. It was observed that 90% of the maximum saturation with diesel fuel came after 10 min and Publisher’s Note: MDPI stays neutral for water after 35 min. with regard to jurisdictional claims in published maps and institutional afﬁl- Keywords: sustainable composites; crumb rubber; devulcanised crumb rubber; cenosphere; peat; iations. biocomposite; hybrid material; bio-binder; oil absorption

Copyright: © 2021 by the authors. 1. Introduction Licensee MDPI, Basel, Switzerland. This article is an open access article In the modern world, human civilisation is suffering from many challenges, such as distributed under the terms and an extensive increase in the generated waste stream by plastic-material pollution and, at conditions of the Creative Commons the same time, lacking new efficient (lightweight, recyclable, or decomposable, made of Attribution (CC BY) license (https:// biosourced or recycled raw materials) materials. −3 creativecommons.org/licenses/by/ Among various waste materials, cenosphere (CS) is a low-density (0.25–0.55 g·cm )[1], 4.0/). inert, nontoxic, nonflammable, powder-like material which is a part of fly ash. Cenospheres

Polymers 2021, 13, 574. https://doi.org/10.3390/polym13040574 https://www.mdpi.com/journal/polymers Polymers 2021, 13, 574 2 of 14

have emerged as beneficial additives for several applications with their hollow structure and lightweight properties. These materials are primarily derived from coal fly ash, which is a significant pollutant all over the world. Thus, the application of cenospheres in composite-materials design contributes to a circular economy concept. Cenospheres have been chosen as a component in previous works for their specific properties such as low bulk density, high thermal resistance, good workability, and high strength [1]. Its addition to composite materials helps make the material lightweight and improves absorption and acoustic properties [2–5]. They may also impose some adverse effects on physical properties such as reduced compressive strength and increased porosity [2,6]. A decision on the trade-off between these various factors, such as lightweight, compressive strength, cost-effectiveness, etc., is essential in developing the material with the desired properties. Every year, millions of tyres are discarded across the world, representing a severe threat to the ecology along with the fly ash. By the year 2030, up to 5000 million tyres could be discarded regularly [7]. Discarded tyres often lead to “black pollution” because they have a long life, are non-biodegradable, and pose a potential threat to the environment [8]. The traditional waste-tyres management methods have been stockpiling, illegal dumping, or landfilling, all of which are short-term solutions. The growing amount of scrap-tyre waste has created a tremendous amount of waste being dumped which is not biodegradable. As Europe is taking the lead in recycling efforts, their use as fuel in the steel industry, cement industry, and incineration facilities is being promoted [9]. In the past, some efforts have been made by developing composite from fly ash and waste-tyre powder [10], and geopolymer from fly ash and waste tyre [11]. Alternatively, waste tyres are also being used to create running tracks, playgrounds, artificial turf, railways, and in road building [12]. The utilisation of crumb rubber is also gaining attention by incorporation into concrete and rubberised asphalt [13]. There is currently more drive in developing sustainable biocomposite materials using fly ash and tyre waste involving other bio-based materials. A biocomposite is a category of biocompatible and environmentally friendly composites that are biopolymers consisting of natural fibres. Biocomposites are composed of a wide range of organic and inorganic components such as natural and synthetic polymers, polysaccharides, proteins, sugars, ceramics, metal particles, and hydrocarbon nanoparticles. Biocomposites come in various forms such as films, membranes, coatings, fibres, and foams. There are several examples of using peat/sapropel binders, such as sapropel concrete, birch-wood fibre, sanding dust, and hemp shives, for composite materials [14,15]. These materials may be in the form of blocks or pellets. Literature studies have shown the possibility of using sapropel/peat as a raw material in ecological construction. They can be considered promising materials for building materials and designing products [16,17]. Extensive research has been carried out to improve materials’ mechanical properties and functionality and develop environmentally friendly composite materials [18–20]. Some related attempts on the recent development of composites with improved performance have been reported [21–24]. The use of bio-binders is essential for developing these biocomposites [25]. Bio-binders, also called biopolymers, are compounds derived from natural resources and are composed of monomer units covalently linked to form larger structures [26,27]. An example of a bio-binder is natural fibres. Natural binders differ in melt flow rate, impact properties, hardness, vapour permeability, and friction and decomposition coefficient. The water absorption of the bio-binder will also vary depending on the chemical composition of the bio-binder’s processing conditions [28]. The production of bio-based polymers using renewable materials has gained significant attention in recent decades because of the United Nation’s Sustainable Development Goals’ achievement. Latvia and the Baltic region are extraordinarily rich in natural peat. One aim of the work is to investigate the possibility of a new application of natural peat as a bio-binder for hybrid composite materials. Through this research, the authors introduce new biocomposite materials made of two recycled materials: a cenosphere and a devulcanised crumb rubber, and a bio-sourced binder made of natural peat. For the first time, this study proposes the use of crumb rubber Polymers 2021, 13, 574 3 of 14

along with cenosphere and a natural binder, peat, in developing a composite material. These solutions are in line with the United Nations sustainable development goals by fostering the conversion of waste materials into value-added products. We describe here the utilisation of devulcanised crumb rubber (DCR), homogenised peat (HP), and cenospheres (CS) for composite-material development with a bio-binder. This research is aimed to answer the question about what effect the main component DCRHP-CS content has on the composite material properties such as density, mechanical properties, and the absorption of water and oil products.

2. Materials and Methods 2.1. Raw Materials and Compositions For the manufacturing of sustainable composite material in two forms, blocks and granules, a bio-binder made of HP, DCR, and CS was used. Three general compositions with a CS content of 0.0, 5.0, and 10.0 wt.% in a wet mixture were used. For each composition, DCR amounts of 0.0, 5.0, 10.0, 15.0, 20.0, and 30.0 wt.% were chosen. Samples designations and composition of the studied materials in blocks and granules are presented in Tables1 and2. For the production of the specimen, the wt.% of HP in wet condition (suspension with water content 85 wt.%) was used, but the real DCR, CS, and HP content after drying is also represented in Tables1 and2 for an understanding of the entire composition of the studied materials.

Table 1. The composition of block and granules in a raw mixture (wet) and after drying, by wt.% (part I).

Designation of the Composition 0–100–0 5–95–0 10–90–0 15–85–0 20–80–0 30–70–0 0–95–5 5–90–5 10–85–5 15–80–5 20–75–5 30–65–5 Wet mixture composition [wt.%] DCR 0.0 5.0 10.0 15.0 20.0 30.0 0.0 5.0 10.0 15.0 20.0 30.0 HP 100 95.0 90.0 85.0 80.0 70.0 95.0 90.0 85.0 80.0 75.0 65.0 CS 0.0 0.0 0.0 0.0 0.0 0.0 5.0 5.0 5.0 5.0 5.0 5.0 Dried composite material formulation [wt.%] DCR 0.0 27.3 44.2 55.8 64.1 75.4 0.0 22.1 37.2 48.1 56.3 68.0 HP 100 72.7 55.8 44.2 35.9 24.6 72.7 55.8 44.2 35.9 29.6 20.6 CS 0.0 0.0 0.0 0.0 0.0 0.0 27.3 22.1 18.6 16.0 14.1 11.3

Table 2. The composition of block and granules in a raw mixture (wet) and after drying, by wt.% (part II).

Designation of the Composition 0–90–10 5–85–10 10–80–10 15–75–10 20–70–10 30–60–10 Wet mixture composition [wt.%] DCR 0.0 5.0 10.0 15.0 20.0 30.0 HP 90.0 85.0 80.0 75.0 70.0 60.0 CS 10.0 10.0 10.0 10.0 10.0 10.0 Dried composite material formulation [wt.%] DCR 0.0 18.6 32.1 42.3 50.3 62.0 HP 55.8 44.2 35.9 29.6 24.6 17.4 CS 44.2 37.2 32.1 28.2 25.1 20.7

For a better understanding, all the studied recipes are represented in a ternary composition diagram in Figure1. Three groups of composition, classiﬁed by a cenosphere (CS) content in the wet composition of 0, 5, and 10 wt.% correspond to the sample series XX–XX–0, XX–XX–5, and XX–XX–10, respectively. Polymers 2020, 12, x 4 of 13 Polymers 2020, 12, x 4 of 13 content in the wet composition of 0, 5, and 10 wt.% correspond to the sample series XX- Polymers 2021, 13, 574 XX-0,content XX-XX-5, in the and wet XX- compositionXX-10, respectively. of 0, 5, and 10 wt.% correspond to the sample4 of 14 series XX- XX-0, XX-XX-5, and XX-XX-10, respectively.

Figure 1. Ternary diagram of the dried composed materi al composition by wt.%. Three groups of compositionFigureFigure 1. 1. TernaryTernaryclassified diagram diagram by CS of content of the the dried dried (XX- composedXX-0, composed XX-XX-5, material materi composition andal XX-XX-10)composition by wt.%. are by Threeindicated. wt.%. groups Three of groups of compositioncomposition classiﬁedclassified by by CS CS content content (XX–XX–0, (XX-XX-0, XX–XX–5, XX-XX-5, and XX–XX–10) and XX-XX-10) are indicated. are indicated. Natural peat (deposition Keizerpurvs, Cesis, Latvia) was preliminarily processed Natural peat (deposition Keizerpurvs, Cesis, Latvia) was preliminarily processed throughNatural a hydrocavitation peat (deposition process Keizerpurvs for use as a, bio-basedCesis, Latvia) binder. was The preliminarily raw peat (humidity processed through a hydrocavitation process for use as a bio-based binder. The raw peat (humidity 65–70%) was mixed with water and processed in a high-speed multidisc mixer–disperser through65–70%) a was hydrocavitation mixed with water process and processed for use inas a a high-speed bio-based multidisc binder. mixer–disperserThe raw peat (humidity (HSMD)65–70%)(HSMD) with was with cavitation mixed cavitation with effect effect water for obtainingobtaining and processed the the homogeneous homogeneous in a high-speed water–peat water–peat multidisc slurry slurry with mixer–disperser dry with dry matter(HSMD)matter contents contents with of cavitation of15 15 ± ±1 wt.%.1 wt.%. effect Raw Raw for peat obtaining agglomeratesagglomerates the homogeneous before, before, and and peat water–peat particlespeat particles (extracted slurry (extracted with dry frommatterfrom the thesuspension) contents suspension) of 15 after after ± 1 treatmentwt.%. treatment Raw by by peat HSMD, HSMD, agglomerates are are shown shown in before, Figure in Figure2 and. peat 2. particles (extracted from the suspension) after treatment by HSMD, are shown in Figure 2.

(a) (b) Figure 2. Peat agglomeratesFigure ( a2.) beforePeat(a) agglomerates and peat particles (a) before (b) after and treatment peat particles by high-speed (b)( afterb) multidisc treatment mixer–disperser by high-speed multi- (HSMD). discFigure mixer–disperser 2. Peat agglomerates (HSMD). (a) before and peat particles (b) after treatment by high-speed multidisc mixer–disperserThe rotation speed (HSMD). of the HSMD used in the experiments was 8500–9000 min−1, and −1 theThe linear rotation velocity speed of the of working the HSMD teeth wasused from in the 70 to experiments 80 m·s−1. Therefore, was 8500–9000 the cavitation min , and -1 the linearconditionsThe velocity rotation required of speedthe for working slurry of the homogenisation teethHSMD was used from were in 70the ensured. to experiments 80 m·sec The technological. Therefore, was 8500–9000 scheme the cavitation min−1, and conditionstheand linear HSMD required velocity standard for of view theslurry working are givenhomogenisation inteeth Figure was3. Thefrom were treatment 70 ensured. to 80 time m·sec byThe-1 HSMD. Therefore,technological was 5 min, the schemecavitation andconditions andHSMD 45 kg standard of required the total view amountfor areslurry ofgiven HP homogenisation was in Figure used to 3. ensure The were treatment a homogenous ensured. time The sludge-like by technological HSMD HP. was 5 schememin, andand 45 HSMDkgThe of CSthe standard used total in amount the view experiments areof HP given was were in us Figure supplieded to 3.ensure The by Biothecha treatment a homogenous Ltd. time (Riga, by sludge-like HSMD Latvia). was HP. 5 min, Chemical composition of the CS is as follows: SiO —53.8 ± 0.5%; Al O —40.7 ± 0.7%; and 45 kg of the total amount of HP was used 2to ensure a homogenous2 3 sludge-like HP. Fe2O3—1.0 ± 0.2%; CaO—1.4 ± 0.2%; MgO—0.6 ± 0.2%; Na2O—0.5 ± 0.1%; and K2O 0.4 ± 0.1%. Loss of ignition is 1.1 ± 0.1%. The grading composition is < 63 µm—1.70 wt.%, 63–75µm—3.86 wt.%, and 75–150—94.30 wt.%. CS average wall thickness is from 7 to 15 µm. A detailed characterisation, including chemical analysis, particle size and morphology, has been published in [2,3,29]. The common appearance of the CS is represented in Figure4.

(a) (b) (a) (b)

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content in the wet composition of 0, 5, and 10 wt.% correspond to the sample series XX- XX-0, XX-XX-5, and XX-XX-10, respectively.

Figure 1. Ternary diagram of the dried composed material composition by wt.%. Three groups of composition classified by CS content (XX-XX-0, XX-XX-5, and XX-XX-10) are indicated.

Natural peat (deposition Keizerpurvs, Cesis, Latvia) was preliminarily processed through a hydrocavitation process for use as a bio-based binder. The raw peat (humidity 65–70%) was mixed with water and processed in a high-speed multidisc mixer–disperser (HSMD) with cavitation effect for obtaining the homogeneous water–peat slurry with dry matter contents of 15 ± 1 wt.%. Raw peat agglomerates before, and peat particles (extracted from the suspension) after treatment by HSMD, are shown in Figure 2.

(a) (b) Figure 2. Peat agglomerates (a) before and peat particles (b) after treatment by high-speed multidisc mixer–disperser (HSMD).

The rotation speed of the HSMD used in the experiments was 8500–9000 min−1, and the linear velocity of the working teeth was from 70 to 80 m·sec-1. Therefore, the cavitation Polymers 2021, 13, 574 conditions required for slurry homogenisation were ensured. The technological5 of 14 scheme and HSMD standard view are given in Figure 3. The treatment time by HSMD was 5 min, and 45 kg of the total amount of HP was used to ensure a homogenous sludge-like HP.

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Figure 3. HSMD common view (a), homogeniser principal scheme (b), where: 1—peat-slurry tank; 2—electric motor; 3— HSMD; and 4—valve for extra component supply funnel (iv); i—water supply; ii—slurry output; and iii—recirculation flow.

The CS used in the experiments were supplied by Biothecha Ltd. (Riga, Latvia). Chemical composition of the CS is as follows: SiO2—53.8 ± 0.5%; Al2O3—40.7 ± 0.7%; Fe2O3—1.0 ± 0.2%; CaO—1.4(a) ± 0.2%; MgO—0.6 ± 0.2%; Na(b2O—0.5) ± 0.1%; and K2O 0.4 ± 0.1%. Loss of ignition is 1.1 ± 0.1%. The grading composition is < 63 µm—1.70 wt.%, 63– Figure 3. a b 75µm—3.86HSMD wt.%, common and 75–150—94.30 view ( ), homogeniser wt.%. principalCS aver schemeage wall ( ), thickness where: 1—peat-slurry is from 7 to tank; 15 µm. 2—electric motor; 3—HSMD; and 4—valve for extra component supply funnel (iv); i—water supply; A detailed characterisation, including chemical analysis, particle size and morphology, ii—slurry output; and iii—recirculation ﬂow. has been published in [2,3,29]. The common appearance of the CS is represented in Figure 4.

(a) (b) (c)

Figure 4. Scanning-electron-microscopyFigure 4. Scanning-electron-microscopy images of CS typical appearance images at × of100 CS times typical magniﬁcation appearance (a), at at ×100×500 times (b) and magnifica- cross-section ×200 timestion magniﬁcation (a), at ×500 (c). (b) and cross-section ×200 times magnification (c).

TheThe DCR DCR used used for for current current research isis provided provided by by company company Rubber Rubber Products Products Ltd. Ltd. (Riga,(Riga, Latvia). Latvia). The The DCR DCR is is produced produced using mechanochemicalmechanochemical technology technology [30 [30].]. The The man- manu- ufacturing process comprises the processing crumb rubber by grinding at 60–70 ◦C with facturing process comprises the processing crumb rubber by grinding at 60–70 °C with devulcanisation agent (urea) addition. The ﬁnal product represents a sponge-like aggre- devulcanisationgate of DCR (average agent (urea) devulcanised addition. rubber The final contents—13.4 product represents wt%). For thea sponge-like DCR milling aggre- gatede-agglomeration, of DCR (average an devulcanised impact-type disintegratorrubber contents—13.4 DESI-15 (Desintegraator wt%). For the Tootmise DCR milling OÜ, deagglomeration,Estonia) at a rotation an impact-type speed of 3000 disintegrator min−1 was DESI-15 used. The (Desintegraator DCR was milled Tootmise in direct modeOÜ, Esto- nia)ﬁve at timesa rotation (passes). speed For theof 3000 present min study,−1 was a0.25–2.0 used. The mm DCR fraction was was milled used (Figurein direct5). Moremode five timesdetails (passes). about DCRFor the milling, present particle study, size a distribution,0.25–2.0 mm and fraction morphology was used are described(Figure 5). by More detailsLapkovskis about etDCR al. [ 31milling,]. particle size distribution, and morphology are described by LapkovskisFor the et productional. [31]. of the block, the components were manually mixed until homogeneous, then placed into plastic moulds of 140 × 180 × 20 mm3. Samples were dried at room temperature for 20 days. After drying, all specimens were demoulded and left for ambient drying for ten days. For removing any residual humidity, samples were dried at 105 ◦C for 48 h.

(a) (b) Figure 5. Digital optical micrographs of devulcanised crumb rubber (DCR) 0.25-0.5 mm (a) and < 0.25 mm (b) size.

For the production of the block, the components were manually mixed until homogeneous, then placed into plastic moulds of 140 × 180 × 20 mm3. Samples were dried at room temperature for 20 days. After drying, all specimens were demoulded and left for

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The CS used in the experiments were supplied by Biothecha Ltd. (Riga, Latvia). Chemical composition of the CS is as follows: SiO2—53.8 ± 0.5%; Al2O3—40.7 ± 0.7%; Fe2O3—1.0 ± 0.2%; CaO—1.4 ± 0.2%; MgO—0.6 ± 0.2%; Na2O—0.5 ± 0.1%; and K2O 0.4 ± 0.1%. Loss of ignition is 1.1 ± 0.1%. The grading composition is < 63 µm—1.70 wt.%, 63– 75µm—3.86 wt.%, and 75–150—94.30 wt.%. CS average wall thickness is from 7 to 15 µm. A detailed characterisation, including chemical analysis, particle size and morphology, has been published in [2,3,29]. The common appearance of the CS is represented in Figure 4.

(a) (b) (c) Figure 4. Scanning-electron-microscopy images of CS typical appearance at ×100 times magnification (a), at ×500 (b) and cross-section ×200 times magnification (c).

The DCR used for current research is provided by company Rubber Products Ltd. (Riga, Latvia). The DCR is produced using mechanochemical technology [30]. The manufacturing process comprises the processing crumb rubber by grinding at 60–70 °C with devulcanisation agent (urea) addition. The final product represents a sponge-like aggregate of DCR (average devulcanised rubber contents—13.4 wt%). For the DCR milling deagglomeration, an impact-type disintegrator DESI-15 (Desintegraator Tootmise OÜ, Esto- nia) at a rotation speed of 3000 min−1 was used. The DCR was milled in direct mode five Polymers 2021, 13, 574 6 of 14 times (passes). For the present study, a 0.25–2.0 mm fraction was used (Figure 5). More details about DCR milling, particle size distribution, and morphology are described by Lapkovskis et al. [31].

Polymers 2020, 12, x 6 of 13 (a) (b)

ambient drying forFigure ten 5. days.DigitalFigure For optical 5. Digitalremoving micrographs optical any micrographs of residual devulcanised of humidity,devulc crumbanised rubber samplescrumb (DCR) rubber were 0.25–0.5 (DCR) dried mm0.25-0.5 at (a) andmm (a) and < 105 °C for 48 h. <0.25 mm (b0.25) size. mm (b) size. For the granules, the components were manually mixed until homogeneous, then For the granules,For the theproduction components of the were block, manually the comp mixedonents until were homogeneous, manually mixed then until homo- placed in a rotary-drumplaced in granulator ageneous, rotary-drum then with granulatorplaced a drum into with plasticdiameter a drum moulds diameterof 950of 140 mm of × 950 180and mm× rotation20 andmm rotation3. Samples speed speed were dried at of 80 s−1. Samplesof were 80 s−1 .driedroom Samples attemperature room were dried temperature for at 20 room days. temperature Afterfor 2 drying,days. for Toall 2 days. specimensremove To remove anywere residual demoulded any residual and left for humidity, specimenshumidity, were specimens dried at were105 dried°C for at 48 105 h.◦C The for 48standard h. The standard production production scheme scheme of of composite blockscomposite and granules blocks is and illustrated granules is in illustrated Figure 6. in Figure6.

FigureFigure 6. Principal 6. Principal scheme scheme for producing for producing the bio-based the binder bio-based composite binder material composite in the shape material of granules in the and shape blocks. of granules and blocks. 2.2. Characterisation Methods 2.2. CharacterisationLiquid Methods Adsorption Determination of liquid (water and oil products) absorption was performed by im- 2.2.1. Liquid Adsorption mersing specimens in the liquid and checking the weight at a speciﬁc interval. The Determinationexperiments of liquid were (water repeated and ﬁve oil timesproducts) for each absorption composition/liquid, was performed with a margin by im- of error mersing specimensrelative in the to theliquid mean and for eachchecking experiment. the weight The liquid at a absorption specific interval. (W) is calculated The exper- according iments were repeatedto Equation five times (1): for each composition/liquid, with a margin of error rela- m1 − m0 tive to the mean for each experiment. The liquid absorptionW = (W) is calculated according to (1) m0 Equation (1): where m1—the mass of the sample saturated − with liquid, g; = m0—dry mass (before immersion) of the sample, g; and (1) W—liquid absorption g/g. where m1—the mass of the sample saturated with liquid, g; m0—dry mass (before immersion) of the sample, g; and W—liquid absorption g/g.

2.3. Used Equipment and Measurement Devices A high-speed multidisc mixer–disperser with cavitation effect (HSMD) [32–34] was used for obtaining a homogeneous water–peat slurry with a dry-matter content of 15 ± 1 wt.%. The moisture content was determined using a moisture analyser Kern MRS 120-3. Measurements were repeated seven times using the standard deviation to determine the standard error from the arithmetic mean. The Clatronic Multi Food Processor KM3350 (Clatronic GmbH, Kempen, Germany) with stainless steel container and a rubber-coated anchor-type mixer was used for the wet-mixture preparation at a rotation speed of 60 min−1. For examining the specimens, a micro-optical inspection digital light microscope Keyence VHX-1000 (Keyence Corp. Osaka, Japan) equipped with digital camera 54MPx and VH-112 Z20R/Z20W lens, scanning electron microscopy (SEM)—field emission SEM Tescan Mira/LMU (Dortmund, Germany), and optical microscopy were used.

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2.3. Used Equipment and Measurement Devices A high-speed multidisc mixer–disperser with cavitation effect (HSMD) [32–34] was used for obtaining a homogeneous water–peat slurry with a dry-matter content of 15 ± 1 wt.%. The moisture content was determined using a moisture analyser Kern MRS 120-3. Measurements were repeated seven times using the standard deviation to determine the standard error from the arithmetic mean. The Clatronic Multi Food Processor KM3350 (Clatronic GmbH, Kempen, Germany) with stainless steel container and a rubber-coated anchor-type mixer was used for the wet-mixture preparation at a rotation speed of 60 min−1. For examining the specimens, a micro-optical inspection digital light microscope Keyence VHX-1000 (Keyence Corp. Osaka, Japan) equipped with digital camera 54MPx and VH-112 Z20R/Z20W lens, scanning electron microscopy (SEM)—ﬁeld emission SEM Tescan Mira/LMU (Dortmund, Germany), and optical microscopy were used.

3. Results 3.1. Morphology of the Obtained Biocomposite Block and Granules The most characteristic differences of the obtained biocomposites morphology in the form of block and granules are shown in Figure7. The most significant difference in the appearance of the obtained composites is noted for the block-shaped material with 0, 5, and 10 wt.% of CS. The specimens containing 100 wt.% of HP (composition 0–100–0) were intensely cracked after drying (Figure7a), demonstrating a high shrinkage. This is attributed to the used HP without any additive containing 85 wt.% of water. Detailed visual inspection of the cracked specimen’s parts, using magnification X50 times (Figure7d) shows a dense non-porous structure with white, crystal-like inclusions—sand particles. After analysis in polarised light, mainly quartz particles and an admixture of limestone were discovered, these being a natural component of the Baltic-region peat. The addition of 5 wt.% of CS and/or 5 wt.% of DCR strongly minimised the shrinkage and cracking. The typical appearance of the 0–95–5, 5–95–0, and 5–90–5 specimens is illustrated in Figure7b. However, in comparison with highly-loaded composition 20–70–10, its geometry differs from mould shape (Figure7b,c) . Nevertheless, it is necessary to consider that the real content of fillers CS and DCR is much higher (Table1, Table2) because the water loss from HP increases the CS and CDR content in the composite. Specimens 0–95–5, 5–95–0, and 5–95–5 after drying have 0–72.7– 27.3, 27.3–72.7–0, and 22.1–55.8–22.1 CDR–HP–CR mass ratio (or weight %), respectively. The shrinkage-ratio decrease has been reported by several works [2,35,36], mainly with a ceramic matrix material where a high shrinkage is usually observed during the drying and firing [2,37]. In contrasts with the block material, the 0-100-0 granules have no significant morpho- logical differences with the other composition specimens (Figure7g–i). All the manufac- tured granules are characterised by a near-spherical shape and the particle-size distribution for all composition was: 1–2 mm—7–15%, 2–6 mm—10–20, and 6–10 mm—60–70 wt.%. Polymers 2020, 12, x 7 of 13

appearance of the obtained composites is noted for the block-shaped material with 0, 5, and 10 wt.% of CS. The specimens containing 100 wt.% of HP (composition 0–100–0) were intensely cracked after drying (Figure 7a), demonstrating a high shrinkage. This is attributed to the used HP without any additive containing 85 wt.% of water. Detailed visual inspection of the cracked specimen’s parts, using magnification X50 times (Figure 7d) shows a dense non-porous structure with white, crystal-like inclusions—sand particles. After analysis in polarised light, mainly quartz particles and an admixture of limestone were discovered, these being a natural component of the Baltic-region peat. The addition of 5 wt.% of CS and/or 5 wt.% of DCR strongly minimised the shrinkage and cracking. The typical appearance of the 0–95–5, 5–95–0, and 5–90–5 specimens is illustrated in Fig- ure 7b. However, in comparison with highly-loaded composition 20–70–10, its geometry differs from mould shape (Figure 7b, 7c). Nevertheless, it is necessary to consider that the real content of fillers CS and DCR is much higher (Table 1, Table 2) because the water loss from HP increases the CS and CDR content in the composite. Specimens 0–95–5, 5–95–0, and 5–95–5 after drying have 0–72.7–27.3, 27.3–72.7–0, and 22.1–55.8–22.1 CDR–HP–CR mass ratio (or weight %), respectively. The shrinkage-ratio decrease has been reported by several works [2,35,36], mainly with a ceramic matrix material where a high shrinkage is usually observed during the drying and firing [2,37]. In contrasts with the block material, the 0-100-0 granules have no significant mor- phological differences with the other composition specimens (Figure 7g–i). All the manu- Polymers 2021, 13, 574 8 of 14 factured granules are characterised by a near-spherical shape and the particle-size distribution for all composition was: 1–2 mm—7–15%, 2–6 mm—10–20, and 6–10 mm—60–70 wt.%.

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 7. Images of CS–DCR–homogenisedFigure 7. Images of peat CS–DCR–homogenised (HP) composite material: peat common (HP) composite appearance material: of dried common block 0–100–0 appearance (a), of 5–90–5 (b), and 20–70–10dried (c); micro-images block 0-100-0 of ( blocksa), 5-90-5 0–100–0 (b), and (d), 5–90–520-70-10 (e ),(c 20–70–10); micro-images (f) fractures: of blocks and granules:0-100-0 (d 0–100–0), 5-90-5 (e), (g), 5–90–5 (h), and 20–70–1020-70-10 (i) common (f) fractures: appearances. and granules: 0-100-0 (g), 5-90-5 (h), and 20-70-10 (i) common appearances.

3.2. Mechanical Properties and Density of the Obtained Biocomposite Block and Granules

The obtained composites in the form of blocks were tested for compression strength and apparent density. The results are represented in a combined diagram in Figure8. It can be seen that the highest compression strength of 79 MPa corresponds to the pure peat-based bio-binder (0–100–0). The second-highest compression strength of 11 MPa corresponds to the 0–100–5 composition with 5 wt.% of CS in the raw wet mixture or 27.3 wt.% in the composite material after drying (Table1). The observation of the parts of the cracked specimens 0–100–5 (with 27.3 wt.% of CS) revealed a dense structure without cracks or voids, the same as 0–100–0 (100 wt.% of HP, Figure7d) specimens. A signiﬁcant difference in mechanical properties (79.3 and 11.1 MPa) could be explained by the presence of the ﬁller with lower mechanical properties than the quartz and limestone particles of the CS. The introduction of 27.3 wt.% of the DCR leads to a decrease in the compression strength to 7.6 MPa. Polymers 2020, 12, x 8 of 13

3.2. Mechanical Properties and Density of the Obtained Biocomposite Block and Granules The obtained composites in the form of blocks were tested for compression strength and apparent density. The results are represented in a combined diagram in Figure 8. It can be seen that the highest compression strength of 79 MPa corresponds to the pure peat- based bio-binder (0–100–0). The second-highest compression strength of 11 MPa corresponds to the 0-100-5 composition with 5 wt.% of CS in the raw wet mixture or 27.3 wt.% in the composite material after drying (Table 1). The observation of the parts of the cracked specimens 0-100-5 (with 27.3 wt.% of CS) revealed a dense structure without cracks or voids, the same as 0–100–0 (100 wt.% of HP, Figure 7d) specimens. A significant difference in mechanical properties (79.3 and 11.1 MPa) could be explained by the presence of the filler with lower mechanical properties than the quartz and limestone particles of the CS. The introduction of 27.3 wt.% of the DCR leads to a decrease in the compression strength to 7.6 MPa. In all the studied cases, an increase of the CDR in the composites leads to a significant Polymers 2021, 13, 574 9 of 14 decrease of compression strength, up to 1.5 ± 0.4 MPa, but not less than 1.1 MPa (10–80– 10 and 20–70–10).

Figure 8. Dependence ofFigure the apparent 8. Dependence density and of compressive the apparent strength density of the and biocomposite compressive in thestrength shape ofof athe block biocomposite on the in the DCR and CS fraction. Theshape composition of a block of DCRon the HP DCR andCS and fraction CS fraction. is indicated The bycomposition weight % in of the DCR wet mixture.HP and CS fraction is indicated by weight % in the wet mixture. In all the studied cases, an increase of the CDR in the composites leads to a signiﬁcant decreaseBy applying of compression the determined strength, up physical–mecha to 1.5 ± 0.4 MPa,nical but not properties less than 1.1 data MPa of (10–80–10 the obtained and 20–70–10). samples to Ashby’s [38] compression strength and density summary diagram (Figure 9), By applying the determined physical–mechanical properties data of the obtained it cansamples be concluded to Ashby’s that [38] the compression obtained strength material and demonstrates density summary a relatively diagram low (Figure density9), and relativelyit can be high concluded strength, that thecharacteristic obtained material of biocomposites, demonstrates which a relatively is one low of density the key and aims of thisrelatively work. Pure high strength,bio-binder characteristic (0–100–0) of composite biocomposites, material which in is units one of MPa—kg·m the key aims− of3, is this charac- terisedwork. by Pure such bio-binder property (0–100–0) combinations composite that material it is located in units near MPa—kg to the·m −three3, is characterised different types of materialsby such (metals, property combinationsceramics, and that polymers), it is located wh nearich to is the a unique three different properties types combination of materials and much(metals, materials ceramics, belong and to polymers), such property’s which is combinations. a unique properties Compositions combination 5–XX–XX and much and 10– XX–XXmaterials with belong 5 and to 10 such wt.% property’s of DCR combinations.content in wet Compositions mixture and 5–XX–XX units MPa—kg·m and 10–XX–XX−3, belong with 5 and 10 wt.% of DCR content in wet mixture and units MPa—kg·m−3, belong to the to the lower zone of the natural material area. lower zone of the natural material area.

Polymers 2021, 13, 574 10 of 14

Polymers 2020, 12, x 10 of 14

Figure 9. Inﬂuence on theFigure compliance 9. Influence of studied on biocomposites the compliance (in theof studied form of a biocompo block) withsites typical (in materialsthe form demonstrated of a block) with typi- in the Ashby classiﬁcationcal diagram materials (adapted demonstrated from [38]). in the Ashby classification diagram (adapted from [38]).

3.3.3.3. Sorption Sorption of ofLiquids Liquids in in the the Stru Structurecture ofof the the Granulated Granulated Biocomposites Biocomposites TheThe obtained obtained biocomposite biocomposite granulesgranules were were used used for for sorption sorption of water of water and oiland products oil products (diesel). Sorption kinetics were estimated for the developed biocomposite using diesel (diesel). Sorption kinetics were estimated for the developed biocomposite using diesel fuel fuel as a model compound, as demonstrated in Figures 10 and 11. All samples reached a as a90% model water-sorbent compound, uptake as demonstrated capacity in 25–30 in min, Figure with 10 maximal and Figure saturation 11. All after samples 35–45 min reached a 90%Figure water-sorbent 10. All the samples’ uptake series capacity demonstrated in 25–30 amin, near with 1.0 g ·gmaximal−1 water-sorption-capacity saturation after 35–45 minsaturation. Figure 10. A All 90% the sorbent samples’ uptake series capacity demonstrated was noted a near for the 1.0 diesel g·g−1 water-sorption-capac- in a shorter time, ity insaturation. 5–10 min, A with 90% a maximalsorbent uptake saturation capacity after 35–45 was minnoted Figure for the11. Thediesel samples’ in a shorter series time, −1 in 5–10demonstrated min, with from a 1.0maximal to 1.5 g ·saturationg diesel sorption after 35–45 capacity min at Figure equilibrium 11. The conditions. samples’ The series · −1 demonstratedhighest adsorption from 1.0 capacity to 1.5 wasg·g-1 1.5 diesel g g sorptionfor specimen capacity 30–65–5, at equilibrium which corresponds conditions. to a The 68.0–20.6–11.3 ratio of the dry composite components. It is necessary to admit that liquid’s highest adsorption capacity was 1.5 g·g-1 for specimen 30–65–5, which corresponds to a maximal saturation was for diesel, with maximal saturation reached within 3–5 min. 68.0–20.6–11.3 ratio of the dry composite components. It is necessary to admit that liquid’s maximal saturation was for diesel, with maximal saturation reached within 3–5 min. Figure 12 illustrates the water and diesel uptake capacity, in g/g, for granules, and it can be seen that for most cases (except 30–70–0, 5–90–5, 15–80–5, and 20–75–5), there is greater sorption for water. For the composition series XX–XX–0 and XX–XX–10, the water uptake is significantly higher than for diesel, from 10 to 50%, but for the XX–XX–5 series, there is no significant difference between the water and diesel uptake. However, considering the sorption-capacity ratio from the mass ratio [g·g-1] of the sorbent mass to the absorbed-liquid volume [cm3·g], the sorbent capacity for diesel is higher by 15%. The diesel density was assumed as 0.85 g·cm−3.

Polymers 2021, 13, 574 11 of 14

PolymersPolymers 20202020,, 1212,, xx 1010 ofof 1313

1.51.5 1.41.4 1.31.3 1.21.2 1.11.1 1.01.0 0.90.9 0.80.8 0.70.7 0.60.6 0.50.5 15-85-015-85-0 Water sorption, g/g Water sorption, Water sorption, g/g Water sorption, 0.40.4 0.30.3 15-80-515-80-5 0.20.2 15-75-1015-75-10 0.10.1 0.00.0 00 5 5 10 10 15 15 20 20 25 25 30 30 35 35 40 40 45 45 Time, min Time, min Figure 10. Water adsorption,FigureFigure in g/g 10.10. for WaterWater granules adsorption,adsorption, compositions inin g/gg/g series forfor granulesgranules with 0 wt.% compositionscompositions (15–85–0), 5 series wt.%series (15–80–5), withwith 00 wt.%wt.% and (15-85-0),(15-85-0), 10 wt.% 55 wt.%wt.% (15–75–10) of CS. (15-80-5),(15-80-5), andand 1010 wt.%wt.% (15-75-10)(15-75-10) ofof CS.CS.

1.51.5 1.41.4 1.31.3 1.21.2 1.11.1 1.01.0 0.90.9 0.80.8 0.70.7 0.60.6 0.50.5 15-85-015-85-0 Diesel sorption, g/g Diesel sorption, Diesel sorption, g/g Diesel sorption, 0.40.4 0.30.3 15-85-515-85-5 0.20.2 0.10.1 15-75-1015-75-10 0.00.0 00 5 5 10 10 15 15 20 20 25 25 30 30 35 35 40 40 45 45 Time, min Time, min Figure 11. Diesel adsorption,FigureFigure in g/g 11.11. for DieselDiesel granules adsorption,adsorption, compositions inin g/gg/g series forfor granulesgranules with 0 wt.% composcompos (15–85–0),itionsitions 5 series wt.%series(15–85–5), withwith 00 wt.%wt.% and (15-85-0),(15-85-0), 10 wt.% 55 wt.%wt.% (15–75–10) of CS. (15-85-5),(15-85-5), andand 1010 wt.%wt.% (15-75-10)(15-75-10) ofof CS.CS.

Figure 12 illustrates the water and diesel uptake capacity, in g/g, for granules, and it can be seen that for most cases (except 30–70–0, 5–90–5, 15–80–5, and 20–75–5), there is greater sorption for water. For the composition series XX–XX–0 and XX–XX–10, the water uptake is signiﬁcantly higher than for diesel, from 10 to 50%, but for the XX–XX–5 series, there is no signiﬁcant difference between the water and diesel uptake. However, considering the sorption-capacity ratio from the mass ratio [g·g−1] of the sorbent mass to the absorbed-liquid volume [cm3·g], the sorbent capacity for diesel is higher by 15%. The diesel density was assumed as 0.85 g·cm−3.

Polymers 2021, 13, 574 12 of 14 Polymers 2020, 12, x 11 of 13

1.7 1.6 1.5 Water Diesel 1.4 1.3 -1 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 Absorption capcity, g·g capcity, Absorption 0.4 0.3 0.2 0.1 0.0

Composition designation

Figure 12. Sorbent water and diesel uptake capacity in g/g for granules compositions. Figure 12. Sorbent water and diesel uptake capacity in g/g for granules compositions.

4. Conclusions 4. Conclusions In the current research, a three-phase composite material containing homogenised In the current research, a three-phase composite material containing homogenised peat as a bio-binder for water and oil products was produced in the form of blocks and peatgranules as a bio-binder for the first for time. water The obtainedand oil products material inwas the produced form of a block in the was form characterised of blocks and granulesby the right for the combination first time. of The compressive obtained strengthmaterial and in the density. form of a block was characterised by the Theright obtained combination granulated of compressive sorbent containing strength 68.0–20.6–11.3 and density. of CDR HP and CS demon- stratedThe upobtained to 1.5 g ·ggranulated−1 maximal sorptionsorbent capacitycontaining for diesel.68.0–20.6–11.3 of CDR HP and CS demonstratedThe composite up to 1.5 material g·g−1 maximal with CS content sorption of 27.3capacity wt.% for is characterised diesel. by the highest valueThe (except composite for the material pure bio-binder) with CS content of compression of 27.3 wt.% strength is characterised of 11.2 MPa andby the at thehighest −3 valuesame (except time an for apparent the pure density bio-binder) of 0.75 gof·cm compre. HPssion as a bio-binderstrength of and 11.2 CS MPa as a and lightweight at the same timefiller an could apparent become density a prospective of 0.75 g·cm material−3. HP for as designing a bio-binder lightweight and CS bio-based as a lightweight structures. filler Further investigations of CS content’s influence on the CS–HP biocomposite are foreseen could become a prospective material for designing lightweight bio-based structures. Fur- and usage for acoustic and thermal insulation to be explored. ther investigations of CS content’s influence on the CS–HP biocomposite are foreseen and usageAuthor for Contributions: acoustic andConceptualisation, thermal insulation A.S. andto be V.L.; explored. methodology, A.S., K.I. and V.L.; validation, A.S., K.I., J.O., J.B., V.K.T. and V.L.; investigation, V.L., A.S., K.I., J.B., G.G.; data curation, A.S. and K.I.; Authorwriting—original Contributions: draft preparation,Conceptualisation, A.S., K.I., A.S. V.K.T. and and V.L.; V.L.; meth writing—reviewodology, A.S., and K.I., editing, and A.S., V.L.; V.L., validation,J.B., A.S., V.K.T. K.I., and J.O., G.G.; J.B., visualisation, V.K.T., and A.S. V.L.; and investigation, K.I.; supervision, V.L., V.M. A.S., and K.I., J.O.; J.B., project G.G.; administration, data curation, A.S.,V.L. and and K.I.; V.M.; writing—original funding acquisition, draft V.L. preparation, and V.M. All A.S., authors K.I. have V.K.T., read and and agreedV.L.; writing—review to the published and editing,version A.S., of the V.L., manuscript. J.B., V.K.T., and G.G.; visualisation, A.S. and K.I.; supervision, V.M. and J.O.; projectFunding: administration,This research V.L. was equallyand V.M.; funded funding by two acquisition, grants: (1) by V.L. Riga and Technical V.M. All University’s authors Doctoralhave read andGrant agreed programme; to the published (2) This project version has of been the supportedmanuscript. by the Latvian Council of Science within the scope of the project “Innovative bio-based composite granules for collecting oil spills from the water Funding: This research was equally funded by two grants: 1) by Riga Technical University’s Doc- surface (InnoGran)” (No. lzp-2020/2-0394). The work resulting within the network collaboration in toral Grant programme; 2) This project has been supported by the Latvian Council of Science the frame of COST Actions CA17133 Circular City (“Implementing nature-based solutions for creating within the scope of the project “Innovative bio-based composite granules for collecting oil spills a resourceful circular city” and CA18224 GREENERING (“Green Chemical Engineering Network fromtowards the water upscaling surface sustainable (InnoGran)” processes”). (No. lzp-2020/ COST Actions2-0394). are The funded work within resulting the EUwithin Horizon the network 2020 collaborationProgramme. in The the authors frame areof COST grateful Actions for the support.CA17133 Circular City ("Implementing nature-based solutions for creating a resourceful circular city" and CA18224 GREENERING (“Green Chemical EngineeringConflicts of Network Interest: Thetowards authors upscaling declare nosustainable conflict of processes”). interest. COST Actions are funded within the EU Horizon 2020 Programme. The authors are grateful for the support. Conflicts of Interest: The authors declare no conflict of interest.

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