Mechanical Properties and Microstructure of Epoxy Mortars Made with Polyethylene and Poly(Ethylene Terephthalate) Waste

materials

Article Mechanical Properties and Microstructure of Epoxy Mortars Made with Polyethylene and Poly(Ethylene Terephthalate) Waste

Bernardeta D˛ebska 1,* and Guilherme Jorge Brigolini Silva 2

1 Department of Building Engineering, Rzeszow University of Technology, ul. Pozna´nska2, 35-959 Rzeszów, Poland 2 Departamento de Engenharia Civil, Universidade Federal de Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto CEP 35.400.000, Brazil; [email protected] * Correspondence: [email protected]; Tel.: +48-177-432-077

Abstract: The article describes the results of a study to determine the simultaneous effect of polyethylene terephthalate waste (PET) and polyethylene (PE) on the strength characteristics and bulk density of epoxy mortars. In these mortars, 9 wt.% of the polymer binder was replaced by glycolysate which was made from PET waste and propylene glycol. Additionally, 0–10 vol.% of the aggregate was substituted with PE agglomerate made from plastic bags waste, respectively. The modification of the composition of epoxy mortar has a special environmental and economic aspect. It also allows to protect natural sources of the aggregate, while reducing the amount of waste and reducing problems arising from the need to store them. The resulting composite has very good strength properties. With the substitution of 9 wt.% of resin and 5 vol.% of sand, a flexural strength of 35.7 MPa and a compressive strength of 101.1 MPa was obtained. The results of the microstructure study of the obtained mortars constitute a significant part of the paper.

Citation: D˛ebska,B.; Brigolini Silva, Keywords: epoxy resin; PET waste; PE waste; design of experiment; profiles of approximated; G.J. Mechanical Properties and composite microstructure Microstructure of Epoxy Mortars Made with Polyethylene and Poly(Ethylene Terephthalate) Waste. Materials 2021, 14, 2203. https:// 1. Introduction doi.org/10.3390/ma14092203 The 21st century is characterized by continuous growth in production and consumption. This behavior consumes the earth’s resources, which are after all limited. In addition, Academic Editor: Gabriele Milani it leads to the extinction of species and the pollution of the atmosphere and oceans with huge amounts of waste, mainly plastic [1,2]. The scale of the problem is well illustrated by Received: 31 March 2021 the fact that billions of plastic objects have formed an artificial island in the Pacific Ocean. Accepted: 22 April 2021 Published: 25 April 2021 Traditional plastic does not break down into natural substances. If left in the ocean, it will decompose into microscopic fragments over years under the influence of seawater

Publisher’s Note: MDPI stays neutral and sunlight, increasing its toxicity [3], as microplastics can also facilitate the transfer of with regard to jurisdictional claims in toxic chemicals and pathogens. Despite these undeniable risks, since the 1960s, global published maps and institutional affil- plastic production has increased 20-fold, reaching 322 million tons in 2015. This number iations. is projected to further double in the next twenty years [4]. If this trend continues then by 2050 plastic could account for 15% of greenhouse gas emissions and there could be more plastic in the sea than fish [5]. Among the plastic waste generated globally, polyolefins (polyethylene (PE) and polypropylene (PP)) and polyethylene terephthalate (PET) have the largest contribution, as can be seen in Figure1a. A significant amount of this waste Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. comprises packaging materials such as PE bags and sacks or PET bottles and packaging, This article is an open access article which are very difficult to dispose of [6,7]. More than 40% of plastic waste comes from distributed under the terms and packaging (Figure1b), but only 40% of this packaging is recycled [ 5,8]. Climate change conditions of the Creative Commons experts show that there will be no future for humanity in a few decades or by the end of this Attribution (CC BY) license (https:// century as it continues to pursue growth while generating high emissions. Solutions to this creativecommons.org/licenses/by/ problem include converting infrastructure to low-carbon, adopting a different concept of 4.0/). transportation, reducing long-distance trade, and thereby strengthening local production.

Materials 2021, 14, 2203. https://doi.org/10.3390/ma14092203 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 18

Materials 2021, 14, 2203 this century as it continues to pursue growth while generating high emissions. Solutions2 of 18 to this problem include converting infrastructure to low-carbon, adopting a different concept of transportation, reducing long-distance trade, and thereby strengthening local production. Long-distance transport has also been reduced in recent months by the outbreak Long-distanceof the COVID-19 transport epidemic. has also been reduced in recent months by the outbreak of the COVID-19 epidemic.

(a) (b)

FigureFigure 1. 1.Plastic Plastic waste waste in in the the world world (2015 (2015 production): production): ( a(a)) by by polymer polymer type, type, ( b(b)) by by sectors. sectors.

ItIt is is therefore therefore advisable advisable to to look look for for waste waste applications applications that that will will enable enable the management the manage- ofment these of troublesomethese troublesom materialse materials at the at placethe place of their of their original original use, use, but but at at the the same same time time reducereduce thethe consumption consumption andand transportation transportation costscosts of of local local natural natural resources. resources. Such Such waste waste applicationsapplications include include the the substitution substitution ofof naturalnatural aggregateaggregatein in concretesconcretes andand mortarsmortars[ [9–15].9–15]. ForFor example, example, ifif a a concrete concrete mixture mixture consists consists of of approximately approximately 30% 30% sand, sand, when when 10% 10% of of this this aggregateaggregate is is replaced replaced by by plastic plastic waste waste and and assuming assuming an annual an annual production production of approximately of approxi- 20mately billion 20 tons,billion approximately tons, approximately 800 million 800 million tons of tons sand of could sand becould saved be saved per year per [year16]. Researches[16]. Researches in this in field,this field, carried carried out out in many in many scientific scientific centers centers in in the the world, world, are are highly highly recommended,recommended, because because they they lead lead to to obtaining obtaining concretes concretes with with various various properties, properties, depending depend- oning many on many features features of the of wastethe waste material, material, including including the typethe type of plastic, of plastic, size size and and shape shape of particles,of particles, degree degree of substitution of substitution [17, 18[17,18].]. At theAt the same same time, time, at this at this point point it is it worth is worth pointing point- outingthat out notthat every not every application applicatio of concreten of concrete elements elements is a structural is a structural one. There one. There are also are such also examplessuch examples of applications of applications where where strength strength is not is the not main the main criterion, criterion, and and what what matters matters is, foris, for example, example, the the weight weight of of the the element, element, chemical chemical resistance, resistance, adhesion adhesion to to other other materials. materials. ConcreteConcrete andand resinresin mortarsmortars seemseem toto bebe particularlyparticularly attractiveattractive inin termsterms ofof wastewaste disposal.disposal. TheyThey areare composedcomposed predominantly of of aggregate aggregate (u (upp to to 90%), 90%), as aswell well as synthetic as synthetic resin resin and andits hardener its hardener and/or and/or additives additives or oradmixtures admixtures [19,20]. [19,20 ].They They have have found found aa specialspecial placeplace amongamong constructionconstruction materialsmaterials inin applicationsapplications suchsuch asas industrial industrial flooring, flooring, production production of of prefabricatedprefabricated elementselements includingincluding bridgebridge andand roadroad drainagedrainage elements,elements, artificialartificial marbles,marbles, machinemachine foundations,foundations, repairrepair systems, systems, among among others. others. These These composites composites are are characterized characterized byby veryvery high high strength strength parameters, parameters, with with chemical chemical and and corrosion corrosion resistance, resistance, in in addition addition to to lowlow water water absorption, absorption, vibration vibration dampingdamping capacity,capacity, thermal thermal stability, stability, and and very very short short time time to achieve operational efficiency [21–23]. Due to the great popularity of cement concretes, to achieve operational efficiency [21–23]. Due to the great popularity of cement concretes, there are many more examples in the literature describing the processes of modification of these composites with plastic waste (including: PE and PET) [24–28] than resin concretes. Resin composites containing plastic waste were studied by Reis et al. [29,30], Vidales et al. [31], D˛ebskaand Lichołai [21], among others. Reis et al. studied epoxy and polyester Materials 2021, 14, 2203 3 of 18

mortars in which 0–20% sand was replaced by washed and shredded PET waste. The strength parameters of these composites deteriorated as the waste content increased, but at the same time a lighter and more ductile product was obtained, exhibiting less brittle damage, which was confirmed by fracture mechanics results. Vidales and his team obtained mortars based on synthetized polyester resin using PET waste chemically processed by glycolysis and PET particles. These authors noted that increasing the resin content in the composite provided better sand wettability and improved the connection between phases [31]. D˛ebskaand Licholai observed an increase in the strength parameters of epoxy mortars containing glycolysates formed from PET waste [21]. This article deals with epoxy mortars modified in two ways. First, 9 wt.% of the epoxy binder was replaced by a glycolysate based on propylene glycol and waste poly(ethylene terephthalate) (PET). Additionally, sand was partially (0–10 vol.%) replaced by a polyethylene (PE) agglomerate obtained by processing waste plastic bags. The proposed material solution is important for both environmental and economic reasons. It also positively influences the flexural strength of epoxy mortars, which is at the level of 30.3–35.7 MPa. A material with excellent compressive strength, varying in the range of 86.0–107.2 MPa, was obtained. The use of PE waste allowed also to reduce the weight of the product. The use of the experimental design methods in the conducted studies facilitated the determination of the most advantageous composition of mortars with respect to the determined properties. It also resulted in a significant reduction in the number of samples of the tested composites, which allowed to shorten the time and decrease the costs of the conducted experiments. Microstructural studies made it possible to correlate the strength test results with the structure of the mortars studied.

2. Materials and Methods 2.1. Materials Epoxy resin (based on bisphenol A) (Epidian 5, CIECH Sarzyna S.A.) was the binder in the composite samples made. The resin was characterized by molecular weight 450 g/mol and epoxy number LE = 0.49 mol/100 g, density of 1.17 g/cm3 and viscosity of 30,000 mPa·s. The resin was partially replaced (9 wt.%) with glycolysate based on propylene glycol and waste poly(ethylene terephthalate) from waste beverage bottles. The process of glycolysis was carried out at 210 ◦C in the presence of zinc acetate as a catalyst, at an assumed Materials 2021, 14, x FOR PEER REVIEWPET/glycol molar ratio of 1/1.7. The glycolysate was in the form of milky-gray semiﬂuid4 of 18

wax (Figure2).

FigureFigure 2. 2.Glycolisate Glycolisate based based on on waste waste PET. PET.

Z-1A partial hardener substitute was used for sand in an was amount an aggl ofomerate 10 wt.% PE of waste, the resin made weight. from waste The plastic main componentbags, made ofavailable the hardener by a local was company triethylenetetramine. that produces The this hardener type of agglomerate. was in the form Nowa- of lightdays, yellow mechanical liquid methods with density are ofmuch 0.981 more g/cm frequently3 and viscosity used in thefoil rangerecycling. of 20–30 Depending mPa·s. on the level of the contamination of the plastic bags, the recycling processes vary. The agglomerate used in the study came from contaminated foils, which were subjected to cyclic agglomeration with separate washing and drying processes of the shredded foil. In the process of agglomerate preparation, the plastic packaging obtained from selective waste collection was ground to produce flakes, cleaned in separation baths and passed through a system of vibrating chutes to drain water. In the next stage carried out at increased temperature (ranging from 150 °C to 170 °C), as a result of the compacting process, the flakes were transformed into an agglomerate, which is a granular form of plastic of irregular shape and varied grain size. Figure 3 shows pictures of different fractions of PE agglomerate.

(a) (b) (c) Figure 3. PE agglomerate: (a) before the selection into fractions, (b) 1 mm fraction, (c) 0.25 mm fraction.

2.2. Methods The epoxy binder was thoroughly mixed with the glycolysate until a homogeneous mixture was obtained. The composition prepared in this way was annealed for 60 min at 85 °C to allow the reaction between the epoxy groups of the resin and the hydroxyl groups of the PET glycolysate. After cooling to ambient temperature, a hardener at 10% in proportion to the weight of the resin was added to the mixture. The components of the mortar were mixed in a laboratory mixer at 140 ± 5 rpm for 3 min. The mortar prepared in this

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Figure 2. Glycolisate based on waste PET. The quartz sand, which density is 2.65 g/cm3, acted as the aggregate. AA partial partial substitute substitute for for sand sand was was an an aggl agglomerateomerate PE PE waste, waste, made made from from waste plastic bags,bags, made made available available by by a a local local company company that that produces produces this this type type of of agglomerate. Nowa- Nowa- days,days, mechanical mechanical methods methods are are much much more frequently used in foil recycling. Depending onon the the level level of the contamination of the plastic bags, the recycling processes vary. The agglomerateagglomerate used used in in the the study study came came from co contaminatedntaminated foils, foils, which were subjected to cycliccyclic agglomeration agglomeration with with separate separate washing washing and and drying drying processes processes of ofthe the shredded shredded foil. foil. In theIn theprocess process of ofagglomerate agglomerate preparation, preparation, the the plastic plastic packaging packaging obtained obtained from from selective wastewaste collection collection was was ground ground to produce flak ﬂakes,es, cleaned in separation baths and passed throughthrough a a system system of of vibrating vibrating chutes chutes to to drain drain water. water. In In the the next next stage stage carried carried out out at at inin- creasedcreased temperature temperature (ranging from 150 °C◦C to to 170 170 °C),◦C), as as a a result result of of the the compacting compacting process, process, thethe flakes ﬂakes were were transformed transformed into into an an agglomerate, agglomerate, which which is is a a granular granular form form of of plastic of of irregularirregular shape shape and and varied varied grain grain size. size. Figure Figure 3 3shows shows pictures pictures of ofdifferent different fractions fractions of PE of agglomerate.PE agglomerate.

(a) (b) (c)

FigureFigure 3. PEPE agglomerate: agglomerate: ( (aa)) before before the the selection selection into into fractions, fractions, ( (bb)) 1 1 mm mm fraction, fraction, ( (c)) 0.25 0.25 mm mm fraction.

2.2.2.2. Methods Methods TheThe epoxy epoxy binder binder was was thoroughly thoroughly mixed wi withth the glycolysate until a homogeneous mixturemixture was was obtained. obtained. The The composition composition prepared prepared in in this this way way was was annealed annealed for for 60 60min min at 85at °C85 to◦C allowto allow the reaction the reaction between between the epoxy the epoxy groups groups of the resin of the and resin the and hydroxyl the hydroxyl groups ofgroups the PET of the glycolysate. PET glycolysate. After cooling After cooling to ambi toent ambient temperature, temperature, a hardener a hardener at 10% at in 10% pro- in portionproportion to the to weight the weight of the of resin the resinwas added was added to the tomixture. the mixture. The components The components of the mortar of the weremortar mixed were in mixed a laboratory in a laboratory mixer at mixer 140 ± at 5 140rpm± for5 rpm3 min. for The 3 min. mortar The mortarprepared prepared in this in this way was molded into steel molds, making it possible to obtain three samples with dimensions of 40 × 40 × 160 mm3. For filling one mold of mortar not modified with PE waste, 1620 g of sand and 414.85 g of resin composition (Epidian 5 epoxy resin + PET glycolysate) were used. For subsequent compositions, corresponding to each point of the experimental plan, sand was replaced by volume with PE waste and the amount of resin composition with glycolysate was modified according to the data in Table1. Additionally, control samples were made which did not contain any type of waste. The composition of such mortar required to fill one mold for strength testing is: 1620 g of standard sand, 414.85 g of epoxy resin, and 41.5 g of the hardener. Three samples from each series (for each point of the experimental plan) were made. The samples were seasoned under laboratory conditions for 7 days. Materials 2021, 14, 2203 5 of 18

Table 1. The composition of the mortar for individual points of the experiment plan.

Experiment Plan Points 1 2 3 4 5 6 7 8 9 10 PE waste content, % vol. 0.0 1.46 1.46 5.0 5.0 5.0 5.0 8.54 8.54 10.0 Ratio of volumes of resin to 0.58 0.40 0.75 0.33 0.82 0.58 0.58 0.4 0.75 0.58 aggregate R/A

The following strength machines were used to determine the flexural and compressive strength: Cometech Testing Machines Co., Taichung, Taiwan and MATEST S.p.A., Arcore, Italy. Special inserts for testing machines were used according to PN-EN 196-1 [32]. The bulk density of epoxy mortars was calculated as the ratio of the dry mass to the total volume. A laboratory shaker with a set of control sieves (LPzE-3e) was used to develop the aggregate particle size distribution. The PE waste was crushed until it passed through a 200 mesh (75 micron) sieve and evaluated by dispersive energy X-ray fluorescence (XRF) in Epsilon-3x-PANalitic equip- ment and X-ray diffraction (XRD) using Bruker D2 Radiation Phaser-CuKa (k = 1.54184 Å) with a Ni filter.

2.3. Central Compositional Plan Methodology Conducting destructive tests involves the need to make a signiﬁcant number of samples. In order to reduce their number, but at the same time obtain full scientiﬁc information regarding the tested mortars, the experiment theory was applied. Following the previous own research [33], a compositional master plan with a response surface was selected. To plan the experiment and to perform subsequent analyses, the Statistica 12 program (StatSoft Inc., Kraków) [34] were used. The composition of the designed mortars was characterized by two input variables:

• x1—waste PE content (% PE)—took values from 0 to 10 vol.% and was a substitute for sand; • x2—the volume ratio of resin to aggregate R/A with values in the range of 0.33–0.82. The experiment plan consisted of 10 points. It was assumed that the focal point study would be repeated (plan points 6 and 7 do not differ in composition). Data characterizing each point in the plan are provided in Table1. For the input variables (z) (bending and compressive strength as well as bulk density), approximating functions (response surfaces) were determined. It was assumed that the approximation function has the form of a second degree polynomial (1). It describes the relationship between output and input quantities.

2 2 zˆ = A0 + A1x1 + A2x1 + A3x2 + A4x2 + A5x1x2 (1)

where: zˆ—value of the test object function for real variable values, x1—percentage share of waste PE (% PE), x2—resin to aggregate ratio (R/A), Ai—coefﬁcients of the equation for real variables. At the end of the research, such a mortar composition was determined that allows obtaining a composite characterized not only by the most favorable values of strength parameters, but also by low volumetric density. For this purpose the utility proﬁle tab of the Statistica program’s was applied, allowing the approximate values of the three tested outputs to be converted into a single value of the total utility of the tested composites. Materials 2021,, 14,, xx FORFOR PEERPEER REVIEWREVIEW 6 of 18

where: ẑ—value of the test object functiontion forfor realreal variablevariable values,values, x11—percentage share of waste PE (% PE), x22—resin to aggregate ratio (R/A), Aii—coefficients of the equation for real variables. At the end of the research, such a mortar compositioncomposition waswas determineddetermined thatthat allowsallows obtaining a composite characterized not only by the most favorable values of strength Materials 2021, 14, 2203 parameters, but also by low volumetric density. For this purpose the utility profile tab6 of of 18 thethe StatisticaStatistica program’sprogram’s waswas applied,applied, allowingallowing thethe approximateapproximate valuesvalues ofof thethe threethree testedtested outputs to be converted into a single value of the total utility of the tested composites.

3. Results Results and and Discussion Discussion Sand grain grain size size distribution distribution curve curve is isshown shown in inFigure Figure 4. 4In. Inaccordance accordance with with the thedis- tributiondistributiontribution shownshown shown inin FigureFigure in Figure 4,4, individualindividual4, individual sandsand sand fractionsfractions fractions werewere were replacedreplaced replaced inin volume involume volume withwith with PEPE waste.PE waste.

Figure 4. SandSand grain grain size distribution curve.

The SEM-SE images of the PE waste are presented inin FigureFigure5 5..

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

FigureFigure 5. 5.SEM SEM images images of of waste waste PE PE at at 50 50×× ((aa),),), 200200×200×× (((bb),),), andand 1000×1000× 1000× ((c()c) )magnifications.magnifications. magniﬁcations.

It can be seen in Figure5b at the ends of the particle (point 1) an elongated shape, possibly obtained after the process of cutting the polyethylene bags. Table2 shows the chemical composition of the quartz sand.

Table 2. Chemical composition of the quartz sand.

Chemical SiO CaO MgO Al O K O Others Composition 2 2 3 2 % 87.14 1.30 0.38 7.80 1.65 1.73 Materials 2021, 14, x FOR PEER REVIEW 7 of 18

It can be seen in Figure 5b at the ends of the particle (point 1) an elongated shape, possibly obtained after the process of cutting the polyethylene bags. Table 2 shows the chemical composition of the quartz sand.

Table 2. Chemical composition of the quartz sand.

Materials 2021, 14, 2203 Chemical Composition SiO2 CaO MgO Al2O3 K2O Others 7 of 18 % 87.14 1.30 0.38 7.80 1.65 1.73

The elementaryThe analysis elementary of the analysis polyethylene of the polyethylene waste identified waste high identified levels of high Ca, levelsTi; of Ca, Ti; average levelsaverage of Al, Si, levels P, S, of Cl, Al, Zn, Si, Fe; P, S, low Cl, levels Zn, Fe; of low K, Mg, levels V, ofCr, K, As, Mg, and V, Pb. Cr, As,The andhigh Pb. The high presence of the Ca element is due to the frequent use of filling additives such as CaCO3. presence of the Ca element is due to the frequent use of filling additives such as CaCO3. The The amount of CaCO3 added to the PE can influence its mechanical properties, such as amount of CaCO3 added to the PE can influence its mechanical properties, such as fracture fracture tensiletensile strength. strength. The tensile The tensile modulus modulus of the ofPE the compound PE compound increases increases with the with ad- the addition dition of CaCO3 [35]. Therefore, PE with different CaCO3 contents influences the mechan- of CaCO3 [35]. Therefore, PE with different CaCO3 contents influences the mechanical ical propertiesproperties of the epoxy-PE of the epoxy-PE compound. compound. The XRD patternsThe XRD of PE patterns waste and of quartz PE waste sand and are presented quartz sand in Figure are presented 6. The diffrac- in Figure6. The togram of PEdiffractogram waste is in agreement of PE waste with is inothe agreementr results found with other in the results literature found [36–38]. in the literatureThe [36–38]. XRD patternThe of PE XRD showed pattern the oftwo PE peaks showed at Bragg the twoangles peaks 2θ = at20.9° Bragg and angles23.1°, character- 2θ = 20.9◦ and 23.1◦, istic of low densitycharacteristic polyethylene of low and density a semi polyethylene crystalline andmaterial. a semi crystalline material.

Figure 6. XRD of the PEFigure waste 6.andXRD quartz of the sand. PE waste and quartz sand.

3.1. Flexural Strength3.1. Flexural Strength The averageThe flexural average strength ﬂexural values strength me valuesasured measuredfor the samples for the samplescorresponding corresponding to to each each point ofpoint the experimental of the experimental plan are plan summarized are summarized in Figure in 7. Figure The standard7. The standard deviations deviations are are also marked.also The marked. values The of the values determined of the determined strength for strength all compositions for all compositions are at a very are at a very high level exceedinghigh level 30 exceeding MPa. The 30lowest MPa. valu Thee lowestof 30.3 valueMPa was of 30.3 recorded MPa was for recordedthe mortar for the mortar containing 8.54%containing of waste 8.54% PE ofat wasteR/A equal PE at to R/A 0.4 (the equal 8th to point 0.4 (the of 8ththe pointplan), ofwhile the plan),the while the highest value of 35.7 MPa characterized the 6th point of the experimental plan (%PE = 5%, R/A = 0.58). In addition, the horizontal line indicates the ﬂexural strength value obtained for epoxy mortars without PET glycolysate and PE waste, characterized by a resin to aggregate ratio of 0.58. Materials 2021, 14, x FOR PEER REVIEW 8 of 18

highest value of 35.7 MPa characterized the 6th point of the experimental plan (%PE = 5%, Materials 2021, 14, 2203R/A = 0.58). In addition, the horizontal line indicates the flexural strength value obtained 8 of 18 for epoxy mortars without PET glycolysate and PE waste, characterized by a resin to aggregate ratio of 0.58.

Figure 7. The compilation of the mean values of the flexural strength at different points of the experimental plan. Figure 7. The compilation of the mean values of the flexural strength at different points of the experimental plan. The proposed modification allows for improving the flexural strength by more than 10 MPa (more than 50%). Comparing the strength values of the test mortar with the The proposed modification allows for improving the flexural strength by more than mortar containing additionally PET glycolysate (the first point of the experimental plan) 10 MPa (more than 50%). Comparing the strength values of the test mortar with the mortar one can confirm the conclusions described in earlier publications [21]. The substitution containing additionally PET glycolysate (the first point of the experimental plan) one can of the part of the resin with PET glycolysate causes a significant increase (in this case by confirm the conclusions described in earlier publications [21]. The substitution of the part 12.4 MPa) of flexural strength of epoxy mortars. SEM images taken for the mortar with of the resin with PET glycolysate causes a significant increase (in this case by 12.4 MPa) of the composition assigned for the first point of the experimental plan (Figure8a,b) show a flexural strengthgood of bond epoxy between mortars. the SEM resin images and PET taken glycolysate, for the mortar the matrix with the is smooth composition and homogeneous. assigned for Further,the first point the strength of the experimental results obtained plan for (Figure compositions 8a,b) show 1, 6a andgood 7 bond and 10,be- i.e., with the tween the resinsame and R/A PET =glycolysate, 0.58, were comparedthe matrix withis smooth the test and mortar. homogeneous. Moreover, Further, the PE the waste (even at strength results10% obtained sand replacement for compositions rate) has 1, a6 favorableand 7 and effect 10, i.e., on with the studiedthe same mechanical R/A = 0.58, property. The were comparedflexural with strengththe test mortar. values obtained Moreover can, the be explainedPE waste (even by the at phenomenon 10% sand replace- of the occurrence of ment rate) hasinteractions a favorable between effect on the the waste studied particles mechanical and the property. polymer The matrix, flexural as shown strength in the paper by values obtainedMart canínez-L be óexplainedpez and theby teamthe phen [39].omenon Such interactions of the occurrence depend onof interactions the morphology of both between the wastethe resin particles and the and waste the particles.polymer Imagesmatrix, ofas theshown PE waste in the taken paper with by Martínez- a scanning microscope López and the(Figure team5 [39].) show Such that interactions the surface depend of the PEon the waste morphology is not smooth. of both The the surface resin roughness and the wasteobserved particles. for Images the PE of particles the PE waste facilitates taken mechanical with a scanning anchoring microscope between (Figure the polymer matrix 5) show that andthe surface the PE particlesof the PE and waste consequently is not smooth. an increase The surface in flexural roughness strength. observed Microphotographs for the PE particles(Figure facilitates8c–f) show mechanical waste particles anchoring that can between have a the fiber-like polymer effect. matrix Figure and9 the shows a crack PE particles andregion consequently in the composition an increase 5 where in flexural it is possible strength. to Microphotographs see a PE particle that (Figure may be behaving 8c–f) show wastelike aparticles fiber. With that acan higher have proportion a fiber-like of effect. waste Figure and a 9 lowershows resin a crack to aggregateregion ratio, the in the compositioncoating 5 ofwhere the aggregate it is possible particles to see bya PE the particle resin may that be may impeded, be behaving resulting like ina some voids fiber. With a thathigher may proportion provide a of place waste for and crack a lower initiation resin during to aggregate mechanical ratio, tests. the coating of the aggregate particles by the resin may be impeded, resulting in some voids that may provide a place for crack initiation during mechanical tests.

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(a) (b)

(e) (f)

Figure 8. BackscatteredFigure 8. Backscattered electron (BSE) electron and Secondary (BSE) and electron Secondary (SE) electron images (SE)of mortar images samples of mortar marked samples in the marked experiment plan as: 1 (BSE—(a), SE—(b)), 10 (BSE—(c,e) ; SE—(d,f)), PE—polyethylene waste, S—quartz sand. in the experiment plan as: 1 (BSE—(a), SE—(b)), 10 (BSE—(c,e) ; SE—(d,f)), PE—polyethylene waste, S—quartz sand.

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Figure 9. SEM-SE micrographs of the Composition 5—PE: PE waste. FigureFigure 9.9. SEM-SESEM-SE micrographsmicrographs ofof thethe CompositionComposition 5—PE:5—PE: PEPE waste.waste.

As it can be observedAsAs it canin Figure be observed observed 10, the in ina dditionFigure Figure 10,of 10 glycolysate,the the addition addition and of of glycolysatePE glycolysate waste signifi- and and PE PEwaste waste signifi- sig- cantly improvesniﬁcantlycantly the plastic improves improves properties the theplastic of plasticmortars properties properties compared of mortars of to mortars the compared control compared samples. to the to control How- the control samples. samples. How- ever, with a higherHowever,ever, degree with with ofa higher sand a higher substituti degree degree ofon sand ofwith sand substituti PE substitution wasteon agglomerate with with PE waste PE (10 waste agglomeratevol.%), agglomerate the (10 (10vol.%), vol.%), the plasticity improvementtheplasticity plasticity is improvementalready improvement at the isexpense already is already of at strength the at theexpense expense reduction. of strength of strength reduction. reduction.

Figure 10. Load vs. displacement curves for flexural strength. Figure 10. Load vs.Figure displacement 10. Load curves vs. displacement for flexural curves strength. for flexural strength. The plasticity was evaluated by displacement testing at maximum load and the results The plasticity wasThe evaluated plasticity by was displacement evaluated by testing displacement at maximum testing load at andmaximum the re- load and the results are shown arein Figure shown 11. in It Figure reveals 11 a. sign It revealsificant improvement a significant improvementin the flexural inductility the flexural ductility ofsults epoxy are shown composites in Figure as the 11. degree It reveals of sanda sign substitutionificant improvement with PE in waste the flexural increases. ductility Both of epoxy compositesof epoxy as the composites degree of as sand the degreesubstitution of sand with su PEbstitution waste increases.with PE waste Both increases. Both types of modifierstypes improve of modifiers the ductility improve and lead the to ductility less brittle and failure lead to of lessmortars. brittle In failureFigure of mortars. In Figuretypes of 10 modifiersfor the control improve sample, the ductility the curve and is lead clearly to less terminated, brittle failure while of mortars. the other In curves Figure 10 for the control10 sample, for the controlthe curve sample, is clearly the terminated,curve is clearly while terminated, the other curveswhile the behave other curves behave differently, whichbehave is due differently, to the fact whichthat ev isen due when to thethe factscratching that even occurs when and the the scratching maxi- occurs and thedifferently, maximum which force is isdue recorded, to the fact the that samples even dowhen not the disintegrate, scratching but occurs remain and somehowthe maximum force is recorded, the samples do not disintegrate, but remain somehow connected connectedmum force by is therecorded, PE waste the particles. samples A do similar not disintegrate, behavior of but the samplesremain somehow was observed connected in the by the PE waste particles. A similar behavior of the samples was observed in the study studyby the performed PE waste particles. by Martínez-L A similarópez behavi and theor team of the [39 samples] for polyester was observed mortars in containing the study performed by Martínez-Lópezperformed by Martínez-Lópezand the team [39] and for the polyester team [39] mortars for polyester containing mortars PET containing PET

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waste such as granules, tire rubber, and polycarbonate, and in the study of Ribeiro et al. [40]. Studies by ReisPET and waste Carneiro such as also granules, confirmed tire rubber, that shredded and polycarbonate, PET waste and particles in the study con- of Ribeiro et al. [40]. Studies by Reis and Carneiro also confirmed that shredded PET waste particles tribute to a significant increase in the elastic modulus of epoxy mortars [29]. However, contribute to a significant increase in the elastic modulus of epoxy mortars [29]. However, these authors didthese not achieve authors such did not high achieve flexural such strength high flexural values strength as described values as in described this paper. in this paper. Also, the displacementAlso, the at displacementultimate flexural at ultimate strength flexural obtained strength by obtainedour team by is ourmuch team higher is much higher (1.19–1.58 mm) than(1.19–1.58 that reported mm) than by that Ma reportedrtínez-López by Mart andínez-L theó pezteam and (0.83–0.89 the team mm). (0.83–0.89 mm).

Figure 11. Displacement at ultimate flexural strength depending on the PE waste content, designated at experiment plan Figure 11. Displacement at ultimate flexural strength depending on the PE waste content, desig- points numbered 1, 6, and 10. nated at experiment plan points numbered 1, 6, and 10. Using the experiment planning method made it possible to find a response surface Using the experimentfunction that planning matched method the results made of the it flexuralpossible strength to find measurements. a response surface In this case, the function that matchedfunction the with results the generalof the flexur Formulaal strength (1) took the measurements. form of (2): In this case, the

function with the general Formula (1) took the form of (2): 2 ff = 14.08 − 0.06(%PE) + 73.55(R/A) − 67.01(R/A) (2) ff = 14.08 − 0.06(%PE) + 73.55(R/A) − 67.01(R/A)2 (2) We can see that the value of the fc function depends linearly on the percentage of We can see thatPE wastethe value content of the and fc its function influence depends is small aslinearly evidenced on the by thepercentage−0.06 coefficient. of PE On the waste content andother its influence hand, the is influence small as of evidenced the second by input the variable, −0.06 coe theffi R/Acient. ratio, On onthe the other values of the hand, the influenceff function of the second is described input byvariab a quadraticle, the R/A relationship, ratio, on butthe values in this caseof the the ff influencefunc- of the tion is described byquadratic a quadratic factor relationship, is small because but thein this values case of the the influence R/A ratio of are the fractional. quadratic The spatial factor is small becauseand contour the values plot of of this the function R/A ratio is shown are fractional. in Figure 12 The. The spatial shape and of these contour plots confirms that the flexural strength is significantly influenced by both input variables, namely the plot of this function is shown in Figure 12. The shape of these plots confirms that the flex- PE waste content and the resin/aggregate ratio. The function has a maximum, which ural strength is significantlyis located in influenced the vicinity by of both the predefined input variables, center namely of the experimental the PE waste plan, con- marked as tent and the resin/aggregatepoints numbered ratio. 6 andThe 7functi (the compositeon has a maximum, samples corresponding which is located to these in plan the points are vicinity of the predefinedcharacterized center by 5%of PEthe waste experimental content and plan, R/A marked equal to as 0.58). points This numbered confirms that 6 the range and 7 (the compositeof variation samples of corresponding the input data was to these correctly plan selected. points are characterized by 5% PE waste content and R/A equal to 0.58). This confirms that the range of variation of the input data was correctly selected.

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(a) (b)

Figure 12. Three-dimensional (a) and contour (b) plot of the response surface for flexural strength. (a) (b)

Figure3.2.Figure Compressive 12. Three-dimensional 12. Three-dimensional Strength (a) ( aand) and contour contour ( (bb)) plot plot ofof thethe response response surface surface for for flexural flexural strength. strength. Mean values of compressive strength and standard deviations of these parameters 3.2. Compressive Strength calculated for3.2. each Compressive composition Strength are shown in Figure 13. The highest results of 107.2 MPa, 101.4 MPa and 94.7Mean MeanMPa values valueswere ofobtained of compressive compressive for samples strengthstrength with and and 0%, standard standard 5%, and deviations deviations 10% of of PE these of waste these parameters parameters content, respectively,calculatedcalculated at for R/A for each eachratio composition composition of 0.58. Three-dimensional are shownshown in in Figure Figure and 13 13..contour The The highest highest plots results of results the of ap- 107.2 of 107.2 MPa, MPa, 101.4 MPa and 94.7 MPa were obtained for samples with 0%, 5%, and 10% of PE waste proximation 101.4function MPa (Figure and 94.7 14) MPaconfirm were the obtained fact of decreasing for samples compressive with 0%, 5%, strength and 10%val- of PE waste content, respectively, at R/A ratio of 0.58. Three-dimensional and contour plots of the content, respectively, at R/A ratio of 0.58. Three-dimensional and contour plots of the ap- ues with increasingapproximation amount of function PE waste (Figure adde 14d.) However, confirm the even fact ofwith decreasing 10% substitution compressive of strength sand by PE wasteproximationvalues (10 point with function increasing of the experiment(Figure amount 14) of confirmplan), PE waste the the added. obtained fact of However, decreasing strength even value, compressive with equal 10% substitution to strength val- 94.7 MPa, is uesstillof withat sand a veryincreasing by PE high waste level amount (10 and point of differs ofPE the waste experimentfrom adde the d.result plan), However, obtained the obtained even for with strength the control10% value, substitution equal of sample (the valuesandto 94.7byof 97.0PE MPa, waste MPa is still corresponding(10 at point a very of high the to levelexperiment the and control differs plan), sample from the the is obtainedmarked result obtained in strength Figure for the13value, control equal to with a horizontal,94.7sample MPa, dashed (theis still line value at) only ofa very 97.0 by MPa high2.3 correspondingMPa. level All and the differs compressive to the from control the samplestrength result is obtained markedvalues inde- forFigure the 13control termined forsample thewith test (the a composite horizontal, value of samples dashed97.0 MPa line) are corresponding onlysignificantly by 2.3 MPa. tohigher the All control than the compressive those sample obtained is marked strength by in values Figure 13 Reis et al. forwith epoxydetermined a horizontal, mortars for modified the dashed test composite line with) only shredded samples by 2.3 PET areMPa. significantly waste All the [29]. compressivehigher Despite than the strength those high obtained values de- by Reis et al. for epoxy mortars modified with shredded PET waste [29]. Despite the degree of crystallinitytermined forand the roughness test composite of PE particles,samples arethey significantly are more susceptible higher than to thosecom- obtained by high degree of crystallinity and roughness of PE particles, they are more susceptible Reis et al. for epoxy mortars modified with shredded PET waste [29]. Despite the high pressive loadingto than compressive quartz sand, loading which than may quartz be sand,the reason which for may the be decrease the reason in forcompres- the decrease in sive strength.degree compressive of crystallinity strength. and roughness of PE particles, they are more susceptible to compressive loading than quartz sand, which may be the reason for the decrease in compressive strength.

Figure 13. The compilation of the mean compressive strength values for the samples corresponding to each point of the Figure 13. The compilation of the mean compressive strength values for the samples correspond- experimental plan. ing to each point of the experimental plan. Figure 13. The compilation of the mean compressive strength values for the samples corresponding to each point of the experimental plan.

Materials 2021, 14, x FOR PEER REVIEW 13 of 18

The response surface function of general Formula (1) adjusted to the results of compressive strength measurements took the form of (3):

fc = 25.34 − 1.77(%PE) + 284.51(R/A) − 253.1(R/A)2 + 2.19(%PE)(R/A) (3) Materials 2021, 14, x FOR PEER REVIEW 13 of 18 Comparing this formula with relation (2) describing the flexural strength, one can notice that there is an additional interaction component, since the last component of the Materials 2021, 14, 2203 sum contains the product of the %PE waste content and the R/A ratio, and the13 resulting of 18 The responsevalue surfaceof the fc functionfunction dependsof general on Foit. rmulaIf one of(1) these adjusted parameters to the increasesresults of and com- the other pressive strengthone decreases,measurements the influence took the of form this ofcomponent (3): on the final value of the compressive strength is minor. fc = 25.34 − 1.77(%PE) + 284.51(R/A) − 253.1(R/A)2 + 2.19(%PE)(R/A) (3) Comparing this formula with relation (2) describing the flexural strength, one can notice that there is an additional interaction component, since the last component of the sum contains the product of the %PE waste content and the R/A ratio, and the resulting value of the fc function depends on it. If one of these parameters increases and the other one decreases, the influence of this component on the final value of the compressive strength is minor.

(a) (b)

FigureFigure 14. Three-dimensional 14. Three-dimensional (a) (anda) and contour contour (b ()b plot) plot of of the the response response surfacesurface for for compressive compressive strength. strength.

3.3. BulkThe Density response surface function of general Formula (1) adjusted to the results of compressive strength measurements took the form of (3): The mean volumetric density values along with the standard deviation calculated at 2 each point infc the= 25.34 experimental− 1.77(%PE) pl +an 284.51(R/A) are summarized− 253.1(R/A) in Figure+ 2.19(%PE)(R/A)15. (3)

Comparing this formula with relation (2) describing the flexural strength, one can notice thatthere is an additional interaction component, since the last component of the sum contains the product of the %PE waste content and the R/A ratio, and the resulting (a) (b) value of the fc function depends on it. If one of these parameters increases and the other one Figure 14. Three-dimensional (a) and decreases,contour (b the) plot influence of the ofresponse this component surface onfor the compressive final value ofstrength. the compressive strength is minor.

3.3. Bulk Density 3.3. Bulk Density The mean volumetricThe mean density volumetric values density along values with along the standard with the standard deviation deviation calculated calculated at at each point in the eachexperimental point in the pl experimentalan are summarized plan are summarized in Figure in 15. Figure 15.

Figure 15. The compilation of mean volumetric density values calculated for each point of the experimental plan.

It can be seen from Figure 16a,b that an increase in the proportion of resin and PE waste content in the epoxy composites reduces the bulk density of these materials.

FigureFigure 15. 15.The The compilation compilation of mean of volumetricmean volumetric density values density calculated values for calculated each point of for the each experimental point of plan. the experimental plan. It can be seen from Figure 16a,b that an increase in the proportion of resin and PE waste content in the epoxy composites reduces the bulk density of these materials. It can be seen from Figure 16a,b that an increase in the proportion of resin and PE waste content in the epoxy composites reduces the bulk density of these materials.

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(a) (b)

FigureFigure 16. Three-dimensional 16. Three-dimensional (a) ( aand) and contour contour ( (bb)) plot plot of responseresponse surface surface for for bulk bulk density. density.

3 3 TheThe volumetric volumetric density density values values rangedranged from from 1.79 1.79 g/cm g/cmto3 2.08to 2.08 g/cm g/cm. The3. The response response surface function of the general Formula (1) adjusted to the volumetric density results took surface function of the general Formula (1) adjusted to the volumetric density results took the form of (4): the form of (4): 2 db = 2.02 − 0.02(%PE) + 0.59(R/A) − 1.04(R/A) (4) Analyzing the parametersdb = 2.02 of− 0.02(%PE) this equation, + 0.59(R/A) again for this− 1.04(R/A) feature, it2 can be seen that (4) theAnalyzing volumetric the density parameters value of of the this samples equation, is more again influenced for this by feature, the resin it tocan aggregate be seen that ratio (R/A) than by the degree of substitution of sand by PE waste (coefficient −0.02). the volumetric density value of the samples is more influenced by the resin to aggregate ratio3.4. (R/A) Multiple than Output by the Optimization: degree of subs A Responsetitution Utility of sand Profile by PE waste (coefficient −0.02). Using the utility profile module of the Statistica program, optimization of all three 3.4. designatedMultiple Output mortar Optimizat propertiesion: (output A Response quantities) Utility was Profile performed for the model under consideration.Using the utility Resin profile mortars module should of be the durable Statistica and atprogram, the same optimization time relatively of light, all three designatedi.e., with mortar a low bulk properties density. (output Mortar quanti compositionties) was should performed be chosen for in the such model a way under as to con- optimize the overall utility of the composite. For the obtained second-order response sideration. Resin mortars should be durable and at the same time relatively light, i.e., with surface models, the program calculated the values of the input quantities corresponding to a lowthe bulk minimum density. and Mortar maximum composition values of the should given surfacebe chosen (i.e., in the such critical a way values as ofto theoptimize given the overallsurface utility together of the with composite. the corresponding For the obta eigenvaluesined second-order and eigenvectors response thus surface describing models, the theprogram curvature calculated and orientation the values of the of responsethe input surface). quantities For corresponding the three properties to the marked minimum and(functions: maximum ff ,values fc, db), of a utility the given function surface was defined(i.e., the that critical reflects values the most of the desirable given valuessurface to- getherof the with output the corresponding quantities and the eigenvalues weight of eachand ofeigenvectors these quantities thus fordescribing the total utility.the curva- tureThis and approach orientation made of it the possible response to plot surface) the profiles. For ofthe the three utility properties function (calculatedmarked (functions: from the values of the approximated output quantities) by assigning to each possible value ff, fc, db), a utility function was defined that reflects the most desirable values of the output quantitiesof the attribute and the under weight study of each a value of inthese the intervalquantities [0, 1] for indicating the total satisfaction utility. This with approach the outcome at that level. A utility of 0.0 (undesirable) was assigned to approximate values for made it possible to plot the profiles of the utility function (calculated from the values of flexural strength below 28.47 MPa, compressive strength below 81.9 MPa, and bulk density the aboveapproximated 2.084 g/cm output3, respectively. quantities) In turn, by assign the utilitying ofto 1.0 each (highly possible desirable) value was of assignedthe attribute underto approximate study a value values in the of flexuralinterval strength [0, 1] indicating above 36.3 satisfaction MPa, compressive with the strength outcome above at that level.110.2 A MPautility, and of a bulk0.0 (undesirable) density below 1.782was g/cmassigned3. Whereas to approximate the utility increasing values linearlyfor flexural strength0.0–1.0 below was defined 28.47 MPa, for flexural compressive strength fromstrength 28.47 below MPa to 81.9 36.3 MPa, compressiveand bulk density strength above 2.084from g/cm 81.93, MParespectively. to 110.2 MPa In turn, and for the decreasing utility of values 1.0 (highly of the volume desirable) density was function assigned from to ap- 3 proximatethe range values 2.084–1.782 of flexural g/cm strength. The search above for 36.3 the compositionMPa, compressive of the mortar strength that above would 110.2 provide the most desirable composite properties was based on the general optimization of MPa, and a bulk density below 1.782 g/cm3. Whereas the utility increasing linearly 0.0–1.0 functions. The results are shown in Figure 17. was defined for flexural strength from 28.47 MPa to 36.3 MPa, compressive strength from 81.9 MPa to 110.2 MPa and for decreasing values of the volume density function from the range 2.084–1.782 g/cm3. The search for the composition of the mortar that would provide the most desirable composite properties was based on the general optimization of functions. The results are shown in Figure 17.

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Figure 17. Profiles of approximated values and utility for the method of general function optimiza- FigureFigure 17. 17.Profiles Profilestion. of of approximated approximated values values and and utility utility for for the the method method of of general general function function optimization. optimization. OptimumOptimum value value of of flexural flexural and and compressiv compressivee strength strength and and volumetric volumetric density density were were Optimumobtainedobtained value offor forflexural the the percentage percentage and compressiv of of PE PE wastee waste strength addi additiontion and at volumetric at 5 5vol.% vol.% and anddensity resin resin towere to aggregate aggregate ratio ratio obtained for (R/A)the(R/A) percentage equals equals 0.58. 0.58.of PEThe The waste values values addi of ofcompressivetion compressive at 5 vol.% strength, strength,and resin flexural flexural to aggregate strength, strength, ratio and and volumetric volumetric 3 (R/A) equals density0.58.density The at at valuesthe the point point of ofcompressive of maximum maximum strength,utility utility are are flexural 101.2 101.2 MPa,MPa, strength, 34.8 34.8 MPa, MPa,and andvolumetric and 1.969 1.969 g/cm g/cm3, respec-, respec- density at thetively. pointtively. Theof The maximum total total utility utility utility index index are reached reached 101.2 MPa, 0.59, 0.59, 34.8 conf confirming MPa,irming and thethe 1.969 relativelyrelatively g/cm3 close,cl respec-ose locationlocation of of critical crit- tively. The totalicalpoints utilitypoints for index for all threeall reached three output output 0.59, variables. conf variables.irming The theThe spatial relatively spatial and contourand close contour location plot ofplot of total crit-of utilitytotal utility is shown is ical points forshown inall Figure three in Figure 18output. 18. variables. The spatial and contour plot of total utility is shown in Figure 18.

(a) (b) (a) (b) FigureFigure 18. 18. Three-dimensionalThree-dimensional (a ()a and) and contour contour (b ()b plot) plot of of total total utility. utility. Figure 18. Three-dimensional (a) and contour (b) plot of total utility. TheThe *-marked *-marked point point in in Figure Figure 1818b,b, indicatingindicating the the centrality centrality of of the the utility utility index index in thein the area The *-markedareadescribed described point byin theFigureby inputthe 18b,input variables, indicating variables, confirms the confir centrality thatms the that experimentalof the the experimental utility planindex was planin the correctly was correctly defined. area describeddefined. by the input variables, confirms that the experimental plan was correctly defined.

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4. Conclusions Based on the research, it can be concluded that: • Epoxy mortars can be successfully modified with such plastic wastes as poly(ethylene terephthalate) and polyethylene. • Both the addition of the glycolysate obtained on the basis of PET waste and the agglomerate of PE waste influence the plasticity of the epoxy mortars obtained and improve the flexural strength. • Flexural strength of the mortars at 5% substitution of sand with PE waste increased by 6.6% and at 10% substitution it was comparable with the values obtained for the mortars without the waste additive and amounted to 33.3 MPa. • The addition of PE waste agglomerate slightly decreased the compressive strength of epoxy mortars, but even at 10% substitution of sand with PE waste the strength remained at a very high level of 94.7 MPa. • Applying the multiple output optimization, it was shown that the most advantageous values of strength parameters and bulk density could be simultaneously obtained for mortars characterized by resin to aggregate ratio (R/A) equaling 0.58 and PE waste content at the level of 5 vol.%. • Environmental concerns cast a shadow over the production, use, and consumption of the plastic. The proposed modification of epoxy mortars with plastic waste may be a way to solve them in accordance with the principles of modern, low-carbon, resource-, and energy-efficient economy, and lead to the implementation of goals adopted in the plans of sustainable development of EU countries by 2030.

Author Contributions: Conceptualization, B.D.; methodology, B.D. and G.J.B.S.; software, B.D. and G.J.B.S.; validation, B.D. and G.J.B.S.; formal analysis, B.D. and G.J.B.S.; investigation, B.D.; resources, B.D. and G.J.B.S.; data curation, B.D.; writing—original draft preparation, B.D.; writing—review and editing, B.D.; visualization, B.D. and G.J.B.S.; supervision, B.D.; project administration, B.D.; funding acquisition, B.D. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. 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.

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